PHOTOLUMINESCENCE FROM RARE EARTH IONS DOPED NANOCRYSTALS

Research and development of nanoscale luminescent and laser materials are part of the rapidly advancing nanoscience and nanotechnology. Besides well-known quantum dots, lanthanide-doped oxide nanocrystals form a new promising class of nanophosphors in view of various application because of unique spectroscopic properties related to two types of electronic transitions: 4 f — 4 f and 4 f — 5d transitions. The subjects of this review are the synthesis of rare earth ions doped nanocrystals and their spectral properties. We begin this review with some introductory comments on the nature of spectroscopy of rare earth ions, and then discuss an important issue that also needs to be addressed is whether the rare earth ions are really being incorporated in the host lattice in the synthesis of rare earth ions doped nanocrystals. A crucial test for the incorporation is an excitation spectrum of the rare earth emission. Citing examples suggest that some of the difficulties encountered in nanodoping are often attributed to "self-purification". Finally, we will discuss the effect of nanostructure on the optical properties of rare earth ions. Specifically, the various mechanisms of spectral properties of rare earth ions from in bulk to in nanocrystals are detailed. The luminescence properties of rare earth doped nanocrystals differ from corresponding bulk materials in several ways: (i) spectroscopic changes, as a broadening and shift(related to the 5d energy level change) of the luminescence bands in nanocrystals, resulting from a structural disorder intrinsic to the small size; (ii) a lower luminescence quantum yield related to the presence of chemical species adsorbed at the surface, especially the OH groups; (iii) a shift towards higher values of the optimum doping concentration, due to the alteration of energy transfers rate from luminescence centers to quenching centers; (iv) new luminescent peak occurs due to the new site occupation of rare earth ion on the surface or another phase; (v) the change of photoluminescence decay time due to surface-bound centers. All the results are emphasis on the photoluminescence from the rare earth-doped oxide nanocrystals in the last 15 years.