Applied Materials Today
Robust two-dimensional topological insulators in derivatives of group-VA oxides with large band gap: Tunable quantum spin Hall states
Graphical abstract
Introduction
In view of the merits of ultrathin, flexibility, transparency and high mobility, two-dimensional (2D) materials are expected to be candidates to satisfy the demands for designing advanced semiconductors in the next generation optoelectronic devices developments [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Recently, a novel type of 2D crystals, group-VA layered materials (phosphorene, arsenene, antimonene, and bismuthene), has drawn increasing research enthusiasm because of excellent properties in various aspects [12], [13], [14], [15], [16], [17], [18], [19]. The semiconducting characteristic of group-VA 2D materials as a precondition makes them as potential candidates for high performance electronics and optoelectronics devices, which distinguishes them from other metallic materials such as group-IIIA (graphene, etc.) and -IVA nanosheets (borophene, etc.). As the first to be investigated in group-VA 2D materials, phosphorene emerged the direct and sizable band gap from 0.3 eV to 2.0 eV and ultrahigh hole mobility exceeding 1 × 104 cm2 V−1 s−1 [20], [21], which render it realizable as suitable applications for field-effect transistor (FET) [22]. However, synthesizing phosphorene monolayer directly is still a giant challenge due to its chemical activity. To solve the stability problems, as the cousins of phosphorene, the semiconducting group-VA nanosheets (arsenene, antimonene, and bismuthene) attract intense research attention. Zhang et al. proposed the arsenene and antimonene firstly in 2015 [23], and the follow-up studies revealed many interesting properties for example strain-modified inversion of conduction bands and thermoelectric response [24], [25], [26]. Moreover, the feasibility of fabricating has been proved by mechanical exfoliation, liquid exfoliation, plasma-assisted process, and vapor deposition techniques [27], [28], [29], [30], [31]. With experimentally proven high stability and ideal band gap as predicted, group-VA 2D materials are expected to be promising and competitive candidates for electronic and optoelectronic applications [32], [33], [34], [35].
Apart from the above-mentioned advantages, the strong spin–orbital coupling (SOC) effect is another virtue of group-VA 2D materials, especially for antimonene and bismuthene [36], [37], [38], [39], [40], [41], [42]. Under applied stress and electric field, or surface functionalization, it is better to exhibit the topologically nontrivial characteristics of group-VA 2D materials. Moreover, the transition from semiconducting to topologically nontrivial state for the group-VA 2D materials is induced by sharing their double electron group. According to our previous researches [43], by oxide atoms decorated, antimonene oxide SbO is a 2D topological insulator (TI) with a band gap of 177 meV. Nevertheless, its nontrivial energy gap is not enough to meet the demands well of promising applications such as spintronics. Therefore, we attempt to the decorate the surface of group-VA monolayers with different group-VIA atoms (i.e. O, S, Se, Te atom) by two-side functionalization. Through this method, we can regulate the structural parameter effectively for instance the heights of wrinkle, and then further modify the electronic properties to improve topologically nontrivial characteristics. Besides, functionalization by group-VIA atoms can enhance the systems’ stability considerably, contributing to achieving tunable quantum spin-Hall (QSH) states and large band gap.
In this work, we provide a systematical investigation on the electronic and topological properties of a novel series 2D compound, namely VA2OX (VA = Bi, Sb, As, P; X = S, Se, Te), based on first-principles calculations. Through the nontrivial Z2-type topological invariants of VA2OX, we confirm that they possess topologically nontrivial characteristics. The 2D TIs VA2OX demonstrate large and tunable nontrivial energy gaps, from 0 to 592 meV, indicating that the tunable QSH effect can be realized in these systems at room temperature. Moreover, as an example of VA2OX systems, the band gap of Bi2OS 2D crystal almost remains unchanged when tensile strain not larger than 8%. Notably, Bi2OS with a large range of strain (±10%) is in inversion-asymmetric topological insulator phases, which indicates the robustness of nontrivial topology against mechanical deformation and makes it competent for realizing new topological phenomena. We believe that other VA2OX crystals also could hold such merits combining with the similarity of structural and electronic properties. The above results are expected to promote further experimental investigation for fundamental exploration and practical application, which will significantly broaden the scientific and technological impact of the QSH effect.
Section snippets
Computational details
The structural optimizations and electronic structure calculations are performed based on density functional theory as implemented in VASP code [44]. Exchange correlation energies are considered by the generalized gradient approximation (GGA) using the Perdew–Burke–Ernzerh (PBE) functional [45]. The wave functions are constructed using a projected augmented wave approach with plane wave cutoff energy of 500 eV. The convergence threshold was set as 10−4 eV in energy and 10−3 eV/Å in force. For all
Results and discussion
The 2D VAene materials, which atoms generate sp3 hybridization to form three bonds with adjacent atoms remaining the nonbonding lone pair electrons and forming a hexagonal network. In Fig. 1a, according to the Lewis structure, the lone pair electrons of VA atom can be donated to the O atom forming a dative bond when connected with O atoms [43]. Consequently, both VA and O atoms fulfill the octet rules and stable VAO can be expected, which is shown in Fig. 1b. The briefly summaries of the
Conclusion
In summary, we reveal that a novel series compounds VA2OX are nontrivial 2D TIs by investigating the electronic and topological properties with first-principles calculations. Through the nontrivial Z2-type topological invariants of VA2OX, we are able to confirm their topologically nontrivial characteristics. The systems of VA2OX reveal large nontrivial energy gaps, and a tunable band gaps from 0 to 592 meV are available with different VA and X atoms, indicating that the tunable QSH effect can be
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (Grant No. 61704082) and Natural Science Foundation of Jiangsu Province (BK20170851, BK20180071). We also acknowledge the Computer Network Information Center (Supercomputing center) of the Chinese Academy of Sciences (CAS) for allocation of computing resources.
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