Fast photoluminescence decay processes of doped ZnO phosphors

Lifetime measurements have been carried out for ZnO doped with killer impurities (Fe, Co or Ni) having doping concentrations 0.05 to 1.00% by weight using a pulsed UV laser (nitrogen laser) as the excitation source having a short pulse width and a high photon flux density. The high photon flux density of the laser is very useful to excite the short-lived shallow trapping states which otherwise would be impossible to excite. Fast photoluminescence emission in the microsecond time domain has been obtained due to killer impurities at room temperature. The effect of killer dopants as well as the effect of their concentrations on lifetime values has been observed. Other optical parameters such as trap depth and decay constant are also reported in the present context. Lifetime values are found to be in the microsecond time domain and a reverse trend is obtained with increase in concentration of killer impurities. PACS 78.55.-m; 78.55.Ap; 78.66.Db; 78.66.Hf     

Large prolongation of free-exciton photoluminescence decay in diamond by two-photon excitation

We report on time-resolved photoluminescence of a free-exciton in IIa chemical vapor deposition diamond crystal. Large difference between decay times for one- and two-photon excitation processes was observed. The longest room-temperature exciton photoluminescence lifetime τ(FE)=220 ns was obtained under two-photon excitation with a photon energy of 4.7 eV. The role of diffusion and surface recombination velocity in exciton photoluminescence dynamics was studied using a new optical method based on two-photon excited time-resolved photoluminescence. The measured room-temperature value of diffusion coefficient in diamond was D=40 cm(2)/s.     

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.     

Kinetics of supersaturation decay in the crystallization of lysozyme

The molecular architecture of proteins can be determined by analysing the X-ray diffraction patterns of their crystals. The technology of X-ray crystallography has reached the point, however, where the determination of the structure of a given crystal is controlled by the limited availability of the crystals themselves. Proteins can often be crystallized from pH buffered aqueous solutions of strong electrolytes. When dissolved protein in solution is more stable than crystalline protein, the appearance of crystals can be said to be under thermodynamic control. If, on the other hand, the crystals are more stable than the dissolved protein, and still crystals are slow to appear, the crystallization can be said to be under kinetic control. Using dilatometry, we have followed the rate of decay of the protein supersaturation in crystallizing solutions of chicken egg-white lysozyme under conditions of kinetic control. We have found that the rate of decay of the supersaturation is first order in the supersaturation and that the rate constant is independent of the initial protein concentration, but increases with increasing pH, decreasing temperature, and with increasing concentrations of sodium chloride and buffer salt. We correlate these observed trends in the rate constant with related trends in the solubility and surface charge density of the crystals. We conclude that the rate constant for supersaturation decay is inversely proportional to the protein solubility.     

Complex photoluminescence decay characteristics of the in-related isoelectronic bound exciton in Si

We present detailed photoluminescence decay measurements on the long-lived P, Q, R lines seen in In-doped Si, which reveal that the transient behaviour of this system is far more interesting than had been previously thought. At temperatures below 10K a double exponential behaviour is observed in the decay curves of all P, Q, R related lines, the long-lived part having a decay constant of up to 3.4 ms at 1.2K. These measurements provide the first evidence that the In-related isoelectronic bound exciton has two distinct configurations, as does the closely related Tl centre.     

A geminate recombination model for photoluminescence decay in plasma-deposited amorphous Si:H

We develop a model of geminate electron-hole recombination, including tunnelling and diffusion, to account for photoluminescence decay in amorphous semiconductors. The model correctly predicts the shape of the decay, the luminescence quantum efficiency, and the microscopic electron mobility.     

Transient photoluminescence decay study of minority carrier lifetime in GaAs heteroface solar cell structures

Transient photoluminescence decay has been studied theoretically and experimentally as a technique for the investigation of GaAs solar cell materials and solar cell structures. The time-dependent continuity equation was solved using two variable boundary conditions modelling the interface between the emitter and hetero-window layer (AlGaAs) and between the emitter and space charge region, respectively. The solution was found with help of the Fourier transform method and the method of residues. There results an analytical expression for the time dependent photoluminescence (PL) intensity. The influence of various solar cell parameters on this photoluminescence transient has been studied in detail. An experimental investigation of transient PL decay was performed using a synchronously pumped mode locked and cavity dumped Nd:YAG/dye laser system for excitation and an optical sampling oscilloscope as the detector. GaAs wafers with and without surface passivation have been measured as well as hetero-window pn-structures and processed solar cells. A fit of the theoretical PL transients to the measured transients allows surface and bulk recombination parameters to be determined.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

Ultrasonic induced photoluminescence decay in sonochemically obtained cauliflower-like ZnO nanostructures with surface 1D nanoarrays

Cauliflower-like ZnO nanostructures with average crystallite size of about 55 nm which have surface one dimensional (1D) nanoarrays with 10 nm diameter were successfully fabricated through a simple sonochemical route. X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and room temperature photoluminescence (PL) characterizations were performed to investigate the morphological and structural properties of the obtained nanostructures. It has been shown that the synthesized cauliflower-like ZnO nanostructures irradiated UV luminescence and a green peak in visible band. Ultrasonic post-treatment of the particles for about 2 h increased the density of surface defects resulted in an increase in the green emission intensity.     

Evidence for fast decay dynamics of the photoluminescence from Ge nanocrystals embedded in SiO2

We have investigated the origin of room temperature photoluminescence from ion-beam synthesized Ge nanocrystals (NCs) embedded in SiO₂ using steady state and time-resolved photoluminescence (PL) measurements. Ge NCs of diameter 4-13 nm were grown embedded in a thermally grown SiO₂ layer by Ge⁺ ion implantation and subsequent annealing. Steady state PL spectra show a peak at ∼2.1 eV originating from Ge NCs and another peak at ∼2.3 eV arising from ion-beam induced defects in the SiO₂ matrix. Time-resolved PL studies reveal double exponential decay dynamics on the nanoseconds time scale. The faster component of the decay with a time constant τ₁∼3.1 ns is attributed to the nonradiative lifetime, since the time constant reduces with increasing defect density. The slower component with time constant τ₂∼10 ns is attributed to radiative recombination at the Ge NCs. Our results are in close agreement with the theoretically predicted radiative lifetime for small Ge NCs.     

Kinetics of annihilation of triplet excitationsand decay of delayed fluorescence for isolated 1,12-benzoperylene pairs

We studied the annihilation kinetics for triplet excitations in isolated pairs of organic molecules and their delayed fluorescence decay using a new mathematical model for the process. According to this model, the intensity of delayed annihilation fluorescence (DAF) for isolated pairs is directly proportional to the number of pairs in which both molecules are found in the excited triplet state. We show that the decrease in such pairs and the decay of their delayed annihilation fluorescence occur according to an exponential law in the absence of random scatter in the deactivation rate constants for the triplet excitations. The results of an experimental study of the DAF decay kinetics for 1,12-benzoperylene in n-hexane at 77 K, where triplet–triplet annihilation occurs in isolated pairs, confirm the validity of the theoretical conclusions.     

Kinetics of electronic excitation decay in LiNbO<Subscript>3</Subscript> crystals

A model for calculation of kinetic characteristics of electronic excitation decay in the impurity subsystem of doped crystals with taking into account the phenomenon of excitation trapping is suggested. Numerical calculations are carried out for LiNbO₃:Yb³⁺ crystals: the probabilities of elementary acts of resonant and non-resonant nonradiative redistributions of electronic excitation energy in the impurity subsystem are calculated and the dependences of excitation lifetime and luminescence quantum yield on the concentration of pair centers in the matrix of LiNbO₃ crystals are determined.     

Stimulated photoluminescence of CdTe

Emission spectra of CdTe at low temperature under high intensity optical pumping by means of a tunable dye laser are reported.We show that the optical gain process is enhanced when a high exciton density is achieved by means of quasi-resonant pumping. The mechanism responsible for the amplification process is found to be the well-known exciton-exciton scattering.     

Stimulated photoluminescence of ZnSe

The stimulated emission from ZnSe under one photon or two photon excitation has been investigated in the temperature range between 20 and 100 K. The spectra obtained under quasi-resonant one photon pumping show a dominant P line, ascribed to exciton-exciton scattering on the basis of its intensity dependence and its optical gain spectrum. The role of many-body processes under non-resonant one photon excitation is discussed. Under two photon pumping the LO-phonon assisted free exciton recombination is shown to be the dominant lasing mechanism.