Orange Photoluminescence Band Research Articles

Donor-acceptor nature of orange photoluminescence in AlN

Recombination dynamics, photoluminescence and photoluminescence excitation spectra have been investigated for 1.9 eV photoluminescence band in AlN in the temperature range of 5–650 K. The recombination dynamics for the 1.9 eV photoluminescence band has been described by a model of donor-acceptor recombination with taking into account a broadening due to electron coupling with local lattice vibrations of a deep level defect. The experimental results have been compared with density functional theory calculations of luminescence peak energies and line shapes of band to defect and donor-acceptor transitions, and possible origin of the orange photoluminescence band in AlN has been discussed.

Correlation among photoluminescence and the electronic and atomic structures of Sr2SiO4:xEu3+ phosphors: X-ray absorption and emission studies

A series of Eu3+-activated strontium silicate phosphors, Sr2SiO4:xEu3+ (SSO:xEu3+, x = 1.0, 2.0 and 5.0%), were synthesized by a sol–gel method, and their crystalline structures, photoluminescence (PL) behaviors, electronic/atomic structures and bandgap properties were studied. The correlation among these characteristics was further established. X-ray powder diffraction analysis revealed the formation of mixed orthorhombic α'-SSO and monoclinic β-SSO phases of the SSO:xEu3+ phosphors. When SSO:xEu3+ phosphors are excited under ultraviolet (UV) light (λ = 250 nm, ~ 4.96 eV), they emit yellow (~ 590 nm), orange (~ 613 nm) and red (~ 652 and 703 nm) PL bands. These PL emissions typically correspond to 4f–4f electronic transitions that involve the multiple excited 5D0 → 7FJ levels (J = 1, 2, 3 and 4) of Eu3+ activators in the host matrix. This mechanism of PL in the SSO:xEu3+ phosphors is strongly related to the local electronic/atomic structures of the Eu3+–O2− associations and the bandgap of the host lattice, as verified by Sr K-edge and Eu L3-edge X-ray absorption near-edge structure (XANES)/extended X-ray absorption fine structure, O K-edge XANES and Kα X-ray emission spectroscopy. In the synthesis of SSO:xEu3+ phosphors, interstitial Eu2O3-like structures are observed in the host matrix that act as donors, providing electrons that are nonradiatively transferred from the Eu 5d and/or O 2p–Eu 4f/5d states (mostly the O 2p–Eu 5d states) to the 5D0 levels, facilitating the recombination of electrons that have transitioned from the 5D0 level to the 7FJ level in the bandgap. This mechanism is primarily responsible for the enhancement of PL emissions in the SSO:xEu3+ phosphors. This PL-related behavior indicates that SSO:xEu3+ phosphors are good light-conversion phosphor candidates for use in near-UV chips and can be very effective in UV-based light-emitting diodes.

Emission and structure variations at aging ZnO:Ag nanocrystals prepared by spray pyrolysis

The emission and structure of ZnO:Ag nanocrystal (NC) films obtained by ultrasonic spray pyrolysis have been studied in the as-grown state and after aging. The scanning electronic microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL) methods have been applied. The films are characterized by the wurtzite crystal structure with the NC sizes about of 8.0–10.0 nm in the as-grown state and 6.0–8.0 nm after aging in ambient air. The Ag content was varied within the range of 1–4 at.% in the ZnO films. PL spectra of as-grown ZnO:Ag films are complex and can be presented as a set of PL bands at 10 K: the near band edge emission with the peak at 3.18 eV, two Ag doping—related PL bands centered at 2.95 and 2.68 eV, and native defect-related green (2.40 eV) and orange (1.90 eV) PL bands. The high PL intensities of Ag doping-related PL bands were detected for the Ag concentrations of 2–3 at.%. PL intensities of high energy PL bands (3.18, 2.95 and 2.68 eV) fall down at aging together with intensity stimulating the orange and green PL bands. The joint analysis of XRD and PL results permit to confirm that the 2.95 eV PL band is connected with the optical transition via the substitutional AgZn defects. It was shown as well that aging in ambient air is owing to the diffusion of oxygen interstitials in the ZnO:Ag NCs that leads to intensity increasing the orange PL band and modification of the AgZn radiative defects.

Morphology, structure and emission of Al-doped ZnO nanocrystal films

The morphology, structure and emission of Al-doped ZnO nanocrystals (NCs) with the different Al contents (1–4 at.%) were studied by means of the scanning electronic microscopy, energy dispersive X ray spectroscopy, X-ray diffraction and photoluminescence (PL) methods. Ultrasonic spray pyrolysis was applied to obtain the ZnO:Al films. To stimulate the crystallization, the ZnO:Al films were annealed at 400 °C for 4 h in a constant nitrogen flow (8 L/min). It is shown that the Al incorporation in the ZnO films with the concentrations of 2–4 at.% stimulates: the reduction of ZnO:Al grain sizes, decreasing the film crystallinity owing to disordering the ZnO:Al crystal lattice, the change of the surface morphology and increasing the surface roughness. Meanwhile, Al-doping the ZnO films at the concentrations ≤ 2 at.% enlarge significantly the PL intensity of the near band edge emission. Last fact testifies to quality improving the ZnO:Al films. Simultaneously, the PL intensities of green and orange PL bands, connected with the native defects: VZn and Oi, fall down. The ZnO NC films with Al-doping ≤ 2 at.% still keep the planar surface morphology that is important for their applications in electronic device structures.

Emission and Structure-Varying ZnO and Carbon Nanocrystal Composite in Mechanical Processing

Morphology, photoluminescence (PL), and Raman scattering spectra have been investigated for mixtures of ZnO+0.1%C nanocrystals (NCs) at different stages of mechanical processing (MP). The transformation of graphite into graphene monolayers covering the ZnO NC surface is revealed for the first MP stage. The interaction with oxygen has been detected in the second MP stage which leads to the dissolution of oxygen interstitials in the ZnO NCs and to the formation of graphene (graphite) oxides. Increasing the concentration of the oxygen interstitials in ZnO NCs also enhances the intensity stimulation of the orange PL band (2.18eV). Simultaneously, the PL band peaking at 2.82–2.90 eV is detected in the PL spectra of the ZnO+0.1%C NC mixture after MP for 9–90 min. Then, the variation of the ZnO NC shape, agglomeration of ZnO NCs, modification of ZnO defects and decreasing PL intensity have been detected after prolonged MP for 390 min. It is expected that short stages of MP can be useful for ZnO NC surface covering by graphene layers or graphene (graphite) oxides.

Silver related emitting defects and aging ZnO nanocrystals

The emission, morphology, structure and chemical composition have been studied in Ag-doped ZnO nanocrystals (NCs) in as-grown state and after aging in ambient air by means of SEM, X-ray diffraction (XRD), photoluminescence (PL) and X-ray photo electronic spectroscopy (XPS) methods. PL spectra of as-grown Ag-doped ZnO NCs include a set of elementary PL bands: near band edge (NBE) emission, green and orange PL bands, as well as Ag related PL bands with the peaks at 2.68 and 2.89 eV. The PL intensity of Ag- related PL bands falls down at aging. Simultaneously, the PL intensity of orange PL band increases. Joint XRD analysis and PL study permit identifying the optical transition connected with the substitutional AgZn defects. At XPS research the variation of AgZn state and the formation of Ag oxide at aging has been revealed. The radiative defect transformations at aging have been analyzed and discussed.

Radiative Defect Varying in Aging ZnO:Ag Nanocrystals

ZnO nanocrystal (NC) films prepared by ultrasonic spray pyrolysis and doped with Ag have been studied in as-grown and aged states. Photoluminescence (PL) and x-ray photo electronic spectroscopy (XPS) have been used in this study. PL spectra of as-grown ZnO:Ag NCs include a number of PL bands peaking at: 3.18 eV, 2.85 eV, 2.70 eV, 2.50 eV and 2.10 eV. It is revealed that the 2.70 eV and 2.85 eV PL bands are connected with Ag-doping ZnO NCs. PL intensities of the first three PL bands decline at aging, but the PL intensity of the orange PL band increases. This last fact indicates that aging at 300 K is connected with increasing the concentration of oxygen interstitials in ZnO NC films. In XPS research, the variation of substitutional AgZn defects and the formation of Ag-O bounds at aging have been detected. The nature of optical transitions and radiative defects varying at aging have been analyzed and discussed.

Emission, Defects and Structure of ZnO Nanocrystals obtained by Electrochemical method

ABSTRACTThe impact of different annealing temperatures on the crystal structure, emission and radiative defects in ZnO nanocrystals (NCs) has been investigated by means of the scanning electron microscopy (SEM), Energy dispersion spectroscopy (EDS), X-ray diffraction (XRD) and photoluminescence (PL) techniques. ZnO NCs were prepared by the anodization of zinc sheets in an electrolyte and thermal annealed at the various temperatures: 200, 240, 280, 320, 360 and 400°C for two hours in ambient air. The XRD study indicates that ZnO NCs are characterized by the hexagonal wurtzite structure.The study of annealing temperature impact on the morphology of ZnO NCs has shown that the NC size enlarges and the film crystallinity improves with increasing annealing temperatures from 200°C up to 400°C. But in the temperature range of 360-400oC the dissolution of oxygen atoms raises essentially in ZnO NC films as it follows from EDS data. Simultaneously, the near band edge (NBE) emission intensity falls down, XRD parameters of ZnO NCs change and the intensity of defect related orange and green PL bands increases owing to increasing the defect concentrations. The optimal temperatures for the ZnO NC oxidation, together with keeping the high NBE emission intensity, are estimated as 360 °C. The nature of native defects responsible for orange and green emissions in ZnO NCs has been discussed.

Radiative defects, emission and structure of ZnO nanocrystals obtained by electrochemical method

Radiative defects, emission and structure of ZnO nanocrystals (NCs) have been studied by means of the scanning electron microscopy (SEM), energy dispersion spectroscopy (EDS), X-ray diffraction (XRD) and photoluminescence (PL) techniques. ZnO NCs were prepared by etching Zn sheets and annealing at 200–400°C in ambient air. The NC size and volume of ZnO crystalline phase increase with annealing temperature rising from 200°C up to 400°C. Simultaneously, XRD parameters and inter-planar distances in ZnO NCs vary no monotonically: decreasing upon annealing at 200–360°C and increasing at 400°C annealing. The PL intensity variation of orange, yellow, green and blue PL bands correlates with the change of XRD parameters. The study of PL band thermal decays, within the range 10–300K, permits to estimate decay activation energies and to analyze the nature of donors and acceptors responsible for the orange, yellow and green PL bands in ZnO NCs.The electrochemical method allows varying the PL intensity of different PL bands by a controllable way that is interesting for ZnO NC film application in “white” light emitting device structures.

Improving color rendering index of Mn-doped ZnO nanorods on silicon-based substrate

Porous silicon pillar array (PSPA) samples which are ideal substantial materials with dominant electronic and luminescence properties were prepared by surface etching method. ZnO nanorods with or without Mn doping grown uniformly and aligned onto PSPA regardless of lattice matching show various photoluminescence (PL) properties. The doped Mn ions in ZnO nanorods were directly observed by X-ray photoelectron spectroscopy (XPS), and ZnO structures were detected by X-ray diffraction (XRD). As the doping concentration increases, XRD peaks of ZnO nanorods shift to low angle. The influences of doping Mn ions on luminescence properties of ZnO nanorods were investigated. Except for the ultraviolet (UV) PL band, the broad PL band is observed at visible region. The band could be divided into three separate bands (orange, green and red) by Lorentzian deconvolution. The intensity of orange PL band firstly increases then decreases, and then gets the maximum at the doping Mn-to-Zn molar ratio of 2.0:100.0 which is the most effective doping concentration. The green PL band is attributed to zinc vacancy of ZnO, the orange PL band to Mn ions recombination of itself, and the red PL band to oxygen vacancy of ZnO, respectively. As the Mn-doped ZnO nanorods could emit yellow green luminescence excited by UV radiation, and doped Mn ions could improve the color rendering index of the luminescence, the nanorods could be used as promising white-light emitters in the future.

An origin of orange (2 eV) photoluminescence in SiO2 films implanted with high Si+-ion doses

The photoluminescence and photoluminescence excitation spectra of SiO2 films implanted with high (3 at %) Si+-ion doses are studied in relation to the temperature of postimplantation annealing. It is shown that two photoluminescence bands with peaks at 2.7 and 2 eV are dominant in the spectra. As the annealing temperature is increased, the relation between the intensities of the 2.7 and 2 eV bands changes in favor of the former one. Both of the photoluminescence bands have their main excitation peak at the energy 5.1 eV. The excitation spectrum of the ~2-eV band exhibits also peaks at 3.8 and 4.6 eV. It is concluded that, in the implanted SiO2 films, the orange photoluminescence band originates from radiative transitions between levels of centers associated with a deficiency of oxygen (≡Si–Si≡ or =Si:) and the levels of nonbridging oxygen (≡Si–O•).

Origin of thermal annealing-induced orange emissions from solution-grown ZnO nanocrystals

ZnO nanocrystals were synthesised via solution route and then thermally annealed in air in the temperature range of 60–900°C. The correlation between the hydroxyl groups on the surface of ZnO nanocrystals and the orange photoluminescence (PL) of ZnO nanocrystals was investigated. The crystal structure, surface chemistry and luminescent properties of thermally annealed ZnO nanocrystals were examined with X-ray diffractometer, Fourier transform infrared (FTIR) spectroscope, PL spectrophotometer, transmission electron microscope (TEM) and selected area electron diffraction technique, respectively. It is found that an orange PL band appeared in the PL spectrum at about 600 nm only when the annealing temperature was higher than 600°C. By contrast, the hydroxyl groups were always present on the solution-grown ZnO nanocrystals as the annealing temperature varied in the range of 60–900°C. Our results demonstrate that the orange PL band has no direct correlation with the hydroxyl groups on the surfaces of the ZnO nanocrystals. Thermally induced defects are proposed to be responsible for the orange PL band.

Temperature-Dependent Photoluminescence of Silicon Nanoporous Pillar Array

Silicon nanoporous pillar array (Si-NPA) is a micron-nanometer hierarchical structure which might be used as functional substrates for constructing optoelectronic nanodevices. This makes understanding the photoluminescence (PL) from Si-NPA important. We measure the PL of Si-NPA in the range of 11–300 K. By analyzing the evolution of the peak energy and intensity with temperature, the ultraviolet, blue, orange and red PL bands from Si-NPA are attributed to the radiative recombination through the deep-levels in silicon oxide, oxygen-related defect states in silicon nanocrystallites (nc-Si), band-to-band transition within nc-Si, and surface/interface states of nc-Si or between nc-Si and SiOx, respectively. At least two non-radiative recombination processes, which are activated at different temperature ranges, are proposed for the PL intensity variation with temperature. These results might provide strong foundations for designing and constructing optoelectronic devices based on silicon nanostructures.

Impact of fluorine admixture, hydrogen loading, and exposure to ArF excimer laser on photoluminescence of bismuth defects in amorphous silica

Photoluminescence (PL) excited by ArF (193nm), KrF (248nm) and N2 (337nm) pulsed lasers is studied in bismuth doped unfused silicon dioxide synthesized on silica substrates by surface-plasma chemical vapor deposition (SPCVD). Additive free and fluorinated (F content ~0.4wt.%) amorphous silica are examined as host materials for bismuth. Three typical PL bands peaking at wavelengths of 650nm (orange), 800nm and 1400nm (near infrared, NIR) were observed. It is found that fluorine additive weakly affects PL detail of as deposited samples. However, hydrogen loading completely deactivates NIR PL in the case of fluorine free sample, but only slightly suppresses the NIR band in fluorinated silicon dioxide. In fluorine free sample quenching of orange PL band caused by hydrogen loading is not as great as for the NIR band. By contrast, the NIR band intensity only slightly goes down in fluorinated silica sample, whereas the orange PL band becomes even more intense as a result of hydrogen loading. Exposure of the hydrogen loaded fluorine free sample to photons of ArF excimer laser renews NIR PL. Storage of fluorine free hydrogen loaded sample in oxygen ambience leads to significant increase of the NIR PL band intensity. The effect of hydrogen loading is explained by collisional deactivation of the excited NIR PL centers. Subsequent outcome of H2 molecules accelerated by UV irradiation and/or storing the sample in oxygen ambience leads to PL recovery. Presence of fluorine in silica network somehow prevents NIR PL centers from collisions with interstitial hydrogen molecules. These molecules at the same time considerably neutralize the negative effect of fluorine on the intensity of the orange PL band. No impact of fluorine on either effectiveness of the orange PL band excitation via electron–hole recombination or inactivity of this process for the NIR PL band excitation is observed.

Acceptors related to group I elements in ZnO ceramics

ZnO ceramics doped with Li, Na or K were sintered in air for 4h at 1000°C. Electrical conductivity as well as photoluminescence (PL), PL excitation and photoconductivity spectra were measured and compared with those in undoped samples. The influence of both fast and slow cooling of the samples from 1000°C on measured characteristics was investigated. The yellow–orange PL bands associated with the deep acceptors LiZn, NaZn and KZn were observed and the corresponding PL excitation spectra were determined. These acceptors were found to form some complexes with other lattice defects.

Simultaneous micro-Raman and photoluminescence study of spark-processed germanium: Report on the origin of the orange photoluminescence emission band

Microstructure and origin of the orange photoluminescence (PL) band of visible luminescing spark-processed germanium (sp-Ge) has been studied using simultaneous micro-Raman and -PL spectroscopy and scanning electron microscope. Instability of the sp-Ge film to energetic electron beams (⩾5 keV) during SEM measurements suggested that sp-Ge is not composed exclusively of purely Ge–Ge bonded material (elemental Ge) but also of other form(s) of material(s) of different chemistry. Indeed, micro-Raman spectra of the films showed that sp-Ge is composed of mainly substoichiometric germanium oxides (GeOxs) and elemental Ge (in the form of nano- or micro-crystals). Further, it is proposed that the Ge particles were embedded in thick GeOx (0<x<2) layers. Sizes of the Ge nanocrystals were estimated using a phonon confinement model and it was found that the spark processing method can generate Ge crystals with sizes as small as 6–8 nm. However, the micro-PL of sp-Ge indicated unambiguously that the orange PL band (near 1.9 eV) does not originate from or involve these Ge nanocrystals but the Ge–O bonded material, i.e., GeOxs.

Intense and stable blue-violet emission from porous silicon modified with alkyls

An intense blue-violet band, centered at 3.0 eV, has been observed besides the red-orange band of the photoluminescence (PL) from the chemically modified porous silicon (PS) samples. The PS samples were formed on 30–50 Ω cm p-type (100) Si wafers by anodic etching at large current densities and then treated with 5-cyano-1-pentynes in toluene solution at 110–120 °C. The intensities of the blue-violet lights increase with the increasing of the etching current densities, while the peak energies are hardly changed. The Fourier transform infrared spectra of the modified samples show that the alkyls have been bonded to the surfaces of the nanometer crystallite silicon particles. The decays of the blue-violet emission with a lifetime about 0.5 μs are different from the nanosecond lifetimes of the blue emissions in the oxidized PS samples reported before, and the 20 μs lifetimes of the concomitant orange PL bands. A carbon-compounds-related luminescence center has been proposed as the origin of this blue-violet PL band.

On the formation process of luminescing centers in spark-processed silicon

Radiative and compositional properties of spark-processed silicon are studied by photoluminescence and x-ray photoelectron spectroscopy measurements. Spark processing of silicon is performed in different atmospheres composed of nitrogen and oxygen. As a result of the process, room-temperature radiative transitions occur at 2.35 eV and vary in intensity over five orders of magnitude depending on the N2/O2 ratio. After processing in pure nitrogen or pure oxygen, however, the green photoluminescence (PL) is wiped out and weak blue (2.7 eV) or orange (1.9 eV) PL bands, respectively, are discernable. The temperature-dependent features of the 2.35 eV emission are characterized by an intensity increase in conjunction with a red shift of the peak position at lowered temperatures. A cross-sectional study reveals that the green PL is mainly generated in a near-surface layer having a chemical composition close to SiO2 and a nitrogen concentration below 1 at. %. Nearly no PL was observed from a deeper SiO2 layer enriched by silicon clusters and with an increased density of nitrogen (up to 7 at. %). The findings do not support a quantum-dot-related PL mechanism in spark-processed silicon. It is proposed that nitrogen additions reduce the density of nonradiative centers introduced by silicon dangling bonds.

Electroluminescence from porous silicon using a conducting polyaniline contact

We have fabricated a p-n junction using a p-type conducting polyaniline layer deposited on an n-type porous silicon layer. This junction shows rectifying behaviour. The I–V curves can be fitted to a Schottky barrier model with a barrier height of 0.8 eV. the porous silicon shows an orange photoluminescence band due to recombination in the silicon quantum structures both before and after coating with polyaniline. The junction emits visible electroluminescence when a forward bias is applied. The emission band is very broad extending from 500 to 1 000 nm with a peak at 790 nm. Mechanisms for charge transfer between the porous silicon and polyaniline are discussed.

Multicolor-Effects of Luminescing, Nanostructured Silicon After Spark-Processing in Pure and Composite Gases

AbstractSpark-processing has been shown recently to generate a silicon-based substance which intensely photoluminesces in the UV/blue or green spectral range, depending on whether the preparation was performed in flowing or stagnant air, respectively. This study reports about significant differences for the radiative properties of spark-processed silicon after preparations in oxygen, nitrogen, and mixtures of both gases. Whereas there is essentially no photoluminescence (PL) after processing in pure nitrogen, an orange PL band at 1.89 eV occurs after preparation in pure oxygen. The orange PL degrades with time and when exposed to laser light. Furthermore, it switches immediately to a more intense blue emission centered at 2.61 eV when the sample is subjected to lower gas pressures. A similar 2.6 eV PL is observed after heat treatments up to 350°C. If, however, spark-processing of Si is carried out in nitrogen and oxygen is added, a green PL arises, centered at 2.35 eV. Moreover, for a volume ratio of about 1:1 N2/O2 a PL band at 3.22 eV emerges, superimposing the green emission. Both bands are about two orders of magnitude more intense than the orange PL, when prepared under otherwise identical processing conditions.