Doping effects of surface functionalization on graphene with aromatic molecule and organic solvents

Aromatic molecule functionalization plays a key role in the development of graphene field-effect transistors (G-FETs) for bio-detection. We have investigated the doping effects of surface functionalization and its influence on the carrier mobility of graphene. The aromatic molecule (1-pyrenebutanoic acid succinimidyl ester, PBASE), which is widely used as a linker to anchor bio-probes, was employed here to functionalize graphene. Dimethyl formamide (DMF) and methanol (CH₃OH) were used as two solvents to dissolve PBASE. Raman spectra showed that both PBASE and these two solvents imposed doping effects on graphene. The PBASE was stably immobilized on the graphene surface, which was confirmed by the new peak at around 1623.5 cm⁻¹ and the disordered D peak at 1350 cm⁻¹. Electrical measurements and Fermi level shift analysis further revealed that PBASE imposes a p-doping effect while DMF and CH₃OH impose an n-doping effect. More importantly, CH₃OH causes a smaller reduction in the carrier mobility of G-FETs (from 1095.6 cm²/V s to 802.4 cm²/V s) than DMF (from 1640.4 cm²/V s to 5.0 cm²/V s). Therefore, CH₃OH can be regarded as a better solvent for the PBASE functionalization. This careful study on the influence of organic solvents on graphene during PBASE functionalization process provides an effective approach to monitor the surface functionalization of graphene.