JOURNAL OF APPLIED PHYSICS 98, 044306 共2005兲
Light emission and degradation of single-walled carbon nanotube filament
Z. G. Zhao, F. Li, C. Liu, and H. M. Chenga兲
Shenyang National Laboratory for Materials Science, Institute of Metal Research,
Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, People’s Republic of China
共Received 25 January 2005; accepted 17 June 2005; published online 18 August 2005兲
Household light bulbs were fabricated using macroscopically long and aligned single-walled carbon
nanotube 共SWNT兲 ropes as filaments. It was found that the SWNT filament could emit bright light
when an electric current was passed through it. The light spectrum from the SWNT filament showed
a nonblackbody characteristic of the thermal emission, and its infrared emission was almost
completely suppressed possibly due to the “photonic band-gap” effect that originates in the loose
fibrous bundle structure of the SWNT filament. The electrical resistance of the SWNT filament was
found to first increase, and then continually decrease during light emission. It was also found that
an electric current could cause degradation and burnout of the SWNT filament and result in
complete amorphization, and that an interesting mushroomlike carbon structure was formed due to
the carbon evaporation of the nanotube filament during light emission. © 2005 American Institute
of Physics. 关DOI: 10.1063/1.1996852兴
I. INTRODUCTION
Carbon nanotubes 共CNTs兲 have been a fascinating subject for both scientific and technological researches.1 Their
unique structure and remarkable mechanical properties,2 high
current carrying ability,3 ballistic electron transport,4 and
field-emission performance5 have opened ways to a number
of possible applications such as field-emission devices,5
interconnectors,6 sensors,7 supercapacitors,8 and battery
electrodes.9 Recently, Wei et al.10 have made a household
light bulb in which the conventional tungsten filament was
replaced by a macroscopically long CNT bundle. The nanotube filament bulbs were found to possess several interesting
advantages over traditional tungsten filament bulbs, such as
lower threshold voltage for light emission and higher brightness at high voltages. However, the overall characteristics of
nanotube bulb filaments, including light spectra, resistance
variation tendency, and the degradation mechanism during
light emission, must be understood for their practical application.
In this paper, light bulbs using macroscopically long and
aligned single-walled carbon nanotube 共SWNT兲 ropes synthesized by an arc discharge method were constructed. The
emitted light spectra were analyzed and compared with the
Planck blackbody radiation distribution. The resistance
variation during light emission was also obtained by the
measurements of two-point current-voltage characteristics of
SWNT filaments. The structural transformation and degradation mechanisms of the SWNT filament during light emission were investigated by scanning electron microscopy
共SEM兲, high-resolution transmission electron microscopy
共HRTEM兲, and Raman spectroscopy.
discharge method.11 Long ropes of SWNT bundles with rope
lengths up to 200 mm could be isolated in the product. The
collected SWNT ropes were lightweight and freestanding.
One of the distinct characteristics of the SWNT ropes is hierarchy structure at different scales, such as isolated SWNTs,
SWNT bundles, and fibers of SWNT bundles. The SEM observations showed that individual SWNTs may form closely
packed bundles 共30–50 nm兲, bundles loosely into fibers 共several hundred nanometers to a few micrometers兲, and fibers
eventually into macroscopic ropes 共50–200 ␮m兲 关Fig.
1共a兲兴.12 These as-grown long ropes of aligned SWNTs were
easily handled and manipulated to make bulb filaments.
The nanotube filaments were connected to electrodes by
conductive carbon paste and sealed in a glass bulb which
was pumped to a vacuum of 5 ⫻ 10−3 Pa. The light of the
SWNT filaments was collected by a charge-coupled device
共CCD兲 detector and the emission spectra of SWNT filaments
were obtained using a micro-Raman system as a spectrometer, which was configured for emission studies between 300
and 1000 nm.
II. EXPERIMENT
Macroscopically long ropes of aligned SWNTs used in
our work were synthesized by the hydrogen and argon arc
a兲
Author to whom correspondence should be addressed; electronic mail:
cheng@imr.ac.cn
0021-8979/2005/98共4兲/044306/4/$22.50
FIG. 1. 共Color online兲 共a兲 SEM image of the as-prepared SWNT ropes,
showing that a SWNT rope is made up of many parallel thinner fibers.
关共b兲–共d兲兴 SWNT filament bulbs emitting incandescent light, whose light
color shifts from orange to white.
98, 044306-1
© 2005 American Institute of Physics
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044306-2
J. Appl. Phys. 98, 044306 共2005兲
Zhao et al.
FIG. 2. 共a兲 Comparison of the experimentally obtained emission spectra of
a SWNT filament induced by an electrical current under different input
power conditions with the curves of blackbody radiation. The light intensity
of the actual spectra and fitted spectra is multiplied by the scale factor. 共b兲
Plot of transmission factor vs wavelength. 共c兲 Plot of the fitted temperature
共T兲 vs input power 共P兲.
III. RESULTS AND DISCUSSION
A. Light-emission effect
Figures 1共b兲–1共d兲 show an illuminating SWNT bulb at
different input powers. The SWNT filament emitted incandescent light evenly along its entire length. It was observed
that the light color gradually varied from orange to white
with increasing input power, indicating a blueshift of visible
spectra 关Figs. 1共b兲–1共d兲兴. At the same time, the light intensity
was also greatly increased with increasing input power. The
SWNT light bulb reached the maximum input power of 35
W before its burnout in our experiment.
The change of the intensity and wavelength of visible
light with input power was also confirmed by the collected
emission spectra. Figure 2共a兲 shows the spectra recorded at
different input powers. The emission spectra are continuous
and featureless over the entire frequency. With the increase
in input power from 1.02 to 9.55 W, the intensity dramatically increased to 8273 times and the peak position shifted
from 836 to 740 nm. Noticeably, infrared emission was
found to be almost completely suppressed in Fig. 2共a兲, indicating a nonblackbody character of the thermal emission
from the SWNT filament. We suggest that the almost complete suppression of infrared emission is due to a “photonic
band-gap” effect that originates in the loose fibrous bundle
structure of the SWNT filament. Kempa et al. reported that
the periodic arrays of aligned CNTs fabricated on Ni dots can
produce photonic band gaps in or around the visible frequency range that are spectrum regions in which the electromagnetic wave propagation is forbidden.13 The aligned
SWNT filament in air background can also be assumed to act
as a two-dimensional photonic band-gap crystal due to the
periodic variation of the dielectric constant. The fibers of the
SWNT bundles in the SWNT filament, which are at a submicrometer length scale 共several hundred nanometers to a
few micrometers兲, can play the role of the primitive cell in a
photonic band-gap crystal. Since the gap position was the
only dependent on the dielectric constant in the gap equation,
Kempa et al.13 also estimated the gap to be centered at ␭
⬇ 500 nm by simply scaling the results obtained by Cassagne et al.14,15 In the same way, the radiation wavelength of
the gap in our SWNT filament can be roughly estimated to
be in or around the infrared frequency range 共several hundred nanometers to a few micrometers兲. If this is the case, on
the short-wavelength side of the emission peak, the emission
spectrum will approach that of a blackbody and its shape can
be used to determine the temperature of the filament, and on
the long-wavelength side of the spectrum, the emission will
be suppressed by the inability of wavelengths longer than a
critical value to propagate through the photonic band-gap
structure.
Figure 2共a兲 also shows that on the short-wavelength side
of the emission peak, the experimentally obtained curves can
be fitted with the curves of blackbody radiation. It was found
that the experimentally obtained curves for higher temperature fit better with the blackbody radiation distribution than
those for lower temperature. In our experiment, color temperatures ranging from 1000 to 1700 K were obtained using
the above fitting. If emissivity is independent of wavelength,
color temperature and true temperature are the same. So the
true temperature of SWNT filaments is estimated to be
1000–1700 K. Figure 2共b兲 was obtained by dividing the actual spectral emissions of Fig. 2共a兲 by the blackbody emissions, and plotting the results as a “transmission factor.” The
shapes of the curves for different temperatures are almost the
same, especially the curves for higher temperature. This
lends support to the hypothesis that the suppression of infrared emission is a purely geometric effect and is approximately independent of temperature. The breadth of the “cutoff” profile is consistent with the expectation that there is a
random distribution of spacings between fibers of SWNT
bundles, and therefore a range of cutoff wavelengths. Moreover, when the fitted temperature 共T兲 was plotted with input
power 共P兲, it was confirmed that the relationship between T
and P is as follows 关Fig. 2共c兲兴:
P ⬀ T2 .
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044306-3
Zhao et al.
FIG. 3. 共a兲 Raman spectra of the SWNT filament before 共curve a兲 and after
light emission 共curves b and c兲. 关共b兲 and 共c兲兴 SEM images of the SWNT
filament after the structural transformation during light emission showing
that a high density of spherical amorphous carbon particles were formed. 共d兲
TEM images of the SWNT filament after the structural transformation during light emission. The inset shows a double-walled carbon nanotube
共DWNT兲 with an outer diameter of 5.6 nm.
B. Structure degradation
Raman spectroscopy was employed to identify the structural transformation of the SWNT filament with an excitation
wavelength of 632.8 nm 关Fig. 3共a兲兴. The pristine sample
共curve a兲 shows the following characteristic SWNT peaks:
radial breathing mode 共RBM兲 共100–300 cm−1兲 , G mode
共1592 cm−1兲, and a very low intensity D band 共1320 cm−1兲.
The main effects of light emission on the evolution of the
Raman spectrum of SWNT filaments are as follows 共curves b
and c兲: 共i兲 In curve c, RBM has almost totally disappeared
indicating degradation and burnout of SWNTs after light
emission. In curve b, though the RBM peak can still be dis-
J. Appl. Phys. 98, 044306 共2005兲
cerned, the intensity of RBM decreases drastically, which
shows that some SWNTs still remained in the sample. 共ii兲
The G band, which is related to the graphite E2g symmetric
interlayer mode,16 decreases drastically in intensity and shifts
from 1592 to 1583 and 1588 cm−1, respectively, for curves b
and c. 共iii兲 The D band, which is characteristic of amorphous
carbon or defects, increases and shifts upwardly from 1320
to 1331 cm−1, indicating structure deformation of SWNTs
after light emission.17 Some similar changes of the Raman
spectrum were also observed in those SWNT samples by
acid or heat treatment.17,18
In Figs. 3共b兲 and 3共c兲, the SEM images show the structural transformation of the SWNT filament induced by an
electric current, which corresponds to Raman spectrum curve
c. It can be observed that numerous spherical particles with a
diameter of 2–5 ␮m are aggregated. The HRTEM observation shows that these spherical particles are mainly made up
of amorphous carbon with randomly embedded catalyst particles 关Fig. 3共d兲兴. Only a few isolated short multiwalled carbon nanotubes 共MWNTs兲 having two or three layers with
larger diameters 共2–6 nm兲 were observed 关the inset in Fig.
3共d兲兴, and no SWNT bundles remained. Since these MWNTs
are never observed in our pristine samples, we believe that
they should result from the coalescence of SWNTs packed in
bundles during the emission. Yudasaka et al.18 has observed
the transformation of SWNT bundles to a multiwalled structure and diameter enlargement induced by heat treatment.
The above observations indicate that, under the condition of
high temperature caused by an electric current, the SWNT
structure will collapse and many disordered carbon clusters
of different sizes will be formed 共amorphization兲.19 However, it seems that the carbon clusters tend to aggregate forming larger spherical particles 共2–5 ␮m兲.
The incomplete structural transformation of the SWNT
filament induced by an electric current, which corresponds to
the Raman spectrum curve b, was also observed. Figure 4共a兲
shows an interesting mushroomlike structure formed during
light emission. This special structure was composed of residual SWNT bundles and those newly transformed catalystembedded amorphous carbon spherical particles attached to
the surface of SWNT bundles. The formation of this mushroomlike carbon structure may be due to the carbon evaporation of the nanotube filament. It was also found that a layer
of carbon was deposited on the interior of the glass bulb,
which indicates that the SWNT filament did suffer from carbon evaporation during light emission. Carbon vapor arising
from the evaporation of carbon atoms can blow off some
SWNT bundles from the SWNT filament to form the mushroomlike structure.
C. Unique electrical resistance change
Normalized electrical resistance R共P兲 / R0 共R0 is the electrical resistance at room temperature兲 versus input power is
shown in Fig. 5, which was obtained by measurements of
two-point current-voltage characteristics of the SWNT filament. At the initial low input power stage, it can be seen that
the resistance of the SWNT rope is rapidly increased, which
seems to indicate that the resistance of the SWNT filament is
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044306-4
J. Appl. Phys. 98, 044306 共2005兲
Zhao et al.
SWNT filament during light emission. As mentioned above,
a high density of spherical amorphous carbon clusters were
formed during light emission, which could produce new conductive paths and increase the mean free path of electrons.
We believe that these should be responsible for the resistance
decrease of the SWNT filament in the light-emission process.
IV. CONCLUSIONS
FIG. 4. SEM images of the SWNT filament after the incomplete structural
transformation during light emission, showing a mushroomlike structure
consisting of the remaining SWNT bundles and newly formed spherical
amorphous carbon particles.
increased with increasing temperature, typical of metallic
conductivity. However, beyond a threshold input power, the
resistance is sharply decreased and then continually decreased. It is not reasonable to consider that this resistance
decrease originated from the semiconducting behavior of the
SWNT filament itself at such high temperatures, which was
proposed by Li et al.20 After decreasing input power to zero,
the resistance of the SWNT filament was measured again. It
was found that the resistance was not restored to the initial
value at room temperature, but remained a very low value
关R共P兲 / R0 = 0.05兴. This fact indicates that the sharp resistance
decrease was caused by the structural transformation of the
FIG. 5. Change of two-point normalized electrical resistance of the SWNT
filament with input power. R0 is the resistance of the SWNT filament at
room temperature.
The SWNT light bulb based on the as-prepared aligned
SWNT ropes synthesized using an arc discharge method was
constructed to explore its light-emission characteristics, and
the SWNT filaments showed a nonblackbody characteristic
of thermal emission. The almost complete suppression of
infrared emission was observed in the emission spectra,
which may be due to a “photonic band-gap” effect that originates in the loose fibrous bundle structure of the SWNT filament. It was also found that an electric current can disintegrate the SWNT walls and result in complete amorphization
of SWNTs, which was responsible for the degradation and
burnout of SWNT filaments. In the case of incomplete structural transformation of the SWNT filament induced by an
electric current, an interesting mushroomlike carbon structure was observed. The electrical resistance of the SWNT
filament was first increased and soon followed by a continuous decrease during light emission because of the structure
transformation of the SWNT filament.
ACKNOWLEDGMENT
This work was supported by National Natural Science
Foundation of China 共Nos. 50025204 and 50202013兲.
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