Structure and electronic factors in benzene coordination to Cr(CO)3 and to cluster models of Ni,Pt, and Ag (111) surfaces

Bonding of benzene to a chromium tricarbonyl fragment and to cluster models of silver, nickel, and platinum (111) surfaces is found by means of molecular orbital calculations to be dominated by a benzene donation bond involving its π (e_1g) orbitals and metal d orbitals. Three-fold hollow sites on the clusters are calculated to be most stable for benzene coordination, a conclusion reached in a number of experimental studies of benzene on metal surfaces. On the Ag, Ni, and pt clusters, the benzene CH bonds are found to bend away from the surface by ?2°, 8° and 19°, respectively, a result of carbon atom hybridization to maximize overlap with metal orbitals. For benzene chromium tricarbonyl, the CH bonds are calculated to bend 3° toward the metal, compared to a 1.7° bend reported in a diffraction study. The direction and magnitude of the CH bending are shown to depend on the metal d orbital occupancy (an electronic factor) and the proximity of metal atoms in the adsorption site (a structure factor). Small Kekulé distortions are calculated for the chromium complex and for the C_3v sites on Pt(111). Finally, recent experimental studies showing a decrease in benzene adsorption energy when potassium is coadsorbed on Pt(111) may be understood to result from decreased π donation which accompanies the shift up of the metal d band with cathodic charging.     

Stabilities,electronic and magnetic properties of Cu-doped nickel clusters: a DFT investigation

In this paper, we investigate the electronic and magnetic properties of Cu-doped nickel clusters by means of density functional theory. The stabilities of these clusters have also been studied in terms of the binding energies, second-order difference of energies, fragmentation energies and HOMO–LUMO energy gaps. The obtained results reveal that the N₄Cu, N₅Cu and Ni₇Cu clusters are found to be more stable that than all other clusters. Higher HOMO–LUMO gap was observed for Ni₅Cu cluster (2.265 eV), indicating its higher chemical stability. A half-metallic behaviour has also been observed for the Ni_nCu clusters, which suggests that these clusters can be employed as nanocatalysts for several catalytic processes, particularly for hydrogenation and dehydrogenation reactions. The magnetism calculations show that the magnetic moment is mostly located on the Ni atoms, and the contribution of the Cu atom to the total magnetic moment in the Ni_nCu clusters is very small. Furthermore, partial density of states analysis indicates that the 3d orbitals in Ni atoms are mostly responsible for the magnetic behaviour of these clusters, and the s orbitals have a very little contribution to the total magnetic moment.     

XPS study of bonding in ligated Au clusters

Ligated metal cluster compounds containing a core of metal atoms with well defined structure surrounded by a variety of organic and inorganic ligands are closely related to the bare metal clusters that are only now becoming available in uniform cluster size. The evolution of band structure and the development of metallic properties as a function of cluster size are of considerable interest. We report here a comparison of these two types of systems based on a study by X-ray photoelectron spectroscopy. The valence band spectra of ligated Au clusters compounds are similar in many respects to those of bare clusters, indicating significant participation of the d electrons in bonding. The core electron binding energy shifts of the central Au atom in Au₁₁(PPh₃)₇Cl₃ corresponds to the loss of approximation one 6 s electron. The total charge transferred to the halogens is accounted for by the shifts of the central and three halogenbonded Au atoms. No indication of metallic behaviour is found in the core of the ligated clusters.     

The catalytic activity of chemisorbed oxygen and carbon monoxide species on palladium and related metals investigated using SCF-Xα-SW calculations

Theoretical calculations of the electronic structure of small clusters modelling the adsorption of carbon monoxide on nickel and palladium and the adsorption of oxygen on palladium and platinum have been carried out using the SCF-Xα-SW method. Our results for the carbon monoxide-nickel clusters are in good agreement with earlier work. Comparing the carbon monoxide-nickel with the carbon monoxide-palladium results, suggests that the ordering of CO-derived cluster orbitals is the same in both cases but that the relative shifts are much different. In addition, the oxygen atom participates more significantly in the cluster containing palladium. An analysis of optical transition energies for Ni₅CO and Pd₅CO clusters is given and discussed in terms of experimental data regarding photodesorption of carbon monoxide. In the case of oxygen atoms in platinum and palldium clusters, we have used two different M₅O geometries: one in which the five metal atoms are in a single plane and oxygen is directly over a single metal atom (Type A) and a second in which the oxygen atom is coordinated to four sur face metal atoms and is directly over a metal atom in the second layer (Type B). The levels calculated for the Pd50 type B cluster are in good agreement with available UPS data. Significant differences in type A and type B clusters are noted for palladium. The results can be correlated with experimental Auger spectroscopic and kinetic data. In particular, the type A oxygen cor relates well with an experimentally observed very reactive species, while type B oxygen cor relates well with a quite unreactive species. These two types also correlate with two species observed by Auger spectroscopy.     

Novel structure and electronic property of Sin (21n30) clusters

Using first-principles simulated annealing generalized gradient approximation density functional calculations based on norm-conserving pseudopotentials, we have investigated the geometric and electronic structures of low-energy silicon clusters (Si_n, n=21–30). We have obtained new low-energy structures not reported previously. Our calculations suggest that the lowest energy structures are spherical ones including core atoms whose number increases with the cluster size. The trend of the binding energy as well as that of the energy difference between the highest occupied and the lowest unoccupied molecular orbitals is studied as a function of the cluster size and the number of core atoms.     

Calculations with the CNDO method on clusters of silver atoms

An attempt has been made to explore the applicability of the complete neglect of differential overlap (CNDO) method for investigating clusters of silver atoms. A new parametrization for silver has been obtained by comparing charge distributions, as well as local and total density of states, from CNDO calculations with those from the X_α scattered wave (X_αSW) method for an Ag₇ cluster which represents a fragment of the silver lattice. These parameters have then been used for making CNDO calculations on four further clusters of the same type, namely Ag₆, Ag₁₀, Ag₁₃ and Ag₁₉, and the results are compared with previous X_αSW calculations. These CNDO calculations give d-band widths in broad agreement with those from the X_αSW method. The most significant difference is that the CNDO method gives less localization on central atoms with high coordination numbers than is found from the X_αSW calculations. It is suggested that this apparent deficiency of the CNDO calculations may be less serious when the clusters are being used for modelling part of a solid metal rather than for specifically investigating the properties of small particles.     

Effect of oxygen content and charge on the structure,stability and optoelectronic properties of yttrium oxide clusters

The electronic and geometrical structures of neutral and charged YOn (n=2–12) clusters have been investigated using density functional theory (DFT) with generalized gradient approximation. The oxygen atom in YOn has been found to be in oxo, peroxo and in superoxo forms. The geometrical structures and topologies of small size anionic clusters resemble that of neutral clusters. Yttrium showed higher coordination number than scandium. Computed results reveal the existence of YO₁₀ cluster to have a penta-peroxo oxygen with a homoleptic Y(η² –O₂)₅ geometrical configuration. The HOMO–LUMO gaps decrease with increasing n due to the increase in 2p orbital population of oxygen atoms. It has been shown that in these clusters bonding are predominantly ionic in nature and anions are thermodynamically more stable, due to the charge delocalization between the metal atom and oxygen ligands. YO₁₀⁺ and YO₁₂⁺ were found to be highly exothermic to release one and two oxygen molecules, while YO₁₁⁺ dissociates though the ozonide dissociation channel. Computed absorption spectra of small clusters are mainly contributed by yttrium metal d and s valence orbitals. The absorbance spectra, shifts towards lower energy with cluster size increase, while charge has no substantial effect on the absorption spectrum.     

Structure,energetic and phase transition of small nickel-palladium heterogeneous clusters

Molecular dynamics simulation (MD) with Sutton-Chen potential for palladium-palladium, nickel-nickel and palladium-nickel interactions has been used to generate the minimum energy structures and to study the thermodynamic and dynamic properties of mixed transition metal cluster motifs of Ni _n Pd_(13?n) for n ≤ 13. Thirteen particle icosahedral clusters of neat palladium and nickel atoms were first reproduced accordingly with the results in literature. Then in the palladium icosahedra, each palladium atom has been successively replaced by nickel atom. Calculation is repeated for both palladium-centered and nickel-centered clusters. It is found that the nickel-centered clusters are more stable than the palladium-centered clusters and cohesive energy increases along the palladium end to nickel end. Phase transition of each cluster from one end-species to the other end-species is studied by means of caloric curve, root mean square bond fluctuation and heat capacity. Trend in variation of melting temperature is opposite to the energy trend. Palladium-centered cluster shows a premelting at low temperature due to the solid-solid structural transition. Species-centric order parameters developed by Hewage and Amar is used to understand the dynamic behavior in the solid-solid transition of palladium-centered cluster to more stable nickel-centered cluster (premelting). This species-centric order parameter calculation further confirmed the stability of nickel-centered species over those of palladium-centered species and solid-solid structural transition at low temperature.     

Density functional theory study of AunMn(n=1-8) clusters

Equilibrium geometries, relative stabilities, and magnetic properties of small Au_nMn (n=1-8) clusters have been investigated using density functional theory at the PW91P86 level. It is found that Mn atoms in the ground state Au_nMn isomers tend to occupy the most highly coordinated position and the lowest energy structure of Au_nMn clusters with even n is similar to that of pure Au_n+1 clusters, except for n=2. The substitution of Au atom in Au_n+1 cluster by a Mn atom improves the stability of the host clusters. Maximum peaks are observed for Au_nMn clusters at n=2, 4 on the size dependence of second-order energy differences and fragmentation energies, implying that the two clusters possess relatively higher stability. The HOMO-LUMO energy gaps of the ground state Au_nMn clusters show a pronounced odd-even oscillation with the number of Au atoms, and the energy gap of Au₂Mn cluster is the biggest among all the clusters. The magnetism calculations indicate that the total magnetic moment of Au_nMn cluster, which has a very large magnetic moment in comparison to the pure Au_n+1 cluster, is mainly localized on Mn atom.     

Ab initio study of formation, migration and binding properties of helium-vacancy clusters in aluminum

Ab initio calculations based on density functional theory have been performed to study the dissolution and migration of helium, and the stability of small helium-vacancy clusters He_nV_m (n, m=0-4) in aluminum. The results indicate that the octahedral configuration is more stable than the tetrahedral. Interstitial helium atoms are predicted to have attractive interactions and jump between two octahedral sites via an intermediate tetrahedral site with low migration energy. The binding energies of an interstitial He atom and an isolated vacancy to a He_nV_m cluster are also obtained from the calculated formation energies of the clusters. We find that the di- and tri-vacancy clusters are not stable, but He atoms can increase the stability of vacancy clusters.     

Structure, dynamic and energetic of mixed transition metal clusters

Classical molecular dynamics simulation (MD) with Sutton-Chen potential has been used to generate the minimum energy and to study the thermodynamic and dynamic properties of mixed transition metal cluster motifs of Ag _n Ni_(13?n) for n ?? 13. Literature results of thirteen particle clusters of neat silver and nickel atoms were first reproduced before the successive replacement of the silver atom by nickel. Calculation was repeated for both silver-centred and nickel-centred clusters. It was found that the nickel-centred clusters were more stable than the silver-centred clusters. Heat capacities and hence the melting points of silver and nickel-centred clusters were determined by using the Histogram method. Species-centric order parameters developed by Hewage and Amar were used to understand the dynamic behaviour in the transition of silver-centred clusters to more stable nickel-centred clusters. This species-centric order parameter calculation further confirmed the stability of nickel-centred clusters over those of silver-centred species.     

Valence band formation in small silver clusters

UV photoemission spectra of valence electrons in small silver clusters have been compared with spectra from bulk silver samples using synchrotron radiation, 16 <hν< 27 eV. Spectra for single silver atoms supported on carbon are indicative of a completely filled 4d¹⁰ initial state configuration. With increased cluster size, both density of states and valence band modulations with respect to photon energy resemble the bulk metal more closely. Spectral modulation, characteristic of conservation of crystal momentum, appears to require a cluster consisting of ～150 atoms.     

A study of some potassium hexachlorometallate complexes using electron spectroscopy

X-ray photoelectron (ESCA) spectra of the core (Cl 2p K 2p and metal 4f, if present) and valence orbitals are reported for K₂ReCl₆, K₂OsCl₆, K₂IrCl₆· 3 H₂O, K₂PtCl₆, K₃MoCl₆, and K₂SnCl₆. The K 2p_{$\text{3}\text{2}$} binding energy was found to be nearly constant (292.7 eV) and that of Cl to increase very slightly with increasing atomic number for the third row transition metals. The chemical shifts of Re(IV), Os(IV), Ir(IV), and Pt(IV) relative to the metals were in qualitative agreement with atomic calculations utilizing configurations obtained from extended Hückel calculations. The valence spectra of the transition metal complexes exhibit a three-band structure. On the basis of MO results and intensity considerations the high binding energy band is assigned as a composite of the a_1g, e_g, 1t_2g MO's. The middle band represents the t_2u, 2t_1g MO's; and the low binding energy band the 2t_2g MO. Calculated nd orbital photoionization cross sections correlate reasonably well with the relative intensifies of the valence manifolds. Comparison of band separations and charge-transfer transition energies suggests that interelectronic repulsion and MO energy separation contribute about equally to the overall charge-transfer energy.     

Probing the structural,electronic and magnetic properties of small Au4M (M = Sc–Zn) clusters

Density-functional method PW91 has been selected to investigate the structural, electronic and magnetic properties of Au₄M (M =Sc–Zn) clusters. Geometry optimisations show that the M atoms in the ground-state Au₄M clusters favour the most highly coordinated position. The ground-state Au₄M clusters possess a solid structure for M = Sc and Ti and a planar structure for M = V–Zn. The characteristic frequency of the doped clusters is much greater than that of pure gold cluster. The relative stability and chemical activity are analysed by means of the averaged binding energy and highest occupied molecular orbital and lowest unoccupied molecular orbital energy gap for the lowest energy Au₄M clusters. It is found that the dopant atoms can enhance the thermal stability of the host cluster except for Zn atom. The Au₄Ti, Au₄Mn and Au₄Zn clusters have relatively higher chemical stability. The vertical detachment energy, electron affinity and photoelectron spectrum are calculated and simulated theoretically for all the ground-state structures. The magnetism calculations reveal that the total magnetic moment of Au₄M cluster is mainly localised on the M atom and vary from 0 to 5 μ_B by substituting an Au atom in Au₅ cluster with different transition-metal atoms.     

The electronic structures of the core and valence ion states of metal oxides

SCF-Xα SW MO calculations on metal core ion hole states and X-ray emission (XES) and X-ray photoelectron (XPS) transition states of the non- transition metal oxidic clusters MgO₆^10?, AlO₄^5? and SiO₄^4? show relative valence orbital energies to be virtually unaffected by the creation of valence orbital or metal core orbital holes. Accordingly, valence orbital energies derived from XPS and XES are directly comparable and may be correlated to generate empirical MO diagrams. In addition, charge relaxation about the metal core hole is small and valence orbital compositions are little changed in the core hole state. On the other hand, for the transition metal oxidic clusters FeO₆^10?, CrO₆^9? and TiO₆^8? relative valence orbital energies are sharply changed by a metal core orbital or crystal field orbital hole, the energy lowering of an orbital increasing with its degree of metal character. Consequently O 2p nonbonding → M 3d-O 2p antibonding (crystal field) energies are reduced, while M 3d bonding → O 2p nonbonding and M 3d-O 2p antibonding → M 4s,p-O 2p antibonding (conduction band) energies increase. Charge relaxation about the core hole is virtually complete in the transition metal oxides and substantial changes are observed in the composition of those valence orbitals with appreciable M 3d character. This change in composition is greater for e _g than for t_2g orbitals and increases as the separation of the e_g crystal field (CF) orbitals and the O 2p nonbonding orbital set decreases. Based on the hole state MO diagrams the higher energy XPS satellite in TiO₂ (at about 13 eV) is assigned to a valence → conduction band transition. The UV PES satellites at 8.2 eV in Cr₂O₃ and 9.3 eV in FeO are tentatively assigned to similar transitions to conduction band orbitals, although the closeness in energy of the crystal field and O 2p nonbonding orbitals in the valence orbital hole state prevents a definite assignment on energy criteria alone. However the calculations do clearly show that charge transfer transitions of the e_g bonding → e_g crystal field orbital type would generally occur at lower energy than is consistent with observed satellite structure.A core electron hole has little effect upon relative orbital energies and is only slightly neutralized by valence electron redistribution for MgO and SiO₂. For the transition metal oxides a core hole lowers the relative energies of M3d containing orbitals by large amounts, reducing O → M charge transfer and increasing M 3d crystal field → conduction band energies. Large and sometimes overcomplete neutralization of the core hole is observed, increasing from CrO₆^9? to FeO₆^10? to TiO₆^8?. as the O → M charge transfer energy declines.High energy XPS satellites in TiO₂ may be assigned to O 2p nonbonding → conduction band transitions while lower energy UV PES satellites in FeO and Cr₂O₃ arise from crystal field or O 2p nonbonding → conduction band excitations. Our “shake-up” assignment for FeO₆^10?, CrO₆^9? and TiO₆^8? are less than definitive because no procedure has yet been developed to calculate “shake-up” intensities resulting from transitions of the type described. However the results do allow a critical evaluation of earlier qualitative predictions of core and valence hole effects. First, we find that the comparison of hole or valence state ionic systems with equilibrium distance systems of higher nuclear and/or cation charge (e.g. the comparison of the FeO₆^10? Fe 2p core hole state to Co₃O₄) is dangerous. For example, larger MO distances in the ion states substantially reduce crystal field splittings. Second, core and CF orbital holes sharply reduce O → M charge transfer energies, giving 2e_g → 3e_g energy separations which are generally too small to match observed satellite energies. Third, highest occupied CF-conduction band energies are only about 4–5 eV in the ground states, but increase to about 7–11 eV in the core and valence hole states of the transition metal oxides studied. The energetic arguments presented thus support the idea of CF and/or O 2p nonbonding → conduction band excitations as assignments for “shake-up” satellites, at least in oxides of metals near the beginning of the transition series.     

Prismane C8: A new form of carbon?

Our numerical calculations on small carbon clusters point to the existence of a metastable three-dimensional eight-atom cluster C₈ which has a shape of a six-atom triangular prism with two excess atoms above and below its bases. We gave this cluster the name “prismane.” The binding energy of prismane is 5.1 eV/atom, i.e., 0.45 eV/atom lower than the binding energy of the stable one-dimensional eight-atom cluster and 2.3 eV/atom lower than the binding energy of bulk graphite or diamond. Molecular dynamics simulations give evidence for a rather high stability of prismane, the activation energy for prismane decay being about 0.8 eV. The prismane lifetime increases rapidly as the temperature decreases, indicating the possibility of experimental observation of this cluster. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 9, 695–699 (10 November 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

UPS spectra of H2O,CH3OH and C5H9OH adsorbed onto Cu(111)/Na and Na(cp)

The present communication presents ultraviolet photoemission spectra (UPS) of three different “alcohols”; water (H₂O), methanol (CH₃OH), and cyclopentanol (C₅H₉OH), chemisorbed onto a Cu(111) surface partially covered by sodium atoms as well as onto closely packed sodium films, a free electron adsorbent. Whereas all three alcohols ROH bind reversibly and associatively to Cu(111) they react with adsorbed sodium atoms to metal bound alcoxides RO. The chemisorption bond, characterized by the interaction between O 2pπ orbitals and metal atoms as an electron donor, the alcoxide being the acceptor, is similar for all groups R. The O 2pπ orbitals shift to higher UPS binding energies with increasing electron density, i.e. decreasing r_s/a_o of the sodium overlayer. Only for HONa, the sterically smallest group R, does the alcoxide growth continue in three dimensions. Although, possibly failing to reproduce the electron density profile of a free electron surface, Hartree-Fock-Slater cluster calculations of small models ROH and RONa₃ enable correlations to be made between UPS intensity peaks and one electron orbitals.     

Structures of Pt clusters on graphene by first-principles calculations

The interactions between Pt_n clusters (n?13) and a graphene sheet have been investigated by first-principles calculations based on density functional theory. For single Pt-atom and Pt₂-dimer adsorptions, the stable adsorption sites are bridge sites between neighboring carbon atoms. When the number of Pt atoms in a cluster increases, the Pt-C interaction energy per contacting Pt atom becomes smaller. For smaller clusters (3?n?7), the adsorption as a vertical planar cluster is more stable than that as parallel planar or three-dimensional (3D) clusters, due to the stability of a planar configuration itself and the stronger planar-edge/graphene interaction, while the adsorption as a parallel planer cluster becomes stable for larger cluster (n?7) via the deformation of the planar configuration so as to attain the planar-edge/graphene contact. For much larger clusters (n?10), the adsorption as a 3D cluster becomes the most stable due to the stability of the 3D configuration itself as well as substantial Pt-C interactions of edge or corner Pt atoms. The interfacial interaction between a Pt cluster and graphene seriously depends on the shape and size of a cluster and the manner of contact on a graphene sheet.     

Positron transport in CF4 and N2/CF4 mixtures

Investigations on freestanding binary and ternary clusters of Fe _x Co _y Ir _z (x + y + z = 5, 6) are carried out using ab initio density functional theory techniques. The geometry, chemical order, binding energy, magnetic moment and electronic structure of the clusters are analyzed for the entire range of composition. Composition dependent structural transition is observed in the five atom clusters, while octahedral geometry prevailed in clusters with six atoms. Both the clusters show increment in binding energy with the increase in number of heterogeneous bonds. Analysis based on the chemical order parameter indicates that clusters favor mixing rather than segregation. The clusters exhibit ferromagnetic ordering and the inter-dependence of optimal cluster geometry to the magnetic moments and electronic structure is observed.     

Structures, stabilities and electronic properties of FePbn (n=1-14) clusters: Density-functional theory investigations

The structures, stabilities and electronic properties of FePb_n (n=1-14) clusters have been studied using the density-functional theory (DFT). Extensive search of the ground-state structures has been carried out by considering a larger number of structural isomers for each cluster size. The Fe atom gradually falls into the interior of the Pb framework as the number of Pb atom increases from 1 to 14. The FePb_n clusters at n=3, 5, 10, 12 have relatively higher stability by analyzing the averaged binding energy and the second-order energy difference. Especially, FePb₁₂ is more stable, owing to its highest symmetrical icosahedron structure. The magnetic moments of FePb_n clusters do not quench when Fe atom is encapsulated in the Pb framework and mostly originate from 3d state of Fe atom.