Epitaxial Core/Shell Nanocrystals of (Europium-Doped) Zirconia and Hafnia

Abstract

A careful design of the nanocrystal architecture can strongly enhance the nanocrystal function. So far, this strategy has faced a synthetic bottleneck in the case of refractory oxides. Here we demonstrate the epitaxial growth of hafnia shells onto zirconia cores and pure zirconia shells onto europium-doped zirconia cores. The core/shell structures are fully crystalline. Upon shelling, the optical properties of the europium dopant are dramatically improved (featuring a more uniform coordination and a longer photoluminescence lifetime), indicating the suppression of nonradiative pathways. These results launch the stable zirconium and hafnium oxide hosts as alternatives for the established NaYF₄ systems.

Figure 1

(A) ZrO₂ core and ZrO₂/HfO₂ core/shell synthesis procedure, where the metal chloride, tetrahydrofuran, propene, and isopropanol are obtained as byproducts.^, (B, C) BF TEM images of (B) ZrO₂ cores and (C) ZrO₂/HfO₂ core/shells. The histograms are based on more than 100 particles. (D) Solution ³¹P NMR spectra of ZrO₂ and ZrO₂/HfO₂ NCs.

Figure 2

(A) HAADF STEM image of a core/shell ZrO₂/HfO₂ nanocrystal, the corresponding power spectrum (FFT) analysis in the reciprocal space along the [010] zone axis, and the frequency-filtered map HAADF STEM image. (B) HAADF STEM image of several ZrO₂/HfO₂ nanocrystals and the corresponding EDX compositional maps featuring hafnium and zirconium.

Figure 3

(A) ZrO₂:Eu core and ZrO₂:Eu/ZrO₂ core/shell syntheses. (B, C) BF TEM images of (B) ZrO₂:Eu cores and (C) ZrO₂:Eu/ZrO₂ core/shells. The histograms are based on more than 100 particles.

Figure 4

(A) Photoluminescence emission spectra and (B) lifetime decays at various emission wavelengths of ZrO₂:Eu cores and ZrO₂:Eu/ZrO₂ core/shells, excited at 238 nm. The measurements were carried out at room temperature in cyclohexane with an absorbance of 0.1 at 238 nm.