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.
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.
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•400nm Luminescence of hBN powder is sensitive to surrounding vacuum or oxygen gas.•Decrease of hBN powder grain size improves its oxygen sensing properties.•hBN powder is proposed as ...prospective material for oxygen gas sensors.
Gas sensing properties of hBN powder bulk and nanosize were studied. It was demonstrated that for hBN powders with grain sizes of 70nm, 1μm and 5μm the native defect-induced luminescence observed at 400nm under 265nm light excitation and room temperature is sensitive to oxygen gas reducing luminescence intensity. The highest value of luminescence intensity is reached when sample is in vacuum. Results obtained allow conclusion that the hBN powder is prospective for sensing of oxygen gas. Some material properties such as dependence of luminescence intensity on vacuum level and pumping time, ratio of luminescence intensity when sample is in vacuum and gas, its dependence on material grain size were studied.
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).
Native luminescent defects were investigated in AlN nanopowder (NP) using spectral characterization methods. Photoluminescence and its excitation spectra were studied within a wide temperature range ...from 8K up to room temperature. It was found that in AlN NP a broad luminescence band appears within a blue spectral region consisting of at least two sub-bands at 415 nm and 390 nm, which can be related to presence of two different but in the same time similar defect types. These luminescent defects are located either inside the bulk material or on the material surface. Interaction of the surface defects with environmental oxygen was found resulting in quenching of the blue luminescence (BL). Three BL mechanisms were revealed, depending on spectral region of the exciting light, resulting in intra-center luminescence, recombination luminescence and exciton caused luminescence. Variations of the F-centers are proposed to be responsible for the BL in AlN NP.
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•In AlN blue luminescence appears caused by intra-center or recombination processes.•The blue luminescence in AlN is related to nitrogen vacancy-based F-centers.•Luminescent F-centers are located either in the bulk of AlN or on its surface.•Oxygen is interacting with F-centers from AlN surface reducing its luminescence.•In AlN nanopowder the blue luminescence from the surface defects is dominant.
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•Determined the dielectric function tensor of orthorhombic single-crystal LiGaO2 in the spectral range from 0.0124eV to 6.5eV.•In the IR region, 100cm−1<ω<800cm−1, εa(ω), εb(ω), and ...εc(ω) dielectric functions manifest a rich structure due to 33 IR-active optical phonons.•The absorption edge of lithium gallate is positioned at about 5.7eV.•The dispersion of εa(ω), εb(ω), and εc(ω) dielectric functions indicates that the LiGaO2 absorption band starts with the excitonic transitions.
The optical dielectric function tensor of orthorhombic single-crystal LiGaO2 was determined for polarizations along a, b, and c crystal-axis in the photon energy range from 0.04eV to 6.5eV by the generalized spectroscopic ellipsometry. In the far-infrared spectral range from 12.4meV to 40meV, the dielectric function was determined from conventional polarized transmittance and polarized reflectance measurements. Lineshape analysis of the dielectric function tensor major components allowed for a determination of the long-wavelength optical phonon characteristics, refractive indices dispersion, and parameters of interband and excitonic optical transitions.
Pyroelectric luminescence was observed in noncentrosymmetrical crystal LiGaO2 with the direct band gap around 6 eV. For the first time spectral and kinetic characteristics of pyroelectric ...luminescence were obtained. The temporal structure of the PEL signal was determined as a sequence of pulses with duration not longer than several nanoseconds. This allowed proposing of the luminescence mechanism: in vacuum conditions in LiGaO2 crystal pyroelectric luminescence occurs inside the sample due to radiative recombination of electrons with the positively charged intrinsic luminescence centres.
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•Pyroelectric luminescence is observed in lithium metagallate LiGaO2.•Spectral and kinetic characteristics of pyroelectric luminescence are obtained.•Emission spectra contain intrinsic luminescence bands of LiGaO2.•Duration of a pyroelectric luminescence pulse is no longer than several nanoseconds.•Mechanism proposed: radiative recombination of electrons with intrinsic hole centres.
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