Surface properties, small clusters

The surface of small clusters, Na Clm, containing ten or so ions, has also been described and explored theoretically where edges and vacancies control the properties of adsorbed molecules [16,17]. Whereas Na" " and Cl" are iso-electronic with the rare gases Ne and Ar, respectively, and are anticipated to be inert, defects consisting of neutrals, Na and Cl, will exhibit chemical reactivity. Vacancies containing electrons, F centers [11], and other defect centers of small clusters have also been described [17]. 

Small metal clusters are also of interest because of their importance in catalysis. Despite the fact that small clusters should consist of mostly surface atoms, measurement of the photon ionization threshold for Hg clusters suggest that a transition from van der Waals to metallic properties occurs in the range of 20-70 atoms per cluster [88] and near-bulk magnetic properties are expected for Ni, Pd, and Pt clusters of only 13 atoms [89] Theoretical calculations on Sin and other semiconductors predict that the stmcture reflects the bulk lattice for 1000 atoms but the bulk electronic wave functions are not obtained [90]. Bartell and co-workers [91] study beams of molecular clusters with electron dirfraction and molecular dynamics simulations and find new phases not observed in the bulk. Bulk models appear to be valid for their clusters of several thousand atoms (see Section IX-3). 

Plieth WJ. 1982. Electrochemical properties of small clusters of metal atoms and their role in surface enhanced Raman scattering. J Phys Chem 86 3166-3170. 

Gas phase transition metal cluster chemistry lies along critical connecting paths between different fields of chemistry and physics. For example, from the physicist s point of view, studies of clusters as they grow into metals will present new tests of the theory of metals. Questions like How itinerant are the bonding electrons in these systems and Is there a metal to non-metal phase transition as a function of size are frequently addressed. On the other hand from a chemist point of view very similar questions are asked but using different terminology How localized is the surface chemical bond and What is the difference between surface chemistry and small cluster chemistry Cluster science is filling the void between these different perspectives with a new set of materials and measurements of physical and chemical properties. 

Classical interaction potentials for molecular systems can be pairwise or take manybody interactions into account, molecules can be rigid or flexible, and both stretching and torsion motions can be considered [28]. Polarizability can be introduced, but for example for water this does not seem to make the potentials mimic reality unambiguously better. The potentials, also called force fields, are fitted to reproduce certain sets of properties, and can fail to reproduce other characteristics. The potentials suffer from problems with transferability for example a water-sulfuric acid potential developed for the two-component mixture does not necessarily describe the interaction between water and sulfuric acid in a three-component mixture of water, sulfuric acid and ammonia. Also, potentials created for bulk liquid do not always work for surface layers or small clusters. 

The study of small, homonuclear clusters of atoms Is Important In understanding nucleatlon because such clusters are Intermediates In the formation of bulk condensed phases. The dynamic process of condensation from a gas must Initially Involve the formation of tiny aggregates of the new phase. This can be Illustrated by the reaction sequence A(g)—A2(g)— A3(g)— . . . — A(1). One of the major weak points In the present day understanding of such nucleatlon phenomena Is the unknown thermodynamic properties of clusters. Certainly, the common practice of treating a 2-200 atom cluster as a tiny piece of the bulk with a large surface Is Inexact. There Is a need for precise thermodynamic data on atomic and molecular clusters to better define nucleatlon kinetics. 

In addition to being coordinatively highly unsaturated, small clusters also are dominated by their surface properties. For example, a 19 atom cluster may have at most one or two interior atoms. Even a 100 atom cluster has only about 28 interior (bulklike ) atoms. Thus the surface sensitive properties become increasingly important as the size becomes smaller. Clusters do not possess the long range periodicity one would have with a bulk crystal. Thus we expect that each small cluster will be a unique entity, and that its ground state structure (or structures if different isomorphic structures are nearly isoenergetic) will depend sensitively on size and likely will not be simply a subunit of the bulk lattice. 

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