Quantum Spin Hall States in Stanene/Ge(111)

Epitaxy of Antimonene Thin Films with Screw Dislocations for the Application of Saturable Absorbers

The exotic optoelectronic properties of antimonene, including strain-induced tunable bandgaps, broad nonlinear refractive response, etc., have evoked profound upsurges for decades. As the screw dislocations break the crystal symmetry and modify interlayer coupling, it is highly desirable to investigate the optical prospects of antimonene with screw dislocations. Herein, controllable epitaxy of spiral β-antimonene is achieved on Fe3GaTe2 substrates. By fine-tuning growth temperatures, the evolutions of spiral β-antimonene with non-centrosymmetric stacking are investigated via scanning tunneling microscopy. The effects of interfacial strain and dislocation motion during screw-dislocation-driven growth are also studied. Additionally, a modulation depth of 40.8% and mode locking at 1558 nm with a pulse width of 290 fs are observed in Er-doped pulsed fiber lasers generated with spiral Sb-based saturable absorbers, revealing superior performance that far outstrips reported Sb-based saturable absorbers to date. Our work sheds light on the preparation of Sb films with screw dislocations and demonstrates a promising approach toward fabricating ultrafast optical devices.

Robust Tunable Large-Gap Quantum Spin Hall States in Monolayer Cu2S on Insulating Substrates

Quantum spin Hall (QSH) insulators with large band gaps and dissipationless edge states are of both technological and scientific interest. Although numerous two-dimensional (2D) systems have been predicted to host the QSH phase, very few of them harbor large band gaps and retain their nontrivial band topology when they are deposited on substrates. Here, based on a first-principles analysis with hybrid functional calculations, we investigated the electronic and topological properties of inversion-asymmetric monolayer copper sulfide (Cu₂S). Interestingly, we found that monolayer Cu₂S possesses an intrinsic QSH phase, Rashba spin splitting, and a large band gap of 220 meV that is suitable for room-temperature applications. Most importantly, we constructed heterostructures of a Cu₂S film on PtTe₂, h-BN, and Cu(111) substrates and found that the topological properties remain preserved upon an interface with these substrates. Our findings suggest Cu₂S as a possible platform to realize inversion-asymmetric QSH insulators with potential applications in low-dissipation electronic devices.

Discovery of asymmetric NaXBi (X= Sn /Pb) monolayers with non-trivial topological properties

Two-dimensional (2D) Bi-based films have attracted intensive attention recently. However, materials with spatial asymmetry are rarely reported, impeding their practical application. In the present work, based on density functional theory (DFT) calculations, we propose a new type of 2D asymmetric NaXBi (X = Sn and Pb) monolayer, which can realize the coexistence of a topological phase and the Rashba effect. The dynamical and thermal stability are confirmed by the phonon spectra and ab initio DFT molecular dynamic simulations. Analysis of the band structures reveals that NaPbBi is an intrinsic 2D topological insulator with a gap as large as 0.35 eV, far beyond room temperature. The non-trivial topology, caused by p xy - p z band inversion, is confirmed by the Z 2 topological index and helical edge states. Remarkably, unlike Bi(111) or BiX (X = H, F, Cl) monolayers, the inversion-symmetry breaking in NaPbBi gives rise to a sizable Rashba splitting energy of 64 meV, which is tunable under external strains (-1 to 7%). Also, an effective tight-binding (TB) model is constructed to understand the origin of the non-trivial topology of NaPbBi. Our work opens a new avenue to designing a feasible 2D asymmetric material platform for application in spintronics.

Functionalization of group-14 two-dimensional materials

The great success of graphene has boosted intensive search for other single-layer thick materials, mainly composed of group-14 atoms arranged in a honeycomb lattice. This new class of two-dimensional (2D) crystals, known as 2D-Xenes, has become an emerging field of intensive research due to their remarkable electronic properties and the promise for a future generation of nanoelectronics. In contrast to graphene, Xenes are not completely planar, and feature a low buckled geometry with two sublattices displaced vertically as a result of the interplay between sp² and sp³ orbital hybridization. In spite of the buckling, the outstanding electronic properties of graphene governed by Dirac physics are preserved in Xenes too. The buckled structure also has several advantages over graphene. Together with the spin-orbit (SO) interaction it may lead to the emergence of various experimentally accessible topological phases, like the quantum spin Hall effect. This in turn would lead to designing and building new electronic and spintronic devices, like topological field effect transistors. In this regard an important issue concerns the electron energy gap, which for Xenes naturally exists owing to the buckling and SO interaction. The electronic properties, including the magnitude of the energy gap, can further be tuned and controlled by external means. Xenes can easily be functionalized by substrate, chemical adsorption, defects, charge doping, external electric field, periodic potential, in-plane uniaxial and biaxial stress, and out-of-plane long-range structural deformation, to name a few. This topical review explores structural, electronic and magnetic properties of Xenes and addresses the question of their functionalization in various ways, including external factors acting simultaneously. It also points to future directions to be explored in functionalization of Xenes. The results of experimental and theoretical studies obtained so far have many promising features making the 2D-Xene materials important players in the field of future nanoelectronics and spintronics.

Strain induced band inversion and topological phase transition in methyl-decorated stanene film

The researches for new quantum spin Hall (QSH) insulators with large bulk energy gap are of much significance for their practical applications at room temperature in electronic devices with low-energy consumption. By means of first-principles calculations, we proposed that methyl-decorated stanene (SnCH₃) film can be tuned into QSH insulator under critical tensile strain of 6%. The nonzero topological invariant and helical edge states further confirm the nontrivial nature in stretched SnCH₃ film. The topological phase transition originates from the s-p_xy type band inversion at the Γ point with the strain increased. The spin-orbital coupling (SOC) induces a large band gap of ~0.24 eV, indicating that SnCH₃ film under strain is a quite promising material to achieve QSH effect. The proper substrate, h-BN, finally is presented to support the SnCH₃ film with nontrivial topology preserved.

Deposition of topological silicene, germanene and stanene on graphene-covered SiC substrates

Growth of X-enes, such as silicene, germanene and stanene, requires passivated substrates to ensure the survival of their exotic properties. Using first-principles methods, we study as-grown graphene on polar SiC surfaces as suitable substrates. Trilayer combinations with coincidence lattices with large hexagonal unit cells allow for strain-free group-IV monolayers. In contrast to the Si-terminated SiC surface, van der Waals-bonded honeycomb X-ene/graphene bilayers on top of the C-terminated SiC substrate are stable. Folded band structures show Dirac cones of the overlayers with small gaps of about 0.1 eV in between. The topological invariants of the peeled-off X-ene/graphene bilayers indicate the presence of topological character and the existence of a quantum spin Hall phase.

Edge states of hydrogen terminated monolayer materials: silicene, germanene and stanene ribbons

We investigate the energy dispersion of the edge states in zigzag silicene, germanene and stanene nanoribbons with and without hydrogen termination based on a multi-orbital tight-binding model. Since the low buckled structures are crucial for these materials, both the π and σ orbitals have a strong influence on the edge states, different from the case for graphene nanoribbons. The obtained dispersion of helical edge states is nonlinear, similar to that obtained by first-principles calculations. On the other hand, the dispersion derived from the single-orbital tight-binding model is always linear. Therefore, we find that the non-linearity comes from the multi-orbital effects, and accurate results cannot be obtained by the single-orbital model but can be obtained by the multi-orbital tight-binding model. We show that the multi-orbital model is essential for correctly understanding the dispersion of the edge states in tetragen nanoribbons with a low buckled geometry.

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.

Structural and electronic properties of two-dimensional stanene and graphene heterostructure

Structural and electronic properties of two-dimensional stanene and graphene heterostructure (Sn/G) are studied by using first-principles calculations. Various supercell models are constructed in order to reduce the strain induced by the lattice mismatch. The results show that stanene interacts overall weakly with graphene via van der Waals (vdW) interactions. Multiple phases of different crystalline orientation of stanene and graphene could coexist at room temperature. Moreover, interlayer interactions in stanene and graphene heterostructure can induce tunable band gaps at stanene's Dirac point, and weak p-type and n-type doping of stanene and graphene, respectively, generating a small amount of electron transfer from stanene to graphene. Interestingly, for model [Formula: see text] , there emerges a band gap about 34 meV overall the band structure, indicating it shows semiconductor feature.

Ab Initio-Based Bond Order Potential to Investigate Low Thermal Conductivity of Stanene Nanostructures

We introduce a bond order potential (BOP) for stanene based on an ab initio derived training data set. The potential is optimized to accurately describe the energetics, as well as thermal and mechanical properties of a free-standing sheet, and used to study diverse nanostructures of stanene, including tubes and ribbons. As a representative case study, using the potential, we perform molecular dynamics simulations to study stanene's structure and temperature-dependent thermal conductivity. We find that the structure of stanene is highly rippled, far in excess of other 2-D materials (e.g., graphene), owing to its low in-plane stiffness (stanene: ∼ 25 N/m; graphene: ∼ 480 N/m). The extent of stanene's rippling also shows stronger temperature dependence compared to that in graphene. Furthermore, we find that stanene based nanostructures have significantly lower thermal conductivity compared to graphene based structures owing to their softness (i.e., low phonon group velocities) and high anharmonic response. Our newly developed BOP will facilitate the exploration of stanene based low dimensional heterostructures for thermoelectric and thermal management applications.

Spin–orbit coupling effects on electronic structures in stanene nanoribbons

SOC effects open the band gaps of stanene sheets and ZSnNRs, but reduce the band gaps of ASnNRs.

Tuning electronic structures of the stanene monolayer via defects and transition-metal-embedding: spin–orbit coupling

The magnetic moments of a transition metal embedded into single and double vacancies of stanene nanosheets.