Nano Lett. 13, 1336–1340 (2013)

Characterizing the motion of photoexcited charge carriers in semiconductor nanostructures is important for optoelectronic and photovoltaic applications. Although electrical measurements can provide spatially averaged information over the whole structure and relatively long timescales, access to time-resolved, local details of the carrier motion is desirable to shed light on carrier diffusion and recombination mechanisms. James Cahoon, John Papanikolas and colleagues at the University of North Carolina at Chapel Hill have now developed a pump–probe microscopy technique that can image and track the motion of charge carriers in individual silicon nanowires with high spatial and temporal resolution.

The key feature of the microscope is that the pump and probe beams can be focused at different positions along the nanowire. Charge carriers are photogenerated in one location by the pump beam and detected as they diffuse along the nanowire by scanning the position of the probe beam. The resulting images record the spatial variation of the transient absorption as a function of time, and show a cloud of free carriers diffusing away from the generation spot up to distances of 2.7 μm within 500 ps and subsequently recombining with a carrier lifetime of 380 ps, which corresponds to a velocity of about 104 cm s−1. The researchers also observed the slower motion of trapped carriers.