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Silicon fullerenes

Silicon fullerenes . Vijay Kumar 1,2 and Yoshiyuki Kawazoe 1 1 Institute for Materials Research Tohoku University, Sendai & 2 VKF, Chennai In Collaboration with C. Majumder, T. M. Briere, A. K. Singh, Q. Sun, Q. Wang, and P. Jena, M.W. Radny, and H. Kawamura. Plan

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Silicon fullerenes

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  1. Silicon fullerenes Vijay Kumar1,2 and Yoshiyuki Kawazoe1 1Institute for Materials Research Tohoku University, Sendai & 2VKF, Chennai In Collaboration with C. Majumder, T. M. Briere, A. K. Singh, Q. Sun, Q. Wang, and P. Jena, M.W. Radny, and H. Kawamura

  2. Plan • Introduction • Novel structures of silicon with metal encapsulation • Silicon Fullerenes and other forms • Metal encapsulated clusters of Germanium • Hydrogenated silicon fullerenes • Metal encapsulated nanotubes of silicon

  3. Introduction • Nanoforms of silicon for atomic-scale engineering - miniature devices • Bright luminescence from nanoparticles of Si. Porous Si, hydrogenated Si clusters • Bulk Si poor emitter of light. • Si laser, integration of photonics and electronics leading to microphotonics integrated circuits.

  4. Bright colors from Hydrogen capped Silicon particles Belomoin et al. Appl. Phys. Lett. 80, 841 (2002)

  5. Elemental silicon clusters • Clusters with N ~ 15-25 atoms prolate, N > 25 → 3D fullerene-like, experiments on H or O passivated nanoparticles or embedded in a matrix, quantum confinement → PL • No strong magic behavior except for Si10. • Often in experiments a distribution of • different sizes

  6. Clusters of Elemental Silicon Si20 Si10 Si25 A similar isomer for Si25 L. Mitas et al. PRL 84, 1479 (2000)

  7. Materials with clusters as superatoms • Clusters with their unique properties can be assembled to develop novel materials with desired properties • Large abundance, stability and size selection important.

  8. Metal encapsulation - a novel approach • A new cluster of silicon: Si12W, hexagonal prism open structure with W at the center. Stability: 18 valence electron rule? • Large abundances of Si15M and Si16M (M = Cr, Mo, and W) reported more than a decade ago. Nucleation conditions play an important role.

  9. Si12W Atomic radius of W larger than Si → open Structure Magnetic moment of M completely quenched Similar behavior for Cr and Mo Interaction of SH4 with M monomers and dimers ions Hexagonal prism with W at the center Hiura et al. PRL 86, 1733 (2001)

  10. About a decade ago experiments by laser evaporation of Si and addition of metal carbonates • Large abundance of Si15M and Si16M (M = Cr, Mo, and W) and little intensity for other M doped clusters, in particular Si12M • Possibilities of size selection • like for C60 S.M. Beck, J. Chem. Phys. 90, 6306 (1989) Also by Bergeron & Castleman, Jr.

  11. We find from computer experiments • High symmetry M encapsulated caged fullerene-like, Frank-Kasper polyhedral and cubic Si clusters M@Sin (n=14-16) • Exceptionally large gap of up to 2.36 eV. • Hydrogenated silicon fullerenes with ~2.8 eV gap, photoluminescence ?

  12. Computational Method • Ab initio plane wave ultrasoft pseudopotential method • Generalized gradient approximation for the exchange-correlation energy • Spin-polarized calculations • Optimizations by conjugate gradient method • Successive cage shrinkage and atom(s) removal method • Dynamic stability of clusters is checked by calculating frequencies using Gaussian program

  13. The Cage Shrinkage Approach for M@Sin (M = Ti, Zr, Hf) – silicon fullerene M@Si16 Kumar and Kawazoe, PRL 87, 045503 (2001) .

  14. ISSPIC-11 Strasbourg Sept. 2002

  15. 8 pentagons and 2 squares, each Si tri-coordinated like in C60 Short bonds 2.25 (double), 2.28 (single), and 2.34 Å (single) sp2-sp3 bonding (double bonds in Si) Small charge transfer from M to Si cage, covalent p-d bonding Possibilities of producing such clusters uniquely in large abundance Silicon fullerene Kumar, Majumder and Kawazoe, CPL 363, 319 (2002)

  16. Frank-Kasper Polyhedral structureM@Si16 M = Ti & Hf ~3e charge transfer from M to Si cage Large Polarizability About 482 a.u. Si-Si bonds 2.45 – 2.66 Å Tetrahedral symmetry Different bonding from fullerene isomer Normally in metal alloys Exceptionally large gap (~2.36 eV) in optical region

  17. Superatom behavior of clusters Cluster IP (eV) EA (eV) Gap (eV) f-Zr@Si16 7.29 2.61 1.58 FK-Ti@Si16 7.39 1.9 2.36 Expt. ~ 1.8eV (green) ↓ True gap ~ 3.2 (eV) Large IPs and low EAs → Superatom

  18. M@Si15 and M@Si16 • Si16M, M= • Cr, Mo, and • W. The f Cage • shrinks b) f-M@Si15 obtained from a) d) Lowest Energy isomer of M@Si15, M = Ti, Zr, Hf, Ru, Os c) Lowest energy isomer M@Si15, M = Cr, Mo, and W Magnetic moment of M quenched Kumar and Kawazoe, Phys. Rev. B 65, 073404 (2002)

  19. Cubic and Fullerenelike M@Si14 • Shrinkage • of f cage b) Fullerene M = Ru, Os, Cr, Mo, W c) Cubic for M = Fe, Ru, Os, Ni, Pd, Pt d) Fullerene M = Os All Si 3-fold coordinated Kumar and Kawazoe, PRL 87, 045503 (2001)

  20. Charge density surfaces of M@Si16 and M@Si14

  21. Binding and Embedding Energies • Large binding energy of M encapsulated Si clusters ~ 4 eV/atom as compared to about 3.5 eV/atom for elemental Si clusters • High embedding energy (EE) (~ 12 – 14 eV) of M atom in the cage. For Fe and Cr, it is significantly lower due to quenching of moments • EE significantly low for M = Pd and Pt presumably due to filled d shell.

  22. Table 1. Binding energy (BE) in eV/atom, embedding energy (EE) in eV and HOMO-LUMO gap (eV) of metal encapsulated silicon clusters. ===================================== Cluster BE EE Gap ===================================== FK-Ti@Si16 4.135 11.269 2.358 f-Ti@Si16 4.089 12.733 1.495 f-Zr@Si16 4.162 13.965 1.580 f-Hf@Si16 4.175 14.176 1.576 FK-Hf@Si16 4.171 12.399 2.352 f-Si16Cr 3.934 8.817 1.244 f-Si16Mo 4.131 12.091 1.195 f-Si16W 4.246 14.053 1.208 f-Si16Fe 4.010 9.426 1.294 f-Si16Ru 4.188 12.445 1.230 f-Si16Os 4.252 13.551 1.246 ======================================

  23. HOMO-LUMO gaps for pure and M doped Si and Ge clusters GGA Clusters with more than 2 eV GGA gap may exhibit visible luminescence

  24. Cluster-cluster interaction between M@Si16 B.E. =1.345 eV Gap = 0.673 eV Fullerene Frank-Kasper B.E. = 0.048 eV Gap = 2.211 eV Self-assembly of clusters, polymerized forms

  25. Stabilization of Si20 fullerene cage All structures dynamically stable. There are distortions, but it is least with Ba. Clathrate compounds of Si with Ba and Na with such cages Low binding energies Importance of d electrons Q. Sun, Q. Wang, T.M. Briere, V. Kumar, Y. Kawazoe, and P. Jena, Phys. Rev. B65, 235417 (2002)

  26. Growth behavior Of SinM clusters M = Cr, Mo, and W N = 15 and 16 are Magic Competing f and FK growths

  27. Metal encapsulated clusters of Ge with Large Gaps 16 15 15 14 Cubic 14 pentagons 14 another view 14 different capping Kumar + Kawazoe, PRL 88, 235504 (2002) M = Ti, Hf, Zr, Cr, Mo, W, Fe, Ru, Os, Pb HOMO-LUMO Gap 1-2 eV

  28. Hydrogenated fullerenes Interaction of hydrogen Si12M and Si18M2 M = Cr, Mo, W Si18M2 a double prism Binding energy per H About 2.4 eV, H2 may not dissociate Kumar and Kawazoe PRL (2002), in press

  29. Hydrogenated silicon fullerenes 12 16 20 Empty center

  30. Hydrogen abundance as a Function of temperature in Si14Hx+ clusters Peaking of the distribution at 1:1 at around 787 K G.A. Rechtsteiner et al. J. Phys. Chem. B105, 4188 (2001)

  31. Excitation energy (optical gap) for hydrogenated Si clusters Optical gap for the FK-Ti@Si16 around 3 eV from time dependent density functional theory

  32. Icosahedral clusters: Zn@Ge12 and Cd@Sn12 Superatom Metal like close packing Such an icosahedral cluster of Ge or Sn found for the first time Zn@Ge12 IP = 6.874 eV EA = 1.735 eV Gap = 2.212 eV B3PW91 gap = 2.97 eV Mn doping 5 µB Magnetic moment Similar result for M = Be, Ca, Mg, Be Perfect icosahedral symmetry and large HOMO-LUMO gaps of about 2.2 eV in the green - blue range V. Kumar and Y. Kawazoe, Appl. Phys. Lett. 80, 859 (2002)

  33. Si12Be Assembly of Nanotubes Chair type 3-fold planar Icosahedron local minimum but not of lowest energy

  34. Assembly of clusters Nanowires Nanotubes Layers Solids

  35. Nanowire of f-Si16Zr Lattice constant = 14.85 Å Semiconducting gap ~0.53 eV Binding energy = 2.98 eV/cluster

  36. Finite nanotubes of elemental silicon Distorted

  37. Assembly of metal encapsulated Si clusters to form nanotubes: Be • Carbon nanotubes or silicon? • Elemental Si tubes distorted. • Metal encapsulation stabilizes • nanotubes to quite symmetric forms Singh, Kumar, Briere, and Kawazoe, Nano Lett. 2, 1243 (2002)

  38. Infinite metal encapsulated Si nanotubes Symmetric, stable, and metallic, could act as nanowires, similar behavior for transition M atoms

  39. Metallic behavior of metal encapsulated Si nanotubes Si24Be4

  40. Infinite Si24Be4 nanotube Excess of charge Depletion of charge

  41. Conclusions • Novel forms of Si with M encapsulation: fullerenelike, cubic and Frank-Kasper, high stability. One metal atom changes the structure and properties drastically. • Strong bonding of M atom leads to compact cages. The dynamic stability of structures has been studied • Size and gap depends upon the M atom. Largest gap of ~ 2.35 eV -> PL. Similar for Ge • Highest symmetry icosahedral clusters of M@Ge12 and M@Sn12 with ~2 eV gap. Mn@Ge12 with 5 µB moment. • Magic behavior of M@Si15(M = Cr, Mo, and W) agrees with experiments • Hydrogenated silicon fullerenes with empty centers • Assemblies: predicted Nanotubes and nanowires, …..

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