Engineering of Alkali Metalorganic Precursors: Synthesis to Mechanistic Aspects

Alkali metal (Li, Na, and K) containing materials have become the holy grail for sustainable energy materials and green functional devices. As a means to transition towards more sustainable technologies, there has been an increase in developing new materials that do not contain harmful, toxic, or geographically limited elements. The strength of the alkali metals lies in their nontoxicity, in addition to their large natural richness and availability, especially for sodium and potassium. Lithium Niobates (LiNbO3), solid solutions of sodium and potassium niobates (KxNa1−xNbO3, KNN), and variants of these alkali niobates family are shown to be viable green alternatives for the current lead-containing piezo- and ferroelectric material, i.e., lead zirconate titanate (PZT). However, the lack of available precursor chemistry for these alkali metals has pushed the scientific community to explore novel routes to synthesize alkali metal precursors. In this regard, metal β-diketonates comprise a unique class of complex where the monovalent bifunctional ligands' strong chelating ability leads to the synthesis of several neutral complexes with predominant covalent characteristics, such as solubility in organic solvents and volatility, thereby often referred to as adding 'wings' to the metals. These characteristics have been extensively exploited in various applications such as precursors for thin-film fabrication of oxides, fluorides, or nitrides in the field of electronics and optoelectronics devices, optical applications, catalysis, corrosion protection, etc. In particular, derivatives having M-O bonds, namely metal alkoxides, carboxylates, or β-diketonates, are the most common sources of metal oxides. Still, the usual thermal stability and good volatility of β-diketonates are most convenient for MOCVD purposes. This has been the primary reason for exploiting β-diketonates such as fluorinated derivatives to be utilized as precursors. Despite these promising advantages, very few alkali metal β-diketonates have been synthesized for vapor phase processes, either MOCVD or atomic layer deposition. The research developed in this dissertation aims to synthesize and engineer alkali metal β-diketonate complexes that provide stability and enough volatility to be utilized as a potential precursor complex to fabricate thin-films/nanostructures of the desired materials with clean decomposition and no secondary phases. In this regard, a one-pot synthesis methodology has been applied in synthesizing 11 novel alkali metal β-diketonate complexes which consist of an alkali metal center, i.e., Li, Na or K with fluorinated β-diketonate, i.e., hexafluoroacetylacetone (1,1,1,5,5,5-Hexafluoro-2,4-pentanedione) and glymes such as monoglyme (1,2-Dimethoxyethane), diglyme (bis(2-methoxyethyl)ether), triglyme (2,5,8,11-tetraoxadodecane) and tetraglyme (2,5,8,11,14-pentaoxapentadecane) which acts as the encapsulating cage to hold the metal β-diketonate structure together and impart the required stability. The characterization investigations of the "Li(hfa)●glyme●H2O" complex reveals impressive coordination of the Li+ ion and also the role of H2O as a potential unit for facilitating dimerization. Thermal and mass transport studies further substantiate these precursors' potential as an alternative to the industrially available precursors due to low residue and high solubility. "Na(hfa)●glyme" complexes show an analogous behaviour, but at a difference with the Li adduct an ionic complex is formed when diglyme is used as a Lewis base. The thermal behavior and mass transport properties reveal high volatility and higher residue, but this is certainly not a drawback for liquid-assisted MOCVD application. Furthermore, "K(hfa)●glyme" complexes exhibit covalent characteristics with a high degree of polymerization. Its thermal robustness and mass transport properties show high volatility and moderate residue formation. The functional validation of Li-precursors was carried out by its application in PI-MOCVD wherein pure phase Lithium Niobate was synthesized on C-sapphire (0001), A-sapphire (1120), R-sapphire (1102), and Si (100). [Li(hfa)]12●monoglyme●4H2O complexes produced highly oriented LiNbO3 thin films on C-sapphire (0001), additionally, "K(hfa)●glyme" and "Na(hfa)●glyme" complexes were utilized through the sol-gel route for the synthesis of (K, Na)NbO3 as thin films on Si(100) and also electrospun as nanofibres. A unique synthesis route was taken using either a Nb β-diketonate, i.e., Nb(tmhd)4, or Nb-alkoxide, i.e., Nb(ethoxide)5, to investigate the effect of β-diketonate vs. alkoxide precursors on the (K, Na)NbO3 formation.