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Peroxide-Induced Tuning of the Conductivity of Nanometer-Thick MoS2 Films for Solid-State Sensors
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ACS Applied Nano Materials

Cite this: ACS Appl. Nano Mater. 2020, 3, 11, 10864–10877
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https://doi.org/10.1021/acsanm.0c02135
Published November 6, 2020

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Abstract

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Applications of molybdenum disulfide (MoS2) in energy storage devices, solar cells, electrocatalysts, and sensors require good electrical conductivity. However, neither of the current ways to prepare conductive MoS2 (lithium intercalation and hydrothermal processes) is easily amenable to scale-up. A possible alternative pathway is the modulation of the electronic properties of the semiconducting form of MoS2 through structural defects. Here, we report the preparation of nanoscale conductive MoS2 flakes by treating exfoliated 2H-MoS2 with dilute aqueous hydrogen peroxide at room temperature. Sheet resistance measurements as well as Raman and photoelectron spectroscopy reveal the partial formation of hydrogen molybdenum bronze (HxMoO3) and substoichiometric MoO3–y, which help tune the conductivity of the nanometer-scale thin films without impacting the sulfur-to-molybdenum ratio. We have cast the material into thin film networks to fabricate highly stable chemiresistive pH sensors. Our work introduces a straightforward and safe way of preparing a conductive form of MoS2 and its application as a low-cost solid-state sensor.

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Introduction

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Two-dimensional (2D) materials have garnered much interest over the past decade due to their demonstrated high performance in nanoelectronic devices. (1,2) The discovery of graphene opened up many opportunities to investigate and explore other 2D materials. Since then, a wide range of materials have been discovered or predicted, (3,4) with molybdenum disulfide (MoS2) being of particular interest. (5,6) The semiconducting phase of MoS2 (2H-MoS2) is one of the most commonly studied among the transition metal dichalcogenides. The direct band gap of monolayer 2H-MoS2 enables its use as a channel material for field-effect transistors (FET), where a high on/off current ratio is desired. (7,8) The presence of a band gap means that the use of 2H-MoS2 requires a relatively high voltage bias to operate. For applications in batteries, (9) supercapacitors, (10) electrocatalytic reactions, (11) and solar cells, (12) however, a substantially increased conductivity is essential in order to achieve practical current densities. The most common conductive form of MoS2 is metallic MoS2 (1T-MoS2) that has been prepared via the lithium intercalation process involving inert atmosphere processing and elaborate safety procedures. (13,14) This procedure requires elevated temperatures (∼100 °C) over long durations (more than 2 days). Another synthesis procedure of conductive MoS2 was reported by using a hydrothermal process (15) in an autoclave under careful temperature control. Again, this process requires sophisticated instrumentation. A safer, cost-effective, and more efficient process to yield conductive MoS2 would be beneficial in a number of intended applications.
MoS2 is most reactive at defect sites, which thus play a very important role in modulating the electrical properties of MoS2. (16−18) Sonication of MoS2 in an appropriate solvent creates many disordered structural defects. The most common defects in MoS2 are sulfur vacancies. (16) Both sulfur and molybdenum vacancies in MoS2 introduce gap states, (19) but sulfur vacancies are energetically favorable. Sulfur vacancies are not desirable for the purpose of device applications as they can result in Fermi level pinning and eventually deteriorate the device performance. (19,20) Various strategies have been proposed to passivate the sulfur vacancy defects using thiols, (21,22) molecular and atomic oxygen, (23) and organic superacids like bis(trifluoromethane)sulfonimide (TFSI) where protons (H+) act as a passivating agent. (24) While past research has mainly focused on ways to modify the defects for photoluminescence efficiency, (25,26) they could also be controlled to improve the conductivity of MoS2 as a safer alternative for applications in batteries, supercapacitors, solar cells, electrocatalysts, and sensors. So far, however, no reliable bulk synthesis of MoS2 with defect-induced conductivity has been reported.
Nanometer-scale thin films of conductive MoS2 can be used as active material for low-cost solid-state chemiresistive pH sensors. Paper-based colorimetric pH sensors are ubiquitous and easy to fabricate and use but are inaccurate and require human intervention. For online monitoring or laboratory applications, ion selective pH electrodes are most popular. However, they are expensive and also require frequent calibration due to their reliance on reference electrodes. Solid-state chemiresistive pH sensors can become a more reliable alternative to ion selective pH electrodes since they are inexpensive to fabricate and do not need any reference electrodes or membranes. In chemiresistive sensors, conductivity changes are observed based on direct interactions between the active material and the analyte. Even though chemiresistive pH sensors based on exfoliated graphene, carbon nanotubes, or graphitic materials are available, their sensing response is limited to <20%. (27−30)
In this work, we demonstrate a facile and safe way to prepare few-layer conductive MoS2 (c-MoS2) at ambient conditions using low concentrations of aqueous hydrogen peroxide (H2O2). The samples were characterized by four-probe conductivity and Hall measurements, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy to elucidate the origin of the enhanced conductivity. It was found that the interaction of H2O2 with 2H-MoS2 causes the formation of small amounts of hydrogen molybdenum bronze (HxMoO3) and substochiometric MoO3–y which are structurally intertwined with MoS2 and act as dopants. While the chemical and structural characteristics of MoS2 are retained in the process, the conductivity of the resulting c-MoS2 material is only about an order of magnitude lower than that of molybdenum bronze or 1T-MoS2. The c-MoS2 flakes can then be cast into percolation networks, forming conductive thin films with a sheet resistance up to 7 orders of magnitude lower than that of 2H-MoS2. We also explored the possibility of surface functionalization by different thiols and demonstrate a practical application of such modified conductive films in environmental sensing by fabricating a pH sensor that is highly stable and has a significant response. Because of the high conductivity of c-MoS2 films, we were able to fabricate chemiresistive pH sensors with centimeter channel length while maintaining low measurement voltages. Our study furthers the understanding of conductive forms of MoS2 and opens a new pathway toward next-generation electronic devices.

Experimental Section

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Materials

Unless otherwise mentioned, all organic solvents were HPLC grade and used without further purification. Bulk molybdenum disulfide (∼6 μm to maximum 40 μm, product number 69860, batch number WXBD2352 V) powder, cysteamine (∼95%, product number M9768), undecanethiol (98%, product number 510467), and 4-chlorothiophenol (97%, product number 125237) were purchased from Sigma-Aldrich and used without further purification. Water used for experiments was ultrapure type I water (18.2 MΩ·cm) from a Millipore Simplicity water purification system. Hydrogen peroxide (H2O2) (30%) was purchased from J.T Baker. A bath sonicator (Elmasonic P60H ultrasonic cleaner) was used for sonication, and an Eppendorf MiniSpin Plus microcentrifuge was used for centrifugation.

Sample Preparation

2H-MoS2 was exfoliated by using 45% (v/v) of ethanol in water via sonication (80 kHz frequency, 100% power and sweep mode) for 12 h, and the temperature (30 °C) was controlled during sonication by cooling the bath. (31) The centrifugation process was then optimized for conditions to consist of a first step with 3500 rpm (820g) for 15 min, and then the supernatant underwent a second step of centrifugation at 4500 rpm (1700g) for 3 min. Thus, the 2H-MoS2 was collected in the form of a gray precipitate. The collected 2H-MoS2 was washed with water, and the supernatant was discarded. Three different concentrations of aqueous H2O2 (0.02, 0.06, and 0.33 vol %) were used to ultrasonically suspend the prepared 2H-MoS2. The suspension was first sonicated (37 kHz frequency, 100% power, and sweep mode) for 2, 10, or 20 min and then centrifuged first at 3500 rpm (820g) for 8 min, followed by centrifugation of the supernatant at 10000 rpm (6708g) for 15 min. Shaken samples were prepared by adding different concentrations of aqueous H2O2 into the exfoliated 2H-MoS2 instead of sonicating with aqueous H2O2 and followed the same centrifugation procedure as described above. Then the supernatant was discarded by using a glass pipet, and the precipitate was collected in the form of a gray solid for further use.
For the functionalization of c-MoS2 surfaces with cysteamine (2-aminoethanethiol) and 4-chlorothiophenol, the air-brushed (NEO for Iwata CN gravity feed dual action brush #N4500 using 20 psi of N2) and dried thin films were immersed for 20 h in either a 2.5% (v/v) aqueous cysteamine solution or a 2.5% (v/v) ethanolic solution of 4-chlorothiophenol. They were then rinsed with water (cysteamine) and pure ethanol (4-chlorothiophenol) and dried in a fume hood for 10 min at room temperature for further experiments.

Characterization

A Renishaw inVia Raman spectrometer was used over the range 100–3000 cm–1, with a spectral resolution of 2 cm–1, using a 20× objective in backscattering configuration. All spectra were obtained on three different spots of the same sample by using a fully focused 633 nm laser on a spot size of about 1.2 μm limited to 1% of laser power to avoid sample damage. A JEOL JSM-7000F scanning electron microscope (SEM) was used to obtain high-resolution images of MoS2 at 20 kV. Talos 120C and Titan LB transmission electron microscopes (TEM) were used to obtain low- and high-resolution images of MoS2 at 120 and 300 kV, respectively. The XPS analysis was performed on a Kratos AXIS Supra X-ray photoelectron spectrometer. XPS survey spectra were obtained from an area of approximately 300 × 700 μm2 by using a pass energy of 160 eV. XPS high-resolution spectra were obtained from an area of approximately 300 × 700 μm2 by using a pass energy of 20 eV. XPS was performed on 2H-MoS2 and all c-MoS2 samples to evaluate the effect of H2O2. Exfoliated 2H-MoS2 was used as a comparison. Survey scans and high-resolution spectra of C 1s, O 1s, S 2p, N 1s, Cl 2p, Mo 3d, and the valence band edge were recorded and analyzed. The thickness of the films was measured on a Bruker Alicona Infinite FocusG5 plus 3D optical measurement system using a 50× objective for height measurement (20 nm vertical resolution). Sheet resistance and Hall mobility of the films were measured by using a Nanometrics HL 5500PC Hall effect measurement system.

Device Fabrication for Sheet Resistance

To measure the conductivity of MoS2, the sheet resistance was calculated by using a four-probe geometry. Silicon wafers (Virginia Semiconductor) were thermally oxidized to grow a 1 μm thick silicon dioxide (SiO2) layer for electrical insulation. After oxidation, the wafers were cut into 1 × 1 cm2 pieces with a dicing saw. The wafer pieces were then rinsed with acetone followed by methanol and DI water, dried with nitrogen gas, and kept in a nitrogen storage box for further experiments. In order to lower the contact resistance, Cr (20 nm)/Au (200 nm) 3 × 3 mm2 pads were sputter-deposited onto the four corners of each sample. Kapton tape was used as a mask for MoS2 deposition onto a 7 × 7 mm2 area in the center of the substrate. A mixture of 95% (v/v) ethanol and 5% (v/v) water was used to suspend c-MoS2 for airbrushing so the solvent can evaporate quickly. Airbrushing was performed onto the masked area of the silicon wafer in order to make a homogeneous film by using a NEO for Iwata CN gravity feed dual action brush #N4500 with 20 psi of N2. Gas flow slowed the solvent evaporation during airbrushing as it cooled the surface of the devices; hence, the hot plate temperature was kept at 80 °C. Once c-MoS2 dried to a solid film, the mask was removed. Two devices were fabricated from each MoS2 sample to ensure reproducibility.

Device Fabrication for Chemiresistive Sensors

All two-terminal sensor devices were fabricated on the frosted end of the microscope glass slides (VWR, catalogue no. CA48323-185, dimensions 1 in. × 3 in., ground edges, precleaned twin-frosted end). Glass slides were cleaned by sonicating first in acetone for 15 min followed by sonicating in methanol for 15 min. The slides were then dried at room temperature and stored in nitrogen storage boxes for further use. Two parallel conductive pads (each 6 mm wide) were drawn about 1 cm apart (channel dimension where c-MoS2 would be drop-casted) by using a 9B pencil on the frosted part of glass slide. Kapton tape (1 mil, 1 in. × 36 yd) was used to mask the area (1.5 cm × 1.5 cm) for drop-casting MoS2. The c-MoS2 dispersion was obtained by suspending the gray solid in DI water, and 150 μL of MoS2 suspension was drop-casted on the mask and dried at 100 °C to get a continuous solid film. Once the film was dry, the Kapton tape was removed, and two strips of conductive adhesive copper (Cu) tape (Adafruit Industries, 6 mm wide) were pasted onto the pencil lines and covered with hot glue (all-purpose glue sticks, thermoplastic adhesive) with a hot glue gun (Aleene’s ultimate glue gun kit). The more involved airbrushing technique is only used for sheet resistance measurements since a homogeneous film thickness is required in order to calculate a meaningful sheet resistance value. Sensor films benefit from the higher surface area and simpler fabrication of drop-casted films despite less exacting dimensions.
For the functionalization of c-MoS2 surfaces with cysteamine (2-aminoethanethiol) and undecanethiol the drop-casted and dried thin films were immersed for 20 h in either a 2.5% (v/v) aqueous cysteamine solution or a 2.5% (v/v) toluene solution of undecanethiol. They were then rinsed with water (cysteamine) and toluene (undecanethiol) and dried in a fume hood for 10 min at room temperature for sensing experiments.

pH Measurement

The conductivity of the chemiresistive devices was measured in air by using a two-probe configuration with a Keithley 2450 source meter at room temperature. The sensing measurements were carried out by using a four-channel eDAQ EPU452 Quad Multifunction isoPod with USB (purchased from eDAQ Inc.). Three channels were used for chemiresistive devices, and one channel was used for a pH electrode. The pH electrode was purchased from eDAQ Inc. and calibrated in both pH 4 and 7 at 25 °C before each experiment. The device responses in liquid medium were recorded by using continuous two-probe measurements in Biosensor mode (100 mV applied bias, current range 2 mA, 6 decimal places, scanning time 1.0 s, and scan rate 30 points/min) at room temperature. The entire experiment was carried in an 800 mL bowl filled with 500 mL of a 200 ppm of NaCl salt solution. The bowl was kept on a stir plate with a continuous stirring. Three devices at a time were run in parallel. 1 M HCl or NaOH solutions were added dropwise into the 200 ppm solution by using glass pipettes in order to adjust the pH in 30 min intervals.

Results and Discussion

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Optimization for Conductivity

In order to achieve high conductivity, the morphology of the percolation network film needs to be optimized. 2H-MoS2 was first exfoliated according to a reported procedure using ethanol in water via sonication at room temperature. (31) The centrifugation process was then optimized for the fraction of well-exfoliated few-layer 2H-MoS2 flakes with minimum damage. These were collected in the form of a gray precipitate which was washed with water to remove ethanol before being used to prepare c-MoS2 by exposure to dilute aqueous H2O2 (Figure 1). The exfoliated c-MoS2 was then airbrushed onto a silicon dioxide (SiO2) substrate with four gold contacts (Figure 2a) to measure sheet resistance and Hall mobility. Scanning electron microscope (SEM) images of these films (Figure 2b) showed the presence of multilayers and homogeneously distributed flakes with sufficient overlap to ensure good conductivity of the film. This kind of film morphology cannot be achieved without first exfoliating the semiconducting form. TEM images further supported the differences in the surface properties of with and without treated with aqueous H2O2 of bulk MoS2 and exfoliated 2H-MoS2 (Figures 3a–d). Films of c-MoS2 obtained directly from bulk powder were more poorly aligned (Figure S1) and had a lower conductivity compared to those obtained from exfoliated 2H-MoS2. (15) The degree of exfoliation may also play a role in increasing the reactive surface area during sonication in aqueous H2O2. The height of the deposited solid films was typically around 9 μm as determined by high-resolution optical microscopy (Figures 2c,d).

Figure 1

Figure 1. Schematic representation of the exfoliation procedure of c-MoS2. Steps are (a) bulk MoS2 sonication in ethanol/water mixture for 12 h; (b) exfoliated 2H-MoS2 suspended in ethanol/water; (c–e) two-stage centrifugation process to collect exfoliated 2H-MoS2; (f) exfoliated 2H-MoS2 as precipitate; (g) exfoliated 2H-MoS2 shaken in 0.06% aqueous hydrogen peroxide; (h) suspended c-MoS2 after sonication; (i–k) two-stage centrifugation process to collect exfoliated c-MoS2; and (l) exfoliated c-MoS2 as precipitate after centrifugation.

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Figure 2

Figure 2. Morphology of MoS2 solid films and sheet resistance as a function of process parameters. (a) Actual image of exfoliated c-MoS2 (gray patch, 7 × 7 mm2) on a SiO2 substrate (1 × 1 cm2) with Au contacts in the four corners of the substrate. (b) SEM images showing overall film distribution of final exfoliated c-MoS2 on the substrate with high magnification showing few-layer flakes of material. The scale bars on the images represent 10 μm and 100 nm, respectively. (c) Alicona optical microscope mapping image of a SiO2 substrate with c-MoS2 film edge (red line is 19.7 mm long). (d) Height profile at the location of the red line in (d). (e) Relationship between the sheet resistance of c-MoS2 samples sonicated for different times in 0.06% and 0.22% aqueous H2O2. Lines are drawn to guide the eye. Solid lines are for 0.22%. The dotted lines are for 0.06%.

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Figure 3

Figure 3. TEM images showing film distribution of different MoS2 flasks: (a) bulk MoS2, (b) exfoliated 2H-MoS2, (c) bulk MoS2 treated with 0.06% aqueous H2O2, and (d) exfoliated MoS2 treated with 0.06% aqueous H2O2. The scale bar on the images represents 0.2 μm.

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In order to achieve the desired degree of oxidation of MoS2 leading to increased conductivity without dissolution by the highly reactive H2O2 at higher concentrations, it is necessary to identify the optimal peroxide concentration and sonication time. Twelve distinct batches of c-MoS2 were prepared by using three different concentrations of aqueous H2O2 (0.02%, 0.06%, and 0.22%) in which exfoliated 2H-MoS2 samples were either briefly shaken or sonicated for 2, 10, or 20 min. Duplicate devices were fabricated from each batch and characterized by using four-probe sheet resistance and Hall probe measurements at room temperature. The sheet resistance data indicate that the lowest resistances were obtained from 0.06% and 0.22% of H2O2 (Figure 2e), whereas the sheet resistances obtained by treatment with 0.02% H2O2 were similar to those of semiconducting 2H-MoS2 (>109 Ω/□). The sonication time was another key factor in controlling the effect of H2O2 on 2H-MoS2. The bulk resistivity of 0.06% and 0.22% c-MoS2 samples at 20 min was 0.43 Ω·cm (the corresponding sheet resistance of a 9 μm thick film was 4.50 × 102 Ω/□) and 0.40 Ω·cm (the corresponding sheet resistance of a 9 μm thick film was 4.34 × 102 Ω/□) (Tables S1–S4). A trend in decreasing sheet resistance with sonication time was observed at 0.06% c-MoS2 samples, whereas the minimum in sheet resistance for 0.22% c-MoS2 was already reached after 10 min of sonication. The 20 min sonicated sample showed no further change in sheet resistance. The measured sheet resistance for c-MoS2 shaken in either of the two concentrations was 5 orders of magnitude less than that of 2H-MoS2, indicating that sonication is not fundamentally required in order to improve the conductivity of 2H-MoS2. Sonication, however, exfoliates and reshuffles the sheets, thus increasing the film conductivity by another 2 orders of magnitude in addition to allowing the material to retain a higher conductivity even if the surface undergoes further modification (as demonstrated below). We also tested higher concentrations of H2O2 but found that MoS2 tended to completely dissolve in those cases following the formation of hydrated molybdenum trioxide and sulfur dioxide. (32,33) Based on the above observations, samples were prepared by using 0.06% H2O2 in all subsequent experiments to maximize yield.
The Hall mobility of the same samples used to measure the sheet resistance was measured at room temperature. It varied between 226 and 355 cm2 V–1 s–1 for different sonication times (Figure S3), which was comparable to some reported values for multilayer MoS2 FETs (34) but higher than other reports for p-doped MoS2 FETs. (35) The Hall mobility depends on several factors such as number of layers, metal contacts, and surface of the materials. (36,37) The positive sign of the Hall mobility values indicates that holes were the majority charge carriers as a result of p-type doping. (36,37)

Identifying the Source of Increased Conductivity

XRD data of the c-MoS2 phase (Figures S3 and S4) show it to be a (doped) 2H-MoS2 phase (a broad (002) peak at 2θ ∼ 14.5°) rather than any of the metallic 1T phases ((001) peak at 2θ ∼ 7.3° for 1T-MoS2) reported in the literature, (38) consistent with the Hall measurements indicating a p-doped semiconductor. No changes in the peak at 2θ of bulk MoS2, 2H-MoS2, and c-MoS2 are observed in the XRD data (Figures S3 and S4). This is also borne out by Raman and XPS data. The Raman spectra for all samples display the two distinct E12g and A1g modes. The in-plane E12g mode at 385 cm–1 originates in the antiparallel vibration of sulfur atoms with respect to the molybdenum atoms, whereas the out-of-plane A1g mode at 410 cm–1 involves the vibration of the sulfur atoms in the opposite direction. These are the most prominent peaks for identification of the properties of 2H-MoS2. The absence of three characteristic Raman peaks at ∼156, ∼226, and ∼333 cm–1 further confirms that our samples do not contain any detectable amounts of 1T-MoS2. (39) The XPS binding energies for Mo 3d5/2, Mo 3d3/2, S 2p3/2, and S 2p1/2 in 1T-MoS2 have been reported to be 228.7, 231.8, 161.6, and 163.7 eV, respectively, about 1 eV lower than those for 2H-MoS2. (13,15) XPS analysis of all H2O2-treated c-MoS2 samples shows the above binding energies to be consistent with 2H-MoS2 (Figure 4). The location of the valence band edge further confirms the semiconducting nature of our samples (Figure S5).

Figure 4

Figure 4. XPS high-resolution spectra of exfoliated 2H-MoS2; shaken c-MoS2; 2 min sonicated c-MoS2; and 20 min sonicated c-MoS2 samples. Spectra are (a) Mo 3d, (b) S 2p in nonfunctionalized MoS2, and (c) S 2p in cysteamine-functionalized MoS2.

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Sulfur vacancies can also have a significant impact on the electronic properties of MoS2 samples. Combined XPS survey and high-resolution data demonstrate that the S2–/Mo4+ ratio does not decrease for MoS2 upon exposure to dilute aqueous H2O2 without sonication when compared to the exfoliated 2H-MoS2 (Table 1). Since the biggest change in conductivity was seen as the result of this step, we conclude that the increase in conductivity also cannot be the result of an increase in sulfur vacancies. There is a small drop in the S2–/Mo4+ ratio for the sonicated samples, likely due to further damage inflicted during sonication, but the change is very small and not correlated with the evolution in conductivity (Table 1). Since no chemical elements beyond H, C, O, S, and Mo have been introduced into our samples during processing, the three most common explanations for conductivity in MoS2 (1T metallic phases, sulfur vacancies, and substitutional doping at the Mo sites) (13,16,40) have thus been excluded from consideration, and we have to look elsewhere to explain the nature of our c-MoS2 phase.
Table 1. Compositional Changes in the Bare Samples from XPS Dataa
sampleS2–/Mo4+Mo4+/MoMo5+/MoMo6+/Mosheet resistance (Ω /□)bulk resisivity (Ω·cm)
2H-MoS21.3590.952 0.0483.0 × 1092.7 × 106
shaken c-MoS21.3720.6840.1640.1512.5 × 10422
2 min c-MoS21.3290.8080.0940.0971.3 × 10412
10 min c-MoS21.3300.8200.0900.0904.1 × 1034.0
20 min c-MoS21.3210.949 0.0514.4 × 1020.43
a

S to Mo atomic ratio of 2H-MoS2 and c-MoS2 samples. The atomic ratio of sulfide to Mo(IV) was calculated from the total atomic percentages of Mo and S as well as high-resolution XPS spectra of S 2p and Mo 3d. The atomic ratios of Mo(IV), Mo(V), and Mo(VI) relative to the total Mo content in 2H-MoS2 and c-MoS2 samples were calculated by using high-resolution XPS spectra of Mo 3d. The bulk resistivity is calculated from the product of the sheet resistance and thickness of ∼9 μm thick films (average of two devices each).

Spectroscopic Determination of the Nature of c-MoS2

Figure 4a shows high-resolution Mo 3d spectra of c-MoS2 shaken or sonicated for 2 or 20 min (Figure S6a for 10 min sonicated c-MoS2) in 0.06% H2O2, while the Mo 3d spectrum of the exfoliated 2H-MoS2 sample is shown in Figure 4a. The Mo 3d doublet shows a Mo 3d5/2 binding energy of 229.7 eV, which is characteristic for Mo4+ in 2H-MoS2. (41,42) A Mo 3d5/2 binding energy of 233.3 eV is characteristic of Mo6+ such as in molybdenum trioxide (MoO3). (41,42) The same +6 oxidation peaks of Mo 3d5/2 are present in the H2O2-treated c-MoS2 samples (shaken, 2, 10, or 20 min sonicated in Figures 4a and Figure S6a). In addition to the +6 and +4 oxidation states, Mo 3d5/2 peaks at 232.1 eV attributable to the formation of Mo5+ were observed for shaken, 2 min, and 10 min c-MoS2 samples. This indicates that while the exfoliated 2H-MoS2 was already partially oxidized during the sonication in the ethanol/water mixture, adding H2O2 causes further oxidation of 2H-MoS2 (Figure 6).
Decomposition of H2O2 can cause formation of atomic hydrogen, (43,44) which may react further with species already present on the 2H-MoS2 surface (e.g., MoO3) and result in the formation of hydrogen molybdenum bronze HxMoO3 and substoichiometric MoO3–y. (45,46) It was reported that the reduction of MoO3 leads to the formation of substoichiometric MoO3–y where an increase in y is correlated with a decrease of the electronic bandgap of MoO3, making the material conductive. (47) The bronze also contains Mo5+, as observed in XPS. HxMoO3 is significantly more conductive than 2H-MoS2 and MoO3. (48,49) The sheet resistance of our c-MoS2 material is only about 1 order of magnitude higher than those reported for MoO2, HxMoO3–y nanobelts, and high-temperature carbon electrodes. (48,50) Hence, we propose the conductivity of the c-MoS2 samples shaken or sonicated up to 10 min in H2O2 to be due to the presence of HxMoO3 and substoichiometric MoO3–y.
The Mo5+ peak is no longer present in the spectrum of the sample sonicated for 20 min since it was the result of H2O2 interacting with the surface, and the small amount of H2O2 will have been consumed by reaction with MoS2 or ultrasonically decomposed by that point in the process. The ratio of Mo5+/Mo in the high-resolution XPS data is highest for shaken samples (0.16) and decreases to 0.09 upon sonication for both 2 and 10 min (Table 1). No Mo5+ signal remains for 20 min sonicated samples, implying that HxMoO3 and MoO3–y are unstable intermediate species that are gradually reduced to MoO2, consistent with growth of the Mo4+ signal in the spectra. (49) The high-resolution Mo 3d peaks should be broader for MoO2 than for MoS2, but the relatively small amounts of MoO2 are likely being obscured by the large Mo4+ signal from MoS2 in the 20 min samples. In addition to the formation of Mo5+, the amount of Mo6+ is found to increase upon exposure to H2O2 coupled with a significant drop in the proportion of Mo4+ (Table 1). Upon sonication, the proportion of Mo4+ gradually recovers over time while Mo5+ and Mo6+ levels gradually decrease back to resemble the 2H-MoS2 starting material, but with a much higher conductivity.
While XPS spectroscopy only probes the top 10 nm from the sample surface, Raman spectroscopy can help to further understand the bulk structure and properties of c-MoS2, which is important because the flakes will be continuously exfoliated and recombined during sonication, thus exposing fresh surfaces to the dilute aqueous peroxide. The gap between E12g and A1g modes is narrowed by 2 cm–1 due to a slight blue shift of the E12g peak combined with a slight red shift of the A1g peak for c-MoS2 as compared to the initial 2H phase (Figure 5a). The shift is very small since our samples consist of multilayers (SEM data, Figure 2b), and both modes have been reported to stiffen as the number of layers increases due to an increase in the restoring force on the atoms. (51) The peaks at 178, 423, 466, 526, 600, and 644 cm–1 for all samples (2H, shaken, and 20 min c-MoS2, Figure 5b–d) are due to resonant Raman scattering (633 nm laser) of MoS2. (52) Two peaks at 570 and 738 cm–1 can be attributed to vibrational modes of MoO2, while the peaks 230 and 492 cm–1 can be assigned to the phonon modes of MoO2. (53,54) The Raman features for MoO2 are very weak, since only a small amount of MoO2 was evidently formed in agreement with our interpretation of the XPS data. The Raman peak at 820 cm–1 is the most prominent peak attributable to MoO3. (46) It is important to note that MoO3 is an insulator and therefore does not contribute to either surface or bulk conductivity. While we did not observe any HxMoO3 peaks (204 cm–1) (46) in the Raman spectrum of either sample, some features around 780 cm–1 were instead detected for all samples. Broad features in this region have previously been attributed to substoichiometric MoO3–y. (47) While insulating MoO3 is clearly present in all samples starting with 2H-MoS2, the presence of minor MoO2 and MoO3–y impurities can be confirmed (although not quantified) by using Raman spectroscopy. The absence of a 440 cm–1 band in the Raman spectra (52) supports the notion that no oxysulfide species were formed during the exfoliation process, in agreement with the high-resolution S 2p XPS data from all MoS2 samples (Figure 4b and Figure S6b). Since the main Raman characteristic peaks of MoS2 still dominate the spectra, we conclude that even after oxidation the quality of the MoS2 material was maintained throughout the sonication process, as suggested earlier by the constant S/Mo ratio (Table 1).

Figure 5

Figure 5. Raman spectra of MoS2 samples. (a) Raman spectra proving the doping effect of H2O2 on 2H-MoS2. Black and violet color curves represent c-MoS2 (20 min sonicated) and 2H-MoS2, respectively, showing the shift in the E12g and A1g modes. (b) Raman spectrum of 2H-MoS2 sample. (c) Raman spectrum of c-MoS2 sample shaken in 0.06% H2O2. (d) Raman spectrum of 20 min sonicated c-MoS2 sonicated sample. All spectra were recorded with a 633 nm laser at 1% power. Each spectrum is normalized based on the highest peak (∼466 cm–1).

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Figure 6

Figure 6. Schematic representation of structure and chemical composition of the formation of c-MoS2.

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Probing the c-MoS2 Surface via Thiol Chemistry

Exposure to H2O2 helped to partially convert the MoO3 (which formed during exfoliation of 2H-MoS2 due to the oxidation of MoS2) to new compounds like hydrogen molybdenum bronze and substochiometric MoO3–y. While the S/Mo ratio did not change significantly as a result of the reaction with H2O2, reactive sulfur vacancy defects may have remained on the c-MoS2 surface or at the edges of the flakes. The c-MoS2 surface was titrated with different thiol molecules, namely cysteamine and 4-chlorothiophenol, to determine the number of residual reactive sites.
The high-resolution S 2p XPS spectra of all 4-chlorothiophenol-treated c-MoS2 samples are dominated by sulfide peaks associated with MoS2, with a binding energy of 162.4 eV observed for 2p3/2 (Figure S8). Thiol-related 2p3/2 peaks at a binding energy of 164.0 eV were only observed in 2H-MoS2 as well as 2 and 10 min sonicated c-MoS2 samples (Figure S8). If the sulfur vacancies had been functionalized by thiols, a thiolate peak would have been expected at a binding energy of 161.8 eV, which is absent in all our spectra. (55) The presence of a small thiol peak suggests the presence of unbound thiol on the surface, (55) but the thiol to total sulfur ratio was very small (Table 2). The very small Cl/Mo ratio (Table 2) further confirms that only a negligible amount of 4-chlorothiophenol remained at the surfaces of 2H-MoS2 and all c-MoS2 samples. The samples had been exposed to ambient conditions prior to thiolation, so oxidation at the sulfur vacancy sites was expected. High-resolution XPS spectra of the Cl 2p peaks of all 4-chlorothiophenol-functionalized c-MoS2 (Figure S7) samples confirm that chlorine remained bonded to organic carbon at a binding energy of 200.7 eV for 2p3/2 and 202.3 eV for 2p1/2. (56) Overall, 4-chlorothiophenol reacted with neither the c-MoS2 nor the 2H-MoS2 surfaces under our given experimental conditions.
Table 2. Compositional Changes in Functionalized Samples from XPS Dataa
 cysteamine4-chlorothiophenol
sampleN/Mothiol/SCl/Mothiol/S
2H-MoS20.4560.1350.0040.021
shaken c-MoS20.2680.0230.007 
2 min c-MoS20.3360.0680.0090.028
10 min c-MoS20.4120.0500.0100.023
20 min c-MoS20.3840.1120.008 
a

Atomic ratios of nitrogen (N) and chlorine (Cl) after functionalization with respect to total Mo for each sample. The atomic ratios were calculated from the total atomic percentages of Mo and S as well as high-resolution XPS spectra of N 1s, Cl 2p, and Mo 3d. Atomic ratios of thiols (cysteamine and 4-chlorothiophenol) were calculated by using high-resolution XPS spectra of S (thiol) 2p with respect to total S for each sample.

In all cysteamine-treated 2H-MoS2 and c-MoS2 samples, S 2p3/2 peaks are observed at 164.0 eV for unbound thiols, and S 2p3/2 peaks associated with sulfide in MoS2 are observed at 162.4 eV (Figure 4c and Figure S10). (55) The unbound thiol to total sulfur ratio is consistently much higher in all cysteamine-treated samples compared to 4-chlorothiophenol-treated samples (Table 2). Unsurprisingly, the N/Mo atomic ratio in these samples is also higher compared to the Cl/Mo atomic ratio in 4-chlorothiophenol-treated samples. Hence, at our given experimental conditions for the respective procedures, cysteamine reacts more easily than 4-chlorothiophenol with both c-MoS2 and 2H-MoS2 samples. It is clear from the XPS data that the thiols did not attach to any reactive sulfur vacancies, as no thiolate peak was detected. Furthermore, we only observed a single N 1 s peak associated with free amines at a binding energy of 399.3 eV (Figure S9). (57) While the amine group may participate in noncovalent interactions with the surface, there is no spectroscopic evidence of it. While the sulfide to Mo(IV) ratios as given in Table 1 for all samples are in the range 1.3–1.4 rather than close to the ideal value of 2.0, the defects have been passivated with oxygen species under the given circumstances and are not accessible to thiol functionalization in organic solvents. The aqueous conditions of cysteamine functionalization appear to have facilitated surface interactions, even though neither the thiol nor the amine group was found to covalently react with the surface. The availability of lone electron pairs at oxygen and sulfur surface sites may nevertheless enable hydrogen-bonding interactions or acid–base chemistry.

Direct In-Solution Measurement of pH Using a c-MoS2-Based Chemiresistive Sensor

A potential application for surface-functionalized thin conductive films (such as those fabricated from c-MoS2) is chemiresistive sensing, for example, of the pH of aqueous solutions (Figure 7). Chemiresistive sensors are solid-state electrical devices based on measuring the change in conductivity of a thin film as a result of the interaction between the active layer and an analyte. (58) While this sensor geometry is generally easy to fabricate and use, MoS2-based chemiresistive sensors operating at low enough voltages to be safe for aqueous analytes are rare due to their low conductivity. We fabricated nanometer thick pristine and cysteamine-functionalized c-MoS2 films on glass substrates as active layers for chemiresistive sensors (Figure S11). The thickness of the c-MoS2 film was calculated based on the bulk resistivity of c-MoS2 material. The sensor films are estimated to be at least 100 nm thick considering their geometry and the reported bulk resistivity of c-MoS2 but neglecting contact resistance and film roughness. The contact resistance cannot be determined in a two-terminal geometry (and will be different from our four terminal measurements due to the different nature of the contacts); the roughness of the drop-cast films will add uncertainty to any film thickness determination, but we can state that the sensor films are a few 100 nm in thickness at most.

Figure 7

Figure 7. Schematic representation of the fabrication and potential applications of exfoliated c-MoS2.

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Before using those chemiresistive devices to measure pH, we recorded current vs voltage graphs in ambient conditions after each fabrication step, including film deposition, passivation of the metal contacts with hot glue, and functionalization with cysteamine (Figure S12). We observed a decrease in current (from 38 to 12 μA at 1 V) after passivation of the metal contacts, whereas after cysteamine functionalization the current dropped by about an order of magnitude. The same devices were then immersed into water to record their pH sensing responses. We used 200 ppm of sodium chloride as a background supporting electrolyte to avoid structural changes in the electrical double layer during acid and base addition for pH adjustment. (59,60)
The transition of the devices from air into an aqueous environment caused another decrease in film conductivity by about 1 order of magnitude, depending on whether deionized water or salt solution was used (Figure S13). This is due to electrostatic gating by the electrical double layer. Current changes over time were recorded for all devices starting at 0 min upon immersion in water. Figure 8e shows the pH sensing response of a pristine c-MoS2 film. While an overall pH ranging from 2.7 to 10 (Figure S14) was tested, no changes in current were observed above pH 7. Even though the pristine device shows a marked response to changes in pH of almost 100% increase in sensor current at pH 4 relative to the baseline at pH 6.8, the sensor response is not very stable upon exposure to alternating higher and lower pH values (Figure 8e). Additional pristine c-MoS2 devices were also fabricated, showing comparable sensor responses and baseline drift (Figure S15). The drift may be the result of pH-related instability of the molybdenum oxide phases. (61) We thus examined the relationship between solution pH and electrochemical open circuit potential of the sensor films against a reference electrode (Ag/AgCl) (Figure S16). This relationship can then be overlaid with established Pourbaix diagrams of molybdenum (oxide, sulfide)/water systems to predict the thermodynamic stability of different species as a function of pH and electrochemical potential. (62,63) The result is that no phase changes are predicted in the potential region transversed by the c-MoS2 films at open circuit potential and relevant pH. Further, Raman spectra were collected of two chemiresistive devices that were exposed to pH 2.5 (Figure S17) or pH 9 (Figure S18) for 40 min, immediately dried with nitrogen gas and transferred to the spectrometer within a minute. Neither sample shows any changes in the spectral features compared to a freshly prepared film of c-MoS2 (Figure 4d). Hence, it is further confirmed that bulk phase changes do not occur during the protonation and deprotonation process, leaving modulations of the doping level due to protonation and deprotonation of surface oxide species as the origin of the sensing response.

Figure 8

Figure 8. pH sensing response of c-MoS2 chemiresistive devices. 100 mV potential bias was applied across the c-MoS2 film to measure the current changes. Cysteamine-functionalized c-MoS2 chemiresisitive pH responses at (a) 3.7, (b) 6.5, (c) 4, and (d) 3.5. (e) Pristine c-MoS2 chemiresistive response to different pH values between 4 and 6.5. (f) Cysteamine-functionalized c-MoS2 chemiresistive response to different pH values between 3 and 6.5. (g) Calibration curve (linear fitting) of functionalized c-MoS2 chemiresistive device response (b). (h) Calibration curve from (g) replotted as a function of pH.

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The pH response of a cysteamine-functionalized device was significantly higher (ranging from 20% at pH 6.5 over 124% at pH 4 to almost 642% at pH 3) (Figures 8a–d,f) and significantly more stable compared to the pristine devices. Because of the previously mentioned increase in film resistivity upon functionalization with cysteamine, the baseline currents of the functionalized devices are noticeably lower than those of the pristine devices. The functionalization of MoS2 with cysteamine has been reported to result in n-type doping. (21) Since the pristine c-MoS2 films are p-doped, this would lead in effect to a reduction in charge carrier density and thus the observed decrease in the conductivity upon functionalization (Figure S12). Protonation of the amino group in the cysteamine at low pH reduces its ability to act as an electron donor (i.e., n-dopant) (Figure 9). The effective doping level of the c-MoS2 films will thus be increased again, leading to the significant sensor response. In contrast, undecanethiol-functionalized devices (containing a thiol group but no amino group) did not exhibit any reproducible pH responses. Moreover, the obtained response was much lower (15% at pH 6.5, 20% at pH 4.2, etc., Figure S19) compared to even pristine devices (30% at pH 6.5, 90% at pH 4, etc.) devices. Not only does the amino group serve to amplify the pH response, but the response of the pristine films can be suppressed by passivation of the defect sites with thiols.

Figure 9

Figure 9. Schematic representation of protonation and deprotonation process of cysteamine-functionalized c-MoS2.

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The sensor response of the cysteamine-functionalized device in Figure 6b was linearly related to the concentration of hydrogen ions in the solution with an R2 value of 0.9864 (Figure 8g). This linear concentration dependence results in a logarithmic sensor response to pH (Figure 8f). Additional devices functionalized with cysteamine were tested and showed qualitatively similar behavior (Figure S20). The simple fabrication method via drop-casting of the c-MoS2 resulted in an uneven distribution of the c-MoS2 flakes on the glass slides and widely varying film resistances. Nevertheless, stable and high sensor responses were observed from all devices upon repeated exposure to pH 4 and 6.5 (Figures S20). The long-term stability of the sensors was further investigated. Functionalized c-MoS2 devices were kept in the 200 ppm of NaCl solution after the pH measurement for about 1 week, and no significant baseline drift was observed (Figure S21), indicating that functionalized devices are stable in aqueous environments over long periods of time.
The new form of conductive MoS2 presented here is a semiconducting 2H-MoS2 phase very heavily p-doped by hydrogen molybdenum bronze (HxMoO3) and substoichiometric MoO3–y, which are formed due to the interaction of very dilute H2O2 with the MoS2 surface. XRD, XPS, Raman, and Hall measurements all corroborate this mechanism. Doping of MoS2 by H2O2 itself cannot be the origin of the conductivity since it would lead to an increase of the band gap by ∼40 meV. (32) The interaction of dilute H2O2 with exfoliated 2H-MoS2 causes HxMoO3 and MoO3–y formation, while sonication in dilute aqueous H2O2 further reduces the sheet resistance of the bulk material down to about 440 Ω /□ (Hall mobilities as high as 355 cm2 V–1 s–1) without compromising the structure of the p-doped 2H-MoS2 phase.
The resulting c-MoS2 material has a wide range of possible applications in batteries, supercapacitors, solar cells, electrocatalysts, or sensors. We have chosen to demonstrate the use of c-MoS2 films in a chemiresistive geometry as pH sensors within a pH range of 3–6.5 for both pristine and functionalized c-MoS2 devices. Functionalized devices showed a stable sensor response of up to 2000% at pH 3 compared to a neutral solution baseline, depending linearly on the hydrogen ion concentration. Pristine devices still have a significant response of over 100%, but the directly exposed substoichiometric oxide phases are not stable under acidic conditions, causing a drift in the sensor baseline. This instability is avoided by passivation of the defect sites with thiols. For unfunctionalized thiols, the passivation also suppresses the pH response, while amino-functionalized thiols (e.g., cysteamine) serve to enhance the pH response. Chemiresistive solid-state sensor devices are cheaper to fabricate, simpler to use, and lower in maintenance than electrochemical sensors since they do not require any reference electrode or gate electrode. (58,64) Most MoS2 sensors reported to date are FET-based gas sensors. (7,65,66) Recently, a MoS2 chemiresistive sensor has been reported to detect the cadmium cations in aqueous environments, but the resistances of the reported 2H-MoS2 devices were much higher the c-MoS2 devices presented here, thus requiring a higher voltage bias that may interfere with measurements in aqueous electrolytes. (67) The lower sheet resistance of c-MoS2 allows for simpler fabrication, more favorable operating parameters (lower voltage and higher current), and a higher sensor response of the chemiresistive films for applications in aqueous environments.

Conclusions

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In summary, a stable conductive form of MoS2 was exfoliated in solution by using a two-step ultrasonication procedure using an ethanol/water mixture in the first step and dilute aqueous H2O2 in the second step. Centrifugation then results in a slurry of micrometer-sized few-layer p-doped 2H-MoS2 flakes that can be processed into conductive nanometer-scale films with <1 Ω·cm in bulk resistivity and good carrier mobility. Chemiresistive devices that perform well and are stable in aqueous environments over a wide pH range have been demonstrated after passivation with cysteamine. While stability of the c-MoS2 under a wider range of conditions still has to be studied, the process is easily scalable for research and industrial applications and may be followed by surface passivation steps.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsanm.0c02135.

  • Additional SEM, additional TEM, sheet resistance, Hall measurement, XRD, XPS, Raman, and pH sensor data (PDF)

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Author Information

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  • Corresponding Author
  • Authors
    • Dipankar Saha - Department of Chemistry and Chemical Biology, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4M1, Canada;  Orcidhttp://orcid.org/0000-0002-6268-2807
    • Ponnambalam Ravi Selvaganapathy - Department of Mechanical Engineering and School of Biomedical Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4L7, Canada;  Orcidhttp://orcid.org/0000-0003-2041-7180
  • Notes
    The authors declare the following competing financial interest(s): The authors have filed for a provisional patent covering the fabrication process described in this manuscript.

Acknowledgments

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We are grateful to Dr. Mark Biesinger (Surface Science Western) for help with XPS; Mohammad Shariful Islam Chowdhury and Ali Aliakbari Khoei (McMaster Manufacturing Research Institute, McMaster University) for help with optical microscopy; Dr. Ross Anthony (Centre for Emerging Device Technologies, McMaster University) for help with oxidation of the silicon wafers; Dr. Shahram Tavakoli and Doris Stevanovic (both from Centre for Emerging Device Technologies, McMaster University) for help with sputtering, sheet resistance, and Hall measurements; Dr. James F. Britten and Victoria Jarvis (both from McMaster University) for help with XRD measurement; Dr. Carmen Andrei (CCEM-McMaster University) for help with TEM; and Vinay Patel, Sreekant Damodara, Jayasree Biswas, Dr. Aditya Aryasomayajula, Ana Zubiarrain Laserna, Marcia Reid, Chris Butcher, Maryam Darestani-Farahani, and Johnson Dalmieda (all from McMaster University) for fruitful discussions. Electronic microscopy was carried out at the Electron Microscopy Facility of the Faculty of Health Sciences (McMaster University) and at the Canadian Centre for Electron Microscopy (CCEM) McMaster University, a national facility. The work was financially supported by the Natural Sciences and Engineering Research Council of Canada through the Discovery Grant Program as well as the Canada First Research Excellence Fund project “Global Water Futures”.

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    Air-stable doping of transition metal dichalcogenides is of fundamental importance to enable a wide range of optoelectronic and electronic devices while exploring their basic material properties. Here the authors demonstrate the use of benzyl viologen (BV), which has one of the highest redn. potentials of all electron-donor org. compds., as a surface charge transfer donor for MoS2 flakes. The n-doped samples exhibit excellent stability in both ambient air and vacuum. Notably, the authors obtained a high electron sheet d. of ∼1.2 × 1013 cm-2, which corresponds to the degenerate doping limit for MoS2. The BV dopant mols. can be reversibly removed by immersion in toluene, providing the ability to control the carrier sheet d. as well as selective removal of surface dopants on demand. By BV doping of MoS2 at the metal junctions, the contact resistances are reduced by a factor of >3. As a proof of concept, top-gated field-effect transistors were fabricated with BV-doped n+ source/drain contacts self-aligned with respect to the top gate. The device architecture, resembling that of the conventional Si transistors, exhibited excellent switching characteristics with a subthreshold swing of ∼77 mV/decade.
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    Sarkar, D.; Liu, W.; Xie, X.; Anselmo, A. C.; Mitragotri, S.; Banerjee, K. MoS2 Field-Effect Transistor for Next-Generation Label-Free Biosensors. ACS Nano 2014, 8 (4), 3992– 4003,  DOI: 10.1021/nn5009148
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    8
    MoS2 Field-Effect Transistor for Next-Generation Label-Free Biosensors
    Sarkar, Deblina; Liu, Wei; Xie, Xuejun; Anselmo, Aaron C.; Mitragotri, Samir; Banerjee, Kaustav
    ACS Nano (2014), 8 (4), 3992-4003CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Biosensors based on field-effect transistors (FETs) have attracted much attention, as they offer rapid, inexpensive, and label-free detection. While the low sensitivity of FET biosensors based on bulk 3D structures has been overcome by using 1D structures (nanotubes/nanowires), the latter face severe fabrication challenges, impairing their practical applications. In this paper, we introduce and demonstrate FET biosensors based on molybdenum disulfide (MoS2), which provides extremely high sensitivity and at the same time offers easy patternability and device fabrication, due to its 2D atomically layered structure. A MoS2-based pH sensor achieving sensitivity as high as 713 for a pH change by 1 unit along with efficient operation over a wide pH range (3-9) is demonstrated. Ultrasensitive and specific protein sensing is also achieved with a sensitivity of 196 even at 100 femtomolar concn. While graphene is also a 2D material, we show here that it cannot compete with a MoS2-based FET biosensor, which surpasses the sensitivity of that based on graphene by more than 74-fold. Moreover, we establish through theor. anal. that MoS2 is greatly advantageous for biosensor device scaling without compromising its sensitivity, which is beneficial for single mol. detection. Furthermore, MoS2, with its highly flexible and transparent nature, can offer new opportunities in advanced diagnostics and medical prostheses. This unique fusion of desirable properties makes MoS2 a highly potential candidate for next-generation low-cost biosensors.
  9. 9
    Stephenson, T.; Li, Z.; Olsen, B.; Mitlin, D. Lithium Ion Battery Applications of Molybdenum Disulfide (MoS2) Nanocomposites. Energy Environ. Sci. 2014, 7 (1), 209– 231,  DOI: 10.1039/C3EE42591F
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    9
    Lithium ion battery applications of molybdenum disulfide (MoS2) nanocomposites
    Stephenson, Tyler; Li, Zhi; Olsen, Brian; Mitlin, David
    Energy & Environmental Science (2014), 7 (1), 209-231CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    A review. This is the first targeted review of the synthesis - microstructure - electrochem. performance relations of MoS2 - based anodes and cathodes for secondary lithium ion batteries (LIBs). Molybdenum disulfide is a highly promising material for LIBs that compensates for its intermediate insertion voltage (∼2 V vs. Li/Li+) with a high reversible capacity (up to 1290 mA h g-1) and an excellent rate capability (e.g. 554 mA h g-1 after 20 cycles at 50 C). Several themes emerge when surveying the scientific literature on the subject: first, we argue that there is excellent data to show that truly nanoscale structures, which often contain a nanodispersed carbon phase, consistently possess superior charge storage capacity and cycling performance. We provide several hypotheses regarding why the measured capacities in such architectures are well above the theor. predictions of the known MoS2 intercalation and conversion reactions. Second, we highlight the growing microstructural and electrochem. evidence that the layered MoS2 structure does not survive past the initial lithiation cycle, and that subsequently the electrochem. active material is actually elemental sulfur. Third, we show that certain synthesis techniques are consistently demonstrated to be the most promising for battery applications, and describe these in detail. Fourth, we present our selection of synthesis methods that we believe to have a high potential for creating improved MoS2 LIB electrodes, but are yet to be tried.
  10. 10
    Cao, L.; Yang, S.; Gao, W.; Liu, Z.; Gong, Y.; Ma, L.; Shi, G.; Lei, S.; Zhang, Y.; Zhang, S.; Vajtai, R.; Ajayan, P. M. Direct Laser-Patterned Micro-Supercapacitors from Paintable MoS2 Films. Small 2013, 9 (17), 2905– 2910,  DOI: 10.1002/smll.201203164
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    10
    Direct Laser-Patterned Micro-Supercapacitors from Paintable MoS2 Films
    Cao, Liujun; Yang, Shubin; Gao, Wei; Liu, Zheng; Gong, Yongji; Ma, Lulu; Shi, Gang; Lei, Sidong; Zhang, Yunhuai; Zhang, Shengtao; Vajtai, Robert; Ajayan, Pulickel M.
    Small (2013), 9 (17), 2905-2910CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Micrometer-sized electrochem. capacitors have recently attracted attention due to their possible applications in micro-electronic devices. Here, a new approach to large-scale fabrication of high-capacitance, two-dimensional MoS2 film-based micro-supercapacitors is demonstrated via simple and low-cost spray painting of MoS2 nanosheets on Si/SiO2 chip and subsequent laser patterning. The obtained micro-supercapacitors are well defined by ten interdigitated electrodes (five electrodes per polarity) with 4.5 mm length, 820 μm wide for each electrode, 200 μm spacing between two electrodes and the thickness of electrode is ∼0.45 μm. The optimum MoS2-based micro-supercapacitor exhibits excellent electrochem. performance for energy storage with aq. electrolytes, with a high area capacitance of 8 mF/cm2 (volumetric capacitance of 178 F/cm3) and excellent cyclic performance, superior to reported graphene-based micro-supercapacitors. This strategy could provide a good opportunity to develop various micro-/nanosized energy storage devices to satisfy the requirements of portable, flexible, and transparent micro-electronic devices.
  11. 11
    Deng, Z. H.; Li, L.; Ding, W.; Xiong, K.; Wei, Z. D. Synthesized Ultrathin MoS2 Nanosheets Perpendicular to Graphene for Catalysis of Hydrogen Evolution Reaction. Chem. Commun. 2015, 51 (10), 1893– 1896,  DOI: 10.1039/C4CC08491H
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    11
    Synthesized Ultrathin MoS2 Nanosheets Perpendicular to Graphene for Catalysis of Hydrogen Evolution Reaction
    Deng, Zihua; Li, Li; Ding, Wei; Xiong, Kun; Wei, Zidong
    Chemical Communications (Cambridge, United Kingdom) (2015), 51 (10), 1893-1896CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    The ultrathin MoS2 nanosheets are synthesized perpendicular to reduced graphene oxides (MoS2 .perp. RGO) as an electrocatalyst, which exhibits excellent catalytic activity and good stability for the hydrogen evolution reaction(HER) in acidic medium.
  12. 12
    Tsai, M.-L.; Su, S.-H.; Chang, J.-K.; Tsai, D.-S.; Chen, C.-H.; Wu, C.-I.; Li, L.-J.; Chen, L.-J.; He, J.-H. Monolayer MoS2 Heterojunction Solar Cells. ACS Nano 2014, 8 (8), 8317– 8322,  DOI: 10.1021/nn502776h
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    12
    Monolayer MoS2 Heterojunction Solar Cells
    Tsai, Meng-Lin; Su, Sheng-Han; Chang, Jan-Kai; Tsai, Dung-Sheng; Chen, Chang-Hsiao; Wu, Chih-I.; Li, Lain-Jong; Chen, Lih-Juann; He, Jr-Hau
    ACS Nano (2014), 8 (8), 8317-8322CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    We realized photovoltaic operation in large-scale MoS2 monolayers by the formation of a type-II heterojunction with p-Si. The MoS2 monolayer introduces a built-in elec. field near the interface between MoS2 and p-Si to help photogenerated carrier sepn. Such a heterojunction photovoltaic device achieves a power conversion efficiency of 5.23%, which is the highest efficiency among all monolayer transition metal dichalcogenide-based solar cells. The demonstrated results of monolayer MoS2/Si-based solar cells hold the promise for integration of two-dimensional materials with com. available Si-based electronics in highly efficient devices.
  13. 13
    Eda, G.; Yamaguchi, H.; Voiry, D.; Fujita, T.; Chen, M.; Chhowalla, M. Photoluminescence from Chemically Exfoliated MoS2. Nano Lett. 2011, 11 (12), 5111– 5116,  DOI: 10.1021/nl201874w
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    13
    Photoluminescence from Chemically Exfoliated MoS2
    Eda, Goki; Yamaguchi, Hisato; Voiry, Damien; Fujita, Takeshi; Chen, Mingwei; Chhowalla, Manish
    Nano Letters (2011), 11 (12), 5111-5116CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    A two-dimensional crystal of molybdenum disulfide (MoS2) monolayer is a photoluminescent direct gap semiconductor in striking contrast to its bulk counterpart. Exfoliation of bulk MoS2 via Li intercalation is an attractive route to large-scale synthesis of monolayer crystals. However, this method results in loss of pristine semiconducting properties of MoS2 due to structural changes that occur during Li intercalation. Here, we report structural and electronic properties of chem. exfoliated MoS2. The metastable metallic phase that emerges from Li intercalation was found to dominate the properties of as-exfoliated material, but mild annealing leads to gradual restoration of the semiconducting phase. Above an annealing temp. of 300°, chem. exfoliated MoS2 exhibit prominent band gap photoluminescence, similar to mech. exfoliated monolayers, indicating that their semiconducting properties are largely restored.
  14. 14
    Xia, J.; Wang, J.; Chao, D.; Chen, Z.; Liu, Z.; Kuo, J.-L.; Yan, J.; Shen, Z. X. Phase Evolution of Lithium Intercalation Dynamics in 2H-MoS2. Nanoscale 2017, 9 (22), 7533– 7540,  DOI: 10.1039/C7NR02028G
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    14
    Phase evolution of lithium intercalation dynamics in 2hydrogen-molybdenum disulfide
    Xia, Juan; Wang, Jin; Chao, Dongliang; Chen, Zhen; Liu, Zheng; Kuo, Jer-Lai; Yan, Jiaxu; Shen, Ze Xiang
    Nanoscale (2017), 9 (22), 7533-7540CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Due to the easy intralayer gliding and weak interlayer van der Waals interaction in transition metal dichalcogenides (TMDs), ion (particularly Li+) intercalation has been used to modify and tune their at. structures to obtain the desired optical, electronic and chem. properties for future optoelectronics and energy storage applications. A good understanding of the transformative structures during intercalation is crit. In this paper, we investigate the structural transformation dynamics of 2H-MoS2 using electrochem. Li+ intercalation for 2H-MoS2. The introduction of Li+ changes the local symmetry of the MoS2 in favor of the dT phase, clearly indicated by the appearance of Raman peaks of the dT phase. Further Li+ insertion causes the samples to become single-layer-like, characterized by the disappearance of the 32 cm-1 Raman peak. We also observe for the first time that the photoluminescence (PL) emission gradually red shifts with decreasing intensity, followed by eventual vanishing of the PL peak in the dT-MoS2 phase. By the nudged elastic band (NEB) calcns., we propose the 2H-1T-dT phase transition mechanism of MoS2 for Li+ intercalated samples. Our claims are supported by high resoln.-transmission electron microscopy (HR-TEM). Our study deepens the understanding of the phase transition dynamics upon lithium intercalation, which is of great value to possible optoelectronic devices based on the phase engineering of TMDs. The new Li-stabilized dT-MoS2 phase does not possess inversion symmetry and may present a feasible way to achieve Weyl state tuning in a single material via phase engineering.
  15. 15
    Geng, X.; Sun, W.; Wu, W.; Chen, B.; Al-Hilo, A.; Benamara, M.; Zhu, H.; Watanabe, F.; Cui, J.; Chen, T.-p. Pure and Stable Metallic Phase Molybdenum Disulfide Nanosheets for Hydrogen Evolution Reaction. Nat. Commun. 2016, 7 (1), 10672,  DOI: 10.1038/ncomms10672
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    15
    Pure and stable metallic phase molybdenum disulfide nanosheets for hydrogen evolution reaction
    Geng, Xiumei; Sun, Weiwei; Wu, Wei; Chen, Benjamin; Al-Hilo, Alaa; Benamara, Mourad; Zhu, Hongli; Watanabe, Fumiya; Cui, Jingbiao; Chen, Tar-pin
    Nature Communications (2016), 7 (), 10672CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    Metallic-phase MoS2 (M-MoS2) is metastable and does not exist in nature. Pure and stable M-MoS2 has not been previously prepd. by chem. synthesis, to the best of our knowledge. Here we report a hydrothermal process for synthesizing stable two-dimensional M-MoS2 nanosheets in water. The metal-metal Raman stretching mode at 146 cm-1 in the M-MoS2 structure, as predicted by theor. calcns., is exptl. obsd. The stability of the M-MoS2 is assocd. with the adsorption of a monolayer of water mols. on both sides of the nanosheets, which reduce restacking and prevent aggregation in water. The obtained M-MoS2 exhibits excellent stability in water and superior activity for the hydrogen evolution reaction, with a c.d. of 10 mA cm-2 at a low potential of -175 mV and a Tafel slope of 41 mV per decade.
  16. 16
    Dabral, A.; Lu, A. K. A.; Chiappe, D.; Houssa, M.; Pourtois, G. A Systematic Study of Various 2D Materials in the Light of Defect Formation and Oxidation. Phys. Chem. Chem. Phys. 2019, 21 (3), 1089– 1099,  DOI: 10.1039/C8CP05665J
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    16
    A systematic study of various 2D materials in the light of defect formation and oxidation
    Dabral, A.; Lu, A. K. A.; Chiappe, D.; Houssa, M.; Pourtois, G.
    Physical Chemistry Chemical Physics (2019), 21 (3), 1089-1099CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The thermodn. aspects of various 2D materials are explored using D. Functional Theory (DFT). Various metal chalcogenides (MX2, M = metal, chalcogen X = S, Se, Te) are investigated with respect to their interaction and stability under different ambient conditions met in the integration process of a transistor device. Their interaction with high-κ dielecs. is also addressed, in order to assess their possible integration in Complementary Metal Oxide Semiconductor (CMOS) field effect transistors. 2D materials show promise for high performance nanoelectronic devices, but the presence of defects (vacancies, grain boundaries,...) can significantly impact their electronic properties. To assess the impact of defects, their enthalpies of formation and their signature levels in the d. of states have been studied. We find, consistently with literature reports, that chalcogen vacancies are the most likely source of defects. It is shown that while pristine 2D materials are in general stable whenever set in contact with different ambient atms., the presence of defective sites affects the electronic properties of the 2D materials to varying degrees. We observe that all the 2D materials studied in the present work show strong reactivity towards radical oxygen plasma treatments while reactivity towards other common gas phase chem. such as O2 and H2O and groups present at the high-κ surface varies significantly between species. While energy band-gaps, effective masses and contact resistivities are key criteria in selection of 2D materials for scaled CMOS and tunneling based devices, the phase and ambient stabilities might also play a very important role in the development of reliable nanoelectronic applications.
  17. 17
    Xie, Y.; Liang, F.; Chi, S.; Wang, D.; Zhong, K.; Yu, H.; Zhang, H.; Chen, Y.; Wang, J. Defect Engineering of Mos2 for Room-Temperature Terahertz Photodetection. ACS Appl. Mater. Interfaces 2020, 12 (6), 7351– 7357,  DOI: 10.1021/acsami.9b21671
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    17
    Defect Engineering of MoS2 for Room-Temperature Terahertz Photodetection
    Xie, Ying; Liang, Fei; Chi, Shumeng; Wang, Dong; Zhong, Kai; Yu, Haohai; Zhang, Huaijin; Chen, Yanxue; Wang, Jiyang
    ACS Applied Materials & Interfaces (2020), 12 (6), 7351-7357CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Two-dimensional (2D) materials have exotic intrinsic electronic band structures and are considered as revolutionary foundations for novel nanodevices. Band engineering of 2D materials may pave a new avenue to overcome numerous challenges in modern technologies, such as room temp. (RT) photodetection of light with photon energy below their band gaps. Here, we reported the pioneering RT MoS2-based photodetection in the terahertz (THz) region via introducing Mo4+ and S2- vacancies for rational band gap engineering. Both the generation and transport of extra carriers, driven by THz electromagnetic radiations, were regulated by the vacancy concn. as well as the resistivity of MoS2 samples. Utilizing the balance between the carrier concn. fluctuation and carrier-scattering probability, a high RT photoresponsivity of 10 mA/W at 2.52 THz was realized in an Mo-vacancy-rich MoS2.19 sample. This work overcomes the challenge in the excessive dark current of RT THz detection and offers a convenient way for further optoelectronic and photonic devices based on band gap-engineered 2D materials.
  18. 18
    Saha, D.; Kruse, P. Editors’ Choice─Review─Conductive Forms of MoS2 and Their Applications in Energy Storage and Conversion. J. Electrochem. Soc. 2020, 167 (12), 126517,  DOI: 10.1149/1945-7111/abb34b
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    18
    Editors' choice-review-conductive forms of MoS2 and their applications in energy storage and conversion
    Saha, Dipankar; Kruse, Peter
    Journal of the Electrochemical Society (2020), 167 (12), 126517CODEN: JESOAN; ISSN:1945-7111. (IOP Publishing Ltd.)
    A review. Conductive forms of MoS2 are important emerging 2D materials due to their unique combination of properties such as high elec. cond., availability of active sites in edge and basal planes for catalytic activity and expanded interlayer distances. Consequently, there has been a drive to find synthetic routes toward conductive forms of MoS2. Naturally occurring or synthetically grown semiconducting 2H-MoS2can either be converted into metallic 1T-MoS2, or various dopants may be introduced to modulate the electronic band gap of the 2H-MoS2phase and increase its cond. Chem. and electrochem. intercalation methods, hydrothermal and solvothermal processes, and chem. vapor deposition have all been developed to synthesize conductive MoS2. Conductive MoS2 finds applications in energy storage devices, electrocatalytic reactions, and sensors. Here, we summarize a detailed understanding of the at. structure and electronic properties of conductive MoS2 which is crucial for its applications. We also discuss various fabrication methods that have been previously reported along with their advantages and disadvantages. Finally, we will give an overview of current trends in different applications in energy storage and electrocatalytic reactions in order to help researchers to further explore the applications of conductive MoS2.
  19. 19
    Kc, S.; Longo, R. C.; Addou, R.; Wallace, R. M.; Cho, K. Impact of Intrinsic Atomic Defects on the Electronic Structure of MoS2 Monolayers. Nanotechnology 2014, 25 (37), 375703,  DOI: 10.1088/0957-4484/25/37/375703
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    There is no corresponding record for this reference.
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    McDonnell, S.; Addou, R.; Buie, C.; Wallace, R. M.; Hinkle, C. L. Defect-Dominated Doping and Contact Resistance in MoS2. ACS Nano 2014, 8 (3), 2880– 2888,  DOI: 10.1021/nn500044q
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    Defect-Dominated Doping and Contact Resistance in MoS2
    McDonnell, Stephen; Addou, Rafik; Buie, Creighton; Wallace, Robert M.; Hinkle, Christopher L.
    ACS Nano (2014), 8 (3), 2880-2888CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Achieving low resistance contacts is vital for the realization of nanoelectronic devices based on transition metal dichalcogenides. Intrinsic defects in MoS2 dominate the metal/MoS2 contact resistance and provide a low Schottky barrier independent of metal contact work function. Also, MoS2 can exhibit both n-type and p-type conduction at different points on a same sample. The authors identify these regions independently by complementary characterization techniques and show how the Fermi level can shift by 1 eV over tens of nanometers in spatial resoln. These variations in doping are defect-chem.-related and are independent of contact metal. This raises questions on previous reports of metal-induced doping of MoS2 since the same metal in contact with MoS2 can exhibit both n- and p-type behavior. These results may provide a potential route for achieving low electron and hole Schottky barrier contacts with a single metal deposition.
  21. 21
    Sim, D. M.; Kim, M.; Yim, S.; Choi, M.-J.; Choi, J.; Yoo, S.; Jung, Y. S. Controlled Doping of Vacancy-Containing Few-Layer MoS2 Via Highly Stable Thiol-Based Molecular Chemisorption. ACS Nano 2015, 9 (12), 12115– 12123,  DOI: 10.1021/acsnano.5b05173
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    21
    Controlled Doping of Vacancy-Containing Few-Layer MoS2 via Highly Stable Thiol-Based Molecular Chemisorption
    Sim, Dong Min; Kim, Mincheol; Yim, Soonmin; Choi, Min-Jae; Choi, Jaesuk; Yoo, Seunghyup; Jung, Yeon Sik
    ACS Nano (2015), 9 (12), 12115-12123CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    MoS2 is considered a promising two-dimensional active channel material for future nanoelectronics. However, the development of a facile, reliable, and controllable doping methodol. is still crit. for extending the applicability of MoS2. Here, the authors report surface charge transfer doping via thiol-based binding chem. for modulating the elec. properties of vacancy-contg. MoS2 (v-MoS2). Although vacancies present in 2-dimensional materials are generally regarded as undesirable components, the elec. properties of MoS2 can be systematically engineered by exploiting the tight binding between the thiol group and sulfur vacancies and by choosing different functional groups. For example, NH2-contg. thiol mols. with lone electron pairs can serve as an n-dopant and achieve a substantial increase of electron d. (Δn = 3.7 × 1012 cm-2). However, fluorine-rich mols. can provide a p-doping effect (Δn = -7.0 × 1011 cm-2) due to its high electronegativity. Also, the n- and p-doping effects were systematically evaluated by photoluminescence (PL), XPS, and elec. measurement results. The excellent binding stability of thiol mols. and recovery properties by thermal annealing will enable broader applicability of ultrathin MoS2 to various devices.
  22. 22
    Förster, A.; Gemming, S.; Seifert, G.; Tománek, D. Chemical and Electronic Repair Mechanism of Defects in MoS2 Monolayers. ACS Nano 2017, 11 (10), 9989– 9996,  DOI: 10.1021/acsnano.7b04162
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    22
    Chemical and electronic repair mechanism of defects in MoS2 monolayers
    Forster, Anja; Gemming, Sibylle; Seifert, Gotthard; Tomanek, David
    ACS Nano (2017), 11 (10), 9989-9996CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Using ab initio d. functional theory calcns., we characterize changes in the electronic structure of MoS2 monolayers introduced by missing or addnl. adsorbed sulfur atoms. We furthermore identify the chem. and electronic function of substances that have been reported to reduce the adverse effect of sulfur vacancies in quenching photoluminescence and reducing electronic conductance. We find that thiol-group-contg. mols. adsorbed at vacancy sites may reinsert missing sulfur atoms. In the presence of addnl. adsorbed sulfur atoms, thiols may form disulfides on the MoS2 surface to mitigate the adverse effect of defects.
  23. 23
    Kc, S.; Longo, R. C.; Wallace, R. M.; Cho, K. Surface Oxidation Energetics and Kinetics on Mos2 Monolayer. J. Appl. Phys. 2015, 117 (13), 135301,  DOI: 10.1063/1.4916536
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    There is no corresponding record for this reference.
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    Lu, H.; Kummel, A.; Robertson, J. Passivating the Sulfur Vacancy in Monolayer Mos2. APL Mater. 2018, 6 (6), 066104,  DOI: 10.1063/1.5030737
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    24
    Passivating the sulfur vacancy in monolayer MoS2
    Lu, Haichang; Kummel, Andrew; Robertson, John
    APL Materials (2018), 6 (6), 066104/1-066104/9CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
    Various methods to passivate the sulfur vacancy in 2D MoS2 are modeled using d. functional theory (DFT) to understand the passivation mechanism at an at. scale. First, the org. super acid, bis(trifluoromethane)sulfonimide (TFSI) is a strong protonating agent, and it is exptl. found to greatly increase the photoluminescence efficiency. DFT simulations find that the effectiveness of passivation depends critically on the charge state and no. of hydrogens donated by TFSI since this dets. the symmetry of the defect complex. A sym. complex is formed by three hydrogen atoms bonding to the defect in a -1 charge state, and this gives no bandgap states and a Fermi level in the midgap. However, a charge state of +1 gives a lower symmetry complex with one state in the gap. One or two hydrogens also give complexes with gap states. Second, passivation by O2 can provide partial passivation by forming a bridge bond across the S vacancy, but it leaves a defect state in the lower bandgap. On the other hand, substitutional addns. do not shift the vacancy states out of the gap. (c) 2018 American Institute of Physics.
  25. 25
    Nan, H.; Wang, Z.; Wang, W.; Liang, Z.; Lu, Y.; Chen, Q.; He, D.; Tan, P.; Miao, F.; Wang, X.; Wang, J.; Ni, Z. Strong Photoluminescence Enhancement of MoS2 through Defect Engineering and Oxygen Bonding. ACS Nano 2014, 8 (6), 5738– 5745,  DOI: 10.1021/nn500532f
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    25
    Strong Photoluminescence Enhancement of MoS2 through Defect Engineering and Oxygen Bonding
    Nan, Haiyan; Wang, Zilu; Wang, Wenhui; Liang, Zheng; Lu, Yan; Chen, Qian; He, Daowei; Tan, Pingheng; Miao, Feng; Wang, Xinran; Wang, Jinlan; Ni, Zhenhua
    ACS Nano (2014), 8 (6), 5738-5745CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    We report on a strong photoluminescence (PL) enhancement of monolayer MoS2 through defect engineering and oxygen bonding. Micro-PL and Raman images clearly reveal that the PL enhancement occurs at cracks/defects formed during high-temp. annealing. The PL enhancement at crack/defect sites could be as high as thousands of times after considering the laser spot size. The main reasons of such huge PL enhancement include the following: (1) the oxygen chem. adsorption induced heavy p doping and the conversion from trion to exciton; (2) the suppression of nonradiative recombination of excitons at defect sites, which was verified by low-temp. PL measurements. First-principle calcns. reveal a strong binding energy of ∼2.395 eV for an oxygen mol. adsorbed on a S vacancy of MoS2. The chem. adsorbed oxygen also provides a much more effective charge transfer (0.997 electrons per O2) compared to phys. adsorbed oxygen on an ideal MoS2 surface. We also demonstrate that the defect engineering and oxygen bonding could be easily realized by mild oxygen plasma irradn. XPS further confirms the formation of Mo-O bonding. Our results provide a new route for modulating the optical properties of two-dimensional semiconductors. The strong and stable PL from defects sites of MoS2 may have promising applications in optoelectronic devices.
  26. 26
    Verhagen, T.; Guerra, V. L. P.; Haider, G.; Kalbac, M.; Vejpravova, J. Towards the Evaluation of Defects in MoS2 Using Cryogenic Photoluminescence Spectroscopy. Nanoscale 2020, 12 (5), 3019– 3028,  DOI: 10.1039/C9NR07246B
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    26
    Towards the evaluation of defects in MoS2 using cryogenic photoluminescence spectroscopy
    Verhagen, Tim; Guerra, Valentino L. P.; Haider, Golam; Kalbac, Martin; Vejpravova, Jana
    Nanoscale (2020), 12 (5), 3019-3028CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Characterization of the type and d. of defects in two-dimensional (2D) transition metal dichalcogenides (TMDs) is important as the nature of these defects strongly influences the electronic and optical properties of the material, esp. its photoluminescence (PL). Defect characterization is not as straightforward as it is for graphene films, where the D and D' Raman scattering modes easily indicate the d. and type of defects in the graphene layer. Thus, in addn. to the Raman scattering anal., other spectroscopic techniques are necessary to perform detailed characterization of atomically thin TMD layers. We demonstrate that PL spectroscopy performed at liq. helium temps. reveals the key fingerprints of defects in TMDs and hence provides valuable information about their origin and concn. In our study, we address defects in chem. vapor deposition (CVD)-grown MoS2 monolayers. A significant difference is obsd. between the as-grown monolayers compared with the CVD-grown monolayers transferred onto a Si/SiO2 substrate, which contain extra defects due to the transfer process. We demonstrate that the temp.-dependent Raman and PL micro-spectroscopy techniques enable disentangling the contributions and locations of various defect types in TMD systems.
  27. 27
    Mohtasebi, A.; Broomfield, A. D.; Chowdhury, T.; Selvaganapathy, P. R.; Kruse, P. Reagent-Free Quantification of Aqueous Free Chlorine Via Electrical Readout of Colorimetrically Functionalized Pencil Lines. ACS Appl. Mater. Interfaces 2017, 9 (24), 20748– 20761,  DOI: 10.1021/acsami.7b03968
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    Reagent-Free Quantification of Aqueous Free Chlorine via Electrical Readout of Colorimetrically Functionalized Pencil Lines
    Mohtasebi, Amirmasoud; Broomfield, Andrew D.; Chowdhury, Tanzina; Selvaganapathy, P. Ravi; Kruse, Peter
    ACS Applied Materials & Interfaces (2017), 9 (24), 20748-20761CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Colorimetric methods are commonly used to quantify free chlorine in drinking water. However, these methods are not suitable for reagent-free, continuous, and autonomous applications. Here, we demonstrate how functionalization of a pencil-drawn film with phenyl-capped aniline tetramer (PCAT) can be used for quant. elec. readout of free chlorine concns. The functionalized film can be implemented in a simple fluidic device for continuous sensing of aq. free chlorine concns. The sensor is selective to free chlorine and can undergo a reagent-free reset for further measurements. Our sensor is superior to electrochem. methods in that it does not require a ref. electrode. It is capable of quantification of free chlorine in the range of 0.1-12 ppm with higher precision than colorimetric (absorptivity) methods. The interactions of PCAT with the pencil-drawn film upon exposure to hypochlorite were characterized spectroscopically. A previously reported detection mechanism relied on the measurement of a baseline shift to quantify free chlorine concns. The new method demonstrated here measures initial spike size upon exposure to free chlorine. It relies on a fast charge built up on the sensor film due to intermittent PCAT salt formation. It has the advantage of being significantly faster than the measurement of baseline shift, but it cannot be used to detect gradual changes in free chlorine concn. without the use of frequent reset pulses. The stability of PCAT was examd. in the presence of free chlorine as a function of pH. While most ions commonly present in drinking water do not interfere with the free chlorine detection, other oxidants may contribute to the signal. Our sensor is easy to fabricate and robust, operates reagent-free, and has very low power requirements and is thus suitable for remote deployment.
  28. 28
    Hoque, E.; Hsu, L. H. H.; Aryasomayajula, A.; Selvaganapathy, P. R.; Kruse, P. Pencil-Drawn Chemiresistive Sensor for Free Chlorine in Water. IEEE Sensors Letters 2017, 1 (4), 1– 4,  DOI: 10.1109/LSENS.2017.2722958
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  29. 29
    Dalmieda, J.; Zubiarrain-Laserna, A.; Ganepola, D.; Selvaganapathy, P. R.; Kruse, P. Chemiresistive Detection of Silver Ions in Aqueous Media. Sens. Actuators, B 2021, 328, 129023,  DOI: 10.1016/j.snb.2020.129023
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    Chemiresistive detection of silver ions in aqueous media
    Dalmieda, Johnson; Zubiarrain-Laserna, Ana; Ganepola, Devanjith; Selvaganapathy, P. Ravi; Kruse, Peter
    Sensors and Actuators, B: Chemical (2021), 328 (), 129023CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
    Silver is used as a water disinfectant in hospital settings as well as in purifiers for potable water. Although there are no strict regulations on the concn. of silver in water, adverse effects such as argyria and respiratory tract irritation have been correlated to excess silver consumption. Based on this, the levels of silver in water are recommended to be maintained below 100 ppb to ensure safety for human consumption. In this work, we present a silver sensor for use in aq. media that utilizes bathocuproine, a silver selective chromophore, adsorbed onto few-layer graphene (FLG) flake networks for the chemiresistive detection of silver. Complexation of silver to bathocuproine modulates the cond. of the FLG film, which can be probed by applying a small voltage bias. The decrease in resistance of the film correlates with the concn. of silver in soln. between 3 ppb and 1 ppm. Exposing the sensor to a lower pH resets the sensor, allowing it to be reused and reset multiple times. This sensor demonstrates a new pathway to chemiresistive cation sensing using known selective complexing agents adsorbed onto graphitic thin films. This concept can be expanded to the detection of other relevant analytes in domestic, industrial and environmental water sources.
  30. 30
    Gou, P.; Kraut, N. D.; Feigel, I. M.; Bai, H.; Morgan, G. J.; Chen, Y.; Tang, Y.; Bocan, K.; Stachel, J.; Berger, L.; Mickle, M.; Sejdić, E.; Star, A. Carbon Nanotube Chemiresistor for Wireless pH Sensing. Sci. Rep. 2015, 4 (1), 4468,  DOI: 10.1038/srep04468
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    Zhou, K.-G.; Mao, N.-N.; Wang, H.-X.; Peng, Y.; Zhang, H.-L. A Mixed-Solvent Strategy for Efficient Exfoliation of Inorganic Graphene Analogues. Angew. Chem., Int. Ed. 2011, 50 (46), 10839– 10842,  DOI: 10.1002/anie.201105364
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    31
    A Mixed-Solvent Strategy for Efficient Exfoliation of Inorganic Graphene Analogues
    Zhou, Kai-Ge; Mao, Nan-Nan; Wang, Hang-Xing; Peng, Yong; Zhang, Hao-Li
    Angewandte Chemie, International Edition (2011), 50 (46), 10839-10842, S10839/1-S10839/6CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The authors demonstrate a versatile and scaleable mixed-solvent strategy for liq. exfoliation of inorg. graphene analogs IGAs such as WS2, MoS2, and BN, in volatile solvents. By choosing solvents with appropriate compn., highly stable IGA suspensions can be obtained in low-boiling solvent mixts., which can then be easily used in further applications. The dispersion of nanomaterials in liqs. can be partially predicted by the theory of Hansen soly. parameters (HSP) which is a semiempirical correlation developed to explain dissoln. behavior. Three HSP parameters are used to describe the character of a solvent or material. The dissoln. process is one of adaptation between the HSP parameters of solvents and solute.
  32. 32
    Su, W.; Dou, H.; Li, J.; Huo, D.; Dai, N.; Yang, L. Tuning Photoluminescence of Single-Layer MoS2 Using H2O2. RSC Adv. 2015, 5 (101), 82924– 82929,  DOI: 10.1039/C5RA12450F
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    Tuning photoluminescence of single-layer MoS2 using H2O2
    Su, Weitao; Dou, Honglei; Li, Jinwei; Huo, Dexuan; Dai, Ning; Yang, Li
    RSC Advances (2015), 5 (101), 82924-82929CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    Enhancing photoluminescence (PL) of single-layer (1L) MoS2 is crit. to its application as the thinnest light-emitting material. In this report, we show that the PL intensity of 1L-MoS2 can be enhanced by 8 times using physisorption of H2O2 mols. as p-type dopants. By using toluene to form the sandwiched structure of H2O2/1L-MoS2/toluene, the PL intensity of 1L-MoS2 can be enhanced up to 27.4 times. Our research proposes a simple but effective method to enhance the light emitting properties of 1L-MoS2.
  33. 33
    Dong, L.; Lin, S.; Yang, L.; Zhang, J.; Yang, C.; Yang, D.; Lu, H. Spontaneous Exfoliation and Tailoring of MoS2 in Mixed Solvents. Chem. Commun. 2014, 50 (100), 15936– 15939,  DOI: 10.1039/C4CC07238C
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    Spontaneous exfoliation and tailoring of MoS2 in mixed solvents
    Dong, Lei; Lin, Shan; Yang, Liu; Zhang, Jiajia; Yang, Chao; Yang, Dong; Lu, Hongbin
    Chemical Communications (Cambridge, United Kingdom) (2014), 50 (100), 15936-15939CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    Spontaneous exfoliation of MoS2 is achieved in H2O2-NMP mixed solvents with a yield of over 60 wt%, operated under mild conditions. H2O2-prompted sheet-tailoring effect induces a size evolution of MoS2 nanosheets from micro-scale to nano-scale. Furthermore, the concurrent dissoln. also affords an approach to introduce structural defects in the nanosheets.
  34. 34
    Pradhan, N. R.; Rhodes, D.; Zhang, Q.; Talapatra, S.; Terrones, M.; Ajayan, P. M.; Balicas, L. Intrinsic Carrier Mobility of Multi-Layered Mos2 Field-Effect Transistors on SiO2. Appl. Phys. Lett. 2013, 102 (12), 123105,  DOI: 10.1063/1.4799172
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    Intrinsic carrier mobility of multi-layered MoS2 field-effect transistors on SiO2
    Pradhan, N. R.; Rhodes, D.; Zhang, Q.; Talapatra, S.; Terrones, M.; Ajayan, P. M.; Balicas, L.
    Applied Physics Letters (2013), 102 (12), 123105/1-123105/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    By fabricating and characterizing multilayered MoS2-based field-effect transistors in a 4 terminal configuration, we demonstrate that the 2 terminal-configurations tend to underestimate the carrier mobility μ due to the Schottky barriers at the contacts. For a back-gated 2-terminal configuration, we observe mobilities as high as 91 cm2 V-1s-1 which is considerably smaller than 306.5 cm2 V-1s-1 as extd. from the same device when using a 4-terminal configuration. The intrinsic mobility of MoS2 on SiO2 is significantly larger than the values previously reported, and provides a quant. method to evaluate the charge transport through the contacts. (c) 2013 American Institute of Physics.
  35. 35
    Laskar, M. R.; Nath, D. N.; Ma, L.; Lee, E. W.; Lee, C. H.; Kent, T.; Yang, Z.; Mishra, R.; Roldan, M. A.; Idrobo, J.-C.; Pantelides, S. T.; Pennycook, S. J.; Myers, R. C.; Wu, Y.; Rajan, S. P-Type Doping of MoS2 Thin Films Using Nb. Appl. Phys. Lett. 2014, 104 (9), 092104,  DOI: 10.1063/1.4867197
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    p-type doping of MoS2 thin films using Nb
    Laskar, Masihhur R.; Nath, Digbijoy N.; Ma, Lu; Lee, Edwin W.; Lee, Choong Hee; Kent, Thomas; Yang, Zihao; Mishra, Rohan; Roldan, Manuel A.; Idrobo, Juan-Carlos; Pantelides, Sokrates T.; Pennycook, Stephen J.; Myers, Roberto C.; Wu, Yiying; Rajan, Siddharth
    Applied Physics Letters (2014), 104 (9), 092104/1-092104/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Reported on the first demonstration of p-type doping in large area few-layer films of (0001)-oriented chem. vapor deposited MoS2. Nb was found to act as an efficient acceptor up to relatively high d. in MoS2 films. For a hole d. of 3.1 × 1020 cm-3, Hall mobility of 8.5 cm2 V-1 s-1 was detd., which matches well with the theor. expected values. X-ray diffraction scans and Raman characterization indicated that the film had good out-of-plane cryst. quality. Absorption measurements showed that the doped sample had similar characteristics to high-quality undoped samples, with a clear absorption edge at 1.8 eV. Scanning transmission electron microscope imaging showed ordered cryst. nature of the Nb-doped MoS2 layers stacked in the [0001] direction. This demonstration of substitutional p-doping in large area epitaxial MoS2 could help in realizing a wide variety of elec. and opto-electronic devices based on layered metal dichalcogenides. (c) 2014 American Institute of Physics.
  36. 36
    Werner, F. Hall Measurements on Low-Mobility Thin Films. J. Appl. Phys. 2017, 122 (13), 135306,  DOI: 10.1063/1.4990470
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    Hall measurements on low-mobility thin films
    Werner, Florian
    Journal of Applied Physics (Melville, NY, United States) (2017), 122 (13), 135306/1-135306/13CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
    We review the conventional measuring std. for dc Hall measurements in van der Pauw configuration with particular focus on the challenges arising from a small Hall signal compared to sizable offset voltages, which is a typical scenario for many material systems, particularly low-mobility thin films. We show that the conventional approach of using a simple field-reversal technique is often unsuited to obtain reliable results, and present an improved correction scheme to extend the accessible measurement range to mobility values well below 1 cm2/(V s). We discuss procedures to limit the impact of temp. fluctuations and long stabilization times for highly resistive materials. We further address potential sources of error due to the presence of grain boundaries in polycryst. specimen and due to multi-carrier conduction, both of which might yield low apparent Hall mobilities significantly underestimating the actual mobility. (c) 2017 American Institute of Physics.
  37. 37
    Rai, A.; Movva, H. C. P.; Roy, A.; Taneja, D.; Chowdhury, S.; Banerjee, S. K. Progress in Contact, Doping and Mobility Engineering of MoS2: An Atomically Thin 2D Semiconductor. Crystals 2018, 8 (8), 316,  DOI: 10.3390/cryst8080316
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    Progress in contact, doping and mobility engineering of MoS2: an atomically thin 2D semiconductor
    Rai, Amritesh; Movva, Hema C. P.; Roy, Anupam; Taneja, Deepyanti; Chowdhury, Sayema; Banerjee, Sanjay K.
    Crystals (2018), 8 (8), 316/1-316/84CODEN: CRYSBC; ISSN:2073-4352. (MDPI AG)
    A review. Atomically thin molybdenum disulfide (MoS2), a member of the transition metal dichalcogenide (TMDC) family, has emerged as the prototypical two-dimensional (2D) semiconductor with a multitude of interesting properties and promising device applications spanning all realms of electronics and optoelectronics. While possessing inherent advantages over conventional bulk semiconducting materials (such as Si, Ge and III-Vs) in terms of enabling ultra-short channel and, thus, energy efficient field-effect transistors (FETs), the mech. flexible and transparent nature of MoS2 makes it even more attractive for use in ubiquitous flexible and transparent electronic systems. However, before the fascinating properties of MoS2 can be effectively harnessed and put to good use in practical and com. applications, several important technol. roadblocks pertaining to its contact, doping and mobility (μ) engineering must be overcome. This paper reviews the important technol. relevant properties of semiconducting 2D TMDCs followed by a discussion of the performance projections of, and the major engineering challenges that confront, 2D MoS2-based devices. Finally, this review provides a comprehensive overview of the various engineering solns. employed, thus far, to address the all-important issues of contact resistance (RC), controllable and area-selective doping, and charge carrier mobility enhancement in these devices. Several key exptl. and theor. results are cited to supplement the discussions and provide further insight.
  38. 38
    Acerce, M.; Voiry, D.; Chhowalla, M. Metallic 1T Phase MoS2 Nanosheets as Supercapacitor Electrode Materials. Nat. Nanotechnol. 2015, 10 (4), 313– 318,  DOI: 10.1038/nnano.2015.40
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    Metallic 1T phase MoS2 nanosheets as supercapacitor electrode materials
    Acerce, Muharrem; Voiry, Damien; Chhowalla, Manish
    Nature Nanotechnology (2015), 10 (4), 313-318CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    Efficient intercalation of ions in layered materials forms the basis of electrochem. energy storage devices such as batteries and capacitors. Recent research has focused on the exfoliation of layered materials and then restacking the two-dimensional exfoliated nanosheets to form electrodes with enhanced electrochem. response. Here, it is shown that chem. exfoliated nanosheets of MoS2 contg. a high concn. of the metallic 1T phase can electrochem. intercalate ions such as H+, Li+, Na+, and K+ with extraordinary efficiency and achieve capacitance values ranging from ∼400 to ∼700 F cm-3 in a variety of aq. electrolytes. It is also demonstrated that this material is suitable for high-voltage (3.5 V) operation in non-aq. org. electrolytes, showing prime volumetric energy and power d. values, coulombic efficiencies in excess of 95%, and stability over 5,000 cycles. As it is shown by X-ray diffraction anal., these favorable electrochem. properties of 1T MoS2 layers are mainly a result of their hydrophilicity and high elec. cond., as well as the ability of the exfoliated layers to dynamically expand and intercalate the various ions.
  39. 39
    Attanayake, N. H.; Thenuwara, A. C.; Patra, A.; Aulin, Y. V.; Tran, T. M.; Chakraborty, H.; Borguet, E.; Klein, M. L.; Perdew, J. P.; Strongin, D. R. Effect of Intercalated Metals on the Electrocatalytic Activity of 1T-MoS2 for the Hydrogen Evolution Reaction. ACS Energy Letters 2018, 3 (1), 7– 13,  DOI: 10.1021/acsenergylett.7b00865
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    Effect of Intercalated Metals on the Electrocatalytic Activity of 1T-MoS2 for the Hydrogen Evolution Reaction
    Attanayake, Nuwan H.; Thenuwara, Akila C.; Patra, Abhirup; Aulin, Yaroslav V.; Tran, Thi M.; Chakraborty, Himanshu; Borguet, Eric; Klein, Michael L.; Perdew, John P.; Strongin, Daniel R.
    ACS Energy Letters (2018), 3 (1), 7-13CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    We show that intercalation of cations (Na+, Ca2+, Ni2+, and Co2+) into the interlayer region of 1T-MoS2 is an effective strategy to lower the overpotential for the hydrogen evolution reaction (HER). In acidic media the onset potential for 1T-MoS2 with intercalated ions is lowered by ∼60 mV relative to that for pristine 1T-MoS2 (onset of ∼180 mV). D. functional theory (DFT) calcns. show a lowering in the Gibbs free energy for H-adsorption (ΔGH) on these intercalated structures relative to intercalant-free 1T-MoS2. The DFT calcns. suggest that Na+ intercalation results in a ΔGH close to zero. Consistent with calcn., expts. show that the intercalation of Na+ ions into the interlayer region of 1T-MoS2 results in the lowest overpotential for the HER.
  40. 40
    Yin, Y.; Han, J.; Zhang, Y.; Zhang, X.; Xu, P.; Yuan, Q.; Samad, L.; Wang, X.; Wang, Y.; Zhang, Z.; Zhang, P.; Cao, X.; Song, B.; Jin, S. Contributions of Phase, Sulfur Vacancies, and Edges to the Hydrogen Evolution Reaction Catalytic Activity of Porous Molybdenum Disulfide Nanosheets. J. Am. Chem. Soc. 2016, 138 (25), 7965– 7972,  DOI: 10.1021/jacs.6b03714
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    40
    Contributions of Phase, Sulfur Vacancies, and Edges to the Hydrogen Evolution Reaction Catalytic Activity of Porous Molybdenum Disulfide Nanosheets
    Yin, Ying; Han, Jiecai; Zhang, Yumin; Zhang, Xinghong; Xu, Ping; Yuan, Quan; Samad, Leith; Wang, Xianjie; Wang, Yi; Zhang, Zhihua; Zhang, Peng; Cao, Xingzhong; Song, Bo; Jin, Song
    Journal of the American Chemical Society (2016), 138 (25), 7965-7972CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Molybdenum disulfide (MoS2) is a promising nonprecious catalyst for the hydrogen evolution reaction (HER) that has been extensively studied due to its excellent performance, but the lack of understanding of the factors that impact its catalytic activity hinders further design and enhancement of MoS2-based electrocatalysts. Here, by using novel porous (holey) metallic 1T phase MoS2 nanosheets synthesized by a liq.-ammonia-assisted lithiation route, we systematically investigated the contributions of crystal structure (phase), edges, and sulfur vacancies (S-vacancies) to the catalytic activity toward HER from five representative MoS2 nanosheet samples, including 2H and 1T phase, porous 2H and 1T phase, and sulfur-compensated porous 2H phase. Superior HER catalytic activity was achieved in the porous 1T phase MoS2 nanosheets that have even more edges and S-vacancies than conventional 1T phase MoS2. A comparative study revealed that the phase serves as the key role in detg. the HER performance, as 1T phase MoS2 always outperforms the corresponding 2H phase MoS2 samples, and that both edges and S-vacancies also contribute significantly to the catalytic activity in porous MoS2 samples. Then, using combined defect characterization techniques of ESR spectroscopy and positron annihilation lifetime spectroscopy to quantify the S-vacancies, the contributions of each factor were individually elucidated. This study presents new insights and opens up new avenues for designing electrocatalysts based on MoS2 or other layered materials with enhanced HER performance.
  41. 41
    Scanlon, D. O.; Watson, G. W.; Payne, D. J.; Atkinson, G. R.; Egdell, R. G.; Law, D. S. L. Theoretical and Experimental Study of the Electronic Structures of MoO3 and MoO2. J. Phys. Chem. C 2010, 114 (10), 4636– 4645,  DOI: 10.1021/jp9093172
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    Theoretical and Experimental Study of the Electronic Structures of MoO3 and MoO2
    Scanlon, David O.; Watson, Graeme W.; Payne, D. J.; Atkinson, G. R.; Egdell, R. G.; Law, D. S. L.
    Journal of Physical Chemistry C (2010), 114 (10), 4636-4645CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The geometric and electronic structures of MoO3 and MoO2 were calcd. using the generalized gradient approxn. to d. functional theory. The calcd. cross-section weighted densities of states were compared with high-resoln. X-ray photoemission spectra. There is very good agreement between the calcd. structures and those detd. previously by X-ray diffraction and between the computed densities of states and the present photoemission measurements. MoO2 is shown to be a metallic material, as is found exptl., but the Fermi level sits in a distinct trough in the d. of states. Satellite peaks found in core photoemission spectra of MoO2 were shown to derive from final state screening effects in this narrow band metallic material.
  42. 42
    Afanasiev, P.; Lorentz, C. Oxidation of Nanodispersed Mos2 in Ambient Air: The Products and the Mechanistic Steps. J. Phys. Chem. C 2019, 123 (12), 7486– 7494,  DOI: 10.1021/acs.jpcc.9b01682
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    Oxidation of Nanodispersed MoS2 in Ambient Air: The Products and the Mechanistic Steps
    Afanasiev, Pavel; Lorentz, Chantal
    Journal of Physical Chemistry C (2019), 123 (12), 7486-7494CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Oxidn. of finely divided MoS2 in ambient air was studied for air exposure times from 10 min to 1 yr to clarify the nature of the reaction products and the mechanistic steps. At the initial steps, for air exposure times from several minutes to several hours, rapid oxidn. of MoS2 edges occurs with simultaneous formation of hydroxyl species and surface disulfide S22- moieties as attested by 1H NMR, x-ray photoelectron spectra, and temp.-programmed redn. Prolonged air exposure of MoS2 nanodispersions leads to deep oxidn. According to the results of x-ray absorption spectroscopy, UV-visible, and ESR spectroscopies, the main oxidn. products are sol. paramagnetic Mo blue species and H2SO4. As shown by EXAS fitting, the major product is oxo-bridged dimolybdenyl Mo(V, VI) species. Ambient moisture plays an important role in the oxidn. process as it contributes to the formation of H2SO4 which leads to liquescence of the material and to deep oxidn. without formation of a protective passivation layer.
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    Ziembowicz, S.; Kida, M.; Koszelnik, P. Sonochemical Formation of Hydrogen Peroxide. Proceedings 2018, 2 (5), 188,  DOI: 10.3390/ecws-2-04957
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    Riesz, P.; Kondo, T. Free Radical Formation Induced by Ultrasound and Its Biological Implications. Free Radical Biol. Med. 1992, 13 (3), 247– 270,  DOI: 10.1016/0891-5849(92)90021-8
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    Free radical formation induced by ultrasound and its biological implications
    Riesz, Peter; Kondo, Takashi
    Free Radical Biology & Medicine (1992), 13 (3), 247-70CODEN: FRBMEH; ISSN:0891-5849.
    A review with 197 refs. The chem. effects of ultrasound in aq. solns. are due to acoustic cavitation, which refers to the formation, growth, and collapse of small gas bubbles in liqs. The very high temps. (several thousand K) and pressures (several hundred atmospheres) of collapsing gas bubbles lead to the thermal dissocn. of water vapor into •OH radicals and •H atoms. Their formation has been confirmed by ESR and spin trapping. The sonochem. of aq. solns. of gases and of volatile and nonvolatile solutes is reviewed. The similarities and differences between sonochem. and radiation chem. of aq. solns. are explained. Some unusual characteristics of aq. sonochem. can be understood by considering the properties of supercrit. water. By the use of rare gases with different thermal conductivities, it is possible to distinguish between temp.-dependent processes such as redox reactions initiated by •OH radicals and •H atoms and pressure-dependent processes which lead to polymer degrdn. and cell lysis. The evidence for free radical formation in aq. solns. by pulsed ultrasound is discussed. This subject is of interest because it is related to the possible deleterious effects of ultrasonic diagnostic devices. The role of free radicals and of mech. effects induced by ultrasound in DNA degrdn., inactivation of enzymes, lipid peroxidn., and cell killing is reviewed.
  45. 45
    Hu, X. K.; Qian, Y. T.; Song, Z. T.; Huang, J. R.; Cao, R.; Xiao, J. Q. Comparative Study on MoO3 and HxMoO3 Nanobelts: Structure and Electric Transport. Chem. Mater. 2008, 20 (4), 1527– 1533,  DOI: 10.1021/cm702942y
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    Comparative Study on MoO3 and HxMoO3 Nanobelts: Structure and Electric Transport
    Hu, Xiao Kai; Qian, Yi Tai; Song, Z. T.; Huang, Jia Rui; Cao, R.; Xiao, John Q.
    Chemistry of Materials (2008), 20 (4), 1527-1533CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    In this study, the suspension of MoO3 nanobelts was first prepd. in a hydrothermal way from Mo powders and H2O2 soln., which could be transformed into the suspension of HxMoO3 nanobelts under an acidic condition using N2H4·H2O as the reducing agent. Three paper-form samples made from MoO3 and HxMoO3 nanobelts (low or high hydrogen content) were then fabricated via a vacuum filtration method, followed by their structural comparative anal. such as FESEM, XRD, Raman spectra, and XPS, etc. The measurement of elec. resistances at room temp. shows that the conductance of HxMoO3 nanobelts is greatly improved because of hydrogen doping. The temp.-dependent resistances of HxMoO3 nanobelts agree with the exponential correlation, supporting that the conducting carriers are the quasi-free electrons released from Mo5+. In addn., the formation process of HxMoO3 nanobelts from MoO3 nanobelts is also discussed.
  46. 46
    Ou, J. Z.; Campbell, J. L.; Yao, D.; Wlodarski, W.; Kalantar-Zadeh, K. In Situ Raman Spectroscopy of H2 Gas Interaction with Layered MoO3. J. Phys. Chem. C 2011, 115 (21), 10757– 10763,  DOI: 10.1021/jp202123a
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    In Situ Raman Spectroscopy of H2 Gas Interaction with Layered MoO3
    Ou, Jian Zhen; Campbell, Jos L.; Yao, David; Wlodarski, Wojtek; Kalantar-zadeh, Kourosh
    Journal of Physical Chemistry C (2011), 115 (21), 10757-10763CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    It is known that the unique layered structure of orthorhombic MoO3 (α-MoO3) facilitates the interaction with H2 gas mols. and that the surface-to-vol. ratios of the crystallites play an important role in the process. MoO3 was deposited on a wide variety of transparent substrates using thermal evapn. in order to alter the surface-to-vol. ratios of the crystallites. In situ Raman spectroscopy was employed to investigate the interaction between MoO3 and 1% H2 in both N2 and synthetic air environments, while incorporating Pd as a catalyst at room temp. This study confirmed that the layered MoO3 with a high surface-to-vol. ratio facilitated the H2 gas interaction. The Raman spectroscopy studies revealed that the H+ ions mainly interacted with the doubly coordinated oxygen atoms and caused the crystal transformation from the original α-MoO3 into the mixed structure of hydrogen molybdenum bronze and substoichiometric MoO3, eventually forming oxygen vacancies and water. It was also found that the presence of O2 during the H2 gas exposure caused the recombination of a no. of oxygen vacancies and reduced the available surface catalytic sites for H2.
  47. 47
    Balendhran, S.; Deng, J.; Ou, J. Z.; Walia, S.; Scott, J.; Tang, J.; Wang, K. L.; Field, M. R.; Russo, S.; Zhuiykov, S.; Strano, M. S.; Medhekar, N.; Sriram, S.; Bhaskaran, M.; Kalantar-Zadeh, K. Enhanced Charge Carrier Mobility in Two-Dimensional High Dielectric Molybdenum Oxide. Adv. Mater. 2013, 25 (1), 109– 114,  DOI: 10.1002/adma.201203346
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    Enhanced Charge Carrier Mobility in Two-Dimensional High Dielectric Molybdenum Oxide
    Balendhran, Sivacarendran; Deng, Junkai; Ou, Jian Zhen; Walia, Sumeet; Scott, James; Tang, Jianshi; Wang, Kang L.; Field, Matthew R.; Russo, Salvy; Zhuiykov, Serge; Strano, Michael S.; Medhekar, Nikhil; Sriram, Sharath; Bhaskaran, Madhu; Kalantar-zadeh, Kourosh
    Advanced Materials (Weinheim, Germany) (2013), 25 (1), 109-114CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The authors propose that 2D semiconducting metal oxides with high dielec. const. (high/c) offer a soln. for obtaining high electron mobility. Advantageously, the electronic properties, in particular the bandgap, of such 2D metal oxides can be largely manipulated using well-known chem. and phys. approaches. Such manipulations, which impose their effects on the 2D environment, categorize these materials as excellent templates for achieving the optimum quantum parameters required for target applications. The charge mobility in a thin layer is calcd.
  48. 48
    Zhang, H.; Wang, H.; Yang, Y.; Hu, C.; Bai, Y.; Zhang, T.; Chen, W.; Yang, S. Hx moo3-Y Nanobelts: An Excellent Alternative to Carbon Electrodes for High Performance Mesoscopic Perovskite Solar Cells. J. Mater. Chem. A 2019, 7 (4), 1499– 1508,  DOI: 10.1039/C8TA10892G
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    HxMoO3-y nanobelts: an excellent alternative to carbon electrodes for high performance mesoscopic perovskite solar cells
    Zhang, Hua; Wang, Huan; Yang, Yinglong; Hu, Chen; Bai, Yang; Zhang, Teng; Chen, Wei; Yang, Shihe
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2019), 7 (4), 1499-1508CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Carbon-based hole-conductor-free perovskite solar cells (C-PSCs) are considered as a promising photovoltaic technol. toward commercialization, owing to their low cost and superior stability. However, efforts to further improve their efficiency have been hampered by the hole-extn. barrier at the Schottky contact between carbon and the perovskite. A conventional approach to resolve this issue is incorporating a hole-transport material (HTM) into the mesoscopic skeleton or chem. modification of the carbon electrode. Here, we show an innovative strategy that uses soln.-processed hydrogen molybdenum bronze (HxMoO3-y) nanobelts, an n-type HTM with high work function and elec. cond., as the sole electrode material to enhance the hole-extn. process and realize efficient PSCs for the first time. The mesoscopic cell configuration of FTO/c-TiO2/m-TiO2/m-Al2O3/HxMoO3-y with perovskite infiltration delivered a champion power conversion efficiency (PCE) of 14.5%, which compares favorably with 13.3% of typical high temp. C-PSCs. This increase in cell efficiency stems primarily from the enhancement in open circuit voltage and short circuit current, which is due to the HxMoO3-y electrode with more favorable energy alignment and higher hole-extn. ability than the carbon electrode. These results show the potential of HxMoO3-y nanobelts as an efficient electrode for realizing high-performance mesoscopic PSCs.
  49. 49
    Borgschulte, A.; Sambalova, O.; Delmelle, R.; Jenatsch, S.; Hany, R.; Nüesch, F. Hydrogen Reduction of Molybdenum Oxide at Room Temperature. Sci. Rep. 2017, 7 (1), 40761,  DOI: 10.1038/srep40761
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    Hydrogen reduction of molybdenum oxide at room temperature
    Borgschulte, Andreas; Sambalova, Olga; Delmelle, Renaud; Jenatsch, Sandra; Hany, Roland; Nuesch, Frank
    Scientific Reports (2017), 7 (), 40761CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
    The color changes in chemo- and photochromic MoO3 used in sensors and in org. photovoltaic (OPV) cells can be traced back to intercalated hydrogen atoms stemming either from gaseous hydrogen dissocd. at catalytic surfaces or from photocatalytically split water. In applications, the reversibility of the process is of utmost importance, and deterioration of the layer functionality due to side reactions is a crit. challenge. Using the membrane approach for high-pressure XPS, we are able to follow the hydrogen redn. of MoO3 thin films using at. hydrogen in a water free environment. Hydrogen intercalates into MoO3 forming HxMoO3, which slowly decomps. into MoO2 +1/2 H2O as evidenced by the fast redn. of Mo6+ into Mo5+ states and slow but simultaneous formation of Mo4+ states. We measure the decrease in oxygen/metal ratio in the thin film explaining the limited reversibility of hydrogen sensors based on transition metal oxides. The results also enlighten the recent debate on the mechanism of the high temp. hydrogen redn. of bulk molybdenum oxide. The specific mechanism is a result of the balance between the redn. by hydrogen and water formation, desorption of water as well as nucleation and growth of new phases.
  50. 50
    Yang, L.; Zhou, W.; Hou, D.; Zhou, K.; Li, G.; Tang, Z.; Li, L.; Chen, S. Porous Metallic MoO2-Supported MoS2 Nanosheets for Enhanced Electrocatalytic Activity in the Hydrogen Evolution Reaction. Nanoscale 2015, 7 (12), 5203– 5208,  DOI: 10.1039/C4NR06754A
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    Porous metallic MoO2-supported MoS2 nanosheets for enhanced electrocatalytic activity in the hydrogen evolution reaction
    Yang, Linjing; Zhou, Weijia; Hou, Dongman; Zhou, Kai; Li, Guoqiang; Tang, Zhenghua; Li, Ligui; Chen, Shaowei
    Nanoscale (2015), 7 (12), 5203-5208CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
    Advanced materials for electrocatalytic water splitting are central to renewable energy research. In this work, MoS2 nanosheets supported on porous metallic MoO2 (MoS2/MoO2) were produced by sulfuration treatments of porous and highly conductive MoO2 for the hydrogen evolution reaction. Porous MoO2 with one-dimensional channel-like structures was prepd. by calcination at elevated temps. using phosphomolybdic acid as the precursor and mesoporous silica (SBA-15) as the template, and the subsequent hydrothermal treatment in the presence of thioacetamide led to the transformation of the top layers to MoS2 forming MoS2/MoO2 composites. Electrochem. studies showed that the obtained composites exhibited excellent electrocatalytic activity for HER with an onset potential of -104 mV (vs. RHE), a large c.d. (10 mA cm-2 at -0.24 V), a small Tafel slope of 76.1 mV dec-1 and robust electrochem. durability. The performance might be ascribed to the high elec. cond. and porous structures of MoO2 with one-dimensional channels of 3 to 4 nm in diam. that allowed for fast charge transport and collection.
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    Lee, C.; Yan, H.; Brus, L. E.; Heinz, T. F.; Hone, J.; Ryu, S. Anomalous Lattice Vibrations of Single- and Few-Layer MoS2. ACS Nano 2010, 4 (5), 2695– 2700,  DOI: 10.1021/nn1003937
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    Anomalous Lattice Vibrations of Single- and Few-Layer MoS2
    Lee, Changgu; Yan, Hugen; Brus, Louis E.; Heinz, Tony F.; Hone, James; Ryu, Sunmin
    ACS Nano (2010), 4 (5), 2695-2700CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Molybdenum disulfide (MoS2) of single- and few-layer thickness was exfoliated on SiO2/Si substrate and characterized by Raman spectroscopy. The no. of S-Mo-S layers of the samples was independently detd. by contact-mode at. force microscopy. Two Raman modes, E12g and A1g, exhibited sensitive thickness dependence, with the frequency of the former decreasing and that of the latter increasing with thickness. The results provide a convenient and reliable means for detg. layer thickness with at.-level precision. The opposite direction of the frequency shifts, which cannot be explained solely by van der Waals interlayer coupling, is attributed to Coulombic interactions and possible stacking-induced changes of the intralayer bonding. This work exemplifies the evolution of structural parameters in layered materials in changing from the three-dimensional to the two-dimensional regime.
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    Li, H.; Zhang, Q.; Yap, C. C. R.; Tay, B. K.; Edwin, T. H. T.; Olivier, A.; Baillargeat, D. From Bulk to Monolayer MoS2: Evolution of Raman Scattering. Adv. Funct. Mater. 2012, 22 (7), 1385– 1390,  DOI: 10.1002/adfm.201102111
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    From Bulk to Monolayer MoS2: Evolution of Raman Scattering
    Li, Hong; Zhang, Qing; Yap, Chin Chong Ray; Tay, Beng Kang; Edwin, Teo Hang Tong; Olivier, Aurelien; Baillargeat, Dominique
    Advanced Functional Materials (2012), 22 (7), 1385-1390CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    MoS2 is systematically studied using Raman spectroscopy with UV and visible laser lines. Only the Raman frequencies of E2g1 and A1g peaks vary monotonously with the layer no. of ultrathin MoS2 flakes, while intensities or widths of the peaks vary arbitrarily. The coupling between electronic transitions and phonons become weaker when the layer no. of MoS2 decreases, attributed to the increased electronic transition energies or elongated intralayer at. bonds in ultrathin MoS2. The asym. Raman peak at 454 cm-1, which was regarded as the overtone of longitudinal optical M phonons in bulk MoS2, is actually a combinational band involving a longitudinal acoustic mode (LA(M)) and an optical mode (A2u). Findings suggest a clear evolution of the coupling between electronic transition and phonon when MoS2 is scaled down from 3- to 2-dimensional geometry.
  53. 53
    Dieterle, M.; Mestl, G. Raman Spectroscopy of Molybdenum Oxides Part II. Resonance Raman Spectroscopic Characterization of the Molybdenum Oxides Mo4O11 and MoO2. Phys. Chem. Chem. Phys. 2002, 4 (5), 822– 826,  DOI: 10.1039/b107046k
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    Raman spectroscopy of molybdenum oxides. Part II. Resonance Raman spectroscopic characterization of the molybdenum oxides Mo4O11 and MoO2
    Dieterle, M.; Mestl, G.
    Physical Chemistry Chemical Physics (2002), 4 (5), 822-826CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    A special sample was prepd. by controlled oxidn. of MoO2, which contained MoO2, Mo4O11 and MoO3, to extend the knowledge about the resonance Raman effect in reduced Mo oxides from those close to MoO3 to those close to MoO2. This knowledge is of paramount importance because tech. partial oxidn. catalysts often contain intermediate Mo oxides of the Magneli type, e.g. Mo4O11, or Mo5O14. The Raman spectra of orthorhombic Mo4O11 and MoO2 were identified in a Raman microspectroscopic image of 100 single spectra recorded of a mixt. of MoO3, MoO2 and Mo4O11. A resonance Raman effect was proven to be responsible for the detection of the Mo oxide phases Mo4O11 and MoO2 in diln. with BN when excited at a laser wavelength of 632.8 nm by comparison with Raman microspectroscopic images of the identical sample when excited at 532 nm. The resonance Raman detection of reduced Mo oxide phases is discussed in the above mentioned context of their active role in catalytic partial oxidn. reactions.
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    Zhang, Q.; Li, X.; Ma, Q.; Zhang, Q.; Bai, H.; Yi, W.; Liu, J.; Han, J.; Xi, G. A Metallic Molybdenum Dioxide with High Stability for Surface Enhanced Raman Spectroscopy. Nat. Commun. 2017, 8 (1), 14903,  DOI: 10.1038/ncomms14903
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    A metallic molybdenum dioxide with high stability for surface enhanced Raman spectroscopy
    Zhang Qiqi; Li Xinshi; Ma Qiang; Zhang Qing; Bai Hua; Han Jing; Xi Guangcheng; Zhang Qiqi; Yi Wencai; Liu Jingyao; Han Jing; Xi Guangcheng
    Nature communications (2017), 8 (), 14903 ISSN:.
    Compared with noble metals, semiconductors with surface plasmon resonance effect are another type of SERS substrate materials. The main obstacles so far are that the semiconducting materials are often unstable and easy to be further oxidized or decomposed by laser irradiating or contacting with corrosive substances. Here, we report that metallic MoO2 can be used as a SERS substrate to detect trace amounts of highly risk chemicals including bisphenol A (BPA), dichloropheno (DCP), pentachlorophenol (PCP) and so on. The minimum detectable concentration was 10(-7) M and the maximum enhancement factor is up to 3.75 × 10(6). To the best of our knowledge, it may be the best among the metal oxides and even reaches or approaches to Au/Ag. The MoO2 shows an unexpected high oxidation resistance, which can even withstand 300 °C in air without further oxidation. The MoO2 material also can resist long etching of strong acid and alkali.
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    Castner, D. G.; Hinds, K.; Grainger, D. W. X-Ray Photoelectron Spectroscopy Sulfur 2p Study of Organic Thiol and Disulfide Binding Interactions with Gold Surfaces. Langmuir 1996, 12 (21), 5083– 5086,  DOI: 10.1021/la960465w
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    X-ray Photoelectron Spectroscopy Sulfur 2p Study of Organic Thiol and Disulfide Binding Interactions with Gold Surfaces
    Castner, David G.; Hinds, Kenneth; Grainger, David W.
    Langmuir (1996), 12 (21), 5083-5086CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    The presence of 2 S species was detected in XPS studies of thiol and disulfide mols. adsorbed on Au surfaces. These species are assigned to bound thiolate (S2p3/2 binding energy 162 eV) and unbound thiol/disulfide (S2p3/2 binding energy from 163.5 to 164 eV). These assignments are consistent with XPS data obtained from different thiols (C12, C16, C18, and C22 alkane thiols, a fluorinated thiol, and a cyclic siloxanethiol) and different adsorption conditions (solvent type, thiol concn., temp., and rinsing). In particular, the use of a poor solvent for thiol adsorption solns. (e.g., EtOH for long chain alkanethiols) and the lack of a rinsing step both resulted in unbound thiol mols. present at the surface of the bound thiolate monolayer. This has implications for recent studies asserting the presence of multiple binding sites for Au-thiolate species in org. monolayers.
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    Moonoosawmy, K. R.; Kruse, P. To Dope or Not to Dope: The Effect of Sonicating Single-Wall Carbon Nanotubes in Common Laboratory Solvents on Their Electronic Structure. J. Am. Chem. Soc. 2008, 130 (40), 13417– 13424,  DOI: 10.1021/ja8036788
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    To Dope or Not To Dope: The Effect of Sonicating Single-Wall Carbon Nanotubes in Common Laboratory Solvents on Their Electronic Structure
    Moonoosawmy, Kevin R.; Kruse, Peter
    Journal of the American Chemical Society (2008), 130 (40), 13417-13424CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Single-wall carbon nanotubes (SWCNTs) are commonly dispersed via sonication in a solvent prior to functionalization. Solvents such as dichloromethane, chloroform, 1,2-dichloroethane, and o-dichlorobenzene lead to an upward shift in the Raman response of the SWCNTs. The authors used o-dichlorobenzene as a model mol. to explain this effect, and an upward shift of 9 cm-1 is obsd. in the D* band. This blue shift is assocd. with p-type doping and is triggered only when the nanotubes are sonicated in the solvent. Sonication decomps. the chlorinated solvents, and (Cl2 and HCl(g)) are formed. The catalytic Fe nanoparticles inherently present in the nanotubes are etched by chlorine and hydrogen chloride to form iron chlorides during sonication in the solvent. The dopant was identified by XPS. With such knowledge of doping, the choice of solvent becomes crucial for any chem. reaction and can be intentionally tuned to produce SWCNTs films for electronics applications.
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    Graf, N.; Yegen, E.; Gross, T.; Lippitz, A.; Weigel, W.; Krakert, S.; Terfort, A.; Unger, W. E. S. XPS and NEXAFS Studies of Aliphatic and Aromatic Amine Species on Functionalized Surfaces. Surf. Sci. 2009, 603 (18), 2849– 2860,  DOI: 10.1016/j.susc.2009.07.029
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    XPS and NEXAFS studies of aliphatic and aromatic amine species on functionalized surfaces
    Graf, Nora; Yegen, Eda; Gross, Thomas; Lippitz, Andreas; Weigel, Wilfried; Krakert, Simone; Terfort, Andreas; Unger, Wolfgang E. S.
    Surface Science (2009), 603 (18), 2849-2860CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
    The chem. constitution of functionalized supports is an important parameter that dets. their performance in a broad range of applications, e.g. for immobilization of biomols. Supports with amino functionalized surfaces are also often used for DNA microarray expts. However, spectral data which were reported for surfaces with amino functionalities suffer from some inconsistencies. A detailed XPS and NEXAFS (Near edge x-ray absorption fine structure) database for amino functionalized surfaces is presented. Amino-terminated surfaces prepd. from aliph. and arom. aminosilanes or aminothiols and a field sample are considered. Effects of aging in air and damage by radiation are addressed as well.
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    Kruse, P. Review on Water Quality Sensors. J. Phys. D: Appl. Phys. 2018, 51 (20), 203002,  DOI: 10.1088/1361-6463/aabb93
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    Review on water quality sensors
    Kruse, Peter
    Journal of Physics D: Applied Physics (2018), 51 (20), 203002/1-203002/25CODEN: JPAPBE; ISSN:0022-3727. (IOP Publishing Ltd.)
    Terrestrial life may be carbon-based, but most of its mass is made up of water. Access to clean water is essential to all aspects of maintaining life. Mainly due to human activity, the strain on the water resources of our planet has increased substantially, requiring action in water management and purifn. Water quality sensors are needed in order to quantify the problem and verify the success of remedial actions. This review summarizes the most common chem. water quality parameters, and current developments in sensor technol. available to monitor them. Particular emphasis is on technologies that lend themselves to reagent-free, low-maintenance, autonomous and continuous monitoring. Chemiresistors and other elec. sensors are discussed in particular detail, while mech., optical and electrochem. sensors also find mentioning. The focus here is on the physics of chem. signal transduction in sensor elements that are in direct contact with the analyte. All other sensing methods, and all other elements of sampling, sample pre-treatment as well as the collection, transmission and anal. of the data are not discussed here. Instead, the goal is to highlight the progress and remaining challenges in the development of sensor materials and designs for an audience of physicists and materials scientists.
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    Zubiarrain-Laserna, A.; Kruse, P. Review─Graphene-Based Water Quality Sensors. J. Electrochem. Soc. 2020, 167 (3), 037539,  DOI: 10.1149/1945-7111/ab67a5
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    Review of graphene based water quality sensors
    Zubiarrain-Laserna, Ana; Kruse, Peter
    Journal of the Electrochemical Society (2020), 167 (3), 037539CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    A review. Water is fundamental to most aspects of civilization and terrestrial life in general. The problem of deteriorating water quality is very real, but often hard to quantify for lack of data. Hence the development of water quality sensors has become an urgently important area of research. Here we summarize an emerging class of water quality sensors based on field effect or chemiresistive geometries, which work completely in the solid state and can operate without ref. electrodes. Such devices are candidates for continuous online monitoring applications of surface, ground, drinking, process, and wastewater streams. Single layer and few layer graphenes are suitable materials for the sensing channels in these devices due to their chem. and mech. robustness and favorable electronic properties. While single layer graphene devices are more sensitive, few layer graphene sensors are easier to manuf. at a lower cost and offer a wider dynamic range. Detection of pH, disinfectants, mercury, lead, chromium, arsenic, potassium, calcium, some anions, as well as org. and biol. species has all been demonstrated at the proof of concept stage, with much more work in progress. One can anticipate the com. availability of such devices in the near future.
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    The electrical double layer and the theory of electro-capillarity
    Grahame, David C.
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    A review with 95 references.
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    Zafir Mohamad Nasir, M.; Sofer, Z.; Pumera, M. Effect of Electrolyte Ph on the Inherent Electrochemistry of Layered Transition-Metal Dichalcogenides (MoS2, MoSe2, WS2, WSe2). ChemElectroChem 2015, 2 (11), 1713– 1718,  DOI: 10.1002/celc.201500259
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    Nishimoto, M.; Muto, I.; Sugawara, Y.; Hara, N. Morphological Characteristics of Trenching around Mns Inclusions in Type 316 stainless Steel: The Role of Molybdenum in Pitting Corrosion Resistance. J. Electrochem. Soc. 2019, 166 (11), C3081– C3089,  DOI: 10.1149/2.0131911jes
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    Morphological characteristics of trenching around MnS inclusions in Type 316 stainless steel: the role of Molybdenum in pitting corrosion resistance
    Nishimoto, Masashi; Muto, Izumi; Sugawara, Yu; Hara, Nobuyoshi
    Journal of the Electrochemical Society (2019), 166 (11), C3081-C3089CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    The role of Mo alloying in the pit initiation process at MnS inclusions in stainless steels was investigated by measuring polarization curves, depassivation pH, and SEM. Molybdenum existed in the steel matrix but was not detected in the MnS inclusions in Type 316 stainless steel. While Mo alloying did not inhibit the dissoln. of the MnS inclusions in a NaCl soln., neither a metastable nor a stable pit occurred in the small areas with the MnS inclusions in the Mo-added specimen. The MnS/steel matrix boundary preferentially dissolved in the Mo-free specimen, with deep trenches formed. However, no deep trench was obsd. in the Mo-added specimen at low potentials. The depassivation pH of the Mo-added specimen in a 0.1 M NaCl-1 mM Na2S2O3 soln. was lower than that of the Mo-free specimen, suggesting that Mo alloying prevents the depassivation of the steel matrix at the boundary and inhibits the formation of trenches. In a 1 M HCl-1 mM Na2S2O3 soln., the active dissoln. rate of the steel was suppressed by Mo alloying. This suggests that, even after trenching at high potentials, Mo alloying inhibits the initiation of pitting inside the trenches.
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    Schulman, D. S.; May-Rawding, D.; Zhang, F.; Buzzell, D.; Alem, N.; Das, S. Superior Electro-Oxidation and Corrosion Resistance of Monolayer Transition Metal Disulfides. ACS Appl. Mater. Interfaces 2018, 10 (4), 4285– 4294,  DOI: 10.1021/acsami.7b17660
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    Superior Electro-Oxidation and Corrosion Resistance of Monolayer Transition Metal Disulfides
    Schulman, Daniel S.; May-Rawding, Dan; Zhang, Fu; Buzzell, Drew; Alem, Nasim; Das, Saptarshi
    ACS Applied Materials & Interfaces (2018), 10 (4), 4285-4294CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Physics of monolayer and few-layer transition metal dichalcogenides (TMDs) and chem. of few-layer TMDs have been well studied in recent years in the context of future electronic, optoelectronic, and energy harvesting applications. However, what has escaped the attention of the scientific community is the unique chem. of monolayer TMDs. It has been demonstrated that the basal plane of multilayer TMDs is chem. inert, whereas edge sites are chem. active. The authors exptl. demonstrate that the edge reactivity of the TMDs can be significantly impeded at the monolayer limit through monolayer/substrate interaction, thus making the monolayers highly resistant to electrooxidn. and corrosion. Few-layer flakes of MoS2 and WS2 exfoliated on conductive TiN substrates are readily corroded beyond a certain pos. electrode potential, while monolayer remnants are left behind unscathed. The electrooxidn. resistance of monolayers was confirmed using a plethora of characterization techniques including at. force microscope (AFM) imaging, Raman spectroscopy, photoluminescence (PL) mapping, scanning/transmission electron microscope (S/TEM) imaging, and selected area electron diffraction (SAED). It is believed that strong substrate monolayer interaction compared to the relatively weak interlayer van der Waals interaction is responsible for the superior monolayers chem. stability in highly corrosive oxidizing environments.
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    Mohtasebi, A.; Kruse, P. Chemical Sensors Based on Surface Charge Transfer. Phys. Sci. Rev. 2018, 3 (2), 20170133,  DOI: 10.1515/psr-2017-0133
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    Wan, H.; Xu, L.; Huang, W.-Q.; Zhou, J.-H.; He, C.-N.; Li, X.; Huang, G.-F.; Peng, P.; Zhou, Z.-G. Band Structure Engineering of Monolayer MoS2: A Charge Compensated Codoping Strategy. RSC Adv. 2015, 5 (11), 7944– 7952,  DOI: 10.1039/C4RA12498G
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    Band structure engineering of monolayer MoS2: a charge compensated codoping strategy
    Wan, Hui; Xu, Liang; Huang, Wei-Qing; Zhou, Jia-Hui; He, Chao-Ni; Li, Xiaofan; Huang, Gui-Fang; Peng, P.; Zhou, Zheng-Gui
    RSC Advances (2015), 5 (11), 7944-7952CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    The monolayer MoS2, possessing an advantage over graphene in that it exhibits a band gap whose magnitude is appropriate for solar applications, has attracted increasing attention because of its possible use as a photocatalyst. Herein, we propose a codoping strategy to tune the band structure of monolayer MoS2 aimed at enhancing its photocatalytic activity using first-principles calcn. The monodoping (halogen element, Nd) introduces impurity states in the gap, thus decreasing the photocatalytic activity of MoS2. Interestingly, the NbMoFS codoping reduces the energy cost of doping as a consequence of the charge compensation between the niobium (p-dopant) and the fluorine (n-dopant) impurities, which eliminates the isolated levels (induced by monodopant) in the band gap. Most importantly, the NbMoFS codoped MoS2 has more active sites for photocatalysis. These results show the proposed NbMoFS codoped monolayer MoS2 is a promising photocatalyst or photosensitizer for visible light in the heterogeneous semiconductor systems.
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    Donarelli, M.; Ottaviano, L. 2d Materials for Gas Sensing Applications: A Review on Graphene Oxide, MoS2, WS2 and Phosphorene. Sensors 2018, 18 (11), 3638,  DOI: 10.3390/s18113638
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    2D materials for gas sensing applications: a review on graphene oxide, MoS2, WS2 and phosphorene
    Donarelli, Maurizio; Ottaviano, Luca
    Sensors (2018), 18 (11), 3638/1-3638/45CODEN: SENSC9; ISSN:1424-8220. (MDPI AG)
    A review. After the synthesis of graphene, in the first year of this century, a wide research field on two-dimensional materials opens. 2D materials are characterized by an intrinsic high surface to vol. ratio, due to their heights of few atoms, and, differently from graphene, which is a semimetal with zero or near zero bandgap, they usually have a semiconductive nature. These two characteristics make them promising candidate for a new generation of gas sensing devices. Graphene oxide, being an intermediate product of graphene fabrication, has been the first graphene-like material studied and used to detect target gases, followed by MoS2, in the first years of 2010s. Along with MoS2, which is now experiencing a new birth, after its use as a lubricant, other sulfides and selenides (like WS2, WSe2, MoSe2, etc.) have been used for the fabrication of nanoelectronic devices and for gas sensing applications. All these materials show a bandgap, tunable with the no. of layers. On the other hand, 2D materials constituted by one at. species have been synthesized, like phosphorene (one layer of black phosphorous), germanene (one atom thick layer of germanium) and silicone (one atom thick layer of silicon). In this paper, a comprehensive review of 2D materials-based gas sensor is reported, mainly focused on the recent developments of graphene oxide, exfoliated MoS2 and WS2 and phosphorene, for gas detection applications. We will report on their use as sensitive materials for conductometric, capacitive and optical gas sensors, the state of the art and future perspectives.
  67. 67
    Bazylewski, P.; Van Middelkoop, S.; Divigalpitiya, R.; Fanchini, G. Solid-State Chemiresistors from Two-Dimensional Mos2 Nanosheets Functionalized with L-Cysteine for in-Line Sensing of Part-Per-Billion Cd2+ Ions in Drinking Water. ACS Omega 2020, 5 (1), 643– 649,  DOI: 10.1021/acsomega.9b03246
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    67
    Solid-State Chemiresistors from Two-Dimensional MoS2 Nanosheets Functionalized with L-Cysteine for In-Line Sensing of Part-Per-Billion Cd2+ Ions in Drinking Water
    Bazylewski, Paul; Van Middelkoop, Sheldon; Divigalpitiya, Ranjith; Fanchini, Giovanni
    ACS Omega (2020), 5 (1), 643-649CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
    Sensing of metal contaminants at ultralow concns. in aq. environments is vital in today's overpopulated world, with an extremely stringent limit (<5 ppb) for Cd2+ ions in drinking water. Here, we utilize sonochem. exfoliated molybdenum disulfide (MoS2) nanosheets functionalized with L-cysteine (Cys) as highly sensitive and selective two-dimensional (2D) materials for solid-state chemiresistors. We specifically targeted Cd2+ ions due to their high toxicity at low concns. MoS2-Cys nanosheets are fabricated using an ad hoc, low-complexity, one-pot synthesis method. Porous MoS2-Cys thin films with a high surface area are assembled from these nanosheets. Two-terminal chemiresistors incorporating MoS2-Cys films are demonstrated to be preferentially sensitive to Cd2+ ions at neutral pH, irresp. of other metal ions present in water flowing through the device. A 5 ppb concn. of the Cd2+ ions in the water stream increases the device resistivity by 20 times. Our devices operate at broad (1-500 ppb) range and fast (∼1 s) response times. Cd2+ is selectively detected because of preferential, size-driven adsorption at the interstitials between L-cysteine functional groups, combined with pH-controlled charge transfer that removes electronic gap states from MoS2. MoS2-Cys-based chemiresistors can be deployed in-line to detect metal ions without any need for addnl. offline measurements.

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  • Abstract

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    Figure 1

    Figure 1. Schematic representation of the exfoliation procedure of c-MoS2. Steps are (a) bulk MoS2 sonication in ethanol/water mixture for 12 h; (b) exfoliated 2H-MoS2 suspended in ethanol/water; (c–e) two-stage centrifugation process to collect exfoliated 2H-MoS2; (f) exfoliated 2H-MoS2 as precipitate; (g) exfoliated 2H-MoS2 shaken in 0.06% aqueous hydrogen peroxide; (h) suspended c-MoS2 after sonication; (i–k) two-stage centrifugation process to collect exfoliated c-MoS2; and (l) exfoliated c-MoS2 as precipitate after centrifugation.

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    Figure 2

    Figure 2. Morphology of MoS2 solid films and sheet resistance as a function of process parameters. (a) Actual image of exfoliated c-MoS2 (gray patch, 7 × 7 mm2) on a SiO2 substrate (1 × 1 cm2) with Au contacts in the four corners of the substrate. (b) SEM images showing overall film distribution of final exfoliated c-MoS2 on the substrate with high magnification showing few-layer flakes of material. The scale bars on the images represent 10 μm and 100 nm, respectively. (c) Alicona optical microscope mapping image of a SiO2 substrate with c-MoS2 film edge (red line is 19.7 mm long). (d) Height profile at the location of the red line in (d). (e) Relationship between the sheet resistance of c-MoS2 samples sonicated for different times in 0.06% and 0.22% aqueous H2O2. Lines are drawn to guide the eye. Solid lines are for 0.22%. The dotted lines are for 0.06%.

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    Figure 3

    Figure 3. TEM images showing film distribution of different MoS2 flasks: (a) bulk MoS2, (b) exfoliated 2H-MoS2, (c) bulk MoS2 treated with 0.06% aqueous H2O2, and (d) exfoliated MoS2 treated with 0.06% aqueous H2O2. The scale bar on the images represents 0.2 μm.

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    Figure 4

    Figure 4. XPS high-resolution spectra of exfoliated 2H-MoS2; shaken c-MoS2; 2 min sonicated c-MoS2; and 20 min sonicated c-MoS2 samples. Spectra are (a) Mo 3d, (b) S 2p in nonfunctionalized MoS2, and (c) S 2p in cysteamine-functionalized MoS2.

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    Figure 5

    Figure 5. Raman spectra of MoS2 samples. (a) Raman spectra proving the doping effect of H2O2 on 2H-MoS2. Black and violet color curves represent c-MoS2 (20 min sonicated) and 2H-MoS2, respectively, showing the shift in the E12g and A1g modes. (b) Raman spectrum of 2H-MoS2 sample. (c) Raman spectrum of c-MoS2 sample shaken in 0.06% H2O2. (d) Raman spectrum of 20 min sonicated c-MoS2 sonicated sample. All spectra were recorded with a 633 nm laser at 1% power. Each spectrum is normalized based on the highest peak (∼466 cm–1).

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    Figure 6

    Figure 6. Schematic representation of structure and chemical composition of the formation of c-MoS2.

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    Figure 7

    Figure 7. Schematic representation of the fabrication and potential applications of exfoliated c-MoS2.

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    Figure 8

    Figure 8. pH sensing response of c-MoS2 chemiresistive devices. 100 mV potential bias was applied across the c-MoS2 film to measure the current changes. Cysteamine-functionalized c-MoS2 chemiresisitive pH responses at (a) 3.7, (b) 6.5, (c) 4, and (d) 3.5. (e) Pristine c-MoS2 chemiresistive response to different pH values between 4 and 6.5. (f) Cysteamine-functionalized c-MoS2 chemiresistive response to different pH values between 3 and 6.5. (g) Calibration curve (linear fitting) of functionalized c-MoS2 chemiresistive device response (b). (h) Calibration curve from (g) replotted as a function of pH.

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    Figure 9

    Figure 9. Schematic representation of protonation and deprotonation process of cysteamine-functionalized c-MoS2.

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      Synthesized Ultrathin MoS2 Nanosheets Perpendicular to Graphene for Catalysis of Hydrogen Evolution Reaction
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      Tsai, M.-L.; Su, S.-H.; Chang, J.-K.; Tsai, D.-S.; Chen, C.-H.; Wu, C.-I.; Li, L.-J.; Chen, L.-J.; He, J.-H. Monolayer MoS2 Heterojunction Solar Cells. ACS Nano 2014, 8 (8), 8317– 8322,  DOI: 10.1021/nn502776h
      12
      Monolayer MoS2 Heterojunction Solar Cells
      Tsai, Meng-Lin; Su, Sheng-Han; Chang, Jan-Kai; Tsai, Dung-Sheng; Chen, Chang-Hsiao; Wu, Chih-I.; Li, Lain-Jong; Chen, Lih-Juann; He, Jr-Hau
      ACS Nano (2014), 8 (8), 8317-8322CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      We realized photovoltaic operation in large-scale MoS2 monolayers by the formation of a type-II heterojunction with p-Si. The MoS2 monolayer introduces a built-in elec. field near the interface between MoS2 and p-Si to help photogenerated carrier sepn. Such a heterojunction photovoltaic device achieves a power conversion efficiency of 5.23%, which is the highest efficiency among all monolayer transition metal dichalcogenide-based solar cells. The demonstrated results of monolayer MoS2/Si-based solar cells hold the promise for integration of two-dimensional materials with com. available Si-based electronics in highly efficient devices.
    13. 13
      Eda, G.; Yamaguchi, H.; Voiry, D.; Fujita, T.; Chen, M.; Chhowalla, M. Photoluminescence from Chemically Exfoliated MoS2. Nano Lett. 2011, 11 (12), 5111– 5116,  DOI: 10.1021/nl201874w
      13
      Photoluminescence from Chemically Exfoliated MoS2
      Eda, Goki; Yamaguchi, Hisato; Voiry, Damien; Fujita, Takeshi; Chen, Mingwei; Chhowalla, Manish
      Nano Letters (2011), 11 (12), 5111-5116CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      A two-dimensional crystal of molybdenum disulfide (MoS2) monolayer is a photoluminescent direct gap semiconductor in striking contrast to its bulk counterpart. Exfoliation of bulk MoS2 via Li intercalation is an attractive route to large-scale synthesis of monolayer crystals. However, this method results in loss of pristine semiconducting properties of MoS2 due to structural changes that occur during Li intercalation. Here, we report structural and electronic properties of chem. exfoliated MoS2. The metastable metallic phase that emerges from Li intercalation was found to dominate the properties of as-exfoliated material, but mild annealing leads to gradual restoration of the semiconducting phase. Above an annealing temp. of 300°, chem. exfoliated MoS2 exhibit prominent band gap photoluminescence, similar to mech. exfoliated monolayers, indicating that their semiconducting properties are largely restored.
    14. 14
      Xia, J.; Wang, J.; Chao, D.; Chen, Z.; Liu, Z.; Kuo, J.-L.; Yan, J.; Shen, Z. X. Phase Evolution of Lithium Intercalation Dynamics in 2H-MoS2. Nanoscale 2017, 9 (22), 7533– 7540,  DOI: 10.1039/C7NR02028G
      14
      Phase evolution of lithium intercalation dynamics in 2hydrogen-molybdenum disulfide
      Xia, Juan; Wang, Jin; Chao, Dongliang; Chen, Zhen; Liu, Zheng; Kuo, Jer-Lai; Yan, Jiaxu; Shen, Ze Xiang
      Nanoscale (2017), 9 (22), 7533-7540CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Due to the easy intralayer gliding and weak interlayer van der Waals interaction in transition metal dichalcogenides (TMDs), ion (particularly Li+) intercalation has been used to modify and tune their at. structures to obtain the desired optical, electronic and chem. properties for future optoelectronics and energy storage applications. A good understanding of the transformative structures during intercalation is crit. In this paper, we investigate the structural transformation dynamics of 2H-MoS2 using electrochem. Li+ intercalation for 2H-MoS2. The introduction of Li+ changes the local symmetry of the MoS2 in favor of the dT phase, clearly indicated by the appearance of Raman peaks of the dT phase. Further Li+ insertion causes the samples to become single-layer-like, characterized by the disappearance of the 32 cm-1 Raman peak. We also observe for the first time that the photoluminescence (PL) emission gradually red shifts with decreasing intensity, followed by eventual vanishing of the PL peak in the dT-MoS2 phase. By the nudged elastic band (NEB) calcns., we propose the 2H-1T-dT phase transition mechanism of MoS2 for Li+ intercalated samples. Our claims are supported by high resoln.-transmission electron microscopy (HR-TEM). Our study deepens the understanding of the phase transition dynamics upon lithium intercalation, which is of great value to possible optoelectronic devices based on the phase engineering of TMDs. The new Li-stabilized dT-MoS2 phase does not possess inversion symmetry and may present a feasible way to achieve Weyl state tuning in a single material via phase engineering.
    15. 15
      Geng, X.; Sun, W.; Wu, W.; Chen, B.; Al-Hilo, A.; Benamara, M.; Zhu, H.; Watanabe, F.; Cui, J.; Chen, T.-p. Pure and Stable Metallic Phase Molybdenum Disulfide Nanosheets for Hydrogen Evolution Reaction. Nat. Commun. 2016, 7 (1), 10672,  DOI: 10.1038/ncomms10672
      15
      Pure and stable metallic phase molybdenum disulfide nanosheets for hydrogen evolution reaction
      Geng, Xiumei; Sun, Weiwei; Wu, Wei; Chen, Benjamin; Al-Hilo, Alaa; Benamara, Mourad; Zhu, Hongli; Watanabe, Fumiya; Cui, Jingbiao; Chen, Tar-pin
      Nature Communications (2016), 7 (), 10672CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      Metallic-phase MoS2 (M-MoS2) is metastable and does not exist in nature. Pure and stable M-MoS2 has not been previously prepd. by chem. synthesis, to the best of our knowledge. Here we report a hydrothermal process for synthesizing stable two-dimensional M-MoS2 nanosheets in water. The metal-metal Raman stretching mode at 146 cm-1 in the M-MoS2 structure, as predicted by theor. calcns., is exptl. obsd. The stability of the M-MoS2 is assocd. with the adsorption of a monolayer of water mols. on both sides of the nanosheets, which reduce restacking and prevent aggregation in water. The obtained M-MoS2 exhibits excellent stability in water and superior activity for the hydrogen evolution reaction, with a c.d. of 10 mA cm-2 at a low potential of -175 mV and a Tafel slope of 41 mV per decade.
    16. 16
      Dabral, A.; Lu, A. K. A.; Chiappe, D.; Houssa, M.; Pourtois, G. A Systematic Study of Various 2D Materials in the Light of Defect Formation and Oxidation. Phys. Chem. Chem. Phys. 2019, 21 (3), 1089– 1099,  DOI: 10.1039/C8CP05665J
      16
      A systematic study of various 2D materials in the light of defect formation and oxidation
      Dabral, A.; Lu, A. K. A.; Chiappe, D.; Houssa, M.; Pourtois, G.
      Physical Chemistry Chemical Physics (2019), 21 (3), 1089-1099CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The thermodn. aspects of various 2D materials are explored using D. Functional Theory (DFT). Various metal chalcogenides (MX2, M = metal, chalcogen X = S, Se, Te) are investigated with respect to their interaction and stability under different ambient conditions met in the integration process of a transistor device. Their interaction with high-κ dielecs. is also addressed, in order to assess their possible integration in Complementary Metal Oxide Semiconductor (CMOS) field effect transistors. 2D materials show promise for high performance nanoelectronic devices, but the presence of defects (vacancies, grain boundaries,...) can significantly impact their electronic properties. To assess the impact of defects, their enthalpies of formation and their signature levels in the d. of states have been studied. We find, consistently with literature reports, that chalcogen vacancies are the most likely source of defects. It is shown that while pristine 2D materials are in general stable whenever set in contact with different ambient atms., the presence of defective sites affects the electronic properties of the 2D materials to varying degrees. We observe that all the 2D materials studied in the present work show strong reactivity towards radical oxygen plasma treatments while reactivity towards other common gas phase chem. such as O2 and H2O and groups present at the high-κ surface varies significantly between species. While energy band-gaps, effective masses and contact resistivities are key criteria in selection of 2D materials for scaled CMOS and tunneling based devices, the phase and ambient stabilities might also play a very important role in the development of reliable nanoelectronic applications.
    17. 17
      Xie, Y.; Liang, F.; Chi, S.; Wang, D.; Zhong, K.; Yu, H.; Zhang, H.; Chen, Y.; Wang, J. Defect Engineering of Mos2 for Room-Temperature Terahertz Photodetection. ACS Appl. Mater. Interfaces 2020, 12 (6), 7351– 7357,  DOI: 10.1021/acsami.9b21671
      17
      Defect Engineering of MoS2 for Room-Temperature Terahertz Photodetection
      Xie, Ying; Liang, Fei; Chi, Shumeng; Wang, Dong; Zhong, Kai; Yu, Haohai; Zhang, Huaijin; Chen, Yanxue; Wang, Jiyang
      ACS Applied Materials & Interfaces (2020), 12 (6), 7351-7357CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Two-dimensional (2D) materials have exotic intrinsic electronic band structures and are considered as revolutionary foundations for novel nanodevices. Band engineering of 2D materials may pave a new avenue to overcome numerous challenges in modern technologies, such as room temp. (RT) photodetection of light with photon energy below their band gaps. Here, we reported the pioneering RT MoS2-based photodetection in the terahertz (THz) region via introducing Mo4+ and S2- vacancies for rational band gap engineering. Both the generation and transport of extra carriers, driven by THz electromagnetic radiations, were regulated by the vacancy concn. as well as the resistivity of MoS2 samples. Utilizing the balance between the carrier concn. fluctuation and carrier-scattering probability, a high RT photoresponsivity of 10 mA/W at 2.52 THz was realized in an Mo-vacancy-rich MoS2.19 sample. This work overcomes the challenge in the excessive dark current of RT THz detection and offers a convenient way for further optoelectronic and photonic devices based on band gap-engineered 2D materials.
    18. 18
      Saha, D.; Kruse, P. Editors’ Choice─Review─Conductive Forms of MoS2 and Their Applications in Energy Storage and Conversion. J. Electrochem. Soc. 2020, 167 (12), 126517,  DOI: 10.1149/1945-7111/abb34b
      18
      Editors' choice-review-conductive forms of MoS2 and their applications in energy storage and conversion
      Saha, Dipankar; Kruse, Peter
      Journal of the Electrochemical Society (2020), 167 (12), 126517CODEN: JESOAN; ISSN:1945-7111. (IOP Publishing Ltd.)
      A review. Conductive forms of MoS2 are important emerging 2D materials due to their unique combination of properties such as high elec. cond., availability of active sites in edge and basal planes for catalytic activity and expanded interlayer distances. Consequently, there has been a drive to find synthetic routes toward conductive forms of MoS2. Naturally occurring or synthetically grown semiconducting 2H-MoS2can either be converted into metallic 1T-MoS2, or various dopants may be introduced to modulate the electronic band gap of the 2H-MoS2phase and increase its cond. Chem. and electrochem. intercalation methods, hydrothermal and solvothermal processes, and chem. vapor deposition have all been developed to synthesize conductive MoS2. Conductive MoS2 finds applications in energy storage devices, electrocatalytic reactions, and sensors. Here, we summarize a detailed understanding of the at. structure and electronic properties of conductive MoS2 which is crucial for its applications. We also discuss various fabrication methods that have been previously reported along with their advantages and disadvantages. Finally, we will give an overview of current trends in different applications in energy storage and electrocatalytic reactions in order to help researchers to further explore the applications of conductive MoS2.
    19. 19
      Kc, S.; Longo, R. C.; Addou, R.; Wallace, R. M.; Cho, K. Impact of Intrinsic Atomic Defects on the Electronic Structure of MoS2 Monolayers. Nanotechnology 2014, 25 (37), 375703,  DOI: 10.1088/0957-4484/25/37/375703
      There is no corresponding record for this reference.
    20. 20
      McDonnell, S.; Addou, R.; Buie, C.; Wallace, R. M.; Hinkle, C. L. Defect-Dominated Doping and Contact Resistance in MoS2. ACS Nano 2014, 8 (3), 2880– 2888,  DOI: 10.1021/nn500044q
      20
      Defect-Dominated Doping and Contact Resistance in MoS2
      McDonnell, Stephen; Addou, Rafik; Buie, Creighton; Wallace, Robert M.; Hinkle, Christopher L.
      ACS Nano (2014), 8 (3), 2880-2888CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Achieving low resistance contacts is vital for the realization of nanoelectronic devices based on transition metal dichalcogenides. Intrinsic defects in MoS2 dominate the metal/MoS2 contact resistance and provide a low Schottky barrier independent of metal contact work function. Also, MoS2 can exhibit both n-type and p-type conduction at different points on a same sample. The authors identify these regions independently by complementary characterization techniques and show how the Fermi level can shift by 1 eV over tens of nanometers in spatial resoln. These variations in doping are defect-chem.-related and are independent of contact metal. This raises questions on previous reports of metal-induced doping of MoS2 since the same metal in contact with MoS2 can exhibit both n- and p-type behavior. These results may provide a potential route for achieving low electron and hole Schottky barrier contacts with a single metal deposition.
    21. 21
      Sim, D. M.; Kim, M.; Yim, S.; Choi, M.-J.; Choi, J.; Yoo, S.; Jung, Y. S. Controlled Doping of Vacancy-Containing Few-Layer MoS2 Via Highly Stable Thiol-Based Molecular Chemisorption. ACS Nano 2015, 9 (12), 12115– 12123,  DOI: 10.1021/acsnano.5b05173
      21
      Controlled Doping of Vacancy-Containing Few-Layer MoS2 via Highly Stable Thiol-Based Molecular Chemisorption
      Sim, Dong Min; Kim, Mincheol; Yim, Soonmin; Choi, Min-Jae; Choi, Jaesuk; Yoo, Seunghyup; Jung, Yeon Sik
      ACS Nano (2015), 9 (12), 12115-12123CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      MoS2 is considered a promising two-dimensional active channel material for future nanoelectronics. However, the development of a facile, reliable, and controllable doping methodol. is still crit. for extending the applicability of MoS2. Here, the authors report surface charge transfer doping via thiol-based binding chem. for modulating the elec. properties of vacancy-contg. MoS2 (v-MoS2). Although vacancies present in 2-dimensional materials are generally regarded as undesirable components, the elec. properties of MoS2 can be systematically engineered by exploiting the tight binding between the thiol group and sulfur vacancies and by choosing different functional groups. For example, NH2-contg. thiol mols. with lone electron pairs can serve as an n-dopant and achieve a substantial increase of electron d. (Δn = 3.7 × 1012 cm-2). However, fluorine-rich mols. can provide a p-doping effect (Δn = -7.0 × 1011 cm-2) due to its high electronegativity. Also, the n- and p-doping effects were systematically evaluated by photoluminescence (PL), XPS, and elec. measurement results. The excellent binding stability of thiol mols. and recovery properties by thermal annealing will enable broader applicability of ultrathin MoS2 to various devices.
    22. 22
      Förster, A.; Gemming, S.; Seifert, G.; Tománek, D. Chemical and Electronic Repair Mechanism of Defects in MoS2 Monolayers. ACS Nano 2017, 11 (10), 9989– 9996,  DOI: 10.1021/acsnano.7b04162
      22
      Chemical and electronic repair mechanism of defects in MoS2 monolayers
      Forster, Anja; Gemming, Sibylle; Seifert, Gotthard; Tomanek, David
      ACS Nano (2017), 11 (10), 9989-9996CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Using ab initio d. functional theory calcns., we characterize changes in the electronic structure of MoS2 monolayers introduced by missing or addnl. adsorbed sulfur atoms. We furthermore identify the chem. and electronic function of substances that have been reported to reduce the adverse effect of sulfur vacancies in quenching photoluminescence and reducing electronic conductance. We find that thiol-group-contg. mols. adsorbed at vacancy sites may reinsert missing sulfur atoms. In the presence of addnl. adsorbed sulfur atoms, thiols may form disulfides on the MoS2 surface to mitigate the adverse effect of defects.
    23. 23
      Kc, S.; Longo, R. C.; Wallace, R. M.; Cho, K. Surface Oxidation Energetics and Kinetics on Mos2 Monolayer. J. Appl. Phys. 2015, 117 (13), 135301,  DOI: 10.1063/1.4916536
      There is no corresponding record for this reference.
    24. 24
      Lu, H.; Kummel, A.; Robertson, J. Passivating the Sulfur Vacancy in Monolayer Mos2. APL Mater. 2018, 6 (6), 066104,  DOI: 10.1063/1.5030737
      24
      Passivating the sulfur vacancy in monolayer MoS2
      Lu, Haichang; Kummel, Andrew; Robertson, John
      APL Materials (2018), 6 (6), 066104/1-066104/9CODEN: AMPADS; ISSN:2166-532X. (American Institute of Physics)
      Various methods to passivate the sulfur vacancy in 2D MoS2 are modeled using d. functional theory (DFT) to understand the passivation mechanism at an at. scale. First, the org. super acid, bis(trifluoromethane)sulfonimide (TFSI) is a strong protonating agent, and it is exptl. found to greatly increase the photoluminescence efficiency. DFT simulations find that the effectiveness of passivation depends critically on the charge state and no. of hydrogens donated by TFSI since this dets. the symmetry of the defect complex. A sym. complex is formed by three hydrogen atoms bonding to the defect in a -1 charge state, and this gives no bandgap states and a Fermi level in the midgap. However, a charge state of +1 gives a lower symmetry complex with one state in the gap. One or two hydrogens also give complexes with gap states. Second, passivation by O2 can provide partial passivation by forming a bridge bond across the S vacancy, but it leaves a defect state in the lower bandgap. On the other hand, substitutional addns. do not shift the vacancy states out of the gap. (c) 2018 American Institute of Physics.
    25. 25
      Nan, H.; Wang, Z.; Wang, W.; Liang, Z.; Lu, Y.; Chen, Q.; He, D.; Tan, P.; Miao, F.; Wang, X.; Wang, J.; Ni, Z. Strong Photoluminescence Enhancement of MoS2 through Defect Engineering and Oxygen Bonding. ACS Nano 2014, 8 (6), 5738– 5745,  DOI: 10.1021/nn500532f
      25
      Strong Photoluminescence Enhancement of MoS2 through Defect Engineering and Oxygen Bonding
      Nan, Haiyan; Wang, Zilu; Wang, Wenhui; Liang, Zheng; Lu, Yan; Chen, Qian; He, Daowei; Tan, Pingheng; Miao, Feng; Wang, Xinran; Wang, Jinlan; Ni, Zhenhua
      ACS Nano (2014), 8 (6), 5738-5745CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      We report on a strong photoluminescence (PL) enhancement of monolayer MoS2 through defect engineering and oxygen bonding. Micro-PL and Raman images clearly reveal that the PL enhancement occurs at cracks/defects formed during high-temp. annealing. The PL enhancement at crack/defect sites could be as high as thousands of times after considering the laser spot size. The main reasons of such huge PL enhancement include the following: (1) the oxygen chem. adsorption induced heavy p doping and the conversion from trion to exciton; (2) the suppression of nonradiative recombination of excitons at defect sites, which was verified by low-temp. PL measurements. First-principle calcns. reveal a strong binding energy of ∼2.395 eV for an oxygen mol. adsorbed on a S vacancy of MoS2. The chem. adsorbed oxygen also provides a much more effective charge transfer (0.997 electrons per O2) compared to phys. adsorbed oxygen on an ideal MoS2 surface. We also demonstrate that the defect engineering and oxygen bonding could be easily realized by mild oxygen plasma irradn. XPS further confirms the formation of Mo-O bonding. Our results provide a new route for modulating the optical properties of two-dimensional semiconductors. The strong and stable PL from defects sites of MoS2 may have promising applications in optoelectronic devices.
    26. 26
      Verhagen, T.; Guerra, V. L. P.; Haider, G.; Kalbac, M.; Vejpravova, J. Towards the Evaluation of Defects in MoS2 Using Cryogenic Photoluminescence Spectroscopy. Nanoscale 2020, 12 (5), 3019– 3028,  DOI: 10.1039/C9NR07246B
      26
      Towards the evaluation of defects in MoS2 using cryogenic photoluminescence spectroscopy
      Verhagen, Tim; Guerra, Valentino L. P.; Haider, Golam; Kalbac, Martin; Vejpravova, Jana
      Nanoscale (2020), 12 (5), 3019-3028CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Characterization of the type and d. of defects in two-dimensional (2D) transition metal dichalcogenides (TMDs) is important as the nature of these defects strongly influences the electronic and optical properties of the material, esp. its photoluminescence (PL). Defect characterization is not as straightforward as it is for graphene films, where the D and D' Raman scattering modes easily indicate the d. and type of defects in the graphene layer. Thus, in addn. to the Raman scattering anal., other spectroscopic techniques are necessary to perform detailed characterization of atomically thin TMD layers. We demonstrate that PL spectroscopy performed at liq. helium temps. reveals the key fingerprints of defects in TMDs and hence provides valuable information about their origin and concn. In our study, we address defects in chem. vapor deposition (CVD)-grown MoS2 monolayers. A significant difference is obsd. between the as-grown monolayers compared with the CVD-grown monolayers transferred onto a Si/SiO2 substrate, which contain extra defects due to the transfer process. We demonstrate that the temp.-dependent Raman and PL micro-spectroscopy techniques enable disentangling the contributions and locations of various defect types in TMD systems.
    27. 27
      Mohtasebi, A.; Broomfield, A. D.; Chowdhury, T.; Selvaganapathy, P. R.; Kruse, P. Reagent-Free Quantification of Aqueous Free Chlorine Via Electrical Readout of Colorimetrically Functionalized Pencil Lines. ACS Appl. Mater. Interfaces 2017, 9 (24), 20748– 20761,  DOI: 10.1021/acsami.7b03968
      27
      Reagent-Free Quantification of Aqueous Free Chlorine via Electrical Readout of Colorimetrically Functionalized Pencil Lines
      Mohtasebi, Amirmasoud; Broomfield, Andrew D.; Chowdhury, Tanzina; Selvaganapathy, P. Ravi; Kruse, Peter
      ACS Applied Materials & Interfaces (2017), 9 (24), 20748-20761CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Colorimetric methods are commonly used to quantify free chlorine in drinking water. However, these methods are not suitable for reagent-free, continuous, and autonomous applications. Here, we demonstrate how functionalization of a pencil-drawn film with phenyl-capped aniline tetramer (PCAT) can be used for quant. elec. readout of free chlorine concns. The functionalized film can be implemented in a simple fluidic device for continuous sensing of aq. free chlorine concns. The sensor is selective to free chlorine and can undergo a reagent-free reset for further measurements. Our sensor is superior to electrochem. methods in that it does not require a ref. electrode. It is capable of quantification of free chlorine in the range of 0.1-12 ppm with higher precision than colorimetric (absorptivity) methods. The interactions of PCAT with the pencil-drawn film upon exposure to hypochlorite were characterized spectroscopically. A previously reported detection mechanism relied on the measurement of a baseline shift to quantify free chlorine concns. The new method demonstrated here measures initial spike size upon exposure to free chlorine. It relies on a fast charge built up on the sensor film due to intermittent PCAT salt formation. It has the advantage of being significantly faster than the measurement of baseline shift, but it cannot be used to detect gradual changes in free chlorine concn. without the use of frequent reset pulses. The stability of PCAT was examd. in the presence of free chlorine as a function of pH. While most ions commonly present in drinking water do not interfere with the free chlorine detection, other oxidants may contribute to the signal. Our sensor is easy to fabricate and robust, operates reagent-free, and has very low power requirements and is thus suitable for remote deployment.
    28. 28
      Hoque, E.; Hsu, L. H. H.; Aryasomayajula, A.; Selvaganapathy, P. R.; Kruse, P. Pencil-Drawn Chemiresistive Sensor for Free Chlorine in Water. IEEE Sensors Letters 2017, 1 (4), 1– 4,  DOI: 10.1109/LSENS.2017.2722958
      There is no corresponding record for this reference.
    29. 29
      Dalmieda, J.; Zubiarrain-Laserna, A.; Ganepola, D.; Selvaganapathy, P. R.; Kruse, P. Chemiresistive Detection of Silver Ions in Aqueous Media. Sens. Actuators, B 2021, 328, 129023,  DOI: 10.1016/j.snb.2020.129023
      29
      Chemiresistive detection of silver ions in aqueous media
      Dalmieda, Johnson; Zubiarrain-Laserna, Ana; Ganepola, Devanjith; Selvaganapathy, P. Ravi; Kruse, Peter
      Sensors and Actuators, B: Chemical (2021), 328 (), 129023CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
      Silver is used as a water disinfectant in hospital settings as well as in purifiers for potable water. Although there are no strict regulations on the concn. of silver in water, adverse effects such as argyria and respiratory tract irritation have been correlated to excess silver consumption. Based on this, the levels of silver in water are recommended to be maintained below 100 ppb to ensure safety for human consumption. In this work, we present a silver sensor for use in aq. media that utilizes bathocuproine, a silver selective chromophore, adsorbed onto few-layer graphene (FLG) flake networks for the chemiresistive detection of silver. Complexation of silver to bathocuproine modulates the cond. of the FLG film, which can be probed by applying a small voltage bias. The decrease in resistance of the film correlates with the concn. of silver in soln. between 3 ppb and 1 ppm. Exposing the sensor to a lower pH resets the sensor, allowing it to be reused and reset multiple times. This sensor demonstrates a new pathway to chemiresistive cation sensing using known selective complexing agents adsorbed onto graphitic thin films. This concept can be expanded to the detection of other relevant analytes in domestic, industrial and environmental water sources.
    30. 30
      Gou, P.; Kraut, N. D.; Feigel, I. M.; Bai, H.; Morgan, G. J.; Chen, Y.; Tang, Y.; Bocan, K.; Stachel, J.; Berger, L.; Mickle, M.; Sejdić, E.; Star, A. Carbon Nanotube Chemiresistor for Wireless pH Sensing. Sci. Rep. 2015, 4 (1), 4468,  DOI: 10.1038/srep04468
      There is no corresponding record for this reference.
    31. 31
      Zhou, K.-G.; Mao, N.-N.; Wang, H.-X.; Peng, Y.; Zhang, H.-L. A Mixed-Solvent Strategy for Efficient Exfoliation of Inorganic Graphene Analogues. Angew. Chem., Int. Ed. 2011, 50 (46), 10839– 10842,  DOI: 10.1002/anie.201105364
      31
      A Mixed-Solvent Strategy for Efficient Exfoliation of Inorganic Graphene Analogues
      Zhou, Kai-Ge; Mao, Nan-Nan; Wang, Hang-Xing; Peng, Yong; Zhang, Hao-Li
      Angewandte Chemie, International Edition (2011), 50 (46), 10839-10842, S10839/1-S10839/6CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The authors demonstrate a versatile and scaleable mixed-solvent strategy for liq. exfoliation of inorg. graphene analogs IGAs such as WS2, MoS2, and BN, in volatile solvents. By choosing solvents with appropriate compn., highly stable IGA suspensions can be obtained in low-boiling solvent mixts., which can then be easily used in further applications. The dispersion of nanomaterials in liqs. can be partially predicted by the theory of Hansen soly. parameters (HSP) which is a semiempirical correlation developed to explain dissoln. behavior. Three HSP parameters are used to describe the character of a solvent or material. The dissoln. process is one of adaptation between the HSP parameters of solvents and solute.
    32. 32
      Su, W.; Dou, H.; Li, J.; Huo, D.; Dai, N.; Yang, L. Tuning Photoluminescence of Single-Layer MoS2 Using H2O2. RSC Adv. 2015, 5 (101), 82924– 82929,  DOI: 10.1039/C5RA12450F
      32
      Tuning photoluminescence of single-layer MoS2 using H2O2
      Su, Weitao; Dou, Honglei; Li, Jinwei; Huo, Dexuan; Dai, Ning; Yang, Li
      RSC Advances (2015), 5 (101), 82924-82929CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      Enhancing photoluminescence (PL) of single-layer (1L) MoS2 is crit. to its application as the thinnest light-emitting material. In this report, we show that the PL intensity of 1L-MoS2 can be enhanced by 8 times using physisorption of H2O2 mols. as p-type dopants. By using toluene to form the sandwiched structure of H2O2/1L-MoS2/toluene, the PL intensity of 1L-MoS2 can be enhanced up to 27.4 times. Our research proposes a simple but effective method to enhance the light emitting properties of 1L-MoS2.
    33. 33
      Dong, L.; Lin, S.; Yang, L.; Zhang, J.; Yang, C.; Yang, D.; Lu, H. Spontaneous Exfoliation and Tailoring of MoS2 in Mixed Solvents. Chem. Commun. 2014, 50 (100), 15936– 15939,  DOI: 10.1039/C4CC07238C
      33
      Spontaneous exfoliation and tailoring of MoS2 in mixed solvents
      Dong, Lei; Lin, Shan; Yang, Liu; Zhang, Jiajia; Yang, Chao; Yang, Dong; Lu, Hongbin
      Chemical Communications (Cambridge, United Kingdom) (2014), 50 (100), 15936-15939CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Spontaneous exfoliation of MoS2 is achieved in H2O2-NMP mixed solvents with a yield of over 60 wt%, operated under mild conditions. H2O2-prompted sheet-tailoring effect induces a size evolution of MoS2 nanosheets from micro-scale to nano-scale. Furthermore, the concurrent dissoln. also affords an approach to introduce structural defects in the nanosheets.
    34. 34
      Pradhan, N. R.; Rhodes, D.; Zhang, Q.; Talapatra, S.; Terrones, M.; Ajayan, P. M.; Balicas, L. Intrinsic Carrier Mobility of Multi-Layered Mos2 Field-Effect Transistors on SiO2. Appl. Phys. Lett. 2013, 102 (12), 123105,  DOI: 10.1063/1.4799172
      34
      Intrinsic carrier mobility of multi-layered MoS2 field-effect transistors on SiO2
      Pradhan, N. R.; Rhodes, D.; Zhang, Q.; Talapatra, S.; Terrones, M.; Ajayan, P. M.; Balicas, L.
      Applied Physics Letters (2013), 102 (12), 123105/1-123105/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      By fabricating and characterizing multilayered MoS2-based field-effect transistors in a 4 terminal configuration, we demonstrate that the 2 terminal-configurations tend to underestimate the carrier mobility μ due to the Schottky barriers at the contacts. For a back-gated 2-terminal configuration, we observe mobilities as high as 91 cm2 V-1s-1 which is considerably smaller than 306.5 cm2 V-1s-1 as extd. from the same device when using a 4-terminal configuration. The intrinsic mobility of MoS2 on SiO2 is significantly larger than the values previously reported, and provides a quant. method to evaluate the charge transport through the contacts. (c) 2013 American Institute of Physics.
    35. 35
      Laskar, M. R.; Nath, D. N.; Ma, L.; Lee, E. W.; Lee, C. H.; Kent, T.; Yang, Z.; Mishra, R.; Roldan, M. A.; Idrobo, J.-C.; Pantelides, S. T.; Pennycook, S. J.; Myers, R. C.; Wu, Y.; Rajan, S. P-Type Doping of MoS2 Thin Films Using Nb. Appl. Phys. Lett. 2014, 104 (9), 092104,  DOI: 10.1063/1.4867197
      35
      p-type doping of MoS2 thin films using Nb
      Laskar, Masihhur R.; Nath, Digbijoy N.; Ma, Lu; Lee, Edwin W.; Lee, Choong Hee; Kent, Thomas; Yang, Zihao; Mishra, Rohan; Roldan, Manuel A.; Idrobo, Juan-Carlos; Pantelides, Sokrates T.; Pennycook, Stephen J.; Myers, Roberto C.; Wu, Yiying; Rajan, Siddharth
      Applied Physics Letters (2014), 104 (9), 092104/1-092104/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Reported on the first demonstration of p-type doping in large area few-layer films of (0001)-oriented chem. vapor deposited MoS2. Nb was found to act as an efficient acceptor up to relatively high d. in MoS2 films. For a hole d. of 3.1 × 1020 cm-3, Hall mobility of 8.5 cm2 V-1 s-1 was detd., which matches well with the theor. expected values. X-ray diffraction scans and Raman characterization indicated that the film had good out-of-plane cryst. quality. Absorption measurements showed that the doped sample had similar characteristics to high-quality undoped samples, with a clear absorption edge at 1.8 eV. Scanning transmission electron microscope imaging showed ordered cryst. nature of the Nb-doped MoS2 layers stacked in the [0001] direction. This demonstration of substitutional p-doping in large area epitaxial MoS2 could help in realizing a wide variety of elec. and opto-electronic devices based on layered metal dichalcogenides. (c) 2014 American Institute of Physics.
    36. 36
      Werner, F. Hall Measurements on Low-Mobility Thin Films. J. Appl. Phys. 2017, 122 (13), 135306,  DOI: 10.1063/1.4990470
      36
      Hall measurements on low-mobility thin films
      Werner, Florian
      Journal of Applied Physics (Melville, NY, United States) (2017), 122 (13), 135306/1-135306/13CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
      We review the conventional measuring std. for dc Hall measurements in van der Pauw configuration with particular focus on the challenges arising from a small Hall signal compared to sizable offset voltages, which is a typical scenario for many material systems, particularly low-mobility thin films. We show that the conventional approach of using a simple field-reversal technique is often unsuited to obtain reliable results, and present an improved correction scheme to extend the accessible measurement range to mobility values well below 1 cm2/(V s). We discuss procedures to limit the impact of temp. fluctuations and long stabilization times for highly resistive materials. We further address potential sources of error due to the presence of grain boundaries in polycryst. specimen and due to multi-carrier conduction, both of which might yield low apparent Hall mobilities significantly underestimating the actual mobility. (c) 2017 American Institute of Physics.
    37. 37
      Rai, A.; Movva, H. C. P.; Roy, A.; Taneja, D.; Chowdhury, S.; Banerjee, S. K. Progress in Contact, Doping and Mobility Engineering of MoS2: An Atomically Thin 2D Semiconductor. Crystals 2018, 8 (8), 316,  DOI: 10.3390/cryst8080316
      37
      Progress in contact, doping and mobility engineering of MoS2: an atomically thin 2D semiconductor
      Rai, Amritesh; Movva, Hema C. P.; Roy, Anupam; Taneja, Deepyanti; Chowdhury, Sayema; Banerjee, Sanjay K.
      Crystals (2018), 8 (8), 316/1-316/84CODEN: CRYSBC; ISSN:2073-4352. (MDPI AG)
      A review. Atomically thin molybdenum disulfide (MoS2), a member of the transition metal dichalcogenide (TMDC) family, has emerged as the prototypical two-dimensional (2D) semiconductor with a multitude of interesting properties and promising device applications spanning all realms of electronics and optoelectronics. While possessing inherent advantages over conventional bulk semiconducting materials (such as Si, Ge and III-Vs) in terms of enabling ultra-short channel and, thus, energy efficient field-effect transistors (FETs), the mech. flexible and transparent nature of MoS2 makes it even more attractive for use in ubiquitous flexible and transparent electronic systems. However, before the fascinating properties of MoS2 can be effectively harnessed and put to good use in practical and com. applications, several important technol. roadblocks pertaining to its contact, doping and mobility (μ) engineering must be overcome. This paper reviews the important technol. relevant properties of semiconducting 2D TMDCs followed by a discussion of the performance projections of, and the major engineering challenges that confront, 2D MoS2-based devices. Finally, this review provides a comprehensive overview of the various engineering solns. employed, thus far, to address the all-important issues of contact resistance (RC), controllable and area-selective doping, and charge carrier mobility enhancement in these devices. Several key exptl. and theor. results are cited to supplement the discussions and provide further insight.
    38. 38
      Acerce, M.; Voiry, D.; Chhowalla, M. Metallic 1T Phase MoS2 Nanosheets as Supercapacitor Electrode Materials. Nat. Nanotechnol. 2015, 10 (4), 313– 318,  DOI: 10.1038/nnano.2015.40
      38
      Metallic 1T phase MoS2 nanosheets as supercapacitor electrode materials
      Acerce, Muharrem; Voiry, Damien; Chhowalla, Manish
      Nature Nanotechnology (2015), 10 (4), 313-318CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      Efficient intercalation of ions in layered materials forms the basis of electrochem. energy storage devices such as batteries and capacitors. Recent research has focused on the exfoliation of layered materials and then restacking the two-dimensional exfoliated nanosheets to form electrodes with enhanced electrochem. response. Here, it is shown that chem. exfoliated nanosheets of MoS2 contg. a high concn. of the metallic 1T phase can electrochem. intercalate ions such as H+, Li+, Na+, and K+ with extraordinary efficiency and achieve capacitance values ranging from ∼400 to ∼700 F cm-3 in a variety of aq. electrolytes. It is also demonstrated that this material is suitable for high-voltage (3.5 V) operation in non-aq. org. electrolytes, showing prime volumetric energy and power d. values, coulombic efficiencies in excess of 95%, and stability over 5,000 cycles. As it is shown by X-ray diffraction anal., these favorable electrochem. properties of 1T MoS2 layers are mainly a result of their hydrophilicity and high elec. cond., as well as the ability of the exfoliated layers to dynamically expand and intercalate the various ions.
    39. 39
      Attanayake, N. H.; Thenuwara, A. C.; Patra, A.; Aulin, Y. V.; Tran, T. M.; Chakraborty, H.; Borguet, E.; Klein, M. L.; Perdew, J. P.; Strongin, D. R. Effect of Intercalated Metals on the Electrocatalytic Activity of 1T-MoS2 for the Hydrogen Evolution Reaction. ACS Energy Letters 2018, 3 (1), 7– 13,  DOI: 10.1021/acsenergylett.7b00865
      39
      Effect of Intercalated Metals on the Electrocatalytic Activity of 1T-MoS2 for the Hydrogen Evolution Reaction
      Attanayake, Nuwan H.; Thenuwara, Akila C.; Patra, Abhirup; Aulin, Yaroslav V.; Tran, Thi M.; Chakraborty, Himanshu; Borguet, Eric; Klein, Michael L.; Perdew, John P.; Strongin, Daniel R.
      ACS Energy Letters (2018), 3 (1), 7-13CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      We show that intercalation of cations (Na+, Ca2+, Ni2+, and Co2+) into the interlayer region of 1T-MoS2 is an effective strategy to lower the overpotential for the hydrogen evolution reaction (HER). In acidic media the onset potential for 1T-MoS2 with intercalated ions is lowered by ∼60 mV relative to that for pristine 1T-MoS2 (onset of ∼180 mV). D. functional theory (DFT) calcns. show a lowering in the Gibbs free energy for H-adsorption (ΔGH) on these intercalated structures relative to intercalant-free 1T-MoS2. The DFT calcns. suggest that Na+ intercalation results in a ΔGH close to zero. Consistent with calcn., expts. show that the intercalation of Na+ ions into the interlayer region of 1T-MoS2 results in the lowest overpotential for the HER.
    40. 40
      Yin, Y.; Han, J.; Zhang, Y.; Zhang, X.; Xu, P.; Yuan, Q.; Samad, L.; Wang, X.; Wang, Y.; Zhang, Z.; Zhang, P.; Cao, X.; Song, B.; Jin, S. Contributions of Phase, Sulfur Vacancies, and Edges to the Hydrogen Evolution Reaction Catalytic Activity of Porous Molybdenum Disulfide Nanosheets. J. Am. Chem. Soc. 2016, 138 (25), 7965– 7972,  DOI: 10.1021/jacs.6b03714
      40
      Contributions of Phase, Sulfur Vacancies, and Edges to the Hydrogen Evolution Reaction Catalytic Activity of Porous Molybdenum Disulfide Nanosheets
      Yin, Ying; Han, Jiecai; Zhang, Yumin; Zhang, Xinghong; Xu, Ping; Yuan, Quan; Samad, Leith; Wang, Xianjie; Wang, Yi; Zhang, Zhihua; Zhang, Peng; Cao, Xingzhong; Song, Bo; Jin, Song
      Journal of the American Chemical Society (2016), 138 (25), 7965-7972CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Molybdenum disulfide (MoS2) is a promising nonprecious catalyst for the hydrogen evolution reaction (HER) that has been extensively studied due to its excellent performance, but the lack of understanding of the factors that impact its catalytic activity hinders further design and enhancement of MoS2-based electrocatalysts. Here, by using novel porous (holey) metallic 1T phase MoS2 nanosheets synthesized by a liq.-ammonia-assisted lithiation route, we systematically investigated the contributions of crystal structure (phase), edges, and sulfur vacancies (S-vacancies) to the catalytic activity toward HER from five representative MoS2 nanosheet samples, including 2H and 1T phase, porous 2H and 1T phase, and sulfur-compensated porous 2H phase. Superior HER catalytic activity was achieved in the porous 1T phase MoS2 nanosheets that have even more edges and S-vacancies than conventional 1T phase MoS2. A comparative study revealed that the phase serves as the key role in detg. the HER performance, as 1T phase MoS2 always outperforms the corresponding 2H phase MoS2 samples, and that both edges and S-vacancies also contribute significantly to the catalytic activity in porous MoS2 samples. Then, using combined defect characterization techniques of ESR spectroscopy and positron annihilation lifetime spectroscopy to quantify the S-vacancies, the contributions of each factor were individually elucidated. This study presents new insights and opens up new avenues for designing electrocatalysts based on MoS2 or other layered materials with enhanced HER performance.
    41. 41
      Scanlon, D. O.; Watson, G. W.; Payne, D. J.; Atkinson, G. R.; Egdell, R. G.; Law, D. S. L. Theoretical and Experimental Study of the Electronic Structures of MoO3 and MoO2. J. Phys. Chem. C 2010, 114 (10), 4636– 4645,  DOI: 10.1021/jp9093172
      41
      Theoretical and Experimental Study of the Electronic Structures of MoO3 and MoO2
      Scanlon, David O.; Watson, Graeme W.; Payne, D. J.; Atkinson, G. R.; Egdell, R. G.; Law, D. S. L.
      Journal of Physical Chemistry C (2010), 114 (10), 4636-4645CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The geometric and electronic structures of MoO3 and MoO2 were calcd. using the generalized gradient approxn. to d. functional theory. The calcd. cross-section weighted densities of states were compared with high-resoln. X-ray photoemission spectra. There is very good agreement between the calcd. structures and those detd. previously by X-ray diffraction and between the computed densities of states and the present photoemission measurements. MoO2 is shown to be a metallic material, as is found exptl., but the Fermi level sits in a distinct trough in the d. of states. Satellite peaks found in core photoemission spectra of MoO2 were shown to derive from final state screening effects in this narrow band metallic material.
    42. 42
      Afanasiev, P.; Lorentz, C. Oxidation of Nanodispersed Mos2 in Ambient Air: The Products and the Mechanistic Steps. J. Phys. Chem. C 2019, 123 (12), 7486– 7494,  DOI: 10.1021/acs.jpcc.9b01682
      42
      Oxidation of Nanodispersed MoS2 in Ambient Air: The Products and the Mechanistic Steps
      Afanasiev, Pavel; Lorentz, Chantal
      Journal of Physical Chemistry C (2019), 123 (12), 7486-7494CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Oxidn. of finely divided MoS2 in ambient air was studied for air exposure times from 10 min to 1 yr to clarify the nature of the reaction products and the mechanistic steps. At the initial steps, for air exposure times from several minutes to several hours, rapid oxidn. of MoS2 edges occurs with simultaneous formation of hydroxyl species and surface disulfide S22- moieties as attested by 1H NMR, x-ray photoelectron spectra, and temp.-programmed redn. Prolonged air exposure of MoS2 nanodispersions leads to deep oxidn. According to the results of x-ray absorption spectroscopy, UV-visible, and ESR spectroscopies, the main oxidn. products are sol. paramagnetic Mo blue species and H2SO4. As shown by EXAS fitting, the major product is oxo-bridged dimolybdenyl Mo(V, VI) species. Ambient moisture plays an important role in the oxidn. process as it contributes to the formation of H2SO4 which leads to liquescence of the material and to deep oxidn. without formation of a protective passivation layer.
    43. 43
      Ziembowicz, S.; Kida, M.; Koszelnik, P. Sonochemical Formation of Hydrogen Peroxide. Proceedings 2018, 2 (5), 188,  DOI: 10.3390/ecws-2-04957
      There is no corresponding record for this reference.
    44. 44
      Riesz, P.; Kondo, T. Free Radical Formation Induced by Ultrasound and Its Biological Implications. Free Radical Biol. Med. 1992, 13 (3), 247– 270,  DOI: 10.1016/0891-5849(92)90021-8
      44
      Free radical formation induced by ultrasound and its biological implications
      Riesz, Peter; Kondo, Takashi
      Free Radical Biology & Medicine (1992), 13 (3), 247-70CODEN: FRBMEH; ISSN:0891-5849.
      A review with 197 refs. The chem. effects of ultrasound in aq. solns. are due to acoustic cavitation, which refers to the formation, growth, and collapse of small gas bubbles in liqs. The very high temps. (several thousand K) and pressures (several hundred atmospheres) of collapsing gas bubbles lead to the thermal dissocn. of water vapor into •OH radicals and •H atoms. Their formation has been confirmed by ESR and spin trapping. The sonochem. of aq. solns. of gases and of volatile and nonvolatile solutes is reviewed. The similarities and differences between sonochem. and radiation chem. of aq. solns. are explained. Some unusual characteristics of aq. sonochem. can be understood by considering the properties of supercrit. water. By the use of rare gases with different thermal conductivities, it is possible to distinguish between temp.-dependent processes such as redox reactions initiated by •OH radicals and •H atoms and pressure-dependent processes which lead to polymer degrdn. and cell lysis. The evidence for free radical formation in aq. solns. by pulsed ultrasound is discussed. This subject is of interest because it is related to the possible deleterious effects of ultrasonic diagnostic devices. The role of free radicals and of mech. effects induced by ultrasound in DNA degrdn., inactivation of enzymes, lipid peroxidn., and cell killing is reviewed.
    45. 45
      Hu, X. K.; Qian, Y. T.; Song, Z. T.; Huang, J. R.; Cao, R.; Xiao, J. Q. Comparative Study on MoO3 and HxMoO3 Nanobelts: Structure and Electric Transport. Chem. Mater. 2008, 20 (4), 1527– 1533,  DOI: 10.1021/cm702942y
      45
      Comparative Study on MoO3 and HxMoO3 Nanobelts: Structure and Electric Transport
      Hu, Xiao Kai; Qian, Yi Tai; Song, Z. T.; Huang, Jia Rui; Cao, R.; Xiao, John Q.
      Chemistry of Materials (2008), 20 (4), 1527-1533CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
      In this study, the suspension of MoO3 nanobelts was first prepd. in a hydrothermal way from Mo powders and H2O2 soln., which could be transformed into the suspension of HxMoO3 nanobelts under an acidic condition using N2H4·H2O as the reducing agent. Three paper-form samples made from MoO3 and HxMoO3 nanobelts (low or high hydrogen content) were then fabricated via a vacuum filtration method, followed by their structural comparative anal. such as FESEM, XRD, Raman spectra, and XPS, etc. The measurement of elec. resistances at room temp. shows that the conductance of HxMoO3 nanobelts is greatly improved because of hydrogen doping. The temp.-dependent resistances of HxMoO3 nanobelts agree with the exponential correlation, supporting that the conducting carriers are the quasi-free electrons released from Mo5+. In addn., the formation process of HxMoO3 nanobelts from MoO3 nanobelts is also discussed.
    46. 46
      Ou, J. Z.; Campbell, J. L.; Yao, D.; Wlodarski, W.; Kalantar-Zadeh, K. In Situ Raman Spectroscopy of H2 Gas Interaction with Layered MoO3. J. Phys. Chem. C 2011, 115 (21), 10757– 10763,  DOI: 10.1021/jp202123a
      46
      In Situ Raman Spectroscopy of H2 Gas Interaction with Layered MoO3
      Ou, Jian Zhen; Campbell, Jos L.; Yao, David; Wlodarski, Wojtek; Kalantar-zadeh, Kourosh
      Journal of Physical Chemistry C (2011), 115 (21), 10757-10763CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      It is known that the unique layered structure of orthorhombic MoO3 (α-MoO3) facilitates the interaction with H2 gas mols. and that the surface-to-vol. ratios of the crystallites play an important role in the process. MoO3 was deposited on a wide variety of transparent substrates using thermal evapn. in order to alter the surface-to-vol. ratios of the crystallites. In situ Raman spectroscopy was employed to investigate the interaction between MoO3 and 1% H2 in both N2 and synthetic air environments, while incorporating Pd as a catalyst at room temp. This study confirmed that the layered MoO3 with a high surface-to-vol. ratio facilitated the H2 gas interaction. The Raman spectroscopy studies revealed that the H+ ions mainly interacted with the doubly coordinated oxygen atoms and caused the crystal transformation from the original α-MoO3 into the mixed structure of hydrogen molybdenum bronze and substoichiometric MoO3, eventually forming oxygen vacancies and water. It was also found that the presence of O2 during the H2 gas exposure caused the recombination of a no. of oxygen vacancies and reduced the available surface catalytic sites for H2.
    47. 47
      Balendhran, S.; Deng, J.; Ou, J. Z.; Walia, S.; Scott, J.; Tang, J.; Wang, K. L.; Field, M. R.; Russo, S.; Zhuiykov, S.; Strano, M. S.; Medhekar, N.; Sriram, S.; Bhaskaran, M.; Kalantar-Zadeh, K. Enhanced Charge Carrier Mobility in Two-Dimensional High Dielectric Molybdenum Oxide. Adv. Mater. 2013, 25 (1), 109– 114,  DOI: 10.1002/adma.201203346
      47
      Enhanced Charge Carrier Mobility in Two-Dimensional High Dielectric Molybdenum Oxide
      Balendhran, Sivacarendran; Deng, Junkai; Ou, Jian Zhen; Walia, Sumeet; Scott, James; Tang, Jianshi; Wang, Kang L.; Field, Matthew R.; Russo, Salvy; Zhuiykov, Serge; Strano, Michael S.; Medhekar, Nikhil; Sriram, Sharath; Bhaskaran, Madhu; Kalantar-zadeh, Kourosh
      Advanced Materials (Weinheim, Germany) (2013), 25 (1), 109-114CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The authors propose that 2D semiconducting metal oxides with high dielec. const. (high/c) offer a soln. for obtaining high electron mobility. Advantageously, the electronic properties, in particular the bandgap, of such 2D metal oxides can be largely manipulated using well-known chem. and phys. approaches. Such manipulations, which impose their effects on the 2D environment, categorize these materials as excellent templates for achieving the optimum quantum parameters required for target applications. The charge mobility in a thin layer is calcd.
    48. 48
      Zhang, H.; Wang, H.; Yang, Y.; Hu, C.; Bai, Y.; Zhang, T.; Chen, W.; Yang, S. Hx moo3-Y Nanobelts: An Excellent Alternative to Carbon Electrodes for High Performance Mesoscopic Perovskite Solar Cells. J. Mater. Chem. A 2019, 7 (4), 1499– 1508,  DOI: 10.1039/C8TA10892G
      48
      HxMoO3-y nanobelts: an excellent alternative to carbon electrodes for high performance mesoscopic perovskite solar cells
      Zhang, Hua; Wang, Huan; Yang, Yinglong; Hu, Chen; Bai, Yang; Zhang, Teng; Chen, Wei; Yang, Shihe
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2019), 7 (4), 1499-1508CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Carbon-based hole-conductor-free perovskite solar cells (C-PSCs) are considered as a promising photovoltaic technol. toward commercialization, owing to their low cost and superior stability. However, efforts to further improve their efficiency have been hampered by the hole-extn. barrier at the Schottky contact between carbon and the perovskite. A conventional approach to resolve this issue is incorporating a hole-transport material (HTM) into the mesoscopic skeleton or chem. modification of the carbon electrode. Here, we show an innovative strategy that uses soln.-processed hydrogen molybdenum bronze (HxMoO3-y) nanobelts, an n-type HTM with high work function and elec. cond., as the sole electrode material to enhance the hole-extn. process and realize efficient PSCs for the first time. The mesoscopic cell configuration of FTO/c-TiO2/m-TiO2/m-Al2O3/HxMoO3-y with perovskite infiltration delivered a champion power conversion efficiency (PCE) of 14.5%, which compares favorably with 13.3% of typical high temp. C-PSCs. This increase in cell efficiency stems primarily from the enhancement in open circuit voltage and short circuit current, which is due to the HxMoO3-y electrode with more favorable energy alignment and higher hole-extn. ability than the carbon electrode. These results show the potential of HxMoO3-y nanobelts as an efficient electrode for realizing high-performance mesoscopic PSCs.
    49. 49
      Borgschulte, A.; Sambalova, O.; Delmelle, R.; Jenatsch, S.; Hany, R.; Nüesch, F. Hydrogen Reduction of Molybdenum Oxide at Room Temperature. Sci. Rep. 2017, 7 (1), 40761,  DOI: 10.1038/srep40761
      49
      Hydrogen reduction of molybdenum oxide at room temperature
      Borgschulte, Andreas; Sambalova, Olga; Delmelle, Renaud; Jenatsch, Sandra; Hany, Roland; Nuesch, Frank
      Scientific Reports (2017), 7 (), 40761CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
      The color changes in chemo- and photochromic MoO3 used in sensors and in org. photovoltaic (OPV) cells can be traced back to intercalated hydrogen atoms stemming either from gaseous hydrogen dissocd. at catalytic surfaces or from photocatalytically split water. In applications, the reversibility of the process is of utmost importance, and deterioration of the layer functionality due to side reactions is a crit. challenge. Using the membrane approach for high-pressure XPS, we are able to follow the hydrogen redn. of MoO3 thin films using at. hydrogen in a water free environment. Hydrogen intercalates into MoO3 forming HxMoO3, which slowly decomps. into MoO2 +1/2 H2O as evidenced by the fast redn. of Mo6+ into Mo5+ states and slow but simultaneous formation of Mo4+ states. We measure the decrease in oxygen/metal ratio in the thin film explaining the limited reversibility of hydrogen sensors based on transition metal oxides. The results also enlighten the recent debate on the mechanism of the high temp. hydrogen redn. of bulk molybdenum oxide. The specific mechanism is a result of the balance between the redn. by hydrogen and water formation, desorption of water as well as nucleation and growth of new phases.
    50. 50
      Yang, L.; Zhou, W.; Hou, D.; Zhou, K.; Li, G.; Tang, Z.; Li, L.; Chen, S. Porous Metallic MoO2-Supported MoS2 Nanosheets for Enhanced Electrocatalytic Activity in the Hydrogen Evolution Reaction. Nanoscale 2015, 7 (12), 5203– 5208,  DOI: 10.1039/C4NR06754A
      50
      Porous metallic MoO2-supported MoS2 nanosheets for enhanced electrocatalytic activity in the hydrogen evolution reaction
      Yang, Linjing; Zhou, Weijia; Hou, Dongman; Zhou, Kai; Li, Guoqiang; Tang, Zhenghua; Li, Ligui; Chen, Shaowei
      Nanoscale (2015), 7 (12), 5203-5208CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
      Advanced materials for electrocatalytic water splitting are central to renewable energy research. In this work, MoS2 nanosheets supported on porous metallic MoO2 (MoS2/MoO2) were produced by sulfuration treatments of porous and highly conductive MoO2 for the hydrogen evolution reaction. Porous MoO2 with one-dimensional channel-like structures was prepd. by calcination at elevated temps. using phosphomolybdic acid as the precursor and mesoporous silica (SBA-15) as the template, and the subsequent hydrothermal treatment in the presence of thioacetamide led to the transformation of the top layers to MoS2 forming MoS2/MoO2 composites. Electrochem. studies showed that the obtained composites exhibited excellent electrocatalytic activity for HER with an onset potential of -104 mV (vs. RHE), a large c.d. (10 mA cm-2 at -0.24 V), a small Tafel slope of 76.1 mV dec-1 and robust electrochem. durability. The performance might be ascribed to the high elec. cond. and porous structures of MoO2 with one-dimensional channels of 3 to 4 nm in diam. that allowed for fast charge transport and collection.
    51. 51
      Lee, C.; Yan, H.; Brus, L. E.; Heinz, T. F.; Hone, J.; Ryu, S. Anomalous Lattice Vibrations of Single- and Few-Layer MoS2. ACS Nano 2010, 4 (5), 2695– 2700,  DOI: 10.1021/nn1003937
      51
      Anomalous Lattice Vibrations of Single- and Few-Layer MoS2
      Lee, Changgu; Yan, Hugen; Brus, Louis E.; Heinz, Tony F.; Hone, James; Ryu, Sunmin
      ACS Nano (2010), 4 (5), 2695-2700CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Molybdenum disulfide (MoS2) of single- and few-layer thickness was exfoliated on SiO2/Si substrate and characterized by Raman spectroscopy. The no. of S-Mo-S layers of the samples was independently detd. by contact-mode at. force microscopy. Two Raman modes, E12g and A1g, exhibited sensitive thickness dependence, with the frequency of the former decreasing and that of the latter increasing with thickness. The results provide a convenient and reliable means for detg. layer thickness with at.-level precision. The opposite direction of the frequency shifts, which cannot be explained solely by van der Waals interlayer coupling, is attributed to Coulombic interactions and possible stacking-induced changes of the intralayer bonding. This work exemplifies the evolution of structural parameters in layered materials in changing from the three-dimensional to the two-dimensional regime.
    52. 52
      Li, H.; Zhang, Q.; Yap, C. C. R.; Tay, B. K.; Edwin, T. H. T.; Olivier, A.; Baillargeat, D. From Bulk to Monolayer MoS2: Evolution of Raman Scattering. Adv. Funct. Mater. 2012, 22 (7), 1385– 1390,  DOI: 10.1002/adfm.201102111
      52
      From Bulk to Monolayer MoS2: Evolution of Raman Scattering
      Li, Hong; Zhang, Qing; Yap, Chin Chong Ray; Tay, Beng Kang; Edwin, Teo Hang Tong; Olivier, Aurelien; Baillargeat, Dominique
      Advanced Functional Materials (2012), 22 (7), 1385-1390CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      MoS2 is systematically studied using Raman spectroscopy with UV and visible laser lines. Only the Raman frequencies of E2g1 and A1g peaks vary monotonously with the layer no. of ultrathin MoS2 flakes, while intensities or widths of the peaks vary arbitrarily. The coupling between electronic transitions and phonons become weaker when the layer no. of MoS2 decreases, attributed to the increased electronic transition energies or elongated intralayer at. bonds in ultrathin MoS2. The asym. Raman peak at 454 cm-1, which was regarded as the overtone of longitudinal optical M phonons in bulk MoS2, is actually a combinational band involving a longitudinal acoustic mode (LA(M)) and an optical mode (A2u). Findings suggest a clear evolution of the coupling between electronic transition and phonon when MoS2 is scaled down from 3- to 2-dimensional geometry.
    53. 53
      Dieterle, M.; Mestl, G. Raman Spectroscopy of Molybdenum Oxides Part II. Resonance Raman Spectroscopic Characterization of the Molybdenum Oxides Mo4O11 and MoO2. Phys. Chem. Chem. Phys. 2002, 4 (5), 822– 826,  DOI: 10.1039/b107046k
      53
      Raman spectroscopy of molybdenum oxides. Part II. Resonance Raman spectroscopic characterization of the molybdenum oxides Mo4O11 and MoO2
      Dieterle, M.; Mestl, G.
      Physical Chemistry Chemical Physics (2002), 4 (5), 822-826CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      A special sample was prepd. by controlled oxidn. of MoO2, which contained MoO2, Mo4O11 and MoO3, to extend the knowledge about the resonance Raman effect in reduced Mo oxides from those close to MoO3 to those close to MoO2. This knowledge is of paramount importance because tech. partial oxidn. catalysts often contain intermediate Mo oxides of the Magneli type, e.g. Mo4O11, or Mo5O14. The Raman spectra of orthorhombic Mo4O11 and MoO2 were identified in a Raman microspectroscopic image of 100 single spectra recorded of a mixt. of MoO3, MoO2 and Mo4O11. A resonance Raman effect was proven to be responsible for the detection of the Mo oxide phases Mo4O11 and MoO2 in diln. with BN when excited at a laser wavelength of 632.8 nm by comparison with Raman microspectroscopic images of the identical sample when excited at 532 nm. The resonance Raman detection of reduced Mo oxide phases is discussed in the above mentioned context of their active role in catalytic partial oxidn. reactions.
    54. 54
      Zhang, Q.; Li, X.; Ma, Q.; Zhang, Q.; Bai, H.; Yi, W.; Liu, J.; Han, J.; Xi, G. A Metallic Molybdenum Dioxide with High Stability for Surface Enhanced Raman Spectroscopy. Nat. Commun. 2017, 8 (1), 14903,  DOI: 10.1038/ncomms14903
      54
      A metallic molybdenum dioxide with high stability for surface enhanced Raman spectroscopy
      Zhang Qiqi; Li Xinshi; Ma Qiang; Zhang Qing; Bai Hua; Han Jing; Xi Guangcheng; Zhang Qiqi; Yi Wencai; Liu Jingyao; Han Jing; Xi Guangcheng
      Nature communications (2017), 8 (), 14903 ISSN:.
      Compared with noble metals, semiconductors with surface plasmon resonance effect are another type of SERS substrate materials. The main obstacles so far are that the semiconducting materials are often unstable and easy to be further oxidized or decomposed by laser irradiating or contacting with corrosive substances. Here, we report that metallic MoO2 can be used as a SERS substrate to detect trace amounts of highly risk chemicals including bisphenol A (BPA), dichloropheno (DCP), pentachlorophenol (PCP) and so on. The minimum detectable concentration was 10(-7) M and the maximum enhancement factor is up to 3.75 × 10(6). To the best of our knowledge, it may be the best among the metal oxides and even reaches or approaches to Au/Ag. The MoO2 shows an unexpected high oxidation resistance, which can even withstand 300 °C in air without further oxidation. The MoO2 material also can resist long etching of strong acid and alkali.
    55. 55
      Castner, D. G.; Hinds, K.; Grainger, D. W. X-Ray Photoelectron Spectroscopy Sulfur 2p Study of Organic Thiol and Disulfide Binding Interactions with Gold Surfaces. Langmuir 1996, 12 (21), 5083– 5086,  DOI: 10.1021/la960465w
      55
      X-ray Photoelectron Spectroscopy Sulfur 2p Study of Organic Thiol and Disulfide Binding Interactions with Gold Surfaces
      Castner, David G.; Hinds, Kenneth; Grainger, David W.
      Langmuir (1996), 12 (21), 5083-5086CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The presence of 2 S species was detected in XPS studies of thiol and disulfide mols. adsorbed on Au surfaces. These species are assigned to bound thiolate (S2p3/2 binding energy 162 eV) and unbound thiol/disulfide (S2p3/2 binding energy from 163.5 to 164 eV). These assignments are consistent with XPS data obtained from different thiols (C12, C16, C18, and C22 alkane thiols, a fluorinated thiol, and a cyclic siloxanethiol) and different adsorption conditions (solvent type, thiol concn., temp., and rinsing). In particular, the use of a poor solvent for thiol adsorption solns. (e.g., EtOH for long chain alkanethiols) and the lack of a rinsing step both resulted in unbound thiol mols. present at the surface of the bound thiolate monolayer. This has implications for recent studies asserting the presence of multiple binding sites for Au-thiolate species in org. monolayers.
    56. 56
      Moonoosawmy, K. R.; Kruse, P. To Dope or Not to Dope: The Effect of Sonicating Single-Wall Carbon Nanotubes in Common Laboratory Solvents on Their Electronic Structure. J. Am. Chem. Soc. 2008, 130 (40), 13417– 13424,  DOI: 10.1021/ja8036788
      56
      To Dope or Not To Dope: The Effect of Sonicating Single-Wall Carbon Nanotubes in Common Laboratory Solvents on Their Electronic Structure
      Moonoosawmy, Kevin R.; Kruse, Peter
      Journal of the American Chemical Society (2008), 130 (40), 13417-13424CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Single-wall carbon nanotubes (SWCNTs) are commonly dispersed via sonication in a solvent prior to functionalization. Solvents such as dichloromethane, chloroform, 1,2-dichloroethane, and o-dichlorobenzene lead to an upward shift in the Raman response of the SWCNTs. The authors used o-dichlorobenzene as a model mol. to explain this effect, and an upward shift of 9 cm-1 is obsd. in the D* band. This blue shift is assocd. with p-type doping and is triggered only when the nanotubes are sonicated in the solvent. Sonication decomps. the chlorinated solvents, and (Cl2 and HCl(g)) are formed. The catalytic Fe nanoparticles inherently present in the nanotubes are etched by chlorine and hydrogen chloride to form iron chlorides during sonication in the solvent. The dopant was identified by XPS. With such knowledge of doping, the choice of solvent becomes crucial for any chem. reaction and can be intentionally tuned to produce SWCNTs films for electronics applications.
    57. 57
      Graf, N.; Yegen, E.; Gross, T.; Lippitz, A.; Weigel, W.; Krakert, S.; Terfort, A.; Unger, W. E. S. XPS and NEXAFS Studies of Aliphatic and Aromatic Amine Species on Functionalized Surfaces. Surf. Sci. 2009, 603 (18), 2849– 2860,  DOI: 10.1016/j.susc.2009.07.029
      57
      XPS and NEXAFS studies of aliphatic and aromatic amine species on functionalized surfaces
      Graf, Nora; Yegen, Eda; Gross, Thomas; Lippitz, Andreas; Weigel, Wilfried; Krakert, Simone; Terfort, Andreas; Unger, Wolfgang E. S.
      Surface Science (2009), 603 (18), 2849-2860CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
      The chem. constitution of functionalized supports is an important parameter that dets. their performance in a broad range of applications, e.g. for immobilization of biomols. Supports with amino functionalized surfaces are also often used for DNA microarray expts. However, spectral data which were reported for surfaces with amino functionalities suffer from some inconsistencies. A detailed XPS and NEXAFS (Near edge x-ray absorption fine structure) database for amino functionalized surfaces is presented. Amino-terminated surfaces prepd. from aliph. and arom. aminosilanes or aminothiols and a field sample are considered. Effects of aging in air and damage by radiation are addressed as well.
    58. 58
      Kruse, P. Review on Water Quality Sensors. J. Phys. D: Appl. Phys. 2018, 51 (20), 203002,  DOI: 10.1088/1361-6463/aabb93
      58
      Review on water quality sensors
      Kruse, Peter
      Journal of Physics D: Applied Physics (2018), 51 (20), 203002/1-203002/25CODEN: JPAPBE; ISSN:0022-3727. (IOP Publishing Ltd.)
      Terrestrial life may be carbon-based, but most of its mass is made up of water. Access to clean water is essential to all aspects of maintaining life. Mainly due to human activity, the strain on the water resources of our planet has increased substantially, requiring action in water management and purifn. Water quality sensors are needed in order to quantify the problem and verify the success of remedial actions. This review summarizes the most common chem. water quality parameters, and current developments in sensor technol. available to monitor them. Particular emphasis is on technologies that lend themselves to reagent-free, low-maintenance, autonomous and continuous monitoring. Chemiresistors and other elec. sensors are discussed in particular detail, while mech., optical and electrochem. sensors also find mentioning. The focus here is on the physics of chem. signal transduction in sensor elements that are in direct contact with the analyte. All other sensing methods, and all other elements of sampling, sample pre-treatment as well as the collection, transmission and anal. of the data are not discussed here. Instead, the goal is to highlight the progress and remaining challenges in the development of sensor materials and designs for an audience of physicists and materials scientists.
    59. 59
      Zubiarrain-Laserna, A.; Kruse, P. Review─Graphene-Based Water Quality Sensors. J. Electrochem. Soc. 2020, 167 (3), 037539,  DOI: 10.1149/1945-7111/ab67a5
      59
      Review of graphene based water quality sensors
      Zubiarrain-Laserna, Ana; Kruse, Peter
      Journal of the Electrochemical Society (2020), 167 (3), 037539CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      A review. Water is fundamental to most aspects of civilization and terrestrial life in general. The problem of deteriorating water quality is very real, but often hard to quantify for lack of data. Hence the development of water quality sensors has become an urgently important area of research. Here we summarize an emerging class of water quality sensors based on field effect or chemiresistive geometries, which work completely in the solid state and can operate without ref. electrodes. Such devices are candidates for continuous online monitoring applications of surface, ground, drinking, process, and wastewater streams. Single layer and few layer graphenes are suitable materials for the sensing channels in these devices due to their chem. and mech. robustness and favorable electronic properties. While single layer graphene devices are more sensitive, few layer graphene sensors are easier to manuf. at a lower cost and offer a wider dynamic range. Detection of pH, disinfectants, mercury, lead, chromium, arsenic, potassium, calcium, some anions, as well as org. and biol. species has all been demonstrated at the proof of concept stage, with much more work in progress. One can anticipate the com. availability of such devices in the near future.
    60. 60
      Grahame, D. C. The Electrical Double Layer and the Theory of Electrocapillarity. Chem. Rev. 1947, 41 (3), 441– 501,  DOI: 10.1021/cr60130a002
      60
      The electrical double layer and the theory of electro-capillarity
      Grahame, David C.
      Chemical Reviews (Washington, DC, United States) (1947), 41 (), 441-501CODEN: CHREAY; ISSN:0009-2665.
      A review with 95 references.
    61. 61
      Zafir Mohamad Nasir, M.; Sofer, Z.; Pumera, M. Effect of Electrolyte Ph on the Inherent Electrochemistry of Layered Transition-Metal Dichalcogenides (MoS2, MoSe2, WS2, WSe2). ChemElectroChem 2015, 2 (11), 1713– 1718,  DOI: 10.1002/celc.201500259
      There is no corresponding record for this reference.
    62. 62
      Nishimoto, M.; Muto, I.; Sugawara, Y.; Hara, N. Morphological Characteristics of Trenching around Mns Inclusions in Type 316 stainless Steel: The Role of Molybdenum in Pitting Corrosion Resistance. J. Electrochem. Soc. 2019, 166 (11), C3081– C3089,  DOI: 10.1149/2.0131911jes
      62
      Morphological characteristics of trenching around MnS inclusions in Type 316 stainless steel: the role of Molybdenum in pitting corrosion resistance
      Nishimoto, Masashi; Muto, Izumi; Sugawara, Yu; Hara, Nobuyoshi
      Journal of the Electrochemical Society (2019), 166 (11), C3081-C3089CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      The role of Mo alloying in the pit initiation process at MnS inclusions in stainless steels was investigated by measuring polarization curves, depassivation pH, and SEM. Molybdenum existed in the steel matrix but was not detected in the MnS inclusions in Type 316 stainless steel. While Mo alloying did not inhibit the dissoln. of the MnS inclusions in a NaCl soln., neither a metastable nor a stable pit occurred in the small areas with the MnS inclusions in the Mo-added specimen. The MnS/steel matrix boundary preferentially dissolved in the Mo-free specimen, with deep trenches formed. However, no deep trench was obsd. in the Mo-added specimen at low potentials. The depassivation pH of the Mo-added specimen in a 0.1 M NaCl-1 mM Na2S2O3 soln. was lower than that of the Mo-free specimen, suggesting that Mo alloying prevents the depassivation of the steel matrix at the boundary and inhibits the formation of trenches. In a 1 M HCl-1 mM Na2S2O3 soln., the active dissoln. rate of the steel was suppressed by Mo alloying. This suggests that, even after trenching at high potentials, Mo alloying inhibits the initiation of pitting inside the trenches.
    63. 63
      Schulman, D. S.; May-Rawding, D.; Zhang, F.; Buzzell, D.; Alem, N.; Das, S. Superior Electro-Oxidation and Corrosion Resistance of Monolayer Transition Metal Disulfides. ACS Appl. Mater. Interfaces 2018, 10 (4), 4285– 4294,  DOI: 10.1021/acsami.7b17660
      63
      Superior Electro-Oxidation and Corrosion Resistance of Monolayer Transition Metal Disulfides
      Schulman, Daniel S.; May-Rawding, Dan; Zhang, Fu; Buzzell, Drew; Alem, Nasim; Das, Saptarshi
      ACS Applied Materials & Interfaces (2018), 10 (4), 4285-4294CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Physics of monolayer and few-layer transition metal dichalcogenides (TMDs) and chem. of few-layer TMDs have been well studied in recent years in the context of future electronic, optoelectronic, and energy harvesting applications. However, what has escaped the attention of the scientific community is the unique chem. of monolayer TMDs. It has been demonstrated that the basal plane of multilayer TMDs is chem. inert, whereas edge sites are chem. active. The authors exptl. demonstrate that the edge reactivity of the TMDs can be significantly impeded at the monolayer limit through monolayer/substrate interaction, thus making the monolayers highly resistant to electrooxidn. and corrosion. Few-layer flakes of MoS2 and WS2 exfoliated on conductive TiN substrates are readily corroded beyond a certain pos. electrode potential, while monolayer remnants are left behind unscathed. The electrooxidn. resistance of monolayers was confirmed using a plethora of characterization techniques including at. force microscope (AFM) imaging, Raman spectroscopy, photoluminescence (PL) mapping, scanning/transmission electron microscope (S/TEM) imaging, and selected area electron diffraction (SAED). It is believed that strong substrate monolayer interaction compared to the relatively weak interlayer van der Waals interaction is responsible for the superior monolayers chem. stability in highly corrosive oxidizing environments.
    64. 64
      Mohtasebi, A.; Kruse, P. Chemical Sensors Based on Surface Charge Transfer. Phys. Sci. Rev. 2018, 3 (2), 20170133,  DOI: 10.1515/psr-2017-0133
      There is no corresponding record for this reference.
    65. 65
      Wan, H.; Xu, L.; Huang, W.-Q.; Zhou, J.-H.; He, C.-N.; Li, X.; Huang, G.-F.; Peng, P.; Zhou, Z.-G. Band Structure Engineering of Monolayer MoS2: A Charge Compensated Codoping Strategy. RSC Adv. 2015, 5 (11), 7944– 7952,  DOI: 10.1039/C4RA12498G
      65
      Band structure engineering of monolayer MoS2: a charge compensated codoping strategy
      Wan, Hui; Xu, Liang; Huang, Wei-Qing; Zhou, Jia-Hui; He, Chao-Ni; Li, Xiaofan; Huang, Gui-Fang; Peng, P.; Zhou, Zheng-Gui
      RSC Advances (2015), 5 (11), 7944-7952CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      The monolayer MoS2, possessing an advantage over graphene in that it exhibits a band gap whose magnitude is appropriate for solar applications, has attracted increasing attention because of its possible use as a photocatalyst. Herein, we propose a codoping strategy to tune the band structure of monolayer MoS2 aimed at enhancing its photocatalytic activity using first-principles calcn. The monodoping (halogen element, Nd) introduces impurity states in the gap, thus decreasing the photocatalytic activity of MoS2. Interestingly, the NbMoFS codoping reduces the energy cost of doping as a consequence of the charge compensation between the niobium (p-dopant) and the fluorine (n-dopant) impurities, which eliminates the isolated levels (induced by monodopant) in the band gap. Most importantly, the NbMoFS codoped MoS2 has more active sites for photocatalysis. These results show the proposed NbMoFS codoped monolayer MoS2 is a promising photocatalyst or photosensitizer for visible light in the heterogeneous semiconductor systems.
    66. 66
      Donarelli, M.; Ottaviano, L. 2d Materials for Gas Sensing Applications: A Review on Graphene Oxide, MoS2, WS2 and Phosphorene. Sensors 2018, 18 (11), 3638,  DOI: 10.3390/s18113638
      66
      2D materials for gas sensing applications: a review on graphene oxide, MoS2, WS2 and phosphorene
      Donarelli, Maurizio; Ottaviano, Luca
      Sensors (2018), 18 (11), 3638/1-3638/45CODEN: SENSC9; ISSN:1424-8220. (MDPI AG)
      A review. After the synthesis of graphene, in the first year of this century, a wide research field on two-dimensional materials opens. 2D materials are characterized by an intrinsic high surface to vol. ratio, due to their heights of few atoms, and, differently from graphene, which is a semimetal with zero or near zero bandgap, they usually have a semiconductive nature. These two characteristics make them promising candidate for a new generation of gas sensing devices. Graphene oxide, being an intermediate product of graphene fabrication, has been the first graphene-like material studied and used to detect target gases, followed by MoS2, in the first years of 2010s. Along with MoS2, which is now experiencing a new birth, after its use as a lubricant, other sulfides and selenides (like WS2, WSe2, MoSe2, etc.) have been used for the fabrication of nanoelectronic devices and for gas sensing applications. All these materials show a bandgap, tunable with the no. of layers. On the other hand, 2D materials constituted by one at. species have been synthesized, like phosphorene (one layer of black phosphorous), germanene (one atom thick layer of germanium) and silicone (one atom thick layer of silicon). In this paper, a comprehensive review of 2D materials-based gas sensor is reported, mainly focused on the recent developments of graphene oxide, exfoliated MoS2 and WS2 and phosphorene, for gas detection applications. We will report on their use as sensitive materials for conductometric, capacitive and optical gas sensors, the state of the art and future perspectives.
    67. 67
      Bazylewski, P.; Van Middelkoop, S.; Divigalpitiya, R.; Fanchini, G. Solid-State Chemiresistors from Two-Dimensional Mos2 Nanosheets Functionalized with L-Cysteine for in-Line Sensing of Part-Per-Billion Cd2+ Ions in Drinking Water. ACS Omega 2020, 5 (1), 643– 649,  DOI: 10.1021/acsomega.9b03246
      67
      Solid-State Chemiresistors from Two-Dimensional MoS2 Nanosheets Functionalized with L-Cysteine for In-Line Sensing of Part-Per-Billion Cd2+ Ions in Drinking Water
      Bazylewski, Paul; Van Middelkoop, Sheldon; Divigalpitiya, Ranjith; Fanchini, Giovanni
      ACS Omega (2020), 5 (1), 643-649CODEN: ACSODF; ISSN:2470-1343. (American Chemical Society)
      Sensing of metal contaminants at ultralow concns. in aq. environments is vital in today's overpopulated world, with an extremely stringent limit (<5 ppb) for Cd2+ ions in drinking water. Here, we utilize sonochem. exfoliated molybdenum disulfide (MoS2) nanosheets functionalized with L-cysteine (Cys) as highly sensitive and selective two-dimensional (2D) materials for solid-state chemiresistors. We specifically targeted Cd2+ ions due to their high toxicity at low concns. MoS2-Cys nanosheets are fabricated using an ad hoc, low-complexity, one-pot synthesis method. Porous MoS2-Cys thin films with a high surface area are assembled from these nanosheets. Two-terminal chemiresistors incorporating MoS2-Cys films are demonstrated to be preferentially sensitive to Cd2+ ions at neutral pH, irresp. of other metal ions present in water flowing through the device. A 5 ppb concn. of the Cd2+ ions in the water stream increases the device resistivity by 20 times. Our devices operate at broad (1-500 ppb) range and fast (∼1 s) response times. Cd2+ is selectively detected because of preferential, size-driven adsorption at the interstitials between L-cysteine functional groups, combined with pH-controlled charge transfer that removes electronic gap states from MoS2. MoS2-Cys-based chemiresistors can be deployed in-line to detect metal ions without any need for addnl. offline measurements.

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