Conformation, and Charge Tunneling through Molecules in SAMs

L Belding and SE Root and Y Li and J Park and M Baghbanzadeh and E Rojas and PF Pieters and HJ Yoon and GM Whitesides, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 143, 3481-3493 (2021).

DOI: 10.1021/jacs.0c12571

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 AuTS/S(CH2)(2)(CONRR2)-R-1//Ga2O3/EGaIn, where R-1 and R-2 are alkyl chains of different length. The lengths of chains R-1 and R-2 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 R-1 = H, the molecules are well ordered and exist predominantly in transextended conformations. When R-1 is an alkyl group (e.g., R-1 not equal 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 R-1 = R-2, the rates of CT decrease (up to 6.3x), relative to rates of CT observed through SAMs having the same total chain lengths, or thicknesses, when R-1 = H. When R-1 not equal H not equal R-2, there is a weaker correlation (relative to that when R-1 = H or R-1 = R-2) between current density and chain length or monolayer thickness, and in some cases the rates of CT through SAMs made from molecules with different R-2 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.

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