Correlation between the Electrical Properties and Formation Temperature of Self-assembled Monolayer-Based Molecular Junctions

Self-assembled monolayers (SAMs) of molecules have been reported to exhibit improved structural quality when formed at elevated temperatures; however, this effect has long been ignored in the field of molecular electronics. In this study, electrical measurements such as current-voltage characteristics were combined with scanning tunneling microscopy (STM) images to analyze the correlation between the SAM formation temperature and the resulting electrical properties of SAM junctions using alkanethiol. Increasing the formation temperature enhanced the conductance of the SAM junctions, which is associated with the improved structural quality of the SAMs with fewer defects and larger domains. Additionally, the rectifying behavior was found to be related to the conductance of the SAMs. A close examination of the current-voltage characteristics revealed that rectification was due to an asymmetrical shift of the highest occupied molecular orbital (HOMO) under bias. Defects in SAMs account for rectification, as well as its correlation with conductance. Based on these observations, it was found that the formation temperature affects the electronic properties of the SAM junctions by controlling the defects. This study elucidates the causal relationship between the SAM formation process and the resulting electrical properties of SAM junctions.