Clemens Barth

Recent Trends in Surface Characterization and Chemistry with High‐Resolution Scanning Force Methods

The current status and future prospects of non-contact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM) for studying insulating surfaces and thin insulating films in high resolution are discussed. The rapid development of these techniques and their use in combination with other scanning probe microscopy methods over the last few years has made them increasingly relevant for studying, controlling, and functionalizing the surfaces of many key materials. After introducing the instruments and the basic terminology associated with them, state-of-the-art experimental and theoretical studies of insulating surfaces and thin films are discussed, with specific focus on defects, atomic and molecular adsorbates, doping, and metallic nanoclusters. The latest achievements in atomic site-specific force spectroscopy and the identification of defects by crystal doping, work function, and surface charge imaging are reviewed and recent progress being made in high-resolution imaging in air and liquids is detailed. Finally, some of the key challenges for the future development of the considered fields are identified.

Atomic Structures of Silicene Layers Grown on Ag(111): Scanning Tunneling Microscopy and Noncontact Atomic Force Microscopy Observations

Silicene, the considered equivalent of graphene for silicon, has been recently synthesized on Ag(111) surfaces. Following the tremendous success of graphene, silicene might further widen the horizon of two-dimensional materials with new allotropes artificially created. Due to stronger spin-orbit coupling, lower group symmetry and different chemistry compared to graphene, silicene presents many new interesting features. Here, we focus on very important aspects of silicene layers on Ag(111): First, we present scanning tunneling microscopy (STM) and non-contact Atomic Force Microscopy (nc-AFM) observations of the major structures of single layer and bi-layer silicene in epitaxy with Ag(111). For the (3 × 3) reconstructed first silicene layer nc-AFM represents the same lateral arrangement of silicene atoms as STM and therefore provides a timely experimental confirmation of the current picture of the atomic silicene structure. Furthermore, both nc-AFM and STM give a unifying interpretation of the second layer (√3 × √3)R ± 30° structure. Finally, we give support to the conjectured possible existence of less stable, ~2% stressed, (√7 × √7)R ± 19.1° rotated silicene domains in the first layer.

Topography and work function measurements of thin MgO(001) films on Ag(001) by nc-AFM and KPFM

The surface topography and local surface work function of ultrathin MgO(001) films on Ag(001) have been studied by noncontact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM). First principles calculations have been used to explain the contrast formation of nc-AFM images. In agreement with literature, thin MgO films grow in islands with a quasi rectangular shape. Contrary to alkali halide films supported on metal surfaces, where the island heights can be correctly measured, small MgO islands are either imaged as depressions or elevations depending on the electrostatic potential of the tip apex. Correct island heights therefore cannot be given without knowing the precise contrast formation discussed in this paper. KPFM shows a silver work function which is reduced by the MgO islands. The values for the work function differences for one and two layer thin films are -1.1 and -1.4 eV, respectively, in good agreement with recent calculations and experiments.

Expeditive syntheses of functionalized pentahelicenes and NC-AFM on Ag(001).

One of the shortest and most efficient routes toward a series of functionalized pentahelicenes is reported. Benzylic (dibromo)methine coupling is an important entry into functional helicene chemistry. It allowed a mono- or a double functionalization by some metal-catalyzed Ar-C, Ar-S, Ar-CN, and Ar-I bond formations. Those functions offer new avenues for further applications. For instance, helicene (4) can be supported on a Ag(001) surface, which was characterized by high-resolution NC-AFM imaging.

Surface preparation of hard ionic crystals by ultrahigh vacuum cleavage

A complete procedure for the preparation of clean surfaces of especially hard ionic crystals by ultrahigh vacuum (UHV) cleavage is given and exemplified by the preparation of the (001) surface of single MgO crystals. All important tools that are needed for the preparation are explained in detail. We present a device that allows precise cleavages in UHV and show how it can be easily integrated into an existing UHV system equipped with a linear manipulator. Cleaving ionic crystals produces charges on and below the fresh surface, which strongly hamper experiments like dynamic scanning force microscopy. In the case of MgO imaging is mostly impossible right after cleavage. We show that it is sufficient to anneal MgO crystals at temperatures higher than 350°C in order to reduce a large part of the charges. For the annealing, an UHV oven is used that can be annealed up to 550°C without leaving the upper 10−10mbar pressure region. Our techniques can be used in principle also for softer ionic crystals such as KBr,...

AFM tip characterization by Kelvin probe force microscopy

Reliable determination of the surface potential with spatial resolution is key for understanding complex interfaces that range from nanostructured surfaces to molecular systems to biological membranes. In this context, Kelvin probe force microscopy (KPFM) has become the atomic force microscope (AFM) method of choice for mapping the local electrostatic surface potential as it changes laterally due to variations in the surface work function or surface charge distribution. For reliable KPFM measurements, the influence of the tip on the measured electrostatic surface potential has to be understood. We show here that the mean Kelvin voltage can be used for a straightforward characterization of the electrostatic signature of neutral, charged and polar tips, the starting point for quantitative measurements and for tip-charge control for AFM manipulation experiments. This is proven on thin MgO(001) islands supported on Ag(001) and is supported by theoretical modeling, which shows that single ions or dipoles at the tip apex dominate the mean Kelvin voltage.

Gold nanoclusters on alkali halide surfaces: Charging and tunneling

Kelvin probe force microscopy measurements of gold nanoclusters (0.04–1.5 ML) deposited on (001) surfaces of UHV-cleaved NaCl and KCl are presented. Groups of clusters exhibit a uniform bright contrast, which varies, however, from group to group. These phenomena are most likely due to charges in the clusters, which got charged from charges of the clean surface produced by the cleavage. The uniform contrast in cluster groups might be due to tunneling processes of charges between clusters. Measurements after a charge injection into a cluster show indeed a possible exchange of charges in cluster rows, which the authors evaluate with simple calculations.

High-resolution imaging of gold clusters on KBr(001) surfaces investigated by dynamic scanning force microscopy

Dynamic scanning force microscopy (dynamic SFM) images of (001) surfaces of KBr which have been prepared by cleavage and further decorated by nanometre-sized gold clusters in UHV are presented. During scanning we could achieve atomic resolution on the KBr substrate surface; however, the clusters mostly appeared as hemisphere-like or fuzzy objects exhibiting only a few details of their internal structure. We performed force spectroscopy measurements above single clusters which might point to a charging of the clusters. We anticipate that mainly a long-range electrostatic interaction between the cluster and the macroscopic tip apex determines the appearance of the clusters. Information on the interior of the clusters can be gained only for short tip–cluster distances in the presence of a short-range, chemical interaction between the cluster and the tip atoms closest to the surface.

Two-dimensional nanostructured growth of nanoclusters and molecules on insulating surfaces.

Noncontact atomic force microscopy (nc-AFM), Kelvin probe force microscopy (KPFM) and first principle calculations show that the nanostructured (001) Suzuki surface of Cd(2+) doped NaCl can be used to confine the growth of palladium clusters and functionalized brominated pentahelicene molecules into only the Suzuki regions, which contain the impurities. The Suzuki surface is an ideal model surface for nanostructuring metal clusters and molecules.

NaCl(001) surfaces nanostructured by Suzuki precipitates: a scanning force microscopy study

Suzuki precipitates on the (001) surfaces of as-cleaved and additionally annealed NaCl:Cd2+ crystals were studied by means of dynamic scanning force microscopy and Kelvin probe force microscopy in ultra-high vacuum. Rectangular precipitates with edge lengths of up to several hundred nanometres can be found on the surfaces of as-cleaved crystals. The precipitates exhibit steps, which are a result of the cleavage of dislocations at the Suzuki–NaCl interfaces. Additional annealing of the crystals below 260 °C leads to surface diffusion and to a restructuring of the whole surface producing atomically flat precipitates at steps, but also on flat terraces in regions of low step density. On surfaces evaporated at 350 °C, the main surface characteristics of evaporated (001) surfaces of pure NaCl can be found. The step edges are merely changed in shape by nanometre-size Suzuki precipitates.