Nanostructures based on graphene and functionalized carbon nanotubes | Grafén és szén nanocső alapú nanoszerkezetek előállítása és jellemzése

In this thesis I have explored the preparation of graphene nanostructures, having crystallographically well defined edges and the scanning probe measurements of graphene and functionalized carbon nanotubes. The results of my research can be summarized in three main parts. I have developed a sample preparation technique, based on a carbon nanotube – few layer graphite composite that provides a simple and effective solution to sample stability issues encountered when measuring functionalized carbon nanotubes with STM. Such a composite has enabled for the first time to measure functionalized carbon nanotubes in atomic resolution, as well as to acquire energy resolved STM images of the tubes. Functionalized and pristine regions of the nanotube surface were made visible and the positions of the functional groups could be correlated with crystal lattice directions. The ease of the sample preparation allows the use of my method to study the properties of other types of functionalized carbon nanotubes. This adds STM to the toolbox of functionalized carbon nanotube characterization techniques, complementing optical spectroscopic methods. I have investigated the source of anomalous thickness measurements of graphene and few layer graphite, obtained by tapping mode AFM. The physical origin of these artefacts was elucidated by measurements and theoretical modeling of the AFM tip oscillation and tip – sample interaction. Numerical calculations and experiments have been used to show the correct experimental parameters needed to image the true thickness of graphene layers on a supporting substrate. The conclusions are general enough so that they can be applied to the measurements of other nanosized objects by AFM. I have demonstrated the existence of a chemical etching procedure that discriminates between the armchair and zigzag type edge termination of graphene layers. Coupled with AFM patterning, I have used this chemical process to pattern graphene sheets into nanostructures having zigzag edges. Raman measurements show that the edge roughness of these nanostructures is low enough that inelastic light scattering processes specific to the zigzag edge could be measured. This is the first study which shows that zigzag edged graphene nanostructures can be prepared in the laboratory in a controlled manner, which have a low enough edge disorder to enable the experimental observation of zigzag edge specific physical processes

Apparent rippling with honeycomb symmetry and tunable periodicity observed by scanning tunneling microscopy on suspended graphene

Suspended graphene is difficult to image by scanning probe microscopy due to the inherent van der Waals and dielectric forces exerted by the tip, which are not counteracted by a substrate. Here, we report scanning tunneling microscopy data of suspended monolayer graphene in constant-current mode, revealing a surprising honeycomb structure with amplitude of 50-200 pm and lattice constant of 10-40 nm. The apparent lattice constant is reduced by increasing the tunneling current I, but does not depend systematically on tunneling voltage V or scan speed v(scan). The honeycomb lattice of the rippling is aligned with the atomic structure observed on supported areas, while no atomic corrugation is found on suspended areas down to the resolution of about 3-4 pm. We rule out that the honeycomb structure is induced by the feedback loop using a changing vscan, that it is a simple enlargement effect of the atomic lattice, as well as models predicting frozen phonons or standing phonon waves induced by the tunneling current. Although we currently do not have a convincing explanation for the observed effect, we expect that our intriguing results will inspire further research related to suspended graphene

Substrate-induced strain in carbon nanodisks

Abstract Graphitic nanodisks of typically 20 – 50 nm in thickness, produced by the so-called Kvaerner Carbon Black and Hydrogen Process were dispersed on gold substrate and investigated by atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), and confocal Raman spectroscopy. The roughness of the gold surface was drastically changed by annealing at 400 °C. AFM measurements show that this change in the surface roughness induces changes also in the topography of the nanodisks, as they closely follow the corrugation of the gold substrate. This leads to strained nanodisks, which is confirmed also by confocal Raman microscopy. We found that the FE-SEM contrast obtained from the disks depends on the working distance used during the image acquisition by In-lens detection, a phenomenon which we explain by the decrease in the amount of electrons reaching the detector due to diffraction. This process may affect the image contrast in the case of other layered materials, like hexagonal boron nitride, and other planar hybrid nanostructures, too

Preparing local strain patterns in graphene by atomic force microscope based indentation

Patterning graphene into various mesoscopic devices such as nanoribbons, quantum dots, etc. by lithographic techniques has enabled the guiding and manipulation of graphene's Dirac-type charge carriers. Graphene, with well-defined strain patterns, holds promise of similarly rich physics while avoiding the problems created by the hard to control edge configuration of lithographically prepared devices. To engineer the properties of graphene via mechanical deformation, versatile new techniques are needed to pattern strain profiles in a controlled manner. Here we present a process by which strain can be created in substrate supported graphene layers. Our atomic force microscope-based technique opens up new possibilities in tailoring the properties of graphene using mechanical strain

Electrostatically Confined Monolayer Graphene Quantum Dots with Orbital and Valley Splittings

The electrostatic confinement of massless charge carriers is hampered by Klein tunneling. Circumventing this problem in graphene mainly relies on carving out nanostructures or applying electric displacement fields to open a band gap in bilayer graphene. So far, these approaches suffer from edge disorder or insufficiently controlled localization of electrons. Here we realize an alternative strategy in monolayer graphene, by combining a homogeneous magnetic field and electrostatic confinement. Using the tip of a scanning tunneling microscope, we induce a confining potential in the Landau gaps of bulk graphene without the need for physical edges. Gating the localized states toward the Fermi energy leads to regular charging sequences with more than 40 Coulomb peaks exhibiting typical addition energies of 7-20 meV. Orbital splittings of 4-10 meV and a valley splitting of about 3 meV for the first orbital state can be deduced. These experimental observations are quantitatively reproduced by tight binding calculations, which include the interactions of the graphene with the aligned hexagonal boron nitride substrate. The demonstrated confinement approach appears suitable to create quantum dots with well-defined wave function properties beyond the reach of traditional techniques