Photoluminescence and Raman scattering from catalytically grown Zn(x)Cd(1-x)Se alloy nanowires

Zn(x)Cd(1-x)Se alloy nanowires, with composition x = 0, 0.2, 0.5, 0.7, and 1, have been successfully synthesized by a chemical vapor deposition (CVD) method assisted with laser ablation. The as-synthesized alloy nanowires, 60-150 nm in diameter and several tens of micrometers in length, complied with a typical vapor-liquid-solid (VLS) growth mechanism. The Zn(x)Cd(1-x)Se nanowires are single crystalline revealed from high-resolution transmission electron microscopic (HRTEM) images, selected area electron diffraction (SAED) patterns, and X-ray diffraction (XRD) measurement. Compositions of the alloy nanowires can be adjusted by varying the precursor ratios of the laser ablated target and the CVD deposition temperature. Crystalline structures of the Zn(x)Cd(1-x)Se nanowires are hexagonal wurtzite at x = 0, 0.2, and 0.5 with the [0 1 -1 0] growth direction and zinc blende at x = 0.7 and 1 with the [1 -1 1] growth direction. Energy gaps of the Zn(x)Cd(1-x)Se nanowires, determined from micro-photoluminescence (PL) measurements, change nonlinearly as a quadratic function of x with a bowing parameter of approximately 0.45 eV. Strong PL from the Zn(x)Cd(1-x)Se nanowires can be tuned from red (712 nm) to blue (463 nm) with x varying from 0 to 1 and has demonstrated that the alloy nanowires have potential applications in optical and sensory nanotechnology. Micro-Raman shifts of the longitudinal optical (LO) phonon mode observed in the Zn(x)Cd(1-x)Se nanowires show a one-mode behavior pattern following the prediction of a modified random element isodisplacement (MREI) model.     

An investigation of the sodium patterning in Na(x)CoO(2) (0.5 < or = x < or = 1) by density functional theory methods

Extensive experimental work has been carried out to characterize the stable Na-vacancy ordering patterns at various compositions of layered Na(x)CoO(2). However, contradictions and debates prevail in the literature, particularly at high Na concentrations x>0.5. Understanding of the exotic electronic properties in this system requires a thorough understanding of the Na-vacancy structural orderings. Using density functional theory in the generalized gradient approximation (GGA), combined with a cluster expansion structure prediction algorithm we have found an intricate set of Na-vacancy ordered ground states in Na(x)CoO(2) (0.5< or =x< or =1). We demonstrate a newly predicted ordering pattern between 0.67< or =x< or =0.71. By comparing the first principles electronic structure methods within the GGA and GGA+U (Hubbard U correction) approximations, we demonstrate that at certain Na concentration the stable ordering is affected by charge localization on the Co layer through coupling between the Na and Co lattices.     

Electron-precise/deficient La(5-x)Ca(x)Ge4 (3.4 < or = x < or = 3.8) and Ce(5-x)Ca(x)Ge4 (3.0 < or = x < or = 3.3): probing low-valence electron concentrations in metal-rich Gd5Si4-type germanides

We report for the first time the syntheses of electron-precise/deficient alloys, Ln5-xCaxGe4 (Ln = La, Ce; x = 3.37, 3.66, 3.82 for La; x = 3.00, 3.20, 3.26 for Ce), in the metal-rich R5Tt4 Zintl system (R = rare earth metal; Tt = Si, Ge). The new alloys extend the phase width from electron-rich to open-shell electron-deficient region in the metal-rich Zintl system and demonstrate possible occurrence of varied electron deficiencies in Zintl phases without structural changes, as a result of other existing structure-forming factors.     

Inorganic single wall nanotubes of SbPS(4-x)Se(x) (0 < or = x < or = 3) with tunable band gap

A new ternary member, SbPS4, has been added to the growing inorganic nanotube family. This material naturally forms bundles of long, single wall nanotubes.     

Chemical size effect on the magnetic and electrical properties in the (Tb(1-)(x)Eu(x))MnO(3) (0 < or = x < or = 1.0) system

The effect of isovalent chemical substitution of Eu3+ into the Tb3+ sites on the magnetic and electrical properties of (Tb1-xEux)MnO3 (0 相似文献   

Guest-dependent negative thermal expansion in nanoporous prussian blue analogues M(II)Pt(IV)(CN)6.x{H2O} (0 < or = x < or = 2; M = Zn, Cd)

The guest-dependent thermal expansion behavior of the nanoporous Prussian Blue analogues MIIPtIV(CN)6.x{H2O} (0 相似文献   

Tuning the magnetic properties of Li(x)CrTi0.25Se2 (0.03 < or = x < or = 0.7) by directed deintercalation of lithium

X-ray diffraction (XRD), in situ energy-dispersive X-ray diffraction (EDXRD), X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and magnetic measurements were applied to investigate the effects of lithium deintercalation on pseudolayered Li(0.70)CrTi(0.25)Se(2). A detailed picture of structural changes during the deintercalation process was obtained by combining the results of EDXRD and EXAFS. Removal of Li from the host-guest complex leads to anisotropic contraction of the unit cell with stronger impact on the c axis, which is the stacking axis of the layers. The EDXRD experiments evidence that the shrinkage of the lattice parameters with decreasing x(Li) in Li(x)CrTi(0.25)Se(2) is nonlinear in the beginning and then becomes linear. Analysis of the EXAFS spectra clearly shows that the Cr/Ti-Se distances are affected in a different manner by Li removal. The Cr-Se bond lengths decrease, whereas the Ti-Se bonds lengthen when the Li content is reduced, which is consistent with XRD data. Magnetic measurements reveal a change from predominantly antiferromagnetic exchange (theta(p) = -300 K) interactions for the pristine material to ferromagnetic exchange interactions (theta = 25 K) for the fully intercalated material. Thus, the magnetic properties can be altered under ambient conditions by directed adjustment of the dominant magnetic exchange. The unusual magnetic behavior can be explained on the basis of the variation of the metal-metal distances and the Cr-Se-Cr angles with x, which were determined by Rietveld refinements. Owing to competing ferromagnetic and antiferromagnetic exchange interactions and disorder, the magnetic ground state of the intercalated materials is characterized by spin-glass or spin-glass-like behavior.     

反胶束法合成Cd1-xMnxS量子点及其发光性能研究

应用反胶束法制备了稀磁半导体Cd1-xMnxS量子点.量子点的大小可通过改变ωo值(wo=[水]/[表面活性剂])来控制.高分辨透射电镜的分析结果表明,量子点呈单分散性,是几乎没有缺陷的单晶体.量子点的大小约为4.8～6nm,随wo值增大而增大.电子能谱(EDS)测定结果表明,Mn2+离子在量子点中的摩尔分数为1.5%.由电子自旋共振(ESR)分析确定一部分Mn2+离子取代Cd2+离子位置而位于晶格,另一部分Mn2+离子位于Cd1-xMnxS的表面或间隙位置.吸收光谱显示,随着量子点变小,吸收带边发生蓝移,显示明显的量子尺寸效应.光致荧光光谱分析表明,发光峰属于Mn2+的4T1-6A1跃迁,而且随着ωo和粒径的增大,发光峰从2.26,2.10,2.05eV红移到1.88eV;其发光峰偏离2.12eV,主要是由于Mn2+离子位于扭曲的四面体晶体场所致.     

Synthesis, structure, and magnetic properties of Li-doped manganese-phthalocyanine, Li(x)[MnPc] (0 < or = x < or = 4)

We report on the first synthesis of Li-intercalated manganese-phthalocyanine (MnPc) in the bulk form and on the evolution of the structural and magnetic properties as a function of Li concentration, x. We find that solid beta-MnPc, which comprises rodlike assemblies of individual planar molecules, is best described as a glassy one-dimensional ferromagnet without three-dimensional ordering and that it can be quasi-continuously intercalated with Li up to x = 4, forming an isosymmetrical series of Li(x)[MnPc] phases. Inserted Li+ ions strongly bond to pyrrole-bridging nitrogen atoms of the Pc rings, thereby disrupting the ferromagnetic Mn-N(a)...Mn superexchange pathways. This gradually induces a crossover of the intrachain exchange interactions from ferromagnetic to antiferromagnetic as the doping level, x, increases coupled with a spin-state transition of the Mn2+ ions from intermediate spin, S = 3/2, to high spin, S = 5/2.     

Structure-property correlations in solid solutions of (CuI)8P12-xAsx, 2.4< or = x < or =6.6

A series of P/As mixed pnicogen phases of composition (CuI)(8)P(12-x)As(x), in which x = 2.4, 4.2, 4.8, 5.4, and 6.6, have been synthesized and characterized by X-ray single crystal and powder diffraction, solid-state NMR spectroscopy, thermal gravimetric analysis, and impedance spectroscopy. These materials are isostructural to (CuI)(8)P(12) and consist of neutral, tubular P/As mixed pnicogen chains associated with Cu(I) and I(-) ions. The As is distributed throughout the pnicogen chains; however, the "roof" sites of the [P8] cage show preferred occupation by As relative to the other sites. Accordingly, the change in cell volume is not a linear function of the As incorporation. Solid-state (31)P NMR spectroscopy of the 40 % As incorporated sample are consistent with the X-ray structural model, with extensive broadening due to (31)P-(75)As coupling and site disorder, and a change in the chemical shifts of the resonances due to the As substitution into the lattice. The degree of copper ion site disorder, probed by single-crystal X-ray diffraction, increases with increasing As content. Although very little change is observed in the copper ionic conductivity of polycrystalline samples, which ranges from 1.8-5.1 x 10(-6) S cm(-1) for (CuI)(8)P(12-x)As(x), x = 0, 4.2, 5.4; a single crystal (x = 4.8) measured along the needle axis has a conductivity of 1.7 x 10(-3) S cm(-1) at 128 degrees C. This represents an order of magnitude improvement in conductivity over (CuI)(8)P(12) at the same temperature.     

CaMn(1-x)Nb(x)O3 (x < or = 0.08) perovskite-type phases as promising new high-temperature n-type thermoelectric materials

Perovskite-type CaMn(1-x)Nb(x)O(3+/-delta) (x = 0.02, 0.05, and 0.08) compounds were synthesized by applying both a "chimie douce" (SC) synthesis and a classical solid state reaction (SSR) method. The crystallographic parameters of the resulting phases were determined from X-ray, electron, and neutron diffraction data. The manganese oxidations states (Mn(4+)/Mn(3+)) were investigated by X-ray photoemission spectroscopy. The orthorhombic CaMn(1-x)Nb(x)O(3+/-delta) (x = 0.02, 0.05, and 0.08) phases were studied in terms of their high-temperature thermoelectric properties (Seebeck coefficient, electrical resistivity, and thermal conductivity). Differences in electrical transport and thermal properties can be correlated with different microstructures obtained by the two synthesis methods. In the high-temperature range, the electron-doped manganate phases exhibit large absolute Seebeck coefficient and low electrical resistivity values, resulting in a high power factor, PF (e.g., for x = 0.05, S(1000K) = -180 microV K(-1), rho(1000K) = 16.8 mohms cm, and PF > 1.90 x 10(-4) W m(-1) K(-2) for 450 K < T < 1070 K). Furthermore, lower thermal conductivity values are achieved for the SC-derived phases (kappa < 1 W m(-1) K(-1)) compared to the SSR compounds. High power factors combined with low thermal conductivity (leading to ZT values > 0.3) make these phases the best perovskitic candidates as n-type polycrystalline thermoelectric materials operating in air at high temperatures.     

Structural study of the T#2-LixCoO2 (0.52 < x < or = 0.72) phase

The metastable O2-LiCoO(2) phase undergoes several reversible phase transitions upon lithium deintercalation. The first transition leads to an unusual oxygen stacking in such layered compounds. This stacking is found to be stable for 0.52 < x < or = 0.72 in Li(x)()CoO(2) and is called T(#)2. We studied this phase from a structural viewpoint using X-ray and neutron diffraction (ab initio method). The new stacking derives from the O2 one by gliding every second CoO(2) slab by (1/3, 1/6, 0). The lithium ions are found to occupy very distorted tetrahedral sites in this structure. We also discuss the possibility of this T(#)2 phase to exhibit stacking faults, whose amount depends on the method used to prepare this deintercalated phase.     

Endohedral silicon fullerenes sinN (27 < or = N < or = 39)

We have performed an unbiased search for the lowest-energy geometric structures of medium-sized silicon clusters SiN (27 < or = N < or = 39) using a genetic algorithm and nonorthogonal-tight-binding method, followed by a refining and biased search using basin-hopping method coupled with density-functional theory. We show that the carbon fullerene cages are most likely generic cage motifs ("magic cages") to form low-lying stuffed-cage silicon clusters (beyond the size N > 27). An empirical rule that provides optimal "stuffing/cage" combinations for constructing low-energy endohedral silicon fullerenes is suggested, with a hope that it can provide guidance to future synthesis of "bucky" silicon.     

Crystal structure, chemical bonding, and phase relations of the novel compound Co4Al(7+x)Si(2-x) (0.27 < or = x < or = 1.05)

The title compound was detected and characterized during a systematic study of the Al-rich part of the Co-Al-Si system. The crystal structure was established via single-crystal X-ray diffraction. It represents a new type of structure of intermetallic compounds (Pearson symbol mC26, space group C2/m). The homogeneity range of the phase Co4Al(7+x)Si(2-x) (0.27(3) < or = x < or = 1.05(2)) and equilibria with neighboring phases were studied by electron probe microanalysis (EPMA) and X-ray powder diffraction. The lattice parameters of the compound were found to vary between Al-poor and Al-rich composition (a = 11.949(1)-12.042(1) A, b = 3.9986(4)-4.0186(4) A, c = 7.6596(8)-7.6637(9) A, and beta = 106.581(7)-106.140(7) degrees). A partial disorder caused by the Al/Si substitution in one of the five main group element positions was found, and different ordering models yielding different Al/Si occupation motifs and different distributions of interatomic distances are discussed in detail. Chemical bonding analysis with the electron localization function (ELF) reveals a covalently bonded Al/Si network and rather ionic interactions between Co and the network.     

Influence of oxygen defects on the crystal structure and magnetic properties of the (Tb1-xNax)MnO3-y (0<or=x<or=0.3) system

The crystallographic and magnetic behaviors of (Tb1-xNax)MnO3-y (0相似文献   

Lithium dimer formation in the Li-conducting garnets Li(5+x)Ba(x)La(3-x)Ta2O12 (0 < x < or =1.6)

The garnet system Li(5+x)Ba(x)La(3-x)Ta2O12 shows an unprecedented Li+ content (x < or = 1.6) and short Li-Li distances of ca 2.44 A between majority occupied sites suggesting that the high Li+ mobility requires a complex cooperative mechanism.     

Lithium exchange processes in the conduction network of the Nasicon LiTi2-xZrx(PO4)3 Series (0 < or = x < or = 2)

Structural sites occupied by lithium in the rhombohedral LiTi2-xZrx(PO4)3 series (0 < or = x < or = 2) have been investigated by 7Li NMR spectroscopy. At room temperature, the XRD patterns of the end-members of the series display rhombohedral R3c symmetry in LiTi2(PO4)3 and triclinic C in LiZr2(PO4)3. In the first compound, Li ions occupy M1 sites; however, in the second one Li occupy intermediate M1/2 sites. As the temperature increases, a first-order displacive transformation is detected in the triclinic phase, but a second-order/disorder transition is detected in the rhombohedral phase. From the temperature dependence of the 7Li NMR quadrupole constant (CQ) of the two compounds, the evolution of M1 and M1/2 sites occupancy in the Nasicon conduction network has been deduced. At high temperatures, analyzed phases tend toward a disordered rhombohedral phase, in which both M1 and M1/2 sites are equally populated and in which lithium mobility is favored by the existence of vacant M1 sites. According to this study, this phase can also be obtained by substituting Ti by Zr in the LiTi2-xZrx(PO4)3 series.     

Syntheses and X-ray diffraction, photochemical, and optical characterization of Cu2SixSn1-xS3 (0.4 < or = x < or = 0.6) for photovoltaic applications

The study of the pseudobinary system Cu(2)SnS(3-)Cu(2)SiS(3) shows that a solid solution (Cu(2)Si(x)Sn(1-x)S(3)) exists in the range 0.4 < or = Si/(Sn+Si) < or = 0.6. Based on diffuse reflectance and photoelectrochemical measurements these compounds show potential as absorber materials for photovoltaic devices. The compounds were prepared at 850 degrees C from copper sulfide, silicon, tin, and sulfur and were analyzed with single-crystal (for x approximately 0.40) and powder diffraction techniques. Optical band gaps of 1.25, 1.35, and 1.45 eV were observed for the three compositions x = 0.39, 0.48, and 0.61; cathodic photocurrent occurring is significant.     

KBi(2-x)Pbx (0 < x <or= 1): a Zintl phase evolving from a distortion of the cubic Laves-phase structure

The quasibinary system KBi(2-x)Pbx has been investigated, both experimentally and theoretically. Phases with compositions 0 < or = x < or = 1.2 were synthesized and structurally characterized by X-ray diffraction experiments. For low values of x (0 < or = x < 0.6), KBi(2-x)Pbx adopts the cubic Laves-phase structure MgCu2 (space group Fdm), which contains a rigid framework of corner-condensed symmetry-equivalent tetrahedra formed by randomly distributed Bi and Pb atoms. For compositions x > or = 0.6, these tetrahedra become alternately elongated and contracted. The distortion of the framework lowers the space-group symmetry to F43m (KBi(1.2)Pb(0.8), F43m, Z = 8, a = 9.572(1) A). Magnetometer measurements show that KBi2 (x = 0) is metallic and goes through a superconducting transition below 3.5 K. First principles calculations reveal that the Fd3m --> F43m distortion is largest for KBiPb (x = 1.0), which at the same time turns into a semiconductor. Thus, F43m KBiPb corresponds to a proper charge-balanced Zintl phase, K+[BiPb]-, with separated polyanionic tetrahedra, (Bi2Pb2)2-. However, it was not possible to prepare F43m KBiPb. Syntheses attempting to increase the Pb content in KBi(2-x)Pbx above x = 0.8 yielded additional, not yet characterized, ternary phases.