Synthesis and resonance Raman spectroscopy of CdS nano-wire arrays

Abstract Highly anisotropic arrays of aligned semiconductor nano-wires with diameters in the range 9–22 nm and lengths up to 1 μm were fabricated by electrodepositing CdS into the pores of anodic aluminum oxide. The value of the exciton energy of the semiconductor nano-wires was determined as a function of wire diameter from the excitation wavelength dependence of their polarized resonance Raman spectra in the vicinity of the CdS absorption edge. The resonance Raman (RR) spectra were dominated by a progression in the longitudinal optical phonon mode. The relative intensities of the members of the progression are both excitation-wavelength and particle-size dependent. The LO phonon frequency was also found to be wire-diameter dependent. The RR spectra were found to be almost independent of the polarization of the light exciting them. The optical exciton energy was determined to vary from 2.36 eV for the largest diameter wires to 2.42 eV for the smallest. The absence of polarization dependence is interpreted to be in part due to the fact that the nano-wires consist of strings of semiconductor crystallites so that the local aspect ratio of the crystallites is both not very great and not uniform in the axial direction. For the smallest nano-wires the radial size uniformity is, however, very high. The dependence of the band-gap energy on the diameter of the nano-wires is in good agreement with theory.