Arturas Katelnikovas

Luminescence and luminescence quenching in Gd3(Ga,Al)5O12 scintillators doped with Ce3+.

The optical properties of gadolinium gallium aluminum garnet, Gd3(Ga,Al)5O12, doped with Ce(3+) are investigated as a function of the Ga/Al ratio, aimed at an improved understanding of the energy flow and luminescence quenching in these materials. A decrease of both the crystal field strength and band gap with increasing content of Ga(3+) is observed and explained by the geometrical influence of Ga(3+) on the crystal field splitting of the 5d level in line with theoretical work of Muñoz-García et al. ( uñoz-García, A. B.; Seijo, L. Phys. Rev. B 2010, 82, 184118 ). Thermal quenching results in shorter decay times as well as reduced emission intensities for all samples in the temperature range from 100 to 500 K. An activation energy for emission quenching is calculated from the data. The band gap of the host is measured upon Ga substitution and the decrease in band gap is related to Ga(3+) substitution into tetrahedral sites after all octahedral sites are occupied in the garnet material. Based on the change in band gap and crystal field splitting, band diagrams can be constructed explaining the low thermal quenching temperatures in the samples with high Ga content. The highest luminescence intensity is found for Gd3(Ga,Al)5O12 with 40% of Al(3+) replaced by Ga(3+).

Synthesis and optical properties of Li3Ba2La3(MoO4)8:Eu3+ powders and ceramics for pcLEDs

A series of polycrystalline Li3Ba2La3−xEux(MoO4)8 samples were prepared by the conventional solid-state reaction. The phase formation of the samples was investigated by X-ray diffraction measurements. The luminescence spectra and decay curves were studied as a function of Eu3+ concentration and temperature. It turned out that the optical band gap of the undoped molybdates is at 3.65 eV. The quantum efficiency (QE) of the Eu3+ doped luminescent materials increases with increasing Eu3+ concentration and almost 100% QE was obtained for those samples doped with 70, 80, or 90% Eu3+. A sample containing 100% Eu3+ showed solely a slight decrease in quantum efficiency. The luminous efficacy (LE) was 330 and 312 lm Wopt−1 for the 10 and 100% Eu3+ doped samples, respectively. The decrease of LE values is caused by a slight shift of the colour point to the red spectral range with increasing Eu3+ content. Temperature dependent measurements revealed that Li3Ba2Eu3(MoO4)8 loses only 15% of efficiency up to 400 K, which demonstrates that the investigated phosphors are attractive for application in pcLEDs.

Luminescence and luminescence quenching of highly efficient Y2Mo4O15:Eu3+ phosphors and ceramics

A good LED phosphor must possess strong enough absorption, high quantum yields, colour purity, and quenching temperatures. Our synthesized Y2Mo4O15:Eu3+ phosphors possess all of these properties. Excitation of these materials with near-UV or blue radiation yields bright red emission and the colour coordinates are relatively stable upon temperature increase. Furthermore, samples doped with 50% Eu3+ showed quantum yields up to 85%, what is suitable for commercial application. Temperature dependent emission spectra revealed that heavily Eu3+ doped phosphors possess stable emission up to 400 K and lose half of the efficiency only at 515 K. In addition, ceramic disks of Y2Mo4O15:75%Eu3+ phosphor with thickness of 0.71 and 0.98 mm were prepared and it turned out that they efficiently convert radiation of 375 and 400 nm LEDs to the red light, whereas combination with 455 nm LED yields purple colour.

Host-sensitized luminescence properties of KLa5O5(VO4)2:Eu3+ for solid-state lighting applications

A novel KLa5O5(VO4)2 blue-emitting phosphor was synthesized and studied in this paper. The introduction of Eu3+ at various concentrations into the host crystal structure is possible through solid-state synthesis and was followed by X-ray diffraction. In this context, KLa5O5(VO4)2 can be considered as an excellent host matrix to develop new phosphors. In the present work, the luminescence properties of KLa5−xEuxO5(VO4)2 (x = 0, 0.05, 0.1, 0.5, 1, 1.5 and 2) were investigated by fluorescence spectroscopy. The photoluminescence (PL) and PL excitation (PLE) spectra were recorded and discussed. KLa5−xEuxO5(VO4)2 phosphors exhibit a bright reddish-orange light emission under UV excitation. The concentration quenching was determined to be at 20 mol% (x = 1) of Eu3+. The KLa4EuO5(VO4)2 phosphor cleverly substituted is an ideal candidate for solid-state lighting and especially, as a red phosphor for near-UV LEDs or to serve as the primary light source for white LEDs.

Synthesis and Optical Properties of Li3Ba2La3(MoO4)8:Sm 3+ Powders for pcLEDs

A series of Sm3+-activated molybdates Li3Ba2(La1−xSmx)3(MoO4)8 with 0<̲x<̲1 (0% to 100% Sm3+) have been prepared by the conventional solid-state synthesis method, and their optical properties were investigated. Reflection, excitation and emission spectra were recorded and put in relation to the various [Xe]4 f 5 → [Xe]4 f 5 transitions of Sm3+. The positions of the charge transfer bands of Sm3+ and Mo6+ were resolved by Gaussian peak fitting. Emission spectra recorded at 100 K revealed the Stark sublevels of the Sm3+ energy levels. Time-dependent emission measurements of the 4G5/2 →6H9/2 transition were performed to disentangle the influence of temperature and activator concentration on the decay constants. The results are discussed in the context of the structure of the host material. Sm3+ occupies two different crystallographic sites at higher activator concentrations, which results in a bi-exponential decay curve. Temperature-dependent emission spectra were recorded to determine the thermal quenching behavior of the material. Internal and external quantum efficiencies (IQE and EQE) have been calculated. The IQE is independent of temperature, while the emission intensity strongly decreases at temperatures higher than 400 K. It is concluded that the photon escape efficiency in Li3Ba2La3(MoO4)8 correlates with temperature. An EQE of 44% was achieved for the 2% Sm3+ sample, which is comparatively high for Sm3+. Color points and luminous efficacies were calculated. The color point is independent of the Sm3+ concentration, but a blue-shift was observed with increasing temperature. This shift may be caused by lattice expansion and a subsequent decrease of spin-orbit coupling. Graphical Abstract Synthesis and Optical Properties of Li3Ba2La3(MoO4)8:Sm3+ Powders for pcLEDs

Photochemical synthesis of CeO2 nanoscale particles using sodium azide as a photoactive material: effects of the annealing temperature and polyvinylpyrrolidone addition

A novel and simple method for CeO2 nanoscale particle synthesis in aqueous solutions via a photochemical route is reported in this paper. To this end, CeCl3·7H2O or Ce(NO3)3·6H2O was used as a Ce precursor, while NaN3 was chosen as the photoactive compound. Synthesis was carried out without any surfactants or by using polyvinylpyrrolidone (PVP). The synthesized samples were subsequently thermally treated at different temperatures between 100 and 900 °C. XRD patterns and Raman spectra indicated that CeO2 samples possess the fluorite structure. TEM analysis revealed that synthesis without surfactants leads to formation of highly agglomerated particles, while adding PVP to the primary solution resulted in decreased agglomeration and reduced particle size. The particle size was calculated from XRD and Raman line broadening and confirmed by TEM analysis. The average crystallite size for the unheated samples prepared without surfactant was hardly radiation exposure dependent and varied from 6.5 to 8.9 nm. Even smaller particles (3.3–7.0 nm) were formed by using PVP. It turned out that an increase of the calcination temperature causes significant crystallite growth. A strong interaction between CeO2 nanoparticles and PVP was revealed by TG analysis. The UV/VIS absorption spectra showed a strong absorption below 400 nm (3.10 eV) with a well-defined absorption peak at around 295–320 nm. The estimated band gap (Eg) of the obtained nanoscale particles was in the range of 2.90–3.57 eV, i.e. the values are higher than that of a bulk CeO2 powder (Eg = 3.19 eV), except for the sample calcined at 900 °C.

Eu3+-Doped Y3−xSmxAl5O12 garnet: synthesis and structural investigation

Sm3+-Doped and Eu3+/Sm3+-co-doped yttrium aluminium garnets (YAG) have been synthesized by an environmentally friendly sol–gel method. The results of X-ray diffraction (XRD) analysis of the powders sintered at 1000 °C showed the high purity of the end products with formation of monophasic compounds. The phase composition of the samples was also investigated by FTIR spectroscopy. The microstructural features of the polycrystalline samples were studied by scanning electron microscopy (SEM). The local environments of Eu3+ and Sm3+ activator ions in the garnet crystal structure compounds were investigated by nuclear magnetic resonance (NMR) and Raman spectroscopy. The luminescence properties were discussed using the results from photoluminescence (PL) study. The effective concentration of Sm in YAG was estimated and optimum ratio between two activators (Eu3+ and Sm3+) was analysed. The correlation between structural features and luminescence was determined. With a constant amount of samarium in Y2.9Sm0.1Al5O12:Eu garnet the strongest luminescence was determined in the sample with 1% of europium when characteristic excitation for europium was used.