2019

Malola, Sami; Kaappa, Sami; Häkkinen, Hannu

There is a wide-spread interest to design ambient-stable gold nanoparticles with tailored physico-chemical properties for applications in several areas such as plasmonics, nanomedicine, catalysis, biological imaging, sensing, and nanoelectronics. It has been known for a long time that optical response of gold nanoparticles changes drastically in a cross-over region from 150 to 250 gold atoms, from a “molecule-like” to “metallic” behavior, but insufficient knowledge of atomic structures has precluded detailed computational studies on the underlying mechanisms. Here, we analyze the electronic structure and optical and chiroptical properties of recently reported gold nanoparticles of 144, 146, and 246 gold atoms, that are made by wet-chemistry methods and whose structures have been resolved to atomic precision. We demonstrate computationally how re-grouping of the quantum states of valence electrons can affect drastically the optical properties of nanoparticles in the crossover-size region, by either generating a multi-band “molecule-like” or a monotonous “plasmon-like” or “metallic” optical absorption. The lower the symmetry of the gold core, the more “metallic” is the nanoparticle. The underlying mechanism arises from symmetry-sensitive distribution of the electronic levels of the nanoparticle close to Fermi energy.