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Download fileOptical Properties and Structural Relationships of the Silver Nanoclusters Ag32(SG)19 and Ag15(SG)11
journal contribution
posted on 2016-12-12, 00:00 authored by Sung Hei Yau, Brian A. Ashenfelter, Anil Desireddy, Adam P. Ashwell, Oleg Varnavski, George C. Schatz, Terry P. Bigioni, Theodore GoodsonThe recent discovery
of stable Ag nanoclusters presents new opportunities
to understand the detailed electronic and optical properties of the
metal core and the ligands using ultrafast spectroscopy. This paper
focuses on Ag32 and Ag15 (with thiolate ligands),
which are stable in solution. The steady state absorption spectra
of Ag nanoclusters show interesting quantum size effects, expected
for this size regime. Using a simple structural model for Ag32, TDDFT calculations show absorption at 480 nm and 680 nm that are
in reasonable correspondence with experiments. Ag32(SG)19 and Ag15(SG)11 have quantum yields
up to 2 orders of magnitude higher than Au nanoclusters of similar
sizes, with an emission maximum at 650 nm, identified as the metal–ligand
state. The emission from both Ag nanoclusters has a common lifetime
of about 130 ps and a common energy transfer rate of KEET ≥ 9.7 × 109 s–1. A “dark state” competing with the emission process
was also observed and was found to be directly related to the difference
in quantum yield (QY) for the two Ag clusters. Two-photon excited
emission was observed for Ag15(SG)11, with a
cross-section of 34 GM under 800 nm excitation. Femtosecond transient
absorption measurements for Ag32 recorded a possible metal
core state at 530 nm, a metal–ligand state at 651 nm, and ground
state bleaches at 485 and 600 nm. The ground state bleach signals
in the transient spectrum for Ag32 are 100 nm blue-shifted
in comparison to Au25. The transient spectrum for Ag15 shows a weak ground state bleach at ∼480 nm and a
broad excited state centered at 610 nm. TDDFT calculations indicate
that the electronic and optical properties of Ag nanoclusters can
be divided into core states and metal–ligand states, and photoexcitation
generally involves a ligand to metal core transition. Subsequent relaxation
leaves the electron in a core state, but the hole can be either ligand
or core-localized. This leads to emission/relaxation that is consistent
with the observed photophysics.
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quantum size effectsstate absorption spectrametal core stateGMEET100 nm blue-shifted800 nm excitationground state bleachesAg 15TDDFT calculations show absorptionenergy transfer rateSilver Nanoclusters Ag 32ground state bleachAg nanoclusters showSGAg nanoclustersmetal core transitionAg 32ground state bleach signalsQY