Oxidation-Induced Topological Phase Transition in Monolayer 1T

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials 2

Oxidation-Induced Topological Phase Transition in Monolayer 1T’-WTe Jiali Yang, Yuanjun Jin, Wangping Xu, Baobing Zheng, Rui Wang, and Hu Xu

J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b01999 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018

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The Journal of Physical Chemistry Letters

Oxidation-induced topological phase transition in monolayer 1T’-WTe2 Jiali Yang1, 2, Yuanjun Jin2, Wangping Xu2, Baobing Zheng2, 3, 4,*, Rui Wang1, 2, *

, Hu Xu2, *

1

Institute for Structure and Function & Department of Physics, Chongqing University, Chongqing 400044, China. 2 Department of Physics & Institute for Quantum Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, China. 3 College of Physics and Optoelectronic Technology, Nonlinear Research Institute, Baoji University of Arts and Sciences, Baoji, 721016, China. 4 School of Physics and Technology, Wuhan University, Wuhan, 430072, China.

Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected]

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ABSTRACT Monolayer (ML) tungsten ditelluride (WTe2) is a well-known quantum spin Hall (QSH) insulator with topologically protected gapless edge states, thus promising dissipationless electronic devices. However, experimental findings exhibit the fast oxidation of ML WTe2 in ambient conditions. To reveal the changes of topological properties of WTe2 arising from oxidation, we systematically study the surface oxidation reaction of ML 1T’-WTe2 using first-principles calculations. The calculated results indicate that the fast oxidation of WTe2 originates from the existence of H2O in air, which significantly promotes the oxidation of ML 1T’-WTe2. More importantly, this low-coverage oxidized WTe2 loses its topological features and is changed into a trivial insulator. Furthermore, we propose a fully oxidized ML WTe2 that can still possess the QSH insulator state. The topological phase transition induced by oxidation provides an exotic insight into understanding the topological features of layered dichalcogenide materials.

Table of Contents (TOC)

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transition-metal

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Since the experimental discovery of graphene, many attempts1-6 have been made to investigate two-dimensional (2D) materials. Layered transition-metal dichalcogenide (TMD) materials,7-9 a class of typical 2D materials, have attracted extensive attention because of their unique sandwich structures and unexpected properties. In particular, their intriguing electronic properties with potential applications in practical devices have been intensively reported in recent years.10-15 The typical bulk phases of TMDs are referred to as 2H, 1T, and 1T’. Most TMD materials crystalize in a semiconducting 2H structure.16 Contrary to common 2H TMDs, the most energetically stable structure of bulk WTe2 is the 1T’ phase,16, 17 which can be considered a distorted 1T phase. The intriguing physical properties of 1T’-WTe2 have prompted researchers to focus on this material extensively, such as in studies of its topological properties,18-22 superconductivity,23 extraordinary magnetoresistance effect,24, 25 and thermoelectric properties,26 etc. Bulk WTe2 is a type-II Weyl semimetal with topologically nontrivial Fermi arcs that connect the electron and the hole pockets.18 Theoretical and experimental studies demonstrate that ML WTe2 is a QSH insulator27, 28 that has a sizable bulk band gap of 55 meV and conductive edge states. The topologically protected edge states make the 1T’-WTe2 a promising material for fabricating spintronic devices due to its low dissipation. However, few-layer 1T’-WTe2 films are relatively unstable under ambient conditions because of their large exposure area.29 The surface of WTe2 film is severely corroded with time when it is exposed to ambient air conditions, especially for single-layer WTe2.30-32 The fast degradation of the WTe2 film in air stems from the surface oxidation reaction only on top of the degraded surface.30 Once the surface layer of the WTe2 film is oxidized, the oxidation products effectively protect the inner layer of WTe2 film from further atmospheric 3



corrosion, resulting in the self-limiting and surface saturating behavior of the WTe2 film. Fast surface oxidation often has detrimental effects on the electronic properties of the ML 1T’-WTe2. Therefore, a crucial issue is how to obtain its predicted electronic phases when we fabricate the corresponding nanoscale devices in a single layer. Thus, the role of surface oxidation on the topological properties of ML 1T’-WTe2 must be further elaborated. In this work, we focus on the influence of surface oxidation on the topological properties of ML 1T’-WTe2. Low-coverage surface oxidation is studied according to the reaction pathway calculations. Oxidation together with the H2O molecule is taken into account in light of the existence of water in air. We essentially elucidate the changes in the electronic properties of oxidized ML 1T’-WTe2 during the surface reaction, including the band structure, density of states, and differential charger density. In addition, to study the topological properties of ML 1T’-WTe2 in a fully oxidized condition, we propose an energetically favorable rectangular structure of 2D WTe2O. The calculated Z2 topological invariant and nontrivial edge states suggest that this 2D WTe2O can continue to possess the QSH insulator states. We performed first-principles calculations as implemented in the Vienna ab initio simulation package (VASP).33, 34 The exchange–correlation energy was described by the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof functional.35 The Kohn-Sham equations were solved self-consistently within the projector augmented wave method.36, 37 The plane wave basis set was truncated with a kinetic energy of 500 eV. We considered van der Waals (vdW) interactions by the DFT-D3 method.38, 39 Spin polarization was included in the calculations. The 1T’-WTe2 in the bulk phase with the optimized lattice constant of a = 6.313 Å and b = 3.490 Å were employed to construct the ML 1T’-WTe2. To avoid interlayer interactions, two neighboring slabs were separated by a perpendicular 4


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vacuum of 18 Å. All the structures were fully relaxed by applying the conjugate gradient method until the total energy and Hellmann-Feynman force convergence values were less than 10-5 eV and 0.02 eV/Å, respectively. The 7×13×1 and 2×3×1 Monkhorst-Pack k-meshes were used for structural relaxation and calculations of reaction, respectively. Additionally, we used Bader charge analysis40 to calculate the amount of charge transfer. The climbing image nudged elastic band (CI-NEB) method41 was employed to study reaction pathways and energies for the surface oxidation of ML WTe2. Furthermore, ab initio molecular dynamics simulations were performed to study the dynamic behavior of O2 dissociation facilitated by an H2O molecule. The top and side views of the ML 1T’-WTe2 with a p(1×1) cell are shown in Figs. 1(a) and 1(b), respectively. W atoms are sandwiched between two Te layers along the c axis, whose Te atoms are asymmetric due to the structural distortion from 1T phase to 1T’ phase. Each W atom is surrounded by six Te atoms, and the nearest-neighboring three W atoms and one Te atom form a triangular pyramidal building block. The interaction between the WTe2 and the adsorbed molecule can be quantitatively described as the adsorption energy (Ead), which is defined as Ead = Eadtot − EWTe2 − Eadmolecule ,

(1)

where Eadtot is the total energy of the supercell with the adsorbed molecule, EWTe2 is the energy of the pure (3×4) supercell WTe2, and Eadmolecule is the

isolated molecule energy. To understand the effects of oxidation on topological properties and for the sake of simplicity, we start with a discussion of the low coverage oxidation of ML 1T’-WTe2. Several possible dissociation configurations (see details in Fig. S1 of the Supporting Information (SI)) of O2 on ML 1T’-WTe2 are considered, and the corresponding adsorption energies in the dissociation 5



form are also listed. We find that the energetically favorable configuration [Fig. S1(f)], in which two O atoms are completely inserted into the structure of ML WTe2 and each O atom coordinates with three adjacent atoms, has the lowest adsorption energy of −1.982 eV per O2. This low adsorption energy implies that 1T’-WTe2 is in favor of the surface oxidation, which supports the fast degradation on the top of the 1T’-WTe2 film observed in the experiment.31 To elucidate the oxidation of ML 1T’-WTe2, we chose the energetically favorable configuration to simulate surface oxidation using the CI-NEB method. The oxidative decomposition process on the ML WTe2 surface, together with the reaction energy barrier, is shown in Fig. 2. During the oxidation reaction, the O2 molecule initially dissociates into two individual O atoms by climbing over a reaction energy barrier of 0.66 eV. The relatively high energy barrier indicates that the decomposition of the O2 molecule on the ML 1T’-WTe2 surface may take place at slow rate. Following the above step, the two separated O atoms draw closer to the WTe2 substrate and bond to the W and Te atoms by overcoming potential barriers of 0.72 eV and 0.50 eV, which are marked in the potential energy profile (see Fig. 2). Consequently, the oxidized ML 1T’-WTe2 reaches the most energetically favorable adsorption structure. The relatively high energy barrier to breaking the O-O covalent bond in the O2 molecule is unfavorable for the oxidative reaction of ML 1T’-WTe2, implying that the oxidative process of the 1T’-WTe2 surface may be slow if only O2 is present. Indeed, because H2O and O2 coexist in ambient conditions, H2O and O2 molecules are simultaneously adsorbed when we expose the WTe2 film in ambient conditions. As a result, the surface reaction of ML WTe2 film in the coexistence of H2O and O2 molecules is much closer to actual condition. Therefore, we will study the oxidation of ML 1T’-WTe2 combined with H2O. 6


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The calculated results show that the H2O molecule can easily move close to O2 by climbing over a very low energy barrier of 0.12 eV [see Fig. 3(a)]. More importantly, once H2O reaches the desirable adsorption site, O2 immediately dissociates into two isolated O atoms by overcoming a lower reaction barrier of only 0.3 eV [as shown in Fig. 3(b)]. Then, two dissociated O atoms move to Te atoms, forming Te-O bonds. Compared with the results in the absence of H2O, the presence of H2O effectively lowers the barrier of O2 dissociation and can significantly accelerate the oxidation of 1T’-WTe2. To further confirm the O2 dissociation promoted by H2O on the ML 1T’-WTe2 surface, we also perform molecular dynamics simulations to study the oxidation of ML 1T’-WTe2 together with H2O (see Fig. S2 in SI). Although the H2O molecule always retains its molecular form, the O2 molecule dissociates into oxygen atoms rapidly in view of the sharp increase in the distance between two oxygen at 8 ps. Combined with the reaction pathway obtained by using the CI-NEB approach, we can further conclude that the presence of H2O accelerates the dissociation of O2 molecules and may account for the fast degradation of the WTe2 flake in air. To gain deeper insight into the influence of oxidation on the ML 1T’-WTe2, we analyzed the electronic properties of the oxidized WTe2 layer. The spin-polarized calculation suggests that the spin state of the adsorbed O2 molecule completely vanishes after this oxidation reaction, and thus the oxidized WTe2 layer is non-magnetic. The calculated band structure, density of states, and differential charge density are illustrated in Fig. 4. The valence band maximum (VBM) is located along the A-X direction, while the conduction band minimum (CBM) is located at Y point, suggesting that the low-coverage oxidized ML 1T’-WTe2 is an indirect band gap semiconductor. It is well known that the pristine ML 1T’-WTe2 is a QSH insulator with a band gap of 55 meV.28 However, all the crystalline symmetries are destroyed after partially oxidation. The space group of the low-coverage 7



oxidized 1T’-WTe2 is C1; that is, there is no any symmetry which can protect the topological features. Therefore, the oxidized ML 1T’-WTe2, whose band structure is no longer sensitive to spin-orbit coupling (SOC), is a trivial insulator with a band gap of 90 meV. The major contribution of the CBM and the VBM arises from the d orbits of W, p orbits of Te, and p orbits of O, as shown in Fig. 4(a). An inspection of DOS for Te and O further demonstrates that the formation of a Te-O bond is more likely than the formation of a W-O bond in oxidized ML 1T’-WTe2, which is consistent with experimental findings that the bond energy of the O-Te bond is much stronger than those of O-W bonds on degraded WTe2.31 Charge transfer between the dissociated O atoms and the ML 1T’-WTe2 can provide a fundamental understanding of atomic bonding during oxidation. Thus, we calculated the differential charge density of the oxidized WTe2 layer. As shown in Fig. 4(b), because of the high electronegativity of O, the existence of dense charge acquisition around the O atoms is clearly visible, indicating that O gains electrons from the ML 1T’-WTe2 during oxidation. Meanwhile, the charge depletion for the oxidized WTe2 layer is found near the O adsorption sites. The Bader charge analysis shows that the O atoms capture almost one electron per O atom, and these electrons mainly originate from neighboring Te and W atoms. Surprisingly, the Te atoms tend to lose electrons more easily than the W atoms despite the high electronegativity of Te atoms. The total amount of electron transfer from the adjacent Te and W atoms is 1.51e and 0.75e, respectively. During the above discussions, we only considered one O2 molecule on a

p(3×4) supercell to investigate the oxidation of ML 1T’-WTe2. This low-coverage oxidation significantly influences the electronic properties of ML 1T’-WTe2 and changes the topologically nontrivial ML 1T’-WTe2 into an indirect band gap semiconductor; that is, it undergoes a phase transition when oxidation occurs under low oxygen coverage. In practice, ML WTe2 is 8


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usually found in oxygen-rich conditions as it is exposed to air. Under the high oxygen coverage condition, ML 1T’-WTe2 is likely to be oxidized completely and then generates new oxidation products, for example, ML WTe2O. To investigate the fully oxidative WTe2, we predicted a 2D stoichiometric WTe2O with a rectangular lattice, which is the most energetically favorable structure. The crystal structure of the ML WTe2O is presented in Fig. 5(a). Clearly, this ML crystal structure is completely different from that of the pristine ML 1T’-WTe2, implying that oxidation may thoroughly change its structure. To verify dynamical stability, we performed the phonon calculation with the frozen-phonon approach, as implemented in the PHONOPY code42; the corresponding result is shown in Fig. 5(b). There is no imaginary frequency in the Brillouin zone, suggesting that this structure is dynamically stable. As shown in Fig. 6(a), the electronic band structures without SOC show that the bands linearly cross along the X-S direction to form a Dirac cone. The ML WTe2O shares the time-reversal T and point group C2v, which contains inversion I, two-fold rotational symmetry C2 along the z axis, and mirror-reflection symmetries mx and my. Without SOC, all the points along X-S share the unitary mirror symmetry Cs, and thus the Dirac cone is protected by mirror-reflection symmetry. Two crossing bands belong to the opposite mirror eigenvalues ±1, and the effective model of low-energy excitations relative to inverted center X point ( k X ) in the ML WTe2O can be described by v y  D q y2  H (q) = vx q x σ x +  q y − D  σ y , q y  2 

(2)

where σ i are Pauli matrixes, q = k − k X are two components of the

momentum vectors, and the two bands linearly cross at ( , ± ) indicating

the positions of Dirac cone in the X-S. The parameters vx and v y are 9



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anisotropic Fermi velocities along x and y axes, respectively.43,

44

The

detailed proof of Eq. (2) is provided in SI. Once SOC is considered, this Dirac cone is destroyed with a gap of 58 meV due to the strong SOC effect. Considering the inversion symmetry of the ML WTe2O, we calculated the Z2 topological invariant (ν) to reveal the band topology based on the parity eigenvalues of the occupied Bloch states at four time-reversal invariant momenta (TRIM) points. The products of parity eigenvalues at TRIM Γ, X, and Y are +1, whereas the product of parity eigenvalues at TRIM S is -1. Therefore, the Z2 topological invariant is ν =1. This result indicates that our predicted ML WTe2O is a good 2D candidate to realize the QSH states. The other typical feature of QSH nature is the topologically protected gapless edge states that connect the valence and conduction bands. Thus, we employed a Wannier tight-binding Hamiltonian45,

46

as implemented in

WannierTools package47 to obtain the surface states. The edge states projected along the ky direction of the Brillouin zone (BZ) are shown in Fig. 6(b). We can clearly observe that the metallic edge states between the valence and conduction bands linearly cross around X point of the projected one-dimensional BZ. This provides powerful evidence that the QSH state is present when ML 1T’-WTe2 is fully oxidative. Note that ML 1T’-WTe2 undergoes a topological transition from low-coverage oxidation to fully oxidative ML WTe2O. In summary, we systematically investigated the effect of surface oxidation on the topologically electronic properties of ML 1T’-WTe2. The oxidation of ML 1T’-WTe2 can be promoted with the existence of H2O, which accounts for the fast surface degradation of WTe2 film. As expected, the oxidation of the ML 1T’-WTe2 under the low oxygen coverage condition is accompanied by the transition from a QSH insulator to a trivial one. Strikingly, the fully oxidized 2D ML WTe2O presents the QSH insulator 10


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state with topologically nontrivial edge states, suggesting the occurrence of topological phase transition from low-coverage oxidation to fully oxidative ML WTe2O. Our findings are expected to shed light on the changes of the topological properties of ML 1T’-WTe2 under oxidation and improve the understanding of oxidation for WTe2.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] Notes Notes the authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC, Grant Nos. 11674148, 11334004, 91634106, and 11404159), the Guangdong Natural Science Funds for Distinguished Young Scholars (No. 2017B030306008), the Basic Research Program of Science, Technology, and Innovation Commission of Shenzhen Municipality

(Grant

Nos.

JCYJ20160531190054083,

JCYJ20170412154426330), the Fundamental Research Funds for the Central Universities of China (Nos. 106112017CDJXY300005 and 11



cqu2018CDHB1B01), the Natural Science Basic Research plan in Shaanxi Province of China (Grant No. 2018JQ1083), and the Scientific Research Program Funded by Shaanxi Provincial Education Department (Grant No. 17JK0041).

ASSOCIATED CONTENT Supporting Information Dissociation structures of O2 and results of molecular dynamics at 300 K, including Figures S1-S2, and the detailed proof of low-energy effective model.

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Figure 1. Top (a) and side (b) views of ML 1T’-WTe2.

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Figure 2. Potential energy profiles for O2 dissociation on ML 1T’-WTe2. The potential energy barriers (Eb) are 0.66 eV, 0.72 eV and 0.50 eV.

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Figure 3. (a) Diffusion behavior of H2O and O2 on ML 1T’-WTe2. (b) Depressed potential energy profile for O2 dissociation together with H2O on ML 1T’-WTe2.

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Figure 4. (a) Band structure and density of states of dissociated O2 on ML 1T’-WTe2. (b) Differential charge dissociation (∆ρ = ρ(total) − ρ(WTe2) − ρ(2Oi)) adsorption of O2. The isosurface value is set to 0.007 e/bohr3 (violet: electron accumulation, green: electron depletion). The Fermi levels are marked by the blue dashed line.

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Figure 5. (a) Top and side views of ML WTe2O. (b) Phonon spectrum of ML WTe2O.

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Figure 6. (a) Band structure of ML WTe2O without and with SOC. (b) The edge states of ML WTe2O linearly cross at X point.

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Oxidation-Induced Topological Phase Transition in Monolayer 1T

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