Nanostructure formation by single particle nanofabrication technique in polymers

High energy charged particle irradiation of a variety of polymer thin films has been shown to cause crosslinking reactions leading to the formation of a polymer gel containing cylindrical nanostructures of regular length and thickness. The deposited energy density in a charged particle track controlled the spatial distribution of crosslinks of polymer molecules. The radial variation in dose along each ion track resulted in the formation of cylindrical structures with nm-sized radius of the cross-section. These cylindrical structures were well visualized as worm-like structures (nanowires). We have developed the formulation defining the size of nanowires in account of the radial dose distribution in an ion track, the efficiency of the crosslinking reactions, and size (shape) of the target polymer molecules. In the present study, this single particle nano-fabrication scheme was applied for the formation of the nanowires based on a variety of polymeric materials including π-conjugated polymers, polymers for hydrogels, and proteins. The present results represent the potential utility of this technique for single-particle fabrication of nano-structures with sub-nanometer spatial resolution. Si based polymer materials, especially polycarbosilane (PCS), have been well known as the polymer precursor materials for β-SiC ceramic fibers. Radiation induced reactions of the PCS were also well investigated in view of the process optimization of the ceramic fiber formation, revealing their very high crosslinking reaction efficiency upon irradiation to electron beams and γ-rays. High energy particle irradiation to the thin films of PCS was performed based on the above scheme, leading to the nanowires in the present study. Figure shows atomic force micrograph of the nanowires isolated on the Si substrate. The thickness and the length of the nanowires are well interpreted by the formulation of particle track reactions. The radii of the cross section of the nanowires varied from 10 to 30 nm, and the sintering process above 1600 K caused shrinking of the nanowires as much as ∼40 %. The AFM image of the shrunk nanowires is also presented in the figure. The shrinkage corresponds to that observed typically in case of the conversion from as-spun PCS to β-SiC ceramic fibers, suggesting effective conversion into ceramic nanowires by this sintering process. The ceramic nanowires showed high thermal stability without deformation under heating at >1600 K. Polymer materials for hydrogels, poly(vinylalcohol) (PVA) and poly(N-vinyl-2-pyrrolidone) (PNVP), are also the choices as the target polymers to prepare nano-hydrogels. The single particle nanofabrication technique also gives the nanowires via crosslinking reaction in solid thin films of these polymer materials, and an image of the nanowires based on PVA is shown in the figure. Typically the crosslinked hydrogels indicate drastic swelling upon impregnation with water molecules, and the degree of impregnation (rate of swelling) depends strongly on the number of crosslinks in unit volume. The swelling rate was observed as very low (<1.5) for the nano-hydrogels based on PVA and PNVP, suggesting high density intra- and inter-molecular crosslinks induced by the present technique. The characteristics of the nanowires based on protein macromolecules and conjugated polymers are also discussed in the present study as well as precise morphological analysis of nanowires prepared from general polymer materials with well-defined molecular structures. (authors)