Charge density wave transition in single-layer titanium diselenide

A single molecular layer of titanium diselenide (TiSe$_{\mathrm{2}})$ is a promising material for advanced electronics beyond graphene-a strong focus of current exploration. Such molecular layers are at the quantum limit of device miniaturization and can show enhanced electronic effects not realizable in thick films. We show that single-layer TiSe$_{\mathrm{2}}$ exhibits a charge density wave (CDW) transition at critical temperature T$_{\mathrm{C}}=$232 K, which is higher than the bulk T$_{\mathrm{C}}=$200 K. Angle-resolved photoemission spectroscopy measurements reveal a small absolute bandgap at room temperature, which grows wider with decreasing temperature T below T$_{\mathrm{C}}$ in conjunction with the emergence of (2x2) ordering. The results are rationalized in terms of first-principles calculations, symmetry breaking and phonon entropy effects. The observed BCS behaviour of the gap implies a mean-field CDW order in the single layer and an anisotropic CDW order in the bulk.