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Bulk gold is a good metal, i.e., conductor of electricity and heat, due toits delocalized electron density that can respond to extremely small externalperturbations such as electric field or temperature gradient. In energy space,the quantum states of the conduction electrons cross over the metal's Fermienergy continuously. But when gold is dispersed in finite nanometer-sizeparticles or 'clusters', the delocalized electronic states are re-grouped inenergy space to 'shells' according to allowed energy levels and the symmetryand shape of the atomic arrangement. This re-grouping generates also an energygap in the vicinity of the Fermi energy akin to the energy gap between occupiedand unoccupied electron orbitals in molecules. How does the formation of theseshells and the energy gap affect the physico-chemical properties of thenanoparticle? How does one get from a 'molecule-like' gold nanoparticle to'metal-like' gold nanoparticle? Here we analyse the electronic structure andoptical and chiroptical properties of recently reported gold nanoparticles of144, 146, and 246 gold atoms, that are made by wet chemistry methods and whosestructures have been resolved to atomic precision. We demonstratecomputationally how re-grouping of the quantum states of valence electrons canaffect drastically the optical properties of nanoparticles in thecrossover-size region, by either generating a multi-band molecule-like or amonotonous metallic optical absorption. The lower the symmetry of the goldcore, the more metallic is the nanoparticle. The underlying mechanism arisesfrom symmetry-sensitive distribution of the electronic levels of thenanoparticle close to Fermi energy. Overall, this work sheds lights onfundamental mechanisms on how molecular nanoparticle properties can changemetallic in the crossover-size region.

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