APPLIED PHYSICS LETTERS
VOLUME 84, NUMBER 24
14 JUNE 2004
Carbon nanotube filaments in household light bulbs
Jinquan Wei,a) Hongwei Zhu, and Dehai Wu
Department of Mechanical Engineering, Tsinghua University, Beijing 100084, People’s Republic of China
Bingqing Weib)
Department of Electrical and Computer Engineering and Center for Computation and Technology,
Louisiana State University, Baton Rouge, Louisiana 70803
共Received 26 January 2004; accepted 18 April 2004; published online 25 May 2004兲
Household light bulbs made from macroscopic single-walled and double-walled carbon nanotube
filaments were fabricated and tested. The nanotube bulbs are found to possess several interesting
features when compared to a conventional tungsten filament in safelight 共36 V, 40 W兲, such as lower
threshold voltage for light emission and higher brightness at high voltages. Electrically induced
excited peaks at 407, 417, 655 nm were identified to be an intrinsic property of nanotubes and these
peaks are observed to become stronger in the light emission spectra at high temperatures which
cannot be explained easily with the concept of blackbody emission. © 2004 American Institute of
Physics. 关DOI: 10.1063/1.1762697兴
Carbon nanotubes 共CNTs兲 have attracted scientific interest due to their unique and outstanding electrical and mechanical properties and their diverse areas of application.1,2 It
has been proved that an individual multiwalled CNT could
carry a current density as high as 109 A/cm2 , which is about
1000 times larger than that of copper, the most popular conductor used in electronic industry.2– 4 CNTs are thus expected
to be one of the most promising candidates in future electronic industry. Extensive efforts have been made to develop
electronic devices based on CNTs and many significant
achievements have been reported.5–9 The tendency of CNTs
to emit light during electron field emission has also been
investigated.10–14 Very recently, it was reported that CNTs
could emit fluorescent light when they were excited by a
laser15,16 of a certain wavelength. Electrically induced optical
emission from a carbon nanotube field effect transistor has
also been addressed.17 Nanotubes could even emit incandescent light when current was allowed to pass through a CNT
bundle.18 One might still remember Thomas Edison’s experiment to make an incandescent lamp using a carbon filament
in a high-vacuum enclosure made of glass. However, the
carbon bulb filaments were very fragile and the bulbs darkened rapidly as a result of the deposition of carbon on the
glass envelop and suffered easily from premature burnout. In
this letter, we report a simple approach for making nanotube
bulbs: the fabrication and testing of household bulb products
made from CNTs, forms of carbon filaments with nanoscale
building blocks. Particularly, the nanotube filaments showed
a lower threshold voltage for operation and higher brightness
at high voltages when compared to tungsten filaments. The
light emission spectra of the carbon nanotubes cannot be
explained using the blackbody radiation concept, especially
at high temperatures.
Single-walled carbon nanotube 共SWNT兲 strands with a
length of about 20 cm and double-walled carbon nanotube
共DWNT兲 films were prepared using an improved chemical
a兲
Electronic mail: jqwei99@mails.tsinghua.edu.cn
Electronic mail: weib@ece.lsu.edu
b兲
vapor deposition method.19,20 The experimental results of
microscopy, micro-Raman and small angle x-ray diffraction
techniques show that the strands are large collections of
well-aligned nanotube bundles, which consist of wellarranged single-walled nanotubes in two-dimensional triangular lattice.21 Our new approach for producing macroscopic
SWNT strands enables us to test the bulk properties of nanotube structures. These very long crystalline strands of
SWNTs could be handled and manipulated easily and macroscopically which greatly facilitated making bulb filaments.
SWNT strands and DWNT films were first immersed in
alcohol, and then assembled into long filaments under the
surface tension when the alcohol evaporated. Tungsten filaments of safelight 共36 V, 40 W兲 were then replaced by the
nanotube filaments. The nanotube filaments were connected
to the electrodes using silver sol and sealed in a glass bulb
after a high vacuum 共above 10⫺7 Torr) was achieved inside
the enclosure.
Figure 1共a兲 shows an illuminating SWNT bulb in contrast to a safelight 共using tungsten filament兲 at the same applied voltage of 20 V. The SWNT filament emits incandescent light evenly along its entire length and has a resistance
of about 18.2 ⍀ at room temperature, while the tungsten
filament has a resistance of about 3 ⍀. The luminance of the
SWNT bulb chimes with that of the safelight from Fig. 1.
It is necessary to point out that all the nanotube filaments
used in the experiments show lower threshold voltage for
incandescent light emission 关as illustrated in Fig. 1共b兲兴 than
the tungsten filament. For instance, a DWNT filament with a
resistance of about 9 ⍀ begins to emit incandescent light at 3
V, a SWNT filament 共18.2 ⍀兲 begins to emit at 5 V, while
tungsten filament 共3 ⍀兲 begins to emit at 6 V. Figure 1共b兲
shows the comparison of irradiance intensity of the DWNT
and the tungsten filaments as a function of voltage. The intensity of irradiance of nanotube bulbs and safelight are recorded by a light meter, keeping the distance between the
bulb and the light meter the same for both cases. It is observed that the nanotube bulbs have lower threshold voltage
than the tungsten bulb 关Fig. 1共b兲兴. The irradiance intensity of
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© 2004 American Institute of Physics
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Appl. Phys. Lett., Vol. 84, No. 24, 14 June 2004
Wei et al.
FIG. 2. Light emission spectra of a DWNT filament at different temperatures. Peaks at 407, 417, and 655 nm can easily be identified and become
stronger as the temperature increases. The blackbody radiation spectra at
1350 and 1600 K are drawn for comparison.
FIG. 1. 共a兲 A SWNT bulb made from SWNT filament compared with a
tungsten bulb operated at same voltage 共20 V兲. The nanotube bulb shows a
high brightness and reliability. 共b兲 Irradiance of nanotube filaments as a
function of voltage. The DWNT filament 共9 ⍀兲 shows a low onset voltage
共marked arrow兲 for the light emission and emits stronger light than tungsten
filament 共3 ⍀兲 at the same voltage.
the nanotube bulb increases quickly with increase in voltage.
In addition, the irradiance intensity of the nanotube filament
is much stronger than that of the tungsten, indicating that the
nanotube filaments can emit more visible light than tungsten
at the same applied voltage.
Light emission spectra of DWNT filaments at different
temperatures are plotted in Fig. 2. Temperatures were measured with a pyrometer and compared with the data obtained
from curve fitting based on the blackbody radiation. It shows
that the emission spectra of the nanotube filaments present a
blueshift as the temperature increases. This phenomenon is
quite similar to the blackbody emission and the spectra at
low temperature (⬍1250 K) can be fitted with blackbody
emission. However, at higher temperatures, nanotube filaments emit stronger visible light than the blackbody, based
on our experimental results. It is worth mentioning that three
peaks at 407, 417, and 655 nm in the visible light region
could be identified. These peaks become stronger as electric
current increases, indicating an electrically induced light
emission property, similar to the fluorescence emission when
excited by a laser.15 These same peaks at the same wavelengths could be identified even for a SWNT filament, indicating that the peaks at about 407, 417, and 655 nm in the
emission spectra are intrinsic properties of the carbon nano-
tubes, which can be triggered by different excitation sources
such as light excitation and electric excitation.
Two different models10–14 have been proposed to support
light emission induced by electron emission from nanotubes,
the blackbody radiation due to resistive Joule heating10,13,14
and photon emission caused by electron transitions between
different electronic levels.11,12 We found that at 1250 K and
below the emission spectra can be explained with the blackbody radiation model, indicating that the light emission from
the nanotubes is mainly due to joule heating at low temperatures. However, at high temperatures, the nanotube filament
presents electro-luminescence behavior; the peaks appearing
at 407, 417, and 655 nm as shown in the spectrum in Fig. 2.
The higher the temperature, the higher the intensity of the
peaks. In addition, at these high temperatures the emission
spectra cannot be explained with the blackbody radiation
model anymore. We proposed that the apparent light spectrum emitted from nanotube filaments is a combination of the
joule heating and the electro-luminescence effect at high
temperatures.
We measured I – V curves of the carbon nanotube bulbs
using two-probe method 关see Fig. 3共a兲兴. The I – V curves of
DWNTs and SWNTs filaments are linear, indicating an
ohmic behavior and a constant resistance of the nanotube
filaments. However, at high voltages of operation 共e.g.,
⬎10 V) the emissive power of the carbon nanotube bulbs is
much higher than that of the tungsten bulb, indicating a
higher efficiency in electrical power consumption of the
nanotube bulbs when compared to the tungsten bulb.
The electrical transport properties of carbon nanotubes
were evaluated at low as well as elevated temperatures 共less
than 300 °C) and both metallic 共electrical resistance increases as temperature increases兲 and semiconducting 共electrical resistance decreases as temperature increases兲 behavior
was reported. However, the temperature dependence of the
electrical resistance of the nanotube filaments at higher temperatures (⬎1000 K) was measured and was found to show
a very different feature 关see Fig. 3共b兲兴. It is surprising that
the resistance of nanotube filaments 共both SWNTs and
DWNTs兲 remains invariant to changes in temperature in a
very large temperature range 共1000–1750 K兲 when com-
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Wei et al.
Appl. Phys. Lett., Vol. 84, No. 24, 14 June 2004
4871
In conclusion, we have fabricated and tested the nanotube bulbs using SWNTs and DWNTs as bulb filaments. The
nanotube bulbs have lower threshold voltage and higher
brightness at the same voltage when compared to the conventional tungsten safelight. The emitted light spectrum at
high temperatures is a combination of blackbody radiation
and electro-luminescence. The carbon nanotube filaments
which are observed to show invariance to changes in temperature at very high temperatures could be used as resistors
in high temperature applications. A household bulb made out
of carbon nanotube filaments is expected in the very near
future.
This work is financially supported by MOST under the
State Key Project for Fundamental Research, Grant No.
G20000264-04. B.Q.W. acknowledges financial support from
LSU Council on Research.
1
FIG. 3. 共a兲 I – V curve comparison of a typical SWNT and a DWNT bulb, as
well as a safelight bulb 共36 V, 40 W兲. 共b兲 Temperature dependence of filament resistance at high temperature. The nanotube filaments 共SWNT and
DWNT兲 show resistance stability over a large temperature region.
pared to a tungsten filament, which shows a typical metallic
behavior as expected. It is crucial to understand the mechanism governing this phenomenon and is under investigation.
The stable resistance of the nanotubes at high temperatures
promises that SWNTs and DWNTs filaments could be used
as highly precise resistors at high temperatures.
We have also investigated the durability of the nanotube
filament bulbs by turning them on and off repeatedly. They
worked well even after being turned on and off for more than
5000 times 共7 V, 2.3A兲, showing great stability of SWNT
filaments. In addition, the nanotube filament could continuously withstand operation at 25 V (⬃1400 K) for more than
360 h without a visible evaporation. The DWNT filaments
however need to be purified in order to remove the amorphous carbon and catalyst content from them before preparing the bulb filaments. This will avoid gradual premature
darkening of the bulbs at high temperatures 共above 1300 K兲
due to the evaporation of amorphous and other disordered
carbon.
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