The photoluminescence (PL) and thermoluminescence (TL) properties of AlN ceramics revealed under UV irradiation are determined mainly by oxygen-related centres, giving rise to the UV (around 3.18 eV) ...and the Blue (2.58 eV) bands. It was found that the UV irradiation-generated donor–acceptor pairs (DAPs), responsible for the UV emission band, are randomly distributed with regard to separation distance. Luminescence properties of AlN are interpreted basing on the model of localised recombination involving electron tunnel transitions from the excited state of D to the ground state of A, proposed by Jain et al. (2012). The observed features of PL, afterglow and TL of AlN ceramics are explained by dependence of tunnelling recombination probability on separation distance between D and A implied by the used model.
•UV and Blue emission in AlN ceramics is due to donor–acceptor pair recombination.•UV emission band is produced by recombination of ON-vAl (acceptor) and ON (donor).•In AlN ceramics donor–acceptor pairs are randomly distributed by separation distance.•Model implies transitions from excited state of donor to ground state of acceptor.•DAP separation determines recombination probability and luminescence features
Native defect-induced photoluminescence around 400nm (blue luminescence - BL) was studied in hBN materials with different size and various origins. The following spectral characterizations were used: ...spectra of luminescence and its excitation, luminescence dependence on temperature, luminescence kinetics, optically stimulated luminescence and infrared absorption. It was found, that the BL is characteristic for all these materials, which were studied. The BL forms a wide, asymmetric and phonon-assisted emission band at 380nm. This luminescence can be excited either through the exciton processes, or with light from two defect-induced excitation bands at 340nm and 265nm. It was found that the BL is caused by two luminescence mechanisms. One of them is intra-center luminescence mechanism (340nm excitation), but the other one is recombination mechanism (265nm excitation). It was considered that the most probable candidates for the defects, which cause the BL in hBN can be related to the nitrogen vacancy type-centers. It was certainly confirmed, that presence of oxygen gas is partly quenching the BL intensity, thus ranking the hBN material among the materials prospective for development of oxygen gas optical sensors.
•Defect-induced luminescence at 380nm is observed for hBN powders.•A phonon-assisted structure is characteristic for 380nm luminescence.•The defect-related excitation of 380nm luminescence is at 340nm and 265nm.•The 380nm luminescence is caused by a recombination or an intra-center mechanism.•The 380nm luminescence is sensitive to oxygen gas environment.
The temperature-dependent polarized photoluminescence spectra of nonpolar ZnO samples were investigated by 263 nm laser. The degree of polarization (DOP) of m-plane quantum wells changes from 76% at ...10 K to 40% at 300 K, which is much higher than that of epilayer. The strong anisotropy was presumably attributed to the enhanced confinement effect of a one-dimension confinement structure formed by the intersection of quantum well and basal stacking fault. The polarization of laser beam also has an influence on the DOP. It is assumed that the luminescence polarization should be affected not only by the in-plane strains but also the microstructural defects, which do modify the electronic band structure.
► Al2O3 nanopowders were produced by calcination in 800–1400°C temperature range. ► In all samples photoluminescence is determined by uncontrolled impurities. ► Phase transition at 1200°C is ...accompanied with modification of emission spectrum. ► Explanation: switching of active luminescence centers. ► Assumption: surface hydroxyl groups transfer excitation energy to impurity centers.
Photoluminescence was studied in six samples of Al2O3 nanopowders produced from the same initial material by calcination in the 800–1400°C temperature range. At temperature around 1200°C phase transition in aluminum oxide lattice occurs; the samples produced at temperatures up to 1200°C contain mainly δ phase, while those obtained at 1400°C contain pure α phase. In all studied samples of nominally pure aluminum oxide nanopowders photoluminescence is determined by trace level concentrations of uncontrolled impurities. It was found that phase transition is accompanied with modification of the emission spectrum: a broad band centered around 750nm presumably ascribed to emission of Fe3+ ions is characteristic for photoluminescence of the samples of δ phase, while narrow band emission of Mn4+ is observed in the samples of α phase. Aside from that emission of Cr3+ ion is observed in all studied samples with the difference that intensity, position and shape of emission bands are characteristic either to transient forms or to α phase of aluminum oxide. Switching of the active luminescence centers in the samples of the same composition with phase transition is tentatively explained by change of the crystal field symmetry affecting probability of electron transitions in impurity centers. An assumption is done about the decisive role of surface hydroxyl groups in energy transfer to impurity luminescence centers.
•A fast and long-lasting recombination luminescence processes in AlN:Mn2+ are observed.•Complex decay processes for 600 nm PersL in AlN:Mn2+ are observed.•The mechanisms of 600 nm recombination ...luminescence in AlN:Mn2+ are proposed.
Luminescence processes resulting in 600 nm emission of Mn2+ ions in AlN:Mn ceramics were studied based on investigations of photoluminescence and its excitation spectra, luminescence kinetics and long-lasting luminescence (PersL) properties. For AlN:Mn2+ nanopowders, the photoluminescence spectra and PersL were studied. Luminescence properties were examined and compared after the samples were irradiated with 520 nm light, resulting in direct excitation of Mn2+ ions, thus causing the intra-center luminescence, or with 263 nm light. As known, in the last case, the oxygen-related defects are primarily excited with the following energy transfer to Mn2+ ions and 600 nm emission, thus forming the recombination luminescence (RecL). Two types of excitations of the 600 nm RecL were used. In the first case, the luminescence response was detected during the sample irradiation with 263 nm light. It was found that at RT, the decay of the RecL is fast and its decay constant τ = 1.2 ms coincides with the value obtained for the intra-center luminescence. A time-dependent rise of the 600 nm luminescence intensity under 263 nm excitation was observed. In the other case, the 600 nm RecL was detected when irradiation of the sample with 263 nm light was ceased, and spectra and decay of PersL were studied. It was found that the decay of 600 nm PersL spectra could be described using three exponential functions, thus manifesting a variety of luminescence processes. The results allow tracing of the luminescence processes and proposal of the mechanisms resulting in the 600 nm light emission of Mn2+ ions. An energy level scheme of AlN:Mn2+ was constructed to elucidate of the luminescence processes and mechanisms.
Background: Elaboration of new luminescent nanomaterials for imaging of biological materials including cells of living organisms and their parts is highly actual. These materials must meet a number ...of requirements such as low toxicity, inherence of intensive luminescence, low costs of raw material and symple synthesis methods. AlN nanopowder is one of such prospective materials fitting the above requirements. Our long time investigations on spectral characteristics for III group element nitrides allows chose of doped AlN nanopowder as prospective candidate for developing of luminescent markers for imaging of biological materials.
Objectives: The aim of the present study is spectral characterization of AlN nanopowder doped with Mn and evaluation of its use as luminescent marker for biological materials.
Materials and methods: AlN nanopowder with average size of polycrystalline grains of 60 nm and the same doped with Mn were sythesized in Institue of Inorganic Chemistry, Riga Technical University. Photoluminescence and its excitation spectra of the materials were studied at room temperature using a self-made set-up.
Results: It was found that in undoped AlN nanopowder at room temperature luminescence of native defects forms a wide and complex band peaking at 415 nm. This blue luminescence can be excited with ultraviolet light from two spectral regions around 315–340 nm and 260 nm. Two luminescence mechanisms are proposed dependent on the spectral region of exciting light. The first of them results in the intra-center luminescence, but the second one is recombination luminescence.
Incorporation of Mn atoms in the crystalline lattice of AlN nanopowder forming AlN:Mn NP results in appearance of intensive red luminescence at 600 nm, which can be excited with light from two excitation bands at 260 and 480 nm. Two mechanisms responsible for an appearence of the red luminescence of Mn are proposed. They are the intra-center luminescence and recombination luminescence mechanisms. In this case the red Mn luminiscence prevails and the blue luminescence characterizing the host material has not been observed.
Conclusion: AlN nanopowder doped with Mn atoms is a prospective material for use as luminescent marker for imaging of biological materials. Properties of this material are in a good agreement with the main requirements obligated to biological materials: i) AlN NP has low toxicity; ii) AlN:Mn NP possesses intensive red luminescence at 600 nm, which can be excited either with the ultraviolet light around 260 nm or with visible light around 480 nm; iii) it is relatively cheep material and it can be synthesized using simple synthesis methods.
Cerium doped yttrium silicates phosphors (YSO:Ce) were prepared by gel combustion using vinyltriethoxysilane (VTEOS) as silicon sources along with aspartic acid as fuel and yttrium-cerium nitrate as ...oxidizer. The study presents the influence of VTEOS amount in the synthesis mixture on the structural and luminescent characteristics of silicate phosphors. The understanding of precursor׳s decomposition was achieved on the basis of thermal analysis in association with gas evolved analysis. XRD, FTIR and XPS were used to reveal the structural changes that occur with VTEOS molar amount variation from 1 to 3mol. It was found that the main crystalline phase was X2-Y2SiO5. The luminescent characteristics of phosphors were measured at room and low temperature (10–300K) based on emission and excitation spectra. Under UV excitation, YSO:Ce exhibits blue emission due to electron transition in Ce3+ from 5d level to the ground state levels (2F5/2, 2F7/2). The emission intensity increases from 70% to 120% along with VTEOS amount, explained by the improvements in structural homogeneity. Incorporation of cerium in different sites (Ce1 and Ce2) is discussed based on PL and PLE spectra measured at low temperature (10K).
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