Investigation of Local Structures Around Luminescent Centers in Doped Nanocrystal Phosphors
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ABSTRACT Extended x-ray absorption fine structure (EXAFS) technique has been employed to investigate the local structures around luminescent centers in nanocrystals of Mn-doped ZnS and Tb-doped Y 20 3. Size-dependent local structural changes around Mn luminescent centers have been found in Mn-doped nanocrystals of ZnS by using Mn K-edge EXAFS. Local structures around Tb investigated by Tb Li-edge EXAFS also show substantial differences between bulk and nanocrystal samples. This structural information is useful for understanding the novel optical properties of doped nanocrystals. INTRODUCTION Doped semiconductor quantum-confined systems can exhibit novel optical properties such as enhanced luminescence efficiency and short recombination time which were found in Mndoped nanocrystals of ZnS'. Considering the possible enhanced effects of symmetry-breaking, crystal field distortion, size quantization, and hybridization of host states with impurity states due to the inclusion of luminescent centers and the small size of the system, the drastic changes of optical properties in the doped nanocrystals are expected to be closely related to the changes of local structure of the luminescent center in the material. It is thus of great interest to investigate the local structures around luminescent centers in the doped nanocrystal phosphors with different
size distributions and compare them with those of conventional bulk phosphors. Since the luminescent centers doped in these nanocrystals lack long-range structural order, conventional structure-determining methods such as x-ray diffraction are not useful for this purpose. However, the EXAFS technique which requires only short-range order is uniquely suited for this task. In this work, we studied the local structural changes around Mn luminescent centers in Mn-doped nanocrystals of ZnS using Mn K-edge EXAFS. 2 Also, in view of the large differences in the emission and excitation spectra between bulk and quantum-confined phosphors in other doped nanocrystal systems such as Y20 3:Tb, Y20 3:Eu, ZnS:Tb, and ZnS:Eu etc., it is of interest to study the local structural variations around luminescent centers in these systems. For the time being, with more experimental work in progress, primary results of Tb-doped nanocrystals of Y 2 0 3 measured from well prepared samples are available and will be presented in this report. EXPERIMENT Samples of nanocrystals of Mn-doped ZnS and Tb-doped Y20 3 were prepared by organometallic chemical processes. .3 In addition, for comparison, a bulk ZnS:Mn powder sample was made by thermal diffusion of Mn at temperatures > 11000 C for several hours and a bulk polycrystalline Y20 3:Tb fiber was made by the method of laser annealing. Two ZnS:Mn 283 Mat. Res. Soc. Symp. Proc. Vol. 405 0 1996 Materials Research Society
nanocrystal materials (Samples A and B with size distributions of 50-55 and 30-35 A, respectively) and two nanocrystal Y20 3 :Tb samples (#222 and #243 with size distribution of 4060 A) were studied in the present EXAFS experiments. Mn K-edg
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