Search

Motivated by the discovery of more and more phosphates with relatively strong nonlinear optic effect, we studied the mechanism of the second-harmonic generation (SHG) effect in several phosphates by band model and first-principles calculations. When the energy of an incident photon is much smaller than the band gap of material, the SHG is almost frequency independent and determined by the combination of Berry connection and a symmetric tensor. The SHG effect in phosphates can be enhanced by the enhancement of orbital hybridization or the reduction of charge-transfer energy, which results in widened bandwidth of occupied state and reduced band gap in the electronic structure, respectively. By the first-principles calculation on the electronic structures of several phosphates - BPO4, LiCs2PO4, β-Li3VO4, and β-Li3PO4 - we interpreted the relatively strong SHG effect in LiCs2PO4 and β-Li3VO4 as the consequence of the reduced charge-transfer energy compared to their parent β-Li3PO4, while the enhanced SHG in BPO4 is resulting from enhanced orbital hybridization. © 2017 American Physical Society.

The migration of Li ions in electrode materials is the main limiting factor to determine the rate capability of Li-ion batteries. In this work, the influences of Jahn-Teller (JT) distortion on Li migration in full delithiated LiMn2O4 (λ-MnO2) were systematically studied by a first-principles computational approach. Our results unravel the direction of JT distortion strongly affecting the activation barrier of Li-ion migration in λ-MnO2. In particular, Li-ion migration has the lowest activation barrier when the two elongated Mn-O bonds of Mn3+ ion are quasi-collinear with the linked Li-O bonds in the transition state, in contrast with the highest barrier when the elongated Mn-O bonds are approximately perpendicular to the linked Li-O bonds. In addition, lattice-strain-induced variation of the Li-migration barrier in λ-MnO2 exhibits either upward or downward trends, depending on the detailed coupling with JT distortion. Further analysis showed that the difference in activation barriers can be explained by the different Li-O distances in terms of the Coulomb interaction energies, which is induced by the different position and direction of JT distortion. Finally, the Li-ion migration in the whole λ-MnO2 system is also discussed by considering the influences of JT polarons. © 2017 The Royal Society of Chemistry.

Two novel full carbon chiral enantiomers, the CHiral Pentagon Only Diamond-like Structures (CHIPODS) are predicted with space groups P6122 and P6522. The enantiomers can be generated by interlayer covalent bonds in multilayer pentagraphene. Both structures are energetically more favorable than their precursor, pentagraphene. The crystal structures of CHIPODS exhibit intriguing three-dimensional space filling only with pentagons. The mass density (3.24 g/cm3) of CHIPODS is lighter than that in diamond (3.51 g/cm3). CHIPODS is a semiconductor with a bandgap of 5.7 eV, comparable to diamond's gap (5.5 eV). Moreover, the tensile and shear strain-stress simulation results demonstrate that CHIPODS are harder than the diamond in the means of ideal strength. This work highlights the importance of structural transformation from carbon-layered structures to novel carbon allotropes, similar strategy can be applied to other two-dimensional materials. © 2017 American Chemical Society.

5-Hydroxymethylfurfural (HMF) is one of the most important and versatile biomass-derived platform chemicals for the synthesis of a wide range of industrial chemicals, including biofuels. HMF can be obtained by dehydration of glucose with acid catalysts. The conversion proceeds through two steps, that is, glucose–fructose isomerization and dehydration of fructose to HMF. During the first step, water plays an important role as a proton shuttle to promote the reaction and reduce the energy barriers, which results in the acceleration of the reaction. Besides, water can also block the side reaction. The role of water joining and catalyzing the reaction is further proved by experimental findings, that is, by using DMSO/D2O as solvent deuterium was found at C1 of HMF from NMR analysis. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

An efficient conversion of CO2 into valuable fuels and chemicals has been hotly pursued recently. Here, for the first time, we have explored a series of M12 x12 nano-cages (M = B, Al, Be, Mg; X = N, P, O) for catalysis of CO2 to HCOOH. Two steps are identified in the hydrogenation process, namely, H2 activation to 2H∗, and then 2H∗ transfer to CO2 forming HCOOH, where the barriers of two H∗ transfer are lower than that of the H2 activation reaction. Among the studied cages, Be12 O12 is found to have the lowest barrier in the whole reaction process, showing two kinds of reaction mechanisms for 2H∗ (simultaneous transfer and a step-wise transfer with a quite low barrier). Moreover, the H2 activation energy barrier can be further reduced by introducing Al, Ga, Li, and Na to B12 N12 cage. This study would provide some new ideas for the design of efficient cluster catalysts for CO2 reduction. © 2017 The Author(s).

Pseudo-topotactic conversion of carbon nanotubes into one-dimensional carbon nanowires is a challenging but feasible path to obtain desired diameters and morphologies. Here, a previously predicted but experimentally unobserved carbon allotrope, T-carbon, has been produced from pseudo-topotactic conversion of a multi-walled carbon nanotube suspension in methanol by picosecond pulsed-laser irradiation. The as-grown T-carbon nanowires have the same diameter distribution as pristine carbon nanotubes, and have been characterized by high-resolution transmission electron microscopy, fast Fourier transform, electron energy loss, ultraviolet-visible, and photoluminescence spectroscopies to possess a diamond-like lattice, where each carbon is replaced by a carbon tetrahedron, and a lattice constant of 7.80 Å. The change in entropy from carbon nanotubes to T-carbon reveals the phase transformation to be first order in nature. The computed electronic band structures and projected density of states are in good agreement with the optical absorption and photoluminescence spectra of the T-carbon nanowires. © 2017 The Author(s).

Ebola virus is a highly lethal filovirus, claimed thousands of people in its recent outbreak. Seven viral proteins constitute ebola viral structure, and four of them (nucleoprotein (NP), polymerase L, VP35 and VP30) participate majorly in viral replication and transcription. We have elucidated a conformation change of NP cleft by VP35 NP-binding protein domains through superimposing two experimental NP structure images and discussed the function of this conformation change in the replication and transcription with polymerase complex (L, VP35 and VP30). The important roles of VP30 in viral RNA synthesis have also been discussed. A "tapping" model has been proposed in this paper for a better understanding of the interplay among the four viral proteins (NP, polymerase L, VP35 and VP30). Moreover, we have pinpointed some key residue changes on NP (both NP N- and C-terminal) and L between Reston and Zaire by computational studies. Together, this paper provides a description of interactions among ebola viral proteins (NP, L, VP35, VP30 and VP40) in viral replication and transcription, and sheds light on the complex system of viral reproduction. © 2017 World Scientific Publishing Company.

The dielectric properties of AC powder electroluminescent devices were measured and analyzed using complex impedance spectroscopy to determine the relaxation processes occurring within the devices. The relaxation processes identified were ascribed to the electrode polarization caused by ion accumulation at the electrode/resin interfaces, the Maxwell-Wagner-Sillars effects at the (ZnS or BaTiO3) particle/resin interfaces, and the dipolar reorientation of polymer chains in the resin matrix. Each relaxation process was represented by its corresponding equivalent circuit component. Space charge polarization at the electrodes were represented by a Warburg element, a resistor, and a constant phase element. The resin matrix, ZnS/resin and BaTiO3/resin interfaces could each be modeled by a resistor and a capacitor in parallel. The simulated equivalent circuits for three different printed structures showed good fitting with their experimental impedance results. © 2016 Elsevier Ltd

High energy photons can affect the dielectric response of AC powder electroluminescent devices (ACPELDs). In this paper, electroluminescent (EL), phosphor and dielectric films are photo-excited at peak wavelengths of 399 nm, 520 nm and 625 nm to identify the dielectric relaxation processes occurring in ACPELDs. The 399 nm illumination changes the frequency-dependent dielectric responses of both EL and phosphor films due to the photo-induced excitation of ZnS:Cu,Al phosphor particles. A higher illumination intensity increases the dipolar polarization in the resin matrix and enhances the Maxwell-Wagner- Sillars (MWS) effect at the particle/resin interfaces. Equivalent circuits relating to the relaxation processes present in the EL and phosphor films are derived. From the analyses of the circuit component values, a charge generation and accumulation process is proposed to explain these opto-impedance behaviors. © 2017 Optical Society of America.

By devising careful electrophoresis, it was shown that at pH below 7.0, the electrophoretic mobility of 3-mercaptopropionic acid (3MPA) coated CdSe-ZnS core-shell quantum dots (denoted as QD-3MPA) was very small. At pH above 7.0, QD-3MPA migrated toward the anode, implying acid dissociation, and the degree of which was proportional to the pH value. QD-3MPA's electrophoretic mobility was impaired after adding sufficient Ca2+ ions to the QD solution and revived when a similar amount of Ca2+ chelators (ethylene glycol tetraacetic acid, EGTA) was added. This demonstrated that acid dissociation and its pH dependence of 3MPA on the QD surface are critical factors in understanding the electric and optical properties of QDs. The acid dissociated QD-3MPA interacted strongly with Ca2+, forming a charge neutral QD-3MPA Ca2+ complex in the absence of EGTA. First-principles study confirmed the observed experimental evidence. The strong ionic interaction between acid dissociated QD-3MPA and Ca2+ is critical for developing reliable QD-based biosensing assays. Moreover, the strategy and techniques reported in this work are easily applicable to other fluorescent biomarkers and therefore can be important for advancing in vivo and in vitro imaging, sensing, and labeling., QC 20160912

By generalizing the traditional single-site strong coupling expansion approach to a cluster one, we study the zero-temperature phase diagram of bosonic atoms in a trimerized Kagom� optical lattice. Some new features are present in this system. Due to the strong intra-trimer hopping interaction, there will be a new Mott insulator (MI), which is by definition incompressible but with a fractional filling per trimer. This is different from the traditional MI, which has an integral filling and originates only from the repulsive interaction between particles. We investigate the MI-to-superfluid transition and the nature of the fractional MI by calculating the critical exponents of phase transitions and the low-lying energy excitation spectra of quasiparticles (quasihole). We will show how the low-energy properties of this system can be understood qualitatively as a Bose-Hubbard model in triangular lattice from the point of view of the cluster strong coupling expansion. We also discuss how our results are related to experiment by studying the Bragg spectroscopy. � 2016 IOP Publishing Ltd.

Dendrite formation on the electrodes of a rechargeable battery during the charge-discharge cycle limits its capacity and application due to short-circuits and potential ignition. However, understanding of the underlying dendrite growth and dissolution mechanisms is limited. Here, the electrochemical growth and dissolution of silver dendrites on platinum electrodes immersed in an aqueous silver nitrate (AgNO3) electrolyte solution was investigated using in situ liquid-cell transmission electron microscopy (TEM). The dissolution of Ag dendrites in an AgNO3 solution with added cetyltrimethylammonium bromide (CTAB) surfactant was compared to the dissolution of Ag dendrites in a pure aqueous AgNO3 solution. Significantly, when CTAB was added, dendrite dissolution proceeded in a step-by-step manner, resulting in nanoparticle formation and transient microgrowth stages due to Ostwald ripening. This resulted in complete dissolution of dendrites and "cleaning" of the cell of any silver metal. This is critical for practical battery applications because "dead" lithium is known to cause short circuits and high-discharge rates. In contrast to this, in a pure aqueous AgNO3 solution, without surfactant, dendrites dissolved incompletely back into solution, leaving behind minute traces of disconnected silver particles. Finally, a mechanism for the CTAB-influenced dissolution of silver dendrites was proposed based on electrical field dependent binding energy of CTA+ to silver. © 2016 American Chemical Society.

We experimentally demonstrate a magnetic field sensor based on long-period fiber grating (LPG) and magnetic fluid. Our proposed sensor possesses a high sensitivity of ∼0.154 dB/Gauss and a low measurement threshold of ∼7.4 Gauss. © 2015 IEEE.

Soft fluidlike nanoscale objects can drive nanoparticle assembly by serving as a scaffold for nanoparticle organization. The intermediate steps in these template-directed nanoscale assemblies are important but remain unresolved. We used real-time in situ transmission electron microscopy to follow the assembly dynamics of platinum nanoparticles into flexible ringlike chains around ethylenediaminetetraacetic acid nanodroplets dispersed in solution. In solution, these nanoring assemblies form via sequential attachment of the nanoparticles to binding sites located along the circumference of the nanodroplets, followed by the rearrangement and reorientation of the attached nanoparticles. Additionally, larger nanoparticle ring assemblies form via the coalescence of smaller ring assemblies. The intermediate steps of assembly reported here reveal how fluidlike nanotemplates drive nanoparticle organization, which can aid the future design of new nanomaterials. © 2016 American Chemical Society.

The selective electrocatalytic conversion of CO2 into useful products is a major challenge in facilitating a closed carbon cycle. Here, on the basis of first-principles calculations combined with computational hydrogen electrode model, we report a curvature-dependent selectivity of CO2 reduction on cobalt-porphyrin nanotubes which are thermodynamically stable, displaying tunable geometric and electronic properties with tube radius. We have found that CO production is preferred on nanotubes with larger diameter, and the predicted current density from microkinetics is larger than that on Au, the best metal catalyst for CO production from CO2 electroreduction. In contrast, highly curved nanotubes with small radii tend to further catalyze CO reduction to CH4 gas and the overpotential is much lower in comparison with the cases on Cu surfaces. The selectivity and the feasibility of synthesis make cobalt-porphyrin nanotubes very promising for CO2 conversion. © 2016 American Chemical Society.

2D materials are the focus of an intense research effort because of their unique properties and their potential for revealing intriguing new phenomena. Phosphorene, a monolayer of black phosphorus, earned its place among the family of 2D semiconductor materials when recent results unveiled its high carrier mobility, high optical and UV absorption, and other attractive properties, which are of particular interest for optoelectronic applications. Unlike graphene, phosphorene has an anisotropic orthorhombic structure that is ductile along one of the in-plane crystal directions but stiff along the other. This results in unusual mechanical, electronic, optical and transport properties that reflect the anisotropy of the lattice. This Review summarizes the physical properties of phosphorene and highlights the recent progress made in the preparation, isolation and characterization of this material. The role of defects and doping is discussed, and phosphorene-based devices are surveyed; finally, the remaining challenges and potential applications of phosphorene are outlined. © 2016 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

The reaction mechanism of 1,2×n-deoxydehydration (DODH; n=1, 2, 3 ...) reactions with 1-butanol as a reductant in the presence of methyltrioxorhenium(VII) catalyst has been investigated by DFT. The reduced rhenium compound, methyloxodihydroxyrhenium(V), serves as the catalytically relevant species in both allylic alcohol isomerization and subsequent DODH processes. Compared with three-step pathway A, involving [1,3]-transposition of allylic alcohols, direct two-step pathway B is an alternative option with lower activation barriers. The rate-limiting step of the DODH reaction is the first hydrogen transfer in methyltrioxorhenium(VII) reduction. Moreover, the increase in the distance between two hydroxyl groups in direct 1,2×n-DODH reactions for C4 and C6 diols results in a higher barrier height. Find the right path: The reaction mechanism of 1,2×n-deoxydehydration (DODH; n=1, 2, 3 ...) reactions with 1-butanol as a reductant in the presence of methyltrioxorhenium(VII) catalyst has been investigated by density functional theory (DFT). Compared with three-step pathway A, involving [1,3]-transposition of allylic alcohols, direct two-step pathway B is an alternative option with lower activation barriers. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The electronic structure, nonlinear electronic polarization induced by a static external electric field, and frequency dependent second-harmonic susceptibility tensor of the borophosphate BPO4 are studied by a first-principles calculation based on density-functional theory. Our calculated results show that the borophosphate BPO4 has a large band gap ∼10.4 eV, which is larger than the band gap of the widely used nonlinear optical crystal KBe2BO3F2. However, BPO4 also has a nonlinear coefficient d36=0.92 pm/V at static limit, which also is larger than the nonlinear coefficient d11=0.47 pm/V of KBe2BO3F2. The unexpected larger nonlinear coefficient of BPO4 can be interpreted by the relatively strong s-p hybridization in BPO4, which can enhance the inter-band Berry connections, while the O 2p orbitals dominating valence bands in KBe2BO3F2 are very flat, resulting from weak s-p hybridization. © 2016 American Physical Society.

Density functional theory is employed to investigate the Fischer-Tropsch mechanism on copper ∑5(310) tilt grain boundary together with Cu(111) surface. For the methanol formation, Cu∑5(310) can effectively reduce each energy barrier through the preferred CH3O intermediate route as compared with Cu(111). Furthermore, Cu∑5(310) provides a synergetic effect to modulate the energy landscape in the methanol synthesis, where the pathway associated with CH2OH (CH2O CH2OH CH3OH) presents the clear kinetic advantage than the CH3O route. Simultaneously, the kinetically assisted intermediate CH2OH enable the CH2 production both thermodynamically and kinetically on Cu∑5(310). The formation of C2 oxygenates from CH2 provides a critical precursor to higher alcohol synthesis. The higher activity of grain boundary is attributed to the presence of low-coordinated sites with flexible local configuration. Consequently, the adsorption strength of small species is enhanced to provide a thermodynamic driving force for C-O bond scission. The adaptive character of ∑5(310) grain boundary facilitates the stabilization of the transition state, therefore lowering the activation barrier. Present work unravels the microscopic origin of the higher catalytic capacity of copper ∑5(310) grain boundary which can be helpful to guide the development of novel Cu-based catalysts for higher alcohol formation from syngas. © 2015 Elsevier B.V. All rights reserved.

A highly active alternative to Pt electrocatalysts for the oxygen reduction reaction (ORR), which is the cathode-electrode reaction of fuel cells, is sought for higher fuel-cell performance. Our theoretical modelling reveals that B-doped Pd (Pd-B) weakens the absorption of ORR intermediates with nearly optimal binding energy by lowering the barrier associated with O2dissociation, suggesting Pd-B should be highly active for ORR. In fact, Pd-B, facile synthesized by an electroless deposition process, exhibits 2.2times and 8.8times higher specific activity and 14times and 35times less costly than commercial pure Pd and Pt catalysts, respectively. Another computational result is that the surface core level of Pd is negatively shifted by B doping, as confirmed by XPS, and implies that filling the density of states related to the anti-bonding of oxygen to Pd surfaces with excess electrons from B doping, weakens the O bonding to Pd and boosts the catalytic activity. Better with a B in its bonnet: Theoretical modelling shows that B doping negatively shifts the surface core level of Pd and lowers the barrier to O2dissociation for the oxygen reduction reaction (ORR). A B-doped Pd nanoparticle catalyst was then rationally designed, synthesized in a facile manner by electroless deposition, and shown to be a highly active ORR catalyst compared to commercial Pd and Pt catalysts. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Understanding the reaction mechanisms of CO2 and CO methanation processes is critical towards the successful development of heterogeneous catalysts with better activity, selectivity, and stability. This review provides detailed mechanisms of methanation processes and undesired catalyst deactivation. We characterize the methanation processes into two categories: (1) associative scheme, in which hydrogen atoms are involved in the C-O bond breaking process, and (2) dissociative scheme, where C-O bond breaking takes place before hydrogenation. For the catalyst deactivation mechanisms, we highlight three important factors, i.e. sulfur poisoning, carbon deposition and metal sintering. © 2016 The Royal Society of Chemistry.

Three low band gap thienopyridazine-based donor-acceptor conjugated polymers, PDTTPTT, PDTTPBT, and PDTTPBDT were synthesized by the Stille copolymerization of thienopyridazine with thienothiophene, benzodithiophene, and bithiophene, respectively. The optical band gaps of PDTTPTT, PDTTPBT, and PDTTPBDT polymers were determined as 1.41 eV, 1.43 eV, and 1.58 eV, respectively by UV absorption. The deep HOMO level of the three polymers was determined by cyclic voltammetry. The three polymers were fabricated to form organic field-effect transistors and their performance as p-type semiconductors were studied. (C) 2016 Published by Elsevier Ltd.