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Cluster
Studies Group
J. Jellinek, M. B. Knickelbein, S. Vajda
Overview
The activities of the Cluster Studies Group encompass studies
of a broad variety of physicochemical properties of cluster systems. The
immense interest in clusters is fueled by the recognition of the central
role they play in many modern technologies and in natural phenomena. The
potential of clusters as a means of optimization of existing technologies,
developing principally new technological processes, materials, and devices,
and in managing natural phenomena, especially those of environmental concern,
is truly unparalleled. To make this potential a reality a comprehensive
understanding of the properties of clusters as a function of their material(s),
size, structure, temperature, etc. is needed. The emphasis in our program
is on fundamental studies leading to such an understanding. Its particular
strength is that it combines both experimental and theoretical components,
which synergistically enhance each other.
Our research focuses on metal clusters because of their
particular technological relevance. The elements studied vary from alkali,
through transition, to coinage metals and their combinations. The properties
investigated include geometrical structures, stability, phases and phase
changes, fragmentation mechanisms, electronic features (e.g., ionization
potentials, electron affinities), chemical reactivity with a broad variety
of molecules, optical and magnetic properties, etc. The cluster features
change, in general, with size, and the different properties may be correlated.
The central goals include uncovering and understanding the size-dependence
of and the correlations between the different features. For example, the
chemical reactivity of a cluster of a given metal with a chosen reactant
molecule may strongly depend on the cluster size, structure, and temperature.
Unraveling the mechanisms through which these latter affect the reaction
pathways and rates is crucial for improving, or even fully optimizing,
the efficiency and selectivity of real catalysts.
Both the experimental and the theoretical parts of the
program represent cutting edge research, which is evidenced by the often
unique techniques, approaches and methodologies utilized, and issues and
systems studied. The experimental set-ups are based on laser-vaporization
cluster sources coupled to continuous or pulsed flow-tube reactors and/or
other devices, e.g., an ion trap. The methodologies include time-of-flight
mass spectrometry, optical and infrared spectroscopy, photofragmentation
measurements, deflection techniques using inhomogeneous magnetic and electric
fields, etc. The experimental studies pioneered and perfected in the group
include elucidation of cluster structure(s) through their chemical reactivity,
separation and quantitative characterization of the thermodynamic and
kinetic aspects of cluster-molecule interactions, the effect of temperature
on cluster reactivity, electronic spectroscopy of metal clusters through
photodissociation spectroscopy of cluster-rare gas complexes, and others.
The theoretical and numerical studies cover a broad range
of cluster properties, which include identification of stable and metastable
isomeric forms, the dynamics of isomerization transitions, cluster phases
and phase changes, multichannel fragmentation, the most detailed to date
qualitative and quantitative characterization of cluster-molecule interactions
and the mechanisms governing them, structural and electronic properties
of cluster-ligand complexes, etc. The theoretical tools used involve first
principles-based electronic structure treatments, the density functional
theory, many-body semiempirical potentials, large-scale dynamical and
statistical simulations, and others. A major part of the theoretical effort
is devoted to developing principally new theoretical concepts, methodologies,
and techniques, which are especially appropriate and efficient for the
small and mesoscale size regime. For example, our group pioneered the
theoretical studies of the effects of rotation on the structural, dynamical,
and phase properties of clusters. For this a new general scheme for separation
of the energy of overall rotation in an arbitrary N-body system has been
developed. The scheme has later been adopted and adapted in many different
areas outside the cluster field and applied to systems ranging from molecules
to continuous media. We have made the first theoretical prediction on
the importance of the state of molecular adsorption as a precursor to
dissociative adsorption of a molecule on a cluster. The other developments
include novel dynamical and statistical analyses schemes, construction
of new, more adequate semiempirical potentials, formulating new fitting
procedures with a greater degree of insight and control, introducing new
notions and concepts for analyses of multicomponent (e.g., alloy) systems,
constructing algorithms that combine accuracy with efficiency, etc.
The presence of both the experimental and theoretical components
in the program has a truly synergistic effect. Our simulation studies
of cluster-molecule reactions were motivated by the ongoing experiments
in the group. Our theoretical prediction of the important role of the
molecularly adsorbed precursor led to subsequent experiments, which confirmed
the prediction. The measurements of the ionization potentials (IPs) and
infrared spectra of (Ag)n(C2H4)m
systems motivated our density functional studies of these systems. The
results of the computations not only confirmed the measured changes in
the IPs of the complexes vs those of the corresponding bare Agn clusters,
but they also identified the otherwise unknown geometric structure of
the dominant species produced in the experiments and pointed out the role
of the IP as a possible structural label. Our theoretical studies of bimetallic
clusters stimulated a major experimental initiative on these systems within
the group.
Our program is alert to changing needs, scientific challenges,
and new opportunities. It responds to these by constantly expanding the
conceptual, technical, and methodological arsenal and the scope of systems
and problems studied. The direction of our evolution can be characterized
as "towards more complex systems and phenomena", where the term "complex"
is used inclusively to mean larger size clusters, multielement (e.g.,
alloy) clusters, clusters in confined geometries, cluster-catalyzed reactions,
interactions of clusters with electric and magnetic fields, cluster magnetism
and its control through chemical means (e.g., ligation), deposited clusters,
cluster-assembled designer nanoparticles, nanopores, thin films and, eventually,
nanomaterials. The current and planned work involves taking full advantage
of the Argonne facilities (e.g., different microscopy probes, Advanced
Photon Source) and contributing to the development of planned new facilities
such as the Center for Nanoscale Materials. It also involves collaborations
both within and outside Argonne. Samples of our recent and current activities
are given below.
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Radiation
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Photochemistry
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Materials Growth Facility
Cluster
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Chemical
Dynamics
Atomic
Physics
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Elements
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