Image courtesy of David H. Cobden
A focused laser is scanned over the sample, a suspended nanobeam of VO2. This material can switch very rapidly between phases with different optical response, visible in the optical image as dark (metallic) and light (insulating) parts. The scanning photocurrent image below reveals a photovoltaic effect peaking at the metal-insulator boundary.
The current generated by light incident on the optical-switching material vanadium dioxide, VO2, was found to be of thermoelectric origin, in contrast with semiconductors like silicon where electrons generated by the light are accelerated by electric fields.
This is the first time the mechanism for the generation of current by light has been unambiguously determined for a complex oxide. This understanding will facilitate the development of these materials for optical switches and for sensing applications that are not possible with semiconductors.
The generation of a current by light is a key process in optoelectronic and photovoltaic devices. In this work, scanning photocurrent microscopy was applied to determine the mechanism for photocurrent generation in vanadium dioxide, a material famous for its dramatic transition between a metallic and insulating state at 65°C. This transition could be useful for optoelectronic detection and switching up to ultraviolet wavelengths. Working with nanobeams (VO2 crystals micrometers in length with nano-dimensioned cross-sections) and micron-scale probes avoids many problems inherent in large samples, such as cracking. Researchers at the University of Washington observed photoresponse peaked at the metal-insulator boundary but extended throughout both insulating and metallic phases. By detailed analysis of many different measurements, it was possible to determine unambiguously that the response is photothermal, implying rapid carrier relaxation to a local equilibrium in a manner consistent with strong electron-electron and electron-phonon correlations. This is in contrast with the response of standard semiconductor like silicon where the photovoltaic response is dominated by photo-generated carriers. The results are relevant for designing and controlling optoelectronic devices employing complex materials.
David H. Cobden
University of Washington
DOE Office of Science, Basic Energy Sciences program (Scanning photocurrent microscopy experiment); National Science Foundation Career Award (lasers and laser expertise); the Army Research Office (support of the early, foundational research).
T. Serkan Kasirga, Dong Sun, Jae H. Park, Jim M. Coy, Xiaodong Xu, and David H. Cobden, "Photoresponse of a strongly correlated material determined by scanning photocurrent microscopy", Nature Nanotechnology 7, 723 (2012).