Semiconductor Nanowires as Efficient Sensing Elements
Dr. Usha Philipose
Department of Physics
University of North Texas
Tuesday, September 10, 2013
3:30 p.m., Room 104, Physics Building
Refreshments
3:15 p.m., Room 104
ABSTRACT: The promise of semiconductor nanowires to provide breakthroughs in the improvement of sensing devices has been the focus of my research program at UNT. In this seminar, I will present progress in three areas. The first of these addresses the challenge of controlling the structure and composition in InSb nanowires, with metallic (In), semiconducting (InSb) and semi-metallic (Sb) nanowires obtained within the framework of the same growth process. This work opens up the possibility of fabricating semiconducting nanowires with metallic ends, which will reduce contact resistance and thus address a challenge faced by nanoscale devices in terms of contact parasitic resistance degrading the device potential of nanomaterials. Over a decade ago, Mildred Dresselhaus and her group at MIT predicted that ZT (a measure of thermoelectric performance) can be increased above bulk values in thin nanowires by tailoring their diameter, composition and carrier concentration. Recent work in our lab shows that InSb nanowires are promising thermoelectric materials and the measured ZT factor is in agreement with recently published works. I will discuss some of my strategies to improve ZT in this materials system. A major requirement in the development of gas-sensing devices is their ability to promptly and reliably detect a broad range of gases in low concentrations. Experimental results obtained in my lab demonstrate the efficiency of individual In2O3 nanowires, functioning as an electronic 'nose', with its conductivity modulated by carrier concentration, carrier mobility, and composition of the sensing gas. Significant changes in the electrical conductance are observed within several seconds of exposure to NH3 and O2 gas molecules at room temperature, thus demonstrating the potential use of these nanowires as efficient miniaturized chemical sensors. Finally, recent investigations on the photo response behavior of a single ZnO nanowire contacted by metal electrodes shows a photocurrent "turn-on" for wavelengths shorter than 370 nm, corresponding to the band edge of ZnO. The enhanced photo response and longer lifetime observed from devices studied implicates the role of nanowire surface in photoconductivity.