Conformation, and Charge Tunneling through Molecules in SAMs

This paper demonstrates that the molecular conformation (in addition to the composition and structure) of molecules making up self-assembled monolayers (SAMs) influences the rates of charge tunneling (CT) through them, in molecular junctions of the form Au^TS/S(CH₂)₂CONR¹R²//Ga₂O₃/EGaIn, where R¹ and R² are alkyl chains of different length. The lengths of chains R¹ and R² were selected to influence the conformations and conformational homogeneity of the molecules in the monolayer. The conformations of the molecules influence the thickness of the monolayer (i.e. tunneling barrier width) and their rectification ratios at ±1.0 V. When R¹ = H, the molecules are well ordered and exist predominantly in trans-extended conformations. When R¹ is an alkyl group (e.g., R¹ ≠ H), however, their conformations can no longer be all-trans-extended, and the molecules adopt more gauche dihedral angles. This change in the type of conformation decreases the conformational order and influences the rates of tunneling. When R¹ = R², the rates of CT decrease (up to 6.3×), relative to rates of CT observed through SAMs having the same total chain lengths, or thicknesses, when R¹ = H. When R¹ ≠ H ≠ R², there is a weaker correlation (relative to that when R¹ = H or R¹ = R²) between current density and chain length or monolayer thickness, and in some cases the rates of CT through SAMs made from molecules with different R² groups are different, even when the thicknesses of the SAMs (as determined by XPS) are the same. These results indicate that the thickness of a monolayer composed of insulating, amide-containing alkanethiols does not solely determine the rate of CT, and rates of charge tunneling are influenced by the conformation of the molecules making up the junction.