Nanodeposition and plasmonically enhanced Raman spectroscopy on individual carbon nanotubes

Abstract

Single-walled carbon nanotubes (SWNTs) exhibit extraordinary properties: mechanical, thermal, optical and, possibly the most interesting, electrical. These all-carbon cylindrical structures can be metallic or semi-conducting depending on their precise structure. They have the potential to allow faster transistor switching speeds and smaller, more closely-packed interconnects in microelectronics. However, such applications are hindered by the difficulties of positioning the correct type of SWNT in a spatially precise location and orientation. In addition, greater understanding of the fundamental limits of SWNTs, such as the limit of current density, is needed for optimum operation in applications. The primary aim of this project was to increase the understanding of current density limitation by using in situ plasmonically enhanced Raman spectroscopy during electrical transport. The use of plasmonic metal nanostructures to enhance the Raman scattering should allow the acquisition of informative spectra from SWNTs away from their intrinsic resonance conditions. To achieve this aim, SWNTs must be integrated with plasmonic metal structures as well as electrical connections. This thesis presents two approaches for the integration of SWNTs with other nanometre-scaled features, in particular plasmonic nanoparticles. Fountain pen nanolithography uses a hollow nanopipette in place of the probe tip in an atomic force microscope (AFM), through which material can be delivered to a spatially precise position on a surface. Aqueous SWNT dispersion was delivered to chemically-functionalised silicon in this way, through pulled quartz pipettes with aperture diameters of 50 nm, 100 nm and 150 nm. The heights, widths and continuity of lines drawn on the surface by the nanopipette depended on the size, setpoint and lateral speed of the tip. A small bias voltage applied between the SWNT dispersion inside the pipette and the substrate allowed the deposition to be switched on or off depending on the polarity of the voltage, through the action of electroosmotic effects within the quartz capillary. The quality and density of the SWNT dispersion was found to be important for successful deposition to occur, since too low a concentration results in the lines deposited from the pipette being only surfactant but too high a concentration of bundles would quickly block the small tip of the pipette. Polarised Raman spectroscopy on SWNT deposited by fountain pen nanolithography showed that they had a high level of alignment parallel to the direction in which the pipette moved. Spherical gold nanoparticles with plasmonic properties suitable for enhancing Raman scattering were dropped onto samples containing individual SWNTs supported on a Si/SiO2 surface. Nanomanipulation with an atomic force microscope was used to push the gold nanoparticles onto the SWNTs. Raman spectra measured with and without the gold particles showed that the gold nanoparticles gave local enhancement factors of 24 for a single 150 nm nanoshell and 130 for a small cluster of 150 nm nanoshells. Polarised Raman studies on the cluster showed that the angle dependence deviated significantly from that expected of a bare SWNT. Electrical transport experiments with in situ plasmonically enhanced Raman spectroscopy may be performed on samples prepared from the methods described here. Such experiments would increase understanding of the electrical properties of SWNTs and how they relate to the vibrational and optical properties.