Ravi Shankar Sundaram
Graphene, an atomically thin sheet of carbon, is the most recent endeavor for the application of carbon nanostructures in conventional electronics. The envisioned creation of devices completely carved out of graphene could lead to the revolution of electronic circuitry. However, the most established technique to obtain high quality graphene sheets, i.e, by micromechanical cleavage cannot be easily upscaled, serving as an impediment towards technological applications. The present thesis is dedicated to the study of graphene prepared via an alternative scalable, high yield and cost effective method which involves the chemical reduction of graphene oxide. The first part of this thesis describes in detail the atomic structure of graphene obtained by this method. Raman spectroscopy was used for this purpose followed by Transmission Electron Microscopy (TEM) and Near Edge X-Ray and Fine Structure (NEXAFS) measurements locally probe these sheets with atomic resolution. This revealed a highly disordered structure of this material, where regions with perfect crystallinity are separated by defect clusters. These defective patches were found to contain remnant oxygenated functional groups. Electrical characterization of chemically derived graphene sheets yielded ambipolar transfer characteristics similar to that of micromechanically cleaved graphene, albeit with moderate performance. The presence of a significant amount of disorder precludes ballistic transport and the charge carriers traverse through the sheet predominantly by thermally assisted variable range hopping in combination with a field driven tunneling mechanism. This is clearly evidenced in the low temperature two-probe conduction measurements performed on devices fabricated from these sheets. Metal contacts in graphene devices play a crucial role in limiting the applicability of these materials. Although the performance of graphene based devices as prepared today is sufficient for a variety of applications, a clear understanding and engineering of contacts is important for utilization of these materials towards high end applications. In the latter part of this thesis a detailed study of contacts is presented; firstly on graphene grown via CVD on copper to understand the physics in play at a simpler carbon-metal interface, followed by a photocurrent microscopy study to understand the more complex interface between metal and reduced graphene oxide and its implications on device characteristics. A strong difference between the two kinds of graphene is found with metal contacts to high quality graphene creating a potential barrier at the interface owing to a charge transfer between them, whereas in the case of the chemically synthesized version contacts are of a non invasive nature stable upto 300 °C. In addition, Raman spectroscopy and Scanning Photocurrent Microscopy investigation of graphene obtained by CVD under the gold, away from the contact edge provided interesting insights towards the phenomena occuring at the gold graphene interface, thus highlighting the importance of understanding and engineering adequate contacts before the successful transition of graphene based devices into the industry. Finally, a novel chemical vapor treatment of graphene oxide was employed to enhance its electrical characteristics. Exposure of reduced graphene oxide to ethylene vapor at 800 °C results in an improvement of conductivities by 2 orders of magnitude lagging behind that of pristine graphene. Electrical and structural characterization revealed that this increase in conductivity occurred inspite of the presence of a considerable amount of defects.
EPFL2011
