Clusters semiconductor elements

Clusters of the semiconductor elements Si and Ge are much more dense than carbon clusters, but they are not spherical, either, as expected for closed packed atomic spheres. Si+ and Ge+ clusters are prolate with geometries based on stacked tricapped trigonal prisms [127-131]. At a certain cluster size (n 25 for SiJ) a structural transition occurs from prolate (n<25) to near spherical (n>25). Interestingly, clusters of tin, which is a metal at room temperature, exhibit very similar structures as Si and Ge, indicating that they are semiconductors as well [131,132]. Bulk Sn does have a semiconductor form (a-tin), that has the same diamond lattice as Si and Ge. Typical metal clusters appear to pack as tightly as possible and exhibit near spherical shapes as observed for the lead [131,133],indium [134],andgold [135] clusters. However, the smaller gold clusters AuJ (n<7) are completely planar [135]. 

To demonstrate the usefulness of the MD/DF approach, we now discuss applications to structure determination in clusters of elements of groups 13, IS, and 16. Clusters of the last two are typically covalently bonded systems. The bulk systems are generally semiconductors or insulators, and there is a substantial energy gap between the highest occupied and lowest unoccupied molecular orbitals. The first, typified by aluminium, show aspects of metallic behaviour. One of the advantages of the DF method is that it can be applied with comparable ease to elements of all atomic numters. 

Silicon is the typical semiconductor element, and Si clusters have attracted abundant attention since the 1980s. Determination of most stable... 

Mo6 octahedron) the cluster is electron-precise, the valence band is fully occupied and the compounds are semiconductors, as, for example, (Mo4Ru2)Se8 (it has two Mo atoms substituted by Ru atoms in the cluster). In PbMo6Sg there are only 22 electrons per cluster the electron holes facilitate a better electrical conductivity below 14 K it becomes a superconductor. By incorporating other elements in the cluster and by the choice of the electron-donating element A, the number of electrons in the cluster can be varied within certain limits (19 to 24 electrons for the octahedral skeleton). With the lower electron numbers the weakened cluster bonds show up in trigonally elongated octahedra. 

Abstract This review highlights how molecular Zintl compounds can be used to create new materials with a variety of novel opto-electronic and gas absorption properties. The generality of the synthetic approach described in this chapter on coupling various group-IV Zintl clusters provides an important tool for the design of new kinds of periodically ordered mesoporous semiconductors with tunable chemical and physical properties. We illustrate the potential of Zintl compounds to produce highly porous non-oxidic semiconductors, and we also cover the recent advances in the development of mesoporous elemental-based, metal-chalcogenide, and binary intermetallic alloy materials. The principles behind this approach and some perspectives for application of the derived materials are discussed. 

Silicon is the element in semiconductor industry. Due to the miniaturization of all semiconductor devices, it is also technological relevant to study the properties of very small structures based on only Si. Therefore, very many studies have been devoted to Si clusters. This also means that it is natural to apply new theoretical developments to Si clusters. Accordingly, Ballone et al 6 used the Car-Parrinello method, shortly after its introduction, to the study of the structural properties of Spy clusters with A up to 10. 

The surfaces of metal oxides and their H2 chemisorption characteristics have been far less studied than the surfaces of elemental metals and semiconductors [113,133]. Cation surface states are formed on ideal oxide surfaces at about 2 eV below the bottom ofthe conduction band. The charge of the surface ions is found to be reduced compared with that of the bulk ions and this leads to an enhanced co valency at the surface. The reduction amounts to less than 10 % for oxides of simple metals such as MgO and to 20-30% for transition metal oxides. Cluster and slab calculations reveal that special surface state bands with metallic character can be formed on polar surfaces by charge compensation effects. To what extent the metallic band accounts for special catalytic activity is not yet known [114]. 

The combination of MD/DF calculations and photodetachment spectroscopy provides a useful approach to the problem of determining the structure of atomic clusters. The PACIS provides a flexible method for generating clusters of many materials and for performing photoelectron spectroscopy on them. The elements include metals (solid and liquid), semiconductors, and highly reactive materials such as phosphorus. There is little doubt that the source will provide interesting data for many other systems. For the analysis of these data, or for predictions of interesting systems for study, the MD/DF approach should prove to be an ideal tool. 

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