Probing Electronic Properties of Carbon Nanotubes

Abstract

Carbon nanotubes are quasi-one-dimensional objects that have many remarkable electronic properties. In Chapter I, an electrostatic force microscopy technique to probe the local density of states of single-walled carbon nanotubes (SWCNTs) under ambient conditions is described. Coupling the atomic force microscope tip motion with the quantum capacitance of nanotubes enables the van Hove singularities in the one-dimensional density of states to be resolved. We utilized this technique to identify individual semiconducting and metallic tubes, and further to estimate the chiral angle of a nanotube. Moreover, in order to realize a SWCNT interferometer, nanotube loop devices where a self-crossing geometry yields two electron paths that is a possible analog of the optical Sagnac interferometer are fabricated and explored in Chapter II. Scanning gate microscopy reveals for semiconducting devices a 0–50% transmission probability into the loop segment at the junction, which can be controlled by applying back gate voltage, hence shifting the Fermi level of the nanotube. Metallic loop devices having low contact resistance showed a large- scale conductance peak with fast oscillations superposed on it. Possible theoretical explanations including Sagnac-type interference, which takes the velocity difference between left and right movers in to account, and Fabry-Perot-type interference are compared with the experimental observations. In Chapter III, in accordance with increasing demand for developing spin-electronic devices, cobalt-filled multi-walled carbon nanotubes (Co–filled MWCNTs) are first synthesized and imaged by transmission electron microscopy, and also characterized by various spectroscopy tools like X–ray diffraction and energy dispersive X–ray spectrometry. Further, a Co–filled MWCNT device having reproducible switching in magnetoresistance was demonstrated. The last topic, in Chapter IV, covers the effects of a transverse electric field in MWCNT devices, where conductance fluctuations as a function of the transverse electric field were observed. The electric field spacing between the peaks of the fluctuations is in agreement with the theoretical predictions of band structure modulation by transverse electric fields. Future work following our experimental studies is proposed and discussed at the end of each chapter.