(5cj) Bottom-up Self-Assembly of Novel Materials and Nanoscale Surface Patterning

Some well known methods for obtaining nanopatterned surfaces through self-assembly are block copolymer phase separation, and directed and templated self-assemblies. We explore phase-separation in self-assembled monolayers, or SAMs, as a possible method for obtaining patterned surfaces. This method exploits immiscibility between two types of molecules combined with a long range repulsion that prevents the two components from bulk phase separating. The source of the long range repulsion is entropic in nature and leads to preference of longer or bulkier molecules to stack next to shorter (or less bulky) molecules despite being immiscible with them [1]. The immiscibility drives phase separation as in block copolymers but is different in that the molecules are grafted at one end on the surface and immiscibility is between two different types of molecules rather than between two sections of the same molecule.

We have performed simulation studies of different compositions of grafted molecular mixtures with varying immiscibilities on flat [1,2], spherical [1] and cylindrical [3,4] surfaces. We have obtained nanoscale domains that are shaped as stripes, spherical patches, elongated patches and combinations of these. In some cases we also find complete bulk phase separation or no phase separation. We have predicted that the shapes and feature sizes of these patterns can be changed by tuning parameters such as immiscibility, composition, surface curvature etc. Some of these predictions have recently been confirmed by experimental studies. We demonstrate how surface stress in the form of nanoscale curvature aids in the formation of stripes as opposed to patches [1-4].

This method for obtaining nanopatterned surfaces can provide a powerful tool for design and fabrication of new nano building blocks [4-5]. Furthermore, these patterns also form on flat substrates [2], with applications in the fields of catalysis, electronics, ceramics and biosensing.

1. C Singh, PK Ghorai, MA Horsch, AM Jackson, RG Larson, F Stellacci and SC Glotzer, ?Entropy-Mediated Patterning of Surfactant-Coated Nanoparticles and Surfaces?, Physical Review Letters 99, 226106 (2007).

2. C Singh, K Kuwabara, F Stellacci and SC Glotzer, ?Nanoscale Striped vs. Patchy Phase Separation in Mixed Self-Assembled Monolayers?, preprint.

3. C Singh, AM Jackson, F Stellacci and SC Glotzer, ?Exploiting Substrate Stress to Modify Nanoscale Phase-Separated Patterns?, preprint.

4. C Singh, Y Hu, F Stellacci and SC Glotzer, ?Striped Phase Separation on Nanowires and Nanotubes?, preprint.

5. AM Jackson, JW Myerson, F Stellacci, ?Spontaneous assembly of subnanometre-ordered domains in the ligand shell of monolayer-protected nanoparticles?, Nature Materials, 3, 330-336 (2004).

6. RP Carney, GA DeVries, C Dubois, H Kim, JY Kim, C Singh, PK Ghorai, JB Tracy, RL Stiles, RW Murray, SC Glotzer and F Stellacci, "Size Limitations for the Formation of Ordered Striped Nanoparticles", Journal of American Chemical Society, (Communication), 130(3), 798-799 (2008).