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.
As circuitry approaches single nanometer length scales, it is important to predict the stability of metals at these scales. The behavior of metals at larger scales can be predicted based on the behavior of dislocations, but it is unclear if dislocations can form and be sustained at single nanometer dimensions. Here, we report the formation of dislocations within individual 3.9 nm Au nanocrystals under nonhydrostatic pressure in a diamond anvil cell. We used a combination of x-2 ray diffraction, optical absorbance spectroscopy, and molecular dynamics simulation to characterize the defects that are formed, which were found to be surface-nucleated partial dislocations. These results indicate that dislocations are still active at single nanometer length scales and can lead to permanent plasticity.
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