Tunable Dipole Moment in Janus Single-Layer MoSSe via Transition

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Tunable Dipole Moment in Janus Single-Layer MoSSe via Transition Metal Atom Adsorption Shengdan Tao, Bo Xu, Jing Shi, Shuying Zhong, Xueling Lei, Gang Liu, and Musheng Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00421 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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The Journal of Physical Chemistry

Tunable Dipole Moment in Janus Single-layer MoSSe via Transition Metal Atom Adsorption Shengdan Tao, Bo Xu*, Jing Shi, Shuying Zhong, Xueling Lei, Gang Liu, Musheng Wu* Department of Physics, Laboratory of Computational Materials Physics, Jiangxi Normal University, Nanchang 330022, China Abstract: 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 molecular sensor. 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 calculations. Our results demonstrate that the dipole moments of Janus MoSSe change with TM atom adsorption. For the adsorption of TM atoms on Se surface, the dipole moments are enhanced when compared to the case of pristine MoSSe, regardless of at M adsorption site or at 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 TM considered, the effect of Sc atom adsorption is the largest, while it is the smallest for Ni atom adsorption. By means of 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 the TM atom adsorption to tune the dipole moments with both magnitude and direction.

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1. INTRODUCTION Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) MX2 (M = Mo, W; X = S, Se) have received wide attention due to their unique structural, physical, and chemical properties,1 consequently as well as their potential in nanoelectronics2,3, valleytronics4,5, optoelectronics6,7, and catalysis8,9. Owing to the simplicity of the pure single-layered TMDs, people turn to investigate the mixed compounds for this kind of materials in order to obtain more and

tunable

properties.

For

example,

in-plane

solid-solution

alloy

of

MoSxSe2-x10,

MoS2(1-x)Se2x11,12, WSxSe2-x13, and MoxW1-xS214 are grown by effective control of the alloy composition of the 2D TMDs. The electronic and optical properties of these alloys are more abundant with respect to single TMDs, such as MoS2, MoSe2, or WS2. On the other hand, various out-of-plane and in-plane TMD heterojunctions have been prepared, such as MoS2/WS2,15 MoS2/MoSe2,16 MoS2/WSe2,17 MoSe2/WSe218, and MoS2/MoS2(1-x)Se2x19,20. Even the metal-TMDs heterojunctions were also be studied.21 The heterojunctions exhibit interesting physical properties owning to the difference of elements in the two layers, not only for TMD systems, but also for other novel 2D systems, such as graphene/GaS system22. Recently, a new type of well-organized Janus single-layer MoSSe has been successfully prepared by using the chemical vapor deposition (CVD) method.23,24 This Janus monolayer MoSSe breaks the out-of-plane structural symmetry by replacing one S (Se) layer with Se (S) atoms within MoS2 (MoSe2) monolayer. Although it was found that the Janus MoSSe has a similar electronic structure to MoS2 (MoSe2),25-28 an intrinsic dipole moment exists in the vertical direction of the MoSSe structure due to the mirror asymmetry.29-31 The appearance of Janus single-layer MoSSe triggers a new upsurge of TMDs research. As we know, atom modification is an effective method to tune the various properties of 2D materials.32 Previous reports demonstrated that the electronic and magnetic properties of graphene,33,34 silicene,35-40 and boron nitride41,42 are profoundly affected by TM atom adsorption. Besides aforementioned graphene-like 2D materials, direct-band-gap semiconducting and nonmagnetic TMDs are often decorated by adsorption doping so that the physical properties of the TMDs host are modulated. For example, topological Chern insulators with parabolic band dispersions are found in the MoS2 monolayer with 3d TM (TM = V, Cr, Mn, and Fe) atoms 2


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adsorbed.43 The valley degeneracy can be lifted and valley characters can be well kept when Sc, Mn, Fe, and Cu atoms are adsorbed on the MoS2 monolayer.44 Besides, adsorption of nonmetal elements (H, B, C, N, O, and F) on the surface of MoSe2, MoTe2, and WS2 monolayer induces local magnetic moments. Especially, H- and F-adsorbed WS2, MoSe2 monolayers exhibit long-range antiferromagnetic coupling between local moments.45 In addition, net magnetic moment upon adsorption of specific TM atoms, as well as Si and Ge atoms, on MoS2 are also obtained.46 Apart from atom adsorption, magnetic doping can also be used to manipulate the physics properties of 2D materials, e.g., valley polarization can be achieved in Janus single-layer MoSSe through magnetic doping.47 Unlike MoS2 (MoSe2) with mirror symmetry, possessing dipole moment is the intrinsic characteristic of Janus single-layer MoSSe. Intrinsic dipole moments is able to induce interesting properties of Janus structure, e.g. out-of-plane piezoelectricity.48 Besides, molecular adsorption on Janus monolayer also might be influenced by the dipole moments. Therefore, adjusting the dipole moment would effectively modify the dipole-related properties for Janus single-layer MoSSe, i.e., electronic, magnetic, and adsorption properties. Accordingly, the effect of atom adsorption on dipole moments is worth studying. For pristine MX2 monolayer, charge transfer happens between MX2 substrate and adatoms when the foreign atoms are adsorbed on the MX2 monolayer. As a result, the dipole moments arise for the atom-adsorbed MX2 system although there are no dipole moments in the original MX2 monolayer. Furthermore, the results are same for the atom adsorption on either surface (top or down) due to the mirror symmetry of pristine MX2 monolayer. In contrast, the situation of Janus single-layer MoSSe is totally different. Due to the intrinsic dipole moments, the additional dipole moments induced by atom adsorption would couple with the original one in the Janus MoSSe system. Therefore, the final dipole moment is more complicated when compared to the case of MX2 monolayer. In other word, the dipole moments are different for the atom adsorption on S surface and Se surface. This brings much room left for people to tune the dipole moment of Janus single-layer MoSSe. Motivated by the reasons mentioned above, in this work, we performed the systematic calculations to study the effect of atom adsorption on the dipole moments of Janus single-layer MoSSe. Herein, the 3d TM atoms (Sc-Ni) were employed in our calculations. Our results show that the dipole moment of Janus MoSSe system is tuned by the adsorption of TM atoms. From Sc 3



to Ni, the atom adsorption on Se surface trends to enhance the dipole moment of Janus MoSSe, while to weaken the dipole moment for the atom adsorption on S surface. Furthermore, based on the analysis of charge transfer between TM atoms and the Janus MoSSe, the variation of the dipole moments could be interpreted by the superposition of the dipole moments from the Janus MoSSe and from the ionic TM atoms. Our study provides a new route to tune the dipole moments in 2D Janus monolayer. 2. COMPUTATIONAL METHODS First-principles calculations were performed by using density functional theory (DFT) method as implemented in Vienna ab initio Simulation Package (VASP) code with a plane wave basis set.49 The electron-ion interaction was described by projected augmented wave (PAW) potentials.50,51 The exchange-correlation functional was described by the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functionals.52 The energy cut-off of 550 eV was employed for the plane wave expansion. The Monkhorst-Pack scheme53 with 3×3×1 k-point mesh was used for the integration in the irreducible Brillouin zone. A 3×3×1 supercell was adopted during the calculations. The tests for the different supercell sizes (4×4×1 and 2×2×1) were also performed. The corresponding results are shown in Supporting Information Table S1 and Figure S1. Such results ensure the convergence of the k-point setting for different supercell sizes. The vacuum space of 20 Å was used to avoid the interactions between the periodic images of the slabs. The convergence criteria for total energies and ionic forces were set to be 10-5 eV and 0.005 eV Å-1 in the formula unit. Due to the magnetism of TM atoms, the spin-polarized calculations were included in this work. Since the correlation effect is important in determining the electronic and magnetic properties of 3d TM, the GGA+U (with Hubbard U interaction) was employed with effective onsite Coulomb terms Ueff = U-J = 2.1 eV for all TM atoms (Sc , Ti ,V , Cr , Mn , Fe , Co , Ni ,Cu), which are the same as other report.43 3. RESULTS AND DISCUSSION 3.1 Atomic structures Figure 1 shows the relaxed Janus single-layer MoSSe atomic structures. In Janus MoSSe, the hexagonal Mo plane is sandwiched by the S and Se layers with unequal distance. In our calculations, the optimized lattice parameter of the primitive cell for Janus single-layer MoSSe is 3.25 Å, which agrees well with the previous reports.28,30 The corresponding Mo-S and Mo-Se 4


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bond lengths are 2.42 and 2.53 Å, respectively. The vertical dipole moment of primitive cell is calculated to be 0.18 Debye, also in agreement with the results of Ji et al.30 As the 3×3×1 supercell of single-layer MoSSe was employed for the adsorption of TM atoms, the final dipole moment for the pristine supercell is 1.63 Debye. Next, we turned to examine the stable adsorption sites of TM atoms on Janus single-layer MoSSe. Generally, there are three typical high-symmetry adsorption sites, regardless of S surface or Se surface, which are H (Hollow), M (Mo-top), and S/Se (S/Se-top), as shown in Figure 1a. The binding energy is used to figure out the most stable adsorption site, which is defined as: 𝐸𝑏 = 𝐸𝑀𝑜𝑆𝑆𝑒 + 𝐸𝑎𝑑𝑎𝑡𝑜𝑚 ― 𝐸𝑡𝑜𝑡𝑎𝑙, where 𝐸𝑀𝑜𝑆𝑆𝑒, 𝐸𝑎𝑑𝑎𝑡𝑜𝑚, and 𝐸𝑡𝑜𝑡𝑎𝑙 are the total energies of the pristine single-layer MoSSe, the single isolated TM atom, and the single-layer MoSSe with TM atoms adsorbed, respectively. According to this definition, positive binding energy means the stable adsorption of TM atoms. The binding energies of TM atoms for all adsorption sites (M, H, S/Se) are given in Supporting Information Table S2. Our calculations show that for each adsorption system (all TM atoms) the binding energy values for M and H sites are close to each other, which are essentially larger than that of the S/Se site, regardless of S surface or Se surface. In other word, the S/Se site is the most unstable adsorption site. As a result, the S/Se site is not considered in the following calculations. Furthermore, except for the case of Sc adsorbed on S surface of MoSSe, the binding energies for each TM atoms on Janus single-layer MoSSe at M sites are larger than those at H sites, regardless of S surface or Se surface, as listed in Table 1 and Table 2. Despite this, to enhance the universality of our results, two kinds of adsorption sites (M site and H site) are both employed in our calculations. As for the exception of Sc, the reason why the adsorption at H site is more stable could be explained by the different magnetic state, which would be discussed in the following section. In addition, the bond lengths between TM atoms and the nearest neighbor Mo or S/Se atoms, which are shown in Figure 1b, are also listed in Table 1 and Table 2. In order to examine the influence of TM atom adsorption on the mechanical properties of MoSSe, we calculated the elastic constants of Janus single-layer MoSSe without and with TM atoms. The calculation method refers to the previous report54. Table 3 lists the elastic constants (C11, C12) and various modulus (in-plane Young’s modulus Y, shear modulus G, and Poisson’s ratio υ) for all TM-MoSSe and pristine MoSSe. Our results show that the adsorption of TM atoms, 5



regardless of adsorption on S surface or Se surface, decreases the in-plane Young’s modulus and shear modulus of MoSSe, whereas increases the Poisson’s ratio. Among all the TM-MoSSe systems, Sc-MoSSe has the smallest in-plane Young’s modulus and shear modulus, while Ni-MoSSe has the largest ones. 3.2 Magnetic moment and electronic structure The 3d electrons within TM atoms generally induce the spin magnetic moments. Different magnetic states correspond to different total energies, thus essentially affecting the determination of ground states. For the pristine single-layer MoSSe, the magnetic moment is 0 μB. After adsorbing one TM atom on it, the distinct magnetic moment appears. Table 1 gives the magnetic moments of TM-adsorbed MoSSe with M adsorption site. The magnetic moments of MoSSe with Sc, Ti, V, Cr, Mn, Fe, Co, and Ni adsorbed on S surface are 3.0, 4.0, 5.0, 6.0, 5.0, 4.0, 1.0, 0.0 μB, respectively. Our results are in good agreement with previous results reported by Wei et al.43 and Chen et al.44, where TM atoms are adsorbed on single-layer MoS2. On the one hand, the magnetic moments change with the type of TM atoms. On the other hand, the adsorption properties of TM atoms on the S surface of MoSSe are similar to that of MoS2. When the TM atoms are adsorbed on the Se surface of MoSSe, the obtained magnetic moments are the same as those on the S surface. In order to analyze the origin of the total magnetic moments, we calculated the total density of states (TDOS) and the projected density of states (PDOS) of the TM atoms and the neighboring Mo atoms, both for adsorption on S surface and Se surface, as shown in Figure 2. Our DOS results show that the magnetic moments mainly originate from the TM atoms, and partly from the d-orbital of the neighboring Mo atoms. Therefore, the adsorption of TM atoms induces the magnetization of Mo atoms in MoSSe. To further compare, the magnetic moments of TM-adsorbed MoSSe with adatoms at H adsorption sites are listed in Table 2. From this table, one can see that the magnetic moments of TM-adsorbed (TM = V, Cr, Mn, Ni) MoSSe are the same as those at M adsorption site, regardless of adsorption on S surface or Se surface. However, the remaining situations with TM atoms (TM = Sc, Ti, Fe, Co) are somewhat different from the cases at M adsorption site. For the adsorption of TM on the S surface, MoSSe with Sc, Ti, and Fe adatoms prefer to be low-spin states when compared with the cases of M adsorption site, whose magnetic moments are 1.0, 2.0, and 2.0 μB, respectively. For the adsorption of TM on Se surface, MoSSe with Sc adatom still has total 6


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magnetic moment of 1.0 μB, while the Co-adsorbed MoSSe is high-spin state with total magnetic moment of 3.0 μB. Actually, Wei et al.43 also found that the different adsorption sites can result in the various total magnetic moments for TM-adsorbed MoS2. They ascribed it to the different initial heights setting for the TM atoms in the structural optimization calculations. Therefore, we tried to adjust the initial heights of the TM atoms in our calculations. Unfortunately, different initial heights are not able to induce the various magnetic moments. Such results could refer to Table S3 in Supporting Information. Based on our further analysis, however, we inferred that the difference of magnetic moments for TM atoms (TM = Sc, Ti, Fe, Co) at H site or M site could stem from the different magnetization of Mo atoms. To confirm this, we calculated the projected magnetic moments of Mo atoms with different adsorption sites of TM atoms, as seen in Supporting Information Table S4. It clearly demonstrates that the magnetic moments of Mo atoms change a lot when the total magnetic moments of TM-MoSSe with M adsorption site is different from that with H adsorption site. In contrast, similar magnetic moments of Mo atoms could be found for the same total magnetic moments with M and H adsorption sites. Despite this, the TDOS and PDOS results of TM atoms at H adsorption sites are basically similar to that at M adsorption sites. 3.3 Dipole moment As dipole moment is the intrinsic characteristic of Janus single-layered MoSSe, we also pay our attention to the change of MoSSe dipole moment after adsorbing TM atoms. As we know, the magnitude of dipole moment (p) is defined as the product of the distance (d) between the center of negative charge and positive charge and the charge quantity (q), p = qd. The dipole moment is directed from the positive charge to the negative one. In our calculations, we set the positive direction of dipole moments along z axis, which is directed from S layer to Se layer, as shown in Figure 1. The dipole moments of TM-adsorbed single-layer MoSSe are listed in Table 1 (M adsorption site) and Table 2 (H adsorption site). To observe conveniently, we also plotted all the dipole moments in Figure 3. Here, Figure 3a corresponds to the case of M adsorption site, while Figure 3b to the case of H adsorption site. As is displayed in Figure 3a, the variation trend of the dipole moments is apparently different for the adsorption of TM atoms on the S surface and Se surface. When TM atoms are adsorbed on the Se surface, the dipole moments of TM-MoSSe range from -3.341 D to -2.188 D for Sc to Ni. Except for Cr and Mn, approximate monotonous 7



trend could be observed for the adsorbed atoms changing from Sc to Ni. Compared to the results of pristine single-layer MoSSe (-1.631 D), TM atoms adsorption on the Se surface enhances the dipole moment of single-layer MoSSe. Furthermore, among all the TM atoms considered in our calculations, the effect of Sc adsorption is the most distinctive, while the situation of Ni adsorption is the least obvious. In contrast, the phenomenon of TM atoms adsorption on the S surface is completely opposite. When the TM atoms are adsorbed on the S surface of single-layer MoSSe, the dipole moments of pristine single-layer MoSSe are weakened. For Ni-adsorbed MoSSe, the dipole moment is reduced from -1.631 D to -0.745 D. With the change of elements from Ni to Sc, the dipole moments weaken more and more. For the case of Cr adsorption, the dipole moment of the Cr-MoSSe reaches -0.052 D, which is close to zero. Interestingly, the dipole moment of V-adsorbed MoSSe changes its sign. That means the directional reversal of dipole moment happens for the V adsorption on the Se surface of single-layer MoSSe. Specifically, the direction of dipole moment of V-adsorbed MoSSe is along the positive z axis. For Ti and Sc adsorption, the direction of dipole moments is kept being along the positive z axis. The dipole moments are 0.903, 1.457, and 1.841 D for V, Ti, and Sc adsorption, respectively. Therefore, the dipole moments exhibit the increasing trend for the adsorbed TM atoms changing from Ni to Sc. The results aforementioned are related with the M adsorption site. For the different adsorption site (H site), the evolution trend of dipole moments with the adsorbed TM atoms are extremely similar, as shown in Figure 3b. The difference is that the directional reversal occurs from Fe to Mn at H adsorption site, while from Cr to V at M adsorption site. Thus, the evolution of dipole moments is insensitive to the specific adsorption site. 3.4 Discussion on dipole moment As mentioned above, the different effects could be observed for the adsorption of TM atoms on the S surface and the Se surface of Janus single-layer MoSSe. A question naturally arises: why do the dipole moments exhibit such interesting behavior? To figure out the mechanism behind this phenomenon, we started with the structural analysis of MoSSe. Unlike the MoS2 with symmetrical structure, Janus single-layer MoSSe is asymmetry along the out-of-plane direction. Therefore, the center of positive charge is different from that of negative charge, thus inducing the non-zero dipole moment of MoSSe, as shown in Figure 4a. After the TM atoms being adsorbed on the MoSSe, the electrons of TM atoms transfer to the MoSSe substrate. As a result, the charge would 8


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redistribute in MoSSe layer. Basically, calculating the positive charge center would include all the ions in MoSSe and TM ions. At the same time, calculating the negative charge center involves the electrons in MoSSe and TM ions. Obviously, compared to the pristine MoSSe, not only the charge quantity, but also the distance between positive charge center and negative charge center changes for the case of TM-adsorbed MoSSe. It is difficult to draw a conclusion with such complicated variables. To qualitatively understand the results of dipole moments, we made a simplification for the TM-adsorbed single-layer MoSSe. By neglecting the redistribution of MoSSe substrate, which is induced by the charge transfer from TM atoms, the dipole moments could be divided into two parts. One is from MoSSe and the other results from the TM atoms. When a TM atom is adsorbed on MoSSe, the electrons would transfer towards the MoSSe substrate, regardless of on the S surface or on the Se surface. Therefore, the dipole moment induced by the TM atom is always directed from TM ion to MoSSe substrate. If the TM atom is adsorbed on the Se surface, the dipole moments of TM atom have the same direction as that of MoSSe substrate, thus also being along the -z axis. Obviously, the dipole moments are strengthened, as shown in Figure 4b. This result is consistent with that in Figure 3. In contrast, when the TM atom is adsorbed on the S surface, the dipole moment is along z axis, which is opposite to that of MoSSe substrate. As a result, the dipole moments are weakened, as shown in Figure 4c. Furthermore, if the value of the dipole moment result from the TM atom is beyond that from MoSSe, the dipole moment from the TM atom is dominant. In this situation, the direction of total dipole moment will change when compared with that case of MoSSe. That is the reason why the positive dipole moment occurs for the TM atom adsorption on the S surface. To further elucidate the effects of different TM atoms, we analyzed the charge transfer from TM atoms to MoSSe substrate, which directly influences the values of dipole moments. In our calculations, bader charges of the TM atoms are calculated, which are shown in Figure 3. It shows that all TM atoms are most likely to lose electrons. The transferred electrons basically decrease with the increase of the atomic number of TM atoms except for the case of Cr. That means that Sc atom loses the most electrons, while Ni atom losing the least electrons. To better understand the charge transfers between adatoms and Janus MoSSe monolayer, the charge density difference of the TM-MoSSe systems is plotted, as shown in Supporting Information Figure S2. Three 9



representative TM atoms are selected, namely, Sc, Mn, and Ni. Regardless of the adsorption on the S surface or the Se surface, the charge transfer between Sc and MoSSe is the most among Sc, Mn, and Ni cases, whereas the case of Ni is the least. Such results agree well with Figure 3. On the other hand, the structural information in Table 1 and Table 2 shows that the distance between Sc and MoSSe surface (either S or Se surface) is significantly larger than that between Ni and MoSSe surface. Combining the charge transfer with the adsorption configuration, one can see that the dipole moment induced by Sc atom is the largest. Therefore, the dipole moments could be reversed when Sc atom is adsorbed on the S surface of single-layer MoSSe. With the increase of atomic number of TM atoms, the dipole moments from TM atoms decrease. As a result, the effect of TM adsorption on the dipole moment of MoSSe decrease more and more. Actually, our conclusions are confirmed by the previous similar work reported by Khan et al.55. For the 2D phosphorene materials, Mg, Cr, and Mo adsorbed systems have large dipole moments, in contrast to Pd, Pt, and Au adsorbed systems, which have small dipole moments. They attribute the original reason to the strong charge transfer between Mg, Cr, Mo atoms and phosphorene substrate, whereas weak charge transfer between Pd, Pt, Au atoms and phosphorene. 4. CONCLUSIONS By using density functional theory method, we studied the structural, magnetic, electronic, and dipole moment properties of TM-adsorbed Janus monolayer MoSSe. Herein, the 3d TM atoms (Sc-Ni) are considered. Total calculations show that MoSSe with TM adatom at M adsorption site has similar energy with that at H adsorption site. Although the magnetic moment and electronic structure of MoSSe with TM adatoms exhibit diversity due to the difference of TM atoms, similarity also can be found for the cases of M adsorption site and H adsorption site. For the pristine monolayer MoSSe, intrinsic dipole moment is observed. After adsorbing the TM atoms on MoSSe, the dipole moments are changed. The adsorption of TM atoms on the Se surface enhances the dipole moment, whereas weakens the dipole moments for the S surface adsorption. For some TM adatoms, the direction of the dipole moments could be reversed when the TM atoms are adsorbed on the S surface. In addition, the results of dipole moments are independent of detailed adsorption sites. Among all the 3d TM atoms considered in our calculations, the effect of Sc is the largest, while the effect of Ni is the smallest. By means of the superposition model, the change of the dipole moments can be clearly explained. 10


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ASSOCIATED CONTENT Supporting Information Binding energy of TM-adsorbed MoSSe with different supercells and different adsorption sites, total energy of TM-adsorbed MoSSe with different initial heights between TM and Mo, total magnetic moments and projected magnetic moments of Mo atoms, and charge density difference of TM-adsorbed MoSSe AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; [email protected] ORCID Bo Xu: 0000-0002-6896-0409 Jing Shi: 0000-0003-3288-3306 Shuying Zhong: 0000-0001-5700-1678 Xueling Lei: 0000-0002-2482-3728 Gang Liu: 0000-0003-3213-3820 Musheng Wu: 0000-0003-1366-8328 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 11664012, 11664013), the Excellent Youth Foundation of Jiangxi Province (Grant No. 20171BCB23035). The computations were partly performed on TianHe-1(A) at the National Supercomputer Center in Tianjin. REFERENCES (1) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.; Zhang, H. The Chemistry of Two-Dimensional Layered Transition Metal Dichalcogenide Nanosheets. Nat. Chem. 2013, 5, 263-275. (2) Radisavljevic, B.; Kis, A. Mobility Engineering and a Metal-Insulator Transition in Monolayer MoS2. Nat. Mater. 2013, 12, 815-820. (3) Gatensby, R.; McEvoy, N.; Lee, K.; Hallam, T.; Berner, N. C.; Rezvani, E.; Winters, S.; O'Brien, M.; Duesberg, G. S. Controlled Synthesis of Transition Metal Dichalcogenide Thin Films for Electronic Applications. Appl. Surf. Sci. 2014, 297, 139-146. (4) Shin, D.; Hubener, H.; De Giovannini, U.; Jin, H.; Rubio, A.; Park, N. Phonon-Driven Spin-Floquet Magneto-Valleytronics in MoS2. Nat. Commun. 2018, 9, 638. (5) Mai, C.; Barrette, A.; Yu, Y.; Semenov, Y. G.; Kim, K. W.; Cao, L.; Gundogdu, K. Many-Body 11



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Table 1. Binding energy (Eb), bond length between TM atoms and the nearest neighbor Mo (dTM-Mo) or S/Se (dTM-S/Se), magnetic moment (MM) and dipole moment (DM) of TM-adsorbed MoSSe with M adsorption site

surface

S

Se

TM

Eb (eV)

dTM-Mo (Å)

dTM-S/Se (Å)

DM (D)

MM (μB)

Sc

1.818

3.042

2.466

1.841

3.0

Ti

2.017

3.032

2.423

1.457

4.0

V

2.097

2.830

2.336

0.903

5.0

Cr

1.847

3.130

2.478

-0.052

6.0

Mn

0.690

2.936

2.373

-0.142

5.0

Fe

1.611

2.958

2.326

-0.216

4.0

Co

2.094

2.546

2.150

-0.374

1.0

Ni

3.466

2.509

2.125

-0.745

0.0

Sc

1.374

3.585

2.692

-3.341

3.0

Ti

1.334

3.336

2.559

-3.003

4.0

V

1.411

3.238

2.523

-3.016

5.0

Cr

1.491

3.600

2.691

-2.388

6.0

Mn

0.321

3.801

2.837

-2.336

5.0

Fe

1.062

2.664

2.261

-2.671

4.0

Co

1.445

2.708

2.231

-2.461

1.0

Ni

2.901

2.668

2.208

-2.188

0.0

Table 2. Binding energy (Eb), bond length between TM atoms and the nearest neighbor Mo (dTM-Mo) or S/Se (dTM-S/Se), magnetic moment (MM) and dipole moment (DM) of TM-adsorbed MoSSe with H adsorption site

surface

S

Se

TM

Eb (eV)

dTM-Mo (Å)

dTM-S/Se (Å)

DM (D)

MM (μB)

Sc

2.060

3.328

2.348

1.889

1.0

Ti

1.939

3.170

2.266

0.505

2.0

V

1.738

3.315

2.309

0.453

5.0

Cr

1.734

3.714

2.513

0.097

6.0

Mn

0.594

3.860

2.616

0.077

5.0

Fe

1.617

2.957

2.123

-0.661

4.0

Co

1.539

2.807

2.061

-0.888

1.0

Ni

3.282

2.797

2.066

-1.089

0.0

Sc

1.107

4.281

2.864

-3.024

1.0

Ti

1.191

4.227

2.773

-3.110

4.0

V

1.246

4.114

2.703

-2.840

5.0

Cr

1.426

4.196

2.782

-2.591

6.0

Mn

0.259

4.316

2.892

-2.481

5.0

Fe

0.926

3.983

2.601

-2.431

4.0

Co

1.187

3.924

2.535

-2.433

3.0

Ni

2.430

2.846

2.142

-1.853

0.0

15



Page 16 of 23

Table 3. Calculated elastic constants C11 and C12, in-plane Young’s modulus Y, shear modulus G, Poisson’s ratio υ of pristine and TM-adsorbed MoSSe

surface

S

Se

TM

C11

C12

Y

G

υ

Pristine

664.08

111.28

645.43

276.4

0.168

Sc

553.77

111.25

531.42

221.26

0.201

Ti

567.21

120.21

541.73

223.5

0.212

V

588.2

125.44

561.45

231.38

0.213

Cr

604.33

136.29

573.59

234.02

0.226

Mn

559.54

100.98

541.32

229.28

0.180

Fe

575.89

107.41

555.86

234.24

0.187

Co

611.25

148.13

575.35

231.56

0.242

Ni

630.23

149.87

594.59

240.18

0.238

Sc

559.12

115.3

535.34

221.91

0.206

Ti

580.4

134.97

549.01

222.72

0.233

V

589.4

134.97

558.49

227.22

0.229

Cr

609.53

139.46

577.62

235.04

0.229

Mn

576.19

103.79

557.49

236.2

0.180

Fe

588.63

123.43

562.75

232.6

0.210

Co

615.55

151.99

578.02

231.78

0.247

Ni

632.97

154.81

595.11

239.08

0.245

Figure 1. (a) Top view of Janus single-layer MoSSe atomic structure with three typical adsorption sites, which are H (Hollow), M (Mo-top), and S/Se (S/Se-top) sites. (b) Side views of Janus single-layer MoSSe atomic structures with TM atoms on H, M, and S/Se adsorption sites. 16


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Figure 2. Spin-polarized total density of states (TDOS) of Janus single-layer MoSSe and projected density of states (PDOS) of the TM atoms and the neighboring Mo atoms for the S surface and Se surface adsorption.

17



Figure 3. Dipole moments and bader charges for the adsorption of TM atoms on the S surface and Se surface with (a) M adsorption site and (b) H adsorption site.

Figure 4. Schematic illustration of the dipole moments for (a) pristine MoSSe, (b) MoSSe with TM atom on the Se surface, and (c) MoSSe with TM atom on the S surface. 18


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TOC Graphic

19



Figure 1 251x188mm (150 x 150 DPI)


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Figure 2 289x508mm (100 x 100 DPI)



Figure 3 238x191mm (150 x 150 DPI)


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Figure 4 222x165mm (150 x 150 DPI)


Tunable Dipole Moment in Janus Single-Layer MoSSe via Transition

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