PERGAMON
Carbon 39 (2001) 1251–1272
Sensitivity of single-wall carbon nanotubes to chemical
processing: an electron microscopy investigation
M. Monthioux a , *, B.W. Smith b , B. Burteaux b , A. Claye b , J.E. Fischer b , D.E. Luzzi b
a
´
et d’ Etudes Structurales, UPR A-8011 CNRS, BP 4347, F-31055 Toulouse Cedex 4, France
Centre d’ Elaboration des Materiaux
b
Department of Materials Science, University of Pennsylvania, 3231 Walnut Street, Philadelphia, PA 19104 -6272, USA
Received 22 July 2000; accepted 13 September 2000
Abstract
Single wall carbon nanotube (SWNT) materials subjected to various chemical treatments including regular, published,
acidic purification treatments, were investigated by high resolution transmission electron microscopy and X-ray diffraction.
Results show that acid purification cannot avoid SWNT structure alteration. The liquid, acidic medium provokes the
gathering of pre-existing fullerenes into crystallised fullerite. A slight temperature increase has a dramatic effect on SWNT
degradation which can result in complete amorphisation. Immersion of some of the SWNT materials in dimethylformamide
(DMF) was also found to be harmful to the SWNT structure. Several observations suggest that as-prepared (not treated)
SWNTs contain structural defects along the tube walls which act as preferred sites for the acid (or DMF) attack, inducing
side wall openings.  2001 Elsevier Science Ltd. All rights reserved.
Keywords: A. Carbon nanotubes; B. Chemical treatment; C. Transmission electron microscopy; D. Defects, Microstructure
1. Introduction
Since the first production of single wall carbon
nanotubes (SWNTs) in 1993 by electric arc discharge by
Iijima et al. [1], Ajayan et al. [2], and Bethune et al. [3],
then in 1995 by pulsed laser vaporisation by Guo et al. [4],
the presence of undesirable impurities has been a challenge. These impurities are both by-products (polyaromatic
carbon shells, amorphous carbon, fullerenes) or remains of
the primary materials (graphite flakes from the arc electrodes or laser target, catalyst crystals). The amount of
impurities is generally large, which prevents the use of the
as-prepared materials for any application, and handicaps
many scientific investigations (those using bulk spectroscopic techniques for example). Fig. 1 qualitatively illustrates the relative amounts of SWNTs and impurities in
as-prepared (5raw) materials obtained from either the arc
or laser methods. Attempts at quantification of the amount
of impurities have yielded variable results, ostensibly
dependent on the preparation conditions and the location in
the reactor where the SWNT materials were gathered. As a
*Corresponding author. Tel.: 133-5-6225-7886; fax: 133-56225-7999.
E-mail address: [email protected] (M. Monthioux).
reasonable approximation, the impurity content can be
considered to be 30% by weight.
As part of synthesis routines, attempts are therefore
currently made to purify the SWNT materials from the
various impurities. It is worth noting that a similar problem
occurred previously regarding the production of C 60 -type
fullerenes using the same methods (i.e. laser vaporisation
[5] and electric arc [6]), but solving it was easy since
fullerene molecules are soluble in solvents such as toluene,
while the impurities are all insoluble in common organic
solvents. On the contrary, SWNTs, which actually are
fullerene molecules but with a variable, large length / width
aspect ratio, are not soluble in common organic solvents,
due to their high molecular weights. They also exhibit poor
sensitivity to inorganic solvents due to the poor reactivity
of the closed polyaromatic structure (graphene).
Most of the purification methods of SWNT-based
materials are therefore based on or include steps involving
oxidising attacks using single mineral acids like HCl [7,8]
or HNO 3 [9–14], or dual attack using HNO 3 / H 2 SO 4
mixture [15], in order to dissolve the remains of the metal
catalysts and the carbon material other than SWNTs (i.e.
amorphous carbon and polyaromatic carbon shells). Mineral acids are known to help in oxidising polyaromatic solids
like graphite, although a mixture with another agent (e.g.
0008-6223 / 01 / $ – see front matter  2001 Elsevier Science Ltd. All rights reserved.
PII: S0008-6223( 00 )00249-9
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M. Monthioux et al. / Carbon 39 (2001) 1251 – 1272
Fig. 1. Low magnification view of an as-prepared SWNT material (sample [1) illustrating the respective amounts of SWNT ropes and
impurities. Also valid for as-prepared material from ARC.
HNO 3 1KClO 3 , so-called Brodie’s reagent [16], or
H 2 SO 4 1Ag 2 Cr 2 O 7 , so-called Simon’s reagent [17]) is
often used. An additional motivation for the use of severe
acidic treatments on raw or previously purified SWNT
materials using HNO 3 / H 2 SO 4 mixtures is the possibility
to open the tubes, in order to enhance their reactivity
through the creation of unsatisfied carbon atoms at the end
[11]. Finally, mineral acids were observed to intercalate
between SWNTs in ropes as they do between the
graphenes sheets of polyaromatic stacks, as a possible first
step towards obtaining isolated SWNTs (i.e. no longer
gathered into ropes) [18].
Another challenge for the subsequent use of pure SWNT
materials is to process them into macroscopic forms that
are easy to handle. In this regard, pure SWNT mats
looking like black paper sheets (so-called ‘buckypaper’)
have been obtained by filtering a suspension [11,15]. The
same approach has been used within a strong magnetic
field to produce buckypaper in which nanotubes are
aligned with a dispersion of 6148 FWHM [19,20]. Beside
such a solid macroscopic form, a ‘liquid’ form, i.e. a
suspension (solution), is likely to be useful too. Functionalisation experiments of purified then shortened
SWNTs have therefore been carried-out involving various
chemical agents like oxysulphurdichloride, carbon disulphide, hydrogen peroxide, dimethylformamide, tetrahydrofuran, surfactants, etc. [11,21,22].
Though ideally expected to be chemically very stable
due to the poor reactivity of the basal aromatic plane from
which SWNTs are built, the question of whether all the
chemicals which are now currently proposed in the literature as purifying, suspending, or grafting agents for
SWNTs actually have a limited effect on the SWNT
integrity has to be addressed. Reliable information is
difficult to gather in this matter, because of the necessity to
use expensive, time-consuming, sophisticated investigation
methods like high resolution transmission electron microscopy to accurately and systematically check the SWNT
structure.
Investigating the effects on SWNT structure of the
various chemical agents proposed in literature and related
parameters such as temperature, time, concentration, and
combination with other chemicals is beyond the scope of a
single paper. In this paper, we report the investigation of
the effects of some commonly used chemical treatments on
SWNT structure by means of high resolution transmission
microscopy (HRTEM). We also report the effect on
purified SWNTs of an organic solvent, dimethylformamide, used to tentatively prepare SWNT suspension [23].
2. Experimental
2.1. Materials and treatments
The materials investigated were as follows:
(1) A raw SWNT material (5‘Raw R’), prepared
using the pulsed laser vaporisation method (PLV), which
M. Monthioux et al. / Carbon 39 (2001) 1251 – 1272
was obtained from Professor Smalley’s laboratory (Rice
University, Houston, Texas, USA). Batch number is
R05017A. Catalysts were Ni and Co. Conditions of
preparation were those given in reference [15] for the 20
apparatus.
(2) A purified material (5‘R / HNO 3 ’), obtained at
Rice University from the former ‘Raw R’ material above
by using a shorter procedure with respect to that described
hereafter. The most relevant difference is the absence of
the acid steps involving sulphuric acid. Therefore, this
sample has gone only through the nitric acid oxidation.
(3) A purified material (5‘R / HNO 3 / H 2 SO 4 1
HNO 3 ’), obtained at Rice University from the former
‘Raw R’ material above after more extensive acid treatments. Treatment conditions are given in Ref. [15] and can
be summarised as follows: The ‘Raw R’ material was
refluxed for 45 h in 2–3 M nitric acid, followed by several
de-ionised water washing / centrifugation cycles, then
powerful sonicated dispersion into a pH 10 NaOH solution
containing 0.5% Triton X-100 (non-ionic surfactant). The
resultant suspension was cross-flow filtered then washed
with methanol. The material obtained was also named
‘buckypaper’ by Liu et al. [11] and Rinzler et al. [15]. The
ultimate purification is achieved with a final acid treatment
using a 3:1 mixture of sulphuric (98%) and nitric (70%)
acids at 708C for 20–30 min, followed by cross-flow
filtration in NaOH as above. A final oxidation was done
with a 4:1 mixture of sulphuric acid (98%) and hydrogen
peroxide (30%) at 708C for 20–30 min followed by NaOH
washing.
(4) A heat-treated, purified material (5‘R / HNO 3 /Annealed’), obtained at Rice University as a result of 12008C
vacuum annealing (10 26 Torr, 14 h) of the ‘R / HNO 3 ’
material (sample [2).
(5) A heat-treated, purified material (5‘R / HNO 3 /
H 2 SO 4 1HNO 3 /Annealed’), obtained at Rice University as
a result of 12008C vacuum annealing (10 26 Torr, 14 h) of
the ‘R / HNO 3 / H 2 SO 4 1HNO 3 ’ material ([3).
(6) An acid-treated material (5‘R / HNO 3 / H 2 SO 4 1
HNO 3 /Annealed /Acid’), obtained by us from the latter
material ([5) using a presumably more severe procedure
than for the ‘R / HNO 3 ’ or ‘R / HNO 3 / H 2 SO 4 1HNO 3 ’
materials above, as described in Ref. [24]. Briefly, the
most relevant steps were an oxidising attack using a 3:1
mixture of sulphuric (90%) and nitric (70%) acids heated
at 908C for 10 min, prior to washing with NaOH.
(7) A raw SWNT material (5‘Raw Arc’), prepared by
the electric arc method (ARC), obtained from Dr Bernier’s
´
laboratory (Groupe de Dynamique des Phases Condensees,
University of Montpellier II, France). Batch number is
[55.
(8) A sample of the same material as above after acid
treatment (5‘Arc /Acid’), obtained by us using an acidic
oxidising treatment whose conditions are basically similar
to that described for sample [6 ‘R / HNO 3 / H 2 SO 4 1
HNO 3 /Annealed /Acid’.
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(9) An acid-treated, dimethylformamide-suspended,
purified material (5‘R / HNO 3 / DMF / H 2 SO 4 1HNO 3 ’),
obtained at Rice University (batch [ L08318) by the
subsequent etching of sample ‘R / HNO 3 / DMF’ (see below
for details on the DMF-treated materials) using the same
nitric1sulphuric acid treatment conditions as described for
sample [3 ‘R / HNO 3 / H 2 SO 4 1HNO 3 ’.
The effect of dimethylformamide (DMF) on SWNT
structure was checked by investigating the following
samples:
(10) A DMF-suspended material (5‘R / HNO 3 /
DMF’), obtained at Rice University from the sonication of
a HNO 3 -purified SWNT material (similar to sample [2
‘R / HNO 3 ’ above) in N,N-dimethylformamide for 15 h
according to the procedure published in Ref. [23].
(11) A material similar to ‘R / HNO 3 / H 2 SO 4 1HNO 3 /
Annealed’ (sample [4, see above) dispersed in DMF by us
using a low-power bath sonication for 30 h [24], resulting
in a sample labelled ‘P/ HNO 3 / H 2 SO 4 1HNO 3 /Annealed /
DMF’.
Genealogical relationships between the samples are
summarised and sketched in Fig. 2.
2.2. TEM operating conditions
All of the samples were provided or obtained as dried
mats, except [10 and [11 (i.e. those whose final step was
a DMF suspension). Dry samples were prepared for TEM
by tearing a piece, which was glued across a slot grid using
carbon tape. Samples [10 and 11 were deposited on a
1000-mesh copper-grid with no carbon film directly from a
drop of the DMF suspension, in order to prevent any
misinterpretation due to the possible occurrence of byproducts originating from the alteration of a carbon film by
the DMF.
TEM investigations were carried-out using a JEOL
4000EX microscope (LaB 6 electron source), with the high
voltage set at 100 kV, and the objective lens current set at
2.64 mA. Current density on the specimen (illumination)
was always adjusted using the condenser lens so that the
exposure time given by the microscope calculator was two
seconds (calculated on the small screen placed on a
specimen-free area), whatever the magnification.
3. Results
3.1. Raw materials
Features of the raw materials either from PLV (‘Raw-R’)
or from ARC (‘Raw-Arc’) were found to be fairly
consistent with observations previously published ([4,25]
and [26], respectively).
Briefly, sample [1 ‘Raw-R’ macroscopically looks like
a loose felt. It is a multi-component material, with the most
abundant constituents being a network of entangled and
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M. Monthioux et al. / Carbon 39 (2001) 1251 – 1272
Fig. 2. Genealogical relationship between samples. Rectangles:
materials obtained from and treated at Rice University. Triangle:
material obtained from Montpellier University. Ellipses: materials
treated at University of Pennsylvania.
Fig. 4. (a and b) Two examples of SWNT rope ‘cross-sections’
(actually the projections of bent ropes whose bent part is oriented
parallel to the electron beam) for as-prepared SWNT material
from PLV (sample [1). Ropes are obviously coated with amorphous material.
Fig. 3. Ropes or catalyst particles are associated with poorly organised carbon phases in as-prepared SWNT material from PLV (sample
[1).
M. Monthioux et al. / Carbon 39 (2001) 1251 – 1272
branched ropes of SWNTs, and remaining catalyst particles (Fig. 1). Minor components are amorphous-like or
poorly organised carbon phases associated with the ropes
or the catalyst (Fig. 3). No fullerene crystals were found
either by X-ray diffraction by us or in reference [15] nor
by TEM.
The SWNT rope diameters range from |4 to |100 nm,
most often below 40 nm, but ropes often exhibit a ribbonlike — instead of cylindrical — morphology (Fig. 1). The
structural organisation of the ropes is heterogeneous, and
they are often irregularly coated with a light-elementcontaining amorphous material, as evidenced from crosssection views (Fig. 4a and b). Such an amorphous coating
of the ropes is not a feature inherent to the PLV method
since ‘clean’ ropes were obtained in 1995 [4]. It is rather
attributed to the experimental modifications brought to the
process to scale-up the production [15]. Some ropes are
poorly organised, i.e. it is difficult to discern the contrast of
individual SWNTs (Fig. 3). They may contain catalyst
crystals or other impurities. Most of the ropes are however
well organised, i.e. SWNTs are obvious and SWNT walls
can be followed as continuous fringes over long distances
(e.g. .150 nm) within the ropes (Fig. 5). This indicates
that SWNTs — or at least many of them — are rather well
aligned parallel to the rope axis within the ropes instead of
being twisted and plaited. Isolated SWNTs can also be
imaged (Fig. 5). SWNT diameters are fairly constant,
1.3–1.4 nm (Fig. 4a and b).
Catalyst particles exhibit round morphologies, with
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diameters ranging from |2 to |50 nm. They are crystallised and always embedded within amorphous carbon
material. The amorphous material often exhibits a
nanoporous texture. Catalysts can also be found associated
with ropes, but good structural organisation of the SWNTs
within the ropes is not achieved in this case, and the ropes
are subsequently mainly made of poorly organised material
and exhibit irregular morphologies (Fig. 3). On the contrary, well-organised ropes are always catalyst-free.
When imaging conditions are suitable, i.e. imaging of
isolated SWNTs with no background material, tiny, weakly
scattering spheroidal objects whose sizes are less than 1
nm are found, suggesting that part of the amorphous
material coating the SWNT surface could be randomly
dispersed fullerene molecules (Fig. 6). Although fullerene
crystals were not found, either in TEM or X-ray diffraction, the accumulation of C 60 molecules at the SWNT
surface might be the most frequent cause of what was
improperly called ‘amorphous coating’ on the ropes, like
that imaged on the isolated SWNT in Fig. 5. This is likely,
since PLV was the first technique used to produce fullerenes [5], and since fullerenes encapsulated in SWNTs
(so-called C 60 @SWNTs [24]) were discovered in sample
‘R / HNO 3 / H 2 SO 4 1HNO 3 /Annealed’ ([27] and Fig. 28 to
come). The occurrence of free fullerene molecules in
‘Raw-R’ has been subsequently experimentally demonstrated to be a preliminary requirement to form
C 60 @SWNTs in ‘R / HNO 3 / H 2 SO 4 1HNO 3 /Annealed’
[28,29].
Fig. 5. Example of a SWNT rope from the as-prepared material from PLV (sample [1). SWNT walls appear defect-free and parallel. An
isolated SWNT is seen at the top of the image.
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M. Monthioux et al. / Carbon 39 (2001) 1251 – 1272
Sample [6 ‘Raw-Arc’ does not significantly differ from
‘Raw-R’ (see Fig. 1 for reference) since it is also a
multi-component material containing SWNT ropes, amorphous carbon, and remaining catalysts. One unique feature
is the frequent occurrence of loop-like ropes (Fig. 7)
similar to that purposely synthesised by Martel et al. [30].
Rope diameter range is smaller, up to |30 nm, also often
exhibiting a flattened morphology, and with less amorphous material around (compare cross-sections in Fig. 8a
and b with Fig. 4a and b). Isolated SWNTs are present but
uncommon. SWNT diameters are also quite homogeneous,
typically |1.3–1.4 nm (Fig. 8a and b). Though some
heterogeneity exists, the structural organisation of the
SWNTs within the ropes is generally good, untwisted (Fig.
9), somewhat better than that of the material from PLV.
Catalyst particles are in the range of 2–50 nm and are
rarely surrounded by polyaromatic carbon shells, but are
embedded in a nanoporous, amorphous carbon phase
instead. Fullerene molecules were not found but are likely,
since ARC is also a major way to prepare fullerenes [6],
and since C 60 @SWNTs were also discovered in raw
SWNT-containing materials prepared from ARC [31].
However, the SWNT / fullerene proportions might differ
with respect to the differences in the plasma characteristics
of the PLV and ARC processes as well as differences in the
catalyst content used in the two processes.
Fig. 6. The occurrence of fullerene molecules (arrows) on the
SWNT surface is shown when isolated SWNTs are imaged (asprepared material from PLV, sample [1).
3.2. Effect of ‘ mid’ (though quite strong!) acid
treatments
Although the acid treatment of sample [3 ‘R / HNO 3 /
Fig. 7. Example of loop-like ropes found in the as-prepared material from ARC (sample [6).
M. Monthioux et al. / Carbon 39 (2001) 1251 – 1272
Fig. 8. (a and b) Two examples of SWNT rope ‘cross-sections’
for the as-prepared material from ARC (sample [6). Compared to
material from PLV (see Fig. 4), ropes are associated with none or
less amorphous material.
H 2 SO 4 1HNO 3 ’ was successful in removing most of the
catalyst particles and the amorphous carbon phase, the
resultant product is still a multi-component material,
whose main component is a network of entangled and
branched bundles of SWNTs (Fig. 10). Compared to
sample ‘Raw-R’, ropes appear to be altered. However, the
material is highly heterogeneous, and the extent of structural degradation is found to be variable, generally depending on the area of the sample at the micrometer scale. In
less altered ropes, fringes corresponding to SWNT walls
within the ropes are still visible but continuous fringe
lengths are much shorter and more distorted than in the
starting material (Fig. 11). Imaging of isolated SWNTs
was possible (Fig. 11) but difficult, since they are much
more sensitive to the electron beam than in ‘Raw R’.
Specifically, isolated SWNTs were found to vibrate, then
finally damage, so that opportunities for appropriate exposure conditions were rare. SWNT walls in ropes appear
damaged, distorted, or even segmented (Fig. 11, compared
to Fig. 5) providing a dotted aspect to the ropes. Again, the
occurrence of 0.7 nm diameter circles is evidence of the
presence of free fullerene molecules (Fig. 11, solid ar-
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Fig. 9. Example of a SWNT rope from the as-prepared material
from ARC (sample [6). SWNT walls appear defect-free and
parallel.
rows), although interpretation can be difficult due to
convolution effects with superimposed materials along the
electron beam path. In more altered ropes, some long
SWNT walls are still visible, but they are associated with
amorphous material (Fig. 12). As the alteration progresses,
SWNTs are often visible at the centre of the rope only.
Surrounding this remaining core is amorphous material
(Fig. 13). Ultimately, the ropes can appear completely
amorphous (Fig. 14). A frequent feature is that the
projected images of the ropes reveal a more severe
alteration on one side (Fig. 15, open arrows). One explanation might be that the acid attack was more efficient on
carbon atoms whose bonds were under tensile stress, i.e.
the more altered parts of the ropes would correspond to
rope portions where ropes were formerly drastically bent.
This interpretation is consistent with simulations that
recently predicted the enhanced chemical reactivity at
regions of local conformational strain on SWNTs [32].
Minor components in this material are catalysts remaining either as widely dispersed, nanometric grains, revealed
in Fig. 15 (solid arrows) by |1–3 nm spots exhibiting
absorption contrast different from that of carbon and
located at the rope surface, or as large crystals up to |100
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M. Monthioux et al. / Carbon 39 (2001) 1251 – 1272
Fig. 10. Low magnification view of the mild-strength acid-treated material from PLV (sample [3). Most of the catalyst particles have been
removed, locally leaving pure SWNT material.
nm in diameter. Large crystals are generally contained
within polyaromatic carbon shells, while nanometric ones
are never associated with carbon shells. Empty carbon
shells are also found. Though well organised regarding the
perfection and parallelism of the graphene sheets (Fig. 16),
these carbon shells are not graphitised but turbostratic, i.e.
the stacking sequence of graphenes does not follow the AB
sequence of graphite but obeys random rotations. This is
indicated by related diffraction patterns that always exhibit
the typical two-indices 10 and 11 bands at |0.213 and
|0.123 nm beside the 00 l reflections. Such carbon shells
are common morphologies of thermally decomposed carbides or catalytically produced polyaromatic carbons and
have been well-known, though they have been improperly
called ‘carbon onions’ in the recent literature dealing with
nanotubes and fullerenes.
Other minor components in this material are nearly
micrometric crystals (dark areas in Fig. 17) which were
identified as fullerite (i.e. crystallised fullerenes) by means
of electron diffraction. An example of an electron diffraction pattern is given as Fig. 18, which exhibits the fcc
structure reflections 111, 220 and 311 at 0.82, 0.50, and
0.43 nm, respectively, among others. The occurrence of
fullerite in the sample was confirmed by us and [15] using
X-ray diffraction.
More rarely, intriguing swollen, nearly amorphous ropelike objects are also found (Fig. 17, arrows), ultimately
exhibiting the appearance of flattened cotton balls piled-up
(Figs. 17 and 19). Since they may contain traces of former
Fig. 11. Example of slightly altered SWNT ropes from the mildstrength acid-treated material from PLV (sample [3). Fringes
representing SWNT walls within the rope appear shorter and
distorted (as compared with Fig. 5). Solid arrows show probable
fullerene molecules.
M. Monthioux et al. / Carbon 39 (2001) 1251 – 1272
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Fig. 12. Example of more altered SWNT ropes from the mild-strength acid-treated material from PLV (sample [3). Short and distorted
SWNT walls are associated with amorphous-like material.
SWNTs (Fig. 20), it is assumed that these peculiar objects
also derive from SWNT ropes. Locally, the swelling of the
rope surface is less pronounced where traces of SWNTs
remain (Figs. 19 and 20). This supports the assumption
that the swollen nanofibre-like objects might be the
ultimate result of the extensive structural transformation of
the SWNT ropes under severe acid treatment. As already
observed on other altered ropes (Fig. 11), they exhibit
specific circular features which are unusual in amorphous
materials, and whose size is consistent with C 60 molecules
(Fig. 20, arrows). Moreover, high magnification imaging
shows that the swollen ropes are not systematically
amorphous, since they may contain periodic features (Figs.
20 and 21) whose directions are no longer oriented parallel
to the rope axis, as the SWNT walls were. Local Fourier

transforms (Digital Micrograph software) were performed

on digitised TEM images (Photoshop software) of the
periodic areas in swollen ropes. Taking the interlayer
distance of a polyaromatic carbon shell (as in Fig. 16)
equal to 0.344 nm as a reference for the magnification, the
results clearly reveal periodic distances of |0.45, 0.51, and
0.83 nm for the most intense reflections (Fig. 22). It is
worth noting that these distances are those of fullerite.
Other possible distances, more difficult to measure due to
their being blurred and faint, are |0.47, 0.62, and 0.95 nm.
These observations are important, since they suggest that
fullerite might be a by-product of SWNT alteration by
acids.
In comparison, sample [2 ‘R / HNO 3 ’ does not look like
much different. However, the average level of SWNT
structural alteration seemed less dramatic and more
homogeneous, and a representative example of the average
SWNT rope aspect is provided in Fig. 11. Swollen ropelike objects were not found, but there is a possibility that
these objects might be present, but were missed due to
rarity and the local heterogeneity of the sample, as is
revealed in sample [3 regarding the phase composition
and extent of rope degradation. Fullerite crystals were not
found by TEM, but were detected by X-ray diffraction.
Finally, the aspect of the ropes in sample [9, i.e.
resulting from the acid attack of a DMF-suspended material (sample [10) does not differ much from sample [3
either, i.e. SWNT ropes appear damaged with about the
same average alteration rate. Surprisingly, SWNT rope
structure looks better in sample [9 than in the sample it is
derived from ([10, see Section 3.5.). It is therefore
believed that the acid treatment has somewhat purified
sample [10 of the most altered and damaged SWNTs.
3.3. Effect of annealing on acid-treated materials
Samples [4 (‘R / HNO 3 /Annealed’) and [5 (‘R /
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M. Monthioux et al. / Carbon 39 (2001) 1251 – 1272
Fig. 13. Example of more altered SWNT ropes from the mildstrength acid-treated material from PLV (sample [3). SWNT
walls only remain at the centre of the rope, surrounded by
amorphous-like material.
Fig. 14. Example of the most altered SWNT ropes from the
mild-strength acid-treated material from PLV (sample [3). Remnants of SWNT walls are seen at the centre of the rope only,
surrounded by amorphous-like material.
HNO 3 / H 2 SO 4 1HNO 3 /Annealed’) are the results of a
12008C annealing treatment of samples [2 and [3,
respectively (see Section 2.1. ‘Materials and treatments’).
The purpose of the annealing treatment was to tentatively
help the SWNTs to recover a defect-free structure, since it
was suspected that the previous acid treatments damage the
SWNTs [15], as it is confirmed here.
Both samples are very similar with a structure illustrated
by Fig. 23, though the ‘quality’ of ropes is possibly better
in the former (i.e. less amorphous material and better
structured SWNTs). Both samples were found to contain
the same components as the non-annealed materials de-
scribed above, i.e. SWNT ropes, amorphous carbon material, catalyst particles, and empty or filled carbon shells.
Also, some flakes of genuine graphite were found, certainly originating from the former graphite-based electrode. Of
course, SWNT ropes are by far the major component.
However, a microtomed cross-section of the dense paperlike ‘R / HNO 3 / H 2 SO 4 1HNO 3 /Annealed’ material obtained after filtration indicates that polyaromatic carbon
shells are very acid-resistant, making them the second
most abundant component of the material (Fig. 24).
Most of the ropes again appear internally discontinuous,
with short fringes, suggesting that SWNTs within the ropes
M. Monthioux et al. / Carbon 39 (2001) 1251 – 1272
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Fig. 15. A peculiar feature is that amorphisation of ropes is sometimes found to affect more severely one side of the ropes (open arrows).
Dark, nanometric particles (arrows) are remnants of catalyst particles, probably redeposited from the solubilized catalysts (sample [3).
are still damaged, though to a lesser extent than before
annealing. A major difference with the non-annealed
materials (specifically sample [3) is that ropes with
obvious extensive amorphisation are no longer found.
However, ropes are still coated with some poorly organised carbon material (Fig. 25).
Imaging of isolated SWNTs was easier than for the
non-annealed acid-treated sample, since they appeared
much less sensitive to electron irradiation. Fig. 26a and b
provide an example of the behaviour of an isolated SWNT
after one minute of electron irradiation. Damage appears as
local distortions of the SWNT walls after |20 s, which
increase slowly under standard illumination conditions.
Distortions are more pronounced, but not fatal after 60 s.
The most astonishing objects discovered were SWNTs
containing C 60 molecules or elongated fullerenes (Fig. 27)
[27]. Contained fullerenes can be as elongated as the
SWNTs appearing as double-wall tubes (Fig. 28), though
with the inner tube still exhibiting the diameter of a C 60
molecule (0.7 nm). These were shown to originate from
the thermally induced coalescence of former contained-C 60
[29]. The occurrence, quantification, and behaviour under
electron irradiation of endotubular (5encapsulated) fullerenes were discussed elsewhere [24,27–29,33]. Consis-
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M. Monthioux et al. / Carbon 39 (2001) 1251 – 1272
Fig. 16. Lattice fringe imaging of a polyaromatic carbon shell
surrounding a catalyst particle.
tently with the high annealing temperature, fullerite is no
longer found either by TEM or X-ray diffraction [15].
3.4. Effect of severe acid treatment
The appearance of samples [6 (‘R / HNO 3 / H 2 SO 4 1
HNO 3 /Annealed /Acid’) and [8 (‘Arc /Acid’) is similar.
The rope network morphology no longer exists and almost
the entire material has become amorphous (Fig. 29).
Amorphisation might not be complete, since the image
contrast of the material slightly differs from the wellknown ‘salt and pepper’ contrast specific to genuine
amorphous materials. In contrast, the amorphisation of the
ropes by the acids results in sphere-like contours whose
dimensions are in the range of C 60 molecules, which are
more easily seen along the edges of the former ropes (Fig.
30, arrows).
Some SWNT ropes remain, however, bridging the
amorphous masses (Fig. 31). When SWNTs within the
ropes are not totally destroyed, they appear highly altered,
with evidence of damage to the tube side-walls, leaving
them open (Fig. 32).
A common observation is the high sensitivity of the
material to electron irradiation within the microscope.
Even some SWNTs that initially appear to be in good
shape (Fig. 33a), rapidly damage under the electron beam
after some 10–20 s, with fragmentation and collapse of the
tube walls occurring (Fig. 33b), though the current density
is maintained at the standard value for imaging conditions.
Fig. 17. Low magnification view of a specific area in mildstrength acid-treated SWNT material (sample [3). Large, dark
area are fullerite crystals. Arrows show some examples of
peculiar, swollen amorphous-like nanofibres.
One discrepancy between sample [6 and [8 is the
presence of well-organised polyaromatic carbon shells in
the former, while they are absent or rare in the latter. This
is consistent with the previous description of the materials
before severe acid treatment. In contrast to the SWNTs, the
polyaromatic carbon shells have survived the acid attack or
Fig. 18. Example of electron diffraction pattern identifying the
dark grains imaged in Fig. 17 as fullerite crystals.
M. Monthioux et al. / Carbon 39 (2001) 1251 – 1272
1263
Fig. 19. Peculiar, swollen amorphous-like nanofibres (arrowed in
Fig. 17) contain traces of former SWNTs.
are just starting to oxidise and are partially damaged (Fig.
34).
3.5. Effect of DMF
Both samples [10 ‘R / HNO 3 / DMF’ and [11 (P/
HNO 3 / H 2 SO 4 1HNO 3 /Annealed / DMF’) appear as a network of entangled nano-ropes (Fig. 35), i.e. not much
different from the material they are derived from, before
suspension in DMF (see Fig. 10 for reference and comparison). However, high magnification images clearly show
that many of the ropes have turned into an amorphous
material. A large number of SWNTs remain, however, as
illustrated in Fig. 35, but they often appear segmented and
highly defective, with open walls (Fig. 36, arrows).
Degradation of the SWNT structure also affects their
Fig. 20. Lattice fringe imaging of an enlarged area from a swollen
amorphous-like nanofibre arrowed in Fig. 17. In addition to
periodic fringes whose spacing are consistent with fullerite, arrows
show circular features consistent with C 60 molecules.
appearance in cross-section, since they often no longer
appear as perfect circles (Fig. 37, to be compared with the
original SWNT cross-sections in Fig. 4). Correspondingly,
the SWNTs are also electron sensitive in these samples.
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M. Monthioux et al. / Carbon 39 (2001) 1251 – 1272
Fig. 22. Example of Fourier transform obtained from area of
swollen amorphous-like nanofibres exhibiting periodic features
like in Figs. 20 and 21. Distance 1 corresponds to |0.83 nm,
distance 2 correspond to |0.51 nm, which both are found in
fullerite.
Fig. 21. Lattice fringe imaging of an enlarged area from a swollen
amorphous-like nanofibre arrowed in Fig. 17. Another example of
lattice fringes consistent with fullerite crystals (see Fig. 22).
4. Discussion
A common observation is that the samples are heterogeneous regarding the effect of the chemicals on the rope and
SWNT structure. However, the average effect of the
various treatments can be summarised with a five-step
scale illustrating the increasing level of damage to the
SWNT-rope network (Table 1). Fig. 2, which places the
samples in a hierarchical experimental relationship and
includes their respective damage index, is also helpful.
Our HRTEM-based study shows that a final annealing
step is useful to cure the SWNT structural defects due to
acid treatment, consistent with previous statements from
the literature [15] (comparison between samples [4 or [5
and sample [2 or [3).
The intermediate strength acid-treatments improve the
average structural state of the material, supposedly by
eliminating heavily damaged SWNTs and ropes (comparison between sample [9 and sample [10).
The dramatic effect of DMF (added with an ultrasonic
treatment), which is able to degrade the rope and SWNT
structures still more severely than intermediate acid-treatments, was unexpected (comparison of samples [10 or
[11 with samples [2 or [3). However, it is likely that
the effect of DMF is enhanced by the previous acid
treatment overcome by the material. This further supports
the hypothesis of the creation of side-openings in the tube
walls under the oxidation by acids, as proposed below.
DMF would thus be able to somewhat dissolve the tubules,
starting from the pit lips created in the tube walls. Under
the assumption that the preservation of the integrity of the
SWNT structure is necessary for the subsequent use of
these materials, DMF should therefore be avoided in any
chemical process, due to its high dissolving power and the
subsequent risk to induce extensive damages from slight
structural defects.
If the severity of the treatments is controlled (temperature is an important factor), the mineral acids (HNO 3 ,
M. Monthioux et al. / Carbon 39 (2001) 1251 – 1272
1265
Fig. 23. Low magnification view of mild-strength acid treated
then annealed material (sample [5). Amorphised ropes and
fullerite-containing nanofibres are no longer found.
Fig. 24. Microtomed cross-section of mild-strength acid treated
then annealed material (sample [5), so-called ‘annealed bucky
paper’, showing that the polyaromatic carbon shells (arrows) have
survived the acidic purification treatment and are an abundant
component of the material. Carbon shells are somewhat aligned in
planes perpendicular to the direction of filtration flow.
Fig. 25. Example of SWNT rope from mild-strength acid treated
then annealed material (sample [5). SWNT structure has improved (as compared to Figs. 11–14).
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M. Monthioux et al. / Carbon 39 (2001) 1251 – 1272
Fig. 26. (a and b) Example of an isolated SWNT in mild-strength
acid treated then annealed material (sample [5) before then after
1 min of electron irradiation illustrating the stability of SWNTs to
the electron beam. Though distorted, the nanotube withstands the
irradiation.
H 2 SO 4 ) purify raw SWNT-containing material from impurities such as remnant catalyst particles and primary
amorphous carbon to some extent (samples [2 or [3).
The removal of other impurities such as polyaromatic
carbon shells requires much higher strength acid treatments, with polyaromatic shells just beginning to be
attacked when acid treatment conditions are such that
nearly complete amorphisation of the SWNT ropes is
achieved (samples [6 or [8).
Under all acid-treatment conditions tested, the acids
were found to alter the SWNT structure in a manner that
subsequent annealing could only partially reverse. Although some individual acid-treated SWNTs appear defectfree in TEM images, their defective structure or the
occurrence of some reactive reaction product at the SWNT
surface is ascertained by their sensitivity to the electron
beam, which induces them to rapidly kink, distort, and
segment within a time period of 5–20 s. These two
limitations, i.e. the inefficiency of removal of polyaromatic
shell impurities and the inevitable alteration of the SWNT
structure, support the assumption that acid treatments are
Fig. 27. Example of endotubular fullerenes (C 60 @SWNT) found
in mild-strength acid treated then annealed material (sample [5).
not the most appropriate route for a future scaled-up
purification process.
Other non-acidic, chemical procedures like controlled
oxidation by O 2 or CO 2 [34,35], or reduction by hydrogen
plasma [35] are interesting alternatives, but more adapted
to MWNT than to SWNT materials. Due to their chemical
similarity with the SWNTs, and that SWNTs are composed
Fig. 28. Example of co-axial tube (i.e. a 0.7-nm wide nanotube
contained into a regular SWNT) found in mild-strength acid
treated then annealed material (sample [5).
M. Monthioux et al. / Carbon 39 (2001) 1251 – 1272
Fig. 29. Appearance of the SWNT material after severe acidtreatment (sample [6). Ropes are highly amorphised and the rope
network morphology is nearly destroyed.
1267
of only a single graphene sheet, getting rid of the polyaromatic carbon shells via chemical routes without strongly affecting the SWNTs is probably an impossible challenge. The ideal solution to this problem would be to use
SWNT formation conditions in which carbon shell formation is not favoured. At the time of this writing, no
comprehensive investigations have been reported in the
literature. Alternatively, physical processes could be suitable. For example, attempts were already made to use heat
treatments under vacuum [36], microfiltration under pressure [37,38], centrifugation [38], etc. None of these
methods has been entirely successful, as they all have
limitations regarding efficiency, harmfulness to the
SWNTs, or selectivity. For instance, heat-treatments are
likely to enhance polyaromatisation mechanisms of amorphous carbon, promoting the formation of multi-wall
structures, and tube coalescence into larger diameter
SWNTs [39]; high power sonication was found to damage
SWNTs [40].
Several evidences reported in the Results section indicate that the degradation of SWNT structure by chemicals
includes the removal of carbon atoms from the graphenebased hexagonal lattice, resulting in the formation of
openings in the SWNT walls. This is supported by the
increase in sensitivity of SWNTs to electron beam damage
after acid (or DMF) treatment, while they are stable either
before acid treatment or when subsequently annealed after
acid treatment. Such sensitivity, which results in distortion,
Fig. 30. Evidence for circular contrasts whose some are consistent with C 60 molecules (arrows) in former SWNT ropes from SWNT
material after severe acid treatment (sample [8).
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M. Monthioux et al. / Carbon 39 (2001) 1251 – 1272
Fig. 31. Remnant ropes in SWNT material after severe acid
treatment (sample [8), bridging amorphous masses.
kinking, and ultimately segmentation of SWNTs, is hard to
explain otherwise than through the previous alteration of
the SWNT lattice. Other indirect evidence is the observed
ability of C 60 fullerenes to enter and get trapped in the
SWNT cavity. As discussed in [24], C 60 @SWNTs are
found at a concentration of 5% in acid-treated SWNTcontaining materials after annealing at 12008C, while they
are not found in raw material, nor in acid-treated, nonannealed material. This is consistent with a process in
which the acid treatment is able to promote the opening of
SWNTs, which then allows C 60 molecules, when vaporised under the effect of a subsequent thermal treatment, to
be attracted to and enter the SWNT cavity. This mechanism is further supported by recent experiments [28,29],
which have shown that, at annealing temperatures much
lower than 12008C in the presence of large amounts of
added C 60 molecules, the yield of C 60 @SWNTs is increased significantly. The low temperatures insure access
to the SWNT cavity before the openings in the tube walls
are annealed out. Considering the huge aspect ratio of the
tubes and the high efficiency of C 60 @SWNT formation in
[28], it is difficult to conceive that C 60 molecules are able
to efficiently fill the tubes only through open ends. This
mechanism presumes that only tube ends would open
under the effect of acids, due to the presence of pentagons
in the graphene structure. It is, therefore, more likely that
other openings are created along the tube walls, as directly
evidenced by HRTEM images in severely acid treated
materials like [6 or [8 (Fig. 32). An extrapolation to
lower damage levels implies the formation on as-prepared
tubes of structural defects that do not affect the projected
image of SWNTs, e.g. heptagon-pentagon pairs, or merely
random atomic vacancies. Actually, it is likely that the
rapid formation rate of SWNTs in ARC or PLV processes
induces faulted structures, since it is a common observation that fast crystal growth promotes dislocations. The
Fig. 32. SWNTs within remnants of ropes of Fig. 31 are either destroyed or altered, with visible openings in the tube side-wall (arrows).
M. Monthioux et al. / Carbon 39 (2001) 1251 – 1272
Fig. 33. (a and b) Example of an isolated SWNT in SWNT
material after severe acid treatment (sample [6) before then after
15–20 s of electron irradiation illustrating the relative sensitivity
of SWNTs to the electron beam. The SWNT is fragmented and
tube walls collapse.
observation that acid-treated SWNTs are able to regain
stability under the electron irradiation after annealing
indicates that heat treatment is able to close the openings
of the SWNTs according to a mechanism which is not yet
ascertained, e.g. through the promotion of carbon atom
rearrangement in the vicinity of the opening, inducing a
slight shortening of the tube and / or a kink if the number of
Fig. 34. Despite the severity of the acid treatment for sample [8,
polyaromatic carbon shells are just starting to be attacked. The
angle between the remnant walls on each side of the opening
suggests that the oxidation has initiated where the graphene sheets
were bent (arrow).
1269
Fig. 35. Low magnification view of a DMF treated SWNT
material (sample [11). The rope network morphology is maintained.
atoms available is not that required to complete the
hexagonal lattice, or through the migration of free carbon
to the defect.
Ultimately, alteration of SWNT ropes was found to
proceed until complete amorphisation, temporarily leaving
the fibrous morphology, then destroying it finally. An
intriguing discovery was the occurrence of periodic features in acid-treated amorphous fibrous-like objects, with
lattice distances consistent with fullerite. Since C 60 molecules were demonstrated to be widespread in the material
and have a strong affinity with the SWNT surface (see
Figs. 5 and 6, and Refs. [28,29]), it is believed that C 60
molecules are randomly bound by weak forces to the
as-prepared SWNTs at the rope surface as well as within
the inter-tube interstices. Correspondingly, no fullerite
crystals are found, either in TEM or X-ray diffraction. As
the SWNTs are altered by the acids, the C 60 loose their
support and gather together in order to lower their energy.
This process occurs while the ropes are being amorphised.
Correspondingly, clusters of C 60 , some of which exhibit
periodic features consistent with the fcc structure of
fullerite are found. Ultimately, the fullerite nano-crystals
gather together into larger fullerite crystals. Correspondingly, fullerite is evidenced by X-ray diffraction and
TEM in mid-acid-treated SWNT materials. The strange
piled-up cotton-ball morphology of some nearly amorphous ropes shown in Figs. 17 and 19, in which fullerite
clusters were found, might be the result of the surface
energetic and kinetic interactions between the transformed
material and the liquid environment in which the ropes are
immersed during the purification process. For the most
severe acid conditions in our samples, neither fullerite
crystals nor swollen nano-fibres were found. We therefore
claim that acid-based purification processes with increasing
strength are likely to promote the transient formation of
fullerite in liquid phase suspensions of nanotubes from the
pre-existing C 60 molecules. Such an effect was not observed in DMF suspensions.
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M. Monthioux et al. / Carbon 39 (2001) 1251 – 1272
Fig. 37. Example of SWNT rope ‘cross-section’ for a DMFtreated SWNT material (sample [11). SWNT cross-sections
appear distorted (to be compared with Fig. 4).
5. Conclusion
Fig. 36. Example of a SWNT rope from a DMF treated SWNT
material (sample [11). SWNTs within the ropes of Fig. 35 are
either destroyed or altered, with visible openings in the tube
side-walls (arrows). Segments of tubes seem sometimes to have
closed back. Attention has to be paid that closing back of tubes
was also found to sometimes occur under the beam (at regular
imaging conditions) from open tubes within times beyond |10 s.
The structure of single wall carbon nanotubes is increasingly altered under the effect of acid treatments with
increasing strength, up to complete amorphisation. The
progression of damages observed in the samples of this
study is best explained by a process in which damage
begins at weak links in the SWNT walls, which are most
likely pre-existent defects in the as-formed nanotubes. The
alteration of the SWNT structure mainly occurs through
the attack at the sites of the pre-existing side defects,
inducing side openings wide enough to ultimately allow
C 60 molecules to enter the tube cavity. Structural defects
and side openings can be cured to some extent by
subsequent annealing. Meanwhile, the specific conditions
of mid-strength purification processes can provoke the
transient formation of fullerite crystals from the pre-existing C 60 molecules.
SWNTs were also unexpectedly found to be sensitive to
organic solvents like DMF, possibly with ultrasonic excitation and / or previous mid-strength acid attack required,
with damage to the tubes occurring in a manner very
similar to extensive acidic oxidation. Results suggest that
transmission electron microscopy analysis should be performed before attempting any experiment or measurement
involving SWNT-based materials treated with new chemicals, either organic or mineral.
Table 1
Scale and description of damages to the SWNT materials with respect to the chemical treatment they were submitted to a
Features
Code
[
Main treatment conditions
Origin
None
Rope network is intact. SWNT structure is intact.
Isolated SWNTs are electron stable.
No fullerite is found (TEM and X-rays).
1
As-synthesised material from
laser pulverisation process
As-synthesised material from
electric arc discharge process
Rice [15]
d
Rope network is intact. SWNT structure is slightly
altered (with respect to raw material).
Isolated SWNTs are electron stable.
No fullerite is found (TEM and X-rays).
7
Montp.
4
HNO 3 1annealing
Rice
5
HNO 3 1(H 2 SO 4 1HNO 3 )
at 708C1annealing
Rice [15]
dd
Rope network is intact. Amorphisation of ropes
has started. Fullerite is found (X-rays).
2
HNO 3
Rice
ddd
Rope network is intact. A full range of ropes exhibiting
increasing amorphisation can be found.
Isolated SWNTs are slightly electron sensitive.
Fullerite is found at least is [3 (TEM and X-rays).
3
HNO 3 1(H 2 SO 4 1HNO 3 ) at 708C
Rice [15]
9
HNO 3 1DMF1(H 2 SO 4 1HNO 3 )
at 708C
Rice
Extensive amorphisation of the SWNT material. The
fibrous network morphology is maintained. Remaining SWNTs
are distorted and / or segmented, with open side walls.
Isolated SWNTs are very electron sensitive.
No fullerite is found (TEM).
10
HNO 3 1DMF
Rice [23]
11
HNO 3 1(H 2 SO 4 1HNO 3 )
at 708C1annealing1DMF
Rice [15]
1 U-Penn [24]
Extensive amorphisation of the whole material. The
fibrous network morphology is destroyed. Remaining SWNTs
are distorted and segmented with open side walls.
Isolated SWNTs are very electron sensitive.
No fullerite is found (TEM).
6
HNO 3 1(H 2 SO 4 1HNO 3 ) at 708C
1 annealing1(H 2 SO 4 1HNO 3 ) at 908C
Rice [15]
1 U-Penn [24]
8
(H 2 SO 4 1HNO 3 ) at 908C
Montp.
1 U-Penn [24]
dddd
ddddd
M. Monthioux et al. / Carbon 39 (2001) 1251 – 1272
Damage extent
a
Number of black dots increases as damage extent increases. Rice, material obtained from Professor Smalley’s laboratory at Rice University (Houston, TE, USA). Montp., starting material
obtained from Dr Bernier’s laboratory at Universite´ de Montpellier II (France). U-Penn, treated by us at the University of Pennsylvania (Philadelphia, PA, USA).
1271
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M. Monthioux et al. / Carbon 39 (2001) 1251 – 1272
Acknowledgements
Financial support was partly provided by the NSF grant
[ DMR 98-02560 (BWS, DEL), the NSF MRSEC Program, DMR96-32598 (AC), and the Department of
Energy, DOE DEFG02-98ER45701 (BB, JEF); MM was
supported by a NATO fellowship. The authors also wish to
thank Drs A. Rinzler and J. Liu (Rice University, Texas),
and P. Bernier (GDPC, France) for providing most of the
samples, Dr E. Snoeck (CEMES, France) for help in the
computerised treatment of the images, and K. Queyssette
(CEMES, France) for the photolab work.
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