Effect of excitation intensity on fluorescence spectra in ZnO nanostructures and its origin

ZnO nanostructures with three kinds of morphologies, namely, tetrapod-, rod-, and sheet-like shape, are synthesized by chemical vapor deposition, conventional solution-phase, and hydrothermal methods, respectively. The fluorescence measurements display that the spectra of these nanostructures exhibit similar unexpected change laws with the altering excitation intensity. It is observed that when the excitation intensity increases, for the green emission band, the peak position shows a small blue-shift, the width turns broader, and the intensity grows first stronger and then weaker; for the UV emission band, the peak position exhibits a significant red-shift, and the width and intensity have the similar behaviors with those of the green band. Additionally, the relative intensity of green emission to UV emission decreases gradually. It is clarified that the origin of this abnormal phenomenon is ascribed to the local laser heating effect and the high sensitivity of nanostructures to this heating effect. Supported by the National Basic Research Program of China (Grant No. 2005CB623603)