We have performed molecular dynamics simulations using a recently developed empirical many-body potential energy function to study the collision of the C60 isomer buckmin-sterfullerene with a hydrogen-terminated diamond surface. The simulations indicate that the cluster can react with the surface and has a larger probability of gaining atoms from the surface than of losing atoms to the surface. We have investigated the dependence of the reaction probability on the initial center-of-mass translational velocity of the cluster. The structures and energy distributions of the product clusters have been determined. Both inelastically and reactively scattered clusters have large amounts of internal energy which suggests that gas-phase dissociation is likely.

This paper reports the most recent results from the cluster chemistry program at the Naval Research Laboratory, in which our efforts in the characterization of gas-phase cluster properties have been extended to studies of condensed-phase species. First, in an attempt to investigate the fundamental interactions between mass-selected cluster ions and surfaces, two tandem mass spectrometers were constructed or modified, and the results of the initial experiments will be discussed. The emphasis will be on illustrating the general utility of ion/surface collisions to study fragmentations, reactions, and deposition. Second, clusters were deposited into a matrix in order to perform spectroscopic analyses. The initial experiments have been directed at optimization of the techniques. Finally, recent measurement of the ionization potentials of large carbon clusters will be reported. These results are especially significant because of the new developments in large-scale production, isolation, and characterization of these species.

We report on the successful softlanding of size selected metal clusters in rare gas matrices. Cluster-ions are sputtered from a metal target, energy filtered and mass selected by a quadrupole mass filter. Deposited clusters are detected by optical spectroscopic methods ( absorption, excitation, fluorescence). Our results show that for dimers and trimers fragmentation decreases with decreasing landing energy. In the case of Ag2 for example fewer than 25% of the dimers fragment when they are deposited at 20 eV. Triatomic clusters appear to fragment less easily than the dimers. New spectroscopic transitions have been found for both diatomic and triatomic clusters. Our results, when compared with previous studies on conventionally prepared clusters, show that some dimer and trimer features are incorrectly assigned.

The interaction between energetic clusters of atoms and solid surfaces has been investigated by molecular-dynamics computer simulation. In this model, the impact of Cu clusters containing 4, 13 or 92 atoms and energies of 326 eV with Cu or Ni substrates is investigated. Either embedded-atom-method (EAM) potentials or a combination of Moliere and EAM potentials were employed for the calculations. The simulations reveal that different physical processes occur depending on cluster size and energy, and the cohesive properties of the substrate.

Atomic scale imaging by STM and TEM of the initial stages of film growth of Ag and Au on graphite substrates indicate that the film nucleation processes are markedly different for ionized cluster beam (ICB) and molecular beam (MBE) deposition. Recent results on measurements of cluster size and formation of epitaxial metal-semiconductor layers by ICB are also discussed.

We present our latest attempts to utilize a Smalley-type cluster beam source, to generate novel thin films. This technique employs entraining, within a high pressure molecular beam expansion, the products generated from laser ablation of a rotating target rod. We will show how such a cluster beam source can be used to generate a high temperature material within a molecular beam and deposit it intact on a relatively cool substrate. By tailoring the various expansion conditions (ie., expansion pressure, laser fluence, type of carrier gas, pulse delay, etc...) one can drastically effect the morphology and chemical nature of the surface generated. This technique has the promise that it may be able to fabricate a wide variety of thin films with obvious industrial applications (superconducting thin films, diamond-like carbon films, patterned or multi-layered thin films, etc...)

Gold clusters (diam. = 1.0 ± .5 nm) are prepared in a gas aggregation source (MECS), expanded into a vacuum chamber to form a neutral cluster beam, and deposited at low impact velocity on room temperature substrates. When several monolayers of these clusters are deposited on clean substrates (nitrocellulose, glass, mica, NaCl), they form smooth ultra-thin films. These cluster-assembled films appear to be similar in quality to those produced by the Takagi-Yamada ion cluster beam process. They exhibit finite electrical conductivity at thicknesses much smaller than is the case with atomically evaporated films. They are extremely uniform and smooth with a surface height that typically varies less than 1 nm across the entire film.

Raman scattering and ultraviolet photoelectron spectroscopy on Ge clusters and ultrathin films before and after H chemisorption reveal the significant changes in the vibrational and electronic densities of states with size. These effects are attributed to the large fraction of surface dangling bonds and the important role of short range order changes on physical properties.

A novel technique for fabricating uniform, ultrafine metal clusters of controlled size and distribution is described. The technique employs conventional adiabatic expansion of an evaporated source material for controlled formation of the clusters, but additionally incorporates electron-impact ionization and acceleration for further control of the size and distribution of the deposited particles. The key control parameters investigated are the diameter of the adiabatic nozzle, the proportion and energy of the ionized particles in the cluster beam, the substrate temperature, and the deposition time. The use of electron irradiation and acceleration of the source beam has proven to be particularly effective in producing monodisperse clusters. Using this new approach, uniform Ni clusters as small as 10 Å were deposited on an amorphous carbon substrate. The size and structure of the deposited Ni clusters were analyzed using high-resolution TEM and STEM.

The evolution of the nanostructure of a-Ge sputter-deposited onto naturally oxidized c-Si and disordered carbon substrates is characterized by in situ ellipsometry, with the focus of attention being on the initial stage when clusters are in evidence. The structural data deduced from ellipsometry are reproducible, are a sensitive function of the preparation parameters, and are also consistent with data deduced from less sensitive Auger electron spectroscopy. The information provided by ellipsometry is of critical importance in studying the effects of average size on the electronic and vibrational properties of clusters by ultraviolet photoelectron spectroscopy and Raman spectroscopy. Other structural probes of clusters for this system, including transmission electron microscopy, have failed to yield reliable results due to the small cluster size.

Oblique depositlon(OD) provides an attractive anisotropic structure In nm-scale as a promising host structure to form anisotropic nm-scale composites(ANSC). Among a variety of possible methods to introduce a guest material Into the host structure of OD, we have attempted simultaneous oblique deposition of a guest and a host material from two different directions. The computer simulation for ballistic deposition, which had greatly contributed to the understanding of the morphology evolution in OD, has been carried out again here to design possible ANSC structures and to survey their variations. A typical ANSC structure obtained here is a composite of slender clusters of the guest and the host materials, which can be regarded as an anisotropic version of the well-known isotropic structures of the metal-insulator or semiconductor-insulator cermets for magneto-optical or nonlinear optical applications. This appears when the host and the guest materials are deposited from the directions of the same polar angle of 70° but from the reverse azimuthal directions. In accordance with this result, a specially designed sputtering apparatus has been constructed. Structures and optical anisotropy of the films of several composites' systems such as ZnTe-SiO2 and Cu-SiO2 formed by this apparatus have indicated that our attempt has hit the mark.

This paper discusses the formation of metal clusters on Xe buffer layers and the deposition of those clusters on GaAs, silica, and Bi2Sr2CaCu2O8 when the Xe is desorbed. These clusters contain hundreds to thousands of atoms, and their interactions with the substrate is different from that encountered when atoms are deposited directly. Since cluster assembly alters the reaction pathway at the surface, novel structures with unique chemical and physical properties can be stabilized.

We present an experimental model where magnetic balls in a box are used to simulate aggregation of small ferromagnetic particles a few nm in diameter. The aggregation can be characterized by a fractal dimension that increases as the magnetic moment of the particles decreases. The magnetization of the system of balls has been measured in an external magnetic field and compared for different values of the magnetic moments.

It is shown that when transparent dielectric particles are deposited on metallic surfaces, excitation of surface plasma waves (SPW) becomes possible. The coupling of light with SPW is done via mirror-image effects. A drop in reflectance is observed at the resonance frequency in a similar fashion as in surface roughness experiments. Experimental evidence is proved by depositing very thin layers of hafnium oxide as well as tin oxide on silver surfaces.

Homogeneous gelled composites of iron and vitreous alumina containing 10-40% Fe have been prepared by room temperature polymerization of aqueous aluminum alkoxide solutions containing ferric nitrate and nitric acid at low pH. Scanning electron microscopy, x-ray diffraction, and Mossbauer spectroscopy demonstrated that this bulk material is comprised of nanometer-sized regions of iron compounds embedded in a vitreous alumina gel matrix. Magnetization data showed that in the as-cured condition these nanocomposites are paramagnetic at room temperature and become either superparamagnetic or ferromagnetic on cooling to 10 K. The magnetic susceptibility increased with the Fe content and with decreasing temperature. Analysis of the temperature dependence of the magnetic susceptibility indicated the magnetic moment per Fe atom was 1.87 µB for the 10% Fe nanocomposite and that it increased linearly with composition to 1.96 µB for the 40% Fe material. Mössbauer effect data showed that subsequent treatment of these materials in a gaseous environment of hydrogen at elevated temperatures (T<400 C) changed the form of the iron in the magnetic regions. These results are compared to that observed for similar nanocomposites prepared using a silica gel matrix.

Materials with nanometer-sized microstructures are solids that contain such a high density of defects (point defects, dislocations, grain boundaries, interphase boundaries etc.) that the volume fraction of the defect cores becomes comparable to the residual, defect free volume. Since 1970, the interest of materials scientists focused on solids with nanometer-sized microstructures when it was recognized that specifically tailored nanometer-sized microstructures permit the generation of materials with new atomic and/or electronics structures. In the area of semiconductors, quantum well structures and superlattices consisting of thin coherent layers with different dopings or compositions have been studied extensively. In these structures quantum size effects prevail because the coherent boundaries structures between the layers act as electron confinements. Research activities aiming towards the synthesis of new atomic (and electronic) structures in metals, ceramics and semiconductors by means of three-dimensional nanometer-sized micro-structure were initiated by the proposal, to generate solids a large volume fraction of which consists of the cores of defects such as incoherent grain and/or interphase boundaries, such solids differ structurally and property-wise from crystals and glasses with the same chemical composition because the atomic arrangements formed in the cores of incoherent grain or interphase boundaries deviate from crystalline or glassy structures. Recent studies of nanometer-sized materials by x-ray/neutron diffraction, EXAFS, different spectroscopies as well as property measurements support this idea. The properties of materials with nanometer-sized microstructures are discussed on the basis of the knowledge available from the numerous studies that have been carried out in the last two decades about the structure and properties of grain and/or interphase boundaries.

Quantitative x-ray diffraction measurements of ultrafine-grained (nanocrystalline) Pd and a coarse-grained polycrystalline reference foil were obtained using synchrotron radiation. The Bragg reflection intensity profiles of nanocrystalline Pd were found to be considerably better represented by Lorentzian functions than by Gaussian functions, indicating that a large fraction of the intensity from the Bragg peaks is located in the tails of the reflections. The intensity that is in the Lorentzian-shaped Bragg peaks differs only slightly for different grain-sized materials; thus, the atomic relaxations in the vicinity of grain boundaries do not produce broadly distributed diffuse scattering other than that found in the Bragg reflections. The results of the present work do not support the previously proposed existence of either a “gas-like” grain boundary phase, or large quantities of vacancies or voids within grains of nanocrystalline Pd.

Nanophase TiO2 and ZnO were investigated with a Raman microprobe in order to determine the spatial variations in material composition, oxygen stoichiometry, and phase. Calibration of the oxygen stoichiometry of nanophase TiO2 was determined through a combination of Raman scattering and thermogravimetric analysis.

Recently, there has been a growing interest on materials called “nanocrystalline solids”. These materials are made of crystals with sizes ranging typically from 1 to 10 nm. Several methods have been used to produce these solids. One of them is the high energy mechanical deformation (HEMD) process. Indeed, we have shown recently that some iron-based amorphous alloys crystallize completely into a nanocrystalline structure when they are subjected to HEMD. In this work, we present a TEM study of the nanocrystals formed by the mechanical crystallization of Fe66Co18si1B15 amorphous metallic ribbons.

Consolidated powders of nanocrystalline Cu and Pd have been studied by x-ray diffraction (XRD) and high resolution electron microscopy (HREM) as part of an investigation of the mechanical behavior of nanocrystalline pure metals. XRD line broadening measurements were made to estimate grain size, qualitative grain size distribution and average long range strains in a number of samples. Mean grain sizes range from 4-60 nm and have qualitatively narrow grain size distributions. Long range lattice strains are of the order of 0.2-3% in consolidated samples. These strains apparently persist and even increase in Cu samples after annealing at 0.35 Tm (498K) for 2h, accompanied by an apparent increase in grain size of e2x. Grain size, grain size distribution width and internal strains vary somewhat among samples produced under apparently identical processing conditions. HREM studies show that twins, stacking faults and low-index facets are abundant in as-consolidated nanocrystalline Cu samples. Methodology, results and analysis of XRD and HREM experiments are presented.