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Towards original quasicrystals built by the self-assembly of gold nanoparticles

In suspension, spherical gold nanoparticles covered by a soft shell of grafted ligands spontaneously self-assemble in crystalline arrays upon concentration increase. These arrays, called superlattices, mainly form classical structures : face centered cubic, body-centered cubic, or hexagonal close-packed. Nevertheless, nanoparticles with a soft shell can also build complex architectures such as the hexagonal Frank and Kasper phase we have recently observed. This result opens the perspective of metallic nanoparticle assemblies with original optical properties as well as quasi-crystalline order in these well controlled systems.

We used nearly spherical gold nanoparticle, with a gold core diameter close to 2.4 nm and covered with hexane-thiol grafted on their surface. These particles are known to self-assemble in a face-centered cubic array. When slowly increasing their concentration, we have observed a hexagonal structure with a large unit cell. Using x-ray scattering and cryo-transmission electron microscopy (cryoTEM) this structure has been identified as a Frank and Kasper structure called MgZn2 or C14. CryoTEM images (Fig. a, scale bar = 10nm) have been recorded on super-lattices formed in emulsion. Fig. b compares the x-ray scattering signal (red curve) given by a suspension of nanoparticles with super-lattices and the computed one (black dotted line) using a C14 cell with parameters a=6.4 nm and c=a√8/3. Fig. c shows the hexagonal cell of the structure, which consists in a stacking of hexagonal and Kagome layers.

The MgZn2 structure had never before been observed in a self-assembly of soft monodisperse colloidal objects, but only in alloys. The presence of this phase has been attributed to the competition between the strong van der Waals interactions between the gold cores and the ligand entropy. This result opens the way towards complex super-lattices with potentially original optical properties. It gives a new perspective on the problem of soft sphere self-assembly since the main feature of the Frank and Kasper phases is local tetrahedral packing, which is the first step towards quasi-crystalline order.

Reference :

Evidence for a C14 Frank–Kasper Phase in One-Size Gold Nanoparticle Superlattices
Stéphanie Hajiw, Brigitte Pansu, and Jean-François Sadoc
ACS Nano 9, 8116 (2015).

Contact :

Brigitte Pansu