Himadri Acharya

Nanopatterning of thin polymer films by controlled dewetting on a topographic pre-pattern

We develop a non-lithographic method for fabricating ordered micro/nanostructures of polymer thin films based on controlled dewetting of the films on topographically pre-patterned substrates with a large area. An ordered nanopattern of polystyrene (PS) is accomplished by thermal treatment of a thin PS film above its Tg spin coated on a topographically patterned substrate. We investigate the influence of pattern geometry on the final morphology of the dewetted polymer films using both mesa and indent patterned substrates. The controlled dewetting, initiated preferentially at the edges of individual pre-patterned mesas, in particular gives rise to spherical cap domains located at the center of the mesas. The domains are much smaller than the individual mesas as a consequence of the significant pattern reduction to nearly 300%. The arrays of 70 nm PS nano-sphere caps are obtained from arrays of 200 nm square pre-patterned mesas. Our method is also applicable for other polymers such as a poly(4-vinyl pyridine) (P4VP) containing Rhodamine 6G (Rh6G) dye on a pre-patterned PS substrate and successfully produced highly fluorescent stable nanopatterned films.

Coassembly of Metal and Titanium Dioxide Nanocrystals Directed by Monolayered Block Copolymer Inverse Micelles for Enhanced Photocatalytic Performance

Functional nanostructures of self-assembled block copolymers (BCPs) incorporated with various inorganic nanomaterials have received considerable attention on account of their many potential applications. Here we demonstrate the two-dimensional self-assembly of anisotropic titanium dioxide (TiO(2)) nanocrystals (NCs) and metal nanoparticles (NPs) directed by monolayered poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) copolymer inverse micelles. The independent position-selective assembly of TiO(2) NCs and silver nanoparticles (AgNPs) preferentially in the intermicelle corona regions and the core of micelles, respectively, for instance, was accomplished by spin-coating a mixture solution of PS-b-P4VP and ex situ synthesized TiO(2) NCs, followed by the reduction of Ag salts coordinated in the cores of micelles into AgNPs. Hydrophobic TiO(2) NCs with a diameter and length of approximately 3 nm and 20-30 nm, respectively, were preferentially sequestered in the intermicelle nonpolar PS corona regions energetically favorable with the minimum entropic packing penalty. Subsequent high-temperature annealing at 550 °C not only effectively removed the block copolymer but also transformed the TiO(2) NCs into connected nanoparticles, thus leading to a two-dimensionally ordered TiO(2) network in which AgNPs were also self-organized. The enhanced photocatalytic activity of the AgNP-decorated TiO(2) networks by approximately 27 and 44 % over that of Ag-free TiO(2) networks and randomly deposited TiO(2) nanoparticles, respectively, was confirmed by the UV degradation property of methylene blue.

Ordered Micropatterns by Confined Dewetting of an Imprinted Polymer Thin Film and Their Microlens Application

We fabricated ordered micro/nano patterns induced by controlled dewetting on the topographically patterned PS/P4VP bilayer thin film. The method is based on utilizing microimprinting lithography to induce a topographically heterogeneous bilayer film that allows the controlled dewetting upon subsequent thermal annealing. The dewetting that was initiated strictly at the boundary of the thicker and thinner regions was guided by the presence of the topographic structure. The dewetting front velocity of the microdomains in the confined regions was linearly proportional to the measurement time, which enabled us to control the size of the dewet domain with annealing time. In particular, the submicron sized dot arrays between lines were generated with ease when the dewetting was confined into geometry with a few microns in size. The kinetically driven, non-lithographical pattern structures accompanied the pattern reduction to 400%. The pattern arrays on a transparent glass substrate were especially useful for noncircular microlens arrays where the focal length of the lens was easily tunable by controlling the thermal annealing.

Block copolymer micelles with near infrared metal phthalocyanine dyes for laser induced writing.

A route has been developed to disperse metal-containing phthalocyanine dyes in a non-polar medium based on amphiphilic block copolymer micelles of poly[styrene-block-(4-vinylpyridine)] (PS-b-P4VP) and poly[styrene-block-(acrylic acid)] (PS-b-PAA) copolymers. Polar P4VP and PAA efficiently encapsulate cobalt(II), manganese(II), and nickel(II) phthalocyanine dyes by axial coordination of nitrogen and µ-oxo bridged dimerization with the transition metals, respectively. Good dispersion of the dyes is confirmed by the linear enhancement of Q-bands in UV-vis absorption spectra with dye concentration. A thin monolayered PS-b-P4VP micelle film that contained a nickel(II) phthalocyanine dye which efficiently adsorbs a laser beam on a localized area to generate a local heat higher than the glass transition temperatures of both blocks. One-dimensional laser writing on the dye-containing film allows the fabrication of a few submicrometer wide line patterns in which the self-assembled nanostructure of the block copolymer is modified by the directional heat arising from laser scanning.