6.2 Chemists and Physicists Are Developing a Mutual
Consensus on Nanoscale Atom Mimicry and Superatoms
Recent dialogue sparked by a plenary presentation to the American Physical
Society in early 2012 [140] has led to the realization that both chemists and
physicists have been thinking and working in parallel worlds concerning the
general concept of nanoscale atom mimicry, nanoscale superatoms, and
nanoclusters [141]. Although physicists have focused primarily on atom mimicry
associated with hard particle, metal cluster-type electron orbital behavior, chemists
have been more interested in heuristic nanoscale atom mimicry based on welldefined nanovalency, nanosterics, nanostoichiometries, and similar issues. Many of
these features and properties have been associated with discrete soft nanoparticles
such as dendrimers, proteins, viral capsids, DNA and RNA, nanolatexes, polymeric
micelles, and monodispersed synthetic polymers.
It is now recognized and generally accepted, that more complex, large nanoscale
collections (i.e., 10
3 times larger than atoms) of discretely organized atoms may
manifest many physico-chemical and building block features that are reminiscent of
individual atoms [142, 143]. These chemically bonded or supramolecularly assembled collections of atoms are generally homogeneous and monodisperse entities
that exhibit well-defined size (i.e., mass), shape, surface chemistry (i.e., valency),
flexibility/rigidity, atomic composition, and architecture. They are often referred to
as nanoscale “superatoms,” [142–145] atom equivalents [146], or heuristic “atom
mimics” [121, 137, 138, 147, 148].
A superatom is defined as any cluster of atoms that seems to exhibit the
properties of elemental atoms. An early example of a hard superatom was the
observed clustering of sodium atoms, when cooled from vapor, to preferentially
form a magic number of cluster atoms (i.e., 2, 8, 20, 40, 58, etc.). The first two
magic numbers (i.e., 2 and 8) are recognized as the number of electrons required to
fill the first and second shells, respectively. Thus, superatom mimicry is related to
the free electrons in the cluster that appear to occupy a new set of orbitals that are
defined by the entire group of atoms involved in the cluster, rather than each
individual atom separately. Superatoms appear to behave chemically in a way
that will allow them to have a closed shell of electrons in this new cluster orbital
counting scheme. Many examples of hard superatoms involving metal atom clusters
have been reported by pioneering physicists such as Khanna, Castleman and
coworkers [143, 144], and others [149].
This atom cluster behavior has also been observed and referred to by others as
“nanoscale atom mimicry,” [137, 138], wherein certain heterogeneous, soft,
non-metal atom clusters appear to exhibit combining patterns that produce welldefined stoichiometries and closed-shell-type behavior that is normally associated
with naked, elemental atoms. More specifically, this nanoscale atom mimicry was
noted in the 1990s [10, 11] for analogous soft superatoms such as dendrimers. For
example, dendrimers possessing unfilled outer monomer shells were observed to be
highly autoreactive, leading to dimer or oligomer formation. In contrast, ideal outer
Twenty-First Century Polymer Science After Staudinger: The Emergence of. . .
353
Consensus on Nanoscale Atom Mimicry and Superatoms
Recent dialogue sparked by a plenary presentation to the American Physical
Society in early 2012 [140] has led to the realization that both chemists and
physicists have been thinking and working in parallel worlds concerning the
general concept of nanoscale atom mimicry, nanoscale superatoms, and
nanoclusters [141]. Although physicists have focused primarily on atom mimicry
associated with hard particle, metal cluster-type electron orbital behavior, chemists
have been more interested in heuristic nanoscale atom mimicry based on welldefined nanovalency, nanosterics, nanostoichiometries, and similar issues. Many of
these features and properties have been associated with discrete soft nanoparticles
such as dendrimers, proteins, viral capsids, DNA and RNA, nanolatexes, polymeric
micelles, and monodispersed synthetic polymers.
It is now recognized and generally accepted, that more complex, large nanoscale
collections (i.e., 10
3 times larger than atoms) of discretely organized atoms may
manifest many physico-chemical and building block features that are reminiscent of
individual atoms [142, 143]. These chemically bonded or supramolecularly assembled collections of atoms are generally homogeneous and monodisperse entities
that exhibit well-defined size (i.e., mass), shape, surface chemistry (i.e., valency),
flexibility/rigidity, atomic composition, and architecture. They are often referred to
as nanoscale “superatoms,” [142–145] atom equivalents [146], or heuristic “atom
mimics” [121, 137, 138, 147, 148].
A superatom is defined as any cluster of atoms that seems to exhibit the
properties of elemental atoms. An early example of a hard superatom was the
observed clustering of sodium atoms, when cooled from vapor, to preferentially
form a magic number of cluster atoms (i.e., 2, 8, 20, 40, 58, etc.). The first two
magic numbers (i.e., 2 and 8) are recognized as the number of electrons required to
fill the first and second shells, respectively. Thus, superatom mimicry is related to
the free electrons in the cluster that appear to occupy a new set of orbitals that are
defined by the entire group of atoms involved in the cluster, rather than each
individual atom separately. Superatoms appear to behave chemically in a way
that will allow them to have a closed shell of electrons in this new cluster orbital
counting scheme. Many examples of hard superatoms involving metal atom clusters
have been reported by pioneering physicists such as Khanna, Castleman and
coworkers [143, 144], and others [149].
This atom cluster behavior has also been observed and referred to by others as
“nanoscale atom mimicry,” [137, 138], wherein certain heterogeneous, soft,
non-metal atom clusters appear to exhibit combining patterns that produce welldefined stoichiometries and closed-shell-type behavior that is normally associated
with naked, elemental atoms. More specifically, this nanoscale atom mimicry was
noted in the 1990s [10, 11] for analogous soft superatoms such as dendrimers. For
example, dendrimers possessing unfilled outer monomer shells were observed to be
highly autoreactive, leading to dimer or oligomer formation. In contrast, ideal outer
Twenty-First Century Polymer Science After Staudinger: The Emergence of. . .
353