A. Scacco

Luminescence of CsPbBr3-like quantum dots in CsBr single crystals

Abstract Luminescence and decay kinetics of the Pb2+ aggregates in CsBr host crystals were measured in the 4–300 K temperature interval and in 10−10–10−3 time scale. Their emission properties are similar to those of CsPbBr3 bulk crystal showing a subnanosecond free exciton emission in the 520–540 nm spectral region and slower trapped exciton emission in the 530–580 nm spectral region. An efficient energy exchange between the free and trapped exciton states is shown by the temperature dependencies of emission spectra. The quantum size effect is demonstrated in the high energy shift and broadening of the absorption and emission spectra and an estimate of the size of the CsPbBr3-like aggregates is provided. Independent evidence of the presence of the CsPbBr3 and Cs4PbBr6 aggregated phases in the CsBr host was obtained by X-ray structural studies.

Quantum size effect in the excitonic luminescence of CsPbX3-like quantum dots in CsX (X = Cl, Br) single crystal host

Copyright (c) 1997 Elsevier Science B.V. All rights reserved.Luminescence of CsPbX 3 -like quantum dots in CsX crystal host (X=Cl, Br) is reported. Single band spectrum is found for as grown or heavily annealed (T=240°C) CsX:Pb, while a family of emisssion bands is observed after annealing at temperatures below 210°C. Anisotropic growth of quantum dots in the early stage is proposed and positions of emission peaks are calculated for CsPbCl 3 quantum dots using a simple model.

Spectrally resolved thermoluminescence of Cu and Eu doped Li2B4O7

Abstract Thermoluminescence studies are carried out on samples in powder form of Li2B4O7 doped with Cu or Eu impurities. Results concerning Li2B4O7:Cu are found in agreement with data in the literature and show that the emitting defects are of intrinsic nature. In Li2B4O7:Eu a further emission is attributed to extrinsic aggregate centres.

Selective production of aggregate centers in LiF crystals by ionizing radiations

Abstract Optical absorption and photoluminescence spectra have been measured in LiF crystals colored for the first time with 5 MeV electrons. The measurements show that this kind of irradiation reduces (much more than γ-rays) the production of complex aggregate defects with respect to that of F + 3 and F 2 centers. Moreover, the laser active F + 3 centers can be preferentially created in place of F 2 centers by decreasing the irradiation temperature.

Impurity-induced thermally stimulated luminescence of KMgF3:Ce3+ crystals

Abstract The thermally stimulated luminescence (TSL) of KMgF 3 doped with Ce 3+ has been measured using a high-sensitivity TSL spectrometer that measures the emitted intensity as a function of both temperature and wavelength. TSL peaks have been detected at 80, 170°C and in the regions of 300–400°C (heating rate 1°C/s). The TSL emissions are at 270 and 360 nm, the latter emission showing a strong dependence on Ce 3+ concentration. The correlation of these emissions with recently published photoluminescence data and the behaviour of the TSL peaks as a function of Ce 3+ concentration allows an interpretation of the responsible defects, in terms of two types of substitutional replacement of K + ions with Ce 3+ impurities.

Optical properties of Pb2+-based aggregated phases in CsBr Thin film and single crystal matrices

Abstract Emission characteristics of CsPbBr3 and Cs4PbBr6 aggregates in CsBr bulk and thin film matrices are reported. The emission of the former aggregated phase is peaking about 520–560 nm. It shows small Stokes shift (50 meV) related to narrow free exciton emission line of sub-nanosecond decay times. Quantum size effect was evidenced for the aggregates of 6–7 nm in diameter. The Cs4PbBr6 aggregates show emission peak at 375 nm and overall emission characteristics are similar to those of KBr: Pb, which is explained by very close local arrangement of emission centres—(PbBr6)4- octahedra—in both structures.

Observation of absorption bands due to the parity-forbidden transitions of ions in crystals

Optical absorption and magnetic circular dichroism spectra of ions in crystals have been measured. The fine structure of the absorption bands due to the and transitions in the spin- and parity-forbidden intraconfigurational electronic transition has been studied at different temperatures. By comparison with the one-photon and two-photon excitation spectra for the luminescence due to the transition, it is shown that the fine structure consists of the absorption peaks due to three kinds of ion which substitutes ions but have different site symmetries, i.e. the cubic, tetragonal and trigonal symmetries.

Colour centres induced in LiF by low-energy electrons

A detailed study has been carried out on the optical properties of intrinsic colour centres, created in LiF crystals under irradiation with low-penetration 3 keV electrons. The kinetics of the production of F and M centres, determined from optical absorption measurements, exhibits a typical three-stage structure and an evident saturation at high doses. The quantitative evaluation of the kinetic parameters is in excellent agreement with the predictions of the most comprehensive theoretical model. A comparison of the effects of irradiation at different temperatures, determined from absorption and photoluminescence spectra, allows one to deduce the conditions for preferential formation of or defects in LiF crystals.

Detailed model of the photostimulated realignment ofF FA centres in KCl:Li+

SummaryThe photostimulated reorientation of theFA centres in KCl:Li+ is described by a simple model that takes into account the off-axis angle of the centres and the overlapping of the tails of the two absorption bands. With this model we have interpreted the dependence of the luminescence intensity on the polarization of the exciting beam. The fitting of the experimental data gives an off-axis angle of approximately 5° and an overlapping of the bands larger than that predicted for Gaussian line shapes.RiassuntoLa riorientazione fotostimolata dei centriFA in KCl:Li+ è descritta con un modello semplice che tiene conto dell’angolo di disallineamento e della sovrapposizione delle code delle due bande di assorbimento. Con questo modello abbiamo interpretato la dipendenza dell’intensità di luminescenza dalla polarizzazione della luce di eccitazione. Il fitting dei dati sperimentali dà un angolo di off-axis di circa 5° ed una sovrapposizione delle bande maggiore di quella prevista per curve di tipo gaussiano.РезюмеС помощью простой модели описывается фотостимулированная переориентацияFA центров в KCl:Li+. Эта модель учитывает угловое удаление центров от оси и перекрытие хвостов двух зон поглощения. С помощью этой модели мы интерпретируем зависимость интенсивности люминесценции от поляризации возбуждающего пучка. Подгонка экспериментальных данных дает угловое удаление от оси порядка 5° и большее перекрытие зон, чем было предсказано для гауссовых форм линий.

New luminescence of the FA centre in RbCl:Li+

Type-I or type-II behaviour of the FA centres in alkali halides has always been thought of as a feature dependent on the crystal lattice parameter and on the impurity size. We have found experimental evidence suggesting for the first time that both behaviours are present in RbCl:Li+, where the FA centre at low temperature (T < 15 K) exhibits a luminescence of type-I switching to type-II at higher temperature. This effect is probably related to the possibility for the Li+ ion of being, at high temperature, a mobile and off-centre ion. Both these conditions seem to be necessary for having a type-II behaviour.