Kai-Felix Braun

Controlled Low-Temperature Molecular Manipulation of Sexiphenyl Molecules on Ag(111) Using Scanning Tunneling Microscopy

A novel scanning tunneling microscope manipulation scheme for a controlled molecular transport of weakly adsorbed molecules is demonstrated. Single sexiphenyl molecules adsorbed on a Ag(111) surface at 6 K are shot towards single silver atoms by excitation with the tip. To achieve atomically straight shooting paths, an electron resonator consisting of linear standing-wave fronts is constructed. The sexiphenyl manipulation signals reveal a pi ring flipping as the molecule moves from the hcp to fcc site. Ab initio calculations show an incorporation of the Ag atom below the center of a pi ring.

Single-Atom Extraction by Scanning Tunneling Microscope Tip Crash and Nanoscale Surface Engineering

We report a novel atom extraction mechanism from the native substrate by means of a scanning tunneling microscope tip crash on a Ag(111) surface at 5 K. Individual atoms are scattered on the surface when a silver coated tip is dipped into the substrate at low tunneling biases. Quantitative analyses reveal that the mechanical energy supplied by the tip crash dominates the atom extraction process. Application of this procedure is demonstrated by constructing quantum structures using the extracted atoms on an atom-by-atom basis.

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.

Atomic-scale structure of dislocations revealed by scanning tunneling microscopy and molecular dynamics.

The intersection between dislocations and a Ag(111) surface has been studied using an interplay of scanning tunneling microscopy (STM) and molecular dynamics. Whereas the STM provides atomically resolved information about the surface structure and Burgers vectors of the dislocations, the simulations can be used to determine dislocation structure and orientation in the near-surface region. In a similar way, the subsurface structure of other extended defects can be studied. The simulations show dislocations to reorient the partials in the surface region leading to an increased splitting width at the surface, in agreement with the STM observations. Implications for surface-induced cross slip are discussed.

Simultaneous and coordinated rotational switching of all molecular rotors in a network

A range of artificial molecular systems has been created that can exhibit controlled linear and rotational motion. In the further development of such systems, a key step is the addition of communication between molecules in a network. Here, we show that a two-dimensional array of dipolar molecular rotors can undergo simultaneous rotational switching when applying an electric field from the tip of a scanning tunnelling microscope. Several hundred rotors made from porphyrin-based double-decker complexes can be simultaneously rotated when in a hexagonal rotor network on a Cu(111) surface by applying biases above 1 V at 80 K. The phenomenon is observed only in a hexagonal rotor network due to the degeneracy of the ground-state dipole rotational energy barrier of the system. Defects are essential to increase electric torque on the rotor network and to stabilize the switched rotor domains. At low biases and low initial rotator angles, slight reorientations of individual rotors can occur, resulting in the rotator arms pointing in different directions. Analysis reveals that the rotator arm directions are not random, but are coordinated to minimize energy via crosstalk among the rotors through dipolar interactions.

Electromigration of single metal atoms observed by scanning tunneling microscopy

The authors show in this letter that single metal atoms on a Ni(111) surface can be pushed by electromigration forces from a scanning tunneling microscope tip. This repulsive interaction is observed over a length scale of 6nm. While for voltages above −300mV the atoms are pulled by the microscope tip, the atoms are pushed away below this threshold. This migration is explained by a resonant scattering of strongly correlated electrons. At small voltages chemical forces are pulling the atom, while for larger voltages the atomic manipulation is assisted by the tunneling current.

Imaging and manipulation of a polar molecule on Ag(1 1 1)

A scanning tunneling microscope (STM) was applied to image and laterally manipulate isolated phosphangulene molecules on Ag(1 1 1) at 6 K. Atomic-resolution images clearly revealed three characteristic types of appearances (three-lobed, fish and bump shape) for the adsorbed molecules, which could correspond to three distinct binding configurations. From a detailed analysis of the relative distance between neighboring three-lobed molecules we determine the adsorption site. Applying the lateral manipulation technique we demonstrate that the molecule can be pulled, slid or pushed by the tip on the surface. Accompanying with the reposition, molecular rotation and/or changing of binding configurations can also be induced. It is found that the dipole moment of the molecule has minor effects on its lateral movement. The results demonstrate that due to many degrees of freedom for large molecules, their manipulating processes can be much more complex in comparison with atoms and small molecules. Such information can shed light on the involved mechanisms on a molecular scale.

Force, Current and Field Effects in Single Atom Manipulation

We present a detailed investigation of the manipulation of Ag and Au atoms with a STM tip on the Ag(111) surface at 5K. The interpretation of the feed‐back loop signal gives a precise picture of the movement of the atom during manipulation. The threshold tunnelling resistance and tip‐height to move a Au/Ag atom have been determined using automated manipulation measurements. The results show that at low biases chemical forces dominate in the manipulation process. Above ≈ ±70 mV tunnelling voltage Au atoms start to move already at greater tip‐atom separation, which can be attributed to a current effect. Tip‐induced diffusion was used to measure the influence of the electric field which can be ruled out at low biases.

Atomistic Constructions by using Scanning Tunneling Microscope Tip

We demonstrate an atomic scale construction scheme, which is performed at an area as small as a few tens of nanometer square. In this atomic scale construction site, all the basic building blocks, single atoms, are extracted locally using a scanning-tunneling-microscope tip from the substrate. These extracted atoms are then precisely positioned on the surface to form desired structures. After the completion of the construction, the remaining debris are removed and the undesired holes near the construction site are filled with atoms/clusters to tidy up the area. This entire construction scheme closely resembles our real world construction process and can be considered as its atomic-scale analog.

MANIPULATION OF ATOMS AND MOLECULES FOR CONSTRUCTION OF NANOSYSTEMS: THE SCANNING TUNNELING MICROSCOPE AS AN OPERATIVE TOOL

Controlled manipulations with scanning tunneling microscope (STM) down to the scale of small molecules and single atoms allow to build molecular and atomic nanosystems, leading to the fascinating possibility of creating manmade structures on atomic scale. Here we present a short review on investigations based on atomic scale manipulation. Upon soft lateral manipulation of adsorbed species, in which only tip/particle forces are used, three different manipulation modes can be discerned: pushing, pulling and sliding. Even the manipulation of strongly bound native substrate atoms is possible. We demonstrate applications as local analytic and synthetic chemistry tools, with important consequences on surface structure research. Vertical manipulation of Xe and CO leads to improved imaging with functionalized tips. With CO deliberately transferred to the tip, we have also succeeded to perform vibrational spectroscopy on single molecules. Furthermore, we describe how we have reproduced a full chemical reaction wit...