The fabrication of functional nanostructured materials requires practical approaches to self-assembly on multiple length scales. Here, the directed self-assembly of functionalized, luminescent nanoparticles at oil-water interface, followed by crosslinking of the associated ligands, affords robust membranes. These composite membranes, nanometers in thickness, are shown to be effective diffusion barriers that have potential applications in controlled encapsulation and release. Cadmium selenide (CdSe) nanoparticles were used, since the photoluminescence of the particles provides a convenient means to monitor the spatial organization of the nanoparticles. The retention of the CdSe photoluminescence demonstrates that the nanoparticles remain unchanged during the manipulation. The concepts shown are non-specific and can be adapted to any nanoparticle and solvent combinations. (Y. Lin, H. Skaff, T. Emrick, A. D. Dinsmore and T. P. Russell, "A Membrane of Cross-Linked Nanoparticles," J. Am. Chem. Soc. 125, 12690-12691 (2003)).

Single Molecule Microscopy

Using single molecule microscopy, an equilibrium configurational description is obtained for flexible polymers confined in molecule-sized pores of defined geometry. Visualizations reveal the dynamic exchange of DNA molecules between adjacent spherical cavities connected by a small opening. At moderate confinements, measured partition coefficients closely track predictions made without fitting parameters for excluded volume chains. At strong confinement, excluded volume creates stable two-cavity “straddling” configurations. (D. Nykypanchuk, H.H. Strey, and D.A. Hoagland, “Brownian Motion of DNA Confined Within a Two-Dimensional Array,” Science 297, 987-990 (2002)).

 Surface Reconstructions in Thin Copolymer Films

Thin films of block copolymers have been used as templates and scaffolds for the fabrication of arrays of nanostructured materials. The preferential interaction of the minor component with a solvent is shown to produce a reconstruction of the block copolymer film, which, upon drying, produces a nanoporous template. The areal density of the pores is identical to that achieved by alternate routes, though the pores are slightly smaller. Since no chemical transformation of the components has occurred, the process is fully reversible. Upon heating the copolymer film above its glass transition temperature, mobility is imparted to the copolymer and the original copolymer film with oriented domains is recovered. The film reconstruction significantly simplifies the generation of nanoporous templates. (T. Xu, J. Stevens, J. Villa, J. Goldbach, K. W. Guarini, C. J. Black, C. Hawker and T. P. Russell, "Block Copolymer Surface Reconstruction: A Reversible Route to Nanoporous Block Copolymer Films," Adv. Func. Mat. 13, 698-702 (2003).).