Fabrication, Structure and Properties of a Single Carbon Nanotube-Based Nano-Electromechanical System

The research work evolved in this dissertation presents (i) a foundational study on the atomic structure, transport property, electromechanical actuation, and inter-shell friction of carbon nanotubes using a nano-electromechanical system based on a single carbon nanotube and (ii) a fabrication technique of the nano-electromechanical system which provides a versatile platform for studies on one-dimensional nano-materials such as nanowires or other types of nanotubes. The geometry of having a free suspended carbon nanotube makes the device capable of in situ electromechanical manipulation and electrical resistance measurement on a single nanotube in a transmission electron microscope. The fabrication and the operation of the device are first described in detail. Experimental results are then presented to report the electrical and mechanical properties of single nanotubes and corresponding device characterization. First, chiral indices of a nanotube and its corresponding electrical resistance at room temperature are measured. A physical model based on the band gap theory is established to correlate the electrical resistivity with the atomic structure of the carbon nanotube. Second, I present a direct measurement of the torsional motion of both shells of a double wall carbon nanotube under an external torque on the outer shell. The measurement is performed by actuating a metal paddle attached to the outer shell of the nanotube while the stains of the nanotube are derived from its electron diffraction patterns. The inner shell is found to twist along with the outer shell with no stiction. The inter-shell friction, both static and kinetic, is inferred from direct measurements of each shell's deformation, van der Waals interactions between the two shells, and a tested model of lattice strain. Finally, the handedness of carbon nanotubes is determined using the same device. The implications are also discussed for potential applications and as directions of future research.Doctor of Philosoph

Low-frequency Current Fluctuations in Individual Semiconducting Single-Wall Carbon Nanotubes

We present a systematic study on low-frequency current fluctuations of nano-devices consisting of one single semiconducting nanotube, which exhibit significant 1/f-type noise. By examining devices with different switching mechanisms, carrier types (electrons vs. holes), and channel lengths, we show that the 1/f fluctuation level in semiconducting nanotubes is correlated to the total number of transport carriers present in the system. However, the 1/f noise level per carrier is not larger than that of most bulk conventional semiconductors, e.g. Si. The pronounced noise level observed in nanotube devices simply reflects on the small number of carriers involved in transport. These results not only provide the basis to quantify the noise behavior in a one-dimensional transport system, but also suggest a valuable way to characterize low-dimensional nanostructures based on the 1/f fluctuation phenomenon

Theoretical and Experimental Studies of Schottky Diodes That Use Aligned Arrays of Single Walled Carbon Nanotubes

We present theoretical and experimental studies of Schottky diodes that use aligned arrays of single walled carbon nanotubes. A simple physical model, taking into account the basic physics of current rectification, can adequately describe the single-tube and array devices. We show that for as grown array diodes, the rectification ratio, defined by the maximum-to-minimum-current-ratio, is low due to the presence of m-SWNT shunts. These tubes can be eliminated in a single voltage sweep resulting in a high rectification array device. Further analysis also shows that the channel resistance, and not the intrinsic nanotube diode properties, limits the rectification in devices with channel length up to ten micrometer.Comment: Nano Research, 2010, accepte

Non-volatile molecular memory elements based on ambipolar nanotube field effect transistors

We have fabricated air-stable n-type, ambipolar carbon nanotube field effect transistors (CNFETs), and used them in nanoscale memory cells. N-type transistors are achieved by annealing of nanotubes in hydrogen gas and contacting them by cobalt electrodes. Scanning gate microscopy reveals that the bulk response of these devices is similar to gold-contacted p-CNFETs, confirming that Schottky barrier formation at the contact interface determines accessibility of electron and hole transport regimes. The transfer characteristics and Coulomb Blockade (CB) spectroscopy in ambipolar devices show strongly enhanced gate coupling, most likely due to reduction of defect density at the silicon/silicon-dioxide interface during hydrogen anneal. The CB data in the ``on''-state indicates that these CNFETs are nearly ballistic conductors at high electrostatic doping. Due to their nanoscale capacitance, CNFETs are extremely sensitive to presence of individual charge around the channel. We demonstrate that this property can be harnessed to construct data storage elements that operate at the few-electron level.Comment: 6 pages text, 3 figures and 1 table of content graphic; available as NanoLetters ASAP article on the we