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Structural Phase Stability Control of Monolayer MoTe with Adsorbed Atoms and Molecules 2

Yao Zhou, and Evan J. Reed J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b05770 • Publication Date (Web): 25 Aug 2015 Downloaded from http://pubs.acs.org on August 31, 2015

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The Journal of Physical Chemistry

Structural Phase Stability Control of Monolayer MoTe2 with Adsorbed Atoms and Molecules Yao Zhou and Evan J. Reed* Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States *E-mail: [email protected]. Tel: [+1] (650) 723 2971. Fax: [+1] (650) 725-4034

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Structural Phase Stability Control of Monolayer MoTe2 with Adsorbed Atoms and Molecules ABSTRACT

We study the adsorption of some common atoms and molecules onto monolayer MoTe2 and the potential for adsorption to induce a structural phase change between the semiconducting 2Hbased and metallic 1T’-based crystal structures of the monolayer. Using density functional theory with spin orbit and van der Waals energy contributions, we determined energetically favorable adsorption positions and orientations on the two crystalline phases of monolayer MoTe2. We then obtained the formation energies for these adsorption reactions and found that atomic adsorption generally favors 1T’ metallic phases while molecular adsorption favors semiconducting 2H phases. The phase sensitivity of this material is due to a relatively small energy difference, approximately 31 meV per MoTe2 formula unit. We further find that the monolayer alloy MoxW1-xTe2 can exhibit some degree of molecular selectivity in phase changes, potentially providing the basis for molecular sensing applications.

Introduction


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The synthesis and subsequent discovery of the unusual properties of graphene1 has created interest in a broader spectrum of materials that can be isolated in a nearly atomically thin form. Transition metal dichalcogenide (TMD) monolayers, where the transition metal can be Mo or W and the chalcogenide can be S, Se or Te, are one such group of monolayers with special structural features. Unlike graphene, these TMD monolayers have the potential to exist in more than one possible crystal structure. One is the semiconducting 2H phase2 in which the transition metal atoms are trigonal prismatically coordinated by the chalcogen atoms. The others are the metallic 1T phase with transition metal atoms in octahedral coordination, and a distorted version of this phase that we will refer to as 1T’ phase where transition metal atoms are also approximately octahedrally coordinated but with lower symmetry3,4. The 2D lattice of a 1T’ monolayer is rectangular while the 2D lattices of the 2H and 1T monolayers are hexagonal. Earlier DFT-based calculations have been reported for these Mo and W-based monolayer TMDs indicating that the 2H phase is the lowest energy structure for all except WTe2, which exhibits lowest energy in the 1T’ phase4. Chemical control over the crystal structure has been reported in some transition metal dichaocogenides5, 6, including the potential for spatial control of the distribution of phases on the monolayer7. The interface between semiconducting and metallic phases on MoS2 has been reported to exhibit favorable electrical contact resistance characteristics, pointing to possible electronic device applications for phase engineering8. Among the Mo- and W- based TMD monolayers, MoTe2 is particularly interesting for phase change applications because it has the smallest energy difference between 2H phase and 1T’ phase as shown in Figure 1. We computed a 31 meV energy difference between these phases for MoTe2 a monolayer at zero stress conditions using a DFT calculation including spin-orbit


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coupling effects as discussed in the Methods section. This relatively small energy difference leads to the reported possibility that tensile uniaxial strains of several percent can change the energy ordering of the phases, leading to a structural phase transition4. There is also experimental evidence that a structural transition from 2H to 1T’ can occur in MoTe2 at high temperatures9,10.

Figure 1. Three crystalline phases of monolayer MoTe2 represented in a 1×1 rectangular unit cell. 2H phase is trigonal prismatic structure and semiconducting, while 1T phase and 1T’ phase are octahedral and a distorted octahedral structure, respectively. 1T phase and 1T’ phase are both metallic. Top view, side view and 3D view of each phase are shown in the figure.

The proximity of monolayer MoTe2 to a structural phase transition suggests the phase stability of this material could be sensitive to the local chemical environment due to its nearly atomic thickness. This property could lead to complexities or opportunities in controlling the phase synthesized by a growth process, e.g. chemical vapor deposition (CVD). Some knowledge of the effect of adsorbed molecules on phase stability may provide guidance for controlling the energetically favored phase during synthesis. For example, one would like to know if there exist


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molecules that can be added to the system during growth that will bias toward a particular phase. DFT-based adsorption sites for H, O, and F atoms on the 2H phase of MoTe2 monolayer without van der Waals corrections and spin orbit coupling have been reported11, indicating minimum energy binding positions similar to those found here. Another motivation to study the phase stability in the presence of molecules or adatoms is for chemical sensing applications. Two-dimensional materials are good candidates for sensing materials due to their high surface to volume ratio. Carbon nanotubes12,13 and graphene14,15 have been reported to provide solid-state gas sensors with exceptional sensitivity. In the TMD family, MoS2 has been reported to be used for glucose and biomolecule detection16, modulation of photoluminescence by molecular physisorption gating17 or ion intercalation18, and gas detection for NH3, NO2, water vapor19, and triethylamine20. Li intercalated in MoS2 has been reported to yield a structural change from 2H to 1T’21,22,23,24. The MoTe2 monolayer has the potential to exhibit more sensitivity to molecular adsorption due to the small energy difference between 2H and 1T’ due to the large expected changes in electronic properties across the transition. DFT calculations of MoTe2 indicate that 2H is semiconducting and 1T’ is metallic or semimetallic,4 and some experimental evidence for significant electronic changes has been reported10. This semiconducting to metallic phase transition induced by molecular or atomic adsorption could provide a mechanism for enhanced chemical sensitivity or specificity. Field effect transistors have been reported using single layer MoTe225. Optical properties of single- and few-layer MoTe2 have been studied and strong photoluminescence has been observed in monolayer MoTe226. These studies indicate that few layer MoTe2 is sufficiently stable to perform experiments and construct devices.


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In this work, we employ density functional methods to explore the potential for adsorbed molecules and atoms to bias the favored thermodynamic phase of MoTe2. We study the formation energies of a range of adsorbed small molecules and adatoms for binding to the surface of 2H and 1T’ MoTe2 to determine the potential for inducing a phase transformation by exposure to these small molecules.

Methods Density functional theory (DFT) was used to study molecule adsorptions on MoTe2 monolayers. All DFT calculations were performed with the Vienna Ab Initio Simulation Package (VASP)27, using the Projected-Augmented Wave (PAW) method28,29. For Mo, Te, H, C, N, O, Cl and F, we used standard PAW pseudopotential implementation methods, treating 6, 6, 1, 4, 5, 6, 7 and 7 valence electrons respectively. Since we expected electrons to transfer from alkali atoms to MoTe2 monolayers, for Li, Na and K, we used PAW pseudopotentials where semi-core electrons are also treated as valence electrons, including 3, 7 and 7 valence electrons respectively. In addition, for oxygen molecule adsorption, we used a harder O pseudopotential for formation energy calculations for O2 in Figure 4 and Figure 5. Electron exchange and correlation effects were treated using the generalized gradient approximation (GGA) functional of Perdew, Burke, and Ernzerhof30. However, GGA tends to underestimate the binding energies31 in the systems where nonlocal correlations play an important role. We have studied a range of methods for including van der Waals interactions within DFT calculations, including the D3 correction method of Grimme (DFT-D3)32, van der Waals density functional (vdW-DF)33 and the Tkatchenko-Scheffler method (TS-SCS)34. In addition, due to the absence of inversion symmetry


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in monolayer MoTe2, it is expected that spin-orbit coupling effects will cause band splitting35. Especially for MoTe2, where we have the heavy element Te, we expect such spin-orbit coupling effect should be strong and non-negligible. In Figure 2, we compared these different methods for the case of oxygen molecule adsorption. In all subsequent calculations (except those involving Li, Na, K) we employ the TS-SCS method which gives results comparable to DFT-D3 for the case of Figure 2. Due to the reported poor performance of TS-SCS method for alkali metals36, we used the DFT-D3 method for Li, Na, K atomic adsorption.

Figure 2. Comparison of formation energies of oxygen molecule adsorption on unit cell of 2HMoTe2 monolayer and 1T’-MoTe2 monolayer for different methods. The methods describing van der Waals interactions (DFT-D3, vdW-DF, TS-SCS) are done with spin-orbit coupling included. We include spin-orbit coupling and van der Waals interactions using TS-SCS or DFT-D3 for all subsequent adsorption cases in this manuscript.

We used a plane-wave basis set with different energy cutoffs for different atomic and molecular adsorption cases. We chose a cutoff greater than 1.3 times the largest ENMAX value given in the potential POTCAR-file for each adsorption case to ensure convergence. We made an exception


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to this choice for systems with O where we used a hard pseudopotential with the 700eV cutoff recommended in the POTCAR-file. For Li and O2 with a hard O pseudoptential, we used a kinetic energy cutoff of 700 eV. For Na, K, H, H2 and Cl we used a 350 eV energy cutoff. For O2 with a regular O pseudopotential, we used a 600 eV energy cutoff. For H2O, NH3, NO, NO2, CO, CO2, N2, O, and F we used a 550 eV energy cutoff. We used the same energy cutoff for calculations of each adsorption case. We sampled the Brillouin zone using an 18×18×1 Monkhorst-Pack37 k-point grid. Also, we used a Gaussian electronic distribution smearing of 50 meV. The computational cell is 20Å along the interlayer direction, which gives a vacuum slab distance of approximately 16Å between monolayers. We did DFT calculations for two systems, one with higher adsorbate concentration including a 1×1 unit cell with two formula units of MoTe2 and one adsorbate atom or molecule, and another with lower adsorbate concentration including a 2×2 unit cell with eight formula units of MoTe2 and one adsorbate atom or molecule. With periodic boundary conditions and when the computational cell is electrically polarized, the requirement for the Kohn-Sham potential to be periodic can introduce undesirable or fictitious electric fields that exist throughout the vacuum region of the cell38,39,40. These were corrected using the method of Neugebauer and Scheffler, which provides a correction for the total energy40. To determine a likely minimum energy configuration for each adsorbed molecule or atom, we performed geometry relaxation calculations starting with the molecule at one of three initial adsorption sites for the higher adsorbate concentration computational cell containing 2 formula units of MoTe2. For the 2H phase, the three sites are top of Mo atom, top of Te atom, and center of hexagon. For the 1T phase, the three sites are the top of Mo atom, and top of two different Te atom sites. Multiple calculations were done with different orientations for some molecules. During relaxations, we found all adsorptions on 1T-MoTe2 relaxed to 1T’-MoTe2.


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Preferential binding positions and orientations for each adsorbate on 2H-MoTe2 and 1T’-MoTe2 were taken to be the lowest energy configuration obtained for all initial conditions. To obtain the configurations at lower concentration with the 8 formula unit MoTe2 cell, we used these lowest energy configurations as initial configuration and performed geometry relaxation calculations. Starting from the lowest energy geometry, we performed further geometry relaxation including van der Waals energy using the TS-SCS method or DFT-D3 method and performed a selfconsistent spin-orbit coupling DFT calculation, except for the H2 case where we only did spin restricted calculation due to convergence difficulties . We use the equilibrium lattice constants of 2H-MoTe2 monolayer for all cases which assumes that substrate friction may prevent lattice constants from changing as discussed in the next section. Allowing the lattice constants to relax to zero stress may be more applicable to a freely-suspended monolayer and may give different results.

Results and Discussion We wish to study the potential for the adsorption of small molecules and atoms to the 2H and 1T’ phases of monolayer MoTe2. In the absence of adsorbates. the 2H phase is energetically stable at ambient conditions and the 1T’ phase of the bulk material is reported to be stable at high temperatures9. We wish to determine the potential for bound molecules and atoms to alter the energetically favored phase, 2H or 1T’. The mechanical constraints on the monolayer are expected to play some role in the phase stability4 and therefore require consideration. At zero stress conditions, the 1T’ and 2H phases of the MoTe2 monolayer exhibit different lattice constants (2.7% and 3.7% strain in the a and b


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lattice directions of the rectangular cell from 2H phase to 1T’ phase). Therefore, a change in area of the monolayer would be expected if a transition from one phase to the other occurs at conditions of zero stress. However, a monolayer on top of a substrate may experience substrate friction that could prevent it from changing area if it remains planar. In the latter case, one might expect that the lattice constants of the material would remain approximately the same when the transition occurs. A third scenario may occur if the monolayer delaminates from the substrate, forming bubbles or wrinkles to relax the internal stress. To obtain a simple estimate of the likelihood of delamination effects, we used DFT-based calculations of elastic moduli of 2H-MoTe2 ଵ

monolayer41 to estimate the strain energy density ‫ = ܧ‬ଶ ‫ܥ‬௜௝ ߝ௜ ߝ௝ for a 2H-MoTe2 monolayer at the lattice constants of the 1T’ phase and obtained a value of 0.064 J/m2. Here, ߝ௜ and ߝ௝ are strains in the monolayer and ‫ܥ‬௜௝ are elastic stiffness coefficients. If this energy density is significantly greater than the energy of adhesion to the substrate, one would expect the monolayer to delaminate or ripple to relieve some of this energy42. While the substrate binding energy likely varies considerably with the type of substrate and the phase of MoTe2, some comparison can be made to the interlayer binding energy of graphite 0.19 J/m2 43 which is greater than the strain energy density. Therefore one might expect the monolayer to remain planar rather than form bubbles or wrinkles, but stress relieving wrinkles could occur for sufficiently small substrate binding energy. In the calculations that follow, we make the assumption that the monolayer remains planar across the transition. We furthermore assume that the lattice constants remain fixed across the


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transition, a condition that may be most appropriate for a monolayer constrained by substrate friction. We use the equilibrium lattice constants of the 2H-MoTe2 monolayer for all cases. Figure 3 shows the preferential binding positions and orientations of molecules and atoms on 2H-MoTe2 and 1T’-MoTe2 monolayer determined as described in Methods. We calculated formation energies for adsorption reactions with 2H-MoTe2 and 1T’-MoTe2 using the energies of these structures including the van der Waals and spin-orbit contributions.


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Figure 3. Most energetically favorable adsorption positions for Li, Na, K, O, Cl, H, F, H2, H2O, NH3, NO, NO2, CO, CO2, N2, O2 on 2×2 depicted supercell of (a) 2H-MoTe2 monolayer, (b) 1T’MoTe2 monolayer. Top view, side view and 3D view for each adsorption case are shown in the figure. Dashed black lines are drawn between nearest atoms to aid in visualization.

(a)

Li Na K

O Cl H2 H2O NH3 NO NO2 CO CO2 N2 O2 no X

(b)

Figure 4. Formation energy of Li, Na, K, O, Cl, H, F, H2, H2O, NH3, NO, NO2, CO, CO2, N2, O2 atomic or molecular adsorption on MoTe2 monolayer with initial structure (a)2H phase (stable phase at room temperature) and (b)1T’ phase (stable phase at high temperature). Two different


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adsorbate concentrations (one atom or molecule per unit cell or 2×2 supercell of MoTe2) are considered. Part (a) indicates that binding of molecules would energetically favor 2H phase while binding of Li, Na, K, H, O at higher concentration, Cl, and F would potentially cause phase transition from 2H to 1T’. The magnitude of formation energy depends on the choice of reference states (alkali metals and isolated molecules are used in this work). Atomic H and F are shown in an inset table due to the large magnitudes associated with the choice of reference states for these atoms (H2 and F2 respectively).

The blue and red bars in Figure 4(a) show the binding energies of molecules to the 2H phase for two different concentrations. These bars indicate the energy of the atom or molecule adsorbed on 2H-MoTe2 monolayer minus the energy of an isolated 2H-MoTe2 monolayer and the energy of the reference states of the atom or molecule. Here the reference states are taken to be Li metal, Na metal, K metal, O2 molecule, Cl2 molecule, H2 molecule, H2O molecule, NH3 molecule, NO molecule, NO2 molecule, CO molecule, CO2 molecule, N2 molecule, and F2 molecule. The choice of reference states impacts the binding energy to the 2H phase. We calculated reference state energies using DFT with the same methods as discussed in the Methods section. The blue and red bars in Figure 4(a) show that the molecules generally bind to the 2H phase while the atoms do not. While we expect typical DFT errors in these energies of tenths of eV34, 36, trends can be established with these calculations. The green and yellow bars in Figure 4(a) show the energy change associated with a species binding to the 2H phase and inducing a phase change to 1T’ for two different concentrations. These bars indicate the energy of the atom or molecule adsorbed on 1T’-MoTe2 monolayer


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minus the energy of an isolated 2H-MoTe2 monolayer minus the energy of the reference states of the atom or molecule. At higher concentrations (green bars), some molecules and atoms have the potential to induce a phase change to 1T’. As the concentration decreases (yellow bars), the energies are positive for all molecules indicating that a phase change to 1T’ is not likely. The trend with concentration results from the fact that the energy required to convert a region of 2H to 1T’ increases with the area. The energy per adsorbate required to convert to 1T’ increases as the concentration decreases. This trend is indicated in the green and yellow bars on the right of Figure 4(a) labeled “No X” that show the energies of these reactions with no adsorbate molecule. The monolayer phase that is energetically favored upon adsorption of a molecule is determined by the difference between the blue and green bars for higher concentrations, and red and yellow bars for lower concentrations. This energy difference is independent of choice of reference state. Figure 4(a) indicates that adsorption of the molecules generally stabilizes the 2H phase, while adsorption of adatoms usually stabilizes the 1T’ phase. Figure 4(b) is analogous to Figure 4(a) for 1T’ as the initial phase. All adsorbates studied are found to bind to the 1T’ phase (green and yellow bars). For the molecules, a subsequent change from the 1T’ to 2H is energetically favored upon binding, while the atoms will generally stabilize the 1T’ phase upon binding. Since the phase change is expected to be accompanied by a significant change in electronic properties, we consider the potential for gas molecular sensing applications and the degree of selectivity. Starting in the 2H phase, the lowest energy phase, Figure 4(a) indicates that the molecules considered here will bind, but none are expected to result in a phase change to the 1T’


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phase. NO2 may be the closest to causing a phase change of the molecules considered. Figure 4(b) indicates that starting in the metastable 1T’ phase, all molecules will bind and favor conversion to the 2H phase. Figure 4 assumes that substrate friction fixes the lattice constants across the transition at the 2H values. If substrate friction is negligible or the monolayer exhibits stress relieving wrinkles (discussed above), a constant stress mechanical constraint may be more relevant. Previous theoretical work has demonstrated that the boundary of a phase transition in monolayer MoTe2 is sensitive to whether the transition occurs at a condition of constant stress or constant area4, in analog with constant pressure or constant volume for bulk materials. The energy difference between the 2H and 1T’ phases of the monolayer at zero stress conditions is 31 meV/f.u., 44 meV/f.u. lower than the value of 75 meV/f.u. when the lattice constant are fixed at the 2H values. The closer proximity of the phase boundary at constant stress conditions has the potential to increase the likelihood that adsorbates that will induce the 2H to 1T’ transition. To explore the role of stress relaxation, we did calculations for N2 and Na adsorption under a zero stress condition at higher concentration. In the case of Na adsorption under zero stress, we find that the 1T’ phase is favored by 193 meV/f.u., 44 meV/f.u. higher than the corresponding energy for the constant area case. In the case of N2 adsorption under zero stress, we find that the 2H phase is favored by 65 meV/f.u., 13 meV/f.u. lower than the corresponding energy for the constant area case. This suggests that the favored phase for both cases is independent of mechanical constraint. However, the zero stress condition pushes the phase boundary toward the 1T’ structure and has the potential to cause the 1T’ phase to be favored for some molecular adsorbates, including NO where the 2H phase is stabilized by 30 meV/f.u., and NO2 where the 1T’ phase is stabilized by 9 meV/f.u..


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A potential route to increasing the molecular selectivity of this material is to consider an alloy designed such that the energies of the 1H and 1T’ phases are closer or even identical. Alloys of single layer materials including MoS2xSe2(x-1) have been reported44,45,46,47. Here we consider the alloy MoxW1-xTe2 because the lattice constant of the 2H phase of WTe2 (3.552Å and 6.154 Å from DFT) is similar to the 2H lattice constant of MoTe2 monolayer4 (3.550Å and 6.149Å from DFT). Unlike monolayer MoTe2, monolayer WTe2 is expected to be 1T’ in the lowest energy state4,3. It is likely that the energy difference between 2H and 1T’ phases of MoxW1-xTe2 can be tuned with fraction x without significant mechanical deformation. Inspired by this, we performed the calculations of Figure 4 for the alloy Mo0.5W0.5Te2 with transition metal atoms randomly distributed, and found that the 1T’ phase is energetically favored, independent of the adsorbed species. However, as the composition is tuned through the range 0 < x < 0.5, one might expect some adsorbed molecules to induce a phase change while others do not. Figure 5 shows the results of Figure 4(a) with the green and yellow bars shifted to reflect a zero energy difference between the 2H and 1T’ phases, i.e. shifted by the green and yellow bars on the right side of Figure 4(a). Figure 5 indicates that H2O, NO and NO2 molecules have the greatest potential to transform the 2H phase to 1T’ phase in a MoxW1-xTe2 monolayer alloy with x chosen to give the same energy for 2H and 1T’ phases. This suggests that alloying may provide some molecular selectivity. The unknown kinetics of the transition could play a role in the utility of this mechanism for molecular sensing.


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Figure 5. Estimated formation energy of small molecule adsorption on a MoxW1-xTe2 monolayer alloy with composition x chosen to yield the same energy for the 2H and 1T’ phases. The binding energies are approximated by those of pure MoTe2 (shown in Figure 4(a)) with the energy difference between the 2H and 1T’ phases set to zero. Two different adsorbate concentrations (one atom or molecule per unit cell or 2×2 supercell of MoTe2) are considered. Of the molecules studied, H2O, NO, and NO2 are the most likely to induce a phase transition by adsorption on the alloy monolayer, pointing to the possibility for some molecule specificity in a sensing application.

It is also possible to speculate that these results could have application in the preferential growth of phases of MoTe2 and its alloys. By introducing gases during the growth or cooling process, it may be possible to bias the growth toward the 2H or 1T’ phases. Potential complications include the spectrum of chemical reactions not considered here that could occur under high temperature growth conditions. Other environmental conditions are expected to impact the favored phase, including strain4 in the monolayer which may result from the growth and cooling process.


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Conclusions We have studied the adsorption of some common atoms and molecules onto monolayer MoTe2 and assessed the potential for adsorption to induce a phase change between the semiconducting 2H and metallic 1T’ crystal structures of the monolayer. We have employed spin orbit effects and studied a variety of van der Waals correction techniques for this purpose. We determined energetically favorable adsorption positions and orientations on the two phases of monolayer MoTe2. We find that atomic adsorption generally induces 1T’ metallic phases while molecular adsorption induces 2H phases. We further find that the monolayer alloy MoxW1-xTe2 has the potential to exhibit some degree of molecular selectivity in phase changes by varying the composition x, potentially providing the basis for molecular sensing applications due to the large electronic contrast between 2H and 1T’ phases. In particular, the calculations indicate that it may be possible to engineer an alloy such that specific molecules including NO2 and NO will induce a phase change to 1T’ while other molecules studied stabilize the 2H phase.

An

additional possible application of this work may be the chemical stabilization of a preferred phase during the growth process.

Acknowledgment This work was supported in part by the U.S. Army Research Laboratory, through the Army High Performance Computing Research Center, Cooperative Agreement W911NF-07-0027, and by computer and software support provided by the U.S. Army Engineer Research and Development Center (ERDC), DoD Supercomputing Resource Center (DSRC) through the DoD High Performance Computing Modernization Program (HPCMP).


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This work was partially supported by NSF grants EECS-1436626 and DMR-1455050. We thank Ludwig Bartels and his research team for helpful discussions. We would like to thank Alexander Duerloo, Yao Li and Qian Yang for useful comments.

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Table of Contents Image


22

Page 23 of 29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 29

top view

side view

3D view 2H

1T'

1T Te


Mo

Page 25 of 29

0.15

Formation Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


0.1 0.05 0 −0.05 −0.1 −0.15

2MoTe2(2H) + O2 → 2MoTe2(O2)0.5(2H)

−0.2

2MoTe2(2H) + O2 → 2MoTe2(O2)0.5(1T’)

−0.25

spin spin-orbit DFT-D3 vdW-DF TS-SCS polarized coupling



(a) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 29

Li

Na

K

O

Cl

H

F

H2

H2O

NH3

NO

NO2

CO

CO2

N2

O2

Li

Na

K

O

Cl

H

F

H2

H2O

NH3

NO

NO2

CO

CO2

N2

O2

(b)

K Li Cl F Te O Environment N H Na Plus MoACS Paragon

C

(a)

Page 27 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Li Na K

O Cl H2 H2O NH3 NO NO2 CO CO2 N2 O2 no X

(b)



0

Formation Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Page 28 of 29

−0.1 −0.2 −0.3 −0.4 −0.5

2MoxW1−xTe2(2H) + X → 2MoxW1−xTe2X0.5(2H) 8MoxW1−xTe2(2H) + X → 8MoxW1−xTe2X0.125(2H) 2MoxW1−xTe2(2H) + X → 2MoxW1−xTe2X0.5(1T’)

−0.6

8MoxW1−xTe2(2H) + X → 8MoxW1−xTe2X0.125(1T’)

H2 H2O NH3 NO NO2 CO CO2


N2

O2

Page 29 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


b

c

a

2H

b

c

a

1T’


Structural Phase Stability Control of Monolayer ... - ACS Publications

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