Luminescence spectra and kinetics of disordered solid solutions

Journal of Luminescence, 1998

A nonperturbative theory of anharmonic multiphonon decay of strong local vibrations in crystals is presented. The theory predicts a slowdown of the decay at large amplitudes, and a jump-like acceleration of the decay near the critical amplitudes of 2 0.3 A. A local vibration with such or larger amplitude is created, e.g. on exciting self-trapped excitons in solid Xe. The time evolution of this vibration manifests itself in the hot luminescence of this crystal. A comparison of the calculated and the observed spectra allows one to conclude that the relaxation rate of the self-trapped exciton in solid Xe has indeed a strong maximum at the amplitude of _ 0.3 A.

physica status solidi (b), 1975

Measurements of the excitation spectra (of the photoluminescence lines below the band gap of CdTe led to the identification of emissions originating from excitons bound to neutral donors, to ionized donors, and to neutral acceptors, as well as from two-electron transitions. Oscillatory variations of the free and bound exciton emission intensity with the energy change of the exciting light above the band gap and their changes with temperature indicated details of the exciton relaxation mechanisms. Mcssungen der Anregnngsspektren der Photolumineszenz von Linien unterhalb der Bandkante von CdTe fiihren zur Tdentifizierung dieser Emissionslinien. Sie werden verursacht durch die Rekombination von Exzit.onen, die an neutrale oder ionisiarte Donatoren oder an neutrale Akzeptoren gebunden sind, sowie von Zwei-Elektronen-Uhergangen. Aus dem oszillatorischen Verhalten der Lumineszenzintensitat von freien und gebundenen Exzitonen bei deran Anregungsspektren oberhalb der Bandkante nnd deren Temperaturabhangigkeit wird auf Einzelheiten der Exzitonen-Relaxationsprozesse gesch!ossen.

Physical Review B, 2005

The dynamics of exciton and electron-hole plasma populations is studied via time-resolved photoluminescence after nonresonant excitation. By comparing the peak emission at the exciton resonance with the emission of the continuum, it is possible to experimentally identify regimes where the emission originates predominantly from exciton and/or plasma populations. The results are supported by a microscopic theory which allows one to extract the fraction of bright excitons as a function of time.

Journal of Applied Spectroscopy, 1994

Earlier [ I-5 ] dips near the maximum or on the shortwave edge of the radiation band of free excitons were discovered in spectra of resonance exciton radiation of ZnSe, ZnTe, and CdTe crystals. It was supposed that this structure is caused by polariton effects or self-absorption of exciton radiation. Upon excitation by continuous radiation from lamps or lasers with hv > Eg at the temperature of liquid nitrogen only exciton radiation lines with a smooth contour of the band are usually observed in spectra of exciton radiation of bulk CdS crystals. An increase in the excitation intensity leads to an increase in the lifetime and length of diffusion of excilons and enhances the reabsorption effect. A radiation peak shift to the longwave side and a decrease in the relative brightness of photoluminescence (PL) in the region of exciton resonance without formation a dip are also observed I6 I. A dip on the resonance band in the PL spectra of ZnTe, CdTe, and ZnSe crystals is considered to be caused by self-absorption of outgoing radiation since according to perturbation theory these crystals show narrow peaks in absorption spectra [5 1. At the same time, owing to the contribution of polariton effects the contour of the exciton bands of absorption of CdS crystals must have a fairly flat trapezoidal form, which is rcsponsible for the absence of a dip in the PL spectra of these crystals. In the present work we detecled and investigated the occurrence of dips at the center of exciton radiation lines of bulk CdS crystals at the temperature of liquid nitrogen. Also, we studied the conditions for occurrence and causes of self-reversal of exciton lines. Photoluminescencc was excited by radiation from "continuous" lamps 0l = 365,436 nm) and He-Cd-lasers with an energy of the quanta that exceeds the forbidden band width and a power density of 0.01-1 W/cm 2 (,;t = 325.0 and 441.6 nm). Samples were cut in form of plates in the plane (0001) or cleaved along one of the prismatic surfaces (1120) or (10]-0). Spectra were recorded from the sidc of the excitation surface (reflection PL) in the temperature range 77-300 K. The PL spectra of the initial high-resistance (Fig. la, curve 1) CdS crystals at the temperature of liquid nitrogen show bands A and B of free excitons, while the PL spectra of low-resistance crystals exhibit band 12 of bound excitons (Fig. lb, curve 1). After heating of the sample to 300-400 K and subsequent rapid cooling to the liquid nitrogen temperature a dip occurs in the peaks of the initially smooth ,4-and B-bands of free excitons (Fig. 1, curves 2, 3). Here, the total intensity of the radiation decreases sharply. A study of the detected effect for a large number of CdS crystals of different quality with a resistivity of 0.1-101~ ~2.cm showed that the dip is larger for low-resistance than for high-resistance samples. For some low-resistance samples, the depth of the dip was 90% of the maximum. After a lapse of time this effect undergoes relaxation. The relaxation time is different for different samples and varies from several hours to several hundreds of hours. The stronger the dip, the slower its relaxation. Repeated heat treatment, weak laser action, the cooling regimes, and the storage conditions for the samples exert a slight

Chemical Physics, 2016

Excitonic state-resolved pump/probe spectroscopy and time correlate single photon counting were used to study exciton dynamics from the femtosecond to nanosecond time scales in CdSe/Cd,Zn,S nanocrystals. These measurements reveal the role of the core/shell interface as well as surface on non-radiative excitonic processes over three time regimes. Time resolved photoluminescence reports on how the interface controls slow non-radiative processes that dictate emission at the single excitonic level. Heterogeneity in decay is minimized by interfacial structure. Pump/probe measurements explore the non-radiative multiexcitonic recombination processes on the picosecond timescale. These Auger based non-radiative processes dictate lifetimes of multiexcitonic states. Finally state-resolved pump/probe measurements on the femtosecond timescale reveal the influence of the interface on electron and hole relaxation dynamics. We find that the interface has a profound influence on all three types of non-radiative processes which ultimately control light emission from nanocrystals.

Journal of Physics: Condensed Matter, 1998

Host luminescence excitation spectra and reflection spectra of PbCl 2 and PbBr 2 crystals at low temperatures have been measured in the energy region of 4-30 eV using synchrotron radiation. High-efficiency host luminescence is connected to radiative decay of self-trapped cation excitons. It is shown that also in the region of excitation multiplication, e.g. if the excitation energy is larger than twice the band gap energy (E > 2E g ), and in the relaxation process of core excitons the radiative self-trapped cation excitons are formed. However, the recombination of hole and self-trapped electrons (band-band excitation) does not give rise to host luminescence.

Low Temperature Physics, 2003

Direct and indirect creation of excitons in rare gas solids has been investigated with reflectivity and luminescence spectroscopy. For the heavy rare gas solids Kr and Xe, new and more reliable exciton parameters have been deduced. With time-resolved luminescence spectroscopy, fast and delayed secondary-exciton creation has been established and separated. Thermalization of photocarriers and their delayed recombination have been analyzed, including a first attempt to investigate the influence of excitation density on the carrier dynamics. The existence of excitonic side bands of ionization limits E i (either band gap or inner-shell ionization limits) in prompt secondary exciton creation has been established. The threshold energies of these side bands are given by E E nE i th ex » + (n is integer, E ex is exciton energy). The side bands are ascribed to the formation of electronic polaron complexes, superimposed to inelastic scattering of photoelectrons. * In we preferred to present a quick scan. During high-resolution scans, which take some hours, the samples degrade due to defect formation. This leads to a continuous decrease of FE luminescence. High-resolution scans are given in .

physica status solidi (a), 2002

We present a theory of time-and energy-resolved photoluminescence (PL) from semiconductors excited by femtosecond laser pulses. Our approach combines quantum kinetics of hot-carrier relaxation and quantum theory of spontaneous emission under consistent inclusion of Coulomb interaction. Model calculations show the transition from PL at the pump frequency via subsequent phonon replicas until the build-up of excitonic PL. We predict hot luminescence to be a sensitive measure of electron-LO-phonon quantum kinetics and bottleneck effects.

Luminescence - An Outlook on the Phenomena and their Applications, 2016

Domestic light providing devices have always been an important component of life and continue to provide us light beyond sunset. These devices continue to be improved frequently to allow ease of use and to enhance their efficiency. The tungsten electric light bulbs are widely used, which are based on incandescence of a continuously heated tungsten element. However, their use will soon be short lived because of the increased usage of fluorescent tubes and light-emitting diode (LED) devices, which are based on luminescence emission. These emission materials that display luminescence are called phosphors, and their emission is based on electron transitions. In the following chapter, we shall look into photoluminescence from both intrinsic and extrinsic defects, covering both down-and upconversion (UP). We will look into the concept of energy transfer and persistent luminescence and lastly provide related applications of luminescence in the modern days.

Journal of Luminescence, 2013

Recently a new kinetic model was presented in the literature, which describes localized electronic recombination in donor-acceptor pairs of luminescent materials. Within this model, recombination is assumed to take place via the excited state of the donor, and nearest-neighbor recombinations take place within a random distribution of centers. Two versions of the model were presented which were found to be in good agreement with each other, namely an exact model that evolves both in space and in time, and an approximate semi-analytical model evolving only in time. The model simulated successfully both thermally stimulated luminescence (TL) and optically stimulated luminescence (OSL), and also demonstrated the power law behavior for simulated OSL signals. This paper shows that the system of simultaneous differential equations in the semi-analytical model can be approximated to an excellent precision by a single differential equation. Furthermore, analytical solutions are obtained for this single differential equation, and for four different experimental modes of stimulation: TL, OSL, linearly modulated OSL (LM-OSL) and isothermal TL processes. The exact form of the power law for the model is found in analytical form for both OSL and isothermal TL processes. The analytical equations are tested by successfully fitting typical infrared stimulated luminescence (IRSL) signals, as well as experimental TL glow curves from feldspar samples. The dimensionless number density of acceptors in the model is estimated from fitting the experimental IRSL and TL data. The analytical expressions derived in this paper apply also to stimulated emission via the excited state of the donor-acceptor system. However, the same analytical expression, with different numerical values for its constants, can also be applied in the case of ground state tunneling, with important implications for luminescence dating.

Physical Review B, 2008

We study the relaxation of excitons in CdZnTe quantum wells by emission of a LO-phonon cascade, ending with a trapping in quantum dots. The state filling of the dots is measured in two-color pump-probe experiments. The observed optical-phonon emission time is found to be smaller ͑130 fs͒ than the phonon oscillation period ͑165 fs͒, showing that the interaction occurs in a quantum kinetic regime. This is evidenced by measuring the buildup of the phonon replica of an initially photocreated electron-hole pair distribution on a subpicosecond time scale.

ACS Photonics, 2017

Selected semiconductor nanostructures provide extremely localized coherent light sources. Here an ensemble of CdS nanostructures was excited by UV/vis femtosecond laser pulses and their ultrafast luminescence characteristics were investigated as functions of the pulse energy fluence and the photon quantum energy. All optical Kerr gating enabled studies of the emission dynamics with a time resolution of 150 fs avoiding any influence on the CdS emission. The initially observed emission built up after a delay of 0.1−3 ps and decayed rapidly in a biexponential way, strongly dependent on both the laser energy fluence and the quantum energy. The central wavelength of the emission spectrum revealed a significant blue-shift within the first few ps followed by a transient red-shift relative to spontaneous excitonic emission of CdS. All findings are mainly attributed to stimulated radiative carrier recombination in the laser excited electron−hole plasma after its thermalization with the CdS lattice.

Journal of Physics: Condensed Matter, 2012

We present a new kinetic model describing localized electronic recombination through the excited state of the donor (d) to an acceptor (a) centre in luminescent materials. In contrast to the existing models based on the localized transition model (LTM) of Halperin and Braner (1960 Phys. Rev. 117 408-15) which assumes a fixed d → a tunnelling probability for the entire crystal, our model is based on nearest-neighbour recombination within randomly distributed centres. Such a random distribution can occur through the entire volume or within the defect complexes of the dosimeter, and implies that the tunnelling probability varies with the donor-acceptor (d-a) separation distance. We first develop an 'exact kinetic model' that incorporates this variation in tunnelling probabilities, and evolves both in spatial as well as temporal domains. We then develop a simplified one-dimensional, semi-analytical model that evolves only in the temporal domain. An excellent agreement is observed between thermally and optically stimulated luminescence (TL and OSL) results produced from the two models. In comparison to the first-order kinetic behaviour of the LTM of Halperin and Braner (1960 Phys. Rev. 117 408-15), our model results in a highly asymmetric TL peak; this peak can be understood to derive from a continuum of several first-order TL peaks. Our model also shows an extended power law behaviour for OSL (or prompt luminescence), which is expected from localized recombination mechanisms in materials with random distribution of centres.

Physical review. B, Condensed matter, 1994