Transcript Prezentace aplikace PowerPoint

Carbon Fullerenes
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
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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
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Collision-free annealing of C60
– Stone-Wales transformation
Formation
•
Fullerene-like cage structures 2500<T<3500
– Extrapolation roughly agrees with experimental conditions
Formation
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C24 flat cluster, 0 s
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
Semispheroid, 50 ps
– Classical equations of motion for
both
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Folding of flat carbon clusters
– Unshared e– rearrange and
form symmetrical sphere layer
outside the fullerene
Fullerene, 150 ps
Formation
•
Another QM and MD simulation
– Density functional theory
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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
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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
•
Fullerenes are made wherever carbon condenses.
It just took us a little while to find out. Smalley
Laser vaporization of graphite
– laser-vaporization supersonic cluster
beam technique (Rice Univ., Texas)
– 1985: H. W. Kroto (Sussex Univ.,
Brighton) & R. E. Smalley (Rice)
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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
Synthesis
• Laser evaporation of doped carbon
Synthesis
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Resistive heating of graphite
– Carbon rod in 100 torr helium
– Kratschmer-Huffman 1990
– First macroscopic quantities of C60
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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
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Pyrolysis of hydrocarbons
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Benzene, acetylene, toluene
Polycyclic aromatic hydrocarbons PAH
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Mechanism
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Removal of hydrogen
Curling of joined rings
Optimum conditions
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Naphtalene
Very low pressure and high temperature
Pyrolysis apparatus
Examples
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Combustion of benzene
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Acetylene/oxygen/argon flame
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Adding Cl2 increases fullerene yield
Torch heating of naphtalene
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Premixed flame of benzene and oxygen with
argon
20 torr, C/O 0.995, 10% Ar, 1800 K
Heating torch
Pyrolysing torch: propane/oxygen 1000 ºC
Laser pyrolysis
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Photosensitizer SF6 + C2H4
CO2 laser 100÷180 W, 300 torr
Mechanism of formation of a partial C60
cage from naphthalene
Synthesis
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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
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Atmospheric pressure combustion
Syringe injector
Oxy-acetylene torch
Benzene, Dicyclopentadiene, Pyridine (C5H5N),
Thiophene (C4H4S)
(Ferrocene (C10H10Fe) – Fe@C60)
Stainless steel plate on water-cooled brass block (< 800 K)
Synthesis
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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
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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
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Face-centered cubic (fcc)
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The most densely packed structure
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Lattice constant a = 14.17 Ǻ
Weak Van der Waals bonds
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Molecules spin nearly freely around
centers
Simple cubic (sc)
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T<261 K
Fixed rotational axis
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4 C60 molecules arranged at vertices of
tetraeder, spinning around different but
fixed axis
Weak coulombic interaction
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Soft
Fixed orientation of molecules
T<90 K: molecules entirely frozen
Polymeric
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Covalent bonds
Photo-excitation, molecular collisions,
high-pressure/temperature, ionization
Insolvable in toluene
Purification
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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
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Separation by chromatograph
Derivatives
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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
7K
K3C60
19 K
Rb3C60
29 K
Cs3C60
30 K
Cs2RbC60
33 K
Polymerized Rb1C60
C60-Fullerene tetrakis(dimethylamino)ethylene - ferromagnet
Derivatives
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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
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Exohedral
– Covalent addition of atom or molecule
– Hydrogenation
• C60H18, C60H36
– Fluorination
• C60F36, C70F34, C60F60 (teflon balls)
– Oxidation
– Organic groups and complexes
(eta2-C70-Fullerene)-carbonyl-chlorobis(triphenylphosphine)-iridium
C60Cl6
Derivatives
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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
Synthesis of microcapsules for medical applications
• Alkali metals
• Lanthanide metals
• N, P (Group V)
N@C60
He@C60
Properties
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C60 electron affinity EA = 2.65 eV (Cl 3.62, )
– more electronegative than hydrocarbons
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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
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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
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Simulation of C60-C240 collision
Kinetic energy = 10 eV
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Kinetic energy = 100 eV
Simulation of C60 melting
David Tomanek
Theoretical Condensed
Matter Physics
Michigan State University
Kinetic energy = 300 eV
Potential applications
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Lubrication
– Molecular-sized ball bearing
• Not economical
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Superconductors
– Intercalation metal fullerides
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(Semi)Conductors
– Excellent conductors when
compressed
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Photoconductors
– add conducting properties to other
polymers as a function of light
intensity
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Optical Limiters
– C60 and C70 solutions absorb high
intensity light: protection for lightsensitive optical sensors
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Atom Encapsulation
– Radioactive waste encapsulation
• Ho@C82
Rh-C60 polymer with vacancies
Excess spin density
Dipole moment of magnitude 2.264 Debye
per C60 unit
Potential applications
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Diamond films
– Smoother than vaporizing graphite
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Novel polymers
Optoelectronic nanomaterials and
buliding blocks for nanotechnology
– Endohedral fullerenes
– Nanobots
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Medical applications
– Magnetic Resonance Imaging markers
• Metal organic complex (toxic Ga)
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contrast agents, tracers
anti-viral (even anticancer) agents
neuroprotective agents
fullerene-based liposome drug delivery
systems
– deployment of fullerene therapeutics to
targeting vehicles
MRI fullerene contrasting agent
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Water soluble tail (red & gray)
Encapsulates 2 gadolinium metal
atoms (purple) and 1 scandium
(green) attached to central
nitrogen atom
H2O molecules (red & yellow)
Potential applications
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Potential AIDS inhibitor
– HIV reproduces by growing
long protein chains
– Protein is cut in the active site
of enzyme HIV-protease
– Derivative of C60 has been
synthesized that is soluable in
water
Model of C60 docked in the binding site of HIV-1 protease