silicon fullerenes vijay kumar 1,2 and yoshiyuki kawazoe 1 1 institute for materials research tohoku...
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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
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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
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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.
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Bright colors fromHydrogen cappedSilicon particles
Belomoin et al.Appl. Phys. Lett.80, 841 (2002)
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Elemental silicon clustersClusters 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
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Clusters of Elemental Silicon
L. Mitas et al. PRL 84, 1479 (2000)
Si10 Si25Si20
A similar isomer for Si25
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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.
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Metal encapsulation - a novel approachA 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.
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Si12W
Hexagonal prism with W at the center Hiura et al. PRL 86, 1733 (2001)
Atomic radiusof W larger thanSi → open Structure
Magnetic momentof M completelyquenched
Similar behavior forCr and Mo
Interaction ofSH4 with M monomers anddimers ions
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S.M. Beck, J. Chem. Phys. 90, 6306 (1989)
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
About a decade ago experimentsby laser evaporation of Si andaddition of metal carbonates
Also by Bergeron & Castleman, Jr.
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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 ?
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Computational MethodAb initio plane wave ultrasoft pseudopotential
method Generalized gradient approximation for the
exchange-correlation energySpin-polarized calculationsOptimizations by conjugate gradient methodSuccessive cage shrinkage and atom(s) removal
methodDynamic stability of clusters is checked by
calculating frequencies using Gaussian program
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The Cage Shrinkage Approach for M@Sin
(M = Ti, Zr, Hf) – silicon fullerene
Kumar and Kawazoe, PRL 87, 045503 (2001).
M@Si16
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ISSPIC-11 Strasbourg Sept. 2002
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Silicon fullerene 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
Kumar, Majumder and Kawazoe, CPL 363, 319 (2002)
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Frank-Kasper Polyhedral structure M@Si16
Exceptionally large gap (~2.36 eV) in optical region
M = Ti & Hf~3e charge transfer from M to Si cage Large
PolarizabilityAbout 482 a.u.
Si-Si bonds2.45 – 2.66 ÅTetrahedralsymmetry
Normally in metal alloys
Different bonding from fullerene isomer
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Superatom behavior of clusters
Cluster IP (eV) EA (eV) Gap (eV)f-Zr@Si16 7.29 2.61 1.58FK-Ti@Si16 7.39 1.9 2.36 Expt. ~ 1.8eV (green) ↓ True gap ~ 3.2 (eV)
Large IPs and low EAs → Superatom
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M@Si15 and M@Si16
a) Si16M, M=Cr, Mo, and W. The f Cage shrinks
b) f-M@Si15
obtained from a)
c) Lowest energy isomerM@Si15, M =Cr, Mo, and W
d) Lowest Energy isomer of M@Si15, M = Ti, Zr, Hf, Ru, Os
Kumar and Kawazoe, Phys. Rev. B 65, 073404 (2002)Magnetic moment of M quenched
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Cubic and Fullerenelike M@Si14
Kumar and Kawazoe, PRL 87, 045503 (2001)
a) Shrinkage of f cage
c) Cubic for M = Fe, Ru, Os, Ni, Pd, Pt
b) Fullerene M = Ru, Os, Cr, Mo, W
d) Fullerene M = Os
All Si 3-foldcoordinated
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Charge density surfaces of M@Si16 and M@Si14
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Binding and Embedding EnergiesLarge 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.
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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 ======================================
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HOMO-LUMO gaps for pure and M doped Si and Ge clusters
Clusters with more than 2 eV GGA gap may exhibit visible luminescence
GGA
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Cluster-cluster interaction between M@Si16
Fullerene
Frank-Kasper
B.E. =1.345 eVGap = 0.673 eV
B.E. = 0.048 eVGap = 2.211 eV
Self-assembly of clusters, polymerized forms
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Stabilization of Si20
fullerene cage All structures dynamically stable.There are distortions, but it is least with Ba.Clathrate compoundsof Si with Ba and Nawith such cages
Q. Sun, Q. Wang, T.M. Briere, V. Kumar,Y. Kawazoe, and P. Jena,Phys. Rev. B65, 235417 (2002)
Low binding energies Importance ofd electrons
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Growth behaviorOf SinM clustersM = Cr, Mo, and W
N = 15 and 16 areMagic
Competing f and FKgrowths
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Metal encapsulated clusters of Ge with Large Gaps
16 15 15
14 Cubic 14 pentagons 14 another view 14 differentcapping
M = Ti, Hf, Zr, Cr, Mo, W, Fe, Ru, Os, Pb Kumar + Kawazoe, PRL88, 235504 (2002)HOMO-LUMO Gap 1-2 eV
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Interaction of hydrogen Si12M and Si18M2
M = Cr, Mo, W
Si18M2 a double prism
Binding energy per HAbout 2.4 eV, H2 maynot dissociateKumar and KawazoePRL (2002), in press
Hydrogenated fullerenes
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Hydrogenated silicon fullerenes
Empty center12 16 20
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Hydrogen abundance as a Function of temperature inSi14Hx
+ clusters
Peaking of the distributionat 1:1 at around 787 K
G.A. Rechtsteiner et al.J. Phys. Chem. B105, 4188(2001)
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Excitation energy (optical gap) for hydrogenated Si clustersOptical gap for the FK-Ti@Si16 around 3 eV from time dependent density functional theory
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Icosahedral clusters: Zn@Ge12 and Cd@Sn12
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)
Zn@Ge12
IP = 6.874 eVEA = 1.735 eVGap = 2.212 eVB3PW91 gap =2.97 eV
Metal like closepacking
Such an icosahedralcluster of Ge or Snfound for the first time
Superatom
Similar result for M = Be, Ca, Mg, Be
Mn doping 5 µB
Magnetic moment
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Si12Be
Chair type 3-fold planar
Icosahedron local minimum but not of lowest energy
Assembly
of N
anotu
bes
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Assembly of clusters
Nanowires
Nanotubes
Layers
Solids
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Nanowire of f-Si16Zr
Lattice constant = 14.85 Å Semiconducting gap ~0.53 eV Binding energy = 2.98 eV/cluster
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Finite nanotubes of elemental silicon
Distorted
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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, NanoLett. 2, 1243 (2002)
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Infinite metal encapsulated Si nanotubes
Symmetric, stable, and metallic, could act as nanowires, similar behavior for transition M atoms
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Metallic behavior of metal encapsulated Si nanotubes
Si24Be4
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Excess ofcharge
Depletionof charge
Infinite Si24Be4 nanotube
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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, …..