Short-Range Charge Density Wave and Bandgap Modulation by Au-Implanted Defects in TiSe₂

Two-dimensional titanium diselenide (TiSe2) thin films are excitonic insulators characterized by a charge density wave (CDW) order and a charge gap below a thermal transition scale TCDW ≈ 200 K. Above 200 K, the charge gap collapses to a small finite value, and incoherent short-range CDW correlations persist as precursors to the low-temperature CDW order, over a small temperature window. Here, we employ low-energy ion implantation to introduce Au, as a disorder potential, in TiSe2 thin films and control the disorder magnitude by the ion beam fluence. By the analysis of the change of in-plane vibrational modes, we demonstrate a remarkable effect, where disorder not only enhances the temperature window of the short-range CDW correlations to 2.5 times the TCDW of the Au-free sample but also systematically increases its charge gap at room temperature. We experimentally demonstrate the tunability of the temperature window of the short-range CDW order and room temperature bandgap with disorder. We provide a qualitative understanding of the results. Such enhancement of quantum mechanical phenomena to high temperatures and tunability of the bandgap are likely to be useful for CDW-based electronic devices.