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Suppressing Defect Formation Pathways in the Direct C-H Arylation Polymerization of Photovoltaic Copolymers
Direct CH arylation polymerization (DARP) holds great promise for the green, atom-efficient synthesis of p-conjugated copolymers for use in high-performance polymer solar cells (PSCs). However, CH arylation regioselectivity control for monomers containing multiple reactive aryl CH bonds is not well understood, and nonselective reactivity results in material defects with unknown effects on PSC performance. Here, the effects of reaction conditions on copolymer molecular mass, dispersity, and PSC performance as well as defect formation pathways occurring during the DARP synthesis of an archetypal benzodithiophene-alt-diketopyrrolopyrrole copolymer, PBDTTDPP, are scrutinized. Small molecule model studies analyzed by HPLC-HRMS elucidate the effects of DARP conditions on trace chemical defect (primarily hydrodehalogenation and beta-CH arylation) formation. Copolymer branching arising from nonselective beta-CH arylation of monomers at the polymer chain end is identified as the principal photovoltaically deleterious defect. Fine-tuning the DARP reaction conditions reduces branching densities to below 1%, with an exceptional CH regioselectivity of >110:1. The optimal copolymers achieve superior PSC performance versus defect-rich DARP-derived copolymers and rival those from Stille polycondensations.
Evolution of the Single-Nanocrystal Photoluminescence Linewidth with Size and Shell: Implications for Exciton–Phonon Coupling and the Optimization of Spectral Linewidths
The optimization of photoluminescence spectral linewidths in semiconductor nanocrystal preparations involves minimizing both the homogeneous and inhomogeneous contributions to the ensemble spectrum. Although the inhomogeneous contribution can be controlled by eliminating interparticle inhomogeneities, far less is known about how to synthetically control the homogeneous, or single-nanocrystal, spectral linewidth. Here, we use solution photon-correlation Fourier spectroscopy (S-PCFS) to measure how the sample-averaged single-nanocrystal emission linewidth of CdSe core and core/shell nanocrystals change with systematic changes in the size of the cores and the thickness and composition of the shells. We find that the single-nanocrystal linewidth at room temperature is heavily influenced by the nature of the CdSe surface and the epitaxial shell, which have a profound impact on the internal electric fields that affect exciton–phonon coupling. Our results explain the wide variations, both experimental and theoretical, in the magnitude and size dependence in previous reports on exciton–phonon coupling in CdSe nanocrystals. Furthermore, our findings offer a general pathway for achieving the narrow spectral linewidths required for many applications of nanocrystals.
Diffusion-Mediated Synthesis of MoS 2 /WS 2 Lateral Heterostructures
The controlled growth of two-dimensional transition metal dichalcogenide (TMD) lateral heterostructures would enable on-demand tuning of electronic and optoelectronic properties in this new class of materials. Prior to this work, compositional modulations in lateral TMD heterostructures have been considered to depend solely on the growth chronology. We show that in-plane diffusion can play a significant role in the chemical vapor deposition of MoS 2 /WS 2 lateral heterostructures leading to a variety of nontrivial structures whose composition does not necessarily follow the growth order. Optical, structural, and compositional studies of TMD crystals captured at different growth temperatures and in different diffusion stages suggest that compositional mixing versus segregation are favored at high and low growth temperatures, respectively. The observed diffusion mechanism will expand the realm of possible lateral heterostructures, particularly ones that cannot be synthesized using traditional methods.
Room-Temperature Micron-Scale Exciton Migration in a Stabilized Emissive Molecular Aggregate
We report 1.6 ± 1 μm exciton transport in self-assembled supramolecular light-harvesting nanotubes (LHNs) assembled from amphiphillic cyanine dyes. We stabilize LHNs in a sucrose glass matrix, greatly reducing light and oxidative damage and allowing the observation of exciton–exciton annihilation signatures under weak excitation flux. Fitting to a one-dimensional diffusion model, we find an average exciton diffusion constant of 55 ± 20 cm 2 /s, among the highest measured for an organic system. We develop a simple model that uses cryogenic measurements of static and dynamic energetic disorder to estimate a diffusion constant of 32 cm 2 /s, in agreement with experiment. We ascribe large exciton diffusion lengths to low static and dynamic energetic disorder in LHNs. We argue that matrix-stabilized LHNS represent an excellent model system to study coherent excitonic transport.
Li 3 PO 4 Matrix Enables a Long Cycle Life and High Energy Efficiency Bismuth-Based Battery
Bismuth is a lithium-ion battery anode material that can operate at an equilibrium potential higher than graphite and provide a capacity twice as high as that of Li 4 Ti 5 O 12 , making it intrinsically free from lithium plating that may cause catastrophic battery failure. However, the potential of bismuth is hampered by its inferior cyclability (limited to tens of cycles). Here, we propose an “ion conductive solid-state matrix” approach to address this issue. By homogeneously confining bismuth nanoparticles in a solid-state γ-Li 3 PO 4 matrix that is electrochemically formed in situ, the resulting composite anode exhibits a reversible capacity of 280 mA hours per gram (mA h/g) at a rate of 100 mA/g and a record cyclability among bismuth-based anodes up to 500 cycles with a capacity decay rate of merely 0.071% per cycle. We further show that full-cell batteries fabricated from this composite anode and commercial LiFePO 4 cathode deliver a stable cell voltage of ~2.5 V and remarkable energy efficiency up to 86.3%, on par with practical batteries (80–90%). This work paves a way for harnessing bismuth-based battery chemistry for the design of high capacity, safer lithium-ion batteries to meet demanding applications such as electric vehicles.
Anderson Localization of Thermal Phonons Leads to a Thermal Conductivity Maximum
Our elastic model of ErAs disordered GaAs/AlAs superlattices exhibits a local thermal conductivity maximum as a function of length due to exponentially suppressed Anderson-localized phonons. By analyzing the sample-to-sample fluctuations in the dimensionless conductance, g, the transition from diffusive to localized transport is identified as the crossover from the multichannel to single-channel transport regime g ≈ 1. Single parameter scaling is shown to hold in this crossover regime through the universality of the probability distribution of g that is independent of system size and disorder strength.
Encapsulation of Metallic Na in an Electrically Conductive Host with Porous Channels as a Highly Stable Na Metal Anode
Room-temperature Na ion batteries (NIBs) have attracted great attention because of the widely available, abundant sodium resources and potentially low cost. Currently, the challenge of the NIB development is due primarily to the lack of a high-performance anode, while the Na metal anode holds great promise considering its highest specific capacity of 1165 mA h/g and lowest anodic potential. However, an uneven deposit, relatively infinite volume change, and dendritic growth upon plating/stripping cycles cause a low Coulombic efficiency, poor cycling performance, and severe safety concerns. In this study, a stable Na carbonized wood (Na–wood) composite anode was fabricated via a rapid melt infusion (about 5 s) into channels of carbonized wood by capillary action. The channels function as a high-surface-area, conductive, mechanically stable skeleton, which lowers the effective current density, ensures a uniform Na nucleation, and restricts the volume change over cycles. As a result, the Na–wood composite anode exhibited flat plating/stripping profiles with smaller overpotentials and stable cycling performance over 500 h at 1.0 mA/cm 2 in a common carbonate electrolyte system. Finally, in sharp comparison, the planar Na metal electrode showed a much shorter cycle life of 100 h under the same test conditions.
Probing Linewidths and Biexciton Quantum Yields of Single Cesium Lead Halide Nanocrystals in Solution
Cesium lead halide (CsPbX 3 , X = Cl, Br, I) perovskite nanocrystals (PNCs) have recently become a promising material for optoelectronic applications due to their high emission quantum yields and facile band gap tunability via both halide composition and size. The spectroscopy of single PNCs enhances our understanding of the effect of confinement on excitations in PNCs in the absence of obfuscating ensemble averaging and can also inform synthetic efforts. However, single PNC studies have been hampered by poor PNC photostability under confocal excitation, precluding interrogation of all but the most stable PNCs, and leading to a lack of understanding of PNCs in the regime of high confinement. Here, we report the first comprehensive spectroscopic investigation of single PNC properties using solution-phase photon-correlation methods, including both highly confined and blue-emitting PNCs, previously inaccessible to single NC techniques. With minimally perturbative solution-phase photon-correlation Fourier spectroscopy (s-PCFS), we establish that the ensemble emission linewidth of PNCs of all sizes and compositions is predominantly determined by the intrinsic single PNC linewidth (homogeneous broadening). The single PNC linewidth, in turn, dramatically increases with increasing confinement, consistent with what has been found for II–VI semiconductor nanocrystals. With solution-phase photon antibunching measurements, we survey the biexciton-to-exciton quantum yield ratio (BX/X QY) in the absence of user-selection bias or photodegradation. Remarkably, the BX/X QY ratio depends both on the PNC size and halide composition, with values between ~2% for highly confined bromide PNCs and ~50% for intermediately confined iodide PNCs. Our results suggest a wide range of underlying Auger rates, likely due to transitory charge carrier separation in PNCs with relaxed confinement.
Tunable UV-Emitters through Graphene Plasmonics
Control over the spontaneous emission of light through tailored optical environments remains a fundamental paradigm in nanophotonics. The use of highly confined plasmons in materials such as graphene provides a promising platform to enhance transition rates in the IR-THz by many orders of magnitude. However, such enhancements involve near-field plasmon modes or other kinds of near-field coupling like quenching, and it is challenging to use these highly confined modes to harness light in the far-field due to the difficulty of plasmonic outcoupling. Here in this paper, we propose that through the use of radiative cascade chains in multilevel emitters, IR plasmons can be used to enhance far field spectra in the visible and UV range, even at energies greater than 10 eV. Combining Purcell-enhancement engineering, graphene plasmonics, and radiative cascade can result in a new type of UV emitter whose properties can be tuned by electrically doping graphene. Varying the distance between the emitter and the graphene surface can change the strength of the far-field emission lines by 2 orders of magnitude. We also find that the dependence of the far-field emission on the Fermi energy is potentially extremely sharp at the onset of interband transitions, allowing the Fermi energy to effectively serve as a “switch” for turning on and off certain plasmonic and far-field emissions.
Exploring Electronic Structure and Order in Polymers via Single-Particle Microresonator Spectroscopy
PEDOT:PSS, a transparent electrically conductive polymer, finds widespread use in electronic devices. While empirical efforts have increased conductivity, a detailed understanding of the coupled electronic and morphological landscapes in PEDOT:PSS has lagged due to substantial structural heterogeneity on multiple length-scales. We use an optical microresonator-based absorption spectrometer to perform single-particle measurements, providing a bottom-up examination of electronic structure and morphology ranging from single PEDOT:PSS polymers to nascent films. Using single-particle spectroscopy with complementary theoretical calculations and ultrafast spectroscopy, we demonstrate that PEDOT:PSS displays bulk-like optical response even in single polymers. Here, we find highly ordered PEDOT assemblies with long-range ordering mediated by the insulating PSS matrix and reveal a preferential surface orientation of PEDOT nanocrystallites absent in bulk films with implications for interfacial electronic communication. Our single-particle perspective provides a unique window into the microscopic structure and electronic properties of PEDOT:PSS.
Ultrafast Graphene Light Emitters
Ultrafast electrically driven nanoscale light sources are critical components in nanophotonics. Compound semiconductor-based light sources for the nanophotonic platforms have been extensively investigated over the past decades. However, monolithic ultrafast light sources with a small footprint remain a challenge. Here in this paper, we demonstrate electrically driven ultrafast graphene light emitters that achieve light pulse generation with up to 10 GHz bandwidth across a broad spectral range from the visible to the near-infrared. The fast response results from ultrafast charge-carrier dynamics in graphene and weak electron-acoustic phonon-mediated coupling between the electronic and lattice degrees of freedom. We also find that encapsulating graphene with hexagonal boron nitride (hBN) layers strongly modifies the emission spectrum by changing the local optical density of states, thus providing up to 460% enhancement compared to the gray-body thermal radiation for a broad peak centered at 720 nm. Furthermore, the hBN encapsulation layers permit stable and bright visible thermal radiation with electronic temperatures up to 2000 K under ambient conditions as well as efficient ultrafast electronic cooling via near-field coupling to hybrid polaritonic modes under electrical excitation. These high-speed graphene light emitters provide a promising path for on-chip light sources for optical communications and other optoelectronic applications.
Self-Templated Formation of P2-type K 0.6 CoO 2 Microspheres for High Reversible Potassium-Ion Batteries
We report that layered metal oxides have been widely used as the best cathode materials for commercial lithium-ion batteries and are being intensively explored for sodium-ion batteries. However, their application to potassium-ion batteries (PIBs) is hampered because of the poor cycling stability and low rate capability due to the larger ionic size of K + than of Li + or Na + . Herein, a facile self-templated strategy was used to synthesize unique P2-type K 0.6 CoO 2 microspheres that consist of aggregated primary nanoplates as PIB cathodes. The unique K 0.6 CoO 2 microspheres with aggregated structure significantly enhanced the kinetics of the K + intercalation/deintercation and also minimized the parasitic reactions between the electrolyte and K 0.6 CoO 2 . The P2-K 0.6 CoO 2 microspheres demonstrated a high reversible capacity of 82 mAh g –1 at 10 mA g –1 , high rate capability of 65 mAh g –1 at 100 mA g –1 , and long cycle life (87% capacity retention over 300 cycles). The high reversibility of the P2-K 0.6 CoO 2 full cell paired with a hard carbon anode further demonstrated the feasibility of PIBs. Lastly, this work not only successfully demonstrates exceptional performance of P2-type K 0.6 CoO 2 cathodes and microspheres K 0.6 CoO 2 ∥hard carbon full cells, but also provides new insights into the exploration of other layered metal oxides for PIBs.
Morphological Engineering of Winged Au@MoS 2 Heterostructures for Electrocatalytic Hydrogen Evolution
Molybdenum disulfide (MoS 2 ) has been recognized as a promising cost-effective catalyst for water-splitting hydrogen production. However, the desired performance of MoS 2 is often limited by insufficient edge-terminated active sites, poor electrical conductivity, and inefficient contact to the supporting substrate. To address these limitations, we developed a unique nanoarchitecture (namely, winged Au@MoS 2 heterostructures enabled by our discovery of the “seeding effect” of Au nanoparticles for the chemical vapor deposition synthesis of vertically aligned few-layer MoS 2 wings). The winged Au@MoS 2 heterostructures provide an abundance of edge-terminated active sites and are found to exhibit dramatically improved electrocatalytic activity for the hydrogen evolution reaction. Theoretical simulations conducted for this unique heterostructure reveal that the hydrogen evolution is dominated by the proton adsorption step, which can be significantly promoted by introducing sufficient edge active sites. Our study introduces a new morphological engineering strategy to make the pristine MoS 2 layered structures highly competitive earth-abundant catalysts for efficient hydrogen production.
Effect of Nucleophilicity on the Kinetics of CO 2 Insertion into Pincer-Supported Nickel Complexes
The insertion of CO 2 into metal-element σ bonds such as M-H, M-OH, and M-NH 2 is often proposed to be a key step in catalytic CO 2 conversion. Nevertheless, there are few experimental studies measuring the kinetics of CO 2 insertion or comparing insertion into L n M-E (E = H, OH, NH 2 ) complexes with the same ancillary ligands. Here, we use a rapid-mixing stopped-flow instrument to measure the rate of CO 2 insertion into ( tBu PCP)Ni(OH) ( tBu PCP = 2,6-C 6 H 3 (CH 2 P t Bu 2 ) 2 ) and show that it proceeds at a faster rate in comparison to insertion into the corresponding metal hydride. We also demonstrate that the rate of CO 2 insertion into ( tBu PCP)Ni(OH) can generally be increased by using solvents with higher acceptor numbers or by modifying the ligand scaffold to reduce steric bulk or increase electron donation to the metal center. We also attempt to measure the rate of CO 2 insertion into ( tBu PCP)Ni(NH 2 ); however, this reaction is too fast to measure with our methodology. This work enables the direct comparison of CO 2 insertion into analogous L n M-E (E = H, OH, NH 2 ) systems and provides quantitative experimental evidence that the rate of insertion into a metal-element σ bond correlates with the nucleophilicity of E.
Chemically Crushed Wood Cellulose Fiber towards High-Performance Sodium-Ion Batteries
Carbon materials have attracted great interest as an anode for sodium-ion batteries (SIBs) due to their high performance and low cost. Furthermore, we studied natural wood fiber derived hard carbon anodes for SIBs considering the abundance and low cost of wood. We discovered that a thermal carbonization of wood fiber led to a porous carbon with a high specific surface area of 586 m 2 g –1 , while a pretreatment with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) could effectively decrease it to 126 m 2 g –1 . When evaluating them as anodes for SIBs, we observed that the low surface area carbon resulted in a high initial Coulombic efficiency of 72% compared to 25% of the high surface area carbon. More importantly, the low surface area carbon exhibits an excellent cycling stability that a desodiation capacity of 196 mAh g –1 can be delivered over 200 cycles at a current density of 100 mA g –1 , indicating a promising anode for low-cost SIBs.