Department of Physics, University of Washington, Seattle, Washington 98195, USA.
Nat Nanotechnol. 2012 Nov;7(11):723-7. doi: 10.1038/nnano.2012.176. Epub 2012 Oct 21.
The generation of a current by light is a key process in optoelectronic and photovoltaic devices. In band semiconductors, depletion fields associated with interfaces separate long-lived photo-induced carriers. However, in systems with strong electron-electron and electron-phonon correlations it is unclear what physics will dominate the photoresponse. Here, we investigate photocurrent in VO(2), an exemplary strongly correlated material known for its dramatic metal-insulator transition at T(c) ≈ 68 °C, which could be useful for optoelectronic detection and switching up to ultraviolet wavelengths. Using scanning photocurrent microscopy on individual suspended VO(2) nanobeams we observe a photoresponse peaked at the metal-insulator boundary but extending throughout both insulating and metallic phases. We determine that the response is photothermal, implying efficient carrier relaxation to a local equilibrium in a manner consistent with strong correlations. Temperature-dependent measurements reveal subtle phase changes within the insulating state. We further demonstrate switching of the photocurrent by optical control of the metal-insulator boundary arrangement. Our work shows the value of applying scanning photocurrent microscopy to nanoscale crystals in the investigation of strongly correlated materials, and the results are relevant for designing and controlling optoelectronic devices employing such materials.
光生电流是光电和光伏器件的关键过程。在能带半导体中，与界面相关的耗尽场会分离长寿命的光致载流子。然而，在具有强电子-电子和电子-声子关联的系统中，尚不清楚哪种物理现象将主导光响应。在这里，我们研究了 VO(2)中的光电流，VO(2)是一种典型的强关联材料，因其在 T(c) ≈ 68°C 时发生剧烈的金属-绝缘体转变而备受关注，这可能对用于光电检测和开关的紫外波长光很有用。我们使用单个悬浮 VO(2)纳米梁的扫描光电显微镜观察到光电流在金属-绝缘体边界处达到峰值，但在整个绝缘和金属相都有延伸。我们确定该响应是光热的，这意味着载流子以与强关联一致的方式有效地松弛到局部平衡。依赖于温度的测量揭示了绝缘态中的细微相变化。我们还通过光学控制金属-绝缘体边界排列来演示了光电流的开关。我们的工作表明，将扫描光电显微镜应用于强关联材料的纳米晶体研究具有重要价值，并且这些结果对于设计和控制采用此类材料的光电设备具有重要意义。
