Solvent-based preferential deposition of functionalized carbon nanotubes on substrates

摘要

Solution-processed deposition of carbon nanotubes (CNTs) provides a cost-effective means to synthesize uniform vertically or horizontally aligned nanostructures on top of substrates. The efficacy of deposition depends on the solubility of CNTs in the solvent as well as the ordering of nanotubes relative to the substrates. These governing factors, which determine the specific morphologies of CNTs that are deposited, are determined by the molecular interactions between the CNTs and the substrate and solvent molecules. In an effort to mimic the conditions during solution-processed deposition of nanotubes on substrates, we employed molecular dynamics (MD) simulations to study systems comprising CNTs and commonly used solvents toluene and acetone sandwiched between silicon substrates. Both charged and uncharged substrates were simulated to evaluate the effect of electrostatic interactions between nanotubes and substrate on deposition. Comparison of simulated systems with pure and functionalized CNTs indicate that the presence of –COOH functional groups in CNTs is necessary for uniform deposition on substrates. Time-averaged distribution of the angles formed between the axes of the CNTs and that between the nanotubes and substrates demonstrate that functionalized carbon nanotubes (FCNTs) align parallel with the substrates to a greater extent when toluene is used as a solvent. Time-averaged density distributions obtained from our simulations illustrate that FCNTs tend to migrate towards the substrates, especially when the substrates have finite charge density. However, polar acetone molecules form layers on the substrates and thus lead to electrostatic shielding between nanotubes and substrates that hinders deposition. Hence, non-polar solvents such as toluene are more effective for deposition of nanotubes. Based on radial distribution functions (RDFs) and coordination numbers of various atoms of CNTs with respect to solvent atoms as well as with respect to that o- other CNTs, we present a hypothesis that relative extent of coordination between solvent molecules and the functional groups and graphitic component of CNTs determine the solubility and the propensity of parallel alignment of FCNTs in solvents. Overall, results presented in this study provide fundamental insight to predict key factors for solution-processed deposition of well-ordered CNTs on silicon substrates.