Two-dimensional (2D) metallic transition metal dichalcogenides (MTMDCs) supply a versatile platform for investigating newfangled physical issues and developing potential applications in electronics/spintronics/electrocatalysis. Among these, NiTe 2 (a type-II Dirac semimetal) possesses a Dirac point near its Fermi level. However, as-prepared 2D MTMDCs are mostly environmentally unstable, and little attention has been paid to synthesizing such materials. Herein, a general chemical vapor deposition (CVD) approach has been designed to prepare thickness-tunable and large-domain (∼1.5 mm) 1T-NiTe 2 on an atomically flat mica substrate. Significantly, ultrahigh conductivity (∼1.15 × 10 6 S m −1 ) of CVD-synthesized 1T-NiTe 2 and high catalytic activity in pH-universal hydrogen evolution reaction have been uncovered. More interestingly, the 2D 1T-NiTe 2 maintains robust environmental stability for more than one year and even after a variety of harsh treatments. These results hereby fill an existing research gap in synthesizing environmentally stable 2D MTMDCs, making fundamental progress in developing 2D MTMDC-based devices/catalysts.

Uncovering the thickness‐dependent electronic property and environmental stability for 2D materials are crucial issues for promoting their applications in high‐performance electronic and optoelectronic devices. Herein, the extrahigh air stability and giant tunable electronic bandgap of chemical vapor deposition (CVD)–derived few‐layer PdSe2 on Au foils, by using scanning tunneling microscope/spectroscopy (STM/STS), are reported. The robust stability of 2D PdSe2 is uncovered by the observation of nearly defect/adsorption‐free atomic lattices on long‐time air‐exposed samples. A one‐to‐one correspondence between the electronic bandgap (from ≈1.15 to ≈0 eV) and thickness of PdSe2/Au (from bilayer to bulk) is established. It is also revealed that few‐layer semiconducting PdSe2 flakes present zero‐gap edges, induced by hybridization of Pd 4d and Se 4p orbitals. This work hereby provides straightforward evidence for the thickness‐tunable electronic property and air stability of 2D semiconductors, thus shedding light on their applications in next‐generation electronic devices.

One of the main challenges in the development of wide bandgap semiconductor devices is to understand the behavior of defects and avoid their harm. Using density-functional theory calculations with hybrid functional, we systematically investigated the neutral and charged native point defects (vacancy, interstitial, and antisite defect) in GaN, AlN, and InN crystals in terms of local geometry relaxation, formation energies, and electronic and diffusion properties. By comparing the defect configuration and transition levels as a function of the Fermi level, we show that Ga interstitial (Gaoc, Gate) in GaN, N vacancy (VN), N interstitial (Ni), In antisite (InN), and In interstitial (Inte, Inoc) in InN can exist stably only in the positive charge states with donor level and VIn is stable in the neutral state, while the other defects exhibit both donor and acceptor behavior. Among them, the most stable defects are identified as VN for p-type nitrides and VGa, VAl for n-type nitrides. These results, providing a mechanism for self-compensation effects, explain the reduced doping efficiencies for both n-type and p-type nitrides due to defects. Moreover, it is also demonstrated that N interstitial diffuses faster than vacancy, which are mainly responsible for the low concentration of N interstitials and N-based defect complexes produced in nitrides. Significantly, the trends of formation energy, transition level, and migration barrier of nitrides are also consistent with their intrinsic atomic size and bandgap. Our study is important for the identification and control of point defects in nitrides, which have a profound impact on device performance and reliability.

In the presence of spin-orbit coupling (SOC), achieving both spin and valley polarized Dirac state is significant to promote the fantastic integration of Dirac physics, spintronics and valleytronics. Based on ab initio calculations, here we demonstrate that a class of spin-valley-coupled Dirac semimetals (svc-DSMs) in the functionalized SbAs monolayers (MLs) can host such desired state. Distinguished from the graphene-like 2D Dirac materials, the Dirac cones in svc-DSMs hold giant spin-splitting induced by strong SOC under inversion symmetry breaking. In the 2.3% strained SbAsH 2 ML, the Dirac fermions in inequivalent valleys have opposite Berry curvature and spin moment, giving rise to Dirac spin-valley Hall effect with constant spin Hall conductivity as well as massless and dissipationless transport. Topological analysis reveals that the svc-DSM emerges at the boundary between trivial and 2D topological insulators, which provides a promising platform for realizing the flexible and controllable tuning among different quantum states.The rise of graphene 1-3 has inspired significant efforts in searching for other 2D Dirac materials (DMs) 4-10 with linear energy dispersion. As the host of massless Dirac fermions, 2D DMs have become the playground for investigating many quantum relativistic phenomena [11][12][13] in the emerging field of Dirac physics. Like graphene, without considering spin-orbit coupling (SOC), the Dirac points in these 2D DMs are protected by symmetry. However, the presence of SOC will open a global bulk gap at the Dirac points and introduce the topologically protected gapless edge states. [13][14][15] Therefore, 2D DMs are formally a quantum spin Hall insulator 13 under time-reversal symmetry [ Fig. 1(a)] or quantum anomalous Hall insulator 12 with time-reversal breaking [ Fig. 1(b)]. In a pioneering theoretical study, Yong and Kane 16 employed symmetry analysis and a two-site tight-binding model to examine the possibility that the Dirac points can not be gapped by SOC [ Fig. 1(c)]. They concluded that nonsymmorphic space group symmetry plays an essential role in protecting the 2D Dirac points against SOC, named as spin-orbit Dirac points (SDPs). Although the first realistic 2D material hosting SDPs was predicted recently 17 , its SDPs are not located at the Fermi level and its Dirac dispersion is contaminated by some extraneous non-Dirac bands. Hence, how to achieve a 2D Dirac semimetal hosting SDPs with clean Dirac bands at the Fermi level remains a great challenge.On the other hand, the discovery of valley-dependent effects in MoS 2 monolayer 18,19 without inversion symmetry aroused an upsurge in the field of 2D valleytronics. 20 For hexagonal 2D materials such as graphene and monolayer group-VI transition metal dichalcogenides, the conically shaped valleys at +K and -K 4 FIG. 1. Schematic electronic band structures without SOC (left) and with SOC (right) for different 2D Dirac materials: (a) quantum spin Hall insulator, (b) quantum anomalous Hall insulator, (c) symmetry-protected Dirac semim...