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Carbon nanotube filaments in household light bulbsJinquan Wei, Hongwei Zhu, Dehai Wu, and Bingqing Wei Citation: Applied Physics Letters 84, 4869 (2004); doi: 10.1063/1.1762697 View online: http://dx.doi.org/10.1063/1.1762697 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/84/24?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Stefan–Boltzmann law for the tungsten filament of a light bulb: Revisiting the experiment Am. J. Phys. 81, 512 (2013); 10.1119/1.4802873 Low voltage energy-saving double-walled carbon nanotube electric lamps J. Appl. Phys. 101, 084306 (2007); 10.1063/1.2717557 Spark light radiation coupled with the field electron emission from carbon nanotube forests J. Appl. Phys. 100, 044327 (2006); 10.1063/1.2335780 Light emission and degradation of single-walled carbon nanotube filament J. Appl. Phys. 98, 044306 (2005); 10.1063/1.1996852 Polarized incandescent light emission from carbon nanotubes Appl. Phys. Lett. 82, 1763 (2003); 10.1063/1.1558900
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Carbon nanotube filaments in household light bulbsJinquan Wei,a) Hongwei Zhu, and Dehai WuDepartment 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
~Received 26 January 2004; accepted 18 April 2004; published online 25 May 2004!
Household light bulbs made from macroscopic single-walled and double-walled carbon nanotubefilaments were fabricated and tested. The nanotube bulbs are found to possess several interestingfeatures when compared to a conventional tungsten filament in safelight~36 V, 40 W!, such as lowerthreshold voltage for light emission and higher brightness at high voltages. Electrically inducedexcited peaks at 407, 417, 655 nm were identified to be an intrinsic property of nanotubes and thesepeaks are observed to become stronger in the light emission spectra at high temperatures whichcannot be explained easily with the concept of blackbody emission. ©2004 American Institute ofPhysics. @DOI: 10.1063/1.1762697#
Carbon nanotubes~CNTs! have attracted scientific inter-est due to their unique and outstanding electrical and me-chanical properties and their diverse areas of application.1,2 Ithas been proved that an individual multiwalled CNT couldcarry a current density as high as 109 A/cm2, which is about1000 times larger than that of copper, the most popular con-ductor used in electronic industry.2–4 CNTs are thus expectedto be one of the most promising candidates in future elec-tronic industry. Extensive efforts have been made to developelectronic devices based on CNTs and many significantachievements have been reported.5–9 The tendency of CNTsto emit light during electron field emission has also beeninvestigated.10–14 Very recently, it was reported that CNTscould emit fluorescent light when they were excited by alaser15,16of a certain wavelength. Electrically induced opticalemission from a carbon nanotube field effect transistor hasalso been addressed.17 Nanotubes could even emit incandes-cent light when current was allowed to pass through a CNTbundle.18 One might still remember Thomas Edison’s experi-ment to make an incandescent lamp using a carbon filamentin a high-vacuum enclosure made of glass. However, thecarbon bulb filaments were very fragile and the bulbs dark-ened rapidly as a result of the deposition of carbon on theglass envelop and suffered easily from premature burnout. Inthis letter, we report a simple approach for making nanotubebulbs: the fabrication and testing of household bulb productsmade from CNTs, forms of carbon filaments with nanoscalebuilding blocks. Particularly, the nanotube filaments showeda lower threshold voltage for operation and higher brightnessat high voltages when compared to tungsten filaments. Thelight emission spectra of the carbon nanotubes cannot beexplained using the blackbody radiation concept, especiallyat high temperatures.
Single-walled carbon nanotube~SWNT! strands with alength of about 20 cm and double-walled carbon nanotube~DWNT! films were prepared using an improved chemical
vapor deposition method.19,20 The experimental results ofmicroscopy, micro-Raman and small angle x-ray diffractiontechniques show that the strands are large collections ofwell-aligned nanotube bundles, which consist of well-arranged single-walled nanotubes in two-dimensional trian-gular lattice.21 Our new approach for producing macroscopicSWNT strands enables us to test the bulk properties of nano-tube structures. These very long crystalline strands ofSWNTs could be handled and manipulated easily and mac-roscopically which greatly facilitated making bulb filaments.
SWNT strands and DWNT films were first immersed inalcohol, and then assembled into long filaments under thesurface tension when the alcohol evaporated. Tungsten fila-ments of safelight~36 V, 40 W! were then replaced by thenanotube filaments. The nanotube filaments were connectedto the electrodes using silver sol and sealed in a glass bulbafter a high vacuum~above 1027 Torr) was achieved insidethe enclosure.
Figure 1~a! shows an illuminating SWNT bulb in con-trast to a safelight~using tungsten filament! at the same ap-plied voltage of 20 V. The SWNT filament emits incandes-cent light evenly along its entire length and has a resistanceof about 18.2V at room temperature, while the tungstenfilament has a resistance of about 3V. The luminance of theSWNT bulb chimes with that of the safelight from Fig. 1.
It is necessary to point out that all the nanotube filamentsused in the experiments show lower threshold voltage forincandescent light emission@as illustrated in Fig. 1~b!# thanthe tungsten filament. For instance, a DWNT filament with aresistance of about 9V begins to emit incandescent light at 3V, a SWNT filament~18.2 V! begins to emit at 5 V, whiletungsten filament~3 V! begins to emit at 6 V. Figure 1~b!shows the comparison of irradiance intensity of the DWNTand the tungsten filaments as a function of voltage. The in-tensity of irradiance of nanotube bulbs and safelight are re-corded by a light meter, keeping the distance between thebulb and the light meter the same for both cases. It is ob-served that the nanotube bulbs have lower threshold voltagethan the tungsten bulb@Fig. 1~b!#. The irradiance intensity of
a!Electronic mail: [email protected]!Electronic mail: [email protected]
APPLIED PHYSICS LETTERS VOLUME 84, NUMBER 24 14 JUNE 2004
48690003-6951/2004/84(24)/4869/3/$22.00 © 2004 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 130.209.6.50
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the nanotube bulb increases quickly with increase in voltage.In addition, the irradiance intensity of the nanotube filamentis much stronger than that of the tungsten, indicating that thenanotube filaments can emit more visible light than tungstenat the same applied voltage.
Light emission spectra of DWNT filaments at differenttemperatures are plotted in Fig. 2. Temperatures were mea-sured with a pyrometer and compared with the data obtainedfrom curve fitting based on the blackbody radiation. It showsthat the emission spectra of the nanotube filaments present ablueshift as the temperature increases. This phenomenon isquite similar to the blackbody emission and the spectra atlow temperature (,1250 K) can be fitted with blackbodyemission. However, at higher temperatures, nanotube fila-ments emit stronger visible light than the blackbody, basedon our experimental results. It is worth mentioning that threepeaks at 407, 417, and 655 nm in the visible light regioncould be identified. These peaks become stronger as electriccurrent increases, indicating an electrically induced lightemission property, similar to the fluorescence emission whenexcited by a laser.15 These same peaks at the same wave-lengths could be identified even for a SWNT filament, indi-cating that the peaks at about 407, 417, and 655 nm in theemission spectra are intrinsic properties of the carbon nano-
tubes, which can be triggered by different excitation sourcessuch as light excitation and electric excitation.
Two different models10–14have been proposed to supportlight emission induced by electron emission from nanotubes,the blackbody radiation due to resistive Joule heating10,13,14
and photon emission caused by electron transitions betweendifferent electronic levels.11,12 We found that at 1250 K andbelow the emission spectra can be explained with the black-body radiation model, indicating that the light emission fromthe nanotubes is mainly due to joule heating at low tempera-tures. However, at high temperatures, the nanotube filamentpresents electro-luminescence behavior; the peaks appearingat 407, 417, and 655 nm as shown in the spectrum in Fig. 2.The higher the temperature, the higher the intensity of thepeaks. In addition, at these high temperatures the emissionspectra cannot be explained with the blackbody radiationmodel anymore. We proposed that the apparent light spec-trum emitted from nanotube filaments is a combination of thejoule heating and the electro-luminescence effect at hightemperatures.
We measuredI –V curves of the carbon nanotube bulbsusing two-probe method@see Fig. 3~a!#. The I –V curves ofDWNTs and SWNTs filaments are linear, indicating anohmic behavior and a constant resistance of the nanotubefilaments. However, at high voltages of operation~e.g.,.10 V) the emissive power of the carbon nanotube bulbs ismuch higher than that of the tungsten bulb, indicating ahigher efficiency in electrical power consumption of thenanotube bulbs when compared to the tungsten bulb.
The electrical transport properties of carbon nanotubeswere evaluated at low as well as elevated temperatures~lessthan 300 °C) and both metallic~electrical resistance in-creases as temperature increases! and semiconducting~elec-trical resistance decreases as temperature increases! behaviorwas reported. However, the temperature dependence of theelectrical resistance of the nanotube filaments at higher tem-peratures (.1000 K) was measured and was found to showa very different feature@see Fig. 3~b!#. It is surprising thatthe resistance of nanotube filaments~both SWNTs andDWNTs! remains invariant to changes in temperature in avery large temperature range~1000–1750 K! when com-
FIG. 1. ~a! A SWNT bulb made from SWNT filament compared with atungsten bulb operated at same voltage~20 V!. The nanotube bulb shows ahigh brightness and reliability.~b! Irradiance of nanotube filaments as afunction of voltage. The DWNT filament~9 V! shows a low onset voltage~marked arrow! for the light emission and emits stronger light than tungstenfilament ~3 V! at the same voltage.
FIG. 2. Light emission spectra of a DWNT filament at different tempera-tures. Peaks at 407, 417, and 655 nm can easily be identified and becomestronger as the temperature increases. The blackbody radiation spectra at1350 and 1600 K are drawn for comparison.
4870 Appl. Phys. Lett., Vol. 84, No. 24, 14 June 2004 Wei et al.
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pared to a tungsten filament, which shows a typical metallicbehavior as expected. It is crucial to understand the mecha-nism governing this phenomenon and is under investigation.The stable resistance of the nanotubes at high temperaturespromises that SWNTs and DWNTs filaments could be usedas highly precise resistors at high temperatures.
We have also investigated the durability of the nanotubefilament bulbs by turning them on and off repeatedly. Theyworked well even after being turned on and off for more than5000 times~7 V, 2.3A!, showing great stability of SWNTfilaments. In addition, the nanotube filament could continu-ously withstand operation at 25 V (;1400 K) for more than360 h without a visible evaporation. The DWNT filamentshowever need to be purified in order to remove the amor-phous carbon and catalyst content from them before prepar-ing the bulb filaments. This will avoid gradual prematuredarkening of the bulbs at high temperatures~above 1300 K!due to the evaporation of amorphous and other disorderedcarbon.
In conclusion, we have fabricated and tested the nano-tube bulbs using SWNTs and DWNTs as bulb filaments. Thenanotube bulbs have lower threshold voltage and higherbrightness at the same voltage when compared to the con-ventional tungsten safelight. The emitted light spectrum athigh temperatures is a combination of blackbody radiationand electro-luminescence. The carbon nanotube filamentswhich are observed to show invariance to changes in tem-perature at very high temperatures could be used as resistorsin high temperature applications. A household bulb made outof carbon nanotube filaments is expected in the very nearfuture.
This work is financially supported by MOST under theState Key Project for Fundamental Research, Grant No.G20000264-04. B.Q.W. acknowledges financial support fromLSU Council on Research.
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FIG. 3. ~a! I –V curve comparison of a typical SWNT and a DWNT bulb, aswell as a safelight bulb~36 V, 40 W!. ~b! Temperature dependence of fila-ment resistance at high temperature. The nanotube filaments~SWNT andDWNT! show resistance stability over a large temperature region.
4871Appl. Phys. Lett., Vol. 84, No. 24, 14 June 2004 Wei et al.
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