From Molecular to Metallic Gold Nanoparticles: The Role of Nanocrystal Symmetry in the Crossover Region

Bulk gold is a good metal, i.e., conductor of electricity and heat, due to its delocalized electron density that can respond to extremely small external perturbations such as electric field or temperature gradient. In energy space, the quantum states of the conduction electrons cross over the metal's Fermi energy continuously. But when gold is dispersed in finite nanometer-size particles or 'clusters', the delocalized electronic states are re-grouped in energy space to 'shells' according to allowed energy levels and the symmetry and shape of the atomic arrangement. This re-grouping generates also an energy gap in the vicinity of the Fermi energy akin to the energy gap between occupied and unoccupied electron orbitals in molecules. How does the formation of these shells and the energy gap affect the physico-chemical properties of the nanoparticle? How does one get from a 'molecule-like' gold nanoparticle to 'metal-like' gold nanoparticle? Here we analyse 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 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. Overall, this work sheds lights on fundamental mechanisms on how molecular nanoparticle properties can change metallic in the crossover-size region.