Substrate induced modulation of electronic, magnetic and chemical properties of MoSe 2 monolayer

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1 Substrate induced modulation of electronic, magnetic and chemical properties of MoSe 2 monolayer A. H. M. Abdul Wasey, Soubhik Chakrabarty, and G. P. Das Citation: AIP Advances 4, (2014); View online: View Table of Contents: Published by the American Institute of Physics Articles you may be interested in Field-effect transistors and intrinsic mobility in ultra-thin MoSe 2 layers Applied Physics Letters 101, (2012); / A WSe 2 /MoSe 2 heterostructure photovoltaic device Applied Physics Letters 107, (2015); / Magnetic properties of : Existence of ferromagnetism Applied Physics Letters 101, (2012); / Structural, mechanical and electronic properties of in-plane 1T/2H phase interface of MoS 2 heterostructures AIP Advances 5, (2015); / Tuning magnetism of monolayer MoS 2 by doping vacancy and applying strain Applied Physics Letters 104, (2014); / Magnetism in MoS 2 induced by proton irradiation Applied Physics Letters 101, (2012); /

2 AIP ADVANCES 4, (2014) Substrate induced modulation of electronic, magnetic and chemical properties of MoSe 2 monolayer A. H. M. Abdul Wasey, Soubhik Chakrabarty, and G. P. Das a Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata , India (Received 18 November 2013; accepted 28 March 2014; published online 9 April 2014) Monolayer of MoSe 2, having a typical direct band gap of 1.5eV,isapromising material for optoelectronic and solar cell applications. When this 2D semiconductor is supported on transition metal substrates, such as Ni(111) and Cu(111), its electronic structure gets modulated. First principles density functional investigation shows the appearance of de-localized mid-gap states in the density of states. The work function of the semiconductor overlayer gets modified considerably, indicating n-type doping caused by the metal contacts. The charge transfer across the metal-semiconductor junction also significantly enhances the chemical reactivity of the MoSe 2 overlayer, as observed by Hydrogen absorption. Furthermore, for Ni contact, there is a signature of induced magnetism in MoSe 2 monolayer. C 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [ I. INTRODUCTION Since the discovery of graphene, 1 4 enormous attentions have been given to exploit its unique properties to realize graphene based devices. Notwithstanding its useful electronic properties, gapless electronic structure of pristine graphene imposes a limitation over its use in nanoelectronics. An appropriate electronic band gap is essential for switching of the electron flow in an electronic device. Accordingly, a number of ways to introduce band gap in graphene have been attempted, such as cutting into nano-ribbons, 5 modulation of edge states, 6 introducing defects, 7 doping with some suitable dopants 8 etc. In order to fulfill the ever increasing demand of 2D flexible, light weight electronic devices, several graphene analogues such as hexagonal boron nitride, silicene, germenene etc. that have similar honeycomb topology, have been probed. More recently, TM dichalcogenides (TMDC), such as MoS 2, has been reported 9 to be a potential candidate whose 2D layered structure with moderate band gap can be used for applications in nanoelectronics. Bulk TMDC s are well known since the seventies for their charge density wave (CDW) behavior and their concomitant electrical and optical properties MoSe 2 is a layered material, structurally similar to MoS 2.The Mo layer is sandwiched between two Se layers along [001] direction. Such a composite MoSe 2 trilayer is stacked together along the [001] direction via weak van der Waals interaction. However, intra planar bonding is strong covalent type which gives the system its mechanical strength. MoSe 2 layer is free of surface states because all the chalcogenides ions are saturated in this compound which reduces its chemical reactivity. As the inter-layer binding is relatively weak, the monolayer of MoSe 2 can be synthesized by techniques like liquid exfoliation or micromechanical cleavage etc. 16, 17 In the post graphene era, attempts are being made to study the monolayers of the same compounds. In order to exploit the device potential of these TMDC monolayers, it is important to investigate how they behave electronically while in contact with the metal. Accordingly, people have investigated MoS 2 -metal contact both experimentally and theoretically 19 for different transition metal a of corresponding author: msgpd@iacs.res.in /2014/4(4)/047107/9 4, C Author(s) 2014

3 Wasey, Chakrabarty, and Das AIP Advances 4, (2014) FIG. 1. Optimized ground state geometries of MoSe 2 /Ni(111) system. (a) Side view and (b) top view of the MoSe 2 /Ni(111) heterostructure. Ni atoms closer to the interface are shown by bigger gray spheres. 11, 20 substrates viz. Au (111) and Ti (0001). However, while bulk MoS 2 has good electron mobility single layer MoS 2 is found to have low mobility when grown on some substrate. 9, 17 Popov et al had argued that it is the contact material which plays the crucial role in controlling the device performance, and a detailed explanation was proposed for the low mobility of MoS 2 monolayer with Au contact. Therefore, search for suitable metal contact to MoS 2 is still on to design hetero-junctions with improved functionality. 18, 21 Monolayer of MoSe 2 is a direct band gap semiconductor with a gap value of 1.5 ev which is same as GaAs and less than that of MoS 2 monolayer ( 1.8 ev), and hence has its own advantages for optoelectronic applications. MoSe 2 is experimentally predicted to be a promising material for solar cells because of its band gap lying within the range of solar spectrum. 22 However, unlike MoS 2, there are very few experimental attempts to fabricate MoSe 2 -metal contact 23, 24 have been reported. In this communication, we report our first-principles based density functional investigation of the electronic and chemical properties of MoSe 2 monolayer on Ni(111) and Cu(111) substrate. We have chosen (111) surfaces of Ni and Cu as substrate, because of their hexagonal symmetry on which MoSe 2 monolayer can be grown epitaxially. We find substantial modulation in the electronic structure of monolayer MoSe 2 while in contact with the transition metal substrates because of charge transfer across the interface. In addition to that, in case of Ni contact signature of induced magnetism in MoSe 2 monolayer has been explored. We also found that the surface property of MoSe 2 monolayer gets considerably modified in presence of the metal contact, which has been probed via estimation of work functions and thereby predicting n-type nature of MoSe 2 overlayer due the charge transfer from the metal substrate. Subsequently, enhancement in the chemical activity has also been observed which is presumably due to the influence of the substrate metal. We have investigated the absorption of H atom on (a) free standing monolayer MoSe 2, (b) MoSe 2 /Ni(111) and (c) MoSe 2 /Cu(111) surfaces for comparing the chemical behavior of MoSe 2 monolayer in the above situations. II. THEORETICAL METHODOLOGY Optimized lattice parameters of 2H-MoSe 2 and Ni(111) are found to be 3.32 Å and 2.46 Å whose corresponding experimental values are 3.29 Å 12 and 2.49 Å respectively. The lattice mismatch between the two is nearly 35% which is too large for epitaxial growth. Since strain modifies the electronic structure and hence the energy band gap of MoSe 2, we have explored the so-called coincidence site epitaxy route for minimizing the interfacial strain arising due to the lattice mismatch. Accordingly, if a 3 3 surface supercell of MoSe 2 monolayer is placed on a 4 4 surface supercell of Ni(111) (see Fig. 1), then the resulting lattice mismatch is reduced considerably to only 1.22%.

4 Wasey, Chakrabarty, and Das AIP Advances 4, (2014) Similarly for MoSe 2 /Cu(111) nearly 31% lattice mismatch can be reduced to +1.97% only (see Table II). The geometry relaxation, total energy calculations and the electronic structure calculations have been performed using spin polarized density functional theory based code Vienna Ab Inito Simulation Package (VASP). 25 The exchange-correlation part is approximated by generalized gradient approximation (GGA) of Perdew-Wang (PW91). 26 The projector augmented wave (PAW) 27 method was employed for describing the electron-ion interactions for the elemental constituents Ni, Cu, Se and Mo. The plane wave basis cut off was 500 ev for all the calculations performed in this work. Ionic relaxations are performed using conjugant gradient (CG) minimization method 28 to minimize the Hellman-Feynman forces among the constituent atoms with the tolerance of ev/å. For all the total energy calculations reported in this paper, self-consistency has been achieved with a 0.01 mev (10 5 ev) convergence. The two dimensional Brillouin Zone (BZ) of the MoSe 2 /Ni(111) and MoSe 2 /Cu(111) slab has been sampled using Monkhorst-Pack methodology. 29 For geometry optimization and electronic structure calculations, we have used and 9 9 1k-mesh respectively. III. RESULTS AND DISCUSSIONS A. Interface relaxation and bonding We have carried out structural optimization by relaxing coordinates of all the atoms on such constructed supercell geometry. Optimized ground state geometry of MoSe 2 /Ni(111) is shown in Fig. 1. Interface structure of MoSe 2 /Ni(111) shows that amongst all the Se atoms on top of Ni(111) surface, only a few atoms form binding with the metal surface. This is because of the fact that, all the interface Se atoms are not equivalent with respect to the subsurface Ni layer, as the interface is being formed by 3:4 ratio of MoSe 2 and Ni surface unit cells. The distance between the Ni atoms at the top layer with nearest Se atoms is 2.35 Å, however the optimized shortest distance between the interface Se layer and the top most Ni layer is found to be 2.21 Å. The sum of covalent radii of Ni (1.15 Å) and Se (1.16 Å) is 2.31 Å which is close to the interfacial Se-Ni distance. This fact signifies that considerable orbital overlap between Se and Ni is possible and thereby leading to chemical bonding at this interface. From our total energy calculations we have estimated the binding energy ( E B ) using the following relation, E B = E tot [E MoSe2 + E M(111) ], where, E tot is the total energy of the system, E MoSe2 is the total energy of the MoSe 2 monolayer and E M(111) is the total energy of the metal substrate M(111) (M = Ni, Cu) slab. The binding energy of each MoSe 2 with the Ni surface is found to be 0.34 ev which is strong enough to form a semiconductor/metal interface of practical importance, whereas MoSe 2 /Cu(111) interface (not shown in Figure) is relatively weaker in strength. The distance between the Cu atom and the nearest Se atom is 2.55 Å and the corresponding inter-planar distance between Se plane and Cu plane at the top is 2.46 Å. This is justified by the smaller interface binding energy which is estimated to be 0.16 ev. The above interface geometries suggest that Se-Ni interaction is slightly stronger than the Se-Cu interaction which is expected since Cu is a noble metal. We have then demonstrated the charge transfer at the interface for MoSe 2 /Ni(111) and MoSe 2 /Cu(111) semiconductor/metal contacts (see Fig. 2 and 3). The iso-surfaces corresponding to the charge density difference for MoSe 2 /Ni(111) and MoSe 2 /Cu(111) have been plotted for same iso-value. The yellow surfaces signify losing of charge and blue surfaces for gaining of charge. Clearly the charge transfer from metal to semiconductor side is more in case of Ni-contact which indicates stronger bonding at the interface than that of Cu-contact. The nature of charge transfer at the interfaces has been further illustrated by plotting the averaged charge density across the semiconductor-metal contacts. Fig. 2(b) shows the planar averaged (average in (111) plane) charge density plotted across the MoSe 2 /Ni(111) junction (black line) coinciding with that of MoSe 2 (red line) and Ni(111) (blue line) respectively on the two sides, excepting at the interface. Four humps in the plot (blue line) correspond to the constituent four Ni layers forming the Ni(111) slab and the

5 Wasey, Chakrabarty, and Das AIP Advances 4, (2014) FIG. 2. (a) Charge density difference plot of MoSe 2 /Ni(111) heterostructure. Yellow and blue regions denote loss (depletion) and gain (accumulation) of charges respectively. (b) Planar averaged (in (111) or X-Y plane) charge density of MoSe 2 /Ni(111) plotted along the direction perpendicular to the surface (along Z-axis). (c) Variation in the averaged charge density difference between MoSe 2 /Ni(111) and the sum of the separate constituents (see text for details). broader hump containing three smaller peaks corresponds to the MoSe 2 layer having three Se-Mo-Se sub layers. The Non-zero value of the averaged charge density of MoSe 2 /Ni(111) at the interface is the manifestation of the bonding between Ni and Se. Similarly, Fig. 3(b) shows the averaged charge density of MoSe 2 /Cu(111). Here also finite charge density at interfacial area is evident from the plot justifying the Cu-Se bonding albeit with less prominence than that of MoSe 2 /Ni(111). Fig. 2(c) and Fig. 3(c) show the difference between the averaged charge density of MoSe 2 /M (111) and the sum of MoSe 2 and M(111) (M = Ni, Cu) separately, i.e. n(z) = n(z) MoSe2/M(111) [n(z) MoSe2 + n(z) M(111) ], where, n(z) is the difference in average charge density, n(z) MoSe2/M (111),n(z) MoSe2 and n(z) M (111) denote the average charge density of MoSe 2 /M(111), MoSe 2 monolayer and M (111) substrate respectively. Salient feature of the plots are very clear for semiconductor/metal hetero-junction. The perturbation is maximum at the interface as expected and gets reduced while going further away from the interface on either side. This is also supported by the charge density difference iso-surface plots shown in Fig. 2(a) and Fig. 3(a). It is worth mentioning here that the presence of point or line defects, which are likely to appear during synthesis, gives rise to some defect states near the Fermi level. The resulting charge transfer

6 Wasey, Chakrabarty, and Das AIP Advances 4, (2014) FIG. 3. Similarly the Charge density plots of MoSe 2 /Cu(111) heterostructure. The iso-value used for the charge density difference plot in this case (Figure-3(a)) is same as that of MoSe 2 /Ni(111) (Figure-2(a)). across such heterostructures will be different from that of the defect free junction considered here in this study. B. Electronic and magnetic properties We performed self-consistent electronic structure calculation on these semiconductor/metal hetero-structures. After obtaining the relaxed ground state geometry, we kept the ionic positions fixed and carried out self-consistent electronic structure calculations. Fig. 4(a) shows the projected density of states (PDOS) of free standing MoSe 2 monolayer. Semiconducting nature of MoSe 2 monolayer is reflected from the PDOS. Strong hybridization between Se-4p and Mo-4d is clearly evident from the DOS spectrum. The energy band gap is found to be 1.3 ev which is in good agreement with the experimental band gap of MoSe 2 monolayer 21 albeit with slightly underestimated value due to the approximation used in the exchange-correlation functional. There is no spin polarization effect observed in the DOS as expected for a non-magnetic semiconductor. Subsequently, we studied the electronic structure of metal supported MoSe 2 monolayer, for which the band gap disappears (see Fig. 4(b)) with some delocalized states appearing around the Fermi level. These gap states arise out of electronic states of the Ni(111) substrate. Presence of these delocalized states in the occupied as well as unoccupied part indicates manifestation of good electron mobility across the

7 Wasey, Chakrabarty, and Das AIP Advances 4, (2014) FIG. 4. (a) Projected density of states (PDOS) of free standing MoSe 2 monolayer. (b) PDOS of MoSe 2 overlayer on Ni(111) substrate. (c) PDOS of MoSe 2 overlayer on Cu(111) substrate. (d) A comparison of total DOS of Ni/Cu supported MoSe 2 monolayer with the free standing one. semiconductor/metal junction which is an important requirement for electronic devices. Injection of charge carrier across such a junction will occur with relative ease. The spin splitting observed in the PDOS is induced by the Ni substrate. In order to underscore the effect of MoSe 2 absorption on Ni(111) on its magnetic moment, we have carried out separate electronic structure calculations on Ni(111) surface. We found that upon absorption of MoSe 2 monolayer on Ni(111) slab, its average magnetic moment gets slightly reduced from ( 0.65 μ B to 0.62 μ B per Ni atom). This quenching of magnetic moment of Ni further justifies the interaction of Se atoms with the Ni atoms at the interface. The above investigation suggests that Ni is a good metal contact to MoSe 2 forming junction with good carrier mobility. Replacing Ni with Cu which is a non-magnetic noble metal, no induced magnetism is expected. However, in this case also Cu-induced delocalized gap states appear around the Fermi level (see Fig. 4(c)) thereby predicting enhancement the electron mobility across the junction. Therefore Cu-contact to MoSe 2 is also a viable candidate for electronic devices, although comparatively less promising than Ni-contact. C. Surface properties Work function is a characteristic property for any solid surface. It is defined as the minimum amount of energy required to remove an electron from the highest occupied level of any solid. In order to bench mark our DFT based theoretical approach we have estimated the work functions of Ni(111) and Cu(111) surfaces. Here we have considered only a single unit cell of metal surfaces instead of using a supercell. It is a well known fact that absolute value of the metal work-function depends sensitively on the choice of exchange-correlation functional. Keeping this in mind, we have estimated the work functions of Ni(111) and Cu(111) by using different exchange-correlation functionals such as LDA, GGA-PW91 and GGA-PBE. Results are tabulated in the Table I. Table

8 Wasey, Chakrabarty, and Das AIP Advances 4, (2014) TABLE I. Table shows the estimated work functions of Ni(111) and Cu(111) surface in comparison with the experimental values. Work function (in ev) System LDA GGA-PW91 GGA-PBE Experimental a Ni(111) Cu(111) a Experimental work functions of Ni(111) and Cu(111) are 5.35 and 4.98 ev respectively. 30 TABLE II. Table shows the results of interface relaxations of MoSe 2 /Ni(111) and MoSe 2 /Cu(111) System a (Å) η (%) d M-Se (Å) d(111) M-Se (Å) E B (ev) (ev) Monolayer MoSe Ni(111) Cu(111) MoSe 2 /Ni(111) ( 1.22) MoSe 2 /Cu(111) (+1.97) Symbols used in the table used explained below: a = lattice parameter. η (%) = ((a substrate a overlayer )/a substrate ) 100; lattice mismatch between overlayer and substrate. Numbers inside the paranthesis are the corresponding lattice mismatch for coincidence site epitaxy. d M-Se = shortest distance between top Metal and Se atoms at the interface. d(111) M-Se = perpendicular distance between top Metal and Se plane. E B = binding energy of MoSe 2 overlayer per MoSe 2 formula unit. = work function or ionization potential of the metal and MoSe 2 (semiconductor) surfaces. shows that GGA-PBE underestimates the values as compared to LDA values for both Ni(111) and Cu(111) surface. However, GGA-PW91 gives intermediate values which reasonably agree with the available experimental values for both the metal surfaces. Therefore, in the rest of our calculations, we have used GGA-PW91 functional. Subsequently, we carried out investigation on MoSe 2 /Ni(111) and MoSe 2 /Cu(111) semiconductor/metal systems. As already discussed that for constructing these hetero-junctions we had to consider 4 4 surface unit cells for representing the metal substrates. Accordingly, we have estimated the work functions of bare Ni(111) and Cu(111) surfaces in the slab geometry and the values are found to be 5.28 ev and 4.93 ev respectively. We repeated the work function calculation for MoSe 2 covered Ni(111) and Cu(111) surface and found considerable reduction in the work function values to 4.90 ev and 4.80 ev respectively (see Table II). For pristine MoSe 2, the calculated ionization potential is 5.45 ev. This indeed justifies the fact that in both the cases for Ni and Cu, the Fermi level shifted upward which implies that MoSe 2 monolayer become n-type doped while supported on Ni(111) and Cu(111) substrate. This is further illustrated by the direction of charge transfer as demonstrated in Fig. 2 and 3. D. Chemical properties From the above analysis it is clear that the electronic property of monolayer of MoSe 2 which is semiconducting in its pristine form gets considerably perturbed when absorbed on Ni(111) and Cu(111) surfaces. This justifies the possibility of some modulation in chemical properties also. Guided by the above fact, we have attempted to study the absorption of H atom on the MoSe 2 surface, before and after depositing on Ni(111) and Cu(111) surfaces. First we placed a single H atom on top of Se atom for this course of investigation. After minimizing the energy and forces our total energy based calculation yield that H atom is bound with the pristine MoSe 2 surface and the binding energy is found to be 0.29 ev using the following relation, E = E tot [E H + E sur f ace ],

9 Wasey, Chakrabarty, and Das AIP Advances 4, (2014) FIG. 5. Charge density plot showing interaction of H atom with the (a) pristine MoSe 2 monolayer and (b) Ni supported MoSe 2 monolayer. The accumulation of more charges along the Se-H line justifies the enhancement of binding of H atom with Se atom while MoSe 2 is supported by Ni(111) substrate. where, E denotes the binding energy of H atom on the surface, E tot, represents the total energy of the system when H atom is absorbed on it, E H is the total energy of a single H atom and E surface is the total energy of the surfaces before absorption of H atom. The estimated Se-H bond length is 1.56 Å. However, strength of this binding increase significantly to 0.77 ev with a concomitant reduction in Se-H bond length to 1.50 Å in case of MoSe 2 / Ni(111). The injection of electrons from metal side, as depicted in the charge density difference plots (see Fig. 2 and 3), enhances the chemical reactivity of the semiconductor. This enhancement of H bonding with the MoSe 2 monolayer is observed also for the Cu supported one, albeit with slightly weaker bonding than that of Ni supported case. In case of MoSe 2 /Cu(111), estimated binding energy of H atom is 0.63 ev with a similar Se-H bond length of 1.51 Å. This slightly weaker binding of H atom as compared to Ni supported case may be rationalized from the fact that Cu is noble metal and is less interactive than Ni. Fig. 5 shows the charge density contour plots exploring the nature of bonding between Se and the on surface H atom. Accumulation of more charges (see Fig. 5(b)) along Se-H bond while MoSe 2 monolayer is supported on Ni(111) dictates stronger binding than that of free standing MoSe 2 monolayer (Fig. 5(a)). The above study reveals that the transition metal support significantly enhances the chemical activity of the otherwise less active MoSe 2 monolayer. IV. CONCLUSION To summarize, we have used state of the art spin polarized density functional approach to carry out investigation on monolayer MoSe 2, a semiconductor having typical direct band gap of 1.5 ev, which is important for optoelectronic applications. We examined its electronic structure upon absorption on the magnetic (Ni(111)) and non-magnetic (Cu(111)) substrate. Delocalized mid-gap states arise in the DOS spectrum of MoSe 2 monolayer for both Ni and Cu contact. In addition to this, there is induced magnetization in case of Ni contact which can be useful for spintronic applications. Work function of metal supported MoSe 2 monolayer gets modified considerably due to the charge transfer across the interface. This injection of electrons from the metal to semiconductor also enhances the chemical reactivity of the surface, thereby suggesting it to be promising materials for surface catalysis. Thus electronic, magnetic and chemical properties of monolayer MoSe 2 can be tailored in controllable fashion using suitable substrate of technological importance. Our

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