Department of Physics and ‡Materials Science Program, Florida State University , Tallahassee, Florida 32306, United States.
National High Magnetic Field Laboratory, Florida State University , Tallahassee, Florida 32310, United States.
Nano Lett. 2015 Oct 14;15(10):7037-42. doi: 10.1021/acs.nanolett.5b03086. Epub 2015 Oct 5.
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.
二氧化钒（VO2）因其在室温附近的独特金属-绝缘体转变而引起了广泛关注。在转变温度（约 68°C）以下，高电阻是晶格（派尔斯）畸变辅助下强电子相关的结果。理论计算表明，强电子间相互作用可能会引起有趣的光电现象，如多激子产生（MEG），这是高效光电和光伏的理想过程。然而，VO2 的电阻对温度非常敏感，因此，VO2 中的光致导电性通常归因于光热效应。在这项工作中，我们通过改变光学照明的斩波频率来区分 VO2 纳米线中的光热和光注入效应。我们发现，在我们的 VO2 纳米线中，当斩波频率大于 2 kHz 时，相对较慢的光热过程可以得到很好的抑制，而快速的光注入成分（载流子的直接光激发）在所有斩波频率下保持不变。通过分离光热和光注入过程，我们的工作为进一步研究强关联材料在光激发下的载流子动力学奠定了基础。
