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

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