Nanogap Junctions and Carbon Nanotube Networks for Chemical Sensing and Molecular Electronics

Abstract

This thesis work may be divided into two parts. The first part (chapters 2-7) focuses on the fabrication of a particular test structure, the electromigration (EM) formed metal nanogap junction, for studying the conduction through single molecules and for hydrogen sensing. The second part (chapters 8 and 9) focuses on carbon nanotube networks as electronic devices for chemical sensing.

Chapters 2-4 discuss the formation of nanogap junctions in thin gold lines fabricated via feedback controlled electromigration. Using a feedback algorithm and experimenting on thin gold lines of different cross sections, I show that the feedback controls nanogap formation via controlling the temperature of the junction. Chapters 5 and 6 discuss the background and my experimental efforts towards fabricating superconducting electrodes for single molecule electronics research.

Chapter 7 discusses the application of the techniques of chapters 2-4 to form palladium nanogaps via electromigration. I show that such devices can be used as hydrogen sensors, but suffer from slow response times (on the order of minutes). The results are discussed in the context of the in-plane stress buildup between the palladium metallization and the SiO2 substrate.

The use of nanotube networks as chemical sensors is discussed in the second part of the thesis (chapters 8 and 9). I show measurements of the resistance and frequency-dependent (50 Hz - 20 KHz) gate capacitance of carbon nanotube thin film transistors (CNT-TFTs) as a function of DC gate bias in ultra-high vacuum as well as low-pressure gaseous environments of water, acetone, and argon. The results are analyzed by modeling the CNT-TFT as an RC transmission line. I show that changes in the measured capacitance as a function of gate bias and analyte pressure are consistent with changes in the capacitive part of the transmission line impedance due to changes in the CNT film resistivity alone, and that the electrostatic gate capacitance of the CNT film does not depend on gate voltage or chemical analyte adsorption to within the resolution of my measurements. However, the resistance of the CNT-TFT is enormously sensitive to small partial pressure (< 10-6 Torr) of analytes, and the gate voltage dependence of the resistance changes upon analyte adsorption show analyte-dependent signatures.