Investigation of gas–surface interactions at self-assembled silicon surfaces acting as gas sensors

This paper reports the results of an investigation aimed at using self-assembled monolayers to modify the supramolecular interactions between Si surfaces and gaseous molecules. The specific goal is that of employing molecularly imprinted silicon surfaces to develop a new class of chemical sensors capable to detect species with enhanced selectivity. Single-crystal p-type (0 0 1) silicon has been modified by grafting organic molecules onto its surface by using wet chemistry synthetic methods. Silicon has been activated toward nucleophilic attack by brominating its surface using a modified version of the purple etch, and aromatic fragments have been bonded through the formation of direct Si–C bonds onto it using Grignard reagents or lithium aryl species. Formation of self-assembled monolayers (SAMs) was verified by using vibrational spectroscopy. Porous metal–SAM–Si diodes have been successfully tested as resistive chemical sensors toward NOx, SOx, CO, NH3 and methane. Current–voltage characteristics measured at different gas compositions showed that the mechanism of surface electron density modulation involves a modification of the junction barrier height upon gas adsorption. Quantum-mechanical simulations of the interaction mechanism were carried out using different computational methods to support such an interaction mechanism. The results obtained appear to open up new relevant applications of the SAM techniques in the area of gas sensing.