Modulated Rectification of Carboxylate-Terminated Self-Assembled Monolayer Junction by Humidity and Alkali Metal Ions: The Coupling and Asymmetric Factor Matter

Abstract

Facile modulation of the rectification behavior of a molecular junction device was achieved by humidity and alkali metal ions using a series of carboxylic acid-terminated self-assembled monolayers (COOH SAMs) on gold and eutectic gallium-indium alloy top contact (EGaIn). The rectification ratio (RR) increased significantly as the humidity (ranging from 1 to 39%) increases for an n-mercaptoalkanoic acid (HSCnO2H, n = 11, 12, 14, 16) SAM device. Meanwhile, K+/Na+, as the counterion of COO– in the same element group with H+, enhanced the RR of the SAM device as well. By the detailed study of the rectifying current–voltage (I–V) response using a theoretical model based on single-level tunneling, we found that the modulated rectification behavior of SAMs was attributed to the change in the coupling strength of the carboxylic group with the EGaIn electrode and the asymmetry factor of the junction in the presence of water and K+/Na+. Our study here suggested that the delicate electronic structure change at the molecule–(GaOx)GaIn interface played the most significant role in the modulation of the rectification performance of the SAM device. The molecule–(GaOx)GaIn interface thus offered a great research opportunity to achieve the desired function of SAM-GaIn-based molecular devices.