FTIR Spectroscopy Study on Rapid Polysulfide Binding via Caffeine in Lithium-Sulfur Batteries.

Lithium-sulfur batteries are limited by the high mobility of polysulfides in the electrolyte, which allows them to migrate from the cathode to the lithium anode. This is known as polysulfide shuttling and simultaneously diminishes the active material and poisons the anode. Various cathode additives have been shown empirically to mitigate this problem, although the mechanism is not often ascertained experimentally.

Continuously tunable negative pressure for engineering high-symmetry nanocrystalline phases.

In this work, the phenomenon of strain induced by a mismatch in thermal expansion coefficients between a thin film and its substrate is harnessed in a new context, replacing the canonical planar support with a three-dimensional (3-D), nanoconfining scaffold in which we embed a material of interest. In this manner, we demonstrate a general approach to exert a continuously tunable, triaxial, tensile strain, defying the Poisson ratio of the embedded material and achieving the exotic condition of "negative pressure." This approach is hypothetically generalizable to materials of low modulus and high thermal expansion coefficient, and we use it here to achieve negative pressure in perovskite-phase CsPbI embedded within the cylindrical pores of anodic aluminum oxide membranes.

Retrograde Solubility of Methylammonium Lead Iodide in γ-Butyrolactone Does Not Enhance the Uniformity of Continuously Coated Films.

Halide perovskite thin films can be the centerpiece of high-performance solar cells, light-emitting diodes, and other optoelectronic devices if the films are of high uniformity and relatively free of pinholes and other defects. A common strategy to form dense films from solution has been to generate a high density of nuclei by rapidly increasing supersaturation, for example, by timely application of an antisolvent or forced convection. In this work, we examine the role of retrograde solubility, wherein solubility decreases with increasing temperature, as a means of increasing the nucleation density and film coverage of slot-die-coated methylammonium lead iodide (MAPbI) from γ-butyrolactone (GBL) solution.

Quasi-2D Model to Predict Solid Microstructure in Drying Thin Films.

The microstructure of solid coatings produced by solution processing is highly dependent on the coupling between growth, solute diffusion, and solvent evaporation. Here, a quasi-2D numerical model coupling drying and solidification is used to predict the transient lateral growth of two adjacent nuclei growing toward each other. Lateral gradients of the solute and solvent influence the evolution of film thickness and solid growth rate.

Interfacial Topochemical Fluoridation of MAPbI by Fluoropolymers.

Herein it is demonstrated that, under conditions relevant to perovskite synthesis (>140 °C in air), fluoride can topochemically react across the interface between a halide perovskite and a fluoropolymer when in close contact, thereby creating a small quantity of strongly bonded lead fluoride species. The quantity increases with temperature and processing duration. Photoinduced charge carrier lifetime provides a metric for the resulting changes in electronic structure of the perovskite.

Fate and Exposure Assessment of Pb Leachate from Hypothetical Breakage Events of Perovskite Photovoltaic Modules.

Emerging lead halide perovskite (LHP) photovoltaics are undergoing intense research and development due to their outstanding efficiency and potential for low manufacturing costs that render them competitive with existing photovoltaic (PV) technologies. While today's efforts are focused on stability and scalability of LHPs, the toxicity of lead (Pb) remains a major challenge to their large-scale commercialization. Here, we present a screening-level, EPA-compliant model of fate and transport of Pb leachate in groundwater, soil, and air, following hypothetical catastrophic breakage of LHP PV modules in conceptual utility-scale sites.

Selective Enhancement of Swine Myocardium with a Novel Ultrasound Enhancing Agent During Transthoracic Echocardiography.

Ultrasound enhancing agents are approved to delineate the endocardial border and opacify the left ventricle cavity (LVC). We present a nested phase change agent (NPCA) designed to enable selective myocardial enhancement without enhancing the LVC by employing a dual-activation mechanism dependent on sufficient ultrasound intensity and the microenvironment of the myocardium. Swine received bolus injections of NPCA while echocardiograms were collected and processed to determine background-subtracted acoustic intensities (AI) in the LVC and septal myocardium.

An Ultrasound Enhancing Agent with Nonlinear Acoustic Activity that Depends on the Presence of an Electric Field.

The nonlinear acoustic properties of microbubble ultrasound enhancing agents have allowed for the development of subharmonic, second harmonic, and contrast-pulse sequence ultrasound imaging modes, which enhance the quality, reduce the noise, and improve the diagnostic capabilities of clinical ultrasound. This study details acoustic scattering responses of perfluorobutane (PFB) microbubbles, an un-nested perfluoropentane (PFP) nanoemulsion, and two nested PFP nanoemulsions-one comprising a negatively charged phospholipid bilayer and another comprising a zwitterionic phospholipid bilayer-when excited at 1 or 2.25 MHz over a peak negative pressure range of 200 kPa to 4 MPa in the absence and presence of a 1-Hz, 1-V/cm electric field.

A Voltage-Sensitive Ultrasound Enhancing Agent for Myocardial Perfusion Imaging in a Rat Model.

Echocardiographers with specialized expertise sometimes perform myocardial perfusion imaging using U.S. Food and Drug Administration-approved microbubbles in an off-label capacity, correlating microbubble replenishment in the near field with blood flow through the myocardium.

Designing Strong Optical Absorbers Continuous Tuning of Interparticle Interaction in Colloidal Gold Nanocrystal Assemblies.

We program the optical properties of colloidal Au nanocrystal (NC) assemblies an unconventional ligand hybridization (LH) strategy to precisely engineer interparticle interactions and design materials with optical properties difficult or impossible to achieve in bulk form. Long-chain hydrocarbon ligands used in NC synthesis are partially exchanged, from 0% to 100%, with compact thiocyanate ligands by controlling the reaction time for exchange. The resulting NC assemblies show transmittance, reflectance, optical permittivity, and direct-current (DC) resistivity that continuously traverse a dielectric-metal transition, providing analog tuning of their physical properties, unlike the digital control realized by complete exchange with ligands of varying length.

Influence of Compact, Inorganic Surface Ligands on the Electrophoretic Deposition of Semiconductor Nanocrystals at Low Voltage.

For electrophoretic deposition (EPD) to achieve its potential as a method for assembling functional semiconductors, it will be necessary to understand both what governs the threshold voltage for deposition and how to reduce that threshold. Herein we demonstrate that postsynthetic modification of the surface chemistry of all-inorganic copper zinc tin sulfide (CZTS) nanocrystals (NCs) enables EPD at voltages of as low as 4 V, which is a 3-fold or greater reduction over previous examples of nonoxide semiconductors. The chemical exchange of the original surfactant-based NC-surface ligands with selenide ions yields essentially bare, highly surface-charged NCs.

Quantitative Phase-Change Thermodynamics and Metastability of Perovskite-Phase Cesium Lead Iodide.

The perovskite phase of cesium lead iodide (α-CsPbI or "black" phase) possesses favorable optoelectronic properties for photovoltaic applications. However, the stable phase at room temperature is a nonfunctional "yellow" phase (δ-CsPbI). Black-phase polycrystalline thin films are synthesized above 330 °C and rapidly quenched to room temperature, retaining their phase in a metastable state.

High Chloride Doping Levels Stabilize the Perovskite Phase of Cesium Lead Iodide.

Cesium lead iodide possesses an excellent combination of band gap and absorption coefficient for photovoltaic applications in its perovskite phase. However, this is not its equilibrium structure under ambient conditions. In air, at ambient temperature it rapidly transforms to a nonfunctional, so-called yellow phase.

Ultrafast electron trapping in ligand-exchanged quantum dot assemblies.

We use time-integrated and time-resolved photoluminescence and absorption to characterize the low-temperature optical properties of CdSe quantum dot solids after exchanging native aliphatic ligands for thiocyanate and subsequent thermal annealing. In contrast to trends established at room temperature, our data show that at low temperature the band-edge absorptive bleach is dominated by 1S3/2h hole occupation in the quantum dot interior. We find that our ligand treatments, which bring enhanced interparticle coupling, lead to faster surface state electron trapping, a greater proportion of surface-related photoluminescence, and decreased band-edge photoluminescence lifetimes.

Gate-induced carrier delocalization in quantum dot field effect transistors.

We study gate-controlled, low-temperature resistance and magnetotransport in indium-doped CdSe quantum dot field effect transistors. We show that using the gate to accumulate electrons in the quantum dot channel increases the "localization product" (localization length times dielectric constant) describing transport at the Fermi level, as expected for Fermi level changes near a mobility edge. Our measurements suggest that the localization length increases to significantly greater than the quantum dot diameter.

Air-stable, nanostructured electronic and plasmonic materials from solution-processable, silver nanocrystal building blocks.

Herein we describe a room-temperature, chemical process to transform silver nanocrystal solids, deposited from colloidal solutions, into highly conductive, corrosion-resistant, optical and electronic materials with nanometer-scale architectures. After assembling the nanocrystal solids, we treated them with a set of simple, compact, organic and inorganic reagents: ammonium thiocyanate, ammonium chloride, potassium hydrogen sulfide, and ethanedithiol. We find that each reagent induces unique changes in the structure and composition of the resulting solid, giving rise to films that vary from insulating to, in the case of thiocyanate, conducting with a remarkably low resistivity of 8.

Solution-based stoichiometric control over charge transport in nanocrystalline CdSe devices.

Using colloidal CdSe nanowire (NW) field-effect transistors (FETs), we demonstrated the dependence of carrier transport on surface stoichiometry by chemically manipulating the atomic composition of the NW surface. A mild, room-temperature, wet-chemical process was devised to introduce cadmium, selenium, or sulfur adatoms at the surface of the NWs in completed devices. Changes in surface composition were tested for by energy dispersive spectroscopy and inductively coupled plasma-atomic emission spectroscopy and through the use of the vibrational reporter thiocyanate.

In situ repair of high-performance, flexible nanocrystal electronics for large-area fabrication and operation in air.

Colloidal semiconductor nanocrystal (NC) thin films have been integrated in light-emitting diodes, solar cells, field-effect transistors (FETs), and flexible, electronic circuits. However, NC devices are typically fabricated and operated in an inert environment since the reactive surface and high surface-to-volume ratio of NC materials render them sensitive to oxygen, water, and many solvents. This sensitivity has limited device scaling and large-scale device integration achievable by conventional fabrication technologies, which generally require ambient air and wet-chemical processing.

Quantitative dissection of hydrogen bond-mediated proton transfer in the ketosteroid isomerase active site.

Hydrogen bond networks are key elements of protein structure and function but have been challenging to study within the complex protein environment. We have carried out in-depth interrogations of the proton transfer equilibrium within a hydrogen bond network formed to bound phenols in the active site of ketosteroid isomerase. We systematically varied the proton affinity of the phenol using differing electron-withdrawing substituents and incorporated site-specific NMR and IR probes to quantitatively map the proton and charge rearrangements within the network that accompany incremental increases in phenol proton affinity.

Chemically tailored dielectric-to-metal transition for the design of metamaterials from nanoimprinted colloidal nanocrystals.

We demonstrate optical metamaterial design using colloidal gold nanocrystal building blocks. In the solid state, chemically exchanging the nanocrystals' surface-capping molecules provides a tailorable dielectric-to-metal transition exhibiting a 10(10) range in DC conductivity and dielectric permittivity ranging from everywhere positive to everywhere negative throughout the visible-to-near-IR. Direct, wide-area nanoimprinting of subwavelength superstructures at room temperature, on plastic and glass substrates, affords plasmonic resonances ranging from 660 to 1070 nm, in agreement with numerical simulations.

Metal-enhanced upconversion luminescence tunable through metal nanoparticle-nanophosphor separation.

We have demonstrated amplification of luminescence in upconversion nanophosphors (UCNPs) of hexagonal phase NaYF(4) (β-NaYF(4)) doped with the lanthanide dopants Yb(3+), Er(3+) or Yb(3+), Tm(3+) by close proximity to metal nanoparticles (NPs). We present a configuration in which close-packed monolayers of UCNPs are separated from a dense multilayer of metal NPs (Au or Ag) by a nanometer-scale oxide grown by atomic layer deposition. Luminescence enhancements were found to be dependent on the thickness of the oxide spacer layer and the type of metal NP with enhancements of up to 5.

Bandlike transport in strongly coupled and doped quantum dot solids: a route to high-performance thin-film electronics.

We report bandlike transport in solution-deposited, CdSe QD thin-films with room temperature field-effect mobilities for electrons of 27 cm(2)/(V s). A concomitant shift and broadening in the QD solid optical absorption compared to that of dispersed samples is consistent with electron delocalization and measured electron mobilities. Annealing indium contacts allows for thermal diffusion and doping of the QD thin-films, shifting the Fermi energy, filling traps, and providing access to the bands.

Quantitative, directional measurement of electric field heterogeneity in the active site of ketosteroid isomerase.

Understanding the electrostatic forces and features within highly heterogeneous, anisotropic, and chemically complex enzyme active sites and their connection to biological catalysis remains a longstanding challenge, in part due to the paucity of incisive experimental probes of electrostatic properties within proteins. To quantitatively assess the landscape of electrostatic fields at discrete locations and orientations within an enzyme active site, we have incorporated site-specific thiocyanate vibrational probes into multiple positions within bacterial ketosteroid isomerase. A battery of X-ray crystallographic, vibrational Stark spectroscopy, and NMR studies revealed electrostatic field heterogeneity of 8 MV/cm between active site probe locations and widely differing sensitivities of discrete probes to common electrostatic perturbations from mutation, ligand binding, and pH changes.

Thiocyanate-capped PbS nanocubes: ambipolar transport enables quantum dot based circuits on a flexible substrate.

We report the use of thiocyanate as a ligand for lead sulfide (PbS) nanocubes for high-performance, thin-film electronics. PbS nanocubes, self-assembled into thin films and capped with the thiocyanate, exhibit ambipolar characteristics in field-effect transistors. The nearly balanced, high mobilities for electrons and holes enable the fabrication of CMOS-like inverters with promising gains of ∼22 from a single semiconductor material.

Thiocyanate-capped nanocrystal colloids: vibrational reporter of surface chemistry and solution-based route to enhanced coupling in nanocrystal solids.

Ammonium thiocyanate (NH(4)SCN) is introduced to exchange the long, insulating ligands used in colloidal nanocrystal (NC) synthesis. The short, air-stable, environmentally benign thiocyanate ligand electrostatically stabilizes a variety of semiconductor and metallic NCs in polar solvents, allowing solution-based deposition of NCs into thin-film NC solids. NH(4)SCN is also effective in replacing ligands on NCs after their assembly into the solid state.