Size Evolution Dynamics of Gold Nanoclusters at an
Atom-Precision Level: Ligand Exchange, Growth Mechanism, Electrochemical,
and Photophysical Properties
posted on 2020-02-19, 20:34authored byManju
P. Maman, Akhil S. Nair, Haritha Cheraparambil, Biswarup Pathak, Sukhendu Mandal
Interpretation
of size evolution is an essential part of nanocluster
transformation processes for unraveling the mechanism at an atom-precision
level. Here we report the transformation of a non-superatomic Au23 to a superatomic Au36 nanocluster via Au28 cluster formation, activated by the bulky 4-tert-butylbenzenethiol ligand. Time-dependent matrix-assisted laser desorption
ionization mass spectrometry data revealed that the conversion proceeds
through ligand exchange followed by the size focusing method, ultimately
leading to size growth. We also validated this transformation through
time-dependent ultraviolet–visible data. Density functional
theory calculations predicted that the kernel of the Au28 cluster evolved through a linear combination of molecular orbitals
of the fragment of 2e– units (Au42+ and Au3+) from the kernel of the Au23 cluster. Periodic growth of gold cores through continuous
growth of Au4 tetrahedral unit leads to the formation of
the Au36 cluster from the Au28 cluster. These
results reinforce the plausibility of size evolution through the growth
mechanism during the transformation process. Differential pulse voltammetry
studies showed that the highest occupied molecular orbital–lowest
unoccupied molecular orbital gap inversely varies with the kernel
size of these clusters. Photophysical experiments support the molecular-like
intersystem crossing rather than core–shell relaxation to these
clusters. The trends of photoluminescence lifetime were found to be
the reverse of those of the energy gap law. The increment of lifetimes
for the larger cluster can be mainly due to the contribution of both
hot carriers and band-edge carriers.