Stefan Fölsch

Two-dimensional water condensation on the NaCl(100) surface

Abstract The adsorption of water on the NaCl(100) surface has been studied by UPS, XPS and LEED, using well-ordered films of NaCl which were grown heteroepitaxially on Ge(100) substrates. In the investigated range of temperature and partial pressure (135–155 K and 1 × 10−4−1 × 10−7 Pa), a first-order phase transition is observed, resulting in the formation of a physisorbed two-dimensional H2O phase (isosteric heat of adsorption qst = 65 kJ/mol). This condensed 2D phase is well ordered, the arrangement of H2O molecules can be described by a c(4 × 2) symmetry in two domains, which are rotated by 90° against each other. According to this symmetry, a structural model is proposed with an ice-like, distorted “bilayer”-structure. Additionally, a phase transition from the 2D H2O phase to a three-dimensional solid ice is observed. This solid ice shows a polycrystalline state of order.

Atomically thin epitaxial films of NaCl on germanium

Abstract On clean Ge(100) and Ge(111) substrates thin epitaxial NaCl layers are generated. At sample temperatures T a 0 = 0.563 nm). On Ge(111) a structure consisting of twinned pyramids is built only at temperatures T > 150K. Measurements of the work function change are in accord with properties concerning film structure and dissociation of NaCl molecules on Ge(111). In contrast to bulk alkali halide crystals, these films do not exhibit any disturbing surface charging effects during photoemission or low energy electron diffraction measurements.

Water adsorption on the NaCl surface

Abstract H2O adsorption on epitaxial NaCl films has been investigated by UPS, XPS and EELS, focused on the dependence of the adsorption behaviour on the defect-structure of the substrate. On the smooth, homogeneous surface only molecular adsorption is observed. A three-dimensional ice-layer grows at a deposition temperature of 130 K. After desorption of the H2O layer (145 K at 1 × 10−6 Pa partial H2O pressure) a small amount of H2O molecules remains on the surface in a modified state of bonding. Surface color centers produced by electron bombardment however induce chemisorption. These defects act as reactive sites which cause dissociation and OH production.

LEED studies of the adsorption of CO2 on thin epitaxial NaCl(100) films

Abstract The adsorption of CO 2 on the NaCl(100) surface was studied with a high-resolution LEED-system. Measurements without charging up at low electron energies and without damage by the e-beam could be performed by using ultrathin epitaxial films on a conducting Ge(100) substrate. The adsorption behavior was recorded as a function of time and pressure at constant substrate temperatures of 78 and 83 K and CO 2 partial pressures from 4 × 10 −8 −2 × 10 −3 Pa. The adsorption system shows a first-order two-dimensional phase transition to a (2 × 1) superstructure including glide planes (herringbone-like structure) at p = 7.2 × 10 −8 Pa ( T = 78 K). The condensation of the CO 2 solid is starting at p = 1.5 × 10 −4 Pa ( T = 78 K). The LEED-pattern shows in this c(2 × 2) superstructure, which corresponds to the pyrite-like structure of the CO 2 solid. Both observed superstructures are commensurable with the NaCl(100) surface. Observation of island growth shows that the domains of the (2 × 1) superstructures have already at coverage of 5% of a monolayer an average lateral size of at least 200 A.

Quantum dots with single-atom precision

Quantum dots are often called artificial atoms because, like real atoms, they confine electrons to quantized states with discrete energies. However, although real atoms are identical, most quantum dots comprise hundreds or thousands of atoms, with inevitable variations in size and shape and, consequently, unavoidable variability in their wavefunctions and energies. Electrostatic gates can be used to mitigate these variations by adjusting the electron energy levels, but the more ambitious goal of creating quantum dots with intrinsically digital fidelity by eliminating statistical variations in their size, shape and arrangement remains elusive. We used a scanning tunnelling microscope to create quantum dots with identical, deterministic sizes. By using the lattice of a reconstructed semiconductor surface to fix the position of each atom, we controlled the shape and location of the dots with effectively zero error. This allowed us to construct quantum dot molecules whose coupling has no intrinsic variation but could nonetheless be tuned with arbitrary precision over a wide range. Digital fidelity opens the door to quantum dot architectures free of intrinsic broadening-an important goal for technologies from nanophotonics to quantum information processing as well as for fundamental studies of confined electrons.

Epitaxial C60 films on CaF2 (111) grown by molecular beam deposition

Epitaxial C60 films grown by molecular beam deposition onto CaF2(111) surfaces are investigated by reflection high‐energy electron diffraction at deposition temperatures of 30–300 °C and coverages corresponding to average thicknesses of 1–50 nm. Over this entire temperature range, C60 forms an incommensurate overgrowth of stacked hexagonal layers exhibiting a characteristic nearest‐neighbor spacing of 0.98 nm. Below 170 °C, unidirectional growth occurs in accordance with the crystallographic directions of the substrate. At higher deposition temperatures, however, two equivalent, rotated domain orientations are observed which are characterized by a significantly lower degree of lattice mismatch.

Controlled manipulation of atoms and small molecules with a low temperature scanning tunneling microscope

With the scanning tunneling microscope (STM) it became possible to perform controlled manipulations down to the scale of small molecules and single atoms, leading to the fascinating aspect of creating manmade structures on atomic scale. Here we present a short review of our work in the last five years on atomic scale manipulation investigations. Upon soft lateral manipulation of adsorbed species, in which only tip/particle forces are used, three different manipulation modes (pushing, pulling, sliding) can be discerned. We show that also manipulation of highly coordinated native substrate atoms is possible and demonstrate the application of these techniques as local analytic and synthetic chemistry tools with important consequences on surface structure research. Vertical manipulation of Xe and CO is presented, leading to improved imaging and even chemical contrast with deliberately functionalized tips. For the transfer of CO it is shown that beside tip voltage current effects play also an important role. This is also the case for the dissociation of molecules. With CO transferred deliberately to the tip we have also succeeded to perform vibrational spectroscopy on single molecules. Furthermore, first experiments aiming for the transfer of all manipulation modes to thin insulating films are described.

Current versus temperature-induced switching in a single-molecule tunnel junction: 1,5 cyclooctadiene on Si(001).

The biconformational switching of single cyclooctadiene molecules chemisorbed on a Si(001) surface was explored by quantum chemical and quantum dynamical calculations and low-temperature scanning tunneling microscopy experiments. The calculations rationalize the experimentally observed switching driven by inelastic electron tunneling (IET) at 5 K. At higher temperatures, they predict a controllable crossover behavior between IET-driven and thermally activated switching, which is fully confirmed by experiment.

Gating a single-molecule transistor with individual atoms

Transistors rely on electrical gates to control conductance but this is challenging on the atomic-scale. It is now shown that individual charged atoms can be used to electrostatically gate a single-molecule transistor with sub-angstrom precision.

Controlled switching within an organic molecule deliberately pinned to a semiconductor surface.

Bistable organic molecules were deposited on a weakly binding III-V semiconductor surface and then pinned into place using individual native adatoms. These pinning atoms, positioned by atomically precise manipulation techniques in a cryogenic scanning tunneling microscope (STM) at 5 K, stabilize the π-conjugated molecule against rotation excited by the tunneling electrons. The pinning allows triggering of the molecules intrinsic switching mechanism (a hydrogen transfer reaction) by the STM tunnel current. Density-functional theory calculations reveal that the energetics of the switching process is virtually unaffected by both the surface and the pinning atoms. Hence, we have demonstrated that individual molecules with predictable, predefined functions can be stabilized and assembled on semiconductor templates.