Two-dimensional hydrogenated molybdenum and tungsten dinitrides MN2H2 (M = Mo, W) as novel quantum spin hall insulators with high stability

Oxygen functionalized InSe and TlTe two-dimensional materials: transition from tunable bandgap semiconductors to quantum spin Hall insulators

From first-principles calculations, we found that oxygen functionalized InSe and TlTe two-dimensional materials undergo the following changes with the increased concentrations of oxygen coverage, transforming from indirect bandgap semiconductors to direct bandgap semiconductors with tunable bandgap, and finally becoming quantum spin hall insulators. The maximal nontrivial bandgap are 0.121 and 0.169 eV, respectively, which occur at 100% oxygen coverage and are suitable for applications at room temperature. In addition, the topological phases are derived from SOC induced p-p bandgap opening, which can be further determined by Z₂ topological invariants and topologically protected gapless edge states. Significantly, the topological phases can be maintained in excess of 75% oxygen coverage and are robust against external strain, making the quantum spin hall effect easy to achieve experimentally. Thus, the oxygen functionalized InSe and TlTe are fine candidate materials for the design and fabrication of topological devices.

Intrinsic ferromagnetism and valley polarization in hydrogenated group V transition-metal dinitride (MN2H2, M = V/Nb/Ta) nanosheets: insights from first-principles

Due to the extraordinary electronic and magnetic properties, transition-metal dinitrides (TMDNs) and their derivatives are gaining importance in low-dimensional layered materials. In this work, through first-principles calculations, we have comprehensively investigated the structural and electronic properties of hydrogenated group V TMDN nanosheets. We find that surface hydrogenation can well stabilize the H-, T- and M-phase structures of group V TMDNs, for which the formed MN2H2 nanosheets have robust energetic, dynamical and thermal stabilities. Different from pristine MN2 systems, the H-phase system has become the most favorable structure of MN2H2 nanosheets. Intrinsic ferromagnetism is present in these H-MN2H2 nanosheets, which even exhibit bipolar magnetic semiconducting behaviors. More interestingly, large spontaneous valley polarization occurs in the H-MN2H2 nanosheets, and is attributed to the coexistence of remarkable spin-orbit coupling and magnetic exchange interactions according to the k·p model analysis. Among them, the H-NbN2H2 nanosheets are found to be a promising ferrovalley material, whose valley polarization value reaches as large as 0.11 eV and the Curie temperature is up to 225 K. Besides that, versatile electronic properties are obtained in the T- and M-phase structures of the MN2H2 nanosheets, which will be magnetic/nonmagnetic metals/semiconductors depending on the metal species and phase structures. Our study demonstrates that the hydrogenation can bring robust structural stabilities and unconventional electronic properties into the group V TMDN nanosheets, which enable many potential applications in spintronics and valleytronics.

Oxygen-functionalized TlTe buckled honeycomb from first-principles study

A sizable band gap is crucial for the applications of topological insulators at room temperature. By first-principles calculations, we found that oxygen-functionalized TlTe buckled honeycomb, namely TlTeO, possessed quantum spin Hall (QSH) state with a sizable band gap of 0.17 eV, which owns potential applications at the room temperature. The QSH phase of TlTeO arose from the SOC-induced p-p band gap opening. In addition, the QSH phase was further confirmed by the topological invariant Z2 and gapless edge state in the bulk gap. Significantly, the QSH phase is robustly against the external strain and possesses more than 75% oxygen coverage, making the QSH effect of TlTeO easy to be achieved experimentally. Thus, the oxygen-functionalized TlTeO film is a fine candidate material for the topological device design and fabrication.

New topological states in HgTe quantum wells from defect patterning

To explore new methods for the realization of the quantum spin Hall (QSH) effect in two-dimensional (2D) materials, we have constructed a honeycomb geometry (HG) by etching rows of hexagonal holes in HgTe quantum wells (QWs). Theoretical calculations show that multiple Dirac cones can be produced by HG, regardless of whether the band inversion occurs or not. Furthermore, the topological states originating from a narrow HG region in a wide ribbon show strong localization at the physical edges of the ribbon, making them easy to manipulate and exploit. When the band inversion condition for QW states is satisfied, the topological states generated by two different mechanisms may coexist. Our studies pave the way to produce and control multiple QSH states in 2D materials as desired for the design of innovative spintronic materials.

Square transition-metal carbides MC6 (M = Mo, W) as stable two-dimensional Dirac cone materials

We identify the existence of Dirac cones in 2D square transition-metal carbides MC₆ (M = Mo, W) with an ultrahigh Fermi velocity comparable to that of graphene.

Dependence of topological and optical properties on surface-terminated groups in two-dimensional molybdenum dinitride and tungsten dinitride nanosheets

Using ab initio methods, the topological and optical properties of surface-functionalized XN₂ sheets (X = Mo, W) were investigated. Based on first principles calculations and the K·p effective model, the existence of topological nodal-line states in potassium-functionalized XN₂ sheets (K₂MoN₂ and K₂WN₂) is reported. This study shows that a nodal line ring exists near the Fermi level in the absence of spin-orbit coupling (SOC). When SOC is included, the band-crossing points are gapped, giving rise to a new nodal ring along Γ-K. This band-crossing is protected due to the existence of mirror reflection and time-reversal symmetry. These calculations demonstrate the inclusion of electron-hole (e-h) interactions, which were further confirmed through the optical absorption of functionalized MoN₂, revealing the presence of strongly bound excitons below the absorption onset where they depend strongly on the terminated surface groups. Moreover, the surface terminated groups change the energy distribution range of the exciton, which can be used to tune the absorption of infrared (IR) and visible light. Interestingly, F₂MoN₂ has several strongly bound excitons, with the first exciton having a binding energy of 1.35 eV, larger than the corresponding one in the transition metal dichalcogenide MoS₂.

Computational Dissection of Two-Dimensional Rectangular Titanium Mononitride TiN: Auxetics and Promises for Photocatalysis

Recently, two-dimensional (2D) transition-metal nitrides have triggered an enormous interest for their tunable mechanical, optoelectronic, and magnetic properties, significantly enriching the family of 2D materials. Here, by using a broad range of first-principles calculations, we report a systematic study of 2D rectangular materials of titanium mononitride (TiN), exhibiting high energetic and thermal stability due to in-plane d-p orbital hybridization and synergetic out-of-plane electronic delocalization. The rectangular TiN monolayer also possesses enhanced auxeticity and ferroelasticity with an alternating order of Possion's Ratios, stemming from the competitive interactions of intra- and inter- Ti-N chains. Such TiN nanosystem is a n-type metallic conductor with specific tunable pseudogaps. Halogenation of TiN monolayer downshifts the Fermi level, achieving the optical energy gap up to 1.85 eV for TiNCl(Br) sheet. Overall, observed electronic features suggest that the two materials are potential photocatalysts for water splitting application. These results extend emerging phenomena in a rich family 2D transition-metal-based materials and hint for a new platform for the next-generation functional nanomaterials.