Paper TF-MoA3
Mechanisms in Organic and Hybrid Organic-Inorganic Molecular Layer Deposition

Monday, October 30, 2017, 2:20 pm, Room 20

Molecular layer deposition (MLD) has seen increased interest in recent years for the deposition of organic and hybrid organic-inorganic ultrathin films. This vapor-phase, layer-by-layer technique, which relies on the same self-limiting precursor saturation as atomic layer deposition, has shown promise for the development of many applications that require conformal organic-containing coatings, such as in photoresists, porous catalysts, and metal organic frameworks. Despite recent developments in MLD, there is a significant gap in our understanding of the mechanisms behind MLD and the microscopic properties of MLD-grown films, such as their molecular-level structure or degradation mechanisms. In this presentation, we will present results of our recent studies to understand such mechanisms, and discuss how that understanding can be used to better control the thermal, mechanical, electrical, and catalytic properties of these materials.

First, we discuss our exploration of the growth behavior of organic 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 (~25° on average for each backbone), or film density (1.0 – 1.2 g/cm³), but are instead caused by an increased frequency of terminations in the more flexible chemistries. Because of the physisorption of precursors, which reintroduce reactive sites, these terminations do not lead a complete cessation of film growth. We further elaborate on the structure of the 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.

Second, we discuss manganese/ethylene glycol hybrid films, which have promise as nanostructured, earth-abundant catalysts for electrochemical water splitting. Time-dependent ellipsometry, infrared absorption, and x-ray photoelectron spectroscopy data indicate that the organic component of the films is unstable in air, as seen in other hybrid MLD materials. The degradation pathway will be discussed, based on the observation that the organic linkers primarily degrade into carboxylate ions. Thermal annealing and chemical treatments, such as exposure to oxygen and water, are further shown to affect this degradation and can be used to control the formation of nanoporous, catalytically active catalysts for the oxygen reduction reaction.

1. D. S. Bergsman, et. al., Chem. Mater, 2017, 29, 1192