6 research outputs found
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
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
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
Geometry-Dependent Plasmonic Tunability and Photothermal Characteristics of Multibranched Gold Nanoantennas
Plasmon resonances of anisotropic
multibranched nanostructures
are governed by their geometry, allowing morphology-directed selective
manipulation of the optical properties. In this work, we have synthesized
multibranched gold nanoantennas (MGNs) of variable geometry by a one-step
seedless approach using 4-(2-hydroxyethyl)-1-piperazineethanesulfonic
acid (HEPES) as a capping and reducing agent. This approach enables
us to modulate the MGNs’ geometry by controlling three different
parameters: concentration of HEPES, concentration of Au<sup>3+</sup>, and pH of HEPES buffer. By altering the MGNs morphology with minimal
increase in the overall dimensions, the plasmon resonances were tuned
from the visible to the near-infrared. The MGNs plasmon resonances
demonstrated a nonintuitive blue-shift when pH > p<i>K</i><sub>a</sub> of HEPES which we attributed to emergence of charge
transfer oscillations formed when MGNs cluster to dimers and trimers.
Further, due to the presence of multiple sharp protrusions, the MGNs
demonstrated a refractive index sensitivity of 373 nm/RIU, which is
relatively high for this class of branched nanostructures of similar
size. Finally, the sharp protrusions of MGNs also give rise to intense
photothermal efficiencies; ∼53 °C was achieved within
5 min of laser illumination, demonstrating the efficacy of MGNs in
therapeutic applications. By modulating the mass density of MGNs,
the laser flux, and time of illumination, we provide a detailed analysis
of the photothermal characteristics of MGNs
Ultrafast Spectral Dynamics of CsPb(Br<sub><i>x</i></sub>Cl<sub>1–<i>x</i></sub>)<sub>3</sub> Mixed-Halide Nanocrystals
In
this work we investigated the spectral dynamics of cesium lead
mixed-halide, CsPbÂ(Br<sub><i>x</i></sub>Cl<sub>1–<i>x</i></sub>)<sub>3</sub> perovskite nanocrystals probed with
complementary spectral techniques: time-resolved photoluminescence
and transient absorption spectroscopy. Mixed-halide perovskite nanocrystals
were synthesized via a hot-injection method followed by anion exchange
reactions. Our results show that increased Cl content in perovskite
nanocrystals (<i>a</i>) diminished the photoluminescence
quantum yield and gave rise to rapid radiative recombination of carriers;
(<i>b</i>) resulted in rapid thermalization of hot carriers
and low carrier temperatures, which suggests weaker hot-phonon bottleneck
and Burstein–Moss effects; (<i>c</i>) decreased the
bandgap renormalization energy, which suggests high exciton binding
energy and poor charge extraction in Cl substituted perovskite nanocrystals;
and (<i>d</i>) increased the number of carriers undergoing
Auger losses, where Auger processes dominate over trap-assisted recombination.
These findings provide a generalized framework to guide researchers
as to when mixed-halide perovskite nanocrystals would be useful for
optoelectronic technologies and when they would be detrimental to
device performance