Effects of Initial Grain Size and Laser Parameters on HfO2 Nanoparticles Prepared Using Femtosecond Laser Ablation in Liquids

The fabrication of an intriguing nano-fiber network interconnected to crystalline spherical shaped nanoparticles of HfO2 has been achieved by femtosecond (fs) pulsed laser ablation in liquids. Understanding the fundamental reasons behind the formation of such heterostructures is important to scale up such process for other varieties of electronic materials. The present work has been designed to verify the impact of initial grain size on the final heterostructures formed. The overall plasma density and its composition were varied since the laser interaction with the matter is affected by the initial grain/particle size. This work covers the effects of initial grain sizes on HfO2 hetero-nanomaterials formed by a controlled ball-milling process. The ablation was performed with fs laser pulses on HfO2 pellets with two different initial grain sizes in distilled water and ethanol. The formed nanoparticles (NPs) had a spherical shape along with an interesting nano-fiber-like structure. The NPs were found to be polycrystalline in nature, and the fiber-like structures were found to be amorphous in nature. Further, the formation of high-temperature and high-pressure phases of HfOx NPs (tetragonal/cubic HfOx) was observed along with a room-temperature phase (monoclinic HfO2). A combination of ball milling and ultrafast laser ablation appears to be a preferred method for synthesizing smaller NPs of exotic non-equilibrium phases.