REPORTS oping better parallel assembly schemes for 14. L Tong et al. Nature 426, 816(2003) 27. L Eldada, Rev. Sci. Instrum. 75. 575(2004). nanowire integration(29) 15. M. Law, H. Kind, B. Messer, F. Kim, P. Yang, Angew. 28. H. ]. Egelhaaf, D. OelkrugJ Growth 161. 190 hem. Int Ed. Engl. 41, 2405(2002)- 6. H. Yan e 17. Z.w. Pan, Z R Dai, Z L Wang, Science 291, 1947 29. A. Tao et al, Nano Lett. 3. 1229(2003) References 30. This work was supported in part by the Camille and Yamamoto. A. Chutinan 18. Materials and methods are available as supporting Dreyfus Foundation, the Alfred P. Sloan Foun- aterial on Science online n, the Beckman Foundation, th 3. W.L. Barnes, A Dereux. T. W. Ebbesen, Nature 424 19. C Gmachl et al. Science 280. 1556(1998) ent of Energy, and NSF. J.G. thanks NSF for duate research fellowship. Work at the Lawrence 2出mm数如mA ver, Boston, 1983). 21. We also have observed extre case of thin (< voluted"wet X Duan et al Nature 425, 274(2003) terials Science of the U. S. Depa M C. McAlpine et al., Nano Lett. 3, 1531(2003)- ed on surfaces. in- We thank H. Yan for the zno 7. M H Huang et al, Science 292, 1897(200 g loops with radi of 100 National Center for Electron Microscopy for the use 8. X Duan, Y. Huang, R. AgarwaL, C. M. Lieber, Nature 421.241(2003) 12.1299(2000) 9. H. Kind, H. Yan, B. Messer, M. Law, P. Yang, Adv. 23. K Kim et al, Rev. Sci Instrum. 74, 4021(2003) Supporting Online Material 24. J C. Johnson, H. Yan, P. Yang, R J. Saykally, J. Phys wsciencemag org/cgi/content/full/305/ 10. D. Psaltis Science 298. 1359 Chem.B107.8816(2003 P. Yang et al, Science 287, 46 25. R. C. Reddick, R.J. Warmack, D. W. Chilcott, S. L Materials and Methods harp, T. LFerrell, Rev. Sci Instrum. 61, 3669(1990). Figs. S1 to $3 13. V.R. Almeida, Q. Xu, C. A Barrios, M. Lipson, Opt. 26. K. Okamoto, Fundamentals of Optical Waveguides let.29.1209(2004) (Academic Press, San Diego, CA, 2000) 2 June 2004: accepted 15 July 2004 Transparent, Conductive Carbon efficiency (4). Our t-SWNT production process is quite simple, comprising three Nanotube films steps:(i) vacuum-filtering a dilute, surfac ant-based suspension of purified nano- ubes onto a filtration membrane(forming Zhuangchun Wu, Zhihong Chen,*t Xu Du, the homogeneous film on the membrane) Jonathan M. Logan, Jennifer Sippel, Maria Nikolou washing away the surfactant with puri Katalin Kamaras, John R. Reynolds, David B. Tanner, 1 fied water; and (iii) dissolving the filtration Arthur F. Hebard, Andrew G. Rinzler'+ membrane in solvent (4). Multiple tech niques for transfer of the film to the desired We describe a simple process for the fabrication of ultrathin, transparent, substrate have been developed. The films can optically homogeneous, electrically conducting films of pure single-walled be made free-standing over appreciable aper- carbon nanotubes and the transfer of those films to various substrates. For tures(-1 cm2)by making the transfer to a equivalent sheet resistance, the films exhibit optical transmittance com substrate with a hole. over which the film is laid arable to that of commercial indium tin oxide in the visible spectrum, but before membrane dissolution (5, 6) far superior transmittance in the technologically relevant 2- to 5-micro This filtration method has several ad- meter infrared spectral band. These characteristics indicate broad applica antages:(1) Homogeneity of the films is bility of the films for electrical coupling in photonic devices. In an example guaranteed by the process itself. As the application, the films are used to construct an electric field-activated optical nanotubes accumulate, they generate a fil- modulator, which constitutes an optical analog to the nanotube-based field ter cake that acts to impede the permeation effect transistor rate. If a region becomes thicker, the local permeation rate and associated deposition Transparent electrical conductors pervade electromagnetic spectrum. Use of the trans- rate slow down, allowing thinner regions to modern technologies, providing a critical parent SWNT films (t-SWNTs) for current catch up. (ii) Because of their extreme ri- component of video displays, video and injection into p-Gan and for blue light- gidity (for objects of such small diameters), still-image recorders, solar cells, lasers, op- emitting GaN/n gan diodes(light extract- the nanotubes have long persistence tical communication devices, and solid- ed through the films) has recently been lengths. They consequently tend to lie tate lighting [for recent reviews, see(I, demonstrated, together with patterning of straight, gaining maximal overlap and in- 2). We describe a class of transparent con- the t-SWNTs by standard microlith- terpenetration within the film as they accu- ducting material based on continuous films graphic techniques (3). Here we elaborate on mulate(the curvature observed in Fig. ID of pure single-walled carbon nanotubes the film production process, transfer to sub- ikely caused by van der Waals forces (SWNTs). These intrinsic electrical con- strates, film morphology, and electrical and dominating as the surfactant is washed are formed into uniform, optically tical properties. We also demonstrate use of the away). This yields maximal electrical con geneous films of controllable thick t-SWNTs as the active element of an optical ductivity and mechanical integrity through- lat are thin enough to be transparent modulator. This constitutes an optical analog to out the films. (iii) The film thickness over technologically relevant regions of the the SWNT-based field-effect transistor(FET), readily controlled, with nanoscale preci- modulating light transmission through the films sion, by the nanotube concer nts of Physics andChemistry, University by application of electric fields. volume of the suspension filtered. of Florida. gainesville, FL 32611 USA. 2MTA SZFKI Other methods of tran Budapest, H 1525, Hungary. parent nano- Examples of the transparent films are be film production include drop-dry shown in Fig. 1. Films of thickness 50 and *These authors contributed equally to this work. from solvent, airbrushing, and Langmuir- 150 nm, as measured by atomic force mi- tPresent address: IBM T. J Watson Research Center, Blodgett deposition. These alternatives, croscopy(AFM) at step edges, display a #To whom correspondence should be addressed. E- owever, present severe limitations in corresponding increase in optical density mail: rinzler@phys. ufL. edu terms of the film quality or production (Fig 1A). Films as large as 10 cm in diam www.sciencemag.orgSciEnceVol30527AugUst2004 1273oping better parallel assembly schemes for nanowire integration (29). References and Notes 1. S. Noda, K. Tomoda, N. Yamamoto, A. Chutinan, Science 289, 604 (2000). 2. C. Lo´pez, Adv. Mater. 15, 1679 (2003). 3. W. L. Barnes, A. Dereux, T. W. Ebbesen, Nature 424, 824 (2003). 4. J. R. Krenn, J.-C. Weeber, Philos. Trans. R. Soc. Lond. A 362, 739 (2004). 5. X. Duan et al., Nature 425, 274 (2003). 6. M. C. McAlpine et al., Nano Lett. 3, 1531 (2003). 7. M. H. Huang et al., Science 292, 1897 (2001). 8. X. Duan, Y. Huang, R. Agarwal, C. M. Lieber, Nature 421, 241 (2003). 9. H. Kind, H. Yan, B. Messer, M. Law, P. Yang, Adv. Mater. 14, 158 (2002). 10. D. Psaltis, Science 298, 1359 (2002). 11. P. Yang et al., Science 287, 465(2000). 12. R. Quidant et al., Phys. Rev. E 64, 066607 (2001). 13. V. R. Almeida, Q. Xu, C. A. Barrios, M. Lipson, Opt. Lett. 29, 1209 (2004). 14. L. Tong et al., Nature 426, 816 (2003). 15. M. Law, H. Kind, B. Messer, F. Kim, P. Yang, Angew. Chem. Int. Ed. Engl. 41, 2405(2002). 16. H. Yan et al., Adv. Mater. 15, 1907 (2003). 17. Z. W. Pan, Z. R. Dai, Z. L. Wang, Science 291, 1947 (2001). 18. Materials and methods are available as supporting material on Science Online. 19. C. Gmachl et al., Science 280, 1556 (1998). 20. A. W. Snyder, D. Love, Optical Waveguide Theory (Kluwer, Boston, 1983). 21. We also have observed extremely convoluted “wet noodle” shapes in the case of thin (50 nm) non￾waveguiding nanoribbons dispersed on surfaces, in￾cluding loops with radii of 100 nm. 22. H. W. C. Postma, A. Sellmeijer, C. Dekker, Adv. Mater. 12, 1299 (2000). 23. K. Kim et al., Rev. Sci. Instrum. 74, 4021 (2003). 24. J. C. Johnson, H. Yan, P. Yang, R. J. Saykally, J. Phys. Chem. B 107, 8816 (2003). 25. R. C. Reddick, R. J. Warmack, D. W. Chilcott, S. L. Sharp, T. L. Ferrell, Rev. Sci. Instrum. 61, 3669 (1990). 26. K. Okamoto, Fundamentals of Optical Waveguides (Academic Press, San Diego, CA, 2000). 27. L. Eldada, Rev. Sci. Instrum. 75, 575 (2004). 28. H. J. Egelhaaf, D. Oelkrug, J. Cryst. Growth 161, 190 (1996). 29. A. Tao et al., Nano Lett. 3, 1229 (2003). 30. This work was supported in part by the Camille and Henry Dreyfus Foundation, the Alfred P. Sloan Foun￾dation, the Beckman Foundation, the U.S. Depart￾ment of Energy, and NSF. J.G. thanks NSF for a graduate research fellowship. Work at the Lawrence Berkeley National Laboratory was supported by the Office of Science, Basic Energy Sciences, Division of Materials Science of the U. S. Department of Energy. We thank H. Yan for the ZnO nanowires and the National Center for Electron Microscopy for the use of their facilities. Supporting Online Material www.sciencemag.org/cgi/content/full/305/5688/1269/ DC1 Materials and Methods Figs. S1 to S3 2 June 2004; accepted 15July 2004 Transparent, Conductive Carbon Nanotube Films Zhuangchun Wu,1 * Zhihong Chen,1 *† Xu Du,1 Jonathan M. Logan,1 Jennifer Sippel,1 Maria Nikolou,1 Katalin Kamaras,2 John R. Reynolds,3 David B. Tanner,1 Arthur F. Hebard,1 Andrew G. Rinzler1 ‡ We describe a simple process for the fabrication of ultrathin, transparent, optically homogeneous, electrically conducting films of pure single-walled carbon nanotubes and the transfer of those films to various substrates. For equivalent sheet resistance, the films exhibit optical transmittance com￾parable to that of commercial indium tin oxide in the visible spectrum, but far superior transmittance in the technologically relevant 2-to 5-micro￾meter infrared spectral band. These characteristics indicate broad applica￾bility of the films for electrical coupling in photonic devices. In an example application, the films are used to construct an electric field–activated optical modulator, which constitutes an optical analog to the nanotube-based field effect transistor. Transparent electrical conductors pervade modern technologies, providing a critical component of video displays, video and still-image recorders, solar cells, lasers, op￾tical communication devices, and solid￾state lighting [for recent reviews, see (1, 2)]. We describe a class of transparent con￾ducting material based on continuous films of pure single-walled carbon nanotubes (SWNTs). These intrinsic electrical con￾ductors are formed into uniform, optically homogeneous films of controllable thick￾ness that are thin enough to be transparent over technologically relevant regions of the electromagnetic spectrum. Use of the trans￾parent SWNT films (t-SWNTs) for current injection into p-GaN and for blue light– emitting GaN/InGaN diodes (light extract￾ed through the films) has recently been demonstrated, together with patterning of the t-SWNTs by standard microlitho￾graphic techniques (3). Here we elaborate on the film production process, transfer to sub￾strates, film morphology, and electrical and op￾tical properties. We also demonstrate use of the t-SWNTs as the active element of an optical modulator. This constitutes an optical analog to the SWNT-based field-effect transistor (FET), modulating light transmission through the films by application of electric fields. Other methods of transparent nano￾tube film production include drop-drying from solvent, airbrushing, and Langmuir￾Blodgett deposition. These alternatives, however, present severe limitations in terms of the film quality or production efficiency (4). Our t-SWNT production process is quite simple, comprising three steps: (i) vacuum-filtering a dilute, surfac￾tant-based suspension of purified nano￾tubes onto a filtration membrane (forming the homogeneous film on the membrane); (ii) washing away the surfactant with puri￾fied water; and (iii) dissolving the filtration membrane in solvent (4). Multiple tech￾niques for transfer of the film to the desired substrate have been developed. The films can be made free-standing over appreciable aper￾tures (
1 cm2 ) by making the transfer to a substrate with a hole, over which the film is laid before membrane dissolution (5, 6). This filtration method has several ad￾vantages: (i) Homogeneity of the films is guaranteed by the process itself. As the nanotubes accumulate, they generate a fil￾ter cake that acts to impede the permeation rate. If a region becomes thicker, the local permeation rate and associated deposition rate slow down, allowing thinner regions to catch up. (ii) Because of their extreme ri￾gidity (for objects of such small diameters), the nanotubes have long persistence lengths. They consequently tend to lie straight, gaining maximal overlap and in￾terpenetration within the film as they accu￾mulate (the curvature observed in Fig. 1D is likely caused by van der Waals forces dominating as the surfactant is washed away). This yields maximal electrical con￾ductivity and mechanical integrity through￾out the films. (iii) The film thickness is readily controlled, with nanoscale preci￾sion, by the nanotube concentration and volume of the suspension filtered. Examples of the transparent films are shown in Fig. 1. Films of thickness 50 and 150 nm, as measured by atomic force mi￾croscopy (AFM) at step edges, display a corresponding increase in optical density (Fig. 1A). Films as large as 10 cm in diam￾Departments of 1 Physics and 3 Chemistry, University of Florida, Gainesville, FL 32611, USA. 2 MTA SzFKI, Budapest, H 1525, Hungary. *These authors contributed equally to this work. †Present address: IBM T. J. Watson Research Center, Yorktown Heights, NY 10598, USA. ‡To whom correspondence should be addressed. E￾mail: rinzler@phys.ufl.edu R EPORTS www.sciencemag.org SCIENCE VOL 30527 AUGUST 2004 1273