Search Results (7 total)

Step edges and low-symmetry faces of metal crystals can restrict the diffusive motion of adsorbates, yet they offer little flexibility with regards to the location and/or direction of the guided motion. We show inherently unidirectional motion of an organic molecule on a high-symmetry thermodynamic-equilibrium metal surface [Cu(111)]. Sequential placement of the substrate linkers of 9,10-dithioanthracene prevents it from rotating or veering off course. A combination of low temperature scanning tunneling microscopy and density functional theory simulations provide atomistic insight.

Scanning tunneling microscopy measurements across isolated straight step edges on a Cu(111) surface were carried out for biases between 100 mV and 5 V. In addition to the well known surface state oscillations, and at lower sample bias than the onset of the two-dimensional surface image state, a sharply defined linear protrusion, was observed at the top of the step faces. This linear feature is interpreted as a one-dimensional image state at the step, with its energy modified by a dipolar potential whose appearance is attributed to Smoluchowski smoothing of the electron density at the step edge.

All elementary steps of a chemical reaction have been successfully induced on individual molecules with a scanning tunneling microscope (STM) in a controlled step-by-step manner utilizing a variety of manipulation techniques. The reaction steps involve the separation of iodine from iodobenzene by using tunneling electrons, bringing together two resultant phenyls mechanically by lateral manipulation and, finally, their chemical association to form a biphenyl molecule mediated by excitation with tunneling electrons. The procedures presented here constitute an important step towards the assembly of individual molecules out of simple building blocks in situ on the atomic scale.

The chemisorption of oxygen on the vicinal Cu(211) surface and the subsequent reconstructions have been studied by means of He-atom scattering, spot profile analysis–low-energy electron diffraction, scanning tunneling microscopy, and Auger-electron spectroscopy. At temperatures below 250 K oxygen adsorbs without any pronounced ordering whereas at elevated temperatures above 300 K the surface starts to reconstruct and the formation of double steps occurs. Several ordered superstructures have been identified that depend on the thermal activation. The double steps are thermally stable up to 800 K. Experiments for very low oxygen coverages indicate that the reconstruction starts initially at the step edges and involves later subsurface oxide formation. Preliminary measurements for a regularly kinked surface reveal a similar oxygen-induced reconstruction.

Electrons tunneling from a scanning tunneling microscope tip to individual CO molecules on Cu(111) can cause their hopping from the surface to the tip if the bias exceeds a threshold of 2.4 V. Polarization- and time-resolved two-photon photoemission identifies the underlying elementary process as intermediate population of a CO 2π^*-derived level, which exhibits an ultrashort lifetime of 0.8–5 fs. From an isotope effect of 2.7_-0.5^+0.3 it can be calculated that ≈0.05% of the tunneling current transiently occupies this level while a desorption of the excited molecule occurs only in 5×10^-9 of the cases.

Detailed tip height measurements during manipulation of single atoms, molecules, and dimers on a Cu(211) surface reveal different manipulation modes depending on tunneling parameters. Both attractive (Cu, Pb, Pb dimers) and repulsive manipulation (CO) are identified. Using attractive forces, discontinuous hopping of Cu and Pb atoms from one adsorption site to the next can be induced (“pulling”). Pb dimers can be pulled with repeated single, double, and triple hops. Pb atoms can also be “slid” continuously. The occurrence of different movement patterns is shown to be a sensitive probe for surface defects.

We report the ability to completely restructure a metal surface by precision manipulation of individual atoms with the scanning tunneling microscope: Besides extracting atoms from kink sites on Cu(211) we are now also able to “dig out” atoms from the even more strongly bound intrinsic step sites and thus to create adatom-vacancy pairs. Together with the processes of moving adatoms along and across intrinsic step edges and the possibility of healing out adatom-vacancy pairs, we have a complete set of lateral manipulation processes at hand. The reliability of all these processes opens up exciting “engineering” possibilities for structuring of extended surface areas.