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Charge Redistribution Mechanisms in SnSe2 Surfaces Exposed to Oxidative and Humid Environments and Their Related Influence on Chemical Sensing
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  • Gianluca D’Olimpio
    Gianluca D’Olimpio
    Department of Physical and Chemical Sciences, University of L’Aquila, via Vetoio, 67100 L’Aquila, AQ, Italy
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  • Francesca Genuzio
    Francesca Genuzio
    Elettra-Sincrotrone S.C.p.A., S.S. 14-km 163.5 in AREA Science Park, 34149 Trieste, Italy
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    Orcidhttp://orcid.org/0000-0003-0699-2525
  • Tevfik Onur Menteş
    Tevfik Onur Menteş
    Elettra-Sincrotrone S.C.p.A., S.S. 14-km 163.5 in AREA Science Park, 34149 Trieste, Italy
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  • Valentina Paolucci
    Valentina Paolucci
    Department of Industrial and Information Engineering and Economics, University of L’Aquila, Via G. Gronchi 18, I-67100 L’Aquila, Italy
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    Orcidhttp://orcid.org/0000-0003-0641-7926
  • Chia-Nung Kuo
    Chia-Nung Kuo
    Department of Physics, National Cheng Kung University, 1 Ta-Hsueh Road, 70101 Tainan, Taiwan
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  • Amjad Al Taleb
    Amjad Al Taleb
    Departamento de Física de la Materia Condensada, Universidad Autónoma de Madrid, 28049 Madrid, Spain
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  • Chin Shan Lue
    Chin Shan Lue
    Department of Physics, National Cheng Kung University, 1 Ta-Hsueh Road, 70101 Tainan, Taiwan
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  • Piero Torelli
    Piero Torelli
    Elettra-Sincrotrone S.C.p.A., S.S. 14-km 163.5 in AREA Science Park, 34149 Trieste, Italy
    Consiglio Nazionale delle Ricerche (CNR)-Istituto Officina dei Materiali (IOM), Laboratorio TASC in Area Science Park S.S. 14 km 163.5, 34149 Trieste, Italy
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  • Daniel Farías
    Daniel Farías
    Departamento de Física de la Materia Condensada, Universidad Autónoma de Madrid, 28049 Madrid, Spain
    Instituto ‘Nicolás Cabrera’, Universidad Autónoma de Madrid, 28049 Madrid, Spain
    Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, Spain
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    Orcidhttp://orcid.org/0000-0002-8537-8074
  • Andrea Locatelli
    Andrea Locatelli
    Elettra-Sincrotrone S.C.p.A., S.S. 14-km 163.5 in AREA Science Park, 34149 Trieste, Italy
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    Orcidhttp://orcid.org/0000-0002-8072-7343
  • Danil W. Boukhvalov
    Danil W. Boukhvalov
    College of Science, Institute of Materials Physics and Chemistry, Nanjing Forestry University, Nanjing 210037, P. R. China
    Theoretical Physics and Applied Mathematics Department, Ural Federal University, Mira Street 19, 620002 Ekaterinburg, Russia
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    Orcidhttp://orcid.org/0000-0002-2286-3443
  • Carlo Cantalini*
    Carlo Cantalini
    Department of Industrial and Information Engineering and Economics, University of L’Aquila, Via G. Gronchi 18, I-67100 L’Aquila, Italy
    *Email: [email protected]
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  • Antonio Politano*
    Antonio Politano
    Department of Physical and Chemical Sciences, University of L’Aquila, via Vetoio, 67100 L’Aquila, AQ, Italy
    CNR-IMM Istituto per la Microelettronica e Microsistemi, VIII strada 5, I-95121 Catania, Italy
    *Email: [email protected]
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    Orcidhttp://orcid.org/0000-0002-4254-2102
Open PDFSupporting Information (1)

The Journal of Physical Chemistry Letters

Cite this: J. Phys. Chem. Lett. 2020, 11, 21, 9003–9011
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https://doi.org/10.1021/acs.jpclett.0c02616
Published October 9, 2020

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Abstract

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Tin diselenide (SnSe2) is a van der Waals semiconductor, which spontaneously forms a subnanometric SnO2 skin once exposed to air. Here, by means of surface-science spectroscopies and density functional theory, we have investigated the charge redistribution at the SnO2–SnSe2 heterojunction in both oxidative and humid environments. Explicitly, we find that the work function of the pristine SnSe2 surface increases by 0.23 and 0.40 eV upon exposure to O2 and air, respectively, with a charge transfer reaching 0.56 e–/SnO2 between the underlying SnSe2 and the SnO2 skin. Remarkably, both pristine SnSe2 and defective SnSe2 display chemical inertness toward water, in contrast to other metal chalcogenides. Conversely, the SnO2–SnSe2 interface formed upon surface oxidation is highly reactive toward water, with subsequent implications for SnSe2-based devices working in ambient humidity, including chemical sensors. Our findings also imply that recent reports on humidity sensing with SnSe2 should be reinterpreted, considering the pivotal role of the oxide skin in the interaction with water molecules.

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After the advent of graphene, (1−3) van der Waals semiconductors are attracting considerable attention, owing to their application capabilities that are often complementary to those of graphene, (4−6) with the subsequent prospect of novel disruptive technologies in different technological areas. (4,7,8) This class of materials is characterized by weak van der Waals bonds between layers enabling their cleavage by mechanical (9) and liquid-phase (10) exfoliation. Among van der Waals semiconductors, several materials show serious drawbacks, limiting their technological potential. Specifically, MoS2 and WS2 display intrinsic electron mobility as low as some tens of cm2 V–1 s–1 at 300 K; (11) black phosphorus rapidly degrades in air due to surface oxidation; (12,13) GaSe is affected by both environmental and laser-induced degradation; (14,15) and PdSe2 (16) has a limited commercial potential, due to the constantly growing price of Pd ($2000–2400/oz), which nearly doubled from 2019 to 2020.
Tin diselenide (SnSe2) is a van der Waals semiconductor with a CdI2-type crystal structure, (17) belonging to the P3̅m1 space group, with tin (Sn) atoms interweaved between two hexagonally packed atomic layers of selenium (Se) (see the atomic structure in Figure S1a,b). (18,19) SnSe2 shows its high intrinsic electron mobility (462.6 cm2 V–1 s–1 at 300 K (20)) and ultralow thermal conductivity (3.82 W m–1 K–1 (20)). It displays pressure-induced periodic lattice distortion, and moreover, it enables novel device functionalities being a phase change memory material; i.e., its atomic structure can reversibly switch from amorphous to crystalline upon laser heating, with consequent remarkable variations in optical reflectivity. Because of these peculiarities, SnSe2 has high application capabilities in numerous fields, including photocatalysis, (21,22) superconductivity, (23,24) Li-ion (18,25,26) and Na-ion (18,26,27) batteries, photodetection, (28) saturable absorbers for eye-safe lasers, (29) and thermoelectricity. (30−32) Furthermore, SnSe2 was used as a co-catalyst for hydrogen evolution reaction. (33)
However, all Sn-based chalcogenides are usually affected by rapid surface degradation with the emergence of tin oxide phases. (34,35) Additionally, the oxidation of starting element Sn during the synthesis can also influence the transport properties of the resulting crystal. Therefore, technological exploitation of Sn-based chalcogenides remains particularly challenging. Especially, the stability of SnSe2-based devices in the ambient atmosphere is related to the chemical reactivity of its surface.
Recently, it has been shown that, though stoichiometric SnSe2 shows outstanding chemical stability under ambient conditions, the presence of Se vacancies drastically affects surface chemical reactivity. (36) The SnSe2–x surface is transformed into SnO2 skin-terminated SnSe2, with the thickness of the SnO2 skin estimated to be subnanometric. (36) Unexpectedly, the self-assembled heterostructure formed by exploiting the natural interaction with air is particularly appropriate for ultrasensitive gas sensing, as demonstrated for NO2 and H2 with sensitivities of (1.06 ± 0.03) and (0.43 ± 0.02) ppm–1. (36) Remarkably, such sensors are effective under dry air conditions, while previously devised SnSe2 sensors used N2 as the carrier gas. (37,38) Moreover, the NO2 sensitivity of the SnO2–SnSe2 heterostructure is significantly higher compared to those of sensors based on other van der Waals semiconductors and their heterostructures. (39,40)
Remarkably, the oxide skin plays a pivotal role in NO2 and H2 sensing, congruently with the abundant literature on SnO2-based sensors. (41−50) The modulation of resistivity upon gas adsorption is strictly connected to charge distribution in the sensing material, ultimately related to the formation of surface dipoles at the SnO2–SnSe2 heterojunction arising from local charge redistribution. Thus, to understand the conduction mechanism ruling chemical sensing, it is crucial to shed light on charge redistribution at the SnO2–SnSe2 heterostructure by measuring work-function changes. Furthermore, sensing experiments in ref (36) were carried out in dry air; thus, stability in a humid environment remains unexplored, although real conditions mandatorily require sensors to work in a changing humidity background (51,52) (not only humidity sensors (53,54)). Despite the relevance of the influence of the humid environment for practical applications, surprisingly it has been scarcely investigated, although previous reports indicated a decrease in resistance under exposure to a humid atmosphere, (51,52) which represents an unambiguous fingerprint that H2O behaves as a reducing gas in the interaction with the SnO2 surface.
In addition, the interaction with water is relevant also for understanding the stability of any other SnSe2-based (opto)electronic device (55) working in ambient humidity, as well as the eventual environmental doping effects in transport properties. (56) Actually, recently different groups have reported that SnSe2 is extremely sensitive to humid environments, (37,38,57) with the possibility of using it in humidity-sensing devices.
Here, we unveil the surface properties of SnSe2 single crystals and their modifications in oxidative and humid environments by means of surface-science experiments and density functional theory (DFT). Definitely, surface oxidation induces an increase in the work function of 0.4 eV, owing to the charge transfer between the substrate and the SnO2 skin of 0.56 e– per SnO2 unit. As opposed to previous reports, (37,38,57) the pristine SnSe2 surface is inert to water at room temperature, while the SnO2–SnSe2 heterostructure displays notable sensitivity to humidity.
The presence of the SnO2 skin in the SnSe2 surface exposed to oxidative environments was ensured by both microscopic evidence from low-energy electron microscopy (LEEM) (Figure S5) and vibrational experiments from high-resolution electron energy loss spectroscopy (HREELS) (Figure S6).
The analysis of the variation of work function ΔΦ probed by LEEM could provide important insights into charge redistribution arising from surface oxidation (Figure 1a), as the total reflectivity threshold in electron backscattering (the MEM–LEEM transition, where MEM stands for mirror electron microscopy) represents a direct measurement of the variation of the surface potential. (58) Explicitly, we find ΔΦ to be 0.23 eV for the SnSe2 surface modified by exposure to 700 L of O2 at room temperature, while air exposure for 15 min induces a further shift in the work function, resulting in a total increase of 0.40 eV. The observed value of ΔΦ can be explained by considering the activation of surface dipoles, due to charge transfer at the interface from substrate to adsorbed oxygen atoms. The electronegativity of oxygen makes its adsorption generally associated with a charge transfer from the substrate to the adsorbate layer, with a subsequent increase in the work function. (59) Considering that the work function of the pristine SnSe2 single crystal is ∼4.6 eV, (60) while that of SnO2 is known to be ∼4.9 eV (61) (although its value can be tuned by reduction reactions (62)), both the sign and the magnitude of the experimental value of ΔΦ are consistent with surface oxidation, involving the formation of a subnanometric SnO2 skin. We can infer that previous experimental studies reporting a work function of SnSe2 of (5.0 ± 0.1) eV (63,64) could be affected by surface oxidation, which generates a self-assembled SnO2–SnSe2 heterostructure with an increased work function. To verify this statement, we calculated ΔΦ for the oxidation of the pristine SnSe2 surface, finding a value of 0.52 eV in qualitative agreement with experimental measurements. We also note that, in the air-exposed sample, variations in the I–V curve associated with electron diffraction from a surface with crystalline order (65) are suppressed, due to the formation of a disordered surface oxide phase.

Figure 1

Figure 1. (a) LEEM I–V curves at the MEM–LEEM transition for the as-cleaved sample (black), after a dose of 700 L of O2 (blue), and after air exposure for 15 min (pink). The shift of the MEM–LEEM transition, characterized by the sharp decrease in intensity, indicates an oxidation-induced modification of the surface potential. (b) Changes in charge density after the formation of the interface between the SnSe2 substrate and SnO2 skin. Sn, Se, and O atoms are represented as dark blue, light green, and red balls, respectively.

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Complementary information about the electronic properties of the SnO2–SnSe2 heterostructure was achieved by comparing the surface excitation spectrum probed by electron energy loss spectroscopy (EELS) with the theoretical density of states (DOS) (section S4 of the Supporting Information). The impact of defects on the DOS is assessed in section S5.
To estimate the amount of charge transfer between the SnSe2 substrate and the SnO2 skin, we calculated the charge density distribution of (i) one SnO2 layer over two layers of SnSe2 (to model the SnO2–SnSe2 heterostructure), (ii) a free-standing SnO2 single unit, and (ii) a bilayer of SnSe2. Then, we calculated the difference between the charge densities of the whole SnO2–SnSe2 interface and those one of its components (single SnO2 unit and bilayer SnSe2). The obtained charge density difference (Figure 1b) illustrates charge redistribution following the formation of the SnO2–SnSe2 interface. The integration of the charge density difference along the c axis provides information regarding the charge transfer between the SnSe2 substrate and the SnO2 skin. Note that the formation of the SnO2–SnSe2 interface provides changes in the charge density difference in not only the outermost SnSe2 layer but also the subsurface area, namely the second SnSe2 layer. Definitely, the charge transfer is estimated to be 0.56 e– per SnO2 unit.
While the adsorption of O2 with further decomposition is energetically favorable on SnSe2 (negative values of ΔG and ΔHdec), as well as on SnSe1.88 and SnSe (Table 1), our theoretical model indicates that water does not adsorb on SnSe2. The energy cost for water adsorption is decreased in the presence of Se vacancies (SnSe1.88) down to ∼3 kJ/mol, although water adsorption (as well as decomposition) remains energetically unfavorable. Similarly, SnSe also shows outstanding chemical inertness toward water.
Table 1. Differential Enthalpies (ΔHads), Differential Gibbs Free Energies of Physisorption (ΔG), and Differential Enthalpies of Decomposition (ΔHdec) for Molecular Oxygen and Water on Pristine SnSe2, SnSe1.88, and SnSe Surfacesa
  physisorptiondecomposition
surfaceadsorbantΔHads (kJ/mol)ΔG (kJ/mol)ΔHdec (kJ/mol)
SnSe2O2–17.46–3.16–42.28 (−161.58/∼−40.2)
 H2O–13.2718.03220.91
SnSe1.88O2–37.58–26.28–135.67 (−99.05/–406.65)
 H2O–27.933.37175.61
SnSeO2–11.53–0.23–236.03 (−323.10/95.4)
 H2O–8.1223.1882.22
SnO2 skinH2O–119.70–106.67–121.31
a

For oxygen decomposition, the table also displays the differential enthalpy of the oxidation of the whole surface with formation of SnO and SnO2-like layers (in parentheses).

Considering that the yield of chemical reactions also depends on the probability of the interactions between reactants, we calculated Langmuir adsorption isotherms (Figure S12). Specifically, the combination of thermodynamic and kinetic calculations evidences that the largest part of the SnSex surface will be oxidized under experimental conditions (72% and 75% for SnSe2 and SnSe1.88, respectively).
On the contrary, the saturation coverage for water at room temperature is just 0.01 ML (with ML being monolayer) for SnSe2 and SnSe1.88, while the full coverage (1 ML) is reached upon exposing the SnO2 skin to only 5 × 10–3 L of H2O below 500 °C, thus evidencing the aptness of the SnO2–SnSe2 interface for ultrasensitive humidity sensing. The increase in temperature corresponds to a decrease in the sticking coefficient, with monolayer saturation reached at 0.05 and 10 L at 500 and 800 °C, respectively. Thus, the SnO2–SnSe2 heterostructure remains rather sensitive even at high operational temperatures.
Therefore, the SnO2–SnSe2 heterostructure shows superior chemical reactivity toward ambient species with respect to SnSe2. On the pristine SnSe2 surface, the local rearrangement of chemical bonds around each adsorbed water molecule is the origin of a redistribution of the charge density in the surface layer of SnSe2 with a charge transfer of 0.17 e– per water molecule (Figure 2a). Correspondingly, water adsorption on SnSe2 and SnSe1.88 surfaces is energetically unfavorable for temperatures above 124 and 264 K, respectively (Figure S11). Hence, we conclude that pristine SnSe2 is stable in a humid environment and, consequently, is unsuitable for humidity sensing, contrarily to conclusions in refs (37), (38), and (57) On the contrary, adsorption of H2O on the SnO2–SnSe2 heterostructure (Figure 2b) is energetically favorable even above room temperature (Figure S11). The values of charge transferred from H2O to the SnO2 skin are 0.43 and 0.30 e– for one and two H2O molecules per supercell, respectively. Correspondingly, DOS (Figure 2c) is modified with a direct correlation with the coverage of the adsorbate, hence proving the appropriateness for humidity sensing also at low concentrations of H2O.

Figure 2

Figure 2. Changes in charge density after adsorption of one water molecule on (a) SnSe2 and (b) SnO2 skin-terminated SnSe2. Panel c represents the DOS of SnO2 skin-terminated SnSe2 (black) and of the same system modified by the adsorption of one (red) and two (blue) water molecules. The Fermi level is set at 0. Panel d shows the response of the SnSe2–SnO2 heterostructure to 20% relative humidity (RH) at an operational temperature (OT) of 150 °C (note that the average residence time of the gas in the cell is approximately 10 min).

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Note that decomposition of a water molecule on the SnO2 skin-terminated SnSe2 is an exothermic process (see Table 1), although the energy gain from this process is moderate (−121.31 kJ/mol) and further water splitting is unfavorable, supporting the possible reversibility of the process.
The SnO2–SnSe2 heterostructure was tested as a humidity sensor (Figure 2d) at an operational temperature of 150 °C. Our devised humidity sensor exhibited (i) full recovery of the baseline resistance after water desorption and (ii) high sensitivity to water molecules, measured as the relative response (RR, the ratio between the resistance in dry air, Ra, and RH2O, the resistance in a humid environment), and an experimental limit of detection (LOD) in terms of relative humidity (RH) as low as 20% (Figure 2d).
Recently, different authors (37,38,57) have reported the outstanding performances of SnSe2 in humidity-sensing devices. Our findings elucidate the key role of the surface oxide skin in the interaction with a humid environment. On the contrary, in refs (37), (38), and (57), surface oxidation was not assessed; thus, the mechanism ruling humidity sensing discussed therein should be reinterpreted.
Theoretical results were validated by surface-science techniques. In particular, HREELS experiments on water-dosed Sn-based selenides (SnSe, SnSe1.4, SnSe1.7, and SnSe2) indicate the absence of chemisorbed water-derived species, as indicated by the lack of O–H stretching at 408–425 meV (molecular water) and 445–460 meV (hydroxyl groups) in spectra in Figure 3 (see ref (66) for more details). These findings are consistent with the positive differential Gibbs free energy of adsorption (corresponding to energetically unfavorable water adsorption) in Table 1. For the sake of comparison, we report in Figure 3 also vibrational data obtained after exposure to the same dose of H2O (105 L, with 1 L = 1 × 10–6 Torr s) at room temperature the surface of other metal chalcogenides, which instead enable the stable adsorption of water molecules (PtTe1.6) and hydroxyl groups (InSe).

Figure 3

Figure 3. HREELS spectra in the region of the O–H band acquired after exposure to 105 L of H2O at room temperature the surfaces of different Sn-based chalcogenides: SnSe2 (orange curve), SnSe1.7 (black), SnSe1.4 (green), and SnSe (blue). To provide a straightforward comparison, the figure also displays data for H2O-dosed InSe (red) and PtTe1.6 (brown) surfaces (105 L at room temperature). The impinging energy is 4 eV.

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The absence of reactivity toward water of Sn-based chalcogenides makes them suitable for catalysis (especially, photocatalytic water splitting (22) and hydrogen evolution reaction (67)) and drug delivery (68) (also considering that neither Sn nor Se is toxic). Congruently, SnSe2 was used as a co-catalyst in combination with TiO2 for hydrogen evolution reaction. (33)
Further information about the surface chemical bonds is gained by the inspection of core levels via X-ray photoelectron spectroscopy (XPS) experiments. Figure 4 shows the Sn-3d and Se-3d core levels of the SnSe2 single-crystal surface cleaved in ultrahigh vacuum and for the same surface modified by O2 and H2O dosage with a total dose of 105 L. The Sn-3d5/2 core level in the as-cleaved surface displayed a binding energy (BE) of 486.8 eV (Figure 4b). Congruently, the Se-3d5/2 core level had a single component at a BE of 54.1 eV, in agreement with previous results for SnSe2 (69) and with a shift of +0.4 eV compared to the case of SnSe. Surface treatments, i.e., 105 L of O2 and H2O exposure, induce only slight changes in Se-3d core levels. A novel doublet appeared in Se-3d (BE = 53.7 eV for 3d5/2), whose total spectral area is 5.4% (for O2 dosage) and 2.6% (for air exposure), arising from Se(0) segregation. (70) In particular, from the analysis of Se-3d core-level spectra (Figure 4c), we can infer the absence of O–Se–O bonds, which would have a BE of ∼59–60 eV. (71−73) Congruently, the intensity of the O-1s peak is especially small in SnSe2 exposed to both an oxidative and humid environment (Figure 4a); thus, we can evaluate the amount of oxygen to be <0.04 ML, due to a particularly weak sticking coefficient for oxygen adsorption at 300 K on SnSe2, with the O2 sticking coefficient being <10–5.

Figure 4

Figure 4. (a) O-1s, (b) Sn-3d, and (c) Se-3d core levels for the pristine surface of SnSe2 cleaved in situ under ultra-high-vacuum conditions and its alteration after exposure to oxidative (105 L of O2) and humid (105 L of H2O) environments at room temperature. The photon energy is 800 eV. We also report in each panel the corresponding spectrum for SnO2–SnSe2–x exposed to a humid environment at room temperature, with x estimated to be 0.29.

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On the contrary, we observed quite distinct peaks in SnO2–SnSe2–x exposed to a humid environment (outermost spectra in the various panels of Figure 4), and a Sn-3d doublet with a J = 5/2 component is present at a BE of 487.8 eV, due to SnO2 (relative amplitude of 54%), which is consistent with previous reports for this system. (74,75) Remarkably, no trace of O–Se–O bonds is present, as suggested by the lack of Se-3d components at 59–60 eV. (72) This result confirms theoretical expectations that Se is involved in only a metastable oxide phase, which represents a precursor for SnO2 formation. Nevertheless, a broad spectral component in Se-3d suggests a different oxidation state for Se. In particular, the peak at 55.0 eV is ascribed to Se2–, while that one at a higher BE should be attributed to Se–2+α (0 < α < 1). (76) The analysis of survey XPS spectra enables us to evaluate α as ∼0.145, corresponding to substoichiometric SnSe1.71. Congruently with the results in ref (36), oxidation is feasible only in substoichiometric SnSe2–x, while perfectly stoichiometric SnSe2 is robust in oxidative environments, thus evidencing the pivotal role of Se vacancies in surface oxidation.
The O-1s spectrum for the SnSe2 surface exposed to a humid environment shows new components arising from −OH groups (relative amplitude of 45%) and H2O (relative amplitude of 9%) at BEs of 531.6 and 533.6 eV, respectively. (77−79) We also exposed the SnO2–SnSe2 heterostructure to the humid environment, with the corresponding O-1s spectrum displaying the SnO2 component (relative amplitude of 50%) at a BE of 531.5 eV, (80,81) overlapped with the −OH component.
In conclusion, we investigated (i) the modifications of surface properties once pristine SnSe2 assumes a subnanometric SnO2 skin upon interaction with oxidative environments and (ii) the subsequent implications for chemical sensors. Definitely, the oxidation process has a direct effect on the work function, which is increased by 0.4 eV, owing to the charge transfer between the substrate and the SnO2 skin of 0.56 e– per SnO2 unit. Though the SnSe2 surface is inert to water at room temperature, upon surface oxidation the SnO2–SnSe2 interface shows a remarkable sensitivity to humidity. The charge transfer from H2O to the SnO2 skin is estimated to be 0.43 and 0.30 e– for one and two H2O molecules per supercell, respectively. Correspondingly, the DOS is correlated with water coverage, hence proving the aptness for humidity sensing also at low concentrations of H2O. Definitely, our findings prove the significant influence of humid environments on the electrical response of the SnO2–SnSe2 heterostructure. Moreover, recent reports regarding the use of SnSe2 in humidity sensors should be reconsidered with regard to the physicochemical mechanism.

Methods

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Theoretical methods are described in section S8.
The single crystals were grown by the Bridgman–Stockbarger method, according to the procedure described in refs (36) and (82) (see also section S1). Their crystalline quality was secured by X-ray diffraction (XRD) (Figures S1c and S2). The analysis of the XPS survey spectrum proves the absence of contaminants in bulk crystals (Figure S3). Samples were exfoliated in situ for surface-science investigations, by using scotch tape. Gas dosage was carried out at a partial pressure of 10–4 mbar.
XPS experiments were carried out at the APE-HE beamline at the Elettra-Trieste synchrotron. Core-level measurements were performed with an Omicron EA125 hemispherical electron energy analyzer, with the sample at room temperature and in normal emission. Linearly polarized light formed an angle of 45° with respect to the perpendicular direction of the surface. After the subtraction of a Shirley background, Sn-3d core-level spectra were analyzed by using a Gaussian line shape convoluted with a Doniach–Sunjic function, (83) while Se-3d and O-1s were fitted by Voigt line shapes.
HREELS experiments were performed with an Ibach-type spectrometer. The primary electron beam energy was 3.5 eV. HREELS spectra were recorded under specular conditions.
Measurements of LEEM images (Figure S4), EELS (Figure S6), and work-function changes ΔΦ (Figure 1a) were carried out at the soft X-ray beamline Nanospectroscopy at Elettra-Trieste synchrotron, using an energy-filtered LEEM–PEEM microscope with a spatial resolution of 10 nm. Specifically, measurements of ΔΦ were carried out by varying the electron beam energy across the total electron reflectivity threshold. This threshold is commonly termed the MEM–LEEM transition, which is characterized by a steep decrease in intensity as a function of a bias voltage applied to the sample (start voltage) as a decelerating potential. The ΔΦ value is identified by the shifts in the bias potential corresponding to the MEM–LEEM transition.
The gas sensing response to humidity at an operating temperature of 150 °C was determined by a volt–amperometric technique, as reported in ref (36). The RH air stream at 20% RH was obtained by mixing dry with saturated water-vapor air. In the analysis of the gas response, the relative response (RR) is defined as the ratio between the measured electrical resistance in dry air (Ra) and that under 20% RH (RH2O).

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpclett.0c02616.

  • Single-crystal growth (section S1), LEEM images (section S2), vibrational spectroscopy (section S3), electronic properties (section S4), density of states in pristine and defective SnSe2 (section S5), temperature dependence of the differential Gibbs free energy for adsorption of ambient gases (section S6), Langmuir isotherm calculations (section S7), and methods (section S8) (PDF)

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Author Information

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  • Corresponding Authors
    • Carlo Cantalini - Department of Industrial and Information Engineering and Economics, University of L’Aquila, Via G. Gronchi 18, I-67100 L’Aquila, Italy;  Email: [email protected]
    • Antonio Politano - Department of Physical and Chemical Sciences, University of L’Aquila, via Vetoio, 67100 L’Aquila, AQ, Italy;  CNR-IMM Istituto per la Microelettronica e Microsistemi, VIII strada 5, I-95121 Catania, Italy;  Orcidhttp://orcid.org/0000-0002-4254-2102;  Email: [email protected]
  • Authors
    • Gianluca D’Olimpio - Department of Physical and Chemical Sciences, University of L’Aquila, via Vetoio, 67100 L’Aquila, AQ, Italy
    • Francesca Genuzio - Elettra-Sincrotrone S.C.p.A., S.S. 14-km 163.5 in AREA Science Park, 34149 Trieste, Italy;  Orcidhttp://orcid.org/0000-0003-0699-2525
    • Tevfik Onur Menteş - Elettra-Sincrotrone S.C.p.A., S.S. 14-km 163.5 in AREA Science Park, 34149 Trieste, Italy
    • Valentina Paolucci - Department of Industrial and Information Engineering and Economics, University of L’Aquila, Via G. Gronchi 18, I-67100 L’Aquila, Italy;  Orcidhttp://orcid.org/0000-0003-0641-7926
    • Chia-Nung Kuo - Department of Physics, National Cheng Kung University, 1 Ta-Hsueh Road, 70101 Tainan, Taiwan
    • Amjad Al Taleb - Departamento de Física de la Materia Condensada, Universidad Autónoma de Madrid, 28049 Madrid, Spain
    • Chin Shan Lue - Department of Physics, National Cheng Kung University, 1 Ta-Hsueh Road, 70101 Tainan, Taiwan
    • Piero Torelli - Elettra-Sincrotrone S.C.p.A., S.S. 14-km 163.5 in AREA Science Park, 34149 Trieste, Italy;  Consiglio Nazionale delle Ricerche (CNR)-Istituto Officina dei Materiali (IOM), Laboratorio TASC in Area Science Park S.S. 14 km 163.5, 34149 Trieste, Italy
    • Daniel Farías - Departamento de Física de la Materia Condensada, Universidad Autónoma de Madrid, 28049 Madrid, Spain;  Instituto ‘Nicolás Cabrera’, Universidad Autónoma de Madrid, 28049 Madrid, Spain;  Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, Spain;  Orcidhttp://orcid.org/0000-0002-8537-8074
    • Andrea Locatelli - Elettra-Sincrotrone S.C.p.A., S.S. 14-km 163.5 in AREA Science Park, 34149 Trieste, Italy;  Orcidhttp://orcid.org/0000-0002-8072-7343
    • Danil W. Boukhvalov - College of Science, Institute of Materials Physics and Chemistry, Nanjing Forestry University, Nanjing 210037, P. R. China;  Theoretical Physics and Applied Mathematics Department, Ural Federal University, Mira Street 19, 620002 Ekaterinburg, Russia;  Orcidhttp://orcid.org/0000-0002-2286-3443
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work has been partially supported by the Spanish Ministerio de Ciencia e Innovación under Project PID2019-109525RB-I00. D.F. acknowledges financial support from the Spanish Ministry of Economy and Competitiveness, through the “María de Maeztu” Programme for Units of Excellence in R&D (CEX2018-000805-M). D.F. and A.A.T. acknowledge the project CALIPSOplus under Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. A.P. and G.D. acknowledge the CERIC–ERIC Consortium for the access to the Nanospectroscopy facility and financial support. G.D. acknowledges funding of a Ph.D. fellowship from PON Ricerca e Innovazione 2014–2020 (Project E12H1800010001) by the Italian Ministry of University and Research (MIUR). D.W.B. acknowledges the support by the Ministry of Science and Higher Education of the Russian Federation (through the basic part of the government mandate, Project No. FEUZ-2020-0060).

References

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    Gallium selenide (GaSe) is a 2D material with a thickness-dependent gap, strong non-linear optical coeffs., and uncommon interband optical selection rules, making it interesting for optoelectronic and spintronic applications. In this work, we monitor the oxidn. dynamics of GaSe with thicknesses ranging from 10 to 200 nm using Raman spectroscopy. In ambient temp. and humidity conditions, the intensity of all Raman modes and the luminescence decrease rapidly with moderate exposure to above-gap illumination. Concurrently, several oxidn. products appear in the Raman spectra: Ga2Se3, Ga2O3, and amorphous and cryst. selenium. We find that no safe measurement power exists for optical measurements on ultrathin GaSe in ambient conditions. We demonstrate that the simultaneous presence of oxygen, humidity, and above-gap illumination is required to activate this photo-oxidn. process, which is attributed to the transfer of photo-generated charge carriers towards aq. oxygen at the sample surface, generating highly reactive superoxide anions that rapidly degrade the sample and quench the optical response of the material. (c) 2017 American Institute of Physics.
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    Shi, L.; Li, Q.; Ouyang, Y.; Wang, J. Effect of Illumination and Se Vacancies on Fast Oxidation of Ultrathin Gallium Selenide. Nanoscale 2018, 10, 12180– 12186,  DOI: 10.1039/C8NR01533C
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    15
    Effect of illumination and Se vacancies on fast oxidation of ultrathin gallium selenide
    Shi, Li; Li, Qiang; Ouyang, Yixin; Wang, Jinlan
    Nanoscale (2018), 10 (25), 12180-12186CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Gallium selenide (GaSe) has recently emerged as a unique platform due to its exciting properties, namely, large and fast photo-response, high carrier mobility and non-linear optical properties. However, exposure for a few days causes the fast oxidn. of ultrathin GaSe under ambient conditions and the oxidn. mechanism remains unclear. By means of d. functional theory calcns. and ab initio mol. dynamics simulations, we comprehensively investigated the possible sources that cause oxidn. of ultrathin GaSe. Our results show that illumination and Se vacancies induce the fast oxidn. of GaSe. Under illumination, photo-excited electrons from the surface of GaSe are effectively transferred to oxygen mols. and thus, superoxide anions (O2-) are generated that react with GaSe. Moreover, Se vacancies directly react with O2. In both the cases, the Ga-Se bonds are continually replaced by Ga-O bonds, which eventually leads to complete degrdn. of GaSe, accompanied with the formation of the oxidn. products Ga2O3 and elemental Se. The comprehensive degrdn. mechanism unveiled herein lays an important foundation for the development of suitable protecting strategies in GaSe-based devices.
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  16. 16
    Fu, M.; Liang, L.; Zou, Q.; Nguyen, G. D.; Xiao, K.; Li, A. P.; Kang, J.; Wu, Z.; Gai, Z. Defects in Highly Anisotropic Transition-Metal Dichalcogenide PdSe2. J. Phys. Chem. Lett. 2020, 11, 740– 746,  DOI: 10.1021/acs.jpclett.9b03312
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    16
    Defects in Highly Anisotropic Transition-Metal Dichalcogenide PdSe2
    Fu, Mingming; Liang, Liangbo; Zou, Qiang; Nguyen, Giang D.; Xiao, Kai; Li, An-Ping; Kang, Junyong; Wu, Zhiming; Gai, Zheng
    Journal of Physical Chemistry Letters (2020), 11 (3), 740-746CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    The at. and electronic structures of pristine PdSe2 as well as various intrinsic vacancy defects in PdSe2 are studied comprehensively by combining scanning tunneling microscopy, spectroscopy, and d. functional theory calcns. Other than the topmost Se atoms, sublayer Pd atoms and the intrinsic Pd and Se vacancy defects are identified. Both VSe and VPd defects induce defect states near the Fermi level. As a result, the vacancy defects can be neg. charged by a tip gating effect. At neg. sample bias, the screened Coulomb interaction between the scanning tunneling microscopy (STM) tip and the charged vacancies creates a disk-like protrusion around the VPd and crater-like features around VSe. The magnification effect of the long-range charge localization at the charged defect site makes sublayer defects as deep as 1 nm visible even in STM images. By gating the probe, scanning probe microscopy can be used as an easy tool for characterizing sublayer defects in a nondestructive way.
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  17. 17
    Li, X.; Luo, N.; Chen, Y.; Zou, X.; Zhu, H. Real-Time Observing Ultrafast Carrier and Phonon Dynamics in Colloidal Tin Chalcogenide Van Der Waals Nanosheets. J. Phys. Chem. Lett. 2019, 10, 3750– 3755,  DOI: 10.1021/acs.jpclett.9b01470
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    17
    Real-Time Observing Ultrafast Carrier and Phonon Dynamics in Colloidal Tin Chalcogenide van der Waals Nanosheets
    Li, Xufeng; Luo, Nannan; Chen, Yuzhong; Zou, Xiaolong; Zhu, Haiming
    Journal of Physical Chemistry Letters (2019), 10 (13), 3750-3755CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Because of their earth-abundant, low-cost, and environmentally benign characteristics, 2-dimensional (2D) Group IV metal chalcogenides (e.g., SnSe2) with layered structures showed great potential in optoelectronic, photovoltaic, and thermoelec. applications. However, the intrinsic motion of excited carriers and their coupling with lattice photons, which fundamentally dictates device operation and optimization, remain yet to be unraveled. Herein, the authors directly follow the ultrafast carrier and photon dynamics of colloidal SnSe2 nanosheets in real time using ultrafast transient absorption spectroscopy. The authors show ∼0.3 ps intervalley relaxation process of photoexcited energetic carriers and ∼3 ps carrier defect trapping process with a long-lived trapped carrier (∼1 ns), highlighting the importance of trapped carriers in optoelectronic devices. Ultrashort laser pulse impulsively drives coherent out-of-plane lattice vibration in SnSe2, indicating strong electron-phonon coupling in SnSe2. This strong electron-phonon coupling could impose a fundamental limit on SnSe2 photovoltaic devices but benefit its thermoelec. applications.
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  18. 18
    Wei, Z.; Wang, L.; Zhuo, M.; Ni, W.; Wang, H.; Ma, J. Layered Tin Sulfide and Selenide Anode Materials for Li- and Na-Ion Batteries. J. Mater. Chem. A 2018, 6, 12185– 12214,  DOI: 10.1039/C8TA02695E
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    18
    Layered tin sulfide and selenide anode materials for Li- and Na-ion batteries
    Wei, Zengxi; Wang, Lei; Zhuo, Ming; Ni, Wei; Wang, Hongxia; Ma, Jianmin
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2018), 6 (26), 12185-12214CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Layered tin sulfides have attracted great interest as high-capacity anode materials in Li-ion batteries (LIBs) and Na-ion batteries (NIBs). In this review, we focus on the recent research progress in the area of design and synthesis of tin sulfide and selenide (SnS, SnS2, SnSe, and SnSe2) based anode materials for LIBs and NIBs. After a brief introduction of the energy concerns and the development prospects of LIBs and NIBs, we further detailed the properties and advantages of tin sulfide and selenide based anode materials for LIBs and NIBs. Besides the material structure design and optimization, the underlying mechanism and theor. anal. for improved electrochem. performance are also presented. Addnl., comparison of tin sulfides and selenides is also provided. Innovative strategies that have demonstrated the effectiveness of enhancing the performance of tin sulfide and selenide based anode materials for LIBs and NIBs are summarized. We hope that this timely review can shed light on the research and development of tin sulfides and selenides as high-performance anode materials which are not only a good supplement to the material pool of commercialized LIBs, but also help facilitate the development of low-cost and sustainable NIBs for stationary energy storage in the future.
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  19. 19
    Huang, Y.; Ling, C.; Liu, H.; Wang, S. Tuning Electronic and Magnetic Properties of SnSe2 Armchair Nanoribbons Via Edge Hydrogenation. J. Mater. Chem. C 2014, 2, 10175– 10183,  DOI: 10.1039/C4TC01919A
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    19
    Tuning electronic and magnetic properties of SnSe2 armchair nanoribbons via edge hydrogenation
    Huang, Yucheng; Ling, Chongyi; Liu, Hai; Wang, Sufan
    Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2014), 2 (47), 10175-10183CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
    First-principles calcns. were carried out to investigate the electronic and magnetic properties of SnSe2 armchair nanoribbons (ANRs) via edge hydrogenation. Interestingly, at different hydrogenation degrees, SnSe2 ANRs exhibit versatile electronic and magnetic properties, i.e., from nonmagnetic-semiconductors to magnetic-semiconductors or nonmagnetic-metals. Through the anal. from spatial spin distribution and d. of states, these transitions are well interpreted. Moreover, the relative stabilities of these ANRs were evaluated by the thermodn. phase diagram where the Gibbs free energies as a function of the chem. potential of the H2 mol. at different temps. were plotted. Our results show that hydrogenation is a well-controlled way to modify the phys. properties of SnSe2 ANRs. Through controlling chem. potential or partial pressure of H2, the different hydrogenation degrees of ANRs are thermodynamically stable, thus, one can arbitrarily steer their electronic and magnetic properties. The diverse electronic phases and magnetic properties endow the hydrogenated SnSe2 ANRs with potential applications in nanoelectronic devices.
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  20. 20
    Shafique, A.; Samad, A.; Shin, Y.-H. Ultra Low Lattice Thermal Conductivity and High Carrier Mobility of Monolayer SnS2 and SnSe2: A First Principles Study. Phys. Chem. Chem. Phys. 2017, 19, 20677– 20683,  DOI: 10.1039/C7CP03748A
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    20
    Ultra low lattice thermal conductivity and high carrier mobility of monolayer SnS2 and SnSe2: a first principles study
    Shafique, Aamir; Samad, Abdus; Shin, Young-Han
    Physical Chemistry Chemical Physics (2017), 19 (31), 20677-20683CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Using d. functional theory, the authors systematically investigate the lattice thermal cond. and carrier mobility of monolayer SnX2 (X = S, Se). The room-temp. ultra low lattice thermal conductivities found in monolayer SnS2 (6.41 W m-1 K-1) and SnSe2 (3.82 W m-1 K-1) are attributed to the low phonon velocity, low Debye temp., weak bonding interactions, and strong anharmonicity in monolayer SnX2. The predicted values of lattice thermal cond. are lower than those of other 2-dimensional materials such as stanene, phosphorene, monolayer MoS2, and bulk SnX2. High phonon-limited carrier mobilities are obtained for the monolayer SnX2. For example, the electron mobility of monolayer SnS2 is 756.60 cm2 V-1 s-1 and the hole mobility is 187.44 cm2 V-1 s-1. The electron mobility of these monolayers is higher than their hole mobility due to the low effective mass of electrons and low deformation consts., which makes them n-type materials. Due to their ultra low lattice thermal conductivities coupled with high carrier mobilities, monolayer SnX2 materials may be promising materials for thermoelec. applications.
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  21. 21
    Tan, P.; Chen, X.; Wu, L.; Shang, Y. Y.; Liu, W.; Pan, J.; Xiong, X. Hierarchical Flower-Like Snse2 Supported Ag3PO4 Nanoparticles: Towards Visible Light Driven Photocatalyst with Enhanced Performance. Appl. Catal., B 2017, 202, 326– 334,  DOI: 10.1016/j.apcatb.2016.09.033
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    21
    Hierarchical flower-like SnSe2 supported Ag3PO4 nanoparticles: Towards visible light driven photocatalyst with enhanced performance
    Tan, Pengfei; Chen, Xi; Wu, Laidi; Shang, Yan Yang; Liu, Wenwen; Pan, Jun; Xiong, Xiang
    Applied Catalysis, B: Environmental (2017), 202 (), 326-334CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
    A novel three-dimensional (3D) hierarchical flower-like Ag3PO4/SnSe2 composite photocatalyst was successfully prepd. via an in situ pptn. method. The compn., microstructure and optical properties of the Ag3PO4/SnSe2 composites were thoroughly investigated. Nano-sized Ag3PO4 particles were uniformly dispersed on the surface of 3D flower-like SnSe2. The obtained Ag3PO4/SnSe2 composites presented enhanced performance for photocatalytic degrdn. of Rhodamine B (RhB) compared with pure Ag3PO4 and SnSe2 under visible light irradn. The Ag3PO4/SnSe2-6 composite exhibited the optimal efficiency for photocatalytic decompn. of RhB, approx. 4.2 and 26 times higher than those of pure Ag3PO4 and SnSe2. Significantly, the superior stability was also obsd. after four cycles. The enhanced performance of the Ag3PO4/SnSe2 composites under visible light was ascribed to a synergistic effect including the matched energy band structures, increased light harvesting and boosted sepn. efficiency of photo-generated carriers. A possible photocatalytic mechanism of the composite was also discussed.
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  22. 22
    Fan, Y.; Wang, J.; Zhao, M. Spontaneous Full Photocatalytic Water Splitting on 2D MoSe2/SnSe2 and WSe2/SnSe2 Vdw Heterostructures. Nanoscale 2019, 11, 14836– 14843,  DOI: 10.1039/C9NR03469B
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    22
    Spontaneous full photocatalytic water splitting on 2D MoSe2/SnSe2 and WSe2/SnSe2 vdW heterostructures
    Fan, Yingcai; Wang, Junru; Zhao, Mingwen
    Nanoscale (2019), 11 (31), 14836-14843CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Spontaneous full photocatalytic water splitting into hydrogen and oxygen under visible light irradn. without the need for sacrificial agents is a challenging task, because suitable band gaps, low overpotentials for both half-reactions and spatially-sepd. catalytic sites should be fulfilled simultaneously in a photocatalytic system. Here, we propose a promising strategy to achieve this goal by constructing van der Waals (vdW) heterostructures of two-dimensional (2D) materials. Using first-principles calcns., we predict two promising photocatalysts, MoSe2/SnSe2 and WSe2/SnSe2 heterostructures, with the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) taking place sep. on the MoSe2 (WSe2) and SnSe2 layers. More excitingly, the Se-vacancy of the MoSe2 (WSe2) monolayer effectively lowers the HER overpotential, making the catalytic reactions occur spontaneously under the potentials solely provided by the photo-generated electrons and holes in pure water. The unique band alignment of these hetero-structured photocatalysts leads to high solar-to-hydrogen (STH) energy conversion efficiencies up to 10.5%, which is quite promising for com. applications. This work opens up an avenue for the design of highly-efficient photocatalysts for full water splitting.
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  23. 23
    Zeng, J.; Liu, E.; Fu, Y.; Chen, Z.; Pan, C.; Wang, C.; Wang, M.; Wang, Y.; Xu, K.; Cai, S.; Yan, X.; Wang, Y.; Liu, X.; Wang, P.; Liang, S. J.; Cui, Y.; Hwang, H. Y.; Yuan, H.; Miao, F. Gate-Induced Interfacial Superconductivity in 1T-SnSe2. Nano Lett. 2018, 18, 1410– 1415,  DOI: 10.1021/acs.nanolett.7b05157
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    23
    Gate-induced interfacial superconductivity in 1T-SnSe2
    Zeng, Junwen; Liu, Erfu; Fu, Yajun; Chen, Zhuoyu; Pan, Chen; Wang, Chenyu; Wang, Miao; Wang, Yaojia; Xu, Kang; Cai, Songhua; Yan, Xingxu; Wang, Yu; Liu, Xiaowei; Wang, Peng; Liang, Shi-Jun; Cui, Yi; Hwang, Harold Y.; Yuan, Hongtao; Miao, Feng
    Nano Letters (2018), 18 (2), 1410-1415CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    Layered metal chalcogenide materials provide a versatile platform to investigate emergent phenomena and two-dimensional (2D) supercond. at/near the atomically thin limit. In particular, gate-induced interfacial supercond. realized by the use of an elec.-double-layer transistor (EDLT) has greatly extended the capability to elec. induce supercond. in oxides, nitrides, and transition metal chalcogenides and enable one to explore new physics, such as the Ising pairing mechanism. Exploiting gate-induced supercond. in various materials can provide us with addnl. platforms to understand emergent interfacial supercond. Here, we report the discovery of gate-induced 2D supercond. in layered 1T-SnSe2, a typical member of the main-group metal dichalcogenide (MDC) family, using an EDLT gating geometry. A superconducting transition temp. Tc ≈ 3.9 K was demonstrated at the EDL interface. The 2D nature of the supercond. therein was further confirmed based on (1) a 2D Tinkham description of the angle-dependent upper crit. field Bc2, (2) the existence of a quantum creep state as well as a large ratio of the coherence length to the thickness of supercond. Interestingly, the in-plane Bc2 approaching zero temp. was found to be 2-3 times higher than the Pauli limit, which might be related to an elec. field-modulated spin-orbit interaction. Such results provide a new perspective to expand the material matrix available for gate-induced 2D supercond. and the fundamental understanding of interfacial supercond.
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  24. 24
    Shao, Z.; Fu, Z.-G.; Li, S.; Cao, Y.; Bian, Q.; Sun, H.; Zhang, Z.; Gedeon, H.; Zhang, X.; Liu, L.; Cheng, Z.; Zheng, F.; Zhang, P.; Pan, M. Strongly Compressed Few-Layered SnSe2 Films Grown on a SrTiO3 Substrate: The Coexistence of Charge Ordering and Enhanced Interfacial Superconductivity. Nano Lett. 2019, 19, 5304– 5312,  DOI: 10.1021/acs.nanolett.9b01766
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    24
    Strongly Compressed Few-Layered SnSe2 Films Grown on a SrTiO3 Substrate: The Coexistence of Charge Ordering and Enhanced Interfacial Superconductivity
    Shao, Zhibin; Fu, Zhen-Guo; Li, Shaojian; Cao, Yan; Bian, Qi; Sun, Haigen; Zhang, Zongyuan; Gedeon, Habakubaho; Zhang, Xin; Liu, Lijun; Cheng, Zhengwang; Zheng, Fawei; Zhang, Ping; Pan, Minghu
    Nano Letters (2019), 19 (8), 5304-5312CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    High pressure has been demonstrated to be a powerful approach of producing novel condensed-matter states, particularly in tuning the superconducting transition temp. (Tc) of the supercond. in a clean fashion without involving the complexity of chem. doping. However, the challenge of high-pressure expt. hinders further in-depth research for underlying mechanisms. Here, we have successfully synthesized continuous layer-controllable SnSe2 films on SrTiO3 substrate using mol. beam epitaxy. By means of scanning tunneling microscopy/spectroscopy (STM/S) and Raman spectroscopy, we found that the strong compressive strain is intrinsically built in few-layers films, with a largest equiv. pressure up to 23 GPa in the monolayer. Upon this, unusual 2 × 2 charge ordering is induced at the occupied states in the monolayer, accompanied by prominent decrease in the d. of states (DOS) near the Fermi energy (EF), resembling the gap states of CDW reported in transition metal dichalcogenide (TMD) materials. Subsequently, the coexistence of charge ordering and the interfacial supercond. is obsd. in bilayer films as a result of releasing the compressive strain. In conjunction with spatially resolved spectroscopic study and first-principles calcn., we find that the enhanced interfacial supercond. with an estd. Tc of 8.3 K is obsd. only in the 1 × 1 region. Such supercond. can be ascribed to a combined effect of interfacial charge transfer and compressive strain, which leads to a considerable downshift of the conduction band min. and an increase in the DOS at EF. Our results provide an attractive platform for further in-depth investigation of compression-induced charge ordering (monolayer) and the interplay between charge ordering and supercond. (bilayer). Meanwhile, it has opened up a pathway to prep. strongly compressed two-dimensional materials by growing onto a SrTiO3 substrate, which is promising to induce supercond. with a higher Tc.
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  25. 25
    Kim, S.; Yao, Z.; Lim, J.-M.; Hersam, M. C.; Wolverton, C.; Dravid, V. P.; He, K. Lithium-Ion Batteries: Atomic-Scale Observation of Electrochemically Reversible Phase Transformations in SnSe2 Single Crystals. Adv. Mater. 2018, 30, 1870393,  DOI: 10.1002/adma.201870393
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    Bai, J.; Wu, H.; Wang, S.; Zhang, G.; Feng, C.; Liu, H. Synthesis of CoSe2-SnSe2 Nanocube-Coated Nitrogen-Doped Carbon (NC) as Anode for Lithium and Sodium Ion Batteries. Appl. Surf. Sci. 2019, 488, 512– 521,  DOI: 10.1016/j.apsusc.2019.05.096
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    26
    Synthesis of CoSe2-SnSe2 nanocube-coated nitrogen-doped carbon (NC) as anode for lithium and sodium ion batteries
    Bai, Jin; Wu, Huimin; Wang, Shiquan; Zhang, Guangxue; Feng, Chuanqi; Liu, Huakun
    Applied Surface Science (2019), 488 (), 512-521CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    CoSe2-SnSe2/NC nanocubes (CSNC@NC) coated by nitrogen-doped carbon (NC) were synthesized successfully by an ordinary pyrazole polymn. and carbonization process. In comparison with bare CSNC, the CSNC@NC composite exhibited good structural stability and improved elec. cond. when used as anode. The CSNC@NC electrode showed a stable Li storage capacity (730.41 mAh g-1 over 100 cycles at 0.2 A g-1) and excellent rate performance (402.10 mAh g-1 at 2 A g-1). For Na storage, the discharge capacity could be maintained 279.3 mAh g-1 over 100 cycles at 0.2 A g-1; the lower capacity than that for Li storage maybe caused by the larger size of Na+ ions. The excellent cycling stability for both Li and Na storage cycle ability may be attributed to the carbon layer, which could tolerated the vol. fluctuations and ensured the structural integrity of the CSNC during the charge/discharge process; Moreover, the improved elec. cond. accelerated the diffusion rate of both Li+ and Na+, which is conducive to the electrochem. reactions in their resp. batteries. This unique structure and preeminent electrochem. performance of CSNC@NC show that CSNC@NC is a promising anode material for high-efficiency Li ion and Na ion batteries.
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  27. 27
    Zhang, F.; Xia, C.; Zhu, J.; Ahmed, B.; Liang, H.; Velusamy, D. B.; Schwingenschlögl, U.; Alshareef, H. N. SnSe2 2D Anodes for Advanced Sodium Ion Batteries. Adv. Energy Mater. 2016, 6, 1601188,  DOI: 10.1002/aenm.201601188
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    Zhou, X.; Zhou, N.; Li, C.; Song, H.; Zhang, Q.; Hu, X.; Gan, L.; Li, H.; Lü, J.; Luo, J.; Xiong, J.; Zhai, T. Vertical Heterostructures Based on SnSe2/MoS2 for High Performance Photodetectors. 2D Mater. 2017, 4, 025048,  DOI: 10.1088/2053-1583/aa6422
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    Vertical heterostructures based on SnSe2/MoS2 for high performance photodetectors
    Zhou, Xing; Zhou, Nan; Li, Chao; Song, Hongyue; Zhang, Qi; Hu, Xiaozong; Gan, Lin; Li, Huiqiao; Lue, Jingtao; Luo, Jun; Xiong, Jie; Zhai, Tianyou
    2D Materials (2017), 4 (2), 025048/1-025048/10CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
    Van der Waals heterostructures from atomically thin 2D materials have opened up new realms in modern semiconductor industry. Recently, 2D layered semiconductors such as MoS2 and SnSe2 have already demonstrated excellent electronic and optoelectronic properties due to their high electron mobility and unique band structures. Such combination of SnSe2 with MoS2 may provide a novel platform for the applications in electronics and optoelectronics. Thus, we constructed SnSe2/MoS2 based van der Waals heterostructures using MoS2 as templates, which may enrich the family of 2D van der Waals heterostructures. We demonstrate that the vdW heterostructures with high symmetry crystallog. directions show efficient interlayer charge transfer due to the strong coupling. This strong coupling is confirmed by theory calcns., low-temp. photoluminescence (PL) spectra, and elec. transport properties. High performance photodetector based on the vdW heterostructure has been demonstrated with a high responsivity of up to 9.1 × 103 A W-1 which is higher by two orders of magnitude than those MoS2-only devices. The improved performance can be attributed to the efficient charge transfer from MoS2 to SnSe2 at the interface.
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    Wang, M.; Wang, Z.; Xu, X.; Duan, S.; Du, C. Tin Diselenide-Based Saturable Absorbers for Eye-Safe Pulse Lasers. Nanotechnology 2019, 30, 265703,  DOI: 10.1088/1361-6528/ab1115
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    Tin diselenide-based saturable absorbers for eye-safe pulse lasers
    Wang, Mengxia; Wang, Zhengping; Xu, Xinguang; Duan, Sihou; Du, Chenlin
    Nanotechnology (2019), 30 (26), 265703CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
    Eye-safe pulse lasers have attracted increasing attention due to their potential wide application in many fields. However, optical modulators with excellent nonlinear optical absorption properties in the range of 1.4-2.1μm are still very scarce. In this study, tin diselenide (SnSe2), a newly-developed 2D layered semiconductor material with facile processability and low cost, was investigated. The nonlinear optical response of SnSe2 was investigated using the open aperture Z-scan method at 1500 and 1800 nm. Using SnSe2 as the saturable absorber, a passive Q-switched solid-state laser was realized at 1.3 and 1.9μm for the first time. This study proved SnSe2 to be an effective optical modulating material for the eye-safe waveband.
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  30. 30
    Zhang, Y.; Liu, Y.; Lim, K. H.; Xing, C.; Li, M.; Zhang, T.; Tang, P.; Arbiol, J.; Llorca, J.; Ng, K. M.; Ibáñez, M.; Guardia, P.; Prato, M.; Cadavid, D.; Cabot, A. Tin Diselenide Molecular Precursor for Solution-Processable Thermoelectric Materials. Angew. Chem., Int. Ed. 2018, 57, 17063– 17068,  DOI: 10.1002/anie.201809847
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    30
    Tin Diselenide Molecular Precursor for Solution-Processable Thermoelectric Materials
    Zhang, Yu; Liu, Yu; Lim, Khak Ho; Xing, Congcong; Li, Mengyao; Zhang, Ting; Tang, Pengyi; Arbiol, Jordi; Llorca, Jordi; Ng, Ka Ming; Ibanez, Maria; Guardia, Pablo; Prato, Mirko; Cadavid, Doris; Cabot, Andreu
    Angewandte Chemie, International Edition (2018), 57 (52), 17063-17068CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    In the present work, we detail a fast and simple soln.-based method to synthesize hexagonal SnSe2 nanoplates (NPLs) and their use to produce crystallog. textured SnSe2 nanomaterials. We also demonstrate that the same strategy can be used to produce orthorhombic SnSe nanostructures and nanomaterials. NPLs are grown through a screw dislocation-driven mechanism. This mechanism typically results in pyramidal structures, but we demonstrate here that the growth from multiple dislocations results in flower-like structures. Crystallog. textured SnSe2 bulk nanomaterials obtained from the hot pressing of these SnSe2 structures display highly anisotropic charge and heat transport properties and thermoelec. (TE) figures of merit limited by relatively low elec. conductivities. To improve this parameter, SnSe2 NPLs are blended here with metal nanoparticles. The elec. conductivities of the blends are significantly improved with respect to bare SnSe2 NPLs, what translates into a three-fold increase of the TE Figure of merit, reaching unprecedented ZT values up to 0.65.
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  31. 31
    Luo, Y.; Zheng, Y.; Luo, Z.; Hao, S.; Du, C.; Liang, Q.; Li, Z.; Khor, K. A.; Hippalgaonkar, K.; Xu, J.; Yan, Q.; Wolverton, C.; Kanatzidis, M. G. N-Type SnSe2 Oriented-Nanoplate-Based Pellets for High Thermoelectric Performance. Adv. Energy Mater. 2018, 8, 1702167,  DOI: 10.1002/aenm.201702167
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    Sun, J.; Liu, S.; Wang, C.; Bai, Y.; Chen, G.; Luo, Q.; Ma, F. Interface Tuning Charge Transport and Enhanced Thermoelectric Properties in Flower-Like SnSe2 Hierarchical Nanostructures. Appl. Surf. Sci. 2020, 510, 145478,  DOI: 10.1016/j.apsusc.2020.145478
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    Interface tuning charge transport and enhanced thermoelectric properties in flower-like SnSe2 hierarchical nanostructures
    Sun, Jun; Liu, Shuai; Wang, Chen; Bai, Yu; Chen, Guanjun; Luo, Qiaomei; Ma, Fei
    Applied Surface Science (2020), 510 (), 145478CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    Thermoelec. properties could be well improved in hierarchical nanostructures due to the selective scattering on electrons and phonons by interfaces. Flower-like SnSe2 nanostructure is synthesized by soln.-based method and, the nanostructure is sintered into pellets by spark plasma sintering (SPS) to evaluate thermoelec. properties. The flower-like SnSe2 nanostructure exhibits the ultralow thermal cond. of 0.44 Wm-1 K-1 due to the strong phonon scattering by high-d. of interface and grain boundaries, which were confirmed by both expts. and simulation. Besides, the elec. transport of the flower-like SnSe2 is optimized synergistically owing to the moderate interfacial potential barrier. The highest power factor of 43μWm-1 K-2 and competitive ZT value are measured at 550 K. The thermoelec. performance of flower-like SnSe2 is better that that of nanoplate and bulk counterparts. It provides an efficient method to improve the thermoelec. properties of SnSe2 based materials.
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  33. 33
    Nasir, M. S.; Yang, G.; Ayub, I.; Wang, X.; Wang, S.; Nasir, A.; Yan, W. Tin Diselenide Nanoflakes Decorated Hierarchical 1D TiO2 Fiber: A Robust and Highly Efficient Co-Catalyst for Hydrogen Evolution Reaction. Appl. Surf. Sci. 2020, 521, 146333,  DOI: 10.1016/j.apsusc.2020.146333
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    Tin diselenide nanoflakes decorated hierarchical 1D TiO2 fiber: A robust and highly efficient co-catalyst for hydrogen evolution reaction
    Nasir, Muhammad Salman; Yang, Guorui; Ayub, Iqra; Wang, Xiaojun; Wang, Silan; Nasir, Abdul; Yan, Wei
    Applied Surface Science (2020), 521 (), 146333CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    Transitional metal dichalcogenide two-dimensional materials have revealed many astonishing properties including as a substitution of noble metals as a co-catalyst for hydrogen evolution reaction. Tin diselenide (SnSe2) is the one who also received substantial consideration in many fields due to low cost, earth-abundant and environment-friendly. However, the great challenge to make heterojunction with other semiconductor material to improve its efficiency for photocatalytic water splitting. For this purpose, a 1D nanofiber of TiO2 is prepd. by electrospinning and produced a needle-like structure on the fiber by following the alkali hydrothermal method. The chem. vapor deposition method (CVD) was adopted to load nanoflakes of SnSe2 on the branched fiber of TiO2 and make a strong heterojunction. The composite interpreted excellent photocatalytic performance by producing hydrogen about 0.95 mmol g-1 h-1 in comparison with branched TiO2 (0.47 mmol g-1 h-1) and conventional TiO2 fiber (0.35 mmol g-1 h-1). The photoluminescence, time decay fluorescent spectra and photoelectrochem. results ratified that SnSe2 not only reduces the charge recombination by increasing the transfer of electron but also provides the active site for hydrogen prodn. as a cocatalyst. This study presents an inexpensive and environmentally friendly photocatalyst for hydrogen prodn. without the use of noble metals.
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    Lee, Y. K.; Luo, Z.; Cho, S. P.; Kanatzidis, M. G.; Chung, I. Surface Oxide Removal for Polycrystalline Snse Reveals near-Single-Crystal Thermoelectric Performance. Joule 2019, 3, 719– 731,  DOI: 10.1016/j.joule.2019.01.001
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    34
    Surface Oxide Removal for Polycrystalline SnSe Reveals Near-Single-Crystal Thermoelectric Performance
    Lee, Yong Kyu; Luo, Zhongzhen; Cho, Sung Pyo; Kanatzidis, Mercouri G.; Chung, In
    Joule (2019), 3 (3), 719-731CODEN: JOULBR; ISSN:2542-4351. (Cell Press)
    Tin selenide (SnSe) has emerged as a surprising new material with exceptional thermal transport and charge transport properties such as ultralow thermal cond., which give it a record-high thermoelec. figure of merit (ZT) of ∼2.5-2.7 at around 800 K. These properties, however, have been only observable in well-prepd. and properly handled single-crystal samples. Polycryst. SnSe samples have markedly inferior properties paradoxically with higher apparent thermal cond. and much lower ZT values than single crystals. The high thermal cond. in polycryst. samples has been attributed to surface tin oxides. Based on this hypothesis, we have employed an oxide-removing strategy that involves a chem. redn. process at 613 K under a 4% H2/Ar atm. This leads to an exceptionally low lattice thermal cond. of ∼0.11 W m-1K-1 in polycryst. hole-doped SnSe alloyed with 5% lead selenide, even lower than that of single crystals, and boosts the ZT to ∼2.5 at 773 K.
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  35. 35
    Lamuta, C.; Campi, D.; Pagnotta, L.; Dasadia, A.; Cupolillo, A.; Politano, A. Determination of the Mechanical Properties of SnSe, a Novel Layered Semiconductor. J. Phys. Chem. Solids 2018, 116, 306– 312,  DOI: 10.1016/j.jpcs.2018.01.045
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    Determination of the mechanical properties of SnSe, a novel layered semiconductor
    Lamuta, Caterina; Campi, Davide; Pagnotta, Leonardo; Dasadia, Abhay; Cupolillo, Anna; Politano, Antonio
    Journal of Physics and Chemistry of Solids (2018), 116 (), 306-312CODEN: JPCSAW; ISSN:0022-3697. (Elsevier Ltd.)
    Tin selenide (SnSe) is one the most promising materials for flexible electronics. However, expts. on the direct detn. of its mech. properties are still missing. By means of depth-sensing nanoindentation expts., we directly evaluate the Youngs modulus of bulk single crystals of tin selenide (25.3 GPa), as well as their hardness (0.82 GPa). Exptl. results are compared with predictions by d. functional theory, performed using eleven different functionals. The discrepancies between the exptl. results and the thoretical predictions can be ascribed to the oxidn. of the SnSe surface, detected by XPS.
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    Paolucci, V.; D’Olimpio, G.; Kuo, C.-N.; Lue, C. S.; Boukhvalov, D. W.; Cantalini, C.; Politano, A. Self-Assembled SnO2/SnSe2 Heterostructures: A Suitable Platform for Ultrasensitive NO2 and H2 Sensing. ACS Appl. Mater. Interfaces 2020, 12, 34362– 34369,  DOI: 10.1021/acsami.0c07901
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    Self-Assembled SnO2/SnSe2 Heterostructures: A Suitable Platform for Ultrasensitive NO2 and H2 Sensing
    Paolucci, Valentina; D'Olimpio, Gianluca; Kuo, Chia-Nung; Lue, Chin Shan; Boukhvalov, Danil W.; Cantalini, Carlo; Politano, Antonio
    ACS Applied Materials & Interfaces (2020), 12 (30), 34362-34369CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    By means of expts. and theory, the gas-sensing properties of tin diselenide (SnSe2) were elucidated. We discover that, while the stoichiometric single crystal is chem. inert even in air, the nonstoichiometric sample assumes a subnanometric SnO2 surface oxide layer once exposed to ambient atm. The presence of Se vacancies induces the formation of a metastable SeO2-like layer, which is finally transformed into a SnO2 skin. Remarkably, the self-assembled SnO2/SnSe2-x heterostructure is particularly efficient in gas sensing, whereas the stoichiometric SnSe2 sample does not show sensing properties. Congruently with the theor. model, direct sensing tests carried out on SnO2/SnSe2-x at an operational temp. of 150°C provided sensitivities of (1.06 ± 0.03) and (0.43 ± 0.02) [ppm]-1 for NO2 and H2, resp., in dry air. The corresponding calcd. limits of detection are (0.36 ± 0.01) and (3.6 ± 0.1) ppm for NO2 and H2, resp. No detectable changes in gas-sensing performances are obsd. in a time period extended above six months. Our results pave the way for a novel generation of ambient-stable gas sensor based on self-assembled heterostructures formed taking advantage on the natural interaction of substoichiometric van der Waals semiconductors with air.
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    Pawar, M.; Kadam, S.; Late, D. J. High-Performance Sensing Behavior Using Electronic Ink of 2D SnSe2 Nanosheets. Chemistry Select 2017, 2, 4068– 4075,  DOI: 10.1002/slct.201700261
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    Pawbake, A. S.; Date, A.; Jadkar, S. R.; Late, D. J. Temperature Dependent Raman Spectroscopy and Sensing Behavior of Few Layer SnSe2 Nanosheets. Chemistry Select 2016, 1, 5380– 5387,  DOI: 10.1002/slct.201601347
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    Chen, X.; Chen, X.; Han, Y.; Su, C.; Zeng, M.; Hu, N.; Su, Y.; Zhou, Z.; Wei, H.; Yang, Z. Two-Dimensional MoSe2 Nanosheets Via Liquid-Phase Exfoliation for High-Performance Room Temperature NO2 Gas Sensors. Nanotechnology 2019, 30, 445503,  DOI: 10.1088/1361-6528/ab35ec
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    Two-dimensional MoSe2 nanosheets via liquid-phase exfoliation for highperformance room temperature NO2 gas sensors
    Chen, Xi; Chen, Xinwei; Han, Yutong; Su, Chen; Zeng, Min; Hu, Nantao; Su, Yanjie; Zhou, Zhihua; Wei, Hao; Yang, Zhi
    Nanotechnology (2019), 30 (44), 445503CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
    Molybdenum selenide (MoSe2) has drawn significant interest due to its typical semiconductor properties. MoSe2 is a relatively novel material in the field of gas sensors esp. at room temp. Herein, we utilize a facile and efficient synthetic method of liq.-phase exfoliation to exfoliate bulk MoSe2 into nanosheets. Anhyd. ethanol is used as dispersant, so the low b.p. makes it easy to be removed from MoSe2 nanosheets, which does not affect the subsequent sensing performance. The exfoliated few-layered MoSe2 nanosheets shows significantly enhanced NO2 gas response (1500% to 10 ppm NO2 which is 18 times greater than pristine bulk MoSe2), a low detection concn. (50 ppb), an outstanding repeatability, a remarkable selectivity, and a reliable long-term device durability (more than 60 d) at room temp. (25°C). The reason of the significant improvement in gas sensing performance can be attributed mainly to the higher surface-to-vol. ratio of exfoliated MoSe2 nanosheets. It promotes the adsorption of gas mols. on the surface of the material, thereby facilitating the charge transfer process. The superior performance of this gas sensor makes MoSe2 nanosheets a potential candidate for room temp. NO2 detection.
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    Guo, R.; Han, Y.; Su, C.; Chen, X.; Zeng, M.; Hu, N.; Su, Y.; Zhou, Z.; Wei, H.; Yang, Z. Ultrasensitive Room Temperature NO2 Sensors Based on Liquid Phase Exfoliated WSe2 Nanosheets. Sens. Actuators, B 2019, 300, 127013,  DOI: 10.1016/j.snb.2019.127013
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    Ultrasensitive room temperature NO2 sensors based on liquid phase exfoliated WSe2 nanosheets
    Guo, Rensong; Han, Yutong; Su, Chen; Chen, Xinwei; Zeng, Min; Hu, Nantao; Su, Yanjie; Zhou, Zhihua; Wei, Hao; Yang, Zhi
    Sensors and Actuators, B: Chemical (2019), 300 (), 127013CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
    Semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDs) are promising gas-sensing materials due to their large surface-to-vol. ratio, layered structures, susceptible surfaces, and excellent conductivities. Herein, a simple and effective method has been developed to exfoliate tungsten diselenide (WSe2) into few-layered nanosheets with N-methyl-2-pyrrolidone (NMP) as a dispersant. The gas sensor based on the as-synthesized WSe2 nanosheets shows an extremely high response (50 ppb, 5.06) under a low detection concn. of nitrogen dioxide (NO2) at room temp. (25°C), which is much higher than many other reports for 2D TMDs-based gas sensors. Addnl., our sensor also exhibits a response of 11.01 to 10 ppm NO2, an excellent selectivity, and a reliable long-term stability within 8 wk. It is expected that the outstanding performances of the gas sensors based on WSe2 nanosheets reported in the present work will make WSe2 a promising candidate for ultrasensitive NO2 sensing applications.
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    Zhong, Y.; Li, W.; Zhao, X.; Jiang, X.; Lin, S.; Zhen, Z.; Chen, W.; Xie, D.; Zhu, H. High-Response Room-Temperature NO2 Sensor and Ultrafast Humidity Sensor Based on SnO2 with Rich Oxygen Vacancy. ACS Appl. Mater. Interfaces 2019, 11, 13441– 13449,  DOI: 10.1021/acsami.9b01737
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    41
    High-Response Room-Temperature NO2 Sensor and Ultrafast Humidity Sensor Based on SnO2 with Rich Oxygen Vacancy
    Zhong, Yujia; Li, WeiWei; Zhao, Xuanliang; Jiang, Xin; Lin, Shuyuan; Zhen, Zhen; Chen, Wenduo; Xie, Dan; Zhu, Hongwei
    ACS Applied Materials & Interfaces (2019), 11 (14), 13441-13449CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    SnO2 nanosheets with abundant vacancies (designated as SnO2-x) were successfully prepd. by annealing SnSe nanosheets in Ar. The TEM results of the prepd. SnO2 nanosheets indicated that high-d. SnO2-x nanoplates with the size of 5-10 nm were distributed on the surface of amorphous C. After annealing, the acquired SnO2-x/amorphous C retained the square morphol. The stoichiometric ratio of Sn/O = 1:1.55 confirmed that O vacancies were abundant in SnO2 nanosheets. The prepd. SnO2-x exhibited excellent performance of sensing NO2 at room temp. The response of the SnO2-x-based sensor to 5 ppm NO2 is 16 with the response time and recovery time of 331 and 1057 s, resp., which is superior to those of most reported room-temp. NO2 sensors based on SnO2 and other materials. When the humidity varied from 30 to 40%, ΔR/R was 0.025. The ultrafast humidity response (52 ms) and recovery (140 ms) are competitive compared with other state-of-art humidity sensors. According to the mechanistic study, the excellent sensing performance of SnO2-x is attributed to its special structure.
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  42. 42
    Vorokhta, M.; Khalakhan, I.; Vondráček, M.; Tomeček, D.; Vorokhta, M.; Marešová, E.; Nováková, J.; Vlček, J.; Fitl, P.; Novotný, M.; Hozák, P.; Lančok, J.; Vrňata, M.; Matolínová, I.; Matolín, V. Investigation of Gas Sensing Mechanism of SnO2 Based Chemiresistor Using near Ambient Pressure Xps. Surf. Sci. 2018, 677, 284– 290,  DOI: 10.1016/j.susc.2018.08.003
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    Investigation of gas sensing mechanism of SnO2 based chemiresistor using near ambient pressure XPS
    Vorokhta, M.; Khalakhan, I.; Vondracek, M.; Tomecek, D.; Vorokhta, M.; Maresova, E.; Novakova, J.; Vlcek, J.; Fitl, P.; Novotny, M.; Hozak, P.; Lancok, J.; Vrnata, M.; Matolinova, I.; Matolin, V.
    Surface Science (2018), 677 (), 284-290CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
    In this article, we present the results of an investigation into chem. processes which take place at the surface of SnO2-based chemiresistor in various atms. (1 mbar of argon, 1 mbar of oxygen, 0.1 mbar of ethanol, 1 mbar of oxygen + 0.1 mbar of ethanol mixt.) at common working temps. (450 and 573 K). The key method for nanoscale anal. was the Near Ambient Pressure XPS. In parallel the resistance and DC-responses of SnO2 layer were in-situ monitored providing information about macroscale processes during gas sensing. The change in the sensor resistance after exposure to the ethanol-contg. atms. together with the disappearance of the band bending effect and observation of different carbonaceous groups including ethoxy groups and acetaldehyde mols. on the sensor surface in the XPS spectra supported the theory of chem. interaction of ethanol with the chemisorbed oxygen. The NAP-XPS spectra also showed that the nanostructured tin oxide is partially reduced even after being exposed to pure oxygen at 573 K. Exposing this surface to the mixt. of O2/EtOH did not significantly increase the surface redn. probably due to slow kinetics of the ethanol redn. process and fast kinetics of the oxygen re-oxidn. process. However, it was demonstrated that the surface of sensor is slowly getting contaminated by carbon.
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    Das, S.; Jayaraman, V. SnO2: A Comprehensive Review on Structures and Gas Sensors. Prog. Mater. Sci. 2014, 66, 112– 255,  DOI: 10.1016/j.pmatsci.2014.06.003
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    SnO2: A comprehensive review on structures and gas sensors
    Das, Soumen; Jayaraman, V.
    Progress in Materials Science (2014), 66 (), 112-255CODEN: PRMSAQ; ISSN:0079-6425. (Elsevier Ltd.)
    A review. Metal oxides possess exceptional potential as base materials in emerging technologies. In recent times, significant amt. of research works is carried out on these materials to assess new areas of applications, including optical, electronic, optoelectronic and biol. domains. In such applications, the response and performance of the devices depend crucially, among other factors, on the size, shape and surface of the active oxide materials. For instance, the electronic and optical properties of oxides depend strongly on the spatial dimensions and compn. [1]. The large no. of atoms on the surface, and the effective van der Waals, Coulombic and interat. coupling significantly modify the phys. and chem. properties of the low dimensional oxide materials vis-a´-vis its bulk counterparts. As a result, low dimensional oxide materials, such as nanoparticles, nanospheres, nanorods, nanowires, nanoribbon/nanobelts, nanotubes, nanodisks, nanosheets evoke vast and diverse interests. Thermal and phys. deposition, hydro/solvothermal process, spray-pyrolysis, assisted self-assembly, oil-in-water microemulsion and template-assisted synthesis are regularly employed to synthesis one-, two- and three-dimensional nanostructures, which have become the focus of intensive research in mesoscopic physics and nanoscale devices. It not only provides good scopes to study the optical, elec. and thermal properties in quantum-confinement, but also offers important insights for understanding the functional units in fabricating electronic, optoelectronic, and magnetic devices of nanoscale dimension. Tin oxide (SnO2) is one such very important n-type oxide and wide band gap (3.6 eV) semiconductor. Its good quality elec., optical, and electrochem. properties are exploited in solar cells, as catalytic support materials, as solid-state chem. sensors and as high-capacity lithium-storage. Previously, Chopra et al. [2] reviewed different aspects of transparent conducting SnO2 thin films. Wang et al. [3] discussed device applications of nanowires and nanobelts of semiconductor oxides, including SnO2. Batzill et al. [4] discussed about the surface of single cryst. bulk SnO2. However, it is understood that neither there is any comprehensive review on various crystallog. phases, polymorphs, bulk modulus, lattice parameters and electronic states of SnO2, nor there is any updated compilation on the recent progress and scope on SnO2 nanostructures. Therefore, the proposed review covers the past and recent progress on the said topics and is summarized in the following manner. The available theor. and exptl. works on crystal structures, bulk modulus, lattice parameters are reviewed in details. The electronic states and the band structures of these phases are discussed next. Active crystal surfaces of SnO2 play vital roles in its many interesting properties, including sensing and catalytic applications. So, a short review is written on its different surfaces, its electronic structures and d. of states. The discussion on the importance of morphol. variations on the properties of SnO2 is followed by a review on different methods for obtaining such structures. A detail survey on the existing literature on techniques and mechanisms for the growth of nanostructures are included. SnO2 is efficiently employed in gas sensing applications. A review on such applications is compiled based on the role of morphol. and performance. The future course of SnO2 as an important material in the contemporary research is also discussed.
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    Li, G.-J.; Kawi, S. High-Surface-Area SnO2: A Novel Semiconductor-Oxide Gas Sensor. Mater. Lett. 1998, 34, 99– 102,  DOI: 10.1016/S0167-577X(97)00142-0
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    High-surface-area SnO2: a novel semiconductor-oxide gas sensor
    Li, G.-J.; Kawi, S.
    Materials Letters (1998), 34 (1,2), 99-102CODEN: MLETDJ; ISSN:0167-577X. (Elsevier Science B.V.)
    High surface area SnO2 sensor materials were systematically synthesized by a surfactant incorporating method. After calcination at 723 K, a high BET surface area of 156.8 m2/g was obtained. The sensing properties of the high surface area SnO2 material were studied using H2 as the probing gas. A linear relation exists between sensor surface area and its sensitivity to H2.
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    Di Giulio, M.; Micocci, G.; Serra, A.; Tepore, A.; Rella, R.; Siciliano, P. SnO2 Thin Films for Gas Sensor Prepared by Rf Reactive Sputtering. Sens. Actuators, B 1995, 25, 465– 468,  DOI: 10.1016/0925-4005(94)01397-7
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    SnO2 thin films for gas sensor prepared by r.f. reactive sputtering
    Di Giulio, M.; Micocci, G.; Serra, A.; Tepore, A.; Rella, R.; Siciliano, P.
    Sensors and Actuators, B: Chemical (1995), 25 (1-3), 465-8CODEN: SABCEB; ISSN:0925-4005. (Elsevier)
    SnO2 thin films were grown by the r.f. reactive sputtering method to be used as gas sensors. The films are deposited onto heated alumina substrates in an Ar-O2 atmosphere starting from an SnO2 target. The authors have optimized the growth parameters to achieve the best thin-film properties. The surface structure and the compn. of the prepd. films are studied by x-ray diffraction and XPS. The effect on the gas-sensing characteristics of dispersing platinum onto the film surface by sputtering from a Pt target followed by a suitable thermal annealing, also was studied. In particular, Pt-added SnO2 thin films show a high sensitivity to carbon monoxide gas at temps. of ∼170°. This temp. is lower than the optimum operating temp. (∼350°) of SnO2 samples without platinum.
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    Li, W.; Kan, K.; He, L.; Ma, L.; Zhang, X.; Si, J.; Ikram, M.; Ullah, M.; Khan, M.; Shi, K. Biomorphic Synthesis of 3D Mesoporous SnO2 with Substantially Increased Gas-Sensing Performance at Room Temperature Using a Simple One-Pot Hydrothermal Method. Appl. Surf. Sci. 2020, 512, 145657,  DOI: 10.1016/j.apsusc.2020.145657
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    Biomorphic synthesis of 3D mesoporous SnO2 with substantially increased gas-sensing performance at room temperature using a simple one-pot hydrothermal method
    Li, Wenna; Kan, Kan; He, Lang; Ma, Laifeng; Zhang, Xueyi; Si, Jiaqi; Ikram, Muhammad; Ullah, Mohib; Khan, Mawaz; Shi, Keying
    Applied Surface Science (2020), 512 (), 145657CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    Biomorphic SnO2 nanoparticles with a mesoporous structure were synthesized using simple one-step hydrothermal method with SnCl2 as the raw material and hemp stems as the bio-template. The uniformly grown SnO2 nanoparticles perfectly inherited the 3D structure of biomass carbon and formed a rich mesoporous structure, which was beneficial to gas sensing and facilitated the transport of the target gas on the surface and inside the sample. Moreover, the high surface area of materials provides more active sites for the adsorption of oxygen and the target gas. The sample with a 450°C annealing temp. exhibited an excellent response (35.83) as a NO2 sensor at room temp. (RT), fast response speed to 100 ppm NO2 (2.67 s), and a detection limit as low as 10 ppb. Furthermore, it displayed long-term stability, excellent selectivity and good repeatability. Therefore, the mesoporous biomorphic SnO2 nanoparticles represent a good candidate as a key green material for an NO2 sensor at RT.
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    Li, W.; Ding, C.; Li, J.; Ren, Q.; Bai, G.; Xu, J. Sensing Mechanism of Sb, S Doped SnO2(110) Surface for CO. Appl. Surf. Sci. 2020, 502, 144140,  DOI: 10.1016/j.apsusc.2019.144140
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    Sensing mechanism of Sb, S doped SnO2 (1 1 0) surface for CO
    Li, Wei; Ding, Chao; Li, Jinze; Ren, Qingying; Bai, Gang; Xu, Jie
    Applied Surface Science (2020), 502 (), 144140CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    First-principles calcn. based on d. functional theory (DFT) was employed to study the adsorption of CO mols. on various SnO2 (1 1 0) surface. For comparison, the adsorption of CO on perfect SnO2, Sb-doped SnO2 and Sb, S co-doped SnO2 surfaces are considered. Adsorption energy, electron population and d. of states show that the sensing properties of SnO2-based sensors in terms of the CO response were improved by Sb, S co-doping.
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    Ko, W. C.; Kim, K. M.; Kwon, Y. J.; Choi, H.; Park, J. K.; Jeong, Y. K. ALD-Assisted Synthesis of V2O5 Nanoislands on SnO2 Nanowires for Improving NO2 Sensing Performance. Appl. Surf. Sci. 2020, 509, 144821,  DOI: 10.1016/j.apsusc.2019.144821
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    ALD-assisted synthesis of V2O5 nanoislands on SnO2 nanowires for improving NO2 sensing performance
    Ko, Woo Chul; Kim, Kang Min; Kwon, Yong Jung; Choi, Heechae; Park, Jin Kuen; Jeong, Young Kyu
    Applied Surface Science (2020), 509 (), 144821CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    Anchoring nanoparticles on the surfaces of metal oxide nanosensors is a promising strategy to improve sensing performance. However, several issues regarding the surface coverage and uniform distribution of nanoislands over base sensing materials have yet to be resolved using conventional synthetic routes. Herein, we present a viable alternative for the decoration of V2O5 nanoislands on SnO2 nanowires: a new two-step process that combines at. layer deposition (ALD) and successive thermal post-treatment. This process enables us to control the surface coverage of V2O5 nanoislands by varying the no. of ALD cycles and to homogeneously disperse the nanoislands on the SnO2 nanowire surfaces. The NO2 response of the V2O5-decorated SnO2 sensor improved as the no. of ALD-V2O5 cycles increased; the highest response, obtained by the sensor prepd. with 50 ALD-V2O5 cycles, was more than 50 times greater than that of the pristine SnO2 nanowires. However, the sensing performance degraded beyond 50 ALD-V2O5 cycles as there was an oversupply of V2O5 nanoislands. Based on d. functional theory calcns., we detd. that V2O5 nanoisland loading is in competition with the exposed SnO2 surface to increase sensing performance, which implies that the surface coverage of V2O5 nanoislands must be precisely optimized.
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    Tombak, A.; Ocak, Y. S.; Bayansal, F. Cu/SnO2 Gas Sensor Fabricated by Ultrasonic Spray Pyrolysis for Effective Detection of Carbon Monoxide. Appl. Surf. Sci. 2019, 493, 1075– 1082,  DOI: 10.1016/j.apsusc.2019.07.087
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    Cu/SnO2 gas sensor fabricated by ultrasonic spray pyrolysis for effective detection of carbon monoxide
    Tombak, A.; Ocak, Y. S.; Bayansal, F.
    Applied Surface Science (2019), 493 (), 1075-1082CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    In this paper, we report results of morphol., structural, optical anal. of ultrasonically sprayed Cu-doped SnO2 thin films and their applications in conductometric gas sensors to detect small traces of CO mols. Effects of Cu-doping on morphol., structural and optical properties of SnO2 nanostructures were investigated by SEM (SEM), X-Ray Diffraction (XRD), and UV-Vis. Spectroscopy measurements. SEM revealed that porosity of the film surfaces is increased with increasing Cu-doping. From the XRD patterns, the size of the crystallites and crystal quality of the films are found to be decreased with Cu-doping. UV-Vis. spectroscopy results presented that the transmittance and bandgap can be manipulated with Cu-doping where both are decreased with Cu-doping. The relation between morphol. and structure of the films with CO response properties are discussed properly. The gas response of the films with different Cu-doping has been investigated at different CO concns. at different operating temps. From the sensing measurements, it is found that Cu-doping improves the SnO2 based sensor response to CO gas. Furthermore, the possible sensing mechanism to enlighten the improved gas sensing behavior of the films is proposed.
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    Han, Y.; Ma, Y.; Liu, Y.; Xu, S.; Chen, X.; Zeng, M.; Hu, N.; Su, Y.; Zhou, Z.; Yang, Z. Construction of MoS2/SnO2 Heterostructures for Sensitive NO2 Detection at Room Temperature. Appl. Surf. Sci. 2019, 493, 613– 619,  DOI: 10.1016/j.apsusc.2019.07.052
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    Construction of MoS2/SnO2 heterostructures for sensitive NO2 detection at room temperature
    Han, Yutong; Ma, Yujie; Liu, Yang; Xu, Shusheng; Chen, Xinwei; Zeng, Min; Hu, Nantao; Su, Yanjie; Zhou, Zhihua; Yang, Zhi
    Applied Surface Science (2019), 493 (), 613-619CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    Molybdenum disulfide (MoS2) has sparked immense interests as a novel gas sensing material due to its inherent layered structure and large surface-to-vol. ratio. However, the obstacles of insufficient sensitivity and poor recoverability for MoS2-based gas sensors need to be resolved for the further applications. Herein, we report the construction of MoS2 nanosheets based p-n heterostructures for the purposes of achieving excellent NO2 detection at room temp. After functionalized with tin oxide (SnO2) nanoparticles, the optimal MoS2/SnO2 heterostructure-based gas sensor exhibits a response of 18.7 to 5 ppm nitrogen dioxide (NO2), an outstanding selectivity compared with other gases, and an excellent long-term stability for 4 wk. The enhanced sensing performance of MoS2/SnO2 heterostructures can be ascribed to the unique 2D/0D nanostructures and the formation of numerous p-n heterojunctions. Therefore, construction of p-n heterostructures provides a versatile soln. to overcome the sensing issues of MoS2-based gas sensors and also paves a new way for others room temp. sensor applications.
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    Barsan, N.; Weimar, U. Understanding the Fundamental Principles of Metal Oxide Based Gas Sensors; the Example of CO Sensing with SnO2 Sensors in the Presence of Humidity. J. Phys.: Condens. Matter 2003, 15, R813,  DOI: 10.1088/0953-8984/15/20/201
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    Understanding the fundamental principles of metal oxide based gas sensors; the example of CO sensing with SnO2 sensors in the presence of humidity
    Barsan, N.; Weimar, U.
    Journal of Physics: Condensed Matter (2003), 15 (20), R813-R839CODEN: JCOMEL; ISSN:0953-8984. (Institute of Physics Publishing)
    This paper investigates the effect of water vapor in CO sensing by using Pd doped SnO2 sensors realized in thick film technol. as an example of the basic understanding of sensing mechanisms applied to sensors. The results of phenomenol. and spectroscopic measurement techniques, all of them obtained under working conditions for sensors, were combined with modeling in order to derive conclusions able to be generalized to the field of metal oxide based gas sensors. The techniques employed were: d.c. conductance, a.c. impedance spectroscopy, work function (by using the Kelvin probe method), catalytic conversion and diffuse reflectance IR Fourier transform measurements. The most important conclusion is that the different parts of the sensor (sensing layer, electrodes, substrate) all influence the gas detection and their role has to be taken into consideration when one attempts to understand how a sensor works.
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    Choi, K.-I.; Hübner, M.; Haensch, A.; Kim, H.-J.; Weimar, U.; Barsan, N.; Lee, J.-H. Ambivalent Effect of Ni Loading on Gas Sensing Performance in SnO2 Based Gas Sensor. Sens. Actuators, B 2013, 183, 401– 410,  DOI: 10.1016/j.snb.2013.04.007
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    Ambivalent effect of Ni loading on gas sensing performance in SnO2 based gas sensor
    Choi, Kwon-Il; Hubner, Michael; Haensch, Alexander; Kim, Hyo-Joong; Weimar, Udo; Barsan, Nicolae; Lee, Jong-Heun
    Sensors and Actuators, B: Chemical (2013), 183 (), 401-410CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
    The gas sensing characteristics of pure and 0.4-2.0 at% Ni-loaded SnO2 nanoparticles were measured in dry and humid atmospheres. Approx. the same response to 50 ppm CO, response/recovery kinetics, and resistance in air regardless of wide range of humidity variation from dry to 80% r.h. were accomplished by loading 1.0 and 2.0 at% Ni to SnO2. The role of Ni related surface species in the decrease of humidity dependence of gas sensing characteristics was elucidated by diffuse-reflectance FTIR spectroscopy. The work function values detd. from the transient of sensor resistance and contact p.d. revealed that Ni loading to SnO2 dets. the appearance of surface electron acceptors responsible for a significant upward energy bands bending even in N2 atmosphere (>0.5 eV), and, ultimately, explains the significant increase of the sensors baseline resistance and the decrease of the sensor signals. In this way, the origins of the ambivalent effect of Ni loading are clarified and the way towards a rational optimization of the sensor performance opened.
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    Shelke, N. T.; Late, D. J. Hydrothermal Growth of MoSe2 Nanoflowers for Photo- and Humidity Sensor Applications. Sens. Actuators, A 2019, 295, 160– 168,  DOI: 10.1016/j.sna.2019.05.045
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    Hydrothermal growth of MoSe2 nanoflowers for photo- and humidity sensor applications
    Shelke, Nitin T.; Late, Dattatray J.
    Sensors and Actuators, A: Physical (2019), 295 (), 160-168CODEN: SAAPEB; ISSN:0924-4247. (Elsevier B.V.)
    In the present investigation, we report the synthesis of molybdenum diselenide (MoSe2) nanoflowers by facile hydrothermal method for photo- and humidity sensor applications. The obtained samples were characterized thoroughly by x-ray diffraction (XRD), Raman spectroscopy, SEM (SEM), and transmission electron microscopy (TEM). The XRD spectrum shows cryst. nature of the sample. Raman spectroscopy shows two prominent vibration modes of E12g and A1g at ∼ 241 and ∼ 283 cm-1 resp. The cryst. nature of the sample confirmed with the TEM. The MoSe2 nanoflowers based sensor shows high photosensitivity and good response to humidity with excellent prolong stability. The max. photoresponsitivity of ∼ 194% along with response of ∼ 40 ms and recovery time of ∼ 48 ms were obsd. for the sample. In case of humidity sensor, response time of ∼ 53 s and recovery time of ∼ 13 s with max. sensitivity -74% were obsd. under humidity environments. It suggests that, MoSe2 nanoflowers appear as a potential candidate for constructing high-performance nanoelectronics devices.
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    Gupta, S. P.; Pawbake, A. S.; Sathe, B. R.; Late, D. J.; Walke, P. S. Superior Humidity Sensor and Photodetector of Mesoporous ZnO Nanosheets at Room Temperature. Sens. Actuators, B 2019, 293, 83– 92,  DOI: 10.1016/j.snb.2019.04.086
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    Superior humidity sensor and photodetector of mesoporous ZnO nanosheets at room temperature
    Gupta, Shobhnath P.; Pawbake, Amit S.; Sathe, Bhaskar R.; Late, Dattatray J.; Walke, Pravin S.
    Sensors and Actuators, B: Chemical (2019), 293 (), 83-92CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
    Miniaturized sensor technol. is vastly demanding multifunctional materials to fulfill many requirements simultaneously; instead of integrating various sensors into a single device. Efficient operation of these miniaturized sensors at room temp. is highly feasible and cost-effective. The humidity sensing and photodetection is precise merit of sensing in special usage like artificial skin. Sensitivity enhancement in both humidity and photodetection required the high surface area for adsorption as well as a high charge transfer mechanism. The two dimensional (2D) zinc oxide nanosheets (ZnO NS) is the ultimate structure for dimensionally confined transport properties owing to the sp. surface at. configuration that results in high sensitivity, low operating temp., fast response and recovery, and improved selectivity. Furthermore, introducing porosity into 2D nanostructures has opened new opportunities to enhance the efficiency of sensors and detectors via increasing large surface area and tunable phys. and chem. properties. Here we report prepn. of mesoporous and highly cryst. 2D ZnO NS by a single step, template free, cost-effective chem. method. The structural and morphol. characterizations of ZnO NS are carried out using XRD, FESEM, XPS, TEM resp. The high-resoln. TEM images emphasize sheet-like morphol. with a thickness of around 18-22 nm.
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    Theillet, P.-O.; Pierron, O. Quantifying Adsorbed Water Monolayers on Silicon Mems Resonators Exposed to Humid Environments. Sens. Actuators, A 2011, 171, 375– 380,  DOI: 10.1016/j.sna.2011.09.002
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    Quantifying adsorbed water monolayers on silicon MEMS resonators exposed to humid environments
    Theillet, P.-O.; Pierron, O. N.
    Sensors and Actuators, A: Physical (2011), 171 (2), 375-380CODEN: SAAPEB; ISSN:0924-4247. (Elsevier B.V.)
    This study investigated the influence of temp. and humidity on the adsorbed water layer on micron-scale monocryst. silicon (Si) films in air, using a Si-MEMS kHz-frequency resonator. Both temp. and relative humidity induced a reversible change in resonant frequency, attributed to the temp.-dependent properties of Si and to a change in adsorbed water layer. The excellent precision in resonant frequency measurement (0.02 Hz, or 0.5 ppm) allowed precise calcn. of the changes in adsorbed water layer thickness over the specimen surface. The increase in water thickness with relative humidity was a function of temp. and could not be described with simple multimol. adsorption theories such as the BET theory. A likely explanation is the presence of hydrocarbon contaminants on the Si surface. Guidelines are provided to accurately measure the influence of temp. and relative humidity on the adsorbed water layer thickness on micron-scale Si surfaces, using this technique.
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    Panchal, V.; Giusca, C. E.; Lartsev, A.; Martin, N. A.; Cassidy, N.; Myers-Ward, R. L.; Gaskill, D. K.; Kazakova, O. Atmospheric Doping Effects in Epitaxial Graphene: Correlation of Local and Global Electrical Studies. 2D Mater. 2016, 3, 015006,  DOI: 10.1088/2053-1583/3/1/015006
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    Atmospheric doping effects in epitaxial graphene: correlation of local and global electrical studies
    Panchal, Vishal; Giusca, Cristina E.; Lartsev, Arseniy; Martin, Nicholas A.; Cassidy, Nathan; Myers-Ward, Rachael L.; Gaskill, D. Kurt; Kazakova, Olga
    2D Materials (2016), 3 (1), 015006/1-015006/10CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
    We directly correlate the local (20 nmscale) and global electronic properties of a device contg. mono-, bi- and tri-layer epitaxial graphene (EG) domains on 6H-SiC(0001) by simultaneously performing local surface potential measurements using Kelvin probe force microscopy and global transport measurements. Using well-controlled environmental conditions we investigate the doping effects of N2, O2, water vapor and NO2 at concns. representative of the ambient air. We show that presence of O2, water vapor and NO2 leads to p-doping of all EG domains. However, the thicker layers of EG are significantly less affected. Furthermore, we demonstrate that the general consensus of O2 and water vapor present in ambient air providing majority of the p-doping to graphene is a common misconception. We exptl. show that even the combined effect of O2, water vapor, and NO2 at concns. higher than typically present in the atm. does not fully replicate p-doping from ambient air. Thus, for EG gas sensors it is essential to consider naturally occurring environmental effects and properly sep. them from those coming from targeted species.
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    Tannarana, M.; Pataniya, P. M.; Bhakhar, S. A.; Solanki, G. K.; Valand, J.; Narayan, S.; Patel, K. D.; Jha, P. K.; Pathak, V. M. Humidity Sensor Based on Two-Dimensional SnSe2/MWCNTs Nanohybrid for the Online Monitoring of Human Respiration and Touchless Positioning Interface. ACS Sustainable Chem. Eng. 2020, 8, 12595– 12602,  DOI: 10.1021/acssuschemeng.0c04027
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    Humidity Sensor Based on Two-Dimensional SnSe2/MWCNT Nanohybrids for the Online Monitoring of Human Respiration and a Touchless Positioning Interface
    Tannarana, Mohit; Pataniya, Pratik M.; Bhakhar, Sanjay A.; Solanki, G. K.; Valand, Jignesh; Narayan, Som; Patel, Kirit D.; Jha, Prafulla K.; Pathak, V. M.
    ACS Sustainable Chemistry & Engineering (2020), 8 (33), 12595-12602CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
    Herein, the authors report the significantly enhanced humidity responsiveness of resistive devices based on two-dimensional SnSe2/MWCNT nanohybrids. The multifunctional humidity sensor was exploited to establish a human-machine interface and for human interactive applications such as breath monitoring and sensing of the humidity of human skin for a touchless positioning interface. The sensor exhibited great potential owing to a high surface-to-vol. ratio of SnSe2/MWCNT nanohybrids. The sensor has good linear response over a broad humidity range from 10-70% with fast response and recovery. The sensor shows the humidity responsivity of 0.148 in the humidity range of 10-70% and 0.063 for the humidity range of 75-95%. The max. current sensitivity of 857% is achieved at 95% relative humidity for SnSe2/MWCNT nanohybrids, which is quite higher than the sensitivity obtained for pristine SnSe2 nanosheets. As a high-performance electronic device, the sensor has extremely low noise level and high recognition power for small humidity variations. The present finding advocates the huge development in humidity monitoring for biomedical, intelligent electronics, and industrial applications. Significantly enhanced humidity responsiveness is realized for SnSe2/MWCNT nanohybrids for real time human respiration monitoring and a touchless positioning interface.
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    Nataf, G. F.; Grysan, P.; Guennou, M.; Kreisel, J.; Martinotti, D.; Rountree, C. L.; Mathieu, C.; Barrett, N. Low Energy Electron Imaging of Domains and Domain Walls in Magnesium-Doped Lithium Niobate. Sci. Rep. 2016, 6, 33098,  DOI: 10.1038/srep33098
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    Low energy electron imaging of domains and domain walls in magnesium-doped lithium niobate
    Nataf, G. F.; Grysan, P.; Guennou, M.; Kreisel, J.; Martinotti, D.; Rountree, C. L.; Mathieu, C.; Barrett, N.
    Scientific Reports (2016), 6 (), 33098CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
    The understanding of domain structures, specifically domain walls, currently attracts a significant attention in the field of (multi)-ferroic materials. In this article, we analyze contrast formation in full field electron microscopy applied to domains and domain walls in the uniaxial ferroelec. lithium niobate, which presents a large 3.8 eV band gap and for which conductive domain walls have been reported. We show that the transition from Mirror Electron Microscopy (MEM - electrons reflected) to Low Energy Electron Microscopy (LEEM - electrons backscattered) gives rise to a robust contrast between domains with upwards (Pup) and downwards (Pdown) polarization, and provides a measure of the difference in surface potential between the domains. We demonstrate that out-of-focus conditions of imaging produce contrast inversion, due to image distortion induced by charged surfaces, and also carry information on the polarization direction in the domains. Finally, we show that the intensity profile at domain walls provides exptl. evidence for a local stray, lateral elec. field.
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    Leung, T.; Kao, C.; Su, W.; Feng, Y.; Chan, C. Relationship between Surface Dipole, Work Function and Charge Transfer: Some Exceptions to an Established Rule. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 68, 195408,  DOI: 10.1103/PhysRevB.68.195408
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    Relationship between surface dipole, work function and charge transfer: Some exceptions to an established rule
    Leung, T. C.; Kao, C. L.; Su, W. S.; Feng, Y. J.; Chan, C. T.
    Physical Review B: Condensed Matter and Materials Physics (2003), 68 (19), 195408/1-195408/6CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
    Local-d.-functional calcns. were used to study the change of work functions induced by a layer of adsorbates. The authors studied and compared the work function of a monolayer of Mo, Ag, Au, Fe, Co, Ni, Nb, Li, N, and O on W(100), W(110), W(211), and W(111) surfaces. While many systems obey the commonly accepted rule that electroneg. adsorbates increase the work function of the surface, the authors find some exceptions. For example, overlayers of Fe, Co, and Ni increase the work function of W(100), W(211), and W(111), but decrease the work function of the W(110) surface, although the charge transfer is the same in all orientations. Even a layer of O can decrease the work function of W(100), although there are always electrons transferred from the W substrate to the O adsorbates. To understand these results, the authors established the relation between surface dipole d. and work function within the framework of local-d. formalism. Subtle details of the charge transfer can det. the sign and magnitude of surface dipole change, leading to a strong dependence on the orientation of the substrate, with the consequence that the work-function changes are not always governed by the sign and quantity of adsorbate induced charge transfer.
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    Roy, T.; Tosun, M.; Hettick, M.; Ahn, G. H.; Hu, C.; Javey, A. 2D-2D Tunneling Field-Effect Transistors Using WSe2/SnSe2 Heterostructures. Appl. Phys. Lett. 2016, 108, 083111,  DOI: 10.1063/1.4942647
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    2D-2D tunneling field-effect transistors using WSe2/SnSe2 heterostructures
    Roy, Tania; Tosun, Mahmut; Hettick, Mark; Ahn, Geun Ho; Hu, Chenming; Javey, Ali
    Applied Physics Letters (2016), 108 (8), 083111/1-083111/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Two-dimensional materials present a versatile platform for developing steep transistors due to their uniform thickness and sharp band edges. We demonstrate 2D-2D tunneling in a WSe2/SnSe2 van der Waals vertical heterojunction device, where WSe2 is used as the gate controlled p-layer and SnSe2 is the degenerately n-type layer. The van der Waals gap facilitates the regulation of band alignment at the heterojunction, without the necessity of a tunneling barrier. ZrO2 is used as the gate dielec., allowing the scaling of gate oxide to improve device subthreshold swing. Efficient gate control and clean interfaces yield a subthreshold swing of ∼100 mV/dec for >2 decades of drain current at room temp., hitherto unobserved in 2D-2D tunneling devices. The subthreshold swing is independent of temp., which is a clear signature of band-to-band tunneling at the heterojunction. A max. switching ratio ION/IOFF of 107 is obtained. Neg. differential resistance in the forward bias characteristics is obsd. at 77 K. This work bodes well for the possibilities of two-dimensional materials for the realization of energy-efficient future-generation electronics. (c) 2016 American Institute of Physics.
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    Li, F.; Gao, X.; Wang, R.; Zhang, T.; Lu, G. Study on TiO2-SnO2 Core-Shell Heterostructure Nanofibers with Different Work Function and Its Application in Gas Sensor. Sens. Actuators, B 2017, 248, 812– 819,  DOI: 10.1016/j.snb.2016.12.009
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    Study on TiO2-SnO2 core-shell heterostructure nanofibers with different work function and its application in gas sensor
    Li, Feng; Gao, Xing; Wang, Rui; Zhang, Tong; Lu, Geyu
    Sensors and Actuators, B: Chemical (2017), 248 (), 812-819CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
    A core-shell heterostructure nanofibers (NFs) have been synthesized via a coaxial electrospinning approach. Two semiconducting metal oxides (SMOX) with different work function were selected to form the core-shell heterostructure NFs. In this work, SnO2 and TiO2 were chosen as the selected SMOXs for the synthesis of the core-shell heterostructure NFs. The electrons in TiO2 will flow to SnO2, because the work function of TiO2 is smaller than SnO2. This phenomenon could result in an increase of the electrons concn. in the SnO2 shell layers and the amt. of adsorbed oxygen species increased. Therefore, the gas sensing properties of the TiO2-SnO2 core-shell heterostructure NFs were enhanced including a high response to the target gas, good selectivity to the target gas and the rapid response/recovery processes. The approach and results proposed in this study may contribute to the realization of more sensitive core-shell heterostructure NFs sensors.
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    Batzill, M.; Katsiev, K.; Burst, J. M.; Losovyj, Y.; Bergermayer, W.; Tanaka, I.; Diebold, U. Tuning Surface Properties of SnO2(101) by Reduction. J. Phys. Chem. Solids 2006, 67, 1923– 1929,  DOI: 10.1016/j.jpcs.2006.05.042
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    Tuning surface properties of SnO2(101) by reduction
    Batzill, Matthias; Katsiev, Khabibulakh; Burst, James M.; Losovyj, Yaroslav; Bergermayer, Wolfgang; Tanaka, Isao; Diebold, Ulrike
    Journal of Physics and Chemistry of Solids (2006), 67 (9-10), 1923-1929CODEN: JPCSAW; ISSN:0022-3697. (Elsevier B.V.)
    The SnO2(1 0 1) surface can be prepd. with a SnO2 or SnO compn. and consequently the surface Sn-atoms are either in a Sn(II) or Sn(IV) charge state. For a Sn(II) surface, Sn-5s derived surface states are identified by resonant, angle resolved photoemission spectroscopy (ARUPS). The differences in the interface properties of the Sn(II) and Sn(IV) surfaces of SnO2(101) are reviewed on the example of benzene and H2O adsorption. The difference in work function of these 2 surfaces causes a shift of the MOs of benzene by ∼1 eV with respect to the Fermi-level of the substrate. D. functional theory calcns. predict dissocn. of H2O on the stoichiometric (Sn(IV)) surface but only weak mol. adsorption on the reduced Sn(II) surface. These predictions are in agreement with ARUPS measurements that show that at 160 K no H2O adsorbs on the reduced surface but adsorbs dissociatively on the stoichiometric surface. A strong adsorbate induced band bending is also obsd. for H2O adsorption on the stoichiometric surface that is likely assocd. with the formation of surface hydroxyls.
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  63. 63
    Serna, M. I.; Hasan, S. M.; Nam, S.; El Bouanani, L.; Moreno, S.; Choi, H.; Alshareef, H. N.; Minary-Jolandan, M.; Quevedo-Lopez, M. A. Low-Temperature Deposition of Layered SnSe2 for Heterojunction Diodes. Adv. Mater. Interfaces 2018, 5, 1800128,  DOI: 10.1002/admi.201800128
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    Zhang, Q.; Li, M.; Lochocki, E. B.; Vishwanath, S.; Liu, X.; Yan, R.; Lien, H.-H.; Dobrowolska, M.; Furdyna, J.; Shen, K. M. Band Offset and Electron Affinity of Mbe-Grown SnSe2. Appl. Phys. Lett. 2018, 112, 042108,  DOI: 10.1063/1.5016183
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    Band offset and electron affinity of MBE-grown SnSe2
    Zhang, Qin; Li, Mingda; Lochocki, Edward B.; Vishwanath, Suresh; Liu, Xinyu; Yan, Rusen; Lien, Huai-Hsun; Dobrowolska, Malgorzata; Furdyna, Jacek; Shen, Kyle M.; Cheng, Guangjun; Hight Walker, Angela R.; Gundlach, David J.; Xing, Huili G.; Nguyen, N. V.
    Applied Physics Letters (2018), 112 (4), 042108/1-042108/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    SnSe2 is currently considered a potential two-dimensional material that can form a near-broken gap heterojunction in a tunnel field-effect transistor due to its large electron affinity which is exptl. confirmed in this letter. With the results from internal photoemission and angle-resolved photoemission spectroscopy performed on Al/Al2O3/SnSe2/GaAs and SnSe2/GaAs test structures where SnSe2 is grown on GaAs by mol. beam epitaxy, we ascertain a (5.2 ± 0.1) eV electron affinity of SnSe2. The band offset from the SnSe2 Fermi level to the Al2O3 conduction band min. is found to be (3.3 ± 0.05) eV and SnSe2 is seen to have a high level of intrinsic electron (n-type) doping with the Fermi level positioned at about 0.2 eV above its conduction band min. It is concluded that the electron affinity of SnSe2 is larger than that of most semiconductors and can be combined with other appropriate semiconductors to form near broken-gap heterojunctions for the tunnel field-effect transistor that can potentially achieve high on-currents. (c) 2018 American Institute of Physics.
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    Bauer, E. Surface Microscopy with Low Energy Electrons; Springer, 2014; Vol. 23.
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    Henderson, M. A. The Interaction of Water with Solid Surfaces: Fundamental Aspects Revisited. Surf. Sci. Rep. 2002, 46, 1– 308,  DOI: 10.1016/S0167-5729(01)00020-6
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    The interaction of water with solid surfaces: fundamental aspects revisited
    Henderson, Michael A.
    Surface Science Reports (2002), 46 (1-8), 1-308CODEN: SSREDI; ISSN:0167-5729. (Elsevier Science B.V.)
    A review. Water is perhaps the most important and most pervasive chem. on our planet. The influence of water permeates virtually all areas of biochem., chem. and phys. importance, and is esp. evident in phenomena occurring at the interfaces of solid surfaces. Since 1987, when Thiel and Madey (TM) published their review titled 'The interaction of water with solid surfaces: fundamental aspects' in Surface Science Reports, there has been considerable progress made in further understanding the fundamental interactions of water with solid surfaces. In the decade and a half, the increased capability of surface scientists to probe at the mol.-level has resulted in more detailed information of the properties of water on progressively more complicated materials and under more stringent conditions. This progress in understanding the properties of water on solid surfaces is evident both in areas for which surface science methodol. has traditionally been strong (catalysis and electronic materials) and also in new areas not traditionally studied by surface scientists such as electrochem., photoconversion, mineralogy, adhesion, sensors, atm. chem. and tribol. Researchers in all these fields grapple with very basic questions regarding the interactions of water with solid surfaces such as how is water adsorbed, what are the chem. and electrostatic forces that constitute the adsorbed layer, how is water thermally or non-thermally activated and how do coadsorbates influence these properties of water. The attention paid to these and other fundamental questions in the past decade and a half has been immense. In this review, exptl. studies published since the TM review are assimilated with those covered by TM to provide a current picture of the fundamental interactions of water with solid surfaces.
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    Inamdar, A. N.; Som, N. N.; Pratap, A.; Jha, P. K. Hydrogen Evolution and Oxygen Evolution Reactions of Pristine and Alkali Metal Doped Snse2 Monolayer. Int. J. Hydrogen Energy 2020, 45, 18657– 18665,  DOI: 10.1016/j.ijhydene.2019.07.093
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    Hydrogen evolution and oxygen evolution reactions of pristine and alkali metal doped SnSe2 monolayer
    Inamdar, Archana N.; Som, Narayan N.; Pratap, Arun; Jha, Prafulla K.
    International Journal of Hydrogen Energy (2020), 45 (37), 18657-18665CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
    Many transition metal di-selenides such as MoSe2 and WSe2 show good catalytic activity on their edges with limited active orientations. These metal di-selenides are actively being used as target material for increasing the no. of electrocatalytic active sites and in turn to improve the H evolution reaction (HER) and O evolution reaction (OER) activities by increasing the ratio of edges to the basal plane. In present work, the authors have studied the activity of pristine and alkali atoms (Na, K and Ca) doped-SnSe2 for HER and OER catalyst. The state-of-art d. functional theory (DFT) based computations were performed for estg. the catalytic activity of the pristine and doped SnSe2 by evaluating the adsorption and Gibbs free energies subjected to H and O adsorption. Further, to get better prediction of adsorption energy on the individual catalytic surface, the authors have included the dispersion correction term to exchange-correlation functional. The pristine SnSe2 is not a good HER catalyst when H is adsorbed on its basal plane. However, edge-sites show the good H adsorption and indicates that the edges of SnSe2 are the most preferential site for H adsorption. As far as the catalytic activity of SnSe2 with dopants is concerned, the Na-doped SnSe2 among all shows the best catalytic activity over its edge-site; whereas K and Ca doped SnSe2 show basal plane as preferred catalytic site. It is interesting to note that the disadvantage of low catalytic activity on basal plane of SnSe2 can be improved by selective doping of alkali metals.
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    Deng, J.; Mo, Y.; Liu, J.; Guo, R.; Zhang, Y.; Xue, W.; Zhang, Y. In Vitro Study of SnS2, BiOCl and SnS2-Incorporated BiOCl Inorganic Nanoparticles Used as Doxorubicin Carrier. J. Nanosci. Nanotechnol. 2016, 16, 5740– 5745,  DOI: 10.1166/jnn.2016.11745
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    In vitro study of SnS2, BiOCl and SnS2-incorporated BiOCl inorganic nanoparticles used as doxorubicin carrier
    Deng, Jiangming; Mo, Yunfei; Liu, Jianghui; Guo, Rui; Zhang, Yi; Xue, Wei; Zhang, Yuanming
    Journal of Nanoscience and Nanotechnology (2016), 16 (6), 5740-5745CODEN: JNNOAR; ISSN:1533-4880. (American Scientific Publishers)
    Inorg. nanoparticles have been widely used in biomedical field. In this paper, we try to study the use of three types of inorg. nanoparticles (i.e., SnS2, BiOCl and SnS2-incorporated BiOCl (SnS2/BiOCl)) as doxorubicin (DOX) carriers. Firstly, SnS2, BiOCl and SnS2/BiOCl were synthesized, then were characterized by TEM, nanoparticles size and zeta potential. Next the drug release and cell viability test were carried out. The cell viability test indicated that the drug carriers can effectively kill HeLa cells while maintaining low cytotoxicity against normal cells-fibroblasts. The results show the potential of SnS2/BiOCl nanoparticles for antitumor applications.
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    Wu, S.; Liu, C.; Wu, Z.; Miao, L.; Gao, J.; Hu, X.; Chen, J.; Zheng, Y.; Wang, X.; Shen, C. Realizing Tremendous Electrical Transport Properties of Polycrystalline SnSe2 by Cl-Doped and Anisotropy. Ceram. Int. 2019, 45, 82– 89,  DOI: 10.1016/j.ceramint.2018.09.136
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    Realizing tremendous electrical transport properties of polycrystalline SnSe2 by Cl-doped and anisotropy
    Wu, Shaohai; Liu, Chengyan; Wu, Zhengsen; Miao, Lei; Gao, Jie; Hu, Xiaokai; Chen, Junliang; Zheng, Yanyan; Wang, Xiuxia; Shen, Chengjin; Yang, Hengquan; Zhou, Xiaoyuan
    Ceramics International (2019), 45 (1), 82-89CODEN: CINNDH; ISSN:0272-8842. (Elsevier Ltd.)
    SnSe2 is regarded as an attractive thermoelec. material for its structural and chem. analogy to SnSe that is claimed with the highest ZT in single crystal. In this study, the pure and Cl-doped SnSe2 polycrystals (3%, 6%, 9% and 12% molar Cl content) were fabricated in four steps that are hydrothermal synthesis, heating purifn., diffusion doping, and spark plasma sintering. The phase structure, lamellar morphol. and crystallite orientation were studied for the synthesized SnSe2 powder and the sintered pellets. The structural evolution was traced from the SnSe2 hexagonal plates of powders to the (001) oriented grains in pellets. The Cl doping into SnSe2 was verified by phase compn., lattice parameter, element distribution, and chem. valance. The doped Cl increased both the carrier concn. and the mobility. The anisotropic thermoelec. properties of SnSe2 bulk materials were investigated as functions of temp. from 50 °C to 300 °C and the doping amt., resp. The Seebeck coeff. was less anisotropic than the elec. and thermal conduction. The grain orientation influenced the anisotropy of the elec. and thermal cond. at a similar ratio. The power factors were less dependent on temp. with an optimum in-plane 1.06 mW m-1 K-2 and out-of-plane 0.41 mW m-1 K-2. The highest ZTs of 0.3 were attained at 300 °C in both directions.
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    Nagaraju, G.; Cha, S. M.; Sekhar, S. C.; Yu, J. S. Metallic Layered Polyester Fabric Enabled Nickel Selenide Nanostructures as Highly Conductive and Binderless Electrode with Superior Energy Storage Performance. Adv. Energy Mater. 2017, 7, 1601362,  DOI: 10.1002/aenm.201601362
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    Dimitriev, Y.; Yordanov, St.; Lakov, L. The Structure of Oxide Glasses Containing SeO2. J. Non-Cryst. Solids 2001, 293–295, 410– 415,  DOI: 10.1016/S0022-3093(01)00836-5
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    The structure of oxide glasses containing SeO2
    Dimitriev, Y.; Yordanov, St.; Lakov, L.
    Journal of Non-Crystalline Solids (2001), 293-295 (), 410-415CODEN: JNCSBJ; ISSN:0022-3093. (Elsevier Science B.V.)
    A review of results for glass formation in selenite systems obtained during the last 20 yr. For all of them, the vitrification regions are situated near the SeO2 corner, which is the main glass-former. The structure of model compns. is studied to elucidate the role of the different building units in the formation of the amorphous network. The IR spectra of binary selenite glasses are compared with those for more complicated compns. It is proven that it is possible to modify the network of selenite glasses by introducing compatible polyhedra TeO4, TeO3, VO5, BiO6, Mo2O8 or BO3 with SeO3 units. The better glass-forming tendency of compns. with 50-90 mol% SeO2 is related to the creation of addnl. disorder in the SeO3 chains by involving other structural units in them. These new glass-forming units are capable of transforming the structure into layers or three-dimensional random networks with a low at. mobility.
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    Bachvarova-Nedelcheva, A.; Iordanova, R.; Kostov, K. L.; Yordanov, S.; Ganev, V. Structure and Properties of a Non-Traditional Glass Containing TeO2, SeO2 and MoO3. Opt. Mater. 2012, 34, 1781– 1787,  DOI: 10.1016/j.optmat.2012.05.002
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    Structure and properties of a non-traditional glass containing TeO2, SeO2 and MoO3
    Bachvarova-Nedelcheva, A.; Iordanova, R.; Kostov, K. L.; Yordanov, St.; Ganev, V.
    Optical Materials (Amsterdam, Netherlands) (2012), 34 (11), 1781-1787CODEN: OMATET; ISSN:0925-3467. (Elsevier B.V.)
    A glass contg. SeO2, TeO2, MoO3 and La2O3 was obtained at high oxygen pressure (P = 36 MPa) using pure oxides as precursors. The real bulk chem. compn. of the glass according to LA-ICP-MS anal. is 17SeO2·50TeO2·32MoO3·1La2O3 (wt.%). The glass was characterized by X-ray diffraction, SEM (SEM), DTA (DTA), UV-Vis, XPS, IR and EPR spectroscopy. According to DTA the glass transition temp. (Tg) is below 300°C. By IR and XPS was detd. the main building units (TeO3, TeO4, SeO3, Mo2O8) and the existing of mixed bridging bonds only, which build up the amorphous network. It was established by UV-Vis that the glass is transparent above 490 nm. As a result of a lengthy heat treatment, crystn. took place and crystals rich in SeO2 and TeO2 were found incorporated into the amorphous part contg. all components.
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    Fan, Y.; Zhuo, Y.; Li, L. Seo2 Adsorption on Cao Surface: Dft and Experimental Study on the Adsorption of Multiple SeO2 Molecules. Appl. Surf. Sci. 2017, 420, 465– 471,  DOI: 10.1016/j.apsusc.2017.04.233
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    SeO2 adsorption on CaO surface: DFT and experimental study on the adsorption of multiple SeO2 molecules
    Fan, Yaming; Zhuo, Yuqun; Li, Liangliang
    Applied Surface Science (2017), 420 (), 465-471CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    SeO2 adsorption mechanisms on CaO surface were investigated by both d. functional theory (DFT) calcns. and adsorption expts. Adsorption of multiple SeO2 on the CaO (001) surface was investigated using slab model. Based on the results of adsorption energy and surface property, a double-layer adsorption mechanisms were proposed. In expts., the SeO2 adsorption products were prepd. in a U-shaped quartz reactor at 200 °C. The surface morphol. was investigated by field emission SEM (FE-SEM). The superficial and total SeO2 mass fractions were measured by XPS and inductively coupled plasma at. emission spectroscopy (ICP-AES), resp. The surface valence state and bulk structure are detd. by XPS and x-ray diffraction (XRD). The exptl. results are in good agreement with the DFT results. In conclusion, the fundamental SeO2 chemisorption mechanisms on CaO surface were suggested.
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    Al-Hada, N. M.; Kamari, H. M.; Baqer, A. A.; Shaari, A. H.; Saion, E. Thermal Calcination-Based Production of SnO2 Nanopowder: An Analysis of Sno2 Nanoparticle Characteristics and Antibacterial Activities. Nanomaterials 2018, 8, 250,  DOI: 10.3390/nano8040250
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    Thermal calcination-based production of SnO2 nanopowder: an analysis of SnO2 nanoparticle characteristics and antibacterial activities
    Al-Hada, Naif Mohammed; Kamari, Halimah Mohamed; Baqer, Anwar Ali; Shaari, Abdul H.; Saion, Elias
    Nanomaterials (2018), 8 (4), 250-267CODEN: NANOKO; ISSN:2079-4991. (MDPI AG)
    SnO2 nanoparticle prodn. using thermal treatment with tin(II) chloride dihydrate and polyvinylpyrrolidone capping agent precursor materials for calcination was investigated. Samples were analyzed using X-ray diffraction (XRD), SEM (SEM), energy dispersive X-ray (EDX), transmission electron microscopy (TEM), Fourier Transform IR Spectroscopy (FT-IR), XPS, diffuse UV-vis reflectance spectra, photoluminescence (PL) spectra and the ESR (ESR). XRD anal. found tetragonal cryst. structures in the SnO2 nanoparticles generated through calcination. EDX and FT-IR spectroscopy phase anal. verified the derivation of the Sn and O in the SnO2 nanoparticle samples from the precursor materials. An av. nanoparticle size of 4-15.5 nm was achieved by increasing calcination temp. from 500 °C to 800 °C, as confirmed through TEM. The valence state and surface compn. of the resulting nanoparticle were analyzed using XPS. Diffuse UV-vis reflectance spectra were used to evaluate the optical energy gap using the Kubelka-Munk equation. Greater calcination temp. resulted in the energy band gap falling from 3.90 eV to 3.64 eV. PL spectra indicated a pos. relationship between particle size and photoluminescence.
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    Zhang, W.; Li, M.; Xiao, X.; Huang, X.; Jiang, Y.; Fan, X.; Chen, L. In Situ Synthesis of Ultrasmall Sno2 Quantum Dots on Nitrogen-Doped Reduced Graphene Oxide Composite as High Performance Anode Material for Lithium-Ion Batteries. J. Alloys Compd. 2017, 727, 1– 7,  DOI: 10.1016/j.jallcom.2017.04.316
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    In situ synthesis of ultrasmall SnO2 quantum dots on nitrogen-doped reduced graphene oxide composite as high performance anode material for lithium-ion batteries
    Zhang, Wei; Li, Meng; Xiao, Xuezhang; Huang, Xu; Jiang, Yiqun; Fan, Xiulin; Chen, Lixin
    Journal of Alloys and Compounds (2017), 727 (), 1-7CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
    SnO2 is considered as one of the anode material for Li-ion batteries in terms of its superiority in high theor. capacity (1494 mAh g-1), low cost and environmental friendly. However, it is suffered from several issues such as rapid capacity deterioration, undesirable aggregation of tin particles and pesky expansion of vol. To conquer these shortcomings, a novel composite of ultrasmall SnO2 quantum dots with an av. particle size of 4-5 nm anchored on nitrogen-doped reduced graphene oxide (SnO2@NRGO) was first in situ synthesized By means of hydrothermal method. The results show that as-prepd. SnO2@NRGO electrode exhibits a greater enhancement in its initial discharge capacity (1678.4 mAh g-1) and reversible capacity (1333.5 mAh g-1 after 450 cycles) at a c.d. of 500 mA g-1, implying a long cycle life. Furthermore, the high rate capability of SnO2@NRGO is superior to SnO2@RGO and SnO2 electrodes. The excellent electrochem. reversibility of SnO2@NRGO electrode can be ascribed to the great cond., ultrahigh sp. surface area and the synergetic effect between ultrasmall SnO2 quantum dots and NRGO.
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    Wakita, T.; Paris, E.; Kobayashi, K.; Terashima, K.; Hacisalihoǧlu, M. Y.; Ueno, T.; Bondino, F.; Magnano, E.; Píš, I.; Olivi, L.; Akimitsu, J.; Muraoka, Y.; Yokoya, T.; Saini, N. L. The Electronic Structure of Ag1-XSn1+XSe2 (X = 0.0, 0.1, 0.2, 0.25 and 1.0). Phys. Chem. Chem. Phys. 2017, 19, 26672– 26678,  DOI: 10.1039/C7CP05369J
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    The electronic structure of Ag1-xSn1+xSe2 (x = 0.0, 0.1, 0.2, 0.25 and 1.0)
    Wakita, Takanori; Paris, Eugenio; Kobayashi, Kaya; Terashima, Kensei; Hacisalihoglu, Muammer Yasin; Ueno, Teppei; Bondino, Federica; Magnano, Elena; Pis, Igor; Olivi, Luca; Akimitsu, Jun; Muraoka, Yuji; Yokoya, Takayoshi; Saini, Naurang L.
    Physical Chemistry Chemical Physics (2017), 19 (39), 26672-26678CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    We have studied the valence electronic structure of Ag1-xSn1+xSe2 (x = 0.0, 0.1, 0.2, 0.25) and SnSe (x = 1.0) by a combined anal. of X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS) measurements. Both XAS and XPS reveal an increase in electron carriers in the system with x (i.e. excess Sn concn.) for 0 ≤ x ≤ 0.25. The core-level spectra (Sn 3d, Ag 3d and Se 3d) show that the charge state of Ag is almost 1+, while that of of Sn splits into Sn2+ and Sn4+ (providing clear evidence of valence skipping for the first time) with a concomitant splitting of Se into Se2- and Se2-δ states. The x dependence of the split components in Sn and Se together with the Se-K edge XAS reveals that the Se valence state may have an essential role in the transport properties of this system.
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    Hoch, L. B.; Wood, T. E.; O’Brien, P. G.; Liao, K.; Reyes, L. M.; Mims, C. A.; Ozin, G. A. The Rational Design of a Single-Component Photocatalyst for Gas-Phase Co2 Reduction Using Both Uv and Visible Light. Adv. Sci. 2014, 1, 1400013,  DOI: 10.1002/advs.201400013
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    The Rational Design of a Single-Component Photocatalyst for Gas-Phase CO2 Reduction Using Both UV and Visible Light
    Hoch Laura B; O'Brien Paul G; Liao Kristine; Reyes Laura M; Ozin Geoffrey A; Wood Thomas E; Mims Charles A
    Advanced science (Weinheim, Baden-Wurttemberg, Germany) (2014), 1 (1), 1400013 ISSN:2198-3844.
    The solar-to-chemical energy conversion of greenhouse gas CO2 into carbon-based fuels is a very important research challenge, with implications for both climate change and energy security. Herein, the key attributes of hydroxides and oxygen vacancies are experimentally identified in non-stoichiometric indium oxide nanoparticles, In2O3-x(OH)y, that function in concert to reduce CO2 to CO under simulated solar irradiation.
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    Detweiler, Z. M.; Wulfsberg, S. M.; Frith, M. G.; Bocarsly, A. B.; Bernasek, S. L. The Oxidation and Surface Speciation of Indium and Indium Oxides Exposed to Atmospheric Oxidants. Surf. Sci. 2016, 648, 188– 195,  DOI: 10.1016/j.susc.2015.10.026
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  • Abstract

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    Figure 1

    Figure 1. (a) LEEM I–V curves at the MEM–LEEM transition for the as-cleaved sample (black), after a dose of 700 L of O2 (blue), and after air exposure for 15 min (pink). The shift of the MEM–LEEM transition, characterized by the sharp decrease in intensity, indicates an oxidation-induced modification of the surface potential. (b) Changes in charge density after the formation of the interface between the SnSe2 substrate and SnO2 skin. Sn, Se, and O atoms are represented as dark blue, light green, and red balls, respectively.

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    Figure 2

    Figure 2. Changes in charge density after adsorption of one water molecule on (a) SnSe2 and (b) SnO2 skin-terminated SnSe2. Panel c represents the DOS of SnO2 skin-terminated SnSe2 (black) and of the same system modified by the adsorption of one (red) and two (blue) water molecules. The Fermi level is set at 0. Panel d shows the response of the SnSe2–SnO2 heterostructure to 20% relative humidity (RH) at an operational temperature (OT) of 150 °C (note that the average residence time of the gas in the cell is approximately 10 min).

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    Figure 3

    Figure 3. HREELS spectra in the region of the O–H band acquired after exposure to 105 L of H2O at room temperature the surfaces of different Sn-based chalcogenides: SnSe2 (orange curve), SnSe1.7 (black), SnSe1.4 (green), and SnSe (blue). To provide a straightforward comparison, the figure also displays data for H2O-dosed InSe (red) and PtTe1.6 (brown) surfaces (105 L at room temperature). The impinging energy is 4 eV.

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    Figure 4

    Figure 4. (a) O-1s, (b) Sn-3d, and (c) Se-3d core levels for the pristine surface of SnSe2 cleaved in situ under ultra-high-vacuum conditions and its alteration after exposure to oxidative (105 L of O2) and humid (105 L of H2O) environments at room temperature. The photon energy is 800 eV. We also report in each panel the corresponding spectrum for SnO2–SnSe2–x exposed to a humid environment at room temperature, with x estimated to be 0.29.

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  • References

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      Deng, Wenjie; Chen, Xiaoqing; Li, Yufo; You, Congya; Chu, Feihong; Li, Songyu; An, Boxing; Ma, Yang; Liao, Lei; Zhang, Yongzhe
      Journal of Physical Chemistry Letters (2020), 11 (11), 4490-4497CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Two-dimensional (2D) materials and their derived quasi one-dimensional structure provide incredible possibilities for the field of photoelec. detection due to their intrinsic optical and elec. properties. However, the photogenerated carriers in atomically thin media are poor due to the low optical absorption, which greatly limits their performance. Here, in the MoS2 nanoscroll photodetector, we meticulously investigated the avalanche multiplication effect. The results show that by employing the nanoscroll structure, the required threshold elec. field for triggering avalanche multiplication is significantly lower than that of MoS2 flake due to the modulation of the energy band and intervalley scattering through the strain effect. Consequently, avalanche multiplication could efficiently enhance the photoresponsivity to >104 A/W. Furthermore, enhanced avalanche multiplication could be generalized to other TMDCs through theor. prediction. The results not only are significant for the understanding of the intrinsic nature of 2D materials but also reveal meaningful advances in high-performance and low-power consumption photodetection.
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    5. 5
      Geng, W. T.; Wang, V.; Liu, Y. C.; Ohno, T.; Nara, J. Moiré Potential, Lattice Corrugation, and Band Gap Spatial Variation in a Twist-Free MoTe2/MoS2 Heterobilayer. J. Phys. Chem. Lett. 2020, 11, 2637– 2646,  DOI: 10.1021/acs.jpclett.0c00605
      5
      Moir´e potential, lattice corrugation, and band gap spatial variation in a twist-free MoTe2/MoS2 heterobilayer
      Geng, W. T.; Wang, V.; Liu, Y. C.; Ohno, T.; Nara, J.
      Journal of Physical Chemistry Letters (2020), 11 (7), 2637-2646CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      To have a fully first-principles description of the moir´e pattern in transition-metal dichalcogenide heterobilayers, we have carried out d. functional theory calcns. on a MoTe2(9 x 9)/MoS2(10 x 10) stacking, which has a superlattice larger than an exciton yet not large enough to justify a continuum model treatment. Lattice corrugation is found to be significant in both monolayers, yet its effect on the electronic properties is marginal. We reveal that the variation of the av. local potential near Mo atoms in both MoTe2 and MoS2 layers displays a conspicuous moir´e pattern. They are the intralayer moir´e potentials correlating closely with the spatial variation of the valence band max. and conduction band min. The interlayer moir´e potential, defined as the difference between the two intralayer moir´e potentials, changes roughly in proportion to the band gap variation in the moir´e cell. This finding might be instructive in chem. engineering of van der Waals bilayers.
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    6. 6
      Zou, X.; Zhang, Z.; Chen, X.; Yakobson, B. I. Structure and Dynamics of the Electronic Heterointerfaces in MoS2 by First-Principles Simulations. J. Phys. Chem. Lett. 2020, 11, 1644– 1649,  DOI: 10.1021/acs.jpclett.0c00147
      6
      Structure and Dynamics of the Electronic Heterointerfaces in MoS2 by First-Principles Simulations
      Zou, Xiaolong; Zhang, Zhuhua; Chen, Xiaobin; Yakobson, Boris I.
      Journal of Physical Chemistry Letters (2020), 11 (5), 1644-1649CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      The transformation of 2H-MoS2 from semiconducting 2H to metallic 1T phases is crit. for its electrochem. and device applications, where the formation and dynamics of electronic heterostructures play a key role. Using first-principles calcns., we explore detailed at. structures and migration processes of such interfaces. While armchair interfacial bonding is severely weakened by the distortion in 1T phase, stable structures form for either Mo- or S-oriented zigzag interfaces with low contact resistance. Different zigzag interfaces have distinct local bonding, which renders interface migration behaviors strongly anisotropic. For Mo-oriented interfaces, both a low formation energy and the migration barrier of the kinks make them prone to fast migration. In contrast, the S-oriented interfaces are more immobile due to the high formation energies of kinks and thus dominate the phys. properties of the whole heterostructures. Our findings not only explain various exptl. observations but also provide insights into phase transition behaviors in 2D MoS2.
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    7. 7
      Guo, B. Y.; Jiang, S. D.; Tang, M. J.; Li, K.; Sun, S.; Chen, P. Y.; Zhang, S. Mos2 Membranes for Organic Solvent Nanofiltration: Stability and Structural Control. J. Phys. Chem. Lett. 2019, 10, 4609– 4617,  DOI: 10.1021/acs.jpclett.9b01780
      7
      MoS2 Membranes for Organic Solvent Nanofiltration: Stability and Structural Control
      Guo, Bing-Yi; Jiang, Shu-Dong; Tang, Ming-Jian; Li, Kerui; Sun, Shipeng; Chen, Po-Yen; Zhang, Sui
      Journal of Physical Chemistry Letters (2019), 10 (16), 4609-4617CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      This paper reveals the chem., structural, and sepn. stability of stacked molybdenum disulfide (MoS2) membranes and establishes a low-cost and facile approach to developing stable, selective membranes for efficient mol. sepn. in an org. solvent. MoS2 nanoflakes that were dominant by monolayer MoS2 sheets as prepd. via direct chem. exfoliation (chem-MoS2) were found to be chem. and structurally instable, with a sharp decrease in the level of solute rejection within a few days. Few-layer MoS2 nanoflakes were then fabricated using a hydrothermal method (hydro-MoS2). A "supportive" drying process involving glycerol pretreatment and drying in an oven was established to allow realignment of nanoflakes and adjustment of interflake spacing. We have shown that the hydro-MoS2 membranes provide a mean interflake free spacing of ∼1 nm, which is ideal for the sepn. of a model solute (Rose Bengal, size of ∼1.45 nm) from the solvent isopropanol (size of 0.58 nm) with good long-term stability over a 7 day test.
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    8. 8
      Hu, C.; Jiang, Z.; Zhou, W.; Guo, M.; Yu, T.; Luo, X.; Yuan, C. Wafer-Scale Sulfur Vacancy-Rich Monolayer MoS2 for Massive Hydrogen Production. J. Phys. Chem. Lett. 2019, 10, 4763– 4768,  DOI: 10.1021/acs.jpclett.9b01399
      8
      Wafer-Scale Sulfur Vacancy-Rich Monolayer MoS2 for Massive Hydrogen Production
      Hu, Ce; Jiang, Zhenzhen; Zhou, Wenda; Guo, Manman; Yu, Ting; Luo, Xingfang; Yuan, Cailei
      Journal of Physical Chemistry Letters (2019), 10 (16), 4763-4768CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      As one of the promising low-cost and high-efficiency catalysts for the electrochem. hydrogen evolution reaction (HER), it is well-known that there are both tiny exposed catalytic active edge sites and large-area inert basal planes in two-dimensional MoS2 structures. For enhancing its HER activity, extensive work has been done to activate the inert basal plane of MoS2. In this article, wafer-scale (2 in.) continuous monolayer MoS2 films with substantial in situ generated sulfur vacancies are fabricated by employing the laser mol. beam epitaxy process benefitting from ultrahigh vacuum growth condition and high substrate temp. The intrinsic sulfur vacancies throughout the wafer-scale basal plane present an ideal electrocatalytic platform for massive hydrogen prodn. The fabricated vacancy-rich monolayer MoS2 can achieve a c.d. of -10 mA/cm2 at an overpotential of -256 mV. The wafer-scale fabrications of sulfur vacancy-rich monolayer MoS2 provide great leaps forward in the practical application of MoS2 for massive hydrogen prodn.
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    9. 9
      Yi, M.; Shen, Z. A Review on Mechanical Exfoliation for the Scalable Production of Graphene. J. Mater. Chem. A 2015, 3, 11700– 11715,  DOI: 10.1039/C5TA00252D
      9
      A review on mechanical exfoliation for the scalable production of graphene
      Yi, Min; Shen, Zhigang
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (22), 11700-11715CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      A review. Mass prodn. and com. availability are prerequisites for the viability and wide application of graphene. The exfoliation of graphite to give graphene is one of the most promising ways to achieve large-scale prodn. at an extremely low cost. This review focuses on discussing different exfoliation techniques based on a common mech. mechanism; because a deep understanding of the exfoliation mechanism can provide fruitful information on how to efficiently achieve high-quality graphene by optimizing exfoliation techniques. The authors highlight the recent progress on mech. exfoliation for graphene prodn. during the last decade. The emphasis is set on the widely used sonication method with the latest insight into sonication-induced defects, the newly explored ball milling method, the fluid dynamics method that has emerged in the last three years, and the innovative supercrit. fluid method. The authors also give an outlook on how to achieve high-quality graphene efficiently using mech. exfoliation techniques. The authors hope this review will point towards a rational direction for the scalable prodn. of graphene.
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    10. 10
      Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; Sun, Z.; De, S.; McGovern, I. T.; Holland, B.; Byrne, M.; Gun’ko, Y. K.; Boland, J. J.; Niraj, P.; Duesberg, G.; Krishnamurthy, S.; Goodhue, R.; Hutchison, J.; Scardaci, V.; Ferrari, A. C.; Coleman, J. N. High-Yield Production of Graphene by Liquid-Phase Exfoliation of Graphite. Nat. Nanotechnol. 2008, 3, 563– 568,  DOI: 10.1038/nnano.2008.215
      10
      High-yield production of graphene by liquid-phase exfoliation of graphite
      Hernandez, Yenny; Nicolosi, Valeria; Lotya, Mustafa; Blighe, Fiona M.; Sun, Zhenyu; De, Sukanta; McGovern, I. T.; Holland, Brendan; Byrne, Michele; Gun'Ko, Yurii K.; Boland, John J.; Niraj, Peter; Duesberg, Georg; Krishnamurthy, Satheesh; Goodhue, Robbie; Hutchison, John; Scardaci, Vittorio; Ferrari, Andrea C.; Coleman, Jonathan N.
      Nature Nanotechnology (2008), 3 (9), 563-568CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      Fully exploiting the properties of graphene will require a method for the mass prodn. of this remarkable material. Two main routes are possible: large-scale growth or large-scale exfoliation. Here, we demonstrate graphene dispersions with concns. up to ∼0.01 mg mL-1, produced by dispersion and exfoliation of graphite in org. solvents such as N-methyl-pyrrolidone. This is possible because the energy required to exfoliate graphene is balanced by the solvent-graphene interaction for solvents whose surface energies match that of graphene. We confirm the presence of individual graphene sheets by Raman spectroscopy, transmission electron microscopy and electron diffraction. Our method results in a monolayer yield of ∼1 wt%, which could potentially be improved to 7-12 wt% with further processing. The absence of defects or oxides is confirmed by x-ray photoelectron, IR, and Raman spectroscopies. We are able to produce semi-transparent conducting films and conducting composites. Soln. processing of graphene opens up a range of potential large-area applications, from device and sensor fabrication to liq.-phase chem. Fully exploiting the properties of graphene will require a method for the mass prodn. of this remarkable material. The dispersion and exfoliation of graphite in org. solvents can produce graphene monolayers with a yield of about 1% by wt. Moreover, these samples are free from defects and oxides, and can be used to produce semi-transparent conducting films and conducting composites.
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    11. 11
      Jin, Z.; Li, X.; Mullen, J. T.; Kim, K. W. Intrinsic Transport Properties of Electrons and Holes in Monolayer Transition-Metal Dichalcogenides. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 90, 045422,  DOI: 10.1103/PhysRevB.90.045422
      11
      Intrinsic transport properties of electrons and holes in monolayer transition-metal dichalcogenides
      Jin, Zhenghe; Li, Xiaodong; Mullen, Jeffrey T.; Kim, Ki Wook
      Physical Review B: Condensed Matter and Materials Physics (2014), 90 (4), 045422CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      Intrinsic electron- and hole-phonon interactions are investigated in monolayer transition-metal dichalcogenides MX2 (M = Mo, W; X = S, Se) based on a d. functional theory formalism. Due to their structural similarities, all four materials exhibit qual. comparable scattering characteristics with the acoustic phonons playing a dominant role near the conduction and valence band extrema at the K point. However, substantial differences are obsd. quant. leading to disparate results in the transport properties. Of those considered, WS2 provides the best performance for both electrons and holes with high mobilities and satn. velocities in the full-band Monte Carlo anal. of the Boltzmann transport equation. It is also found that monolayer MX2 crystals with an exception of MoSe2 generally show hole mobilities comparable to or even larger than the value for bulk silicon at room temp., suggesting a potential opportunity in p-type devices. The anal. is extended to est. the effective deformation potential consts. for a simplified treatment as well.
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    12. 12
      Edmonds, M. T.; Tadich, A.; Carvalho, A.; Ziletti, A.; O’Donnell, K. M.; Koenig, S. P.; Coker, D. F.; Özyilmaz, B.; Neto, A. H. C.; Fuhrer, M. S. Creating a Stable Oxide at the Surface of Black Phosphorus. ACS Appl. Mater. Interfaces 2015, 7, 14557– 14562,  DOI: 10.1021/acsami.5b01297
      12
      Creating a Stable Oxide at the Surface of Black Phosphorus
      Edmonds, M. T.; Tadich, A.; Carvalho, A.; Ziletti, A.; O'Donnell, K. M.; Koenig, S. P.; Coker, D. F.; Ozyilmaz, B.; Castro Neto, A. H.; Fuhrer, M. S.
      ACS Applied Materials & Interfaces (2015), 7 (27), 14557-14562CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      The stability of the surface of in situ cleaved black phosphorus crystals upon exposure to atm. is investigated with synchrotron-based photoelectron spectroscopy. After 2 days atm. exposure a stable subnanometer layer of primarily P2O5 forms at the surface. The work function increases by 0.1 eV from 3.9 eV for as-cleaved black phosphorus to 4.0 eV after formation of the 0.4 nm thick oxide, with phosphorus core levels shifting by <0.1 eV. The results indicate minimal charge transfer, suggesting that the oxide layer is suitable for passivation or as an interface layer for further dielec. deposition.
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    13. 13
      Kumar, A.; Telesio, F.; Forti, S.; Al-Temimy, A.; Coletti, C.; Serrano-Ruiz, M.; Caporali, M.; Peruzzini, M.; Beltram, F.; Heun, S. STM Study of Exfoliated Few Layer Black Phosphorus Annealed in Ultrahigh Vacuum. 2D Mater. 2019, 6, 015005,  DOI: 10.1088/2053-1583/aadd20
      13
      STM study of exfoliated few layer black phosphorus annealed in ultrahigh vacuum
      Kumar, Abhishek; Telesio, F.; Forti, S.; Al-Temimy, A.; Coletti, C.; Serrano-Ruiz, M.; Caporali, M.; Peruzzini, M.; Beltram, F.; Heun, S.
      2D Materials (2019), 6 (1), 015005CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
      Black phosphorus (bP) has emerged as an interesting addn. to the category of two-dimensional materials. Surface-science studies on this material are of great interest, but they are hampered by bP's high reactivity to oxygen and water, a major challenge to scanning tunneling microscopy (STM) expts. As a consequence, the large majority of these studies were performed by cleaving a bulk crystal in situ. Here we present a study of surface modifications on exfoliated bP flakes upon consecutive annealing steps, up to 550°C, well above the sublimation temp. of bP. In particular, our attention is focused on the temp. range 375°C-400°C, when sublimation starts, and a controlled desorption from the surface occurs alongside with the formation of characteristic well-aligned craters. There is an open debate in the literature about the crystallog. orientation of these craters, whether they align along the zigzag or the armchair direction. Thanks to the at. resoln. provided by STM, we are able to identify the orientation of the craters with respect to the bP crystal: the long axis of the craters is aligned along the zigzag direction of bP. This allows us to solve the controversy, and, moreover, to provide insight in the underlying desorption mechanism leading to crater formation.
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    14. 14
      Bergeron, A.; Ibrahim, J.; Leonelli, R.; Francoeur, S. Oxidation Dynamics of Ultrathin GaSe Probed through Raman Spectroscopy. Appl. Phys. Lett. 2017, 110, 241901,  DOI: 10.1063/1.4986189
      14
      Oxidation dynamics of ultrathin GaSe probed through Raman spectroscopy
      Bergeron, Alaric; Ibrahim, John; Leonelli, Richard; Francoeur, Sebastien
      Applied Physics Letters (2017), 110 (24), 241901/1-241901/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Gallium selenide (GaSe) is a 2D material with a thickness-dependent gap, strong non-linear optical coeffs., and uncommon interband optical selection rules, making it interesting for optoelectronic and spintronic applications. In this work, we monitor the oxidn. dynamics of GaSe with thicknesses ranging from 10 to 200 nm using Raman spectroscopy. In ambient temp. and humidity conditions, the intensity of all Raman modes and the luminescence decrease rapidly with moderate exposure to above-gap illumination. Concurrently, several oxidn. products appear in the Raman spectra: Ga2Se3, Ga2O3, and amorphous and cryst. selenium. We find that no safe measurement power exists for optical measurements on ultrathin GaSe in ambient conditions. We demonstrate that the simultaneous presence of oxygen, humidity, and above-gap illumination is required to activate this photo-oxidn. process, which is attributed to the transfer of photo-generated charge carriers towards aq. oxygen at the sample surface, generating highly reactive superoxide anions that rapidly degrade the sample and quench the optical response of the material. (c) 2017 American Institute of Physics.
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    15. 15
      Shi, L.; Li, Q.; Ouyang, Y.; Wang, J. Effect of Illumination and Se Vacancies on Fast Oxidation of Ultrathin Gallium Selenide. Nanoscale 2018, 10, 12180– 12186,  DOI: 10.1039/C8NR01533C
      15
      Effect of illumination and Se vacancies on fast oxidation of ultrathin gallium selenide
      Shi, Li; Li, Qiang; Ouyang, Yixin; Wang, Jinlan
      Nanoscale (2018), 10 (25), 12180-12186CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Gallium selenide (GaSe) has recently emerged as a unique platform due to its exciting properties, namely, large and fast photo-response, high carrier mobility and non-linear optical properties. However, exposure for a few days causes the fast oxidn. of ultrathin GaSe under ambient conditions and the oxidn. mechanism remains unclear. By means of d. functional theory calcns. and ab initio mol. dynamics simulations, we comprehensively investigated the possible sources that cause oxidn. of ultrathin GaSe. Our results show that illumination and Se vacancies induce the fast oxidn. of GaSe. Under illumination, photo-excited electrons from the surface of GaSe are effectively transferred to oxygen mols. and thus, superoxide anions (O2-) are generated that react with GaSe. Moreover, Se vacancies directly react with O2. In both the cases, the Ga-Se bonds are continually replaced by Ga-O bonds, which eventually leads to complete degrdn. of GaSe, accompanied with the formation of the oxidn. products Ga2O3 and elemental Se. The comprehensive degrdn. mechanism unveiled herein lays an important foundation for the development of suitable protecting strategies in GaSe-based devices.
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    16. 16
      Fu, M.; Liang, L.; Zou, Q.; Nguyen, G. D.; Xiao, K.; Li, A. P.; Kang, J.; Wu, Z.; Gai, Z. Defects in Highly Anisotropic Transition-Metal Dichalcogenide PdSe2. J. Phys. Chem. Lett. 2020, 11, 740– 746,  DOI: 10.1021/acs.jpclett.9b03312
      16
      Defects in Highly Anisotropic Transition-Metal Dichalcogenide PdSe2
      Fu, Mingming; Liang, Liangbo; Zou, Qiang; Nguyen, Giang D.; Xiao, Kai; Li, An-Ping; Kang, Junyong; Wu, Zhiming; Gai, Zheng
      Journal of Physical Chemistry Letters (2020), 11 (3), 740-746CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      The at. and electronic structures of pristine PdSe2 as well as various intrinsic vacancy defects in PdSe2 are studied comprehensively by combining scanning tunneling microscopy, spectroscopy, and d. functional theory calcns. Other than the topmost Se atoms, sublayer Pd atoms and the intrinsic Pd and Se vacancy defects are identified. Both VSe and VPd defects induce defect states near the Fermi level. As a result, the vacancy defects can be neg. charged by a tip gating effect. At neg. sample bias, the screened Coulomb interaction between the scanning tunneling microscopy (STM) tip and the charged vacancies creates a disk-like protrusion around the VPd and crater-like features around VSe. The magnification effect of the long-range charge localization at the charged defect site makes sublayer defects as deep as 1 nm visible even in STM images. By gating the probe, scanning probe microscopy can be used as an easy tool for characterizing sublayer defects in a nondestructive way.
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    17. 17
      Li, X.; Luo, N.; Chen, Y.; Zou, X.; Zhu, H. Real-Time Observing Ultrafast Carrier and Phonon Dynamics in Colloidal Tin Chalcogenide Van Der Waals Nanosheets. J. Phys. Chem. Lett. 2019, 10, 3750– 3755,  DOI: 10.1021/acs.jpclett.9b01470
      17
      Real-Time Observing Ultrafast Carrier and Phonon Dynamics in Colloidal Tin Chalcogenide van der Waals Nanosheets
      Li, Xufeng; Luo, Nannan; Chen, Yuzhong; Zou, Xiaolong; Zhu, Haiming
      Journal of Physical Chemistry Letters (2019), 10 (13), 3750-3755CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Because of their earth-abundant, low-cost, and environmentally benign characteristics, 2-dimensional (2D) Group IV metal chalcogenides (e.g., SnSe2) with layered structures showed great potential in optoelectronic, photovoltaic, and thermoelec. applications. However, the intrinsic motion of excited carriers and their coupling with lattice photons, which fundamentally dictates device operation and optimization, remain yet to be unraveled. Herein, the authors directly follow the ultrafast carrier and photon dynamics of colloidal SnSe2 nanosheets in real time using ultrafast transient absorption spectroscopy. The authors show ∼0.3 ps intervalley relaxation process of photoexcited energetic carriers and ∼3 ps carrier defect trapping process with a long-lived trapped carrier (∼1 ns), highlighting the importance of trapped carriers in optoelectronic devices. Ultrashort laser pulse impulsively drives coherent out-of-plane lattice vibration in SnSe2, indicating strong electron-phonon coupling in SnSe2. This strong electron-phonon coupling could impose a fundamental limit on SnSe2 photovoltaic devices but benefit its thermoelec. applications.
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    18. 18
      Wei, Z.; Wang, L.; Zhuo, M.; Ni, W.; Wang, H.; Ma, J. Layered Tin Sulfide and Selenide Anode Materials for Li- and Na-Ion Batteries. J. Mater. Chem. A 2018, 6, 12185– 12214,  DOI: 10.1039/C8TA02695E
      18
      Layered tin sulfide and selenide anode materials for Li- and Na-ion batteries
      Wei, Zengxi; Wang, Lei; Zhuo, Ming; Ni, Wei; Wang, Hongxia; Ma, Jianmin
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2018), 6 (26), 12185-12214CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Layered tin sulfides have attracted great interest as high-capacity anode materials in Li-ion batteries (LIBs) and Na-ion batteries (NIBs). In this review, we focus on the recent research progress in the area of design and synthesis of tin sulfide and selenide (SnS, SnS2, SnSe, and SnSe2) based anode materials for LIBs and NIBs. After a brief introduction of the energy concerns and the development prospects of LIBs and NIBs, we further detailed the properties and advantages of tin sulfide and selenide based anode materials for LIBs and NIBs. Besides the material structure design and optimization, the underlying mechanism and theor. anal. for improved electrochem. performance are also presented. Addnl., comparison of tin sulfides and selenides is also provided. Innovative strategies that have demonstrated the effectiveness of enhancing the performance of tin sulfide and selenide based anode materials for LIBs and NIBs are summarized. We hope that this timely review can shed light on the research and development of tin sulfides and selenides as high-performance anode materials which are not only a good supplement to the material pool of commercialized LIBs, but also help facilitate the development of low-cost and sustainable NIBs for stationary energy storage in the future.
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    19. 19
      Huang, Y.; Ling, C.; Liu, H.; Wang, S. Tuning Electronic and Magnetic Properties of SnSe2 Armchair Nanoribbons Via Edge Hydrogenation. J. Mater. Chem. C 2014, 2, 10175– 10183,  DOI: 10.1039/C4TC01919A
      19
      Tuning electronic and magnetic properties of SnSe2 armchair nanoribbons via edge hydrogenation
      Huang, Yucheng; Ling, Chongyi; Liu, Hai; Wang, Sufan
      Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2014), 2 (47), 10175-10183CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
      First-principles calcns. were carried out to investigate the electronic and magnetic properties of SnSe2 armchair nanoribbons (ANRs) via edge hydrogenation. Interestingly, at different hydrogenation degrees, SnSe2 ANRs exhibit versatile electronic and magnetic properties, i.e., from nonmagnetic-semiconductors to magnetic-semiconductors or nonmagnetic-metals. Through the anal. from spatial spin distribution and d. of states, these transitions are well interpreted. Moreover, the relative stabilities of these ANRs were evaluated by the thermodn. phase diagram where the Gibbs free energies as a function of the chem. potential of the H2 mol. at different temps. were plotted. Our results show that hydrogenation is a well-controlled way to modify the phys. properties of SnSe2 ANRs. Through controlling chem. potential or partial pressure of H2, the different hydrogenation degrees of ANRs are thermodynamically stable, thus, one can arbitrarily steer their electronic and magnetic properties. The diverse electronic phases and magnetic properties endow the hydrogenated SnSe2 ANRs with potential applications in nanoelectronic devices.
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    20. 20
      Shafique, A.; Samad, A.; Shin, Y.-H. Ultra Low Lattice Thermal Conductivity and High Carrier Mobility of Monolayer SnS2 and SnSe2: A First Principles Study. Phys. Chem. Chem. Phys. 2017, 19, 20677– 20683,  DOI: 10.1039/C7CP03748A
      20
      Ultra low lattice thermal conductivity and high carrier mobility of monolayer SnS2 and SnSe2: a first principles study
      Shafique, Aamir; Samad, Abdus; Shin, Young-Han
      Physical Chemistry Chemical Physics (2017), 19 (31), 20677-20683CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Using d. functional theory, the authors systematically investigate the lattice thermal cond. and carrier mobility of monolayer SnX2 (X = S, Se). The room-temp. ultra low lattice thermal conductivities found in monolayer SnS2 (6.41 W m-1 K-1) and SnSe2 (3.82 W m-1 K-1) are attributed to the low phonon velocity, low Debye temp., weak bonding interactions, and strong anharmonicity in monolayer SnX2. The predicted values of lattice thermal cond. are lower than those of other 2-dimensional materials such as stanene, phosphorene, monolayer MoS2, and bulk SnX2. High phonon-limited carrier mobilities are obtained for the monolayer SnX2. For example, the electron mobility of monolayer SnS2 is 756.60 cm2 V-1 s-1 and the hole mobility is 187.44 cm2 V-1 s-1. The electron mobility of these monolayers is higher than their hole mobility due to the low effective mass of electrons and low deformation consts., which makes them n-type materials. Due to their ultra low lattice thermal conductivities coupled with high carrier mobilities, monolayer SnX2 materials may be promising materials for thermoelec. applications.
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    21. 21
      Tan, P.; Chen, X.; Wu, L.; Shang, Y. Y.; Liu, W.; Pan, J.; Xiong, X. Hierarchical Flower-Like Snse2 Supported Ag3PO4 Nanoparticles: Towards Visible Light Driven Photocatalyst with Enhanced Performance. Appl. Catal., B 2017, 202, 326– 334,  DOI: 10.1016/j.apcatb.2016.09.033
      21
      Hierarchical flower-like SnSe2 supported Ag3PO4 nanoparticles: Towards visible light driven photocatalyst with enhanced performance
      Tan, Pengfei; Chen, Xi; Wu, Laidi; Shang, Yan Yang; Liu, Wenwen; Pan, Jun; Xiong, Xiang
      Applied Catalysis, B: Environmental (2017), 202 (), 326-334CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
      A novel three-dimensional (3D) hierarchical flower-like Ag3PO4/SnSe2 composite photocatalyst was successfully prepd. via an in situ pptn. method. The compn., microstructure and optical properties of the Ag3PO4/SnSe2 composites were thoroughly investigated. Nano-sized Ag3PO4 particles were uniformly dispersed on the surface of 3D flower-like SnSe2. The obtained Ag3PO4/SnSe2 composites presented enhanced performance for photocatalytic degrdn. of Rhodamine B (RhB) compared with pure Ag3PO4 and SnSe2 under visible light irradn. The Ag3PO4/SnSe2-6 composite exhibited the optimal efficiency for photocatalytic decompn. of RhB, approx. 4.2 and 26 times higher than those of pure Ag3PO4 and SnSe2. Significantly, the superior stability was also obsd. after four cycles. The enhanced performance of the Ag3PO4/SnSe2 composites under visible light was ascribed to a synergistic effect including the matched energy band structures, increased light harvesting and boosted sepn. efficiency of photo-generated carriers. A possible photocatalytic mechanism of the composite was also discussed.
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    22. 22
      Fan, Y.; Wang, J.; Zhao, M. Spontaneous Full Photocatalytic Water Splitting on 2D MoSe2/SnSe2 and WSe2/SnSe2 Vdw Heterostructures. Nanoscale 2019, 11, 14836– 14843,  DOI: 10.1039/C9NR03469B
      22
      Spontaneous full photocatalytic water splitting on 2D MoSe2/SnSe2 and WSe2/SnSe2 vdW heterostructures
      Fan, Yingcai; Wang, Junru; Zhao, Mingwen
      Nanoscale (2019), 11 (31), 14836-14843CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Spontaneous full photocatalytic water splitting into hydrogen and oxygen under visible light irradn. without the need for sacrificial agents is a challenging task, because suitable band gaps, low overpotentials for both half-reactions and spatially-sepd. catalytic sites should be fulfilled simultaneously in a photocatalytic system. Here, we propose a promising strategy to achieve this goal by constructing van der Waals (vdW) heterostructures of two-dimensional (2D) materials. Using first-principles calcns., we predict two promising photocatalysts, MoSe2/SnSe2 and WSe2/SnSe2 heterostructures, with the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) taking place sep. on the MoSe2 (WSe2) and SnSe2 layers. More excitingly, the Se-vacancy of the MoSe2 (WSe2) monolayer effectively lowers the HER overpotential, making the catalytic reactions occur spontaneously under the potentials solely provided by the photo-generated electrons and holes in pure water. The unique band alignment of these hetero-structured photocatalysts leads to high solar-to-hydrogen (STH) energy conversion efficiencies up to 10.5%, which is quite promising for com. applications. This work opens up an avenue for the design of highly-efficient photocatalysts for full water splitting.
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    23. 23
      Zeng, J.; Liu, E.; Fu, Y.; Chen, Z.; Pan, C.; Wang, C.; Wang, M.; Wang, Y.; Xu, K.; Cai, S.; Yan, X.; Wang, Y.; Liu, X.; Wang, P.; Liang, S. J.; Cui, Y.; Hwang, H. Y.; Yuan, H.; Miao, F. Gate-Induced Interfacial Superconductivity in 1T-SnSe2. Nano Lett. 2018, 18, 1410– 1415,  DOI: 10.1021/acs.nanolett.7b05157
      23
      Gate-induced interfacial superconductivity in 1T-SnSe2
      Zeng, Junwen; Liu, Erfu; Fu, Yajun; Chen, Zhuoyu; Pan, Chen; Wang, Chenyu; Wang, Miao; Wang, Yaojia; Xu, Kang; Cai, Songhua; Yan, Xingxu; Wang, Yu; Liu, Xiaowei; Wang, Peng; Liang, Shi-Jun; Cui, Yi; Hwang, Harold Y.; Yuan, Hongtao; Miao, Feng
      Nano Letters (2018), 18 (2), 1410-1415CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      Layered metal chalcogenide materials provide a versatile platform to investigate emergent phenomena and two-dimensional (2D) supercond. at/near the atomically thin limit. In particular, gate-induced interfacial supercond. realized by the use of an elec.-double-layer transistor (EDLT) has greatly extended the capability to elec. induce supercond. in oxides, nitrides, and transition metal chalcogenides and enable one to explore new physics, such as the Ising pairing mechanism. Exploiting gate-induced supercond. in various materials can provide us with addnl. platforms to understand emergent interfacial supercond. Here, we report the discovery of gate-induced 2D supercond. in layered 1T-SnSe2, a typical member of the main-group metal dichalcogenide (MDC) family, using an EDLT gating geometry. A superconducting transition temp. Tc ≈ 3.9 K was demonstrated at the EDL interface. The 2D nature of the supercond. therein was further confirmed based on (1) a 2D Tinkham description of the angle-dependent upper crit. field Bc2, (2) the existence of a quantum creep state as well as a large ratio of the coherence length to the thickness of supercond. Interestingly, the in-plane Bc2 approaching zero temp. was found to be 2-3 times higher than the Pauli limit, which might be related to an elec. field-modulated spin-orbit interaction. Such results provide a new perspective to expand the material matrix available for gate-induced 2D supercond. and the fundamental understanding of interfacial supercond.
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    24. 24
      Shao, Z.; Fu, Z.-G.; Li, S.; Cao, Y.; Bian, Q.; Sun, H.; Zhang, Z.; Gedeon, H.; Zhang, X.; Liu, L.; Cheng, Z.; Zheng, F.; Zhang, P.; Pan, M. Strongly Compressed Few-Layered SnSe2 Films Grown on a SrTiO3 Substrate: The Coexistence of Charge Ordering and Enhanced Interfacial Superconductivity. Nano Lett. 2019, 19, 5304– 5312,  DOI: 10.1021/acs.nanolett.9b01766
      24
      Strongly Compressed Few-Layered SnSe2 Films Grown on a SrTiO3 Substrate: The Coexistence of Charge Ordering and Enhanced Interfacial Superconductivity
      Shao, Zhibin; Fu, Zhen-Guo; Li, Shaojian; Cao, Yan; Bian, Qi; Sun, Haigen; Zhang, Zongyuan; Gedeon, Habakubaho; Zhang, Xin; Liu, Lijun; Cheng, Zhengwang; Zheng, Fawei; Zhang, Ping; Pan, Minghu
      Nano Letters (2019), 19 (8), 5304-5312CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      High pressure has been demonstrated to be a powerful approach of producing novel condensed-matter states, particularly in tuning the superconducting transition temp. (Tc) of the supercond. in a clean fashion without involving the complexity of chem. doping. However, the challenge of high-pressure expt. hinders further in-depth research for underlying mechanisms. Here, we have successfully synthesized continuous layer-controllable SnSe2 films on SrTiO3 substrate using mol. beam epitaxy. By means of scanning tunneling microscopy/spectroscopy (STM/S) and Raman spectroscopy, we found that the strong compressive strain is intrinsically built in few-layers films, with a largest equiv. pressure up to 23 GPa in the monolayer. Upon this, unusual 2 × 2 charge ordering is induced at the occupied states in the monolayer, accompanied by prominent decrease in the d. of states (DOS) near the Fermi energy (EF), resembling the gap states of CDW reported in transition metal dichalcogenide (TMD) materials. Subsequently, the coexistence of charge ordering and the interfacial supercond. is obsd. in bilayer films as a result of releasing the compressive strain. In conjunction with spatially resolved spectroscopic study and first-principles calcn., we find that the enhanced interfacial supercond. with an estd. Tc of 8.3 K is obsd. only in the 1 × 1 region. Such supercond. can be ascribed to a combined effect of interfacial charge transfer and compressive strain, which leads to a considerable downshift of the conduction band min. and an increase in the DOS at EF. Our results provide an attractive platform for further in-depth investigation of compression-induced charge ordering (monolayer) and the interplay between charge ordering and supercond. (bilayer). Meanwhile, it has opened up a pathway to prep. strongly compressed two-dimensional materials by growing onto a SrTiO3 substrate, which is promising to induce supercond. with a higher Tc.
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    25. 25
      Kim, S.; Yao, Z.; Lim, J.-M.; Hersam, M. C.; Wolverton, C.; Dravid, V. P.; He, K. Lithium-Ion Batteries: Atomic-Scale Observation of Electrochemically Reversible Phase Transformations in SnSe2 Single Crystals. Adv. Mater. 2018, 30, 1870393,  DOI: 10.1002/adma.201870393
      There is no corresponding record for this reference.
    26. 26
      Bai, J.; Wu, H.; Wang, S.; Zhang, G.; Feng, C.; Liu, H. Synthesis of CoSe2-SnSe2 Nanocube-Coated Nitrogen-Doped Carbon (NC) as Anode for Lithium and Sodium Ion Batteries. Appl. Surf. Sci. 2019, 488, 512– 521,  DOI: 10.1016/j.apsusc.2019.05.096
      26
      Synthesis of CoSe2-SnSe2 nanocube-coated nitrogen-doped carbon (NC) as anode for lithium and sodium ion batteries
      Bai, Jin; Wu, Huimin; Wang, Shiquan; Zhang, Guangxue; Feng, Chuanqi; Liu, Huakun
      Applied Surface Science (2019), 488 (), 512-521CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      CoSe2-SnSe2/NC nanocubes (CSNC@NC) coated by nitrogen-doped carbon (NC) were synthesized successfully by an ordinary pyrazole polymn. and carbonization process. In comparison with bare CSNC, the CSNC@NC composite exhibited good structural stability and improved elec. cond. when used as anode. The CSNC@NC electrode showed a stable Li storage capacity (730.41 mAh g-1 over 100 cycles at 0.2 A g-1) and excellent rate performance (402.10 mAh g-1 at 2 A g-1). For Na storage, the discharge capacity could be maintained 279.3 mAh g-1 over 100 cycles at 0.2 A g-1; the lower capacity than that for Li storage maybe caused by the larger size of Na+ ions. The excellent cycling stability for both Li and Na storage cycle ability may be attributed to the carbon layer, which could tolerated the vol. fluctuations and ensured the structural integrity of the CSNC during the charge/discharge process; Moreover, the improved elec. cond. accelerated the diffusion rate of both Li+ and Na+, which is conducive to the electrochem. reactions in their resp. batteries. This unique structure and preeminent electrochem. performance of CSNC@NC show that CSNC@NC is a promising anode material for high-efficiency Li ion and Na ion batteries.
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    27. 27
      Zhang, F.; Xia, C.; Zhu, J.; Ahmed, B.; Liang, H.; Velusamy, D. B.; Schwingenschlögl, U.; Alshareef, H. N. SnSe2 2D Anodes for Advanced Sodium Ion Batteries. Adv. Energy Mater. 2016, 6, 1601188,  DOI: 10.1002/aenm.201601188
      There is no corresponding record for this reference.
    28. 28
      Zhou, X.; Zhou, N.; Li, C.; Song, H.; Zhang, Q.; Hu, X.; Gan, L.; Li, H.; Lü, J.; Luo, J.; Xiong, J.; Zhai, T. Vertical Heterostructures Based on SnSe2/MoS2 for High Performance Photodetectors. 2D Mater. 2017, 4, 025048,  DOI: 10.1088/2053-1583/aa6422
      28
      Vertical heterostructures based on SnSe2/MoS2 for high performance photodetectors
      Zhou, Xing; Zhou, Nan; Li, Chao; Song, Hongyue; Zhang, Qi; Hu, Xiaozong; Gan, Lin; Li, Huiqiao; Lue, Jingtao; Luo, Jun; Xiong, Jie; Zhai, Tianyou
      2D Materials (2017), 4 (2), 025048/1-025048/10CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
      Van der Waals heterostructures from atomically thin 2D materials have opened up new realms in modern semiconductor industry. Recently, 2D layered semiconductors such as MoS2 and SnSe2 have already demonstrated excellent electronic and optoelectronic properties due to their high electron mobility and unique band structures. Such combination of SnSe2 with MoS2 may provide a novel platform for the applications in electronics and optoelectronics. Thus, we constructed SnSe2/MoS2 based van der Waals heterostructures using MoS2 as templates, which may enrich the family of 2D van der Waals heterostructures. We demonstrate that the vdW heterostructures with high symmetry crystallog. directions show efficient interlayer charge transfer due to the strong coupling. This strong coupling is confirmed by theory calcns., low-temp. photoluminescence (PL) spectra, and elec. transport properties. High performance photodetector based on the vdW heterostructure has been demonstrated with a high responsivity of up to 9.1 × 103 A W-1 which is higher by two orders of magnitude than those MoS2-only devices. The improved performance can be attributed to the efficient charge transfer from MoS2 to SnSe2 at the interface.
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    29. 29
      Wang, M.; Wang, Z.; Xu, X.; Duan, S.; Du, C. Tin Diselenide-Based Saturable Absorbers for Eye-Safe Pulse Lasers. Nanotechnology 2019, 30, 265703,  DOI: 10.1088/1361-6528/ab1115
      29
      Tin diselenide-based saturable absorbers for eye-safe pulse lasers
      Wang, Mengxia; Wang, Zhengping; Xu, Xinguang; Duan, Sihou; Du, Chenlin
      Nanotechnology (2019), 30 (26), 265703CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
      Eye-safe pulse lasers have attracted increasing attention due to their potential wide application in many fields. However, optical modulators with excellent nonlinear optical absorption properties in the range of 1.4-2.1μm are still very scarce. In this study, tin diselenide (SnSe2), a newly-developed 2D layered semiconductor material with facile processability and low cost, was investigated. The nonlinear optical response of SnSe2 was investigated using the open aperture Z-scan method at 1500 and 1800 nm. Using SnSe2 as the saturable absorber, a passive Q-switched solid-state laser was realized at 1.3 and 1.9μm for the first time. This study proved SnSe2 to be an effective optical modulating material for the eye-safe waveband.
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    30. 30
      Zhang, Y.; Liu, Y.; Lim, K. H.; Xing, C.; Li, M.; Zhang, T.; Tang, P.; Arbiol, J.; Llorca, J.; Ng, K. M.; Ibáñez, M.; Guardia, P.; Prato, M.; Cadavid, D.; Cabot, A. Tin Diselenide Molecular Precursor for Solution-Processable Thermoelectric Materials. Angew. Chem., Int. Ed. 2018, 57, 17063– 17068,  DOI: 10.1002/anie.201809847
      30
      Tin Diselenide Molecular Precursor for Solution-Processable Thermoelectric Materials
      Zhang, Yu; Liu, Yu; Lim, Khak Ho; Xing, Congcong; Li, Mengyao; Zhang, Ting; Tang, Pengyi; Arbiol, Jordi; Llorca, Jordi; Ng, Ka Ming; Ibanez, Maria; Guardia, Pablo; Prato, Mirko; Cadavid, Doris; Cabot, Andreu
      Angewandte Chemie, International Edition (2018), 57 (52), 17063-17068CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      In the present work, we detail a fast and simple soln.-based method to synthesize hexagonal SnSe2 nanoplates (NPLs) and their use to produce crystallog. textured SnSe2 nanomaterials. We also demonstrate that the same strategy can be used to produce orthorhombic SnSe nanostructures and nanomaterials. NPLs are grown through a screw dislocation-driven mechanism. This mechanism typically results in pyramidal structures, but we demonstrate here that the growth from multiple dislocations results in flower-like structures. Crystallog. textured SnSe2 bulk nanomaterials obtained from the hot pressing of these SnSe2 structures display highly anisotropic charge and heat transport properties and thermoelec. (TE) figures of merit limited by relatively low elec. conductivities. To improve this parameter, SnSe2 NPLs are blended here with metal nanoparticles. The elec. conductivities of the blends are significantly improved with respect to bare SnSe2 NPLs, what translates into a three-fold increase of the TE Figure of merit, reaching unprecedented ZT values up to 0.65.
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    31. 31
      Luo, Y.; Zheng, Y.; Luo, Z.; Hao, S.; Du, C.; Liang, Q.; Li, Z.; Khor, K. A.; Hippalgaonkar, K.; Xu, J.; Yan, Q.; Wolverton, C.; Kanatzidis, M. G. N-Type SnSe2 Oriented-Nanoplate-Based Pellets for High Thermoelectric Performance. Adv. Energy Mater. 2018, 8, 1702167,  DOI: 10.1002/aenm.201702167
      There is no corresponding record for this reference.
    32. 32
      Sun, J.; Liu, S.; Wang, C.; Bai, Y.; Chen, G.; Luo, Q.; Ma, F. Interface Tuning Charge Transport and Enhanced Thermoelectric Properties in Flower-Like SnSe2 Hierarchical Nanostructures. Appl. Surf. Sci. 2020, 510, 145478,  DOI: 10.1016/j.apsusc.2020.145478
      32
      Interface tuning charge transport and enhanced thermoelectric properties in flower-like SnSe2 hierarchical nanostructures
      Sun, Jun; Liu, Shuai; Wang, Chen; Bai, Yu; Chen, Guanjun; Luo, Qiaomei; Ma, Fei
      Applied Surface Science (2020), 510 (), 145478CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      Thermoelec. properties could be well improved in hierarchical nanostructures due to the selective scattering on electrons and phonons by interfaces. Flower-like SnSe2 nanostructure is synthesized by soln.-based method and, the nanostructure is sintered into pellets by spark plasma sintering (SPS) to evaluate thermoelec. properties. The flower-like SnSe2 nanostructure exhibits the ultralow thermal cond. of 0.44 Wm-1 K-1 due to the strong phonon scattering by high-d. of interface and grain boundaries, which were confirmed by both expts. and simulation. Besides, the elec. transport of the flower-like SnSe2 is optimized synergistically owing to the moderate interfacial potential barrier. The highest power factor of 43μWm-1 K-2 and competitive ZT value are measured at 550 K. The thermoelec. performance of flower-like SnSe2 is better that that of nanoplate and bulk counterparts. It provides an efficient method to improve the thermoelec. properties of SnSe2 based materials.
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    33. 33
      Nasir, M. S.; Yang, G.; Ayub, I.; Wang, X.; Wang, S.; Nasir, A.; Yan, W. Tin Diselenide Nanoflakes Decorated Hierarchical 1D TiO2 Fiber: A Robust and Highly Efficient Co-Catalyst for Hydrogen Evolution Reaction. Appl. Surf. Sci. 2020, 521, 146333,  DOI: 10.1016/j.apsusc.2020.146333
      33
      Tin diselenide nanoflakes decorated hierarchical 1D TiO2 fiber: A robust and highly efficient co-catalyst for hydrogen evolution reaction
      Nasir, Muhammad Salman; Yang, Guorui; Ayub, Iqra; Wang, Xiaojun; Wang, Silan; Nasir, Abdul; Yan, Wei
      Applied Surface Science (2020), 521 (), 146333CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      Transitional metal dichalcogenide two-dimensional materials have revealed many astonishing properties including as a substitution of noble metals as a co-catalyst for hydrogen evolution reaction. Tin diselenide (SnSe2) is the one who also received substantial consideration in many fields due to low cost, earth-abundant and environment-friendly. However, the great challenge to make heterojunction with other semiconductor material to improve its efficiency for photocatalytic water splitting. For this purpose, a 1D nanofiber of TiO2 is prepd. by electrospinning and produced a needle-like structure on the fiber by following the alkali hydrothermal method. The chem. vapor deposition method (CVD) was adopted to load nanoflakes of SnSe2 on the branched fiber of TiO2 and make a strong heterojunction. The composite interpreted excellent photocatalytic performance by producing hydrogen about 0.95 mmol g-1 h-1 in comparison with branched TiO2 (0.47 mmol g-1 h-1) and conventional TiO2 fiber (0.35 mmol g-1 h-1). The photoluminescence, time decay fluorescent spectra and photoelectrochem. results ratified that SnSe2 not only reduces the charge recombination by increasing the transfer of electron but also provides the active site for hydrogen prodn. as a cocatalyst. This study presents an inexpensive and environmentally friendly photocatalyst for hydrogen prodn. without the use of noble metals.
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    34. 34
      Lee, Y. K.; Luo, Z.; Cho, S. P.; Kanatzidis, M. G.; Chung, I. Surface Oxide Removal for Polycrystalline Snse Reveals near-Single-Crystal Thermoelectric Performance. Joule 2019, 3, 719– 731,  DOI: 10.1016/j.joule.2019.01.001
      34
      Surface Oxide Removal for Polycrystalline SnSe Reveals Near-Single-Crystal Thermoelectric Performance
      Lee, Yong Kyu; Luo, Zhongzhen; Cho, Sung Pyo; Kanatzidis, Mercouri G.; Chung, In
      Joule (2019), 3 (3), 719-731CODEN: JOULBR; ISSN:2542-4351. (Cell Press)
      Tin selenide (SnSe) has emerged as a surprising new material with exceptional thermal transport and charge transport properties such as ultralow thermal cond., which give it a record-high thermoelec. figure of merit (ZT) of ∼2.5-2.7 at around 800 K. These properties, however, have been only observable in well-prepd. and properly handled single-crystal samples. Polycryst. SnSe samples have markedly inferior properties paradoxically with higher apparent thermal cond. and much lower ZT values than single crystals. The high thermal cond. in polycryst. samples has been attributed to surface tin oxides. Based on this hypothesis, we have employed an oxide-removing strategy that involves a chem. redn. process at 613 K under a 4% H2/Ar atm. This leads to an exceptionally low lattice thermal cond. of ∼0.11 W m-1K-1 in polycryst. hole-doped SnSe alloyed with 5% lead selenide, even lower than that of single crystals, and boosts the ZT to ∼2.5 at 773 K.
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    35. 35
      Lamuta, C.; Campi, D.; Pagnotta, L.; Dasadia, A.; Cupolillo, A.; Politano, A. Determination of the Mechanical Properties of SnSe, a Novel Layered Semiconductor. J. Phys. Chem. Solids 2018, 116, 306– 312,  DOI: 10.1016/j.jpcs.2018.01.045
      35
      Determination of the mechanical properties of SnSe, a novel layered semiconductor
      Lamuta, Caterina; Campi, Davide; Pagnotta, Leonardo; Dasadia, Abhay; Cupolillo, Anna; Politano, Antonio
      Journal of Physics and Chemistry of Solids (2018), 116 (), 306-312CODEN: JPCSAW; ISSN:0022-3697. (Elsevier Ltd.)
      Tin selenide (SnSe) is one the most promising materials for flexible electronics. However, expts. on the direct detn. of its mech. properties are still missing. By means of depth-sensing nanoindentation expts., we directly evaluate the Youngs modulus of bulk single crystals of tin selenide (25.3 GPa), as well as their hardness (0.82 GPa). Exptl. results are compared with predictions by d. functional theory, performed using eleven different functionals. The discrepancies between the exptl. results and the thoretical predictions can be ascribed to the oxidn. of the SnSe surface, detected by XPS.
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    36. 36
      Paolucci, V.; D’Olimpio, G.; Kuo, C.-N.; Lue, C. S.; Boukhvalov, D. W.; Cantalini, C.; Politano, A. Self-Assembled SnO2/SnSe2 Heterostructures: A Suitable Platform for Ultrasensitive NO2 and H2 Sensing. ACS Appl. Mater. Interfaces 2020, 12, 34362– 34369,  DOI: 10.1021/acsami.0c07901
      36
      Self-Assembled SnO2/SnSe2 Heterostructures: A Suitable Platform for Ultrasensitive NO2 and H2 Sensing
      Paolucci, Valentina; D'Olimpio, Gianluca; Kuo, Chia-Nung; Lue, Chin Shan; Boukhvalov, Danil W.; Cantalini, Carlo; Politano, Antonio
      ACS Applied Materials & Interfaces (2020), 12 (30), 34362-34369CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      By means of expts. and theory, the gas-sensing properties of tin diselenide (SnSe2) were elucidated. We discover that, while the stoichiometric single crystal is chem. inert even in air, the nonstoichiometric sample assumes a subnanometric SnO2 surface oxide layer once exposed to ambient atm. The presence of Se vacancies induces the formation of a metastable SeO2-like layer, which is finally transformed into a SnO2 skin. Remarkably, the self-assembled SnO2/SnSe2-x heterostructure is particularly efficient in gas sensing, whereas the stoichiometric SnSe2 sample does not show sensing properties. Congruently with the theor. model, direct sensing tests carried out on SnO2/SnSe2-x at an operational temp. of 150°C provided sensitivities of (1.06 ± 0.03) and (0.43 ± 0.02) [ppm]-1 for NO2 and H2, resp., in dry air. The corresponding calcd. limits of detection are (0.36 ± 0.01) and (3.6 ± 0.1) ppm for NO2 and H2, resp. No detectable changes in gas-sensing performances are obsd. in a time period extended above six months. Our results pave the way for a novel generation of ambient-stable gas sensor based on self-assembled heterostructures formed taking advantage on the natural interaction of substoichiometric van der Waals semiconductors with air.
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    37. 37
      Pawar, M.; Kadam, S.; Late, D. J. High-Performance Sensing Behavior Using Electronic Ink of 2D SnSe2 Nanosheets. Chemistry Select 2017, 2, 4068– 4075,  DOI: 10.1002/slct.201700261
      There is no corresponding record for this reference.
    38. 38
      Pawbake, A. S.; Date, A.; Jadkar, S. R.; Late, D. J. Temperature Dependent Raman Spectroscopy and Sensing Behavior of Few Layer SnSe2 Nanosheets. Chemistry Select 2016, 1, 5380– 5387,  DOI: 10.1002/slct.201601347
      There is no corresponding record for this reference.
    39. 39
      Chen, X.; Chen, X.; Han, Y.; Su, C.; Zeng, M.; Hu, N.; Su, Y.; Zhou, Z.; Wei, H.; Yang, Z. Two-Dimensional MoSe2 Nanosheets Via Liquid-Phase Exfoliation for High-Performance Room Temperature NO2 Gas Sensors. Nanotechnology 2019, 30, 445503,  DOI: 10.1088/1361-6528/ab35ec
      39
      Two-dimensional MoSe2 nanosheets via liquid-phase exfoliation for highperformance room temperature NO2 gas sensors
      Chen, Xi; Chen, Xinwei; Han, Yutong; Su, Chen; Zeng, Min; Hu, Nantao; Su, Yanjie; Zhou, Zhihua; Wei, Hao; Yang, Zhi
      Nanotechnology (2019), 30 (44), 445503CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
      Molybdenum selenide (MoSe2) has drawn significant interest due to its typical semiconductor properties. MoSe2 is a relatively novel material in the field of gas sensors esp. at room temp. Herein, we utilize a facile and efficient synthetic method of liq.-phase exfoliation to exfoliate bulk MoSe2 into nanosheets. Anhyd. ethanol is used as dispersant, so the low b.p. makes it easy to be removed from MoSe2 nanosheets, which does not affect the subsequent sensing performance. The exfoliated few-layered MoSe2 nanosheets shows significantly enhanced NO2 gas response (1500% to 10 ppm NO2 which is 18 times greater than pristine bulk MoSe2), a low detection concn. (50 ppb), an outstanding repeatability, a remarkable selectivity, and a reliable long-term device durability (more than 60 d) at room temp. (25°C). The reason of the significant improvement in gas sensing performance can be attributed mainly to the higher surface-to-vol. ratio of exfoliated MoSe2 nanosheets. It promotes the adsorption of gas mols. on the surface of the material, thereby facilitating the charge transfer process. The superior performance of this gas sensor makes MoSe2 nanosheets a potential candidate for room temp. NO2 detection.
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    40. 40
      Guo, R.; Han, Y.; Su, C.; Chen, X.; Zeng, M.; Hu, N.; Su, Y.; Zhou, Z.; Wei, H.; Yang, Z. Ultrasensitive Room Temperature NO2 Sensors Based on Liquid Phase Exfoliated WSe2 Nanosheets. Sens. Actuators, B 2019, 300, 127013,  DOI: 10.1016/j.snb.2019.127013
      40
      Ultrasensitive room temperature NO2 sensors based on liquid phase exfoliated WSe2 nanosheets
      Guo, Rensong; Han, Yutong; Su, Chen; Chen, Xinwei; Zeng, Min; Hu, Nantao; Su, Yanjie; Zhou, Zhihua; Wei, Hao; Yang, Zhi
      Sensors and Actuators, B: Chemical (2019), 300 (), 127013CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
      Semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDs) are promising gas-sensing materials due to their large surface-to-vol. ratio, layered structures, susceptible surfaces, and excellent conductivities. Herein, a simple and effective method has been developed to exfoliate tungsten diselenide (WSe2) into few-layered nanosheets with N-methyl-2-pyrrolidone (NMP) as a dispersant. The gas sensor based on the as-synthesized WSe2 nanosheets shows an extremely high response (50 ppb, 5.06) under a low detection concn. of nitrogen dioxide (NO2) at room temp. (25°C), which is much higher than many other reports for 2D TMDs-based gas sensors. Addnl., our sensor also exhibits a response of 11.01 to 10 ppm NO2, an excellent selectivity, and a reliable long-term stability within 8 wk. It is expected that the outstanding performances of the gas sensors based on WSe2 nanosheets reported in the present work will make WSe2 a promising candidate for ultrasensitive NO2 sensing applications.
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    41. 41
      Zhong, Y.; Li, W.; Zhao, X.; Jiang, X.; Lin, S.; Zhen, Z.; Chen, W.; Xie, D.; Zhu, H. High-Response Room-Temperature NO2 Sensor and Ultrafast Humidity Sensor Based on SnO2 with Rich Oxygen Vacancy. ACS Appl. Mater. Interfaces 2019, 11, 13441– 13449,  DOI: 10.1021/acsami.9b01737
      41
      High-Response Room-Temperature NO2 Sensor and Ultrafast Humidity Sensor Based on SnO2 with Rich Oxygen Vacancy
      Zhong, Yujia; Li, WeiWei; Zhao, Xuanliang; Jiang, Xin; Lin, Shuyuan; Zhen, Zhen; Chen, Wenduo; Xie, Dan; Zhu, Hongwei
      ACS Applied Materials & Interfaces (2019), 11 (14), 13441-13449CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      SnO2 nanosheets with abundant vacancies (designated as SnO2-x) were successfully prepd. by annealing SnSe nanosheets in Ar. The TEM results of the prepd. SnO2 nanosheets indicated that high-d. SnO2-x nanoplates with the size of 5-10 nm were distributed on the surface of amorphous C. After annealing, the acquired SnO2-x/amorphous C retained the square morphol. The stoichiometric ratio of Sn/O = 1:1.55 confirmed that O vacancies were abundant in SnO2 nanosheets. The prepd. SnO2-x exhibited excellent performance of sensing NO2 at room temp. The response of the SnO2-x-based sensor to 5 ppm NO2 is 16 with the response time and recovery time of 331 and 1057 s, resp., which is superior to those of most reported room-temp. NO2 sensors based on SnO2 and other materials. When the humidity varied from 30 to 40%, ΔR/R was 0.025. The ultrafast humidity response (52 ms) and recovery (140 ms) are competitive compared with other state-of-art humidity sensors. According to the mechanistic study, the excellent sensing performance of SnO2-x is attributed to its special structure.
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    42. 42
      Vorokhta, M.; Khalakhan, I.; Vondráček, M.; Tomeček, D.; Vorokhta, M.; Marešová, E.; Nováková, J.; Vlček, J.; Fitl, P.; Novotný, M.; Hozák, P.; Lančok, J.; Vrňata, M.; Matolínová, I.; Matolín, V. Investigation of Gas Sensing Mechanism of SnO2 Based Chemiresistor Using near Ambient Pressure Xps. Surf. Sci. 2018, 677, 284– 290,  DOI: 10.1016/j.susc.2018.08.003
      42
      Investigation of gas sensing mechanism of SnO2 based chemiresistor using near ambient pressure XPS
      Vorokhta, M.; Khalakhan, I.; Vondracek, M.; Tomecek, D.; Vorokhta, M.; Maresova, E.; Novakova, J.; Vlcek, J.; Fitl, P.; Novotny, M.; Hozak, P.; Lancok, J.; Vrnata, M.; Matolinova, I.; Matolin, V.
      Surface Science (2018), 677 (), 284-290CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
      In this article, we present the results of an investigation into chem. processes which take place at the surface of SnO2-based chemiresistor in various atms. (1 mbar of argon, 1 mbar of oxygen, 0.1 mbar of ethanol, 1 mbar of oxygen + 0.1 mbar of ethanol mixt.) at common working temps. (450 and 573 K). The key method for nanoscale anal. was the Near Ambient Pressure XPS. In parallel the resistance and DC-responses of SnO2 layer were in-situ monitored providing information about macroscale processes during gas sensing. The change in the sensor resistance after exposure to the ethanol-contg. atms. together with the disappearance of the band bending effect and observation of different carbonaceous groups including ethoxy groups and acetaldehyde mols. on the sensor surface in the XPS spectra supported the theory of chem. interaction of ethanol with the chemisorbed oxygen. The NAP-XPS spectra also showed that the nanostructured tin oxide is partially reduced even after being exposed to pure oxygen at 573 K. Exposing this surface to the mixt. of O2/EtOH did not significantly increase the surface redn. probably due to slow kinetics of the ethanol redn. process and fast kinetics of the oxygen re-oxidn. process. However, it was demonstrated that the surface of sensor is slowly getting contaminated by carbon.
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    43. 43
      Das, S.; Jayaraman, V. SnO2: A Comprehensive Review on Structures and Gas Sensors. Prog. Mater. Sci. 2014, 66, 112– 255,  DOI: 10.1016/j.pmatsci.2014.06.003
      43
      SnO2: A comprehensive review on structures and gas sensors
      Das, Soumen; Jayaraman, V.
      Progress in Materials Science (2014), 66 (), 112-255CODEN: PRMSAQ; ISSN:0079-6425. (Elsevier Ltd.)
      A review. Metal oxides possess exceptional potential as base materials in emerging technologies. In recent times, significant amt. of research works is carried out on these materials to assess new areas of applications, including optical, electronic, optoelectronic and biol. domains. In such applications, the response and performance of the devices depend crucially, among other factors, on the size, shape and surface of the active oxide materials. For instance, the electronic and optical properties of oxides depend strongly on the spatial dimensions and compn. [1]. The large no. of atoms on the surface, and the effective van der Waals, Coulombic and interat. coupling significantly modify the phys. and chem. properties of the low dimensional oxide materials vis-a´-vis its bulk counterparts. As a result, low dimensional oxide materials, such as nanoparticles, nanospheres, nanorods, nanowires, nanoribbon/nanobelts, nanotubes, nanodisks, nanosheets evoke vast and diverse interests. Thermal and phys. deposition, hydro/solvothermal process, spray-pyrolysis, assisted self-assembly, oil-in-water microemulsion and template-assisted synthesis are regularly employed to synthesis one-, two- and three-dimensional nanostructures, which have become the focus of intensive research in mesoscopic physics and nanoscale devices. It not only provides good scopes to study the optical, elec. and thermal properties in quantum-confinement, but also offers important insights for understanding the functional units in fabricating electronic, optoelectronic, and magnetic devices of nanoscale dimension. Tin oxide (SnO2) is one such very important n-type oxide and wide band gap (3.6 eV) semiconductor. Its good quality elec., optical, and electrochem. properties are exploited in solar cells, as catalytic support materials, as solid-state chem. sensors and as high-capacity lithium-storage. Previously, Chopra et al. [2] reviewed different aspects of transparent conducting SnO2 thin films. Wang et al. [3] discussed device applications of nanowires and nanobelts of semiconductor oxides, including SnO2. Batzill et al. [4] discussed about the surface of single cryst. bulk SnO2. However, it is understood that neither there is any comprehensive review on various crystallog. phases, polymorphs, bulk modulus, lattice parameters and electronic states of SnO2, nor there is any updated compilation on the recent progress and scope on SnO2 nanostructures. Therefore, the proposed review covers the past and recent progress on the said topics and is summarized in the following manner. The available theor. and exptl. works on crystal structures, bulk modulus, lattice parameters are reviewed in details. The electronic states and the band structures of these phases are discussed next. Active crystal surfaces of SnO2 play vital roles in its many interesting properties, including sensing and catalytic applications. So, a short review is written on its different surfaces, its electronic structures and d. of states. The discussion on the importance of morphol. variations on the properties of SnO2 is followed by a review on different methods for obtaining such structures. A detail survey on the existing literature on techniques and mechanisms for the growth of nanostructures are included. SnO2 is efficiently employed in gas sensing applications. A review on such applications is compiled based on the role of morphol. and performance. The future course of SnO2 as an important material in the contemporary research is also discussed.
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    44. 44
      Li, G.-J.; Kawi, S. High-Surface-Area SnO2: A Novel Semiconductor-Oxide Gas Sensor. Mater. Lett. 1998, 34, 99– 102,  DOI: 10.1016/S0167-577X(97)00142-0
      44
      High-surface-area SnO2: a novel semiconductor-oxide gas sensor
      Li, G.-J.; Kawi, S.
      Materials Letters (1998), 34 (1,2), 99-102CODEN: MLETDJ; ISSN:0167-577X. (Elsevier Science B.V.)
      High surface area SnO2 sensor materials were systematically synthesized by a surfactant incorporating method. After calcination at 723 K, a high BET surface area of 156.8 m2/g was obtained. The sensing properties of the high surface area SnO2 material were studied using H2 as the probing gas. A linear relation exists between sensor surface area and its sensitivity to H2.
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    45. 45
      Di Giulio, M.; Micocci, G.; Serra, A.; Tepore, A.; Rella, R.; Siciliano, P. SnO2 Thin Films for Gas Sensor Prepared by Rf Reactive Sputtering. Sens. Actuators, B 1995, 25, 465– 468,  DOI: 10.1016/0925-4005(94)01397-7
      45
      SnO2 thin films for gas sensor prepared by r.f. reactive sputtering
      Di Giulio, M.; Micocci, G.; Serra, A.; Tepore, A.; Rella, R.; Siciliano, P.
      Sensors and Actuators, B: Chemical (1995), 25 (1-3), 465-8CODEN: SABCEB; ISSN:0925-4005. (Elsevier)
      SnO2 thin films were grown by the r.f. reactive sputtering method to be used as gas sensors. The films are deposited onto heated alumina substrates in an Ar-O2 atmosphere starting from an SnO2 target. The authors have optimized the growth parameters to achieve the best thin-film properties. The surface structure and the compn. of the prepd. films are studied by x-ray diffraction and XPS. The effect on the gas-sensing characteristics of dispersing platinum onto the film surface by sputtering from a Pt target followed by a suitable thermal annealing, also was studied. In particular, Pt-added SnO2 thin films show a high sensitivity to carbon monoxide gas at temps. of ∼170°. This temp. is lower than the optimum operating temp. (∼350°) of SnO2 samples without platinum.
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    46. 46
      Li, W.; Kan, K.; He, L.; Ma, L.; Zhang, X.; Si, J.; Ikram, M.; Ullah, M.; Khan, M.; Shi, K. Biomorphic Synthesis of 3D Mesoporous SnO2 with Substantially Increased Gas-Sensing Performance at Room Temperature Using a Simple One-Pot Hydrothermal Method. Appl. Surf. Sci. 2020, 512, 145657,  DOI: 10.1016/j.apsusc.2020.145657
      46
      Biomorphic synthesis of 3D mesoporous SnO2 with substantially increased gas-sensing performance at room temperature using a simple one-pot hydrothermal method
      Li, Wenna; Kan, Kan; He, Lang; Ma, Laifeng; Zhang, Xueyi; Si, Jiaqi; Ikram, Muhammad; Ullah, Mohib; Khan, Mawaz; Shi, Keying
      Applied Surface Science (2020), 512 (), 145657CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      Biomorphic SnO2 nanoparticles with a mesoporous structure were synthesized using simple one-step hydrothermal method with SnCl2 as the raw material and hemp stems as the bio-template. The uniformly grown SnO2 nanoparticles perfectly inherited the 3D structure of biomass carbon and formed a rich mesoporous structure, which was beneficial to gas sensing and facilitated the transport of the target gas on the surface and inside the sample. Moreover, the high surface area of materials provides more active sites for the adsorption of oxygen and the target gas. The sample with a 450°C annealing temp. exhibited an excellent response (35.83) as a NO2 sensor at room temp. (RT), fast response speed to 100 ppm NO2 (2.67 s), and a detection limit as low as 10 ppb. Furthermore, it displayed long-term stability, excellent selectivity and good repeatability. Therefore, the mesoporous biomorphic SnO2 nanoparticles represent a good candidate as a key green material for an NO2 sensor at RT.
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    47. 47
      Li, W.; Ding, C.; Li, J.; Ren, Q.; Bai, G.; Xu, J. Sensing Mechanism of Sb, S Doped SnO2(110) Surface for CO. Appl. Surf. Sci. 2020, 502, 144140,  DOI: 10.1016/j.apsusc.2019.144140
      47
      Sensing mechanism of Sb, S doped SnO2 (1 1 0) surface for CO
      Li, Wei; Ding, Chao; Li, Jinze; Ren, Qingying; Bai, Gang; Xu, Jie
      Applied Surface Science (2020), 502 (), 144140CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      First-principles calcn. based on d. functional theory (DFT) was employed to study the adsorption of CO mols. on various SnO2 (1 1 0) surface. For comparison, the adsorption of CO on perfect SnO2, Sb-doped SnO2 and Sb, S co-doped SnO2 surfaces are considered. Adsorption energy, electron population and d. of states show that the sensing properties of SnO2-based sensors in terms of the CO response were improved by Sb, S co-doping.
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    48. 48
      Ko, W. C.; Kim, K. M.; Kwon, Y. J.; Choi, H.; Park, J. K.; Jeong, Y. K. ALD-Assisted Synthesis of V2O5 Nanoislands on SnO2 Nanowires for Improving NO2 Sensing Performance. Appl. Surf. Sci. 2020, 509, 144821,  DOI: 10.1016/j.apsusc.2019.144821
      48
      ALD-assisted synthesis of V2O5 nanoislands on SnO2 nanowires for improving NO2 sensing performance
      Ko, Woo Chul; Kim, Kang Min; Kwon, Yong Jung; Choi, Heechae; Park, Jin Kuen; Jeong, Young Kyu
      Applied Surface Science (2020), 509 (), 144821CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      Anchoring nanoparticles on the surfaces of metal oxide nanosensors is a promising strategy to improve sensing performance. However, several issues regarding the surface coverage and uniform distribution of nanoislands over base sensing materials have yet to be resolved using conventional synthetic routes. Herein, we present a viable alternative for the decoration of V2O5 nanoislands on SnO2 nanowires: a new two-step process that combines at. layer deposition (ALD) and successive thermal post-treatment. This process enables us to control the surface coverage of V2O5 nanoislands by varying the no. of ALD cycles and to homogeneously disperse the nanoislands on the SnO2 nanowire surfaces. The NO2 response of the V2O5-decorated SnO2 sensor improved as the no. of ALD-V2O5 cycles increased; the highest response, obtained by the sensor prepd. with 50 ALD-V2O5 cycles, was more than 50 times greater than that of the pristine SnO2 nanowires. However, the sensing performance degraded beyond 50 ALD-V2O5 cycles as there was an oversupply of V2O5 nanoislands. Based on d. functional theory calcns., we detd. that V2O5 nanoisland loading is in competition with the exposed SnO2 surface to increase sensing performance, which implies that the surface coverage of V2O5 nanoislands must be precisely optimized.
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    49. 49
      Tombak, A.; Ocak, Y. S.; Bayansal, F. Cu/SnO2 Gas Sensor Fabricated by Ultrasonic Spray Pyrolysis for Effective Detection of Carbon Monoxide. Appl. Surf. Sci. 2019, 493, 1075– 1082,  DOI: 10.1016/j.apsusc.2019.07.087
      49
      Cu/SnO2 gas sensor fabricated by ultrasonic spray pyrolysis for effective detection of carbon monoxide
      Tombak, A.; Ocak, Y. S.; Bayansal, F.
      Applied Surface Science (2019), 493 (), 1075-1082CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      In this paper, we report results of morphol., structural, optical anal. of ultrasonically sprayed Cu-doped SnO2 thin films and their applications in conductometric gas sensors to detect small traces of CO mols. Effects of Cu-doping on morphol., structural and optical properties of SnO2 nanostructures were investigated by SEM (SEM), X-Ray Diffraction (XRD), and UV-Vis. Spectroscopy measurements. SEM revealed that porosity of the film surfaces is increased with increasing Cu-doping. From the XRD patterns, the size of the crystallites and crystal quality of the films are found to be decreased with Cu-doping. UV-Vis. spectroscopy results presented that the transmittance and bandgap can be manipulated with Cu-doping where both are decreased with Cu-doping. The relation between morphol. and structure of the films with CO response properties are discussed properly. The gas response of the films with different Cu-doping has been investigated at different CO concns. at different operating temps. From the sensing measurements, it is found that Cu-doping improves the SnO2 based sensor response to CO gas. Furthermore, the possible sensing mechanism to enlighten the improved gas sensing behavior of the films is proposed.
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    50. 50
      Han, Y.; Ma, Y.; Liu, Y.; Xu, S.; Chen, X.; Zeng, M.; Hu, N.; Su, Y.; Zhou, Z.; Yang, Z. Construction of MoS2/SnO2 Heterostructures for Sensitive NO2 Detection at Room Temperature. Appl. Surf. Sci. 2019, 493, 613– 619,  DOI: 10.1016/j.apsusc.2019.07.052
      50
      Construction of MoS2/SnO2 heterostructures for sensitive NO2 detection at room temperature
      Han, Yutong; Ma, Yujie; Liu, Yang; Xu, Shusheng; Chen, Xinwei; Zeng, Min; Hu, Nantao; Su, Yanjie; Zhou, Zhihua; Yang, Zhi
      Applied Surface Science (2019), 493 (), 613-619CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      Molybdenum disulfide (MoS2) has sparked immense interests as a novel gas sensing material due to its inherent layered structure and large surface-to-vol. ratio. However, the obstacles of insufficient sensitivity and poor recoverability for MoS2-based gas sensors need to be resolved for the further applications. Herein, we report the construction of MoS2 nanosheets based p-n heterostructures for the purposes of achieving excellent NO2 detection at room temp. After functionalized with tin oxide (SnO2) nanoparticles, the optimal MoS2/SnO2 heterostructure-based gas sensor exhibits a response of 18.7 to 5 ppm nitrogen dioxide (NO2), an outstanding selectivity compared with other gases, and an excellent long-term stability for 4 wk. The enhanced sensing performance of MoS2/SnO2 heterostructures can be ascribed to the unique 2D/0D nanostructures and the formation of numerous p-n heterojunctions. Therefore, construction of p-n heterostructures provides a versatile soln. to overcome the sensing issues of MoS2-based gas sensors and also paves a new way for others room temp. sensor applications.
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    51. 51
      Barsan, N.; Weimar, U. Understanding the Fundamental Principles of Metal Oxide Based Gas Sensors; the Example of CO Sensing with SnO2 Sensors in the Presence of Humidity. J. Phys.: Condens. Matter 2003, 15, R813,  DOI: 10.1088/0953-8984/15/20/201
      51
      Understanding the fundamental principles of metal oxide based gas sensors; the example of CO sensing with SnO2 sensors in the presence of humidity
      Barsan, N.; Weimar, U.
      Journal of Physics: Condensed Matter (2003), 15 (20), R813-R839CODEN: JCOMEL; ISSN:0953-8984. (Institute of Physics Publishing)
      This paper investigates the effect of water vapor in CO sensing by using Pd doped SnO2 sensors realized in thick film technol. as an example of the basic understanding of sensing mechanisms applied to sensors. The results of phenomenol. and spectroscopic measurement techniques, all of them obtained under working conditions for sensors, were combined with modeling in order to derive conclusions able to be generalized to the field of metal oxide based gas sensors. The techniques employed were: d.c. conductance, a.c. impedance spectroscopy, work function (by using the Kelvin probe method), catalytic conversion and diffuse reflectance IR Fourier transform measurements. The most important conclusion is that the different parts of the sensor (sensing layer, electrodes, substrate) all influence the gas detection and their role has to be taken into consideration when one attempts to understand how a sensor works.
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    52. 52
      Choi, K.-I.; Hübner, M.; Haensch, A.; Kim, H.-J.; Weimar, U.; Barsan, N.; Lee, J.-H. Ambivalent Effect of Ni Loading on Gas Sensing Performance in SnO2 Based Gas Sensor. Sens. Actuators, B 2013, 183, 401– 410,  DOI: 10.1016/j.snb.2013.04.007
      52
      Ambivalent effect of Ni loading on gas sensing performance in SnO2 based gas sensor
      Choi, Kwon-Il; Hubner, Michael; Haensch, Alexander; Kim, Hyo-Joong; Weimar, Udo; Barsan, Nicolae; Lee, Jong-Heun
      Sensors and Actuators, B: Chemical (2013), 183 (), 401-410CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
      The gas sensing characteristics of pure and 0.4-2.0 at% Ni-loaded SnO2 nanoparticles were measured in dry and humid atmospheres. Approx. the same response to 50 ppm CO, response/recovery kinetics, and resistance in air regardless of wide range of humidity variation from dry to 80% r.h. were accomplished by loading 1.0 and 2.0 at% Ni to SnO2. The role of Ni related surface species in the decrease of humidity dependence of gas sensing characteristics was elucidated by diffuse-reflectance FTIR spectroscopy. The work function values detd. from the transient of sensor resistance and contact p.d. revealed that Ni loading to SnO2 dets. the appearance of surface electron acceptors responsible for a significant upward energy bands bending even in N2 atmosphere (>0.5 eV), and, ultimately, explains the significant increase of the sensors baseline resistance and the decrease of the sensor signals. In this way, the origins of the ambivalent effect of Ni loading are clarified and the way towards a rational optimization of the sensor performance opened.
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    53. 53
      Shelke, N. T.; Late, D. J. Hydrothermal Growth of MoSe2 Nanoflowers for Photo- and Humidity Sensor Applications. Sens. Actuators, A 2019, 295, 160– 168,  DOI: 10.1016/j.sna.2019.05.045
      53
      Hydrothermal growth of MoSe2 nanoflowers for photo- and humidity sensor applications
      Shelke, Nitin T.; Late, Dattatray J.
      Sensors and Actuators, A: Physical (2019), 295 (), 160-168CODEN: SAAPEB; ISSN:0924-4247. (Elsevier B.V.)
      In the present investigation, we report the synthesis of molybdenum diselenide (MoSe2) nanoflowers by facile hydrothermal method for photo- and humidity sensor applications. The obtained samples were characterized thoroughly by x-ray diffraction (XRD), Raman spectroscopy, SEM (SEM), and transmission electron microscopy (TEM). The XRD spectrum shows cryst. nature of the sample. Raman spectroscopy shows two prominent vibration modes of E12g and A1g at ∼ 241 and ∼ 283 cm-1 resp. The cryst. nature of the sample confirmed with the TEM. The MoSe2 nanoflowers based sensor shows high photosensitivity and good response to humidity with excellent prolong stability. The max. photoresponsitivity of ∼ 194% along with response of ∼ 40 ms and recovery time of ∼ 48 ms were obsd. for the sample. In case of humidity sensor, response time of ∼ 53 s and recovery time of ∼ 13 s with max. sensitivity -74% were obsd. under humidity environments. It suggests that, MoSe2 nanoflowers appear as a potential candidate for constructing high-performance nanoelectronics devices.
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    54. 54
      Gupta, S. P.; Pawbake, A. S.; Sathe, B. R.; Late, D. J.; Walke, P. S. Superior Humidity Sensor and Photodetector of Mesoporous ZnO Nanosheets at Room Temperature. Sens. Actuators, B 2019, 293, 83– 92,  DOI: 10.1016/j.snb.2019.04.086
      54
      Superior humidity sensor and photodetector of mesoporous ZnO nanosheets at room temperature
      Gupta, Shobhnath P.; Pawbake, Amit S.; Sathe, Bhaskar R.; Late, Dattatray J.; Walke, Pravin S.
      Sensors and Actuators, B: Chemical (2019), 293 (), 83-92CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
      Miniaturized sensor technol. is vastly demanding multifunctional materials to fulfill many requirements simultaneously; instead of integrating various sensors into a single device. Efficient operation of these miniaturized sensors at room temp. is highly feasible and cost-effective. The humidity sensing and photodetection is precise merit of sensing in special usage like artificial skin. Sensitivity enhancement in both humidity and photodetection required the high surface area for adsorption as well as a high charge transfer mechanism. The two dimensional (2D) zinc oxide nanosheets (ZnO NS) is the ultimate structure for dimensionally confined transport properties owing to the sp. surface at. configuration that results in high sensitivity, low operating temp., fast response and recovery, and improved selectivity. Furthermore, introducing porosity into 2D nanostructures has opened new opportunities to enhance the efficiency of sensors and detectors via increasing large surface area and tunable phys. and chem. properties. Here we report prepn. of mesoporous and highly cryst. 2D ZnO NS by a single step, template free, cost-effective chem. method. The structural and morphol. characterizations of ZnO NS are carried out using XRD, FESEM, XPS, TEM resp. The high-resoln. TEM images emphasize sheet-like morphol. with a thickness of around 18-22 nm.
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    55. 55
      Theillet, P.-O.; Pierron, O. Quantifying Adsorbed Water Monolayers on Silicon Mems Resonators Exposed to Humid Environments. Sens. Actuators, A 2011, 171, 375– 380,  DOI: 10.1016/j.sna.2011.09.002
      55
      Quantifying adsorbed water monolayers on silicon MEMS resonators exposed to humid environments
      Theillet, P.-O.; Pierron, O. N.
      Sensors and Actuators, A: Physical (2011), 171 (2), 375-380CODEN: SAAPEB; ISSN:0924-4247. (Elsevier B.V.)
      This study investigated the influence of temp. and humidity on the adsorbed water layer on micron-scale monocryst. silicon (Si) films in air, using a Si-MEMS kHz-frequency resonator. Both temp. and relative humidity induced a reversible change in resonant frequency, attributed to the temp.-dependent properties of Si and to a change in adsorbed water layer. The excellent precision in resonant frequency measurement (0.02 Hz, or 0.5 ppm) allowed precise calcn. of the changes in adsorbed water layer thickness over the specimen surface. The increase in water thickness with relative humidity was a function of temp. and could not be described with simple multimol. adsorption theories such as the BET theory. A likely explanation is the presence of hydrocarbon contaminants on the Si surface. Guidelines are provided to accurately measure the influence of temp. and relative humidity on the adsorbed water layer thickness on micron-scale Si surfaces, using this technique.
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    56. 56
      Panchal, V.; Giusca, C. E.; Lartsev, A.; Martin, N. A.; Cassidy, N.; Myers-Ward, R. L.; Gaskill, D. K.; Kazakova, O. Atmospheric Doping Effects in Epitaxial Graphene: Correlation of Local and Global Electrical Studies. 2D Mater. 2016, 3, 015006,  DOI: 10.1088/2053-1583/3/1/015006
      56
      Atmospheric doping effects in epitaxial graphene: correlation of local and global electrical studies
      Panchal, Vishal; Giusca, Cristina E.; Lartsev, Arseniy; Martin, Nicholas A.; Cassidy, Nathan; Myers-Ward, Rachael L.; Gaskill, D. Kurt; Kazakova, Olga
      2D Materials (2016), 3 (1), 015006/1-015006/10CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
      We directly correlate the local (20 nmscale) and global electronic properties of a device contg. mono-, bi- and tri-layer epitaxial graphene (EG) domains on 6H-SiC(0001) by simultaneously performing local surface potential measurements using Kelvin probe force microscopy and global transport measurements. Using well-controlled environmental conditions we investigate the doping effects of N2, O2, water vapor and NO2 at concns. representative of the ambient air. We show that presence of O2, water vapor and NO2 leads to p-doping of all EG domains. However, the thicker layers of EG are significantly less affected. Furthermore, we demonstrate that the general consensus of O2 and water vapor present in ambient air providing majority of the p-doping to graphene is a common misconception. We exptl. show that even the combined effect of O2, water vapor, and NO2 at concns. higher than typically present in the atm. does not fully replicate p-doping from ambient air. Thus, for EG gas sensors it is essential to consider naturally occurring environmental effects and properly sep. them from those coming from targeted species.
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    57. 57
      Tannarana, M.; Pataniya, P. M.; Bhakhar, S. A.; Solanki, G. K.; Valand, J.; Narayan, S.; Patel, K. D.; Jha, P. K.; Pathak, V. M. Humidity Sensor Based on Two-Dimensional SnSe2/MWCNTs Nanohybrid for the Online Monitoring of Human Respiration and Touchless Positioning Interface. ACS Sustainable Chem. Eng. 2020, 8, 12595– 12602,  DOI: 10.1021/acssuschemeng.0c04027
      57
      Humidity Sensor Based on Two-Dimensional SnSe2/MWCNT Nanohybrids for the Online Monitoring of Human Respiration and a Touchless Positioning Interface
      Tannarana, Mohit; Pataniya, Pratik M.; Bhakhar, Sanjay A.; Solanki, G. K.; Valand, Jignesh; Narayan, Som; Patel, Kirit D.; Jha, Prafulla K.; Pathak, V. M.
      ACS Sustainable Chemistry & Engineering (2020), 8 (33), 12595-12602CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
      Herein, the authors report the significantly enhanced humidity responsiveness of resistive devices based on two-dimensional SnSe2/MWCNT nanohybrids. The multifunctional humidity sensor was exploited to establish a human-machine interface and for human interactive applications such as breath monitoring and sensing of the humidity of human skin for a touchless positioning interface. The sensor exhibited great potential owing to a high surface-to-vol. ratio of SnSe2/MWCNT nanohybrids. The sensor has good linear response over a broad humidity range from 10-70% with fast response and recovery. The sensor shows the humidity responsivity of 0.148 in the humidity range of 10-70% and 0.063 for the humidity range of 75-95%. The max. current sensitivity of 857% is achieved at 95% relative humidity for SnSe2/MWCNT nanohybrids, which is quite higher than the sensitivity obtained for pristine SnSe2 nanosheets. As a high-performance electronic device, the sensor has extremely low noise level and high recognition power for small humidity variations. The present finding advocates the huge development in humidity monitoring for biomedical, intelligent electronics, and industrial applications. Significantly enhanced humidity responsiveness is realized for SnSe2/MWCNT nanohybrids for real time human respiration monitoring and a touchless positioning interface.
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    58. 58
      Nataf, G. F.; Grysan, P.; Guennou, M.; Kreisel, J.; Martinotti, D.; Rountree, C. L.; Mathieu, C.; Barrett, N. Low Energy Electron Imaging of Domains and Domain Walls in Magnesium-Doped Lithium Niobate. Sci. Rep. 2016, 6, 33098,  DOI: 10.1038/srep33098
      58
      Low energy electron imaging of domains and domain walls in magnesium-doped lithium niobate
      Nataf, G. F.; Grysan, P.; Guennou, M.; Kreisel, J.; Martinotti, D.; Rountree, C. L.; Mathieu, C.; Barrett, N.
      Scientific Reports (2016), 6 (), 33098CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
      The understanding of domain structures, specifically domain walls, currently attracts a significant attention in the field of (multi)-ferroic materials. In this article, we analyze contrast formation in full field electron microscopy applied to domains and domain walls in the uniaxial ferroelec. lithium niobate, which presents a large 3.8 eV band gap and for which conductive domain walls have been reported. We show that the transition from Mirror Electron Microscopy (MEM - electrons reflected) to Low Energy Electron Microscopy (LEEM - electrons backscattered) gives rise to a robust contrast between domains with upwards (Pup) and downwards (Pdown) polarization, and provides a measure of the difference in surface potential between the domains. We demonstrate that out-of-focus conditions of imaging produce contrast inversion, due to image distortion induced by charged surfaces, and also carry information on the polarization direction in the domains. Finally, we show that the intensity profile at domain walls provides exptl. evidence for a local stray, lateral elec. field.
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    59. 59
      Leung, T.; Kao, C.; Su, W.; Feng, Y.; Chan, C. Relationship between Surface Dipole, Work Function and Charge Transfer: Some Exceptions to an Established Rule. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 68, 195408,  DOI: 10.1103/PhysRevB.68.195408
      59
      Relationship between surface dipole, work function and charge transfer: Some exceptions to an established rule
      Leung, T. C.; Kao, C. L.; Su, W. S.; Feng, Y. J.; Chan, C. T.
      Physical Review B: Condensed Matter and Materials Physics (2003), 68 (19), 195408/1-195408/6CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
      Local-d.-functional calcns. were used to study the change of work functions induced by a layer of adsorbates. The authors studied and compared the work function of a monolayer of Mo, Ag, Au, Fe, Co, Ni, Nb, Li, N, and O on W(100), W(110), W(211), and W(111) surfaces. While many systems obey the commonly accepted rule that electroneg. adsorbates increase the work function of the surface, the authors find some exceptions. For example, overlayers of Fe, Co, and Ni increase the work function of W(100), W(211), and W(111), but decrease the work function of the W(110) surface, although the charge transfer is the same in all orientations. Even a layer of O can decrease the work function of W(100), although there are always electrons transferred from the W substrate to the O adsorbates. To understand these results, the authors established the relation between surface dipole d. and work function within the framework of local-d. formalism. Subtle details of the charge transfer can det. the sign and magnitude of surface dipole change, leading to a strong dependence on the orientation of the substrate, with the consequence that the work-function changes are not always governed by the sign and quantity of adsorbate induced charge transfer.
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    60. 60
      Roy, T.; Tosun, M.; Hettick, M.; Ahn, G. H.; Hu, C.; Javey, A. 2D-2D Tunneling Field-Effect Transistors Using WSe2/SnSe2 Heterostructures. Appl. Phys. Lett. 2016, 108, 083111,  DOI: 10.1063/1.4942647
      60
      2D-2D tunneling field-effect transistors using WSe2/SnSe2 heterostructures
      Roy, Tania; Tosun, Mahmut; Hettick, Mark; Ahn, Geun Ho; Hu, Chenming; Javey, Ali
      Applied Physics Letters (2016), 108 (8), 083111/1-083111/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Two-dimensional materials present a versatile platform for developing steep transistors due to their uniform thickness and sharp band edges. We demonstrate 2D-2D tunneling in a WSe2/SnSe2 van der Waals vertical heterojunction device, where WSe2 is used as the gate controlled p-layer and SnSe2 is the degenerately n-type layer. The van der Waals gap facilitates the regulation of band alignment at the heterojunction, without the necessity of a tunneling barrier. ZrO2 is used as the gate dielec., allowing the scaling of gate oxide to improve device subthreshold swing. Efficient gate control and clean interfaces yield a subthreshold swing of ∼100 mV/dec for >2 decades of drain current at room temp., hitherto unobserved in 2D-2D tunneling devices. The subthreshold swing is independent of temp., which is a clear signature of band-to-band tunneling at the heterojunction. A max. switching ratio ION/IOFF of 107 is obtained. Neg. differential resistance in the forward bias characteristics is obsd. at 77 K. This work bodes well for the possibilities of two-dimensional materials for the realization of energy-efficient future-generation electronics. (c) 2016 American Institute of Physics.
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    61. 61
      Li, F.; Gao, X.; Wang, R.; Zhang, T.; Lu, G. Study on TiO2-SnO2 Core-Shell Heterostructure Nanofibers with Different Work Function and Its Application in Gas Sensor. Sens. Actuators, B 2017, 248, 812– 819,  DOI: 10.1016/j.snb.2016.12.009
      61
      Study on TiO2-SnO2 core-shell heterostructure nanofibers with different work function and its application in gas sensor
      Li, Feng; Gao, Xing; Wang, Rui; Zhang, Tong; Lu, Geyu
      Sensors and Actuators, B: Chemical (2017), 248 (), 812-819CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
      A core-shell heterostructure nanofibers (NFs) have been synthesized via a coaxial electrospinning approach. Two semiconducting metal oxides (SMOX) with different work function were selected to form the core-shell heterostructure NFs. In this work, SnO2 and TiO2 were chosen as the selected SMOXs for the synthesis of the core-shell heterostructure NFs. The electrons in TiO2 will flow to SnO2, because the work function of TiO2 is smaller than SnO2. This phenomenon could result in an increase of the electrons concn. in the SnO2 shell layers and the amt. of adsorbed oxygen species increased. Therefore, the gas sensing properties of the TiO2-SnO2 core-shell heterostructure NFs were enhanced including a high response to the target gas, good selectivity to the target gas and the rapid response/recovery processes. The approach and results proposed in this study may contribute to the realization of more sensitive core-shell heterostructure NFs sensors.
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    62. 62
      Batzill, M.; Katsiev, K.; Burst, J. M.; Losovyj, Y.; Bergermayer, W.; Tanaka, I.; Diebold, U. Tuning Surface Properties of SnO2(101) by Reduction. J. Phys. Chem. Solids 2006, 67, 1923– 1929,  DOI: 10.1016/j.jpcs.2006.05.042
      62
      Tuning surface properties of SnO2(101) by reduction
      Batzill, Matthias; Katsiev, Khabibulakh; Burst, James M.; Losovyj, Yaroslav; Bergermayer, Wolfgang; Tanaka, Isao; Diebold, Ulrike
      Journal of Physics and Chemistry of Solids (2006), 67 (9-10), 1923-1929CODEN: JPCSAW; ISSN:0022-3697. (Elsevier B.V.)
      The SnO2(1 0 1) surface can be prepd. with a SnO2 or SnO compn. and consequently the surface Sn-atoms are either in a Sn(II) or Sn(IV) charge state. For a Sn(II) surface, Sn-5s derived surface states are identified by resonant, angle resolved photoemission spectroscopy (ARUPS). The differences in the interface properties of the Sn(II) and Sn(IV) surfaces of SnO2(101) are reviewed on the example of benzene and H2O adsorption. The difference in work function of these 2 surfaces causes a shift of the MOs of benzene by ∼1 eV with respect to the Fermi-level of the substrate. D. functional theory calcns. predict dissocn. of H2O on the stoichiometric (Sn(IV)) surface but only weak mol. adsorption on the reduced Sn(II) surface. These predictions are in agreement with ARUPS measurements that show that at 160 K no H2O adsorbs on the reduced surface but adsorbs dissociatively on the stoichiometric surface. A strong adsorbate induced band bending is also obsd. for H2O adsorption on the stoichiometric surface that is likely assocd. with the formation of surface hydroxyls.
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    63. 63
      Serna, M. I.; Hasan, S. M.; Nam, S.; El Bouanani, L.; Moreno, S.; Choi, H.; Alshareef, H. N.; Minary-Jolandan, M.; Quevedo-Lopez, M. A. Low-Temperature Deposition of Layered SnSe2 for Heterojunction Diodes. Adv. Mater. Interfaces 2018, 5, 1800128,  DOI: 10.1002/admi.201800128
      There is no corresponding record for this reference.
    64. 64
      Zhang, Q.; Li, M.; Lochocki, E. B.; Vishwanath, S.; Liu, X.; Yan, R.; Lien, H.-H.; Dobrowolska, M.; Furdyna, J.; Shen, K. M. Band Offset and Electron Affinity of Mbe-Grown SnSe2. Appl. Phys. Lett. 2018, 112, 042108,  DOI: 10.1063/1.5016183
      64
      Band offset and electron affinity of MBE-grown SnSe2
      Zhang, Qin; Li, Mingda; Lochocki, Edward B.; Vishwanath, Suresh; Liu, Xinyu; Yan, Rusen; Lien, Huai-Hsun; Dobrowolska, Malgorzata; Furdyna, Jacek; Shen, Kyle M.; Cheng, Guangjun; Hight Walker, Angela R.; Gundlach, David J.; Xing, Huili G.; Nguyen, N. V.
      Applied Physics Letters (2018), 112 (4), 042108/1-042108/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      SnSe2 is currently considered a potential two-dimensional material that can form a near-broken gap heterojunction in a tunnel field-effect transistor due to its large electron affinity which is exptl. confirmed in this letter. With the results from internal photoemission and angle-resolved photoemission spectroscopy performed on Al/Al2O3/SnSe2/GaAs and SnSe2/GaAs test structures where SnSe2 is grown on GaAs by mol. beam epitaxy, we ascertain a (5.2 ± 0.1) eV electron affinity of SnSe2. The band offset from the SnSe2 Fermi level to the Al2O3 conduction band min. is found to be (3.3 ± 0.05) eV and SnSe2 is seen to have a high level of intrinsic electron (n-type) doping with the Fermi level positioned at about 0.2 eV above its conduction band min. It is concluded that the electron affinity of SnSe2 is larger than that of most semiconductors and can be combined with other appropriate semiconductors to form near broken-gap heterojunctions for the tunnel field-effect transistor that can potentially achieve high on-currents. (c) 2018 American Institute of Physics.
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    65. 65
      Bauer, E. Surface Microscopy with Low Energy Electrons; Springer, 2014; Vol. 23.
      There is no corresponding record for this reference.
    66. 66
      Henderson, M. A. The Interaction of Water with Solid Surfaces: Fundamental Aspects Revisited. Surf. Sci. Rep. 2002, 46, 1– 308,  DOI: 10.1016/S0167-5729(01)00020-6
      66
      The interaction of water with solid surfaces: fundamental aspects revisited
      Henderson, Michael A.
      Surface Science Reports (2002), 46 (1-8), 1-308CODEN: SSREDI; ISSN:0167-5729. (Elsevier Science B.V.)
      A review. Water is perhaps the most important and most pervasive chem. on our planet. The influence of water permeates virtually all areas of biochem., chem. and phys. importance, and is esp. evident in phenomena occurring at the interfaces of solid surfaces. Since 1987, when Thiel and Madey (TM) published their review titled 'The interaction of water with solid surfaces: fundamental aspects' in Surface Science Reports, there has been considerable progress made in further understanding the fundamental interactions of water with solid surfaces. In the decade and a half, the increased capability of surface scientists to probe at the mol.-level has resulted in more detailed information of the properties of water on progressively more complicated materials and under more stringent conditions. This progress in understanding the properties of water on solid surfaces is evident both in areas for which surface science methodol. has traditionally been strong (catalysis and electronic materials) and also in new areas not traditionally studied by surface scientists such as electrochem., photoconversion, mineralogy, adhesion, sensors, atm. chem. and tribol. Researchers in all these fields grapple with very basic questions regarding the interactions of water with solid surfaces such as how is water adsorbed, what are the chem. and electrostatic forces that constitute the adsorbed layer, how is water thermally or non-thermally activated and how do coadsorbates influence these properties of water. The attention paid to these and other fundamental questions in the past decade and a half has been immense. In this review, exptl. studies published since the TM review are assimilated with those covered by TM to provide a current picture of the fundamental interactions of water with solid surfaces.
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    67. 67
      Inamdar, A. N.; Som, N. N.; Pratap, A.; Jha, P. K. Hydrogen Evolution and Oxygen Evolution Reactions of Pristine and Alkali Metal Doped Snse2 Monolayer. Int. J. Hydrogen Energy 2020, 45, 18657– 18665,  DOI: 10.1016/j.ijhydene.2019.07.093
      67
      Hydrogen evolution and oxygen evolution reactions of pristine and alkali metal doped SnSe2 monolayer
      Inamdar, Archana N.; Som, Narayan N.; Pratap, Arun; Jha, Prafulla K.
      International Journal of Hydrogen Energy (2020), 45 (37), 18657-18665CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
      Many transition metal di-selenides such as MoSe2 and WSe2 show good catalytic activity on their edges with limited active orientations. These metal di-selenides are actively being used as target material for increasing the no. of electrocatalytic active sites and in turn to improve the H evolution reaction (HER) and O evolution reaction (OER) activities by increasing the ratio of edges to the basal plane. In present work, the authors have studied the activity of pristine and alkali atoms (Na, K and Ca) doped-SnSe2 for HER and OER catalyst. The state-of-art d. functional theory (DFT) based computations were performed for estg. the catalytic activity of the pristine and doped SnSe2 by evaluating the adsorption and Gibbs free energies subjected to H and O adsorption. Further, to get better prediction of adsorption energy on the individual catalytic surface, the authors have included the dispersion correction term to exchange-correlation functional. The pristine SnSe2 is not a good HER catalyst when H is adsorbed on its basal plane. However, edge-sites show the good H adsorption and indicates that the edges of SnSe2 are the most preferential site for H adsorption. As far as the catalytic activity of SnSe2 with dopants is concerned, the Na-doped SnSe2 among all shows the best catalytic activity over its edge-site; whereas K and Ca doped SnSe2 show basal plane as preferred catalytic site. It is interesting to note that the disadvantage of low catalytic activity on basal plane of SnSe2 can be improved by selective doping of alkali metals.
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    68. 68
      Deng, J.; Mo, Y.; Liu, J.; Guo, R.; Zhang, Y.; Xue, W.; Zhang, Y. In Vitro Study of SnS2, BiOCl and SnS2-Incorporated BiOCl Inorganic Nanoparticles Used as Doxorubicin Carrier. J. Nanosci. Nanotechnol. 2016, 16, 5740– 5745,  DOI: 10.1166/jnn.2016.11745
      68
      In vitro study of SnS2, BiOCl and SnS2-incorporated BiOCl inorganic nanoparticles used as doxorubicin carrier
      Deng, Jiangming; Mo, Yunfei; Liu, Jianghui; Guo, Rui; Zhang, Yi; Xue, Wei; Zhang, Yuanming
      Journal of Nanoscience and Nanotechnology (2016), 16 (6), 5740-5745CODEN: JNNOAR; ISSN:1533-4880. (American Scientific Publishers)
      Inorg. nanoparticles have been widely used in biomedical field. In this paper, we try to study the use of three types of inorg. nanoparticles (i.e., SnS2, BiOCl and SnS2-incorporated BiOCl (SnS2/BiOCl)) as doxorubicin (DOX) carriers. Firstly, SnS2, BiOCl and SnS2/BiOCl were synthesized, then were characterized by TEM, nanoparticles size and zeta potential. Next the drug release and cell viability test were carried out. The cell viability test indicated that the drug carriers can effectively kill HeLa cells while maintaining low cytotoxicity against normal cells-fibroblasts. The results show the potential of SnS2/BiOCl nanoparticles for antitumor applications.
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    69. 69
      Wu, S.; Liu, C.; Wu, Z.; Miao, L.; Gao, J.; Hu, X.; Chen, J.; Zheng, Y.; Wang, X.; Shen, C. Realizing Tremendous Electrical Transport Properties of Polycrystalline SnSe2 by Cl-Doped and Anisotropy. Ceram. Int. 2019, 45, 82– 89,  DOI: 10.1016/j.ceramint.2018.09.136
      69
      Realizing tremendous electrical transport properties of polycrystalline SnSe2 by Cl-doped and anisotropy
      Wu, Shaohai; Liu, Chengyan; Wu, Zhengsen; Miao, Lei; Gao, Jie; Hu, Xiaokai; Chen, Junliang; Zheng, Yanyan; Wang, Xiuxia; Shen, Chengjin; Yang, Hengquan; Zhou, Xiaoyuan
      Ceramics International (2019), 45 (1), 82-89CODEN: CINNDH; ISSN:0272-8842. (Elsevier Ltd.)
      SnSe2 is regarded as an attractive thermoelec. material for its structural and chem. analogy to SnSe that is claimed with the highest ZT in single crystal. In this study, the pure and Cl-doped SnSe2 polycrystals (3%, 6%, 9% and 12% molar Cl content) were fabricated in four steps that are hydrothermal synthesis, heating purifn., diffusion doping, and spark plasma sintering. The phase structure, lamellar morphol. and crystallite orientation were studied for the synthesized SnSe2 powder and the sintered pellets. The structural evolution was traced from the SnSe2 hexagonal plates of powders to the (001) oriented grains in pellets. The Cl doping into SnSe2 was verified by phase compn., lattice parameter, element distribution, and chem. valance. The doped Cl increased both the carrier concn. and the mobility. The anisotropic thermoelec. properties of SnSe2 bulk materials were investigated as functions of temp. from 50 °C to 300 °C and the doping amt., resp. The Seebeck coeff. was less anisotropic than the elec. and thermal conduction. The grain orientation influenced the anisotropy of the elec. and thermal cond. at a similar ratio. The power factors were less dependent on temp. with an optimum in-plane 1.06 mW m-1 K-2 and out-of-plane 0.41 mW m-1 K-2. The highest ZTs of 0.3 were attained at 300 °C in both directions.
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    70. 70
      Nagaraju, G.; Cha, S. M.; Sekhar, S. C.; Yu, J. S. Metallic Layered Polyester Fabric Enabled Nickel Selenide Nanostructures as Highly Conductive and Binderless Electrode with Superior Energy Storage Performance. Adv. Energy Mater. 2017, 7, 1601362,  DOI: 10.1002/aenm.201601362
      There is no corresponding record for this reference.
    71. 71
      Dimitriev, Y.; Yordanov, St.; Lakov, L. The Structure of Oxide Glasses Containing SeO2. J. Non-Cryst. Solids 2001, 293–295, 410– 415,  DOI: 10.1016/S0022-3093(01)00836-5
      71
      The structure of oxide glasses containing SeO2
      Dimitriev, Y.; Yordanov, St.; Lakov, L.
      Journal of Non-Crystalline Solids (2001), 293-295 (), 410-415CODEN: JNCSBJ; ISSN:0022-3093. (Elsevier Science B.V.)
      A review of results for glass formation in selenite systems obtained during the last 20 yr. For all of them, the vitrification regions are situated near the SeO2 corner, which is the main glass-former. The structure of model compns. is studied to elucidate the role of the different building units in the formation of the amorphous network. The IR spectra of binary selenite glasses are compared with those for more complicated compns. It is proven that it is possible to modify the network of selenite glasses by introducing compatible polyhedra TeO4, TeO3, VO5, BiO6, Mo2O8 or BO3 with SeO3 units. The better glass-forming tendency of compns. with 50-90 mol% SeO2 is related to the creation of addnl. disorder in the SeO3 chains by involving other structural units in them. These new glass-forming units are capable of transforming the structure into layers or three-dimensional random networks with a low at. mobility.
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    72. 72
      Bachvarova-Nedelcheva, A.; Iordanova, R.; Kostov, K. L.; Yordanov, S.; Ganev, V. Structure and Properties of a Non-Traditional Glass Containing TeO2, SeO2 and MoO3. Opt. Mater. 2012, 34, 1781– 1787,  DOI: 10.1016/j.optmat.2012.05.002
      72
      Structure and properties of a non-traditional glass containing TeO2, SeO2 and MoO3
      Bachvarova-Nedelcheva, A.; Iordanova, R.; Kostov, K. L.; Yordanov, St.; Ganev, V.
      Optical Materials (Amsterdam, Netherlands) (2012), 34 (11), 1781-1787CODEN: OMATET; ISSN:0925-3467. (Elsevier B.V.)
      A glass contg. SeO2, TeO2, MoO3 and La2O3 was obtained at high oxygen pressure (P = 36 MPa) using pure oxides as precursors. The real bulk chem. compn. of the glass according to LA-ICP-MS anal. is 17SeO2·50TeO2·32MoO3·1La2O3 (wt.%). The glass was characterized by X-ray diffraction, SEM (SEM), DTA (DTA), UV-Vis, XPS, IR and EPR spectroscopy. According to DTA the glass transition temp. (Tg) is below 300°C. By IR and XPS was detd. the main building units (TeO3, TeO4, SeO3, Mo2O8) and the existing of mixed bridging bonds only, which build up the amorphous network. It was established by UV-Vis that the glass is transparent above 490 nm. As a result of a lengthy heat treatment, crystn. took place and crystals rich in SeO2 and TeO2 were found incorporated into the amorphous part contg. all components.
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    73. 73
      Fan, Y.; Zhuo, Y.; Li, L. Seo2 Adsorption on Cao Surface: Dft and Experimental Study on the Adsorption of Multiple SeO2 Molecules. Appl. Surf. Sci. 2017, 420, 465– 471,  DOI: 10.1016/j.apsusc.2017.04.233
      73
      SeO2 adsorption on CaO surface: DFT and experimental study on the adsorption of multiple SeO2 molecules
      Fan, Yaming; Zhuo, Yuqun; Li, Liangliang
      Applied Surface Science (2017), 420 (), 465-471CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      SeO2 adsorption mechanisms on CaO surface were investigated by both d. functional theory (DFT) calcns. and adsorption expts. Adsorption of multiple SeO2 on the CaO (001) surface was investigated using slab model. Based on the results of adsorption energy and surface property, a double-layer adsorption mechanisms were proposed. In expts., the SeO2 adsorption products were prepd. in a U-shaped quartz reactor at 200 °C. The surface morphol. was investigated by field emission SEM (FE-SEM). The superficial and total SeO2 mass fractions were measured by XPS and inductively coupled plasma at. emission spectroscopy (ICP-AES), resp. The surface valence state and bulk structure are detd. by XPS and x-ray diffraction (XRD). The exptl. results are in good agreement with the DFT results. In conclusion, the fundamental SeO2 chemisorption mechanisms on CaO surface were suggested.
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    74. 74
      Al-Hada, N. M.; Kamari, H. M.; Baqer, A. A.; Shaari, A. H.; Saion, E. Thermal Calcination-Based Production of SnO2 Nanopowder: An Analysis of Sno2 Nanoparticle Characteristics and Antibacterial Activities. Nanomaterials 2018, 8, 250,  DOI: 10.3390/nano8040250
      74
      Thermal calcination-based production of SnO2 nanopowder: an analysis of SnO2 nanoparticle characteristics and antibacterial activities
      Al-Hada, Naif Mohammed; Kamari, Halimah Mohamed; Baqer, Anwar Ali; Shaari, Abdul H.; Saion, Elias
      Nanomaterials (2018), 8 (4), 250-267CODEN: NANOKO; ISSN:2079-4991. (MDPI AG)
      SnO2 nanoparticle prodn. using thermal treatment with tin(II) chloride dihydrate and polyvinylpyrrolidone capping agent precursor materials for calcination was investigated. Samples were analyzed using X-ray diffraction (XRD), SEM (SEM), energy dispersive X-ray (EDX), transmission electron microscopy (TEM), Fourier Transform IR Spectroscopy (FT-IR), XPS, diffuse UV-vis reflectance spectra, photoluminescence (PL) spectra and the ESR (ESR). XRD anal. found tetragonal cryst. structures in the SnO2 nanoparticles generated through calcination. EDX and FT-IR spectroscopy phase anal. verified the derivation of the Sn and O in the SnO2 nanoparticle samples from the precursor materials. An av. nanoparticle size of 4-15.5 nm was achieved by increasing calcination temp. from 500 °C to 800 °C, as confirmed through TEM. The valence state and surface compn. of the resulting nanoparticle were analyzed using XPS. Diffuse UV-vis reflectance spectra were used to evaluate the optical energy gap using the Kubelka-Munk equation. Greater calcination temp. resulted in the energy band gap falling from 3.90 eV to 3.64 eV. PL spectra indicated a pos. relationship between particle size and photoluminescence.
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    75. 75
      Zhang, W.; Li, M.; Xiao, X.; Huang, X.; Jiang, Y.; Fan, X.; Chen, L. In Situ Synthesis of Ultrasmall Sno2 Quantum Dots on Nitrogen-Doped Reduced Graphene Oxide Composite as High Performance Anode Material for Lithium-Ion Batteries. J. Alloys Compd. 2017, 727, 1– 7,  DOI: 10.1016/j.jallcom.2017.04.316
      75
      In situ synthesis of ultrasmall SnO2 quantum dots on nitrogen-doped reduced graphene oxide composite as high performance anode material for lithium-ion batteries
      Zhang, Wei; Li, Meng; Xiao, Xuezhang; Huang, Xu; Jiang, Yiqun; Fan, Xiulin; Chen, Lixin
      Journal of Alloys and Compounds (2017), 727 (), 1-7CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
      SnO2 is considered as one of the anode material for Li-ion batteries in terms of its superiority in high theor. capacity (1494 mAh g-1), low cost and environmental friendly. However, it is suffered from several issues such as rapid capacity deterioration, undesirable aggregation of tin particles and pesky expansion of vol. To conquer these shortcomings, a novel composite of ultrasmall SnO2 quantum dots with an av. particle size of 4-5 nm anchored on nitrogen-doped reduced graphene oxide (SnO2@NRGO) was first in situ synthesized By means of hydrothermal method. The results show that as-prepd. SnO2@NRGO electrode exhibits a greater enhancement in its initial discharge capacity (1678.4 mAh g-1) and reversible capacity (1333.5 mAh g-1 after 450 cycles) at a c.d. of 500 mA g-1, implying a long cycle life. Furthermore, the high rate capability of SnO2@NRGO is superior to SnO2@RGO and SnO2 electrodes. The excellent electrochem. reversibility of SnO2@NRGO electrode can be ascribed to the great cond., ultrahigh sp. surface area and the synergetic effect between ultrasmall SnO2 quantum dots and NRGO.
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    76. 76
      Wakita, T.; Paris, E.; Kobayashi, K.; Terashima, K.; Hacisalihoǧlu, M. Y.; Ueno, T.; Bondino, F.; Magnano, E.; Píš, I.; Olivi, L.; Akimitsu, J.; Muraoka, Y.; Yokoya, T.; Saini, N. L. The Electronic Structure of Ag1-XSn1+XSe2 (X = 0.0, 0.1, 0.2, 0.25 and 1.0). Phys. Chem. Chem. Phys. 2017, 19, 26672– 26678,  DOI: 10.1039/C7CP05369J
      76
      The electronic structure of Ag1-xSn1+xSe2 (x = 0.0, 0.1, 0.2, 0.25 and 1.0)
      Wakita, Takanori; Paris, Eugenio; Kobayashi, Kaya; Terashima, Kensei; Hacisalihoglu, Muammer Yasin; Ueno, Teppei; Bondino, Federica; Magnano, Elena; Pis, Igor; Olivi, Luca; Akimitsu, Jun; Muraoka, Yuji; Yokoya, Takayoshi; Saini, Naurang L.
      Physical Chemistry Chemical Physics (2017), 19 (39), 26672-26678CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      We have studied the valence electronic structure of Ag1-xSn1+xSe2 (x = 0.0, 0.1, 0.2, 0.25) and SnSe (x = 1.0) by a combined anal. of X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS) measurements. Both XAS and XPS reveal an increase in electron carriers in the system with x (i.e. excess Sn concn.) for 0 ≤ x ≤ 0.25. The core-level spectra (Sn 3d, Ag 3d and Se 3d) show that the charge state of Ag is almost 1+, while that of of Sn splits into Sn2+ and Sn4+ (providing clear evidence of valence skipping for the first time) with a concomitant splitting of Se into Se2- and Se2-δ states. The x dependence of the split components in Sn and Se together with the Se-K edge XAS reveals that the Se valence state may have an essential role in the transport properties of this system.
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    77. 77
      Hoch, L. B.; Wood, T. E.; O’Brien, P. G.; Liao, K.; Reyes, L. M.; Mims, C. A.; Ozin, G. A. The Rational Design of a Single-Component Photocatalyst for Gas-Phase Co2 Reduction Using Both Uv and Visible Light. Adv. Sci. 2014, 1, 1400013,  DOI: 10.1002/advs.201400013
      77
      The Rational Design of a Single-Component Photocatalyst for Gas-Phase CO2 Reduction Using Both UV and Visible Light
      Hoch Laura B; O'Brien Paul G; Liao Kristine; Reyes Laura M; Ozin Geoffrey A; Wood Thomas E; Mims Charles A
      Advanced science (Weinheim, Baden-Wurttemberg, Germany) (2014), 1 (1), 1400013 ISSN:2198-3844.
      The solar-to-chemical energy conversion of greenhouse gas CO2 into carbon-based fuels is a very important research challenge, with implications for both climate change and energy security. Herein, the key attributes of hydroxides and oxygen vacancies are experimentally identified in non-stoichiometric indium oxide nanoparticles, In2O3-x(OH)y, that function in concert to reduce CO2 to CO under simulated solar irradiation.
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    78. 78
      Detweiler, Z. M.; Wulfsberg, S. M.; Frith, M. G.; Bocarsly, A. B.; Bernasek, S. L. The Oxidation and Surface Speciation of Indium and Indium Oxides Exposed to Atmospheric Oxidants. Surf. Sci. 2016, 648, 188– 195,  DOI: 10.1016/j.susc.2015.10.026
      78
      The oxidation and surface speciation of indium and indium oxides exposed to atmospheric oxidants
      Detweiler, Zachary M.; Wulfsberg, Steven M.; Frith, Matthew G.; Bocarsly, Andrew B.; Bernasek, Steven L.
      Surface Science (2016), 648 (), 188-195CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
      Metallic indium and its oxides are useful in electronics applications, in transparent conducting electrodes, as well as in electrocatalytic applications. In order to understand more fully the speciation of the indium and oxygen compn. of the indium surface exposed to atm. oxidants, XPS, HREELS, and TPD were used to study the indium surface exposed to water, oxygen, and carbon dioxide. Clean In and authentic samples of In2O3 and In(OH)3 were examd. with XPS to provide std. spectra. Indium was exposed to O2 and H2O, and the ratio of O2- to OH- in the O1s XPS region was used to monitor oxidn. and speciation of the surface. HREELS and TPD indicate that water dissocs. on the indium surface even at low temp., and that In2O3 forms at higher temps. Initially, OH- is the major species at the surface. Pure In2O3 is also OH- terminated following water exposure. Ambient pressure XPS studies of water exposure to these surfaces suggest that high water pressures tend to passivate the surface, inhibiting extensive oxide formation.
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    79. 79
      Nappini, S.; Matruglio, A.; Naumenko, D.; Dal Zilio, S.; Bondino, F.; Lazzarino, M.; Magnano, E. Graphene Nanobubbles on TiO2 for in-Operando Electron Spectroscopy of Liquid-Phase Chemistry. Nanoscale 2017, 9, 4456– 4466,  DOI: 10.1039/C6NR09061C
      79
      Graphene nanobubbles on TiO2 for in-operando electron spectroscopy of liquid-phase chemistry
      Nappini, S.; Matruglio, A.; Naumenko, D.; Dal Zilio, S.; Bondino, F.; Lazzarino, M.; Magnano, E.
      Nanoscale (2017), 9 (13), 4456-4466CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      XPS and X-Ray Absorption Spectroscopy (XAS) provide unique knowledge on the electronic structure and chem. properties of materials. Unfortunately this information is scarce when investigating solid/liq. interfaces and chem. or photochem. reactions under ambient conditions because of the short electron inelastic mean free path (IMFP) that requires a vacuum environment, which poses serious limitation on the application of XPS and XAS to samples present in the atm. or in the presence of a solvent. One promising approach is the use of graphene (Gr) windows transparent to both photons and electrons. This paper proposes an innovative system based on sealed Gr nanobubbles (GNBs) on a titanium dioxide TiO2 (100) rutile single crystal filled with the soln. of interest during the fabrication stage. The GNBs were successfully employed to follow in-operando the thermal-induced redn. of FeCl3 to FeCl2 in aq. soln. The electronic states of chlorine, iron and oxygen were obtained through a combination of electron spectroscopy methods (XPS and XAS) in different phases of the process. The interaction of various components in soln. with solid surfaces constituting the cell was obtained, also highlighting the formation of a covalent C-Cl bond in the Gr structure. For the easiness of GNB fabrication and straightforward extension to a large variety of solns., we envisage a broad application of the proposed approach to investigate in detail electronic mechanisms that regulate liq./solid electron transfer in catalytic and energy conversion related applications.
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    80. 80
      Hong, X.; Li, S.; Wang, R.; Fu, J. Hierarchical SnO2 Nanoclusters Wrapped Functionalized Carbonized Cotton Cloth for Symmetrical Supercapacitor. J. Alloys Compd. 2019, 775, 15– 21,  DOI: 10.1016/j.jallcom.2018.10.099
      80
      Hierarchical SnO2 nanoclusters wrapped functionalized carbonized cotton cloth for symmetrical supercapacitor
      Hong, Xiaodong; Li, Shunli; Wang, Rui; Fu, Jiawei
      Journal of Alloys and Compounds (2019), 775 (), 15-21CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
      As an excellent free-standing carbon source, commercialized cotton cloth can be carbonized to fabricate conductive carbon cloth composed of hollow carbon microfibers. In view of the hydrophobicity of carbon cloth, a functionalized carbonized cotton cloth (FCC) is prepd. by acidification treatment, and acted as a flexible substrate to grow hierarchical SnO2 nanoclusters by a solvothermal reaction and calcination process. In this SnO2 wrapped FCC (FCC@SnO2) composite, the oxygen-contg. groups in carbon microfibers provide numerous anchoring sites for growing SnO2 nanoparticles, meanwhile, the carbon microfibers provide conductive channels for the fast transfer of electrons and ions. The SnO2 nanoclusters effectively contribute their pseudocapacitance. As free-standing electrodes in a sym. two-electrode supercapacitor, the FCC@SnO2 composite exhibits a higher capacitance of 197.7 F g-1 or 1265.3 mF cm-2 at 1 A g-1, much higher than that of FCC (100.3 F g-1 or 411.2 mF cm-2); furthermore, its capacitance remains 95.5% after cycling for 5000 cycles at 15 A g-1. The FCC@SnO2 composite is easier prepn. and low cost, which can be utilized as self-supporting flexible electrodes for high performance supercapacitors.
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    81. 81
      Xu, H.; Ju, J.; Li, W.; Zhang, J.; Wang, J.; Cao, B. Superior Triethylamine-Sensing Properties Based on TiO2/SnO2 N–N Heterojunction Nanosheets Directly Grown on Ceramic Tubes. Sens. Actuators, B 2016, 228, 634– 642,  DOI: 10.1016/j.snb.2016.01.059
      81
      Superior triethylamine-sensing properties based on TiO2/SnO2 n-n heterojunction nanosheets directly grown on ceramic tubes
      Xu, Hongyan; Ju, Judianxing; Li, Wenru; Zhang, Jun; Wang, Jieqiang; Cao, Bingqiang
      Sensors and Actuators, B: Chemical (2016), 228 (), 634-642CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
      A highly sensitive and selective gas sensor towards triethylamine (TEA) has been successfully fabricated by designing n-n heterojunctions consisting of SnO2 nanosheets and TiO2 nanoparticles. The SnO2 nanosheets with the thickness about 15 nm were directly grown on Al2O3 ceramic tubes by a simple hydrothermal process. After the formation of n-n heterojunctions by employing PLD method, the TiO2 nanoparticle/SnO2 nanosheets heterojunctions exhibit high sensing properties to TEA gas. The as-prepd. ST3 (SnO2/TiO2 3000) sensor response could reach to 52.3 at relatively low temp. (260°C) when exposed to 100 ppm TEA gas, which is much higher than that of pure SnO2 nanosheet sensor (∼3@100 ppm TEA@320°C). Compared with the pure SnO2 nanosheet sensor (S sensor), the depletion layer formed at the n-n heterojunctions interface in TiO2/SnO2 sensor can greatly increase the resistance in air and decrease the resistance in TEA gas. Due to the general working principle and controllable growth strategy, this work provides a way for developing the chemiresistive gas sensors.
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    82. 82
      Guo, C.; Guo, W.; Xu, H.; Zhang, L.; Chen, G.; D'Olimpio, G.; Kuo, C.-N.; Lue, C. S.; Wang, L.; Politano, A.; Chen, X.; Lu, W. Ultrasensitive Ambient-Stable SnSe2-Based Broadband Photodetectors for Room-Temperature IR/THz Energy Conversion and Imaging. 2D Mater. 2020, 7, 035026,  DOI: 10.1088/2053-1583/ab8ec0
      82
      Ultrasensitive ambient-stable SnSe2-based broadband photodetectors for room-temperature IR/THz energy conversion and imaging
      Guo, Cheng; Guo, Wanlong; Xu, Huang; Zhang, Libo; Chen, Gang; D'olimpio, Gianluca; Kuo, Chia-Nung; Lue, Chin Shan; Wang, Lin; Politano, Antonio; Chen, Xiaoshuang; Lu, Wei
      2D Materials (2020), 7 (3), 035026CODEN: DMATB7; ISSN:2053-1583. (IOP Publishing Ltd.)
      The advent of tin diselenide (SnSe2) enables novel pathways for optoelectronics, due to its reduced cost, ultralow thermal cond. and high potential for thermoelectricity. To date, SnSe2-based optoelectronic devices have been focused on the visible and IR range of the electromagnetic spectrum, with efficiency sharply decreasing at longer wavelength. Here, we present SnSe2 photodetectors with exfoliated SnSe2 nanosheets extended in the range of THz frequency, exhibiting high responsivity (170 V W-1), fast speed (2.2μs), as well as room-temp. operation, based on efficient prodn. of hotelectrons under deep-subwavelength electromagnetic focus, which outperform thermal-based photodetectors. Our SnSe2-based detectors show high-contrast imaging from terahertz (THz) up to visible. The outstanding ambient stability of our broadband photodetectors in a timescale of months is due to the chem. inertness of stoichiometric SnSe2 crystals, validated by surface-science expts. Our results demonstrate the suitability of SnSe2 for multispectral sensing and real-time imaging.
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    83. 83
      Doniach, S.; Sunjic, M. Many-Electron Singularity in X-Ray Photoemission and X-Ray Line Spectra from Metals. J. Phys. C: Solid State Phys. 1970, 3, 285,  DOI: 10.1088/0022-3719/3/2/010
      83
      Many-electron singularity in x-ray photoemission and x-ray line spectra from metals
      Doniach, Sebastian; Sunjic, M.
      Journal of Physics C: Solid State Physics (1970), 3 (2), 285-91CODEN: JPSOAW; ISSN:0022-3719.
      The spectra of x-ray-induced fast photoelectrons from metal should have a characteristic skew line shape resulting from Kondo-like many-electron interactions of the metallic conduction electrons with the accompanying deep hole in the final state. The same line shape should also occur for the discrete line spectra of x-rays emitted from metals. This mechanism could account for the well known asymmetries obsd. for Kα lines.
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpclett.0c02616.

    • Single-crystal growth (section S1), LEEM images (section S2), vibrational spectroscopy (section S3), electronic properties (section S4), density of states in pristine and defective SnSe2 (section S5), temperature dependence of the differential Gibbs free energy for adsorption of ambient gases (section S6), Langmuir isotherm calculations (section S7), and methods (section S8) (PDF)


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