Effect of silicon doping in HfO2 nanoparticles from an atomic view

Abstract

Nanoparticles have attracted a great deal of interest due to their desirable properties suited for technological and medical applications. Hafnium dioxide (HfO2) can be used in both areas. Simple and low-cost synthesis of HfO2 as thin films or nanoparticles are, therefore, very important for applicability of these materials. The sol-gel method of synthesis besides fulfilling these characteristics also allows an efficient controlled doping of HfO2 with different elements to improve its properties. In this work, we investigated the effects of the doping with 5 at.% of silicon in HfO2 nanoparticles prepared by the sol-gel method by measuring hyperfine interactions at 181Ta probe nuclei on Hf sites using the 􀀀 perturbed angular correlations (PAC) spectroscopy. The HfO2 powder was obtained by sol-gel method from high purity Hf (99.99%) and Si (99.99%) elements. Initial colloid includes Hf and Si dissolved in appropriated acids, citric acid and ethylene glycol all in stoichiometric proportion. The solution (sol) was heated to 100 C until gel aggregation. After that, the solution was calcined in air at 550 C for 14 hours in order to evaporate organic materials present in the gel. Part of the resulting powder had their structure investigated by X-ray diffraction (XRD). Another part was irradiated with neutrons in the IEA-R1 research reactor of IPEN to produce radioactive 181Hf(181Ta) to carry out hyperfine interactions measurements by PAC. This methodology has the advantage to assure an extremely low concentration and highly homogenous distribution of probe nuclei along with a very well defined location of them. These features enable the investigation of different regions inside the nanoparticles, within an atomic resolution, concerning point defects and formation of other phases. Hyperfine paramenters were measured within the range from 200 C to 900 C. XRD results showed a single phase with the expected monoclinic structure for the as-prepared samples indicating that Si atoms are at substitutional Hf sites. However, PAC results for the electric field gradient and asymmetry parameter measured with 181Ta probe at 600 C indicate that Si dopants can induce dislocation of Hf atoms from their native location in HfO2 throughout the lattice.