Pressure-Induced Enhancement of the Photoluminescence in Ca 1−x Sr x F 2 :Mn 2+

The surprising disappearance of the Mn 2+ photoluminescence (PL) on passing from CaF 2 to SrF 2 through the Ca 1 m x Sr x F 2 :Mn 2+ series is investigated through pressure spectroscopy, and as a function of the temperature. The PL quenching which is observed along the series is explained on the basis of multiphonon relaxation mechanisms, which are described by a thermally activated process. In non-luminescent crystals, PL can be recovered by applying pressure. The results are accounted for through an universal equation relating the PL lifetime as a function of P (or V ) and T , using an activation energy and a transition rate that are strongly dependent on the lattice parameter (crystal volume).     

Abnormal Energy Dependence of Photoluminescence Decay Time in InGaN Epilayer

We employ photoluminescence (PL) and time-resolved PL to study exciton localization effect in InGaN epilayers.By measuring the exciton decay time as a function of the monitored emission energy at different temperatures,we have found unusual behaviour of the energy dependence in the PL decay process. At low temperature, the measured PL decay time increases with the emission energy. It decreases with the emission energy at 200K, and remains nearly constant at the intermediate temperature of 12OK. We have studied the dot size effect on the radiative recombination time by calculating the temperature dependence of the exciton recombination lifetime in quantum dots, and have found that the observed behaviour can be well correlated to the exciton localization in quantum dots. This suggestion is further supported by steady state PL results.     

Polariton thermalization in GaN microcavities in the strong light–matter coupling regime

We discuss the thermalization of the polariton population from a bulk GaN λ/2 microcavity at room temperature. Simultaneous optical measurements of reflectivity and photoluminescence (PL), as well as time resolved measurements, reveal strong light–matter coupling. Transfer matrix theory is used to calculate reflectivity, absorption, and transmission coefficients for the structure. The PL emission is found to be thermalized, despite its very short lifetime, suggesting the existence of very fast energy relaxation channels.     

Excitation process and photoluminescence properties of Tb and Eu ions in SnO2 and in SnO2: Porous silicon hosts

Tin oxide (SnO₂)-layers-doped terbium and europium ions are elaborated by the sol-gel method on silicon substrates. After annealing at 500 °C, the transmission electron microscopy revealed a crystallization of tin oxide.The emission properties of rare-earth in SnO₂ are studied systematically against temperature annealing and Tb³⁺ concentration. The PL spectrum is optimal after annealing at 900 °C and the corresponding photoluminescence (PL) decay is nearly exponential, showing that the sample is homogenous and the PL process can be described by two levels system.The concentration effect shows a quenching of the PL intensity for Tb³⁺ concentration above 4%. From the investigation of the decay rate from the ⁷F₅ state within terbium concentration, we show that self-quenching is insured by dipole - dipole interaction. The evolutions of both PL intensity and PL lifetime versus temperature are studied. The PL intensity and PL lifetime are enhanced by deposing SnO₂:Tb³⁺ and SnO₂:Eu³⁺ in porous silicon. We show that an efficient excitation transfer from Si nanocrystallites to RE ions can occur.     

Fermi edge singularity evidence from photoluminescence spectroscopy of AlGaAs/InGaAs/GaAs pseudomorphic HEMTs grown on (3 1 1)A GaAs substrates

InGaAs/AlGaAs/GaAs pseudomorphic high electron mobility transistor (P-HEMT) structures were grown by Molecular Beam Epitaxy (MBE) on (3 1 1)A GaAs substrates with different well widths, and studied by photoluminescence (PL) spectroscopy as a function of temperature and excitation density.The PL spectra are dominated by one or two spectral bands, corresponding, respectively, to one or two populated electron sub-bands in the InGaAs quantum well. An enhancement of PL intensity at the Fermi level energy (E_F) in the high-energy tail of the PL peak is clearly observed and associated with the Fermi edge singularity (FES). This is practically detected at the same energy for all samples, in contrast with energy transitions in the InGaAs channel, which are shifted to lower energy with increasing channel thickness. PL spectra at low temperature and low excitation density are used to optically determine the density of the two-dimensional electron gas (2DEG) in the InGaAs channel for different thicknesses. The results show an enhancement of the 2DEG density when the well width increases, in good agreement with our previous theoretical study.     

Size‐Dependent Structural and Optical Characteristics of Glucose‐Derived Graphene Quantum Dots

It is of scientific importance to obtain graphene quantum dots (GQDs) with narrow‐size distribution in order to unveil their size‐dependent structural and optical properties, thereby further to explore the energy band diagram of GQDs. Here, a soft‐template microwave‐assisted hydrothermal method to prepare GQDs with diameters less than 5 nm ± 0.55 nm is reported. The size‐dependent photoluminescence (PL) quantum yield (QY) decay lifetime and electron energy loss spectroscopy (EELS) of the GQDs are studied systematically. The QY of the GQDs with an average diameter of 2 nm is the highest (15%) among all the samples investigated and the QY decreases with increasing diameter of the GQDs. The size‐dependence of the PL decay lifetime is also observed. The result suggests that spatial confinement effects related to radiative relaxation play an important role in the size‐dependent decay lifetime. A realistic energy band diagram of the GQDs is deduced from the experimental results.     

Size-dependent radiative emission of PbS quantum dots embedded in Nafion membrane

PbS quantum dots (QDs) have been incorporated in a Nafion membrane, where the QD sizes were adjusted by changing the reaction time due to the steady growing process. The radiative emissions of the samples were investigated by optical absorption, photoluminescence (PL), and time-resolved PL spectroscopy. Size-tunable emissions are shown by the PL spectrum in a range of 1.84–1.65 eV, and the emission mechanism was investigated based on a four-band envelope-function model. Possible energy transitions for the radiative emission are listed. The PL lifetime depending on the particle size is about one microsecond, and PL decay curves exhibit a trend of decreasing decay time with an increase of the PbS QD size.     

Dielectric relaxation of CdSe nanoparticles

Nanoparticles of cadmium selenide (CdSe) have been synthesized by soft chemical route using mercaptoethanol as a capping agent. X-ray diffraction and transmission electron microscope measurements show that the prepared sample belongs to sphalerite structure with the average particle size of 25 nm. The band gap of the material is found to be 2.1 eV. The photoluminescence (PL) emission spectra of the sample are measured at various excitation wavelengths. The PL spectra appear in the visible region, and the emission feature depends on the wavelength of the excitation. Impedance spectroscopy is applied to investigate the dielectric relaxation of the sample in a temperature range from 323 to 473 K and in a frequency range from 42 Hz to 1.1 MHz. The complex impedance plane plot has been analyzed by an equivalent circuit consisting of two serially connected R-CPE units, each containing a resistance (R) and a constant phase element (CPE). The dielectric relaxation of the sample is investigated in the electric modulus formalism. The temperature dependent relaxation times obey the Arrhenius law. The Havriliak–Negami model is used to investigate the dielectric relaxation mechanism in the sample. The frequency dependent conductivity spectra are found to obey the power law.     

Temperature dependence of the photoluminescence and phosphorescence time decay of magnesia-stabilized zirconia

Temperature dependence of photoluminescence (PL) spectra and time decay ranging from 90 to 330 K are investigated in magnesia-stabilized zirconia single crystals. The emission PL spectra can be decomposed into two bands. The prominent one is centered in the blue-green region of the spectrum whereas the secondary one is centered in the yellow-orange region. The temperature dependence of these bands are analyzed in terms of the so-called configuration coordinate model. The Huang-Rhys parameter for the prominent band is found near 40 and the effective phonon at about 0.030 eV. Thermal quenching energy is determined to be 0.24 eV from the decreasing part of the I(T) curve. Luminescent decays were satisfactorily fitted by two exponentials over the whole temperature range investigated. Total lifetime temperature dependence can be accounted for by assuming a radiative decay from two metastable levels with a separation energy of 0.073 eV. Results are discussed on the basis of the major defects, oxygen vacancies and complex defects.     

Temperature-dependent optical properties of wurtzite InN

Optical properties and carrier recombination dynamics of a series of InN epilayers, with varying free electron concentrations, grown by molecular beam epitaxy were studied by steady-state photoluminescence (PL) and time-resolved differential transmission spectroscopy. At room temperature strong PL around 0.7 eV was observed. Temperature-dependent PL measurements show a redshift of the peak energy and a linear increase of the emission linewidth with temperature. Furthermore, our results demonstrate that room temperature carrier lifetimes are inversely proportional to the free electron concentrations for theses samples. Carrier lifetime as long as 1.3 ns was observed in the best quality sample, indicating a highly improved crystalline quality.     

Localized exciton dynamics in AlInGaN alloy

Carrier recombination dynamics in AlInGaN alloy has been studied by photoluminescence (PL) and time-resolved PL (TRPL) at various temperatures. The fast red-shift of PL peak energy is observed and well fitted by a physical model considering the thermal activation and transfer processes. This result provides evidence for the exciton localization in the quantum dot (QD)-like potentials in our AlInGaN alloy. The TRPL signals are found to be described by a stretched exponential function of exp[(−t/τ)^β], indicating the presence of a significant disorder in the material. The disorder is attributed to a randomly distributed QDs or clusters caused by indium fluctuations. By studying the dependence of the dispersive exponent β on temperature and emission energy, we suggest that the exciton hopping dominate the diffusion of carriers localized in the disordered QDs. Furthermore, the localized states are found to have 0D density of states up to 250 K, since the radiative lifetime remains almost unchanged with increasing temperature.     

Depolarization of the photoluminescence and spin relaxation in n-doped GaAs

Optically oriented electron spin lifetime in n-doped gallium arsenide was measured via depolarization of the photoluminescence (PL) in a transverse magnetic field (Hanle effect). In order to measure the PL polarization, a time-resolved pump-probe experiment, where a pump pulse generates spin-polarized electrons and a probe pulse monitors their polarization, was employed. The PL polarization in dependences of the pump-probe delay, external magnetic field as well as of the sample temperature was studied. The PL polarization was found to decay exponentially with the pump-probe delay, from which the spin lifetime of the electrons was measured. The measured value was found to depend on the strength of the magnetic field and sample temperature.     

基于分子束外延生长的1.05 eV InGaAsP的超快光学特性研究

利用分子束外延方法制备了应用于四结光伏电池的1.05 eV InGaAsP薄膜, 并对其超快光学特性进行了研究. 温度和激发功率有关的发光特性表明: InGaAsP材料以自由激子发光为主. 室温下InGaAsP材料的载流子发光弛豫时间达到10.4 ns, 且随激发功率增大而增大. 发光弛豫时间随温度升高呈现S形变化, 在低于50 K时随温度升高而增大, 在50–150 K之间时减小, 而温度高于150 K时再次增大. 基于载流子弛豫动力学, 分析并解释了温度及非辐射复合中心浓度对样品材料载流子发光弛豫时间S形变化的影响.     

Effects of natural oxidation on the photoluminescence properties of Si nanocrystals prepared by pulsed laser ablation

In this work, Si nanocrystals (Si-NCs) have been prepared by pulsed laser ablation technique in dichloromethane, and the microstructure and photoluminescence (PL) properties of the Si-NCs before and after natural oxidation were investigated. Transmission electron microscopy and Raman results show that the average diameter of the Si-NCs is 2.42 nm in the dichloromethane solution. Blue–violet PL with a lifetime of 4.6 ns is observed at room temperature, and the PL peak shifts toward longer wavelength with the red shift of excitation wavelength. The PL excitation spectrum indicates that the bandgap of the Si-NCs in solution is 2.64 eV, which confirms that the blue–violet PL originates from interband transition of Si-NCs caused by quantum confinement effect. The PL peak red shifts to 607 nm after natural oxidation, and the peak lifetime of which is slow down to 13.1 μs. The fixed PL peak excited by different wavelengths and the slow PL decay time indicate that interface defects become the main PL mechanism after natural oxidation. The results will add new information for understanding the PL mechanism of Si-NCs in different environments.     

双模自组织量子点光致发光的温度依赖性

采用稳态速率方程模型,对双模自组织量子点光致发光的温度依赖性进行了研究,模拟获得了不同温度下双模自组织量子点的光致发光光谱,并进一步研究了两组量子点分布的光致发光强度比的温度依赖性。研究表明:在低温下(<75K),两组量子点分布的发光强度比基本保持不变;随着温度的升高(75K相似文献   

Photoluminescence investigation of ZnO:P nanoneedle arrays on InP substrate by pulsed laser deposition

Phosphorus-doped ZnO nanoneedle arrays were prepared by phosphorus diffusion from InP substrate using a pulsed laser deposition (PLD) technique. The optical properties of ZnO nanoneedle were investigated by photoluminescence (PL) spectroscopy. Low-temperature photoluminescence spectrum measurements exhibited five acceptor-related emission peaks. The excitation intensity and temperature dependent photoluminescence spectra confirmed that the emission peaks corresponded to neutral-acceptor bound exciton, free electron to acceptor, donor-acceptor pairs, and their first and second photon replicas transitions. Acceptor-binding energy was determined to be 135-167 meV, which agrees well with the best-fitting result of the temperature dependent photoluminescence measurements and is reasonable in terms of theoretic prediction in ZnO.     

Resonant electronic energy transfer from excitons confined in silicon nanocrystals to oxygen molecules

We demonstrate efficient resonant energy transfer from excitons confined in silicon nanocrystals to molecular oxygen (MO). Quenching of photoluminescence (PL) of silicon nanocrystals by MO physisorbed on their surface is found to be most efficient when the energy of excitons coincides with triplet-singlet splitting energy of oxygen molecules. The dependence of PL quenching efficiency on nanocrystal surface termination is consistent with short-range resonant electron exchange mechanism of energy transfer. A highly developed surface of silicon nanocrystal assemblies and a long radiative lifetime of excitons are favorable for achieving a high efficiency of this process.     

Synthesis and dielectric characteristics of La0.5Bi0.5MnO3 ceramics

La_0.5Bi_0.5MnO₃ ceramics with a single phase were prepared by a solid-state reaction method, and their dielectric properties were characterized. Two dielectric relaxations with a giant dielectric constant were identified in the temperature range from 125 to 350 K. The electron hopping between Mn³⁺ and Mn⁴⁺ was found to be the origin of the dielectric relaxation at low temperatures (125–200 K) with an activation energy of 0.18 eV. The high temperature (200–350 K) dielectric relaxation can be attributed to the conduction.     

Effect of electron-hole spatial correlation on spin relaxation dynamics in InAs submonolayer

Using time-resolved photoluminescence and time-resolved Kerr rotation spectroscopy, we explore the unique electron spin behavior in an InAs submonolayer sandwiched in a GaAs matrix, which shows very different spin characteristics under resonant and non-resonant excitations. While a very long spin relaxation lifetime of a few nanoseconds at low temperature is observed under non-resonant excitation, it decreases dramatically under resonant excitation. These interesting results are attributed to the difference in electron-hole interactions caused by non-geminate or geminate capture of photo-generated electron-hole pairs in the two excitation cases, and provide a direct verification of the electron-hole spatial correlation effect on electron spin relaxation.     

Luminescence and EPR studies of YBO3:U

Yttrium borate doped with uranium was prepared by mixing and heating yttrium oxide obtained through oxalate precipitation route, boric acid and requisite amount of nuclear-grade uranium oxide at high temperature. Photoluminescence (PL), thermally stimulated luminescence (TSL) and electron paramagnetic resonance (EPR) studies were carried out on gamma-irradiated doped/undoped yttrium borate samples in the temperature range 300-600 K. TSL studies showed the presence of two glow peaks at 414 and 471 K. PL studies along with lifetime decay investigation suggested uranium goes in the matrix as UO₂²⁺. EPR studies showed the presence of O₂⁻radical ion along with electron trapped in defect centres, which might have been produced for charge compensation. Apart from this, CO₂⁻ radical was also observed in the system having its origin from residual oxalate ion. Temperature dependence EPR studies of the observed radical confirmed the involvement of the CO₂⁻ and dioxide radical ion in the observed glow peaks. By correlating the TSL, PL and ESR data, probable mechanism is proposed for the observed TSL glow in the system.