Pressure-Induced Optical Transitions in Metal Nanoclusters

Abstract

Currently, a comprehensive understanding of the relationship between atomic structures and optical properties of ultrasmall metal nanoclusters with diameters between 1 and 3 nm is lacking. To address this challenge, it is necessary to develop tools for perturbing the atomic structure and modulating the optical properties of metal nanoclusters beyond what can be achieved using synthetic chemistry. Here, we present a systematic high-pressure study on a series of atomically precise ligand-protected metal nanoclusters. A diamond anvil cell is used as a high-pressure chamber to gradually compress the metal nanoclusters, while their optical properties are monitored in situ. Our experimental results show that the photoluminescence (PL) of these nanoclusters is enhanced by up to 2 orders of magnitude at pressures up to 7 GPa. The absorption onset red-shifts with increasing pressure up to ∼12 GPa. Density functional theory calculations reveal that the red-shift arises because of narrowing of the spacing between discrete energy levels of the cluster due to delocalization of the core electrons to the carbon ligands. The pressure-induced PL enhancement is ascribed to (i) the enhancement of the near-band-edge transition strength, (ii) suppression of the nonradiative vibrations, and (iii) hindrance of the excited-state structural distortions. Overall, our results demonstrate that high pressure is an effective tool for modulating the optical properties and improving the luminescence brightness of metal nanoclusters. The insights into structure–property relations obtained here also contribute to the rational design of metal nanoclusters for various optical applications.

Hierarchical Assembly of Atomically Precise Metal Clusters as a Luminescent Strain Sensor

“…This is proposed as one of the possible reasons for the emergence of the peak at 575 nm. Under high pressures, red shifts of the absorption and the emission onset of the Au 21 (SR) 12 (dppm) 2 (SR = cyclohexanethiolate; dppm = bisdiphenylphosphinemethane) and Au 28 (S-TBBT) 20 (S-TBBT = tert -butyl-benzenethiolate) NCs were shown in a recent report . This red shift is due to the narrowing of the energy levels of NCs because of the delocalization of the core electrons to the ligands under compression.…”

Source of Bright Near-Infrared Luminescence in Gold Nanoclusters

“…This explains why their measured PL lifetimes are generally from hundreds of nanoseconds to several microseconds. Meanwhile, the low absorption coefficient could make the band-edge transition “invisible” in the absorption spectrum; thus, the observed the large Stokes shifts in many nanoclusters are actually the energy difference between higher energy absorption transition and the PL. , Second, many gold nanoclusters show rich structure–relaxation/transformation processes under excitation, and these significant excited-state processes could increase the Stokes shifts.…”

“…In addition, the Aikens group conducted a series of computational studies on the mechanism of the PL enhancement in gold nanoclusters. ,, In our previous studies, significant enhancement of the PL QY was achieved by tailoring the surface structures, , embedding nanoclusters into polymers, and dissolving the nanocluster in viscous solvents . Most notably, our recent results show that the PL of gold nanoclusters can be significantly enhanced by increasing pressure …”

“…10 Most notably, our recent results show that the PL of gold nanoclusters can be significantly enhanced by increasing pressure. 34 Despite these research advances, there is still a pressing need to elucidate the fundamental origins of common PL characteristics (e.g., large Stokes shifts and long PL lifetime) toward the precise control of gold nanoclusters' PL properties and to determine some general structure−design strategy for a rational improvement of their PL efficiencies. Furthermore, although QYs up to 20−60% have been recently reported under specific conditions (e.g., in viscous solvents, 10 micelles, 15 and solid matrix 12 ), NIR-emitting gold nanoclusters, especially thiolate-protected gold nanoclusters (Au m (SR) n ), commonly show low QY (<1%) in normal solvents and ambient environments.…”

Superatomic Au25(SC2H5)18 Nanocluster under Pressure

“…35 But so far only a couple of experimental studies have explored their high-pressure properties. Gu et al 36 reported the first highpressure optical study of a series of fcc and bitetrahedral nanoclusters, including Au21(SR)12, Au28(SR)20, Ag28Pt(SR)18, Au24(SR)20, and Au14Cd(SR)12. They observed a red shift in the absorption onset with increasing pressure up to ∼12 GPa in all the studied ulatrasmall nanoclusters and an up to 200-fold enhancement in the photoluminescence at pressures around 7 GPa.…”