Raman spectroscopy and electrical transport in suspended carbon nanotube field effect transistors under applied bias and gate voltages.

摘要

One-dimensional materials exhibit striking, unique phenomena that are not found in two or three dimensions. For the last twenty years, single walled carbon nanotubes (CNTs) have served as the prototypical experimental one-dimensional system. In this thesis, I investigate experimental data and theoretical models of spatially and electrically isolated single-walled CNTs field-effect transistors.;Carbon nanotubes are grown using chemical vapor deposition, which relies on small percentage of as-grown CNTs landing across pre-defined Pt electrodes that define the transistor. A Landauer model is developed, which explains the gate and bias voltage dependence of the electrical transport in these devices, and which serves as the basis for much of the analysis of the experimental electrical transport and Raman data.;Raman spectra are collected from CNTs under high applied bias voltages. When heated with an electrical current, the Raman spectra of CNTs downshift, and this shift can be used as an in-situ temperature probe. In some CNTs, the various Raman bands are observed to downshift unequally, indicating non-equilibrium phonon populations caused by threshold optical phonon emission. The spatial temperature profile of several CNTs is measured, and shorter CNTs ( L 2mum) are found to exhibit pronounced non-equilibrium phonon populations, while longer CNTs (L = 5mum) exhibit thermal equilibrium behavior.;In addition to bias voltage effects (heating), gate voltage effects are also investigated. When metallic CNTs are doped with an applied gate voltage, two primary effects are observed. First, the one-dimensional Kohn anomaly is shut off, and secondly, large modulations are observed in the Raman intensity.;The Kohn anomaly is a phonon damping phenomenon unique to one-dimensional systems, and caused by electron-phonon coupling at the Fermi energy. In metallic CNTs, a Kohn anomaly can be observed in the Raman G band spectrum. Doping the CNTs causes this Kohn anomaly to go away, which results in spectral shifts of the G band. A phonon renormalization model is implemented to fit the results. This effect is used to experimentally confirm, for the first time, the theoretically predicted breakdown of the adiabatic Born-Oppenheimer approximation in individual CNTs.;In addition to the spectral shifts associated with the Kohn anomaly, large variations of up to two orders of magnitude are observed in the Raman intensity of pristine, suspended quasi-metallic single-walled CNTs in response to applied gate potentials. No change in the resonance condition is observed, and all Raman bands exhibit the same changes in intensity, regardless of phonon energy or laser excitation energy. The electronic energy gaps correlate with the drop in the Raman intensity, and the recently observed Mott insulating behavior in CNTs presented as an explanation for the effect.;Finally, the combined effects of heating and doping are investigated. It is found that doping can change the high bias electrical behavior of CNTs from Ohmic (linear) to negative differential conductance (NDC). In addition, in some CNTs, there is an accompanying change in phonon population distribution, from equilibrium in the Ohmic regime, to non-equilibrium in the NDC regime. Threshold phonon emission is identified as the mechanism behind this phenomenon, and a model is presented which utilizes a new formulation of Matthiessen's rule for threshold phonon emission processes.