Length-dependent transport in molecular junctions based on SAMs of alkanethiols and alkanedithiols: Effect of metal work function and applied bias on tunneling efficiency and contact resistance

Nanoscopic tunnel junctions were formed by contacting Au-, Pt-, or Ag-coated atomic force microscopy (AFM) tips to self-assembled monolayers (SAMs) of alkanethiol or alkanedithiol molecules on polycrystalline Au, Pt, or Ag substrates. Current - voltage traces exhibited sigmoidal behavior and an exponential attenuation with molecular length, characteristic of nonresonant tunneling. The length-dependent decay parameter, β, was found to be approximately 1.1 per carbon atom (C^-1) or 0.88 Å^-1 and was independent of applied bias (over a voltage range of ±1.5 V) and electrode work function. In contrast, the contact resistance, R₀, extrapolated from resistance versus molecular length plots showed a notable decrease with both applied bias and increasing electrode work function. The doubly bound alkanedithiol junctions were observed to have a contact resistance approximately 1 to 2 orders of magnitude lower than the singly bound alkanethiol junctions. However, both alkanethiol and dithiol junctions exhibited the same length dependence (β value). The resistance versus length data were also used to calculate transmission values for each type of contact (e.g., Au-S-C, Au/CH₃, etc.) and the transmission per C-C bond (T_c-c).