Photoresponse in self-assembled films of carbon nanotubes

Water-soluble single-walled carbon nanotubes (SWNT; with carboxylic functionalities) were self-assembled on a water-pentane interface. Ultrathin (< 30 nm) films consisting of two distinct types of nanotube morphologies, wide bundles of nanotubes interwoven with a network of thin bundled (or individual) tubes, were obtained. The films were highly homogeneous and sufficiently robust to be transferred from the liquid interface and deposited onto solid substrates such as silica, quartz, or plastic. Under normal conditions the electrical conductance of such films is metal-like and is dominated by the transport of charge carriers through bundles of SWNT. However, semiconducting SWNT within the structure can readily accept charge carriers (holes) from excitons generated in optically active conjugated polymers upon absorption of visible light. This increase in concentration of charge carriers stimulated by light in interfacial SWNT-polymer films results in a measurable conductance change that depends linearly on the applied voltage and the intensity of light when the light intensity is low. Critically, neither drop-cast SWNT samples nor interfacial films of multiwalled carbon nanotube (MWNT) exhibit any photoresponsive behavior and thus the photoresponse of the SWNT-polymer composite depends on the morphology of the nanotubes and their assembly methodology.