Karl-Heinz Ernst

Electrically driven directional motion of a four-wheeled molecule on a metal surface

Propelling single molecules in a controlled manner along an unmodified surface remains extremely challenging because it requires molecules that can use light, chemical or electrical energy to modulate their interaction with the surface in a way that generates motion. Nature’s motor proteins have mastered the art of converting conformational changes into directed motion, and have inspired the design of artificial systems such as DNA walkers and light- and redox-driven molecular motors. But although controlled movement of single molecules along a surface has been reported, the molecules in these examples act as passive elements that either diffuse along a preferential direction with equal probability for forward and backward movement or are dragged by an STM tip. Here we present a molecule with four functional units—our previously reported rotary motors—that undergo continuous and defined conformational changes upon sequential electronic and vibrational excitation. Scanning tunnelling microscopy confirms that activation of the conformational changes of the rotors through inelastic electron tunnelling propels the molecule unidirectionally across a Cu(111) surface. The system can be adapted to follow either linear or random surface trajectories or to remain stationary, by tuning the chirality of the individual motor units. Our design provides a starting point for the exploration of more sophisticated molecular mechanical systems with directionally controlled motion.

Amplification of chirality in two-dimensional enantiomorphous lattices

The concept of chirality dates back to 1848, when Pasteur manually separated left-handed from right-handed sodium ammonium tartrate crystals. Crystallization is still an important means for separating chiral molecules into their two different mirror-image isomers (enantiomers), yet remains poorly understood. For example, there are no firm rules to predict whether a particular pair of chiral partners will follow the behaviour of the vast majority of chiral molecules and crystallize together as racemic crystals, or as separate enantiomers. A somewhat simpler and more tractable version of this phenomenon is crystallization in two dimensions, such as the formation of surface structures by adsorbed molecules. The relatively simple spatial molecular arrangement of these systems makes it easier to study the effects of specific chiral interactions; moreover, chiral assembly and recognition processes can be observed directly and with molecular resolution using scanning tunnelling microscopy. The enantioseparation of chiral molecules in two dimensions is expected to occur more readily because planar confinement excludes some bulk crystal symmetry elements and enhances chiral interactions; however, many surface structures have been found to be racemic. Here we show that the chiral hydrocarbon heptahelicene on a Cu(111) surface does not undergo two-dimensional spontaneous resolution into enantiomers, but still shows enantiomorphism on a mesoscopic length scale that is readily amplified. That is, we observe formation of racemic heptahelicene domains with non-superimposable mirror-like lattice structures, with a small excess of one of the heptahelicene enantiomers suppressing the formation of one domain type. Similar to the induction of homochirality in achiral enantiomorphous monolayers by a chiral modifier, a small enantiomeric excess suffices to ensure that the entire molecular monolayer consists of domains having only one of two possible, non-superimposable, mirror-like lattice structures.

Switching the chirality of single adsorbate complexes.

Pumped up: Propene molecules form chiral complexes when adsorbed on a copper surface. Inelastically scattered tunneling electrons from the tip of a scanning tunneling microscope induce rotation or diffusion of the adsorbate on the surface. Higher tunneling currents can lead to conversion of the adsorbate into the opposite enantiomer.

Surface analysis of chemically-etched and plasma-treated polyetheretherketone (PEEK) for biomedical applications

Abstract Surface modifications of polyetheretherketone (PEEK) made by chemical etching or oxygen plasma treatment were examined in this study. Chemical etching caused surface topography to become irregular with higher roughness values R a and R q . Oxygen plasma treatment also affected surface topography, unveiling the spherulitic structure of PEEK. R a , R q and surface area significantly increased after plasma treatment; topographical modifications were, nonetheless, moderate. Wetting angle measurements and surface energy calculations revealed an increase of wettability and surface polarity due to both treatments. XPS measurements showed an increase of surface oxygen concentration after both treatments. An O:C ratio of 3.10 for the plasma-treated PEEK surface and 4.41 for the chemically-etched surface were determined. The results indicate that surface activation by oxygen plasma treatment for subsequent coating processes in supersaturated physiological solutions to manufacture PEEK for biomedical appiications is preferable over the chemical etching treatment.

Kinetics of the reverse water-gas shift reaction over Cu(110)

The reverse water-gas shift reaction (CO2 + H2 → H2O + CO) has been studied over a clean Cu(110) single-crystal model catalyst at temperatures between 573 and 723 K. The steady-state kinetic measurements were carried out at medium pressures (10–2000 Torr) in a microreactor after cleaning and characterization of the sample under UHV conditions. The H2/CO2-pressure ratios varied from 1000: 1 to 1 : 10. The product buildup was monitored with a gas chromatograph (GC). The apparent activation energy is about 18 kcal/mol, and the reaction orders in H2 and CO2 depend strongly on the H2/CO2 ratio and temperature. The steady-state kinetics are compared favorably with the rates of elementary steps potentially involved in a “surface redox” reaction mechanism of the reverse and forward water-gas shift reaction involving the formation and removal of oxygen adatoms. Kinetic evidence that is tentatively attributed to a hydrogen-induced surface phase transition that affects the reaction rate, is also presented.

Lifetime of biomolecules in polymer-based hybrid nanodevices

Prolonging the lifetime of biomolecules in their functional states is critical for many applications where biomolecules are integrated into synthetic materials or devices. A simplified molecular shuttle system, which consists of fluorescently labelled microtubules propelled by kinesin motor proteins bound to the surface of a flowcell, served here as a model system to probe the lifetime of a hybrid device. In this system, the functional decay can easily be assayed by utilizing optical microscopy to detect motility and disintegration of microtubules. We found that the lifetimes of these hybrid systems were mainly limited by the stability of microtubules (MTs), rather than of kinesin. To determine the biocompatibility of polymers widely used in microfabrication, we assembled flowcells with glass bottom surfaces and covers fabricated from glass, poly(urethane) (PU), poly(methyl-methacrylate) (PMMA), poly(dimethylsiloxane) (PDMS) and ethylene-vinyl alcohol copolymer (EVOH). Without illumination, only PU had a substantial negative impact on MT stability, while PMMA, PDMS and EVOH showed stabilities comparable to glass. Under the influence of light, however, the MTs degraded rapidly in the presence of PDMS or PMMA, even in the presence of oxygen scavengers. A similar effect was observed on glass if oxygen scavengers were not added to the medium. Strong bleaching of the fluorophores was again only found on the polymer substrates and photobleaching coincided with an accelerated depolymerization of the MTs.

Orientation of chiral heptahelicene C30H18 on copper surfaces: An x-ray photoelectron diffraction study

The orientation and the intramolecular relaxation due to adsorption of the chiral phenanthrene-derivative heptahelicene, C30H18, on Cu(111) and Cu(332) surfaces have been investigated by means of angle-scanned full-hemispherical x-ray photoelectron diffraction. Although the C 1s diffraction patterns of the adsorbed submonolayer coverage helicene films exhibit scattering anisotropies of less than two percent, a detailed analysis involving simple molecular mechanics calculations of the atomic coordinates, photoelectron diffraction single-scattering cluster calculations and an R-factor analysis permits the determination of the helicene molecular orientation. On Cu(111), the molecules are found to bind to the substrate surface via their terminal phenanthrene group oriented parallel to the surface plane, while on Cu(332) the three terminal C-6 rings are oriented parallel to the (111) terrace plane. Six azimuthal molecular orientations are found to coexist on Cu(111), on Cu(332), however, the step–molecule inte...

Building 2D crystals from 5-fold-symmetric molecules

Concepts of close packing in monolayers of 5-fold-symmetric buckybowls are discussed. When the symmetry of lattice and molecular building blocks are incompatible, new strategies evolve. Corannulene forms a hexagonal lattice on Cu(111) by tilting away from the C(5) symmetry and aligning one hexagonal ring parallel to the surface. The chiral 5-fold-substituted chloro and methyl derivatives do not show this tilt and maintain the 5-fold symmetry as adsorbates. Consequently, a nonperfect tiling is observed. Their lattices are quasi-hexagonal: one in an antiparallel fashion with almost pm symmetry and the other with azimuthal and positional disorder on the hexagonal grid. Our results are in remarkable agreement with computational and mechanical modeling experiments of close packing of hard pentagonal discs in macroscopic two-dimensional systems and prove the validity of such modeling strategies.

Reversible phase transitions in a buckybowl monolayer.

Like penguins on ice, buckybowl molecules move closer together when cooled on a copper surface (see model of a corannulene molecule adsorbed on Cu(111)). Upon heating, the molecules spread out into the original crystal phase again. The lower density at room temperature can be explained by the increase in entropy owing to the excitation of bowl vibrations at the surface.

The interaction of glycine with a platinum (111) surface

Abstract The (complex) interaction between glycine (α-amino acetic acid) and Pt(111) was studied in vacuo at 100–600 K by means of LEED, AES, TDS and UPS. Up to ∼ 350 K the glycine molecule is molecularly adsorbed in a configuration in which the lone-pair orbitais of the hydroxyl and the amino group both contribute to the bonding to the Pt surface as indicated by the relatively large chemical shifts of the corresponding orbitais in UPS. In the multilayer range there is evidence of the zwitterion configuration. Gentle heating of the chemisorbed layer leads to a partial ordering of the adsorbed glycine, and a 2 × 2 LEED pattern is formed at 370 K. Heating to higher temperatures causes partial thermal desorption of undecomposed species around 380 K, but most of the glycine remains in the chemisorbed state and undergoes decomposition. A c(4 × 2) LEED structure is formed at 400 K which remains stable up to 460 K until at 520 K. the surface species decompose abruptly and desorb in a reaction-controlled kinetics. The main route of the decomposition is the rupture of the C-C bond and the formation of two fragments with methyl amine and formic acid character. These then transform further into H 2 O and CO on the one hand and H 2 , N 2 and HCN on the other. The two main reaction channels are supported by separate adsorption experiments on Pt(111) using methyl amine and formic acid.