Alfons Schulte

Luminescence properties of europium-doped cerium oxide nanoparticles: role of vacancy and oxidation states.

Enhancing the optical emission of cerium oxide nanoparticles is essential for potential biomedical applications. In the present work, we report a simple chemical precipitation technique to synthesize europium-doped cerium oxide nanostructures to enhance the emission properties. Structural and optical properties showed an acute dependence on the concentration of oxygen ion vacancy and trivalent cerium, which, in turn, could be modified by dopant concentration and the annealing temperature. Results from X-ray photoelectron spectroscopy showed an increase in tetravalent cerium concentration to 85% on annealing at 900 degrees C. The concentration of oxygen ion vacancy increased from 1.7x10(20) cm(-3) to 4.1x10(20) cm(-3) with the increase in dopant concentration. Maximum emission at room temperature was obtained for 15 mol % Eu-doped ceria, which improved with annealing temperature. The role of oxygen ion vacancies and trivalent cerium in modifying the emission properties is discussed.

Direct femtosecond laser writing of waveguides in As2S3 thin films

Single-channel waveguides and Y couplers were fabricated in chalcogenide thin films by use of femtosecond laser pulses from a 25-MHz repetition rate Ti:sapphire laser. Refractive-index differentials (delta n > 10(-2)) were measured through interferometric microscopy and are higher than the typical values reported for oxide glasses. The dependence of the index differential on the peak intensity reveals the nonlinear nature of the photosensitivity in arsenic trisulfide below its bandgap energy, and the refractive-index change is correlated to the photoinduced structural changes inferred by Raman spectroscopy data. A free-electron model to predict the parametric dependence of delta n is proposed.

Role of trivalent La and Nd dopants in lattice distortion and oxygen vacancy generation in cerium oxide nanoparticles

Nanocrystalline Ce1−xRExO2−y (RE=La and Nd) powders were synthesized with a controlled size in the range of 3–5nm using microemulsion technique. The nanoceria retains its cubic fluorite structures for the complete doping range. X-ray diffraction and Raman spectroscopic studies showed that the lattice distortion increased with the doping amount and was found to be higher for La-doped samples compared to the Nd doping. Spatial correlation model used for Raman analysis suggested increased defect concentration for the doped samples compared to nanoceria.

Tellurite glasses with peak absolute Raman gain coefficients up to 30 times that of fused silica.

An experimental system has been assembled to measure the absolute values of the Raman gain spectrum for millimeter-thick glass samples. Results are reported for two new oxide glasses with Raman gain coefficients as much as 30 times larger than that of fused silica and more than twice its spectral coverage.

Dopant-mediated oxygen vacancy tuning in ceria nanoparticles

Ceria nanoparticles with 20 and 40 at.% RE (RE = Y, Sm, Gd, and Yb) dopants were synthesized through a microemulsion method. Independently of the dopant nature and concentration, nearly monodispersed nanoparticles of size 3-5 nm were observed in high resolution transmission electron microscopic analysis. The ceria lattice either expands or contracts depending on the dopant cation ionic radii, as indicated by x-ray diffraction studies. X-ray photoelectron and Raman spectroscopic studies were used to quantify the cerium oxidation state and oxygen vacancy concentration. The results show the tunability of the oxygen vacancy and Ce(3+) concentrations based on the dopant properties. First principles simulations using the free energy density functional theory method support the observed experimental trends. The reported results establish a relationship between the oxygen vacancies and oxidation states in doped ceria required for tailoring properties in catalytic and biomedical applications.

Comparative study of CdS thin films deposited by single, continuous, and multiple dip chemical processes

We have used Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD), Raman, and photoconductivity to characterize CdS thin films grown by single, continuous, and multiple dip chemical processes. XRD has further shown, without ambiguity, that grown CdS films, independent of the process, in an almost homogeneous reaction free basic aqueous bath have a zincblende crystal structure where reflections from (111), (200), (220), and (311) planes are clearly identified. RBS, Raman, and photoconductivity confirm the high stoichiometry and excellent structural properties with low optically active trap state density of single and continuous dip CdS films. However, they collectively suggest that multiple dip CdS films suffer from defects that act as carrier traps and lead to prolong photoconductivity decay in these films.

Role of S/Se ratio in chemical bonding of As-S-Se glasses investigated by raman, x-ray photoelectron, and extended x-ray absorption fine structure spectroscopies

Chalcogenide glasses have attracted considerable attention and found various applications due to their infrared transparency and other optical properties. The As–S–Se chalcogenide glass, with its large glass-formation domain and favorable nonlinear property, is a promising candidate system for tailoring important optical properties through modification of glass composition. In this context, a systematic study on ternary As–S–Se glass, chalcogen-rich versus well-studied stochiometric compositions, has been carried out using three different techniques: Raman spectroscopy, x-ray photoelectron spectroscopy, and extended x-ray absorption fine structure spectroscopy. These complementary techniques lead to a consistent understanding of the role of S∕Se ratio in chalcogen-rich As–S–Se glasses, as compared to stochiometric composition, and to provide insight into the structural units (such as the mixed pyramidal units) and evidence for the existence of homopolar bonds (such as Se–Se, S–S, and Se–S), which are the ...

Electron Beam and Optical Depth Profiling of Quasibulk GaN

Electron beam and optical depth profiling of thick (5.5–64 μm) quasibulk n-type GaN samples, grown by hydride vapor-phase epitaxy, were carried out using electron beam induced current (EBIC), microphotoluminescence (PL), and transmission electron microscopy (TEM). The minority carrier diffusion length, L, was found to increase linearly from 0.25 μm, at a distance of about 5 μm from the GaN/sapphire interface, to 0.63 μm at the GaN surface, for a 36-μm-thick sample. The increase in L was accompanied by a corresponding increase in PL band-to-band radiative transition intensity as a function of distance from the GaN/sapphire interface. We attribute the latter changes in PL intensity and minority carrier diffusion length to a reduced carrier mobility and lifetime at the interface, due to scattering at threading dislocations. The results of EBIC and PL measurements are in good agreement with the values for dislocation density obtained using TEM.

Investigation of chemical bath deposition of ZnO thin films using six different complexing agents

Chemical bath deposition of ZnO thin films using six different complexing agents, namely ammonia, hydrazine, ethanolamine, methylamine, triethanolamine and dimethylamine, is investigated. As-grown films were mainly ZnO2 with a band gap around 4.3?eV. Films annealed at 400??C were identified as ZnO with a band gap around 3.3?eV. X-ray diffraction and micro-Raman spectroscopy revealed that as-grown films consist mainly of cubic zinc peroxide that was transformed into hexagonal ZnO after annealing. Rutherford backscattering spectroscopy (RBS) detected excess oxygen content in ZnO films after annealing. Fourier transform infrared spectroscopy of as-grown films showed a broad absorption band around 3300?cm?1 suggesting that the as-grown films may consist of a mixture of zinc peroxide and zinc hydroxide. X-ray photoelectron spectroscopy multiplex spectra of the O 1s peak were found to be consistent with film stoichiometry revealed by RBS. High-resolution transmission electron micrographs showed small variations of the order of 10?nm in film thickness which corresponds to the average grain size. A carrier density as high as 2.24?1019?cm?3 and a resistivity as low as 6.48 ? 10?1???cm were obtained for films annealed at 500??C in argon ambient.

Defects and symmetry influence on visible emission of Eu doped nanoceria

Europium doped cerium oxide particles of 10nm were synthesized by room temperature chemical precipitation technique and annealed at 500 and 900°C to study its effect on luminescence. X-ray photoelectron spectroscopic result shows an increase in Ce3+ concentration from 20% to 23% on Eu doping but decreases to 8% on annealing. Raman studies show a progressive blueshift from 461to464cm−1 due to local symmetry ordering with temperature. Emission intensity varies with the wavelength of excitation and observed transitions indicate the presence of Eu3+ in different symmetry environments.