Electron transfer induced thermochromism in a VO₂-graphene-Ge heterostructure

As a marvelous thermochromic material, vanadium dioxide (VO₂) holds great promise for applications in smart devices due to its semiconductor-metal transition (SMT), which is an active response to external temperature stimuli and near-infrared irradiation. Herein, we demonstrate for the first time that the phase transition temperature of a VO₂ film can be effectively manipulated using graphene as an interlayer. A high quality graphene monolayer was deposited using chemical vapor deposition on a Ge underlayer, followed by the growth of a VO₂ film onto the graphene to form a semimetal-semiconductor contact, namely, a VO₂-graphene-Ge junction. The thermochromic properties of the VO₂ film were demonstrated with ∼20% infrared reflectance contrast. Furthermore, the transition temperature of the VO₂ film was effectively reduced from 340 K to 330 K. On the basis of the Mott-Hubbard phase transition theory, a plausible mechanism is proposed here for the first time from the perspective of charge transfer to elucidate the experimental phenomenon, which indicates that the electron transfer can increase the electron concentration in the VO₂ film and destabilize the semiconductor phase of the VO₂ film, thus decreasing the SMT temperature of the thermochromic VO₂ film. In addition to stimulating scientific interest, this study may also contribute to thermochromic VO₂-based applications in sensors, optical and electrical switches, and other nanodevices.