26 research outputs found
Molecular Ligands Control Superlattice Structure and Crystallite Orientation in Colloidal Quantum Dot Solids
Colloidal quantum
dot solids represent a new materials platform
that has garnered interest for a variety of electronic, optoelectronic,
and photovoltaic applications. In such solids, individual quantum
dots must be coupled with each other to facilitate charge transport
through the solid. Past improvements on charge transport of colloidal
quantum dot solids have been achieved primarily through the control
of the interparticle spacing. However, the role of morphological ordering
of the crystalline facets of individual quantum dots on the charge
transport of the quantum dot solid is unknown. Here, we show for the
first time that small passivating ligand molecules around the quantum
dots can control the arrangement of different facets of quantum dots
within the quantum dot solid. The insights from this study provide
important directions for future enhancement in orientation of quantum
dots in colloidal quantum dot solids
<i>In Situ</i> Xâray Scattering Guides the Synthesis of Uniform PtSn Nanocrystals
Compared to monometallic nanocrystals
(NCs), bimetallic ones often
exhibit superior properties due to their wide tunability in structure
and composition. A detailed understanding of their synthesis at the
atomic scale provides crucial knowledge for their rational design.
Here, exploring the PtâSn bimetallic system as an example,
we study in detail the synthesis of PtSn NCs using <i>in situ</i> synchrotron X-ray scattering. We show that when PtÂ(II) and SnÂ(IV)
precursors are used, in contrast to a typical simultaneous reduction
mechanism, the PtSn NCs are formed through an initial reduction of
PtÂ(II) to form Pt NCs, followed by the chemical transformation from
Pt to PtSn. The kinetics derived from the <i>in situ</i> measurements shows fast diffusion of Sn into the Pt lattice accompanied
by reordering of these atoms into intermetallic PtSn structure within
300 s at the reaction temperature (âź280 °C). This crucial
mechanistic understanding enables the synthesis of well-defined PtSn
NCs with controlled structure and composition via a seed-mediated
approach. This type of <i>in situ</i> characterization can
be extended to other multicomponent nanostructures to advance their
rational synthesis for practical applications
Activity of Silica-Alumina for the Conversion of Polyethylene into Tunable Aromatics Below Pyrolytic Temperatures
Plastic waste is a mounting problem that lacks global
strategy,
largely due to inadequate recycling capabilities. One alternative
gaining traction in the heterogeneous catalysis community is the upcycling
of polyolefins into value-added products, such as hydrogen-free conversion
to alkylaromatic compounds over Pt/Al2O3. Here,
we examined the activity of nominally metal-free, mesoporous silica-alumina
mixed oxide materials (SiO2âAl2O3) for the conversion of polyethylene into aromatic compounds
at temperatures of and below 280 °C. Yields with the silica-alumina
catalysts are comparable to those obtained over Pt(1 wt %)/Al2O3 under identical conditions, and product selectivity
can be tuned by altering reaction conditions or the acid site density
of the SiO2âAl2O3. Notably,
the fraction of polyaromatic products increases with the Brønsted
acid site density of the catalyst, as does the degree of polymer deconstruction.
These catalysts can be reused without regeneration, and their activity
improves with each recycling event, producing soluble product yields
up to 83%. Preliminary work on the mechanism of the reaction suggests
that acid sites are responsible for initiating depolymerization and
aromatization reactions, in analogy to previous work in the literature.
This work showcases the activity of SiO2âAl2O3 for polyolefin deconstruction/aromatization
at subpyrolytic temperatures and lays the foundation for future studies
involving solid acid and bifunctional catalysts
Chemical Annealing of Zinc Tetraphenylporphyrin Films: Effects on Film Morphology and Organic Photovoltaic Performance
We present a chemical annealing process for organic thin
films.
In this process, a thin film of a molecular material, such as zinc
tetraphenylporphyrin (ZnTPP), is exposed to a vapor of nitrogen-based
ligand (e.g., pyrazine, pz, and triazine, tz), forming a film composed
of the metalâligand complex. Fast and quantitative formation
of the complex leads to marked changes in the morphology and optical
properties of the film. X-ray diffraction studies show that the chemical
annealing process converts amorphous ZnTPP films to crystalline ZnTPP¡ligand
films, whose porphryin planes lie nearly parallel to the substrate
(average deviation is 8° for the ZnTPP¡pz film). Organic
solar cells were prepared with ZnTPP donor and C<sub>60</sub> acceptor
layers. Devices were prepared with and without chemical annealing
of the ZnTPP layer with a pyrazine ligand. The devices with chemically
annealed ZnTPP donor layer show an increase in short-circuit current
(<i>J</i><sub>SC</sub>) and fill factor (<i>FF</i>) relative to analogous unannealed devices, presumably because of
enhanced exciton diffusion length and improved charge conductivity.
The open circuit voltages (<i>V</i><sub>OC</sub>) of the
chemically annealed devices are lower than their unannealed counterpart
because of enhanced polaron pair recombination at the donor/acceptor
heterojunction. A net improvement of 5â20% in efficiency has
been achieved, after chemical annealing of ZnTPP films with pyrazine
Effect of Backbone Chemistry on the Structure of Polyurea Films Deposited by Molecular Layer Deposition
An experimental investigation
into the growth of polyurea films
by molecular layer deposition was performed by examining trends in
the growth rate, crystallinity, and orientation of chains as a function
of backbone flexibility. Growth curves obtained for films containing
backbones of aliphatic and phenyl groups indicate that an increase
in backbone flexibility leads to a reduction in growth rate from 4
to 1 Ă
/cycle. Crystallinity measurements collected using grazing
incidence X-ray diffraction and Fourier transform infrared spectroscopy
suggest that some chains form paracrystalline, out-of-plane stacks
of polymer segments with packing distances ranging from 4.4 to 3.7
Ă
depending on the monomer size. Diffraction intensity is largely
a function of the homogeneity of the backbone. Near-edge X-ray absorption
fine structure measurements for thin and thick samples show an average
chain orientation of âź25° relative to the substrate across
all samples, suggesting that changes in growth rate are not caused
by differences in chain angle but instead may be caused by differences
in the frequency of chain terminations. These results suggest a model
of molecular layer deposition-based chain growth in which films consist
of a mixture of upward growing chains and horizontally aligned layers
of paracrystalline polymer segments
Systematic Identification of Promoters for Methane Oxidation Catalysts Using Size- and Composition-Controlled Pd-Based Bimetallic Nanocrystals
Promoters enhance the performance
of catalytic active phases by
increasing rates, stability, and/or selectivity. The process of identifying
promoters is in most cases empirical and relies on testing a broad
range of catalysts prepared with the random deposition of active and
promoter phases, typically with no fine control over their localization.
This issue is particularly relevant in supported bimetallic systems,
where two metals are codeposited onto high-surface area materials.
We here report the use of colloidal bimetallic nanocrystals to produce
catalysts where the active and promoter phases are colocalized to
a fine extent. This strategy enables a systematic approach to study
the promotional effects of several transition metals on palladium
catalysts for methane oxidation. In order to achieve these goals,
we demonstrate a single synthetic protocol to obtain uniform palladium-based
bimetallic nanocrystals (PdM, M = V, Mn, Fe, Co, Ni, Zn, Sn, and potentially
extendable to other metal combinations) with a wide variety of compositions
and sizes based on high-temperature thermal decomposition of readily
available precursors. Once the nanocrystals are supported onto oxide
materials, thermal treatments in air cause segregation of the base
metal oxide phase in close proximity to the Pd phase. We demonstrate
that some metals (Fe, Co, and Sn) inhibit the sintering of the active
Pd metal phase, while others (Ni and Zn) increase its intrinsic activity
compared to a monometallic Pd catalyst. This procedure can be generalized
to systematically investigate the promotional effects of metal and
metal oxide phases for a variety of active metal-promoter combinations
and catalytic reactions
Tuning Precursor Reactivity toward Nanometer-Size Control in Palladium Nanoparticles Studied by in Situ Small Angle Xâray Scattering
Synthesis of monodisperse
nanoparticles (NPs) with precisely controlled
size is critical for understanding their size-dependent properties.
Although significant synthetic developments have been achieved, it
is still challenging to synthesize well-defined NPs in a predictive
way due to a lack of in-depth mechanistic understanding of reaction
kinetics. Here we use synchrotron-based small-angle X-ray scattering
(SAXS) to monitor in situ the formation of palladium (Pd) NPs through
thermal decomposition of PdâTOP (TOP: trioctylphosphine) complex
via the âheat-upâ method. We systematically study the
effects of different ligands, including oleylamine, TOP, and oleic
acid, on the formation kinetics of Pd NPs. Through quantitative analysis
of the real-time SAXS data, we are able to obtain a detailed picture
of the size, size distribution, and concentration of Pd NPs during
the syntheses, and these results show that different ligands strongly
affect the precursor reactivity. We find that oleylamine does not
change the reactivity of the PdâTOP complex but promote the
formation of nuclei due to strong ligandâNP binding. On the
other hand, TOP and oleic acid substantially change the precursor
reactivity over more than an order of magnitude, which controls the
nucleation kinetics and determines the final particle size. A theoretical
model is used to demonstrate that the nucleation and growth kinetics
are dependent on both precursor reactivity and ligandâNP binding
affinity, thus providing a framework to explain the synthesis process
and the effect of the reaction conditions. Quantitative understanding
of the impacts of different ligands enables the successful synthesis
of a series of monodisperse Pd NPs in the broad size range from 3
to 11 nm with nanometer-size control, which serve as a model system
to study their size-dependent catalytic properties. The in situ SAXS
probing can be readily extended to other functional NPs to greatly
advance their synthetic design
Monitoring a Silent Phase Transition in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> Solar Cells via <i>Operando</i> Xâray Diffraction
The
relatively modest temperature of the tetragonal-to-cubic phase
transition in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> perovskite
is likely to occur during real world operation of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> solar cells. In this work, we simultaneously
monitor the structural phase transition of the active layer along
with solar cell performance as a function of the device operating
temperature. The tetragonal to cubic phase transition is observed
in the working device to occur reversibly at temperatures between
60.5 and 65.4 °C. In these <i>operando</i> measurements,
no discontinuity in the device performance is observed, indicating
electronic behavior that is insensitive to the structural phase transition.
This decoupling of device performance from the change in long-range
order across the phase transition suggests that the optoelectronic
properties are primarily determined by the local structure in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>. That is, while the average
crystal structure as probed by X-ray diffraction shows a transition
from tetragonal to cubic, the local structure generally remains well
characterized by uncorrelated, dynamic octahedral rotations that order
at elevated temperatures but are unchanged locally
Polypeptide Composite Particle-Assisted Organization of ĎâConjugated Polymers into Highly Crystalline âCoffee Stainsâ
We demonstrate that homopolypeptides
covalently tethered to anisotropically shaped silica particles induce
crystalline ordering of representative semiconducting polymers. Films
drop-cast from chloroform dispersions of polyÂ(Îł-stearyl-l-glutamate) (PSLG) composite particles and polyÂ(3-hexythiophene)
(P3HT) led to highly ordered crystalline structures of P3HT. Hydrophobicâhydrophobic
interactions between the alkyl side chains of P3HT and PSLG were the
main driving force for P3HT chain ordering into the crystalline assemblies.
It was found that the orientation of rigid P3HT fibrils on the substrate
adopted the directionality of the evaporating front. Regardless of
the PSLG-coated particle dimensions used, the drop-cast films displayed
patterns that were shaped by the coffee ring and Marangoni effects.
PSLG-coated particles of high axial ratio (4.2) were more efficient
in enhancing the electronic performance of P3HT than low axial ratio
(2.6) homologues. Devices fabricated from the ordered assemblies displayed
improved charge-carrier transport performance when compared to devices
fabricated from P3HT alone. These results suggest that PSLG can favorably
mediate the organization of semiconducting polymers
Protein-Assisted Assembly of ĎâConjugated Polymers
In
an aqueous suspension process, protein dispersions facilitated improved
alignment and organization of polyÂ(3-hexylthiophene) (P3HT) chains
into highly ordered crystalline structures. A solution of P3HT in
1,2,4-trichlorobenzene (TCB) was added to an aqueous dispersion of
the hydrophobin, Cerato ulmin (CU). Upon gentle agitation, the semiconductor
solution was readily confined within CU membrane-stabilized microstructures,
often with extended shapes. UVâvis and polarized micro-Raman
spectroscopy suggested complex, enhanced molecular alignment due to
a transition from isotropic to liquid crystalline fluid to polycrystalline
states. Grazing-incidence X-ray diffraction corroborates this interpretation.
On aging, the initial CU:P3HT/TCB structures develop dendritic architectures
that slowly release polymer-containing capsules. The counterintuitive
evolution from large structures to smaller ones suggests the initial
structures were nonequilibrium, and it opens the door to latex-like
processing of semiconducting polymers into crystalline, high-performance
thin films for device applications. Preliminary studies using an organic
field-effect transistor architecture suggest that optimized processing
and device configuration will enable highly crystalline active materials
with efficient charge transport characteristics