PERSPECTIVE pubs.acs.org/JPCL
From Elements to Clusters: The Periodic Table Revisited A. W. Castleman, Jr.* Departments of Chemistry and Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ABSTRACT: Cluster science has given rise to the possibility of forming “superatoms”, species that mimic elements of the periodic table but often display multiple functionalities. The ability to tailor the properties of these species opens up a new approach to forming nanoscale materials from the bottom up via cluster assembly. Recent success in designing these superatoms composing a “3D periodic table” and understanding the fundamentals governing their properties and stability are discussed, as well as prospects for the future of this field.
V
irtually everyone who has been exposed to a scientific subject has heard the name “Mendeleev”, the individual credited with devising the periodic law. Concepts leading to the periodic table1 can be traced to a number of early observations, among the first being curious relationships between the molecular weights of various compounds determined by a German chemist, Johann Doebereiner (Goethe’s chemistry teacher). His findings, which predated Mendeleev’s discovery by some 50 years, were a prelude to the periodicity noted. Prior to J. J. Thomson’s discovery of the electron, all of the observations were based on atomic weights and the occasional discovery of numerous elements whose weights or properties fit the periodic relationships based on Mendeleev’s predictions. Clearly, the discovery of the electron paved the way for the fundamental understanding of chemical bonding.2 Seminal work by G. N. Lewis and Irving Langmuir further established the overall basis for the modern version of the periodic table and enabled likely missing pieces of the “puzzle” to be identified. Finally, taking into consideration Bohr’s work on the atom, the basis for the ordering of the elements and the explanation for trends with respect to atomic/molecular weights was resolved. Surprisingly, even to this day, new elements continue to be discovered, but due to the fact that recently identified ones are not naturally occurring and stable, having very short half-lives, their study is a difficult task. Assigning them to their proper place in the periodic table is often accomplished by determining their chemical properties, a wellaccepted method employed by nuclear physicists and chemists to ascertain the group to which they belong.3 Except for the occasional rare discovery of a new metastable short-lived element which is associated with an island of stability, the table available in the 20th century seemed to be effectively complete. However, if one accepts emerging new ideas about what constitutes “an element”, that supposition is not quite true. Expanding the definition of an “element” to include species that behave as composite entities, which mimic the properties of some element atoms and yet retain their integrity while serving as r 2011 American Chemical Society
building blocks of new materials, broadens the scope of the “periodic table” to a third dimension.4 The implications can be profound, and a consideration of these is the subject of this invited Perspective. These entities have become known as “superatoms” based on findings from the field of cluster science. Specifically, their concept emerged as an outgrowth of the observed behavior of certain magic
Expanding the definition of an “element” to include species that behave as composite entities broadens the scope of the “periodic table” to a third dimension. number species,5 which are found under situations where there is an abrupt change in an otherwise relatively smooth varying distribution of cluster intensities with the degree of clustering or aggregation. In cases where collective electronic effects interplay with the magic numbers, an entity may sometimes arise whose stability and properties can be accorded to electronic effects and in that way mimics the properties of certain elements. The first of the cluster species taken to be representative of an element was one comprised of 13 aluminum atoms.6 Although variations in electron affinities of aluminum with cluster size had been previously noted,7 the first evidence that certain aluminum clusters can reveal element-like characteristics was based on a pioneering study of their chemical reactivity, conducted in 1989 by Leuchtner in the Castleman group at Penn State (see Table 1).6 Received: February 17, 2011 Accepted: April 8, 2011 Published: April 15, 2011 1062
dx.doi.org/10.1021/jz200215s | J. Phys. Chem. Lett. 2011, 2, 1062–1069
The Journal of Physical Chemistry Letters
PERSPECTIVE
Table 1. Some of the Element Mimics That We Have Identified in Our Studies cluster superatom Al13
rare gas “mimic” or superhalogen anion
Al13 Al14
superhalogen alkaline earth
Al7/þ(x) a
group III; multivalent “element”
As7/þ(x) a
phosphorus
K3O
alkali metal
TiO
Ni
Pt
ZrO
Pd
ZrO2
PdO
WC
a
element mimic
The superscripts on Al7 and As7 denote possible valence states.
Considering the 3N number of electrons associated with aluminum (3s23p1) of cluster size N, and the confining potential, upon the addition of an extra electron to Al13, a very stable 40-electron anionic species was identified. It displayed properties of a rare gas in terms of an unreactive closed electronic shell atom on the one hand or as a halogen-like anionic species on the other. Although in earlier studies the 13-mer had been found to display a larger electron affinity than proximate species, to the best of our knowledge, this cluster is the first one found to display unique reactive characteristics such as being found to be unreactive with triplet oxygen.810 This chemical inertness arises due to filled electron shell effects and concomitant nonconservation of spin among the reactants and products. Subsequently, aluminum 23 anions (70 electrons) and aluminum 37 anions (112 electrons) were also found to be unreactive with triplet oxygen, and these species also conform to considerations in accord with the shell model. This model, termed jellium, is based on an extension by Cohen, Chou, Knight, and co-workers,11 of the nuclear-physics-independent nucleon model. The superatom concept is based on the idea that an aggregate/cluster of atoms contributes free electrons to occupy a set of orbitals chemically mimicking an element. The original idea proposed to account for the lack of chemical reactivity was based on the united (or unified) atom concept, a terminology first used but now replaced by “superatom” in the more recent literature.4,1214 The term is particularly appropriate for systems having completed electronic and geometric shells. Many of the systems that were among the first to be described as superatoms and to be in accord with considerations of the jellium model were comprised at least partly of an aluminum component, examples being ones also containing C, Nb, and V.15,16 Al4Nb is an 18-electron species, expected to be stable in terms of the shell model; a similar situation arises for the 18-electron cluster Al7C, which gives rise to the aluminum component manifesting a trivalent oxidation state.17,18 Even though these are clearly not all free-electron systems, the shell model still accounts well for their observed behavior. Interestingly, in the context of the aforementioned findings is other reported evidence19 that a salt-like cluster molecule can be formed from potassium bound to Al13, where the latter species is considered as being metallic and treatable in terms of the jellium model, yet evidently, the composite system is more ionic than metallic. Studies of various superatoms continued to reveal interesting and surprising facts. One example was the finding that the aluminum-13 retained its integrity even upon interacting with the other compo-
Figure 1. Sketch of Al14I3, which we find is a viable mimic of a divalent alkaline earth metal superatom bound with three iodine atoms.21
Figure 2. Aluminumhalogen clusters formed by the reaction of aluminum clusters with I2 and then subsequently oxidized with triplet oxygen. Adapted from refs 4 and 12.
nents of a cluster compound. This is evidenced by the fact that Al13X persists as a stable entity, where X is a halogen such as iodine having an electron affinity approximately equal to or less than that of the aluminum complex.8,20 Experiments in which aluminum was reacted with halogen-containing molecules such as I2 and HI showed that the aluminum-containing component behaved like a halogen bound as a di- or trihalogen species; indeed, Al13I and Al13I2 are analogues of well-known gas-phase species such as XY and XY2, where X and Y designate halogens.8,12,20,21 Furthermore, more recent measurements of the electron affinity of Al13 placed it close to that of chlorine.22 Neutral Al13 and its anion seemed to mimic a (super) halogen in the former case, and in the context of a closed-shell “atom”, they behaved like an unreactive or rare gas species. The findings provide further evidence that the Al13 retains its integrity, even in the presence of reactive ligands, namely, iodine.8 Interestingly, an aluminum-14 species shown in Figure 1, as represented by a simplified drawing, displays characteristics reminiscent of an alkaline earth metal, the second of the cluster-element mimics discovered in our laboratory. A particularly revealing finding is seen from the aluminum halide distributions in Figure 2, which show two cluster series corresponding to Al13In, with n being an even number, and Al14In, where n is odd. The figure shows the stability arising in the odd/even aluminumhalogen complexes that have distributions similar to that known for analogous species in the 1063
dx.doi.org/10.1021/jz200215s |J. Phys. Chem. Lett. 2011, 2, 1062–1069
The Journal of Physical Chemistry Letters
PERSPECTIVE
Figure 3. Energy level diagrams, binding energy (BE) spectra, and raw photoelectron images for Pt and WC at a photon energy of 2.33 eV (532 nm). The inset to the Pt binding energy spectrum displays weaker intensity transitions from anion excited states. Isosurface plots of the highest occupied 16σ and 4δ molecular orbitals appear as insets to the WC binding energy spectrum. See details in ref 25.
condensed phase, though the resulting geometrical structures are not identical. Subsequent investigations of other cluster species showed multivalent behavior17 analogous to that displayed by various elements of the periodic table, for example, group III elements mimicked by Al7/þ(x). Recent studies have further revealed the fact that the superatom concept can extend far beyond application to aluminum-containing systems, as evidenced by examples of other superatom elements including As73 and As113, which bear some analogy to phosphorus (valence 5, 3,3), and an alkali metal superatom analogue, based on K3O.4,23,24 Efforts to Quantify the Superatom Concept.25,26 The aforemen-
Recent studies have further revealed the fact that the superatom concept can extend far beyond application to aluminum-containing systems. tioned findings revealed the prospect that virtually any element could be simulated given that an appropriate assembly of atoms was acquired. However, the question arose whether there were any guiding principles to follow in designing an element mimic. If the cluster is a viable mimic, “its chemistry” should bear some resemblance to that of the corresponding element. This raised the prospect that one approach would be to look for related electronic (excited) states, hence providing a way to quantify the superatom concept. It is a well-known principle in chemistry that compounds having isoelectronic character27 will yield species with somewhat similar properties. However, “isoelectronic” implies similar structure as well as valence electrons; lacking similar structural geometry, the term “isovalent” is more appropriate. Some hint that atomic and isovalent species might display similar chemistry was evident from early work of Boudart and co-workers who, during the early 70s, reported that WC and platinum, which have a similar electronic distribution, could both affect some similar catalytic reactions.28 The vertical electron detachment energies of WC and Pt and the electron affinity were expected to be
similar, a fact that we experimentally established via photoelectron measurements. Employing the technique of velocity map imaging enabled us to quantify the electronically excited state characteristics of
Employing the technique of velocity map imaging enabled us to quantify the electronically excited state characteristics of cluster element mimics, including their anisotropies. cluster element mimics, including their anisotropies. Extending these quantitative measurements to a variety of systems yielded similar findings, namely, that in a number of cases, various other atom combinations also led to element mimics. The quantitative similarity of the electronic states was determined from electron affinity and vertical detachment values, as well as in some cases findings of very similar anisotropy values. Figure 3 shows that WC does display electronic transitions closely akin to that of Pt, accounting for the fact that WC reveals similar surface catalytic behavior.28 Although the spectra are not quantitatively identical, it is clearly evident that the measured differences in the electronic excited states are similar, as are the anisotropy parameters (β). Evidence that this is not a mere coincidence in the Pt/WC case is apparent by comparing spectra for other isovalent pairs such as TiO, which reveals remarkable similarity25 to that of Ni, and similarly, ZrO compares well with Pd. Examples are shown in Figure 4a (Ni/TiO) and b (ZrO/Pd). It is found that even species with differing compositions but instead having similar structural arrangements (hence isoelectronic) are quantitatively quite similar. This is seen from comparison of the nearly identical vertical detachment energies and electron affinities of a series of other large clusters such as four-atom species comprised of Bi, Sn, Sb, and Pb.29 See Figure 5 for a comparison of two clusters containing differing numbers of Pb, Sb, and In yet the same number of valence electrons. 1064
dx.doi.org/10.1021/jz200215s |J. Phys. Chem. Lett. 2011, 2, 1062–1069
The Journal of Physical Chemistry Letters
PERSPECTIVE
Figure 4. (a) Energy level diagrams, binding energy (BE) spectra, and raw photoelectron images for Ni and TiO at a photon energy of 2.33 eV (532 nm). Surface plots of the highest occupied 9σ and 1δ molecular orbitals from ab initio calculations appear in the inset of the binding energy spectrum of TiO. Note their resemblance to the associated 3d and 4s atomic orbitals of Ni. See ref 25 for details. (b) Energy level diagrams, BE spectra, and raw photoelectron images for Pd and ZrO at a photon energy of 2.33 eV (532 nm). The insets to the BE spectra display higher-resolution reconstruction slices of the indicated energy regions. Peak C0 and the 3Δ1 r 2Σ component of D0 appear as unresolved shoulders to more intense transitions. See ref 25.
The finding of similar electronic properties also suggests the likelihood of similar chemical behavior, a fact that we have found from studies of the products of a number of small organic molecules interacting with various isovalent ions (unpublished results). Devising New Material. The preceding section showed that insights into potential superatom element mimics can be deduced from measurements of electronic states, suggesting the prospect that materials with desired functionality might be designed and constructed from appropriate building blocks. Materials formed via cluster assembly are usually composed of varying numbers of assemblies of the building blocks, and they offer a route to the formation of new nanoscale materials. Of course, it is important that the individual clusters do not coalesce; designing an approach to effectively impede this is currently one of the major challenges in the superatom field. One can raise the issue of why there may be an advantage in materials formed via the assembly of element mimics rather than employing ones simply produced using the elements themselves. The key reason is that through an assembly process, it may be possible to acquire materials with more than a single functionality, for example, detection followed by destruction. Indeed, the prime objective of the studies presented here is to acquire the ability to tailor the chemical nature of superatom cluster building blocks for devising new nanoscale materials with desired functionality. If one can attain the ability to form a material with certain chosen chemical characteristics that perform differently
when perturbed, a wide variety of new avenues would be opened up. At this juncture, the jellium shell model provided the motivation for conceiving new superatoms. Our first foray into producing materials employing our superatom concepts involved the use of arsenic in combination with alkali metal complexes such as potassium ions in conjunction with cryptates.23 On the basis of these components, a nanoscale complex was produced utilizing a three step protocol. This involved the study of individual complexes in the gas phase to identify promising species, followed by a study of their properties. Thereafter, an identification of their bonding and stability was accomplished via theory and, finally, production via condensed-phase chemical techniques. This approach led to the formation of well identified materials. For example, an As-seven and an As-eleven atom species in combination with cryptated alkali atoms was produced, somewhat similar in structure to one already mentioned in the literature;30,31 see Figure 6. Subsequent theoretical and experimental studies revealed the possibility to produce materials having selected band gaps. Other groups are also undertaking work on superatoms. For example, through an extension of the jellium model, Jadzinski and co-workers32 were able to create other stable complexes employing the superatom concept. They placed particular emphasis on ligand-protected gold clusters, whereupon the gold atom complexes formed stable closed-shell entities. Surprisingly, in this context, a p-(MBA) protected nanoparticle comprised of 102 gold atoms and attached thiol groups led to complexes of a 1065
dx.doi.org/10.1021/jz200215s |J. Phys. Chem. Lett. 2011, 2, 1062–1069
The Journal of Physical Chemistry Letters
PERSPECTIVE
Figure 5. Note the similarity of the electronic properties of Pb2In2 compared to those of In3Sb; these two cluster systems are isoelectronic.27
Figure 6. Condensed-phase cluster materials with differing band gaps produced using the protocol deduced in our studies and discussed above. Also see refs 4, 23, 30, and 31.
precise number of atoms rather than ones of a varying number, as might be expected for a cluster comprised of this many atoms; donation of 44 gold atoms led to a core of 58 interacting atoms of gold, hence conforming to the jellium model as a closed-shell entity. There are a number of examples of such condensed-phase species developed employing colloidal chemical methods, building from the bottom up rather than the top down by subdividing bulk material. That there can be a major difference between the properties of matter of different sizes is clear and is one of the prime reasons for the large interest in cluster/nanoscale science, “systems where each atom counts”. Another example of matter having selected properties, also developed from the “bottom up” rather than the “top down”, comes
from the work of Perez et al.33 They demonstrated the possibility of forming films of a wide variety of materials using clusters, laying them down in a “paving” type process. A shortcoming of this method is the fact that the acquired material does not have a specific nor homogeneous composition, and the individual clusters do not function as an element mimic. Another approach that has met with some success is one in which zeolites have served as a skeleton framework to enable the establishment of a compositional arrangement of metal atoms, for example. One interesting finding was the ability to arrange 13 Pt atoms into a specific geometry employing the zeolite channels, giving rise to an emerging magnetic moment for a system that does not display a magnetic moment in the bulk state.34 More recently, Khanna and co-workers35 theoretically explored the possibility of inducing exchange splitting in supershells of a composite cluster comprised of a central transition metal surrounded by nearly free electron metal atoms, thereby acquiring a magnetic superatom. New Approach to Nanoscale Catalysts. One area where cluster assemblies are likely to be of particular use is related to nanocatalysis. The Castleman group has had a long-standing interest in the application of cluster science in unraveling certain catalytic mechanisms such as oxygen transfer employing specific clusters as model surface sites.3638 A number of findings related to the influence of size, composition, and charge state and electron density on reactive/ catalytic behavior have been acquired, which yield new insights into the role of nanoscale clusters as catalysts. It is worth noting that early debates raised issues such as questions whether charged clusters were suitable catalyst mimics, but the large number of recent findings that charged centers play a role in the functioning of many classes have put these concerns to rest. In the context of our own findings, this issue was also settled in the course of comparing cluster reactions for a variety of classes of reactions with the findings of bulk materials. In various studies, we found that clusters of selected sizes 1066
dx.doi.org/10.1021/jz200215s |J. Phys. Chem. Lett. 2011, 2, 1062–1069
The Journal of Physical Chemistry Letters
PERSPECTIVE
Table 2. Similarities in Chemical Behavior between Various Clusters and Bulk Catalysts
can serve as surface sites, where their structure may have geometries akin to steps, ledges, or corners, for example.36,38 In many cases, the finding of a similarity in chemical behavior of numerous cluster structures compared to those of various bulk catalysts is striking, as seen in Table 2 (bulk/cluster comparison). Employing the superatom concept, it has been realized that element cluster mimics may provide a valuable tool to tailor the design of new nanoscale materials having selected properties, as presented in an earlier section above. Even though mimics having similar electronic states have been found, a question regarding their reactive/catalytic behavior arises. To shed light on this possible issue, we commenced a detailed study of some palladium ion reactions with small organic molecules, finding nearly identical behavior for chemical reactions of Pd and the mimic ZrO, even for
In many cases, the finding of a similarity in chemical behavior of numerous cluster structures compared to those of various bulk catalysts is striking. species of differing charge states. This unexpected but promissing behavior does still require further investigation for other systems. Prospects for the Future. The fields of cluster science and nanoscale science have, in some respects, grown up separately. The former nearly always involved a study of species formed from the bottom up, while in the beginning, nanoscale materials most often had birth arising from the subdivision of bulk materials. Nevertheless, it is becoming increasingly obvious that the two have much in common and that cluster science enables considerable new insight into the fundamental properties and behavior of matter of small size. Cluster science offers many advantages in
that as the clusters grow, they approach the properties of a surface, and ones can often be produced that model various surface sites and defects; they are reproducible in character, which is frequently not the case for bulk catalysts. Small clusters are amenable to theoretical treatment, allowing comparison with experimental findings for more in-depth interpretation of properties, and the influence of charge density can be determined because clusters can usually be produced in any of the three charge states, as cations, anions, and neutrals. A regime of particular interest is one where each atom counts, giving rise to catalysts (and materials) with differing properties and reactivity.39 The most exciting prospect is one in which material properties can be tailor designed to display valuable and useful properties; we envision catalysts, electronic and spintronic materials, detectors, and energetic materials to be the most promising, warranting considerable effort to attain such goals. The discovery of naturally occurring elements has taken chemistry and physics a long way from the possibilities brought about by conceiving of earth, air, water, and fire to be the basis of all substances. Now, adopting selected combinations of elements to produce complex molecules and assemblies that both produce compounds that mimic individual elements and also give rise to species of multifunctionality offers uncountable new prospects. Playing a role in helping further develop this emerging area of science is an exciting intellectual challenge.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ BIOGRAPHY A. W. Castleman, Jr. is Evan Pugh Professor of Chemistry and Physics and Eberly Distinguished Chair in Science at Penn State University. Among his honors include member of the National Academy of Sciences, fellow of the American Academy of Arts 1067
dx.doi.org/10.1021/jz200215s |J. Phys. Chem. Lett. 2011, 2, 1062–1069
The Journal of Physical Chemistry Letters and Sciences, and recipient of the ACS Irving Langmuir Award in Chemical Physics. His research interests span numerous areas of cluster science. http://research.chem.psu.edu/awcgroup/Castleman Homepage.html
’ ACKNOWLEDGMENT The agencies which sponsored the research that led to the advances noted in this invited Perspective are gratefully acknowledged: AFOSR Grant FA9550-10-1-0071, AFOSR MURI Grant No. 10006288-51, U.S. Department of the Army MURI Grant No. W911NF-06-1-0280, and U.S. Department of Energy Grant DE-FG02-92ER14258. The author sincerely thanks the many present and past students and postdocs, as well as theoretical collaborators at VCU and the Humboldt University of Berlin, whose work has contributed to the findings described herein. The author gives special thanks to Mr. Joshua Melko for helpful discussions and assistance in preparing the final manuscript. ’ REFERENCES (1) Morris, R. The Last Sorcerers; Joseph Henry Press: Washington, DC, 2003. (2) Coffey, Patrick Cathedrals of Science: The Personalities and Rivalries That Made Modern Chemistry; Oxford University Press: New York, 2008. (3) Seaborg, G. T.; Wahl, A. C. The Chemical Properties of Elements 94 and 93. J. Am. Chem. Soc. 1948, 70, 1128–1134. (4) Castleman, A. W., Jr.; Khanna, S. N. Clusters, Superatoms, and Building Blocks of New Materials. J. Phys. Chem. C 2009, 113, 2664–2675. (5) Castleman, A. W., Jr.; Bowen, K. H. Clusters: Structures, Energetics, and Dynamics of Intermediate States of Matter. J. Phys. Chem. 1996, 100, 12911–12944. (6) Leuchtner, R. E.; Harms, A. C.; Castleman, A. W., Jr. Thermal Metal Cluster Anion Reactions: Behavior of Aluminum Clusters with Oxygen. J. Chem. Phys. 1989, 91, 2753–2754. (7) Gantef€or, G.; Meiwes-Br€or, K. H.; Lutz, H. O. Photodetachment Spectroscopy of Cold Aluminum Cluster Anions. Phys. Rev. A 1988, 37, 2716–2718. (8) Bergeron, D. E.; Castleman, A. W., Jr.; Morisato, T.; Khanna, S. N. Formation of Al13I: Evidence for the Superhalogen Character of Al13. Science 2004, 304, 84–87. (9) Reber, A. C.; Khanna, S. N.; Roach, P. J.; Woodward, W. H; Castleman, A. W., Jr. Spin Accommodation and Reactivity of Aluminum Based Clusters with O2. J. Am. Chem. Soc. 2007, 129, 16098–16101. (10) Burgert, R.; Schnoeckel, H.; Grubisic, A.; Li, X.; Stokes, S. T.; Bowen, K. H.; Gantef€or, G. F.; Kiran, B.; Jena, P. Spin Conservation Accounts for Aluminum Cluster Anion Reactivity Pattern with O2. Science 2008, 319, 438–442. (11) Knight, W. D.; Clemenger, K.; deHeer, W. A.; Saunders, W. A.; Chou, M. Y.; Cohen, M. L. Electronic Shell Structure and Abundances of Sodium Clusters. Phys. Rev. Lett. 1984, 52, 2141–2143. (12) Bergeron, D. E.; Roach, P. J.; Castleman, A. W., Jr.; Jones, N. O.; Khanna, S. N. Al Cluster Superatoms as Halogens in Polyhalides and as Alkaline Earths in Iodide Salts. Science 2005, 307, 231–235. (13) Khanna, S. N.; Jena, P. Assembling Crystals from Clusters. Phys. Rev. Lett. 1992, 69, 1664–1667. (14) “A rose by any other name is still a rose”. Shakespeare, W. Romeo and Juliet; 1594. (15) Harms, A. C.; Leuchtner, R. E.; Sigsworth, S. W.; Castleman, A. W., Jr. Gas-Phase Reactivity of Metal Alloy Clusters. J. Am. Chem. Soc. 1990, 112, 5673–5674. (16) Wagner, R. L.; Vann, W. D.; Castleman, A. W., Jr. A Technique for Efficiently Generating Bimetallic Clusters. Rev. Sci. Instrum. 1997, 68, 3010–3013.
PERSPECTIVE
(17) Reveles, J. U.; Khanna, S. N.; Roach, P. J.; Castleman, A. W., Jr. Multiple Valence Superatoms. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 18405–18410. (18) Leskiw, B. D.; Castleman, A. W., Jr. The Interplay Between the Electronic Structure and Reactivity of Aluminum Clusters: Model Systems as Building Blocks for Cluster Assembled Materials. Chem. Phys. Lett. 2000, 316, 31–36. (19) Zheng, W.-J.; Thomas, O. C.; Lippa, T. P.; Xu, S.-J.; Bowen, K. H. The Ionic KAl13 Molecule: A Stepping Stone to ClusterAssembled Materials. J. Chem. Phys. 2006, 124, 1443041–1443045. (20) Bergeron, D. E.; Castleman, A. W., Jr.; Morisato, T.; Khanna, S. N. Formation and Properties of Halogenated Aluminum Clusters. J. Chem. Phys. 2004, 121, 10456–10466. (21) Bergeron, D. E.; Roach, P. J.; Castleman, A. W., Jr.; Jones, N. O.; Reveles, J. U.; Khanna, S. N. Reactions of AlnIx with Methyl Iodide: The Enhanced Stability of Al7I and the Chemical Significance of Active Centers. J. Am. Chem. Soc. 2005, 127, 16048–16053. (22) Li, X.; Wu, H.; Wang, X.-B.; Wang, L.-S. sp Hybridization and Electron Shell Structures in Aluminum Clusters: A Photoelectron Spectroscopy Study. Phys. Rev. Lett. 1998, 81, 1909–1912. (23) Castleman, A. W., Jr.; Khanna, S. N.; Sen, A.; Reber, A. C.; Qian, M.; Davis, K. M.; Peppernick, S. J.; Ugrinov, A.; Merritt, M. D. From Designer Clusters to Synthetic Crystalline Nanoassemblies. Nano Lett. 2007, 7, 2734–2741. (24) Reber, A. C.; Khanna, S. N.; Castleman, A. W., Jr. Superatom Compounds, Clusters, and Assemblies: Ultra Akali Motifs and Architectures. J. Am. Chem. Soc. 2007, 129, 10189–10194. (25) Peppernick, S. J.; Gunaratne, K. D.; Castleman, A. W., Jr. Superatom Spectroscopy and the Electronic State Correlation Between Elements and Isoelectronic Molecular Counterparts. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 975–980. (26) Peppernick, S. J.; Gunaratne, K. D. D.; Castleman, A. W., Jr. Towards Comprehending the Superatomic State of Matter. Chem. Phys. Lett. 2010, 489, 1–11. (27) IUPAC Gold Book, IUPAC definition of isoelectronic, PAC, 1994, 66, 1077; (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1128. (28) Levy, R. B.; Boudart, M. Platinum-like Behavior of Tungsten Carbide in Surface Catalysis. Science 1973, 181, 547–549. (29) Melko, J. J.; Ong, S. V.; Gupta, U.; Reveles, J. U.; D’Emidio, J.; Khanna, S. N.; Castleman, A. W., Jr. Anion Photoelectron Spectroscopy and First-Principles Study of PbxIny Clusters. J. Phys. Chem. C 2010, 114, 20907–20916. (30) Materials with band gaps of 2.60 and 1.81 eV were produced at Penn State University, A. Sen. (31) Materials with a band gap of 1.35 eV: Emmerling, F.; R€ohr, C. Alkaline Metal Arsenides A3As7 and AAs (A = K, Rb, Cs). Preparation, Crystal Structure, Vibrational Spectroscopy. Z. Naturforsch., B: Chem. Sci. 2002, 57, 963–975. (32) Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Bushnell, D. A.; Kornberg, R. D. Structure of a Thiol Monolayer-Protected Gold Nanoparticle at 1.1 Å Resolution. Science 2007, 318, 430–433. (33) Perez, A.; Melinon, P.; Dupuis, V.; Jensen, P.; Prevel, B.; Tuaillon, J.; Bardotti, L.; Martet, C.; Treilleux, M.; Broyer, M.;Cluster Assembled Materials: A Novel Class of Nanostructured Solids with Original Structures and Properties. J. Phys. D: Appl. Phys. 1997, 30, 709–721. (34) Bartolome, J.; Bartolome, F.; García, L. M.; Roduner, E.; Akdogan, Y.; Wilhelm, F.; Rogalev, A. Phys. Rev. B 2009, 80, 014404/ 1–014404/10. (35) Reveles, J. U.; Clayborne, P. A.; Reber, A. C.; Khanna, S. N.; Pradhan, K.; Sen, P.; Pederson, M. R. Designer Magnetic Superatoms. Nat. Chem. 2009, 132, 310–315. (36) Zemski, K. A.; Justes, D. R.; Castleman, A. W., Jr. Studies of Metal Oxide Clusters: Elucidating Reactive Sites Responsible for the Activity of Transition Metal Oxide Catalysts. J. Phys. Chem. B 2002, 106, 6136–6148. 1068
dx.doi.org/10.1021/jz200215s |J. Phys. Chem. Lett. 2011, 2, 1062–1069
The Journal of Physical Chemistry Letters
PERSPECTIVE
(37) Reilly, N. M.; Johnson, G. E.; Castleman, A. W., Jr. The Reactivity of Gas Phase Metal Oxide Clusters: Systems for Understanding the Mechanisms of Heterogeneous Catalysts. In Model Systems in Catalysis: From Single Crystals and Size-selected Clusters to Supported Enzyme Mimics; Rioux, R. M., Ed.; Springer: New York, 2010. (38) Johnson, G. E.; Mitric, R.; Bonacic-Koutecky , V.; Castleman, A. W., Jr. Clusters as Model Systems for Investigating Nanoscale Oxidation Catalysis. Chem. Phys. Lett. 2009, 475, 1–9. (39) Heiz, U.; Landman, U. Nanocatalysis; Springer: New York, 2006.
1069
dx.doi.org/10.1021/jz200215s |J. Phys. Chem. Lett. 2011, 2, 1062–1069
