J. L. Lensch

Near-field scanning photocurrent microscopy of a nanowire photodetector

A near-field scanning optical microscope was used to image the photocurrent induced by local illumination along the length of a metal-semiconductor-metal (MSM) photodetector made from an individual CdS nanowire. Nanowire MSM photodetectors exhibited photocurrents ∼105 larger than the dark current (<2pA) 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.

Temperature dependent photoluminescence of single CdS nanowires

Temperature dependent photoluminescence (PL) is used to study the electronic properties of single CdS nanowires. At low temperatures, both near band edge (NBE) 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 the nanowires follow closely the temperature dependent band edge, similar to that observed in bulk CdS.

Local photocurrent mapping as a probe of contact effects and charge carrier transport in semiconductor nanowire devices

Three types of two-terminal CdS nanowire devices with distinct current versus voltage characteristics were fabricated by forming Schottky and/or Ohmic contacts in a controlled manner. Argon ion bombardment of CdS nanowires increased the carrier concentration allowing the formation of Ohmic Ti–CdS contacts. Scanning photocurrent microscopy (SPCM) was used to explore the influence of the contacts on the spatially resolved photoresponse in two-terminal devices and to analyze charge carrier transport processes. Modeling of the spatial profiles of the local photocurrent images enabled the quantitative extraction of electron and hole mobility-lifetime products in Ohmic devices and the hole mobility-lifetime product in Schottky devices. Analysis of the evolution of SPCM images with bias suggests that the electric field is localized to the optical generation region in the Ohmic devices and localized beneath the contacts in the Schottky devices.

Low-temperature photoluminescence imaging and time-resolved spectroscopy of single CdS nanowires

Time-resolved photoluminescence (PL) and micro-PL imaging were used to study single CdS nanowires at 10K. The low-temperature PL of all CdS nanowires exhibits 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 nonradiative recombination at the surface of the nanowires.

Resonant Raman scattering from CdS nanowires

Resonant Raman scattering (RRS) was used to probe the electronic states of CdS nanowires (∼10nm diameter) grown by chemical vapor deposition. Individual Ar+ laser lines were used to vary the excitation energy while observing scattering from CdS phonons; strong 1-longitudinal optical (LO) and 2-LO Raman resonances were readily observed within the broader photoluminescence emission. The energy separation between the peaks of the 1-LO and 2-LO resonance was found to be 34 meV, in good agreement with bulk values. These results demonstrate that RRS is a powerful tool for probing the electronic and vibrational properties of semiconductor nanowires.

Spatially-resolved photoluminescence imaging of CdS and GaAs/AlGaAs nanowires

We use low temperature spatially resolved photoluminescence imaging to study optical properties and electronic states of single CdS and GaAs/AlGaAs core‐shell nanowires.

Low-Temperature Optical Characterization of Single CdS Nanowires

We use spatially resolved micro-PL imaging at low temperature to study optical properties of two sets of CdS nanowires grown using 20 nm and 50 nm catalysts. We find that low temperature PL of single nanowires is an ideal technique to gauge the quality of a given growth run, and moreover enables the collection of detailed spatial information on single wire electronic states.

Raman Spectroscopy as a Probe of Single Semiconductor Nanowires

We report on room temperature resonant Raman scattering measurements on individual CdS nanowires. We show that spatially-resolved Raman spectroscopy on single nanowires can be used as a tool to study structural and compositional uniformity of the nanowire.