Large-Scale Molecular Dynamics Simulations of Energetic Ni Nanocluster Impact onto the Surface

On the atomic scale, Molecular Dynamics (MD) Simulation of Nano Ni cluster impact on Ni (100) substrate surface have been carried out for energies of E _a = 1–5 eV/atom and total energy of E _T = 195 eV (the total energy of cluster is E _T = nE _a, n is the number of cluster atoms) to understand quantitatively the interaction mechanisms between the cluster atoms and the substrate atoms. The many-body Embedded Atom Method (EAM) was used in this simulation. We investigated the maximum substrate temperature T _max and the time t _max within which this temperature is reached as a function of cluster sizes and the total energy E _T. The temperature T _max is linearly proportional to total cluster energy. For the constant energy per atom and for the cluster size increase, the correlated collisions rapidly transfers energy to the substrate, and the time t _max approached a constant value. For constant total energy the temperature T _max and the time t _max versus different cluster sizes was studied. We showed that the cluster implantation and sputtering atoms from the surface are affected by the cluster size and total kinetic energy of the clusters. Finally time dependence of the number N _dis of disordered atoms in the substrate was observed.     

Electronic and atomic structure of Na,Mg, Al and Pb clusters

Density functional theory is used to study the electronic and atomic structure of small clusters of Na, Mg, Al and Pb. We study the quantityE _N?1–E _N , which has relevance to the processes of cluster growth and evaporation (E _N is the total energy of the cluster withN atoms). By comparing the results of the jellium model with those of a more realistic model (although still simple) we are able to appreciate “structural” effects beyond the “electronic-shell effects” which form the essence of the predictions of the jellium model. The calculations predict formation of atomic shells and appreciable reconstruction as the cluster grows.     

Magic numbers in the mass distribution of the benzene+-argonn ions: evaporation dynamics and cluster structure

The intensity distribution of benzene⁺-Ar_n cluster ions formed by laser ionization of neutral clusters has been investigated: two main intensity anomalies (magic numbers atn=20 and 45) have been observed in the 15–60 size range. The evaporation dynamics of these species in the 2–50 microsecond time window following ionization has been studied using the electrostatic mirror of a reflectron time-of-flight mass spectrometer as a kinetic energy analyser capable to distinguish parent and daughter ions. The magic numbers are interpreted in terms of size dependent evaporation behaviors: beyondn=20, a sudden decrease of the evaporation energy is observed; in then=45–47 size range, the magic number is accounted for by the specific dynamics of then=46 and 47 clusters, in particular the possible loss of two argon atoms forn=47 within the experimental time window. These results and their implications on the cluster structure are discussed in the light of the evaporative ensemble model and compared to the evaporation characteristics of similar species, in particular the neat rare gas clusters.     

The stability and structure of (N<Subscript>2</Subscript>)<Subscript>j</Subscript>(H<Subscript>2</Subscript>O)<Subscript>i</Subscript> and (Ar)<Subscript>j</Subscript>(H<Subscript>2</Subscript>O)<Subscript>i</Subscript> clusters

The stability and structure of water clusters absorbing nitrogen molecules or argon atoms was analyzed by molecular dynamics simulation at 233 K. The (?μ/?i)_{V, T} derivative of the chemical potential, a value characterizing the stability of a cluster with respect to its size, depends linearly on the number of molecules i. According to this criterion, the clusters under study become stable near i = 40. The average length of H-bonds increases monotonically in the growing cluster of pure water and exhibits oscillatory behavior if the growing cluster contains N₂ molecules or Ar atoms. The number of H-bonds per molecule oscillates between one and six as the cluster size changes. These oscillations are damped in pure water and sustained for clusters containing impurities, especially argon.     

Collisional dynamics of Ag19 on Pd(100): a molecular dynamics study

By means of molecular dynamics simulations based on realistic n-body potentials we investigate structural and dynamical features inherent to the energetic collision of a silver cluster (Ag₁₉) on the Pd(100) substrate. Both the system and the impact energy (E_i = 95 eV) adopted have been chosen to parallel an experimental study of size selected Ag cluster deposition on Pd(100). Our results indicate that the experimental cross section obtained via thermal energy atom scattering at the same collision energy is well reproduced by the simulations.The modeling allows to rationalize the collision outcome in terms of defect production and cluster atoms implantation. The adsorbed structures have an heterogenous nature and are mostly two-dimensional.     

A density functional molecular dynamics study of the bonding and stability of Mg n clusters (n=2−13)

A study of the structure and the bonding nature of Mg clusters having 2 to 13 atoms has been made using the density functional molecular dynamics method within the local density approximation. The calculated lowest energy structures can be described in terms of a tetrahedron and a trigonal prism. Mg₄ and Mg₁₀ are magic clusters and Mg₁₃ is neither an icosahedron nor a cuboctahedron. The bonding nature varies from atom to atom in a cluster and the transition from weakly bonded dimer to bulk like metallic behaviour is oscillatory and slow.     

Immersion of hydrogen atoms in aluminium clusters

The immersion of a H atom at the centre of Al _N clusters with 8≦N≦21 has been studied using the density functional formalism and a pseudopotential averaged about the cluster centre. The immersion energy is negative forN≦12, due to the fact that the corresponding pure Al clusters contain a central vacancy, which shows a strong affinity for the H atom. For larger sizes the immersion energy is positive (with a few exceptions), since the H atom must displace a central Al atom to the cluster surface, and it shows non negligible size oscillations.     

Evaporation of tetramers in Sb4n clusters and conditions for the formation of Sb2n+1 clusters

Antimony clusters are produced by the inert gas condensation technique. They are found to be built from Sb₄ units. The fragmentation by evaporation of Sb₄ units is studied as a function of the excess energy in the cluster. By this way the binding energy of the Sb₄ units in the cluster is found to be about 1.5 eV, well below the binding energy of a Sb atom in the bulk and in Sb₄(?3eV). The evolution of ionization potentials of Sb_4n clusters confirms that their structure is probably non metallic. Finally the possible metastable character of this Sb_4n structure is discussed.     

Unimolecular dissociation of trivalent metal cluster ions (Al n + , Ga n + , In n + ): evidence for a transition from covalent to metallic bonding

Unimolecular dissociation of aluminum, gallium and indium clusters is investigated. Small sizes dissociate into two channels: either the evaporation of a neutral or a charged monomer. Above a given size n _c, only dissociation of a neutral atom subsists. The evaporation of a charged monomer is characteristic of trivalent metal clusters and is consistent with the size evolution of the ionization potential towards the atomic value. The experiments are interpreted in the framework of the statistical R.R.K. model. For smaller sizes (n < n _c), as two evaporation processes are in competition, we have evaluated cluster relative dissociation energies and ionization potentials. The competition between the two evaporation channels is well mirrored by the evolution of the ionization potentials independently measured by near-threshold photoionization experiments. For gallium, our measurements have revealed that the covalent to metal transition occurs for larger sizes (n = 30–50 atoms) than for aluminum clusters.     

Laser photoionization and spectroscopy of gas phase silver clusters

Silver clusters containing up to 40–50 atoms are produced by laser vaporization in a pulsed-nozzle molecular beam source and studied with laser photoionization mass spectroscopy. A variety of Nd:YAG pumped dye laser and UV excimer laser wavelengths are used to achieve ionization. Ionization dynamics are studied by varying the laser wavelength and fluence. Bracketing experiments under single-photon ionization conditions are used to estimate ionization potentials as a function of cluster size. An even-odd ionization potential alternation is observed with odd-numbered clusters (N=3, 5, 7 ...) having lower ionization potentials than adjacent even-numbered species. Shell closings at clusters containing 2, 8 20 and 40 electrons are observed consistent with a one-electron shell model picture of cluster electronic structure. Resonance-enhanced ionization produces a vibrationally resolved spectrum for the trimer, Ag₃, yielding an electronic state assignment and excited state vibrational frequencies. Fragmentation in dimer ionization via theE state at 249 nm establishes the dissociation energy of Ag ₂ ⁺ to be <2.1 eV.     

Phase change of Lennard-Jones nano clusters containing non magic numbers

Phase changes of Lennard-Jones clusters containing 4N ³ (N= 1?20) identical atoms in terms of solid and liquid phase-like forms have been studied by performing molecular dynamics (MD) simulation at sharply-bounded range of temperatures between freezing temperature (T _f) and melting temperature (T _m) and at constant pressure. The small differences between the free energies of clusters in different phase-like forms and also the non-rigidity of the cluster (0 ≤ γ ≤ 1) as an order-parameter, which characterizes the phase transition, have been calculated. Plots of the free energy of phase change versus the non-rigidity indicate that the free energy is a continuous function of the non-rigidity and also different crystalline-like cores with different free energies correspond to the same non-rigidity factor at any given temperature.     

Structure and Magnetism of Hybrid Fe and Co Nanoclusters up to N?≤?7 Atoms

The structural and magnetic properties of small gas-phase Fe _m Co _n clusters with m?+?n ranging between 2 and 7 atoms are investigated using spin-polarized density functional theory. For a given cluster size possible compositions are subject to optimization using a variety of initial structures. The geometry, bond lengths, binding energies and magnetization are reported for the lowest energy structures. The results show that a magnetization peak occurs for Fe₄, while for hybrid clusters, switching a cobalt atom with an iron atom increases the cluster’s total magnetization by 1?μ _B . Our structural predictions are generally in agreement with other theoretical results; the origin of the discrepancies arising in some cases is discussed.     

Fragmentation dynamics of ammonia cluster ions after single photon ionisation

A reflecting time of flight mass spectrometer (RETOF) is used to study unimolecular and collision induced fragmentation of ammonia cluster ions. Synchrotron radiation from the BESSY electron storage ring is used in a range of photon energies from 9.08 up to 17.7 eV for single photon ionisation of neutral clusters in a supersonic beam. The threshold photoelectron photoion coincidence technique (TPEPICO) is used to define the energy initially deposited into the cluster ions. Metastable unimolecular decay (µs range) is studied using the RETOF's capacity for energy analysis. Under collision free conditions the by far most prominent metastable process is the evaporation of one neutral NH₃ monomer from protonated clusters (NH₃) _{n ? 2}NH ₄ ⁺ . Abundance of homogeneous vs. protonated cluster ions and of metastable fragments are reported as a function of photon energy and cluster size up ton=10.     

Structure of hydrogenated silicon clusters. Small clusters

The ground state structures of silicon hydride clusters Si_nH_m containing up to 12 silicon atoms are obtained by numerical modeling. The cluster geometry is optimized for a wide set of initial structures using the MINDO/3 approximation for Monte Carlo simulation of interatomic interactions. The energy of the cluster depending on the content of hydrogen is studied, and it is shown that the Si-H and Si-Si bond energies depend little on the cluster size.     

Theoretical study of the stability of small triply charged carbon clusters Cn3+ (n = 3–12)

Using density functional theory, coupled cluster and multireference methods, dissociation energies and 3rd ionization potentials for, respectively, triply charged and neutral carbon clusters have been evaluated. The results show that the smaller C_n³⁺ clusters are metastable, i.e., they present a fragmentation channel with negative dissociation energy. The lowest dissociation channel always corresponds to evaporation of a singly charged carbon atom. Good agreement with available experimental data is found for most two-fragment channels. The third ionization potential of the corresponding neutral species decreases with cluster size.     

Structures,stabilities, and electronic properties of Al n Au (n = 1–15) clusters: A density functional study

We present density functional calculations of Al _n Au clusters for n = 1–15. The growth pattern for Al _n Au (n = 1–7, 12, 14, 15) clusters is the Au atom occupying a peripheral position of Al _n clusters, and the growth pattern for Al _n Au (n = 8, 10 and 13) clusters is Au-substituted Al _n+1 clusters. It is found that the Au atom replaces the surface atom of an Al _n+1 cluster and occupies a peripheral position. In addition, the ground state structures of Al _n Au clusters are more stable than pure Aln clusters. It is found that the Al₁₃Au cluster exhibits high stability.     

Structure, stability and electronic property of the gold-doped germanium clusters: AuGe n (n = 2–13)

The structure, stability and electronic property of the AuGe _n (n = 2–13) clusters with different spin configurations are systematically investigated with density-functional theory approach at UB3LYP/LanL2DZ level. In examining the lowest energy structures, it is found that the growth behaviors for the small-sized AuGe _n (n = 2–9) clusters and relatively large-sized AuGe _n (n = 10–13) clusters are different. As the number of Ge atom increases, the Au atom would gradually move from convex to surface and to interior sites. For the most stable structures of AuGe _n (n = 10–13) clusters, the Au atom would be completely surrounded by the Ge atoms to form Au-encapsulated Ge _n cages. Natural population analysis shows that the charges always transfer from the Au atom to the Ge _n framework except for the AuGe₂ cluster. This indicates that the Au atom acts as electron donor even the 5d orbitals of the Au atom are not significantly involved in chemical bonding. The analyses of the average atomic binding energies as well as the dissociation energies and the second-order differences of total energy show that the AuGe _n clusters with n = 5, 9 and 12 are more stable than their neighboring ones, in which the bicapped pentagonal prism AuGe₁₂ in D _2d symmetry is most stable. The highest occupied molecular orbital–lowest unoccupied molecular orbital gaps are explored to be in the region of semiconductors and the more stable clusters have slightly smaller gaps. It could be expected that the stable clusters might be considered as the novel building blocks in practical applications, e.g., the cluster-assembled semiconductors or optoelectronic material.     

The local electronic and magnetic properties of Fe impurity in Al clusters

The local magnetic property,d electronic structure and the charge transfer effect of Fe impurity in Al clusters have been studied by using a tight-binding model Hamiltonian in the unrestricted Hartree-Fock approximation, which includes intra-atomic and interatomic Coulomb interactions. We have obtained that local magnetic moment of Fe impurity in FeAl _N clusters decreases with increasing cluster size and convergences to zero (that of bulk given by Anderson) withN larger than 12, meanwhile, the local magnetic moment for smaller clusters depends on the clusters size and it is a monotonous descent function of cluster size. We have also found that the spin splitting of the localizedd states decreases as the cluster size increases, which mainly results from the interaction between the localized electrons of Fe atom and the delocalized electrons of Al atoms.     

Diffraction and vibrational dynamics of large clusters from He atom scattering

The vibrational dynamics of large Ar _n clusters from n=30 to n=4500 is investigated by measuring the energy loss of He atoms in a high resolution scattering experiment. The clusters are generated by adiabatic expansion through conical nozzles and contain a distribution of cluster sizes. The He supersonic nozzle beam provides a resolution of better than 1 meV. The results are compared with calculated spectral density functions for single cluster sizes and bulk phonon spectra.