Blue PL and EL emissions from Bi-activated binary oxide thin-film phosphors

The photoluminescent (PL) and electroluminescent (EL) characteristics in the thin films of various Bi-activated binary oxide phosphors have been investigated. La₂O₃:Bi, Gd₂O₃:Bi and Y₂O₃:Bi phosphor thin films were prepared on thick BaTiO₃ ceramic sheet substrates by r.f. magnetron sputtering depositions followed by postannealing. Intense blue PL emissions were observed from all Bi-activated binary oxide phosphor thin films postannealed at a high temperature. Blue, whitish blue-green or blue-green emissions were observed from thin-film electroluminescent (TFEL) devices fabricated with a La₂O₃:Bi, a Gd₂O₃:Bi or a Y₂O₃:Bi thin-film emitting layer, respectively. In addition, high luminance with good color purity in blue EL was obtained in a TFEL device using a La₂O₃:Bi thin film prepared under optimized conditions.     

Color control of emissions from rare earth-co-doped La2O3:Bi phosphor thin films prepared by magnetron sputtering

Multicolor photoluminescence (PL) and electroluminescence (EL) were observed from developed Bi- and rare earth (RE)-co-doped La₂O₃ (La₂O₃:Bi,RE) phosphor thin films. The phosphor thin films were prepared with various contents of co-doped RE, such as Dy, Er, Eu, Tb and Tm, by a combinatorial radio frequency magnetron sputtering deposition method. The obtainable luminance in EL and PL intensities changed considerably as the kind and content of RE were varied. Color changes from blue and blue-green to various colors in PL and EL emissions, respectively, were obtained in postannealed La₂O₃:Bi,RE phosphor thin films: films prepared by co-doping Bi at a constant content with various REs at varying levels of content. However, all of the observed emission peaks in PL and EL from La₂O₃:Bi,RE phosphor thin films were assigned to either the broad emission originating from the transition in Bi³⁺ or the visible emission peaks originating from the transition in the co-doped trivalent RE ion. The difference between PL and EL characteristics in La₂O₃:Bi,RE phosphor thin films is mainly attributed to the difference in the excitation mechanism.     

Blue-violet phosphate phosphor thin films for EL

Photoluminescence (PL) and electroluminescence (EL) in blue-violet emission were observed in newly developed phosphate phosphor thin films such as Ba₃(PO₄)₂:Eu and Ba₃(PO₄)₂:Ti. These phosphate phosphor thin films were first deposited on thick BaTiO₃ ceramic sheets by r.f. magnetron sputtering using powder targets and then post-annealed in various atmospheres. Blue-violet PL and EL emissions were obtained in Ba₃(PO₄)₂:Eu and Ba₃(PO₄)₂:Ti phosphor thin films that were deposited in an Ar + H₂ (10%) gas atmosphere and then post-annealed above about 900 °C in an Ar + H₂ (10%) gas atmosphere. In particular, the EL observed in Ba₃(PO₄)₂:Eu thin films exhibited two peaks, a red emission peaking at about 615 nm and a blue-violet emission peaking at about 420 nm. A luminance of 2.0 cd/m² in blue-violet emission was obtained in a thin-film EL device using a two step post-annealed Ba₃(PO₄)₂:Eu thin-film emitting layer: step 1, post-annealing at 1000 °C in air for 1 h, and step 2, post-annealing at 1000 °C in an Ar + H₂ atmosphere.     

PL and EL properties of Tm-activated vanadium oxide-based phosphor thin films

The preparation of high-luminance blue emitting Tm-activated multicomponent oxide phosphor thin films is described. The phosphor thin films were deposited on thick BaTiO₃ ceramic sheets by r.f. magnetron sputtering using powder targets. A very high photoluminescence (PL) intensity in blue emission could be observed in postannealed Tm-activated gadolinium oxide-vanadium oxide Gd_0.5-V_0.5-O:Tm thin films prepared using Tm₂O₃-doped (Gd₂O₃)_0.5-(V₂O₅)_0.5 targets (V₂O₅ content of 50 mol.%). It should be noted that thin films of this phosphor postannealed at 1100 °C in air exhibited higher PL intensity than those from commercially available blue BaMgAl₁₄O₂₃:Eu (BAM) phosphor powder and Gd_0.5-V_0.5-O:Tm powders. In addition, a Gd_0.5-V_0.5-O:Tm thin-film electroluminescent device exhibited a blue emission that was the same as the PL emission.     

Luminescence mechanisms of organic/inorganic hybrid organic light-emitting devices fabricated utilizing a Zn2SiO4:Mn color-conversion layer

Zn₂SiO₄:Mn phosphor layers used in this study were synthesized by using the sol-gel method and printed on the glass substrates by using a vehicle solution and a heating process. Organic/inorganic hybrid organic light-emitting devices (OLEDs) utilizing a Zn₂SiO₄:Mn color-conversion layer were fabricated. X-ray diffraction data for the synthesized Zn₂SiO₄:Mn phosphor films showed that the Zn ions in the phosphor were substituted into Mn ions. The electroluminescence (EL) spectrum of the deep blue OLEDs showed that a dominant peak at 461 nm appeared. The photoluminescence spectrum for the Zn₂SiO₄:Mn phosphor layer by using a 470 nm excitation source showed that a dominant peak at 527 nm appeared, which originated from the ⁴T₁-⁶A₁ transitions of Mn ions. The appearance of the peak around 527 nm of the EL spectra for the OLEDs fabricated utilizing a Zn₂SiO₄:Mn phosphor layer demonstrated that the emitted blue color from the deep blue OLEDs was converted into a green color due to the existence of the color-conversion layer. The luminescence mechanisms of organic/inorganic hybrid OLEDs fabricated utilizing a Zn₂SiO₄:Mn color-conversion layer are described on the basis of the EL and PL spectra.     

Cathodoluminescence of CaWO4 and SrWO4 thin films prepared by spray pyrolysis

Thin films of CaWO₄ and SrWO₄ were prepared on glass substrates by spray pyrolysis. The effects of preparation conditions and monovalent, bivalent and trivalent cation doping on cathodoluminescence (CL) properties of the films were studied. Polycrystalline CaWO₄ and SrWO₄ films formed a scheelite structure after being annealed above 300°C. They exhibited analogous cathodoluminescence consisting of a blue emission band at 447 nm and a blue-green emission band at 487 nm. The blue and blue-green emission intensities increased with substrate and annealing temperature. Annealing atmosphere and doping with Ag⁺, Pb²⁺ and La³⁺ did not influence the characteristics of the blue and blue-green emissions, whereas Eu³⁺ did. The results indicated both the blue and blue-green emissions originated from the WO₄²⁻ molecular complex. The luminance and efficiency for CaWO₄ film were 150 cd/m² and 0.7 lm/W at 5 kV and 57 μA/cm².     

Effect of the ligand on the properties of emitting materials: Pentacoordinated 8-hydroxyquinoline aluminum complexes

Two series of pentacoordinated complexes, AlMq₂q′ and Alq₂q′ (q is 8-hydroxyquinoline, Mq is 2-methyl-8-hydroxyquinoline and q′ is phenolato ligand), were synthesized, and assessed as potential emitting materials in thin film (photoluminescence, PL) and electroluminescent (EL) device. EL devices with a configuration of ITO/NPB(50 nm)/Complex(50 nm)/Mg/Ag(10:1) were fabricated. Strong blue and green emission were observed from thin solid film and EL device of AlMq₂q′ and Alq₂q′, respectively. The results revealed that the PL and EL emission wavelengths of these materials are primarily determined by Mq and q (first ligand), whereas phenolato ligand (second ligand) has little effect on the emission wavelength but remarkable influence on EL efficiency.     

MBE n-ZnO/MOCVD p-GaN heterojunction light-emitting diode

The growth, fabrication, and subsequent electroluminescence (EL) characterization of an n-ZnO/p-GaN heterojunction light-emitting diode prepared on c-Al₂O₃ substrate are presented. The diode-like I-V characteristics and room temperature EL spectrum with an intense broadband emission in the yellow-green spectral region has been observed with forward bias applied. Photoluminescence (PL) and Raman spectra of the n-ZnO and p-GaN films were also measured. By comparing PL and EL spectra, it was concluded that the deep-level defect-related emission mainly originated from the GaN epitaxial layer.     

Effects of ambient oxygen pressures and substrate temperatures in pulsed laser deposited Zn0.85Li0.15O thin films

Li doped zinc oxide Zn_1−xLi_xO (x = 0.15) thin films were grown by using the pulsed laser deposition method. The depositions were done onto Pt(111)/Ti/SiO₂/Si(100) substrate set at temperatures ranging from 300 °C to 700 °C, with varying the ambient O₂ pressure range of 3-20 mTorr. The effects of substrate temperatures and ambient O₂ pressures on the surface morphology and structural properties of the Zn_0.85Li_0.15O thin films were investigated by using the scanning probe microscopy and X-ray diffraction spectra, respectively. Also the chemical structures of the films were investigated by observing the X-ray photoelectron spectra of the core and shallower levels. We observed the deep blue PL emissions centered at about 390 nm (3.20 eV) from the Zn_0.85Li_0.15O thin films. It was investigated with respect to the ambient O₂ pressures during the deposition. It is considered that the deep blue PL emission in the Zn_0.85Li_0.15O thin film may be related to the incorporation of oxygen vacancies.     

Pechinin synthesis and luminescence properties of Y3Ga5O12(YGG):Tb thin film

YGG:Tb thin films were successfully prepared by Pechni sol-gel process. The structure, surface morphology, evolution of film crystallization and their luminescent properties were investigated. We find the annealing process is very important to the YGG:Tb film crystallization. Uniform crack-free films can be obtained by conventional annealing for 1 h after rapid thermal treating for 10 min at 800 °C. The excitation and emission spectra of photoluminescence (PL) were used to characterize the luminescent properties. The excitation spectrum of YGG:Tb is dominated by the 4f-4f5d transition of Tb³⁺at 260 nm. The emission peaks of phosphor thin film lie at 488 and 543 nm. It can be used as a promising phosphor material for FED or ELD application.     

Synthesis,photoluminescence properties of Sr<Subscript>1.95</Subscript>Ba<Subscript>0.05</Subscript>CeO<Subscript>4</Subscript>:Eu<Superscript>3+</Superscript> for LED applications

The Sr_1.95Ba_0.05 CeO₄:Eu³⁺ phosphors are synthesized by the solid-state reaction method. The samples are characterized using X-ray diffraction (XRD), diffuse reflectance spectroscopy and photoluminescence (PL) spectra. The XRD results reveal that the synthesized phosphors are genuine crystalline and belong to the orthorhombic structure. The intense PL emission is optimized from the PL spectra at various doping concentrations of europium ions. The results indicates that the phosphor can be effectively excited under 264 nm wavelength producing on intense emission spectrum of the synthesis material at 484 nm (blue region). The color purity of the phosphor is confirmed by CIE coordinates (x = 0.217, y = 0.265). The experimental data indicate that the prepared phosphors can be used as blue-emitting material in the field of illuminations and display devices.     

Combustion synthesis of Eu2?+? and Dy3?+? activated Sr3(VO4)2 phosphor for LEDs

Combustion synthesis and photoluminescence (PL) characterization of Sr₃(VO₄)₂:Eu,Dy phosphors are presented in this paper. PL emission of Sr₃(VO₄)₂:Eu phosphor shows green broad emission band centring at 511 nm and a red sharp band at 614 nm by excitation wavelength of 342 nm. The PL emission spectrum of Sr₃(VO₄)₂:Dy phosphor exhibits an intense blue emission peak at 479 nm, yellow broad band centring at 573 nm and red band at 644 nm by the excitation wavelength of 426 nm in near visible blue region. The excitation wavelength of Eu (342 nm) and Dy (426 nm) activated Sr₃(VO₄)₂ phosphor are well matched with the excitation of near UV excited solid state lighting and blue chip excitation of light emitting diodes, respectively. The effect of Eu^2 +and Eu^3 +ions concentration on the emission intensity of Sr₃(VO₄)₂ was also investigated. The Sr₃(VO₄)₂:Eu is a potential green and red emitting phosphor as well as Sr₃(VO₄)₂:Dy is blue and yellow emitting phosphor for solid state lighting i.e. white LEDs. The XRD and SEM characteristics of Sr₃(VO₄)₂ materials was also reported in this paper.     

Photoluminescence and Raman studies of graphene thin films prepared by reduction of graphene oxide

Photoluminescence (PL) and Raman studies have been performed to investigate the optical properties of graphene thin films prepared by chemical and thermal reductions of graphene oxide (GO). The G peak in Raman spectra red-shifted after reduction of GO. Thermal reduction resulted in a more red-shift of the G peak than chemical reduction. A strong intensity of the D peak indicated that the prepared graphene films have significant structural disorders. A blue-shifted emission in PL spectra suggested that sp² clusters are embedded in a sp³ matrix that acts as a tunnel barrier, causing a strong fluctuation in the local band gap.     

Novel blue-emitting Sr3Ga2O5Cl2:Eu phosphor for UV-pumped white LEDs

A novel blue-emitting Sr₃Ga₂O₅Cl₂:Eu²⁺ phosphor has been synthesized by a two-step solid-state reaction. The luminescence properties have been investigated by photoluminescence (PL) spectra, and temperature-dependent PL spectra. It shows an efficient broad absorption band around 400 nm, which matches well with the commercial near-ultraviolet light-emitting chips, and an efficient blue emission. It shows a higher thermal quenching temperature than that of Sr₃Al₂O₅Cl₂:Eu²⁺ phosphor. Sr₃Ga₂O₅Cl₂:Eu²⁺ phosphor is a promising blue-emitting component for UV chip excited white light-emitting-diodes.     

Effect of the oxygen partial pressure on the microstructure and optical properties of ZnO:Cu films

Cu-doped zinc oxide (ZnO:Cu) films were deposited on Si substrates using radio frequency reactive magnetron sputtering at different oxygen partial pressures. The effect of oxygen partial pressure on the microstructures and optical properties of ZnO:Cu thin films were systematically investigated by X-ray diffraction (XRD), atomic force microscopy (AFM) and fluorescence spectrophotometer. The results indicated that the grain orientation of the films was promoted by appropriate oxygen partial pressures. And with increasing oxygen partial pressure, the compressive stress of the films increased first and then decreased. The photoluminescence (PL) of the samples were measured at room temperature. A violet peak, two blue peaks and a green peak were observed from the PL spectra of the four samples. The origin of these emissions was discussed and the mechanism of violet emission of ZnO:Cu thin films were suggested.     

Green electroluminescence from AlN:Tb thin film devices on glass

Electroluminescence (EL) properties of AlN:Tb thin film EL device (TFELD) prepared on a glass substrate by a rf-magnetron sputtering method have been studied. The AlN:Tb emission layer consists of hexagonal (110)-oriented poly-crystals of AlN with a high transparency in visible region. Four emission peaks originating from ⁵D₄ → ⁷F _j (j = 6, 5, 4, 3) transitions of Tb³⁺ were found in both photoluminescence (PL) and EL spectra of the AlN:Tb thin film. The peak emission intensity of the ⁵D₄ → ⁷F₆ transitions is almost the same magnitude with that of the ⁵D₄ → ⁷F₅ transitions, being largely different from the intensity ratio of Tb³⁺in other host materials.     

Photoluminescence properties of MgAl2O4:Dy powder phosphor

Trivalent dysprosium (Dy³⁺) activated magnesium alluminate phosphors were synthesized by high temperature solid state reaction method. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FT-IR), photoluminescence (PL) spectra as well as lifetimes were used to characterize the resulting phosphors. The results show that the obtained MgAl₂O₄:Dy³⁺ phosphors have good crystallinity, spherical morphology with sizes ranged from 120 to 140 nm and strong blue emission under an excitation of 258 nm. The emission spectrum of this phosphor consists of two emission bands: blue band and yellow band, and the emission intensity of the former is stronger than that of the later. Luminescence quenching is explained and the corresponding luminescence mechanisms have been proposed.     

Effect of annealing temperature on optical and electrical properties of ZrO2–SnO2 nanocomposite thin films

ZrO₂–SnO₂ nanocomposite thin films were deposited onto quartz substrate by sol–gel dip-coating technique. Films were annealed at 500, 800 and 1,200 °C respectively. X-ray diffraction pattern showed a mixture of three phases: tetragonal ZrO₂ and SnO₂ and orthorhombic ZrSnO₄. ZrSnO₄ phase and grain size increased with annealing temperature. Fourier transform infra-red spectroscopy spectra indicated the reduction of –OH groups and increase in ZrO₂–SnO₂, by increasing the treatment temperature. Scanning electron microscopy observations showed nucleation and particle growth on the films. The electrical conductivity decreased with increase in annealing temperature. An average transmittance greater than 80 % (in UV–visible region) was observed for all the films. The optical constants of the films were calculated. A decrease in optical band gap from 4.79 to 4.59 eV was observed with increase in annealing temperature. Photoluminescence (PL) spectra revealed an emission peak at 424 nm which indicates the presence of oxygen vacancy in ZrSnO₄. PL spectra of the films exhibited an increase in the emission intensity with increase in temperature which substantiates enhancement of ZrSnO₄ phase and reduction in the non-radiative defects in the films. The nanocomposite modifies the structure of the individual metal oxides, accompanied by the crystallite size change and makes it ideal for gas sensor and optical applications.     

Luminescent properties of Gd2Ti2O7: Eu phosphor films prepared by sol-gel process

Gd₂Ti₂O₇: Eu³⁺ thin film phosphors were fabricated by a sol-gel process. X-ray diffraction (XRD), atomic force microscopy (AFM) and photoluminescence (PL) spectra as well as lifetimes were used to characterize the resulting films. The results of XRD indicated that the films began to crystallize at 800 °C and the crystallinity increased with the elevation of annealing temperatures. Uniform and crack free phosphor films were obtained, which mainly consisted of grains with an average size of 70 nm. The doped Eu³⁺ showed orange-red emission in crystalline Gd₂Ti₂O₇ phosphor films due to an energy transfer from Gd₂Ti₂O₇ host to them. Both the lifetimes and PL intensity of the Eu³⁺ increased with increasing the annealing temperature from 800 to 1000 °C, and the optimum concentrations for Eu³⁺ were determined to be 9 at.%. of Gd³⁺ in Gd₂Ti₂O₇ film host.     

Preparation and properties of thin ZnS:Cu films phosphors

Thin films of ZnS and ZnS:Cu were prepared by an original metalorganic chemical vapour deposition (MOCVD) method under atmospheric pressure onto a glass substrate heated up to 230–250 °C. The film thickness varied from 0.6 to 1 μm. The thin films were doped with Cu and Cl by the thermal treatment during 1 h at 600 °C at atmospheric pressure in the blend composed of a ZnS powder with Cu and Cl compounds. These films were used for fabrication of the thin film electroluminescent (TFEL) devices with a conventional double insulating structure. The structural properties were investigated by use of X-ray diffraction (XRD) techniques and atomic force microscopy (AFM). Electroluminescent (EL) spectra, electrical and EL characteristics were investigated. The EL spectra and characteristics as well as structural parameters depend on the growth conditions and significantly modified after the annealing. Blue color emission with brightness of 10 cd m^− 2 under a sine wave excitation at 60 V and 5 kHz was obtained. The degradation behavior of the TFEL devices with ZnS:[Cu, Cl] films fabricated using an original non-vacuum methods of deposition and annealing is the same as that of commercial thin film phosphor.