Wilfred T. Tysoe
Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee, Milwaukee, WI 53211 USA

Richard Feynman proposed in 1959 in a talk entitled “There’s Plenty of Room at the Bottom” that it would be preferable to fabricate electronic devices by assembling atoms “from the bottom up” to make more complex nanoscale assemblies instead of etching macroscale objects to make smaller ones in a “top-down” approach. Accomplishing this goal will require molecules with the appropriate properties that can self-assemble into more complex structures but to be relatively small so that the size of the resulting device is not too large. One of the simplest molecular architectures consists of a conductive, π-conjugated core with at least two attachment groups. Molecules that fulfill these requirements are 1,4-phenylene diisocyanide (PDI) with isocyanide (NC) attachment groups, and 1,4-benzenedithiol (BDT) with terminal thiols (SH). PDI self-assembles on a Au(111) substrate by forming extended, oligomeric structures that grow parallel to the surface to form long, one-dimensional chains. This occurs because the isocyanide functionality binds strongly enough to gold to extract gold atoms from low-coordination sites on a Au(111) surface to form structures that incorporate gold adatoms such that the π-conjugation is maintained throughout the oligomer, effectively forming molecular wires. Analogous chemistry occurs with thiol-functionalized analogs of PDI, for example, 1,4-benzenedithiol, which forms zig-zag rather than linear chains. This behavior is exploited to fabricate simple molecular-electronic circuits. In the first, it is found that one of the isocyanide-metal linkages can be displaced by co-adsorbing carbon monoxide to disrupt the molecular conductivity thus acting as a molecular-scale carbon monoxide detector. It is also found that the oligomers are capable of bridging between gold nanoparticles on an insulating substrate thereby providing a conductive linkage between them. Devices can be fabricated using this property by judiciously depositing gold nanoparticle between gold nanoelectrodes that are ~30 nm apart to make either molecular voltage regulators or temperature sensors.