Properties of Functionalized Carbon Nanotubes and Their Interaction with a Metallic Substrate Investigated by Scanning Tunneling Microscopy

Noncovalent functionalization of carbon nanotubes (CNTs) allows the combination of the remarkable physical properties of these one-dimensional systems with the properties of the functional molecules and, at the same time, modifies the physicochemical properties of nanotubes for specific applications. The use of functionalized carbon nanotubes in electronics often requires the deposition of the nanotubes on a substrate, and eventually an annealing step, which can modify their properties due to molecule-surface interactions. Using scanning tunneling microscopy/spectroscopy (STM/STS) and classical molecular dynamics (MD) simulations we studied the physical properties of carbon nanotubes functionalized with porphyrin derivatives and discuss the effect of physisorption and sample annealing on the nanotube surface. The results reveal that the functionalized parts exhibit nonperiodic structure with a significant modification of the local density of states. The coverage degree can be estimated from STM images. When the sample is annealed, STM data clearly show an unwrapping of the functionalizing species leading to a lowering of the coverage degree. In addition, periodic structures were observed that correspond to the surfactants originally present in the CNT sample, revealing that the surfactants are still present in such functionalized nanotubes. These results provide information on the structure and properties of polymer-functionalized nanotubes and the effect of substrate interaction and sample annealing that can markedly modify the structure and properties of the functionalized nanotubes.