R. Pascal

STM study of carbon-induced reconstructions on W(110): strong evidence for a surface lattice deformation

Abstract We report on a STM study of clean W(110) and the so-called R(15 × 3) and R(15 × 12) carbon-induced reconstructions of the W(110) surface on the atomic scale. The dimensions of the unit cells of both reconstructions, until now only known from diffraction experiments, are confirmed. Depending on the tunnelling gap resistance the inner structure of the R(15 × 3) unit cell is resolved. The experimental results provide strong evidence, that this structure is formed by a periodic surface lattice deformation.

Scanning tunneling spectroscopy of Fe/W(110) using iron covered probe tips

We have used iron covered tungsten probe tips to study the electronic structure of thin films of Fe on a W(110) substrate by means of scanning tunneling spectroscopy. In contrast to earlier experiments which were performed with tungsten tips we have found a minimum in the differential conductivity dI/dV at submonolayer Fe coverage just above the Fermi level. At a coverage of 1.3 ML a pronounced peak is measured above Fe islands of the second layer approximately 0.9 eV above the Fermi level, i.e., in the empty sample states. This empty state peak was found to be extremely sensitive to contamination from the residual gas which is also known to influence the magnetic state of the second layer islands.We have used iron covered tungsten probe tips to study the electronic structure of thin films of Fe on a W(110) substrate by means of scanning tunneling spectroscopy. In contrast to earlier experiments which were performed with tungsten tips we have found a minimum in the differential conductivity dI/dV at submonolayer Fe coverage just above the Fermi level. At a coverage of 1.3 ML a pronounced peak is measured above Fe islands of the second layer approximately 0.9 eV above the Fermi level, i.e., in the empty sample states. This empty state peak was found to be extremely sensitive to contamination from the residual gas which is also known to influence the magnetic state of the second layer islands.

ISSUES OF ATOMIC-RESOLUTION STRUCTURE AND CHEMICAL ANALYSIS BY SCANNING PROBE MICROSCOPY AND SPECTROSCOPY

The ultimate goal of high‐resolution microscopy, i.e., the determination of structure and chemistry on the atomic scale, has not been achieved yet. In the case of scanning tunneling microscopy (STM), the superposition of atomic and electronic structure information was found to be a critical issue, particularly for semiconductor surfaces. Here, we show that, in the case of multicomponent systems with large unit cell size, the structural analysis is further complicated by the decay behavior of the surface wave functions, which drastically influences the distance dependence of STM images. The chemical analysis at the atomic level is generally prevented by the inaccessibility of core levels due to the low‐energy scale of scanning probe microscopies. However, fingerprints for particular chemical species at the atomic level can be obtained from tunneling spectroscopy independent of the chosen substrate. Here, we will focus on a particular example of iron adsorbates on different metal substrates. Finally, the po...

STM-study of GdW(110) at submonolayer coverages

Abstract We have studied the growth of Gd on a W(110) substrate at submonolayer coverages by means of scanning tunneling microscopy (STM). Gd forms domains of equidistant chains along the [110] direction of the substrate. The distance between the chains decreases in discrete steps with increasing coverage. These chain-like superstructures coincide with the appearance of (n × 2) LEED-patterns (n = 10, 8, 7, 6, 5). Atomically resolved STM images support an earlier model which explains the chains by periodic adsorption in on-top and non-top sites. The apparent corrugation of the chains exhibits a strong bias dependence which can be explained by adsorption-site-dependent local electronic properties. Spatially resolved spectroscopy of the local barrier height is in excellent qualitative agreement with earlier spatially averaging experiments.

Magnetic nanostructures studied by scanning probe microscopy and spectroscopy

Scanning tunneling microscopy and spectroscopy (STM/STS) were applied to study the nanostructural and nanoelectronic properties of ultrathin magnetic films, from individual adatoms and clusters (zero-dimensional systems) to three-dimensional bulklike islands. Complementary investigations of the nanomagnetic structure of ultrathin films were performed by magnetic force microscopy (MFM). Quantum magnetic structures were prepared by electron-beam lithography and nanosphere lithography and analyzed by STM/STS as well as by MFM. Magnetization switching of single-domain Co dots induced by the MFM tip is demonstrated, and its potential application for quantum magnetic storage is shown. Finally, tunneling spectroscopy performed with magnetic probe tips yields new information about the spin-resolved nanoelectronic properties of magnetic nanostructures.

Chemical-specific imaging of multicomponent metal surfaces on the nanometer scale by scanning tunneling spectroscopy

The topographic and chemical surface structure of a submonolayer iron film on a W(110) substrate has been studied by combined scanning tunneling microscopy and spectroscopy. Local tunneling spectra revealed a pronounced difference in the electronic structure between nanometer-scale iron islands of monolayer height and the bare W(110) substrate. In particular, a pronounced empty-state peak at 0.2 eV above the Fermi level has been identified for the iron islands. Based on the pronounced difference in the local tunneling spectra measured above the iron islands and the tungsten substrate, chemical-specific imaging was achieved by performing spatially resolved measurements of the differential tunneling conductivitydl/dU (x, y) at selected sample bias voltages.

Lattice relaxation of Gd on W(110)

A combined scanning tunneling microscopy and low energy electron diffraction study enables to propose a model of the atomic arrangement in the first close-packed monolayer of Gd on W(110). The lattice parameter of the Gd/W(110) epitaxial system in the first two layers is larger than in the bulk Gd. This lattice mismatch amounts to 2% for the first and ca. 0.3% for the second monolayer. From the third monolayer on the lattice parameter corresponds to the one of the bulk Gd crystal.

GdFe2 alloy formation observed by STM

Abstract Ultra thin films of an alloy consisting of Gd and Fe were prepared under ultra high vacuum conditions on top of a W(110) surface. The films were characterized using scanning tunneling microscopy and low energy electron diffraction. We succeeded in preparing the initial stages of well ordered GdFe 2 films. Basing on atomically resolved STM images and LEED studies, a structure model for the thin GdFe 2 films was deduced. The crystallographic structure of the first and second ML GdFe 2 on W(110) was found to be different from the bulk structure (Laves phase C15).

The adsorption process of hydrogen on Gd(0001)

The development of the adsorption processes of hydrogen on Gd(0001) was studied by means of photoelectron spectroscopy in combination with scanning tunneling microscopy and spectroscopy, respectively. Due to its high spatial resolution the visualization of the adsorption process is obtained. It was found that the adsorption occurs in two steps. It starts at crystallographic surface imperfections. From these nucleation centers, a domain-like spreading is present which is strongly hindered at surface steps. A short duration high exposure dose results in a different behavior; areas on the surface without imperfections now become affected. These areas with adsorbed hydrogen are not connected to each other.