Photoluminescence Reveals Properties of Single CdS Nanowires

Lauren I. Rugani

Although advancements in CdS nanowire synthesis and device fabrication have led to such optically driven applications as lasers, photodetectors and waveguides, there is little information on the fundamental properties of the nanowires and their morphological dependence.

Researchers from the University of Cincinnati, from Miami University in Oxford, Ohio, and from Northwestern University in Evanston, Ill., have combined low-temperature microphotoluminescence imaging and time-resolved photoluminescence to provide information on the electronic states and recombination modes of single CdS nanowires.

The time-resolved photoluminescence spectrum from a uniform wire (a) shows a single broad peak and the corresponding time decay (c) taken at the near-band-edge emission (NBE) energy, 2.525 eV. Similarly, a nonuniform wire exhibits a peak at 2.525 eV, along with peaks at 2.486 and 2.472 eV (b), with time decays illustrating the longer recombination lifetimes at lower energies (d). Reprinted with permission from Applied Physics Letters.

The nanowires, with diameters ranging from 50 to 200 nm and lengths of 10 to 15 μm, were excited with 432-nm, 200-fs pulses from a Coherent Ti:sapphire laser for time-resolved photoluminescence measurements. A Hamamatsu microchannel photomultiplier tube and time-correlated single-photon counting were used to detect photoluminescence with 80-ps resolution. Slit-confocal microscopy then was used to obtain spatially resolved photoluminescence by exciting the nanowires with a 458-nm beam from a Spectra-Physics CW Ar⁺ laser. A liquid-nitrogen-cooled CCD detector from Jobin Yvon provided information about each nanowire with 70-μeV spectral resolution and 1.2-μm spatial resolution.

Direct optical imaging revealed both a spectral band emitting at energies associated with free and bound exciton emissions and lower-energy spatially localized states that the researchers associated with morphological aberrations in the nanowire. The team performed time-resolved photoluminescence and microphotoluminescence measurements on 10 nanowires that demonstrated a variety of behaviors.

Varies with position

One largely uniform and straight wire showed a single broad spectral peak at 2.525 eV, which then decayed rapidly. A two-dimensional photoluminescence image of the wire showed no evidence of localized states, signifying the dominance of the near-band emission peak on its spectra. The intensity of the photoluminescence emission varied at different positions along the length of the wire, and the peak position varied by nearly 10 meV.

The researchers then tested a nonuniform nanowire that had many kinks, bends and lobes. A time-resolved photoluminescence measurement revealed the same short-lived emission at 2.525 eV. However, unlike the uniform wire, the nonuniform nanowire exhibited a series of sharp peaks with lower energies and longer time decays. These secondary peaks were found to be associated with particular positions along the wire.

Low-temperature and time-resolved photoluminescence measurements have proved useful for providing information regarding the electrical and morphological properties of single CdS nanowires. The scientists believe that passivating the surface states of the nanowires could increase the quantum efficiency by at least an order of magnitude, and that the technique will be useful in identifying high-quality single nanowires for further examination.

Applied Physics Letters, published online Aug. 3, 2006.