Time-structuring in the evolution of 2D nanopatterns through interactions with substrate

Hydrophobic dodecanethiol capped gold nanoparticles (AuNPs) are found to self-assemble into two-dimensional patterns in monolayers of amphiphiles spread at the air-water interface of a Langmuir trough. In this communication we investigate the role of the nanoparticle-monolayer (FNMA) and monolayer-monolayer (FMMA) lipophilic attraction in influencing morphology and dynamics of AuNP cluster patterns in fatty acid monolayers. FNMA and FMMA are progressively varied by changing n, where n is the number of -CH2 groups in the alkyl tails of the amphiphilic fatty acid (CH3(CH2)nCOOH) molecules forming the monolayer. Compressibility measurements on the pristine and nanoparticle-laden monolayers show that, while the compressibility of the pristine monolayer decreases with increasing n, pointing to a progressive increase in FMMA, the effect of nanoparticles (increase in compressibility or lowering of FMMA) is discernible only for 14 < n < 22. The corresponding pattern morphology, observed with a Brewster Angle Microscope (BAM) at an in-plane resolution of 450 nm for 6 hours, reveals that there are essentially three stages in pattern evolution, lamellae of Au nanoclusters spread over the fatty-acid monolayer background (the λ state) followed by a network of nanoclusters with high node density (the ν state) and finally rings (circular/elongated) of random sizes with very low node density (the ρ state), evolving from an initial unsegregated state, without appreciable change in the average nanoparticle number density over the field of view. Increasing FNMA alongwith FMMA is found to shift a certain state to later times, thus playing the role of a viscous drag and introducing a delay in the timeline. The mean square fluctuation of BAM intensity remains flat and then decays as f(ξ) = ξ(2H) over smaller length scales, where ξ is the spatial separation and H the Hurst exponent. The study of f(ξ) over time reveals the growth of a sub-diffusive regime (H < 0.5) at the intermediate length scale, in almost all the films coinciding with the emergence of the ρ state. The growth of this sub-diffusive regime is slower for stronger FNMA and FMMA, the interactions thus acting as control parameters in dictating the time structure of the spatio-temporal patterns.