Pauliac-Vaujour, Emmanuelle
(2008)
Patterns and instabilities in colloidal nanoparticle assemblies.
PhD thesis, University of Nottingham.
Abstract
Colloidal nanoparticles exhibit unusual individual and collective behaviour, often associated with interesting electrical, optical or electromagnetic properties. Thiol-passivated colloidal gold nanoparticles possess in addition a self-organising property, which, when the particles are deposited on a substrate, yields a plethora of fascinating patterns. The conditions of formation of these patterns are investigated, in order to understand the principles of - and gain control over - non-equilibrium self-organisation following drop evaporation.
The work presented in this thesis relies mostly on experimental observations, although the results are supported by numerical simulations carried out in the group and based on modified versions of the model developed by Rabani et al. in 2003 [1]. A novel deposition method is introduced, which provides controllable conditions for the occurrence of a wide variety of patterns, including close-packed monolayers of nanoparticles. Pattern and surface characterisation is achieved by combined microscopy techniques - atomic force microscopy (AFM) and real-time contrast-enhanced optical microscopy. The influence on pattern formation of the nanoparticle-solvent-substrate interactions is studied by altering the physical properties of all three components (substrate, solvent and nanoparticles).
The experimental set-up allows a meniscus-driven evaporation of the solvent of the nanoparticle solution and enables monitoring of drying front instabilities during the dewetting process. The effects of these instabilities on pattern formation are investigated and highlight a strong contribution of free excess ligands.
We have focused on two specific types of patterns which emerge in these experiments : fingering structures and nanoparticle rings. The former are reminiscent of patterns that form in a number of other systems, a process usually called "viscous fingering". A thorough investigation reveals that the mechanism of formation of such patterns involves the combination of specific experimental conditions and at least two different dewetting processes, with different time and length scales. A "pseudo-3D" Monte Carlo model recreates such conditions and yields simulated results which are in good qualitative and quantitative agreement with experimental results. On the other hand, nanoparticle rings, although they are a recurrent type of pattern observed in nanoparticle assemblies [2, 3], form according to a mechanism which is not yet fully understood. We show however that wetting properties play a central role in ring formation and growth. As in the case of fingering structures, a narrow range of parameters has been determined, via an exhaustive experimental investigation, which favours the occurrence of nanoparticle rings.
For all the nanoparticle assemblies studied in this thesis (close-packed monolayers, fingering structures and nanoparticle rings), the deduction of pattern formation mechanisms from experimental observation (and simulations) relies on the very high degree of reproducibility that it is possible to attain using the combination of a meniscus-driven evaporation, a very fine tuning of experimental conditions and nanoparticle-solvent-substrate interactions, and a systematic cross-characterisation by complementary imaging techniques.
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