Effects of metal-molecule contact and molecular structure on molecular electronic conduction in nonresonant tunneling regime: Alkyl versus conjugated molecules

The effect of metal-molecule contacts and molecular structures on the electronic conduction of alkyl and conjugated molecular junctions in nonresonant tunneling regime was investigated based on a proposed multibarrier tunneling (MBT) model, where the molecular junction was decomposed into three individual barriers through the molecular body and metal-molecule contacts on either side of the molecule. The resistance-area product (R_A), specific contact resistance (R_C), and decay coefficients (β_C, β_P, β_Body, and β_o) were estimated for alkyl and two selected conjugated molecules (oligoacene and oligophenylene) between Au contacts, and the result of R_A (alkyl) > R_A (oligoacene) ≥ R_A (oligophenylene) for a given molecular length was obtained from different decay coefficients for different molecules. By assuming the tunneling around the highest occupied molecular orbital of alkyl molecules (like a hole type tunneling), R_C for alkanemonothiol and alkanedithiol junctions with various metallic (Ag, Cu, and Au) contacts was observed to decrease when metal work function was increased.