C. Vijayan

Single- and few-layer graphene growth on stainless steel substrates by direct thermal chemical vapor deposition

Increasing interest in graphene research in basic sciences and applications emphasizes the need for an economical means of synthesizing it. We report a method for the synthesis of graphene on commercially available stainless steel foils using direct thermal chemical vapor deposition. Our method of synthesis and the use of relatively cheap precursors such as ethanol (CH(3)CH(2)OH) as a source of carbon and SS 304 as the substrate proved to be economically viable. The presence of single- and few-layer graphene was confirmed using confocal Raman microscopy/spectroscopy. X-ray photoelectron spectroscopic measurements were further used to establish the influence of various elemental species present in stainless steel on graphene growth. The role of cooling rate on surface migration of certain chemical species (oxides of Fe, Cr and Mn) that promote or hinder the growth of graphene is probed. Such analysis of the chemical species present on the surface can be promising for graphene based catalytic research.

Optical absorption and photoluminescence studies on CdS quantum dots in Nafion

Excitonic effects are observed in the optical absorption and photoluminescence of strongly confined CdS quantum dots embedded in the polymer Nafion. The three bands identified in the optical absorption spectra could be attributed to 1se-1sh, 1pe-1ph, and 2se-2sh transitions of the noninteracting particle model. Photoluminescence spectra show a strong emission band corresponding to electron–hole recombination and a weak band due to emission from defect states. The strength of electron–phonon coupling is small in the regime of strong confinement and decreases with decreasing particle size.

Excitonic Transitions and Off-resonant Optical Limiting in CdS Quantum Dots Stabilized in a Synthetic Glue Matrix

Stable films containing CdS quantum dots of mean size 3.4 nm embedded in a solid host matrix are prepared using a room temperature chemical route of synthesis. CdS/synthetic glue nanocomposites are characterized using high resolution transmission electron microscopy, infrared spectroscopy, differential scanning calorimetry and thermogravimetric analysis. Significant blue shift from the bulk absorption edge is observed in optical absorption as well as photoacoustic spectra indicating strong quantum confinement. The exciton transitions are better resolved in photoacoustic spectroscopy compared to optical absorption spectroscopy. We assign the first four bands observed in photoacoustic spectroscopy to 1se–1sh, 1pe–1ph, 1de–1dhand 2pe–2phtransitions using a non interacting particle model. Nonlinear absorption studies are done using z-scan technique with nanosecond pulses in the off resonant regime. The origin of optical limiting is predominantly two photon absorption mechanism.

Sequential Electrochemical Unzipping of Single-Walled Carbon Nanotubes to Graphene Ribbons Revealed by in Situ Raman Spectroscopy and Imaging

We report an in situ Raman spectroscopic and microscopic investigation of the electrochemical unzipping of single-walled carbon nanotubes (SWNTs). Observations of the radial breathing modes (RBMs) using Raman spectral mapping reveal that metallic SWNTs are opened up rapidly followed by gradual unzipping of semiconducting SWNTs. Consideration of the resonant Raman scattering theory suggests that two metallic SWNTs with chiralities (10, 4) and (12, 0) get unzipped first at a lower electrode potential (0.36 V) followed by the gradual unzipping of another two metallic tubes, (9, 3) and (10, 1), at a relatively higher potential (1.16 V). The semiconducting SWNTs with chiralities (11, 7) and (12, 5), however, get open up gradually at ±1.66 V. A rapid decrease followed by a subsequent gradual decrease in the metallicity of the SWNT ensemble as revealed from a remarkable variation of the peak width of the G band complies well with the variations of RBM. Cyclic voltammetry also gives direct evidence for unzipping in terms of improved capacitance after oxidation followed by more important removal of oxygen functionalities during the reduction step, as reflected in subtle changes of the morphology confirming the formation of graphene nanoribbons. The density functional-based tight binding calculations show additional dependence of chirality and diameter of nanotubes on the epoxide binding energies, which is in agreement with the Raman spectroscopic results and suggests a possible mechanism of unzipping determined by combined effects of the structural characteristics of SWNTs and applied field.

Design of a low power optical limiter based on a new nanocomposite material incorporating silica-encapsulated phthalocyanine in Nafion

We report on the design of a stable optical limiter in the low laser power regime based on the thermal variation of refractive index in a novel nanocomposite material. The optical material, chloroaluminium-phthalocyanine (ClAlPc), is embedded in SiO2-Nafion nanocomposite membrane (ClSNf) and its thermally induced nonlinear refractive index is characterized by the Z-scan technique with a low power cw He–Ne laser as the source. The value of nonlinear refractive index coefficient, n2, is found to be about 1.11 × 10−11 m2 W−1. The experiment is repeated with the dye doped in pure Nafion membrane (ClNf) and the results are compared with those of ClAlPc doped SiO2–Nafion nanocomposite membrane. The value of n2 is found to be 1.36 × 10−11 m2 W−1 and is larger than that of the ClAlPc embedded SiO2–Nafion nanocomposite membrane. The photostability of the dye-embedded membrane is studied by exposing the sample to cw He–Ne laser and monitoring its fluorescence emission intensity continuously. The samples are found to show large thermal lens effect and demonstrated to be good optical limiters in the low power regime. Whereas the optical properties of dye-doped Nafion appear to be slightly better than those of the dye embedded in silica and incorporated in Nafion, the latter is found to offer excellent photostability.

Quantum size effects on the third order optical nonlinearity of CdS quantum dots in Nafion

Abstract Optical nonlinearities of CdS quantum dots in the strongly confined regime are probed through degenerate four-wave-mixing experiments in an attempt to study the size dependence of the third order optical susceptibility χ (3) . The results indicate that χ (3) is strongly dependent on the diameter of the dots and that there is a large increase in χ (3) values as the diameter decreases below 2 nm. Polarization discrimination studies point to nonthermal contributions to the mechanism of optical nonlinearity.

Approaching Sensitivity of Tens of Ions Using Atomically Precise Cluster−Nanofiber Composites

A new methodology has been demonstrated for ultratrace detection of Hg(2+), working at the limit of a few tens of metal ions. Bright, red luminescent atomically precise gold clusters, Au@BSA (BSA, bovine serum albumin), coated on Nylon-6 nanofibers were used for these measurements. A green emitting fluorophore, FITC (fluorescein isothiocyanate), whose luminescence is insensitive to Hg(2+) was precoated on the fiber. Exposure to mercury quenched the red emission completely, and the green emission of the fiber appeared which was observed under dark field fluorescence microscopy. For the sensing experiment at the limit of sensitivity, we have used individual nanofibers. Quenching due to Hg(2+) ions was fast and uniform. Adaptation of such sensors to pH paper-like test-strips would make affordable water quality sensors at ultralow concentrations a reality.

Enhanced optical nonlinearity in β-AgVO3 nanobelts on decoration with Ag nanoparticles

This paper reports on the optical nonlinearity of β-AgVO3 nanobelts and the modification in the physical mechanisms of nonlinear response on decorating with silver nanoparticles. The nanobelts are synthesized by a hydrothermal technique and characterized using SEM, TEM, and XRD etc. The nanobelts are found to exhibit large nonlinear optical absorption as revealed by open Z-scan measurements using 5 ns laser pulses at 532 nm. Nonlinearity appears to be arising from a combination of mechanisms of two photon absorption and saturable absorption (SA). Decoration with Ag nanoparticles is found to enhance the saturable absorption and alter the coefficient of nonlinear absorption of the nanobelts. Efficient optical limiting is demonstrated in both the nanobelt systems.

Effect of the organic solvent on the formation and stabilization of CdS and PbS nanoclusters

Nanoclusters of CdS and PbS were prepared using two different organic solvents as stabilizers in order to understand the factors affecting their formation and stabilization. Growth of the nanoclusters was monitored by optical absorption spectroscopy at regular intervals of time. Mean cluster size was characterized by X-ray diffraction (XRD). The surface structure of nanoclusters was analyzed using infrared (IR) spectroscopy. Spectroscopic studies under identical experimental conditions reveal interesting correlations between the stability of the nanoclusters formed, the nature of the solvent and the size of metal ion involved, leading to a better understanding of nanocluster formation.

Efficient ultrafast optical limiting using single walled carbon nanotubes functionalized noncovalently with free base and metalloporphyrins

The present work is on the linear and third order nonlinear optical properties of single walled carbon nanotubes functionalized noncovalently with free base porphyrin and a metalloporphyrin separately. The functionalized carbon nanotubes (CNTs) are soluble in dimethylformamide, and the microscopic images show that the porphyrin molecules are attached to the surface of the CNTs. The interaction between CNTs and porphyrins is noncovalent and probably due to π-π interaction as both CNTs and porphyrins have π-electron rich structures. The samples exhibit large ultrafast nonlinear absorption as evident from the open aperture z-scan studies performed using mode-locked Ti:Sapphire femtosecond pulses at 780 nm. The mechanisms of optical nonlinearity of the functionalized CNTs appear to be two photon absorption along with nonlinear scattering with a small contribution from saturable absorption, whereas nonfunctionalized CNTs are known to exhibit saturable absorption. The value of the nonlinear absorption coefficie...