Distinguishing the Photothermal and Photoinjection Effects in Vanadium Dioxide Nanowires.

Vanadium dioxide (VO2) has drawn significant attention for its unique metal-to-insulator transition near the room temperature. The high electrical resistivity below the transition temperature (∼68 °C) is a result of the strong electron correlation with the assistance of lattice (Peierls) distortion. Theoretical calculations indicated that the strong interelectron interactions might induce intriguing optoelectronic phenomena, such as the multiple exciton generation (MEG), a process desirable for efficient optoelectronics and photovoltaics. However, the resistivity of VO2 is quite temperature sensitive, and therefore, the light-induced conductivity in VO2 has often been attributed to the photothermal effects. In this work, we distinguished the photothermal and photoinjection effects in VO2 nanowires by varying the chopping frequency of the optical illumination. We found that, in our VO2 nanowires, the relatively slow photothermal processes can be well suppressed when the chopping frequency is >2 kHz, whereas the fast photoinjection component (direct photoexcitation of charge carriers) remains constant at all chopping frequencies. By separating the photothermal and photoinjection processes, our work set the basis for further studies of carrier dynamics under optical excitations in strongly correlated materials.