Origin of green luminescence in PbWO4 crystals

Characteristics of two green emission bands, G(I) and G(II), and their origin were investigated within 0.4-300 K under photoexcitation in the 3.4-6.0 eV energy range for undoped and Mo⁶⁺-, Mo⁶⁺ , Y³⁺-, Mo⁶⁺, Nb⁵⁺-, Mo⁶⁺, Ce³⁺-, Cr⁶⁺-, La³⁺-, Ba²⁺- and Cd²⁺-doped PbWO₄ crystals with different concentrations of impurity and intrinsic defects, grown by different methods and annealed at different conditions. The G(I) emission band, observed at low temperatures, located around 2.3-2.4 eV and excited around 3.9 eV, is usually a superposition of many closely positioned bands. The G(I) emission of undoped crystals is assumed to arise from the WO₄²⁻ groups located in the crystal regions of lead-deficient structure. In Mo⁶⁺-doped crystals, this emission arises mainly from the MoO₄²⁻ groups themselves. The G(II) emission band located at 2.5 eV is observed only in the crystals, containing the isolated oxygen vacancies — WO₃ groups. This emission appears at T>160 K under excitation around 4.07 eV as a result of the photo-thermally stimulated disintegration of localized exciton states and subsequent recombination of the produced electron and hole centres near WO₃ groups. The G(II) emission accompanies also thermally stimulated recombination processes in PbWO₄ crystals above 150 K. Mainly the G(II) emission is responsible for the slow decay of the green luminescence in PbWO₄ crystals.     

Luminescence and defects creation under photoexcitation of CsI : Tl crystals in Tl-related absorption bands

Characteristics of the defects created at 4.2 K by the UV-irradiation of CsI : Tl crystals in the Tl⁺-related absorption bands (by photons of 5.8-4.8 eV energy) have been studied. The dependences of the intensities of the thermally stimulated luminescence peaks appearing near 60, 90 and 125 K and of the recombination luminescence photostimulation bands peaking at 2.35, 1.92, 1.33 and 0.89 eV on the irradiation energy and duration, uniaxial stress and thallium concentration have been examined. The mechanisms of the processes, responsible for the appearance of the intense visible (2.55 and 2.25 eV) luminescence of excitons localized near Tl⁺ ions and creation of defects pairs of the type of Tl⁰-V_K and Tl⁺-V_K with various distances between the components, have been discussed.     

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.     

Temperature-dependent photoluminescence in La2/3Ca1/3MnO3

Visible photoluminescence and its temperature dependence of La_2/3Ca_1/3MnO₃ in the temperature range 138-293 K were measured. It was observed that the main broad band centered at ∼1.77 eV with the shoulders at ∼1.57 and ∼1.90 eV existed in the entire temperature range. It can be well fitted by three Gaussian curves B₁, B₂ and B₃ centered at ∼1.52, ∼1.75 and ∼1.92 eV, respectively. The intensities of the peak B₁ and B₂ vary as temperature increases. In the entire temperature range, the intensity of B₁ increases with increasing temperature, whereas that of B₂ decreases. The photoluminescence mechanisms for La_2/3Ca_1/3MnO₃ are presented based on the electronic structures formed by the interactions among spin, charge and lattice, in which B₁ was identified with the charge transfer excitation of an electron from the lower Jahn-Teller split e_g level of a Mn³⁺ ion to the e_g level of an adjacent Mn⁴⁺ ion, B₂ is assigned to the transition between the spin up and spin down e_g bands separated by Hund's coupling energy E_J and B₃ is attributed to the transition, determined by the crystal field energy E_C, between a t_2g core electron of Mn³⁺ to the spin up e_g bands of Mn⁴⁺ by a dipole allowed charge transfer process.     

Interference of resonance luminescence of exciton polaritons in CuGaS2 crystals

The nonmodulated and wavelength-modulated reflection spectra of CuGaS₂ crystals for the polarization EIIc of 10 K are studied. The states n = 1, 2 and 3 of the excitons Γ₄ (A-excitons) and n = 1, n = 2 of B- and C-excitons are found. The nonmodulated absorption spectra for the polarization E⊥c at 10 K have been studied. The states n = 1, 2 and 3 of Γ₅ excitons are found. The main parameters of the A (Γ₄, Γ₅) and B, C exciton series at the energies of the longitudinal and transverse excitons Γ₄ for the states n = 1 and n = 2, the effective masses of electrons and holes are determined. The photoluminescence peaks were observed at n = 3 and n = 4 of the excitons Γ₅ in the luminescence spectra excited by the line 4880 Å of Ar⁺ laser. In the luminescence spectra the interference is found.     

Strongly enhanced free-exciton luminescence in microcrystalline CsPbCl3 films produced via the amorphous phase

High-quality CsPbCl₃ films composed of crystallites with narrow size distributions are achieved for various size levels, from microcrystalline to polycrystalline, by a novel heat-treatment method applied to the same amorphous films. Their photoluminescence is dominated by free-exciton emission at every size level without showing trapped-exciton emission in great contrast to the case for single crystals. The microcrystalline state shows more than an order of magnitude stronger free-exciton emission than the polycrystalline state, and exhibits intense stimulated emission under high-power excitation.     

Low-temperature photoluminescence spectra of highly excited quantum wires

Optical spectra of highly excited quantum wires at low temperatures have been studied within the dynamical screening approximation. We found a strong Fermi-edge singularity (FES) in the photoluminescence spectra. The spectral shape and FES intensity strongly depend on temperature in agreement with recent experimental results.     

Luminescence and Tb-Ce-Eu ion energy transfer in single-crystalline films of Tb3Al5O12:Ce,Eu garnet

The paper is devoted to investigation of the processes of excitation energy transfer between the host cations (Tb³⁺ ions) and the activators (Ce³⁺ and Eu³⁺ ions) in single-crystalline films of Tb₃Al₅O₁₂:Ce,Eu (TbAG:Ce,Eu) garnet which is considered as a promising luminescent material for the conversion of LED's radiation. The cascade process of excitation energy transfer is shown to be realized in TbAG:Ce,Eu: (i) from Tb³⁺ ions to Ce³⁺ and Eu³⁺ ions; (ii) from Ce³⁺ ions to Eu³⁺ ions by means of dipole-dipole interaction and through Tb³⁺ ion sublattice.     

Exciton condensation in coupled quantum wells

Bound electron-hole pairs—excitons—are Bose particles with small mass. Exciton Bose-Einstein condensation is expected to occur at a few degrees Kelvin—a temperature many orders of magnitude higher than for atoms. Experimentally, an exciton temperature well below 1 K is achieved in coupled quantum well (CQW) semiconductor nanostructures. In this contribution, we review briefly experiments that signal exciton condensation in CQWs: a strong enhancement of the indirect exciton mobility consistent with the onset of exciton superfluidity, a strong enhancement of the radiative decay rate of the indirect excitons consistent with exciton condensate superradiance, strong fluctuations of the indirect exciton emission consistent with critical fluctuations near the phase transition, and a strong enhancement of the exciton scattering rate with increasing concentration of the indirect excitons revealing bosonic stimulation of exciton scattering. Novel experiments with exciton condensation in potential traps, pattern formation in exciton system and macroscopically ordered exciton state will also be reviewed briefly.     

Microstructure related photoluminescence in a-CNx films deposited by reactive rf magnetron sputtering

We investigated the photoluminescence (PL) properties of carbon nitride films (CN_x) deposited by rf magnetron sputtering and compared them to their microstructure depending on the target self-bias. While many of the data are compatible with ‘a-C:H like’ PL properties the observed variation of the PL efficiency η with respect to the target bias cannot be easily explained by the standard models. It is suggested that the observed variation of η is rather dominated by a change in microstructure which depends on the bombardment intensity during growth than by the concentration of non-radiative centres.     

Synthesis and luminescence properties of Tb:NaGd(WO4)2 novel green phosphors

Tb³⁺:NaGd(WO₄)₂ (Tb:NGW) phosphors with different Tb³⁺ concentrations have been synthesized by a mild hydrothermal process directly without further sintering treatment. X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence excitation and emission spectra and decay curve were used to characterize the Tb:NGW phosphors. XRD analysis confirmed the formation of NGW with scheelite structure. SEM study showed that the obtained Tb:NGW phosphors appeared to be nearly spherical and their sizes ranged from 1 to 1.5 μm. The excitation spectra of these systems showed an intense broad band with maximum at 270 nm related to the O→W ligand-to-metal charge-transfer state. Photoluminescence spectra indicated the phosphors emitted strong green light centered at 545 nm under UV light excitation. Analysis of the photoluminescence spectra with different Tb³⁺ concentrations revealed that the optimum dopant concentration for Tb³⁺ is about 15 at% of Tb³⁺ ions in Tb:NGW phosphors.     

Synthesis, microstructure and photoluminescence of Eu/Tb activated Y2SiO5 nanophosphors by new silicate sources

Y_2−xTb_xSiO₅ and Y_2−xEu_xSiO₅ nanophosphors with seven different kinds of silicate sources were synthesized by sol-gel method. The structures have been investigated to be composed of nanometer-size grains of 30-60 nm through X-ray diffraction (XRD) and scanning electron microscopy (SEM) was used to compare the different morphology of patterns from seven different silicon sources. The photoluminescence of Y_2−xTb_xSiO₅ was investigated as a function of silicate sources and the results revealed that these nanometer materials showed the characteristic emission ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) of Tb ions. The characteristic emission ⁵D₀ → ⁷F_J (J = 1, 2, 4) of Eu ions was also found in the materials of Y_2−xEu_xSiO₅.     

Novel ultraviolet photoluminescence of ZnO/ZnGa2O4 composite layers

ZnO/ZnGa₂O₄ composite layers were synthesized by simple thermal oxidation of ZnS substrates with gallium in the air. The continuous-wave and time-resolved photoluminescence measurements for the composites were performed at room temperature. It is found that the visible deep level emission from ZnO in ZnO/ZnGa₂O₄ composite layer was almost suppressed. In addition, the UV emission with long lifetime was also observed in comparison with that of pure ZnO layer without ZnGa₂O₄.     

Fast and efficient VUV/UV emissions from (Ba,La)F2:Er crystals

We report observation of fast and efficient VUV/UV luminescence from the mixed (Ba,La)F₂:Er crystals. The broad bands, peaking at 162.5, 181.9, 194.2, 202.8, 216.1, 233.5 and 281.5 nm and decaying, at 10 and 293 K, with time constants of 46 and 35 ns respectively, are due to spin-allowed transitions from the low-spin (LS) state of the 4f¹⁰5d configuration.We also observed a weak and slow broad band emission peaking at 170 nm due to the spin-forbidden transition from the high-spin (HS) state of the 4f¹⁰5d configuration.While at room temperature the excitation into any of the three identified LS bands (J=8, 7 and 6) dominating the excitation spectrum yields fast VUV and UV emissions, at 10 K the excitation into higher lying J=7 and 6 bands generates slow and sharp line emissions. The positions of these lines fit energies of transitions originating from the ²G_7/2 multiplet at 66140 cm⁻¹. The emission from the ²G_7/2 multiplet has been never, to the best of our knowledge, observed before.The efficient and fast VUV and UV emissions from the higher (LS, J=8) with almost no contribution from the lower (HS, J=8) level of the 4f¹⁰5d configuration are possible because the modified crystal field in (Ba,La)F₂ shifts the level of the (LS, J=8) state below the ²F_5/2 multiplet which, therefore, does not contribute to nonradiative relaxation between the LS and HS levels.We conclude that the ²G_7/2 and ²F_5/2 levels have major impact on VUV and UV emissions from the Er³⁺ ion in (Ba,La)F₂ contributing to complex emission pattern described in this report Their key role, elucidated by the VUV and UV luminescence spectroscopy, is consistent with predictions from a simple configuration coordinate model based on experimental results and calculations of the 4f¹¹ energy levels.     

The effects of crystal structure on optical absorption/photoluminescence of bis(8-hydroxyquinoline)zinc

In this paper, the transformation processes of two types of bis(8-hydroxyquinoline)zinc: Znq₂ dihydrate and anhydrous (Znq₂)₄ were investigated by X-ray diffraction (XRD), infrared spectra (IR), scanning electron microscope (SEM), differential scanning calorimetry (DSC) and thermogravimetry (TG). The effects of crystal structure on optical properties of bis(8-hydroxyquinoiline)zinc were analyzed. Znq₂ dihydrate can be transformed into anhydrous (Znq₂)₄ during heating under vacuum. Reversal transformation occurs by the interaction between chloroform and (Znq₂)₄. But (Znq₂)₄ was partially transformed into Znq₂ dihydrate by the interaction between ethanol and (Znq₂)₄. The different molecular structure results in different crystal stacking and electronic structure, thereby affect its optical properties.     

New red phosphors BaZr(BO3)2 and SrAl2B2O7 doped with Eu for PDP applications

Vacuum ultraviolet (VUV) excitation and photoluminescence (PL) characteristics of Eu³⁺ ion doped borate phosphors; BaZr(BO₃)₂:Eu³⁺ and SrAl₂B₂O₇:Eu³⁺ are studied. The excitation spectra show strong absorption in the VUV region with the absorption band edge at ca. 200 nm for BaZr(BO₃)₂:Eu³⁺ and 183 nm for SrAl₂B₂O₇:Eu³⁺, respectively, which ensures the efficient absorption of the Xe plasma emission lines. In BaZr(BO₃)₂:Eu³⁺, the charge transfer band of Eu³⁺ does not appear strongly in the excitation spectrum, which can be enhanced by co-doping Al³⁺ ion into the BaZr(BO₃)₂ lattices. The luminescence intensity of BaZr(BO₃)₂:Eu³⁺ is also increased by Al³⁺ incorporation into the lattices. The PL spectra show the strongest emission at 615 nm corresponding to the electric dipole ⁵D₀→⁷F₂ transition of Eu³⁺ in both BaZr(BO₃)₂ and SrAl₂B₂O₇, similar to that in YAl₃(BO₃)₄, which results in a good color purity for display applications.     

Synthesis and photoluminescence of CeO2:Eu phosphor powders

The crystalline structure and photoluminescence (PL) properties of europium-doped cerium dioxide synthesized by the solid-state reaction method were analyzed. CeO₂:Eu³⁺ phosphor powders exhibit the pure cubic fluorite phase up to 10 mol% doping concentration of Eu³⁺. With indirect excitation of CeO₂ host at 373 nm, the PL intensity quickly increases with increasing Eu³⁺ concentration, up to about 1 mol%, and then decreases indicating the concentration quenching. While with direct excitation (467 nm), much more stronger PL emissions, especially the electric dipole emission ⁵D₀-⁷F₂ at 612 nm, are observed and no concentration quenching occurs up to 10 mol% doping concentration of Eu³⁺. The nature of this behavior and the cause of the concentration quenching were discussed.     

Correlation of photoluminescence of (Y, Ln)VO4:Eu (Ln=Gd and La) phosphors with their crystal structures

We have studied the photoluminescence (PL) of (Y, Ln)VO₄:Eu³⁺ (Ln=La and Gd) phosphors and the correlation of the PL of those phosphor with their crystal structure. It is found that (Y, Gd)VO₄:Eu³⁺ phosphors have the same crystal structure as YVO₄:Eu³⁺, which is tetragonal with a little different lattice parameters. In the case of (Y, La)VO₄:Eu³⁺ phosphors, however, the gradual change from tetragonal to monoclinic structure of host lattice was observed as the amount of La ion increased. To investigate the PL property of (Y, Ln)VO₄:Eu³⁺ (Ln=La and Gd) phosphors, vacuum ultraviolet (VUV) and ultraviolet (UV) excitation were used. The favorable crystal structure for the PL intensity of orthovanadate phosphor under 147 and 254 nm excitation was tetragonal containing Gd ion and under 365 nm excitation was monoclinic containing La ion which might have the lowest site symmetry for Eu³⁺ ion.     

Synthesis and optical properties of KZnLa0.99Nd0.01(VO4)2 triple vanadate(V)—New promising laser materials

In an attempt to find a neodymium-vanadate system with long lifetime of ⁴F_3/2 level and relatively strong ⁴F_3/2→⁴I_11/2 emission for laser applications, the optical properties of Nd³⁺ in a new KZnLa(VO₄)₂ host is reported. The crystalline samples were obtained at 900 °C in air. The samples were crystallized in monoclinic system and were isostructural with KZnLa(PO₄)₂. KZnLa_0.99Nd_0.01(VO₄)₂ strongly emits in the near infrared range with the maxima at 871.6 and 1057 nm upon excitation through the ⁴F_5/2 level (808 nm) or by the charge transfer bands of VO₄³⁻. The lifetime of ⁴F_3/2 level of Nd³⁺ ion is larger than that observed in other neodymium-vanadates systems.     

Potential PDP phosphors with strong absorption around 172 nm: Rare earth doped NaLaP2O7 and NaGdP2O7

NaLaP₂O₇ and NaGdP₂O₇ powder samples are prepared by solid-state reactions at 750 and 600 °C, respectively, and the VUV-excited luminescence properties of Ln³⁺ (Ln=Ce, Pr, Tb, Tm, Eu) in both diphosphates are studied. Ln³⁺ ions in both hosts show analogous luminescence. For Ce³⁺-doped samples, the five Ce³⁺ 5d levels can be clearly identified. As for Pr³⁺ and Tb³⁺-doped samples, strong 4f-5d absorption band around 172 nm is observed, which matches well with Xe-He excimer in plasma display panel (PDP) devices. As a result, Pr³⁺ can be utilized as sensitizer to absorb 172 nm VUV photon and transfer energy to appropriate activators, and Tb³⁺-doped NaREP₂O₇(RE=La, Gd) are potential 172 nm excited green PDP phosphors. For Tm³⁺ and Eu³⁺-doped samples, the Tm³⁺-O²⁻ charge transfer band (CTB) is observed to be at 177 nm, but the CTB of Eu³⁺ is observed at abnormally low energy position, which might originate from multi-position of Eu³⁺ ions. The similarity in luminescence properties of Ln³⁺ in both hosts indicates certain structural resemblance of coordination environment of Ln³⁺ in the two sodium rare earth diphosphates.