Ankur Verma

Enhanced Self-Organized Dewetting of Ultrathin Polymer Films Under Water-Organic Solutions: Fabrication of Sub-micrometer Spherical Lens Arrays

Field-induced self-organized patterning in ultrathin (< 100 nm) polymer films produces resolutions of the order of 10 {\mu}m or more because of the high energy penalty for the surface deformations on small scales. We propose here a very simple but versatile method to fabricate sub-micron (~100 nm) ordered and tunable polymeric structures by self-organized room temperature dewetting of ultrathin polystyrene films by minimizing the surface tension limitation. We illustrate this technique by fabricating sub-micron lens arrays of tunable curvature. This is achieved by switching to controlled room temperature dewetting under an optimal mix of water, acetone and methyl-ethyl ketone (MEK). Organic solvents used decrease the glass transition temperature, greatly decrease the interfacial tension, intensify the field and increase the contact angle/aspect ratio of the resulting tunable nano-structures, without a concurrent solubilization of PS owing to water being the majority phase in the outside mixture.

Self-organized nano-lens arrays by intensified dewetting of electron beam modified polymer thin-films

Sub-100 nm polymeric spherical plano-convex nano-lens arrays are fabricated using short electron beam exposures to selectively modify the ultrathin (<30 nm) polymer films, followed by their intensified self-organized dewetting under an aqueous–organic mixture. A short exposure to e-beam locally modifies the polymer chains to effectively change the viscosity of the film in small domains, thus bringing in the dynamical dewetting contrast in the film that produces aligned and ordered dewetted nanostructures. Both negative and positive e-beam tone polymers are thus used to produce an array of nano-lenses. The intensified self-organized dewetting under a water–organic solvent mixture overcomes the limitations on surface tension and dewetting force and thus facilitates the formation of sub-100 nm diameter polymer nano-lenses of tunable curvature. By varying the extent of e-beam exposure, various configurations from isolated to connected nano-lens arrays can be fabricated.

Kinetics of p-Nitrophenol Reduction Catalyzed by PVP Stabilized Copper Nanoparticles

Copper nanoparticles (CuNPs) in aqueous medium were prepared with alkaline hydrazine hydrate as the reductant and PVP as the stabilizing agent without any inert gas protection. Careful variation in the amount of reductant resulted in formation of two stable CuNPs dispersions with different average sizes and consequently different localized surface plasmon resonance absorbances. These two as-synthesized CuNPs dispersions were used to catalyze the model p-nitrophenol reduction reaction. The kinetics of reduction was monitored as a function of concentration and temperature of reactants. Using this we determined the activation energy, pre-exponential factor and the entropy of activation for the two types of CuNPs samples. Catalytic activities of the CuNPs were found to be affected mainly by their activation energies.Graphical Abstract

Ultrafast Large-Area Micropattern Generation in Nonabsorbing Polymer Thin Films by Pulsed Laser Diffraction

An ultrafast, parallel, and beyond-the-master micropatterning technique for ultrathin (30-400 nm) nonabsorbing polymer films by diffraction of laser light through a 2D periodic aperture is reported. The redistribution of laser energy absorbed by the substrate causes self-organization of polymer thin films in the form of wrinklelike surface relief structures caused by localized melting and freezing of the thin film. Unlike conventional laser ablation and laser writing processes, low laser fluence is employed to only passively swell the polymer as a pre-ablative process without loss of material, and without absorption/reaction with incident radiation. Self-organization in the thin polymer film, aided by the diffraction pattern, produces microstructures made up of thin raised lines. These regular microstructures have far more complex morphologies than the mask geometry and very narrow line widths that can be an order of magnitude smaller than the openings in the mask. The microstructure morphology is easily modulated by changing the film thickness, aperture size, and geometry, and by changing the diffraction pattern.

Microfabrication of carbon structures by pattern miniaturization in resorcinol-formaldehyde gel.

A simple and novel method to fabricate and miniaturize surface and subsurface microstructures and micropatterns in glassy carbon is proposed and demonstrated. An aqueous resorcinol-formaldehyde (RF) sol is employed for micromolding of the master pattern to be replicated, followed by controlled drying and pyrolysis of the gel to reproduce an isotropically shrunk replica in carbon. The miniaturized version of the master pattern thus replicated in carbon is about 1 order of magnitude smaller than original master by repeating three times the above cycle of molding and drying. The microfabrication method proposed will greatly enhance the toolbox for a facile fabrication of a variety of carbon-MEMS and C-microfluidic devices.

Sub-40 nm polymer dot arrays by self-organized dewetting of electron beam treated ultrathin polymer films

Sub-40 nm size ordered nanodroplet arrays of polystyrene are fabricated by a low dose selective electron beam exposure of an ultrathin polymer film followed by its intensified self organized dewetting under a mixture of water and polar organic solvents. The self organization speeds up patterning and reduces the feature size, both by more than 10 times.

Glycerol as green hydrogen source for catalytic reduction over anisotropic silver nanoparticles

The search for nanomaterials for catalysis of transfer hydrogenation reactions with green hydrogen sources is an important topical issue. In this work we show that anisotropic silver nanoparticles act as efficient catalysts for p-nitrophenol (Nip) reduction with glycerol, a renewable biomass byproduct, as the green hydrogen source. Anisotropic silver nanoparticles (AgNPs) were prepared using polyol method in presence of Cu2+ salt as an etchant. These AgNPs are found to be a combination of mixture of anisotropic shapes and spherical faceted nanoparticles. The percentage of anisotropic shapes in the distribution changes with amount of etchant used. Effect of shape and size of AgNPs on the catalytic kinetics of Nip reduction using glycerol as a hydrogen source was investigated. The AgNPs sample having more nanoparticles with anisotropy and finer average particle size exhibits better kinetics, lower activation energy and better turnover frequency (TOF).