Large-gap two-dimensional topological insulator in oxygen functionalized MXene

Abstract

Two-dimensional (2D) topological insulator (TI) have been recognized as a new class of quantum state of matter. They are distinguished from normal 2D insulators with their nontrivial bandstructure topology identified by the Z 2 number as protected by time-reversal symmetry (TRS).2D TIs have intriguing spin-velocity locked conducting edge states and insulating properties in the bulk. In the edge states, the electrons with opposite spins propagate in opposite directions and the backscattering is fully prohibited when the TRS is conserved. This leads to quantized dissipationless "two-lane highway" for charge and spin transportation and promises potential applications. Up to now, only very few 2D systems have been discovered to possess this property. The lack of suitable material obstructs the further study and application. Here, by using first-principles calculations, we propose that the functionalized MXene with oxygen, M 2 CO 2 (M=W, Mo and Cr), are 2D TIs with the largest gap of 0.194 eV in W case. They are dynamically stable and natively antioxidant. Most importantly, they are very likely to be easily synthesized by recent developed selective chemical etching of transition-metal carbides (MAX phase). This will pave the way to tremendous applications of 2D TIs, such as "ideal" conducting wire, multifunctional spintronic device, and the realization of topological superconductivity and Majorana modes for quantum computing.2

Electronic properties and applications of MXenes: a theoretical review

“…The induced band gap is larger as the SOC is larger [68]. It is found that the band gaps can be as large as 0.194 eV (0.472 eV) for W 2 CO 2 and 0.285 (0.401 eV) for W 2 HfC 2 O 2 within the generalized gradient approximation (hybrid functional HSE06) [67,68]. Although the similar features are observed for M…”

“…The trivial and nontrivial topological phases are identified with ν = 0 and 1, respectively. The parity analysis of the occupied bands at the TRIM points finds that ν = 1 for all the systems discussed here, and therefore M 2 CO 2 (M= Mo, W) and M [67,68]. The 2D topological insulators display intriguing conducting edge states in which electrons with opposite spins propagate in the opposite directions and thus they are robust against nonmagnetic impurities and disorders [123][124][125][126].…”

“…However, upon surface functionalization, some of the MXenes become semiconducting. As will be discussed below, theoretical investigations have shown that the MXenes can be divided into two categories, topologically trivial [39] and nontrivial [67][68][69][70][71] metal/semimetal or semiconductor, depending on the strength of relativistic SOC in these systems. [39].…”

“…5) The surfaces of MXenes can be functionalized with various chemical groups, which offers possibilities for surface state engineering. 6) Similar to graphene, some of MXenes exhibit massless Dirac dispersions in their band structures near the Fermi level [67][68][69][70]77]. This opens broad possibilities for Dirac-based physics and applications [67][68][69][70].…”

“…Theoretically, many applications have been proposed for MXenes in electronic [39][40][41][42][43], magnetic [44][45][46][47][48], optical [49,50], thermoelectric [51][52][53][54][55][56], and sensing devices [57], as well as being new potential materials for catalytic and photocatalytic reactions [58][59][60][61][62][63], hydrogen storage media [64,65], and nanoscale superconductivity [66]. Some of MXenes are predicted to be topological insulators with large band gaps involving only d orbitals [67][68][69][70][71]. MXenes are also expected to be used as ultralow work function materials [72] and Schottky barrier junctions [73][74][75][76].…”