Displacement Of Cu Atoms Research Articles

Tuning valley splitting and magnetic anisotropy of multiferroic CuMP2X6 (M=Cr,V;X=S,Se) monolayer

Monolayer $\mathrm{Cu}M{\mathrm{P}}_{2}{X}_{6}$ $(M=\mathrm{Cr},\mathrm{V};X=\mathrm{S},\mathrm{Se})$ has been attracting increasing attention due to the ferromagnetism derived from the indirect exchange interaction between Cr (V) atoms and the ferroelectricity originating from the spontaneous displacement of Cu atoms. Using first-principles calculations, we predicted the intrinsic valley splittings and anomalous valley Hall effect in $\mathrm{Cu}M{\mathrm{P}}_{2}{X}_{6}$ monolayers. In addition, the valley splittings and Berry curvature are independent of the orientation of out of plane (OOP) ferroelectric polarization, which makes it a promising storage device with dual degrees of freedom in ferroelectric polarization and ferrovalleys. Due to the variations in the energy of the spin-dependent valence-band maximum (VBM) and orbital compositions of the valleys, the magnitude of valley splitting at VBM can be effectively tuned by the biaxial strain and external electric field. Large OOP magnetic anisotropy can be obtained by changing the energy and occupation (unoccupation) states of $d$ orbital compositions through strain and charge doping, respectively, and the magnetic anisotropy of $\mathrm{Cu}M{\mathrm{P}}_{2}{X}_{6}$ increases with increasing tensile biaxial strain from \ensuremath{-}5% to 5%. Our research results are expected to tune the valley splittings and magnetic anisotropy of $\mathrm{Cu}M{\mathrm{P}}_{2}{X}_{6}$ and to help understand the physical mechanism.

Electric conductivity of remotely heated Cu nanofilaments in Cu/TaOx/Pt ReRAM cells

A memory cell, initially set to a conductive state, of a resistive resistive random access memory array is heated remotely and controllably by a neighboring heat source cell, both cells having either a common active or an inert electrode. Upon switching off the heating of the heat source cell, the conductivity of the filament of the investigated cell is characterized by sufficiently small voltages to make sure that the filament does not suffer any structural reconstruction such as removal, addition, or permanent displacement of Cu atoms. One observes that immediately after the heating of the neighboring heat source device is switched off, the cell, initially set to an on state and characterized by a low resistance Ron, is found to be reset by remote heating to an off state, characterized by R off ≫ R on. In some instances, after a cooling off time of approximately 2 min or more, one observes the cell's spontaneous recovery from the Roff state to the original on state or very close to the initial Ron value. The process of the spontaneous recovery of the electric conductivity of the filament is interpreted as an attenuation of Cu atom vibrations in the filament with the attendant increase of electron tunneling effects for which the variation of the average tunneling distance between the vibrating Cu atoms is roughly proportional to the square root of the absolute temperature. At high temperatures, the average tunneling distance increases, leading to a sharp decrease of the tunneling probability and, consequently, to a sharp increase in transient resistance.

Platinum Monolayer Electrocatalysts

Explained here is the new approach of designing electrocatalysts with dramatically reduced Pt content while enhancing catalytic performance that entails a nanoparticle of another metal covered by a monolayer of platinum. A monolayer of Cu is formed by underpotential deposition (UPD) on a metal core, followed by galvanic displacement of Cu atoms by Pt ions. Pt monolayer on Pd substrate can be formed by controlled reduction of Pt2+ by ethanol, and by controlled surface-mediated growth by cycling potential between Cu and Pt bulk deposition. Besides the ultimately low Pt content and complete utilization of Pt atoms in the reaction, this method allows tunable activity induced by substrate electronic and geometric structure. The morphology of nanoparticles and their performance were characterized by X-ray absorption spectroscopy, infrared spectroelectrochemistry, and rotating disk electrode, and the principles of operation were summarized.

Shear Softening in a Metallic Glass: First-Principles Local-Stress Analysis.

Metallic glasses deform elastically under stress. However, the atomic-level origin of elastic properties of metallic glasses remain unclear. In this Letter using abinitio molecular dynamics simulations of the Cu_{50}Zr_{50} metallic glass under shear strain, we show that the heterogeneous stress relaxation results in the increased charge transfer from Zr to Cu atoms, enhancing the softening of the shear modulus. Changes in compositional short-range order and atomic position shifts due to the nonaffine deformation are discussed. It is shown that the Zr subsystem exhibits a stiff behavior, whereas the displacements of Cu atoms from their initial positions, induced by the strain, provide the stress drop and softening.

Methanol synthesis revisited: The nature of the active site of Cu in industrial Cu/ZnO/Al2O3 catalyst and Cu-Zn synergy

In this report, we shed an illuminating light on the recent developments on the nature of active site in copper and the microstructural analysis (based on HRTEM/EDX/X-ray techniques) of the Cu/ZnO methanol catalyst. We investigate/analyze two phenomena frequently invoked to explain the key promotional role of Zn in Cu-based methanol catalysts: Formation of a near-surface Cu-Zn alloy by a substitutional displacement of Cu atoms by Zn, especially at the perimeter interface and polarization of electron densities, which can lead additional charge transfer from Zn, from ionic vacancies in the ZnO lattice. With these 2 mechanisms and the ubiquitous role of ZnO as a structural promoter, we explain how these results can be used to design better/more active catalysts from the standpoint of chemical kinetics.

Correlation between thermal-vibration-induced large displacement of Cu atoms and phase transition in Cu4SnS4: First-principles investigation

First-principles (FP) calculations and first-principles molecular dynamics (FPMD) simulations for Cu4SnS4 were performed to clarify the origin of the structural phase transition at 232 K between the high-temperature phase (HP) and low-temperature phase (LP), which leads to an experimentally measured drastic change in the transport properties of Cu4SnS4. The results of the FP and PFMD calculations indicated that, rather than being caused by the so-called freezing of soft modes, the key driving force behind the phase transition in Cu4SnS4 is a large-scale displacement of the Cu atoms located at particular sites due to thermal vibration. In fact, tetrahedrally coordinated CuS4 is stabilized by the effect of the thermal vibration of Cu atoms in the HP whereas CuS3, which is in a trigonal planar environment, is stabilized in the LP.

Atomic-scale simulation study of structural changes of Fe-Cu binary system containing Cu clusters embedded in the Fe matrix during heating

Nano-size Cu precipitates are the main products of irradiation embrittlement of nuclear reactor pressure vessel steels. Molecular dynamics simulation within the framework of embedded atom method is performed to study atomic packing change in Fe-Cu binary system, where the small Cu clusters are embedded in the crystal body centered cubic (BCC) Fe lattices. As the temperature increases, atomic packing change occurs in the Fe-Cu binary system. The mean square displacement of Cu atom, pair distribution function of the Cu atoms, and the atomic density profile along the radial direction are calculated. The atom packing structures in pure Cu region, Fe-Cu interface region, and pure Fe matrix are analyzed. The simulation results show that the packing structures in the Cu cluster and the Fe matrix are greatly affected by the sizes of these clusters and the volume of the Fe matrix containing these clusters. The structural changes present apparent differences, for the Fe matrixes contain these confined Cu clusters with different atom numbers during heating. As the Fe matrix can only provide small space to accommodate the Cu atoms, packing patterns in many Cu atoms are disordered for the Febulk-Cu135 system. In this binary system, strain region in the Fe matrix is adjacent to the Cu cluster. In the meantime, there are a lot of vacancy defects and strain regions in the matrix. For the Febulk-Cu141 system, although the Cu cluster contains more atoms, the Fe matrix can accommodate Cu atoms in a larger space, and the majority of these Cu atoms are located at the BCC crystal lattices. With increasing the temperature, the changes can be observed that the number of the strain regions decrease, whereas the sizes of some strain regions increase.

High-resolution transmission electron microscopy study on reversion of Al2CuMg precipitates in Al–Cu–Mg alloys under irradiation

Image deconvolution analyses showed that reversion of S-Al2CuMg precipitates occurred in an Al–Cu–Mg alloy during high-resolution transmission electron microscopy observations. A fraction of Mg and Cu atoms in the precipitates diffused into Al matrix due to electron beam irradiation at 300kV, resulting in structural/chemical reversion of the precipitates. The structural reversion of the S-Al2CuMg precipitates is closely related with irradiation-induced displacement of atoms. The strong attraction between Cu and Mg atoms might assist the sub-threshold displacement of Cu atoms. One transitional structure is determined to be S′′-Al10Cu3Mg3, a precursor of S-Al2CuMg. Two other transitional structures, Al3CuMg and Al18Cu5Mg5 which have the same lattice parameters of a=c=0.405nm as that of S′′-Al10Cu3Mg3, but different b values, are suggested.

Platinum Monolayer Perfection with the Assistance of Citric Acid in Galvanic Displacement Reaction

The slow kinetics of oxygen reduction reaction (ORR) is one of the main obstacles hindering the commercialization of low-temperature fuel cells.1 Core-shell catalysts consisting of a Pd-based core and a Pt shell (Pt/Pd/C) showed higher Pt mass activity than commercial Pt/C catalysts due to a synergistic effect between the core and Pt shell and substantially complete utilization of Pt atoms.2 The synthesis of a core-shell catalyst involves two main steps: (1) underpotential deposition (UPD) of a Cu monolayer on the surface of Pd nanoparticles; (2) displacement of the Cu atoms with Pt via a simple oxidation-reduction reaction: Cu + Pt2+ → Pt + Cu2+. The challenges associated with production of core-shell catalysts in a large scale are the uniform deposition of a Pt overlayer on the Pd core during the displacement of Cu atoms. Herein, we report our study on the production of core-shell catalysts at the gram scale by introducing citric acid in the Pt-Cu displacement reaction solution. It was found that the quality of the Pt monolayer shell could be significantly improved by adding citric acid.3,4 In the presentation, we will discuss the role of citric acid in smoothing the Pt overlayer based on the experimental and density functional theory calculation results. The characterization and evaluation results of core-shell structures will be shown as well.

Surface relaxation and near-surface atomic displacements in the N/Cu(1 0 0) self-ordered system

The atomic displacements of Cu atoms induced by nitrogen adsorption on Cu(1 0 0) have been studied by channelling–blocking of swift 4 He ions. This study has been performed at two adsorption stages. The first one corresponds to the formation of a dense, two-dimensional, self-ordered array of square-shaped islands covered by nitrogen. The second one corresponds to uniform coverage at saturation. We have determined by nuclear reaction analysis the absolute quantity of nitrogen adsorbed at these two stages. The values obtained, when confronted to previous observations of these stages by low energy electron diffraction and by scanning tunnelling microscopy, demonstrate that nitrogen remains mostly at the sample surface and that the N concentration in bulk Cu could not exceed 1%. However, channelling measurements show that this surface adsorption generates atomic displacements of Cu atoms down to depths of a few ten (1 0 0) interplanar distances. In the mean time, blocking measurements reveal that nitrogen adsorption induces a strong surface expansion: the interplanar distance between the first two (1 0 0) planes increases of about 0.2 Å, in contrast with the weak contraction observed on bare Cu(1 0 0) surfaces. This observation supports the hypothesis that, when nitrogen is adsorbed, the surface is submitted to stress variations, from tensile to compressive stress for, respectively, bare and nitrogen-covered surface regions. The surface forces corresponding to such variations have been introduced in molecular dynamics simulations. For coverage leading to self-ordering, these simulations do indeed predict displacements of subsurface Cu atoms. The adjustment of these displacements to those measured by channelling gives the amplitude of the stress variation.

Synthesis and structure determination of [Cu(α-L)(N 3)]NO 3 and [Cu(α-L)(NCS) 2] complexes with α-L= trans-5( R),7( R),12( S),14( S)-tetramethyl-6,13-dinitro-1,4,8,11-tetraazacyclotetradecane

The synthesis, structure, IR and UV–Vis spectra of [Cu(α-L)(N 3)]NO 3 ( 1), and [Cu(α-L)(NCS) 2] ( 2) complexes are reported, where α-L= trans-5( R),7( R),12( S),14( S)-tetramethyl-6,13-dinitro-1,4,8,11-tetraazacyclotetradecane. The complex 1 crystallizes in monoclinic space group P2 1/ c (no.14), with a=18.1269(1), b=9.6761(1), c=13.0378(1) Å, β=102.84(1)°, Z=4. A distorted square pyramidal (SP) geometry is found for the complex 1 and the apical position is occupied by the N 3 − ligand. The basal displacement of Cu atom in 1 is 0.16 Å. The crystals of [Cu(α-L)(NCS) 2] ( 2) are monoclinic, P2 1/ n (no.14), with a=11.3912(1), b=8.8444(1), c=12.0482(1) Å, β=110.17(1)°, Z=2. The coordination environment for complex 2 is Cu(N 4)(N 2) having a tetragonally distorted octahedral geometry in which the axial positions are occupied by two (N-bonded) NCS − ligands (Cu–N ax=2.5051(15) Å).

Bond-valence approach to the coppercopper and coppernitrogen bonding in binuclear copper(II) complexes: Structure of tetrakis(2-iodobenzoato)bis(caffeine)dicopper(II) at 210 K

The crystal structure of tetrakis(2-iodobenzoato)bis(caffeine)dicopper(II) at 210 K is monoclinic, space group P2 1/ n (No.14), unit cell parameters a=13.6090(4), b=13.5900(2), c=14.5370(4) Å, β=116.394(1)°, Z=2 and D calc=2.07 g cm −3. The structure consists of centrosymmetric [Cu 2(2IbzO) 4(caf) 2] units (2IbzO=2-iodobenzoate, caf=caffeine). These binuclear units are mutually held by van der Waals forces. Cu atoms are pentacoordinated by four carboxylic O atoms in the basal plane and one apical N atom from the caffeine molecule. CuO bond lengths are 1.961(3), 1.969(3), 1.969(2), 1.974(3) Å, and CuN bond length is 2.238(3) Å. The displacement of Cu atom from the basal plane is 0.2354(4) Å. The calculated Cu(II)N bond length by bond-valence sum model is 2.31 Å. The Cu…Cu distance is 2.7041(6) Å. The non-bonding character of Cu(II)…Cu(II) contact in 26 binuclear carboxylic complexes with CuO 4N chromophores is proved.

Bond-valence approach to the copper-copper and copper-oxygen bonding in binuclear copper(II) complexes: Structure of tetrakis(2-fluoro-benzoato-O,O´)-bis(2-fluorobenzoate-O) dicopper(II)

The crystal and molecular structure of tetrakis(2-fluorobenzoato-O,O´)-bis(2-fluorobenzoate-O) dicopper(II) - C42H26Cu2F60O12 - has been determined at room temperature. The blue-green crystals are monoclinic, space group P21/n (No.14), with unit cell parameters a=10.633(3) Å, b= 12.528(3) Å, c=14.729(3) Å, β = 91.62(4)°, Z= 2, Dm= 1.62 and Dx= 1.64 g/cm3. The final R value for independent reflections is equal to 0.045. The structure consists of two centrosymmetric units [Cu2(2FbzO)4(2HFbzO)2] (abbreviation 2FbzO stands for 2-fluorobenzoate, abbr. 2HFbzO for 2-fluorobezoic acid). These units are mutually held by Van der Waals forces. Within the binuclear unit the Cu atom is pentacoordinated by four carboxylic O atoms in the basal plane of tetragonal pyramid (Cu–O bond lengths being 1.962(3) Å, 1.963(2) Å, 1.958(2) Å and 1.996(2) Å and one apical O atom from HFbzO molecule, with Cu-O bond length 2.193(2) Å. The apical Cu(II)–O bond length predicted by bond valence method is 2.16 Å. The Cu...Cu distance is 2.6086(8) Å and the displacement of Cu atom from the basal plane is 0.1905(6) Å. Nonbonding character of Cu(II)...Cu(II) contact in 16 binuclear carboxylic complexes is proved.

Main- and superstructure by X-ray scattering for annealed single crystal of YBa 2Cu 3O 6.5

A single crystal of YBa 2Cu 3O 6.5 was annealed at 430, 510, 570 and 590 K in the furnace of a four-circle X-ray diffractometer, cooled down slowly to room temperature and investigated structurally. From the dataset of about 200 reflections with 40 superstructure reflections, the main- and superstructure models have been derived. Displacements of Cu atoms, changes of oxygen populations in the basal plane and the small displacements of Y, Ba and apex-oxygens were observed. The changes of atom positions have been correlated with the superconducting transition temperature T c. With increasing annealing temperature, the phase separation, a characteristic diffuse scattering in the 00 l-direction, changes of oxygen population in the basal plane and after annealing at 430 K, the disappearance of twinned reflections in the a*– b* plane have been seen.

Is the central executive intact in schizophrenia in the context of impaired working memory?

The atomic displacements of Cu atoms induced by nitrogen adsorption on Cu(1 0 0) have been studied by channelling–blocking of swift 4He ions. This study has been performed at two adsorption stages. The first one corresponds to the formation of a dense, two-dimensional, self-ordered array of square-shaped islands covered by nitrogen. The second one corresponds to uniform coverage at saturation. We have determined by nuclear reaction analysis the absolute quantity of nitrogen adsorbed at these two stages. The values obtained, when confronted to previous observations of these stages by low energy electron diffraction and by scanning tunnelling microscopy, demonstrate that nitrogen remains mostly at the sample surface and that the N concentration in bulk Cu could not exceed 1%. However, channelling measurements show that this surface adsorption generates atomic displacements of Cu atoms down to depths of a few ten (1 0 0) interplanar distances. In the mean time, blocking measurements reveal that nitrogen adsorption induces a strong surface expansion: the interplanar distance between the first two (1 0 0) planes increases of about 0.2 Å, in contrast with the weak contraction observed on bare Cu(1 0 0) surfaces. This observation supports the hypothesis that, when nitrogen is adsorbed, the surface is submitted to stress variations, from tensile to compressive stress for, respectively, bare and nitrogen-covered surface regions. The surface forces corresponding to such variations have been introduced in molecular dynamics simulations. For coverage leading to self-ordering, these simulations do indeed predict displacements of subsurface Cu atoms. The adjustment of these displacements to those measured by channelling gives the amplitude of the stress variation.

A channeling study of lead submonolayers deposited onto Cu(100)

We have studied the location of Pb atoms on Cu(100) at submonolayer coverages using a few hundred keV He + ion beams in channeling experiments under single- and double-alignment geometry. Different shadowing effects of Pb on Cu are experimentally observed according to the structure of the deposit, which was evidenced by LEED (in particular the c(5√2 × √2)R45° and c(2 × 2) structures). These effects disappear around the melting temperature of the structures. For a Pb coverage of θ ≈ 0.5, when a c(2 × 2) structure is formed, the distance d Pb between the Pb plane and the first (100) Cu plane, measured under double alignment geometry (channeling along [100], blocking along [110] 45° off the normal to the surface), is d Pb = 2.05 ± 0.05 A ̊ . We also find that the Pb atoms induce a static disorder in the first Cu surface plane. This disorder may be described by isotropic Gaussian displacements of Cu atoms, with a one-dimensional projected RMS deviation u d of around 0.12 Å. It depends little on the Pb structure: in particular, there is no specific disordering in the alloyed state corresponding to low Pb coverages. From this observation, we deduce that for low coverage, the Pb atoms lie about 1 Å above the first Cu plane.

Surface reconstruction of Cu (111) upon oxygen adsorption

Low-energy He + ion scattering (IS) in combination with AES, LEED and work function measurements has been applied for the determination of surface reconstruction of Cu(111) upon oxygen adsorption. IS data clearly indicate that oxygen is not significant incorporated into the bulk at room temperature adsorption, however the surface shows reconstruction by displacement of Cu atoms by 0.3 Å. The disappearance of structure of both Cu IS and O IS in the “♥ in pattern” demonstrate the development of a disordered layer of reconstruction centres. At saturation coverage, a rough and dis-ordered oxygen-copper surface layer is present.

Neutron diffraction study of partial radial densities in γ-CuCl, including an appendix on the effect of instrumental resolution on radial density analysis

Neutron diffraction has been used to study the structural disorder in γ-CuCl at 25 and 366°C. Powder patterns of 63CuNCl, NCuNCl and 65CuNCl (N= natural abundance) were treated by integrated intensity (II) and radial density (RD) analyses. Using II, a satisfactory fit to these 366°C data was made with the anharmonic model described by Sakata, Hoshino & Harada [Acta Cryst. (1974). A30, 655-661]. This fit resulted in an anharmonic parameter βCu = 1.2 ± 0.6 × 10-12 erg Å-3, which is in excellent agreement with the value of βCu = 1.15 ± 0.66 × 10-12 erg Å-3 obtained from a single-crystal study at room temperature by Sakata et al. (1974). To eliminate the indistinguishability of the anharmonic and statistically disordered models inherent in II, an energy analysis was made of the diffuse scattering. It was found that at 25 °C, 50 to 70% of the diffuse scattering is elastic. The non-monotonic κ dependence of this elastic diffuse scattering implies a model of correlated static displacements of Cu atoms and is inconsistent with the anharmonic model. Radial density analyses of the three spectra were fitted to a model based on harmonic vibration to obtain partial radial densities qCuCu, qClCl and qCuCl,. Although theqClCl, was typical of that expected for a solid structure, qCuCu,, at 366°C was found to be more 'liquid-like'. Quantitative conclusions from this radial density analysis are limited by the strong dependence of the results on instrumental resolution. An estimate of the effect of instrumental resolution on the measured radial density function is contained in Appendix B.