First Principles Study of Reactions in Alucone Growth: the Role of the Organic Precursor

Abstract

Organic-inorganic hybrid materials are a unique class of materials with properties driven by the organic and inorganic components, making them useful for flexible devices. Molecular layer deposition (MLD) offers novel pathways for the fabrication of such hybrids by using inorganic metal precursors and the vast range of organic molecules with tunable properties. To investigate and understand the mechanism of growth a combination of theoretical and experimental data is needed. In this contribution, we present a first principles investigation of the molecular mechanism of the growth of hybrid organic−inorganic thin films of aluminium alkoxides, known as "alucones" grown by MLD. We explore the interactions between precursors by analyzing the MLD reaction products of the alumina surface terminated with Al(CH3) groups after the trimethyl aluminium pulse; this yields monomethyl-Al2O3 (Al-CH3-Al2O3) and dimethyl-Al2O3 (Al(CH3)2-Al2O3) terminated surfaces. The organic precursors are ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG) and tetraethylene glycol (FEG). A detailed comparison with alucones grown with ethylene glycol (EG) and glycerol (GL) precursors is presented to assist the interpretation of experimental findings regarding the differences in the hybrid films grown by EG and GL. The results show that Al-O formation with release of methane is favorable for all precursors. EG and GL can lie flat and create so-called double reactions through the reaction of the two terminal hydroxyl groups with the surface fragments. This phenomenon removes active hydroxyl sites for EG. However, for GL the third hydroxyl group is available and growth can proceed. This analysis shows the origin of differences in thickness of alucones found for EG and GL.

Role of Terminal Groups in Aromatic Molecules on the Growth of Al2O3-Based Hybrid Materials

“…The hydroxylated Al2O3 surface terminated with one methyl group (MMA-Al2O3) that results from the adsorption of trimethylaluminium (TMA) on the Al2O3 surface was taken from our previous work with alucones. [37] We performed fundamental investigations on the interactions between the post-TMA pulse methyl-terminated Al2O3 surface and a selection of aromatic molecules with very different chemical character and these are hydroquinone (HQ), p-phenylenediamine (PD) and 4-aminophenol (AP), Figure 1. This set of aromatic molecules allows us to examine the reactivity of hydroxyl (OH) and amino (NH2) groups towards TMA as well as the preferred orientation of the organic species.…”

“…It was also found that the reaction energy for TMA with NH2 or NO2 functional groups is considerably less favourable than with OH. To investigate the double reaction phenomenon for the aromatic molecules in more detail, we examine interactions of the Al2O3 surface terminated with two surface bound Al(CH3) species (2MMA-Al2O3), used in our previous work [37], with HQ, PD and AP molecules in the upright and flat lying configuration. Minimising double reactions of the organic precursor is considered an important advantage of using stiff aromatic molecules as organic precursors.…”

“…Density functional theory (DFT) has proven to be a powerful tool that allows us to explore and understand the reactions between the organometallic precursors and organic molecules that lead to the formation of metalcones and to address aspects of MLD experiments. [32,35,36,37] In ref 32 DFT calculations were used to explore the growth mechanism of magnesium containing hybrid films known as "magnesicone" through the reaction of ethylene glycol (EG) and glycerol (GL) at MgCp-terminated MgO as a model system. This study shows that while the ligand elimination process is favourable for both organic molecules, EG prefers to orient in a flat configuration and interacts at the MgO surface and GL species prefer to lie in an upright position.…”