Roland Bennewitz

Local work function measurements of epitaxial graphene

The work function difference between single layer and bilayer graphene grown epitaxially on 6H-SiC(0001) has been determined to be 135±9 meV by means of the Kelvin probe force microscopy. Bilayer films are found to increase the work function as compared to single layer films. This method allows an unambiguous distinction between interface layer, single layer, and bilayer graphene. In combination with high-resolution topographic imaging, the complex step structure of epitaxial graphene on SiC can be resolved with respect to substrate and graphene layer steps.

Friction experiments on the nanometre scale

In this review, we present various results obtained by friction force microscopy in the last decade. Starting with material-specific contrast, commonly observed in friction force maps, we discuss how the load dependence of friction and the area of contact have been estimated and compared to elasticity theories. The features observed in a sliding process on the atomic scale can be interpreted within the Tomlinson model. An extension of the model, including thermal effects, predicts a smooth velocity dependence of friction, which recent experiments have confirmed. Other subjects like anisotropy of friction, role of environment, topographical effects, electronic friction and tip modifications are also discussed. The growing importance of molecular dynamics simulations in the study of tribological processes on the atomic scale is outlined.

Atomically accurate Si grating with 5.73 nm period

A vicinal surface of silicon is found that exhibits an atomically accurate step pattern with a period of 5.73 nm, corresponding to 17 atomic rows per (111) terrace. It can be viewed as reconstructed Si(557) surface, where a triple step is combined with a single Si(111)7×7 unit. The driving forces for establishing regular step patterns are discussed.

Ultrathin films of NaCl on Cu(111) : a LEED and dynamic force microscopy study

Ultrathin films of NaCl on Cu(111) have been studied with low-energy electron diffraction (LEED) and Dynamic force microscopy (DFM). The orientation and the lattice constant of the films are revealed by LEED while DFM allows a real space view on their growth modes. The ability of the DFM to image local mechanical surface properties is demonstrated at a substrate step which is covered by a continuous NaCl film.

Ferroelectric domain characterisation and manipulation : A challenge for scanning probe microscopy

Domain writing and reading on the nanometer scale is addressed with scanning force microscopy (SFM) Compared to other scanning probe methods, SFM provides broad possibilities for the on-line data controlling. i.e. three-dimensional mapping of polarisation distribution, differentiation between polarisation and topography, nanoscale domain switching of domains with a 60 nm diameter, recording of nanoscale hysteresis loops, phase transition mapping. domain wall imaging with 9 nm resolution, atomic resolution of ferroelectric surfaces, etc. All these issues are reported in this paper. The challenging result of such a concerted investigation is the possibility of using SFM for nanoscale domain writing and reading with nanometer resolution. Fig. 1 illustrates such an example where line shaped c - domains are purposely written into a ferroelectric Barium-Titanate single crystal with a 400 nm line-width. With this figure we highly appreciate and honour the work of Bob Newnham passing our best nano-wishes for his future.

Atomic scale memory at a silicon surface

The limits of pushing storage density to the atomic scale are explored with a memory that stores a bit by the presence or absence of one silicon atom. These atoms are positioned at lattice sites along self-assembled tracks with a pitch of five atom rows. The memory can be initialized and reformatted by controlled deposition of silicon. The writing process involves the transfer of Si atoms to the tip of a scanning tunnelling microscope. The constraints on speed and reliability are compared with data storage in magnetic hard disks and DNA.

One-dimensional electronic states at surfaces

One-dimensional electron systems can now be synthesized at stepped surfaces by self-assembly of atomic and molecular chains. A wide variety of adsorbate and substrate combinations provides opportunities for systematically tailoring electronic properties, such as the intra-chain and inter-chain coupling, the electron count, magnetic moment and the Coulomb interaction. Angle-resolved photoemission with synchrotron radiation is an ideal probe to reveal the complete set of quantum numbers for electrons at an ordered surface, i.e. energy, momentum parallel to the surface, spin and point group symmetry. Interesting electronic features are discussed, such as spin-charge separation in a Luttinger liquid, charge density waves, the Peierls gap, mixed dimensionality and one-dimensional quantum well states.

Atomic Scale Mechanisms of Friction Reduction and Wear Protection by Graphene

We study nanoindentation and scratching of graphene-covered Pt(111) surfaces in computer simulations and experiments. We find elastic response at low load, plastic deformation of Pt below the graphene at intermediate load, and eventual rupture of the graphene at high load. Friction remains low in the first two regimes, but jumps to values also found for bare Pt(111) surfaces upon graphene rupture. While graphene substantially enhances the load carrying capacity of the Pt substrate, the substrates intrinsic hardness and friction are recovered upon graphene rupture.

Friction and Wear on Single-Layer Epitaxial Graphene in Multi-Asperity Contacts

Friction and wear of single layers of graphene have been studied at the micrometer scale. Epitaxial graphene grown by thermal decomposition on SiC-6H(0001) is found to have an initial friction coefficient of 0.02, significantly lower than graphite under the same experimental conditions. During reciprocal sliding the graphene layer is damaged. The evolving friction coefficient of 0.08 for the carbon-rich interface layer terminating the SiC layer is still lower than that of graphite and five times lower than that of the hydrogen-etched SiC substrate. Micrometer-sized patches within the sliding track retain the low friction coefficient of graphene even after hundred sliding cycles.

Cu-TBPP and PTCDA molecules on insulating surfaces studied by ultra-high-vacuum non-contact AFM

The adsorption of two kinds of porphyrin (Cu-TBPP) and perylene (PTCDA) derived organic molecules deposited on KBr and Al2O3 surfaces has been studied by non-contact force microscopy in ultra-high vacuum, our goal being the assembly of ordered molecular arrangements on insulating surfaces at room temperature. On a Cu(100) surface, well ordered islands of Cu-TBPP molecules were successfully imaged. On KBr and Al2O3 surfaces, it was found that the same molecules aggregate in small clusters at step edges, rather than forming ordered monolayers. First measurements with PTCDA on KBr show that nanometre-scale rectangular pits in the surface can act as traps to confine small molecular assemblies.