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

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

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.

Bright colors fromHydrogen cappedSilicon particles

Belomoin et al.Appl. Phys. Lett.80, 841 (2002)

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

Clusters of Elemental Silicon

L. Mitas et al. PRL 84, 1479 (2000)

Si10 Si25Si20

A similar isomer for Si25

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.

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.

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

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.

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 ?

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

The Cage Shrinkage Approach for M@Sin

(M = Ti, Zr, Hf) – silicon fullerene

Kumar and Kawazoe, PRL 87, 045503 (2001).

M@Si16

ISSPIC-11 Strasbourg Sept. 2002

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)

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

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

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

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

Charge density surfaces of M@Si16 and M@Si14

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.

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 ======================================

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

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

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

Growth behaviorOf SinM clustersM = Cr, Mo, and W

N = 15 and 16 areMagic

Competing f and FKgrowths

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

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

Hydrogenated silicon fullerenes

Empty center12 16 20

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)

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

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

Si12Be

Chair type 3-fold planar

Icosahedron local minimum but not of lowest energy

Assembly

of N

anotu

bes

Assembly of clusters

Nanowires

Nanotubes

Layers

Solids

Nanowire of f-Si16Zr

Lattice constant = 14.85 Å Semiconducting gap ~0.53 eV Binding energy = 2.98 eV/cluster

Finite nanotubes of elemental silicon

Distorted

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)

Infinite metal encapsulated Si nanotubes

Symmetric, stable, and metallic, could act as nanowires, similar behavior for transition M atoms

Metallic behavior of metal encapsulated Si nanotubes

Si24Be4

Excess ofcharge

Depletionof charge

Infinite Si24Be4 nanotube

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, …..