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  • Structural control of nonnative ligand binding in engineered mutants of phosphoenolpyruvate carboxykinase

    Tang, Henry Yue Hin; Shin, David S; Hura, Gregory L.; Yang, Yue; Hu, Xiaoyu; Lightstone, Felice C; McGee, Matthew D; Padgett, Hal S; Yannone, Steven M; Tainer, John A. (American Chemical Society (ACS), 2018-10-30)
    Protein engineering to alter recognition underlying ligand binding and activity has enormous potential. Here, ligand binding for E. coli phosphoenolpyruvate carboxykinase (PEPCK), which converts oxaloacetate into CO2 and phosphoenolpyruvate as the first committed step in gluconeogenesis, was engineered to accommodate alternative ligands as an exemplary system with structural information. From our identification of bicarbonate binding in the PEPCK active site at the supposed CO2 binding site, we probed binding of nonnative ligands with three oxygen atoms arranged to resemble bicarbonate geometry. Crystal structures of PEPCK and point mutants with bound nonnative ligands thiosulfate and methanesulfonate along with strained ATP plus reoriented oxaloacetate intermediates and unexpected bicarbonate were solved and analyzed. The mutations successfully altered the bound ligand position and orientation, as well as its specificity: mutated PEPCKs bound either thiosulfate or methanesulfonate, but never both. Computational calculations predicted a methanesulfonate binding mutant and revealed that release of active site ordered solvent exerts a strong influence on ligand binding. Besides nonnative ligand binding, one mutant altered the Mn2+ coordination sphere: instead of the canonical octahedral ligand arrangement, the mutant in question only had a five-coordinate arrangement. From this work, critical features of ligand binding, position, and metal ion co-factor geometry required for all downstream events can be engineered with small numbers of mutations to provide insights into fundamental underpinnings of protein-ligand recognition. Through structural and computational knowledge, the combination of designed and random mutations aids robust design of predetermined changes to ligand binding and activity in order to engineer protein function.
  • Synthesis and Characterization of Cu Decorated Zeolite A@void@Et-PMO Nanocomposites for Removal of Methylene Blue by a Heterogeneous Fenton Reaction

    Li, Xiayu; Zeng, Shangjing; Qu, Xuejian; Dai, Jinyu; Liu, Xiaofang; Wang, Runwei; Zhang, Zongtao; Qiu, Shilun (MDPI AG, 2018-10-29)
    The development of novel porous composite materials for organic dye degradation and removal has received increasing attention due to water contamination problem. In this paper, hydrothermal synthesized nano zeolite A have been encapsulated with porous periodic mesoporous organosilica (PMO) through a simple modified StÖber method an organosilane-directed growth-induced etching strategy, the obtained yolk-shell structured sample was further functionalized by the impregnation of copper, realizing the composite material with hierarchical porous and catalytic properties. The morphology, porosity and metal content of the zeolite Cu/A and Cu/A@Et-PMO were fully characterized. As compared to the parent material, the composite Cu/A@Et-PMO have an efficient adsorption and catalytic degradation performance on methylene blue (MB), the removal efficiency reached as high as 95% of 60 mg/L MB with 10min. These novel structured porous composites may have great potential for adsorption and degradation application including waste effluents.
  • Structure regulation of amino acids derived nitrogen doped porous carbon nanosheet through facile solid state assembly method

    Wang, Yu; Pan, Ying; Zhu, Liangkui; Guo, Ningning; Wang, Runwei; Zhang, Zongtao; Qiu, Shilun (Elsevier BV, 2018-10-16)
    Carbon nanosheets are widely used in electrocatalysis. Structure control is a great challenge in preparation of carbon materials. Here, a facial solid state assembly method is applied to prepare carbon nanosheets. By choosing amino acids as precursors, structure regulation of carbon nanosheets could be realized via switching the side chains of organic linkers, showing structure evolution from dense monolithic to porous nanosheets. Graphene-like carbon nanosheet constructed foam superstructure was also obtained. Owing to high porosity (BET surface area 1680 m2 g−1 with hierarchical pore structure) and nitrogen (8.23 wt%) modified graphitic frameworks, glutamic acid derived nitrogen doped porous carbon nanosheet superstructure showed high ORR activity (onset potential 0.9 V vs. RHE).
  • Three dimensional simulation on the transport and quantum efficiency of UVC-LEDs with random alloy fluctuations

    Chen, Hung-Hsiang; Speck, James S.; Weisbuch, Claude; Wu, Yuh-Renn (AIP Publishing, 2018-10-11)
    The active regions of ultraviolet light emitting diodes (UVLEDs) for UVB and ultra-violet band C wavelengths are composed of AlGaN alloy quantum barriers (QBs) and quantum wells (QWs). The use of alloy QBs and QWs facilitates the formation of percolative paths for carrier injection but also decreases carrier confinement within the QWs. We applied the recently developed Localization Landscape (LL) theory for a full 3D simulation of the LEDs. LL theory describes the effective quantum potential of the quantum states for electrons and holes in a random disordered system with a high computational speed. The results show that the potential fluctuations in the n-AlGaN buffer layer, QWs, and QBs provide percolative paths for carrier injection into the top (p-side) QW. Several properties due to compositional disorder are observed: (1) The peak internal quantum efficiency is larger when disorder is present, due to carrier localization, than for a simulation without fluctuations. (2) The droop is larger mainly due to poor hole injection and weaker blocking ability of the electron blocking layer caused by the fluctuating potentials. (3) Carriers are less confined in the QW and extend into the QBs due to the alloy potential fluctuations. The wave function extension into the QBs enhances TM emission as shown from a k·p simulation of wave-functions admixture, which should then lead to poor light extraction.
  • Design and techno-economic optimization of a rotary chemical looping combustion power plant with CO2 capture

    Iloeje, Chukwunwike O.; Zhao, Zhenlong; GHONIEM, AHMED F. (Elsevier BV, 2018-09-29)
    The rotary chemical looping combustion reactor design - which utilizes oxygen carriers in a matrix of micro channels for indirect fuel conversion - provides a viable path for fossil-based electric power generation with CO2 capture. Its thermally integrated matrix of micro channels minimizes irreversibilities associated with heat transfer in the reactor, and establishes multiscale coupling between oxygen carrier kinetics, reactor geometry and plant operating conditions. In this study, we implement an optimization framework that exploits this multiscale coupling for simultaneous reactor design and power plant economic optimization. Results for the methane-fueled power plant reveal optimized thermal efficiencies of 54–56% for a rotary chemical looping recuperative Brayton cycle plant, with compressor pressure ratio in the 3–7 range. By switching from an efficiency to an economic objective, we identified solutions that reduced electricity cost by about 11%; by performing scaling and technology maturity projections, we show competitive economics for the rotary chemical looping plant with CO2 capture.
  • Clustering algorithms to analyze molecular dynamics simulation trajectories for complex chemical and biological systems

    Peng, Jun-hui; Wang, Wei; Yu, Ye-qing; Gu, Han-lin; Huang, Xuhui (AIP Publishing, 2018-09-25)
    Molecular dynamics (MD) simulation has become a powerful tool to investigate the structure-function relationship of proteins and other biological macromolecules at atomic resolution and biologically relevant timescales. MD simulations often produce massive datasets containing millions of snapshots describing proteins in motion. Therefore, clustering algorithms have been in high demand to be developed and applied to classify these MD snapshots and gain biological insights. There mainly exist two categories of clustering algorithms that aim to group protein conformations into clusters based on the similarity of their shape (geometric clustering) and kinetics (kinetic clustering). In this paper, we review a series of frequently used clustering algorithms applied in MD simulations, including divisive algorithms, agglomerative algorithms (single-linkage, complete-linkage, average-linkage, centroid-linkage and ward-linkage), center-based algorithms (K-Means, K-Medoids, K-Centers, and APM), density-based algorithms (neighbor-based, DBSCAN, density-peaks, and Robust-DB), and spectral-based algorithms (PCCA and PCCA+). In particular, differences between geometric and kinetic clustering metrics will be discussed along with the performances of different clustering algorithms. We note that there does not exist a one-size-fits-all algorithm in the classification of MD datasets. For a specific application, the right choice of clustering algorithm should be based on the purpose of clustering, and the intrinsic properties of the MD conformational ensembles. Therefore, a main focus of our review is to describe the merits and limitations of each clustering algorithm. We expect that this review would be helpful to guide researchers to choose appropriate clustering algorithms for their own MD datasets.
  • Functional Nanomaterials and Nanostructures Enhancing Electrochemical Biosensors and Lab-on-a-Chip Performances: Recent Progress, Applications, and Future Perspective

    Wongkaew, Nongnoot; Simsek, Marcel; Griesche, Christian; Baeumner, Antje J. (American Chemical Society (ACS), 2018-09-24)
    Electrochemical biosensors and associated lab-on-a-chip devices are the analytical system of choice when rapid and on-site results are needed in medical diagnostics and food safety, for environmental protection, process control, wastewater treatment, and life sciences discovery research among many others. A premier example is the glucose sensor used by diabetic patients. Current research focuses on developing sensors for specific analytes in these application fields and addresses challenges that need to be solved before viable commercial products can be designed. These challenges typically include the lowering of the limit of detection, the integration of sample preparation into the device and hence analysis directly within a sample matrix, finding strategies for long-term in vivo use, etc. Today, functional nanomaterials are synthesized, investigated, and applied in electrochemical biosensors and lab-on-a-chip devices to assist in this endeavor. This review answers many questions around the nanomaterials used, their inherent properties and the chemistries they offer that are of interest to the analytical systems, and their roles in analytical applications in the past 5 years (2013–2018), and it gives a quantitative assessment of their positive effects on the analyses. Furthermore, to facilitate an insightful understanding on how functional nanomaterials can be beneficial and effectively implemented into electrochemical biosensor-based lab-on-a-chip devices, seminal studies discussing important fundamental knowledge regarding device fabrication and nanomaterials are comprehensively included here. The review ultimately gives answers to the ultimate question: “Are they really needed or can bulk materials accomplish the same?” Finally, challenges and future directions are also discussed.
  • Nitridation of optimised TiO2 nanorods through PECVD towards neural electrode application

    Sait, Roaa; Govindarajan, Sridhar; Cross, Richard (Elsevier BV, 2018-09-18)
    A neural electrode interface material is a key component for effective stimulation and recording of neural activity. The fundamental requirement of a neural electrode is for it to be able to deliver adequate charge to targeted neuronal population. Coating electrode surfaces with nanostructured material not only provides an increase in surface area, providing relatively more active sites for charge delivery than planar systems, but also allows for the reduction of electrode dimension to reduce invasiveness and increase selectivity. In this work, titanium nitride nanowires (TiN-NWs) synthesised by novel nitridation process in Plasma Enhanced Chemical Vapour Deposition (PECVD) is suggested as an enhanced coating material for neural electrodes. The synthesis involved the solution growth of crystalline titanium oxide nanorods (TiO2-NRs) from a sputtered TiN nucleation layer followed by nitridation. TiO2-NRs exhibited high aspect ratio of 23.1 and were converted into TiN after one hour of nitridation at 600°C. Evidence of conversion was studied by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and Transmission electron microscopy (TEM). The nitridation temperature and time reported here are the lowest and shortest as compared to the literature. The near-stoichiometric TiN-NWs (x=0.49) achieved in this work were used subsequently for electrochemical characterisation through Cyclic Voltammetry (CV). The capacitance of relatively high aspect TiN-NWs was 3.78 mF/cm2, which was a 5-fold enhancement compared to thin film of TiN layer (0.7 mF/cm2). A stability test of the nanowires were performed in which the capacitance remained relatively unchanged.
  • Elastic-Beam Triboelectric Nanogenerator for High-Performance Multifunctional Applications: Sensitive Scale, Acceleration/Force/Vibration Sensor, and Intelligent Keyboard

    Chen, Yuliang; Wang, Yi-Cheng; Zhang, Ying; Zou, Haiyang; Lin, Zhiming; Zhang, Guobin; Zou, Chongwen; Wang, Zhong Lin (Wiley, 2018-09-03)
    Exploiting novel devices for either collecting energy or self-powered sensors is vital for Internet of Things, sensor networks, and big data. Triboelectric nanogenerators (TENGs) have been proved as an effective solution for both energy harvesting and self-powered sensing. The traditional triboelectric nanogenerators are usually based on four modes: contact-separation mode, lateral sliding mode, single-electrode mode, and freestanding triboelectric-layer mode. Since the reciprocating displacement/force is necessary for all working modes, developing efficient elastic TENG is going to be important and urgent. Here, a kind of elastic-beam TENG with arc-stainless steel foil is developed, whose structure is quite simple, and its working states depend on the contact area and separating distance as proved by experiments and theoretical calculations. This structure is different from traditional structures, e.g., direct sliding or contact-separation structures, whose working states mainly depend on contact area or separating distance. This triboelectric nanogenerator shows advanced mechanical and electrical performance, such as high sensitivity, elasticity, and ultrahigh frequency response, which encourage applications as a force sensor, sensitivity scale, acceleration sensor, vibration sensor, and intelligent keyboard.
  • Chemical functional group descriptor for ignition propensity of large hydrocarbon liquid fuels

    Dussan, Karla; Won, Sang Hee; Ure, Andrew D.; Dryer, Frederick L.; Dooley, Stephen (Elsevier BV, 2018-08-30)
    The chemical functional group approach is investigated to verify the fundamental applicability of low-dimensional descriptors in the prediction of global combustion behavior, as described by homogeneous reflected shock ignition delay times. Three key chemical functional groups, CH2, CH3 and benzyl-type, are used to represent n-alkyl, iso-alkyl, and aromatic functionalities, respectively. To examine whether such descriptors can appropriately reflect the influences of these functionalities on ignition delay, Quantitative Structure-Property Relationship (QSPR) regression analysis is performed with the formulation of analytical models based on a fundamental Arrhenius-type description. The models are trained using literature measurements of reflected shock ignition delay times for stoichiometric fuel/air mixtures at 20 atm. Sensitivity analyses applied to the QSPR regression models show that the CH2 functional group dominates chemical kinetic behaviors at low temperature, while the chemical kinetic impacts of CH2, CH3, and benzyl-type functional groups all diminish as temperature increases. Further analyses of constant-volume adiabatic ignition delay predictions using detailed chemical kinetic models demonstrate influences of n-alkyl, iso-alkyl, and aromatic functionalities at both low and high temperature, consistent with those found for the QSPR regression models. Finally, 1H and 13C Nuclear Magnetic Resonance (NMR) spectroscopy is used to directly quantify the chemical functional group compositions of both petroleum-derived and alternative jet fuels. Combining the QSPR model with NMR spectra interpretation, the applicability of current approach as an expeditious tool to accurately characterize the ignition propensity of real transportation fuels is demonstrated by comparison with experimental measurements.
  • Architecture and Preparation of Hollow Catalytic Devices

    Li, Bowen; Zeng, Hua Chun (Wiley, 2018-08-30)
    Since pioneering work done in the late 1990s, synthesis of functional hollow materials has experienced a rapid growth over the past two decades while their applications have been proven to be advantageous across many technological fields. In the field of heterogeneous catalysis, the development of micro- and nanoscale hollow materials as catalytic devices has also yielded promising results, because of their higher activity, stability, and selectivity. Herein, the architecture and preparation of these catalysts with tailorable composition and morphology are reviewed. First, synthesis of hollow materials is introduced according to the classification of template mediated, template free, and combined approaches. Second, different architectural designs of hollow catalytic devices, such as those without functionalization, with active components supported onto hollow materials, with active components incorporated within porous shells, and with active components confined within interior cavities, are evaluated respectively. The observed catalytic performances of this new class of catalysts are correlated to structural merits of individual configuration. Examples that demonstrate synthetic approaches and architected configurations are provided. Lastly, possible future directions are proposed to advance this type of hollow catalytic devices on the basis of our personal perspectives.
  • Absolute adsorption of light hydrocarbons and carbon dioxide in shale rock and isolated kerogen

    Wu, Tianhao; Zhao, Huangjing; Tesson, Stéphane; Firoozabadi, Abbas (Elsevier BV, 2018-08-28)
    Natural gas production from shale formations has changed the energy landscape. Knowledge of adsorption in the subsurface shale formations improves resource assessment. The excess adsorption is directly measurable from experiments. Evaluation of fluid content in shale is based on the absolute adsorption. At high pressure relevant to subsurface conditions, the computation of absolute adsorption from excess adsorption has shortcomings when the conventional models are used. In this work, we first present the excess sorption data of light hydrocarbons and carbon dioxide in subsurface shale rock and in isolated kerogen. Gravimetric method was used in our measurements. The results show that, at high pressure, the excess adsorption of ethane and carbon dioxide decreases significantly as pressure increases. Excess adsorption of ethane at 60 °C for the shale sample investigated becomes negative at high pressure. The conventional models may provide a non-monotonic absolute adsorption and even magnify the unphysical negative adsorption. In addition to the proposed model based on adsorbed layer volume, we also account for effective sample volume due to the pore volume accessibility by different molecules, as well as the swelling of kerogen. The adsorption data from subsurface shale and the method for analysis presented in this work set the stage for prediction capability in hydrocarbon production from shale reservoirs.
  • A bilayered PVA/PLGA-bioresorbable shuttle to improve the implantation of flexible neural probes

    Pas, Jolien; Rutz, Alexandra L.; Quilichini, Pascale; Slézia, Andrea; Ghestem, Antoine; Kaszas, Attila; Donahue, Mary; Curto, Vincenzo; O’Connor, Rodney P.; bernard, christophe; Williamson, Adam; Malliaras, George (IOP Publishing, 2018-08-22)
    Objective: Neural electrophysiology is often conducted with traditional, rigid depth probes. The mechanical mismatch between these probes and soft brain tissue is unfavorable for tissue interfacing. Making probes compliant can improve biocompatibility, but as a consequence, they become more difficult to insert into the brain. Therefore, new methods for inserting compliant neural probes must be developed. \n Approach: Here, we present a new bioresorbable shuttle based on the hydrolytically degradable poly(vinyl alcohol) (PVA) and poly(lactic-co-glycolic acid) (PLGA). We show how to fabricate the PVA/PLGA shuttles on flexible and thin parylene probes. The method consists of PDMS molding used to fabricate a PVA shuttle aligned with the probe and to also impart a sharp tip necessary for piercing brain tissue. The PVA shuttle is then dip-coated with PLGA to create a bi-layered shuttle. \n Main results: While single layered PVA shuttles are able to penetrate agarose brain models, only limited depths were achieved and repositioning was not possible due to the fast degradation. We demonstrate that a bilayered approach incorporating a slower dissolving PLGA layer prolongs degradation and enables facile insertion for at least several millimeters depth. Impedances of electrodes before and after shuttle preparation were characterized and showed that careful deposition of PLGA is required to maintain low impedance. Facilitated by the shuttles, compliant parylene probes were also successfully implanted into anaesthetized mice and enabled the recording of high quality local field potentials. \n Significance: This work thereby presents a solution towards addressing a key challenge of implanting compliant neural probes using a two polymer system. PVA and PLGA are polymers with properties ideal for translation: commercially available, biocompatible with FDA-approved uses and bioresorbable. By presenting new ways to implant compliant neural probes, we can begin to fully evaluate their chronic biocompatibility and performance compared to traditional, rigid electronics.
  • Diffusivity of Mono- and Divalent Salts and Water in Polyelectrolyte Desalination Membranes

    Aryal, Dipak; Ganesan, Venkat (American Chemical Society (ACS), 2018-08-14)
    The dynamics of ions and solvent molecules in polyelectrolyte desalination membranes is key to water purification technologies in which selective transport of the different components is desired. Recent experimental and our computational results have shown that nontrivial mechanisms underlie the transport properties of salt ions and water in charged polymer membranes. Explicitly, in polymer electrolytes, we found a reversal in the salt concentration dependence of the mobilities of Na+, Cl– salt ions and water molecules when compared with aqueous solutions. Motivated by such results, in this study, we have used atomistic molecular dynamics simulations to probe whether the mechanisms deduced in our earlier work apply to other salt systems and to mixtures of salts. Specifically, we report results for the ion diffusivities in aqueous KCl, MgCl2, and a 1:1 mixture of NaCl and MgCl2 salt solutions at different concentrations (ranging from 0.06 to 1 M) and investigate, at the molecular level, the mechanisms underlying the behaviors of salt and water transport properties. Our results show that diffusion of salt ions and water in charged polymer membranes are in general influenced by their association with polymer charge groups and ion pairing effects. Divalent ions are more strongly coupled with the polymeric ionic groups than monovalent salt ions and exhibit diffusivity trends that are distinct relative to monovalent salts. Further, we demonstrate that the mobilities of water molecules are influenced by coordination of water with polymer charge groups and their ion pairing tendencies and also exhibit distinct trends in monovalent and divalent salt solutions.
  • Translating Catalysis to Chemiresistive Sensing

    Schroeder, Vera; Swager, Timothy M. (American Chemical Society (ACS), 2018-08-14)
    Activating molecules or functional groups with high chemoselectivity in complex environments is the central goal of transition-metal-based catalysis. Promoting strong interactions between a selected substrate and a catalytic system can also be used to create highly selective and customizable sensors, and these concepts are widely recognized for enzymatic processes. We demonstrate the successful translation of organometallic reactions to sensing capability. Specifically, we have developed single-walled carbon nanotube (SWCNT) chemiresistive sensors for the highly selective detection of acrylates using conditions for the aerobic oxidative Heck reaction. The sensors mirror the catalytic processes and selectively respond to electron-deficient alkenes by adapting a catalytic reaction system to modulate the doping levels in carbon nanotubes. The sensors readily detect acrylates at parts per million (ppm) levels in untreated air. The concepts presented here are generally applicable and can guide future sensor development based upon known catalytic processes.
  • Ammonium Removal from Domestic Wastewater Using Selective Battery Electrodes

    Kim, Taeyoung; Gorski, Christopher A.; Logan, Bruce (American Chemical Society (ACS), 2018-08-13)
    Conventional technologies for ammonium removal from wastewaters are based on biological conversion to nitrogen gas, eliminating the possibility for ammonium recovery. A new electrochemical approach was developed here to selectively remove ammonium using two copper hexacyanoferrate (CuHCF) battery electrodes separated by an anion exchange membrane, at low applied voltages (0.1 to 0.3 V). The CuHCF battery electrodes removed NH4+ from a synthetic wastewater with a selectivity >5 (i.e., percent removed of NH4+/percent removed of Na+) when operated with a 0.1 V applied voltage, despite the much higher initial Na+ concentration in the sample (20 mM) than NH4+ (5 mM). In contrast, we observed only negligible selective removal of NH4+ over Na+ (<2) when using nonselective electrodes or ion-selective membranes (10 mM Na+, 5 mM NH4+, 0.1 V). The selectivity further increased to 9 when using equimolar concentrations of NH4+ and Na+ (10 mM). With an actual domestic wastewater, the CuHCF electrodes removed 85% of NH4+ (3.4 to 0.5 mM) with a selectivity >4 versus Na+ in the presence of other competing cations. These results demonstrate that CuHCF electrodes can be used to selectively remove NH4+ from various waters containing multiple ions.
  • Spin control in reduced-dimensional chiral perovskites

    Long, Guankui; Jiang, Chongyun; Sabatini, Randy; Yang, Zhenyu; Wei, Mingyang; Quan, Li Na; Liang, Qiuming; Rasmita, Abdullah; Askerka, Mikhail; Walters, Grant; Gong, Xiwen; Xing, Jun; Wen, Xinglin; Quintero-Bermudez, Rafael; Yuan, Haifeng; Xing, Guichuan; Wang, X. Renshaw; Song, Datong; Voznyy, Oleksandr; Zhang, Mingtao; Hoogland, Sjoerd; Gao, Weibo; Xiong, Qihua; Sargent, E. (Springer Nature America, Inc, 2018-08-10)
    Hybrid organic–inorganic perovskites exhibit strong spin–orbit coupling1, spin-dependent optical selection rules2,3 and large Rashba splitting4,5,6,7,8. These characteristics make them promising candidates for spintronic devices9 with photonic interfaces. Here we report that spin polarization in perovskites can be controlled through chemical design as well as by a magnetic field. We obtain both spin-polarized photon absorption and spin-polarized photoluminescence in reduced-dimensional chiral perovskites through combined strategies of chirality transfer and energy funnelling. A 3% spin-polarized photoluminescence is observed even in the absence of an applied external magnetic field owing to the different emission rates of σ+ and σ− polarized photoluminescence. Three-dimensional perovskites achieve a comparable degree of photoluminescence polarization only under an external magnetic field of 5 T. Our findings pave the way for chiral perovskites as powerful spintronic materials.
  • Electrically induced 2D half-metallic antiferromagnets and spin field effect transistors

    Gong, Shi-Jing; Gong, Cheng; Sun, Yu-Yun; Tong, Wen-Yi; Duan, Chun-Gang; Chu, Jun-Hao; Zhang, Xiang (Proceedings of the National Academy of Sciences, 2018-08-03)
    Engineering the electronic band structure of material systems enables the unprecedented exploration of new physical properties that are absent in natural or as-synthetic materials. Half metallicity, an intriguing physical property arising from the metallic nature of electrons with singular spin polarization and insulating for oppositely polarized electrons, holds a great potential for a 100% spin-polarized current for high-efficiency spintronics. Conventionally synthesized thin films hardly sustain half metallicity inherited from their 3D counterparts. A fundamental challenge, in systems of reduced dimensions, is the almost inevitable spin-mixed edge or surface states in proximity to the Fermi level. Here, we predict electric field-induced half metallicity in bilayer A-type antiferromagnetic van der Waals crystals (i.e., intralayer ferromagnetism and interlayer antiferromagnetism), by employing density functional theory calculations on vanadium diselenide. Electric fields lift energy levels of the constituent layers in opposite directions, leading to the gradual closure of the gap of singular spin-polarized states and the opening of the gap of the others. We show that a vertical electrical field is a generic and effective way to achieve half metallicity in A-type antiferromagnetic bilayers and realize the spin field effect transistor. The electric field-induced half metallicity represents an appealing route to realize 2D half metals and opens opportunities for nanoscale highly efficient antiferromagnetic spintronics for information processing and storage.
  • Chirped-probe-pulse femtosecond CARS thermometry in turbulent spray flames

    Lowe, A.; Thomas, L.M.; Satija, A.; Lucht, R.P.; Masri, A.R. (Elsevier BV, 2018-07-31)
    This paper presents temperature measurements in turbulent dilute and dense spray flames using single-laser-shot chirped-probe-pulse femtosecond coherent anti-Stokes Raman spectroscopy (CPP-fs-CARS). This ultrafast technique, with a repetition rate of 5 kHz, is applied to the piloted Sydney Needle Spray Burner (SYNSBURNTM). The burner system features air-blast atomization of liquid injected from a needle that can be translated within a co-flowing air stream. The pilot-stabilized spray flames can range between the two extremes of dense and dilute by physically translating the needle tip relative to the burner's exit plane. The CPP-fs-CARS set-up has achieved integration times of 3 picoseconds (ps) as well as spatial resolution of approximately 800 µm along beam propagation and 60 µm in the transverse dimension. Brief details of the technique, calibration, correction of interferences, and spectral fitting processes are presented along with estimates of the associated error. The measurements are compared against well-established, line Raman–Rayleigh data for temperature collected in a turbulent CH4/air jet diffusion flame, which is largely non-sooting. At peak gaseous flame temperatures of up to 2512 K, the relative accuracy and precision were 2.8% and ±3.4%, respectively. Measurements in turbulent spray flames are shown after applying the relevant corrections based on non-resonant background (NRB) behavior and camera saturation effects on the shape of the CARS signal spectrum. Preliminary mapping of the temperature fields demonstrates the wealth of information available in this dataset which will provide insights into the spatio-temporal structure of spray flames once relevant statistical analysis is applied.
  • Dipolar cations confer defect tolerance in wide-bandgap metal halide perovskites

    Tan, Hairen; Che, Fanglin; Wei, Mingyang; Zhao, Yicheng; Saidaminov, Makhsud I.; Todorović, Petar; Broberg, Danny; Walters, Grant; Tan, Furui; Zhuang, Taotao; Sun, Bin; Liang, Zhiqin; Yuan, Haifeng; Fron, Eduard; Kim, Junghwan; Yang, Zhenyu; Voznyy, Oleksandr; Asta, Mark; Sargent, E. (Springer Nature America, Inc, 2018-07-31)
    Efficient wide-bandgap perovskite solar cells (PSCs) enable high-efficiency tandem photovoltaics when combined with crystalline silicon and other low-bandgap absorbers. However, wide-bandgap PSCs today exhibit performance far inferior to that of sub-1.6-eV bandgap PSCs due to their tendency to form a high density of deep traps. Here, we show that healing the deep traps in wide-bandgap perovskites—in effect, increasing the defect tolerance via cation engineering—enables further performance improvements in PSCs. We achieve a stabilized power conversion efficiency of 20.7% for 1.65-eV bandgap PSCs by incorporating dipolar cations, with a high open-circuit voltage of 1.22 V and a fill factor exceeding 80%. We also obtain a stabilized efficiency of 19.1% for 1.74-eV bandgap PSCs with a high open-circuit voltage of 1.25 V. From density functional theory calculations, we find that the presence and reorientation of the dipolar cation in mixed cation–halide perovskites heals the defects that introduce deep trap states.

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