J. Kröger

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.

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.

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.

Supramolecular patterns controlled by electron interference and direct intermolecular interactions.

Whereas all 230 three-dimensional space groups occur in organic crystals, out of only 17 plane groups some highly symmetric ones such as p31m have not yet been observed in two-dimensional (2D) crystals of organic molecules. Here a kagome network with p31m symmetry is reported for cobalt phthalocyanine on Cu(111). This unusual structure results from substrate-induced reduction of molecular symmetry and substrate-mediated interaction via quantum interference of surface electrons. These interactions provide additional control over the symmetry of 2D crystals of phthalocyanines and lead to a variety of other symmetries in self-assembled arrays.

Surface-state localization at adatoms.

Low-temperature scanning tunneling spectroscopy of magnetic and nonmagnetic metal atoms on Ag(111) and on Cu(111) surfaces reveals the existence of a common electronic resonance at an energy below the binding energies of the surface states. Using an extended Newns-Anderson model, we assign this resonance to an adsorbate-induced bound state, split off from the bottom of the surface-state band, and broadened by the interaction with bulk states. A line shape analysis of the bound state indicates that Ag and Cu adatoms on Ag(111) and Cu(111), respectively, decrease the surface-state lifetime, while a cobalt adatom causes no significant change.

Conductance of oriented C60 molecules.

C60 molecules adsorbed to Cu(100) are contacted with the tip of a cryogenic scanning tunneling microscope. Images with submolecular resolution reveal distinct orientations of the molecules. We find that the orientation significantly affects the conductance of the contact despite the high symmetry of C60.

Review Article: Structures of phthalocyanine molecules on surfaces studied by STM

This review mainly focuses on progress recently achieved in the growth of phthalocyanine molecules on single-crystal surfaces of sub-monolayer up to few-monolayer thin films studied by scanning tunneling microscopy in our groups. On metallic surfaces such as Au(111), Ag(111) and Cu(111), molecular superstructures are determined by combining directional intermolecular interactions caused by symmetry reduction, molecule-substrate interactions and indirect long-range interactions due to quantum interference of surface state electrons. On semiconducting TiO2 surface, molecular assembling structures are dictated by the strong molecule-substrate interaction. However, on insulating NaCl film, molecule-molecule interaction dominates over the molecule-NaCl coupling, leading to molecular growth behavior. Knowledge obtained from these studies would help people better understand the physicochemical properties of the phthalocyanine molecules at surfaces so that their new applications could be further explored and unco...

Spatially resolved conductance of oriented C60

The conductance of oriented C60 molecules on Au(111) and Cu(100) was investigated using a low-temperature scanning tunnelling microscope. A remarkable dependence of spectra of the differential conductance on the adsorption orientation is observed for Au(111) that is almost absent for Cu(100). For C60 adsorbed on Au(111), the spatial distribution of the three lowest unoccupied molecular orbitals is revealed by maps of the differential conductance.

Fullerene nanowires on a vicinal gold surface

Long and narrow fullerene stripes have been fabricated on a vicinal gold surface by utilizing faceting of Au(433) substrates upon C60 adsorption and subsequent annealing. The observed step bunching is attributed to a lowering of the facet-free energy. This surface morphology is a promising template candidate for subsequent deposition of functional units.