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

One-dimensional Si chains embedded in Pt(111)and protected by a hexagonal boron-nitride monolayer

Using scanning tunneling microscopy, we show that Si deposition on Pt(111) at 300K leads to a network of one-dimensional Si chains. On the bare Pt(111) surface, the chains, embedded into the Pt surface, are orientated along the -direction. They disappear within a few hours in ultrahigh vacuum due to the presence of residual gas. Exposing the chains to different gases deliberately reveals that CO is largely responsible for the disappearance of the chains. The chains can be stabilized by a monolayer of hexagonal boron nitride, which is deposited prior to the Si deposition. The resulting Si chains are rotated by 30{\deg} with respect to the chains on the bare Pt(111) surface and survive even an exposure to air for 10 minutes.Comment: 8 pages, 4 Figure

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 $v_{\rm scan}$. 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 $v_{\rm scan}$, that it is a simple enlargement effect of the atomic resolution as well as models predicting frozen phonons or standing phonon waves induced by the tunneling current. Albeit 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.Comment: 10 pages, 7 figures, modified, more detailed discussion on errors in vdW parameter

Tuning the electronic structure of graphene by ion irradiation

Mechanically exfoliated graphene layers deposited on SiO2 substrate were irradiated with Ar+ ions in order to experimentally study the effect of atomic scale defects and disorder on the low-energy electronic structure of graphene. The irradiated samples were investigated by scanning tunneling microscopy and spectroscopy measurements, which reveal that defect sites, besides acting as scattering centers for electrons through local modification of the on-site potential, also induce disorder in the hopping amplitudes. The most important consequence of the induced disorder is the substantial reduction in the Fermi velocity, revealed by bias-dependent imaging of electron-density oscillations observed near defect sites

Graphene nanoribbons with zigzag and armchair edges prepared by scanning tunneling microscope lithography on gold substrates

The properties of graphene nanoribbons are dependent on both the nanoribbon width and the crystallographic orientation of the edges. Scanning tunneling microscope lithography is a method which is able to create graphene nanoribbons with well defined edge orientation, having a width of a few nanometers. However, it has only been demonstrated on the top layer of graphite. In order to allow practical applications of this powerful lithography technique, it needs to be implemented on single layer graphene. We demonstrate the preparation of graphene nanoribbons with well defined crystallographic orientation on top of gold substrates. Our transfer and lithography approach brings one step closer the preparation of well defined graphene nanoribbons on arbitrary substrates for nanoelectronic applications

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