A microwave molecular solution based approach towards high-κ-tantalum(V)oxide nanoparticles: synthesis, dielectric properties and electron paramagnetic resonance spectroscopic studies of their defect chemistry
Stable dispersions of tantalum oxide nanoparticles are accessible from solutions of tantalum(V) complexes with a mixed malonato and alkanolato ligand sphere in ethoxyethanol by microwave processing. The malonato ligand is cleaved during decomposition and acetic acid or acetic acid esters are formed as derived from in situ spectroscopic studies. The solubility of the tantalum precursor and the obtained particle size therefrom depend strongly on the type of alkanolato ligand moiety. Dispersions of the molecular complexes possess good film forming properties. Films with low surface roughness can be obtained by spincoating. These exhibited a dielectric constant of about 15 and disruptive strengths above 1.5 MV cm−1. The electrical measurements indicate that the presence of moisture is detrimental with respect to the dielectric performance of the films. After removal of the solvent from the suspensions of the nanoparticles, the residue can be redispersed in aprotic solvents. The particles can be isolated therefrom by precipitation with pentane. XRD and HRTEM indicate that the material remains amorphous up to temperatures of 750 °C. XPS proved that only Ta2O5 is formed as lower oxidation states of Ta cannot be detected. A detailed EPR study allows us to gain insight into the surface defect chemistry. Multiple types of oxygen vacancies exist at the surface of the Ta2O5 particles which are influenced by additional calcination and annealing in a vacuum.