6 research outputs found
Optical Properties and Electronic Energy Relaxation of Metallic Au<sub>144</sub>(SR)<sub>60</sub> Nanoclusters
Electronic energy relaxation of Au<sub>144</sub>(SR)<sub>60</sub><sup>q</sup> ligand-protected nanoclusters,
where SR = SC<sub>6</sub>H<sub>13</sub> and <i>q</i> = −1,
0, +1, and +2,
was examined using femtosecond time-resolved transient absorption
spectroscopy. The observed differential transient spectra contained
three distinct components: (1) transient bleaches at 525 and 600 nm,
(2) broad visible excited-state absorption (ESA), and (3) stimulated
emission (SE) at 670 nm. The bleach recovery kinetics depended upon
the excitation pulse energy and were thus attributed to electron–phonon
coupling typical of metallic nanostructures. The prominent bleach
at 525 nm was assigned to a core-localized plasmon resonance (CLPR).
ESA decay kinetics were oxidation-state dependent and could be described
using a metal-sphere charging model. The dynamics, emission energy,
and intensity of the SE peak exhibited dielectric-dependent responses
indicative of Superatom charge transfer states. On the basis of these
data, the Au<sub>144</sub>(SR)<sub>60</sub> system is the smallest-known
nanocluster to exhibit quantifiable electron dynamics and optical
properties characteristic of metals
Ligand- and Solvent-Dependent Electronic Relaxation Dynamics of Au<sub>25</sub>(SR)<sub>18</sub><sup>–</sup> Monolayer-Protected Clusters
Electronic relaxation
dynamics of Au<sub>25</sub>(PET)<sub>18</sub><sup>–1</sup> and
Au<sub>25</sub>(PET*)<sub>18</sub><sup>–1</sup> monolayer-protected
clusters (MPCs) were examined using femtosecond
time-resolved transient absorption spectroscopy (fsTA). The use of
two different excitation wavelengths (400 and 800 nm) allowed for
quantification of state-resolved and ligand-dependent carrier dynamics
for gold MPCs. Specifically, one-photon 400 nm (3.1 eV) and two-photon
800 nm (1.55 eV) interband excitations promoted electrons from the
MPC ligand band into gold superatom d states. Following rapid internal
conversion, carriers generated by interband excitation exhibited picosecond
relaxation dynamics that depended upon both ligand structure and the
dielectric of the dispersing medium. These solvent- and ligand-dependent
effects were attributed to charge-transfer processes mediated by the
manifold of ligand-based states. In contrast, one-photon intraband
(gold sp–sp) excitation by 800 nm light resulted in solvent-
and ligand-independent relaxation dynamics. The observed solvent independences
of these data were attributed to internal relaxation via superatom
p and d states localized to the MPC core. Effectively, these core-based
transitions were screened from dielectric influences of the dispersing
medium by the MPC gold–thiolate protecting units. Additionally,
a low frequency (2.4 THz) modulation of TA signal amplitude was detected
following intraband excitation. The 2.4 THz mode was consistent with
Au–Au expansion in
the MPC core. Based on these data, we conclude that intraband relaxation
among the MPC Superatom states is mediated by low-frequency vibrations
of the gold core. Structure-specific and state-resolved descriptions
of MPC electron dynamics are necessary for integration of metal clusters
as functional components in photonic materials
On the pH-Dependent Quenching of Quantum Dot Photoluminescence by Redox Active Dopamine
We investigated the charge transfer interactions between
luminescent
quantum dots (QDs) and redox active dopamine. For this, we used pH-insensitive
ZnS-overcoated CdSe QDs rendered water-compatible using poly (ethylene
glycol)-appended dihydrolipoic acid (DHLA-PEG), where a fraction of
the ligands was amine-terminated to allow for controlled coupling
of dopamine–isothiocyanate onto the nanocrystal. Using this
sample configuration, we probed the effects of changing the density
of dopamine and the buffer pH on the fluorescence properties of these
conjugates. Using steady-state and time-resolved fluorescence, we
measured a pronounced pH-dependent photoluminescence (PL) quenching
for all QD-dopamine assemblies. Several parameters affect the PL loss.
First, the quenching efficiency strongly depends on the number of
dopamines per QD-conjugate. Second, the quenching efficiency is substantially
increased in alkaline buffers. Third, this pH-dependent PL loss can
be completely eliminated when oxygen-depleted buffers are used, indicating
that oxygen plays a crucial role in the redox activity of dopamine.
We attribute these findings to charge transfer interactions between
QDs and mainly two forms of dopamine: the reduced catechol and oxidized
quinone. As the pH of the dispersions is changed from acidic to basic,
oxygen-catalyzed transformation progressively reduces the dopamine
potential for oxidation and shifts the equilibrium toward increased
concentration of quinones. Thus, in a conjugate, a QD can simultaneously
interact with quinones (electron acceptors) and catechols (electron
donors), producing pH-dependent PL quenching combined with shortening
of the exciton lifetime. This also alters the recombination kinetics
of the electron and hole of photoexcited QDs. Transient absorption
measurements that probed intraband transitions supported those findings
where a simultaneous pronounced change in the electron and hole relaxation
rates was measured when the pH was changed from acidic to alkaline
Dynamic Diglyme-Mediated Self-Assembly of Gold Nanoclusters
We report the assembly of gold nanoclusters by the nonthiolate ligand diglyme into discrete and dynamic assemblies. To understand this surprising phenomenon, the assembly of Au<sub>20</sub>(SC<sub>2</sub>H<sub>4</sub>Ph)<sub>15</sub>-diglyme into Au<sub>20</sub>(SC<sub>2</sub>H<sub>4</sub>Ph)<sub>15</sub>-diglyme-Au<sub>20</sub>(SC<sub>2</sub>H<sub>4</sub>Ph)<sub>15</sub> is explored in detail. The assembly is examined by high-angle annular dark field scanning transmission electron microscopy, size exclusion chromatography, mass spectrometry, IR spectroscopy, and calorimetry. We establish a dissociation constant for dimer to monomer conversion of 20.4 μM. Theoretical models validated by transient absorption spectroscopy predict a low-spin monomer and a high-spin dimer, with assembly enabled through weak diglyme oxygen–gold interactions. Close spatial coupling allows electron delocalization between the nanoparticle cores. The resulting assemblies thus possess optical and electronic properties that emerge as a result of assembly
Optomechanics of Single Aluminum Nanodisks
Aluminum nanostructures
support tunable surface plasmon resonances and have become an alternative
to gold nanoparticles. Whereas gold is the most-studied plasmonic
material, aluminum has the advantage of high earth abundance and hence
low cost. In addition to understanding the size and shape tunability
of the plasmon resonance, the fundamental relaxation processes in
aluminum nanostructures after photoexcitation must be understood to
take full advantage of applications such as photocatalysis and photodetection.
In this work, we investigate the relaxation following ultrafast pulsed
excitation and the launching of acoustic vibrations in individual
aluminum nanodisks, using single-particle transient extinction spectroscopy.
We find that the transient extinction signal can be assigned to a
thermal relaxation of the photoexcited electrons and phonons. The
ultrafast heating-induced launching of in-plane acoustic vibrations
reveals moderate binding to the glass substrate and is affected by
the native aluminum oxide layer. Finally, we compare the behavior
of aluminum nanodisks to that of similarly prepared and sized gold
nanodisks
Polycrystallinity of Lithographically Fabricated Plasmonic Nanostructures Dominates Their Acoustic Vibrational Damping
The
study of acoustic vibrations in nanoparticles provides unique
and unparalleled insight into their mechanical properties. Electron-beam
lithography of nanostructures allows precise manipulation of their
acoustic vibration frequencies through control of nanoscale morphology.
However, the dissipation of acoustic vibrations in this important
class of nanostructures has not yet been examined. Here we report,
using single-particle ultrafast transient extinction spectroscopy,
the intrinsic damping dynamics in lithographically fabricated plasmonic
nanostructures. We find that in stark contrast to chemically synthesized,
monocrystalline nanoparticles, acoustic energy dissipation in lithographically
fabricated nanostructures is solely dominated by intrinsic damping.
A quality factor of <i>Q</i> = 11.3 ± 2.5 is observed
for all 147 nanostructures, regardless of size, geometry, frequency,
surface adhesion, and mode. This result indicates that the complex
Young’s modulus of this material is independent of frequency
with its imaginary component being approximately 11 times smaller
than its real part. Substrate-mediated acoustic vibration damping
is strongly suppressed, despite strong binding between the glass substrate
and Au nanostructures. We anticipate that these results, characterizing
the optomechanical properties of lithographically fabricated metal
nanostructures, will help inform their design for applications such
as photoacoustic imaging agents, high-frequency resonators, and ultrafast
optical switches