Roland Goh

Effects of substrate curvature on the adsorption of poly(3-hexylthiophene) on single-walled carbon nanotubes

A detailed study of poly(alkylthiophene) self-assembly and organization on single-walled carbon nanotubes (SWNTs) is presented. Monolayers of regioregular poly(3-hexyl thiophene) (rrP3HT) adsorbed on SWNTs have been imaged by using scanning tunneling microscopy. Our results show that the rrP3HT interchain distance is greater for rrP3HT monolayers adsorbed onto the curved surfaces of SWNTs than on the flat surfaces of highly ordered pyrolytic graphite samples. Comparisons between the native polymer deposited on graphite and the composite structure confirmed that the presence of carbon nanotubes in rrP3HT produces a new material with a high degree of order at the molecular level.

Self-organization in composites of poly(3-hexylthiophene) and single-walled carbon nanotubes designed for use in photovoltaic applications

A detailed study of poly(alkylthiophene) self-assembly and organization on single-walled carbon nanotubes (SWNTs) is presented. Experimental evidence for self-assembly and organization of regioregular poly(3-hexyl thiophene) (rrP3HT) on single-walled carbon nanotubes was obtained using scanning tunneling microscopy (STM) and transmission electron microscopy (TEM). TEM images of drop-cast rrP3HT/SWNT composites displayed strong evidence that SWNTs were isolated from each other in a polymer matrix and coated with between 1-3 layers rrP3HT. STM measurements of adsorbed monolayers of rrP3HT on SWNT surfaces were compared to rrP3HT monolayer deposition on highly ordered pyrolytic graphite (HOPG) surfaces. The results show that average inter-lamellar distances of adsorbed polymer are greater for rrP3HT monolayers adsorbed onto the curved surfaces of SWNTs than on the flat surfaces of HOPG samples. Analysis of STM images yielded the chiral angles at which the thiophene polymer chains wrap around individual carbon nanotubes (41-48° with respect to nanotube axis) while the interchain spacings of adsorbed macromolecules was 1.68 ± 0.02 nm. Comparisons between the native polymer deposited on graphite and the composite structure confirmed that the presence of carbon nanotubes in rrP3HT produces a material with a high degree of order at the molecular level. This high level of order and close coupling of the two components of the composite are prerequisites for its use as the active layer of an organic photovoltaic.

Molecular-resolved imaging of conductive polymer self-organization at single-walled carbon nanotube interfaces

Molecular-resolved real-space images of self-assembled structures of the conductive polymer regioregular poly(3- hexylthiophene) (rrP3HT) on single-walled carbon nanotubes (SWNT) were obtained using scanning tunneling microscopy (STM). The STM images revealed that the adsorbed polymer typically formed a 10 nm thick coating on SWNTs. This is in agreement with transmission electron microscopy (TEM) results for drop-cast composite films that provided strong evidence that SWNTs were isolated in a polymer matrix and coated with rrP3HT multilayers. A 10 nm thick deposit corresponds to a coating of ~25 layers of polymer on SWNT, assuming that π-π interactions between rrP3HT layers determine deposition and that the underlying SWNT directs the polymer self-assembly process. STM measurements of adsorbed monolayers and multilayers of rrP3HT on SWNT surfaces were compared to rrP3HT monolayer and multilayer deposition on highly ordered pyrolytic graphite (HOPG) surfaces. The average inter-lamellar distances of adsorbed polymer was greater for both rrP3HT monolayer and multilayer films adsorbed onto the curved surfaces of SWNTs than on the flat surfaces of HOPG samples. Analysis of STM images yielded the interchain spacings of adsorbed macromolecules, dcc = 1.55 - 1.68 ± 0.02 nm. The polymer was observed to wrap around some SWNTs at an angle with respect to the SWNT long-axis, which indicated that the rrP3HT self-assembly is hierarchical. The conductive polymers deposition appears to occur with epitaxy and is directed by the underlying SWNT chiral structure.

Influence of dispersed carbon nanotubes on the optical and structural properties of a conjugated polymer

We studied composites of single walled carbon nanotubes and poly(3hexylthiophene) by optical absorption, X-ray diffraction and transmission electron and scanning tunneling microscopy. Dispersing single walled carbon nanotubes in poly(3hexylthiophene) leads to sharpening of vibronic structure and enhanced optical absorbance near the band edge. We show that the enhanced order in the polymer is due to templating of the polymer chains by the surface of the carbon nanotubes leading to increased electronic delocalization.