Conductance of Alkanediisothiocyanates: Effect of Headgroup−Electrode Contacts

The realization of molecular electronics requires comprehension of single-molecule I−V characteristics. Aside from the electron-transport properties of the molecular framework, the molecule−electrode binding contributes significantly to the contact resistance, Rn=0, and thus to the values of single-molecule resistance. Isothiocyanate (−NCS), a versatile ligand for organometallics, can bind to a metal substrate to complete a metal−molecule−metal configuration for external measurements. Isothiocyanate has the advantage of being a π-conjugated moiety that presumably exhibits a relatively smaller impedance than the commonly used methylene thiol headgroup (−CH2SH) in many molecular wires. For example, this study shows that the single-molecule conductance of n-butanediisothiocyanate is an order of magnitude better than that of n-octanedithiol even though they both contain 10 atoms counted from sulfur to sulfur. For a homologous series of molecules, Rn=0 can be extrapolated from the intercept of the resistance obtained by the repeated formation of molecular junctions using scanning tunneling microscopy. To isolate the contact effect of the −NCS−Au electrode from other factors, alkanediisothiocyanates were studied because the large HOMO−LUMO gap of alkyl chains is not sensitive to the number of methylene units. The results show two sets of Rn=0 values, with the smaller set being 128 kΩ, about 1/12 the other value. A detailed examination of the results suggests that the preferential adsorption site for isothiocyanate on gold is the atop site rather than the 3-fold-hollow sites of thiol on gold.