Cluster-assembled materials offer the ability to tune component properties, lattice parameters, and thus coupling of physical properties through the careful selection and assembly of building blocks. Multi-atom clusters have been found to exhibit physical properties beyond those available from the standard elements in the periodic table; classification of the properties of such clusters effectively enables expansion of the periodic table to a third dimension. Using clusters as superatomic building blocks for hierarchically assembled materials allows these properties to be incorporated into designer materials with tailored properties. Cluster-assembled materials are currently being explored and methods developed to control their design and function. Here, we discuss examples of building block syntheses, assembly strategies, and property control achieved to date.

The physical and chemical properties of cluster systems at the subnano and nanoscale are often found to differ from those of the bulk and display a unique dependence on size, geometry, and composition. Indeed, most interesting are systems which have properties that vary discontinuously with the number of atoms and composition, rather than scale linearly with size. This realm of cluster science where “one atom makes a difference” is undergoing an explosive growth in activity, and as a result of extensive collaborative activities through theory at VCU and experiment at PSU, our groups are recognized as pioneers in this area in which we have been active for many years. Herein we provide an overview of the field with primary focus on our joint undertakings which have spawned the superatom concept, giving rise to a 3-D periodic table of cluster elements and the prospect of using these as building blocks of new nanoscale materials with tailored properties.

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Al13- is a cluster known for the pronounced stability that arises from coincident closures of its geometric and electronic shells. We present experimental evidence for a very stable cluster corresponding to Al13I-. Ab initio calculations show that the cluster features a structurally unperturbed Al13- core and a region of high charge density on the aluminum vertex opposite from the iodine atom. This ionically bound magic cluster can be understood by considering that Al13 has an electronic structure reminiscent of a halogen atom. Comparisons to polyhalides provide a sound explanation for our chemical observations.