Two-dimensional (2D) topological insulators (TIs) have been recognized as a new class of quantum state of matter. They are distinguished from normal 2D insulators with their nontrivial band-structure topology identified by the $Z_2$ number as protected by time-reversal symmetry (TRS). Two-dimensional 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 a 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 further study and application. Here, by using first-principles calculations, we propose that functionalized MXenes with oxygen, ${\mathrm{M}}_2{\mathrm{CO}}_2$ (M=W, Mo, and Cr), are 2D TIs with the largest gap of 0.194 eV in the W case. They are dynamically stable and natively antioxidant. Most importantly, they are very likely to be easily synthesized by recently developed selective chemical etching of transition-metal carbides (the ${\mathrm{M}}_{n+1}{\mathrm{AX}}_n$ phase). This will pave the way to tremendous applications of 2D TIs, such as “ideal” conducting wire, multifunctional spintronic devices, and the realization of topological superconductivity and Majorana modes for quantum computing.

• Received 6 July 2015

DOI:https://doi.org/10.1103/PhysRevB.92.075436