P.R. Biju

Energy transfer in Sm3+:Eu3+ system in zinc sodium phosphate glasses

The mechanism of nonradiative energy transfer process in zinc sodium phosphate glass system co-doped with samarium and europium ion has been examined under cw laser excitation. Donor–acceptor distance and quantum efficiency of transfer have been evaluated using the relevant theoretical expressions. Transfer probabilities have been determined using the overlap integral and relative fluorescence methods. The Forster–Dexter theoretical predictions are found to be in excellent agreement with experimental results. The nonresonant energy transfer assisted by phonons is the dominant transfer mechanism in the concentration range taken. Excitation spectra and the decay profile of the samarium ion also support the energy transfer from samarium to europium.

Structural studies and luminescence properties of CeO2:Eu3+ nanophosphors synthesized by oxalate precursor method

A novel synthesis strategy to prepare CeO2:Eu3+ nanophosphors and its luminescence behavior is reported. Different structural characterization techniques such as X-ray diffraction, transmission electron microscope and thermogravimetric analysis reveal that thermal decomposition of oxalate precursor is an effective pathway to produce rare earth oxide nanocrystals. Optical characterizations of the CeO2:Eu3+ were done by UV–Visible absorption, photoluminescence excitation and emission spectra. The presence of structural defects and their role on the band gap and luminescence were discussed on the basis of absorption and emission studies. Luminescence study of the CeO2:Eu3+ ensures that the strong charge transfer band of CeO2 makes it a suitable host material for efficiently exciting Eu3+ ions by subsequent energy transfer. The dependence of luminescence efficiency of the CeO2:Eu3+ with varying concentrations of Eu3+ was also studied and discussed. The results show that Eu3+-doped CeO2 nanophosphor is a potential candidate in ultraviolet-based LEDs.

Structural and spectral investigation of terbium molybdate nanophosphor

Terbium molybdate nanophosphors were synthesized through a facile sol-gel route. The structure of the phosphors was characterized by X-ray diffraction, Raman spectra and Fourier transform infrared spectroscopy analysis. The X-ray diffraction studies revealed that the structure of the nanophosphor gradually changes from monoclinic to orthorhombic phase as heated from 700 to 900 °C. High resolution transmission electron microscopy, SAED and EDS were also employed to characterize the size, crystallinity and composition of the samples. Detailed spectroscopic investigations were carried out by Judd-Ofelt analysis based on UV-Visible-NIR absorption and emission spectra. The luminescence spectra suggest that phosphors with orthorhombic structure have better luminescence properties than the monoclinic structure. The phosphors showed intense green emission under near-UV excitation due to the energy transfer from the host lattice to Tb(3+) ions. The CIE coordinates suggest enhanced color purity for green emission and short fluorescence decay values proposes the suitability for LED applications. These phosphors can be applied as promising candidates for blue and near-UV excited WLEDs.

Synthesis, structural and spectroscopic investigations of nanostructured samarium oxalate crystals.

Nanostructured samarium oxalate crystals were prepared via microwave assisted co-precipitation method. The crystal structure and morphology of the sample were analyzed using X-ray powder diffraction, Scanning electron microscopy and Transmission electron microscopy. The presence of functional groups is ascertained by Fourier transform infrared spectroscopy. Samarium oxalate nanocrystals of average size 20 nm were aggregated together to form nano-plate structure in sub-microrange. Detailed spectroscopic investigation of the prepared phosphor material was carried out by Judd-Ofelt analysis based on the UV-Visible-NIR absorption spectra and photoluminescence emission spectra. The analysis reveals that the transition from energy level (4)G5/2 to (6)H7/2 of Sm(3+) ion has maximum branching ratio and the corresponding orange emission can be used for display applications.

NIR emission studies and dielectric properties of Er(3+)-doped multicomponent tellurite glasses.

Multicomponent tellurite glasses containing altered concentrations of Er2O3 (ranging from 0 to 1 mol%) were prepared by the standard melt quenching technique. Investigations through energy dispersive X-ray spectroscopy (EDS), Raman scattering spectroscopy, Fourier transform infrared (FTIR) spectroscopy, near-infrared (NIR) emission studies and dielectric measurement techniques were done to probe their compositional, structural, spectroscopic and dielectric characteristics. The broad emission together with the high values of the effective linewidth (~63 nm), stimulated emission cross-section (9.67 × 10(-21) cm(2)) and lifetime (2.56 ms) of (4)I13/2 level for 0.5 mol% of Er(3+) makes these glasses attractive for broadband amplifiers. From the measured capacitance and dissipation factor, the relative permittivity, dielectric loss and the conductivity were computed; which furnish the dielectric nature of the multicomponent tellurite glasses that depend on the applied frequency. Assuming the ideal Debye behavior as substantiated by Cole-Cole plot, an examination of the real and imaginary parts of impedance was performed. The power-law and Cole-Cole parameters were resolved for all the glass samples. From the assessment of the emission analysis and dielectric properties of the glass samples, it was obvious that the Er(3+) ion concentration had played a vital role in tuning the optical and dielectric properties and the 0.5 mol% of Er(3+) -doped glass was confirmed as the optimum composition.

Optical analysis of samarium doped sodium bismuth silicate glass.

Samarium doped sodium bismuth silicate glass was synthesized using the melt quenching method. Detailed optical spectroscopic studies of the glassy material were carried out in the UV-Vis-NIR spectral range. Using the optical absorption spectra Judd-Ofelt (JO) parameters are derived. The calculated values of the JO parameters are utilized in evaluating the various radiative parameters such as electric dipole line strengths (Sed), radiative transition probabilities (Arad), radiative lifetimes (τrad), fluorescence branching ratios (β) and the integrated absorption cross- sections (σa) for stimulated emission from various excited states of Sm3+‡ ion. The principal fluorescence transitions are identified by recording the fluorescence spectrum. Our analysis revealed that the novel glassy system has the optimum values for the key parameters viz. spectroscopic quality factor, optical gain, stimulated emission cross section and quantum efficiency, which are required for a high performance optical amplifier. Calculated chromaticity co-ordinates (0.61, 0.38) also confirm its application potential in display devices.

Synthesis and luminescence characterization of Pr3+ doped Sr1.5Ca0.5SiO4 phosphor

Luminescence properties of Pr(3+) activated Sr1.5Ca0.5SiO4 phosphors synthesized by solid state reaction method are reported in this work. Blue, orange red and red emissions were observed in the Pr(3+) doped sample under 444nm excitation and these emissions are assigned as (3)P0→(3)H4, (3)P0→(3)H6 and (3)P0→(3)F4 transitions. The emission intensity shows a maximum corresponding to the 0.5wt% Pr(3+) ion. The decay analysis was done for 0.05 and 0.5wt% Pr(3+) doped samples for the transition (3)P0→(3)H6. The life times of 0.05 and 0.5wt% Pr(3+) doped samples were calculated by fitting to exponential and non-exponential curve respectively, and are found to be 156 and 105μs respectively. The non-exponential behaviour arises due to the statistical distribution of the distances between the ground state Pr(3+) ions and excited state Pr(3+) ions, which cause the inhomogeneous energy transfer rate. The XRD spectrum confirmed the triclinic phase of the prepared phosphors. The compositions of the samples were determined by the energy dispersive X-ray spectra. From the SEM images it is observed that the particles are agglomerated and are irregularly shaped. IR absorption bands were assigned to different vibrational modes. The well resolved peaks shown in the absorption spectra are identical to the excitation spectra of the phosphor samples. Pr(3+) activated Sr1.5Ca0.5SiO4 phosphors can be efficiently excited with 444nm irradiation and emit multicolour visible emissions. From the CIE diagram it can be seen that the prepared phosphor samples give yellowish-green emission.

Synthesis and luminescence characterization of Sr0.5Ca0.5TiO3:Sm3+ phosphor

The spectroscopic properties of trivalent samarium doped Sr0.5Ca0.5TiO3 perovskite phosphor material (Sr0.5Ca0.5TiO3:xSm(3+), x=0.05, 0.1, 0.5, 1, 1.5) synthesized by the solid state method have been studied. The X-Ray Diffraction profile confirms the orthorhombic perovskite Sr0.5Ca0.5TiO3 structure of the prepared samples. The SEM study reveals the surface morphology. The Judd-Ofelt intensity parameters were calculated for 0.5 wt% Sm(3+) doped Sr0.5Ca0.5TiO3. Transition probabilities, branching ratios and radiative lifetime were evaluated by using Judd-Ofelt analysis. The emission spectra under 405 nm excitation shows five emission peaks at 564 nm, 599 nm, 645 nm, 707 nm and 776 nm corresponding to the transitions (4)G5/2→(6)Hj (j=5/2, 7/2, 9/2, 11/2 and 13/2) respectively. The higher values of branching ratio and stimulated emission cross-section for (4)G5/2→(6)H7/2 transition of Sr0.5Ca0.5TiO3:0.5 wt% Sm(3+) shows its suitability in the field of visible lasers and optical fiber amplifiers. The experimental lifetimes of Sm(3+) doped samples were estimated using the decay curves corresponding to (4)G5/2→(6)H7/2 transition upon 405 nm excitation. Concentration dependence on emission intensity and experimental lifetime were also studied. From the CIE diagram we can see that as the concentration of Sm(3+) ions increases from 0.05 wt% to 1.5 wt% the CIE color co-ordinates changes from greenish yellow to yellowish orange.

Spectroscopic and photoluminescence characterization of Dy3+ in Sr0.5Ca0.5TiO3 phosphor

The spectroscopic and photoluminescence characteristics of trivalent dysprosium (Dy(3+))-doped Sr0.5Ca0.5TiO3 phosphor materials synthesized via solid-state reaction method were studied. The X-ray diffraction profile confirmed the orthorhombic perovskite structure of the prepared samples. Judd-Ofelt analysis was carried out to obtain the intensity parameters and predicted radiative properties of Sr0.5Ca0.5TiO3:2wt%Dy(3+). The photoluminescence spectrum of Dy(3+)-doped Sr0.5Ca0.5TiO3 showed three emission peaks at 481, 574 and 638 nm corresponding to (4)F9/2 →(6)H15/2, (4)F9/2 →(6)H13/2 and (4)F9/2 →(6)H11/2 transitions respectively. The variation of luminescence intensity with different excitation wavelengths and Dy(3+) concentrations is discussed. The decay profiles of (4)F9/2 excited levels of Dy(3+) ions show bi-exponential behaviour and also a decrease in average lifetime with increase in Dy(3+) concentration. Yellow to blue luminescence intensity ratio, CIE chromaticity co-ordinates and correlated color temperature were also calculated for different concentrations of Dy(3+)-doped Sr0.5Ca0.5TiO3 phosphor at different λex.

Influences of Annealing Temperature and Doping Concentration on Microstructural and Optical Properties of CeO2:Sm3+ Nanocrystals

Nanocrystals of CeO2 with different doping concentrations of Sm3+ were synthesized by a novel and cost-effective method. The crystal structure, morphology and particle size were systematically investigated by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Effects of the annealing temperature and doping concentrations on the microstructural properties of the crystals were studied. X-ray diffraction analysis indicates that the cubic structure of the CeO2 is not affected by the doping of Sm3+ up to a doping concentration of 20%. Different structural parameters such as lattice constant, surface area, bulk density and porosity of the crystal were determined and discussed. Microscopic images of the CeO2:Sm3+ suggest that the thermal decomposition of oxalate precursor is a suitable synthesis pathway to produce uniform-sized microparticles and nanoparticles. The influences of annealing temperature and doping concentration of Sm3+ on the optical properties of the nanocrystals were also discussed. The photoluminescence excitation spectra reveal that the charge transfer band is redshifted with increasing annealing temperatures. Emission attains its maximum intensity for Sm3+ concentration of 1%, and higher concentrations lead to emission quenching.