Going beyond the self-assembled monolayer: Metal intercalated dithiol multilayers and their conductance

A study of the self-assembly of silver atom intercalated 5,5′-bis(mercaptomethyl)-2,2′-bipyridine (BPD; HS-CH₂-(C₅H₃N)₂-CH₂-SH) and 1,4-benzenedimethanethiol (BDMT; HS-CH₂-(C₆H₄)-CH₂-SH)) dithiol (DT) multilayers on gold is presented. The bilayer of these SAMs can be obtained starting from the exposure of a DT monolayer to a concentrated silver ion solution. After grafting the silver atoms on the sulfur end group, the incubation of the resulting DT-Ag SAM in a DT solution leads to the formation of a DT-Ag-DT bilayer. This process was extended to make a multilayer structure. The corresponding changes in these self-assembled layers on Au are characterized by X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE) measurements, and I-V characteristics. Our interpretation of evolution in the absorbed layer are based on changes in intensities of peaks in XPS related to S bound to substrate or Ag and in -SH groups, as well as changes in thickness and absorption features in SE measurements. The latter show the evolution in absorbance wavelength as a function of thickness and indicate a decrease in the HOMO-LUMO gap from about 4.5 eV to 4 eV. The I-V characteristics show a significant bias dependence on the number of the BPD layers and there appears to be a transition from tunneling to a hopping regime when going from the single to the multiple layers.