Cun Clusters Research Articles

Small copper clusters: Study of the local atomic and electronic structure by the XANES and DFT methods

The Cu L3 edge X-ray absorption spectra of free Cun clusters containing 5–15 atoms were obtained for the first time. It is shown that the geometry of small clusters, including the bond length and bond angle, can be studied by analyzing the X-ray absorption spectra. The experimental X-ray absorption spectra of Cu13 clusters were theoretically interpreted using the self-consistent method of full multiple scattering, the finite difference method in the muffin-tin potential approximation, and the full potential method for the icosahedral cluster structure. Good agreement between the theoretical and experimental spectra is achieved for the calculation in the full potential approximation.

Structural changes during freezing and coalescing of small sized clusters on atomic scale

Structural changes of three molten CuN（N=57,58,59） clusters during freezing and two Cu55 clusters with perfect icosahedral geometries in the coalescence processing at 300 K are investigated by molecular dynamics simulations based on the embedded atom method. Simulation results show that both freezing and coalescing processes have distinct stages. There exist great differences in atomic movement and microstructure change among the three CuN（N=57,58,59） clusters during freezing, which result in the different patterns of atomic packing in the three clusters. Of the three clusters, the ordered degree of the Cu59 cluster is the lowest. Initially structural changes of the two Cu55 cluster during coalescence result from large position displacements of atoms due to the deformation, then the atomic diffusion plays a mainly role in changing structure. The atoms far from the contact region between the two clusters can remain their origin structures.

Adsorption Behavior of CH2 and CH3 on Metal Clusters Cun (n= 16)

Using density functional theory with generalized gradient approximation and hybrid functional, we studied the properties of energy, charge population, and vibration of CH2 and CH3 adsorbed on Cun (n = 16) clusters. The results show that the DFT calculation with the hybrid functional matches the experimental results better in both cases. The calculation results indicate that the adsorption of CH2 is stronger than that of CH3. During adsorption, the charges transfer from Cu to CH2 or CH3. The obtained vibrational frequencies for different modes of CH2 and CH3 adsorbed on Cun agree well with the experimental results for the adsorption on Cu(lll) surface.

High-Resolution Infrared Spectroscopy of HCN−Agn (n = 1−4) Complexes Solvated in Superfluid Helium Droplets

High-resolution infrared spectroscopy has been used to determine the structures, C-H stretching frequencies, and dipole moments of the HCN-Agn (n = 1-3) complexes formed in superfluid helium droplets. The HCN-Ag4 cluster was tentatively assigned based upon pick-up cell pressure dependencies and harmonic vibrational shift calculations. Ab initio and density functional theory calculations were used in conjunction with the high-resolution spectra to analyze the bonding nature of each cluster. All monoligated species reported here are bound through the nitrogen end of the HCN molecule. The HCN-Agn complexes are structurally similar to the previously reported HCN-Cun clusters, with the exception of the HCN-Ag binary complex. Although the interaction between the HCN and the Agn clusters follows the same trends as the HCN-Cun clusters, the more diffuse nature of the electrons surrounding the silver atoms results in a much weaker interaction.

Ranges of implanted atoms under polyatomic cluster bombardment of the Cu (111) surface

A computer simulation of the bombardment of the Cu (111) surface with CuN and AuN polyatomic clusters differing in size (N = 1, 6, and 13) with energies of 0.5 and 5 keV/atom has been performed in the framework of classical molecular dynamics. The spatial distribution of the implanted atoms, their ranges, and range fluctuations (straggling) depending on the size N and energy E/N of the incident cluster has been investigated. It has been shown that an increase in the mean range and range straggling is observed at a fixed energy per incident atom as the cluster size N increases. At the same time, the effect of an increase in the range (at a specified value of E/N) gradually disappears with increasing cluster energy, whereas the effect of an increase in straggling is retained. These tendencies qualitatively agree with the available experimental observations. It has been shown that the dominating contribution to the increase in the atom range of the implanted cluster is made by the so-called clearing-the-way effect, which is weakened with increasing the incident cluster energy. The effect of the range straggling increasing is significantly due to the presence of nonlinear “spike” effects at the bombarded target.

The initial sputtering of Cu(1 0 0) by 1 keV O2 projectiles

Molecular dynamics simulations of the sputter erosion of a Cu(100) crystal by 1keV O2 projectiles up to a nominal fluence of 1.7×1015 O2 molecules cm−2 are reported for both normal and oblique projectile incidence. O–Cu attractive interactions have been described in the simulations using a many-body Gupta potential that was fitted to ab initio structural and energy predictions for O–Cu clusters. The simulations predict that implanted oxygen atoms segregate from octahedral sites in the bulk to hollow sites at the surface, and that the sputter yield of the target is enhanced by a factor of two due to the efficient formation of CunO clusters. The predicted yields of CunO clusters display a non-monotonic abundance distribution, with a maximum at n=2–3, in contrast to the monotonic, power-law abundance distribution predicted for Cun clusters.

Structure and energetics of nickel, copper, and gold clusters

The most stable structures of CuN, NiN, and AuN clusters with 2≤N≤60 have been determined using a combination of the embedded-atom (EAM), the quasi-Newton, and our own Aufbau/Abbau methods for the calculation of the total energy for a given structure, the structures of the local total-energy minima, and the structure of the global total-energy minimum, respectively. We have employed two well-known versions of the EAM: (1) the ‘bulk’ version of Daw, Baskes, and Foiles and (2) the Voter-Chen version which takes into account also properties of the dimer in the parameterization. The lower-energy structures (also for the smallest) of CuN and NiN clusters (i.e., structural details as well as symmetry) obtained with the two versions are very similar. Thus, our study supports an universality of the bulk embedding functions for copper and nickel. But for gold clusters the differences between structures calculated with the two different versions of the EAM are significant, even for larger clusters.

Structural properties of small copper clusters with a nickel impurity

The structural and magnetic stability of small NiCuN−1 clusters is determined by performing global structural optimizations within Kohn-Sham density-functional theory. The calculated structures for N⩽5 atoms are compact and resemble, besides some interesting changes in bond length, the structures of the corresponding CuN clusters. The Ni ion occupies the most-coordinated atomic position, which reflects the dominant role of the sd hybridizations. For the optimal NiCuN−1 geometry the ground-state corresponds to a minimum-spin configuration (Sz=0 or 1/2). Varying the total spin moment of the cluster reveals interesting correlations between cluster structure and magnetism. Low-energy spin excitations are found which involve similar energies as isomerizations. Structure and magnetic behavior are found to depend strongly on each other. The possible consequences of electron-correlation and finite-temperature effects are briefly discussed in the framework of the single-impurity Anderson model.

Ab Initio Calculation of Endohedal Fullerenes with Copper and Silver Clusters

A series of models built from fullerene molecules (C60 and C82) and Agn and Cun (n<5) clusters embedded inside fullerenes are calculated with ab initio SCF Hartree‐Fock methods of full geometry optimization. The effect of the “fullerene‐shell” was shown to be very significant upon features of the clusters. Monoatomic Ag‐ and Cu‐ endo‐fullerenes are stable species, while 2‐4‐atomic endo‐C60 structures reveal the stability with copper clusters only, and Cu4@C60 with tetrahedron Cu4 is more favorable than one with flat rhombus isomer. The stable structures calculated with these models indicate the effect of charge transfer between the fullerene cage and metal atoms, resulting in positive charge for copper clusters. However, a negative charge for silver clusters corresponds to unstable structures. In the case of the C82 cage, both Ag@ C82 and Ag2@C82 are stable, but the first appears to be strongly distorted.

Electronic properties of small bimetallic LinCum (n, m ≤ 4) clusters. A comparison with Lin and Cun clusters

AbstractThe electronic structure and geometry of clusters of the type LinCum, LinCu (n, m ≤ 4), Lin, and Cun (n ≤ 8) were theoretically investigated using density functional methods. The LinCum bimetallic system is important to understand the promotion effects of the alkali atoms on the copper surface. Some clear trends have been found. The inclusion of lithium atoms on a bare copper cluster tends to break the CuCu bond favoring the formation of polar LiCu bonds. In comparison with the bare Lin and Cun clusters the geometrical shape and relative stability follow the same trend. However, the interatomic distances show strong variations. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Geometry and electronic properties of Cu n ( n 9)

The equilibrium geometries and the atomization energies of Cun(n≤9) clusters have been calculated using the B3LYP/LANL2DZ method. It is shown that the clusters do not copy the bulk structures and undergo significant geometrical changes with size and the atomization energy per atom increases monotonically with size. By analysing the energy level distribution, the Fermi level, HOMO–LUMO gaps, the electron affinities and the ionization potentials are calculated and the results are in reasonable agreement with experiment. These electronic properties are found to be strongly structure dependent, which can be used to determine which of the low-lying structures is observed experimentally.

Characterization of Cu-complexes in smectite with different layer charge location: chemical, thermal and EXAFS studies

The retention of Cu and Cu-amino acid complexes by montmorillonite and beidellite, before and after repeated acidified aqueous solution treatments, was studied using X-ray diffraction, chemical and thermal analyses, mass spectrometry and synchrotron-based X-ray absorption spectroscopy (XAS). The results indicate that the extraction of metal complexes from smectites depends on the nature of the layer charge and on the kind of organic species. Cu-cysteine complexes are strongly retained in the interlayer position, whereas Cu-glycine complexes are mostly adsorbed in cationic form which can be easily removed from the silicate layer. The layer periodicity for Cu-smectites treated with glycine shows little or no layer expansion, whereas significant swelling of the layer periodicity is observed in smectites treated with cysteine. Thermal decomposition of both smectites with sorbed Cu-amino acid species shows the evolution of H 2O, NO, CH 3CH 3, and CO 2. In Cu-cysteine treated smectites, the release of H 2S, NO 2, SO 2, and N 2O 3 also occurs. X-ray absorption spectroscopy (XAS) was used to assess the relationships between the structure of the Cu complexes and their desorption from smectites. In Cu-exchanged smectites, the first coordination shell agrees with the hypothesis that the Cu coordinates to oxygen atoms to form monomer and/or dimer complexes. The first coordination shell of Cu in smectites treated with glycine shows four atoms at distances of ∼2 Å. Two of these bonds are with nitrogen and two with oxygen atoms. For copper-cysteine complexes XAS data are compatible with the existence of Cu-N clusters, thus suggesting that Cu links to the amino acid by the aminic group.

STUDY OF SMALL METALLIC NANOPARTICLES: AN AB-INITIO FULL-POTENTIAL MUFFIN-TIN ORBITALS BASED MOLECULAR DYNAMICS STUDY OF SMALL Cu CLUSTERS

The equilibrium structures and cohesive energies of small Cun clusters using the full-potential muffin-tin orbitals (FP-LMTO) based molecular dynamics (MD) have been studied for n ≤ 9. The results obtained have been compared with other chemical methods. We propose the FP-LMTO-MD technique to be a useful starting point of more empirical methods which can efficiently deals with larger cluster sizes relevant for nanoparticles.

Comparison of static polarizabilities of Cun, Nan, and Lin (n⩽9) clusters

This paper presents the first study of static polarizabilities and polarizability anisotropies of copper clusters up to nine atoms calculated in the framework of density functional theory. The calculations were of all-electron type and have been performed by using a finite field approach implemented in the density functional program ALLCHEM. A newly developed first-order field induced copper basis set for density functional calculation was employed. A gradient-corrected exchange-correlation functional has been used. All cluster structures were fully optimized. The calculated polarizabilities of copper clusters are compared with experimental polarizabilities of sodium and lithium clusters. This comparison shows that the size dependency of the static polarizabilities per atom of copper clusters posseses the same trend as that observed in sodium clusters. However, the absolute polarizabilities of the copper clusters are considerably smaller as those of the sodium clusters.

Size-selected Agn and Cun clusters supported on MgO(100) films

Small Ag n (n ≤ 13) and Cu n (n ≤ 7) clusters, generated by sputtering with a new ultrahigh vacuum (UHV)-compatible Xe-ion gun and size-selected by a quadrupole mass spectrometer, are deposited in situ in submonolayer quantities on a magnesium oxide film supported on a molybdenum single crystal. At low substrate temperature (T = 45 K), the measured electron energy loss spectra display characteristic losses within the MgO band gap. These novel results are discussed in terms of electronic transitions and plasmon excitations within the cluster.

Ab Initio Theory of Metal Deposition on SiO2. 1. Cun (n = 1−5) Clusters on Nonbridging Oxygen Defects

The nucleation of small Cu clusters at the surface of SiO2 has been investigated by means of ab initio quantum chemical calculations using cluster models. The interaction of Cun clusters (n =1−5) with a nonbridging oxygen, NBO, a paramagnetic point defect of silica, has been studied by means of DFT calculations using the B3LYP exchange-correlation potential. Different from the regular nondefective surface, characterized by a weak adhesion with metal species, strong metal−oxide interface bonds form at the NBO centers. The Cu clusters are bound with one or two Cu atoms with the NBO forming covalent polar bonds. The partial charge transfer to the oxide favors the formation of electrostatic interactions between the metal cluster and the two-coordinated O atoms of the silica surface. Some observable consequences of the cluster nucleation have been studied, including core level shifts and the appearance of new states in the gap of the oxide.

Structural and electronic properties of small Cun clusters using generalized-gradient approximations within density functional theory

Neutral and anionic Cun clusters (Cu2, Cu3, Cu6 and Cu7−) are studied within density functional theory via the local density approximation (LDA) and the generalized-gradient approximation (GGA) of Perdew and Wang for exchange and correlation. Three different levels of improvement upon the LDA are considered. In the first level, the GGA correction to the exchange-correlation energy is evaluated using the electronic density and the atomic coordinates obtained in the LDA calculation. In the second level, the electronic density is obtained self-consistently within the GGA while keeping the LDA structural configurations. In the third level, both electronic density and ionic positions are obtained fully self-consistently within the GGA. We found that the first level of approximation is already sufficient to correct the overbinding found in the LDA. With respect to the LDA, the self-consistent GGA enhances the electron charge accumulation around the nuclei by depleting the interatomic bonding regions.

A first principles study of small Cun clusters based on local-density and generalized-gradient approximations to density functional theory

Neutral and anionic Cun clusters (Cu2, Cu3, Cu6 and Cu7−) are studied within density functional theory via (a) the local-density approximation (LDA) and (b) the generalized-gradient approximation (GGA) of Perdew and Wang (GGA-PW) for exchange and correlation. GGA reduces by ~20% the binding energies, while the bond lengths are increased by ~3–4%. The different levels of GGA approximation, involving optimization of the electronic density and/or of the geometry, are shown in detail. In the case of Cu6 the GGA configurational ground state is a planar structure of D3h symmetry. This result differs from the one obtained by LDA, where the three different isomers (one two-dimensional and two three-dimensional) were found to lie within 0.04 eV.

Molecular-dynamics simulation of the structural stability, energetics, and melting of $Cu_{n}\ (n=13-135)$ clusters

Cluster properties of copper have been investigated using the Molecular-Dynamics md technique. The structural stability and energetics of spherical Cun (n = 13–135) clusters have been investigated at temperatures T = 1 K and T = 300 K. It has been found that the average interaction energy per atom in the cluster decreases and reaches an asymptotic value as cluster size increases. The melting behaviour of clusters n = 13 and n = 55 have been investigated. It has been found that the melting temperature decreases as cluster size increases, and for clusters with multishell structures melting starts from the outermost shell. In the simulation an emprical potential energy function (PEF) proposed by Erkoc has been used, which contains two-body atomic interactions.

ELECTRON–PHONON INTERACTION AND ELECTRONIC STRUCTURE OF SMALL METAL CLUSTERS

The Su–Schrieffer–Heeger (SSH) Hamiltonian has been extended to study the electron–phonon interaction and the electronic structures of the alkali-like metal clusters. The eigen-energy levels of s valence electrons are obtained from a Hückel-like Hamiltonian including the correction of the electron–phonon interaction in the hopping integral, which is proportional to the variable of bond length. The self-consistent equations for electrons and phonons are solved adiabatically through an iteration process. The energy-level structures of an octahedral Cu6 cluster are calculated with variable electron–phonon coupling constant λ to investigate the influence of electron–phonon interaction on the lattice distortion and electronic structures of metal clusters. The size-dependent ionization potential for small Cun clusters are calculated and compared with the experimental results.