Hans-Peter Rust

Selectivity in vibrationally mediated single-molecule chemistry

The selective excitation of molecular vibrations provides a means to directly influence the speed and outcome of chemical reactions. Such mode-selective chemistry has traditionally used laser pulses to prepare reactants in specific vibrational states to enhance reactivity or modify the distribution of product species. Inelastic tunnelling electrons may also excite molecular vibrations and have been used to that effect on adsorbed molecules, to cleave individual chemical bonds and induce molecular motion or dissociation. Here we demonstrate that inelastic tunnelling electrons can be tuned to induce selectively either the translation or desorption of individual ammonia molecules on a Cu(100) surface. We are able to select a particular reaction pathway by adjusting the electronic tunnelling current and energy during the reaction induction such that we activate either the stretching vibration of ammonia or the inversion of its pyramidal structure. Our results illustrate the ability of the scanning tunnelling microscope to probe single-molecule events in the limit of very low yield and very low power irradiation, which should allow the investigation of reaction pathways not readily amenable to study by more conventional approaches.

Measuring the Charge State of Point Defects on MgO/Ag(001)

A detailed understanding of surface defects is highly desirable, e.g. to clarify their role as active sites in catalysis. Here localized defects on the surface of MgO films deposited on Ag(001) are investigated. Since the electronic structure of color centers depends on their local position, spectroscopic signals are highly convoluted and often difficult to disentangle. In this study we aimed to obtain morphological and spectroscopic information on single color centers at a microscopic level with frequency modulated dynamic force microscopy (FM-DFM) and scanning tunneling microscopy (STM) in an ultrahigh vacuum and at low temperature. Four of the major and in literature mostly discussed defect types on MgO have been characterized by their charge state and finally identified by the complementary application of FM-DFM and STM in combination with density functional theory results.

Au dimers on thin MgO(001) films: flat and charged or upright and neutral?

A combination of low temperature scanning tunneling microscopy (STM) and theoretical calculations is used to investigate Au dimers, supported on thin MgO(001) films, whose thickness was chosen such that charge transfer from the Ag substrate to the deposited Au is possible. Au dimers exist not only in an upright geometry--as theoretically predicted to be the most stable configuration--but also as flat lying dimers which populate a manifold of different azimuthal orientations. Apart from the difference in adsorption configurations, these two isomers exhibit rather different electronic structures: while upright dimers are neutral, flat ones are charged.

Role of Spin in Quasiparticle Interference

Quasiparticle interference patterns measured by scanning tunneling microscopy can be used to study the local electronic structure of metal surfaces and high-temperature superconductors. Here, we show that even in nonmagnetic systems the spin of the quasiparticles can have a profound effect on the interference patterns. On Bi(110), where the surface state bands are not spin degenerate, the patterns are not related to the dispersion of the electronic states in a simple way. In fact, the features which are expected for the spin-independent situation are absent and the observed interference patterns can be interpreted only by taking spin-conserving scattering events into account.

Probing adsorption sites on thin oxide films by dynamic force microscopy

Site specific atomically resolved dynamic force spectroscopy measurements were performed on a thin MgO film grown on Ag(001). The microscope is operated in ultrahigh vacuum at low temperature to ensure defined imaging condition, high stability, and drift reduction. Atomically resolved dynamic force microscopy images have been combined with site specific frequency shift versus distance measurements. The frequency shift is measured as a function of z and the lateral displacement. With these measurement characteristics the authors are currently probing inequivalent surface sites on thin MgO films to extract atomic-scale information on surface chemical reactivity and possible adsorption sites for metal atoms and small clusters.

Double quartz tuning fork sensor for low temperature atomic force and scanning tunneling microscopy

A double quartz tuning fork sensor for low temperature ultrahigh vacuum atomic force and scanning tunneling microscopy is presented. The features of the new sensor are discussed and compared to a single asymmetric tuning fork assembly. In addition, a low temperature ac signal amplifier has been developed to pick up the oscillation amplitude of the tuning fork. Current consumption and amplification factor versus the supply voltage of the amplifier as well as the magnitude response of the sensor have been measured at room temperature, 77 and 4 K. Atomically resolved images of a Ag(111) surface and single Ag atoms on Ag(111) were recorded in the scanning tunneling microscopy mode. Initial atomic force measurements are shown that reveal step resolution on a NiAl(110) surface.

A double lamellae dropoff etching procedure for tungsten tips attached to tuning fork atomic force microscopy/scanning tunneling microscopy sensors

We present an electrochemical etching scheme for producing sharp tungsten tips for use in scanning probe microscopes. The motivation behind the development of this particular method comes from the need to have an etched probe attached to a quartz tuning fork. Comparisons with existing etching methods are made. This rather simple scheme incorporates the key advantages of previously established techniques to give reproducible and controlled etching cycles.

Structure investigation of the topmost layer of a thin ordered alumina film grown on NiAl(110) by low temperature scanning tunneling microscopy

A thin Al2O3 layer grown on NiAl(1 1 0) has been studied by scanning tunneling microscopy at about 4 K. The film exhibited monocrystalline ordering compatible with the (1 1 1) face of c-alumina with a slightly extended surface unit cell. Although the topographical images change substantially with bias voltage we show that the autocorrelation transform of the images clearly reveals the unit cell of the alumina lattice. By using particular tunneling conditions we have identified almost all oxygen atom positions within the unit cell. The parameters applied lead to a considerable interaction between tip and top atoms of the substrate reflected in the image as local perturbations denoted as scratching. 2002 Elsevier Science B.V. All rights reserved.

High precision mechanical approach mechanism for a low temperature scanning tunneling microscope

A novel mechanical micropositioner has been developed for producing exact linear displacements in the nm range. Incorporated in a low temperature scanning tunneling microscope (STM), it is used as a coarse approach mechanism for the tunneling tip. The advantage of the design is the high accuracy of the linear motion and the absence of backlash. The design principle is discussed and its positioning accuracy and stability for STM imaging are demonstrated.

Atomic resolution on MgO(001) by atomic force microscopy using a double quartz tuning fork sensor at low-temperature and ultrahigh vacuum

Atomic resolution with a double quartz tuning fork sensor for low-temperature ultrahigh vacuum atomic force and scanning tunneling microscopy is presented. The new features of the force sensor assembly are discussed. Atomically resolved images of MgO on Ag(001) have been obtained. Images acquired in the attractive and the repulsive regimes controlled to a constant frequency shift are shown, revealing contrast changes.