C4: Molecular Nanostructures on Surfaces

We develop procedures to controllably position and contact molecular nanostructures on surfaces while providing for well-defined nanostructure-substrate interactions. These approaches are used to study optical/electronic properties of individual species with emphasis on robust and switchable molecular building blocks.

Nanoelectrode Gaps and Electrically Contacted Molecules

Nanoelectrodes allow probes of electronic transport through individual molecules –potentially also providing novel (opto)electronic devices and sensors. Aiming for better-defined device geometries, we have tailored gaps into nanoelectrodes and have bridged these gaps by depositing molecules. Specifically, various carbon nanotube//molecule//carbon nanotube and STM tip//molecule//surface junctions were studied. Electroluminescence from bridging organic molecules which were dielectrophoretically deposited across gaps in metallic semiconducting carbon nanotubes has been observed and characterized (see subproject C4.1: Site-Selective Coupling to Electrodes and Transport through Single Molecules).

Furthermore, we have attempted to merge molecular electronics with spintronics aiming at implementing molecular spintronics (i.e., combined spin and charge transport across single molecules). Specifically, electronic transport through metal-organic molecules adsorbed on metallic surfaces was studied using low-temperature STM with spin resolution. This allowed detailed comparison with ab initio calculations describing charge and spin transport. In specific cases, ferromagnetic tip//molecule//ferromagnetic island heterostructures were found to act as molecular giant- magnetoresistance (GMR) devices with a GMR effect of > 60% (see: C4.10: Electron Spin Transport through Single Molecules with STM and C4.11: Theory and Simulation of Molecular Matrials and Functionality).

Recently, a new FEI Titan Cubed 80-300 aberration-corrected transmission-electron microscope (TEM) has been installed and used to image transition metal atoms and small metal clusters on self-assembled monolayer (SAM) substrates. TEM allows, e.g., studies of the time evolution of particle size distributions prepared by soft-landing of mass-selected cluster ions - providing information on the mechanism and activation enthalpy of surface diffusion (see: C4.5: Electron Microscopy Studies of the Properties of Nanoparticles). In order to understand cluster-substrate interactions, reference structures of the corresponding clusters in gas phase are required. These are being determined using a novel combination of ion-mobility mass-spectrometry, trapped-ion electron diffraction, and quantum-chemical calculations. Results have so far been obtained for cluster ions of boron, copper, silver, gold, tin, lead, and bismuth (see: C4.6: Support Interactions and Thermal Stability of Size Selected Cluster Deposited onto Single Crystal Surfaces).

Low-energy deposition of mass-selected cluster ions was also used to generate tunable thin-film cluster-assembled materials. These were characterized using photoelectron- and thermal desorption spectroscopy. Work concentrated on non-IPR fullerenes. Their annelated pentagon rings allow for significantly stronger inter-cage bonding than for van der Waals-bound IPR fullerenes. Nevertheless, non-IPR fullerenes can be thermally desorbed without fragmentation. To understand competing solid-state reactions, vibrational properties of thermally-treated films have also been examined. This was complemented by low-temperature UHV-STM studies.

Other novel methods to prepare metal nanostructures on surfaces have also been explored. In particular, electrochemical patterning as well as direct reduction of metal ions in a thin (non-volatile) electrolyte film using a focused electron beam have been pursued – and resulting structures characterized by e-beam induced cathodoluminescence (see subprojects: C4.8: Fabrication and Optical Characterization of Metal Nanostructures and C4.9: Electrochemistry with an electron beam – local metal deposition in ionic-liquid and molten-salt thin films).