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[1]. 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 Au ₂₀ , Au ₅₅ and Au ₁₄₇ 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 Au ₅₅ exhibiting cavities below their external shell [2] (Figure 1a) and we showed that Au ₁₄₇ amorphous geometries are relevant low energy candidates [3], 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 Au ₂₀ that, for such a size, a single charge quantitatively affects the thermal properties such as the melting temperature [4]. 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 [5]. These studies build the bases for further investigations on gold NPs embedded in organic media with a quantum level of theory [6]. This highlight reports a joint CEMES-LCPQ/IRSAMC research work.

Figure 1: Cavity observed inside a low-energy isomer of Au55+ from [2]. Full structure of the cluster (left). The front atom in blue has been cleared for easier observation (right).

© CEMES-CNRS

Figure 2: Core-shell like organization of the lowest energy Au147 isomer found in [3] (left) : surface atoms (middle) and core atoms (right) only.

© CEMES-CNRS

References

1. "Benchmarking DFTB for silver and gold materials: from small clusters to bulk" L. Oliveira, N. Tarrat, J. Cuny, J. Morillo, D. Lemoine, F. Spiegelman and M. Rapacioli, J. Phys. Chem. A, 2016 , 120, 8469 .
2. " Global Optimization of neutral and charged 20- and 55-atom Silver and Gold Clusters at the DFTB level" N. Tarrat, M. Rapacioli, J. Cuny, J. Morillo, J-L Heully and F. Spiegelman, Comp. Theor. Chem., 2017 , 1107, 102.
3. " Au ₁₄₇ nanoparticles: ordered or amorphous?" N. Tarrat, M. Rapacioli and F. Spiegelman, J. Chem. Phys., 2018 , 148, 204398 .
4. " Melting of the Au ₂₀ gold cluster: does charge matter? " M. Rapacioli, N. Tarrat and F. Spiegelman, J. Phys. Chem. A, 2018 , 122, 4092.
5. " Surface-charge dependent orientation of water at the interface of a gold electrode: a cluster study" G. Fazio, G. Seifert, M. Rapacioli, N. Tarrat and J-O. Joswig, Z. Phys. Chem., 2018 , ahead of print, DOI https://doi.org/10.1515/zpch-2018-1136 .
6. " Density-Functional Tight-Binding Approach for Metal Clusters, Surfaces and Bulk: Application to Silver and Gold" J. Cuny, N. Tarrat, F. Spiegelman, A. Huguenot, M. Rapacioli, J. Phys. Condens. Matter (Invited Review), in press.

Contact

Dr. Nathalie Tarrat , CEMES (CNRS)
nathalie.tarrat at cemes.fr