Gold Nanoparticle Decorated Carbon Nanotube Field Effect Transistors for Glucose and Gas Sensing The research presented in this thesis focuses on the fabrication, functionalization and characterization of carbon nanotube field effect transistors (CNTFETs) with gold (Au) nanoparticles for gas and glucose sensing applications.; Carbon nanotube field effect transistors are made of an individual carbon nanotube (CNT) in this work. Carbon nanotubes are synthesized using chemical vapor deposition (CVD) on a p-doped silicon/silicon oxide substrate. This process provides mostly single-walled carbon nanotubes (SWCNTs) that are either metallic or semiconducting. Au is deposited onto the carbon nanotube using electron-beam lithography to make source and drain electrodes. Electrical characterization is performed using software controlled pico-amp meter/DC voltage source connecting to a four-probe micromanipulator system.; A simple in-situ electrochemical method to target the deposition of gold, as well as other metallic nanoparticles along a CNTFET is introduced in this research. The transistors are passivated by a thin layer of poly(methyl-methacrylate), or PMMA. Areas of the PMMA along the carbon nanotube are exposed using electron-beam lithography to target the locations where Au nanoparticles need to be placed. An appropriate potential difference is applied between an in-situ sacrificial gold electrode and the CNT, all immersed under a droplet of electrolyte solution. By adjusting the applied voltage and time of deposition, the size of the Au nanoparticle can be controlled from 10 nm to over 100 nm. This method provides better control and is much easier to carry out compared to other site-specific deposition techniques. Such decorated Au nanoparticle/CNTFET heterostructures will allow for a better understanding of CNTFET gas sensing behavior, as well as finding application in site-specific biomolecule anchoring for the development of highly sensitive and selective biosensors, which will be demonstrated in this research work.; This research suggests that when a Au nanoparticle is deposited onto a semiconducting CNT, a local Schottky barrier is created on the CNT side wall. This Schottky barrier changes when exposed to gas molecules, which is the main mechanism for gas sensing of the CNTFET. For glucose detection, Au nanoparticles provide a preferable orientation of the glucose oxidase molecules, which potentially facilitates the electron transfer between glucose oxidase and the CNT. Ph.D.
Mechanism of Gas Sensing in Carbon Nanotube Field Effect Transistors Gas sensors based on carbon nanotubes in the field effect transistor configuration have exhibited impressive sensitivities compared to the existing technologies. However, the lack of an understanding of the gas sensing mechanism in these carbon nanotube field effect transistors (CNTFETs) has impeded setting-up a calibration standard and customization of these nano-sensors for specified gas sensing application. Calibration requires identifying fundamental transistor parameters and establishing how they vary in the presence of a gas and influence the overall sensing behavior. This work focuses on modeling the sensing behavior of a CNTFET in the presence of oxidizing (NO2) and reducing (NH3) gases and determining how each of the transistor parameters, namely: the Schottky barrier height, Schottky barrier width and doping level of the nanotube are affected by the presence of these gases.; Earlier experiments have shown that the carbon nanotube-metal interface is responsible for the observed change in the CNTFET response. The interface consists of the metal contact and the depletion region in the carbon nanotube. A change in the metal work function will change the Schottky barrier height, whereas doping of the depletion region will affect the Schottky barrier width and the doping level of the carbon nanotube. A theoretical model containing these parameters was systematically fitted to the experimental transfer characteristics for different concentrations of NO2 and NH3. A direct correlation between the measured changes in the CNTFET saturated conductance and the Schottky barrier height was found. These changes are directly related to the changes in the metal work function of the electrodes that I determined experimentally, independently, with a Kelvin probe system. The overall change in the CNTFET characteristics were explained and quantified by also including changes due to doping from molecules adsorbed at the carbon nanotube-metal interface through the parameters Schottky barrier width and the doping level. Ph.D.