Raman Spectroscopy And Electron Transport Of Single Walled Carbon Nanotubes

This thesis presents the main findings in my PhD project on the Raman spectrum and the electron transport properties of single-walled carbon nanotubes (SWNTs). Resonance Raman spectroscopy is firstly used to characterize the structural properties of SWNTs under various external perturbations, including strain, nanoparticles decorated, electrostatic gating, electrical heating, et al. Afterwards, the electron transport mechanisms in two novel SWNTs hybrid structures, SWNT@Au-NCs (gold nanocrystals) and SWNT@Cn (carbon chains), are investigated. At last, stable n-type SWNT field effect transistors (FETs) are successfully fabricated. One conventional optical lithography compatible method is developed for sub-10 nm nanogap electrode arrays fabrication, which has been proven an excellent way for nano-sized devices integration.Abnormal Raman scattering from a large diameter ultralong SWNT is studied in detail. Along the SWNT, the Raman spectra show the frequencies of 1553,1563, and 2597 cm-1 for G+, G-, and G'peaks, respectively, much lower than the corresponding frequencies well reported both experimentally and theoretically. The significant downshifts in the peaks frequencies can be attributed to self-built tensile strain which is likely caused by carbon nanodots decorated on the tube. After infrared laser heating is performed to one point of it, all the Raman modes are found to shift to higher frequencies and approach to their conventional values. We suggest that the SWNTs with larger diameters easily possess such self-built strain compared to small-diameter SWNTs because of the weaker curvature effect for the larger ones.In situ Raman measurements have been carried out on a thin bundle of SWNTs in FET configuration at various gate voltages. Two excitation lasers with the photon energy of 1.96 eV and 2.41 eV are selected to excite the Raman scattering modes of metallic and semiconducting SWNTs in the bundle, respectively. For the metallic SWNTs, the G-Raman mode is found to shift to higher frequencies and narrow down its line shape at negative gate voltages, but be insensitive to positive gate voltages. These findings confirm that the Kohn anomaly exists in a thin SWNTs bundle and that the LO phonon mode changes along with the position of the Fermi level in the metallic SWNTs. In contrast, semiconducting SWNTs do not show any observable changes in the Raman spectra.Electrical bias voltages tuned Raman spectroscopic of SWNT-FET show that the G peaks shift gradually to lower frequency, become broader and decrease in intensity, along with the increasing of electrical power per unit length on the SWNT. This result indicates that the generation of hot phonons and the underestimation of the interaction between the SWNTs and the SiO2 substrate.Self-assembly gold nanocrystals (Au-NCs) are formed on the surface of SWNTs on a platform of FET by electron-beam evaporation and post rapid thermal annealing processes. Strong oscillations in the SWNTs channel (～2000 nm long) current occur below 150 K. The Raman scattering from the SWNTs suggests that the electronic structure of the SWNTs has been significantly altered by the strong Coulombic interaction with attached Au-NCs. Two possible mechanisms are presented to explain the observations:(1) the charging process of Au-NCs is dominated by the Coulomb blockade effect. Thus the electrostatic potential of charged Au-NCs modifies the Schottky barrier at the SWNT/Au electrode contacts and subsequently affects the SWNT channel current (or the Schottky barrier modulation) and (2) the charged Au-NCs serve as scattering centers, which modify the local potential along the SWNTs channel and then induce the oscillations in the current (i.e. energy band modulation).Pure sp-hybridized linear carbon chains (Cn) encapsulated within vertically aligned SWNTs (Cn@SWNT) is observed using Raman spectroscopy and electrical measurements. For the first time Cn@SWNT is shown to possess unique quantum wire properties, including nonlinear electrical property and conductance power law behaving characteristic of a Luttinger-liquid. Single electron tunneling (SET) and Coulomb blockade phenomena were clearly recorded at 40 K with a series of different bias voltages from 1 to 10 mV. The high bias voltage 10 mV indicates that SET can occur on Cn@SWNT at temperature up to 110 K, providing many potential applications in computer industry, single electron memory, highly sensitive electrometer among others.We have developed a fabrication method for nanogap electrodes without employing electron-beam lithography to measure the electrical characteristics of nanostructured molecules. This side layer-controlled method enables us to reproducibly fabricate down to～5 nm nanogap electrodes. The fully compatibility with traditional microfabrication make us easily fabricate nanogap electrode array for higher density integration. Combined with the AC dielectrophresis method, one example for short channel SWNT-FETs array is demonstrated.Owing to the unique 1-D structure of SWNTs with fully saturated surface bonds and no interface states, resulting in no Fermi-level pinning, stable n-type SWNT-FETs are fabricated using the low workfunction metals, Sc, Sm, or even Al as source/drain electrodes. The well matching between the Fermi level of Sc and the conductance band of SWNTs make the SWNT-FETs show nice ohmic contact property and with quite high Ion/Ioff ratio (more than 3 orders) and conductance (～0.02 Go in 5μm channel).