Simulation of DNA- and Polymer-Mediated Nanoscale Assembly
An impressive variety of nano building blocks--including nanospheres, nanorods, nanocubes, nanoplates, nanotetrapods, and nanoprisms-- exists and continues to grow with breakthroughs in synthesis techniques. The application of nanotechnology to areas such as photonics and electronics, chemical and biological sensors, energy storage and catalysis requires the manipulation of these nano-objects into functional materials and devices, and this remains a fundamental challenge. Self-assembly is generally regarded as the most promising means for designing and controlling bottom-up assembly of nanometer-scale objects into structures such as sheets, tubes, wires, and shells needed as scaffolds and structures for catalysis, hydrogen storage, nanoelectronic devices, and drug delivery. While many nanoparticle assembly demonstrations have appeared in the literature, few approaches offer a comprehensive, predictable, and generally applicable scheme. Increasingly, synthetic chemists are turning their attention to the functionalization of nano building blocks (both nanocrystals and supramolecular entities) with flexible oligomeric, polymeric, and biomolecular tethers with specific and non-specific interactions, to direct their assembly. We have begun a comprehensive simulation study of self-assembly in which we consider tethered nano building blocks as a new class of “macromolecule” with which to control nanoparticle assembly for new nanomaterials. We are exploring how tuning thermodynamic parameters and architectural features of the nano building blocks can control aspects of local and global ordering of the nanoparticles, and how the additional packing constraints introduced by the nanoparticle geometry and the nano building block topology, combined with tether and nanoparticle immiscibility, lead to structures far richer than those known for conventional block copolymer, surfactant, and liquid crystal systems.

*Funded by the Department of Energy

Theory and Simulation of Patchy Particles
We are carrying out an exploratory investigation of the effectiveness of different molecular and mesoscale simulation methodologies for simulating self-assembly of arrays of nanoparticles functionalized by biomolecules. The highly specific, “lock-and-key” recognitive interaction capability of biomolecules like DNA, proteins, and antibodies makes them ideal for directing the assembly of gold nanoparticles, quantum dots, Buckyballs, nanorods, and other nano building blocks into higher order structures needed for applications such as sensors, catalysts, photonics, molecular electronics, and materials design.

Integrated Multiscale Modeling of Molecular Computing Devices
A major challenge in the fabrication of molecular electronics (ME) devices is the nanopatterning of millions of molecular wires necessary in constructing transistors and other devices to realize complex circuitry. Two approaches to ordering molecules into arrays are nanolithography and SA. While nanolithography may ultimately attain pattern sizes near 10 nm, 1-2 nm patterns containing well separated, individual molecular wires are not expected to be achieved with this approach. Instead, self-assembly approaches in which thiolated molecules tightly bind and form well-ordered arrays on gold surfaces is being pursued for fabrication of (ME) devices. The details of the assembly process and the resulting patterns will depend intimately on the interplay between entropic and energetic forces between the molecules and between the molecules and substrate. By appropriately tuning these forces, different nanopatterns that persist over large distances and compartmentalize millions of molecular wires can potentially be obtained. Key to fabrication processes relying on self-assembly is strict control over the assembled structures. ME devices involving chemical assembly in the fabrication process are likely to be imperfect, resulting in potential defects that, if excessive, could cause failure of the device. For this reason, is it important to quantify the degree of order attained by various assembly routes, and the propensity for certain assembly methods to produce defect-free or low-defect ordered structures. As ME devices become increasingly complex, interfacing with arrays of quantum dots and other nanocomponents to yield complex nanoelectronic machines, quantification of order and the development of strategies to control order will become increasingly important.

We are collaborating with researchers at Vanderbilt University, North Carolina State University, Princeton and Oakridge National Laboratory to develop multiscale modeling and simulation methods capable of modeling the synthesis, assembly, and operation of molecular electronics devices. Our role in this project is the development of coarse-grained molecular and mesoscale models and simulation methods capable of simulating the assembly of millions of organic conducting molecules and other molecular components into nanowires, crossbars, and other organized patterns.

*Funded by the Department of Energy.

Surfactant Assembly on Nanostructured Surfaces
We are developing multiscale simulation approaches to investigate the assembly behavior of surfactants on nanostructured surfaces including nanotubes and Buckyballs. This work has application to the design and synthesis of patchy particles -- particles with designed patterns of interactions that mediate the forces between particles so as to induce assembly into target structures. It also has application to the surfactant-mediated nanocrystal growth and nanoparticle and nanotube dispersion. Collaborators include Keith Gubbins, Jerry Bernholc and Don Brenner at NCSU.
*Funded by the National Science Foundation under the Nanoscale Interdisciplinary Research Teams (NIRT) program.