Kinetic and photon induced effects in SWNT solutions

Abstract

Single walled carbon nanotubes (SWNTs) have potential application in fabrication of nanodevices and sensors. SWNTs are mostly studied as dispersed colloidal solution in non-aqueous solvents like DMF, due to ease in chemical modification and transferability required in the fabrication of such devices. These kinetically stable dispersions help us to understand intermolecular interactions, their solubility and rate of aggregation. These rates depend upon electrical double layer and surface charges of SWNTs. The present work is to determine the stability of SWNTs dispersions in the presence of various coagulants. The stability is discussed keeping in mind the Schulze-Hardy rule and DLVO theory, which has been traditionally used to understand colloidal solutions. While stability of SWNT dispersion is typically desired, aggregation of SWNTs with coagulants and interaction of SWNTs among themselves are affected by size, shape and ionic strength of coagulants; which leads to changes in electrodynamic repulsive and van der Walls attractive forces acting between SWNTs. The study ascertains the relation between Critical Coagulation Concentration (CCC) of SWNT solutions with various inorganic coagulants (like NaBr, CuCl2, FeCl3) and that of traditionally considered spherical ions in colloidal solutions. CCC is the concentration above which onset of rapid aggregation occurs. Along with experimental results of Schulze-Hardy law for nanotubes, novel photon enhanced aggregation in these solutions is also observed. We observed dependence of aggregation rate on optical excitation and band gap energy of individual nanotubes. Photon enhanced aggregation of SWNTs is observed and characterized using spectroscopic techniques. This, dependence of stability of dispersion [CCC] on optical excitation and dielectric property of SWNTs (metallic and semiconducting) is experimentally demonstrated.