Luminescence quenching via non-radiative recombination channels limits the efficiency of optical materials such as phosphors and scintillators and therefore has implications for conversion efficiency and device lifetimes. In materials such as Ce-doped yttrium aluminum garnet (YAG:Ce), quenching shows a strong dependence on both temperature and activator concentration, limiting the ability to fabricate high-intensity white-light emitting diodes with high operating temperatures. Here, we reveal by means of first-principles calculations an efficient recombination mechanism in YAG:Ce that involves oxygen vacancies and gives rise to thermally activated concentration quenching. We demonstrate that the key requirements for this mechanism to be active are localized states with strong electron-phonon coupling. These conditions are commonly found for intrinsic defects such as anion vacancies in wide band-gap materials. The present findings are therefore relevant to a broad class of optical materials and shine light on thermal quenching mechanisms in general.
The various decay mechanisms of excitons in cuprous oxide (Cu2O) are highly sensitive to defects which can relax selection rules. Here we report cryogenic hyperspectral imaging of exciton luminescence from cuprous oxide crystals grown via the floating zone method showing the samples have few defects. Some locations, however, show strain splitting of the 1s orthoexciton triplet polariton luminescence. Strain is reduced by annealing. In addition, annealing causes annihilation of oxygen and copper vacancies, which leads to a negative correlation between luminescence of unlike vacancies.
We report, for the first time, the luminescence property of the hydroxyl group surface functionalized quantum dots (QDs) and nanoparticles (NPs) of SnO_2 using low energy excitations of 2.54 eV (488 nm) and 2.42 eV (514.5 nm). This luminescence is in addition to generally observed luminescence from 'O' defects. The as-prepared SnO_2 quantum dots (QDs) are annealed at different temperatures in ambient conditions to create varied NPs in size. Subsequently, average size of the NPs is calculated from the acoustic vibrations observed at low frequencies in the Raman spectra and by the transmission e…
We study from first principles the luminescence of Lu$_2$SiO$_5$:Ce$^{3+}$ (LSO:Ce), a scintillator widely used in medical imaging applications, and establish the crucial role of oxygen vacancies (V$_O$) in the generated spectrum. The excitation energy, emission energy and Stokes shift of its luminescent centers are simulated through a constrained density-functional theory method coupled with a $Δ$SCF analysis of total energies, and compared with experimental spectra. We show that the high-energy emission band comes from a single Ce-based luminescent center, while the large experimental spread…
Optical photoluminescence studies are performed in self-ion (Ga+)-implanted nominally doped n-GaN nanowires. A 50-keV Ga+ focused ion beam (FIB) in the fluence range of 1x1014 -2x10^16 ions cm^-2 is used for the irradiation process. A blueshift is observed for the yellow luminescence (YL) band with increasing fluence. Donor-acceptor pair (DAP) model with emission involving shallow donor introduced by point-defect clusters related to nitrogen vacancies and probable deep acceptor created by gallium interstitial clusters is made responsible for the shift. High temperature annealing in nitrogen am…
Using temperature dependent photoluminescence (PL), polarization resolved PL and Raman spectroscopy, we investigate the effect of in situ vacuum annealing as well as the relaxation of strain on the luminescence and the valley polarization properties of large area strictly monolayer (1L)-MoS$_2$, grown on sapphire and SiO$_2$/Si substrates by a microcavity based chemical vapor deposition (CVD) technique. The study shows that the strain as well as the physisorption of air molecules at the sulfur vacancy ($V_S$) sites play key roles in governing the optical quality of CVD grown 1L-MoS$_2$. Remova…
Phonons are produced when an excited vacancy in cuprous oxide (Cu$_2$O) relaxes. Time resolved luminescence was used to find the excited copper vacancy (acceptor) and oxygen vacancy (donor) trap levels and lifetimes. It was also used to determine the typical energy and number of phonons in the phonon pulses emitted by vacancies. The vacancy properties of cuprous oxide are controlled by several synthesis parameters and by the temperature. We directly demonstrate the absorption of light by oxygen vacancies with transient absorption. Copper and oxygen vacancies behave differently, in part because…
First-principle study of possible bismuth-related centers in SiO2 and GeO2 hosts is performed and the results are compared with the experimental data. The following centers are modeled: trivalent and divalent Bi substitutional centers; BiO interstitial molecule; interstitial ion, Bi^+, and atom, Bi^0; Bi...Si-Si and Bi...Ge-Ge complexes formed by interstitial Bi atoms and glass intrinsic defects, Si-Si or Ge-Ge oxygen vacancies; interstitial dimers, Bi0 and Bi_2^-. Experimental data available on bismuth-related IR luminescence in SiO2:Bi and GeO2:Bi glasses, visible (red) luminescence in SiO2:…
Experimental and theoretical studies of spectral properties of chalcogenide Ge-S and As-Ge-S glasses and fibers are performed. A broad infrared (IR) luminescence band which covers the 1.2-2.3~$μ$m range with a lifetime about 6~$μ$s is discovered. Similar luminescence is also present in optical fibers drawn from these glasses. Arsenic addition to Ge-S glass significantly enhances both its resistance to crystallization and the intensity of the luminescence. Computer modeling of Bi-related centers shows that interstitial Bi$^+$ ions adjacent to negatively charged S vacancies are most likely respo…
ZrO$_2$ nanotubular arrays with intrinsic defects are a promising solid-state basis for the development of devices for detecting, storing, and converting energy. Layers of the self-ordered zirconia nanotubes of 5 $μ$m length and 30 nm diameter, containing oxygen vacancies and their complexes, have been synthesized by anodic oxidation. The spectrally resolved TSL (thermally stimulated luminescence) above room temperature for the samples exposed by UV-irradiation with an energy of 4.1 eV have been studied. Two emission bands with maxima near 2.5 and 2.8 eV, associated with radiative relaxation o…
Structure and energetics of rare-earth (RE) defects and luminescence of RE and related defects in zincblende zinc sulfide (ZnS) are investigated using hybrid density-functional defect calculations. We find that europium (Eu) is stable predominantly as the divalent Eu$^{2+}$ ion in bulk ZnS. The trivalent Eu$^{3+}$ is structurally and electronically stable, but energetically unfavorable compared to Eu$^{2+}$ due to the presence of low-energy native defects and Eu$^{2+}$-related defect complexes. Other RE dopants, dysprosium (Dy) and erbium (Er), are stable only as Dy$^{3+}$ and Er$^{3+}$, respe…
Zinc sulfide (ZnS) based materials are widely used in many applications. Yet, due to a lack of detailed knowledge of defect energy levels, the electrical properties and luminescence mechanisms in the materials still give rise to debate. Here, we report a first-principles study of native point defects and impurities in zincblende ZnS using hybrid density-functional calculations. We find that cation and anion vacancies and antisite defects introduce deep defect levels in the band gap and can act as donors or acceptors depending on the position of the Fermi level. The substitutional impurity Cu$_…
Photo- and thermally stimulated luminescence characteristics of Gd3(Ga,Al)5O12:Ce single crystals co-doped with W and Mo are investigated in the 85 - 510 K temperature range and compared with the corresponding characteristics of the undoped and Ce3+ - doped Gd3(Ga,Al)5O12 single crystals of similar composition. A strong effect of the W and Mo impurity ions appears in the photoluminescence spectra and temperature dependences of the photoluminescence intensity, afterglow intensity and decay kinetics, thermally stimulated luminescence (TSL) intensity and TSL glow curves, excitation spectra of the…
We investigate native point defects and rare-earth (co)dopants in SrAl$_2$O$_4$ using hybrid density-functional defect calculations. Europium (Eu) and dysprosium (Dy) are found to be mixed valence and energetically most favorable at the Sr lattice sites. However, unlike Eu where both Eu$^{2+}$ and Eu$^{3+}$ can be realized in synthesis, Dy is stable predominantly as Dy$^{3+}$, and the divalent Dy$^{2+}$ may only be photogenerated under irradiation. On the basis of an analysis of Eu-related band-defect (including charge-transfer) and interconfigurational $5d$-$4f$ optical transitions, we assign…
Defect pyrochlore oxides have attracted a great interest as promising luminescent materials due to their flexible composition and high electron/hole mobility. In this work, we investigate the structural and electronic properties of lanthanide-doped (Ln) defect pyrochlore oxides \ce{KNbWO6}:0.125Ln$^{3+}$ by using first-principles calculations. We perform structural optimizations of various defect pyrochlore models and calculate their electronic structures, revealing that hydration has a significant influence on both local symmetry around Eu$^{3+}$ ion and band structures with an alteration of …
The blue-green photoluminescence emitted by pure and electron-doped strontium titanate under intense pulsed near-ultraviolet excitation is studied experimentally, as a function of excitation intensity and temperature. Both emission spectra and time-resolved decays of the emission are measured and analyzed in the framework of simple phenomenological models. We find an interesting blue-to-green transition occurring for increasing temperatures in pure samples, which is instead absent in doped materials. The luminescence yield and decay rate measured as a function of temperature can be modeled wel…
First-principle study of bismuth-related oxygen-deficient centers ($=$Bi$\cdots$Ge$\equiv$, $=$Bi$\cdots$Si$\equiv$, and $=$Bi$\cdots$Bi$=$ oxygen vacancies) in Bi$_2$O$_3$-GeO$_2$, Bi$_2$O$_3$-SiO$_2$, Bi$_2$O$_3$-Al$_2$O$_3$-GeO$_2$, and Bi$_2$O$_3$-Al$_2$O$_3$-SiO$_2$ hosts is performed. A comparison of calculated spectral properties of the centers with the experimental data on luminescence emission and excitation spectra suggests that luminescence in the 1.2-1.3 $μ$m and 1.8-3.0 $μ$m ranges in Bi$_2$O$_3$-GeO$_2$ glasses and crystals is likely caused by $=$Bi$\cdots$Ge$\equiv$ and $=$Bi$\c…
We report on the systematic characterization of photoluminescence (PL) lifetimes in NV- and NV0 centers in 2 MeV H+ implanted type Ib diamond samples by means of a time correlated single photon counting (TCSPC) microscopy technique. A dipole-dipole resonant energy transfer model was applied to interpret the experimental results, allowing a quantitative correlation of the concentration of both native (single substitutional nitrogen atoms) and ion-induced (isolated vacancies) PL-quenching defects with the measured PL lifetimes. The TCSPC measurements were carried out in both frontal (i.e. laser …
Defect physics of europium (Eu) doped GaN is investigated using first-principles hybrid density-functional defect calculations. This includes the interaction between the rare-earth dopant and native point defects (Ga and N vacancies) and other impurities (O, Si, C, H, and Mg) unintentionally present or intentionally incorporated into the host material. While the trivalent Eu$^{3+}$ ion is often found to be predominant when Eu is incorporated at the Ga site in wurtzite GaN, the divalent Eu$^{2+}$ is also stable and found to be predominant in a small range of Fermi-level values in the band-gap r…
ZnO is co-doped with Na+ and Si4+ in the ratio 2:1. The ratio was intentionally chosen so that net valence state of dopant theoretically matches that of host. This is to avoid dependence in the amount of oxygen vacancies/interstitials arising out of cationic valence state of the dopant. With such a combination, modifications in structural and optical properties do not depend on excess or deficit of the dopant charge state. For lower doping, Na+ ions behave as interstitial sites which enhance strain, lattice disorder and thereby creating defects. Formation of interstitial defects leads to reduc…
ZnO films have been grown on gold (111) by electrodeposition using two different OH- sources, nitrate and peroxide, in order to obtain a comparative study between these films. The morphology, structural and optical characterization of the films were investigated depending on the solution used (nitrate and peroxide) and the applied potential. Scanning Electron Microscopy pictures show different morphologies in each case. X-Ray Diffraction confirms that the films are pure ZnO oriented along the (0002) direction. ZnO films have been studied by photoluminescence to identify the emission of defects…
Device contacts as spin-state selectors for silicon vacancies in 4H-SiC
University of Oslo（奥斯陆大学）；ETH Zürich（苏黎世联邦理工学院）；Paul Scherrer Institute（保罗谢尔研究所）；California Institute of Technology（加州理工学院）；NASA Jet Propulsion Laboratory（美国国家航空航天局喷气推进实验室）
The signature of defects in the optical spectra of hexagonal boron nitride (BN) is investigated using many body perturbation theory. A single BN-sheet serves as a model for different layered BN- nanostructures and crystals. In the sheet we embed prototypical defects such as a substitutional impurity, isolated Boron and Nitrogen vacancies, and the di-vacancy. Transitions between the deep defect levels and extended states produce characteristic excitation bands that should be responsible for the emission band around 4 eV, observed in luminescence experiments. In addition, defect bound excitons o…
It was recently shown that the energy resolution of Ce-doped LaBr$_3$ scintillator radiation detectors can be crucially improved by co-doping with Sr, Ca, or Ba. Here we outline a mechanism for this enhancement on the basis of electronic structure calculations. We show that (i) Br vacancies are the primary electron traps during the initial stage of thermalization of hot carriers, prior to hole capture by Ce dopants; (ii) isolated Br vacancies are associated with deep levels; (iii) Sr doping increases the Br vacancy concentration by several orders of magnitude; (iv) $\text{Sr}_\text{La}$ binds …
Understanding the luminescence of GaN doped with erbium (Er) requires a detailed knowledge of the interaction between the rare-earth dopant and the nitride host, including intrinsic defects and other impurities that may be present in the host material. We address this problem through a first-principles hybrid density functional study of the structure, energetics, and transition levels of the Er impurity and its complexes with N and Ga vacancies, substitutional C and O impurities, and H interstitials in wurtzite GaN. We find that, in the interior of the material, Er$_{\rm Ga}$ is the dominant E…
Rare-earth doped barium zirconate (BaZrO$_3$) ceramics are of interest as proton-conducting and luminescent materials. Here, we report a study of dysprosium (Dy) and other relevant point defects in BaZrO$_3$ using hybrid density-functional defect calculations. The tetravalent Dy$^{4+}$ is found to be structurally and electronically stable at the Zr lattice site (i.e., as Dy$_{\rm Zr}^0$), but most often energetically less favorable than the trivalent Dy$^{3+}$ (i.e., Dy$_{\rm Zr}^-$) in as-synthesized BaZrO$_3$, due to the formation of low-energy, positively charged oxygen vacancies and the mi…
