Florian Banhart

Structural Defects in Graphene

Graphene is one of the most promising materials in nanotechnology. The electronic and mechanical properties of graphene samples with high perfection of the atomic lattice are outstanding, but structural defects, which may appear during growth or processing, deteriorate the performance of graphene-based devices. However, deviations from perfection can be useful in some applications, as they make it possible to tailor the local properties of graphene and to achieve new functionalities. In this article, the present knowledge about point and line defects in graphene are reviewed. Particular emphasis is put on the unique ability of graphene to reconstruct its lattice around intrinsic defects, leading to interesting effects and potential applications. Extrinsic defects such as foreign atoms which are of equally high importance for designing graphene-based devices with dedicated properties are also discussed.

Irradiation effects in carbon nanostructures

The paper reviews the principles of interaction of energetic particles with solid carbon and carbon nanostructures. The reader is first introduced to the basic mechanisms of radiation effects in solids with particular emphasis on atom displacements by knock-on collisions. The influence of various parameters on the displacement cross sections of carbon atoms is discussed. The types of irradiation-induced defects and their migration are described as well as ordering phenomena which are observable under the non-equilibrium conditions of irradiation. The main part of this review deals with alterations of carbon nanostructures by the electron beam in an electron microscope. This type of experiment is of paramount importance because it allows in situ observation of dynamic processes on an atomic scale. In the second part, radiation effects in the modifications of elemental carbon, in particular in graphite which forms the crystallographic basis of most carbon nanostructures, are treated in detail. It follows a review of the available experimental results on radiation defects in carbon nanostructures such as fullerenes, nanotubes and carbon onions. Finally, the phenomena of structure formation under irradiation, in particular the self-assembling of spherical carbon onions and the irradiation-induced transformation of graphitic nanoparticles into diamond, are presented and discussed qualitatively in the context of non-equilibrium structure formation.

One- and Two-Dimensional Diffusion of Metal Atoms in Graphene**

In the present work, individual Au or Pt atoms in layersconsisting of one or twographene planes have been monitoredin real time at high temperature by high-resolution TEM. Weobtain information about the location of metal atoms ingrapheneandthediffusionmechanisms.Activationenergiesfordiffusion are obtained in a temperature range close to thetemperature of the technically important metal-assisted CVDprocess.Thematerialwassynthesizedinanarcdischarge

Carbon Nanotubes as High-Pressure Cylinders and Nanoextruders

Closed-shell carbon nanostructures, such as carbon onions, have been shown to act as self-contracting high-pressure cells under electron irradiation. We report that controlled irradiation of multiwalled carbon nanotubes can cause large pressure buildup within the nanotube cores that can plastically deform, extrude, and break solid materials that are encapsulated inside the core. We further showed by atomistic simulations that the internal pressure inside nanotubes can reach values higher than 40 gigapascals. Nanotubes can thus be used as robust nanoscale jigs for extruding and deforming hard nanomaterials and for modifying their properties, as well as templates for the study of individual nanometer-sized crystals under high pressure.

Trapping of metal atoms in vacancies of carbon nanotubes and graphene.

Lattice defects in carbon nanotubes and graphene are created by focusing an electron beam in a scanning transmission electron microscope onto a 0.1 nm spot on the objects. Metal atoms migrating on the graphenic surfaces are observed to be trapped by these defects. Depending on the size of the defect, single metal atoms or clusters of several atoms can be localized in or on nanotubes or graphene layers. Subsequent escape of the metal atoms from the trapping centers gives information about the bonding between the metal atom and the defect. The process of trapping and detrapping is studied in a temperature range of 20-670 degrees C. The technique allows one to place metal atoms with almost atomic precision in graphenic structures and to create a predefined pattern of foreign atoms in graphene or carbon nanotubes.

In situ nucleation of carbon nanotubes by the injection of carbon atoms into metal particles

The synthesis of carbon nanotubes (CNTs) of desired chiralities and diameters is one of the most important challenges in nanotube science and achieving such selectivity may require a detailed understanding of their growth mechanism. We report the formation of CNTs in an entirely condensed phase process that allows us, for the first time, to monitor the nucleation of a nanotube on the spherical surface of a metal particle. When multiwalled CNTs containing metal particle cores are irradiated with an electron beam, carbon from graphitic shells surrounding the metal particles is ingested into the body of the particle and subsequently emerges as single-walled nanotubes (SWNTs) or multiwalled nanotubes (MWNTs) inside the host nanotubes. These observations, at atomic resolution in an electron microscope, show that there is direct bonding between the tubes and the metal surface from which the tubes sprout and can be readily explained by bulk diffusion of carbon through the body of catalytic particles, with no evidence of surface diffusion.

The formation, annealing and self-compression of carbon onions under electron irradiation

Abstract Electron irradiation-induced basal plane disordering in single- and multi-shell carbon nanotubes and onions is found to be inhibited at irradiation temperatures above 300°C. Irradiation-induced defects anneal out due to the thermally activated migration of interstitials at elevated temperatures and leave behind perfectly coherent shells of high tensile stability. Continuous loss of atoms as a result of sputtering induces a surface tension which can be identified as the origin of the formation and self-compression of spherical concentric-shell carbon onions.

Low Temperature Casting of Graphene with High Compressive Strength

has attracted attention due to its fascinating properties such as high carrier mobility, [ 6–8 ] high thermal conductivity, [ 9 , 10 ] extraordinary elasticity and stiffness [ 11 ] and other properties. While mechanical exfoliation, [ 6 ] liquid exfoliation, [ 12 ] and epitaxial growth [ 13 ] can produce pristine graphene, graphene yields are currently too low for large-scale production of macrostructures. In contrast, chemical reduction of graphene oxide provides ‘graphene’ sheets in large scale for graphene macrostructures. [ 14–16 ] Graphene-based macrostructures prepared to date have been relatively weak mechanically, given their fl exible and often relatively porous or open structures, [ 17–26 ] particularly with respect to compressive strength when compared with commercial graphite products. [ 27–29 ] Achieving highly compacted and thus “fully dense” macrostructures based on graphene and measuring the physical properties of such material(s) is thus an important goal. Here, we report a pH-mediated hydrothermal reduction which is combined with moulding methods and allows controllable fabrication of compact high density graphene macrostructures with various shapes. The compact graphene (CG) product that is fabricated in this study shows great advantages over hitherto reported 3-D graphene products, [ 17–26 ] e.g. , a solid microstructure and a high density ( ∼ 1.6 g cm − 3 ) which is comparable to conventional graphite products [ 27–29 ] and an ultrahigh compressive strength ( ∼ 361 Mpa) which is 6 times higher than

Electrical Transport Measured in Atomic Carbon Chains

The first electrical-transport measurements of monatomic carbon chains are reported in this study. The chains were obtained by unraveling carbon atoms from graphene ribbons while an electrical current flowed through the ribbon and, successively, through the chain. The formation of the chains was accompanied by a characteristic drop in the electrical conductivity. The conductivity of the chains was much lower than previously predicted for ideal chains. First-principles calculations using both density functional and many-body perturbation theory show that strain in the chains has an increasing effect on the conductivity as the length of the chains increases. Indeed, carbon chains are always under varying nonzero strain that transforms their atomic structure from the cumulene to the polyyne configuration, thus inducing a tunable band gap. The modified electronic structure and the characteristics of the contact to the graphitic periphery explain the low conductivity of the locally constrained carbon chain.

Creation of individual vacancies in carbon nanotubes by using an electron beam of 1 A diameter.

The focused electron beam of an aberration-corrected scanning transmission electron microscope is used to create individual vacancies in predefined positions of carbon nanotubes. Vacancies in single-wall tubes are unstable and cause an immediate reconstruction of the lattice between 20 and 700 degrees C. In double-wall tubes, vacancies are stable and observable up to at least 235 degrees C, whereas above 480 degrees C a relaxation of the lattice occurs.