atom cluster

Atom cluster

Clusters are aggregates of atoms (or molecules) containing between three and a few thousand atoms that have properties intermediate between those of the isolated monomer (atom or molecule) and the bulk or solid-state material. The study of such species has been an increasingly active research field since about 1980. This activity is due to the fundamental interest in studying a completely new area that can bridge the gap between atomic and solid-state physics and also shows many analogies to nuclear physics. However, the research is also done for its potential technological interest in areas such as catalysis, photography, and epitaxy. A characteristic of clusters which is responsible for many of their interesting properties is the large number of atoms at the surface compared to those in the cluster interior. For many kinds of atomic clusters, all atoms are at the surface for sizes of up to 12 atoms. As the clusters grow further in size, the relative number of atoms at the surface scales as approximately 4N-1/3, where N is the total number of atoms. Even in a cluster as big as 105 atoms, almost 10% of the atoms are at the surface. Clusters can be placed in the following categories:

1. Microclusters have from 3 to 10–13 atoms. Concepts and methods of molecular physics are applicable.

2. Small clusters have from 10–13 to about 100 atoms. Many different geometrical isomers exist for a given cluster size with almost the same energies. Molecular concepts lose their applicability.

3. Large clusters have from 100 to 1000 atoms. A gradual transition is observed to the properties of the solid state.

4. Small particles or nanocrystals have at least 1000 atoms. These bodies display some of the properties of the solid state.

The most favored geometry for rare-gas (neon, argon, and krypton) clusters of up to a few thousand atoms is icosahedral. However, the preferred cluster geometry depends critically on the bonding between the monomers in the clusters. For example, ionic clusters such as those of sodium chloride [(NaCl)N] very rapidly assume the cubic form of the bulk crystal lattice, and for metallic clusters it is the electronic structure rather than the geometric structure which is most important. See Crystal structure

There are two main types of sources for producing free cluster beams. In a gas-aggregation source, the atoms or molecules are vaporized into a cold, flowing rare-gas atmosphere. In a jet-expansion source, a gas is expanded under high pressure through a small hole into a vacuum.

In most situations, the valence electrons of the atoms making up the clusters can be regarded as being delocalized, that is, not attached to any particular atom but with a certain probability of being found anywhere within the cluster. The simplest and most widely used model to describe the delocalized electrons in metallic clusters is that of a free-electron gas, known as the jellium model. The positive charge is regarded as being smeared out over the entire volume of the cluster, while the valence electrons are free to move within this homogeneously distributed, positively charged background.

atom cluster

[′ad·əm ‚kləs·tər]
(physical chemistry)
An assembly of between three and a few thousand atoms or molecules that are weakly bound together and have properties intermediate between those of the isolated atom or molecule and the bulk or solid-state material.
References in periodicals archive ?
12(a) we show the 87 atom cluster used to calculate the XANES of crystalline Ge.
The molecule easily scarfed up as many as five loose copper atoms and then delivered the atom cluster to a desired location, the team reports in the December Nature Materials.
We have represented our system as a 208 atom cluster consisting of a top hexagonal layer of 33 chloride ions, a middle layer of 33 copper atoms and five hexagonal layers of gold atoms which contained 142 gold atoms in total.
In atom clusters, it is not unusual for hydrogen atoms to occupy a bridge site between two atoms/he says.
They found that the 11- and 13-gold atom clusters form closed electronic shells with 8 electrons and the 39-atom gold clusters with 34.
2]', Nature, 254:503-504, 1975; 'Transition Metal Atoms in the Synthesis of Binuclear Complexes', Angewandte Chemie International Edition, 14:292-303, 1975, 'Metal Atom Matrix Chemistry -- Correlation of Bonding with Chemisorbed Molecules', Accounts of Chemical Research, 10:21, 1977; 'Relative Extinction Coefficient Measurements for Naked Silver Atom Clusters Ag1,2,3 by Photoaggregation Techniques', Journal of the American Chemical Society, 100:6776-6777, 1978; 'Cobalt, Nickel and Copper Naked Metal Clusters and Olefin Chemisorption Models, Coordination Chemistry Reviews, 28:117-147, 1979; 'Selective Photoaggregation of Metal Ato ms to Small, Bimetallic Clusters of Known Size -- The Chromium-Molybdenum System [Cr.
Fullerenes are a recently discovered form of carbon consisting of 60 or 70 atom clusters arranged in soccer-ball-like cage structures, which allows them to be combined with other molecules to form carbon- based derivatives with possible uses as lubricants, coatings, catalysts and in electronic devices.
In the March 2 NATURE, Alivisatos, Peng, and their coworkers described how they created rod-shaped atom clusters.
By focusing flickers of energy lasting much less than a trillionth of second into an extremely small volume, the laser beam heats atom clusters to tens of millions of degrees Celsius, report Todd Ditmire and his colleagues at Lawrence Livermore (Calif.
In addition, the ability to pick up and move small numbers of atoms will make it easier for chemists to study the properties of atom clusters, says Avouris.