Low Frequency Raman spectroscopy of small gold clusters: limits of Lamb model ?

Metallic nanoclusters present interesting optical, vibrational, electronic and catalytic properties and explain why they have numerous fundamental research and applications. Properties of metallic clusters are due, like for... [ view full abstract ]

Metallic nanoclusters present interesting optical, vibrational, electronic and catalytic properties and explain why they have numerous fundamental research and applications. Properties of metallic clusters are due, like for nanocrystals, to their size and shape as well as to their discrete electronic structure alike organometallics systems. Therefore it is essentially to explain the correlation between the geometrical structure, i.e. the spatial confinement, and its properties.  Studies of the vibrational behaviors of clusters are great approaches to accomplish this objective. The theory of elasticity by Horace Lamb regarding a free homogeneous sphere¹, perfectly predicts the frequency of the vibration of a nanoparticle as a whole. These modes, coming from the confinement of the acoustic waves, allow to probe their size, shape, crystallinity and surrounding environment. Furthermore recent works² have shown that this theory based on the linear elastic theory is still valid for clusters as small as Au₇₅. In this study, we investigate the vibrational modes of ultrasmall gold clusters (Au₉ and Au₆), capped with phosphorous ligands, by acquiring their low frequency Raman scattering to determine if continuum mechanics remains still accurate for such sizes. Two distinct vibrational modes involving the gold core and depending on the size of the clusters have been identified and calculations taking into account the real position of the atoms have been developed. Moreover, another vibration due to the mechanical coupling³ among clusters is observed for the first time in such sample.

Reference:

(1)      Lamb, H. On the Vibrations of an Elastic Sphere. Proc. London Math. Soc. 1881, s1-13, 189–212.

(2)      Sauceda, H. E.; Mongin, D.; Maioli, P.; Crut, A.; Pellarin, M.; Fatti, N. Del; Vallée, F.; Garzón, I. L. Vibrational Properties of Metal Nanoparticles: Atomistic Simulation and Comparison with Time-Resolved Investigation. J. Phys. Chem. C 2012, 116, 25147–25156.

(3)      Girard, A.; Gehan, H.; Crut, A.; Mermet, A.; Saviot, L.; Margueritat, J. Mechanical Coupling in Gold Nanoparticles Supermolecules Revealed by Plasmon-Enhanced Ultralow Frequency Raman Spectroscopy. Nano Lett. 2016, 16, 3843–3849.