Decrease In Luminescence Efficiency Research Articles

Structural, optical and thermoluminescence properties of amazonite

This study presents an analysis of the structural and thermoluminescence properties of amazonite, a feldspar species. The characterization was done using X-ray diffraction (XRD), X-ray fluorescence (XRF), optical spectroscopy, and vibrational spectroscopies such as Raman and Fourier Transform Infrared (FTIR). The sample exhibits a partial (Al, Si) ordenation, and contains an albite phase in addition to microcline. FTIR and Raman indicated the presence of water and OH– vibration modes. Optical absorption measurements highlighted the influence of the hydroxyl groups as catalyst for Pb transitions, contributing to the coloration of amazonite. Thermoluminescence (TL) studies were performed within a dose range from 0.1 to 10 Gy using a built-in beta source (90Sr/90Y; delivering 10 mGy/s), showing seven TL glow peaks based on non-first kinetic order. The sample exhibited a linear dose–response, with TL signals proving to be both repeatable and reproducible (CV ∼ 2%). Fading analysis showed the first two TL peaks disappear after 16 days of dark storage. Thermal quenching behavior was investigated under various heating rates, demonstrating a decrease in luminescence efficiency with increasing heating rates. Additionally, the activation energies were determined using different methods, and the amazonite TL glow curve presented activation energies values within 0.7 to 2 eV.

Photochemical optimization of fluorescent dye-doped PDMS for enhanced luminescent solar concentrator performance

Luminescent solar concentrators (LSCs) that incorporate organic dyes face challenges such as self-absorption loss and aggregation-caused quenching (ACQ) as doping concentration increases, limiting the dye loading capacity. Particularly for dyes with a small Stokes shift, losses due to self-absorption or quenching are prominent even at low concentrations, hindering the attainment of high power conversion efficiency (PCE) in LSCs. Additionally, exposure of the dye-impregnated polymer matrix to oxygen, moisture, UV light, and other factors leads to a decrease in luminescence efficiency and stability due to the photooxidation reaction of the phosphor. This study presents a facile approach for enhancement of the efficiency and environmental stability of coumarin 6 (C6) dye-doped polydimethylsiloxane (PDMS) LSC through ultraviolet ozone (UVO) treatment. Photocleavage of the C6 dimer into a C6 monomer through UVO treatment leads to a significant enhancement in luminescence. Additionally, thin SiOx layers formed on both sides of the LSC not only assist in capturing luminescent light more efficiently but also block the penetration of oxygen and moisture into the LSC, resulting improved device stability. UVO-treated LSC shows approximately 32 % improvement in PCE compared to bare LSCs and exhibits significantly better stability during the 30-day long-term performance test.

Functional design and implementation of multilayer Ce:YAG/Cr:YAG composite transparent ceramics by tape casting for white LEDs/LDs

The shortage of red-light emission in Ce3+:Y3Al5O12 (Ce:YAG) luminescent ceramics is a bottleneck problem to realize the key applications of ceramics based white light-emitting diode (LED) and laser diode (LD) devices. However, the energy band-gap engineering for Ce3+ ion via host substitution or red-emission ions co-doping always leads to its decrease of luminescence efficiency (LE), while a spectrum overlay strategy would be simple and effective. In this study, multilayer composite structure Ce:YAG/Cr:YAG transparent ceramics (TCs) with high color rendering index (CRI) were fabricated by tape casting and vacuum sintering. An organic tape casting protocol with low toxicity and environmental protection was developed and the amounts of dispersants, binders, and plasticizers were systematically optimized to produce good quality tapes with no defects. The transmittance of ceramic reached 78% at 800 nm. By using a 455 nm blue chip, the CRI of the Ce:YAG/Cr:YAG TCs based LED device was increased from 26.7 to 77.5, and the LE was decreased from 181.5 to 10.4 lm/W. Additionally, the optimized LE and CRI for LD device were 130.2 lm/W and 78.1, respectively. Therefore, this research displays that the design and implementation of ceramic functional layer to achieve the essential enhancement of CRI in TC based white lighting is a promising method through the tape casting forming technology.

Investigation of Thermal Quenching Effect for Lithium Fluoride (LiF) Type Dosimeters

Thermal quenching is described as a decrease in luminescence efficiency with increasing measurement temperature. Luminescence intensity decreases with increasing heating rates in the presence of thermal quenching. In such a case, the heating rate to be used in the measurements becomes important. Lithium fluoride (LiF) type dosimeters have been widely used in radiation dosimetry for many years. In this study, thermal quenching effect was investigated for LiF:Mg,Ti (TLD-100) and LiF:Mg,Cu,P (TLD-100H), 6LiF:Mg,Ti(TLD-600) and 7LiF:Mg,Ti (TLD-700) at two different doses (10, 1000mGy) using 90Sr/90Y beta source. TLD-100, TLD-600 and TLD-700 showed different thermal quenching behaviors according to dose values, while TLD-100H had the same characteristics at both doses. On the other hand, other dosimeters showed thermal quenching based on the total area at 10mGy, while they did not show thermal quenching when ROI was used. Again, thermal quenching was not observed at 1000mGy for all dosimeters. In conclusion, it is recommended to use ROI or low heating rate during measurements at a low dose (in the order of mGy) for TLD-100, TLD-600 and TLD-700, while desired heating rate can be used at a high dose (Gy) for all dosimeters.

Wurtzite quantum well structures under high pressure

Quantum well systems based on semiconductors with the wurtzite crystalline structure have found widespread applications in photonics and optoelectronic devices, such as light-emitting diodes, laser diodes, or single-photon emitters. In these structures, the radiative recombination processes can be affected by (i) the presence of strain and polarization-induced electric fields, (ii) quantum well thickness fluctuations and blurring of a well–barrier interface, and (iii) the presence of dislocations and native point defects (intentional and unintentional impurities). A separate investigation of these phenomena is not straightforward since they give rise to similar effects, such as a decrease of luminescence efficiency and decay rate, enhancement of the Stokes shift, and strong blueshift of the emission with increasing pump intensity. In this Perspective article, we review the usefulness of measurements of the quantum well luminescence as a function of the hydrostatic pressure for both scientific research and the development of light-emitting technologies. The results presented here show that high-pressure investigations combined with ab initio calculations can identify the nature of optical transitions and the main physical factors affecting the radiative efficiency in quantum well systems. Finally, we will discuss an outlook to the further possibilities to gain new knowledge about the nature of recombination processes in quantum wells using high-pressure spectroscopy.

Thermal quenching studies in LiMgPO4 based OSL phosphors

LiMgPO4 based optically stimulated luminescence (OSL) phosphors are found to have good dosimetric characteristics. Thermal quenching is the decrease in luminescence efficiency of the phosphor at high temperature. In this paper we report the thermal quenching behaviour of LiMgPO4:Tb, LiMgPO4:Tb,B and LiMgPO4:B (LMP-N) OSL phosphors. The quenching has been investigated by recording the thermoluminescence (TL) glow curves with increasing heating rates. To have some knowledge about the centres responsible for quenching the luminescence was recorded at selective wavelengths. The results suggest that the centres associated with B are responsible for the quenching.

Eu3+ ternary and tetrakis complexes with carbazole and methyl group substituted dibenzoylmethane derivatives: Induction of aggregation enhanced emission

Two dibenzoylmethane (DBM) derivatives with methyl (Me-DBM) or carbazole (CBZ-DBM) substituents at the para-position of the phenyl ring and their four novel ternary and tetrakis Eu3+ complexes with 1,10-phenanthroline (PHEN) as a secondary ligand or tetraethylamonium ion (N+(Et)4) as the counter-cation were synthesized and characterized. The investigation of the optical properties of the complexes revealed that Me-DBM based compounds exhibit aggregation enhanced emission (AEE), while in the case of CBZ-DBM this effect is not observed. At the same time, the introduction of a carbazole substituent reduces the emission quantum yield of the complexes. The decrease in luminescence efficiency for CBZ-DBM based Eu3+ complexes is mainly attributed to the closely situated S1 and T1 energy levels of the ligand that obstruct the intersystem crossing process.

Photon energy (8–250 keV) response of optically stimulated luminescence: Implications for luminescence geochronology

Understanding the energy response of dosimeters and scintillators for measurements of ionising radiation is an important area in the field of solid state dosimetry. Such an understanding is particularly critical for source calibration and for quantifying effective dose (rates) in wide-energy mixed-radiation fields. While there have been several investigations on characterising the ionising- energy response of artificial dosimeters (e.g. LiF, Al2O3:C, etc.), there exist no studies on natural dosimeters such as quartz and feldspar which are extensively used in luminescence geochronology and retrospective dosimetry. Currently, luminescence geochronology assumes that all beta and gamma irradiations in nature (from the decay of K, the U and Th radioactive chains) are equivalent in terms of luminescence production per unit dose to each other, and to the laboratory 137Cs, 60Co (gamma) and 90Sr/90Y (beta) irradiations used for calibration and/or equivalent dose determination. In this study, we experimentally determine the quartz Optically Stimulated Luminescence (OSL) and feldspar Infra-Red Stimulated Luminescence (IRSL: IRSL at 50 °C and post-IR IRSL at 290 °C) response to irradiations with photons of in the energy range 8–250 keV. We show that microdosimetric effects lead to a steady, significant decrease in quartz OSL production efficiency for < 80 keV photons. Similar to quartz, the local saturation of the electron traps also leads to a systematic decrease in luminescence efficiency for < 120 keV photons in feldspar; however, this effect competes with an increase in recombination efficiency due an increased number density of hole centres in the vicinity of the trap, thus giving a peak-shaped IRSL efficiency response in the range of 250–8 keV. Our results suggest that dose-rate conversion factors, specific to radiation type and radioisotope, should be used for age calculation, and significant care should be taken for luminescence based calibration of the dose rate of the laboratory irradiation source.

Radiofluorescence as a detection tool for quartz luminescence quenching processes

Thermal quenching is a well-known phenomenon in quartz, which describes the decrease in luminescence efficiency (light output) with sample temperature. In the present work, the UV radiofluorescence (RF) signals of three different quartz samples during cooling from 500 °C to room temperature were monitored and analysed. Resulting thermal quenching parameters W (activation energy) and K (constant) agree with published values, except for one sample. Another quenching process in quartz is the reduction of luminescence sensitivity following irradiation (dose quenching), mainly known for TL and OSL of old samples with large palaeodoses. Here, the intensity of the 110 °C TL peak and the OSL signal were used to monitor the dose quenching effect. UV-OSL and UV-TL signals are analysed and found to be very similar. The UV-RF recorded during irradiation in between repeated cycles of TL and OSL measurements differs at high doses from a continuously recorded reference signal.Furthermore, numerical simulations are presented to decipher the charge transport processes in quartz. In summary, thermal quenching simulations are capable of mimicking experimental findings and confirm that UV-RF is a valuable tool to determine thermal quenching parameters. Dose quenching simulations differ from experimental results in the high dose range but help to understand the basic principle of dose quenching: charge competition of different centres.

Application of Al 2 O 3 :C+fibre dosimeters for 290 MeV/n carbon therapeutic beam dosimetry

Abstract The capability of radioluminescence (RL) dosimeters composed of carbon-doped aluminium oxide (Al 2 O 3 :C) detectors+optical fibre has been verified for absorbed dose-rate measurements during carbon radiotherapy. The RL signals from two separate Al 2 O 3 :C detectors (single crystal 'CG' and droplet 'P1') have been systematically measured and compared along the Bragg-curve and Spread-Out Bragg-Peak of 290 MeV/n carbon beams in the water. The absorbed dose response was assessed for the range of 0.5–10 Gy. For doses up to 6 Gy, we observed a linear response for both types of detectors, while for higher doses CG presented a more prominent supraliearity than P1. The RL response for low-LET protons in the entrance from the curve was found to closely resemble that observed for a clinical 6 MV X-ray beam, while it was found that P1 has a better agreement with the reference data from standard ionization chamber than CG. We observed a significant decrease in luminescence efficiency with LET in the Bragg peak region. The Al 2 O 3 :C RL luminescence efficiency differs from Al 2 O 3 :C OSL results, which implies that the signal can be corrected for LET dependency to match the correct SOBP and Bragg Peak.

Luminescent Properties of Multilayered Eu2O3 and TiO2 Grown by Atomic Layer Deposition**

Atomic layer deposition (ALD) is used to control the interatomic interactions of Eu and Ti in multilayered structures, as measured by characterizing the luminescent properties of the deposited material. Luminescent multilayer structures of Eu2O3 and TiO2 are deposited as thin films by ALD at 300 °C using Eu(thd)3/O3 and TiCl4/H2O (thd = 2,2,6,6‐tetramethyl‐3,5‐heptanedione) as precursor systems. The individual layer thickness of the multilayered structure is produced from first N ALD cycles Eu2O3 and then N ALD cycles TiO2 (N = 1 to 50), while the total film thickness is kept constant. The thinnest distinct layers are detected for N = 10, where each layer is measured to be less than 0.4 nm thick. The as‐deposited films are smooth (root mean square (rms) roughness &lt; 0.4 nm) and amorphous, independent of the layer thickness, N. The refractive index and extinction coefficient are also independent of N, while the luminescence efficiency is constant for N up to 10 cycles, and decreases for thicker superlayers. Annealing deteriorates the layered structures, causing a decrease in luminescence efficiency for thin superlayers, while thick superlayers increase in efficiency upon annealing. The films are characterized by spectroscopic ellipsometry (SE), photoluminescence (PL), X‐ray diffraction (XRD), X‐ray fluorescence (XRF), X‐ray reflectivity (XRR), and atomic force microscopy (AFM).

Morphology and chain aggregation dependence of optical gain in thermally annealed films of the conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene

Aggregate formation in conjugated polymer films is one of the most important phenomena thought to influence the photophysical properties of optical devices based on these materials. In the current work, we report the results of a detailed investigation on the morphology and chain aggregation dependence of optical gain in spin-coated thin films of the conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV). Extensive gain measurements are performed using the variable stripe length technique with picosecond pulse excitation. The polymer morphology and extent of aggregate formation in the films are controlled by thermal annealing, which is relevant to the fabrication and optimization of conjugated polymer-based optical devices. The aggregation state of the polymer chains increases with the annealing temperature, which results in a decrease in luminescence efficiency at low excitation density (≤1018 cm−3). However, the increase in aggregate formation with increasing annealing temperature does not significantly alter the optical gain; very large gain coefficients are still achieved in films containing a relatively large fraction of aggregates. Although the largest gain coefficients, 450 cm−1, are observed for as-cast (non-annealed) MEH-PPV films, very large gain coefficients of 315 and 365 cm−1 are also demonstrated for MEH-PPV films annealed at 60 and 80 °C, respectively, in spite of the enhanced packing morphology and conformational order of the polymer chains. These results are contrary to the commonly held view that aggregate formation has a detrimental effect on the amplified spontaneous emission behavior of polymer-based devices operating in the stimulated emission regime, as would be characteristic of lasers and optical amplifiers. Moreover, because aggregates promote favorable charge transport properties, our data have important implications for future development of electrically driven polymer lasers; improving carrier mobility through controlled increases in chain aggregation should provide a viable path for enhancing injection efficiency without significantly degrading optical gain.

New numerical model for thermal quenching mechanism in quartz based on two-stage thermal stimulation of thermoluminescence model

The effect of thermal quenching plays an important role in the thermoluminescence (TL) of quartz on which many applications of TL are based. The studies of the stability and kinetics of the 325°C thermoluminescence peak in quartz are described by Wintle (1975), which show the occurrence of thermal quenching, the decrease in luminescence efficiency with rise in temperature. The thermal quenching of thermoluminescence in quartz was studied experimentally by several authors. The simulations work presented in the literature is based on the single-stage thermal stimulation model of thermoluminescence, in spite of that the mechanisms of this effect remain incomplete. This paper presents a new numerical model for thermal quenching in quartz, using the previously published two-stage thermal stimulation of thermoluminescence model.

He+ ion implantation and electron irradiation effects on cathodoluminescence of plagioclase

Cathodoluminescence (CL) spectra of unirradiated, He+ ion-implanted and electron-irradiated plagioclase minerals contain the following emission bands: (1) below 300 nm due to Pb2+, (2) at ~320 and ~350 nm to Ce3+, (3) at 380–420 nm to Eu2+, Ti4+ and/or Al–O−–Al/Ti defects, (4) at 560–580 nm to Mn2+ and (5) at 720–760 nm to Fe3+. During the implantation of He+ ion, much of their energy may be dissipated by partial destruction and strain of the feldspar framework, resulting in quenching of CL. Deconvolution of CL spectra acquired from albite and oligoclase reveals an emission component at 1.86 eV (666 nm) assigned to a radiation-induced defect center associated with Na+ atoms. As its intensity increases with radiation dose, this emission component has potential for geodosimetry and geochronometry. Electron irradiation causes Na+ migration in plagioclase, and then a considerable reduction in intensity of emissions assigned to impurity centers, which is responsible for an alteration in the energy state or a decrease in luminescence efficiency following the change of activation energy. Emission intensity at 1.86 eV positively correlates with electron irradiation time for unimplanted and He+ ion-implanted albite and oligoclase, but negatively for the implanted albite above 1.07 × 10−4 C/cm2. It implies that radiation halo produced by α-particles should not be measured using CL spectroscopy to estimate β radiation dose on albite in the high radiation level.

Mechanoluminescence induced by elastic deformation of coloured alkali halide crystals using pressure steps

During the elastic deformation of coloured alkali halide crystals, the bending segments of dislocations capture F-centre electrons lying in the expansion region of edge dislocations, to the states of dislocation band. After the separation from interacting F-centres, the captured electrons move together with the bending segments of dislocations and also drift along the axis of dislocations and subsequently the radiative electron–hole recombinations, owing to both the processes of captured-electron movement, give rise to the light emission. The generation rate of electrons in the dislocation band and the mechanoluminescence (ML) intensity initially increase with time, attain maximum value at a particular time, and then they decrease with time. The intensity I m corresponding to the peak of ML intensity versus time curve and the total intensity I T of ML increase with the applied pressure and also with the density of F-centres in the crystals. At low temperature, both I m and I T increase with temperature and at higher temperature they decrease with increasing temperature due to the thermal bleaching of F-centres and also due to the decrease in luminescence efficiency. Thus, both I m and I T are optimum for a particular temperature of the crystals. For longer time duration, the ML intensity decreases exponentially with time in which the decay time is equal to the lifetime of interacting F-centres. Expressions derived for the different characteristics of ML are able to explain the experimental results. It is shown that the time constant for rise of pressure, lifetime of the interacting F-centres or damping time of dislocation segments, and the activation energy can be determined from the ML measurements.

Absorption, luminescence, and Raman spectroscopic properties of thin films of benzo-annelated metal-free porphyrazines

Thin film properties of the set of metal-free linearly benzo-annelated porphyrazines, tetra( tert-butyl)porphyrazine ( tBu-TAPH 2), tetra( tert-butyl)phthalocyanine ( tBu-PcH 2), tetra( tert-butyl)naphthalocyanine ( tBu-NpH 2), and octaphenyltetraanthraporphyrazine (AnH 2) were studied by laser excitation using micro-Raman and -luminescence spectroscopy. With the exception of tBu-TAPH 2 all compounds are characterized by a broad absorption pattern mainly due to excitonic coupling. A strong bathochromic energy shift for the Q-band absorption per annelated benzo-moiety was found in solution as well as in the solid state in going from tBu-TAPH 2 to AnH 2. Films of tBu-TAPH 2, tBu-PcH 2, and tBu-NpH 2 exhibit a single broad featureless luminescence band with a large Stokes shift, which originates from an excitonic state. In addition, tBu-TAPH 2 and tBu-NpH 2 exhibit photoluminescence peaks emerging from monomer-like molecules. These porphyrazines are the only ones known to show two luminescence peaks each. A systematic decrease in luminescence efficiency of thin films is observed and can be attributed to exciton coupling and migration leading to efficient quenching of the radiative decay channels. It also allows for Raman spectroscopic characterization of the thin films and for the case of tBu-NpH 2 in highly concentrated solutions.The different resonance Raman spectra of the compounds can be used as fingerprint for distinguishing the degree of benzo-annelation.

Medical proton dosimetry using radioluminescence from aluminium oxide crystals attached to optical-fiber cables

The prime objective of this study is to investigate if radioluminescence (RL) from carbon-doped aluminum oxide ( Al 2 O 3 : C ) crystals potentially can be used for absorbed dose-rate measurements during proton radiotherapy. The RL from two separate (2 mg) Al 2 O 3 : C crystals attached to optical-fiber cables were recorded during irradiations in water in a 175 MeV clinical proton beam. The RL response for low-LET protons in the plateau region of the Bragg curve was found to closely resemble that observed for a clinical 6 MV X-ray beam. An identical response was found in the Bragg peak (where the dose-averaged LET is about 4 keV / μ m ) for absorbed doses less than 0.3 Gy. For doses in the range of 0.3–3 Gy, we observed a significant decrease in luminescence efficiency with LET. At 3 Gy, the luminescence efficiency was about 60% in the Bragg-peak region. The study implies that the RL-signal from Al 2 O 3 : C could potentially be suitable for medical proton dosimetry in the 0–0.3 Gy range even without any LET-dependent correction factors.

PACs

LiMgPO4 based optically stimulated luminescence (OSL) phosphors are found to have good dosimetric characteristics. Thermal quenching is the decrease in luminescence efficiency of the phosphor at high temperature. In this paper we report the thermal quenching behaviour of LiMgPO4:Tb, LiMgPO4:Tb,B and LiMgPO4:B (LMP-N) OSL phosphors. The quenching has been investigated by recording the thermoluminescence (TL) glow curves with increasing heating rates. To have some knowledge about the centres responsible for quenching the luminescence was recorded at selective wavelengths. The results suggest that the centres associated with B are responsible for the quenching.

Molecular beam epitaxy growth of GaAsN layers with high luminescence efficiency

(In)GaAsN bulk layers and quantum wells usually demonstrate lower photoluminescence intensity than the nitrogen-free compositions. In the present work we have carefully optimized both conductance and operation of a nitrogen plasma source as well as growth parameters of GaAsN layers. We found conditions when incorporation of nitrogen did not lead to formation of additional nonradiative recombination. There is some minimum growth rate to obtain good crystal and optical quality of GaAsN. At growth rates below this value the pattern of reflection high energy electron diffraction turns spotty and the growth proceeds in a three-dimensional mode. This leads to a steep decrease in luminescence efficiency of the grown layer. The minimum value of growth rate depends on nitrogen content and growth temperature. Defects caused by low temperature growth are removed by post-growth annealing. We achieved the same radiative efficiency of GaAsN samples with nitrogen content up to about 1.5% grown at 520 °C as that of a reference layer of GaAs grown at 600 °C. Compositional fluctuation in the GaAsN layers leads to the S-shape temperature dependence of photoluminescence peak position. Post-growth annealing reduces compositional fluctuation.

Correlation between oxygen distribution and luminescence efficiency in SiGe/Si layer structures measured by cathodoluminescence imaging and spectroscopy

SiGe layers and quantum wells grown on Si by low-pressure chemical vapour deposition (LPCVD) have been investigated by cathodoluminescence (CL) imaging and spectroscopy. Monochromatic imaging of the shallow bound exciton luminescence from the SiGe layers showed that in some cases it was not uniform. Dark regions were observed with dimensions mainly in the range 100 - 200 . As the density and size of these regions increased, the overall luminescence efficiency decreased. In this paper we report an investigation of the possibility that oxygen incorporation in the SiGe layer structures is responsible for the decrease in luminescence efficiency. Luminescence systems created in Si by radiation damage, known to be associated with carbon - oxygen and carbon - carbon complexes, were used to investigate the depth and spatial distribution of oxygen. A SiGe sample which contained a high density of dark regions was shown to have a high oxygen concentration close to the interface, either in the Si buffer layer or the substrate. Monochromatic CL images showed that the oxygen distribution was non-uniform, and that the areas where the oxygen concentration increased could be correlated with the regions where the shallow bound exciton luminescence efficiency decreased. This implies that oxygen incorporation in the SiGe layers during growth creates the non-radiative regions.