Real-time Atomistic Observation of Structural Phase Transformations in Individual Hafnia Nanorods

High-temperature phases of hafnium dioxide have exceptionally high dielectric constants and large bandgaps, but quenching them to room temperature remains a challenge. Scaling the bulk form to nanocrystals, while successful in stabilizing the tetragonal phase of isomorphous ZrO2, has produced nanorods with a twinned version of the room temperature monoclinic phase in HfO2. Here we use in situ heating in a scanning transmission electron microscope to observe the transformation of an HfO2 nanorod from monoclinic to tetragonal, with a transformation temperature suppressed by over 1000°C from bulk. When the nanorod is annealed, we observe with atomic-scale resolution the transformation from twinned-monoclinic to tetragonal, starting at a twin boundary and propagating via coherent transformation dislocation; the nanorod is reduced to hafnium on cooling. Unlike the bulk displacive transition, nanoscale size-confinement enables us to manipulate the transformation mechanism, and we observe discrete nucleation events and sigmoidal nucleation and growth kinetics

Stabilizing metastable tetragonal HfO \u3c inf\u3e 2 using a non-hydrolytic solution-phase route: Ligand exchange as a means of controlling particle size

This journal is © 2016 The Royal Society of Chemistry. There has been intense interest in stabilizing the tetragonal phase of HfO2 since it is predicted to outperform the thermodynamically stable lower-symmetry monoclinic phase for almost every application where HfO2 has found use by dint of its higher dielectric constant, bandgap, and hardness. However, the monoclinic phase is much more thermodynamically stable and the tetragonal phase of HfO2 is generally accessible only at temperatures above 1720 °C. Classical models comparing the competing influences of bulk free energy and specific surface energy predict that the tetragonal phase of HfO2 ought to be stable at ultra-small dimensions below 4 nm; however, these size regimes have been difficult to access in the absence of synthetic methods that yield well-defined and monodisperse nanocrystals with precise control over size. In this work, we have developed a modified non-hydrolytic condensation method to precisely control the size of HfO2 nanocrystals with low concentrations of dopants by suppressing the kinetics of particle growth by cross-condensation with less-reactive precursors. This synthetic method enables us to stabilize tetragonal HfO2 while evaluating ideas for critical size at which surface energy considerations surpass the bulk free energy stabilization. The phase assignment has been verified by atomic resolution high angle annular dark field images acquired for individual nanocrystals

In a Different Light: Deciphering Optical and X‑ray Sensitization Mechanisms in an Expanded Palette of LaOCl Phosphors

The conversion and numerical amplification of X-ray photons to visible light is at the heart of numerous technological applications spanning the range from X-ray detectors and scintillators to radiographic medical imaging devices. The need for increased sensitivity and spatial resolution to reduce radiation exposure and provide better differentiation of specimens presenting an X-ray contrast has been a strong driving force in the search for novel X-ray phosphors. However, the current palette of X-ray phosphors is rather sparse. The development of color tunable phosphors necessitates the incorporation of multiple dopants, which in turn interact through complex sensitization mechanisms that are poorly understood for high-energy excitation. In this work, we describe the stabilization of multiply alloyed LaOCl nanocrystals incorporating Tb³⁺ cations in conjunction with either divalent or trivalent europium ions, yielding phosphors emitting in the blue–green and green–red regions of the electromagnetic spectrum, respectively. The choice of the coordinating ligand (tri-<i>n</i>-octylphosphine oxide versus oleylamine) dictates the oxidation state of the incorporated Eu-ions. Pronounced differences are observed in sensitization mechanisms upon optical and X-ray excitation thereby considerably modifying the perceived color of the X-ray phosphors as compared to stimulation by ultraviolet illumination. Upon UV illumination, strong Tb³⁺ → Eu³⁺ sensitization is observed for LaOCl nanocrystals incorporating trivalent europium ions; however, upon X-ray excitation, this sensitization pathway is instead supplanted by independent La³⁺ → Eu³⁺ and La³⁺ → Tb³⁺ sensitization routes. In contrast, strong Eu²⁺ → Tb³⁺ sensitization is observed for LaOCl nanocrystals incorporating divalent europium ions upon both optical and X-ray excitation (La³⁺ → Eu²⁺ → Tb³⁺ and La³⁺ → Tb³⁺ pathways are also observed in X-ray excited optical luminescence spectra). The increased efficacy of the Eu²⁺ → Tb³⁺ as compared with Tb³⁺→ Eu³⁺ sensitization pathway derives from the parity allowed nature and orders of magnitude higher absorption cross-section for the excitation of the divalent Eu-ion. A strong suppression of luminescence intensity is observed upon excitation at the giant resonance absorption for all of the observed emission bands and corresponds to a change in mechanism from the creation of multiple thermal electron–hole pairs to a nonradiative Coster–Kronig process dominated by Auger photoionization