Molecular Layer Deposition of Metal Phosphonates: A New Class of Hybrid Coating Layers
Authors: Dhara Arpan , Chalishazar Aditya , Dendooven Jolien , Detavernier Christophe 
Published: February 2024

Atomic layer deposition (ALD) is a vapor phase thin film deposition technique where materials can be grown atom-by-atom in a layer-by-layer manner. Over the course of just few years, it has become to be one of the most sophisticated methods of producing high quality thin films with control at the sub-nanometer scale. Molecular layer deposition (MLD) is a variant of ALD where molecular fragments are intentionally built into the growing films.
MLD is one of the techniques which allows to grow organic-inorganic hybrid materials with thickness control at sub-nanometer level.1 Nowadays, organic-inorganic hybrid materials have been found to attract a lot of attention because of their distinctive properties obtained by combining chemical and physical properties of organic and inorganic segments. Numerous hybrid materials have been synthesized by MLD over the time; however, commercial application of those materials are still a challenge due to their hydrolytic instability in most cases. Perhaps, the most widely used hybrid materials in our daily life is polysiloxanes, or commonly called as silicones, a silicon-based hybrid material . The hydrolytic stability of Si-C and Si-O-Si bonds is the main reason of their practical usage as hybrid materials. Similarly, organophosphorus compounds are an important class of materials because they offer several alternatives to silicon-based linking in the field of hybrid materials. Most of the hybrid films produced by MLD technique contain C, O, N, and S elements in the organic part. Hybrid materials containing P-atoms are yet to be explored by MLD.
Among others, metal phosphonates, in particular, an important class of organophosphorus compound, show promising application covering from ion-exchange, proton conductors, catalysts, sensors, membranes, fire retardants etc.2,3 Examples of metal-organic compounds based on phosphonic acid derivatives can be found in literature stemming several decades back. Phosphonates are widely considered as an effective metal chelating ligand. Their dianionic tetrahedral form (RPO3
-2) enables different modes of coordination with metal which is not possible with the extensively used contemporary carboxylate (RCO2
-2) ligands. P-O-M and P-C linkages are quite stable, and therefore organophosphorous compounds with phosphonate ligands have been used to produce thermally and chemically stable hybrid materials by several other techniques.4

Here, we report the synthesis of novel aluminum phosphonate hybrid layers obtained with MLD method. Trimethyl aluminum (TMA) has been used as the metal source and two different novel phosphonate ester precursors with two different organic groups have been used to grow the aluminum phosphonate layers. The growth characteristics of the layers have been studied by in situ ellipsometry, whereas the as-grown films have been characterized with different ex situ techniques like FTIR, XPS, XRD, SEM and AFM. As-deposited metal phosphonate layers show superior thermal (>500 0C in air) and complete water stability. We believe, this kind of metal phosphonate layers may open up new opportunities in the field of organic-inorganic hybrid coatings for various applications.

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