Self-Assembled, Molecularly Aligned Conjugated Polymer Nanowires via Dewetting

High aspect-ratio poly(9,9'-dioctylfluorene) (F8) nanowires are fabricated on top of silicon/amorphous Teflon substrates, by dewetting of F8 thin-films. The conjugated nature of the polymers enables the self-assembly of continuous molecularly-aligned nanowires of several micrometers in length and tens of nanometers width, without using a template. The sizescale of the dewetted morphology can be controlled by varying the thickness of the F8 and amorphous Teflon layers. As predicted by van der Waals theory the characteristic sizescale of the dewetted morphology increases with F8 film thickness. However, the dependence of the characteristic sizescale on amorphous Teflon thickness is not accounted for, even qualitatively, by standard spinodal theory with van der Waals forces as the de-stabilizing force across the F8 film. The Rayleigh instability is strongly suppressed in the F8 nanowires in the late stages of dewetting, compared to isotropic, amorphous polymers. Polarized Raman measurements show a systematic increase in molecular alignment along the axis of the nanowires as their width is reduced below the typical liquid-crystalline domain size in polyfluorene films. Thus the dewetting process aligns the polymers, and the aligned polymers suppress the Rayleigh instability and enable the formation of high aspect-ratio continuous nanowires.