Computational Study of Dynamic Susceptibility and Phase-Matching Angle by Two-Photon Entangled Generation.

Two-photon entangled generation is used to produce an entangled photon source which is a key and core element concerning the technology applications of quantum computing, quantum communication, and quantum precision measurement. In this work, we have deduced the formulas of dynamic susceptibility and phase-matching angle of two-photon entangled generation in nonlinear optical crystals. The formulas are employed to compute the susceptibilities and phase-matching angles of these optical processes for uniaxial and biaxial crystals. The susceptibility magnitude and phase-matching condition of two-photon entangled generation affect the performance of the source. The calculated results by these formulas are employed to study properties and estimate the performance of an entangled photon source. In this way, we discuss the phase matching among waves and working wavelength in an entangled source that affects the efficiency of satellite communication with the ground during the day and night.

Heusler alloy catalysts for electrochemical CO2 reduction.

Developing efficient catalysts for electrochemical CO2 reduction reaction (ECO2RR) to hydrocarbons is becoming increasingly important but still challenging due to their high overpotential and poor selectivity. Here, the famous Heusler alloys are investigated as ECO2RR catalysts for the first time by means of density functional theory calculations. The linear scaling relationship between the adsorption energies of CHO (and COOH) and CO intermediates is broken and, thus, the overpotential can be tuned regularly by chemically permuting different 3d, 4d, or 5d transition metals (TMs) in Heusler alloy Cu2TMAl. Cu2ZnAl shows the best activity among all the 30 Heusler alloys considered in the present study, with 41% improvement in energy efficiency compared to pure Cu electrode. Cu2PdAl, Cu2AgAl, Cu2PtAl, and Cu2AuAl are also good candidates. The calculations on the competition between hydrogen evolution reaction and CO2RR indicate that Cu2ZnAl is also the one having the best selectivity toward hydrocarbons. This work identifies the possibility of applying the Heusler alloy as an efficient ECO2RR catalyst. Since thousands of Heusler alloys have been found in experiments, the present study also encourages the search for more promising candidates in this broad research area.

Ba4GeSb2Se11: An Infrared Nonlinear Optical Crystal with a V-Shaped Se32- Group Possessing a Large Contribution to the SHG Response.

The quaternary selenide Ba4GeSb2Se11 was prepared by a high-temperature solid state reaction method. Ba4GeSb2Se11 crystallizes in an acentric orthorhombic space group Cmc21 with the lattice constants a = 9.370(11) Å, b = 25.850(0) Å, and c = 8.798(10) Å. The compound is composed of a [SbSe3]3- trigonal pyramid, [GeSbSe5]3- dimers, V-shaped Se32-, and the adjacent Ba2+ ions. It has indirect band gap of 1.35 eV and exhibits a second harmonic generation intensity of about 0.2 times that of the benchmark compound AgGaS2 at the same particle size. Interestingly, theoretical analyses show that the central Se atom of Se32- has the largest contribution (8.1%) to d31 compared to that of other Se atoms, which may be due to its easy swing in the a-axis direction.

Conductive Two-Dimensional Phthalocyanine-based Metal-Organic Framework Nanosheets for Efficient Electroreduction of CO2.

The electrocatalytic conversion of CO2 into value-added chemicals is a promising approach to realize a carbon-energy balance. However, low current density still limits the application of the CO2 electroreduction reaction (CO2 RR). Metal-organic frameworks (MOFs) are one class of promising alternatives for the CO2 RR due to their periodically arranged isolated metal active sites. However, the poor conductivity of traditional MOFs usually results in a low current density in CO2 RR. We have prepared conductive two-dimensional (2D) phthalocyanine-based MOF (NiPc-NiO4 ) nanosheets linked by nickel-catecholate, which can be employed as highly efficient electrocatalysts for the CO2 RR to CO. The obtained NiPc-NiO4 has a good conductivity and exhibited a very high selectivity of 98.4 % toward CO production and a large CO partial current density of 34.5 mA cm-2 , outperforming the reported MOF catalysts. This work highlights the potential of conductive crystalline frameworks in electrocatalysis.

Highly Selective CO2 Electroreduction to CH4 by In†Situ Generated Cu2 O Single-Type Sites on a Conductive MOF: Stabilizing Key Intermediates with Hydrogen Bonding.

It is still a great challenge to achieve high selectivity of CH4 in CO2 electroreduction reactions (CO2 RR) because of the similar reduction potentials of possible products and the sluggish kinetics for CO2 activation. Stabilizing key reaction intermediates by single type of active sites supported on porous conductive material is crucial to achieve high selectivity for single product such as CH4 . Here, Cu2 O(111) quantum dots with an average size of 3.5 nm are in situ synthesized on a porous conductive copper-based metal-organic framework (CuHHTP), exhibiting high selectivity of 73 % towards CH4 with partial current density of 10.8 mA cm-2 at -1.4 V vs. RHE (reversible hydrogen electrode) in CO2 RR. Operando infrared spectroscopy and DFT calculations reveal that the key intermediates (such as *CH2 O and *OCH3 ) involved in the pathway of CH4 formation are stabilized by the single active Cu2 O(111) and hydrogen bonding, thus generating CH4 instead of CO.

Ba10In6Zn7S26- nZnS: An Inorganic Composite System with Interface Phase-Matching Tuned for High-Performance Infrared Nonlinear Optical Materials.

Mid- and far-infrared nonlinear optical (MFIR NLO) materials are important in modern laser technologies. However, it is very challenging to develop materials that can achieve a subtle balance between the key requirements, such as large NLO response, high laser-induced damage threshold (LIDT), wide IR transparency, and phase-matching. In this work, a new wide IR transparency (0.38-15.3 µm) NLO crystal Ba10In6Zn7S26 (SS26) is synthesized. Further, its composite system Ba10In6Zn7S26- nZnS is synthesized by eutectic reaction. In particular, Ba10In6Zn7S26-14ZnS (SS40) shows excellent balanced NLO performance that includes a large band gap of 3.05 eV, high LIDT (13.3 × AgGaS2), large second harmonic generation (SHG) response (2.1 × AgGaS2 at 2050 nm, 5.2 × KDP at 1064 nm), and wide optical transmission window (0.37-15.4 µm). Importantly, the phase-matching condition is realized for SS40 by interfaces formed between the crystal face (112) of matrix SS26 and the crystal face (111) of reinforcement cubic ZnS by topological chemical reaction, and the NLO performance can be tuned by different concentrations of ZnS. First-principles simulations are employed to study NLO properties of SS26 and the interfaces. This work demonstrates that SS40 is a promising MFIR NLO material, and tuning components of the composite material system is a useful way to develop new MFIR NLO materials with excellent comprehensive performance.

Nitrogen-Mediated Graphene Oxide Enables Highly Efficient Proton Transfer.

Two-dimensional (2D) graphene and graphene oxide (GO) offer great potential as a new type of cost-efficient proton-exchange membranes (PEM) for electrochemical devices. However, fundamental issues of proton transfer mechanism via 2D membranes are unclear and the transfer barrier for perfect graphene are too high for practical application. Using ab initio molecular dynamic simulations, we screened the proton transfer barrier for different un-doped and nitrogen doped GO membranes, and clarified the corresponding transfer mechanisms. More significantly, we further identify that N-mediated GO can be built into a highly efficient PEM with a proton transfer rate of seven orders of magnitude higher than an un-doped case via. a proton relay mechanism between a ketone-like oxygen and a pyridine-like nitrogen across the vacancy site. The N-doped 2D GO is also impermeable to small molecules, and hence a highly efficient PEM for practical applications.

Theoretical Evaluation of Terahertz Sources Generated From SnGa4 Q7 (Q=S, Se) as Infrared Nonlinear Optical Materials.

We theoretically evaluated the integrated knowledge that contributes to conversion efficiency, including the phonon, photon, and electron properties of infrared nonlinear optical materials such as SnGa4 Q7 (Q=S, Se), which are terahertz (THz) sources. Specifically, we developed a new formula to calculate the susceptibility of the difference frequency generation (DFG) optical process. By evaluating the characteristics of the materials themselves in the THz region, we found that a larger nonlinear susceptibility or a large figure of merit resulted in a large efficiency of the THz source by comparing the findings of SnGa4 Se7 and SnGa4 S7 under the same experimental conditions; furthermore, THz absorption was found to reduce the efficiency of the THz source for the two SnGa4 Q7 (Q=S, Se) materials. The efficiency of the THz source also depended on the experimental conditions. A large crystal size, strong pump intensity, and small THz wavelength resulted in better efficiency of the THz source based on the DFG process. The efficiency was found to be a comprehensive index to evaluate the THz source based on the DFG process.

Highly effective sites and selectivity of nitrogen-doped graphene/CNT catalysts for CO2 electrochemical reduction.

Metal-free catalysts, such as graphene/carbon nanostructures, are highly cost-effective to replace expensive noble metals for CO2 reduction if fundamental issues, such as active sites and selectivity, are clearly understood. Using both density functional theory (DFT) and ab initio molecular dynamic calculations, we show that the interplay of N-doping and curvature can effectively tune the activity and selectivity of graphene/carbon-nanotube (CNT) catalysts. The CO2 activation barrier can be optimized to 0.58 eV for graphitic-N doped graphene edges, compared with 1.3 eV in the un-doped counterpart. The graphene catalyst without curvature shows strong selectivity for CO/HCOOH production, whereas the (6, 0) CNT with a high degree of curvature is effective for both CH3OH and HCHO production. Curvature is also very influential to tune the overpotential for a given product, e.g. from 1.5 to 0.02 V for CO production and from 1.29 to 0.49 V for CH3OH production. Hence, the graphene/CNT nanostructures offer great scope and flexibility for effective tunning of catalyst efficiency and selectivity, as shown here for CO2 reduction.

Active sites and mechanisms for oxygen reduction reaction on nitrogen-doped carbon alloy catalysts: Stone-Wales defect and curvature effect.

Carbon alloy catalysts (CACs) are promising oxygen reduction reaction (ORR) catalysts to substitute platinum. However, despite extensive studies on CACs, the reaction sites and mechanisms for ORR are still in controversy. Herein, we present rather general consideration on possible ORR mechanisms for various structures in nitrogen doped CACs based on the first-principles calculations. Our study indicates that only a particular structure of a nitrogen pair doped Stone-Wales defect provides a good active site. The ORR activity of this structure can be tuned by the curvature around the active site, which makes its limiting potential approaching the maximum limiting potential (0.80 V) in the volcano plot for the ORR activity of CACs. The calculated results can be compared with the recent experimental ones of the half-wave potential for CAC systems that range from 0.60 to 0.80 V in the reversible-hydrogen-electrode (RHE) scale.

Efficient [FeFe] hydrogenase mimic dyads covalently linking to iridium photosensitizer for photocatalytic hydrogen evolution.

Two [FeFe] hydrogenase mimics, [Fe(2)(µ-pdt)(CO)(5)L1] (L1 = PPh(2)SPhNH(2)) (Ph = phenyl) (2) and [Fe(2)(µ-pdt)(CO)(5)L2] (L2 = PPh(2)PhNH(2)) (3), and two molecular photocatalysts, [(CO)(5)(µ-pdt)Fe(2)PPh(2)SPhNHCO(bpy)(ppy)(2)Ir]PF(6) (bpy = bipyridine, ppy = 2-phenylpyridine) (2a) and [(CO)(5)(µ-pdt)Fe(2)PPh(2)PhNHCO(bpy)(ppy)(2)Ir](PF(6)) (3a), have been designed and synthesized, anchoring Ir(ppy)(2)(mbpy)PF(6) (mbpy = 4-methyl-4'-carbonyl-2,2'-bipyridine) (PS) to one of the iron centers of complexes 2 and 3 by forming amide bonds. Molecular dyads 2a, 3a and the intermolecular systems 2, 3 with PS have also been successfully constructed for photoinduced H(2) production using triethylamine (TEA) as a sacrificial electron donor by visible light (>400 nm) in CH(3)CN-H(2)O solution. The time-dependence of H(2) generation and spectroscopic studies suggest that the activity of H(2) evolution can be tuned by addition of a S atom to the phosphane ligand. The highest turnover numbers (TON) of hydrogen evolution obtained are 127, using 2a as a photocatalyst in a supramolecular system, and 138, based on catalyst 2 in a multi-component system. Density functional theory (DFT) computational studies demonstrate that the S atom in the second coordination sphere makes complex 2 accept an electron more easily than 3 and improves the activity in light-induced hydrogen production.

Nonlinear optical properties of carbon nitride nanotubes.

The second order polarizabilities (ß) of the C(3)N(4) NT systems were investigated in this study. The ß values of end groups substituted C(3)N(4) NTs were calculated to find their most favorable paradigm for nonlinear optical design. It was found that their electric dipole transitions are only allowed along the tube axis direction and the position of terminal groups has a great effect on NLO properties of substituted C(3)N(4) NTs. The obtained results provide us details to understand the relation between the structure and nonlinear optical properties. The results indicate that the second-order polarizabilities originate from charge transfer from a donor (-NH(2)) to an acceptor (-O(2)N) and the electron density redistribution in heptazine units. We employ a one-dimensional two-state model to analyze the nature of the second-order polarizabilities of studied materials. The frequency-dependent second-order polarizabilities were also calculated. The second-order polarizability of the O(2)N-C(3)N(4)-NH(2) NT is 2.51 × 10(-27) esu when the input photon energy is 2.232 eV, which is much larger (about two orders of magnitude) than static second-order polarizability (2.54 × 10(-29)).

First-principles study of ZnO cluster-decorated carbon nanotubes.

We have investigated the structural, electronic and carbon monoxide (CO) detection properties of the ZnO cluster-decorated single-walled carbon nanotubes (SWCNTs) by using density functional theory (DFT). The stable structures of hybrid ZnO/SWCNT materials are that the ZnO cluster plane is perpendicular to the surface of SWCNTs with the Zn atoms towards the SWCNTs (Zn atom above axial C-C bond or above the C atom). For the ZnO cluster-decorated semiconducting SWCNTs, the SWCNTs present p-type characteristics which may lead to the decrease of conductance upon illumination with ultraviolet (UV) light. The CO can be adsorbed on the hybrid ZnO/SWCNT materials due to the charge transfer between them. Compared with isolated ZnO clusters or bare SWCNTs, the ZnO/SWCNT network would have excellent CO detection ability due to their suitable adsorption energy and conductivity.

A series of novel rare-earth bismuth tungstate compounds LnBiW2O9 (Ln=Ce, Sm, Eu, Er): synthesis, crystal structure, optical and electronic properties.

The structural, optical, and electronic properties of four rare-earth bismuth tungstate compounds, LnBiW(2)O(9) (Ln = Ce, Sm, Eu, Er), have been investigated by means of single-crystal X-ray diffraction, elemental analyses, and spectral measurements. For some of the compounds, the calculations of energy band structures and density of states have also been made by the density functional theory. The structure of CeBiW(2)O(9) features a three-dimensional (BiW(2)O(9))(3-) anionic framework with interesting channels where Ce atoms are located. The framework is constructed by one-dimensional BiO(9) polyhedra chains and one-dimensional zigzag W(2)O(9) chains via edge- and face-sharing. LnBiW(2)O(9) (Ln = Sm, Eu, Er) are isostructural and their structures feature a three-dimensional network based on alternating (BiO(2))(-) layers and (Ln(2)W(2)O(12))(6-) layers connected by corner-linked chains of WO(6) octahedra. Results of spectral measurements indicate that EuBiW(2)O(9) exhibit the characteristic yellow-red light emission under excitation at 395 nm, and it will be a red phosphor in designing white light-emitting diode device. The calculated results of band structures by using the density functional theory (DFT) show that the solid-state compound CeBiW(2)O(9) and SmBiW(2)O(9) are indirect band gap materials.