Low temperature optical properties of amorphous oxide nanoclusters in polymethyl methacrylate matrix

We studied the temperature-dependent steady-state and time-resolved fluores cence properties of very small (1-2 nm) ZnO, CdO, and PbO amorphous nanoclu sters prepared in AOT reverse micelles and imbedded in polymethyl methacryl ate(PMMA) films. X-ray diffraction and electron diffraction and imaging ind icate that these structures are amorphous. These amorphous oxide nanocluste rs demonstrate similar structural, electronic, and optical properties. Prop erties of steady-state fluorescence spectra indicate the unique localizatio n of electronic states due to the amorphous structure. ZnO and CdO show dou ble-band fluorescence structure, which is due to the spin-orbital splitting , similar to Cu2O. Time-resolved fluorescence studies of the nanoclusters i n the polymer reveal two lifetime components, as found in solution. The slo w component reflects relaxation processes from band-tail states while the f ast component may be related to high-lying extended states. The temperature dependence of fast fluorescence component reveals the presence of exciton hopping between anharmonic wells at temperatures higher than 200K. We corre late the barrier height between two wells formed around local atoms with th e inter-atomic distance and bond ionicity.