Solid Electrolyte Coated Graphene Transistors For Sensing Applications

The arrival of the Internet of Things(Io T)era brings unprecedented development opportunities for sensors,as well as new challenges.Sensors in Io T network are expected to have different features,including miniaturization,low power consumption,and high performance,to meet the requirements of the new scene.Therefore,high-performance sensors based on new materials,new structures,and new principles are urgent for investigation.Graphene with high carrier mobility,high specific surface area,broad absorption spectral,good flexibility,and ultrathin thickness is a promising material in sensing applications.Moreover,graphene-based transistors are featured of high speed and miniaturization.High-capacitance solid electrolytes are an ideal candidate for gate dielectrics in transistors,and these electrolytes with unique properties are also widely used in flexible sensors.In the Io T era,strain sensors,tactile sensors,humidity sensors,ethanol sensors,and photodetectors are indispensable.However,these devices are currently suffering some low sensing performance and poor functionality.Based on the purpose of improving the sensing performance and functions,this dissertation studied the physical and chemical properties of solid electrolyte and graphene,and combined them in the form of transistor to realize strain and tactile sensing,humidity and ethanol sensing,and photoelectric sensing applications based on the new working mechanisms.The main contents are as follows:(1)A strain sensor with the ability to distinguish strain polarity was realized by coupling piezoionic effect of solid electrolyte and the resistance variation of the mechanically doped graphene.Under the application of compressive strain(or tensile strain),piezoionic potential generated inside the solid electrolyte caused Dirac point voltage of the transistor to move to the left(or to the right).The sensor showed a stable output response to the stepwise strain,with a response time of approximately 250 ms.The operating range of the device is 0.045 %–0.312 %,and its response can be divided into two regions.In region I(tensile strain < 0.23 %),the sensor showed approximately linear output performance to the variation of tensile strain,and its gauge factor(GF)is about-16.In region II(tensile strain > 0.23 %),the GF is about-30.The strain sensor can be attached to human hand to monitor the joint movement.A tactile sensor with the ability to distinguish contact objects was realized by utilizing the triboelectric effect of solid electrolyte.The device showed different response signals when contacted with nylon film and PFA film.(2)A fast-response humidity sensor with the ability to tune the response value and response polarity was realized by coupling the attraction effect of the hydrophilic polymer PEGDA in [EMIM][TFSI]-based ion gel to polar water molecules and the external gate effect of the polar water molecules.The sensor's response to humidity has two sensitivity regions.When the relative humidity is < 71 %,the sensitivity is ?0.0014;when the relative humidity is > 71%,the sensitivity is 0.0135.The device showed a stable response to different humidity conditions over an extended period of four weeks.The device is capable to monitor the human breath when placed near the mouth.The response signal of the sensor is about 23% to each exhalation,and the response time is 350 ms.Through the modulation effect of small external gate voltages,higher response output signals and opposite polarity response signals were observed.In addition,due to the presence of water molecules at the fingertips,the humidity sensor can also be used to detect the distance between the fingertips and the device.An ethanol gas sensor was realized by utilizing the external gate effect that arises from the attachment of ethanol molecules.The Dirac point voltage showed a significant negative shift when the device was placed in an ethanol atmosphere.The response signal was-11% to the ethanol gas concentration of 78.51 g/L,the response time and recovery time were about 10 s and 18 s,respectively.The detection range of the ethanol gas sensor was from 19.76 g/L to 785.1 g/L.(3)The enhancement of ion gel on the negative photoconductivity of graphene was explored.In graphene-based negative photoconductivity detectors,the intrinsic CVD graphene without electrolyte coating showed a negative current change under the illumination,and the current recovered slowly after the light was switched off.This negative photoconductivity effect exhibited irradiation wavelength dependency and voltage bias dependency.Compared with the non-electrolyte coated sample,electrolyte coated graphene photodetector with increased carrier mobility showed stronger negative photoconductivity effect,e.g.larger negative current variation.Moreover,the current recovery speed of the electrolyte coated graphene photodetector was faster.The enhancement of ion gel on the graphene/molybdenum disulfide/graphene heterostructure-based photodetector was explored.The heterostructure photodetector generated a high positive photocurrent under the application of illumination and voltage bias,and exhibited irradiation wavelength dependency and voltage bias dependency.Compared with the non-electrolyte coated sample,the dark current and photocurrent of the electrolyte coated heterostructure sample were significantly increased,and the response speed was faster.