Piezotronic Effect on Ultrasensitive Chemical and Biochemical Sensors

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Nontechnical Description: Sensors have numerous applications such as monitoring carbon monoxide near home furnaces or fireplaces, detecting explosives for airport security, monitoring harmful chemical vapors in industrial plants, and tracking air pollution sources. Sensors fabricated using nanomaterials have been demonstrated to be more sensitive than sensors made of large-size materials. This research project combines piezoelectric semiconductor nanomaterials (i.e., materials that can generate electric charges when subjected to mechanical forces) in the sensor designs with the ultimate goal to achieve ultrahigh sensitivity. The fundamental understanding on sensing mechanisms of these nanomaterials developed in this research project is expected to further advance the sensor designs. Graduate and undergraduate students in this project are trained in the growth and characterization of nanomaterials as well as sensor device fabrication and testing. The research findings are incorporated into two existing courses: Nanomaterials and Nanotechnology and Advanced Nanomaterials, which enriches the materials science curriculum at Georgia Tech. Outreach activities include involving K-12 teachers in research through the existing programs on the Georgia Tech campus. Technical Description: The typical functioning component of chemical and biochemical nanosensors is a field-effect transistor structure made of nanowires. The current change in the nanowires after exposing to gas molecules gives rise to the sensitivity. The high sensitivity of these nanowire-based sensors results from the high surface-to-volume ratio and a large number of available molecular binding sites to the nanowires to change its conductance. Ohmic contacts are typically applied between the semiconducting nanowire/nanotube and the metal electrodes in these sensors. Instead, this project focuses on the Schottky contact in nanowire sensors because of the ultrahigh sensitivity and fast-response compared to the nanowire nanosensors with the Ohmic contact. Combining the piezotronic effect, the transport of charge carriers across a metal-semiconductor barrier or p-n junction is modulated by piezoelectric charges. Specifically, the localized piezoelectric charges alter the free charge carrier redistribution and band structures near the interface, and further impact the charge carrier transport properties. Electron holography and in-situ transmission electron microscopy (TEM) are used to quantitatively map the piezoelectric charges/potentials at the metal/semiconductor interface. This study provides fundamental understanding of the sensing mechanisms of these nanosenor materials, by providing key microstructural and electrical properties at the metal-semiconductor interfaces.