theory and simulation in understanding the nanoscale



ChemNEWS editor, Dr Mat Todd, sat down with ARC Postdoctoral Research Fellow, Dr Asaph Widmer-Cooper to talk about Asaph's research in the School of Chemistry.

"Complex fluids are fascinating materials. They include the liquid crystals in your computer screen, the colloidal suspensions that give birth to opals, and the soapy solutions that we use to wash the dishes. I want to understand the beautiful structures that spontaneously appear in these systems, how this affects their properties, and how to control their formation." - Asaph Widmer-Cooper


What made you interested in complex fluids?

I have been fascinated with how materials order themselves and how this affects their properties since I was an undergraduate: from the folding of proteins to the structure of surfactant phases and abalone shells. This naturally combined with an interest in mathematics and programming and guided me to the type of research that I am doing now. More recently, my work has been motivated by a desire to find more sustainable ways for us to live on this planet, so that the natural environment on which we all depend can continue to sustain us.

What are you working on at the moment?

I am spending most of my time on three projects: Understanding how to direct the assembly of rod-shaped nanoparticles into structures that will be useful for converting solar energy into electricity and fuels; Understanding how polyhedral silver nanoparticles grow and self-assemble in order to make new materials with potential applications in sensing and catalysis; And understanding how long-lived spatial variations in structure affect the way that viscous materials, like honey and molten glass, flow (see Fig. 1). In contrast to normal liquids, long-lived aggregates can appear in these systems driven by intermolecular and surface forces, and remarkably also by an increase in disorder of the system as a whole.

Figure 1

The pattern of structurally soft regions in a glass-forming liquid.

For more information abut this work see [1]

What are nanoparticles and why is their assembly an exciting area to work in at the moment?

Nanoparticles are tiny inorganic crystals that are usually grown in solution in the presence of molecules like surfactants and adsorbing polymers that bind to their surfaces and help keep them dispersed. These molecules can also be used to direct the growth of the crystal away from its natural shape, which is actually very similar to what happens in biological mineralisation like shell growth where proteins play a similar role. Because of their small size and high surface to volume ratio nanoparticles typically have quite different properties from crystals that you can see with the naked eye. People can now make such particles in a staggering range of sizes, shapes, patterns and materials (see Fig. 2 for some examples), and incredible advances have been made in controlling their optical and electronic properties. However, organizing them into extended structures that could revolutionize technology remains a challenge that is just starting to be understood. One of the major difficulties is that nanoparticles sit between the atomistic and mesoscopic scales, and typically have both molecular and crystalline components. Their interactions and self-assembly therefore depend on both bulk and surface properties as well as fluctuation effects on multiple lengthscales. This can lead to complex phase behavior – reminiscent of liquid crystals in the case of non-spherical particles - and dependence on external driving forces like temperature, gravity, electric fields, and solvent evaporation. Of course this also means that a wide range of driving forces can be manipulated to direct their assembly, which makes this a fascinating area to work in, as well as one that can benefit from close interaction between theoretical studies of simplified models and detailed experiments.

Figure 2

(left) Simulation image of a 4x20nm CdS nanocrystal covered in ligands and (right) scanning electron microscopy images of silver nano-polyhedra (scale bar is 100nm, images courtesy of Dr Joel Henzie, UC Berkeley)

Tell me more about your research in this area.

Among the barriers to making cheaper solar cells is the high cost of the single crystalline silicon and vapor deposition methods commonly used today. One possible solution is to print solar cells using an ink of semiconducting nanoparticles. This may also allow for the use of new materials, such as pyrite (fool's gold), and harnessing quantum effects to improve performance. To this end we are using self-assembly to make large 'carpets' of aligned nanorods. This involves understanding how the nanorods interact with one another and with interfaces, how this influences their phase behavior, and how kinetics affects their aggregation and ordering. By combining insight from theory and experiments we were recently able to report the first cm2 films and explain why film formation works better on some substrates [2]. We are now exploring better ways to make these films using amphiphilic rods (see Fig. 3).

Figure 3

Assembling semiconducting nanorods into monolayer 'carpets' that can serve as the basis for printable solar cells and photoelectrochemical devices


Another project involves understanding how to control the self-assembly of silver nano-polyhedra into 3D superlattices – i.e. crystals built from crystals - in order to make materials with possible application in photocatalysis, chemical and biological sensing, and as contrast agents for surface enhanced Raman spectroscopy. When light strikes a material like silver that has delocalised electrons it can induce electron density waves on the surface. This can lead to absorption at selective wavelengths and very strong enhancement of local electromagnetic fields. By varying the particle shape, their spacing, and how they are packed, it should be possible to make materials with unique and tunable properties. Recently we demonstrated that it is possible to make highly uniform mm-sized superlattices using a relatively simple sedimentation process. We explained the forces that must be balanced to achieve this, and showed that it is possible to make the densest known packings of a wide range of polyhedral shapes including cubes, cubeoctahedra, and octahedra [3]. Finally, we showed that it is possible to change the crystal lattice by introducing polymers into the solution, and discovered an entirely new packing of octahedra with complex helical motifs (Fig. 4). We are now working on understanding how to control the growth of the particles in order to make any shape in any size, and on making porous structures using mixtures of oppositely charged particles.

Figure 4

A new packing of octahedra formed by the self-assembly of silver nanoparticles

As a theoretician what does your work typically involve?

This typically involves building simplified models using chemical insight and mathematics and investigating the properties of these models using the tools of statistical mechanics and computer simulation. By comparing our results with detailed experiments in an iterative manner, we can tease out the dominant interactions and driving forces that are responsible for the observed behavior and make real progress in this way. This process, where fascinating experiments inspire our work, and where our results in turn inspire new experiments, is something that I really enjoy. By working closely with experimentalists, often at the interface between disciplines, I am constantly learning new things, being stimulated by new ideas, and sometimes have the pleasure of seeing my theoretical insight make a direct impact in the real world. For example, we have been able to suggest changes in solution composition for assembly experiments and reactant ratios for nanoparticle synthesis that have led to increased yield of novel assemblies and greater control of particle size and shape. Interactions like these make research not only exciting, but also fun.

References
  1. Widmer-Cooper, A; Perry, H; Harrowell, P and Reichman, DR. Irreversible reorganization in a supercooled liquid originates from localized soft modes. Nature Physics, 4 (2008) 711. DOI: 10.1038/nphys1025
  2. Baker, JL; Widmer-Cooper, A; Toney, M; Geissler, P and Alivisatos, AP. Device-scale perpendicular alignment of colloidal nanorods. Nano Lett., 10 (2010) 195. DOI: 10.1021/nl903187v
  3. Henzie, J; Grünwald, M; Widmer-Cooper, A; Geissler, PL and Yang, P. Self-assembly of uniform polyhedral silver nanocrystals into densest packings and exotic superlattices, submitted (2011).
Biography

Dr Asaph Widmer-Cooper graduated from The University of Sydney in 2000 with Honours in Chemistry and a University Medal. After 6 months of traveling around the world, he spent a year as a DAAD Fellow working at the German National Research Center for Information Science in Bonn, before returning to The University of Sydney for postgraduate study, completing his PhD on structure and dynamics in glass-forming materials in 2006.

He then spent a year working part-time as a research assistant while exploring his interest in sustainable development, studying environmental management, helping analyse the environmental impact of The University of Sydney's operations, and contributing to research on sustainability indicators.

In 2008 he took up a postdoctoral fellowship at the University of California at Berkeley and Lawrence Berkeley National Lab, where he spent 2.5 years as part of the Helios Solar Energy Research Center, before commencing his independent career in late 2010 with the award of an ARC Australian Postdoctoral Fellowship. Asaph's research interests include the structure and dynamics of complex fluids, nucleation, self-assembly and crystal growth, and statistical thermodynamics.
When not working he can usually be found climbing up a rock, skiing down a mountain, or exploring canyons in the Blue Mountains.

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