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Sample records for early transition metal

  1. Transition metals

    PubMed Central

    Rodrigo-Moreno, Ana; Poschenrieder, Charlotte; Shabala, Sergey

    2013-01-01

    Transition metals such as Iron (Fe) and Copper (Cu) are essential for plant cell development. At the same time, due their capability to generate hydroxyl radicals they can be potentially toxic to plant metabolism. Recent works on hydroxyl-radical activation of ion transporters suggest that hydroxyl radicals generated by transition metals could play an important role in plant growth and adaptation to imbalanced environments. In this mini-review, the relation between transition metals uptake and utilization and oxidative stress-activated ion transport in plant cells is analyzed, and a new model depicting both apoplastic and cytosolic mode of ROS signaling to plasma membrane transporters is suggested. PMID:23333964

  2. Early diagenesis in differing depositional environments: The response of transition metals in pore water

    SciTech Connect

    Shaw, T.J.; Gieskes, J.M. ); Jahnke, R.A. )

    1990-05-01

    The cycling of Fe, Mn, Ni, Co, Cu, Cr, V, and Mo during early diagenesis was investigated in sediments from five different depositional environments in the California Borderland. Dissolved O{sub 2}, NO{sup {minus}}{sub 3}, NO{sup {minus}}{sub 2}, and SO{sup 2{minus}}{sub 4} were also measured at each site to establish the position of redox boundaries pertinent to this study. Sites were chosen to allow the comparison of several parameters of potential importance to the cycling of these metals: bottom water O{sub 2} concentration; sediment redox conditions; cycling of metal oxide carrier phases and the relative contribution of biogenic and terrigenous material to the detrital metal flux. In general, metal cycling associated with the early diagenesis of sediments was observed to decouple transport processes from burial processes for the transition metals measured in this study.

  3. Studies of high temperature ternary phases in mixed-metal-rich early transition metal sulfide and phosphide systems

    SciTech Connect

    Marking, G.A.

    1994-01-04

    Investigations of ternary mixed early transition metal-rich sulfide and phosphide systems resulted in the discovery of new structures and new phases. A new series of Zr and Hf - group V transition metal - sulfur K-phases was synthesized and crystallographically characterized. When the group V transition metal was Nb or Ta, the unit cell volume was larger than any previously reported K-phase. The presence of adventitious oxygen was determined in two K-phases through a combination of neutron scattering and X-ray diffraction experiments. A compound Hf{sub 10}Ta{sub 3}S{sub 3} was found to crystallize in a new-structure type similar to the known gamma brasses. This structure is unique in that it is the only reported {open_quotes}stuffed{close_quotes} gamma-brass type structure. The metal components, Hf and Ta, are larger in size and more electropositive than the metals found in normal gamma brasses (e.g. Cu and Zn) and because of the larger metallic radii, sulfur can be incorporated into the structure where it plays an integral role in stabilizing this phase relative to others. X-ray single-crystal, X-ray powder and neutron powder refinements were performed on this structure. A new structure was found in the ternary Nb-Zr-P system which has characteristics in common with many known early transition metal-rich sulfides, selenides, and phosphides. This structure has the simplest known interconnection of the basic building blocks known for this structural class. Anomalous scattering was a powerful tool for differentiating between Zr and Nb when using Mo K{alpha} X-radiation. The compounds ZrNbP and HfNbP formed in the space group Prima with the simple Co{sub 2}Si structure which is among the most common structures found for crystalline solid materials. Solid solution compounds in the Ta-Nb-P, Ta-Zr-P, Nb-Zr-P, Hf-Nb-P, and Hf-Zr-S systems were crystallographically characterized. The structural information corroborated ideas about bonding in metal-rich compounds.

  4. Single-step preparation and consolidation of reduced early-transition-metal oxide/metal n-type thermoelectric composites

    NASA Astrophysics Data System (ADS)

    Gaultois, Michael W.; Douglas, Jason E.; Sparks, Taylor D.; Seshadri, Ram

    2015-09-01

    Reduced early transition metal oxides/metal composites have been identified here as interesting thermoelectric materials. Numerous compositions in the Nb-rich portion of the WO3-Nb2O5 system have been studied, in composite formulations with elemental W. Spark plasma sintering (SPS) has been employed to achieve rapid preparation and consolidation of composite materials containing W metal precipitates with characteristic length scales that range from under 20 nm to a few microns, that exhibit thermal conductivities that are constant from 300 K to 1000 K, approximately 2.5 W m-1 K-1. Thermoelectric properties of these n-type materials were measured, and the highest-performing compositions were found to reach figure of merit zT values close to 0.1 at 950 K. The measurements point to higher zT values at yet-higher temperatures.

  5. Increased induction in FeCo-based nanocomposite materials with reduced early transition metal growth inhibitors

    NASA Astrophysics Data System (ADS)

    Miller, K. J.; Leary, A.; Kernion, S. J.; Wise, A.; Laughlin, D. E.; McHenry, M. E.; Keylin, Vladimir; Huth, Joe

    2010-05-01

    We report on new high-saturation induction, high-temperature nanocomposite alloys with reduced glass formers. The amounts of the magnetic transition metals and early transition metal growth inhibitors were systematically varied to determine trade-offs between higher inductions and fine microstructures with consequently lower magnetic losses. Alloys of nominal composition (Fe65Co35)79.5+xNb4-xB13Si2Cu1.5 (x=0-4) were cast into a 28 mm wide, 20 μm thick ribbon from which toroidal cores were wound. Inductions and magnetic losses were measured after nanocrystallization and stress relief. We report technical magnetic properties: permeability, maximum induction, remanence ratio, coercive field, and high frequency magnetic losses as a function of composition and annealing temperature for these alloys. Of note is the development of maximum inductions in excess of 1.76 T in cores made of alloys with the x=4 composition and maximum inductions in excess of 1.67 T in alloys with the x=3 composition, which also exhibit power losses smaller than 10 W/kg at 0.2 T induction levels in 20 kHz fields. We discuss optimization of induction with chemistry and correlate the microstructures with losses.

  6. Correlation between the spin Hall angle and the structural phases of early 5d transition metals

    NASA Astrophysics Data System (ADS)

    Liu, Jun; Ohkubo, Tadakatsu; Mitani, Seiji; Hono, Kazuhiro; Hayashi, Masamitsu

    2015-12-01

    We have studied the relationship between the structure and the spin Hall angle of the early 5d transition metals in X/CoFeB/MgO (X = Hf, Ta, W, and Re) heterostructures. Spin Hall magnetoresistance (SMR) is used to characterize the spin Hall angle of the heavy metals. Transmission electron microscopy images show that all underlayers are amorphous-like when their thicknesses are small, however, crystalline phases emerge as the thickness is increased for certain elements. We find that the heavy metal layer thickness dependence of the SMR reflects these changes in structure. The largest spin Hall angle |θSH| of Hf, Ta, W, and Re (˜0.11, 0.10, 0.23, and 0.07, respectively) is found when the dominant phase is amorphous-like. We find that the amorphous-like phase not only possesses large resistivity but also exhibits sizeable spin Hall conductivity, which both contribute to the emergence of the large spin Hall angle.

  7. Correlation between the spin Hall angle and the structural phases of early 5d transition metals

    SciTech Connect

    Liu, Jun; Ohkubo, Tadakatsu; Mitani, Seiji; Hono, Kazuhiro; Hayashi, Masamitsu

    2015-12-07

    We have studied the relationship between the structure and the spin Hall angle of the early 5d transition metals in X/CoFeB/MgO (X = Hf, Ta, W, and Re) heterostructures. Spin Hall magnetoresistance (SMR) is used to characterize the spin Hall angle of the heavy metals. Transmission electron microscopy images show that all underlayers are amorphous-like when their thicknesses are small, however, crystalline phases emerge as the thickness is increased for certain elements. We find that the heavy metal layer thickness dependence of the SMR reflects these changes in structure. The largest spin Hall angle |θ{sub SH}| of Hf, Ta, W, and Re (∼0.11, 0.10, 0.23, and 0.07, respectively) is found when the dominant phase is amorphous-like. We find that the amorphous-like phase not only possesses large resistivity but also exhibits sizeable spin Hall conductivity, which both contribute to the emergence of the large spin Hall angle.

  8. Synthesis of early transition metal and non-equilibrium intermetallic nanoparticles using n-butyllithium

    NASA Astrophysics Data System (ADS)

    Bondi, James F.

    Over the past decade, the role of inorganic nanomaterials has become an essential cornerstone for modern research applications. Despite these applications becoming progressively more advanced, the field of nanoscience is dependent on a material's physical and chemical properties which are affected by factors such as size, shape, composition, and crystal structure. One synthetic approach to yield inorganic nanomaterials with great control is solution-based methods, particularly the reduction of metal salt precursors. Non-equilibrium phases and early transition metals represent one class of materials that may result in new and enhanced properties at the nanoscale but are challenging to synthesize. In this dissertation, I present my studies on synthesizing non-equilibrium intermetallics and early transition metal nanoparticles using n-butyllithium and solution-based methods. By utilizing a template-driven approach, I first report an optimized synthesis for the non-equilibrium L12-type Au 3M1-x ( M = Fe, Co, or Ni) intermetallics with morphological, compositional, and structural control. Modifying a previous n-butyllithium procedure, it was possible to identify key variables (solvent, order of reagent addition, stabilizer, and heating rate) which led to the generation of high phase purity and increased sample sizes. Aliquot studies showed that the intermetallic nanoparticles were formed through the initial nucleation of Au nanoparticles, followed by subsequent incorporation of the 3d transition metal. Property studies of the non-equilibrium phases found that Au3Fe1- x and Au3Co1-x nanoparticles are superparamagnetic with TB = 7.9 K and 2.4 K, respectively, while Au3Ni 1-x is weakly paramagnetic down to 1.8 K. Elemental analysis by energy dispersive X-ray spectroscopy and refinement of electron diffraction patterns confirmed Au3Fe1- x with a composition of approximately Au3Fe 0.7. The 3d transition metal deficiency in the non-equilibrium Au3 M1-x phases was studied by reacting Au nanoparticle seeds with n-butyllithium. The reaction yielded the thermodynamically stable phase Au3Li, a polar intermetallic which adopts the L12 structure type. Interestingly, the Au3Li nanoparticles decompose in water to regenerate Au. The Au3Li phase gives insight to a plausible template-driven reaction pathway for the non-equilibrium Au3M 1-x phases. The synthetic achievement of both non-equilibrium phases and polar intermetallics shows that n-butyllithium is capable of affecting nucleation kinetics and lithium intercalation. Finally, n-butyllithium was used as a strong reducing agent in the solution-based synthesis of elemental Mn nanoparticles. The particles were synthesized using air-free techniques by reacting n-butyllithium with MnCl2 and oleic acid in diphenyl ether. The nanoparticles were found to adopt the alpha-Mn structure and contained a thin amorphous MnO layer bound by oleate ligands to help render them air-stable. Unlike antiferromagnetic bulk Mn, the as-made nanoparticles were paramagnetic. With little modification, crystalline Mo and amorphous W nanoparticles were synthesized using the same n-butyllithium procedure. Using the thermal decomposition of metal-carbonyls was shown to yield W, Mo-based alloys, and tetrapod-like MnO nanoparticles.

  9. C-H Bond Activation by Early Transition Metal Carbide Cluster Anion MoC3 (-).

    PubMed

    Li, Zi-Yu; Hu, Lianrui; Liu, Qing-Yu; Ning, Chuan-Gang; Chen, Hui; He, Sheng-Gui; Yao, Jiannian

    2015-12-01

    Although early transition metal (ETM) carbides can activate CH bonds in condensed-phase systems, the electronic-level mechanism is unclear. Atomic clusters are ideal model systems for understanding the mechanisms of bond activation. For the first time, CH activation of a simple alkane (ethane) by an ETM carbide cluster anion (MoC3 (-) ) under thermal-collision conditions has been identified by using high-resolution mass spectrometry, photoelectron imaging spectroscopy, and high-level quantum chemical calculations. Dehydrogenation and ethene elimination were observed in the reaction of MoC3 (-) with C2 H6 . The CH activation follows a mechanism of oxidative addition that is much more favorable in the carbon-stabilized low-spin ground electronic state than in the high-spin excited state. The reaction efficiency between the MoC3 (-) anion and C2 H6 is low (0.23±0.05) %. A comparison between the anionic and a highly efficient cationic reaction system (Pt(+) +C2 H6 ) was made. It turned out that the potential-energy surfaces for the entrance channels of the anionic and cationic reaction systems can be very different. PMID:26490554

  10. Cyclopentadienyl-containing low-valent early transition metal olefin polymerization catalysts

    DOEpatents

    Marks, Tobin J.; Luo, Lubin; Yoon, Sung Cheol

    2003-04-08

    A catalyst system useful to polymerize and co-polymerize polar and non-polar olefin monomers is formed by in situ reduction with a reducing agent of a catalyst precursor comprising {Cp*MRR'.sub.n }.sup.+ {A}.sup.- wherein Cp* is a cyclopentadienyl or substituted cyclopentadienyl moiety; M is an early transition metal; R is a C.sub.1 -C.sub.20 hydrocarbyl; R' are independently selected from hydride, C.sub.1 -C.sub.20 hydrocarbyl, SiR".sub.3, NR".sub.2, OR", SR", GeR".sub.3, and SnR".sub.3 containing groups (R"=C.sub.1 -C.sub.10 hydrocarbyl); n is an integer selected to balance the oxidation state of M; and A is a suitable non-coordinating anionic cocatalyst or precursor. This catalyst system may form stereoregular olefin polymers including syndiotactic polymers of styrene and methylmethacrylate and isotactic copolymers of polar and nonpolar olefin monomers such as methylmethacrylate and styrene.

  11. Cyclopentadienyl-containing low-valent early transition metal olefin polymerization catalysts

    DOEpatents

    Marks, Tobin J.; Luo, Lubin; Yoon, Sung Cheol

    2007-01-09

    A catalyst system useful to polymerize and co-polymerize polar and non-polar olefin monomers is formed by in situ reduction with a reducing agent of a catalyst precursor comprising {Cp*MRR'.sub.n}.sup.+{A}.sup.- wherein Cp* is a cyclopentadienyl or substituted cyclopentadienyl moiety; M is an early transition metal; R is a C.sub.1 C.sub.20 hydrocarbyl; R' are independently selected from hydride, C.sub.1 C.sub.20 hydrocarbyl, SiR''.sub.3, NR''.sub.2, OR'', SR'', GeR''.sub.3, SnR''.sub.3, and C.dbd.C-containing groups (R''=C.sub.1 C.sub.10 hydrocarbyl); n is an integer selected to balance the oxidation state of M; and A is a suitable non-coordinating anionic cocatalyst or precursor. This catalyst system may form stereoregular olefin polymers including syndiotactic polymers of styrene and methylmethacrylate and isotactic copolymers of polar and nonpolar olefin monomers such as methylmethacrylate and styrene.

  12. Cyclopentadienyl-containing low-valent early transition metal olefin polymerization catalysts

    DOEpatents

    Marks, Tobin J.; Luo, Lubin; Yoon, Sung Cheol

    2003-12-30

    A catalyst system useful to polymerize and co-polymerize polar and non-polar olefin monomers is formed by in situ reduction with a reducing agent of a catalyst precursor comprising wherein Cp* is a cyclopentadienyl or substituted cyclopentadienyl moiety; M is an early transition metal; R is a C.sub.1 -C.sub.20 hydrocarbyl; R' are independently selected from hydride, C.sub.1 -C.sub.20 hydrocarbyl, SiR".sub.3, NR".sub.2, OR", SR", GeR".sub.3, SnR".sub.3, and C.dbd.C containing groups (R".dbd.C.sub.1 -C.sub.10 hydrocarbyl); n is an integer selected to balance the oxidation state of M; and A is a suitable non-coordinating anionic cocatalyst or precursor. This catalyst system may form stereoregular olefin polymers including syndiotactic polymers of styrene and methylmethacrylate and isotactic copolymers of polar and nonpolar olefin monomers such as methylmethacrylate and styrene.

  13. Cyclopentadienyl-containing low-valent early transition metal olefin polymerization catalysts

    DOEpatents

    Marks, Tobin J.; Luo, Lubin; Yoon, Sung Cheol

    2006-10-10

    A catalyst system useful to polymerize and co-polymerize polar and non-polar olefin monomers is formed by in situ reduction with a reducing agent of a catalyst precursor comprising {Cp*MRR'.sub.n}.sup.+{A}.sup.- wherein Cp* is a cyclopentadienyl or substituted cyclopentadienyl moiety; M is an early transition metal; R is a C.sub.1 C.sub.20 hydrocarbyl; R' are independently selected from hydride, C.sub.1 C.sub.20 hydrocarbyl, SiR''.sub.3, NR''.sub.2, OR'', SR'', GeR''.sub.3, SnR''.sub.3, and C.dbd.C-containing groups (R''=C.sub.1 C.sub.10 hydrocarbyl); n is an integer selected to balance the oxidation state of M; and A is a suitable non-coordinating anionic cocatalyst or precursor. This catalyst system may form stereoregular olefin polymers including syndiotactic polymers of styrene and methylmethacrylate and isotactic copolymers of polar and nonpolar olefin monomers such as methylmethacrylate and styrene.

  14. Cyclopentadienyl-Containing Low-Valent Early Transition Metal Olefin Polymerization Catalysts

    DOEpatents

    Marks, Tobin J.; Luo, Lubin; Yoon, Sung Cheol

    2004-06-08

    A catalyst system useful to polymerize and co-polymerize polar and non-polar olefin monomers is formed by in situ reduction with a reducing agent of a catalyst precursor comprising {Cp*MRR'.sub.n }.sup.+ {A}.sup.- wherein Cp* is a cyclopentadienyl or substituted cyclopentadienyl moiety; M is an early transition metal; R is a C.sub.1 -C.sub.20 hydrocarbyl; R' are independently selected from hydride, C.sub.1 -C.sub.20 hydrocarbyl, SiR".sub.3, NR".sub.2, OR", SR", GeR".sub.3, SnR".sub.3, and C.dbd.C-containing groups (R"=C.sub.1 -C.sub.10 hydrocarbyl); n is an integer selected to balance the oxidation state of M; and A is a suitable non-coordinating anionic cocatalyst or precursor. This catalyst system may form stereoregular olefin polymers including syndiotactic polymers of styrene and methylmethacrylate and isotactic copolymers of polar and nonpolar olefin monomers such as methylmethacrylate and styrene.

  15. Early Transition Metal Oxides as Catalysts: Crossing Scales from Clusters to Single Crystals to Functioning Materials

    SciTech Connect

    Lai-Sheng Wang

    2009-07-07

    The overall goal of this program is to investigate the electronic structure and chemical bonding of early transition metal oxide clusters and use them as well-defined molecular models to obtain insight into properties and mechanisms of oxide catalysts, as well as to provide accurate spectroscopic and molecular information to verify theoretical methods used to predict materials properties. A laser vaporization cluster source is used to produce metal oxide clusters with different sizes, structures, and compositions. Well-defined inorganic polyoxometalate clusters in solution are transported in the gas phase using electrospray. Two state-of-the-art photoelectron spectroscopy apparatuses are used to interrogate the oxide clusters and polyoxometalate anions in the gas phase to obtain spectroscopic and electronic structure information. The experimental effort is assisted by theoretical calculations to understanding the structures, chemical bonding, and catalytical properties of the transition metal oxide clusters. The research approach combines novel and flexible experimental techniques and advanced theoretical/computational methodologies and seeks molecular-level information to aiding the design of new catalysts, as well as mechanistic understanding. We have focused on the investigation of tungsten oxide clusters containing three W atoms: W{sub 3}O{sub x}{sup -} (x = 7-11). A number of interesting findings have been made. We observed that the oxygen-poor W{sub 3}O8 cluster contains a localized W{sup 4+} center, which can be used as a molecular model for O-deficient defect sites. A chemisorption energy was obtained through density functional calculations for W{sub 3}O8 + O{sub 2} {yields} W{sub 3}O{sub 10} as -78 kcal/mol. We further found that the neutral stoichiometric W{sub 2}O{sub 6} and W{sub 3}O{sub 9} clusters do not react with O{sub 2} and they only form physi-sorbed complexes, W{sub 2}O{sub 6}(O{sub 2}) and W{sub 3}O{sub 9}(O{sub 2}). However, the negatively charged W{sub 2}O{sub 6}{sup -} and W{sub 3}O{sub 9}{sup -} clusters are found to form chemisorbed complexes due to the presence of the extra electron. Thus, the W{sub 2}O{sub 6}{sup -} and W{sub 3}O{sub 9}{sup -} negative clusters can be viewed as models for O{sub 2} interaction with a reduced W site (W{sup 5+}) on the oxide surface. These studies also led to the surprising observation of the first d-orbital aromatic clusters in W{sub 3}O{sub 9}{sup 2-} and Mo{sub 3}O{sub 9}{sup 2-}, which each contains a completely delocalized three-center two-electron bond made entirely made of the metal d orbitals. This last result was highlighted in both Chem & Eng. News and Nature. We further studied a series of small metalate anions using electrospray, including the hydroxo and methoxo oxometalate MO{sub 3}(OH){sup -} and MO{sub 3}(OCH{sub 3}){sup -}, and the dimetalates: M{sub 2}O{sub 7}{sup 2-}, MM{prime}O{sub 7}{sup 2-}, and M{sub 2}O{sub 7}{sup -} (M, M{prime} = Cr, Mo, and W).

  16. Electronic and magnetic properties of early transition-metal substituted iron-cyclopentadienyl sandwich molecular wires: Parity-dependent half-metallicity

    NASA Astrophysics Data System (ADS)

    Li, Yuanchang; Zhou, Gang; Wu, Jian; Duan, Wenhui

    2011-07-01

    Electronic and magnetic properties of early transition metals (V, Ti, Sc)-FekCpk + 1 sandwich molecular wires (SMWs) are investigated by means of ab initio calculations. It is found that all SMWs favor a ferromagnetic ground state. Significantly, V-FekCpk + 1 SMWs are either half-metallic or semiconducting, dependent upon the parity (even or odd) of the number (k) of Fe atoms in the unit cell of SMWs. This parity oscillation of conductive properties results from the combined effects of the band-folding and gap-opening at the Brillouin-zone boundary of one-dimensional materials. In contrast, Sc-FekCpk + 1 and Ti-FekCpk + 1 SMWs are always semiconducting. Our work may open up the way toward half metal/semiconductor heterostructures with perfect atomic interface.

  17. Low-Coordinate First Row Early Transition Metal Complexes Stabilized by Modified Terphenyl Ligands

    NASA Astrophysics Data System (ADS)

    Boynton, Jessica Nicole

    The research in this dissertation is focused on the synthesis, structural, and magnetic characterization of two-coordinate open shell (d1-d4) transition metal complexes. Background information on this field of endeavor is provided in Chapter 1. In Chapter 2 I describe the synthesis and characterization of the mononuclear chromium (II) terphenyl substituted primary amido complexes and a Lewis base adduct. These studies suggest that the two-coordinate chromium complexes have significant spin-orbit coupling effects which lead to moments lower than the spin only value of 4.90 muB owing to the fact that lambda (the spin orbit coupling parameter) is positive. The three-coordinated complex 2.3 had a magnetic moment of 3.77 muB. The synthesis and characterization of the first stable two-coordinate vanadium complexes are described in Chapter 3. The values suggest a significant spin orbital angular momentum contribution that leads to a magnetic moment that is lower than their spin only value of 3.87 muB. DFT calculations showed that the major absorptions in their UV-Vis spectra were due to ligand to metal charge transfer transitions. The titanium synthesis and characterization of the bisamido complex along with its three-coordinate titanium(III) precursor are described in Chapter 4. Compound 4.1 was obtained via the stoichiometric reaction of LiN(H)AriPr 6 with the Ti(III) complex TiCl3 *2NMe 3 in trimethylamine. The precursor 4.1 has trigonal pyramidal coordination at the titanium atom, with bonding to two amido nitrogens and a chlorine as well as a secondary interaction to a flanking aryl ring of a terphenyl substituent. Compound 4.2 displays a very distorted four-coordinate metal environment in which the titanium atom is bound to two amido nitrogens and to two carbons from a terphenyl aryl ring. This structure is in sharp contrast to the two-coordinate linear structure that was observed in its first row metal (V-Ni) analogs. The synthesis and characterization of mononuclear chromium(II) terphenyl primary substituted thiolate complexes are described in Chapter 5. Reaction of the terphenyl primary thiolate lithium derivatives LiSAriPr4 and LiSArMe6 with CrCl2THF2 in a 2:1 ratio afforded complexes 5.1 and 5.2, which are the very rare examples of chromium(II) thiolates with quasi-two-coordination at the metal center. Both deviate from linearity and have S-Cr-S angles of 111.02(3)° and 107.86(3) ° with secondary Cr-C(aryl ring) interactions of ca. 2.115 A and 1.971 A respectively. The initial work on titanium and vanadium terphenyl thiolates is described in Appendix I and II. In Chapter 6 I show that the reaction of K2COT (COT= 1,3,5,7-cyclooctatetraene, C8H8) with an aryl chromium(II) halide gave (CrAriPr 4)2(mu2-n3:n4-COT) (6.1) in which a non-planar COT ring is complexed between two CrAriPr4 moieties -- a configuration previously unknown for chromium complexes of COT. OneCr2+ ion is bonded primarily to three COT carbons (Cr--C= 2.22-2.30 A ) as well as an ipso carbon (Cr-C= ca. 2.47 A) from a flanking aryl ring of its terphenyl substituent. The otherCr2+ ion bonds to an ipso carbon (Cr-C= ca. 2.53 A) from its terphenyl substituent as well as four COT carbons (Cr--C= 2.24-2.32 A). The COT carbon-carbon distances display an alternating pattern, consistent with the non-planarity and non-aromatic character of the ring. The magnetic properties of 6.1 indicate that theCr2+ ions have a high-spin d4 configuration with S = 2. The temperature dependence of the magnetism indicates that their behavior is due to zero-field splitting of the S = 2 state. Attempts to prepare 6.1 by the direct reaction of quintuple-bonded (CrAriPr4)2 with COT were unsuccessful. (Abstract shortened by UMI.)

  18. FINAL TECHNICAL REPORT Synthetic, Structural and Mechanistic Investigations of Olefin Polymerization Catalyzed by Early Transition Metal Compounds

    SciTech Connect

    Bercaw, John E.

    2014-05-23

    The goal of this project is to develop new catalysts and provide understanding of ligand effects on catalyst composition in order to guide development of superior catalyst systems for polymerization of olefins. Our group is designing and synthesizing new “LX2”,“pincer” type ligands and complexing early transition metals to afford precatalysts. In a collaboration with Hans Brintzinger from the University of Konstanz, we are also examining the structures of the components of catalyst systems obtained from reaction of zirconocene dichlorides with aluminum alkyls and aluminum hydrides. Such systems are currently used commercially to produce polyolefins, but the nature of the active and dormant species as well as the mechanisms of their interconversions are not understood. New information on catalyst design and performance may lead to new types of polymers and/or new chemical transformations between hydrocarbons and transition metal centers, ultimately contributing to the development of catalytic reactions for the production of fuels, commodity and polymeric materials.

  19. Oligocyclopentadienyl transition metal complexes

    SciTech Connect

    de Azevedo, Cristina G.; Vollhardt, K. Peter C.

    2002-01-18

    Synthesis, characterization, and reactivity studies of oligocyclopentadienyl transition metal complexes, namely those of fulvalene, tercyclopentadienyl, quatercyclopentadienyl, and pentacyclopentadienyl(cyclopentadienyl) are the subject of this account. Thermal-, photo-, and redox chemistries of homo- and heteropolynuclear complexes are described.

  20. Superconductivity in transition metals.

    PubMed

    Slocombe, Daniel R; Kuznetsov, Vladimir L; Grochala, Wojciech; Williams, Robert J P; Edwards, Peter P

    2015-03-13

    A qualitative account of the occurrence and magnitude of superconductivity in the transition metals is presented, with a primary emphasis on elements of the first row. Correlations of the important parameters of the Bardeen-Cooper-Schrieffer theory of superconductivity are highlighted with respect to the number of d-shell electrons per atom of the transition elements. The relation between the systematics of superconductivity in the transition metals and the periodic table high-lights the importance of short-range or chemical bonding on the remarkable natural phenomenon of superconductivity in the chemical elements. A relationship between superconductivity and lattice instability appears naturally as a balance and competition between localized covalent bonding and so-called broken covalency, which favours d-electron delocalization and superconductivity. In this manner, the systematics of superconductivity and various other physical properties of the transition elements are related and unified. PMID:25666075

  1. Synthesis, characterization and reactivity of early transition metal neo-pentoxides

    SciTech Connect

    Boyle, T.J.; Alam, T.M.; Scott, B.; Ziller, J.W.

    1997-12-31

    Titanium neo-pentoxide was isolated by the alcoholysis exchange between Ti(OCHMe{sub 2}){sub 4} and ONep. The molecule, [Ti(ONep){sub 4}]{sub 2}, was characterized using X-ray analysis and solution {sup 17}O, and {sup 47},{sup 49}Ti NMR spectroscopy. This dinuclear complex is the smallest Ti(OR){sub 4} isolated wherein each metal center is 5-coordinated. The molecule is highly soluble in standard solvents and volatile. The reactivity of this compound has been undertaken to compare with the ubiquitous Ti(O-I-Pr){sub 4}. The various compounds isolated and further NMR studies will be reported. Analogous routes to other M(ONep){sub n} will be reported as well.

  2. Transition Metal Switchable Mirror

    ScienceCinema

    None

    2013-05-29

    The switchable-mirrors technology was developed by Tom Richardson and Jonathan Slack of Berkeley Lab's Environmental Energy Technologies Division. By using transition metals rather than the rare earth metals used in the first metal-hydride switchable mirrors, Richardson and Slack were able to lower the cost and simplify the manufacturing process. Energy performance is improved as well, because the new windows can reflect or transmit both visible and infrared light. Besides windows for offices and homes, possible applications include automobile sunroofs, signs and displays, aircraft windows, and spacecraft. More information at: http://windows.lbl.gov/materials/chromogenics/default.htm

  3. Transition Metal Switchable Mirror

    ScienceCinema

    None

    2010-01-08

    The switchable-mirrors technology was developed by Tom Richardson and Jonathan Slack of Berkeley Lab's Environmental Energy Technologies Division. By using transition metals rather than the rare earth metals used in the first metal-hydride switchable mirrors, Richardson and Slack were able to lower the cost and simplify the manufacturing process. Energy performance is improved as well, because the new windows can reflect or transmit both visible and infrared light. Besides windows for offices and homes, possible applications include automobile sunroofs, signs and displays, aircraft windows, and spacecraft.

  4. Transition Metal Switchable Mirror

    SciTech Connect

    2009-08-21

    The switchable-mirrors technology was developed by Tom Richardson and Jonathan Slack of Berkeley Lab's Environmental Energy Technologies Division. By using transition metals rather than the rare earth metals used in the first metal-hydride switchable mirrors, Richardson and Slack were able to lower the cost and simplify the manufacturing process. Energy performance is improved as well, because the new windows can reflect or transmit both visible and infrared light. Besides windows for offices and homes, possible applications include automobile sunroofs, signs and displays, aircraft windows, and spacecraft.

  5. Transition Metal Switchable Mirror

    SciTech Connect

    2009-01-01

    The switchable-mirrors technology was developed by Tom Richardson and Jonathan Slack of Berkeley Lab's Environmental Energy Technologies Division. By using transition metals rather than the rare earth metals used in the first metal-hydride switchable mirrors, Richardson and Slack were able to lower the cost and simplify the manufacturing process. Energy performance is improved as well, because the new windows can reflect or transmit both visible and infrared light. Besides windows for offices and homes, possible applications include automobile sunroofs, signs and displays, aircraft windows, and spacecraft. More information at: http://windows.lbl.gov/materials/chromogenics/default.htm

  6. Melting of Transition Metals

    SciTech Connect

    Ross, M; Japel, S; Boehler, R

    2005-04-11

    We review the transition melting studies carried out at Mainz, and describe a recently developed model used to explain that the relatively low melting slopes are due to the partially filled d-bands, and the persistence of the pressure induced s-d transition. The basic tenets of the model have now been reconfirmed by new measurements for Cu and Ni. The measurements show that Cu which has a filled 3d-band, has a melt slope that is about 2.5 greater than its neighbor Ni. In the case of Mo, the apparent discrepancy of DAC melting measurements with shock melting can be explained by accounting for the change in melt slope due to the bcc-cp transition observed in the shock studies. The Fe melt curve is revisited. The possible relevance of the Jahn-Teller effect and recently observed transition metal melts with Icosahedral Short-Range Order (ISRO) is discussed.

  7. Semiconducting transition metal oxides

    NASA Astrophysics Data System (ADS)

    Lany, Stephan

    2015-07-01

    Open shell transition metal oxides are usually described as Mott or charge transfer insulators, which are often viewed as being disparate from semiconductors. Based on the premise that the presence of a correlated gap and semiconductivity are not mutually exclusive, this work reviews electronic structure calculations on the binary 3d oxides, so to distill trends and design principles for semiconducting transition metal oxides. This class of materials possesses the potential for discovery, design, and development of novel functional semiconducting compounds, e.g. for energy applications. In order to place the 3d orbitals and the sp bands into an integrated picture, band structure calculations should treat both contributions on the same footing and, at the same time, account fully for electron correlation in the 3d shell. Fundamentally, this is a rather daunting task for electronic structure calculations, but quasi-particle energy calculations in GW approximation offer a viable approach for band structure predictions in these materials. Compared to conventional semiconductors, the inherent multivalent nature of transition metal cations is more likely to cause undesirable localization of electron or hole carriers. Therefore, a quantitative prediction of the carrier self-trapping energy is essential for the assessing the semiconducting properties and to determine whether the transport mechanism is a band-like large-polaron conduction or a small-polaron hopping conduction. An overview is given for the binary 3d oxides on how the hybridization between the 3d crystal field symmetries with the O-p orbitals of the ligands affects the effective masses and the likelihood of electron and hole self-trapping, identifying those situations where small masses and band-like conduction are more likely to be expected. The review concludes with an illustration of the implications of the increased electronic complexity of transition metal cations on the defect physics and doping, using as an example the diversity of possible atomic and magnetic configurations of the O vacancy in TiO2, and the high levels of hole doping in Co2ZnO4 due to a self-doping mechanism that originates from the multivalence of Co.

  8. Semiconducting transition metal oxides.

    PubMed

    Lany, Stephan

    2015-07-22

    Open shell transition metal oxides are usually described as Mott or charge transfer insulators, which are often viewed as being disparate from semiconductors. Based on the premise that the presence of a correlated gap and semiconductivity are not mutually exclusive, this work reviews electronic structure calculations on the binary 3d oxides, so to distill trends and design principles for semiconducting transition metal oxides. This class of materials possesses the potential for discovery, design, and development of novel functional semiconducting compounds, e.g. for energy applications. In order to place the 3d orbitals and the sp bands into an integrated picture, band structure calculations should treat both contributions on the same footing and, at the same time, account fully for electron correlation in the 3d shell. Fundamentally, this is a rather daunting task for electronic structure calculations, but quasi-particle energy calculations in GW approximation offer a viable approach for band structure predictions in these materials. Compared to conventional semiconductors, the inherent multivalent nature of transition metal cations is more likely to cause undesirable localization of electron or hole carriers. Therefore, a quantitative prediction of the carrier self-trapping energy is essential for the assessing the semiconducting properties and to determine whether the transport mechanism is a band-like large-polaron conduction or a small-polaron hopping conduction. An overview is given for the binary 3d oxides on how the hybridization between the 3d crystal field symmetries with the O-p orbitals of the ligands affects the effective masses and the likelihood of electron and hole self-trapping, identifying those situations where small masses and band-like conduction are more likely to be expected. The review concludes with an illustration of the implications of the increased electronic complexity of transition metal cations on the defect physics and doping, using as an example the diversity of possible atomic and magnetic configurations of the O vacancy in TiO(2), and the high levels of hole doping in Co(2)ZnO(4) due to a self-doping mechanism that originates from the multivalence of Co. PMID:26126022

  9. Electrical Conductivity in Transition Metals

    ERIC Educational Resources Information Center

    Talbot, Christopher; Vickneson, Kishanda

    2013-01-01

    The aim of this "Science Note" is to describe how to test the electron-sea model to determine whether it accurately predicts relative electrical conductivity for first-row transition metals. In the electron-sea model, a metal crystal is viewed as a three-dimensional array of metal cations immersed in a sea of delocalised valence

  10. Electrical Conductivity in Transition Metals

    ERIC Educational Resources Information Center

    Talbot, Christopher; Vickneson, Kishanda

    2013-01-01

    The aim of this "Science Note" is to describe how to test the electron-sea model to determine whether it accurately predicts relative electrical conductivity for first-row transition metals. In the electron-sea model, a metal crystal is viewed as a three-dimensional array of metal cations immersed in a sea of delocalised valence…

  11. Selenophene transition metal complexes

    SciTech Connect

    White, C.J.

    1994-07-27

    This research shows that selenophene transition metal complexes have a chemistry that is similar to their thiophene analogs. Selenophene coordination has been demonstrated and confirmed by molecular structure in both the {eta}{sup 5}- and the {eta}{sup 1}(Se)-coordination modes. The reaction chemistry of selenophene complexes closely resembles that of the analogous thiophene complexes. One major difference, however, is that selenophene is a better donor ligand than thiophene making the selenophene complexes more stable than the corresponding thiophene complexes. The {sup 77}Se NMR chemical shift values for selenophene complexes fall within distinct regions primarily depending on the coordination mode of the selenophene ligand. In the final paper, the C-H bond activation of {eta}{sup 1}(S)-bound thiophenes, {eta}{sup 1}(S)-benzothiophene and {eta}{sup 1}(Se)-bound selenophenes has been demonstrated. The deprotonation and rearrangement of the {eta}{sup 1}(E)-bound ligand to the carbon bound L-yl complex readily occurs in the presence of base. Reprotonation with a strong acid gives a carbene complex that is unreactive towards nucleophilic attack at the carbene carbon and is stable towards exposure to air. The molecular structure of [Cp(NO)(PPh{sub 3})Re(2-benzothioenylcarbene)]O{sub 3}SCF{sub 3} was determined and contains a Re-C bond with substantial double bond character. Methyl substitution for the thienylcarbene or selenylcarbene gives a carbene that rearranges thermally to give back the {eta}{sup 1}(E)-bound complex. Based on these model reactions, a new mechanism for the H/D exchange of thiophene over the hydrodesulfurization catalyst has been proposed.

  12. Living Ziegler-Natta polymerization by early transition metals: synthesis and evaluation of cationic zirconium alkyl complexes bearing beta-hydrogens as models for propagating centers.

    PubMed

    Harney, Matthew B; Keaton, Richard J; Fettinger, James C; Sita, Lawrence R

    2006-03-15

    The synthesis and characterization of a series of cationic zirconium and hafnium complexes with alkyl substituents bearing beta-hydrogens of general formula {(eta(5)-C5Me5)MR[N(Et)C(Me)N(t-Bu)]}[B(C6F5)4] [M = Zr; R = Et, n-Pr, i-Pr, n-Bu, i-Bu, and 2-ethylbutyl (5a-f) and M = Hf; R = i-Bu and t-Bu (6 and 7, respectively)] is described, including several isotopically labeled derivatives. The ability of these complexes to serve as model complexes for the living Ziegler-Natta polymerization of olefins that can be effected using the initiator 2a (R = Me in 5) has been addressed. The results obtained shed additional light on the steric and electronic factors that can contribute to the living character of a Ziegler-Natta polymerization based on an early transition metal initiator. PMID:16522123

  13. DFT+U study of electrical levels and migration barriers of early 3 d and 4 d transition metals in silicon

    NASA Astrophysics Data System (ADS)

    Marinopoulos, A. G.; Santos, P.; Coutinho, J.

    2015-08-01

    Owing to their strong interaction with carriers, early 3 d -row (Ti, V, and Cr) and 4 d -row (Zr, Nb, and Mo) transition metals (TMs) are undesired contaminants in solar- and electronic-grade Si. The increasing stringent control of contamination levels is urging an accurate picture of their electronic structure. In the present work, the electrical levels and migration energies of these TMs are determined by means of standard density-functional theory (DFT) and a rotationally invariant formulation of DFT+U . The latter approach improves on the treatment of electronic correlations at the TM sites and relies on on-site Hubbard Coulomb and Hund's exchange parameters U and J , respectively. These are calculated self-consistently from linear-response theory without fitting to experimental data. The effect of correlation was found more pronounced for Ti and V, with a strong impact on the location of their electrical levels. In most cases, the agreement with the experimental data is satisfactory allowing the identification of the type and character of the levels. For Cr and Mo in particular, the results resolve longstanding controversies concerning the type and position of the levels. The obtained migration barriers display moderate charge-state and correlation dependency. High barriers were found for all metals studied, with the exception of Cr, confirming them as slow diffusers in silicon among the whole TM family.

  14. Alkali and transition metal phospholides

    NASA Astrophysics Data System (ADS)

    Bezkishko, I. A.; Zagidullin, A. A.; Milyukov, V. A.; Sinyashin, O. G.

    2014-06-01

    Major tendencies in modern chemistry of alkali and transition metal phospholides (phosphacyclopentadienides) are systematized, analyzed and generalized. Basic methods of synthesis of these compounds are presented. Their chemical properties are considered with a special focus on their complexing ability. Potential applications of phospholides and their derivatives are discussed. The bibliography includes 184 references.

  15. Transition metal sulfide loaded catalyst

    DOEpatents

    Maroni, Victor A.; Iton, Lennox E.; Pasterczyk, James W.; Winterer, Markus; Krause, Theodore R.

    1994-01-01

    A zeolite based catalyst for activation and conversion of methane. A zeolite support includes a transition metal (Mo, Cr or W) sulfide disposed within the micropores of the zeolite. The catalyst allows activation and conversion of methane to C.sub.2 + hydrocarbons in a reducing atmosphere, thereby avoiding formation of oxides of carbon.

  16. Transition metal sulfide loaded catalyst

    DOEpatents

    Maroni, V.A.; Iton, L.E.; Pasterczyk, J.W.; Winterer, M.; Krause, T.R.

    1994-04-26

    A zeolite-based catalyst is described for activation and conversion of methane. A zeolite support includes a transition metal (Mo, Cr or W) sulfide disposed within the micropores of the zeolite. The catalyst allows activation and conversion of methane to C[sub 2]+ hydrocarbons in a reducing atmosphere, thereby avoiding formation of oxides of carbon.

  17. Surface chemistry on transition metal carbides

    NASA Astrophysics Data System (ADS)

    Hwu, Henry Hanyuan

    The carbides of Groups IVB-VIB early transition metals have been shown to exhibit catalytic properties similar to those observed for Pt-Group metals in reactions with hydrocarbon molecules. Various electrochemical studies revealed tungsten carbides (WC and W2C) to be remarkably resistant to acidic corrosion at anodic potentials, though their effectiveness as electrocatalysts remained inconclusive. The first part of this research therefore focused on the fundamental mechanistic studies of the reactivity of the clean and carbide-modified W(111), W(110), and Mo(110) surfaces toward the direct methanol fuel cell (DMFC) molecules, methanol, water, and carbon monoxide. Temperature-programmed desorption and electron energy loss spectroscopy results showed that the carbide-modified surfaces were highly active toward the decomposition of methanol and water. Furthermore, the desorption of CO from both carbide surfaces occurred at near room temperature. To better characterize the carbide surfaces, the second part of this research concentrated on identifying the various parameters influencing the surface chemistry of carbides. Studies comparing the reaction of ethylene on carbide-modified W(111) and W(110) showed that, though both surfaces readily decomposed ethylene, only the C/W(110) surface was able to form the ethylidyne intermediate commonly observed on Pt-group metals. Additionally, the Pt-like reactivity of C/Mo(110) was modified after exposure to oxygen. Similar to many carbide-modified early transition metal surfaces, C/Ti(0001) also demonstrated Pt-like reactivity toward cyclohexene and ethylene. Lastly, the effects of carbon-modification was examined on Ni(111), a late transition metal surface. Although the carbide-modified Ni(111) surface was similar to other early transition metal carbide surfaces in the selective dehydrogenation of cyclohexene to benzene, the carbide layer converted to graphitic carbon at higher temperatures. This dissertation showed that tungsten carbide materials may be considered as a potential electrocatalyst for the direct methanol fuel cell on the basis that, when compared to Pt-group metal surfaces, they exhibited higher oxidation activity toward methanol and water, and that they were able to desorb carbon monoxide at lower temperatures. In addition, this study also demonstrated that substrate structure, surface modifications, and the position of the transition metal in the periodic table could influence the reactivity of the carbide surfaces.

  18. Transition metals in superheat melts

    NASA Technical Reports Server (NTRS)

    Jakes, Petr; Wolfbauer, Michael-Patrick

    1993-01-01

    A series of experiments with silicate melts doped with transition element oxides was carried out at atmospheric pressures of inert gas at temperatures exceeding liquidus. As predicted from the shape of fO2 buffer curves in T-fO2 diagrams the reducing conditions for a particular oxide-metal pair can be achieved through the T increase if the released oxygen is continuously removed. Experimental studies suggest that transition metals such as Cr or V behave as siderophile elements at temperatures exceeding liquidus temperatures if the system is not buffered by the presence of other oxide of more siderophile element. For example the presence of FeO prevents the reduction of Cr2O3. The sequence of decreasing siderophility of transition elements at superheat conditions (Mo, Ni, Fe, Cr) matches the decreasing degree of depletion of siderophile elements in mantle rocks as compared to chondrites.

  19. Early metal bis(phosphorus-stabilised)carbene chemistry.

    PubMed

    Liddle, Stephen T; Mills, David P; Wooles, Ashley J

    2011-05-01

    Since the discovery of covalently-bound mid- and late-transition metal carbenes there has been a spectacular explosion of interest in their chemistry, but their early metal counterparts have lagged behind. In recent years, bis(phosphorus-stabilised)carbenes have emerged as valuable ligands for metals across the periodic table, and their use has in particular greatly expanded covalently-bound early metal carbene chemistry. In this tutorial review we introduce the reader to bis(phosphorus-stabilised)carbenes, and cover general preparative methods, structure and bonding features, and emerging reactivity studies of early metal derivatives (groups 1-4 and the f-elements). PMID:21321726

  20. Approximating metal-insulator transitions

    NASA Astrophysics Data System (ADS)

    Danieli, Carlo; Rayanov, Kristian; Pavlov, Boris; Martin, Gaven; Flach, Sergej

    2015-12-01

    We consider quantum wave propagation in one-dimensional quasiperiodic lattices. We propose an iterative construction of quasiperiodic potentials from sequences of potentials with increasing spatial period. At each finite iteration step, the eigenstates reflect the properties of the limiting quasiperiodic potential properties up to a controlled maximum system size. We then observe approximate Metal-Insulator Transitions (MIT) at the finite iteration steps. We also report evidence on mobility edges, which are at variance to the celebrated Aubry-André model. The dynamics near the MIT shows a critical slowing down of the ballistic group velocity in the metallic phase, similar to the divergence of the localization length in the insulating phase.

  1. Synthesis of transition metal carbonitrides

    DOEpatents

    Munir, Zuhair A. R.; Eslamloo-Grami, Maryam

    1994-01-01

    Transition metal carbonitrides (in particular, titanium carbonitride, TiC.sub.0.5 N.sub.0.5) are synthesized by a self-propagating reaction between the metal (e.g., titanium) and carbon in a nitrogen atmosphere. Complete conversion to the carbonitride phase is achieved with the addition of TiN as diluent and with a nitrogen pressure .gtoreq.0.6 MPa. Thermodynamic phase-stability calculations and experimental characterizations of quenched samples provided revealed that the mechanism of formation of the carbonitride is a two-step process. The first step involves the formation of the nonstoichiometric carbide, TiC.sub.0.5, and is followed by the formation of the product by the incorporation of nitrogen in the defect-structure carbide.

  2. Radiative transitions in metallic nanoclusters

    NASA Astrophysics Data System (ADS)

    Shalin, A. S.

    2008-02-01

    In this article, a new theoretical approach to studying light-scattering characteristics of nanosized objects based on the solution to the Thomas-Fermi equation and quasi-classical approximation is considered. It is shown that the distribution of valence electrons in the volume of metallic clusters exhibits a specific structure of "spatial zones." With the aid of quasi-classical wave functions, expressions for the appropriate dipole moments of the transitions between the ground and excited states are obtained; the behavior of the spectrum of gold clusters depending on their sizes is studied; a comparison with existing experimental data is carried out.

  3. Method for dry etching of transition metals

    DOEpatents

    Ashby, C.I.H.; Baca, A.G.; Esherick, P.; Parmeter, J.E.; Rieger, D.J.; Shul, R.J.

    1998-09-29

    A method for dry etching of transition metals is disclosed. The method for dry etching of a transition metal (or a transition metal alloy such as a silicide) on a substrate comprises providing at least one nitrogen- or phosphorus-containing {pi}-acceptor ligand in proximity to the transition metal, and etching the transition metal to form a volatile transition metal/{pi}-acceptor ligand complex. The dry etching may be performed in a plasma etching system such as a reactive ion etching (RIE) system, a downstream plasma etching system (i.e. a plasma afterglow), a chemically-assisted ion beam etching (CAIBE) system or the like. The dry etching may also be performed by generating the {pi}-acceptor ligands directly from a ligand source gas (e.g. nitrosyl ligands generated from nitric oxide), or from contact with energized particles such as photons, electrons, ions, atoms, or molecules. In some preferred embodiments of the present invention, an intermediary reactant species such as carbonyl or a halide ligand is used for an initial chemical reaction with the transition metal, with the intermediary reactant species being replaced at least in part by the {pi}-acceptor ligand for forming the volatile transition metal/{pi}-acceptor ligand complex.

  4. Transition metals activate TFEB in overexpressing cells

    PubMed Central

    Peña, Karina A.; Kiselyov, Kirill

    2015-01-01

    Transition metal toxicity is an important factor in the pathogenesis of numerous human disorders, including neurodegenerative diseases. Lysosomes have emerged as important factors in transition metal toxicity because they handle transition metals via endocytosis, autophagy, absorption from the cytoplasm and exocytosis. Transcription factor EB (TFEB) regulates lysosomal biogenesis and the expression of lysosomal proteins in response to lysosomal and/or metabolic stresses. Since transition metals cause lysosomal dysfunction, we proposed that TFEB may be activated to drive gene expression in response to transition metal exposure and that such activation may influence transition metal toxicity. We found that transition metals copper (Cu) and iron (Fe) activate recombinant TFEB and stimulate the expression of TFEB-dependent genes in TFEB-overexpressing cells. In cells that show robust lysosomal exocytosis, TFEB was cytoprotective at moderate levels of Cu exposure, decreasing oxidative stress as reported by the expression of heme oxygenase-1 (HMOX1) gene. However, at high levels of Cu exposure, particularly in cells with low levels of lysosomal exocytosis, activation of overexpressed TFEB was toxic, increasing oxidative stress and mitochondrial damage. Based on these data, we conclude that TFEB-driven gene network is a component of the cellular response to transition metals. These data suggest limitations and disadvantages of TFEB overexpression as a therapeutic approach. PMID:26251447

  5. Method for dry etching of transition metals

    DOEpatents

    Ashby, Carol I. H. (Edgewood, NM); Baca, Albert G. (Albuquerque, NM); Esherick, Peter (Albuquerque, NM); Parmeter, John E. (Albuquerque, NM); Rieger, Dennis J. (Tijeras, NM); Shul, Randy J. (Albuquerque, NM)

    1998-01-01

    A method for dry etching of transition metals. The method for dry etching of a transition metal (or a transition metal alloy such as a silicide) on a substrate comprises providing at least one nitrogen- or phosphorous-containing .pi.-acceptor ligand in proximity to the transition metal, and etching the transition metal to form a volatile transition metal/.pi.-acceptor ligand complex. The dry etching may be performed in a plasma etching system such as a reactive ion etching (RIE) system, a downstream plasma etching system (i.e. a plasma afterglow), a chemically-assisted ion beam etching (CAIBE) system or the like. The dry etching may also be performed by generating the .pi.-acceptor ligands directly from a ligand source gas (e.g. nitrosyl ligands generated from nitric oxide), or from contact with energized particles such as photons, electrons, ions, atoms, or molecules. In some preferred embodiments of the present invention, an intermediary reactant species such as carbonyl or a halide ligand is used for an initial chemical reaction with the transition metal, with the intermediary reactant species being replaced at least in part by the .pi.-acceptor ligand for forming the volatile transition metal/.pi.-acceptor ligand complex.

  6. Superconducting Metallic Glass Transition-Edge-Sensors

    NASA Technical Reports Server (NTRS)

    Hays, Charles C. (Inventor)

    2013-01-01

    A superconducting metallic glass transition-edge sensor (MGTES) and a method for fabricating the MGTES are provided. A single-layer superconducting amorphous metal alloy is deposited on a substrate. The single-layer superconducting amorphous metal alloy is an absorber for the MGTES and is electrically connected to a circuit configured for readout and biasing to sense electromagnetic radiation.

  7. Metal-metal bond lengths in complexes of transition metals*

    PubMed Central

    Pauling, Linus

    1976-01-01

    In complexes of the transition metals containing clusters of metal atoms the cobalt-cobalt bond lengths are almost always within 1 pm of the single-bond value 246 pm given by the enneacovalent radius of cobalt, whereas most of the observed iron-iron bond lengths are significantly larger than the single-bond value 248 pm, the mean being 264 pm, which corresponds to a half-bond. A simple discussion of the structures of these complexes based on spd hybrid orbitals, the electroneutrality principle, and the partial ionic character of bonds between unlike atoms leads to the conclusion that resonance between single bonds and no-bonds would occur for iron and its congeners but not for cobalt and its congeners, explaining the difference in the bond lengths. PMID:16592368

  8. Metal-semiconductor-metal transition in zigzag carbon nanoscrolls

    NASA Astrophysics Data System (ADS)

    Dong, Haixia; Zhang, Yang; Fang, Dangqi; Gong, Baihua; Zhang, Erhu; Zhang, Shengli

    2016-01-01

    Similar to rolling up paper, carbon nanoscrolls (CNSs) can be rolled from graphene nanoribbons (GNRs) using physical approaches. Owing to their peculiar one-dimensional nanostructures, CNSs have attracted great attention over the past few years. In this study, we have investigated the effects of bending deformation on the electronic properties of zigzag CNSs (ZCNSs) during the rolling process from zigzag GNRs (ZGNRs) by means of first-principles calculations. It is found that a metal-semiconductor-metal transition is observed. By analyzing charge density and density of states, the origin of this electronic property transition is discussed. Furthermore, we find that the metal-semiconductor-metal transition in ZCNSs is independent of ribbon width as well as spin-orbit interaction. Our results of the metal-semiconductor-metal transition in the ZCNSs are robust and may open potential applications in nano-electromechanical devices based on the ZCNSs.Similar to rolling up paper, carbon nanoscrolls (CNSs) can be rolled from graphene nanoribbons (GNRs) using physical approaches. Owing to their peculiar one-dimensional nanostructures, CNSs have attracted great attention over the past few years. In this study, we have investigated the effects of bending deformation on the electronic properties of zigzag CNSs (ZCNSs) during the rolling process from zigzag GNRs (ZGNRs) by means of first-principles calculations. It is found that a metal-semiconductor-metal transition is observed. By analyzing charge density and density of states, the origin of this electronic property transition is discussed. Furthermore, we find that the metal-semiconductor-metal transition in ZCNSs is independent of ribbon width as well as spin-orbit interaction. Our results of the metal-semiconductor-metal transition in the ZCNSs are robust and may open potential applications in nano-electromechanical devices based on the ZCNSs. Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr07628e

  9. Protein-Transition Metal Ion Networks

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Proteins obtained from agricultural sources were blended with divalent metal ions. Feather keratin, egg albumin, and wheat gluten showed increases of 2-3 times in modulus with addition of divalent transition metal ions Cu2+ and Zn2+. Increasing concentrations of ions resulted in increased stiffnes...

  10. PROTEIN-TRANSITION METAL ION NETWORKS

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Proteins obtained from agricultural sources were blended with divalent metal ions. Feather keratin, egg albumin, and wheat gluten had low, medium, and high levels of aspartic and glutamic acid, respectively, and FT-IR showed that the divalent transition metal ions Mn2+, Cu2+, and Zn2+ were tightly ...

  11. EARLY SOCIAL TRANSITION AND THE DROPOUT PROBLEM.

    ERIC Educational Resources Information Center

    DEUTSCH, MARTIN

    AN ATTEMPT IS MADE TO RELATE THE PROBLEM OF THE HIGH SCHOOL DROPOUT TO HIS EARLY SCHOOL EXPERIENCES. ALTHOUGH NO DATA ARE AVAILABLE AT PRESENT TO SUPPORT THE IDEA THAT THE PRESCHOOL PROGRAM COULD REDUCE THE INCIDENCE OF LATER DROPOUT, IT IS SUGGESTED THAT THE PROBLEM OF THE HIGH SCHOOL DROPOUT SHOULD BE FOCUSED ON THE TRANSITION BETWEEN PRESCHOOL…

  12. Transition metal contacts to graphene

    NASA Astrophysics Data System (ADS)

    Politou, Maria; Asselberghs, Inge; Radu, Iuliana; Conard, Thierry; Richard, Olivier; Lee, Chang Seung; Martens, Koen; Sayan, Safak; Huyghebaert, Cedric; Tokei, Zsolt; De Gendt, Stefan; Heyns, Marc

    2015-10-01

    Achieving low resistance contacts to graphene is a common concern for graphene device performance and hybrid graphene/metal interconnects. In this work, we have used the circular Transfer Length Method (cTLM) to electrically characterize Ag, Au, Ni, Ti, and Pd as contact metals to graphene. The consistency of the obtained results was verified with the characterization of up to 72 cTLM structures per metal. Within our study, the noble metals Au, Ag and Pd, which form a weaker bond with graphene, are shown to result in lower contact resistance (Rc) values compared to the more reactive Ni and Ti. X-ray Photo Electron Spectroscopy and Transmission Electron Microscopy characterization for the latter have shown the formation of Ti and Ni carbides. Graphene/Pd contacts show a distinct intermediate behavior. The weak carbide formation signature and the low Rc values measured agree with theoretical predictions of an intermediate state of weak chemisorption of Pd on graphene.

  13. Method of boronizing transition-metal surfaces

    SciTech Connect

    Koyama, K.; Shimotake, H.

    1981-08-28

    A method is presented for preparing a boride layer on a transition metal substrate for use in corrosive environments or as a harden surface in machine applications. This method is particularly useful in treating current collectors for use within a high temperature and corrosive electrochemical cell environment. A melt of a alkali metal boride tetrafluoride salt including such as KF to lower its melting point is prepared including a dissolved boron containing material, for instance NiB, MnB/sub 2/, or CrB/sub 2/. A transition metal to be coated is immersed in the melt at a temperature of no more than 700/sup 0/C and a surface boride layer of that transition metal is formed within a period of about 24 hours on the substrate surface.

  14. Method of boronizing transition metal surfaces

    DOEpatents

    Koyama, Koichiro; Shimotake, Hiroshi.

    1983-08-16

    A method is presented for preparing a boride layer on a transition metal substrate for use in corrosive environments or as a harden surface in machine applications. This method is particularly useful in treating current collectors for use within a high temperature and corrosive electrochemical cell environment. A melt of a alkali metal boride tetrafluoride salt including such as KF to lower its melting point is prepared including a dissolved boron containing material, for instance NiB, MnB[sub 2], or CrB[sub 2]. A transition metal to be coated is immersed in the melt at a temperature of no more than 700 C and a surface boride layer of that transition metal is formed within a period of about 24 hours on the substrate surface. 4 figs.

  15. Method of boronizing transition metal surfaces

    DOEpatents

    Koyama, Koichiro; Shimotake, Hiroshi

    1983-01-01

    A method is presented for preparing a boride layer on a transition metal substrate for use in corrosive environments or as a harden surface in machine applications. This method is particularly useful in treating current collectors for use within a high temperature and corrosive electrochemical cell environment. A melt of a alkali metal boride tetrafluoride salt including such as KF to lower its melting point is prepared including a dissolved boron containing material, for instance NiB, MnB.sub.2, or CrB.sub.2. A transition metal to be coated is immersed in the melt at a temperature of no more than 700.degree. C. and a surface boride layer of that transition metal is formed within a period of about 24 hours on the substrate surface.

  16. Understanding topological phase transition in monolayer transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Choe, Duk-Hyun; Sung, Ha-Jun; Chang, K. J.

    2016-03-01

    Despite considerable interest in layered transition metal dichalcogenides (TMDs), such as M X2 with M =(Mo ,W ) and X =(S ,Se ,Te ) , the physical origin of their topological nature is still poorly understood. In the conventional view of topological phase transition (TPT), the nontrivial topology of electron bands in TMDs is caused by the band inversion between metal d - and chalcogen p -orbital bands where the former is pulled down below the latter. Here, we show that, in TMDs, the TPT is entirely different from the conventional speculation. In particular, M S2 and M S e2 exhibits the opposite behavior of TPT such that the chalcogen p -orbital band moves down below the metal d -orbital band. More interestingly, in M T e2 , the band inversion occurs between the metal d -orbital bands. Our findings cast doubts on the common view of TPT and provide clear guidelines for understanding the topological nature in new topological materials to be discovered.

  17. Microwave-assisted synthesis of transition metal phosphide

    SciTech Connect

    Viswanathan, Tito

    2014-12-30

    A method of synthesizing transition metal phosphide. In one embodiment, the method has the steps of preparing a transition metal lignosulfonate, mixing the transition metal lignosulfonate with phosphoric acid to form a mixture, and subjecting the mixture to a microwave radiation for a duration of time effective to obtain a transition metal phosphide.

  18. Chemical vapour deposition: Transition metal carbides go 2D

    NASA Astrophysics Data System (ADS)

    Gogotsi, Yury

    2015-11-01

    The unique properties of 2D materials, such as graphene or transition metal dichalcogenides, have been attracting much attention in the past decade. Now, metallically conductive and even superconducting transition metal carbides are entering the game.

  19. Transition metal based anticancer drugs.

    PubMed

    Garbutcheon-Singh, K Benjamin; Grant, Maxine P; Harper, Benjamin W; Krause-Heuer, Anwen M; Manohar, Madhura; Orkey, Nikita; Aldrich-Wright, Janice R

    2011-01-01

    With an ageing baby-boomer population in the Western World, cancer is becoming a significant cause of death. The prevalence of cancer and all associated costs, both in human and financial terms, drives the search for new therapeutic drugs and treatments. Platinum anticancer agents, such as cisplatin have been highly successful but there are several disadvantages associated with their use. What is need are new compounds with different mechanisms of action and resistance profiles. What needs to be recognised is that there are many other metal in the periodic table with therapeutic potential. Here we have highlighted metal complexes with activity and have illustrate the different approaches to the design of anticancer complexes. PMID:21189131

  20. Transition metal sulfide sites in industry

    SciTech Connect

    Stiefel, E.I.

    1996-10-01

    Transition metal sulfide (TMS) catalysts are used industrially in the hydrotreating of petroleum fractions. In particular, hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) catalysis involve the use of supported transition metal sulfide catalysts usually involving molybdenum, sulfur, and cobalt. The cobalt is said to promote the activity of the molybdenum sulfide. A broad survey of the activity of TMS catalysts for HDS (by Chianelli et al.) reveals periodic trends in activity and offers insight into the material properties necessary for activity. Additionally, studies of molecular systems by several research groups have led to increased understanding of the chemical nature and reactivity trends of TMS systems. Studies on mononuclear {open_quotes}sites{close_quotes} and heteronuclear clusters have each provided food for thought and new approaches to catalyst systems. Related heteronuclear TMS clusters and complexes remain of great interest with respect to transition metal sulfide sites in enzymes.

  1. From Early Intervention to Early Childhood Programs: Timeline for Early Successful Transitions (TEST)

    ERIC Educational Resources Information Center

    Brandes, Joyce A.; Ormsbee, Christine K.; Haring, Kathryn A.

    2007-01-01

    More than one million transitions between early intervention services and early childhood programs are facilitated annually for youngsters with special needs. To be successful, these transitions require planning and ongoing communication between all parties. This article substantiates the need for a timeline/checklist and provides a model of…

  2. Holographic lattices and metal-insulator transition

    NASA Astrophysics Data System (ADS)

    Ling, Yi

    2015-10-01

    This paper is an extension of the talk given at the conference on Gravitation and Cosmology/The Fourth Galileo-Xu Guangqi Meeting. We intend to present a short review on recent progress on the construction of holographic lattices and its application to metal-insulator transition (MIT), which is a fundamentally important phenomenon in condensed matter physics. We will firstly implement the Peierls phase transition by constructing holographic charge density waves which are induced by the spontaneous breaking of translational symmetry. Then we turn to the holographic realization of metal-insulator transition as a quantum critical phenomenon with many strongly correlated electrons involved. The holographic entanglement entropy as a diagnostic for such quantum phase transitions will be briefly mentioned.

  3. OH-transition metal bonding

    NASA Technical Reports Server (NTRS)

    Bauschlicher, Charles W., Jr.

    1986-01-01

    The bonding in both CuOH and AgOH has a covalent component that leads to a bent structure. The larger electrostatic stabilization in CuOH leads to a larger D(e) (2.83 eV) compared with AgOH (2.20 eV). Using Ni5OH to model chemisorption of OH on a Ni surface, it is found that OH adsorption in the fourfold hollow of Ni(100) leads to an OH normal to the surface, while adsorption directly above a Ni atom leads to a tilted OH. These qualitative Ni5OH calculations allow for speculation on the observed variation of OH on metal surfaces.

  4. Dimensional diversity in transition metal trihalides

    SciTech Connect

    Jianhua Lin; Miller, G.J. )

    1993-04-14

    Structural variations of the second- and third-row transition metal trihalides are rationalized via tight-binding band calculations and evaluation of Madelung energetic factors. The observed structure for a given metal halide is controlled by both the coordination geometry at the anion and the d electron configuration at the metal. As the polarizability of the halide increases, the M-X-M angle, in general, decreases so that three-dimensional frameworks occur for the fluorides, while layer and chain structures are found for the chlorides, bromides, and iodides. Within a particular halide system, systematic structural trends also occur as the d electron configuration changes. 56 refs., 23 figs., 4 tabs.

  5. Magnetic and Metal-Insulator Transition in natural Transition Metal Sulfides

    NASA Astrophysics Data System (ADS)

    Wang, Renxiong; Metz, Tristin; Liu, I.-Lin; Wang, Kefeng; Wang, Xiangfeng; Jeffries, J. R.; Saha, S. R.; Greene, R. L.; Paglione, J.; Santelli, C. C.; Post, J.,

    In collaboration with the Smithsonian Institution's National Museum of Natural History, we present detailed studies of a class of natural minerals with potential to harbor correlated behavior. Transition metal sulfide minerals, such as Bornite (Cu5FeS4), are an important family of compounds known for their thermoelectric properties. We will present low temperature experimental studies of magnetic transitions and focus on a compound that exhibits a metal to insulator transition concident with entrance to an antiferromagnetic ground state, suggesting a potentially interesting system with promise for realizing new correlated states of matter in a naturally occurring mineral.

  6. Cascade morphology transition in bcc metals.

    PubMed

    Setyawan, Wahyu; Selby, Aaron P; Juslin, Niklas; Stoller, Roger E; Wirth, Brian D; Kurtz, Richard J

    2015-06-10

    Energetic atom collisions in solids induce shockwaves with complex morphologies. In this paper, we establish the existence of a morphological transition in such cascades. The order parameter of the morphology is defined as the exponent, b, in the defect production curve as a function of cascade energy (N(F) ~ E(MD)(b)). Response of different bcc metals can be compared in a consistent energy domain when the energy is normalized by the transition energy, μ, between the high- and the low-energy regime. Using Cr, Fe, Mo and W data, an empirical formula of μ as a function of displacement threshold energy, E(d), is presented for bcc metals. PMID:25985256

  7. Cascade morphology transition in bcc metals

    SciTech Connect

    Setyawan, Wahyu; Selby, A.; Juslin, Niklas; Stoller, Roger E.; Wirth, Brian D.; Kurtz, Richard J.

    2015-06-10

    Energetic atom collisions in solids induce shockwaves with complex morphologies. In this paper, we establish the existence of a morphological transition in such cascades. The order parameter of the morphology is defined as the exponent, $b$, in the defect production curve as a function of cascade energy ($N_F$$ \\sim$$E_{MD}^b$). Response of different bcc metals can be compared in a consistent energy domain when the energy is normalized by the transition energy, $\\mu$, between the high- and the low-energy regime. Using Cr, Fe, Mo and W data, an empirical formula of $\\mu$ as a function of displacement threshold energy, $E_d$, is presented for bcc metals.

  8. (Electronic structure and reactivities of transition metal clusters)

    SciTech Connect

    Not Available

    1992-01-01

    The following are reported: theoretical calculations (configuration interaction, relativistic effective core potentials, polyatomics, CASSCF); proposed theoretical studies (clusters of Cu, Ag, Au, Ni, Pt, Pd, Rh, Ir, Os, Ru; transition metal cluster ions; transition metal carbide clusters; bimetallic mixed transition metal clusters); reactivity studies on transition metal clusters (reactivity with H{sub 2}, C{sub 2}H{sub 4}, hydrocarbons; NO and CO chemisorption on surfaces). Computer facilities and codes to be used, are described. 192 refs, 13 figs.

  9. Single-layer transition metal sulfide catalysts

    DOEpatents

    Thoma, Steven G.

    2011-05-31

    Transition Metal Sulfides (TMS), such as molybdenum disulfide (MoS.sub.2), are the petroleum industry's "workhorse" catalysts for upgrading heavy petroleum feedstocks and removing sulfur, nitrogen and other pollutants from fuels. We have developed an improved synthesis technique to produce SLTMS catalysts, such as molybdenum disulfide, with potentially greater activity and specificity than those currently available. Applications for this technology include heavy feed upgrading, in-situ catalysis, bio-fuel conversion and coal liquefaction.

  10. Semiconductor-metal transition in semiconducting bilayer sheets of transition-metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Bhattacharyya, Swastibrata; Singh, Abhishek K.

    2012-08-01

    Using first-principles calculations we show that the band gap of bilayer sheets of semiconducting transition-metal dichalcogenides (TMDs) can be reduced smoothly by applying vertical compressive pressure. These materials undergo a universal reversible semiconductor-to-metal (S-M) transition at a critical pressure. The S-M transition is attributed to lifting of the degeneracy of the bands at the Fermi level caused by interlayer interactions via charge transfer from the metal to the chalcogen. The S-M transition can be reproduced even after incorporating the band gap corrections using hybrid functionals and the GW method. The ability to tune the band gap of TMDs in a controlled fashion over a wide range of energy opens up the possibility for its usage in a range of applications.

  11. Aging of Transition Metal Dichalcogenide Monolayers.

    PubMed

    Gao, Jian; Li, Baichang; Tan, Jiawei; Chow, Phil; Lu, Toh-Ming; Koratkar, Nikhil

    2016-02-23

    Two-dimensional sheets of transition metal dichalcogenides are an emerging class of atomically thin semiconductors that are considered to be "air-stable", similar to graphene. Here we report that, contrary to current understanding, chemical vapor deposited transition metal dichalcogenide monolayers exhibit poor long-term stability in air. After room-temperature exposure to the environment for several months, monolayers of molybdenum disulfide and tungsten disulfide undergo dramatic aging effects including extensive cracking, changes in morphology, and severe quenching of the direct gap photoluminescence. X-ray photoelectron and Auger electron spectroscopy reveal that this effect is related to gradual oxidation along the grain boundaries and the adsorption of organic contaminants. These results highlight important challenges associated with the utilization of transition metal dichalcogenide monolayers in electronic and optoelectronic devices. We also demonstrate a potential solution to this problem, featuring encapsulation of the monolayer sheet by a 10-20 nm thick optically transparent polymer (parylene C). This strategy is shown to successfully prevent the degradation of the monolayer material under accelerated aging (i.e., high-temperature, oxygen-rich) conditions. PMID:26808328

  12. Ferroelectric control of metal-insulator transition

    NASA Astrophysics Data System (ADS)

    He, Xu; Jin, Kui-juan; Ge, Chen; Ma, Zhong-shui; Yang, Guo-zhen

    2016-03-01

    We propose a method of controlling the metal-insulator transition of one perovskite material at its interface with another ferroelectric material based on first principle calculations. The operating principle is that the rotation of oxygen octahedra tuned by the ferroelectric polarization can modulate the superexchange interaction in this perovskite. We designed a tri-color superlattice of (BiFeO3)N/LaNiO3/LaTiO3, in which the BiFeO3 layers are ferroelectric, the LaNiO3 layer is the layer of which the electronic structure is to be tuned, and LaTiO3 layer is inserted to enhance the inversion asymmetry. By reversing the ferroelectric polarization in this structure, there is a metal-insulator transition of the LaNiO3 layer because of the changes of crystal field splitting of the Ni eg orbitals and the bandwidth of the Ni in-plane eg orbital. It is highly expected that a metal-transition can be realized by designing the structures at the interfaces for more materials.

  13. Hydrogenating properties of unsupported transition metal sulfides

    SciTech Connect

    Lacroix, M.; Boutarfa, N.; Guillard, C.; Vrinat, M.; Breysse, M. )

    1989-12-01

    Group VI and Group VIII transition metal sulfides (TMS) have been widely used in hydrotreating catalysis. In addition to hydrogenation reactions, these processes involve removal of sulfur (hydrodesulfurization: HDS) and nitrogen (hydrodenitrogenation: HDN). Until now, HDN has not received as much attention as HDS, probably because sulfur compounds have historically been of prime importance. As petroleum feedstocks dwindle, the need for hydroprocessing oils containing larger amounts of heteromolecules will increase and industry will require more active materials. For this purpose, researchers may either improve the current catalysts of develop a new generation of catalysts based on transition metal sulfides presenting higher activities, stabilities, and selectivities toward desired compounds. The objective of the present work is to classify the hydrogenation performances of well-defined unsupported transition metal sulfides. These catalysts have been chosen since their characterization by physiochemical techniques, mainly X-ray diffraction (XRD), is easier than that for supported materials. Actually, it has been shown in several cases that the catalytic properties are closely related to the crystalline structure and the stoichiometry of the different phases which can be encountered for a given element. The catalyst were tested in the hydrogenation of biphenyl (HN of BP) as well as in the hydrodesulfurization of dibenzothiophene (HDS of DBT) in order to compare the present results to previous studies. To evaluate the hydrogenating function, the HN of BP has been chosen preferably to the consecutive HN of BP resulting from the HDS of DBT.

  14. Corrosion behavior of mesoporous transition metal nitrides

    SciTech Connect

    Yang, Minghui; Allen, Amy J.; Nguyen, Minh T.; Ralston, Walter T.; MacLeod, Michelle J.; DiSalvo, Francis J.

    2013-09-15

    Transition metal nitrides (TMN) have many desirable characteristics such as high hardness and good thermal stability under reducing conditions. This work reports an initial survey of the chemical stability of mesoporous TMNs (TM=Nb, V, Cr and Ti) in water at 80 °C at neutral, acidic and alkaline pH. The mesoporous TMNs had specific surface areas of 25–60 m{sup 2}/g with average pore sizes ranging from 10 to 50 nm. The high surface areas of these materials enhance the rate of corrosion per unit mass over that of a bulk material, making detection of corrosion much easier. The products were characterized by Rietveld refinement of powder X-ray diffraction (PXRD) patterns and by scanning electron microscopy (SEM). Several nitrides have corrosion rates that are, within error, not distinguishable from zero (±1 Å/day). Of the nitrides examined, CrN appears to be the most corrosion resistant under acidic conditions. None of the nitrides studied are corrosion resistant under alkaline conditions. - Graphical abstract: Corrosion behavior of mesoporous transition metal nitrides (TM=Nb, V, Cr and Ti) in acidic and alkaline solutions at 80 °C for 2 weeks. Display Omitted - highlights: • Corrosion rates of mesoporous transition metal nitrides in aqueous solution is reported. • The mesoporous TMNs had surface areas of 25–60 m{sup 2}/g. • CrN is the most corrosion resistant under the conditions studied.

  15. Pseudopotentials for quantum Monte Carlo studies of transition metal oxides

    NASA Astrophysics Data System (ADS)

    Krogel, Jaron T.; Santana, Juan A.; Reboredo, Fernando A.

    2016-02-01

    Quantum Monte Carlo (QMC) calculations of transition metal oxides are partially limited by the availability of high-quality pseudopotentials that are both accurate in QMC and compatible with major plane-wave electronic structure codes. We have generated a set of neon-core pseudopotentials with small cutoff radii for the early transition metal elements Sc to Zn within the local density approximation of density functional theory. The pseudopotentials have been directly tested for accuracy within QMC by calculating the first through fourth ionization potentials of the isolated transition metal (M) atoms and the binding curve of each M-O dimer. We find the ionization potentials to be accurate to 0.16(1) eV, on average, relative to experiment. The equilibrium bond lengths of the dimers are within 0.5(1)% of experimental values, on average, and the binding energies are also typically accurate to 0.18(3) eV. The level of accuracy we find for atoms and dimers is comparable to what has recently been observed for bulk metals and oxides using the same pseudopotentials. Our QMC pseudopotential results also compare well with the findings of previous QMC studies and benchmark quantum chemical calculations.

  16. Pseudopotentials for quantum Monte Carlo studies of transition metal oxides

    DOE PAGESBeta

    Krogel, Jaron T.; Santana Palacio, Juan A.; Reboredo, Fernando A.

    2016-02-22

    Quantum Monte Carlo (QMC) calculations of transition metal oxides are partially limited by the availability of high-quality pseudopotentials that are both accurate in QMC and compatible with major plane-wave electronic structure codes. We have generated a set of neon-core pseudopotentials with small cutoff radii for the early transition metal elements Sc to Zn within the local density approximation of density functional theory. The pseudopotentials have been directly tested for accuracy within QMC by calculating the first through fourth ionization potentials of the isolated transition metal (M) atoms and the binding curve of each M-O dimer. We find the ionization potentialsmore » to be accurate to 0.16(1) eV, on average, relative to experiment. The equilibrium bond lengths of the dimers are within 0.5(1)% of experimental values, on average, and the binding energies are also typically accurate to 0.18(3) eV. The level of accuracy we find for atoms and dimers is comparable to what has recently been observed for bulk metals and oxides using the same pseudopotentials. Our QMC pseudopotential results compare well with the findings of previous QMC studies and benchmark quantum chemical calculations.« less

  17. Highly Polar Metal-Metal Bonds in "Early-Late" Heterodimetallic Complexes.

    PubMed

    Gade

    2000-08-01

    Metal-metal bond polarity in its extreme form involving transition elements is found in di- or polynuclear complexes in which molecular fragments containing metal atoms from the two ends of the d block in the periodic table are combined. This linkage by direct metal-metal bonds of metal centers having very different oxidation states has been a challenge to the synthetic chemist. The suppression of degradative reaction channels caused by intramolecular single-electron transfer and the protection of the highly Lewis acidic early transition metal center by an appropriately designed ligand shell have opened up the systematic investigation of such systems. Concomitant with this development, advances in the conceptual framework for the quantitative description of bond polarity have led to a refined understanding of the nature of this type of metal-metal bonding. The greatest stimulus for the development of this field of research is the investigation of the cooperative reactivity of two or more coordination centers in their interaction with and transformation of organic substrates. This cooperativity, which is characterized by the different functions adopted by the metal centers in these conversions offers attractive perspectives in stoichiometric or even catalytic transformations. PMID:10934392

  18. Peculiar magnetism of transition metal cluster compounds

    NASA Astrophysics Data System (ADS)

    Malik, Vikas; Yadav, C. S.; Rastogi, Ashok; Kumar, Deepak

    2013-03-01

    We discuss a class of solids which have in their structure fairly separated tetrahederal clusters of transition metals: vanadium and molybednum. With decreasing temperature these compounds undergo a structural transition from cubic to rhombohederal phase at a temperature Ts, followed by a transition to a ferromagnetic phase at a lower temperarure Tc. Magnetic measurements indicate the presence of moments of spin half per cluster. Between Tc and Ts susceptibility shows large departures from Curie-Weiss behavior and specific heat has an unusually large non-phononic contribution. We present a model in which each cluster acts as a unit associated with a magnetic moment of spin half and a structural degree of freedom related to distortions of the tetrahedron. Our model accounts for all the observed features including effects due to external pressure and magnetic field.

  19. Orbital ordering in transition-metal spinels

    NASA Astrophysics Data System (ADS)

    Radaelli, Paolo G.

    2006-03-01

    Transition-metal spinels (general formula AB2X4) have been for many years the subject of intense experimental and theoretical activity. Structurally, the most interesting feature of these systems is the fact that the B cation occupies the nodes of a pyrochlore lattice, which is known to be geometrically frustrated. Therefore, one can explore how their natural tendency to order in the charge, magnetic and orbital sectors is affected by geometrical frustration. Orbital Ordering (OO) occurs when the orbital degeneracy of an extended concentrated system is lifted, typically through a distortion of the lattice. This may result in either single orbital occupation or alternation/modulation of the orbital occupancy, with or without an associated charge ordering. The degree of charge localization is another important issue: in highly localized systems, one can speak of a cooperative Jahn-Teller (JT) effect, but this paradigm is no longer sufficient in the present of significant electronic hopping. Both eg and t2g orbitals are relevant for transition-metal spinels; eg systems, such as ZnMn2O4, tend to display the strongest JT effects, with direct coupling to the lattice. More recently, significant attention has been devoted to the possibility of orbital ordering in ions with partial t2g occupation, such as Ti^3+ and V^4+ (3d^1) and V^3+ (3d^2) and their electron/hole-symmetric counterparts. With respect to eg systems, t2g systems have a greater degeneracy and weaker coupling to the lattice, and can give rise to significant hopping, due to the direct overlap of the t2g orbitals along the <110> crystallographic directions of the spinel structure. Recent results on Ti, V, Fe and Ir spinels will be discussed, with particular reference to the relation between the complex crystallographic superstructures and the changes in transport (metal-insulator transitions) and magnetic (paramagnetic-diamagnetic transitions) properties at the ordering temperatures.

  20. Synthesis of arsenic transition metal sulfides and metal arsenides

    SciTech Connect

    Singhal, G.H.; Brown, L.D.; Ryan, D.F.

    1993-12-31

    One of the chief problems in upgrading shale oil is the presence of inherent arsenic which is known to poison downstream catalysts. Highly dispersed transition metal sulfides formed in situ from the decomposition of dithiocarbamate (DTC) complexes of transition metals show excellent potential as dearsenation agents. The authors have studied the reaction of these sulfides with various arsenic compositions and characterized the metal arsenides and arsenic metal sulfides formed as well as the ease of their formation. Thus, the reaction of bis(butyldithiocarbamato)Ni, (NiBuDTC) with model compounds was very facile and gave NiAs, NiAsS, and NiAs2=xSx. In general the effectiveness of the sulfides for dearsenation followed the sequence Ni>Mo{much_gt}Co, while iron sulfides were totally ineffective. Based upon these results, tests were run in autoclaves (as well as a fixed-bed flow-through unit) with NiBuDTC and shale oil having 73 ppm inherent As. Under optimum conditions, dearsenation down to les than 1 ppm was obtained.

  1. Gas-phase thermochemistry of the early cationic transition-metal sulfides of the second row: YS+, ZrS+, and NbS+

    NASA Astrophysics Data System (ADS)

    Kretzschmar, Ilona; Schröder, Detlef; Schwarz, Helmut; Armentrout, P. B.

    2006-03-01

    The gas-phase reactivity of YS+, ZrS+, and NbS+ towards oxygen-transferring reagents such as H2O, CO, CO2, and COS is investigated using guided-ion beam (GIB) and Fourier-transform ion cyclotron resonance (FTICR) mass spectrometry. A lower limit of D0(M+-S) > 4.50 +/- 0.04 eV for the sulfur binding energy is derived from the exothermic formation of MS+ in the reaction of all three atomic metal ions, M+, with CS2. Upper limits for D0(M+-S) are determined by collision-induced dissociation of MS+ with Xe. The O-transfer experiments carried out in the GIB instrument lead to several estimates for D0(M+-S), which are further refined by equilibrium constants Keq derived from the reaction MS+ + H2O --> MO+ + H2S and its reverse. Overall assessment of the results from ion-molecule reactions, collision-induced dissociation, and equilibrium measurements yields the 0 K bond energies of D0(Y+-S) = 5.49 +/- 0.18 eV, D0(Zr+-S) = 5.69 +/- 0.10 eV, D0(Nb+-S) = 5.20 +/- 0.21 eV, D0(Y+-CS) = 1.42 +/- 0.08 eV, D0(Zr+-CS) = 2.67 +/- 0.11 eV, and D0(Nb+-CS) = 2.51 +/- 0.11 eV, and heats of formation for [Delta]fH0(YOS+) = 6.59 +/- 0.37 eV, [Delta]fH0(ZrOS+) = 8.45 +/- 0.33 eV, [Delta]fH0(NbOS+) = 9.05 +/- 0.27 eV, [Delta]fH0(YS2+) = 6.96 +/- 0.70 eV, and [Delta]fH0(ZrS2+) = 9.34 +/- 0.73 eV.

  2. Pseudopotentials for quantum Monte Carlo calculations of transition metal oxides

    NASA Astrophysics Data System (ADS)

    Krogel, Jaron; Santana, Juan; Kent, Paul; Reboredo, Fernando

    2015-03-01

    Quantum Monte Carlo calculations of transition metal oxides are partially limited by the availability of high quality pseudopotentials that are both accurate in QMC and compatible with major electronic structure codes, e.g. by not being overly hard in the standard planewave basis. Following insight gained from recent GW calculations, a set of neon core pseudopotentials with small cutoff radii have been created for the early transition metal elements Sc to Zn within the local density approximation of DFT. The pseudopotentials have been tested for energy consistency within QMC by calculating the first through fourth ionization potentials of the isolated transition metal (TM) atoms and the binding curve of each TM-O dimer. The vast majority of the ionization potentials fall within 0.3 eV of the experimental values, with exceptions occurring mainly for atoms with multiple unpaired d electrons where multireference effects are the strongest. The equilibrium bond lengths of the dimers are within 1% of experimental values and the binding energy errors are typically less than 0.3 eV. Given the uniform treatment of the core, the larger deviations occasionally observed may primarily reflect the limitations of a Slater-Jastrow trial wavefunction. This work is supported by the Materials Sciences & Engineering Division of the Office of Basic Energy Sciences, U.S. DOE. Research by PRCK was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

  3. Vibrational scaling factors for transition metal carbonyls

    NASA Astrophysics Data System (ADS)

    Assefa, M. K.; Devera, J. L.; Brathwaite, A. D.; Mosley, J. D.; Duncan, M. A.

    2015-11-01

    Vibrational frequencies for a selected set of transition metal carbonyl complexes are computed with various forms of density functional theory (B3LYP, BP86, M06, and M06-L), employing several different basis sets. The computed frequencies for the carbonyl stretches are compared to the experimental values obtained from gas phase infrared spectra of isolated neutrals and ions. Recommended carbonyl-stretch scaling factors which are developed vary significantly for different functionals, but there is little variation with basis set. Scaled frequencies compared to experimental spectra for cobalt and tantalum carbonyl cations reveal additional variations in multiplet patterns and relative band intensities for different functionals.

  4. Electrical Conduction in Transition-Metal Salts

    NASA Astrophysics Data System (ADS)

    Grado-Caffaro, M. A.; Grado-Caffaro, M.

    2016-04-01

    We predict that a given transition-metal salt as, for example, a K2CuCl4·2H2O-type compound, can behave as an electrical conductor in the paramagnetic case. In fact, we determine the electrical conductance in a salt of this type. This conductance is found to be quantised in agreement with previous well-known results. Related mathematical expressions in the context of superexchange interaction are obtained. In addition, we determine the corresponding (macroscopically viewed) current density and the associated electron wave functions.

  5. Tunable magnetocaloric effect in transition metal alloys.

    PubMed

    Belyea, Dustin D; Lucas, M S; Michel, E; Horwath, J; Miller, Casey W

    2015-01-01

    The unpredictability of geopolitical tensions and resulting supply chain and pricing instabilities make it imperative to explore rare earth free magnetic materials. As such, we have investigated fully transition metal based "high entropy alloys" in the context of the magnetocaloric effect. We find the NiFeCoCrPdx family exhibits a second order magnetic phase transition whose critical temperature is tunable from 100 K to well above room temperature. The system notably displays changes in the functionality of the magnetic entropy change depending on x, which leads to nearly 40% enhancement of the refrigerant capacity. A detailed statistical analysis of the universal scaling behavior provides direct evidence that heat treatment and Pd additions reduce the distribution of exchange energies in the system, leading to a more magnetically homogeneous alloy. The general implications of this work are that the parent NiFeCoCr compound can be tuned dramatically with FCC metal additives. Together with their relatively lower cost, their superior mechanical properties that aid manufacturability and their relative chemical inertness that aids product longevity, NiFeCoCr-based materials could ultimately lead to commercially viable magnetic refrigerants. PMID:26507636

  6. Single Layer Transition Metal Dichalcogenides Transistors

    NASA Astrophysics Data System (ADS)

    Jiang, Yuhang; Mao, Jinhai; Andrei, Eva; Rutgers University, Department of Physics; Astronomy Team

    2014-03-01

    Single layer Transition Metal Dichalcogenides (TMDs), such as MoS2, WS2 and TaS2, are atomically thin two-dimensional materials with unique electronic properties different from their bulk counterparts. The lack of inversion symmetry, high mass of the components and the 2D geometry lead to strong spin-orbit coupling and to a metal insulator transition (MIT). Recent progressing ultrathin sample preparation and nanodevice fabrication has opened new opportunities to explore the transport properties of these layers for potential applications in nanoelectronics. In particular the ability to gate these samples across the MIT, carries the promise of sharp switching characteristics that defeat the thermodynamically imposed limiton the sub-threshold slope in standard field effect transistors. We will report on the electronic properties of field effect transistors fabricated with monolayer in the TMD family under conditions of extreme doping achieved by ionic liquid gating. Supported by DOE-FG02-99ER45742 and NSF DMR 1207108.

  7. Tunable magnetocaloric effect in transition metal alloys

    NASA Astrophysics Data System (ADS)

    Belyea, Dustin D.; Lucas, M. S.; Michel, E.; Horwath, J.; Miller, Casey W.

    2015-10-01

    The unpredictability of geopolitical tensions and resulting supply chain and pricing instabilities make it imperative to explore rare earth free magnetic materials. As such, we have investigated fully transition metal based “high entropy alloys” in the context of the magnetocaloric effect. We find the NiFeCoCrPdx family exhibits a second order magnetic phase transition whose critical temperature is tunable from 100 K to well above room temperature. The system notably displays changes in the functionality of the magnetic entropy change depending on x, which leads to nearly 40% enhancement of the refrigerant capacity. A detailed statistical analysis of the universal scaling behavior provides direct evidence that heat treatment and Pd additions reduce the distribution of exchange energies in the system, leading to a more magnetically homogeneous alloy. The general implications of this work are that the parent NiFeCoCr compound can be tuned dramatically with FCC metal additives. Together with their relatively lower cost, their superior mechanical properties that aid manufacturability and their relative chemical inertness that aids product longevity, NiFeCoCr-based materials could ultimately lead to commercially viable magnetic refrigerants.

  8. Methyl Complexes of the Transition Metals.

    PubMed

    Campos, Jesús; López-Serrano, Joaquín; Peloso, Riccardo; Carmona, Ernesto

    2016-05-01

    Organometallic chemistry can be considered as a wide area of knowledge that combines concepts of classic organic chemistry, that is, based essentially on carbon, with molecular inorganic chemistry, especially with coordination compounds. Transition-metal methyl complexes probably represent the simplest and most fundamental way to view how these two major areas of chemistry combine and merge into novel species with intriguing features in terms of reactivity, structure, and bonding. Citing more than 500 bibliographic references, this review aims to offer a concise view of recent advances in the field of transition-metal complexes containing M-CH3 fragments. Taking into account the impressive amount of data that are continuously provided by organometallic chemists in this area, this review is mainly focused on results of the last five years. After a panoramic overview on M-CH3 compounds of Groups 3 to 11, which includes the most recent landmark findings in this area, two further sections are dedicated to methyl-bridged complexes and reactivity. PMID:26991740

  9. Transition Metal Nitrides: A First Principles Study

    NASA Astrophysics Data System (ADS)

    Pathak, Ashish; Singh, A. K.

    2016-04-01

    The present work describes the structural stability and electronic and mechanical properties of transition metal nitrides (TmNs: B1 cubic structure (cF8, Fm ‾ overline 3 m)) using first principles density functional theory (DFT) within generalized gradient approximation (GGA). The lattice constant of TmNs increases with increasing the atomic radii of the transition metals. Stability of the TmNs decreases from IVB to VIB groups due to increase in formation energy/atom. The bonding characteristics of these nitrides have been explained based on electronic density of states and charge density. All the TmNs satisfy Born stability criteria in terms of elastic constants except CrN and MoN that do not exist in equilibrium binary phase diagrams. The groups IVB and V-VIB nitrides are associated with brittle and ductile behaviour based on G/B ratios, respectively. The estimated melting temperatures of these nitrides exhibit reasonably good agreement with calculated with B than those of the C11 for all nitrides.

  10. Tunable magnetocaloric effect in transition metal alloys

    PubMed Central

    Belyea, Dustin D.; Lucas, M. S.; Michel, E.; Horwath, J.; Miller, Casey W.

    2015-01-01

    The unpredictability of geopolitical tensions and resulting supply chain and pricing instabilities make it imperative to explore rare earth free magnetic materials. As such, we have investigated fully transition metal based “high entropy alloys” in the context of the magnetocaloric effect. We find the NiFeCoCrPdx family exhibits a second order magnetic phase transition whose critical temperature is tunable from 100 K to well above room temperature. The system notably displays changes in the functionality of the magnetic entropy change depending on x, which leads to nearly 40% enhancement of the refrigerant capacity. A detailed statistical analysis of the universal scaling behavior provides direct evidence that heat treatment and Pd additions reduce the distribution of exchange energies in the system, leading to a more magnetically homogeneous alloy. The general implications of this work are that the parent NiFeCoCr compound can be tuned dramatically with FCC metal additives. Together with their relatively lower cost, their superior mechanical properties that aid manufacturability and their relative chemical inertness that aids product longevity, NiFeCoCr-based materials could ultimately lead to commercially viable magnetic refrigerants. PMID:26507636

  11. Metal Insulator transition in Vanadium Dioxide

    NASA Astrophysics Data System (ADS)

    Jovaini, Azita; Fujita, Shigeji; Suzuki, Akira; Godoy, Salvador

    2012-02-01

    MAR12-2011-000262 Abstract Submitted for the MAR12 Meeting of The American Physical Society Sorting Category: 03.9 (T) On the metal-insulator-transition in vanadium dioxide AZITA JOVAINI, SHIGEJI FUJITA, University at Buffalo, SALVADOR GODOY, UNAM, AKIRA SUZUKI, Tokyo University of Science --- Vanadium dioxide (VO2) undergoes a metal-insulator transition (MIT) at 340 K with the structural change from tetragonal to monoclinic crystal. The conductivity _/ drops at MIT by four orders of magnitude. The low temperature monoclinic phase is known to have a lower ground-state energy. The existence of the k-vector k is prerequisite for the conduction since the k appears in the semiclassical equation of motion for the conduction electron (wave packet). The tetragonal (VO2)3 unit is periodic along the crystal's x-, y-, and z-axes, and hence there is a three-dimensional k-vector. There is a one-dimensional k for a monoclinic crystal. We believe this difference in the dimensionality of the k-vector is the cause of the conductivity drop. Prefer Oral Session X Prefer .

  12. Precursors in the preparation of transition metal nitrides and transition metal carbonitrides and their reaction intermediates

    SciTech Connect

    Maya, L.

    1991-06-11

    This patent describes a composition comprising: a transition metal bound to; a first ligand selected from the group inorganic amide and imide ligands; and a second ligand being acetylide ligands. This patent also describes a process for making ceramics comprising: pyrolyzing the precursor in an inert atmosphere.

  13. High-Pressure Thermodynamic Properties of f-electron Metals, Transition Metal Oxides, and Half-Metallic Magnets

    SciTech Connect

    Richard T. Scalettar; Warren E. Pickett

    2005-08-02

    This project involves research into the thermodynamic properties of f-electron metals, transition metal oxides, and half-metallic magnets at high pressure. These materials are ones in which the changing importance of electron-electron interactions as the distance between atoms is varied can tune the system through phase transitions from localized to delocalized electrons, from screened to unscreened magnetic moments, and from normal metal to one in which only a single spin specie can conduct. Three main thrusts are being pursued: (i) Mott transitions in transition metal oxides, (ii) magnetism in half-metallic compounds, and (iii) large volume-collapse transitions in f-band metals.

  14. Recent advances in metathesis-derived polymers containing transition metals in the side chain

    PubMed Central

    Demonceau, Albert; Fischer, Helmut

    2015-01-01

    Summary This account critically surveys the field of side-chain transition metal-containing polymers as prepared by controlled living ring-opening metathesis polymerization (ROMP) of the respective metal-incorporating monomers. Ferrocene- and other metallocene-modified polymers, macromolecules including metal-carbonyl complexes, polymers tethering early or late transition metal complexes, etc. are herein discussed. Recent advances in the design and syntheses reported mainly during the last three years are highlighted, with special emphasis on new trends for superior applications of these hybrid materials. PMID:26877797

  15. Recent advances in metathesis-derived polymers containing transition metals in the side chain.

    PubMed

    Dragutan, Ileana; Dragutan, Valerian; Simionescu, Bogdan C; Demonceau, Albert; Fischer, Helmut

    2015-01-01

    This account critically surveys the field of side-chain transition metal-containing polymers as prepared by controlled living ring-opening metathesis polymerization (ROMP) of the respective metal-incorporating monomers. Ferrocene- and other metallocene-modified polymers, macromolecules including metal-carbonyl complexes, polymers tethering early or late transition metal complexes, etc. are herein discussed. Recent advances in the design and syntheses reported mainly during the last three years are highlighted, with special emphasis on new trends for superior applications of these hybrid materials. PMID:26877797

  16. Defect-Tolerant Monolayer Transition Metal Dichalcogenides.

    PubMed

    Pandey, Mohnish; Rasmussen, Filip A; Kuhar, Korina; Olsen, Thomas; Jacobsen, Karsten W; Thygesen, Kristian S

    2016-04-13

    Localized electronic states formed inside the band gap of a semiconductor due to crystal defects can be detrimental to the material's optoelectronic properties. Semiconductors with a lower tendency to form defect induced deep gap states are termed defect-tolerant. Here we provide a systematic first-principles investigation of defect tolerance in 29 monolayer transition metal dichalcogenides (TMDs) of interest for nanoscale optoelectronics. We find that the TMDs based on group VI and X metals form deep gap states upon creation of a chalcogen (S, Se, Te) vacancy, while the TMDs based on group IV metals form only shallow defect levels and are thus predicted to be defect-tolerant. Interestingly, all the defect sensitive TMDs have valence and conduction bands with a very similar orbital composition. This indicates a bonding/antibonding nature of the gap, which in turn suggests that dangling bonds will fall inside the gap. These ideas are made quantitative by introducing a descriptor that measures the degree of similarity of the conduction and valence band manifolds. Finally, the study is generalized to nonpolar nanoribbons of the TMDs where we find that only the defect sensitive materials form edge states within the band gap. PMID:27027786

  17. Routines and Transitions: A Guide for Early Childhood Professionals

    ERIC Educational Resources Information Center

    Malenfont, Nicole

    2006-01-01

    In early childhood settings, children spend over 50 percent of their time on handwashing, dressing, napping, and other routines and transitions. "Routines and Transitions" is a guide to help turn these routine daily activities into learning experiences. By using transitions wisely, providers not only help children develop skills, but also run a…

  18. Diagnostic Transitions from Childhood to Adolescence to Early Adulthood

    ERIC Educational Resources Information Center

    Copeland, William E.; Adair, Carol E.; Smetanin, Paul; Stiff, David; Briante, Carla; Colman, Ian; Fergusson, David; Horwood, John; Poulton, Richie; Costello, E. Jane; Angold, Adrian

    2013-01-01

    Background: Quantifying diagnostic transitions across development is needed to estimate the long-term burden of mental illness. This study estimated patterns of diagnostic transitions from childhood to adolescence and from adolescence to early adulthood. Methods: Patterns of diagnostic transitions were estimated using data from three prospective,…

  19. Cosmological Consequences of QCD Phase Transition(s) in Early Universe

    SciTech Connect

    Tawfik, A.

    2009-04-17

    We discuss the cosmological consequences of QCD phase transition(s) on the early universe. We argue that our recent knowledge about the transport properties of quark-gluon plasma (QGP) should thraw additional lights on the actual time evolution of our universe. Understanding the nature of QCD phase transition(s), which can be studied in lattice gauge theory and verified in heavy ion experiments, provides an explanation for cosmological phenomenon stem from early universe.

  20. Magnetic localization in transition-metal nanowires

    SciTech Connect

    Skomski, R.; Zeng, H.; Zheng, M.; Sellmyer, D. J.

    2000-08-01

    Magnetization reversal in transition-metal nanowires is investigated. Model calculations explain why magnetization reversal is localized, as opposed to the sometimes assumed delocalized coherent-rotation and curling modes. The localization is a quite general phenomenon caused by morphological inhomogenities and occurring in both polycrystalline and single-crystalline wires. In the polycrystalline limit, the competition between interatomic exchange and anisotropy gives rise to a variety of random-anisotropy effects, whereas nearly single-crystalline wires exhibit a weak localization of the nucleation mode. Model predictions are used to explain the coercive and magnetic-viscosity behavior of Co (and Ni) nanowires electrodeposited in self-assembled alumina pores. (c) 2000 The American Physical Society.

  1. Radiation damage of transition metal carbides

    SciTech Connect

    Dixon, G.

    1991-01-01

    In this grant period we have investigated electrical properties of transition metal carbides and radiation-induced defects produced by low-temperature electron irradiation in them. Special attention has been given to the composition VC[sub 0.88] in which the vacancies on the carbon sublattice of this fcc crystal order to produce a V[sub 8]C[sub 7] superlattice. The existence of this superlattice structure was found to make the crystal somewhat resistant to radiation damage at low doses and/or at ambient temperature. At larger doses significant changes in the resistivity are produced. Annealing effects were observed which we believe to be connected with the reconstitution of the superlattice structure.

  2. Energy bands in some transition metals

    NASA Astrophysics Data System (ADS)

    Laurent, D. G.

    1981-08-01

    Self consistent linear combination of Gaussian orbitals energy band calculations were performed for the two paramagnetic 3d transition metals, chromium and vanadium. The energy bands densities of states and Fermi surfaces were obtained using the two most popular local exchange correlation potentials (Kohn-Sham-Gaspar and von Barth-Hedin) for chromium and the Kohn-Sham-Gaspar potential alone for vanadium. A comparison was made with the available experimental data. New interpretations for some of the neutron scattering data are made in the chromium case. Results are also presented for the Compton profiles and optical conductivities. These correlate well with the experiments if appropriate angular averages (for the Compton profile) and lifetime effcts (for the optical conductivity) are included. The electron energy loss spectrum, computed over the range 0-6.5 eV agreed well with experiment.

  3. Transitions for Young Children: Creating Connections across Early Childhood Systems

    ERIC Educational Resources Information Center

    Kagan, Sharon Lynn, Ed.; Tarrant, Kate, Ed.

    2010-01-01

    Smooth early childhood transitions are key to ensuring positive outcomes for young children the world over--but in today's fragmented early education systems, it's difficult to ensure continuity among programs and services. Early childhood professionals will help change that with this book, the first to propose a comprehensive, practical framework…

  4. Spin-orbit damping in transition metals

    NASA Astrophysics Data System (ADS)

    Gilmore, Keith

    2008-03-01

    Magnetization dynamics are routinely described with the Landau-Lifshitz-Gilbert (LLG) equation. However, it is expected that the LLG equation fails to properly describe the large amplitude dynamics that occur during magnetization reversal. Improving switching speeds in nanoscale devices by tailoring materials requires both a qualitative understanding of the relaxation processes that contribute to damping and the ability to quantitatively calculate the resulting damping rates. We consider small amplitude LLG damping in transition metals as a prelude to approaching the more complicated mechanisms expected in complete reversal events. LLG damping rates in pure transition metal systems have non-monotonic temperature dependencies that have been empirically shown by Heinrich et al. [1] to have one part proportional to the conductivity and one part proportional to the resistivity. Kambersky [2] postulated that both contributions result from a torque between the spin and orbital moments. We have conducted first-principles calculations that validate this claim for single element systems [3]. Our calculations for Fe, Co, and Ni both qualitatively match the two trends observed in measurements and quantitatively agree with the observed damping rates. We will discuss how the spin-orbit interaction produces two contributions to damping with nearly opposite temperature dependencies and compare calculations of the damping rate versus resistivity with experimental results. [1] B. Heinrich, D.J. Meredith, and J.F. Cochran, J. Appl. Phys., 50(11), 7726 (1979). [2] V. Kambersky, Czech. J. Phys. B, 26, 1366 (1976). [3] K. Gilmore, Y.U. Idzerda, and M.D. Stiles, Phys. Rev. Lett., 99, 027204 (2007).

  5. Isomorphic introduction of d(0) transition metals to mesoporous silica

    NASA Astrophysics Data System (ADS)

    Morey, Mark Sanson

    1998-12-01

    Early transition metals (Ti, V, Zr, Mo, W) have been incorporated in the mesoporous silicate system by hydrothermal and post-synthesis treatments. The materials were studied by diffraction, adsorption and spectroscopic techniques and were found to have catalytic, halogenation activity toward organic substrates at neutral pH. Intense interest has been shown in the new class of silica-based, mesoporous materials due to their high potential for catalytic applications. Their synthesis occurs via a cooperative self-assembly of surfactant/silicate pairs to form numerous, extended network structures upon silicate condensation based on liquid crystal phases of surfactant/water systems. Surfactant micelle removal by calcination generates a well defined pore system with a narrow pore size distribution. Of these phases, the cubic MCM-48 form possesses a high surface area (1200-1500msp2/g), a 3-D array of pores, and a large pore diameter (20-100A) so that selectivity for large (>10A kinetic diameter) molecules is possible. For this work, the MCM-48 phase was chosen since its branched, bi-continuous pore array would be less likely to clog during use than a one dimensional array. Two techniques for transition metal incorporation are compared consisting of hydrothermal and post-synthesis treatment. A brief description of an attempt to synthesize mesoporous molybdena will be included. The first approach consists of combining various metal and Si precursors in the starting gel, while exploring a broad region of the multi-component phase diagram. The second pathway involves grafting metal species on a pure silica, MCM-48 support by anchoring them to surface silanols using reactive metal alkoxides. Bulk structural characterization by X-ray powder diffraction and nitrogen adsorption shows that the pore structure is maintained after incorporation of metal species by both methods. Spectroscopic methods (FTIR/RAMAN and UV/VIS) are used to gain insight into the local metal/silica environments. The post-synthesis and grafted materials have been shown to exhibit activity in the bromination of large, organic molecules such as phenol red at neutral pH and is a first for a non-biological system. In nature, vanadium bromoperoxidase accomplishes this task, effectively producing brominated, marine natural products with interesting pharmacological properties. The five metals were meticulously chosen based on their well known oxidative properties and ease of incorporation into the silica matrix. A discussion of the relative rates of halogenation will be based on the metal's ability to coordinate and activate peroxide ligands.

  6. Electronic transitions and multiferroicity in transition metal oxides

    NASA Astrophysics Data System (ADS)

    Zhou, Haidong

    Four systems have been studied for the localized-itinerant electronic transition in transition-metal oxides: (i) In CaV1- xTixO3, substitution of Ti(IV) introduces Anderson-localized states below a mobility edge mu c that increases with x, crossing epsilon F in the range 0.2 < x< 0.4 and also transforms the strong-correlation fluctuations to localized V(IV): t1e0 configurations for x ≥ 0.1. (ii) The properties of LaTiO3+delta reveal that a hole-poor, strongly correlated electronic phase coexists with a hole-rich, itinerant-electron phase. With delta ≥ 0.03, the hole-rich phase exists as a minority phase of isolated, mobile itinerant-electron clusters embedded in the hole-poor phase. With delta ≥ 0.08, isolated hole-poor clusters are embedded in an itinerant-electron matrix. As delta > 0.08 increases, the hole-poor clusters become smaller and more isolated until they are reduced to super-paramagnetic strong-correlation fluctuations by delta = 0.12. (iii) The data of Y1-xLaxTiO 3 appears to distinguish an itinerant-electron antiferromagnetic phase in the La-rich samples from a localized-electron ferromagnetic phase with a cooperative Jahn-Teller distortion in the Y-rich phase. (iv) The transition at Tt in Mg[Ti2]O4 is a semiconductor-semiconductor transition associated with Ti-Ti dimerization instabilities. The dimerization is caused by lattice instabilities resulting from a double-well Ti-Ti bond potential at a crossover from localized to itinerant electronic behavior. RMn1-xGaxO 3 (R = Ho, Y) and Ho1-xY xMnO3 have been studied for the multiferroicity of RMnO3. Ga doping raises the ferrielectric Curie temperature TC and the Mn-spin reorientation temperature TSR while lowering TN of the Mn spins and the Ho magnetic ordering temperature T 2. The data show an important coupling between the Mn3+-ion and HO3+-ion spins as well as a TSR that is driven by a cooperative MnO5 site rotation and R 3+-ion displacements that modify the c lattice parameter. The data also support an enhanced spin-lattice interaction in the geometrically frustrated (GF) Mn-spin system. Y doping enhances the temperature region for the P6'3cm' magnetic phase and thereby increases TSR for Ho1-xY xMnO3. The studies of several oxygen non-stoichiometric Fe4+/Fe 3+ oxoperovskite show that two mechanisms, the formation of Fe 3+-O-Fe4+ pair and the disproportionation reaction 2Fe(IV)O6/2 = Fe3+ + Fe(V)O6, dominate the electronic behavior. The properties of DyBaCo2O5.5 reveal a spin-state transition from the low-spin t 6e0 ground state to higher spin-state at octahedral-site Co3+, which is also accounted for the metamagnetism in the sample.

  7. Polytypism in superhard transition-metal triborides.

    PubMed

    Liang, Yongcheng; Yang, Jiong; Yuan, Xun; Qiu, Wujie; Zhong, Zheng; Yang, Jihui; Zhang, Wenqing

    2014-01-01

    The quest of novel compounds with special structures and unusual functionalities continues to be a central challenge to modern materials science. Even though their exact structures have puzzled scientists for decades, superhard transition-metal borides (TMBs) have long been believed to exist only in simple crystal structures. Here, we report on a polytypic phenomenon in superhard WB3 and MoB3 with a series of energetically degenerate structures due to the random stacking of metal layers amongst the interlocking boron layers. Such polytypism can create a multiphase solid-solution compound with a large number of interfaces amongst different polytypes, and these interfaces will strongly hinder the interlayer sliding movement within each polytype, thereby further increase the hardness of this particular material. Furthermore, in contrast to the conventional knowledge that intrinsically strong chemical bonds in superhard materials should lead to high lattice thermal conductivity, the polytypic TMB3 manifest anomalously low lattice thermal conductivity due to structural disorders and phonon folding. These findings promise to open a new avenue to searching for novel superhard materials with additional functionalities. PMID:24863493

  8. Polytypism in superhard transition-metal triborides

    PubMed Central

    Liang, Yongcheng; Yang, Jiong; Yuan, Xun; Qiu, Wujie; Zhong, Zheng; Yang, Jihui; Zhang, Wenqing

    2014-01-01

    The quest of novel compounds with special structures and unusual functionalities continues to be a central challenge to modern materials science. Even though their exact structures have puzzled scientists for decades, superhard transition-metal borides (TMBs) have long been believed to exist only in simple crystal structures. Here, we report on a polytypic phenomenon in superhard WB3 and MoB3 with a series of energetically degenerate structures due to the random stacking of metal layers amongst the interlocking boron layers. Such polytypism can create a multiphase solid-solution compound with a large number of interfaces amongst different polytypes, and these interfaces will strongly hinder the interlayer sliding movement within each polytype, thereby further increase the hardness of this particular material. Furthermore, in contrast to the conventional knowledge that intrinsically strong chemical bonds in superhard materials should lead to high lattice thermal conductivity, the polytypic TMB3 manifest anomalously low lattice thermal conductivity due to structural disorders and phonon folding. These findings promise to open a new avenue to searching for novel superhard materials with additional functionalities. PMID:24863493

  9. Transition Metal Phosphide Hydroprocessing Catalysts: A review

    SciTech Connect

    Oyama, S.; Gott, T; Zhao, H; Lee, Y

    2009-01-01

    The diminishing quality of oil feedstocks coupled with increasingly more stringent environmental regulations limiting the content of sulfur in transportation fuels have given rise to a need for improved hydroprocessing technology. This review begins with a summary of the major improvements in hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) catalysts and processes that have been reported in recent years. It then describes a new class of hydroprocessing catalysts, the transition metal phosphides, which have emerged as a promising group of high-activity, stable catalysts. The phosphides have physical properties resembling ceramics, so are strong and hard, yet retain electronic and magnetic properties similar to metals. Their crystal structures are based on trigonal prisms, yet they do not form layered structures like the sulfides. They display excellent performance in HDS and HDN, with the most active phosphide, Ni{sub 2}P, having activity surpassing that of promoted sulfides on the basis of sites titrated by chemisorption (CO for the phosphides, O{sub 2} for the sulfides). In the HDS of difficult heteroaromatics like 4,6-dimethyldibenzothiophene Ni{sub 2}P operates by the hydrogenation pathway, while in the HDN of substituted nitrogen compounds like 2-methylpiperidine it carries out nucleophilic substitution. The active sites for hydrogenation in Ni{sub 2}P have a square pyramidal geometry, while those for direct hydrodesulfurization have a tetrahedral geometry. Overall, Ni{sub 2}P is a promising catalyst for deep HDS in the presence of nitrogen and aromatic compounds.

  10. Pristine and intercalated transition metal dichalcogenide superconductors

    NASA Astrophysics Data System (ADS)

    Klemm, Richard A.

    2015-07-01

    Transition metal dichalcogenides (TMDs) are quasi-two-dimensional layered compounds that exhibit strongly competing effects of charge-density wave (CDW) formation and superconductivity (SC). The weak van der Waals interlayer bonding between hexagonal layers of octahedral or trigonal prismatic TMD building blocks allows many polytypes to form. In the single layer 1 T polytype materials, one or more CDW states can form, but the pristine TMDs are not superconducting. The 2 H polytypes have two or more Fermi surfaces and saddle bands, allowing for dual orderings, which can be coexisting CDW and SC orderings, two SC gaps as in MgB2, two CDW gaps, and possibly even pseudogaps above the onset TCDW s of CDW orderings. Higher order polytypes allow for multiple CDW gaps and at least one superconducting gap. The CDW transitions TCDW s usually greatly exceed the superconducting transitions at their low Tc values, their orbital order parameters (OPs) are generally highly anisotropic and can even contain nodes, and the SC OPs can be greatly affected by their simultaneous presence. The properties of the CDWs ubiquitously seen in TMDs are remarkably similar to those of the pseudogaps seen in the high-Tc cuprates. In 2H-NbSe2, for example, the CDW renders its general s-wave SC OP orbital symmetry to be highly anisotropic and strongly reduces its Josephson coupling strength (IcRn) with the conventional SC, Pb. Hence, the pristine TMDs are highly "unconventional" in comparison with Pb, but are much more "conventional" than are the ferromagnetic superconductors such as URhGe. Applied pressure and intercalation generally suppress the TMD CDWs, allowing for enhanced SC formation, even in the 1 T polytype materials. The misfit intercalation compound (LaSe)1.14(NbSe2) and many 2 H -TMDs intercalated with organic Lewis base molecules, such as TaS2(pyridine)1/2, have completely incoherent c-axis transport, dimensional-crossover effects, and behave as stacks of intrinsic Josephson junctions. Except for the anomalously large apparent violation of the Pauli limit of the upper critical field of (LaSe)1.14(NbSe2), these normal state and superconducting properties of these intercalation compounds are very similar to those seen in the high-Tc superconductor, Bi2Sr2CaCu2O8+δ and in the organic layered superconductor, κ-(ET)2Cu[N(CN)2]Br, where ET is bis(ethylenedithio) tetrathiafulvalene. Electrolytic intercalation of TMDs with water and metallic ions leads to compounds with very similar properties to cobaltates such as NaxCoO2 · y H2O.

  11. Spectroscopic investigations of complex transition metal oxides

    NASA Astrophysics Data System (ADS)

    Cao, Jinbo

    In this dissertation, I present spectroscopic studies of several model electronic and magnetic materials. Compounds of interest include VO x nanoscrolls, VOHPO4·1/2H2O, and (La0:4Pr0:6)1:2Sr1:8Mn 2O7. These materials are attractive systems for the investigation of optical gap tuning, lattice and charge dynamics, spin-lattice-charge coupling, and hydrogen bonding effects. I measured the optical properties of VO x nanoscrolls and the ion-exchanged derivatives to investigate the lattice and charge degrees of freedom. Selected V-O-V stretching modes sharpen and redshift with increasing amine size, which are microscopic manifestations of strain. We observed bound carrier localization in the metal exchanged nanoscrolls, indicating they are weakly metallic in their bulk form. I also investigated the variable temperature vibrational properties of single crystals of the S = 1/2 Heisenberg antiferromagnet VOHPO4·1/2H 2O. In order to explain the activation and polarization dependence of the singlet-to-triplet gap in the far-infrared response, we invoke a dynamic Dzyaloshinskii-Moriya mechanism and we identify the low-energy phonons that likely facilitate this coupling. Vibrational mode splitting of VOHPO 4·1/2H2O also points toward a weak local symmetry breaking near 180 K, and the low-temperature redshift of V-O and H-O related modes demonstrates enhanced low-temperature hydrogen bonding. Finally, I measured the magneto-optical response of (La0:4Pr0:6)1:2 Sr1:8Mn2O7 to investigate the microscopic aspects of the magnetic field driven spin-glass insulator to ferromagnetic metal transition. Application of a magnetic field recovers the ferromagnetic state with an overall redshift of the electronic structure, growth of the bound carrier localization associated with ferromagnetic domains, development of a pseudogap, and softening of the Mn-O stretching and bending modes that indicate a structural change. By exploiting the electronic mechanisms, we can induce large high energy magnetodielectric contrast in (La0:4Pr 0:6)1:2Sr1:8Mn2O7. The dielectric contrast is over 100% near 0.8 eV at 4.2 K. Remnants of the transition also drive the high energy magnetodielectric effect at room temperature.

  12. Transition metal oxide nanowires synthesized by heating metal substrates

    SciTech Connect

    Yan, Hui; Sun, Yi; He, Lin; Nie, Jia-Cai

    2011-11-15

    Highlights: {center_dot} This paper describes a simple and general method to synthesize 3d metal oxide nanowires. {center_dot} Self-catalysis growth mechanism was proposed to explain the growth of the nanowires. {center_dot} The temperature range for the growth of nanowires was estimated by taking into account the Gibbs free energy of reaction. {center_dot} This synthesis approach could be applied to synthesize other one-dimensional structures, such as FeSe and Bi{sub 2}Te{sub 3} nanowires. -- Abstract: Here we reported a simple method to synthesize transition metal oxide nanowires. Copper oxide (CuO), zinc oxide (ZnO), and cobalt oxide (Co{sub 3}O{sub 4}) nanowires were synthesized by heating the copper, zinc, and cobalt substrates under atmosphere condition. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to analyze the morphology and microstructure of the nanowires. According to our experimental results, self-catalysis growth mechanism was proposed to explain the growth of the nanowires. The temperature window for the growth of nanowires was estimated by taking into account the Gibbs free energy of reaction. The synthesis approach observed in our experiment could be applied to synthesize other one-dimensional structures, such as FeSe and Bi{sub 2}Te{sub 3} nanowires.

  13. Metallicity gradients in early-type galaxies

    NASA Technical Reports Server (NTRS)

    Schombert, James M.; Hanlan, Patricia C.; Barsony, Mary; Rakos, Karl D.

    1993-01-01

    A study of medium-to-bright early-type galaxies in six bandpasses from 3500 A to 2.2 microns is presented in order to quantify their colors and color gradients and relate these to metallicity and properties of the underlying stellar population. The Stromgren filter system chosen makes it possible to introduce a new calibration to the Mg(2) system from the present narrow-band v - y indices. A comparison is presented of narrow-band colors centered on particular spectral features vs a color dominated by the mean temperature of the giant branch (i.e., J - K) to test the effects of light vs heavy element abundances on knowledge of the total system metallicity, Z, and the effects of reddening. A good correlation is found between v - y and Mg(2); it provides a connection between one light element metallicity indicator (v - y centers on the CN blend) and another, Mg. The color-magnitude relations for all five optical and near-IR colors are shown. The strongest correlation exists for the metallicity colors, v - y and J - K.

  14. Trends in Ionization Energy of Transition-Metal Elements

    ERIC Educational Resources Information Center

    Matsumoto, Paul S.

    2005-01-01

    A rationale for the difference in the periodic trends in the ionization energy of the transition-metal elements versus the main-group elements is presented. The difference is that in the transition-metal elements, the electrons enter an inner-shell electron orbital, while in the main-group elements, the electrons enter an outer-shell electron

  15. Trends in Ionization Energy of Transition-Metal Elements

    ERIC Educational Resources Information Center

    Matsumoto, Paul S.

    2005-01-01

    A rationale for the difference in the periodic trends in the ionization energy of the transition-metal elements versus the main-group elements is presented. The difference is that in the transition-metal elements, the electrons enter an inner-shell electron orbital, while in the main-group elements, the electrons enter an outer-shell electron…

  16. Nanostructured transition metal oxides useful for water oxidation catalysis

    DOEpatents

    Frei, Heinz M; Jiao, Feng

    2013-12-24

    The present invention provides for a composition comprising a nanostructured transition metal oxide capable of oxidizing two H.sub.2O molecules to obtain four protons. In some embodiments of the invention, the composition further comprises a porous matrix wherein the nanocluster of the transition metal oxide is embedded on and/or in the porous matrix.

  17. Critical behavior in the hydrogen insulator-metal transition

    NASA Technical Reports Server (NTRS)

    Hemley, R. J.; Mao, H. K.

    1990-01-01

    The vibrational Raman spectrum of solid hydrogen has been measured from 77 to 295 K in the vicinity of the recently observed insulator-metal transition and low-temperature phase transition at 150 gigapascals. The measurements provide evidence for a critical point in the pressure-temperature phase boundary of the low-temperature transition. The result suggests that below the critical temperature the insulator-metal transition changes from continuous to discontinuous, consistent with the general criteria originally proposed by Mott (1949) for metallization by band-gap closure. The effect of temperature on hydrogen metallization closely resembles that of the lower-pressure insulator-metal transitions in doped V2O3 alloys.

  18. Hydrodenitrogenation of quinoline and coal using transition metal sulfides

    SciTech Connect

    Curtis, C.W.; Cahela, D.R.

    1988-01-01

    This study investigates the effectiveness of unsupported, precipitated transition metal sulfides as HYD and HDN catalysts in both a quinoline system and a coal liquefaction system. The transition metal sulfides of moderate surface areas were produced by a method developed by Chianelli and Dines in the late 1970's. These materials crystallize in weakly interacting layers which allow for ready intercalation of appropriate species. A number of different transition metal sulfides have been tested for HDS activity using dibenzothiophene and are good candidates for hydrodenitrogenation (HDN) studies. The degree of HYD and HDN of quinoline and the reaction products from coal liquefaction were determined using precipitated transition metal sulfides and compared to commercial transition metal sulfides, commercial hydrotreating catalysts such as CoMo/Al/sub 2/O/sub 3/ and NiMo/Al/sub 2/O/sub 3/, and platinum containing catalysts such as Pt/SiO/sub 2/ and PtS/sub 2/.

  19. Nutritional immunity: transition metals at the pathogen-host interface

    PubMed Central

    Hood, M. Indriati; Skaar, Eric P.

    2013-01-01

    Transition metals occupy an essential niche in biological systems. Their electrostatic properties stabilize substrates or reaction intermediates in the active sites of enzymes, while their heightened reactivity is harnessed for catalysis. However, the latter property renders transition metals toxic at high concentrations. Bacteria, like all living organisms, must regulate the levels of these elements to satisfy their physiological needs while avoiding harm. It is therefore not surprising that the host capitalizes on both the essentiality and toxicity of transition metals to defend against bacterial invaders. This review will discuss established and emerging paradigms in nutrient metal homeostasis at the pathogen-host interface. PMID:22796883

  20. Properties of Transition Metal Doped Alumina

    NASA Astrophysics Data System (ADS)

    Nykwest, Erik; Limmer, Krista; Brennan, Ray; Blair, Victoria; Ramprasad, Rampi

    Crystallographic texture can have profound effects on the properties of a material. One method of texturing is through the application of an external magnetic field during processing. While this method works with highly magnetic systems, doping is required to couple non-magnetic systems with the external field. Experiments have shown that low concentrations of rare earth (RE) dopants in alumina powders have enabled this kind of texturing. The magnetic properties of RE elements are directly related to their f orbital, which can have as many as 7 unpaired electrons. Since d-block elements can have as many as 5 unpaired electrons the effects of substitutional doping of 3d transition metals (TM) for Al in alpha (stable) and theta (metastable) alumina on the local structure and magnetic properties, in addition to the energetic cost, have been calculated by performing first-principles calculations based on density functional theory. This study has led to the development of general guidelines for the magnetic moment distribution at and around the dopant atom, and the dependence of this distribution on the dopant atom type and its coordination environment. It is anticipated that these findings can aid in the selection of suitable dopants help to guide parallel experimental efforts. This project was supported in part by an internship at the Army Research Laboratory, administered by the Oak Ridge Institute for Science and Education, along with a grant of computer time from the DoD High Performance Computing Modernization Program.

  1. Translational diffusion of transition metal complexes

    NASA Astrophysics Data System (ADS)

    Kowert, Bruce A.; Hughes, Angela M.; Dang, Nhan C.; Martin, Michael B.; Cheung, Gia H.; Tran, Hung D.; Reed, Joshua P.

    1999-09-01

    Capillary flow techniques have been used to measure DT, the translational diffusion constant at temperature T, for three transition metal complexes in several low-viscosity liquids. The translational radius, rt, for each of the complexes was obtained from DT using the Stokes-Einstein relation. The common value of rt of Ni(mnt) 2- in n-butyl alcohol, acetonitrile (ACN), acetone, and ethyl alcohol (EtOH) indicates that the apparently different rotational radii (from ESR) in a series of polar solvents are due to solute-solvent interactions and not to solvation or ion pairing. Another complex, Ni(mnt) 22-, was also studied in ACN; our values of DT for Ni(mnt) 22- and Ni(mnt) 2- in ACN are in agreement with electrochemical values. The reproducibility of our techniques was checked by using three different capillaries to make four separate determinations of DT for Ni(mnt) 2- in EtOH; the values are in good agreement. Values of DT were measured for Mn(Cat-N-SQ) 2, MnR 2, in tetrahydrofuran, 2-methyltetrahydrofuran, diethyl ether, and acetone. This complex is of interest because of its screw-propeller geometry. The values of rt in our solvents are in agreement with each other and are consistent with ESR studies of the reorientational motion of MnR 2.

  2. Trion formation in monolayer transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Kezerashvili, Roman Ya.; Tsiklauri, Shalva M.

    We present three-body calculations for trions binding energy in monolayer transition metal dichalcogenides using the method of hyperspherical harmonics (HH). In numerical calculations for a proper treatment of Coulomb screening in two dimensions we assume that electrons and holes are interacted via Keldysh potential. The convergences of binding energy calculations for the ground state of the trion as a function of the grand angular momentum are studied. For the trion binding energy in MoS2 we obtain 19.2 mev. This value is remarkably close to the experimental one of 18 meV. A comparison with results of other calculations are presented. We also study solutions of a hyperradial equation in a minimal approximation for the ground angular momentum to examine two regimes: a long range and a short range cases when the inter particle distance is much greater and much less than the screening length. For these cases, we find analytical expressions for the energy and wave function for trion states

  3. Energetic characteristics of transition metal complexes.

    PubMed

    Wojewódka, Andrzej; Bełzowski, Janusz; Wilk, Zenon; Staś, Justyna

    2009-11-15

    Ten transition metal nitrate and perchlorate complexes of hydrazine and ethylenediamine were synthesized, namely [Cu(EN)(2)](ClO(4))(2), [Co(EN)(3)](ClO(4))(3), [Ni(EN)(3)](ClO(4))(2), [Hg(EN)(2)](ClO(4))(2), [Cr(N(2)H(4))(3)](ClO(4))(3), [Cd(N(2)H(4))(3)](ClO(4))(2), [Ni(N(2)H(4))(3)](NO(3))(2), [Co(N(2)H(4))(3)](NO(3))(3), [Zn(N(2)H(4))(3)](NO(3))(2), and [Cd(N(2)H(4))(3)](NO(3))(2) based on the lines of the literature reported methods. All of them were tested with applying underwater detonation test and further compared to the typical blasting explosives: RDX, HMX, TNT and PETN. From the above presented complexes [Ni(N(2)H(4))(3)](NO(3))(2) (called NHN) and [Co(N(2)H(4))(3)](NO(3))(3) (called CoHN) are known as primary explosives and can be used as the standard explosives. Explosion parameters, such as shock wave overpressure, shock wave energy equivalent and bubble energy equivalent, were determined. Evaluated energetic characteristics of the tested compounds are comparable to those of the classic high explosives and are even enhanced in some cases. PMID:19631466

  4. Impact Electrochemistry of Layered Transition Metal Dichalcogenides.

    PubMed

    Lim, Chee Shan; Tan, Shu Min; Sofer, Zdeněk; Pumera, Martin

    2015-08-25

    Layered transition metal dichalcogenides (TMDs) exhibit paramount importance in the electrocatalysis of the hydrogen evolution reaction. It is crucial to determine the size of the electrocatalytic particles as well as to establish their electrocatalytic activity, which occurs at the edges of these particles. Here, we show that individual TMD (MoS2, MoSe2, WS2, or WSe2; in general MX2) nanoparticles impacting an electrode surface provide well-defined current "spikes" in both the cathodic and anodic regions. These spikes originate from direct oxidation of the nanoparticles (from M(4+) to M(6+)) at the anodic region and from the electrocatalytic currents generated upon hydrogen evolution in the cathodic region. The positive correlation between the frequency of the impacts and the concentration of TMD nanoparticles is also demonstrated here, enabling determination of the concentration of TMD nanoparticles in colloidal form. In addition, the size of individual TMD nanoparticles can be evaluated using the charge passed during every spike. The capability of detecting both the "indirect" catalytic effect of an impacting TMD nanoparticle as well as "direct" oxidation indicates that the frequency of impacts in both the "indirect" and "direct" scenarios are comparable. This suggests that all TMD nanoparticles, which are electrochemically oxidizable (thus capable of donating electrons to electrodes), are also capable of catalyzing the hydrogen reduction reaction. PMID:26241193

  5. Transition metal-free olefin polymerization catalyst

    DOEpatents

    Sen, Ayusman; Wojcinski, II, Louis M.; Liu, Shengsheng

    2001-01-01

    Ethylene and/or propylene are polymerized to form high molecular weight, linear polymers by contacting ethylene and/or propylene monomer, in the presence of an inert reaction medium, with a catalyst system which consists essentially of (1) an aluminum alkyl component, such as trimethylaluminum, triethylaluminum, triisobutylaluminum, tri-n-octylaluminum and diethylaluminum hydride and (2) a Lewis acid or Lewis acid derivative component, such as B (C.sub.6 F.sub.5).sub.3, [(CH.sub.3).sub.2 N (H) (C.sub.6 H.sub.5)].sup.+ [B (C.sub.6 F.sub.5)4].sup.-, [(C.sub.2 H.sub.5).sub.3 NH].sup.+ [B C.sub.6 F.sub.5).sub.4 ],.sup.-, [C(C.sub.6 F.sub.5).sub.3 ].sup.+ [B(C.sub.6 F.sub.5).sub.4 ].sup.-, (C.sub.2 H.sub.5).sub.2 Al(OCH.sub.3), (C.sub.2 H.sub.5).sub.2 Al(2,6-di-t-butyl-4-methylphenoxide), (C.sub.2 H.sub.5)Al(2,6 -di-t-butylphenoxide).sub.2, (C.sub.2 H.sub.5).sub.2 Al(2,6-di-t-butylphonoxide) , 2,6 -di-t-butylphenol.multidot.methylaluminoxane or an alkylaluminoxane, and which may be completely free any transition metal component(s).

  6. Patterning Superatom Dopants on Transition Metal Dichalcogenides.

    PubMed

    Yu, Jaeeun; Lee, Chul-Ho; Bouilly, Delphine; Han, Minyong; Kim, Philip; Steigerwald, Michael L; Roy, Xavier; Nuckolls, Colin

    2016-05-11

    This study describes a new and simple approach to dope two-dimensional transition metal dichalcogenides (TMDCs) using the superatom Co6Se8(PEt3)6 as the electron dopant. Semiconducting TMDCs are wired into field-effect transistor devices and then immersed into a solution of these superatoms. The degree of doping is determined by the concentration of the superatoms in solution and by the length of time the films are immersed in the dopant solution. Using this chemical approach, we are able to turn mono- and few-layer MoS2 samples from moderately to heavily electron-doped states. The same approach applied on WSe2 films changes their characteristics from hole transporting to electron transporting. Moreover, we show that the superatom doping can be patterned on specific areas of TMDC films. To illustrate the power of this technique, we demonstrate the fabrication of a lateral p-n junction by selectively doping only a portion of the channel in a WSe2 device. Finally, encapsulation of the doped films with crystalline hydrocarbon layers stabilizes their properties in an ambient environment. PMID:27082448

  7. Corrosion behavior of mesoporous transition metal nitrides

    NASA Astrophysics Data System (ADS)

    Yang, Minghui; Allen, Amy J.; Nguyen, Minh T.; Ralston, Walter T.; MacLeod, Michelle J.; DiSalvo, Francis J.

    2013-09-01

    Transition metal nitrides (TMN) have many desirable characteristics such as high hardness and good thermal stability under reducing conditions. This work reports an initial survey of the chemical stability of mesoporous TMNs (TM=Nb, V, Cr and Ti) in water at 80 °C at neutral, acidic and alkaline pH. The mesoporous TMNs had specific surface areas of 25-60 m2/g with average pore sizes ranging from 10 to 50 nm. The high surface areas of these materials enhance the rate of corrosion per unit mass over that of a bulk material, making detection of corrosion much easier. The products were characterized by Rietveld refinement of powder X-ray diffraction (PXRD) patterns and by scanning electron microscopy (SEM). Several nitrides have corrosion rates that are, within error, not distinguishable from zero (±1 Å/day). Of the nitrides examined, CrN appears to be the most corrosion resistant under acidic conditions. None of the nitrides studied are corrosion resistant under alkaline conditions.

  8. Two-dimensional, ordered, double transition metals carbides (MXenes)

    NASA Astrophysics Data System (ADS)

    Kent, Paul; Anasori, Babak; Xie, Yu; Beidaghi, Majid; Lu, Jun; Hosler, Brian; Hultman, Lars; Gogotsi, Yury; Barsoum, Michel

    We use density functional theory to predict the existence of two new families of 2D ordered carbides (MXenes), M'2M''C2 and M'2M''2C3, where each M is a different early transition metal. Synthesizing Mo2TiC2Tx, Mo2Ti2C3Tx, and Cr2TiC2Tx (where T is a surface termination), we validated the DFT predictions. Since the Mo and Cr atoms are on the outside, they control the 2D flakes' chemical and electrochemical properties. The latter was proven by showing quite different electrochemical behavior of Mo2TiC2Tx and Ti3C2Tx. This work further expands the family of 2D materials, offering additional choices of structures, chemistries, and ultimately useful properties.

  9. Evaluating transition-metal catalysis in gas generation from the Permian Kupferschiefer by hydrous pyrolysis

    NASA Astrophysics Data System (ADS)

    Lewan, M. D.; Kotarba, M. J.; Więcław, D.; Piestrzyński, A.

    2008-08-01

    Transition metals in source rocks have been advocated as catalysts in determining extent, composition, and timing of natural gas generation (Mango, F. D. (1996) Transition metal catalysis in the generation of natural gas. Org. Geochem.24, 977-984). This controversial hypothesis may have important implications concerning gas generation in unconventional shale-gas accumulations. Although experiments have been conducted to test the metal-catalysis hypothesis, their approach and results remain equivocal in evaluating natural assemblages of transition metals and organic matter in shale. The Permian Kupferschiefer of Poland offers an excellent opportunity to test the hypothesis with immature to marginally mature shale rich in both transition metals and organic matter. Twelve subsurface samples containing similar Type-II kerogen with different amounts and types of transition metals were subjected to hydrous pyrolysis at 330° and 355 °C for 72 h. The gases generated in these experiments were quantitatively collected and analyzed for molecular composition and stable isotopes. Expelled immiscible oils, reacted waters, and spent rock were also quantitatively collected. The results show that transition metals have no effect on methane yields or enrichment. δ 13C values of generated methane, ethane, propane and butanes show no systematic changes with increasing transition metals. The potential for transition metals to enhance gas generation and oil cracking was examined by looking at the ratio of the generated hydrocarbon gases to generated expelled immiscible oil (i.e., GOR), which showed no systematic change with increasing transition metals. Assuming maximum yields at 355 °C for 72 h and first-order reaction rates, pseudo-rate constants for methane generation at 330 °C were calculated. These rate constants showed no increase with increasing transition metals. The lack of a significant catalytic effect of transition metals on the extent, composition, and timing of natural gas generation in these experiments is attributed to the metals not occurring in the proper form or the poisoning of potential catalytic microcosms by polar-rich bitumen, which impregnates the rock matrix during the early stages of petroleum formation.

  10. Evaluating transition-metal catalysis in gas generation from the Permian Kupferschiefer by hydrous pyrolysis

    USGS Publications Warehouse

    Lewan, M.D.; Kotarba, M.J.; Wieclaw, D.; Piestrzynski, A.

    2008-01-01

    Transition metals in source rocks have been advocated as catalysts in determining extent, composition, and timing of natural gas generation (Mango, F. D. (1996) Transition metal catalysis in the generation of natural gas. Org. Geochem.24, 977–984). This controversial hypothesis may have important implications concerning gas generation in unconventional shale-gas accumulations. Although experiments have been conducted to test the metal-catalysis hypothesis, their approach and results remain equivocal in evaluating natural assemblages of transition metals and organic matter in shale. The Permian Kupferschiefer of Poland offers an excellent opportunity to test the hypothesis with immature to marginally mature shale rich in both transition metals and organic matter. Twelve subsurface samples containing similar Type-II kerogen with different amounts and types of transition metals were subjected to hydrous pyrolysis at 330° and 355 °C for 72 h. The gases generated in these experiments were quantitatively collected and analyzed for molecular composition and stable isotopes. Expelled immiscible oils, reacted waters, and spent rock were also quantitatively collected. The results show that transition metals have no effect on methane yields or enrichment. δ13C values of generated methane, ethane, propane and butanes show no systematic changes with increasing transition metals. The potential for transition metals to enhance gas generation and oil cracking was examined by looking at the ratio of the generated hydrocarbon gases to generated expelled immiscible oil (i.e., GOR), which showed no systematic change with increasing transition metals. Assuming maximum yields at 355 °C for 72 h and first-order reaction rates, pseudo-rate constants for methane generation at 330 °C were calculated. These rate constants showed no increase with increasing transition metals. The lack of a significant catalytic effect of transition metals on the extent, composition, and timing of natural gas generation in these experiments is attributed to the metals not occurring in the proper form or the poisoning of potential catalytic microcosms by polar-rich bitumen, which impregnates the rock matrix during the early stages of petroleum formation.

  11. Solid-solid phase transitions via melting in metals.

    PubMed

    Pogatscher, S; Leutenegger, D; Schawe, J E K; Uggowitzer, P J; Löffler, J F

    2016-01-01

    Observing solid-solid phase transitions in-situ with sufficient temporal and spatial resolution is a great challenge, and is often only possible via computer simulations or in model systems. Recently, a study of polymeric colloidal particles, where the particles mimic atoms, revealed an intermediate liquid state in the transition from one solid to another. While not yet observed there, this finding suggests that such phenomena may also occur in metals and alloys. Here we present experimental evidence for a solid-solid transition via the formation of a metastable liquid in a 'real' atomic system. We observe this transition in a bulk glass-forming metallic system in-situ using fast differential scanning calorimetry. We investigate the corresponding transformation kinetics and discuss the underlying thermodynamics. The mechanism is likely to be a feature of many metallic glasses and metals in general, and may provide further insight into phase transition theory. PMID:27103085

  12. Solid–solid phase transitions via melting in metals

    PubMed Central

    Pogatscher, S.; Leutenegger, D.; Schawe, J. E. K.; Uggowitzer, P. J.; Löffler, J. F.

    2016-01-01

    Observing solid–solid phase transitions in-situ with sufficient temporal and spatial resolution is a great challenge, and is often only possible via computer simulations or in model systems. Recently, a study of polymeric colloidal particles, where the particles mimic atoms, revealed an intermediate liquid state in the transition from one solid to another. While not yet observed there, this finding suggests that such phenomena may also occur in metals and alloys. Here we present experimental evidence for a solid–solid transition via the formation of a metastable liquid in a ‘real' atomic system. We observe this transition in a bulk glass-forming metallic system in-situ using fast differential scanning calorimetry. We investigate the corresponding transformation kinetics and discuss the underlying thermodynamics. The mechanism is likely to be a feature of many metallic glasses and metals in general, and may provide further insight into phase transition theory. PMID:27103085

  13. Transition Metal and Vacancy Defect Complexes in Phosphorene

    NASA Astrophysics Data System (ADS)

    Kabir, Mukul; Babar, Rohit

    Inducing magnetic moment in otherwise nonmagnetic two-dimensional semiconducting materials is the first step to design spintronic material. Here, we study the adsorption of transition-metals on pristine and defected phosphorene, within density functional theory. We predict that increased transition-meal diffusivity on the pristine phosphorene would hinder controlled magnetism. In contrast, point-defects anchor the transiton-metal to reduce metal diffusivity. The di-vacancy complex is more important in this context due to their increased thermodynamic stability over the mono-vacancy. For most cases, the defect-transition metal complexes retain the intrinsic semiconducting properties, and induce a local moment. We provide a simple microscopic model which describe the local moment of these transition metal and defect complexes.

  14. Steam Reforming on Transition-metal Carbides from Density-functional Theory

    SciTech Connect

    Vojvodic, Aleksandra

    2012-05-11

    A screening study of the steam reforming reaction on clean and oxygen covered early transition-metal carbides surfaces is performed by means of density-functional theory calculations. It is found that carbides provide a wide spectrum of reactivities, from too reactive via suitable to too inert. Several molybdenum-based systems are identified as possible steam reforming catalysts. The findings suggest that carbides provide a playground for reactivity tuning, comparable to the one for pure metals.

  15. Crystal structure and chemical reactivity of transition metal carbides and nitrides

    NASA Astrophysics Data System (ADS)

    Ted Oyama, S.

    1992-02-01

    Trends in melting points, stoichiometry, and crystal structures in early transition metal carbides and nitrides are discussed in terms of the Engel-Brewer valence bond theory. It is deduced that carbon and nitrogen combine their valence sp electrons with the metal spd bands. As a consequence crystal structure and chemical reactivity of the compounds resemble those of elements further to the right in the Periodic Table. This is in accord with the original suggestion of Levy and Boudart

  16. Process for making transition metal nitride whiskers

    DOEpatents

    Bamberger, Carlos E. (Oak Ridge, TN)

    1989-01-01

    A process for making metal nitrides, particularly titanium nitride whiskers, using a cyanide salt as a reducing agent for a metal compound in the presence of an alkali metal oxide. Sodium cyanide, various titanates and titanium oxide mixed with sodium oxide react to provide titanium nitride whiskers that can be used as reinforcement to ceramic composites.

  17. Process for making transition metal nitride whiskers

    DOEpatents

    Bamberger, C.E.

    1988-04-12

    A process for making metal nitrides, particularly titanium nitride whiskers, using a cyanide salt as a reducing agent for a metal compound in the presence of an alkali metal oxide. Sodium cyanide, various titanates and titanium oxide mixed with sodium oxide react to provide titanium nitride whiskers that can be used as reinforcement to ceramic composites. 1 fig., 1 tab.

  18. Environmentally relevant metal and transition metal ions enhance Fc epsilon RI-mediated mast cell activation.

    PubMed Central

    Walczak-Drzewiecka, Aurelia; Wyczlkowska, Janina; Dastych, Jaroslaw

    2003-01-01

    Upon contact with allergen, sensitized mast cells release highly active proinflammatory mediators. Allergen-mediated mast cell activation is an important mechanism in the pathogenesis of atopic asthma. Asthmatic patients are especially susceptible to air pollution. Epidemiologic studies found a positive correlation between severity of symptoms among asthmatic patients and the level of particulate matter (PM) in the air. Among the constituents of PM are metals and transition metals, which could mediate some of its adverse effects on human health. We sought to determine the effect of metal and transition metal ions on allergen-mediated mast cell activation. We observed that several metal and transition metal ions activated mast cells and enhanced allergen-mediated mast cell activation. Thus, Al(3+), Cd(2+), and Sr(2+) induced release of granule-associated N-acetyl-ss-d-hexosaminidase, and Al(3+) and Ni(2+) enhanced antigen-mediated release. Metal and transition metal ions also induced significant secretion of interleukin (IL)-4 and increased antigen-mediated IL-4 secretion in mast cells. These effects of metal and transition metal ions on mast cells were observed at concentrations that do not result in direct cytotoxicity and might be relevant for environmental exposure. Thus, metals and transition metals could increase the level of allergen-mediated mast cell activation, which might be one of the mechanisms mediating exacerbation of allergen-driven asthma symptoms by air pollution. PMID:12727598

  19. Mechanisms of transition-metal gettering in silicon

    SciTech Connect

    MYERS JR.,SAMUEL M.; SEIBT,M.; SCHROTER,W.

    2000-03-23

    The atomic process, kinetics, and equilibrium thermodynamics underlying the gettering of transition-metal impurities in Si are reviewed from a mechanistic perspective. Methods for mathematical modeling of gettering are reviewed and illustrated. Needs for further research are discussed.

  20. Electrical properties of transition metal hydrogen complexes in silicon

    SciTech Connect

    Weber, J.

    1998-12-31

    A summary is given on the electrical properties of transition-metal hydrogen complexes in silicon. Contrary to the general understanding, hydrogen leads not only to passivation of deep defect levels but also creates several new levels in the band gap due to electrically active transition-metal complexes. The author presents detailed data for Pt-H complexes and summarize briefly the results on the transition metals Ti, Co, Ni, Pd, and Ag. The introduction of hydrogen at room temperature by wet chemical etching, followed by specific annealing steps allows us to study the formation of the different complexes. In particular, depth profiles of the defect concentrations give an estimate of the number of hydrogen atoms involved in the complexes. Transition-metals binding up to four hydrogen atoms are identified.

  1. Theory of semiconductor and transition metal alloys

    NASA Astrophysics Data System (ADS)

    Zunger, Alex

    1997-03-01

    There is a lot of talk about computer-aided discoveries/design of new materials, but the simple fact is that even limiting oneself to materials made of just two elements, (e.g., Cu-Au or Si-Ge), and to a substitutional system, there can exist as many as 2^N configurations that include compounds, alloys, superlattices, and impurities. Indeed, even for modest number of sites N, this is an astronomical number. Thus, the conventional energy minimization approach (e.g., first-principles pseudopotentials) for selecting the most stable crystal structure for A_qBq is hopeless if one considers the full 2^N space configurations of A, B on a lattice of N points. Molecular-dynamics is of no help, since it does not explore effectively the space of lattice configurations. Thus, contemporary energy minimization approaches use instead the method of ``rounding-up the usual suspects'': selecting the lowest energy from only a small number of well-known candidate configurations. The potential for missing new and important structures is obviously large. This talk gives an outline of the solution. It addresses the questions of (i) finding the lowest energy configuration of substitutional systems, (ii) calculating their composition-temperature phase diagram, and (iii) their finite-temperature thermodynamic properties, using the first-principles local density approximation (LDA). Mapping of the LDA energies of only 10-20 A_qBq compounds onto an Ising-like ``cluster-expansion'' enables use of lattice statistical mechanics techniques that elegantly solve the above problems. This extends the utility of the LDA from simple, perfectly-ordered compounds to truly complex structures, beyond the reach of direct LDA calculations. I will illustrate the method for semiconductor systems and transition-metal intermetallic systems, showing how previously unsuspected structures and materials are predicted, and how one can calculate free energies, short range-order and phase-diagrams of alloys. See literature in http://www.sst.nrel.gov under alloys.

  2. Importance of electronegativity differences and surface structure in molecular dissociation reactions at transition metal surfaces.

    PubMed

    Crawford, Paul; Hu, P

    2006-12-14

    The dissociative adsorption of N2 has been studied at both monatomic steps and flat regions on the surfaces of the 4d transition metals from Zr to Pd. Using density functional theory (DFT) calculations, we have determined and analyzed the trends in both straight reactivity and structure sensitivity across the periodic table. With regards to reactivity, we find that the trend in activation energy (Ea) is determined mainly by a charge transfer from the surface metal atoms to the N atoms during transition state formation, namely, the degree of ionicity of the N-surface bond at the transition state. Indeed, we find that the strength of the metal-N bond at the transition state (and therefore the trend in Ea) can be predicted by the difference in Mulliken electronegativity between the metal and N. Structure sensitivity is analyzed in terms of geometric and electronic effects. We find that the lowering of Ea due to steps is more pronounced on the right-hand side of the periodic table. It is found that for the early transition metals the geometric and electronic effects work in opposition when going from terrace to step active site. In the case of the late 4d metals, however, these effects work in combination, producing a more marked reduction in Ea. PMID:17149914

  3. Transition metal bimetallic oxycarbides: Synthesis, characterization, and activity studies

    SciTech Connect

    Oyama, S.T.; Yu, C.C.; Ramanathan, S.

    1999-06-10

    A new family of bimetallic oxycarbide compounds M{sup I}-M{sup II}-O-C (M{sup I} = Mo, W; M{sup II} = V, Nb, Cr, Fe, Co, Ni) has been synthesized by carburizing bimetallic oxide precursors using a temperature-programmed method. The oxide precursors are prepared by conventional solid-state reaction between two appropriate monometallic oxides. The synthesis involves passing a 20 mol% CH{sub 4} in H{sub 2} mixture over the oxide precursors while raising the temperature at a linear rate of 8.3 {times} 10{sup {minus}2} K/s (5 K/min) to a final temperature (T{sub max}) which is held for a period of time (t{sub hold}). The synthesis, chemisorption properties, and reactivation of the materials indicate that the compounds can be divided into two groups of different reducibility (high and low). Their surface activity and surface area are evaluated based on CO chemisorption and N{sub 2} physisorption measurements. It is found that the CO number density correlates with the reducibility of the compounds. The catalysts were evaluated for hydroprocessing in a three-phase trickle-bed reactor operated at 3.1 MPa and 643 K. The feed was a model liquid mixture containing 3000 ppm sulfur (dibenzothiophene), 2000 ppm nitrogen (quinoline), 500 ppm oxygen (benzofuran), 20 wt% aromatics (tetralin), and balance aliphatics (tetradecane). The bimetallic oxycarbides had moderate activity for HDN of quinoline, with Nb-Mo-O-C showing higher HDN than a commercial sulfided Ni-Mo/Al{sub 2}O{sub 3} catalyst tested at the same conditions. X-ray diffraction of the spent catalysts indicated that the oxycarbides of the early transition metals were tolerant of sulfur, while those involving the late transition metals showed bulk sulfide phases.

  4. Bi-phase transition diagrams of metallic thin multilayers

    SciTech Connect

    Li, J.C.; Liu, W.; Jiang, Q. . E-mail: jiangq@jlu.edu.cn

    2005-02-01

    Phase transitions of metallic multilayers induced by differences in interface energy are considered thermodynamically, based on a thermodynamic model for interface energy and the Goldschmidt premise for lattice contraction. Bi-phase transition diagrams of Co/Cr, Zr/Nb, Ti/Nb and Ti/Al multilayers are constructed, which are in agreement with experimental results.

  5. The transition to the metallic state in low density hydrogen.

    PubMed

    McMinis, Jeremy; Morales, Miguel A; Ceperley, David M; Kim, Jeongnim

    2015-11-21

    Solid atomic hydrogen is one of the simplest systems to undergo a metal-insulator transition. Near the transition, the electronic degrees of freedom become strongly correlated and their description provides a difficult challenge for theoretical methods. As a result, the order and density of the phase transition are still subject to debate. In this work, we use diffusion quantum Monte Carlo to benchmark the transition between paramagnetic and anti-ferromagnetic body centered cubic atomic hydrogen in its ground state. We locate the density of the transition by computing the equation of state for these two phases and identify the phase transition order by computing the band gap near the phase transition. These benchmark results show that the phase transition is continuous and occurs at a Wigner-Seitz radius of rs = 2.27(3) a0. We compare our results to previously reported density functional theory, Hedin's GW approximation, and dynamical mean field theory results. PMID:26590549

  6. Fluctuation-driven electroweak phase transition. [in early universe

    NASA Technical Reports Server (NTRS)

    Gleiser, Marcelo; Kolb, Edward W.

    1992-01-01

    We examine the dynamics of the electroweak phase transition in the early Universe. For Higgs masses in the range 46 less than or = M sub H less than or = 150 GeV and top quark masses less than 200 GeV, regions of symmetric and asymmetric vacuum coexist to below the critical temperature, with thermal equilibrium between the two phases maintained by fluctuations of both phases. We propose that the transition to the asymmetric vacuum is completed by percolation of these subcritical fluctuations. Our results are relevant to scenarios of baryogenesis that invoke a weakly first-order phase transition at the electroweak scale.

  7. An analytic model for phase transitions in the early universe

    NASA Astrophysics Data System (ADS)

    Diosi, L.; Lukacs, B.; Keszthelyi, B.; Paal, G.

    A simplified model of the very early universe is presented for studying the effects of the GUT phase transition on cosmic evolution and vice versa. An exact description of a strongly first order phase transition is given. Both inflationary and non-inflationary solutions are obtained. The cosmological scale factor and the temperature are determined and graphically depicted. The self-consistency of the simplifying assumptions needed to obtain analytic solutions is checked. Contrary to expectations spread in the literature, the strongly first order character of the phase transition in itself is found to be insufficient to solve the monopole problem inherent in most GUT cosmologies.

  8. Configuring bonds between first-row transition metals.

    PubMed

    Eisenhart, Reed J; Clouston, Laura J; Lu, Connie C

    2015-11-17

    Alfred Werner, who pioneered the field of coordination chemistry, envisioned coordination complexes as a single, transition metal atom at the epicenter of a vast ligand space. The idea that the locus of a coordination complex could be shared by multiple metals held together with covalent bonds would eventually lead to the discovery of the quadruple and quintuple bond, which have no analogues outside of the transition metal block. Metal-metal bonding can be classified into homometallic and heterometallic groups. Although the former is dominant, the latter is arguably more intriguing because of the inherently larger chemical space in which metal-metal bonding can be explored. In 2013, Lu and Thomas independently reported the isolation of heterometallic multiple bonds with exclusively first-row transition metals. Structural and theoretical data supported triply bonded Fe-Cr and Fe-V cores. This Account describes our continued efforts to configure bonds between first-row transition metals from titanium to copper. Double-decker ligands, or binucleating platforms that brace two transition metals in proximity, have enabled the modular synthesis of diverse metal-metal complexes. The resulting complexes are also ideal for investigating the effects of an "ancillary" metal on the properties and reactivities of an "active" metal center. A total of 38 bimetallic complexes have been compiled comprising 18 unique metal-metal pairings. Twenty-one of these bimetallics are strictly isostructural, allowing for a systematic comparison of metal-metal bonding. The nature of the chemical bond between first-row metals is remarkably variable and depends on two primary factors: the total d-electron count, and the metals' relative d-orbital energies. Showcasing the range of covalent bonding are a quintuply bonded (d-d)(10) Mn-Cr heterobimetallic and the singly bonded late-late pairings, e.g., Fe-Co, which adopt unusually high spin states. A long-term goal is to rationally tailor the properties and reactivities of the bimetallic complexes. In some cases, synergistic redox and magnetic properties were found that are different from the expected sum of the individual metals. Intermetal charge transfer was shown in a Co-M series, for M = Mn to Cu, where the transition energy decreases as M is varied across the first-row period. The potential of using metal-metal complexes for multielectron reduction of small-molecules is addressed by N2 binding studies and a mechanistic study of a dicobalt catalyst in reductive silylation of N2 to N(SiMe3)3. Finally, metal-ion exchange reactions with metal-metal complexes can be selective under appropriate reaction conditions, providing an alternative synthetic route to metal-metal species. PMID:26492331

  9. Theory of metal insulator transition in strongly correlated electron systems

    SciTech Connect

    Gulacsi, M.; Bedell, K.S.

    1994-06-01

    A comprehensive theory of the correlation driven metal insulator transition in 1D and 2D strongly correlated electron systems is given. In both the 1D and 2D Hubbard model the metal insulator transition encountered close to half filling is of Pokrovsky-Talapov type. An important consequence of this in the 2D Hubbard model is the break down of the Fermi liquid theory. We also describe in detail the properties of the Pokrovsky-Talapov transition in 2D ferroelectrics.

  10. Melting of bcc Transition Metals and Icosahedral Clustering

    SciTech Connect

    Ross, M; Boehler, R; Japel, S

    2006-05-26

    In contrast to polyvalent metals, transition metals have low melting slopes(dT/dP) that are due to partially filled d-bands that allow for a lowering of liquid phase energy through s-d electron transfer and the formation of local structures. In the case of bcc transition metals we show the apparent discrepancy of DAC melting measurements with shock melting of Mo can be understood by reexamining the shock data for V and Ta and introducing the presence of an icosahedral short range order (ISRO) melt phase.

  11. Electronic and magnetic engineering of transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Tang, Youjian; Crespi, Vincent; Vincent Crespi group Team

    Transition metal dichalcogenides (TMDs) have moderate bandgaps and great potential in electronic and optoelectronic applications. We show that by intercalation and compensated doping of transition metal ions, we could generate a ``half-semiconductor'', half-metal or doped magnetic semiconductor. We will also show that covalently connecting a single layer of WS2 to a small aromatic molecule with appropriate electronegativity, it is possible to align the molecular energy levels with the WS2 conduction band edge, yielding an electronic structure of potential interest for thermoelectric applications, and covalently connecting single-layer WS2 to magnetic coordination compounds could introduce magnetization into the WS2 layer.

  12. Application of Two Dimensional Flourescence Spectroscopy to Transition Metal Clusters.

    NASA Astrophysics Data System (ADS)

    Kokkin, Damian L.; Steimle, Timothy

    2014-06-01

    Determining the physical properties (bond lengths, angles, dipole moments, etc) of transition metal oxides and dioxides is relevant to catalysis, high temperature chemistry, materials science and astrophysics. Analysis of optical spectra is a convenient method for extraction of physical properties, but can be difficult because of the density of electronic states and in the case of the dioxides, presence of both the oxide and superoxide forms. Here we demonstrate the application of two dimensional fluorescence spectroscopy for aiding in the assignment and analysis. Particular attention will be paid to the spectroscopy of first row transition metal monoxides and dioxides of Nickel, NiO and NiO_2, and Manganese, MnO. Furthermore, the application of this technique to discovering the spectrum of other transition metal systems such as Metal-dicarbides will be outlined. N.J. Reilly, T.W. Schmidt, S.H. Kable, J. Phys. Chem. A., 110(45), 12355-12359, 2006

  13. Kansas Early Childhood Research Institute on Transitions: Executive Summary.

    ERIC Educational Resources Information Center

    Rice, Mabel L.; O'Brien, Marion

    This executive summary reviews activities over the past 5 years of the Kansas Early Childhood Research Institute (KECRI). The Institute has addressed transition issues faced by infants and young children (and their families) who have a disability or are at risk for developmental delay. KECRI goals are stated and the importance and impact of the…

  14. Early warning signals of ecological transitions: methods for spatial patterns.

    PubMed

    Kéfi, Sonia; Guttal, Vishwesha; Brock, William A; Carpenter, Stephen R; Ellison, Aaron M; Livina, Valerie N; Seekell, David A; Scheffer, Marten; van Nes, Egbert H; Dakos, Vasilis

    2014-01-01

    A number of ecosystems can exhibit abrupt shifts between alternative stable states. Because of their important ecological and economic consequences, recent research has focused on devising early warning signals for anticipating such abrupt ecological transitions. In particular, theoretical studies show that changes in spatial characteristics of the system could provide early warnings of approaching transitions. However, the empirical validation of these indicators lag behind their theoretical developments. Here, we summarize a range of currently available spatial early warning signals, suggest potential null models to interpret their trends, and apply them to three simulated spatial data sets of systems undergoing an abrupt transition. In addition to providing a step-by-step methodology for applying these signals to spatial data sets, we propose a statistical toolbox that may be used to help detect approaching transitions in a wide range of spatial data. We hope that our methodology together with the computer codes will stimulate the application and testing of spatial early warning signals on real spatial data. PMID:24658137

  15. Be Prepared: Tips for Transitioning into Early Childhood Education

    ERIC Educational Resources Information Center

    Oliver, Laura Ann

    2008-01-01

    The first days of school can be scary for parents, too. As a child reaches an age where she is ready to begin school, the very thought can overwhelm any parent. As a parent of a child with a disability there are added considerations. Nonetheless, transitions through the early years of school can become relatively easier for both the parents and…

  16. West Virginia Early Childhood Transition Initiative: Operation Tadpole. 1995 Report.

    ERIC Educational Resources Information Center

    West Virginia State Dept. of Education, Charleston. Office of Special Education Programs and Assurances.

    The West Virginia Early Childhood Transition Initiative is an interagency systems change effort started in 1993 to develop a seamless system of service for children, ages birth to 5 years. The initiative's training and technical assistance component is known as Operation Tadpole. A state level steering committee coordinates the initiative on an…

  17. Kansas Early Childhood Research Institute on Transitions. Final Report.

    ERIC Educational Resources Information Center

    Rice, Mabel L.; O'Brien, Marion

    This final report describes research projects and other activities of the Kansas Early Childhood Research Institute (KECRI), a multi-investigator, cross-disciplinary Institute focusing on successful transitions for young (birth to age 8) children with disabilities or developmental delays. Interventions were developed, evaluated, and disseminated

  18. Early Warning Signals of Ecological Transitions: Methods for Spatial Patterns

    PubMed Central

    Brock, William A.; Carpenter, Stephen R.; Ellison, Aaron M.; Livina, Valerie N.; Seekell, David A.; Scheffer, Marten; van Nes, Egbert H.; Dakos, Vasilis

    2014-01-01

    A number of ecosystems can exhibit abrupt shifts between alternative stable states. Because of their important ecological and economic consequences, recent research has focused on devising early warning signals for anticipating such abrupt ecological transitions. In particular, theoretical studies show that changes in spatial characteristics of the system could provide early warnings of approaching transitions. However, the empirical validation of these indicators lag behind their theoretical developments. Here, we summarize a range of currently available spatial early warning signals, suggest potential null models to interpret their trends, and apply them to three simulated spatial data sets of systems undergoing an abrupt transition. In addition to providing a step-by-step methodology for applying these signals to spatial data sets, we propose a statistical toolbox that may be used to help detect approaching transitions in a wide range of spatial data. We hope that our methodology together with the computer codes will stimulate the application and testing of spatial early warning signals on real spatial data. PMID:24658137

  19. Facilitating Family Involvement in Early Intervention to Preschool Transition

    ERIC Educational Resources Information Center

    Pang, Yanhui

    2010-01-01

    Active family involvement and important family roles in the early intervention to preschool transition have been mandated by laws, recognized by the position statements of professional organizations, and validated through evidence-based research. In order to involve families in this process, reduce stresses, and conquer the challenges families may…

  20. [Transition metal mediated transformations of small molecules

    SciTech Connect

    Sen, A.

    1992-01-01

    Work on organotransition metal chemistry, homogeneous and heterogeneous catalysis is summarized. Several cationic palladium(II) complexes with bulky phosphine or pyridine ligands were discovered that are highly selective catalysts for linear dimerization of vinyl monomers and linear polymerization of p-divinylbenzene, the reactions proceeding through a carbocationic mechanism. Our studies were continued on alternating olefin-carbon monoxide copolymers. The copolymerization reaction and reactivity of copolymers were examined. New catalytic systems for alternating copolymerization of [alpha]-olefins with CO were discovered. In the case of styrene derivatives, tactic copolymers were obtained. Poly(ethylenepyrrolediyl) derivatives were synthesized from alternating ethylene-carbon monoxide copolymer and become electronic conductors when doped with iodine. A catalytic system for direct synthesis of polyureas and polyoxamides from and diamines was also discovered. Pt metal catalyzed the oxidation of ethers, esters, and amines to carboxylic acids and the oxidation of olefins to 1,2-diols. Anaerobic and aerobic decomposition of molybdenum(VI)-oxoalkyl compounds were studied for heterogeneous oxidation of alkanes and olefins on Mo(VI)-oxide surfaces. Synthesis of polymer-trapped metal, metal oxide, and metal sulfide nanoclusters (size <1--10 nm) was studied.

  1. Flexible metallic seal for transition duct in turbine system

    SciTech Connect

    Flanagan, James Scott; LeBegue, Jeffrey Scott; McMahan, Kevin Weston; Dillard, Daniel Jackson; Pentecost, Ronnie Ray

    2014-04-22

    A turbine system is disclosed. In one embodiment, the turbine system includes a transition duct. The transition duct includes an inlet, an outlet, and a passage extending between the inlet and the outlet and defining a longitudinal axis, a radial axis, and a tangential axis. The outlet of the transition duct is offset from the inlet along the longitudinal axis and the tangential axis. The transition duct further includes an interface member for interfacing with a turbine section. The turbine system further includes a flexible metallic seal contacting the interface member to provide a seal between the interface member and the turbine section.

  2. (S)TEM analysis of functional transition metal oxides

    NASA Astrophysics Data System (ADS)

    Chi, Miaofang

    Perovskite vanadates (AVO3) form an ideal family to study the structure-property relationships in transition metal oxides because their physical properties can easily be tailored by varying the A-site cations. (S)TEM is an ideal tool for this type of study due to its capacity for simultaneous imaging and chemical analysis. Determination of the oxidation state of vanadium in complex oxides have been carried out by electron energy loss spectroscopy. SrVO3/LaAlO3 is then studied both experimentally and theoretically as a prototype system. Extra electrons have been detected on the interface layer, and further proven to originate mainly from a change in the local bonding configuration of V at the La-O terminated substrate surface. Cr-containing stainless steel deposited with a LaCrO3 thin-film layer is a promising interconnect material of Solid Oxide Fuel Cells (SOFC). Our investigation on its microstructural evolution reveals that the LaCrO 3 thin film plays a role in inhibiting the growth of an oxide layer on the metal surface and thus protects the surface of the stainless steel. Ca-doped LaCoO3 is a promising SOFC cathode material. The domain structures and the oxidation state of Co in Ca-doped LaCoO3, which are directly related to its mechanical properties and electronic conductivity, are investigated by in-situ TEM and EELS. The formation of microcracks is observed during thermal cycles. Ca-doping in LaCoO3 is shown to not only improve the electronic conductivity of the material, but is also likely to strengthen the grain boundaries. The realization of its application in SOFCs depends on depressing the ferroelastisity to reduce strain formation during thermal cycles. The application of the (S)TEM techniques used for studying the perovskite systems are further extended to other compounds containing transition metal elements. The refractory minerals from Comet 81 P/Wild-2 are studied to investigate the formation of the early solar system. A relatively high Ti3+/Ti 4+ ratio in fassaite and the presence of osbornite indicate that the Comet refractory minerals formed in the inner solar nebula and were later transported to the outer solar system where the comet formed. This implies a much more dynamic and perhaps more violent solar nebula than was previously suspected.

  3. The Metallicities of Stars with and without Transiting Planets

    NASA Astrophysics Data System (ADS)

    Buchhave, Lars A.; Latham, David W.

    2015-08-01

    Host star metallicities have been used to infer observational constraints on planet formation throughout the history of the exoplanet field. The giant planet metallicity correlation has now been widely accepted, but questions remain as to whether the metallicity correlation extends to the small terrestrial-sized planets. Here, we report metallicities for a sample of 518 stars in the Kepler field that have no detected transiting planets and compare their metallicity distribution to a sample of stars that hosts small planets ({R}p\\lt 1.7 {R}\\oplus ). Importantly, both samples have been analyzed in a homogeneous manner using the same set of tools (Stellar Parameters Classification tool). We find the average metallicity of the sample of stars without detected transiting planets to be {[{{m}}/{{H}}]}{SNTP,{dwarf}}=-0.02+/- 0.02 {dex} and the sample of stars hosting small planets to be {[{{m}}/{{H}}]}{STP}=-0.02+/- 0.02 {dex}. The average metallicities of the two samples are indistinguishable within the uncertainties, and the two-sample Kolmogorov-Smirnov test yields a p-value of 0.68 (0.41σ), indicating a failure to reject the null hypothesis that the two samples are drawn from the same parent population. We conclude that the homogeneous analysis of the data presented here supports the hypothesis that stars hosting small planets have a metallicity similar to stars with no known transiting planets in the same area of the sky.

  4. The metal-insulator transition in magnetite.

    NASA Technical Reports Server (NTRS)

    Cullen, J. R.; Callen, E.

    1972-01-01

    We describe an electronic model for the low temperature transition in magnetite, in which the average number of electrons on a site is non-integral. The solution of the one-dimensional problem is reviewed, and the connection of the model with the Verwey ordering is discussed. Some of the implication of the three dimensional problem are discussed.

  5. Flexible transition metal dichalcogenide nanosheets for band-selective photodetection

    PubMed Central

    Velusamy, Dhinesh Babu; Kim, Richard Hahnkee; Cha, Soonyoung; Huh, June; Khazaeinezhad, Reza; Kassani, Sahar Hosseinzadeh; Song, Giyoung; Cho, Suk Man; Cho, Sung Hwan; Hwang, Ihn; Lee, Jinseong; Oh, Kyunghwan; Choi, Hyunyoug; Park, Cheolmin

    2015-01-01

    The photocurrent conversions of transition metal dichalcogenide nanosheets are unprecedentedly impressive, making them great candidates for visible range photodetectors. Here we demonstrate a method for fabricating micron-thick, flexible films consisting of a variety of highly separated transition metal dichalcogenide nanosheets for excellent band-selective photodetection. Our method is based on the non-destructive modification of transition metal dichalcogenide sheets with amine-terminated polymers. The universal interaction between amine and transition metal resulted in scalable, stable and high concentration dispersions of a single to a few layers of numerous transition metal dichalcogenides. Our MoSe2 and MoS2 composites are highly photoconductive even at bending radii as low as 200 μm on illumination of near infrared and visible light, respectively. More interestingly, simple solution mixing of MoSe2 and MoS2 gives rise to blended composite films in which the photodetection properties were controllable. The MoS2/MoSe2 (5:5) film showed broad range photodetection suitable for both visible and near infrared spectra. PMID:26333531

  6. Flexible transition metal dichalcogenide nanosheets for band-selective photodetection

    NASA Astrophysics Data System (ADS)

    Velusamy, Dhinesh Babu; Kim, Richard Hahnkee; Cha, Soonyoung; Huh, June; Khazaeinezhad, Reza; Kassani, Sahar Hosseinzadeh; Song, Giyoung; Cho, Suk Man; Cho, Sung Hwan; Hwang, Ihn; Lee, Jinseong; Oh, Kyunghwan; Choi, Hyunyoug; Park, Cheolmin

    2015-09-01

    The photocurrent conversions of transition metal dichalcogenide nanosheets are unprecedentedly impressive, making them great candidates for visible range photodetectors. Here we demonstrate a method for fabricating micron-thick, flexible films consisting of a variety of highly separated transition metal dichalcogenide nanosheets for excellent band-selective photodetection. Our method is based on the non-destructive modification of transition metal dichalcogenide sheets with amine-terminated polymers. The universal interaction between amine and transition metal resulted in scalable, stable and high concentration dispersions of a single to a few layers of numerous transition metal dichalcogenides. Our MoSe2 and MoS2 composites are highly photoconductive even at bending radii as low as 200 μm on illumination of near infrared and visible light, respectively. More interestingly, simple solution mixing of MoSe2 and MoS2 gives rise to blended composite films in which the photodetection properties were controllable. The MoS2/MoSe2 (5:5) film showed broad range photodetection suitable for both visible and near infrared spectra.

  7. Flexible transition metal dichalcogenide nanosheets for band-selective photodetection.

    PubMed

    Velusamy, Dhinesh Babu; Kim, Richard Hahnkee; Cha, Soonyoung; Huh, June; Khazaeinezhad, Reza; Kassani, Sahar Hosseinzadeh; Song, Giyoung; Cho, Suk Man; Cho, Sung Hwan; Hwang, Ihn; Lee, Jinseong; Oh, Kyunghwan; Choi, Hyunyoug; Park, Cheolmin

    2015-01-01

    The photocurrent conversions of transition metal dichalcogenide nanosheets are unprecedentedly impressive, making them great candidates for visible range photodetectors. Here we demonstrate a method for fabricating micron-thick, flexible films consisting of a variety of highly separated transition metal dichalcogenide nanosheets for excellent band-selective photodetection. Our method is based on the non-destructive modification of transition metal dichalcogenide sheets with amine-terminated polymers. The universal interaction between amine and transition metal resulted in scalable, stable and high concentration dispersions of a single to a few layers of numerous transition metal dichalcogenides. Our MoSe2 and MoS2 composites are highly photoconductive even at bending radii as low as 200 μm on illumination of near infrared and visible light, respectively. More interestingly, simple solution mixing of MoSe2 and MoS2 gives rise to blended composite films in which the photodetection properties were controllable. The MoS2/MoSe2 (5:5) film showed broad range photodetection suitable for both visible and near infrared spectra. PMID:26333531

  8. Memristor using a transition metal nitride insulator

    DOEpatents

    Stevens, James E; Marinella, Matthew; Lohn, Andrew John

    2014-10-28

    Apparatus is disclosed in which at least one resistive switching element is interposed between at least a first and a second conducting electrode element. The resistive switching element comprises a metal oxynitride. A method for making such a resistive switching element is also disclosed.

  9. Metallization and charge-transfer gap closure of transition-metal iodides under pressure

    SciTech Connect

    Chen, A. Li-Chung

    1993-05-01

    It is shown with resistivity and near-IR absorption measurements that NiI{sub 2}, CoI{sub 2}, and FeI{sub 2} metallize under pressure by closure of the charge-transfer energy gap at pressures of 17, 10, and 23 GPa, respectively, which is close to the antiferromagnetic-diamagnetic transition in NiI{sub 2} and CoI{sub 2}. Thus, the magnetic transitions probably are caused by the metallization; in NiI{sub 2} and CoI{sub 2}, the insulator-metal transitions are first order. Moessbauer and XRD data were also collected. Figs, 46 refs.

  10. Early warning of atmospheric regime transitions using transfer operators

    NASA Astrophysics Data System (ADS)

    Tantet, Alexis; Dijkstra, Henk

    2015-04-01

    The existence of persistent midlatitude atmospheric regimes, such as blocking events, with time scales larger than 5-10 days and indications of preferred transition paths between them motivates the development of early-warning indicators of regime transitions. Here, we use a barotropic model of the northern midlatitudes winter flow to study such meta-stable regimes. We look at estimates of transfer operators acting on densities evolving on a reduced phase space spanned by the first Empirical Orthogonal Functions of the streamfunction and develop an early-warning indicator of zonal to blocked flow transition. The study of the spectra of transfer operators estimated for different lags reveals a multi-level structure in the flow as well as the effect of memory on the reduced dynamics due to past interactions between the resolved and unresolved variables. The slowest motions in the reduced phase space are thereby found to have time scales larger than 8 days and to behave as Markovian for larger lags. These motions are associated with meta-stable regimes and their transitions and can be detected as almost-invariant sets of the transfer operator. The early-warning indicator is based on the action on an initial density of products of the transfer operators estimated for sufficiently long lags, making use of the semi-group property of these operators and shows relatively good Peirce skill score. From the energy budget of the model, we are able to explain the meta-stability of the regimes and the existence of preferred transition paths as the manifestation of barotropic instability. Finally, even though the model is highly simplified, the skill of the early warning indicator is promising, suggesting that the transfer operator approach can be used in parallel to an operational deterministic model for stochastic prediction or to assess forecast uncertainty.

  11. Anharmonic Vibrational Spectroscopy on Metal Transition Complexes

    NASA Astrophysics Data System (ADS)

    Latouche, Camille; Bloino, Julien; Barone, Vincenzo

    2014-06-01

    Advances in hardware performance and the availability of efficient and reliable computational models have made possible the application of computational spectroscopy to ever larger molecular systems. The systematic interpretation of experimental data and the full characterization of complex molecules can then be facilitated. Focusing on vibrational spectroscopy, several approaches have been proposed to simulate spectra beyond the double harmonic approximation, so that more details become available. However, a routine use of such tools requires the preliminary definition of a valid protocol with the most appropriate combination of electronic structure and nuclear calculation models. Several benchmark of anharmonic calculations frequency have been realized on organic molecules. Nevertheless, benchmarks of organometallics or inorganic metal complexes at this level are strongly lacking despite the interest of these systems due to their strong emission and vibrational properties. Herein we report the benchmark study realized with anharmonic calculations on simple metal complexes, along with some pilot applications on systems of direct technological or biological interest.

  12. Transition metal-substituted cobalt ferrite nanoparticles for biomedical applications.

    PubMed

    Sanpo, Noppakun; Berndt, Christopher C; Wen, Cuie; Wang, James

    2013-03-01

    Transition metals of copper, zinc, chromium and nickel were substituted into cobalt ferrite nanoparticles via a sol-gel route using citric acid as a chelating agent. The microstructure and elemental composition were characterized using scanning electron microscopy combined with energy-dispersive X-ray spectroscopy. Phase analysis of transition metal-substituted cobalt ferrite nanoparticles was performed via X-ray diffraction. Surface wettability was measured using the water contact angle technique. The surface roughness of all nanoparticles was measured using profilometry. Moreover, thermogravimetric analysis and differential scanning calorimetry were performed to determine the temperature at which the decomposition and oxidation of the chelating agents took place. Results indicated that the substitution of transition metals influences strongly the microstructure, crystal structure and antibacterial property of the cobalt ferrite nanoparticles. PMID:23137676

  13. Nanodisperse transition metal electrodes (NTME) for electrochemical cells

    SciTech Connect

    Striebel, Kathryn A.; Wen, Shi-Jie

    1998-12-01

    Disclosed are transition metal electrodes for electrochemical cells using gel-state and solid-state polymers. The electrodes are suitable for use in primary and secondary cells. The electrodes (either negative electrode or positive electrode) are characterized by uniform dispersion of the transition metal at the nanoscale in the polymer. The transition metal moiety is structurally amorphous, so no capacity fade should occur due to lattice expansion/contraction mechanisms. The small grain size, amorphous structure and homogeneous distribution provide improved charge/discharge cycling performance, and a higher initial discharge rate capability. The cells can be cycled at high current densities, limited only by the electrolyte conductivity. A method of making the electrodes (positive and negative), and their usage in electrochemical cells are disclosed.

  14. Nanodisperse transition metal electrodes (NTME) for electrochemical cells

    DOEpatents

    Striebel, Kathryn A.; Wen, Shi-Jie

    2000-01-01

    Disclosed are transition metal electrodes for electrochemical cells using gel-state and solid-state polymers. The electrodes are suitable for use in primary and secondary cells. The electrodes (either negative electrode or positive electrode) are characterized by uniform dispersion of the transition metal at the nanoscale in the polymer. The transition metal moiety is structurally amorphous, so no capacity fade should occur due to lattice expansion/contraction mechanisms. The small grain size, amorphous structure and homogeneous distribution provide improved charge/discharge cycling performance, and a higher initial discharge rate capability. The cells can be cycled at high current densities, limited only by the electrolyte conductivity. A method of making the electrodes (positive and negative), and their usage in electrochemical cells are disclosed.

  15. Chirality Recognition and Transition Mechanism of prochiral Molecules on metals

    NASA Astrophysics Data System (ADS)

    Wang, Yeliang; Yang, Bing; Cun, Huanyao; Jiang, Nan; Du, Shixuan; Gao, Hong-Jun

    2012-02-01

    The self-assemble behavior of prochiral species and the induced high-order chirality by 2D confinement on solid surfaces, including (1) QA16C molecules on a Au(111) surface and (2) molecule-metallic (TPA-Fe) complex on Cu(110) as well as their transferring process will be presented. Initial stages of a chiral phase transition in the molecule monolayer on metal surfaces were investigated by scanning tunneling microscopy (STM) at submolecular resolution. The prochiral QA16C molecules form a homochiral lamella phase at low coverages upon adsorption. A transition to a racemate lattice is observed with increasing coverage. Enantiomers of a homochiral lamella line become specifically substituted by opposite enantiomers such that a heterochiral structure evolves. A ``chiral replacement'' model is proposed for the transition: enantiomers replace QA molecules in enantiopure phase, leading to racemic one. Our findings are significant for the understanding and control of chiral phase transitions in related molecular systems like liquid crystals.

  16. Anderson localization effects near the Mott metal-insulator transition

    NASA Astrophysics Data System (ADS)

    Bragança, Helena; Aguiar, M. C. O.; Vučičević, J.; Tanasković, D.; Dobrosavljević, V.

    2015-09-01

    The interplay between Mott and Anderson routes to localization in disordered interacting systems gives rise to different transitions and transport regimes. Here, we investigate the phase diagram at finite temperatures using dynamical mean-field theory combined with typical medium theory, which is an effective theory of the Mott-Anderson metal-insulator transition. We mainly focus on the properties of the coexistence region associated with the Mott phase transition. For weak disorder, the coexistence region is found to be similar to that in the clean case. However, as we increase disorder, Anderson localization effects are responsible for shrinking the coexistence region, and at sufficiently strong disorder (approximately equal to twice the bare bandwidth) it drastically narrows, the critical temperature Tc abruptly goes to zero, and we observe a phase transition in the absence of a coexistence of the metallic and insulating phases. In this regime, the effects of interaction and disorder are found to be of comparable importance for charge localization.

  17. Transition Metal Complexes of Expanded Porphyrins

    PubMed Central

    Sessler, Jonathan L.; Tomat, Elisa

    2008-01-01

    Over the last two decades, the rapid development of new synthetic routes for the preparation of expanded porphyrin macrocycles has allowed exploration of a new frontier consisting of “porphyrin-like” coordination chemistry. In this Account, we summarize our exploratory forays into the still relatively poorly explored area of oligopyrrolic macrocycle metalation chemistry. Specifically, we describe our successful formation of both mono- and binuclear complexes and in doing so highlight the diversity of coordination modes available to expanded porphyrin-type ligands. The nature of the inserted cation, the emerging role of tautomeric equilibria, and the importance of hydrogen-bonding interactions in regulating this chemistry are also discussed. PMID:17397134

  18. Mechanisms of transition-metal gettering in silicon

    SciTech Connect

    Myers, S. M.; Seibt, M.; Schroeter, W.

    2000-10-01

    The atomic process, kinetics, and equilibrium thermodynamics underlying the gettering of transition-metal impurities in Si are reviewed. Methods for mathematical modeling of gettering are discussed and illustrated. Needs for further research are considered. (c) 2000 American Institute of Physics.

  19. Epoxy nanocomposites with two-dimensional transition metal dichalcogenide additives.

    PubMed

    Eksik, Osman; Gao, Jian; Shojaee, S Ali; Thomas, Abhay; Chow, Philippe; Bartolucci, Stephen F; Lucca, Don A; Koratkar, Nikhil

    2014-05-27

    Emerging two-dimensional (2D) materials such as transition metal dichalcogenides offer unique and hitherto unavailable opportunities to tailor the mechanical, thermal, electronic, and optical properties of polymer nanocomposites. In this study, we exfoliated bulk molybdenum disulfide (MoS2) into nanoplatelets, which were then dispersed in epoxy polymers at loading fractions of up to 1% by weight. We characterized the tensile and fracture properties of the composite and show that MoS2 nanoplatelets are highly effective at enhancing the mechanical properties of the epoxy at very low nanofiller loading fractions (below 0.2% by weight). Our results show the potential of 2D sheets of transition metal dichalcogenides as reinforcing additives in polymeric composites. Unlike graphene, transition metal dichalcogenides such as MoS2 are high band gap semiconductors and do not impart significant electrical conductivity to the epoxy matrix. For many applications, it is essential to enhance mechanical properties while also maintaining the electrical insulation properties and the high dielectric constant of the polymer material. In such applications, conductive carbon based fillers such as graphene cannot be utilized. This study demonstrates that 2D transition metal dichalcogenide additives offer an elegant solution to such class of problems. PMID:24754702

  20. Transition Metal Photoredox Catalysis of Radical Thiol-Ene Reactions

    PubMed Central

    Tyson, Elizabeth L.; Ament, Michael S.

    2012-01-01

    We describe the anti-Markovnikov hydrothiolation of olefins using visible light absorbing transition metal photocatalysts. The key thiyl radical intermediates are generated upon quenching of photoexcited Ru*(bpz)32 with a variety of thiols. The adducts of a wide variety of olefins and thiols are formed in excellent yield (73–99%). PMID:23094660

  1. On the thermodynamics of phase transitions in metal hydrides

    NASA Astrophysics Data System (ADS)

    Vita, Andrea

    2012-02-01

    Metal hydrides are solutions of hydrogen in a metal, where phase transitions may occur depending on temperature, pressure etc. We apply Le Chatelier's principle of thermodynamics to a particular phase transition in TiHx, which can approximately be described as a second-order phase transition. We show that the fluctuations of the order parameter correspond to fluctuations both of the density of H+ ions and of the distance between adjacent H+ ions. Moreover, as the system approaches the transition and the correlation radius increases, we show -with the help of statistical mechanics-that the statistical weight of modes involving a large number of H+ ions (`collective modes') increases sharply, in spite of the fact that the Boltzmann factor of each collective mode is exponentially small. As a result, the interaction of the H+ ions with collective modes makes a tiny suprathermal fraction of the H+ population appear. Our results hold for similar transitions in metal deuterides, too. A violation of an -insofar undisputed-upper bound on hydrogen loading follows.

  2. On the thermodynamics of phase transitions in metal hydrides

    NASA Astrophysics Data System (ADS)

    di Vita, Andrea

    2012-02-01

    Metal hydrides are solutions of hydrogen in a metal, where phase transitions may occur depending on temperature, pressure etc. We apply Le Chatelier's principle of thermodynamics to a particular phase transition in TiH x , which can approximately be described as a second-order phase transition. We show that the fluctuations of the order parameter correspond to fluctuations both of the density of H+ ions and of the distance between adjacent H+ ions. Moreover, as the system approaches the transition and the correlation radius increases, we show -with the help of statistical mechanics-that the statistical weight of modes involving a large number of H+ ions (`collective modes') increases sharply, in spite of the fact that the Boltzmann factor of each collective mode is exponentially small. As a result, the interaction of the H+ ions with collective modes makes a tiny suprathermal fraction of the H+ population appear. Our results hold for similar transitions in metal deuterides, too. A violation of an -insofar undisputed-upper bound on hydrogen loading follows.

  3. Oxide Electronics Utilizing Ultrafast Metal-Insulator Transitions

    NASA Astrophysics Data System (ADS)

    Yang, Zheng; Ko, Changhyun; Ramanathan, Shriram

    2011-08-01

    Although phase transitions have long been a centerpiece of condensed matter materials science studies, a number of recent efforts focus on potentially exploiting the resulting functional property changes in novel electronics and photonics as well as understanding emergent phenomena. This is quite timely, given a grand challenge in twenty-first-century physical sciences is related to enabling continued advances in information processing and storage beyond conventional CMOS scaling. In this brief review, we discuss synthesis of strongly correlated oxides, mechanisms of metal-insulator transitions, and exploratory electron devices that are being studied. Particular emphasis is placed on vanadium dioxide, which undergoes a sharp metal-insulator transition near room temperature at ultrafast timescales. The article begins with an introduction to metal-insulator transition in oxides, followed by a brief discussion on the mechanisms leading to the phase transition. The role of materials synthesis in influencing functional properties is discussed briefly. Recent efforts on realizing novel devices such as field effect switches, optical detectors, nonlinear circuit components, and solid-state sensors are reviewed. The article concludes with a brief discussion on future research directions that may be worth consideration.

  4. Electrocatalysis using transition metal carbide and oxide nanocrystals

    NASA Astrophysics Data System (ADS)

    Regmi, Yagya N.

    Carbides are one of the several families of transition metal compounds that are considered economic alternatives to catalysts based on noble metals and their compounds. Phase pure transition metal carbides of group 4-6 metals, in the first three periods, were synthesized using a common eutectic salt flux synthesis method, and their electrocatalytic activities compared under uniform electrochemical conditions. Mo2C showed highest hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) activities among the nine metal carbides investigated, but all other metal carbides also showed substantial activities. All the metal carbides showed remarkable enhancement in catalytic activities as supports, when compared to traditional graphitic carbon as platinum support. Mo2C, the most active transition metal carbide electrocatalyst, was prepared using four different synthesis routes, and the synthesis route dependent activities compared. Bifunctional Mo 2C that is HER as well as oxygen evolution reaction (OER) active, was achieved when the carbide was templated on a multiwalled carbon nanotube using carbothermic reduction method. Bimetallic carbides of Fe, Co, and Ni with Mo or W were prepared using a common carbothermic reduction method. Two different stoichiometries of bimetallic carbides were obtained for each system within a 60 °C temperature window. While the bimetallic carbides showed relatively lower electrocatalytic activities towards HER and ORR in comparison to Mo2C and WC, they revealed remarkably higher OER activities than IrO2 and RuO2, the state-of-the-art OER catalysts. Bimetallic oxides of Fe, Co, and Ni with Mo and W were also prepared using a hydrothermal synthesis method and they also revealed OER activities that are much higher than RuO2 and IrO2. Additionally, the OER activities were dependent on the degree and nature of hydration in the bimetallic oxide crystal lattice, with the completely hydrated, as synthesized, cobalt molybdate and nickel molybdate showing the highest OER activities.

  5. Hydrophosphination reactions with transition metal ferrocenylphosphine complexes.

    PubMed

    Pritzwald-Stegmann, Julian Rodger Frederic; Lönnecke, Peter; Hey-Hawkins, Evamarie

    2016-02-01

    The group 6 metal mono-, bis- and tris-ferrocenylphosphine complexes [M(CO)5(PH2Fc)] (1a, M = Cr; 1b, M = Mo; 1c, M = W), cis-[M(CO)4(PH2Fc)2] (2a, M = Cr; 2b, M = Mo; 2c, M = W) and fac-[M(CO)3(PH2Fc)3] (3a, M = Cr; 3b, M = Mo; 3c, M = W) [Fc = Fe(η(5)-C5H4)(η(5)-C5H5)] were prepared and fully characterised. IR and NMR spectroscopy and single-crystal X-ray diffraction analysis indicate that FcPH2 is as good a σ donor as PhPH2 but is easier to handle and furthermore has a redox-active ferrocenyl group. Complex 1c was employed in the hydrophosphination of acrylonitrile and methyl acrylate in the presence of catalytic amounts of KOtBu giving the secondary phosphine complexes [W(CO)5{PH(Fc)(CH2CH2CN)}] (4a) and [W(CO)5{PH(Fc)(CH2CH2C(O)OMe)}] (4b). In addition, FcP(CH2CH2CN)2 (5) was prepared by a similar method from FcPH2 and acrylonitrile. These hydrophosphination products represent a convenient method for the modification of phosphines. PMID:26579617

  6. Mechanical failure and glass transition in metallic glasses

    SciTech Connect

    Egami, Takeshi

    2011-01-01

    The current majority view on the phenomenon of mechanical failure in metallic glasses appears to be that it is caused by the activity of some structural defects, such as free-volumes or shear transformation zones, and the concentration of such defects is small, only of the order of 1%. However, the recent results compel us to revise this view. Through molecular dynamics simulation it has been shown that mechanical failure is the stress-induced glass transition. According to our theory the concentration of the liquid-like sites (defects) is well over 20% at the glass transition. We suggest that the defect concentration in metallic glasses is actually very high, and percolation of such defects causes atomic avalanche and mechanical failure. In this article we discuss the glass transition, mechanical failure and viscosity from such a point of view.

  7. Preparation of nanoporous metal foam from high nitrogen transition metal complexes

    DOEpatents

    Tappan, Bryce C.; Huynh, My Hang V.; Hiskey, Michael A.; Son, Steven F.; Oschwald, David M.; Chavez, David E.; Naud, Darren L.

    2006-11-28

    Nanoporous metal foams are prepared by ignition of high nitrogen transition metal complexes. The ammonium salts of iron(III) tris[bi(tetrazolato)-amine], cobalt(III) tris(bi(tetrazolato)amine), and high nitrogen compounds of copper and silver were prepared as loose powders, pressed into pellets and wafers, and ignited under an inert atmosphere to form nanoporous metal foam monoliths having very high surface area and very low density.

  8. Metal-insulator transition in films of doped semiconductor nanocrystals

    NASA Astrophysics Data System (ADS)

    Chen, Ting; Reich, K. V.; Kramer, Nicolaas J.; Fu, Han; Kortshagen, Uwe R.; Shklovskii, B. I.

    2016-03-01

    To fully deploy the potential of semiconductor nanocrystal films as low-cost electronic materials, a better understanding of the amount of dopants required to make their conductivity metallic is needed. In bulk semiconductors, the critical concentration of electrons at the metal-insulator transition is described by the Mott criterion. Here, we theoretically derive the critical concentration nc for films of heavily doped nanocrystals devoid of ligands at their surface and in direct contact with each other. In the accompanying experiments, we investigate the conduction mechanism in films of phosphorus-doped, ligand-free silicon nanocrystals. At the largest electron concentration achieved in our samples, which is half the predicted nc, we find that the localization length of hopping electrons is close to three times the nanocrystals diameter, indicating that the film approaches the metal-insulator transition.

  9. Transition Metal Nitrides for Electrocatalytic Energy Conversion: Opportunities and Challenges.

    PubMed

    Xie, Junfeng; Xie, Yi

    2016-03-01

    Electrocatalytic energy conversion has been considered as one of the most efficient and promising pathways for realizing energy storage and energy utilization in modern society. To improve electrocatalytic reactions, specific catalysts are needed to lower the overpotential. In the search for efficient alternatives to noble metal catalysts, transition metal nitrides have attracted considerable interest due to their high catalytic activity and unique electronic structure. Over the past few decades, numerous nitride-based catalysts have been explored with respect to their ability to drive various electrocatalytic reactions, such as the hydrogen evolution reaction and the oxygen evolution reaction to achieve water splitting and the oxygen reduction reaction coupled with the methanol oxidation reaction to construct fuel cells or rechargeable Li-O2 batteries. This Minireview provides a brief overview of recent progress on electrocatalysts based on transition metal nitrides, and outlines the current challenges and future opportunities. PMID:26494184

  10. Metal-insulator transition in films of doped semiconductor nanocrystals.

    PubMed

    Chen, Ting; Reich, K V; Kramer, Nicolaas J; Fu, Han; Kortshagen, Uwe R; Shklovskii, B I

    2016-03-01

    To fully deploy the potential of semiconductor nanocrystal films as low-cost electronic materials, a better understanding of the amount of dopants required to make their conductivity metallic is needed. In bulk semiconductors, the critical concentration of electrons at the metal-insulator transition is described by the Mott criterion. Here, we theoretically derive the critical concentration nc for films of heavily doped nanocrystals devoid of ligands at their surface and in direct contact with each other. In the accompanying experiments, we investigate the conduction mechanism in films of phosphorus-doped, ligand-free silicon nanocrystals. At the largest electron concentration achieved in our samples, which is half the predicted nc, we find that the localization length of hopping electrons is close to three times the nanocrystals diameter, indicating that the film approaches the metal-insulator transition. PMID:26618885

  11. Synthesis, characterization and reactivity of transition-metal-containing zeolites

    SciTech Connect

    Rossin, J.A.

    1986-01-01

    Transition metal containing zeolites (zeolite A and ZSM-5) were prepared by addition of various transition metal containing substrates to zeolite synthesis gels. Crystal growth data were recorded in order to determine the influence of the transition metal species on the rate of crystal growth. X-ray diffraction, oxygen adsorption, FTIR and SEM were utilized to evaluate crystal purity. X-ray photoelectron spectroscopy (XPS), chemical analysis and electron microprobe analysis were performed in order to ascertain the position (intrazeolitic versus surface) and homogeneity of the transition metal. It was concluded that intrazeolitic transition metals are produced by the novel procedure presented in this work. 1-Hexane hydroformylation by rhodium zeolite A showed intrazeolitic rhodium to migrate to the external surface of the zeolite. However, in the presence of a solution and surface rhodium poison, intrazeolitic rhodium was found to hydroformylate 1-hexene exclusively to heptanal. Ruthenium containing zeolite A was evaluated under CO-hydrogenation conditions. No migration of intrazeolitic ruthenium to the external surface of the zeolite was observed over the course of the reaction. The product distribution obtained for this catalyst did not follow a log normal behavior. Also, loss of zeolite crystallinity was observed following the reaction. Cobalt ZSM-5 was evaluated under CO-hydrogenation conditions. No migration of cobalt to the external surface of the zeolite occurred. XPS analysis of the catalyst following various stages of the reaction indicated that intrazeolitic cobalt was not reduced to the zero valent state. Consequently, the non-zero valent cobalt was not capable of hydrogenating carbon monoxide.

  12. The transition to the metallic state in low density hydrogen

    SciTech Connect

    McMinis, Jeremy; Morales, Miguel A.; Ceperley, David M.; Kim, Jeongnim

    2015-11-18

    Solid atomic hydrogen is one of the simplest systems to undergo a metal-insulator transition. Near the transition, the electronic degrees of freedom become strongly correlated and their description provides a difficult challenge for theoretical methods. As a result, the order and density of the phase transition are still subject to debate. In this work we use diffusion quantum Monte Carlo to benchmark the transition between the paramagnetic and anti-ferromagnetic phases of ground state body centered cubic atomic hydrogen. We locate the density of the transition by computing the equation of state for these two phases and identify the phase transition order by computing the band gap near the phase transition. These benchmark results show that the phase transition is continuous and occurs at a Wigner-Seitz radius of rs = 2.27(3)a0. As a result, we compare our results to previously reported density functional theory, Hedin s GW approximation, and dynamical mean field theory results.

  13. Transition Metal d-Orbital Splitting Diagrams: An Updated Educational Resource for Square Planar Transition Metal Complexes

    ERIC Educational Resources Information Center

    Borgel, Jonas; Campbell, Michael G.; Ritter, Tobias

    2016-01-01

    The presentation of d-orbital splitting diagrams for square planar transition metal complexes in textbooks and educational materials is often inconsistent and therefore confusing for students. Here we provide a concise summary of the key features of orbital splitting diagrams for square planar complexes, which we propose may be used as an updated

  14. Transition Metal d-Orbital Splitting Diagrams: An Updated Educational Resource for Square Planar Transition Metal Complexes

    ERIC Educational Resources Information Center

    Bo¨rgel, Jonas; Campbell, Michael G.; Ritter, Tobias

    2016-01-01

    The presentation of d-orbital splitting diagrams for square planar transition metal complexes in textbooks and educational materials is often inconsistent and therefore confusing for students. Here we provide a concise summary of the key features of orbital splitting diagrams for square planar complexes, which we propose may be used as an updated…

  15. Early warnings and missed alarms for abrupt monsoon transitions

    NASA Astrophysics Data System (ADS)

    Thomas, Z. A.; Kwasniok, F.; Boulton, C. A.; Cox, P. M.; Jones, R. T.; Lenton, T. M.; Turney, C. S. M.

    2015-12-01

    Palaeo-records from China demonstrate that the East Asian Summer Monsoon (EASM) is dominated by abrupt and large magnitude monsoon shifts on millennial timescales, switching between periods of high and weak monsoon rains. It has been hypothesized that over these timescales, the EASM exhibits two stable states with bifurcation-type tipping points between them. Here we test this hypothesis by looking for early warning signals of past bifurcations in speleothem δ18O records from Sanbao Cave and Hulu Cave, China, spanning the penultimate glacial cycle. We find that although there are increases in both autocorrelation and variance preceding some of the monsoon transitions during this period, it is only immediately prior to the abrupt monsoon shift at the penultimate deglaciation (Termination II) that statistically significant increases are detected. To supplement our data analysis, we produce and analyse multiple model simulations that we derive from these data. We find hysteresis behaviour in our model simulations with transitions directly forced by solar insolation. However, signals of critical slowing down, which occur on the approach to a bifurcation, are only detectable in the model simulations when the change in system stability is sufficiently slow to be detected by the sampling resolution of the data set. This raises the possibility that the early warning "alarms" were missed in the speleothem data over the period 224-150 kyr and it was only at the monsoon termination that the change in the system stability was sufficiently slow to detect early warning signals.

  16. Quantifying covalency and metallicity in correlated compounds undergoing metal-insulator transitions

    NASA Astrophysics Data System (ADS)

    Chainani, Ashish; Yamamoto, Ayako; Matsunami, Masaharu; Eguchi, Ritsuko; Taguchi, Munetaka; Takata, Yasutaka; Takagi, Hidenori; Shin, Shik; Nishino, Yoshinori; Yabashi, Makina; Tamasaku, Kenji; Ishikawa, Tetsuya

    2013-01-01

    The tunability of bonding character in transition-metal compounds controls phase transitions and their fascinating properties such as high-temperature superconductivity, colossal magnetoresistance, spin-charge ordering, etc. However, separating out and quantifying the roles of covalency and metallicity derived from the same set of transition-metal d and ligand p electrons remains a fundamental challenge. In this study, we use bulk-sensitive photoelectron spectroscopy and configuration-interaction calculations for quantifying the covalency and metallicity in correlated compounds. The method is applied to study the first-order temperature- (T-) dependent metal-insulator transitions (MITs) in the cubic pyrochlore ruthenates Tl2Ru2O7 and Hg2Ru2O7. Core-level spectroscopy shows drastic T-dependent modifications which are well explained by including ligand-screening and metallic-screening channels. The core-level metallic-origin features get quenched upon gap formation in valence band spectra, while ionic and covalent components remain intact across the MIT. The results establish temperature-driven Mott-Hubbard MITs in three-dimensional ruthenates and reveal three energy scales: (a) 4d electronic changes occur on the largest (˜eV) energy scale, (b) the band-gap energies/charge gaps (Eg˜160-200 meV) are intermediate, and (c) the lowest-energy scale corresponds to the transition temperature TMIT (˜10 meV), which is also the spin gap energy of Tl2Ru2O7 and the magnetic-ordering temperature of Hg2Ru2O7. The method is general for doping- and T-induced transitions and is valid for V2O3, CrN, La1-xSrxMnO3, La2-xSrxCuO4, etc. The obtained transition-metal-ligand (d-p) bonding energies (V˜45-90 kcal/mol) are consistent with thermochemical data, and with energies of typical heteronuclear covalent bonds such as C-H, C-O, C-N, etc. In contrast, the metallic-screening energies of correlated compounds form a weaker class (V*˜10-40 kcal/mol) but are still stronger than van der Waals and hydrogen bonding. The results identify and quantify the roles of covalency and metallicity in 3d and 4d correlated compounds undergoing metal-insulator transitions.

  17. Buckley Prize Talk: The Suprerconductor-(Metal)-Insulator Transition

    NASA Astrophysics Data System (ADS)

    Kapitulnik, Aharon

    2015-03-01

    While the classical theory of phase transitions has been extraordinarily successful, there are several reasons to exercise caution when applying this approach to the zero temperature superconducting transition. First, experimental identification of the relevant phases requires extrapolation to zero temperature, which becomes complicated, especially when one needs to identify sources of dissipation. In addition, since superconductivity may be highly inhomogeneous as appreciable superconducting order parameter may be concentrated in ``superconducting puddles'' due to disorder and/or spontaneous phase separation, the nature of the quantum phase transition to a superconducting state may be highly anomalous, where the system attempts to optimizes the formation of puddles with the Josephson coupling among them to obtain global superconductivity. In this talk we will review some of the consequences of these considerations, emphasizing the possible emergence of anomalous metallic phases close to the superconductor-insulator transition.

  18. Transition-metal-catalyzed C-S bond coupling reaction.

    PubMed

    Lee, Chin-Fa; Liu, Yi-Chen; Badsara, Satpal Singh

    2014-03-01

    Sulfur-containing molecules such as thioethers are commonly found in chemical biology, organic synthesis, and materials chemistry. While many reliable methods have been developed for preparing these compounds, harsh reaction conditions are usually required in the traditional methods. The transition metals have been applied in this field, and the palladium-catalyzed coupling of thiols with aryl halides and pseudo halides is one of the most important methods in the synthesis of thioethers. Other metals have also been used for the same purpose. Here, we summarize recent efforts in metal-catalyzed C-S bond cross-coupling reactions, focusing especially on the coupling of thiols with aryl- and vinyl halides based on different metals. PMID:24443103

  19. The transition to the metallic state in low density hydrogen

    DOE PAGESBeta

    McMinis, Jeremy; Morales, Miguel A.; Ceperley, David M.; Kim, Jeongnim

    2015-11-18

    Solid atomic hydrogen is one of the simplest systems to undergo a metal-insulator transition. Near the transition, the electronic degrees of freedom become strongly correlated and their description provides a difficult challenge for theoretical methods. As a result, the order and density of the phase transition are still subject to debate. In this work we use diffusion quantum Monte Carlo to benchmark the transition between the paramagnetic and anti-ferromagnetic phases of ground state body centered cubic atomic hydrogen. We locate the density of the transition by computing the equation of state for these two phases and identify the phase transitionmore » order by computing the band gap near the phase transition. These benchmark results show that the phase transition is continuous and occurs at a Wigner-Seitz radius of rs = 2.27(3)a0. As a result, we compare our results to previously reported density functional theory, Hedin s GW approximation, and dynamical mean field theory results.« less

  20. Structural transitions and metallization in dense GeS

    NASA Astrophysics Data System (ADS)

    Dias, Ranga P.; Kim, Minseob; Yoo, Choong-Shik

    2016-03-01

    We have studied the pressure-induced structural and electronic transitions of GeS to 49 GPa, using diamond anvil cells, microconfocal Raman spectroscopy, synchrotron angle-resolved x-ray diffraction, and electrical resistance measurements. The results indicate that layered GeS phase I (Pnma) undergoes two structural phase transitions: first to phase II (P 21/m ) at ˜9 GPa and then to phase III (Cmcm) at ˜32 GPa. These transitions occur sluggishly over large pressure ranges and accompany an increase of the nearest coordination number of Ge and S atoms from three in phase I and II to five in phase III, resulting in an extended network of densely packed layer structures in phase III. We suggest anisotropic compression enhanced shears to be responsible for the transitions. Phase II also undergoes an electronic insulator-metal transition and becomes metallic above 20 GPa, The temperature-dependent resistance change of phase II further indicates a gradual band-gap closing of phase II (P 21/m ) .

  1. Thermal properties of zirconium diboride -- transition metal boride solid solutions

    NASA Astrophysics Data System (ADS)

    McClane, Devon Lee

    This research focuses on the thermal properties of zirconium diboride (ZrB2) based ceramics. The overall goal was to improve the understanding of how different transition metal (TM) additives influence thermal transport in ZrB2. To achieve this, ZrB2 with 0.5 wt% carbon, and 3 mol% of individual transition metal borides, was densified by hot-press sintering. The transition metals that were investigated were: Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, and Re. The room temperature thermal diffusivities of the compositions ranged from 0.331 cm2/s for nominally pure ZrB2 to 0.105 cm2/s for (Zr,Cr)B2 and converged around 0.155cm2/s at higher temperatures for all compositions. Thermal conductivities were calculated from the diffusivities, using temperature-dependent values for density and heat capacity. The electron contribution to thermal conductivity was calculated from measured electrical resistivity according to the Wiedemann-Franz law. The phonon contribution to thermal conductivity was calculated by subtracting the electron contribution from the total thermal conductivity. Rietveld refinement of x-ray diffraction data was used to determine the lattice parameters of the compositions. The decrease in thermal conductivity for individual additives correlated directly to the metallic radius of the additive. Additional strain appeared to exist for additives when the stable TM boride for that metal had different crystal symmetries than ZrB2. This research provided insight into how additives and impurities affect thermal transport in ZrB2. The research potentially offers a basis for future modeling of thermal conductivity in ultra-high temperature ceramics based on the correlation between metallic radius and the decrease in thermal conductivity.

  2. Cross-linking proteins with bimetallic tetracarboxylate compounds of transition metals

    DOEpatents

    Kostic, N.M.; Chen, J.

    1991-03-05

    Stable cross-linked complexes of transition-metal tetracarboxylates and proteins are formed. The preferred transition-metal is rhodium. The protein may be collagen or an enzyme such as a proteolytic enzyme. No Drawings

  3. Cross-linking proteins with bimetallic tetracarboxylate compounds of transition metals

    DOEpatents

    Kostic, Nenad M.; Chen, Jian

    1991-03-05

    Stable cross-linked complexes of transition-metal tetracarboxylates and proteins are formed. The preferred transition-metal is rhodium. The protein may be collagen or an enzyme such as a proteolytic enzyme.

  4. An Alternative Approach to the Teaching of Systematic Transition Metal Chemistry.

    ERIC Educational Resources Information Center

    Hathaway, Brian

    1979-01-01

    Presents an alternative approach to teaching Systematic Transition Metal Chemistry with the transition metal chemistry skeleton features of interest. The "skeleton" is intended as a guide to predicting the chemistry of a selected compound. (Author/SA)

  5. Continuity and Respect for Diversity: Strengthening Early Transitions in Peru. Working Papers in Early Childhood Development, No. 56. Studies in Early Childhood Transitions

    ERIC Educational Resources Information Center

    Ames, Patricia; Rojas, Vanessa; Portugal, Tamia

    2010-01-01

    This working paper is part of a series on early transitions from "Young Lives," a 15-year longitudinal study of childhood poverty in Ethiopia, India, Peru and Vietnam. It explores the diverse experiences of 28 children from four contrasting communities in Peru as they start school. These detailed case studies highlight common problems: exclusion…

  6. Resonant Ultrasound Studies of Complex Transition Metal Oxides

    SciTech Connect

    Dr. Henry Bass; Dr. J. R. Gladden

    2008-08-18

    Department of Energy EPSCoR The University of Mississippi Award: DE-FG02-04ER46121 Resonant Ultrasound Spectroscopy Studies of Complex Transition Metal Oxides The central thrust of this DOE funded research program has been to apply resonant ultrasound spectroscopy (RUS), an elegant and efficient method for determining the elastic stiffness constants of a crystal, to the complex and poorly understood class of materials known as transition metal oxides (TMOs). Perhaps the most interesting and challenging feature of TMOs is their strongly correlated behavior in which spin, lattice, and charge degrees of freedom are strongly coupled. Elastic constants are a measure of the interatomic potentials in a crystal and are thus sensitive probes into the atomic environment. This sensitivity makes RUS an ideal tool to study the coupling of phase transition order parameters to lattice strains. The most significant result of the project has been the construction of a high temperature RUS apparatus capable of making elastic constant measurements at temperatures as high as 1000 degrees Celsius. We have designed and built novel acoustic transducers which can operate as high as 600 degrees Celsius based on lithium niobate piezoelectric elements. For measurement between 600 to 1000 C, a buffer rod system is used in which the samples under test and transducers are separated by a rod with low acoustic attenuation. The high temperature RUS system has been used to study the charge order (CO) transition in transition metal oxides for which we have discovered a new transition occurring about 35 C below the CO transition. While the CO transition exhibits a linear coupling between the strain and order parameter, this new precursor transition shows a different coupling indicating a fundamentally different mechanism. We have also begun a study, in collaboration with the Jet Propulsion Laboratory, to study novel thermoelectric materials at elevated temperatures. These materials include silicon germanium with various doping and Zintl phase materials. Such materials show promise for increased figures of merit, vital to making thermolectrics competitive with traditional power generation mechanisms.

  7. Electroforming and switching in oxides of transition metals: The role of metal-insulator transition in the switching mechanism

    SciTech Connect

    Chudnovskii, F.A.; Odynets, L.L.; Pergament, A.L.; Stefanovich, G.B.

    1996-02-15

    Electroforming and switching effects in sandwich structures based on anodic films of transition metal oxides (V, Nb, Ti, Fe, Ta, W, Zr, Hf, Mo) have been studied. After being electroformed, some materials exhibited current-controlled negative resistance with S-shaped V-I characteristics. For V, Fe, Ti, and Nb oxides, the temperature dependence of the threshold voltage has been measured. As the temperature increased, V{sub th} decreased to zero at a critical temperature T{sub O}, which depended on the film material. Comparison of the T{sub O} values with the temperatures of metal-insulator phase transition for some compounds (T{sub t}=120 K for Fe{sub 3}O{sub 4}, 340 K for VO{sub 2}, {approximately}500 K for Ti{sub 2}O{sub 3}, and 1070 K for NbO{sub 2}) showed that switching was related to the transition in the applied electric field. Channels consisting of the above-metioned lower oxides were formed in the initial anodic films during the electroforming. The possibility of formation of these oxides with a metal-insulator transition was confirmed by thermodynamic calculations.

  8. Transition metal catalysis in the generation of natural gas

    SciTech Connect

    Mango, F.D.

    1995-12-31

    The view that natural gas is thermolytic, coming from decomposing organic debris, has remained almost unchallenged for nearly half a century. Disturbing contradictions exist, however: Oil is found at great depth, at temperatures where only gas should exist and oil and gas deposits show no evidence of the thermolytic debris indicative of oil decomposing to gas. Moreover, laboratory attempts to duplicate the composition of natural gas, which is typically between 60 and 95+ wt% methane in C{sub 1}-C{sub 4}, have produced insufficient amounts of methane (10 to 60%). It has been suggested that natural gas may be generated catalytically, promoted by the transition metals in carbonaceous sedimentary rocks. This talk will discuss experimental results that support this hypothesis. Various transition metals, as pure compounds and in source rocks, will be shown to generate a catalytic gas that is identical to natural gas. Kinetic results suggest robust catalytic activity under moderate catagenetic conditions.

  9. Dynamics and Control in Complex Transition Metal Oxides

    NASA Astrophysics Data System (ADS)

    Zhang, J.; Averitt, R. D.

    2014-07-01

    Advances in the synthesis, growth, and characterization of complex transition metal oxides coupled with new experimental techniques in ultrafast optical spectroscopy have ushered in an exciting era of dynamics and control in these materials. Experiments utilizing femtosecond optical pulses can initiate and probe dynamics of the spin, lattice, orbital, and charge degrees of freedom. Major goals include (a) determining how interaction and competition between the relevant degrees of freedom determine macroscopic functionality in transition metal oxides (TMOs) and (b) searching for hidden phases in TMOs by controlling dynamic trajectories in a complex and pliable energy landscape. Advances in creating intense pulses from the far-IR spectrum through the visible spectrum enable mode-selective excitation to facilitate exploration of these possibilities. This review covers recent developments in this emerging field and presents examples that include the cuprates, manganites, and vanadates.

  10. A supramolecular microenvironment strategy for transition metal catalysis.

    PubMed

    Kaphan, David M; Levin, Mark D; Bergman, Robert G; Raymond, Kenneth N; Toste, F Dean

    2015-12-01

    A self-assembled supramolecular complex is reported to catalyze alkyl-alkyl reductive elimination from high-valent transition metal complexes [such as gold(III) and platinum(IV)], the central bond-forming elementary step in many catalytic processes. The catalytic microenvironment of the supramolecular assembly acts as a functional enzyme mimic, applying the concepts of enzymatic catalysis to a reactivity manifold not represented in biology. Kinetic experiments delineate a Michaelis-Menten-type mechanism, with measured rate accelerations (k(cat)/k(uncat)) up to 1.9 × 10(7) (here k(cat) and k(uncat) are the Michaelis-Menten enzymatic rate constant and observed uncatalyzed rate constant, respectively). This modality has further been incorporated into a dual catalytic cross-coupling reaction, which requires both the supramolecular microenvironment catalyst and the transition metal catalyst operating in concert to achieve efficient turnover. PMID:26785485

  11. Recent Advances in Transition Metal-Catalyzed Glycosylation

    PubMed Central

    McKay, Matthew J.; Nguyen, Hien M.

    2012-01-01

    Having access to mild and operationally simple techniques for attaining carbohydrate targets will be necessary to facilitate advancement in biological, medicinal, and pharmacological research. Even with the abundance of elegant reports for generating glycosidic linkages, stereoselective construction of α- and β-oligosaccharides and glycoconjugates is by no means trivial. In an era where expanded awareness of the impact we are having on the environment drives the state-of-the-art, synthetic chemists are tasked with developing cleaner and more efficient reactions for achieving their transformations. This movement imparts the value that prevention of waste is always superior to its treatment or cleanup. This review will highlight recent advancement in this regard by examining strategies that employ transition metal catalysis in the synthesis of oligosaccharides and glycoconjugates. These methods are mild and effective for constructing glycosidic bonds with reduced levels of waste through utilization of sub-stoichiometric amounts of transition metals to promote the glycosylation. PMID:22924154

  12. Mechanical properties of 4d transition metals in molten state

    NASA Astrophysics Data System (ADS)

    Singh, Deobrat; Sonvane, Yogesh; Thakor, P. B.

    2016-05-01

    Mechanical properties of 4d transition metals in molten state have been studied in the present study. We have calculated mechanical properties such as isothermal bulk modulus (B), modulus of rigidity (G), Young's modulus (Y) and Hardness have also been calculated from the elastic part of the Phonon dispersion curve (PDC). To describe the structural information, we have used different structure factor S(q) using Percus-Yevick hard sphere (PYHS) reference systems along with our newly constructed parameter free model potential.To see the influence of exchange and correlation effect on the above said properties of 3d liquid transition metals, we have used Sarkar et al (S)local field correction functions. Present results have been found good in agreement with available experimental data.

  13. Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Mak, Kin Fai; Shan, Jie

    2016-04-01

    Recent advances in the development of atomically thin layers of van der Waals bonded solids have opened up new possibilities for the exploration of 2D physics as well as for materials for applications. Among them, semiconductor transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se), have bandgaps in the near-infrared to the visible region, in contrast to the zero bandgap of graphene. In the monolayer limit, these materials have been shown to possess direct bandgaps, a property well suited for photonics and optoelectronics applications. Here, we review the electronic and optical properties and the recent progress in applications of 2D semiconductor transition metal dichalcogenides with emphasis on strong excitonic effects, and spin- and valley-dependent properties.

  14. Wetting Transitions of Inert Gases on Alkali Metal Surfaces

    NASA Astrophysics Data System (ADS)

    Bojan, M. J.; McDonald, I. A.; Cole, M. W.; Steele, W. A.

    1996-03-01

    Theoretical and experimental discoveries have been made recently of wetting and prewetting transitions of helium and hydrogen films on alkali metal surfaces [1,2]. New experiments show anomalous nonwetting behavior of Ne on Rb and Cs [3]. Building on earlier work [4], we have done and will describe results from the first Monte Carlo simulations showing wetting transitions for classical gases on alkali metal surfaces. * Research supported by an NSF Materials Research Group grant. 1. R. B.Hallock, J. Low Temp. Phys. 101, 31, 1995 2. M. W. Cole, J. Low Temp. Phys. 101, 25, 1995. 3. G. B. Hess, M. Sabatini, and M. H. W. Chan, unpublished 4. J. E. Finn and P. A. Monson, Phys. Rev. A 39, 6402, 1989.

  15. Ductile-to-brittle transition in spallation of metallic glasses

    SciTech Connect

    Huang, X.; Ling, Z.; Dai, L. H.

    2014-10-14

    In this paper, the spallation behavior of a binary metallic glass Cu{sub 50}Zr{sub 50} is investigated with molecular dynamics simulations. With increasing the impact velocity, micro-voids induced by tensile pulses become smaller and more concentrated. The phenomenon suggests a ductile-to-brittle transition during the spallation process. Further investigation indicates that the transition is controlled by the interaction between void nucleation and growth, which can be regarded as a competition between tension transformation zones (TTZs) and shear transformation zones (STZs) at atomic scale. As impact velocities become higher, the stress amplitude and temperature rise in the spall region increase and micro-structures of the material become more unstable. Therefore, TTZs are prone to activation in metallic glasses, leading to a brittle behavior during the spallation process.

  16. [Spectroscopic studies on transition metal ions in colored diamonds].

    PubMed

    Meng, Yu-Fei; Peng, Ming-Sheng

    2004-07-01

    Transition metals like nickel, cobalt and iron have been often used as solvent catalysts in high pressure high temperature (HPHT) synthesis of diamond, and nickel and cobalt ions have been found in diamond lattice. Available studies indicated that nickel and cobalt ions could enter the lattice as interstitial or substitutional impurities and form complexes with nitrogen. Polarized microscopy, SEM-EDS, EPR, PL and FTIR have been used in this study to investigate six fancy color natural and synthetic diamonds in order to determine the spectroscopic characteristics and the existing forms of transition metal ions in colored diamond lattice. Cobalt-related optical centers were first found in natural chameleon diamonds, and some new nickel and cobalt-related optical and EPR centers have also been detected in these diamond samples. PMID:15766067

  17. Metallic ferromagnetism-insulating charge order transition in doped manganites

    NASA Astrophysics Data System (ADS)

    Phan, Van-Nham; Ninh, Quoc-Huy; Tran, Minh-Tien

    2016-04-01

    We show that an interplay of double exchange and impurity randomness can explain the competition between metal-ferromagnetic and insulating charge ordered states in doped manganites. The double exchange is simplified in the Ising type, whereas the randomness is modeled by the Falicov-Kimball binary distribution. The combined model is considered in a framework of dynamical mean-field theory. Using the Kubo-Greenwood formalism, the transport coefficients are explicitly expressed in terms of single-particle spectral functions. Dividing the system into two sublattices we have pointed out a direct calculation to the checkerboard charge order parameter and the magnetizations. Numerical results show us that the checkerboard charge order can settle inside the ferromagnetic state at low temperature. An insulator-metal transition is also found at the point of the checkerboard charge order-ferromagnetic transition.

  18. Multireference Character for 4d Transition Metal-Containing Molecules.

    PubMed

    Wang, Jiaqi; Manivasagam, Sivabalan; Wilson, Angela K

    2015-12-01

    Four diagnostic criteria have been examined to identify the suitability of single-reference wave function-based quantum chemistry methods for a set of 118 4d transition metal species. These diagnostics include the weight of the leading configuration of the CASSCF wave function, C0(2); the Frobenius norm of the coupled cluster amplitude vector related to single excitations, T1; the matrix 2-norm of the coupled cluster T1 amplitude vector arising from coupled cluster calculations, D1; and the percent total atomization energy, %TAE, corresponding to a relationship between energies determined with CCSD and CCSD(T) calculations. New criteria, namely, T1 ≥ 0.045, D1 ≥ 0.120, and %TAE ≥ 10%, are herein proposed as a gauge for 4d transition metal-containing molecules to predict the possible need to employ multireference (MR) wave function-based methods to describe energetic and spectroscopic properties. PMID:26642991

  19. Metal-insulator and charge ordering transitions in oxide nanostructures

    NASA Astrophysics Data System (ADS)

    Singh, Sujay Kumar

    Strongly correlated oxides are a class of materials wherein interplay of various degrees of freedom results in novel electronic and magnetic phenomena. Vanadium oxides are widely studied correlated materials that exhibit metal-insulator transitions (MIT) in a wide temperature range from 70 K to 380 K. In this Thesis, results from electrical transport measurements on vanadium dioxide (VO2) and vanadium oxide bronze (MxV 2O5) (where M: alkali, alkaline earth, and transition metal cations) are presented and discussed. Although the MIT in VO2 has been studied for more than 50 years, the microscopic origin of the transition is still debated since a slew of external parameters such as light, voltage, and strain are found to significantly alter the transition. Furthermore, recent works on electrically driven switching in VO2 have shown that the role of Joule heating to be a major cause as opposed to electric field. We explore the mechanisms behind the electrically driven switching in single crystalline nanobeams of VO2 through DC and AC transport measurements. The harmonic analysis of the AC measurement data shows that non-uniform Joule heating causes electronic inhomogeneities to develop within the nanobeam and is responsible for driving the transition in VO2. Surprisingly, field assisted emission mechanisms such as Poole-Frenkel effect is found to be absent and the role of percolation is also identified in the electrically driven transition. This Thesis also provides a new insight into the mechanisms behind the electrolyte gating induced resistance modulation and the suppression of MIT in VO2. We show that the metallic phase of VO2 induced by electrolyte gating is due to an electrochemical process and can be both reversible and irreversible under different conditions. The kinetics of the redox processes increase with temperature; a complete suppression of the transition and the stabilization of the metallic phase are achievable by gating in the rutile metallic phase. First principles calculations show that the destabilization of the insulating phase during the gating arises due to the formation of oxygen vacancies in VO2; the rutile phase is far more amenable to electrochemical reduction as compared to the monoclinic phase, likely due to its higher electrical conductivity. The generation of oxygen vacancies appears thermodynamically favorable if the removed oxygen atoms from VO2 oxidize the anions in the ionic liquid. Finally, electronic properties of single crystalline, individual nanowires of vanadium oxide bronzes (MxVO 2O5) are presented. The intercalation effects of metal cation and the stoichiometry (x) are explored and discussed. These nanowires exhibit thermally and electrically driven charge ordering and metal to insulator transitions. The electrolyte gating measurements show resistance modulations across the phase transition but the effect is not as dramatic as in VO2.

  20. Electronic and bonding analysis of hardness in pyrite-type transition-metal pernitrides

    NASA Astrophysics Data System (ADS)

    Liu, Z. T. Y.; Gall, D.; Khare, S. V.

    2014-10-01

    Most commonly known hard transition-metal nitrides crystallize in rocksalt structure (B1). The discovery of ultraincompressible pyrite-type PtN2 10 years ago has raised a question about the cause of its exceptional mechanical properties. We answer this question by a systematic computational analysis of the pyrite-type PtN2 and other transition-metal pernitrides (MN2) with density functional theory. Apart from PtN2, the three hardest phases are found among them in the 3d transition-metal period. They are MnN2, CoN2, and NiN2, with computed Vickers hardness (HV) values of 19.9 GPa, 16.5 GPa, and 15.7 GPa, respectively. Harder than all of these is PtN2, with a HV of 23.5 GPa. We found the following trends and correlations that explain the origin of hardness in these pernitrides. (a) Charge transfer from M to N controls the length of the N-N bond, resulting in a correlation with bulk modulus, dominantly by providing Coulomb repulsion between the pairing N atoms. (b) Elastic constant C44, an indicator of mechanical stability and hardness is correlated with total density of states at EF, an indicator of metallicity. (c) Often cited monotonic variation of HV and Pugh's ratio with valence electron concentration found in rocksalt-type early transition-metal nitrides is not evident in this structure. (d) The change in M-M bond strength under a shearing strain indicated by crystal orbital Hamilton population is predictive of hardness. This is a direct connection between a specific bond and shear related mechanical properties. This panoptic view involving ionicity, metallicity, and covalency is essential to obtain a clear microscopic understanding of hardness.

  1. Pressure induced structural phase transition in IB transition metal nitrides compounds

    NASA Astrophysics Data System (ADS)

    Soni, Shubhangi; Kaurav, Netram; Jain, A.; Shah, S.; Choudhary, K. K.

    2015-06-01

    Transition metal mononitrides are known as refractory compounds, and they have, relatively, high hardness, brittleness, melting point, and superconducting transition temperature, and they also have interesting optical, electronic, catalytic, and magnetic properties. Evolution of structural properties would be an important step towards realizing the potential technological scenario of this material of class. In the present study, an effective interionic interaction potential (EIOP) is developed to investigate the pressure induced phase transitions in IB transition metal nitrides TMN [TM = Cu, Ag, and Au] compounds. The long range Coulomb, van der Waals (vdW) interaction and the short-range repulsive interaction upto second-neighbor ions within the Hafemeister and Flygare approach with modified ionic charge are properly incorporated in the EIOP. The vdW coefficients are computed following the Slater-Kirkwood variational method, as both the ions are polarizable. The estimated value of the phase transition pressure (Pt) and the magnitude of the discontinuity in volume at the transition pressure are consistent as compared to the reported data.

  2. The chemistry and physics of transition metal clusters

    SciTech Connect

    Parks, E.K.; Jellinek, J.; Knickelbein, M.B.; Riley, S.J.

    1994-06-01

    In this program the authors study the fundamental properties of isolated clusters of transition metal atoms. Experimental studies of cluster chemistry include determination of cluster structure, reactivity, and the nature of cluster-adsorbate interactions. Studies of physical properties include measurements of cluster ionization potentials and photoabsorption cross sections. Theoretical studies focus on the structure and dynamics of clusters, including isomers, phases and phase changes, interactions with molecules, and fragmentation process.

  3. Zwitterionic Group VIII transition metal initiators supported by olefin ligands

    DOEpatents

    Bazan, Guillermo C.; Chen, Yaofeng

    2011-10-25

    A zwitterionic Group VIII transition metal complex containing the simple and relatively small 3-(arylimino)-but-1-en-2-olato ligand that catalyzes the formation of polypropylene and high molecular weight polyethylene. A novel feature of this catalyst is that the active species is stabilized by a chelated olefin adduct. The present invention also provides methods of polymerizing olefin monomers using zwitterionic catalysts, particularly polypropylene and high molecular weight polyethylene.

  4. The energetics of ordered intermetallic alloys (of the transition metals)

    SciTech Connect

    Watson, R.E.; Weinert, M.; Davenport, J.W. ); Fernando, G.W. . Dept. of Physics); Bennett, L.H. . Metallurgy Div.)

    1992-01-01

    The atomically ordered phases in ordered transition metal alloys are discussed. This chapter is divided into: physical parameters controlling phase stability (Hume-Rothery, structural maps, Miedema Hamiltonian), wave functions band theory, comment on entropy terms, cohesive energies (electron promotion energies, Hund's rule on orbital effects), structural energies/stabilities of elemental solids, total energies and atomic positions, charge transfer (Au alloys, charge tailing), heats of formation of ordered compounds.

  5. The energetics of ordered intermetallic alloys (of the transition metals)

    SciTech Connect

    Watson, R.E.; Weinert, M.; Davenport, J.W.; Fernando, G.W.; Bennett, L.H.

    1992-10-01

    The atomically ordered phases in ordered transition metal alloys are discussed. This chapter is divided into: physical parameters controlling phase stability (Hume-Rothery, structural maps, Miedema Hamiltonian), wave functions & band theory, comment on entropy terms, cohesive energies (electron promotion energies, Hund`s rule on orbital effects), structural energies/stabilities of elemental solids, total energies and atomic positions, charge transfer (Au alloys, charge tailing), heats of formation of ordered compounds.

  6. Stability of Multiferroic Transition in FeVO4 against Transition Metal Doping

    NASA Astrophysics Data System (ADS)

    Kumarasiri, Akila; Dixit, Ambesh; Lawes, Gavin

    2012-02-01

    FeVO4 is a recently discovered multiferroic material which undergoes successive antiferromagnetic phase transitions at TN1 ˜ 22 K and TN2 ˜ 15 K, with ferroelectricity developing at the TN2 transition. FeVO4 is a type II multiferroic where the ferroelectricity is magnetically driven. We have studied the effect of transition metal doping on these two phase transitions in order to explore how the multiferroic order is affected by introducing perturbations into the lattice. We synthesized polycrystalline MxFe1-xVO4 samples (M = Zn, Mn, Cr) using a standard solid state reaction method, and we used magnetic, dielectric, and heat capacity measurements to track the transition temperatures. Both magnetic and heat capacity measurements show clear peaks at the two transitions, enabling us to map how the transitions are suppressed as the doping fraction is changed. On doping with non magnetic Zn, we find only a minimal suppression of both transition temperatures, indicating the magnetic interactions producing the multiferroic order are surprisingly robust against non-magnetic perturbations. We will also present preliminary results of the effects of magnetic dopants, specifically Mn and Cr.

  7. Kentucky Early Childhood Transition Project. Evaluation Report: Quality and Impact of Transition Training and Technical Assistance Services

    ERIC Educational Resources Information Center

    Interdisciplinary Human Development Institute, 2002

    2002-01-01

    The Kentucky Early Childhood Transition Project (KECTP) was funded in 1992 by the Kentucky Department of Education and later expanded with support from the Kentucky Early Intervention System, to provide training and technical assistance to local communities in the state of Kentucky on effective transitions. KECTP was an outgrowth of Project STEPS,…

  8. Thermodynamics of the structural transition in metal-organic frameworks.

    PubMed

    Rodriguez, J; Beurroies, I; Coulet, M-V; Fabry, P; Devic, T; Serre, C; Denoyel, R; Llewellyn, P L

    2016-03-01

    A thermodynamic study of the structural large-pore (LP) to narrow pore (NP) transition in various Metal Organic Frameworks (MOFs) is presented. First, the pressure induced transition at a constant temperature is investigated using a Tian-Calvet microcalorimeter set-up equipped with a high pressure cell. This device permits simultaneous measurements of the mechanical work and heat associated with the LP → NP transition. It is shown that MIL-53(Al) and MIL-53(Cr) have similar thermodynamic and mechanical behaviour whilst the MIL-47(V) system is characterized by much higher transition energy and mechanical work. Second, the temperature induced transition at ambient pressure is studied by means of differential scanning calorimetry (DSC) combined with X-ray absorption spectroscopy. This set-up enables one to follow simultaneously the structural changes associated with the phase transition detected by DSC. The MIL-53(Cr)-Br functionalized MOF is chosen here as a case study where both energetics and structural changes are discussed. PMID:26574728

  9. Speed limit of the insulator-metal transition in magnetite

    NASA Astrophysics Data System (ADS)

    de Jong, S.; Kukreja, R.; Trabant, C.; Pontius, N.; Chang, C. F.; Kachel, T.; Beye, M.; Sorgenfrei, F.; Back, C. H.; Bruer, B.; Schlotter, W. F.; Turner, J. J.; Krupin, O.; Doehler, M.; Zhu, D.; Hossain, M. A.; Scherz, A. O.; Fausti, D.; Novelli, F.; Esposito, M.; Lee, W. S.; Chuang, Y. D.; Lu, D. H.; Moore, R. G.; Yi, M.; Trigo, M.; Kirchmann, P.; Pathey, L.; Golden, M. S.; Buchholz, M.; Metcalf, P.; Parmigiani, F.; Wurth, W.; Fhlisch, A.; Schler-Langeheine, C.; Drr, H. A.

    2013-10-01

    As the oldest known magnetic material, magnetite (Fe3O4) has fascinated mankind for millennia. As the first oxide in which a relationship between electrical conductivity and fluctuating/localized electronic order was shown, magnetite represents a model system for understanding correlated oxides in general. Nevertheless, the exact mechanism of the insulator-metal, or Verwey, transition has long remained inaccessible. Recently, three-Fe-site lattice distortions called trimerons were identified as the characteristic building blocks of the low-temperature insulating electronically ordered phase. Here we investigate the Verwey transition with pump-probe X-ray diffraction and optical reflectivity techniques, and show how trimerons become mobile across the insulator-metal transition. We find this to be a two-step process. After an initial 300?fs destruction of individual trimerons, phase separation occurs on a 1.50.2?ps timescale to yield residual insulating and metallic regions. This work establishes the speed limit for switching in future oxide electronics.

  10. Magnetocrystalline anisotropy of 3d transition metal atoms on graphen

    NASA Astrophysics Data System (ADS)

    Hu, Jun; Wu, Ruqian

    2012-02-01

    Graphene has attracted most attention in the field of condensed matter physics, chemistry and material science since the first day when it was produced in lab. It's an ideal material for two-dimensional electron gas (2DEG). Most intriguingly, the electronic and magnetic properties can be easily engineered by decorating with external elemental atoms ranging from nonmetal to transition metal. Especially, graphene is attractive for spintronics due to its long spin life time and high mobility. So far, ultrathin Co films have been deposited on graphene which exhibit perpendicular magnetic anisotropy. In this work, first-principles calculations are performed to systematically study the magnetic properties of graphene decorated by 3d transition metal atoms with several covering patterns. We find that Fe/graphene always exhibits in-plane anisotropy regardless the coverage, while Mn/graphene and Co/graphene tend to have perpendicular easy axis for most range of coverage. The spin moments of Ni atoms are largely quenched for Ni/graphene. Moreover, the Mn atoms on graphene prefer ferromagnetic exchange coupling which are totally different from the pure counterpart without graphene support. The strong hybridization between 3d orbitals of transition metal atoms and pi states of graphene are responsible for the modification of magnetic properties. Acknowledgement. This work was supported by DOE Grant DE-FG02-05ER46237.

  11. Optical properties of transition metal oxide quantum wells

    SciTech Connect

    Lin, Chungwei; Posadas, Agham; Choi, Miri; Demkov, Alexander A.

    2015-01-21

    Fabrication of a quantum well, a structure that confines the electron motion along one or more spatial directions, is a powerful method of controlling the electronic structure and corresponding optical response of a material. For example, semiconductor quantum wells are used to enhance optical properties of laser diodes. The ability to control the growth of transition metal oxide films to atomic precision opens an exciting opportunity of engineering quantum wells in these materials. The wide range of transition metal oxide band gaps offers unprecedented control of confinement while the strong correlation of d-electrons allows for various cooperative phenomena to come into play. Here, we combine density functional theory and tight-binding model Hamiltonian analysis to provide a simple physical picture of transition metal oxide quantum well states using a SrO/SrTiO{sub 3}/SrO heterostructure as an example. The optical properties of the well are investigated by computing the frequency-dependent dielectric functions. The effect of an external electric field, which is essential for electro-optical devices, is also considered.

  12. Sensitivity of FRDA lymphoblasts to salts of transition metal ions.

    PubMed

    Wong, A; Yang, J; Danielson, S; Gellera, C; Taroni, F; Cortopassi, G

    2000-01-01

    Friedreich's ataxia (FRDA) is an autosomal recessive neurodegenerative disease resulting from decreased expression of the nuclear-encoded mitochondrial protein, frataxin. FRDA patients have characteristic iron deposits and dysfunction of mitochondrial enzymes in the heart. Inactivation of the frataxin homologue in yeast causes dysregulation of both mitochondrial iron levels and iron export. Previously, we have observed sensitivity of FRDA fibroblasts to FeCl3 and hydrogen peroxide, results consistent with the hypothesis that FRDA cells may experience increased Fenton chemistry. To determine whether the sensitivity of FRDA cells to transition metal ions is a general or specific property, we have compared the sensitivity of lymphoblasts from FRDA patients and healthy controls to the transition metal salts CoCl2, CuSO4 FeCl3 FeSO4, MnCl2, and ZnCl2. FRDA lymphoblasts were significantly more sensitive to FeCl3 and MnCl2 than control cells. However, there were no significant differences observed in sensitivity to CoCl2, CuSO4, FeSO4 and ZnCl2 in the concentration ranges studied. Thus, the sensitivity of FRDA lymphoblasts exposed to transition metals appears to be specific, and could be relevant to the pathophysiological mechanism, which is discussed. PMID:11229359

  13. Exploiting Semiconductor to Metallic Phase Transformation in Layered Transition Metal Dichalcogenides for Ohmic contact Contacts

    NASA Astrophysics Data System (ADS)

    Kappera, Rajesh; Voiry, Damien; Jen, Wesley; Yalcin, Sibel Ebru; Gupta, Gautam; Mohite, Aditya; Chhowalla, Manish; Material Science department, Rutgers University, Piscataway, NJ, 08854, USA Team; CenterIntegrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, NM, 87544, US Team

    2014-03-01

    Achieving ohmic contacts to transition metal dichalcogenides (MoS2, WS2, WSe2 and MoSe2) has been a challenge for researchers owing to the formation of a large Schottky barrier between metal and semiconductor. This results in low on-currents, mobilities and sub-threshold swings in the devices made with these materials. Here we report a universal strategy using chemical approach to reversibly transform the semiconducting phase (2H) to metallic phase (1T). Taking advantage of the metallic phase, we have fabricated hybrid transistors, which have 1T phase contacts and semiconducting 2H phase of the material as the channel. The metallic phase dramatically reduces the Schottky barrier between the metal and the semiconductor thereby mitigating the high contact resistance issues. This strategy should be applicable to several other applications such as catalysis, supercapacitors and batteries. Detailed synthesis, structural, electrical and optical characterization will be described.

  14. Atomically Thin Transition-Metal Dinitrides: High-Temperature Ferromagnetism and Half-Metallicity.

    PubMed

    Wu, Fang; Huang, Chengxi; Wu, Haiping; Lee, Changhoon; Deng, Kaiming; Kan, Erjun; Jena, Puru

    2015-12-01

    High-temperature ferromagnetic two-dimensional (2D) materials with flat surfaces have been a long-sought goal due to their potential in spintronics applications. Through comprehensive first-principles calculations, we show that the recently synthesized MoN2 monolayer is such a material; it is ferromagnetic with a Curie temperature of nearly 420 K, which is higher than that of any flat 2D magnetic materials studied to date. This novel property, made possible by the electron-deficient nitrogen ions, render transition-metal dinitrides monolayers with unique electronic properties which can be switched from the ferromagnetic metals in MoN2, ZrN2, and TcN2 to half-metallic ones in YN2. Transition-metal dinitrides monolayers may, therefore, serve as good candidates for spintronics devices. PMID:26575002

  15. Soldering of Carbon Materials Using Transition Metal Rich Alloys.

    PubMed

    Burda, Marek; Lekawa-Raus, Agnieszka; Gruszczyk, Andrzej; Koziol, Krzysztof K K

    2015-08-25

    Joining of carbon materials via soldering has not been possible up to now due to lack of wetting of carbons by metals at standard soldering temperatures. This issue has been a severely restricting factor for many potential electrical/electronic and mechanical applications of nanostructured and conventional carbon materials. Here we demonstrate the formation of alloys that enable soldering of these structures. By addition of several percent (2.5-5%) of transition metal such as chromium or nickel to a standard lead-free soldering tin based alloy we obtained a solder that can be applied using a commercial soldering iron at typical soldering temperatures of approximately 350 °C and at ambient conditions. The use of this solder enables the formation of mechanically strong and electrically conductive joints between carbon materials and, when supported by a simple two-step technique, can successfully bond carbon structures to any metal terminal. It has been shown using optical and scanning electron microscope images as well as X-ray diffraction patterns and energy dispersive X-ray mapping that the successful formation of carbon-solder bonds is possible, first, thanks to the uniform nonreactive dispersion of transition metals in the tin-based matrix. Further, during the soldering process, these free elements diffuse into the carbon-alloy border with no formation of brazing-like carbides, which would damage the surface of the carbon materials. PMID:26256042

  16. Thermophysical Property Measurements of Silicon-Transition Metal Alloys

    NASA Technical Reports Server (NTRS)

    Banish, R. Michael; Erwin, William R.; Sansoucie, Michael P.; Lee, Jonghyun; Gave, Matthew A.

    2014-01-01

    Metals and metallic alloys often have high melting temperatures and highly reactive liquids. Processing reactive liquids in containers can result in significant contamination and limited undercooling. This is particularly true for molten silicon and it alloys. Silicon is commonly termed "the universal solvent". The viscosity, surface tension, and density of several silicon-transition metal alloys were determined using the Electrostatic Levitator system at the Marshall Space Flight Center. The temperature dependence of the viscosity followed an Arrhenius dependence, and the surface tension followed a linear temperature dependence. The density of the melts, including the undercooled region, showed a linear behavior as well. Viscosity and surface tension values were obtain for several of the alloys in the undercooled region.

  17. Transition-metal prion protein attachment: Competition with copper

    NASA Astrophysics Data System (ADS)

    Hodak, Miroslav; Bernholc, Jerry

    2012-02-01

    Prion protein, PrP, is a protein capable of binding copper ions in multiple modes depending on their concentration. Misfolded PrP is implicated in a group of neurodegenerative diseases, which include ``mad cow disease'' and its human form, variant Creutzfeld-Jacob disease. An increasing amount of evidence suggests that attachment of non-copper metal ions to PrP triggers transformations to abnormal forms similar to those observed in prion diseases. In this work, we use hybrid Kohn-Sham/orbital-free density functional theory simulations to investigate copper replacement by other transition metals that bind to PrP, including zinc, iron and manganese. We consider all known copper binding modes in the N-terminal domain of PrP. Our calculations identify modes most susceptible to copper replacement and reveal metals that can successfully compete with copper for attachment to PrP.

  18. Method for treating rare earth-transition metal scrap

    DOEpatents

    Schmidt, Frederick A.; Peterson, David T.; Wheelock, John T.; Jones, Lawrence L.

    1992-12-29

    Rare earth-transition metal (e.g., iron) scrap (e.g., Nd-Fe-B scrap) is flux (slag) remelted to reduce tramp non-metallic impurities, such as oxygen and nitrogen, and metallic impurities, such as Li, Na, Al, etc., picked up by the scrap from previous fabrication operations. The tramp impurities are reduced to concentrations acceptable for reuse of the treated alloy in the manufacture of end-use articles, such as permanent magnets. The scrap is electroslag or inductoslag melted using a prefused, rare earth fluoride-bearing flux of CaF.sub.2, CaCl.sub.2 or mixtures thereof or the slag resulting from practice of the thermite reduction process to make a rare earth-iron alloy.

  19. Method for treating rare earth-transition metal scrap

    DOEpatents

    Schmidt, F.A.; Peterson, D.T.; Wheelock, J.T.; Jones, L.L.

    1992-12-29

    Rare earth-transition metal (e.g., iron) scrap (e.g., Nd-Fe-B scrap) is flux (slag) remelted to reduce tramp non-metallic impurities, such as oxygen and nitrogen, and metallic impurities, such as Li, Na, Al, etc., picked up by the scrap from previous fabrication operations. The tramp impurities are reduced to concentrations acceptable for reuse of the treated alloy in the manufacture of end-use articles, such as permanent magnets. The scrap is electroslag or inductoslag melted using a rare earth fluoride-bearing flux of CaF[sub 2], CaCl[sub 2] or mixtures thereof or the slag resulting from practice of the thermite reduction process to make a rare earth-iron alloy. 3 figs.

  20. Capacitive network near the metal insulator transition in Vanadium Dioxide

    NASA Astrophysics Data System (ADS)

    Ramirez, J. G.; Patino, E. J.; Schmidt, R.; Sharoni, A.; Gomez, M. E.; Schuller, I. K.

    2011-03-01

    Recent infrared spectroscopy and transport measurements in nano-scaled junction of VO2 have revealed the existence of phase separation into metallic and insulating phases. Here we present Impedance spectroscopy measurements performed in high quality Vanadium dioxide (VO2) thin films for the first time. This technique allows distinguishing between the resistive and capacitive response of the VO2 films and provides the dielectric properties across the metal-insulator transition (MIT). The film capacitance exhibits an unusual increase close to the MIT which implies the formation of a capacitor network produced by the nanoscale phase separation of metallic and insulating phases. This work has been supported by AFOSR, COLCIENCIAS, CENM and Ramon y Cajal Fellowship.

  1. Quantifying limits to detection of early warning for critical transitions

    PubMed Central

    Boettiger, Carl; Hastings, Alan

    2012-01-01

    Catastrophic regime shifts in complex natural systems may be averted through advanced detection. Recent work has provided a proof-of-principle that many systems approaching a catastrophic transition may be identified through the lens of early warning indicators such as rising variance or increased return times. Despite widespread appreciation of the difficulties and uncertainty involved in such forecasts, proposed methods hardly ever characterize their expected error rates. Without the benefits of replicates, controls or hindsight, applications of these approaches must quantify how reliable different indicators are in avoiding false alarms, and how sensitive they are to missing subtle warning signs. We propose a model-based approach to quantify this trade-off between reliability and sensitivity and allow comparisons between different indicators. We show these error rates can be quite severe for common indicators even under favourable assumptions, and also illustrate how a model-based indicator can improve this performance. We demonstrate how the performance of an early warning indicator varies in different datasets, and suggest that uncertainty quantification become a more central part of early warning predictions. PMID:22593100

  2. A Transition to Metallic Hydrogen: Evidence of the Plasma Phase Transition

    NASA Astrophysics Data System (ADS)

    Silvera, Isaac; Zaghoo, Mohamed; Salamat, Ashkan

    The insulator-metal transition in hydrogen is one of the most outstanding problems in condensed matter physics. The high-pressure metallic phase is now predicted to be liquid atomic from T =0 K to very high temperatures. We have conducted measurements of optical properties of hot dense hydrogen in the region of 1.1-1.7 Mbar and up to 2200 K in a diamond anvil cell using pulsed laser heating of the sample. We present evidence in two forms: a plateau in the heating curves (average laser power vs temperature) characteristic of a first-order phase transition with latent heat, and changes in transmittance and reflectance characteristic of a metal for temperatures above the plateau temperature. For thick films the reflectance saturates at ~0.5. The phase line of this transition has a negative slope in agreement with theories of the so-called plasma phase transition. The NSF, Grant DMR-1308641, the DOE Stockpile Stewardship Academic Alliance Program, Grant DE-FG52-10NA29656, and NASA Earth and Space Science Fellowship Program, Award NNX14AP17H supported this research.

  3. Polaronic Transport in Phosphate Glasses Containing Transition Metal Ions

    NASA Astrophysics Data System (ADS)

    Henderson, Mark

    The goal of this dissertation is to characterize the basic transport properties of phosphate glasses containing various amounts of TIs and to identify and explain any electronic phase transitions which may occur. The P2 O5-V2O5-WO3 (PVW) glass system will be analyzed to find the effect of TI concentration on conduction. In addition, the effect of the relative concentrations of network forming ions (SiO2 and P2O5) on transport will be studied in the P2O5-SiO2-Fe2O 3 (PSF) system. Also presented is a numerical study on a tight-binding model adapted for the purposes of modelling Gaussian traps, mimicking TI's, which are arranged in an extended network. The results of this project will contribute to the development of fundamental theories on the electronic transport in glasses containing mixtures of transition oxides as well as those containing multiple network formers without discernible phase separation. The present study on the PVW follows up on previous investigation into the effect on mixed transition ions in oxide glasses. Past research has focused on glasses containing transition metal ions from the 3d row. The inclusion of tungsten, a 5d transition metal, adds a layer of complexity through the mismatch of the energies of the orbitals contributing to localized states. The data have indicated that a transition reminiscent of a metal-insulator transition (MIT) occurs in this system as the concentration of tungsten increases. As opposed to some other MIT-like transitions found in phosphate glass systems, there seems to be no polaron to bipolaron conversion. Instead, the individual localization parameter for tungsten noticeably decreases dramatically at the transition point as well as the adiabaticity. Another distinctive feature of this project is the study of the PSF system, which contains two true network formers, phosphorous pentoxide (P2O 5) and silicon dioxide (SiO2). It is not usually possible to do a reliable investigation of the conduction properties of such glasses because the two network formers will tend to separate into different phases, making it difficult to obtain homogenous samples. The PSF system proved easier to study than other systems. The hopping in this system seems to be dominated by the Greaves mid-range mechanism. In addition, in samples containing the same proportion of iron, conductivities were found to not depend noticeably on composition, supporting the use of models focusing on the transition metal ions in calculating conductivity. Despite ostensibly changing the structural and metrical properties of the network, the ratio of the concentration of the network formers only appears to have an effect on the conductivity through changing the inter-atomic distance of iron. The numerical model adds to the evidence for the dominating contribution on the nearest-neighbor ordering of TI ions on the electrical properties of a glass; especially interesting is the reproducibility of the mixed-transition ion effect (MTE) in a numerical model where ensemble averages are taken over possible arrangements. It was also determined that the disorder arising from the spread between two types of traps can lead to a MIT as function of population. Finally, an outline of the notion of invariance in TI glasses is extended from work done by other authors, creating an opportunity for further research.

  4. Localization Transitions in 2D Normal Metal Films

    NASA Astrophysics Data System (ADS)

    Valles, James M., Jr.

    1996-03-01

    Much work has shown that no zero temperature metal to insulator transition exists in quasi-two dimensional (2d) metal films. All films exhibit arbitrarily high resistances when cooled to sufficiently low temperatures or made sufficiently thin. Nevertheless, a gradual transition from a metallic state of weakly localized electrons to an insulating state of strongly localized electrons occurs at finite temperatures. We have investigated( This work was performed in collaboration with Shih-Ying Hsu and James A. Chervenak. We gratefully acknowledge support from the Office of Naval Research and the National Science Foundation.) this transition in ultrathin quench condensed films of Au, Ag, Cu on Ge using the magnetoconductance (MC) as a probe of the nature, extended or localized, of the electronic states involved in transport. In the weakly localized regime, the phase coherent multiple scattering of the conduction electrons leads to the well known MC<0 in the presence of spin-orbit (so) scattering. In contrast, the effects of so scattering on the sign of the MC in the strongly localized regime has been controversial. We present evidence that the MC>0 but smaller, in this regime in the presence of so scattering. The conductance at which the MC changes sign appears independent of temperature or material. This result, taken with recent theory, suggests that there is a well defined conductance separating the weak and strong localization regimes. I will discuss our latest efforts to test the universality of this result, and its implications for the superconductor to insulator transition that occurs in similar films.

  5. Disorder and Metal-Insulator Transitions in Weyl Semimetals.

    PubMed

    Chen, Chui-Zhen; Song, Juntao; Jiang, Hua; Sun, Qing-feng; Wang, Ziqiang; Xie, X C

    2015-12-11

    The Weyl semimetal (WSM) is a newly proposed quantum state of matter. It has Weyl nodes in bulk excitations and Fermi arc surface states. We study the effects of disorder and localization in WSMs and find three novel phase transitions. (i) Two Weyl nodes near the Brillouin zone boundary can be annihilated pairwise by disorder scattering, resulting in the opening of a topologically nontrivial gap and a transition from a WSM to a three-dimensional quantum anomalous Hall state. (ii) When the two Weyl nodes are well separated in momentum space, the emergent bulk extended states can give rise to a direct transition from a WSM to a 3D diffusive anomalous Hall metal. (iii) Two Weyl nodes can emerge near the zone center when an insulating gap closes with increasing disorder, enabling a direct transition from a normal band insulator to a WSM. We determine the phase diagram by numerically computing the localization length and the Hall conductivity, and propose that the novel phase transitions can be realized on a photonic lattice. PMID:26705648

  6. Mixed-Ligand Approach to Changing the Metal Ratio in Bismuth-Transition Metal Heterometallic Precursors.

    PubMed

    Lieberman, Craig M; Wei, Zheng; Filatov, Alexander S; Dikarev, Evgeny V

    2016-04-18

    A new series of heteroleptic bismuth-transition metal β-diketonates [BiM(hfac)3(thd)2] (M = Mn (1), Co (2), and Ni (3); hfac = hexafluoroacetylacetonate, thd = tetramethylheptanedionate) with Bi:M = 1:1 ratio have been synthesized by stoichiometric reactions between homometallic reagents [Bi(III)(hfac)3] and [M(II)(thd)2]. On the basis of analysis of the metal-ligand interactions in heterometallic structures, the title compounds were formulated as ion-pair {[Bi(III)(thd)2](+)[M(II)(hfac)3](-)} complexes. The direct reaction between homometallic reagents proceeds with a full ligand exchange between main group and transition metal centers, yielding dinuclear heterometallic molecules. In heteroleptic molecules 1-3, the Lewis acidic, coordinatively unsaturated Bi(III) centers are chelated by two bulky, electron-donating thd ligands and maintain bridging interactions with three oxygen atoms of small, electron-withdrawing hfac groups that chelate the neighboring divalent transition metals. Application of the mixed-ligand approach allows one to change the connectivity pattern within the heterometallic assembly and to isolate highly volatile precursors with the proper Bi:M = 1:1 ratio. The mixed-ligand approach employed in this work opens broad opportunities for the synthesis of heterometallic (main group-transition metal) molecular precursors with specific M:M' ratio in the case when homoleptic counterparts either do not exist or afford products with an incorrect metal:metal ratio for the target materials. Heteroleptic complexes obtained in the course of this study represent prospective single-source precursors for the low-temperature preparation of multiferroic perovskite-type oxides. PMID:27054922

  7. Effect of Transition Metals on Polysialic Acid Structure and Functions.

    PubMed

    Murthy, Raghavendra Vasudeva; Bharate, Priya; Gade, Madhuri; Sangabathuni, Sivakoti; Kikkeri, Raghavendra

    2016-04-01

    Polysialic acid (PSA) is one of the most abundant glycopolymer present in embryonic brain, and it is known to be involved in key roles such as plasticity in the central nervous system, cell adhesion, migration and localization of neurotrophins. However, in adult brain, its expression is quite low. The exception to this is in Alzheimer's disease (AD) brain, where significantly increased levels of polysilylated neural cell adhesion molecule (PSA-NCAM) have been reported. Here, we confirm the role of PSA as a metal chelator, allowing it to decrease cytotoxicity caused by high levels of transition metals, commonly found in AD brain, and as a regulator of cell behavior. UV-visible (UV-vis) and circular dichroism (CD) spectroscopy, atomic force microscopy (AFM), and isothermal titration calorimetry (ITC) techniques were used to investigate the assembly of PSA-metals complexes. These PSA-metal complexes exhibited less toxicity compared to free metal ions, and in particular, the PSA-Cu(2+) complex synergistically promoted neurite outgrowth in PC12 cells. PMID:26990355

  8. Probing early-universe phase transitions with CMB spectral distortions

    NASA Astrophysics Data System (ADS)

    Amin, Mustafa A.; Grin, Daniel

    2014-10-01

    Global, symmetry-breaking phase transitions in the early universe can generate scaling seed networks which lead to metric perturbations. The acoustic waves in the photon-baryon plasma sourced by these metric perturbations, when Silk damped, generate spectral distortions of the cosmic microwave background (CMB). In this work, the chemical potential distortion (μ ) due to scaling seed networks is computed and the accompanying Compton y -type distortion is estimated. The specific model of choice is the O (N ) nonlinear σ -model for N ≫1 , but the results remain the same order of magnitude for other scaling seeds. If CMB anisotropy constraints to the O (N ) model are saturated, the resulting chemical potential distortion μ ≲2 ×1 0-9 .

  9. Comparative study of the synthesis of layered transition metal molybdates

    SciTech Connect

    Mitchell, S.; Gomez-Aviles, A.; Gardner, C.; Jones, W.

    2010-01-15

    Mixed metal oxides (MMOs) prepared by the mild thermal decomposition of layered double hydroxides (LDHs) differ in their reactivity on exposure to aqueous molybdate containing solutions. In this study, we investigate the reactivity of some T-Al containing MMOs (T=Co, Ni, Cu or Zn) towards the formation of layered transition metal molybdates (LTMs) possessing the general formula AT{sub 2}(OH)(MoO{sub 4}){sub 2}.H{sub 2}O, where A=NH{sub 4}{sup +}, Na{sup +} or K{sup +}. The phase selectivity of the reaction was studied with respect to the source of molybdate, the ratio of T to Mo and the reaction pH. LTMs were obtained on reaction of Cu-Al and Zn-Al containing MMOs with aqueous solutions of ammonium heptamolybdate. Rehydration of these oxides in the presence of sodium or potassium molybdate yielded a rehydrated LDH phase as the only crystalline product. The LTM products obtained by the rehydration of MMO precursors were compared with LTMs prepared by direct precipitation from the metal salts in order to study the influence of preparative route on their chemical and physical properties. Differences were noted in the composition, morphology and thermal properties of the resulting products. - Graphical abstract: Mixed metal oxides (MMOs) derived from layered double hydroxide precursors differ in their reactivity on exposure to aqueous molybdate containing solutions. We investigate the influence of the molybdate source, the rehydration pH and the ratio of T/Mo on the reactivity of some T-Al containing MMOs (T=Co, Ni, Cu or Zn) towards the formation of layered transition metal molybdates of general formula AT{sub 2}(OH)(MoO{sub 4}){sub 2}.H{sub 2}O (where A{sup +}=NH{sub 4}{sup +}, K{sup +} or Na{sup +}).

  10. Mechanically induced metal-insulator transition in carbyne.

    PubMed

    Artyukhov, Vasilii I; Liu, Mingjie; Yakobson, Boris I

    2014-08-13

    First-principles calculations for carbyne under strain predict that the Peierls transition from symmetric cumulene to broken-symmetry polyyne structure is enhanced as the material is stretched. Interpretation within a simple and instructive analytical model suggests that this behavior is valid for arbitrary 1D metals. Further, numerical calculations of the anharmonic quantum vibrational structure of carbyne show that zero-point atomic vibrations eliminate the Peierls distortion in the mechanically free chain, preserving the cumulene symmetry. The emergence and increase of Peierls dimerization under tension then implies a qualitative transition between the two forms, which our computations place around 3% strain. Thus, the competition between the zero-point vibrations and mechanical strain determines a switch in symmetry resulting in the transition from metallic state to a dielectric, with a small effective mass and a high carrier mobility. In any practical realization, it is important that the effect is also chemically modulated by the choice of terminating groups. These findings are promising for applications such as electromechanical switching and band gap tuning via strain, and besides carbyne itself, they directly extend to numerous other systems that show Peierls distortion. PMID:24991984

  11. XO-2b: Transiting Hot Jupiter in a Metal-rich Common Proper Motion Binary

    NASA Astrophysics Data System (ADS)

    Burke, Christopher J.; McCullough, P. R.; Valenti, Jeff A.; Johns-Krull, Christopher M.; Janes, Kenneth A.; Heasley, J. N.; Summers, F. J.; Stys, J. E.; Bissinger, R.; Fleenor, Michael L.; Foote, Cindy N.; García-Melendo, Enrique; Gary, Bruce L.; Howell, P. J.; Mallia, F.; Masi, G.; Taylor, B.; Vanmunster, T.

    2007-12-01

    We report on a V=11.2 early K dwarf, XO-2 (GSC 03413-00005), that hosts a Rp=0.98+/-0.030.01 RJ, Mp=0.57+/-0.06 MJ transiting extrasolar planet, XO-2b, with an orbital period of 2.615857+/-0.000005 days. XO-2 has high metallicity, [Fe/H]=0.45+/-0.02, high proper motion, μtot=157 mas yr-1, and a common proper motion stellar companion with 31" separation. The two stars are nearly identical twins, with very similar spectra and apparent magnitudes. Due to the high metallicity, these early K dwarf stars have a mass and radius close to solar, M*=0.98+/-0.02 Msolar and R*=0.97+/-0.020.01 Rsolar. The high proper motion of XO-2 results from an eccentric orbit (Galactic pericenter, Rper<4 kpc) well confined to the Galactic disk (Zmax~100 pc). In addition, the phase-space position of XO-2 is near the Hercules dynamical stream, which points to an origin of XO-2 in the metal-rich, inner thin disk and subsequent dynamical scattering into the solar neighborhood. We describe an efficient Markov chain Monte Carlo algorithm for calculating the Bayesian posterior probability of the system parameters from a transit light curve.

  12. Energy transitions in the early 21st Century

    SciTech Connect

    Paul Meakin

    2013-01-01

    We are in the early stages of a long and complex transition from a global economy based on fossil energy to an economy based on low carbon renewable energy. However, fossil fuel resources are abundant and widely distributed, and they will remain the dominant source of primary energy for at least the next quarter century. In the United States, displacement of coal by natural gas for electric power generation has done more to reduce CO2 emissions than all new renewables combined, and this may occur globally for the next decade or two, even if the European Union does not take advantage of its large unconventional natural gas resources. Greater energy efficiency (not including the efficiencies associated with displacement of coal by gas) will also be more important than new renewables. Cost/benefit ratios are important for sustainability of the transition, and some energy efficiency technologies and displacement of coal by natural gas have lower cost/benefit ratios than wind power, solar power or biofuels. Money spent on the large scale deployment of wind, solar and especially biofuels would be better spent on research, development and demonstration of a broader suite of technologies that would support the energy transition, with a focus on improving the cost benefit ratios of already deployed technologies and developing alternatives. Advanced nuclear reactors, engineered geothermal systems, fossil fuel recovery coupled with CO2 sequestration and pre-combustion or post-combustion decarbonation of fossil fuels with geological CO2 sequestration are among the technologies that might be more cost effective than wind, solar or biofuels, and biofuels have serious adverse societal and environment consequences.

  13. Metal-Insulator Transition of Correlated Systems and Origin of Unusual Metal

    NASA Astrophysics Data System (ADS)

    Imada, Masatoshi

    1995-08-01

    Transitions between the Mott insulator and metals in cleansystems are analyzed with the scaling theory in terms of quantumcritical phenomena. In the case of generic transitions controlled bythe filling, the scaling theory is established from analyses of theDrude weight and the compressibility based on hyperscaling. In thetransition to the Mott insulator, a new universality class with thecorrelation length exponent ?<1/2 and the dynamical exponent z>2 isderived, which is in contrast to the transition to the band insulatorcharacterized by ?=1/2 and z=2. The unusual exponents lead tovarious anomalous characters of metals near the Mott insulator such asstrong suppression of the degeneracy temperature, and nonlineardependence of the Drude weight on the doping concentration. Remarkableproperties are also found in the specific heat, the compressibilityand spin correlations. A mechanism of high temperaturesuperconductivity is discussed in terms of the release from thesuppressed coherence.

  14. Enhancing conductivity of metallic carbon nanotube networks by transition metal adsorption

    SciTech Connect

    Ketolainen, T. Havu, V.; Puska, M. J.

    2015-02-07

    The conductivity of carbon nanotube thin films is mainly determined by carbon nanotube junctions, the resistance of which can be reduced by several different methods. We investigate electronic transport through carbon nanotube junctions in a four-terminal configuration, where two metallic single-wall carbon nanotubes are linked by a group 6 transition metal atom. The transport calculations are based on the Green’s function method combined with the density-functional theory. The transition metal atom is found to enhance the transport through the junction near the Fermi level. However, the size of the nanotube affects the improvement in the conductivity. The enhancement is related to the hybridization of chromium and carbon atom orbitals, which is clearly reflected in the character of eigenstates near the Fermi level. The effects of chromium atoms and precursor molecules remaining adsorbed on the nanotubes outside the junctions are also examined.

  15. Roles of transition metals interchanging with lithium in electrode materials.

    PubMed

    Kawaguchi, Tomoya; Fukuda, Katsutoshi; Tokuda, Kazuya; Sakaida, Masashi; Ichitsubo, Tetsu; Oishi, Masatsugu; Mizuki, Jun'ichiro; Matsubara, Eiichiro

    2015-06-01

    Roles of antisite transition metals interchanging with Li atoms in electrode materials of Li transition-metal complex oxides were clarified using a newly developed direct labeling method, termed powder diffraction anomalous fine structure (P-DAFS) near the Ni K-edge. We site-selectively investigated the valence states and local structures of Ni in Li0.89Ni1.11O2, where Ni atoms occupy mainly the NiO2 host-layer sites and partially the interlayer Li sites in-between the host layers, during electrochemical Li insertion/extraction in a lithium-ion battery (LIB). The site-selective X-ray near edge structure evaluated via the P-DAFS method revealed that the interlayer Ni atoms exhibited much lower electrochemical activity as compared to those at the host-layer site. Furthermore, the present analyses of site-selective extended X-ray absorption fine structure performed using the P-DAFS method indicates local structural changes around the residual Ni atoms at the interlayer space during the initial charge; it tends to gather to form rock-salt NiO-like domains around the interlayer Ni. The presence of the NiO-like domains in the interlayer space locally diminishes the interlayer distance and would yield strain energy because of the lattice mismatch, which retards the subsequent Li insertion both thermodynamically and kinetically. Such restrictions on the Li insertion inevitably make the NiO-like domains electrochemically inactive, resulting in an appreciable irreversible capacity after the initial charge but an achievement of robust linkage of neighboring NiO2 layers that tend to be dissociated without the Li occupation. The P-DAFS characterization of antisite transition metals interchanging with Li atoms complements the understanding of the detailed charge-compensation and degradation mechanisms in the electrode materials. PMID:25959625

  16. Charge-ordering transition in ultrathin metal films

    NASA Astrophysics Data System (ADS)

    Carpinelli, Joseph Michael

    It is well established that charge-ordering transitions are more likely to exist in materials of reduced dimensions, such as the quasi-one-dimensional and quasi-two-dimensional crystals. One might therefore expect the crystal surface environment (˜two dimensional) to likewise invite such phenomena, but few (if any) examples have yet been discovered. In this thesis, I present the first direct observation of a charge density wave (CDW) localized at a vacuum interface, and independent of bulk phenomena. With decreasing temperatures, low density overlayers of both Sn and Pb atop Ge(111) transform from the room-temperature metallic (surd 3 × surd 3)R30sp° structure to the low-temperature 3 x 3 structure gradually and reversibly. This new phase displays significant charge ordering (as observed with scanning tunneling microscopy) accompanied by a lattice distortion (as observed with electron diffraction). The transitions are driven by electron-phonon coupling resulting from a strongly nested Fermi surface. Although these iso-valent and iso-structural interfaces share many common properties, they have radically different electronic ground states (as observed with energy loss spectroscopy): the low-temperature Pb overlayer is semiconducting while the Sn overlayer remains metallic. Electron correlation effects must be considered in order to understand the Pb overlayer's metal-nonmetal transition. These systems have allowed for thorough investigation into the origin of charge ordering. Also, the interaction of CDW domains with defects and each other (CDW domain boundaries) were studied. My observation of room-temperature Friedel oscillations with the same symmetry as the low-temperature CDW relates the two as different limits of the same phenomenon, verifying that Fermi contour shape dictates screening at all temperatures.

  17. Low temperature synthesis of transition metal oxides containing surfactant ions

    NASA Astrophysics Data System (ADS)

    Janauer, Gerald Gilbert

    1998-11-01

    Recently there has been much interest in reacting vanadium oxides hydrothermally with cationic surfactants to form novel layered compounds. A series of new transition metal oxides, however, has also been formed at or near room temperature in open containers. Synthesis, characterization, and proposed mechanisms of formation are the focus of this work. Low temperature reactions of vanadium pentoxide and ammonium transition metallates with long chain amine surfactants, such as dodecyltrimethylammonium bromide yielded interesting new products many of which are layered phases. DTAsb4\\ Hsb2Vsb{10}Osb{28}. 8Hsb2O, a layered highly crystalline phase, is the first such phase for which a single crystal X-ray structure has been determined. The unit cell for this material was found to be triclinic with space group P1-, cell parameters a=9.8945(3)A, b=11.5962(1)A, c=21.9238(2)A, alpha=95.153(2)sp°,\\ beta=93.778(1)sp°, and gamma=101.360(1)sp°. Additionally, a novel tungsten, a molybdenum and a dichromate phase will be discussed. Both the tungsten and the dichromate materials were indexed from their powder diffraction patterns yielding monoclinic unit cells. The tungsten material was found to have a=50.56(4)A, b=54.41(4)A, c=13.12(1)A, and beta=99.21sp°. The dichromate compound was determined to have a=26.757(5)A, b=10.458(2)A, c=14.829(3)A and beta=98.01(1)sp°. Interlayer spacings for the lamellar dichromate and molybdenum phases were d001 = 28.7 A, and d001 = 22.9 A. The synthesis, characterization, composition, and structure of these transition metal oxide-surfactant materials will be discussed.

  18. Storing excitons in transition-metal dichalcogenides using dark states

    NASA Astrophysics Data System (ADS)

    Gunlycke, Daniel; Tseng, Frank; Simsek, Ergun

    Monolayer transition-metal dichalcogenides exhibit strongly bound excitons confined to two dimensions. One challenge in exploiting these excitons is that they have a finite life time and collapse through electron-hole recombination. We propose that the exciton life time could be extended by transitioning the exciton population into dark states. The symmetry of these dark states require the electron and hole to be spatially separated, which not only causes these states to be optically inactive but also inhibits electron-hole recombination. Based on an atomistic model we call the Triangular Lattice Exciton (3ALE) model, we derive transition matrix elements and approximate selection rules showing that excitons could be transitioned into and out of dark states using a pulsed infrared laser. For illustration, we also present exciton population scenarios based on different recombination decay constants. Longer exciton lifetimes could make these materials candidates for applications in energy management and quantum information processing. This work was supported by the Office of Naval Research, directly and through the Naval Research Laboratory.

  19. Spin-exchange interaction between transition metals and metalloids in soft-ferromagnetic metallic glasses.

    PubMed

    Das, Santanu; Choudhary, Kamal; Chernatynskiy, Aleksandr; Choi Yim, Haein; Bandyopadhyay, Asis K; Mukherjee, Sundeep

    2016-06-01

    High-performance magnetic materials have immense industrial and scientific importance in wide-ranging electronic, electromechanical, and medical device technologies. Metallic glasses with a fully amorphous structure are particularly suited for advanced soft-magnetic applications. However, fundamental scientific understanding is lacking for the spin-exchange interaction between metal and metalloid atoms, which typically constitute a metallic glass. Using an integrated experimental and molecular dynamics approach, we demonstrate the mechanism of electron interaction between transition metals and metalloids. Spin-exchange interactions were investigated for a Fe-Co metallic glass system of composition [(Co1-x Fe x )0.75B0.2Si0.05]96Cr4. The saturation magnetization increased with higher Fe concentration, but the trend significantly deviated from simple rule of mixtures. Ab initio molecular dynamics simulation was used to identify the ferromagnetic/anti-ferromagnetic interaction between the transition metals and metalloids. The overlapping band-structure and density of states represent 'Stoner type' magnetization for the amorphous alloys in contrast to 'Heisenberg type' in crystalline iron. The enhancement of magnetization by increasing iron was attributed to the interaction between Fe 3d and B 2p bands, which was further validated by valence-band study. PMID:27143686

  20. Spin-exchange interaction between transition metals and metalloids in soft-ferromagnetic metallic glasses

    NASA Astrophysics Data System (ADS)

    Das, Santanu; Choudhary, Kamal; Chernatynskiy, Aleksandr; Choi Yim, Haein; Bandyopadhyay, Asis K.; Mukherjee, Sundeep

    2016-06-01

    High-performance magnetic materials have immense industrial and scientific importance in wide-ranging electronic, electromechanical, and medical device technologies. Metallic glasses with a fully amorphous structure are particularly suited for advanced soft-magnetic applications. However, fundamental scientific understanding is lacking for the spin-exchange interaction between metal and metalloid atoms, which typically constitute a metallic glass. Using an integrated experimental and molecular dynamics approach, we demonstrate the mechanism of electron interaction between transition metals and metalloids. Spin-exchange interactions were investigated for a Fe–Co metallic glass system of composition [(Co1‑x Fe x )0.75B0.2Si0.05]96Cr4. The saturation magnetization increased with higher Fe concentration, but the trend significantly deviated from simple rule of mixtures. Ab initio molecular dynamics simulation was used to identify the ferromagnetic/anti-ferromagnetic interaction between the transition metals and metalloids. The overlapping band-structure and density of states represent ‘Stoner type’ magnetization for the amorphous alloys in contrast to ‘Heisenberg type’ in crystalline iron. The enhancement of magnetization by increasing iron was attributed to the interaction between Fe 3d and B 2p bands, which was further validated by valence-band study.

  1. Transition metal complexes of an isatinic quinolyl hydrazone

    PubMed Central

    2011-01-01

    Background The importance of the isatinic quinolyl hydrazones arises from incorporating the quinoline ring with the indole ring in the same compound. Quinoline ring has therapeutic and biological activities. On the other hand, isatin (1H-indole-2,3-dione) and its derivatives exhibit a wide range of biological activities. Also, the indole ring occurs in Jasmine flowers and Orange blossoms. Recently, the physiological and biological activities of quinolyl hydrazones arise from their tendency to form metal chelates with transition metal ions. In this context, we have reported to isolate, characterize and study the biological activity of some transition metal complexes of an isatinic quinolyl hydrazone; 3-[2-(4-methyl quinolin-2-yl)hydrazono] indolin-2-one. Results Mono- and binuclear as well as dimeric chelates were obtained from the reaction of a new isatinic quinolyl hydrazone with Fe(III), Co(II), Ni(II), Cu(II), VO(II) and Pd(II) ions. The ligand showed a variety of modes of bonding viz. (NNO)2-, (NO)- and (NO) per each metal ion supporting its ambidentate and flexidentate characters. The mode of bonding and basicity of the ligand depend mainly on the type of the metal cation and its counter anion. All the obtained Pd(II)- complexes have the preferable square planar geometry (D4h- symmetry) and depend mainly on the mole ratio (M:L). Conclusion The effect of the type of the metal ion for the same anion (Cl-) is obvious from either structural diversity of the isolated complexes (Oh, Td and D4h) or the various modes of bonding. The isatinic hydrazone uses its lactim form in all complexes (Cl-) except complex 5 (SO42-) in which it uses its lactam form. The obtained Pd(II)- complexes (dimeric, mono- and binuclear) are affected by the mole ratio (M:L) and have the square planar (D4h) geometry. Also, the antimicrobial activity is highly influenced by the nature of the metal ion and the order for S. aureus bacteria is as follows: Nickel(II) > Vanadyl(II) > Cobalt(II) > Copper(II) ≈ Palladium(II) >> Iron(III). PMID:21708023

  2. Theoretical study of transition-metal ions bound to benzene

    NASA Technical Reports Server (NTRS)

    Bauschlicher, Charles W., Jr.; Partridge, Harry; Langhoff, Stephen R.

    1992-01-01

    Theoretical binding energies are reported for all first-row and selected second-row transition metal ions (M+) bound to benzene. The calculations employ basis sets of at least double-zeta plus polarization quality and account for electron correlation using the modified coupled-pair functional method. While the bending is predominantly electrostatic, the binding energies are significantly increased by electron correlation, because the donation from the metal d orbitals to the benzene pi* orbitals is not well described at the self-consistent-field level. The uncertainties in the computed binding energies are estimated to be about 5 kcal/mol. Although the calculated and experimental binding energies generally agree to within their combined uncertainties, it is likely that the true binding energies lie in the lower portion of the experimental range. This is supported by the very good agreement between the theoretical and recent experimental binding energies for AgC6H6(+).

  3. Control of plasmonic nanoantennas by reversible metal-insulator transition

    PubMed Central

    Abate, Yohannes; Marvel, Robert E.; Ziegler, Jed I.; Gamage, Sampath; Javani, Mohammad H.; Stockman, Mark I.; Haglund, Richard F.

    2015-01-01

    We demonstrate dynamic reversible switching of VO2 insulator-to-metal transition (IMT) locally on the scale of 15 nm or less and control of nanoantennas, observed for the first time in the near-field. Using polarization-selective near-field imaging techniques, we simultaneously monitor the IMT in VO2 and the change of plasmons on gold infrared nanoantennas. Structured nanodomains of the metallic VO2 locally and reversibly transform infrared plasmonic dipole nanoantennas to monopole nanoantennas. Fundamentally, the IMT in VO2 can be triggered on femtosecond timescale to allow ultrafast nanoscale control of optical phenomena. These unique features open up promising novel applications in active nanophotonics. PMID:26358623

  4. Augmenting Molecular Junctions with Different Transition Metal Contacts

    NASA Astrophysics Data System (ADS)

    Kaur, Rupan Preet; Sawhney, Ravinder Singh; Engles, Derick

    2013-11-01

    In this research paper, the effect of the material of electrodes at the nanometer scale was elucidated towards measuring the electron transport properties of a single molecular junction comprising of anthracenedithiol molecule (ADT) stringed to two semi-infinite metallic electrodes using Extended Huckle Theory (EHT)-based semi-empirical modelling approach. The electron transport parameters i.e., I-V curves, Conductance-Voltage curves and transmission spectrum were investigated through ADT molecule by buffering it between different electrodes composed of rhodium, palladium, nickel and copper, all from transition metals series, under finite bias voltages within Keldysh's non equilibrium green function formulism (NEGF). The simulated results revealed that the copper electrodes showed maximum conduction whereas palladium showed least. The maximum conductance of 0.82 G0 and 43 μA current was exhibited by copper and thus affirmed to be the most effective electrode at nanometre scale when compared with other electrodes viz. nickel, rhodium and palladium.

  5. Dirac cones in transition metal doped boron nitride

    SciTech Connect

    Feng, Min; Cao, Xuewei; Shao, Bin; Zuo, Xu

    2015-05-07

    The transition metal (TM) doped zinc blende boron nitride (c-BN) is studied by using the first principle calculation. TM atoms fill in the interstitials in c-BN and form two-dimensional honeycomb lattice. The generalized gradient approximation and projector augmented wave method are used. The calculated density of states and band structures show that d electrons of TM atoms form impurity bands in the gap of c-BN. When the TM-BN system is in ferromagnetic or non-magnetic state, Dirac cones emerge at the K point in Brillouin zone. When TM is Ti and Co, the Dirac cones are spin polarized and very close to the Fermi level, which makes them promising candidates of Dirac half-metal [H. Ishizuka and Y. Motome, Phys. Rev. Lett. 109, 237207 (2012)]. While TM is Ni and Cu, the system is non-magnetic and Dirac cones located above the Fermi level.

  6. Dinuclear transition metal complexes in carbon nanostructured materials synthesis

    NASA Astrophysics Data System (ADS)

    Ayuso, J. I.; Hernández, E.; Delgado, E.

    2013-06-01

    Carbon nanomaterials (CNMs) were prepared with two similar techniques using organometallic complexes as catalysts precursors. Chemical vapour deposition (CVD) and pyrolysis with chlorine gas approaches were employed in order to explore the effect of dinuclear transition metal compounds [Fe2(CO)6(μ-S2C6H2X2), (X=OH, Cl)] in synthesis of CNMs. Our to-date results have shown these complexes generate different carbonaceous materials when they are used in bulk, it was also observed that their performances in synthesis differ even though these compounds are analogous. With X=OH complex used in CVD process, metal nanoparticles of ca. 20-50 nm in size and embedded in carbon matrix were obtained. X=C1 complex has been used in pyrolysis experiments and showed an entire volatilisation or no reaction, depending on selected temperature. Furthermore, obtaining of a new tetranuclear iron cluster is presented in this work.

  7. Theory of the pairbreaking superconductor-metal transition in nanowires

    SciTech Connect

    Del Maestro, Adrian Rosenow, Bernd; Sachdev, Subir

    2009-03-15

    We present a detailed description of a zero temperature phase transition between superconducting and diffusive metallic states in very thin wires due to a Cooper pair breaking mechanism. The dissipative critical theory contains current reducing fluctuations in the guise of both quantum and thermally activated phase slips. A full cross-over phase diagram is computed via an expansion in the inverse number of complex components of the superconducting order parameter (one in the physical case). The fluctuation corrections to the electrical ({sigma}) and thermal ({kappa}) conductivities are determined, and we find that {sigma} has a non-monotonic temperature dependence in the metallic phase which may be consistent with recent experimental results on ultra-narrow wires. In the quantum critical regime, the ratio of the thermal to electrical conductivity displays a linear temperature dependence and thus the Wiedemann-Franz law is obeyed, with a new universal experimentally verifiable Lorenz number.

  8. Ising superconductivity and Majorana fermions in transition-metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Zhou, Benjamin T.; Yuan, Noah F. Q.; Jiang, Hong-Liang; Law, K. T.

    2016-05-01

    In monolayer transition-metal dichalcogenides (TMDs), electrons in opposite K valleys are subject to opposite effective Zeeman fields, which are referred to as Ising spin-orbit coupling (SOC) fields. The Ising SOC, originating from in-plane mirror symmetry breaking, pins the electron spins to the out-of-plane directions, and results in Ising superconducting states with strongly enhanced upper critical fields. Here, we show that the Ising SOC generates equal-spin-triplet Cooper pairs with spin polarized in the in-plane directions. Importantly, the spin-triplet Cooper pairs can induce superconducting pairings in a half-metal wire placed on top of the TMD and result in a topological superconductor with Majorana end states. Direct ways to detect equal-spin triplet Cooper pairs and the differences between Ising superconductors and Rashba superconductors are discussed.

  9. Structural and Electronic Transitions in Ytterbium Metal to 202 GPa

    SciTech Connect

    Chesnut, G.N.; Vohra, Y.K.

    1999-02-01

    Ytterbium (Yb), a member of the rare earth group metals with a filled f shell and a divalent electronic character, has been studied under static high pressures in a diamond anvil cell to 202thinspthinspGPa. We report a new transformation from a face-centered-cubic structure to a hexagonal phase with 3thinspthinspatoms/cell (space group P3{sub 1}21 referred to as hP3 ) at 98{plus_minus}5 GPa . This hP3 phase is similar to that observed in trivalent neodymium and samarium at pressures near 40thinspthinspGPa. Our high pressure study thus clearly demonstrates that Yb is no longer an {open_quotes}irregular{close_quotes} member of the rare earth metals, and divalent to trivalent electronic transition is completed at megabar pressures. {copyright} {ital 1999} {ital The American Physical Society}

  10. Control of plasmonic nanoantennas by reversible metal-insulator transition.

    PubMed

    Abate, Yohannes; Marvel, Robert E; Ziegler, Jed I; Gamage, Sampath; Javani, Mohammad H; Stockman, Mark I; Haglund, Richard F

    2015-01-01

    We demonstrate dynamic reversible switching of VO2 insulator-to-metal transition (IMT) locally on the scale of 15 nm or less and control of nanoantennas, observed for the first time in the near-field. Using polarization-selective near-field imaging techniques, we simultaneously monitor the IMT in VO2 and the change of plasmons on gold infrared nanoantennas. Structured nanodomains of the metallic VO2 locally and reversibly transform infrared plasmonic dipole nanoantennas to monopole nanoantennas. Fundamentally, the IMT in VO2 can be triggered on femtosecond timescale to allow ultrafast nanoscale control of optical phenomena. These unique features open up promising novel applications in active nanophotonics. PMID:26358623

  11. The metal-to-insulator transition in disordered lithium titanate

    NASA Astrophysics Data System (ADS)

    Fazileh, Farhad

    The electronic and magnetic properties of the superconducting spinel lithium titanate have attracted substantial interest during the last three decades. This compound is one of the four spinel systems among more than 300 known spinel compounds that exhibit superconducting properties at low temperatures. It is still an open question whether electronic correlations have any significant role in the electronic and superconducting properties of this material. In this thesis, we provide some supporting evidence for the presence and importance of strong electronic correlations in this compound by studying the composition-induced metal-insulator transitions in Li1+ xTi2-xO4 and LiAlyTi2-yO 4 systems. In this study, first we introduce a one electron model for the conducting electrons, a so-called quantum site percolation (QSP) model. Experimentally, the non-stoichiometric compounds of lithium titanate, Li1+xTi2-x O4 and LiAlyTi 2-yO4, show metal-to-insulator transitions at critical doping concentrations of xc = 0.15 and yc = 0.33. We performed systematic numerical calculations based on the above-mentioned quantum site percolation model on a corner-sharing tetrahedral lattice, and our numerical results led to predictions for the critical doping concentrations of xc = 0.32 and yc = 0.83, more than twice the experimental values. Thus, this discrepancy indirectly supports the idea that perhaps electronic correlations are important, and their omission is the source of this discrepancy. The hypothesis that the screened Coulomb potentials of the doping cations could explain this disagreement was also rejected by our detailed study of this effect. We showed that this effect can only slightly change the critical doping concentrations of the metal-to-insulator transition. In the final part of our study we take into account the electronic interactions in a real-space Hartree-Fock approximation. In this model the QSP disorder is treated exactly, but the interactions are included in a mean-field approximation. Our numerical results based on this model show that an electron-electron interaction of Uc/t = 11 provides a critical doping concentration close to the experimental number for the LiAlyTi 2-yO4 system. Indeed, experimental estimates of the Hubbard interactions for d orbitals in titanium atoms for other transition-metal compounds are consistent with this value of U/t, and therefore this study provides strong supporting evidence for the presence of strong electronic interactions in this material.

  12. Ionically-mediated electromechanical hysteresis in transition metal oxides

    SciTech Connect

    Kim, Yunseok; Kumar, Amit; Jesse, Stephen; Kalinin, Sergei V

    2012-01-01

    Electromechanical activity, remanent polarization states, and hysteresis loops in paraelectric TiO2 and SrTiO3 are observed. The coupling between the ionic dynamics and incipient ferroelectricity in these materials is analyzed using extended Ginsburg Landau Devonshire (GLD) theory. The possible origins of electromechanical coupling including ionic dynamics, surface-charge induced electrostriction, and ionically-induced ferroelectricity are identified. For the latter, the ionic contribution can change the sign of first order GLD expansion coefficient, rendering material effectively ferroelectric. These studies provide possible explanation for ferroelectric-like behavior in centrosymmetric transition metal oxides.

  13. Atomic healing of defects in transition metal dichalcogenides.

    PubMed

    Lu, Junpeng; Carvalho, Alexandra; Chan, Xinhui Kim; Liu, Hongwei; Liu, Bo; Tok, Eng Soon; Loh, Kian Ping; Castro Neto, A H; Sow, Chorng Haur

    2015-05-13

    As-grown transition metal dichalcogenides are usually chalcogen deficient and therefore contain a high density of chalcogen vacancies, deep electron traps which can act as charged scattering centers, reducing the electron mobility. However, we show that chalcogen vacancies can be effectively passivated by oxygen, healing the electronic structure of the material. We proposed that this can be achieved by means of surface laser modification and demonstrate the efficiency of this processing technique, which can enhance the conductivity of monolayer WSe2 by ∼400 times and its photoconductivity by ∼150 times. PMID:25923457

  14. Low temperature luminescence of transition metal-doped beryls

    NASA Astrophysics Data System (ADS)

    Chithambo, M. L.; Raymond, S. G.; Calderon, T.; Townsend, P. D.

    1995-01-01

    This paper reports the thermo- and radio-luminescence of transition metal-doped natural beryls. The luminescence was measured between 20 and 300K. Mn 3+ and Cr 3+ have been found to activate the thermoluminescence with a high degree of efficiency. Comparison of beryl:Ni, beryl:Co and aquamarine emission spectra with that of quartz suggests that in some respects the beryl behaves as though it is a quartz and this has been concluded to follow from similarities in the framework nature of both lattices.

  15. The nature of the bonding in the transition metal trimers

    NASA Technical Reports Server (NTRS)

    Walch, Stephen P.; Bauschlicher, Charles W., Jr.

    1986-01-01

    The electronic structure of the transition metal (TM) trimers was studied by comparing the bonding in the Ca3, Sc3, and Cu3 molecules. The complete active space SCF/externally contracted configuration interaction (CI) ratio for the low-lying states of Sc3 and Sc3(+) and the SCF/CI ratio for Ca3 and Cu3 trimers, all for near equilateral triangle geometries, were calculated. In addition, vertical excitation energies for Cu3 were computed, leading to a new assignment of the upper state in the resonant two-photon ionization spectrum. Based on these studies, bonding in other TM trimers was discussed.

  16. Cross-plane thermal properties of transition metal dichalcogenides

    SciTech Connect

    Muratore, C.; Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433 ; Varshney, V.; Universal Technology Corporation, Dayton, Ohio 45432 ; Gengler, J. J.; Spectral Energies LLC, Dayton, Ohio 45431 ; Hu, J. J.; Bultman, J. E.; University of Dayton Research Institute, Dayton, Ohio 45469 ; Smith, T. M.; Shamberger, P. J.; Roy, A. K.; Voevodin, A. A.; Qiu, B.; Ruan, X.

    2013-02-25

    In this work, we explore the thermal properties of hexagonal transition metal dichalcogenide compounds with different average atomic masses but equivalent microstructures. Thermal conductivity values of sputtered thin films were compared to bulk crystals. The comparison revealed a >10 fold reduction in thin film thermal conductivity. Structural analysis of the films revealed a turbostratic structure with domain sizes on the order of 5-10 nm. Estimates of phonon scattering lengths at domain boundaries based on computationally derived group velocities were consistent with the observed film microstructure, and accounted for the reduction in thermal conductivity compared to values for bulk crystals.

  17. Correlated electron pseudopotentials for 3d-transition metals

    SciTech Connect

    Trail, J. R. Needs, R. J.

    2015-02-14

    A recently published correlated electron pseudopotentials (CEPPs) method has been adapted for application to the 3d-transition metals, and to include relativistic effects. New CEPPs are reported for the atoms Sc − Fe, constructed from atomic quantum chemical calculations that include an accurate description of correlated electrons. Dissociation energies, molecular geometries, and zero-point vibrational energies of small molecules are compared with all electron results, with all quantities evaluated using coupled cluster singles doubles and triples calculations. The CEPPs give better results in the correlated-electron calculations than Hartree-Fock-based pseudopotentials available in the literature.

  18. Correlated electron pseudopotentials for 3d-transition metals

    NASA Astrophysics Data System (ADS)

    Trail, J. R.; Needs, R. J.

    2015-02-01

    A recently published correlated electron pseudopotentials (CEPPs) method has been adapted for application to the 3d-transition metals, and to include relativistic effects. New CEPPs are reported for the atoms Sc - Fe, constructed from atomic quantum chemical calculations that include an accurate description of correlated electrons. Dissociation energies, molecular geometries, and zero-point vibrational energies of small molecules are compared with all electron results, with all quantities evaluated using coupled cluster singles doubles and triples calculations. The CEPPs give better results in the correlated-electron calculations than Hartree-Fock-based pseudopotentials available in the literature.

  19. Rare earth-transition metal scrap treatment method

    DOEpatents

    Schmidt, Frederick A.; Peterson, David T.; Wheelock, John T.; Jones, Lawrence L.; Lincoln, Lanny P.

    1992-02-11

    Rare earth-transition metal (e.g. iron) scrap (e.g. Nd-Fe-B scrap) is melted to reduce the levels of tramp oxygen and nitrogen impurities therein. The tramp impurities are reduced in the melt by virtue of the reaction of the tramp impurities and the rare earth to form dross on the melt. The purified melt is separated from the dross for reuse. The oxygen and nitrogen of the melt are reduced to levels acceptable for reuse of the treated alloy in the manufacture of end-use articles, such as permanent magnets.

  20. Rare earth-transition metal scrap treatment method

    DOEpatents

    Schmidt, F.A.; Peterson, D.T.; Wheelock, J.T.; Jones, L.L.; Lincoln, L.P.

    1992-02-11

    Rare earth-transition metal (e.g. iron) scrap (e.g. Nd-Fe-B scrap) is melted to reduce the levels of tramp oxygen and nitrogen impurities therein. The tramp impurities are reduced in the melt by virtue of the reaction of the tramp impurities and the rare earth to form dross on the melt. The purified melt is separated from the dross for reuse. The oxygen and nitrogen of the melt are reduced to levels acceptable for reuse of the treated alloy in the manufacture of end-use articles, such as permanent magnets. 3 figs.

  1. He-He and He-metal interactions in transition metals from first-principles

    NASA Astrophysics Data System (ADS)

    Zhang, Pengbo; Zou, Tingting; Zhao, Jijun

    2015-12-01

    We investigated the atomistic mechanism of He-He and He-metal interactions in bcc transition metals (V, Nb, Ta, Cr, Mo, W, and Fe) using first-principles methods. We calculated formation energy and binding energy of He-He pair as function of distance within the host lattices. The strengths of He-He attraction in Cr, Mo, W, and Fe (0.37-1.11 eV) are significantly stronger than those in V, Nb, and Ta (0.06-0.17 eV). Such strong attractions mean that He atoms would spontaneously aggregate inside perfect Cr, Mo, W, and Fe host lattices in absence of defects like vacancies. The most stable configuration of He-He pair is <100> dumbbell in groups VB metals, whereas it adopts close <110> configuration in Cr, Mo, and Fe, and close <111> configuration in W. Overall speaking, the He-He equilibrium distances of 1.51-1.55 Å in the group VIB metals are shorter than 1.65-1.70 Å in the group VB metals. Moreover, the presence of interstitial He significantly facilitates vacancy formation and this effect is more pronounced in the group VIB metals. The present calculations help understand the He-metal/He-He interaction mechanism and make a prediction that He is easier to form He cluster and bubbles in the groups VIB metals and Fe.

  2. First-Principles Study of Electronic Structure and Hydrogen Adsorption of 3d Transition Metal Exposed Paddle Wheel Frameworks

    SciTech Connect

    Bak, J. H.; Le, V. D.; Kang, J.; Wei, S. H.; Kim, Y. H.

    2012-04-05

    Open-site paddle wheels, comprised of two transition metals bridged with four carboxylate ions, have been widely used for constructing metal-organic frameworks with large surface area and high binding energy sites. Using first-principles density functional theory calculations, we have investigated atomic and electronic structures of various 3d transition metal paddle wheels before and after metal exposure and their hydrogen adsorption properties at open metal sites. Notably, the hydrogen adsorption is impeded by covalent metal-metal bonds in early transition metal paddle wheels from Sc to Cr and by the strong ferromagnetic coupling of diatomic Mn and Fe in the paddle wheel configurations. A significantly enhanced H{sub 2} adsorption is predicted in the nonmagnetic Co{sub 2} and Zn{sub 2} paddle wheel with the binding energy of {approx}0.2 eV per H{sub 2}. We also propose the use of two-dimensional Co{sub 2} and Zn{sub 2} paddle wheel frameworks that could have strongly adsorbed dihydrogen up to 1.35 wt % for noncryogenic hydrogen storage applications.

  3. Origin of Transitions between Metallic and Insulating States in Simple Metals

    NASA Astrophysics Data System (ADS)

    Naumov, Ivan I.; Hemley, Russell J.

    2015-04-01

    Unifying principles that underlie recently discovered transitions between metallic and insulating states in elemental solids under pressure are developed. Using group theory arguments and first-principles calculations, we show that the electronic properties of the phases involved in these transitions are controlled by symmetry principles. The valence bands in these systems are described by simple and composite band representations constructed from localized Wannier functions centered on points unoccupied by atoms, and which are not necessarily all symmetrical. The character of the Wannier functions is closely related to the degree of s -p (-d ) hybridization and reflects multicenter chemical bonding in these insulating states. The conditions under which an insulating state is allowed for structures having an integer number of atoms per primitive unit cell as well as reentrant (i.e., metal-insulator-metal) transition sequences are detailed, resulting in predictions of behavior such as phases having band-contact lines. The general principles developed are tested and applied to the alkali and alkaline earth metals, including elements where high-pressure insulating phases have been reported (e.g., Li, Na, and Ca).

  4. Anderson metal-insulator transitions with classical magnetic impurities

    NASA Astrophysics Data System (ADS)

    Jung, Daniel; Kettemann, Stefan

    2014-08-01

    We study the effects of classical magnetic impurities on the Anderson metal-insulator transition (AMIT) numerically. In particular we find that while a finite concentration of Ising impurities lowers the critical value of the site-diagonal disorder amplitude Wc, in the presence of Heisenberg impurities, Wc is first increased with increasing exchange coupling strength J due to time-reversal symmetry breaking. The resulting scaling with J is compared to analytical predictions by Wegner [1]. The results are obtained numerically, based on a finite-size scaling procedure for the typical density of states [2], which is the geometric average of the local density of states. The latter can efficiently be calculated using the kernel polynomial method [3]. Although still suffering from methodical shortcomings, our method proves to deliver results close to established results for the orthogonal symmetry class [4]. We extend previous approaches [5] by combining the KPM with a finite-size scaling analysis. We also discuss the relevance of our findings for systems like phosphor-doped silicon (Si:P), which are known to exhibit a quantum phase transition from metal to insulator driven by the interplay of both interaction and disorder, accompanied by the presence of a finite concentration of magnetic moments [6].

  5. Stabilization of Small Boron Cage by Transition Metal Encapsulation

    NASA Astrophysics Data System (ADS)

    Zhang, Lijun; Lv, Jian; Wang, Yanchao; Ma, Yanming

    2015-03-01

    The discovery of chemically stable fullerene-like structures formed by elements other than carbon has been long-standing desired. On this aspect significant efforts have centered around boron, only one electron deficient compared with carbon. However, during the past decade a large number of experimental and theoretical studies have established that small boron clusters are either planar/quasi-planar or forming double-ring tubular structures. Until recently, two all-boron fullerenes have been independently discovered: B38 proposed by our structure searching calculations and B40 observed in a joint experimental and theoretical study. Here we extend our work to the even smaller boron clusters and propose an effective routine to stabilize them by transition metal encapsulation. By combining swarm-intelligence structure searching and first-principles calculations, we have systematically investigated the energy landscapes of transition-metal-doped MB24 clusters (M = Ti, Zr, Hf, Cr, Mo, W, Fe, Ru and Os). Two stable symmetric endohedral boron cages, MoB24 and WB24 are identified. The stability of them can be rationalized in terms of their unique 18-electron closed-shell electronic structures. Funded by Recruitment Program of Global Experts of China and China Postdoctoral Science Foundation.

  6. Exfoliation of large-area transition metal chalcogenide single layers

    PubMed Central

    Magda, Gábor Zsolt; Pető, János; Dobrik, Gergely; Hwang, Chanyong; Biró, László P.; Tapasztó, Levente

    2015-01-01

    Isolating large-areas of atomically thin transition metal chalcogenide crystals is an important but challenging task. The mechanical exfoliation technique can provide single layers of the highest structural quality, enabling to study their pristine properties and ultimate device performance. However, a major drawback of the technique is the low yield and small (typically < 10 μm) lateral size of the produced single layers. Here, we report a novel mechanical exfoliation technique, based on chemically enhanced adhesion, yielding MoS2 single layers with typical lateral sizes of several hundreds of microns. The idea is to exploit the chemical affinity of the sulfur atoms that can bind more strongly to a gold surface than the neighboring layers of the bulk MoS2 crystal. Moreover, we found that our exfoliation process is not specific to MoS2, but can be generally applied for various layered chalcogenides including selenites and tellurides, providing an easy access to large-area 2D crystals for the whole class of layered transition metal chalcogenides. PMID:26443185

  7. Anderson metal-insulator transitions with classical magnetic impurities

    SciTech Connect

    Jung, Daniel; Kettemann, Stefan

    2014-08-20

    We study the effects of classical magnetic impurities on the Anderson metal-insulator transition (AMIT) numerically. In particular we find that while a finite concentration of Ising impurities lowers the critical value of the site-diagonal disorder amplitude W{sub c}, in the presence of Heisenberg impurities, W{sub c} is first increased with increasing exchange coupling strength J due to time-reversal symmetry breaking. The resulting scaling with J is compared to analytical predictions by Wegner [1]. The results are obtained numerically, based on a finite-size scaling procedure for the typical density of states [2], which is the geometric average of the local density of states. The latter can efficiently be calculated using the kernel polynomial method [3]. Although still suffering from methodical shortcomings, our method proves to deliver results close to established results for the orthogonal symmetry class [4]. We extend previous approaches [5] by combining the KPM with a finite-size scaling analysis. We also discuss the relevance of our findings for systems like phosphor-doped silicon (Si:P), which are known to exhibit a quantum phase transition from metal to insulator driven by the interplay of both interaction and disorder, accompanied by the presence of a finite concentration of magnetic moments [6].

  8. Wannier function analysis of charge states in transition metal oxides

    NASA Astrophysics Data System (ADS)

    Quan, Yundi; Pickett, Warren

    2015-03-01

    The charge (or oxidation) state of a cation has been a crucial concept in analyzing the electronic and magnetic properties of oxides as well as interpreting ``charge ordering'' metal-insulator transitions. In recent years a few methods have been proposed for the objective identification of charge states, beyond the conventional (and occasionally subjective) use of projected densities of states, weighted band structures (fatbands), and Born effective charges. In the past two decades Wannier functions (WFs) and particularly maximally localized WFs (MLWFs), have become an indispensable tool for several different purposes in electronic structure studies. These developments have motivated us to explore the charge state picture from the perspective of MLWFs. We will illustrate with a few transition metal oxide examples such as AgO and YNiO3 that the shape, extent, and location of the charge centers of the MLWFs provide insights into how cation-oxygen hybridization determines chemical bonding, charge distribution, and ``charge ordering.'' DOE DE-FG02-04ER46111.

  9. Anderson metal-insulator transitions with classical magnetic impurities

    NASA Astrophysics Data System (ADS)

    Jung, Daniel; Kettemann, Stefan; Slevin, Keith

    2016-04-01

    We study numerically the effects of classical magnetic impurities on the Anderson metal-insulator transition. We find that a small concentration of Heisenberg impurities enhances the critical disorder amplitude Wc with increasing exchange coupling strength J . The resulting scaling with J is analyzed which supports an anomalous scaling prediction by Wegner due to the combined breaking of time-reversal and spin-rotational symmetry. Moreover, we find that the presence of magnetic impurities lowers the critical correlation length exponent ν and enhances the multifractality parameter α0. The new value of ν improves the agreement with the value measured in experiments on the metal-insulator transition (MIT) in doped semiconductors like phosphor-doped silicon, where a finite density of magnetic moments is known to exist in the vicinity of the MIT. The results are obtained by a finite-size scaling analysis of the geometric mean of the local density of states which is calculated by means of the kernel polynomial method. We establish this combination of numerical techniques as a method to obtain critical properties of disordered systems quantitatively.

  10. Transition metal oxide as anode interface buffer for impedance spectroscopy

    NASA Astrophysics Data System (ADS)

    Xu, Hui; Tang, Chao; Wang, Xu-Liang; Zhai, Wen-Juan; Liu, Rui-Lan; Rong, Zhou; Pang, Zong-Qiang; Jiang, Bing; Fan, Qu-Li; Huang, Wei

    2015-12-01

    Impedance spectroscopy is a strong method in electric measurement, which also shows powerful function in research of carrier dynamics in organic semiconductors when suitable mathematical physical models are used. Apart from this, another requirement is that the contact interface between the electrode and materials should at least be quasi-ohmic contact. So in this report, three different transitional metal oxides, V2O5, MoO3 and WO3 were used as hole injection buffer for interface of ITO/NPB. Through the impedance spectroscopy and PSO algorithm, the carrier mobilities and I-V characteristics of the NPB in different devices were measured. Then the data curves were compared with the single layer device without the interface layer in order to investigate the influence of transitional metal oxides on the carrier mobility. The careful research showed that when the work function (WF) of the buffer material was just between the work function of anode and the HOMO of the organic material, such interface material could work as a good bridge for carrier injection. Under such condition, the carrier mobility measured through impedance spectroscopy should be close to the intrinsic value. Considering that the HOMO (or LUMO) of most organic semiconductors did not match with the work function of the electrode, this report also provides a method for wide application of impedance spectroscopy to the research of carrier dynamics.

  11. Effects of interband transitions on Faraday rotation in metallic nanoparticles.

    PubMed

    Wysin, G M; Chikan, Viktor; Young, Nathan; Dani, Raj Kumar

    2013-08-14

    The Faraday rotation in metallic nanoparticles is considered based on a quantum model for the dielectric function ϵ(ω) in the presence of a DC magnetic field B. We focus on effects in ϵ(ω) due to interband transitions (IBTs), which are important in the blue and ultraviolet for noble metals used in plasmonics. The dielectric function is found using the perturbation of the electron density matrix due to the optical field of the incident electromagnetic radiation. The calculation is applied to transitions between two bands (d and p, for example) separated by a gap, as one finds in gold at the L-point of the Fermi surface. The result of the DC magnetic field is a shift in the effective optical frequency causing IBTs by ±μBB/ħ, where opposite signs are associated with left/right circular polarizations. The Faraday rotation for a dilute solution of 17 nm diameter gold nanoparticles is measured and compared with both the IBT theory and a simpler Drude model for the bound electron response. Effects of the plasmon resonance mode on Faraday rotation in nanoparticles are also discussed. PMID:23846610

  12. Ultrafast exciton relaxation in monolayer transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Thilagam, A.

    2016-04-01

    We examine a mechanism by which excitons undergo ultrafast relaxation in common monolayer transition metal dichalcogenides. It is shown that at densities ≈1 × 1011 cm-2 and temperatures ≤60 K, excitons in well known monolayers (MoS2, MoSe2, WS2, and WSe2) exist as point-like structureless electron-hole quasi-particles. We evaluate the average rate of exciton energy relaxation due to acoustic phonons via the deformation potential and the piezoelectric coupling mechanisms and examine the effect of spreading of the excitonic wavefunction into the region perpendicular to the monolayer plane. Our results show that the exciton relaxation rate is enhanced with increase in the exciton temperature, while it is decreased with increase in the lattice temperature. Good agreements with available experimental data are obtained when the calculations are extrapolated to room temperatures. A unified approach taking into account the deformation potential and piezoelectric coupling mechanisms shows that exciton relaxation induced by phonons is as significant as defect assisted scattering and trapping of excitons by surface states in monolayer transition metal dichalcogenides.

  13. Chemical Trends for Transition Metal Compound Bonding to Graphene

    NASA Astrophysics Data System (ADS)

    Lange, Bjoern; Blum, Volker

    2015-03-01

    Transition metal compounds are of interest as catalysts for the hydrogen evolution reaction (HER). However, a perfect candidate to replace expensive platinum has not yet been identified. To tailor a specific compound, several properties come into play. One is the bonding to the underlying substrate, for which π-bonded carbon nanostructures are promising candidates. Here we analyze the bonding of small transition metal compound nanoclusters to a graphene layer for a range of chemical compositions: MxAy (M = Mo, Ti; A = S, O, B, N, C). The clusters are generated by an unbiased random search algorithm. We perform total energy calculations based on density functional theory to identify lowest energy clusters. We calculate binding energies using the PBE and HSE functionals with explicit van der Waals treatment and benchmark those against RPA cluster calculations. Our results indicate that molybdenum-carbides and -nitrides tend to bond tightly to graphene. Mo-oxides and -sulfides show small binding energies, indicating van der Waals bonding.

  14. Transition metal-mediated desulfurization of aromatic sulfur compounds

    SciTech Connect

    Eisch, J.J.; Im, K.R.; Hallenbeck, L.E.

    1980-03-01

    The presence of organic sulfur in various coals and crude petroleum stocks constitutes a serious problem in producing environmentally acceptable fuels. Accordingly, a worthy objective of fuel research would be to discover energy-efficient ways of cleaving organic sulfur bonds in coal, petroleum or ther derived liquids, so as to minimize atmospheric pollution caused by organic sulfide or mercaptan contaminants in fuels. Since existing desulfurization procedures generally require elevated temperatures, high pressures of hydrogen and the use of heterogeneous catalysts, a search has been launched for desulfurization processes effective under milder conditions, namely those involving homogeneous transition-metal agents. In order to learn more about the fundamental chemistry of carbon-sulfur bond cleavages by transition metals, we have chosen as substrates those aromatic sulfur compounds that are typical of the stable organic sulfur constituents found in coal tar or petroleum stocks, namely dibenzothiophene (Ia), phenoxathiin (Ib), phenothiazine (Ic) and thianthrene (Id), as well as various open-chain sulfides (Ar-S-Ar) and mercaptans (ArSH). Although some soluble nickel (O) complexes, such as bis (1,5-cyclooctadiene) nickel (O), are unreactive in desulfurization, a change of the ligands about nickel causes a striking rise in reactivity. With the aromatic sulfur heterocycles (Ib-Id), either desulfurization with ring contraction or hydrodesulfurization could be achieved, depending on the reaction conditions (1,2). Recent findings on the mechanisms of these unusual desulfurizations are reported.

  15. Ethanol adsorption on transition-metal surfaces: A DFT investigation

    NASA Astrophysics Data System (ADS)

    da Silva, Juarez L. F.

    2010-03-01

    The development of low-cost and long-term stability catalyst compounds for the production of hydrogen from ethanol is one of the main problems to be solved for large scale use of direct- ethanol fuel cells. Steam reforming, which is one of the main routes to obtain hydrogen from ethanol, as well as ethanol oxidation, are critically dependent on the choice of the catalyst devices. Therefore, an atom-level understanding of the interaction of ethanol with catalysts systems is on the first problems to be addressed. In this talk, we will report first- principles calculations based on density functional theory for the adsorption of ethanol on close-packed transition-metal surfaces at the limit of low-coverage. In particular, we will report the following properties, namely, adsorption energy, work function changes, and structural parameters for a large number of substrates, which will be used to build up a simple picture to describe the interaction of ethanol with transition-metal surfaces. This work is supported by FAPESP.

  16. Transitions in critical size in metal (100) and metal (111) homoepitaxy

    NASA Astrophysics Data System (ADS)

    Amar, Jacques G.; Family, Fereydoon

    1997-06-01

    Transitions in the critical island size i in submonolayer growth from i = 1 to i = 2 and from i = 1 to i = 3, corresponding to homoepitaxial growth on metal (111) and (100) surfaces, are studied using kinetic Monte Carlo simulations of a restricted pairbond model, both with and without island relaxation, and are compared with rate-equation predictions. In both cases, the rate equations significantly underestimate the transition temperature from i = 1 behavior to a higher critical island size. The difference is due to the neglect of spatial correlations in the standard rate-equation approach. A partial solution involving the use of effective detachment rates is proposed.

  17. Investigating Metallicity Variations in Early-type Galaxies with Chandra

    NASA Astrophysics Data System (ADS)

    Dahlin, Patrick; Hodges-Kluck, Edmund J.; Bregman, Joel N.

    2016-04-01

    Some simulations of galaxy formation predict large variations in the metallicity of the hot X-ray emitting galaxy atmospheres, producing a higher-emission weighted metallicity than the true mass-weighted metallicity when a spectrum is fit. Since the variations may be detectable in existing data, we searched for the predicted variations using X-ray intensity maps from Chandra and color-color analysis, which can constrain the metallicity of an isothermal plasma. Applying this analysis to 5 early-type galaxies revealed variations in the surface brightness distribution but these variations are not simply due to changes in metallicity. NGC 5846 provides an important case study as the intensity of photons is high enough to fit spectra to small regions of the galaxy, providing a comparison to the results of the color-color analysis. Although the spectra of small regions in NGC 5846 are consistent with two temperature (2T) models, further analysis indicates that the metallicities and temperatures of the 2T models must fall within a certain range of the isothermal values predicted from color-color analysis. The contribution of undetected X-ray binaries to the diffuse X-rays were investigated, but they are likely not the cause of the variations.

  18. Early and Late Stage Metals and Sulfides in Diogenites

    NASA Technical Reports Server (NTRS)

    Sideras, L. C.; Domanik, K. J.; Lauretta, D. S.

    2004-01-01

    Diogenites are typically highly brecciated orthopyroxenites that contain 84-100 vol.% orthopyroxene. Common accessory minerals include olivine, chromite, Ca-pyroxene, plagioclase, silica, troilite and Fe-Ni metal. Metal and sulfides are minor phases in diogenites with an average abundance of < 1 vol.% and 0-2 vol.% respectively. However their presence is important, as they could provide information on T-fO2-fS2 conditions and the evolution of the diogenite parent magma during crystallization and/or later metamorphism. We have examined the occurrence of Fe-Ni metal and sulfides in thin sections of several diogenites including, Johnstown, Manegaon, Roda, Shalka, Bilanga, and Tatahouine using optical microscopy and the electron microprobe. Here, we describe three features of metals and sulfides that are common in most of these diogenites. These are: i) The widespread occurrence of pentlandite associated with copper and copper sulfide minerals; ii) Textural evidence that at least some of the metal and sulfide occurring interstitially between, and as inclusions within, orthopyroxene formed from an early immiscible sulfide-oxide liquid; iii) That this sulfide- oxide liquid subsequently fractionated into assemblages containing either Fe-Ni metal, troilite, and chromite or pentlandite, troilite, and copper-bearing sulfide.

  19. XO-2b: A Transiting Hot Jupiter in a Metal-rich Common Proper Motion Binary

    NASA Astrophysics Data System (ADS)

    Burke, Christopher J.; McCullough, P. R.; Valenti, J. A.; Summers, F. J.; Stys, J. E.; Johns-Krull, C. M.; Janes, K. A.; Heasley, J. N.; Bissinger, R.; Fleenor, M.; Foote, C. N.; Garcia-Melendo, E.; Gary, B. L.; Howell, P. J.; Mallia, F.; Masi, G.; Vanmunster, T.

    2007-05-01

    XO-2b, the second transiting extrasolar planet from the XO Project (McCullough et al. 2005), is approximately Jupiter-size and 0.6 Jupiter-mass with an orbital period of 2.6 days. The stellar host, XO-2, is a V=11.2, early K dwarf which is metal rich, [Fe/H]=+0.44. XO-2 has a high proper motion, 157 mas/yr, and has a common proper motion stellar companion with half arcmin separation. The two stars are nearly identical twins, with very similar spectra and apparent magnitudes. The global network of amateur and professional astronomers organized by the XO project confirmed the XO-2b transit light curve two days after being notified that it was a high-priority candidate, and radial velocities confirmed its planetary mass eight days after that.

  20. Volume-Collapse Transitions in the Rare Earth Metals

    NASA Astrophysics Data System (ADS)

    McMahan, A. K.

    2000-03-01

    A number of the rare earth metals (Ce, Pr, Gd) exhibit pressure-induced phase transitions characterized by unusually large volume changes (5--15%), which are believed to be driven by f electron correlation effects. The present talk will review attempts to calculate these transitions by realistic methods which include all valence orbitals. It is suggested that while corrected local density methods (orbital polarization, self-interaction correction, LDA+U) can approximate these transitions, such solutions bear similarities to Hartree-Fock mean field, raising worries about incorrect temperature dependence of the total energy and entropy in the low-pressure ``localized'' regimes, and too-large predicted temperature-pressure domains for the localized phases, among other concerns [A.K. McMahan et. al., J. Comp.-Aided Mat. Des. 5, 131 (1998)]. These points are made by comparing Hartree-Fock and self-energy based solutions of all-valence-orbital effective Hamiltonians, as well as Hartree-Fock and correlated solutions of simpler model Hamiltonians [C. Huscroft et. al., Phys. Rev. Lett. 82, 2342 (1999); K. Held et. al., cond-mat/9905011].

  1. Quantum capacitance in thin film vanadium dioxide metal insulator transition

    NASA Astrophysics Data System (ADS)

    Wu, Zhe; Knighton, Talbot; Tarquini, Vinicio; Torres, David; Wang, Tongyu; Sepulveda, Nelson; Huang, Jian

    We present capacitance measurements of the electronic density of states performed in high quality vanadium dioxide (VO2) thin films on sapphire (Al2O3) substrate. These films show the expected metal insulator transition near 60 °C with resistivity changing by 3 orders of magnitude with a hysteresis of 10 °C. To make a capacitive probe, a gate is suspended above the film surface using a flip-chip method with microfabricated supports. The geometric capacitance per-area reached is 40 pF/mm2. Such a large capacitance can be significantly modified by electron interaction and band charging/discharging which appear as an extra term known as the quantum capacitance (Cq). An AC signal applied to the gate allows measurement of the changing density of states (DOS) across the MIT. The DOS abruptly increases as the sample is heated through the transition point. Conversely the low temperature drop of d μ / d n is consistent with an energy gap opening in the insulating phase. These parameters shed light on the transition mechanism. NSF DMR-1105183, NSF ECCS 1306311.

  2. Polynuclear transition metal complexes with thiocarbohydrazide and dithiocarbamates

    NASA Astrophysics Data System (ADS)

    Siddiqi, K. S.; Khan, Sadaf; Nami, Shahab A. A.; El-ajaily, M. M.

    2007-07-01

    Sn(tch) 2{MCl 2} 2 was prepared from the precursor Sn(tch) 2 and MCl 2. It was subsequently allowed to react with diethyldithiocarbamate which yielded the trinuclear complexes of the type Sn(tch) 2{M 2(dtc) 4}, where tch = thiocarbohydrazide, M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and dtc = diethyldithiocarbamate. They were characterized on the basis of microanalytical, thermal (TGA/DSC), spectral (IR, UV-vis, EPR, 1H NMR) studies, conductivity measurement and magnetic moment data. On the basis of spectral data a tetrahedral geometry has been proposed for the halide complexes, Sn(tch) 2{MCl 2} 2 except for Cu(II) which exhibits a square planar coordination although the transition metal ion in Sn(tch) 2{M 2(dtc) 4} achieves an octahedral geometry where the dithiocarbamato moiety acts as a symmetrical bidentate ligand. The bidentate nature has been established by the appearance of a sharp single ν(C-S) around 1000 cm -1. A downfield shift observed in NH a and NH b protons on moving from Sn(tch) 2 to Sn(tch) 2{MCl 2} 2 is due to the drift of electrons toward metal atoms. A two-step pyrolysis has been observed in the Sn(tch) 2{MCl 2} 2 complexes while their dithiocarbamato derivatives exhibit a three-stage degradation pattern. Finally, the in vitro antibacterial activity of Sn(tch) 2{M 2(dtc) 4} and the mononuclear Sn(tch) 2 has been carried out on bacterial strains Escherichia coli and Salmonella typhi. The compounds were found to be active against the test organisms. The activity of the complexes is enhanced with increasing concentration. The maximum activity in both the strains was achieved by cobalt(II) dithiocarbamate complex. Minimum activity was found for Sn(tch) 2 which generally increases with the introduction of transition metal ion in the complex.

  3. Materials with intermediate valence ; a comparison with transition metals

    NASA Astrophysics Data System (ADS)

    Mott, N. F.

    A discussion of metallic intermediate valence materials is given, particularly of hybridisation between the 4f and the conduction band δ. If n, 1 - n are the numbers of ions in each of two charge states, the variation of n with temperature is described. Resistivity is ascribed to scattering of the conduction electrons into the 4f band. The mechanism is compared with that in transition metals and their alloys, particularly Pd1-xAg x. The resistivity can be very large, of order of the Ioffe-Regel value 1/3 e2/ħa. It is argued that both here and in metallic alloys, this can only occur with a two-band model. At high temperatures there is some evidence that s-f scattering does not occur. On discute des matériaux métalliques à valence intermédiaire, en particulier de l'hybridation entre la bande 4f et la bande de conduction. Notant n et 1 - n les nombres d'ions dans chacun des deux états de charge, on décrit la variation de n avec la température. Le mécanisme est comparé avec celui des métaux de transition et de leurs alliages, particulièrement Pd1-xAg x. La résistivité peut être très grande, de l'ordre de la valeur de Ioffe-Regel 1/3 e2/ħa. On donne des arguments tendant à prouver que ceci ne peut se produire que dans le cadre d'un modèle à 2 bandes. Il y a des évidences qu'à haute température la diffusion s-f n'a pas lieu.

  4. Spectroscopic Studies of Diatomic Transition Metal Oxides and Fluorides.

    NASA Astrophysics Data System (ADS)

    McCord, John Edward

    Wavelength selected fluorescence excitation spectroscopy (WSFES) techniques and ligand field theory (LFT) calculations have been applied to the following transition metal diatomics: CeO, UO, LaF, YF, ScF, HfO, TiO, and ZrO. All of the rotational spectra recorded for these molecules were at a resolution of 0.03 cm^{-1}, and, with few exceptions, Omega assignments for electronic states were unambiguously determined from observations of the first lines in at least two rotational branches. Accurate term energies and rotational constants are reported. Thirty one electronic transitions of CeO were recorded, and all of the sixteen states that correlate with rm Ce^{2+}(4f6s)O^ {2-} were characterized. The results are in good accord with a ligand field theory model of the low-lying states. New assignments were established for four previously observed transitions, and spectra for three new excited states were analyzed. LFT calculations have been used in an attempt to provide configurational assignments for the excited states (including those from previous studies). Twenty two states are tentatively assigned to the rm Ce^{2+}(4f6p)O ^{2-} configuration. Twelve others are tentatively assigned to rm Ce^ {2+}(4f5d)O^{2-}.. Rotation-electronic interactions between states of the 4f6s configuration of CeO, mediated by the operator {-}B(R)( J^+ cdot J_sp{a}{-}+ J^-cdot J_sp{a} {+}), have been calculated. Second-order perturbation theory was used to account for the effect of heterogeneous interactions on the rotation constants within a single configuration. Thirty-three electronic transitions of UO were analyzed, and nine low-lying electronic states that correlated with either rm U^{2+}(5f ^37s)O^{2-} or rm U^{2+}(5f^27s^2)O ^{2-} were characterized. Ligand field theory calculations were also used in an attempt to provide configurational assignments for the excited states of UO and ThO. Experimentally derived values for Delta B_sp{0} {0}(nl/n^' l^') parameters were used to predict the electronic structures of UO and UO^+.. Eight electronic transitions of LaF and nine transitions of YF were studied. LFT calculations (and in the case of LaF, hyperfine constants) were used to suggest electronic configurational assignments for the excited states of these molecules. WSFE spectra of the 0-0 and 1-1 bands of the ScF F^1Phi-A^1 Delta transition were observed, and perturbations in the F^1Phi state, caused by interactions with the h^3Pi_2 state, were analyzed. Using a method based on Franck-Condon factors, the vibrational numbering in the perturbing h^3Pi_2 state was estimated. The 5698 A band of HfO has been re examined. The wavelength-resolved fluorescence excitation technique was used to record the (2,0) band of the C ^3Delta_3-a^1 Delta transition, and the (2,3) and (2,4) bands of the C^3Delta_3 -X^3Delta_3 electronic transition of TiO. The (0,0) bands of the satellite e^3 Pi_1-a^3Delta _1 and e^3Pi_2 -a^3Delta_2 transitions of ZrO were investigated, and these data were combined with previous high resolution analyses of the ZrO beta-system and with the analysis of the intercombination e^3Pi_1 -X^1Sigma ^+ transition to obtain accurate term energies for the triplet states of ^{90 }Zr^{16}O. (Abstract shortened by UMI.).

  5. DNA nuclease activity of Rev-coupled transition metal chelates.

    PubMed

    Joyner, Jeff C; Keuper, Kevin D; Cowan, J A

    2012-06-01

    Artificial nucleases containing Rev-coupled metal chelates based on combinations of the transition metals Fe(2+), Co(2+), Ni(2+), and Cu(2+) and the chelators DOTA, DTPA, EDTA, NTA, tripeptide GGH, and tetrapeptide KGHK have been tested for DNA nuclease activity. Originally designed to target reactive transition metal chelates (M-chelates) to the HIV-1 Rev response element mRNA, attachment to the arginine-rich Rev peptide also increases DNA-binding affinity for the attached M-chelates. Apparent K(D) values ranging from 1.7 to 3.6 µM base pairs for binding of supercoiled pUC19 plasmid DNA by Ni-chelate-Rev complexes were observed, as a result of electrostatic attraction between the positively-charged Rev peptide and negatively-charged DNA. Attachment of M-chelates to the Rev peptide resulted in enhancements of DNA nuclease activity ranging from 1-fold (no enhancement) to at least 13-fold (for Cu-DTPA-Rev), for the rate of DNA nicking, with second order rate constants for conversion of DNA(supercoiled) to DNA(nicked) up to 6 × 10(6) M(-1) min(-1), and for conversion of DNA(nicked) to DNA(linear) up to 1 × 10(5) M(-1) min(-1). Freifelder-Trumbo analysis and the ratios of linearization and nicking rate constants (k(lin)/k(nick)) revealed concerted mechanisms for nicking and subsequent linearization of plasmid DNA for all of the Rev-coupled M-chelates, consistent with higher DNA residency times for the Rev-coupled M-chelates. Observed rates for Rev-coupled M-chelates were less skewed by differing DNA-binding affinities than for M-chelates lacking Rev, as a result of the narrow range of DNA-binding affinities observed, and therefore relationships between DNA nuclease activity and other catalyst properties, such as coordination unsaturation, the ability to consume ascorbic acid and generate diffusible radicals, and the identity of the metal center, are now clearly illustrated in light of the similar DNA-binding affinities of all M-chelate-Rev complexes. This work paints a clearer picture of the factors governing DNA nuclease activity by redox active M-chelates than was previously possible. The results demonstrate enhancement of DNA cleavage by use of a targeting sequence, but also clearly underscore that significant orientational factors are required for optimal reactivity at the metal center. Moreover, the studies confirm high selectivity for the target HIV RRE RNA at the most likely dosage concentrations, lending further support to the feasibility of designing and applying targeted catalytic metallodrugs. PMID:22450234

  6. Epithelial-to-Mesenchymal Transition in Early Transplant Tubulointerstitial Damage

    PubMed Central

    Vitalone, Matthew J.; O'Connell, Philip J.; Jimenez-Vera, Elvira; Yuksel, Aysen; Wavamunno, Moses; Fung, Caroline L.-S.; Chapman, Jeremy R.; Nankivell, Brian J.

    2008-01-01

    It is unknown whether epithelial-to-mesenchymal transition (EMT) leads to tubulointerstitial fibrosis in renal transplants. In this study, interstitial fibrosis and markers of EMT were followed in protocol transplant biopsies in 24 patients. Tubulointerstitial damage (TID) increased from 34 to 54% between 1 and 3 mo after transplantation. Detection of EMT depended on the marker used; low levels of α-smooth muscle actin were found in 61% of biopsies, but the less specific marker S100 calcium binding protein-A4 (also known as Fsp1) suggested a higher incidence of EMT. The presence or development of TID did not correlate with EMT but instead significantly correlated with subclinical immune activity (P < 0.05). Among biopsies showing TID, microarray analysis revealed differential regulation of 127 genes at 1 mo and 67 genes at 3 mo compared with baseline; these genes were predominantly associated with fibrosis, tissue remodeling, and immune response. Of the 173 EMT-associated genes interrogated, however, only 8.1% showed an expression pattern consistent with EMT at 1 mo and 6.3% at 3 mo. The remainder were not differentially altered, or their changes in expression were opposite those expected to promote EMT. Quantitative reverse transcriptase–PCR revealed that the expression pattern of 12 EMT-associated genes was inconsistent over time, opposite that expected, or consistent with subclinical rejection or inflammation. In conclusion, EMT does not seem to play a significant role in the development of early allograft fibrosis. PMID:18480317

  7. Discovery of elusive structures of multifunctional transition-metal borides.

    PubMed

    Liang, Yongcheng; Wu, Zhaobing; Yuan, Xun; Zhang, Wenqing; Zhang, Peihong

    2016-01-14

    A definitive determination of crystal structures is an important prerequisite for designing and exploiting new functional materials. Even though tungsten and molybdenum borides (TMBx) are the prototype for transition-metal light-element compounds with multiple functionalities, their elusive crystal structures have puzzled scientists for decades. Here, we discover that the long-assumed TMB2 phases with the simple hP3 structure (hP3-TMB2) are in fact a family of complex TMB3 polytypes with a nanoscale ordering along the axial direction. Compared with the energetically unfavorable and dynamically unstable hP3-TMB2 phase, the energetically more favorable and dynamically stable TMB3 polytypes explain the experimental structural parameters, mechanical properties, and X-ray diffraction (XRD) patterns better. We demonstrate that such a structural and compositional modification from the hP3-TMB2 phases to the TMB3 polytypes originates from the relief of the strong antibonding interaction between d electrons by removing one third of metal atoms systematically. These results resolve the longstanding structural mystery of this class of metal borides and uncover a hidden family of polytypic structures. Moreover, these polytypic structures provide an additional hardening mechanism by forming nanoscale interlocks that may strongly hinder the interlayer sliding movements, which promises to open a new avenue towards designing novel superhard nanocomposite materials by exploiting the coexistence of various polytypes. PMID:26660270

  8. Solar selective coatings based on carbon: transition metal nanocomposites

    NASA Astrophysics Data System (ADS)

    Heras, Irene; Guillén, Elena; Krause, Matthias; Pardo, Ainhoa; Endrino, Jose L.; Escobar Galindo, Ramón

    2015-09-01

    The design of an efficient and stable solar selective coating for Concentrating Solar Power central receivers requires a complex study of the materials candidates that compose the coating. Carbon-transition metal nanocomposites were studied in this work as absorber materials because they show appropriate optical properties with high absorption in the solar region and low thermal emittance in the infrared. Furthermore metal carbides are thermal and mechanical stable in air at high temperatures. In this work a solar selective coating was grown by a dual source filtered cathodic vacuum arc. The complete stack consists on an infrared reflection layer, an absorber layer of carbon-zirconium carbide nanocomposites and an antireflection layer. The aim of this research is optimize the absorber layer and for that, the metal content was controlled by adjusting the pulse ratio between the two arc sources. The elemental composition was determined by Ion Beam Analysis, X-Ray diffraction measurements show the crystal structure and the optical properties were characterized by spectroscopic ellipsometry measurements. The reflectance spectra of the complete selective coating were simulated with the optical software CODE. Bruggeman effective medium approximation was employed to average the dielectric functions of the two components which constitute the nanocomposite in the absorber layer. The optimized coating exhibited a solar absorptance of 95.41% and thermal emittance of 3.5% for 400°C. The simulated results were validated with a deposited multilayer selective coating.

  9. K-CO on transition metals: A local ionic interaction

    NASA Astrophysics Data System (ADS)

    Patterson, Charles H.; Schultz, Peter A.; Messmer, Richard P.

    1987-05-01

    The nature of the K-CO interaction on a transition metal surface is addressed when the K:CO stoichiometry is approx. 1. The interaction proposed is transfer of the K 4s electron to CO. A BORN-Haber cycle for this process on a surface has been calculated. The reference state is K plus CO, bound separately to the surface (a). A key point in the cycle is that removal of the electron from K bound to a metal surface (b) is less costly in energy (approx. metal/K surface, i.e., approx. +2.0 eV) than from K in the gas phase (IP = +4.3 eV). This means that the complex is significantly stabilized on the surface. The removed K electron is then transported to the isolated adsorbed CO (c) at a cost of the electron affinity of CO (approx. = 1.5 eV). When the isolated ions are brought together (d) the net stabilization at the equilibrium geometry is approx. - 1.8 eV.

  10. Discovery of elusive structures of multifunctional transition-metal borides

    NASA Astrophysics Data System (ADS)

    Liang, Yongcheng; Wu, Zhaobing; Yuan, Xun; Zhang, Wenqing; Zhang, Peihong

    2015-12-01

    A definitive determination of crystal structures is an important prerequisite for designing and exploiting new functional materials. Even though tungsten and molybdenum borides (TMBx) are the prototype for transition-metal light-element compounds with multiple functionalities, their elusive crystal structures have puzzled scientists for decades. Here, we discover that the long-assumed TMB2 phases with the simple hP3 structure (hP3-TMB2) are in fact a family of complex TMB3 polytypes with a nanoscale ordering along the axial direction. Compared with the energetically unfavorable and dynamically unstable hP3-TMB2 phase, the energetically more favorable and dynamically stable TMB3 polytypes explain the experimental structural parameters, mechanical properties, and X-ray diffraction (XRD) patterns better. We demonstrate that such a structural and compositional modification from the hP3-TMB2 phases to the TMB3 polytypes originates from the relief of the strong antibonding interaction between d electrons by removing one third of metal atoms systematically. These results resolve the longstanding structural mystery of this class of metal borides and uncover a hidden family of polytypic structures. Moreover, these polytypic structures provide an additional hardening mechanism by forming nanoscale interlocks that may strongly hinder the interlayer sliding movements, which promises to open a new avenue towards designing novel superhard nanocomposite materials by exploiting the coexistence of various polytypes.

  11. Environmental Effects on the Metallicities of Early-Type Galaxies

    NASA Technical Reports Server (NTRS)

    Jones, Christine; Oliversen, Ronald (Technical Monitor)

    2004-01-01

    We completed and published two papers in the Astrophysical Journal based on research from grant. In the first paper we analyzed nine X-ray-bright Virgo early-type galaxies observed by both ASCA and ROSAT. Through spatially resolved spectroscopy, we determined the radial temperature profiles and abundances of Mg, Si, and Fe for six galaxies. The temperature profiles are consistent with isothermal temperatures outside of cooler regions at the galaxies' centers. We present new evidence for iron abundance gradients in NGC 4472 and NGC 4649 and confirm the previous results on NGC 4636. Mg and Si abundance gradients on average are flatter than those of iron and correspond to an underabundance of α-process elements at high Fe values, while at low iron the element ratios favor enrichment by Type II supernovae (SNe). We explain the observed trend using the metallicity dependence of SN Ia metal production and present constraints on the available theoretical modeling for low-metallicity inhibition of SNe Ia. In the second paper We analyzed nine X-ray-bright Virgo early-type galaxies observed by both ASCA and ROSAT. Through spatially resolved spectroscopy, we determined the radial temperature profiles and abundances of Mg, Si, and Fe for six galaxies. The temperature profiles are consistent with isothermal temperatures outside of cooler regions at the galaxies' centers. We present new evidence for iron abundance gradients in NGC 4472 and NGC 4649 and confirm the previous results on NGC 4636. Mg and Si abundance gradients on average are flatter than those of iron and correspond to an underabundance of α-process elements at high Fe values, while at low iron the element ratios favor enrichment by Type I1 supernovae (SNe). We explain the observed trend using the metallicity dependence of SN Ia metal production and present constraints on the available theoretical modeling for low-metallicity inhibition of SNe Ia.

  12. Catalytic mechanism of transition-metal compounds on Mg hydrogen sorption reaction.

    PubMed

    Barkhordarian, Gagik; Klassen, Thomas; Bormann, Rüdiger

    2006-06-01

    The catalytic mechanisms of transition-metal compounds during the hydrogen sorption reaction of magnesium-based hydrides were investigated through relevant experiments. Catalytic activity was found to be influenced by four distinct physico-thermodynamic properties of the transition-metal compound: a high number of structural defects, a low stability of the compound, which however has to be high enough to avoid complete reduction of the transition metal under operating conditions, a high valence state of the transition-metal ion within the compound, and a high affinity of the transition-metal ion to hydrogen. On the basis of these results, further optimization of the selection of catalysts for improving sorption properties of magnesium-based hydrides is possible. In addition, utilization of transition-metal compounds as catalysts for other hydrogen storage materials is considered. PMID:16771356

  13. Electronic Relaxation Processes of Transition Metal Atoms in Helium Nanodroplets

    NASA Astrophysics Data System (ADS)

    Kautsch, Andreas; Lindebner, Friedrich; Koch, Markus; Ernst, Wolfgang E.

    2014-06-01

    Spectroscopy of doped superfluid helium nanodroplets (He_N) gives information about the influence of this cold, chemically inert, and least interacting matrix environment on the excitation and relaxation dynamics of dopant atoms and molecules. We present the results from laser induced fluorescence (LIF), photoionization (PI), and mass spectroscopy of Cr and Cu doped He_N. From these results, we can draw a comprehensive picture of the complex behavior of such transition metal atoms in He_N upon photo-excitation. The strong Cr and Cu ground state transitions show an excitation blueshift and broadening with respect to the bare atom transitions which can be taken as indication for the solvation inside the droplet. From the originally excited states the atoms relax to energetically lower states and are ejected from the He_N. The relaxation processes include bare atom spin-forbidden transitions, which clearly bears the signature of the He_N influence. Two-color resonant two-photon ionization (2CR2PI) also shows the formation of bare atoms and small Cr-He_n and Cu-He_n clusters in their ground and metastable states ^c. Currently, Cr dimer excitation studies are in progress and a brief outlook on the available results will be given. C. Callegari and W. E. Ernst, Helium Droplets as Nanocryostats for Molecular Spectroscopy - from the Vacuum Ultraviolet to the Microwave Regime, in Handbook of High-Resolution Spectroscopy, eds. M. Quack and F. Merkt, John Wiley & Sons, Chichester, 2011. A. Kautsch, M. Koch, and W. E. Ernst, J. Phys. Chem. A, 117 (2013) 9621-9625, DOI: 10.1021/jp312336m F. Lindebner, A. Kautsch, M. Koch, and W. E. Ernst, Int. J. Mass Spectrom. (2014) in press, DOI: 10.1016/j.ijms.2013.12.022 M. Koch, A. Kautsch, F. Lackner, and W. E. Ernst, submitted to J. Phys. Chem. A

  14. State Efforts to Meet the Early Childhood Transition Requirements of IDEA. inForum

    ERIC Educational Resources Information Center

    Muller, Eve; Whaley, Kathy; Rous, Beth

    2009-01-01

    In March 2008, the National Early Childhood Transition Initiative released a document developed collaboratively over several months titled "Designing and Implementing Effective Early Childhood Transition Processes". The document was created as a resource for improving state and local performance on the State Performance Plans (SPP) and Annual…

  15. Functionalization of transition metal dichalcogenides with metallic nanoparticles: implications for doping and gas-sensing.

    PubMed

    Sarkar, Deblina; Xie, Xuejun; Kang, Jiahao; Zhang, Haojun; Liu, Wei; Navarrete, Jose; Moskovits, Martin; Banerjee, Kaustav

    2015-05-13

    Transition metal dichalcogenides (TMDs), belonging to the class of two-dimensional (2D) layered materials, have instigated a lot of interest in diverse application fields due to their unique electrical, mechanical, magnetic, and optical properties. Tuning the electrical properties of TMDs through charge transfer or doping is necessary for various optoelectronic applications. This paper presents the experimental investigation of the doping effect on TMDs, mainly focusing on molybdenum disulfide (MoS2), by metallic nanoparticles (NPs), exploring noble metals such as silver (Ag), palladium (Pd), and platinum (Pt) as well as the low workfunction metals such as scandium (Sc) and yttrium (Y) for the first time. The dependence of the doping behavior of MoS2 on the metal workfunction is demonstrated and it is shown that Pt nanoparticles can lead to as large as 137 V shift in threshold voltage of a back-gated monolayered MoS2 FET. Variation of the MoS2 FET transfer curves with the increase in the dose of NPs as well as the effect of the number of MoS2 layers on the doping characteristics are also discussed for the first time. Moreover, the doping effect on WSe2 is studied with the first demonstration of p-type doping using Pt NPs. Apart from doping, the use of metallic NP functionalized TMDs for gas sensing application is also demonstrated. PMID:25723363

  16. The Timing of School Transitions and Early Adolescent Problem Behavior

    ERIC Educational Resources Information Center

    Lippold, Melissa A.; Powers, Christopher J.; Syvertsen, Amy K.; Feinberg, Mark E.; Greenberg, Mark T.

    2013-01-01

    This longitudinal study investigates whether rural adolescents who transition to a new school in sixth grade have higher levels of risky behavior than adolescents who transition in seventh grade. Our findings indicate that later school transitions had little effect on problem behavior between sixth and ninth grades. Cross-sectional analyses found…

  17. Dense organic-inorganic framework materials containing transition metal ions

    NASA Astrophysics Data System (ADS)

    Feller, Russell Kenneth

    2008-10-01

    Hybrid inorganic-organic framework materials built upon metal cations and polyfunctional anionic ligands have arrived at the forefront of chemical research, boasting unique properties that derive from their diverse structures. The majority of these compounds are prepared through mild hydrothermal or solvothermal synthesis, at temperatures below 225°C. There has been a great deal of emphasis on porous coordination polymers, or "metal-organic frameworks" (MOFs), which exhibit a wide range of useful sorption and catalytic properties, but there is growing interest in a second class of hybrid frameworks which are denser and often have extended inorganic connectivity, which may be thought of as hybrid metal oxides. These denser compounds provide access to more oxide-like properties such as magnetism, along with improved thermal stability. The flourishing diversity of structures and dimensionalities seen in these materials has led to their growth into a major field of research. Here, the hydrothermal syntheses and crystal structures of fourteen new hybrid framework materials are reported, and additional properties are measured for several of them. The terminology used to describe this class of materials is laid out, and a classification scheme is introduced based on the dimensionalities of their structures. While a combinatorial approach for new compound discovery is often used, strategies for rational synthesis are also discussed. A family of isostructural 3,4,5-trihydroxybenzoate (gallate) hybrid frameworks of the first-row transition metals is produced, with chiral structures imparted by helical packing arrangements. Complex magnetic behavior is characterized in a pillared layered cobalt ethanedisulfonate, the structure of which features intricate metal-oxygen-metal connectivity. Frameworks of new, sometimes unreported, ligand molecules are created through the reactivities of aspartic acid and 5-hydroxyisophthalic acid under hydrothermal conditions. A family of two-ligand frameworks are created using 5-hydroxyisophthalic acid and 4,4'-bipyridyl, which form very different structures when reacted with Mn, Ni, Cu and Zn. Finally, reactions of Cu and Zn together with these two ligands lead to a family of four- and five-component hybrid frameworks showing even greater degrees of structural complexity.

  18. Early hominin speciation at the Plio/Pleistocene transition.

    PubMed

    Cameron, D W

    2003-01-01

    Over the last half-decade or so, there has been an explosion in the recognition of hominin genera and species. We now have the late Miocene genera Orrorin and Sahelanthropus, the mid Pliocene genus Kenyanthropus, three new Pliocene species of Australopithecus (A. anamensis, A. garhi and A. bahrelghazali) and a sub species of Ardipithecus (Ar. r. kadabba) to contend with. Excepting also the more traditional species allocated to Paranthropus, Australopithecus and early Homo we are approaching around 15 species over 5 million years (excluding hominin evolution over the last one million years). Can such a large number of hominin species be justified? An examination of extant hominid (Gorilla gorilla, Pan troglodytes, and Pan paniscus) anatomical variability indicates that the range of fossil hominin variability supports the recognition of this large number of fossil species. It is also shown that not all hominins are directly related to the emergence of early Homo and as such have become extinct. Indeed the traditional australopithecine species 'A'. anamensis, 'A'. afarensis and 'A'. garhi are considered here to belong to a distinct genus Praeanthropus. They are also argued not be hominins, but rather an as yet undefined hominid group from which the more derived hominins evolved. The first hominin is represented by A. africanus or a hominin very much like it. The Paranthropus clade is defined by a derived heterochronic condition of peramorphosis, associated with sequential progenesis (contraction of successive growth stages) in brain and dental development, but a mixture of peramorphic and paedomorphic features in its craniofacial anatomy. Conversely, Kenyanthropus and Homo both share a pattern of peramorphosis, associated with sequential hypermorphosis (prolongation of successive growth stages) in brain development, and paedomorphosis processes in cranial, facial and dental development. This suggests, that these two clades share an important synapomorphy not recognised in the parsimony analyses, suggesting that they may form a sister group relationship to the exclusion of Paranthropus. This highlights the need to re-interpret phylogenetic results in terms of function and development. The rapid speciation and extinction as argued here is in keeping with other fossil groups in Africa at the Plio/Pleistocene transition. This emphasises that we must approach the pre-australopithecines and hominins as part of the endemic African fauna, and not in isolation to the evolutionary and climatic processes that were operating all around them. PMID:12968420

  19. Parents' Marital Distress, Divorce, and Remarriage: Links with Daughters' Early Family Formation Transitions

    ERIC Educational Resources Information Center

    Amato, Paul R.; Kane, Jennifer B.

    2011-01-01

    The authors used data from the Add Health study to estimate the effects of parents' marital status and relationship distress on daughters' early family formation transitions. Outcomes included traditional transitions (marriage and marital births) and nontraditional transitions (cohabitation and nonmarital births). Relationship distress among…

  20. Parents' Marital Distress, Divorce, and Remarriage: Links with Daughters' Early Family Formation Transitions

    ERIC Educational Resources Information Center

    Amato, Paul R.; Kane, Jennifer B.

    2011-01-01

    The authors used data from the Add Health study to estimate the effects of parents' marital status and relationship distress on daughters' early family formation transitions. Outcomes included traditional transitions (marriage and marital births) and nontraditional transitions (cohabitation and nonmarital births). Relationship distress among

  1. Tailoring Nanoscale Friction in MX2 Transition Metal Dichalcogenides.

    PubMed

    Cammarata, Antonio; Polcar, Tomáš

    2015-06-15

    Lattice dynamics of MX2 transition metal dichalcogenides (M = Mo, W; X = S, Se, Te) have been studied with density functional theory techniques to control the macroscopic tribological behavior. Long-range van der Waals forces have been modeled with Grimme correction to capture the interlayer interactions. A new lattice dynamic metric, named cophonicity, is proposed and used in combination with electronic and geometric descriptors to relate the stability of the lattice distortions with the electro-structural features of the system. The cophonicity analysis shows that the distortion modes relevant to the microscopic friction can be controlled by tuning the relative M/X atomic contributions to the phonon density of states. Guidelines on how to engineer macroscopic friction at nanoscale are formulated, and finally applied to design a new Ti-doped MoS2 phase with enhanced tribologic properties. PMID:26000720

  2. Spin and pseudospins in layered transition metal dichalcogenides

    SciTech Connect

    Xu, Xiaodong; Yao, Wang; Xiao, Di; Heinz, Tony F.

    2014-01-01

    The recent emergence of two-dimensional layered materials in particular the transition metal dichalcogenides provides a new laboratory for exploring the internal quantum degrees of freedom of electrons and their potential for new electronics. These degrees of freedom are the real electron spin, the layer pseudospin, and the valley pseudospin. New methods for the quantum control of the spin and these pseudospins arise from the existence of Berry phase-related physical properties and strong spin orbit coupling. The former leads to the versatile control of the valley pseudospin, whereas the latter gives rise to an interplay between the spin and the pseudospins. Here, we provide a brief review of both theoretical and experimental advances in this field.

  3. Continuously tunable electronic structure of transition metal dichalcogenides superlattices

    PubMed Central

    Zhao, Yong-Hong; Yang, Feng; Wang, Jian; Guo, Hong; Ji, Wei

    2015-01-01

    Two dimensional transition metal dichalcogenides have very exciting properties for optoelectronic applications. In this work we theoretically investigate and predict that superlattices comprised of MoS2 and WSe2 multilayers possess continuously tunable electronic structure with direct bandgaps. The tunability is controlled by the thickness ratio of MoS2 versus WSe2 of the superlattice. When this ratio goes from 1:2 to 5:1, the dominant K-K direct bandgap is continuously tuned from 0.14 eV to 0.5 eV. The gap stays direct against −0.6% to 2% in-layer strain and up to −4.3% normal-layer compressive strain. The valance and conduction bands are spatially separated. These robust properties suggest that MoS2 and WSe2 multilayer superlattice should be a promising material for infrared optoelectronics. PMID:25677917

  4. Improved Versions of Fielek Model for BCC Transition Metals

    NASA Astrophysics Data System (ADS)

    Khanna, R. N.; Rathore, R. P. S.

    1980-09-01

    Three improved versions of the Fielek Model are developed and discussed. The first model uses a modified Bhatia scheme for the volume interaction in place of the Krebs scheme. The second and third model assumes the core-core couplings to be central pairwise effective upto first neighbours only. The second model describes the d-shell-d-shell interaction on the same lines as those adopted for core-core interactions, while the third model assumes these interactions to be of the many body type. Suitable equilibrium conditions for the second and the third model are developed. These improved versions are employed to derive dispersion relations in tantalum. Comparison of these relations with-each other and also with the experimental findings leads to some important conclusions regarding the nature and range of the interactions coupling the various constituents of a bcc transition metal like tantalum.

  5. Radiation damage of transition metal carbides. Final technical report

    SciTech Connect

    Dixon, G.

    1991-12-31

    In this grant period we have investigated electrical properties of transition metal carbides and radiation-induced defects produced by low-temperature electron irradiation in them. Special attention has been given to the composition VC{sub 0.88} in which the vacancies on the carbon sublattice of this fcc crystal order to produce a V{sub 8}C{sub 7} superlattice. The existence of this superlattice structure was found to make the crystal somewhat resistant to radiation damage at low doses and/or at ambient temperature. At larger doses significant changes in the resistivity are produced. Annealing effects were observed which we believe to be connected with the reconstitution of the superlattice structure.

  6. Theoretical ultra-fast spectroscopy in transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Molina-Sanchez, Alejandro; Sangalli, Davide; Marini, Andrea; Wirtz, Ludger

    Semiconducting 2D-materials like the transition metal dichalcogenides (TMDs) MoS2, MoSe2, WS2, WSe2 are promising alternatives to graphene for designing novel opto-electronic devices. The strong spin-orbit interaction along with the breaking of inversion symmetry in single-layer TMDs allow using the valley-index as a new quantum number. The practical use of valley physics depends on the lifetimes of valley-polarized excitons which are affected by scattering at phonons, impurities and by carrier-carrier interactions. The carrier dynamics can be monitored using ultra-fast spectroscopies such as pump-probe experiments. The carrier dynamics is simulated using non-equilibrium Green's function theory in an ab-initio framework. We include carrier relaxation through electron-phonon interaction. We obtain the transient absorption spectra of single-layer TMD and compare our simulations with recent pump-probe experiments

  7. Transition Metals Catalyzed Element-Cyano Bonds Activations

    PubMed Central

    Wang, Rui; Falck, John R.

    2014-01-01

    Cyano group as a versatile functionalized intermediate has been explored for several decades, as it readily transfers to many useful functionalization groups such as amine, amide, acid, etc., which make it possess high popularization and use value in organic synthesis. Reactions involved with element-cyano bond cleavage can provide not only a new cyano group but also a freshly functionalized skeleton in one-pot, consequently making it of high importance. The highlights reviewed herein include H-CN, Si-CN, C-CN, B-CN, Sn-CN, Ge-CN, S-CN, Halo-CN, N-CN, and O-CN bonds cleavages and will summarize progress in such an important research area. This review article will focus on transition metal catalyzed reactions involving element-cyano bond activation. PMID:25558119

  8. Noncollinear exchange interaction in transition metal dichalcogenide edges

    NASA Astrophysics Data System (ADS)

    Ávalos-Ovando, Oscar; Mastrogiuseppe, Diego; Ulloa, Sergio E.

    2016-04-01

    We study the Ruderman-Kittel-Kasuya-Yosida effective exchange interaction between magnetic impurities embedded on the edges of transition metal dichalcogenide flakes, using a three-orbital tight-binding model. Electronic states lying midgap of the bulk structure have a strong one-dimensional (1D) character, localized on the edges of the crystallite. This results in exchange interactions with 1 /r (or slower) decay with distance r , similar to other 1D systems. Most interestingly, however, the strong spin-orbit interaction in these materials results in sizable noncollinear Dzyaloshinskii-Moriya interactions between impurities, comparable in size to the usual Ising and in-plane components. Varying the relevant Fermi energy by doping or gating may allow one to modulate the effective interactions, controlling the possible helical ground state configurations of multiple impurities.

  9. Transition metal oxide hierarchical nanotubes for energy applications

    NASA Astrophysics Data System (ADS)

    Wei, Wei; Wang, Yongcheng; Wu, Hao; Al-Enizi, Abdullah M.; Zhang, Lijuan; Zheng, Gengfeng

    2016-01-01

    We report a general synthetic method for transition metal oxide (TMO) hierarchical nanotube (HNT) structures by a solution-phase cation exchange method from Cu2O nanowire templates. This method leads to the formation of hollow, tubular backbones with secondary, thin nanostructures on the tube surface, which substantially increases the surface reactive sites for electrolyte contacts and electrochemical reactions. As proofs-of-concept, several representative first-row TMO HNTs have been synthesized, including CoO x , NiO x , MnO x , ZnO x and FeO x , with specific surface areas much larger than nanotubes or nanoparticles of corresponding materials. An example of the potential energy storage applications of CoO x HNTs as supercapacitors is also demonstrated.

  10. Exchange coupling in transition metal monoxides: Electronic structure calculations

    SciTech Connect

    Fischer, Guntram; Daene, Markus W; Ernst, Arthur; Bruno, Patrick; Lueders, Martin; Szotek, Zdzislawa; Temmerman, Walter M; Wolfam, Hergert

    2009-01-01

    An ab initio study of magnetic-exchange interactions in antiferromagnetic and strongly correlated 3d transition metal monoxides is presented. Their electronic structure is calculated using the local self-interaction correction approach, implemented within the Korringa-Kohn-Rostoker band-structure method, which is based on multiple scattering theory. The Heisenberg exchange constants are evaluated with the magnetic force theorem. Based on these the corresponding Neel temperatures TN and spin-wave dispersions are calculated. The Neel temperatures are obtained using mean-field approximation, random-phase approximation and Monte Carlo simulations. The pressure dependence of TN is investigated using exchange constants calculated for different lattice constants. All the calculated results are compared to experimental data.

  11. Ionically-mediated electromechanical hysteresis in transition metal oxides.

    PubMed

    Kim, Yunseok; Morozovska, Anna N; Kumar, Amit; Jesse, Stephen; Eliseev, Eugene A; Alibart, Fabien; Strukov, Dmitri; Kalinin, Sergei V

    2012-08-28

    Nanoscale electromechanical activity, remanent polarization states, and hysteresis loops in paraelectric TiO(2) and SrTiO(3) thin films are observed using scanning probe microscopy. The coupling between the ionic dynamics and incipient ferroelectricity in these materials is analyzed using extended Landau-Ginzburg-Devonshire (LGD) theory. The possible origins of electromechanical coupling including ionic dynamics, surface-charge induced electrostriction, and ionically induced ferroelectricity are identified. For the latter, the ionic contribution can change the sign of first order LGD expansion coefficient, rendering material effectively ferroelectric. The lifetime of these ionically induced ferroelectric states is then controlled by the transport time of the mobile ionic species and well above that of polarization switching. These studies provide possible explanation for ferroelectric-like behavior in centrosymmetric transition metal oxides. PMID:22845698

  12. Theory of strain in single-layer transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Rostami, Habib; Roldán, Rafael; Cappelluti, Emmanuele; Asgari, Reza; Guinea, Francisco

    2015-11-01

    Strain engineering has emerged as a powerful tool to modify the optical and electronic properties of two-dimensional crystals. Here we perform a systematic study of strained semiconducting transition metal dichalcogenides. The effect of strain is considered within a full Slater-Koster tight-binding model, which provides us with the band structure in the whole Brillouin zone (BZ). From this, we derive an effective low-energy model valid around the K point of the BZ, which includes terms up to second order in momentum and strain. For a generic profile of strain, we show that the solutions for this model can be expressed in terms of the harmonic oscillator and double quantum well models, for the valence and conduction bands respectively. We further study the shift of the position of the electron and hole band edges due to uniform strain. Finally, we discuss the importance of spin-strain coupling in these 2D semiconducting materials.

  13. Phase stability and electronic structure of transition-metal aluminides

    SciTech Connect

    Carlsson, A.E.

    1992-12-31

    This paper will describe the interplay between die electronic structure and structural energetics in simple, complex, and quasicrystalline Al-transition metal (T) intermetallics. The first example is the Ll{sub 2}-DO{sub 22} competition in Al{sub 3}T compounds. Ab-initio electronic total-energy calculations reveal surprisingly large structural-energy differences, and show that the phase stability of both stoichiometric and ternary-substituted compounds correlates closely with a quasigap in the electronic density of states (DOS). Secondly, ab-initio calculations for the structural stability of the icosahedrally based Al{sub 12}W structure reveal similar quasigap effects, and provide a simple physical explanation for the stability of the complex aluminide structures. Finally, parametrized tight-binding model calculations for the Al-Mn quasicrystal reveal a large spread in the local Mn DOS behavior, and support a two-site model for the quasicrystal`s magnetic behavior.

  14. Phase stability and electronic structure of transition-metal aluminides

    SciTech Connect

    Carlsson, A.E.

    1992-01-01

    This paper will describe the interplay between die electronic structure and structural energetics in simple, complex, and quasicrystalline Al-transition metal (T) intermetallics. The first example is the Ll[sub 2]-DO[sub 22] competition in Al[sub 3]T compounds. Ab-initio electronic total-energy calculations reveal surprisingly large structural-energy differences, and show that the phase stability of both stoichiometric and ternary-substituted compounds correlates closely with a quasigap in the electronic density of states (DOS). Secondly, ab-initio calculations for the structural stability of the icosahedrally based Al[sub 12]W structure reveal similar quasigap effects, and provide a simple physical explanation for the stability of the complex aluminide structures. Finally, parametrized tight-binding model calculations for the Al-Mn quasicrystal reveal a large spread in the local Mn DOS behavior, and support a two-site model for the quasicrystal's magnetic behavior.

  15. Extraction of exchange parameters in transition-metal perovskites

    NASA Astrophysics Data System (ADS)

    Furrer, A.; Podlesnyak, A.; Krämer, K. W.

    2015-09-01

    The extraction of exchange parameters from measured spin-wave dispersion relations has severe limitations particularly for magnetic compounds such as the transition-metal perovskites, where the nearest-neighbor exchange parameter usually dominates the couplings between the further-distant-neighbor spins. Very precise exchange parameters beyond the nearest-neighbor spins can be obtained by neutron spectroscopic investigations of the magnetic excitation spectra of isolated multimers in magnetically diluted compounds. This is exemplified for manganese trimers in the mixed three- and two-dimensional perovskite compounds KM nxZ n1 -xF3 and K2M nxZ n1 -xF4 , respectively. It is shown that the small exchange couplings between the second-nearest-neighbor and the third-nearest-neighbor spins can be determined unambiguously and with equal precision as the dominating nearest-neighbor exchange coupling.

  16. Observation of Interlayer Phonons in Transition Metal Dichalcogenide Heterostructures

    NASA Astrophysics Data System (ADS)

    He, Rui; Ye, Zhipeng; Ji, Chao; Means-Shively, Casie; Anderson, Heidi; Kidd, Tim; Chiu, Kuan-Chang; Chou, Cheng-Tse; Wu, Jenn-Ming; Lee, Yi-Hsien; Andersen, Trond; Lui, Chun Hung

    Interlayer phonon modes in transition metal dichalcogenide (TMD) heterostructures are observed for the first time. We measured the low-frequency Raman response of MoS2/WSe2 and MoSe2/MoS2 heterobilayers. We discovered a distinct Raman mode (30 - 35 cm-1) that cannot be found in any individual monolayers. By comparing with Raman spectra of Bernal bilayer (2L) MoS2, 2L MoSe2 and 2L WSe2, we identified the new Raman mode as the layer breathing vibration arising from the vertical displacement of the two TMD layers. The layer breathing mode (LBM) only emerges in bilayer regions with atomically close layer-layer proximity and clean interface. In addition, the LBM frequency exhibits noticeable dependence on the rotational angle between the two TMD layers, which implies a change of interlayer separation and interlayer coupling strength with the layer stacking.

  17. Transition Metal Dichalcogenide Growth via Close Proximity Precursor Supply

    PubMed Central

    O'Brien, Maria; McEvoy, Niall; Hallam, Toby; Kim, Hye-Young; Berner, Nina C.; Hanlon, Damien; Lee, Kangho; Coleman, Jonathan N.; Duesberg, Georg S.

    2014-01-01

    Reliable chemical vapour deposition (CVD) of transition metal dichalcogenides (TMDs) is currently a highly pressing research field, as numerous potential applications rely on the production of high quality films on a macroscopic scale. Here, we show the use of liquid phase exfoliated nanosheets and patterned sputter deposited layers as solid precursors for chemical vapour deposition. TMD monolayers were realized using a close proximity precursor supply in a CVD microreactor setup. A model describing the growth mechanism, which is capable of producing TMD monolayers on arbitrary substrates, is presented. Raman spectroscopy, photoluminescence, X-ray photoelectron spectroscopy, atomic force microscopy, transmission electron microscopy, scanning electron microscopy and electrical transport measurements reveal the high quality of the TMD samples produced. Furthermore, through patterning of the precursor supply, we achieve patterned growth of monolayer TMDs in defined locations, which could be adapted for the facile production of electronic device components. PMID:25487822

  18. Proximity-induced magnetism in transition-metal substituted graphene

    SciTech Connect

    Crook, Charles B.; Constantin, Costel; Ahmed, Towfiq; Zhu, Jian -Xin; Balatsky, Alexander V.; Haraldsen, Jason T.

    2015-08-03

    We investigate the interactions between two identical magnetic impurities substituted into a graphene superlattice. Using a first-principles approach, we calculate the electronic and magnetic properties for transition-metal substituted graphene systems with varying spatial separation. These calculations are compared for three different magnetic impurities, manganese, chromium, and vanadium. We determine the electronic band structure, density of states, and Millikan populations (magnetic moment) for each atom, as well as calculate the exchange parameter between the two magnetic atoms as a function of spatial separation. We find that the presence of magnetic impurities establishes a distinct magnetic moment in the graphene lattice, where the interactions are highly dependent on the spatial and magnetic characteristic between the magnetic and carbon atoms, which leads to either ferromagnetic or antiferromagnetic behavior. Furthermore, through an analysis of the calculated exchange energies and partial density of states, it is determined that interactions between the magnetic atoms can be classified as an RKKY interaction.

  19. Transition-metal dimers and physical limits on magnetic anisotropy.

    PubMed

    Strandberg, Tor O; Canali, Carlo M; MacDonald, Allan H

    2007-09-01

    Recent advances in nanoscience have raised interest in the minimum bit size required for classical information storage. This bit size is determined by the necessity for bistability with suppressed quantum tunnelling and energy barriers that exceed ambient temperatures. In the case of magnetic information storage, much attention has centred on molecular magnets with bits consisting of about 100 atoms, magnetic uniaxial anisotropy energy barriers of about 50 K and very slow relaxation at low temperatures. Here, we draw attention to the remarkable magnetic properties of some transition-metal dimers, which have energy barriers approaching 500 K with only two atoms. The spin dynamics of these ultrasmall nanomagnets is strongly affected by a Berry phase, which arises from quasi-degeneracies at the electronic highest occupied molecular orbital energy. In a giant-spin approximation, this Berry phase makes the effective reversal barrier thicker. PMID:17660824

  20. Proximity-induced magnetism in transition-metal substituted graphene

    NASA Astrophysics Data System (ADS)

    Crook, Charles B.; Constantin, Costel; Ahmed, Towfiq; Zhu, Jian-Xin; Balatsky, Alexander V.; Haraldsen, Jason T.

    2015-08-01

    We investigate the interactions between two identical magnetic impurities substituted into a graphene superlattice. Using a first-principles approach, we calculate the electronic and magnetic properties for transition-metal substituted graphene systems with varying spatial separation. These calculations are compared for three different magnetic impurities, manganese, chromium, and vanadium. We determine the electronic band structure, density of states, and Millikan populations (magnetic moment) for each atom, as well as calculate the exchange parameter between the two magnetic atoms as a function of spatial separation. We find that the presence of magnetic impurities establishes a distinct magnetic moment in the graphene lattice, where the interactions are highly dependent on the spatial and magnetic characteristic between the magnetic and carbon atoms, which leads to either ferromagnetic or antiferromagnetic behavior. Furthermore, through an analysis of the calculated exchange energies and partial density of states, it is determined that interactions between the magnetic atoms can be classified as an RKKY interaction.

  1. The Role of Carbon in Catalytically Stabilized Transition Metal Sulfides

    SciTech Connect

    Kelty,S.; Berhault, G.; Chianelli, R.

    2007-01-01

    Since WWII considerable progress has been made in understanding the basis for the activity and the selectivity of molybdenum and tungsten based hydrotreating catalysts. Recently, the focus of investigation has turned to the structure of the catalytically stabilized active catalyst. The surface of the catalytically stabilized MoS2 has been shown to be carbided with the formula MoSxCy under hydrotreating conditions. In this paper we review the basis for this finding and present new data extending the concept to the promoted TMS (transition metal sulfides) systems CoMoC and NiMoC. Freshly sulfided CoMoS and NiMoS catalyst have a strong tendency to form the carbided surface phases from any available carbon source.

  2. Proximity-induced magnetism in transition-metal substituted graphene

    PubMed Central

    Crook, Charles B.; Constantin, Costel; Ahmed, Towfiq; Zhu, Jian-Xin; Balatsky, Alexander V.; Haraldsen, Jason T.

    2015-01-01

    We investigate the interactions between two identical magnetic impurities substituted into a graphene superlattice. Using a first-principles approach, we calculate the electronic and magnetic properties for transition-metal substituted graphene systems with varying spatial separation. These calculations are compared for three different magnetic impurities, manganese, chromium, and vanadium. We determine the electronic band structure, density of states, and Millikan populations (magnetic moment) for each atom, as well as calculate the exchange parameter between the two magnetic atoms as a function of spatial separation. We find that the presence of magnetic impurities establishes a distinct magnetic moment in the graphene lattice, where the interactions are highly dependent on the spatial and magnetic characteristic between the magnetic and carbon atoms, which leads to either ferromagnetic or antiferromagnetic behavior. Furthermore, through an analysis of the calculated exchange energies and partial density of states, it is determined that interactions between the magnetic atoms can be classified as an RKKY interaction. PMID:26235646

  3. Proximity-induced magnetism in transition-metal substituted graphene.

    PubMed

    Crook, Charles B; Constantin, Costel; Ahmed, Towfiq; Zhu, Jian-Xin; Balatsky, Alexander V; Haraldsen, Jason T

    2015-01-01

    We investigate the interactions between two identical magnetic impurities substituted into a graphene superlattice. Using a first-principles approach, we calculate the electronic and magnetic properties for transition-metal substituted graphene systems with varying spatial separation. These calculations are compared for three different magnetic impurities, manganese, chromium, and vanadium. We determine the electronic band structure, density of states, and Millikan populations (magnetic moment) for each atom, as well as calculate the exchange parameter between the two magnetic atoms as a function of spatial separation. We find that the presence of magnetic impurities establishes a distinct magnetic moment in the graphene lattice, where the interactions are highly dependent on the spatial and magnetic characteristic between the magnetic and carbon atoms, which leads to either ferromagnetic or antiferromagnetic behavior. Furthermore, through an analysis of the calculated exchange energies and partial density of states, it is determined that interactions between the magnetic atoms can be classified as an RKKY interaction. PMID:26235646

  4. Transition metal oxide hierarchical nanotubes for energy applications.

    PubMed

    Wei, Wei; Wang, Yongcheng; Wu, Hao; Al-Enizi, Abdullah M; Zhang, Lijuan; Zheng, Gengfeng

    2016-01-15

    We report a general synthetic method for transition metal oxide (TMO) hierarchical nanotube (HNT) structures by a solution-phase cation exchange method from Cu2O nanowire templates. This method leads to the formation of hollow, tubular backbones with secondary, thin nanostructures on the tube surface, which substantially increases the surface reactive sites for electrolyte contacts and electrochemical reactions. As proofs-of-concept, several representative first-row TMO HNTs have been synthesized, including CoOx, NiOx, MnOx, ZnOx and FeOx, with specific surface areas much larger than nanotubes or nanoparticles of corresponding materials. An example of the potential energy storage applications of CoOx HNTs as supercapacitors is also demonstrated. PMID:26629880

  5. Intrinsic disorder in graphene on transition metal dichalcogenide heterostructures

    NASA Astrophysics Data System (ADS)

    Yankowitz, Matthew; Larentis, Stefano; Kim, Kyounghwam; Xue, Jiamin; McKenzie, Devin; Huang, Shengqiang; Paggen, Marina; Ali, Mazhar; Cava, Robert; Tutuc, Emanuel; Leroy, Brian J.

    2015-03-01

    Recently, semiconducting materials in the transition metal dichalcogenide (TMD) family have gained great popularity for use in novel graphene-based heterostructure devices such as tunneling transistors, highly efficient flexible photovoltaic devices, and nonvolatile memory cells. TMDs have also been explored as alternatives to hexagonal boron nitride (hBN) as substrates for pristine graphene devices. However, their quality has thus far been significantly worse than comparable hBN devices. We examine graphene on numerous TMD substrates (MoS2, WS2, WSe2, MoTe2) with scanning tunneling microscopy and spectroscopy and find that point and line defects intrinsic to all TMD crystals (both of natural and synthetic origin) result in scattering of electrons in graphene. Our findings suggest that the quality of graphene on TMD heterostructures is limited by the intrinsic crystalline quality of the TMDs.

  6. The Timing of School Transitions and Early Adolescent Problem Behavior.

    PubMed

    Lippold, Melissa A; Powers, Christopher J; Syvertsen, Amy K; Feinberg, Mark E; Greenberg, Mark T

    2013-08-01

    This longitudinal study investigates whether rural adolescents who transition to a new school in sixth grade have higher levels of risky behavior than adolescents who transition in seventh grade. Our findings indicate that later school transitions had little effect on problem behavior between sixth and ninth grades. Cross-sectional analyses found a small number of temporary effects of transition timing on problem behavior: Spending an additional year in elementary school was associated with higher levels of deviant behavior in the Fall of Grade 6 and higher levels of antisocial peer associations in Grade 8. However, transition effects were not consistent across waves and latent growth curve models found no effects of transition timing on the trajectory of problem behavior. We discuss policy implications and compare our findings with other research on transition timing. PMID:24089584

  7. Towards understanding a mechanism for reversible hydrogen storage: theoretical study of transition metal catalysed dehydrogenation of sodium alanate.

    PubMed

    Ljubić, Ivan; Clary, David C

    2010-04-28

    On the basis of density functional theory and coupled-cluster CCSD(T) calculations we propose a mechanism of the dehydrogenation of transition metal doped sodium alanate. Insertion of two early 3d-transition metals, scandium and titanium, both of which are promising catalysts for reversible hydrogen storage in light metal hydrides, is compared. The mechanism is deduced from studies on the decomposition of a model system consisting of one transition metal atom and two NaAlH(4) units. Subsequently, the significance of such minimal cluster model systems to the real materials is tested by embedding the systems into the surface of the NaAlH(4) crystal. It is found that the dehydrogenation proceeds via breaking of the bridge H-Al bond and consequent formation of intermediate coordination compounds in which the H(2) molecule is side-on (eta(2)-) bonded to the transition metal centre. The total barrier to the H(2) release is thus dependent upon both the strength of the Al-H bond to be broken and the depth of the coordinative potential. The analogous mechanism applies for the recognized three successive dehydrogenation steps. The gas-phase model structures embedded into the surface of the NaAlH(4) crystal exhibit an unambiguous kinetic stability and their general geometric features remain largely unchanged. PMID:20379493

  8. Electron-energy-loss spectroscopy of transition-metal carbides

    SciTech Connect

    Allison, C.

    1983-01-01

    Electron-energy-loss spectroscopy (EELS) was applied to several problems dealing with the transition-metal carbides. First, narrow (20/sup 0/A) precipitates of TiC were identified in a single crystal of titanium diboride. Second, the carbon concentration in individual stacking faults of TaC was determined to be less than in the surrounding matrix, and the implications of this finding for crystal structure and mechanical properties were discussed. Next, thin films of amorphous carbon and TiO were studied with EELS. Experimental cross sections for C and Ti were obtained from these spectra and used for quantitative analysis of a material of known stoichiometry, V/sub 6/C/sub 5/ (by assuming V and Ti cross sections equal), and one of unknown stoichiometry, a cemented carbide composed of 1 to 5 ..mu..m grains of TiC/sub x/ embedded in a Ni-Mo binder. These experimental cross sections gave a C/V ratio of 0.84, very close to the expected value of 0.833, and a C/Ti ratio of 0.81 for a grain of TiC/sub x/, a reasonable value of x in these materials. Two other methods of determining cross sections were applied: calculated cross sections based on a hydrogenic model gave inaccurate results, perhaps due to the inadequacy of the model for these transition metals; and efficiency factors applied to the Bethe cross section gave reasonable results, but inferior to those from the experimental cross sections.

  9. Synthesis and characterization of transition metal doped semiconducting nanowires

    NASA Astrophysics Data System (ADS)

    Kaszpurenko, Jason Michael

    The abundance of semiconductors in everyday life has exploded because of their cheapness, ability to do massive calculations, harvest energy and more. For all their utility semiconductors used in calculations suffer because they need an auxiliary way to store the data they've calculated. Magnetic storage has traditionally been the answer to this problem but suffers from slower speeds. Since the 1960's a class of materials known as dilute magnetic semiconductors has tried to combine the advantages of semiconductors with the non-volatile storage properties found in magnets. Often the easiest way to make these materials is by doping semiconductors with transition metal ions. In this study I worked with PbS and ZnSe to create transition metal doped semiconducting nanostructures. The initial studies focus on the synthesis and characterization of PbS nanowires doped with Mn. The wires revealed high quality nanowires with uniform doping concentrations, both axially and radially, with atomic concentrations of 0.18 and 0.01 atomic %. The Mn didn't create any secondary phases and was substitutionally introduced. Zn1-xMn xSe nanostructures were grown with the hopes of achieving a higher Mn doping concentration where we succeeded in achieving dopant levels of x~0.3. To increase carrier concentrations, estimated to be~1016cm -3 for pure ZnSe samples, Al was doped with ZnSe and co-doped with Mn. ZnAlSe nanowires showed carrier concentration ~1019cm -3. Optical studies revealed hole traps with a characteristic time on the order of 1ms in ZnAlSe nanowire samples

  10. Transition Metal Substitution Effects on Metal-to-Polyoxometalate Charge Transfer.

    PubMed

    Glass, Elliot N; Fielden, John; Huang, Zhuangqun; Xiang, Xu; Musaev, Djamaladdin G; Lian, Tianquan; Hill, Craig L

    2016-05-01

    A series of hetero-bimetallic transition metal-substituted polyoxometalates (TMSPs) were synthesized based on the Co(II)-centered ligand [Co(II)W11O39](10-). The eight complex series, [Co(II)(M(x)OHy)W11O39]((12-x-y)-) (M(x)OHy = V(IV)O, Cr(III)(OH2), Mn(II)(OH2), Fe(III)(OH2), Co(II)(OH2), Ni(II)(OH2), Cu(II)(OH2), Zn(II)(OH2)), of which six are reported for the first time, was synthesized starting from [Co(III)W11O39](9-) and studied using spectroscopic, electrochemical, and computational techniques to evaluate the influence of substituted transition metals on the photodynamics of the metal-to-polyoxometalate charge transfer (MPCT) transition. The bimetallic complexes all show higher visible light absorption than the plenary [Co(II)W12O40](6-) and demonstrate the same MPCT transition as the plenary complex, but they have shorter excited-state lifetimes (sub-300 ps in aqueous media). The decreased lifetimes are rationalized on the basis of nonradiative relaxation due to coordinating aqua ligands, increased interaction with cations due to increased negative charge, and the energy gap law, with the strongest single factor appearing to be the charge on the anion. The most promising results are from the Cr- and Fe-substituted systems, which retain excited-state lifetimes at least 50% of that of [Co(II)W12O40](6-) while more than tripling the absorbance at 400 nm. PMID:27082443

  11. Oligomeric rare-earth metal cluster complexes with endohedral transition metal atoms

    SciTech Connect

    Steinberg, Simon; Zimmermann, Sina; Brühmann, Matthias; Meyer, Eva; Rustige, Christian; Wolberg, Marike; Daub, Kathrin; Bell, Thomas; Meyer, Gerd

    2014-11-15

    Comproportionation reactions of rare-earth metal trihalides (RX{sub 3}) with the respective rare-earth metals (R) and transition metals (T) led to the formation of 22 oligomeric R cluster halides encapsulating T, in 19 cases for the first time. The structures of these compounds were determined by single-crystal X-ray diffraction and are composed of trimers ((T{sub 3}R{sub 11})X{sub 15}-type, P6{sub 3}/m), tetramers ((T{sub 4}R{sub 16})X{sub 28}(R{sub 4}) (P-43m), (T{sub 4}R{sub 16})X{sub 20} (P4{sub 2}/nnm), (T{sub 4}R{sub 16})X{sub 24}(RX{sub 3}){sub 4} (I4{sub 1}/a) and (T{sub 4}R{sub 16})X{sub 23} (C2/m) types of structure) and pentamers ((Ru{sub 5}La{sub 14}){sub 2}Br{sub 39}, Cc) of (TR{sub r}){sub n} (n=2–5) clusters. These oligomers are further enveloped by inner (X{sup i}) as well as outer (X{sup a}) halido ligands, which possess diverse functionalities and interconnect like oligomers through i–i, i–a and/or a–i bridges. The general features of the crystal structures for these new compounds are discussed and compared to literature entries as well as different structure types with oligomeric T centered R clusters. Dimers and tetramers originating from the aggregation of (TR{sub 6}) octahedra via common edges are more frequent than trimers and pentamers, in which the (TR{sub r}) clusters share common faces. - Graphical abstract: Rare earth-metal cluster complexes with endohedral transition metal atoms (TR{sub 6}) may connect via common edges or faces to form dimers, trimers, tetramers and pentamers of which the tetramers are the most prolific. Packing effects and electron counts play an important role. - Highlights: • Rare-earth metal cluster complexes encapsulate transition metal atoms. • Oligomers are built via connection of octahedral clusters via common edges or faces. • Dimers through pentamers with closed structures are known. • Tetramers including a tetrahedron of endohedral atoms are the most prolific.

  12. Ternary alkali-metal and transition metal or metalloid acetylides as alkali-metal intercalation electrodes for batteries

    DOEpatents

    Nemeth, Karoly; Srajer, George; Harkay, Katherine C; Terdik, Joseph Z

    2015-02-10

    Novel intercalation electrode materials including ternary acetylides of chemical formula: A.sub.nMC.sub.2 where A is alkali or alkaline-earth element; M is transition metal or metalloid element; C.sub.2 is reference to the acetylide ion; n is an integer that is 0, 1, 2, 3 or 4 when A is alkali element and 0, 1, or 2 when A is alkaline-earth element. The alkali elements are Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Cesium (Cs) and Francium (Fr). The alkaline-earth elements are Berilium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba), and Radium (Ra). M is a transition metal that is any element in groups 3 through 12 inclusive on the Periodic Table of Elements (elements 21 (Sc) to element 30 (Zn)). In another exemplary embodiment, M is a metalloid element.

  13. Photoinduced Magnetism of Ternary Transition Metal Prussian Blue Analogs

    NASA Astrophysics Data System (ADS)

    Pajerowski, D. M.; Meisel, M. W.; Gardner, J. E.; Talham, D. R.

    2009-03-01

    The magnetism of Prussian blue analog materials (PBAs) can be tuned with external stimuli such as temperature, pressure, and light. Recently, novel effects have been seen in PBAs with substitutionally mixed ternary and quaternary transition metals, rather than the usual binary analogs. One noteworthy material we have studied is a NaaNi1-xCox[Fe(CN)6]b.nH2O powder, which can show either a photoinduced increase or decrease in magnetization depending upon the Ni substitution, the applied magnetic field, and the temperature. This result is the first example of a photoinduced decrease in magnetization while generating new spins via a charge transfer induced spin transition (CTIST) in a bulk material. Constrastingly, the photodecrease observed in PBA binary thin films has different microscopic origins [1-2]. Insight into the underlying mechanisms can be obtained by using mean field models, which qualitatively reproduce the experimental data. SQUID magnetometer, FT-IR, TEM, and EDS data will be presented.[1] J.-H. Park, et al., Appl. Phys. Lett. 85, 3797 (2004). [2] F. A. Frye et al., Chem. Mater. 20, 5706 (2008).

  14. Magneto-photoconductivitity of atomically thin transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Eginligil, M.; Zou, C.; Peimyou, N.; Cao, B.; Shen, X.; Shang, J.; Cong, C.; Yu, T.

    2015-03-01

    Photoinduced effects of two-dimensional (2D) transition metal dichalcogenides (TMDs) are of great interest since the bandgap of these materials corresponds to visible range of spectrum. For instance, in molybdenum disulphide (MoS2) - a 2D semiconductor TMD and a non-centrosymmertic crystal, inherent broken inversion symmetry in monolayers leads to a large spin-orbit interaction which splits the valence band (VB) by 160 meV and gives rise to strong excitonic transitions due to the direct band gap at low energy K and -K valleys. The same broken inversion symmetry together with time reversal symmetry is responsible for spin-valley coupling in monolayer MoS2 and similar TMDs (such as tungsten disulphide, WS2) . Spin-valley coupled band edges in TMDs result in different localization behaviors for different scattering mechanisms. In this work, we present our magneto-photoconductivity studies of mono- and bilayer field-effect transistor devices of MoS2 and WS2, and discuss our results in terms of localization effects. Supported in part by the Singapore Ministry of Education under MOE2013-T1-2-235, MOE2012-T2-2-049, and MOE2013-T2-1-044.

  15. Orbital reconstruction in nonpolar tetravalent transition-metal oxide layers

    NASA Astrophysics Data System (ADS)

    Bogdanov, Nikolay A.; Katukuri, Vamshi M.; Romhányi, Judit; Yushankhai, Viktor; Kataev, Vladislav; Büchner, Bernd; van den Brink, Jeroen; Hozoi, Liviu

    2015-06-01

    A promising route to tailoring the electronic properties of quantum materials and devices rests on the idea of orbital engineering in multilayered oxide heterostructures. Here we show that the interplay of interlayer charge imbalance and ligand distortions provides a knob for tuning the sequence of electronic levels even in intrinsically stacked oxides. We resolve in this regard the d-level structure of layered Sr2IrO4 by electron spin resonance. While canonical ligand-field theory predicts g||-factors less than 2 for positive tetragonal distortions as present in Sr2IrO4, the experiment indicates g|| is greater than 2. This implies that the iridium d levels are inverted with respect to their normal ordering. State-of-the-art electronic-structure calculations confirm the level switching in Sr2IrO4, whereas we find them in Ba2IrO4 to be instead normally ordered. Given the nonpolar character of the metal-oxygen layers, our findings highlight the tetravalent transition-metal 214 oxides as ideal platforms to explore d-orbital reconstruction in the context of oxide electronics.

  16. Photoinduced energy transfer in transition metal complex oligomers

    SciTech Connect

    1997-06-01

    The work done over the past three years has been directed toward the preparation, characterization and photophysical examination of mono- and bimetallic diimine complexes. The work is part of a broader project directed toward the development of stable, efficient, light harvesting arrays of transition metal complex chromophores. One focus has been the synthesis of rigid bis-bidentate and bis-tridentate bridging ligands. The authors have managed to make the ligand bphb in multigram quantities from inexpensive starting materials. The synthetic approach used has allowed them to prepare a variety of other ligands which may have unique applications (vide infra). They have prepared, characterized and examined the photophysical behavior of Ru(II) and Re(I) complexes of the ligands. Energy donor/acceptor complexes of bphb have been prepared which exhibit nearly activationless energy transfer. Complexes of Ru(II) and Re(I) have also been prepared with other polyunsaturated ligands in which two different long lived (> 50 ns) excited states exist; results of luminescence and transient absorbance measurements suggest the two states are metal-to-ligand charge transfer and ligand localized {pi}{r_arrow}{pi}* triplets. Finally, the authors have developed methods to prepare polymetallic complexes which are covalently bound to various surfaces. The long term objective of this work is to make light harvesting arrays for the sensitization of large band gap semiconductors. Details of this work are provided in the body of the report.

  17. Orbital reconstruction in nonpolar tetravalent transition-metal oxide layers

    PubMed Central

    Bogdanov, Nikolay A.; Katukuri, Vamshi M.; Romhányi, Judit; Yushankhai, Viktor; Kataev, Vladislav; Büchner, Bernd; van den Brink, Jeroen; Hozoi, Liviu

    2015-01-01

    A promising route to tailoring the electronic properties of quantum materials and devices rests on the idea of orbital engineering in multilayered oxide heterostructures. Here we show that the interplay of interlayer charge imbalance and ligand distortions provides a knob for tuning the sequence of electronic levels even in intrinsically stacked oxides. We resolve in this regard the d-level structure of layered Sr2IrO4 by electron spin resonance. While canonical ligand-field theory predicts g||-factors less than 2 for positive tetragonal distortions as present in Sr2IrO4, the experiment indicates g|| is greater than 2. This implies that the iridium d levels are inverted with respect to their normal ordering. State-of-the-art electronic-structure calculations confirm the level switching in Sr2IrO4, whereas we find them in Ba2IrO4 to be instead normally ordered. Given the nonpolar character of the metal-oxygen layers, our findings highlight the tetravalent transition-metal 214 oxides as ideal platforms to explore d-orbital reconstruction in the context of oxide electronics. PMID:26105992

  18. Orbital textures and charge density waves in transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Ritschel, T.; Trinckauf, J.; Koepernik, K.; Büchner, B.; Zimmermann, M. V.; Berger, H.; Joe, Y. I.; Abbamonte, P.; Geck, J.

    2015-04-01

    Low-dimensional electron systems, as realized in layered materials, often tend to spontaneously break the symmetry of the underlying nuclear lattice by forming so-called density waves; a state of matter that at present attracts enormous attention. Here we reveal a remarkable and surprising feature of charge density waves, namely their intimate relation to orbital order. For the prototypical material 1T-TaS2 we not only show that the charge density wave within the two-dimensional TaS2 layers involves previously unidentified orbital textures of great complexity. We also demonstrate that two metastable stackings of the orbitally ordered layers allow manipulation of salient features of the electronic structure. Indeed, these orbital effects provide a route to switch 1T-TaS2 nanostructures from metallic to semiconducting with technologically pertinent gaps of the order of 200 meV. This new type of orbitronics is especially relevant for the ongoing development of novel, miniaturized and ultrafast devices based on layered transition metal dichalcogenides.

  19. Photoinduced energy transfer in transition metal complex oligomers

    SciTech Connect

    1997-04-01

    The work we have done over the past three years has been directed toward the preparation, characterization and photophysical examination of mono- and bimetallic diimine complexes. The work is part of a broader project directed toward the development of stable, efficient, light harvesting arrays of transition metal complex chromophores. One focus has been the synthesis of rigid bis-bidentate and bis-tridentate bridging ligands. We have managed to make the ligand bphb in multigram quantities from inexpensive starting materials. The synthetic approach used has allowed us prepare a variety of other ligands which may have unique applications (vide infra). We have prepared, characterized and examined the photophysical behavior of Ru(II) and Re(I) complexes of the ligands. Energy donor/acceptor complexes of bphb have been prepared which exhibit nearly activationless energy transfer. Complexes of Ru(II) and Re(I) have also been prepared with other polyunsaturated ligands in which two different long lived ( > 50 ns) excited states exist; results of luminescence and transient absorbance measurements suggest the two states are metal-to-ligand charge transfer and ligand localized {pi}{r_arrow}{pi}* triplets. Finally, we have developed methods to prepare polymetallic complexes which are covalently bound to various surfaces. The long term objective of this work is to make light harvesting arrays for the sensitization of large band gap semiconductors. Details of this work are provided in the body of the report.

  20. Antiferromagnetic Metal and Mott Transition on Shastry-Sutherland Lattice

    PubMed Central

    Liu, Hai-Di; Chen, Yao-Hua; Lin, Heng-Fu; Tao, Hong-Shuai; Liu, Wu-Ming

    2014-01-01

    The Shastry-Sutherland lattice, one of the simplest systems with geometrical frustration, which has an exact eigenstate by putting singlets on diagonal bonds, can be realized in a group of layered compounds and raises both theoretical and experimental interest. Most of the previous studies on the Shastry-Sutherland lattice are focusing on the Heisenberg model. Here we opt for the Hubbard model to calculate phase diagrams over a wide range of interaction parameters, and show the competing effects of interaction, frustration and temperature. At low temperature, frustration is shown to favor a paramagnetic metallic ground state, while interaction drives the system to an antiferromagnetic insulator phase. Between these two phases, there are an antiferromagnetic metal phase and a paramagnetic insulator phase (which should consist of a small plaquette phase and a dimer phase) resulting from the competition of the frustration and the interaction. Our results may shed light on more exhaustive studies about quantum phase transitions in geometrically frustrated systems. PMID:24777282

  1. Synthesis, characterization and formation process of transition metal oxide nanotubes using carbon nanofibers as templates

    SciTech Connect

    Ogihara, Hitoshi; Masahiro, Sadakane; Nodasaka, Yoshinobu; Ueda, Wataru

    2009-06-15

    Mono and binary transition metal oxide nanotubes could be synthesized by the immersion of carbon nanofiber templates into metal nitrate solutions and removal of the templates by heat treatment in air. The transition metal oxide nanotubes were composed of nano-crystallites of metal oxides. The functional groups on the carbon nanofiber templates were essential for the coating of these templates: they acted as adsorption sites for the metal nitrates, ensuring a uniform metal oxide coating. During the removal of the carbon nanofiber templates by calcination in air, the metal oxide coatings promoted the combustion reaction between the carbon nanofibers and oxygen. - Graphical abstract: Mono and binary transition metal-oxide nanotubes could be synthesized by the immersion of carbon nanofiber templates into metal nitrate solutions and removal of the templates by heat treatment in air.

  2. Transition metal activation and functionalization of carbon-hydrogen bonds

    SciTech Connect

    Jones, W.D.

    1990-07-01

    This project is directed toward the continued investigation of the fundamental thermodynamic and kinetic factors that influence carbon-hydrogen bond activation at homogeneous transition metal centers. The project is also directed toward the conversion of hydrocarbons into functionalized products of potential use to chemical industry. In the past year, advances have been made in both understanding the interactions of hydrocarbons with metals and in the functionalized of hydrocarbons. Major advanced that have been made include: (1) We have found that RhCl(PR{sub 3}){sub 2}(CNR) complexes can catalyze the insertion of isonitriles into the C-H bonds of arenes upon photolysis. (2) We have also examined reactions of a series of arenes with (C{sub 5}Me{sub 5})Rh(PMe{sub 3})PhH and begun to map out the kinetic and thermodynamic preferences for arene coordination. (3) We have begun to examine the reactions of rhodium pyrazolylborates for isonitrile functionalization of C-H bonds. (4) We have completed studies of the reactions of (C{sub 5}Me{sub 5})Rh(PMe{sub 3})H{sub 2} with D{sub 2} and PMe{sub 3} that indicate that both {eta}{sup 5} {yields} {eta}{sup 3} ring slippage and metal to ring hydride migration occur more faciley than reductive elimination of H{sub 12}. (5) We have examined the reactions of heterocycles with (C{sub 5}Me{sub 5})Rh(PMe{sub 3})PhH and found that pyrrole and furan undergo C-H or N-H activation.

  3. Transition metal activation and functionalization of carbon-hydrogen bonds

    SciTech Connect

    Jones, W.D.

    1991-06-01

    This project is directed towards the continued investigation of the fundamental thermodynamic and kinetic factors that influence carbon-hydrogen bond activation at homogeneous transition metal centers. The project is also directed towards the conversion of hydrocarbons into functionalized products of potential use to the chemical industry. In the past two years, advances have been made in both understanding the interactions of hydrocarbons with metals and in the functionalization of hydrocarbons. Major advances that have been made include: (1) We have found that RhCl(PR{sub 3}){sub 2}(CNR) complexes can catalyze the insertion of isonitriles into the C-H bonds of arenes upon photolysis. (2) We have also examined reactions of a series of arenes with (C{sub 5}Me{sub 5})Rh(PMe{sub 3})PhH and begun to map out the kinetic and thermodynamic preferences for arene coordination. (3) We have begun to examine the reactions of rhodium isonitrile pyrazolylborates for alkane and arene C-H bond activation. A new, labile, carbodiimide precursor has been developed for these studies. (4) We have completed studies of the reactions of (C{sub 5}Me{sub 5)}Rh(PMe{sub 3})H{sub 2} with D{sub 2} and PMe{sub 3} that indicate that both {eta}{sup 5} {yields} {eta}{sup 3} ring slippage and metal to ring hydride migration occur more facilely than thermal reductive elimination of H{sub 2}. (5) We have examined the reactions of heterocycles with (C{sub 5}Me{sub 5})Rh(PMe{sub 3})PhH and found that pyrrole and furan undergo C-H or N-H activation. Thiophene, however, undergoes C-S bond oxidative addition, and the mechanism of activation has been shown to proceed through sulfur coordination prior to C-S insertion. 43 refs., 9 figs., 4 tabs.

  4. Formation of transition metal-doxorubicin complexes inside liposomes.

    PubMed

    Abraham, Sheela Ann; Edwards, Katarina; Karlsson, Göran; MacIntosh, Scott; Mayer, Lawrence D; McKenzie, Cheryl; Bally, Marcel B

    2002-09-20

    Doxorubicin complexation with the transition metal manganese (Mn(2+)) has been characterized, differentiating between the formation of a doxorubicin-metal complex and doxorubicin fibrous-bundle aggregates typically generated following ion gradient-based loading procedures that rely on liposome encapsulated citrate or sulfate salts. The physical and chemical characteristics of the encapsulated drug were assessed using cryo-electron microscopy, circular dichroism (CD) and absorbance spectrophotometric analysis. In addition, in vitro and in vivo drug loading and release characteristics of the liposomal formulations were investigated. Finally, the internal pH after drug loading was measured with the aim of linking formation of the Mn(2+) complex to the presence or absence of a transmembrane pH gradient. Doxorubicin was encapsulated into either 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)/cholesterol (Chol) or 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC)/Chol liposomes, where the entrapped salts were citrate, MnSO(4) or MnCl(2). In response to a pH gradient or a Mn(2+) ion gradient, doxorubicin accumulated inside to achieve a drug-to-lipid ratio of approximately 0.2:1 (wt/wt). Absorbance and CD spectra of doxorubicin in the presence of Mn(2+) suggested that there are two distinct structures captured within the liposomes. In the absence of added ionophore A23187, drug loading is initiated on the basis of an established pH gradient; however, efficient drug uptake is not dependent on maintenance of the pH gradient. Drug release from DMPC/Chol is comparable regardless of whether doxorubicin is entrapped as a citrate-based aggregate or a Mn(2+) complex. However, in vivo drug release from DSPC/Chol liposomes indicate less than 5% or greater than 50% drug loss over a 24-h time course when the drug was encapsulated as an aggregate or a Mn(2+) complex, respectively. These studies define a method for entrapping drugs possessing coordination sites capable of complexing transition metals and suggest that drug release is dependent on lipid composition, internal pH, as well as the nature of the crystalline precipitate, which forms following encapsulation. PMID:12225851

  5. Transition metal catalysis in the generation of petroleum and natural gas. Final report

    SciTech Connect

    Mango, F.D.

    1997-01-21

    This project originated on the premise that natural gas could be formed catalytically in the earth rather than thermally as commonly believed. The intention was to test this hypothetical view and to explore generally the role of sedimentary metals in the generation of light hydrocarbons (C1 - C9). We showed the metalliferous source rocks are indeed catalytic in the generation of natural gas. Various metal compounds in the pure state show the same levels of catalytic activity as sedimentary rocks and the products are identical. Nickel is particularly active among the early transition metals and is projected to remain catalytically robust at all stages of catagenesis. Nickel oxide promotes the formation of n-alkanes in addition to natural gas (NG), demonstrating the full scope of the hypothetical catalytic process: The composition of catalytic gas duplicates the entire range of natural gas, from so-called wet gas to dry gas (60 to 95+ wt % methane), while gas generated thermally is consistently depleted in methane (10 to 60 wt % methane). These results support the view that metal catalysis is a major pathway through which natural gas is formed in the earth.

  6. Metal-Semiconductor Barrier Modulation for High Photoresponse in Transition Metal Dichalcogenide Field Effect Transistors

    PubMed Central

    Li, Hua-Min; Lee, Dae-Yeong; Choi, Min Sup; Qu, Deshun; Liu, Xiaochi; Ra, Chang-Ho; Yoo, Won Jong

    2014-01-01

    A gate-controlled metal-semiconductor barrier modulation and its effect on carrier transport were investigated in two-dimensional (2D) transition metal dichalcogenide (TMDC) field effect transistors (FETs). A strong photoresponse was observed in both unipolar MoS2 and ambipolar WSe2 FETs (i) at the high drain voltage due to a high electric field along the channel for separating photo-excited charge carriers and (ii) at the certain gate voltage due to the optimized barriers for the collection of photo-excited charge carriers at metal contacts. The effective barrier height between Ti/Au and TMDCs was estimated by a low temperature measurement. An ohmic contact behavior and drain-induced barrier lowering (DIBL) were clearly observed in MoS2 FET. In contrast, a Schottky-to-ohmic contact transition was observed in WSe2 FET as the gate voltage increases, due to the change of majority carrier transport from holes to electrons. The gate-dependent barrier modulation effectively controls the carrier transport, demonstrating its great potential in 2D TMDCs for electronic and optoelectronic applications. PMID:24509565

  7. Environmental Effects on the Metallicities of Early-Type Galaxies

    NASA Technical Reports Server (NTRS)

    Oliversen, Ronald J. (Technical Monitor); Jones, Christine

    2004-01-01

    In this multi-year project to investigate the metal enrichment of early-type galaxies, we have used ROSAT, ASCA and now Chandra observations to study samples of galaxies. We have published two papers and a third paper that incorporates Chandra archival observations is nearing completion. Below, we briefly describe our findings. Our first paper "SN IA Enrichment in Virgo Early-type Galaxies from ROSAT and ASCA Observations" was published in the Astrophysical Journal (vol 539, 603) reported on the properties of nine X-ray bright elliptical galaxies in the Virgo cluster observed by ROSAT and ASCA. We measured iron abundance gradients as a function of radius in three galaxies. We found that the magnesium and silicon abundance gradients were in general flatter than those of iron. We suggest this is due to a metallicity dependence in the metal production rates of SN Ia's. We calculate SN Ia rates in the center of these galaxies that are comparable to those measured optically. Our second paper "ASCA Observations of Groups at Radii of Low Overdensity: Implications for Cosmic Preheating" also was published in the Astrophysical Journal (vol 578, 74). This paper reported on the ASCA spectroscopy of nine groups of galaxies. We found that the entropy profile in groups is driven by nongravitational heating processes, and could be explained by a short period of preheating by galactic winds. The third paper (in preparation) uses a sample of about 200 galaxies from both ROSAT and Chandra observations. In this paper we characterize both the nuclear and the extended X-ray emission for this sample. We will use these observations to determine the "on-time" of the X-ray emitting AGN and the fraction of "fossil groups" as well as to investigate how large AGN outbursts can sweep the galaxy of its hot ISM, thus leading to changes in the ISM metal enrichment.

  8. Generic trend of work functions in transition-metal carbides and nitrides

    SciTech Connect

    Yoshitake, Michiko

    2014-11-15

    Transition-metal carbides and nitrides (TMCs and TMNs) are promising electrode materials for various electronic devices such as metal-oxide-semiconductor field-effect transistors and metal-insulator-metal capacitors. In this paper, the work functions of TMCs and TMNs are discussed systematically. Based upon the origin of the work function, the effect upon transition metal species by different periodic table groups is explained, carbides are compared with nitrides for the same transition metal, and the effect of carbon or nitrogen vacancies is discussed. In addition, a method to estimate the generic trend of the work function is proposed for TMC{sub x}, TMN{sub x}, TMC{sub 1−y}N{sub y} (transition metal carbonitrides), and TM{sub 1−z}TM′{sub z}C (alloy carbides)

  9. Ammonia and hydrazine. Transition-metal-catalyzed hydroamination and metal-free catalyzed functionalization

    SciTech Connect

    Bertrand, Guy

    2012-06-29

    The efficient and selective preparation of organic molecules is critical for mankind. For the future, it is of paramount importance to find catalysts able to transform abundant and cheap feedstocks into useful compounds. Acyclic and heterocyclic nitrogen-containing derivatives are common components of naturally occurring compounds, agrochemicals, cosmetics, and pharmaceuticals; they are also useful intermediates in a number of industrial processes. One of the most widely used synthetic strategies, allowing the formation of an N-C bond, is the addition of an N-H bond across a carbon-carbon multiple bond, the so-called hydroamination reaction. This chemical transformation fulfills the principle of “green chemistry” since it ideally occurs with 100% atom economy. Various catalysts have been found to promote this reaction, although many limitations remain; one of the most prominent is the lack of methods that permit the use of NH3 and NH2NH2 as the amine partners. In fact, ammonia and hydrazine have rarely succumbed to homogeneous catalytic transformations. Considering the low cost and abundance of ammonia (136 million metric tons produced in 2011) and hydrazine, catalysts able to improve the reactivity and selectivity of the NH3- and NH2NH2-hydroamination reaction, and more broadly speaking the functionalization of these chemicals, are highly desirable. In the last funded period, we discovered the first homogeneous catalysts able to promote the hydroamination of alkynes and allenes with ammonia and the parent hydrazine. The key feature of our catalytic systems is that the formation of catalytically inactive Werner complexes is reversible, in marked contrast to most of the known ammonia and hydrazine transition metal complexes. This is due to the peculiar electronic properties of our neutral ancillary ligands, especially their strong donating capabilities. However, our catalysts currently require high temperatures and long reaction times. To address this issue, we have developed several new families of carbon- and boron-based ligands, which are even better donors. The corresponding metal complexes (particularly gold, rhodium, iridium, and ruthenium) of all these species will be tested in the Markovnikov and anti-Markovnikov hydroamination of alkynes, allenes, and also alkenes with ammonia and hydrazine. We will also develop metal-free catalytic processes for the functionalization of ammonia and hydrazine. By possessing both a lone pair of electrons and an accessible vacant orbital, singlet carbenes resemble and can mimic the chemical behavior of transition metals. Our preliminary results demonstrate that specially designed carbenes can split the N–H bond of ammonia by an initial nucleophilic activation that prevents the formation of Lewis acid-base adducts, which is the major hurdle for the transition metal catalyzed functionalization of NH3. The use of purely organic compounds as catalysts will eliminate the major drawbacks of transition-metal-catalysis technology, which are the excessive cost of metal complexes (metal + ligands) and in many cases the toxicity of the metal.

  10. Bad Metallic Behavior in Model Hamiltonian Studies and in Transition Metal Oxides

    NASA Astrophysics Data System (ADS)

    Kotliar, Gabriel

    2014-03-01

    We investigate the transport properties of a correlated metal within dynamical mean-field theory. Canonical Fermi liquid behavior emerges only below a very low temperature scale TFL. Surprisingly the quasiparticle scattering rate follows a quadratic temperature dependence up to much higher temperatures and crosses over to saturated behavior around a temperature scale Tsat indicating the existence of ``hidden'' Fermi liquid behavior. The non-Fermi-liquid transport above TFL, in particular the linear-in- T resistivity, is shown to be a result of a strongly temperature dependent band dispersion. We derive simple expressions for the resistivity, Hall angle, thermoelectric power and Nernst coefficient in terms of a temperature dependent renormalized band structure and the quasiparticle scattering rate. We discuss the implications of the results for numerous transition metal oxides and other correlated materials connecting the non Fermi liquid transport with anomalous transfer of spectral weight.

  11. Ternary Amides Containing Transition Metals for Hydrogen Storage: A Case Study with Alkali Metal Amidozincates.

    PubMed

    Cao, Hujun; Richter, Theresia M M; Pistidda, Claudio; Chaudhary, Anna-Lisa; Santoru, Antonio; Gizer, Gökhan; Niewa, Rainer; Chen, Ping; Klassen, Thomas; Dornheim, Martin

    2015-11-01

    The alkali metal amidozincates Li4 [Zn(NH2)4](NH2)2 and K2[Zn(NH2)4] were, to the best of our knowledge, studied for the first time as hydrogen storage media. Compared with the LiNH2-2 LiH system, both Li4 [Zn(NH2)4](NH2)2-12 LiH and K2[Zn(NH2)4]-8 LiH systems showed improved rehydrogenation performance, especially K2[Zn(NH2)4]-8 LiH, which can be fully hydrogenated within 30 s at approximately 230 °C. The absorption properties are stable upon cycling. This work shows that ternary amides containing transition metals have great potential as hydrogen storage materials. PMID:26463124

  12. Mott metal-insulator transition in a metallic liquid - Gutzwiller molecular dynamics simulations

    NASA Astrophysics Data System (ADS)

    Barros, Kipton; Chern, Gia-Wei; Batista, Cristian D.; Kress, Joel D.; Kotliar, Gabriel

    2015-03-01

    Molecular dynamics (MD) simulations are crucial to modern computational physics, chemistry, and materials science, especially when combined with potentials derived from density-functional theory. However, even in state of the art MD codes, the on-site Coulomb repulsion is only treated at the self-consistent Hartree-Fock level. This standard approximation may miss important effects due to electron correlations. The Gutzwiller variational method captures essential correlated-electron physics yet is much faster than, e.g., the dynamical-mean field theory approach. We present our efficient Gutzwiller-MD implementation. With it, we investigate the Mott metal-insulator transition in a metallic fluid and uncover several surprising static and dynamic properties of this system.

  13. Charge density wave, superconductivity, and anomalous metallic behavior in 2D transition metal dichalcogenides.

    PubMed

    Castro Neto, A H

    2001-05-01

    We propose a theory for quasi-two-dimensional transition metal dichalcogenides that provides a unified microscopic picture of the charge density wave (CDW) and superconducting phases. We show, based on the electron-phonon coupling and Fermi surface topology, that a CDW order parameter with sixfold symmetry and nodes ( f-wave) gives a consistent description of the available experimental data. The elementary excitations in the CDW phase are Dirac electrons. The superconducting state has its origin on the attractive interaction mediated by phonons. The theory predicts strong deviations from Fermi liquid theory in the CDW phase. PMID:11328180

  14. Solubility Behavior and Phase Stability of Transition Metal Oxides in Alkaline Hydrothermal Environments

    SciTech Connect

    S.E. Ziemniak

    2000-05-18

    The solubility behavior of transition metal oxides in high temperature water is interpreted by recognizing three types of chemical reaction equilibria: metal oxide hydration/dehydration, metal oxide dissolution and metal ion hydroxocomplex formation. The equilibria are quantified using thermodynamic concepts and the thermochemical properties of the metal oxides/ions representative of the most common constituents of construction metal alloys, i.e., element shaving atomic numbers between Z = 22 (Ti) and Z = 30 (Zn), are summarized on the basis of metal oxide solubility studies conducted in the laboratory. Particular attention is devoted to the uncharged metal ion hydrocomplex, M{sup Z}(OH){sub Z}(aq), since its thermochemical properties define minimum solubilities of the metal oxide at a given temperature. Experimentally-extracted values of standard partial molal entropy (S{sup 0}) for the transition metal ion neutral hydroxocomplex are shown to be influenced by ligand field stabilization energies and complex symmetry.

  15. Ligational behavior of Schiff bases towards transition metal ion and metalation effect on their antibacterial activity

    NASA Astrophysics Data System (ADS)

    Devi, Jai; Batra, Nisha; Malhotra, Rajesh

    2012-11-01

    New Schiff bases pyrazine-2-carboxylicacid (phenyl-pyridin-2-yl-methylene)-hydrazide (Hpch-bp) HL1 and pyrazine-2-carboxylicacid (pyridin-2-ylmethylene)-hydrazide (Hpch-pc) HL2 derived from condensation of pyrazine carboxylic hydrazide (Hpch) with 2-benzoyl pyridine (bp) or pyridine 2-carbaldehyde (pc) and their transition metal complexes of type ML(1-2)2 have been synthesized, where M = Mn(II), Co(II), Ni(II), Cu(II) and Zn(II). Characterization of ligands and their metal complexes was carried out by elemental analysis, conductimetric studies, magnetic susceptibility, spectroscopic techniques (IR, UV-VIS, NMR, ESR, Mass) and thermogravimetric analysis. The physico-chemical studies revealed octahedral geometry or distorted octahedral geometry around metal ion. These azomethine Schiff base ligands acted as tridentate ? coordinating through carbonyl, azomethine and pyridine nitrogen present in the ligand. The thermodynamic and thermal properties of the complexes have been investigated and it was observed on the basis of these studies that thermal stability of complexes follows the order Mn < Zn < Cu < Co < Ni. The ligands and their complexes were tested for in vitro antibacterial activity at different concentrations against bacteria viz. Gram positive Bacillus subtilis, Micrococcus luteus and Gram negative Pseudomonas aeruginosa, Pseudomonas mendocina. A marked enhancement in biocidal activity of the ligands under similar experimental conditions was observed as a consequence of coordination with metal ions. The trend of growth inhibition in the complexes was found to be in the order: Cu > Mn > Ni > Co > Zn.

  16. Goodbyes and Hellos: Effective Strategies for Bridging Early Services. Project APPLES. Bridging Early Services Transition Project.

    ERIC Educational Resources Information Center

    Rosenkoetter, Sharon; Shotts, Cynthia

    This packet of information items presents a variety of resources for the effective transition of young children with disabilities from one program or service to another. Typical items include: challenges and truths of transition; six key points of transition; regulations from relevant federal laws; a flowchart of interagency transition planning; a…

  17. Metallic-like to nonmetallic transitions in a variety of heavily oxygen deficient ferroelectrics

    NASA Astrophysics Data System (ADS)

    Bock, Jonathan A.; Lee, Soonil; Trolier-McKinstry, Susan; Randall, Clive A.

    2015-08-01

    The coupling between ferroelectric distortions and electron transport is an important factor in understanding ferroelectric/noncentrosymmetric materials with metallic conductivities and ferroelectric-based thermoelectrics. Here, multiple d0 ferroelectrics with a variety of crystal structures are doped via oxygen deficiency, resulting in metallic-like conduction in the paraelectric state. It is found that most of the studied systems show a metallic-like to nonmetallic transition near the paraelectric-ferroelectric transition. The metallic-like to nonmetallic transition temperature can be shifted using mechanisms that shift the paraelectric-ferroelectric transition temperature. It was found that the metallic-like to nonmetallic transition temperature could be shifted from 373 K to 273 K by varying (Ba1-xSrx)TiO3-? from x = 0 to x = 0.3 and x = 1. The most probable mechanism for ferroelectric-electron transport coupling was determined to be Anderson localization associated with polarization with short-range order.

  18. Spin-polarized charge transfer induced by transition metal adsorption on graphene

    NASA Astrophysics Data System (ADS)

    del Castillo, Elisabetta; Cargnoni, Fausto; Soave, Raffaella; Trioni, Mario I.

    2016-05-01

    The electronic properties of transition metal atoms adsorbed on a graphene sheet are analyzed in the framework of the quantum theory of atoms in molecules (QTAIM). Particular attention is devoted to the spin dependence of the charge rearrangement upon the adsorption of magnetic adatoms. A comparison between the band structures and the charges in the atomic basins makes it possible to shed light on the different roles that the spin components play in the bond formation. This aspect is likely to be crucial in determining the spin-dependent properties required in spintronics applications and could help in tailoring them. We found that for early (Sc, Ti, and V) and late (Fe, Co) transition metals the two spin populations behave very differently, being involved in the bonding only the majority or the minority spin component for the lighter and heavier adsorbates, respectively. We expect that the response properties, in particular those related to the states at the Fermi level, will be very different for the two spin components. As a by-product of our study it becomes apparent that the QTAIM analysis is not the most suitable tool for catching the charge transfer phenomenon in the case of weakly interacting electronic populations.

  19. Voltage controlled magnetism in 3d transitional metals

    NASA Astrophysics Data System (ADS)

    Wang, Weigang

    2015-03-01

    Despite having attracted much attention in multiferroic materials and diluted magnetic semiconductors, the impact of an electric field on the magnetic properties remains largely unknown in 3d transitional ferromagnets (FMs) until recent years. A great deal of effort has been focused on the voltage-controlled magnetic anisotropy (VCMA) effect where the modulation of anisotropy field is understood by the change of electron density among different d orbitals of FMs in the presence of an electric field. Here we demonstrate another approach to alter the magnetism by electrically controlling the oxidation state of the 3d FM at the FM/oxide interface. The thin FM film sandwiched between a heavy metal layer and a gate oxide can be reversibly changed from an optimally-oxidized state with a strong perpendicular magnetic anisotropy to a metallic state with an in-plane magnetic anisotropy, or to a fully-oxidized state with nearly zero magnetization, depending on the polarity and time duration of the applied electric fields. This is a voltage controlled magnetism (VCM) effect, where both the saturation magnetization and anisotropy field of the 3d FM layer can be simultaneously controlled by voltage in a non-volatile fashion. We will also discuss the impact of this VCM effect on magnetic tunnel junctions and spin Hall switching experiments. This work, in collaboration with C. Bi, Y.H. Liu, T. Newhouse-Illige, M. Xu, M. Rosales, J.W. Freeland, O. Mryasov, S. Zhang, and S.G.E. te Velthuis, was supported in part by NSF (ECCS-1310338) and by C-SPIN, one of six centers of STARnet, a Semiconductor Research Corporation program, sponsored by MARCO and DARPA.

  20. Sub-surface alloying largely influences graphene nucleation and growth over transition metal substrates.

    PubMed

    Zhang, Liying; Zhao, Xingju; Xue, Xinlian; Shi, Jinlei; Li, Chong; Ren, Xiaoyan; Niu, Chunyao; Jia, Yu; Guo, Zhengxiao; Li, Shunfang

    2015-11-11

    Sub-surface alloying (SSA) can be an effective approach to tuning surface functionalities. Focusing on Rh(111) as a typical substrate for graphene nucleation, we show strong modulation by SSA atoms of both the energetics and kinetics of graphene nucleation simulated by first-principles calculations. Counter-intuitively, when the sub-surface atoms are replaced by more active solute metal elements to the left of Rh in the periodic table, such as the early transition metals (TMs), Ru and Tc, the binding between a C atom and the substrate is weakened and two C atoms favor dimerization. Alternatively, when the alloying elements are the late TMs to the right of Rh, such as the relatively inert Pd and Ag, the repulsion between the two C atoms is enhanced. Such distinct results can be well addressed by the delicately modulated activities of the surface host atoms in the framework of the d-band theory. More specifically, we establish a very simple selection rule for optimizing the metal substrate for high quality graphene growth: the introduction of an early (late) solute TM in the SSA lowers (raises) the d-band center and the activity of the top-most host metal atoms, weakening (strengthening) the C-substrate binding, meanwhile both energetically and kinetically facilitating (hindering) the graphene nucleation, and simultaneously promoting (suppressing) the orientation disordering the graphene domains. Importantly, our preliminary theoretical results also show that such a simple rule is also proposed to be operative for graphene growth on the widely invoked Cu(111) catalytic substrate. PMID:26257125

  1. Transition metal activation and functionalization of carbon-hydrogen bonds

    SciTech Connect

    Jones, W.D.

    1992-06-01

    We are investigating the fundamental thermodynamic and kinetic factors that influence carbon-hydrogen bond activation at homogeneous transition metal centers and the conversion of hydrocarbons into functionalized products of potential use to the chemical industry. Advances have been made in both understanding the interactions of hydrocarbons with metals and in the functionalization of hydrocarbons. We have found that RhCl(PR{sub 3}){sub 2}(CNR) complexes can catalyze the insertion of isonitriles into the C-H bonds or arenes upon photolysis. The mechanism of these reactions was found to proceed by way of initial phosphine dissociation, followed by C-H activation and isonitrile insertion. We have also examined reactions of a series of arenes with (C{sub 5}Me{sub 5})Rh(PMe{sub 3})PhH and begun to map out the kinetic and thermodynamic preferences for arene coordination. The effects of resonance, specifically the differences in the Hueckel energies of the bound vs free ligand, are now believed to fully control the C-H activation/{eta}{sup 2}-coordination equilibria. We have begun to examine the reactions of rhodium isonitrile pyrazolylborates for alkane and arene C-H bond activation. A new, labile, carbodiimide precursor has been developed for these studies. We have completed studies of the reactions of (C{sub 5}Me{sub 5})Rh(PMe{sub 3})H{sub 2} with D{sub 2} and PMe{sub 3} that indicate that both {eta}{sup 5} {yields} {eta}{sup 3} ring slippage and metal to ring hydride migration occur more facilely than thermal reductive elimination of H{sub 2}. We have examined the reactions of heterocycles with (C{sub 5}Me{sub 5})Rh(PMe{sub 3})PhH and found that pyrrole and furan undergo C-H or N-H activation. Thiophene, however, undergoes C-S bond oxidative addition, and the mechanism of activation has been shown to proceed through sulfur coordination prior to C-S insertion.

  2. Induce magnetism into silicene by embedding transition-metal atoms

    SciTech Connect

    Sun, Xiaotian; Wang, Lu E-mail: yyli@suda.edu.cn; Lin, Haiping; Hou, Tingjun; Li, Youyong E-mail: yyli@suda.edu.cn

    2015-06-01

    Embedding transition-metal (TM) atoms into nonmagnetic nanomaterials is an efficient way to induce magnetism. Using first-principles calculations, we systematically investigated the structural stability and magnetic properties of TM atoms from Sc to Zn embedded into silicene with single vacancy (SV) and double vacancies (DV). The binding energies for different TM atoms correlate with the TM d-shell electrons. Sc, Ti, and Co show the largest binding energies of as high as 6 eV, while Zn has the lowest binding energy of about 2 eV. The magnetic moment of silicene can be modulated by embedding TM atoms from V to Co, which mainly comes from the 3d orbitals of TM along with partly contributions from the neighboring Si atoms. Fe atom on SV and Mn atom on DV have the largest magnetic moment of more than 3 μB. In addition, we find that doping of N or C atoms on the vacancy site could greatly enhance the magnetism of the systems. Our results provide a promising approach to design silicene-based nanoelectronics and spintronics device.

  3. High current density field emission from transition metal carbides

    NASA Astrophysics Data System (ADS)

    Mackie, William A.; Hartman, Robert L.; Davis, Paul R.

    1993-04-01

    We report on fabrication, performance and modeling of field emission cathodes made from refractory transition metal carbides. The primary carbides under study include HfC, TaC, and ZrC. Single crystal carbide specimens were prepared by arc floating zone refinement from sintered stock. Emission patterns are shown along with emission stability data. Work function ordering of various crystal planes determined by means of field emission microscopy is reported and comparisons are made with results from thermionic projection microscopy. Typically, the carbides show that ? 210,310 2400 h are reported. Stable pulse field emission of > mA for a single emitter at pulse lengths of 10 ?s and duty factors of 0.01-0.001 are reported. Single-pulse currents as high as 48 mA were observed from a single emitter tip. Current densities have been calculated to be in the high 10 8 A/cm 2 range.

  4. Spin-orbit engineering in transition metal dichalcogenide alloy monolayers

    PubMed Central

    Wang, Gang; Robert, Cedric; Suslu, Aslihan; Chen, Bin; Yang, Sijie; Alamdari, Sarah; Gerber, Iann C.; Amand, Thierry; Marie, Xavier; Tongay, Sefaattin; Urbaszek, Bernhard

    2015-01-01

    Binary transition metal dichalcogenide monolayers share common properties such as a direct optical bandgap, spin-orbit splittings of hundreds of meV, light–matter interaction dominated by robust excitons and coupled spin-valley states. Here we demonstrate spin-orbit-engineering in Mo(1−x)WxSe2 alloy monolayers for optoelectronics and applications based on spin- and valley-control. We probe the impact of the tuning of the conduction band spin-orbit spin-splitting on the bright versus dark exciton population. For MoSe2 monolayers, the photoluminescence intensity decreases as a function of temperature by an order of magnitude (4–300 K), whereas for WSe2 we measure surprisingly an order of magnitude increase. The ternary material shows a trend between these two extreme behaviours. We also show a non-linear increase of the valley polarization as a function of tungsten concentration, where 40% tungsten incorporation is sufficient to achieve valley polarization as high as in binary WSe2. PMID:26657930

  5. Surface Modification of Nanoclays by Catalytically Active Transition Metal Ions

    SciTech Connect

    Nawani,P.; Gelfer, M.; Hsiao, B.; Frenkel, A.; Gilman, J.; Khalid, S.

    2007-01-01

    A unique class of nanoclays was prepared by modification of pristine clays or organoclays (Cloisite C20A) with transition metal ions (TMIs). The composition, structure, morphology and thermal properties of TMI-modified nanoclays were investigated by atomic absorption spectroscopy (AAS), elemental analysis (EA), scanning electron microscopy (SEM), small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and X-ray absorption near-edge structure (XANES) spectroscopy. The content of TMIs in modified clays was found to be close to the limiting value of ion exchange capacity. SEM and X-ray results confirmed that TMIs were located between the mineral layers instead of being adsorbed on the surface of clay particles. TGA results indicated that the TMI treatment of organoclays could significantly increase the thermal stability, which was more pronounced in air than in nitrogen. Temperature-resolved SAXS measurements revealed that the presence of TMIs increased the onset temperature of structural degradation. The higher thermal stability of TMI-modified organoclays can be attributed to the change in the thermal degradation mechanism, resulting in a decrease in the yield of volatile products and the formation of char facilitated by the presence of catalytically active TMIs.

  6. Polymer Nanocomposites Based on Transition Metal Ion Modified Organoclays

    SciTech Connect

    Nawani,P.; Desai, P.; Lundwall, M.; Gelfer, M.; Hsiao, B.; Rafailovich, M.; Frenkel, A.; Tsou, A.; Gilman, J.; Khalid, S.

    2007-01-01

    A unique class of nanocomposites containing organoclays modified with catalytically active transition metal ions (TMI) and ethylene vinyl acetate (EVA) copolymers was prepared. The morphology, thermal and rheological properties of these nanocomposites were studied by thermal gravimetric analysis (TGA), transmission electron microscopy (TEM), extended X-ray absorption fine structure (EXAFS) spectroscopy, X-ray scattering/diffraction and oscillatory shear rheometry. TMI-modified organoclays were thought to possess pillaring of multivalent TMI in the interlayer silicate gallery, leading to a notable reduction of the interlayer d-spacing. The resulting nanocomposites exhibited significantly improved thermal stability and fire retardation properties, but similar morphology (i.e., an intercalated-exfoliated structure) and rheological properties comparable with EVA nanocomposites containing unmodified organoclays. It appears that the compressed organic component in the TMI-modified organoclay can still facilitate the intercalation/exfoliation processes of polymer molecules, especially under extensive shearing conditions. The improved fire retardation in nanocomposites with TMI-modified organoclays can be attributed to enhanced carbonaceous char formation during combustion, i.e., charring promoted by the presence of catalytically active TMI.

  7. Surface modification of nanoclays by catalytically active transition metal ions.

    PubMed

    Nawani, Pranav; Gelfer, Mikhail Y; Hsiao, Benjamin S; Frenkel, Anatoly; Gilman, Jeffrey W; Khalid, Syed

    2007-09-11

    A unique class of nanoclays was prepared by modification of pristine clays or organoclays (Cloisite C20A) with transition metal ions (TMIs). The composition, structure, morphology and thermal properties of TMI-modified nanoclays were investigated by atomic absorption spectroscopy (AAS), elemental analysis (EA), scanning electron microscopy (SEM), small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and X-ray absorption near-edge structure (XANES) spectroscopy. The content of TMIs in modified clays was found to be close to the limiting value of ion exchange capacity. SEM and X-ray results confirmed that TMIs were located between the mineral layers instead of being adsorbed on the surface of clay particles. TGA results indicated that the TMI treatment of organoclays could significantly increase the thermal stability, which was more pronounced in air than in nitrogen. Temperature-resolved SAXS measurements revealed that the presence of TMIs increased the onset temperature of structural degradation. The higher thermal stability of TMI-modified organoclays can be attributed to the change in the thermal degradation mechanism, resulting in a decrease in the yield of volatile products and the formation of char facilitated by the presence of catalytically active TMIs. PMID:17705406

  8. Interactions between lasers and two-dimensional transition metal dichalcogenides.

    PubMed

    Lu, Junpeng; Liu, Hongwei; Tok, Eng Soon; Sow, Chorng-Haur

    2016-05-01

    The recent increasing research interest in two-dimensional (2D) layered materials has led to an explosion of in the discovery of novel physical and chemical phenomena in these materials. Among the 2D family, group-VI transition metal dichalcogenides (TMDs), such as represented by MoS2 and WSe2, are remarkable semiconductors with sizable energy band gaps, which make the TMDs promising building blocks for new generation optoelectronics. On the other hand, the specificity and tunability of the band gaps can generate particularly strong light-matter interactions between TMD crystals and specific photons, which can trigger complex and interesting phenomena such as photo-scattering, photo-excitation, photo-destruction, photo-physical modification, photochemical reaction and photo-oxidation. Herein, we provide an overview of the phenomena explained by various interactions between lasers and the 2D TMDs. Characterizations of the optical fundamentals of the TMDs via laser spectroscopies are reviewed. Subsequently, photoelectric conversion devices enabled by laser excitation and the functionality extension and performance improvement of the TMDs materials via laser modification are comprehensively summarized. Finally, we conclude the review by discussing the prospects for further development in this research area. PMID:27141556

  9. Spin-orbit engineering in transition metal dichalcogenide alloy monolayers.

    PubMed

    Wang, Gang; Robert, Cedric; Suslu, Aslihan; Chen, Bin; Yang, Sijie; Alamdari, Sarah; Gerber, Iann C; Amand, Thierry; Marie, Xavier; Tongay, Sefaattin; Urbaszek, Bernhard

    2015-01-01

    Binary transition metal dichalcogenide monolayers share common properties such as a direct optical bandgap, spin-orbit splittings of hundreds of meV, light-matter interaction dominated by robust excitons and coupled spin-valley states. Here we demonstrate spin-orbit-engineering in Mo(1-x)WxSe2 alloy monolayers for optoelectronics and applications based on spin- and valley-control. We probe the impact of the tuning of the conduction band spin-orbit spin-splitting on the bright versus dark exciton population. For MoSe2 monolayers, the photoluminescence intensity decreases as a function of temperature by an order of magnitude (4-300 K), whereas for WSe2 we measure surprisingly an order of magnitude increase. The ternary material shows a trend between these two extreme behaviours. We also show a non-linear increase of the valley polarization as a function of tungsten concentration, where 40% tungsten incorporation is sufficient to achieve valley polarization as high as in binary WSe2. PMID:26657930

  10. APCVD Transition Metal Oxides - Functional Layers in "Smart windows"

    NASA Astrophysics Data System (ADS)

    Gesheva, K. A.; Ivanova, T. M.; Bodurov, G. K.

    2014-11-01

    Transition metal oxides (TMO) exhibit electrochromic effect. Under a small voltage they change their optical transmittance from transparent to collored (absorbing) state. The individual material can manifest its electrochromic properties only when it is part of electrochromic (EC) multilayer system. Smart window is controlling the energy of solar flux entering the building or car and makes the interiors comfortable and energy utilization more effective. Recently the efforts of material researchers in this field are directed to price decreasing. APCVD technology is considered as promissing as this process permits flowthrough large-scale production process. The paper presents results on device optimization based on WO3-MoO3 working electrode. Extensive research reveals that WO3-MoO3 structure combines positive features of single oxides: excellent electrochromic performance of WO3 and better kinetic properties of MoO3 deposition. The achieved color efficiency of APCVD WO3-MoO3 films is 200cm2/C and optical modulation of 65-70% are practically favorable electrochromic characteristics. To respond to low cost requirement, the expensive hexacarbonyl can be replaced with acetylacetonate. We have started with this precursor to fabricate mixed WxV1-xO3 films. The films possess excellent surface coverage and high growth-rate. CVD deposition of VO2, a promissing thermochromic thin film material is also presented.

  11. Transition-metal-based perovskite oxides for enhanced thermopower

    NASA Astrophysics Data System (ADS)

    Kolesnik, Stanislaw; Dabrowski, Bogdan; Wojciechowski, Krzysztof; Swierczek, Konrad

    2012-02-01

    Due to the enhancement of thermopower by spin and orbital degrees of freedom, transition-metal-based perovskite oxides are good candidates for stable and nontoxic materials with a large thermoelectric figure of merit ZT. We have investigated the most promising Mn-, Co- and Ti-based perovskite oxides. Electron doping of SrMnO3 materials on either Mn or Sr sites induces a rapid decrease in both electrical resistivity and thermopower with the doping level due to the introduction of itinerant charge carriers. The thermopower of electron-doped SrTiO3 materials satisfy the basic Heikes description, however, no additional enhancement is observed. The hole-doped RCoO3 perovskites exhibit limited solubility of alkaline earth's for small rare earth ion sizes. The dependence of thermopower on charge doping and temperature appears to follow the extended Heikes formulation only at low doping and below 300 K, which indicates that Co^3+ and Co^4+ exist in several spin states beyond that range. Among all investigated compounds the largest ZT˜0.3 values were observed for 3-8% Nb-substituted SrTiO3 materials at about 700 K. Supported by the U.S. DOE-BES DE-AC02-06CH11357.

  12. Effective tight-binding model for transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Ho, Yen-Hung; Cazalilla, Miguel; Ochoa, Hector

    For transition metal dichalcogenides, various band models have been developed to describe the novel subband features. In this work, we propose a new effective minimum-band model by preforming a canonical transformation on the full-band Hamiltonian. We found that, depending on the form of transformation, both the Γ- and K-valley electrons can be well captured, including the frequency and band effective mass. And, for the full-band parameters used, starting from Wannier function basis set leads to a better result than from Slater-Koster basis set. A close inspection of the transformation projection also enables us to extract the modification on the site energy, as well as the orbital hopping between several nearest neighboring atoms. Instead of pure empirical fitting, our effective models preserve rich orbital physics inside, which is shown to be versatile in studying a variety of fundamental physical properties. Ministry of Science and Technology of Taiwan (NSC 102-2112-M-007-024-MY5).

  13. Self-Limiting Layer Synthesis of Transition Metal Dichalcogenides

    NASA Astrophysics Data System (ADS)

    Kim, Youngjun; Song, Jeong-Gyu; Park, Yong Ju; Ryu, Gyeong Hee; Lee, Su Jeong; Kim, Jin Sung; Jeon, Pyo Jin; Lee, Chang Wan; Woo, Whang Je; Choi, Taejin; Jung, Hanearl; Lee, Han-Bo-Ram; Myoung, Jae-Min; Im, Seongil; Lee, Zonghoon; Ahn, Jong-Hyun; Park, Jusang; Kim, Hyungjun

    2016-01-01

    This work reports the self-limiting synthesis of an atomically thin, two dimensional transition metal dichalcogenides (2D TMDCs) in the form of MoS2. The layer controllability and large area uniformity essential for electronic and optical device applications is achieved through atomic layer deposition in what is named self-limiting layer synthesis (SLS); a process in which the number of layers is determined by temperature rather than process cycles due to the chemically inactive nature of 2D MoS2. Through spectroscopic and microscopic investigation it is demonstrated that SLS is capable of producing MoS2 with a wafer-scale (~10 cm) layer-number uniformity of more than 90%, which when used as the active layer in a top-gated field-effect transistor, produces an on/off ratio as high as 108. This process is also shown to be applicable to WSe2, with a PN diode fabricated from a MoS2/WSe2 heterostructure exhibiting gate-tunable rectifying characteristics.

  14. Mechanism of Uniaxial Magnetocrystalline Anisotropy in Transition Metal Alloys

    NASA Astrophysics Data System (ADS)

    Kota, Yohei; Sakuma, Akimasa

    2014-03-01

    Magnetocrystalline anisotropy in transition metal alloys (FePt, CoPt, FePd, MnAl, MnGa, and FeCo) was studied using first-principles calculations to elucidate its specific mechanism. The tight-binding linear muffin-tin orbital method in the local spin-density approximation was employed to calculate the electronic structure of each compound, and the anisotropy energy was evaluated using the magnetic force theorem and the second-order perturbation theory in terms of spin-orbit interactions. We systematically describe the mechanism of uniaxial magnetocrystalline anisotropy in real materials and present the conditions under which the anisotropy energy can be increased. The large magnetocrystalline anisotropy energy in FePt and CoPt arises from the strong spin-orbit interaction of Pt. In contrast, even though the spin-orbit interaction in MnAl, MnGa, and FeCo is weak, the anisotropy energies of these compounds are comparable to that of FePd. We found that MnAl, MnGa, and FeCo have an electronic structure that is efficient in inducing the magnetocrystalline anisotropy in terms of the selection rule of spin-orbit interaction.

  15. Metal-insulator transitions in IZO, IGZO, and ITZO films

    SciTech Connect

    Makise, Kazumasa; Hidaka, Kazuya; Ezaki, Syohei; Asano, Takayuki; Shinozaki, Bunju; Tomai, Shigekazu; Yano, Koki; Nakamura, Hiroaki

    2014-10-21

    In this study, we measured the low-temperature resistivity of amorphous two- and three-dimensional (2D and 3D) indium-zinc oxide, indium-gallium-zinc oxide, and indium-tin-zinc oxide films with a wide range of carrier densities. To determine their critical characteristics at the metal-insulator transition (MIT), we used the Ioffe–Regel criterion. We found that the MIT occurs in a narrow range between k{sub F}ℓ =0.13 and k{sub F}ℓ =0.25, where k{sub F} and ℓ are the Fermi wave number and electron mean free path, respectively. For films in the insulating region, we analyzed ρ(T) using a procedure proposed by Zabrodskii and Zinov'eva. This analysis confirmed the occurrence of Mott and Efros–Shklovskii (ES) variable-range hopping. The materials studied show crossover behavior from exp(T{sub Mott}/T){sup 1/4} or exp(T{sub Mott}/T){sup 1/3} for Mott hopping conduction to exp(T{sub ES}/T){sup 1/2} for ES hopping conduction with decreasing temperature. For both 2D and 3D materials, we found that the relationship between T{sub Mott} and T{sub ES} satisfies T{sub ES}∝T{sub Mott}{sup 2/3}.

  16. Spinodal nanodecomposition in semiconductors doped with transition metals

    NASA Astrophysics Data System (ADS)

    Dietl, T.; Sato, K.; Fukushima, T.; Bonanni, A.; Jamet, M.; Barski, A.; Kuroda, S.; Tanaka, M.; Hai, Pham Nam; Katayama-Yoshida, H.

    2015-10-01

    This review presents the recent progress in computational materials design, experimental realization, and control methods of spinodal nanodecomposition under three- and two-dimensional crystal-growth conditions in spintronic materials, such as magnetically doped semiconductors. The computational description of nanodecomposition, performed by combining first-principles calculations with kinetic Monte Carlo simulations, is discussed together with extensive electron microscopy, synchrotron radiation, scanning probe, and ion beam methods that have been employed to visualize binodal and spinodal nanodecomposition (chemical phase separation) as well as nanoprecipitation (crystallographic phase separation) in a range of semiconductor compounds with a concentration of transition metal (TM) impurities beyond the solubility limit. The role of growth conditions, codoping by shallow impurities, kinetic barriers, and surface reactions in controlling the aggregation of magnetic cations is highlighted. According to theoretical simulations and experimental results the TM-rich regions appear in the form of either nanodots (the dairiseki phase) or nanocolumns (the konbu phase) buried in the host semiconductor. Particular attention is paid to Mn-doped group III arsenides and antimonides, TM-doped group III nitrides, Mn- and Fe-doped Ge, and Cr-doped group II chalcogenides, in which ferromagnetic features persisting up to above room temperature correlate with the presence of nanodecomposition and account for the application-relevant magneto-optical and magnetotransport properties of these compounds. Finally, it is pointed out that spinodal nanodecomposition can be viewed as a new class of bottom-up approach to nanofabrication.

  17. Dislocations and Plasticity in bcc Transition Metals at High Pressure

    SciTech Connect

    Yang, L H; Tang, M; Moriarty, J A

    2009-01-23

    Using first-principles electronic structure calculations, quantum-based atomistic simulations and atomistically informed dislocation dynamics (DD) simulations, we have studied individual dislocation behavior and the multiscale modeling of single-crystal plasticity in the prototype bcc transition metals Ta, Mo and V under both ambient and high pressure conditions. The primary focus in this work is on the pressure-dependent structure, mobility and interaction of a/2<111> screw dislocations, which dominate the plastic deformation properties of these materials. At the electronic scale, first-principles calculations of elasticity, ideal strength and generalized stacking fault energy surfaces have been used to validate quantum-based multi-ion interatomic potentials. At the atomistic scale, these potentials have been used in flexible Green's function boundary condition simulations to study the core structure, Peierls stress {tau}{sub P}, thermally activated kink-pair formation and mobility below {tau}{sub P}, and phonon-drag mobility above {tau}{sub P}. These results have then been distilled into analytic velocity laws and used directly in predictive microscale DD simulations of flow stress and resolved yield stress over wide ranges of pressure, temperature and strain rate.

  18. Transitional liquid metal duct flow near a magnetic dipole

    NASA Astrophysics Data System (ADS)

    Tympel, Saskia; Boeck, Thomas; Schumacher, Joerg

    2013-11-01

    The flow transformation and the generation of vortex structures by a strong magnetic dipole field in a liquid metal duct flow is studied by means of three-dimensional direct numerical simulations. The dipole is considered as the paradigm for a magnetic obstacle which will deviate the streamlines due to Lorentz forces which act on the fluid elements. The duct is of square cross-section. The dipole is located above the top wall and is centered in spanwise direction. Our model uses the quasi-static approximation which is applicable in the limit of small magnetic Reynolds numbers. The analysis covers the stationary flow regime at small hydrodynamic Reynolds numbers Re as well as the transitional time-dependent regime at higher values which may generate a turbulent flow in the wake of the magnetic obstacle. We present a systematic study of these two basic flow regimes on Re and on the Hartmann number Ha, a measure of the strength of the magnetic dipole field. Furthermore, several orientations and positions of the dipole are compared. The most efficient generation of turbulence at a fixed distance above the duct follows for the spanwise orientation which is caused by a certain configuration of Hartmann layers and reversed flow at the top plate.

  19. Strain engineered optoelectronic properties of transition metal dichalcogenides lateral heterostructures

    NASA Astrophysics Data System (ADS)

    Lee, Jaekwang; Yoon, Mina

    2015-03-01

    Most three-dimensional bulk-scale materials rarely survive beyond 1% strain, while recently spotlighted two-dimensional (2-D) materials can sustain a high elastic strain (up to 10%) to optimize optical quantities such as band gaps and absorption spectra governing optoelectronic device performance. Despite the enormous interest in strained 2-D materials, most researches are focused on single materials or vertical heterostructures where precise control of stacking orientation is challenging. Here, using first-principles density-functional calculations, we explore how uniaxial tensile strains modify overall electronic and optical properties of transition metal dichalcogenides lateral heterostructures, such as MoX2/WX2 (X =S, Se). Based on the detailed optoelectronic information, we predict the optimal strain condition for maximal power efficiency. Furthermore, we find that uniaxial tensile strain readily develops a continuously varying direct-bandgap across the lateral heterojunctions, which results in the broad range absorption of solar spectrum useful for future optoelectronic devices. This research was conducted at the CNMS, which is sponsored at Oak Ridge National Laboratory (ORNL) by the Office of Basic Energy Sciences, U.S. Department of Energy; a portion of theory work was supported by the LDRD Program of ORNL.

  20. Correlations in rare-earth transition-metal permanent magnets

    NASA Astrophysics Data System (ADS)

    Skomski, R.; Manchanda, P.; Kashyap, A.

    2015-05-01

    It is investigated how electron-electron correlations affect the intrinsic properties of rare-earth transition-metal magnets. Focusing on orbital moment and anisotropy, we perform model calculations for 3d-4f alloys and density-functional theory (DFT) calculations for NdCo5. On an independent-electron level, the use of a single Slater determinant with broken spin symmetry introduces Hund's rule correlations, which govern the behavior of rare-earth ions and of alloys described by the local spin density approximation (LSDA) and LSDA + U approximations to DFT. By contrast, rare-earth ions in intermetallics involve configuration interactions between two or more Slater determinants and lead to phenomena such as spin-charge distribution. Analyzing DFT as a Legendre transformation and using Bethe's crystal-field theory, we show that the corresponding density functionals are very different from familiar LSDA-type expressions and outline the effect of spin-charge separation on the magnetocrystalline anisotropy.

  1. Magnetic ground state of semiconducting transition-metal trichalcogenide monolayers

    NASA Astrophysics Data System (ADS)

    Sivadas, Nikhil; Daniels, Matthew W.; Swendsen, Robert H.; Okamoto, Satoshi; Xiao, Di

    2015-06-01

    Layered transition-metal trichalcogenides with the chemical formula A B X3 have attracted recent interest as potential candidates for two-dimensional magnets. Using first-principles calculations within density functional theory, we investigate the magnetic ground states of monolayers of Mn- and Cr-based semiconducting trichalcogenides. We show that the second and third nearest-neighbor exchange interactions (J2 and J3) between magnetic ions, which have been largely overlooked in previous theoretical studies, are crucial in determining the magnetic ground state. Specifically, we find that monolayer CrSiTe3 is an antiferromagnet with a zigzag spin texture due to significant contribution from J3, whereas CrGeTe3 is a ferromagnet with a Curie temperature of 106 K. Monolayers of Mn compounds (MnPS3 and MnPSe3) always show antiferromagnetic Néel order. We identify the physical origin of various exchange interactions, and demonstrate that strain can be an effective knob for tuning the magnetic properties. Possible magnetic ordering in the bulk is also discussed. Our study suggests that A B X3 can be a promising platform to explore two-dimensional magnetic phenomena.

  2. Magnetic ground state of semiconducting transition metal trichalcogenide monolayers

    DOE PAGESBeta

    Sivadas, Mr. Nikhil; Daniels, Matthew W.; Swendsen, Robert H.; Okamoto, Satoshi; Xiao, Di

    2015-01-01

    Layered transition-metal trichalcogenides with the chemical formula ABX3 have attracted recent interest as potential candidates for two-dimensional magnets. Using first-principles calculations within density functional theory, we investigate the magnetic ground states of monolayers of Mn- and Cr-based semiconducting trichalcogenides.We show that the second and third nearest-neighbor exchange interactions (J2 and J3) between magnetic ions, which have been largely overlooked in previous theoretical studies, are crucial in determining the magnetic ground state. Specifically, we find that monolayer CrSiTe3 is an antiferromagnet with a zigzag spin texture due to significant contribution from J3, whereas CrGeTe3 is a ferromagnet with a Curie temperaturemore » of 106 K. Monolayers of Mn compounds (MnPS3 and MnPSe3) always show antiferromagnetic N eel order. We identify the physical origin of various exchange interactions, and demonstrate that strain can be an effective knob for tuning the magnetic properties. Possible magnetic ordering in the bulk is also discussed. Our study suggests that ABX3 can be a promising platform to explore two-dimensional magnetic phenomena.« less

  3. Theoretical study of first-row transition metal oxide cations

    NASA Astrophysics Data System (ADS)

    Nakao, Yoshihide; Hirao, Kimihiko; Taketsugu, Tetsuya

    2001-05-01

    The equilibrium geometries, dissociation energies, and electronic structures of the ground and low-lying excited states for the first-row transition metal oxide cations, MO+ (M=Sc to Zn), have been studied using the multireference singles and doubles configuration interaction (MR-SDCI) and the multireference second-order Møller-Plesset methods. To investigate the applicability of the density functional theory (DFT) to an electronic structure system with a multiconfigurational character, the Becke exchange functional with the Lee-Yang-Parr correlation functional, the Becke exchange functional with the one-parameter progressive correlation functional (BOP), and the Becke three-parameter hybrid exchange functional with the Lee-Yang-Parr correlation functional (B3LYP) methods have also been applied. The DFT predicts the ground state M-O bond lengths in good agreement with the multireference-based methods except for MnO+ and CuO+, which have a multiconfigurational electronic structure. With respect to the dissociation energies, the B3LYP results are in good agreement with the multireference-based methods, while the DFT with pure functionals overestimates the energetics by about 20 kcal/mol compared to the MR-SDCI method.

  4. Self-Limiting Layer Synthesis of Transition Metal Dichalcogenides

    PubMed Central

    Kim, Youngjun; Song, Jeong-Gyu; Park, Yong Ju; Ryu, Gyeong Hee; Lee, Su Jeong; Kim, Jin Sung; Jeon, Pyo Jin; Lee, Chang Wan; Woo, Whang Je; Choi, Taejin; Jung, Hanearl; Lee, Han-Bo-Ram; Myoung, Jae-Min; Im, Seongil; Lee, Zonghoon; Ahn, Jong-Hyun; Park, Jusang; Kim, Hyungjun

    2016-01-01

    This work reports the self-limiting synthesis of an atomically thin, two dimensional transition metal dichalcogenides (2D TMDCs) in the form of MoS2. The layer controllability and large area uniformity essential for electronic and optical device applications is achieved through atomic layer deposition in what is named self-limiting layer synthesis (SLS); a process in which the number of layers is determined by temperature rather than process cycles due to the chemically inactive nature of 2D MoS2. Through spectroscopic and microscopic investigation it is demonstrated that SLS is capable of producing MoS2 with a wafer-scale (~10 cm) layer-number uniformity of more than 90%, which when used as the active layer in a top-gated field-effect transistor, produces an on/off ratio as high as 108. This process is also shown to be applicable to WSe2, with a PN diode fabricated from a MoS2/WSe2 heterostructure exhibiting gate-tunable rectifying characteristics. PMID:26725854

  5. Spin-orbit engineering in transition metal dichalcogenide alloy monolayers

    NASA Astrophysics Data System (ADS)

    Wang, Gang; Robert, Cedric; Suslu, Aslihan; Chen, Bin; Yang, Sijie; Alamdari, Sarah; Gerber, Iann C.; Amand, Thierry; Marie, Xavier; Tongay, Sefaattin; Urbaszek, Bernhard

    2015-12-01

    Binary transition metal dichalcogenide monolayers share common properties such as a direct optical bandgap, spin-orbit splittings of hundreds of meV, light-matter interaction dominated by robust excitons and coupled spin-valley states. Here we demonstrate spin-orbit-engineering in Mo(1-x)WxSe2 alloy monolayers for optoelectronics and applications based on spin- and valley-control. We probe the impact of the tuning of the conduction band spin-orbit spin-splitting on the bright versus dark exciton population. For MoSe2 monolayers, the photoluminescence intensity decreases as a function of temperature by an order of magnitude (4-300 K), whereas for WSe2 we measure surprisingly an order of magnitude increase. The ternary material shows a trend between these two extreme behaviours. We also show a non-linear increase of the valley polarization as a function of tungsten concentration, where 40% tungsten incorporation is sufficient to achieve valley polarization as high as in binary WSe2.

  6. Superconductivity Series in Transition Metal Dichalcogenides by Ionic Gating

    PubMed Central

    Shi, Wu; Ye, Jianting; Zhang, Yijin; Suzuki, Ryuji; Yoshida, Masaro; Miyazaki, Jun; Inoue, Naoko; Saito, Yu; Iwasa, Yoshihiro

    2015-01-01

    Functionalities of two-dimensional (2D) crystals based on semiconducting transition metal dichalcogenides (TMDs) have now stemmed from simple field effect transistors (FETs) to a variety of electronic and opto-valleytronic devices, and even to superconductivity. Among them, superconductivity is the least studied property in TMDs due to methodological difficulty accessing it in different TMD species. Here, we report the systematic study of superconductivity in MoSe2, MoTe2 and WS2 by ionic gating in different regimes. Electrostatic gating using ionic liquid was able to induce superconductivity in MoSe2 but not in MoTe2 because of inefficient electron accumulation limited by electronic band alignment. Alternative gating using KClO4/polyethylene glycol enabled a crossover from surface doping to bulk doping, which induced superconductivities in MoTe2 and WS2 electrochemically. These new varieties greatly enriched the TMD superconductor families and unveiled critical methodology to expand the capability of ionic gating to other materials. PMID:26235962

  7. Correlations in rare-earth transition-metal permanent magnets

    SciTech Connect

    Skomski, R. Manchanda, P.; Kashyap, A.

    2015-05-07

    It is investigated how electron-electron correlations affect the intrinsic properties of rare-earth transition-metal magnets. Focusing on orbital moment and anisotropy, we perform model calculations for 3d-4f alloys and density-functional theory (DFT) calculations for NdCo{sub 5}. On an independent-electron level, the use of a single Slater determinant with broken spin symmetry introduces Hund's rule correlations, which govern the behavior of rare-earth ions and of alloys described by the local spin density approximation (LSDA) and LSDA + U approximations to DFT. By contrast, rare-earth ions in intermetallics involve configuration interactions between two or more Slater determinants and lead to phenomena such as spin-charge distribution. Analyzing DFT as a Legendre transformation and using Bethe's crystal-field theory, we show that the corresponding density functionals are very different from familiar LSDA-type expressions and outline the effect of spin-charge separation on the magnetocrystalline anisotropy.

  8. Magnetic ground state of semiconducting transition metal trichalcogenide monolayers

    SciTech Connect

    Sivadas, Mr. Nikhil; Daniels, Matthew W.; Swendsen, Robert H.; Okamoto, Satoshi; Xiao, Di

    2015-01-01

    Layered transition-metal trichalcogenides with the chemical formula ABX3 have attracted recent interest as potential candidates for two-dimensional magnets. Using first-principles calculations within density functional theory, we investigate the magnetic ground states of monolayers of Mn- and Cr-based semiconducting trichalcogenides.We show that the second and third nearest-neighbor exchange interactions (J2 and J3) between magnetic ions, which have been largely overlooked in previous theoretical studies, are crucial in determining the magnetic ground state. Specifically, we find that monolayer CrSiTe3 is an antiferromagnet with a zigzag spin texture due to significant contribution from J3, whereas CrGeTe3 is a ferromagnet with a Curie temperature of 106 K. Monolayers of Mn compounds (MnPS3 and MnPSe3) always show antiferromagnetic N eel order. We identify the physical origin of various exchange interactions, and demonstrate that strain can be an effective knob for tuning the magnetic properties. Possible magnetic ordering in the bulk is also discussed. Our study suggests that ABX3 can be a promising platform to explore two-dimensional magnetic phenomena.

  9. Defect engineering of two-dimensional transition metal dichalcogenides

    NASA Astrophysics Data System (ADS)

    Lin, Zhong; Carvalho, Bruno R.; Kahn, Ethan; Lv, Ruitao; Rao, Rahul; Terrones, Humberto; Pimenta, Marcos A.; Terrones, Mauricio

    2016-06-01

    Two-dimensional transition metal dichalcogenides (TMDs), an emerging family of layered materials, have provided researchers a fertile ground for harvesting fundamental science and emergent applications. TMDs can contain a number of different structural defects in their crystal lattices which significantly alter their physico-chemical properties. Having structural defects can be either detrimental or beneficial, depending on the targeted application. Therefore, a comprehensive understanding of structural defects is required. Here we review different defects in semiconducting TMDs by summarizing: (i) the dimensionalities and atomic structures of defects; (ii) the pathways to generating structural defects during and after synthesis and, (iii) the effects of having defects on the physico-chemical properties and applications of TMDs. Thus far, significant progress has been made, although we are probably still witnessing the tip of the iceberg. A better understanding and control of defects is important in order to move forward the field of Defect Engineering in TMDs. Finally, we also provide our perspective on the challenges and opportunities in this emerging field.

  10. Proximity-induced magnetism in transition-metal substituted graphene

    DOE PAGESBeta

    Crook, Charles B.; Constantin, Costel; Ahmed, Towfiq; Zhu, Jian -Xin; Balatsky, Alexander V.; Haraldsen, Jason T.

    2015-08-03

    We investigate the interactions between two identical magnetic impurities substituted into a graphene superlattice. Using a first-principles approach, we calculate the electronic and magnetic properties for transition-metal substituted graphene systems with varying spatial separation. These calculations are compared for three different magnetic impurities, manganese, chromium, and vanadium. We determine the electronic band structure, density of states, and Millikan populations (magnetic moment) for each atom, as well as calculate the exchange parameter between the two magnetic atoms as a function of spatial separation. We find that the presence of magnetic impurities establishes a distinct magnetic moment in the graphene lattice, wheremore » the interactions are highly dependent on the spatial and magnetic characteristic between the magnetic and carbon atoms, which leads to either ferromagnetic or antiferromagnetic behavior. Furthermore, through an analysis of the calculated exchange energies and partial density of states, it is determined that interactions between the magnetic atoms can be classified as an RKKY interaction.« less

  11. Superconductivity Series in Transition Metal Dichalcogenides by Ionic Gating.

    PubMed

    Shi, Wu; Ye, Jianting; Zhang, Yijin; Suzuki, Ryuji; Yoshida, Masaro; Miyazaki, Jun; Inoue, Naoko; Saito, Yu; Iwasa, Yoshihiro

    2015-01-01

    Functionalities of two-dimensional (2D) crystals based on semiconducting transition metal dichalcogenides (TMDs) have now stemmed from simple field effect transistors (FETs) to a variety of electronic and opto-valleytronic devices, and even to superconductivity. Among them, superconductivity is the least studied property in TMDs due to methodological difficulty accessing it in different TMD species. Here, we report the systematic study of superconductivity in MoSe2, MoTe2 and WS2 by ionic gating in different regimes. Electrostatic gating using ionic liquid was able to induce superconductivity in MoSe2 but not in MoTe2 because of inefficient electron accumulation limited by electronic band alignment. Alternative gating using KClO4/polyethylene glycol enabled a crossover from surface doping to bulk doping, which induced superconductivities in MoTe2 and WS2 electrochemically. These new varieties greatly enriched the TMD superconductor families and unveiled critical methodology to expand the capability of ionic gating to other materials. PMID:26235962

  12. Superconductivity Series in Transition Metal Dichalcogenides by Ionic Gating

    NASA Astrophysics Data System (ADS)

    Shi, Wu; Ye, Jianting; Zhang, Yijin; Suzuki, Ryuji; Yoshida, Masaro; Miyazaki, Jun; Inoue, Naoko; Saito, Yu; Iwasa, Yoshihiro

    2015-08-01

    Functionalities of two-dimensional (2D) crystals based on semiconducting transition metal dichalcogenides (TMDs) have now stemmed from simple field effect transistors (FETs) to a variety of electronic and opto-valleytronic devices, and even to superconductivity. Among them, superconductivity is the least studied property in TMDs due to methodological difficulty accessing it in different TMD species. Here, we report the systematic study of superconductivity in MoSe2, MoTe2 and WS2 by ionic gating in different regimes. Electrostatic gating using ionic liquid was able to induce superconductivity in MoSe2 but not in MoTe2 because of inefficient electron accumulation limited by electronic band alignment. Alternative gating using KClO4/polyethylene glycol enabled a crossover from surface doping to bulk doping, which induced superconductivities in MoTe2 and WS2 electrochemically. These new varieties greatly enriched the TMD superconductor families and unveiled critical methodology to expand the capability of ionic gating to other materials.

  13. Strain Engineering for Transition Metal Dichalcogenides Based Field Effect Transistors.

    PubMed

    Shen, Tingting; Penumatcha, Ashish V; Appenzeller, Joerg

    2016-04-26

    Using electrical characteristics from three-terminal field-effect transistors (FETs), we demonstrate substantial strain induced band gap tunability in transition metal dichalcogenides (TMDs) in line with theoretical predictions and optical experiments. Devices were fabricated on flexible substrates, and a cantilever sample holder was used to apply uniaxial tensile strain to the various multilayer TMD FETs. Analyzing in particular transfer characteristics, we argue that the modified device characteristics under strain are clear evidence of a band gap reduction of 100 meV in WSe2 under 1.35% uniaxial tensile strain at room temperature. Furthermore, the obtained device characteristics imply that the band gap does not shrink uniformly under strain relative to a reference potential defined by the source/drain contacts. Instead, the band gap change is only related to a change of the conduction band edge of WSe2, resulting in a decrease in the Schottky barrier (SB) for electrons without any change for hole injection into the valence band. Simulations of SB device characteristics are employed to explain this point and to quantify our findings. Last, our experimental results are compared with DFT calculations under strain showing excellent agreement between theoretical predictions and the experimental data presented here. PMID:27043387

  14. New catalysts for hydroprocessing: Transition metal carbides and nitrides

    SciTech Connect

    Ramanathan, S.; Oyama, S.T. |

    1995-11-02

    A series of moderate surface area transition metal carbides and nitrides of molybdenum, tungsten, vanadium, niobium, and titanium were prepared by temperature-programmed reaction of the oxide precursor with a reactant gas (20% CH{sub 4}/H{sub 2} for the carbides and 100% NH{sub 3} for the nitrides). The phase purity and composition of the samples were established by X-ray diffraction photoelectron spectroscopy (XPS), while surface properties were determined by N{sub 2} BET and CO chemisorption measurements. The catalysts were tested in three-phase trickle-bed reactor for their activity in hydrodenitrogenation (HDN), hydrodesulfurization, and hydrodeoxygenation, with particular emphasis on HDN. The catalytic tests were carried out using a model liquid feed mixture containing 3000 ppm sulfur (dibenzothiophene), 2000 ppm nitrogen (quinoline), 500 ppm oxygen (benzofuran), 20 wt% aromatics (tetralin), and balance aliphatics (tetradecane). The carbides and nitrides were found to be active for HDN of quinoline with activity following the order group 6 > group 5 > group 4. Notably, Mo{sub 2}C showed superior areal HDN activity than a commercial sulfided Ni-Mo/Al{sub 2} O{sub 3} catalyst (shell 324). The XRD analysis of the spent catalysts indicated no change in the bulk structure, while XPS results showed little incorporation of sulfur in the surface region of the catalysts, suggesting that these materials are tolerant of sulfur. 42 refs., 11 figs., 7 tabs.

  15. Transition Metal Oxide Alloys as Potential Solar Energy Conversion Materials

    SciTech Connect

    Toroker, Maytal; Carter, Emily A.

    2013-02-21

    First-row transition metal oxides (TMOs) are inexpensive potentia alternative materials for solar energy conversion devices. However, some TMOs, such as manganese(II) oxide, have band gaps that are too large for efficiently absorbing solar energy. Other TMOs, such as iron(II) oxide, have conduction and valence band edges with the same orbital character that may lead to unfavorably high electron–hole recombination rates. Another limitation of iron(II) oxide is that the calculated valence band edge is not positioned well for oxidizing water. We predict that key properties, including band gaps, band edge positions, and possibly electron–hole recombination rates, may be improved by alloying TMOs that have different band alignments. A new metric, the band gap center offset, is introduced for simple screening of potential parent materials. The concept is illustrated by calculating the electronic structure of binary oxide alloys that contain manganese, nickel, iron, zinc, and/or magnesium, within density functional theory (DFT)+U and hybrid DFT theories. We conclude that alloys of iron(II) oxide are worth evaluating further as solar energy conversion materials.

  16. Exciton radiative lifetime in transition metal dichalcogenide monolayers

    NASA Astrophysics Data System (ADS)

    Robert, C.; Lagarde, D.; Cadiz, F.; Wang, G.; Lassagne, B.; Amand, T.; Balocchi, A.; Renucci, P.; Tongay, S.; Urbaszek, B.; Marie, X.

    2016-05-01

    We have investigated the exciton dynamics in transition metal dichalcogenide monolayers using time-resolved photoluminescence experiments performed with optimized time resolution. For MoS e2 monolayer, we measure τrad0=1.8 ±0.2 ps at T =7 K that we interpret as the intrinsic radiative recombination time. Similar values are found for WS e2 monolayers. Our detailed analysis suggests the following scenario: at low temperature (T ≲50 K ), the exciton oscillator strength is so large that the entire light can be emitted before the time required for the establishment of a thermalized exciton distribution. For higher lattice temperatures, the photoluminescence dynamics is characterized by two regimes with very different characteristic times. First the photoluminescence intensity drops drastically with a decay time in the range of the picosecond driven by the escape of excitons from the radiative window due to exciton-phonon interactions. Following this first nonthermal regime, a thermalized exciton population is established gradually yielding longer photoluminescence decay times in the nanosecond range. Both the exciton effective radiative recombination and nonradiative recombination channels including exciton-exciton annihilation control the latter. Finally the temperature dependence of the measured exciton and trion dynamics indicates that the two populations are not in thermodynamical equilibrium.

  17. Valency configuration of transition metal impurities in ZnO

    SciTech Connect

    Petit, Leon; Schulthess, Thomas C; Svane, Axel; Temmerman, Walter M; Szotek, Zdzislawa; Janotti, Anderson

    2006-01-01

    We use the self-interaction corrected local spin-density approximation to investigate the ground state valency configuration of transition metal (TM=Mn, Co) impurities in n- and p-type ZnO. We find that in pure Zn{sub 1-x}TM{sub x}O, the localized TM{sup 2+} configuration is energetically favored over the itinerant d-electron configuration of the local spin density (LSD) picture. Our calculations indicate furthermore that the (+/0) donor level is situated in the ZnO gap. Consequently, for n-type conditions, with the Fermi energy {epsilon}F close to the conduction band minimum, TM remains in the 2+ charge state, while for p-type conditions, with {epsilon}F close to the valence band maximum, the 3+ charge state is energetically preferred. In the latter scenario, modeled here by co-doping with N, the additional delocalized d-electron charge transfers into the entire states at the top of the valence band, and hole carriers will only exist, if the N concentration exceeds the TM impurity concentration.

  18. Density functional localized orbital corrections for transition metals

    PubMed Central

    Rinaldo, David; Tian, Li; Harvey, Jeremy N.; Friesner, Richard A.

    2008-01-01

    This paper describes the development of the B3LYP localized orbital correction model which improves the accuracy of the B3LYP thermochemical predictions for compounds containing transition metals. The development of this model employs a large data set containing 36 experimental atomic energies and 71 bond dissociation energies. B3LYP calculations were carried out on these systems with different basis sets. Based on an electronic structure analysis and physical arguments, we built a set of 10 parameters to correct atomic data and a set of 21 parameters to correct bond dissociation energies. Using the results from our biggest basis set, the model was shown to reduce the mean absolute deviation from 7.7 to 0.4 kcal∕mol for the atomic data and from 5.3 to 1.7 kcal∕mol for the bond dissociation energies. The model was also tested using a second basis set and was shown to give relatively accurate results too. The model was also able to predict an outlier in the experimental data that was further investigated with high level coupled-cluster calculations. PMID:19045248

  19. Exciton radiative lifetimes in two-dimensional transition metal dichalcogenides.

    PubMed

    Palummo, Maurizia; Bernardi, Marco; Grossman, Jeffrey C

    2015-05-13

    Light emission in two-dimensional (2D) transition metal dichalcogenides (TMDs) changes significantly with the number of layers and stacking sequence. While the electronic structure and optical absorption are well understood in 2D-TMDs, much less is known about exciton dynamics and radiative recombination. Here, we show first-principles calculations of intrinsic exciton radiative lifetimes at low temperature (4 K) and room temperature (300 K) in TMD monolayers with the chemical formula MX2 (X = Mo, W, and X = S, Se), as well as in bilayer and bulk MoS2 and in two MX2 heterobilayers. Our results elucidate the time scale and microscopic origin of light emission in TMDs. We find radiative lifetimes of a few picoseconds at low temperature and a few nanoseconds at room temperature in the monolayers and slower radiative recombination in bulk and bilayer than in monolayer MoS2. The MoS2/WS2 and MoSe2/WSe2 heterobilayers exhibit very long-lived (∼20-30 ns at room temperature) interlayer excitons constituted by electrons localized on the Mo-based and holes on the W-based monolayer. The wide radiative lifetime tunability, together with the ability shown here to predict radiative lifetimes from computations, hold unique potential to manipulate excitons in TMDs and their heterostructures for application in optoelectronics and solar energy conversion. PMID:25798735

  20. Strain engineering of electronic properties of transition metal dichalcogenide monolayers

    NASA Astrophysics Data System (ADS)

    Maniadaki, Aristea E.; Kopidakis, Georgios; Remediakis, Ioannis N.

    2016-02-01

    We present Density Functional Theory (DFT) results for the electronic and dielectric properties of single-layer (2D) semiconducting transition metal dichalcogenides MX2 (M=Mo, W; X=S, Se, Te) under isotropic, uniaxial (along the zigzag and armchair directions), and shear strain. Electronic band gaps decrease while dielectric constants increase for heavier chalcogens X. The direct gaps of equilibrium structures often become indirect under certain types of strain, depending on the material. The effects of strain and of broken symmetry on the band structure are discussed. Gaps reach maximum values at small compressive strains or in equilibrium, and decrease with larger strains. In-plane dielectric constants generally increase with strain, reaching a minimum value at small compressive strains. The out-of-plane constants exhibit a similar behavior under shear strain but under isotropic and uniaxial strain they increase with compression and decrease with tension, thus exhibiting a monotonic behavior. These DFT results are theoretically explained using only structural parameters and equilibrium dielectric constants. Our findings are consistent with available experimental data.

  1. Magnetic Exchange Couplings in Transition Metal Complexes from DFT

    NASA Astrophysics Data System (ADS)

    Peralta, Juan

    In this talk I will review our current efforts for the evaluation of magnetic exchange couplings in transition metal complexes from density functional theory. I will focus on the performance of different DFT approximations, including a variety of hybrid density functionals, and show that hybrid density functionals containing approximately 30% Hartree-Fock type exchange are in general among the best choice in terms of accuracy. I will also describe a novel computational method to evaluate exchange coupling parameters using analytic self-consistent linear response theory. This method avoids the explicit evaluation of energy differences, which can become impractical for large systems. Our approach is based on the evaluation of the transversal magnetic torque between two magnetic centers for a given spin configuration using explicit constraints of the local magnetization direction via Lagrange multipliers. This method is applicable in combination with any modern density functional with a noncollinear spin generalization and can be utilized as a ``black-box''. I will show proof-of-concept calculations in frustrated Fe7IIIdisk-shaped clusters, and dinuclear CuII, FeIII, and heteronuclear complexes. NSF DMR-1206920.

  2. Defect-induced semiconductor to metal transition in graphene monoxide.

    PubMed

    Woo, Jungwook; Yun, Kyung-Han; Cho, Sung Beom; Chung, Yong-Chae

    2014-07-14

    This study investigates the influence of point defects on the geometric and electronic structure of graphene monoxide (GMO) via density functional theory calculations. In aspects of defect formation energy, GMOs with oxygen vacancies and bridge interstitial defects are more likely to form when compared to GMOs with defects such as carbon vacancies and hollow interstitial defects. It was also found that the oxygen vacancy or the hollow interstitial defect induces local tensile strain around the defective site and this strain increases the band gap energy of the defective GMO. In addition, the band gaps of GMO with carbon vacancies or bridge interstitial defects decreased mainly due to the dangling bonds, not due to the strain effect. It is noted that the dangling bond derived from the defects forms the defect-level in the band gap of GMO. The semiconductor to metal transition by the band gap change (0-0.7 eV) implies the possibility for band gap engineering of GMO by vacancies and interstitial defects. PMID:24886723

  3. Hybrid functional studies of defects in layered transition metal oxides

    NASA Astrophysics Data System (ADS)

    Hoang, Khang; Johannes, Michelle

    2014-03-01

    Layered oxides LiMO2 (M is a transition metal) have been studied extensively for Li-ion battery cathodes. It is known that defects have strong impact on the electrochemical performance. A detailed understanding of native point defects in LiMO2 is however still lacking, thus hindering rational design of more complex materials for battery applications. In fact, first-principles defect calculations in LiMO2 are quite challenging because standard density functional theory (DFT) calculations using the generalized gradient approximation (GGA) of the exchange-correlation functional fail to reproduce the correct physics. The GGA+U extension can produce reasonable results, but the transferability of U across the compounds is limited. In this talk, we present our DFT studies of defects in LiMO2 (M=Co, Ni) using the Heyd-Scuseria-Ernzerhof (HSE) screened hybrid functional. The dominant point defects will be identified and compared with experiment; and their impact on the structural stability and the charge (electronic and ionic) and mass transport will be addressed. We will also discuss possible shortcomings of the HSE functional in the study of these electron-correlated materials.

  4. Recognition- and Reactivity-Based Fluorescent Probes for Studying Transition Metal Signaling in Living Systems

    PubMed Central

    2015-01-01

    Conspectus Metals are essential for life, playing critical roles in all aspects of the central dogma of biology (e.g., the transcription and translation of nucleic acids and synthesis of proteins). Redox-inactive alkali, alkaline earth, and transition metals such as sodium, potassium, calcium, and zinc are widely recognized as dynamic signals, whereas redox-active transition metals such as copper and iron are traditionally thought of as sequestered by protein ligands, including as static enzyme cofactors, in part because of their potential to trigger oxidative stress and damage via Fenton chemistry. Metals in biology can be broadly categorized into two pools: static and labile. In the former, proteins and other macromolecules tightly bind metals; in the latter, metals are bound relatively weakly to cellular ligands, including proteins and low molecular weight ligands. Fluorescent probes can be useful tools for studying the roles of transition metals in their labile forms. Probes for imaging transition metal dynamics in living systems must meet several stringent criteria. In addition to exhibiting desirable photophysical properties and biocompatibility, they must be selective and show a fluorescence turn-on response to the metal of interest. To meet this challenge, we have pursued two general strategies for metal detection, termed “recognition” and “reactivity”. Our design of transition metal probes makes use of a recognition-based approach for copper and nickel and a reactivity-based approach for cobalt and iron. This Account summarizes progress in our laboratory on both the development and application of fluorescent probes to identify and study the signaling roles of transition metals in biology. In conjunction with complementary methods for direct metal detection and genetic and/or pharmacological manipulations, fluorescent probes for transition metals have helped reveal a number of principles underlying transition metal dynamics. In this Account, we give three recent examples from our laboratory and collaborations in which applications of chemical probes reveal that labile copper contributes to various physiologies. The first example shows that copper is an endogenous regulator of neuronal activity, the second illustrates cellular prioritization of mitochondrial copper homeostasis, and the third identifies the “cuprosome” as a new copper storage compartment in Chlamydomonas reinhardtii green algae. Indeed, recognition- and reactivity-based fluorescent probes have helped to uncover new biological roles for labile transition metals, and the further development of fluorescent probes, including ones with varied Kd values and new reaction triggers and recognition receptors, will continue to reveal exciting and new biological roles for labile transition metals. PMID:26215055

  5. Recognition- and reactivity-based fluorescent probes for studying transition metal signaling in living systems.

    PubMed

    Aron, Allegra T; Ramos-Torres, Karla M; Cotruvo, Joseph A; Chang, Christopher J

    2015-08-18

    Metals are essential for life, playing critical roles in all aspects of the central dogma of biology (e.g., the transcription and translation of nucleic acids and synthesis of proteins). Redox-inactive alkali, alkaline earth, and transition metals such as sodium, potassium, calcium, and zinc are widely recognized as dynamic signals, whereas redox-active transition metals such as copper and iron are traditionally thought of as sequestered by protein ligands, including as static enzyme cofactors, in part because of their potential to trigger oxidative stress and damage via Fenton chemistry. Metals in biology can be broadly categorized into two pools: static and labile. In the former, proteins and other macromolecules tightly bind metals; in the latter, metals are bound relatively weakly to cellular ligands, including proteins and low molecular weight ligands. Fluorescent probes can be useful tools for studying the roles of transition metals in their labile forms. Probes for imaging transition metal dynamics in living systems must meet several stringent criteria. In addition to exhibiting desirable photophysical properties and biocompatibility, they must be selective and show a fluorescence turn-on response to the metal of interest. To meet this challenge, we have pursued two general strategies for metal detection, termed "recognition" and "reactivity". Our design of transition metal probes makes use of a recognition-based approach for copper and nickel and a reactivity-based approach for cobalt and iron. This Account summarizes progress in our laboratory on both the development and application of fluorescent probes to identify and study the signaling roles of transition metals in biology. In conjunction with complementary methods for direct metal detection and genetic and/or pharmacological manipulations, fluorescent probes for transition metals have helped reveal a number of principles underlying transition metal dynamics. In this Account, we give three recent examples from our laboratory and collaborations in which applications of chemical probes reveal that labile copper contributes to various physiologies. The first example shows that copper is an endogenous regulator of neuronal activity, the second illustrates cellular prioritization of mitochondrial copper homeostasis, and the third identifies the "cuprosome" as a new copper storage compartment in Chlamydomonas reinhardtii green algae. Indeed, recognition- and reactivity-based fluorescent probes have helped to uncover new biological roles for labile transition metals, and the further development of fluorescent probes, including ones with varied Kd values and new reaction triggers and recognition receptors, will continue to reveal exciting and new biological roles for labile transition metals. PMID:26215055

  6. Watching Transitions Unfold: A Mixed-Method Study of Transitions within Early Childhood Care and Education Settings

    ERIC Educational Resources Information Center

    O'Farrelly, Christine; Hennessy, Eilis

    2014-01-01

    Unlike the transitions children make between settings, those they undertake between age groups within early childhood care and education (ECCE) settings are seldom studied. Accordingly, this exploratory study followed seven preschool children (three boys and four girls) as they moved to new rooms in five ECCE settings. Structured observations of…

  7. Measurements of Schottky barrier heights formed from metals and 2D transition metal dichalcogedides

    NASA Astrophysics Data System (ADS)

    Kim, Changsik; Moon, Inyong; Nam, Seunggeol; Cho, Yeonchoo; Shin, Hyeon-Jin; Park, Seongjun; Yoo, Won Jong

    Schottky barrier height (SBH) is an important parameter that needs to be considered for designing electronic devices. However, for two dimensional (2D) materials based devices, SBH control is limited by 2D structure induced quantum confinement and 2D surface induced Fermi level pinning. In this work, we explore differences in measuring SBH between 2D and 3D materials. Recently, low temperature I-V measurement has been reported to extract SBH based on thermionic emission equation for Schottky diode. However, 2D devices are not real Schottky diode in that both source and drain metal electrodes make Schottky contact. According to our experimental results, SBH extracted from linear slope of ln (I/T3/2) against 1/T show widely diverse values, dependent on applied voltage bias and tested temperature which affect carrier transport including tunneling or thermionic emission across the metal-2D material interface. In this work, we wish to demonstrate the method to determine SBH and Fermi level pinning which are attributed to 2D transition metal dichalcogedides, differently from conventional 3D materials. .

  8. The preparation of transition metal carbides and phosphides from metallic fluxes

    SciTech Connect

    Jeitschko, W.; Albering, J.H.; Dewalski, M.V.; Jakubowski-Ripke, U.; Kahnert, G.E.; Musanke, U.E.; Poettgen, R.; Wallinda, J.; Zimmer, B.I.

    1993-12-31

    Metallic fluxes can be used for the synthesis of binary and ternary carbides, phosphides, and related materials to obtain well crystallized products or when the direct reaction of the elemental components is too slow. Among the requirements for a flux is a moderate affinity (neither too low nor too high) of the flux for all components and the possibility to dissolve the flux without attacking the desired compounds. Lithium is well suited for the growth of carbides, while tin is optimal to synthesize many transition metal phosphides. Recent examples of compounds synthesized via a metallic flux include Yb{sub 2}Cr{sub 2}C{sub 3}, Dy{sub 10.1}Mn{sub 12.9}C{sub 18}, Ln{sub 4}Ni{sub 2}C{sub 5} (Ln = Er, Tm, Yb, Lu), YbCoC, YbAl{sub 3}C{sub 3}, La{sub 2}Fe{sub 25}P{sub 12}, ThFe{sub 4}P{sub 2}, Th{sub 5}Fe{sub 19}P{sub 19}, Ln{sub 6}Co{sub 30}P{sub 19} (Ln = Er-Lu), VFe{sub 2}P{sub 12}, TNi{sub 4}P{sub 12} (T = V, Nb, W), CrMnP{sub 8}, MoNiP{sub 8}, and WNiP{sub 8}. The structural chemistry of these compounds will briefly be discussed.

  9. A Mixed Methods Investigation of Maternal Perspectives on Transition Experiences in Early Care and Education

    ERIC Educational Resources Information Center

    Swartz, Rebecca Anne; Speirs, Katherine Elizabeth; Encinger, Amy Johnson; McElwain, Nancy L.

    2016-01-01

    Research Findings: Strong relationships among children, families, and early care and education (ECE) providers are key to quality infant-toddler care. These relationships are shaped during the initial transition period to group care. We used a mixed methods approach to (a) assess maternal perspectives on the transition to group care, (b) explore

  10. Early warning signs for saddle-escape transitions in complex networks.

    PubMed

    Kuehn, Christian; Zschaler, Gerd; Gross, Thilo

    2015-01-01

    Many real world systems are at risk of undergoing critical transitions, leading to sudden qualitative and sometimes irreversible regime shifts. The development of early warning signals is recognized as a major challenge. Recent progress builds on a mathematical framework in which a real-world system is described by a low-dimensional equation system with a small number of key variables, where the critical transition often corresponds to a bifurcation. Here we show that in high-dimensional systems, containing many variables, we frequently encounter an additional non-bifurcative saddle-type mechanism leading to critical transitions. This generic class of transitions has been missed in the search for early-warnings up to now. In fact, the saddle-type mechanism also applies to low-dimensional systems with saddle-dynamics. Near a saddle a system moves slowly and the state may be perceived as stable over substantial time periods. We develop an early warning sign for the saddle-type transition. We illustrate our results in two network models and epidemiological data. This work thus establishes a connection from critical transitions to networks and an early warning sign for a new type of critical transition. In complex models and big data we anticipate that saddle-transitions will be encountered frequently in the future. PMID:26294271

  11. Early warning signs for saddle-escape transitions in complex networks

    NASA Astrophysics Data System (ADS)

    Kuehn, Christian; Zschaler, Gerd; Gross, Thilo

    2015-08-01

    Many real world systems are at risk of undergoing critical transitions, leading to sudden qualitative and sometimes irreversible regime shifts. The development of early warning signals is recognized as a major challenge. Recent progress builds on a mathematical framework in which a real-world system is described by a low-dimensional equation system with a small number of key variables, where the critical transition often corresponds to a bifurcation. Here we show that in high-dimensional systems, containing many variables, we frequently encounter an additional non-bifurcative saddle-type mechanism leading to critical transitions. This generic class of transitions has been missed in the search for early-warnings up to now. In fact, the saddle-type mechanism also applies to low-dimensional systems with saddle-dynamics. Near a saddle a system moves slowly and the state may be perceived as stable over substantial time periods. We develop an early warning sign for the saddle-type transition. We illustrate our results in two network models and epidemiological data. This work thus establishes a connection from critical transitions to networks and an early warning sign for a new type of critical transition. In complex models and big data we anticipate that saddle-transitions will be encountered frequently in the future.

  12. A Mixed Methods Investigation of Maternal Perspectives on Transition Experiences in Early Care and Education

    ERIC Educational Resources Information Center

    Swartz, Rebecca Anne; Speirs, Katherine Elizabeth; Encinger, Amy Johnson; McElwain, Nancy L.

    2016-01-01

    Research Findings: Strong relationships among children, families, and early care and education (ECE) providers are key to quality infant-toddler care. These relationships are shaped during the initial transition period to group care. We used a mixed methods approach to (a) assess maternal perspectives on the transition to group care, (b) explore…

  13. Bridging Early Services Transition Project--Outreach July, 1993 - June, 1997. Final Report.

    ERIC Educational Resources Information Center

    Rosenkoetter, Sharon; Shotts, Cynthia

    This report discusses the outcomes of a project designed to help young children with special needs and their families prepare for and adjust to new service settings. The Bridging Early Service Transition (BEST) Project has helped administrators, service providers, and families plan and coordinate transitions for young children with disabilities or…

  14. Early warning signs for saddle-escape transitions in complex networks

    PubMed Central

    Kuehn, Christian; Zschaler, Gerd; Gross, Thilo

    2015-01-01

    Many real world systems are at risk of undergoing critical transitions, leading to sudden qualitative and sometimes irreversible regime shifts. The development of early warning signals is recognized as a major challenge. Recent progress builds on a mathematical framework in which a real-world system is described by a low-dimensional equation system with a small number of key variables, where the critical transition often corresponds to a bifurcation. Here we show that in high-dimensional systems, containing many variables, we frequently encounter an additional non-bifurcative saddle-type mechanism leading to critical transitions. This generic class of transitions has been missed in the search for early-warnings up to now. In fact, the saddle-type mechanism also applies to low-dimensional systems with saddle-dynamics. Near a saddle a system moves slowly and the state may be perceived as stable over substantial time periods. We develop an early warning sign for the saddle-type transition. We illustrate our results in two network models and epidemiological data. This work thus establishes a connection from critical transitions to networks and an early warning sign for a new type of critical transition. In complex models and big data we anticipate that saddle-transitions will be encountered frequently in the future. PMID:26294271

  15. Recent advances in transition metal-catalyzed Csp(2)-monofluoro-, difluoro-, perfluoromethylation and trifluoromethylthiolation.

    PubMed

    Landelle, Grégory; Panossian, Armen; Pazenok, Sergiy; Vors, Jean-Pierre; Leroux, Frédéric R

    2013-01-01

    In the last few years, transition metal-mediated reactions have joined the toolbox of chemists working in the field of fluorination for Life-Science oriented research. The successful execution of transition metal-catalyzed carbon-fluorine bond formation has become a landmark achievement in fluorine chemistry. This rapidly growing research field has been the subject of some excellent reviews. Our approach focuses exclusively on transition metal-catalyzed reactions that allow the introduction of -CFH2, -CF2H, -C n F2 n +1 and -SCF3 groups onto sp² carbon atoms. Transformations are discussed according to the reaction-type and the metal employed. The review will not extend to conventional non-transition metal methods to these fluorinated groups. PMID:24367416

  16. Recent advances in transition metal-catalyzed Csp2-monofluoro-, difluoro-, perfluoromethylation and trifluoromethylthiolation

    PubMed Central

    Landelle, Grégory; Panossian, Armen; Pazenok, Sergiy; Vors, Jean-Pierre

    2013-01-01

    Summary In the last few years, transition metal-mediated reactions have joined the toolbox of chemists working in the field of fluorination for Life-Science oriented research. The successful execution of transition metal-catalyzed carbon–fluorine bond formation has become a landmark achievement in fluorine chemistry. This rapidly growing research field has been the subject of some excellent reviews. Our approach focuses exclusively on transition metal-catalyzed reactions that allow the introduction of –CFH2, –CF2H, –CnF2 n +1 and –SCF3 groups onto sp² carbon atoms. Transformations are discussed according to the reaction-type and the metal employed. The review will not extend to conventional non-transition metal methods to these fluorinated groups. PMID:24367416

  17. Chemistry of Two-Dimensional Transition Metal Carbides (MXenes)

    NASA Astrophysics Data System (ADS)

    Mashtalir, Olha

    With consumer trends pushing toward smaller, faster, more flexible, multitasking devices, researchers striving to meet these needs have targeted two-dimensional (2D) materials---and graphene in particular---as holding the most promise for use in advanced applications. But in 2011, a significant interest has been triggered by a newly discovered family of novel 2D materials---layered transitional metal carbides and carbonitrides, named MXenes. Those compounds were of general formula Mn+1 XnTx, where M stands for metal atom, X is C and/or N, n = 1, 2 or 3, and Tx represents surface groups. Being initially suggested as a material for electrical energy storage systems, MXenes' properties and their potential applications have not been explored. This work is the first complete study of MXenes' chemistry that sheds light on the chemical composition, structure and properties of these novel materials and possible routes of its modification. The research was focused on 2D titanium carbide, Ti3C2Tx, chosen as the representative of the MXene family. The kinetic study of Ti 3C2Tx synthesis discovered the main synthesis parameters, viz. temperature, time and particle size, that affect the etching process and define the quality of final product. MXenes were found to be able to spontaneously accommodate various ions and small organic molecules between the layers leading to preopening of the structure. A major challenge of large scale production of delaminated, atomically thin 2D MXene layers was solved with two delamination techniques involving dimethyl sulfoxide and isopropyl amine pre-intercalation followed by sonication in water. Ti3C2Tx was also found to possess adsorptive and photocatalytic properties, revealing its potential for environmental applications. It also showed limited stability in water and in the presence of oxygen, providing important practical information on proper handling and storage of MXene materials. Completion of this work allowed the performance of energy storage devices to be improved significantly, viz. Li-ion batteries and electrochemical capacitors, and gave rise to many other potential applications such as MXene-polymer composites, sorption, and catalysis. More importantly, it opened a path to the large-scale synthesis of thin, single-layer MXene sheets and led to establishing MXenes as fully-fledged members of the growing family of 2D materials.

  18. Porous nanoarchitectures of spinel-type transition metal oxides for electrochemical energy storage systems.

    PubMed

    Park, Min-Sik; Kim, Jeonghun; Kim, Ki Jae; Lee, Jong-Won; Kim, Jung Ho; Yamauchi, Yusuke

    2015-12-14

    Transition metal oxides possessing two kinds of metals (denoted as AxB3-xO4, which is generally defined as a spinel structure; A, B = Co, Ni, Zn, Mn, Fe, etc.), with stoichiometric or even non-stoichiometric compositions, have recently attracted great interest in electrochemical energy storage systems (ESSs). The spinel-type transition metal oxides exhibit outstanding electrochemical activity and stability, and thus, they can play a key role in realising cost-effective and environmentally friendly ESSs. Moreover, porous nanoarchitectures can offer a large number of electrochemically active sites and, at the same time, facilitate transport of charge carriers (electrons and ions) during energy storage reactions. In the design of spinel-type transition metal oxides for energy storage applications, therefore, nanostructural engineering is one of the most essential approaches to achieving high electrochemical performance in ESSs. In this perspective, we introduce spinel-type transition metal oxides with various transition metals and present recent research advances in material design of spinel-type transition metal oxides with tunable architectures (shape, porosity, and size) and compositions on the micro- and nano-scale. Furthermore, their technological applications as electrode materials for next-generation ESSs, including metal-air batteries, lithium-ion batteries, and supercapacitors, are discussed. PMID:26549729

  19. Ligational behavior of Schiff bases towards transition metal ion and metalation effect on their antibacterial activity.

    PubMed

    Devi, Jai; Batra, Nisha; Malhotra, Rajesh

    2012-11-01

    New Schiff bases pyrazine-2-carboxylicacid (phenyl-pyridin-2-yl-methylene)-hydrazide (Hpch-bp) HL(1) and pyrazine-2-carboxylicacid (pyridin-2-ylmethylene)-hydrazide (Hpch-pc) HL(2) derived from condensation of pyrazine carboxylic hydrazide (Hpch) with 2-benzoyl pyridine (bp) or pyridine 2-carbaldehyde (pc) and their transition metal complexes of type ML((1-2)2) have been synthesized, where M=Mn(II), Co(II), Ni(II), Cu(II) and Zn(II). Characterization of ligands and their metal complexes was carried out by elemental analysis, conductimetric studies, magnetic susceptibility, spectroscopic techniques (IR, UV-VIS, NMR, ESR, Mass) and thermogravimetric analysis. The physico-chemical studies revealed octahedral geometry or distorted octahedral geometry around metal ion. These azomethine Schiff base ligands acted as tridentate coordinating through carbonyl, azomethine and pyridine nitrogen present in the ligand. The thermodynamic and thermal properties of the complexes have been investigated and it was observed on the basis of these studies that thermal stability of complexes follows the order Mnmetal ions. The trend of growth inhibition in the complexes was found to be in the order: Cu>Mn>Ni>Co>Zn. PMID:22813991

  20. On the valence fluctuation in the early actinide metals

    SciTech Connect

    Soderlind, P.; Landa, A.; Tobin, J. G.; Allen, P.; Medling, S.; Booth, C. H.; Bauer, E. D.; Cooley, J. C.; Sokaras, D.; Weng, T. -C.; Nordlund, D.

    2015-12-15

    In this study, recent X-ray measurements suggest a degree of valence fluctuation in plutonium and uranium intermetallics. We are applying a novel scheme, in conjunction with density functional theory, to predict 5f configuration fractions of states with valence fluctuations for the early actinide metals. For this purpose we perform constrained integer f-occupation calculations for the α phases of uranium, neptunium, and plutonium metals. For plutonium we also investigate the δ phase. The model predicts uranium and neptunium to be dominated by the f3 and f4 configurations, respectively, with only minor contributions from other configurations. For plutonium (both α and δ phase) the scenario is dramatically different. Here, the calculations predict a relatively even distribution between three valence configurations. The δ phase has a greater configuration fraction of f6 compared to that of the α phase. The theory is consistent with the interpretations of modern X-ray experiments and we present resonant X-ray emission spectroscopy results for α-uranium.

  1. Environmental Effects on the Metallicities of Early-Type Galaxies

    NASA Technical Reports Server (NTRS)

    Jones, Christine; Oliversen, Ronald J. (Technical Monitor)

    2003-01-01

    We have completed and published two papers based on research from this grant. Our first paper "SN IA Enrichment in Virgo Early-type Galaxies from ROSAT and ASCA Observations" was published in the Astrophysical Journal (vol 539,603) reported on the properties of nine X-ray bright elliptical galaxies in the Virgo cluster observed by ROSAT and ASCA. We measured iron abundance gradients as a function of radius in three galaxies. We found that the magnesium and silicon abundance gradients were in general flatter than those of iron. We suggest this is due to a metallicity dependence in the metal production rates of SN Ia's. We calculate SN Ia rates in the center of these galaxies that are comparable to those measured optically. Our second paper "ASCA Observations of Groups at Radii of Low Overdensity: Implications for Cosmic Preheating" also was published in the Astrophysical Journal (vol 578, 74). This paper reported on the ASCA spectroscopy of nine groups of galaxies. We found that the entropy profile in groups is driven by nongravitational heating processes, and could be explained by a short period of preheating by galactic winds.

  2. On the valence fluctuation in the early actinide metals

    DOE PAGESBeta

    Soderlind, P.; Landa, A.; Tobin, J. G.; Allen, P.; Medling, S.; Booth, C. H.; Bauer, E. D.; Cooley, J. C.; Sokaras, D.; Weng, T. -C.; et al

    2015-12-15

    In this study, recent X-ray measurements suggest a degree of valence fluctuation in plutonium and uranium intermetallics. We are applying a novel scheme, in conjunction with density functional theory, to predict 5f configuration fractions of states with valence fluctuations for the early actinide metals. For this purpose we perform constrained integer f-occupation calculations for the α phases of uranium, neptunium, and plutonium metals. For plutonium we also investigate the δ phase. The model predicts uranium and neptunium to be dominated by the f3 and f4 configurations, respectively, with only minor contributions from other configurations. For plutonium (both α and δmore » phase) the scenario is dramatically different. Here, the calculations predict a relatively even distribution between three valence configurations. The δ phase has a greater configuration fraction of f6 compared to that of the α phase. The theory is consistent with the interpretations of modern X-ray experiments and we present resonant X-ray emission spectroscopy results for α-uranium.« less

  3. Structural and Electrochemical Characterization of Lithium Transition Metal Phosphates

    NASA Astrophysics Data System (ADS)

    Hashambhoy, Ayesha Maria

    The lithium ion battery has emerged as one of the most promising hybrid vehicle energy storage systems of the future. Of the potential cathode chemistries explored, lithium transition metal phosphates have generated a significant amount of interest due to their low-cost precursors, potential ease of synthesis, stability, and their environmentally friendly nature. This is in contrast to layered oxide systems such as LiCoO2, which have long been considered state of the art, but are now being reevaluated due to their structural instability at elevated temperatures, and higher cost. In particular, LiFePO4 has an operating potential comparable to those batteries available on the market (˜3.5V vs. Li/Li+), and higher theoretical specific capacity (170mAh/g vs. that of LiCoO2 which is 140mAh/g). The manganese analog to LiFePO4, LiMnPO4, exhibits a higher operating potential (˜4.1V v Li/Li+), and the same theoretical capacity, however Li-ion diffusion through this structure is much more rate limited and its theoretical capacity cannot be realized at rates suitable for commercial applications. The purpose of this work was threefold: 1) To explore the impact of Fe substitution on Mn sites in LiMnPO 4. 2) To examine the effects of alterations to the particle/electrolyte interface on rate capability. 3) To explore a novel fabrication route for LiMnPO4 using microwaves, and determine an optimal power and time combination for best performance. The coexistence of Fe and Mn on the transition metal site M, of LiMPO 4 resulted in an improved apparent Li-ion diffusivity in both Fe and Mn regimes as compared to that observed for LiFePO4 and LiMnPO 4 respectively. Calculations made from two different analysis methods, cyclic voltammetry (CV) and galvanostatic intermittent titration (GITT) drew this same conclusion. The signature characteristics observed from the CVs pertaining to single and dual phase reactions led to a delithiation model of LiFe0.5Mn0.5PO4 proposing the localization of half the Li atoms with Fe and half with Mn. Following this work, pure LiMnPO 4 was explored. LiMnPO4 was successfully fabricated using rapid, solid state microwave irradiation. Three classes of materials were prepared to study the effects of particle size, carbon coating, and electrolytic environment on rate capability. Reduction in particle size, carbon coating, and the aqueous electrolyte environment provided the most favorable conditions for performance enhancement. Based on the initial promising results of solid state microwave synthesis, further studies were conducted to optimize irradiation parameters. C-LiMnPO 4 fabricated at 200W for 5.0 minutes demonstrated the most superior rate capability. This material attained its full theoretical specific capacity, showing promise for the advancement of materials fabricated via this method.

  4. A Qualitative Study of Early Family Histories and Transitions of Homeless Youth

    ERIC Educational Resources Information Center

    Tyler, Kimberly A.

    2006-01-01

    Using intensive qualitative interviews with 40 homeless youth, this study examined their early family histories for abuse, neglect, and other family problems and the number and types of transitions that youth experienced. Multiple forms of child maltreatment, family alcoholism, drug use, and criminal activity characterized early family histories…

  5. Relationship of an Early Placement Program to the Transition from School to Full-Time Employment.

    ERIC Educational Resources Information Center

    Foreman, Ronald L.

    To evaluate the relationship of an early placement program to the initial transition from school to full-time employment and to determine whether actual work experience as part of the high school curriculum affects attitudes toward work and/or self-esteem, 100 senior students received a 6-9 week early placement as part of their vocational…

  6. A Qualitative Study of Early Family Histories and Transitions of Homeless Youth

    ERIC Educational Resources Information Center

    Tyler, Kimberly A.

    2006-01-01

    Using intensive qualitative interviews with 40 homeless youth, this study examined their early family histories for abuse, neglect, and other family problems and the number and types of transitions that youth experienced. Multiple forms of child maltreatment, family alcoholism, drug use, and criminal activity characterized early family histories

  7. IR analysis of polyvinylidene fluoride doped with transition metal halides

    NASA Astrophysics Data System (ADS)

    El Hefnawy, Somia M.; Aboelkher, Mervet M.; Abdelkader, H.

    2009-05-01

    Pure and doped polyvinylidene fluoride (PVDF) films were prepared by casting. Films with various concentrations of transition metal halides TMHs (AlCl3, ZnCl2, and CoCl2) were prepared. The microstructure and physical properties of these films were studied by IR analysis. The two factors affecting the interaction between the PVDF and MHs are (i) the dopant weight fraction (Hc) (0.05% -30%) by weght, and (ii) precasting time (tpc) which is the time during which the PVDF pellets are maintained solved with the halides added before casting. From the IR quantitative analysis, it is evident that the addition of the three MH to the undoped PVDF film makes β-phase as the dominant crystalline structures in the doped films without the need for mechanical drawing treatment. The precasting time plays a role for new crystalline structures to appear which becomes strong for CoCl2 doping, moderate in ZnCl2 doping and weak in AlCl3 doping. This phase is maximum for the relatively low doping levels < 5%. The stability of these structures in the samples doped with CoCl2 is high compared to the doping with ZnCl2 and AlCl3. This result is extremely important hence the β-phase is that one which is electrically active compared with the other two phases and it is needed in all the samples used in the useful applications of the PVF2 films. Remembering, that β-phase is obtained in the crystallization from melt samples by the uneasy mechanical stress and elevated temperature, it becomes evident the importance of the present result.

  8. Normal spectral emittance of crystalline transition metal carbides

    NASA Astrophysics Data System (ADS)

    Mackie, William A.; Carleson, Pete; Filion, Janice; Hinrichs, C. H.

    1991-05-01

    We report on experiments to determine the normal spectral emittance of crystalline forms of five transition metal carbides: hafnium carbide, niobium carbide, tantalum carbide, titanium carbide, and zirconium carbide. These emittance measurements were taken at the commonly used pyrometer wavelength of 0.65 ?m. The specimens were cylindrical with 0.10-cm-diameter blackbody holes drilled axially to a depth of 1.0 cm. The crystalline specimens were prepared from sintered stock by floating zone arc refinement and then centerless ground to the desired diameter. Measurements were made in vacuum in the temperature range 1200

  9. Destabilization effect of transition metal fluorides on sodium borohydride.

    PubMed

    Kalantzopoulos, Georgios N; Guzik, Matylda N; Deledda, Stefano; Heyn, Richard H; Muller, Jiri; Hauback, Bjørn C

    2014-10-14

    The effect of transition metal fluorides on the decomposition of NaBH4 has been investigated for NaBH4 ball milled with TiF3, MnF3 or FeF3. The compounds were examined by thermal programmed desorption with residual gas analysis, thermo gravimetric analysis and volumetric measurements using a Sieverts-type apparatus. The phase formation process during thermal decomposition was studied by in situ synchrotron radiation powder X-ray diffraction on the as-milled powders. NaBF4 was among the products in all mechano-chemical reactions. (11)B-NMR spectra analysis gave NaBF4 : NaBH4 ratios of 1 : 150 for Na-Ti, 1 : 40 for Na-Mn, and 1 : 10 for Na-Fe. Pure NaBH4 possessed a hydrogen release onset temperature of 430 °C. The hydrogen release in the NaBH4-MnF3 system began as low as 130 °C. FeF3 decreased the onset temperature to 161 °C and TiF3 to 200 °C. TiF3 reacted completely with NaBH4 below 320 °C. All the examined systems have negligible emissions of diborane species. H-sorption studies performed at selected temperatures above 300 °C exhibited relatively fast desorption kinetics. Partial hydrogen re-absorption was observed for the Na-Mn and Na-Fe samples. PMID:25140831

  10. Anaerobic Killing of Oral Streptococci by Reduced, Transition Metal Cations

    PubMed Central

    Dunning, J. C.; Ma, Y.; Marquis, R. E.

    1998-01-01

    Reduced, transition metal cations commonly enhance oxidative damage to cells caused by hydroperoxides formed as a result of oxygen metabolism or added externally. As expected, the cations Fe2+ and Cu+ enhanced killing of Streptococcus mutans GS-5 by hydroperoxides. However, unexpectedly, they also induced lethal damage under fully anaerobic conditions in a glove box with no exposure to O2 or hydroperoxides from initial treatment with the cations. Sensitivities to anaerobic killing by Fe2+ varied among the organisms tested. The oral streptococci Streptococcus gordonii ATCC 10558, Streptococcus rattus FA-1, and Streptococcus sanguis NCTC 10904 were approximately as sensitive as S. mutans GS-5. Enterococcus hirae ATCC 9790, Actinomyces viscosus OMZ105E, and Actinomyces naeslundii WVU45 had intermediate sensitivity, while Lactobacillus casei ATCC 4646 and Escherichia coli B were insensitive. Killing of S. mutans GS-5 in response to millimolar levels of added Fe2+ occurred over a wide range of temperatures and pH. The organism was able to take up ferrous iron, but ferric reductase activity could not be detected. Chelators, uric acid, and thiocyanate were not effective inhibitors of the lethal damage. Sulfhydryl compounds, ferricyanide, and ferrocyanide were protective if added prior to Fe2+ exposure. Fe2+, but not Fe3+, acted to reduce the acid tolerance of glycolysis by intact cells of S. mutans. The reduction in acid tolerance appeared to be related directly to Fe2+ inhibition of F-ATPase, which could be assayed with permeabilized cells, isolated membranes, or F1 enzyme separated from membranes. Cu+ and Cu2+ also inhibited F-ATPase and sensitized glycolysis by intact cells to acid. All of these damaging actions occurred anaerobically and thus did not appear to involve reactive oxygen species. PMID:9435058

  11. Late transition metal. mu. -oxo and. mu. -imido complexes

    SciTech Connect

    Sharp, P.R.

    1990-01-01

    The synthesis and reactions of late-transition-metal oxo and imido complexes was explored. The deprotonation of platinum(II) hydroxo complexes yielded new oxo complexes. Attempted deprotonation of Cp*Rh(III) hydroxo complexes did not give oxo complexes but complex mixtures probably resulting from reduction of the Rh(III) center. The reaction of Na/Hg with (Cp*RhCl{sub 2}){sub 2} gave the very reactive Rh(II) dimer, (Cp*RhCl){sub 2}. Rhodium(I) imido complexes with the bis(dimethylphosphino)methane ligand were prepared and found to be similar to the previously prepared bis(diphenylphosphino)methane complexes. Attempts to prepare bis(diphenylphosphino)methylamine, bis(diphenylphosphino)phenylamine, PMe{sub e} and NO{sup +} analogues were not successful. Attempts to prepare Cp*Rh(III) imido complexes resulted in amido complexes and reduction. Rhodium (III) tris(3.5-dimethylpyrazoyl)borate analogues are reduction resistant but have not yet yielded imido complexes. The first imido complexes of Au were prepared by treating a Au oxo complex with amines or isocyanates. Dimeric Cp*Rh dioxygen and nitrosobenzene complexes were prepared by insertion into the Rh-Rh bond of (Cp*RhCl){sub 2}. The dioxygen complex activates a C-H bond of the Cp* ligand on treatment with PMe{sub 3}. Imido and oxo complexes nitrene and oxygen atom transfer product in reactions with CO. A novel electrophilic ring addition was observed with sterically protected aryl imido complexes. 15 refs.

  12. Scaling Relationships for Adsorption Energies of C2 Hydrocarbons on Transition Metal Surfaces

    SciTech Connect

    Jones, G

    2011-08-18

    Using density functional theory calculations we show that the adsorption energies for C{sub 2}H{sub x}-type adsorbates on transition metal surfaces scale with each other according to a simple bond order conservation model. This observation generalizes some recently recognized adsorption energy scaling laws for AH{sub x}-type adsorbates to unsaturated hydrocarbons and establishes a coherent simplified description of saturated as well as unsaturated hydrocarbons adsorbed on transition metal surfaces. A number of potential applications are discussed. We apply the model to the dehydrogenation of ethane over pure transition metal catalysts. Comparison with the corresponding full density functional theory calculations shows excellent agreement.

  13. Phase transition into the metallic state in hypothetical (without molecules) dense atomic hydrogen

    SciTech Connect

    Khomkin, A. L. Shumikhin, A. S.

    2013-10-15

    A simple physical model of the metal-dielectric (vapor-liquid) phase transition in hypothetical (without molecules) atomic hydrogen is proposed. The reason for such a transition is the quantum collective cohesive energy occurring due to quantum electron-electron exchange similar to the cohesive energy in the liquid-metal phase of alkali metals. It is found that the critical parameters of the transition are P{sub c} ? 41000 atm, ?{sub c} ? 0.1 g/cm{sup 3}, and T{sub c} ? 9750 K.

  14. Microalloying of transition metal silicides by mechanical activation and field-activated reaction

    DOEpatents

    Munir, Zuhair A.; Woolman, Joseph N.; Petrovic, John J.

    2003-09-02

    Alloys of transition metal suicides that contain one or more alloying elements are fabricated by a two-stage process involving mechanical activation as the first stage and densification and field-activated reaction as the second stage. Mechanical activation, preferably performed by high-energy planetary milling, results in the incorporation of atoms of the alloying element(s) into the crystal lattice of the transition metal, while the densification and field-activated reaction, preferably performed by spark plasma sintering, result in the formation of the alloyed transition metal silicide. Among the many advantages of the process are its ability to accommodate materials that are incompatible in other alloying methods.

  15. Critical behavior at a dynamic vortex insulator-to-metal transition

    NASA Astrophysics Data System (ADS)

    Poccia, Nicola; Baturina, Tatyana I.; Coneri, Francesco; Molenaar, Cor G.; Wang, X. Renshaw; Bianconi, Ginestra; Brinkman, Alexander; Hilgenkamp, Hans; Golubov, Alexander A.; Vinokur, Valerii M.

    2015-09-01

    An array of superconducting islands placed on a normal metal film offers a tunable realization of nanopatterned superconductivity. This system enables investigation of the nature of competing vortex states and phase transitions between them. A square array creates the eggcrate potential in which magnetic field-induced vortices are frozen into a vortex insulator. We observed a vortex insulator-vortex metal transition driven by the applied electric current and determined critical exponents that coincided with those for thermodynamic liquid-gas transition. Our findings offer a comprehensive description of dynamic critical behavior and establish a deep connection between equilibrium and nonequilibrium phase transitions.

  16. Mechanism of phase transitions affecting intramolecular electron transfer in trinuclear mixed-valence transition-metal compounds

    NASA Astrophysics Data System (ADS)

    Kambara, Takeshi; Hendrickson, David N.; Sorai, Michio; Oh, Seung M.

    1986-09-01

    Intramolecular electron delocalization in discrete mixed-valence transition metal complexes in the condensed phase depends not only on the electronic structure of a single complex but also sensitively on the details of the packing arrangement [D. N. Hendrickson, S. M. Oh, T.-Y. Dong, T. Kambara, M. J. Cohn, and M. F. Moore, Comments Inorg. Chem. 4, 329 (1985)]. The problem of how the cooperative properties of mixed-valence complexes in the solid state depend on the electron localization and/or delocalization in a single complex is studied theoretically. A phenomenological intermolecular interaction which depends on the sense and the magnitude of molecular distortion arising from the electron localization is introduced. A theoretical model is developed based on molecular field theory in order to show what types of phase transitions relating to the electron delocalization are possible in the trinuclear mixed-valence compounds and how the electronic structure of constituent molecules determines the type of phase transition. There are three types of phase transitions: (1) Order-disorder transition with respect to the alignment of the sense of molecular distortion associated with the electron localization; (2) static localization-delocalization transition, where the molecular distortion disappears above the transition temperature and electrons are coherently delocalized on three transition metal ions; (3) dynamical localization-delocalization transition in which the delocalization comes from fast electron transfer between three transition-metal ions and the molecular structure is changed from a static distortion to a dynamical distortion. The theoretical model is used to explain the observed temperature dependencies of heat capacity and Mössbauer spectra for the trinuclear mixed-valence complex [Fe3O(O2CCH3)6(py)3](py), where (py) is pyridine [S. M. Oh. T. Kambara, D. N. Hendrickson, M. Sorai, K. Kaji, S. E. Woehler, and R. J. Wittebort, J. Am. Chem. Soc. 107, 5540 (1985); M. Sorai, K. Kaji, D. N. Hendrickson, and S. M. Oh, ibid. 108, 702 (1986)]. The first-order phase transition at ˜112 K is assigned as an order-disorder transition and the higher-order transition at ˜190 K is assigned as a dynamical localization-delocalization transition.

  17. Methods for detecting early warnings of critical transitions in time series illustrated using simulated ecological data.

    PubMed

    Dakos, Vasilis; Carpenter, Stephen R; Brock, William A; Ellison, Aaron M; Guttal, Vishwesha; Ives, Anthony R; Kéfi, Sonia; Livina, Valerie; Seekell, David A; van Nes, Egbert H; Scheffer, Marten

    2012-01-01

    Many dynamical systems, including lakes, organisms, ocean circulation patterns, or financial markets, are now thought to have tipping points where critical transitions to a contrasting state can happen. Because critical transitions can occur unexpectedly and are difficult to manage, there is a need for methods that can be used to identify when a critical transition is approaching. Recent theory shows that we can identify the proximity of a system to a critical transition using a variety of so-called 'early warning signals', and successful empirical examples suggest a potential for practical applicability. However, while the range of proposed methods for predicting critical transitions is rapidly expanding, opinions on their practical use differ widely, and there is no comparative study that tests the limitations of the different methods to identify approaching critical transitions using time-series data. Here, we summarize a range of currently available early warning methods and apply them to two simulated time series that are typical of systems undergoing a critical transition. In addition to a methodological guide, our work offers a practical toolbox that may be used in a wide range of fields to help detect early warning signals of critical transitions in time series data. PMID:22815897

  18. Methods for Detecting Early Warnings of Critical Transitions in Time Series Illustrated Using Simulated Ecological Data

    PubMed Central

    Dakos, Vasilis; Carpenter, Stephen R.; Brock, William A.; Ellison, Aaron M.; Guttal, Vishwesha; Ives, Anthony R.; Kéfi, Sonia; Livina, Valerie; Seekell, David A.; van Nes, Egbert H.; Scheffer, Marten

    2012-01-01

    Many dynamical systems, including lakes, organisms, ocean circulation patterns, or financial markets, are now thought to have tipping points where critical transitions to a contrasting state can happen. Because critical transitions can occur unexpectedly and are difficult to manage, there is a need for methods that can be used to identify when a critical transition is approaching. Recent theory shows that we can identify the proximity of a system to a critical transition using a variety of so-called ‘early warning signals’, and successful empirical examples suggest a potential for practical applicability. However, while the range of proposed methods for predicting critical transitions is rapidly expanding, opinions on their practical use differ widely, and there is no comparative study that tests the limitations of the different methods to identify approaching critical transitions using time-series data. Here, we summarize a range of currently available early warning methods and apply them to two simulated time series that are typical of systems undergoing a critical transition. In addition to a methodological guide, our work offers a practical toolbox that may be used in a wide range of fields to help detect early warning signals of critical transitions in time series data. PMID:22815897

  19. Controlled synthesis of transition metal/conducting polymer nanocomposites.

    PubMed

    Liu, Zhen; Liu, Yang; Zhang, Lin; Poyraz, Selcuk; Lu, Ning; Kim, Moon; Smith, James; Wang, Xiaolong; Yu, Yajiao; Zhang, Xinyu

    2012-08-24

    A novel displacement reaction has been observed to occur between conducting polymers (CP) and metal salts which can be used to fabricate nanostructured CP-metal composites in a one-pot manner. Vanadium pentoxide (V(2)O(5)) nanofiber is used during the synthesis as the reactive seeds to induce the nanofibril CP-metal network formation. The CP-metal nanocomposites exhibit excellent sensory properties for hydrogen peroxide (H(2)O(2)) detection, where both high sensitivity and a low detection limit can be obtained. The sensory performance of the CP-metal composite can be further enhanced by a facile microwave treatment. It is believed that the CP-metal nanofibril network can be converted to a carbon-metal network by a microwave-induced carbonization process and result in the sensory enhancement. PMID:22842608

  20. Electron Scattering at Surfaces and Interfaces of Transition Metals

    NASA Astrophysics Data System (ADS)

    Zheng, Pengyuan

    The effect of surfaces on the electron transport at reduced scales is attracting continuous interest due to its broad impact on both the understanding of materials properties and their application for nanoelectronics. The size dependence of for conductor's electrical resistivity rho due to electron surface scattering is most commonly described within the framework of Fuchs and Sondheimer (FS) and their various extensions, which uses a phenomenological scattering parameter p to define the probability of electrons being elastically (i.e. specularly) scattered by the surface without causing an increase of rho at reduced size. However, a basic understanding of what surface chemistry and structure parameters determine the specularity p is still lacking. In addition, the assumption of a spherical Fermi surface in the FS model is too simple for transition metals to give accurate account of the actual surface scattering effect. The goal of this study is to develop an understanding of the physics governing electron surface/interface scattering in transition metals and to study the significance of their Fermi surface shape on surface scattering. The advancement of the scientific knowledge in electron surface and interface scattering of transition metals can provide insights into how to design high-conductivity nanowires that will facilitate the viable development of advanced integrated circuits, thermoelectric power generation and spintronics. Sequential in situ and ex situ transport measurements as a function of surface chemistry demonstrate that electron surface/interface scattering can be engineered by surface doping, causing a decrease in the rho. For instance, the rho of 9.3-nm-thick epitaxial and polycrystalline Cu is reduced by 11--13% when coated with 0.75 nm Ni. This is due to electron surface scattering which exhibits a specularity p = 0.7 for the Cu-vacuum interface that transitions to completely diffuse (p = 0) when exposed to air. In contrast, Ni-coated surfaces exhibit partial specularity with p = 0.3 in vacuum and p = 0.15 in air, as Cu2O formation is suppressed, leading to a smaller surface potential perturbation and a lower density of localized surface states, yielding less diffuse electron scattering. The localized surface density of states (LDOS) at the Fermi level N(Ef) as a primary parameter determining the surface scattering specularity is further confirmed by a different surface dopant. In particular, the measured sheet resistance of 9-25-nm-thick epitaxial Cu(001) layers increases when coated with dTi = 0.1-4.0 monolayers (ML) of Ti, but decreases again during exposure to 37 Pa of O 2. The corresponding changes in rho are a function of d Cu and dTi and are due to a transition from partially specular electron scattering at the Cu surface to completely diffuse scattering at the Cu-Ti interface, and the recovery of surface specularity as the Ti is oxidized. X-ray reflectivity and photoelectron spectroscopy indicate the formation of a 0.47+/-0.03 nm thick Cu2O surface layer on top of the TiO2-Cu2O during air exposure, while density functional calculations of TiOx cap layers as a function of x = 0-2 and dTi = 0.25-1.0 ML show a reduction of N(Ef) by up to a factor of four. This reduction is proposed to be the key cause for the recovery of surface specularity and results in electron confinement and channeling in the Cu layer upon Ti oxidation. Transport measurements at 293 and 77 K confirm the electron channeling and demonstrate the potential for high-conductivity metal nanowires by quantifying the surface specularity parameter p = 0.67+/-0.05, 0.00+/-0.05, and 0.35+/-0.05 at the Cu-vacuum, Cu-Ti, and Cu-TiO 2 interfaces. In order to determine the effect of the Fermi surface shape on the size effect, experimental and simulation results are combined to study how the resistivity changes with film thickness dw on monocrystalline W layers with different surface orientation, W(001) and W(110). As the first step of the experiments, the growth of epitaxial W(001) layers on MgO(001) substrates by ultra-high vacuum magnetron sputtering is studied, in order to obtain an alternative W layer orientation in addition to the well-known growth of epitaxial W(011) on Al2O3 substrates. X-ray diffraction o-2theta scans, o-rocking curves, and pole figures show that 5-400 nm thick W(001) layers grown at Ts = 900 °C are monocrystalline with a relaxed lattice constant of 3.167+/-0.001 nm, as determined from high resolution reciprocal space mapping. The magnitude of the residual in-plane compressive strain decreases from -1.2+/-0.1% to 0.1+/-0.1% with increasing dw. This is attributed to the glide of threading dislocations which increases the average misfit dislocation length, causing relaxation of the stress associated with differential thermal contraction. X-ray reflectivity measurements indicate smooth surfaces with a root-mean-square surface roughness ≤1.0 nm and a roughness exponent of 0.38 for dw below 20 nm. Secondly, the effect of surface roughness on surface scattering is investigated to ensure its contribution to the resistivity size effect is properly included when comparing W films grown on different substrates. In fact it is found the rho of in situ annealed 4-20 nm thick epitaxial W(001) layers grown on MgO(001) samples show weaker dw dependence than that of unannealed samples in vacuum and air at both 295 and 77 K although completely diffuse surface scattering are present on both sets of films. No significant change in the structural quality of the samples after annealing is observed for d ≤ 20 nm. While a combination of X-ray reflectivity and Atomic Force Microscope study on surface morphology shows flatter surface mounds after annealing. Consequently, in situ annealing treatment is carried out on both epitaxial W(110) and W(001) from dw =4-320 nm to obtain surface with comparable rms roughness and lateral correlation length. Thus the rho increase due to the surface roughness is estimated in similar degree for the two types of films. Finally, a transport model for thin films with anisotropic Fermi surfaces is presented, which includes the effect of electron surface scattering. Simulations done using the calculated W Fermi surface show the resistivity rho to be 1.15-2.23 and 1.21-3.14 times larger than that of bulk W for (011) and (001) oriented thin films, respectively at a layer thickness d = 37.5- 3.75 nm, indicating an orientation dependent surface scattering effect on rho. The resistivity of epitaxial W(110) increases from 5.77+/-0.03 to 13.24+/-0.24 microO-cm as d decreases from 320 to 5.7 nm, but increases stronger for epitaxial W(001) from rho = 5.77+/-0.03 to 24.42+/-0.58 microO-cm for d = 320 and 4.5 nm. This orientation dependence is quantified with a different effective mean free path lambda(110) = 18.5+/-0.3 nm vs lambda(001) = 33+/-0.4 nm at 295 K by fitting using rho vs t with the Fuchs-Sondheimer (FS) model for spherical Fermi surfaces since their surface scattering is found completely diffuse by sequential in situ and ex situ electron transport measurements. Similarly, the rho from simulation can be fitted to obtain another set of lambda(110) and lambda(001). The ratio lambda (110)/lambda(001) = 0.57+/-0.01 from simulations, in good agreement with 0.56+/-0.01 from experiment. The orientation dependent size effect is the result of (1) the projected Fermi surface area along the surface normal and (2) the rate of electrons approaching the surfaces due to the anisotropic electron Fermi velocity distribution along different directions.

  1. Suppression of Structural Phase Transition in VO2 by Epitaxial Strain in Vicinity of Metal-insulator Transition

    PubMed Central

    Yang, Mengmeng; Yang, Yuanjun; Bin Hong; Wang, Liangxin; Hu, Kai; Dong, Yongqi; Xu, Han; Huang, Haoliang; Zhao, Jiangtao; Chen, Haiping; Song, Li; Ju, Huanxin; Zhu, Junfa; Bao, Jun; Li, Xiaoguang; Gu, Yueliang; Yang, Tieying; Gao, Xingyu; Luo, Zhenlin; Gao, Chen

    2016-01-01

    Mechanism of metal-insulator transition (MIT) in strained VO2 thin films is very complicated and incompletely understood despite three scenarios with potential explanations including electronic correlation (Mott mechanism), structural transformation (Peierls theory) and collaborative Mott-Peierls transition. Herein, we have decoupled coactions of structural and electronic phase transitions across the MIT by implementing epitaxial strain on 13-nm-thick (001)-VO2 films in comparison to thicker films. The structural evolution during MIT characterized by temperature-dependent synchrotron radiation high-resolution X-ray diffraction reciprocal space mapping and Raman spectroscopy suggested that the structural phase transition in the temperature range of vicinity of the MIT is suppressed by epitaxial strain. Furthermore, temperature-dependent Ultraviolet Photoelectron Spectroscopy (UPS) revealed the changes in electron occupancy near the Fermi energy EF of V 3d orbital, implying that the electronic transition triggers the MIT in the strained films. Thus the MIT in the bi-axially strained VO2 thin films should be only driven by electronic transition without assistance of structural phase transition. Density functional theoretical calculations further confirmed that the tetragonal phase across the MIT can be both in insulating and metallic states in the strained (001)-VO2/TiO2 thin films. This work offers a better understanding of the mechanism of MIT in the strained VO2 films. PMID:26975328

  2. Suppression of Structural Phase Transition in VO2 by Epitaxial Strain in Vicinity of Metal-insulator Transition.

    PubMed

    Yang, Mengmeng; Yang, Yuanjun; Bin Hong; Wang, Liangxin; Hu, Kai; Dong, Yongqi; Xu, Han; Huang, Haoliang; Zhao, Jiangtao; Chen, Haiping; Song, Li; Ju, Huanxin; Zhu, Junfa; Bao, Jun; Li, Xiaoguang; Gu, Yueliang; Yang, Tieying; Gao, Xingyu; Luo, Zhenlin; Gao, Chen

    2016-01-01

    Mechanism of metal-insulator transition (MIT) in strained VO2 thin films is very complicated and incompletely understood despite three scenarios with potential explanations including electronic correlation (Mott mechanism), structural transformation (Peierls theory) and collaborative Mott-Peierls transition. Herein, we have decoupled coactions of structural and electronic phase transitions across the MIT by implementing epitaxial strain on 13-nm-thick (001)-VO2 films in comparison to thicker films. The structural evolution during MIT characterized by temperature-dependent synchrotron radiation high-resolution X-ray diffraction reciprocal space mapping and Raman spectroscopy suggested that the structural phase transition in the temperature range of vicinity of the MIT is suppressed by epitaxial strain. Furthermore, temperature-dependent Ultraviolet Photoelectron Spectroscopy (UPS) revealed the changes in electron occupancy near the Fermi energy EF of V 3d orbital, implying that the electronic transition triggers the MIT in the strained films. Thus the MIT in the bi-axially strained VO2 thin films should be only driven by electronic transition without assistance of structural phase transition. Density functional theoretical calculations further confirmed that the tetragonal phase across the MIT can be both in insulating and metallic states in the strained (001)-VO2/TiO2 thin films. This work offers a better understanding of the mechanism of MIT in the strained VO2 films. PMID:26975328

  3. Suppression of Structural Phase Transition in VO2 by Epitaxial Strain in Vicinity of Metal-insulator Transition

    NASA Astrophysics Data System (ADS)

    Yang, Mengmeng; Yang, Yuanjun; Bin Hong; Wang, Liangxin; Hu, Kai; Dong, Yongqi; Xu, Han; Huang, Haoliang; Zhao, Jiangtao; Chen, Haiping; Song, Li; Ju, Huanxin; Zhu, Junfa; Bao, Jun; Li, Xiaoguang; Gu, Yueliang; Yang, Tieying; Gao, Xingyu; Luo, Zhenlin; Gao, Chen

    2016-03-01

    Mechanism of metal-insulator transition (MIT) in strained VO2 thin films is very complicated and incompletely understood despite three scenarios with potential explanations including electronic correlation (Mott mechanism), structural transformation (Peierls theory) and collaborative Mott-Peierls transition. Herein, we have decoupled coactions of structural and electronic phase transitions across the MIT by implementing epitaxial strain on 13-nm-thick (001)-VO2 films in comparison to thicker films. The structural evolution during MIT characterized by temperature-dependent synchrotron radiation high-resolution X-ray diffraction reciprocal space mapping and Raman spectroscopy suggested that the structural phase transition in the temperature range of vicinity of the MIT is suppressed by epitaxial strain. Furthermore, temperature-dependent Ultraviolet Photoelectron Spectroscopy (UPS) revealed the changes in electron occupancy near the Fermi energy EF of V 3d orbital, implying that the electronic transition triggers the MIT in the strained films. Thus the MIT in the bi-axially strained VO2 thin films should be only driven by electronic transition without assistance of structural phase transition. Density functional theoretical calculations further confirmed that the tetragonal phase across the MIT can be both in insulating and metallic states in the strained (001)-VO2/TiO2 thin films. This work offers a better understanding of the mechanism of MIT in the strained VO2 films.

  4. Electronic Principles Governing the Stability and Reactivity of Ligated Metal and Silicon Encapsulated Transition Metal Clusters

    NASA Astrophysics Data System (ADS)

    Abreu, Marissa Baddick

    A thorough understanding of the underlying electronic principles guiding the stability and reactivity of clusters has direct implications for the identification of stable clusters for incorporation into clusters-assembled materials with tunable properties. This work explores the electronic principles governing the stability and reactivity of two types of clusters: ligated metal clusters and silicon encapsulated transition metal clusters. In the first case, the reactivity of iodine-protected aluminum clusters, Al13Ix - (x=0-4) and Al14Iy- (y=0-5), with the protic species methanol was studied. The symmetrical ground states of Al13Ix- showed no reactivity with methanol but reactivity was achieved in a higher energy isomer of Al 13I2- with iodines on adjacent aluminum atoms -- complementary Lewis acid-base active sites were induced on the opposite side of the cluster capable of breaking the O-H bond in methanol. Al 14Iy- (y=2-5) react with methanol, but only at the ligated adatom site. Reaction of methanol with Al14 - and Al14I- showed that ligation of the adatom was necessary for the reaction to occur there -- revealing the concept of a ligand-activated adatom. In the second case, the study focused heavily on CrSi12, a silicon encapsulated transition metal cluster whose stability and the reason for that stability has been debated heavily in the literature. Calculations of the energetic properties of CrSi n (n=6-16) revealed both CrSi12 and CrSi14 to have enhanced stability relative to other clusters; however CrSi12 lacks all the traditional markers of a magic cluster. Molecular orbital analysis of each of these clusters showed the CNFEG model to be inadequate in describing their stability. Because the 3dz2 orbital of Cr is unfilled in CrSi12, this cluster has only 16 effective valence electrons, meaning that the 18-electron rule is not applicable. The moderate stability of CrSi 12 can be accounted for by the crystal-field splitting of the 3d orbitals, which pushes the 3dz2 orbital up in energy. CrSi14, on the other hand, has 18 effective valence electrons on Cr, minimal 3d-orbital splitting, and does follow the 18-electron rule. A repetition of these calculations with WSin (n=6-16) showed similar results, except WSi12 shows all the markers of a magic cluster, due to the greater crystal-field splitting of 5d orbitals.

  5. Magnetic silicon carbide nanotubes by 3d transition metal atom functionalization

    NASA Astrophysics Data System (ADS)

    Adhikari, Kapil; Ray, Asok K.

    2013-11-01

    Interaction of 3d transition metal atoms with (3,3), (5,5), (7,7), and (9,9) SiC nanotubes has been studied using hybrid density functional PBE0 and an all electron basis set 6-31G. The interaction energy between transition metal and silicon carbide nanotubes depends both on the number of d-electrons and on the curvature of the nanotubes, with this energy, in general, increasing with increase in curvature. Except for the nanotubes functionalized by Ni and Zn, all 3d transition metal-functionalized nanotubes indicate magnetic ground states. The silicon carbide nanotubes doped with transition metals have significantly lower band gaps, in general, than those of bare nanotubes.

  6. Probing the electron states and metal-insulator transition mechanisms in molybdenum disulphide vertical heterostructures

    NASA Astrophysics Data System (ADS)

    Chen, Xiaolong; Wu, Zefei; Xu, Shuigang; Wang, Lin; Huang, Rui; Han, Yu; Ye, Weiguang; Xiong, Wei; Han, Tianyi; Long, Gen; Wang, Yang; He, Yuheng; Cai, Yuan; Sheng, Ping; Wang, Ning

    2015-01-01

    The metal-insulator transition is one of the remarkable electrical properties of atomically thin molybdenum disulphide. Although the theory of electron-electron interactions has been used in modelling the metal-insulator transition in molybdenum disulphide, the underlying mechanism and detailed transition process still remain largely unexplored. Here we demonstrate that the vertical metal-insulator-semiconductor heterostructures built from atomically thin molybdenum disulphide are ideal capacitor structures for probing the electron states. The vertical configuration offers the added advantage of eliminating the influence of large impedance at the band tails and allows the observation of fully excited electron states near the surface of molybdenum disulphide over a wide excitation frequency and temperature range. By combining capacitance and transport measurements, we have observed a percolation-type metal-insulator transition, driven by density inhomogeneities of electron states, in monolayer and multilayer molybdenum disulphide. In addition, the valence band of thin molybdenum disulphide layers and their intrinsic properties are accessed.

  7. Activation of methane by transition metal-substituted aluminophosphate molecular sieves

    DOEpatents

    Iton, Lennox E.; Maroni, Victor A.

    1991-01-01

    Aluminophosphate molecular sieves substituted with cobalt, manganese or iron and having the AlPO.sub.4 -34 or AlPO.sub.4 -5, or related AlPO.sub.4 structure activate methane starting at approximately 350.degree. C. Between 400.degree. and 500.degree. C. and at methane pressures .ltoreq.1 atmosphere the rate of methane conversion increases steadily with typical conversion efficiencies at 500.degree. C. approaching 50% and selectivity to the production of C.sub.2+ hydrocarbons approaching 100%. The activation mechanism is based on reduction of the transition metal(III) form of the molecular sieve to the transition metal(II) form with accompanying oxidative dehydrogenation of the methane. Reoxidation of the - transition metal(II) form to the transition metal(III) form can be done either chemically (e.g., using O.sub.2) or electrochemically.

  8. Reactivity of halide and pseudohalide ligands in transition-metal complexes

    SciTech Connect

    Kukushkin, Yu.N.; Kukushkin, V.Yu.

    1985-10-01

    The experimental material on the reactions of coordinated halide ligands, as well as cyanide, azido, thiocyanato, and cyanato ligands, in transition-metal complexes has been generalized in this review.

  9. Selective exchange of divalent transition metal ions in cryptomelane-type manganic acid with tunnel structure

    SciTech Connect

    Tsuji, M. ); Komarneni, S. )

    1993-03-01

    The ion-exchange selectivity of divalent transition metal ions on cryptomelane-type manganic acid (CMA) with tunnel structure has been studied using the distribution coefficients ([ital K][sub [ital d

  10. Molecular early main group metal hydrides: synthetic challenge, structures and applications.

    PubMed

    Harder, Sjoerd

    2012-11-25

    Within the general area of early main group metal chemistry, the controlled synthesis of well-defined metal hydride complexes is a rapidly developing research field. As group 1 and 2 metal complexes are generally highly dynamic and lattice energies for their [MH](∞) and [MH(2)](∞) salts are high, the synthesis of well-defined soluble hydride complexes is an obvious challenge. Access to molecular early main group metal hydrides, however, is rewarding: these hydrocarbon-soluble metal hydrides are highly reactive, have found use in early main group metal catalysis and are potentially also valuable molecular model systems for polar metal hydrides as a hydrogen storage material. The article focusses specifically on alkali and alkaline-earth metal hydride complexes and discusses the synthetic challenge, molecular structures, reactivity and applications. PMID:23012695

  11. The electronic structure of lithium transition metal oxides

    NASA Astrophysics Data System (ADS)

    Kocher, Michael P.

    Currently, LiCoO2 is the cathode in the majority of the batteries used in cellular phones and laptop computers. Due to the low abundance of cobalt, thermal instability and environmental concerns, there has been a strong effort to find an alternative material. This dissertation focuses on ab initio calculations of the electronic structure of several lithium transition metal oxides used as cathode material in Li-ion batteries, especially layered LiMn1/2Ni1/2O2 and LiMn1/3,Ni 1/3,Co1/3O2, and olivine structure LiFePO 4 and FePO4. These materials offer substantial increases in energy density and cycle life, and could be used in electric vehicles. Density Functional Theory (DFT) was used to calculate the electronic structure of LixMn1/2Ni1/2O2 and LixMn1/3,Ni1/3,Co1/3O2. To understand the fundamental characteristics of these materials, the spherical integrated charge and spin density, and angular momentum projected density of states was calculated to investigate the effect of insertion of Li. The observed changes of the integrated spin density suggest Ni is changing valence state as Li is removed. However, the integrated charge density shows no dependence on the Li concentration, which suggests that Ni remains in the same charge state. The electronic density of states reveals that the hybridized O p near the Fermi level are key component to charge compensation mechanism. This provides evidence that the O has a key role in the charge regulation during delithiation/lithiation and Ni is not changing from Ni2+ to Ni4+. The calculated projected density of states was compared with EELS measurements to further validate these findings. The electronic structure of LiFePO4 and FePO4 was calculated using DFT and DFT+U. The spherically integrated spin and charge densities show a small dependency of the Li concentration, but do not suggest a change from Fealpha to Fealpha+1. The projected density of states shows an increase in the hybridization of the O p and Fe d states as Li is removed, suggesting the covalent bonding within the material is a key component to the charge compensation mechanism.

  12. Transition Metal-MoS2 Reactions: Review and Thermodynamic Predictions

    NASA Astrophysics Data System (ADS)

    Domask, A. C.; Gurunathan, R. L.; Mohney, S. E.

    2015-11-01

    Molybdenum disulfide is a layered transition-metal dichalcogenide semiconductor that is attracting renewed attention for its potential use in future nanoscale electronics, optoelectronics, catalysis, tribology, and other fields. In all of these cases, the interaction between MoS2 and various transition metals is very important. In this work we survey the thermodynamics of the metal-Mo-S systems and the anticipated reaction products from transition metals and MoS2. We examined over 200 references on the reactions between transitions metals (M) and MoS2, compiled thermodynamic data, and used the thermodynamic data to predict M-Mo-S ternary phase diagrams for systems without experimentally determined diagrams. Where possible, experimental literature on the interactions between metals and MoS2 was used to corroborate our predicted diagrams and stable reaction products. Both the previously reported and newly predicted M-Mo-S phase diagrams fall into three categories. In the first category, the metal is in thermodynamic equilibrium with MoS2. In another, there is a driving force for the metal to reduce MoS2, with tie lines to sulfides of the contact metal dominating the phase diagram. In a final category, there is a very stable solid solution or ternary phase that dominates the phase diagram. Better understanding of the phase equilibria in the M-Mo-S systems will aid research on the use of MoS2 in a variety of fields.

  13. Transition-Metal Additives For Long-Life Na/NiCI(2) Cells

    NASA Technical Reports Server (NTRS)

    Bugga, Ratnakumar V.; Surampudi, Subbarao; Halpert, Gerald

    1995-01-01

    Transition-metal additives in cathodes of Na/NiCI(2) high-temperature, rechargeable electrochemical cells found to slow premature fading of charge/discharge capacity. Decline in capacity of cell attributed to agglomeration of Ni particles at cathode: this agglomeration reduces electrochemical area of cathode. Depending on choice of transition-metal additive for particular cell, additive might even participate in desired electrochemical reactions in cell, contributing to specific energy of cell.

  14. Conversion and displacement reaction types of transition metal compounds for sodium ion battery

    NASA Astrophysics Data System (ADS)

    Chen, Guo-Ying; Sun, Qian; Yue, Ji-Li; Shadike, Zulipiya; Yang, Yin; Ding, Fei; Sang, Lin; Fu, Zheng-Wen

    2015-06-01

    Transition metal compounds of FeSe and CuWO4 thin films have been successfully fabricated by using R.F. sputtering method. Although two kinds of transition metal compounds of FeSe and CuWO4 thin films can react with sodium electrochemically, they exhibit different electrochemical features. The nanosized metal Fe is highly dispersed into Na2Se matrix and metal Cu is extruded from Na2WO4 mixture after the FeSe/Na and CuWO4/Na cells are discharged, respectively. The conversion reaction mechanism between FeSe and Na2Se is proposed for the FeSe/Na cell. While the displacement reaction mechanism for CuWO4/Na cell is proposed for the first time based on the transmission electron microscopy (TEM) and selected area electron diffraction (SAED) data. These various mechanisms make transition metal compounds interesting materials for rechargeable sodium ion batteries.

  15. Bridged transition-metal complexes and uses thereof for hydrogen separation, storage and hydrogenation

    DOEpatents

    Lilga, M.A.; Hallen, R.T.

    1990-08-28

    The present invention constitutes a class of organometallic complexes which reversibly react with hydrogen to form dihydrides and processes by which these compounds can be utilized. The class includes bimetallic complexes in which two cyclopentadienyl rings are bridged together and also separately [pi]-bonded to two transition metal atoms. The transition metals are believed to bond with the hydrogen in forming the dihydride. Transition metals such as Fe, Mn or Co may be employed in the complexes although Cr constitutes the preferred metal. A multiple number of ancillary ligands such as CO are bonded to the metal atoms in the complexes. Alkyl groups and the like may be substituted on the cyclopentadienyl rings. These organometallic compounds may be used in absorption/desorption systems and in facilitated transport membrane systems for storing and separating out H[sub 2] from mixed gas streams such as the producer gas from coal gasification processes. 3 figs.

  16. Bridged transition-metal complexes and uses thereof for hydrogen separation, storage and hydrogenation

    DOEpatents

    Lilga, Michael A.; Hallen, Richard T.

    1991-01-01

    The present invention constitutes a class of organometallic complexes which reversibly react with hydrogen to form dihydrides and processes by which these compounds can be utilized. The class includes bimetallic complexes in which two cyclopentadienyl rings are bridged together and also separately .pi.-bonded to two transition metal atoms. The transition metals are believed to bond with the hydrogen in forming the dihydride. Transition metals such as Fe, Mn or Co may be employed in the complexes although Cr constitutes the preferred metal. A multiple number of ancilliary ligands such as CO are bonded to the metal atoms in the complexes. Alkyl groups and the like may be substituted on the cyclopentadienyl rings. These organometallic compounds may be used in absorption/desorption systems and in facilitated transport membrane systems for storing and separating out H.sub.2 from mixed gas streams such as the product gas from coal gasification processes.

  17. Bridged transition-metal complexes and uses thereof for hydrogen separation, storage and hydrogenation

    DOEpatents

    Lilga, M.A.; Hallen, R.T.

    1991-10-15

    The present invention constitutes a class of organometallic complexes which reversibly react with hydrogen to form dihydrides and processes by which these compounds can be utilized. The class includes bimetallic complexes in which two cyclopentadienyl rings are bridged together and also separately [pi]-bonded to two transition metal atoms. The transition metals are believed to bond with the hydrogen in forming the dihydride. Transition metals such as Fe, Mn or Co may be employed in the complexes although Cr constitutes the preferred metal. A multiple number of ancillary ligands such as CO are bonded to the metal atoms in the complexes. Alkyl groups and the like may be substituted on the cyclopentadienyl rings. These organometallic compounds may be used in absorption/desorption systems and in facilitated transport membrane systems for storing and separating out H[sub 2] from mixed gas streams such as the product gas from coal gasification processes. 3 figures.

  18. Bridged transition-metal complexes and uses thereof for hydrogen separation, storage and hydrogenation

    DOEpatents

    Lilga, Michael A.; Hallen, Richard T.

    1990-01-01

    The present invention constitutes a class of organometallic complexes which reversibly react with hydrogen to form dihydrides and processes by which these compounds can be utilized. The class includes bimetallic complexes in which two cyclopentadienyl rings are bridged together and also separately .pi.-bonded to two transition metal atoms. The transition metals are believed to bond with the hydrogen in forming the dihydride. Transition metals such as Fe, Mn or Co may be employed in the complexes although Cr constitutes the preferred metal. A multiple number of ancilliary ligands such as CO are bonded to the metal atoms in the complexes. Alkyl groups and the like may be substituted on the cyclopentadienyl rings. These organometallic compounds may be used in absorption/desorption systems and in facilitated transport membrane systems for storing and separating out H.sub.2 from mixed gas streams such as the produce gas from coal gasification processes.

  19. Preparation of transition metal nanoparticles and surfaces modified with (co)polymers synthesized by RAFT

    DOEpatents

    McCormick, III, Charles L.; Lowe, Andrew B.; Sumerlin, Brent S.

    2011-12-27

    A new, facile, general one-phase method of generating thiol-functionalized transition metal nanoparticles and surfaces modified by (co)polymers synthesized by the RAFT method is described. The method includes the steps of forming a (co)polymer in aqueous solution using the RAFT methodology, forming a colloidal transition metal precursor solution from an appropriate transition metal; adding the metal precursor solution or surface to the (co)polymer solution, adding a reducing agent into the solution to reduce the metal colloid in situ to produce the stabilized nanoparticles or surface, and isolating the stabilized nanoparticles or surface in a manner such that aggregation is minimized. The functionalized surfaces generated using these methods can further undergo planar surface modifications, such as functionalization with a variety of different chemical groups, expanding their utility and application.

  20. Preparation of transition metal nanoparticles and surfaces modified with (CO)polymers synthesized by RAFT

    DOEpatents

    McCormick, III., Charles L.; Lowe, Andrew B.; Sumerlin, Brent S.

    2006-11-21

    A new, facile, general one-phase method of generating thio-functionalized transition metal nanoparticles and surfaces modified by (co)polymers synthesized by the RAFT method is described. The method includes the stops of forming a (co)polymer in aqueous solution using the RAFT methodology, forming a colloidal transition metal precursor solution from an appropriate transition metal; adding the metal precursor solution or surface to the (co)polymer solution, adding a reducing agent into the solution to reduce the metal colloid in situ to produce the stabilized nanoparticles or surface, and isolating the stabilized nanoparticles or surface in a manner such that aggregation is minimized. The functionalized surfaces generated using these methods can further undergo planar surface modifications, such as functionalization with a variety of different chemical groups, expanding their utility and application.

  1. Preparation of transition metal nanoparticles and surfaces modified with (CO) polymers synthesized by RAFT

    DOEpatents

    McCormick, III, Charles L.; Lowe, Andrew B.; Sumerlin, Brent S.

    2006-10-25

    A new, facile, general one-phase method of generating thiol-functionalized transition metal nanoparticles and surface modified by (co)polymers synthesized by the RAFT method is described. The method includes the steps of forming a (co)polymer in aqueous solution using the RAFT methodology, forming a collidal transition metal precursor solution from an appropriate transition metal; adding the metal precursor solution or surface to the (co)polymer solution, adding a reducing agent into the solution to reduce the metal colloid in situ to produce the stabilized nanoparticles or surface, and isolating the stabilized nanoparticles or surface in a manner such that aggregation is minimized. The functionalized surfaces generated using these methods can further undergo planar surface modifications, such as fuctionalization with a variety of different chemical groups, expanding their utility and application.

  2. Correlation between oxygen adsorption energy and electronic structure of transition metal macrocyclic complexes

    NASA Astrophysics Data System (ADS)

    Liu, Kexi; Lei, Yinkai; Wang, Guofeng

    2013-11-01

    Oxygen adsorption energy is directly relevant to the catalytic activity of electrocatalysts for oxygen reduction reaction (ORR). In this study, we established the correlation between the O2 adsorption energy and the electronic structure of transition metal macrocyclic complexes which exhibit activity for ORR. To this end, we have predicted the molecular and electronic structures of a series of transition metal macrocyclic complexes with planar N4 chelation, as well as the molecular and electronic structures for the O2 adsorption on these macrocyclic molecules, using the density functional theory calculation method. We found that the calculated adsorption energy of O2 on the transition metal macrocyclic complexes was linearly related to the average position (relative to the lowest unoccupied molecular orbital of the macrocyclic complexes) of the non-bonding d orbitals (d_{z^2 }, d_{xy}, d_{xz}, and dyz) which belong to the central transition metal atom. Importantly, our results suggest that varying the energy level of the non-bonding d orbitals through changing the central transition metal atom and/or peripheral ligand groups could be an effective way to tuning their O2 adsorption energy for enhancing the ORR activity of transition metal macrocyclic complex catalysts.

  3. Constant-volume pair potential for Al-transition-metal compounds

    NASA Astrophysics Data System (ADS)

    Zou, J.; Carlsson, A. E.

    1993-02-01

    We treat the problem of two transition-metal atoms embedded in Al by means of a simple s-d model Hamiltonian with localized d orbitals. A Green's-function analysis gives the electronic density of states and total energy as functions of the separation between the two transition-metal atoms. The pair potential thus obtained is strong and has Ruderman-Kittel-Kasuga-Yosida-type oscillations as its asymptotic behavior. It is applicable to cases in which transition metals are not nearest neighbors. With this pair potential, we calculate the structural energy differences of Al3M compounds in the L12 and DO22 structures, where M includes all of the group III, IV, V transition metals and the whole 4d row. Comparison with ab initio results reveals good agreement for the transition metals in group V and beyond, but not for the earlier transition metals, in which the p-d covalent bonding and three-body interactions are likely more important. We also calculate the (100) antiphase boundary (APB) energies for Al3V, Al3Nb, and Al3La, and find a strong correlation between the APB energy and the structural energy difference. The low-order moment-expansion method is used to obtain short-ranged potentials in an effort to obtain better convergence for the structural energies. This approach fails, giving magnitudes for the structural energy differences that are much too small.

  4. Correlation between oxygen adsorption energy and electronic structure of transition metal macrocyclic complexes

    SciTech Connect

    Liu, Kexi; Lei, Yinkai; Wang, Guofeng

    2013-11-28

    Oxygen adsorption energy is directly relevant to the catalytic activity of electrocatalysts for oxygen reduction reaction (ORR). In this study, we established the correlation between the O{sub 2} adsorption energy and the electronic structure of transition metal macrocyclic complexes which exhibit activity for ORR. To this end, we have predicted the molecular and electronic structures of a series of transition metal macrocyclic complexes with planar N{sub 4} chelation, as well as the molecular and electronic structures for the O{sub 2} adsorption on these macrocyclic molecules, using the density functional theory calculation method. We found that the calculated adsorption energy of O{sub 2} on the transition metal macrocyclic complexes was linearly related to the average position (relative to the lowest unoccupied molecular orbital of the macrocyclic complexes) of the non-bonding d orbitals (d{sub z{sup 2}}, d{sub xy}, d{sub xz}, and d{sub yz}) which belong to the central transition metal atom. Importantly, our results suggest that varying the energy level of the non-bonding d orbitals through changing the central transition metal atom and/or peripheral ligand groups could be an effective way to tuning their O{sub 2} adsorption energy for enhancing the ORR activity of transition metal macrocyclic complex catalysts.

  5. NEXAFS investigations of transition metal oxides, nitrides, carbides, sulfides and other interstitial compounds

    NASA Astrophysics Data System (ADS)

    Chen, J. G.

    Owing to their unique physical and chemical properties, transition metal compounds, especially transition metal oxides, nitrides, carbides and sulfides, have been the subject of many surface science investigations. In this article we will review applications of the near-edge X-ray absorption fine structure (NEXAFS) technique in the investigations of electronic and structural properties of transition metal compounds. This review covers NEXAFS studies of compounds in various physical forms, including bulk single crystals, well-characterized overlayers on surfaces of corresponding parent metals, and amorphous powder materials. In addition to transition metal oxides, nitrides, carbides and sulfides, we will also briefly discuss NEXAFS studies of interstitial compounds containing other 2p and 3p non-metal components, namely boron, fluorine, silicon, phosphorus and chlorine. We will discuss the correlation between experimental NEXAFS spectra and the local bonding environment of these compounds, such as the number of d-electrons, spin configurations, ligand-field splitting, coordination numbers, local symmetries, and crystal structures. In addition, NEXAFS investigations of the adsorption and reaction of probing molecules will also be discussed to reveal the underlying chemical reactivities of these materials. We will use many examples to demonstrate the importance of NEXAFS studies in the overall understanding of the physical and chemical properties of transition metal compounds. Finally, we will conclude this review by summarizing the current applications, as well as potential research opportunities, of NEXAFS in several technologically important research areas, including materials science, catalysis, biological science, earth science and environmental science.

  6. Insulator-metal transition of highly compressed carbon disulfide

    SciTech Connect

    Dias, Ranga P.; Yoo, Choong-Shik; Kim, Minseob; Tse, John S.

    2012-04-24

    We present integrated spectral, structural, resistance, and theoretical evidences for simple molecular CS{sub 2} transformations to an insulating black polymer with threefold carbon atoms at 9 GPa, then to a semiconducting polymer above 30 GPa, and finally to a metallic solid above 50 GPa. The metallic phase is a highly disordered three-dimensional network structure with fourfold carbon atoms at the carbon-sulfur distance of {approx}1.70 {angstrom}. Based on first-principles calculations, we present two plausible structures for the metallic phase: {alpha}-chalcopyrite and tridymite, both of which exhibit metallic ground states and disordered diffraction features similar to that measured. We also present the phase and chemical transformation diagram for carbon disulfide, showing a large stability field of the metallic phase to 100 GPa and 800 K.

  7. Automatic quantification of early transition points in biofilm formation

    NASA Astrophysics Data System (ADS)

    Thatcher, Travis; Bienvenu, Samuel; Strain, Shinji; Gordon, Vernita

    2010-10-01

    Biofilms are multicellular, dynamic communities of interacting single-cell organisms, like bacteria. Biofilms are responsible for many infectious diseases as well as for significant damage in industrial settings, yet many aspects of biofilm formation are not well understood. Identifying and quantifying the interactions leading to biofilm formation will not only be important for understanding the basic science of these and other multicellular systems, but it will also be essential for designing targeted strategies to prevent or disrupt biofilms. In particular, it is not clear what physical interactions, and corresponding biological mechanisms, are responsible for the early steps in biofilm formation. Because of this, we are developing high-throughput software techniques to analyze micrograph movies of biofilm formation, from attachment to surfaces through the development of microcolonies. This work will focus on developing software tools to identify and quantify key steps in biofilm formation, first in non-chemotacting systems and later in chemotacting (and autotacting) systems.

  8. Neutron diffraction studies of transition metal hydrides and chemistry of metal hydride clusters

    NASA Astrophysics Data System (ADS)

    Drabnis, Mary Helen

    In this dissertation we start by presenting a survey of single-crystal neutron diffraction results on molecular transition metal hydrides. Powder neutron diffraction results from stoichiometric ternary (solid state) hydrides which contain transition metal coordination complexes in their structures are also reviewed. Tabulations of all known neutron diffraction transition metal-hydrogen distances from both the molecular compounds and the ternary hydrides are summarized in the first chapter. In the second chapter, we describe the main achievement of our doctoral research, the discovery of five-coordinate hydrogen via the single-crystal neutron diffract on analysis of the cluster complex (NMesb4rbracksb3(Rhsb{13}Hsb2(CO)sb{24}rbrack*(Me)CO(Et). This work definitively shows that both hydrides in the (Rhsb{13}Hsb2(CO)sb{24}rbracksp{3-} anion are located in square pyrmaidal cavities. The shortest Rh-H bonds are those involving the central Rh atom, 1.827(2)A and 1.861(2)A, while the surface Rh-H bond length average is 1.975(2)A. A large number of crystallizations of the related trihydride cluster (Rhsb{13}Hsb3(CO)sb{24}rbracksp{2-} anion were also set up, of which two yielded crystals suitable for data collection. The x-ray analysis of the ((Phsb3PCHsb2)sb2Csb6Hsb4rbracksp{2+} salt shows the same Rhsb{13} cage and pattern of bridging CO ligands as all previous (Rhsb{13}Hsbn(CO)sb{24}rbracksp(5-n)- (n = 1,2,3) structures. Ligand exchange reactions with phosphines were tried with the (Rhsb{13}Hsb{3}(CO)sb{24}rbracksp{2-} cluster; (Phsb2PCHsb2)sb2, P(cyclohexyl)Phsb2, and P(Csb6Fsb5)sb3 appeared to add successfully. Syntheses of both (Rhsb{13}Hsb1(CO)sb{24}rbracksp{4-} and (Rhsb{13}Hsb4(CO)sb{24}rbracksp- were attempted but did not yield acceptable products. The (Nisb9Ptsb3H(CO)sb{24}rbracksp{3-} anion was synthesized, but crystallization attempts yielded only powders. Attempts to dissolve and crystallize some ternary hydrides are also briefly mentioned. In the final chapter, the neutron diffraction analysis of (K(18-crown-6)) ((PPhsb3)sb2ReHsb6Cr(CO)sb3rbrack is described. The results of this structure determination revealed three hydrides bridging the Re-Cr bond and three terminal hydrides bound to Re. This compound can be described as a donor-acceptor complex, in which the bridging Re-H bonds of ((PPhsb3)sb2ReHsb6rbracksp- act as donors to the Cr(CO)sb3 fragment. Based on a comparison of the Re-musb2H and Cr-musb2H distances with those previously measured, as well as comparisons to the other known M-H donor-acceptor complexes, ((PPhsb3)sb2ReHsb6Cr(CO)sb3rbracksp- is found to be a weak donor-acceptor complex. The Re-Cr separation in ((PPhsb3)sb2ReHsb6Cr(CO)sb3rbracksp- is shorter than previously-determined values for Re-Cr bonds, suggesting that the Re-Cr interaction is strong.

  9. Manipulation of a Schlenk Line: Preparation of Tetrahydrofuran Complexes of Transition-Metal Chlorides

    ERIC Educational Resources Information Center

    Davis, Craig M.; Curran, Kelly A.

    2007-01-01

    Before taking an inorganic laboratory course few students have experience handling air-sensitive materials using Schlenk techniques. This exercise introduces them to techniques they will employ in later syntheses. The procedure involves the formation of anhydrous tetrahydrofuran complexes of transition-metal chlorides from metal-chloride hydrates;…

  10. Approaches for reducing the insulator-metal transition pressure in hydrogen

    NASA Technical Reports Server (NTRS)

    Carlsson, A. E.; Ashcroft, N. W.

    1983-01-01

    Two possible techniques for reducing the external pressure required to induce the insulator-metal transition in solid hydrogen are described. One uses impurities to lower the energy of the metallic phase relative to that of the insulating phase. The other utilizes a negative pressure induced in the insulating phase by electron-hole pairs, created either with laser irradiation or pulsed synchrotron sources.

  11. Manipulation of a Schlenk Line: Preparation of Tetrahydrofuran Complexes of Transition-Metal Chlorides

    ERIC Educational Resources Information Center

    Davis, Craig M.; Curran, Kelly A.

    2007-01-01

    Before taking an inorganic laboratory course few students have experience handling air-sensitive materials using Schlenk techniques. This exercise introduces them to techniques they will employ in later syntheses. The procedure involves the formation of anhydrous tetrahydrofuran complexes of transition-metal chlorides from metal-chloride hydrates;

  12. Complex transition metal hydrides: linear correlation of countercation electronegativity versus T-D bond lengths.

    PubMed

    Humphries, T D; Sheppard, D A; Buckley, C E

    2015-06-30

    For homoleptic 18-electron complex hydrides, an inverse linear correlation has been established between the T-deuterium bond length (T = Fe, Co, Ni) and the average electronegativity of the metal countercations. This relationship can be further employed towards aiding structural solutions and predicting physical properties of novel complex transition metal hydrides. PMID:26077621

  13. Predicting metal-to-metal charge transfer in closed-shell transition metal oxides doped with Bi 3+ or Pb 2+

    NASA Astrophysics Data System (ADS)

    Boutinaud, Philippe; Cavalli, Enrico

    2011-02-01

    An empirical model is proposed to predict the energy position of the metal-to-metal charge transfer (MMCT) bands in closed shell d 0 transition metal complex oxides doped with Bi 3+ or Pb 2+ ions. The model is constructed on the basis of optical data compiled from the literature and from the investigation of the luminescence properties of a series of compounds (titanates, vanadates, niobates, tantalates, molybdates, and zirconates) prepared and characterized in this work.

  14. Insulator to Correlated Metal Transition in V1−xMoxO2

    SciTech Connect

    Holman, K.; McQueen, T; Williams, A; Klimczuk, T; Stephens, P; Zandbergen, H; Xu, Q; Ronning, F; Cava, R

    2009-01-01

    Although many materials display the transition from insulating to metallic behavior on doping, only a few, such as VO2, have the right combination of crystal structure and physical properties to serve as model systems. Here we report the electronic and structural characteristics of the insulator to metal transition in V1-xMoxO2, which we have studied over the range 0.0=x=0.50 through characterization of the electrical resistivity, magnetic susceptibility, specific heat, and average- and short-range crystal structures. We find that metal-metal pairing exists in small domains in the doped metallic phases and an unexpected phenomenology for the crossover between a Curie-Weiss insulating regime and an intermediate mass metallic regime. An electronic phase diagram is presented.

  15. Transition metal ions at the crossroads of mucosal immunity and microbial pathogenesis

    PubMed Central

    Diaz-Ochoa, Vladimir E.; Jellbauer, Stefan; Klaus, Suzi; Raffatellu, Manuela

    2013-01-01

    Transition metal ions are essential micronutrients for all living organisms. In mammals, these ions are often protein-bound and sequestered within cells, limiting their availability to microbes. Moreover, in response to infection, mammalian hosts further reduce the availability of metal nutrients by activating epithelial cells and recruiting neutrophils, both of which release metal-binding proteins with antimicrobial function. Microorganisms, in turn, have evolved sophisticated systems to overcome these limitations and acquire the metal ions essential for their growth. Here we review some of the mechanisms employed by the host and by pathogenic microorganisms to compete for transition metal ions, with a discussion of how evading “nutritional immunity” benefits pathogens. Furthermore, we provide new insights on the mechanisms of host-microbe competition for metal ions in the mucosa, particularly in the inflamed gut. PMID:24478990

  16. Heterobimetallic coordination polymers involving 3d metal complexes and heavier transition metals cyanometallates

    SciTech Connect

    Peresypkina, Eugenia V.; Samsonenko, Denis G.; Vostrikova, Kira E.

    2015-04-15

    The results of the first steps in the design of coordination polymers based on penta- and heptacyanometallates of heavier d transitions metals are presented. The 2D structure of the coordination polymers: [(Mn(acacen)){sub 2}Ru(NO)(CN){sub 5}]{sub n} and two complexes composed of different cyanorhenates, [Ni(cyclam)]{sub 2}[ReO(OH)(CN){sub 4}](ClO{sub 4}){sub 2}(H{sub 2}O){sub 1.25} and [Cu(cyclam)]{sub 2}[Re(CN){sub 7}](H{sub 2}O){sub 12}, was confirmed by single crystal XRD study, the rhenium oxidation state having been proved by the magnetic measurements. An amorphism of [M(cyclam)]{sub 3}[Re(CN){sub 7}]{sub 2} (M=Ni, Cu) polymers does not allow to define strictly their dimensionality and to model anisotropic magnetic behavior of the compounds. However, with high probability a honey-comb like layer structure could be expected for [M(cyclam)]{sub 3}[Re(CN){sub 7}]{sub 2} complexes, studied in this work, because such an arrangement is the most common among the bimetallic assemblies of hexa- and octacyanometallates with a ratio [M(cyclam)]/[M(CN){sub n}]=3/2. For the first time was prepared and fully characterized a precursor (n-Bu{sub 4}N){sub 2}[Ru(NO)(CN){sub 5}], soluble in organic media. - Graphical abstract: The very first results in the design of 2D coordination polymers based on penta- and heptacyanometallates of 4d and5d transitions metals are presented. - Highlights: • Design of coordination polymers based on penta- and heptacyanometallates. • New Ru and Re cyanide based heterobimetallic coordination complexes. • Hydrolysis and ox/red processes involving [Re(CN){sub 7}]{sup 3+} during crystallization. • High magnetic anisotropy of [M(cyclam)]{sub 3}[Re(CN){sub 7}]{sub 2}(H{sub 2}O){sub n}, M=Cu, Ni, complexes.

  17. A macrocyclic approach to transition metal and uranyl Pacman complexes.

    PubMed

    Love, Jason B

    2009-06-14

    Multielectron redox chemistry involving small molecules such as O2, H2O, N2, CO2, and CH4 is intrinsic to the chemical challenges surrounding sustainable, low-carbon energy generation and exploitation. Compounds with more than one metal reaction site facilitate this chemistry by providing both unique binding environments and combined redox equivalents. However, controlling the aggregation of metal cations is problematic, as both the primary coordination spheres of the metals and the metal-metal separations have to be defined carefully. We described recently a series of pyrrole-based macrocyclic ligands designed to manage metal aggregation and form molecular multimetallic complexes. In particular, we have shown that these compartmentalised Schiff-base calixpyrroles generally form rigid Pacman complexes that prescribe well-defined, metallo microenvironments within the molecular cleft. This article will review the development of this chemistry and its context, and will highlight structural facets and reaction chemistry of metal complexes from across the periodic table. PMID:19587900

  18. Transition Metal Configurations and Limitations of the Orbital Approximation.

    ERIC Educational Resources Information Center

    Scerri, Eric R.

    1989-01-01

    Points out a misconception that is reinforced in many elementary and advanced chemistry texts. Discusses the general limitations of the orbital concept. Notes misconceptions related to the transition elements and their first ionization energies. (MVL)

  19. Adsorption of alkali, alkaline-earth, and 3d transition metal atoms on silicene

    NASA Astrophysics Data System (ADS)

    Sahin, H.; Peeters, F. M.

    2013-02-01

    The adsorption characteristics of alkali, alkaline-earth, and transition metal adatoms on silicene, a graphene-like monolayer structure of silicon are analyzed by means of first-principles calculations. In contrast to graphene, interaction between the metal atoms and the silicene surface is quite strong due to its highly reactive buckled hexagonal structure. In addition to structural properties, we also calculate the electronic band dispersion, net magnetic moment, charge transfer, work function, and dipole moment of the metal adsorbed silicene sheets. Alkali metals, Li, Na, and K, adsorb to hollow sites without any lattice distortion. As a consequence of the significant charge transfer from alkalis to silicene, metalization of silicene takes place. Trends directly related to atomic size, adsorption height, work function, and dipole moment of the silicene/alkali adatom system are also revealed. We found that the adsorption of alkaline-earth metals on silicene is entirely different from their adsorption on graphene. The adsorption of Be, Mg, and Ca turns silicene into a narrow gap semiconductor. Adsorption characteristics of eight transition metals Ti, V, Cr, Mn, Fe, Co, Mo, and W are also investigated. As a result of their partially occupied d orbital, transition metals show diverse structural, electronic, and magnetic properties. Upon the adsorption of transition metals, depending on the adatom type and atomic radius, the system can exhibit metal, half-metal, and semiconducting behavior. For all metal adsorbates, the direction of the charge transfer is from adsorbate to silicene, because of its high surface reactivity. Our results indicate that the reactive crystal structure of silicene provides a rich playground for functionalization at nanoscale.

  20. A Vanadium Dioxide Metamaterial Disengaged from Insulator-to-Metal Transition.

    PubMed

    Jeong, Young-Gyun; Han, Sanghoon; Rhie, Jiyeah; Kyoung, Ji-Soo; Choi, Jae-Wook; Park, Namkyoo; Hong, Seunghun; Kim, Bong-Jun; Kim, Hyun-Tak; Kim, Dai-Sik

    2015-10-14

    We report that vanadium dioxide films patterned with λ/100000 nanogaps exhibit an anomalous transition behavior at millimeter wavelengths. Most of the hybrid structure's switching actions occur well below the insulator to metal transition temperature, starting from 25 °C, so that the hysteresis curves completely separate themselves from their bare film counterparts. It is found that thermally excited intrinsic carriers are responsible for this behavior by introducing enough loss in the context of the radically modified electromagnetic environment in the vicinity of the nanogaps. This phenomenon newly extends the versatility of insulator to metal transition devices to encompass their semiconductor properties. PMID:26352780