Mechanical Vibrations of Atomically Defined Metal Clusters: From Nano- to Molecular-Size Oscillators

Abstract

Acoustic 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 Au n (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.

Anomalous phonon relaxation in Au 333 (SR) 79 nanoparticles with nascent plasmons

“…S2); in other words, the additional 4-to 5-ps process is not due to e-p coupling, whereas the 1-ps process is. We believe that the unique flat shape of Au 333 should be a key to the intriguing 4-to 5-ps ph-ph process, albeit it remains to be confirmed by theoretical simulation (which is very demanding for such a large-sized cluster) (45).…”

Advances in thermodynamic modelling of nanoparticles

“…experimentally that vibrations of nanoparticles are still ruled by continuum mechanics laws down to sizes of a few nanometers [44,45]. At frequencies higher than the Debye frequency, the 'lattice' is unable to 'see' the vibrations because the wavelength of the vibrations is smaller than the lattice parameter; therefore, the vibrations become independent from the lattice.…”

Atomically Precise Bimetallic Nanoclusters as Photosensitizers in Photoelectrochemical Cells

“…In these demonstrations, one of the most important roles of the NCs is acting as a photosensitizer to extend the photoresponse of the semiconductor (i.e., TiO 2 ) from the UV to the Vis‐IR region . Advanced techniques such as time‐resolved optical spectrometry have been used to study the excited states of NCs . Several NCs (i.e., Au 25 and Ag 44 ) are reported to possess long‐lived excited states, which could favor the electron injection from the excited state of NCs to the conduction band of the semiconductor .…”

“…[24][25][26][27][28] Advanced techniquess uch as time-resolved opticals pectrometry have been used to studyt he excited states of NCs. [29][30][31][32][33][34][35][36][37][38][39] Several NCs (i.e., Au 25 and Ag 44 )a re reportedt op ossess long-livede xcited states, which could favor the electroni njection from the excited state of NCs to the conduction band of the semiconductor. [29][30][31] However,i ti ss till unclear whether there are other important factors that affect the overall efficiency of the system.…”