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.     

First-principles studies on initial growth of Ni on MgO(0 0 1) surface

To elucidate the initial growth of metal on oxide surface, we studied adsorption of small nickel clusters, Ni_n (n = 1-5), on MgO(0 0 1) surface using first-principles method based on density-functional theory. It was found that the preferential adsorption site for an isolated Ni atom is directly above the surface oxygen atom. A strong covalent bond with partial ionic character is formed between the Ni adatom and the surface oxygen atom. Various structures were considered for the Ni_n isomers and 3D structures were found to be energetically more stable than 2D structures for clusters of more than two atoms. For the 2D clusters, metal-metal bonds prevail over metal-substrate bonds with increasing Ni coverage. The calculated work function and ionization energy were found to vary with Ni coverage which is attributed to the change of the surface dipole moment upon metal adsorption, while the evolution of Schottky barrier height at the initial growth stage is dominated by the adatom-induced gap states.     

Dopant dependent stability of Co n TM+ (TM = Ti,V, Cr,and Mn) clusters

In this paper, we present a photofragmentation study of mass-selected transition metal-doped cobalt cluster cations Co _n TM⁺  (n = 8–18, TM = Ti, V, Cr, and Mn). Time-of-flight spectra recorded after laser excitation of mass-selected clusters in the gas phase show that the evaporation of a cobalt atom is the most facile dissociation channel for clusters with TM = Ti and V, suggesting an enhanced stability of the doped clusters compared to the bare ones. In contrast, for Co _n TM⁺ with TM = Cr and Mn, the loss of the dopant atom is found to be the preferred dissociation channel. Co₁₃Cr⁺ is a notable exception and favors dissociation by loss of a neutral Co atom. It is implied that substituting Mn and Cr generally destabilizes the cobalt clusters with the exception of Co₁₂Cr⁺, which is relatively more stable than Co ₁₃ ⁺ . Additional measurements of V _n Co⁺ (n = 9–16) show that the loss of a Co atom is still the most facile dissociation channel, which is in agreement with the predicted stronger V?V bond compared to the V?Co one.     

Metastable fragmentation of silver bromide clusters

The abundance spectra and the fragmentation channels of silver bromide clusters have been measured and analyzed. The most abundant species are Ag_nBr_{n - 1} ⁺ and Ag_nBr_{n + 1} ^- and Ag₁₄Br₁₃ ⁺ is a magic number, revealing their ionic nature. However, some features depart from what is generally observed for alkali-halide ionic clusters. From a certain size, Ag_nBr_{n - 1} ⁺ is no more the main series, and Ag_nBr _{n - 2, 3} ⁺ series become almost as important. The fast fragmentation induced by a UV laser makes the cations lose more bromine than silver ions and lead to more silver-rich clusters. Negative ions mass spectra contain also species with more silver atoms than required by stoichiometry. We have investigated the metastable fragmentation of the cations using a new experimental method. The large majority of the cations release mainly a neutral Ag₃Br₃ cluster. These decay channels are in full agreement with our recent ab initio DFT calculations, which show that Ag⁺-Ag⁺ repulsion is reduced due to a globally attractive interaction of their d orbitals. This effect leads to a particularly stable trimer (AgBr)₃ and to quasi-planar cyclic structures of (AgBr)_n clusters up to n = 6. We have shown that these two features may be extended to other silver halides, to silver hydroxides (AgOH)_n, and to cuprous halide compounds. Received 9 November 2000 and Received in final form 25 January 2001     

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.     

First-principles study on the structure,electronic and magnetic properties of HoSin （n=1-12, 20） clusters

The structure, electronic and magnetic properties of HoSin（n= 1 - 12, 20） clusters have been widely investigated by first-principles calculation method based on density flmctional theory （DFT）. From our calculation results, we find that for HoSin（n=1- 12） clusters except n = 7.10, the most stable structures are a replacement of Si atom in the corresponding pure Sin＋1 clusters by Ho atom. The doping of Ho atom makes the stability of Si clusters enhance remarkably, and HoSin（n = 2, 5, 8, 11） clusters are more stable than their neighboring clusters. The magnetic moment of Ho atom in HoSin （n = 1 - 12, 20） clusters mainly comes from of electron of tto, and never quenches.     

Structural, electronic and magnetic properties of Mn, Co, Ni in Gen for (n=1-13)

The structural, electronic and magnetic properties of TMGe_n (TM=Mn, Co, Ni; n=1-13) have been investigated using spin polarized density functional theory. The transition metal (TM) atom prefers to occupy surface positions for n<9 and endohedral positions for n≥9. The critical size of the cluster to form endohedral complexes is at n=9, 10 and 11 for Mn, Co and Ni respectively. The binding energy of TMGe_n clusters increases with increase in cluster size. The Ni doped Ge_n clusters have shown higher stability as compared to Mn and Co doped Ge_n clusters. The HOMO-LUMO gap for spin up and down electronic states of Ge_n clusters is found to change significantly on TM doping. The magnetic moment in TMGe_n is introduced due to the presence of TM. The magnetic moment is mainly localized at the TM site and neighbouring Ge atoms. The magnetic moment is quenched in NiGe_n clusters for all n except for n=2, 4 and 8.     

密度泛函理论对ZrnPd团簇结构和性质的研究

用B3LYP/LANL2DZ方法对Zr_nPd(n =1—13)团簇的平衡几何结构、能量、频率、电子性质和磁性进行了计算.研究表明,Pd原子位于表面的异构体更为稳定,其中Zr₇Pd,Zr₁₂Pd团簇稳定性高,是幻数团簇,此外,相对于Zr_nCo与Zr_nFe团簇,Zr_nPd团簇参与化学反应的能力较弱,化学稳定性更     

Specific features of the luminescence of silicate glasses with silver introduced by ion exchange

The luminescence spectra of silicate glasses with silver introduced by ion exchange have been investigated. It is shown that silver introduced into glass by ion exchange exists not only in the form of ions, but also as neutral atoms and charged and neutral molecular clusters, which provide intense luminescence in the visible spectral range. Cerium ions in glass facilitate the formation of neutral molecular silver clusters, due to which the luminescence intensity increases. It is shown that Ag _n -Ce ^x+ complexes can be formed in glass containing cerium ions and neutral molecular silver clusters.     

Structural and electronic properties of Snn-1Pb and Pbn-1Sn clusters: a theoretical investigation through first principles calculations

Here we report a systematic theoretical study of the structure and electronic properties of Sn_n-1Pb and Pb_n-1Sn (n = 2-13) clusters and compare these results with pure Sn_n and Pb_n to understand the influence of the dopant elements. The calculations were carried out using the density functional theory with generalized gradient approximation for the exchange-correlation potential. Extensive search based on large number of initial configurations has been carried out to locate the stable isomers of Sn_n-1Pb and Pb_n-1Sn (n = 2-13) clusters. The relative stability of Sn_n-1Pb and Pb_n-1Sn (n = 2-13) clusters is analyzed based on the calculated binding energies and second difference in energy. The stability analysis of these clusters suggests that, while the substitution of Sn by Pb lowers the stability of Sn_n clusters, presence of Sn enhances the stability of the Pb_n clusters. The results suggest that while for Sn_n-1Pb, n=4, 7, 10, 12 clusters are more stable than their respective neighbors, Pb_n-1Sn clusters with n = 4, 7 and 9 are found to be more stable. Based on the fragmentation pattern it is seen that for Sn_n-1Pb and Pb_n-1Sn clusters favor monomer evaporation of the Pb atom up to n =11 and n =12, respectively. Unlike this trend, the Sn₁₁Pb undergoes fission type fragment into Sn₅Pb and Sn₆ clusters. A comparison between our theoretical results and surface induced dissociation experiment shows good agreement, which gives confidence on the prediction of the ground state geometries.     

Geometries and electronic properties of NbnV(0, ±1) (n = 1−6) clusters studied by density-functional theory

Using density functional theory (DFT) with valence basis set LANL2TZ to study the relative stabilities and electronic properties of the most stable structures of Nb _n V^(0,?±1) (n = 1?6) clusters. The ground state structures of Nb _n V ^(0,?±1) keep the similar geometric structure as the host Nb _n clusters. The doping of vanadium atom enhances the chemical activities of Nb _n clusters. The Nb₃V and Nb₆V are more stable than other clusters. The average binding energy of charged systems (Nb _n V⁺ and Nb _n V^? clusters) are generally larger than neutral Nb _n V clusters natural population analysis shows that there are charge transfers from niobium to vanadium atoms in the small Nb_1?4V, however, for larger clusters (Nb₅V and Nb₆V), the charge transfers are from vanadium to niobium atoms. The vertical and adiabatic ionization potentials (VIP and AIP) are estimated and the vertical one is more close to experimental results.     

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.     

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.     

The stabilization mechanism of titanium cluster

A systematic and comparative theoretical study on the stabilization mechanism of titanium cluster has been performed by selecting the clusters Ti_n (n=3, 4, 5, 7, 13, 15 and 19) as representatives in the framework of density-functional theory. For small clusters Ti_n (n=3, 4 and 5), the binding energy gain due to spin polarization is substantially larger than that due to structural distortion. For medium clusters Ti₁₃ and Ti₁₅, both have about the same contribution. For Ti_n (n=4, 5, 13 and 15), when the undistorted high symmetric structure with spin-polarization is changed into the lowest energy structure, the energy level spelling due to distortion fails to reverse the level order of occupied and unoccupied molecular orbital (MO) of two type spin states, the spin configuration remains unchanged. In spin restricted and undistorted high symmetric structure, d orbitals participate in the hybridization in MOs, usually by way of a less distorted manner, and weak bonds are formed. In contrast, d orbitals take part in the formation of MOs in the ground state structure, usually in a distorted manner, and strong covalent metallic bonds are formed.     

DFT study of size-dependent geometries,stabilities and electronic properties of Si2Agn clusters: comparison with pure silver clusters

The equilibrium geometric structures, stabilities and electronic properties of Si₂Ag_n (n = 1–8) and pure silver Ag_n (n ≤ 10) clusters have been systematically investigated by using meta-generalised gradient approximation. Due to sp³ hybridisation, the lowest energy structures of Si₂Ag_n clusters for n > 2 favour the three-dimensional structure. The silicon atoms prefer to be located at the surface of the host silver clusters. By analysing the relative stabilities, it is found that the di-bridged structure Si₂Ag₂ isomer is the most stable structure for Si₂Ag_n (n = 1–8) clusters. The highest occupied–lowest unoccupied molecular orbital gaps, exhibiting a pronounced even–odd alternation, indicate that the doped clusters with even number of atoms have enhanced chemical stability than those with odd number of atoms. The results of Wiberg bond indices and electronic localisation function show that the stronger Si–Si and Si–Ag interaction may be the main driving force for the higher stability of Si₂Ag_n clusters.     

Effect of alloying on the stabilities and catalytic properties of Pt–Au bimetallic subnanoclusters: a theoretical investigation

Density functional theory (DFT) has been applied to study the geometrical and electronic structures and the catalytic properties for NO oxidation of pure Pt and PtAu clusters. The calculated results suggest that Pt₁₀ clusters shows the most stable structure among the pure Pt _n (n = 2–13) clusters with the local maximum Δ₂ E value. The doping of Au atoms reduces the stability of the clusters, and Pt₆Au₄ cluster has the most stable structure among Pt_10?n Au _n (n = 1–7) clusters, due to the closest band centers between Pt and Au atoms (0.83 eV) and the obvious s–p resonance peaks near the Fermi level. Pt₆Au₄ cluster displays the strongest activation of O₂ molecules among Pt_10?n Au _n (n = 0–7) clusters, owing to the clear overlap between O 2p and Pt 6 s and Au 6 s near the Fermi level, and the more positive d band center than the others. The interaction between NO and metals changes slightly in NO/Pt_10-nAu_n (n = 2–7) systems, which is weaker than that in NO/Pt₉Au system, as a result of the decreasing resonance peaks of sp hybridization near the Fermi level. Compared to pure Pt₁₀ cluster, the lower energy barriers and larger reaction energies on Pt₆Au₄ cluster suggest a higher catalytic activity of PtAu cluster for the O₂ dissociation and NO oxidation reactions. Our study provides atomic-scale insights into the nature of the interfacial effect that determines NO oxidation on PtAu cluster catalysts.     

Ga_nN_3(n=1～8)团簇几何结构及光电子能谱的研究

用密度泛函理论的B3LYP方法在6-31G*的水平上,对GanN3(n=1～8)团簇的结构进行优化,并对体系的成键特性、光电子能谱及稳定性进行了计算与分析,得到了GanN3(n=1～8)团簇的最稳定结构.结果表明,当n≤5时,其基态几何结构为平面结构,N-N键在这些团簇的形成过程中起着决定性的作用;当n≥6时,其基态几何结构为立体结构,Ga-N键起主导作用;在所研究的团簇中,Ga4N3、Ga7N3的基态结构最稳定;随着n值的增大,平均极化率逐渐增强;通过对光电子能谱的分析,得到Ga-N键的振动频率与六方晶系纤锌矿结构GaN的光学声子峰值相近.     

Synthesis and characterization of gold atomic clusters by the two-phase method

In this work we present a simple procedure for the production of gold atomic clusters by means of a modification of the Brust’s two-phase method, using tetrahexylammonium bromide as capping agent. The gold atomic clusters obtained have the following formula: Au_n, with $3 \leq \textit{n} \leq 13$ , where n is the number of gold atoms per cluster. We report the formation of different species at different stages of the reduction reaction. Au clusters were characterized by TEM, MALDI-TOF mass spectrometry and UV-vis spectroscopy. Gold clusters are stable in solution after several years of preparation.     

密度泛函理论研究ZrnCo(n=1—13)团簇的结构和磁性

利用密度泛函理论中的广义梯度近似对Zr_nCo(n=1—13)团簇进行了结构优化、能量和频率的计算,研究了Zr_nCo团簇的平衡几何结构、稳定性、电子性质和磁性.结果表明：Zr₄Co,Zr₇Co,Zr₉Co和Zr₁₂Co团簇的基态稳定性较高,是幻数团簇,尤其是Zr₁₂Co团簇基态为I_{h< 关键词： n}Co团簇')" href="#">Zr_nCo团簇 平衡几何结构 稳定性和磁性     

Structures and its evolution of Ba n ( n =2 ∼ 14) clusters

We have studied the atomic structure and the electronic properties of Ba_n clusters by the ab initio molecular dynamics method. We find that a structural transition to the bulk-like structure begins at Ba₉ cluster, and the structures of the clusters are transferred to be icosahedral-like around n = 13. The relatively high stability for Ba₄, Ba₁₀ and Ba₁₃ clusters are observed. Received 1st December 2000