Zhang Xiao-Song

Effects of Annealing Temperature on Structural and Optical Properties of ZnO Thin Films

The effects of annealing temperature on the structural and optical properties of ZnO films grown on Si (100) substrates by sol-gel spin-coating are investigated. The structural and optical properties are characterized by x-ray diffraction, scanning electron microscopy and photoluminescence spectra. X-ray diffraction analysis shows the crystal quality of ZnO films becomes better after annealing at high temperature. The grain size increases with the temperature increasing. It is found that the tensile stress in the plane of ZnO films first increases and then decreases with the annealing temperature increasing, reaching the maximum value of 1.8 GPa at 700°C. PL spectra of ZnO films annealed at various temperatures consists of a near band edge emission around 380 nm and visible emissions due to the electronic defects, which are related to deep level emissions, such as oxide antisite (OZn), interstitial oxygen (Oi), interstitial zinc (Zni) and zinc vacancy (V−Zn), which are generated during annealing process. The evolution of defects is analyzed by PL spectra based on the energy of the electronic transitions.

Structural and Magnetic Properties of Fe-Doped Anatase TiO2 Films Annealed in Vacuum

Structural and magnetic properties of Fe-doped anatase TiO2 films fabricated by sol-gel spin coating are investigated. X-ray diffraction measurements reveal that Fe ions are incorporated into the TiO2 lattice. No ferromagnetism-related secondary phases and magnetic nanoparticles are observed in the films. The presence of electron paramagnetic resonance signals at g ~ 2.0 supports oxygen vacancies and/or defects generated in the films after annealing in vacuum. Magnetic measurements indicate that Fe-doped anatase TiO2 films are ferromagnetic at room temperature. These observations suggest that oxygen vacancies and/or defects are energetically favorable for the long range Fe3+-Fe3+ ferromagnetic coupling in Fe-doped anatase TiO2 films.

Energy transfer mechanism of GdPO4: RE3+ (RE = Tb, Tm) under VUV-UV excitation

Polycrystal of GdPO4:RE3+ (RE=Tb, Tm) phosphors were prepared by solid-state method. Vacuum ultraviolet excitation and emission spectrum and the energy transfer mechanism between the host and dopants of Tb3+ and Tm3+ were investigated respectively. The emission of Gd3+ at 313 nm was enhanced by the strong absorption of CTS of Tm3+ at 180 nm in GdPO4: Tm. It has also been concluded that the excitation of Gd3+ is transferred to Tb3+ and then emission peaks of 5DJ→7FJ of Tb3+ were observed.

White Emitting ZnS Nanocrystals: Synthesis and Spectrum Characterization

Spherical organic-bonded ZnS nanocrystals with 4.0?0.2 nm in diameter are synthesized by a liquid-solid-solution method. The photoluminescence spectrum of sample ([S2?]/[Zn2+] = 1.0) shows a strong white emission with a peak at 490 nm and ~ 170 nm full widths at half maximum. By Gauss fitting, the white emission is attributed to the overlap of a blue emission and a green-yellow emission, originating from electronic transitions from internal S2? vacancies level to valence band and to the internal Zn2+ vacancy level, respectively. After sealingZnS nanocrystals onto InGaN chips, the device shows CIE coordinates of (0.29,0.30), which indicates their potential applications for white light emitting diodes.

Fabrication and temperature-dependent photoluminescence spectra of Zn—Cu—In—S quaternary nanocrystals

A series of Zn—Cu—In—S nanocrystals (ZCIS NCs) are prepared and the optical properties of the ZCIS NCs are tuned by adjusting the reaction time. It is interesting to observe that the temperature-dependent photoluminescence (PL) spectra of the ZCIS NCs show a redshift with decreasing intensity at low temperature (50–280 K) and a blueshift at high temperature (318–403 K). The blueshift can be explained by the thermally active phonon-assisted tunneling from the excited states of the low-energy emission band to the excited states of the high-energy emission band.

Infrared Luminescent Properties of a Pr-Doped KBr Submicron Rod

KBr:Pr with a submicron rod structure is successfully synthesized by a solid-state reaction using absolute alcohol as the abrasive. X-ray diffraction, scanning electron microscopy, photoluminescence spectra and fluorescence decay curves are used to characterize the resulting materials. The influences of Pr3+ dopant concentration on the luminescence and lifetime are discussed. Furthermore, luminescent measurements show that KBr:Pr has a high emission intensity compared with other Pr-doped matrixes, which is related to the low phonon energy of KBr. The results suggest that the phonon energy of the host is important in determining the luminescent efficiency.

Influence of Al3+doping on the energy levels and thermal property of the 3.5MgO·0.5MgF2·GeO2:Mn4+ red-emitting phosphor*

A series of Al3+ -doped 3.5MgO0.5MgF2GeO2:Mn4+ red-emitting phosphors is synthesized by high temperature solid-state reaction. The broad excitation band at 300 nm–380 nm, resulting from the 4A2 → 4T1 transition of Mn4+, exhibits a blue shift with the increase of Al2O3 content. The observation of the decreased Mn4+O2 − distance is explained by the crystal field theory. The temperature-dependent photoluminescence spectra with various amounts of Al2O3 content are comparatively measured and the calculation shows that the activation energy increases up to 0.41 eV at the Al2O3 content of 0.1 mol. The maximum phonon densities of state for these samples are calculated from Raman spectra and they are correlated with the thermal properties.

The influence of cation additives on the NIR luminescence intensity of Er3+-doped borate glasses

Er3+-doped 25BaO-(25-x)SiO2-xAl2O3-25B2O3 transparent glasses are prepared with x = 0, 12.5 and 25 by a solid-state reaction. The Er-related NIR luminescence intensity, which corresponds to the transition of 4I15/2-4I13/2, is obviously altered with different silicon/aluminum ratios. The Judd-Ofelt parameters of the Er3+ ions are adopted to explain the intensity change in the NIR fluorescence, and the Raman scattering intensity versus the amount of Al and/or Si components are discussed. The spectra of the three samples are quite similar in the peak positions, but different in intensity. The maximal phonon density of state for the samples is calculated from the Raman spectra and is correlated to the NIR luminescence efficiency.

Thermal Behaviour for InGaAsP/InP Multi-Quantum-Well Superluminescent Diodes

Using a two-dimensional thermal flow model, we calculate the thermal resistance and the temperature distribution of InGaAsP/InP multi-quantum-well superluminescent diodes. The influence of lateral chip size and composition are evaluated. The results reveal that when the injection power reaches 1 W, temperatures in the active region rises up to almost 50 K. The width and length of the chip also have strong influence on the thermal resistance that can reach two orders of magnitude. The thermal resistance will change from 290 K/W to 68 K/W when the chip width increases from 500 μm to 2500 μm, and a similar result exists for the length. There is small effect on thermal resistance for active width. In view of the characteristics of output power versus the input current under pulsed and continues currents, the fitted experimental thermal resistance matches well with the measured results.