Ti<sub>3</sub>O M<i><b>O</b></i>enes:\nQuantum Spin Hall Effect and Promising Semiconductors for Light-Harvesting

The continuous pursuit of novel two-dimensional (2D)\nmaterials\nwith intriguing properties has been a driving force in advancing various\nscientific and technological frontiers. Here, based on a wide range\nof first-principles calculations, we predicted the existence of a\nnovel family of 2D transition-metal oxides, the Ti<sub>3</sub>O M<b><i>O</i></b>enes (MXene-like 2D transition oxides),\nand determined its distinctive electronic and topological properties.\nA pair of 2D antiferromagnetic (AFM) Dirac points precisely located\nat the Fermi level in the absence of spin–orbit coupling (SOC)\nis observed in the 1T-Ti<sub>3</sub>O monolayer. Moreover, upon halogenation\non a bare monolayer, 1T-Ti<sub>3</sub>OCl<sub>3</sub> and 1T-Ti<sub>3</sub>OBr<sub>3</sub> monolayers display the quantum spin Hall (QSH)\neffect with nontrivial helical edge states within the gapless bulk\nstates. Specifically, single layer 1T-Ti<sub>3</sub>OF<sub>3</sub> behaves as an indirect semiconductor with a gap of 0.81 eV, exhibiting\na strong light-harvesting capability. The indirect-gap feature can\nbe switched to a direct one by only exerting a small tensile strain\nof 1.5%. These findings broaden emerging phenomena in a rich family\nof M<b><i>O</i></b>enes, suggesting a novel platform\nfor the development of next-generation nanodevices.