Size dependent structural and polymorphic transitions in ZnO: from nanocluster to bulk

Area of Science:

• Materials Science
• Nanotechnology
• Computational Chemistry

Background:

• Understanding the structure-property relationships of zinc oxide (ZnO) nanostructures is crucial for their application.
• The energetic preferences and polymorphism of ZnO nanoclusters and nanoparticles are not fully understood across various sizes.

Purpose of the Study:

• To investigate the size-dependent energetic stability and structural evolution of ZnO nanostructures.
• To compare bottom-up nanoclusters with top-down bulk-cut nanoparticles.
• To identify critical size regimes for structural and polymorphic transitions.

Main Methods:

• Extensive survey of (ZnO)N nanostructures (N=10-1026) using all-electron, relativistic density functional theory (DFT).
• Optimization of single-layered and multi-layered nanocages, and bulk-cut nanoparticles from sodalite (SOD), body-centered tetragonal (BCT), and wurtzite (WZ) polymorphs.
• Interpolation and extrapolation of data to assess size-dependent energetic stabilities.

Main Results:

• Energetic stability shifts from single-layered to multi-layered cage-like nanoclusters with increasing size.
• A transitional region around 2.6 nm diameter shows similar stability for multi-layered cages, SOD, and BCT nanostructures.
• Wurtzite (WZ) ZnO polymorph becomes energetically dominant for N ~ 2200 (approx. 4.7 nm diameter).

Conclusions:

• The study elucidates the size-driven evolution of ZnO nanostructure stability and polymorphism.
• Bottom-up and top-down approaches converge around 2.6 nm.
• Experimental observations of wurtzite crystallinity in larger ZnO nanoparticles are consistent with theoretical predictions.