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,with themain 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
|Series:||Science and Technology of Atomic, Molecular, Condensed Matter & Biological Systems Series , #1|
|Product dimensions:||5.90(w) x 9.10(h) x 1.40(d)|
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NanoclustersA Bridge across Disciplines
By Purusottam Jena A. Welford Castleman, Jr.
ELSEVIERCopyright © 2010 Elsevier B.V.
All right reserved.
Chapter OneIntroduction to Atomic Clusters
P. Jena and A. W. Castleman, Jr.
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.
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. 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.
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. Some of the unidentified infrared bands in interstellar matter are attributed to metal–organic clusters. 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. Reference to the formation of aggregates and related nucleation phenomena in smoke and aerosols can be found in the literature dating from the 1930s and earlier. Clusters were also used as models to study properties of extended systems 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 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, 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 were mostly phenomenological in nature and first-principles calculations dealt with very small number of atoms or molecules. With advancement in computer technology and development of efficient computer codes based on density functional theory, one is now able to model clusters containing as many as a thousand atoms.
The ability to synthesize and characterize clusters consisting of up to a few thousand atoms has given birth to a new field that forms a bridge not only between atoms, molecules, nanoparticles, and bulk matter but also between the disciplines of physics, chemistry, materials science, biology, medicine, and environmental science. The limited size and tunable composition allow clusters to have unusual combinations of physical and chemical properties. Metallic elements can become insulating, semiconductor elements can become metallic, nonmagnetic materials can become magnetic, opaque materials can become transparent, and inert materials can become reactive. It has also been suggested that clusters can be designed and synthesized by varying their size and composition such that they mimic the electronic properties of atoms. These clusters, originally termed unified atoms, are now commonly referred to as superatoms; they can form the basis of a new three-dimensional periodic table with superatoms constituting the third dimension. A new class of cluster-assembled materials where clusters instead of atoms form the building blocks can usher an exciting era in materials science with unlimited possibilities for new materials.
A first step in realizing this lofty goal is to understand how the properties of clusters evolve with size and composition and when they mimic properties of their corresponding bulk matter. How large does a cluster have to be before it can resemble a crystal? When does a metal become a metal? Is the evolution of the properties monotonic or does it vary widely with size? It was expected that by systematically studying the structure and properties of this new phase of matter as a function of size, one atom at a time, one can finally answer these fundamental questions. While much work has been done to achieve this understanding, studies of clusters have raised more questions than answers. Different properties evolve differently, and in most cases, the limiting value is not reached even for the largest clusters studied thus far. The field of atomic and molecular clusters has become a new and growing field of research in its own right. This book describes some of the unique properties of clusters and how they have helped to bridge our understanding in many disciplines.
II. ATOMIC STRUCTURE OF CLUSTERS
Crystals exhibit 14 different lattice symmetries. Among these, body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal close-packed (hcp) structures are among the most prevalent ones. The alkali metals such as Na, for example, form the bcc structure, while alkaline-earth elements such as Mg form the hcp structure. The coinage metals such as Cu, on the other hand, form the fcc structure. An understanding of how these structures evolve and how many atoms are needed for the crystal structure to emerge has been a fundamental question. Studies of the geometries of clusters as a function of size are expected to illustrate this point. However, with existing experimental techniques it is difficult to unambiguously determine cluster structure. Many clusters are too large for precise study by most spectroscopic techniques and often too small for diffraction techniques. Determination of cluster geometries is now possible through a synergy between theory and experiment. First-principles theory and well-developed computer codes allow researchers to determine the geometry of the clusters, their isomers, and relative stability up to a hundred atoms. Calculated electronic and vibrational properties of these clusters can be compared with experiments and a good agreement can provide a level of confidence on the theoretically determined structures. The experimental techniques that are frequently used for this comparison are photoelectron spectroscopy (PES), trapped ion electron diffraction (TIED), ion mobility, and infrared spectroscopy. We should note that there are many isomers of a given cluster and often the energy differences between low lying isomers are within the accuracy of theoretical methods. Thus, it is again difficult to predict with absolute certainty the ground-state geometry of a cluster. To make things more complicated, it is not always true that experimentally one observes the ground-state structure. Higher energy isomers with a large catchment area in the potential energy surface or having a spin multiplicity that differs from its ground-state spin may be present. In spite of these difficulties, considerable progress has been made and one has a reasonable understanding how the structures evolve.
To demonstrate this evolution, we plot in Figures 2–7 geometries of clusters of nearly free-electron metals such as Na and Be, noble metal Au, transition metal Ni, and semiconductor C and Si. We also discuss structures of compound clusters.
Excerpted from Nanoclusters by Purusottam Jena A. Welford Castleman, Jr. Copyright © 2010 by Elsevier B.V. . Excerpted by permission of ELSEVIER. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of Contents
1. Introduction to Atomic Clusters, P. Jena and A. W. Castleman, Jr 2. Clusters: An Embryonic Form of Crystals and Nano-structures, K. Hoang, M. S. Lee, S. D. Mahanti, and P. Jena 3. Applications of the Cluster Method for Biological Systems, R. H. Scheicher, M. Pujari, K. Ramini Lata, N. Sahoo and T. P. Das 4. Cluster Structures: Bridging Experiment and Theory, F. Janetzko, A. Goursot, T. Mineva, P. Calaminici, R. Flores-Morenocd, A.M. Köster, D.R. Salahub 5. Multiple Aromaticity, Multiple Antiaromaticity, and Conflicting Aromaticity in Planar Clusters, D. Yu. Zubarev and A. I. Boldyrev 6. Reactivity and Thermochemistry of Transition Metal Cluster Cations, P. B. Armentrout 7. Hydrogen and Hydrogen Clusters Across Disciplines, I. Cabria, M. Isla, M. J. Lόpez, J. I. Martínez, L. M. Molina and J. A. Alonso 8. Laser Induced Crystallization, A. Fischer, R.M. Pagni, R.N. Compton, and D. Kondepudi 9. Superatoms: From Motifs to Materials, A. C. Reber, S. N. Khanna and A. W. Castleman, Jr 10. Silica as an Exceptionally Versatile Nanoscale Building Material: (SiO2)N Clusters to Bulk, S. T. Bromley 11. Uncovering New Magnetic Phenomena in Metal Clusters , Mark B. Knickelbein 12. Metal clusters, Quantum Dots and Trapped Atoms – From Single-Particle Models to Correlation, M. Manninen and S.M. Reimann 13. Tailoring Functionality of Clusters and Their Complexes with Biomolecules by Size, Structures, and Lasers, V. Bonačić-Koutecký, R. Mitríc, C. Bürgel, and J. Petersen 14.Interfacing Cluster Physics with Biology at the Nanoscale, C. Leung and R. Palmer 15. Dynamics of Biomolecules from Firstprinciples, I. M. Degtyarenko and R. M. Nieminen