Band gap opening of metallic single-walled carbon nanotubes via noncovalent symmetry breaking.

Covalent bonding interactions determine the energy-momentum (E-k) dispersion (band structure) of solid-state materials. Here, we show that noncovalent interactions can modulate the E-k dispersion near the Fermi level of a low-dimensional nanoscale conductor. We demonstrate that low energy band gaps may be opened in metallic carbon nanotubes through polymer wrapping of the nanotube surface at fixed helical periodicity. Electronic spectral, chiro-optic, potentiometric, electronic device, and work function data corroborate that the magnitude of band gap opening depends on the nature of the polymer electronic structure. Polymer dewrapping reverses the conducting-to-semiconducting phase transition, restoring the native metallic carbon nanotube electronic structure. These results address a long-standing challenge to develop carbon nanotube electronic structures that are not realized through disruption of π conjugation, and establish a roadmap for designing and tuning specialized semiconductors that feature band gaps on the order of a few hundred meV.

Photoelectrochemical Proton-Coupled Electron Transfer of TiO2 Thin Films on Silicon.

TiO2 thin films are often used as protective layers on semiconductors for applications in photovoltaics, molecule-semiconductor hybrid photoelectrodes, and more. Experiments reported here show that TiO2 thin films on silicon are electrochemically and photoelectrochemically reduced in buffered acetonitrile at potentials relevant to photoelectrocatalysis of CO2 reduction, N2 reduction, and H2 evolution. On both n-type Si and irradiated p-type Si, TiO2 reduction is proton-coupled with a 1e-:1H+ stoichiometry, as demonstrated by the Nernstian dependence of the Ti4+/3+ E1/2 on the buffer pKa. Experiments were conducted with and without illumination, and a photovoltage of ∼0.6 V was observed across 20 orders of magnitude in proton activity. The 4 nm films are almost stoichiometrically reduced under mild conditions. The reduced films catalytically transfer protons and electrons to hydrogen atom acceptors, based on cyclic voltammogram, bulk electrolysis, and other mechanistic evidence. TiO2/Si thus has the potential to photoelectrochemically generate high-energy H atom carriers. Characterization of the TiO2 films after reduction reveals restructuring with the formation of islands, rendering TiO2 films as a potentially poor choice as protecting films or catalyst supports under reducing and protic conditions. Overall, this work demonstrates that atomic layer deposition TiO2 films on silicon photoelectrodes undergo both chemical and morphological changes upon application of potentials only modestly negative of RHE in these media. While the results should serve as a cautionary tale for researchers aiming to immobilize molecular monolayers on "protective" metal oxides, the robust proton-coupled electron transfer reactivity of the films introduces opportunities for the photoelectrochemical generation of reactive charge-carrying mediators.

Amorphous K-Co-Mo-Sx Chalcogel: A Synergy of Surface Sorption and Ion-Exchange.

Chalcogel represents a unique class of meso- to macroporous nanomaterials that offer applications in energy and environmental pursuits. Here, the synthesis of an ion-exchangeable amorphous chalcogel using a nominal composition of K2CoMo2S10 (KCMS) at room temperature is reported. Synchrotron X-ray pair distribution function (PDF), X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) reveal a plausible local structure of KCMS gel consisting of Mo5+ 2 and Mo4+ 3 clusters in the vicinity of di/polysulfides which are covalently linked by Co2+ ions. The ionically bound K+ ions remain in the percolating pores of the Co-Mo-S covalent network. XANES of Co K-edge shows multiple electronic transitions, including quadrupole (1s→3d), shakedown (1s→4p + MLCT), and dipole allowed 1s→4p transitions. Remarkably, despite a lack of regular channels as in some crystalline solids, the amorphous KCMS gel shows ion-exchange properties with UO2 2+ ions. Additionally, it also presents surface sorption via [S∙∙∙∙UO2 2+] covalent interactions. Overall, this study underscores the synthesis of quaternary chalcogels incorporating alkali metals and their potential to advance separation science for cations and oxo-cationic species by integrating a synergy of surface sorption and ion-exchange.

Surface-Doped Zinc Gallate Colloidal Nanoparticles Exhibit pH-Dependent Radioluminescence with Enhancement in Acidic Media.

As abnormal acidic pH symbolizes dysfunctions of cells, it is highly desirable to develop pH-sensitive luminescent materials for diagnosing disease and imaging-guided therapy using high-energy radiation. Herein, we explored near-infrared-emitting Cr-doped zinc gallate ZnGa2O4 nanoparticles (NPs) in colloidal solutions with different pH levels under X-ray excitation. Ultrasmall NPs were synthesized via a facile hydrothermal method by controlling the addition of ammonium hydroxide precursor and reaction time, and structural characterization revealed Cr dopants on the surface of NPs. The synthesized NPs exhibited different photoluminescence and radioluminescence mechanisms, confirming the surface distribution of activators. It was observed that the colloidal NPs emit pH-dependent radioluminescence in a linear relationship, and the enhancement reached 4.6-fold when pH = 4 compared with the colloidal NPs in the neutral solution. This observation provides a strategy for developing new biomaterials by engineering activators on the nanoparticle surfaces for potential pH-sensitive imaging and imaging-guided therapy using high-energy radiation.

Superradiance and Directional Exciton Migration in Metal-Organic Frameworks.

Crystalline metal-organic frameworks (MOFs) are promising synthetic analogues of photosynthetic light-harvesting complexes (LHCs). The precise assembly of linkers (organic chromophores) around the topology-defined pores offers the evolution of unique photophysical behaviors that are reminiscence of LHCs. These include MOF excited states with photoabsorbed energy that is spatially dispersed over multiple linkers defining the molecular excitons. The multilinker molecular excitons display superradiance─a hallmark of coupled oscillators seen in LHCs─with radiative rate constant (krad) exceeding that of a single linker. Our theoretical model and experimental results on three zirconium MOFs, namely, PCN-222(Zn), NU-1000, and SIU-100, with similar topology but varying linkers suggest that the size of such molecular excitons depends on the electronic symmetry of the linker. This multilinker exciton model effectively predicts the energy transfer rate constant; corresponding single-step exciton hopping time, ranging from a few picoseconds in SIU-100 and NU-1000 to a few hundreds of picoseconds in PCN-222(Zn), matches well with the experimental data. The model also predicts the anisotropy of exciton displacement with preferential migration along the crystallographic c-axis. Overall, these findings establish various missing links defining the exciton size and dynamics in MOF-assembled linkers. The understandings will provide design principles, especially, positioning the catalysts or electrode relative to the linker orientation for low-density solar energy conversion systems.

How to reprogram the excitonic properties and solid-state morphologies of π-conjugated supramolecular polymers.

The development of supramolecular tools to modulate the excitonic properties of non-covalent assemblies paves the way to engineer new classes of semicondcuting materials relevant to flexible electronics. While controlling the assembly pathways of organic chromophores enables the formation of J-like and H-like aggregates, strategies to tailor the excitonic properties of pre-assembled aggregates through post-modification are scarce. In the present contribution, we combine supramolecular chemistry with redox chemistry to modulate the excitonic properties and solid-state morphologies of aggregates built from stacks of water-soluble perylene diimide building blocks. The n-doping of initially formed aggregates in an aqueous medium is shown to produce π-anion stacks for which spectroscopic properties unveil a non-negligible degree of electron-electron interactions. Oxidation of the n-doped intermediates produces metastable aggregates where free exciton bandwidths (ExBW) increase as a function of time. Kinetic data analysis reveals that the dynamic increase of free exciton bandwidth is associated with the formation of superstructures constructed by means of a nucleation-growth mechanism. By designing different redox-assisted assembly pathways, we highlight that the sacrificial electron donor plays a non-innocent role in regulating the structure-function properties of the final superstructures. Furthermore, supramolecular architectures formed via a nucleation-growth mechanism evolve into ribbon-like and fiber-like materials in the solid-state, as characterized by SEM and HRTEM. Through a combination of ground-state electronic absorption spectroscopy, electrochemistry, spectroelectrochemistry, microscopy, and modeling, we show that redox-assisted assembly provides a means to reprogram the structure-function properties of pre-assembled aggregates.

Synthesis of Core@Shell Cu-Ni@Pt-Cu Nano-Octahedra and Their Improved MOR Activity.

Fabrication of 3d metal-based core@shell nanocatalysts with engineered Pt-surfaces provides an effective approach for improving the catalytic performance. The challenges in such preparation include shape control of the 3d metallic cores and thickness control of the Pt-based shells. Herein, we report a colloidal seed-mediated method to prepare octahedral CuNi@Pt-Cu core@shell nanocrystals using CuNi octahedral cores as the template. By precisely controlling the synthesis conditions including the deposition rate and diffusion rate of the shell-formation through tuning the capping ligand, reaction temperature, and heating rate, uniform Pt-based shells can be achieved with a thickness of <1 nm. The resultant carbon-supported CuNi@Pt-Cu core@shell nano-octahedra showed superior activity in electrochemical methanol oxidation reaction (MOR) compared with the commercial Pt/C catalysts and carbon-supported CuNi@Pt-Cu nano-polyhedron counterparts.

Imaging Excited State Dynamics in Layered 2D Perovskites with Transient Absorption Microscopy.

Two-dimensional (2D) hybrid perovskites are generating broad scientific interest because of their potential for use in photovoltaics and microcavity lasers. It has recently been demonstrated that mixtures of quantum wells with different thicknesses can be assembled in films with heterogeneous quantum well distributions. Large (small) quantum wells are concentrated at the air side (substrate side) of the films, thereby promoting directional energy and/or electron transfer. However, profiles of the quantum well concentrations have not been directly measured throughout the full thicknesses of the films. Similarly, the lateral motions of the excitations in these systems are not well-characterized. In this work, we perform focused ion beam milling tests to establish quantum well concentrations as a function of depth in layered 2D perovskite films. In addition, transient absorption microscopy is used to investigate carrier diffusion and two-body recombination processes. Comparisons of the layered films with phase-pure single crystals reveal that diffusion is suppressed by grain boundaries in the films, which in turn promotes two-body recombination. Similar behaviors were previously observed in bulk perovskite films and single crystals. These studies suggest that the morphology of the film, rather than the identity of the material, is the primary factor that governs the two-body recombination dynamics. Enhancement of the two-body recombination processes is desirable for applications such as microcavity lasers.

Self-Catalyzed Vapor-Liquid-Solid Growth of Lead Halide Nanowires and Conversion to Hybrid Perovskites.

Lead halide perovskites (LHPs) have shown remarkable promise for use in photovoltaics, photodetectors, light-emitting diodes, and lasers. Although solution-processed polycrystalline films are the most widely studied morphology, LHP nanowires (NWs) grown by vapor-phase processes offer the potential for precise control over crystallinity, phase, composition, and morphology. Here, we report the first demonstration of self-catalyzed vapor-liquid-solid (VLS) growth of lead halide (PbX2; X = Cl, Br, or I) NWs and conversion to LHP. We present a kinetic model of the PbX2 NW growth process in which a liquid Pb catalyst is supersaturated with halogen X through vapor-phase incorporation of both Pb and X, inducing growth of a NW. For PbI2, we show that the NWs are single-crystalline, oriented in the ⟨1̅21̅0⟩ direction, and composed of a stoichiometric PbI2 shaft with a spherical Pb tip. Low-temperature vapor-phase intercalation of methylammonium iodide converts the NWs to methylammonium lead iodide (MAPbI3) perovskite while maintaining the NW morphology. Single-NW experiments comparing measured extinction spectra with optical simulations show that the NWs exhibit a strong optical antenna effect, leading to substantially enhanced scattering efficiencies and to absorption efficiencies that can be more than twice that of thin films of the same thickness. Further development of the self-catalyzed VLS mechanism for lead halide and perovskite NWs should enable the rational design of nanostructures for various optoelectronic technologies, including potentially unique applications such as hot-carrier solar cells.

Unambiguous Diagnosis of Photoinduced Charge Carrier Signatures in a Stoichiometrically Controlled Semiconducting Polymer-Wrapped Carbon Nanotube Assembly.

Single-walled carbon nanotube (SWNT)-based nanohybrid compositions based on (6,5) chirality-enriched SWNTs ([(6,5) SWNTs]) and a chiral n-type polymer (S-PBN(b)-Ph4 PDI) that exploits a perylenediimide (PDI)-containing repeat unit are reported; S-PBN(b)-Ph4 PDI-[(6,5) SWNT] superstructures feature a PDI electron acceptor unit positioned at 3 nm intervals along the nanotube surface, thus controlling rigorously SWNT-electron acceptor stoichiometry and organization. Potentiometric studies and redox-titration experiments determine driving forces for photoinduced charge separation (CS) and thermal charge recombination (CR) reactions, as well as spectroscopic signatures of SWNT hole polaron and PDI radical anion (PDI(-.) ) states. Time-resolved pump-probe spectroscopic studies demonstrate that S-PBN(b)-Ph4 PDI-[(6,5) SWNT] electronic excitation generates PDI(-.) via a photoinduced CS reaction (τCS ≈0.4 ps, ΦCS ≈0.97). These experiments highlight the concomitant rise and decay of transient absorption spectroscopic signatures characteristic of the SWNT hole polaron and PDI(-.) states. Multiwavelength global analysis of these data provide two charge-recombination time constants (τCR ≈31.8 and 250 ps) that likely reflect CR dynamics involving both an intimately associated SWNT hole polaron and PDI(-.) charge-separated state, and a related charge-separated state involving PDI(-.) and a hole polaron site produced via hole migration along the SWNT backbone that occurs over this timescale.

Mo3S13 Chalcogel: A High-Capacity Electrode for Conversion-Based Li-Ion Batteries.

Despite large theoretical energy densities, metal-sulfide electrodes for energy storage systems face several limitations that impact the practical realization. Here, we present the solution-processable, room temperature (RT) synthesis, local structures, and application of a sulfur-rich Mo3S13 chalcogel as a conversion-based electrode for lithium-sulfide batteries (LiSBs). The structure of the amorphous Mo3S13 chalcogel is derived through operando Raman spectroscopy, synchrotron X-ray pair distribution function (PDF), X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) analysis, along with ab initio molecular dynamics (AIMD) simulations. A key feature of the three-dimensional (3D) network is the connection of Mo3S13 units through S-S bonds. Li/Mo3S13 half-cells deliver initial capacity of 1013 mAh g-1 during the first discharge. After the activation cycles, the capacity stabilizes and maintains 312 mAh g-1 at a C/3 rate after 140 cycles, demonstrating sustained performance over subsequent cycling. Such high-capacity and stability are attributed to the high density of (poly)sulfide bonds and the stable Mo-S coordination in Mo3S13 chalcogel. These findings showcase the potential of Mo3S13 chalcogels as metal-sulfide electrode materials for LiSBs.

Single-handed helical wrapping of single-walled carbon nanotubes by chiral, ionic, semiconducting polymers.

We establish the requisite design for aryleneethynylene polymers that give rise to single-handed helical wrapping of single-walled carbon nanotubes (SWNTs). Highly charged semiconducting polymers that utilize either an (R)- or (S)-1,1'-bi-2-naphthol component in their respective conjugated backbones manifest HRTEM and AFM images of single-chain-wrapped SWNTs that reveal significant preferences for the anticipated helical wrapping handedness; statistical analysis of these images, however, indicates that ∼20% of the helical structures are formed with the "unexpected" handedness. CD spectroscopic data, coupled with TDDFT-based computational studies that correlate the spectral signatures of semiconducting polymer-wrapped SWNT assemblies with the structural properties of the chiral 1,1'-binaphthyl unit, suggest strongly that two distinct binaphthalene SWNT binding modes, cisoid-facial and cisoid-side, are possible for these polymers, with the latter mode responsible for inversion of helical chirality and the population of polymer-SWNT superstructures that feature the unexpected polymer helical wrapping chirality at the nanotube surface. Analogous aryleneethynylene polymers were synthesized that feature a 2,2'-(1,3-benzyloxy)-bridged (b)-1,1'-bi-2-naphthol unit: this 1,1'-bi-2-naphthol derivative is characterized by a bridging 2,2'-1,3 benzyloxy tether that restricts the torsional angle between the two naphthalene subunits along its C1-C1' chirality axis to larger, oblique angles that facilitate more extensive van der Waals contact of the naphthyl subunits with the nanotube. Similar microscopic, spectroscopic, and computational studies determine that chiral polymers based on conformationally restricted transoid binaphthyl units direct preferential facial binding of the polymer with the SWNT and thereby guarantee helically wrapped polymer-nanotube superstructures of fixed helical chirality. Molecular dynamics simulations provide an integrated picture tying together the global helical superstructure and conformational properties of the binaphthyl units: a robust, persistent helical handedness is preferred for the conformationally restricted transoid binaphthalene polymer. Further examples of similar semiconducting polymer-SWNT superstructures are reported that demonstrate that the combination of single-handed helical wrapping and electronic structural modification of the conjugated polymer motif opens up new opportunities for engineering the electro-optic functionality of nanoscale objects.

Ionic self-assembly provides dense arrays of individualized, aligned single-walled carbon nanotubes.

Wrap and stack: Polyanionic [arylene]ethynylene polymers that helically wrap single-walled carbon nanotubes (SWNTs) enable the production of functionalized SWNTs that are soluble in organic solvents. These SWNTs can assemble into structures featuring aligned nanotubes that maintain the optoelectronic properties of individual SWNTs.

Reversible Kirkwood-Alder transition observed in Pt3Cu2 nanoctahedron assemblies under controlled solvent annealing/drying conditions.

We report a Kirkwood-Alder transition in a system of nonspherical Pt(3)Cu(2) nanoctahedra coated with oleic acid and oleylamine ligands. Using both transmission electron microscopy tomography with 3D reconstruction analysis and synchrotron-based in-situ grazing-incidence small-angle X-ray scattering (GISAXS) techniques, we specifically determined that these nanoctahedra can assemble into an open structure in which the nanoctahedra are arranged tip-to-tip to form a bcc superlattice with a low packing efficiency. Using in-situ and real-time GISAXS, we further observed a "nanoctahedron crystallization" as a soft Kirkwood-Alder transition, that is, the soft nanoactahedra crystallize at a critical concentration and possess continuous crystalline states during a period of solvent evaporation. Finally, we found a reversible change of the superlattice constant during the solvent annealing and evaporation/drying processes.

Mesoporous carbon/zirconia composites: a potential route to chemically functionalized electrically-conductive mesoporous materials.

Mesoporous nanocomposite materials in which nanoscale zirconia (ZrO(2)) particles are embedded in the carbon skeleton of a templated mesoporous carbon matrix were prepared, and the embedded zirconia sites were used to accomplish chemical functionalization of the interior surfaces of mesopores. These nanocomposite materials offer a unique combination of high porosity (e.g., ∼84% void space), electrical conductivity, and surface tailorability. The ZrO(2)/carbon nanocomposites were characterized by thermogravimetric analysis, nitrogen-adsorption porosimetry, helium pychnometry, powder X-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. Comparison was made with templated mesoporous carbon samples prepared without addition of ZrO(2). Treatment of the nanocomposites with phenylphosphonic acid was undertaken and shown to result in robust binding of the phosphonic acid to the surface of ZrO(2) particles. Incorporation of nanoscale ZrO(2) surfaces in the mesoporous composite skeleton offers unique promise as a means for anchoring organophosphonates inside of pores through formation of robust covalent Zr-O-P bonds.

Low packing density self-assembled superstructure of octahedral Pt3Ni nanocrystals.

We present a structural study of Pt(3)Ni nanoctahedron superlattice, prepared through both drop-casting and controlled solvent evaporation approaches. In this superlattice system containing ∼10.6 nm side-length Pt(3)Ni nanoctahedra, we observed a body-centered cubic (bcc) packing structure in both local superlattices and statistically averaged superlattice ensembles using transmission electron microscopic tomography and grazing-incidence small-angle X-ray scattering techniques, respectively. Within the superstructure, it was directly observed that nanoctahedra are orientated along the superstructure axes through sharing their vertices. We found that this arrangement of a bcc superstructure with nanoctahedra connecting through their vertices is dependent on neither the processing pathway nor the substrate under our experimental conditions. With such a very low packing density and ultrahigh surface area, this type of self-organized superstructure possesses unique features for future applications.

Nonaqueous synthesis and photoluminescence of ITO nanoparticles.

SnO(2) has successfully been doped into octahedral In(2)O(3) nanoparticles using a high-temperature nonaqueous reaction. The resultant ITO nanoparticles exhibit a particle/crystal decrease in size, sphericity in morphology, and enhancement in photoluminescence.

Irreversible Shear-Activated Gelation of a Liquid Crystalline Polyelectrolyte.

We report irreversible, shear-activated gelation in liquid crystalline solutions of a rigid polyelectrolyte that forms rodlike assemblies (rods) in salt-free solution. At rest, the liquid crystalline solutions are kinetically stable against gelation and exhibit low viscosities. Under steady shear at, or above, a critical shear rate, a physically cross-linked, nematic gel network forms due to linear growth and branching of the rods. Above a critical shear rate, the time scale of gelation can be tuned from hours to nearly instantaneously by varying the shear rate and solution concentration. The shear-activated gels are distinct in their structure and rheological properties from thermoreversible gels. At a fixed concentration, the induction time prior to gelation decreases exponentially with the shear rate. This result indicates that shear-activated thermalization of the electrostatically stabilized rods overcomes the energy barrier for rod-rod contact, enabling rod fusion and subsequent irreversible network formation.

A carbon nano-onion-ferrocene donor-acceptor system: synthesis, characterization and properties.

We describe the synthesis and characterization of a novel ferrocene-carbon onion derivative, where ferrocene acts as an electron-donating moiety, while the carbon nano-onion (CNO) serves as the electron acceptor. CNOs were functionalized by 1,3-dipolar cycloaddition and the resulting products were characterized by transmission electron microscopy, thermogravimetric analysis, Raman and energy dispersive spectroscopies. The electronic properties of the Fc-CNO derivative were investigated by electrochemical and photophysical techniques, complemented by quantum chemical calculations. On average, the CNOs have a spherical appearance with six shells. Functionalization saturates one carbon atom in 36 carbon atoms on the outer cage of the CNO. Through-space interactions between the Fc moiety and the CNO core were detected electrochemically. Fluorescence was observed at low and high energies with an intrinsic decay that is faster at lower energies. Based on theory and experiment, we conclude that, after absorption of a photon at low energy, there is emission from CNOs characterized by larger external shells and a lower degree of functionalization. At high energy, emission comes from CNOs with smaller external shells and a higher degree of functionalization.

Nanofibrillar Ionic Polymer Composites Enable High-Modulus Ion-Conducting Membranes.

Polymer electrolyte membranes (PEMs) with high volume fractions of ionic liquids (IL) and high modulus show promise for enabling next-generation gas separations, and electrochemical energy storage and conversion applications. Herein, we present a conductive polymer-IL composite based on a sulfonated all-aromatic polyamide (sulfo-aramid, PBDT) and a model IL, which we term a PBDT-IL composite. The polymer forms glassy and high-aspect-ratio hierarchical nanofibrils, which enable fabrication of PEMs with both high volume fractions of IL and high elastic modulus. We report direct evidence for nanofibrillar networks that serve as matrices for dispersed ILs using atomic force microscopy and small- and wide-angle X-ray scattering. These supramolecular nanofibrils form through myriad noncovalent interactions to produce a physically cross-linked glassy network, which boasts the best combination of room-temperature modulus (0.1-2 GPa) and ionic conductivity (8-4 mS cm-1) of any polymer-IL electrolyte reported to date. The ultrahigh thermomechanical properties of our PBDT-IL composites provide high moduli (∼1 GPa) at temperatures up to 200 °C, enabling a wide device operation window with stable mechanical properties. Together, the high-performance nature of sulfo-aramids in concert with the inherent properties of ILs imparts PBDT-IL composites with nonflammability and thermal stability up to 350 °C. Thus, nanofibrillar ionic networks based on sulfo-aramids and ILs represent a new design paradigm affording PEMs with exceptionally high moduli at exceedingly low polymer concentrations. This new design strategy will drive the development of new high-performance conductive membranes that can be used for the design of gas separation membranes and in electrochemical applications, such as fuel cells and Li-metal batteries.