Article pubs.acs.org/JPCC
Tuning the Electronic and Magnetic Properties of MoS2 Nanoribbons by Strain Engineering Hui Pan* and Yong-Wei Zhang Institute of High Performance Computing, A*STAR, Singapore 138632 ABSTRACT: First-principles calculations are carried out to study the effects of strain on the electronic and magnetic properties of MoS2 nanoribbons. We predict that MoS2 nanoribbons are stretchable up to a strain of 10%. The band structures of the nonmagnetic armchair MoS2 nanoribbons change from direct character to indirect with the increase of strain due to the shift of the energy states near the Fermi level. The ferromagnetic states of metallic zigzag MoS2 nanoribbons are greatly improved because the energy difference between the nonmagnetic and magnetic states is increased up to 4.9 times, and the magnetic moments are increased up to 2 times under a strain up to 10%. Our calculations show that the electronic and magnetic properties of MoS2 nanoribbons can be controlled by applying strain, indicating their potential applications to spintronics and photovoltaic cells. have been obtained by using an electrochemical method.44 Theoretical studies showed that the nanoribbons exhibited unusual electronic and magnetic properties: the zigzag nanoribbon was ferromagnetic and metallic, whereas the armchair nanoribbon was nonmagnetic and semiconducting.45−48 The Raman active modes of the two-dimensional structure were partially softened due to the absence of the interlayer interaction.49 To date, the effects of strain on the physical properties of MoS2 nanoribbons with various edge structures have never been touched. In this work, we present our first-principles calculations on the effects of strain on the electronic and magnetic properties of MoS2 nanoribbons with different edge structures. We show that the bandgaps of armchair nanoribbons decrease with the increase of strain. We further show that the magnetic states and moments of zigzag MoS2 nanoribbons are strongly enhanced under strain.
1. INTRODUCTION The discovery of two-dimensional material, graphene, has triggered extensive study on the two-dimensional materials for applications in next-generation nanodevices because of their easy fabrication.1−4 Experimentally and theoretically, a graphene-analogous inorganic layer and monolayer materials, such as BN, SiC, GaN, ZnO, V2O5, SiC, and even 2D MOFs, have been reported.5−14 Belonging to the family of layered transition metal dichalcogenides, molybdenum disulfide (MoS2) has a crystal structure consisting of weakly coupled sandwich layers S−Mo−S, where a Mo atom layer is enclosed within two S layers, the atoms in layers are hexagonally packed, and these layers are held together by van der Walls interaction.15 MoS2 has been widely used in numerous areas, such as hydrodesulfurization catalyst, photovoltaic cell, photocatalyst, nanotribology, lithium battery, and dry lubrication, due to their distinctive electronic, optical, and catalytic properties.15−27 Bulk MoS2 is a semiconductor with an indirect bandgap of 1.2 eV.28 The report on synthesis of transition metal dichalcogenide nanotubes has triggered extensive research of the inorganic nanostructures, including nanotubes, quantum dots, nanowires, and single layer, due to the outstanding physical and chemical properties.29−38 The monolayer MoS2 has recently attracted great interests because of its potential applications in two-dimensional nanodevices,37,38 although it had been obtained and studied in the past several decades.39−42 The monolayer MoS2 is a direct gap semiconductor with a bandgap of 1.8 eV37 and can be easily synthesized by using scotch tap or lithium-based intercalation.38−43 The mobility of the monolayer MoS2 can be at least 200 cm2 V−1 s−1 at room temperature using a halfnium oxide as gate dielectric, and the monolayer transistor has roomtemperature current on/off ratios of 1 × 108 and ultralow standby power dissipation.38 Recently, the MoS2 nanotribbons © 2012 American Chemical Society
2. METHODS The first-principles calculations based on the density functional theory (DFT)50 and the Perdew−Burke−Eznerhof generalized gradient approximation (PBE-GGA)51 are carried out to study the effects of strain on the electronic and magnetic properteis of the MoS2 nanoribbons. The projector augmented wave (PAW) scheme52,53 as incorporated in the Vienna ab initio simulation package (VASP)54 is used in the study. The Monkhorst and Pack scheme of k point sampling is used for integration over the first Brillouin zone.55 A 3 × 1 × 1 and 5 × 1 × 1 grids for kpoint sampling for geometry optimization and calculations of density of states, respectively, and an energy cutoff of 400 eV are consistently used for the nanoribbons in our calculations. Received: February 17, 2012 Revised: May 9, 2012 Published: May 10, 2012 11752
dx.doi.org/10.1021/jp3015782 | J. Phys. Chem. C 2012, 116, 11752−11757
The Journal of Physical Chemistry C
Article
(Figure 1), two ac-MoS2−NRs can be constructed: symmetrical (ac-MoS2−NR-s; Figure 1a) and asymmetrical (ac-MoS2−NRu; Figure 1b). Similarly, eight zz-MoS2−NRs can be formulated by considering the atoms at the edges and symmetry. For zzMoS2−NRs, one of the two edges can be terminated by Mo atoms and another by S atoms (zz-MoS2−NR-s and zz-MoS2− NR-u; Figure 1c,d), both of the two edges can be Mo atoms (zz-MoS2−NR-Mo-s and zz-MoS2−NR-Mo-u; Figure 1e,f) or 100% S atoms (zz-MoS2−NR-S-s and zz-MoS2−NR-S-u; Figure 1g,h), and one of the edges is terminated by 50% and another 100% S atoms (zz-MoS2−NR-S-h-s and zz-MoS2−NR-S-h-u; Figure 1i,j). The two edges of each zigzag nanoribbon are classified as edge 1 (up edge) and edge 2 (down edge), respectively. The edge states of nanoribbons have not been saturated by hydrogen when investigating the strain effects. The width of the nanoribbons is about 15 Ǻ . A large supercell dimension with a layer-to-layer distance of 16 Å in the plane perpendicular to the layer and an edge-to-edge distance of 20 Å in the plane parallel to the layer are used to avoid interaction between the 1-D nanostructure and its images in neighboring cells. The strain is realized by stretching the nanoribbons, i.e., increasing the lattice constant (a) by x (x = Δa/a × 100%; Figure 2).
Good convergence is obtained with these parameters and the total energy is converged to 2.0 × 10−5 eV/atom. Both of the calculations with and without spin polarization are carried out to investigate the magnetic properties of the nanoribbons. The MoS2 nanoribbons (MoS2−NRs) can be directly obtained by cutting the MoS2 monolayer (Figures 1). According to the directions of termination, there exist two kinds of nanoribbons: armchair (ac-MoS2−NR) and zigzag (zzMoS2−NR). Based on the symmetry of the S layer or Mo layer
Figure 2. Representative scheme of a MoS2 nanoribbon with the applied strain along its axis.
3. RESULTS AND DISCUSSION The structure of MoS2 monolayer is first optimized to obtain the lattice constants. The calculated S−Mo bond length, Mo− Mo distance, and S−S distance in one layer are 2.42, 3.19, and 3.13 Å, respectively. The S−Mo bond length, Mo−Mo distance, and S−S distance are almost equal to the bulk’s values,56 which are in good agreement with the reported results.45 The MoS2 monolayer is a direct semiconductor with a gap of 1.72 eV, which is consistent with the recent experimental result.37 The nanoribbons are constructed based on these lattice parameters. 3.1. Strain Effects on Structures. The MoS2 nanoribbons with different chirality and edge structures are first optimized without strain. The relaxed structures show that only the atoms at the edges distort strongly after the geometry optimizations, whereas the lattice parameters within the ribbons keep almost unchanged. Then, the optimized nanoribbons are further relaxed after applying the strain to investigate its effects on the electronic properties. The calculated lattice parameters (taken from the centers of the nanoribbons) as a function of the strain are shown in Figure 3. The S−Mo bonds are extended under the strain, but the extension is only about 3.7 and 2.6% for armchair and zigzag ribbons, respectively, with the maximum strain applied (10%), indicating that the nanoribbons keep the lattice structures and are reversible after releasing the strain. The S−S distance perpendicular to the monolayer is linearly shortened by increasing the strain and reduced by 2.6% at the strain of 10%.
Figure 1. Structures of MoS2 nanoribbons. (a) Armchair nanoribbon with symmetrical S/Mo layer (ac-MoS2−NR-s), (b) armchair nanoribbon with asymmetrical S/Mo layer (ac-MoS2−NR-u), (c and d) symmetrical and asymmetrical zigzag nanoribbon with two edges terminated by S and Mo atoms, respectively (zz-MoS2−NR-s and zzMoS2−NR-u), (e and f) symmetrical and asymmetrical zigzag nanoribbon with two edges terminated by Mo atoms (zz-MoS2− NR-Mo-s and zz-MoS2−NR-Mo-u), (g and h) symmetrical and asymmetrical zigzag nanoribbon with two edges terminated by 100% S atoms (zz-MoS2−NR-S-s and zz-MoS2−NR-S-u), and (i and j) symmetrical and asymmetrical zigzag nanoribbon with one edge terminated by 50% S atoms and another by 100% S atoms, respectively (zz-MoS2−NR-S-h-s and zz-MoS2−NR-S-h-u). For zigzag nanoribbons, both top and side views are shown. 11753
dx.doi.org/10.1021/jp3015782 | J. Phys. Chem. C 2012, 116, 11752−11757
The Journal of Physical Chemistry C
Article
Figure 3. Calculated changes of atom−atom distances in MoS2 nanoribbons (a) armchair MoS2 nanoribbon and (b) zigzag MoS2 nanoribbon as a function of applied strain. S−Mo is the bond length; S−S (V) is the interlayer distance between two vertical S atoms. Figure 5. Calculated bandgaps of armchair MoS2 nanoribbons as a function of applied strain.
3.2. Strain Effects on Electronic Properties of Armchair Nanoribbons. The electronic and magnetic properties of MoS2 nanoribbons without strain are chirality dependent.45−48 The bandgap of the semiconducting armchair MoS2 nanoribbon and the magnetic moment of the metallic zigzag nanoribbon also depend on their edge structures. The calculated electronic structures show that the armchair MoS2 nanoribbons are semiconducting and nonmagnetic under the applied strain in the range from 1% to 10% (Figure 4). However, the armchair MoS2 nanoribbon with a direct bandgap at the Γ point under weak strain (
