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Graphene, the atomic layer of carbon, is one of the most intensely studied objects since it was isolated for the first time in 2004. However, its Dirac bands are made up of p z orbitals, which creates certain limitations in correlated physics. Here, we replace carbon with transition-metal atoms (M: Ti, Zr, and Hf) to form a corrugated honeycomb lattice. Moreover, both the topside and downside are passivized by halogen atoms (X: F, Cl, Br, and I), making sure that all of them are thermodynamically stable. Due to the irregularly octahedral crystalline field on M-d orbitals, two energy bands derived from d x y and d x 2 − y 2 orbitals intersect each other at the center of the Brillouin zone. Unlike graphene, this intersection is not conical but bell-shaped, further showing the Mexican-hat-shaped dispersion slightly higher than the Fermi level for TiCl, TiBr, and MI (M: Ti, Zr, and Hf). Moreover, the Mexican-hat coefficient could be controllably modulated by the biaxial strain. More interestingly, these intersecting points are robust on the on-site Coulomb interaction, but could be split by the spin-orbital coupling (SOC). Due to the strong SOC strength of correlated d x y and d x 2 − y 2 orbitals, the gap induced by SOC is three orders of magnitude larger than that of p z orbitals in graphene. Furthermore, the topological invariant and edge-states spectrum are calculated to suggest that transition-metal halogenides are non-trivial topological insulators with the topological invariant Z 2 = 1. [ABSTRACT FROM AUTHOR]

Inorganic molecular crystal films, particularly α-Sb2O3, have emerged as promising van der Waals (vdW) dielectrics for the large-scale integration of two-dimensional (2D) semiconductors in field-effect transistors (FETs) [K. L. Liu, B. Fin, W. Han, X. Chen, P. L. Gong and L. Huang, et al., Nat. Electron., 2021, 4, 906.]. Nevertheless, a notable gap exists in understanding the electronic and dielectric characteristics of few-layer α-Sb2O3 and the underlying physics governing its interaction with common 2D semiconductors. Herein, we address such issues through first-principles calculations. As the layer number increases, the electronic properties (e.g., band gap and band edges) of α-Sb2O3 exhibit minimal variations, resembling the electronic decoupling behavior, while the out-of-plane high dielectric constant significantly rises, indicating significant surface ionic behavior. These features stem from the weak interlayer quasi-bonding interaction, small atomic Born effective charge at the surface, and the influence of surface-to-volume ratio. Furthermore, exploring device physics, with a focus on complementary metal-oxide-semiconductor FETs, demonstrates that the leakage currents between the N-layer α-Sb2O3 (N ≥ 4) and all our studied 2D semiconducting channels adhere to international standards and the dielectrics with 4 and 5 layers meet the criteria for small equivalent oxide thickness. Unlike other layered vdW dielectrics, few-layer α-Sb2O3 emerges as a novel high-k vdW dielectric with exceptional dielectric performance and distinctive electronic characteristics, inspiring further exploration of inorganic molecular crystals for vdW dielectrics in integrated 2D semiconductor devices. [ABSTRACT FROM AUTHOR]

Cobalt has the highest Curie temperature ( T c ) among the elemental ferromagnetic metals and has a hexagonal close-packed (HCP) structure at room temperature. In this study, HCP Co was thinned to the thickness of several ( n ) unit cells along the c -axis and then passivated by halogen atoms, thus being named Co 2 n X 2 (X = F, Cl, Br and I). For Co 2 X 2 and Co 3 X 2 , all of them are not only kinetically but also thermodynamically stable from the viewpoint of the phonon spectra and molecular dynamics. Similar to HCP Co, two-dimensional (2D) Co 2 F 2 , Co 2 Cl 2 and Co 3 X 2 (X = Cl, Br and I) are still ferromagnetic metals within the Stoner model but Co 2 X 2 (X = Br and I) is a ferromagnetic half-metal with the coexistence of the metallic behavior for one spin and the insulating behavior for the other spin. Taking into account the spin-orbital coupling (SOC), the easy-magnetization axis is within the plane where the magnetization is isotropic, making it look like a 2D XY magnet. Applying a critical biaxial strain could lead to an easy-magnetization axis changing from the in-plane to the out-of-plane direction. Finally, we use classical Monte Carlo simulations to estimate the Curie temperature ( T c ) which is as high as 957 and 510 K for Co 2 F 2 and Co 2 Cl 2 , respectively, because of the strong direct exchange interaction. Different from being obtained by mechanical or liquid exfoliation from van der Waals layered structures, our study opens up new possibilities to search for novel 2D ferromagnets from the elemental ferromagnets and provides opportunities for realizing realistic ultra-thin spintronic devices.

Recently, two-dimensional (2D) van der Waals heterostructures (vdWHs) have exhibited emergent electronic and optical properties due to their peculiar phonons and excitons, which lay the foundation for the development of photoelectronic devices. The dielectric environment plays an important role in the interlayer coupling of vdWHs. Here, we studied the interlayer and extra-layer dielectric effects on phonon and exciton properties in WS 2 /MoS 2 and MoS 2 /WS 2 vdWHs by Raman and photoluminescence (PL) spectroscopy. The ultralow frequency (ULF) Raman modes are insensitive to atomic arrangement at the interface between 1LW and 1LM and dielectric environments of neighboring materials, and the layer breathing mode (LBM) frequency follows that of WS 2 . The shift of high-frequency (HF) Raman modes is attributable to interlayer dielectric screening and charge transfer effects. Furthermore, the energy of interlayer coupling exciton peak I is insensitive to atomic arrangement at the interface between 1LW and 1LM and its energy follows that of MoS 2 , but the slight intensity difference in inversion vdWHs means that the substrate's dielectric properties may induce doping on the bottom layer. This paper provides fundamental understanding of phonon and exciton properties of such artificially formed vdWHs structures, which is important for new insights into manipulating the performances of potential devices.