K. P. Ramesh

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.

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 g\cong 2.02 exhibits a six line hyperfine structure. In addition to this, a prominent peak with g\cong 4.64, with a shoulder around g\cong 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 g\cong 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.

Low Resistive Micrometer-Thick SnS : Ag Films for Optoelectronic Applications

Ag-doped SnS films have been grown with a thickness of ∼0.5 μm by thermal evaporation technique on Coring 7059 glass substrates at a substrate temperature of 275°C. The effects of doping on the physical properties of the films have been investigated. The physical characteristics of the films are discussed and correlated to the microstructural and electro-optical properties. Electro-optical studies show that undoped SnS films have an electrical resistivity and optical bandgap of 35.6 Ω cm and 1.35 eV at room temperature. With the increase of Ag dopant concentration, the resistivity of the SnS layers initially decreased, reached a minimum value of 6.98 Ω cm at 15 atom % of Ag and again increased thereafter. However, optical bandgap (Eg) of the films decreased nonlinearly with increase of Ag percentage. An empirical formula Eg = 1.345-0.0014 X + 5.952 × 10 -5 X 2 , which describes the energy gap as a function of the film composition, has been derived. The doping effect on the surface structure of SnS films was also studied.

Thickness Effect on the Physical Properties of Evaporated SnS Films

SnS films with different thicknesses have been deposited on glass substrates at a constant substrate temperature of 300°C. The physical properties of the films were investigated using energy dispersive analysis of X-rays, X-ray diffraction, scanning electron microscopy, atomic force microscopy, van der Pauw method, and Fourier transform infrared spectroscopy measurements at room temperature. The deposited films exhibit only SnS phase with different orientations. We show that the electrical resistivity, activation energy, and optical bandgap of the films depend strongly on the preferred orientation of the SnS films. The electrical resistivity of films decreased with the increase of film thickness. The optical and electrical data of the SnS film are well interpreted with the composition, crystal, and surface structure data.

Synthesis and Characterization of Nanocrystalline SnS Films Grown by Thermal Evaporation Technique

The SnS films were grown on glass substrates using the thermal evaporation technique at different substrate temperatures

Optimization of the distance between source and substrate for device-grade SnS films grown by the thermal evaporation technique

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

varied from 20 to

Probing disorder and transport properties in polypyrrole thin-film devices by impedance and Raman spectroscopy

300 ^0C

Crystallization studies on glasses

, and their physical properties were studied with appropriate techniques. While increasing

Effect of fuel on the formation structure, transport and magnetic properties of LaMnO3+nanopowders

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