Bilayer Heterosystem

Transcription

1 Journal of Physics: Conference Series PAPER OPEN ACCESS First-Principles Stuy of Asorption of Halogen Molecules on Graphene-MoS 2 Bilayer Heterosystem To cite this article: S. Lamichhane et al 2016 J. Phys.: Conf. Ser View the article online for upates an enhancements. Relate content - First-Principles Stuies for the Electronic Structures of Dilute Magnetic Semiconuctors (Ga, Fe)As * Wei Shu-Yi, Wang Tian-Xing, Yang Zong- Xian et al. - First-Principles Stuy of Structural, Magnetic, Electronic an Elastic Properties of PuC2 Rong Yang, Bin Tang, Tao Gao et al. - First-principles stuy on the electronic, elastic an thermoynamic properties of three novel germanium nitries Cang Yuping, Yao Xiaoling, Chen Dong et al. This content was ownloae from IP aress on 19/12/2017 at 21:48

2 First-Principles Stuy of Asorption of Halogen Molecules on Graphene-MoS 2 Bilayer Hetero-system S. Lamichhane 1, P. Lage 1, G. B. Khatri 1, N. Pantha 1, N. P. Ahikari 1* an B. Sanyal 2 1 Central Department of Physics, Tribhuvan University, Kathmanu, Nepal. 2 Division of Materials Theory, Department of Physics an Astronomy, Uppsala University, Sween. * npahikari@tucp.eu.np Abstract We have performe ensity functional theory base first-principles calculations to stuy the stability, geometrical structures an electronic properties of pure 3 3 supercell of MoS 2, 4 4 supercell of graphene, graphene-mos 2 bilayer heterosystem, F 2, Cl 2, Br 2 an I 2 molecules on hetero-system within the DFT- D 2 level of approximations. The preferable site an asorption energy of halogen molecules are stuie. The most stable geometries are consiere to stuy their electronic ban structure, Density of states an magnetic properties with reference to iniviual 2D components, graphene an MoS Introuction The strongest, thinnest an most stretchable material arrange in honeycomb lattice of thick sheet of sp 2 -bone carbon atoms terme as graphene is at focus of many research works worlwie ue to its peculiar properties like observable quantum Hall effect at room temperature an existence of twoimensional gas of massless Dirac fermions [1-5]. These nobel properties of graphene with its applications in the areas of spintronics, hyrogen storage, sensing [6-9] have attracte the most moern consent of researchers to preict other esirable properties. On complementary to graphene, MoS 2 belonging to 2D transition metalichalcogenie, well known as ban gap semiconuctor is becoming most stuie material in the last 5 years [10-12]. MoS 2 is a layere transition metal ichalcogenie semiconuctor with an inirect ban gap in which Mo an S atoms are stacke together to give S-Mo-S sanwiches coorinate in a triangular prismatic arrangement [13]. In recent time, MoS 2 is attracting the attention of many researchers as it is excessively available in the form of a natural mineral, molybenite [14]. MoS 2 monolayer is wiely use to prouce transistors [15], integrate logic circuits, signal amplifiers [16], photoetectors an flexible optoelectronic evices. It is a promising material with highly potential applications in solar cells [17], photocatalysts, signal amplifiers an lubrication [18]. However, its properties are limite by ban gap. The graphene-mos 2 bilayer hetero-structure as shown in figure [1] offers the excellent mechanical flexibility, electronic properties, optical transparency, photoconuctivity an favorable transport properties [19]. Content from this work may be use uner the terms of the Creative Commons Attribution 3.0 licence. Any further istribution of this work must maintain attribution to the author(s) an the title of the work, journal citation an DOI. Publishe uner licence by Lt 1

3 The association of two materials with their istinct electronic properties is believe to rag the esire properties in practical applications. Moreover, the asorption of foreign atoms on hetrostructure is one of the promising approach to moify an exploit unwante properties of any constituent. Combination of two materials staning for a bran new family with halogen, commonly known as reactive non metal an highly electronegative elements of perioic table is highly expecte to start new irection towars mate-rials science research.heterostructure may bee important to stuy the properties beyon the capacities of constituents as well as to eliminate the negative properties an rag esirable one. Although the graphene-mos 2 hetero-structure is use to stuy the asorption of ifferent metals on nano-scale channels [20], it has not been teste for the ability of heterostructure with reactive nonmetals. Previous work for the asorption of halogens on graphene layers opens very small gap (3-75 mev, highest in fluorine). Since MoS 2 is ban gap material, we are intereste to test halogens on graphene-mos 2 bilayer to obtain high ban gaps an particularly the role of fluorine to change the electronic structure. From both scientific an technological point of view, investigation of halogen asorption on semiconucting surface is very important as halogen an its compouns are wiely use in the etching technologies. To make comparison for the interaction of halogens an semiconuctor surface is of prime importance. In this work, we consier asorption of iatomic halogen molecules on graphene-mos 2 bilayer hetero-system. The remaining part of the paper is organize in the following way. In Sec. II, we iscuss the computational etails an the systems uner our stuy. Sec. III gives a brief escription of results of asorption of F 2, Cl 2, Br 2 an I 2 molecules on graphene-mos 2 bilayer hetero-system. Here, we present the analysis of asorption sites, bining energies, ban structures an ensity of states. The last conclusion section summarizes the major finings an possible extensions of the ongoing research. Figure 1: 3 3 supercell of MoS 2 Monolayer an 4 4 super cell of graphene to form a MoS 2 -graphene bilayer hetero-system, (a) Top-view an (b) Sie-view respectively. 2. Computational Details In the present work, we have incorporate Density Functional Theory (DFT) [21, 22] implemente in the Quantum Espresso (QE) [23] coe to stuy the geometrical an structural properties of 3 3 supercell of MoS 2, 4 4 supercell of graphene, graphene-mos 2 bilayer hetero-structure an halogen molecules asorbe graphene- MoS 2 bilayer hetero-structure. The Perew-Burke-Ernzerhof (PBE) form of generalize graient approximation (GGA) [24] with van Der Waals interaction via Grimme s moel [25, 26] is use to treat inter electron interaction. The bfgs algorithm with Rappe-Rabe-Kaxiras- Joannopoulos (RRKJ) moel of ultrasoft pseuopotential is use to account the interaction between the ion cores an valence electrons. The unit cell is optimize with respect to lattice parameter a, kinetic energy cut-off (E cut ) for plane wave an the number of k-points along x- an y-axes respectively. Base on these convergence tests, we obtaine the lattice constant a for the unit cell of MoS 2 as A which agrees with the previous results [13, 14]. For the 3 3 supercell of MoS 2 the lattice constant is three times that of the unit cell. Further, the plot of the total energy verses the number of k-points reveals that the energy of the unit cell of monolayer MoS 2 is almost constant after n kx equal to 10. Hence a mesh of k-points, in case of unit cell, 2

4 coul be use for the Brillouin-zone integration. The mesh was reuce to for 3 3 as per relations of irect an reciprocal lattice geometries, which helps to minimize the computational cost. A plane wave basis set with energy cut-off values of 476 ev (35 Ry) for the wave-function an 4760 ev (350 Ry ) for the charge ensity is use for the expansion of the groun state electronic wave functions. 3. Results an Discussion 3.1 Asorption of halogen molecules on Graphene-MoS 2 bilayer hetero-structure: The MoS 2 /graphene heterostructures is mae using 3 3 supercell of MoS 2 an 4 4 supercell of graphene by consiering the lattice mismatch. There are two possibilities for the construction of the heterostructures accounting the ifference in lattice parameters between Bohrs of MoS 2 an Bohrs of graphene. If lattice constant of MoS 2 is use (i.e MoS 2 is taken reference an graphene is moelle on it), there will be compressive strain of 3.3 % on graphene sheet. If lattice constant of graphene is use i.e graphene as reference an MoS 2 is moelle on it, there will be 3.2 % tensile strain to MoS 2. In this stuy, Both cases are taken with the ifferent stacking an configurations to fin the minimum energy state for the calculations of the electronic structurs of the heterostructures. Vaccum length is mae greater than 20 A along z- axis to avoi the interactions between the ajacent supercells. We have consiere six ifferent occupation sites of high symmetry : the hollow site, at the center of hexagonal plane of MoS 2, top of molybenum atom (Mo-Top), top of sulphur atom (S-Top) from MoS 2 sie an hollow site at the center of hexagon of graphene (C-Hollow), brige between two carbon atoms (C-Brige) an top of carbon atom (C-Top) from graphene sie as shown in figure [2]. The asorbates (halogens) were kept in two ifferent orientations before optimization, viz. an Perpenicular to the graphene an MoS 2 sheets. Reverse to the orer of bon issociation energy in halogen molecules (Cl 2 >Br 2 >F 2 >I 2 ), the B.E. of halogens on MoS 2 sheet is foun in the orer Cl 2 <Br 2 <F 2 <I 2. The reason is obvious, the halogen molecule with the stronger intra-atomic boning gain its inert state an becomes less reactive towars MoS 2 monolayer. Figure 2: Different possible asorption sites for the asorption of halogen molecules on graphene-mos 2 bilayer hetero-structure. The bining energy (asorption energy) of a halogen molecule on graphene-mos 2 bilayer heterosystem is calculate by using the relation 3

5 = E hal + E graphene MoS2 E hal graphene MoS2 (1) where E hal is groun state energy of a fully relaxe halogen molecule, E graphene MoS2 is the groun state energy of graphene-mos 2 bilayer hetero-structure an E hal graphene MoS2 is the groun state energy of halogen asorbe graphene-mos 2 bilayer hetero-structure. The positive bining energy reflects the stability of the system. The total energy of a halogen molecule asorbe graphene- MoS 2 bilayer hetero-structure is calculate in 3 3 supercell of MoS 2 an 4 4 supercell of graphene. Out of the six asorption sites, the site with the largest bining energy is inicate as the most favourable site for asorption of the halogen molecule. The ifferent parameters obtaine from our calcula-tions are shown in tables below. Table 1: Table for Bining Energy (E), istance of centre of asorbe molecule from sulphur plane of MoS 2 ( ), reference ( )*[27], istortion on MoS 2 sheet ( ) an observe bon length of asorbe halogen molecule () along graphene sie of graphene- MoS 2 bilayer heterostructure Halogen Orientation C-hollow C-brige C-top F * * * Cl Br I Table 2: Table for Bining Energy (E), istance of centre of asorbe molecule from sulphur plane of MoS 2 ( ), istortion on MoS 2 sheet ( ) an observe bon length of asorbe halogen molecule () along MoS 2 sie of graphene-mos 2 bilayer heterostructure Halogen Orientation hollow Mo-top S-top F Cl Br I

6 Tables 1 & 2 show the bining energy values of ifferent structures at ifferent occupation sites. The istance of centre of asorbe molecule from sulphur plane of MoS 2 ( ) is foun to agree well with the reference values of ( ) [27]. The bon length of asorbe molecule on graphene- MoS 2 bilayer hetero-structure is not obtaine to be significantly change uring asorption process because the obtaine molecule-surface interaction is much weaker than the intra molecular boning in halogens. Here, we consier the system with the highest B.E. or the least total energy is most stable one. The fluorine asorbe graphene- MoS 2 bilayer hetero-structure is most stable at the hollow site (C- Hollow) at the center of hexagon of the graphene sheet in the parallel configuration. But all the remaining halogen molecules asorbe hetero-structures are obtaine to be most stable while they are to the sheet an above the sulphur atom of MoS 2 sheet (S-Top). The highest value of B.E. are ev, ev, ev an ev respectively for fluorine atom, Cl 2, Br 2, an I 2 molecules asorbe hetero-structures. The geometrical structures with the highest B.E. are consiere to raw the various properties of the respective systems. In aition, the istance between two F-atoms increases when it is boun more strongly to MoS 2 sheet of graphene- MoS 2 bilayer hetero-structure. This result inicates that the boning of F or F 2 with the substrate happens on the cost of weakening of F-F interaction. The asorption geometries of molecular halogens (i.e. an parallel orientations) above the graphene sie of hetero-structure is shown in figure [3], while the optimize structures of ifferent halogens on hetero-structure at the most stable occupation site are shown in figure [4]. (a) (b) (c) () (e) (f) 5

7 Figure 3: Top an Sie view respectively for the asorption of halogen molecules with ifferent orientations of asorbates in the irection parallel ((a), (c), (e)) an (fig. (b), (), (f)) to the graphene sheet. Figures (a) an (b) show the asorption in C-Hollow (H) region, (c) an () at C- Brige an (e) an (f) at C-Top, respectively. Similarly, the asorption of halogen molecules with ifferent orientations of asorbates in the parallel an irection to the Mo 2 sheet are teste to know the most occupation site. (m) (n) (o) (p) Figure 4: Optimize geometries of a halogen molecule asorbe graphene-mos 2 bilayer heterostructure with top an sie-view respectively. The figures, (m) fluorine atoms parallel to graphene plane at Hollow-site (C- hollow), (n) Cl 2 molecule to MoS 2 plane at S-Top site (o) Br 2 molecule to MoS 2 plane at S-Top site, (p) I 2 molecule to MoS 2 plane at S- Top site Electronic structure calculations: In orer to unerstan the impact of asorption of halogen molecules, we first nee to unerstan the electronic properties of 3 3 sheet of MoS 2 monolayer as shown in figure [5(a)], 4 4 sheet of graphene as in figure [5(b)] an graphene-mos 2 bilayer hetero-structure as in figure [5(c)]. Figure [5(a)] shows the ban structure of 3 3 sheet of MoS 2 monolayer. Since, the conuction ban minimum an valence ban maximum lie at the same symmetric point, it is a irect ban gap semiconuctor with ban gap of 1.65 ev. In case of 4 4 sheet of graphene, there is no ban gap as conuction ban an valence ban coincies at Fermi level inicating that the graphene is a zero ban gap semiconuctor. Figure [5(c)] shows the ban structure of graphene-mos 2 bilayer hetero-structure. Here, the Dirac point shifts about 0.40 ev above the Fermi level which shows that the graphene-mos 2 bilayer hetero-structure is metallic in nature. Afterwars, we have stuie the effect of asorption of halogen molecules on the heterostructure along graphene plane an MoS 2 plane. The ban structures of ifferent systems are shown in figures [5()], [5(e)], [5(f)], an [(5(g). 6

8 (a) (b) (c) () (e) (f) 7

9 (g) Figure 5: Ban structures of (a) 3 3 sheet of MoS 2 monolayer (b) 4 4 sheet of graphene (c) graphene- MoS 2 bilayer () fluorine atoms (e) Cl 2 (f) Br 2 (g) I 2 molecule asorbe graphene-mos 2 bilayer hetero-structure. The moifications in ban structures is illustrate in table [3]: Table 3: Electronic properties of halogen asorbe Graphene/MoS2 system. Optimize structure Ban Gap Fermi Energy Dirac Point Dirac Shift Graphene-MoS 2 bilayer F 2 -graphene MoS Cl 2 -graphene MoS Br 2 -graphene MoS I 2 -graphene MoS From table [3], it is clearly seen that the ban gap of ev is foun in the case of fluorine atoms asorbe graphene-mos 2 bilayer but in other halogen asorbe bilayer, Dirac point shifte above the Fermi level. The formation of ban gap in fluorine asorbe system is ue to the boning an antiboning states of the electrons in the orbitals ans also ue to ban foling property of electrons in the orbitals. The ban structures of other halogen asorbe systems resembles the ban structure of graphene-mos 2 bilayer. The shift of Dirac point increases from Cl 2 to I 2 asorbe graphene-mos 2 bilayer. Also the Fermi energy is seen to be in the increasing orer from Cl 2 to I 2 asorbe graphene- MoS 2 bilayer. Since fluorine atom has the highest electronegative value among halogens, it coul easily go for interaction in presence of other reacting species. This coul be seen at the B.E. values in table [1]. Our results strongly show the favourable conition to absorb halogens particularly fluorine molecule in excite moe as one to store hyrogen molecule/s on metal ecorate graphene [28]. Similarly, the magnetic properties of our system are stuie with the help of information isplaye by ensity of states (DOS) as shown in figure [6] 8

10 Figure 6: DOS of graphene- MoS 2 bilayer hetero-structure an halogen asorbe graphene- MoS 2 bilayer hetero-structure. The curve above zero x-axis in each plot represents up-dos, an that below the axis represents own-dos. The plot of DOS shows that both up-dos an own-dos are symmetrical about the axis in graphene- MoS 2 bilayer hetero-system as well as halogen molecules asorbe bilayer hetero-structures. This means, the non magnetic nature of graphene-mos 2 bilayer hetero-structure is still preserve with asorption of halogens on it. From figure [6], it is clear that the DOS shift above the Fermi level ue to asorption of halogen molecules on hetero-structure. 4. Conclusions: We have stuie a first-principles stuy of geometrical an electronic properties of graphene-mos 2 bilayer hetero-structure an halogen molecules asorbe hetero-structures. The asorption sites an orientations are teste to raw the most stable configurations. The fluorine atoms asorbe system is stable at hollow site of hexagonal plane of graphene (C-Hollow) in parallel configuration with the bining energy ev. The Cl 2, B 2 an I 2 asorbe hetero-structures all are most stable in the configuration above the sulphur atoms of MoS 2 plane (S-Top) with bining energies ev, ev an ev respectively. Ban structure unalters in case of Cl 2, B 2 an I 2 asorbe graphene-mos 2 bilayer hetero-structure but Dirac point shifts above the Fermi level. The ban gap of ev is foun in fluorine atoms asorbe hetero-structure. Our work suggests a way to tune the electronic properties of heterostructure with asorption of halogens. We then analyze DOS of respective systems an conclue that monolayer MoS 2 an halogen asorbe system are non magnetic. 5. Acknowlegements: The authors acknowlege to Sweish Research Links for partial support. 9

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