Chemical functionalization of carbon nanotubes for controlled optical, electrical and dispersion properties

摘要

A carbon nanotube is a graphitic sheet, rolled into a one-dimensional, hollow tube. This structure provides certain individual nanotubes with high conductivity and near-infrared optical activity. These properties are not necessarily translated at the macroscale, however, due to strong van der Waals attractive forces that determine the behavior at the bulk level - exemplified by aggregation of nanotubes into bundles with significantly attenuated functionality. Different methods of carbon nanotube covalent functionalization are studied to improve dispersion while simultaneously maintaining intrinsic electrical and optical properties. In addition to retention of known behavior, new carbon nanotube photoluminescence pathways are also revealed as a result of this same covalent functionalization strategy. With various wet chemistries, including super-acid oxidation, the Billups-Birch reaction, and various diazonium based reactions, that utilize strong reducing or oxidizing conditions to spontaneously exfoliate aggregated carbon nanotubes, we are able to covalently functionalize individually dispersed nanotubes in a highly scalable manner. Covalent addition to the nanotube sidewalls converts the native sp2 hybridized carbon atoms to sp3 hybridization, which helps disrupt inter-tube van der Waals forces. However, this change in hybridization also perturbs the carbon nanotube electronic structure, resulting in an undesired loss of electrical conductivity and optical activity. We observe that controlling the location of functionalization, such as to the outer-walls of double-walled carbon nanotubes or as discrete functional "bands," we avert the loss of desirable properties by leaving significant tracts of sp2 carbon atoms unperturbed. We also demonstrate that such functional groups can act as electron and hole traps through the creation of a potential well deviation in the carbon nanotube electronic structure. This defect-activated carrier trapping primes the formation of charged excitons (trions) which are observed as redshifted photoluminescence in the near-infrared region. Implications and impacts of these covalent functionalization strategies will be discussed.