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A series of long-lived excitons in a monolayer of tungsten disulphide are found to have strong binding energy and an energy dependence on orbital momentum that significantly deviates from conventional, three-dimensional, behaviour. Excitonic processes in monolayer WS.sub.2 The emergence of graphene optoelectronics has stimulated the development of near-transparent two-dimensional semiconductor materials. Much attention is focusing on the potentially extremely versatile transition metal dichalcogenides, such as molybdenum disulphide and tungsten disulphide, as components for ultrathin electronic devices. The physical origins of the unusually strong light-matter interactions in these materials remain unclear. An active topic in this area is how excitons (electron-hole pairs generated by light) behave in these low-dimensional systems. Here Xiang Zhang and colleagues report the discovery of a series of two-dimensional excitonic dark states in monolayer tungsten disulphide that have strong binding energy and an energy dependence on orbital momentum that significantly deviates from conventional (3D) behaviour. The findings open new avenues for fundamental research and opportunities to design devices such as photodetectors and photovoltaic cells. Transition metal dichalcogenide (TMDC) monolayers have recently emerged as an important class of two-dimensional semiconductors with potential for electronic and optoelectronic devices.sup.1,2. Unlike semi-metallic graphene, layered TMDCs have a sizeable bandgap.sup.3. More interestingly, when thinned down to a monolayer, TMDCs transform from indirect-bandgap to direct-bandgap semiconductors.sup.4,5, exhibiting a number of intriguing optical phenomena such as valley-selective circular dichroism.sup.6,7,8, doping-dependent charged excitons.sup.9,10 and strong photocurrent responses.sup.11. However, the fundamental mechanism underlying such a strong light-matter interaction is still under intensive investigation. First-principles calculations have predicted a quasiparticle bandgap much larger than the measured optical gap, and an optical response dominated by excitonic effects.sup.12,13,14. In particular, a recent study based on a GW plus Bethe-Salpeter equation (GW-BSE) approach, which employed many-body Green's-function methodology to address electron-electron and electron-hole interactions, theoretically predicted a diversity of strongly bound excitons.sup.14. Here we report experimental evidence of a series of excitonic dark states in single-layer WS.sub.2 using two-photon excitation spectroscopy. In combination with GW-BSE theory, we prove that the excitons are of Wannier type, meaning that each exciton wavefunction extends over multiple unit cells, but with extraordinarily large binding energy (~0.7 electronvolts), leading to a quasiparticle bandgap of 2.7 electronvolts. These strongly bound exciton states are observed to be stable even at room temperature. We reveal an exciton series that deviates substantially from hydrogen models, with a novel energy dependence on the orbital angular momentum. These excitonic energy levels are experimentally found to be robust against environmental perturbations. The discovery of excitonic dark states and exceptionally large binding energy not only sheds light on the importance of many-electron effects in this two-dimensional gapped system, but also holds potential for the device application of TMDC monolayers and their heterostructures.sup.15 in computing, communication and bio-sensing., Author(s): Ziliang Ye [sup.1] , Ting Cao [sup.2] [sup.3] , Kevin O'Brien [sup.1] , Hanyu Zhu [sup.1] , Xiaobo Yin [sup.1] , Yuan Wang [sup.1] , Steven G. Louie [sup.2] [...]

Diffuse reflection spectroscopy is used to study color reactions of tungsten and molybdenum with pyrocatechol violet (PV) and phenylfluorone (PF) on a solid phase of polyacrylonitrile fiber impregnated with AN-31 (PANV-AN-31) or AV-17 (PANV-AV-17) anion exchangers. The relations of the analytical signals of complexes to sorption conditions in the batch mode, such as the acidity of solutions, presence of masking substances, and metal concentration, were found. A system was selected for the sorption-spectroscopy determination of 0.02-0.12 µg/mL of molybdenum in the presence of a fivefold excess of tungsten by sorption on PANV-AV-17 from a boric acid solution with pH 5 and reaction with PF. The sorption on PANV-AN-321 from boric acid solutions with pH 3 made possible the determination of the total concentration (0.8-4.2) x [10.sup.-6] M of molybdenum and tungsten by the reaction with PV. Keywords: diffuse reflection spectroscopy, molybdenum, tungsten, pyrocatechol violet, phenylfluorone, fiber impregnated with AV-17 and AN-31 anion exchangers DOI: 10.1134/S1061934813120101, Molybdenum and tungsten are relatively rare chemical elements that acquired great importance in contemporary technology. Both metals are strategically important and necessary for fulfilling the needs of state defense, in [...]

The chemical similarity between molybdenum and tungsten makes the direct spectrophotometric determination of these metals impossible. Usually the determination is preceded by a separation step. In order to find out a selective and quantitative isolation method, coprecipitation with thioacetamide and Cu(II) as a carrier; Mn[O.sub.2]; cupferron, tannin and crystal violet; quinolin-8-ol, tannin and thioacetamide, were examined. Molybdenum(VI) could be determined in the presence of 100-fold mass excess of tungsten after precipitation with thioacetamide and Cu(II). The remaining methods could only be applied if mass excess of W is equal to or lower with respect to Mo. For the resolution of this problem, the derivative spectrophotometry was used. The studies of different order spectra of gallein complexes of molybdenum, tungsten and their mixtures have shown that the fifth-derivative spectra allows one to eliminate the interfering effects of W on the determination of Mo. At 650 nm the spectral features of tungsten is zeroing while the value of the fifth derivative spectrum of mixture of Mo and W corresponds only to the concentration of molybdenum in the examined solution. Beer's law is obeyed in the range 0.32-0.80 µg/mL of Mo. The developed derivative spectrophotometric method and the most selective pre-separation method, based on the precipitation of molybdenum(VI) sulphide, were applied to the determination of Mo in Armco iron and steel. The accuracy of the elaborated methods was confirmed by comparison of the determined content of Mo with certified values as well as with the result obtained by the reference ICP-OES technique. Keywords: molybdenum and tungsten, separation methods, spectrophotometry, derivative spectrophotometry, molybdenum in steel, gallein DOI: 10.1134/S1061934813010115, Molybdenum and tungsten occur in nature in the form of their ores, the most important once are molybdenite and wulfenite, wolframite and scheelite, respectively. Metals are used chiefly in alloying [...]

Breaking the chain The transformation of petroleum-derived feedstocks into useful chemicals often requires controllable cleavage of carbon-hydrogen or carbon-carbon bonds. There are many examples achieving this through the oxidative addition of C-H bonds to metal centres, but analogous transformations of C-C bonds are rare. Aaron Sattler and Gerard Parkin show that by using a tungsten centre and exploiting the formation of an unusual ligand, a strong C-C bond can be cleaved. This raises the prospect that many other metal centres with suitable ligands can be developed for the same task, providing new ways of turning feedstocks into valuable chemicals. The transformation of petroleum-derived feedstocks into useful chemicals often requires controllable cleavage of C-H or C-C bonds. There are many examples of achieving this through the oxidative addition of C-H bonds to metal centres, but analogous transformations of C-C bonds are rare. Here, using a tungsten centre and exploiting the formation of an unusual chelating ligand, a strong C-C bond is cleaved; other metal centres with suitable ancillary ligands could perform the same function. The cleavage of C-H and C-C bonds by transition metal centres is of fundamental interest and plays an important role in the synthesis of complex organic molecules from petroleum feedstocks.sup.1,2,3,4,5,6. But while there are many examples for the oxidative addition of C-H bonds to a metal centre, transformations that feature oxidative addition of C-C bonds are rare. The paucity of transformations that involve the cleavage of C-C rather than C-H bonds is usually attributed to kinetic factors arising from the greater steric hindrance and the directional nature of the sp .sup.n hybrids that form the C-C bond, and to thermodynamic factors arising from the fact that M-C bonds are weaker than M-H bonds.sup.2,3,4,5. Not surprisingly, therefore, most examples of C-C bond cleavage either avoid the kinetic limitations by using metal compounds in which the C-C bond is held in close proximity to the metal centre, or avoid the thermodynamic limitations by using organic substrates in which the cleavage is accompanied by either a relief of strain energy or the formation of an aromatic system.sup.2,3,4,5. Here, we show that a tungsten centre can be used to cleave a strong C-C bond that is a component of an unstrained 6-membered aromatic ring. The cleavage is enabled by the formation of an unusual chelating di(isocyanide) ligand, which suggests that other metal centres with suitable ancillary ligands could also accomplish the cleavage of strong C-C bonds of aromatic substrates and thereby provide new ways of functionalizing such molecules., Author(s): Aaron Sattler [sup.1] , Gerard Parkin [sup.1] Author Affiliations: (1) Department of Chemistry, Columbia University, New York, New York 10027, USA, Main We are currently developing the coordination chemistry [...]

Copper-tungsten composite materials are developed for applications such as electrical contacts, resistance electrodes, and contact tips in welding guns as well as for components requiring higher wear resistance. In addition to the aspect of improved performance, it is scientifically interesting to assess the tribological properties, and therefore the objectives of the present work include, to determine the role of W additions in improving the fretting wear resistance of Cu for electrical applications, to determine the optimum concentration for W additions, and to identify the mechanisms responsible for fretting wear improvements. In addressing these issues, a planned set of fretting wear tests were conducted on powder metallurgically processed Cu-W composites (maximum W content of 20 wt %) against steel counterbody under varying load (up to 10 N) for 10,000 cycles. It has been observed that at lower loads of 2 N, the coefficient of friction (COF) recorded was ~0.9 for the Cu-20 wt % W/steel, whereas it was ~0.85 for a pure Cu/steel couple. Under similar operating conditions with the increase in load, the COF decreases to 0.5 at 10 N load, irrespective of the composition of the Cu-W composite. Furthermore, the incorporation of 5 wt % W has reduced the volumetric wear loss by 4-6 folds in comparison to unreinforced Cu. The addition of even higher percentage of W has led to increase its wear resistance by ~10 folds. Under the investigated conditions, the wear rate systematically decreases with the increase in load for all the tested Cu-W composites. Based on the topographical observation of worn surfaces, it is observed that wear mechanisms for the Cu and Cu-W composites are tribochemical wear, adhesive wear, and abrasive wear. The incorporation of harder W particles (5 wt % or more) help in abrading the steel ball and in forming a dense tribolayer of [Fe.sub.x][O.sub.y], which effectively reduces wear rate and hence, increases wear resistance of the Cu-W composite surface in reference to unreinforced Cu. [DOI: 10.1115/1.3204776] Keywords: Cu, W, repeated pressing and sintering, fretting, wear, tribolayers, powder metallurgy

Results of a solar thermophotovoltaic (STPV) system study are reported. Modeling of the STPV module performance and the analysis of various parameters influencing the system are presented. The ways for the STPV system efficiency to increase and their magnitude are considered such as: improvement of the emitter radiation selectivity and application of selective filters for better matching the emitter radiation spectrum and cell photoresponse; application of the cells with a back side reflector for recycling the sub-band gap photons; and development of low-band gap tandem TPV cells for better utilization of the radiation spectrum. Sunlight concentrator and STPV modules were designed, fabricated, and tested under indoor and outdoor conditions. A cost-effective sunlight concentrator with Fresnel lens was developed as a primary concentrator and a secondary quartz meniscus lens ensured the high concentration ratio of ~4000X, which is necessary for achieving the high efficiency of the concentrator--emitter system owing to trap escaping radiation. Several types of STPV modules have been developed and tested under concentrated sunlight. Photocurrent density of 4.5 A/c[m.sup.2] was registered in a photoreceiver based on 1 X 1 c[m.sup.2] GaSb cells under a solar powered tungsten emitter [DOI: 10.1115/1.2734576]

An investigation of the parameters governing both the binding of small molecules to different tungsten complexes and agostic bonding in the W[(CO).sub.3][(P[Cy.sub.3]).sub.2] complex applying ab initio MP2 calculations with and without basis set superposition error (BSSE) correction is described. It is suggested that BSSE correction are a must for quantitative predictions for bimolecular reactions.

A single-crystal X-ray diffraction, IR and elemental analysis are used to synthesize and characterize four new lanthanide-substituted polyoxotung-states. It is indicated that the [P.sub.8][W.sub.48] anion are occupied by lacunary polytungstate cations.

The mechanism of enhanced adhesion between hydrogenated amorphous carbon films and tungsten preimplanted steel substrates was systematically investigated. The results showed that preimplantation created a graded WC interface that diminishes diffusion of the dissociated carbon atoms on the surface and the films deposited on tungsten implanted steel substrates show improved adhesion strength.

The synthesis and properties of metal--metal (MM) quadruply bonded compounds of molybdenum and tungsten in liquid crystalline phases are described. Covalently linked MM quadruply bonded complexes, dimers of dimers, are shown to give rise to mixed valence complexes upon oxidation and the degree of electron delocalization is shown to be correlated with the [M.sub.2][delta] bridge [pi] interactions. Class III behavior or complete delocalization is observed to [M.sub.2] to [M.sub.2] distances of up to 14 [Angstrom]. When the [M.sub.2] unit is attached to oligothiophenes via a carboxylate tether, the photophysical properties of the thienyl units are greatly modified due to [M.sub.2][delta]-thienyl [pi] conjugation and spin--orbit coupling. This leads to long-lived emissive states and electroluminescence in solid-state devices. mesogens | mixed valency

The sunlight-induced photosynthesis and characterization of (dihapto-[60]fullerene) pentacarbonyl tungsten(0) [eta.sup.2]-C60)5 is proposed as a laboratory experience that promotes student understanding of kinetics, mechanisms, temperature dependence of rate constant values, and graphical interpretations of chemical behavior. In this experience, students learn to estimate the W-C60 bond enthalpy or the W-C60 bond energy form the experimentally determined enthalpy of activation.

The well-known photoreduction of polyoxometalates by alcohols are exploited to produce the reduced species, which cannot be directly produced electrochemically in nonaqueous solvents. Photochemistry is a reliable means to produce stable reduced transition metal substituted heteropoly tungstates in nonpolar media, where they hold promise as multielectron reduction catalysts.

The interaction of acetic acid (AA, CH3COOH), with solid water, deposited on metals, tungsten and gold, at 80 K, is investigated. Acid-water interfaces at 80K, namely acid layers on thin films of solid water and H2O adilayers on thin acid films are prepared and annealed between 80 and 200 K.

The construction of heteroaromatic compounds by cyclization of o-aklynylphenyl derivatives that is a highly useful method for the preparation of compounds like indoles is discussed. Iodine-promoted cyclization of o-alkynylphenyl carbonyl derivatives was reported to give 1H-isohromenes.

The reactions of W(CO)(sub 6) with formamidines contrast with those of Mo(CO)(sub 6) and Cr(CO)(sub 6) in that the former do not yield quadruply bonded dimetal species. The DFT calculations are obtained for W2(mu-CO)(sub 2) complex.

Experimental and computational studies on cationic molybdenocene trihydride complexes, namely [cp2MoH3]+, [(Cp(Super But)2MoH3]+, [Cp*2MoH3]+, and {[Me2Si(C5Me4)2]MoH3}+, demonstrate that the most stable form for the ansa molybdenocene derivative is a nonclassical dihydrogen-hydride isomer, {[Me2Si(C5Me4)2]Mo(n2-H2)(H)}+, whereas the stable forms for the non-ansa complexes are classical triydrides, [Cp2Mo(H)3]+, [(Cp(Super But)2Mo(H)3]+, and [Cp*2MoH3].

Tris(1,2ethanediolato)tungsten(VI), [W(eg)3] reacts with methylamino-N,N-bis(2-methylene-4,6-di-methylphenol) and methylamino-N,N-bis(2-methylene-4-methyl-6-tert-butylphenol) to form oxotungsten complexes. The prepared oxotungsten(VI) complexes can be activated by Et2AlCl to catalyze ring-opening metathesis polymerization of norbornene.

Vanadium environments in Keggin oxopolytungstates were characterized by (super 51)V solid-state MAS NMR spectroscopy. These (super 51)V fine structure constants in the solid state are determined by the number of vanadium atoms present in the oxoanion core.

A thermal desorption and vibrational spectra which monitors the decomposition of ethylene on clean and carbide-modified W(111) and W(110) is presented. The results show that clean W(111) is twice as active as W(110) towards the decomposition of ethylene.

The interaction of CsCl with solid water, deposited on tungsten at 130 K, is investigated using metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS). Above the critical stoichiometry, besides water, Cl and Cs species are found in the top layer, indicating that the migration of CsCl species are largely suppressed.

The deprotonation of substituted (methylthiophenylcarbene)pentacarbonyl tungsten(0) ((CO)5W=C(SC6H4Z)CH3) and (benzoxymethylcarbene)pentacarbonyl tungsten(0) ((CO)5W=C(OCH2C6H4Z)CH3) in aqueous acetonitrile is studied. This indicates a transition state imbalance in which the loss of the carbene complex lags behind the proton transfer.

Wire desorbed radicals present during hot wire chemical vapor deposition growth are measured by quadrupole mass spectrometry. Calculation of the relative rates for the various wire kinetic processes, coupled with experimental observations, reveals that silicon diffusion through the silicide is the slowest process, followed by Si evaporation, with SiH(sub 4) decomposition being the fastest.

The decomposition and dehydrogenation of cyclohexene (c-C6H10) are used s probe reaction to compare the surface reactivities of clean and carbide-modified W(111). It is concluded that the desorption of the benzene product is reaction-limited on C/W(111), but it is desorption-limited on Pt(111).

The reaction of dehydrated H-ZSM5 with WCl(sub 6) vapor at 673K were prepared with exchanged WO(sub x)/H-ZSM5, samples. By hydrolysis in H2O-containg streams, exchanged WCl(sub x) precursors were converted to WO(sub x) species.

Rh, Pt, and Pd are formed to cause a reconstruction of the W(211) surface similar to oxygen. When covered with a thin film of Pt, Pd or Rh on W(211) surface, an nx1 type of superstructure is formed upon annealing.

The chemical vapor deposition (CVD) of tungsten nitride from bis(tertbutylimido)bis(tertbutylamido)tungsten is examined. The examination is done by focusing on the mechanisms and energetics involved in the activation and thermal decomposition of the CVD precursor.

A combination of [super 1]H and [super 183]W NMR evidence, elemental analysis, acid titration measurements, and H/D isotopomer assignments has established that in nonaqueous media the internal cryptand cavity of [alpha]-[[([H.sub.2])[W.sub.12][O.sub.40]].sup.6-] reversibly accommodates only one more proton to form [alpha]-[Q.sub.5]-[([H.sub.3])[W.sub.12][O.sub.40]]. The results have shown that the transformation is instantaneous by [super 1]H NMR, whereas the reverse process is slow.

Abstract -- The complexation of molybdenum(VI) with Bromopyrogallol Red in the presence of the nonionic surfactant Triton X-114 is studied spectrophotometrically. A mixed-ligand complex is formed with the component ratio [...]

The electronic characteristics and redox properties of isomer PW12O(sub 40)(super 3-) depends on the arrangement adopted by the metal-oxide framework. At the DFT/BP86 level, the structures of the five isomers of PW12O(sub 40)(super 3-) were computed in oxidized form.

The N-acetylpyridinium complex of {TpW(NO)(P[Me.sub.3])} has undergone regio- and stereoselective reactions with a broad range of common organic nucleophiles, which as provided a family of 1,2-dihydropyridine (DHP) complexes of the form TpW(NO)(P[Me.sub.3])(3,4-[[eta].sup.2]DHP). The subsequent oxidative decomplexation of the materials has produced many piperidinamides with unconventional substitution patterns.

A study was conducted to establish that the enzyme purified from cells grown with 4.5 [mu]M [WO.sub.4.sup.2-] which contains W as the metal cofactor is otherwise identical to the Mo-Nar purified from Pyrobaculum aerophilum grown at low [WO.sub.4.sup.2-] concentrations. The findings for the active W-containing Nar demonstrated the unique ability of hyperthermophiles to adapt to their high-[WO.sub.4.sup.2-] environment.

Tris(hydroxymethyl)aminomethane (Tris) is grafted onto the surface of a [Ni.sub.6]-substituted polyoxotungstate formed in situ to generate a three-connected polyoxometalate building block. The cooperative assembly of Tris functionalized three-connected building blocks and rigid 1,3,5-benzenetricarboxylate has provided an unprecedented cubic polyoxometalate-organic molecular cage with high thermal and hydrothermal stability.

The structure-property relationships of zirconia-supported tungsten oxide (W[O.sub.x]/Zr[O.sub.2]), considered as an important supported metal oxide model acid catalyst is explored for numerous acid-catalyzed reactions. Analysis of tungsten oxide for n-pentane isomerization activity and selectivity as a function of tungsten surface density, catalyst support type, and calcination temperature revealed (W[O.sub.x]/Zr[O.sub.2]) to exhibit isomerization activity that peaked within the first 2 h time-on-stream, which might be due to Zr-W[O.sub.x] clusters undergoing an activation process.

The mechanisms involved in the self-assembly of the nanosized polyoxometalate anion, [[[H.sub.34][W.sub.119][Se.sub.8][Fe.sub.2][O.sub.420]].sup.54- ] are discussed. The generation of the compound from the pentagonal building blocks is also analyzed.

The unique CO-tolerant property of Pt nanoparticles supported on [Ti.sub.0.7][W.sub.0.3][O.sub.2], which is prepared by a sol-gel process and platinized using an impregnation-reduction technique, is described. The studies have shown that Pt/[Ti.sub.0.7][W.sub.0.3][O.sub.2] is a promising new anode catalyst for proton-exchange membrane fuel cell (PEMFC) applications.

The tungsten oxide covered tungsten (W) tip of a scanning tunneling microscope which has the ability to act as a catalyst to catalyze the S-H dissociative adsorption of phenylthiol and 1-octanethiol molecules onto a Ge(100) surface is reported. The local tip-catalyzed adsorption might be used in positive lithographic methods to produce nanoscale patterning on semiconductor substrates.

Nanoparticulate zirconium tungstate prepared through hydrothermal methods is autohydrated under ambient conditions. The studies have shown that the cause of autohydration is a result of structural defects present in the nanoparticles, while kinetics are governed by surface area, indicating that processing methods are used for minimizing or overcoming the problem.

The article discusses a new [beta]-elimination reaction that involves the zinc(II) and LiCl compounds. The reaction is shown to be extremely beneficial in the conversion of a zinc disilazide to a zinc hydride.