Trends in precursor design for conventional and aerosol‐assisted CVD of high‐Tc superconductors

Abstract

Conventional MOCVD techniques require molecules displaying volatility and constant vapor pressure. Metal oxide precursors, i.e., b-diketonates, or classical or functionalized metal alkoxides are mostly solids. The various approaches used to tailor volatility are discussed with barium derivatives as an example. The relationships between sublimation temperature and molecular weight suggest that volatility can be optimized on the basis of molecular weight.Aerosol-assisted CVD (AACVD) can use a larger range of precursors since volatility is no longer crucial. The solvent is an undesired ballast in a CVD process. High solubility of the precursors in the selected solvent is thus desirable. 'Stability' here includes the absence of precipitation which would change the stoichiometry of the feed solution for multicomponent oxides. Precipitation is often promoted by hydrolysis; stability toward moisture is thus desirable. The use of mixtures of precursors based on different ligands (b-diketonates, bketoesterates, alkoxides) can lead to ligand exchange reactions giving homometallic species, sometimes of low solubility, or mixed-metal species by self-assembly, thus improving solubility and stability toward moisture. These aspects are illustrated in compositions related to high-T c superconductors.

Effect of the Ligand Structure on Chemical Vapor Deposition of WNxCy Thin Films from Tungsten Nitrido Complexes of the Type WN(NR2)3

“…Reduced symmetry about the metal center also has the expected effect on precursor volatility. 12,33,34 Homoleptic complexes 6 and 7 have volatilities (as measured by the sublimation temperature) that are similar to that of 2, whereas heteroleptic complexes 8−13 were similar to 3, 4, and the previously reported complexes 15 and 20 (Figure 2). 31 The highest volatilities were found for the unsymmetrical complexes 16−19, which all sublimed at temperatures below 50 °C at 150−200 mTorr (Table 1).…”

“…The increase in volatility in going from homoleptic ligand sets to heteroleptic ones has been previously noted and ascribed to perturbation of crystal packing forces by lowered symmetry. 12,33,34 It has also been postulated that steric bulk around the metal center contributes to volatility by hindering intermolecular interactions. The higher volatility of 3 and 4 led us to synthesize additional complexes containing heteroleptic sets of ligands, in which the lowered symmetry is complemented by varying degrees of steric bulk in the dialkylamide ligands.…”

Effect of the Ligand Structure on Chemical Vapor Deposition of WNxCy Thin Films from Tungsten Nitrido Complexes of the Type WN(NR2)3

“…Reduced symmetry about the metal center also has the expected effect on precursor volatility. 12,33,34 Homoleptic complexes 6 and 7 have volatilities (as measured by the sublimation temperature) that are similar to that of 2, whereas heteroleptic complexes 8−13 were similar to 3, 4, and the previously reported complexes 15 and 20 (Figure 2). 31 The highest volatilities were found for the unsymmetrical complexes 16−19, which all sublimed at temperatures below 50 °C at 150−200 mTorr (Table 1).…”

“…The increase in volatility in going from homoleptic ligand sets to heteroleptic ones has been previously noted and ascribed to perturbation of crystal packing forces by lowered symmetry. 12,33,34 It has also been postulated that steric bulk around the metal center contributes to volatility by hindering intermolecular interactions. The higher volatility of 3 and 4 led us to synthesize additional complexes containing heteroleptic sets of ligands, in which the lowered symmetry is complemented by varying degrees of steric bulk in the dialkylamide ligands.…”

Cubane Variations: Syntheses, Structures, and Magnetic Property Analyses of Lanthanide(III)−Copper(II) Architectures with Controlled Nuclearities

“…This is an important result that should be stressed here as one concern for the future of SMMs is the ability to design predictable polynuclear architectures . While this synthetic approach of 3d−4f polynuclear systems strengthens the design of SMMs, it may be also relevant to other field such as the oxide precursors of the YBaCuO superconductor types, for example …”

Halide Effects on the Sublimation Temperature of X–Au–L Complexes: Implications for Their Use as Precursors in Vapor Phase Deposition Methods

“…Complexes with sterically crowded metal centers that are of low symmetry and therefore tend to have irregular crystal packing patterns exhibit a tendency to sublime at much lower temperatures than highly symmetric molecules with orderly crystal packing. 3,32,33 For Au complexes, the crystal packing can be further complicated by the tendency for the molecules to aggregate within the crystal lattice. Aurophilic bonds usually fall in the range of 2.9−3.6 Å, and the complexes aggregate to form parallel, antiparallel, and cross-linked oligomers.…”

Effect of the Ligand Structure on Chemical Vapor Deposition of WNxCy Thin Films from Tungsten Nitrido Complexes of the Type WN(NR2)3

“…Reduced symmetry about the metal center also has the expected effect on precursor volatility. 12,33,34 Homoleptic complexes 6 and 7 have volatilities (as measured by the sublimation temperature) that are similar to that of 2, whereas heteroleptic complexes 8−13 were similar to 3, 4, and the previously reported complexes 15 and 20 (Figure 2). 31 The highest volatilities were found for the unsymmetrical complexes 16−19, which all sublimed at temperatures below 50 °C at 150−200 mTorr (Table 1).…”

“…The increase in volatility in going from homoleptic ligand sets to heteroleptic ones has been previously noted and ascribed to perturbation of crystal packing forces by lowered symmetry. 12,33,34 It has also been postulated that steric bulk around the metal center contributes to volatility by hindering intermolecular interactions. The higher volatility of 3 and 4 led us to synthesize additional complexes containing heteroleptic sets of ligands, in which the lowered symmetry is complemented by varying degrees of steric bulk in the dialkylamide ligands.…”

Cubane Variations: Syntheses, Structures, and Magnetic Property Analyses of Lanthanide(III)−Copper(II) Architectures with Controlled Nuclearities

“…This is an important result that should be stressed here as one concern for the future of SMMs is the ability to design predictable polynuclear architectures . While this synthetic approach of 3d−4f polynuclear systems strengthens the design of SMMs, it may be also relevant to other field such as the oxide precursors of the YBaCuO superconductor types, for example …”

Halide Effects on the Sublimation Temperature of X–Au–L Complexes: Implications for Their Use as Precursors in Vapor Phase Deposition Methods

“…Complexes with sterically crowded metal centers that are of low symmetry and therefore tend to have irregular crystal packing patterns exhibit a tendency to sublime at much lower temperatures than highly symmetric molecules with orderly crystal packing. 3,32,33 For Au complexes, the crystal packing can be further complicated by the tendency for the molecules to aggregate within the crystal lattice. Aurophilic bonds usually fall in the range of 2.9−3.6 Å, and the complexes aggregate to form parallel, antiparallel, and cross-linked oligomers.…”

Effect of the Ligand Structure on Chemical Vapor Deposition of WNxCy Thin Films from Tungsten Nitrido Complexes of the Type WN(NR2)3

“…Reduced symmetry about the metal center also has the expected effect on precursor volatility. 12,33,34 Homoleptic complexes 6 and 7 have volatilities (as measured by the sublimation temperature) that are similar to that of 2, whereas heteroleptic complexes 8−13 were similar to 3, 4, and the previously reported complexes 15 and 20 (Figure 2). 31 The highest volatilities were found for the unsymmetrical complexes 16−19, which all sublimed at temperatures below 50 °C at 150−200 mTorr (Table 1).…”

“…The increase in volatility in going from homoleptic ligand sets to heteroleptic ones has been previously noted and ascribed to perturbation of crystal packing forces by lowered symmetry. 12,33,34 It has also been postulated that steric bulk around the metal center contributes to volatility by hindering intermolecular interactions. The higher volatility of 3 and 4 led us to synthesize additional complexes containing heteroleptic sets of ligands, in which the lowered symmetry is complemented by varying degrees of steric bulk in the dialkylamide ligands.…”

Cubane Variations: Syntheses, Structures, and Magnetic Property Analyses of Lanthanide(III)−Copper(II) Architectures with Controlled Nuclearities

“…This is an important result that should be stressed here as one concern for the future of SMMs is the ability to design predictable polynuclear architectures . While this synthetic approach of 3d−4f polynuclear systems strengthens the design of SMMs, it may be also relevant to other field such as the oxide precursors of the YBaCuO superconductor types, for example …”

Halide Effects on the Sublimation Temperature of X–Au–L Complexes: Implications for Their Use as Precursors in Vapor Phase Deposition Methods

“…Complexes with sterically crowded metal centers that are of low symmetry and therefore tend to have irregular crystal packing patterns exhibit a tendency to sublime at much lower temperatures than highly symmetric molecules with orderly crystal packing. 3,32,33 For Au complexes, the crystal packing can be further complicated by the tendency for the molecules to aggregate within the crystal lattice. Aurophilic bonds usually fall in the range of 2.9−3.6 Å, and the complexes aggregate to form parallel, antiparallel, and cross-linked oligomers.…”