Carbon
Onions and Diamond Nucleation
1.
Carbon Onions
Self-compression
of a carbon onion under electron irradiation
at
700°C in the electron microscope
The formation of carbon onions
Intense electron irradiation of amorphous
or graphitic specimens in an electron microscope results first in
graphitisation (when an amorphous precursor is irradiated), then
curling of the graphene planes and finally closure, leaving perfectly
spherical concentric-shell graphitic onions.
When such an irradiation experiment is
carried out at specimen temperatures above 400 °C, no defect
clustering takes place and the shells of the onions are perfectly
coherent . Careful examination of such onions reveals the unique
property of self-compression during irradiation. This manifests itself
by the reduction of the spacing between the shells below the
usual layer spacing of crystalline graphite (0.335 nm). This phenomenon
can be explained by the permanent loss of atoms in the outer shells as
a result of sputtering by the electron beam. When two adjacent carbon
atoms in a shell are missing (a divacancy) the shell can close again by
reducing the number of faces. Rearrangement via the Stone–Wales
transformation can create pentagons and heptagons (see the figure
below); an appropriate combination of both ensures the uniform
spherical curvature of the onion. The shrinkage of the shells creates a
surface tension that tends to make the object spherical and generates
pressure within the onion. The migration of interstitials from inner
shells to the outer shells leads to an increase in compression towards
the centre.
Model
structure of a spherical multi-shell carbon onion.
Combination
of pentagons, hexagons, and heptagons
(calculated by H.
Terrones & M. Terrones)
Publications:
F. Banhart, T. Füller, Ph. Redlich
and P.M. Ajayan
"The formation and self-compression of
carbon onions"
Chemical Physics Letters 269,
349-355 (1997)
Ph. Redlich, F. Banhart, Y. Lyutovich
and P.M. Ajayan
"EELS study of the irradiation-induced
transformation of carbon onions to diamond"
Carbon 36, 561-563 (1998)
F. Banhart
"Irradiation
effects in carbon
nanostructures"
Reports on
Progress in Physics 62,
1181-1221 (1999)
F.
Banhart
"Formation
and transformation of carbon nanoparticles under electron irradiation"
Philosophical
Transactions, in the press (2004)
T.
Cabioc'h, E. Thune, M. Jaouen and F. Banhart
"Electron
microscopy study of carbon onions synthesised by ion implantation"
Philosophical Magazine A, 82,
1509-1520 (2002)
2.
Diamond Nucleation
Diamond
nucleation in the core of a carbon onion
after
~1h of intense electron irradiation
The nucleation of diamond crystals
in carbon onions
The generation of high pressure in
carbon onions under sustained irradiation can lead to the nucleation of
diamond crystals in their cores. This phenomenon is observed when
carbon onions are irradiated with an intense electron beam (~100 A/cm2)
for about one hour at temperatures above 600 °C. The nucleating
crystal appears in the microscope image as a set of straight fringes
that represent the (111) planes of diamond. This phase transformation
from curved graphitic material to diamond is assumed to be induced by
the high pressure in the core of the onions and is probably facilitated
by the increased sp3 bonding character as the
curvature of the graphitic structure becomes more pronounced.
Publications:
F. Banhart and P.M. Ajayan
"Carbon onions as nanoscopic pressure
cells for diamond formation"
Nature 382, 433-435 (1996)
F. Banhart
"The transformation of graphitic onions to
diamond under electron irradiation"
Journal of Applied Physics 81,
3440-3445 (1997)
F. Banhart
"Irradiation effects in carbon
nanostructures"
Reports on Progress in Physics 62,
1181-1221 (1999)
F. Banhart
"Diamantbildung in Kohlenstoffzwiebeln"
Physikalische Blätter 53,
33-35 (1997)
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