Albert CASTLEMAN, Jr., et al.

Superatom Clusters


Superatoms are clusters of atoms that seem to exhibit some of the properties of elemental atoms.

Sodium atoms, when cooled from vapor, naturally condense into clusters, more so into clusters of 2, 8, 20, 40, 58 or 82 atoms (the magic numbers), than into the other numbers. The first two of these can be recognized as the numbers of electrons needed to fill s and p orbitals, respectively. The superatom suggestion is that free electrons in the cluster occupy a new set of orbitals that are defined by the entire group of atoms, i.e. cluster, rather than each individual atom separately (non-spherical or doped clusters show deviations in the number of electrons that form a closed shell as the potential is defined by the shape of the positive nuclei.) Superatoms tend to behave chemically in a way that will allow them to have a closed shell of electrons, in this new counting scheme. Therefore, a superatom with one more electron than a full shell should give up that electron very easily, similar to an alkali metal, and a cluster with one electron short of full shell should have a large electron affinity, such as a halogen.

Aluminium clusters

Certain aluminium clusters have superatom properties. These aluminium clusters are generated as anions (Aln- with n = 1,2,3...) in helium gas and reacted with a gas containing iodine. When analyzed by mass spectrometry one main reaction product turns out to be Al13I-.[1] These clusters of 13 aluminium atoms with an extra electron added do not appear to react with oxygen when it is introduced in the same gas stream. Assuming each atom liberates its 3 valence electrons, this means that there are 40 electrons present, which is one of the magic numbers noted above for sodium, and implies that these numbers are a reflection of the noble gases. Calculations show that the additional electron is located in the aluminium cluster at the location directly opposite from the iodine atom. The cluster must therefore have a higher electron affinity for the electron than iodine and therefore the aluminium cluster is called a superhalogen. The cluster component in Al13I- ion is similar to an iodine ion or better still a bromine atom. The related Al13I2- cluster is expected to behave chemically like the triiodide ion.

Similarly it has been noted that Al14 clusters with 42 electrons (2 more than the magic numbers) appear to exhibit the properties of an alkaline earth metal which typically adopt +2 valence states. This is only known to occur when there are at least 3 iodine atoms attached to an Al14- cluster, Al14I3-. The anionic cluster has a total of 43 itinerant electrons, but the three Iodine atoms each remove one of the itinerant electrons to leave 40 electrons in the jellium shell.[2][3]

It is particularly easy and reliable to study atomic clusters of inert gas atoms by computer simulation because interaction between two atoms can be approximated very well by the Lennard-Jones potential. Other methods are readily available and it has been established that the magic numbers are 13, 19, 23, 26, 29, 32, 34, 43, 46, 49, 55, etc. [I. A. Harris et al. Phys. Rev. Lett. Vol. 53, 2390-94 (1984).]

Aluminum clusters

* Al7 = the property is similar to germanium atoms.
* Al13 = the property is similar to halogen atoms, more specifically, chlorine.
          o Al13Ix-, where x = 1-13.[4]
* Al14 = the property is similar to alkaline metals.
          o Al14Ix-, where x = 1-14.[4]
* Al23
* Al37

Other clusters

* Li(HF)3Li = the (HF)3 interior causes 2 valence electrons from the Li to orbit the entire molecule as if it were an atom's nucleus.[5]
* VSi16F = has ionic bonding.[6]
* A cluster of 13 platinum becomes magnetic.[7]
* A cluster of 2000 Rubidium atoms.[8]


1. ^ Formation of Al13I-: Evidence for the Superhalogen Character of Al13 D. E. Bergeron, A.W. Castleman Jr., T. Morisato, S. N. Khanna Science, Vol 304, Issue 5667, 84-87 , 2 April 2004

2. ^ Philip Ball, "A New Kind of Alchemy", New Scientist, 2005-04-16.

3. ^ Al Cluster Superatoms as Halogens in Polyhalides and as Alkaline Earths in Iodide Salts D. E. Bergeron, P. J. Roach, A.W. Castleman Jr., N.O. Jones, S. N. Khanna Science, Vol 307, Issue 5707, 231-235 , 14 January 2005

4. ^ a b Naiche Owen Jones, 2006. --

5. ^ Extraordinary superatom containing double shell nucleus: Li(HF)3Li connected mainly by intermolecular interactions, Sun, Xiao-Ying, Li, Zhi-Ru, Wu, Di, & Sun, Chia-Chung, 2007. --

6. ^ Electronic and geometric stabilities of clusters with transition metal encapsulated by silicon, Kiichirou Koyasu et al. --

7. ^ Platinum nanoclusters go magnetic,, 2007 --

8. ^ Ultra Cold Trap Yields Superatom, NIST, 1995 --

External links

* Expanding the periodic table? The Scientist, 2005 --

* On the Aluminum Cluster Superatoms acting as Halogens and Alkaline-earth Metals, Bergeron, Dennis E, 2006 --

* Research Reveals Halogen Characteristics innovations report, 2004. Have pictures of Al13. --

* Clusters of Aluminum Atoms Found to Have Properties of Other Elements Reveal a New Form of Chemistry, innovations report, 2005. Have a picture of Al14. --

* Beyond The Periodic Table, Computational Chemistry Portal, 2006 --

* Clusters of Aluminum Atoms Found to Have Properties of Other Elements Reveal a New Form of Chemistry, Penn State, Eberly College of Science, 2005 --
J. Phys. Chem. C, 2009, 113 (7), pp 2664–2675
January 23, 2009

Clusters, Superatoms, and Building Blocks of New Materials

A. W. Castleman, Jr.
Departments of Chemistry and Physics, The Pennsylvania State University, University Park, Pennsylvania 16802

S. N. Khanna
Department of Physics, Virginia Commonwealth University, Richmond, Virginia 23284
J. Phys. Chem. C, 2009, 113 (7), pp 2664–2675
Publication Date (Web): January 23, 2009


A. W. Castleman, Jr. is Evan Pugh Professor of Chemistry and Physics and Eberly Distinguished Chair in Science at The Pennsylvania State University, having been professor of chemistry and member of CIRES at the University of Colorado (1975?1882) and previously on the staff at the Brookhaven National Laboratory. He served as a Senior Editor of the Journal of Physical Chemistry from 1988 to 1998. He is a member of the National Academy of Sciences, and a fellow of the American Academy of Arts and Sciences, as well as of the American Physical Society, the American Association for the Advancement of Science, the New York Academy of Sciences and the Royal Society of Chemistry. Included among his many awards and honors are the Doktors Honoris Causa from the University of Innsbruck, Austria, the American Chemical Society Award for Creative Advances in Environmental Science and Technology, the Wilhelm Jost Memorial Lectureship Award from the German Chemical Society, U. S. Senior Scientist von Humboldt Award, Fulbright Senior Scholar award, Senior Fellow of the Japanese Society for the Promotion of Science, Rensselaer distinguished Thomas W. Phelan Fellows alumni award, and Sherman Fairchild Distinguished Scholar at Cal Tech. He is engaged in studies to bridge the gas and condensed phase and to elucidate the fundamentals of solvation dynamics through investigation of cluster photophysics. He is particularly interested in exploring the properties of matter of finite dimension using ultrafast laser techniques, elucidating the physical basis for catalysis and surface phenomena at the molecular level, and developing with his theoretical collaborator S. N. Khanna of VCU the unique characteristics of superatom clusters as building blocks to cluster assembled nanoscale materials. Castleman has published over 600 papers dealing with these subjects.

Shiv N. Khanna is a Professor of Physics at Virginia Commonwealth University, having been a visiting associate professor at the Northeastern University (1983?84) and a scientific collaborator at the Swiss Federal Institute of Technology in Switzerland (1980?1983). He is a Fellow of the American Physical Society and has been twice the recipient of the Distinguished Scholar Award of the College of Humanities and Sciences at VCU. He is a member of the Advisory Board of the “Materials Science Forum” from Trans Tech Publications and “Journal of Mathematics and Sciences: Collaborative Explorations”. He has co-authored more than 200 research publications in refereed journals, has edited six monographs, and has chaired/co-chaired several International Conferences. Dr. Khanna and his group are involved in theoretical studies of the electronic structure, magnetic properties, and catalytic properties of atomic clusters, cluster assemblies, and nanoscale materials. Along with A. W. Castleman, Jr. at PSU, they have proposed “superatoms” that extend the “Periodic Chart” to a third dimension and could lead to novel materials with tunable properties and potential for applications in numerous areas.

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.

Artist’s rendition of an aluminum-iodine "Superatom" identified by the Castleman group at Penn State and the Khanna group at Virginia Commonwealth University.

Credit: D.E. Bergeron, P.J. Roach, A.W. Castleman, N.O. Jones, and S.N. Khanna
Science 14 January 2005: Vol. 307. no. 5707, pp. 231 - 235
DOI: 10.1126/science.1105820

Al Cluster Superatoms as Halogens in Polyhalides and as Alkaline Earths in Iodide Salts

D. E. Bergeron, P. J. Roach, A. W. Castleman, Jr., N. O. Jones, S. N. Khanna

Two classes of gas-phase aluminum-iodine clusters have been identified whose stability and reactivity can be understood in terms of the spherical shell jellium model. Experimental reactivity studies show that the Al13I –x clusters exhibit pronounced stability for even numbers of I atoms. Theoretical investigations reveal that the enhanced stability is associated with complementary pairs of I atoms occupying the on-top sites on the opposing Al atoms of the Al13– core. We also report the existence of another series, Al14I –x, that exhibits stability for odd numbers of I atoms. This series can be described as consisting of an Al14I –3 core upon which the I atoms occupy on-top locations around the Al atoms. The potential synthetic utility of superatom chemistry built upon these motifs is addressed.[all%3A+Castleman+superatom]

J. Am. Chem. Soc., 2008, 130 (1), pp 2–3
December 08, 2007 (Communication)
DOI: 10.1021/ja074225m

Does the “Superatom” Exist in Halogenated Aluminum Clusters?

Young-Kyu Han and Jaehoon Jung

Does the “Superatom” Exist in Halogenated Aluminum Clusters? ... We have shown that Aln clusters do not show any characteristics of a superatom ... The enhanced stability of halogenated Al clusters can be explained by the magic nature of the clusters, not by superatom chemistry. ...

Inorg. Chem., Articles ASAP (As Soon As Publishable)
February 23, 2009 (Communication)
DOI: 10.1021/ic8024588

From Superatomic Au25(SR)18? to Superatomic M@Au24(SR)18q Core?Shell Clusters

De-en Jiang and Sheng Dai

Au25(SR)18? belongs to a new type of superatom ... This superatom ... By applying this superatom ...

J. Am. Chem. Soc., 2007, 129 (33), pp 10189–10194
Publication Date (Web): July 27, 2007 (Article)
DOI: 10.1021/ja071647n

Superatom Compounds, Clusters, and Assemblies:  Ultra Alkali Motifs and Architectures

Arthur C. Reber, Shiv N. Khanna, and A. Welford Castleman, Jr.

J. Am. Chem. Soc., 2005, 127 (14), pp 4998–4999
March 18, 2005
DOI: 10.1021/ja045380t

Selective Formation of MSi16 (M = Sc, Ti, and V)

Kiichirou Koyasu, Minoru Akutsu, Masaaki Mitsui, and Atsushi Nakajima

We present experimental evidence for a highly stable cluster corresponding to M@Si16 (M = Sc, Ti, and V). Mass spectrometry and anion photoelectron spectroscopy show that the cluster features an electronically closed TiSi16 neutral core which undergoes a change in the number of valence electrons involving (i) substitution of neighboring metals with Sc and V, or (ii) addition of a halogen atom to the TiSi16 anion, and that VSi16F is predicted to form an ionically bound superatom complex. ...

ACS Nano, 2009, 3 (2), pp 244–255
February 5, 2009 (Review)
DOI: 10.1021/nn800820e

Cluster-Assembled Materials

Shelley A. Claridge, A. W. Castleman, , Shiv N. Khanna, Christopher B. Murray, Ayusman Sen and Paul S. Weiss

Inorg. Chem., 2008, 47 (21), pp 9773–9778
October 3, 2008
DOI: 10.1021/ic800184z

Compounds of Superatom Clusters: Preferred Structures and Significant Nonlinear Optical Properties of the BLi6-X (X = F, LiF2, BeF3, BF4) Motifs

Ying Li, Di Wu and Zhi-Ru Li

J. Am. Chem. Soc., 2007, 129 (7), pp 1900–1901
January 27, 2007
DOI: 10.1021/ja068334x

Assembly and Stabilization of a Planar Tetracoordinated Carbon Radical CAl3Si:  A Way To Design Spin-Based Molecular Materials

Li-ming Yang, Yi-hong Ding, and Chia-chung Sun

To capture and stabilize the ptC radical, we proposed a scheme “heterodecked sandwich”, in which way CAl3Si can act as a spin-embedded “superatom” because of the well conservation of the radical's spin, structural and electronic integrity during the cluster assembly. ...

J. Phys. Chem. A, 2006, 110 (44), pp 12073–12076
October 19, 2006
DOI: 10.1021/jp065161p

Experimental and Theoretical Characterization of Aluminum-Based Binary Superatoms of Al12X and Their Cluster Salts

Minoru Akutsu, Kiichirou Koyasu, Junko Atobe, Natsuki Hosoya, Ken Miyajima, Masaaki Mitsui, and Atsushi Nakajima

The geometric and electronic structures of aluminum binary clusters, AlnX (X = Si and P), have been investigated, using mass spectrometry, anion photoelectron spectroscopy, photoionization spectroscopy, and theoretical calculations. Both experimental and ...

J. Am. Chem. Soc., 2009, 131 (7), pp 2490–2492
January 29, 2009
DOI: 10.1021/ja809157f

Reversible Switching of Magnetism in Thiolate-Protected Au25 Superatoms

Manzhou Zhu, Christine M. Aikens, Michael P. Hendrich, Rupal Gupta, Huifeng Qian, George C. Schatz and Rongchao Jin

Interestingly, the HOMO orbital exhibits distinct P-like character, reminiscent of the superatom model for bare metal clusters. ...

J. Phys. Chem. A, 2007, 111 (42), pp 10675–10681
October 3, 2007
DOI: 10.1021/jp071054z

Theoretical Study on the Assembly and Stabilization of a Magic Cluster Al4N-

Li-ming Yang, Yi-hong Ding, and Chia-chung Sun

The good structural and electronic integrity of the Al4N- unit within the designed assembled systems leads us to propose that the magic unit Al4N- might act as a new kind of “superatom”8d-f,11 in combinational chemistry. ...

J. Phys. Chem. A, 2007, 111 (20), pp 4378–4383
April 21, 2007
DOI: 10.1021/jp068591o

Structures and Electronic Properties of Al7X0,- and Al13X1,2,12- Clusters with XF, Cl, and Br

Jiao Sun, Wen-Cai Lu, Li-Zhen Zhao, Wei Zhang, Ze-Sheng Li, and Chia-Chung Sun

Among the systems studied, Al7 and Al13 clusters in Al7X and Al13X- reveal alkali-like and halogen-like superatom characters, respectively. ... However, when adding more halogens, the superatom structure would be destroyed, resulting in low-symmetry compounds with the center Al atom moving toward the cluster surface. ...

J. Phys. Chem. A, 2007, 111 (1), pp 42–49
December 14, 2006
DOI: 10.1021/jp066757f

Electronic and Geometric Stabilities of Clusters with Transition Metal Encapsulated by Silicon

Kiichirou Koyasu, Junko Atobe, Minoru Akutsu, Masaaki Mitsui, and Atsushi Nakajima

The reactivity of a halogen atom with the MSi16 clusters reveals that VSi16F forms a superatom complex with ionic bonding. ...

J. Am. Chem. Soc., 2005, 127 (46), pp 16048–16053
October 28, 2005
DOI: 10.1021/ja050338z

Stability of Al7I and the Chemical Significance of Active Centers

Denis E. Bergeron, Patrick J. Roach, A.Welford Castleman, Jr., Naiche O. Jones, J. Ulises Reveles, and Shiv N. Khanna

We show that the relative inertness of the cluster is derived from the stability of the neutral Al7I which can be looked upon as a “jellium compound” formed by the interaction between a Al7 superatom and an I atom. ...

J. Phys. Chem. A, 2008, 112 (51), pp 13316–13325
November 24, 2008
DOI: 10.1021/jp804667d

AlnBi Clusters: Transitions Between Aromatic and Jellium Stability

Charles E. Jones, , Pene A. Clayborne, J. Ulises Reveles, Joshua J. Melko, Ujjwal Gupta, Shiv N. Khanna and A. W. Castleman,

The electron count (20) combined with a compact, three-dimensional geometry makes the stable Al5Bi a possible superatom candidate. ...

J. Chem. Theory Comput., 2008, 4 (12), pp 2011–2019
November 5, 2008
DOI: 10.1021/ct800232b

Al5O4: A Superatom with Potential for New Materials Design

Ujjal Das and Krishnan Raghavachari

Al5O4: A Superatom with Potential for New Materials Design ... For example, Castleman and co-workers have shown that the size of the Al13 superatom(4) is too big to fit with counterions such as the alkali metals. ... For example, the HOMO?LUMO energy gap in Al13K, which also behaves like a stable diatomic ionic molecule as shown by Bowen and co-workers, is observed to be 1.3 eV.(38) As7K3 is another stable cluster where this gap is measured to be 2.2 eV by Castleman et al.(15) Using TDDFT calculations, we have computed the energy required to excite an electron from the doubly filled HOMO of Al5O4K to the LUMO without allowing any geometric changes. ...

J. Phys. Chem. A, 2007, 111 (37), pp 9122–9129
August 29, 2007
DOI: 10.1021/jp074645y

Investigation of the Typical Triangular Structure B3 in Boron Chemistry:  Insight into Bare All-Boron Clusters Used as Ligands or Building Blocks

Li-ming Yang, Jian Wang, Yi-hong Ding, and Chia-chung Sun

Additionally, the electronic and structural properties of B3- are well conserved during cluster-assembly, characteristic of a “superatom” feature. ...

J. Am. Chem. Soc., 2006, 128 (24), pp 7904–7908
Publication Date (Web): May 28, 2006 (Article)
DOI: 10.1021/ja060613x

Primary Reaction Steps of Al13- Clusters in an HCl Atmosphere:  Snapshots of the Dissolution of a Base Metal

Ralf Burgert, Sarah T. Stokes, Kit H. Bowen, and Hansgeorg Schnöckel

Recently, the icosahedral Al13- cluster has been shown to possess some unusual characteristics due to its special stability (Bergeron, D. E.; et al. Science 2004, 304, 84?87; 2005, 307, 231?235). Here we present reactions of isolated Al13- clusters with ...

J. Phys. Chem. B, 2006, 110 (41), pp 20098–20101
September 20, 2006
DOI: 10.1021/jp064821n

Aromatic Superclusters from All-Metal Aromatic and Antiaromatic Monomers, [Al4]2- and [Al4]4-

Sairam S. Mallajosyula, Ayan Datta, and Swapan K. Pati

In fact, within these superclusters, each monomer cluster acts as a superatom, leading to the formation of highly stable three-dimensional structures. ...

J. Am. Chem. Soc., 2005, 127 (45), pp 15680–15681
October 20, 2005
DOI: 10.1021/ja055407o

Gold-Caged Metal Clusters with Large HOMO?LUMO Gap and High Electron Affinity

Yi Gao, Satya Bulusu, and Xiao Cheng Zeng

In particular, highly stable clusters with large energy gap (>1.5 eV) between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) may be perceived as a “superatom”, analogous to fullerene C60 (with a large HOMO?LUMO gap ? = 1.57 eV)4 that tends to retain its structure integrity and chemical identity in cluster-assembled solids. ...

Clusters of Aluminum Atoms Found to Have Properties of Other Elements Reveal a New Form of Chemistry

Barbara K. Kennedy

Source: EurekAlert!
Further information:

A research team has discovered clusters of aluminum atoms that have chemical properties similar to single atoms of metallic and nonmetallic elements when they react with iodine. The discovery opens the door to using ’superatom chemistry’ based on a new periodic table of cluster elements to create unique compounds with distinctive properties never seen before. The results of the research, headed jointly by Shiv N. Khanna, professor of physics at Virginia Commonwealth University and A. Welford Castleman Jr., the Evan Pugh Professor of Chemistry and Physics and the Eberly Family Distinguished Chair in Science at Penn State University, will be reported in the 14 January 2005 issue of the journal Science.

"Depending on the number of aluminum atoms in the cluster, we have demonstrated ’superatoms’ exhibiting the properties of either halogens or alkaline earth metals," says Castleman. "This result suggests the intriguing potential of this chemistry in nanoscale synthesis." The discovery could have practical applications in the fields of medicine, food production and photography.

The researchers examined the chemical properties, electronic structure, and geometry of aluminum clusters both theoretically and experimentally in chemical compounds with iodine atoms. They found that a cluster of 13 aluminum atoms behaves like a single iodine atom, while a cluster of 14 aluminum atoms behaves like an alkaline earth atom. "The discovery of these new iodine compounds, which include aluminum clusters, is critical because it reveals a new form of ’superatom’ chemistry," said Khanna. "In the future, we may apply this chemistry, building on our previous knowledge, to create new materials for energy applications and even medical devices."
To make their discovery, the research team replaced iodine atoms with the aluminum clusters in naturally occurring chains or networks of iodine atoms and molecules known as polyiodides. When the researchers substituted the iodine atom with the aluminum cluster, Al13, they observed that the entire chemistry of the compound changed--causing the other iodine molecules to break apart and bind individually to the cluster. The researchers then were able to bind 12 iodine atoms to a single Al13 cluster, forming a completely new class of polyiodides. "Our production of such a species is a stirring development that may lead to new compounds with a completely new class of chemistry and applications," says Castleman. "Along with the discovery that Al14 clusters appear to behave similarly to alkaline earth atoms when combined with iodine, these new results give further evidence that we are really on our way to the development of a periodic table of the ’cluster elements’."

The researchers conducted experimental reactivity studies that indicate that certain aluminum-cluster superatoms are highly stable by nature. The team’s related theoretical investigations reveal that the enhanced stability of these superatoms is associated with a balance in their atomic and electronic states. While the clusters resemble atoms of other elements in their interactions, their chemistry is unique, creating stable compounds with bonds that are not identical to those of single atoms.

Using stable clusters provides a possible route to an adaptive chemistry that introduces the aluminum-cluster species into nanoscale materials, tailoring them to create desirable properties. "The flexibility of an Al13 cluster to act as an iodine atom shows that superatoms can have synthetic utility, providing an unexplored ’third dimension’ to the traditional periodic table of elements," said Khanna. "Applications using Al13 clusters instead of iodine in polymers may lead to the development of improved conducting materials. Assembling Al13I units may provide aluminum materials that will not oxidize, and may help overcome a major problem in fuels that burn aluminum particles."

The theoretical investigations for this project were conducted by Khanna with N.O. Jones, a graduate student in the physics department at Virginia Commonwealth University, and the experimental work was conducted by Castleman with Denis Bergeron and Patrick J. Roach, graduate students in the chemistry department at Penn State.

This research was supported by the U. S. Air Force Office of Scientific Research and the U. S. Department of Energy.
New Scientist # 2495 ( 16 April 2005 ), page 30

A New Kind of Alchemy

Philip Ball

The Transformative Power of the nano-clustered Superatom generates a new form of Chemistry with super-catalytic effects and implications for metal colloidals.

LET'S hear it for Dmitri Mendeleev. His periodic table has done a remarkable job of making sense of the elements, arranging them neatly into families whose members share similar properties. For more than a century it has been chemists' guiding light. But Mendeleev's classic layout is starting to prove inadequate at describing the unexpected ways in which chemical elements behave when divvied up into small chunks. And now some chemists think it may be time to build a whole new table, this time from something much stranger than atoms: superatoms.

According to Mendeleev's roll call, an element's chemistry can be deduced from where it sits in the periodic table. Reactive metals like sodium and calcium occupy the two columns on the left. The inert "noble" gases make up the column on the far right, flanked by typical non-metals such as chlorine and sulphur.

Now this neat picture is being disrupted by superatoms - clusters of atoms of a particular chemical element that can take on the properties of entirely different elements. The chemical behaviour can be altered, sometimes drastically, by the addition of just one extra atom. "We can take one element and have it mimic several different elements in the periodic table," says Welford Castleman, an inorganic chemist at Pennsylvania State University who has studied the chemistry of aluminium superatoms.

It is a finding that is challenging our entire understanding of chemical reactivity. Adding superatoms to the periodic table would transform it from a flatland to a three-dimensional landscape in which each element is drawn out into a series of super-elements. Superatoms could have practical uses too: they could be combined into super-molecules to make new materials. And their unusual chemistry could be harnessed to make efficient fuels.

According to conventional thinking, the chemical properties of an atom depend on the way the electrons orbiting its nucleus are arranged in a series of shells. This in turn is determined by the number of electrons it possesses - just one in the case of hydrogen, for example, but up to 92 for an atom of the heavy metal uranium. The structure of the periodic table is explained by the gradual filling of the shells. Atoms with completely filled shells - the noble gases, such as helium, argon and xenon - are particularly unreactive. The most reactive elements are often those with atoms that are just one electron short of a filled shell and so occupy the column next to the noble gases in the periodic table, or those with one electron too many, which make up the left-most column of the table.

This simple picture was thrown into disarray in the early 1980s, when evidence started appearing that clusters of atoms of one element could behave like another. Thomas Upton at the California Institute of Technology in Pasadena discovered that clusters of six aluminium atoms could catalyse the splitting of hydrogen molecules in much the same way as ruthenium, a metal used as a catalyst in the chemical industry. This quickly led to thoughts of extending the periodic table. "Some of us started giving talks with Mendeleev in the title," recalls Robert Whetten, a cluster chemist at the Georgia Institute of Technology in Atlanta.

What was so special about these six-atom clusters? Research carried out around the same time by Walter Knight and his colleagues at the University of California, Berkeley, on another type of cluster started to provide some clues. Knight's team was working with a cool gas of sodium atoms and noticed clusters of atoms condensing out of the gas, rather like water droplets in a steamy room. Close inspection led to an unexpected discovery: rather than being made up of random numbers of atoms, the clusters mostly contained 8, 20, 40, 58 or 92 atoms. But why these numbers over others?

Atomic alter ego

Knight and his colleagues suspected it was down to the arrangement of electrons in the clusters. In a large lump of any metal, including sodium, some of each atom's electrons are free to move through the solid lattice. That's why metals conduct electricity. But Knight suspected that if these electrons are confined to a small number of atoms they might behave differently. To find out more, he borrowed a model used in nuclear physics and applied it to the cluster of atoms. Known as the "jellium" model, it treats the cluster of atoms as though they were a blob of jelly. Inside the blob, one electron from each sodium atom becomes free to roam through the blob.

According to Knight's calculations, the electrons in the blob arrange themselves in shells, just as the electrons of a single atom do, making the cluster behave as a giant atom. And when his team calculated the number of electrons that would make complete shells in a jellium cluster, the answer turned out to be 8, 20, 40 and so on. Since each sodium atom contributes one electron to the jelly, this explains why sodium clusters tended to be made of 8, 20 and 40 atoms. Clusters of this size can be thought of as the superatom counterparts of the noble gases, because their jellium electron shells are completely filled.

Knight's jellium model explains why stable clusters form. But could it explain why clusters of one element mimic another as Upton had found? Fast-forward to the mid-1990s, when Castleman was investigating what happens when oxygen reacts with aluminium cluster-ions - clusters that had been given an extra electron. Castleman saw the oxygen stripping away aluminium atoms from the clusters one at a time, steadily shrinking them down to nothing as the reaction progressed.

“ We can take one element and have it mimic several different elements in the periodic table ”

But when he did the experiment with clusters of various sizes, he noticed that the reaction would suddenly stop, leaving behind a depleted cluster. When he looked more closely, he found that the leftover clusters contained 13, 23 and 37 aluminium atoms. It seemed that there was something about these clusters that made them unwilling to react with oxygen.

To understand what that was, Castleman and his colleagues turned to the jellium model and used it to calculate the arrangement of electrons in the Al13, Al23 and Al37 clusters. They found something similar to what Knight had seen in sodium clusters. Aluminium cluster-ions made of 13, 23 and 37 atoms - plus an extra electron - have just the right number of electrons to form closed electron shells. In effect, aluminium cluster ions with this number of atoms behave more like a noble gas than aluminium, at least as far as the reaction with oxygen is concerned. The numbers are different from the numbers in Knight's clusters because aluminium atoms contribute more electrons to the jelly than sodium does.

Castleman then wondered what would happen if he removed the extra electron from the clusters. Elements with one electron fewer than the noble gases are the halogens - fluorine, chlorine, bromine and iodine - which are highly reactive. Sure enough, his team found that if they removed an electron, the neutral Al13 clusters underwent the same chemical reactions as the halogens. What's more, they found that Al13 cluster-ions, with their extra electron, behave much like the bromide ions that form when bromine atoms gain an electron. So it certainly looks as if aluminium, which is a typical metal, can be made to behave like a classic non-metal if it is in superatom form.

How far does the similarity go? To test the chemistry of the aluminium superatom, Castleman's team investigated how it reacts with a halogen molecule such as iodine. Bromide ions are known to stick to iodine gas molecules to create BrI2- ions. Similarly, iodine ions latch onto iodine molecules to form tri-iodide ions, I3-, and further iodine molecules can then be added to create I5- and I7-. Castleman thought that if Al13 cluster-ions really do mimic halide ions, then they should undergo the same reaction too. So his group tried it. Sure enough, they found that they could make Al13I2- and Al13I4-.

It certainly looked promising. "We then started to work with other aluminium clusters," says Castleman, and that's when they discovered that they could get aluminium to mimic another element too. In reactions with iodine gas, they found that a cluster of 14 aluminium atoms behaves like an alkaline earth metal, the family in the second column of the periodic table that includes calcium and magnesium.

Scouring for superatoms

These discoveries have prompted Castleman and his colleagues to scour the periodic table for more superatoms. So far, they have found hints that the chemical reactivity of clusters combining vanadium and oxygen atoms changes dramatically with the number of atoms in the cluster.

But curiosity aside, what's the point? What can be gained from making a compound with a superatom mimicking an element like bromine, rather than with bromine itself?

One answer is that superatoms could provide entirely new types of material, including "expanded" crystals. In a solid such as sodium chloride, the atoms are stacked together like oranges in a market display. In an expanded crystal, the atoms would be replaced by a stack of giant superatoms.

Expanded crystals could have useful properties. In the early 1990s, it was discovered that the superconducting properties of carbon-60 crystals doped with metal ions could be maintained at ever higher temperatures by squeezing larger and larger ions into the crystal lattice. Even so, the temperature at which the material ceased to act as a superconductor was still not very high - and was certainly a long way from the room-temperature superconductivity that researchers would love to achieve. Perhaps superatoms could hold the answer here and in related applications. Shiv Khanna, a physicist at Virginia Commonwealth University in Richmond who works with Castleman, hopes that replacing iodine in conducting polymers with aluminium superatoms could improve their conductivity.

Not all researchers share his optimism. "There is scepticism, mostly expressed by physicists and theorists, that a crystalline material composed of large aluminium clusters could ever be achieved," Whetten admits. "But my opinion is that one of these projects will eventually succeed." Castleman is confident that chemists' ingenuity will win through. "Physicists lack appreciation for the immense variety of chemical approaches to synthesising new materials," he says. He looks forward to being able to use clusters to build materials with tailor-made properties.

Another of the hopes for superatoms is that they could be used to disguise an element's normal chemistry. Aluminium could be a useful additive to solid fuels because it releases huge amounts of energy when it burns. But there is a problem: fine aluminium power is so reactive that the grains often oxidise before they even reach the ignition chamber, making them useless for boosting fuel.

Castleman thinks the solution might lie with noble-gas-like Al13 cluster-ions, which do not react with oxygen. His plan is to combine them with some kind of combustible organic molecule and mix the resulting compound with the fuel. "It would be totally stable," he says, "until a flame kicks out the extra electron." At that moment, the cluster's disguise would fall away, returning it to its reactive neutral form.

The idea "is just getting started", Castleman says, and he cautions that he doesn't know yet if it will work. But it is looking promising enough to have attracted the US air force, which is funding him to do further research.

Applications like these are not the main point, however, at least as far as chemists are concerned. For them, superatoms could provide a means to change something they had previously accepted as given: the chemical properties of the elements. Now they are on the verge of being able to control and alter the way the elements react. It is a kind of alchemy, but it has no need of magic. All you have to do is count the right number of atoms.

Size does matter

FOR nearly two centuries, researchers have known that when matter is divided into very small lumps it behaves in new and sometimes surprising ways. One of the most recent examples is seen in the change in the colour of light produced by some fluorescent materials if they are diced into nanoscale specks.

When the semiconductor cadmium selenide is illuminated with white light, it normally fluoresces in the infrared part of the spectrum. But prepare it in the form of grains just a few tens of nanometres wide, and the wavelength of the light it emits becomes shorter, putting it into the red or yellow part of the visible range.

The light is emitted when electrons in the semiconductor jump between quantised energy levels. Confining the electrons within nanoscale particles changes the energy levels, making the gap between them larger. As a result, the photons of fluorescent light have more energy, which in turn means that their wavelength is shorter. This effect allows the colour of the light emitted by the nanoparticles to be tuned simply by changing their size. The particles are already being used as glowing tags for labelling cells and could be turned into tiny light sources for optical communications.
Jan 10, 2007
Phys. Rev. Lett.

Platinum Nanoclusters Go Magnetic

Belle Dumé

Platinum atoms, which are not magnetic in the bulk, become magnetic when grouped together in small clusters, according to new experiments by physicists in Germany. The result, which confirms theoretical predictions, is not only of fundamental interest but could find applications in information storage and spintronics in the future.

13-atom Pt cluster

Clusters of atoms form a type of matter that is intermediate between single atoms and bulk matter. Metallic clusters are widely used as catalysts because they have a very high surface-to-volume ratio, which allows them to speed up chemical reactions. Researchers believe that magnetic clusters might also be used in information storage or in spintronic devices that exploit the spin of the electron as well as its charge.

Scientists had already predicted that nanosized samples of platinum are highly paramagnetic (i.e. they are attracted to a magnet). The new experimental results, from Emil Roduner of the University of Stuttgart and colleagues, confirm these predictions and could help us to understand better how magnetism can develop in a normally non-magnetic element.

Roduner and co-workers began their experiment by preparing the platinum clusters in the pores of a zeolite – a crystalline aluminosilicate that resembles a highly regular sponge with a network of pores measuring 1.3 nm in diameter. They then stabilized the clusters in these pores so that they did not grow any further. Next the team identified the clusters using a special X-ray technique called EXAFS, which is selective for specific elements (in this case platinum) and is particularly suitable for small species like these clusters. Finally the physicists measured the magnetization of the clusters as a function of temperature and magnetic field using an extremely sensitive magnetometer known as a superconducting quantum interference device.

The Germany team found that each cluster, which consists of 13 atoms, has a magnetic moment as high as 0.65 Bohr magnetons per atom. By comparison the figure for iron is 2.2 Bohr magnetons per atom.

"The main significance of our work at this point is a fundamental understanding of magnetism," Roduner told "However, since small, isolated, high magnetic moments - in this case corresponding to eight unpaired electrons per 13-atom cluster - are also of general interest to information storage or spintronics, these materials might be further developed to suit such advanced applications."

Another intriguing feature of the platinum clusters is that they are best described as "superatoms", said Roduner. This means that each 13-atom cluster has properties that are very similar to those of individual atoms. "It is fascinating to imagine that new periodic tables of superatoms might be drawn and that this may lead to a new chemistry of superatoms, offering a fantastic perspective for the future of young chemists," he added.

The researchers reported their work in Phys. Rev. Lett..

Proc Natl Acad Sci U S A. 2006 December 5; 103(49): 18405–18410.
doi: 10.1073/pnas.0608781103.

Multiple valence superatoms

J. U. Reveles, S. N. Khanna, P. J. Roach, and A. W. Castleman, Jr.

We recently demonstrated that, in gas phase clusters containing aluminum and iodine atoms, an Al13 cluster behaves like a halogen atom, whereas an Al14 cluster exhibits properties analogous to an alkaline earth atom. These observations, together with our findings that Al13? is inert like a rare gas atom, have reinforced the idea that chosen clusters can exhibit chemical behaviors reminiscent of atoms in the periodic table, offering the exciting prospect of a new dimension of the periodic table formed by cluster elements, called superatoms. As the behavior of clusters can be controlled by size and composition, the superatoms offer the potential to create unique compounds with tailored properties. In this article, we provide evidence of an additional class of superatoms, namely Al7?, that exhibit multiple valences, like some of the elements in the periodic table, and hence have the potential to form stable compounds when combined with other atoms. These findings support the contention that there should be no limitation in finding clusters, which mimic virtually all members of the periodic table.


Fig. 2.

Structure and energetics of aluminum compound clusters. (a) Al7C?-optimized geometry. (b) Energy gained by adding an Al atom to Aln-1C? species and HOMO–LUMO gap for the AlnC? clusters. (c) Electron charge density of the HOMO in Al7C? clusters. (d) Al7O?-optimized geometry. (e) Energy gained by adding an Al atom to Aln-1O? species and HOMO–LUMO gap for the AlnO? clusters. (f) Electron charge density of the HOMO in Al7O?.

Research Reveals Halogen Characteristics Of Cluster Of Metal Atoms

A stable cluster of aluminum atoms, Al13, acts as a single entity in chemical reactions, demonstrating properties similar to those of a halogen, reports a research team led by A Welford Castleman Jr., the Evan Pugh Professor of Chemistry and Physics and the Eberly Family Distinguished Chair in Science at Penn State, in a paper to be published in the 2 April 2004 issue of the journal Science. Experimental results and theoretical calculations indicate that the cluster chemically resembles a "superhalogen" atom, retaining its properties during the reaction and in reaction products. Other team members include Denis E. Bergeron of the Penn State departments of chemistry and physics and Shiv N. Khanna of the Virginia Commonwealth University department of Physics. One implication of the research is the possibility of using such clusters as building blocks in nanoscale fabrication. The project focused on experimental evidence of the existence of a very stable cluster anion, Al13I-, prepared by the gas-phase reaction of aluminum clusters with HI gas. Mass spectrometric analysis indicated that the reaction produced relatively few products, the most abundant corresponding to Al13I-. Energy calculations to determine the bonding mechanism between the aluminum cluster and the iodine atom indicate that the extra electron is localized on the Al13 cluster, meaning that the cluster maintains its integrity throughout the reaction. Because the cluster has a greater electron affinity in the compound, or attraction to the free electron, than does iodine, it can be considered a "superhalogen."

"One of the themes of our research is using the clusters as building blocks for new nanoscale materials," says Castleman. "In many cases, people have worked from the top down; that is, subdividing matter to get it smaller and smaller. We're trying to work with atoms and molecules and put them together--working our way from the bottom up. If we can retain the properties of aggregates, as we put them together, perhaps we will be able to construct new nanoscale materials." The key to using the aggregates as building blocks is that they retain their individual properties during the reaction and do not coalesce into a large aggregate.

One goal of the research is to test the Jellium model of stable clusters, which treats metal atoms in a small system as positive cores surrounded by the valence electrons. The model predicts certain closed-shell arrangements with high stability, called magic clusters. In the Jellium model, the cluster's atomic nuclei and inner electrons are seen as a single, spherical, positively charged core, surrounded by valence electrons in electronic shells similar to those of atoms. Essentially, the magic clusters can be viewed as superatoms, capable of forming compounds.

"When we started looking at reactions, Al13 turned out to be a very interesting species for several reasons, " says Castleman. "It behaves very much like a halogen, somewhere between iodine and bromine the way it wants to bind an electron. If we could put an iodine atom in contact with Al13, the Al13 has a little higher electron affinity than iodine, which could allow the Al13 to retain the electron, thereby bonding the Al13 and I together."

Experimental observations indicated that the stability of the Al13I- ion is comparable to that of BrI-, a well-known and very stable molecular halogen ion. The ability of a cluster of aluminum atoms to behave like a halogen opens up the prospect that Al13 and other magic clusters can retain their properties as a building block for assembling new materials.

"This superhalogen is not disrupted even in the presence of the very reactive iodine atom in close proximity, but still keeps its properties," says Castleman. "Now that we have shown that this is possible, we see potential ways to make other clusters, maybe involving other metals or alloys. It should be possible to construct something in the Jellium framework that would have the properties not only of a halogen, but of other types of atoms as well. For example, the Al13- ion itself resembles a rare gas atom because it is so unreactive. Ideally, we could have a whole series of clusters--a 'three dimensional' periodic table, not of elements but rather of clusters simulating the properties of the elements." The goal is to use these clusters as building blocks to tailor the design and formation of nanoscale materials with selected properties.

This research was supported by the U. S. Air Force Office of Scientific Research and the U. S. Department of Energy.

Charge density map of the highest occupied molecular orbital for the Al13I- cluster. Note the preservation of Al13I- icosahedral geometry, and the localized charge density on the
aluminum cluster moiety. Color code: blue=aluminum; red=iodine. (Image courtesy Penn State)
Clusters Of Aluminum Atoms Found To Have Properties Of Other Elements Reveal A New Form Of Chemistry
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