carbon fullerenes. formation basic model –clustering chains, rings, tangled poly-cyclic structures...
TRANSCRIPT
Formation
• Basic model– Clustering
• Chains, rings, tangled poly-cyclic structures or graphite sheets
– Annealing (no collisions)• Random cage, open cage, closed
cage structures– Elimination of dangling bonds
• Fullerenes– Stone-Wales transformation
» Migration of pentagons» Rearrangement to lower
energy
• Critical parameters– Annealing time– Annealing temperature
• 10-1 ms; 1000-1500 K for the laser method
• 100 s; 1000 K for the arc discharge method
Formation• Picture models
– Pentagon road (1)• Addition of dimers and trimers
leaving pentagons as a deffect• Reduction of dangling bonds,
adjacent pentagons too much stress
– Ring pentagon road (2)• Stacking of proper size of C rings• Pentagon annealing
– Fullerene road (3)• Linear chains up to C10
• Rings C10 to C20, fullerene from C30,• Addition of C2 at two neighboring p-s
– Ring annealing (4)• Big rings, bi/tri-cyclic structures
(C60+) anneal under high T conditions
– Chain annealing (5)• Long chain with spiral structure
– Graphite road (6)• C10 clusters, graphite sheet, curling
– Nanotube road (7)• Chips of carbon nanotubes
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Formation
• Molecular dynamics (MD) simulations– Many-body potential function– Kinetic energy of clusters
• Classical mechanics
• translation, vibration and rotation
– Clustering• Collisions of atoms or clusters: grow and fragmentation of cluster
• Cooling: collisions with buffer gas and radiation
• Annealing between collisions
T = 3000 K
Formation
• Temperature dependence of cluster structures
• Collision-free annealing of C60
– Stone-Wales transformation
Formation
• Fullerene-like cage structures 2500<T<3500– Extrapolation roughly agrees with experimental conditions
Formation• Model of charges at bonds
– Molecules: classical dynamics– Electrons: quantum mechanics
• Ground and excited states
– Interaction potentials• Covalent bonds, rotation,
torsional vibration• Interaction between atoms and
electrons– bonding electron pairs at the
centers of the covalent bonds– unshared electrons at
approximately the same distance from the carbon atoms
– Classical equations of motion for both
• Folding of flat carbon clusters– Unshared e– rearrange and form
symmetrical sphere layer outside the fullerene
C24 flat cluster, 0 s
Semispheroid, 50 ps
Fullerene, 150 ps
Formation
• Another QM and MD simulation– Density functional theory
• Ring fusion spiral zipper mechanism– C atoms combine to C2 and C3
– n<10: linear chain Cn
• sp hybrid prefer linear geometry
– 10<n<30: ring• Energy gain in killing dangling bonds
overcompensates for strain energy caused by folding
– n>30: ring structure can grow in fullerene
Synthesis
• Graphite vaporization or ablation– Laser
– Resistive heating
– AC or DC arc
• Pyrolysis of hydrocarbons– Flame combustion
– Laser
– Torch or tube furnace
• Ion implantation
• Temperature of condensation and annealing– 1000÷1500 K
• C60 $30/gram
The first published mass spectrum of carbon clusters in a supersonic beam produced by laser vaporization of a carbon target in a pulsed supersonic nozzle operating with a helium carrier gas.
Synthesis• Laser vaporization of graphite
– laser-vaporization supersonic cluster beam technique (Rice Univ., Texas)
– 1985: H. W. Kroto (Sussex Univ., Brighton) & R. E. Smalley (Rice)
• Experiment– Nd:YAG
• 300 mJ, 535 nm, 5ns
– Rotating graphite disk– Plasma of vaporized carbon atoms
• 10 000 K
– High-density helium pulse• Condensation and transport
– “Integration cup”• Adjusts the time of clustering
– Supersonic expansion• Frizzing out the reactions
– Ionization by excimer laser– Mass spectrometer
Fullerenes are made wherever carbon condenses.It just took us a little while to find out. Smalley
Synthesis
• Resistive heating of graphite– Carbon rod in 100 torr helium– Kratschmer-Huffman 1990
– First macroscopic quantities of C60
• Carbon arc– AC or DC arc in 100 torr helium– 60 Hz, 100÷200 A, 10÷20 V rms– Continuous graphite rod feedeing
The generator design based on the Kratschmer-Huffman apparatus.
Synthesis• Pyrolysis of hydrocarbons
– Benzene, acetylene, toluene
– Polycyclic aromatic hydrocarbons PAH• Naphtalene
– Mechanism• Removal of hydrogen• Curling of joined rings
– Optimum conditions• Very low pressure and high temperature
• Examples– Combustion of benzene
• Premixed flame of benzene and oxygen with argon
• 20 torr, C/O 0.995, 10% Ar, 1800 K
– Acetylene/oxygen/argon flame• Adding Cl2 increases fullerene yield
– Torch heating of naphtalene• Heating torch• Pyrolysing torch: propane/oxygen 1000 ºC
– Laser pyrolysis• Photosensitizer SF6 + C2H4
• CO2 laser 100÷180 W, 300 torr
Mechanism of formation of a partial C60
cage from naphthalene
Pyrolysis apparatus
Synthesis
• Low-pressure benzene/oxygen diffusion flame
– p = 12 ÷ 40 torr, Tmax = 1500 ÷1700 K
– Precursor PAH• Elimination of CO from oxidized PAH
thought to be a source of C pentagons
– Highest yield of fullerenes• High soot formation
• High dilution with argon
Synthesis
• Atmospheric pressure combustion
Oxy-acetylene torch(Ferrocene (C10H10Fe) – Fe@C60)
Syringe injectorBenzene, Dicyclopentadiene, Pyridine (C5H5N), Thiophene (C4H4S)
Stainless steel plate on water-cooled brass block (< 800 K)
Synthesis
• DC arc torch dissociation of C2Cl4 (tetrachlorethylene)
Operating conditions:Torch power: 56 kW
He flow rate: 225 slm
C2Cl4 feed rate: 0.29 mol/min
Synthesis
• Ion implantation– Carbon ions 120 keV– Copper substrates 700÷1000 ºC– Thin film (diamond, fullerenes,
onions)– Endohedral fullerenes
• Evaporation of fullerene (C60) onto a substrate
• Ions of dopant
N@C60
Solid State C60 - Fullerite
• Face-centered cubic (fcc)– The most densely packed structure
– Lattice constant a = 14.17 Ǻ– Weak Van der Waals bonds
• Soft
– Molecules spin nearly freely around centers
• Simple cubic (sc)– T<261 K
– Fixed rotational axis• 4 C60 molecules arranged at vertices of
tetraeder, spinning around different but fixed axis
– Weak coulombic interaction• Fixed orientation of molecules
– T<90 K: molecules entirely frozen
• Polymeric– Covalent bonds
– Photo-excitation, molecular collisions, high-pressure/temperature, ionization
– Insolvable in toluene
Purification
• Extraction from carbon soot– Cn<100 solvable in aromatic solvents
• Toluene, benzene, hexane, chloroform
– C60 magenta
– C70 dark red
– Cn>100 high boiling-solvents
• trichlorbenzene
• Separation by chromatograph
Derivatives
• Intercalation (fullerides)– Octahedral or tetrahedral inter. sites– Alkali or alkaline-earth metal atoms
• Na, K, Rb, Cs, Ca, Sr and Ba)
– Charge transfer to the cage– Superconductors– Polymers
Ba6C60 7 K
K3C60 19 K
Rb3C60 29 K
Cs3C60 30 K
Cs2RbC60 33 K
Polymerized Rb1C60
C60-Fullerene tetrakis(dimethylamino)ethylene - ferromagnet
Derivatives
• Heterofullerenes– Substitution of an impurity
atom with a different valence for C on the cage
• B, N, BN Nb
• C59X (X=B,N): nonlinear optical properties
– Deformation of the electronic structure, strong enhancement of chemical activity
– Radicals which can be stabilized by dimerization Azafullerenes: (a) C59N, (b) C59HN, and (c) (C59N)2
C48N12
Derivatives
• Exohedral– Covalent addition of atom or molecule– Hydrogenation
• C60H18, C60H36
– Fluorination• C60F36, C70F34, C60F60 (teflon balls)
– Oxidation– Organic groups and complexes
C60Cl6
(eta2-C70-Fullerene)-carbonyl-chloro-bis(triphenylphosphine)-iridium
Derivatives
• Endohedral– Synthesis
• Evaporation of doped carbon– Arc, laser
• Ion implantation
– M@C60• Noble gases
– without overlap of Van der Waals radii
• Metallofullerenes– B, Al, Ga, Y, In, La– Stabilize cages not fulfilling
isolated pentagon’s rule (n<60)– With permanent dipole moment
form di/trimers and large aggregates on metal surfaces and C60 films
• Alkali metals
• Lanthanide metals
• N, P (Group V)
Synthesis of microcapsules for medical applications
N@C60 He@C60
Properties
• C60 electron affinity EA = 2.65 eV (Cl 3.62, )
– more electronegative than hydrocarbons
• Dissolves in common solvents like benzene, toluene, hexane
• Readily sublimes in vacuum around 400°C
• Low thermal conductivity
• Pure C60 is an electrical insulator
• C60 doped with alkali metals shows a range of electrical conductivity:
– Insulator (K6 C60) to superconductor (K3 C60) < 30 K
• Interesting magnetic and optical properties– Ferromagnetism
• At high pressure C60 transfoms to diamond
• C60 soft and compressible brown/black odorless powder/solid
• Flexible chemical reactivity
breathing vibrational mode
Pentagonal pinch mode
Properties
• Simulation of C60-C240 collision
• Simulation of C60 melting
Kinetic energy = 10 eV Kinetic energy = 100 eV Kinetic energy = 300 eV
David TomanekTheoretical CondensedMatter PhysicsMichigan State University
Potential applications• Lubrication
– Molecular-sized ball bearing• Not economical
• Superconductors– Intercalation metal fullerides
• (Semi)Conductors– Excellent conductors when
compressed
• Photoconductors– add conducting properties to other
polymers as a function of light intensity
• Optical Limiters– C60 and C70 solutions absorb high
intensity light: protection for light-sensitive optical sensors
• Atom Encapsulation– Radioactive waste encapsulation
• Ho@C82
Rh-C60 polymer with vacanciesExcess spin densityDipole moment of magnitude 2.264 Debye per C60 unit
Potential applications
• Diamond films– Smoother than vaporizing graphite
• Novel polymers
• Optoelectronic nanomaterials and buliding blocks for nanotechnology
– Endohedral fullerenes– Nanobots
• Medical applications– Magnetic Resonance Imaging markers
• Metal organic complex (toxic Ga)
– contrast agents, tracers– anti-viral (even anticancer) agents– neuroprotective agents– fullerene-based liposome drug delivery
systems– deployment of fullerene therapeutics to
targeting vehicles
• Water soluble tail (red & gray)• Encapsulates 2 gadolinium
metal atoms (purple) and 1 scandium (green) attached to central nitrogen atom
• H2O molecules (red & yellow)
MRI fullerene contrasting agent