Density Functional Theory Study of Adsorption and Selection Behavior of Harmful Gas Molecules on the Surface of SWNTs: Implications for Gas Sensing

Abstract

To explore the adsorption and selection behavior of harmful gas molecules on the surface of single-walled nanotubes (SWNTs) within field-effect transistors (FETs), NO 2 , SO 2 , NO, CO 2 , H 2 S, and NH 3 adsorption effects on the electronic character of SWNTs (three different armchair SWNTs with indexes (6,6), (9,9), and (10,10)) were explored theoretically by density functional theory (DFT) calculations in this work. Two factors determining the sensing mechanism of real devices, namely, the electrostatic gate modulation caused by internal carrier mobility within SWNT networks and the interfacial Schottky barrier effects caused by the work function changes of SWNTs, will be examined. Calculated results show that the adsorption process of gas molecules on the surface of SWNTs is exothermic and energetically favored. From the work function results, it is predicted that the interfacial Schottky barrier of the contact between SWNTs and neighboring materials must be affected significantly upon adsorption of different gas molecules, which may lead to distinct electrical resistance changes of SWNT-FETs. Moreover, it is observed that electron density distribution of SWNTs is increased by NH 3 adsorption while it is decreased by other gas adsorptions. It means that the internal carrier mobility of SWNTs must be increased by NH 3 adsorption while it must be decreased by other gas adsorptions with a trend NO 2 > SO 2 > NO > CO 2 > H 2 S. Therefore, it is confirmed that SWNTs should be highly electrically sensitive and responsive to harmful gas molecules, which is favorable for their application in gas sensor areas.