Speaker: John M. Papanikolas, Department of Chemistry, University of North Carolina, Chapel Hill, and NSF Program Director, Chemical Structure, Dynamics and Mechanisms
Title: Visualizing Charge Carrier Dynamics in Nanowires with Pump-Probe Microscopy
Abstract: A detailed understanding of the factors that govern the motion of mobile charge carriers through nanostructures is critical to many emerging nanotechnologies in electronics, optoelectronics and solar energy conversion. While the motion of charge carriers at low carrier densities is uncorrelated and easy to understand, many active electronic components operate at high carrier concentrations resulting from heavy doping or high injection. In this regime, carrier-carrier interactions and other many body effects (e.g. dopant/carrier interactions, electron screening, and electron-hole scattering) must be considered. We have combined ultrafast pump-probe spectroscopy with optical microscopy to directly image the charge carrier dynamics in individual Si nanowires (NWs) with both spatial and temporal resolution. In these experiments, an individual NW is excited by a femtosecond pump pulse that has been focused to a diffraction limited spot by a microscope objective, producing photogenerated carriers (electrons and holes) in a localized region of the structure. Motion of the photogenerated carriers is observed using a configuration in which carriers are created in one location and detected in another. In this configuration the pump beam is held fixed and the position of the probe beam is scanned by varying the angle of the probe beam as it enters the objective, resulting in a spatial map of the free carriers that provides a direct visualization of carrier diffusion.