J. W. M. Frenken

Pushing the limits of SPM

In the two decades since the invention of the scanning tunneling microscope (STM) 1 , the family of local probing techniques known as scanning probe microscopy (SPM) has come to full maturity. Nowadays, the quality with which nanoscale images can be obtained and local spectroscopic information acquired using these instruments is spectacular. In addition, the ease of use of these machines has improved so much that they have found their way into the laboratories, not just of physicists, but also chemists, biologists, and engineers.

Scanning probe microscopes go video rate and beyond

In this article we introduce a, video-rate, control system that can be used with any type of scanning probe microscope, and that allows frame rates up to 200images∕s. These electronics are capable of measuring in a fast, completely analog mode as well as in the more conventional digital mode. The latter allows measurements at low speeds and options, such as, e.g., atom manipulation, current–voltage spectroscopy, or force–distance curves. For scanning tunneling microscope (STM) application we implemented a hybrid mode between the well-known constant-height and constant-current modes. This hybrid mode not only increases the maximum speed at which the surface can be imaged, but also improves the resolution at lower speeds. Acceptable image quality at high speeds could only be obtained by pushing the performance of each individual part of the electronics to its limit: we developed a preamplifier with a bandwidth of 600kHz, a feedback electronics with a bandwidth of 1MHz, a home-built bus structure for the fas...

The Active Phase of Palladium during Methane Oxidation

The active phase of Pd during methane oxidation is a long-standing puzzle, which, if solved, could provide routes for design of improved catalysts. Here, density functional theory and in situ surface X-ray diffraction are used to identify and characterize atomic sites yielding high methane conversion. Calculations are performed for methane dissociation over a range of Pd and PdOx surfaces and reveal facile dissociation on either under-coordinated Pd sites in PdO(101) or metallic surfaces. The experiments show unambiguously that high methane conversion requires sufficiently thick PdO(101) films or metallic Pd, in full agreement with the calculations. The established link between high activity and atomic structure enables rational design of improved catalysts.

Scanning tunnelling microscopy study of the growth of small palladium particles on TiO2(110)

Abstract We studied the thermal stability of small palladium clusters on a TiO 2 (110) surface. Upon heating the number of particles decreases and the average size of the particles increases. This corresponds to a decrease of palladium surface area of this model catalyst. The decrease in particle density cannot be used to distinguish between different kinds of mass transport over the surface, but on the basis of the resulting size distribution we conclude that the coalescence of diffusing particles is the main mechanism. This is supported by a direct observation of mobile clusters. During growth the palladium clusters change their shape.

Theory and simulation of high-energy ion scattering experiments for structure analysis of surfaces and interfaces

Abstract Monte Carlo computer calculations form an indispensable tool in the preparation and analysis of high-energy (50 keV–5 MeV) ion shadowing and blocking experiments. In this paper we discuss the theory of such calculations and describe a new algorithm, with which ion backscattering measurements from relaxed or reconstructed crystal surfaces, overlayer systems, interfaces, etc., can be accurately simulated. A comparison is made with other calculations, and some results are presented as illustration.

Surface structure and reactivity of Pd(100) during CO oxidation near ambient pressures

The surface structure of Pd(100) during CO oxidation was measured using a combination of a flow reactor and in situ surface X-ray diffraction coupled to a large-area 2-dimensional detector. The surface structure was measured for P(O(2))/P(CO) ratios between 0.6 and 10 at a fixed total gas pressure of 200 mbar and a fixed CO pressure of 10 ± 1 mbar. In conjunction with the surface structure the reactivity of the surface was also determined. For all P(O(2))/P(CO) ratios the surface was found to oxidize above a certain temperature. Three different types of oxides were observed: the surface oxide, an epitaxial layer of bulk-like PdO, and a non-epitaxial layer of bulk-like PdO. As soon as an oxide was present the reactivity of the surface was found to be mass transfer limited by the flux of CO molecules reaching the surface.

Static and dynamic displacements of nickel atoms in clean and oxygen covered Ni(001) surfaces

Abstract Ion shadowing and blocking measurements show that the clean Ni(001) surface is inwardly relaxed by −3.2 ± 0.5% of the interlayer spacing. Oxygen chemisorption induces a sign reversal in the relaxation resulting in net expansions of + 2.0 ± 1.0% and + 5.2 ± 1.5% for the p(2 × 2) and c(2 × 2) structures respectively. Within the range of coverages studied ( ⩽ 0.46 ± 0.04 monolayer) no evidence was found for an oxidic c(2 × 2) phase. A comparison of the surface blocking data with Monte Carlo simulations shows that the surface thermal vibration amplitude is significantly enhanced with respect to the bulk value. This enhancement persists for oxygen chemisorption.

Ni‐Si(111) interface: Growth of Ni2Si islands at room temperature

Ultrathin films (0–20 A) of Ni have been deposited on atomically clean Si(111) surfaces at room temperature. The composition and morphology of the films have been determined, employing the high depth resolution obtainable in medium energy ion scattering. Disordered Ni2Si islands are formed, which grow laterally and in thickness with increasing Ni coverage. The silicide formation ends when the islands coalesce into a continuous film, at a Ni coverage of ≊8×1015 Ni atoms/cm2. During the silicide growth, the surfaces of the islands are rich in Si.

Formation of epitaxial β-FeSi2 films on Si(001) as studied by medium-energy ion scattering

Ultrathin (∼1.3 nm) epitaxial films of β‐FeSi2 were grown on Si(001) by room temperature (RT) deposition of Fe followed by annealing. During the various stages of the growth process, the lattice structure, composition, and morphology of the films were investigated by medium‐energy ion scattering in conjunction with shadowing and blocking. At RT, the deposited Fe reacts with the Si(001) substrate and forms a continuous film of average composition FeSi. After annealing to 670 K, a conversion into β‐FeSi2 has taken place and the film is no longer continuous. Further annealing at higher temperatures results in the formation of islands of increasing height. The β‐FeSi2 films grown are composites of two azimuthal orientations with respect to the substrate: The predominant A orientation with β‐FeSi2 [010]∥ Si〈110〉 and the B orientation with β‐FeSi2 [010] ∥ Si〈100〉. The lattice strain in the films is partially relaxed. At the interface, the Fe atoms are found to be displaced from bulk lattice sites. These displac...

Silicon strained layers grown on GaP(001) by molecular beam epitaxy

Mismatch‐induced lattice strain in thin Si films grown by molecular beam epitaxy on GaP(001) substrates has been measured using transmission electron microscopy, Raman spectroscopy, and Rutherford backscattering. The perpendicular strain in the topmost part of the layers is found to be enhanced in comparison to elasticity theory. Relaxation of the strain occurs by the formation of misfit dislocations at significantly larger thickness than predicted by equilibrium theory.