Synthesis and optical properties of colloidal CuO nanoparticles

Copper oxide nanoparticles of sizes ranging from 1 to 25 nm were synthesized using a colloid microwave-thermal method, where the average size of CuO nanoparticles can be tailored by controlled microwave treatment time. The particle size was found to significantly decrease as the microwave processing time increases and can be controlled to have narrow size distributions. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the sizes of the prepared nanoparticles. The UV-visible absorption spectra of the nanoparticles are blue-shifted with the size reduction, and this is attributed to quantum-confinement (QC) effect. Furthermore, the photoluminescence spectra showed UV and visible emissions, and were red shifted with increasing particles size and excitation wavelength. While the former observation confirms the QC effect and corroborates the results of UV-visible absorption spectra, the latter one is attributed to selective near band-edge excitonic transitions associated with defect states.