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Superatoms : A M acroatomic Perspective

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  1. Superatoms: A Macroatomic Perspective Cory Camasta

  2. A little bit of history… • Walter Knight and his team discovered “magic clusters” of sodium in 1984 by vaporizing sodium and passing it through a supersonic hose. • The mass spectra of the sodium clusters showed distinct peaks corresponding to clusters of 2, 8, 18, 20, 34, 40, 58, and 92 atoms. • Since sodium has only one valence electron, these magic numbers also corresponded to the superatomic clusters’ closed-shell valence electron counts. • Subsequent experiments showed high ionization potentials, low electron affinities, and high stabilities of the magic species; as well as a strong tendency to form magic clusters over non-magic clusters. • The team found that they could explain the electronic properties of the clusters with the Jellium Model, in which the entire cluster is treated as a single, uniform point charge.

  3. What is a Superatom? • The true superatom is a subset in the class of compounds known as “atomic clusters.” • Clusters are particularly stable congregations of one or more elements whose properties are better characterized when treated as a single entity. • In a superatom, the valence electrons orbit the molecule as they would an atom, though the supershells are filled slightly differently. • These supershells almost always correlate with a “magic number” of valence electrons when closed. • Magic numbers: 2, 8, 18, 20, 34, 40, 58… etc. (corresponding to closed-shell electron configurations in the supershell filling order).

  4. Why should you care? • Superatoms usher in a new frontier of nanotechnology • They are large enough to manipulate individually, small enough to act on the atomic level, and stable enough to hold their form, thus their properties can be fine-tuned to almost any application • They can be made magnetic when they contain a transition metal, allowing for applications in the realm of spintronics – a subset in the electronics industry allowing for massive increases in data storage and efficient information processing (read: quantum computers) • They can also be made to mimic properties of elemental atoms without actually incorporating any of the atom(s) that they are designed to act like (read: cheaper catalysts made from mimic clusters of platinum, etc.) • Superatoms have shown much promise in the pharmaceutical industry as biologically inert drug-delivery apparatuses

  5. How can I make one? • It’s actually relatively easy – superatoms tend to form spontaneously in the right chemical environment. • For example, in the synthesis of Gold nanoparticles (superatoms with the inclusion of stabilizing/protecting membranes), all you need to do is reduce HAuCl4 in aqueous medium (traditionally with citrate as a reducing/stablizing agent) with added heat and stirring. The gold will begin to cluster immediately, and continue in the days following. • For more specialized applications or geometries, the synthesis would be a bit more difficult, requiring very high-precision equipment, but the clusters still form spontaneously when they can.

  6. Supershells • The valent molecular orbits of superatoms strongly resemble atomic orbitals • They tend to fill up in the order: 1S2, 1P6, 1D10, 2S2, 1F14, 2P6, 1G18, 2D10, 3S2, 1H22, etc. (on a side-note, this also may lead to a better way of accommodating orbits of higher angular momentum since the stabilities of single atoms that can accommodate them drops off dramatically in the actinide series). • The filling pattern is not definite, and is subject to macroatomicJahn-Teller distortion, just as with the filling of atomic d orbitals. • A supershell is more likely to be stable in an open-shell electron configuration if the cluster contains a transition metal that provides the angular momentum necessary to “breed” the shapes of higher orbits. Superatomsakin to this do not necessarily need a magic number of valent electrons to exist as stable clusters (although many of them are still more stable with closed-shell “magic” electron counts).

  7. Ab initio studies • Ab initio is Latin for “from the beginning” • Density Functional Theory (DFT) uses various different types of electron density/interactionalgorithms in combination with one or more basis sets in order to fit the many-electron wavefunction,Ψ(r), to its density functional, n(r), with constraints defined by the basis. • This is based mostly on the idea outlined by Hohenberg-KohnTheory, stating that the wavefunction approaches its corresponding density functional when energy is minimized (i.e. in the ground state). • Once the wavefunction is generated, any desired observable can be interpolated from it. • The above explaination is a bit over-simplified, but one must start somewhere…

  8. Results Due to constraints in time and computing power, I was forced to use methods that provide values that are “good enough” for a relative analysis, but not exceedingly accurate. I also tried to get some pictures of gold and silver clusters, but alas, as of now (more than two weeks since I submitted the jobs), the optimizations still have not finished. Bearing these things in mind, here is what I came up with…

  9. Miniature Superatom: BLi5 (D4h) 1S *Optimized @ MP2(full)/cc-pVDZ *Rendered with isovalue = 0.03

  10. Miniature Superatom: BLi5 (D4h) 1P *Optimized @ MP2(full)/cc-pVDZ *Optimized @ MP2(full)/cc-pVDZ *Rendered with isovalue = 0.03 HOMO

  11. Miniature Superatom: BLi5 (D4h) 2S *Optimized @ MP2(full)/cc-pVDZ *Optimized @ MP2(full)/cc-pVDZ *Rendered with isovalue = 0.03 HOMO-LUMO gap energy = ̴4.35 eV LUMO

  12. BLi5 Natural Vibrational Mode *Optimized @ MP2(full)/cc-pVDZ

  13. Superatomic Cluster: Na8 (C2v) 1S *Optimized @ B3LYP/6-311G* *Rendered with isovalue = 0.03

  14. Superatomic Cluster: Na8 (C2v) 1P *Optimized @ B3LYP/6-311G* *Rendered with isovalue = 0.03 HOMO

  15. Superatomic Cluster: Na8 (C2v) 1D *Optimized @ B3LYP/6-311G* *Rendered with isovalue = 0.03 HOMO-LUMO gap energy = ̴1.63 eV LUMO

  16. Na8 Natural Vibrational Mode *Optimized @ B3LYP/6-311G*

  17. Magnetic Superatom: VNa6- (C5v) 1S *Optimized @ B3LYP/pc-2 as a septet *Rendered with isovalue = 0.02 *pc-2 is a triple zeta basis set that includes diffuse and polarization functions – obtained from the EMSL Basis Set Exchange, uploaded by Frank Jensen (et. al?)

  18. Magnetic Superatom: VNa6- (C5v) 1D *Optimized @ B3LYP/pc-2 as a septet *Rendered with isovalue = 0.02 SOMO SOMO SOMO

  19. Magnetic Superatom: VNa6- (C5v) 1P *Optimized @ B3LYP/pc-2 as a septet *Rendered with isovalue = 0.02 SOMO SOMO (S)HOMO

  20. Magnetic Superatom: VNa6- (C5v) 2S *Optimized @ B3LYP/pc-2 as a septet *Rendered with isovalue = 0.02 HOMO-LUMO gap energy = ̴1.09 eV LUMO

  21. VNa6- Natural Vibrational Mode *Optimized @ B3LYP/pc-2 as a septet

  22. Noble Superatom: Al13- (Ih) 1S *Optimized @ MP2(full)/aug-cc-pVDZ *Rendered with isovalue = 0.03

  23. Noble Superatom: Al13- (Ih) 1P *Optimized @ MP2(full)/aug-cc-pVDZ *Rendered with isovalue = 0.03

  24. Noble Superatom: Al13- (Ih) 1D *Optimized @ MP2(full)/aug-cc-pVDZ *Rendered with isovalue = 0.03

  25. Noble Superatom: Al13- (Ih) 2S *Optimized @ MP2(full)/aug-cc-pVDZ *Rendered with isovalue = 0.03

  26. Noble Superatom: Al13- (Ih) 1F *Optimized @ MP2(full)/aug-cc-pVDZ *Rendered with isovalue = 0.03

  27. Noble Superatom: Al13- (Ih) 2P *Optimized @ MP2(full)/aug-cc-pVDZ *Rendered with isovalue = 0.03

  28. Noble Superatom: Al13- (Ih) 1F *Optimized @ MP2(full)/aug-cc-pVDZ *Rendered with isovalue = 0.03 HOMO

  29. Noble Superatom: Al13- (Ih) 1G *Optimized @ MP2(full)/aug-cc-pVDZ *Rendered with isovalue = 0.03 HOMO-LUMO gap energy = ̴2.45 eV LUMO

  30. Al13- Natural Vibrational Mode *Optimized @ MP2(full)/aug-cc-pVDZ

  31. References Castleman, A et al. “ ’Superatoms’ Mimic Elements: Research Gives New Perspective on Periodic Table”. Penn State Science. December 2009. <http://science.psu.edu/news-and-events/2009-news/Castleman12- 2009.htm>. Medel, V et al. "Hund's Rule in Superatoms With Transition Metal Impurities". PNAS 108: 10062-10066. 2011. Purusottam, Jena and Castleman, Albert Jr. (editors). “Nanoclusters: A Bridge Across Disciplines”. Print. 2010. Accessed electronically via: <http://books.google.com/books?id=aK-c04g70tQC>. Varnes, R and Strange, P. “Super-atom Properties of 13 Atom Clusters of Group 13 Elements”. Phys. Status Solidi B 249: 2179-2189. 2012. Zeiger, B. “Superatoms”. Literature Seminar. October 2008. <http://www.chemistry.illinois.edu/research/inorganic/seminar_abstracts/200 8-2009/Brad_Zeiger_Lit_Seminar_Abstract.pdf>.