Luminophores based on doped Li2B4O7 have been studied by a variety of methods (photoluminescence, pulsed cathodoluminescence and kinetics thereof, thermoluminescence, and electron paramagnetic resonance). Different impurities exhibit both interplay and competition. The interplay of impurities is expressed in the luminescence sensitization due to effective excitation energy transfer which occurs in composite impurity centers. Such centers consist of two differently incorporated impurities, one at a cationic site, another imbedded in the anionic structure. The impurities can compete for a particular position in the crystal lattice, and the priority of doping affects the result. The luminescence and thermoluminescence properties observed in the samples are connected with the structure of impurity centers. It is shown that the loss of trapped charge carriers due to instant recombination on the luminescence centers and low-temperature thermoluminescence (below 400 K) diminishes the luminescence yield for the high-temperature (above 400 K) thermoluminescence peak used in dosimetry.

The radiation stability of polymer test objects fabricated by direct laser writing is studied. The two groups of 6 cubes (50×50×50 μm3) were printed on a common silicon substrate. 6 cubes were irradiated with X-rays in the synchrotron radiation beam (12 keV, 1010 photons/mm2/s), the other 6 kept non-irradiated for the reference. In each group, the fabrication parameters (laser power and slicing step) were uniformly varied from cube to cube. Each irradiated cube was exposed with the dose of 1013 X-ray photons. The calibration of this dose in the radiological units (Gy) was carried out using the tissue-equivalent thermoluminescent detectors (TLD-800). The corresponding exposition dose was estimated as 30 kGy for each cube. The radiation effects were evaluated by Raman spectroscopy and by morphological changes observed with the raster electron microscope. The primary radiation effect appears in the essential increase of the polymerization degree with consuming carbon-carbon double bonds in favor of the polymer cross-linking. Only one cube of 6 has changed the size and shape due to this effect, while the others retained the initial shape. Morphological changes are connected with the improper fabrication parameters (laser power and slicing step). Properly fabricated objects withstand the X-rays of the synchrotron radiation beam.

Nanoparticles (NPs) exhibiting X-ray-excited UV-C luminescence can be used in radiation therapy to deactivate cancer cells through photochemical reactions of DNA with UV-C quanta. Colloidal solutions of monoclinic La1−xPrxPO4 NPs (x = 0.01–0.3), luminescent in the UV-C range (220–280 nm), with different morphologies, from nanofiber (diameter and length not larger than 15 and 600 nm, respectively) to short nanorod (diameter and length not larger than 8 and 35 nm, respectively), were obtained by a microwave-assisted hydrothermal method. For possible biomedical use, the synthesis parameters (pH = 8, anion excess coefficient = 2) were determined, at which nanorods of suitable sizes (diameter and length not larger than 10 and 80 nm, respectively) with the brightest UV-C luminescence among all synthesized nanorods were obtained. A gradual increase in the optimal concentration of Pr3+ ions with maximum luminescence brightness from 4 mol-

Based on an analysis of published data, we formulate an idea that a cascade two-photon transfer of holes from traps to recombination centers occurs in borate phosphors used in dosimetry. The previously described kinetic model is supplemented with the absorption of a second photon by a hole in a metastable state. It is concluded that the intensity of the stimulating light flux should be accurately stabilized if readout occurs in the two-photon regime, since in this case the maximum amplitude of the optically stimulated glow curve has a complex dependence on the intensity of the light flux.

We studied the physical principles of creating 3D microstructures using the method of femtosecond two-photon photopolymerization. We present 3D microstructures of complex topology and demonstrate a wide range of optical applications: elements of photonic integrated circuits, a new generation of micro-optical devices, elements of X-ray optics, and devices with single-photon sources.

Different types of ultrafast radiative transitions are considered. The most interesting among them is the case when the radiative transition is accelerated by the configurational transformation of a structural unit where it occurs. Impurity-induced VUV excitation bands of doped Li2B4O7 are attributed to the creation of impurity-bound excitons. When Mn2+ is involved into exciton recombination, the radiative transition in the Mn2+ 3d5 configuration is accelerated and occurs on a sub-nanosecond time scale. Excitation within the UV bands is connected with energy transfer from the structural units formed by the sensitizers (Cu, Sn) and oxygen to Mn2+. In this case, Mn2+ transitions are not accelerated since its excited state appears after complete relaxation of excitation in the corresponding sensitizer’s unit. Pulsed cathodoluminescence decays are rather slow due to very slow transport of electron–hole pairs and excitons in Li2B4O7.

The possibility of optical readout has been studied for the detectors based on MgB4O7:Dy,Na. It is shown that due to the release of electrons from traps instead of holes under illumination by light, optically stimulated luminescence is not observed, despite effective erasing of thermally stimulated luminescence glow curves by light. By analogy with MgB4O7:Tm, a model of the processes occurring during thermoluminescence is presented.

Cathodoluminescent UV-light (peaked at 315 nm and 355 nm) sources have been created using commercially available phosphors, with photocatalytic activity observed at irradiation of titanium dioxide. The power density of UV radiation is higher than 10 mW/cm2. The photocatalytic efficiency was estimated by measuring the oxidation rate of acetone vapors (reaction rate exceeds 2 ppm/min).

Sub-nanosecond luminescence of Mn2+ in Mn-, Sn/Mn-, and Cu/Mn-doped Li2B4O7 ceramics is observed under laser excitation. Picosecond pulsed laser radiation (350-370 nm) generates the frequency-doubled radiation directly in a single microcrystal selected in a ceramic sample, with this secondary radiation falling within the impurity-induced VUV excitation bands of doped Li2B4O7. These excitation bands are attributed to the creation of an impurity-bound intra-anionic exciton in MeiO4-BO3-Mn-Li complex centres (Me-i = Mn, Sn, or Cu). The exciton, in turn, interacts with a nearby Mn2+ ion due to mixing of the Mn2+ excited states with the excitonic states. Exciton annihilation involves Mn2+ energy levels as transient states and causes a sub-nanosecond radiative transition which is accelerated, being accompanied by rearrangement of the surrounding atoms. Excitation within the lower-energy bands is related to the other part of the complex luminescence centre, this is metal-to-ligand intra-anionic charge transfer, which makes an electron accessible to Mn-Li, while the hole is transferred only during relaxation of the excited structure. Mn2+ radiative transitions are not forced in the latter case, and slow Mn2+ luminescence decay is observed. (C) 2021 Elsevier B.V. All rights reserved.

Ultraviolet radiation is widely used in biomedical practice. In some areas, its use is limited by the lack of sufficiently cheap sources of the desired spectrum and power. The prototypes of mercury-free cathodoluminescent UV-radiation sources are manufactured. They use field emission cathodes on the basis of carbon fiber. These sources exhibit various UV spectra depending on the phosphors used. New types of UV-emitting cathode-ray-tube phosphors are suggested.

A simplified model of thermally stimulated luminescence (TSL) and optically stimulated luminescence (OSL) kinetics is considered for the case of impeded carrier transport. A response function is obtained for the OSL with transport barrier, being of special interest for the extreme case of saturation mode (when the optical term in the model exceeds all other terms). In this case, the maximum of the response function does not depend on the stimulating light intensity but is proportional to the recombination rate, which in turn, depends on the transport barrier and temperature. A high barrier makes the OSL ineffective, whereas the elevated temperatures can increase OSL response for moderate barriers. If carriers released by light are transferred deeper to the respective energy band, they can overcome the transport barrier during their thermalization, which decreases the temperature required for OSL.

Samples of inverted photonic-crystal films, containing planar and rolled-up (in the form of scrolls) CdSe nanocrystals, are studied. The transmission spectra of these structures are recorded. These spectra (along with the change in colour) confirm the incorporation of nanocrystals into the films. The photoluminescence decay dynamics is investigated. It is shown that the photonic-crystal matrix affects significantly the luminescence kinetics of nanostructures. The differences in the decay curves measured for nanocrystals in a photonic-crystal matrix and for their ensemble on a glass substrate are explained by the influence of the photonic-crystal stop band and the orientational effect of crystalline matrix, which orients anisotropic nanocrystals and prevents them from aggregation. The results obtained may be important for potential applications in optoelectronic devices.

Manganese clustering effects were studied in Li2B4O7 (LTB) ceramics. The samples LTB:Me + Mn (Me = Sn, Cu, Mn, Zn, Be) were prepared by a two-stage doping method (except for LTB doped with Mn solely). EPR, pulsed cathodoluminescence, thermoluminescence (TL) were measured. Mn clustering causes supralinearity of the material response to the radiation dose, reversible shift of the dosimetric TL peak, and irreversible radiation damage. Radiation damage of the LTB crystal lattice is accompanied with the formation of a peroxide bridge instead of a single oxygen at the common vertex of two BO4 tetrahedra. Repelled by an excessive positive charge of a Mn cluster, holes are localized around at traps including the peroxide bridges where they are stable up to 630 K. The extent of the radiation damage can be estimated by the intensity of the high-temperature TL at 630 K, when the released holes recombine with the electrons near regular Mn2+. The most stable against the radiation damage and also suitable for TL dosimetry is LTB:Sn + Mn.

AbstractThe results of the study of the degradation of thermoluminescent materials Li_2B_4O_7:Be + Mn and Li_2B_4O_7:Zn + Mn under the effect of radiation (pulsed electron beam) and laser radiation are presented. As a result of exposure to high doses of radiation, the structure of the samples under study partially acquires an amorphous character, while the effect of radiation exposure is manifested in the optical properties in the appearance of green luminescence due to manganese centers in the tetrahedral environment. With subsequent irradiation with a laser at a wavelength of 350 nm, luminescence centers decay due to photochemical oxidation of manganese by the reaction of Mn^2+ → Mn^3+. It is shown that Li_2B_4O_7:Be + Mn has a lower radiation resistance than Li_2B_4O_7:Zn + Mn.

ZnO:W luminescent material was prepared and characterized. We compare nano-sized and bulk material composition and luminescence. Nanoparticles were prepared by a novel acoustoplasma technique (plasma discharge in a liquid at an intensive ultrasonic field above the cavitation threshold). Bulk ZnO:W was prepared by classical annealing way from the mixture of ZnO powder and H2WO4. Under excitation with a pulsed electron beam, the ZnO:W nanoparticles demonstrated very low luminescence efficiency while the bulk sample produced intensive green cathodoluminescence. The most intensive green luminescence was observed after annealing of the sample in air, which excludes a well-known mechanism involving anion vacancies. Probably, tungsten impurity tunes excitonic luminescence towards longer wavelengths, but this point requires further investigations for verification.

The results of the study of the degradation of thermoluminescent materials Li 2 B 4 O 7 :Be + Mn and Li 2 B 4 O 7 :Zn + Mn under the effect of radiation (pulsed electron beam) and laser radiation are presented. As a result of exposure to high doses of radiation, the structure of the samples under study partially acquires an amorphous character, while the effect of radiation exposure is manifested in the optical properties in the appearance of green luminescence due to manganese centers in the tetrahedral environment. With subsequent irradiation with a laser at a wavelength of 350 nm, luminescence centers decay due to photochemical oxidation of manganese by the reaction of Mn 2+ → Mn 3+ . It is shown that Li 2 B 4 O 7 :Be + Mn has a lower radiation resistance than Li 2 B 4 O 7 :Zn + Mn.

The effect of doping conditions on the properties of Li2B4O7-based thermoluminescent materials is studied. It is shown that suppression of Mn clustering improves the linearity of the radiation dose response. Moreover, the order of doping is essential to achieve the formation of trapping and luminescence centers: simultaneous co-doping with Mg degrades the sensitivity of detectors by a factor of 10, while sequential Mg co-doping doesn't diminish the sensitivity but decreases the dispersion of properties in the batch. The origin of the observed technological effects is discussed.