Structural phase instability,mixed-phase,and energy band gap change in BiFeO3 under lattice strain effect from first-principles investigation

Lattice strain effects drive a variety of novel functional responses in epitaxial BiFeO₃ thin films and have attracted significant interest and attention from researchers in experimental and theoretical studies. However, the difficulty in designing experimental techniques in addition to facing problems in the first principles approach, such as output accuracy and high computational costs, constitute the discovery of new functional responses in epitaxial BiFeO₃ thin films not entirely understood. Therefore, in this study, we perform a first principles calculation based on the less expensive LDA+U method to investigate the structural phase instability and electronic properties change in BiFeO₃ under the lattice strain effect. The structural phase transformation of BiFeO₃ under volumetric and compressive/tensile lattice strain was examined established on the calculated lower energy phases. Importantly, we demonstrated that the change of crystal structure phases of BiFeO₃ was extremely sensitive to the volumetric and compressive/tensile lattice strain, comparable with various experiment data, as reported in the literature. Moreover, we revealed for the first time from the first principles prediction the coexistence of mixed R-T phases in the region of moderate compressive ζ_in-plane of ?2.9% (e.g. LaAlO₃ substrates with ɑ = 3.79 Å). From the prediction of electronic properties obtained by the LDA+U and PBE0 methods, we found that the energy band gap increased when the compressive in-plane lattice strain is increased while, in contrast, the energy band gap decreased when BiFeO₃ was under the tensile in-plane lattice strain effect. We also demonstrate that our computational technique based on the first principles study was sufficiently accurate enough, helping to speed up the process of designing new materials having an excellent multifunctional response (piezoelectric, magnetic, photovoltaic, spintronic).     

Synthesis, crystal structures and photoluminescence properties of Sm-based enantiomeric pair

Novel Sm-based enantiomeric pair, generally formulated Sm(DBM)₃L (L_R,R in 1, L_S,S in 2, DBM = dibenzoylmethanate) have been successfully prepared via the reaction of Sm(DBM)₃·2H₂O with chiral ligands L_R,R (−)-4,5-pinene bipyridine and L_S,S (+)-4,5-pinene bipyridine (Scheme 1), respectively. The crystal structure analysis of 1 and 2 reveal that they crystallize in chiral space group P2₁ of monoclinic system. The central Sm(III) ion is octacoordinate with six β-diketonate oxygen atoms and two chiral pinene bipyridine nitrogen atoms, forming a coordination polyhedron best described as the distorted square antiprism (SA). The CD spectra (Fig. S1) further confirm that 1 and 2 are enantiomers. The photoluminescence investigations of 1 and 2 demonstrate that they display deep-red luminescence characteristic of the Sm³⁺_.     

First-principles study on predicting the crystal structures,mechanical properties and electronic structures of HfCxN1-x

The crystal structures, mechanical properties and electronic structures of HfC_xN_1-x have been predicted by using evolutionary structure search followed by the first-principles calculations in this study. The crystal structures predicted indicate that there are 10 thermodynamic stable phases for HfC_xN_1-x, of which 8 are newly discovered crystal structures and 2 are already known. We investigated the mechanical properties, including the bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio and Vickers hardness, of all 10 stable phases, and further established the relationship between such properties and the ratio of nitrogen to carbon content. Besides, the Fermi energy level and electronic density of states of these 10 stable phases are calculated as well, and the results reveal the fundamental reason why the mechanical properties change with the ratio of nitrogen to carbon. The predictions of this study agree well with both the experimental data and the previous theoretical evaluations.     

Lanthanide-doped nanocrystals: synthesis, optical-magnetic properties, and applications

Because of the potential applications of lanthanide-doped nanocrystals in display devices, optical communication, solid-state lasers, catalysis, and biological labeling, the controlled synthesis of these new nanomaterials has sparked considerable interest. Nanosized phosphorescent or optoelectronic devices usually exhibit novel properties, depending on their structures, shapes, and sizes, such as tunable wavelengths, rapid responses, and high efficiencies. Thus, the development of facile synthetic methods towards high-quality lanthanide-doped nanocrystals with uniform size and shape appears to be of key importance both for the exploration of their materials properties and for potential applications. This Account focuses on the recent development in our laboratory of the synthesis and applications of lanthanide-doped nanocrystals. Since 2005, when we proposed a general strategy for nanocrystal synthesis via a liquid-solid-solution process, a range of monodisperse and colloidal lanthanide-doped fluoride, oxide, hydroxide, orthovanadate, thiooxide, borate, and phosphate nanocrystals have been successfully prepared. By rationally tuning the reaction conditions, we have readily synthesized nanostructures, such as hollow microspheres, nanorods, nanowires, hexagonal nanoplates, and nanobelts. By adjusting the different colloidal nanocrystal mixtures, we fabricated unique binary nanostructures with novel dual-mode luminescence properties through a facile ultrasonic method. By tridoping with lanthanide ions that had different electronic structures, we successfully achieved β-NaYF(4) nanorods that were paramagnetic with tuned upconversion luminescence. We have also used NaYF(4):Yb(3+)/Er(3+) conbined with magnetite nanoparticles as a sensitive detection system for DNA: NaYF(4):Yb(3+)/Er(3+) and Fe(3)O(4) nanoparticles were modified with two different DNA sequences. Then, the modified NaYF(4):Yb(3+)/Er(3+) nanoparticles were conjugated to the modified Fe(3)O(4) nanoparticles. These binary nanoparticles can be hybridized with a third DNA (target DNA) molecule and separated with the assistance of a magnetic field. In addition, a novel fluorescence resonance energy transfer (FRET) method for nonenzymatic glucose determination has been developed by using the glucose-modified LaF(3):Ce(3+)/Tb(3+) nanocrystals. By using bioconjugated NaYF(4):Yb(3+)/Er(3+) nanoparticles as the energy donor and bioconjugated gold nanoparticles as the energy acceptor, we successfully developed a simple and sensitive fluorescence resonance energy transfer (FRET) biosensor for avidin. Meanwhile, we also carried out preliminary studies to investigate possible applications of lanthanide-doped nanocrystals in catalysis and in dye-sensitized solar cells.     

Ligand substitution in sandwich-type complexes of germanotungstates: Syntheses,crystal structures and magnetic properties

Two new sandwich-type germanotungstates K₂H₂{Zn(en)₂}{Zn(en)₂}₂{(GeW₉O₃₄)₂Zn₄(Hen)₂}·17H₂O (1) and K₂H₂{Ni(en)₂}{Ni(en)₂}₂{(GeW₉O₃₄)₂Ni₄(Hen)₂}·12H₂O (2) (en = ethylenediamine) have been hydrothermally synthesized. The polyanions of the two compounds are composed of a [Ge₂W₁₈M₄(Hen)₂O₆₈]^10? (M = Zn, Ni) group decorated by two [M(en)₂(H₂O)]²⁺ units. The luminescence of compound 1 exhibits an intense UV radiation emission maximum at λ ≈ 449 nm upon excitation of 372 nm. The magnetic investigation of compound 2 demonstrates the presence of ferromagnetic interactions between the Ni²⁺ ions of the Ni₄O₁₄(Hen)₂ clusters in 2.     

Syntheses, characterization, and energy transfer properties of benzothiadiazole-based hyperbranched polyfluorenes

A series of benzothiadiazole-based (BT) hyperbranched polyfluorene copolymers with various branching degrees (5-40%) were designed and synthesized. TGA and film annealing tests showed the substantial thermal stability of these highly branched polymers. The optical performance of the polymers in solutions and as films, and their electrochemical properties were characterized. The energy transfer (ET) processes in these hyperbranched conjugated polymers, both in solutions and in the solid state, were also investigated. With the change of the solution concentration and the branching degree, the energy transfer efficiency of the polymers varied in solutions and the main photoluminescence (PL) peaks changed from blue to green region. As films, only green light emitted from BT units. In addition, the PL efficiency of the films decreased dramatically with the increase of branching degrees. All these features demonstrated that highly branched structure would effectively impede the intra- and interchain energy migration, especially in solutions, and remarkably influence the ET process in the solid state, which resulted in low PL efficiency.     

Doping dependent crystal structures and optoelectronic properties of n-type CdSe:Ga nanowries

Although CdSe nanostructures possess excellent electrical and optical properties, efforts to make nano-optoelectronic devices from CdSe nanostructures have been hampered by the lack of efficient methods to rationally control their structural and electrical characteristics. Here, we report CdSe nanowires (NWs) with doping dependent crystal structures and optoelectronic properties by using gallium (Ga) as the efficient n-type dopant via a simple thermal co-evaporation method. The phase change of CdSe NWs from wurtzite to zinc blende with increased doping level is observed. Systematical measurements on the transport properties of the CdSe:Ga NWs reveal that the NW conductivity could be tuned in a wide range of near nine orders of magnitude by adjusting the Ga doping level and a high electron concentration up to 4.5 × 10(19) cm(-3) is obtained. Moreover, high-performance top-gate field-effect transistors are constructed based on the individual CdSe:Ga NWs by using high-κ HfO(2) as the gate dielectric. The great potential of the CdSe:Ga NWs as high-sensitive photodetectors and nanoscale light emitters is also exploited, revealing the promising applications of the CdSe:Ga NWs in new-generation nano-optoelectronics.     

Influence of crystal lattice defectiveness on the properties of solid solutions based on ZrO2

Conclusions It is shown that the parameter of the crystal lattice of cubic solid solutions based on zirconium dioxide, stabilized with oxides of yttrium, ytterbium, and calcium, is increased with a rise in their concentrations, and hardly alters for the solid solution ZrO₂-Sc₂O₃.In the solid solutions of Y₂O₃, Sc₂O₃, and CaO in zirconium dioxide we note a reduction in the x-ray densities with an increase in the concentration of stabilizing additives. For the solid solutions ZrO₂-Yb₂O₃ we note an increase in the x-ray density with rise in the concentration of ytterbium oxide.The introduction into the zirconia of 9–13% stabilizing oxides ensures the highest density characteristics for the ceramics.Translated from Ogneupory, No. 1, pp. 12–15, January, 1987.     

Two cobalt(II) coordination polymers based on carboxylate ligand: Preparation,crystal structures and magnetic properties

Two new coordination polymers, [Co₂(L)_1.5(OH)(H₂O)₂]·2H₂O (1) and [CoL(bpp)] (2) (H₂L = 2,5-dibenzoylterephthalic acid and bpp = 1,3-bis(4-pyridyl)propane), have been synthesized and characterized by elemental analyses, IR spectra, and thermogravimetric analyses. Their structures were determined by single crystal X-ray diffraction analyses. Complex 1 exhibits a 3D framework assembled from tetra-nuclear cobalt-oxygen bridged by carboxylic O atoms with a (4¹²·6³) topology. Complex 2 possesses a 2D layered structure which extends into a 3D architecture via C–H···π interaction between propane proton and the phenyl ring or pyridine ring of an adjacent sheets. Furthermore, the magnetic properties of them are discussed.     

Self-assembled, optically responsive nematic liquid crystal/polymer core-shell fibers: Formation and characterization

We report here the formation and structural characterization of optically responsive, highly birefringent electrospun nematic liquid crystal (LC) microfibers. The LC microfibers are electrospun from a solution of polylactic acid (PLA) and low molecular weight 4-pentyl-4^’-cyanobiphenyl (5CB) in chloroform/acetone solvent. In the electrospinning process, the low molecular weight 5CB phase-separates and self-assembles to form a planarly aligned nematic core within a PLA shell. The solubility limit of 5CB in PLA and the degree of phase separation of LC in the 5CB/PLA core/sheath fibers is determined using the phase transition enthalpies associated with LC and polymer components. Structural analysis revealed that the LC core and dissolved LC in the fibers promote the formation of the α-form of PLA crystals and increase the degree of crystallinity of the PLA shell in 5CB/PLA fibers from 6.6% to 52%. Competition between 5CB droplet formation and PLA fiber formation is observed as a function of spinning solution composition and applied electrospinning voltage. Alignment of the 5CB within the PLA core is confirmed by polarizing optical microscopy.     

Monodisperse NaYbF4:Tm3+/NaGdF4 core/shell nanocrystals with near-infrared to near-infrared upconversion photoluminescence and magnetic resonance properties

We report core/shell NaYbF(4):Tm(3+)/NaGdF(4) nanocrystals to be used as probes for bimodal near infrared to near infrared (NIR-to-NIR) upconversion photoluminescence (UCPL) and magnetic resonance (MR) imaging. The NaYbF(4):Tm(3+) nanocrystals were previously reported to produce the intense NIR-to-NIR UCPL peaked at ～800 nm under excitation at ～975 nm. We have found that the growth of a NaGdF(4) shell on surface of the NaYbF(4):Tm(3+) nanocrystals results in the increase in the intensity of UCPL of Tm(3+) ions by about 3 times. Unlike biexponential PL decay of NaYbF(4):Tm(3+) nanocrystals, the PL decay of NaYbF(4):Tm(3+)/NaGdF(4) core/shell nanocrystals is single exponential and of longer lifetime due to the suppression of surface quenching effects for Tm(3+) PL. The growth of a NaGdF(4) shell on surface of the NaYbF(4):Tm(3+) nanocrystals also provides high MR relaxivity from paramagnetic Gd(3+) ions contained in the shell. The T1-weighted MR signal of the (NaYbF(4):2% Tm(3+))/NaGdF(4) nanoparticles was measured to be about 2.6 mM(-1)s(-1). Due to the combined presence of efficient optical and MR imaging capabilities, nanoprobes based on NaYbF(4):Tm(3+)/NaGdF(4) fluoride nanophosphors can be considered as a promising platform for simultaneous bimodal PL and MR bioimaging.     

The synthesis,photophysical properties and energy transfer of a coumarin-based bichromophoric compound

The energy transfer between two coumarin compounds having donor emission/acceptor absorption spectra overlap was studied firstly in solution by applying an excess of the acceptor. The observed transfer was then confirmed in the bichromophoric compound resulting from the covalent binding of the two fluorophores via a short alkyl chain.     

Preparation and optical properties of dispersible ZnSe nanocrystals synthesized by high energy ball milling

ZnSe nanocrystals have been successfully synthesized by high energy ball milling method. X-ray diffraction patterns show a single zinc blende structure formed in the milling process. HRTEM images confirm that the formation of the ZnSe nanocrystals synthesized by high energy ball milling have a wide crystals distribution (3–20 nm). Using the aqueous solutions of Na₃PO₄, (NaPO₃)₆ and Na₄P₂O₇ to disperse the 40 h-milled samples, we have observed the gradual blue-shift of the absorption edge along with the different centrifuging speed. In PL spectras, two main bands peaked at about 1.95 and 2.35 eV are observed, the former band is related to the V_Zn defects emission; and the latter is related to the V_Se defects emission. The V_Se defects emission does not depend on the dispersants, but the V_Zn defects emission changes in different dispersants.     

Morphology and phase control of fluorides nanocrystals activated by lanthanides with two-model luminescence properties

The morphology, size and phase control of luminescent fluoride nanocrystals through doping has become a new research hotspot due to their improved properties. In this work, Yb(3+) ions, as one of the most efficient sensitizers for various lanthanide activators, were doped in NaGd(Y)F(4) nanocrystals. The results show that no obvious influence was observed for Yb(3+)-doped NaYF(4) nanocrystals, while the influence of Yb(3+) doping on NaGdF(4) nanocrystals was remarkable. The NaGd(1-x)Yb(x)F(4) nanocrystals were synthesized by a hydrothermal route and had a morphology of rice-like nanorods. By controlling the synthesis parameters, the average size and slenderness of the nanocrystals increased gradually with addition of Yb(3+) ions. In contrast, the NaGd(1-x)Yb(x)F(4) nanocrystals maintained a hexagonal phase, which is more beneficial for application as a luminescent host, until the content of Yb(3+) ions reached x = 0.9. The growth and transformation mechanism of NaGd(1-x)Yb(x)F(4) nanocrystals was proposed to be a result of the competition between ion diffusion and an Oswald ripening process. Photoluminescence (PL) spectra confirm the efficient up-conversion and near-infrared (NIR) two-model luminescence properties of Er(3+) (Tm(3+)) activated NaGd(Y)(1-x)Yb(x)F(4) nanocrystals. Simulated analysis results indicate that a colloidal solution of mixed luminescent nanocrystals is expected to find application as the activated medium of three dimensional displays and a broadband optical amplifier.     

Infrared colloidal lead chalcogenide nanocrystals: synthesis, properties, and photovoltaic applications

Simple solution phase, catalyst-free synthetic approaches that offer monodispersed, well passivated, and non-aggregated colloidal semiconductor nanocrystals have presented many research opportunities not only for fundamental science but also for technological applications. The ability to tune the electrical and optical properties of semiconductor nanocrystals by manipulating the size and shape of the crystals during the colloidal synthesis provides potential benefits to a variety of applications including photovoltaic devices, light-emitting diodes, field effect transistors, biological imaging/labeling, and more. Recent advances in the synthesis and characterization of colloidal lead chalcogenide nanocrystals and the achievements in colloidal PbS or PbSe nanocrystals solar cells have demonstrated the promising application of infrared-emitting colloidal lead chalcogenide nanocrystals in photovoltaic devices. Here, we review recent progress in the synthesis and optical properties of colloidal lead chalcogenide nanocrystals. We focus in particular upon the size- and shape-controlled synthesis of PbS, PbSe, and PbTe nanocrystals by using different precursors and various stabilizing surfactants for the growth of the colloidal nanocrystals. We also summarize recent advancements in the field of colloidal nanocrystals solar cells based on colloidal PbS and PbSe nanocrystals.     

Effect of polymer structures on electro-optical properties of polymer stabilized liquid crystal films

The polymer stabilized liquid crystal (PSLC) film is a relatively novel electro-optical material, which is generally obtained by dissolving a small amount of a bifunctional photoreactive monomer in a low molecular mass liquid crystal. In this paper, the PSLC films were prepared with photoreactive biphenyl methacrylate monomers by photopolymerization induced phase separation. The effects of liquid crystal concentration, curing time, monomer structures and alignment layer on the electro-optical properties of PSLC films were investigated. The results show that the transmittance in the OFF state (T _OFF) increased with the liquid crystal concentration, but the driving voltage decreased. T _OFF was also influenced by the curing time. Furthermore, when polyimide was used as alignment layer, the films prepared from the bifunctional monomer shows a higher T _OFF, while those from the single functional monomer exhibited a deformed electro-optical curve due to the unsteady polymer networks. __________ Translated from Polymer Materials Science and Engineering, 2008, 24(1): 63–66 [译自: 高分子材料科学与工程]     

The continuous phase heat and mass transfer properties of dispersions

Techniques have been developed for measuring the interfacial area in gas-liquid dispersions. It has thus been possible to measure the liquid-phase mass transfer coefficients in gas-liquid dispersions such as are produced in aerated mixing vessels, and sieve and sintered plate columns. The results have been combined with other published data for heat and mass transfer in liquid-liquid and solid-gas dispersions in which the dispersed phases are free to move under the action of gravity, and also with data on transfer by free convection from spheres. These data can all be correlated by

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Aluminum,gallium and indium thiobenzoates: synthesis,characterization and crystal structures

Compounds R₂M[S(O)CPh] [where R?=?^tBu, M?=?Al (1); R?=?^tBu, M?=?Ga (2); R?=?Me, M?=?Ga (3)] have been synthesized in reactions of R₃M with thiobenzoic acid in a 1:1 molar ratio of reagents. The reaction of Me₃Ga with three equivalents of thiobenzoic acid yielded the compound Ga[S(O)CPh]₃ (4), in which thiobenzoate moieties act as bidentate SO ligands. In the presence of Et₃N, InCl₃ reacted with thiobenzoic acid with formation of an ionic compound {In[S(O)CPh]₄}^?(HNEt₃)⁺ (5). The thiobenzoate ligands are bonded with the metallic center via the sulfur atoms only. The compounds 4 and 5 have been structurally and thermally studied. The thermal decomposition pathways of compounds 4 and 5 are proposed.     

Plasmon enhanced upconversion luminescence of NaYF(4):Yb,Er@SiO(2)@Ag core-shell nanocomposites for cell imaging

NaYF(4):Yb,Er@SiO(2)@Ag core-shell nanocomposites were prepared to investigate metal-enhanced upconversion luminescence. Two sizes (15 and 30 nm) of Ag nanoparticles were used. The emission intensity of the upconversion nanocrystals was found to be strongly modulated by the presence of Ag nanoparticles (NPs) on the outer shell layer of the nanocomposites. The extent of modulation depended on the separation distance between Ag NPs and upconversion nanocrystals. The optimum upconversion luminescence enhancement was observed at a separation distance of 10 nm for Ag NPs with two different sizes (15 and 30 nm). A maximum upconversion luminescence enhancement of 14.4-fold was observed when 15 nm Ag nanoparticles were used and 10.8-fold was observed when 30 nm Ag NPs were used. The separation distance dependent emission intensity is ascribed to the competition between energy transfer and enhanced radiative decay rates. The biocompatibility of the nanocomposites was significantly improved by surface modification with DNA. The biological imaging capabilities of these nanocomposites were demonstrated using B16F0 cells.     

Bond ionicity,lattice energy,bond energy,and microwave dielectric properties of Ca1-xSrxWO4 ceramics

The Ca_1-xSr_xWO₄ (x?=?0, 0.02, 0.04, 0.06, 0.08, 0.10) ceramics were fabricated through solid-state reaction, and the relationships among microwave dielectric properties of Ca_1-xSr_xWO₄, bond ionicity, lattice energy and bond energy were systematically investigated for the first time. The patterns of X-ray diffractions of Ca_1-xSr_xWO₄ presented tetragonal scheelite structure and no second phase appeared throughout the entire compositions. Dielectric properties of Ca_1-xSr_xWO₄ were proved to be related to the microstructures: dielectric constant (ε_r) of Ca_1-xSr_xWO₄ was dependent on the bond ionicity; the quality factor (Q×f₀) of Ca_1-xSr_xWO₄ was affected by W-site lattice energy when intrinsic loss is dominant; the temperature coefficient of resonant frequency (|τ_f|) would increase if B-site bond energy decreased. Ca_1-xSr_xWO₄ ceramic showed excellent microwave dielectric properties, ε_r =?9.42, Q×f₀ =?79876?GHz and τ_f =??18.8?ppm/°C when x?=?0.08 and sintered at 1100?°C for 4?h.