Exploring potential for semiconductor to quantum anomalous Hall insulator transitions via substrate-induced structural modifications in Ti₃Se₄ monolayers

The quantum anomalous Hall (QAH) effect in two-dimensional (2D) topological materials has attracted widespread attention due to its potential for dissipationless chiral edge transport without an external magnetic field, which is highly promising for low-power electronic applications. However, identifying materials that exhibit these properties remains particularly challenging, as only a limited number of such materials are known, raising the intriguing question of whether it is possible to induce the QAH effect in materials with ordinary properties through structural modifications. In this work, we grow an unreported 2D titanium selenide (Ti₃Se₄) on a Cu(111) substrate using molecular beam epitaxy. Low-energy electron diffraction and scanning tunneling microscopy characterizations reveal a $\sqrt{\text{7}}\times \sqrt{\text{7}}$ brick-like structure. First-principles calculations and X-ray photoelectron spectroscopy measurements confirm its composition to be Ti₃Se₄. Our calculations further demonstrate that monolayer Ti₃Se₄, in its grown form on Cu(111), has the potential to host the QAH effect. Interestingly, when we examine its freestanding form, the monolayer transitions from a QAH insulator candidate into a conventional semiconductor, despite only minor differences in their atomic structures. This transition enlightens us that subtle lattice adjustments can induce a transition from semiconductor to QAH properties in freestanding Ti₃Se₄. This discovery provides a potential route to engineering practical materials that may exhibit the QAH effect.