RESEARCH NUGGETS
Interfacial Assembly of Nanoparticles
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).).
PARTNERSHIP NUGGETS
Solvent-Induced Ordering in Thin Films of Diblock Copolymer/Homopolymer Mixtures
Using solvent evaporation, long-range lateral order over many square microns has been achieved in mixtures of homopolymers with asymmetric diblock copolymers having a cylindrical microdomain morphology. Arrays of cylindrical microdomains ~20 nm in diameter with average separation distances of ~40 nm with near-perfect hexagonal packing in films several tenths of a micron in thickness were found. These results point to a simple route to produce nanoscopic templates for the generation of addressable media. (S. H. Kim, M. J. Misner, M. Kimura, T. Xu and T. P. Russell, "Highly Oriented and Ordered Arrays from Block Copolymers Via Solvent Evaporation," Adv. Mats. 16(3), 226-231 (2004).).
Flash Memory
Block copolymers, two distinct polymers joined at one end, self-assemble into well-defined, well-ordered arrays of nanoscopic domains where the type of domain (spheres, cylinders or sheets) depends on the relative volume fraction of the components. For spincoated films, by balancing the interactions of each block with the substrate, by attaching random copolymers to the underlying substrate to mediate interfacial interactions. the With cylindrical domains, by removing one of the block, e.g. by exposure to ultraviolet radiation, a densely packed array of nanopores (1.2 Terapores/in2) is produced in a crosslinked thin polymer films that serves as templates where, as shown in the image, using established ion beam etching techniques pattern transfer. This simple process is being used by IBM to produce arrays of floating gates in flash memory. (IBM Press Release (2004)).
EDUCATION NUGGETS
Decorated Breath Figures
The formation of so-called “breath figures” by condensation of micron-sized water droplets on the surface of a polymer solution, and the self-assembly of CdSe nanoparticles at the polymer solution-water droplet interface have been combined to form hierarchically structured nanoparticle arrays. Complete evaporation of the solvent and water forms an array of nanoparticle decorated spherical cavities and allows for ex-situ investigation. This process opens new ways for the generation of highly functionalized ordered microarrays of nanoparticles, potentially useful for applications such as sensors, separation membranes or catalysis. (A. Böker, Y. Lin, K. Chiapperini, R. Horowitz, M. Thompson, V. Carreon, T. Xu, C. Abetz, H. Skaff, A. D. Dinsmore, T. Emrick and T. P. Russell, "Nanoparticle Self-Assembly in Polymer Film by Solvent Evaporation," Nature Mats. advance online publication(18/04/04 (doi: 10.1038/nmat1110).) (2004)).
VISUAL
The recently launched VISUAL program, where images obtained by researchers at the Center in the course of their studies, are framed along with a description of the image in plain English, has received nationally acclaim from Nikon in their Small World Exhibition. One image (Frazzled) from the VISUAL program has been included within the Nikon 2004-2005 Gallery. Images from VISUAL have been used in the New Jersey State Science Museum which included their Super Science Weekend and are now being hung in the Franklin Medical Center, Greenfield, MA, reaching directly to the public. VISUAL was also highlighted by the Optical Society of America in Optics and Photonics News and the American Chemical Society in Chemical and Engineering News for its originality in bringing science to the public sector. (L.Strzegowski, T.P. Russell, Microscopy Today, May 2004)
