Optically Excited Acoustic Vibrations in Quantum-Sized Monolayer-Protected Gold Clusters

Abstract

We report a systematic investigation of the optically excited vibrations in monolayer-protected gold clusters capped with hexane thiolate as a function of the particle size in the range of 1.1-4 nm. The vibrations were excited and monitored in transient absorption experiments involving 50 fs light pulses. For small quantum-sized clusters (< or =2.2 nm), the frequency of these vibrations has been found to be independent of cluster size, while for larger clusters (3 and 4 nm), we did not observe detectable optically excited vibrations in this regime. Possible mechanisms of excitation and detection of the vibrations in nanoclusters in the course of the transient absorption are discussed. The results of the current investigation support a displacive excitation mechanism associated with the presence of finite optical energy gap in the quantum-sized nanoclusters. Observed vibrations provide a new valuable diagnostic tool for the investigations of quantum size effects and structural studies in metal nanoclusters.

Acoustic vibrations of metal nano-objects: Time-domain investigations

“…Time-resolved investigations were recently extended to nanospheres with diameters smaller than 3 nm [28,71,[175][176][177]. In this size regime, the interface effects are expected to significantly impact the particle properties, more than half of the atoms being located at their surface.…”

“…Surface effect for particles in the nanometer range is emphasized by the very different size behaviors of the vibrational mode period of particles with surface bound molecules prepared by chemical synthesis, as compared to bare ones [175][176][177]. Size-independent vibrational frequencies around 2.2 THz were detected in Au l (SR) m thiol-protected gold nanoclusters smaller than 2 nm [177].…”

Viscoelastic Flows in Simple Liquids Generated by Vibrating Nanostructures

“…For smaller objects with higher vibrational frequencies, viscoelastic effects are expected to become important even for lower-viscosity liquids with faster shear relaxation times. Measurements of vibrations in gold nanoparticles with diameters close to 1 nm, for example, have shown vibrational periods of less than 0.5 ps [27,28], which means that they should induce strong viscoelastic effects in pure water. The experiments reported here represent the first time that the intrinsic viscoelasticity of simple bulk liquids is observed to affect the mechanical response of vibrating solids.…”

Mechanistic Investigations of Organic Aggregates with Different Topologies for Optical Limiting Applications

“…21 Utilizing the quantum size effects in novel gold and silver nanoclusters will lead to cheap and efficient optical limiters as well as to the development of new generation of two-photon probes for imaging applications. [21][22][23][24] Novel cubic 3-D systems based on octasilsesquioxanes investigated in my lab demonstrated strong enhancement in two-photon absorption cross-section as compared to single stilbene building block (corner). 14,25 This result opens the way to the development real 3-D building bocks which are capable of forming highly dense threedimensional structures with strong nonlinear optical absorption.…”

“…Measurements with these materials have been carried out in conjunction with the design and fabrication of new quinoidal oligothiophenes, [11][12][13] circular molecular systems, [2][3][4][5][6][7][8] Fe-, Ru-, 8 Pt-complexes, 19,20 and small metal nanoparticle systems [21][22][23][24] as to provide a direct feed back to the understanding of the structure/function relationships. Due to biradical character of the ground state for quinoidal oligomers these systems open new avenue in the design of highly responsive optical limiting materials covering very broad spectral range up to 2200nm.…”