Pronounced structure in x-ray excited luminescence (XEL) has been observed in dilute Tb-doped Y2O3 (Y2O3:Tb) nanocrystals. This effect affords a means to assess different energy transfer mechanisms in the nanocrystals and also an opportunity for novel device applications. Sharp jumps and oscillations are found in the XEL output with the incident x-ray energy around the absorption edges of Y and Tb. When compared with a bulk Y2O3:Tb sample, these effects are attributed to some unique electronic and optical properties of doped nanocrystals related to quantum confinement of charge carriers, and the main features can be explained by a proposed model of multichannel energy transfer. Extended x-ray absorption fine structure techniques have also been employed to study the effect of size variation and chemical doping on the local structures in Y2O3 and Y2O3:Tb nanocrystals. The local environment surrounding Y and Tb in the nanocrystals is compared with that in the respective bulk material. The results indicate that Tb impurity atoms substitute for Y sites in bulk Y2O3, while doping in the nanocrystals is complicated by the large fraction of surface atoms and local disorder.

We report for the first time how the optical transitions associated with localized levels of a rareearth impurity can be modulated by quantum confinement. The effect of quantum confinement on impurity critically depends on the size of the host crystal. The variation of absorption and luminescence efficiency of Tb 3+ -ion doped Y 2 O 3 is discussed within the framework of the quantum confined atoms.Introduction. The localized states are useful centers in semiconductors and insulators, since they yield a well-defined`characteristic' optical signature. Usually such emission characteristics are independent of the host in which these dopants have been incorporated. For example, the localized centers such as rare earth and transition metal impurities are the best line emitting phosphors for display and lighting industry. The biggest drawback of these localized states is that the transfer of carriers from the host to the center is rather slow. This slow capture process thus limits the luminescence efficiency provided by these localized states.These localized states are similar to deep states in many respects. For example they possess the following characteristics.1. The energy levels associated with the localized levels are typically much deeper than the normal effective mass energy.2. The thermalization rates from these states are small. 3. The localized states similar to deep states can dominate the recombination kinetics, yet they do not play a significant role in modulating the conductivity of the semiconductor.4. In semiconductors or insulators the localized states act more like deep states but frequently are neutral centers. The charge neutrality has been exploited to yield excellent phosphors.In this paper, we report for the first time how the optical transitions associated with localized levels of a rare-earth impurity can be modulated by quantum confinement. The effect of the quantum confinement on impurity critically depends on the size of the host crystal. As the size of the host decreases, the degree of the confinement and its effect increase. This type of quantum confinement allows us to change the properties of a localized impurity. In future using similar size control, we hope to modulate the known deep states in semiconductors. Before we discuss the recent results, we want to describe the evolutionary background work of modulating the luminescence properties of the localized or deep states and which eventually led to the model of quantum confined atoms.

Local environment surrounding Y atoms in Y2O3:Tb nanocrystals under various heat treatment conditions has been investigated by using the extended x-ray absorption fine structure (EXAFS) technique. X-ray excited luminescence (XEL) with the incident x-ray energy near Y K edge and Tb L edges has also been measured to investigate the mechanisms of x-ray-to-visible down conversion in these doped nanoparticles. The observed changes in EXAFS, XEL, and photoluminescent data can be explained on the basis of increased average size of the nanoparticles as confirmed by transmission electron microscopy studies. Our results thus demonstrate that the doped nanoparticles can agglomerate to a controllable degree by varying the heat treatment temperature. At higher temperatures, the local environment surrounding Y atoms in the nanoparticles is found to become similar to that in bulk Y2O3 while the XEL output still shows the characteristics of nanocrystals. These results indicate that appropriate heat treatment can afford an effective means to control the intensity and signal-to-background ratio of green luminescence output of these doped nanocrystal phosphors, potentially useful for some device applications.

Photoluminescence spectra were measured as a function of temperature and excitation intensity in two periodic multiple narrow asymmetric coupled quantum-well structures. The peaks of the spectra have been identified and the temperature-dependent natures of the recombination processes have been determined. The half-widths at half maximum of the line shapes were deduced by fitting the low-energy sides to Gaussian functions. The excellent fits indicate that inhomogeneous broadening is dominant. The rate at which the linewidth broadens with temperature is more dramatic below 140 K due to the sharp changes of the ionized impurity and of the longitudinal-optical-phonon scattering mechanisms. Above 140 K, the linewidth has a linear dependence on temperature due to the thermal excitation of the longitudinal optical phonons. By using the model proposed by Lee et al. we have determined the scattering rates of the longitudinal acoustic and optical phonons, of the impurities, and of the inhomogeneous broadening. Our results show that unless the growth is interrupted at each interface, the inhomogeneous broadening remains dominant up to room temperature. Measurement of the temperature dependence of the photoluminescence linewidth proves to be a simple and effective technique to investigate the scattering mechanisms of the photogenerated carriers and the origins of the emission peak s linewidth broadening in these structures.

Articles you may be interested inRadiative and nonradiative recombination in an ultraviolet GaN/AlGaN multiple-quantum-well laser diodeWe have investigated the dependence of the continuous-wave photoluminescence on the laser intensity, which is affected by the trap saturation and the dominant nonradiative recombination in multiple intrinsic quantum wells. Ifthe laser intensity is relatively low, the photoluminescence is proportional to the 1.5 power of the laser intensity. As the laser intensity increases, however, and above a critical intensity, the photoluminescence increases at a rate that is higher than square-law and finally approaches this dependence. This is the result of trap saturation. In order to examine the trap saturation in a more sensitive scale, we have calculated the dependence of the effective nonradiative decay time on the laser intensity. In addition, the laser-intensity dependences of the trapping efficiency and of the ratio of the electron and hole nonradiative decay times, have been determined for different ratios of the trapping and nonradiative recombination rates. The laser-intensity dependences of the electron and hole nonradiative decay times have also been determined.