Declan Ryan

Using self-assembly for the fabrication of nano-scale electronic and photonic devices

Challenges facing the scaling of microelectronics to sub-50 nm dimensions and the demanding material and structural requirements of integrated photonic and microelectromechanical systems suggest that alternative fabrication technologies are needed to produce nano-scale devices. Inspired by complex, functional, self-assembled structures and systems found in Nature we suggest that self-assembly can be employed as an effective tool for nanofabrication. We define a self-assembling system as one in which the elements of the system interact in pre-defined ways to spontaneously generate a higher order structure. Self-assembly is a parallel fabrication process that, at the molecular level, can generate three-dimensional structures with sub-nanometer precision. Guiding the process of self-assembly by external forces and geometrical constraints can reconfigure a system dynamically on demand. We survey some of the recent applications of self-assembly for nanofabrication of electronic and photonic devices. Five self-assembling systems are discussed: 1) self-assembled molecular monolayers; 2) self-assembly in supramolecular chemistry; 3) self-assembly of nanocrystals and nanowires; 4) self-assembly of phase-separated block copolymers; 5) colloidal self-assembly. These techniques can generate features ranging in size from a few angstroms to a few microns. We conclude with a discussion of the limitations and challenges facing self-assembly and some potential directions along which the development of self-assembly as a nanofabrication technology may proceed.

A magnetic trap for living cells suspended in a paramagnetic buffer

This manuscript describes the fabrication and use of a three-dimensional magnetic trap for diamagnetic objects in an aqueous solution of paramagnetic ions; this trap uses permanent magnets. It demonstrates trapping of polystyrene spheres, and of various types of living cells: mouse fibroblast (NIH-3T3), yeast (Saccharomyces cerevisiae), and algae (Chlamydomonas reinhardtii). For a 40mM solution of gadolinium (III) diethylenetriaminepentaacetic acid (Gd·DTPA) in aqueous buffer, the smallest cell (particle) that could be trapped had a radius of ∼2.5μm. The trapped particle and location of the magnetic trap can be translated in three dimensions by independent manipulation of the permanent magnets. This letter a1so characterizes the biocompatibility of the trapping solution.

Dip coating for the alignment of carbon nanotubes on curved surfaces

The preferential orientation of carbon nanotubes (CNTs) on a substrate using a dip-coating approach is described. The extraction of a steel fiber from a dispersion of CNTs provides the necessary forces required for preferential orientation of the CNTs on the fiber surface. We study the effect of the orientation of the long axis of the fiber relative to the direction of the extraction. We conclude that shorter CNTs are not as susceptible to these shear forces. The preferential orientation of CNTs is expected to provide a simple method for isolating one-dimensional nanostructures for imaging and analysis.

Self-assembly of alkane capped silver and silica nanoparticles

Silver and silica nanoparticles stabilised by long chain alkanes have been prepared and characterised using 1H-NMR, transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). Upon solvent evaporation, the alkane stabilised silica and silver nanoparticles self-assemble into close-packed two- and three-dimensional structures. Furthermore, it is shown that a mixture of large silica (90 A) and small silver (48 A) nanoparticles self-assemble into mixed well-defined close-packed structures. It has been demonstrated that particle size, core material and solvent composition can be employed to control the forces between individual nanoparticles and, consequently, the structure of the self-assembled arrays. These mixed structures have also been studied using UV-visible spectroscopy and their collective optical properties are shown to depend on their overall composition.