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Title:Physical mechanism of a capacitance-based graphene humidity sensor
Author(s):Sun, Tao
Department / Program:Mechanical Science & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
molecular dynamics (MD)
density functional theory (DFT)
Abstract:The interactions of ambient molecules with graphene-based devices, espe- cially sensors, is of great importance as such interactions could impact the operation of the device, often producing unpredictable effects. In this project, we focus on a graphene humidity sensor based on capacitance measurement to uncover 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 and the substrate (Hf O 2 ). Both of these phenomena can have large effects on graphene sensing behavior. When the device is in vacuum, the oxygen va- cancies (VOs) on the surface of the substrate can induce n-type doing effect to graphene. Then the device is brought into dry air, where O 2 molecules will enter between graphene and the substrate and fill the vacancies, which eliminates the n-type doping effect. O 2 molecules also appear on the top of graphene, acting as electron acceptors and causing p-type doping effect on graphene. After that the device is brought into the atmosphere, and the intercalation of H 2 O molecules underneath graphene is observed. The inter- facial distance between graphene and the substrate is enlarged, thus changing the measured capacitance. At the same time, H 2 O molecules appear above graphene will displace some of the originally existing O 2 molecules, which causes graphene to be less p-type doped than before. Our simulations un- cover how a capacitance-based graphene humidity sensor works, which is due to change of the interlayer distance caused by intercalated water molecules underneath graphene. Also through the interactions between graphene sensor system and ambient molecules, we understand the doping effect on graphene during the operation process.
Issue Date:2015-07-17
Rights Information:Copyright 2015 Tao Sun
Date Available in IDEALS:2015-09-29
Date Deposited:August 201

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