n/p-Type Doping Modulation of the Adsorption and Selection Behavior of Harmful Gas Molecules on the Surface of SWCNTs for Enhanced Gas-Sensing Performance

Single-walled carbon nanotubes (SWCNTs) are a promising candidate material for detecting harmful gases due to their unique advanced character, but their gas-sensing properties still need to be improved. With the aim of exploring more effective modulation ways to improve the gas-sensing behavior of SWCNTs, the surface doping effects of the sodium (Na) atom, a typical n-type dopant, and tetracyanoethylene (TCNE), a typical p-type dopant, on the electronic and sensing properties of (7,3), (6,5), and (7,5) SWCNTs for NO₂, SO₂, NO, CO₂, H₂S, and NH₃ were examined theoretically with density functional theory (DFT) calculations. It is found that the decoration of SWCNTs with Na/TCNE dopant is energetically favorable, with enhanced/lowered frontier energy levels. Therefore, the energy-level alignment among the frontier orbitals of SWCNTs and gas molecules can be regulated effectively. The interfacial charge transfer that occurs from the occupied valence band maximum (VBM) of SWCNTs to the empty lowest unoccupied molecular orbital (LUMO) of gas molecules is much more significant than that between the occupied VBM of SWCNTs and the highest occupied molecular orbital (HOMO) of gas molecules. As a result, among the gas-adsorbed cases considered here, carrier concentration increments and the frontier energy level of gas-adsorbed SWCNTs (i.e., the internal carrier mobility of SWCNTs and interfacial Schottky barrier of the contact between SWCNTs and neighboring materials within single-walled carbon nanotube field-effect transistors (SWCNT-FETs)) are changed more significantly for NO₂- and SO₂-adsorbed pristine SWCNTs, for NO₂-, SO₂-, and NO-adsorbed n-type SWCNTs, and for NO₂-adsorbed p-type SWCNTs. Our study highlights the key role that a controlled electronic character of dopants can play in regulating the gas adsorption and selection behaviors for their practical gas sensor applications.