Showing papers by "Bobby G. Sumpter published in 2017"

PdSe2: Pentagonal Two-Dimensional Layers with High Air Stability for Electronics.

Abstract: Most studied two-dimensional (2D) materials exhibit isotropic behavior due to high lattice symmetry; however, lower-symmetry 2D materials such as phosphorene and other elemental 2D materials exhibit very interesting anisotropic properties. In this work, we report the atomic structure, electronic properties, and vibrational modes of few-layered PdSe2 exfoliated from bulk crystals, a pentagonal 2D layered noble transition metal dichalcogenide with a puckered morphology that is air-stable. Micro-absorption optical spectroscopy and first-principles calculations reveal a wide band gap variation in this material from 0 (bulk) to 1.3 eV (monolayer). The Raman-active vibrational modes of PdSe2 were identified using polarized Raman spectroscopy, and a strong interlayer interaction was revealed from large, thickness-dependent Raman peak shifts, agreeing with first-principles Raman simulations. Field-effect transistors made from the few-layer PdSe2 display tunable ambipolar charge carrier conduction with a high elec...

Low-Frequency Shear and Layer-Breathing Modes in Raman Scattering of Two-Dimensional Materials.

Abstract: Ever since the isolation of single-layer graphene in 2004, two-dimensional layered structures have been among the most extensively studied classes of materials. To date, the pool of two-dimensional materials (2DMs) continues to grow at an accelerated pace and already covers an extensive range of fascinating and technologically relevant properties. An array of experimental techniques have been developed and used to characterize and understand these properties. In particular, Raman spectroscopy has proven to be a key experimental technique, thanks to its capability to identify minute structural and electronic effects in nondestructive measurements. While high-frequency (HF) intralayer Raman modes have been extensively employed for 2DMs, recent experimental and theoretical progress has demonstrated that low-frequency (LF) interlayer Raman modes are more effective at determining layer numbers and stacking configurations and provide a unique opportunity to study interlayer coupling. These advantages are due to...

Big Effect of Small Nanoparticles: A Shift in Paradigm for Polymer Nanocomposites

Abstract: Polymer nanocomposites (PNCs) are important materials that are widely used in many current technologies and potentially have broader applications in the future due to their excellent property tunability, light weight, and low cost. However, expanding the limits in property enhancement remains a fundamental scientific challenge. Here, we demonstrate that well-dispersed, small (diameter ∼1.8 nm) nanoparticles with attractive interactions lead to unexpectedly large and qualitatively different changes in PNC structural dynamics in comparison to conventional nanocomposites based on particles of diameters ∼10–50 nm. At the same time, the zero-shear viscosity at high temperatures remains comparable to that of the neat polymer, thereby retaining good processability and resolving a major challenge in PNC applications. Our results suggest that the nanoparticle mobility and relatively short lifetimes of nanoparticle-polymer associations open qualitatively different horizons in the tunability of macroscopic propertie...

Interfacial Properties of Polymer Nanocomposites: Role of Chain Rigidity and Dynamic Heterogeneity Length Scale

Abstract: While it is known that the properties of polymer nanocomposites are largely dominated by the interfacial layer around nanoparticles, the molecular parameters controlling the interfacial layer structure and dynamics remain unknown. In this work we combine small-angle X-ray scattering, differential scanning calorimetry, and broadband dielectric spectroscopy to analyze the dependence of the interfacial layer thickness, lint, on polymer rigidity defined through the characteristic ratio, C∞. This analysis revealed a value of C∞ ∼ 5–7, beyond which lint increases substantially with C∞. Moreover, lint grows upon approaching the glass transition temperature from above, and the rate of this growth seems to correlate with polymer fragility. Most important, our analysis revealed that lint is comparable to the characteristic length scale of dynamic heterogeneities in the studied materials. These results provide new understandings of molecular parameters controlling the interfacial layer and are important not only for...

Focus: Structure and dynamics of the interfacial layer in polymer nanocomposites with attractive interactions.

Abstract: In recent years it has become clear that the interfacial layer formed around nanoparticles in polymer nanocomposites (PNCs) is critical for controlling their macroscopic properties. The interfacial layer occupies a significant volume fraction of the polymer matrix in PNCs and creates strong intrinsic heterogeneity in their structure and dynamics. Here, we focus on analysis of the structure and dynamics of the interfacial region in model PNCs with well-dispersed, spherical nanoparticles with attractive interactions. First, we discuss several experimental techniques that provide structural and dynamic information on the interfacial region in PNCs. Then, we discuss the role of various microscopic parameters in controlling structure and dynamics of the interfacial layer. The analysis presented emphasizes the importance of the polymer-nanoparticle interactions for the slowing down dynamics in the interfacial region, while the thickness of the interfacial layer appears to be dependent on chain rigidity, and has been shown to increase with cooling upon approaching the glass transition. Aside from chain rigidity and polymer-nanoparticle interactions, the interfacial layer properties are also affected by the molecular weight of the polymer and the size of the nanoparticles. In the final part of this focus article, we emphasize the important challenges in the field of polymer nanocomposites and a potential analogy with the behavior observed in thin films.

Influence of Chain Rigidity and Dielectric Constant on the Glass Transition Temperature in Polymerized Ionic Liquids.

Abstract: Polymerized ionic liquids (PolyILs) are promising candidates for a wide range of technological applications due to their single ion conductivity and good mechanical properties. Tuning the glass transition temperature (Tg) in these materials constitutes a major strategy to improve room temperature conductivity while controlling their mechanical properties. In this work, we show experimental and simulation results demonstrating that in these materials Tg does not follow a universal scaling behavior with the volume of the structural units Vm (including monomer and counterion). Instead, Tg is significantly influenced by the chain flexibility and polymer dielectric constant. We propose a simplified empirical model that includes the electrostatic interactions and chain flexibility to describe Tg in PolyILs. Our model enables design of new functional PolyILs with the desired Tg.

Strain-engineered optoelectronic properties of 2D transition metal dichalcogenide lateral heterostructures

Abstract: Compared with their bulk counterparts, 2D materials can sustain much higher elastic strain at which optical quantities such as bandgaps and absorption spectra governing optoelectronic device performance can be modified with relative ease. Using first-principles density functional theory and quasiparticle GW calculations, we demonstrate how uniaxial tensile strain can be utilized to optimize the electronic and optical properties of transition metal dichalcogenide lateral (in-plane) heterostructures such as MoX2/WX2 (X = S, Se, Te). We find that these lateral-type heterostructures may facilitate efficient electron–hole separation for light detection/harvesting and preserve their type II characteristic up to 12% of uniaxial strain. Based on the strain-dependent bandgap and band offset, we show that uniaxial tensile strain can significantly increase the power conversion efficiency of these lateral heterostructures. Our results suggest that these strain-engineered lateral heterostructures are promising for optimizing optoelectronic device performance by selectively tuning the energetics of the bandgap.

Enhancing Ion Migration in Grain Boundaries of Hybrid Organic–Inorganic Perovskites by Chlorine

Abstract: Ionicity plays an important role in determining material properties, as well as optoelectronic performance of organometallic trihalide perovskites (OTPs). Ion migration in OTP films has recently been under intensive investigation by various scanning probe microscopy (SPM) techniques. However, controversial findings regarding the role of grain boundaries (GBs) associated with ion migration are often encountered, likely as a result of feedback errors and topographic effects common in to SPM. In this work, electron microscopy and spectroscopy (scanning transmission electron microscopy/electron energy loss spectroscopy) are combined with a novel, open-loop, band-excitation, (contact) Kelvin probe force microscopy (BE-KPFM and BE-cKPFM), in conjunction with ab initio molecular dynamics simulations to examine the ion behavior in the GBs of CH3NH3PbI3 perovskite films. This combination of diverse techniques provides a deeper understanding of the differences between ion migration within GBs and interior grains in OTP films. This work demonstrates that ion migration can be significantly enhanced by introducing additional mobile Cl− ions into GBs. The enhancement of ion migration may serve as the first step toward the development of high-performance electrically and optically tunable memristors and synaptic devices.

Controllable conversion of quasi-freestanding polymer chains to graphene nanoribbons.

Abstract: In the bottom-up synthesis of graphene nanoribbons (GNRs) from self-assembled linear polymer intermediates, surface-assisted cyclodehydrogenations usually take place on catalytic metal surfaces. Here we demonstrate the formation of GNRs from quasi-freestanding polymers assisted by hole injections from a scanning tunnelling microscope (STM) tip. While catalytic cyclodehydrogenations typically occur in a domino-like conversion process during the thermal annealing, the hole-injection-assisted reactions happen at selective molecular sites controlled by the STM tip. The charge injections lower the cyclodehydrogenation barrier in the catalyst-free formation of graphitic lattices, and the orbital symmetry conservation rules favour hole rather than electron injections for the GNR formation. The created polymer-GNR intraribbon heterostructures have a type-I energy level alignment and strongly localized interfacial states. This finding points to a new route towards controllable synthesis of freestanding graphitic layers, facilitating the design of on-surface reactions for GNR-based structures.

Aminopolymer functionalization of boron nitride nanosheets for highly efficient capture of carbon dioxide

Abstract: Boron nitrides (BNs) are a class of materials with unique properties that exhibit promise for applications in CO2 capture. However, the surface electron-deficiency of BNs makes their interaction with Lewis acidic CO2 very weak. By utilizing the strong interaction between electron-deficient boron atoms and electron-donating amine groups, BN nanosheets were functionalized with polyethyleneimine (PEI) which is rich in amine density, through simple impregnation to improve their performance for CO2 capture. The important roles of the boron–amine interaction in the incorporation, distribution and stabilization of PEI, as well as the facilitation of CO2 adsorption and desorption were both experimentally and theoretically investigated. It is demonstrated that after functionalization with PEI, the capacity of pure CO2 on BN nanosheets was significantly improved (3.12 mmol g−1 for BN functionalized with 54.9 wt% of PEI vs. 0.29 mmol g−1 for pristine BN at 75 °C). Furthermore, the adsorbed CO2 can be facilely released through N2 purge at 75 °C, and the PEI-functionalized BN nanosheets exhibit high stability throughout consecutive cycles.

UV-activated ZnO films on a flexible substrate for room temperature O 2 and H 2 O sensing.

Abstract: We demonstrate that UV-light activation of polycrystalline ZnO films on flexible polyimide (Kapton) substrates can be used to detect and differentiate between environmental changes in oxygen and water vapor. The in-plane resistive and impedance properties of ZnO films, fabricated from bacteria-derived ZnS nanoparticles, exhibit unique resistive and capacitive responses to changes in O2 and H2O. We propose that the distinctive responses to O2 and H2O adsorption on ZnO could be utilized to statistically discriminate between the two analytes. Molecular dynamic simulations (MD) of O2 and H2O adsorption energy on ZnO surfaces were performed using the large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) with a reactive force-field (ReaxFF). These simulations suggest that the adsorption mechanisms differ for O2 and H2O adsorption on ZnO, and are governed by the surface termination and the extent of surface hydroxylation. Electrical response measurements, using DC resistance, AC impedance spectroscopy, and Kelvin Probe Force Microscopy (KPFM), demonstrate differences in response to O2 and H2O, confirming that different adsorption mechanisms are involved. Statistical and machine learning approaches were applied to demonstrate that by integrating the electrical and kinetic responses the flexible ZnO sensor can be used for detection and discrimination between O2 and H2O at low temperature.

New Insights on Electro-Optical Response of Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) Film to Humidity.

Abstract: Understanding the relative humidity (RH) response of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is critical for improving the stability of organic electronic devices and developing selective sensors. In this work, combined gravimetric sensing, nanoscale surface probing, and mesoscale optoelectronic characterization are used to directly compare the RH dependence of electrical and optical conductivities and unfold connections between the rate of water adsorption and changes in functional properties of PEDOT:PSS film. We report three distinct regimes where changes in electrical conductivity, optical conductivity, and optical bandgap are correlated with the mass of adsorbed water. At low (RH 60%) humidity levels, dramatic changes in electrical, optical, and structural properties occur, while changes are insignificant in mid-RH (25 < RH < 60%) conditions. We associate the three regimes with water adsorption at hydrophilic moieties at low RH, diffusion and swelling throughout the film at mid-RH, and saturation of the film by water at high RH. Optical film thickness increased by 150% as RH was increased from 9 to 80%. Low frequency (1 kHz) impedance increased by ∼100%, and film capacitance increased by ∼30% as RH increased from 9 to 80% due to an increase in the film dielectric constant. Changes in electrical and optical conductivities concomitantly decrease across the full range of RH tested.

Engineering the bacterial microcompartment domain for molecular scaffolding applications

Abstract: As synthetic biology advances the intricacy of engineered biological systems, the importance of spatial organization within the cellular environment must not be marginalized. Increasingly, biological engineers are investigating means to control spatial organization within the cell, mimicking strategies used by natural pathways to increase flux and reduce cross-talk. A modular platform for constructing a diverse set of defined, programmable architectures would greatly assist in improving yields from introduced metabolic pathways and increasing insulation of other heterologous systems. Here, we review recent research on the shell proteins of bacterial microcompartments and discuss their potential application as “building blocks” for a range of customized intracellular scaffolds. We summarize the state of knowledge on the self-assembly of BMC shell proteins and discuss future avenues of research that will be important to realize the potential of BMC shell proteins as predictively assembling and programmable biological materials for bioengineering.

High Conduction Hopping Behavior Induced in Transition Metal Dichalcogenides by Percolating Defect Networks: Toward Atomically Thin Circuits

Abstract: Atomically thin circuits have recently been explored for applications in next-generation electronics and optoelectronics and have been demonstrated with 2D lateral heterojunctions. In order to form true 2D circuitry from a single material, electronic properties must be spatially tunable. Here, tunable transport behavior is reported which is introduced into single layer tungsten diselenide and tungsten disulfide by focused He+ irradiation. Pseudometallic behavior is induced by irradiating the materials with a dose of ≈1 × 1016 He+ cm−2 to introduce defect states, and subsequent temperature-dependent transport measurements suggest a nearest neighbor hopping mechanism is operative. Scanning transmission electron microscopy and electron energy loss spectroscopy reveal that Se is sputtered preferentially, and extended percolating networks of edge states form within WSe2 at a critical dose of 1 × 1016 He+ cm−2. First-principle calculations confirm the semiconductor-to-metallic transition of WSe2 after pore and edge defects are introduced by He+ irradiation. The hopping conduction is utilized to direct-write resistor loaded logic circuits in WSe2 and WS2 with a voltage gain of greater than 5. Edge contacted thin film transistors are also fabricated with a high on/off ratio (>106), demonstrating potential for the formation of atomically thin circuits.

Interlayer bond polarizability model for stacking-dependent low-frequency Raman scattering in layered materials

Abstract: Two-dimensional (2D) layered materials have been extensively studied owing to their fascinating and technologically relevant properties. Their functionalities can be often tailored by the interlayer stacking pattern. Low-frequency (LF) Raman spectroscopy provides a quick, non-destructive and inexpensive optical technique for stacking characterization, since the intensities of LF interlayer vibrational modes are sensitive to the details of the stacking. A simple and generalized interlayer bond polarizability model is proposed here to explain and predict how the LF Raman intensities depend on complex stacking sequences for any thickness in a broad array of 2D materials, including graphene, MoS2, MoSe2, NbSe2, Bi2Se3, GaSe, h-BN, etc. Additionally, a general strategy is proposed to unify the stacking nomenclature for these 2D materials. Our model reveals the fundamental mechanism of LF Raman response to the stacking, and provides general rules for stacking identification.

Triphasic 2D Materials by Vertically Stacking Laterally Heterostructured 2H‐/1T′‐MoS2 on Graphene for Enhanced Photoresponse

Abstract: Recently the applications of 2D materials have been broadened by engineering their mechanical, electronic, and optical properties through either lateral or vertical hybridization. Here, the successful design and fabrication of a novel triphasic 2D material by vertically stacking lateral 2H-/1T′-molybdenum disulfide (MoS2) heterostructures on graphene with the assistance of supercritical carbon dioxide is reported. This triphasic structure is experimentally shown to significantly enhance the photocurrent densities for hydrogen evolution reactions. First-principles theoretical analyses reveal that the improved photoresponse should be ascribed to the beneficial band alignments of the triphasic heterostructure. More specifically, electrons can efficiently hop to the 1T′-MoS2 phase via the highly conductive graphene layer as a result of their strong vertical interfacial electronic coupling. Subsequently, the electrons acquired on the 1T′-MoS2 phase are exploited to fill the photoholes on the photoexcited 2H-MoS2 phase through the lateral heterojunction structure, thereby suppressing the recombination process of the photoinduced charge carriers on the 2H-MoS2 phase. This novel triphasic concept promises to open a new avenue to widen the molecular design of 2D hybrid materials for photonics-based energy conversion applications.

Effects of counterion size and backbone rigidity on the dynamics of ionic polymer melts and glasses

Abstract: It is well-known that the nature and size of the counterions affect the ionic conductivity and glass transition temperature of ionic polymers in a significant manner. However, the microscopic origin of the underlying changes in the dynamics of chains and counterions is far from completely understood. Using coarse-grained molecular dynamics simulations of flexible and semi-flexible ionic polymers, we demonstrate that the glass transition temperature of ionic polymeric melts depends on the size of monovalent counterions in a non-monotonic manner. The glass transition temperature is found to be the highest for the smallest counterions and decreases with an increase in the counterion radii up to a point, after which the glass transition temperature increases with a further increase in the radii. This behavior is because the counterions have significant effects on the coupled dynamics of the charges on the chains and counterions. In particular, increase in the radii of the counterions leads to strongly coupled dynamics between the charges on the chains and the counterions. The static dielectric constant of the polymer melts also has a significant effect on the coupling and the glass transition temperature. The glass transition temperature is predicted to decrease with an increase in the dielectric constant. This, in turn, leads to an increase in the diffusion constant of the counterions at a given temperature. Backbone rigidity is shown to increase the glass transition temperature and decrease the coupling. Furthermore, faster counterion dynamics is predicted for the melts of semi-flexible chains in comparison with flexible chains at the same segmental relaxation time. As the semi-flexible chains tend to have a longer segmental relaxation time, semi-flexible polymers with high dielectric constants are predicted to have diffusion constants of counterions comparable with flexible polymers.

Unraveling the Agglomeration Mechanism in Charged Block Copolymer and Surfactant Complexes

Abstract: We report a molecular dynamics simulation investigation of self-assembly and complex formation of charged–neutral double hydrophilic and hydrophobic–hydrophilic block copolymers (BCP) with oppositely charged surfactants. The structure of the surfactant micelles and the BCP aggregation on the micelle surface is systematically studied for five different BCP volume fractions that also mimics a reduction of the surfactant concentration. The local electrostatic interactions between the oppositely charged species encourage the formation of core–shell structures between the surfactant micelles where the surfactants form the cores and the charged blocks of the BCP form the corona. The emergent morphologies of these aggregates are contingent upon the nature of the BCP neutral blocks. The hydrophilic neutral blocks agglomerate with the micelles as hairy colloidal structures while the hydrophobic neutrals agglomerate in lamellar structures with the surfactant micelles. The distribution of counterion charges along th...

Relevance of the Nuclear Quantum Effects on the Proton/Deuteron Transmission through Hexagonal Boron Nitride and Graphene Monolayers

Abstract: According to recent experiments, atomically thin hexagonal boron nitride and graphene are permeable to protons and deuterons (and not to other atomic species), and the experimental estimates of the activation energy are lower than the theoretical values by about 0.5 eV for the isolated proton–membrane transfer model. Our analysis of the electronic potential energy surfaces along the normal to the transmission direction, obtained using correlated electronic structure methods, suggests that the aqueous environment is essential to stabilize the proton transmission, as opposed to the hydrogen atom. Therefore, the process is examined within a molecular model of H2O–H(D)+–material–H2O. Exact quantum-mechanical scattering calculations are performed to assess the relevance of the nuclear quantum effects, such as tunneling factors and the kinetic isotope effect (KIE). Deuteration is found to affect the thermal reaction rate constants (KIE of 3–4 for hexagonal boron nitride and 20–30 for the graphene) and to effect...

Aminopolymer Mobility and Support Interactions in Silica-PEI Composites for CO2 Capture Applications: A Quasielastic Neutron Scattering Study.

Abstract: Composite gas sorbents, formed from an active polymer phase and a porous support, are promising materials for the separation of acid gases from a variety of gas streams Significant changes in sorption performance (capacity, rate, stability etc) can be achieved by tuning the properties of the polymer and the nature of interactions between polymer and support Here we utilize quasielastic neutron scattering (QENS) and coarse-grained molecular dynamics (MD) simulations to characterize the dynamic behavior of the most commonly reported polymer in such materials, poly(ethylenimine) (PEI), both in bulk form and when supported in a mesoporous silica framework The polymer chain dynamics (rotational and translational diffusion) are characterized using two neutron backscattering spectrometers that have overlapping time scales, ranging from picoseconds to a few nanoseconds Two modes of motion are detected for the PEI molecule in QENS At low energy transfers, a “slow process” on the time scale of ∼200 ps is foun

Effects of partial La filling and Sb vacancy defects on CoS b 3 skutterudites

Abstract: Over the past decade, the open frame (``cagey'') structure of $\mathrm{CoS}{\mathrm{b}}_{3}$ skutterudite has invited intensive filling studies with various rare-earth elements for delivering state-of-the-art midtemperature thermoelectric performance. To rationalize previously reported experimental results and provide new insight into the underexplored roles of La fillers and Sb vacancies, ab initio density functional theory studies, along with semiclassical Boltzmann transport theory calculations, are performed for pristine $\mathrm{CoS}{\mathrm{b}}_{3}$ of different lattice settings and La-filled $\mathrm{CoS}{\mathrm{b}}_{3}$ with and without Sb's mono- and divacancy defects. The effects of spin-orbit coupling (SOC), partial La filling, Sb vacancy defects, and spin polarization on the electronic and thermoelectric properties are systematically examined. The SOC shows minor effects on the electronic and thermoelectric properties of $\mathrm{CoS}{\mathrm{b}}_{3}$. The peculiar quasi-Dirac band in the pristine $\mathrm{CoS}{\mathrm{b}}_{3}$ largely survives La filling but not Sb vacancies, which instead introduce dispersive bands in the band gap region. The non-spin-polarized and spin-polarized solutions of La-filled $\mathrm{CoS}{\mathrm{b}}_{3}$ are nearly degenerate. Importantly, the band structure, density of states, and Fermi surface of the latter are significantly spin polarized, giving rise to spin-dependent thermoelectric properties. Seebeck coefficients directly calculated as a function of chemical potential are interpreted in connection with the electronic structures. Temperature-dependent Seebeck coefficients derived for the experimentally studied materials agree well with available experimental data. Seebeck coefficients obtained as a function of charge carrier concentration corroborate the thermoelectrically favorable role at high filling fractions played by the Fermi electron pockets associated with the degenerate valleys in the conduction bands, and also point toward a similar role of the Fermi hole pockets associated with the degenerate hills in the valence bands. These results serve to advance the understanding of $\mathrm{CoS}{\mathrm{b}}_{3}$ skutterudite, a class of materials with important fundamental and application implications for thermoelectrics and spintronics.

A Computational Approach for Modeling Neutron Scattering Data from Lipid Bilayers

Abstract: Biological cell membranes are responsible for a range of structural and dynamical phenomena crucial, which are crucial to a cell’s well-being and its associated functions. Due to the complexity of cell membranes, lipid bilayer systems are often used as biomimetic models. These systems have led to significant insights into vital membrane phenomena such as domain formation, passive permeation, and protein insertion. Experimental observations of membrane structure and dynamics are, however, limited in resolution, both spatial and temporal. Importantly, computer simulations are starting to play a more prominent role in interpreting experimental results, enabling a molecular understanding of lipid membranes. In particular, the synergy between scattering experiments and simulations offers opportunities for new discoveries in membrane physics, as the length and time scales probed by molecular dynamics (MD) simulations parallel those of experiments. Here, we describe a coarse-grained MD simulation approach that m...

Nanoporous poly(3-hexylthiophene) thin film structures from self-organization of a tunable molecular bottlebrush scaffold

Abstract: The ability to widely tune the design of macromolecular bottlebrushes provides access to self-assembled nanostructures formed by microphase segregation in melt, thin film and solution that depart from structures adopted by simple linear copolymers. A series of random bottlebrush copolymers containing poly(3-hexylthiophene) (P3HT) and poly(D,L-lactide) (PLA) side chains grafted on a poly(norbornene) backbone were synthesized via ring-opening metathesis polymerization (ROMP) using the grafting through approach. P3HT side chains induce a physical aggregation of the bottlebrush copolymers upon solvent removal by vacuum drying, primarily driven by attractive π–π interactions; however, the amount of aggregation can be controlled by adjusting side chain composition or by adding linear P3HT chains to the bottlebrush copolymers. Coarse-grained molecular dynamics simulations reveal that linear P3HT chains preferentially associate with P3HT side chains of bottlebrush copolymers, which tends to reduce the aggregation. The nanoscale morphology of microphase segregated thin films created by casting P3HT–PLA random bottlebrush copolymers is highly dependent on the composition of P3HT and PLA side chains, while domain spacing of nanostructures is mainly determined by the length of the side chains. The selective removal of PLA side chains under alkaline conditions generates nanoporous P3HT structures that can be tuned by manipulating molecular design of the bottlebrush scaffold, which is affected by molecular weight and grafting density of the side chains, and their sequence. The ability to exploit the unusual architecture of bottlebrushes to fabricate tunable nanoporous P3HT thin film structures may be a useful way to design templates for optoelectronic applications or membranes for separations.

Multi-mode humidity sensing with water-soluble copper phthalocyanine for increased sensitivity and dynamic range.

Abstract: Aqueous solubility of copper phthalocyanine-3,4′,4″,4″′-tetrasulfonic acid tetrasodium salt (CuPcTs) enables fabrication of flexible electronic devices by low cost inkjet printing. We (1) investigate water adsorption kinetics on CuPcTs for better understanding the effects of relative humidity (RH) on hydrophilic phthalocyanines, and (2) assess CuPcTs as a humidity-sensing material. Reaction models show that H2O undergoes 2-site adsorption which can be represented by a pair of sequentially-occurring pseudo-first order reactions. Using high frequency (300–700 THz) and low frequency (1–8 MHz) dielectric spectroscopy combined with gravimetric measurements and principal component analysis, we observe that significant opto-electrical changes in CuPcTs occur at RH ≈ 60%. The results suggest that rapid H2O adsorption takes place at hydrophilic sulfonyl/salt groups on domain surfaces at low RH, while slow adsorption and diffusion of H2O into CuPcTs crystallites leads to a mixed CuPcTs-H2O phase at RH > 60%, resulting in high frequency dielectric screening of the film by water and dissociation of Na+ from CuPc(SO3 −)4 ions. The CuPcTs-H2O interaction can be tracked using a combination of gravimetric, optical, and electrical sensing modes, enabling accurate ( ± 2.5%) sensing in the ~0–95% RH range with a detection limit of less than 0.1% RH.

A Rayleighian approach for modeling kinetics of ionic transport in polymeric media.

Abstract: We report a theoretical approach for analyzing impedance of ionic liquids (ILs) and charged polymers such as polymerized ionic liquids (PolyILs) within linear response. The approach is based on the Rayleigh dissipation function formalism, which provides a computational framework for a systematic study of various factors, including polymer dynamics, in affecting the impedance. We present an analytical expression for the impedance within linear response by constructing a one-dimensional model for ionic transport in ILs/PolyILs. This expression is used to extract mutual diffusion constants, the length scale of mutual diffusion, and thicknesses of a low-dielectric layer on the electrodes from the broadband dielectric spectroscopy measurements done for an IL and three PolyILs. Also, static dielectric permittivities of the IL and the PolyILs are determined. The extracted mutual diffusion constants are compared with the self-diffusion constants of ions measured using pulse field gradient (PFG) fluorine nuclear magnetic resonance (NMR). For the first time, excellent agreement between the diffusivities extracted from the Electrode Polarization spectra (EPS) of IL/PolyILs and those measured using the PFG-NMR are found, which allows the use of the EPS and the PFG-NMR techniques in a complimentary manner for a general understanding of the ionic transport.

Emerging materials for lowering atmospheric carbon

Abstract: CO 2 emissions from anthropogenic sources and the rate at which they increase could have deep global ramifications such as irreversible climate change and increased natural disasters. Because greater than 50% of anthropogenic CO 2 emissions come from small, distributed sectors such as homes, offices, and transportation sources, most renewable energy systems and on-site carbon capture technologies for reducing future CO 2 emissions cannot be effectively utilized. This problem might be mediated by considering novel materials and technologies for directly capturing/removing CO 2 from air. However, compared to materials for capturing CO 2 at on-site emission sources, materials for capturing CO 2 directly from air must be more selective to CO 2 , and should operate and be stable at near ambient conditions. In this review article, we briefly summarize the recent developments in materials for capturing carbon dioxide directly from air. We discuss the challenges in this field and offer a perspective for developing the current state-of-art and also highlight the potential of a few recent discoveries in materials science that show potential for advanced application of air capture technology.

Deuteration as a Means to Tune Crystallinity of Conducting Polymers

Abstract: The effects of deuterium isotope substitution on conjugated polymer chain stacking of poly(3-hexylthiophene) is studied experimentally by X-ray diffraction (XRD) in combination with gel permeation chromatography and theoretically using density functional theory and quantum molecular dynamics. For four P3HT materials with different levels of deuteration (pristine, main-chain deuterated, side-chain deuterated, and fully deuterated), the XRD measurements show that main-chain thiophene deuteration significantly reduces crystallinity, regardless of the side-chain deuteration. The reduction of crystallinity due to the main-chain deuteration is a quantum nuclear effect resulting from a static zero-point vibrational energy combined with a dynamic correlation of the dipole fluctuations. The quantum molecular dynamics simulations confirm the interchain correlation of the proton–proton and deuteron–deuteron motions but not of the proton–deuteron motion. Thus, isotopic purity is an important factor affecting stabilit...

Multicomponent Gas Storage in Organic Cage Molecules

Abstract: Porous liquids are a promising new class of materials featuring nanoscale cavity units dispersed in liquids that are suitable for applications such as gas storage and separation In this work, we use molecular dynamics simulations to examine the multicomponent gas storage in a porous liquid consisting of crown-ether-substituted cage molecules dissolved in a 15-crown-5 solvent We compute the storage of three prototypical small molecules including CO2, CH4, and N2 and their binary mixtures in individual cage molecules For porous liquids in equilibrium with a binary 1:1 gas mixture bath with partial gas pressure of 275 bar, a cage molecule shows a selectivity of 43 and 131 for the CO2/CH4 and CO2/N2 pairs, respectively We provide a molecular perspective of how gas molecules are stored in the cage molecule and how the storage of one type of gas molecule is affected by other types of gas molecules Our results clarify the molecular mechanisms behind the selectivity of such cage molecules toward different