The Role of Superatoms - American Chemical Society

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Beyond the Periodic Table of Elements: Role of Superatoms Puru Jena J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/jz400156t • Publication Date (Web): 11 Apr 2013 Downloaded from http://pubs.acs.org on April 14, 2013

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The Journal of Physical Chemistry Letters

Beyond the Periodic Table of Elements: Role of Superatoms

Puru Jena Physics Department, Virginia Commonwealth University, Richmond, Virginia 23284

Abstract

Atomic clusters composed of homo or hetero-atomic species constitute an intermediate phase of matter where every atom counts and whose properties depend on their size, shape, composition, and charge. If specific clusters mimicking the chemistry of atoms can be produced, they can be thought of as man-made superatoms forming the building blocks of a new three dimensional periodic table. Novel materials with tailored properties can then be synthesized by assembling these superatoms. This invited perspective presents a brief summary of the pioneering works that led to this concept, and highlights the recent breakthroughs that hold promise for a new era in materials science.

Key words: Magic clusters, superatoms, cluster assembly, electron counting rules, superhalogens 1 ACS Paragon Plus Environment


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In a formal presentation to the Russian Chemical Society on March 6, 1869 Dmitri Ivanovich Mendeleev revealed the periodic table he had developed to illustrate periodic trends in the properties of the then known elements. It is not until the discovery of electron in 1897 and the subsequent development of quantum mechanics in the early 20th century that a fundamental understanding of the chemistry of these elements and their justification for the sites they occupy in the periodic table was achieved. While new short lived elements are being added to the periodic table, the numbers of naturally occurring stable elements from which materials are made remain at 90. Any new material that is being synthesized is done so by varying the composition of elements and synthetic techniques, hither to un-attempted, but atoms are generally the building blocks of matter. The realization that matter at nanometer length scale can behave very differently from their bulk and when self-assembled can lead to fundamentally new materials, has changed this paradigm. This possibility was first brought into focus by Richard Feynman1 in a famous speech to the American Physical Society in 1959, entitled “There is plenty of room at the bottom”. He argued that quantum confinement of electrons can enable nano particles to have uncommon properties and, thus, there is plenty of room at the bottom to synthesize novel materials. Around the same time work on clusters of atoms/molecules2 in the gas phase had shown that their size could be controlled one atom/molecule at a time. Development of new experimental techniques3 in the early 1980’s allowed researchers to further tune their composition with atomic precision. The discovery of C60 fullerene4 in the gas phase, its subsequent large scale synthesis in solutions5 and its assembly leading to the formation of fulleride crystal have confirmed that materials composed of clusters are very different from those composed of atoms, in this case diamond and graphite. If new stable clusters like C60 can be synthesized by following some specific design rules, it will have immense impact on materials science as clusters may be able to replace some 2 ACS Paragon Plus Environment

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of the precious elements in the periodic table. In 1992 Khanna and Jena6 proposed such a possibility. In this perspective I describe some of the pioneering works in cluster science that led to this proposition and discuss the recent breakthroughs that provide hope that novel materials with clusters as building blocks can indeed be realized. One of the first experiments was the seminal work of Knight and coworkers7 in 1984. The authors observed conspicuous peaks in the mass spectra of Na clusters containing 2, 8, 20, 40, --atoms and showed that their enhanced stability can be explained by electronic shell closure much as the stability of magic nuclei was explained earlier by nuclear shell closure. Assuming that the electrons in Na clusters are free as they are in the bulk, Knight et al7 proposed a simple jellium model for the clusters where the Na ions are replaced by a sphere of uniform distribution of positive charge density (Fig. 1). The electrons in Na clusters containing 2, 8, 20, 40, --- atoms are confined in quantized orbitals 1S2, 1S2 1P6, 1S2 1P6 1D10 2S2, 1S2 1P6 1D10 2S2 1F14 2P6, …respectively, and successive electronic shell closure accounts for the enhanced stability of the magic clusters. Recalling that the noble gas atoms due to their closed electronic shells are also chemically inert, one can imagine that the magic clusters would be less reactive than their neighbors. This was shown to be the case by Castleman and coworkers8 who observed that Al13was much less reactive towards O2 than their neighbors (Fig. 2). Note that Al being tri-valent, Al13- contains 40 valence electrons and hence satisfies the electron shell closure rule. Subsequent mass spectra of other simple homo as well as hetero-atomic metal clusters have confirmed the link between electronic shell closure and enhanced stability. In later experiments of the mass spectra of large Na clusters Martin and coworkers9 observed that clusters with atomic shell closure are also more stable than their neighbors.

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Fig. 1. Schematic diagram of an atom and atomic orbitals (left panel) where the positively charged nucleus is localized at a point and jellium model of a cluster where the positive charge is smeared over a sphere of finite radius with corresponding electronic orbitals (right panel)

Fig. 2. Series of mass spectra showing progression of the etching reaction of Al anions with oxygen (Ref. 8). 4 ACS Paragon Plus Environment

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That both atomic and electronic shell closure can enhance the stability of a cluster led Khanna and Jena6 to critically examine the Al13- cluster. Its structure is that of an icosahedron with one atom at the center and 12 atoms on the surface. Hence, Al13- as well as Si@Al12 are simultaneously atomic and electronic closed shell systems and gain enhanced stability. Furthermore, since only one electron is needed to close the electronic shells of Al13, one can expect neutral Al13 to mimic the chemistry of a halogen atom which too requires only one electron to close its (ns2 np5) electronic shells. In that case the electron affinity of Al13 should be close to that of a halogen atom and it can react with an alkali atom such as K to form a salt-like molecule, KAl13 just as Cl does with K, namely KCl. This prediction10 was confirmed experimentally by Li et al11 and Zheng et al12; electron affinity of Al13 is 3.62 eV and KAl13 is an ionic “molecule”. If a cluster with suitable size and composition could be synthesized such that it mimics the chemistry of an atom in the periodic table, such a cluster could be regarded as a manmade “element”. Khanna and Jena13 named such clusters as “superatoms” and suggested that they can form the building blocks of a new three dimensional periodic table just as atoms are the building blocks of Mendeleev’s two dimensional periodic table. In principle, there will be no limit to this third dimension as there are endless possibilities for varying the size and composition of clusters. Assuming that these superatoms remain intact when brought into the vicinity of each other, a new era in materials science could emerge where novel materials with tailored properties can be synthesized with clusters, instead of atoms, as building blocks. In the last decade considerable amount of work has been carried out to explore the fundamental science behind the “superatom” concept and its role in forming new materials14- 26. In addition to superatoms exhibiting the same chemistry as one of the atoms, the concept also suggests that the free electrons in the cluster occupy a new set of orbitals that are defined by the entire group of atoms behaving as a single unit rather than by each individual atom separately 5 ACS Paragon Plus Environment


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14,15, 18

. For example, it was shown14 that Al13I2- behaves chemically as a triiodide ion (Al13-.2I)

with Al13 mimicking a halogen. Similarly, Al14I3- can be viewed as Al142+.3I- with Al14 mimicking an alkaline-earth element. That ligands can be used to manipulate the total electron count of a metallic core has further enriched the field of superatoms opening new possibilities for their design and synthesis. In the interest of history it should be mentioned that Saito and Ohnishi27 had earlier described Na8 and Na19 clusters as “giant atoms”. This terminology was based on the analogy between atoms and clusters with equivalent properties. Using the jellium model they showed that Na8 cluster, due to its electronic closed shells, scarcely reacts while Na19 with electronic configuration of 1S2 1P6 1D10 2S1 behaves like an alkali atom. However, a later calculation28 that considered explicit atomic structure of Na8 showed that its structural stability depends on the surroundings. While Na8 is able to retain its structure up to 600K on an insulating NaCl (001) surface, it spontaneously collapses on the Na (110) surface, forming an epitaxial adlayer. In the vacuum, interaction with another Na8 cluster destroys its shell structure giving rise to a deformed Na16. Clusters mimicking the chemistry of halogens or alkali atoms have also been discussed in early 1980’s by Gutsev and Boldyrev29. This followed the experimental work of Bartlett and coworkers30 who showed that a Xe atom as well as an O2 molecule can be oxidized by PtF6. The authors estimated the electron affinity of PtF6 to be 6.8 eV, almost a factor of two larger than that of Cl, the element with the highest electron affinity in the periodic table. Gutsev and Boldyrev29 showed that any cluster with composition MXk+1 where M is a metal atom with maximal valence k, and X is a halogen atom can have electron affinities larger than that of the halogen and named such species as “superhalogens”. They also showed31 that clusters with ionization energies (IE) lower than that of alkali metal atoms can be designed to behave as “superalkalies”. The lowest experimentally measured ionization potential was found for Li3O32, namely, 3.54±0.30 eV and 6 ACS Paragon Plus Environment

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the highest electron affinity of 13.87 eV was predicted33 for H12F13. The low IE of superalkali species can be used in the synthesis of a new class of charge-transfer salts in which the corresponding anions are formed by species with low electron affinity, while superhalogens with very large electron affinities can be used to access the high oxidation states of metal atoms34 otherwise inaccessible in conventional chemistry. One can regard superalkalies and superhalogens as belonging to a special class of superatoms since for a cluster to be a superatom it is sufficient that it mimics the chemistry of an atom. In this context it should be mentioned that Al13 has often been referred to in the literature14-18 as a superhalogen. This is inconsistent with the superhalogen terminology discussed in the above since its electron affinity is not larger than that of Cl; a correct terminology for Al13 is that it is a superatom mimicking the chemistry of halogens. Why should one expect that properties of materials with clusters/superatoms as building blocks would be different from those where atoms are the building blocks, assuming, of course, that clusters/superatoms retain their structure and identity when assembled? The reasons are simple: a cluster assembled material has two length scales; the inter-cluster and intra-cluster distance while conventional crystals have only one length scale, namely the lattice constant. The energy bands in a conventional crystal is made up of overlap between atomic orbitals while in a cluster assembled crystal the energy bands form due to the overlap of molecular orbitals and these are very different (Fig. 3). Similarly, the phonon spectra are expected to be different due to the coupling between intra-cluster and inter-cluster vibrational modes. As mentioned before, a classic example that supports the above discussion is fulleride crystal made from C60 as the building block and graphite and diamond where carbon atoms are the building blocks.



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Fig. 3. Spin polarized electron orbitals of Al atom (left panel) and the Al13 cluster (right panel).

The strategy for identifying stable clusters as superatoms requires the synergy between theory and experiment. First, simple electron counting rules and first principles theory are used to tailor the size, composition, and charge state of a cluster that simultaneously renders it stability and allows it to mimic the chemistry of a specified atom. Second, the theoretical prediction is validated by synthesizing these clusters in the gas phase and analyzing their electronic properties experimentally. Third, clever techniques are devised to assemble such clusters into bulk form. I begin with the electron counting rules. As has already been discussed in the above, the electronic shell closure rule combined with the jellium model has been enormously successful in understanding the stability and chemical properties of clusters of simple metal atoms. For example, researchers have shown that Al13 has the same chemistry as that of a halogen atom. The question then is: Can Al13 be used in place of a Cl atom to synthesize a new material? Or, are there more robust electron counting rules that can be used to design very stable moieties? In the 8 ACS Paragon Plus Environment

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following I discuss some of the widely used electron counting rules and show how they can be used to design superatoms.

Fig. 4. Direct atomic imaging and dynamical fluctuations of the tetrahedral Au20 cluster soft landed on amorphous carbon substrate (Ref. 35)

Jellium Rule: The jellium rule is an electron shell closure rule that accounts for the stability of magic clusters based on the jellium model. Although this model correctly accounts for the stability of free-electron metal clusters in the gas phase, it is not immediately clear if these clusters can keep their structural identity when deposited on a non-interacting substrate. The experimental proof came recently from Palmer’s group35 showing that the Au20 cluster soft landed on an amorphous carbon surface (Fig. 4) retains its gas-phase pyramidal structure36. It is, however, unlikely that a crystal with Au20 as the building block can be synthesized as the intercluster interaction is likely to destroy its pyramidal structure. On the contrary, one may expect that KAl13 may serve as a building block of a new salt made of only metal atoms since it has been demonstrated to be an ionic molecule (K+Al13-), both theoretically10 and experimentally12. When assembled, the charge on the Al13- moiety is expected to keep them apart. This was addressed theoretically37 by examining the stability and electronic structure of KAl13 crystal. It 9 ACS Paragon Plus Environment


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was found to be metastable and metallic. In addition, the distance between Al atoms belonging to two different Al13 moieties was closer than the intra-cluster Al-Al distance. Thus, neither the electronic nor the structural integrity of Al13- is retained when it is assembled into a crystal lattice. One can also expect that Al13- once deposited on a substrate may lose its charge to the substrate and hence having a metal cluster that is stable as an ion may not bring much advantage in a self-assembly process. In the author’s view bare metal clusters designed by the jellium electronic shell closure rule, in general, may not be suitable candidates for cluster assembled materials. Atomic clusters can, however, be prevented from coalescing if they are protected by ligands. This field has attracted considerable interest in recent years when it was realized that the thermodynamic stability of ligated clusters can be controlled by manipulating both the number of ligands as well as the size of the metallic core. As a simple approximation, consider that all the valence electrons in the metallic core not satisfying the electronic shell closure rule can be transferred to suitable ligands or localize them into covalent bonds, thus restoring the magic number of the metallic core. Kiran et al38 demonstrated this possibility by examining the structure and stability of a large number of AlnHm clusters and showed that in the Al-rich phase both n and m can be varied so as to design stable clusters using the jellium rule. The predicted composition and structure of these clusters were verified by simultaneously carrying out photoelectron spectroscopy experiments38. Hakkinen and coworkers26,39 have effectively used the superatom concept to explain the stability of ligated Al and Au clusters. For example, Clayborne et al26 showed that the unusual stability of Al50Cp*12 cluster arises from the electron shell closure. Here one can regard Al50Cp*12 cluster to be composed of 38 Al-atom core surrounded with 12 AlCp* moiety with each Cp* taking away one electron from the metal atoms. This charge transfer allows the cluster to have 50x3-12 = 138 electrons, just enough to 10 ACS Paragon Plus Environment

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close the 1I26 superatom orbital. In assembling ligated clusters to form a bulk material one must also ensure that the protective ligand has a sterically complete shell compatible with a compact atomic shell structure of the metallic core. Achieving a compact geometry, electron shell closing in the metal core, and complete steric shielding simultaneously is a challenge. Recently Hakkinen and coworkers39 have shown that ligand protected gold clusters satisfying electron shell closure at 8, 34, and 58 can indeed be synthesized. Similarly, As7 and As11 species in combination with cryptated alkali atoms have been designed and bulk materials have been produced15,17 using condensed-phase chemical techniques. Experimental verfification of the superatom concept in ligated clusters has also come from a recent experiment by Tofanelli and Ackerson20. Using differential scanning calorimetry the authors showed that oxidizing the Au25(SR)18- superatom from the noble-gas-like 1S2 1P6 electron configuration to the open shell radical 1S2 1P5 and diradical 1S2 1P4 configuration results in decreased thermal stability of the compound. Similar conclusion has also been arrived by Zhu et al24 who observed reversible switching of magnetism in Au25(SR)18- superatom. In parallel investigations Duncan and coworkers40 have succeeded in capturing gas-phase magic vanadium oxide clusters in solution by ligand coating. Using acetonitrile and tetrahydrofuran as ligands, they have coated clusters containing 10-30 atoms of oxide core. Clusters coated by acetonitrile have the vanadium oxide stoichiometries exactly matching those found to be particularly stable in the gas phase. This is important since the prominent magic number stoichiometries found in the gas phase are quite different from the most common V2O5 bulk phase. This provides the possibility that an entirely new form of metal oxide materials can be produced with potential applications in catalysis, chemical sensors, fuel cells, and other magnetic materials. While metal clusters with suitable ligands have been used for synthesizing bulk materials, reader is to be cautioned that extending the superatom idea to cluster-ligand interaction is not 11 ACS Paragon Plus Environment


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immune from controversy. Much of it depends upon how strongly the ligands interact with the metal core and how much of the metal core geometry changes as a result. For example, Han and Jung18 have noted that “it remains unclear whether superatom chemistry really exists, particularly for metal cluster systems with a few electronegative elements attached, such as Al13I2- and Al14I3-. The authors further point out that “the geometrical symmetry of a cluster is a crucial factor in determining the electron counting picture for that cluster”. Similar conclusion has been reached by Zhang et al41 who showed that Al13 cannot be simply considered as a superatom when it interacts with sulfur for AlnS- (n=3-15) and AlnS2- (n=7-15). Reimers et al21 have also reported that superatom model for nanoparticle is inadequate for the prediction of the thermodynamic stability of gold nanoparticles. Thus, using ligands to manipulate the addition or removal of electrons from the metallic superatom orbitals and creating species electronically analogous to atomic orbitals depend crucially on the effect ligands have on the core metal cluster geometry. For example, Shafai et al23 have shown that the Au13 cluster when ligated by phosphines, has the icosahedric structure as its lowest energy structure while pure Au13 is planar. In addition, the jellium model has other limitations for describing the properties of clusters. First, it can only be used for simple metals such as Na, Mg, and Al. Second, it ignores the effect real atomic structure of a cluster can have on its electronic structure and energies. For example, silicon and germanium clusters do not adhere to the jellium model nor do they form fullerene cage structures as carbon does. However, doping of metal atoms can change their electronic structure and stability completely. This was demonstrated by Kumar and Kawazoe42,43 who showed through density functional theory based calculations that metal encapsulation allows Si and Ge clusters to form cage structures. In particular, they found that Ti@Si16 has the Frank-Kasper tetrahedral structure known to exist for metallic systems. In addition, this has an exceptionally large HUMO-LUMO gap of 2.36 eV and interacts weakly with itself, raising the 12 ACS Paragon Plus Environment

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possibility that it can serve as a building block of cluster assembled materials. Note that this cluster has 20 valence electrons assuming that each Si and Ti atom is regarded as contributing one and four electrons, respectively, to the valence pool. The chemical stability and the large HOMO-LUMO gap was later confirmed experimentally by Nakajima and coworkers44. The authors measured the HOMO-LUMO gap to be 1.9 eV and the cluster was found to be unreactive towards F2. They further showed that Ti@Si16 can be formed selectively by fine tuning the experimental conditions very similar to that done for C60. Mass spectra of Sc@Si16and V@Si16+ which are isoelectronic with Ti@Si16 also show magic behavior (Fig. 5). The authors pointed out that V@Si16 can be thought of as an alkali atom and its interaction with a halogen atom such as F can lead to an ionically bound superatom complex of (VSi16)+F-. That non-free electron clusters can sometimes adhere to the jellium shell closure rule is probably an exception, and not a rule.

Fig. 5. Mass spectra showing size-selective formation of (A) TiSi16 neutrals, (B) ScSi16 anions, and (c) VSi16 cations (reprinted from Ref. 44)



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Octet Rule: The octet rule45-47 is a simple electron counting rule that was developed in early 1900’s to show that atoms of low ( 1). The electron affinity of Au(BO2)n as a function of n plotted in Fig. 6 demonstrates this behavior. The discovery of these highly electronegative moieties termed “hyperhalogens” opens the door for the synthesis of a new class of salts which can best be described as “hypersalts”. To demonstrate how the “bottom up” approach can lead to the synthesis of new salts with super or hyperhalogens as building blocks we consider the following example involving BH4. The electron affinity of BH4 is 3.17 eV57. While it is not larger than that of Cl, it is substantially larger than the electron affinity of H, 15 ACS Paragon Plus Environment


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namely 0.7 eV. When counter balanced by a metal ion, BH4 can form a salt. And NaBH4, KBH4, Mg(BH4)2, and Al(BH4)3 are all known salts. In particular, Al(BH4)3 is a highly volatile pyrophoric liquid. Adding another BH4 moiety to Al(BH4)3 can transform Al(BH4)4 to a hyperhalogen. The calculated electron affinity of Al(BH4)4, namely, 5.56 eV confirms this. When counter balanced by a K cation, a “hypersalt” can be formed with the composition KAl(BH4)4. One can imagine that such a salt may be synthesized by mixing KBH4 and Al(BH4)3 under appropriate conditions. This has just been accomplished by Knight and Zidan of Savannah River National Laboratory (private communication). These two forms of borohydrides [Al(BH4)3 and KAl(BH4)4] are shown in Fig. 7. The crystal structure of KAl(BH4)4 confirms that the gas phase structure of BH4- remains intact in the bulk solid. It needs to be emphasized that KAl(BH4)4 which still contains a large amount of hydrogen is safer and more stable than Al(BH4)3. Similarly, Kasuya et al58 have demonstrated from their mass spectrometry experiments of CdSe that ultra-stable nanoclusters such as (CdSe)34 (Fig. 8) can be macroscopically produced in solution at “precisely specified numbers of constituent atoms with their stoichiometric composition identical to the bulk solids”.


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Fig. 7. Al(BH4)3 (left panel) and KAl(BH4)4 (right panel) (courtesy D. Knight and R. Zidan, private communication)

Fig. 8. Structure of (CdSe)34 (Ref. 58) An analogous electronic counting rule can be seen to play a role in the recent photoelectron imaging experiment of Peppernick et al59. The authors observed that photoelectron spectroscopy of negatively charged ions of group 10 elements such as Ni-, Pd-, and Pt- are very similar to their isoelectronic molecular counterparts, MX- (M=Ti, Zr, W; X=O, C). Taking Pd and ZrO as an example, we note that electronic configuration of Zr and Pd are, respectively, [Kr] 5s2 4d2 and [Kr] 4d10. Thus, Zr lacks six electrons to be isoelectronic with Pd. If O can be viewed as contributing these six electrons (electronic configuration of O is [He] 2s2 2p4), ZrO can be expected to mimic the chemistry of Pd. One may then expect that ZrO may be used in place of Pd as a catalyst. A recent study by Castleman and coworkers60 involving the interaction of Pd+ and ZrO+ with organic molecules provides some hope. However, the full potential of this discovery will require further studies. For example, does Pd atom react the same way as ZrO does with gases such as CO, O2, and H2? In addition, since catalysts contain more than one 17 ACS Paragon Plus Environment


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atom, one has to demonstrate that large clusters of Pd and ZrO show similar chemical behavior. Until further work is done, it is premature to assume that diatomic molecules, once assembled, can perform the same chemistry as the precious elements. 18 Electron Rule: This rule applies to stable metal complexes containing transition metal atoms which require 18 electrons to fill s2p6d10 orbitals61. When a metal complex has 18 valence electrons, it is said to have achieved the same closed electron shell configuration as noble gas atoms. Typical examples of transition metal complexes that obey the 18-electron rule are Cr(C6H6)2, Fe(C5H5)2, [Co(NH3)5Cl]2+, Mo(CO)6, and [Fe(CN)6]4-. Pyykko and Runenberg62 showed that an icosahedral WAu12 cluster can be stabilized by aurophilic attraction and the 18electron rule. Using density functional theory they showed that ag, t1u, and hg molecular orbitals are filled and the cluster has a large HOMO-LUMO gap of 3.0 eV at the B3LYP/LANL1DZ level of theory. Wang and coworkers63 verified this prediction through photoelectron spectroscopy experiments and showed that the preferred structure of WAu12 is indeed icosahedral, but the measured HOMO-LUMO gap is 1.68 eV. Although it is smaller than the predicted value, the gap is large enough to render the cluster stability. Note that the HOMOLUMO gap of C60 is 1.57 eV64. These authors further showed that MoAu12 is a stable cluster with icosahedric symmetry as Mo and W atoms are isoelectronic. Pyykko and Runenberg62 also showed that TaAu12- is a stable cluster as it too contains 18-valence electrons. Since the extra electron can delocalize over 12 Au atoms and Au has the largest electron affinity among all metal atoms in the periodic table, one can expect TaAu12 to be an all-metal superhalogen. This was confirmed experimentally65 and the measured electron affinity of TaAu12 is 3.76 eV. It remains to be seen if WAu12 can retain its structure and properties when assembled into a crystal and if TaAu12- can form the anion component of a salt.


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The 18-electron rule giving rise to stable clusters has also been confirmed for transition metal encapsulated Si clusters. Using an ion trap Hiura et al66 reported the formation of a series of hydrogenated Si clusters, MSinHx, interacting with a single metal atom, M (M=Hf, Ta, W, Re, Ir, etc.). For specific compositions the dehydrogenated clusters are so stable that, according to these authors, the clusters can be used as tunable building block for cluster-assembled materials. One of these clusters is WSi12 where W is encapsulated between two Si hexagons. This structure is reminiscent of Cr(C6H6)2 where Cr atom is encapsulated between two benzene molecules. Assuming that each Si atom contributes one electron to the valence pool, as was the case with Ti@Si16 discussed earlier, WSi12 comprises an 18-electron system. The electronic closed shell configuration of CrSi12 was later shown67 to completely quench the 6 µB magnetic moment of Cr, the highest in the 3d-transition metal series. Thus, metal doping of Si clusters can not only change the HOMO-LUMO gap but also the magnetic character of the complex, thus raising the possibility that metal encapsulated Si and Ge clusters can lead to new cluster assembled materials for electronic applications. Polyhedral skeletal electron pair theory (PSEPT) rule: This theory originally formulated by Wade68,69 and further developed by Mingos70,71 provides electron counting rules useful for predicting the structures of electron deficient clusters such as boranes and carboranes. Consider a borane cluster BnHn where Bn, with n number of atoms, forms a polyhedron with n vertices. B and H together contribute four valence electrons of which two are involved in the covalent bond between the B and its radially bonded H atom. The other two electrons are contributed towards cage bonding. Wade-Mingos rule68-71 requires that for cage bonding (2n+1) pairs of electrons are needed. Thus BnHn2-, referred to as closo-borane, is a stable cluster. While numerous examples of borane based clusters are known, similar clusters involving Al instead of B were not known. This prompted Bowen and coworkers72 to carry out an experiment where Al clusters produced in a 19 ACS Paragon Plus Environment


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pulsed arc cluster ion source were reacted with hydrogen. They discovered literally hundreds of AlnHm clusters previously unknown. Among these, a particularly noteworthy cluster is Al4H6.

Fig. 9. PES spectrum of Al4H6 anion (left panel) and mass spectra of Al4Hm-. The insert shows the geometry of Al4H6- (Ref. 72) The structure of this cluster turned out to be a tetrahedron with four Al atoms occupying the vertex sites (Fig. 9). Four of the six hydrogen atoms are radially bonded while the other two form bridge-bonded configuration on opposite side of the tetrahedron. The total number of electrons contributed to this bonding is 10 of which eight come from the four radially bonded AlH pairs and the two bridge-bonded H atoms contribute the remaining two electrons. Thus, Al4H6 can be viewed as Al4H42- which following the Wade-Mingos rule, is a stable cluster. Both mass ion and photoelectron spectroscopy studies proved this to be case. Large peak in the mass ion intensity distribution (Fig. 9) combined with a large HOMO-LUMO gap of 1.9 eV confirms Al4H6 to be a stable cluster which can serve as the building block of new materials. This was later accomplished by Henke et al73 who were able to synthesize Al4R6 cluster compound where H atom was replaced by a ligand, R, namely Br dimethylphosphine. If Al4H6 can be similarly synthesized, the energy cost to remove two of the bridge-bonded hydrogen atoms is expected to


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be smaller than those that are radially bonded. Unlike alanates (eg. NaAlH4) from which it is difficult to desorb hydrogen, Al4H6 may prove to be a better material for hydrogen storage. Other candidates for cluster assembled materials include metal-encapsulated icosahedral superatoms of Zn@Ge12 and Cd@Sn12. Using density functional theory Kumar and Kawazoe74 had shown that these clusters possess perfect icosahedral symmetry and large HOMO-LUMO gaps. This prediction was experimentally confirmed by Cui et al75. During a systematic study to understand the semiconductor-to-metal transition in Sn clusters, these authors75 found the photoelectron spectra of Sn12- to be remarkably simple and totally different from the corresponding spectra of its isoelectronic cousin Ge12-. Structural optimization led to a slightly distorted icosahedral cage with C5v symmetry. However, Sn122- synthesized in the form of KSn12(K+Sn122-) was shown to be a closed shell icosahedral cluster with Ih symmetry. Its PES spectrum was found to be similar to that of Sn12-. The authors found the electronic structure of this unusually stable stannasperene, Sn122- to be characterized by “four delocalized radial π bonds and nine delocalized on-sphere tangential σ bonds from the 5p orbitals of the Sn atoms, whereas the 5s2 electrons remain largely localized and non-bonding”. The bonding pattern in Sn122- is similar to that of B12H122- discussed in the above. However, unlike B12H122-, the diameter of the Sn122cage is 6.1 Å which is only slightly smaller than that of C60 (7.1 Å) and can host a transition metal atom. Later studies by these authors76 showed that M@Sn12 icosahedral cages (M= Ti, V, Cr, Fe, Co, Ni, Cu, Y, Nb, Gd, Hf, Ta, Pt, Au) can be synthesized where the metal atoms occupy endohedral position. These endohedral stannasperenes containing 3d-transition metal atoms are magnetic with high spins and can form the building blocks of cage clusters with tunable magnetic and optical properties. This possibility was examined by Kandalam et al77 who studied the interaction of two Mn@Sn12 clusters. Note that Mn is predominantly divalent, although it can exhibit oxidation states ranging from 0 to +7. Thus, Mn@Sn12 can be viewed as Mn2+@Sn122-. 21 ACS Paragon Plus Environment


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Since Mn2+ carries a magnetic moment of 5 µB, study of the interaction of the dimer can reveal not only the stability of the cage but also the coupling between the magnetic moments. The authors77 found that the cage structure is maintained, although each icosahedron undergoes a small distortion resulting in a dimer with C2h symmetry. The ground state of the dimer is antiferromagnetic, but a high spin state with 10 µB lies only 0.11 eV above the ground state. Wang and coworkers78 also discovered another hollow cage cluster called plumbaspherene Pb122- with a diameter of 6.29 Å. The cage is large enough to trap an atom inside to form endohedral plumbaspeherene. This is consistent with the gas phase synthesis79 of AlPb12+ which can be viewed as Al3+Pb122-. In addition, a series of endohedral cage compounds, M@Pb122- (M=Ni, Pd, Pt) have been synthesized in solution and crystalline form with K+ as counter ions. X-ray diffraction and NMR experiments by Eichorn and coworkers80 have confirmed that the clusters possess icosahedral symmetry. Inspired by the Eichorn compounds, Wang and coworkers81 were able to synthesize a new Pd2@Sn184- cluster (Fig. 10) which was crystallized as a [(2,2,2-crypt)K]4(Pd2@Sn18).3ED salt and characterized by X-ray diffraction.

Fig. 10. Comparison of the structural evolution from Pd@Sn12- to Pd2@Sn184- to that from C60 to C70 (reprinted from Ref. 81)


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Future Challenges: From the discussion above, it is clear that present theoretical methods are capable of predicting the size and composition of clusters that mimic the chemistry of atoms. Many of these predictions have been verified by experiments in the gas phase, suggesting that the level of theory is adequate in guiding focused discovery of clusters as superatoms. A few cluster assembled materials with superatoms as the building blocks have even been synthesized. However, the above studies have raised more questions and challenges than they have solved. These range from both fundamental to applied in nature. For example, how does one arrange the superatoms in the third dimension of the periodic table? Note that in the 2D periodic table atoms are arranged in terms of their atomic number. Since dozens of clusters are now known that mimic the chemistry of halogens, how their sites in the three dimensional periodic table can be identified? Second, where does C60 belong in the hierarchy of superatoms? Clearly, fullerenes are very stable and form a solid, but which atom’s chemistry they emulate? We note that in solid C60 the fullerenes are weakly bonded as would be the case with solids made from noble gas elements. But unlike noble gas atoms, C60 clusters are reactive. Third, to synthesize cluster assembled materials, superatoms need to be produced in large quantities and once synthesized, they must not coalesce. This is not often the case with neutral clusters and they need to be coated with suitable ligands. As mentioned in the above, this also poses problems as ligands can alter the structure and property of the bare metal clusters. When ligated clusters are assembled to form bulk materials, they eventually have to be removed to expose metal sites for performing catalytic action. This in turn will have adverse effect on the stability of the material. In spite of these challenges, recent advances in experimental techniques provide hope. Some of these coated clusters do retain their gas phase properties and can even be collected in a “bottle” while others have been prepared in solution. Methods of depositing these clusters on non-interacting substrates may also pave the way for synthesizing cluster assembled materials and substantial 23 ACS Paragon Plus Environment


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progress has already been made in this direction. One has to keep in mind though that metal clusters ions (stabilized because of their charge) may lose their charge to the substrate. What is really needed for a paradigm shift in synthesizing cluster assembled materials is finding clever techniques where “bottom up” design meets with “top down” synthesis. This is what led to the synthesis of fulleride when macroscopic quantities of C60 were produced in arc vaporization of graphite and isolated in solution.

Fig. 11. Three dimensional periodic table with super and hyperhalogens occupyimg the third dimension above the halogen atoms. Also given in the figure are examples of clusters as superatoms designed using different electron counting rules; Al13- and Al20 (jellium rule), MnCl3 and PtF6 (octet rule), WSi12 (18 electron rule), and Al4H6 (Wade-Mingos rule).

Summary: In this perspective I hope to have provided a balanced view of how advancements in our understanding of the size, shape, and composition specific properties of atomic clusters have led to the discovery of clusters that mimic the properties of atoms and how these superatoms can 24 ACS Paragon Plus Environment

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be thought of as man-made “elements” forming the building blocks of a three dimensional periodic table (Fig. 11). Some of the superatoms discussed in this perspective are also summarized in Fig. 11. They are: Al13- and Au20 that obey the jellium shell closure rule, MnCl3 and PtF6 superhalogens that obey the octet electron rule, WSi12 that obeys the 18-electron rule, and Al4H6 that obeys the Wade-Mingos rule. The center of the figure features C60 even though it does not fit the superatom definition strictly, but it embodies the spirit of the superatom concept as building blocks of novel cluster assembled materials. The goal of current research is to use the synergy between theory and experiment for focused discovery of stable clusters like C60 that are suitable for forming cluster assembled materials. Some examples are provided where such a scheme works. One hopes that the strategy of using theory to predict stable clusters with tailored properties, validating theoretical prediction by experiments in the gas phase, and then finding ways to synthesize these clusters into bulk materials can usher a new era in materials science. Acknowledgement: I am thankful to Professors Qiang Sun and Qian Wang for a critical reading of the manuscript. Mr. L. Zhou and Prof. Sun also helped in preparing the original figures. This perspective is the result of long standing collaboration with many colleagues, postdoctoral fellows and students over the past 25 years and I am grateful to them for their collaboration and friendship. Support of the Department of Energy during these years is gratefully acknowledged. Biographies Puru Jena is Distinguished Professor of Physics at Virginia Commonwealth University and Jefferson Science Fellow alumnus at the US Department of State. He is a Fellow of the American Physical Society and recipient of Virginia Commonwealth University’s Presidential Medallion, Award of Excellence, and Distinguished Scholar Award and Outstanding Faculty Award of the Commonwealth of Virginia. His research interests include clusters, cluster 25 ACS Paragon Plus Environment


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assembled materials, surfaces, interfaces, and hydrogen storage. http://www.has.vcu.edu/phy/jenagroup/puru.htm

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Quotes The discovery of C60 fullerene in the gas phase, its subsequent large scale synthesis in solutions and its assembly leading to the formation of fulleride crystal have confirmed that materials composed of clusters are very different from those composed of atoms, in this case diamond and graphite.

If a cluster with suitable size and composition could be synthesized such that it mimics the chemistry of an atom in the periodic table, such a cluster could be regarded as a man-made “element”.

What is really needed for a paradigm shift in synthesizing cluster assembled materials is finding clever techniques where “bottom up” design meets with “top down” synthesis.



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One hopes that the strategy of using theory to predict stable clusters with tailored properties, validating theoretical prediction by experiments in the gas phase, and then finding ways to synthesize these clusters into bulk materials can usher a new era in materials science.


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The Role of Superatoms - American Chemical Society

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