Correlation between luminescent characteristics and phase composition of ZnS:Cu powder prepared by self-propagating high temperature synthesis

a b s t r a c t The powder-like ZnS:Cu grown by self-propagating high temperature synthesis from the mixture of Zn, S and CuCl is investigated before and after annealing at 800 1C by photoluminescence (PL) and X-ray diffraction (XRD) techniques. It is found that after synthesis the ZnS:Cu powder consists of a mixture of cubic and hexagonal ZnS phases as well as crystalline Cu x Zn 1 À x solid solution. PL spectrum shows a wide PL band which is the superposition of green and blue Cu-related bands as well as self-activated one. It is shown that annealing at 800 1C gives rise to three processes, controlled by the heating time to annealing temperature: (i) phase transformation of ZnS hexagonal phase to cubic one; (ii) oxidation processes resulting in ZnO formation; (iii) the non-monotonic changes of Cu x Zn 1 À x phase composition and decrease of its content. These changes are accompanied by the non-monotonic variation of the blue to green Cu-related PL band intensities ratio which correlates with the variation of Cu x Zn 1 À x phase composition. The model that explains the changes of ZnS:Cu PL characteristics by indiffusion of Zn and Cu from Cu x Zn 1 À x phase is proposed. The anisotropic character of ZnS phase transformation and oxidation process is found

Photoluminescence and structural properties of CdSe quantum dot-gelatin composite films

a b s t r a c t Optical and structural properties of composite films of CdSe quantum dots (QDs) embedded in gelatin matrix have been investigated by photoluminescence (PL), optical absorption and X-ray diffraction (XRD) methods. The optical absorption of the composite in the visible spectral range is found to be determined mainly by light absorption in the QDs. The decrease of the film transparency and the shift of the absorption edge to lower energies observed upon thermal annealing of the films at 140-160 1C are ascribed to the formation of chromophore groups in gelatin matrix. XRD patterns of the composite revealed helix to coil transition in gelatin matrix under thermal annealing of the composite at 100-160 1C. It is found that PL spectra of the composite are dominated by exciton and defect-related emission of the QDs and also contain weak emission of gelatin matrix. It is found that thermal annealing of the composite at 100-160 1C changes PL intensity and produces the shift of the PL bands to lower energies. As the annealed composite was kept in air for several months, the shift of exciton-related PL band position restored partially and the PL intensity increased. It is proposed that the increase of the PL intensity upon the thermal annealing of composite at 140 1C can be used for enhancement of the QDrelated PL. Changes that occurred in the PL spectra of composite are ascribed to structural and chemical transformations in gelatin matrix and at the QD/gelatin interface

The effect of TiO2 crystalline phase on microstructure and optical features of Zn2TiO4 doped with Mn

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Study of Mn ion charge state in Zn2TiO4 and its impact on the photoluminescence and optical absorption spectra

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Effect of Li+ co-doping on structural and luminescence properties of Mn4+ activated magnesium titanate films

International audienceThe effect of Li+ co-doping on crystal phase formation and photoluminescence (PL) of Mn4+ activated magnesium titanate films produced by a solid state reaction method at different temperatures (800–1200 °C) has been investigated by using X-ray diffraction (XRD), diffuse reflectance and PL spectroscopy. The chemical composition of sintered films was estimated by energy dispersive X-ray spectroscopy. The concentration of Mn impurity estimated by Electron spin resonance was about 5 × 1016 cm−3. The XRD study of the annealed films revealed several magnesium titanate crystal phases, such as Mg2TiO4, MgTiO3 and MgTi2O5. The contribution of each phase depended strongly on the annealing temperature and the presence of Li+ additive. Furthermore, Li+ co-doping facilitated the formation of both MgTiO3 and Mg2TiO4 phases, especially at lower annealing temperatures. The PL spectra showed two bands centered at 660 and 710 nm and ascribed to the $^{2}E$ → ${^4}$A$_{2}$ spin-forbidden transition of the Mn4+ ion in the Mg2TiO4 and MgTiO3, respectively. In Li co-doped films, the integrated intensity of Mn4+ luminescence was found several times stronger compared to Li-undoped films that was ascribed mainly to flux effect of lithium

The role of excess MgO in the intensity increase of red emission of Mn4+-activated Mg2TiO4 phosphors

International audienceThe influence of magnesium oxide (MgO) content on the intensity of red photoluminescence (PL) of Mn4+ ions in Mn-doped phosphors Mg2TiO4:Mn produced by solid-state reaction at 1200 °C has been investigated by PL, optical absorption, X-ray diffraction, and electron paramagnetic resonance methods. The phosphors synthesized with excess MgO show an increase of Mn4+ red emission compared with those of stoichiometric composition. The magnitude of this increase depends on both MgO and Mn content. The largest increase of PL intensity is found for the phosphors synthesized under 3:1 molar ratios of MgO to TiO2. For these phosphors, the PL intensity increases from time 1.1 to time 3 when Mn concentration decreases from 1.0 to 0.0001 mol%. The phosphors produced under 6:1 molar ratios demonstrate a decrease of PL intensity at any Mn concentration. It is shown that excess MgO promotes stabilization of Mg2TiO4 phase against decomposition, hinders formation of Mn2+ centers, and enhances Mn4+ ions incorporation in the Mg2TiO4 crystal lattice. The latter together with reduced concentration quenching are supposed to be the main reasons of PL enhancement, which leads to the conclusion that excess MgO is necessary to produce an efficient red phosphor

Influence of annealing on luminescence and energy transfer in ZnO multilayer structure co-doped with Tb and Eu

International audienceThe influence of rapid thermal annealing (RTA) on structural and optical properties of ZnO multilayer structures co-doped with Tb and Eu ions has been investigated by X-ray diffraction, Time-of-Fligth Secondary Ion Mass Spectrometry, Specular Infrared reflection, micro-Raman and photoluminescence (PL) methods. It is shown that incorporation of rare earth (RE) ions in ZnO host is accompanied by the formation of intrinsic defects in oxygen and zinc sub-lattices of ZnO. The appearance of intense Raman mode at 275 cm−1 is ascribed mainly to Eu ion incorporation on Zn site in ZnO matrix. The PL of RE ions localized in ZnO and other crystal phases is revealed. The effect of energy transfer from Tb3+ to Eu3+ ions in ZnO is identified. It is shown that the RTA improves crystal structure of ZnO host, i.e. stimulates the increase of coherent domain sizes and strain relaxation, as well as promotes the redistribution of RE ions across the structure. The effect of RTA on RE ion PL depends strongly on the annealing temperature. It is found that RTA at 500 °C promotes Eu incorporation into ZnO and the enhancement of Eu3+ PL due to energy transfer from Tb3+ to Eu3+, while RTA at 800 °C stimulates segregation of RE ions and the decrease of their PL

Optical and structural properties of Mn-doped magnesium titanates fabricated with excess MgO

International audienceOptical and structural properties of ceramics based on Mn-doped magnesium titanates synthesized by sintering in air at 1200 °C of MgO and TiO$_2$ powders of different molar ratio ranging from MgTiO$_3$ to Mg$_2$TiO$_4$ stoichiometric compositions were studied. The influence of excess MgO on Mn incorporation in crystal lattice of MgTiO$_3$ was also investigated. The Mn$^{4+}$ ions substituted Ti$^{4+}$ sites were controlled by the photoluminescence (PL) and diffuse reflectance spectroscopy, and the Mn$^{2+}$ ions on Mg$^{2+}$ sites were monitored by electron paramagnetic resonance (EPR). The ceramics produced using equimolar ratio of MgO and TiO$_2$ composed of a major MgTiO$_3$ and a minor MgTi$_2$O$_5$ crystal phases, and those made with excess MgO contained MgTiO$_3$ and Mg$_2$TiO$_4$ phases in different proportions. The EPR study showed that Mn incorporated in MgTiO$_3$ synthesized under 1:1 molar ratio as Mn$^{2+}$ ion mainly. This agreed with low intensity of Mn$^{4+}$ red PL ascribed to low concentration of Mn$^{4+}$ centers and partial absorption of the UV excitation light by the MgTi$_2$O$_5$ phase. The Mn-doped MgTiO$_3$ synthesized with excess MgO of 19 and 50 mol.% showed increased Mn$^{4+}$ red PL by a factor 30-50, enhanced Mn$^{4+}$ optical absorption and more than ten times decreased Mn$^{2+}$ EPR signal. The Mg$_2$TiO$_4$ phase was found to be under compressive strains attributed to the presence of Mg vacancies and demonstrated Mn$^{4+}$ red PL with modified spectrum shape and decay behavior. It is concluded that in MgTiO$_3$ the excess MgO facilitates the incorporation of Mn onto Ti$^{4+}$ site and can be used for the increasing of Mn$^{4+}$ PL intensity