Fully exohydrogenated Si60 fullerene cage

Using first-principles calculations based on density functional theory, we demonstrate that Si₆₀ fullerene cage can be stabilized by exohydrogenated method. In contrast to previous theoretical studies that Si₆₀ fullerene geometry construction will be seriously distorted when it is bare or encapsulated by metal atom clusters, exohydrogenated scheme shows that Si₆₀H₆₀ cage will be able to keep perfect fullerene structure similar to C₆₀.     

The structures and stabilities of small diarsenic-doped silicon clusters

The structures and stabilities of As₂-doped Si_n (n = 1-7) clusters have been investigated at the B3LYP level of theory, incorporating the 6-311+G^∗ basis set. An isosceles triangle is predicted to be the lowest-energy structure of the As₂Si cluster, whereas the global minimum of As₂Si₂ possesses an As-As-butterfly structure. The ground state structures for As₂Si₃, As₂Si₄ and As₂Si₅ are all bipyramids: trigonal, tetragonal and pentagonal, respectively, which could have important applications as building blocks to synthesize silicon nanowires. The most stable isomer of As₂Si₆ possesses a tricapped trigonal bipyramid structure. The lowest energy structure of As₂Si₇ can be viewed as a substitutional structure of the tricapped trigonal prism Si₉ isomer. In the majority of the lowest energy isomers, the two As atoms tend to be separated from each other, in order to maximize the number of Si-As bonds, and therefore locate at the axial vertex or face-capping atomic positions, especially for As₂Si₄-As₂Si₇. According to results of the incremental binding energies, the HOMO-LUMO gaps and the vertical ionization potentials, the As₂Si₃ and As₂Si₆ clusters are relatively stable compared to their neighbors. Natural bond orbital analyses suggest that delocalized electrons and multi-centered bonds play an important role in stabilizing the low-energy As₂Si_n structures.     

A F420-dependent NADP reductase in the extremely thermophilic sulfate-reducing Archaeoglobus fulgidus

Archaeoglobus fulgidus, a sulfate-reducing Archaeon with a growth temperature optimum of 83°C, uses the 5-deazaflavin coenzyme F₄₂₀ rather than pyridine nucleotides in catabolic redox processes. The organism does, however, require reduced pyridine nuclcotides for biosynthetic purposes. We describe here that the Archaeon contains a coenzyme F₄₂₀-dependent NADP reductase which links anabolism to catabolism. The highly thermostable enzyme was purfied 3600-fold by affinity chromatography to apparent homogeneity in a 60% yield. The native enzyme with an apparent molecular mass of 55 kDa was composed of only one type of subunit of apparent molecular mass of 28 kDa. Spectroscopic analysis of the enzyme did not reveal the presence of any chromophoric prosthetic group. The purified enzyme catalyzed the reversible reduction of NADP (apparent K _M 40 M) with reduced F₄₂₀ (apparent K _M 20M) with a specific activity of 660 U/mg (apparent V _max) at pH 8.0 (pH optimum) and 80°C (temperature optimum). It was specific for both coenzyme F₄₂₀ and NADP. Sterochemical investigations showed that the F₄₂₀-dependent NADP reductase was Si face specific with respect to C5 of F₄₂₀ and Si face specific with respect to C4 of NADP.Abbreviations F₄₂₀ coenzyme F₄₂₀ - F₄₂₀H₂ 1,5-dihydrocoenzyme F₄₂₀ - H₄MPT tetrahydromethanopterin - CH=H₄MPT N⁵, N¹⁰-methylenetetrahydromethanopterin - MFR methanofuran - HPLC high performance liquid chromatography - methylene-H₄MPT dehydrogenase N⁵, N¹⁰-methylenetetrahydromethanopterin dehydrogenase - 1 U = 1 mol/min     

Arsenic interactions with a fullerene-like BN cage in the vacuum and aqueous phase

Adsorption of arsenic ions, As (III and V), on the surface of fullerene-like B₁₂N₁₂ cage has been explored in vacuum and aqueous phase using density functional theory in terms of Gibbs free energies, enthalpies, geometry, and density of state analysis. It was found that these ions can be strongly chemisorbed on the surface of the cluster in both vacuum and aqueous phase, resulting in significant changes in its electronic properties so that the cluster transforms from a semi-insulator to a semiconductor. The solvent significantly affects the geometry parameters and electronic properties of the As/B₁₂N₁₂ complexes and the interaction between components is considerably weaker in the aqueous phase than that in the vacuum.     

The structure and stability of B36N36 cages: a computational study

The structure and stability of 22 B₃₆N₃₆ cage molecules containing four-membered (F ₄), five-membered (F ₅), six-membered (F ₆), eight-membered (F ₈) and 12-membered (F ₁₂) rings have been computed at the B3LYP/6-31G* level of density functional theory. The most stable structure (1) has T _d symmetry with six F ₄ and 32 F ₆ rings, following the isolated square rule, while the fullerene-like structures (12 F ₅ and 26 F ₆) and also structures with F ₈ and F ₁₂ are much higher in energy. Figure The T _d symmetrical structure (1) with six F ₄ and thirty-two F ₆ rings is the most stable B₃₆N₃₆ cage.     

The effect of tree height on crown level stomatal conductance

Variation in stomatal conductance is typically explained in relation to environmental conditions. However, tree height may also contribute to the variability in mean stomatal conductance. Mean canopy stomatal conductance of individual tree crowns (G_Si) was estimated using sap flux measurements in Fagus sylvatica L., and the hypothesis that G_Si decreases with tree height was tested. Over 13 d of the growing season during which soil moisture was not limiting, G_Si decreased linearly with the natural logarithm of vapour pressure deficit (D), and increased exponentially to saturation with photosynthetic photon flux density (Q_o). Under conditions of D = 1 kPa and saturating Q_o, G_Si decreased by approximately 60% with 30 m increase in tree height. Over the same range in height, sapwood‐to‐leaf area ratio (A_S:A_L) doubled. A simple hydraulic model explained the variation in G_Si based on an inverse relationship with height, and a linear relationship with A_S:A_L. Thus, in F. sylvatica, adjustments in A_S:A_L partially compensate for the negative effect of increased flow‐path length on leaf conductance. Furthermore, because stomata with low conductance are less sensitive to D, gas exchange of tall trees is reduced less by high D. Despite these compensations, decreasing hydraulic conductance with tree height in F. sylvatica reduces carbon uptake through a corresponding decrease in stomatal conductance.     

A Low Resistance Calcium/Reduced Titania Passivated Contact for High Efficiency Crystalline Silicon Solar Cells

Recent advances in the efficiency of crystalline silicon (c‐Si) solar cells have come through the implementation of passivated contacts that simultaneously reduce recombination and resistive losses within the contact structure. In this contribution, low resistivity passivated contacts are demonstrated based on reduced titania (TiO_x) contacted with the low work function metal, calcium (Ca). By using Ca as the overlying metal in the contact structure we are able to achieve a reduction in the contact resistivity of TiO_x passivated contacts of up to two orders of magnitude compared to previously reported data on Al/TiO_x contacts, allowing for the application of the Ca/TiO_x contact to n‐type c‐Si solar cells with partial rear contacts. Implementing this contact structure on the cell level results in a power conversion efficiency of 21.8% where the Ca/TiO_x contact comprises only ≈6% of the rear surface of the solar cell, an increase of 1.5% absolute compared to a similar device fabricated without the TiO_x interlayer.     

2D quantum mechanical device modeling and simulation: Centre-channel (CC) and double-gate (DG) MOSFET

In this paper, a novel device structure (Si_1 ? x Ge _x /Si/Si_1 ? x Ge _x hetero-structure), which is named as “center-channel (CC) double-gate (DG) MOSFET,” is proposed. Device performance of the proposed FET structure was investigated with our two-dimensional quantum-mechanical simulator which produces a self-consistent solution of Poisson–Schrödinger equations and the current continuity equation. The CC operation of CC-NMOS is confirmed from the electron density distribution and the band lineups as well as the lowest energy wave function. Current–voltage characteristics including the trans-conductance (G _m) of CC-MOSFET are carefully compared with those of the conventional DG-NMOS to evaluate the distinct feature of the proposed FET structure. Our simulation revealed that the proposed FET demonstrates the enhanced (about (～1.6 × ) current drive and 60% G _m. Finally, the short-channel effects of CC and DG MOSFET, both of which demonstrate excellent sub-threshold behaviors and open the possibility of device scaling down to sub-20 nm.     

A Novel Phase of Li15Si4 Synthesized under Pressure

Li₁₅Si₄, the only crystalline phase that forms during lithiation of the Si anode in lithium‐ion batteries, is found to undergo a structural transition to a new phase at 7 GPa. Despite the large unit cell of Li₁₅Si₄ (152 atoms in the unit cell), ab initio evolutionary metadynamics (using the USPEX code) successfully predicts the atomic structure of this new phase (β‐Li₁₅Si₄), which has an orthorhombic structure with an Fdd2 space group. In the new β‐Li₁₅Si₄ phase Si atoms are isolated by Li atoms analogous to the original cubic phase (α‐Li₁₅Si₄), whereas the atomic packing is more efficient owing to the higher Si? Li coordination number and shorter Si? Li, Li? Li bonds. β‐Li₁₅Si₄ has substantially larger elastic moduli compared with α‐Li₁₅Si₄, and has a good electrical conductivity. As a result, β‐Li₁₅Si₄ has superior resistance to deformation and fracture under stress. The theoretical volume expansion of Si would decrease 25% if it transforms to β‐Li₁₅Si₄, instead of α‐Li₁₅Si₄, during lithiation. Moreover, β‐Li₁₅Si₄ can be recovered back to ambient pressure, providing opportunities to further investigate its properties and potential applications.     

Ab initio molecular dynamics simulation on the formation process of He@C60 synthesized by explosion

The applications of endohedral non-metallic fullerenes are limited by their low production rate. Recently, an explosive method developed in our group shows promise to prepare He@C₆₀ at fairly high yield, but the mechanism of He inserting into C₆₀ cage at explosive conditions was not clear. Here, ab initio molecular dynamics analysis has been used to simulate the collision between C₆₀ molecules at high-temperature and high-pressure induced by explosion. The results show that defects formed on the fullerene cage by collidsion can effectively decrease the reaction barrier for the insertion of He into C₆₀, and the self-healing capability of the defects was also observed.

Computational synthesis of C<Subscript>60</Subscript> cyano- and azopolyderivatives

The cyanation of C₆₀ to C₆₀(CN)₁₈ and the aziridination of C₆₀ to C₆₀(NH)₉ were studied by an unrestricted broken spin symmetry Hartree–Fock approach implemented in semiempirical codes based on the AM1 technique. The calculations focused on the successive addition of CN and NH moieties to the fullerene cage following the identification of the target cage atoms as those with the highest atomic chemical susceptibilities calculated at each step. The results obtained were analyzed from the viewpoint of the parallelism between these derivatives as well as C₆₀ fluorides and hydrides. The difference between the first-stage C₆₀ chlorination and other sterically free processes is discussed.     

Computational synthesis of hydrogenated fullerenes from C<Subscript>60</Subscript> to C<Subscript>60</Subscript>H<Subscript>60</Subscript>

Hydrogenation from C₆₀ to C₆₀H₆₀ was studied by an unrestricted broken spin symmetry Hartree–Fock approach implemented in semiempirical codes based on the AM1 technique. The calculations focused on the successive addition of hydrogen molecules to the fullerene cage following the identification of the cage target atoms by calculating the highest atomic chemical susceptibility at each step. The results obtained are analyzed from energy, symmetry, and composition perspectives.     

Peculiarities of the antioxidant and radioprotective effects of hydrated C60 fullerene nanostuctures in vitro and in vivo

Aqueous solutions of highly stable supramolecular donor–acceptor complexes of chemically nonmodified pristine C₆₀ fullerene molecules with H₂O molecules (hydrated C₆₀ fullerene–C₆₀HyFn) and their labile nano-sized clusters were examined for their antioxidant effects on removal of hydroxyl radicals (OH) and protecting DNA against oxidative damage induced by ionizing radiation in vitro. The suppressing influence of C₆₀HyFn on the formation of OH-radicals in water exposed to X-rays at doses of 1–7 Gy was assessed by determination of oxidation levels of coumarin-3-carboxylic acid. C₆₀HyFn demonstrates apparent antiradical activity in vitro in the range of concentrations of 10⁻¹¹–10⁻⁶ M. Paradoxically, the OH-removing efficacy of C₆₀HyFn was in reverse correlation with fullerene concentration. It was hypothesized that the antiradical action of C₆₀HyFn in water medium generally is due to a “nonstoichiometric” mechanism, supposedly to a hydrated free radical recombination (self-neutralization), which is catalyzed by specific water structures ordered by C₆₀HyFn. With the use of 8-oxoguanine as a marker of oxidative damage to DNA, it has been demonstrated that C₆₀HyFn in concentrations of 10⁻⁷–10⁻⁶ M protects nucleic acids against radical-induced damage. The second part of the present study was aimed to evaluate the overall radioprotective efficacy of C₆₀HyFn in doses of 0.1 or 1 mg/kg b.w. injected intraperitoneally to mice either 1 h before or 15 min after lethal dose exposure of the X-ray (7 Gy) irradiation. Survival rate of the mice was observed at 30 day intervals after irradiation, while the weight gains of experimental animals were monitored as well. The most significant protective effect was demonstrated when 1 mg/kg dosage of C₆₀HyFn was administered before irradiation. The outcome of the substance testing is 15% survival rate of irradiated animals at 30 days of observation, and prevention of noticeable weight loss characteristic for radiation impact, versus unprotected control animals. In conclusion, results of the study obviate that the apparent protective action of C₆₀HyFn in vivo is determined by its considerable ability to decrease X-ray-generated reactive oxygen species. Based on the results and that neat C₆₀ is nontoxic, actually in the hydrated form, without side effects and with sufficient radioprotective effects in low doses, C₆₀HyFn may be considered as a novel antioxidant agent, which substantially diminishes the harmful effects of ionizing radiation.     

Approaching the Minimum Thermal Conductivity in Rhenium‐Substituted Higher Manganese Silicides

Higher manganese silicides (HMS) made of earth‐abundant and non‐toxic elements are regarded as promising p‐type thermoelectric materials because their complex crystal structure results in low lattice thermal conductivity. It is shown here that the already low thermal conductivity of HMS can be reduced further to approach the minimum thermal conductivity via partial substitution of Mn with heavier rhenium (Re) to increase point defect scattering. The solubility limit of Re in the obtained Re_xMn_1‐xSi_1.8 is determined to be about x = 0.18. Elemental inhomogeneity and the formation of ReSi_1.75 inclusions with 50?200 nm size are found within the HMS matrix. It is found that the power factor does not change markedly at low Re content of x ≤ 0.04 before it drops considerably at higher Re contents. Compared to pure HMS, the reduced lattice thermal conductivity in Re_xMn_1‐xSi_1.8 results in a 25% increase of the peak figure of merit ZT to reach 0.57 ± 0.08 at 800 K for x = 0.04. The suppressed thermal conductivity in the pure Re_xMn_1‐xSi_1.8 can enable further investigations of the ZT limit of this system by exploring different impurity doping strategies to optimize the carrier concentration and power factor.     

In Situ Grain Boundary Functionalization for Stable and Efficient Inorganic CsPbI2Br Perovskite Solar Cells

The phase instability and large energy loss are two obstacles to achieve stable and efficient inorganic‐CsPbI_3?xBr_x perovskite solar cells. In this work, stable cubic perovskite (α)‐phase CsPbI₂Br is successfully achieved by Pb(Ac)₂ functioning at the grain boundary under low temperature. Ac^? strongly coordinates with CsPbI₂Br to stabilize the α‐phase and also make the grain size smaller and film uniform by fast nucleation. PbO is formed in situ at the grain boundary by decomposing Pb(Ac)₂ at high‐temperature annealing. The semiconducting PbO effectively passivates the surface states, reduces the interface recombination, and promotes the charge transport in CsPbI₂Br perovskite solar cells. A 12% efficiency and good stability are obtained for in situ PbO‐passivated CsPbI₂Br solar cells, while Pb(Ac)₂‐passivated device exhibits 8.7% performance and the highest stability, much better than the control device with 8.5% performance and inferior stability. This article highlights the extrinsic ionic grain boundary functionalization to achieve stable and efficient inorganic CsPbI_3?xBr_x materials and the devices.     

Measurement of the unstained biological sample by a novel scanning electron generation X-ray microscope based on SEM

We introduced a novel X-ray microscope system based on scanning electron microscopy using thin film, which enables the measurement of unstained biological samples without damage. An unstained yeast sample was adsorbed under a titanium (Ti)-coated silicon nitride (Si₃N₄) film 90 nm thick. The X-ray signal from the film was detected by an X-ray photodiode (PD) placed below the sample. With an electron beam at 2.6 kV acceleration and 6.75 nA current, the yeast image is obtained using the X-ray PD. The image is created by soft X-rays from the Ti layer. The Ti layer is effective in generating the characteristic 2.7-nm wavelength X-rays by the irradiation of electrons. Furthermore, we investigated the electron trajectory and the generation of the characteristic X-rays within the Ti-coated Si₃N₄ film by Monte Carlo simulation. Our system can be easily utilized to observe various unstained biological samples of cells, bacteria, and viruses.     

模拟SO42-沉降对闽江口淡水感潮野慈姑湿地甲烷排放通量的影响

2015年12月—2016年10月,每月小潮日原位定期向闽江口塔礁洲淡水感潮野慈姑(Sagittaria trifolia L.)湿地施加剂量为60、120 kg S hm~(-2)a~(-1)的K_2SO_4溶液(分别记做S-60和S-120),探讨模拟硫酸根(SO_4~(2-))沉降对河口淡水感潮湿地甲烷(CH4)排放通量及间隙水SO_4~(2-)浓度的影响。对照、S-60和S-120处理组CH_4排放通量年均值分别为(7.88±1.00)mg h~(-1)m~(-2)、(6.55±0.97)mg h~(-1)m~(-2)和(6.66±1.49)mg h~(-1)m~(-2)。在年尺度上,两个高强度模拟SO_4~(2-)沉降处理组均未显著降低闽江口淡水感潮野慈姑湿地CH_4排放通量(P0.05),即高强度SO_4~(2-)沉降不会对河口淡水感潮湿地CH_4排放通量产生类似于其对泥炭湿地和水稻田的显著抑制效应。在年尺度以及秋、冬季,两个施加K_2SO_4溶液处理显著增加了野慈姑湿地10 cm深度土壤间隙水SO_4~(2-)浓度。对于各个处理组,温度较高的夏、秋季CH_4排放通量均显著高于温度相对较低的冬、春季(P0.05)。不同处理组CH_4排放通量均与土壤温度呈显著正相关关系,温度仍然是影响亚热带河口淡水感潮湿地CH_4排放通量的重要环境因子。     

Decomposition of CH4 hydrate: effects of temperature and salt from molecular simulations

We report a molecular simulation study to investigate the decomposition of CH₄ hydrate. The decomposition is revealed to be stepwise from the outer to inner layers. Upon decomposition, the number of 5¹²6² cages drops faster than that of 5¹² cages. CH₄ molecules are released, dissolved in water, then enter gas phase; meanwhile, CH₄ bubbles may form particularly at a high temperature. Based on the variations of potential energy, order parameter, cage number and density profile of CH₄ at different temperatures (300, 330, 345 and 360 K) and NaCl concentrations (0, 0.6 and 1.8 M), the effects of temperature and salt are comprehensively examined. With increasing temperature, the decomposition in pure water is accelerated, whereas two opposite effects are observed in NaCl solution. At 330 K, the decomposition is retarded at a higher NaCl concentration, as attributed to the reduced CH₄ solubility in NaCl solution and the participation of ions in cage formation; at 360 K, however, the decomposition is accelerated when NaCl concentration increases due to bubble formation. This simulation study provides microscopic insights into hydrate decomposition, which might be useful towards the optimisation of operating conditions for CH₄ production from CH₄ hydrate.     

Flurbiprofen,A Cyclooxygenase Inhibitor,Reduces the Brain Arachidonic Acid Signal in Response to the Cholinergic Muscarinic Agonist,Arecoline, in Awake Rats

Cholinergic muscarinic receptors, when stimulated by arecoline, can activate cytosolic phospholipase A₂ (cPLA₂) to release arachidonic acid (AA) from membrane phospholipid. This signal can be imaged in the brain in vivo using quantitative autoradiography following the intravenous injection of radiolabeled AA, as an increment in a regional brain AA incorporation coefficient k*. Arecoline increases k* significantly in brain regions having muscarinic M_1,3,5 receptors in wild-type but not in cyclooxygenase (COX)-2 knockout mice. To further clarify the roles of COX enzymes in the AA signal, in this paper we imaged k* following arecoline (5 mg/kg i.p.) or saline in each of 81 brain regions of unanesthetized rats pretreated 6 h earlier with the non-selective COX inhibitor flurbiprofen (FB, 60 mg/kg s.c.) or with vehicle. Baseline values of k* were unaffected by FB treatment, which however reduced by 80% baseline brain concentrations of prostaglandin E₂ (PGE₂) and thromboxane B₂ (TXB₂), eicosanoids preferentially derived from AA via COX-2 and COX-1, respectively. In vehicle-pretreated rats, arecoline increased the brain PGE₂ but not TXB₂ concentration, as well as values for k* in 77 of the 81 brain regions. FB-pretreatment prevented these arecoline-provoked changes. These results and those reported in COX-2 knockout mice suggest that the AA released in brain following muscarinic receptor-mediated activation is lost via COX-2 to PGE₂ but not via COX-1 to TXB₂, and that increments in k* following arecoline largely represent replacement by unesterified plasma AA of this loss.     

Spectral‐converting study of Ba9–3(m+n)/2ErmYbnY2Si6O24 (m = 0.005 – 0.2, n = 0 – 0.3) orthosilicate phosphors

Optical materials composed of Ba_9–3(m+n)/2Er_mYb_nY₂Si₆O₂₄ (m = 0.005–0.2, n = 0–0.3) were prepared using a solid‐state reaction. The X‐ray diffraction patterns of the obtained phosphors were examined to index the peak positions. The photoluminescence (PL) excitation and emission spectra of the Er³⁺‐activated phosphors and the critical emission quenching as a function of Er³⁺ content in the Ba_9–3m/2Er_mY₂Si₆O₂₄ structure were monitored. The spectral conversion properties of Er³⁺ and Er³⁺–Yb³⁺ ions doped in Ba₉Y₂Si₆O₂₄ phosphors were elucidated under diode‐laser irradiation at 980 nm. Up‐conversion emission spectra and the dependence of the emission intensity on pump power for the Ba_8.55Er_0.1Yb_0.2Y₂Si₆O₂₄ phosphor were investigated. The desired up‐conversion of the emitted light, which passed through the green, yellow, orange and red regions of the spectrum, was achieved through the use of appropriate Er³⁺ and/or Yb³⁺ concentrations in the host structure and 980 nm excitation light. The up‐conversion mechanism in the phosphors is described by an energy‐level schematic. Copyright © 2015 John Wiley & Sons, Ltd.