Now showing items 1-20 of 20176

    • Distributed Processing for Multi-Relay Assisted OFDM with Index Modulation

      Dang, Shuping; Li, Jun; Wen, Miaowen; Mumtaz, Shahid (IEEE, 2019-01-16)
      Orthogonal frequency-division multiplexing with index modulation (OFDM-IM) has become a high-profile modulation scheme for the fifth generation (5G) wireless communications, and thus been extended to multi-hop scenarios in order to improve the network coverage and energy efficiency. However, the extension of OFDM-IM to multi-relay cooperative networks is not trivial, since it is required that a complete OFDM block should be received and decoded as an entity in one node. This requirement prevents the employment of multiple relays to forward fragmented OFDM blocks on individual subcarriers. In this regard, we propose a distributed processing scheme for multirelay assisted OFDM-IM, by which multiple relays are selected to forward signals in a per-subcarrier manner to provide optimal error performance for two-hop decode-and-forward (DF) OFDMIM systems. Specifically, a single selected relay only needs to decode partial information carried on certain active subcarriers and forward just as for traditional OFDM systems without IM. After receiving all signals on active subcarriers forwarded by different relays, the destination can reconstruct the complete OFDM block and retrieve the full information. We analyze the average block error rate (BLER) and modulation capacity of the two-hop OFDM-IM system employing the proposed distributed DF protocol and verify the analysis by numerical simulations.
    • Projected Changes in Photosynthetic Picoplankton in a Warmer Subtropical Ocean

      Agusti, Susana; Lubián, Luis M.; Moreno-Ostos, Enrique; Estrada, Marta; Duarte, Carlos M. (Frontiers Media SA, 2019-01-15)
      The oligotrophic subtropical gyres are the largest biome on Earth, where picoplankton constitute the dominant autotrophs. The trend for autotrophic picoplankton to increase with sea temperature has led to predictions that picophytoplankton abundance will increase with warming. Here we conducted a global survey in the open subtropical-tropical ocean to resolve the functional relationships between picophytoplankton abundance and oceanic properties (water temperature, chlorophyll a concentration, nutrient concentrations, and underwater visible and ultraviolet B radiation). We then used these relationships to build models projecting the future changes of Synechococcus, Prochlorococcus, and eukaryotic picoautotrophs populations in the subtropical gyres with warming. Our goal is to refine the forecasts for this large biome and implement the analysis by including the picoeukaryotes, absent in previous models, but a relevant component of picophytoplankton. The data obtained and the relationships found in our global survey of the subtropical-tropical ocean between picophytoplankton abundance and ocean properties differed from previous global studies including colder (temperate, subpolar, and polar) and coastal waters. These differences included a lower abundance of Synechococcus populations, significant negative relationships between Prochlorococcus abundance and nutrient concentrations, and positive relationships for picoeukaryotes and no relationship for Synechococcus abundance, and, a moderate response to temperature in the warm waters of the tropical-subtropical open ocean. A model based on temperature increase alone forecasts a general increase in picoautotrophs by year 2100, although minimal for picoeukaryotes, and much more moderate for Synechococcus than previously forecasted. However, a global change model linking the thermal increase with the associated decline in chlorophyll a, and increased underwater solar radiation penetration, projected a decline in the abundance of autotrophic picoplankton. The decline was larger at the surface layer and partially compensated by the increased importance of deep picophytoplankton blooms, especially those of Prochlorococcus. The global change model predicted an increased dominance of Prochlorococcus sp. in the subtropical-tropical ocean with future warming. Our results, based on current patterns of picophytoplankton distribution, help improve existing projections by considering feedbacks affecting picophytoplankton abundance in the future subtropical and tropical ocean, the larger biome on Earth.
    • OFDM with Index Modulation Assisted by Multiple Amplify-and-Forward Relays

      Sheng, Guo; Dang, Shuping; Zhang, Zhenrong; Kocan, Enis; Pejanovic-Djurisic, Milica (IEEE, 2019-01-15)
      A multi-relay assisted orthogonal frequency division multiplexing (OFDM) system with index modulation (OFDM-IM) is proposed in this letter, assuming that amplify-and-forward (AF) relaying protocol is adopted at relays. Two commonly used AF protocols are considered: fixed-gain (FG) AF and variable-gain (VG) AF relaying protocols. To utilize relays in an efficient manner, we also employ two multi-carrier relay selection schemes termed bulk and per-subcarrier (PS) relay selection in the proposed system. We analyze the outage performance of the proposed system and derive closed-form expressions of the average outage probabilities (AOPs) for all cases with different AF relaying protocols and multi-carrier relay selection schemes. In addition, we obtain the asymptotic expressions of AOPs in the high signal-to-noise ratio (SNR) region, by which it is proven that the full diversity gain equaling the number of relays is attainable.
    • Experimental and Numerical Investigation of the Response of a Swirled Flame to Flow Modulations in a Non-Adiabatic Combustor

      Chatelier, Adrien; Guiberti, Thibault; Mercier, Renaud; Bertier, Nicolas; Fiorina, Benoît; Schuller, Thierry (Springer Nature, 2019-01-11)
      Turbulent combustion models for Large Eddy Simulation (LES) aims at predicting the flame dynamics. So far, they have been proven to predict correctly the mean flow and flame properties in a wide range of configurations. A way to challenge these models in unsteady situations is to test their ability to recover turbulent flames submitted to harmonic flow modulations. In this study, the Flame Transfer Function (FTF) of a CH4/H2/air premixed swirled-stabilized flame submitted to harmonic flowrate modulations in a non-adiabatic combustor is compared to the response computed using the Filtered TAbulated Chemistry for LES (F-TACLES) formalism. Phase averaged analysis of the perturbed flow field and flame response reveal that the velocity field determined with Particle Image Velocimetry measurements, the heat release distribution inferred from OH* images and the probability of presence of burnt gases deduced from OH-Planar Laser Induced Fluorescence measurements are qualitatively well reproduced by the simulations. However, noticeable differences between experiments and simulations are also observed in a narrow frequency range. A detailed close-up view of the flow field highlight differences in experimental OH* and numerical volumetric heat release rate distributions which are at the origin of the differences observed between the numerical and experimental FTF. These differences mainly originate from the outer shear layer of the swirling jet where a residual reaction layer takes place in the simulations which is absent in the experiments. Consequences for turbulent combustion modeling are suggested by examining the evolution of the perturbed flame brush envelope along the downstream distance of the perturbed flames. It is shown that changing the grid resolution and the flame subgrid scale wrinkling factor in these regions does not alter the numerical results. It is finally concluded that the combined effects of strain rate and enthalpy defect due to heat losses are the main factors leading to small but sizable differences of the flame response to coherent structures synchronized by the acoustic forcing in the outer shear layer of the swirling flow. These small differences in flame response lead in turn to a misprediction of the FTF at specific forcing frequencies.
    • PN3(P)-Pincer Complexes: Cooperative Catalysis and Beyond

      Li, Huaifeng; Goncalves, Theo; Lupp, Daniel; Huang, Kuo-Wei (American Chemical Society (ACS), 2019-01-11)
    • Characterization and Engineering Properties of Dry and Ponded Class-F Fly Ash

      Bachus, R. C.; Terzariol, Marco; Pasten, C.; Chong, S. H.; Dai, S.; Cha, M. S.; Kim, S.; Jang, Junbong; Papadopoulos, E.; Roshankhah, S.; Lei, L.; Garcia, Adrian; Park, J.; Sivaram, A.; Santamarina, F.; Ren, X.; Santamarina, Carlos (American Society of Civil Engineers (ASCE), 2019-01-10)
      Characterization studies conducted on Class-F fly-ash specimens gathered from different producers in the southeastern United States confirm general trends reported for fly ash worldwide. Additional tests and detailed analyses explain the spread in specific gravity (interparticle porosity cenospheres), highlight the tendency to segregation and layering, and show marked ferromagnetism. Furthermore, data show that early diagenetic cementation—within days after wetting—hinders densification and produces a fabric that is prone to collapse. New procedures are specifically developed to diagnose and characterize early diagenesis, including (1) pH measurements as an indicator of diagenetic potential, (2) test protocols to assess early diagenesis using oedometer tests and shear-wave velocity, and (3) procedures to determine realizable unit weights as reference values for the analyses of contractive or dilative tendencies and instability. In the absence of early diagenetic cementation, dilative fly-ash behavior is expected in the upper ≈20 m under monotonic shear loading. Flow instability may follow the failure of the containment structure if the ponded ash is saturated and has experienced hindered densification.
    • Experimental Mixed-Gas Permeability, Sorption and Diffusion of CO2-CH4 Mixtures in 6FDA-mPDA Polyimide Membrane: Unveiling the Effect of Competitive Sorption on Permeability Selectivity

      Genduso, Giuseppe; Ghanem, Bader; Pinnau, Ingo (MDPI AG, 2019-01-10)
      The nonideal behavior of polymeric membranes during separation of gas mixtures can be quantified via the solution-diffusion theory from experimental mixed-gas solubility and permeability coefficients. In this study, CO₂-CH₄ mixtures were sorbed at 35 °C in 4,4'-(hexafluoroisopropylidene)diphthalic dianhydride (6FDA)-m-phenylenediamine (mPDA)-a polyimide of remarkable performance. The existence of a linear trend for all data of mixed-gas CO₂ versus CH₄ solubility coefficients-regardless of mixture concentration-was observed for 6FDA-mPDA and other polymeric films; the slope of this trend was identified as the ratio of gas solubilities at infinite dilution. The CO₂/CH₄ mixed-gas solubility selectivity of 6FDA-mPDA and previously reported polymers was higher than the equimolar pure-gas value and increased with pressure from the infinite dilution value. The analysis of CO₂-CH₄ mixed-gas concentration-averaged effective diffusion coefficients of equimolar feeds showed that CO₂ diffusivity was not affected by CH₄. Our data indicate that the decrease of CO₂/CH₄ mixed-gas diffusion, and permeability selectivity from the pure-gas values, resulted from an increase in the methane diffusion coefficient in mixtures. This effect was the result of an alteration of the size sieving properties of 6FDA-mPDA as a consequence of CO₂ presence in the 6FDA-mPDA film matrix.
    • Single-camera 3D PTV using particle intensities and structured light

      Aguirre-Pablo, A. A.; Aljedaani, Abdulrahman Barakat; Xiong, J.; Idoughi, Ramzi; Heidrich, Wolfgang; Thoroddsen, Sigurdur T (Springer Nature, 2019-01-10)
      We use structured monochromatic volume illumination with spatially varying intensity profiles, to achieve 3D intensity particle tracking velocimetry using a single video camera. The video camera records the 2D motion of a 3D particle field within a fluid, which is perpendicularly illuminated with depth gradients of the illumination intensity. This allows us to encode the depth position perpendicular to the camera, in the intensity of each particle image. The light intensity field is calibrated using a 3D laser-engraved glass cube containing a known spatial distribution of 1100 defects. This is used to correct for the distortions and divergence of the projected light. We use a sequence of changing light patterns, with numerous sub-gradients in the intensity, to achieve a resolution of 200 depth levels.
    • Unified Statistical Channel Model for Turbulence-Induced Fading in Underwater Wireless Optical Communication Systems

      Zedini, Emna; Oubei, Hassan M.; Kammoun, Abla; Hamdi, Mounir; Ooi, Boon S.; Alouini, Mohamed-Slim (IEEE, 2019-01-09)
      A unified statistical model is proposed to characterize turbulence-induced fading in underwater wireless optical communication (UWOC) channels in the presence of air bubbles and temperature gradient for fresh and salty waters, based on experimental data. In this model, the channel irradiance fluctuations are characterized by the mixture Exponential-Generalized Gamma (EGG) distribution. We use the expectation maximization (EM) algorithm to obtain the maximum likelihood parameter estimation of the new model. Interestingly, the proposed model is shown to provide a perfect fit with the measured data under all channel conditions for both types of water. The major advantage of the new model is that it has a simple mathematical form making it attractive from a performance analysis point of view. Indeed, we show that the application of the EGG model leads to closed-form and analytically tractable expressions for key UWOC system performance metrics such as the outage probability, the average bit-error rate, and the ergodic capacity. To the best of our knowledge, this is the first-ever comprehensive channel model addressing the statistics of optical beam irradiance fluctuations in underwater wireless optical channels due to both air bubbles and temperature gradient.
    • Self-Patterned CsPbBr3 Nanocrystals for High-Performance Optoelectronics

      Xin, Bin; Pak, Yusin; Mitra, Somak; Almalawi, Dhaifallah; Alwadai, Norah Mohammed Mosfer; Zhang, Yuhai; Roqan, Iman S. (American Chemical Society (ACS), 2019-01-09)
      All-inorganic lead halide perovskites are promising materials for many optoelectronic applications. However, two issues that arise during device fabrication hinder their practical use, namely inadequate continuity of coated inorganic perovskite films across large areas and inability to integrate these films with traditional photolithography due to poor adhesion to wafers. Herein, for the first time, to address these issues, we show a room-temperature synthesis process employed to produce of CsPbBr3 perovskite nanocrystals with two-dimensional (2D) nanosheet features. Due to the unique properties of these 2D nanocrystals, including the “self-assembly” characteristic, and “double solvent evaporation induced self-patterning” strategy are used to generate high-quality patterned thin films in selected areas automatically after-drop-casting, enabling fabrication of high-performance devices without using complex and expensive fabrication processing techniques. The films are free from micro-cracks. In a proof-of-concept experiment, photodetector arrays are used to demonstrate the superior properties of such films. We provide evidence of both high responsivity (9.04 A/W) and high stability across large areas. The photodetectors fabricated on flexible substrate exhibit outstanding photo-response stability. Advanced optical and structural studies reveal the possible mechanism. Our simple and cost-effective method paves the way for the next-generation nanotechnology based on high-performance, cost-effective optoelectronic devices.
    • Passive seismic event estimation using multi-scattering waveform inversion

      Song, Chao; Wu, Zedong; Alkhalifah, Tariq Ali (Society of Exploration Geophysicists, 2019-01-09)
      Passive seismic monitoring has become an effective method to understand underground processes. Time-reversal-based methods are often used to locate passive seismic events directly. However, these kinds of methods are strongly dependent on the accuracy of the velocity model. Full waveform inversion (FWI) has been employed on passive seismic data to invert the velocity model and source image, simultaneously. However, waveform inversion of passive seismic data utilizes mainly the transmission energy, which results in poor illumination and low resolution. We develop a waveform inversion using multi-scattered energy for passive seismic to extract more information from the data than conventional FWI. Using transmission wavepath information from single- and double-scattering, computed from a predicted scatterer field acting as secondary sources, the proposed method provides better illumination of the velocity model than conventional FWI. Using a new objective function, we optimize the source image and velocity model including multi-scattered energy, simultaneously. As we conduct our method in the frequency domain with a complex source function including both spatial and wavelet information, we mitigate the uncertainties of the source wavelet and source origin time. Inversion results from the Marmousi model show that by taking advantage of multi-scattered energy and starting from a reasonably acceptable frequency (single source at 3 Hz and multiple sources at 5 Hz), the proposed method yields better inverted velocity models and source images compared with the conventional FWI.
    • Low Abundances but High Growth Rates of Coastal Heterotrophic Bacteria in the Red Sea

      Silva, Luis; Calleja, Maria L.; Huete-Stauffer, Tamara Megan; Ivetic, Snjezana; Ansari, Mohd Ikram; Viegas, Miguel; Moran, Xose Anxelu G. (Frontiers Media SA, 2019-01-07)
      Characterized by some of the highest naturally occurring sea surface temperatures, the Red Sea remains unexplored regarding the dynamics of heterotrophic prokaryotes. Over 16 months, we used flow cytometry to characterize the abundance and growth of four physiological groups of heterotrophic bacteria: membrane-intact (Live), high and low nucleic acid content (HNA and LNA) and actively respiring (CTC+) cells in shallow coastal waters. Chlorophyll a, dissolved organic matter (DOC and DON) concentrations, and their fluorescent properties were also measured as proxies of bottom-up control. We performed short-term incubations (6 days) with the whole microbial community (Community treatment), and with the bacterial community only after removing predators by filtration (Filtered treatment). Initial bacterial abundances ranged from 1.46 to 4.80 × 105 cells mL-1. Total specific growth rates in the Filtered treatment ranged from 0.76 to 2.02 d-1. Live and HNA cells displayed similar seasonal patterns, with higher values during late summer and fall (2.13 and 2.33 d-1, respectively) and lower in late spring (1.02 and 1.01 d-1, respectively). LNA cells were outgrown by the other physiological groups (0.33–1.08 d-1) while CTC+ cells (0.28–1.85 d-1) showed weaker seasonality. The Filtered treatment yielded higher bacterial abundances than the Community treatment in all but 2 of the incubations, and carrying capacities peaked in November 2016 (1.04 × 106 cells mL-1), with minimum values (3.61 × 105 cells mL-1) observed in May 2017. The high temperatures experienced from May through October 2016 (33.4 ± 0.4∘C) did not constrain the growth of heterotrophic bacteria. Indeed, bacterial growth efficiencies were positively correlated with environmental temperature, reflecting the presence of more labile compounds (high DON concentrations resulting in lower C:N ratios) in summer. The overall high specific growth rates and the consistently higher carrying capacities in the Filtered treatment suggest that strong top-down control by protistan grazers was the likely cause for the low heterotrophic bacteria abundances.
    • Manipulating the Topology of Nanoscale Skyrmion Bubbles by Spatially Geometric Confinement

      Hou, Zhipeng; Zhang, Qiang; Xu, Guizhou; Zhang, Senfu; GONG, CHEN; Ding, Bei; Li, Hang; Xu, Feng; Yao, Yuan; Liu, Enke; Wu, Guangheng; Zhang, Xixiang; Wang, Wenhong (American Chemical Society (ACS), 2019-01-07)
    • Prediction of Ignition Regimes in DME/Air Mixtures with Temperature and Concentration Fluctuations

      Luong, Minh Bau; Hernandez Perez, Francisco E.; Sow, Aliou; Im, Hong G. (American Institute of Aeronautics and Astronautics, 2019-01-07)
      The objective of the present study is to establish a theoretical prediction of the autoignition behavior of a reactant mixture for a given initial bulk mixture condition. The ignition regime criterion proposed by Im and coworkers based on the Sankaran number (Sa), which is a ratio of the laminar flame speed to the spontaneous ignition front speed, is extended to account for both temperature and equivalence ratio fluctuations. The extended ignition criterion is then applied to predict the autoignition characteristics of dimethyl ether (DME)/air mixtures and validated by two-dimensional direct numerical simulations (DNS). The response of the ignition mode of DME/air mixtures to three initial mean temperatures of 770, 900 K, and 1045 K lying within/outside the NTC regime, two levels of temperature and concentration fluctuations at a pressure of 30 atm and equivalence ratio of 0.5 is systematically investigated. The statistical analysis is performed, and a newly developed criterion –the volumetric fraction of Sa < 1.0, FSa,S, is proposed as a deterministic criterion to quantify the fraction of heat release attributed to strong ignition. It is found that the strong and weak ignition modes are well captured by the predicted Sa number and FSa,S regardless of different initial mean temperatures and the levels of mixture fluctuations and correlations. Sap and FSa,S demonstrated under a wide range of initial conditions as a reliable criterion in determining a priori the ignition modes and the combustion intensity.
    • Ultrafast Spectroscopy of Polymer: Non-fullerene Small Molecule Acceptor Bulk Heterojunction Organic Solar Cells

      Alamoudi, Maha A (2019-01-07)
      Organic photovoltaics has emerged as a promising technology for electricity generation. The essential component in an organic solar cell is the bulk heterojunction absorber layer, typically a blend of an electron donor and an electron acceptor. Efforts have been made to design new materials such as donor polymers and novel acceptors to improve the power conversion efficiencies. New fullerene free acceptors providing low cost synthesis routes and tenability of their optoelectronic and electrochemical properties have been designed. Despite the efforts, still not much is known about the photopysical processes in these blends that limit the performance. In this respect, time-resolved spectroscopy such as transient absorption and time-resolved photoluminescence, can provide in-depth insight into the various (photo) physical processes in bulk heterojunction solar cell. In this thesis, PCE10 was used as donor and paired with different non fullerene acceptors. In the first part of this thesis the impact of the core structure (cyclopenta-[2, 1-b:3, 4-b’]dithiophene (CDT) versus indacenodithiophene (IDTT)) of malononitrile (BM)-terminated acceptors, abbreviated as CDTBM and IDTTBM, on the photophysical characteristics of BHJ solar cells is reported. The IDTT-based acceptor achieves power conversion efficiencies of 8.4%, higher than the CDT-based acceptor (5.6%), due to concurrent increase in short-circuit current and open-circuit voltage. Using (ultra)fast transient spectroscopy we demonstrate that reduced geminate recombination in PCE10: IDTTBM blends is the reason for the difference in short-circuit currents. External quantum efficiency measurements indicate that the higher energy of interfacial charge-transfer states observed for the IDTT-based acceptor blends is the origin of the higher open-circuit voltage. In the second part of this thesis, I report the impact of acceptor side chains on the photo-physical processes of BHJ solar cells using three different IDT-based acceptors, namely O-IDTBR, EH-IDTBR and O-IDTBCN blended with PCE10. Power conversion efficiencies as high as 10 % were achieved. The transient absorption spectroscopy experiments provide insight into sub-picosecond exciton dissociation and charge generation which is followed by nanosecond triplet state formation in PCE10:O-DTBR and PCE10:EH-IDTBR blends, while in O-IDTBCN triplets are not observed. Time delayed collection field experiments (TDCF) were performed to address the charge carrier generation and examine its dependence on the electric field.
    • Development of bismuth (oxy)sulfide-based materials for photocatalytic applications

      BaQais, Amal (2019-01-07)
      Technologies based on alternative and sustainable energy sources present a vital solution in the present and for the future. These technologies are strongly driven by the increased global energy demand and need to reduce environmental issues created by fossil fuel. Solar energy is an abundant, clean and free-access resource, but it requires harvesting and storage for a sustainable future. Direct conversion and storage of solar energy using heterogeneous photocatalysts have been identified as parts of a promising paradigm for generating green fuels from sunlight and water. This thesis focused on developing semiconductor absorbers in a visible light region for photocatalytic hydrogen production reaction. In addition, theoretical studies are combined with experimental results for a deep understanding of the intrinsic optoelectronic properties of the obtained materials. The study presents a novel family of oxysulfide BiAgOS, produced by applying a full substitution strategy of Cu by Ag in BiCuOS. I was interested to address how the total substitution of Cu by Ag in a BiCuOS system affects its crystal structure, optical and electronic properties using experimental characterizations and theoretical calculations. Single-phase bismuth silver oxysulfide BiAgOS was prepared via a hydrothermal method. Rietveld refinement of the powder confirmed that BiAgOS is an isostructural BiCuOS. The diffraction peak positions of BiAgOS, relative to those of BiCuOS, were shifted toward lower angles, indicating an increase in the cell parameters. BiCuOS and BiAgOS were found to have indirect bandgaps of 1.1 and 1.5 eV, respectively. The difference in the bandgap results from the difference in the valence band compositions. The hybrid level of the S and Ag orbitals in BiAgOS is located at a more positive potential than that of S and Cu, leading to a widened bandgap. Both materials possess high dielectric constants and low electron and hole effective masses, making them interesting for photoconversion applications. BiAgOS has a potential for photocatalytic hydrogen evolution reaction in the presence of sacrificial reagents; however, it is inactive toward water oxidation. BiCuOS and BiAgOS can be considered interesting starting compositions for the development of new semiconductors for PV or Z-scheme photocatalytic applications. The second study investigates the synthesis and characterization of NaBiS2, this contains Bi3+, which belongs to the p-block electronic configuration Bi3+ 6s26p0, and NaLaS2, which contains La3+ with electronic configuration 6s05d0. Solid-state reactions from oxide precursor starting materials were applied for synthesis the materials. The sulfurization process was conducted by pressurizing a saturated vapor of CS2. The obtained black material of NaBiS2 has an indirect transition with high absorption coefficients in the visible region of the spectrum and the absorption edge is determined at 1.21 eV. However, NaBiS2 did not show photocatalytic activity toward hydrogen production. NaLaS2 is characterized by an indirect transition with a bandgap in the UV region at 3.15 eV and can drive the photocatalytic hydrogen evolution reaction in Na2S/Na2SO3 solution. Utilizing the solid solution NaLa1-xBixS2 strategy, the absorption properties and band edge position for photocatalytic hydrogen evolution reaction were optimized. The results indicated that the bismuth content is critical parameter for maintaining the photocatalytic activity. The incorporation of low Bi content up to 6% in NaLaS2 leads to extending the photon absorption from the UV to the visible region and enhancing the photocatalytic activity of hydrogen production. In contrast, all the solid solutions that have Bi content of more than 12% present absorption edges close to that of pure NaBiS2, and they are inactive for photocatalytic hydrogen production. Combining the experimental measurements with density functional theory calculations, such behavior can be explained by the degree of overlapping of Bi and La states on the conduction band minimum (CBM). Finally, self-assembly of Bi2S3 nanorods were grown on FG or FTO substrates. Bi2S3 thin films were prepared by sulfurization of Bi metal layer using the hydrothermal method. The results show that Bi2S3 has absorption up to 1.3 eV and has a moderate absorption coefficient in the visible region. The ultraviolet photoelectron spectroscopy and photoelectron spectroscopy in air results showed that the conduction band minimum of Bi2S3 is located slightly above the hydrogen redox potential. However, Pt/Bi2S3 did not evolve a detectable amount of hydrogen, suggesting the presence of surface states that can hinder the hydrogen reduction reaction.
    • Fast Poynting-Vector based wave-mode separation and RTM in 2D elastic TI media

      Liu, Qiancheng; Zhang, Jianfeng; Lu, Yongming; Gao, Hongwei; Liu, Shaolin; Zhang, Hao (Elsevier BV, 2019-01-05)
      The wave-modes in isotropic elastic media are easy to get separated by using Helmholtz decomposition. This method, however, fails in TI (transverse isotropic) media due to the anisotropy, and more sophisticated operators are required. Most of these existing operators are implemented and limited in FD (finite-difference) stencil. We propose a Poynting-vector based method, which separates wave-modes pointwisely, independent of the modeling stencils. In TI media, the Poynting-vector indicates the group-velocity direction while the wave-modes get separated in the polarization-vector direction. We write the relationship between these two directions into a small numerical table by exploiting the phase-velocity direction as a bridge prior to wavefield propagation. During the modeling process, it is easy to estimate the group-velocity direction from the Poynting vector, and then we can get the polarization-vector direction to separate wave-modes by checking the numerical table. We test our method on several TI models. We furthermore apply our method to elastic reverse-time migration (RTM) in TI media.
    • Molecular Simulation Study of Montmorillonite in Contact with Water

      Li, Yiteng; Nair, Arun Kumar Narayanan; Kadoura, Ahmad Salim; Yang, Yafan; Sun, Shuyu (American Chemical Society (ACS), 2019-01-05)
      Grand canonical Monte Carlo and molecular dynamics simulations were applied to understand the molecular mechanism of ion and water transport in montmorillonite clays as a function of relative humidity (RH). The variation of basal spacings of montmorillonite as a function of RH predicted based on the swelling free energy profiles was consistent with X-ray data. The hydration of the montmorillonite shows the following well-known order: Mg2+ > Ca2+ > Sr2+ > Li+ > Na+ > K+. The relative contribution of water on external surfaces only becomes significant close to the saturation pressure. However, this behavior for K-montmorillonite starts to occur well below the saturation pressure due to the clay-swelling inhibition by potassium ions. The diffusion of water and ions generally increases with RH in all samples. However, for samples with weakly hydrated ions, the water mobility in thin films adsorbed on external basal surfaces of clay can be higher than that in the water-saturated mesopores. For a given RH, mobility of interlayer species is typically lower than that from the external surfaces. The results of the simulations were applied to interpret recent laboratory measurements of ion mobility with changing RH. We also assess the effect of layer charge distribution on such sorption and diffusion processes.
    • Polyoxometalate−Cyclodextrin Metal−Organic Frameworks: From Tunable Intrinsic Microporosity to Customized Storage Functionality

      Yang, Peng; zhao, Wenli; Shkurenko, Aleksander; Belmabkhout, Youssef; Eddaoudi, Mohamed; Dong, Xiaochen; Alshareef, Husam N.; Khashab, Niveen M. (American Chemical Society (ACS), 2019-01-04)
      Self-assembly allows structures to organize themselves into regular patterns by using local forces to find the lowest-energy configuration. However, assembling organic and inorganic building blocks in an ordered framework is hampered by the difficulties of interfacing two dissimilar materials. Herein, the ensemble of polyoxometalates (POMs) and cyclodextrins (CDs) as molecular building blocks (MBBs) has yielded two unprecedented POM-CD-MOFs, namely [PW12O40]3− & α-CD MOF (POT-CD) and [P10P15.5O50]19− & γ-CD MOF (POP-CD), with distinct properties not shared by their isolated parent MBBs. Markedly, the POT-CD features a nontraditional enhanced Li storage behavior by virtue of a unique “amorphization & pulverization” process. This opens the door to a new generation of hybrid materials with tuned structures and customized functionalities.
    • Design of intense nanoscale stray fields and gradients at magnetic nanorod interfaces

      Ivanov, Yurii P.; Leliaert, Jonathan; Crespo, Adrian; Pancaldi, Matteo; Tollan, Christopher; Kosel, Jürgen; Chuvilin, Andrey; Vavassori, Paolo (American Chemical Society (ACS), 2019-01-04)
      We explore electrodeposited ordered arrays of Fe, Ni and Co nanorods embedded in anodic alumina membranes as a source of intense magnetic stray field gradients localized at the nanoscale. We perform a multiscale characterization of the stray fields using a combination of experimental methods (Magneto-optical Kerr effect, Virtual Bright Field Differential Phase Contrast Imaging) and micromagnetic simulations, and establish a clear correlation between the stray fields and the magnetic configurations of the nanorods. For uniformly magnetized Fe and Ni wires the field gradients vary following saturation magnetization of corresponding metal and the diameter of the wires. In the case of Co nanorods, very localized (~10 nm) and intense (> 1T) stray field sources are associated with the cores of magnetic vortexes. Confinement of that strong field at extremely small dimensions leads to exceptionally high field gradients up to 108 T/m. These results demonstrate a clear path to design and fine-tune nanoscale magnetic stray field ordered patterns with a broad applicability in key nanotechnologies, such as nanomedicine, nanobiology, nanoplasmonics and sensors.