Géza I. Márk

Scanning tunnelling microscopy (STM) imaging of carbon nanotubes

Carbon nanotubes prepared by thermal decomposition of hydrocarbons on supported Co catalysts were investigated by STM in air. An interpretation of the STM images is proposed which accounts for specific distortions taking place while scanning three-dimensional objects whose dimensions are of the order of the curvature radius of the tip. These distortions have both geometric and electronic origins, and cannot be neglected. The distortion mechanism was found to be different for nanotube diameters in the ranges of 1 nm and 10 nm. The 1 nm tubes are more strongly affected by their apparent broadening, reflecting the finite size of the tip apex. Here the distortion can reach up to 300% of the geometric diameter, whereas for 10 nm tubes the distortions are in the range of 50% of the geometric diameter. An apparent flattening of the nanotubes in the vertical direction was also found, which is attributed to differences in electronic densities of states between the substrate and the nanotube, and to an additional tunnelling barrier between the nanotube and the substrate. STM images with atomic resolution and line cut topographic profiles show similar structures as for the case of HOPG. However, the atomic corrugation was found to be five times smaller on the 1 nm diameter tubules than for the 10 nm family, the latter being close to the value obtained with HOPG. Coiled nanotubes have been imaged by STM for the first time. Here both the electrical resistance of the coiled nanotube and its elastic deformation play a significant role in the image formation process, these effects being more important than for straight nanotubes.

Electronic transport through ordered and disordered graphene grain boundaries

The evolution of electronic wave packets (WPs) through grain boundaries (GBs) of various structures in graphene was investigated by the numerical solution of the time-dependent Schrodinger equation. WPs were injected from a simulated STM tip placed above one of the grains. Electronic structure of the GBs was calculated by ab-initio and tight-binding methods. Two main factors governing the energy dependence of the transport have been identified: the misorientation angle of the two adjacent graphene grains and the atomic structure of the GB. In case of an ordered GB made of a periodic repetition of pentagon−heptagon pairs, it was found that the transport at high and low energies is mainly determined by the misorientation angle, but the transport around the Fermi energy is correlated with the electronic structure of the GB. A particular line defect with zero misorientation angle Lahiri et al., behaves as a metallic nanowire and shows electron–hole asymmetry for hot electrons or holes. To generate disordered GBs, found experimentally in CVD graphene samples, a Monte-Carlo-like procedure has been developed. Results show a reduced transport for the disordered GBs, primarily attributed to electronic localized states caused by C atoms with only two covalent bonds.

AFM and STM investigation of carbon nanotubes produced by high energy ion irradiation of graphite

Carbon nanotubes (CNT) were produced by high energy, heavy ion irradiation (215 MeV Ne, 246 MeV Kr, 156 MeV Xe) of graphite. On samples irradiated with Kr and Xe ions large craters were found by atomic force microscopy, these are attributed to sputtering. Frequently one or several CNTs emerge from the craters. Some of the observed CNTs showed a regular vibration pattern. No other carbon based materials, like amorphous carbon or fullerenes were evidenced. Focused ion beam cuts were used to compare CNTs with surface folds on graphite. ” 1999 Elsevier Science B.V. All rights reserved.

Carbon nanotubes produced by high energy (E > 100 MeV), heavy ion irradiation of graphite

Abstract Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) were used to investigate the surface of highly oriented pyrolytic graphite (HOPG) irradiated with 209 MeV Kr or 830 MeV U ions. The density of hillocks found on samples irradiated by Kr and U ions indicates synergism of electronic and nuclear stopping processes. Carbon nanotubes (CNTs) were found on all of the investigated samples, STM images show an atomic arrangement identical with that of graphite. AFM revealed sputtering craters from which emerge CNTs, the vibration of some CNTs was observed.

Bilayered semiconductor graphene nanostructures with periodically arranged hexagonal holes

We present a theoretical study of new nanostructures based on bilayered graphene with periodically arranged hexagonal holes (bilayered graphene antidots). Our ab initio calculations show that fabrication of hexagonal holes in bigraphene leads to connection of the neighboring edges of the two graphene layers with formation of a hollow carbon nanostructure sheet which displays a wide range of electronic properties (from semiconductor to metallic), depending on the size of the holes and the distance between them. The results were additionally supported by wave packet dynamical transport calculations based on the numerical solution of the time-dependent Schrödinger equation.

Regularly coiled carbon nanotubes

Regularly coiled carbon nanotubes, their structure, and formation mechanism are puzzling questions. The first models were based on the very regular incorporation of a small fraction (of the order of 10%) of nonhexagonal (n-Hx) rings: (pentagons and heptagons) in a perfect hexagonal (Hx) lattice. It is difficult to understand by which mechanism takes place such a regular incorporation of isolated n-Hx rings. In this paper, a new family of Haeckelite nanotubes is generated in a systematic way by rolling up a two-dimensional three-fold coordinated carbon network composed of pentagon-heptagon pairs and hexagons in proportion 2 : 3. In this model, the n-Hx rings are treated like regular building blocks of the structure. Cohesion energy calculation shows that the stability of the generated three-dimensional Haeckelite structures falls between that of straight carbon nanotubes and that of C/sub 60/. Electronic density of states of the Haeckelite computed with a tight-binding Hamiltonian that includes the C-/spl pi/ orbitals only shows that the structures are semiconductor. The relation of the structures with experimental observations is discussed.

Scanning probe method investigation of carbon nanotubes produced by high energy ion irradiation of graphite

Carbon nanotubes were evidenced by atomic force microscopy and scanning tunneling microscopy on highly oriented pyrolytic graphite irradiated with high energy ions (215 MeV Ne, 209 MeV Kr, 246 MeV Kr, and 156 MeV Xe). On the samples irradiated with Kr and Xe ions, craters attributed to sputtering were found. Frequently, one or several nanotubes emerge from these sputtering craters. Some of the observed nanotubes vibrate when scanned with the AFM. Except nanotubes, no other deposits were observed.

Living Photonic Crystals: Nanostructure of the Scales of Cyanophrys Remus Butterfly

The complex photonic crystal type nanoarchitectures found in the wing scales of the male butterfly Cyanophrys remus were investigated structurally by electron microscopy and optically by reflectance spectroscopy. Both the vivid metallic blue of the dorsal scales and the matt, pea-green coloration of the ventral scales are attributed to photonic crystal type structures composed of chitin and air. The dorsal scales are single crystalline, while the ventral ones contain a large number of randomly oriented micron size single crystalline grains of face centered cubic inverse opal. The remarkable complexity and efficiency of biologic photonic crystals may provide clues in designing artificial structures with similar parameters.

Forming electronic waveguides from graphene grain boundaries

Abstract. If graphene is a promising material in many respects, its remarkable properties may be impaired by unavoidable defects. Chemical vapor deposition-grown graphene samples are polycrystalline in nature, with many grain boundaries. Those extended defects influence the global electronic structure and the transport properties of graphene in a way that remains to be clarified. As a step forward in this direction, we have undertaken quantum mechanical calculations of electron wave-packet dynamics in a multigrain self-supported graphene layer. Our computer simulations show that a grain boundary may act as a reflector at some energies and for some incidences of the Bloch waves. In addition, our calculations reveal that when two grain boundaries run parallel to each other, the graphene ribbon confined between them may behave like a channel for the charge carriers. We emphasize therefore the possibility of creating nanoscale electronic waveguides and nanowires on the graphene surface by a controlled engineering of its grain boundaries.

Quasiordered photonic band gap materials of biologic origin: butterfly scales

Individual, unsupported scales of two male butterflies with dorsal blue and ventral green color were compared by microscpectrometric measurements, optical and electronic microscopy. All the scales are colored by photonic band gap type materials built of chitin (n = 1.58) and air. The different scales are characterized by different degrees of order from fully ordered single crystalline blue scales of the Cyanophrys remus butterfly through polycrystalline green scales on the ventral side of the same butterfly, to the most disordered dorsal blue scales of the Albulina metallica, where only the distance of the first neighbors is constant. The different scale nanoarchitectures and their properties are compared.