16 research outputs found
Directional Scattering and Sensing with Bimetallic Fanocubes: A Complex Fano-Resonant Plasmonic Nanostructure
Concentric
nanostructures provide a unique architecture to manipulate
light by modification of their internal geometry with minimal changes
to their overall size. In this work, we have theoretically examined,
using finite difference time domain simulations, the plasmonic properties
of a concentric cubic nanostructure consisting of a silver (Ag) core,
silica (SiO<sub>2</sub>) interlayer, and gold (Au) shell. These “bimetallic
fanocubes” display two separate geometry dependent Fano resonances
in the visible and in the near-infrared. We employed a plasmon hybridization
model to understand the origin of the spectral features and observe
distinct hybridized modes contributed by the edges and corners, which
is unique to the cubic geometry. Specifically, we note that the “nonbonding”
mode that is essentially dark and not observable in spherical concentric
nanostructures is enhanced in the bimetallic fanocubes. We show the
far-field properties, and Fano resonances of the fanocubes can be
tuned by altering the thickness of the silica layer, the thickness
of the Au shell, and by breaking symmetry. Further, we have examined
the refractive index sensing and directional scattering abilities
of the fanocubes to ultimately enable their use in a range of applications,
harnessing their absorption and scattering properties
Size-Dependent Phononic Properties of PdO Nanocrystals Probed by Nanoscale Optical Thermometry
With the advent of novel nanoscale
devices, fast and reliable thermal
mapping with high spatiotemporal resolution is imperative for probing
the characteristics of phonons and evaluating the local temperature
at the nanoscale. In this work, Raman spectroscopy is employed as
a rapid and noncontact optical thermometry technique to investigate
phononic properties of macroscopic assemblies of monodisperse palladium
oxide (PdO) nanocrystals. PdO has been extensively employed in high
temperature catalytic devices; however, the phonon behavior which
determines the thermal stability of PdO remains unexplored thus far.
Our study focuses on homogeneous, large-scale assemblies of monodisperse
4 and 10 nm nanocrystals synthesized using colloidal chemistry to
understand size-dependent effects on the measured thermal properties.
By monitoring the Raman peak shifts, peak broadening, and alterations
in peak intensities as a function of laser power and particle concentration,
a size-dependent trend is observed attributable to confinement of
optical phonons within nanocrystal grain boundaries and laser-induced
heating, both influenced by nanocrystal size. This study correlates
size-dependent single-particle heating effects with size-dependent
interparticle heat transfer under laser irradiation and is enabled
by controlled nanocrystal synthesis
Solvent-Assisted Self-Assembly of CsPbBr<sub>3</sub> Perovskite Nanocrystals into One-Dimensional Superlattice
The
self-assembly of colloidal nanocrystals into ordered architectures
has attracted significant interest enabling innovative methods of
manipulating physicochemical properties for targeted applications.
This study reports the self-assembly of CsPbBr<sub>3</sub> perovskite
nanocrystals (NCs) in one-dimensional (1D) superlattice chains mediated
by ligand–solvent interactions. CsPbBr<sub>3</sub> NCs synthesized
at ≥170 °C and purified in a nonpolar solvent, hexane,
self-assembled into 1D chains, whereas those purified in polar solvents,
including toluene and ethyl acetate, were disordered or formed short-range
two-dimensional (2D) assemblies. The NCs assembled into 1D chains
showed red shifts in both the absorbance and photoluminescence spectra
relative to those of disordered NCs purified in a 50/50 hexane/ethyl
acetate mixture. Microscopy and X-ray diffraction results confirmed
the formation of polymeric nanostrands in hexane followed by organization
of the NCs into 1D chains along the nanostrands. Our results suggest
that excess aliphatic ligands remaining after purification of the
NCs complex with ionic Cs<sup>+</sup> and Br<sup>–</sup> species
through a hydrophobic effect; further, the alkyl chains of these ligands
interlace with each other through van der Waals forces. Collectively,
these interactions give rise to the nanostrands and subsequent self-assembly
of CsPbBr<sub>3</sub> into 1D chains. In polar solvents, the minimization
of repulsive forces between the solvent and the ligands drives proximal
CsPbBr<sub>3</sub> NCs together into short-range 2D assemblies or
disordered clusters. Our solvent-assisted self-assembly approach provides
a general strategy for designing 1D superlattice chains of nanocrystals
of any geometry, dimension, and composition by simply tuning the ligand–solvent
interactions
Ultrafast Excited-State Dynamics in Shape- and Composition-Controlled Gold–Silver Bimetallic Nanostructures
In
this work, we have examined the ultrafast dynamics of shape-
and composition-controlled bimetallic Au/Ag core/shell nanostructures
with transient absorption spectroscopy (TAS) as a function of Ag layer
thickness (0–15 nm) and pump excitation fluence (50–500
nJ/pulse). Our synthesis approach generated both bimetallic nanocubes
and nanopyramids with distinct dipolar plasmon resonances and plasmon
dephasing behavior at the resonance. Lifetimes obtained from TAS at
low powers (50 nJ/pulse) demonstrated minimal dependence on the Ag
layer thickness, whereas at high power (500 nJ/pulse) a rise in electron–phonon
coupling lifetime (Ď„<sub>1</sub>) was observed with increasing
Ag shell thickness for both nanocubes and nanopyramids. This is attributable
to the stronger absorption of the 400 nm pump pulse with higher Ag
content, which induced higher electron temperatures. The phonon–phonon
scattering lifetime (Ď„<sub>2</sub>) also rises with increasing
Ag layer, contributed both by the increasing size of the Au/Ag nanostructures
as well as by surface chemistry effects. Further, we observed that
even the thinnest, 2 nm, Ag shell strongly impacts both Ď„<sub>1</sub> and Ď„<sub>2</sub> at high power despite minimal change
in overall size, indicating that the nanostructure composition also
strongly impacts the thermalization temperature following absorption
of 400 nm light. We also observed a shape-dependent trend at high
power, where Ď„<sub>2</sub> increased for the nanopyramids with
increasing Ag shell thickness and nanostructure size, but bimetallic
nanocubes demonstrated an unexpected decrease in Ď„<sub>2</sub> for the thickest, 15 nm, Ag shell. This was attributed to the larger
number of corners and edges in the nanocubes relative to the nanopyramids
Engineered Porous Silicon Counter Electrodes for High Efficiency Dye-Sensitized Solar Cells
In this work, we demonstrate for
the first time, the use of porous
silicon (P-Si) as counter electrodes in dye-sensitized solar cells
(DSSCs) with efficiencies (5.38%) comparable to that achieved with
platinum counter electrodes (5.80%). To activate the P-Si for triiodide
reduction, few layer carbon passivation is utilized to enable electrochemical
stability of the silicon surface. Our results suggest porous silicon
as a promising sustainable and manufacturable alternative to rare
metals for electrochemical solar cells, following appropriate surface
modification
Enhanced Efficiency in Dye-Sensitized Solar Cells with Shape-Controlled Plasmonic Nanostructures
In this work, we demonstrate enhanced
light harvesting in dye-sensitized
solar cells (DSSCs) with gold nanocubes of controlled shape. Silica-coated
nanocubes (Au@SiO<sub>2</sub> nanocubes) embedded in the photoanodes
of DSSCs had a power conversion efficiency of 7.8% relative to 5.8%
of reference (TiO<sub>2</sub> only) devices, resulting in a 34% improvement
in DSSC performance. Photocurrent behavior and incident photon to current efficiency spectra revealed
that device performance is controlled by the particle density of Au@SiO<sub>2</sub> nanocubes and monotonically decreases at very high nanocube
concentration. Finite difference time domain simulations suggest that,
at the 45 nm size regime, the nanocubes predominantly absorb incident
light, giving rise to the lightning rod effect, which results in intense
electromagnetic fields at the edges and corners. These intense fields
increase the plasmonic molecular coupling, amplifying the carrier
generation and DSSC efficiency
Electrochemical and Corrosion Stability of Nanostructured Silicon by Graphene Coatings: Toward High Power Porous Silicon Supercapacitors
We
demonstrate the electrochemical stability of nanostructured
silicon in corrosive aqueous, organic, and ionic liquid media enabled
by conformal few-layered graphene heterogeneous interfaces. We demonstrate
direct gas-phase few-layered graphene passivation (<i>d</i> = 0.35 nm) at temperatures that preserve the structural integrity
of the nanostructured silicon. This passivation technique is transferrable
both to silicon nanoparticles (Si-NPs) as well as to electrochemically
etched porous silicon (P-Si) materials. For Si-NPs, we find the graphene-passivated
silicon to withstand physical corrosion in NaOH aqueous conditions
where unpassivated Si-NPs spontaneously dissolve. For P-Si, we demonstrate
electrochemical stability with widely different electrolytes, including
NaOH, enabling these materials for electrochemical supercapacitors.
This leads us to develop high-power on-chip porous silicon supercapacitors
capable of up to 10 Wh/kg and 65 kW/kg energy and power densities,
respectively, and 5 Wh/kg energy density at 35 kW/kgî—¸comparable
to many of the best high-power carbon-based supercapacitors. As surface
reactivity wholly dictates the utilization of nanoscale silicon in
diverse applications across electronics, energy storage, biological
systems, energy conversion, and sensing, we emphasize direct formation
of few-layered graphene on nanostructured silicon as a means to form
heterogeneous on-chip interfaces that can maintain stability in even
the most reactive of environments
Pulsed Current for Diameter-Controlled Carbon Nanotubes and Hybrid Carbon Nanostructures in Electrolysis of Captured Carbon Dioxide
Here, we demonstrate how temporally controlled pulses
of current
can control the physical properties of multiwalled carbon nanotubes
(MWCNTs) synthesized from the electrolysis of air-captured carbon
dioxide. Our findings demonstrate that a transient 1 min 7.5-fold
rate increase of current or carbonate reduction during nucleation
of MWCNTs leads to 2.5 times smaller average MWCNT diameters, a higher
degree of graphitization in the walls, and an overall 10% lower energy
consumption over the full growth duration. Conversely, when identical
transient current pulses are applied after MWCNT nucleation in the
middle of CNT growth, our findings indicate the deposition of noncatalytic
onionlike carbons on the surface of the MWCNTs to form hybrid nanostructured
materials, but no changes are observed to MWCNT diameters, energy
consumption, or wall graphitization of the MWCNTs. A detailed study
of this system by three-electrode cyclic voltammetry, imaging, X-ray
diffraction (XRD), and Raman spectroscopy supports the mechanistic
role of current pulses in nucleation to facilitate rapid catalyst
reduction and minimize coarsening to sustain catalysts with high activity.
This work demonstrates how temporally controlled electrochemical current
density, and hence carbon flux, in molten carbonate electrolysis is
a powerful tool to engineer the production of carbon nanostructures
with tailored physical properties and a total energy consumption footprint
Morphology-Directed Catalysis with Branched Gold Nanoantennas
We
synthesized multibranched gold nanoantennas (MGNs) of two morphologies
by varying the core-to-branch ratio. We compared their efficacy in
catalytic reduction of <i>p</i>-nitrophenol (PNP) to <i>p</i>-aminiphenol (PAP). We observed that MGNs with shorter
protrusions had a faster induction time and higher apparent rate constant, <i>k</i><sub>app</sub>, for PNP catalysis relative to the MGNs
with longer protrusions. By examining the reaction as a function of
temperature, we observed significantly lower activation energy for
the MGNs with shorter protrusions (80 J/g) compared to MGNs with longer
protrusions (200 J/g). The Langmuir–Hinshelwood model was used
to fit the change in <i>k</i><sub>app</sub> as a function
of increasing [PNP], which demonstrated more efficient PNP adsorption
on the surfaces of MGNs with shorter protrusions. For the MGNs with
longer protrusions, PNP adsorption is affected by the heterogeneity
of the surface sites resulting in a lower adsorption coefficient.
We attributed the improved efficiency of the MGNs with shorter protrusions
to the presence of {100} and {110} crystal planes, which have a high
density of atomic steps and kinks that promote higher catalytic activity
for PNP degradation. MGNs with long protrusions are bound by low index
{111} facets; the highly coordinated atoms of {111} reduce the adsorption
efficiency of PNP
Solution Assembled Single-Walled Carbon Nanotube Foams: Superior Performance in Supercapacitors, Lithium-Ion, and Lithium–Air Batteries
We demonstrate a surfactant-free,
solution processing route to
form three-dimensional freestanding foams of pristine single-walled
carbon nanotubes (SWCNTs) and explore the diverse electrochemical
energy storage applications of these materials. This route utilizes
SWCNT dispersions in organic <i>n</i>-methylpyrrolidone
solvents and subsequent electrophoretic assembly onto a metal foam
sacrificial template which can be dissolved to yield surfactant-free,
binder-free freestanding SWCNT foams. We further provide a comparison
between surfactant-free foams and conventional surfactant-based solvent
processing routes and assess performance of these foams in supercapacitors,
lithium-ion batteries, and lithium–air batteries. For pristine
SWCNT foams, we measure up to 83 F/g specific capacitance in supercapacitors,
specific capacity up to 2210 mAh/g for lithium-ion batteries with
up to 50% energy efficiency, and specific discharge capacity up to
8275 mAh/g in lithium–air batteries. For lithium–air
batteries, this corresponds to a total energy density of 21.2 and
3.3 kWh/kg for the active mass and total battery device, respectively,
approaching the 12.7 kWh/kg target energy density of gasoline. In
comparison, SWCNT foams prepared with surfactant exhibit poorer gravimetric
behavior in all devices and compromised Faradaic storage that leads
to depreciated amounts of usable, stored energy. This work demonstrates
the broad promise of SWCNTs as lightweight and highly efficient energy
storage materials but also emphasizes the importance of clean nanomanufacturing
routes which are critical to achieve optimized performance with nanostructures