(604e) Molecular Layer Deposition: Mechanisms of Vapor-Phase Organic Ultrathin Film Synthesis

Molecular layer deposition (MLD) is an increasingly popular process for the growth of organic ultrathin films. This vapor-phase, layer-by-layer technique, which relies on the same principle of self-limiting surface reactions as atomic layer deposition, has shown promise for the development of many applications that require conformal organic-containing coatings, such as in diffusion barriers, photoresists, and porous catalysts. Despite recent developments in MLD, there is still a significant gap in our understanding of the mechanisms behind MLD growth and the microscopic properties of MLD-grown films, such as the origin of the film growth rate or their molecular-level structure. In this presentation, we present results of our recent studies to understand such mechanisms, and discuss how that understanding can be used to better control the properties of these materials.

First, we discuss our exploration of the growth behavior of MLD films by examining trends in film properties as a function of backbone flexibility [1]. Our results suggest that changes in growth rate between the most rigid and most flexible backbones (4 Å/cycle vs 1 Å/cycle) are not caused by differences in length of molecular precursors, chain orientation, or film density, but are instead caused by an increased frequency of terminations in the more flexible chemistries. These terminations likely result from monomers reacting with more than one functional group on the underlying surface, which reduces the total number of available reactive sites. We further elaborate on the relationship between the number of reactive sites and the film growth rate by modeling growth behavior after an intentional reduction in the number of reactive sites [2]. We show that terminations caused by dual-reacting monomers reduce the film growth rate; however, the adsorption of monomers likely reintroduces reactive sites, preventing the complete cessation of film growth. Finally, we discuss the structure of films by examining crystal orientation and infrared absorption data, which suggest that films consist of a mixture of upward growing chains and horizontally aligned layers of paracrystalline polymer segments [1]. Combined, these results provide a clearer picture of MLD film growth and provide insight for the design of future ultrathin film synthesis chemistries.

1. D. S. Bergsman, et al., Chem. Mater, 2017, 29, 1192
2. D. S. Bergsman, et al., Chem. Mater, 2018, 30, 5087