Andrew Stannard

Dewetting-mediated pattern formation in nanoparticle assemblies.

The deposition of nanoparticles from solution onto solid substrates is a diverse subfield of current nanoscience research. Complex physical and chemical processes underpin the self-assembly and self-organization of colloidal nanoparticles at two-phase (solid-liquid, liquid-air) interfaces and three-phase (solid-liquid-air) contact lines. This review discusses key recent advances made in the understanding of nonequilibrium dewetting processes of nanoparticle-containing solutions, detailing how such an apparently simple experimental system can give rise to such a strikingly varied palette of two-dimensional self-organized nanoparticle array morphologies. Patterns discussed include worm-like domains, cellular networks, microscale rings, and fractal-like fingering structures. There remain many unresolved issues regarding the role of the solvent dewetting dynamics in assembly processes of this type, with a significant focus on how dewetting can be coerced to produce nanoparticle arrays with desirable characteristics such as long-range order. In addition to these topics, methods developed to control nanofluid dewetting through routes such as confining the geometries of drying solutions, depositing onto pre-patterned heterogeneous substrates, and post-dewetting pattern evolution via local or global manipulation are covered.

Front instabilities in evaporatively dewetting nanofluids

Various experimental settings that involve drying solutions or suspensions of nanoparticles-often called nanofluids-have recently been used to produce structured nanoparticle layers. In addition to the formation of polygonal networks and spinodal-like patterns, the occurrence of branched structures has been reported. After reviewing the experimental results we use a modified version of the Monte Carlo model first introduced by Rabani [Nature 426, 271 (2003)] to study structure formation in evaporating films of nanoparticle solutions for the case that all structuring is driven by the interplay of evaporating solvent and diffusing nanoparticles. After introducing the model and its general behavior we focus on receding dewetting fronts which are initially straight but develop a transverse fingering instability. We analyze the dependence of the characteristics of the resulting branching patterns on the driving effective chemical potential, the mobility and concentration of the nanoparticles, and the interaction strength between liquid and nanoparticles. This allows us to understand the underlying instability mechanism.

Broken symmetry and the variation of critical properties in the phase behaviour of supramolecular rhombus tilings

The tiling of surfaces has long attracted the attention of scientists, not only because it is intriguing intrinsically, but also as a way to control the properties of surfaces. However, although random tiling networks are studied increasingly, their degree of randomness (or partial order) has remained notoriously difficult to control, in common with other supramolecular systems. Here we show that the random organization of a two-dimensional supramolecular array of isophthalate tetracarboxylic acids varies with subtle chemical changes in the system. We quantify this variation using an order parameter and reveal a phase behaviour that is consistent with long-standing theoretical studies on random tiling. The balance between order and randomness is driven by small differences in intermolecular interaction energies, which can be related by numerical simulations to the experimentally measured order parameter. Significant variations occur with very small energy differences, which highlights the delicate balance between entropic and energetic effects in complex self-assembly processes.

Molecular random tilings as glasses

We have recently shown that p-terphenyl-3,5,3′,5′-tetracarboxylic acid adsorbed on graphite self-assembles into a two-dimensional rhombus random tiling. This tiling is close to ideal, displaying long-range correlations punctuated by sparse localized tiling defects. In this article we explore the analogy between dynamic arrest in this type of random tilings and that of structural glasses. We show that the structural relaxation of these systems is via the propagation–reaction of tiling defects, giving rise to dynamic heterogeneity. We study the scaling properties of the dynamics and discuss connections with kinetically constrained models of glasses.

Mechanical Stiffening of Porphyrin Nanorings through Supramolecular Columnar Stacking

Solvent-induced aggregates of nanoring cyclic polymers may be transferred by electrospray deposition to a surface where they adsorb as three-dimensional columnar stacks. The observed stack height varies from single rings to four stacked rings with a layer spacing of 0.32 ± 0.04 nm as measured using scanning tunneling microscopy. The flexibility of the nanorings results in distortions from a circular shape, and we show, through a comparison with Monte Carlo simulations, that the bending stiffness increases linearly with the stack height. Our results show that noncovalent interactions may be used to control the shape and mechanical properties of artificial macromolecular aggregates offering a new route to solvent-induced control of two-dimensional supramolecular organization.

Random-walk statistics and the spherical harmonic representation of cosmic microwave background maps

We investigate the properties of the (complex) coefficients obtained in a spherical harmonic representation of temperature maps of the cosmic microwave background (CMB). We study the effect of the coefficient phase only, as well as the combined effects of phase and amplitude. The method used to check for anomalies is to construct a ‘random walk’ trajectory in the complex plane where the step length and direction are given by the amplitude and phase (respectively) of the harmonic coefficient. If the fluctuations comprise a homogeneous and isotropic Gaussian random field on the sky, the path so obtained should be a classical ‘Rayleigh flight’ with very well known statistical properties. We illustrate the use of this random-walk representation by using the net walk length as a test statistic, and apply the method to the coefficients obtained from a Wilkinson Microwave Anisotropy Probe (WMAP) preliminary sky temperature map. Ke yw ords: galaxies: clustering ‐ cosmology: theory ‐ large-scale structure of the Universe.

Dewetting of Au nanoparticle assemblies

Atomic force microscopy measurements as a function of annealing temperature, time of exposure to a high relative humidity environment, and scan duration/parameters have been used to ascertain the stability of assemblies of thiol-passivated Au nanoparticles on silicon substrates. Striking changes in the morphology of self-organised nanoparticle patterns are observed following the exposure of samples to atmospheres with a relative humidity of 80%. The nanoparticle film dewets the underlying silicon substrate on exposure to water, forming locally thicker regions. Time-lapse imaging shows that the dewetting proceeds via layer-by-layer growth, and there is no evidence for classical coarsening mechanisms involving self-similar film morphologies. Annealing at temperatures between 100 °C and 160 °C produces a rather different dewetting effect for the highest temperatures and/or annealing times, where significant nanoparticle sintering promotes the break-up of the two-dimensional assembly. The morphology of the initial 2D film plays a key role in determining the time scale on which annealing promotes nanoparticle dewetting. Dewetting can also be induced by a scanning probe such that localised (micron-scale) areas of the nanoparticle assembly can be converted from 2D to 3D character.

Uncertainties in forces extracted from non-contact atomic force microscopy measurements by fitting of long-range background forces

Summary In principle, non-contact atomic force microscopy (NC-AFM) now readily allows for the measurement of forces with sub-nanonewton precision on the atomic scale. In practice, however, the extraction of the often desired ‘short-range’ force from the experimental observable (frequency shift) is often far from trivial. In most cases there is a significant contribution to the total tip–sample force due to non-site-specific van der Waals and electrostatic forces. Typically, the contribution from these forces must be removed before the results of the experiment can be successfully interpreted, often by comparison to density functional theory calculations. In this paper we compare the ‘on-minus-off’ method for extracting site-specific forces to a commonly used extrapolation method modelling the long-range forces using a simple power law. By examining the behaviour of the fitting method in the case of two radically different interaction potentials we show that significant uncertainties in the final extracted forces may result from use of the extrapolation method.

Directing the formation of nanostructured rings via local oxidation.

We provide compelling evidence that ring formation in solutions of thiol-passivated Au nanoparticles is driven by breath figure dynamics. A method for the controlled placement of rings of nanoparticles on a solid substrate, which exploits variations in substrate wettability to fix the positions of the submicrometer water droplets formed in the breath figure process, has been developed. This is achieved by heterogeneously patterning hydrogen-terminated silicon substrates with oxide regions that act as adsorption sites for the droplets. The droplets in turn template the formation of thiol-passivated Au nanoparticle rings during spin-casting from volatile solvents.

Measuring Si-C60 chemical forces via single molecule spectroscopy

We measure the short-range chemical force between a silicon-terminated tip and individual adsorbed C(60) molecules using frequency modulation atomic force microscopy. The interaction with an adsorbed fullerene is sufficiently strong to drive significant atomic rearrangement of tip structures.