Thermal Behavior of Transparent Film Heaters Made of Single-Walled Carbon Nanotubes

Duckjong Kim, Hyun-Chang Lee, Ju Yeon Woo, and Chang-Soo Han*Nano Mechanical Systems Research Center, Korea Institute of Machinery and Materials, 171 Jang-dong,Yuseong-gu, Daejeon 305-343, Korea

ReceiVed: NoVember 13, 2009; ReVised Manuscript ReceiVed: February 5, 2010

We investigate thermal behavior of transparent film heaters (TFH) made of single-walled carbon nanotubes.We fabricate the TFH by using the spray coating method. We studied the temperature dependence of theelectrical resistance of the TFH in terms of Joule and external heating in various gas environments. Testresults show that the effect of the electrical current through the TFH on the temperature dependence of theelectrical resistance is not important and that the humidity and the degree of vacuum significantly affect theshape of the resistance-temperature curve. We discuss the physical meanings underlying the experimentalresults and how to make use of these findings. This study improves the understanding of the heating effecton electrical conductance of the TFH made of single-walled carbon nanotubes which could be a good candidatefor the heater in many applications requiring both transparency and heating function.

Introduction

Since the discovery of carbon nanotubes (CNTs), theirsuperior physical properties have led to many practicalapplications.1,2 In particular, films made of CNTs have thepotential of next generation transparent conducting film (TCF)due to high conductivity, mechanical flexibility, simple fabrica-tion, and abundance of raw carbon materials.3-5 CNT-TCFs havebeen studied for use as transistors, electrodes in photovoltaicsand organic light-emitting diodes, sensors, and actuators.6-10

These applications generally require high transparency and lowelectrical resistance.

Recently, we reported the possibility of creating a transparentfilm heater (TFH) using CNT-TCF.11 We made the TCF byusing the vacuum filtration method and demonstrated itspotential as a vehicle defroster. Although small test sampleswere successfully fabricated by using the vacuum filtrationmethod, the TCF size is limited by the filter dimensions for thefiltration method and this is a drawback for applicationsrequiring TCF with large area. In this context, the spray coatingmethod, which is a robust approach for TCFs with large area,could be a breakthrough in the commercialization of the TFH.

Understanding the thermal behavior of CNT-TCFs is alsocrucial in a commercially valuable achievement. Severalresearchers have reported the effect of temperature on theelectrical transport properties of the TCFs. The resistivity ofTCFs made of single-walled carbon nanotubes (SWCNTs)decreased as the temperature increased at low temperature. Thistemperature dependence of electrical resistance is called negativetemperature dependence. Above a certain transition temperature,the resistivity showed opposite temperature dependence, positivetemperature dependence.12-16 Kaiser et al. explained that thisso-called U-shaped temperature dependence was related to thetransition from semiconducting to metallic behavior.12 Barneset al. attributed the positive temperature dependence of theresistance to the dopant desorption from their experimentalresults on the temperature dependence of resistivity for metallicand semiconducting SWCNTs-enriched transparent networks.13

However, the previous studies mostly focused on the thermalbehavior of the TCFs below room temperature. They did notuse Joule heating of the TCFs and just relied on externaltemperature controller to regulate the temperature of the TCFs.To the best knowledge of the authors, there has been no reporton the thermal behavior of the heater using the TCF.

In this research, we fabricated TFHs by using the spraycoating method and investigated the relationship betweentemperature and the electrical resistance of the TFH in termsof Joule and external heating in various gas environments. Wealso discussed physical meanings underlying the experimentalresults and how to use them.

Experimental Details

We fabricated TCFs made of SWCNTs by using the spraycoating method. We used thermal annealing and acid treatmentto purify SWCNTs prepared by the arc-discharge method. Usingfield emission scanning electron microscopy (FESEM, Model:FEI NOVA 200) and field emission transmission electronmicroscopy (FETEM, Model: JEOL JEM-2100F), we verifiedthat the purified SWCNTs had almost no large impurities asshown in the inset of Figure 1a. We dispersed SWCNTs indeionized water with 1 wt % sodium dodecyl sulfate (SDS) andsonicated for several hours. The concentration of the SWCNTsolution was 1 µg/mL. We sprayed the SWCNT solution on aglass substrate to form the TCF. The size of the glass substrateis 50 mm × 50 mm × 0.5 mm. As a result, we made TCFswith transparency of about 70% at 550 nm. The sheet resistanceof the TCFs was in the range of 130-190 Ω/sq. We measuredthe optical transmittance and the sheet resistance by using theabsorption spectroscopy (Optizen 2120UV Plus) and the four-point probe method (Jandel CMT 100), respectively. Finally,on the TCF, we formed electrodes by using silver paste to makea low-resistance electrical contact. Figure 1a shows the preparedsample. The Raman spectrum shown in the inset of Figure 1aclearly demonstrates the most characteristic features of SWCNTsin terms of the radial breathing mode (RBM), the D-band, andthe G-band.

We heated the prepared samples either by Joule heating orexternal heating. Figure 1b shows the setup for the Joule heating

* To whom correspondence should be addressed. E-mail: cshan@kimm.re.kr. Phone: +82-42-868-7126. Fax: +82-42-868-7884.

J. Phys. Chem. C 2010, 114, 5817–5821 5817

10.1021/jp910799a 2010 American Chemical SocietyPublished on Web 03/12/2010

experiment. For Joule heating, we applied the voltage differenceof 60 V across the electrodes by using a DC power supply(Agilent E3649A). We measured the voltage difference and thesurface temperature of the TFH by using a data acquisition unit(Agilent 34970A) and monitored the electrical current throughthe TFH by using a current meter (Fluke 189). For temperature

measurement, we used T-type thermocouples. We calculatedthe electrical resistance by dividing the applied voltage differ-ence by the measured current. For external heating, we heatedthe samples on a hot plate. For this case, we measured theelectrical resistance directly by using the data acquisition unit(Agilent 34970A). In the present work, we collected measureddata once every second. For experiments under vacuum or aspecific gas environment, we tested TFHs in a vacuum chamber.For vacuum environment, we decreased the chamber pressureto 3 × 10-6 Torr. For a specific gas environment, we infusedthe specific gas into the chamber keeping the maximum chamberpressure to the atmospheric pressure as soon as the chamberreached the desired vacuum level. When we filled the chamberwith dry air, we dehumidified the air using an air dryer system(Keumsung NRD-12).

Results and Discussion

To obtain the resistance-temperature (R-T) curve, weincreased the film temperature from room temperature to 160°C, and then immediately cooled the film to the initialtemperature by natural convection. Once the film temperaturereturned to the initial value, we reapplied heat to the filmimmediately. We repeated this heating and cooling cycle threetimes. Parts a and b of Figure 2 show the results for Joule andexternal heating in the air, respectively. We normalized the TCFresistance by the initial TCF resistance, R0. The results show amixed behavior with a transition from the negative temperaturedependence to the positive temperature dependence, the so-called U-shaped curve for both cases. Although the U-shape ismore apparent for the Joule heating case, the effect of the heatingmethod on the transition temperature is not important. Abovethe transition temperature, the resistance of the TCFs increasesas the temperature increases, i.e., positive temperature depen-

Figure 1. (a) Prepared TFH made of SWCNTs. The inset includesFESEM, FETEM images, and the Raman spectrum of the SWCNTsfilm. (b) Setup for the Joule heating experiment.

Figure 2. Resistance-temperature curves for (a) Joule heating and(b) external heating in the air.

5818 J. Phys. Chem. C, Vol. 114, No. 13, 2010 Kim et al.

dence. These results show that the temperature dependence ofthe resistance is not significantly affected by the electrical currentthrough the TFH. Throughout the thermal cycles, the temper-ature dependence of the resistance showed a repeated patternexcept for the first heating cycle. After this cyclic experiment,we left the TFH at room temperature in the air for about oneweek, after which the electrical conductance of the heaterrecovered.

To clarify the effect of the air on the electrical resistance ofthe TCF, we performed the Joule heating experiment undervacuum. We heated the TFH to 160 °C. Figure 3 clearly showsthat there is no U-shaped temperature dependence of theresistance. Only positive temperature dependence appears in thegraph. Initially, the resistance increases rapidly with thetemperature rise. The slope of resistance change decreases asthe temperature increases. At around 60 °C, the slope of theresistance increase reaches a steady value. In the vacuumenvironment, the heater resistance increase during the firstheating cycle is much larger than that in the air. The experi-mental results clearly show that some components in the airinfluence the electrical resistance of the TCF seriously. Toinvestigate the effect of each component in the air, we testedthe TFH in various gas environments. Dry air contains 78.1%nitrogen, 21.0% oxygen, 0.9% argon, and so on. Air alsocontains a variable amount of water vapor. Figure 4 shows theresults from the experiments in dry air, nitrogen, oxygen, andargon. Figure 4a shows that the TFH tested in the dry air doesnot show the U-shaped temperature dependence of the resis-tance. This indicates that water molecules in the air cause theU-shaped temperature dependence. Some researchers havereported that water molecules act as a donor doping theSWCNTs and that sufficient exposure to humidity decreasesthe electrical resistance of SWCNTs.17,18 We developed anexplanation on the U-shaped temperature dependence of theresistance assuming two conflicting effects. As the temperatureincreases, the kinetic energy of water molecules in the vicinityof the TFH increases and the adsorption of water molecules toSWCNTs caused by collision between water molecules andSWCNTs increases. On the other hand, as the temperatureincreases, desorption of water molecules from SWCNTs be-comes more active. For temperatures below the transitiontemperature, the former effect is dominant and the resistancedecreases as the temperature increases. For temperatures abovethe transition temperature, the latter effect is dominant and thepositive temperature dependence of the resistance appears.Panels b-d of Figure 4 show that the R-T curve does notseriously depend on the gas filling the test chamber. However,there is a trivial difference among them. For nitrogen and oxygenenvironments, the temperature dependence of the resistance

shows a repeated pattern except for the first heating cycle asshown in parts b and c of Figure 4. On the other hand, in theargon gas, the resistance consistently increases even after thefirst heating cycle as shown in Figure 4d. The resistance increaseis more apparent in the R-T curve for vacuum as shown inFigure 3. Therefore we could infer that the nitrogen and oxygenmolecules in the air reduce the electrical resistance of the CNT-TCFs as reported by several researchers.19-21 To clearly showthe effect of each component in the air on the R-T curve ofthe TFH, we summarized the experimental results as shown inFigure 5. Figure 5a shows the resistance increase during thefirst heating cycle. The R-T curves in Figure 5a show that thechamber pressure is a predominant factor determining theresistance increase during the initial heating and that thehumidity is another one. Except for the case of vacuumenvironment, we maintained the chamber pressure to theatmospheric pressure. Figure 5a shows that the resistanceincrease due to the initial heating is less than 20% for theatmospheric pressure. Only for the vacuum environment doesthe resistance increase by about 150% during the first heatingcycle. It is well-known by the Langmuir isotherm that adsorptionof molecules on a solid surface decreases as the gas pressuredecreases.22 Hence, our results indicate that the resistanceincrease seriously depends on desorption of some dopants aspointed out by Barnes et al.13 In addition, the R-T curves forair with water vapor and dry air show that the humidity in theair suppresses the resistance increase during the initial heating.The initial resistance increase appears just once during thethermal cyclic test. Therefore, to guarantee the repeatability ofthe TFH, warming of the TFH would be necessary. Figure 5bshows the R-T curves during the third heating cycle and thistemperature dependence of the resistance is repeatable as shownin Figures 2-4. The R-T curves repeated during thermal cycliccondition show that the humidity is the most important factorand that the chamber pressure is another one. The humidityreduces the resistance up to 5%. In the vacuum environment,the resistance increase due to the temperature rise becomes moreapparent as the temperature increases. Because the effect of gasfilling the test chamber on the temperature dependence of theresistance is negligible as shown in Figure 5, sensing thehumidity or the degree of vacuum from the R-T curve wouldbe much easier than gas sensing. For temperatures less thanabout 100 °C, the humidity effect is predominant and the R-Tcurve could be used for humidity sensing. For higher temper-atures, the effect of the chamber pressure becomes moreimportant and the R-T curve could be used for measurementof the degree of vacuum. Since the humidity in the air seriouslyaffects the R-T curve of the TFH, humidity control orwaterproofing the CNT-TCFs would be essential for repeat-ability and reliability of the TFH. Figure 5b also shows thatanother effect could become apparent when all dopants areremoved from SWCNTs. The positive temperature dependenceof the electrical resistance maintains in vacuum environmenteven after warm-up. This strongly indicates that the positivetemperature dependence is also caused by the intrinsic propertyof the SWCNTs, the so-called metallic behavior, for tempera-tures above room temperature, as pointed out by Kaiser et al.12

Therefore, the temperature dependence of the electrical resis-tance of the TFH depends on both the dopant adsorption andintrinsic electrical behavior of the SWCNTs.

After the thermal test, we measured the resistance of the TFHfor about 20 h in vacuum without heating to determine whetherthe TFH would recover their initial conductance. As shown inFigure 6, there was no significant recovery. However, when we

Figure 3. Resistance changes of the heater for the thermal cyclicexperiment in vacuum.

TFH Made of Single-Walled Carbon Nanotubes J. Phys. Chem. C, Vol. 114, No. 13, 2010 5819

placed the TCF in the air, the electrical conductance recoveredappreciably after a few hours. This conductance recoveryindicates that the dopants seriously reducing the resistance ofthe TFH would be some components of the air. From theexperimental results discussed above, the candidates for the

dopants would be nitrogen, oxygen, and water molecules. Thereduction of the film resistance gradually proceeded over a fewdays.

Conclusion

We experimentally studied the thermal behavior of TFH usingSWCNTs. We used the spray coating method in heater fabrica-tion. We investigated the temperature dependence of theelectrical resistance of the TFH in terms of Joule and externalheating in various gas environments. There was no significantdifference in the temperature dependence of the resistancebetween Joule and external heating in the air; we observedsimilar temperature dependence of the resistance including theU-shaped curve for both heating conditions. However, for theexperiments conducted in environments without water vapor,there was only positive temperature dependence in the R-Tcurve. We could come to the conclusion that the water moleculescause the U-shaped temperature dependence of the resistance.The experimental results show that the humidity and the degreeof vacuum are the main parameters affecting the R-T curve.Hence, humidity control or waterproofing the CNT-TCFs would

Figure 4. Resistance-temperature curves from thermal cyclic experiments in (a) dry air, (b) nitrogen, (c) oxygen, and (d) argon.

Figure 5. Resistance-temperature curves for (a) the first heating cycle and (b) the third heating cycle.

Figure 6. Resistance changes of the heater for long-term cooling.

5820 J. Phys. Chem. C, Vol. 114, No. 13, 2010 Kim et al.

be necessary to guarantee the repeatability in the temperaturedependence of the electrical resistance of the CNT-TCFs. Thisstudy provides useful information on the thermal behavior ofthe TFH made of SWCNTs and could be a cornerstone forachievements of commercially valuable TCF applications.

Acknowledgment. This research was supported by the Centerfor Nanoscale Mechatronics and Manufacturing, one of the 21stCentury Frontier Programs.

References and Notes

(1) Baughman, R. H.; Zakhidov, A. A.; Heer, W. D. Science 2002,297, 787.

(2) Collins, P. G.; Avouris, P. Sci. Am. 2000, 62.(3) Hu, L.; Hecht, D. S.; Gruner, G. Nano Lett. 2004, 4, 2513.(4) Saran, N.; Parikh, K.; Suh, D.-S.; Munoz, E.; Kolla, H.; Manohar,

S. K. J. Am. Chem. Soc. 2004, 126, 4462.(5) Wu, Z. C.; Chen, Z. H.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou,

M.; Kamaras, K.; Reynolds, J. R.; Tanner, D. B.; Herbard, A. F.; Rinzler,A. G. Science 2004, 305, 1273.

(6) Artukovic, E.; Kaempgen, M.; Hecht, D. S.; Roth, S.; Gruner, G.Nano Lett. 2005, 5, 757.

(7) Pasquier, A. D.; Unalan, H. E.; Kanwal, A.; Miller, S.; Chhowalla,M. Appl. Phys. Lett. 2005, 87, 203511.

(8) Kaempgen, M.; Roth, S. J. Electroanal. Chem. 2006, 586, 72.

(9) Li, J.; Hu, L.; Wang, L.; Zhou, Y.; Gruner, G.; Marks, T. J. NanoLett. 2006, 6, 2472.

(10) Yu, X.; Rajamani, R.; Stelson, K. A.; Cui, T. Sens. Actuators, A2006, 132, 626.

(11) Yoon, Y. H.; Song, J. W.; Kim, D.; Kim, J.; Park, J.; Oh, S.; Han,C. S. AdV. Mater. 2007, 19, 4284.

(12) Kaiser, A. B.; Dusberg, G.; Roth, S. Phys. ReV. B 1998, 57, 1418.(13) Barnes, T. M.; Blackburn, J. L.; Lagemaat, J.; Coutts, T. J.; Heben,

M. J. ACS Nano 2008, 2, 1968.(14) Fischer, J. E.; Dai, H.; Thess, A.; Lee, R.; Hanjani, N. M.; Dehaas,

D. L.; Smalley, R. E. Phys. ReV. B 1997, 55, 4921.(15) Itkis, M. E.; Borondics, F.; Yu, A.; Haddon, R. C. Science 2006,

312, 413.(16) Skakalova, V.; Kaiser, A. B.; Woo, Y. S.; Roth, S. Phys. ReV. B

2006, 74, 085403.(17) Zahab, A.; Spina, L.; Poncharal, P. Phys. ReV. B 2000, 62, 10000.(18) Na, P. S.; Kim, H.; So, H.-M.; Kong, K.-J.; Chang, H.; Ryu, B. H.;

Choi, Y.; Lee, J.-O.; Kim, B.-K.; Kim, J.-J.; Kim, J. Appl. Phys. Lett. 2005,87, 093101.

(19) Collins, P. G.; Bradley, K.; Ishigami, M.; Zettl, A. Science 2000,287, 1801.

(20) Jhi, S. H.; Louie, S. G.; Cohen, M. L. Phys. ReV. Lett. 2000, 85,1710.

(21) Mowbray, D. J.; Morgan, C.; Thygesen, K. S. Phys. ReV. B 2009,79, 195431.

(22) Langmuir, I. J. Am. Chem. Soc. 1916, 38, 2221.

JP910799A

TFH Made of Single-Walled Carbon Nanotubes J. Phys. Chem. C, Vol. 114, No. 13, 2010 5821