Structure and dynamics of graphite-supported bimetallic nanoclusters

Molecular dynamics simulations are used to analyze the structure and dynamics of isolated bimetallic nanoclusters of 343 (Cu-Ni) and 1000 atoms (Cu-Ni and Pt-Au) deposited on a graphite substrate. The metal-metal interactions are modeled with the many-body Sutton-Chen potential, and a Lennard-Jones potential is used to describe the metal-carbon interactions. The nanocluster melting temperature is determined from caloric and heat capacity curves, and the atomic distribution is studied layer-by-layer as a function of temperature in a direction perpendicular to the substrate plane. Changes in the nanocluster shape as temperature increases are monitored through deformation parameters that show clear evidence of structural and melting transitions as well as of atomic surface diffusion in the cluster. Dynamic properties such as atomic and whole-cluster diffusion, and the motion of the metal atoms at the interface metal/graphite are characterized as a function of temperature.     

激光熔覆铜锰合金选择性脱合金制备纳米多孔涂层的研究

本文通过激光加工结合电化学腐蚀脱合金法, 成功实现了纳米多孔涂层的制备. 采用激光熔覆首先在45钢表面制备了成形良好、稀释率低的铜锰合金熔覆层, 并通过快速重熔工艺实现了初始材料组织细化. 研究表明, 在不同的电解液下,铜锰合金的临界腐蚀电位出现了明显的偏移; 在不同的腐蚀电流下,铜锰合金腐蚀后的形貌迥异. 最终,通过选择性腐蚀成功实现了纳米多孔铜和纳米多孔锰涂层的制备, 并利用电位-pH图对脱合金的选择性腐蚀进行了详细的理论解释.     

Novel synthesis, structure and catalysis of inverted core/shell structured Pd/Pt bimetallic nanoclusters

The catalytic properties of Pd-core/Pt-shell (inverted core/shell) structured bimetallic nanoclusters, synthesized by a successive addition method using sacrificial hydrogen, were investigated for hydrogenation of methyl acrylate. Partial deposition of Pt atoms on the surface of Pd nanoclusters can enhance the catalytic activity of the Pd atoms remaining in the surface of the inverted core/shell structured Pd/Pt bimetallic nanoclusters. Received 29 November 2000     

Cooperative superfluorescence in nanoclusters

The features of superfluorescence (SF) in nanoclusters, which consist of a few neighboring particles interacting with each other by exchange or/and electrostatic forces, are investigated theoretically. It is shown that there are two types of cooperatively SF depending on the ratio between the linear size of a nanocluster and the cooperative interaction range. Experiments on cooperative luminescence in Ytterbium-doped silica fibers are analyzed. It is shown that the intensity of cooperative SF in such a system should be proportional to the fourth degree of Ytterbium concentration.     

Exchange bias in iron oxide nanoclusters

Iron oxide nanoclusters have been prepared by the gas-phase aggregation technique to form thin film structures with very high exchange bias values (up to 3000?Oe at low temperatures). Composition has been analysed by x-ray absorption and M?ssbauer spectroscopies in order to elucidate the actual origin of the observed magnetic behaviour. The formation of a metal-oxide core-shell arrangement to explain the observed exchange bias has to be discarded since results show no metallic iron content and the main presence of α-Fe(2)O(3). The observed weak ferromagnetism and exchange bias are in agreement with the obtained size of α-Fe(2)O(3) nanoparticles: weak ferromagnetism because of the well-known spin canting in this antiferromagnetic structure and exchange bias because of the interaction between different spin sublattice configurations promoted by the modification of iron coordination in α-Fe(2)O(3) nanoparticles. Moreover, the preparation method is proposed for tuning both magnetization and exchange bias values by modification of the preparation conditions of α-Fe(2)O(3) nanoparticles, which open new possibilities in the design of new materials with required properties.     

Copper nanoclusters in amorphous hydrogenated carbon

A study of the geometric characteristics of copper nanoclusters incorporated in an amorphous hydrogenated carbon matrix is reported. It makes use of small-angle x-ray scattering and transmission electron microscopy (100 keV). The fractal dimension and nanocluster diameter have been determined from the x-ray scattering indicatrix for different copper concentrations. TEM images of copper nanoclusters have been analyzed, and a cluster distribution function in size constructed. The shape of the distribution function is discussed in terms of the theory of nucleation of a new phase. Fiz. Tverd. Tela (St. Petersburg) 40, 568–572 (March 1998)     

Shellwise Mackay transformation in iron nanoclusters

Structure and magnetism of iron clusters with up to 641 atoms have been investigated by means of density functional theory calculations including full geometric optimizations. Body-centered cubic (bcc) isomers are found to be lowest in energy when the clusters contain more than about 100 atoms. In addition, another stable conformation has been identified for magic-number clusters, which lies well within the range of thermal energies as compared to the bcc isomers. Its structure is characterized by a close-packed particle core and an icosahedral surface, while intermediate shells are partially transformed along the Mackay path between icosahedral and cuboctahedral geometry. The gradual transformation results in a favorable bcc environment for the subsurface atoms. For Fe55, the shellwise Mackay-transformed morphology is a promising candidate for the ground state.     

Novel magnetism in 3He nanoclusters

The magnetic susceptibility of 3He nanoclusters embedded in a 4He matrix has been measured from 0.5 to 10 mK at pressures from 2.88 to 3.54 MPa. Even the lowest pressure clusters have a solid fraction in the region of the phase diagram where bulk solid is unstable. At 3.54 MPa, straight theta = -250 microK, equal to that of bulk 3He for v = 21.3 cm3/mole. For 2.88 MPa, straight theta = 140 microK, indicating a ferromagnetic tendency, similar to 2D films at some coverages. At intermediate pressures, chi has a peak near 1.05 mK, but with no discontinuity. Magnetic ordering in nanoclusters appears to be different than the U2D2 phase of bulk 3He.     

Radiation effects in nanoclusters embedded in solids

Nanoclusters embedded in hard solid materials such as silica show great promise forincreased practical applications, as they combine the exciting nanosize effects with veryhigh structural stability. Ion irradiation can be used to tailor the properties of theseclusters, the perhaps most dramatic example being the use of swift heavy ions to reshapespherical metal nanoparticles to have antennas or become rods. In this article we reviewexperimental and simulation studies of ion beam processing of nanoclusters embedded insolids.     

Broken even-odd symmetry in self-selection of distances between nanoclusters due to the presence or absence of topological solitons

Depositing particles randomly on a 1D lattice is expected to result in an equal number of particle pairs separated by even or odd lattice units. Unexpectedly, the even-odd symmetry is broken in the self-selection of distances between indium magic-number clusters on a Si(100)-2×1 reconstructed surface. Cluster pairs separated by even units are less abundant because they are linked by silicon atomic chains carrying topological solitons, which induce local strain and create localized electronic states with higher energy. Our findings reveal a unique particle-particle interaction mediated by the presence or absence of topological solitons on alternate lattices.     

Inverse Jahn-Teller transition in bimetallic oxalates

Because of the competition between the spin-orbit coupling and the Jahn-Teller (JT) energies in Fe(II)Fe(III) bimetallic oxalates, we theoretically predict that an undistorted phase with C3 symmetry about each Fe site may be recovered at low temperatures. Both lower and upper JT transitions bracketing the ferrimagnetic transition temperature Tc are predicted for compounds that exhibit magnetic compensation. Comparisons with recent measurements and first-principles calculations provide strong evidence for the inverse JT transition below Tc.     

Spin transport in bimetallic pentalene complexes

Spin transport in bimetallic pentalene complexes (CpM(pentalene)M′Cp;M,M′=Fe,Co,Ni) between two gold electrodes was investigated, using a Green’s function formalism under density functional theory. Variation of the metal atom species in the complexes gives a considerable change in their spin properties, with hetero-bimetallic complexes containing an odd number of electrons exhibiting spin filter behaviour. In contrast, alternation in the contact condition, whether Cp-anchoring or adducting by sulphur-gold bonds, had almost no effect on spin filter behaviour, but did lead to variation in electrical conduction. We examined suitable bimetallic pentalene complexes in order to enhance their spin filter efficiency.     

Shock shells in Coulomb explosions of nanoclusters

We predict that Coulomb explosion of a nanoscale cluster, which is ionized by high-intensity laser radiation and has a naturally occurring spatial density profile, will invariably produce shock waves. In most typical situations, two shocks, a leading and a trailing one, form a shock shell that eventually encompasses the entire cluster. Being the first example of shock waves on the nanometer scale, this phenomenon promises interesting effects and applications, including high-rate nuclear reactions inside each individual cluster.     

Cubic and hexagonal symmetries in LiCl nanoclusters

Cubic and hexagonal symmetries are observed in molecular dynamics simulations of lithium chloride unconstrained nanoclusters, using the Born-Mayer-Huggins (BMH) potential model. Phase changes between the two solid phases, and solid-liquid coexistences, are studied for LiCl clusters with a number of ions ranging from 1000 to 5292. A stability analysis of the clusters and bulk systems, at 0K, is presented, using the BMH and the Michielsen-Woerlee-Graaf (MWG) potential models. The cubic structure from the BMH model is slightly more stable than the hexagonal one for cluster sizes between 1000 and ~10 000 ions. For higher cluster sizes and bulk LiCl the opposite is true. Moreover, at 0K, the bulk cubic phase from the MWG potential is significantly more stable than the hexagonal one. Thus, the BMH potential model seems unrealistic for large clusters and the bulk as far as a comparison with experiment is concerned. Finally, a fairly good correlation of the simulation results is obtained by means of a theoretical model recently reported by us.     

Isomorphous substitution in bimetallic oxide clusters

The geometric and electronic structure of bimetallic oxide clusters is studied as a function of their composition with gas phase vibrational spectroscopy. Infrared multiple photon dissociation spectra of titanium-vanadium oxide cluster anions are measured in the 500 to 1200 wave number range and assigned on the basis of harmonic frequencies calculated using density functional theory. Singly substituted (V(2)O(5))(n-1)(VTiO(5))(-) (n=2-4) cluster anions are shown to form polyhedral caged structures similar to those predicted for their isoelectronic counterparts, the neutral (V(2)O(5))(n) clusters. Upon systematic exchange of V by Ti atoms in V(4-n)Ti(n)O(-)(10) (n=1-4), the structure does not change. The stress induced by the isomorphous substitution results in an increased number of unpaired electrons (n-1) for the Ti-rich systems, leading to a quartet ground state for Ti(4)O(-)(10).     

Magnetic phase transitions in nanoclusters and nanostructures

Four nanosystems possessing the first order magnetic phase transitions (FOMPT) are considered: (1) isolated iron oxide nanoclusters, (2) iron oxide nanostructures in the beginning of sintering, (3) nanostructures induced by shear stress under high-pressure action, (4) ordered iron oxide nanostructures. The thermodynamic models for FOMPT appearance and cluster critical size are proposed. The role of surface tension, defect density and nanocluster organization are discussed.     

Formation of structural modifications in copper nanoclusters

The melting and crystallization of copper nanoclusters are investigated using the molecular dynamics simulation with tight-binding potentials. The formation of a cluster structure depends on the conditions used for cooling from the liquid phase. Slow cooling results predominantly in the formation of a face-centered cubic structure, whereas rapid cooling in the majority of cases leads to the formation of an icosahedral structure. Therefore, the simulation performed has demonstrated the possibility of controlling the formation of a structure of copper nanoclusters during crystallization.     

Josephson effect between nanoclusters in resonance conditions

A general expression is derived for the Josephson current between nanoclusters. It is shown that, in the resonance conditions between electron levels of clusters, the expression for the current obtained in the tunnel Hamiltonian model becomes invalid. In the case of degeneracy or close to degeneracy of energy levels in isolated clusters, the critical Josephson current may exceed the value obtained in the model of tunnel Hamiltonian in the large parameter, viz., the ratio of the order parameter |Δ| to the distance between the resonance level and the levels closest to it.     

Giant strengthening of superconducting pairing in metallic nanoclusters

The presence of shell structure and the accompanying high level degeneracy leads to a strengthening of the pairing interaction in some metallic nanoclusters. It is predicted that for specific systems one can expect a large increase in the values of the critical temperature and other parameters.