Effects Of Solution Environment On The Performance Of Graphene Devices

Graphene (Gr) has attracted great research interest since its rediscovery in2004. Researchers inthe fields of physics, chemistry, materials, electronics and others have put a lot of work in this areadue to its unique properties such as high mobility of Dirac carriers, superior thermal conductivity,high strength, high transparency and flexibility. Known as the thinnest material with very highmechanical and chemical stability, graphene has extensive applications in smart sensors.Graphene-based electronic sensors—such as gas, PH, metal ions and biomolecules sensors and DNAdetectors have been proposed or simply fabricated, most of these sensors worked in solutionenvironments. However, the basic properties of graphene devices in solution environment—such astransfer transport properties, Hall sensitivity, magnetoresistance and others are still lack of systematicresearch and clear understanding. In this thesis, we have systematically investigated the controllingand adjusting methods of graphene properties in different solution environment. The main results areconcluded as follows:CVD growth of monolayer graphene and fabrication of graphene devices. Graphene films weregrown on Cu foil by chemical vapor deposition (CVD) method using methane as a carbon source, thescanning electron microscope (SEM) and Raman charactarization show that the large area of singleatomic layer graphene could be formed under appropriate conditions. The effects of the surfacemorphology of Cu foil on the quality of graphene have been investigated using surface oxidationmethod. There are more defects and fragments in the graphene grown on roughed Cu surface thanthose grown on well-ordered Cu surface. The quality of graphene can be significantly improved whenroughed Cu foils were pre-treated by electrochemical polishing process. After CVD growth, thegraphene layers were transferred onto other insulated substrates like PET using PMMA coating andsubstrate etching methods. The graphene on insulated substrates were then used to fabricatesolution-gated graphene FET and Hall device samples. The current-voltage (I-V) characterizationsshowed that the graphene samples are in good ohmic contact with their metal electrodes.Effects of different solution environment on the graphene Dirac point and the transfercharacteristics. The transport properties of the graphene devices were measured in DI water、NaCl andPEI solution by solution-gated controlling method. Under normal circumstances, graphene is p-typedoped by water or oxygen moleculars. In NaCl solution, graphene shows p-type characteristics too.With the increase of NaCl solution concentration, the Dirac point moved to the right and the transfer curve became more asymmetry, which revealed the deeper p-type doping of graphene. However,when the graphene device was put in PEI solution, the Dirac point was moving to the left with theincrease of solution concentration, which showed that the p-type doping was gradually reduced ingraphene.Hall sensitivity and magnetoresistance property of graphene in solution environment. We havesystematically measured the Hall response and magnetoresistance changes of graphene in differentsolution at room temperature and atmospheric pressure. The hall response can be over200VA-1T-1inDI water or some solutions. In NaCl solution, with the increase of concentration, the Hall responsewas decreasing gradually, which caused by the enhancement of graphene doping and interfacescattering by solution ions. In the contrary, the Hall response was increasing gradually with theincrease of PEI solution concentration, indicating the effectively neutralization of p doping by PEI.Magnetoresistance effect in graphene usually caused by weak localization can also be dramaticallycontrolled by solution: the magnetoresistance was decreasing with the increase of NaCl concentration,while increasing with the increase of PEI concentration. The magnetoresistance (MR) can be adjustedfrom1.7%to13%at1.17T in different solution.