R.S. Srinivasa

Orange-red luminescence from Cu doped CdS nanophosphor prepared using mixed Langmuir–Blodgett multilayers

Cu doped CdS nanophosphors were fabricated through Langmuir-Blodgett route for the first time. Precursors mixed Langmuir-Blodgett multilayers of cadmium arachidate-copper arachidate were used to grow doped sulfide nanoparticles within the organic matrix through postdeposition treatment with H(2)S gas. Changes in composition and layered structure of precursor multilayers were studied using Fourier transform infrared and x-ray reflection. Uptake of Cu in the multilayers was analyzed by inductively coupled plasma atomic emission spectroscopy measurements. Unannealed H(2)S exposed multilayers containing CdS nanoparticles show strong surface state emission centered at approximately 570 nm, whereas Cu doped CdS nanoparticles show orange-red luminescence. Photoluminescence (PL) spectra of annealed-Cu doped CdS nanoparticles show distinct Cu-related emission compared to annealed-undoped CdS nanoparticles. Power dependent PL measurements of annealed samples show that an efficient carrier recombination takes place at T(2) level of Cu(++). The carrier relaxation from the excitonic states to T(2) level results in the strong orange-red luminescence.

Study of simultaneous reduction and nitrogen doping of graphene oxide Langmuir?Blodgett monolayer sheets by ammonia plasma treatment

Graphene oxide (GO) monolayer sheets, transferred onto Si by the Langmuir-Blodgett technique, were subjected to ammonia plasma treatment at room temperature with the objective of simultaneous reduction and doping. Scanning electron microscopy and atomic force microscopy studies show that plasma treatment at a relatively low power (∼10 W) for up to 15 min does not affect the morphological stability and monolayer character of GO sheets. X-ray photoelectron spectroscopy has been used to study de-oxygenation of GO monolayers and the incorporation of nitrogen in graphitic-N, pyrrolic-N and pyridinic-N forms due to the plasma treatment. The corresponding changes in the valence band electronic structure, density of states at the Fermi level and work function have been investigated by ultraviolet photoelectron spectroscopy. These studies, supported by Raman spectroscopy and electrical conductivity measurements, have shown that a short duration plasma treatment of up to 5 min results in an increase of sp²-C content along with a substantial incorporation of the graphitic-N form, leading to the formation of n-type reduced GO. Prolonged plasma treatment for longer durations results in a decrease of electrical conductivity, which is accompanied by a substantial decrease of sp²-C and an increase in defects and disorder, primarily attributed to the increase in pyridinic-N content.

Growth of CdS nanocrystallites on graphene oxide Langmuir–Blodgett monolayers

Large area GO-Cd composite Langmuir-Blodgett monolayers were transferred onto Si substrate by introducing Cd(2+) ions into the subphase. The changes in the behaviour of the Langmuir monolayer isotherm in the presence of Cd(2+) ions are attributed to changes in the microstructure and density of the GO sheets on the subphase surface. The uptake of Cd onto the GO monolayers and the effect of subsequent sulphidation were investigated by AFM, FTIR, Raman, XPS and HRTEM techniques. The incorporation of Cd into the GO monolayers causes some overlapping of sheets and extensive formation of wrinkles. Sulphidation of the GO-Cd sheets results in the formation of uniformly distributed CdS nanocrystallites on the entire basal plane of the GO monolayers. The de-bonding of Cd with oxygen functional groups results in a reduction of the wrinkles. The GO sheets function primarily as a platform for the interaction of metal ions with oxygen functionalities and their structure and characteristic features are not affected by either uptake of Cd or formation of CdS.

Spectroscopic ellipsometry studies of GaN films deposited by reactive rf sputtering of GaAs target

GaN films have been deposited by reactive rf sputtering of GaAs target in 100% nitrogen ambient on quartz substrates at different substrate temperatures ranging from room temperature to 700°C. A series of films, from arsenic-rich amorphous to nearly arsenic-free polycrystalline hexagonal GaN, has been obtained. The films have been characterized by phase modulated spectroscopic ellipsometry to obtain the optical parameters, viz., fundamental band gap, refractive index, and extinction coefficient, and to understand their dependence on composition and microstructure. A generalized optical dispersion model has been used to carry out the ellipsometric analysis for amorphous and polycrystalline GaN films and the variation of the optical parameters of the films has been studied as a function of substrate temperature. The refractive index values of polycrystalline films with preferred orientation of crystallites are slightly higher (2.2) compared to those for amorphous and randomly oriented films. The dominantly ...

Effect of ammonia plasma treatment on graphene oxide LB monolayers

Graphene oxide monolayer sheets were transferred on Si and SiO2/Si substrates by Langmuir-Blodgett technique and were exposed to ammonia plasma at room temperature. The monolayer character of both graphene oxide and plasma treated graphene oxide sheets were ascertained by atomic force microscopy. X-ray photoelectron spectroscopy and Raman spectroscopy revealed that ammonia plasma treatment results in enhancement of graphitic carbon content along with the incorporation of nitrogen. The conductivity of graphene oxide monolayers, which was in the range of 10−6-10−7 S/cm, increased to 10−2-10−3 S/cm after the ammonia plasma treatment. These results indicate that the graphene oxide was simultaneously reduced and N-doped during ammonia plasma treatment, without affecting the morphological stability of sheets.

GO and RGO based FETs fabricated with Langmuir-Blodgett grown monolayers

Graphene oxide (GO) monolayers were transferred onto SiO2/Si substrates by Langmuir-Blodgett (LB) technique and were converted to reduced graphene oxide (RGO) by exposure to hydrazine vapors followed by various durations of heat treatment at 400 °C in Ar atmosphere. Bottom gated FETs were fabricated with LB grown monolayers before and after reduction and were electrically characterized. The conductivity of RGO monolayers has been found to be in the range of 3 to 5 Scm−1. The RGO devices showed p-type behavior with a hole mobility of 0.07cm2/Vs and Ion/Ioff ratio of 2.

Structural Investigation of Polyurethane Based Gel Polymer Electrolytes Using Small Angle X-ray Scattering (SAXS)

Cross linked polymers PU (polyurethane) gel electrolytes have been synthesized from polyethylene oxide triol (PEO triol) and 4,4-methylenediphenyl diisocyanate (MDI). The minimum and maximum intake of liquid electrolyte by this gel is 35 and 65 wt % respectively. WAXD patterns for PU gel electrolytes shows featureless behaviour indicating absence of crystalline phase. SAXS pattern of polyurethane gel exhibits a broad peak at lower q value (∼0.7 nm -1 ) that is attributed to the presence of two phases, namely hard domains and soft domains. The structure of the gel was inferred as a continuous soft phase with dispersed hard domains. The shape and interface of the hard domains were determined from the slope of the curve in log ( I ( q )) versus log ( q ) plot. The slope of the curve decreases with increase in propylene carbonate (PC) content. The decrease in slope from -1.5 to -0.2 suggest that the shape of the hard domains changed from diffused ellipsoidal to the diffused spherical when the PC content was i...

Graphene oxide based hybrid nanostructures with ZnO thin films and nanorods

Graphene oxide (GO) monolayer sheets were transferred on to sputtered ZnO films and chemically grown ZnO nanorods by Langmuir-Blodgett technique. SEM images show that the GO sheets are well defined and uniformly distributed over both surfaces, namely, ZnO film and ZnO nanorods. Raman studies show that the ZnO film has a higher reducing effect on GO monolayers compared to ZnO nanorods, with the most effective reduction at intermediate temperatures in the range of 400 - 600 °C, and a degradation of GO sheets on both structures, after heat treatment at 800 °C. Photoluminescence spectra of heat treated GO/ZnO film and GO/ZnO nanorods show characteristic near-band-edge emission of ZnO, which is not significantly affected by the presence of RGO sheets.

Effect of subphase pH on Langmuir-Blodgett deposition of graphene oxide monolayers on Si and SiO2/Si substrates

Graphene oxide monolayers were transferred onto Si and SiO2/Si substrates by Langmuir-Blodgett technique. Effect of subphase pH changes on the GO Langmuir isotherm and the morphology of monolayers deposited on RCA-1 treated and untreated Si and SiO2/Si substrates is reported. No GO transfer was observed on untreated Si substrate but well defined GO sheets could be transferred at subphase pH ∼ 5.5 on RCA-1 treated Si. GO sheets transferred on untreated SiO2/Si surface exhibit curled up edges, implying poor adhesion. Adherent GO sheets with planar morphology were transferred on RCA-1 treated SiO2/Si substrates at all subphase pH values in the range 3.5-6.5.

Effect of substrate temperature on microstructure of epitaxial ZnO films grown on sapphire by sputtering

The microstructure of epitaxial ZnO films on (0001) sapphire substrates deposited by reactive sputtering in substrate temperature (Ts) range of 100 – 500 °C were investigated by High-resolution X-ray diffraction. The studies show the presence of a strained 2D layer along with 3D crystallites at low Ts, whereas 3D growth is dominant at higher Ts, resulting in formation of mosaic structure with lesser point and line defects.