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Atomic Structure and Electronic Properties of Janus SeMoS Monolayers on Au(111)
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Cite this: Nano Lett. 2025, 25, 8, 3330–3336
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https://doi.org/10.1021/acs.nanolett.4c06543
Published February 18, 2025

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Janus SeMoS monolayers (MLs) are synthetic 2D materials with unique electronic properties, as theory predicts, but their experimental exploration has been hindered by the low quality of the samples. Here we report a synthesis of high-quality Janus MLs on gold substrates by thermal exchange reaction taking place at the ML/Au(111) interface. The synthesized Janus SeMoS MLs were characterized by complementary techniques, and insights into the topography and electronic properties of the system were obtained. Specifically, due to the lattice mismatch with the Au(111), a moiré pattern with a periodicity of 2.9 nm was observed. A precise experimental determination of the lattice constant of Janus SeMoS of 3.22 ± 0.01 Å was obtained, and the measured spin–orbit splitting at the K point of the valence band was found to be 170 ± 15 meV, matching well the results of the density functional theory calculations.

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Transition metal dichalcogenide (TMD) monolayers (MLs) including 1H MoS2, MoSe2, WS2, WSe2, etc. are two-dimensional (2D) materials with unique electronic and photonic properties continuously attracting a significant research interest. (1,2) Recently, a new class of TMD MLs, the so-called Janus TMDs, came into the focus of research. (3−6) In contrast to the conventional TMD MLs, in which transition metal atoms are sandwiched between chemically similar chalcogens, in Janus TMD MLs the chalcogen atoms differ between the different faces of a ML. This asymmetry results in the broken out-of-plane mirror symmetry and leads to an intrinsic dipole which may cause new physical properties. (7) Numerous theoretical studies of Janus TMDs predict strong Rashba splitting, (8,9) piezoelectric, (10,11) catalytic, (12) novel excitonic, (13) valleytronic (14) and spintronic phenomena, (15) which have to be proven experimentally yet. Experimental studies of Janus TMD MLs are still limited (see, e.g., refs (16−26)). Unlike conventional TMD MLs, which can be exfoliated from bulk crystals, these nanomaterials can only be obtained by bottom-up synthetic approaches, which limit their availability to the experimentalists. The reported synthesis methods are often based on complex methodologies including stripping of the topmost chalcogen layer of TMD ML by laser pulses, (18,26) H2 plasma (19) and refilling of the formed vacancies with chalcogen atoms of the second type. Such approaches typically result in a high defect density in the formed Janus TMD MLs, which complicates the revealing of the theoretically predicted properties. Beyond Janus TMD monolayers several other Janus materials with distinct electronic properties have been studied so far including Janus IV–VI structures, (27,28) Janus transition metal trichalcogenide monolayers, (29) Janus transition metal dichalcogenide oxides, (30) or Janus transition metal oxyhalides. (31)

Recently, a method for the large-area synthesis of high-quality Janus SeMoS MLs was introduced, which is based on the exchange of the chalcogen atoms of the parent TMD ML at the ML/substrate interface via intercalation of chalcogen atoms of the second type. (16) In this method, a MoSe2 ML is first grown on gold foils by chemical vapor deposition (CVD). Next, this monolayer is exposed to sulfur vapor at elevated temperatures. Due to the high affinity of sulfur to gold, sulfur atoms can intercalate between the MoSe2 ML/gold interface leading to an exchange of the bottom Se layer by S and the formation of a SeMoS ML, as schematically depicted in Figure 1a. The quality of the formed Janus SeMoS MLs is characterized at low temperatures by a narrow photoluminescence line width of only 18 meV, high circular polarization of the excitonic emission as well as by the valley Zeeman splitting. (16)

Figure 1

Figure 1. (a) Cross section of Janus SeMoS ML on Au(111) based on DFT calculations. Selenium, molybdenum, and sulfur atoms are highlighted in orange, blue, and yellow, respectively. The gold substrate atoms are represented as larger balls and colored according to their depth. (b) Raman spectrum (excitation wavelength, 532 nm) and (c) Mo 3d/S 2s/Se 3s (left), S 2p/Se 3p (middle), and Se 3d/Mo 4s/Au 5p (right) XP spectral regions of Janus SeMoS on Au(111) recorded at an emission angle of 0° (top) and 70° (bottom). Mo, S, Se, and Au peaks are highlighted in blue, green, orange, and yellow, respectively.

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Here we present the structural and electronic characterization down to the nanoscale of the grown Janus SeMoS MLs on Au(111). Due to the lattice mismatch between the Janus SeMoS and Au(111), we observe a moiré superstructure by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) and precisely determine the lattice parameters of the Janus SeMoS from the LEED pattern. Further, we measure the band structure by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and extract the value of the spin–orbit splitting of the valence band at the K point. To the best of our knowledge, such structural and electronic properties of the Janus TMD MLs are obtained experimentally for the first time. Our findings are backed by density functional theory (DFT) calculations on the SeMoS/Au(111) system.

The synthesis of Janus SeMoS MLs on Au(111) starts with the growth of MoSe2 MLs on Au(111) by ambient pressure CVD. Afterward, the as-grown MoSe2 MLs were sulfurized at 700 °C to exchange the Se layer at the Au(111) interface with S atoms. Details regarding the preparation of Janus SeMoS are given in the Experimental Section. To prove the formation of Janus SeMoS on Au(111) we carried out Raman spectroscopy measurements at ambient conditions. The spectrum shown in Figure 1b reveals a characteristic peak at ∼290 cm–1 corresponding to the A11 mode of the Janus SeMoS. (16,32)

Next, we introduced the sample into a UHV chamber and studied it by angle-resolved X-ray photoelectron spectroscopy (XPS) measurements. The relevant XP spectra measured with an emission angle of 0° (top) and 70° (bottom) are shown in Figure 1c. A detailed peak analysis is presented in Table S1. First, we focus on the XP spectra measured at normal emission. In the Mo 3d region we observe a sharp, high intensity doublet at binding energies (BEs) of 228.9 ± 0.1 eV (Mo 3d5/2) and 232.0 ± 0.1 eV (Mo 3d3/2), which is due to the formed Janus SeMoS ML (see Figure 1c, left). The other low intensity Mo doublets, at BEs of 229.3 ± 0.1 eV/232.5 ± 0.1 eV and 231.0 ± 0.2 eV/234.2 ± 0.2 eV, originate from remaining nonconverted MoSe2 and MoO3, respectively. Since the intensity of the Janus SeMoS features is about 4 times higher than that of the MoSe2 features, we conclude that about 80% of the as-grown MoSe2 is converted into Janus SeMoS. In the BE region of 171–156 eV (see Figure 1c, middle), dominated by the Se 3p peaks, a high intensity S 2p doublet at the BEs of 162.2 ± 0.1 eV (S 2p3/2) and 163.4 ± 0.1 eV (S 2p1/2), corresponding to the formed Janus SeMoS ML, is clearly recognized, which is accompanied by an additional low intensity S 2p doublet at a higher BE resulting most probably from excess unreacted sulfur clusters. (16,33) In comparison to the S 2p peaks, the Se 3p peaks have a larger spin–orbit splitting and a broader full width at half-maximum. The Se species result also in the 3d doublet at BEs of 54.5 ± 0.1 eV (Se 3d5/2) and 55.4 ± 0.1 eV (Se 3d3/2) (see Figure 1c, right). In comparison to the Mo 3d spectrum, one cannot discriminate Se in the Janus SeMoS and MoSe2 MLs in this spectrum. However, if we consider the SeMoS to MoSe2 ratio obtained from the Mo 3d spectrum, we can estimate the amount of Se related to the SeMoS ML to be ∼70%. This estimation results in an elemental ratio for Se/Mo/S of (1.4 ± 0.3):1.0:(1.2 ± 0.2), which is in good agreement with the expected one of 1:1:1. The presence of the Se atoms on top and of the S atoms on the bottom of the SeMoS ML is further confirmed by the angular dependence XP intensities. Due to the surface sensitivity of XPS, as seen in Figure 1c and Table S1, the intensity ratio of Se/S is higher for an emission angle of 70° compared to normal emission (0°). These results conclusively confirm the formation of the SeMoS ML with its S face oriented toward the Au(111) substrate.

Next, we characterize the structural properties of the Janus SeMoS ML on Au(111) by LEED measurements. The distortion-corrected LEED pattern presented in Figure 2a shows a 6-fold symmetry. The reciprocal unit cells and the LEED spots of the Janus SeMoS and Au(111) are marked in cyan and red, respectively. The first-order spots of the Janus SeMoS reveal the highest intensity. Around each of these spots, six further spots are visible. As we demonstrate below, such a LEED pattern arises from a moiré structure, due to the lattice mismatch between the Janus SeMoS ML and the Au(111) substrate. Note that the formation of moiré structures is also observed for MoS2 and MoSe2 on Au(111). (34) From the distortion-corrected LEED pattern, we precisely determined the lattice parameters of Janus SeMoS MLs on Au(111). For this quantitative analysis, we fitted all visible spots considering multiple scattering with the hexagonal structure of Au(111) with a lattice constant of 2.884 Å (35) as a reference. The obtained lattice constant, aSeMoS, and the enclosing angle between both lattice vectors ∠(a⃗1,a⃗2) are 3.22 ± 0.01 Å and 120.00 ± 0.02°, respectively. Interestingly, the value of aSeMoS is between the experimental values for MoS2 (3.15 ± 0.01 Å) and MoSe2 (3.28 ± 0.01 Å) MLs on Au(111) which were determined in the same way previously. (36) The optimized freestanding SeMoS ML structure (without Au(111) substrate) calculated by DFT results in the lattice parameters of |a⃗1| = |a⃗2| = 3.22 Å = aSeMoS and ∠(a⃗1,a⃗2) = 120° which are in a perfect agreement with our experimental results and previous theoretical reports. (37) We further compared the values of the Janus SeMoS ML with the DFT-calculated values of freestanding MoS2 and MoSe2 MLs (see Table S2). Therewith, the optimized lattice constant of SeMoS (3.22 Å) lies between the values of MoS2 (3.15 Å) and MoSe2 (3.28 Å) and matches the experimental values obtained from LEED analysis very well. In the Janus ML, the Mo–S bond distances become longer by 0.01 Å than the corresponding bonds in a MoS2 ML, whereas the Mo–Se bond gets shorter by 0.01 Å than the corresponding bonds in MoSe2 monolayers. At the same time, the angle of Se–Mo–S bonds falls between those values obtained for the pristine materials. We calculated atomic Bader charges and averaged the electrostatic profile perpendicular to the Janus monolayer and compared it with that in the MoS2 and MoSe2 monolayer (Figures S1, S2 and Table S3). The Mo atoms became more positive in the SeMoS ML compared with MoS2, while the S atoms became more negative. On the other hand, the Se atoms have less negative charge than those in a MoSe2 ML. This leads to a higher charge density around the S atoms than the Se atoms as shown in Figure S1b. The electrostatic potential variation at different chalcogen layers is consistent with the charge transfer from the Mo to S or Se atoms. These results suggest a higher polarization in the Janus material in comparison with the pristine MoS2 and MoSe2 MLs.

Figure 2

Figure 2. (a) LEED pattern of a Janus SeMoS ML on Au(111) (118 eV, 293 K). The unit cell and the corresponding LEED spots of the Janus SeMoS and Au(111) are highlighted in light blue and red, respectively. (b) Top view of an atomic ball model of the Janus SeMoS ML on Au(111). Due to the different adsorption sites of the sulfur layer with respect to the topmost Au layer, a moiré contrast appears. We distinguish three different regions which are discussed in the text. (c) STM (−1.1 V, 0.5 nA, 4.2 K) and (d) simulated STM images of a Janus SeMoS ML on Au(111). A fast Fourier transform (FFT) of the STM image in panel c is shown in the inset (scale bar ≈ 1 Å–1).

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In Figure 3, we compare the structural models of MoS2, Janus SeMoS, and MoSe2 on Au(111). We simulated the lattices of MoS2 (blue), Janus SeMoS (cyan) and MoSe2 (green) and presented them with respect to the lattice of Au(111) (red). For the simulation, we used the following lattice constants obtained from LEED analysis: 3.15 Å (36) (MoS2), 3.22 Å (Janus SeMoS), and 3.28 Å (36) (MoSe2). These values lie within the uncertainties of the LEED analyses. The lattice constant of Au(111) is 2.884 Å. (35) Due to the lattice mismatches between TMDs and Au(111) we observe different moiré structures. Although the lattice constants of the TMDs differ only slightly from each other, we observe more distinct differences in the corresponding moiré patterns, which allows for a more precise determination of the lattice constants. The moiré lattice constants are 34.6 Å for MoS2/Au(111), 28.9 Å for Janus SeMoS/Au(111), and 23.1 Å for MoSe2/Au(111). Thereby, an (11 × 11) MoS2 supercell matches very well a (12 × 12) Au(111) supercell. (38) A (9 × 9) Janus SeMoS supercell matches very well a (10 × 10) Au(111) supercell. The same is true for a (7 × 7) MoSe2 and an (8 × 8) Au(111) supercell. In addition, we optimized the structure of the Janus SeMoS ML on Au(111) using DFT and extracted a moiré periodicity of 28.99 Å. This result is in excellent agreement with our estimated moiré periodicity based on LEED data.

Figure 3

Figure 3. Representation of the lattices of (a) MoS2 (3.15 Å, (36) blue), (b) Janus SeMoS (3.22 Å, light blue), and (c) MoSe2 (3.28 Å, (36) green) on Au(111) (2.884 Å, (35) red). Due to the lattice mismatch between the TMD and Au(111) a moiré contrast appears. The small differences in the lattice constant of the different TMDs results in large differences in the moiré lattice constant periodicities which are 34.6 Å for MoS2/Au(111), 28.9 Å for Janus SeMoS/Au(111), and 23.1 Å for MoSe2/Au(111).

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To probe the Janus SeMoS structure on Au(111) in real space, we carried out low-temperature STM measurements. An atomically resolved STM image with the moiré contrast is shown in Figure 2c. The fast Fourier transform of the STM image confirms the hexagonal structure (FFT, inset of Figure 2b). We estimated the moiré periodicity from STM line scans to be ∼2.9 nm in agreement with the DFT calculations. Note that we analyzed the original STM data without any drift correction. Furthermore, an STM image was simulated by integrating occupied states within a narrow energy window (−0.1 eV) below the valence band edge which is shown in Figure 2d. The energy range of the simulated STM images can only be roughly compared to the bias voltages in the experiment due to the differences in the simulated and experimental bandgaps and the different zero energy levels, which corresponds to the Fermi level in the experiments and the highest occupied energy level in the calculations. Nevertheless, the simulated STM image displays good agreement with the experimental one. In the moiré unit cell, we can distinguish three different regions with slightly different electronic properties. These regions differ in the adsorption sites of the bottom S layer to the Au(111) atoms: (i) S atoms on top sites of Au(111), (ii) S atoms on fcc hollow sites, (iii) S atoms on hcp hollow sites. Region (i) has the largest STM height and appears brightest in the STM image. Regions (ii) and (iii) exhibit a significantly darker contrast whereby the STM height of region (iii) is slightly higher than for region (ii). The topography of the optimized Janus ML was carefully analyzed. The structure of SeMoS exhibits a slight buckling on Au(111) in different moiré regions of the interface. As a result, the equilibrium distance between SeMoS and substrate in region (i), i.e., 2.51 Å, is smaller compared to region (ii) (2.61 Å) and region (iii) (2.66 Å), suggesting stronger interaction between the lower S layer and Au atoms in region (i) than other regions. These results are consistent with the experimental STM image indicating high tunneling contrast in region (i).

Furthermore, we also observed point defects as shown in the STM image in Figure S3. These are likely Se vacancies in the top layer. We carefully evaluated the defect density, which proved to be about 1012 vacancies/cm2 confirming the high quality of our Janus SeMoS samples. To help identify the defects, STM images are simulated for Se vacancies in a SeMoS ML (Figure S4). The electronic structure characterization shows that Se vacancies lead to the formation of several in-gap states, including those being close to the conduction and valence band edges of the pristine SeMoS ML. The simulated image shows a single depression at the Se-vacancy site, which agrees well with the experimental STM data.

The electronic structure of Janus SeMoS MLs was characterized using DFT and ARUPS. The calculated band structure of a freestanding Janus SeMoS (without the Au(111) substrate) is shown in Figure 4a. We observe the valence band maximum (VBM) and the conduction band minimum (CBM) at the K point. This finding identifies the Janus SeMoS ML as a direct band semiconductor with a calculated band gap of 1.57 eV, which aligns closely with the previously reported theoretical value of 1.56 eV. (37) The spin–orbit coupling (SOC) leads to a split of the valence and conduction band at the K point (Figure 4a). The energetic distance of the split valence bands is about 168 meV. Our calculated SOC splitting for MoS2 and MoSe2 was found to be 147 and 184 meV, respectively. The projected densities of states suggest that the VBM and CBM mainly result from Mo 4d and chalcogen p orbitals, whereas the sulfur orbitals have a larger contribution than the selenium states at the VBM (Figure S5). Specifically, the valence band splitting at the K point is primarily attributed to the in-plane Mo 4dxy and Mo 4dx2–y2 orbitals. (39,40) We also analyzed the effect of the substrate on the electronic structure of the Janus monolayer. The calculated interlayer binding energy of SeMoS on Au (111) is 1.1 J/m2, similar to those for MoS2 on metal substrates. (41) A comparison between the projected DOS of freestanding and Au-supported SeMoS indicates that the interaction with the Au surface perturbs the electronic states of SeMoS close to the Fermi level as shown in Figure S4. It is also evident that the presence of the substrate shifts the Fermi level toward the conduction band minimum, suggesting a charge transfer at the interface. The calculated charge transfer from the substrate to SeMoS is presented in the charge density difference plots and is analyzed for each moiré region using Bader analyses (Figure 5a). The variation of the charge transfer at different moiré regions is caused by the different interactions between S atoms and Au atoms underneath (Figure 5b). Overall, the results indicate an electron transfer from gold to the Janus ML which is consistent with the observed n-type behavior of SeMoS in our electronic structure calculation (Figure S6).

Figure 4

Figure 4. (a) Electronic band structure and the corresponding density of states of Janus SeMoS MLs calculated by DFT. (b) ARUPS data along the Γ–K direction and around the K point of Janus SeMoS MLs on Au(111).

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Figure 5

Figure 5. (a) Average Bader charges for all atoms at each moiré region in the SeMoS/Au(111) interface, as compared to the free-standing SeMoS monolayer. (b) Charge transfer between SeMoS and Au(111) calculated as the difference between the charges in the interface and the isolated Au(111) and SeMoS monolayer. Blue (red) colors indicate charge accumulation (depletion).

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In Figures 4b and S7, the band structure of Janus SeMoS MLs on Au(111) measured by ARUPS is shown. From energy distribution curves (EDCs) at Γ and K points, we estimate the valence band maxima to be at BEs of 1.20 ± 0.10 eV and 1.06 ± 0.10 eV, respectively. These results mean that the valence band maximum has a slightly lower BE at the K point than at the Γ point which agrees with the DFT calculations. By zooming in the band structure (right part of Figure 4b), the spin–orbit split valence band at the K point can be recognized even more clearly. The splitting is 170 ± 15 meV in agreement with the DFT calculations of freestanding Janus SeMoS. From the maxima plot of each EDC as a function of their k value (Figure S8), we estimate the effective mass m* of the electron in the valence band to be 0.99 ± 0.10 me. Using the calculated electronic band structure, the effective masses of the electron and the hole are calculated to be 0.60 me and 0.54 me, respectively. Due to substrate interaction and hybridization, the measured effective mass of electrons in the valence band of Janus SeMoS MLs on Au(111) is larger than the calculated effective mass for freestanding Janus SeMoS.

The electronic structure of Janus SeMoS monolayers was characterized for the first time using ARUPS, enabling experimental determination of spin–orbit splitting and valence band effective mass, which are critical for understanding its electronic properties. DFT calculations further reveal that different regions of the Janus SeMoS monolayer interact distinctly with the Au(111) substrate, highlighting the complexity of substrate effects on its electronic structure.

In summary, we grew Janus SeMoS MLs with a high structural quality on Au(111) by CVD, which enabled a detailed investigation of their structural and electronic properties by various complementary experimental techniques. The experimental findings were supported by the results of DFT calculations. The formation of Janus SeMoS MLs was unambiguously confirmed by Raman spectroscopy and XPS, whereas the LEED measurements allowed us to precisely determine their lattice constant of 3.22 ± 0.01 Å. This value lies between the values of MoS2 and MoSe2 MLs and coincides with the theoretically predicted value. The observed moiré pattern of the Janus SeMoS ML on Au(111) with a moiré lattice constant of ∼2.9 nm is in line with the locally different charge transfer. The spin–orbit splitting of the valence band measured at the K point by ARUPS is 170 ± 15 meV and is in good agreement with the DFT predictions as well. To summarize, our experimental and theoretical results demonstrate that the Janus SeMoS ML is a direct band gap semiconductor, exhibiting structural and electronic properties that are intermediate between those of MoS2 and MoSe2 MLs. This study represents a significant step toward advancing the understanding of the structural and electronic characteristics of these novel 2D quantum materials.

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  • Details of the growth and methods used for characterization, XPS fit details, DFT calculations, STM data, ARUPS data and analyses, and LEED data (PDF)

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Acknowledgments

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This research was supported by the Deutsche Forschungsgemeinschaft (DFG) through a research infrastructure grant INST 275/257-1 FUGG (313713174), CRC 1375 NOA (Project B2, 398816777), SPP2244 (Project TU149/21-1, 535253440), and DFG individual grant TU149/16-1 (464283495). The authors thank Stephanie Höppener and Ulrich S. Schubert for enabling Raman spectroscopy studies at the Jena Center for Soft Matter (JCSM). A.V.K. thanks the German Research Foundation (DFG) for support through Project KR 4866/9-1 and the collaborative research center “Chemistry of Synthetic 2D Materials” SFB-1415-417590517. The authors further thank the HZDR Computing Center, HLRS, Stuttgart, Germany, and TU Dresden Cluster “Taurus” for generous grants of CPU time. M.S. acknowledges financial support from the Studienstiftung des deutschen Volkes through a PhD scholarship.

References

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This article references 41 other publications.

  1. 1
    Manzeli, S.; Ovchinnikov, D.; Pasquier, D.; Yazyev, O. V.; Kis, A. 2D transition metal dichalcogenides. Nat. Rev. Mater. 2017, 2 (8), 17033,  DOI: 10.1038/natrevmats.2017.33
    Google Scholar
    1
    2D transition metal dichalcogenides
    Manzeli, Sajedeh; Ovchinnikov, Dmitry; Pasquier, Diego; Yazyev, Oleg V.; Kis, Andras
    Nature Reviews Materials (2017), 2 (2), 17033CODEN: NRMADL; ISSN:2058-8437. (Nature Publishing Group)
    A review. Graphene is very popular because of its many fascinating properties, but its lack of an electronic bandgap has stimulated the search for 2D materials with semiconducting character. Transition metal dichalcogenides (TMDCs), which are semiconductors of the type MX2, where M is a transition metal atom (such as Mo or W) and X is a chalcogen atom (such as S, Se or Te), provide a promising alternative. Because of its robustness, MoS2 is the most studied material in this family. TMDCs exhibit a unique combination of at.-scale thickness, direct bandgap, strong spin-orbit coupling and favorable electronic and mech. properties, which make them interesting for fundamental studies and for applications in high-end electronics, spintronics, optoelectronics, energy harvesting, flexible electronics, DNA sequencing and personalized medicine. In this Review, the methods used to synthesize TMDCs are examd. and their properties are discussed, with particular attention to their charge d. wave, superconductive and topol. phases. The use of TMCDs in nanoelectronic devices is also explored, along with strategies to improve charge carrier mobility, high frequency operation and the use of strain engineering to tailor their properties.
  2. 2
    Joseph, S.; Mohan, J.; Lakshmy, S.; Thomas, S.; Chakraborty, B.; Thomas, S.; Kalarikkal, N. A review of the synthesis, properties, and applications of 2D transition metal dichalcogenides and their heterostructures. Mater. Chem. Phys. 2023, 297, 127332  DOI: 10.1016/j.matchemphys.2023.127332
    Google Scholar
    2
    A review of the synthesis, properties, and applications of 2D transition metal dichalcogenides and their heterostructures
    Joseph, Saju; Mohan, Jainy; Lakshmy, Seetha; Thomas, Simil; Chakraborty, Brahmananda; Thomas, Sabu; Kalarikkal, Nandakumar
    Materials Chemistry and Physics (2023), 297 (), 127332CODEN: MCHPDR; ISSN:0254-0584. (Elsevier B.V.)
    A review. One of the most nascent and universally accepted categories among the 2D materials is the transition metal dichalcogenides (TMDs). The absence of bandgap in graphene fomented the researchers to quest for an alternative semiconducting 2D materials. The aberrant optical and electronic properties helped 2D TMDs to excel in fundamental research and novel device applications. This article overviews the recent advances in the prepn. methods, properties, characterization, and novel multifunctional applications of TMDs and their corresponding heterostructures with special emphasis on their biosensing applications. The major portion of the review underlines the TMDs based heterostructures, which find potential applications in optoelectronics, sensors, and photovoltaics, that are devised to boost the functionality and flexibility and there by surmount the existing limitations of monolayered materials. At the end, the advancement and elevation of TMDs as biosensors over other 2D materials are also explored.
  3. 3
    Cheng, Y.; Zhu, Z.; Tahir, M.; Schwingenschlögl, U. Spin-orbit–induced spin splittings in polar transition metal dichalcogenide monolayers. EPL 2013, 102 (5), 57001,  DOI: 10.1209/0295-5075/102/57001
    Google Scholar
    There is no corresponding record for this reference.
  4. 4
    Yin, W.-J.; Tan, H.-J.; Ding, P.-J.; Wen, B.; Li, X.-B.; Teobaldi, G.; Liu, L.-M. Recent advances in low-dimensional Janus materials: theoretical and simulation perspectives. Mater. Adv. 2021, 2 (23), 7543– 7558,  DOI: 10.1039/D1MA00660F
    Google Scholar
    4
    Recent advances in low-dimensional Janus materials: theoretical and simulation perspectives
    Yin, Wen-Jin; Tan, Hua-Jian; Ding, Pei-Jia; Wen, Bo; Li, Xi-Bo; Teobaldi, Gilberto; Liu, Li-Min
    Materials Advances (2021), 2 (23), 7543-7558CODEN: MAADC9; ISSN:2633-5409. (Royal Society of Chemistry)
    A review. Owing to highly peculiar properties such as tunable electronic band gaps and coexistence of Rashba, excitonic and piezoelec. effects, low-dimensional Janus transition metal chalcogenides (TMDs) have received growing attention across different research and technol. areas. Exptl. and theor. investigations have shown that these emerging properties originate directly or indirectly from breaking of the mirror-asymmetry in the Janus TMD structure, resulting in an intrinsic dipole moment perpendicular to the system's layers. Despite substantial exptl. and computational research in many different Janus materials and their properties, partially covered in a limited no. of earlier reviews, an up-to-date comprehensive overview of the theor. and computational advances in the field is currently lacking. To fill this gap, here we review recent theor. and computational work on competing phases and properties of Janus TMD materials, covering their monolayers, bilayers, multilayers and hetero-structures. For each of these systems, we collate and discuss the calcd. results and trends on electronic properties such as band gaps, carrier mobility, electrostatic dipole moments, ensuing work-function differences, Schottky barriers, and solar-to-hydrogen energy conversion efficiencies. Based on the computational results, we then discuss the potential of low dimensional Janus materials for a diversified set of potential applications ranging from IR-visible photocatalytic water splitting and hydrogen evolution reactions, to gas sensing, field-effective transistors, and piezoelec. devices. We conclude the review with a crit. perspective on residual theor., computational, and exptl. challenges in the field.
  5. 5
    Tang, X.; Kou, L. 2D Janus transition metal dichalcogenides: Properties and applications. Phys. Status Solidi B 2022, 259 (4), 2100562  DOI: 10.1002/pssb.202100562
    Google Scholar
    There is no corresponding record for this reference.
  6. 6
    Ahmad, W.; Wang, Y.; Kazmi, J.; Younis, U.; Mubarak, N. M.; Aleithan, S. H.; Channa, A. I.; Lei, W.; Wang, Z. Janus 2D Transition Metal Dichalcogenides: Research Progress, Optical Mechanism and Future Prospects for Optoelectronic Devices. Laser Photonics Rev. 2024, 2400341  DOI: 10.1002/lpor.202400341
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    There is no corresponding record for this reference.
  7. 7
    Tao, S.; Xu, B.; Shi, J.; Zhong, S.; Lei, X.; Liu, G.; Wu, M. Tunable Dipole Moment in Janus Single-Layer MoSSe via Transition-Metal Atom Adsorption. J. Phys. Chem. C 2019, 123 (14), 9059– 9065,  DOI: 10.1021/acs.jpcc.9b00421
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    7
    Tunable Dipole Moment in Janus Single-Layer MoSSe via Transition-Metal Atom Adsorption
    Tao, Shengdan; Xu, Bo; Shi, Jing; Zhong, Shuying; Lei, Xueling; Liu, Gang; Wu, Musheng
    Journal of Physical Chemistry C (2019), 123 (14), 9059-9065CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Intrinsic dipole moment is an important characteristic of Janus single-layer MoSSe. Tuning the dipole moment would broaden the potential applications of Janus MoSSe in the field of piezoelectricity and mol. sensing. In this study, the dipole moments of Janus single-layer MoSSe with 3d transition-metal (TM) adatoms (Sc-Ni) are explored by using first-principles calcns. Our results demonstrate that the dipole moments of Janus MoSSe change with TM atom adsorption. For the adsorption of TM atoms on the Se surface, the dipole moments are enhanced when compared to the case of pristine MoSSe, regardless of at the M adsorption site or at the H adsorption site. However, in the case of S surface adsorption, the dipole moments are weakened or even reversed for some TM atoms. Among all the 3d TMs considered, the effect of Sc atom adsorption is the largest, while it is the smallest for Ni atom adsorption. By means of a simplified model, the total dipole moments can be regarded as the superposition of the dipole moments from the Janus MoSSe and the ionic TM atoms. Strengthening and weakening of the dipole moments depend on the direction of dipole moments from the ionic TM atoms. Thus, we could utilize TM atom adsorption to tune the dipole moments with both magnitude and direction.
  8. 8
    Hu, T.; Jia, F.; Zhao, G.; Wu, J.; Stroppa, A.; Ren, W. Intrinsic and anisotropic Rashba spin splitting in Janus transition-metal dichalcogenide monolayers. Phys. Rev. B 2018, 97 (23), 235404  DOI: 10.1103/PhysRevB.97.235404
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    8
    Intrinsic and anisotropic Rashba spin splitting in Janus transition-metal dichalcogenide monolayers
    Hu, Tao; Jia, Fanhao; Zhao, Guodong; Wu, Jiongyao; Stroppa, Alessandro; Ren, Wei
    Physical Review B (2018), 97 (23), 235404CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    Transition-metal dichalcogenides (TMDs) monolayers have been considered as important two-dimensional semiconductor materials for the study of fundamental physics in the field of spintronics. However, the out-of-plane mirror symmetry in TMDs may constrain electrons'degrees of freedom and it may limit spin-related applications. Recently, a newly synthesized Janus TMDs MoSSe was found to intrinsically possess both the in-plane inversion and the out-of-plane mirror-symmetry breaking. Here we performed first-principles calcns. in order to systematically investigate the electronic band structures of a series of Janus monolayer TMDs with chem. formula MXY(M=Mo,WandX,Y=S,Se,Te). It is found that they possess robust electronic properties like their parent phases. We explored also the effect of perpendicular external elec. field and in-plane biaxial strain on the Rashba spin splittings. The Zeeman-type spin splitting and valley polarization at K(K') point are well preserved and we obsd. a Rashba-type spin splitting around the G point for all the MXY systems. We have also found that these spin splittings can be enhanced by an external elec. field collinear with the local elec. field derived by the polar bonds and by the compressive strain. The Rashba parameters change linearly with the external elec. field, but nonlinearly with the biaxial strain. The compressive strain is found to enhance significantly the anisotropic Rashba spin splitting.
  9. 9
    Chen, J.; Wu, K.; Ma, H.; Hu, W.; Yang, J. Tunable Rashba spin splitting in Janus transition-metal dichalcogenide monolayers via charge doping. RSC Adv. 2020, 10 (11), 6388– 6394,  DOI: 10.1039/D0RA00674B
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    9
    Tunable Rashba spin splitting in Janus transition-metal dichalcogenide monolayers via charge doping
    Chen, Jiajia; Wu, Kai; Ma, Huanhuan; Hu, Wei; Yang, Jinlong
    RSC Advances (2020), 10 (11), 6388-6394CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    Two-dimensional (2D) Janus transition-metal dichalcogenides (TMDs) (MXY, M = Mo, W; X, Y = S, Se, Te; X ≠ Y) have desirable energy gaps and high stability in ambient conditions, similar to traditional 2D TMDs with potential applications in electronics. But different from traditional 2D TMDs, 2D Janus TMDs possess intrinsic Rashba spin splitting due to out-of-plane mirror symmetry breaking, with promising applications in spintronics. Here we demonstrate a new and effective way to manipulate the Rashba effect in 2D Janus TMDs, i.e., charge doping, by using first-principles d. functional theory (DFT) calcns. We find that electron doping can effectively strengthen the Rashba spin splitting at the valence band max. (VBM) and conduction band min. (CBM) in 2D Janus TMDs without const. energy consumption, superior to traditional techniques (elec. fields and strain engineering), but hole doping would weaken the Rashba effect in 2D Janus TMDs. By combining the DFT calcns. with the elec.-triple-layer model, we also reveal the intrinsic mechanism of tuning the Rashba effect in 2D Janus TMDs by charge doping, and find that the charge transfer plays an important role in tuning the Rashba spin splitting in 2D polar semiconductors. In particular, the Rashba consts. are linearly dependent on the charge transfer between X (or Y) and M atoms in 2D Janus TMDs. These results enrich the fundamental understanding of the Rashba effect in 2D semiconductors, which can be promising candidates for spin field-effect transistors (FETs) in expts.
  10. 10
    Dong, L.; Lou, J.; Shenoy, V. B. Large in-plane and vertical piezoelectricity in Janus transition metal dichalchogenides. ACS Nano 2017, 11 (8), 8242– 8248,  DOI: 10.1021/acsnano.7b03313
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    10
    Large in-plane and vertical piezoelectricity in Janus transition metal dichalchogenides
    Dong, Liang; Lou, Jun; Shenoy, Vivek B.
    ACS Nano (2017), 11 (8), 8242-8248CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Piezoelectricity in 2D van der Waals materials has received considerable interest because of potential applications in nanoscale energy harvesting, sensors, and actuators. However, in all the systems studied to date, strain and elec. polarization are confined to the basal plane, limiting the operation of piezoelec. devices. In this paper, based on ab initio calcns., we report a 2D materials system, namely, the recently synthesized Janus MXY (M = Mo or W, X/Y = S, Se, or Te) monolayer and multilayer structures, with large out-of-plane piezoelec. polarization. For MXY monolayers, both strong in-plane and much weaker out-of-plane piezoelec. polarizations can be induced by a uniaxial strain in the basal plane. For multilayer MXY, we obtain a very strong out-of-plane piezoelec. polarization when strained transverse to the basal plane, regardless of the stacking sequence. The out-of-plane piezoelec. coeff. d33 is found to be strongest in multilayer MoSTe (5.7-13.5 pm/V depending on the stacking sequence), which is larger than that of the commonly used 3D piezoelec. material AlN (d33 = 5.6 pm/V); d33 in other multilayer MXY structures are a bit smaller, but still comparable. Our study reveals the potential for utilizing piezoelec. 2D materials and their van der Waals multilayers in device applications.
  11. 11
    Mohanta, M. K.; De Sarkar, A. Interfacial hybridization of Janus MoSSe and BX (X= P, As) monolayers for ultrathin excitonic solar cells, nanopiezotronics and low-power memory devices. Nanoscale 2020, 12 (44), 22645– 22657,  DOI: 10.1039/D0NR07000A
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    There is no corresponding record for this reference.
  12. 12
    Ma, X.; Wu, X.; Wang, H.; Wang, Y. A Janus MoSSe monolayer: a potential wide solar-spectrum water-splitting photocatalyst with a low carrier recombination rate. J. Mater. Chem. A 2018, 6 (5), 2295– 2301,  DOI: 10.1039/C7TA10015A
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    12
    A Janus MoSSe monolayer: a potential wide solar-spectrum water-splitting photocatalyst with a low carrier recombination rate
    Ma, Xiangchao; Wu, Xin; Wang, Haoda; Wang, Yucheng
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2018), 6 (5), 2295-2301CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    For realizing efficient solar to hydrogen energy conversion based on photocatalytic technol., it is important to explore a photocatalyst with wide-range solar absorption and high electron-hole sepn. efficiency. With a built-in elec. field, the recently synthesized Janus MoSSe is intrinsically beneficial for promoting the sepn. of photo-generated electrons and holes. Thus in this work, we examine the possibility of MoSSe as an efficient water-splitting photocatalyst and the effects of isotropic and uniaxial strains by the first-principles calcns. It is interesting to find that MoSSe exhibits pronounced visible-light absorption efficiency, proper valence and conduction band positions for initializing the redox reactions of H2O, and high carrier mobilities. Moreover, the band gap of MoSSe is decreased and the direct-indirect band gap transition occurs upon tensile strain, which can not only extend the light absorption range, but also reduce the recombination of photo-generated carriers. Furthermore, H2O mols. adsorb more strongly on the MoSSe monolayer surface than on the MoS2 surface, which is also beneficial for the surface water-splitting reactions. These insights provide eloquent evidence that the Janus MoSSe monolayer is potentially an efficient and wide solar-spectrum water-splitting photocatalyst.
  13. 13
    Zheng, T.; Lin, Y.-C.; Yu, Y.; Valencia-Acuna, P.; Puretzky, A. A.; Torsi, R.; Liu, C.; Ivanov, I. N.; Duscher, G.; Geohegan, D. B. Excitonic dynamics in Janus MoSSe and WSSe monolayers. Nano Lett. 2021, 21 (2), 931– 937,  DOI: 10.1021/acs.nanolett.0c03412
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    13
    Excitonic Dynamics in Janus MoSSe and WSSe Monolayers
    Zheng, Ting; Lin, Yu-Chuan; Yu, Yiling; Valencia-Acuna, Pavel; Puretzky, Alexander A.; Torsi, Riccardo; Liu, Chenze; Ivanov, Ilia N.; Duscher, Gerd; Geohegan, David B.; Ni, Zhenhua; Xiao, Kai; Zhao, Hui
    Nano Letters (2021), 21 (2), 931-937CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
    We report here details of steady-state and time-resolved spectroscopy of excitonic dynamics for Janus transition metal dichalcogenide monolayers, including MoSSe and WSSe, which were synthesized by low-energy implantation of Se into transition metal disulfides. Absorbance and photoluminescence spectroscopic measurements detd. the room-temp. exciton resonances for MoSSe and WSSe monolayers. Transient absorption measurements revealed that the excitons in Janus structures form faster than those in pristine transition metal dichalcogenides by about 30% due to their enhanced electron-phonon interaction by the built-in dipole moment. By combining steady-state photoluminescence quantum yield and time-resolved transient absorption measurements, we find that the exciton radiative recombination lifetime in Janus structures is significantly longer than in their pristine samples, supporting the predicted spatial sepn. of the electron and hole wave functions due to the built-in dipole moment. These results provide fundamental insight in the optical properties of Janus transition metal dichalcogenides.
  14. 14
    Peng, R.; Ma, Y.; Zhang, S.; Huang, B.; Dai, Y. Valley Polarization in Janus Single-Layer MoSSe via Magnetic Doping. J. Phys. Chem. Lett. 2018, 9 (13), 3612– 3617,  DOI: 10.1021/acs.jpclett.8b01625
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    14
    Valley Polarization in Janus Single-Layer MoSSe via Magnetic Doping
    Peng, Rui; Ma, Yandong; Zhang, Shuai; Huang, Baibiao; Dai, Ying
    Journal of Physical Chemistry Letters (2018), 9 (13), 3612-3617CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Two-dimensional valleytronic systems can provide information storage and processing advantages that complement or surpass those of conventional charge- and spin-based semiconductor technologies. The major challenge currently is to realize valley polarization for manipulating the valley degree of freedom. Here, we propose that valley polarization can be readily achieved in Janus single-layer MoSSe through magnetic doping, which is highly feasible in expt. Due to inversion symmetry breaking combined with strong spin-orbit coupling (SOC), the pure single-layer MoSSe harbors an intriguing multivalleyed band structure and strong coupled spin and valley physics. After doping Cr/V, the long-sought valley polarization is successfully achieved with a remarkable energy difference of ∼0.06 eV upon switching on SOC. Furthermore, the valley polarization in Cr/V-doped single-layer MoSSe is tunable via strain engineering. Our work thus provides a promising platform for exptl. studies and applications of the valleytronics.
  15. 15
    Vojáček, L.; Medina Dueñas, J. n.; Li, J.; Ibrahim, F.; Manchon, A.; Roche, S.; Chshiev, M.; García, J. H. Field-Free Spin–Orbit Torque Switching in Janus Chromium Dichalcogenides. Nano Lett. 2024, 24 (38), 11889– 11894,  DOI: 10.1021/acs.nanolett.4c03029
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    There is no corresponding record for this reference.
  16. 16
    Gan, Z.; Paradisanos, I.; Estrada-Real, A.; Picker, J.; Najafidehaghani, E.; Davies, F.; Neumann, C.; Robert, C.; Wiecha, P.; Watanabe, K. Chemical Vapor Deposition of High-Optical-Quality Large-Area Monolayer Janus Transition Metal Dichalcogenides. Adv. Mater. 2022, 34 (38), 2205226  DOI: 10.1002/adma.202205226
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    There is no corresponding record for this reference.
  17. 17
    Kim, S. W.; Choi, S. Y.; Lim, S. H.; Ko, E. B.; Kim, S.; Park, Y. C.; Lee, S.; Kim, H. H. Understanding Solvent-Induced Delamination and Intense Water Adsorption in Janus Transition Metal Dichalcogenides for Enhanced Device Performance. Adv. Funct. Mater. 2024, 34, 2308709  DOI: 10.1002/adfm.202308709
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    There is no corresponding record for this reference.
  18. 18
    Lin, Y.-C.; Liu, C.; Yu, Y.; Zarkadoula, E.; Yoon, M.; Puretzky, A. A.; Liang, L.; Kong, X.; Gu, Y.; Strasser, A. Low energy implantation into transition-metal dichalcogenide monolayers to form Janus structures. ACS Nano 2020, 14 (4), 3896– 3906,  DOI: 10.1021/acsnano.9b10196
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    18
    Low Energy Implantation into Transition-Metal Dichalcogenide Monolayers to Form Janus Structures
    Lin, Yu-Chuan; Liu, Chenze; Yu, Yiling; Zarkadoula, Eva; Yoon, Mina; Puretzky, Alexander A.; Liang, Liangbo; Kong, Xiangru; Gu, Yiyi; Strasser, Alex; Meyer, Harry M.; Lorenz, Matthias; Chisholm, Matthew F.; Ivanov, Ilia N.; Rouleau, Christopher M.; Duscher, Gerd; Xiao, Kai; Geohegan, David B.
    ACS Nano (2020), 14 (4), 3896-3906CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    Atomically thin two-dimensional (2D) materials face significant energy barriers for synthesis and processing into functional metastable phases such as Janus structures. Here, the controllable implantation of hyperthermal species from pulsed laser deposition (PLD) plasmas is introduced as a top-down method to compositionally engineer 2D monolayers. The kinetic energies of Se clusters impinging on suspended monolayer WS2 crystals were controlled in the <10 eV/atom range with in situ plasma diagnostics to det. the thresholds for selective top layer replacement of sulfur by selenium for the formation of high quality WSSe Janus monolayers at low (300°C) temps. and bottom layer replacement for complete conversion to WSe2. Atomic-resoln. electron microscopy and spectroscopy in tilted geometry confirm the WSSe Janus monolayer. Mol. dynamics simulations reveal that Se clusters implant to form disordered metastable alloy regions, which then recrystallize to form highly ordered structures, demonstrating low-energy implantation by PLD for the synthesis of 2D Janus layers and alloys of variable compn.
  19. 19
    Lu, A.-Y.; Zhu, H.; Xiao, J.; Chuu, C.-P.; Han, Y.; Chiu, M.-H.; Cheng, C.-C.; Yang, C.-W.; Wei, K.-H.; Yang, Y. Janus monolayers of transition metal dichalcogenides. Nat. Nanotechnol. 2017, 12 (8), 744– 749,  DOI: 10.1038/nnano.2017.100
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    19
    Janus monolayers of transition metal dichalcogenides
    Lu, Ang-Yu; Zhu, Hanyu; Xiao, Jun; Chuu, Chih-Piao; Han, Yimo; Chiu, Ming-Hui; Cheng, Chia-Chin; Yang, Chih-Wen; Wei, Kung-Hwa; Yang, Yiming; Wang, Yuan; Sokaras, Dimosthenis; Nordlund, Dennis; Yang, Peidong; Muller, David A.; Chou, Mei-Yin; Zhang, Xiang; Li, Lain-Jong
    Nature Nanotechnology (2017), 12 (8), 744-749CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
    Structural symmetry-breaking plays a crucial role in detg. the electronic band structures of 2-dimensional materials. Tremendous efforts have been devoted to breaking the in-plane symmetry of graphene with elec. fields on AB-stacked bilayers or stacked van der Waals heterostructures. But transition metal dichalcogenide monolayers are semiconductors with intrinsic in-plane asymmetry, leading to direct electronic bandgaps, distinctive optical properties and great potential in optoelectronics. Apart from their in-plane inversion asymmetry, an addnl. degree of freedom allowing spin manipulation can be induced by breaking the out-of-plane mirror symmetry with external elec. fields or, as theor. proposed, with an asym. out-of-plane structural configuration. Here, the authors report a synthetic strategy to grow Janus monolayers of transition metal dichalcogenides breaking the out-of-plane structural symmetry. In particular, based on a MoS2 monolayer, the authors fully replace the top-layer S with Se atoms. The authors confirm the Janus structure of MoSSe directly by scanning TEM and energy-dependent XPS, and prove the existence of vertical dipoles by 2nd harmonic generation and piezoresponse force microscopy measurements.
  20. 20
    Schmeink, J.; Musytschuk, V.; Pollmann, E.; Sleziona, S.; Maas, A.; Kratzer, P.; Schleberger, M. Evaluating strain and doping of Janus MoSSe from phonon mode shifts supported by ab initio DFT calculations. Nanoscale 2023, 15 (25), 10834– 10841,  DOI: 10.1039/D3NR01978K
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    There is no corresponding record for this reference.
  21. 21
    Zhang, J.; Jia, S.; Kholmanov, I.; Dong, L.; Er, D.; Chen, W.; Guo, H.; Jin, Z.; Shenoy, V. B.; Shi, L.; Lou, J. Janus Monolayer Transition-Metal Dichalcogenides. ACS Nano 2017, 11 (8), 8192– 8198,  DOI: 10.1021/acsnano.7b03186
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    21
    Janus Monolayer Transition-Metal Dichalcogenides
    Zhang, Jing; Jia, Shuai; Kholmanov, Iskandar; Dong, Liang; Er, Dequan; Chen, Weibing; Guo, Hua; Jin, Zehua; Shenoy, Vivek B.; Shi, Li; Lou, Jun
    ACS Nano (2017), 11 (8), 8192-8198CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    The crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2, the top layer of selenium atoms is substituted by sulfur atoms, while the bottom selenium layer remains intact. The structure of this material is systematically investigated by Raman, photoluminescence, transmission electron microscopy, and XPS and confirmed by time-of-flight secondary ion mass spectrometry. D. functional theory (DFT) calcns. are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding exptl. results. Finally, high basal plane hydrogen evolution reaction activity is discovered for the Janus monolayer, and DFT calcn. implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.
  22. 22
    Trivedi, D. B.; Turgut, G.; Qin, Y.; Sayyad, M. Y.; Hajra, D.; Howell, M.; Liu, L.; Yang, S.; Patoary, N. H.; Li, H. Room-temperature synthesis of 2D Janus crystals and their heterostructures. Adv. Mater. 2020, 32 (50), 2006320  DOI: 10.1002/adma.202006320
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    22
    Room-Temperature Synthesis of 2D Janus Crystals and their Heterostructures
    Trivedi, Dipesh B.; Turgut, Guven; Qin, Ying; Sayyad, Mohammed Y.; Hajra, Debarati; Howell, Madeleine; Liu, Lei; Yang, Sijie; Patoary, Naim Hossain; Li, Han; Petric, Marko M.; Meyer, Moritz; Kremser, Malte; Barbone, Matteo; Soavi, Giancarlo; Stier, Andreas V.; Mueller, Kai; Yang, Shize; Esqueda, Ivan Sanchez; Zhuang, Houlong; Finley, Jonathan J.; Tongay, Sefaattin
    Advanced Materials (Weinheim, Germany) (2020), 32 (50), 2006320CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Janus crystals represent an exciting class of 2D materials with different at. species on their upper and lower facets. Theories have predicted that this symmetry breaking induces an elec. field and leads to a wealth of novel properties, such as large Rashba spin-orbit coupling and formation of strongly correlated electronic states. Monolayer MoSSe Janus crystals have been synthesized by two methods, via controlled sulfurization of monolayer MoSe2 and via plasma stripping followed thermal annealing of MoS2. However, the high processing temps. prevent growth of other Janus materials and their heterostructures. Here, a room-temp. technique for the synthesis of a variety of Janus monolayers with high structural and optical quality is reported. This process involves low-energy reactive radical precursors, which enables selective removal and replacement of the uppermost chalcogen layer, thus transforming classical transition metal dichalcogenides into a Janus structure. The resulting materials show clear mixed character for their excitonic transitions, and more importantly, the presented room-temp. method enables the demonstration of first vertical and lateral heterojunctions of 2D Janus TMDs. The results present significant and pioneering advances in the synthesis of new classes of 2D materials, and pave the way for the creation of heterostructures from 2D Janus layers.
  23. 23
    Sayyad, M.; Kopaczek, J.; Gilardoni, C. M.; Chen, W.; Xiong, Y.; Yang, S.; Watanabe, K.; Taniguchi, T.; Kudrawiec, R.; Hautier, G. The Defects Genome of Janus Transition Metal Dichalcogenides. Adv. Mater. 2024, 36, 2403583  DOI: 10.1002/adma.202403583
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    There is no corresponding record for this reference.
  24. 24
    Lakshmy, S.; Mondal, B.; Kalarikkal, N.; Rout, C. S.; Chakraborty, B. Recent developments in synthesis, properties, and applications of 2D Janus MoSSe and MoSexS(1-x) alloys. Adv. Powder Mater. 2024, 3, 100204  DOI: 10.1016/j.apmate.2024.100204
    Google Scholar
    There is no corresponding record for this reference.
  25. 25
    Bian, C.; Shi, J.; Liu, X.; Yang, Y.; Yang, H.; Gao, H. Optical second-harmonic generation of Janus MoSSe monolayer. Chin. Phys. B 2022, 31 (9), 097304  DOI: 10.1088/1674-1056/ac6db4
    Google Scholar
    There is no corresponding record for this reference.
  26. 26
    Harris, S. B.; Lin, Y.-C.; Puretzky, A. A.; Liang, L.; Dyck, O.; Berlijn, T.; Eres, G.; Rouleau, C. M.; Xiao, K.; Geohegan, D. B. Real-Time diagnostics of 2D crystal transformations by pulsed laser deposition: Controlled synthesis of Janus WSSe monolayers and alloys. ACS Nano 2023, 17 (3), 2472– 2486,  DOI: 10.1021/acsnano.2c09952
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    26
    Real-Time Diagnostics of 2D Crystal Transformations by Pulsed Laser Deposition: Controlled Synthesis of Janus WSSe Monolayers and Alloys
    Harris, Sumner B.; Lin, Yu-Chuan; Puretzky, Alexander A.; Liang, Liangbo; Dyck, Ondrej; Berlijn, Tom; Eres, Gyula; Rouleau, Christopher M.; Xiao, Kai; Geohegan, David B.
    ACS Nano (2023), 17 (3), 2472-2486CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    A feedback approach to reveal and control the transformation pathways in materials synthesis by pulsed laser deposition (PLD) is reported and applied to study the transformation kinetics of monolayer WS2 crystals into Janus WSSe and WSe2 by implantation of Se clusters with different max. kinetic energies (<42 eV/Se-atom) generated by laser ablation of a Se target. Real-time Raman spectroscopy and luminescence are used to assess the structure, compn., and optoelectronic quality of the monolayer crystal as it is implanted with well-controlled fluxes of Se for different kinetic energies that are regulated with in situ intensified CCD imaging, ion probe, and spectroscopy diagnostics. First-principles calcns., XPS, and at.-resoln. HAADF STEM imaging are used to understand the intermediate alloy compns. and their vibrational modes to identify transformation pathways. The real-time kinetics measurements reveal highly selective top-layer conversion as WS2 transforms through WS2(1-x)Se2x alloys to WSe2 and provide the means to adjust processing conditions to achieve fractional and complete Janus WSSe monolayers as metastable transition states. The general approach demonstrates a real-time feedback method to achieve Janus layers or other metastable alloys of the desired compn., and a general means to adjust the structure and quality of materials grown by PLD, addressing priority research directions for precision synthesis with real-time adaptive control.
  27. 27
    Zheng, K.; Vegge, T.; Castelli, I. E. Giant In-Plane Flexoelectricity and Radial Polarization in Janus IV–VI Monolayers and Nanotubes. ACS Appl. Mater. Interfaces 2024, 16 (15), 19369– 19378,  DOI: 10.1021/acsami.4c01527
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    There is no corresponding record for this reference.
  28. 28
    Liu, M.-Y.; Gong, L.; He, Y.; Cao, C. Tuning Rashba effect, band inversion, and spin-charge conversion of Janus XSn2Y monolayers via an external field. Phys. Rev. B 2021, 103 (7), 075421  DOI: 10.1103/PhysRevB.103.075421
    Google Scholar
    There is no corresponding record for this reference.
  29. 29
    Ahammed, R.; Jena, N.; Rawat, A.; Mohanta, M. K.; Dimple; De Sarkar, A. Ultrahigh out-of-plane piezoelectricity meets giant Rashba effect in 2D Janus monolayers and bilayers of group IV transition-metal trichalcogenides. J. Phys. Chem. C 2020, 124 (39), 21250– 21260,  DOI: 10.1021/acs.jpcc.0c05134
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    29
    Ultrahigh Out-of-Plane Piezoelectricity Meets Giant Rashba Effect in 2D Janus Monolayers and Bilayers of Group IV Transition-Metal Trichalcogenides
    Ahammed, Raihan; Jena, Nityasagar; Rawat, Ashima; Mohanta, Manish K.; Dimple; De Sarkar, Abir
    Journal of Physical Chemistry C (2020), 124 (39), 21250-21260CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The simultaneous occurrence of gigantic piezoelectricity and Rashba effect in two-dimensional (2D) materials is unusually scarce. Inversion symmetry occurring in MX3 (M = Ti, Zr, Hf; X = S, Se) monolayers is broken upon constructing their Janus monolayer structures MX2Y (X ≠ Y =S, Se), thereby inducing a large out-of-plane piezoelec. const. d33 (~68 pm/V) in them. d33 can be further enhanced to a super high value of ~ 1000 pm/V upon applying vertical compressive strain in the van der Waals bilayers constituted by interfacing these Janus monolayers. Therefore, d33 in these Janus transition-metal trichalcogenide (TMTC) bilayers reach more than 4-fold times that of bulk ceramic PZT material (~ 268 pm/V). The absence of a horizontal mirror symmetry and the presence of strong spin-orbit coupling cause Rashba spin-splitting in electronic bands in these Janus 2D monolayers, which shows up as an ultrahigh Rashba parameter, αR ~1.1 eV Å. It can be raised to 1.41 eV Å via compressive strain. Most of the 2D materials reported to date mainly show in-plane elec. polarization, which severely limits their prospects in piezotronic devices. In this present work, the piezoelectricity shown by the Janus monolayers of group IV TMTCs and their bilayers is significantly higher than the ones generally utilized in the form of three-dimensional bulk piezoelec. solids, for example, α-quartz (d11 = 2.3 pm/V), wurtzite-GaN (d33 = 3.1 pm/V), and wurtzite-AlN (d33 = 5.6 pm/V). It is exceedingly higher than that in Janus multilayer/bulk structures of Mo- and W-based transition-metal dichalcogenides, for example, MoSTe (d33 ~ 10 pm/V). The 2D Janus TMTC monolayers and their bilayers reported herewith straddle giant Rashba spin-splitting and ultrahigh piezoelectricity, thereby making them immensely promising candidates in the next-generation electronics, piezotronics, spintronics, flexible electronics, and piezoelec. devices.
  30. 30
    Varjovi, M. J.; Yagmurcukardes, M.; Peeters, F. M.; Durgun, E. Janus two-dimensional transition metal dichalcogenide oxides: First-principles investigation of WXO monolayers with X = S, Se, and Te. Phys. Rev. B 2021, 103 (19), 195438  DOI: 10.1103/PhysRevB.103.195438
    Google Scholar
    30
    Janus two-dimensional transition metal dichalcogenide oxides: First-principles investigation of WXO monolayers with X=S, Se, and Te
    Varjovi, M. Jahangirzadeh; Yagmurcukardes, M.; Peeters, F. M.; Durgun, E.
    Physical Review B (2021), 103 (19), 195438CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    Structural symmetry breaking in two-dimensional materials can lead to superior phys. properties and introduce an addnl. degree of piezoelectricity. In the present paper, we propose three structural phases (1H, 1T, and 1T') of Janus WXO (X=S, Se, and Te) monolayers and investigate their vibrational, thermal, elastic, piezoelec., and electronic properties by using first-principles methods. Phonon spectra anal. reveals that while the 1H phase is dynamically stable, the 1T phase exhibits imaginary frequencies and transforms to the distorted 1T' phase. Ab initio mol. dynamics simulations confirm that 1H- and 1T'-WXO monolayers are thermally stable even at high temps. without any significant structural deformations. Different from binary systems, addnl. Raman active modes appear upon the formation of Janus monolayers. Although the mech. properties of 1H-WXO are found to be isotropic, they are orientation dependent for 1T'-WXO. It is also shown that 1H-WXO monolayers are indirect band-gap semiconductors and the band gap narrows down the chalcogen group. Except 1T'-WSO, 1T'-WXO monolayers have a narrow band gap correlated with the Peierls distortion. The effect of spin-orbit coupling on the band structure is also examd. for both phases and the alteration in the band gap is estd. The versatile mech. and electronic properties of Janus WXO monolayers together with their large piezoelec. response imply that these systems are interesting for several nanoelectronic applications.
  31. 31
    Yang, Q.; Wang, D.; Zeng, Z.-Y.; Geng, H.-Y.; Chen, X.-R. High-performance photocatalytic and piezoelectric properties of two-dimensional transition metal oxyhalide ZrO X 2 (X= Br, I) and their Janus structures. Phys. Rev. B 2024, 109 (3), 035411  DOI: 10.1103/PhysRevB.109.035411
    Google Scholar
    There is no corresponding record for this reference.
  32. 32
    Petrić, M. M.; Kremser, M.; Barbone, M.; Qin, Y.; Sayyad, Y.; Shen, Y.; Tongay, S.; Finley, J. J.; Botello-Méndez, A. R.; Müller, K. Raman spectrum of Janus transition metal dichalcogenide monolayers WSSe and MoSSe. Phys. Rev. B 2021, 103 (3), 035414  DOI: 10.1103/PhysRevB.103.035414
    Google Scholar
    32
    Raman spectrum of janus transition metal dichalcogenide monolayers tungsten selenide sulfide and molybdenum selenide sulfide
    Petric, Marko M.; Kremser, Malte; Barbone, Matteo; Qin, Ying; Sayyad, Yasir; Shen, Yuxia; Tongay, Sefaattin; Finley, Jonathan J.; Botello-Mendez, Andres R.; Mueller, Kai
    Physical Review B (2021), 103 (3), 035414CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
    Janus transition metal dichalcogenides (TMDs) lose the horizontal mirror symmetry of ordinary TMDs, leading to the emergence of addnl. features, such as native piezoelectricity, Rashba effect, and enhanced catalytic activity. While Raman spectroscopy is an essential nondestructive, phase- and compn.-sensitive tool to monitor the synthesis of materials, a comprehensive study of the Raman spectrum of Janus monolayers is still missing. Here, we discuss the Raman spectra of WSSe and MoSSe measured at room and cryogenic temps., near and off resonance. By combining polarization-resolved Raman data with calcns. of the phonon dispersion and using symmetry considerations, we identify the four first-order Raman modes and higher-order two-phonon modes. Moreover, we observe defect-activated phonon processes, which provide a route toward a quant. assessment of the defect concn. and, thus, the crystal quality of the materials. Our work establishes a solid background for future research on material synthesis, study, and application of Janus TMD monolayers.
  33. 33
    Yu, M.; Ascolani, H.; Zampieri, G.; Woodruff, D. P.; Satterley, C. J.; Jones, R. G.; Dhanak, V. R. The Structure of Atomic Sulfur Phases on Au(111). J. Phys. Chem. C 2007, 111 (29), 10904– 10914,  DOI: 10.1021/jp072088+
    Google Scholar
    33
    The Structure of Atomic Sulfur Phases on Au(111)
    Yu, Miao; Ascolani, H.; Zampieri, G.; Woodruff, D. P.; Satterley, C. J.; Jones, Robert G.; Dhanak, V. R.
    Journal of Physical Chemistry C (2007), 111 (29), 10904-10914CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The structural phases formed by at. sulfur on Au(111) due to reaction with mol. S2 have been investigated by qual. LEED, scanning tunneling microscopy, and normal incidence X-ray standing wavefield absorption (NIXSW) combined with XPS. Three phases are identified with increasing coverage, namely, a newly identified (5 × 5) phase, a (√3 × √3)R30° phase, and a "complex" phase. The (5 × 5) phase, with a LEED pattern having the appearance of a "split-spot" (√3 × √3)R30° pattern, is interpreted in terms of local (√3 × √3)R30° ordering within a (5 × 5) ordered domain structure. The S atoms in the (5 × 5) phase occupy fcc hollow sites 1.56 Å above the outermost extended Au(111) bulk at. scatterer plane. A specific model of the ordering in this phase is proposed that, together with the obsd. marginal stability of the true, long-range-ordered, (√3 × √3)R30° phase, indicates significant short-range S-S repulsion and probably compressive surface stress. The complex phase, that coexists in a poorly ordered state with the lower coverage at. chemisorption phases, is interpreted in terms of an incommensurate long-range periodicity, but the NIXSW data shows clear evidence of local commensuration, with the S atoms mainly close to atop sites relative to the underlying Au(111) substrate; these data provide strong support for a previously proposed model based on a sulfide layer of stoichiometry AuS.
  34. 34
    Yasuda, S.; Takahashi, R.; Osaka, R.; Kumagai, R.; Miyata, Y.; Okada, S.; Hayamizu, Y.; Murakoshi, K. Out-of-Plane Strain Induced in a Moiré Superstructure of Monolayer MoS2 and MoSe2 on Au(111). Small 2017, 13 (31), 1700748  DOI: 10.1002/smll.201700748
    Google Scholar
    There is no corresponding record for this reference.
  35. 35
    Dutta, B.; Dayal, B. Lattice Constants and Thermal Expansion of Gold up to 878° C by X-Ray Method. Phys. Status Solidi B 1963, 3 (3), 473– 477,  DOI: 10.1002/pssb.19630030312
    Google Scholar
    There is no corresponding record for this reference.
  36. 36
    Picker, J.; Schaal, M.; Gan, Z.; Gruenewald, M.; Neumann, C.; George, A.; Otto, F.; Forker, R.; Fritz, T.; Turchanin, A. Structural and electronic properties of MoS2 and MoSe2 monolayers grown by chemical vapor deposition on Au(111). Nanoscale Adv. 2023, 6 (1), 92– 101,  DOI: 10.1039/D3NA00475A
    Google Scholar
    There is no corresponding record for this reference.
  37. 37
    Wen, Y.-N.; Xia, M.-G.; Zhang, S.-L. Bandgap engineering of Janus MoSSe monolayer implemented by Se vacancy. Comput. Mater. Sci. 2018, 152, 20– 27,  DOI: 10.1016/j.commatsci.2018.05.023
    Google Scholar
    37
    Bandgap engineering of Janus MoSSe monolayer implemented by Se vacancy
    Wen, Yan-Ni; Xia, Ming-Gang; Zhang, Sheng-Li
    Computational Materials Science (2018), 152 (), 20-27CODEN: CMMSEM; ISSN:0927-0256. (Elsevier B.V.)
    Vacancy defects in 2D materials provide opportunities to tailor local phys., structural and electronic properties of point and linear vacancies in Janus MoSSe monolayers. In this paper, we studied vacancy formation in Janus MoSSe monolayers using ab initio d. functional theory and obsd. that all vacancies are preferentially formed in the Se layer. In the case of point defects with more than two vacancies, a zigzag line feature is obtained for the lowest formation energy. In the case of infinite linear defects, the zigzag vacancy lines preferred to be distant from each other. The bandgap of a Janus MoSSe monolayer can be modulated by the concn. of vacancies. The bandgap energy decreased from 1.080 eV to 0.675 eV with the increase in the no. of point vacancies. However, it is obsd. to oscillate around 0.530 eV with the increase of distance between the vacancy lines in the case of linear vacancies. This work is very useful in bandgap engineering of optical and electronic devices based on MoS2.
  38. 38
    Silva, C. C.; Dombrowski, D.; Atodiresei, N.; Jolie, W.; Farwick zum Hagen, F.; Cai, J.; Ryan, P. T.; Thakur, P. K.; Caciuc, V.; Blügel, S. Spatial variation of geometry, binding, and electronic properties in the moiré superstructure of MoS2 on Au(111). 2D Mater. 2022, 9 (2), 025003  DOI: 10.1088/2053-1583/ac4958
    Google Scholar
    There is no corresponding record for this reference.
  39. 39
    Xiao, D.; Liu, G.-B.; Feng, W.; Xu, X.; Yao, W. Coupled spin and valley physics in monolayers of MoS2 and other group-VI dichalcogenides. Phys. Rev. Lett. 2012, 108 (19), 196802  DOI: 10.1103/PhysRevLett.108.196802
    Google Scholar
    39
    Coupled spin and valley physics in monolayers of MoS2 and other Group-VI dichalcogenides
    Xiao, Di; Liu, Gui-Bin; Feng, Wanxiang; Xu, Xiaodong; Yao, Wang
    Physical Review Letters (2012), 108 (19), 196802/1-196802/5CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    We show that inversion symmetry breaking together with spin-orbit coupling leads to coupled spin and valley physics in monolayers of MoS2 and other group-VI dichalcogenides, making possible controls of spin and valley in these 2D materials. The spin-valley coupling at the valence-band edges suppresses spin and valley relaxation, as flip of each index alone is forbidden by the valley-contrasting spin splitting. Valley Hall and spin Hall effects coexist in both electron-doped and hole-doped systems. Optical interband transitions have frequency-dependent polarization selection rules which allow selective photoexcitation of carriers with various combination of valley and spin indexes. Photoinduced spin Hall and valley Hall effects can generate long lived spin and valley accumulations on sample boundaries. The physics discussed here provides a route towards the integration of valleytronics and spintronics in multivalley materials with strong spin-orbit coupling and inversion symmetry breaking.
  40. 40
    Yu, S.-B.; Zhou, M.; Zhang, D.; Chang, K. Spin Hall effect in the monolayer Janus compound MoSSe enhanced by Rashba spin-orbit coupling. Phys. Rev. B 2021, 104 (7), 075435  DOI: 10.1103/PhysRevB.104.075435
    Google Scholar
    There is no corresponding record for this reference.
  41. 41
    Calis, M.; Lloyd, D.; Boddeti, N.; Bunch, J. S. Adhesion of 2D MoS2 to Graphite and Metal Substrates Measured by a Blister Test. Nano Lett. 2023, 23 (7), 2607– 2614,  DOI: 10.1021/acs.nanolett.2c04886
    Google Scholar
    There is no corresponding record for this reference.

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  1. Haomai Yang, Changhao Ding, Zhifu Duan, Jiang Zeng, Li-Ming Tang, Nannan Luo, Ke-Qiu Chen. High thermoelectric performance of monolayer Janus Zn AX Te (A = Ge, Sn; X = S, Se) induced by large band degeneracy and low lattice thermal conductivity. Applied Physics Letters 2025, 126 (20) https://doi.org/10.1063/5.0264169
  2. D. Szczęśniak, J.T. Gnyp, M. Kielak. Semiconducting and Superconducting Properties of 2D Hexagonal Materials. Acta Physica Polonica A 2025, 147 (3) , 191-195. https://doi.org/10.12693/APhysPolA.147.191
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  • Abstract

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    Figure 1

    Figure 1. (a) Cross section of Janus SeMoS ML on Au(111) based on DFT calculations. Selenium, molybdenum, and sulfur atoms are highlighted in orange, blue, and yellow, respectively. The gold substrate atoms are represented as larger balls and colored according to their depth. (b) Raman spectrum (excitation wavelength, 532 nm) and (c) Mo 3d/S 2s/Se 3s (left), S 2p/Se 3p (middle), and Se 3d/Mo 4s/Au 5p (right) XP spectral regions of Janus SeMoS on Au(111) recorded at an emission angle of 0° (top) and 70° (bottom). Mo, S, Se, and Au peaks are highlighted in blue, green, orange, and yellow, respectively.

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    Figure 2

    Figure 2. (a) LEED pattern of a Janus SeMoS ML on Au(111) (118 eV, 293 K). The unit cell and the corresponding LEED spots of the Janus SeMoS and Au(111) are highlighted in light blue and red, respectively. (b) Top view of an atomic ball model of the Janus SeMoS ML on Au(111). Due to the different adsorption sites of the sulfur layer with respect to the topmost Au layer, a moiré contrast appears. We distinguish three different regions which are discussed in the text. (c) STM (−1.1 V, 0.5 nA, 4.2 K) and (d) simulated STM images of a Janus SeMoS ML on Au(111). A fast Fourier transform (FFT) of the STM image in panel c is shown in the inset (scale bar ≈ 1 Å–1).

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    Figure 3

    Figure 3. Representation of the lattices of (a) MoS2 (3.15 Å, (36) blue), (b) Janus SeMoS (3.22 Å, light blue), and (c) MoSe2 (3.28 Å, (36) green) on Au(111) (2.884 Å, (35) red). Due to the lattice mismatch between the TMD and Au(111) a moiré contrast appears. The small differences in the lattice constant of the different TMDs results in large differences in the moiré lattice constant periodicities which are 34.6 Å for MoS2/Au(111), 28.9 Å for Janus SeMoS/Au(111), and 23.1 Å for MoSe2/Au(111).

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    Figure 4

    Figure 4. (a) Electronic band structure and the corresponding density of states of Janus SeMoS MLs calculated by DFT. (b) ARUPS data along the Γ–K direction and around the K point of Janus SeMoS MLs on Au(111).

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    Figure 5

    Figure 5. (a) Average Bader charges for all atoms at each moiré region in the SeMoS/Au(111) interface, as compared to the free-standing SeMoS monolayer. (b) Charge transfer between SeMoS and Au(111) calculated as the difference between the charges in the interface and the isolated Au(111) and SeMoS monolayer. Blue (red) colors indicate charge accumulation (depletion).

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  • References

    This article references 41 other publications.

    1. 1
      Manzeli, S.; Ovchinnikov, D.; Pasquier, D.; Yazyev, O. V.; Kis, A. 2D transition metal dichalcogenides. Nat. Rev. Mater. 2017, 2 (8), 17033,  DOI: 10.1038/natrevmats.2017.33
      1
      2D transition metal dichalcogenides
      Manzeli, Sajedeh; Ovchinnikov, Dmitry; Pasquier, Diego; Yazyev, Oleg V.; Kis, Andras
      Nature Reviews Materials (2017), 2 (2), 17033CODEN: NRMADL; ISSN:2058-8437. (Nature Publishing Group)
      A review. Graphene is very popular because of its many fascinating properties, but its lack of an electronic bandgap has stimulated the search for 2D materials with semiconducting character. Transition metal dichalcogenides (TMDCs), which are semiconductors of the type MX2, where M is a transition metal atom (such as Mo or W) and X is a chalcogen atom (such as S, Se or Te), provide a promising alternative. Because of its robustness, MoS2 is the most studied material in this family. TMDCs exhibit a unique combination of at.-scale thickness, direct bandgap, strong spin-orbit coupling and favorable electronic and mech. properties, which make them interesting for fundamental studies and for applications in high-end electronics, spintronics, optoelectronics, energy harvesting, flexible electronics, DNA sequencing and personalized medicine. In this Review, the methods used to synthesize TMDCs are examd. and their properties are discussed, with particular attention to their charge d. wave, superconductive and topol. phases. The use of TMCDs in nanoelectronic devices is also explored, along with strategies to improve charge carrier mobility, high frequency operation and the use of strain engineering to tailor their properties.
    2. 2
      Joseph, S.; Mohan, J.; Lakshmy, S.; Thomas, S.; Chakraborty, B.; Thomas, S.; Kalarikkal, N. A review of the synthesis, properties, and applications of 2D transition metal dichalcogenides and their heterostructures. Mater. Chem. Phys. 2023, 297, 127332  DOI: 10.1016/j.matchemphys.2023.127332
      2
      A review of the synthesis, properties, and applications of 2D transition metal dichalcogenides and their heterostructures
      Joseph, Saju; Mohan, Jainy; Lakshmy, Seetha; Thomas, Simil; Chakraborty, Brahmananda; Thomas, Sabu; Kalarikkal, Nandakumar
      Materials Chemistry and Physics (2023), 297 (), 127332CODEN: MCHPDR; ISSN:0254-0584. (Elsevier B.V.)
      A review. One of the most nascent and universally accepted categories among the 2D materials is the transition metal dichalcogenides (TMDs). The absence of bandgap in graphene fomented the researchers to quest for an alternative semiconducting 2D materials. The aberrant optical and electronic properties helped 2D TMDs to excel in fundamental research and novel device applications. This article overviews the recent advances in the prepn. methods, properties, characterization, and novel multifunctional applications of TMDs and their corresponding heterostructures with special emphasis on their biosensing applications. The major portion of the review underlines the TMDs based heterostructures, which find potential applications in optoelectronics, sensors, and photovoltaics, that are devised to boost the functionality and flexibility and there by surmount the existing limitations of monolayered materials. At the end, the advancement and elevation of TMDs as biosensors over other 2D materials are also explored.
    3. 3
      Cheng, Y.; Zhu, Z.; Tahir, M.; Schwingenschlögl, U. Spin-orbit–induced spin splittings in polar transition metal dichalcogenide monolayers. EPL 2013, 102 (5), 57001,  DOI: 10.1209/0295-5075/102/57001
      There is no corresponding record for this reference.
    4. 4
      Yin, W.-J.; Tan, H.-J.; Ding, P.-J.; Wen, B.; Li, X.-B.; Teobaldi, G.; Liu, L.-M. Recent advances in low-dimensional Janus materials: theoretical and simulation perspectives. Mater. Adv. 2021, 2 (23), 7543– 7558,  DOI: 10.1039/D1MA00660F
      4
      Recent advances in low-dimensional Janus materials: theoretical and simulation perspectives
      Yin, Wen-Jin; Tan, Hua-Jian; Ding, Pei-Jia; Wen, Bo; Li, Xi-Bo; Teobaldi, Gilberto; Liu, Li-Min
      Materials Advances (2021), 2 (23), 7543-7558CODEN: MAADC9; ISSN:2633-5409. (Royal Society of Chemistry)
      A review. Owing to highly peculiar properties such as tunable electronic band gaps and coexistence of Rashba, excitonic and piezoelec. effects, low-dimensional Janus transition metal chalcogenides (TMDs) have received growing attention across different research and technol. areas. Exptl. and theor. investigations have shown that these emerging properties originate directly or indirectly from breaking of the mirror-asymmetry in the Janus TMD structure, resulting in an intrinsic dipole moment perpendicular to the system's layers. Despite substantial exptl. and computational research in many different Janus materials and their properties, partially covered in a limited no. of earlier reviews, an up-to-date comprehensive overview of the theor. and computational advances in the field is currently lacking. To fill this gap, here we review recent theor. and computational work on competing phases and properties of Janus TMD materials, covering their monolayers, bilayers, multilayers and hetero-structures. For each of these systems, we collate and discuss the calcd. results and trends on electronic properties such as band gaps, carrier mobility, electrostatic dipole moments, ensuing work-function differences, Schottky barriers, and solar-to-hydrogen energy conversion efficiencies. Based on the computational results, we then discuss the potential of low dimensional Janus materials for a diversified set of potential applications ranging from IR-visible photocatalytic water splitting and hydrogen evolution reactions, to gas sensing, field-effective transistors, and piezoelec. devices. We conclude the review with a crit. perspective on residual theor., computational, and exptl. challenges in the field.
    5. 5
      Tang, X.; Kou, L. 2D Janus transition metal dichalcogenides: Properties and applications. Phys. Status Solidi B 2022, 259 (4), 2100562  DOI: 10.1002/pssb.202100562
      There is no corresponding record for this reference.
    6. 6
      Ahmad, W.; Wang, Y.; Kazmi, J.; Younis, U.; Mubarak, N. M.; Aleithan, S. H.; Channa, A. I.; Lei, W.; Wang, Z. Janus 2D Transition Metal Dichalcogenides: Research Progress, Optical Mechanism and Future Prospects for Optoelectronic Devices. Laser Photonics Rev. 2024, 2400341  DOI: 10.1002/lpor.202400341
      There is no corresponding record for this reference.
    7. 7
      Tao, S.; Xu, B.; Shi, J.; Zhong, S.; Lei, X.; Liu, G.; Wu, M. Tunable Dipole Moment in Janus Single-Layer MoSSe via Transition-Metal Atom Adsorption. J. Phys. Chem. C 2019, 123 (14), 9059– 9065,  DOI: 10.1021/acs.jpcc.9b00421
      7
      Tunable Dipole Moment in Janus Single-Layer MoSSe via Transition-Metal Atom Adsorption
      Tao, Shengdan; Xu, Bo; Shi, Jing; Zhong, Shuying; Lei, Xueling; Liu, Gang; Wu, Musheng
      Journal of Physical Chemistry C (2019), 123 (14), 9059-9065CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Intrinsic dipole moment is an important characteristic of Janus single-layer MoSSe. Tuning the dipole moment would broaden the potential applications of Janus MoSSe in the field of piezoelectricity and mol. sensing. In this study, the dipole moments of Janus single-layer MoSSe with 3d transition-metal (TM) adatoms (Sc-Ni) are explored by using first-principles calcns. Our results demonstrate that the dipole moments of Janus MoSSe change with TM atom adsorption. For the adsorption of TM atoms on the Se surface, the dipole moments are enhanced when compared to the case of pristine MoSSe, regardless of at the M adsorption site or at the H adsorption site. However, in the case of S surface adsorption, the dipole moments are weakened or even reversed for some TM atoms. Among all the 3d TMs considered, the effect of Sc atom adsorption is the largest, while it is the smallest for Ni atom adsorption. By means of a simplified model, the total dipole moments can be regarded as the superposition of the dipole moments from the Janus MoSSe and the ionic TM atoms. Strengthening and weakening of the dipole moments depend on the direction of dipole moments from the ionic TM atoms. Thus, we could utilize TM atom adsorption to tune the dipole moments with both magnitude and direction.
    8. 8
      Hu, T.; Jia, F.; Zhao, G.; Wu, J.; Stroppa, A.; Ren, W. Intrinsic and anisotropic Rashba spin splitting in Janus transition-metal dichalcogenide monolayers. Phys. Rev. B 2018, 97 (23), 235404  DOI: 10.1103/PhysRevB.97.235404
      8
      Intrinsic and anisotropic Rashba spin splitting in Janus transition-metal dichalcogenide monolayers
      Hu, Tao; Jia, Fanhao; Zhao, Guodong; Wu, Jiongyao; Stroppa, Alessandro; Ren, Wei
      Physical Review B (2018), 97 (23), 235404CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      Transition-metal dichalcogenides (TMDs) monolayers have been considered as important two-dimensional semiconductor materials for the study of fundamental physics in the field of spintronics. However, the out-of-plane mirror symmetry in TMDs may constrain electrons'degrees of freedom and it may limit spin-related applications. Recently, a newly synthesized Janus TMDs MoSSe was found to intrinsically possess both the in-plane inversion and the out-of-plane mirror-symmetry breaking. Here we performed first-principles calcns. in order to systematically investigate the electronic band structures of a series of Janus monolayer TMDs with chem. formula MXY(M=Mo,WandX,Y=S,Se,Te). It is found that they possess robust electronic properties like their parent phases. We explored also the effect of perpendicular external elec. field and in-plane biaxial strain on the Rashba spin splittings. The Zeeman-type spin splitting and valley polarization at K(K') point are well preserved and we obsd. a Rashba-type spin splitting around the G point for all the MXY systems. We have also found that these spin splittings can be enhanced by an external elec. field collinear with the local elec. field derived by the polar bonds and by the compressive strain. The Rashba parameters change linearly with the external elec. field, but nonlinearly with the biaxial strain. The compressive strain is found to enhance significantly the anisotropic Rashba spin splitting.
    9. 9
      Chen, J.; Wu, K.; Ma, H.; Hu, W.; Yang, J. Tunable Rashba spin splitting in Janus transition-metal dichalcogenide monolayers via charge doping. RSC Adv. 2020, 10 (11), 6388– 6394,  DOI: 10.1039/D0RA00674B
      9
      Tunable Rashba spin splitting in Janus transition-metal dichalcogenide monolayers via charge doping
      Chen, Jiajia; Wu, Kai; Ma, Huanhuan; Hu, Wei; Yang, Jinlong
      RSC Advances (2020), 10 (11), 6388-6394CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      Two-dimensional (2D) Janus transition-metal dichalcogenides (TMDs) (MXY, M = Mo, W; X, Y = S, Se, Te; X ≠ Y) have desirable energy gaps and high stability in ambient conditions, similar to traditional 2D TMDs with potential applications in electronics. But different from traditional 2D TMDs, 2D Janus TMDs possess intrinsic Rashba spin splitting due to out-of-plane mirror symmetry breaking, with promising applications in spintronics. Here we demonstrate a new and effective way to manipulate the Rashba effect in 2D Janus TMDs, i.e., charge doping, by using first-principles d. functional theory (DFT) calcns. We find that electron doping can effectively strengthen the Rashba spin splitting at the valence band max. (VBM) and conduction band min. (CBM) in 2D Janus TMDs without const. energy consumption, superior to traditional techniques (elec. fields and strain engineering), but hole doping would weaken the Rashba effect in 2D Janus TMDs. By combining the DFT calcns. with the elec.-triple-layer model, we also reveal the intrinsic mechanism of tuning the Rashba effect in 2D Janus TMDs by charge doping, and find that the charge transfer plays an important role in tuning the Rashba spin splitting in 2D polar semiconductors. In particular, the Rashba consts. are linearly dependent on the charge transfer between X (or Y) and M atoms in 2D Janus TMDs. These results enrich the fundamental understanding of the Rashba effect in 2D semiconductors, which can be promising candidates for spin field-effect transistors (FETs) in expts.
    10. 10
      Dong, L.; Lou, J.; Shenoy, V. B. Large in-plane and vertical piezoelectricity in Janus transition metal dichalchogenides. ACS Nano 2017, 11 (8), 8242– 8248,  DOI: 10.1021/acsnano.7b03313
      10
      Large in-plane and vertical piezoelectricity in Janus transition metal dichalchogenides
      Dong, Liang; Lou, Jun; Shenoy, Vivek B.
      ACS Nano (2017), 11 (8), 8242-8248CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Piezoelectricity in 2D van der Waals materials has received considerable interest because of potential applications in nanoscale energy harvesting, sensors, and actuators. However, in all the systems studied to date, strain and elec. polarization are confined to the basal plane, limiting the operation of piezoelec. devices. In this paper, based on ab initio calcns., we report a 2D materials system, namely, the recently synthesized Janus MXY (M = Mo or W, X/Y = S, Se, or Te) monolayer and multilayer structures, with large out-of-plane piezoelec. polarization. For MXY monolayers, both strong in-plane and much weaker out-of-plane piezoelec. polarizations can be induced by a uniaxial strain in the basal plane. For multilayer MXY, we obtain a very strong out-of-plane piezoelec. polarization when strained transverse to the basal plane, regardless of the stacking sequence. The out-of-plane piezoelec. coeff. d33 is found to be strongest in multilayer MoSTe (5.7-13.5 pm/V depending on the stacking sequence), which is larger than that of the commonly used 3D piezoelec. material AlN (d33 = 5.6 pm/V); d33 in other multilayer MXY structures are a bit smaller, but still comparable. Our study reveals the potential for utilizing piezoelec. 2D materials and their van der Waals multilayers in device applications.
    11. 11
      Mohanta, M. K.; De Sarkar, A. Interfacial hybridization of Janus MoSSe and BX (X= P, As) monolayers for ultrathin excitonic solar cells, nanopiezotronics and low-power memory devices. Nanoscale 2020, 12 (44), 22645– 22657,  DOI: 10.1039/D0NR07000A
      There is no corresponding record for this reference.
    12. 12
      Ma, X.; Wu, X.; Wang, H.; Wang, Y. A Janus MoSSe monolayer: a potential wide solar-spectrum water-splitting photocatalyst with a low carrier recombination rate. J. Mater. Chem. A 2018, 6 (5), 2295– 2301,  DOI: 10.1039/C7TA10015A
      12
      A Janus MoSSe monolayer: a potential wide solar-spectrum water-splitting photocatalyst with a low carrier recombination rate
      Ma, Xiangchao; Wu, Xin; Wang, Haoda; Wang, Yucheng
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2018), 6 (5), 2295-2301CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      For realizing efficient solar to hydrogen energy conversion based on photocatalytic technol., it is important to explore a photocatalyst with wide-range solar absorption and high electron-hole sepn. efficiency. With a built-in elec. field, the recently synthesized Janus MoSSe is intrinsically beneficial for promoting the sepn. of photo-generated electrons and holes. Thus in this work, we examine the possibility of MoSSe as an efficient water-splitting photocatalyst and the effects of isotropic and uniaxial strains by the first-principles calcns. It is interesting to find that MoSSe exhibits pronounced visible-light absorption efficiency, proper valence and conduction band positions for initializing the redox reactions of H2O, and high carrier mobilities. Moreover, the band gap of MoSSe is decreased and the direct-indirect band gap transition occurs upon tensile strain, which can not only extend the light absorption range, but also reduce the recombination of photo-generated carriers. Furthermore, H2O mols. adsorb more strongly on the MoSSe monolayer surface than on the MoS2 surface, which is also beneficial for the surface water-splitting reactions. These insights provide eloquent evidence that the Janus MoSSe monolayer is potentially an efficient and wide solar-spectrum water-splitting photocatalyst.
    13. 13
      Zheng, T.; Lin, Y.-C.; Yu, Y.; Valencia-Acuna, P.; Puretzky, A. A.; Torsi, R.; Liu, C.; Ivanov, I. N.; Duscher, G.; Geohegan, D. B. Excitonic dynamics in Janus MoSSe and WSSe monolayers. Nano Lett. 2021, 21 (2), 931– 937,  DOI: 10.1021/acs.nanolett.0c03412
      13
      Excitonic Dynamics in Janus MoSSe and WSSe Monolayers
      Zheng, Ting; Lin, Yu-Chuan; Yu, Yiling; Valencia-Acuna, Pavel; Puretzky, Alexander A.; Torsi, Riccardo; Liu, Chenze; Ivanov, Ilia N.; Duscher, Gerd; Geohegan, David B.; Ni, Zhenhua; Xiao, Kai; Zhao, Hui
      Nano Letters (2021), 21 (2), 931-937CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)
      We report here details of steady-state and time-resolved spectroscopy of excitonic dynamics for Janus transition metal dichalcogenide monolayers, including MoSSe and WSSe, which were synthesized by low-energy implantation of Se into transition metal disulfides. Absorbance and photoluminescence spectroscopic measurements detd. the room-temp. exciton resonances for MoSSe and WSSe monolayers. Transient absorption measurements revealed that the excitons in Janus structures form faster than those in pristine transition metal dichalcogenides by about 30% due to their enhanced electron-phonon interaction by the built-in dipole moment. By combining steady-state photoluminescence quantum yield and time-resolved transient absorption measurements, we find that the exciton radiative recombination lifetime in Janus structures is significantly longer than in their pristine samples, supporting the predicted spatial sepn. of the electron and hole wave functions due to the built-in dipole moment. These results provide fundamental insight in the optical properties of Janus transition metal dichalcogenides.
    14. 14
      Peng, R.; Ma, Y.; Zhang, S.; Huang, B.; Dai, Y. Valley Polarization in Janus Single-Layer MoSSe via Magnetic Doping. J. Phys. Chem. Lett. 2018, 9 (13), 3612– 3617,  DOI: 10.1021/acs.jpclett.8b01625
      14
      Valley Polarization in Janus Single-Layer MoSSe via Magnetic Doping
      Peng, Rui; Ma, Yandong; Zhang, Shuai; Huang, Baibiao; Dai, Ying
      Journal of Physical Chemistry Letters (2018), 9 (13), 3612-3617CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Two-dimensional valleytronic systems can provide information storage and processing advantages that complement or surpass those of conventional charge- and spin-based semiconductor technologies. The major challenge currently is to realize valley polarization for manipulating the valley degree of freedom. Here, we propose that valley polarization can be readily achieved in Janus single-layer MoSSe through magnetic doping, which is highly feasible in expt. Due to inversion symmetry breaking combined with strong spin-orbit coupling (SOC), the pure single-layer MoSSe harbors an intriguing multivalleyed band structure and strong coupled spin and valley physics. After doping Cr/V, the long-sought valley polarization is successfully achieved with a remarkable energy difference of ∼0.06 eV upon switching on SOC. Furthermore, the valley polarization in Cr/V-doped single-layer MoSSe is tunable via strain engineering. Our work thus provides a promising platform for exptl. studies and applications of the valleytronics.
    15. 15
      Vojáček, L.; Medina Dueñas, J. n.; Li, J.; Ibrahim, F.; Manchon, A.; Roche, S.; Chshiev, M.; García, J. H. Field-Free Spin–Orbit Torque Switching in Janus Chromium Dichalcogenides. Nano Lett. 2024, 24 (38), 11889– 11894,  DOI: 10.1021/acs.nanolett.4c03029
      There is no corresponding record for this reference.
    16. 16
      Gan, Z.; Paradisanos, I.; Estrada-Real, A.; Picker, J.; Najafidehaghani, E.; Davies, F.; Neumann, C.; Robert, C.; Wiecha, P.; Watanabe, K. Chemical Vapor Deposition of High-Optical-Quality Large-Area Monolayer Janus Transition Metal Dichalcogenides. Adv. Mater. 2022, 34 (38), 2205226  DOI: 10.1002/adma.202205226
      There is no corresponding record for this reference.
    17. 17
      Kim, S. W.; Choi, S. Y.; Lim, S. H.; Ko, E. B.; Kim, S.; Park, Y. C.; Lee, S.; Kim, H. H. Understanding Solvent-Induced Delamination and Intense Water Adsorption in Janus Transition Metal Dichalcogenides for Enhanced Device Performance. Adv. Funct. Mater. 2024, 34, 2308709  DOI: 10.1002/adfm.202308709
      There is no corresponding record for this reference.
    18. 18
      Lin, Y.-C.; Liu, C.; Yu, Y.; Zarkadoula, E.; Yoon, M.; Puretzky, A. A.; Liang, L.; Kong, X.; Gu, Y.; Strasser, A. Low energy implantation into transition-metal dichalcogenide monolayers to form Janus structures. ACS Nano 2020, 14 (4), 3896– 3906,  DOI: 10.1021/acsnano.9b10196
      18
      Low Energy Implantation into Transition-Metal Dichalcogenide Monolayers to Form Janus Structures
      Lin, Yu-Chuan; Liu, Chenze; Yu, Yiling; Zarkadoula, Eva; Yoon, Mina; Puretzky, Alexander A.; Liang, Liangbo; Kong, Xiangru; Gu, Yiyi; Strasser, Alex; Meyer, Harry M.; Lorenz, Matthias; Chisholm, Matthew F.; Ivanov, Ilia N.; Rouleau, Christopher M.; Duscher, Gerd; Xiao, Kai; Geohegan, David B.
      ACS Nano (2020), 14 (4), 3896-3906CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Atomically thin two-dimensional (2D) materials face significant energy barriers for synthesis and processing into functional metastable phases such as Janus structures. Here, the controllable implantation of hyperthermal species from pulsed laser deposition (PLD) plasmas is introduced as a top-down method to compositionally engineer 2D monolayers. The kinetic energies of Se clusters impinging on suspended monolayer WS2 crystals were controlled in the <10 eV/atom range with in situ plasma diagnostics to det. the thresholds for selective top layer replacement of sulfur by selenium for the formation of high quality WSSe Janus monolayers at low (300°C) temps. and bottom layer replacement for complete conversion to WSe2. Atomic-resoln. electron microscopy and spectroscopy in tilted geometry confirm the WSSe Janus monolayer. Mol. dynamics simulations reveal that Se clusters implant to form disordered metastable alloy regions, which then recrystallize to form highly ordered structures, demonstrating low-energy implantation by PLD for the synthesis of 2D Janus layers and alloys of variable compn.
    19. 19
      Lu, A.-Y.; Zhu, H.; Xiao, J.; Chuu, C.-P.; Han, Y.; Chiu, M.-H.; Cheng, C.-C.; Yang, C.-W.; Wei, K.-H.; Yang, Y. Janus monolayers of transition metal dichalcogenides. Nat. Nanotechnol. 2017, 12 (8), 744– 749,  DOI: 10.1038/nnano.2017.100
      19
      Janus monolayers of transition metal dichalcogenides
      Lu, Ang-Yu; Zhu, Hanyu; Xiao, Jun; Chuu, Chih-Piao; Han, Yimo; Chiu, Ming-Hui; Cheng, Chia-Chin; Yang, Chih-Wen; Wei, Kung-Hwa; Yang, Yiming; Wang, Yuan; Sokaras, Dimosthenis; Nordlund, Dennis; Yang, Peidong; Muller, David A.; Chou, Mei-Yin; Zhang, Xiang; Li, Lain-Jong
      Nature Nanotechnology (2017), 12 (8), 744-749CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)
      Structural symmetry-breaking plays a crucial role in detg. the electronic band structures of 2-dimensional materials. Tremendous efforts have been devoted to breaking the in-plane symmetry of graphene with elec. fields on AB-stacked bilayers or stacked van der Waals heterostructures. But transition metal dichalcogenide monolayers are semiconductors with intrinsic in-plane asymmetry, leading to direct electronic bandgaps, distinctive optical properties and great potential in optoelectronics. Apart from their in-plane inversion asymmetry, an addnl. degree of freedom allowing spin manipulation can be induced by breaking the out-of-plane mirror symmetry with external elec. fields or, as theor. proposed, with an asym. out-of-plane structural configuration. Here, the authors report a synthetic strategy to grow Janus monolayers of transition metal dichalcogenides breaking the out-of-plane structural symmetry. In particular, based on a MoS2 monolayer, the authors fully replace the top-layer S with Se atoms. The authors confirm the Janus structure of MoSSe directly by scanning TEM and energy-dependent XPS, and prove the existence of vertical dipoles by 2nd harmonic generation and piezoresponse force microscopy measurements.
    20. 20
      Schmeink, J.; Musytschuk, V.; Pollmann, E.; Sleziona, S.; Maas, A.; Kratzer, P.; Schleberger, M. Evaluating strain and doping of Janus MoSSe from phonon mode shifts supported by ab initio DFT calculations. Nanoscale 2023, 15 (25), 10834– 10841,  DOI: 10.1039/D3NR01978K
      There is no corresponding record for this reference.
    21. 21
      Zhang, J.; Jia, S.; Kholmanov, I.; Dong, L.; Er, D.; Chen, W.; Guo, H.; Jin, Z.; Shenoy, V. B.; Shi, L.; Lou, J. Janus Monolayer Transition-Metal Dichalcogenides. ACS Nano 2017, 11 (8), 8192– 8198,  DOI: 10.1021/acsnano.7b03186
      21
      Janus Monolayer Transition-Metal Dichalcogenides
      Zhang, Jing; Jia, Shuai; Kholmanov, Iskandar; Dong, Liang; Er, Dequan; Chen, Weibing; Guo, Hua; Jin, Zehua; Shenoy, Vivek B.; Shi, Li; Lou, Jun
      ACS Nano (2017), 11 (8), 8192-8198CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      The crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2, the top layer of selenium atoms is substituted by sulfur atoms, while the bottom selenium layer remains intact. The structure of this material is systematically investigated by Raman, photoluminescence, transmission electron microscopy, and XPS and confirmed by time-of-flight secondary ion mass spectrometry. D. functional theory (DFT) calcns. are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding exptl. results. Finally, high basal plane hydrogen evolution reaction activity is discovered for the Janus monolayer, and DFT calcn. implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.
    22. 22
      Trivedi, D. B.; Turgut, G.; Qin, Y.; Sayyad, M. Y.; Hajra, D.; Howell, M.; Liu, L.; Yang, S.; Patoary, N. H.; Li, H. Room-temperature synthesis of 2D Janus crystals and their heterostructures. Adv. Mater. 2020, 32 (50), 2006320  DOI: 10.1002/adma.202006320
      22
      Room-Temperature Synthesis of 2D Janus Crystals and their Heterostructures
      Trivedi, Dipesh B.; Turgut, Guven; Qin, Ying; Sayyad, Mohammed Y.; Hajra, Debarati; Howell, Madeleine; Liu, Lei; Yang, Sijie; Patoary, Naim Hossain; Li, Han; Petric, Marko M.; Meyer, Moritz; Kremser, Malte; Barbone, Matteo; Soavi, Giancarlo; Stier, Andreas V.; Mueller, Kai; Yang, Shize; Esqueda, Ivan Sanchez; Zhuang, Houlong; Finley, Jonathan J.; Tongay, Sefaattin
      Advanced Materials (Weinheim, Germany) (2020), 32 (50), 2006320CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Janus crystals represent an exciting class of 2D materials with different at. species on their upper and lower facets. Theories have predicted that this symmetry breaking induces an elec. field and leads to a wealth of novel properties, such as large Rashba spin-orbit coupling and formation of strongly correlated electronic states. Monolayer MoSSe Janus crystals have been synthesized by two methods, via controlled sulfurization of monolayer MoSe2 and via plasma stripping followed thermal annealing of MoS2. However, the high processing temps. prevent growth of other Janus materials and their heterostructures. Here, a room-temp. technique for the synthesis of a variety of Janus monolayers with high structural and optical quality is reported. This process involves low-energy reactive radical precursors, which enables selective removal and replacement of the uppermost chalcogen layer, thus transforming classical transition metal dichalcogenides into a Janus structure. The resulting materials show clear mixed character for their excitonic transitions, and more importantly, the presented room-temp. method enables the demonstration of first vertical and lateral heterojunctions of 2D Janus TMDs. The results present significant and pioneering advances in the synthesis of new classes of 2D materials, and pave the way for the creation of heterostructures from 2D Janus layers.
    23. 23
      Sayyad, M.; Kopaczek, J.; Gilardoni, C. M.; Chen, W.; Xiong, Y.; Yang, S.; Watanabe, K.; Taniguchi, T.; Kudrawiec, R.; Hautier, G. The Defects Genome of Janus Transition Metal Dichalcogenides. Adv. Mater. 2024, 36, 2403583  DOI: 10.1002/adma.202403583
      There is no corresponding record for this reference.
    24. 24
      Lakshmy, S.; Mondal, B.; Kalarikkal, N.; Rout, C. S.; Chakraborty, B. Recent developments in synthesis, properties, and applications of 2D Janus MoSSe and MoSexS(1-x) alloys. Adv. Powder Mater. 2024, 3, 100204  DOI: 10.1016/j.apmate.2024.100204
      There is no corresponding record for this reference.
    25. 25
      Bian, C.; Shi, J.; Liu, X.; Yang, Y.; Yang, H.; Gao, H. Optical second-harmonic generation of Janus MoSSe monolayer. Chin. Phys. B 2022, 31 (9), 097304  DOI: 10.1088/1674-1056/ac6db4
      There is no corresponding record for this reference.
    26. 26
      Harris, S. B.; Lin, Y.-C.; Puretzky, A. A.; Liang, L.; Dyck, O.; Berlijn, T.; Eres, G.; Rouleau, C. M.; Xiao, K.; Geohegan, D. B. Real-Time diagnostics of 2D crystal transformations by pulsed laser deposition: Controlled synthesis of Janus WSSe monolayers and alloys. ACS Nano 2023, 17 (3), 2472– 2486,  DOI: 10.1021/acsnano.2c09952
      26
      Real-Time Diagnostics of 2D Crystal Transformations by Pulsed Laser Deposition: Controlled Synthesis of Janus WSSe Monolayers and Alloys
      Harris, Sumner B.; Lin, Yu-Chuan; Puretzky, Alexander A.; Liang, Liangbo; Dyck, Ondrej; Berlijn, Tom; Eres, Gyula; Rouleau, Christopher M.; Xiao, Kai; Geohegan, David B.
      ACS Nano (2023), 17 (3), 2472-2486CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      A feedback approach to reveal and control the transformation pathways in materials synthesis by pulsed laser deposition (PLD) is reported and applied to study the transformation kinetics of monolayer WS2 crystals into Janus WSSe and WSe2 by implantation of Se clusters with different max. kinetic energies (<42 eV/Se-atom) generated by laser ablation of a Se target. Real-time Raman spectroscopy and luminescence are used to assess the structure, compn., and optoelectronic quality of the monolayer crystal as it is implanted with well-controlled fluxes of Se for different kinetic energies that are regulated with in situ intensified CCD imaging, ion probe, and spectroscopy diagnostics. First-principles calcns., XPS, and at.-resoln. HAADF STEM imaging are used to understand the intermediate alloy compns. and their vibrational modes to identify transformation pathways. The real-time kinetics measurements reveal highly selective top-layer conversion as WS2 transforms through WS2(1-x)Se2x alloys to WSe2 and provide the means to adjust processing conditions to achieve fractional and complete Janus WSSe monolayers as metastable transition states. The general approach demonstrates a real-time feedback method to achieve Janus layers or other metastable alloys of the desired compn., and a general means to adjust the structure and quality of materials grown by PLD, addressing priority research directions for precision synthesis with real-time adaptive control.
    27. 27
      Zheng, K.; Vegge, T.; Castelli, I. E. Giant In-Plane Flexoelectricity and Radial Polarization in Janus IV–VI Monolayers and Nanotubes. ACS Appl. Mater. Interfaces 2024, 16 (15), 19369– 19378,  DOI: 10.1021/acsami.4c01527
      There is no corresponding record for this reference.
    28. 28
      Liu, M.-Y.; Gong, L.; He, Y.; Cao, C. Tuning Rashba effect, band inversion, and spin-charge conversion of Janus XSn2Y monolayers via an external field. Phys. Rev. B 2021, 103 (7), 075421  DOI: 10.1103/PhysRevB.103.075421
      There is no corresponding record for this reference.
    29. 29
      Ahammed, R.; Jena, N.; Rawat, A.; Mohanta, M. K.; Dimple; De Sarkar, A. Ultrahigh out-of-plane piezoelectricity meets giant Rashba effect in 2D Janus monolayers and bilayers of group IV transition-metal trichalcogenides. J. Phys. Chem. C 2020, 124 (39), 21250– 21260,  DOI: 10.1021/acs.jpcc.0c05134
      29
      Ultrahigh Out-of-Plane Piezoelectricity Meets Giant Rashba Effect in 2D Janus Monolayers and Bilayers of Group IV Transition-Metal Trichalcogenides
      Ahammed, Raihan; Jena, Nityasagar; Rawat, Ashima; Mohanta, Manish K.; Dimple; De Sarkar, Abir
      Journal of Physical Chemistry C (2020), 124 (39), 21250-21260CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The simultaneous occurrence of gigantic piezoelectricity and Rashba effect in two-dimensional (2D) materials is unusually scarce. Inversion symmetry occurring in MX3 (M = Ti, Zr, Hf; X = S, Se) monolayers is broken upon constructing their Janus monolayer structures MX2Y (X ≠ Y =S, Se), thereby inducing a large out-of-plane piezoelec. const. d33 (~68 pm/V) in them. d33 can be further enhanced to a super high value of ~ 1000 pm/V upon applying vertical compressive strain in the van der Waals bilayers constituted by interfacing these Janus monolayers. Therefore, d33 in these Janus transition-metal trichalcogenide (TMTC) bilayers reach more than 4-fold times that of bulk ceramic PZT material (~ 268 pm/V). The absence of a horizontal mirror symmetry and the presence of strong spin-orbit coupling cause Rashba spin-splitting in electronic bands in these Janus 2D monolayers, which shows up as an ultrahigh Rashba parameter, αR ~1.1 eV Å. It can be raised to 1.41 eV Å via compressive strain. Most of the 2D materials reported to date mainly show in-plane elec. polarization, which severely limits their prospects in piezotronic devices. In this present work, the piezoelectricity shown by the Janus monolayers of group IV TMTCs and their bilayers is significantly higher than the ones generally utilized in the form of three-dimensional bulk piezoelec. solids, for example, α-quartz (d11 = 2.3 pm/V), wurtzite-GaN (d33 = 3.1 pm/V), and wurtzite-AlN (d33 = 5.6 pm/V). It is exceedingly higher than that in Janus multilayer/bulk structures of Mo- and W-based transition-metal dichalcogenides, for example, MoSTe (d33 ~ 10 pm/V). The 2D Janus TMTC monolayers and their bilayers reported herewith straddle giant Rashba spin-splitting and ultrahigh piezoelectricity, thereby making them immensely promising candidates in the next-generation electronics, piezotronics, spintronics, flexible electronics, and piezoelec. devices.
    30. 30
      Varjovi, M. J.; Yagmurcukardes, M.; Peeters, F. M.; Durgun, E. Janus two-dimensional transition metal dichalcogenide oxides: First-principles investigation of WXO monolayers with X = S, Se, and Te. Phys. Rev. B 2021, 103 (19), 195438  DOI: 10.1103/PhysRevB.103.195438
      30
      Janus two-dimensional transition metal dichalcogenide oxides: First-principles investigation of WXO monolayers with X=S, Se, and Te
      Varjovi, M. Jahangirzadeh; Yagmurcukardes, M.; Peeters, F. M.; Durgun, E.
      Physical Review B (2021), 103 (19), 195438CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      Structural symmetry breaking in two-dimensional materials can lead to superior phys. properties and introduce an addnl. degree of piezoelectricity. In the present paper, we propose three structural phases (1H, 1T, and 1T') of Janus WXO (X=S, Se, and Te) monolayers and investigate their vibrational, thermal, elastic, piezoelec., and electronic properties by using first-principles methods. Phonon spectra anal. reveals that while the 1H phase is dynamically stable, the 1T phase exhibits imaginary frequencies and transforms to the distorted 1T' phase. Ab initio mol. dynamics simulations confirm that 1H- and 1T'-WXO monolayers are thermally stable even at high temps. without any significant structural deformations. Different from binary systems, addnl. Raman active modes appear upon the formation of Janus monolayers. Although the mech. properties of 1H-WXO are found to be isotropic, they are orientation dependent for 1T'-WXO. It is also shown that 1H-WXO monolayers are indirect band-gap semiconductors and the band gap narrows down the chalcogen group. Except 1T'-WSO, 1T'-WXO monolayers have a narrow band gap correlated with the Peierls distortion. The effect of spin-orbit coupling on the band structure is also examd. for both phases and the alteration in the band gap is estd. The versatile mech. and electronic properties of Janus WXO monolayers together with their large piezoelec. response imply that these systems are interesting for several nanoelectronic applications.
    31. 31
      Yang, Q.; Wang, D.; Zeng, Z.-Y.; Geng, H.-Y.; Chen, X.-R. High-performance photocatalytic and piezoelectric properties of two-dimensional transition metal oxyhalide ZrO X 2 (X= Br, I) and their Janus structures. Phys. Rev. B 2024, 109 (3), 035411  DOI: 10.1103/PhysRevB.109.035411
      There is no corresponding record for this reference.
    32. 32
      Petrić, M. M.; Kremser, M.; Barbone, M.; Qin, Y.; Sayyad, Y.; Shen, Y.; Tongay, S.; Finley, J. J.; Botello-Méndez, A. R.; Müller, K. Raman spectrum of Janus transition metal dichalcogenide monolayers WSSe and MoSSe. Phys. Rev. B 2021, 103 (3), 035414  DOI: 10.1103/PhysRevB.103.035414
      32
      Raman spectrum of janus transition metal dichalcogenide monolayers tungsten selenide sulfide and molybdenum selenide sulfide
      Petric, Marko M.; Kremser, Malte; Barbone, Matteo; Qin, Ying; Sayyad, Yasir; Shen, Yuxia; Tongay, Sefaattin; Finley, Jonathan J.; Botello-Mendez, Andres R.; Mueller, Kai
      Physical Review B (2021), 103 (3), 035414CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)
      Janus transition metal dichalcogenides (TMDs) lose the horizontal mirror symmetry of ordinary TMDs, leading to the emergence of addnl. features, such as native piezoelectricity, Rashba effect, and enhanced catalytic activity. While Raman spectroscopy is an essential nondestructive, phase- and compn.-sensitive tool to monitor the synthesis of materials, a comprehensive study of the Raman spectrum of Janus monolayers is still missing. Here, we discuss the Raman spectra of WSSe and MoSSe measured at room and cryogenic temps., near and off resonance. By combining polarization-resolved Raman data with calcns. of the phonon dispersion and using symmetry considerations, we identify the four first-order Raman modes and higher-order two-phonon modes. Moreover, we observe defect-activated phonon processes, which provide a route toward a quant. assessment of the defect concn. and, thus, the crystal quality of the materials. Our work establishes a solid background for future research on material synthesis, study, and application of Janus TMD monolayers.
    33. 33
      Yu, M.; Ascolani, H.; Zampieri, G.; Woodruff, D. P.; Satterley, C. J.; Jones, R. G.; Dhanak, V. R. The Structure of Atomic Sulfur Phases on Au(111). J. Phys. Chem. C 2007, 111 (29), 10904– 10914,  DOI: 10.1021/jp072088+
      33
      The Structure of Atomic Sulfur Phases on Au(111)
      Yu, Miao; Ascolani, H.; Zampieri, G.; Woodruff, D. P.; Satterley, C. J.; Jones, Robert G.; Dhanak, V. R.
      Journal of Physical Chemistry C (2007), 111 (29), 10904-10914CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The structural phases formed by at. sulfur on Au(111) due to reaction with mol. S2 have been investigated by qual. LEED, scanning tunneling microscopy, and normal incidence X-ray standing wavefield absorption (NIXSW) combined with XPS. Three phases are identified with increasing coverage, namely, a newly identified (5 × 5) phase, a (√3 × √3)R30° phase, and a "complex" phase. The (5 × 5) phase, with a LEED pattern having the appearance of a "split-spot" (√3 × √3)R30° pattern, is interpreted in terms of local (√3 × √3)R30° ordering within a (5 × 5) ordered domain structure. The S atoms in the (5 × 5) phase occupy fcc hollow sites 1.56 Å above the outermost extended Au(111) bulk at. scatterer plane. A specific model of the ordering in this phase is proposed that, together with the obsd. marginal stability of the true, long-range-ordered, (√3 × √3)R30° phase, indicates significant short-range S-S repulsion and probably compressive surface stress. The complex phase, that coexists in a poorly ordered state with the lower coverage at. chemisorption phases, is interpreted in terms of an incommensurate long-range periodicity, but the NIXSW data shows clear evidence of local commensuration, with the S atoms mainly close to atop sites relative to the underlying Au(111) substrate; these data provide strong support for a previously proposed model based on a sulfide layer of stoichiometry AuS.
    34. 34
      Yasuda, S.; Takahashi, R.; Osaka, R.; Kumagai, R.; Miyata, Y.; Okada, S.; Hayamizu, Y.; Murakoshi, K. Out-of-Plane Strain Induced in a Moiré Superstructure of Monolayer MoS2 and MoSe2 on Au(111). Small 2017, 13 (31), 1700748  DOI: 10.1002/smll.201700748
      There is no corresponding record for this reference.
    35. 35
      Dutta, B.; Dayal, B. Lattice Constants and Thermal Expansion of Gold up to 878° C by X-Ray Method. Phys. Status Solidi B 1963, 3 (3), 473– 477,  DOI: 10.1002/pssb.19630030312
      There is no corresponding record for this reference.
    36. 36
      Picker, J.; Schaal, M.; Gan, Z.; Gruenewald, M.; Neumann, C.; George, A.; Otto, F.; Forker, R.; Fritz, T.; Turchanin, A. Structural and electronic properties of MoS2 and MoSe2 monolayers grown by chemical vapor deposition on Au(111). Nanoscale Adv. 2023, 6 (1), 92– 101,  DOI: 10.1039/D3NA00475A
      There is no corresponding record for this reference.
    37. 37
      Wen, Y.-N.; Xia, M.-G.; Zhang, S.-L. Bandgap engineering of Janus MoSSe monolayer implemented by Se vacancy. Comput. Mater. Sci. 2018, 152, 20– 27,  DOI: 10.1016/j.commatsci.2018.05.023
      37
      Bandgap engineering of Janus MoSSe monolayer implemented by Se vacancy
      Wen, Yan-Ni; Xia, Ming-Gang; Zhang, Sheng-Li
      Computational Materials Science (2018), 152 (), 20-27CODEN: CMMSEM; ISSN:0927-0256. (Elsevier B.V.)
      Vacancy defects in 2D materials provide opportunities to tailor local phys., structural and electronic properties of point and linear vacancies in Janus MoSSe monolayers. In this paper, we studied vacancy formation in Janus MoSSe monolayers using ab initio d. functional theory and obsd. that all vacancies are preferentially formed in the Se layer. In the case of point defects with more than two vacancies, a zigzag line feature is obtained for the lowest formation energy. In the case of infinite linear defects, the zigzag vacancy lines preferred to be distant from each other. The bandgap of a Janus MoSSe monolayer can be modulated by the concn. of vacancies. The bandgap energy decreased from 1.080 eV to 0.675 eV with the increase in the no. of point vacancies. However, it is obsd. to oscillate around 0.530 eV with the increase of distance between the vacancy lines in the case of linear vacancies. This work is very useful in bandgap engineering of optical and electronic devices based on MoS2.
    38. 38
      Silva, C. C.; Dombrowski, D.; Atodiresei, N.; Jolie, W.; Farwick zum Hagen, F.; Cai, J.; Ryan, P. T.; Thakur, P. K.; Caciuc, V.; Blügel, S. Spatial variation of geometry, binding, and electronic properties in the moiré superstructure of MoS2 on Au(111). 2D Mater. 2022, 9 (2), 025003  DOI: 10.1088/2053-1583/ac4958
      There is no corresponding record for this reference.
    39. 39
      Xiao, D.; Liu, G.-B.; Feng, W.; Xu, X.; Yao, W. Coupled spin and valley physics in monolayers of MoS2 and other group-VI dichalcogenides. Phys. Rev. Lett. 2012, 108 (19), 196802  DOI: 10.1103/PhysRevLett.108.196802
      39
      Coupled spin and valley physics in monolayers of MoS2 and other Group-VI dichalcogenides
      Xiao, Di; Liu, Gui-Bin; Feng, Wanxiang; Xu, Xiaodong; Yao, Wang
      Physical Review Letters (2012), 108 (19), 196802/1-196802/5CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      We show that inversion symmetry breaking together with spin-orbit coupling leads to coupled spin and valley physics in monolayers of MoS2 and other group-VI dichalcogenides, making possible controls of spin and valley in these 2D materials. The spin-valley coupling at the valence-band edges suppresses spin and valley relaxation, as flip of each index alone is forbidden by the valley-contrasting spin splitting. Valley Hall and spin Hall effects coexist in both electron-doped and hole-doped systems. Optical interband transitions have frequency-dependent polarization selection rules which allow selective photoexcitation of carriers with various combination of valley and spin indexes. Photoinduced spin Hall and valley Hall effects can generate long lived spin and valley accumulations on sample boundaries. The physics discussed here provides a route towards the integration of valleytronics and spintronics in multivalley materials with strong spin-orbit coupling and inversion symmetry breaking.
    40. 40
      Yu, S.-B.; Zhou, M.; Zhang, D.; Chang, K. Spin Hall effect in the monolayer Janus compound MoSSe enhanced by Rashba spin-orbit coupling. Phys. Rev. B 2021, 104 (7), 075435  DOI: 10.1103/PhysRevB.104.075435
      There is no corresponding record for this reference.
    41. 41
      Calis, M.; Lloyd, D.; Boddeti, N.; Bunch, J. S. Adhesion of 2D MoS2 to Graphite and Metal Substrates Measured by a Blister Test. Nano Lett. 2023, 23 (7), 2607– 2614,  DOI: 10.1021/acs.nanolett.2c04886
      There is no corresponding record for this reference.

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