Fabrication of Monolayers Containing Internal Molecular Scaffolding: Effect of Substrate Preparation

Abstract

The nanoscale design and fabrication of monolayer assemblies is becoming increasingly important for research areas ranging from adhesion to chemical sensors. The formation of molecular scaffolding within a single molecular layer by linking adjacent molecules provides an important means for the nano- to microscale fabrication of such interfacial assemblies. Unfortunately, key factors in the design and fabrication of these internally linked monolayers are often overlooked due to direct analogy with the often studied n-alkyl monolayer systems. In this investigation, the impact of substrate preparation on the resulting monolayer structure is compared for n-alkyl (C18) and internally linked polydiacetylene monolayer assemblies formed on evaporated, sputtered, and colloidal gold surfaces. Polydiacetylene monolayers are fabricated by the spontaneous assembly of diacetylene-containing disulfides followed by photoinduced polymerization. The resulting polydiacetylene monolayers exhibit systematic variations in the degree of polymerization, alkyl chain crystallinity, advancing contact angle, and electron-transfer inhibition with substrate preparation. By all these measures, both the short- and long-range order of these polymerized monolayers are observed to increase substantially on evaporated gold substrates. In contrast, the n-alkyl-based monolayers formed under identical conditions show minimal structural variation. Moreover, surface pretreatment is demonstrated to have a significant impact on the long-range order for both the n-alkyl and polydiacetylene monolayers. These experimental observations implicate domain size as a significant parameter in the fabrication of polydiacetylene monolayers, while exhibiting little or no impact on the apparent structure of n-alkyl-based monolayer assemblies. Ultimately, the successful fabrication of monolayer structures containing internal molecular scaffolding is made possible by the judicious choice of substrate preparation conditions.