CARBON-BASED ALLOTROPES
AND THEIR PROPERTIES
A.A. 2011-2012
Fullerenes.
The compounds of pure Carbon
Carbon is a non-metallic element whose most
abundant pure allotropes are graphite and
diamond, two hypothetically infinite lattices.
Only synthetic diamonds are free from impurities.
Graphite
The most abundant natural allotrope of C.
• Color and aspect: Black or gray,
opaque, non crystalline, non fluorescent
• Number of interatomic bonds: 3 (sp2
orbitals, lenght: 1.421 Å)
• Atomic density: 1.14 * 1023
• Distance between sheets:
335 nm, no covalent bonds.
The arrangement in multiple
sheets confers to the graphite high
anisotropy, and explains its optical,
acoustic and magnetic properties.
Diamond
The second most abundant natural
allotrope form of C.
• Color and aspect: colourless if
pure, transparent, fluorescent and
phosphorescent.
• Number of interatomic bonds:
4 (sp3 orbitals, length: 1.54 Å,
angle: 109.47°)
• Atomic density: 1.77*1023
Diamond crystal have tetrahedral
symmetry (A).
When hexagonal symmetry is
displayed, the allotrope is called
lonsdaleite (B).
Properties of Graphite and Diamond
Diamond
Graphite
Specific
gravity
3.52 gm/cc
2.25 gm/cc.
Hardness
10
1.5-2.0
Optics
isotropic, transparent;
Abs band at 415.5 nm
(weaker lines at 478,
465, 452, 435, 423 nm)
extreme anisotropy, optically
uniaxial (-), high absorption of
visible light.
Acoustic insulator.
Electrical
properties
Insulator (band gap: 5.47 eV).
Semimetal, (band gap -0.04 eV),
high conductivity.
Thermic
properties
Expansion Coeff, linear: 1.18 α
(10−6 K−1); metastable, no
decay at room temperature
Expansion Coeff, linear: 6.5-0.5 α
(10−6 K−1), stable
Radiation,
magnetism
phosphorescent and
fluorescent.
non fluorescent,
diamagnetic (caused by the
itinerant  electron in semimetal).
Other carbon-based allotropes:
nanomaterials.
Nanodiamonds and buckyballs are discrete nanosized
molecules
Graphene, a single layer of graphite with the thickness of a
single atom
Carbon nanotubes, Single Walled (SWCNT) or MultiWalled
(MWCNT). SWCNTs can be considered as the result of the
folding of a graphene sheet.
With the only exception of graphene, these compounds are
found in traces in nature, as in vulcanic rocks and soots.
Despite their chemical similarities, the physical chemistry of
the nanosized allotropes of the C, changes dramatically in
relation to properties like simmetry and size.
Fullerenes, graphene and CNTs are compared herein.
Nanodiamonds
Nanodiamonds generate
when TNT/RDX detonate in
the absence of oxygen.
Their size if up to 5 nm (1.5
nm in the exemple here
shown).
Thanks to their properties,
and their propensity to be
functionalized at the
surface, the nanodiamonds act very well as drug carriers.
Their potential toxicity in living systems is under study, and
seems to be low.
Buckyballs fullerenes are
closed cells of carbon
clusters, with icosahedral
cubic symmetry. Found in
small amounts in soots
They are odourless,
generally soluble at room
temperature in organic
solvent, not in water.
The smallest stable
representative is C20, while
bucky balls larger than C100
are known.
C atoms are arranged in
series of hexagonal and
pentagonal faces.
The hexagonal faces are
diamagnetic and aromatic,
the pentagonal ones are paramagnetic and antiaromatics: their properties
mutually cancell inside the cluster. The functionalization of the surface is
easy.
The fullerenes
Solubility of bucky balls fullerenes.
The smallest buckyball is C20, however the smallest stable
fullerene is C36.
Bucky balls larger than C100 are known.
At room temperature, the fullerenes are soluble in organic
solvent, not in water.
The colour of the solution varies for different clusters,
according to their symmetry.
C60 is purple, C70 is reddish brown, C76 and C84 have
different colours for their different isomers.
Small band gap fullerenes lack solubility, when pure.
These allotropes, including C36, C50 and C72, are highly
reactive and bind to other fullerenes, to soot particles, or
can be functionalized with a lantanide in their core.
C60
C70
1,2 dichlorobenzene
24
36
carbon disulfide
8
10
xylene
5
4
toluene
3
1
benzene
1
1
mesitylene
1
1
carbon tetrachloride
0.5
0.1
dichloromethane
0.3
0.1
dodecane
0.1
0.1
decane
0.1
0.05
n-hexane
0.05
0.01
cyclohexane
0.04
0.08
octane
0.03
0.04
pentane
<0.01
<0.01
Solvent
Solubility of C60 and C70 related to
properties of the solvent.
The solubility of C60 and C70 in various solvents (see the previous slide) is
related to the surface tension of the solvent (graph), not to its octanol/water
partition coefficient, nor to its specific gravity/density.
Physical properties of C60
Black solid, odourless.
Density: 1.65 g cm-3
Standard heat of formation:
9.08 k cal mol-1
Index of refraction: 2.2
(600nm)
Boiling point: Sublimes at
800K
Resistivity: 1014 ohms m-1
Vapour density: N/A
Aromatic, Superconductor,
Ferromagnetic (polarized at
room T°C)
Graphene.
Graphene is a compound of pure
carbon, arranged as an hexagonal
lattice in which the C atoms are
bound by sp2.
The sheet has the thickness of a
single atom; in this respect
graphene differentiates from
graphite.
Graphene and CNTs as derivatives of
graphite
Diamond and graphite are stable structures, at least at room
temperature and atmospheric pressure. To transform them
into each other high energy is requiered.
The production of graphene, a single sheet of the graphene
allotrope of C, requires instead the separation of only weak,
non-covalent bonds.
Graphene is an hexagonal lattice of carbon atoms, bound
each other as in graphite, whose thickness is that of a single
atom.
Though a graphene sheet is potentially infinite in the other two
dimensions, its thermodynamic stability depends on the
number of atoms and on the shape.
Thermodynamic stability of graphene
To be thermodynamically stable the graphene must reach a
minimum size of 6000 atoms, that is a sheet of at least 20 x
20 nm. Most stable structures are larger than 24,000 C.
The sheet’s shape is curled, not flat, as shown in the picture,
which was obtained “in silico” by using the tool for mimicking
the energy minimization, implemented in the NanoRex
software.
Electrical properties of graphene
Graphene is a
conductor, even
powerful than Cu
Engineered nanomaterials:
Single Walled Carbon NanoTubes
(SWCNT).
Engineered SWCNTs can have different symmetries:
Armchair, m=n; n,n = 5,5
Chiral: mn: m,n = 10,5 (in the example, see next slides)
Zigzag, m,n = 9,0
The three allotropes can be conceived as generating from
a graphene ribbon, rolled up with different axes of
symmetry.
SWCNT simmetry….
A
B
C
A: Armchair (m,n=5,5), B: Zigzag (m,n=9,0), C: Chiral (m,n=10,5)
and some consequences.
Anti-Bonding
B
A
C
Bonding
A: Armchair (m,n=5,5), B: Zigzag (m,n=9,0), C: Chiral (m,n=10,5)
Scanning Tunnelling Microscope and
conductance measurement.
Keeping the tip of the
STM close to the surface
of CNTs, electrons from
the tip can jump to the
nanotube. A s.c.
"tunnelling current“
establishes.
The plot of dI/dV by the
voltage measures the
number of electronic
states available for
electrons to tunnel at a
certain energy (V).
Metallic or semiconducting SWCNTs.
The chemicals deals into two main classes for their electrical
properties:
1. metals, in which the electric current generally flows freely
and there is no energy gap between the valence and the
conducting states.
2. semiconductors, in which an energy gap exists and
therefore a higher voltage is needed to make electric current
flow.
For most materials the metallic or semiconducting nature
depends on the chemical composition and 2-D arrangement of
atoms and molecules.
SWCNTs can show metallic or semiconducting properties in
relation to their chirality (n,m) and diameter.
•
A new exploitation attempt.
Acceptor (porphyrine)
Transmitters (short
nucelotides)
Electrochemical
solar cells
mimicking
photosynthesis.
SWCNT
Choi et al., 2010, doi:DOI: 10.1117/2.1201007.003130
Fullerene-like structures not
Carbon-based
Boron bucky balls
B112, isomers
B16N16
B80
•De et al., 2011, DOI: 10.1103/PhysRevLett.106.225502
•Muya et al., Phys. Chem. Chem. Phys., 2011, 13, 7524–7533
Boronnitride
nanotubes
Armchair
simmetry
(n,m=5,5)
Boron: rosa,
Nitride: blu.
Aknowledgments
All the figures, if not otherways indicated, have been constructed with the help of
the following softwares, freely distributed:
•
•
•
Nanoegineering 1, version 1.1.1, by Nanorex
Ninithi 1, by Lanka Software Foundation
UCSF Chimera, by the University of California.
Further readings
1. Makarova T. 2004. Magnetism in polymerized fullerenes. In “Frontiers of
Multifunctional Integrated Nanosystems” (E. Buzaneva and P. Scharff eds.),
Kluwer Academic Publishers, the Netherlands, pag. 331-342.
2.Małolepsza E, Witek HA. 2007. Comparison of Geometric, Electronic, and
Vibrational Properties for Isomers of Small Fullerenes C20-C36. J. Phys.
Chem. A 2007, 111, 6649-6657
Armchair SWCNT (m,n = 5,5):
Electrical properties
Chiral SWCNT (m,n = 10,5):
Electrical properties
Chiral SWCNT (m,n = 10,5):
Electrical properties