Electronic and Magnetic Properties of Vanadium Dichalcogenides

May 27, 2014 - Institute of Applied Physics and Materials Engineering, Faculty of Science and Technology, University of Macau, Macau SAR, China. J. Ph...
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Electronic and Magnetic Properties of Vanadium Dichalcogenides Monolayers Tuned by Hydrogenation Hui Pan* Institute of Applied Physics and Materials Engineering, Faculty of Science and Technology, University of Macau, Macau SAR, China ABSTRACT: First-principles calculations based on density-functional theory are carried out to systematically investigate the effects of hydrogenation on the electronic and magnetic properties of vanadium dichalcogenides (VX2, X = S, Se, and Te) monolayers. We find that semimetallic and ferromagnetic VX2 monolayers can be tuned to be nonmagnetic/antiferromagnetic and intrinsically semiconducting by functionalizing with hydrogen atoms on one of their surfaces, and ferromagnetic/antiferromagnetic and n-type semiconducting when both of their two surfaces are fully covered by hydrogen atoms. The ferromagnetism and antiferromagnetism are contributed to carrier-mediated doubleexchange and superexchange, respectively. Ferromagnetism is obtained in semimetallic or lightly doped semiconducting monolayers due to doubleexchange of localized spins with static localized states, while antiferromagnetism can be achieved in intrinsic or heavily doped semiconducting monolayer duo to the superexchange interaction. These vanadium dichalcogenides monolayers with hydrogenation may apply to nanodevices, sensors, and spintronics.

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We find that the metallic and magnetic vanadium dichalcogenides monolayers can be semiconducting, nonmagnetic, or antiferromagnetic by hydrogen functionalization. We show that vanadium dichalcogenides monolayers with versatile functions can be achieved by hydrogenation.

INTRODUCTION Two-dimensional (2D) transition metal dichalcogenides monolayers have attracted increasing attention because of their unusual physical and chemical properties.1−6 Transition metal dichalcogenides monolayers have been widely used in numerous areas, such as nanodevices, hydrodesulfurization catalyst, photovoltaic cell, photocatalyst, nanotribology, lithium battery, sensor, and dry lubrication, due to their distinctive electronic, optical, and catalytic properties.6−13 The transition metal dichalcogenides are a class of materials with the formula MX2, where M is a transition metal element from group IV, group V, or group VI, and X is a chalcogen (S, Se or Te). These materials have crystal structures consisting of weakly coupled sandwich layers X−M−X, where one M atom layer is enclosed within two X layers and the atoms in layers are hexagonally packed.3 In contrast to the graphene, pure transition metal dichalcogenides can be semiconducting, metallic, and magnetic due to the abundant configurations of MX2.2,14 Particularly, their physical and chemical properties can be easily tuned by controlling the composition, functionalizing, and applying external fields.9,15−22 For example, the magnetic properties of 2D MX2 nanostructures can be efficiently enhanced by applying strain due to their superflexibility.15,16 By combining hydrogenation with external tension, Shi et al. reported that magnetic properties of MoS2 monolayer can be tuned from nonmagnetism, to ferromagnetism, and further to nonmagnetism with the increase of tension.17 Different from MoS2, VS2 and VSe2 monolayers are metallic and magnetic in ground states.21,23−25 To date, the effects of hydrogenation on VX2 (X = S, Se, and Te) monolayers have not been investigated. Here, we report the first-principles study on the electronic and magnetic properties of VX2 monolayers with hydrogenation. © 2014 American Chemical Society

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METHODS

First-principles calculations are carried out to investigate the electronic and magnetic properties of vanadium dichalcogenide monolayers. The calculations are based on the density functional theory (DFT)26 and the Perdew-Burke-Eznerhof generalized gradient approximation (PBE-GGA).27 The projector augmented wave (PAW) scheme28,29 as incorporated in the Vienna ab initio simulation package (VASP)30 is used in the study. The Monkhorst and Pack scheme of k point sampling is used for integration over the first Brillouin zone.31 A 15 × 15 × 1 grid for k-point sampling for geometry optimization of unit cells, and an energy cutoff of 450 eV are consistently used in our calculations. Sufficiently large supercells are used so that the monolayers in neighboring cells in the vertical direction are separated by a vacuum region of at least 20 Å. A 2 × 2 × 1 cell is used to study the spin alignments. Spin-polarized calculations are employed. Good convergence is obtained with these parameters, and the total energy was converged to 2.0 × 10−5 eV/atom. Received: March 27, 2014 Revised: May 25, 2014 Published: May 27, 2014 13248

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RESULTS AND DISCUSSION In our calculations, we focus on 2H vanadium dichalcogenides monolayers because 2H phase is more stable. The VX2 unit cells with trigonal prismatic (2H) coordination (Figure 1a) are

and the X-M bonds of the monolayers are reduced by the Hcoverage on their surfaces (Table 1). The calculated bond lengths are about 1.37, 1.51, and 1.72 Å for S−H, Se−H, and Te−H bonding in VX2-1H monolayer, respectively (Table 1). For VX2-2H monolayers, the X-H bonds are 1.39, 1.54, and 1.74 Å for X = S, Se, and Te, respectively. To find out the ground states of VX2 monolayers with and without hydrogenation, the exchange energy (Eex) is calculated as Eex = (EAFM − EFM)/N, where EFM and EAFM are their energies at ferromagnetic and antiferromagnetic states, and N (= 4) is the number of units in the used supercell. The calculated exchange energies are 40, 70, and 80 meV for pure VS2, VSe2, and VTe2 monolayers, respectively (Figure 3). The positive Eex shows that they have ferromagnetic ground states because the ferromagnetic energies are lower than the antiferromagnetic ones, and the higher Eex illustrates that the pure monolayers may show high Curie temperatures. Our calculations further confirm that all VX2 (X = S, Se, and Te) monolayers are ferromagnetic in their ground states. We also note that Eex of VX2 monolayer increases as X changing from S, to Se, and further to Te, indicating their Curie temperatures follow the same trend. On the basis of mean field theory and Heisenberg model, their Curie temperatures (TC) can be estimated from kBTC = (2/3) Eex.32 The estimated Curie temperatures are 309, 541, and 618 K for VS2, VSe2, and VTe2 monolayers, respectively, indicating that they can be used in spintroincs at high temperature. Although the calculated exchange energies of VS2 and VSe2 are much larger than those in ref 25, our estimated Curie temperature of VS2 from the calculated exchange energy is consistent with the experimental result.24 The calculated band structures of VX2 monolayers without hydrogenation clearly show that their spinup bands are asymmetrical to the spin-down bands (Figure 3a,d,g), confirming their ferromagnetic ground states. Different from the metallic characters of VS2 monolayer reported in refs 24 and 25, the calculated band structures show that VX2 monolayers are semimetals or ultranarrow-band semiconductors because the band edges (conduction band bottom, valence band top, or both of them) in spin-up or spin-down band structures are very close to or almost on the Fermi levels. The ferromagnetism of VX2 monolayers is strongly affected by hydrogenation. The calculated exchange energies show that VX2 monolayer with one surface of VX2 monolayer fully covered by hydrogen atoms (VX2-1H) can be nonmagnetic or antiferromagnetic (Figure 2). We see that VS2-1H and VSe2-1H

Figure 1. Representative structures of (a) VX2 monolayer, (b) VX2 monolayer with one side fully covered by hydrogen atoms, and (c) VX2 monolayer with two sides fully covered by hydrogen atoms.

first optimized to obtain the lattice parameters. To investigate the effects of hydrogenation on the lattice parameters of MX2 monolayers, the geometries of VX2 monolayers with one surface (VX2-1H) (Figure 1b) and with two surfaces (VX2-2H) (Figure 1c) fully covered by hydrogen atoms are relaxed. The hydrogen atoms are adsorbed on the top of X atoms (Figure 1), where is the most stable position.9,17 The optimized structures of VX2 (X = S, Se, and Te) (Table 1) show that the lattice constants (a) are extended by hydrogenation. The optimized structures (Table 1) show that the lattice constants (a) are extended by 3.4 to 4.5% for VX2-1H, and 5.5 to 7.5% for VX22H, respectively, compared with pure VX2 monolayers. The thicknesses (or the vertical distance between two X atoms) (c) Table 1. Lattice Parameters of VX2 (X = S, Se, and Te) without Hydrogenation and with One Surface (-1H) or Two Surfaces (-2H) Fully Covered by Hydrogen Atomsa

VS2 VS2-1H VS2-2H VSe2 VSe21H VSe22H VTe2 VTe21H VTe22H

a (Å)

c (Å)

X−V (Å)

∠X−V−X (deg)

∠V−X−V (deg)

X−H (Å)

3.167 3.274 3.341 3.326 3.460

2.971 2.797 2.776 3.190 2.975

2.356 2.320 2.376 2.496 2.453

84.45/78.19 89.75/72.96 89.33/71.48 83.49/79.16 89.68/73.32

84.45 89.75 89.33 83.49 89.68

---1.370 1.394 ---1.514

3.531

2.933

2.511

89.33/71.47

89.33

1.543

3.572 3.734

3.507 3.224

2.707 2.640

82.57/80.74 90.04/73.51

80.47 90.04

---1.719

3.842

3.115

2.711

90.28/70.13

90.28

1.743

Figure 2. Calculated energy difference between antiferromagnetic and ferromagnetic states of VX2 monolayer with and without hydrogenation.

a

a is neighbouring V−V distance, c is the X−X distance in a vertical line, and X−V is the bond length.). 13249

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VTe2-2H are antiferromagnetic with the exchange energies of −13 and −60 meV, respectively (Figure 2). The ferromagnetism of VS2-2H monolayer is very weak and may not be stable (probably nonmagnetic) because the calculated exchange energy of 2 meV is within the error bar of the DFT calculation. The calculated band structures show that VX2-2H monolayers are n-type semiconductors (Figure 3c,f,i) because H-doping introduces electrons into the intrinsic VX2-1H semiconductors (Figure 3b,e,h). The mechanism of magnetism in materials, including the phenomenological Zener/RKKY, superexchange, double exchange, p−d hybridization exchange and magnetic polarons for oxide insulators, and semiconductors doped with magnetic elements, have been proposed and widely discussed.33−46 To reveal the origin of the magnetic transition of VX2 monolayers upon hydrogenation, the partial density of states (PDOSs) and their magnetic moments are calculated. The calculated PDOSs show that the d electrons of V atoms near the Fermi levels in pure VX2 monolayers are spin-polarized (Figure 4a,d,g), leading to the magnetic moments of 0.86, 0.95, and 0.97 μB/V for VS2, VSe2, and VTe2, respectively (Table 2). The p electrons of X

monolayers are nonmagnetic because their exchange energies are almost zero (Figure 2). The VS2-1H monolayer is intrinsic semiconductor with an indirect band gap of 0.75 eV (Figure 3b), and the semiconducting VSe2-1H monolayer has a direct

Table 2. Magnetic Moments of VX2 (X = S, Se, and Te) without Hydrogenation and with One Surface (-1H) or Two Surfaces (-2H) Fully Covered by Hydrogen Atoms

Figure 3. Calculated band structures of (a) VS2, (b) VS2-1H, (c) VS22H, (d) VSe2, (e) VSe2-1H, (f) VSe2-2H, (g) VTe2, (h) VTe2-1H, and (i) VTe2-2H monolayers. The Fermi level is at 0 eV. The red and black lines in panels a, c, d, and g represent spin-up and spin-down band structures, respectively.

VS2 VS2-1H VS2-2H VSe2 VSe2-1H VSe2-2H VTe2 VTe2-1H VTe2-2H

band gap of 0.61 eV (Figure 3e). The VTe2-1H monolayer is antiferromagnetic with an exchange energy of −13 meV (Figure 2) and intrinsic semiconductor with a direct band gap of 0.48 eV (Figure 3h). The electronic properties of VX2-1H (X = S, Se, and Te) monolayers (intrinsic semiconductors) are different from that of MoS2-1H monolayer (n-type semiconductor) under zero strain.9,17 The VX2-1H (X = S and Se) and MoS21H monolayers are nonmagnetic, while VTe2-1H monolayer is antiferromagnetic. When the two surfaces of the monolayer are fully covered by hydrogen atoms (VX2-2H), VS2-2H monolayer is ferromagnetic with a weak Eex of 2 meV, and VSe2-2H and

V (μB)

X (μB)

0.858 0 0.463 0.951 0 1.384 0.986 0.789 1.620

0.047 0 0.008 0.062 0 0.012 0.096 0.025 0.023

atoms in pure VX2 monolayers are also weakly spin-polarized (Figures 4a,d,g), leading to smaller magnetic moments of 0.047 μB/S, 0.062 μB/Se, and 0.096 μB/Te (Table 2). Interestingly, the moments of V atoms are antiparallel to those of X (X = S,

Figure 4. Calculated partial density of states of (a) VS2, (b) VS2-1H, (c) VS2-2H, (d) VSe2, (e) VSe2-1H, (f) VSe2-2H, (g) VTe2, (h) VTe2-1H, and (i) VTe2-2H monolayers. The Fermi level is at 0 eV. 13250

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Upon the one-side hydrogenation on the monolayers, the PDOSs of VS2-1H and VSe2-1H monolayers show that the electrons are spin-unpolarized (Figure 4b,e) because the Hintroduced electrons may occupy the empty orbitals, leading to their semiconducting characters (Figure 3b,e) and vanishing of magnetic moment (Table 2). However, the calculated exchange energy and PDOSs show that VTe2-1H monolayer is antiferromagnetic, where the d electrons of V atoms and p electrons of Te atoms are spin-polarized (Figure 4h). The spinpolarized electrons may be induced by the strong ionic V−Te bond, which leads to charge redistribution. The calculated magnetic moments of V atom and Te atom are 0.79 and 0.025 μB, respectively, which are smaller than those in pure VX2 monolayers (Table 2), and confirms the redistribution of charge. The PDOSs analysis further shows that not only the moments of V atoms, but those of Te atoms are antiparallel among neighboring cells (Figure 5b). From the antiparallel alignment of moments and its semiconducting character, we see that superexchange, the mechanism of antiferromagnetic oxide insulators,43,44 plays a dominant role on the antiferromagnetism of VTe2-1H monolayer. When the two sides of the monolayers are fully covered by hydrogen atoms, the calculated PDOSs (Figure 4c) shows that the weak ferromagnetism of VS2-2H monolayer is mainly attributed to the spin-polarized d electrons of V atoms within the conduction band. The calculated magnetic moments are 0.46 μB/V and 0.008 μB/S, respectively, which are much smaller than those in VX2 monolayers (Table 2). The moments of V and S atoms in VS2-2H monolayer are antiparallel (Figure 5a). We also note that the VS2-2H monolayer is n-type magnetic semiconductor with the band gap of 0.5 eV in spin-up band structure (red lines in Figure 3c), and a low density of carriers, while heavily doped n-type semiconductor with the band gap of 0.5 eV in spin-down band structure (black lines in Figure 3c). The weak ferromagnetism can be, therefore, explained by double exchange,34 which is related to the exchange interaction between the moments and carriers within conduction band at low density. The carrier density in VS2-2H monolayer is much larger than that in VS2 monolayer, resulting in lower exchange energy and weak ferromagnetism. However, for VSe2-2H and VTe2-2H monolayers, the electronic structures show that they are heavily doped n-type semiconductors with high carrier density and narrowed band gaps or even be metals (Figure 4f,i). The magnetic moments of V atoms are 0.97 and 1.62 μB/V for

Se, and Te) atoms (Figure 5a). The calculated band structures (Figure 3a,d,g) and PDOSs (Figure 4a,d,g) shows that the pure

Figure 5. Representative alignment of magnetic moments for (a) ferromagnetism and (b) antiferromagnetism.

VX2 monolayers are semimetals or ultranarrow-band semiconductors. The antiparallel alignment between the magnetic moments and V and X atoms, and the lower density of carriers demonstrate that the double exchange is the dominant mechanism for their ferromagnetism,34,40,42 where the exchange is realized by the hopping of localized carriers. That is, given the incomplete filling of bands due to lower density of carrier, when the exchange splitting is bigger than the bandwidth, the band energy of the ferromagnetic state is lower than that of the antiferromagnetic state if a sufficient (usually rather small) number of carriers exist.34 We see that the intensities of spinpolarized electrons in VSe2 and VTe2 monolayers are stronger than that in VS2 monolayers due to the enhanced strength of V−X (X = Se and Te) ionic bond, resulting in their relatively larger magnetic moments. The reduced density of carriers in pure VX2 monolayers as X changing from S → Se → Te also leads to the increment of exchange energy (Figure 2), further confirming the dominant role of double-exchange for their ferromagnetism.

Figure 6. (a) The calculated energy difference between antiferromagnetic and ferromagnetic states of VS2-2H as a function of applied strain, and (b) the calculated partial density of states of VS2-2H under a strain of 8%. 13251

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Macau. The DFT calculations were performed at the High Performance Computing Cluster (HPCC) of Information and Communication Technology Office (ICTO) at University of Macau.

VSe2-2H and VTe2-2H, respectively, and the magnetic moments of Se and Te atoms are 0.012 and 0.023 μB/atom (Table 2). The alignment of the moments of V and X in the VX2-2H (X = Se and Te) monolayer is similar to that in the VTe2-1H monolayer (Figure 5b). The mobile carriers at high density enhance the superexchange interaction of antiferromagnetic metals,45 which leads to the antiferromagnetism in VSe2-2H and VTe2-2H monolayers. It is, therefore, predicted that the double-exchange in VS2-2H monolayer is not stable enough, and the weak ferromagnetic VS2-2H monolayer should become antiferromagnetic by increasing the carrier density due to superexchange. To confirm this prediction, we apply a tension of 0.5−8% to the monolayer. The VS2-2H monolayer is easily tuned to antiferromagnetism with the negative exchange energy enhanced by increasing tension (Figure 6a). We see that its band gap is reduced, and the carrier density is increased (Figure 6b). The enhanced mobile carriers promote the superexchange interaction of antiferromagnetic metals,45 and result in the antiferromagnetic VS2-2H monolayer under tension. The calculated electronic and magnetic properties of VX2 monolayers with and without hydrogenation show their magnetism strongly depends on the carrier density and electronic structures. Three regions can be clarified as the increment of carrier density: (1) superexchange induced antiferromagnetism in intrinsic semiconductor (VTe2-1H monolayer); (2) double-exchanged induced ferromagnetism under low carrier density, and semimetallic/doped conductivity (VX2 and VS2-2H monolayers); and (3) superexchange induced antiferromagnetism under high carrier density and in heavily doped semiconductors or metals (VSe2-2H and VTe22H monolayers, and VS2-2H monolayer under tension).

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CONCLUSIONS We perform first-principles calculations on the electronic and magnetic properties of VX2 monolayers with and without hydrogenation based on density-functional theory. We find that the hydrogenation can tune the ferromagnetic and semimetallic monolayers to nonmagnetic/antiferromagnetic and intrinsic semiconductor, or antiferromagnetic and n-type doped semiconductors. We show that double-exchange is the origin of ferromagnetism, and superexchange is that of antiferromagnetism. We further show that the exchange interaction is determined by the carrier density and band gap. Our results show that the electronic and magnetic properties of the monolayer can be controlled by doping and applying external field to adjust the carrier density and band gap. We expect that the monolayers with and without hydrogenation may find applications in spintronics, nanodevices, and sensors.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Hui Pan acknowledges the support of the Science and Technology Development Fund from Macao SAR (FDCT076/2013/A), and a Multi-Year Research Grant (MYRG201400159-FST) and Start-up Research Grant (SRG-2013-00033FST) from the Research & Development Office at University of 13252

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dx.doi.org/10.1021/jp503030b | J. Phys. Chem. C 2014, 118, 13248−13253