Optical properties of conductive carbon-based nanomaterials

Abstract

The interaction of light with carbon nanomaterials is the main focus of this thesis. I explore several nanostructured systems involving different allotropes of carbon, and characterize them both electrically, if applicable, and optically. Special attention is paid to search for plasmon-like excitations on the systems, or utilizing surface plasmons on characterization. The first objective is to achieve control of carbon nanotube (CNT) conductivity with surface plasmon polaritons (SPPs), which resulted in the first CNT field-effect transistor (FET) that can be gated definitively with SPPs. The second objective is the investigation of optical properties of various thin carbon-based molecular networks. Recently developed methods allow separation of different types of CNTs. Inspired by that, films consisting of only metallic-type single-walled (SW)CNTs were studied, which led to the discovery of a dispersive collective optical resonance in these thin films.With similar methods, conductive polymer films were also measured. To pursue the first goal, a FET was fabricated using a semiconducting-type SWCNT and a thin silver film as a backgate, on which SPPs were excited close to the CNT via the Kretschmann total internal reflection (TIR) configuration. As a result, the CNT FET could be gated at a low optical excitation power using SPPs, which most likely trigger desorption on the device, alter the Schottky barriers on CNT contacts and modulate the current. A scanning near-field optical microscope was also used to measure the local photosensitivity of the CNT FETs. Thin films of chirality-selected SWCNTs were measured with optical spectroscopy in TIR conditions, and a new collective excitation was discovered in metallictype SWCNTs. This dispersive phenomenon appeared only with a polarization not able to excite regular SPPs, and was linked to the excitonic transitions of the tubes. It shared features with SPPs such as the dependence on both the film thickness and the properties of the surrounding medium. Transparent conductive polymer films, some with graphene flakes, were also characterized, and their optical properties evaluated with TIR spectroscopy. No plasmonic or other peculiar resonances were detected, but the study led to a method to evaluate the optical anisotropy in thin polymer films. Using this method, it was possible to measure thick and uneven films, that are unsuitable for ellipsometry.