16 research outputs found
Colloidal Random Terpolymers: Controlling Reactivity Ratios of Colloidal Comonomers via Metal Tipping
We report on a versatile
synthetic method of preparing colloidal
copolymers and terpolymers composed of dipolar Au@Co core–shell
nanoparticles (NPs) in the backbone, along with semiconductor CdSe@CdS
nanorod (NR), or tetrapod (TP) side chain groups. A seven-step colloidal
total synthesis enabled the synthesis of well-defined colloidal comonomers
composed of a dipolar Au@CoNP attached to a single CdSe@CdS NR, or
TP, where magnetic dipolar associations between Au@CoNP units promoted
the formation of colloidal co- or terpolymers. The key step in this
synthesis was the ability to photodeposit a single AuNP tip onto CdSe@CdS
NR or TP that enables selective seeding of a dipolar CoNP onto the
AuNP seed. We show that the variation of the AuNP size directly controlled
the size and dipolar character of the CoNP tip, where the size modulation
of the Au and Au@CoNP tips is analogous to control of comonomer reactivity
ratios in classical copolymerization processes
Hybrids by Cluster Complex-Initiated Polymerization
Hybrids by Cluster Complex-Initiated
Polymerizatio
Waveguide-Based Spectroelectrochemical Characterization of Band Edge Energies in Submonolayers of CdSe Quantum Dots Tethered to Indium–Tin Oxide Electrodes
We
present here high sensitivity attenuated total reflectance (ATR)
spectroelectrochemical studies of electron injection (reduction) into
surface-tethered, submonolayer to monolayer coverages of CdSe quantum
dots (QDs) linked to indium–tin oxide (ITO) electrodes using
a strong X-type bifunctional phosphonic acid (PA) surface linker,
octanediphosphonic acid (ODiPA). Estimates of conduction band energies
(ECB) were obtained from the onset of
absorbance bleaching as a function of QD diameter (3.2–6.4
nm) and as a function of the supporting electrolyte (LiClO4) concentration. For CdSe QDs created from combinations of moderately
strong stearic acid, hexadecylamine, trioctylphosphine oxide, and
trioctylphosphine ligands, surface-tethering was accompanied by decreases
in QD diameter and loss of up to 25% volume for the largest QDs. For
QDs prepared with PA ligands, followed by aggressive (3×) pyridine
exchange to produce QDs with weak capping ligands, no size reduction
was observed as a result of adsorption to the ODiPA/ITO surface. For
both types of tethered CdSe QDs, significant stabilization of the
reduction product of the surface-tethered QD was observed with ca.
700 meV lowering of ECB relative to estimates
of ECB obtained from our recent in vacuuo
UV-photoemission studies of bare CdSe QDs tethered to Au surfaces.
A sizeable fraction of that stabilization is proposed to arise from
the tethering of these asymmetric QDs to a complex, high dielectric
constant interface region. At least 200 meV of the stabilization arises
from concentration-dependent charge screening by the solution counter
ion (Li+), with no evidence for the incorporation of Li+ as a result of the electron injection process. The overall
stabilization in the reduced form of these tethered QDs is larger
than seen for previous spectroelectrochemical studies of QD reduction,
in solution, tethered at higher coverages to transparent electrodes,
or as electrophoretically deposited multilayer QD thin films. This
waveguide ATR spectroelectrochemical approach to estimating energetics
for QDs tethered to semiconductor or oxide substrates at low surface
coverages is likely to be relevant for a wide array of energy conversion
and energy storage processes using nanomaterials and may be especially
useful for studying the effects of surface coverage, type of surface
linker, contacting solvent/electrolytes, and adsorbed molecular reactants
Conformal Polymeric Multilayer Coatings on Sulfur Cathodes via the Layer-by-Layer Deposition for High Capacity Retention in Li–S Batteries
We
report on the conformal coating of thickness-tunable multilayers
directly onto the sulfur (S<sub>8</sub>) cathodes by the layer-by-layer
(LbL) deposition for the significant improvement in the performances
of Li–S batteries even without key additives (LiNO<sub>3</sub>) in the electrolyte. Poly(ethylene oxide) (PEO)/poly(acrylic acid)
(PAA) multilayers on a single poly(allylamine hydrochloride) (PAH)/PAA
priming bilayer, deposited on the S<sub>8</sub> cathodes, effectively
protected from the polysulfide leakage, while providing a Li<sup>+</sup> ion diffusion channel. As a result, PAH/PAA/(PEO/PAA)<sub>3</sub> multilayer-coated cathodes exhibited the highest capacity retention
(806 mAh g<sup>–1</sup>) after 100 cycles at 0.5 C, as well
as the high C-rate capability up to 2.0 C. Furthermore, the multilayer
coating effectively mitigated the polysulfide shuttle effect in the
absent of LiNO<sub>3</sub> additives in the electrolyte
Band Edge Energetics of Heterostructured Nanorods: Photoemission Spectroscopy and Waveguide Spectroelectrochemistry of Au-Tipped CdSe Nanorod Monolayers
Conduction and valence band energies (<i>E</i><sub>CB</sub>, <i>E</i><sub>VB</sub>) for CdSe nanorods (NRs) functionalized with Au nanoparticle (NP) tips are reported here, referenced to the vacuum scale. We use (a) UV photoemission spectroscopy (UPS) to measure <i>E</i><sub>VB</sub> for NR films, utilizing advanced approaches to secondary electron background correction, satellite removal to enhance spectral contrast, and correction for shifts in local vacuum levels; and (b) waveguide-based spectroelectrochemistry to measure <i>E</i><sub>CB</sub> from onset potentials for electron injection into NR films tethered to ITO. For untipped CdSe NRs, both approaches show <i>E</i><sub>VB</sub> = 5.9–6.1 eV and <i>E</i><sub>CB</sub> = 4.1–4.3 eV. Addition of Au tips alters the NR band edge energies and introduces midgap states, in ways that are predicted to influence the efficiency of these nanomaterials as photoelectrocatalysts. UPS results show that Au tipping shifts <i>E</i><sub>VB</sub> closer to vacuum by up to 0.4 eV, shifts the apparent Fermi energy toward the middle of the band gap, and introduces additional states above <i>E</i><sub>VB</sub>. Spectroelectrochemical results confirm these trends: Au tipping shifts <i>E</i><sub>CB</sub> closer to vacuum, by 0.4–0.6 eV, and introduces midgap states below <i>E</i><sub>CB</sub>, which are assigned as metal–semiconductor interface (MSI) states. Characterization of these band edge energies and understanding the origins of MSI states is needed to design energy conversion systems with proper band alignment between the light absorbing NR, the NP catalyst, and solution electron donors and acceptors. The complementary characterization protocols presented here should be applicable to a wide variety of thin films of heterogeneous photoactive nanomaterials, aiding in the identification of the most promising material combinations for photoelectrochemical energy conversion
Universal Length Dependence of Rod-to-Seed Exciton Localization Efficiency in Type I and Quasi-Type II CdSe@CdS Nanorods
A critical step involved in many applications of one-dimensional seeded CdSe@CdS nanorods, such as luminescent solar concentrators, optical gains, and photocatalysis, is the localization of excitons from the light-harvesting CdS nanorod antenna into the light-emitting CdSe quantum dot seed. We report that the rod-to-seed exciton localization efficiency decreases with the rod length but is independent of band alignment between the CdSe seed and CdS rod. This universal dependence can be well modeled by the competition between exciton one-dimensional diffusion to the CdSe seed and trapping on the CdS rod. This finding provides a rational approach for optimizing these materials for their various device applications
Preparation of Dynamic Covalent Polymers via Inverse Vulcanization of Elemental Sulfur
The synthesis of dynamic covalent
polymers with controllable amounts of sulfur–sulfur (S–S)
bonds in the polymer backbone via inverse vulcanization of elemental
sulfur (S<sub>8</sub>) and 1,3-diisopropenylbenzene (DIB) is reported.
An attractive feature of the inverse vulcanization process is the
ability to control the number and dynamic nature of S–S bonds
in poly(sulfur-<i>random</i>-(1,3-diisopropenylbenzene))
(poly(S-<i>r</i>-DIB) copolymers by simple variation of
S<sub>8</sub>/DIB feed ratios in the copolymerization. S–S
bonds in poly(S-<i>r</i>-DIB) copolymers of high sulfur
content and sulfur rank were found to be more dynamic upon exposure
to either heat, or mechanical stimuli. Interrogation of dynamic S–S
bonds was conducted in the solid-state utilizing electron paramagnetic
resonance spectroscopy and <i>in situ</i> rheological measurements.
Time-dependent rheological property behavior demonstrated a compositional
dependence of the healing behavior in the copolymers, with the highest
sulfur (80 wt % sulfur) content affording the most rapid dynamic response
and recovery of rheological properties
Multimodal Characterization of the Morphology and Functional Interfaces in Composite Electrodes for Li–S Batteries by Li Ion and Electron Beams
We
report the characterization of multiscale 3D structural architectures
of novel poly[sulfur-<i>random</i>-(1,3-diisopropenylbenzene)]
copolymer-based cathodes for high-energy-density Li–S batteries
capable of realizing discharge capacities >1000 mAh/g and long
cycling lifetimes >500 cycles. Hierarchical morphologies and interfacial
structures have been investigated by a combination of focused Li ion
beam (LiFIB) and analytical electron microscopy in relation to the
electrochemical performance and physicomechanical stability of the
cathodes. Charge-free surface topography and composition-sensitive
imaging of the electrodes was performed using recently introduced
low-energy scanning LiFIB with Li<sup>+</sup> probe sizes of a few
tens of nanometers at 5 keV energy and 1 pA probe current. Furthermore,
we demonstrate that LiFIB has the ability to inject a certain number
of Li cations into the material with nanoscale precision, potentially
enabling control of the state of discharge in the selected area. We
show that chemical modification of the cathodes by replacing the elemental
sulfur with organosulfur copolymers significantly improves its structural
integrity and compositional homogeneity down to the sub-5-nm length
scale, resulting in the creation of (a) robust functional interfaces
and percolated conductive pathways involving graphitic-like outer
shells of aggregated nanocarbons and (b) extended micro- and mesoscale
porosities required for effective ion transport
Improving the Charge Conductance of Elemental Sulfur via Tandem Inverse Vulcanization and Electropolymerization
The synthesis of polymeric materials
using elemental sulfur (S<sub>8</sub>) as the chemical feedstock has
recently been developed using
a process termed inverse vulcanization. The preparation of chemically
stable sulfur copolymers was previously prepared by the inverse vulcanization
of S<sub>8</sub> and 1,3-diisopropenylbenzene (DIB); however, the
development of synthetic methods to introduce new chemical functionality
into this novel class of polymers remains an important challenge.
In this report the introduction of polythiophene segments into poly(sulfur-<i>random</i>-1,3-diisopropenylbenzene) is achieved by the inverse
vulcanization of S<sub>8</sub> with a styrenic functional 3,4-propylenedioxythiophene
(ProDOT-Sty) and DIB, followed by electropolymerization of ProDOT
side chains. This methodology demonstrates for the first time a facile
approach to introduce new functionality into sulfur and high sulfur
content polymers, while specifically enhancing the charge conductivity
of these intrinsically highly resistive materials
One Dimensional Photonic Crystals Using Ultrahigh Refractive Index Chalcogenide Hybrid Inorganic/Organic Polymers
We
report on the fabrication of wholly polymeric one-dimensional
(1-D) photonic crystals (i.e., Bragg reflectors, Bragg mirrors) via
solution processing for use in the near (NIR) and the short wave (SWIR)
infrared spectrum (1–2 μm) with very high reflectance
(<i>R</i> ∼ 90–97%). Facile fabrication of
these highly reflective films was enabled by direct access to solution
processable, ultrahigh refractive index polymers, termed, Chalcogenide
Hybrid Inorganic/Organic Polymers (CHIPs). The high refractive index
(<i>n</i>) of CHIPs materials (<i>n</i> = 1.75–2.10)
allowed for the production of narrow band IR Bragg reflectors with
high refractive index contrast (Δ<i>n</i> ∼
0.5) when fabricated with low <i>n</i> polymers, such as
cellulose acetate (<i>n</i> = 1.47). This is the highest
refractive index contrast (Δ<i>n</i> ∼ 0.5)
demonstrated for an all-polymeric Bragg mirror which directly enabled
high reflectivity from films with 22 layers or less. Facile access
to modular, thin, highly reflective films from inexpensive CHIPs materials
offers a new route to IR Bragg reflectors and other reflective coatings
with potential applications for IR photonics, commercial sensing,
and LIDAR applications