A near-field scanning optical microscope was used to image the photocurrent induced by local illumination along the length of a metal-semiconductor-metal ͑MSM͒ photodetector made from an individual CdS nanowire. Nanowire MSM photodetectors exhibited photocurrents ϳ10 5 larger than the dark current ͑Ͻ2 pA͒ under uniform monochromatic illumination; under local illumination, the photoresponse was localized to the near-contact regions. Analysis of the spatial variation and bias dependence of the local photocurrent allowed the mechanisms of photocarrier transport and collection to be identified, highlighting the importance of near-field scanning photocurrent microscopy to elucidating the operating principles of nanowire devices.

The mobility−lifetime products (μτ) for electrons and holes in CdS nanowires were quantitatively determined by scanning photocurrent microscopy of devices with ohmic contacts. Ohmic contacts were fabricated by ion bombardment of the contact regions. By analyzing the spatial profiles of the local photoconductivity maps, we determined that electron transport (μeτe ≈ 5 × 10-7 cm2/V) was more efficient than hole transport (μhτh ≈ 10-7 cm2/V). The results demonstrate that photocurrent mapping can provide quantitative insight into intrinsic carrier transport properties of semiconductor nanostructures.

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Time-resolved photoluminescence (PL) and micro-PL imaging were used to study single CdS nanowires at 10 K. The low-temperature PL of all CdS nanowires exhibit spectral features near energies associated with free and bound exciton transitions, with the transition energies and emission intensities varying along the length of the nanowire.In addition, several nanowires show spatially localized PL at lower energies which are associated with morphological irregularities in the nanowires. Time-resolved PL measurements indicate that exciton recombination in all CdS nanowires is dominated by non-radiative recombination at the surface of the nanowires.

Temperature dependent photoluminescence (PL) is used to study the electronic properties of single CdS nanowires. At low temperatures, both near-band edge (NBE) photoluminescence (PL) and spatially-localized defect-related PL are observed in many nanowires. The intensity of the defect states is a sensitive tool to judge the character and structural uniformity of nanowires. As the temperature is raised, the defect states rapidly quench at varying rates leaving the NBE PL which dominates up to room temperature.All PL lines from nanowires follow closely the temperature-dependent band edge, similar to that observed in bulk CdS.