P. Kulis

Colour centres in LiBaF3 crystals

Abstract The origin of the absorption bands in LiBaF 3 created by X-ray irradiation at RT has been investigated. It is found that three absorption bands at 270, 320 and 430 nm represent different electron transitions within a radiation defect effectively created in LiBaF 3 single crystals. Following the Mollwo–Ivey relation we discuss investigations of optical dichroism, magnetic optical dichroism, as well as the assumptions regarding the accumulation kinetics of these absorption bands, the F centre being the main radiation defect created by X-rays in undoped LiBaF 3 crystals at RT.

Colour centres in LiBaF3

Abstract The X-ray-induced optical absorption (OA) and the spectra of magnetic circular dichroism (MCD) followed by optically detected EPR (ODEPR) have been investigated in undoped LiBaF3 crystals. According to ODEPR, the absorption bands in the spectral region 470–770 nm correspond to the F-type centres, whereas the absorption band at 420 nm is assigned to the hole centres. The redistribution of electrons to the thermal more stable F-type centres indicates that all types of colour centres are annealed in the course of the thermal release of electrons from F-type centres, accompanied by a thermostimulated luminescence.

F-type centres in LiBaF3 crystals

Abstract The origin of the absorption bands in LiBaF3 created by X-ray irradiation at RT has been investigated. It is found that three absorption bands at 270, 320 and 430 nm represent different electron transitions within the same radiation defect effectively created in LiBaF3. The Mollwo-Ivey relation as well as investigations of optical dichroism, magnetic circular dichroism and the assumptions regarding the accumulation kinetics of these absorption bands lead us to suggest that the F-type centre being the main radiation defect created by X-rays in LiBaF3 crystals at RT.

Photo- and thermostimulated processes in α-Al2O3

Abstract We reported on the recombination processes determined by the release of electrons from defects connected with the dosimetric 430 K thermostimulated luminescence (TSL) peak as well as with the 260 K TSL peak. These TSL peaks appear in thermochemically reduced α-Al 2 O 3 crystals containing hydrogen and emission of these TSL peaks corresponds to luminescence of the F-center. The X-ray exposure or UV excitation in the absorption band of F-centers at 6.0 eV of reduced α-Al 2 O 3 crystals doped with acceptor impurities results in the appearance of a broad anisotropic complex absorption band in the spectral region 2.5–3.5 eV and in the appearance of a predominant TSL peak at 430 K. Above 430 K the above-mentioned broad absorption band disappears. Optical bleaching of the 2.5–3.5 eV band is accompanied by the disappearance of the 430 K TSL peak and results in F-center emission. The X-ray or UV excitation of reduced α-Al 2 O 3 crystals with donor-type impurities results in the appearance of an anisotropic absorption band at 4.2 eV and the appearance of a dominant TSL peak at 260 K. Above 260 K the 4.2 eV absorption disappears and photostimulated luminescence (PSL) of the F-center recombination luminescence in the 4.2 eV region is no longer observed. Optical bleaching of the 4.2 eV absorption band is accompanied by the disappearance of the 260 K TSL peak. The successful use of reduced α-Al 2 O 3 in dosimetry needs the optimization of the concentration of all components (acceptors, hydrogen, intrinsic defects) involved in the thermo- and photostimulated processes.

Self-trapped holes and recombination luminescence in LiBaF3 crystals

Abstract We investigated electron paramagnetic resonance (EPR) angular dependencies, recombination afterglow and thermostimulated luminescence of undoped LiBaF3 crystals, X-irradiated at low temperatures. EPR parameters of the F2− molecule oriented along the [1 1 0] direction have been obtained. Based on the value of the g-shift Δg=g⊥−gII=0.02, characteristic for the VK-centres in similar perovskites, we propose that we indeed observed the VK-centres, not the H-centres. X-irradiation below 170 K results in creation of a long-time temperature-independent afterglow due to the tunnelling recombination between close electron and hole centres. The F-type electron centres and the VK as well as probably O2− centres are proposed to be the tunnelling recombination partners, responsible for the 4.1 and 3.15 eV luminescence bands, respectively.

Ionic charge transport, thermoactivated delocalization and interaction with radiation-induced defects in LiBaF3 crystals

Abstract The ionic and ion-diffusion controlled thermally stimulated relaxation (TSR) processes in CaF2, BaF2 and LiBaF3 crystals have been investigated at 290–650 K by means of the ionic conductivity, ionic thermally stimulated depolarization current (TSDC) and X-ray induced optical absorption spectra TS bleaching (TSB) techniques. In the region of the extrinsic ionic conductivity a number of the ionic TSDC peaks (anion vacancy and interstitial detrapping) and correlated TSB stages have been detected for the first time. The TSR and TSB kinetics above RT in fluoride crystals (X-ray irradiated at RT) are initiated and controlled by the anion interaction with the radiation-induced defects, i.e., the anion diffusion controlled processes take place.

Defect assisted intrinsic luminescence in Al2O3 crystals

Abstract The origin of the luminescence bands at 7.5 eV anv 3.8 eV appearing additionaly to the luminescence of F- and F+- centres in pure Al2O3 are investigated. The time - resolved luminescence spectra, absorption and luminescence excitation spectra as well as trap spectroscopy data depending on deviation from the stochiometry of crystals are discussed in terms of self - trapping of excitons in two configurations. The role of defects due to annihilation of excitons is considered.

Thermal relaxation of colour centres in LiBaF3 crystals

Abstract Processes in LiBaF3 crystals caused by the thermal decay of F-type centres created by X-irradiation at room temperature have been examined. It is shown that the thermal decay of F-type centres results in the formation of two kinds of electron centres peaking at 630 nm and 740 nm differing in thermal stability. Weak TSL intensity, accompanying the decay of F-centres, also observed as well as the low value of the process activation energy suggest that due to the presence of moving anion vacancies a random walk of the F-centres occur. We propose that in course of the random walk of the F-centres both the aggregate F-centres are created and the annihilation with some complementary radiation defects take place.

Work function of colloidal semiconducting nanocrystals measured by Kelvin probe

Colloidal suspensions of CdS, PbS, Cu2S, Ag2S and ZnO nanocrystals were prepared by chemical route in presence of organic capping ligands. The formation of nanocrystals was studied by using UV-visible absorption spectroscopy. Kelvin probe measurements of work function were performed on nanocrystals thin film on ITO (indium tin oxide) coated glass.

Annealing of color centers in LiBaF 3

Results of the glow rate technique to analyze the activation energy of thermostimulated annealing of X-ray created F -type color centers in LiBaF 3 crystals are presented, showing pure and containing oxygen centers. It is shown that depending on the impurity composition two alternative mechanisms are involved in the annealing of color centers. It is proposed that either the anion vacancy governed migration of F -centers resulting in recombination with complementary defects, or the thermal delocalization of radiation created fluorine ( F i ) interstitials captured by anti-structure defects followed by recombination with all kinds of complementary F -type centers are responsible for the recombination of radiation defects above RT.