The electronic structure and properties of small clusters are strongly dependent on their size. These size specific properties are illustrated by confining the discussion to three different topics (1) interaction of hydrogen with clusters (2) effect of temperature and magnetic field on the magnetization of clusters and (3) reaction of clusters with gas atoms. It is shown that the electronic energy levels that are strongly dependent on cluster size give rise to the size specific electronic properties of clusters. In cluster hydrides certain elements are found to absorb more hydrogen per atom in cluster form than in a crystalline form. The magnetization of clusters increases as a function of cluster size and externally applied magnetic field. In clusters reacting with gas atoms, it is possible to find the product cluster in an electronically excited state. The energetics of these states are strongly dependent on the cluster size and the reacting gas.

Experiments are described which illustrate that resonant neutralization of mass-selected cluster ions provides a universal method to produce neutral mass-selected metal cluster beams. Specifically, lead cluster ions, Pbn+ with n d 12, were neutralized by charge exchange reactions in atomic sodium and other target materials. Absolute charge exchange cross sections are on the order of 40 Å2 and up to 30% of the incident ions were neutralized. Possible fragmentation of the clusters associated with neutralization was investigated by translational spectroscopy. No indication of fragmentation was found for resonant electron transfer with atomic Na as target gas.. Thus, neutral metal cluster beams with ultra-narrow size distribution are produced. Deposition of such size-selected metal clusters on substrates opens new perspectives in materials science.

The size and size distribution of small metal particles on surfaces can be manipulated with laser light. This has become possible by use of a nonthermal desorption process based on surface plasmon excitation in the particles. Most importantly, a considerable narrowing of a given size distribution on a surface can be achieved. Recent experiments on the desorption mechanism and the size manipulation are summarized.

Electronic structure calculations on neutral and charged Mgn (n≤7) clusters and KnMgm mixed clusters have been carried out within a linear combination of atomic orbitals molecular orbital approach. The exchange correlaton effects have been treated via local spin density functional and non-local pseudopotentials have been used to replace the core. Our studies on charged Mgn clusters focus on the electron affinity and atomic binding in anionic clusters and the stability and observability of doubly ionized clusters. In KnMgm clusters, we have considered the evolution of the electronic spectrum and the variation of the binding energy with size and composition.

High-spin 13-atom transition metal cluster are studied using the linear-combination-of-Gaussian-type-orbital, local-density-functional, all-electron method. The high degeneracy associated with high symmetry is exploited to find geometry optimized clusters that have the highest contribution by spin to the magnetic moment subject to a constraint. The constraint is that the cluster not distort by the Jahn-Teller mechanism and thus have the lowest possible orbital-angular-momentum component of the magnetic moment. Pure 13-atom nickel and iron clusters are approximately icosahedral because of increased symmetry-required orbital degeneracy for electrons of different spins in this largest point group. The central atom of the icosahedral iron cluster has been varied to optimize the spin of the cluster keeping the orbital contribution to the magnetic moment quenched. Varying the central atom under this constraint can alter the magnetic moment by 20%, relative to the pure iron cluster.

Over the past years, theoretical and experimental investigations have shown that Ce plays an important role in determining the electronic and magnetic properties in CeFe2- Recent experiments indicate that Ce carries a small moment in contrast to the earlier notion of its being non-magnetic. It is also been found that various additives to the intermetallic CeFe2 alloy behave differently when substituted at the Fe site. The magnetization in some cases remains nearly the same whereas in other cases drops off drastically. We report here the effect of Ti impurity substituted at the Fe site in CeFe2.

Our studies of small (n=2−40) gas phase transition metal clusters (cations, neutrals and anions) have revealed a number of size-dependent chemical and physical properties. This paper will discuss results involving activated dissociative molecular chemisorption of hydrogen (deuterium) and small alkanes on cationic, neutral and anionic platinum and gold clusters. For example, we have yet to find any size gold cluster anion to exhibit measurable reactivity towards di-deuterium, but reactivity is observed on small (n<15) gold cations and very small (n<9) neutral gold clusters. Methane or ethane chemisorption occurs most readily only on small Pt and Au cluster cations and least rapidly, if at all, on cluster anions. The larger Pt and Au clusters are found to be non-reactive towards the alkanes under our experimental conditions.

At the other extreme, at steady state, small clusters of Pt, Rh, Ni, Pd, V, Nb, and Ta are hydrogen “rich” exhibiting H/M stoichiometrics >> 1. These results are consistent with a limited data base on supported clsutes obtained via EXAFS measurements and have important implications for catalytic reactions involving hydrogen and light hydrocarbons.

Raman scattering measurements of Bi clusters formed on disordered C films at 110K exhibit a phase transformation from nanocrystalline rhombohedral structure to a suggested disordered phase. XPS measurements on this phase indicate core level shifts attributed to intrinsic, initial state effects on cluster electronic states.

A vapor deposition technique has been used to prepare nano-size particles of Fe, Co and Ni using argon gas. The particles were passivated from further oxidation using a small volume of air. The range of particle size obtained in these systems was 47–200 Å. The saturation magnetization of Fe particles varied between 25–200 emu/g with the higher values corresponding to larger particles and the highest coercivity achieved at room temperature was 1050 Oe. In the case of Co and Ni, the magnetization varied in the range 35–100 emu/g and 14–45 emu/g, respectively. The highest room temperature coercivity was 1200 and 41 Oe for Co and Ni, respectively. A shell-type structure consisting of a metallic core surrounded by an oxide shell has been proposed for the particles.

Reaction studies with trimethylamine on positively charged laser-annealed silicon clusters of 39, 43, and 48 atoms magnetically levitated in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer reveal the same relative reaction pattern as previously observed for identical studies with ammonia and ethylene. The 39th cluster was found to be considerably less reactive than the other positive cluster sizes, and significantly, its negative ion is also relatively less reactive than the 43rd and 48th. The continued observation of this reactivity trend provides additional experimental evidence that the annealed clusters adopt well-organized structures, and that these crystal forms are similar for both charge states.

In this report we discuss the photoelectron spectrum of Si2− obtained at 355 nm. The spectrum shows a series of transitions to the low-lying triplet states of Si2 as well as transitions to the higher-lying singlet states. Several possible assignments of the anion and neutral triplet states are presented.

The lattice vibrational properties of two clusters of Si atoms are studied using the Stillinger-Weber phenomenological interatomic potential to calculate the dynamical matrix. The clusters contain 33 and 45 atoms, and their structure was postulated from considerations of surface reconstructions in analogy to crystalline Si surfaces. A remarkable similarity to the vibrational density of states of amorphous Si is demonstrated especially for the 45 atom cluster.

Electronic contributions to the optical (hyper)polarizabilities of small silicon clusters are theoretically determined. Geometries and the electronic structures of the clusters are established using the tight-binding model. The nonlinear polarizabilities are found to depend primarily on the symmetry of the cluster and prove to be high for the low-symmetry clusters. Possible experiments and applications are discussed.

An electrostatic deposition process which utilizes charged particle motion in an electric field has been successfully developed as a method for depositing coatings of silicon films. Coatings have been deposited on conducting, Insulating, and semiconducting surfaces after making suitable changes in deposition parameters. This process along with parameters Important to the deposition process which Include particle charge and size characteristics are discussed. These coatings are subsequently heat treated. Experimental results for depositions on silicon substrates are presented. Characterization in terms of chemical, electrical and physical properties of the film are presented.

We combine the principles of phonon and electron quantization in confined systems and breakdown of wavevector conservation in the photon-electron and phonon-electron couplings to describe the Raman spectrum of nanocrystals. The approach proposed here is consistent with experimental results and models developed for Raman scattering from quantum wells and superlattices.

We have studied the formation of molecular scale particles or “clusters” of alkali halides and semiconductors during laser vaporization of solids. By measuring the abundances of cluster ions produced in several different source configurations, we have determined that clusters are ejected directly from the source sample and do not need to grow from atomic or molecular vapor. Using samples of mixed alkali halide powders, we have found that unalloyed clusters are easily produced in a source that prevents growth from occurring after the clusters leave the sample surface. However, melting the sample or encouraging growth after vaporization lead to the production of alloyed cluster species.

The sizes of the ejected clusters are initially random, but the population spectrum quickly becomes structured as hot, unstable-sized clusters decay into smaller particles. In carbon, large clusters with odd numbers of atoms decay almost immediately. The hot even clusters also decay, but much more slowly. The longest lived clusters are the magic C50 and C60 fullerenes. The mass spectrum of large carbon clusters evolves in time from structureless, to only the even clusters, to primarily C50 and C60. If cluster growth is encouraged, the odd clusters reappear and the population spectrum again becomes relatively structureless.

The laser vaporization process in non-metallic systems closely resembles multiphoton laser photodissociation of clusters themselves. As a result, the best cluster source samples are powders or powder-like materials. We have found that carbon powder or exfoliated graphite sheet (Union Carbide Grafoil) are the best samples for the production of large carbon clusters. Many previous studies of carbon clusters formed in vacuum may have relied on the vaporization of carbon dust produced by repeated vaporizations in a small region of the sample.

I report the electronic structures of “buckminsterfullerene”, C60 cluster with truncated-icosahedron geometry, and C60X where X is a foreign atom trapped at the center of the C60 cage (X=K, O, and Cl). The local-density approximation with Xα exchange-correlation potential is used. The obtained electronic structure of C60 has a rather large energy gap between the completely occupied highest-occupied state and the lowest-unoccupied state, in accord with previous calculations. In the case of C60K, a valence electron of an alkali-metal atom is found to transfer to the outer C60 surface. On the other hand, in C60O and C60Cl, 2p state of O and 3p state of Cl appear inside the energy gap of the pure C60 cluster. Such variety of results indicates great possibilities of C60X as constructing units of new materials.

The results of boron cluster ion reactions with SiH4 and N2O are presented. For N2O reactions, significant size effects were observed in the branching between the BnO+ and BnN+ product channels. Carbon cluster ion reactions with D2 reveal the effect of structural isomers on cluster reactivity.

Small-particle clusters of CdS, CdSe, CdTe, PbS, and PbSe have been synthesized in aqueous solution. Absorption spectra with very sharp maxima were obtained for extremely small clusters (diameters < 30 Å). However, when clusters grow the excitonic absorption bands gradually decrease and the sharp maxima are replaced by broad shoulders. Cadmium chalcogenide clusters capped with 3-mercapto-l, 2-propanediol can be collected as stable solids with the same properties as original clusters. A method for the incorporation of semiconductor clusters in transparent silicate glasses has been developed. This technique relies on the low temperature sol-gel process, and silane alkoxide as the starting material. The absorption spectra of the clusters in the glasses are identical to those of the starting aqueous solutions. Excess charge carriers on PbS and CdS clusters lead to a blue shift in the optical absorption edge of semiconductors. The appearance of this shift depends critically on the method of colloid preparation and surface chemistry.

We examine the effects of finite crystallite size on the energies of optical transitions originating from states located at two different critical points of the semiconductor Brillouin zone. Using a versatile, dual source, R.F. sputtering technique, CdTe-glass composite thin films have been produced possessing average crystal sizes ranging from 24 to 125 Å in films containing up to 30 vol% semiconductor. Analysis of quantum-size induced transition energy shifts, monitored by optical absorption, indicates the persistence of significant Coulomb interactions between carriers at the gamma point of CdTe in crystallite sizes 0.3 times the size of the bulk exciton. L-point transition energy shifts support the existence of 2-D bound electron-hole pair states whose center-of-mass motion is confined in the semiconductor crystals.