It is well known that nanometric systems present different properties compared with those of the bulk material. Atomic clusters of non-magnetic materials become magnetic, semiconductor materials present metallic properties, the color of the particles change with the size, and noble metals become highly reactive. These properties originate from the unusual structure of the clusters, and because of the confinement of the electrons of the system.
One of the most exciting moments in cluster science was the discovery that some stable clusters mimic the electronic and chemical properties of elements of the periodic tabled. As the electronic states in metallic clusters are grouped in shells like in atoms, the filling of the shells lead to stable clusters with common chemical characteristics to atoms, further extending the periodic table to a third dimension with these clusters known as superatoms.
Al13 is one example of a superatom cluster. Al13 has 39 valence electrons, and within the jellium model, clusters with 40 electrons have large HOMO-LUMO Gaps and enhanced stability. Because of this, the cluster has a very high electron affinity of 3.4 eV, and the anion has a large HOMO-LUMO Gap of 1.87 eV. Because the chemical and electronic properties are dominated by the enhanced stability it recieves by moving to a valence of -1, the Al13 cluster is considered a Superhalogen Superatom.
Determining the electronic and structural principles that rule the stability of clusters that can be used as building units of nanostructure materials with tailored properties is one of the most important research areas of cluster science. These new materials with desirable magnetic, catalytic, geometric, and electronic properties are expected to revolutionize the science of the 21th century, being able to efficiently store energy and information, and to reduce the pollution by replacing the expensive catalyst used today.
Following the recent research that has discovered other superatom clusters like Al14 (mimic of an alkaline earth atom), Al7- (mimic of a main group element like Ge, and presenting multiple valences), and VCs8 (the first known magnetic superatom); We performed theoretical studies on the electronic structure and stability of neutral and anionic Ti-doped sodium clusters to examine the role of electronic and geometric shells on the physical and chemical behavior. Our studies delineate the effect of the filling of atomic electronic states, electronic shells in a two- and three-dimensional confined electronic gas, and the completion of geometrical shells on the energetic stability and chemical reactivity.
In particular, a TiNa9 cluster is shown to be marked by a filled 1S2 1P6 super shell, and a half-filled atomic electronic d-level on the Ti atom, with the composite closure leading to a magnetic superatom mimic of a Mn atom.
Electronic structure studies of clusters: Discovery of multiple valence
and magnetic cluster superatoms.
Designer Magnetic Superatoms,
J.U. Reveles, P. Claybone, A.C. Reber, S.N. Khanna, K. Pradhan, P. Sen, and M.R. Pederson Nat. Chem. 1, 310 (2009).
Research Highlights papers in Nature Chemistry1, 260 (2009),
and New Scientist, June 2009.
Featured on the Physorg., Sciencedaily, and more than 200 other websites.