A core-shell strategy for constructing a single-molecule junction

Understanding the effects of intermolecular interactions on the charge-transport properties of metal/ molecule/metal junctions is an impor- tant step towards using individual mol- ecules as building blocks for electronic devices. This work reports a systematic electron-transport investigation on a series of "core-shell"-structured oligo(- phenylene ethynylene) (Gn-OPE) mo- lecular wires. By using dendrimers of different generations as insulating "shells", the intermolecular p-p inter- actions between the OPE "cores" can be precisely controlled in single-com- ponent monolayers. Three techniques are used to evaluate the electron-trans- port properties of the Au/Gn-OPE/Au molecular junctions, including crossed- wire junction, scanning tunneling spec- troscopy (STS), and scanning tunneling microscope (STM) break-junction tech- niques. The STM break-junction mea- surement reveals that the electron- transport pathways are strongly affect- ed by the size of the side groups. When the side groups are small, electron transport could occur through three pathways, including through single- molecule junctions, double-molecule junctions, and molecular bridges be- tween adjacent molecules formed by aromatic p-p coupling. The dendrimer shells effectively prohibit the p-p cou- pling effect, but at the same time, very large dendrimer side groups may hinder the formation of AuS bonds. A first-generation dendrimer acts as an optimal shell that only allows electron transport through the single-molecule junction pathway, and forbids the other undesired pathways. It is demonstrated that the dendrimer-based core-shell strategy allows the single-molecule conductance to be probed in a homo- genous monolayer without the influ- ence of intermolecular p-p interac- tions.