Template-directed Molecular Self-Assembly

Due to its ubiquitous nature, van der Waals (vdW) interactions account for a significant part of free energies in systems such as solid-liquid interfaces, noble-gas atoms, bio-molecules, physisorption, liquid crystals, solute-solvent dynamics and, broadly speaking, soft-condensed matter. While one can write down a suitable form of second-order perturbation theory for the interaction between two well-separated neutral atoms, a macroscopic continuum theory also exists to compute vdW forces in simple geometries \cite{Lifshitz56}, an efficient and accurate description of vdW interactions remains elusive when two systems of irregular shapes are overlapped or separated by a short distance. Among widely used methods in molecular simulations, some fail to capture the essence of vdW energies (e.g., LDA and GGA within density-functional theory), and others are based on pairwise summation premise, which ignores important many-body effects in dense media. Indeed, the lack of quantitatively reliable predictive method has yet to be remedied; fundamental work is needed.

Surfactant micelles spontaneously adsorb on a substrate with orientational order dictated by the crystal structure. In particular, on a gold surface it happens despite the screening effects of delocalized electron clouds in metallic systems. To understand the van der Waals forces that provide organization on metallic substrates, we developed a formalism wherein the dielectric response acquires directional dependence through phonon dispersion relations. In metals, ionic screening is enhanced along certain directions and a crystalline metallic substrate generates both torque and attraction on geometrically asymmetric objects. The advance could lead to a new `bottom-up' tool in the nanofabrication and give scientists refined control over how nanoscale building blocks assemble on a substrate.