Fabrication and Optoelectronic Properties of Graphene Nanoribbons

Graphene, a single layer of carbon atoms, exhibits excellent charge transport properties. However, due to the absence of a band gap in this two-dimensional carbon nanostructure, graphene-based field effect transistors cannot be turned off. One strategy to increase the on/off ratio relies on patterning graphene into narrow stripes, so-called graphene nanoribbons (GNRs). The present thesis aimed at developing novel fabrication and chemical functionalization methods for GNRs. Along these lines, electrical transport studies and spectroscopic investigations were envisioned as a major tool to monitor the changes brought about by the functional groups attached to the GNRs. GNRs were fabricated with the aid of V2O5 nanofibers or CdSe nanowires as an etching mask during the plasma etching of graphene. The resulting GNRs exhibited good electrical conductivity for ribbon widths as small as 10 nm. Moreover, the transport gap in the GNRs was found to scale inversely with the ribbon width. Scanning tunneling microscopy and surface enhanced Raman spectroscopy testified a good structural quality of the GNRs. Electrical characterization of GNRs revealed a pronounced hysteresis, which was exploited for the fabrication of electrically switchable GNR memory cells. Dynamic pulse response measurements demonstrated reliable switching between two conductivity states for clock frequencies of up to 1 kHz and pulse durations as short as 500 ns for >10^7 cycles. As the most likely switching mechanism, charge trapping within a water layer in the GNR surrounding could be identified. Furthermore, the optoelectronic properties of individual GNRs were studied by scanning photocurrent microscopy. The pronounced photocurrent signal close to the nanoribbon/metal contacts was found to be directly proportional to the conductance of the devices, suggesting that a local voltage source is generated at the nanoribbon/metal interface by the photo-thermoelectric Seebeck effect. The dominance of this mechanism over charge separation via built-in electrical fields was attributed to strong local heating of the GNRs by the laser spot, combined with the reduced thermal conduction capability of the nanoribbons in comparison to extended graphene sheets. Chemical functionalization of graphene and GNRs was attempted via different gas- and liquid-phase approaches. While electrical transport and Raman measurements revealed the presence of significant doping effects, it was not possible to unequivocally prove the covalent attachment of atoms at the GNR edges, neither through changes in the charge transport characteristics, nor via scanning microscopy studies.

Electrical Properties of Chemically Derived Graphene

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

Atomic Structure of Reduced Graphene Oxide

Marko Burghard, Klaus Kern, Ravi Shankar Sundaram

Using high resolution transmission electron microscopy. we identify the specific atomic scale features in chemically derived graphene monolayers that originate from the oxidation-reduction treatment of graphene. The layers are found to comprise defect-free graphene areas with sizes of a few nanometers interspersed with defect areas dominated by clustered pentagons and heptagons. Interestingly, all carbon atoms in these defective areas are bonded to three neighbors maintaining a planar sp(2)-configuration, which makes them undetectable by spectroscopic techniques. Furthermore, we observe that they introduce significant in-plane distortions and strain in the surrounding lattice.

2010

Scanning near-field infrared microscopy and spectromicroscopy applied to nano-systems and cells

Dusan Vobornik

This thesis further explores the possibilities of scanning near-field optical microscopy (SNOM) in both materials and life sciences. Two experimental SNOM setups were developed: one designed for infrared spectroscopy applications and the other for the imaging of fluorescently labeled samples. The results of the experiments that were conducted with both setups are presented and analyzed in this thesis. Diffraction limits the resolution of lens-based microscopes to a value close to that of the wavelength of the light that is used to illuminate the samples. SNOM instruments overcome this limit by probing the near-field light — the light that remains within close vicinity of the sample and decays exponentially away from it. The SNOM concept and instrumental design are described. A comparison of SNOM with other novel microscopies (electron, atomic force, and scanning tunneling microscopies) outlines the main advantages of SNOM — the sole optical microscopy technique with a nanometer resolution. Aperture-based infrared SNOM (IR-SNOM), performed in the spectroscopic mode using the Vanderbilt University free electron laser, recently started delivering spatially resolved information on the distribution of chemical species and on other laterally fluctuating properties. The IR-SNOM combines the IR spectroscopy's chemical specificity and the SNOM's high optical and topographical resolutions. The IR-SNOM experimental setup is described in detail and results from the study of cells, boron nitride and lithium fluoride thin films are presented and analyzed. They demonstrate the great potential of this new technique. An overview of the development of IR-SNOM, as well as a consideration of its future possibilities are presented. The newly built SNOM instrument at EPFL was used for experiments that are complementary to the aforementioned IR-SNOM experiments. Specifically, the new SNOM enabled the study of fluorescently marked samples. Images of fluorescently labeled cells and carbon nanotubes are presented and analyzed. The results further demonstrate the exciting possibilities of SNOM.