Optical and Electrical Properties of Single-walled Carbon Nanotubes with Known Chiralities

Abstract

Carbon nanotube (CNT) is a hollow structure consisted by one-atom-thick sheet of carbon atoms, which can be considered as a rolled-up graphene sheet. The diameter and rolling angle (chirality) uniquely determines its electronic structure. Over two decades of study, due to the difficulty of synthesizing clean individual CNTs and the limitation of accurate chirality characterization, there are still unveiled questions towards the intrinsic properties of this 1-D material at single molecular level. In this thesis, I will discuss the approaches of fabricating chirality assigned CNT device and the experimental results of its optical and electrical properties. In the first part, I describe using 'fast heating' chemical vapor deposition (CVD) method to achieve the high quality suspended CNT growth. Combining Rayleigh and Raman spectroscopy, I demonstrate the accurate assignment of chirality for each suspended individual CNT. With the ability of chirality identification, a series of optical and electrical experiments were conducted on the selected CNTs of interest. In the following part, I first discuss the probe of many-body effect in a semiconducting CNT by observing the elastic scattering (Rayleigh spectra) with electrostatic gating. We found the dominant short-range interaction is reduced to 85% of its intrinsic strength for doping level of ρ=0.4e/nm, demonstrating the possible control of sub-band exciton resonance frequency without rely on Pauli-blocking effect in CNTs. In order to study the substrate effect in electrical transport of CNTs, I improved the transfer technique to accurately place individual CNT on a specific substrate. With this technique, I've achieved transferring individual CNT on 20µmx20µm thin layer of hexagonal-boron nitride (h-BN) substrate with a ± 5µm error. The low field electrical transport studies were conducted on both metallic and semiconducting CNTs with known chiralities on h-BN. Temperature dependent measurement shows the resistivity becomes super-linear around 250K, consistent with the prediction that the surface polar phonon of h-BN couples with electrons in CNT at higher phonon energy than SiO₂. Moreover, the FET devices of CNT on h-BN with graphite local back gate show hysteresis free feature in vacuum, and the subthreshold swing of 118mV/dec is comparable to high κ dielectric HfO₂ based device.