Structures of bimetallic clusters

We report an optimization algorithm for studying bimetallic nanoclusters. The algorithm combines two state-of-the-art methods, the genetic algorithm and the basin hopping approach, widely employed in the literature for predicting structures of pure metallic and nonmetallic clusters. To critically test the present algorithm and its use in determining the lowest-energy structures of bimetallic nanoclusters, we apply it to study the bimetallic clusters Cu(n)Au(38-n) (0< or =n< or =38). It is predicted that the Au atoms, being larger in size than the Cu atoms, prefer to occupy surface sites showing thus the segregating behavior. As the atom fraction of Cu increases, the bimetallic cluster Cu(n)Au(38-n), as a whole, first takes on an amorphous structure and is followed by dramatic changes in structure with the Cu atoms revealing hexagonal, then assuming pentagonal, and finally shifting to octahedral symmetry in the Cu-rich range.     

Optimization of bimetallic Cu–Au and Ag–Au clusters by using a modified adaptive immune optimization algorithm

A modified adaptive immune optimization algorithm (AIOA) is designed for optimization of Cu–Au and Ag–Au bimetallic clusters with Gupta potential. Compared with homoatom clusters, there are homotopic isomers in bimetallic cluster, so atom exchange operation is presented in the modified AIOA. The efficiency of the algorithm is tested by optimization of Cu_nAu_38‐n (0 ≤ n ≤ 38). Results show that all the structures with the putative global minimal energies are successfully located. In the optimization of Ag_nAu_55‐n (0 ≤ n ≤ 55) bimetallic clusters, all the structures with the reported minimal energies are obtained, and 36 structures with even lower potential energies are found. On the other hand, with the optimized structures of Cu_nAu_55‐n, it is shown that all 55‐atom Cu–Au bimetallic clusters are Mackay icosahedra except for Au₅₅, which is a face‐centered cubic (fcc)‐like structure; Cu₅₅, Cu₁₂Au₄₃, and Cu₁Au₅₄ have two‐shell Mackay icosahedral geometries with I_h point group symmetry. © 2009 Wiley Periodicals, Inc. J Comput Chem 2009     

Structural optimization of Cu-Ag-Au trimetallic clusters by adaptive immune optimization algorithm

The putative global minimum structures of Cu-Ag-Au trimetallic clusters with 19 and 55 atoms are obtained by adaptive immune optimization algorithm (AIOA) with the Gupta potential. For the 19-atom trimetallic clusters, the results indicate that all of them have double-icosahedral motifs. For the optimized structures of Cu(13)Ag(n)Au(42-n) (n = 1-41), the clusters can be categorized into 19 Mackay icosahedral structures, 1 6-fold pancake structure, and 21 ring-like structures linked by three face-sharing double-icosahedra. Furthermore, the segregation phenomena of the Cu, Ag, and Au atoms in the Cu-Ag-Au trimetallic clusters are studied to provide useful information for geometric character. Results show that Cu and Ag atoms prefer to locate in the inner-shell and on the surface, respectively, whereas Au atoms mainly locate in the middle-shell and tend to solve into Cu and Ag atoms.     

Equilibrium geometries, stabilities, and electronic properties of the bimetallic M2-doped Au(n) (M = Ag, Cu; n = 1-10) clusters: comparison with pure gold clusters

The density functional method with relativistic effective core potential has been employed to investigate systematically the geometrical structures, relative stabilities, growth-pattern behaviors, and electronic properties of small bimetallic M(2)Au(n) (M = Ag, Cu; n = 1-10) and pure gold Au(n) (n ≤ 12) clusters. The optimized geometries reveal that M(2) substituted Au(n+2) clusters and one Au atom capped M(2)Au(n-1) structures are dominant growth patterns of the stable alloyed M(2)Au(n) clusters. The calculated averaged atomic binding energies, fragmentation energies, and the second-order difference of energies as a function of the cluster size exhibit a pronounced even-odd alternation phenomenon. The analytic results exhibit that the planar structure Ag(2)Au(4) and Cu(2)Au(2) isomers are the most stable geometries of Ag(2)Au(n) and Cu(2)Au(n) clusters, respectively. In addition, the HOMO-LUMO gaps, charge transfers, chemical hardnesses and polarizabilities have been analyzed and compared further.     

Structural and energetic properties of Ni-Cu bimetallic clusters

The lowest-energy structures for all compositions of Ni n Cu m bimetallic clusters with N = n + m up to 20 atoms, N = 23, and N = 38 atoms have been determined using a genetic algorithm for unbiased structure optimization in combination with an embedded-atom method for the calculation of the total energy for a given structure. Comparing bimetallic clusters with homoatomic clusters of the same size, it is shown that the most stable structures for each cluster size are composed entirely of Ni atoms. Among the bimetallic clusters in the size range N = 2-20, the Ni N-1 Cu 1 clusters possess the highest stability. Further, it has been established that most of the bimetallic cluster structures have geometries similar to those of pure Ni clusters. The size N = 38 presents a special case, as the bimetallic clusters undergo a dramatic structural change with increasing atom fraction of Cu. Moreover, we have identified an icosahedron, a double, and a triple icosahedron with one, two, and three Ni atoms at the centers, respectively, as particularly stable structures. We show that in all global-minimum structures Ni atoms tend to occupy mainly high-coordination inner sites, and we confirm the segregation of Cu on the surface of Ni-Cu bimetallic clusters predicted in previous studies. Finally, it is observed that, in contrast to the bulk, the ground-state structures of the 15-, 16-, and 17-atom bimetallic clusters do not experience a smooth transition between the structures of the pure copper and the pure nickel clusters as a function of the relative number of the two types of atoms. For these sizes, the concentration effect on energy is more important than the geometric one.     

Determination of structures, stabilities, and electronic properties for bimetallic cesium-doped gold clusters: a density functional theory study

The equilibrium geometric structures, stabilities, and electronic properties of bimetallic Au(n)Cs (n = 1-10) and pure gold Au(n) (n ≤ 11) clusters have been systematically investigated by using density functional theory with meta-generalized gradient approximation. The optimized geometries show that one Au atom capped on Au(n-1)Cs structures and Cs atom capped Au(n) structures for different sized Au(n)Cs (n = 1-10) clusters are two dominant growth patterns. Theoretical calculated results indicate that the most stable isomers have three-dimensional structures at n = 4 and 6-10. Averaged atomic binding energies, fragmentation energies, and second-order difference of energies exhibit a pronounced even-odd alternations phenomenon. The same even-odd alternations are found in the highest occupied-lowest unoccupied molecular orbital gaps, vertical ionization potential, vertical electron affinity, and hardnesses. In addition, it is found that the charge in corresponding Au(n)Cs clusters transfers from the Cs atom to the Au(n) host in the range of 0.851-1.036 electrons.     

Structural, electronic, bonding, magnetic, and optical properties of bimetallic [Ru(n)Au(m)](0/+) (n + m ≤ 3) clusters

The structural, electronic, bonding, magnetic, and optical properties of bimetallic [Ru(n)Au(m)](0/+) (n + m ≤ 3; n, m = 0-3) clusters were computed in the framework of the density functional theory (DFT) and time-dependent DFT (TD-DFT) using the full-range PBE0 non local hybrid GGA functional combined with the Def2-QZVPP basis sets. Several low-lying states have been investigated and the stability of the ground state spinomers was estimated with respect to all possible fragmentation schemes. Molecular orbital and population analysis schemes along with computed electronic parameters illustrated the details of the bonding mechanisms in the [Ru(n)Au(m)](0/+) clusters. The TD-DFT computed UV-visible absorption spectra of the bimetallic clusters have been fully analyzed and compared to those of pure gold and ruthenium clusters. Assignments of all principal electronic transitions are given and interpreted in terms of contribution from specific molecular orbital excitations.     

Bimetallic clusters Pt6Au: geometric and electronic structures within density functional theory

Within density functional theory at the general gradient approximation for exchange and correlation (BPW91) and the relativistic 19-electron Los Alamos National Laboratory effective core pseudopotentials and basis sets (3s3p2d), the geometric and electronic structures of Pt(6)Au bimetallic clusters have been studied in detail in comparison with Pt(7). A total of 38 conformations for Pt(6)Au are located. The most stable conformation for Pt(6)Au is a sextet with an edge- and face-capped trigonal bipyramid, in which the Au atom caps an edge of the trigonal bipyramid. Pt(6)Au, in general, prefers a three-dimensional geometry and high spin electronic state with multireference character. The electronic impact of the doping of Au in Pt clusters on the overall chemical activity of the doped bimetallic cluster is not as significant as that of the doping of Pt in Au clusters; however, the doping of Au lowers the chemical activity, thus enhancing the chemoselectivity in the gas phase, of PtAu bimetallic clusters.     

Density functional study of structural, electronic, and optical properties of small bimetallic ruthenium-copper clusters

The structural, electronic, bonding, magnetic, and optical properties of bimetallic [Cu(n)Ru(m)](+/0/-) (n + m ≤ 3; n, m = 0-3) clusters were computed in the framework of the density functional theory (DFT) and time-dependent DFT (TD-DFT) using the full-range PBE0 nonlocal hybrid GGA functional combined with the Def2-QZVPP basis sets. Several low-lying states have been investigated and the stability of the ground state spinomers was estimated with respect to all possible fragmentation schemes. Molecular orbital and population analysis schemes along with computed electronic parameters illustrated the details of the bonding mechanisms in the [Cu(n Ru(m)](+/0/-) clusters. The TD-DFT computed UV-visible absorption spectra of the bimetallic clusters have been fully analyzed and assignments of all principal electronic transitions were made and interpreted in terms of contribution from specific molecular orbital excitations.     

Structures and relative stability of neutral gold clusters: Aun (n=15-19)

We performed a global-minimum search for low-lying neutral clusters (Au(n)) in the size range of n=15-19 by means of basin-hopping method coupled with density functional theory calculation. Leading candidates for the lowest-energy clusters are identified, including four for Au(15), two for Au(16), three for Au(17), five for Au(18), and one for Au(19). For Au(15) and Au(16) we find that the shell-like flat-cage structures dominate the population of low-lying clusters, while for Au(17) and Au(18) spherical-like hollow-cage structures dominate the low-lying population. The transition from flat-cage to hollow-cage structure is at Au(17) for neutral gold clusters, in contrast to the anion counterparts for which the structural transition is at Au(16) (-) [S. Bulusu et al., Proc. Natl. Acad. Sci. U.S.A. 103, 8362 (2006)]. Moreover, the structural transition from hollow-cage to pyramidal structure occurs at Au(19). The lowest-energy hollow-cage structure of Au(17) (with C(2v) point-group symmetry) shows distinct stability, either in neutral or in anionic form. The distinct stability of the hollow-cage Au(17) calls for the possibility of synthesizing highly stable core/shell bimetallic clusters M@Au(17) (M=group I metal elements).     

Study of Structures and Electronic Properties of Pdn-1Pb and Pdn(n≤8) Clusters

Geometric and electronic properties of Pdn-1Pb and Pdn(n≤8) clusters have been studied by using density functional theory with effective core potentials, focusing on the differences between mono- and bimetallic clusters. The average bond length of Pdn-1Pb (n≤8) bimetallic clusters is longer than that of pure palladium clusters except for n = 2 and 3. The most stable structure of Pdn-1Pb (n≤7) is the singlet where there is at least a Pd or Pb atom on its excited state. The energy gaps of Pd-Pb binary clusters are narrower than those of Pdn clusters, and then the chemical activity is strengthened when Pdn clusters are doped with Pb.     

Geometries, stabilities, and electronic properties of Pt-group-doped gold clusters, their relationship to cluster size, and comparison with pure gold clusters

A systematic study of bimetallic Au(n)M(2) (n = 1-6, M = Ni, Pd, and Pt) clusters is performed by using density functional theory at the B3LYP level. The geometric structures, relative stabilities, HOMO-LUMO gaps, natural charges and electronic magnetic moments of these clusters are investigated, and compared with pure gold clusters. The results indicate that the properties of Au(n)M(2) clusters for n = 1-3 diverge more from pure gold clusters, while those for n = 4-6 show good agreement with Au(n) clusters. The dissociation energies, the second-order difference of energies, and HOMO-LUMO energy gaps, exhibiting an odd-even alternation, indicate that the Au(4)M(2) clusters are the most stable structures for Au(n)M(2) (n = 1-6, M = Ni, Pd, and Pt) clusters. Moreover, we predict that the average atomic binding energies of these clusters should tend to a limit in the range 1.56-2.00 eV.     

Cun、Agn和Aun(n≤9)团簇的静电极化率

The static polarizabilities and polarizability anisotropies of Cun,Agn and Aun (n≤9)clusters have been calculated by the B3LYP density functional method,which is a three parameter mixture of density functional and"exact" Hartree Fock exchange. The calculated results are compared with experimental polarizabilities of sodium clusters. It is shown that the size dependency of the static polarizabilities per atom of Cun,and Agn clusters possesses the same trend as that observed in sodium clusters exception of the Aunclusters while the polarizability of Au atom is much smaller than these of Cu and Ag.The(α-)of Au atom is the smallest and the(α-)per atom of Au approach to the values of Cu from the dimmer to the hexamer. It indicates that in Au clusters the electrons are more strongly attracted by the nuclei because of the more electrons. However,the absolute polarizabilities of the noble mental clusters are considerably smaller than those of the sodium clusters and the electronic structures of the noblemental are much more compact.     

小尺寸金属团簇熔化过程的分子动力学模拟

运用分子动力学方法模拟了小尺寸金属团簇的熔化过程, 原子之间的作用采用嵌入原子法(EAM)模型, 计算了均方根键长涨落δ随温度的变化, 以及升温过程中团簇热容的变化. 包含55、56个原子的面心立方(FCC)结构Au团簇的熔化过程是基本相同的. 而同样结构和数目Cu团簇的熔化过程却呈现出不同的趋势. Cu55、Cu56在模拟过程中都出现了FCC结构到二十面体结构的转变. 但由于表面多出了一个原子, Cu56的热容曲线比Cu55多了一个峰, 体系出现了预熔化现象. 这表明小尺寸团簇的固液转变的过程与团簇的原子类型、几何结构和原子数目密切相关.     

Formation and emission of gold and silver carbide cluster ions in a single C60- surface impact at keV energies: experiment and calculations

Impact of fullerene ions (C(60)(-)) on a metallic surface at keV kinetic energies and under single collision conditions is used as an efficient way for generating gas phase carbide cluster ions of gold and silver, which were rarely explored before. Positively and negatively charged cluster ions, Au(n)C(m)(+) (n = 1-5, 1 ≤ m ≤ 12), Ag(n)C(m)(+) (n = 1-7, 1 ≤ m ≤ 7), Au(n)C(m)(-) (n = 1-5, 1 ≤ m ≤ 10), and Ag(n)C(m)(-) (n = 1-3, 1 ≤ m ≤ 6), were observed. The Au(3)C(2)(+) and Ag(3)C(2)(+) clusters are the most abundant cations in the corresponding mass spectra. Pronounced odd/even intensity alternations were observed for nearly all Au(n)C(m)(+/-) and Ag(n)C(m)(+/-) series. The time dependence of signal intensity for selected positive ions was measured over a broad range of C(60)(-) impact energies and fluxes. A few orders of magnitude immediate signal jump instantaneous with the C(60)(-) ion beam opening was observed, followed by a nearly constant plateau. It is concluded that the overall process of the fullerene collision and formation∕ejection of the carbidic species can be described as a single impact event where the shattering of the incoming C(60)(-) ion into small C(m) fragments occurs nearly instantaneously with the (multiple) pickup of metal atoms and resulting emission of the carbide clusters. Density functional theory calculations showed that the most stable configuration of the Au(n)C(m)(+) (n = 1, 2) clusters is a linear carbon chain with one or two terminal gold atoms correspondingly (except for a bent configuration of Au(2)C(+)). The calculated AuC(m) adiabatic ionization energies showed parity alternations in agreement with the measured intensity alternations of the corresponding ions. The Au(3)C(2)(+) ion possesses a basic Au(2)C(2) acetylide structure with a π-coordinated third gold atom, forming a π-complex structure of the type [Au(π-Au(2)C(2))](+). The calculation shows meaningful contributions of direct gold-gold bonding to the overall stability of the Au(3)C(2)(+) complex.     

包含两个平面五配位碳原子C_(2+n)B_(10-n)(n=0～10)团簇的结构的理论研究

采用B3LYP/6-311+G**方法,我们优化了初始构型中包含两个平面五配位碳原子(ppCs)的C2+nB10-n(n=0～10)团簇的结构并计算了它们的振动频率.计算结果表明,C2+nB10-n(n=0～2)团簇是稳定的,而且这三个结构中ppC—B键的Wiberg键级介于0.511～0.909之间,ppC—C键的Wiberg键级为0.2254(n=1)和0.8586(n=2),ppC的键级介于3.778到3.879之间,即这三个结构中存在两个ppCs,而且ppC遵循八隅规则;C2+nB10-n(n=3～6)团簇的最稳定结构包含一个ppC;C2+nB10-n(n6)团簇能量最低结构中不存在ppC.而且只有团簇C2+nB10-n(n=0～2)中没有悬键,它们的π电子数分别为:6,7和8,计算它们的NICS(0)值表明强芳香性一般位于局部的三元环中心,表明局部离域有利于平面结构的形成.C2+nB10-n(n=0～2)团簇的第一垂直激发能分别为:1.91,0.56和3.12eV.     

Copper doping of small gold cluster cations: influence on geometric and electronic structure

The effect of Cu doping on the properties of small gold cluster cations is investigated in a joint experimental and theoretical study. Temperature-dependent Ar tagging of the clusters serves as a structural probe and indicates no significant alteration of the geometry of Au(n) (+) (n = 1-16) upon Cu doping. Experimental cluster-argon bond dissociation energies are derived as a function of cluster size from equilibrium mass spectra and are in the 0.10-0.25 eV range. Near-UV and visible light photodissociation spectroscopy is employed in conjunction with time-dependent density functional theory calculations to study the electronic absorption spectra of Au(4-m)Cu(m) (+) (m = 0, 1, 2) and their Ar complexes in the 2.00-3.30 eV range and to assign their fragmentation pathways. The tetramers Au(4) (+), Au(4) (+)[middle dot]Ar, Au(3)Cu(+), and Au(3)Cu(+)[middle dot]Ar exhibit distinct optical absorption features revealing a pronounced shift of electronic excitations to larger photon energies upon substitution of Au by Cu atoms. The calculated electronic excitation spectra and an analysis of the character of the optical transitions provide detailed insight into the composition-dependent evolution of the electronic structure of the clusters.     

Chromatographic isolation of "missing" Au55 clusters protected by alkanethiolates

We report on the first synthesis of alkanethiolate-protected Au55 (11 kDa), which has been a "missing" counterpart of Schmid's Au55(PR3)12Cl6. Au:SCx clusters (x = 12, 18) were prepared by the reaction of alkanethiol (CxSH) with polymer-stabilized Au clusters ( approximately 1.3 nm) and subsequently incubated in neat CxSH. The resulting clusters were successfully fractionated by recycling gel permeation chromatography into Au approximately 38:SCx and Au approximately 55:SCx and identified by laser-desorption ionization mass spectrometry. The Au approximately 55:SCx clusters exhibited structured optical spectra, suggesting molecular-like properties. The thiolate monolayers were found to be liquid-like on the basis of the IR spectrum and the monolayer thickness, which was estimated from the hydrodynamic diameter.     

Growth format, electronic architecture, magnetic, and optical properties of aromatic cyclo-Cu3Au3 homotops

Bimetallic Cu(3)Au(3) clusters have been investigated using electronic structure calculation techniques (DFT) to understand their electronic, magnetic, and optical properties as well as the geometrical structures. The most stable homotop is the planar cyclo-[Cu(3)(micro-Au)(3)] form consisting of a triangular positively charged Cu(3) structural core with negatively charged Au atoms occupying exposed positions. This structure is characterized by the maximum number of heterobonds and peripheral positions of Au atoms. Possible growth formats of the cyclo-[Cu(3)(micro-Au)(3)] homotops have been explored following both the edge-capping and the stepwise metal atom substitution mechanism. The bonding pattern along with the density of states (DOS) plots of the cyclo-[Cu(3)(micro-Au)(3)] homotop are thoroughly analyzed and compared with those of the pure cyclo-[Cu(3)(micro-Cu)(3)] and cyclo-[Au(3)(micro-Au)(3)] clusters. Particular attention was paid on the stability of these bimetallic clusters in relation with the ring-shaped electron density distribution (aromaticity). It was found that all 3-membered metal rings exhibit significant aromatic character, which was verified by a number of established criteria of aromaticity, such as structural, energetic, magnetic (NICS profiles), and out-of-plane ring deformability criteria. The NICS (1) values correlate well with the out-of-plane ring deformation energy. Finally, a comprehensive analysis of the optical spectra of the CuAu, Cu(2), and Au(2) diatomics and the cyclo-[Cu(3)(micro-Au)(3)], cyclo-[Cu(3)(micro-Cu)(3)], and cyclo-[Au(3)(micro-Au)(3)] clusters placed the electronic assignments of the optical transitions on a firm footing.     

Synthesis, molecular structure and properties of the [H6-nNi30C4(CO)34(CdCl)2]n- (n=3-6) bimetallic carbide carbonyl cluster: a model for the growth of noncompact interstitial metal carbides

Reaction of the [Ni(9)C(CO)(17)](2-) dianion with CdCl(2)2.5 H(2)O in THF affords the novel bimetallic Ni--Cd carbide carbonyl clusters [H(6-n)Ni(30)C(4)(CO)(34)(micro(5)-CdCl)(2)](n-) (n=3-6), which undergo several protonation-deprotonation equilibria in solution depending on the basicity of the solvent or upon addition of acids or bases. Although the occurrence in solution of these equilibria complicates the pertinent electrochemical studies on their electron-transfer activity, they clearly indicate that the clusters [H(6-n)Ni(30)C(4)(CO)(34)(micro(5)-CdCl)(2)](n-) (n=3-6), as well as the structurally related [H(6-n)Ni(34)C(4)(CO)(38)](n-) (n=4-6), undergo reversible or partially reversible redox processes and provide circumstantial and unambiguous evidence for the presence of hydrides for n=3, 4 and 5. Three of the [H(6-n)Ni(30)C(4)(CO)(34)(micro(5)-CdCl)(2)](n-) anions (n=4-6) have been structurally characterized in their [NMe(3)(CH(2)Ph)](4)[H(2)Ni(30)C(4)(CO)(34)(CdCl)(2)]2 COMe(2), [NEt(4)](5)[HNi(30)C(4)(CO)(34)(CdCl)(2)]2 MeCN and [NMe(4)](6)[Ni(30)C(4)(CO)(34)(CdCl)(2)]6 MeCN salts, respectively. All three anions display almost identical geometries and bonding parameters, probably because charge effects are minimized by delocalization over such a large metal carbonyl anion. Moreover, the Ni(30)C(4) core in these Ni-Cd carbide clusters is identical within experimental error to those present in the [HNi(34)C(4)(CO)(38)](5-) and [Ni(35)C(4)(CO)(39)](6-) species, suggesting that the stepwise assembly of their nickel carbide cores may represent a general pathway of growth of nickel polycarbide clusters. The fact that the [H(6-n)Ni(30)C(4)(CO)(34)(micro(5)-CdCl)(2)](n-)(n=4-6) anions display two valence electrons more than the structurally related [H(6-n)Ni(34)C(4)(CO)(38)](n-) (n=4-6) species has been rationalized by extended Hückel molecular orbital (EHMO) analysis.