The energetics and structures of metallic clusters containing up to 55 atoms are studied by the Car-Parrinello (quantum-molecular dynamics) method with Al chosen as a paradigm. It is found that the structural energy differences between small cluster (icosahedral) and bulk (fee) structures are surprisingly small, indicating that the icosahedron-fee structural transition can occur very early in Al clusters. Simulated annealing studies for 13- and 55-atom clusters, where both perfectly symmetric icosahedral and cuboctahedral (fee) structures exist, show that the distortions from the ideal structures are substantial. For the 55-atom cluster several inequivalent but energetically nearly degenerate structures are found, which is likely to lead to floppiness at finite temperatures as well as a low melting point. Although the structure factors for the annealed structures are nearly identical, they are very different from those of either of the ideal structures. The computed IPs and EAs differ by less than 0.1 eV for weakly annealed cuboctahedral and icosahedral clusters, making the structural identification on the basis of IP and EA measurements very difficult. The quantum-mechanical results are also used to develop a classical potential for long-time molecular dynamics simulations of large clusters.

Reflection spectroscopy techniques have been developed with a sensitivity of detecting a change in surface reflectivity of ∼1 part in 106. This has provided the opportunity to measure spectra of ∼1010isolated clusters with an average cluster size as small as 〈N〉=10 atoms. This paper will describe the application of these techniques to the study of alkali clusters emphasizing the size and shape dependence of cluster spectra as well as measurement of surface kinetics determining cluster growth and desorption.

CdS crystallites of 40 and 45 Å average diameter have been studied under hydrostatic high pressure by optical absorption. The optical absorption onset is observed to shift to higher energy with pressure according to

This is exactly the same shift with pressure as is observed in the bulk. The solid-solid phase transition from the zincblende to the NaCl phase is observed as an abrupt change in the intensity, intercept, and functional form of the absorption onset. The phase transition, which occurs at 27Kbar in bulk CdS, is shifted to 85 Kbar in nanocrystals stabilized with polyphosphate, and to 60 Kbar in nanocrystals surface derivatized with EDTA. It thus appears possible to control the relative stability of the crystalline phases of a nanocrystal by chemical manipulation of the surface.

Surface melting on clusters is investigated by a combination of analytic modeling and computer simulation. Homogeneous, argon-like clusters bound by Lennard-Jones forces and Cu-like clusters bound by ‘embedded atom’ potentials are the systems considered. Molecular dynamics (MD) calculations have been carried out for clusters with 40–147 atoms. Well below the bulk melting temperature, the surfaces become very soft, exhibiting well-defined diffusion constants even while the cores remain nearly rigid and solid-like. The simulations, particularly animations, of atomic motion reveal that the surface melting is associated not with amorphous, random surface structures in constant, irregular motion, but rather in large-amplitude, organized, collective motion of most of the surface atoms accompanied by a few “floaters” and holes. At any time, a few of the surface atoms move out of the surface layer, leaving vacancies; these promoted particles wander diffusively, the holes also but less so, and occasionally exchange with atoms in the surface layer. This result is the basis for an analytic, statistical model. The caloric curves, particularly the latent heats, show that surface melting of clusters is a “phase change” different from the bulk melting of clusters.

The factors which promote stability of icosahedrally twinned clusters are discussed. Results for the statics and dynamics of Lennard-Jones clusters are summarized, with special focus on the unusual vibrational properties of Icosahedrally twinned clusters. In addition, we present results from density-functional -based electronic structure calculations on for 13– and 55– atom aluminum clusters with icosahedral and octahedral symmetries.

We observe thermionic emission from tungsten clusters Wn, n ≥ 5, following excitation by a Q-switched Nd:YAG laser. The ion yield from this process dominates over prompt ionization if the laser fluence is chosen properly. Clusters originating from thermionic emission are significantly colder than expected if evaporative cooling would prevail.

Within a tight binding framework, the interplay between local atomic arrangement, orbital directionality and phase stability properties is discussed for a series of tetrahedrally close packed (tcp) structures. The common features which characterize the local densities of states of tcp phases can be explained by a moment analysis up to fourth order terms. On the contrary, it is shown that structural energy differences between simple and tcp crystalline structures requires the knowledge of at least the fifth order moments, a fact which has been underestimated in the past. Finally, the relative stability of various representative geometries, including the ones of simple crystalline structures is examined as a function of the number of valence electrons.

Temperature dependent electron diffraction studies on 30 Å diameter CdS nanocrystals are described. The linear thermal expansion coefficient of the nanocrystals is 2.75 * 10−5 Å/K,and the melting point is 575 K. These data are in contrast to bulk CdS which has a melting point of 1750 K and a linear expansion coefficient of 5.5 * 10−6Å/K. The observed depression in the melting point of these semiconductor clusters is similar to effects observed in metals and molecular crystals, indicating that the phenomenon of reduced melting point in small systems is a general one regardless of the type of material. The observation of melting point depression in these clusters also has far reaching implications for the preparation of highly crystalline clusters of CdS, as well as for the use of these nanocrystals as precursors to thin films.

We report on the further development of an ion source for producing intense, continuous beams of large positive and negative cluster ions comprised of high temperature materials. This device, the Smoke-Ion Source, is the result of combining inert gas condensation methods with techniques for injecting electrons directly into expanding jets. We demonstrate the capability of this ion source to generate strong beams of cluster ions comprised of materials including metals, semiconductors, and metal oxide ceramics.

The preparation of nanocrystalline particles or clusters (sizes 1–50 nm) is of interest to the study of small systems and for use in sintering or compacting of high purity bulk materials. Recently, a method whereby these crystals can be fabricated using a sputter discharge has been reported. We have developed a computer model to simulate the formation of homogeneous (e.g., Si, Cu, Ti) gas phase clusters in these devices as precursors to larger nanocrystals. The model combines Monte Carlo and drift-diffusion algorithms to simulate the sputtering of atoms from the target, their thermalization in the buffer gas, and gas phase nucleation reactions. Densities of clusters having one to many hundreds of atoms are obtained as a function of position in the discharge. We find that the experimentally observed particle sizes cannot be explained by clustering involving solely neutral reactants due to their short residence times in the plasma. Negatively charged clusters which are trapped in the plasma have correspondingly longer residence times and most likely are responsible for the growth of large particles. Scaling laws for the growth of homogeneous clusters will be presented based on the results of the model.

Atoms are gas discharge sputtered from a solid target. They are condensed to form clusters using the gas aggregation technique. An intense beam of clusters of all solid materials can be obtained. Up to 80 % of the clusters can be ionised without using additional electron impact ionisation. Total deposition rates vary between 1 and 1000 Å per second depending on cluster diameter, which can be varied between 3 and 500 nm. Thin films of Al, Cu, and Mo have been produced so far. For non accelerated beams a weakly adhering mostly coulored deposit is obtained. Accelerating the cluster ions this changes to a strongly adhering film, having a shiny metallic appearance, and a very sharp and plane surface as seen in an electron microscope. The advantages compared to Kyoto ICB-method are: easy control of the cluster size, no electron impact ionisation, high degree of ionisation, and sputtering is used instead of thermal evaporation, which allows the use of high melting point materials.

A computer simulation that includes the space charge of electrons and ions has been done for the Eaton ionized cluster beam source in order to better understand how the potential fields affect film deposition. It is found that the space charge effects dominate the dynamics of the ionized clusters, painting a picture that differs radically from what the Laplace fields predict.

Photochemical and chemical reduction methods are described for the controlled formation of metallic silver in aqueous solutions. The former approach is capable of generally depositing a number of metals. When spherical silver bromide particles are the substrates, the resulting silver coated composite particles exhibit optical absorption spectra which vary with the coat thickness as theoretically predicted. In the case of spherical silica particles of uniform size, it was possible to produce both quantum size silver particles supported on silica, as well as a silver coat of variable thickness, depending on the rate of the deposition process. In addition to silica, substrates such as latex and chromium hydroxide could be used.

CdS particles with two different particle diameters (50–200 Å and < 30 Å) were subjected to 308 nm excimer laser irradiation at 77 K, and the subsequent charge carrier processes studied by ESR. Dramatic differences in the ESR signals as a function of decreasing particle size could be observed, consistent with the localization of charge carriers on numerous surface sites.

In the synthesis of polyacrylates, a necessary step involves the conversion of acroleins to acrylic acids- a process catalysed by Mo oxides. We have directed our work towards the elucidation of the electronic structure of the most stable structure of 12-heteromolybdate, ie. the α-keggin unit, using EPR spectroscopy, Cyclic Voltammetry, and molecular orbital calculations based on the extended Huckel theory. Metal ion substitution has been shown to have negligible effect on the electronic structure of the keggin unit thus rationalising previous arguments of structural rigidity. Electronic exchange has been shown to be facilitated through metal-metal interaction.

Some factors influencing the formation, growth and convective transport of the particles by the rapid condensation of a vapor produced by high pressure sputtering in an inert gas are discussed.

Silver aggregates find their technological applications in electricity conducting cements and as catalysts in biomedical sciences. We have developped a method for synthesis of colloidal silver aggregates, based in redox reactions. Aggregate size and structure are strongly influenced by preparation physico-chemical conditions. We have investigated the pH influence over the growth mechanism through dynamic and static light scattering techniques. We introduce here an analysis of the sedimentation rate from which the aggregation regime can also be inferred.

Amorphous Fe-B alloys can be prepared by reacting aqueous solutions of Fe salts and NaBH4. In this work the effect of the addition rate of the NaBH4 solution to the FeSO4 solution on the precipitate is investigated. The chemical composition of the amorphous alloys formed varies between Fe79B21 and Fe68B32. The hyperfine parameters of the alloys, derived from Mössbauer spectra, show a decrease from 29 to 25 T of the magnetic hyperfine field and an increase from 0.19 to 0.28 mms−1 of the isomer shift with increasing NaBH4 addition rate. The results suggest that alloys with different structures but identical composition may be produced by chemical reduction.

We discuss a novel approach to colloid synthesis in which high speed gas jets inject colloid forming species directly into liquids.

Collision phenomena involving the smallest solid objects and inert solid surfaces have been explored by the method of mass-selective time-of-flight and angular measurements of the surface-collision dynamics of ionized cluster beams. At lowest energy, both transient physisorption (soft-landing) and intact scattering have been observed. Impact of nanocrystalline sodium-fluoride clusters (NanFn-1+) against graphite at intermediate energy (0.2 - 1 eV per atom) leads efficiently to fracture-like processes. At higher energy, a featureless evaporative cascade dominates the fragment distribution. Energy- and angle-resolved scattering of mass-selected clusters from oriented graphite is shown to be a versatile method for investigation of cluster structure-energetics, cluster-surface interactions, and sticking phenomena.

Using high energy rare gas ion sputtering of metal targets, we are able to produce nanoamps of mass selected transition metal clusters. Mono-sized cluster ions are deposited at low kinetic energy upon substrates, e.g. silica or carbon, and are then characterized using UV and x-ray photoemission. In this paper we will discuss photoemission measurements of the 4f7/2 core level energies of Au (1–5,7 atom samples) clusters deposited on silica. From such studies we are beginning to understand how electronic structure, cluster stability and mobility depend on (deposited) cluster size, surface coverage, and substrate temperature.