T. P. Radhakrishnan

Polymer thin films embedded with in situ grown metal nanoparticles

Metal nanoparticle-polymer composites are versatile materials which not only combine the unique characteristics of the components, but also manifest mutualistic effects between the two. Embedding inside polymer thin films facilitates immobilization and organization of the metal nanoparticles and tuning of their electronic and optical responses by the dielectric environment. The embedded metal nanoparticles in turn can impact upon the various material attributes of the polymer matrix. Some of the most convenient and attractive routes to the fabrication of metal nanoparticle-embedded polymer thin films involve in situ generation of the nanoparticles through reduction or decomposition of appropriate precursors inside the solid film. In this tutorial review we present an overview of the different methodologies developed using this general concept and describe the environment-friendly protocol we have optimized for the fabrication of noble metal nanostructures inside polymer thin films, using aqueous media for the synthesis and deploying the polymer itself as the reducing as well as stabilizing agent. A variety of techniques that have been exploited to characterize the precursor to product transformation inside the polymer film are discussed. The unique control provided by the in situ fabrication route on the size, shape and distribution of the nanostructures, and application of the polymer thin films with the in situ generated metal nanoparticles in areas such as nonlinear optics, surface enhanced Raman scattering, e-beam lithography, microwave absorption, non-volatile memory devices and random lasers, illustrate the versatility of these materials. A brief appraisal of the avenues for future developments in this area is presented.

Optical power limiting in the femtosecond regime by silver nanoparticle–embedded polymer film

A simple methodology is developed for the fabrication of freestanding polymer films with embedded silver nanoparticles grown in situ. Strong nonlinear absorption, positive nonlinear refraction, and efficient optical limiting in the femtosecond regime are demonstrated with these films. Additional investigations with supported films as well as parallel studies in the nanosecond regime are also presented. The freestanding nature of the films is not only of potential interest from the application perspective but also facilitates the unambiguous determination of the nonlinear coefficients of the metal nanoparticles embedded in polymer matrix without complications arising due to the contributions from the substrate.

Polygonal gold nanoplates in a polymer matrix

Polygonal gold nanoplates are generated in situ in poly(vinyl alcohol) film through thermal treatment, the polymer serving as the reducing agent and stabilizer for the nanoparticle formation and enforcing preferential orientation of the plates. The rare pentagonal as well as the more commonly observed hexagonal, triangular and square/rectangle shapes are obtained by fine-tuning the Au/PVA ratio and the time and temperature of fabrication.

An evaluation of water cluster geometries derived from semi-empirical AM1 calculations

Abstract The calculated geometries of water clusters, (H 2 O) n with n = 2–6, obtained from the semi-empirical AM1 procedure differ from geometries based on ab initio calculations. The AM1 hydrogen-bonded linear dimer with C s symmetry reported by Dewar et al. [J. Am. Chem. Soc. 107 (1985) 3902], which corresponds to the most stable ab initio or experimental geometry, is only a local minimum. The AM1 C s dimer is found to be 2.0 kcal less stable than an unusual cyclic bifurcated structure (approximately C 2h symmetry) in which one of the lone pairs of electrons from each water molecule bisects the HOH bond angle of the other molecule. An additional local minimum, 1.7 kcal more stable than the reported C s structure, consists of a C 2v framework in which the two hydrogen atoms of one water molecule coordinate to both lone pairs of the second molecule. These results lead to questions regarding the use of the present AM1 parameterization in studies of hydrogen bonding.

Molecules to materials

Molecular materials represent an area of fruitful interaction between synthetic chemistry, solid state physics and materials science. This article discusses the development of magnetic materials based on metal complexes, organometallic compounds and organic radicals.

Tuning the SERS Response with Ag-Au Nanoparticle-Embedded Polymer Thin Film Substrates

Development of facile routes to the fabrication of thin film substrates with tunable surface enhanced Raman scattering (SERS) efficiency and identification of the optimal conditions for maximizing the enhancement factor (EF) are significant in terms of both fundamental and application aspects of SERS. In the present work, polymer thin films with embedded bimetallic nanoparticles of Ag-Au are fabricated by a simple two-stage protocol. Ag nanoparticles are formed in the first stage, by the in situ reduction of silver nitrate by the poly(vinyl alcohol) (PVA) film through mild thermal annealing, without any additional reducing agent. In the second stage, aqueous solutions of chloroauric acid spread on the Ag-PVA thin film under ambient conditions, lead to the galvanic displacement of Ag by Au in situ inside the film, and the formation of Ag-Au particles. Evolution of the morphology of the bimetallic nanoparticles into hollow cage structures and the distribution of Au on the nanoparticles are revealed through electron microscopy and energy dispersive X-ray spectroscopy. The localized surface plasmon resonance (LSPR) extinction of the nanocomposite thin film evolves with the Ag-Au composition; theoretical simulation of the extinction spectra provides insight into the observed trends. The Ag-Au-PVA thin films are found to be efficient substrates for SERS. The EF follows the variation of the LSPR extinction vis-à-vis the excitation laser wavelength, but with an offset, and the maximum SERS effect is obtained at very low Au content; experiments with Rhodamine 6G showed EFs on the order of 10(8) and a limit of detection of 0.6 pmol. The present study describes a facile and simple fabrication of a nanocomposite thin film that can be conveniently deployed in SERS investigations, and the utility of the bimetallic system to tune and maximize the EF.

Amorphous‐to‐Crystalline Transformation with Fluorescence Enhancement and Switching of Molecular Nanoparticles Fixed in a Polymer Thin Film

Growth in a confined environment: A protocol for the amorphous-to-crystalline transformation (ACT) of nanoparticles has been developed, the critical step being the partial confinement of the particles by fixing in a polymer thin film and solvent vapor fuming. The ACT of a diaminodicyanoquinodimethane-based nanoparticle is accompanied by substantial fluorescence enhancement and switching of the emission color.

Steering molecular dipoles from centrosymmetric to a noncentrosymmetric and SHG active assembly using remote functionality and complexation

Utilization of flexible pendant groups bearing a remote functionality and complexation with an appropriate partner leading to the fabrication of a noncentrosymmetric molecular crystal lattice of interest in quadratic nonlinear optical applications is explored. 7,7-Diamino-8,8-dicyanoquinodimethane, which possesses a large molecular hyperpolarizability but shows an overriding tendency towards centrosymmetric crystal lattice formation, is chosen as a critical test case of a nonlinear optical chromophore. N,N-Dimethylaminoethylene moieties attached to the amino groups of diaminodicyanoquinodimethane serve as the pendant groups and terephthalic acid as the complexing partner. Parallel investigation of the crystal structures of the pure chromophore and its complex demonstrates the crucial role of the pendant group and the complexing partner in the formation of the noncentrosymmetric lattice of the complex with concomitant induction of optical second harmonic generation capability. This study suggests the possibility of developing a simple chemical approach towards the fabrication of acentric crystals for quadratic nonlinear optical applications.

Extensively Reversible Thermal Transformations of a Bistable, Fluorescence-Switchable Molecular Solid: Entry into Functional Molecular Phase-Change Materials.

Functional phase-change materials (PCMs) are conspicuously absent among molecular materials in which the various attributes of inorganic solids have been realized. While organic PCMs are primarily limited to thermal storage systems, the amorphous-crystalline transformation of materials like Ge-Sb-Te find use in advanced applications such as information storage. Reversible amorphous-crystalline transformations in molecular solids require a subtle balance between robust supramolecular assembly and flexible structural elements. We report novel diaminodicyanoquinodimethanes that achieve this transformation by interlinked helical assemblies coupled with conformationally flexible alkoxyalkyl chains. They exhibit highly reversible thermal transformations between bistable (crystalline/amorphous) forms, along with a prominent switching of the fluorescence emission energy and intensity.

Ground-state spin of conjugated non-Kekulé molecules

A simple rule is presented to predict the ground-state spin of π-conjugated non-Kekule molecules and polymers. It is compared with previous rules, computational studies and experimental observations. Predictions of the new rule for novel systems of interest in the pursuit of organic ferromagnetism are tested using semi-empirical calculations.