Present Photoluminescence Research Articles

Synthesis and photoluminescence characteristics of a novel Eu and Tb doped Li2MoO4 phosphor

Li2MoO4:x Eu3+ and Li2MoO4:xTb3+ phosphors, where x = 0.5, 1, 2, 3, 5 and 7 wt%, were synthesized through a gel-combustion method. The XRD data reveals that Eu3+ and Tb3+ doped Li2MoO4 phosphors exhibit a Rhombohedral structure belonging to the space group R3 which matched well with the standard JCPDS files (No.012-0763). We present photoluminescence (PL) spectra from Eu and Tb doped Li2MoO4 under 349 nm Nd:YLF pulses laser excitation over the temperature range of 10–300 K. Undoped Li2MoO4 shows a wide broad band around 600 nm because of the intrinsic PL emission of tetrahedral of MoO42− which was in good agreement with previous findings. Under the excitation of 394 nm, the as-synthesized phosphors exhibited sharp and strong intensity PL emission signals in the red (612 nm, 5D0→7F2 transition) and green (544 nm, 5D4→7F5 transition), respectively. The critical doping concentration of Eu3+ and Tb3+ ions in the Li2MoO4 were estimated to be 2 wt%. The concentration quenching phenomena were discussed, and the critical distances for energy transfer have also been evaluated by the concentration quenching.

Complex features of the photoluminescence from ZnO nanorods grown by vapor-phase transport method

We present photoluminescence (PL) studies of ZnO nanorods synthesized by high-temperature vapor-phase transport method under pulse laser excitation. The PL spectrum contains a strong near band edge (NBE) emission located at around 387 nm and no emission bands in the visible range were observed. Our studies show that an increase in power density of pulse UV laser results in appearance of a so-called P-band centered at 389 nm and it overlaps the NBE emission. The variations in excitation intensity and detection angles revealed that the band at 389 nm can be deconvoluted into Lorentzian components at ~382 and ~395 nm. Excitation intensity dependence of the band at 389 nm exhibits a clear threshold behavior while the intensity of the band at ~395 nm grows almost linearly with the increase in the excitation intensity. It is possible that the emission band at 395 nm is associated with emission centers caused by zinc vacancy defects or their complexes in ZnO nanorods.

Luminescence Mechanism in Amorphous Silicon Oxynitride Films: Band Tail Model or N-Si-O Bond Defects Model

Silicon oxynitride films are one kind of important gate dielectric materials for applications in the fabrication of silicon CMOS integrated circuits (ICs), which have been widely and deeply studied. However, with the significant demand of the technologies for Si-based monolithic optoelectronic ICs, the research efforts on the optoelectronic applications of these materials have been continually increasing, particularly in the study of light emission properties and recombination mechanisms. In this paper, we first briefly outline the present photoluminescence (PL) mechanisms in amorphous silicon oxynitride (a-SiOxNy) films. Since the PL properties and recombination processes are affected by both structural disorder and chemical disorder, the PL mechanism has been still unclear and even controversial until now. Among these various PL recombination models, the band tail states and defect states models have gained general consensus. Recently a N-Si-O bond defect model has been reported, which depends on relative atom concentration of oxygen and nitrogen in the silicon oxynitride materials. It has been revealed that oxygen bonding plays a key role not only in reducing the structural disorder, but also in creating N-Si-O (Nx) defect states in the band gap. The characteristics of two models, namely band tail and N-Si-O bond defects, have been discussed in detail. Finally it has been shown that by controlling the chemical composition of these non-stoichiometric silicon oxynitride materials, the optical and electronic properties can be improved.

Photoluminescence properties of CaWO4 and CdWO4 thin films deposited on SiO2/Si substrates

In this study, we present photoluminescence (PL) analyses under UV and X-ray excitations of thin layers of CaWO4 and CdWO4 deposited on SiO2/Si substrates, by radio-frequency sputtering method. The main objective was to determine the efficiency of PL emissions in the case of these specific SiO2/Si substrates. Polycrystalline CaWO4 and CdWO4 phases were used as standards for PL emission analyses. Characterizations of films were carried out by X-ray diffraction, scanning electron microscopy, and atomic force microscopy. The PL experiments were carried out under monochromatic UV and polychromatic X-ray excitations. The PL intensities varied with tungstate film thicknesses. In the case of X-ray excitation, oscillations of PL intensities were observed. These oscillations corresponded to interferences of PL emissions, strongly correlated with the thickness of intermediate SiO2 layers. A Fabry-Perot model simulating these oscillations is applied allowing differentiating the PL responses of these films.

Photoluminescence study of deep donor- deep acceptor pairs in Cu2ZnSnS4

We present photoluminescence (PL) studies of high quality Cu2ZnSnS4 single crystals and thin films. At T = 10 K two PL bands (D1 and D2) were detected in both samples at about 1.35 eV and 1.27 eV. The temperature and laser power dependencies indicate that the properties of PL bands can be explained by deep donor-deep acceptor pair model, where the D1 and D2 bands result from a recombination between pairs of the closest neighbors, and between pairs of the next-closest neighbors, respectively. The donor defect in these pairs is suggested to be an interstitial Zn atom and located in either of the two possible interstitial positions. The most probable deep acceptor defect in this pair is CuZn.

Mn doped AIZS/ZnS nanocrystals: Synthesis and optical properties

In this work, Mn doped AIZS/ZnS (Mn:AIZS/ZnS) nanocrystals (NCs) have been synthesized in an approach using heat-up and drop-wise addition of precursors. On the basis of the characterization of these doped NCs on their optical properties and materials, it is found that: (1) as more Mn atoms are doped into NCs, the doped NCs present photoluminescence (PL) red-shift and quantum yield quenching; (2) the doped NCs possess a short PL lifetime in tens of microseconds and a long PL lifetime in hundreds of microseconds, and the short lived PL is more dominant than the long lived one; and (3) the doped NCs present a reversible PL thermal quenching in a range from room temperature to 170 °C. Possible PL mechanisms of these NCs were discussed by analyzing their time-resolved PL spectra and thermal stability.

Effect of Structural Phase Transition on Charge-Carrier Lifetimes and Defects in CH3NH3SnI3 Perovskite.

Methylammonium tin triiodide (MASnI3) has been successfully employed in lead-free perovskite solar cells, but overall power-conversion efficiencies are still significantly lower than for lead-based perovskites. Here we present photoluminescence (PL) spectra and time-resolved PL from 8 to 295 K and find a marked improvement in carrier lifetime and a substantial reduction in PL line width below ∼110 K, indicating that the cause of the hindered performance is activated at the orthorhombic to tetragonal phase transition. Our measurements therefore suggest that targeted structural change may be capable of tailoring the relative energy level alignment of defects (e.g., tin vacancies) to reduce the background dopant density and improve charge extraction. In addition, we observe for the first time an above-gap emission feature that may arise from higher-lying interband transitions, raising the prospect of excess energy harvesting.

Green photoluminescence in GdAlO3−δ powders

Nanocrystalline GdAlO3−δ powders were prepared by means of solid state-reaction process. The parent compound GdAlO3 (δ=0.0) has a broad electronic band gap of 5.0eV and do not present photoluminescence (PL) emission. This work aims to study the effect of the introduction of oxygen deficiencies in the GdAlO3−δ compounds on the establishing of PL signal. XRD patterns revealed that all samples present an orthorhombic structure with the absence of structural transition when increasing δ.The optical study exposed a very intense green PL emission, observed at room temperature and centered at 530nm using a 350nm excitation line. The PL emission was related to the oxygen deficiencies doubly ionized vacancy VO••, created in the orthorhombic structure of GdAlO3−δ compounds, which may be responsible for the formation of intermediary energy levels into the band gap.

Defects in 700 keV oxygen ion irradiated ZnO

It is well known that energetic oxygen ions induce heavy crystalline disorder in ZnO, however, systematic study on this regard is very much limited. Here, we present photoluminescence (PL), optical absorption and sheet resistance measurements on poly and single crystalline ZnO samples irradiated with 700keV O ions. Results have been compared with the effects of 1.2MeV Ar irradiation on similar ZnO target. Colour change of the samples with increasing O irradiation fluence has also been noted. Non-monotonic variation of room temperature sheet resistance with the increase of fluence has been observed for polycrystalline ZnO. Such an outcome has been understood as point defects transforming to bigger size clusters. Near band edge (NBE) PL emission is largely reduced due to O ion irradiation. However, at 10K NBE emission can be observed for irradiated polycrystalline samples. Irradiated ZnO single crystal does not show any band to band transition even at 10K. It is evident that dynamic recovery of defects is more effective in polycrystalline samples. Ultraviolet–visible absorption spectrum of the irradiated ZnO crystal show pronounced sub-band gap absorption. Oxygen irradiation generated new absorption band in ZnO is at 3.05eV. In the light of earlier reports, this particular band can be ascribed to absorption by neutral oxygen vacancy defects.

Carrier dynamics and photoluminescence quenching mechanism of strained InGaSb/AlGaSb quantum wells

GaSb based quantum wells (QWs) show promising optical properties in near-infrared spectral range. In this paper, we present photoluminescence (PL) spectroscopies of InxGa1−xSb/AlyGa1−ySb QWs and discuss the possible thermal quenching and non-radiative carrier recombination mechanisms of the QW structures. The In and Al concentrations as well as the QW thicknesses were precisely determined with the X-ray diffraction measurements. Temperature dependent time-integrated and time-resolved PL spectroscopies resulted in the thermal activation energies of ∼45 meV, and the overall self-consistent calculation of the band parameters based on the measured physical values confirmed that the activation energies are due to the hole escape from the QW to the barriers. The relation of the present single carrier escape mechanism with the other escape mechanisms reported with other material systems was discussed based on the estimated band offset. The relation of the present thermal hole escape to the Auger recombination was also discussed.

Tuning Intermolecular Interactions in Dioctyl-Substituted Polyfluorene via Hydrostatic Pressure

Polyfluorenes (PFs) represent a unique class of poly-para-phenylene-based blue-emitting polymers with intriguing structure-property relationships. Slight variations in the choice of functionalizing side chains result in dramatic differences in the inter- and intrachain structures in PFs. Dioctyl-substituted PF (PF8) is characterized by different backbone conformations that depend upon the torsion angle between the monomers. We present photoluminescence (PL) and Raman scattering studies of bulk samples and thin films of dioctyl-substituted PF (PF8) under hydrostatic pressure. The bulk sample was further thermally annealed and studied as a function of pressure. The PL energies of the as-is and thermally annealed samples both red shift but at very different rates, and the difference between their pressure coefficients elucidates the role of the backbone torsional angle. This is further corroborated by density functional theoretical calculations of a fluorene oligomer, where the energy gap is calculated as a function of both the torsion angle as well as compression. The Raman peaks harden with increasing pressures; the intraring C-C stretch frequency at 1600 cm(-1) has a pressure coefficient of 7.2 cm(-1)/GPa and exhibits asymmetric line shapes at higher pressures, characteristic of a strong electron-phonon interaction.

Photoluminescence Spectroscopy of CdTe/ZnTe Self‐Assembled Quantum Dots

We present photoluminescence (PL) measurements of two different, 3 monolayers and 12 monolayers (ml), CdTe self‐assembled quantum dot (SAQD) samples. The spectra were recorded in the temperature range 20 K–300 K, with photoexcitation over the ZnTe barrier layer. PL spectra displayed two main emission bands. High‐energy PL emission (E1) is ZnTe LO like phonon‐ (ωLO = 204.2 cm−1 (3 ml), ωLO = 207.3 cm−1 (12 ml)) assisted deexcitation. Dominant low‐energy band (E2) presents the direct deexcitation to ground state of the CdTe quantum dots.

Photoluminescence and Electron Paramagnetic Resonance Studies of Bulk GaN Doped with Gadolinium

Theoretical predictions of possible room temperature ferromagnetism in GaN diluted with manganese strongly motivated studies of nitrides based diluted magnetic semiconductors (DMS) with transition metals or rare-earths. Over the last years, above room temperature ferromagnetism for MBE grown GaN layers weakly doped with Gd has been reported by different researchers [for example see ref.1]. The estimated average value of magnetic moment per Gd ion has been obtained as high as 500 times that of Gd atomic value. It has been argued that the observed enhancement of the average magnetic moment resulted from strong contribution of GaN host lattice to macroscopic magnetization of the whole material [1]. The origin of such behavior remains still puzzling and more studies of GaN:Gd are required to clarify this issue. In this paper we present photoluminescence (PL) and electron paramagnetic resonance (EPR) experiments on strain free GaN bulk crystals of wurtzite structure doped with gadolinium. The samples were grown from the solution of nitrogen in liquid gallium under high (1.5GPa) pressure (HP) of N2 and at elevated temperatures of about 1500 0 C. Doping with Gd was obtained by adding this element to the solution. Both PL and EPR techniques are well known as a powerful experimental tools to study electronic and magnetic properties of transition metals and rare-earth in semiconductors, and they have not been exploited up to now for GaN:Gd system. In EPR experiment sharp absorption lines were detected, which could be attributed to Gd 3+ (4f 7 ) state. Identification of the lines was possible by comparison with other semiconductors doped with Gd. The observed Dysonian shape of the lines originated from presence of high concentration of free electrons in measured GaN. However, the estimated value of free electron concentration was definitively lower than in undoped HP GaN, what indicated purification role of Gd from unintentional donors (presumably oxygen). PL studies revealed intensive spectra in two energy range: close to GaN band gap (3.13.6 eV) and in the “red” range of 1.6-1.8 eV. Band-to band transitions, typical for GaN doped with free electron concentration at a high 10 18 cm -3 level, were clearly seen. They were followed by several sharp peaks in direction of lower energy. In the “red” range sharp structure with main line at 1.77eV energy was detected. The FWHM of this line was as small as 5meV. This structure was related to Gd 3+ (4f 7 ) state. Judging from EPR and PL results Gd dopant was present in the studied samples. However, no ferromagnetic behavior of the samples was observed by means of EPR. This creates some doubts if ferromagnetism observed by others was really related to GaGdN system.

A hydrogen-related shallow donor in GaN?

We present photoluminescence (PL) data for deliberately O-doped, high-resistive GaN samples where a new shallow donor-bound exciton (DBE) peak at about 3.4746 eV (corrected for strain shift) at 2 K appears. This DBE is strongly enhanced upon annealing in the entire range 450–900 °C. The possible relation of this DBE to a metastable H donor state is discussed.

Study of optical properties of TRIS (8-hydroxyquinoline) aluminum (III)

Abstract In this article, we present photoluminescence (PL) properties of tris (8-hydroxyquinoline) aluminum(III) (Alq3) layers. The refractive index, absorption coefficient and layer thickness were determined from reflection (R) and transmission (T) spectra.

Photoluminescence dynamics in ensembles of wide-band-gap nanocrystallites and powders

We present photoluminescence (PL) studies of GaN and ZnO nanocrystallites and powders. Our studies show that in addition to the intrinsic photoluminescence characteristics, the photoluminescence properties of the porous media are also a strong function of conditions such as ensemble size and powder density, ultraviolet-laser excitation power, and vacuum state. PL redshifts up to 120 meV were observed for GaN and ZnO crystallites and were attributed to laser heating and heat trapping in the ensemble. The electron-phonon interaction model for GaN indicated ensemble temperature ∼550 K, which is consistent with the finding obtained via high-temperature PL and Raman experiments. The PL in the vacuum state exhibited a significant redshift, ∼80 meV relative to that in air, and the PL of a dense ZnO pellet was found to resemble that of the bulk more than does a loose powder. The PL analyses indicated an excitonic emission at room temperature for both GaN and ZnO crystallites with intensity saturation occurring for large ensembles at high laser power.

Enhancement of 1.54 μm photoluminescence observed in Al-doped β-FeSi2

We present photoluminescence (PL) properties of impurity (Al, Mn, Co)-doped β-FeSi2 produced by ion-beam synthesis. The β-Fe(Si1−xAlx)2 sample showed an increase of both the PL intensity and activation energy (Ea) for a nonradiative recombination path. On the contrary, the β-(Fe1−xMnx)Si2 and the β-(Fe1−xCox)Si2 samples reduced the PL intensity and Ea. We found that the doping of Al atoms occupying the Si sites in β-FeSi2 is effective to enhance the 1.54 μm photoluminescence.

Structural and optical characterisation of ZnO nanocrystals embedded in bulk KBr single crystal

The ZnO nanocrystals (NCs) embedded in KBr single crystal matrix has been fabricated using the Czochralski (Cz) method. X-ray diffraction has confirmed the embedding of ZnO nanocrystals inside the bulk KBr matrix. A blue shift of the absorption edge of the obtained samples has been observed. Moreover, ZnO nanocrystals embedded in bulk KBr single crystals, present photoluminescence (PL) bands, at 1.6 K, associated with free and bound excitons. These results confirm that bulk KBr single crystal is a suitable matrix-host for ZnO nanocrystals.

Effect of temperature and pressure on the optical properties of polyfluorene

We present photoluminescence (PL) and Raman scattering studies from polyfluorene as a function of hydrostatic pressure and temperature. The PL backbone emission blue shifts and changes dramatically around 20 kbar becoming primarily a single peak. This is related to the emission from keto defects in the sample. The Raman modes shift to higher energies and exhibit unexpected antiresonance lineshapes at higher pressures, indicating a strong electron-phonon interaction.

Optical and electrical transport mechanisms in Si-nanocrystal-based LEDs

In this work, we present photoluminescence (PL) and electroluminescence studies on thermal SiO 2 Si-implanted light-emitting devices (LEDs) containing crystallites of size 3 nm . The PL measurements reveal a bulk-type behavior of nc-Si-associated PL confirming that the PL obtained at ∼1.6 eV is closely related to Q-confined nc-Si. Analysis of electrical transport mechanisms confirms that the current is related to a tunneling process as expected and not to Fowler–Nordheim regime. Finally, threshold electric field for light emission obtained was as low as 5 MV/cm , confirming the real potential of a such Si-implanted material for LEDs based on Si.