Richard Berndt

Highly ordered structures and submolecular scanning tunnelling microscopy contrast of PTCDA and DM-PBDCI monolayers on Ag(111) and Ag(110)

Abstract Highly ordered monolayers of two perylene derivatives (perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) and N,N′-dimethylperylene-3,4,9,10-bis(dicarboximide) (DM-PBDCI)) were prepared by vacuum sublimation on Ag(111) and Ag(110) surfaces. Using scanning tunnelling microscopy (STM) and low energy electron diffraction, the nucleation, growth, step site interaction, and superstructures were investigated and compared. All adsorbate structures are commensurate, with the exception of DM-PBDCI on Ag(111). A similar, but not identical, submolecular STM resolution was obtained for both molecules and on both surfaces. The local distribution shows ten dominant intensity maxima per molecule and corresponds to that of the lowest unoccupied orbital. From these results we draw conclusions concerning the tunnelling mechanism, the adsorbate–substrate, and the adsorbate–adsorbate interactions, which are discussed in relation to the formerly published results on the electronic structure of PTCDA/Ag(111).

Photon Emission at Molecular Resolution Induced by a Scanning Tunneling Microscope

The tip-surface region of a scanning tunneling microscope (STM) emits light when the energy of the tunneling electrons is sufficient to excite luminescent processes. These processes provide access to dynamic aspects of the local electronic structure that are not directly amenable to conventional STM experiments. From monolayer films of carbon-60 fullerenes on gold(110) surfaces, intense emission is observed when the STM tip is placed above an individual molecule. The diameter of this emission spot associated with carbon-60 is approximately 4 angstroms. These results demonstrate the highest spatial resolution of light emission to date with a scanning probe technique.

Pushing and pulling a Sn ion through an adsorbed phthalocyanine molecule.

Molecule-based functional devices on surfaces may take advantage of bistable molecular switches. The conformational dynamics and efficiency of switches are radically different on surfaces compared to the liquid phase. We present a design of molecular layers which enables bistable switching on a surface and, for the first time, demonstrate control of a single switch in a dense and ordered array at the spatial limit. Up and down motion of a central Sn ion through the frame of a phthalocyanine molecule is achieved via resonant electron or hole injection into molecular orbitals.

Separation of a Racemic Mixture of Two‐Dimensional Molecular Clusters by Scanning Tunneling Microscopy

Adsorption of sub-monolayer amounts of 1-nitronaphthalene (NN) onto Au(111) leads to the aggregation of NN decamers, which exhibit two-dimensional chirality and represent a racemic mixture. In analogy to Pasteurs experiment of 1848 a scanning tunneling microscope can be used to discriminate and separate the enantiomers on a molecular scale.

Controlled Contact to a C60 Molecule

The tip of a low-temperature scanning tunneling microscope is approached towards a C60 molecule adsorbed at a pentagon-hexagon bond on Cu(100) to form a tip-molecule contact. The conductance rapidly increases to approximately 0.25 conductance quanta in the transition region from tunneling to contact. Ab-initio calculations within density functional theory and nonequilibrium Greens function techniques explain the experimental data in terms of the conductance of an essentially undeformed C60. The conductance in the transition region is affected by structural fluctuations which modulate the tip-molecule distance.

Atom transfer and single-adatom contacts

The point contact of a tunnel tip approaching towards Ag(111) and Cu(111) surfaces is investigated with a low temperature scanning tunneling microscope. A sharp jump to contact, random in nature, is observed in the conductance. After point contact, the tip-apex atom is transferred to the surface, indicating that a one-atom contact is formed during the approach. In sharp contrast, the conductance over single silver and copper adatoms exhibits a smooth and reproducible transition from tunneling to contact regime. Numerical simulations show that this is a consequence of the additional dipolar bonding between the adatom and the surface atoms.

Helium atom scattering studies of thermal energy vibrations on the clean and adsorbate-covered NI(100) surfaces

Abstract Vibrations at a Ni(100) surface have been studied by Helium atom scattering. Adsorption of various adsorbates (CO,H,O) leads to characteristic changes and new features in the phonon spectrum of the clean surface. The dispersion curves for the clean substrate are compared to recent EELS data. Experimentally determined values for the frequency of the hindered translation of adsorbed CO-molecules strongly deviate from theoretical predictions.

Passing current through touching molecules.

The charge flow from a single C(60) molecule to another one has been probed. The conformation and electronic states of both molecules on the contacting electrodes have been characterized using a cryogenic scanning tunneling microscope. While the contact conductance of a single molecule between two Cu electrodes can vary up to a factor of 3 depending on electrode geometry, the conductance of the C(60)-C(60) contact is consistently lower by 2 orders of magnitude. First-principles transport calculations reproduce the experimental results, allow a determination of the actual C(60)-C(60) distances, and identify the essential role of the intermolecular link in bi- and trimolecular chains.

Structural and Electronic Properties of Ultrathin Tin–Phthalocyanine Films on Ag(111) at the Single-Molecule Level†

Heads or tails? The evolution of structural and electronic properties of tin-phthalocyanine films has been analyzed for sub-monolayer to multilayer coverage using low-temperature scanning tunneling microscopy. Two molecular conformations are observed: randomly dispersed for the first layer, and islands with a single conformation in subsequent layers.

Photon emission scanning tunneling microscope

The analysis of photons emitted from the tip–surface region of an STM has produced new information on local dissipative processes in tunneling. A key objective to extend our work to a general spectroscopic and microscopic method is to collect and detect photons as efficiently as possible and here we describe our experimental arrangements, one of which incorporates an ellipsoidal mirror and the other a lens system to collect the photons. The systems operate in UHV and have in situ sample and tip transfer facilities. Examples of a variety of photon spectroscopy modes including photon spectra, isochromat spectra, angular distributions, and photon mapping are presented.