Bartosz Such

Functionalized Truxenes: Adsorption and Diffusion of Single Molecules on the KBr(001) Surface

In this work, we have studied the adsorption and diffusion of large functionalized organic molecules on an insulating ionic surface at room temperature using a noncontact atomic force microscope (NC-AFM) and theoretical modeling. Custom designed syn-5,10,15-tris(4-cyanophenylmethyl)truxene molecules are adsorbed onto the nanoscale structured KBr(001) surface at low coverages and imaged with atomic and molecular resolution with the NC-AFM. The molecules are observed rapidly diffusing along the perfect monolayer step edges and immobilized at monolayer kink sites. Extensive atomistic simulations elucidate the mechanisms of adsorption and diffusion of the molecule on the different surface features. The results of this study suggest methods of controlling the diffusion of adsorbates on insulating and nanostructured surfaces.

Atomic-resolution images of radiation damage in KBr

The first steps of electron irradiation induced modification of a KBr(1 0 0) surface have been studied by dynamic force microscopy with atomic resolution. Rectangular pits of monatomic depth with not more than one kink site per pit have been found. The atomic structure of KBr(1 0 0) is preserved at the bottom of the pits. Possible deexcitation and desorption mechanisms are discussed based on these results

Determination of effective tip geometries in Kelvin probe force microscopy on thin insulating films on metals

In scanning probe techniques, accurate height measurements on heterogeneous surfaces are a major requirement. Different electrostatic potentials of various materials have a significant influence on the measured force/current and therefore a direct influence on the tip-sample distance. Kelvin probe force microscopy (KPFM) is based on a dynamic compensation of the electrostatic force while performing non-contact atomic force microscopy measurements. Thus, the influence of the electrostatic potentials can be minimized and accurate height measurements become possible. Here, the study of ultra-thin alkali halide films on Cu(111) investigated by KPFM is presented. This work is focused on the interface between areas of bare Cu(111) and the first layers of salt. The compensation of the electrostatic potential allow us to determine layer heights with high accuracy. The second objective was to elaborate on the characterization of tip geometries across suitable nanostructures. Simulations of measured images are performed with different input parameters, which gives a direct estimation of the effective tip radius and geometry used for the measurements.

Contacting a conjugated molecule with a surface dangling bond dimer on a hydrogenated Ge(001) surface allows imaging of the hidden ground electronic state.

Fabrication of single-molecule logic devices requires controlled manipulation of molecular states with atomic-scale precision. Tuning molecule-substrate coupling is achieved here by the reversible attachment of a prototypical planar conjugated organic molecule to dangling bonds on the surface of a hydrogenated semiconductor. We show that the ground electronic state resonance of a Y-shaped polyaromatic molecule physisorbed on a defect-free area of a fully hydrogenated surface cannot be observed by scanning tunneling microscopy (STM) measurements because it is decoupled from the Ge bulk states by the hydrogen-passivated surface. The state can be accessed by STM only if the molecule is contacted with the substrate by a dangling bond dimer. The reversibility of the attachment processes will be advantageous in the construction of surface atomic-scale circuits composed of single-molecule devices interconnected by the surface dangling bond wires.

Frenkel defect interactions at surfaces of irradiated alkali halides studied by non-contact atomic-force microscopy

Irradiation of alkali halide crystal leads to production of Frenkel defects in the crystal bulk. Subsequent diffusion and interactions of these defects with the surface results in desorption processes at the surface. We have studied surface topography of electron bombarded alkali halide crystals (KBr, NaCI) and the desorption fluxes. It is found that the desorption proceeds in a layer-by-layer mode by growth and linking of pits of monoatomic depth, which results in modulation of surface step density. Electron-stimulated desorption fluxes are correlated with the surface step density. Based on these results it is concluded that H-centers decay at the (100) surface plane with the emission of halogen atoms, F * -centers recombine with the terrace edge and initiate emission of alkali atoms, and F-centers accumulate in the sub-surface region. Above certain temperature (∼450 K for KBr, 370 K for NaCI) the desorption proceeds in more complex, multilayer mode as a result of combination of the above described Frenkel defect mediated mechanism with simultaneous thermal restructuring of the surface.

Interplay of the tip-sample junction stability and image contrast reversal on a Cu(111) surface revealed by the 3D force field.

Non-contact atomic force microscopy is used to measure the 3D force field on a dense-packed Cu(111) surface. An unexpected image contrast reversal is observed as the tip is moved towards the surface, with atoms appearing first as bright spots, whereas hollow and bridge sites turn bright at smaller tip-sample distances. Computer modeling is used to elucidate the nature of the image contrast. We find that the contrast reversal is essentially a geometrical effect, which, unlike in gold, is observable in Cu due to an unusually large stability of the tip-sample junction over large distances.

Nonacene Generated by On-Surface Dehydrogenation

The on-surface synthesis of nonacene has been accomplished by dehydrogenation of an air-stable partially saturated precursor, which could be aromatized by using a combined scanning tunneling and atomic force microscope as well as by on-surface annealing. This transformation allowed the in-detail analysis of the electronic properties of nonacene molecules physisorbed on Au(111) by scanning tunneling spectroscopy measurements. The spatial mapping of molecular orbitals was corroborated by density functional theory calculations. Furthermore, the thermally induced dehydrogenation uncovered the isomerization of intermediate dihydrononacene species, which allowed for their in-depth structural and electronic characterization.

Characterization of individual molecular adsorption geometries by atomic force microscopy: Cu-TCPP on rutile TiO2 (110).

Functionalized materials consisting of inorganic substrates with organic adsorbates play an increasing role in emerging technologies like molecular electronics or hybrid photovoltaics. For such applications, the adsorption geometry of the molecules under operating conditions, e.g., ambient temperature, is crucial because it influences the electronic properties of the interface, which in turn determine the device performance. So far detailed experimental characterization of adsorbates at room temperature has mainly been done using a combination of complementary methods like photoelectron spectroscopy together with scanning tunneling microscopy. However, this approach is limited to ensembles of adsorbates. In this paper, we show that the characterization of individual molecules at room temperature, comprising the determination of the adsorption configuration and the electrostatic interaction with the surface, can be achieved experimentally by atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM). We demonstrate this by identifying two different adsorption configurations of isolated copper(ii) meso-tetra (4-carboxyphenyl) porphyrin (Cu-TCPP) on rutile TiO2 (110) in ultra-high vacuum. The local contact potential difference measured by KPFM indicates an interfacial dipole due to electron transfer from the Cu-TCPP to the TiO2. The experimental results are verified by state-of-the-art first principles calculations. We note that the improvement of the AFM resolution, achieved in this work, is crucial for such accurate calculations. Therefore, high resolution AFM at room temperature is promising for significantly promoting the understanding of molecular adsorption.

Organic Molecules Reconstruct Nanostructures on Ionic Surfaces

Modification and functionalization of the atomic-scale structure of insulating surfaces is fundamental to catalysis, self-assembly, and single-molecule technologies. Specially designed syn-5,10,15-tris(4-cyanophenylmethyl)truxene molecules can reshape features on an ionic KBr (001) surface. Atomic force microscopy images demonstrate that both KBr monolayer islands and pits can reshape from rectangular to round structures, a process which is directly facilitated by molecular adsorption. Simulations reveal that the mechanism of the surface reconstruction consists of collective atomic hops of ions on the step edges of the islands and pits, which correlate with molecular motion. The energy barriers for individual processes are reduced by the presence of the adsorbed molecules, which cause surface structural changes. These results show how appropriately designed organic molecules can modify surface morphology on insulating surfaces. Such strongly adsorbed molecules can also serve as anchoring sites for building new nanostructures on inert insulating surfaces.

Cutting and self-healing molecular wires studied by dynamic force microscopy

Tip-induced deformations of meso-(4-cyanophenyl)-substituted Zn(II) porphyrin molecular wires self-assembled on KBr(001) were studied by frequency modulation dynamic force microscopy. Since the wires are weakly bonded to the KBr substrate and to the neighboring molecules, they can easily be cut by the scanning tip. We found that the damaged molecular wires self-healed at room temperature.