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Multi-layer LB films of single-wall carbon nanotubes
Physica B 323 (2002) 235–236
Multi-layer LB films of single-wall carbon nanotubes Yinzhong Guoa, Nobutsugu Minamia,*, Said Kazaouia, Junbiao Penga, Masaru Yoshidaa, Tokuji Miyashitab a
National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan b Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan
Abstract We propose a new processing technique of single-wall carbon nanotubes (SWNTs), namely, multi-layer Langmuir– Blodgett (LB) films. Solubilized SWNTs dispersed in an amphiphilic polymer matrix were spread on a water surface and vertically deposited on substrates. Optical absorbance at 1820 nm was perfectly proportional to the number of layers, demonstrating the successful, layer-by-layer growth of SWNT thin films. Moreover, polarized absorption and Raman spectra indicated a certain degree of tube orientation in the dipping direction. It was also possible to deposit LB films without the polymer matrix, though the film stability was somewhat lowered. Realization of SWNT thin films with precisely controlled thicknesses and optical transparency can be an important breakthrough for both basic understanding and technological applications of this novel form of carbon. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Nanotubes; LB films; Amphiphilic polymers; Orientation; Absorption; Raman spectrum
Realization of homogeneous thin films of singlewall carbon nanotubes (SWNTs) with precisely controlled nanostructures is an important pre-requisite for the development of basic understanding as well as in the technological applications of SWNTs. SWNT thin films were fabricated by various methods: spraying of ethanol dispersion [1,2], Langmuir–Blodgett (LB) mono-layers made from surfactant dispersion [3], etc. However, it has been difficult to simultaneously meet different requirements such as optical homogeneity, precise control of thickness, and control of tube orientation, all of which are indispensable for the characterization of their optoelectronic properties and for device applications of SWNT assem*Corresponding author. Tel.: +81-298-61-6309; fax: +81298-61-6243. E-mail address: [email protected] (N. Minami).
blies. To address this problem, a new technique to fabricate SWNT thin films must be developed. In the present paper, we report the successful deposition of multi-layer LB films of solubilized SWNTs with or without an amphiphilic polymer matrix, poly(N-dodecylacrylamide) (PDDA). Purification and solubilization of SWNTs were carried out using SWNT-containing raw soot (Carbolex, AP Grade) as a starting material. Shortening and solubilization of SWNTs were carried out following the literature [4,5]. The solubility in common solvents was brought about by the attachment of octadecylamine groups at both ends of each tube. The preparation of PDDA was described elsewhere [6]. SWNT/PDDA mixtures at various weight ratios were spread on the water surface from chloroform solutions and p2A isotherms were measured at 201C. All of the isotherms showed steep rises in the
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 0 9 7 5 - 4
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Y. Guo et al. / Physica B 323 (2002) 235–236
surface pressure and had high collapse pressures, just like a PDDA mono-layer itself. Such stability may be attributed to the good miscibility between the functional groups attached to the polymer backbone (dodecylamine) and the SWNTs (octadecylamine). We also suggest that hydrogen bonds, likely to be formed between these functional groups, should play an important role. All of these condensed mixed films on the water surface could be transferred vertically onto either hydrophobic or hydrophilic surfaces of solid substrates, such as glass, quartz, and silicon, forming Y-type films with a transfer ratio of almost unity. The films were optically homogeneous and no light scattering could be seen. Fig. 1 shows the visible to near infrared absorption spectra of mixed LB films with increasing number of layers, featuring the three absorption bands at 1820, 1000, and 700 nm, all characteristic of semiconducting and metallic SWNTs. As shown in the inset, the peak absorbance at 1820 nm is perfectly proportional to the number of layers at least up to 100 layers (50 layers on each side of the substrate). This demonstrates that the layer-bylayer deposition of homogeneous, multi-layer SWNT films with a precisely controlled thickness has been achieved. Fig. 2 shows the polarized Raman spectra of a 60 layer, mixed multi-layer film deposited on hydrophobic quartz. Both lines at 180 and 1590 cm 1 were stronger for the laser excitation
Fig. 2. Polarized Raman spectra of a 60 layer SWNT (37 wt%)/ PDDA mixed film deposited on hydrophobic quartz. The incident laser beam was polarized parallel (01) or perpendicular (901) to the dipping direction.
polarized parallel (01) to the dipping direction than that polarized perpendicular (901). The results show that SWNTs are preferentially oriented in the dipping direction to a certain extent. This was corroborated by polarized absorption spectra whose absorbance (at 1820 nm) was stronger for 01 than for 901. Multi-layer films consisting only of solubilized SWNTs could also be deposited on hydrophilic substrates with somewhat less stability, and a tube orientation similar to that in mixed films was obtained. The details will be reported elsewhere. To summarize, we have succeeded in depositing homogeneous multi-layer thin films of solubilized SWNTs using the LB technique. The tube orientation can be controlled to a certain extent, though its improvement remains an important problem to address. The availability of these thin films should accelerate the understanding of the electronic and optoelectronic properties of SWNTs, as various optical and electrical characterization techniques can now be applied. References
Fig. 1. Visible to near infrared absorption spectra of SWNT (37 wt%)/PDDA mixed films on hydrophobic quartz for increasing number of layers. Inset: Linear relationship between the absorbance at 1820 nm and the number of layers.
[1] [2] [3] [4] [5] [6]
H. Kataura, et al., Synth. Met. 103 (1999) 2555. S. Kazaoui, et al., Phys. Rev. B 60 (1999) 13339. V. Krstic, et al., Chem. Mater. 10 (1998) 2338. J. Liu, et al., Science 280 (1998) 1253. J. Chen, et al., Science 282 (1998) 95. T. Miyashita, et al., Br. Polym. J. 22 (1990) 327.
Multi-layer LB films of single-wall carbon nanotubes Yinzhong Guoa, Nobutsugu Minamia,*, Said Kazaouia, Junbiao Penga, Masaru Yoshidaa, Tokuji Miyashitab a
National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan b Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan
Abstract We propose a new processing technique of single-wall carbon nanotubes (SWNTs), namely, multi-layer Langmuir– Blodgett (LB) films. Solubilized SWNTs dispersed in an amphiphilic polymer matrix were spread on a water surface and vertically deposited on substrates. Optical absorbance at 1820 nm was perfectly proportional to the number of layers, demonstrating the successful, layer-by-layer growth of SWNT thin films. Moreover, polarized absorption and Raman spectra indicated a certain degree of tube orientation in the dipping direction. It was also possible to deposit LB films without the polymer matrix, though the film stability was somewhat lowered. Realization of SWNT thin films with precisely controlled thicknesses and optical transparency can be an important breakthrough for both basic understanding and technological applications of this novel form of carbon. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Nanotubes; LB films; Amphiphilic polymers; Orientation; Absorption; Raman spectrum
Realization of homogeneous thin films of singlewall carbon nanotubes (SWNTs) with precisely controlled nanostructures is an important pre-requisite for the development of basic understanding as well as in the technological applications of SWNTs. SWNT thin films were fabricated by various methods: spraying of ethanol dispersion [1,2], Langmuir–Blodgett (LB) mono-layers made from surfactant dispersion [3], etc. However, it has been difficult to simultaneously meet different requirements such as optical homogeneity, precise control of thickness, and control of tube orientation, all of which are indispensable for the characterization of their optoelectronic properties and for device applications of SWNT assem*Corresponding author. Tel.: +81-298-61-6309; fax: +81298-61-6243. E-mail address: [email protected] (N. Minami).
blies. To address this problem, a new technique to fabricate SWNT thin films must be developed. In the present paper, we report the successful deposition of multi-layer LB films of solubilized SWNTs with or without an amphiphilic polymer matrix, poly(N-dodecylacrylamide) (PDDA). Purification and solubilization of SWNTs were carried out using SWNT-containing raw soot (Carbolex, AP Grade) as a starting material. Shortening and solubilization of SWNTs were carried out following the literature [4,5]. The solubility in common solvents was brought about by the attachment of octadecylamine groups at both ends of each tube. The preparation of PDDA was described elsewhere [6]. SWNT/PDDA mixtures at various weight ratios were spread on the water surface from chloroform solutions and p2A isotherms were measured at 201C. All of the isotherms showed steep rises in the
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 0 9 7 5 - 4
236
Y. Guo et al. / Physica B 323 (2002) 235–236
surface pressure and had high collapse pressures, just like a PDDA mono-layer itself. Such stability may be attributed to the good miscibility between the functional groups attached to the polymer backbone (dodecylamine) and the SWNTs (octadecylamine). We also suggest that hydrogen bonds, likely to be formed between these functional groups, should play an important role. All of these condensed mixed films on the water surface could be transferred vertically onto either hydrophobic or hydrophilic surfaces of solid substrates, such as glass, quartz, and silicon, forming Y-type films with a transfer ratio of almost unity. The films were optically homogeneous and no light scattering could be seen. Fig. 1 shows the visible to near infrared absorption spectra of mixed LB films with increasing number of layers, featuring the three absorption bands at 1820, 1000, and 700 nm, all characteristic of semiconducting and metallic SWNTs. As shown in the inset, the peak absorbance at 1820 nm is perfectly proportional to the number of layers at least up to 100 layers (50 layers on each side of the substrate). This demonstrates that the layer-bylayer deposition of homogeneous, multi-layer SWNT films with a precisely controlled thickness has been achieved. Fig. 2 shows the polarized Raman spectra of a 60 layer, mixed multi-layer film deposited on hydrophobic quartz. Both lines at 180 and 1590 cm 1 were stronger for the laser excitation
Fig. 2. Polarized Raman spectra of a 60 layer SWNT (37 wt%)/ PDDA mixed film deposited on hydrophobic quartz. The incident laser beam was polarized parallel (01) or perpendicular (901) to the dipping direction.
polarized parallel (01) to the dipping direction than that polarized perpendicular (901). The results show that SWNTs are preferentially oriented in the dipping direction to a certain extent. This was corroborated by polarized absorption spectra whose absorbance (at 1820 nm) was stronger for 01 than for 901. Multi-layer films consisting only of solubilized SWNTs could also be deposited on hydrophilic substrates with somewhat less stability, and a tube orientation similar to that in mixed films was obtained. The details will be reported elsewhere. To summarize, we have succeeded in depositing homogeneous multi-layer thin films of solubilized SWNTs using the LB technique. The tube orientation can be controlled to a certain extent, though its improvement remains an important problem to address. The availability of these thin films should accelerate the understanding of the electronic and optoelectronic properties of SWNTs, as various optical and electrical characterization techniques can now be applied. References
Fig. 1. Visible to near infrared absorption spectra of SWNT (37 wt%)/PDDA mixed films on hydrophobic quartz for increasing number of layers. Inset: Linear relationship between the absorbance at 1820 nm and the number of layers.
[1] [2] [3] [4] [5] [6]
H. Kataura, et al., Synth. Met. 103 (1999) 2555. S. Kazaoui, et al., Phys. Rev. B 60 (1999) 13339. V. Krstic, et al., Chem. Mater. 10 (1998) 2338. J. Liu, et al., Science 280 (1998) 1253. J. Chen, et al., Science 282 (1998) 95. T. Miyashita, et al., Br. Polym. J. 22 (1990) 327.