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. Given their large surface‐to‐volume ratio, silicon nanowires are excellent for sensing of toxic gases at a molecular level. The effect of the diameter on the detection properties of these systems, however, has not been sufficiently explored. In this work, these effects are studied from first principles, analyzing how adsorption and electronic properties of Si nanowires of three different diameters change on the adsorption of CO, NO, NO2, or SO2 molecules.