Bryan L. Bennett

Crystal growth and optical characterization of cerium-doped Lu1.8Y0.2SiO5

Czochralski growth of cerium-doped Lu1.8Y0.2SiO5 (LYSO) from a 90/10 solution of Lu2SiO5 (LSO) and Y2SiO5 (YSO) is demonstrated. The alloyed scintillator retains the favorable growth properties of YSO and the desirable physical and optical scintillator properties of LSO. Radioluminescence, thermally stimulated luminescence, optical absorption, and lifetime measurements confirm the equivalence of LYSO and LSO optical properties. Advantages of LYSO Czochralski growth relative to LSO include reduced melting point, less propensity for formation of crystalline inclusions, lower cost of starting material, and easier incorporation of cerium into the host lattice. This material offers an attractive alternative to LSO for scintillator applications.

Y2O3:Bi nanophosphor: Solution combustion synthesis, structure, and luminescence

Photoluminescence (PL), radioluminescence (RL), and thermoluminescence (TL) investigation of Y2O3:Bi nanophosphors prepared by solution combustion synthesis using urea, glycine, and hexamethylenetetramine (HMT) as fuels was carried out. The as-prepared nanopowders have increasing crystallinity and average crystallite sizes for urea, glycine, and HMT, respectively. Luminescence is composed of two emission bands centered at 408 and 505 nm due to two nonequivalent Bi3+ sites with symmetry S6 and C2, respectively. The occupancy of these sites depends on the synthesis conditions, in agreement with theoretical predictions. Annealing at 1000 °C for 1 h improves PL and RL efficiency due to enhanced crystallinity of the nanopowders and activation of recombination centers (Bi3+ ions). No shift in the PL peak position was observed as a function of average crystallite size. The concentration quenching was experimentally determined to have a maximum emission of around 3 mol % of the dopant. TL spectra present several ...

Luminescent properties and reduced dimensional behavior of hydrothermally prepared Y2SiO5:Ce nanophosphors

Hydrothermally prepared nanophosphor Y2SiO5:Ce crystallizes in the P21∕c structure, rather than the B2∕b structure observed in bulk material. Relative to bulk powder, nanophosphors of particle size ∼25–100nm diameter exhibit redshifts of the photoluminescence excitation and emission spectra, reduced self absorption, enhanced light output, and medium-dependent radiative lifetime. Photoluminescence data are consistent with reduced symmetry of the P21∕c structure and are not necessarily related to reduced dimensionality of the nanophosphor. In contrast, medium-dependent lifetime and enhanced light output are attributed to nanoscale behavior. Perturbation of the Ce ion electric field is responsible for the variable lifetime.

Thermally stimulated luminescence from x-irradiated porous silicon

We have measured thermally stimulated luminescence from light-emitting porous silicon that has been x irradiated at room temperature and heated to 400 °C. The glow curve exhibits peaks at 103, 155, 219, and 271 °C, with additional maxima occurring above 400 °C. Each of the peaks emits similar emission spectra characterized by a band with a maximum near 720 nm and 0.39 eV full width at half-maximum. Following x irradiation at room temperature, the sample exhibits well-known photoluminescence, but after heating to 400 °C, the loss of hydrogen renders the sample nonphotoluminescent. However, thermally stimulated luminescence can be repeatedly induced. Observation of thermally stimulated luminescence is unambiguous evidence for the existence of an insulating surface layer on porous silicon.

SiOx luminescence from light‐emitting porous silicon: Support for the quantum confinement/luminescence center model

Measurements of hydrogen loss and luminescence as a function of annealing temperature in porous silicon suggest that luminescence is attributable to electron‐hole recombination in SiOx surface layers with an intensity that is dependent upon the surface hydrogen content. The luminescence is composed of three Gaussian bands similar to those found in amorphous SiO2. X‐ray photoelectron spectroscopy and scanning electron microscopy show porous silicon has SiOx on the surface, which is comprised of many particles of about 10 nm size. Collectively, the data strongly support the previously proposed quantum confinement/luminescence center model.

Thermally stimulated luminescence from commonly occuring impurity phases in high-temperature superconductors

Abstract The insulating materials Y2O3, Y2BaCuO5, BaCO3, Ba3CuO4, and BaCuO2, which frequently occur as secondary phases in high temperature superconductors (HTS), have been investigated using thermally stimulated luminescence (TSL). When irradiated with X- or γ-rays at room temperature and subsequently heated to 400°C they produce TSL whose glow-peak maxima occur between 115 and 280°C. Analyses of the TSL glow curves reveal that the trapped charges require energies from 0.7 to 1.2 eV for their release from metastable states following mixed-order kinetics ranging from 1.3 to 3 with frequency factors varying from 107 to 1014 s-1. TSL in these nominally pure, single-phase insulating materials are attributed to impurities and radiation-induced ionic radicals.

Intrinsic trapping sites in rare-earth and yttrium oxyorthosilicates

Similarity among the thermally stimulated luminescence glow curves of undoped Lu2SiO5 and Ce3+-doped oxyorthosilicates possessing the monoclinic C2/c structure strongly suggests the luminescence traps are intrinsic in origin. They are most likely associated with the configuration of oxygen ions in the vicinity of not only the Ce3+ ion, as suggested in previous work, but also the host lanthanide ion. The optical absorption spectrum of pristine Lu2SiO5 shows the presence of intrinsic absorption centers that are enhanced upon x irradiation as seen in other oxides containing oxygen related point defects.

Multifunction Gd2O3:Eu nanocrystals produced by solution combustion synthesis: Structural, luminescent, and magnetic characterization

The synthesis, structural, luminescent, and magnetic properties of multifunction 8 at. % Eu-doped Gd2O3 nanocrystals were investigated. The material was synthesized by the solution combustion technique and characterized by x-ray diffraction (XRD), transmission electron microscopy, and photoluminescence and magnetization measurements. The as-prepared material presents predominant base-centered monoclinic structure with average crystallite size of 35 nm. Isothermal annealing at 1000 °C for up to 152 h induced gradual structural transition toward the body-centered cubic structure. Debye–Scherrer analysis of XRD results showed that annealing did not induce grain growth of monoclinic nanocrystals, while cubic nanocrystals reached 47 nm after annealing for 152 h. The luminescent behavior was monitored as a function of annealing time and related to the structural transformation. In particular, an inversion of the predominant photoluminescence emission line from 621.4 to 609.5 nm was observed and related to chang...

Science and Application of Oxyorthosilicate Nanophosphors

Nanophosphor Y2SiO5:Ce (n-YSO), Lu2SiO5:Ce (n-LSO), and Gd2SiO5:Ce (n-GSO) were prepared by solution-combustion synthesis yielding nanophosphor crystallite sizes between 20 nm - 80 nm. Ce dopant concentrations were varied between 0.1%-10% for each the nanophosphors and concentration quenching curves were measured by radioluminescence (RL) and photoluminescence (PL). n-YSO exhibits concentration quenching at 1 at% and 4 at% under UV and X-ray excitation, respectively. Red shifted emission with a larger Stokes shift is observed for nanophosphors as compared to bulk crystals. The measured PL lifetime depended on the refractive index of the media, indicating that the PL originates from the surface. Measurements of the RL/PL intensity indicate that the light output of these materials is comparable to the bulk crystal.

Quantum confinement contribution to porous silicon photoluminescence spectra

Photoluminescence (PL) spectra of freshly etched and ambient-aged porous silicon specimens have been measured and analyzed to extract spectral contributions due to quantum confinement (QC) and non-QC effects. It is shown that all spectra can be deconvolved into five Gaussian bands with typical peak energies 1.59, 1.76, 1.84, 1.94, and 2.07 eV. Ambient aging induces ∼0.1 eV blueshift in each of the three highest energy peaks, which is attributed to QC effects. In contrast, the two lowest energy peaks remain unshifted as expected for non-QC effects. Because size of the porous silicon nanoparticles is deduced from the magnitude of blueshift, it is imperative to correctly identify the spectral components associated solely with QC. The three closely spaced Gaussian bands are summed and the resulting single Gaussian band is analyzed with the model of Islam and Kumar [J. Appl. Phys. 93, 1753 (2003)]. Results show that peak energy of the freshly etched sample is 1.86 eV with average nanoparticle size L0=3.90 nm a...