An Investigation On The Electrical And Mechanical Properties Of Graphene Oxide And Reduced Graphene Oxide Sheets Based On Atomic Force Microscope

Graphene is a monolayer of carbon atoms arranged in a two-dimensional (2D)hexagonal network, which has attracted great interest from scientific communitiesbecause of its unique electrical, mechanical, optical and thermal properties. Fromgraphene oxide (GO) to reduced graphene oxide (rGO) is a cost-effective route formass production of graphene-based materials. The GO and rGO are attractivealternatives to pristine graphene due to their tunable electronic and mechanicalproperties, as well as compatibility to various substrates. Therefore, they have becomeversatile building blocks in numerous applications, such as field effect transistors(FET), ultracapacitors, sensors, memory devices, and lithium batteries. Owing to thefact that graphene-based materials are as thin as only one atomic layer, with lateralsize up to centimetre level, their physical properties are susceptible to environment,substrate, their own size and deformation.Scanning probe-based microscopes (SPM) including atomic force microscope(AFM) are among the most intuitive instruments in the research of graphene-basedmaterials. SPM provides not only an imaging means, which can be used forcharacterization of the surface morphology, charge transferring and local dielectricconstant, but also a manipulation tool to construct featured structures on thenanometer scale. In this dissertation, the effects of relative humidity (RH) andsubstrate on charge transferring on single layer GO and rGO sheets were investigatedby AFM-based charging and discharging techniques. In addition, an innovationalPeakForce Quantitative Nano-Mechanics (PF-QNM) mode of AFM was used tomeasure the adhesion change with varied tip bias and the threshold of cutting force ofGO/rGO sheets, based on which rGO nanostructures induced by DNA molecules wasfabricated by AFM manipulation. The main results are as following:1. The charge migration on the surface of single layer graphene oxide (GO)sheets was investigated by sample-charged scanning polarization force microscopy (SP-SPFM), which uses apparent height to characterize local charge distribution.Electrostatic charges were observed to transfer over one micrometer across the surfaceof the insulating single layer GO sheets with the help of water molecules condensedfrom the environment. Humidity-dependent charge migration on GO sheets wasvisualized on the nanometer scale and the migration velocity was plotted at a relativehumidity range of10%-40%. When the relative humidity reached50%or higher,water film would form on the whole GO sheet surface, which made the GO sheetconductive to the electrostatic charges. Our results indicate that the interfacial waterinduced charge mobility should be taken into account in engineering devicescomposed of graphene-based materials which would work under ambient conditions.2. When graphene-based materials are integrated into realistic devices, theirelectronic properties should be heavily influenced by the substrate. Through chargingand discharging of individual single layer rGO sheets on SiO2/Si surface, it was foundthat SiO2/Si substrate could “trap” electrostatic charges from rGO which bondedweakly via the van der Waals interaction with it. By using Kelvin probe microscopy(KPFM) and SC-SPFM, we discovered that, only when the surface potential of adischarged rGO sheet was above±0.5V, the conductive AFM tip outside rGO sheetcould be polarized. This result means single layer rGO sheet could screen internalcharges. A comprehension of the charge screening is essential for nanoscale electronicapplication, which determines how the number of carriers depends on the gate voltageand the effectiveness with which substrate noise is screened.3. A novel method to distinguish nano-objects with different dielectric constantwas developed, which based on the adhesion measurement in PF-QNM mode of AFM.It was found that the adhesion between a single layer rGO sheet and AFM tip was inproportion to the AFM tip bias and reduction extent of rGO sheet. Due to thereduction extent of rGO is closely related to its dielectric constant, this feature can beused to detect local dielectric distribution. We made a comparison of this method withscanning polarization force microscopy (SPFM), a conventional technique tocharacterize relative dielectric constant distribution by measuring the apparent height. Results indicated that our method could offer higher sensitivity and lateral spatialresolution, and insusceptible to ambient conditions.4. Tailoring the geometry of graphene sheet is very important for fundamentalresearch and potential applications. AFM-based nanolithography is an effective meansto construct pre-designated graphene nanostructures. We developed a manipulationmethod based on PF-QNM mode of AFM, which could conveniently, intuitively andprecisely control the load applied by AFM tip. Using this method, the threshold ofcutting force of GO and rGO sheets was investigated. In addition, cutting, pushingand sweeping manipulations were realized on DNA nanostructures covered by singlelayer rGO sheets, which would help to construct diverse graphene nanostructuresinduced by biomolecules.