Layer-by-layer deposition of ultra-thin films of carbon nanotubes

This paper reports about the characterisation of single-walled carbon nanotubes (SWNT) ultra-thin films fabricated through an automated layer-by-layer process. This consists of dipping the substrate alternately into a polycation namely, poly(diallyldimethylammonium) (PDDA), and an SWNT suspension. The morphology and roughness of samples containing varying numbers of PDDA/SWNT layers were studied, showing that the multi-layers are made of uniform nanotube networks with an average surface roughness between 2.3 and 7.6 nm. The average thickness of a single PDDA/SWNT layer is around 5 nm. The Raman spectra, using the 633 nm excitation line, show a significant change in the shape of radiant breathing mode (RBM) features and a blue-shift of the G-mode for PDDA/SWNT networks compared to the pristine SWNT material. This proves that PDDA is acting as an acceptor. The electrical conductance of the PDDA/SWNT networks is order of magnitude higher than that of equivalent networks fabricated without PDDA. Moreover, the temperature dependence of conductance differs from that measured in ultra-thin SWNT networks; the typical thermally activated electron transport mechanism changes towards low temperatures so that, close to zero temperature, a final value of conductance is observed. These results show that the use of a polyelectrolyte as adsorption promoter improves the quality of the film in terms of uniformity and density of nanotubes in the network.