Local spectroscopy of image-potential-derived states: from single molecules to monolayers of benzene on Cu(111).
ABSTRACT Stark-shifted image-potential states were measured with an STM tip for benzene adsorbed on a Cu(111) surface. A single benzene molecule locally shifts the position of the first image state toward the Fermi level by 0.2 eV relative to its position on the clean surface. The energetic position of this molecule-modified state shifts to lower energy with increasing coverage of benzene on the surface. This is attributed to local surface potential changes that are correlated with the lowering of the crystal work function due to adsorption of benzene.
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ABSTRACT: Electron transport through metal-molecule contacts greatly affects the operation and performance of electronic devices based on organic semiconductors and is at the heart of molecular electronics exploiting single-molecule junctions. Much of our understanding of the charge injection and extraction processes in these systems relies on our knowledge of the potential barrier at the contact. Despite significant experimental and theoretical advances a clear rationale of the contact barrier at the single-molecule level is still missing. Here, we use scanning tunnelling microscopy to probe directly the nanocontact between a single molecule and a metal electrode in unprecedented detail. Our experiments show a significant variation on the submolecular scale. The local barrier modulation across an isolated 4-[trans-2-(pyrid-4-yl-vinyl)] benzoic acid molecule bound to a copper(111) electrode exceeds 1 eV. The giant modulation reflects the interaction between specific molecular groups and the metal and illustrates the critical processes determining the interface potential. Guided by our results, we introduce a new scheme to locally manipulate the potential barrier of the molecular nanocontacts with atomic precision.Nature Material 04/2010; 9(4):320-3. · 32.84 Impact Factor
Article: Electrons, photons, and force: quantitative single-molecule measurements from physics to biology.[show abstract] [hide abstract]
ABSTRACT: Single-molecule measurement techniques have illuminated unprecedented details of chemical behavior, including observations of the motion of a single molecule on a surface, and even the vibration of a single bond within a molecule. Such measurements are critical to our understanding of entities ranging from single atoms to the most complex protein assemblies. We provide an overview of the strikingly diverse classes of measurements that can be used to quantify single-molecule properties, including those of single macromolecules and single molecular assemblies, and discuss the quantitative insights they provide. Examples are drawn from across the single-molecule literature, ranging from ultrahigh vacuum scanning tunneling microscopy studies of adsorbate diffusion on surfaces to fluorescence studies of protein conformational changes in solution.ACS Nano 02/2011; 5(2):693-729. · 10.77 Impact Factor