Madhumita Choudhuri

Two dimensional mixtures at water surface

Thiol capped gold nanoparticles (Au NPs) form a simple two dimensional (2D) liquid on water surface but this thin film is unstable under compression. Amphiphilic stearic acid (StA) molecules on water surface, on the other hand, form a complex and more stable 2D liquid. We have initiated a study on a mixture of StA and Au NPs in a monolayer through Surface Pressure (π) – Specific Molecular Area (A) isotherms and Brewster Angle Microscopy (BAM). A mixture of Stearic Acid and Au nanoparticles (10% by weight) produces a monolayer on water surface that acts as a 2D liquid with phases that are completely reversible with negligible hysteresis.

Surfactant assisted Au nanoparticle layering in titanium oxide thin films

Au Nanoparticle (NP) decorated TiO2 thin films, prepared by a unique surfactant assisted 2D self-assembling technique with molecular level control, showed significant decrease in optical band gap as well as enhanced crystallinity compared to its sol-gel prepared pristine counterpart. Spin coated Au NP overlayers on titania in absence of surfactant, on the other hand, had no appreciable effect on either band gap or crystal structure compared to undoped TiO2 films. Apart from exhibiting band gap tuning of TiO2, this cheap, scalable technique of surfactant aided deposition of 2D layers of Au NPs on semiconducting oxides, may be used for development of multilayered structures with promising light harvesting and unidirectional energy transfer (LUET) applications.

Gold nanoparticle patterning on titanium dioxide thin films by hydrophilic and hydrophobic interactions effect on band gap

Thin films of patterned, thiol capped Au nanoparticle (NP) over layer on titanium dioxide (TiO2) matrix (Au-TO-Ov), prepared by a unique surfactant assisted 2D self-assembling technique with molecular level control, in conjuncture with sol-gel route showed significant decrease in optical band gap and enhanced crystallinity compared to sol-gel prepared pristine TiO2. Combined results of scanning electron microscopy, UV-Vis spectroscopy, Raman spectroscopy and near edge x-ray absorption fine structure spectroscopy indicated that decrease in optical band gap was associated with changes in local electronic structure of Au-TO films. No such changes were observed for the film with AuNP as underlayer (Au-TO-Un). Dependence of structure and band gap, on order of Au NP layer is attributed to difference in hydrophilic and hydrophobic interactions occurring at AuNP-TiO2 interface. Unlike spin coated Au NPs on TiO2, which resulted in Au NP agglomeration, or in-situ doping of Au NPs in titanium dioxide precursor by sol-gel method, which indicated a slight increase in band gap, Langmuir-Schaefer method caused significant changes in local structure and hence band gap of the system brought about by these weak Van der Waals interactions. This cheaper, scalable technique of surfactant aided deposition of 2D layers of Au NPs on semiconducting oxides may be used for development of multilayered structures with promising light harvesting and unidirectional energy transfer (LUET) applications.

Long-timescale dynamics of thiol capped Au nanoparticle clusters at the air-water interface

A two-dimensional network of thiol-capped Au nanoparticle (AuNP) clusters is self-organized on a Stearic Acid (amphiphilic fatty acid) Langmuir monolayer on water surface. The AuNP clusters are found to form a pattern of connected and enclosed microspaces in the stearic acid template. The network features can be controlled by changing the surface pressure of the monolayer during compression. The two-dimensional dynamics of this network has been studied over a long timescale using Brewster Angle Microscopy (BAM). The dynamics is very slow, indicating the stability of the network system, and is essentially driven by the tendency to lower the number of nodes or joints in the network.