242
14 Creating Genetic Materials of Metal Clusters
The electronic and geometric structures of the deposited clusters were investigated
although a notable difference from the nascent clusters in gas phase [17, 19, 29, 30].
The deposition clusters can be a size distribution or a mass-selected species having
an exact number of atoms, feasibly grown on diverse surfaces composed of metals,
metal oxides, graphite and silicon etc. generally with controlled defect sites [31–
33]. Because of the small sizes, high specific surface area, likely fluorescence and
flexible chemistry of the cluster assembly, the properties of as-prepared materials
can be tuned through quantum confinement by adsorbing surface-active species, or
by altering the elemental composition of the clusters building blocks. It is anticipated
that superatom clusters will be a significant aim in creating materials of metal clusters
via assembly.
14.2 Building Blocks Identified from Gas Phase
One of the most exciting developments in cluster science is the realization that chosen
stable clusters can mimic the chemical behavior of an atom or group of the periodic
table, known as superatoms [34]. The major finding in the field of metal cluster
science pertaining to the development of the superatom concept, originally termed
a unified atom [35], came from study of aluminum cluster reactivity conducted in
the Castleman group in 1989 [7], where they found a dramatic size-dependence of
reactivity that cluster anions containing 13, 23, and 37 atoms were unreactive toward
oxygen although the other species were etched away. This observation was accounted
for by shell closings at 40, 70, and 112 electrons predicted by the well-established
jellium shell model [36–38]. Replacement of the term “superatom” for unified atom,
and the initial conceptual framework behind the idea that clusters mimicking different
elements of the periodic table could be designed by changing size, composition, and
the charged state was introduced by Khanna and Jena in a series of pioneering papers
starting in 1990s [39–50]. Among them, the recognition of superatoms with one less
electron than a full shell has been ascertained as superhalogens, such as Al 13 which
has an adiabatic electron affinity of 3.40 eV exhibiting behaviour reminiscent to a
halogen atom [34]. Extensive studies in this field have made the stability of such
superatomic clusters being understood within a few models, including the jellium
model, aromaticity (e.g., a planar-structured boron cluster B 19
− [51]), and WadeMingos rules (e.g., organometallic clusters) depending on the geometry and metallicity of the cluster [1, 40, 52, 53]. The cluster sizes and electron counts of these
clusters are quite stable relative to others of similar size, and hence their physical
and chemical properties are dominated as expectation to reach a certain valence state
[54].
While the behavior of metals is determined by the electronic levels near the Fermi
energy, the entire valence electronic structure in simple metal clusters is made of
discrete electronic shells which are multiple highly degenerate states and much like
atoms. For such clusters a simplified framework to explain their behavior is the
jellium model which was first proposed by Martins et al. in 1981 [14] and verified
14 Creating Genetic Materials of Metal Clusters
The electronic and geometric structures of the deposited clusters were investigated
although a notable difference from the nascent clusters in gas phase [17, 19, 29, 30].
The deposition clusters can be a size distribution or a mass-selected species having
an exact number of atoms, feasibly grown on diverse surfaces composed of metals,
metal oxides, graphite and silicon etc. generally with controlled defect sites [31–
33]. Because of the small sizes, high specific surface area, likely fluorescence and
flexible chemistry of the cluster assembly, the properties of as-prepared materials
can be tuned through quantum confinement by adsorbing surface-active species, or
by altering the elemental composition of the clusters building blocks. It is anticipated
that superatom clusters will be a significant aim in creating materials of metal clusters
via assembly.
14.2 Building Blocks Identified from Gas Phase
One of the most exciting developments in cluster science is the realization that chosen
stable clusters can mimic the chemical behavior of an atom or group of the periodic
table, known as superatoms [34]. The major finding in the field of metal cluster
science pertaining to the development of the superatom concept, originally termed
a unified atom [35], came from study of aluminum cluster reactivity conducted in
the Castleman group in 1989 [7], where they found a dramatic size-dependence of
reactivity that cluster anions containing 13, 23, and 37 atoms were unreactive toward
oxygen although the other species were etched away. This observation was accounted
for by shell closings at 40, 70, and 112 electrons predicted by the well-established
jellium shell model [36–38]. Replacement of the term “superatom” for unified atom,
and the initial conceptual framework behind the idea that clusters mimicking different
elements of the periodic table could be designed by changing size, composition, and
the charged state was introduced by Khanna and Jena in a series of pioneering papers
starting in 1990s [39–50]. Among them, the recognition of superatoms with one less
electron than a full shell has been ascertained as superhalogens, such as Al 13 which
has an adiabatic electron affinity of 3.40 eV exhibiting behaviour reminiscent to a
halogen atom [34]. Extensive studies in this field have made the stability of such
superatomic clusters being understood within a few models, including the jellium
model, aromaticity (e.g., a planar-structured boron cluster B 19
− [51]), and WadeMingos rules (e.g., organometallic clusters) depending on the geometry and metallicity of the cluster [1, 40, 52, 53]. The cluster sizes and electron counts of these
clusters are quite stable relative to others of similar size, and hence their physical
and chemical properties are dominated as expectation to reach a certain valence state
[54].
While the behavior of metals is determined by the electronic levels near the Fermi
energy, the entire valence electronic structure in simple metal clusters is made of
discrete electronic shells which are multiple highly degenerate states and much like
atoms. For such clusters a simplified framework to explain their behavior is the
jellium model which was first proposed by Martins et al. in 1981 [14] and verified