FROM FULLERENES TO
GRAPHENE PASSING THROUGH
CARBON NANOTUBES:
SYNTHESIS, PROPERTIES AND
APPLICATIONS OF quasi-NEW
ALLOTROPES OF CARBON.
Simone Musso
sp2 and sp3 hybridizations
Carbon, having an electron configuration 1s2 2s2 2p2, forms a great variety of
crystalline and disordered structures because it can exist in three different
hybridizations, sp3, sp2, sp1.
sp2
sp3
Planar geometry
Tetrahedral geometry
Allotropic forms of C
3.4 Å
Planar, sp2
hybridization
Graphite
Cubic, sp3
hybridization
Diamond
Planar geometry
Tetrahedral geometry
Allotropic forms of C
3.4 Å
Planar, sp2
hybridization
Graphite
Cubic, sp3
hybridization
Diamond
Nanotube
Fullerene
Physical and chemical properties are
a direct consequence of the carbon-carbon
bonds and lattice configuration
Nanotube/Fullerene sp2 + sp3 character
Allotropic forms of C
3.4 Å
Planar, sp2
hybridization
Graphene
Graphite
Cubic, sp3
hybridization
Diamond
Fullerene
Carbon Nanotube
Nanotube/Fullerene sp2 + sp3 character
Allotropic forms of C
3.4 Å
Planar, sp2
hybridization
Graphene
Graphite
Cubic, sp3
hybridization
Diamond
Fullerene
Carbon Nanotube
Nanotube/Fullerene sp2 + sp3 character
Discovery of fullerene or buckyball
H. W. Kroto, J. R. Heath, S. C. O’Brien, R. F. Curl and R. E. Smalley, Nature 318 (1985) 162–63.
Discovery of fullerene
Before the C60 structure was considered beyond doubt, a sufficient
amount of material had to be prepared for detailed spectroscopic
analysis, but a lack of funds stopped the research6..
Then, in 1990 Kratschmer and Huffman developed an arc discharge
technique:
DC Arc-discharge
ddp ≈ 20 V
I = 100-200 A
T° → 4000-6000°C
He or Ar (600mbar)
W. Krätschmer, L. D. Lamb, K. Fostiropoulos and D. R.
Huffman, Nature 347 (1990) 354–58.
Characterization of fullerene
XRD
MS
FT-IR
W. Krätschmer, L. D. Lamb, K. Fostiropoulos and D. R.
Huffman, Nature 347 (1990) 354–58.
UV-Vis
Fullerenes
8.34 Å ; 7.66 Å
C240
C540
C960
C70 (rugby-ball
shaped) fullerenes can be separated
Different
by means of chromatographic
column.
d= 7.1 Å
C60 (bucky ball)
Fullerene properties
CHEMICAL REACTIVITY
Fullerenes are non-toxic and are soluble in several organic solvents.
Fullerenes are quite stable. However the spherical curvature produces
angle strain that allows fullerenes to undergo characteristic reactions of
addition to double bonds (hybridization turns from sp2 to sp3).
C60 in organic solvents exhibits 5 stages of reversible oxidation/reduction,
hence fullerene can work either as electrophiles or nucleophiles.
C60F18
Image created by Chris Ewels, www.ewels.info
Fullerene properties
DERIVATIVES: EXOHEDRAL, ENDOHEDRAL and ON-SITE DOPING
Fullerenes, packed in fcc structure, can be intercalated with alkali and
alkaline-earth metal atoms which provide electrons to the conduction
band (from semi-conducting to metallic behavior). In 1991 potassiumdoped fullerene (K3C60) revealed superconducting behavior at 18K.
L. Forrò and L. Mihaly, Rep. Prog. Phys. 64 (2001) 649-699.
Fullerene properties
DERIVATIVES: EXOHEDRAL, ENDOHEDRAL and ON-SITE DOPING
In the endohedral doping a foreign atom is inserted in the inner cavity
(M @ C60). This doping can be performed either by ion implantation or
CNTs !!!
coevaporation of C and metal in arc discharge system.
The on-site doping is achieved by replacing one carbon atom with a
nitrogen atom ((C59N)2 azafullerene).
Image created by Chris Ewels, www.ewels.info
L. Forrò and L. Mihaly, Rep. Prog. Phys. 64 (2001) 649-699.
Fullerenes applications
Hydrogen or oxygen storage: hydrogenation of fullerene produces
hydrides. The reaction is reversible and can be catalyzed with metals (low
pressure).
Catalyst: fullerene promotes the conversion of methane into higher
hydrocarbons and inhibits coking reactions.
Sensor: fullerene based capacitors can be used to detect ppm of H2S
in N2, ppm of water in isopropanol.
Diamond precursor: fullerene can be transformed to diamond at high
pressure (RT) or can be used as a diamond nucleation center during CVD.
Alloy strengthening/hardening
conductivity of Cu alloys.
(Ti),
improvement
of
electrical
Biomedical field: inhibition of human HIV replication and HIV-1
protease. Biological antioxidant (radical sponge).
Allotropic forms of C
3.4 Å
Planar, sp2
hybridization
Graphite
Cubic, sp3
hybridization
Diamond
Fullerene
Carbon Nanotube
Nanotube/Fullerene sp2 + sp3 character
Discovery of CNTs
Arc-discharge: from endohedral fullerene to CNTs
Many scientific papers start citing ‘‘the discovery of
carbon nanotubes by Iijima in 1991. . .’’
S. Iijima, Nature 354, (1991) 56
Discovery of CNTs
First TEM evidences of nanonano-sized carbon tubes
L.V. Radushkevich, V.M. Lukyanovich, Zurn. Fisic. Chim. 26, (1952) 88
Discovery of CNTs
What are CNTs?
Rolled graphene sheet
Diameter: 0,7 - 100 nm
Length: from few tens of nm up to several mm
Even if the curvature causes a higher strain energy, a defect free CNT
has an overall lower energy state than graphite because dangling
bonds are removed
removed.. This explains high thermal stability and chemical
inertness..
inertness
CNTs classification
SWCNTs classification
A CNT is characterized by its Chiral Vector (C
(Ch)= n â1 + m â2
and by the Chrial Angle (θ
(θ) with respect to the zig zag axis
Zig-zag
n − m ≠ 3 × integer
Semi-conductor behavior
depending on the diameter.
Chiral
Armchair
n − m = 3 × integer Metallic
conductor behavior
SWCNTs classification
Armchair (5,5)
θ = 30°°
Zig Zag (9,0)
θ = 0°°
Chiral (10,5)
0°° θ < 30°
Characterization
HR-TEM
SEM
TGA
Air 20°C/min
1.4
Weight [%]
80
1.2
1.0
60
0.8
0.6
40
0.4
20
0.2
Weight derivative [%/°C]
1.6
100
0.0
0
-0.2
200
400
600
800
Temperature [°C]
Raman spectroscopy
XRD
1800
1591 cm-1
G peak
1600
(100)
(002)
Intensity (arb.unit)
Intensity (a.u.)
1400
1200
cm-1
1344
D peak
1000
800
600
400
RBM
(*)
(*)
(004)
200
10
0
500
100 0
1500
-1
R am an shift (cm )
2000
20
30
40
50
60
2θ (°)
70
80
90
100
Properties of defect free CNTs
Young’s modulus
(GPa)
Tensile Strength
(GPa)
Density
(g/cm3)
MWCNT
SWCNT
SWCNT bundle
Graphite (in-plane)
1200
∼150
2.6
1054
75
1.3
563
∼150
1.3
350
2.5
2.2-2.6
Steel
208
0.4
7.8
Thermal conductivity
(W/mK) RT
Electrical conductivity
(A/cm2)
MWCNT
∼3000
≤109
SWCNT
∼6000
Depends on the chirality
SWCNT bundle
∼3000
≤109
Graphite (in-plane)
∼1700
≤109
Diamond
2000 - 2500
Copper
401
106
Functionalization
Chemical functionalization can be used to tune CNTs properties.
Composite – improving compatibility matrix/filler
Biomedical – improving biocompatibility, drug delivery, diagnosis6
Chemical (KMnO4, HNO3-H2SO4) or plasma oxidation change surface
properties while producing lattice defects (removable by annealing).
Van der Waals interactions with porphyrin or pyrene derivatives are less
effective.
CNT properties/applications
CNT synthesis
High temperature methods
Arc Discharge
Laser ablation
These techniques generate small amount of high quality CNTs by sublimation of
graphite in presence of catalyst particles.
Gas phase (vapor phase) methods
Chemical Vapor Deposition (CVD)
and plasma enhanced-CVD (PECVD)
During CVD a conventional heat source is used to form CNTs by thermal cracking of
hydrocarbons. A plasma source is used in PE-CVD to create a glow discharge which
provokes a low temperature precursor dissociation
Nanoparticles of Co, Ni or Fe are necessary to catalyze the growth
CNT synthesis
CVD
Gas or low bp liquids can be
used. The CVD technique is
simple, low-cost, easily scalable
for commercial production and
allows to produce large amounts
of CNTs with high purity.
PECVD
Plasma discharge sources:
- direct current (DC),
- alternating current (AC),
V= -600V
T° → 400-800°C
Ar or N2 (0.1-30 Torr)
- radio frequency (RF),
- hot-filament aided with DC,
- microwaves
We do like gas phase techniques
CVD allows to produce cm long CNTs. With direct spinning is also possible the
synthesis of very long CNT ropes in situ.
Ya-Li Li et al., Science 304 (2004) 276
S. Musso et al. Carbon 45 (2007) 1133-1136
CNT growth mechanism
Two theories for the carbon-metal interaction:
Dissolution and saturation of carbon atoms in metal nanoparticles and
precipitation of carbon.
The catalyst provokes dehydroaromatization (DHA) of cyclic molecules of
hydrocarbon.
Tip growth mechanism is due to a
weak adhesion of the catalyst to
the substrate.
Base growth mechanism is due
to a strong adhesion of the
catalyst to the substrate.
Tip growth mechanism
Stephan Hofmann et al., Nanoletters 7 (2007) 602
TOXICITY
Factors influencing the safety of CNTs in vivo.
Small CNT
Big CNT
Allotropic forms of C
3.4 Å
Planar, sp2
hybridization
Graphene
Graphite
Cubic, sp3
hybridization
Diamond
Fullerene
Carbon Nanotube
Nanotube/Fullerene sp2 + sp3 character
Allotropic forms of C
Graphene
Graphite
Graphene was largely studied from a theoretical point of view.
The unsuccessful attempts to synthesize it seemed to confirm the theory that truly twodimensional crystals (any kind of crystal) could not exist.
Fullerene
In Nanotube
2-D the thermal vibration of the atoms should lead
to a displacement
comparable to interatomic distance (thermodynamically unstable).
The very large perimeter-to-surface ratio of 2-D crystals promotes a collapse.
Discovery of Graphene?... Not yet6
Epitaxial Growth by CVD
Graphene and few layers of graphene are grown on Ni
Graphene is not free-standing and its properties are strongly affected by
substrate:
Significant charge transfer from the substrate to the epitaxial graphene
Hybridization between the d orbitals of the substrate atoms and π orbitals of
graphene, which significantly alters the electronic structure of the epitaxial
graphene.
Discovery of Graphene
Free-standing graphene films (10 µm in size) were
prepared by mechanical exfoliation (repeated peeling) of
small mesas of highly oriented pyrolytic graphite (HOPG).
K.S. Novoselov et al., Science 306 (2004) 666-669
Multi--layers graphene
Multi
The electronic structure of graphene rapidly evolves with the number of
layers, approaching the 3D limit of graphite at 10 layers.
Only graphene and its bi-layer has simple electronic spectra: they are both
zero-gap semiconductors (or zero-overlap semimetals).
For three or more layers, the spectra become more complicated, several
charge carriers appear, and the conduction and valence bands start notably
overlapping. This allows single-, double and few (3 to <10) layer graphene to be
distinguished as three different types of 2D crystals (‘graphenes’).
Thicker structures should be considered as thin films of graphite.
J.C. Meyer et al., Nature 446 (2007) 60-63
Characterization
HR-TEM
500 nm
2 nm
1 nm
J.C. Meyer et al., Nature 446 (2007) 60-63
Characterization
AFM-STM topography
STM
AFM
A. Charrier et al., J. Appl. Phys. 92 (2002) 2479-2484
M.I. Katsnelson, MaterialsToday 10 (2007) 20-27
Characterization
Nanobeam e-diffraction
Since graphene is microscopically corrugated the ediffraction peaks of graphene become broader while
increasing the tilt angle.
The
elastic
strain
can
intrinsically
thermodynamic stability of the film.
J.C. Meyer et al., Nature 446 (2007) 60-63
provide
the
Characterization
Raman analysis
2D (G’)
Raman spectra of graphene flakes. 2D (G') Raman peak changes in shape width and
position for an increasing number of layers reflecting a change in electron band
structure and electron-phonon interactions.
A.C. Ferrari et al., Phys. Rev. Lett. 97 (2006) 187401
Characterization
Confocal Raman
Confocal Raman map (2D band center of mass position).
1, 2, 3 and 4- layered flakes can be easily distinguished
when using a color palette scale.
High yield graphene synthesis
Chemical Vapor Deposition
CVD (or PE-CVE) of carbon precursor is used to deposit graphene on thin layer of
nickel (1 1 1) over silicon substrate. The graphene can be transferred to various
substrates, demonstrating viability for numerous electronic applications.
K.S. Kim et al., Nature 457 (2009) 706-710
Other graphene synthesis
Silicon carbide sublimation
Silicon carbide (6H-SiC) is heated between 1080 and 1320°C giving the
growth of graphene. The face of the SiC used for graphene creation, Siterminated or C-terminated, highly influences the thickness, mobility and
carrier density of the graphene.
Hydrazine reduction
Graphene oxide paper is reduced to single-layer graphene in a solution of
pure hydrazine (NH2-NH2).
Sodium reduction of ethanol
Gram-quantities of graphene are produced by the reduction of ethanol by
sodium metal, followed by pyrolysis of the ethoxide product, and washing
with water to remove sodium salts.
Unfolding CNTs
Graphene ribbons are produced by cutting and unfolding CNTs via
chemical etching (KMnO4-H2SO4) o plasma etching.
Graphene properties/applications
Electrical properties
Charge carriers (electrons and holes) have high density (1013 cm-2) and high
mobility (15000 cm2V-1s-1) at RT, and the mobility weakly depends on
temperature (only impurity scattering):
Ballistic field effect transistor (high speed electronic devices)
Rather modest on-off conductance ratio of ~30 at RT
K.S. Novoselov et al., Science 306 (2004) 666-669
Graphene properties/applications
Electrical properties
The transistor conductance can be significantly altered (106 on-off ratio at RT)
by a reversible chemical modification. In presence of humidity the electric field
changes the graphene to graphane (hydrogenated derivative, 3.5 eV of band
gap) and graphene oxide (-OH, insulator).
Non volatile memory application.
T.J. Echtermeyer et al., IEEE Electronic Device Letters 29 (2008) 952
Graphene properties/applications
Electrical properties
Graphene chip can double the frequency of an electromagnetic signal.
Graphene chips can transmit data faster than standard silicon chips,
consuming less energy and having a higher S/N ratio.
Graphene has high electrical conductivity, high optical transparency, mechanical
strength and flexibility.
Touchscreens, liquid
stretchable electrodes.
crystal
displays,
organic
photovoltaic
cells,
K.S. Kim et al., Nature 457 (2009) 706-710
Graphene properties/applications
Electrical properties
Graphene has an incredibly high surface area to mass ratio.
Ultra-capacitors with great energy storage density.
Graphene has electrical conductivity can be tuned by gas molecule absorption.
High sensitive gas sensors.
Graphene stripes, called graphene nanoribbons (GNRs), have electrical
properties that depends on the un-bonded edges.
Armchair GNRs are semiconductors with a band gap that increases with
the decreasing of the width.
Research is being done to create quantum dots by changing the width of
GNRs at select points along the ribbon, creating quantum confinement.
Graphene properties/applications
High thermal conductivity (~ 5000 Wm-1K-1)
Heat sinks.
Graphenen is very strong (Young’s modulus 0.5 TPa) and stiff (elastic constant
1-5N/m).
NEMS applications as pressure sensors or resonators.
Nanocomposites
Small spin-orbit interaction and near absence of nuclear magnetic moments in
carbon. GNRs in the zig-zag orientation, at low temperatures, show spinpolarized edge currents.
Spintronics (magnetoelectronics) applications
Graphene properties/applications
Graphene oxide is obtained by oxidizing (KMnO4-H2SO4) and chemically
processing graphite/graphene. The graphene oxide flakes dispersed in water
can give well-ordered structure with exceptional mechanical properties.
Nanocomposites
S. Stankovich et al., J. Mater. Chem. 16 (2006) 155
H. Chen et al., Adv. Mater. 20 (2008) 3557-3561
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