Marko Burghard

Atomic structure of reduced graphene oxide.

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.

Elastic properties of chemically derived single graphene sheets.

The elastic modulus of freely suspended graphene monolayers, obtained via chemical reduction of graphene oxide, was determined through tip-induced deformation experiments. Despite their defect content, the single sheets exhibit an extraordinary stiffness ( E = 0.25 TPa) approaching that of pristine graphene, as well as a high flexibility which enables them to bend easily in their elastic regime. Built-in tensions are found to be significantly lower compared to mechanically exfoliated graphene. The high resilience of the sheets is demonstrated by their unaltered electrical conductivity after multiple deformations. The electrical conductivity of the sheets scales inversely with the elastic modulus, pointing toward a 2-fold role of the oxygen bridges, that is, to impart a bond reinforcement while at the same time impeding the charge transport.

Contact and edge effects in graphene devices

Electrical transport studies on graphene have been focused mainly on the linear dispersion region around the Fermi level and, in particular, on the effects associated with the quasiparticles in graphene behaving as relativistic particles known as Dirac fermions. However, some theoretical work has suggested that several features of electron transport in graphene are better described by conventional semiconductor physics. Here we use scanning photocurrent microscopy to explore the impact of electrical contacts and sheet edges on charge transport through graphene devices. The photocurrent distribution reveals the presence of potential steps that act as transport barriers at the metal contacts. Modulations in the electrical potential within the graphene sheets are also observed. Moreover, we find that the transition from the p- to n-type regime induced by electrostatic gating does not occur homogeneously within the sheets. Instead, at low carrier densities we observe the formation of p-type conducting edges surrounding a central n-type channel.

Electrical Conduction Mechanism in Chemically Derived Graphene Monolayers

We have performed a detailed study of the intrinsic electrical conduction process in individual monolayers of chemically reduced graphene oxide down to a temperature of 2 K. The observed conductance can be consistently interpreted in the framework of two-dimensional variable-range hopping in parallel with electric-field-driven tunneling. The latter mechanism is found to dominate the electrical transport at very low temperatures and high electric fields. Our results are consistent with a model of highly conducting graphene regions interspersed with disordered regions, across which charge carrier hopping and tunneling are promoted by strong local electric fields.

Organic n-Channel Transistors Based on Core-Cyanated Perylene Carboxylic Diimide Derivatives

Five core-cyanated perylene carboxylic diimides end-functionalized with fluorine-containing linear and cyclic substituents have been synthesized and employed in the fabrication of air-stable n-channel organic thin-film field-effect transistors with carrier mobilities up to 0.1 cm2/Vs. The relationships between molecular structure, thin-film morphology, substrate temperature during vacuum deposition, transistor performance, and air stability have been investigated. Our experiments led us to conclude that the role of the fluorine functionalization in the air-stable n-channel operation of the transistors is different than previously thought.

Carbon nanotube memory devices of high charge storage stability

Molecular memory devices with semiconducting single-walled carbon nanotubes constituting a channel of 150 nm in length are described. Data storage is achieved by sweeping gate voltages in the range of 3 V, associated with a storage stability of more than 12 days at room temperature. By annealing in air or controlled oxygen plasma exposure, efficient switching devices could be obtained from thin nanotube bundles that originally showed only a small gate dependence of conductance.

Field-effect transistor made of individual V2O5 nanofibers

A field-effect transistor (FET) with a channel length of ∼100 nm was constructed from a small number of individual V2O5 fibers of the cross section 1.5 nm×10 nm. At low temperature, the conductance increases as the gate voltage is changed from negative to positive values, characteristic of a FET with n-type enhancement mode. The carrier mobility, estimated from the low-field regime, is found to increase from 7.7×10−5 cm2/V s at T=131 K to 9.6×10−3 cm2/V s at T=192 K with an activation energy of Ea=0.18 eV. The nonohmic current/voltage dependence at high electric fields was analyzed in the frame of small polaron hopping conduction, yielding a nearest-neighbor hopping distance of ∼4 nm.

Electrochemical Modification of Single Carbon Nanotubes

However, up to now all of thesechemicalmodificationmethodswereperformedonlyonbulkSWCNTmaterial,notonindividualtubes.Herein we describe a versatile approach to the electro-chemicalmodificationofindividualcarbonnanotubes(smallSWCNT bundles), as demonstrated bythe attachment ofsubstituted phenyl groups. The electrochemical approach isparticularlysuitable for the chemical alteration of single-molecular objects, as their electrochemical potential, whichdeterminestheextentofreaction,canbedirectlyadjustedbyanappliedpotential.Ourresultsshowthatfunctionalgroupscan be controllablyattached to appropriatelycontactednanotubes,whichresultsinhomogeneousmolecularcoatingsofuptoseveralnanometersinthickness.Wepresenttwotypesof coupling reactions, working under oxidative (anodic) orreductive (cathodic) conditions. The corresponding electro-chemical reactions are illustrated schematicallyin Figure1.Inbothcasesaradicalspeciesisproducedonthesurfaceofthe nanotube, which attacks the carbon lattice to form acovalent bond. When the potential is applied for a suitablelength of time, polymerization of the radicals leads tomultilayer coating. This situation is similar to the multi-layer growth of aryl films observed in the electrochemicalmodification of highly oriented pyrolytic graphite(HOPG).

Polymer nanofibers via nozzle-free centrifugal spinning

We report on the unexpected finding of nanoscale fibers with a diameter down to 25 nm that emerge from a polymer solution during a standard spin-coating process. The fiber formation relies upon the Raleigh-Taylor instability of the spin-coated liquid film that arises due to a competition of the centrifugal force and the Laplace force induced by the surface curvature. This procedure offers an attractive alternative to electrospinning for the efficient, simple, and nozzle-free fabrication of nanoscale fibers from a variety of polymer solutions.

Photoelectronic transport imaging of individual semiconducting carbon nanotubes

Photoconductivity in individual semiconducting single-wall carbon nanotubes was investigated using a confocal scanning optical microscope. The magnitude of the photocurrent was found to increase linearly with the laser intensity, and to be maximum for parallel orientation between the light polarization and the tube axis. Larger currents were obtained upon illuminating the tubes at 514.5 nm in comparison to those at 647.1 nm, consistent with the semiconducting tubes having a resonant absorption energy at the former wavelength. Moreover, the determination of the photoresponse as a function of position along single nanotubes has proven to be a useful tool to monitor local electronic structure effects.