Exploring two-dimensional valleytronic crystals with large valley-polarized state is of considerable importance due to the promising applications in next-generation information related devices. Here, we show first-principles evidence that single-layer NbX₂ (X = S, Se) is potentially the long-sought two-dimensional valleytronic crystal. Specifically, the valley-polarized state is found to occur spontaneously in single-layer NbX₂, without needing any external tuning, which arises from their intrinsic magnetic exchange interaction and inversion asymmetry. Moreover, the strong spin-orbit coupling strength within Nb-d orbitals renders their valley- polarized states being of remarkably large (NbS₂ ~ 156 meV/NbSe₂ ~ 219 meV), enabling practical utilization of their valley physics accessible. In additional, it is predicted that the valley physics (i.e., anomalous valley Hall effect) in single-layer NbX₂ is switchable via applying moderate strain. These findings make single-layer NbX₂ tantalizing candidates for realizing high-performance and controllable valleytronic devices.

Fabrication of lateral heterostructures (LHS) is promising for a wide range of next-generation devices and could sufficiently unlock the potential of two-dimensional materials. Herein, we demonstrate the design of lateral heterostructures based on new building materials, namely 1S-MX₂ LHS, using first-principles calculations. 1S-MX₂ LHS exhibits excellent stability, demonstrating high feasibility in the experiment. The desired bandgap opening can endure application at room temperature and was confirmed in 1S-MX₂ LHS with spin-orbit coupling (SOC). A strain strategy further resulted in efficient bandgap engineering and an intriguing phase transition. We also found that black phosphorus can serve as a competent substrate to support 1S-MX₂ LHS with a coveted type-Ⅱ band alignment, allowing versatile functionalized bidirectional heterostructures with built-in device functions. Furthermore, the robust electronic features could be maintained in the 1S-MX₂ LHS with larger components. Our findings will not only renew interest in LHS studies by enriching their categories and properties, but also highlight the promise of these lateral heterostructures as appealing materials for future integrated devices.

Quantum spin Hall (QSH) insulator is a new class of materials that is quickly becoming mainstream in condensed-matter physics. The main obstacle for the development of QSH insulators is that their strong interactions with substrates make them difficult to study experimentally. In this study, using density functional theory, we discovered that MoTe₂ is a good match for a GeI monolayer. The thermal stability of a van der Waals GeI/MoTe₂ heterosheet was examined via molecular-dynamics simulations. Simulated scanning tunneling microscopy revealed that the GeI monolayer perfectly preserves the bulked honeycomb structure of MoTe₂. The GeI on MoTe₂ was confirmed to maintain its topological band structure with a sizable indirect bulk bandgap of 0.24 eV by directly calculating the spin Chern number to be -1. As expected, the electron mobility of the GeI is enhanced by MoTe₂ substrate restriction. According to deformation-potential theory with the effective-mass approximation, the electron mobility of GeI/MoTe₂ was estimated as 372.7 cm²·s^-1·V^-1 at 300 K, which is 20 times higher than that of freestanding GeI. Our research shows that traditional substrates always destroy the topological states and hinder the electron transport in QSH insulators, and pave way for the further realization and utilization of QSH insulators at room temperature.

A new family of two-dimensional (2D) topological insulators (TIs) comprising g-TlA (A = N, P, As, and Sb) monolayers constructed by Tl and group-Ⅴ elements is predicted by first-principles calculations and molecular-dynamics (MD) simulations. The geometric stability, band inversion, nontrivial edge states, and electric polarity are investigated to predict the large-gap quantum spin Hall insulator and Rashba-Dresselhaus effects. The MD results reveal that the g-TlA monolayers remain stable even at room temperature. The g-TlA (A = As, Sb) monolayers become TIs under the influence of strong spin-orbit couplings with large bulk bandgaps of 131 and 268 meV, respectively. A single band inversion is observed in each g-TlA (A = As, Sb) monolayer, indicating a nontrivial topological nature. Furthermore, the topological edge states are described by introducing a sufficiently wide zigzag-nanoribbon. A Dirac point in the middle of the bulk gap connects the valence- and conduction-band edges. The Fermi velocity near the Dirac point with a linear band dispersion is ~0.51 × 10⁶ m/s, which is comparable to that of many other 2D nanomaterials. More importantly, owing to the broken inversion symmetry normal to the plane of the g-TlA films, a promising Rashba-Dresselhaus effect with the parameter up to 0.85 eV·? is observed in the g-TlA (A = As, Sb) monolayers. Our findings regarding 2D topological g-TlA monolayers with room-temperature bandgaps, intriguing topological edge states, and a promising Rashba-Dresselhaus effect are of fundamental value and suggest potential applications in nanoelectronic devices.