Bulk metallic glass-like structure of small icosahedral metallic nanoparticles

TL;DR

This study reveals that very small Ni nanoparticles adopt a bulk metallic glass-like structure with local icosahedral packing, bridging the understanding between nanoparticles and bulk metallic glasses, and highlighting surface stabilization effects.

Contribution

It demonstrates the structural transition of Ni nanoparticles to a BMG-like form at sizes below 5 nm and explains this with local icosahedral packing, connecting nanoparticle and bulk glass structures.

Findings

Nanoparticles <5 nm exhibit BMG-like structure

Local icosahedral packing stabilizes the structure

Surface effects are crucial for stabilization

Abstract

We demonstrate a remarkable equivalence in structure measured by total X-ray scattering methods between very small metallic nanoparticles and bulk metallic glasses (BMGs), thus connecting two disparate fields, shedding new light on both. Our results show that for nanoparticle diameters <5 nm the structure of Ni nanoparticles changes from fcc to the characteristic BMG-like structure, despite them being formed from a single element, an effect we call nano-metallic glass (NMG) formation. However, high-resolution TEM images of the NMG clusters exhibit lattice fringes indicating a locally well-ordered, rather than glassy, structure. These seemingly contradictory results may be reconciled by finding a locally ordered structure that is highly isotropic and we show that local icosahedral packing within 5 atomic shells explains this. Since this structure is stabilized only in the vicinity of a surface which highlights the importance of the presence of free volume in BMGs for stabilizing similar local clusters.