Controlling the semiconductor–metal transition in Cu-intercalated TiSe2 by applying stress

Abstract

A possibility of efficiently controlling the optoelectronic properties of quasi-two-dimensional transition metal chalcogenides could greatly expand their innovative applications. Titanium diselenide (TiSe₂) 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 TiSe₂ 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_xTiSe₂ (0 ≤ x < 0.6) single crystals at room temperature by measurements of thermoelectric power (Seebeck coefficient). We found that the Cu_xTiSe₂ 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 dE_g/dP = −17 meV GPa⁻¹ for x = 0.002 to −60 meV GPa⁻¹ for x = 0.1. Crystals of binary TiSe₂ 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_xTiSe₂ crystals in various optoelectronic micro-devices in which characteristics of elements can be tuned or controlled by stress or strain.