The field of cluster research can trace its origins back to the mid-nineteenth century when early studies of colloids, aerosols, and nucleation phenomena were reported. The field underwent a resurgence of interest several decades ago when well-defined clusters were observed in supersonic expansions that could be investigated using mass spectrometers. The advent of the laser provided a new dimension, enabling detailed spectroscopic observations through the probing of systems of varying size and degree of solvation. Modern interest derives from recognition that interrogating clusters provides a way of studying the energetics and dynamics of intermediate states of matter as cluster systems evolve from the gas toward the condensed state. Herein, we endeavor to highlight some of the significant advances which have been made during the past several decades that have led to a nearly explosive growth of interest in the field of cluster science. Finally, we conclude that the field will continue to expand through i...

Clusters: Structure, Energetics, and Dynamics of Intermediate States of Matter

Ultraviolet photoelectron spectra (UPS) were recorded for mass‐selected negative clusters of copper (1–411 atoms), silver (1–60 atoms), and gold (1–233 atoms), using photodetachment lasers at 6.4 and 7.9 eV photon energy. The results provide a direct estimate of the vertical electron affinity (EA) of these clusters and information on the evolution of the d bands of copper and gold as a function of cluster size. The large even/odd alternation of EA in small clusters of these metals in earlier work is found to largely disappear as the cluster size exceeds 40 atoms. The ellipsoidal shell model is shown to be consistent with the observed EA behavior of all three metals, the predicted spherical shell closing at cluster 58 being evident for silver and gold. The UPS data show a smooth evolution of the d band toward that of the bulk metal.

Ultraviolet photoelectron spectra of coinage metal clusters

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.

Cluster-Assembled Materials

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.

https://science.sciencemag.org/content/sci/304/5667/84.full.pdf

Formation of Al13I-: evidence for the superhalogen character of Al13.

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.

Clusters, Superatoms, and Building Blocks of New Materials†

Aluminum anions, (Al−n,n=5–>40), were generated using laser vaporization and equilibrated to room temperature in a flow tube apparatus. The behavior of these species, in general, show a propensity to lose the neutral oxides. In sharp contrast, several anions, Al−13, Al−23, and Al−37, show special inertness to these etching reactions and indeed, the etching reactions even contribute to their further production. The jellium model predicts these species to have closed shells. There are some exceptions in reactivity, however, which indicate the likelihood that the local structure may also play a role. An odd/even alternation, where even atom clusters react more quickly than successive odd ones, is also apparent. This is attributed to electron pairing within the cluster.

Thermal metal cluster anion reactions: Behavior of aluminum clusters with oxygen

Aluminum clusters, both anion and cation, are produced using laser vaporization and reacted under thermal conditions with oxygen in a flow tube reactor. An etching reaction is observed and bimolecular rate constants are reported for Al+n, n=1–33, and Al−n, n=5–37. For certain clusters, namely Al+7, Al−13, and Al−23, no apparent reactivity is observed (they are found to be produced from larger species). Interestingly, these correspond to predicted jellium shell closings with 20, 40, and 70 electrons, respectively. Besides these exceptions, and a small odd/even alternation in reaction rates, the overall reactivity is relatively insensitive to cluster size, and is found to range between about 1 × 10−12 and 5 × 10−12 cm3/s.

Aluminum cluster reactions

Copper cluster cations (Cu+n,n=1–14) were produced in a laser vaporization/flow tube apparatus and equilibrated to room temperature. The association rate constants of carbon monoxide onto these ions were measured; low‐pressure, termolecular behavior was observed for the smaller species while for clusters greater than Cu+7, the longer lifetimes due to the increased number of degrees of freedom leads to pressure independence (>0.3 Torr) of the effective bimolecular rates. Unimolecular decay theory (RRKM) is used to explain the overall trend and when intrinsic surface site reactivity is taken into account, excellent agreement with measured reactivity is obtained.

Metal cluster cation reactions: Carbon monoxide association to Cu+n ions

Rates were measured for the association of CO, CH4, CH3F, NH3, ND3, CH3Cl, and CH3Br onto Ag+ and Cu+ at 298 K. In the order given above, the three‐body association rate constants for Ag+ range from 2.5×10−30 to 3.9×10−27 cm6 s−1. The rate constants for Cu+ are about four to six times larger for a given neutral reactant. The rate constants display trends in the order expected considering the relative bond energies of the clusters, although the enormous range of reactivity is not reflected simply by differences in dipole moments and polarizabilities. There is a very large isotope effect, where the rate constant for ND3 association was found to be about three times greater than NH3 in the case of both ions. (Results for Na+ follow the same trend.) This suggests a coupling of ligand vibrations with orbiting motion which leads to enhanced lifetimes of the cluster intermediates. The trend found for the methyl halides also supports the involvement of this effect.

R. E. Leuchtner

Papers

Thermal metal cluster anion reactions: Behavior of aluminum clusters with oxygen

Aluminum cluster reactions

Metal cluster cation reactions: Carbon monoxide association to Cu+n ions

Considerations of the rates and lifetimes of intermediate complexes for the association of various ligands to metal ions: Ag+ and Cu+

Gas-phase reactivity of metal alloy clusters