Palash Gangopadhyay

Ultrathin organic bulk heterojunction solar cells: Plasmon enhanced performance using Au nanoparticles

The plasmonic effect of gold nanoparticles (AuNPs) enhances light absorption and, thus, the efficiency of organic bulk heterojunction solar cells with poly (3-hexylthiophene) (P3HT): [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as active layer. We report optimization of this enhancement by varying the attachment density of the self-assembled AuNPs on silanized ITO using N1-(3-trimethoxysilylpropyl)diethylenetriamine. Using finite difference time domain simulations, the thicknesses of poly (3,4-ethylenedioxythiophene) (PEDOT): poly (styrenesulfonate) (PSS) and P3HT:PCBM layers were suitably varied to ensure broadband optical absorption enhancement and minimal exciton quenching within the active layer. Our experimental results demonstrate that for solar cell structures with 20% surface coverage, absorption is increased by 65% as predicted by simulations. Further, we show that AuNPs increase the efficiency by 30% and that silanization of ITO positively impacts device performance.

Modification of symmetrically substituted phthalocyanines using click chemistry: Phthalocyanine nanostructures by nanoimprint lithography

Phthalocyanines (Pcs) are commonly applied to advanced technologies such as optical limiting, photodynamic therapy (PDT), organic field-effect transistors (OFETs), and organic photovoltaic (OPV) devices, where they are used as the p-type layer. An approach to Pc structural diversity and the incorporation of a functional group that allows fabrication of solvent resistant Pc nanostructures formed by using a newly developed nanoimprint by melt processing (NIMP) technique, a variant of standard nanoimprint lithography (NIL), is reported. Copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC), a click chemistry reaction, serves as an approach to structural diversity in Pc macrocycles. We have prepared octaalkynyl Pc 1b and have modified this Pc using the CuAAC reaction to yield four Pc derivatives 5a-5d with different peripheral substituents on the macrocycle. One of these derivatives, 5c, has photo-cross-linkable cinnamate residues, and we have demonstrated the fabrication of robust cross-linked photopatterned and imprinted nanostructures from this material.

Faraday rotation in magnetite-polymethylmethacrylate core-shell nanocomposites with high optical quality

Wavelength dependent Faraday rotation measured at room temperature in a polymer containing varying concentrations of crosslinked magnetite core poly(3-methyl)methacrylate (PMMA) shell nanocomposite particles is reported. The magnitude of the FR in these composites appears to be strongly dependent on the concentration of the particles, the wavelength of measurement, and the size and shape of the nanoparticles. Comparison of the figure of merit of 2.1° in these core-shell nanocomposites to that of 4.2° for yttrium iron garnet at 980 nm indicates that these nanocomposites may find applications in various magnetic and magneto-optic sensors and devices.

Enhanced magnetism in highly ordered magnetite nanoparticle-filled nanohole arrays.

A new approach to develop highly ordered magnetite (Fe3O4) nanoparticle-patterned nanohole arrays with desirable magnetic properties for a variety of technological applications is presented. In this work, the sub-100 nm nanohole arrays are successfully fabricated from a pre-ceramic polymer mold using spin-on nanoprinting (SNAP). These nanoholes a then filled with monodispersed, spherical Fe3O4 nanoparticles of about 10 nm diameter using a novel magnetic drag and drop procedure. The nanohole arrays filled with magnetic nanoparticles a imaged using magnetic force microscopy (MFM). Magnetometry and MFM measurements reveal room temperature ferromagnetism in the Fe3O4-filled nanohole arrays, while the as-synthesized Fe3O4 nanoparticles exhibit superparamagnetic behavior. As revealed by MFM measurements, the enhanced magnetism in the Fe3O4-filled nanohole arrays originates mainly from the enhanced magnetic dipole interactions of Fe3 O4 nanoparticles within the nanoholes and between adjacent nanoholes. Nanoparticle filled nanohole arrays can be highly beneficial in magnetic data storage and other applications such as microwave devices and biosensor arrays that require tunable and anisotropic magnetic properties.

Nanoimprinting by Melt Processing: An Easy Technique to Fabricate Versatile Nanostructures

Insights gained from rheological and contact angle measurements of plasticized and non-plasticized polymers have led to the development of a simple method to print densely packed micro- and nanoscale features without proximity effects. Versatile large-area nanopatterns and landscapes with a high degree of fidelity are successfully imprinted. This technique promises a variety of polymer nanostructures to a wide spectrum of scientific fields.

Printed sub-100 nm polymer-derived ceramic structures.

We proposed an unconventional fabrication technique called spin-on nanoprinting (SNAP) to generate and transfer sub-100 nm preceramic polymer patterns onto flexible and rigid substrates. The dimensions of printed nanostructures are almost the same as those of the mold, since the ceramic precursor used is a liquid. The printed patterns can be used as a replica for printing second-generation structures using other polymeric materials or they can be further converted to desirable ceramic structures, which are very attractive for high-temperature and harsh environment applications. SNAP is an inexpensive parallel process and requires no special equipment for operation.

Cobalt ferrite nanoparticles polymer composites based all-optical magnetometer

A method has been developed to prepare cobalt ferrite particle core polymer shell nanoparticles. These engineered nanoparticles can be further embedded into a polymer host matrix to develop highly transparent polymer based magneto-optic materials. A proof-of-principle all-optical magnetometer has been constructed based on the cobalt ferrite core polymer shell based nanocomposite material. A noise equivalent magnetic field sensitivity of 50nT/√Hz was observed using a 3μT 500Hz control magnetic field.

Titanium oxide sol-gel films with tunable refractive index

Glycidylmethacrylate and propylene oxide were used in the epoxide initiated formation of titanium oxide sols which were spun to form thin films. Glycidylmethacrylate can be used to tune the refractive index of the resulting composite and allowed us to photo-pattern the material. The refractive index of the films can be controlled between 1.76 and 2.05 at 589 nm. The thicknesses of the films ranged between 80 and 200 nm and the rms roughness below 2 nm. The films were characterized by atomic force microscopy (AFM), electric force microscopy (EFM), x-ray photoelectron spectroscopy (XPS) and ellipsometry, among other techniques.

Efficient Faraday rotation in conjugated polymers

Very efficient Faraday rotation has been observed in regioregular polyalkoxythiophene. These observations could be a first step towards a new type of application of conjugated polymers.

Multiphoton microscopy as a detection tool for photobleaching of EO materials

Multi-photon microscopy operating at 1550 nm is employed as a rapid characterization tool for studying the photostability of three well-known electro-optical materials. Different nonlinear optical responses such as multi-photon excitation fluoresence, second-, and third-harmonic generation can be used as detection probes to reveal the degradation mechanisms. This technique is rapid, accurate, and can be used to study the photostability of a broad range of materials.