J.L. Rao

Solution combustion derived nanocrystalline Zn2SiO4:Mn phosphors: A spectroscopic view

Manganese doped nanocrystalline willemite powder phosphors Zn(2-x)Mn(x)SiO(4) (0.1<or=x<or=0.5) have been synthesized by a low-temperature initiated, self-propagating, gas producing solution combustion process. The phosphors have been characterized by using x-ray diffraction (XRD), energy dispersive spectroscopy, scanning electron microscopy, Fourier transform infrared spectroscopy (FTIR), electron paramagnetic resonance (EPR), and photo luminescence (PL) spectroscopic techniques. The lattice parameters calculated from XRD confirm that Zn(2-x)Mn(x)SiO(4) has a rhombohedral space group R3H. The XRD patterns confirm that Zn(2-x)Mn(x)SiO(4) phosphor samples undergo a phase transformation from beta-willemite to alpha-willemite phase at 950 degrees C. The EPR spectra of Mn(2+) ions exhibit resonance signals at g approximately = 3.24 and g approximately = 2.02, with a sextet hyperfine structure centered around g approximately = 2.02. The EPR signals of Mn(2+) give a clear indication of the presence of two different Mn(2+) sites. The magnitude of the hyperfine splitting (A) indicates that the Mn(2+) is in an ionic environment. The number of spins participating in resonance (N), the paramagnetic susceptibility (chi), and the zero-field splitting parameter (D) have been evaluated as function of x. It is interesting to observe that the variation of N with temperature obeys Boltzmann. The paramagnetic susceptibility is calculated from the EPR data at various temperatures and the Curie constant and Curie paramagnetic temperature was evaluated from the 1/chi versus T graph. The luminescence of Mn(2+) ion in Zn(2)SiO(4) shows a strong green emission peak around 520 nm from the synthesized phosphor particles under UV excitation (251 nm). The luminescence is assigned to a transition from the upper (4)T(1)-->(6)A(1) ground state. The mechanism involved in the generation of a green emission has been explained in detail. The effect of Mn content on luminescence has also been studied.

Combustion synthesis, characterization and Raman studies of ZnO nanopowders

Spherical shaped ZnO nanopowders (14-50 nm) were synthesized by a low temperature solution combustion method in a short time <5 min. Rietveld analysis show that ZnO has hexagonal wurtzite structure with lattice constants a=3.2511(1) Å, c=5.2076(2) Å, unit cell volume (V)=47.66(5) (Å)(3) and belongs to space group P63mc. SEM micrographs reveal that the particles are spherical in shape and the powders contained several voids and pores. TEM results also confirm spherical shape, with average particle size of 14-50 nm. The values are consistent with the grain sizes measured from Scherrers method and Williamson-Hall (W-H) plots. A broad UV-vis absorption spectrum was observed at ∼375 nm which is a characteristic band for the wurtzite hexagonal pure ZnO. The optical energy band gap of 3.24 eV was observed for nanopowder which is slightly lower than that of the bulk ZnO (3.37 eV). The observed Raman peaks at 438 and 588 cm(-1) were attributed to the E(2) (high) and E(1) (LO) modes respectively. The broad band at 564 cm(-1) is due to disorder-activated Raman scattering for the A(1) mode. These bands are associated with the first-order Raman active modes of the ZnO phase. The weak bands observed in the range 750-1000 cm(-1) are due to small defects.

Mixed alkali effect in borate glasses-EPR and optical absorption studies in xNa2O.(30-x)K2O-70B2O3 glasses doped with Mn2+

The mixed alkali borate xNa2O–(30−x)K2O–70B2O3 (5≤x≤25) glasses doped with 1 mol% of manganese ions were investigated using EPR and optical absorption techniques as a function of alkali content to look for ‘mixed alkali effect’ (MAE) on the spectral properties of the glasses. The EPR spectra of all the investigated samples exhibit resonance signals which are characteristic of the Mn2+ ions. The resonance signal at gcong 2.02 exhibits a six line hyperfine structure. In addition to this, a prominent peak with gcong 4.64, with a shoulder around gcong 4.05 and 2.98, was also observed. From the observed EPR spectrum, the spin-Hamiltonian parameters g and A have been evaluated. It is interesting to note that some of the EPR parameters do show MAE. It is found that the ionic character increases with x and reaches a maximum around x=20 and thereafter it decreases showing the MAE. The number of spins participating in resonance (N) at gcong 2.02 decreases with x and reaches a minimum around x=20 and thereafter it increases showing the MAE. It is also observed that the zero-field splitting parameter (D) increases with x, reaches a maximum around x=15 and thereafter decreases showing the MAE. The optical absorption spectrum exhibits a broad band around ~20,000 cm−1 which has been assigned to the transition 6A1g(S)→4T1g(G). From ultraviolet absorption edges, the optical bandgap energies and Urbach energies were evaluated. It is interesting to note that the Urbach energies for these glasses decrease with x and reach a minimum around x=15. The optical band gaps obtained in the present work lie in the range 3.28–3.40 eV for both the direct and indirect transitions. The physical parameters of all the glasses were also evaluated with respect to the composition.

Mixed alkali effect in borate glasses - electron paramagnetic resonance and optical absorption studies in Cu2+ doped xNa2O– (30 − x)K2O– 70B2O3 glasses

The mixed alkali borate glasses xNa2O–(30 − x)K2O–70B2O3 (5 ≤ x ≤ 25), doped with 0.5 mol% of CuO, have been investigated, using electron paramagnetic resonance (EPR) and optical absorption techniques, as a function of mixed alkali content, to look for the mixed alkali effect (MAE) on the spectral properties of the glasses. The EPR spectra of all the investigated samples exhibit resonance signals which are characteristic of the Cu2+ ions in octahedral sites with tetragonal distortion. From the observed EPR spectra, the spin-Hamiltonian parameters have been determined. It is observed that the spin-Hamiltonian parameter g∥ goes through a minimum around x = 10– 15 whereas A∥ goes through a maximum around x = 15 showing the MAE. The number of spins participating in resonance (N2) and the calculated paramagnetic susceptibilities (χ) exhibit a shallow minimum around x = 20 showing the MAE in these glasses. The optical absorption spectrum of the x = 5 glass exhibits two bands: a strong band centred at 14 240 cm−1 corresponding to the transition (2B1g →2B2g) and a weak band on the higher energy side at 22 115 cm−1 corresponding to the transition (2B1g →2Eg). With x > 5, the higher energy band disappears and the lower energy band shifts slightly to the lower energy side. By correlating the EPR and optical absorption data, the molecular orbital coefficients α2 and β12 are evaluated for the different glasses investigated. The values indicate that the in-plane σ bonding is moderately covalent while the in-plane π bonding is significantly ionic in nature; these exhibit a minimum with x = 15, showing the MAE. The theoretical values of optical basicity of the glasses have also been evaluated. From optical absorption edges, the optical bandgap energies have been calculated and are found to lie in the range 3.00–3.40 eV. The physical properties of the glasses studied have also been evaluated with respect to the composition.

EPR, thermo and photoluminescence properties of ZnO nanopowders.

Nanocrystalline ZnO powders have been synthesized by a low temperature solution combustion method. The photoluminescence (PL) spectrum of as-formed and heat treated ZnO shows strong violet (402, 421, 437, 485 nm) and weak green (520 nm) emission peaks respectively. The PL intensities of defect related emission bands decrease with calcinations temperature indicating the decrease of Zn(i) and V(o)(+) caused by the chemisorptions of oxygen. The results are correlated with the electron paramagnetic resonance (EPR) studies. Thermoluminescence (TL) glow curves of gamma irradiated ZnO nanoparticles exhibit a single broad glow peak at ∼343°C. This can be attributed to the recombination of charge carriers released from the surface states associated with oxygen defects, mainly interstitial oxygen ion centers. The trapping parameters of ZnO irradiated with various γ-doses are calculated using peak shape method. It is observed that the glow peak intensity increases with increase of gamma dose without changing glow curve shape. These two characteristic properties such as TL intensity increases with gamma dose and simple glow curve structure is an indication that the synthesized ZnO nanoparticles might be used as good TL dosimeter for high temperature application.

EPR and photoluminescence studies of ZnO:Mn nanophosphors prepared by solution combustion route

Nanocrystalline ZnO:Mn (0.1 mol%) phosphors have been successfully prepared by self propagating, gas producing solution combustion method. The powder X-ray diffraction of as-formed ZnO:Mn sample shows, hexagonal wurtzite phase with particle size of ∼40 nm. For Mn doped ZnO, the lattice parameters and volume of unit cell (a=3.23065 Å, c=5.27563 Å and V=47.684 (Å)(3)) are found to be greater than that of undoped ZnO (a=3.19993 Å, c=5.22546 Å and V=46.336 (Å)(3)). The SEM micrographs reveal that besides the spherical crystals, the powders also contained several voids and pores. The TEM photograph also shows the particles are approximately spherical in nature. The FTIR spectrum shows two peaks at ∼3428 and 1598 cm(-1) which are attributed to O-H stretching and H-O-H bending vibration. The PL spectra of ZnO:Mn indicate a strong green emission peak at 526 nm and a weak red emission at 636 nm corresponding to (4)T(1)→(6)A(1) transition of Mn(2+) ions. The EPR spectrum exhibits fine structure transition which will be split into six hyperfine components due to (55)Mn hyperfine coupling giving rise to all 30 allowed transitions. From EPR spectra the spin-Hamiltonian parameters have been evaluated and discussed. The magnitude of the hyperfine splitting (A) constant indicates that there exists a moderately covalent bonding between the Mn(2+) ions and the surrounding ligands. The number of spins participating in resonance (N), its paramagnetic susceptibility (χ) have been evaluated.

The effect of host glass on optical absorption and fluorescence of Nd3+ in xNa2O?(30? x)K2O?70B2O3 glasses

The effect of host glass composition on the optical absorption and fluorescence spectra of Nd3+ has been studied in mixed alkali borate glasses of the type xNa2O?(30? x)K2O?69.5B2O3?0.5Nd2O3 (x = 5, 10, 15, 20 and 25). Various spectroscopic parameters such as Racah (E1, E2 and E3), spin?orbit (?4f) and configuration interaction (?,?) parameters have been calculated. The Judd?Ofelt intensity parameters (??) have been calculated and the radiative transition probabilities (Arad),radiative lifetimes (?r), branching ratios (?) and integrated absorption cross sections (?) have been obtained for certain excited states of the Nd3+ ion and are discussed with respect to x. From the fluorescence spectra, the effective fluorescence line widths (??eff) and stimulated emission cross sections (?p) have been obtained for the three transitions , and of Nd3+. The stimulated emission cross section (?p) values are found to be in the range (2.0?4.8) ? 10?20?cm2 and they are large enough to indicate that the mixed alkali borate glasses could be potential laser host materials.

Mixed alkali effect in borate glasses: optical absorption studies in Ho3+ doped x(Na2O) . (30 - x)(K2O) . 70(B2O3) glasses

Mixed alkali effect (MAE) in xNa2O · (30 − x)K2O · 70B2O3 (x = 5, 10, 15, 20 and 25) glasses doped with 0.5Ho2O3 has been investigated by measuring the optical properties of Ho3+. From the optical absorption spectra, optical band gaps (Eopt) for both direct and indirect transitions have been calculated using Davis and Mott theory and are found to exhibit a minimum when the two alkalies are in equal concentration (due to mixed alkali effect). Spectroscopic parameters like Racah (E1, E2, E3), spin-orbit (ξ4f), configuration interaction (α, β) and Judd-Ofelt intensity parameters (Ω2, Ω4 and Ω6) have been calculated as a function of x. Also radiative and non-radiative transition probabilities (AT and WMPR), radiative lifetimes (τR), branching ratios (β) and integrated absorption cross sections (Σ) have been obtained. The spectral profile of the hypersensitive transition has been correlated to the site symmetry of the rare earth ion. The trends observed in the intensity parameters, radiative lifetimes and stimulated emission cross sections as a function of x in these borate glasses have been discussed, keeping in view the mixed alkali effect.

The effect of mixed alkalis on the absorption and fluorescence properties of Ho3+ ions in borate glasses

Spectroscopic properties of Ho3+ in 67B2O3?xLi2O ?(32? x)Na2O, 67B2O3?xLi2O ?(32? x)K2O and 67B2O3?xLi2O ?(32? x)Cs2O (where x = 8, 12, 16, 20 and 24) glasses were investigated on the basis of Judd?Ofelt theory. Various spectroscopic parameters (E1,E2,E3, ?4f, ? and ?) and Judd?Ofelt intensity parameters (?2, ?4 and ?6) are calculated as a function of x in the three glass matrices. The obtained Judd?Ofelt intensity parameter ?2 increases with the addition of a second alkali and also shows minima or maxima at x = 16?20?mol % in the above glasses. Using Judd?Ofelt intensity parameters (?2, ?4 and ?6), radiative transition probabilities (AT), radiative lifetimes (?R), branching ratios (?) and integrated absorption cross sections (?) have been calculated for certain excited states of Ho3+. The non-radiative transition rates of different excited states of Ho3+ are also estimated from energy gap law. From the luminescence spectra, stimulated emission cross-sections (?p) have been obtained for the two transitions and of Ho3+. The trend of all these parameters observed as a function of x in these glass systems have been discussed, keeping in mind the effect of mixed alkalis.

MIXED ALKALI EFFECT IN BORATE GLASSES — EPR AND OPTICAL BAND GAP STUDIES IN xNa2O-(30-x)K2O-70 B2O3 GLASSES DOPED WITH Gd3+ IONS

Mixed alkali borate xNa2O-(30 - x)K2O-70 B2O3 (5 leq x leq 25) glasses doped with 0.5 mol% of gadolinium ions have been investigated by using electron paramagnetic resonance (EPR) and optical absorption techniques, as a function of alkali content, to look for the mixed alkali effect on the spectral properties of the glasses. The EPR spectrum consists of three prominent features with effective g-values, g approx 5:6, 2.8 and 2.0, and two weak features at g approx 3:3 and 4.3. The three EPR signals at g approx 2:0, g approx 2:8 and g approx 5:6 are attributed to Gd3+ ions located at sites with weak, intermediate and strong cubic symmetry fields, respectively. In principle these sites may be of network forming and network modifying types. Ionic radius considerations suggest that gadolinium ions cannot substitute the much smaller boron ions and thus only the network modifier site is acceptable. The number of spins (N) participating in resonance and its paramagnetic susceptibility (chi) for g approx 5:6 resonance line have been calculated. It is interesting to note that N and chi increase with x and reach a maximum around x = 15 and thereafter decrease showing the mixed alkali effect in these glasses. From ultraviolet absorption edges, the optical band gap energies were evaluated. It is interesting to note that the optical band gap energies for these glasses decrease slightly with increasing x and reach a minimum around x = 10, and thereafter increase showing the mixed alkali effect. Optical band gap energies (Eopt) obtained in the present work vary from 2.20-3.35 Ev for both the direct and indirect transitions. The physical parameters of the glasses have been evaluated with respect to the composition.