Nucleation in atomic, molecular, and colloidal systems
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The emerging science of materials synthesis Research directed toward the predictive science of materials has been largely focused on understanding and manipulating the relationship between structure and function. The goal has been to predict “where the atoms should be placed” in order to achieve a set of properties. In contrast, making materials has generally been pursued through Edisonian approaches, sometimes with the aid of combinatorial techniques. Much less effort has been directed toward the predictive science of materials synthesis, that is, understanding “how to get the atoms where they need to go” in order to achieve a specified structure. In recent years, the advent of a suite of in situ characterization techniques that can probe synthetic processes at the molecular-to-nanometer scale,1 and computational approaches that can simulate processes of cluster formation and particle assembly,2 has altered the research landscape, bringing efforts to develop a predictive understanding of synthesis to the forefront. Because nucleation is the seminal event in materials formation, much of the research has focused on this process.3 Ordered assemblies of small molecules, macromolecules, or particles that form in solutions from their component parts via the processes of nucleation are materials of major scientific and industrial importance. Such materials include nanowires, nanoparticles and their superlattices, crystalline optical
materials and scintillators, porous framework materials, protein crystals, and pharmaceuticals. In order to fabricate better versions of these materials and to predict conditions under which new materials will form, an understanding of the microscopic dynamics of nucleation is crucial. Although nucleation is a topic whose study dates back to the days of Gibbs,4 much of what researchers thought they knew in the last century has been called into question thanks to recent in situ and computational studies. Multistep pathways3 involving polynuclear clusters and metastable crystalline, amorphous, or dense liquid states are just some of the phenomena being explored today. These pathways were never envisioned in the classical theory of nucleation.
Adding complexity to the classical theory of nucleation Certain basic considerations apply to essentially all molecular systems undergoing nucleation. Moving molecules from the solution into a cluster of molecules (Figure 1a) gives rise to a decrease in free energy that is proportional to the number of molecules in the cluster, scaling with the volume of the cluster.5 However, the cluster–liquid interface is, in general, thermodynamically costly, because molecules that sit at the interface possess less entropy than molecules in solution and less favorable energy than molecules in the bulk of the cluster. These factors result in a surface tension or free-energy penalty
Jim De Yoreo, Physical Sciences Division, Pacific Northwest National Laboratory, USA; [email protected] Stephen Whitelam, Molecular Foundry, Lawrence Berkeley National Laboratory, USA; s
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