Carrier-doping by oxygen/water adsorption in graphene on substrates modified with self-assembled monolayers of different acidity

Abstract

The carrier-doping of monolayer graphene induced by molecular adsorption was investigated by changing the interfacial proton activity by chemical modification of the supporting substrate surface with self-assembled monolayers (SAMs) of different acidity. Graphene field-effect transistors were fabricated on SiO₂/Si substrates functionalized with n-octyltriethoxysilane (OTS) and 3-aminopropyltriethoxysilane (APS), which respectively contain neutral alkyl and basic amino terminal groups. Surface characterization through water contact angle measurements and X-ray photoelectron spectroscopy confirmed the formation of a SAM. Electrical transport measurements showed that oxygen exposure induced hole-doping in graphene on OTS, whereas graphene on APS showed electron-doping by both oxygen and water vapor adsorption. The type and density of carriers doped by oxygen/water adsorption were explained by a water-mediated electrochemical doping mechanism, in which the redox potential is changed by the interfacial proton activity governed by the acidity of the SAM. Systematic shifts of the G and G’ bands in the Raman spectra supported the carrier-type variation. These findings indicate that adsorption-induced charge transfer in graphene can be electrochemically changed by changes in the substrate surface chemistry. The ability to tailor oxygen- and water-mediated doping through interfacial proton control provides a versatile platform for the design of graphene-based sensing devices with different carrier characteristics.