First-principle study of hydrogenation on monolayer MoS2

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1 First-principle study of hydrogenation on monolayer MoS2 Yong Xu, Yin Li, Xi Chen, Chunfang Zhang, Ru Zhang, and Pengfei Lu Citation: AIP Advances 6, (2016); View online: View Table of Contents: Published by the American Institute of Physics Articles you may be interested in Structural, mechanical and electronic properties of in-plane 1T/2H phase interface of MoS 2 heterostructures AIP Advances 5, (2015); / Surface oxidation energetics and kinetics on MoS 2 monolayer Journal of Applied Physics 117, (2015); / Thermal conductivity and phonon linewidths of monolayer MoS 2 from first principles Applied Physics Letters 103, (2013); / p-type doping of MoS 2 thin films using Nb Applied Physics Letters 104, (2014); / A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu The Journal of Chemical Physics 132, (2010); / Sulfur vacancies in monolayer MoS 2 and its electrical contacts Applied Physics Letters 103, (2013); /

2 AIP ADVANCES 6, (2016) First-principle study of hydrogenation on monolayer MoS2 Yong Xu, 1,2 Yin Li, 1,2 Xi Chen, 1,3 Chunfang Zhang, 4 Ru Zhang, 1,3 and Pengfei Lu 1,a 1 State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing , China 2 School of science, Beijing University of Posts and Telecommunications, Beijing , China 3 School of Ethnic Minority Education, Beijing University of Posts and Telecommunications, Beijing , China 4 Beijing Computational Science Research Center, Beijing , China (Received 23 April 2016; accepted 23 June 2016; published online 1 July 2016) The structural and electronic properties of hydrogenation on 1H-MoS 2 and 1T-MoS 2 have been systematically explored by using density functional theory (DFT) calculations. Our calculated results indicate an energetically favorable chemical interaction between H and MoS 2 monolayer for H adsorption when increasing concentration of H atoms. For 1H-MoS 2, single H atom adsorption creates midgap approaching the fermi level which increases the n-type carrier concentration effectively. As a consequence, its electrical conductivity is expected to increase significantly. For 1T-MoS 2, H atoms adsorption can lead to the opening of a direct gap of 0.13eV compared to the metallic pristine 1T-MoS 2. C 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license ( [ I. INTRODUCTION Two-dimensional (2D) atomic crystals such as graphane family, 2D chalcogenides, and 2D oxides monolayer have been a topic of current interest due to their exotic electronic properties and potential applications in electronics. 1 3 Among these 2D monolayer materials, monolayer MoS 2 attracts much attention because of its important role in inflexible electronic device, 4 photoluminescence, 5 valleytronics, 6,7 and field effect transistors. MoS 2 is classified to the layered transition-metal dichalcogenide family. Similar to graphene, sandwiched S-Mo-S layers are held together by weak van der Waals interactions, and monolayer MoS 2 can be extracted to atomic thin nanosheets by using mechanical or solvent-based exfoliation methods. 8,9 The bulk structure of MoS 2 is composed of two molecular layers of MoS 2 with trigonal prismatic phase named as 2H-MoS 2. The mechanical method always leads to a 2D trigonal prismatic phase, commonly denoted as 1H-MoS 2, which is found to be a semiconductor with a direct band gap of 1.80eV, contrast to the zero gap of graphene. 10 The 1H-MoS 2 belongs to the space group of P6/mmc and is believed to be stable under normal condition. 11 On the other hand, the chemical solvent-based exfoliation method make different octahedral coordinated phases, which is referred to as 1T-MoS 2. Unlike 1H and 2H phases which is semiconducting, the 1T phase are supposed to be metallic But the exact structure of 1T-MoS 2 is not yet firmly established, and it is still complicated and confused. Schematics of two basic MoS 2 polytypes, namely 1H and 1T, are shown in Fig. 1. MoS 2 is known for its good ability to absorb hydrogen and the catalytic ability of its active hydrogen molecule. 16 The surface-sorbed atomic hydrogen is believed to move from the edge sites to the basal plane through a replenishing mechanism. 17 Recently, Cai et al. has reported the a Corresponding author: photon.bupt@gmail.com /2016/6(7)/075001/9 6, Author(s) 2016.

3 Xu et al. AIP Advances 6, (2016) FIG. 1. Structure used for the calculations. (a) A H-MoS 2 single layer supercell with possible substitutional and adsorbing positions. Tags A and B are the possible substitutional doping sites for H, and tags A, B, C and D are possible adsorption sites for H. (b) A T-MoS 2 single layer supercell with possible adsorption sites A, B, C, D. Subfigure (c) and (d) are side views of 1H-MoS 2 and 1T-MoS 2, respectively. Blue, yellow and green ball represents Mo, S and H atom respectively. hydrogenation on the surface of MoS2 monolayer which induces a semiconductor-metal transition. 18 Also, the effects of nonmetal (H, B, N, F) atom substitutions on electronic and magnetic properties of monolayer MoS 2 have been investigated. 19 Recently, Zhang et al. calculated the hydrogenation on 1H-MoS2 with the number of H atoms from 1 to While the investigation on concentration-dependent hydrogenation on whole monolayer MoS 2 polytypes is still insufficient. In this study, first-principles calculations within density functional theory (DFT) is introduced to investigate the energetics of hydrogenation on the 1H-MoS 2 and 1T-MoS 2 monolayer and the resulting change in electronic properties with aim to explore the effect of hydrogen concentration on electronic and crystal structure. We systematically explore the possibility of band structure engineering of monolayer MoS 2 by replacing atom or adsorbing H atoms on MoS 2 surface. The motivation of hydrogenating 1H-MoS 2 here is in contrast to that of hydrogenating 1T-MoS 2. Hydrogenating 1T-MoS2 causes its band gap opening, a metal to semiconductor transition, which can be used to create clusters on 1T-MoS 2. Hydrogenating 1H-MoS 2 leads an enhancement in electrical conductivity, which can be used to create surface metallization on monolayer MoS 2. II. METHODS AND MODELS Our calculations were carried out using Vienna ab-initio simulation package (VASP). The DFT was used to perform geometry relaxation, total energy calculations, and the electronic structure calculations. The exchange-correlation potential are performed by the generalized gradient approximation (GGA) with Perdew Burke Ernzerhof (PBE) functional and the plane-wave cutoff energy is 400eV. A set of Monkhorst-Pack mesh is used for geometry optimization and static total energy calculations. The structural optimization is continued until the maximum Hellmann-Feynman forces acting on each atom and the total energy is less than 0.01eV/Å and 10 5 ev, respectively with a maximum of 100 steps. The supercell of H-MoS 2 and 1T-MoS 2 sheet with Å lateral dimensions are constructed to be the calculated models and doping is introduced by replacing/adding atoms in the supercell, as shown in Fig. 1, which contain 32 S atoms and 16 Mo atoms. The distance between adjacent monolayers is larger than 15Å to eliminate the interaction between them.

4 Xu et al. AIP Advances 6, (2016) III. RESULTS AND DISCUSSION A. Stability and crystal structure Before discussing crystal and electronic structure, the energetics and stability of hydrogenated MoS 2 are investigated firstly. The relative stability of the doped structure is determined by the formation energy and relates to the realization in experiments. We divide substitutional doping 1H-MoS 2 into two steps, which includes vacancy creation and then embedding dopants into the vacancy site, as suggested that the electron-beam can realize substitutional doping scheme. Formation energy of vacancy E f is defined as: E f = E v (E MoS2 E i ), (1) where E MoS2 and E v represent the total energy of 1H-MoS2 with and without vacancy, and E i is the energy of single S or Mo atom. The calculated formation energy of single S vacancy (V s ) and single Mo vacancy (V m ) are 5.55 ev and ev, respectively. The V s value is in good agreement with the reported value of 5.89eV. 21 Obviously, it is energetically easier to produce a V s than V m under electron-beam irradiation, 22 so an S vacancy is chosen for calculation. Through substitutional doping strategy, the formation energy can be calculated by the following general equation which is inferred from some experiences of other semiconductor materials: E f sub = [E(nH : MoS 2 ) E nv (MoS 2 ) ne H ]/n, (2) where E(nH : MoS 2 ) and E nv (MoS 2 ) represent the total energy doped with impurities and the total energy of defective MoS 2 with n atoms vacancy, respectively, n means the number of H atoms which are doped into the system. E H represents the energy of a single H atom. Meanwhile, the formation energy for adsorption systems is defined as: E f ad = [E(nH : MoS 2 ) E(MoS 2 ) ne H ]/n, (3) where E(nH : MoS 2 ) and E(MoS 2 ) are total energy of n H atoms adsorbed system and total energy of the primitive MoS 2 monolayer, respectively. Several adsorption possibilities of such hydrogenation in 1H-MoS 2 and 1T-MoS 2 are under consideration, as shown in Fig. 1. The H atom can be adsorbed directly on the top of a S atom (A site) or a Mo atom (B site), over the top of the bond of S and Mo atoms (C site) or the top of the hollow (D site), which is shown in Fig. 1(a), 1(b). For each configuration, the formation energies of doped and adsorbed positions are listed in Table I. The minimum formation energy eV of 1H-MoS2 indicates the H atom adsorbed on C site is the most stable configuration among 4 options. In this configuration, the adsorbed H atom has a bond with the nearby S atom, and the bond length is 1.410Å. Meanwhile, A site is the most stable position of 1T-MoS2 with the minimum formation energy of ev. The adsorbed H atom has formed a bond with S atom and the bond length is 1.37 Å. The formation energies of the selected sites are negative, which reveal H atom can be chemically absorbed on MoS 2 monolayer even at room temperature. In order to investigate the influence of the concentration x, two H atoms substitutional and adsorbed configurations are under consideration in the next discussion. Fig. 2 shows the schematics of two H atoms adsorbed on 1H-MoS 2, two different types labeled as To (two H atoms occupy the opposite sites) and Ta (two H atoms occupy the adjacent sites), with one H atom fixed, and there are two sites for the second individual H atoms in Fig. 2(a). TABLE I. The formation energy of doped and adsorbed MoS 2 monolayers. Configuration A Site B Site C Site D Site 1H-MoS 2,substitutional(eV) H-MoS 2,adsorption(eV) T-MoS 2,adsorption(eV)

5 Xu et al. AIP Advances 6, (2016) FIG. 2. (a) Possible adsorption sites with two H atoms for 1H-MoS2, red arrow pointed green dotted ball indicates the possible sites for the second H atom, and (b-e) are the structures of the most stable 1H-MoS2 with 6.3%,12.5%,18.8%,25% concentrations of H atoms. The calculated formation energy with different configurations are shown in Table II. The formation energies of To type are smaller than that of Ta, which indicates the opposite site is more stable than the adjacent site when two H atoms are doped. Then the concentration of H continues to increase, for a simple consideration, all the possible H distributions in calculation are kept symmetry. The H atoms are introduced by pairs, and the process is repeated until the number of H atoms increases to 8 in a (4 4) supercell (see Fig. 2(b)-2(e)). As the number of H atoms added, clustering of Mo increases only in 1T-MoS 2, which is shown in Fig. 3. Because there are 32 most stable sites for H atoms on supercell of monolayer MoS 2, the concentration of H is defined as the number of H atoms n divided by 32 (n/32*100%). The stability is under consideration when multiple H atoms are added into the system. Fig. 3 gives the variation of the total energy and formation energy for H@MoS 2 as the concentration x increasing. As shown in Fig. 3, the black, red and blue dot lines show the total energy of 1H-MoS 2 substitution, 1H-MoS 2 adsorption and 1T-MoS 2 adsorption changes with the concentration x. The black dot line increases with the enhancement of concentration, but the red and blue dot line show an opposite trend, which indicates it is energetically unstable to increase the concentration when substitutional doping H atoms into 1H-MoS 2, while the adsorbing strategy is more reasonable. Therefore, the substitutional doping configuration is ignored in the following discussion of the concentration-dependent crystal and electronic structure. The optimized geometrical parameters are given in Table III. Due to 1H-MoS 2 monolayer has a sandwich structure, the distance of two Mo atoms (d Mo-Mo) and the bond length of S and Mo atoms (d S-Mo) are under consideration. The calculated results show that the optimized pristine 1H-MoS 2 bond length between two adjacent Mo atoms is 3.16Å and the bond length between Mo and S atoms is 2.41Å, which are in good agreement with the results of Ciraci et al Å and 2.42Å, respectively. Table III shows the crystal parameters of hydrogenated 1H-MoS 2 and 1T-MoS 2 with the concentration of H atoms increasing. We find that for both hexagonal and tetragonal polytypes, the d S-Mo decreases marginally with single H atom adsorbed. The variations of the d Mo-Mo and d S-Mo reveal that the structure of 1H-MoS 2 single layers has been slightly distorted. On the other hand, the d Mo-Mo has an obvious decrease from pristine 3.16Å to 2.80Å after single H atom adsorbed TABLE II. Calculated formation energy of To and Ta configurations. Configuration To Ta 1H-MoS 2,substitutional(eV) H-MoS 2,adsorption(eV) T-MoS 2,adsorption(eV)

6 Xu et al. AIP Advances 6, (2016) FIG. 3. Variation of the total energy and formation energy (see subfigure) for monolayer MoS 2 as the concentration of H changes. into 1T-MoS 2, which means monolayer 1T-MoS 2 has a significant distortion, adjacent Mo atoms bonding together. It is known that repetitive structural changes are related to ion intercalation and deintercalation can lead to capacity fade and a loss of electrochemical performance of the electrodes. 24 To investigate the influence of H atoms doped or adsorbed into MoS 2 monolayer, we have analyzed the variation of crystal parameters with H concentration. Table III shows the formation energies across the entire composition range for various stable binding configurations within 1H-MoS 2 and 1T-MoS 2. It is evident that formation energies for all configurations in 1H-MoS 2 and 1T-MoS 2 polytypes are negative, indicating an energetically favorable chemical interaction between H and MoS 2 monolayer. For both 1H-MoS 2 and 1T-MoS 2 polytypes, the distance between S and Mo atoms (d S-Mo) changes marginally in a narrow range of Å, and the distance between the adatom and surface (d H-S) increases slightly all the time. On the contrary, the variations of d Mo-Mo are more significant. When H atoms enter the surface, for tetragonal polytypes, d Mo-Mo decreases obviously from 2.80 to 2.65Å upon further increasing the concentration of H atom and Mo-Mo bonding like a diamond chain, with the clusterization of Mo atoms, shown in Fig. 3. These findings are in good agreement with the results of Kan et al., 25 who reported the phenomenon that Mo-Mo cluster TABLE III. Theoretically calculated formation energies(e form ) and the distance of two adjacent Mo atoms(d Mo-Mo ),the bond distances of S-Mo(d S-Mo ), H-Mo(d H-Mo ), H-S (d H-S ) for adsorbed 1H-MoS2 and 1T-MoS 2 monolayer with the increase of concentration x. Configuration x d Mo-Mo (Å) d S-Mo (Å) d H-S (Å) E form (ev) 1H-MoS 2 1T-MoS

7 Xu et al. AIP Advances 6, (2016) transformed from zigzag chain to diamond chain with the increase of concentration of Li. These fluctuations are associated with the H-H interactions. B. Electronic structure Among the three possible substitution and adsorption systems, the most stable one is chosen for presenting the results of the electronic band edges, which is close to the valance band maximum (VBM) and the conduction band minimum (CBM). In Fig. 4 and 5, the band structure and some PDOS of pristine and hydrogenated 1H- MoS 2 and 1T-MoS 2 are presented. The primitive monolayer 1H-MoS 2 is a direct semiconductor with a band gap 1.81eV and the VBM and CBM situated at K point of the Brillouin zone, as shown in Fig. 4(a), which is in good agreement with previous theoretical results with GGA-PBE. 26,27 The calculated band gap is consistent well with the experimentally-measured optical band gap among the range of ev. 28,29 The CBM and VBM are mainly ruled by the 4d states of Mo, while other states of Mo playing a minor role. 30 Then the electronic properties of single layer MoS 2 are calculated under the condition of H atom substitution and adsorption, respectively. The validated DFT parameters are introduced to discuss the findings. According to the electronic band structure of H substitutional doping MoS 2, as shown in Fig. 4(b), there are midgap energy levels close to the Fermi level and the bandgap is 1.06eV, which is smaller than that of the primitive monolayer 1H-MoS 2. When single H atom adsorbed by 1H-MoS 2 monolayer, H adsorption creates midgap energy levels across the Fermi level and very close to the CBM. Although H does not directly enhance the free electron density, the partially filled midgap energy levels are close to the CBM, and these electrons can be excited to the conduction band, effectively increasing the n-type carrier concentration in 1H-MoS 2. As a consequence, its electrical conductivity is expected to increase significantly. It is clear that the narrow pick in the bandgap are formed by the hybridizing of the 1s state of H atom, 3p state of S atom, and 4d state of adjacent Mo atoms in the H-adsorbed 1H-MoS 2, shown in Fig. 4(c) and 4(f). FIG. 4. Band structure of (a) pristine, (b) one H atom substitutionally doped, and (c)-(e) adsorbed 1H-MoS 2 monolayer (4 4). In which (c), (d) and (e) represent the band structure of 3.1%, 6.3%, 12.5% H atoms adsorbed 1H-MoS2 respectively. (f)-(h) are corresponding partial density of states of (c)-(e). The Fermi level is set to zero ev in each case.

8 Xu et al. AIP Advances 6, (2016) FIG. 5. Band structure of (a) pristine, and (b)-(d) adsorbed 1T-MoS 2 monolayer (4 4). In which (b), (c) and (d) represent the band structure of 3.1%, 6.3%, 12.5% H atoms concentration adsorbed 1T-MoS2 respectively, (e) and (f) indicate the PDOS of (c) and (d). Compared to the bandgap shift that caused by H substitutional doping into 1H-MoS 2, single H atom adsorption makes MoS 2 to n-type semiconductor, which is in good agreement with the results in the literature. 18 As the concentration x increasing to 6.3% and 12.5%, the band structure and PDOS are shown in Fig. 4(d)-4(h). It is shown that the impurity states appear in the band gaps of them, and make them to be direct semiconductor with band gap of 0.43 ev and 0.15 ev respectively. The impurity states are derived from the hybridization of 1s state of H atom, 3p state of S atom, and 4d state of Mo atoms, and we can find that the composition of the impurity states is same with single H atom adsorption but the width of the impurity states is extended. The band structure and PDOS calculated of 1T-MoS 2 along the lines connecting the highsymmetry points of Brillouin zone is shown in Fig. 5. Before the H atom adsorption, the 1T-MoS 2 shows its metallic characteristic according to the band structure (see Fig. 5(a)). As shown in Fig. 5(b), with single H adsorption, there is a gap opened between CBM and CVM. In order to analyze the influence of adsorption concentration, the electronic structures of multiple H atoms hydrogenated 1T-MoS 2 are introduced. Band structure and density of states are calculated to analyze the changes in electronic structure of MoS 2 upon H adsorption as a result of the charge transfer. Fig. 5(c) - 5(f) shows the concentration-dependent band structures and PDOS for nh@1h-mos 2 and 1T-MoS 2 monolayer, where n stands for the number of H atoms. In this study, n equals 2 and 4 correspond to the concentration of 6.25% and 12.5%, respectively. When 2 H atoms join in, 1T-MoS 2 monolayer opens a narrow direct band gap of 0.13 ev, which shows a transformation from metal to semiconductor (see Fig. 5(c)). When continually adding the concentration of H atoms to 12.5%, 1T-MoS 2 monolayer becomes an indirect semiconductor, shown in Fig. 5(d). In Fig. 5(e) and 5(f), it is found that the CVM and CBM are composited by Mo 4d states and S 3p states, and H atom does not contribute to the CBM and CVM, which means H atom adsorption just

9 Xu et al. AIP Advances 6, (2016) induce the transformation from metal to semiconductor for 1T-MoS2. The transformation can be related to the clusterization of Mo coordination which provides a large concentration of available electrons. 31 It is worth mentioning that when the concentration of H atoms is more than 12.5%, both hexagonal and tetragonal polytypes have metallic behavior. IV. CONCLUSIONS In summary, we have systematically investigated single H atom and multiple H atoms hydrogenation on 1H- and 1T-MoS 2 monolayers. (1) We calculated the possible sites for single H atom doped or adsorbed into 1H-MoS 2 and 1T-MoS 2 monolayer. We find that H atoms are favorable to the top of Mo-S bond (C position) when adsorbed into 1H-MoS 2, while the H atom prefers to occupy sulfur (A site) when substitutional doping into 1H-MoS 2 or adsorbed into 1T-MoS 2. The formation energy shows that all these positions can chemically absorb on MoS 2 monolayer even at room temperature. When the concentration of H increases, the energy of the substitution is not stable, but the adsorption is relatively stable. (2) For 1H-MoS 2, with single H atom adsorption, H creates midgap level close to Fermi Level, which effectively increases the n-type carrier concentration, and as a consequence its electrical conductivity is expected to increase significantly. The impurity states are formed by the hybridization of 1s state of H atom, 3p state of S atom, and 4d state of Mo atoms. For 6.25% and 12.5% concentration, 1H-MoS 2 becomes small band gap semiconductor. When the concentration over 12.5%, 1H-MoS 2 monolayer transforms to be metallic. For 1T-MoS 2, hydrogenation causes its band gap opening, which results a metal to semiconductor transition with concentration up to 6.25%, which can be used to create Mo-Mo clusters on 1T-MoS 2. As the concentration over 12.5%, 1T-MoS 2 monolayer transforms to be metallic. This study gives the relation between the characteristic of monolayer MoS 2 and the increase of concentration H atoms, and can be used in selecting strategies for further work on monolayer MoS 2. ACKNOWLEDGEMENTS This work is supported by the National Basic Research Program of China (973 Program) under Grant No. 2014CB643900, the Open Fund of IPOC (BUPT), the Open Program of State Key Laboratory of Functional Materials for Informatics, the Fundamental Research Funds for the Central Universities (No. 2015RC28), and the Fund of State Key Laboratory of Information Photonics and Optical Communications (BUPT) (No. IPOC2015ZT05). 1 C N R Rao, A K Sood, K S Subrahmanyam et al., Graphene: the new two - dimensional nanomaterial, Angewandte Chemie International Edition 48(42), (2009). 2 J O Sofo, A S Chaudhari, and G D. Barber, Graphane: A two-dimensional hydrocarbon, Physical Review B 75(15), (2007). 3 C Jin, F Lin, K Suenaga et al., Fabrication of a freestanding boron nitride single layer and its defect assignments, Physical review letters 102(19), (2009). 4 S Bertolazzi, J Brivio, and A. Kis, Stretching and breaking of ultrathin MoS2, ACS nano 5(12), (2011). 5 G Eda, H Yamaguchi, D Voiry et al., Photoluminescence from chemically exfoliated MoS2, Nano letters 11(12), (2011). 6 T Cao, G Wang, W Han et al., Valley-selective circular dichroism of monolayer molybdenum disulphide, Nature communications 3, 887 (2012). 7 K F Mak, K He, J Shan et al., Control of valley polarization in monolayer MoS2 by optical helicity, Nature nanotechnology 7(8), (2012). 8 B Radisavljevic, A Radenovic, J Brivio et al., Single-layer MoS2 transistors, Nature nanotechnology 6(3), (2011). 9 C Lee, Q Li, W Kalb et al., Frictional characteristics of atomically thin sheets, Science 328(5974), (2010). 10 G Eda, H Yamaguchi, D Voiry et al., Photoluminescence from chemically exfoliated MoS2, Nano letters 11(12), (2011). 11 X R Qin, D Yang, R F Frindt et al., Real-space imaging of single-layer MoS 2 by scanning tunneling microscopy, Physical Review B Condensed Matter 44(7), (1991). 12 D Voiry, H Yamaguchi, J Li et al., Enhanced catalytic activity in strained chemically exfoliated WS2 nanosheets for hydrogen evolution, Nature materials 12(9), (2013). 13 M A Lukowski, A S Daniel, F Meng et al., Enhanced hydrogen evolution catalysis from chemically exfoliated metallic MoS2 nanosheets, Journal of the American Chemical Society 135(28), (2013). 14 G. Eda, T. Fujita, H. Yamaguchi, D. Voiry, M. W. Chen, and M. Chhowalla, ACS Nano 6, (2012).

10 Xu et al. AIP Advances 6, (2016) 15 G Eda, H Yamaguchi, D Voiry et al., Photoluminescence from chemically exfoliated MoS2, Nano letters 11(12), (2011). 16 D C Sorescu, D S Sholl, and A V. Cugini, Density functional theory studies of the interaction of H, S, Ni-H, and Ni-S complexes with the MoS2 basal plane, The Journal of Physical Chemistry B 108(1), (2004). 17 P Sundberg, R B Moyes, and J. Tomkinson, Inelastic neutron scattering spectroscopy of Hydrogen adsorbed on powdered - MoS2, MoS2 - Alumina and Nickel - promoted MOS2, Bulletin des Sociétés Chimiques Belges 100(11-12), (1991). 18 Y Cai, Z Bai, H Pan et al., Constructing metallic nanoroad on MoS2 monolayer via hydrogenation, Nanoscale 6(3), (2013). 19 Q Yue, S Chang, S Qin et al., Functionalization of monolayer MoS 2 by substitutional doping: a first-principles study, Physics Letters A 377(19), (2013). 20 W Zhang, Z Zhang, and W. Yang, Stability and electronic properties of hydrogenated MoS2 monolayer: A first-principles study, Journal of nanoscience and nanotechnology 15(10), (2015). 21 C Ataca and S. Ciraci, Functionalization of single-layer MoS2 honeycomb structures, The Journal of Physical Chemistry C 115(27), (2011). 22 H P Komsa, J Kotakoski, S Kurasch et al., Two-dimensional transition metal dichalcogenides under electron irradiation: defect production and doping, Physical review letters 109(3), (2012). 23 C Ataca, H Sahin, E Akturk et al., Mechanical and electronic properties of MoS2 nanoribbons and their defects, The Journal of Physical Chemistry C 115(10), (2011). 24 L Y Beaulieu, K W Eberman, R L Turner et al., Colossal reversible volume changes in lithium alloys, Electrochemical and Solid-State Letters 4(9), A137-A140 (2001). 25 M Kan, J Y Wang, X W Li et al., Structures and phase transition of a MoS2 monolayer, The Journal of Physical Chemistry C 3, (2014). 26 P Johari and V B. Shenoy, Tuning the electronic properties of semiconducting transition metal dichalcogenides by applying mechanical strains, ACS nano 6(6), (2012). 27 Y Ding, Y Wang, J Ni et al., First principles study of structural, vibrational and electronic properties of graphene-like MX 2 (M= Mo, Nb, W, Ta; X= S, Se, Te) monolayers, Physica B: Condensed Matter 406(11), (2011). 28 K F Mak, C Lee, J Hone et al., Atomically thin MoS 2: a new direct-gap semiconductor, Physical Review Letters 105(13), (2010). 29 A Splendiani, L Sun, Y Zhang et al., Emerging photoluminescence in monolayer MoS2, Nano letters 10(4), (2010). 30 C Ataca and S. Ciraci, Functionalization of single-layer MoS2 honeycomb structures, The Journal of Physical Chemistry C 115(27), (2011). 31 M Mortazavi, C Wang, J Deng et al., Ab initio characterization of layered MoS 2 as anode for sodium-ion batteries, Journal of Power Sources 268, (2014).