Quantum Capacitance of Graphene and Its Gas Sensing Applications

Abstract

Based on graphene and quantum capacitance (CQ), two types of graphene field-effect transistors (G-FETs) with dual-gated and back-gated systems are fabricated, and their electronic properties and capacitive NO2 sensing performance are investigated. Capacitance of a graphene-based capacitor is the series connection of quantum capacitance and geometrical capacitance (Cgeo). Quantum capacitance provides its intrinsic properties, and to measure quantum capacitance, geometrical capacitance should be comparable to the minimum quantum capacitance of graphene. Dual-gated G-FETs are fabricated using mechanically exfoliated graphene from natural graphite flakes. To enhance geometrical capacitance, several thin top-gate dielectric layers are introduced such as cross-linked PMMA, a bilayer of AlOx and cross-linked PMMA, and AlOx formed by Al top-gate electrodes. Cgeo of the AlOx formed from the Al top-gate shows the highest value in above top-gate dielectrics. Acquired CQ is well in agreement with a theory for ideal graphene except for near the Dirac point. The discrepancy in values near the Dirac point is then adjusted by considering extra residual carrier density induced from charged impurities at finite temperatures. The modified theory with residual carriers shows a great agreement with CQ measurements with the extracted residual carrier density of n* = 3.58×1011 cm-2. Furthermore, a new device design of G-FETs for capacitive sensing application is developed by incorporating Al back-gate electrode on sapphire substrates, which gives approximately 1 μF/cm2 of Cgeo. Graphene is known to be one of the promising sensing materials due to its high surface-to-volume ratio, low electrical noise, and high electrical conductivity. Molecules that adhere to graphene act as impurities that affect the electron transport within graphene. Two of the common ways to evaluate such change are measuring changes in resistance and change in quantum capacitance. Previous research has been largely focused on using resistive measurement due to restrictions from device design even though the capacitive measurement can be cost-effective. To overcome the obstacles, G-FET with high capacitance and a large exposed channel area was developed by incorporating Al back-gate electrodes with naturally oxidized AlOx surface as an insulating layer. The measured capacitance was well-modulated in vacuum by the gate voltage owing to the quantum capacitance effect. When the graphene, the top-side of the device, is exposed to NO2, the quantum capacitance of graphene, and thus, the measured capacitance of the device changed in accordance with NO2 concentrations of 1-100 parts per million. The operational principle of the proposed device is also explained by the changes in gate voltage-dependent capacitance of the device exposed to various concentrations of NO2. Further analyses regarding carrier density changes and potential variances under various concentrations of NO2 are also presented to strengthen the argument. The results demonstrate the feasibility of capacitive NO2 sensing using graphene and the operational principle of capacitive NO2 sensing.