Experimental evidence for fluctuating, chiral-type Au55 clusters by direct atomic imaging.

We report the atomic-scale structures and fluctuating dynamical behavior of size-selected Au(55) clusters obtained by aberration-corrected scanning transmission electron microscopy (STEM) coupled with systematic STEM simulations. No high-symmetry structures (face-centered cubic polyhedron, icosahedron, or decahedron) were observed in our statistical investigation. We find Au(55) clusters that are characteristic of the theoretically predicted chiral structure and similar sister isomers (which together we define as the chiral structural zone). The chiral structural zone was found to arise repeatedly in the time-lapse sequences of images we measured, though other amorphous-like structures are also frequently observed. The approach demonstrated here can be applied to identify specific low-symmetry atomic structures in other small clusters and distinguish them unambiguously from high-symmetry isomers.