Full text of DOI:10.1557/JMR.2002.0325

Formation of iodine-doped C60 whiskers by the use of
liquid–liquid interfacial precipitation method
Kun’ichi Miyazawa
Advanced Materials Laboratory, National Institute for Materials Science, Namiki 1-1,
Tsukuba, 305-0044, Japan
Koichi Hamamoto
Department of Materials Engineering, School of Engineering, University of Tokyo, 7-3-1, Hongo,
Bunkyo-ku, Tokyo, 113-8656, Japan
(Received 13 March 2002; accepted 27 June 2002)
Iodine-doped whiskers of C60 (I–C60 whiskers) with diameters ranging from
submicrometers to micrometers and lengths longer than 100 ␮m were successfully
obtained by the use of the liquid–liquid interfacial precipitation method. Transmission
electron microscopy observations showed that the I–C60 whiskers were single
crystalline and had a growth axis parallel to the close-packed direction of C60
molecules and expanded (002) lattice planes indicative of the intercalation of iodine
and oxygen atoms between the (002) planes of a body-centered-tetragonal crystal
system. The I–C60 whiskers showed nonlinear I-V curves. The electrical resistivity of
the I–C60 whiskers was more than three orders of magnitude lower than that of pristine
face-centered-cubic C60 crystals.
Since the discovery of C60 in 1985,1 tremendous ef-
forts have been made to understand its fundamental
physical and chemical properties.2,3 C60 has been inves-
tigated in various forms such as films, powders, bulk
single crystals, and composites with metals, ceramics,
and polymer matrices.4–7 Linear-shaped structures of C60
have been also reported. For example, Michaud et al.8
obtained needlelike solvated C60 crystals by slowly
evaporating solutions of C60 in 1,2-dichloroethane
(DECAN) at room temperature. The needlelike crys-
tals had a composition of C60:DECAN ס 1:1 in molar
ratio and exhibited a decagonal shape with a length of
500 ␮m and a diameter of 150 ␮m. Ce´olin et al.9 formed
hexagonal needlelike crystals of C60 with a length of
200 ␮m and a diameter of about 20 ␮m by evaporating
the dichloromethane solutions of C60. Further, Ogawa
et al.10 formed a needlelike substance with a length of
20 mm and a diameter of 100 ␮m through a slow evapo-
ration of a toluene solution of C60. This substance had a
structure composed of thin layers with thickness of about
30 nm and was found to be amorphous by transmission
electron microscopy (TEM).
nucleation of perovskite PZT crystals at a low tempera-
ture. The C60 fibers with submicrometer diameters and
lengths more than 100 ␮m were proved to be single-
crystalline whiskers of C60 and called “C60 nanowhis-
kers” (C60NWs).11
Since isopropyl alcohol was used in the PZT-C60 sol as
the solvent, we further considered that the C60 nanowhis-
kers would be produced in a system of a C60-saturated
toluene and isopropyl alcohol.12 A toluene solution with
saturated C60 was put into a glass bottle, and then iso-
propyl alcohol was gently added into the bottle to form a
liquid–liquid interface of C60–saturated toluene and iso-
propyl alcohol at a temperature of about 21 °C. Fine
whiskers of C60 nucleated at the interface, and the bottle
was aged at room temperature for 30 days to grow the
C60 whiskers. The C60 whiskers fabricated by this
“liquid–liquid interfacial precipitation (LLIP) method”
had diameters ranging from submicrometers to microme-
ters and lengths of more than 100 ␮m. The C60 whiskers
are considered to nucleate at the toluene–isopropyl alco-
hol interface with supersaturated C60 molecules. The su-
persaturated state is assumed to be caused by a loss of
toluene, which diffuses into isopropyl alcohol.
Recently, on the other hand, we accidentally dis-
covered fine fibrous structures of C60 in the course of
preparing a mixture of lead zirconate titanate (PZT) sol–
toluene with dissolved C60.11 This experiment was done
with the expectation that addition of a small amount of
C60 into a PZT sol prepared from lead acetate and al-
coxides of Zr and Ti would effectively enhance the
An example of the single-crystalline whiskers of C60 is
shown in Fig. 1(a), where we can see straight or flexibly
curved C60 whiskers with constant diameters along their
growth axes. The C60 whiskers were well dispersed in the
toluene–isopropyl alcohol solution. We also found
bundled C60 whiskers, as shown in Fig. 1(b). This result
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indicates that the flexible C60 whiskers can be spun into
a yarn. The yarns of C60 whiskers would be processed
further into various cages, nets, and cloths.
(60 K) higher than that of the tetrakis (dimethylamino)
ethylene (TDAE)-C60 and IBr–C60 systems,15 and Yue
et al. showed that the photoconductivity of a toluene
derivative of C60 increased by one order of magnitude
after iodine doping.16
The LLIP method was repeated for C70 and single-
crystalline whiskers of C70 with diameters ranging from
submicrometers to micrometers were successfully ob-
tained.13 The C70 single-crystalline (nano)whiskers had a
curved structure, and the C70 molecules polymerized
along their close-packed direction parallel to the growth
axis as well as the C60 NWs.
The second stage of our study was confined to exami-
nation of the possibility that the C60 (nano)whiskers can
incorporate impurity elements, since we found an ex-
ample in which CHBr3-intercalated C60 single crystals
with expanded lattice parameters exhibited a high-Tc
superconductivity.14
This paper shows an attempt to obtain single-
crystalline whiskers of C60 doped with impurity atoms by
the LLIP method. Iodine was selected for this purpose
because it can be easily dissolved into isopropyl alcohol,
and iodine-doped C60 crystals have been reported to ex-
hibit interesting optical and magnetic properties. For
example, Grigoryan et al. showed that iodine-doped C60
samples exhibited a magnetic transition temperature
The experimental procedure was as follows. First, a
toluene (99.5%, Wako Pure Chemical Industries, Ltd.,
Japan) solution of C60 (99%, MER Corp., Tucson, AZ)
with a concentration of 0.2 mass% C60 was prepared.
Secondly, an isopropyl alcohol (99.5%, Wako) solution
of iodine with a concentration of 2.3 mass% iodine was
prepared by ultrasonically dissolving iodine (99.9%,
Wako). A 5-mL aliquot of the toluene with dissolved C60
was poured into a 10-ml glass bottle, and a 5-ml aliquot
of the isopropyl alcohol with dissolved iodine was gently
added into the bottle. The C60 whiskers nucleated in-
stantly at the toluene–isopropyl alcohol interface when
the iodine–isopropyl alcohol solution was added into the
C60–toluene solution. This glass bottle was kept still for
two days at about 21 °C to allow the precipitates of C60
whiskers to settle on the bottom of the glass bottle,
shaken by hand to disperse the C60 whisker precipitates
that were expected to act as the seed crystals for longer
C60 whiskers, and then aged for 30 days at room tem-
perature to grow the iodine-doped C60 whiskers.
Specimens for structural investigation for TEM
(JEOL, JEM-4000FXII, 400kV, Tokyo, Japan) and scan-
ning electron microscopy (SEM; Hitachi S-4200, Tokyo,
Japan) with energy dispersive x-ray spectrometry (EDX;
Kevex Instruments, SuperDry, Scotts Valley, CA) were
prepared by placing droplets of the aged solution containing
the iodine-doped C60 (nano)whiskers on TEM microgrids.
Straight iodine-doped C60 (I–C60) whiskers were suc-
cessfully fabricated as shown in the SEM image of
Fig. 2(a). Figure 2(b) shows that the whisker had clear
habit planes. The I–C60 whiskers had diameters ranging
submicrometers to micrometers. Long I–C60 whiskers had
lengths of more than several hundreds of micrometers.
The chemical composition of an I–C60 whisker was
analyzed by EDX of the SEM and was calculated to
be C:I:O ס 74:2:24 in atomic percentage by a semi-
quantitative analytical program. The C:I atomic percent-
age ratio was calculated as C:I ס 60:1.6, and this ratio is
15
the same as the reported value of a compound C60I1.6
.
A semi-quantitative SEM-EDX analysis of a single crys-
talline C60 whisker without iodine doping showed
C:O ס 87.5:12.5 ס 1.0:0.14 in atomic percentage,
while the above I–C60 whisker had the atomic per-
centage ratio of C:O ס 1.0:0.32. This increase in the
oxygen content in the I–C60 whisker may be explained by
the expanded (002) plane spacing that will be discussed
later. A SEM-EDX analysis of the C60 powder used for
the preparation of the C60 and I–C60 whiskers showed
C:O ס 92.4:7.6 ס 1:0.08 in atomic percentage. The C60
powder must have absorbed impurity oxygen from air.
FIG. 1. (a) SEM image of single-crystalline C60 (nano)whiskers on a
paper filter. (b) SEM image of bundled C60 (nano)whiskers.
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The possible sources of oxygen contained in the C60 and
I–C60 whiskers are air and isopropyl alcohol. The oxygen
atoms are assumed to have been incorporated into the
I–C60 whiskers through a strong interaction with iodine.
Single-crystalline I–C60 whiskers with diameters less
than 1 ␮m were also fabricated as shown in the TEM
image in Fig. 3. The crystalline nature of the I–C60 whis-
ker of Fig. 3(a) is shown by the areas of differing contrast
as marked by arrows. The single crystallinity of the I–C60
whisker was confirmed by the selected-area electron dif-
fraction pattern shown in Fig. 3(b).
A high-resolution TEM observation is shown in
Fig. 4(a), which was taken at the edge of an I–C60 whis-
ker. Figure 4(b) shows a magnified image for the
enclosed area of Fig. 4(a), and the (001) and (100)
lattice-plane spacings are indicated as 2.10 nm and
FIG. 2. (a) SEM image of iodine-doped C60 whiskers. (b) Magnified
SEM image of the arrowed whisker of photo (a).
FIG. 4. (a) HRTEM image of a single-crystalline iodine-doped C60
nanowhisker taken at an underfocusing condition. (b) Enlarged image
of the enclosed area of photo (a). (c) The a (ס1.01 nm) and c
(ס2.10 nm) axes of the bct unit cell. (d) Filtered inverse FFT image
for photo (c) (d1ס2.10 nm, d2ס1.17 nm, d3ס0.93 nm). (e,f) FFT
patterns for photos (c) and (d), respectively.
FIG. 3. (a) TEM image of an iodine-doped single-crystalline C60
nanowhisker with a diameter of 280 nm, and (b) its selected-area elec-
tron diffraction pattern. Bend contours are indicated by arrows.
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analyzer, Hewlett Packard 4192A, Tulsa, OK). The elec-
trical resistivity of the iodine-doped C60 whiskers was
measured to be about 1.4 × 106, 8.1 × 105 and 1.3 × 106
⍀ cm for the whiskers with diameters of 2, 3, and 5 ␮m,
respectively. These values of electrical resistivity were
more than three orders of magnitude lower than that of
pristine C60 crystals in the range 108 to 1014 ⍀ cm.18 The
nonlinear I-V curves indicated that the doped iodine
acted as the source of carriers. A detailed study is nec-
essary to clarify the origin of the nonlinear I-V curves.
In summary, the above results demonstrate that the
LLIP method is a promising technique to fabricate
halogen-doped single-crystalline fullerene whiskers and
may be applicable to the formation of fulerene whiskers
doped with the other elements such as alkali metals.
ACKNOWLEDGMENTS
The authors are grateful to Prof. M. Kuwabara for
the use of laboratory facilities, Mr. M. Nakamura, Mr.
H. Tsunakawa, and Dr. C. Iwamoto (University of To-
kyo) for the use of SEM and high-resolution TEM.
FIG. 5. I-V curves for the iodine-doped C60 whiskers with diameters
of 2, 3, and 5 ␮m.
1.01 nm, respectively. The Miller indices are tentatively
given by a body-centered-tetragonal (bct) system whose
unit cell is indicated in the magnified image in Fig. 4(c),
with the lattice constants a ס 1.01 nm and c ס 2.10 nm
(סd1). The arrows in Fig. 4(c) show the structure image
of C60 molecules. Figure 4(d) is a filtered inverse fast
fourier transform (FFT) image for Fig. 4(c), which was
obtained by using the FFT spots of the 00n series (n ס
0, 1, 2, . . .). The (002) planes modulated and con-
sisted of the wider planes with a spacing of 1.17 nm
(סd2) and the narrower planes with a spacing of 0.93 nm
(סd3). The corresponding (002) lattice plane spacing
was 0.71 nm in face-centered-cubic (fcc) pristine C60
crystals with a lattice constant a ס 1.4166 nm.17 The
wider planes with the spacing d2 showed darker contrast
than the narrower planes with the spacing d3. Hence, it is
conjectured that more impurity atoms were incorporated
between the wider planes than the narrower planes.
The center-to-center distance between adjoining C60
molecules was 1.01 nm and very close to that of a pris-
tine C60 crystal measured along its close-packed direc-
tion (1.00 nm). The above results suggest that impurity
atoms were not interstitially incorporated between the
adjoining C60 molecules polymerized along the close-
packed direction of the I–C60 whisker. The expansion of
(002) plane spacing is of great interest with respect to
superconductivity, since higher superconducting transi-
tion temperature Tc could be obtained by increasing the
lattice constant of C60 crystals.14
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