Controlling the semiconductor-metal transition in Cu-intercalated TiSe by applying stress

A possibility of efficiently controlling the optoelectronic properties of quasi-two-dimensional transition metal chalcogenides could greatly expand their innovative applications. Titanium diselenide (TiSe 2 ) is a scientifically and industrially important representative of this class, serving as a model for others. Here, we experimentally discovered a room-temperature semiconductor-metal transition in Cu-intercalated TiSe 2 single crystals ( x ≤ 0.1) controlled by applying a mechanical stress. We investigated the effect of applied high pressure on the electronic transport properties of Cu x TiSe 2 (0 ≤ x < 0.6) single crystals at room temperature by measurements of thermoelectric power (Seebeck coefficient). We found that the Cu x TiSe 2 crystals with a small copper content ( x ≤ 0.1) are semiconductors with narrow band gaps of E g 40-50 meV. An applied stress of 1-3 GPa, depending on the composition, turned them to metals. This transition was reversible and well reproducible under multiple pressure cycling. The difference in the metallization pressures was related to the difference in pressure coefficients of their band gaps that increased with the copper content, from d E g /d P = −17 meV GPa −1 for x = 0.002 to −60 meV GPa −1 for x = 0.1. Crystals of binary TiSe 2 demonstrated a reversible phase transition to a metal above 4 GPa, which was accompanied by a p-n inversion of the conductivity type. These findings suggest emerging potential applications of Cu x TiSe 2 crystals in various optoelectronic micro-devices in which characteristics of elements can be tuned or controlled by stress or strain. An abrupt semiconductor-metal transition in Cu x TiSe 2 single crystals with x ≤ 0.1 under a high pressure of 1-3 GPa has been revealed. Band gaps and their pressure coefficients of the crystals have been determined.