Copper Neighbours Research Articles

An important atomic process in the CVD growth of graphene: Sinking and up-floating of carbon atom on copper surface

By density functional theory (DFT) calculations, the early stages of the growth of graphene on copper (111) surface are investigated. At the very first time of graphene growth, the carbon atom sinks into subsurface. As more carbon atoms are adsorbed nearby the site, the sunken carbon atom will spontaneously form a dimer with one of the newly adsorbed carbon atoms, and the formed dimer will up-float on the top of the surface. We emphasize the role of the co-operative relaxation of the co-adsorbed carbon atoms in facilitating the sinking and up-floating of carbon atoms. In detail: when two carbon atoms are co-adsorbed, their co-operative relaxation will result in different carbon–copper interactions for the co-adsorbed carbon atoms. This difference facilitates the sinking of a single carbon atom into the subsurface. As a third carbon atom is co-adsorbed nearby, it draws the sunken carbon atom on top of the surface, forming a dimer. Co-operative relaxations of the surface involving all adsorbed carbon atoms and their copper neighbors facilitate these sinking and up-floating processes. This investigation is helpful for the deeper understanding of graphene synthesis and the choosing of optimal carbon sources or process.

Single crystal EPR study of electronic structure and exchange interactions for copper(II)( l-arginine) 2(SO 4) · (H 2O) 6: a model system to study exchange interactions between unpaired spins in proteins

We report EPR measurements at 9.77 and 34.1 GHz in powder and single crystal samples of the ternary copper amino acid complex Cu( l-arginine) 2(SO 4) · (H 2O) 6. The single crystal Electron Paramagnetic Resonance spectra display a single resonance for all magnetic field orientations in the ca and cb crystal planes. In the ab plane they display two resonances for most orientations of the magnetic field, and only one resonance for orientations close to the crystal axes. This behavior is a result of the selective collapse of the resonances corresponding to the four copper sites in the unit cell produced by the exchange interactions between copper ions. From the characteristics of the collapse and the angular dependences of the position and width of the resonances we evaluate the g-tensors of the copper molecules and estimate exchange interactions | J 1/ k B| = 0.9 K and | J 2/ k B| = 0.009 K between copper neighbors at 5.908 Å and at 15.684 Å, respectively. J 1 is assigned to a syn– anti equatorial–apical carboxylate bridge with a total bond length of 7.133 Å. J 2 is assigned to a long bridge of 12 atoms with a total bond length of 19.789 Å, that includes two hydrogen bonds. The results are discussed in terms of the crystal and electronic structure of Cu( l-arginine) 2(SO 4) · (H 2O) 6. We show that J 2 is in excellent agreement with the observed magnetic interaction between the reduced quinone acceptors in the photosynthetic reaction center protein of the bacterium Rb. sphaeroides, which is transmitted along a similar chemical path containing two hydrogen bonds. Our findings indicate that it is valid to estimate values for the exchange interactions between redox centers in proteins transmitted along long chemical paths containing sigma and H-bonds, from data obtained in model systems, and emphasize the importance of measuring exchange interactions in biologically relevant model systems.

Tetrathiomolybdate causes formation of hepatic copper-molybdenum clusters in an animal model of Wilson's disease.

Wilson's disease is an autosomal recessive human illness in which large quantities of copper accumulate in various organs, including the brain and the liver. If left untreated, it results in hepatitis, neurological complications, and death. Long-Evans Cinnamon (LEC) rats have a homologous mutation to Wilson's disease and thus provide an animal model. Liver lysosomes from tetrathiomolybdate-treated LEC rats were isolated and analyzed by Cu and Mo K-edge X-ray absorption spectroscopy. The lysosomes contained a Cu-Mo-S cluster in which the Mo is coordinated by four sulfurs at 2.24 A with approximately three copper neighbors at 2.70 A. Each Cu is coordinated to 3-4 sulfurs at 2.28 A with approximately one Mo neighbor at 2.70 A. These results indicate the formation of a biologically novel molybdenum-copper-sulfur cluster.

Pi-conjugated ligand polymers entwined around copper centres

We describe conjugated polymers entwined around Cu1 with alternating alpha-quaterthienyl (poly[Cu(T2)2]) or 3',4',3'''',4''''-tetrahexyl-alpha-sexythienyl (poly[Cu(T3)2]) moieties and 1,10-phenanthroline complexing sites. Our strategy is to synthesise the 2,9-bis(oligothienyl)-1,10-phenanthroline precursors, then to assemble these ligands by means of Cu1 templating followed by electropolymerisation. Poly[Cu(T2)2] shows separate electroactivities for Cu redox centres and conjugated backbones, whereas the electroactivities overlap in the case of poly[Cu(T3)2]. An X-ray absorption study on these polymers in their reduced state at the Cu-K edge identifies, in both cases, four nitrogen atoms as the closest copper(I) neighbours. For poly[Cu(T2)2], the Cu1 environment is a distorted tetrahedron similar to a monomer model compound, but with a slightly higher number of steric constraints. The Cu1 environment for poly[Cu(T3)2] is a less distorted tetrahedron with an unusually short Cu1-N average bond length. Cu1 removal in poly[Cu(T2)2] induces an irreversible collapse of the structure, whereas the reversibility of Cu1 binding is almost perfect for poly[Cu(T3)2], as the hexyl chains prevent irreversible gliding of the wires after copper removal. Combined electrochemical and resistance measurements reveal that the Cu centres in poly[Cu(T2)2] play the role of mechanical support for the structure with no significant electronic interactions with the conjugated backbone, whereas in the case of poly[Cu(T3)2] copper centres contribute to the conductivity of the structure.

The exchange interactions and magnetic behaviour of Cu(L-alanine)2: specific heat measurements

We report specific heat measurements in the temperature range 0.05 K<or=T<or=1.95 K for the copper-amino acid complex Cu(L-alanine)2. The data display a peak at 0.068 K, and a broad maximum at 0.55 K. Above 0.2 K they reflect a one-dimensional antiferromagnetic spin-chain behaviour. The temperature dependence of the specific heat is compared to that calculated for Heisenberg finite spin chains in order to evaluate the magnitude and characteristics of the exchange interaction. We obtain an antiferromagnetic exchange coupling parameter J0/k=(-0.57+or-0.01) K between nearest-neighbour copper ions. The peak observed at 0.068 K indicates a transition to a phase with three-dimensional magnetic order. The effect of this phase transition on the specific heat measured at higher temperatures is analysed. The experimental results, together with existing magnetic susceptibility and EPR data, are discussed considering the crystal structure and the superexchange paths connecting the copper ions in Cu(L-alanine)2. The nature of the spin chains and the possibility of competing exchange interactions between first and second copper neighbours along the chains are analysed in order to model the magnetic behaviour of Cu(L-alanine)2.

In situ x-ray absorption spectroscopy study of underpotential deposition of copper on platinum (100)

X-Ray absorption spectroscopy (XAS) spectra have been recorded for the electrochemical adsorption of Cu on Pt(100) in the fluorescence detection mode at grazing incidence, with polarization parallel to the surface. During data collection, the Pt(100) surface was under potentiostatic control in a 10 −4 M Cu 2+ electrolytic solution, in order to maintain a monolayer coverage. The near-edge structure of the copper K-edge resembles that of Cu +. The Fourier transform of the EXAFS spectra shows a first peak at 0.15 nm, and a broad unresolved peak with maximum at 0.30 and a shoulder at 0.37 nm. The first single peak is associated with oxygen neighbours at 0.197 nm. The unresolved peak is built up from contributions of copper first and second neighbours. The distance of first copper neighbours is 0.309 nm, and that of second copper neighbours is 0.393 nm, equal to the substrate lattice parameter (0.3929 nm). This suggests that a first copper adlayer of half monolayer density is in registry with the platinum unreconstructed (100) surface net. A second copper adlayer of the same density lies over the first adlayer, at a distance of 0.135 nm. The copper atoms in this second layer are in the fourfold geometry with respect to the underlying copper half-monolayer. These layers form a c(2x2) periodicity on the Pt (100) substrate. The copper—oxygen distance is about half the second copper—copper distance. The sharing of oxygen between the two copper adlayers is unclear.

Exchange interactions and magnetic dimension in Cu(L-alanine)2.

A study of the magnetic properties of the copper (II) complex of the amino acid l-alanine [Cu(l-alanine${)}_{2}$] is reported. The susceptibility of a powder sample has been measured between 0.013 and 240 K. A linear-spin-chain model with antiferromagnetic exchange coupling J=-0.52 K fits well the susceptibility data above 0.3 K. Room-temperature electron paramagnetic resonance (EPR) spectra of single crystals of Cu(l-alanine${)}_{2}$ at 9 and 35 GHz show a single, exchange-narrowed resonance. The g tensor, with principal values ${\mathit{g}}_{1}$=2.0554\ifmmode\pm\else\textpm\fi{}0.0005, ${\mathit{g}}_{2}$=2.1064\ifmmode\pm\else\textpm\fi{}0.0005, and ${\mathit{g}}_{3}$=2.2056\ifmmode\pm\else\textpm\fi{}0.0005, reflects the crystal structure of Cu(l-alanine${)}_{2}$ and the electronic properties of the copper ions. The observed angular variation of the linewidth is attributed to the magnetic interactions, narrowed by the exchange coupling between copper ions, and shows a contribution characteristic of the dipole-dipole interaction in a spin system with a predominant two-dimensional spin dynamics. Considering the exchange-collapsed resonance corresponding to the two lattice sites for copper in Cu(l-alanine${)}_{2}$, we evaluate an exchange constant \ensuremath{\Vert}J(${\mathit{AB}}_{1}$)\ensuremath{\Vert}=0.47 K between nonequivalent copper neighbors in a spin chain, similar to the value obtained from the susceptibility data. The one-dimensional magnetic behavior suggested by the susceptibility data in Cu(l-alanine${)}_{2}$, where the metal ions are distributed in layers, is explained by proposing that carboxylate bridges provide electronic paths for superexchange interactions between coppers. Considering the characteristics of the molecular structure of Cu(l-alanine${)}_{2}$, the layers seem to be magnetically split off into one-dimensional zigzag ribbons. The apparent disagreement between the one-dimensional behavior suggested by the susceptibility data and the two-dimensional behavior of the spin dynamics suggested by the EPR linewidth is analyzed.

ChemInform Abstract: In‐Plane Structural and Electronic Characteristics of Underpotentially Deposited Copper on Gold(100) Probed by in‐situ X‐Ray Absorption Spectroscopy.

Abstract In-situ X-ray absorption spectra of an electrochemically deposited monolayer and bilayer of copper on a gold (100) surface immersed in an electrolytic solution and under potential control were obtained by using fluorescence detection under grazing incidence, the X-rays being polarized parallel to the plane of the gold surface. This study revealed that two distinct structural configurations are obtained for copper on gold (100) depending on the film thickness. For a full-coverage monolayer of copper, the X-ray absorption near edge structure (XANES) revealed clearly that a charge-transfer process occurred from the copper atoms to the gold substrate where the oxidation state of copper was close to Cu+. The Fourier transforms of the Cu K edge extended X-ray absorption spectroscopy (EXAFS) oscillations exhibited (i) an unresolved split peak in the range 0.1–0.25 nm and (ii) a second well-defined peak at around 0.35 nm. The nearest neighbours were determined to be oxygen at 0.197 ± 0.003 nm and copper at 0.266 ± 0.003 nm. The copper-copper distance lies in between the gold-gold lattice spacing (0.288 nm) and the copper-copper one (0.254 nm), which seems to indicate that structural rearrangement of the first gold atom layer occurs. The observation of copper backscattering suggests only that the copper atoms sit in an “ atop ” site. This geometry associated with the charge-transfer process is stabilized by oxygen atoms coming from the water molecules or from the sulphate ions. The second peak in the Fourier transform is connected mainly with the second copper neighbours located at 0.372 ± 0.003 nm, in good agreement with the “ atop ” geometry. When two copper layers were deposited electrochemically on gold (100), strong structural modifications were observed. The Fourier transform exhibited only one split peak in the range 0.15–0.35 nm. Under these conditions, no oxygen is detected and the near neighbours correspond to copper at 0.259 ± 0.003 nm and copper at 0.290 ± 0.003 nm. The second copper-copper distance is very close to the usual gold-gold lattice spacing, suggesting that the first copper layer forms a commensurate phase on the gold (100) surface. The first distance due to the second copper layer is close to the copper-copper lattice spacing but a small discrepancy is still observed. These results reveal that a reconstruction process occurs when one copper layer is added onto the first one where the copper atoms evolve from an “ atop ” site stabilized by oxygen to a 4-fold hollow site.

In-plane structural and electronic characteristics of underpotentially deposited copper on gold (100) probed by in-situ X-ray absorption spectroscopy

In-situ X-ray absorption spectra of an electrochemically deposited monolayer and bilayer of copper on a gold (100) surface immersed in an electrolytic solution and under potential control were obtained by using fluorescence detection under grazing incidence, the X-rays being polarized parallel to the plane of the gold surface. This study revealed that two distinct structural configurations are obtained for copper on gold (100) depending on the film thickness. For a full-coverage monolayer of copper, the X-ray absorption near edge structure (XANES) revealed clearly that a charge-transfer process occurred from the copper atoms to the gold substrate where the oxidation state of copper was close to Cu +. The Fourier transforms of the Cu K edge extended X-ray absorption spectroscopy (EXAFS) oscillations exhibited (i) an unresolved split peak in the range 0.1–0.25 nm and (ii) a second well-defined peak at around 0.35 nm. The nearest neighbours were determined to be oxygen at 0.197 ± 0.003 nm and copper at 0.266 ± 0.003 nm. The copper-copper distance lies in between the gold-gold lattice spacing (0.288 nm) and the copper-copper one (0.254 nm), which seems to indicate that structural rearrangement of the first gold atom layer occurs. The observation of copper backscattering suggests only that the copper atoms sit in an “ atop ” site. This geometry associated with the charge-transfer process is stabilized by oxygen atoms coming from the water molecules or from the sulphate ions. The second peak in the Fourier transform is connected mainly with the second copper neighbours located at 0.372 ± 0.003 nm, in good agreement with the “ atop ” geometry. When two copper layers were deposited electrochemically on gold (100), strong structural modifications were observed. The Fourier transform exhibited only one split peak in the range 0.15–0.35 nm. Under these conditions, no oxygen is detected and the near neighbours correspond to copper at 0.259 ± 0.003 nm and copper at 0.290 ± 0.003 nm. The second copper-copper distance is very close to the usual gold-gold lattice spacing, suggesting that the first copper layer forms a commensurate phase on the gold (100) surface. The first distance due to the second copper layer is close to the copper-copper lattice spacing but a small discrepancy is still observed. These results reveal that a reconstruction process occurs when one copper layer is added onto the first one where the copper atoms evolve from an “ atop ” site stabilized by oxygen to a 4-fold hollow site.

Non-secular ESR broadening in a copper-amino acid complex

Single crystal ESR measurements at 33.6 GHz (Q band) in the layered copper-amino acid complex bis (L-Phenylalaninato) copper (II), Cu (L-Phe)2, are reported and compared with earlier results at 9.7 GHz (X band). Strong frequency-dependent effects were observed in the linewidth which were isolated by subtracting the Q-band data from the X-band ones. The difference data were least-squares fitted to a theoretical expression including non-secular contributions of the different interactions present in this exchange-coupled system, plus a secular contribution ('residual Zeeman') due to the existence of two copper sites with different orientations. The weight of non-secular contributions allowed us to estimate an exchange frequency omega e/2 pi =(10+or-2) GHz in Cu(L-Phe)2, which involves the interaction with the six copper neighbours. The frequency related with the exchange interaction between non-equivalent copper neighbours only, omega eAB/2 pi =(10.7+or-0.6)GHz, was calculated from the secular residual Zeeman contribution. These values indicate that the spin dynamics in Cu(L-Phe)2 is mainly determined by the exchange interaction between non-equivalent copper neighbours.

Exchange interactions in bis(L-leucinato)copper(II)

In order to evaluate the exchange interaction parameters for interaction between copper ions, the authors measured the angular variation in the gyromagnetic factor and the linewidth of the single electron paramagnetic resonance line of bis(L-leucinato)Cu(II) single crystals, at 9.7 and 33.6 GHz, and 300 K. They obtain mod J mod =0.17 K and mod J' mod =0.23 K for the root mean square values of the exchange interaction of one copper with its six nearest copper neighbours and with its four nonequivalent copper neighbours, respectively. These results are analysed in terms of the chemical pathways for superexchange and compared with those obtained in other similar systems.

Arguments for a lone-pair model for high temperature superconductivity

Structure determination of copper-oxygen planes in high temperture oxide superconductor materials by neutron scattering have shown that the Cu-O-Cu bond angles strongly deviate from 180°. This is explainable by the Cu-O bond having a strong covalent character that allows the valence-alternating pair behaviour of copper neighbours. In this case a pair of electrons — the lone pair of oxygen — oscillates between the copper ions. This mechanism allows lone pairs to mediate supercurrent like Cooper pairs. An interesting consequence of this valence alternating pair behaviour is quasi-ferroelectricity, which has an important contribution to the conducting mechanism of the supercurrent.

EPR of layered magnetic metal-amino acid salts. II. Cu(L-Met) 2

Single crystals of bis(L-methioninato)copper(II), Cu(L-Met) 2, were studied by EPR at 9.7 and 33.6 GHz, at 300 K. The position and the peak-to-peak linewidth of the single observed EPR line were measured in three perpendicular planes of the samples. This single resonance is due to the collapse of the resonances of the two magnetically inequivalent copper ions in the lattice caused by the exchange interaction. The components of the molecular g tensor for isolated copper ions were obtained using a model which assumes axial symmetry. The results indicate that the unpaired electron occupies the d( x 2 − y 2) orbital, and the orientation of the molecule obtained from the EPR data agrees with the crystallographic result. The linewidth data support a model which assumes exchange narrowing of the magnetic dipolar interaction in a two-dimensional magnetic lattice, an incomplete collapse of the hyperfine structure, and a frequency-dependent contribution in the planes where the g factors for the two sites of copper are different. An analysis of this latter contribution, allows to evaluate an exchange coupling constant | J′| = 0.10 K between inequivalent copper neighbors. Besides, the analysis of the hyperfine contribution to the linewidth gives | J| = 0.18 K for the average value of the exchange interaction of one copper ion with its six nearest neighbors within the layer. Concerning the possible superexchange paths between inequivalent copper ions, we suggest they can be of two types: the CuNH…OCu one, involving hydrogen bonds between equatorial nitrogens and equatorial oxygens, and the other consisting of CuOCOCu carboxylate bridges involving apical and equatorial oxygens. They are discussed in view of the experimental results.

EPR of layered magnetic metal—amino acid salts. I. Cu(L-PHE)2

EPR measurements in single crystals of Cu(L-PHE)2, the copper derivative of the amino acid L-phenylalanine, were performed at 300 K and 9.7 GHz. The gyromagnetic factor, g, and the linewidth of the single EPR line were measured in three perpendicular planes of the sample. The gyromagnetic tensor was obtained, and its principal values are g1 = 2.211, g2 = 2.134, and g3 = 2.075. The single resonance is explained by the collapse of the resonances of the two magnetically non-equivalent copper sites due to the exchange interaction. The molecular g-factor of isolated copper ions is obtained from a model assuming axial symmetry. The results are g| = 2.266 and g⊥ = 2.075, indicating a d(x2 - y2) ground orbital. The orientations of the molecules in the crystal obtained by this model are in agreement with the crystallographic values. The linewidth data support a model which assumes exchange narrowing of the magnetic dipolar interaction in a two-dimensional magnetic structure, an incomplete collapse of the hyperfine structure, and contributions arising from non-isotropic exchange. A mean value ¦J¦=0.19 K is calculated for the isotropic exchange interactions between one copper and its six copper neighbors in Cu(L-PHE)2. Also, a lower limit ¦J′¦ > 0.02 K for the exchange coupling J′ between non-equivalent copper neighbors is obtained.

Electric Field Gradients in Dilute Copper Alloys

The electric-quadrupole-tensor parameters of copper neighbors to the impurities Zn, Cd, Ag, and Au have been measured by studying the angular dependence of the second-order splitting of the $\frac{1}{2}\ensuremath{\leftrightarrow}\ensuremath{-}\frac{1}{2}$ nuclear-magnetic-resonance transitions in single crystals of copper metal. The parameters for the first neighbors were determined for all the impurities, and for the second neighbors to Cd, Ag, and Au. The results for the charged impurities complement the work of Jensen, Nevald, and Williams on Ge, In, Sn, and Sb in copper, and show a systematic change in the electric field gradient at nearest-neighbor sites as a function of impurity valence for which present theory is inadequate.