Abstract
A driving goal of the research community in recent years has been to develop various methodologies of producing novel materials as uniform films with high transparency T coupled with high conductivity σ. Most transparent electrodes are made from indium-doped tin oxides (ITO). This is due to the materials having high electrical conductivity Rs < 100 Ω/sq and transparency, T = 90%. However, using ITO involves several drawbacks. Primarily, it is brittle and expensive. In recent years, many candidates have been proposed as potential substitutes for ITO such as, nanotubes, nanowires, graphene and hybrids of these materials. The use of the Langmuir technique has already been shown to allow the user to produce high quality films by being able to manipulate various factors i. e. the surface pressure of the subphase and number of isotherm cycling. Here we use a modified method to produce graphene, hybrid of graphene/silver nanowires and single-walled carbon nanotubes based thin films in order to compete with ITO and viable alternative. Single or few layer graphene can be considered an exciting pseudo-two-dimensional molecular material that potentially has a wide range of applications. A critical performance bottleneck may arise with issues in their controlled assembly into macroscopic ensembles over large areas both in two and three dimensions. Langmuir-type assembly is a particularly useful method to control and manipulate the distribution of graphene at the air-water interface via edge-edge interactions. In this study, pristine graphene suspended in organic solvent was prepared through adaptation of a previously developed process involving the non-invasive exfoliation of graphite. Successful deposition of graphene at the air-water interface was achieved by manipulating the vapor-pressure of the graphene dispersion through solvent mixing. The film density is precisely controlled by following the pressure-area isotherm during monolayer compression/rarefaction. Moreover, the resulting assemblies can be easily transferred to glass and other substrates producing thin films with tunable electrical conductivity using the Langmuir-Schaefer horizontal deposition method. A major advantage of this process is that the conducting films require no further treatment unlike their graphene-oxide counterparts. Moreover, the physical properties of these assemblies can be easily controlled which is a precursor for graphene-based electronic applications. Although silver-nanowires demonstrate comparable electrical conductivity and optical transmittance to ITO, challenges remain. For example, silver nanowires are expensive and in order to achieve high electrical conductivity relatively dense films are required. Moreover, the resulting films are often hazy and require a protective coating to prevent eventual oxidation. Numerous studies have investigated silver nanowires and graphene individually as transparent conductors, but little research has been done on hybrid systems of the two. We report a simple, scalable and relatively inexpensive method to prepare transparent conducting films based on silver-nanowire/graphene hybrids. We use a combination of spray deposition and Langmuir-based techniques to produce ultrathin films with controlled nanowire and graphene densities. Surface morphology of the hybrid films was observed by SEM, AFM and optical microscopies. We demonstrate that adsorption of graphene at nanowire junctions markedly affects the macroscopic conductivity without significantly reducing the optical transmittance. Our optimised films which have comparable properties to commercial ITO contains reduced densities of silver-nanowires in comparison to films made of pristine silver-nanowires with the same properties. The results indicate that these graphene/nanowire hybrid films may serve as a cheap replacement for existing technologies in electronic devices. Another nanostructure material that has been touted for applications is carbon nanotubes. Carbon nanotubes are materials with a variety of fascinating properties related to their interesting quasi-one-dimensional electronic structure. There is considerable interest in developing economical and practical methods for producing aligned carbon nanotubes and utilizing them in electronic components, optical waveguides and optoelectronic devices. A method for assembling monolayers of aligned single-walled carbon nanotubes is described. Dispersions of nanotubes were prepared as a function of concentration in solutions of 1,2-Dichloroethene and the conjugated polymer, poly (mphenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene). The helical structures of this conjugated polymer forms a hydrophobic cavity that becomes a suitable host for nanotube bundles and consequently enhances their dispersability. The Langmuir-Blodgett technique was used to prepare monolayers of oriented polymer wrapped nanotubes. The alignment of the assembles on the substrates is controlled by varying different parameters in the Langmuir-Blodgett process. SEM, AFM microscopies and polarized Raman spectroscopy reveal a crucial relationship between pressure cycling and the formation of aligned assemblies of nanotubes. Specifically, we have discovered that alignment can be improved if the surface pressure during deposition and the number of deposition cycles is increased. Electrical conductivity of the resulting films again show percolation-type behaviour as a function of density modification in the prepared films. The calculated percolation threshold is found to be unexpectedly high which may result from the local mesoscopic ordering of nanotubes in the film facilitated by the presence of the polymer.