Langmuir 2008, 24, 2655-2662
2655
Comparative Study on Different Carbon Nanotube Materials in Terms of Transparent Conductive Coatings Zhongrui Li,*,† Hom R. Kandel,‡ Enkeleda Dervishi,† Viney Saini,† Yang Xu,† Alexandru R. Biris,§ Dan Lupu,§ Gregory J. Salamo,| and Alexandru S. Biris† Nanotechnology Center and Applied Science Department, UniVersity of Arkansas at Little Rock, Little Rock, Arkansas 72204, Department of Physics and Astronomy, UniVersity of Arkansas at Little Rock, Little Rock, Arkansas 72204, National Institute for Research and DeVelopment of Isotopic and Molecular Technologies, P.O. Box 700, R-400293 Cluj-Napoca, Romania, and Physics Department, UniVersity of Arkansas, FayetteVille, Arkansas 72701 ReceiVed June 24, 2007. In Final Form: December 3, 2007 We compared conductive transparent carbon nanotube coatings on glass substrates made of differently produced single-wall (SWNT), double-wall, and multiwall carbon nanotubes. The airbrushing approach and the vacuum filtration method were utilized for the fabrication of carbon nanotube films. The optoelectronic performance of the carbon nanotube film was found to strongly depend on many effects including the ratio of metallic-to-semiconducting tubes, dispersion, length, diameter, chirality, wall number, structural defects, and the properties of substrates. The electronic transportability and optical properties of the SWNT network can be significantly altered by chemical doping with thionyl chloride. Hall effect measurements revealed that all of these thin carbon nanotube films are of p-type probably due to the acid reflux-based purification and atmospheric impurities. The competition between variable-range hoping and fluctuation-assisted tunneling in the functionized carbon nanotube system could lead to a crossover behavior in the temperature dependence of the network resistance.
I. Introduction High transportability and large aspect ratio of carbon nanotubes (CNTs) are attractive properties for producing conductive composites. As a new class of materials, individual variations in chirality and length are ensemble averaged to yield uniform physical properties. This feature can be used in coatings to get transparent and conductive two-dimensional networks on various types of transparent substrates.1 Utilizing carbon nanotubes as transparent conductor can produce materials that closely match the properties of conductive metal oxides in optoelectronic applications, and are more cost effectively produced, and are far more flexible and environmentally resistant. There is an increasing interest in thin transparent networks, since it has been demonstrated that they can be used for more than basic investigations.2 There is also an upcoming potential for several applications such as transistors,3,4 diodes,5 sensors,6 optical modulators,7 or as a conductive backbone for electrochemical polymer coating.8 Recently, it is found possible to replace the indium tin oxide * Corresponding author. E-mail:
[email protected]. † Nanotechnology Center and Applied Science Department, University of Arkansas at Little Rock. ‡ Department of Physics and Astronomy, University of Arkansas at Little Rock. § National Institute for Research and Development of Isotopic and Molecular Technologies. | University of Arkansas. (1) Parekh, B. B.; Fanchini, G.; Eda, G.; Chhowalla, M. J. Appl. Phys. 2007, 90, 121913. (2) Zhang, M.; Fang, S. L.; Zakhidov, A. A.; Lee, S. B.; Aliev, A. E.; Williams, C. D.; Atkinson, K. R.; Baughman, R. H. Science 2005, 309, 1215. (3) Lay, M. D.; Novak, J. P.; Snow, E. S. Nano Lett. 2004, 4 (4), 603. (4) Shiraishia, M.; Takenobub, T.; Iwaid, T.; Iwasab, Y.; Kataurae, H.; Ata. M. Chem. Phys. Lett. 2004, 394, 110. (5) Zhou, Y.; Gaur, A.; Hur, S.-H.; Kocabas, C.; Meitl, M. A.; Shim, M.; Rogers, J. A. Nano Lett. 2004, 4 (10) 2031. (6) Sayago, I.; Terrado, E.; Lafuente, E.; Horrillo, M. C.; Maser, W. K.; Benito, A. M.; Navarro, R.; Urriolabeitia, E. P.; Martinez, M. T.; Gutierrez, J. Synth. Met. 2005, 148 (1), 15-19. (7) Wu, Z.; Chen, Z.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.; Reynolds, J. R.; Tanner, D. B.; Hebard, A. F.; Rinzler, A. G. Science 2004, 305, 1273.
(ITO) electrode in an organic photovoltaic device with transparent and conducting SWNT thin-film electrodes with efficiencies exceeding those of reference devices.9 So far, several methods are used in order to make thin CNT networks such as filtration,7 spin coating,10 drying from solvent,11 or Langmuir-Blodgett12 deposition. However, the controlled deposition of a large area of highly conducting carbon nanotube films with high homogeneity required for applications remains elusive. Direct deposition using spin coating, spraying, or incubation produces films with conductivity much lower than that of commercially used ITO.13 Deposition by vacuum filtration14,15 is limited by filter size and requires consistent transfer to flat substrates for further applications. According to the shell numbers, CNTs can be classified as single-wall (SWNT), double-wall (DWNT), and multiwall (MWNT) carbon nanotubes. The electronic transport properties of an individual SWNT16 and MWNT17,18 have been extensively investigated and found to be strongly relying on their structure. DWNT is a type of one-dimensional material between SWNT (8) Ferrer-Anglada, N.; Kaempgen, M.; Ska´kalova´, V.; Dettlaf-Weglikowska, U.; Roth. S. Diam. Relat. Mater. 2004, 13 (2), 256. (9) Du Pasquier, A.; Unalan, H. E.; Kanwal, A.; Miller, S.; Chhowalla, M. Appl. Phys. Lett. 2005, 87, 203511. (10) Meitl, M. A.; Zhou, Y. X.; Gaur, A.; Jeon, S.; Usrey, M. L.; Strano, M. S.; Rogers, J. A. Nano Lett. 2004, 4, 1643. (11) Sreekumar, T. V.; Liu, T.; Kumar, S.; Ericson, L. M.; Hauge, R. H.; Smalley, R. E. Chem. Mater. 2003, 15, 175. (12) Krstic, V.; Muster, J.; Duesberg, G. S.; Philipp, G.; Burghard, M.; Roth, S. Synth. Met. 2000, 110 (3), 245-249. (13) Bekyarova, E.; Itkis, M. E.; Cabrera, N.; Zhao, B.; Yu, A. P.; Gao, J. B.; Haddon, R. C. J. Am. Chem. Soc. 2005, 127, 5990. (14) Hu, L.; Hecht, D. S.; Gru¨ner, G. Nano Lett. 2004, 4, 2513. (15) Armitage, N. P.; Gabriel, J. C. P.; Gru¨ner, G. J. Appl. Phys. 2004, 95, 3228. (16) Saito, R.; Dresselhaus, G.; Dressselhaus, M. S. Physical Properties of Carbon Nanotubes; Imperial College Press, London, 1998. (17) Ebbesen, T. W.; Lezec, H. J.; Hiura, H.; Bennett, J. W.; Ghaemi, H. F.; Thio, T. Nature (London) 1996, 382, 54. (18) Li, H. J.; Lu, W. G.; Li, J. J.; Bai, X. D.; Gu, C. Z. Phys. ReV. Lett. 2005, 95, 086601.
10.1021/la701880h CCC: $40.75 © 2008 American Chemical Society Published on Web 02/06/2008
2656 Langmuir, Vol. 24, No. 6, 2008
and MWNT. Early calculation shows that the band structure of the DWNT depends little on the interaction between the outer and inner tubes.19,20 A recent theoretical work revealed that the potential barrier of DWNTs depends not on the diameters but significantly on the chirality pairs.21 Since electronic and optical properties vary with CNT material, it is necessary to start a comparative study on different CNT materials in terms of simple transparent conductive coatings. In this work, we compared two different fabrication approaches: airbrushing and vacuum filtration. Transparent conductive CNT films were coated on conventional glass substrates by using different carbon nanotube species. We further investigated many effects (including preparation method, metallicity, dispersion, diameter, length, chemical modification and defects) on the optical and electric properties of carbon tubes films. Temperature-dependent conductance study was employed to understand the underlying physics of the electric properties of the CNT thin films. This fundamental study would provide some guidance in the fabrication of carbon nanotube based optically transparent and electrically conductive coatings. II. Experimental Section Differently produced SWNT, DWNT, and MWNT materials were used to fabricate electrically conductive thin coatings on optically transparent glass substrates. The small diameter SWNT material (C-SWNT) was synthesized from CO disproportionation over CoMo/SiO2 catalyst.22 Laser ablation produced single-walls, tubes@Rice (L-SWNT), were used for comparison.23 The DWNT material was synthesized by the thermal pyrolysis of n-hexane over a Fe-Mo/ MgO catalyst.24 Two different MWNT materials were produced by chemical vapor deposition of acetylene on Fe-Co/CaCO3 catalyst under external furnace heating (EF-MWNT) and radio frequency (RF-MWNT) excitation,25,26 respectively (see the Supporting Information). All these CNT materials were purified in a similar procedure: thermally oxidize the amorphous carbon around 300 °C, then wash twice with NaOH and HF (for SWNT)/hydrochloric acid (for DWNT and MWNT) under sonication, and finally rinse with nanopure water (doubly deionized water with resistivity larger than 18.2 MΩ cm). To evaluate the overall quality of the carbon nanotube materials, which were used for the films fabrication, we characterized them by thermogravimetric analysis (TGA), and Raman scattering spectroscopy. TGA was performed under air flow at 150 mL/min using a Mettler Toledo TGA/SDTA 851e instrument. Raman scatterings from the CNTs were collected at room temperature using a Horiba Jobin Yvon LabRam HR800 spectrometer equipped with a charge-coupled detector and a spectrometer with grating of 600 lines/mm. He-Ne laser (633 nm) and Ar+ (514 nm) were used as excitation sources. The laser beam intensity measured at the sample was kept constant around 1 mW. The microscope focused the incident beam to a spot size of about 1 micron, and the backscattered signal was collected backward from the direction of incidence through an Olympus optical microscope. Raman shifts were calibrated with a silicon wafer at the 521 cm-1 peak. (19) Saito, R.; Dresselhaus, G.; Dresselhaus, M. S. J. Appl. Phys. 1993, 73, 494-500. (20) Charlier, J. C.; Michenaud, J. P.; Phys. ReV. Lett. 1993, 70, 1858-61. (21) Saito R, Matsuo R, Kimura T, Dresselhaus G, Dresselhaus MS. Chem. Phys. Lett. 2001, 348, 187-93. (22) Alvarez, W. E.; Pompeo, F.; Herrera, J. E.; Balzano, L.; Resasco, D. E. Chem. Mater. 2002, 14, 1853-1858. (23) Lebedkin, S.; Schweiss, P.; Renker, B.; Malik, S.; Hennrich, F.;Neumaier, M.; Stoermer, C.; Kappes, M. M., Carbon 2002, 40, 417-423. (24) Jung, S. I.; Jo, S. H.; Moon, H. S.; Kim, J. M.; Zang, D.-S.; Lee, C. J. J. Phys. Chem. C 2007, 111, 4175-4179. (25) Biris, A. S.; Schmitt, T. C.; Little, R. B.; Li, Z.; Xu, Y.; Biris, A. R.; Lupu, D.; Dervishi, E.; Trigwell, S.; Miller, D. W.; Rahman, Z. J. Phys. Chem. 2007, in press. (26) Dervishi, E.; Li, Z.; Biris, A. R.; Lupu, D.; Trigwell, S.; Biris, A. S. Chem. Mater. 2007, 19, 179-184.
Li et al. Basically, CNTs consist of seamlessly rolled-up graphene sheets of carbon with π-conjugative and highly hydrophobic sidewalls but that can interact with, for example, surfactants and some kinds of aromatic compounds through hydrophobic or π-π electronic interaction(s).27 So, the CNTs can be made into uniform solutions after proper surface treatment. Two different fabrication approaches, airbrushing and vacuum filtration methods, were utilized to prepare the CNT films. (1) Airbrushing: The purified CNTs were first dispersed in dimethylformamide (DMF, 0.5 mg/mL) with the assistance of sonication, and the uniform solution was directly sprayed on glass substrates by means of an airbrush spray using compressed dry air as carrier gas at 2 bar. Glass substrate was placed on a heating platform and heated up to 150 °C in order to evaporate DMF solvent from the film. (2) Filtration: The purified CNTs was first dispersed in sodium cholate (NaCh)28 aqueous solution (CNT:sodium cholate ) 1:1 wt, 5 mg/L). First, a dilute, surfactant-based suspension of purified nanotubes was vacuum-filtered onto a filtration membrane; then the surfactant was washed away with deionized water; and finally the filtration membrane was dissolved in an organic solvent. The film was transferred onto the glass substrate and the membrane was dissolved by using chloroform. Finally, these films were immersed into pure water for 30 min to remove the residual surfactant and dried in air again on the hot plate. Optical transmission on a range of wavelengths from 200 to 3300 nm was performed with an appropriate method by averaging the structural irregularities and characterizing the thickness of various thin carbon nanotube networks prepared on a glass substrate, using a UV-vis-NIR Varian Cary 5000 spectrometer. The transmittance values were measured at 550 nm wavelength for all the transparent networks. The resistance was measured with a standard dc fourterminal method using Keithley 2000 digital multimeters. The observed temperature gradient across the sample was typically 1-2 K and monitored by the silicon diode thermosensor. Since, in many cases, the resistance behavior at low-temperature strongly depends on the distance of the electrodes because of the random potentials set up by grain boundaries and/or defects. The electrode gap for measuring the dc conductivity (resistance) was set at 3 mm for all samples. Furthermore, the current for measuring the conductivity was in the range of 0.1-50 mA, and all film samples exhibited linear current-voltage characteristics in the current range at room temperature. The Hall Effect measurements were performed under magnetic field of 0.2 T by using standard van der Pauw geometry. Electrical contacts were prepared using indium and annealed at 200 °C. The contacts were found to be ohmic. All electrical measurements were performed by using four probe technique.
III. Results and Discussion 3.1. Characterization of CNT Materials. Thermogravimetric analysis is a useful technique for characterizing the purity of carbon nanotubes. The weight loss profile in Figure 1a was obtained by heating the purified CNTs from room temperature to 850 °C at a rate of 5 °C/min. The normalized TGA curves and their first derivatives (dW/dT) indicate significant mass drops around 420, 427, 560, 574, and 588 °C for C-SWNT, L-SWNT, DWNT, EF-MWNT, and RF-MWNT, respectively. The width of the derivative (dW/dT) shown in Figure 1b also reflects the crystallinity of the material as well as the distribution of tube diameters, since thin and defective tubes usually have lower combustion temperature. The quantitative analysis revealed that the purities of the C-SWNT, L-SWNT, DWNT, EF-MWNT, and RF-MWNT materials are 98.5%, 98.1%, 97.3%, 98.6%, and 98.2%, respectively. The impurities are trace amount of amorphous carbon ( DWNT > RF-MWNT. The conductance-temperature dependence analysis revealed that the defects in CNT could lead to crossover behavior in the temperature-dependence of the film resistance. All these thin CNT films are of p-type probably due to the acid reflux-based purification and atmospheric impurities. Acknowledgment. This work was financially supported by the U.S. Department of Energy (Grant No. DE-FG 36-06 GO 86072). We are also grateful to Prof. Tetsuro Majima for his enlightening suggestions. Supporting Information Available: The cross sectional SEM images of filtration film, the synthesis and characterization of the EFand RF-MWNTs. This material is available free of charge via the Internet at http://pubs.acs.org. LA701880H
