Stabilization of Cubic Phase at Room Temperature and Photoluminescence Properties of Dy and Sm Co-Doped HfO₂ Nanoparticles

Abstract

HfO 2 shows the monoclinic phase at room temperature (RT), whereas the technologically important high-k tetragonal and cubic phases are observed at ∼1700 °C and 2600 °C, respectively. Herein, we reveal that the high-temperature cubic phase of HfO 2 is stabilized at RT after incorporating Dy and Sm codopant total concentration up to 13 at%. Below 13 at%, the monoclinic and cubic phases coexist, evidenced by Le-Bail profile refinement of the X-ray diffraction patterns. Transmission electron micrographs demonstrate average particle size as ∼31 and ∼10 nm for the monoclinic and cubic phase, respectively, which agrees with the crystallite size estimated from Debye-Scherrer equation. The monoclinic to cubic phase transformation is explained in terms of the oxygen vacancies formation and difference in ionic radii of Sm 3+ , Dy 3+ , and Hf 4+ ions. Interestingly, electron spin resonance spectroscopy analysis indicates that while HfO 2 exhibits oxygen vacancies, Dy and Sm co-doped HfO 2 shows formation of magnetically inactive defect complexes. Moreover, low Dy and Sm co-dopant concentration in HfO 2 produces strong emissions in green, yellow, and orange-red color regions under different excitation wavelength induced via exchange of excited electrons between nearby energy levels of Dy 3+ and Sm 3+ . Such a weak energy transfer phenomenon is primarily governed through multipolar interaction mechanism.

Impact of Europium and Niobium Doping on Hafnium Oxide (HfO2): Comparative Analysis of Sol–Gel and Combustion Synthesis Methods

“…HfO 2 exists in different polymorph phases-cubic (space group: Fm3m), tetragonal (P42/nmc), orthorhombic (Pca21), and monoclinic (P21/s) [13]. The monoclinic phase is stable from RT up to 2000 K. The tetragonal phase is observed at 2000 K-2900 K. The highest symmetry cubic phase is stabilized between 2900 K and 3100 K. [13] The orthorhombic phase is typically not a stable phase under normal pressure conditions [14]. Various synthesis methods have been explored to optimize the physicochemical properties of HfO 2 and tailor its application-specific attributes [15][16][17][18].…”