Pei-ji Wang

New family of room temperature quantum spin Hall insulators in two-dimensional germanene films

Searching for two-dimensional (2D) group IV films with high structural stability and large-gaps is crucial for the realization of a dissipationless transport edge state using the quantum spin Hall effect (QSHE). Based on first-principles calculations, we predict that 2D germanene decorated with ethynyl-derivatives (GeC2X; X = H, F, Cl, Br, I) can be a topological insulator (TI) with a large band-gap for room-temperature applications. Both GeC2I and GeC2Br films are intrinsic TIs with a gap reaching up to 180 meV over a wide range, while GeC2H, GeC2F, and GeC2Cl transform from trivial to nontrivial phases under tensile strain. This topological characteristic can be confirmed by s–pxy band inversion, topological invariant Z2, and time-reversal symmetry protected helical edge states. Notably, the characteristic properties of edge states, such as the Fermi velocity and edge shape, can be tuned by edge modifications. Furthermore, we demonstrate that the h-BN sheet is an ideal substrate for the experimental realization of GeC2X, maintaining their nontrivial topology. Considering their higher thermo-stability, these GeC2X films may be good QSHE platforms for topological electronic device design and fabrication in spintronics.

Two-dimensional arsenene oxide: A realistic large-gap quantum spin Hall insulator

Searching for two-dimensional (2D) realistic materials that are able to realize room-temperature quantum spin Hall effects is currently a growing field. Here, through ab initio calculations, we identify arsenene oxide, AsO, as an excellent candidate, which demonstrates high stability, flexibility, and tunable spin-orbit coupling gaps. In contrast to known pristine or functionalized arsenene, the maximum nontrivial bandgap of AsO reaches 89u2009meV and can be further enhanced to 130u2009meV under biaxial strain. By sandwiching 2D AsO between boron nitride sheets, we propose a quantum well in which the band topology of AsO is preserved with a sizeable bandgap. Considering that AsO having fully oxidized surfaces are naturally stable against surface oxidization and degradation, this functionality provides a viable strategy for designing topological quantum devices operating at room temperature.

Robust room-temperature inversion-asymmetry topological transitions in functionalized HgSe monolayer

The new quantum materials known as inversion-asymmetry topological insulators (IATIs) have recently drawn intense attention because these structures possess distinct properties from those of inversion-symmetry insulators. However, the reported IATIs are rare in monolayer structures thus far. On the basis of first-principles calculations, we predict that the two-dimensional I-decorated HgSe monolayer (HgSeI2) is a new IATI with a large gap of 59.1 meV, as well as large Rashba spin splitting (RSS) of 37.2 meV, which derives from the polarity of atoms. The topological characteristic is confirmed by the s–pxy band inversion, topological invariant Z2, and time-reversal symmetry-protected helical edge states. The topologically nontrivial band gap and RSS of HgSeI2 can be effectively modulated with a wide range of strain (−12 to 6%) and external electric field (−10 to 10 V nm−1), and the maximum band gap can be improved to 156 meV under compressive strain, while other F-, Cl-, and Br-decorated cases can transform from trivial to nontrivial TIs with respect to appropriate strain. These novel IATIs with controlled band gap and RSS provide excellent platforms to realize topological spintronic devices based on inversion-asymmetry films.

The electronic properties of the stanene/MoS2 heterostructure under strain

The effect of a MoS2 substrate on the structural and electronic properties of stanene were systematically investigated by first-principles calculations. The Brillouin zone of isolated stanene has a Dirac cone at the K point. MoS2 helps to open an energy gap at the K point, whereas contributes no additional transport channels near the Fermi level. Our results suggest that the carrier mobility remains large, which makes the stanene/MoS2 heterostructure a competitive material for electronic applications. Subsequently, strain engineering study by changing the interlayer spacing between stanene and MoS2 layer and changing lattice constants indicates that the energy gap at K point can be effectively tuned to meet the demands of experiments and device design in nanoelectronics. Moreover, a large enough strain leads to a metal–semiconductor phase transition to make the intrinsic semiconductor turn into self-doping phase. Our study indicates that MoS2 is a good substrate to promote the development of Sn-based nanoelectronics.

Prediction of tunable quantum spin Hall effect in methyl-functionalized tin film

The quantum spin Hall (QSH) effect may promote revolutionary device development due to dissipationless propagation of spin currents. The bottleneck preventing applications from the QSH effect, however, is a lack of large nontrivial bulk gap and highly stable two-dimensional (2D) films. In this work, we design a novel 2D honeycomb lattice, namely a SnCH3 monolayer, using comprehensive density-functional theory (DFT) computations. The structural stability is confirmed using a phonon spectrum and molecular dynamics simulations. Interestingly, its nontrivial bulk gap can reach up to 0.34 eV, which is further tunable via external strain. The nontrivial topology stems mainly from band inversion between the s–px,y orbitals, demonstrated by the nonzero topological invariant Z2 and a single pair of gapless helical edge states located in the bulk gap. The effects of a growth substrate on the QSH effect are also checked by hydrogen bonding on a single side in stanene, showing the robustness of the observed QSH phase. Considering its compatibility with the current electronics industry, these findings present an efficient platform to enrich topological phenomena and expand potential applications of 2D stanene at high temperature.

A planar C3Ca2 film: a novel 2p Dirac half metal

The exploration of Dirac materials is a great challenge in condensed matter physics and material chemistry. In this paper we present a novel 2D magnetic graphene-like Dirac material with a Honeycomb–Kagome (HK) lattice, named as C3Ca2. The ground state of C3Ca2 is a half-metal with a 100% spin polarized Dirac cone locating exactly at the Fermi level in the metallic spin channel, and has a large band gap in the insulating spin channel. In particular, C3Ca2 has 2p magnetism with the Dirac cones mainly contributed by pxy orbitals of C atoms, instead of 3d magnetism or pz dominated Dirac cones in other HK or Kagome materials, and it is robust against spin–orbit coupling and biaxial strains. The mechanism of magnetism could be understood by double exchange between carbon anions, using Ca2+ cations as bridges. These outstanding properties of C3Ca2 indicate it to be a promising 2D material for applications in spintronics.

The effects of biaxial strain and electric field on the electronic properties in stanene

The effects of biaxial strain (?2%?<???<?6%) and electric field (E-field) on structural and electronic properties in stanene are systematically investigated by first-principles calculations. We find that, with the increase of biaxial strain, the conduction bands at the high symmetric ? point in the first Brillouin zone shift towards the Fermi level in stanene. In addition, the biaxial strain also affects the position of Dirac cone. The E-field changes the band dispersions near the ? with a small band gap opening at the K point. Remarkably, the band gap opening in stanene can be effectively modulated by the external E-field and strain. These results present a flexible method toward modulating the electronic and band properties of stanene and shed light on its experimental applications.

First-principles prediction of a giant-gap quantum spin Hall insulator in Pb thin film

The quantum spin Hall (QSH) effect is promising for achieving dissipationless transport devices due to the robust gapless states inside the insulating bulk gap. However, QSH insulators currently suffer from requiring extremely high vacuums or low temperatures. Here, using first-principles calculations, we predict cyanogen-decorated plumbene (PbCN) to be a new QSH phase, with a large gap of 0.92 eV, that is robust and tunable under external strain. The band topology mainly stems from s-pxy band inversion related to the lattice symmetry, while the strong spin-orbit coupling (SOC) of the Pb atoms only opens a large gap. When halogen atoms are incorporated into PbCN, the resulting inversion-asymmetric PbFx(CN)1-x can host the QSH effect, accompanied by the presence of a sizable Rashba spin splitting at the top of the valence band. Furthermore, the Te(111)-terminated BaTe surface is proposed to be an ideal substrate for experimental realization of these monolayers, without destroying their nontrivial topology. These findings provide an ideal platform to enrich topological quantum phenomena and expand the potential applications in high-temperature spintronics.

Tunable Electronic and Topological Properties of Germanene by Functional Group Modification

Electronic and topological properties of two-dimensional germanene modified by functional group X (X = H, F, OH, CH3) at full coverage are studied with first-principles calculation. Without considering the effect of spin-orbit coupling (SOC), all functionalized configurations become semiconductors, removing the Dirac cone at K point in pristine germanene. We also find that their band gaps can be especially well tuned by an external strain. When the SOC is switched on, GeX (X = H, CH3) is a normal insulator and strain leads to a phase transition to a topological insulator (TI) phase. However, GeX (X = F, OH) becomes a TI with a large gap of 0.19 eV for X = F and 0.24 eV for X = OH, even without external strains. More interestingly, when all these functionalized monolayers form a bilayer structure, semiconductor-metal states are observed. All these results suggest a possible route of modulating the electronic properties of germanene and promote applications in nanoelectronics.

Quantum spin Hall phase transitions in two-dimensional SbBi alloy films

Bismuth (Bi) and antimony (Sb) have similar electronic structures but distinct topological properties in their two-dimensional (2D) films, due to their different spin–orbit coupling strengths. An Sb/Bi hetero-junction is a good candidate for the formation of normal band insulator (NI)–topological insulator (TI) boundary states where dissipationless spin currents exist. However, the topological properties of 2D Bi/Sb alloy films, forming at the boundaries of their hetero-junctions, have not been well studied yet. Here, first-principles calculations are performed to study the geometric and band structures of buckled and puckered SbBi alloy 2D films. The transition point between the TI and NI phases is at x = 5 in buckled BixSb8−x and at x = 3 in puckered BixSb4−x. The topological transition of buckled SbBi can be explained using the well-known band inversion mechanism between p orbits, caused by the variation of either components or lattice parameters. According to the analysis on the SOC strength, we propose an experiential ratio rule for the topological phase transition point of SbBi, by simply comparing the number of Bi and Sb atoms, which will be useful in the designing of topological materials and devices for experiment.