Photoluminescence spectra of nitrogen implanted GaSe crystals

GaSe single crystals were N-implanted along c-axis with ion beams of 10¹⁴ and 10¹⁶ ions/cm² doses having energy values of 60 and 100 keV. The photoluminescence (PL) spectra of undoped and N-implanted GaSe crystals were measured at different temperatures. The PL intensity was observed to decrease with increasing implantation dose while the FWHM of the exciton peaks increased. In heavily doped crystals, due to the interaction with the radiation induced disorders, the wave vector selection rules are satisfied and an indirect exciton PL band is observed 36 meV below the direct exciton states.     

Exciton photoluminescence, photoconductivity and absorption in GaSe0.9Te0.1 alloy crystals

The photoluminescence, photoconductivity and absorption in GaSe_0.9Te_0.1 alloy crystals have been investigated as a function of temperature and external electric field. It has been observed that the exciton peaks shift to lower energy in GaSe_0.9Te_0.1 alloy crystals compared to GaSe crystals. The long wavelength tails of interband photoluminescence, photoconductivity and absorption spectra are determined by the free exciton states and show an Urbach-Martienssen-type dependence to the photon energy. The maxima of the extrinsic photoluminescence and photoconductivity spectra were found to be determined by the acceptor centers with an energy of E_A=E_V+0.19 eV formed by the polytypism and defects complexes that include Se and Te anions.     

Growth and spectral properties of Nd:GdVO4 laser crystal

In this paper, the Czochralski growth, absorption spectra, and photoluminescence spectra of Nd:GdVO₄ crystals are studied. From its absorption spectra, Nd:GdVO₄ is found to exhibit an anisotropic optical absorption effect, and its effective Judd-Ofelt parameters are calculated: Ω₂=10.281×10⁻²⁰ cm², Ω₄=5.426×10⁻²⁰ cm² and Ω₆=9.943×10⁻²⁰ cm². By these parameters, the absorption oscillator strengths, emission oscillator strengths, transition probabilities, fluorescence branch ratios, energy lifetimes, and integrated emission cross-sections are also derived. The photoluminescence spectra of Nd:GdVO₄ crystal consist of a wide emission band of host and the characteristic emission bands of Nd³⁺. Based on the excitation spectrum, both the two evident peaks locating at 345 and 371 nm are ascribed to the characteristic excitation of Nd³⁺, and an energy transfer from the host to its doping Nd³⁺ ions is indicated.     

Tuning of photoluminescence in cadmium selenide nanoparticles grown in CTAB based quaternary water-in-oil microemulsions

Cadmium selenide nanoparticles were synthesized in cetyl trimethyl ammonium bromide (CTAB) microemulsion under ambient conditions and were found to be stable for months. The as-grown cadmium selenide nanoparticles exhibit strong room temperature photoluminescence with two emission peaks at 500 and 580 nm. The photo-excitation at wavelength above the exciton absorption peak gives rise to the sole band gap photoluminescence with the maximum at 500 nm whereas, the photo-excitation below the exciton absorption peak, furnishes only the trap state photoluminescence with the maximum at 580 nm. The photo-excitations at wavelengths closer to the exciton absorption peak give rise to both the emission peaks simultaneously. Intensities of these two emission peaks could be clearly tuned by controlling the precursor concentrations and the size of water pool in the microemulsions.     

Preparation and spectroscopic properties of Nb2O5 nanorods

Nb₂O₅ nanorods have been prepared using water/ethanol media. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-visible absorption and photoluminescence spectroscopy. The as-prepared Nb₂O₅ nanorods appeared to be single pseudohexagonal (TT-Nb₂O₅) phase. From the photoluminescence spectrum, two emission bands at 407 and 496 nm, respectively, were observed. The origin of the luminescence was discussed in detail.     

Bleaching in the region of exciton resonance of layered GaSe crystals

Light absorption in the region of exciton resonance of GaSe crystal is studied experimentally at high levels of optical excitation. A picosecond YAG:Nd³⁺ laser emitting 30-ps light pulses and a dye laser with a pulse width of ～3 ns tunable within the range 594–643 nm were used as light sources. It was found that, at high levels of optical excitation, the exciton absorption line of the GaSe crystal disappeared, which was attributed to increasing exciton density with arising mechanisms of their decay: exciton-exciton interactions and screening of excitons by the free charge-carrier plasma. It is shown that these mechanisms are also responsible for the arising new emission band in the long-wavelength region of the photoluminescence spectrum.     

Crystalline and photoluminescence characteristics of YVO4:Sm thin films grown by pulsed laser deposition under oxygen pressure

YVO₄:Sm³⁺ films were deposited on Al₂O₃ (0 0 0 1) substrates at various oxygen pressures changing from 13.3 to 46.6 Pa by using the pulsed laser deposition method. The crystallinity and surface morphology of these films were investigated by means of X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. The XRD pattern confirmed that YVO₄:Sm³⁺ film has zircon structure and the AFM study revealed that the films consist of homogeneous grains ranging from 100 to 400 nm. The room temperature photoluminescence (PL) spectra showed that the emitted radiation was dominated by a reddish-orange emission peak at 602 nm radiating from the transition of (⁴G_5/2→⁶H_7/2). The crystallinity, surface morphology, and photoluminescence spectra of thin-film phosphors were highly dependent on the deposition conditions, in particular, the substrate temperature. The surface roughness and photoluminescence intensity of these films showed similar behavior as a function of oxygen pressure.     

Synthesis and luminescence investigation of RE (Eu, Tb and Ce)-doped lithium silicate (Li2SiO3)

Synthesis and photoluminescence (PL) investigations of lithium metasilicate doped with Eu³⁺, Tb³⁺ and Ce³⁺ were carried out. PL spectra of Eu-doped sample showed peaks corresponding to the ⁵D₀→⁷F_j (j=1, 2, 3 and 4) transitions under ultraviolet excitation. Strong red emission coming from the hypersensitive ⁵D₀→⁷F₂ transition of Eu³⁺ ion suggested the presence of the dopant ion in structurally disordered environment. Tb³⁺-doped silicate sample showed blue-green emission corresponding to the ⁵D₄→⁷F_j (j=6, 5 and 4) transitions. Ce-doped sample under excitation from UV, showed a broad emission band in the region 350-370 nm with shoulders around 410 nm. The fluorescence lifetimes of Eu³⁺ and Tb³⁺ ions were found out to be 790 and 600 μs, respectively. For Ce³⁺, the lifetime was of the order of 45 ns. PL spectra of the europium- and terbium-doped samples were compared with commercial red (Y₂O₃:Eu³⁺) and green (LaPO₄:Tb³⁺) phosphors, respectively. It was found that the emission from the doped silicate sample was 37% of the commercial phosphor in case of the Tb-doped sample and 8% of the commercial phosphor in case of the Eu-doped sample.     

Synthesis of bamboo-leaf-shaped ZnO nanostructures by oxidation of Zn/SiO2 composite films deposited with radio frequency magnetron co-sputtering

Bamboo-leaf-shaped ZnO nanostructures were synthesized by oxidation of metal Zn/SiO₂ matrix composite thin films deposited on Si(1 1 1) substrates with radio frequency magnetron co-sputtering. The synthesized bamboo-leaf-shaped ZnO are single crystalline in nature with widths ranging from 30 to 60 nm and lengths of up to 5-10 μm, room temperature photoluminescence spectrum of the nanostructures shows a strong and sharp UV emission band at 372 nm and a weak and broad green emission band at about 520 nm which indicates relatively excellent crystallization and optical quality of the ZnO nanostructures synthesized by this novel method.     

Photoluminescence properties of trace amounts of Pr and Tb in yttria-stabilized zirconia

We have investigated the photoluminescence (PL) properties of trace amounts of Pr and Tb in single-crystal samples of yttria-stabilized zironia (YSZ), and found that Pr of the order of 10⁻⁶ mass% and Tb of the order of 10⁻⁵ mass% in YSZ can be detected by the PL spectroscopy. The PL spectra of the YSZ samples for the 280 nm excitation were comprised of several peaks and a broad emission. The peaks were attributed to transitions of Pr³⁺ and Tb³⁺ in the YSZ samples, whereas the broad emission seemed to be attributed to the yttria-associated oxygen vacancies. The peak intensities corresponded to the amounts of Pr and Tb in the YSZ samples, the amounts of which were analyzed by glow discharge mass spectrometry. In the PL excitation spectra, but not in the photoabsorption spectra, small peaks at 376 and 381 nm were observed, and were attributed to the transitions of Tb³⁺ in the YSZ samples. The results of the PL excitation spectra corresponding to the Pr³⁺ line emissions suggest that the charge transfer occurs between the YSZ and Pr ion in it. The trace amounts of these lanthanoids in YSZ would disturb the decay process of the photoinduced electrons to the yttria-associated oxygen vacancies.     

Z-Scan studies of KYW, KYbW, KGW, and Ba(NO3)2 crystals

We present the studies of nonlinear refraction and nonlinear absorption in promising crystals which are extensively used in Raman lasers or as solid-state laser host materials: Ba(NO₃)₂, KGW, KYW, and KYbW. The single-beam z-scan technique with 1 ps laser pulses at 790 and 395 nm has been applied for the study. Nonlinear refraction-index intensity-coefficients and two-photon absorption coefficients have been determined for the crystals. The considerable enhancement of nonlinear refraction is observed in the crystals at 395 nm.     

Fabrication and optical properties of SnS thin films by SILAR method

Although the fabrication of tin disulfide thin films by SILAR method is quiet common, there is, however, no report is available on the growth of SnS thin film using above technique. In the present work, SnS films of 0.20 μm thickness were grown on glass and ITO substrates by SILAR method using SnSO₄ and Na₂S solution. The as-grown films were smooth and strongly adherent to the substrate. XRD confirmed the deposition of SnS thin films. Scanning electron micrograph revealed almost equal distribution of the particle size well covered on the surface of the substrate. EDAX showed that as-grown SnS films were slightly rich in tin component while UV-vis transmission spectra exhibited high absorption in the visible region. The intense and sharp emission peaks at 680 and 825 nm (near band edge emission) dominated the photoluminescence spectra.     

Simple room temperature synthesis and optical studies on Mg doped ZnO nanostructures

We report simple room temperature synthesis of Mg doped ZnO nanostructures through the sol–gel method. X-ray diffraction shows the prepared ZnO particles are in wurtzite structure and replacement of Zn²⁺ by Mg²⁺ alters the position of the X-ray diffraction peak slightly towards higher angle. Measured optical absorption spectra show the exciton peaks of ZnO present around 366, 296 and 235 nm. Room temperature photoluminescence measurements show strong peaks around 385, 394 nm are attributed to band edge exciton emission; other peaks found at 469 and 558 are attributed to oxygen ion vacancy and formation of Vo⁺ and Vo⁺⁺ centers in nanostructures.     

Photoluminescence and Raman spectra of the ordered vacancy compound CuGa5Se8

We studied the photoluminescence (PL) and Raman properties of the ordered defect compound CuGa₅Se₈. Twelve peaks were detected from the room-temperature Raman spectra with the A₁ mode around 160 cm⁻¹. Due to the stress in the polycrystalline thin film the corresponding frequencies of the Raman modes of a CuGa₅Se₈ single crystal were slightly shifted. One broad asymmetric PL band at 1.788 and 1.765 eV was observed at 10 K in the PL spectra of CuGa₅Se₈ single crystal and polycrystalline layer, respectively. The temperature and laser power dependencies of the PL spectra were also studied. The shape and properties of the PL band assure the presence of potential fluctuations and the analyses of the PL data suggest that the emission is due to band-to-tail (BT) or band-to-impurity (BI) recombination.     

Photoluminescence studies of red-emitting NaEu(WO4)2 as a near-UV or blue convertible phosphor

Sodium europium double tungstate [NaEu(WO₄)₂] phosphor was prepared by the solid-state reaction method. Its crystal structure, photoluminescence properties and thermal quenching characteristics were investigated aiming at the potential application in the field of white light-emitting diodes (LEDs). The influences of Sm doping on the photoluminescence properties of this phosphor were also studied. It is found that this phosphor can be effectively excited by 394 or 464 nm light, which nicely match the output wavelengths of near-ultraviolet (UV) or blue LED chips. Under 394 or 464 nm light excitation, this phosphor exhibits stronger emission intensity than the Y₂O₂S:Eu³⁺ or Eu²⁺-activated sulfide phosphor. The introduction of Sm³⁺ ions can broaden the excitation peaks at 394 and 464 nm of the NaEu(WO₄)₂ phosphor and significantly enhance its relative luminance under 400 and 460 nm LEDs excitation. Furthermore, the relative luminance of NaEu(WO₄)₂ phosphor shows a superior thermal stability compared with the commercially used sulfide or oxysulfide phosphor, and make it a promising red phosphor for solid-state lighting devices based on near-UV or blue LED chips.     

Infrared superbroadband emission of Bi ion doped germanium-aluminum-sodium glass

The 96GeO-(3-χ)Al₂O₃-χNa₂O-1NaBiO₃ (χ = 0, 0.5, 1.5 molar percent designated as A1, A2 and A3) and 96GeO-(3.5-ψ)Al₂O₃-ψNa₂O-0.5Bi₂O₃ (ψ = 0.5, 1, 2 molar percent designated as B1, B2 and B3) glasses were prepared by conventional melting method with the measurement of their DTA curve, fluorescence decay curve, transmission, absorption and emission spectra. The near infrared superbroadband emission characteristics of the A1, A2, B1 and B2 glasses peak at ∼1220 nm were observed when pumped by an 800 nm laser diode. The stimulated emission cross section (σ_p) was obtained from the emission spectra. The result indicated that the introduction of Bi⁵⁺ in NaBiO₃ into raw materials could increase the emission intensity of the obtained glasses by 5.6 times than that of Bi³⁺ in Bi₂O₃, and the FWHM (Δλ) and emission lifetime (τ) at 1220 nm increased from 195 nm to 275 nm, and 280 μs to 434 μs. Meanwhile, it was found that the absorption edges were blue-shifted from 486 to 447 nm by comparing those of A1 and B1. The absorption edges were considered to be ascribed to the charge transfer from Bi³⁺ 6s² to Bi⁵⁺ 6s⁰. Therefore we could conclude that the content of Bi⁵⁺ ions in A1 was more than that in B1 glasses. It could be deduced from the emission and absorption spectra that the stronger emission intensity and wider FWHM were due to the higher concentration of Bi⁵⁺ ion in glass. In particular, the increase of Na₂O content was in proportion to the thermal stability and the value of σ_p × τ and σ_p × Δλ of glasses.     

Photoluminescence investigations of Zn2SiO4 co-doped with Eu and Tb ions

Zinc silicate phosphors co-doped with Eu³⁺ ions and also with both Eu³⁺ and Tb³⁺ ions were prepared by high temperature solid state reaction in air or reducing atmosphere. The luminescence characteristics of the prepared phosphors were investigated. While in the samples prepared in air, Eu³⁺ emission was found to be dominant over Tb³⁺ emission, in the samples prepared in reducing atmosphere, intense Eu²⁺ emission at 448 nm was found to be predominant over narrow Tb³⁺ emission. Luminescence studies showed that Eu³⁺ ions occupy asymmetric sites in Zn₂SiO₄ lattice. The intense f-f absorption peak of Eu³⁺ at 395 nm observed in these phosphors suggests their potential as red emitting phosphors for near ultra-violet light emitting diodes.     

Anomalous optical transitions in AlInGaN/GaN heterostructures grown by metalorganic chemical vapor deposition

Using temperature-dependent photoluminescence (PL) measurements, we report a comprehensive study on optical transitions in Al_yIn_xGa_1−x−yN epilayer with target composition, x=0.01 and y=0.07 and varying epilayer thickness of 40, 65 and 100 nm. In these quaternary alloys, we have observed an anomalous PL temperature dependence such as an S-shape band-edge PL peak shift and a W-shape spectral broadening with an increase in temperature. With an increase in excitation power density, the emission peak from the AlInGaN epilayers shows a blue shift at 100 K and a substantial red shift at room temperature. This is attributed to the localization of excitons at the band-tail states at low temperature. Compared to 40 and 65 nm thick epilayers, the initial blue shift observed with low excitation power from 100 nm thick AlInGaN epilayer at room temperature is caused by the existence of deeper localized states due to confinement effects arising from higher In and Al incorporation. The subsequent red shift of the PL peak can be attributed by free motion of delocalized carriers that leads to bandgap renormalization by screening. Due to competing effects of exciton and free carrier recombination processes, such behavior of optical transitions leads to two different values of exponent ‘k’ in the fitting of PL emission intensity as a function of excitation power.     

Growth and radioluminescence of metal elements doped LiCaAlF6 single crystals for neutron scintillator

The ns²-type metal elements (Pb and Sn) doped LiCaAlF₆ single crystals were grown by a micro-pulling-down (μ-PD) method. Pb doped LiCaAlF₆ [Pb:LiCAF] crystals showed high transparency and single phase of the LiCAF structure. However, we could not obtain Sn:LiCAF crystals due to the evaporation of SnF₂ during the crystal growth. There was an absorption peak around 193 nm in the transmittance spectrum of Pb:LiCAF crystal. In the radioluminescence spectrum of the Pb:LiCAF crystal under X-ray irradiation, two emission peaks around 200 and 830 nm were observed.     

Luminescent properties of LiCl-Cu single crystals

The absorption, photoluminescence, x-ray luminescence, and thermoluminescence spectra, and the photoluminescence excitation spectra of LiCl-Cu single crystals with different activator concentration were investigated at temperatures of 79–450°K. The absorption spectrum at room temperature has two bands with maxima at 237 and 259 nm. The absorption coefficient of the 237 nm band is proportional to the copper concentration in the crystal (C_Cu ≤ 7·10^?4 mole %). The photoluminescence and x-ray luminescence spectrum at room temperature consists of one emission band at 324 nm, which conforms with the Mollwo-Ivey rule in the homologous series RbCl → KCl → NaCl → LiCl. The copper ions create trapping levels for electrons and holes at different depths in the forbidden band of the LiCl crystal. The correlation between the thermoluminescence peaks and the recombination-luminescence excitation bands (infrared stimulation) is investigated.