Adapting an approximate quantum method (DFTB), we modelled gold nanoparticles (NPs) up to a few hundred atoms. The theoretical identification and structural analysis of a variety of low-energy isomers in the range Au55-147 show a predominance for amorphous character. Original structuration is found for such monometallic NPs: occurrence of cavities or core-shell like organization (disordered core and regular surface). We also evidence on Au20 that, while varying the charge (0,+1,-1) does not change the pyramidal equilibrium structure, it strongly affects the melting temperature. Moreover, a negative branch in the microcanonical heat capacity is found for neutral Au20, a thermodynamical behaviour specific of finite systems. This highlight reports a joint CEMES-LCPQ/IRSAMC research work.
Gold nanoparticles (NPs) have deserved a lot of attention motivated by their unique properties that make them materials of choice in a large number of applications. The generic simulation methods to deal with nano-objects can roughly be classified into three schemes: (i) wavefunction methods for small clusters; (ii) DFT for intermediate sizes and (iii) phenomenological force fields or potentials for large NPs. At the frontier between schemes (ii) and (iii), approximate quantum methods have emerged. Among them, the Density Functional based Tight Binding (DFTB) approach allows for an explicit treatment of the electronic structure while reducing the computational cost thanks to the use of parametrized integrals and a minimal valence basis set. As a consequence, DFTB can tackle problems requiring an explicit description of the electronic structure even for large systems consisting of several hundreds of atoms. We adjusted and benchmarked DFTB for gold. We showed that this method is satisfactory in reproducing essential small cluster properties (low-energy isomers structures, 2D−3D structural transition and its dependency upon cluster charge) and energetic ordering of medium-size isomers. By running global optimization on Au20, Au55 and Au147 systems using a Parallel-Tempering Molecular Dynamics algorithm at the DFTB level without pre-screening combined with a DFT refinement, we reported for the first time low-energy isomers of Au55 exhibiting cavities below their external shell  (Figure 1a) and we showed that Au147 amorphous geometries are relevant low energy candidates , likely to contribute in finite temperature dynamics and thermodynamics (Figure 1b). Concerning the dependence of the properties upon charge, we highlighted on the magic cluster Au20 that, for such a size, a single charge quantitatively affects the thermal properties such as the melting temperature . Finally, taking into account the surrounding solvent, we have demonstrated the applicability of this method to study the structural/dynamical properties of gold/water interfaces . These studies build the bases for further investigations on gold NPs embedded in organic media with a quantum level of theory . This highlight reports a joint CEMES-LCPQ/IRSAMC research work.
Dr. Nathalie Tarrat, CEMES (CNRS)
nathalie.tarrat at cemes.fr