Oliver Gröning

Porous graphenes: two-dimensional polymer synthesis with atomic precision.

We demonstrate, by surface-assisted coupling of specifically designed molecular building blocks, the fabrication of regular two-dimensional polyphenylene networks with single-atom wide pores and sub-nanometer periodicity.

Controlled synthesis of single-chirality carbon nanotubes

Over the past two decades, single-walled carbon nanotubes (SWCNTs) have received much attention because their extraordinary properties are promising for numerous applications. Many of these properties depend sensitively on SWCNT structure, which is characterized by the chiral index (n,m) that denotes the length and orientation of the circumferential vector in the hexagonal carbon lattice. Electronic properties are particularly strongly affected, with subtle structural changes switching tubes from metallic to semiconducting with various bandgaps. Monodisperse ‘single-chirality’ (that is, with a single (n,m) index) SWCNTs are thus needed to fully exploit their technological potential. Controlled synthesis through catalyst engineering, end-cap engineering or cloning strategies, and also tube sorting based on chromatography, density-gradient centrifugation, electrophoresis and other techniques, have delivered SWCNT samples with narrow distributions of tube diameter and a large fraction of a predetermined tube type. But an effective pathway to truly monodisperse SWCNTs remains elusive. The use of template molecules to unambiguously dictate the diameter and chirality of the resulting nanotube holds great promise in this regard, but has hitherto had only limited practical success. Here we show that this bottom-up strategy can produce targeted nanotubes: we convert molecular precursors into ultrashort singly capped (6,6) ‘armchair’ nanotube seeds using surface-catalysed cyclodehydrogenation on a platinum (111) surface, and then elongate these during a subsequent growth phase to produce single-chirality and essentially defect-free SWCNTs with lengths up to a few hundred nanometres. We expect that our on-surface synthesis approach will provide a route to nanotube-based materials with highly optimized properties for applications such as light detectors, photovoltaics, field-effect transistors and sensors.

Vacuum arc discharges preceding high electron field emission from carbon films

Field emission measurements on chemical vapor deposition diamond and laser ablated a‐C films show an activation step after reaching a certain critical electric field. At this field a vacuum arc of some hundred ns duration initiates. While high current arcing leads to the evaporation of the spot surface melting, amorphization or cracking of the film is encountered for lower currents. In any case, much higher electron emission can be observed after this activation procedure due possibly to tip formation resulting in an electric field enhancement. By using a 1 GΩ resistance the discharge current can be limited nevertheless, an activation is observed.

Self-assembly and conformation of tetrapyridyl-porphyrin molecules on Ag(111)

We present a low-temperature scanning tunneling microscopy (STM) study on the supramolecular ordering of tetrapyridyl-porphyrin (TPyP) molecules on Ag(111). Vapor deposition in a wide substrate temperature range reveals that TPyP molecules easily diffuse and self-assemble into large, highly ordered chiral domains. We identify two mirror-symmetric unit cells, each containing two differently oriented molecules. From an analysis of the respective arrangement it is concluded that lateral intermolecular interactions control the packing of the layer, while its orientation is induced by the coupling to the substrate. This finding is corroborated by molecular mechanics calculations. High-resolution STM images recorded at 15 K allow a direct identification of intramolecular features. This makes it possible to determine the molecular conformation of TPyP on Ag(111). The pyridyl groups are alternately rotated out of the porphyrin plane by an angle of 60 degrees.

Field emission from DLC films

Abstract Field emission measurements on diamond like carbon (DLC) films with different amounts of sp3 and sp2 carbon were carried out. Depending on the amount of sp2 carbon in the film, activated and non-activated Fowler-Nordheim like emission could be observed. The emission spots were investigated using a combination of AFM and STM, by simultaneously measuring the topography and the conductivity of the samples. In the case of sp2 rich DLC films we could observe that the emission originates from highly conducting inclusions of sp2 carbon in a matrix of insulating sp3 carbon. These inclusions are already existing on the sp2 sample by the deposition process itself and are formed by the activation on the sp3 rich sample.

C60/corannulene on Cu(110): a surface-supported bistable buckybowl-buckyball host-guest system.

Corannulene (COR) buckybowls were proposed as near ideal hosts for fullerene C60, but direct complexation of C60 and COR has remained a challenge in supramolecular chemistry. We report the formation of surface-supported COR-C60 host-guest complexes by deposition of C60 onto a COR lattice on Cu(110). Variable-temperature scanning tunneling microscopy studies reveal two distinctly different states of C60 on the COR host lattice, with different binding energies and bowl-ball separations. The transition from a weakly bound precursor state to a strongly bound host-guest complex is found to be thermally activated. Simple model calculations show that this bistability originates from a subtle interplay between homo- and heteromolecular interactions.

Electronic band dispersion of graphene nanoribbons via Fourier-transformed scanning tunneling spectroscopy

The electronic structure of atomically precise armchair graphene nanoribbons of width N=7 (7-AGNRs) are investigated by scanning tunneling spectroscopy (STS) on Au(111). We record the standing waves in the local density of states of finite ribbons as a function of sample bias and extract the dispersion relation of frontier electronic states by Fourier transformation. The wave-vector-dependent contributions from these states agree with density functional theory calculations, thus enabling the unambiguous assignment of the states to the valence band, the conduction band, and the next empty band with effective masses of 0.41±0.08me,0.40±0.18me, and 0.20±0.03me, respectively. By comparing the extracted dispersion relation for the conduction band to corresponding height-dependent tunneling spectra, we find that the conduction band edge can be resolved only at small tip-sample separations and has not been observed before. As a result, we report a band gap of 2.37±0.06 eV for 7-AGNRs adsorbed on Au(111).

Investigation of field emission properties of carbon nanotube arrays defined using nanoimprint lithography

We demonstrate the use of nanoimprint lithography as an alternative low-cost fabrication route for the production of ordered arrays of individual carbon nanotube field emitters. A high emission site density of 4×105cm−2 was observed and is well within the specification of the cathode for a field emission display. The measured field enhancement values from the geometry of the nanotubes were in reasonable agreement with the values obtained through electrical measurements. We also show that the distribution of the field enhancement factor is Gaussian, indicative of the presence of well ordered arrays of carbon nanotube field emitters.

Electron field emission from diamond tips prepared by ion sputtering

In this letter we report the field emission from cones etched into a synthetic‐type IIb (100) oriented boron‐doped diamond crystal. The cones were produced with an ion sputtering process. With a high‐resolution scanning electron microscope we found the curvature radius at the end of the cones to be less than 10 nm. The length of these cones is in the range of 10 μm giving a field enhancement of approx. 1000. Field emission started at field strength of 2 V/μm, and at 3.8 V/μm 10 nA were measured using a spherical anode of 4 mm diam. From a Fowler‐Nordheim fit the work function could be deduced to be 3–4 eV. Hence, electrons are emitted from the valence band.

Resolving Atomic Connectivity in Graphene Nanostructure Junctions

We report on the structural characterization of junctions between atomically well-defined graphene nanoribbons (GNRs) by means of low-temperature, noncontact scanning probe microscopy. We show that the combination of simultaneously acquired frequency shift and tunneling current maps with tight binding (TB) simulations allows a comprehensive characterization of the atomic connectivity in the GNR junctions. The proposed approach can be generally applied to the investigation of graphene nanomaterials and their interconnections and is thus expected to become an important tool in the development of graphene-based circuitry.