Transition-Metal Dihydride Monolayers: A New Family of Two

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Transition Metal Dihydride Monolayers: A New Family of Two-Dimensional Ferromagnetic Materials with Intrinsic Room-Temperature Half-Metallicity Qisheng Wu, Yehui Zhang, Qionghua Zhou, Jinlan Wang, and Xiao Cheng Zeng J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 13, 2018

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The Journal of Physical Chemistry Letters

Transition Metal Dihydride Monolayers: A New Family of Two-Dimensional Ferromagnetic Materials with Intrinsic Room-Temperature HalfMetallicity Qisheng Wu†,‡, Yehui Zhang,† Qionghua Zhou,† Jinlan Wang†,§,*, Xiao Cheng Zeng‡,*

†

School of Physics, Southeast University, Nanjing 211189, P. R. China

‡

Department of Chemistry, University of Nebraska-Lincoln, Lincoln NE 68588, United States

§

Synergetic Innovation Center for Quantum Effects and Applications (SICQEA), Hunan Normal

University, Changsha, Hunan 410081, China

Corresponding Author * J.W. (Email: [email protected]) * X. C. Z. (Email: [email protected])

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The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Two-dimensional (2D) ferromagnetic materials with intrinsic half-metallicity are highly desirable for nanoscale spintronic applications. Here, we predict a new and stable family of 2D transition metal dihydride (MH2; M=Sc, Ti, V, Cr, Fe, Co, Ni) monolayers with novel properties. Our density-functional-theory computation shows that CoH2 and ScH2 monolayers are ferromagnetic metals, while the others are antiferromagnetic semiconductors. In particular, CoH2 monolayer is perfect half-metal with a wide spin gap of 3.48 eV. ScH2 monolayer can also possess half-metallicity through hole doping. Most importantly, our Monte Carlo simulations show that CoH2 monolayer possesses an above-room-temperature Curie point (339 K), while that of ScH2 monolayer can also reach 160 K. A synthetic approach is proposed to realize CoH2 and ScH2 monolayers in the laboratory. Notably, their half-metallicity can be well maintained on substrates. The new family of MH2 monolayers are promising functional materials for spintronic applications due to their novel magnetic properties.

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Spin electronics (or spintronics) that makes use of the spin degree freedom of electrons is of significance for future information technologies owing to its great promise in markedly enhancing data processing speed and integration densities.1,

2

To build practical spintronic

devices, the selection of spintronic materials is crucial and yet challenging.3 Like graphene,4 various two-dimensional (2D) materials have been exploited for using their spin degree of freedom in electrons5 to keep pace with the demand of continuous miniaturization of spintronic devices.6 Extensive efforts have been made for inducing magnetism in 2D systems through defect engineering,7 ribbon engineering,8 doping,9 adsorption,10 and chemical functionalization.11 However, success is still limited since the induced magnetism in 2D materials reported so far is mostly quite weak and lacks robustness.3 To realize the long-range ferromagnetic (FM) coupling in 2D materials, it is essential to design the material with intrinsic ferromagnetism. CrI3 and Cr2Ge2Te6 atomic layers12, 13 were reported to be intrinsically ferromagnetic, but with low Curie temperatures. Very recently, 2D room-temperature ferromagnetism was independently observed in VSe2/MoS2 van der Waals heterostructure14 and epitaxial manganese selenide films,15 opening new ways to achieve 2D room-temperature spintronics. As a special kind of ferromagnet, half-metal is any substance that acts a metal in one spin channel and an insulator (or semiconductor) in the other one.16,

17

Half-metals can offer

completely spin-polarized carriers, and thus are recognized as perfect spintronic materials for pure spin injection18 and spin transport.19 The band gap for the insulating channel is termed as spin gap. To prevent spin leakage, the spin gap needs to be as wide as possible.20 Until now, intrinsic half-metallicity and wide spin gap are still absent in 2D ferromagnetic materials



reported. Hence, it is highly desirable to find out a 2D material with room-temperature halfmetallicity and wide spin gap.20-27 Previous experiments28, 29 and theories30-32 have investigated various molecular and onedimensional transition metal trihydrides, in which transition metal and hydrogen atoms bind with each other in a pyramidal form. In this work, for the first time, we report stable 2D transition metal hydride (MH2; M=Sc, Ti, V, Cr, Fe, Co, Ni) monolayers with novel magnetism featuring pyramidal symmetry (C3v). Our computations show that CoH2 and ScH2 monolayers are FM metals, while TiH2, VH2, CrH2, FeH2, and NiH2 monolayers are predicted to be antiferromagnetic (AFM) and semiconducting. Moreover, CoH2 monolayer exhibits intrinsic half-metallicity with 100% spin filtering and a spin gap of 3.48 eV. Half-metallicity can be observed in ScH2 monolayer as well by slight hole doping. Monte Carlo simulations, based on 2D Heisenberg Hamiltonian model, suggest that CoH2 and ScH2 monolayers possess Curie temperatures of 339 K and 160 K, respectively. Geometric structures for MH2 monolayers with various magnetic ground states are fully optimized and displayed in Figure 1. Here, FM (Figure 1a), three AFM ground states (AFM1, AFM2 and AFM3; Figure 1b-d), and nonmagnetic (NM) state, are considered in the geometric optimizations (Figure 1e). Relative energies among different magnetic ground states are listed in Table S1, showing that CoH2 and ScH2 monolayers are FM metals with magnetic moments of 1.19 and 0.59 µB on the metal atoms, respectively (Table S2). TiH2, VH2, CrH2, FeH2 and NiH2 monolayers have AFM ground states. The lattice parameters and atomic positions for the MH2 monolayers are given in Table S2. The magnetism associated with the MH2 monolayer stems from the transition-metal atoms, as shown in the spin density distributions (Figure S1). All MH2 monolayers considered are in the T phase, in which each transition atom is bonded with six


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hydrogen atoms, forming octahedral ligand field (Figure 1f). To probe the chemical bonding in MH2 monolayers, we compute the electron localization functions (ELF),33-35 which can properly describe electron localization in solid. The electron localization (Figure S2) centers around H atoms and is absent in the mid-point between transition metal and H atoms, indicating that ionic bonding feature in the MH2 monolayers. Our Bader charge analyses36 further reveal that electrons are apparently transferred from transition metal atoms to H atoms (Table S2).

Figure 1. Top views (a-d) and side view (e) of geometric structures for ferromagnetic and three different antiferromagnetic states for the 2D transition-metal dihydride monolayers. (f) Top and perspective views of octahedral ligand field. Dark cyan and white spheres denote transition metal and hydrogen atoms, respectively. Red and blue arrows denote two opposite spin orientations. Dashed lines indicate primitive cells.



Figure 2. Electronic band structures and projected density of states (DOS) of CoH2 (a, b) and ScH2 (c, d) monolayers. Red and blue lines (arrows) represent spin-up and spin-down, respectively. Dashed lines refer to the Fermi level set to zero. Next, the electronic structures of MH2 monolayers are computed based on the HSE06 functional, which can give reasonably accurate band gap for semiconductors.37 The electronic band structures and projected density of states (DOS) for CoH2 and ScH2 monolayers are shown in Figure 2. In particular, CoH2 monolayer is intrinsic half-metal. As shown in Figure 2a, the spin gap for the semiconducting channel is 3.48 eV, wide enough to block the thermally induced spin-flip transition, allowing the CoH2 monolayer to serve as an ideal spintronic material. Note that for the spin-down band structures of ScH2 monolayer, only the lowest conduction band crosses the Fermi level (Figure 2c). Slight hole doping of -0.5 e can lower the Fermi level,


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thereby inducing the half-metallicity (Figure S3). The novel magnetic and electronic properties for CoH2 and ScH2 monolayers can be understood from the crystal field theory. In an octahedral coordination field (Figure S4), 3d orbitals are split into a three-fold degenerate band t2g ( , , ) and a two-fold degenerate band eg ( , ). The projected electronic density of states clearly shows that contributions near the Fermi level are mainly from 3d orbitals of transition-metal atoms, as shown in Figure 2b and 2d. This means that the d electronic states of CoH2 and ScH2 monolayer around the Fermi level are itinerant, which usually leads to ferromagnetism.38 For TiH2, VH2, CrH2, FeH2, and NiH2 monolayers, our computed band structures indicate that all of them are antiferromagnetic and semiconducting, with their band gaps ranging from 0.09 eV to 2.43 eV (Figure S5). It is important to understand the temperature effect on the magnetism before implementing CoH2 and ScH2 monolayers into practical spintronic devices. Monte Carlo simulations on the basis of 2D Heisenberg Hamiltonian model have been performed to examine the spin dynamics (see Computational methods for details). Here, the spin Hamiltonian is given by = − ∑ − ∑ − , where and are the nearest and next-nearest magnetic exchange interaction parameters (Figure S6), is the spin vector of each atom, is the anisotropy energy parameter, and is the z component of spin vector. The computed and for 2D CoH2 monolayer are 187.4 and -36.4 , respectively. The values for ScH2 monolayer are 17.8 and 22.7 , respectively. Anisotropy energy parameter is calculated to be -1.77 and 81.66 for CoH2 and ScH2 monolayers. Supercells of 100 × 100 × 1 grids are adopted for both CoH2 and ScH2 monolayers. The magnetic susceptibility and magnetic moment as a function of temperature are shown in Figure 3. The Curie temperatures can be accurately determined by locating the peak of magnetic



susceptibilities. CoH2 monolayer has an estimated Curie temperature of 339 K (Figure 3a), notably above the room temperature (~300 K). The Curie temperature of CoH2 monolayer is 160 K (Figure 3b), still much higher than that (20 K) recently reported for 2D Cr2Ge2Te612, or CrI3 (45 K)13 and the boiling point of liquid nitrogen (77 K). Overall, CoH2 and ScH2 monolayers are apparently outstanding candidates for spintronic materials.

Figure 3. The magnetic susceptibility (blue) and magnetic moment (red) versus the temperature for CoH2 (a) and ScH2 (b) monolayers, obtained from Monte Carlo simulations on the basis of the 2D Heisenberg Hamiltonian model. Curie temperatures for both systems are marked with vertical dashed line.


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Since MH2 monolayers have novel magnetism and promising spintronic applications, it is natural to further probe whether they have excellent stabilities as well. To address this issue, we first compute the formation energies, defined as Eform = (E2D – EM – EH2)/3, where E2D is the energy of MH2 monolayer, EM is the energy of transition metal atom, and EH2 is the energy of H2 molecule. Calculated formation energies range from -5.30 eV/atom to -2.71 eV/atom (Table S2). In particular, absolute value of the formation energy for ScH2 monolayer is larger than that of ScH3 nanowire,32 suggesting its high stability. Note that MH2 monolayers have atomic structures similar to that of T-MoS2. The latter is energetically less favorable to H-MoS2.39 This fact urged us to compare relative energies between T and H phases of MH2 monolayers. As shown in Table S3, the T phase is always energetically more favorable than the H phase for MH2 monolayers. Moreover, we have performed comprehensive structure searches using the particle swarm optimization scheme algorithm as implemented in CALYPSO code,40-42 to further confirm that the predicted MH2 monolayers are indeed the global minima, giving more credence on the predicted structures. We have also conducted phonon dispersion calculations and ab initio molecular dynamics (AIMD) simulations with PBE functional43 to examine dynamic and thermal stabilities of the MH2 monolayers. To approximately describe the strongly-correlated interactions we adopt the Dudarev implementation44 with an on-site Coulomb interaction Ueff = U−J for the 3d orbitals (see Figure S7 and computational methods). As displayed in Figure 4 and Figure S8, the absence of imaginary frequencies in the phonon dispersion indicates dynamic stabilities of the MH2 monolayers. AIMD simulations (Figure 4 and Figure S9) show that their atomic structures are robust after 10 ps at 300 K, suggesting thermal stabilities at ambient condition. Even after heating up to 500 K for 10 ps, most MH2 monolayers are still intact except for NiH2 monolayer.



Note that transition metal series from Sc to Ni are fully considered. However, the MnH2 monolayer is unstable according to our phonon dispersion calculations (see Figure S10), and thus it is not discussed hereafter.

Figure 4. Phonon spectra and ab initio molecular dynamics snapshots for CoH2 (a, b) and ScH2 (c, d) monolayers. To inspire the probably subsequent experimental fabrications,45 we theoretically present practical synthesis approaches for CoH2 and ScH2 monolayers. It is suggested that CoH2 monolayer be fabricated on Cu(111) surface by using chemical vapor deposition, which has been extensively used for exploring many other 2D materials.46 Then, CoH2 monolayer can be easily transferred to BN substrate (Figure 5a) since the exfoliation energy is rather small (1.98 J/m2), which is on the same order of magnitude as that of graphite.47 For ScH2 monolayer, it can be exfoliated from its 3D bulk counterpart, like for MXenes,48 followed by a transfer to MoS2


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substrate for utilization (Figure 5c). The Cu(111) surface, BN substrates MoS2 substrates are chosen here since the mismatch can be very small (

Transition-Metal Dihydride Monolayers: A New Family of Two

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