Towards Understanding the lntertwinement between Chemical Modification and Electronic Properties of Single-Wall Carbon Nanotubes

Abstract

Single Wall Carbon Nanotubes (SWCNTs) are often synthesized as bundles and are chemically modified via either covalent or non-covalent approaches to prevent aggregation, improve their dispersability and tune their physical properties for a potential application. The spatial distribution and effect of covalent addends on the electronic properties of SWCNTs was characterized using a Scanning Tunneling Microscope but with limited success. The effect of sample preparation was questioned as it often involves sonicating the SWCNTs in either an organic or an aqueous medium. Sonication of SWCNTs in certain common solvents was found to alter their electronic properties. The solvent molecules are broken down via a radical pathway during sonicating leading to the formation of new species that interact with the SWCNTs and in some cases with the catalytic material present. New species such as iron chlorides and oxygen, which were formed for example in o-dichlorobenzene and water respectively, caused p-type doping. Doping was characterized by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). A blue shift in the D* mode along with a shift to lower binding energy in the C1s peak was observed from Raman spectroscopy and XPS respectively. The reactivity of the radicals (formed during sonication) towards the structure of the SWCNTs was also investigated. Radicals formed during sonication of certain chlorinated aromatic solvents lead to the formation of sonochemical polymers, which interacted heavily with the SWCNTs. These interactions, which can be the source of features commonly associated with covalent functionalization, were thwarted by a washing protocol and were found to be non-covalent in nature. The observations are of relevance towards understanding an inadvertent chemical modification during chemical processing, which can impact reproducibility of results that involve wet-chemical processes. However, with such knowledge of the chemistry involved during sample preparation the occurrence of doping can be either circumvented or appropriately used.