The Panel was charged with assessing the present scientific understanding of the size-dependent physical and chemical properties of clusters, the methods of synthesis of macroscopic amounts of size-selected clusters with desired properties, and most importantly, the possibility of their controlled assembly into new materials with novel properties. The Panel was composed of both academic and industrial scientists from the physics, chemistry, and materials science communities, and met in January 1988.In materials (insulators, semiconductors, and metals) with strong chemical bonding, there is extensive spatial delocalization of valence electrons, and therefore the bulk physical properties which depend upon these electrons develop only gradually with cluster size. Recent research using supersonic-jet, gas-aggregation, colloidal, and chemical-synthetic methods indeed clearly establishes that intermediate size clusters have novel and hybrid properties, between the molecular and bulk solid-state limits. A scientific understanding of these transitions in properties has only been partially achieved, and the Panel believes that this interdisciplinary area of science is at the very heart of the basic nature of materials. In Sec. V (Future Challenges and Opportunities), a series of basic questions for future research are detailed. Each question has an obvious impact on our potential ability to create new materials.Present methods for the synthesis of useful amounts of size-selected clusters, with surface chemical properties purposefully controlled and/or modified, are almost nonexistent, and these fundamentally limit our ability to explore the assembly of clusters into potentially novel materials. While elegant spectroscopic and chemisorption studies of size-selected clusters have been carried out using molecular-beam technologies, there are no demonstrated methods for recovery and accumulation of such samples. Within the past year, the first reports of the chemical synthesis of clusters with surfaces chemically modified have been reported for limited classes of materials. Apparatus for the accumulation and consolidation of nanophase materials have been developed, and the first promising studies of their physical properties are appearing. In both the chemical and nanophase synthesis areas, clusters with a distribution of sizes and shapes are being studied. Progress on macroscopic synthetic methods for size-selected clusters of controlled surface properties is the most important immediate goal recognized by the Panel. Simultaneous improvement in physical characterization will be necessary to guide synthesis research.Assuming such progress will occur, the Panel suggests that self-assembly of clusters into new elemental polymorphs and new types of nanoscale heterogeneous materials offers an area of intriguing technological promise. The electrical and optical properties of such heterogeneous materials could be tailored in very specific ways. Such ideas are quite speculative at this time; their implementation critically depends upon controlled modification of cluster surfaces, and upon development of characterization and theoretical tools to guide experiments.The Panel concluded that a number of genuinely novel ideas had been enunciated, and that in its opinion some would surely lead to exciting new science and important new materials.

Research opportunities on clusters and cluster-assembled materials—A Department of Energy, Council on Materials Science Panel Report

A higher resolution magnetic bottle photoelectron spectrometer for the study of the electronic structure of size‐selected metal clusters is presented. The initial study on Fe−n (n=3–24) is reported at a photon energy of 3.49 eV. The photoelectron spectra of these clusters exhibit sharp features throughout the size range. The spectra for Fe−3–8 show large size dependence with many resolved features. The spectra for Fe−9–15 exhibit some similarity with each other, all with a rather sharp feature near the threshold. An abrupt spectral change occurs at Fe−16, then again at Fe−19 and Fe−23. These photoelectron spectral changes coincide remarkably with changes of the cluster reactivity with H2. Extended Huckel molecular orbital (EHMO) calculations are performed for all the clusters to aid the spectral interpretations. The calculations yield surprisingly good agreement with the experiment for clusters beyond Fe9 when body‐centered cubic (bcc) structures are assumed for Fe9–15 and a similarly close‐packed structu...

http://casey.brown.edu/chemistry/research/LSWang/publications/32.pdf

Photoelectron spectroscopy of size‐selected transition metal clusters: Fe−n, n=3–24

We summarize here the research advances on the reactivity of metal clusters. After a simple introduction of apparatuses used for gas-phase cluster reactions, we focus on the reactivity of metal clusters with various polar and nonpolar molecules in the gas phase and illustrate how elementary reactions of metal clusters proceed one-step at a time under a combination of geometric and electronic reorganization. The topics discussed in this study include chemical adsorption, addition reaction, cleavage of chemical bonds, etching effect, spin effect, the harpoon mechanism, and the complementary active sites (CAS) mechanism, among others. Insights into the reactivity of metal clusters not only facilitate a better understanding of the fundamentals in condensed-phase chemistry but also provide a way to dissect the stability and reactivity of monolayer-protected clusters synthesized via wet chemistry.

Reactivity of Metal Clusters.

Studies of the chemistry that occurs on the nanosurfaces of size-selected silicon clusters reveal a number of fascinating qualitative similarities to the behavior of bulk surfaces. However, silicon clusters containing up to 70 atoms appear to be much less reactive than bulk silicon surfaces. This unexpected result suggests that these large silicon clusters are not just small crystals of bulk silicon, but have much more compact geometric structures

https://science.sciencemag.org/content/sci/252/5009/1085.full.pdf

Nanosurface Chemistry on Size-Selected Silicon Clusters

Cross sections for collision‐induced dissociation (CID) of Fe+n with Xe, 2≤n≤10, are presented Experiments were performed on a newly constructed guided ion beam mass spectrometer, the design and capabilities of which are described in detail The single mechanism for dissociation of iron cluster ions is sequential loss of iron atoms with increasing collision energies There is no evidence for fission to molecular neutral products The cross section threshold energy dependences are analyzed to give the bond dissociation energies (BDEs), D0(Fe+n−1–Fe) Data analysis employs an empirical model that incorporates RRKM theory to account for inefficient dissociation on the time scale of the experiment Results show that Fe+6 has the strongest BDE, D0(Fe+5–Fe) =344±018 eV, while Fe+3 is the most weakly bound, D0(Fe+2–Fe) =164±015 eV Neutral cluster BDEs are derived from ionic binding energies and known ionization potentials Branching ratios and other cross section features are also discussed with respect to

Collision‐induced dissociation of Fe+n (n=2–10) with Xe: Ionic and neutral iron binding energies

Evidence is presented for structural changes in iron clusters in the Fe13 to Fe23 size range. Abrupt changes with cluster size are found for several chemical properties, including reactivity with hydrogen and binding energies of ammonia and water. These changes often come at the same cluster sizes, pointing to a common origin—fundamental changes in the structure of the bare iron clusters. In addition, changes in structure as a consequence of adsorbate binding are suggested. The experimental observations leading to these conclusions are detailed, and possible structures for clusters in this size range are proposed.

Chemical probes of metal cluster structure: Reactions of iron clusters with hydrogen, ammonia, and water

The kinetics of reactions of nickel clusters with hydrogen and deuterium are studied in a laser-vaporization cluster source coupled to a continuous-flow reactor. The abslute rate constants for the addition of the first H2 (D2) molecule to nickel clusters Ni
 n
 (n=7→36 for H2 andn=7→60 for D2) have been measured. Rate constants are found to be only weakly dependent onn forn≧14, showing a gradual increase with size that scales approximately withn
(2/3), i.e., the cluster geometrical cross section. Reaction probabilities for clusters in this size range are approximately 0.6 for H2 and 0.3 for D2. Belown=14, there is a stronger dependence of reactivity on size, with Ni9 being far less reactive than any other cluster studied. These results are compared to bulk nickel studies, and a discussion of possible correlation of reactivity to cluster structure is presented.

The kinetics of reactions of nickel clusters with hydrogen and deuterium

Studies of two‐reagent reactions of iron clusters with ammonia and deuterium are described. The compositions of clusters saturated with both ammonia and deuterium have been determined, and indicate that the ammonia binds nondissociatively to iron clusters and to different sites than the deuterium. Rate constants for the reaction of the first deuterium molecule with ammoniated clusters have been measured. In general, low ammonia coverage (one to three molecules) has either little effect on cluster reactivity, or in the case of unreactive bare clusters, increases reactivity dramatically. Clusters saturated with ammonia show general reactivity decreases, although again the least reactive bare clusters show dramatic increases in reactivity when covered with ammonia molecules. Interesting cluster activation and deactivation effects are seen. Possible mechanisms for these reactions are discussed.

Two‐reagent reactions of iron clusters with ammonia and deuterium: Saturated compositions and the kinetics of reactions of deuterium with ammoniated clusters

W. F. Hoffman

Papers

Chemical probes of metal cluster structure: Reactions of iron clusters with hydrogen, ammonia, and water

The kinetics of reactions of nickel clusters with hydrogen and deuterium

Two‐reagent reactions of iron clusters with ammonia and deuterium: Saturated compositions and the kinetics of reactions of deuterium with ammoniated clusters