Colloidal-to-Crystalline Transition in HfO₂ Driven by Interfacial Aggregation Pathways and Multiscale Characterization

Hafnium oxide is a benchmark material for advanced dielectric, optical, and radiation-resistant technologies; however, its functional behavior is critically governed by its microstructural state and defect landscape. Here, hafnium oxide particles synthesized via non-hydrothermal nanoprecipitation (NPN) and hydrothermal processing (NPH) were investigated to elucidate how synthesis-driven structural consolidation influences thermal stability, morphological features, and electronic structure. A comprehensive multi-technique approach combining X-ray diffraction, dynamic light scattering, electron microscopy, semi-quantitative EDS, thermal analysis (TGA/DSC), and UV-Vis spectroscopy was employed. The non-hydrothermally synthesized material (NPN) exhibits a highly disordered amorphous network characterized by an elevated optical band gap (∼$5.5\mathrm{eV}$), pronounced sub-gap defect states, and an extended Urbach absorption tail. In contrast, hydrothermal processing (NPH) promotes local dehydroxylation and crystal phase consolidation, yielding a stable framework where structural ordering narrows the optical gap to $5.0\mathrm{eV}$ and restores a well-defined fundamental absorption edge. These findings demonstrate that tailored synthesis pathways enable precise tuning of the structural, chemical, and electronic properties of hafnium oxide, providing a clear roadmap for engineering materials optimized either for defect-driven functional applications or for high-stability dielectric devices.