Structural aspects of redox-mediated electron tunneling (Dedicated to the memory of professor Alexander M. Kuznetsov)

We investigated structural aspects of electron transfer (ET) in tunneling junctions (Au(1 1 1)vertical bar FcN vertical bar solution gap vertical bar Au STM tip) with four different redox-active N-thioalk(ano)ylferrocenes (FcN) embedded. The investigated molecules consist of a redox-active ferrocene (Fc) moiety connected via alkyl spacers with N = 4, 6, 8 and 11 carbon atoms to a thiol anchoring group. We found that for short FcNs (N = 4, 6,8) the redox-mediated ET response increases with the increase of the alkyl chain length, while no enhancement of the ET was observed for Fc1 1. The model of two-step ET with partial vibrational relaxation by Kuznetsov and Ulstrup was used to rationalize these results. The theoretical ET steps were assigned to two processes: (1) electron tunneling from the Fc group to the Au tip through the electrolyte layer and (2) electron transport from the Au(1 1 1) substrate to the Fc group through the organic adlayer. We argue that for the three short FcNs, the first process represents the rate-limiting step. The increase of the length of the alkyl chain leads to an approach of the Fc group to the STM tip, and consequently accelerates the first El' step. In case of the Fcl 1 junctions the rather high thickness of the organic layer leads to a decrease of the rate of the second ET step. In consequence, the contribution of the redox-mediated current enhancement to the total tunneling current appears to be insignificant. Our work demonstrates the importance of combined structural and transport approaches for the understanding of Er processes in electrochemical nanosystems. (C) 2010 Elsevier B.V. All rights reserved.

Resolving Atomic Connectivity in Graphene Nanostructure Junctions

We report on the structural characterization of junctions between atomically well-defined graphene nanoribbons (GNRs) by means of low-temperature, noncontact scanning probe microscopy. We show that the combination of simultaneously acquired frequency shift and tunneling current maps with tight binding (TB) simulations allows a comprehensive characterization of the atomic connectivity in the GNR junctions. The proposed approach can be generally applied to the investigation of graphene nanomaterials and their interconnections and is thus expected to become an important tool in the development of graphene-based circuitry.

From In Situ towards In Operando Conditions: Scanning Tunneling Microscopy Study of Hydrogen Intercalation in Cu(111) during Hydrogen Evolution

We used electrochemical scanning tunneling microscopy to study the intercalation of hydrogen into a Cu(111) model electrode under reactive (in operando) conditions. Hydrogen evolution causes hydrogen intermediates to migrate into the copper lattice as function of the applied potential and the resulting current density. This H-inclusion is demonstrated to be reversible. The presence of subsurface hydrogen leads to a significant surface relaxation/reconstruction affecting both the geometric and electronic structure of the electrode surface.