Organic mass spectrometry with low-energy projectiles

Molecular dynamics computer simulations have been employed to elucidate mechanisms responsible for uplifting of a monolayer of benzene and polystyrene molecules adsorbed on Ag{1 1 1} by low-energy atomic and cluster Ar projectiles. The sputtering yield and mass distributions of ejected particles are analyzed depending on the type and the kinetic energy of a projectile. It is shown that the relative contribution of intact molecules can be greatly enhanced if the kinetic energy of atomic projectile is reduced below 60 eV. At these energies, however, the efficiency of desorption is low and the ejection process is limited only to loosely bound molecules. Much better results can be obtained for cluster projectiles containing hundreds of Ar atoms with the incident energy of a few eV per atom. The impact of such particles leads to a gentle and very efficient removal of intact organic molecules originally adsorbed at the surface.     

On the background damping in the vicinity of the grain-boundary damping peak in zinc

The background damping in the vicinity of the grain-boundary damping decreases with increasing grain size. An analysis of the strain amplitude dependence of this damping shows that as the grain size increases the distance between solute pinning atoms on dislocation decreases. This can be explained in terms of a movement of solute away from grain boundaries and to dislocations. Thus as the grain size increases the total number of solute atoms at grain boundaries decreases and is rejected both into the lattice and to dislocations. A mathematical model is used to explain this result. As a consequence an activation energy of 0.05 eV is obtained for the binding energy of the solute to the dislocation.     

The nature of the ordering of atoms in a melt and the surface properties of simple eutectic systems

Using the models reconstructed from the experimental structure factor curves by the reverse Monte-Carlo and Voronoy-Delaunay methods the local atomic structures of the Sn-Ge, Ag-Ge, and Ni-C simple eutectic systems has been analyzed. It has been found that the nature of the atom ordering in melts is responsible not only for the melt bulk properties, but for its surface properties (surface tension, wetting) as well. Clusters that form from atoms of the same sort in melts and whose binding energy inside the clusters exceeds the binding energy between the atoms of a solvent and a cluster, exhibit the surface activity in the melt, which explains the extremes in isotherms of the density and surface tension of the melts. Clusters with a chemical ordering of atoms patterned after the Me₃C electronic compound revealed in the Ni-C and Ag-Ge systems indicate that the equilibrium phase diagrams of these systems at high pressures transform from diagrams of a simple eutectic type to diagrams with a compound, i.e. an increase in pressure contributes to the metallization of bonds in a melt.     

Hierarchical modeling of C and Si nano-cluster nucleation utilizing quantum and statistical mechanics

A hierarchical, multi-scale computer model for the nucleation of nano-phase materials from the vapor phase is presented. The model utilizes full solutions to quantum mechanics cluster energy equations for sizes up to 10 atoms, and statistical rate theory for larger cluster sizes. Ab initio and semi-empirical quantum mechanics methods are used to investigate the energetics of Si and C clusters. The results of binding energy and most stable configurations show significant differences between C and Si nano-clusters. Atomic cluster size distributions are obtained from reaction rate theory on the basis of collision frequencies in the vapor phase. Cluster reaction rates are determined from the energetics and vibrational modes, as investigated by quantum mechanics for small sizes. The nucleation and further evolution of the cluster size distribution is modeled by solutions to detailed kinetic equations. This multi-scale model is shown to be a useful computer simulation tool, which can be utilized to design experiments on nano-phase materials with minimum adjustable parameters. This revised version was published online in July 2006 with corrections to the Cover Date.     

Transition-metal dimers and physical limits on magnetic anisotropy

Recent advances in nanoscience have raised interest in the minimum bit size required for classical information storage. This bit size is determined by the necessity for bistability with suppressed quantum tunnelling and energy barriers that exceed ambient temperatures. In the case of magnetic information storage, much attention has centred on molecular magnets with bits consisting of about 100 atoms, magnetic uniaxial anisotropy energy barriers of about 50 K and very slow relaxation at low temperatures. Here, we draw attention to the remarkable magnetic properties of some transition-metal dimers, which have energy barriers approaching 500 K with only two atoms. The spin dynamics of these ultrasmall nanomagnets is strongly affected by a Berry phase, which arises from quasi-degeneracies at the electronic highest occupied molecular orbital energy. In a giant-spin approximation, this Berry phase makes the effective reversal barrier thicker.     

An Atomistic View of Platinum Cluster Growth on Pristine and Defective Graphene Supports (Small 10/2023)

Density functional theory (DFT) is used to systematically investigate the electronic structure of platinum clusters grown on different graphene substrates. Platinum clusters with 1 to 10 atoms and graphene vacancy defect supports with 0 to 5 missing C atoms are investigated. Calculations show that Pt clusters bind more strongly as the vacancy size increases. For a given defect size, increasing the cluster size leads to more endothermic energy of formation, suggesting a templating effect that limits cluster growth. The opposite trend is observed for defect-free graphene where the formation energy becomes more exothermic with increasing cluster size. Calculations show that oxidation of the defect weakens binding of the Pt cluster, hence it is suggested that oxygen-free graphene supports are critical for successful attachment of Pt to carbon-based substrates. However, once the combined material is formed, oxygen adsorption is more favorable on the cluster than on the support, indicating resistance to oxidative support degradation. Finally, while highly-symmetric defects are found to encourage formation of symmetric Pt clusters, calculations also reveal that cluster stability in this size range mostly depends on the number of and ratio between Pt C, Pt Pt, and Pt O bonds; the actual cluster geometry seems secondary.     

The effects of oxygen on the surface passivation of InP nanowires

The effects of surface passivation on the electronic and structural properties of InP nanowires have been investigated by first-principles calculations. We compare the properties of nanowires whose surfaces have been passivated in several ways, always by H atoms and OH radicals. Taking as the initial reference nanowires that are fully passivated by H atoms, we find that the exchange of these atoms at the surface by OH radicals is always energetically favorable. A nanowire fully passivated by OH radicals is about 2.5?eV per passivated dangling bond more stable than a nanowire fully passivated by H atoms. However, the energetically most stable passivated surface is predicted to have all In atoms bonded to OH radicals and all P atoms bonded to H atoms. This mixed passivation is 2.66?eV per passivated dangling bond more stable than a nanowire fully passivated by H atoms. Our results show that, in comparison with the fully H-saturated nanowire, the fully OH-saturated nanowire has a smaller energy band gap and localized states near the energy band edges. Also, more interestingly, concerning optical applications, the most stable H+OH passivated nanowire has a well-defined energy band gap, only 10% smaller than the H-saturated nanowire energy gap, and few localized states always close to the valence band maximum.     

Size determination of silver clusters by time-of-flight mass spectroscopy and electron microscopy

The determination of size distribution is sought via time-of-flight mass spectroscopy and electron microscopy of neutral silver clusters produced by the inert gas condensation method. By adjusting suitable parameters the mean size could be varied between a few tens and several hundreds of atoms per cluster. In particular, the influence exerted on the cluster size by ionization (fragmentation) and by deposition onto an amorphous carbon support film (migration) was assessed. The stabilization of clusters sampled on amorphous carbon films is discussed in detail.     

Metal Clusters on Semiconductor Surfaces and Application in Catalysis with a Focus on Au and Ru

Metal clusters typically consist of two to a few hundred atoms and have unique properties that change with the type and number of atoms that form the cluster. Metal clusters can be generated with a precise number of atoms, and therefore have specific size, shape, and electronic structures. When metal clusters are deposited onto a substrate, their shape and electronic structure depend on the interaction with the substrate surface and thus depend on the properties of both the clusters and those of the substrate. Deposited metal clusters have discrete, individual electron energy levels that differ from the electron energy levels in the constituting individual atoms, isolated clusters, and the respective bulk material. The properties of clusters with a focus on Au and Ru, the methods to generate metal clusters, and the methods of deposition of clusters onto substrate surfaces are covered. The properties of cluster-modified surfaces are important for their application. The main application covered here is catalysis, and the methods for characterization of the cluster-modified surfaces are described.     

Evolution of the electronic structure and properties of neutral and charged aluminum arsenide clusters: A comprehensive analysis

The B3LYP-DFT/6-311+G(d) method has been used to optimize the geometries of (AlAs)_n neutrals and charged ions in the size range of n = 1–15. Frequency analyses are performed at the B3LYP/6-31G(d) level to check whether the optimized structures are transition states or true minima on the potential energy surfaces of corresponding clusters. The total energies of these clusters are then used to study the evolution of their binding energy, relative stability, and electronic properties as a function of size. The geometries are found to undergo a structural change from two-dimensional to three-dimensional when the cluster contains 6 atoms. The medium size clusters (n = 6–15) display the hollow globular conformers with large surface effect, which may cause the bulk limit still far from my computed results. The geometrical changes are companied by corresponding changes in the nearest-neighbor distances and coordination numbers. For medium size clusters (n = 6–15), both ionization potential and electron affinity have the tendency of decrease when the number of AlAs units in the cluster increases. Some magic clusters in neutral, cationic, and anionic form compared to its neighboring clusters are argued according to the calculated results of the second energy difference and electronic properties.     

Influence of ultrasonication energy on the dispersion consistency of Al2O3–glycerol nanofluid based on viscosity data,and model development for the required ultrasonication energy density

Achieving homogenised and stable suspensions has been one of the important research topics in nanofluid investigations. Preparing nanofluids, especially from the two-step method, is often accompanied with varying degrees of agglomerations depending on some parameters. These parameters include the physical structure of the nanoparticle, the prevalent particle charge, the strength of van der Waals forces of attraction and repulsiveness strength. Amongst the methods of deagglomeration, the use of ultrasonic vibration is most popular for achieving uniform dispersion. However, there are very few works related to its effect on the thermo-physical properties of nanofluids, and above all, standardising the minimum required ultrasonication time/energy for nanofluids synthesis. In this work, the optimum energy required for uniform and initially stable nanofluid has been investigated through experimental study on the combined influence of ultrasonication time/energy, nanoparticle size, volume fraction and temperature on the viscosity of alumina–glycerol nanofluids. Three different sizes of alumina nanoparticles were synthesised with glycerol using ultrasonication-assisted two-step approach. The viscosities of the nanofluid samples were measured between temperatures of 20–70?°C for volume fractions up to 5%. Based on the present experimental results, the viscosity characteristics of the nanofluid samples were dependent on particle size, volume fraction and working temperature. Using viscometry, the optimum energy density required for preparing homogenous nanofluid was obtained for all particle sizes and volume fractions. Finally, an energy density model was derived using dimensionless analysis based on the consideration of nanoparticle binding/interaction energy in base fluid, particle size, volume fraction, temperature and other base fluid properties. The model's empirical constants were obtained using nonlinear regression based on the present experimental data.     

碳纳米管的压缩屈曲机理和电子结构

用分子动力学方法模拟了碳纳米管的受压屈曲变形过程，分析了碳纳米管结构屈曲的微观机理，从能量和微观结构变化研究了碳纳米管屈曲变形的机理；用紧束缚分子动力学方法研究了碳纳米管的结构屈曲对其电子结构和输运特性的影响，在压缩过程中，随着能量的不断积累，碳纳米管局部区域原子的温度上升，结构软化，产生失稳；在压缩变形过程中，随着碳纳米管变形的发展，在结构失稳之前，原子间的相互结合作用增强，高结合能区域的电子能态分布增大；而随着失稳的发生，原子结构松弛，原子间相互结合作用减弱，导致电子能态朝低结合能区域发展，碳纳米管受压的弹性屈曲变形对其电子结构不产生较大的影响，其输运特性也不发生本质的改变。     

Doping effects on the stability of stacking faults in silicon crystals

In silicon crystals annealed at 1173 K, n-type dopant atoms segregate nearby a stacking fault ribbon bound by a pair of partial dislocations and the width of the ribbon is increased. The origin of the width increase is the reduction of the stacking fault energy due to an electronic interaction between the ribbon and the dopant atoms segregating at the ribbon, rather than the reduction of the strain energy around the partial dislocations due to the dopant atoms segregating at the partials.     

The effect of joint size on the creep properties of microscale lead-free solder joints at elevated temperatures

Solder joints in electronic packaging systems are becoming smaller and smaller to meet the miniaturization requirements of electronic products and high density interconnect technology. Furthermore, many properties of the real solder joints at the microscale level are obviously different from that of bulk solder materials. Creep, as one of the key mechanical properties at elevated temperatures, can impair the reliability of miniature solder joints in electronic devices. However, there is a lack of knowledge about the comparative creep properties of microscale solder joints of different sizes. Most previous studies have focused on the creep properties of bulk solder materials or solder joints of the same size. In this research, to determine whether a size effect exists for creep properties of solder joints or not, we characterized the creep behaviors of Sn–3.0Ag–0.5Cu lead-free solder joints under tensile loading modes using microscale butt-joint specimens with a copper-wire/solder/copper-wire sandwich structure with two different sizes. Also, the creep failure mechanisms were investigated. Experimental results show that the creep activation energy and creep stress exponent are very similar for both sizes of solder joint. However, under the same testing conditions, the joints with a larger size exhibit a much higher steady-state creep rate and a shorter creep lifetime than the smaller joints.     

On the Nature of Disorder in Solid 4He

We apply a modified Debye approach to calculate the Gibbs free energy for different structural phases and crystallite sizes in ⁴He. Atoms are assumed to interact via the Aziz potential. We have found that some intermediate (between hcp and bcc) phase predicted previously is more favorable than hcp at low temperatures and for small sizes. We show that it can exist in a wide pressure range up to 60 bar in ⁴He for crystallite sizes about 3,000 atoms. For larger sizes (10,000 atoms or more) this phase becomes unfavorable. In multidomain structures the intermediate phase competes with hcp and metastable fcc that can be a reason for disorder in solid ⁴He.     

Screening of excitons in single, suspended carbon nanotubes

Resonant Raman spectroscopy of single carbon nanotubes suspended across trenches displays red-shifts of up to 30 meV of the electronic transition energies as a function of the surrounding dielectric environment. We develop a simple scaling relationship between the exciton binding energy and the external dielectric function and thus quantify the effect of screening. Our results imply that the underlying particle interaction energies change by hundreds of meV.     

Geometry, stability and properties of metallo-carbohederenes

The geometry, electronic structure and the stability of the newly discovered metallo-carbohedrene Ti₈C₁₂ (met-car) has been investigated using the density functional methods. We show that the structure of Ti₈C₁₂ is a caged structure with a binding energy of 6.7 eV/atom. This unusual stability is derived from the C₂ and the TiC bonds which decorate the structure. The density of states at the Fermi energy is low and these states are formed from a strong hybridization between the Ti d and the carbon sp-states. We also investigate alternate cubic structure containing Ti and C atoms and show that these fragments of the bulk carbide structure have binding energies per atom comparable to the met-cars. These structures were however not observed in original experiments but have been observed in some of the recent experiments. Conditions favoring the formation of met-cars or the cubic structures are examined.     

Information on the mechanism of mechanochemical reaction from detailed studies of the reaction kinetics

Mechanochemical and mechanical alloying processes take place between colliding surfaces in the heavy container of a ball mill, where the in situ examination of the reaction mechanism is extremely challenging. As shown in this paper, useful indirect information can be obtained from detailed analysis of the reaction kinetics. A shaker mill with a single ball was used, so that time could be replaced with the number of collisions as the variable of kinetics. A simple stochastic model was developed that is capable of describing the kinetics of gradual mechanochemical reactions and the variation of physical properties such as grain size. The kinetic constant is directly related to the fraction of powder processed in a single collision, and its value indicates that only a few micrograms of powder are processed in a single collision. Measuring the kinetic constant as a function of impact energy revealed that a minimum impact energy, on the order of a few hundreds of a Joule, is needed to initiate chemical change. The model was also applied to the self-sustaining reaction between Ti and graphite. In that case, the critical number of collisions required for ignition characterizes the speed of mechanical activation.     

Ultra-small fluorescent metal nanoclusters: Synthesis and biological applications

Recent advances in nanotechnology have given rise to a new class of fluorescent labels, fluorescent metal nanoclusters, e.g., Au and Ag. These nanoclusters are of significant interest because they provide the missing link between atomic and nanoparticle behavior in metals. Composed of a few to a hundred atoms, their sizes are comparable to the Fermi wavelength of electrons, resulting in molecule-like properties including discrete electronic states and size-dependent fluorescence. Fluorescent metal nanoclusters have an attractive set of features, such as ultrasmall size, good biocompatibility and excellent photostability, making them ideal fluorescent labels for biological applications. In this review, we summarize synthesis strategies of water-soluble fluorescent metal nanoclusters and their optical properties, highlight recent advances in their application for ultrasensitive biological detection and fluorescent biological imaging, and finally discuss current challenges for their potential biomedical applications.     

‘The crystal structure problem’ in noble gas nanoclusters

Calculations of the energetics of multiply twinned particles (MTPs) such as icosahedra and decahedra with fivefold symmetry as well as face-centered cubic (fcc) and hexagonal close-packed (hcp) particles in the size interval from 13 up to ∼ 45,000 atoms were made applying Lennard-Jones potentials. We essentially extended the size interval comparatively with previous studies and included shape-optimized hcp clusters in the global energy analysis that gives rise to the new insight into the basic fcc/hcp problem. For the cluster size N from minimal up to N ∼ 2000 atoms the binding energy is highest for icosahedra, in the size interval from 2000 up to ∼ 11,500 atoms decahedra prevail, above N ∼ 11,500 atoms decahedra and optimized fcc clusters were found to alternate. The hcp structure was revealed to become favorable above N ∼ 34,000 atoms. Thus, hcp clusters can attain their preference with respect to MTPs (comprising fcc fragments) and optimized fcc clusters only for very large sizes. The comparison with several other models is suggested and the opportunity of experimental observations is discussed.