Designing Volatile Molybdenum Compounds for Vapour-Phase Deposition
The vapor phase deposition of many molybdenum-containing films relies on the delivery of volatile compounds with the general bis(tert-butylimido)molybdenum(VI) framework. Herein the preparation and thermal characterization of several adducts of (tBuN)2MoCl2 with various neutral ligands is described. All of the compounds were characterized using NMR and IR spectroscopy, high-resolution mass spectrometry, and elemental analysis, and their solid-state structures have been determined using single-crystal X-ray crystallography. The volatility and thermal stability of all compounds have also been assessed using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively. Although many of the compounds described herein are volatile, the focus of this thesis relates to the demystification of their mechanism(s) of thermal decomposition and systematic modifications to increase their thermal stabilities. When weakly coordinating ligands, such as ethers, are used, the ligands dissociate from the metal complex upon heating forming the dimeric compound, [(tBuN)Mo(μ-NtBu)Cl2]2, which has been isolated and characterized in the solid state. Notably, when neutral N,N’-chelating ligands were employed the ligands remained coordinated to the metal complex during its evaporation process. These compounds were found to all undergo analogous decomposition pathways: first the ligands dissociated from the metal complex forming (tBuN)2MoCl2 in the gas phase, which subsequently underwent γ-hydrogen transfer yielding a nitrido-amido adduct (tBuNH)MoNCl2 and isobutylene. Ultimately, the molybdenum species undergoes further decomposition to yield tert-butylamine and Mo2N. The mechanism of thermolysis has been investigated using computational methods, and the computed activation barrier was confirmed using an experimental Eyring analysis. The volatile decomposition products were confirmed using a combination of 1H NMR spectroscopy and gas-chromatography mass spectrometry and the non-volatile residue was analyzed using electron diffraction. Various N,N’-chelating ligands have been used, which either increased the volatility, or the thermal stability, of the (tBuN)2MoCl2 framework, and some of these properties have been combined using rational ligand design strategies. Finally, using the design strategies described herein, as well as understanding the mechanism decomposition for the (tBuN)2MoCl2 framework, a highly volatile adduct was prepared that was easily delivered into a deposition chamber and was used as a single-source precursor for the chemical vapor deposition of pure Mo2N thin films.
Networked Digital Library of Theses and Dissertations