We have developed a pump-probe microscope capable of exciting a single semiconductor nanostructure in one location and probing it in another with both high spatial and temporal resolution. Experiments performed on Si nanowires enable a direct visualization of the charge cloud produced by photoexcitation at a localized spot as it spreads along the nanowire axis. The time-resolved images show clear evidence of rapid diffusional spreading and recombination of the free carriers, which is consistent with ambipolar diffusion and a surface recombination velocity of ∼10(4) cm/s. The free carrier dynamics are followed by trap carrier migration on slower time scales.

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Ultrafast charge carrier dynamics in silicon nanowires (NWs) grown by a vapor−liquid−solid mechanism were interrogated with optical pump−probe microscopy. The high time and spatial resolutions achieved by the experiments provide insight into the charge carrier dynamics of single nanostructures. Individual NWs were excited by a femtosecond pump pulse focused to a diffraction-limited spot, producing photogenerated carriers (electrons and holes) in a localized region of the structure. Photoexcited carriers undergo both electron−hole recombination and diffusional migration away from the excitation spot on similar time scales. The evolution of the carrier population is monitored by a delayed probe pulse that is also focused to a diffraction-limited spot. When the pump and probe are spatially overlapped, the transient signal reflects both recombination and carrier migration. Diffusional motion is directly observed by spatially separating the pump and probe beams, enabling carriers to be generated in one location and detected in another. Quantitative analysis of the signals yields a statistical distribution of carrier lifetimes from a large number of individual NWs. On average, the lifetime was found to be linearly proportional to the diameter, consistent with a surface-mediated recombination mechanism. These results highlight the capability of pump−probe microscopy to quantitatively evaluate key recombination characteristics in semiconductor nanostructures, which are important for their implementation in nanotechnologies.

Pump–probe transient absorption microscopy was used to follow the electron–hole (e–h) recombination dynamics at different points within individual needle-shaped ZnO rods to characterize spatial differences in dynamical behavior. The results from pump–probe experiments are correlated with spatially resolved ultrafast emission measurements, and scanning electron microscopy provides structural details. Dramatically different e–h recombination dynamics are observed in the narrow tips compared to the interior, with the ends exhibiting a greater propensity for electron–hole plasma (EHP) formation and faster recombination of carriers across the band gap that stem from a physical confinement of the charge carriers. In the interior of the rod, a greater fraction of the e–h recombination is trap-mediated and occurs on a significantly longer time scale.

Ultrafast carrier dynamics in silicon nanowires with average diameters of 40, 50, 60, and 100 nm were studied with transient absorption spectroscopy. After 388 nm photoexcitation near the direct band gap of silicon, broadband spectra from 400 to 800 nm were collected between 200 fs and 1.3 ns. The transient spectra exhibited both absorptive and bleach features that evolved on multiple time scales, reflecting contributions from carrier thermalization and recombination as well as transient shifts of the ground-state absorption spectrum. The initially formed “hot” carriers relaxed to the band edge within the first ∼300 fs, followed by recombination over several hundreds of picoseconds. The charge carrier lifetime progressively decreased with decreasing diameter, a result consistent with a surface-mediated recombination process. Recombination dynamics were quantitatively modeled using the diameter distribution measured from each sample, and this analysis yielded a consistent surface recombination velocity of ∼2 × 104 cm/s across all samples. The results indicate that transient absorption spectroscopy, which interrogates thousands of individual nanostructures simultaneously, can be an accurate probe of material parameters in inhomogeneous semiconductor samples when geometrical differences within the ensemble are properly analyzed.