nl8b02717_si_001.mpg (8.31 MB)
Mechanical Vibrations of Atomically Defined Metal Clusters: From Nano- to Molecular-Size Oscillators
Version 2 2018-10-02, 16:48
Version 1 2018-10-01, 18:35
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posted on 2018-09-24, 00:00 authored by Paolo Maioli, Tatjana Stoll, Huziel E. Sauceda, Israel Valencia, Aude Demessence, Franck Bertorelle, Aurélien Crut, Fabrice Vallée, Ignacio L. Garzón, Giulio Cerullo, Natalia Del FattiAcoustic
vibrations of small nanoparticles are still ruled by continuum
mechanics laws down to diameters of a few nanometers. The elastic
behavior at lower sizes (<1–2 nm), where nanoparticles become
molecular clusters made by few tens to few atoms, is still little
explored. The question remains to which extent the transition from
small continuous-mass solids to discrete-atom molecular clusters affects
their specific low-frequency vibrational modes, whose period is classically
expected to linearly scale with diameter. Here, we investigate experimentally
by ultrafast time-resolved optical spectroscopy the acoustic response
of atomically defined ligand-protected metal clusters Aun(SR)m with a number n of atoms ranging from 10 to 102 (0.5–1.5 nm diameter
range). Two periods, corresponding to fundamental breathing- and quadrupolar-like
acoustic modes, are detected, with the latter scaling linearly with
cluster diameters and the former taking a constant value. Theoretical
calculations based on density functional theory (DFT) predict in the
case of bare clusters vibrational periods scaling with size down to
diatomic molecules. For ligand-protected clusters, they show a pronounced
effect of the ligand molecules on the breathing-like mode vibrational
period at the origin of its constant value. This deviation from classical
elasticity predictions results from mechanical mass-loading effects
due to the protecting layer. This study shows that clusters characteristic
vibrational frequencies are compatible with extrapolation of continuum
mechanics model down to few atoms, which is in agreement with DFT
computations.
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Atomically Defined Metal ClustersTheoretical calculationsbreathing-like mode vibrational periodclusters vibrational periodsligand-protected clustersMechanical Vibrationscontinuum mechanics lawsvibrational modesligand-protected metal clusterscontinuous-mass solidsMolecular-Size Oscillators Acoustic vibrationsligand moleculescluster diametersvibrational frequencieselasticity predictions resultsultrafast time-resolvedSRmass-loading effectsnumber ncontinuum mechanics modelDFT computations
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