Evolution of ligand-capped nanoparticle multilayers toward a near unique thickness.

The structural evolution of thiol-capped Au-nanoparticle (AuNP) multilayers on a H-passivated Si substrate, formed through a Langmuir-Schaefer (LS) deposition process, has been investigated using complementary grazing incidence X-ray scattering techniques. The fractional coverage multilayers of AuNPs, formed through a multi-transfer process, are found to be quite unstable under ambient conditions. The thickness of these decreases with time and tends to saturate toward a near unique thickness (NUT ≈ 6 nm). Although initial low coverage and their instability create hindrance in the control and formation of desired 3D-nanostructures in the bottom-up approach, the formation of a NUT-layer, through time-evolution, is quite distinctive, thus interesting. It is clear from the evolution that the thermodynamically driven monolayer structures (of AuNPs) at the air-water interface become thermodynamically unstable when transferred sequentially onto the solid substrate. The thermal energy (kT) and the partial change in the substrate surface energy (Δγ) create the instability and induce diffusion in the AuNPs, which in the presence of a net attractive force towards the substrate (arising from anisotropic interaction of the top AuNPs with the other AuNPs and/or hydrophobic substrate) tries to create a thermodynamically favourable and relatively stable NUT-layer through reorganization for a different duration. This happens if the number of AuNPs is less than or equal to the maximum number that can be accommodated within the NUT. The value of the NUT mainly depends on the particle size and a kT-energy related fluctuation of particles. Furthermore, the formation of the NUT-layer indicates that the hydrophobic-hydrophobic interaction mediated net attraction towards the substrate is long range, while the hydrophilic-hydrophobic interaction mediated repulsion and/or kT-energy induced fluctuations are short range.

Interfacial and thermal energy driven growth and evolution of Langmuir-Schaefer monolayers of Au-nanoparticles.

Structures of Langmuir-Schaefer (LS) monolayers of thiol-coated Au-nanoparticles (DT-AuNPs) deposited on H-terminated and OTS self-assembled Si substrates (of different hydrophobic strength and stability) and their evolution with time under ambient conditions, which plays an important role for their practical use as 2D-nanostructures over large areas, were investigated using the X-ray reflectivity technique. The strong effect of substrate surface energy (γ) on the initial structures and the competitive role of room temperature thermal energy (kT) and the change in interfacial energy (Δγ) at ambient conditions on the evolution and final structures of the DT-AuNP LS monolayers are evident. The strong-hydrophobic OTS-Si substrate, during transfer, seems to induce strong attraction towards hydrophobic DT-AuNPs on hydrophilic (repulsive) water to form vertically compact partially covered (with voids) monolayer structures (of perfect monolayer thickness) at low pressure and nearly covered buckled monolayer structures (of enhanced monolayer thickness) at high pressure. After transfer, the small kT-energy (in absence of repulsive water) probably fluctuates the DT-AuNPs to form vertically expanded monolayer structures, through systematic exponential growth with time. The effect is prominent for the film deposited at low pressure, where the initial film-coverage and film-thickness are low. On the other hand, the weak-hydrophobic H-Si substrate, during transfer, appears to induce optimum attraction towards DT-AuNPs to better mimic the Langmuir monolayer structures on it. After transfer, the change in the substrate surface nature, from weak-hydrophobic to weak-hydrophilic with time (i.e. Δγ-energy, apart from the kT-energy), enhances the size of the voids and weakens the monolayer/bilayer structure to form a similar expanded monolayer structure, the thickness of which is probably optimized by the available thermal energy.

Chitosan-fatty acid interaction mediated growth of Langmuir monolayer and Langmuir-Blodgett films.

The interaction of chitosan with bio-membranes, which plays important role in deciding its use in biological applications, is realized by investigating the interaction of chitosan with stearic acid (fatty acid) in Langmuir monolayers (at air-water interface) and Langmuir-Blodgett (LB) films (after transferring it onto solid substrate). It is found from the pressure-area isotherms that the chitosan insertion causes an expansion of chitosan-fatty acid hybrid monolayers, which reduces the elasticity and make the film heterogeneous. It is likely that at low surface pressure chitosan is situated at the interface, interacting with stearic acid molecules via electrostatic and hydrophobic interactions whereas at high pressure chitosan mainly located at subsurface beneath stearic acid molecules. In the latter case the interaction is predominantly electrostatic yielding very small contribution to the surface pressure. The reduction of temperature of the subphase water allows more number of chitosan molecules to reach surface to increase the pressure/interaction. On the other hand, although pure chitosan is found difficult to relocate on the substrate from air-water interface due to its hydrophilic-like nature, it alongside stearic acid (amphiphilic molecules) can be transferred onto substrate using LB technique as evident from infrared spectra. Their out-of-plane and in-plane structures, as extracted from two complementary surface sensitive techniques- X-ray reflectivity and atomic force microscopy, are found strongly dependent on the chitosan mole fraction and the deposition pressure. These analysis of the film-structure will essentially allow one to model the system better and provide better insight into the interaction.