Ferroelectric freestanding hafnia membranes with metastable rhombohedral structure down to 1-nm-thick.

Two-dimensional freestanding membranes of materials, which can be transferred onto and make interfaces with any material, have attracted attention in the search for functional properties that can be utilized for next-generation nanoscale devices. We fabricated stable 1-nm-thick hafnia membranes exhibiting the metastable rhombohedral structure and out-of-plane ferroelectric polarizations as large as 13 µC/cm2. We also found that the rhombohedral phase transforms into another metastable orthorhombic phase without the ferroelectricity deteriorating as the thickness increases. Our results reveal the key role of the rhombohedral phase in the scale-free ferroelectricity in hafnia and also provide critical insights into the formation mechanism and phase stability of the metastable hafnia. Moreover, ultrathin hafnia membranes enable heterointerfaces and devices to be fabricated from structurally dissimilar materials beyond structural constrictions in conventional film-growth techniques.

Novel Route for Enhancing Piezoelectricity of Ferroelectric Films: Controlling Nontrivial Polarization States in Pb(Zr, Ti)O3 Monodomain Superlattice Structure.

Artificial superlattice films made of Pb(Zr0.4Ti0.6)O3 and Pb(Zr0.6Ti0.4)O3 were investigated for their polarization states and piezoelectric properties theoretically and experimentally in this study. The developed theory predicts nontrivial polarization along neither [001] nor [111] directions in (111)-epitaxial monodomain superlattice films with uniform compressive strain. Such films were achieved via pulsed laser deposition. When the layer thickness is reduced to 3 nm, d33 becomes 128 ± 3.8 pm/V at 100 kV/cm and 71.3 ± 2.83 pm/V at 600 kV/cm, comparable to that of (111)-oriented Pb(Zr0.4Ti0.6)O3 or Pb(Zr0.6Ti0.4)O3 bulks and clearly exceeding that of the typical clamped films. The measurement agrees with the theoretical analysis, which reveals that the enhanced piezoelectricity is due to rotation of the nontrivial polarization. Furthermore, the theoretical study predicts an even larger d33 exceeding 300 pm/V for specific parameters in superlattice films with uniform tensile strain, which is promising for applications of microelectromechanical systems.

Electro-exfoliated PdTe2 nanosheets for enhanced methanol electrooxidation performance in alkaline media.

Hydrogen-free cathodic exfoliation was utilized to obtain PdTe2 nanosheets (PTNS) with size 300 nm × 100 nm. Abundant highly exposed active sites and a strong electronic effect between Pd and Te endow PTNS with simultaneous superior methanol oxidation performance in alkaline media, which delivers a low onset potential, high mass and specific activity, and efficient CO elimination ability.

The role of oxygen vacancies in water oxidation for perovskite cobalt oxide electrocatalysts: are more better?

We explore the role of oxygen vacancies in the oxygen evolution reaction (OER) for double perovskite PrBaCo2O6-δ. Interestingly, we find that largely increasing oxygen vacancies leads to a significant reduction in the intrinsic OER activity. Structural studies reveal that oxygen vacancies tend to orderly align in PrO1-δ. This ordered structure not only lowers the cobalt oxidation states but also triggers a spin-state transition from high-spin to low-spin states for cobalt ions, both greatly slowing the OER kinetics.