Electrical investigations of layer-by-layer films of carbon nanotubes

Abstract

Single-wall carbon nanotubes (SWNTs) with anionic or cationic coatings have been prepared by exploiting the ability of certain surfactants to form a monolayer shell around the nanotube. The presence of electrically charged functional groups on the surface of the SWNT allows thin film deposition to proceed via the electrostatic layer-by-layer method. This self-assembly process was monitored using the quartz microbalance technique and Raman spectroscopy, while the morphology of the resulting thin layers was studied with atomic force microscopy. A variety of different architectures has been built up. In one arrangement, a single species of a modified SWNT (anionic or cationic) was alternated with a passive polymer to form a composite structure. A ‘superlattice’ architecture comprising alternating anionic and cationic modified nanotubes was also fabricated. The in-plane and out-of-plane dc conductivities of the films were measured at room temperature and contrasted with reference architectures (i.e. those containing no nanotubes). The results showed clearly that the incorporation of SWNTs into the multilayer assemblies provided electrically conductive thin films. It is suggested that the current versus voltage behaviour, particularly in the out-of-plane direction, is controlled by quantum mechanical tunnelling of carriers between the nanotubes.

Poole–Frenkel conduction in single wall carbon nanotube composite films built up by electrostatic layer-by-layer deposition

“…2͑b͒, to ascertain the presence of SWCNTs in the composites. 9 The relative increase in the sp 3 ͑ϳ1350 cm −1 ͒ compared to sp 2 band ͑ϳ1580 cm −1 ͒ in the LbL SWCNT film may have resulted from the surfactant wrapping of the SWCNTs in the matrix.…”

“…This report follows up our previous work 9 and aims to elucidate the conduction process͑es͒ occurring in LbL films of single wall carbon nanotubes ͑SWCNTs͒. Monolayers of anionic ͑SWCNT a ͒ and cationic ͑SWCNT c ͒ SWCNTs are stacked up as many times as necessary to produce the nanometer-thick devices.…”

“…In brief, the SWCNTs were subjected to a thermal oxidation for 90 min at 300°C, followed by stirring in a concentrated HCl bath overnight, before finally rinsing the nanotubes with deionized water ͑until the pH of the solution is to that of deionized water͒ and drying overnight at 120°C. 18,19 The purified SWCNTs were positively and negatively charged by noncovalent surface coatings 8 with SDS and DTAB to produce SWCNT a and SWCNT c , 9 respectively. Solutions of SDS ͑1 % wt͒ and DTAB ͑0.65 % wt͒ were prepared by dissolving the surfactants in de-ionized water.…”

“…This is based on the alternate adsorption of negatively and positively charged species in order to build up multilayer device architectures. 8,9 In LbL deposition, nanometer-thick amorphous polymer shells are formed around individual carbon nanotubes to provide an electrically charged surface; the reversal of the electrostatic charges is then responsible for the build up of multilayers. 10,11 Most of these research initiatives have focused on the chemical modification of the nanotubes.…”

“…The film structure and electrical response have also been investigated. [6][7][8][9][10][11] However, the electrical characteristics are often limited to recording the current versus voltage ͑I-V͒ characteristics, assessing the potential applications, and to reporting the improved conductivity achieved within the polymer composites as a result of the nanotube blending. 6,7 Very little is known about the electrical transport mechanism in the LbL nanotube composites.…”