Development and application of graphene ？based devices

Abstract

Graphene is two dimensional honeycomb lattice structure consisting of carbon atoms, and is a basic building block for graphitic materials of all other dimensionalities. The transport characteristics of graphene, for example high charge carrier mobility and current density, tunability of electrical properties by external potential or chemical treatments, make this material attractive for applications in nanoelectronics.In chapter 2, the assembly methods of GO pieces which allowed us to mass-produce a uniform array of graphene-based memory devices using only conventional microfabrication facilities are introduced. As a proof of concept, we demonstrated the large-scale assembly of reduced GO for the fabrication of the conductivity-switching and type-switching memory devices. Significantly, we successfully demonstrated the real-time type-switching of carrier types in the reduced GO-FETs and reconfigurable inverter logic gates by controlling the charge in the Au NPs. We expect our results will provide enormous flexibility to the future electronic industry and will pave the way for advanced hybrid memory devices.In chapter 3, the growth characteristics of pentacene films, which depend on the surface properties of graphene, in an effort toward fabricating high-performance OFETs have been revealed. Polymer residues are inevitably physisorbed onto graphene surfaces during transfer and patterning processes. These residues induce a stand-up molecular orientation when growing a pentacene film. In contrast, clean graphene surfaces, from which polymer residues are removed by thermal treatment, are capable of π-interactions with pentacene that induce epitaxial pentacene growth to form pentacene crystals where molecules are arranged in a lying-down orientation. Interestingly, pentacene FETs using monolayer graphene source/drain electrodes with polymer residues exhibit an outstanding field-effect mobility of 1.2 cm2/Vs, which arise directly from continuous grain growth that benefits from identical molecular orientations of pentacene between channel and electrode.In chapter 4, High-performance pentacene FETs with graphene source/drain electrodes by transferring and patterning CVD-grown monolayer graphene films were fabricated. Systematic studies of the contact characteristics between source/drain electrodes and pentacene films revealed that our patterned monolayer graphene electrodes exhibited properties that were superior to those of common metal electrodes. By designing a new process (patterning of graphene on a Cu foil) for the fabrication of patterned graphene electrodes, transparent/flexible organic transistors that employed monolayer graphene electrodes were successfully assembled on plastics substrates with excellent performance properties. Monolayer graphene, with a high transparency and good conductivity, provides an alternative electrode material for use in next-generation flexible electronic devices assembled on plastic substrates.In chapter 5, a method to optimize the performance of graphene-based OFETs utilizing the work-function engineering by functionalizing the substrate with SAMs, showing ~10 times enhancements in the charge carrier mobility and the on-off ratio of OFETs was described. The doping type, carrier concentration, and Fermi energy level from the Dirac point are tuned by the surface characteristics of SAMs inserted between graphene and silicon oxide substrate, which were confirmed by XPS, UPS, Raman spectroscopy, and graphene FETs. Considering the patternability and robustness of SAMs, the present method would find numerous applications in graphene-based organic electronics and optoelectronic devices such as organic light emitting diodes and organic photovoltaic devices.In chapter 6, the systematic studies of the effects of various hydrophobic SAMs inserted between CVD-grown large-area graphene and SiO2 substrates were provided. The chemical and physical properties of graphene and the electrical characteristics of graphene FETs were investigated for various SAMs. As the SAM alkyl chain length increased, substrate-induced doping of the graphene decreased and, thus, the graphene FETs exhibited higher hole/electron mobilities with lower Dirac point voltages. Furthermore, changes in the electrical performances in the presence SAMs could be explained by charged impurity scattering, indicating that charged impurities in graphene could be tuned by the alkyl chain length of a SAM inserted between the graphene and the SiO2 substrate. Our study demonstrated that charged impurity scattering arises from the chemical and physical properties of a substrate surface as well as from the intrinsic quality of graphene. Thus, appropriate surface chemistries at graphene/dielectric interfaces are essential for enhancing the electrical properties of graphene FETs.In chapter 7, the effect of dual doping on the electrical properties of bilayer graphene FETs was invesgated. Single-gate bilayer graphene FETs having high current on/off current ratio were successfully fabricated by dual molecular doping. Our method provides that bandgap opening of bilayer graphene can be facilely accomplished by dual molecular doping which does not require a complicated fabrication step for preparing the device with dual-gate structure.