Large Built-In Fields Control the Electronic Properties of Nanoscale Molecular Devices with Dipolar Structures

Large area molecular junctions are nanoscale materials made of a molecular layer sandwiched between two metal electrodes. In this study, a fundamental property of these systems is demonstrated: the emergence of a large built-in field, typically as high as 109 V m−1, and susceptible to appear in any junction possessing an asymmetry. This field originates from permanent dipoles in the contacted molecular layer and is responsible for a large renormalization of the band alignment in the junction. To illustrate this effect, a predictive theoretical characterization of two realistic gold-thiolate-based organic layers is carried out. The parameters leading to the emergence of the built-in field are given and the very large renormalization effect is interpreted in terms of an applied voltage drop. Despite a high significance, this generic feature of molecular junctions has widely been neglected hitherto in the design and interpretation of molecular device characteristics. Built-in fields provide a powerful way to achieve structure–property relationships in molecular junctions, by taking advantage of dipolar nanometric building blocks.