Physical mechanism of a capacitance-based graphene humidity sensor

graphene;capacitance;humidity;molecular dynamics (MD);density functional theory (DFT)

Sun, Tao

关键词: graphene;    capacitance;    humidity;    molecular dynamics (MD);    density functional theory (DFT);   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/88203/SUN-THESIS-2015.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

The interactions of ambient molecules with graphene-based devices, espe-cially sensors, is of great importance as such interactions could impact theoperation of the device, often producing unpredictable effects. In this project,we focus on a graphene humidity sensor based on capacitance measurement touncover fundamental physical mechanisms governing the operation of the de-vice. Using molecular dynamics (MD) and density functional theory (DFT)simulations, we show that ambient molecules (mainly O 2 and H 2 O) can ap-pear on the top of graphene, and get intercalated between graphene andthe substrate (Hf O 2 ). Both of these phenomena can have large effects ongraphene sensing behavior. When the device is in vacuum, the oxygen va-cancies (VOs) on the surface of the substrate can induce n-type doing effectto graphene. Then the device is brought into dry air, where O 2 moleculeswill enter between graphene and the substrate and fill the vacancies, whicheliminates the n-type doping effect. O 2 molecules also appear on the topof graphene, acting as electron acceptors and causing p-type doping effecton graphene. After that the device is brought into the atmosphere, and theintercalation of H 2 O molecules underneath graphene is observed. The inter-facial distance between graphene and the substrate is enlarged, thus changingthe measured capacitance. At the same time, H 2 O molecules appear abovegraphene will displace some of the originally existing O 2 molecules, whichcauses graphene to be less p-type doped than before. Our simulations un-cover how a capacitance-based graphene humidity sensor works, which is dueto change of the interlayer distance caused by intercalated water moleculesunderneath graphene. Also through the interactions between graphene sensorsystem and ambient molecules, we understand the doping effect on grapheneduring the operation process.