  A wireless vapor sensor based upon the quantum capacitance effect in graphene
is demonstrated. The sensor consists of a metal-oxide-graphene variable
capacitor (varactor) coupled to an inductor, creating a resonant oscillator
circuit. The resonant frequency is found to shift in proportion to water vapor
concentration for relative humidity (RH) values ranging from 1% to 97% with a
linear frequency shift of 5.7 +- 0.3 kHz / RH%. The capacitance values
extracted from the wireless measurements agree with those determined from
capacitance-voltage measurements, providing strong evidence that the sensing
arises from the variable quantum capacitance in graphene. These results
represent a new sensor transduction mechanism and pave the way for graphene
quantum capacitance sensors to be studied for a wide range of chemical and
biological sensing applications.
