S.Y. Wu

Mechanism of light emission and electronic properties of a Eu3+-doped Bi2SrTa2O9 system determined by coupled X-ray absorption and emission spectroscopy

The origin of light emission from newly discovered orange-red UV light emitting diodes, and their electronic properties are critical issues yet to be understood. In this study, X-ray absorption spectroscopy (XAS) and emission spectroscopy (XES) are utilized to examine the electronic structure of the Eu3+-doped Bi2SrTa2O9 system. While no significant change in the electronic structure is observed around the Bi and Ta sites, variation around the Eu and Sr atoms is observed, along with even more significant changes in the O 2p states in the conduction band. Upon UV irradiation, Eu-induced states within the conduction band are observed and found to shift to the conduction band minimum upon substitution of Sr with Eu. This phenomenon is the result of the creation by Eu of an excitable state and the fact that Eu is more electronegative than Sr, such that the substitution lowers the Eu 4f5d–O 2p hybridized states. Consequently, the substitution reduces the energy of electron recombination between the valence and conduction bands, which is consistent with the red shift in the photoluminescence spectra. The presence of the newly formed hole states distributed over the O 2p states in the conduction band is strongly correlated with the emission intensity. The results and analyses demonstrate that Eu can be introduced to tailor the Eu–O hybridized states within the conduction band and change the route of recombination, suggesting that Eu is critically involved in light emission in these UV-induced orange-red emitting LED materials.

Tuning the dimensionality of ZnO nanowires through thermal treatment: An investigation of growth mechanism

In this study, we synthesized various dimensionalities of ZnO nanowires using the Ti grid-assisted chemical vapor deposition process. Energy dispersive X-ray spectroscopic mapping technique accompanied with a lattice diffusion model was used to characterize the growth mechanism. A diffusion ratio γ, defined by short-circuit and lattice diffusion activation energies, was obtained to describe the growth mechanism of ZnO nanowires. The tunable dimensionalities of ZnO nanowires allow us to modify the morphology of ZnO nanocrystals by developing well-controlled potential applications.

Low-temperature phase separation in GaN nanowires: An in situ x-ray investigation

In this study, we in situ employed low-temperature x-ray diffraction to investigate phase separation in GaN nanowires. Our observations showed that a distinct phase separation developed below 260K, the zinc-blende phase, which is related to short range ordering. The correlation lengths of the zinc-blende phase reached their maximum at 140K but correlation length was still revealed at around 23nm. Our results may be understood using the finite size model and support the conclusion that the phase separation was reversible and occurred through the interaction of the characteristic size of the ordered domain of the GaN nanowires.

Quantum thermal contraction of Au nanoparticles

We report on the successful fabrication of a fine Au powder with a mean particle size of 4 nm. A crossover from a positive thermal expansion at low temperatures to a negative thermal expansion at high temperatures was observed in the fcc cell parameter at around 125 K. Using a discrete electronic energy level scheme for nanoparticles, we introduce a weakly temperature dependent level spacing Δ(T) to show how the potential energy is lowered by lattice expansion at low temperatures and by contraction at high temperatures. Anomalies were also observed in the AC magnetic susceptibility and the heat capacity measurements, which can be interpreted by the same model.