Electrically Stable Self-Assembled Monolayers Achieved through Repeated Surface Exchange of Molecules

Conspectus Self-assembled monolayers (SAMs) are two-dimensional molecular ensembles. In molecular electronics, SAMs serve as active components for exploring structure-tunneling relationships due to their easy and simple fabrication and atomic-detailed (supra)molecular modifications. Single-component pure SAMs are commonly incorporated into tunneling junctions. However, pure SAMs are defective to some extent, which results in low electrical breakdown voltages and allows access to narrow bias windows through the junctions. Narrow bias windows ultimately limit the possible charge-transport channels to shallow molecular orbital energy levels such as the highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO), making it difficult to investigate charge transport through deeper molecular orbital (MO) energy levels such as sub-HOMOs and post-LUMOs. This Account describes a simple method to enhance the breakdown voltages of SAMs, repeated surface exchange of molecules (ReSEM). The ReSEM method is based on molecular mixing. Namely, it relies on the exchange reaction of high-energy adsorbates within a SAM with free molecules in a solution. This exchange reaction is repeated alternatively with two different molecules until nanoscale pinhole defects are minimized, affording mixed SAMs with remarkably increased breakdown voltages compared to analogous single-component SAMs. The two molecules used in the ReSEM method are defined as the matrix and reinforcement molecules. While the overall electronic function of the junction is governed by the matrix molecule, the reinforcement molecule plays a role in improving the packing quality of the monolayer. As a proof-of-concept, the ReSEM method was initially validated with an organic molecular diode, 2,2′-bipyridyl-terminated n-undecanethiol (HSC₁₁BIPY), and a skinny dummy molecule, n-octanethiol (HSC₈), for the matrix and reinforcement molecules, respectively. When the pure HSC₁₁BIPY SAM is diluted with HSC₈ via ReSEM, the breakdown voltage is enhanced from 1.4 to 3.3 V. The original function is retained after the molecular mixing. The increased breakdown voltage allows the study of molecular electronics in a high-voltage regime that is hardly accessible with pure SAMs. In the ReSEM-processed HSC₁₁BIPY-HSC₈ mixed SAM, in situ control of rectification size and polarity through variation of the size of the external bias voltage is possible. This unexpected finding is attributed to the widened bias window due to the ReSEM, allowing deep MO energy levels to actively participate in charge transport. ReSEM can be more powerful for SAMs composed of electro-inactive molecules, as their molecular orbital energy levels are not accessible within narrow bias windows. As a model study, mercaptoalkanoic acid (HSC₁₅COOH) was tested as the matrix molecule. A ReSEM-processed mixed SAM formed with mercaptoalkanoic acid and n-alkanethiol (HSC₁₂) shows an unprecedentedly high breakdown voltage of 4.6 V and dynamic variation of rectification as a function of external voltage. These findings highlight that the ReSEM method is an attractive supramolecular approach to imbuing new electrical properties in electro-inactive molecules.