Gargi Raina

B-C-N, C-N AND B-N NANOTUBES PRODUCED BY THE PYROLYSIS OF PRECURSOR MOLECULES OVER CO CATALYSTS

Abstract B–C–N, C–N and B–N nanotubes, prepared by the pyrolysis of the appropriate precursor compounds around 1000°C in an atmosphere of argon, have been examined by electron microscopy and other techniques. The compositions of the nanotubes have been analysed by electron energy loss spectroscopy and X-ray photoelectron spectroscopy. There is significant compositional variation in B–C–N and C–N nanotubes. Properties of the B–C–N and C–N nanotubes have been compared with those of carbon nanotubes, particularly with respect to the I – V characteristics as obtained from UHV–STM studies. The near absence of B–N nanotubes on pyrolysing the appropriate precursor compound, and other observations made in the present study, indicate the crucial role played by carbon in the initial nucleation and growth of nanotubes.

Structures and Images of Novel Derivatives of Carbon Nanotubes, Fullerenes and Related New Carbon Forms

In the light of the finding that carbon nanotubes get functionalized on reaction with acid and other oxidizing agents, the structures and shapes of nanotube derivatives resulting from possible reaction between functionalized nanotubes, are obtained by an energy minimization procedure and the structures, so obtained, are compared with the observed microscopic images. The shapes of fullerene-like and nanotube-like structures containing seven-membered rings, in addition to six and five-membered rings, are depicted along with the structures of bent nanotubes containing similar ring systems. Diamond-graphite hybrid structures which constitute an important class of carbon materials are also investigated.

Preparation and evaluation of nimesulide-loaded ethylcellulose and methylcellulose nanoparticles and microparticles for oral delivery.

The present study was designed to assess and compare with a range of surfactant-coated, nimesulide-free, and nimesulide-loaded ethylcellulose/methylcellulose (EC/MC) nanoparticles that were prepared by varying drug concentration (ED/MD), polymer concentration (EP/MP), and surfactant concentration (ES/MS). EC/MC nanoparticles prepared by desolvation method produced discrete particles and they were characterized by SEM, AFM, and FTIR studies. The particles mean size diameter (nm) ranged from 244 to 1056 nm and 1065 to 1710 nm for EC and MC nanoparticles, respectively. Studies on drug: polymer ratio showed a linear relationship between drug concentration and percentage of loading in nanoparticles. The encapsulation efficiency decreased with the increase of nimesulide concentration with respect to polymer concentration. Encapsulation efficiency of drug-loaded nanoparticles was varied between 32.8% and 64.9%. The in vitro release of drug-loaded nanoparticles was found to be a first order. This was significantly increased in EC nanoparticles (95.50%) in comparison with MC nanoparticles (95.12%) after 12 h in 24 h long study. Nimesulide release from EC nanoparticles was much slower at slightly alkaline pH 7.4. The in vitro hemolysis tests of nanoparticles were carried out to ascertain the hemocompatibility and shown to be insignificant for EC nanoparticles. In comparison, ES4 from EC formulations with nimesulide was found to be promising with slow and sustained drug release.

Mass spectrometric determination of the surface compositions of ethanol-water mixtures

Vapor-phase clusters from ethanol-water liquid mixtures of varying compositions, generated by sampling the binary vapor in equilibrium with the liquid mixture, were examined with the aid of a time-of-flight mass spectrometer. The variation of the mole fraction of the alcohol in the cluster beam with the liquid mole fraction was found to be identical to that of the surface concentration obtained from surface tension measurements. The results also compare well with those obtained from neutron reflection. The mass spectrometric method of determining surface compositions of liquid mixtures is more direct and model independent and is applicable over the entire range of compositions.

A molecular beam study of water–methanol clusters

Clusters generated by injecting vapor from water–methanol liquid mixtures of varying compositions through a pulsed valve, have been analyzed using a time-of-flight mass spectrometer. Neat clusters of water and methanol as well as mixed species have been observed, the relative populations varying with composition. The mole fractions in the vapor deviate significantly from the values calculated assuming ideality of the liquid except at water mole fraction of 0.67, the composition around which the excess enthalpy is minimum. An anomaly observed in the cluster populations at this composition is related to an extended hydrogen bonded interaction of molecules in the liquid.

An indigenous cluster beam apparatus with a reflectron time-of-flight mass spectrometer

The design and fabrication of a Smalley-type cluster source in combination with a reflectron based time-of-flight (TOF) mass spectrometer are reported. The generation of clusters is based on supersonic jet expansion of the sampling plume. Sample cells for both liquid and solid targets developed for this purpose are described. Two pulsed Nd-YAG lasers are used in tandem, one (532 nm) for target vapourization and the other (355 nm) for cluster ionization. Methanol clusters of nuclearity up to 14 (mass 500 amu) were produced from liquid methanol as the test sample. The clusters were detected with a mass resolution of ∼ 2500 in the R-TOF geometry. Carbon clusters up to a nuclearity of 28 were obtained using a polyimide target. The utility of the instrument is demonstrated by carrying out experiments to generate mixed clusters from alcohol mixtures.

Cellular network formation of hydrophobic alkanethiol capped gold nanoparticles on mica surface mediated by water islands.

Dendritic and cellular networks of nanoparticles are known to form commonly either by random diffusion-limited aggregation or by solvent evaporation dynamics. Using alkanethiol capped gold nanoparticles deposited on mica imaged under ambient and controlled water vapor conditions by atomic force microscope and in situ scanning electron microscope, respectively, we show a third mechanism in action. The cellular network consisting of open and closed polygons is formed by the nucleation and lateral growth of adsorbed water islands, the contact lines of which push the randomly distributed hydrophobic nanoparticles along the growth directions, eventually leading to the polygonal structure formation as the boundaries of the growing islands meet. Such nanoparticle displacement has been possible due to the weakly adhering nature of the hydrophilic substrate, mica. These results demonstrate an important but hitherto neglected effect of adsorbed water in the structure formation on hydrophilic substrates and provide a facile tool for the fabrication of nanoparticle networks without specific particle or substrate modifications and without a tight control on particle deposition conditions during the solvent evaporation.

(HCN)m(NH3)nH+ clusters formed by the reaction of carbon vapor with jet-cooled ammonia

Abstract New cluster species of the type (HCN)m(NH3)nH+ with m up to 4 and n up to 7 alongwith (NH3)nH+ species are observed using mass spectrometry, on the reaction of carbon vapor with jet-cooled NH3 in admixture with helium. The most preponderant species correspond to m=1 and 2, viz. (HCN)1(NH3)4H+, (HCN)1(NH3)3H+ and (HCN)2(NH3)3H+. These clusters involving the tetrahedral coordination in the first solvation shell of the NH4+ ion are the most stable species, as corroborated by molecular-orbital calculations. The incremental complexation energy due to successive addition of the NH3 molecule to (HCN)(NH3)nH+, decreases monotonically with increasing n, for n>2.

Study of Stability of Local Anodic Oxidation on HOPG and Few Layer Graphene Using AFM in Ambient

Local anodic oxidation (LAO) has been performed on highly oriented pyrolytic graphite (HOPG) and few layer graphene (FLG) using contact-mode (CM) atomic force microscopy (AFM) in a controlled humidity chamber. Different types of LAO patterns, namely, protrusion and trench features were observed for different tip speeds under similar conditions of LAO patterning, in both CM and noncontact-mode (NCM) AFM images. Observed LAO patterns show a variation in their dimensions over time. This paper carries out a study performed on the observed changes in dimensions of LAO patterns made on HOPG and FLG over several days, pointing to the dynamics of these LAO patterns. The stability of the LAO patterns on HOPG is compared with that for FLG. LAO patterns on HOPG show a widening and reduction in depth over a day. LAO patterns on FLG show, in contrast, a random variation in the lateral dimension “width” and stabilization in the vertical dimension “depth” over several days.

An atomic force microscope study of carbon onions and related nanoparticles

Carbon onions are found along with carbon nanotubes and other carbon nanoparticles in the cathodic deposit in the arc-vaporization of graphite. Atomic force microscopy has been used to characterize these particles on the basis of their sizes and shapes. Onion-like particles have three-dimensional, near spherical structure and are distinct from two-dimensional graphitic particles. The spherical shape and height to diameter ratios obtained using atomic force microscope, afford a distinction between onion-like structures and other carbon nanoparticles.