Jena Nanoclusters

A Bridge across Disciplines
1. Auflage 2010
ISBN: 978-0-08-096422-5
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, Band Volume 1, 700 Seiten

Reihe: Science and Technology of Atomic, Molecular, Condensed Matter & Biological Systems

This comprehensive book on Nanoclusters comprises sixteen authoritative chapters written by leading researchers in the field. It provides insight into topics that are currently at the cutting edge of cluster science,ÿwith theÿmain focus on metal and metal compound systems that are of particular interest in materials science, and also on aspects related to biology and medicine. While there are numerous books on clusters, the focus on clusters as a bridge across disciplines sets this book apart from others. - Delivers cutting edge coverage of cluster science - Covers a broad range of topics in physics, chemistry, and materials science - Written by leading researchers in the field

Autoren/Hrsg.

Jena, Purusottam

Leseproben

Chapter 1 Introduction to Atomic Clusters P. Jena*, A.W. Castleman, Jr. † * Department of Physics, Virginia Commonwealth University, Richmond, Virginia, USA † Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania, USA Abstract This chapter provides a brief overview of the field of nanoclusters and the manner in which it has evolved. It starts with a definition of clusters and nanoclusters and provides a description of their main characteristics that distinguish them from molecules and nanoparticles. Atomic and electronic structure of clusters, as well as their properties, is highlighted with examples. The field of atomic clusters is vast and this short introduction does not do justice to all that is known. Rather, we have focused on some important developments. The reader is encouraged to read the remaining chapters that go in depth to various topics and show how clusters have been able to bridge many disciplines. Keywords Atomic clusters, magic numbers, electronic structure, magnetic properties, reactivity, melting properties, compound clusters, stability A cluster is defined by the American heritage dictionary as “a group of same or similar elements gathered together.” Consequently, clusters have different meanings depending on the “elements” of which they are composed. A few common examples include cluster cereals, cluster bombs, cluster headache, computer clusters, musical clusters, and clusters of stars and galaxies. However, in the physics and chemistry communities, the term “clusters” is typically used to describe an aggregate of atoms or molecules. Clusters can be formed when a hot plume of atoms or molecules in a gas are cooled by collision with rare-gas atoms much as droplets of water are formed when hot steam cools and condenses. Clusters composed of a finite number of atoms and molecules are an embryonic form of matter and have become a robust field of research in the last four decades. Molecules and nanoparticles also represent an aggregate of atoms as do clusters. For example, molecules can consist of as few as two atoms, that is, H2, to as many as a few thousand atoms, for example, proteins. In contrast, nanoparticles may consist of hundreds of thousands of atoms. In the early stage of development of these fields, nanoparticles were large, typically of the order of 10–100 nm, and clusters were small, typically less than 1 nm. With the progress in synthesis techniques, these size differences have now narrowed: clusters as large as a few thousand atoms or molecules and nanoparticles as small as 1–2 nm can now be produced. What then differentiates a cluster from a molecule or a nanoparticle? To distinguish clusters from molecules, we provide in Table 1 a summary of some of their properties. As pointed out before, both clusters and molecules are aggregates of atoms and may contain as few as two atoms to as many as thousands of atoms. However, molecules such as H2, O2, and N2 exist in nature under ambient pressures and temperatures, while clusters are made in the laboratory under vacuum or cold flow conditions. Unlike molecules that interact weakly with each other, clusters, in general, interact more strongly and often coalesce to form larger clusters. The size and composition of clusters can be varied easily whereas the composition of molecules is fixed by nature. A given cluster can exhibit numerous isomers where the atoms are arranged in different geometric patterns. The atomic structures of molecules, on the other hand, have specific geometries and only rarely exhibit isomeric forms. The electronic bond between atoms in a molecule is primarily covalent where atoms forming the bonds share their electrons. Clusters, on the other hand, show a variety of bonding schemes starting with weak van der Waals to metallic and strong covalent or ionic bonds. Molecules are abundant in nature whereas clusters need to be formed under special experimental conditions and their stability varies widely depending upon their size and composition. Thus, molecules are different from clusters. One exception may be C60, which, although discovered as a cluster, has most of the properties of a molecule. Table 1 Clusters Versus Molecules Molecules Clusters Available in nature Synthesized in a laboratory Stable in ambient environment Atomic clusters are stable only in vacuum or in inert environment Weakly interact with each other Interaction varies from weak to strong Size and composition are fixed Size and composition can be varied Typically very few isomers Numerous isomers Primarily covalent or ionic bonding Bonding can be weak van der Waals, metallic, ionic, or covalent Stable against coalescence Metastable and coalesce To distinguish between clusters and nanoparticles, we note that the size and composition of clusters can be controlled one atom at a time while in general the number of atoms in a nanoparticle cannot be determined with the same precision. Thus, clusters are the ultimate nanoparticles where the size and composition are known with atomic precision and the evolution of their properties can be studied one atom at a time. In Figure 1, we show a schematic plot of how a given property, be it the interatomic distance or electronic, magnetic, and optical property, varies as a function of size [1]. In clusters consisting of a few atoms, the properties change nonmonotonically, often varying widely with the addition of a single atom. As the cluster size reaches a few hundred to a few thousand atoms, the variations of properties with size become less drastic, and eventually the properties smoothly approach the bulk value. The fields of clusters and nanoparticles have been developing over the years in a parallel way. As clusters became large and nanoparticles became small, the distinctions between the two fields have narrowed and consequently clusters are often referred to as nanoclusters. Thus, nanoclusters can provide complimentary understanding of properties in nanoparticles and in some bulk materials. Figure 1 The cluster size dependence of a cluster property ?(n) on the number n of the cluster constituents. The data are plotted versus n–ß where 0 = ß = 1. “Small” clusters reveal specific size effects, while “large” clusters are expected to exhibit for many properties a “smooth” dependence of ?(n) which converges for n ? 8 to the bulk value ?(8) (see Ref. [1]). I A Brief History
The history of atomic and molecular clusters dates back to very early times. For example, it has been suggested that in the creation of the universe, very stable clusters such as C60 may have been formed [2]. Some of the unidentified infrared bands in interstellar matter are attributed to metal–organic clusters [3]. Similar examples of clusters in nature may be found in biology; ferritin is a shell of proteins that surround an Fe core of up to 4500 atoms [4]. Reference to the formation of aggregates and related nucleation phenomena in smoke and aerosols can be found in the literature [5] dating from the 1930s and earlier. Clusters were also used as models to study properties of extended systems [6-10] such as crystals and proteins by replacing these systems with a few atoms confined to the geometry of their bulk counterpart. This is particularly helpful in studying defects in crystals since carrying out band structure calculations without periodic boundary conditions was not possible due to limited computing power. Here, one assumed that the properties of defects are governed primarily by their interaction with a few neighboring atoms and a finite cluster where the atoms occupied the positions given by their parent crystal structure serves as a good model. In semiconducting or ionic systems, the dangling bonds of the atoms were saturated by hydrogen while in metals this was not necessary due to delocalized nature of the conduction electrons. How large a cluster has to be to account for the defect properties in the bulk remained as a nagging question which could only be solved by increasing the cluster size until the properties converged. However, the origin of clusters as we know it today can be traced to the first set of experiments [11] in mass spectrometer ion sources in the 1950s and 1960s when intense molecular beams at low temperatures were used to produce clusters by supersonic expansion. Most of early work on clusters involved molecular clusters, clusters of inert gas atoms, and of low-melting-point metals. With the advent of laser vaporization techniques [12], clusters of a vast majority of the elements in the periodic table can now be produced. Since the 1980s, we have witnessed work on clusters of transition and refractory metals as well as semiconductor elements and compound clusters consisting of binary and ternary elements. The early theoretical works...