Interactions of Water Molecules Adsorbed on Hydrophilic Substrates Clarified by Transport Characteristics of Graphene

Hydrophobic treatment of SiO₂/Si substrates is indispensable for achieving excellent transport characteristics in graphene devices. Although the interaction of the carriers with electric dipole moments of water molecules is believed to degrade the transport characteristics, the configuration and electric-field-induced control of molecular alignment remain to be elucidated. In this study, we investigate the electric polarization of the water molecules adsorbed on hydrophilic SiO₂ by measuring the conductivity of graphene field-effect transistors (FETs). The conductivity is found to be highly sensitive to polarization-induced changes in carrier accumulation. Consequently, we demonstrate that carrier accumulation can be enhanced by interfacial water at room temperature. Furthermore, the temperature and gate voltage dependences of the conductivity reveal the phase diagram of the interfacial water/ice layer. Notably, the freezing point is suppressed at zero gate voltage due to the electric field induced by negative charges on the SiO₂, whereas it is enhanced with increasing gate voltage. The stable ice phase without electric polarization is also observed at high gate voltages.