Electrochemical modification and photoelectronic properties of carbon nanotubes

Abstract

This thesis reports on the investigations performed on electrochemical functionalization and photoelectronic transport properties of individual single-wall carbon nanotubes (SWCNTs). The first part of the thesis is concerned with the controlled modification of individual nanotubes through electrochemistry. The electrochemical modification has been performed using reductive and oxidative coupling schemes, resulting in thin molecular coatings around the SWCNTs. The former scheme is based on a reactive diazonium salt, while the latter involved a substituted aromatic amine. The characterization methods included electrical transport measurements and confocal Raman spectroscopy, both of which can address selected single SWCNTs or bundles. These studies were performed separately on metallic and semiconducting nanotubes, in both cases comparing the effect of the oxidative and reductive coupling schemes. While the oxidative scheme was found to yield non-covalently attached layers on the nanotubes, the reductive modification led to covalently grafted phenyl groups. The reductive coupling scheme was utilized to address a fundamental problem in the creation of nanotube field-effect transistors (FETs). The fabrication of FETs using carbon nanotubes has been impeded as all current production procedures yield a mixture of metallic and semiconducting tubes. In this work, a generic approach employing electrochemistry for selective covalent modification of metallic nanotubes was devised, resulting in exclusive electrical transport through the unmodified semiconducting tubes. Towards this goal, the semiconducting tubes were rendered non-conductive by application of an appropriate gate voltage prior to the electrochemical modification. The FETs fabricated in this manner were found to display favourable hole mobilities and a high ratio approaching 106 between the current in the ON and OFF state. The second part of the thesis deals with electronic transport through individual carbon nanotubes under local photo-illumination. The source of excitation was a diffraction-limited laser spot (diameter ≈ λexc/2) generated by a confocal scanning optical microscope. Using this setup, photoconductivity in individual semiconducting SWCNTs was investigated in detail. The magnitude of the photocurrent was found to increase linearly with the laser intensity, and was maximum for parallel orientation between the light polarization and the tube axis. Larger currents were obtained upon illuminating the tubes at 514.5 nm in comparison to those at 647.1 nm, consistent with the investigated semiconducting tubes having a resonant absorption energy at the former wavelength. Furthermore, due to the relatively small diameter of the laser spot, the photoresponse could be measured as a function of position, which allowed the acquisition of photoelectronic transport (PET) images of individual nanotubes. During the course of this work, the PET imaging technique was developed into a useful tool to monitor local electronic structure effects, including the observation of Schottky barriers at the contacts with semiconducting SWCNTs. Subsequently, metallic SWCNTs were investigated by recording PET images. The locally induced photocurrents directly reflect the existence of built-in electric fields associated with the presence of depletion layers at the contacts or structural defects along the tubes. These observations have strong implications on the realisation of high-performance electrical devices incorporating carbon nanotubes, which critically depends on the minimisation of charge transport barriers along the tubes and at the contacts.