The transition from molecular to collective properties as small metal clusters grow in size is of fundamental interest. While small entities with direct metal-metal bonds can be described as molecules, metal nanoparticles are understood as tiny pieces of the bulk metal. Metal clusters form a transition regime with electron delocalization, yet discrete energy levels. As clusters grow larger, the density of states increases. At which point is it fair to call a metal clusters 'molecule-like' or metallic? We adress this question using wavelength-dependent second-order nonlinear optical scattering of thiolate-protected gold clusters (Monolayer-protected clusters, MPCs). These can be prepared with atomic precision, and in a number of cases, the single-crystal structure is available. For small MPCs, non-scalable properties are observed due to their molecular nature. We established the fundamental second-order nonlinear optical properties of MPCs with Second-Harmonic Generation and Hyper-Rayleigh Scattering. The latter techniques provides access to the first hyperpolariyability. In a combination of experimental and computational efforts, we established several symmetry-breaking mechanisms that lead to significant first hyperpolariyabilities. In larger clusters (containig more than 100 gold atoms), wavelength-dependent HRS measurements were used to detect resonance enhancement due to metallic interband transitions. In combination with multiphoton-excited fluorescence, MPCs are interesting candidates for deep tissue imaging applications, due to their small size (&lt; 2 nm), good stability in solution and availability for bio-conjugation.

Molecular and collective properties of monolayer-protected gold clusters revealed by nonlinear optical scattering spectroscopy

256th National Meeting (2018)

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