Quantum confinement effects on the harmful-gas-sensing properties of silicon nanowires

In this work, the effects of the adsorption of different toxic gas molecules CO, NO, NO₂, and SO₂ on the electronic structure of hydrogen-passivated, [111]-oriented, silicon nanowires (H-SiNWs), are studied through density functional theory. To analyze the effects of quantum confinement, three nanowire diameters are considered. The results show that the adsorption energies are almost independent of the nanowire diameter with NO₂ being the most strongly adsorbed molecule (∼3.44 eV). The electronic structure of small-diameter H-SiNWs is modified due to the creation of isolated defect-like states on molecule adsorption. However, these discrete levels are eventually hybridized with the former nanowire states as the nanowire diameter increases and quantum confinement effects become less evident. Hence, there is a range of small nanowire diameters with distinctive band gaps and adsorption energies for each molecule species.