Large-Gap Quantum Spin Hall States in the Bilayer Hexagonal Structure of Rhenium and Technetium Dinitrides: A First-Principles Study

Abstract

Layered transition-metal dinitrides are the recently proposed cousins of transition-metal dichalcogenides, which exhibit extraordinary electronic and magnetic properties. In this work, through first-principles calculations, we have identified the lowest energy geometry of the ReN2 nanosheet, which is a bilayer hexagonal structure (BHS) akin to the MoN2 system. Such a BHS-ReN2 nanosheet possesses robust dynamical, thermal, and mechanical stabilities at the free-standing state. Interestingly, without spin–orbit coupling (soc), this BHS-ReN2 system exhibits a semimetallic feature with a d-character Dirac cone, for which a band gap is opened at the Dirac point when the soc effect is taken into account. As a result, the BHS-ReN2 nanosheet becomes a quantum spin Hall (QSH) insulator in the presence of soc, whose bulk gap reaches up to 384 meV, sufficiently large for the room-temperature QSH effect. The nontrivial topological feature is characterized by a Z 2 = 1 invariant and a pair of topologically protected edge states in the BHS-ReN2 nanosheet, and such a topological feature is robust against tensile strain. In addition to ReN2, the TcN2 nanosheet, another group VIIB dinitride, also has a stable BHS-phase structure and behaves as a QSH insulator with a sizeable bulk gap of 274 meV. Our study demonstrates that large-gap QSH states exist in the highly stable structure of ReN2 and TcN2 nanosheets, which will be superior candidate materials for the QSH-based devices and applications.

Computational Exploration of Stable 4d/5d Transition-Metal MSi2N4 (M = Y–Cd and Hf–Hg) Nanosheets and Their Versatile Electronic and Magnetic Properties

“…When the magnetization is altered to the in-plane direction, the intriguing Weyl semimetallic and quantum anomalous Hall states can be alternately switched by rotating the magnetization angle of T-YN 2 nanosheets . It would be noticed that in addition to the H and T phases, several non-MoS 2 -like geometries, such as the ortho-phase, tetra-phase, and bilayer hexagonal structures, , have been proposed for the pristine MN 2 nanosheets. These non-MoS 2 -like phases have better structural stability than the H- and T-phase ones due to the high activity of surface N atoms .…”

Bilayer hexagonal structure MnN2 nanosheets with room-temperature ferromagnetic half-metal behavior and a tunable electronic structure

“…Studies show that BHS-ReN 2 nanosheets can become a quantum spin Hall (QSH) insulator in the presence of SOC, with a volume gap of up to 384 meV, which is sufficient to satisfy the room temperature QSH effect. 25 These studies indicate that the BHS phase may also be applicable to other TMDN systems. We consider combining Mn atoms with the energy stable structure BHS of MoN 2 nanocrystals, which may yield a new material with strong ferromagnetism.…”

Structures, properties and applications of two-dimensional metal nitrides: from nitride MXene to other metal nitrides

“…The particle swarm optimization (PSO) algorithm is usually used as the effective tool to theoretically predict low-energy stable structures of 2D MNs [71][72][73][74]. To confirm their dynamical, thermal and mechanical stability, theoretical approaches such as phonon frequencies, abinitio molecular dynamics (AIMD) simulation and elastic constants based on first-principles calculations are usually employed on the predicted 2D MNs [74][75][76][77][78]. Herein, we gathered the reported 2D MNs that have been experimentally prepared or theoretically predicted, and sorted them into different element groups, as indicated by figure 1.…”