Quantum Phenomena in Clusters and Nanostructures

276 Pages·2003·9.744 MB·English

by  Prof. S. N. Khanna, Prof. A. W. Castleman Jr. (auth.)

Springer Series in
CLUSTER PHYSICS
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Springer Series in
CLUSTER PHYSICS
Se ries Editors:
A. W. Castleman, Jr. R. S. Berry H. Haberland J. Jortner T. Kondow
The intent of the Springer Series in Cluster Physics is to provide systematic infor
mation on developments in this rapidly expanding field of physics. In comprehen
sive books prepared by leading scholars, the current state-of-the-art in theory and
experiment in cluster physics is presented.
Mesoscopic Materials and Clusters
Their Physical and Chemical Properties
Editors: T. Arai, K. Mihama, K. Yamamoto and S. Sugano
Cluster Beam Synthesis ofNanostructured Materials
By P. Milani and S. Iannotta
Theory of Atomic and Molecular Clusters
With a Glimpse at Experiments
Editor: J. Jellinek
Metal Clusters at Surfaces
Structure, Quantum Properties, Physical Chemistry
Editor: K.-H. Meiwes-Broer
Clusters and Nanomaterials
Theory and Experiment
Editors: Y. Kawazoe, T. Kondow and K. Ohno
Quantum Phenomena in Clusters and Nanostructures
By S.N. Khanna and A.W. Castleman, Jr.
Water in Confining Geometries
By V. Buch and J.P. Devlin
Series homepage - hup:// www.springer.de/phys/books/cluster-physics/
S.N. Khanna A.W. Castleman, Jr.
Quantum Phenomena
in Clusters
and Nanostruetures
With 115 Figures and 8 Tables
Springer
Prof. S.N. Khanna
Department of Physies, Virginia Commonwealth University
Riehmond, VA 23284-2000, USA
[e-mail]
Prof. A.W. Castleman, Jr.
Eberly Distinguished Chair in Scienee
Evan Pugh Professor, Department of Chemistry and Physies
152 Davey Laboratory, The Pennsylvania State Univesity
University Park, PA 16802, USA
[e-mail]
ISSN 1437-0395
ISBN 978-3-642-05503-4 ISBN 978-3-662-02606-9 (eBook)
DOI 10.1007/978-3-662-02606-9
Library of Congress Cataloging-in-Publication Data
Khanna, S. N.
Quantum phenomena in clusters and nanostruetures / S.N. Khanna, A.W. Castleman, jr. p. em. --(Springer
series in cluster physies, ISSN 1437-0395)
1. Nanostruetures. 2. Quantum theory. I. Castleman, Y. W. (Albert Welford), 1936-11. Title. III. Series.
QC176.8.N35 K43 2003
530.12--de2l 2002034365
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Preface
One of the greatest triumphs of the last century was the development of
quantum mechanics. The sharp lines and their position in the atomic spectra
were the prime motivation that led to the Bohr model of the hydrogen atom,
with the development of the Schrödinger equation ultimately providing the
framework for understanding the properties of matter at the molecular level.
In the experimental arena, technical developments over the past thirty years
have enabled researchers to devise methods to fabricate structures that are so
small that the energy levels of the systems present a discrete energy spectrum
where the stability and the reactivity are determined by the nature of these
electronic levels and the degree to which they are filled. In these miniature
solids, clusters in some cases or assemblies that constitute nanoscale materials
in others, the properties are controlled by the discrete quantum conditions
associated with reduced size. Consequently, they are ideal systems for observ
ing quantum effects. Indeed, numerous novel phenomena, e.g., magie numbers
in the mass spectra, macroscopic quantum tunneling of magnetization, and
quantum corrals, have provided novel examples for observing quantum effects
at the nanoscopic scale. Of equal importance are the developments in experi
mental techniques that are now enabling researchers to unravel the quantum
evolution by probing the excitation/relaxation dynamics of electronic states
in real time. Theoretical techniques have developed to the extent of not
only providing a fundamental understanding of the properties of nanoscale
systems, but also having predictive capability. These unprecedented devel
opments are going to guide scientific thinking and material designs well into
the next century.
The field of clusters and nano-scale materials is itself a rapidly developing
area of research for other reasons. This is due in part to developments in
experimental techniques ranging from supersonic molecular beams, sol-gel
formation, sputtering, ball milling, to ones used for the formation of micelles
and polymers. Indeed, it has been possible to generate free and embedded
clusters of controlled size and composition, nanoscale particles containing
up to several million atoms, nanocomposites, and nanocrystalline materials.
The enormous interest in these systems stems from the fact that they display
a totally new class of physical, chemieal, electronic, magnetic and catalytic
properties attributable to the reduced size and related aspects of quantum
VI Preface
confinement. Further, the properties change with size and composition. One
can thus understand how the quantum character evolves from atoms to the
bulk solids. These types of behavior offer not only challenges for their funda
mental understanding, but also avenues for new technologies. For example, it
is envisioned that small clusters/nano-structures could serve as the building
blocks of a new class of cluster assemblies. Three examples are the fullerides
made from fullerenes, cluster-assembled solids made from Met-Cars, and
nanostructures made from quantum dots.
In this volume, we have collected a diverse set of topics to highlight the
influence of quantum constraints at reduced sizes. It is difficult to cover all
aspects in a single volume but we hope that the reader will find the limited
material interesting and stimulating.
Richmond, VA; University Park, PA B.N. Khanna
August 2002 A. W. Castleman
Contents
1 Cluster and Nanoscale Science: Overview and Perspective
A.W. Castleman, Jr., S.N. Khanna. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Introduction............................................... 1
1.2 Cluster Types: Formation and Study. . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 Theoretical Developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.4 Theme of the Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Quantum and Classical Size Effects
in Thermodynamic Properties
R.S. Berry ....... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1 Introduction............................................... 7
2.2 Quantum Properties of Small Systems ........................ 10
2.3 Phases of Finite Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 12
2.4 Phase Diagrams of Finite Systems. . . . . . . . . . . . . . . . . . . . . . . . . . .. 19
2.5 Conclusion................................................ 26
References ..................................................... 26
3 Photoelectron Spectroscopy
G. Ganteför . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 29
3.1 Introduction............................................... 29
3.2 Physics of Photoelectron Spectroscopy . . . . . . . . . . . . . . . . . . . . . . .. 31
3.3 Experimental Set Up ....................................... 36
3.3.1 Cluster Source. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 37
3.3.2 Time-of-Flight Mass Spectrometer . . . . . . . . . . . . . . . . . . . . .. 37
3.3.3 Laser............................................... 37
3.3.4 Photoelectron Spectrometer ........................... 38
3.4 Results.................................................... 39
3.4.1 Example: Electronic Shells in Clusters of Simple Metals . .. 39
3.4.2 Example: The Size Dependence of the Band Gap . . . . . . . .. 42
3.4.3 Example: Chemical reactivity and electronic structure. . . .. 45
3.4.4 Example: Dynamics .................................. 48
3.5 Conclusion and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 51
VIII Contents
4 Quantulll Tunneling of the Magnetization
in Molecular Nanoclusters
R. Sessoli, D. Gatteschi, W. Wernsdorfer. . . . . . . . . . . . . . . . . . . . . . . . . .. 55
4.1 Introduction............................................... 55
4.2 The Magnetic Anisotropy of Molecular Clusters. . . . . . . . . . . . . . .. 57
4.3 The Superparamagnetic Behavior. . . . . . . . . . . . . . . . . . . . . . . . . . . .. 61
4.4 Longitudinal Field Dependence of the Relaxation Rate:
The Stepped Hysteresis ...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 63
4.5 Transverse Field Dependence of the Relaxation Rate:
The Berry Phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 65
4.6 The Role of Dipolar Fields: The Non-Exponential Relaxation. . .. 69
4.7 The Role of the Nuclear Magnetic Moments: The Isotope Effect .. 74
4.8 Conclusions................................................ 78
References ..................................................... 79
5 Magnetislll of Free and Supported Metal Clusters
J .P. Bucher .................................................... 83
5.1 Introduction............................................... 84
5.2 Simple Considerations ...................................... 86
5.2.1 Common Ideas on Magnetism. . . . . . . . . . . . . . . . . . . . . . . . .. 86
5.2.2 Implications for Cluster Magnetism . . . . . . . . . . . . . . . . . . . .. 87
5.3 The Stern-Gerlach Experiments. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 89
5.3.1 Experimental Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 89
5.3.2 Magnetic Moment Measurements of Unsupported Clusters. 91
5.3.3 A High Resolution Experiment: Nickel. . . . . . . . . . . . . . . . .. 92
5.3.4 Temperature Dependence of the Giant Moments. . . . . . . . .. 93
5.3.5 Clusters of Non-Ferromagnetic Transition Metals . . . . . . . .. 96
5.3.6 Locked Moment Clusters and Spin Canting . . . . . . . . . . . . .. 97
5.4 Interpretation of the Beam Experiments. . . . . . . . . . . . . . . . . . . . . .. 99
5.4.1 Magnetic Anisotropy .................................. 100
5.4.2 Coupling Between Magnetic Moment and Lattice:
Deflection Profile. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
5.5 Growth Kinetics of Clusters on Surfaces ....................... 102
5.5.1 Tuning the Clusters Density ........................... 102
5.5.2 Island Shapes ........................................ 104
5.6 Thermodynamic Growth Modes .............................. 106
5.6.1 Growth Criteria ...................................... 106
5.6.2 Elastic and Structural Considerations ................... 107
5.7 Organized Growth .......................................... 108
5.7.1 Incommensurate Modulated Layers ..................... 108
5.7.2 Atomic Scale Template ................................ 110
5.7.3 Self Organization ..................................... 112
5.7.4 Periodic Patterning by Stress Relaxation ................ 113
Contents IX
5.7.5 Organization on Vieinal Surfaees ....................... 115
5.7.6 Low Temperature Growth ............................. 115
5.8 Magnetie Properties of Nanostruetures ........................ 116
5.8.1 Isolated Clusters on Surfaces ........................... 117
5.8.2 Interaeting Islands and Chains ......................... 120
5.8.3 The Two-Dimensional Limit ........................... 125
5.9 Conclusion and Outlook ..................................... 130
Referenees ..................................................... 132
6 Magnetism in Free Clusters
and in Mn12012-Acetate Nanomagnets
S.N. Khanna, C. Ashman, M.R. Pederson, J. Kortus 139
6.1 Introduction ............................................... 139
6.2 Magnetie Moment of Free Clusters in Beams ................... 140
6.3 Oseillatory Change in the Magnetie Moment
of Ni Clusters upon H Adsorption ........................... 143
n
6.4 Quantum Tunneling and Atomie, Eleetronie
and Magnetie Structure of Mn12012-Aeetate ................... 146
6.5 Details of Theoretical Studies ................................ 149
6.6 Geometry and Electronie Strueture of Isolated Mn12012 Clusters. 150
6.6.1 Vibrational Frequencies of the Hexagonal Tower .......... 151
6.7 Electronie Structure of Mn12012-Aeetate ...................... 152
6.8 Magnetie Anisotropy Energy ................................. 154
6.9 Conclusions and Extension to Fe8 Nanomagnets ................ 156
Referenees ..................................................... 157
7 Size Effects in Catalysis by Supported Metal Clusters
A.A. Kolmakov, D.W. Goodman .................................. 159
7.1 Introduetion ............................................... 159
7.2 Methodology ............................................... 160
7.2.1 Thin Oxide Films as a Model Support .................. 160
7.2.2 Cluster Deposition: Density, Size and Control
of Morphology ....................................... 161
7.2.3 Analytieal Tools: Speetroseopy and Mieroseopy ........... 164
7.3 Cluster Size and Reaetivity .................................. 170
7.3.1 Geometrie Factors .................................... 170
7.3.2 Eleetronie Faetors .................................... 174
7.4 Examples of Size Effects in Cluster Reactivity .................. 181
7.4.1 Onset of the Reaetivity of Au/Ti02
with Metal-Nonmetal Transitions
and the Dimensionality of Supported Clusters ............ 181
7.4.2 CO Dissoeiation on Struetural Defects
of Rh/ Alz03/NiAI (110) .............................. 186
7.4.3 CO Oxidation Over a Pt/MgO Monodispersed Catalyst ... 189
X Contents
7.5 Concluding Remarks and Future Prospects .................... 192
References ..................................................... 193
8 Delayed Ionization
E.E.B. Campbell, R.D. Levine .................................... 199
8.1 Introduction ............................................... 199
8.2 Transition State Theory ..................................... 200
8.3 Detailed Balance ........................................... 203
8.4 Experimental: The Rate of Thermionic Emission ............... 210
8.5 Experimental: Dynamics .................................... 213
8.6 Kinetic Model ............................................. 217
8.7 Concluding Remarks ........................................ 219
9 Cluster Dynamies: Influences of Solvation and Aggregation
Q. Zhong, A.W. Castleman, Jr .................................... 223
9.1 Introduction ............................................... 223
9.2 Charge-Transfer Reactions ................................... 224
9.2.1 Photo-Induced Electron-Transfer Reactions .............. 224
9.2.2 Excited-State Proton-Transfer ......................... 226
9.2.3 Excited-State Double Proton-Transfer ................... 229
9.3 Caging Dynamics ........................................... 231
9.3.1 Caging Dynamics in Neutral Clusters ................... 232
9.3.2 Caging Dynamics in Anionic Clusters ................... 234
9.4 Coulomb Explosion Process in Clusters ....................... 238
9.4.1 Role of Clusters in the Coulomb Explosion Process ....... 238
9.4.2 Modeling of Coulomb Explosion Process ................. 240
9.4.3 Coulomb Explosion Imaging ........................... 243
9.5 Electronic Excitation, Relaxation and Ionization of Met-Cars .... 245
9.5.1 Met-Cars: A Unique Molecular Cluster System ........... 246
9.5.2 Delayed Ionization .................................... 246
9.5.3 Ultrafast Spectroscopy ................................ 248
9.6 Conclusion ................................................ 251
References ..................................................... 252
10 Future Directions
A.W. Castleman, Jr., S.N. Khanna ................................ 259
Index ......................................................... 263