Surface crystallization effects on the optical and electric properties of CdS nanorods.

We synthesized CdS nanorods through a conventional solvothermal method and studied their photoluminescence and electric transport properties before and after annealing. High-resolution transmission electron microscopy indicated that the surface layer of the annealed CdS nanorods was well crystallized, while that of the unannealed nanorods was amorphous. Energy-dispersive x-ray spectrography, x-ray photoelectron spectrography and thermogravimetric analysis were used to demonstrate that the amorphous layer at the surface of the as-prepared nanorods was pure CdS. The photoluminescence spectra showed that after annealing the intensity of the band-edge emission increased several times and the surface state emission at 548 nm disappeared. The unannealed CdS nanorods had approximately linear I-V characteristics and the conductance suddenly increased about 100 times upon visible light illumination by a halogen lamp. The annealed CdS nanorods exhibited nonlinear conductance with a turn-on voltage at about 2.2 V. These properties show that CdS nanorods have potential applications in nanophotoelectric or sensing devices.