Decay Of Self-trapped Excitons Research Articles

Ultrafast Dynamics of Self-Trapped Excitons in Cs2AgInCl6:Al3+ Double Perovskite Nanocrystals.

It is a huge challenge to increase the photoluminescence (PL) of lead-free halide perovskites, and understanding the mechanism behind exciton dynamics can provide a valuable solution. Herein, we achieved enhanced broad-band emission at ambient conditions in Cs2AgInCl6 by tuning self-trapped excitons (STEs) through Al3+ doping. Cryogenic measurements showed an inhomogeneous nature of STE emission due to the presence of defect states and is subject to thermal quenching. An increased Huang-Rhys factor (S-factor) resulted in better electron-phonon coupling and high-density STE states post Al3+ doping. Femtosecond transient absorption (fs-TA) results provided insights into the distribution dynamics of excitons, which occurs through gradient energy levels from free excitons (FE) to STEs, where each STE state potentially possesses higher quantized energy states. Overall, this study aims to comprehend the origins of self-trapping and decay of STEs in Cs2AgInCl6:Al3+ and emphasizes the potential of compositional engineering to mitigate self-trapping in this material.

Ionoluminescence Spectra of a ZnO Single Crystal Irradiated with 2.5 MeV H+ Ions

The ionoluminescence (IL) spectra of a ZnO single crystal irradiated with 2.5 MeV H+ ions reveal that its intensity decreases with increasing the ion fluence, which indicates that the concentration of luminescence centers decreases with irradiation. The Gaussian decomposition results of the ZnO IL spectrum with a fluence of 1.77×1011 ions/cm2 show that the spectrum is a superposition of energy levels centered at 1.75 eV, 2.10 eV, 3.12 eV and 3.20 eV. The four peaks are associated with electronic transitions from CB to VZn, CB to Oi, Zni to VB and the decay of self-trapped excitons, respectively. The results of single-exponential fitting demonstrate that different luminescent centers have different radiation resistance, which may explain why the emission decreases more slowly in the NBE band than in the DBE band. The agglomeration of larger point clusters accounts for the decrease in the concentration of luminescence centers and the increase in the concentration of non-luminescence centers, which indicates that the defect clusters induced by ion implantation act as nonradiative recombination centers and suppress light emission. The results of the photoluminescence spectra of a virgin ZnO single crystal and a ZnO single crystal irradiated with a fluence of 3.4×1014 ions/cm2 show that compared with the virgin ZnO, the emission intensity of irradiated ZnO decreases by nearly two orders of magnitude, which demonstrates that the irradiation effect reduces radiative recombination and enhances nonradiative recombination. The conclusions of photoluminescence are consistent with the IL results.

Formation of the defect halo of swift heavy ion tracks in LiF due to spatial redistribution of valence holes

The fundamental effect of the relaxation kinetics of the ensemble of generated valence holes on structure transformations in swift heavy ions tracks in alkali halides is demonstrated. The parameters of the spatial distributions of F-color centers formed in the nanometric vicinities of the trajectories of different ions (from C to U) decelerated in LiF in the electronic stopping regime were extracted from the in situ spectroscopy experiments. Formation of these defects results from self-trapping of valence holes appearing during material ionizations followed by creation and decay of self-trapped excitons. The Monte-Carlo model is applied in order to determine the initial radial distributions of valence holes in tracks of different ions. The detected large difference between the hole and defect distributions indicates fast and long-range diffusion of valence holes from the ion trajectory before their self-trapping. The averaged diffusion coefficient (Dh) of hot holes at the subpicosecond time scale after the projectile passage as well as the conversion efficiencies of valence holes into color centers (β) in relaxing tracks of different ions are estimated (Dh = 0.016–1.4 cm2 s−1, β = 0.01–0.04).

Effect of spatial redistribution of valence holes on the formation of a defect halo of swift heavy-ion tracks in LiF

In-situ spectroscopy is applied to analyze the spatial distribution of point defects ($F$-color centers) formed in the nanometric vicinities of the trajectories of different swift heavy ions (from C to U) decelerated in LiF crystals in the electronic stopping regime. The formation of these defects results from the decay of self-trapped excitons appearing due to self-trapping and further relaxation of valence holes generated during beam induced ionization processes. A Monte Carlo model is applied to estimate the initial radial distribution of valence holes in the ion tracks by the time the ionization cascades are finished. It is demonstrated that there exists a large difference between this and the radial distribution of point defects detected in the experiments. This difference is described by fast and long-range diffusion of valence holes before their self-trapping. The effect illustrates the fundamental importance of the relaxation kinetics of the valence band on structure transformations in tracks. The average diffusion coefficient of holes ${D}_{h}$ and their conversion efficiencies into color centers \ensuremath{\beta} in relaxing tracks of different ions in LiF are estimated (${D}_{h}$ = 0.016--1.4 cm${}^{2}$/s, \ensuremath{\beta} = 0.01--0.04).

LUMINESCENCE MECHANISM STUDY OF A POTENTIAL SCINTILLATION CRYSTAL YBa3B9O18

The luminescence mechanisms of a potential scintillation crystal YBa 3 B 9 O 18 were investigated by experimental methods. As a first step, the optical band gap energy Eg was estimated to be 7.3 eV from the optical absorption spectrum and vacuum ultraviolet excited luminescence spectra. The X-ray photoelectron spectroscopy measured in the valence region coincided with the first principle energy band calculation. Based on the analysis of valence band structure and vacuum ultraviolet excited luminescence spectra the origins of emission bands of YBa 3 B 9 O 18 were studied. The intrinsic luminescence of YBa 3 B 9 O 18 was attributed to the radiative decay of self-trapped excitons. The elucidation of the luminescence mechanism provides a physical picture on the property-structure correlation and may help to accelerate the applications of the potential scintillation crystal.

Electronic excitations and luminescence of SrAlF5 crystals doped with Ce3+ ions

Abstract This paper reports the results of a time-resolved photoluminescence and energy transfer processes study in Ce 3+ doped SrAlF 5 single crystals. Several Ce 3+ centers emitting near 4 eV due to 5d-4f transitions of Ce 3+ ions substituting for Sr 2+ in non-equivalent lattice sites were identified. The lifetime of these transitions is in the range of 25–35 ns under intra-center excitation in the energy region of 4–7 eV at T = 10 K. An effective energy transfer from lattice defects to dopant ions was revealed in the – 7–11 eV energy range. Both direct and indirect excitation channels are efficient at room temperature. Excitons bound to dopants are revealed at T = 10 K under excitation in the fundamental absorption region above 11 eV, as well as radiative decay of self-trapped excitons resulting in luminescence near 3 eV.

Exciton emission and defect formation in yttrium trifluoride

Two intrinsic emission bands at 220 and 280 nm have been detected in nominally pure YF3 powders at 10 K. Excitation spectra for both emissions have a sharp peak at 12 eV near the edge of interband transition. Observed emissions are assigned to the radiative decay of self-trapped excitons in YF3. The strong thermal quenching of intrinsic luminescence was observed at temperature above 120 K. It was supposed that non-radiative decay of self-trapped exciton at high temperatures lead to defect formation in YF3. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence of RE‐doped thin metal oxide films

Photoluminescence (PL) of pure and Sm3+-doped TiO2, ZrO2 and HfO2 thin films was studied at temperatures of 6–300 K under interband excitation by using pulsed lasers and synchrotron radiation in the energy range of 4–20 eV. PL excitation spectra and decay kinetics were recorded. The thin films were prepared by using both the atomic layer deposition (ALD) and the sol-gel process. The ion implantation was applied to dope the ALD-grown films with Sm3+ ions, whereas an in-situ doping was used in the sol-gel process.The main emission of undoped materials was attributed to the radiative decay of self-trapped excitons (STE). In doped materials, a broadband emission superposed by Sm3+ emission lines was observed. The broadband emission was attributed to the decay of STE as well as various bound excitonic states. The differencies in the PL excitation spectra of studied films were assigned to the damage produced by ion implantation and uncontrolled impurities incorporated into the films in sol-gel process. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Time-dependent luminescence of self-trapped excitons in alkaline-earth fluorides excited by femtosecond laser pulses

We studied the decay of self-trapped excitons (STE's) in ${\mathrm{BaF}}_{2},$ ${\mathrm{SrF}}_{2},$ and ${\mathrm{CaF}}_{2}$ excited at room temperature by two-photon absorption of femtosecond laser pulses by recording time-resolved triplet-luminescence spectra. For ${\mathrm{BaF}}_{2}$ we detected significant spectral changes as a function of decay time but this effect was much less pronounced for ${\mathrm{SrF}}_{2}$ and barely to be identified in ${\mathrm{CaF}}_{2}.$ A careful analysis of ${\mathrm{BaF}}_{2}$ data revealed two principal decay times of 0.6 \ensuremath{\mu}s and approximately 4 \ensuremath{\mu}s, respectively, where the latter is actually a range of decay times correlating with photon energy within the luminescence band. We attribute this spectral characteristic to a superposition of contributions from several relaxed STE configurations while the two principal decay times are believed to result from the zero-field splitting of the ${}^{3}{\ensuremath{\Sigma}}_{u}^{+}$ state of the STE.

Magnetic resonance studies on radiation-induced point defects in mixed oxide glasses. II. Spin centers in alkali silicate glasses

Irradiation of alkali silicate glasses results in the formation of metastable spin centers, such as oxygen hole centers (OHC 1 and OHC 2), silicon peroxy radicals and a silicon dangling bond (E ′ center). In this work, electron paramagnetic resonance (EPR) and electron nuclear double magnetic resonance (ENDOR) are used to study these spin-1/2 defects. It is shown that in a subset of the OHC 1 centers the ≡SiO  radical is strongly coupled to a single alkali cation. Thermally activated swinging motion of this cation causes asymmetric T 2 relaxation and changes electron spin echo envelop modulation (ESEEM) spectra. It is argued that trapping of the hole by non -bridging oxygen atoms does not result in the release of a compensating alkali cation. Rather, the O–Alk bond elongates and the whole structure relaxes. This view is supported by semi-empirical and ab initio calculations. The observed axial symmetry of the g-tensor for OHC 2 is the result of rapid tunneling of the electron between two degenerate sites with rate (0.2−50)×10 11 s −1 and activation energy ∼10 meV. This center is a hole trapped on a tetrahedral >SiO 2 2− unit or a planar –SiO 2 − unit. It is demonstrated that silicon peroxy radicals are not formed by charge trapping. Their generation is temperature-independent and occurs via the decay of self-trapped excitons. It seems likely that the same process yields silicon dangling bond centers.

Experiment of D-T neutron induced luminescence on window materials

A photon measurement system has been developed to analyze wavelength spectra of photons emitted from window material during D-T neutron irradiation, and the system consisted of a sample holder, a radiation-resistant optical fiber, a polychromator system and other electronic devices. High-purity silica glass, synthesized quartz and other optical materials were irradiated with D-T neutrons. The luminescence spectrum of the silica glass had a large peak around 450 nm, which was concerned with decay of self-trapped excitons in oxygen vacancies. The intensity of photon emission was proportional to the D-T neutron flux, though the neutron fluence was too low to damage the sample. 60Co γ-ray irradiation experiments were also performed with the same photon measurement system. The luminescence efficiency for D-T neutrons has been found to be 5±3 photons MeV −1 in visible range, while that for 60Co γ-rays to be 135±50 photons MeV −1. These data and the related discussions should be useful for the estimation of the photon-noise level on optical windows used for fusion plasma diagnostic systems.

Femtosecond Transient Absorption Spectroscopy of Nanocrystalline Polydiacetylene Colloids

Femtosecond transient absorption spectral measurements have been performed for size-controlled polydiacetylene microcrystals (30, 100, and 150 nm mean diameter) dispersed in water. Ultrafast relaxation processes within 5 ps after excitation were attributed to formation and decay of self-trapped exciton, which was independent on the size. A time-dependent spectral shift of transient bleaching, corresponding to the stationary excitonic absorption band, was observed in the time region from 10 ps to a few ns. It is explained as a thermalization of absorbed light energy within the microcrystal and the succeeding cooling by comparing transient absorption spectra with temperature difference spectra of the ground state absorption. Size-dependent cooling dynamics of a “hot” microcrystal was successfully demonstrated; the time constant increases with the crystal size.

Stabilization of H centres in irradiated LiF:Mg crystals

Abstract The paper is dedicated to investigation of possible ways of stabilization of the H centres created together with F and Mg+ centres in a process of decay of selftrapped exciton. As a main mode of the H centres stabilizationa capture of the H centre by magnesium-vacancy dipole and creation of V-type centre was suggested. This process was theoretically studied in the frames of model of molecular cluster which electron structure was calculated by the semi-empirical version of the Hartree-Fock method. For experimental corroboration of this process the relations between dependences of magnesium dipoles decrease on dose of gamma irradiation and corresponding dose dependences of the Mg+, Mg°, and F centres radiation creation were used.

The experimental observation of the potential barrier for self-trapped exciton decay into F-H pair in KCl-Na crystals

Abstract The optical absorption induced by the electron pulse irradiation of Na+ doped KCl has been measured. Transient optical absorption band of FA centers was observed at 80 K (LNT). The temperature dependence of FA center formation was studied. It is proposed that the obtained activation energy originates from the potential barrier between the STE perturbed by the cation impurity and the nearest neighbour FA-H pair. The mechanism of the suppression of the defect formation by the monovalent cation impurity in alkali halide crystals is discussed.

Femtosecond transient absorption saturation in poly(alkyl-thiophene-vinylene)s.

We report on ultrafast transient absorption measurements on solutions of poly(3-decyl-thiophene-2,5-diyl-vinylene), poly(3,4-dibutyl-thiophene-2,5-diyl-vinylene), and poly(3,3'-dibutyl-2,2'-dithiophene-5,5'-diyl-vinylene). In these polymers, a stiffening of the thiophenic backbone has been accomplished by inserting intrachain double bonds (vinyl groups). The transient absorption data seem to be consistent with the formation and decay of self-trapped excitons. The bleaching decays are independent of the size and location of the side groups, while they are sensitive to the backbone rigidity. A random-walk process on a finite polymer chain with recombination centers at the end is proposed to contribute to the relaxation mechanism of self-trapped excitons.

Femtosecond pump and probe experiments on thiophene-based polymers by a hybridly mode-locked cavity-dumped dye laser

Results on the bleaching decay observed in low-band gap thiophene-based polymers by femtosecond pump and probe experiments are presented. The laser used for excitation and probe is a newly developed cavity dumped femtosecond dye laser. The excited state kinetics of these materials presents a fast biexponential time behavior that may be related to the formation and decay of self-trapped excitons. >

Mechanism of Storage of Ionising Radiation Energy in LiF:Mg, Ti Crystals

Abstract Ionising radiation energy is stored in LiF:Mg,Ti as a result of radiative reduction of initial divalent magnesium ion-cation vacancy dipoles (Mg2+ centres) to univalent (Mg+ centres) and atomic (Mg0 centres) states. The Mg+ centre is created through decay of self-trapped excitons near Mg2+ centres. The Mg0 centre is formed owing to electron capture by a Mg+ centre. Calculations of electronic and spatial structure and dose dependences of radiative creation of these centres have been made. The dose dependences of the F, Mg+ and Mg0 centres, radiative creation have been obtained using LiF:Mg (0.003 - 0.3 wt %) crystals irradiated by gamma rays. Comparisons of the experimental results with computed ones confirm the validity of the model used.

Radiation defect production processes in activated KCl crystals

An investigation is made of radiation defect production processes in KCl, KClTl and KClPb crystals under X-ray irradiation at 290 and 85 K, respectively, as well as by irradiation with electron pulses with lengths of 50 ps and 5 ns at 290 K. The activator is found to localize electronic excitations near itself in the region of a capture radius Rc ≈ 4a. Besides, the activator leads to an increase of both, the probability of self-trapped exciton decay into FH pairs and that of survival of F centres due to stabilization of H centres. Thus, the above three effects lead to a nonuniform distribution of F centres with respect to the activator. A mathematical model is developed to explain the experimental results obtained. The existence of Pb Vk centre is also reported. [Russian Text Ignored].