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Synthesis of carbon nanotubes with identical dimensions using an anodic aluminum oxide template on a silicon wafer
Synthetic Metals 148 (2005) 263–266
Synthesis of carbon nanotubes with identical dimensions using an anodic aluminum oxide template on a silicon wafer Ok-Joo Leea,∗,1 , Sun-Kyu Hwangb,1 , Soo-Hwan Jeongc , Pyung Soo Leeb , Kun-Hong Leeb a
b
Basic Research Laboratory, Electronics and Telecommunications Research Institute (ETRI), Gajeong-dong, Yuseong-gu, Daejeon 305-350, South Korea Department of Chemical Engineering, Computer and Electrical Engineering Division, Pohang University of Science and Technology (POSTECH), Pohang 790-784, South Korea c U team, Samsung Advanced Institute of Technology (SAIT), Suwon 440-600, South Korea Received 27 July 2004; received in revised form 4 October 2004; accepted 5 October 2004 Available online 23 November 2004
Abstract Anodic aluminum oxide (AAO) templates with uniform channels of sub-micron length were fabricated on silicone wafers. Carbon was deposited on the wall of the pores via decomposition of acetylene at 800 ◦ C. The synthesized carbon nanotubes have identical dimensions of 900 nm in length and 70 nm in diameter. Raman spectrum showed that the crystallinity of these CNTs is relatively high though no catalyst was used. The proposed technique can be applied to the fabrication of vacuum microelectronic devices. © 2004 Elsevier B.V. All rights reserved. Keywords: Carbon nanotube; Anodic aluminium oxide (AAO); Thin film aluminium; Thin film alumina; Raman spectrum
1. Introduction Carbon nanotubes (CNTs) have drawn much attention because of their unique physical properties and wide variety of applications [1,2]. Various methods for the synthesis of CNTs have been reported: arc-discharge [3], laser ablation [4], chemical vapor deposition [5], high-pressure CO (HiPCO) process [6] and microwave synthesis [7]. Although the ability to control the morphology and the crystallinity of CNTs have been progressing continuously, current state-ofthe-art still needs much improvement to be applied to the fabrication of real electronic devices. In particular, a technique which is able to synthesize CNTs with a uniform size and high purity is highly desirable and is thought to be an ∗ Corresponding authors. Present address: Max Planck Institute for Polymer Research, Ackermanweg 10, D-55128 Mainz, Germany. Tel. +49 6131 379 366; fax: +49 6131 379 100. E-mail address: [email protected] (O.-J. Lee). 1 These authors contributed equally to this work.
0379-6779/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2004.10.005
important technical element in the application of CNTs in electronic devices. A simple and effective method to control the dimensions of CNTs is to use templates. Mesoporous silica was used as the template in the synthesis of CNTs with similar diameters [8]. Anodic aluminum oxide (AAO) templates have also been widely used [9,10]. An AAO template has uniform and straight channels, the dimensions of which can be controlled by applied voltage, type of electrolyte and temperature [11]. Various metal nanowires as well as CNTs have been synthesized, taking advantage of the straight channel structure of AAO templates [12–15]. Research on the synthesis of CNTs in AAO templates can be classified into two categories: works in the first category use high-purity Al sheets [16,17]. The temperature of CNT synthesis, however, is limited to below 660 ◦ C by the presence of the unanodized aluminum under the AAO template, resulting in poor crystallinity of CNTs. Complete anodization of the Al sheet can avoid this problem, though the synthesized CNTs are usually too long to be used in electronic devices [9].
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Alternatively, an AAO template with a thickness of less than a few microns can be separated from the underlying Al layer, but it would be very difficult to handle. The last and most effective method is to deposit metal catalysts at the bottom of the pores in AAO templates, then to synthesize CNTs. CNTs with high crystallinity can now be obtained under 660 ◦ C, though they usually outgrow from the pores, resulting in a wide distribution in length. The second category contains the works to fabricate AAO templates on silicon wafers [18,19]. Thin layer of Al is deposited on a Si wafer, and then the Al layer is completely converted to alumina through anodization. The temperature of CNT synthesis goes up much higher than 660 ◦ C, resulting in higher crystallinity of CNTs even without catalysts. In this paper, we demonstrate the synthesize CNTs with identical dimensions and good crystallinity. This method belongs to the second category. We have previously reported a method in the first category with some distribution in length [20], but present method produces almost identical CNTs.
2. Experimental Thin film aluminum (Al) was prepared for the anodization. Fig. 1 schematically depicts the overall fabrication process. At first, a niobium (Nb) layer of 200 nm thick was deposited on an oxidized silicon wafer using the magnetron sputter, and an Al layer of 4.4 m thick was deposited on it using the thermal evaporator as seen in Fig. 1(a). The initial pressure
of the evaporation was lower than 1.2 × 10−6 Torr and the average working pressure was 4 × 10−6 Torr. The Si wafer was rotated at a speed of 48 rpm during the evaporation. Subsequently, the Al layer was anodized at 40 V in 0.3 M oxalic acid at 15 ◦ C for 10 min with the Nb layer as the working electrode and a cylindrical carbon counter electrode. Etching of the anodized film was followed in a mixture of chromic acid and phosphoric acid for 30 min at 70 ◦ C. The anodizationetching cycle was repeated more than three times until the mirror surface was obtained. The thickness of the residual Al just before the final anodization should be enough for a desirable AAO thickness. In our experiments, the thickness ratio of the Al and the AAO was 1.45. The final anodization was carried out until the deposited Al was completely converted to AAO to avoid the temperature limitation in the synthesis of CNTs as shown in Fig. 1(b). The pores were further widened through wet etching in 0.1 M phosphoric acid for 50 min at 30 ◦ C after the final anodization (Fig. 1(c)). CNTs were synthesized using the AAO template on the Si wafer through the decomposition of acetylene (C2 H2 ) in 200 sccm flow rate of 10% C2 H2 in argon (Ar) at 800 ◦ C for 20 min in the infrared tube furnace (P65C, ULVAC Co.), which was presented in Fig. 1 (d). The oxygen plasma treatment was followed at 3 × 10−2 Torr for 10 min at a RF power of 100 W to remove the deposited carbon on the top surface of the AAO template (Fig. 1(e)). The CNTs were obtained by removing the AAO template in a mixture of chromic acid and phosphoric acid at 70 ◦ C overnight, and filtrating the CNTs-dispersed solution with an Isopore® (Millipore Co.) membrane filter (Fig. 1(f)). Morphology and crystallinity of the synthesized CNTs were examined using field emission scanning electron microscopy (FE-SEM; S-4200, Hitachi) and the Raman spectroscope (Raman System 3000, Renishaw). The Raman spectrum was obtained using an Ar-ion laser of 632.8 nm as an excitation source.
3. Results and discussion
Fig. 1. Schematic diagrams for the fabrication of AAO film and carbon nanotubes on a silicon wafer: (a) deposition of Al layer by a thermal evaporation on a silicon wafer; (b) formation of straight channels by multiple anodizations and etchings; (c) pore widening for 50 min in a phosphoric acid; (d) decomposition of acetylene at 800 ◦ C; (e) removal of surface carbon by O2 plasma treatment; (f) collection of CNTs by etching AAO film.
Fig. 2 shows the SEM images of the cross section of the AAO film. Straight pores with a parallel arrangement are observed. Geometrical perfection is usually better with the AAO templates on aluminum sheets than with the thin film AAO templates on silicon wafers. This is because of the technical difficulty of the deposition of thick aluminum layer on a silicon wafer, so that self-alignment of pores is limited. The pores of the present sample are 940 nm long and about 70 nm in diameter, though they seem to be somewhat smaller in the figure. This is simply because the cleavage for the analysis of SEM did not line the center of the pores. Although the diameters of pores appear slightly different in this figure due to the uneven cleavage during the sample preparation, the CNTs in Fig. 3 indirectly prove the existence of uniform and straight channels. The current decreased to zero at the end of the final
O.-J. Lee et al. / Synthetic Metals 148 (2005) 263–266
Fig. 2. SEM images for the cross-sectional view of anodic aluminum oxide films formed on the silicon wafer were shown. They had straight channels with the identical dimension of length and diameter. This sample was prepared by the three repetitions of anodization and etching cycle. The part of pore bottom was presented in inset, which has hemispherical pore structure. A scale bar in inset is 100 nm length.
anodization due to the formation of niobium oxide (Nb2 O5 ) layer. It took 11 min to fabricate this particular sample from the beginning to zero current. Note the columnar structure of the Nb2 O5 layer. Pores can be further widened by additional etching in phosphoric acid. The length of the pores also increases slightly during this isotropic etching, and the hemispherical shape of pore bottoms becomes more prominent. This hemispherical structure would be replicated into the morphology of the synthesized CNTs. The Al layer must be completely anodized to Al2 O3 . Otherwise, residual Al can diffuse into AAO templates in the synthesis of CNTs at high temperatures [21]. Solubility of AAO templates in the etchant solutions, such as a mixture of
Fig. 3. SEM images of CNTs synthesized in the AAO widened for 50 min, which are collected on the filter membrane by a vacuum filtration system after etching AAO. All the CNTs have the identical dimension. An individual CNT was shown in inset—it has the uniform diameter. A scale bar in inset is 300 nm length. The pores of the membrane are shown in the left-upper part of the figure and the inset.
265
chromic acid and phosphoric acid or a concentrated sodium hydroxide, rapidly decreases by the diffusion of Al into the AAO templates, thus the separation of CNTs from the AAO templates becomes difficult. This is the main reason why bulk Al sheets is not a good choice as the templates for the synthesis of CNTs. An AAO template can be separated from the underlying aluminum layer even with a bulk Al sheet provided the AAO template is separated from the underlying Al layer in a saturated mercuric chloride solution. However, a standalone AAO template with a thickness of a few microns is very fragile and difficult to handle. When the thick Al layer under the AAO template is not removed for the convenience of handling, the low melting point of Al limits the temperature of CNT synthesis below 660 ◦ C, resulting in poor crystallinity. Fig. 3 shows the SEM images of collected CNTs. The CNTs in this figure have almost identical dimensions. The average length of CNTs is about 900 nm, which is shorter than that of the AAO channels in Fig. 2. It can be attributed to the burning out of CNTs at the mouths of pores by reactive oxygen ions during oxygen plasma treatment. The average diameter of CNTs is 70 nm. The inset is the SEM image of an individual CNT with a uniform diameter. Note that this CNT has different morphologies at each end. The right end has a hemispherical shape and is believed to be synthesized from the bottom part of the AAO template. On the other hand, the left end has a relatively rough structure, which comes from the top part of the AAO template through burning out of carbon by reactive oxygen ions. The reactivity in the two ends of the CNT is expected to be different and this fact can be utilized for the selective functionalization of only one end. In fact, CNTs can be aligned on a gold substrate through the functionalization of the reactive end by using a self-assembly monolayer technique [22]. Raman spectrum, shown in Fig. 4, was used to investigate the crystallinity of CNTs. The shifted signal of a reflected beam from the CNTs had some noise because the thickness of CNTs lying on the membrane was very thin and some part of an incident beam struck a bare membrane. However, the peak configurations of the signal were enough to give information on the graphite wall. As seen in Fig. 4, the spectrum was separated into three distinctive peaks in the region from 1000 to 2000 cm−1 by deconvolution fitting. The peak at 1582 cm−1 (G-band) is a feature of two carbons stretching to opposite directions, which is known to be a characteristic of graphitic walls. The peak at 1337 cm−1 (D-band) is attributed
Fig. 4. Raman spectrum of the CNTs on the filter membrane.
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to the vibrating carbons at bent positions. The small peak at 1610 cm−1 also arises due to the bent carbons. The size of the peak at 1337 cm−1 is about the same as that at 1582 cm−1 , that is to say, the crystallinity is limited. This is typical for CNTs synthesized without metal catalysts [9], though our CNTs possess relatively high crystallinity. From a practical point of view, the ability to control the dimensions of CNTs is very important to fabricate efficient field emitters with triode structures. The leakage current would be large if the heights of the CNT emitters are lower than the gate position, while the control of anode current would be difficult when the heights of the CNT emitters are higher than the gate position. The distance between the tip of an emitter and the gate is only a few microns in modern vacuum microelectronic devices, so that control of the dimensions of CNTs is an important technological element in the fabrication of microelectronic devices. For example, if the CNTs of the same length were attached to the hole of the gold bottom with the self-assembled monolayer technique, the triode structure with the emitter tips located at the surface of the gate electrode can be fabricated [22]. 4. Conclusions CNTs with an identical dimension and a sub-micrometer length were synthesized using the AAO templates on Si wafers. Two ends of the CNTs were distinctively different in morphology and possibly in chemical nature, so that selective functionalization of one end is likely to be achieved. Proposed technique is simple, straightforward, yet an evolutionary step toward the fabrication of vacuum microelectronic devices. Acknowledgements This work was supported by the National R&D Project for Nano Science and Technology and by the Brain Korea 21 Project in 2003.
References [1] W.A. de Heer, A. Chatelain, D. Ugarte, Science 270 (1995) 1179. [2] P.G. Collins, A. Zettle, H. Bando, A. Thess, R.E. Smally, Science 278 (1997) 100. [3] P.M. Ajayan, J.M. Lambert, P. Bernier, L. Barbedette, C. Colliex, J.M. Planeix, Chem. Phys. Lett. 215 (1993) 509. [4] L.-C. Qin, S. Iijima, Chem. 1: Phys. Lett. 269 (1997) 65. [5] C.J. Lee, D.W. Kim, T.J. Lee, Y.C. Choi, Y.S. Park, Y.H. Lee, W.B. Choi, N.S. Lee, G.-S. Park, J.M. Kim, Chem. Phys. Lett. 312 (1999) 461. [6] J.H. Hafner, M.J. Bronikowski, B.R. Azamian, P. Nikolaev, A.G. Rinzler, D.T. Colbert, K.A. Smith, R.E. Smalley, Chem. Phys. Lett. 196 (1998) 195. [7] E.-H. Hong, K.-H. Lee, S.H. Oh, C.-G. Park, Adv. Mater. 14 (2002) 676. [8] W.Z. Li, S.S. Xie, L.X. Qian, B.H. Chang, B.S. Zou, W.Y. Zhou, R.A. Zhao, G. Wang, Science 274 (1996) 1701. [9] T. Kyotani, L. Tsai, A. Tomita, Chem. Mater. 8 (1996) 2109. [10] J. Li, C. Papadopoulos, J.M. Xu, Appl. Phys. Lett. 75 (1999) 367. [11] F. Keller, M.S. Hunter, D.L. Robinson, J. Electrochem. Soc. 100 (1953) 411. [12] S.R. Nicewarner-Pe˜na, R.G. Freeman, B.D. Reiss, L. He, D.J. Pe˜na, I.D. Walton, R. Cromer, C.D. Keating, M.J. Natan, Science 294 (2001) 137. [13] D. AlMawlawi, N. Coombs, M. Moskovits, J. Appl. Phys. 70 (1991) 4421. [14] D.H. Qin, M. Lu, H.L. Li, Chem. Phys. Lett. 350 (2001) 51. [15] S. Yang, H. Zhu, D. Yu, Z. Jin, S. Tang, Y. Du, J. Magn. Magn. Mater. 222 (2000) 97. [16] J.S. Suh, K.S. Jeong, J.S. Lee, I. Han, Appl. Phys. Lett. 80 (2002) 2392. [17] W.B. Choi, B.-H. Cheong, J.J. Kim, J. Ju, E. Bae, Adv. Funct. Mater. 12 (2002) 1. [18] T. Iwasaki, T. Motoi, T. Den, Appl. Phys. Lett. 75 (1999) 2044. [19] Jeong, H.-Y. Hwang, K.-H. Lee, Y. Jeong, Appl. Phys. Lett. 78 (2001) 2052. [20] S.-H. Jeong, O.-J. Lee, K.-H. Lee, S.H. Oh, C.-G. Park, Chem. Mater. 14 (2002) 1859. [21] M. Garcia-Mendez, N. Vallas-Villarreal, G.A. Hirata-Flares, M.H. Farias, Appl. Surf. Sci. 151 (1999) 139. [22] O.-J. Lee, K.-H. Lee, Appl. Phys. Lett. 82 (2003) 3770.
Synthesis of carbon nanotubes with identical dimensions using an anodic aluminum oxide template on a silicon wafer Ok-Joo Leea,∗,1 , Sun-Kyu Hwangb,1 , Soo-Hwan Jeongc , Pyung Soo Leeb , Kun-Hong Leeb a
b
Basic Research Laboratory, Electronics and Telecommunications Research Institute (ETRI), Gajeong-dong, Yuseong-gu, Daejeon 305-350, South Korea Department of Chemical Engineering, Computer and Electrical Engineering Division, Pohang University of Science and Technology (POSTECH), Pohang 790-784, South Korea c U team, Samsung Advanced Institute of Technology (SAIT), Suwon 440-600, South Korea Received 27 July 2004; received in revised form 4 October 2004; accepted 5 October 2004 Available online 23 November 2004
Abstract Anodic aluminum oxide (AAO) templates with uniform channels of sub-micron length were fabricated on silicone wafers. Carbon was deposited on the wall of the pores via decomposition of acetylene at 800 ◦ C. The synthesized carbon nanotubes have identical dimensions of 900 nm in length and 70 nm in diameter. Raman spectrum showed that the crystallinity of these CNTs is relatively high though no catalyst was used. The proposed technique can be applied to the fabrication of vacuum microelectronic devices. © 2004 Elsevier B.V. All rights reserved. Keywords: Carbon nanotube; Anodic aluminium oxide (AAO); Thin film aluminium; Thin film alumina; Raman spectrum
1. Introduction Carbon nanotubes (CNTs) have drawn much attention because of their unique physical properties and wide variety of applications [1,2]. Various methods for the synthesis of CNTs have been reported: arc-discharge [3], laser ablation [4], chemical vapor deposition [5], high-pressure CO (HiPCO) process [6] and microwave synthesis [7]. Although the ability to control the morphology and the crystallinity of CNTs have been progressing continuously, current state-ofthe-art still needs much improvement to be applied to the fabrication of real electronic devices. In particular, a technique which is able to synthesize CNTs with a uniform size and high purity is highly desirable and is thought to be an ∗ Corresponding authors. Present address: Max Planck Institute for Polymer Research, Ackermanweg 10, D-55128 Mainz, Germany. Tel. +49 6131 379 366; fax: +49 6131 379 100. E-mail address: [email protected] (O.-J. Lee). 1 These authors contributed equally to this work.
0379-6779/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2004.10.005
important technical element in the application of CNTs in electronic devices. A simple and effective method to control the dimensions of CNTs is to use templates. Mesoporous silica was used as the template in the synthesis of CNTs with similar diameters [8]. Anodic aluminum oxide (AAO) templates have also been widely used [9,10]. An AAO template has uniform and straight channels, the dimensions of which can be controlled by applied voltage, type of electrolyte and temperature [11]. Various metal nanowires as well as CNTs have been synthesized, taking advantage of the straight channel structure of AAO templates [12–15]. Research on the synthesis of CNTs in AAO templates can be classified into two categories: works in the first category use high-purity Al sheets [16,17]. The temperature of CNT synthesis, however, is limited to below 660 ◦ C by the presence of the unanodized aluminum under the AAO template, resulting in poor crystallinity of CNTs. Complete anodization of the Al sheet can avoid this problem, though the synthesized CNTs are usually too long to be used in electronic devices [9].
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O.-J. Lee et al. / Synthetic Metals 148 (2005) 263–266
Alternatively, an AAO template with a thickness of less than a few microns can be separated from the underlying Al layer, but it would be very difficult to handle. The last and most effective method is to deposit metal catalysts at the bottom of the pores in AAO templates, then to synthesize CNTs. CNTs with high crystallinity can now be obtained under 660 ◦ C, though they usually outgrow from the pores, resulting in a wide distribution in length. The second category contains the works to fabricate AAO templates on silicon wafers [18,19]. Thin layer of Al is deposited on a Si wafer, and then the Al layer is completely converted to alumina through anodization. The temperature of CNT synthesis goes up much higher than 660 ◦ C, resulting in higher crystallinity of CNTs even without catalysts. In this paper, we demonstrate the synthesize CNTs with identical dimensions and good crystallinity. This method belongs to the second category. We have previously reported a method in the first category with some distribution in length [20], but present method produces almost identical CNTs.
2. Experimental Thin film aluminum (Al) was prepared for the anodization. Fig. 1 schematically depicts the overall fabrication process. At first, a niobium (Nb) layer of 200 nm thick was deposited on an oxidized silicon wafer using the magnetron sputter, and an Al layer of 4.4 m thick was deposited on it using the thermal evaporator as seen in Fig. 1(a). The initial pressure
of the evaporation was lower than 1.2 × 10−6 Torr and the average working pressure was 4 × 10−6 Torr. The Si wafer was rotated at a speed of 48 rpm during the evaporation. Subsequently, the Al layer was anodized at 40 V in 0.3 M oxalic acid at 15 ◦ C for 10 min with the Nb layer as the working electrode and a cylindrical carbon counter electrode. Etching of the anodized film was followed in a mixture of chromic acid and phosphoric acid for 30 min at 70 ◦ C. The anodizationetching cycle was repeated more than three times until the mirror surface was obtained. The thickness of the residual Al just before the final anodization should be enough for a desirable AAO thickness. In our experiments, the thickness ratio of the Al and the AAO was 1.45. The final anodization was carried out until the deposited Al was completely converted to AAO to avoid the temperature limitation in the synthesis of CNTs as shown in Fig. 1(b). The pores were further widened through wet etching in 0.1 M phosphoric acid for 50 min at 30 ◦ C after the final anodization (Fig. 1(c)). CNTs were synthesized using the AAO template on the Si wafer through the decomposition of acetylene (C2 H2 ) in 200 sccm flow rate of 10% C2 H2 in argon (Ar) at 800 ◦ C for 20 min in the infrared tube furnace (P65C, ULVAC Co.), which was presented in Fig. 1 (d). The oxygen plasma treatment was followed at 3 × 10−2 Torr for 10 min at a RF power of 100 W to remove the deposited carbon on the top surface of the AAO template (Fig. 1(e)). The CNTs were obtained by removing the AAO template in a mixture of chromic acid and phosphoric acid at 70 ◦ C overnight, and filtrating the CNTs-dispersed solution with an Isopore® (Millipore Co.) membrane filter (Fig. 1(f)). Morphology and crystallinity of the synthesized CNTs were examined using field emission scanning electron microscopy (FE-SEM; S-4200, Hitachi) and the Raman spectroscope (Raman System 3000, Renishaw). The Raman spectrum was obtained using an Ar-ion laser of 632.8 nm as an excitation source.
3. Results and discussion
Fig. 1. Schematic diagrams for the fabrication of AAO film and carbon nanotubes on a silicon wafer: (a) deposition of Al layer by a thermal evaporation on a silicon wafer; (b) formation of straight channels by multiple anodizations and etchings; (c) pore widening for 50 min in a phosphoric acid; (d) decomposition of acetylene at 800 ◦ C; (e) removal of surface carbon by O2 plasma treatment; (f) collection of CNTs by etching AAO film.
Fig. 2 shows the SEM images of the cross section of the AAO film. Straight pores with a parallel arrangement are observed. Geometrical perfection is usually better with the AAO templates on aluminum sheets than with the thin film AAO templates on silicon wafers. This is because of the technical difficulty of the deposition of thick aluminum layer on a silicon wafer, so that self-alignment of pores is limited. The pores of the present sample are 940 nm long and about 70 nm in diameter, though they seem to be somewhat smaller in the figure. This is simply because the cleavage for the analysis of SEM did not line the center of the pores. Although the diameters of pores appear slightly different in this figure due to the uneven cleavage during the sample preparation, the CNTs in Fig. 3 indirectly prove the existence of uniform and straight channels. The current decreased to zero at the end of the final
O.-J. Lee et al. / Synthetic Metals 148 (2005) 263–266
Fig. 2. SEM images for the cross-sectional view of anodic aluminum oxide films formed on the silicon wafer were shown. They had straight channels with the identical dimension of length and diameter. This sample was prepared by the three repetitions of anodization and etching cycle. The part of pore bottom was presented in inset, which has hemispherical pore structure. A scale bar in inset is 100 nm length.
anodization due to the formation of niobium oxide (Nb2 O5 ) layer. It took 11 min to fabricate this particular sample from the beginning to zero current. Note the columnar structure of the Nb2 O5 layer. Pores can be further widened by additional etching in phosphoric acid. The length of the pores also increases slightly during this isotropic etching, and the hemispherical shape of pore bottoms becomes more prominent. This hemispherical structure would be replicated into the morphology of the synthesized CNTs. The Al layer must be completely anodized to Al2 O3 . Otherwise, residual Al can diffuse into AAO templates in the synthesis of CNTs at high temperatures [21]. Solubility of AAO templates in the etchant solutions, such as a mixture of
Fig. 3. SEM images of CNTs synthesized in the AAO widened for 50 min, which are collected on the filter membrane by a vacuum filtration system after etching AAO. All the CNTs have the identical dimension. An individual CNT was shown in inset—it has the uniform diameter. A scale bar in inset is 300 nm length. The pores of the membrane are shown in the left-upper part of the figure and the inset.
265
chromic acid and phosphoric acid or a concentrated sodium hydroxide, rapidly decreases by the diffusion of Al into the AAO templates, thus the separation of CNTs from the AAO templates becomes difficult. This is the main reason why bulk Al sheets is not a good choice as the templates for the synthesis of CNTs. An AAO template can be separated from the underlying aluminum layer even with a bulk Al sheet provided the AAO template is separated from the underlying Al layer in a saturated mercuric chloride solution. However, a standalone AAO template with a thickness of a few microns is very fragile and difficult to handle. When the thick Al layer under the AAO template is not removed for the convenience of handling, the low melting point of Al limits the temperature of CNT synthesis below 660 ◦ C, resulting in poor crystallinity. Fig. 3 shows the SEM images of collected CNTs. The CNTs in this figure have almost identical dimensions. The average length of CNTs is about 900 nm, which is shorter than that of the AAO channels in Fig. 2. It can be attributed to the burning out of CNTs at the mouths of pores by reactive oxygen ions during oxygen plasma treatment. The average diameter of CNTs is 70 nm. The inset is the SEM image of an individual CNT with a uniform diameter. Note that this CNT has different morphologies at each end. The right end has a hemispherical shape and is believed to be synthesized from the bottom part of the AAO template. On the other hand, the left end has a relatively rough structure, which comes from the top part of the AAO template through burning out of carbon by reactive oxygen ions. The reactivity in the two ends of the CNT is expected to be different and this fact can be utilized for the selective functionalization of only one end. In fact, CNTs can be aligned on a gold substrate through the functionalization of the reactive end by using a self-assembly monolayer technique [22]. Raman spectrum, shown in Fig. 4, was used to investigate the crystallinity of CNTs. The shifted signal of a reflected beam from the CNTs had some noise because the thickness of CNTs lying on the membrane was very thin and some part of an incident beam struck a bare membrane. However, the peak configurations of the signal were enough to give information on the graphite wall. As seen in Fig. 4, the spectrum was separated into three distinctive peaks in the region from 1000 to 2000 cm−1 by deconvolution fitting. The peak at 1582 cm−1 (G-band) is a feature of two carbons stretching to opposite directions, which is known to be a characteristic of graphitic walls. The peak at 1337 cm−1 (D-band) is attributed
Fig. 4. Raman spectrum of the CNTs on the filter membrane.
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to the vibrating carbons at bent positions. The small peak at 1610 cm−1 also arises due to the bent carbons. The size of the peak at 1337 cm−1 is about the same as that at 1582 cm−1 , that is to say, the crystallinity is limited. This is typical for CNTs synthesized without metal catalysts [9], though our CNTs possess relatively high crystallinity. From a practical point of view, the ability to control the dimensions of CNTs is very important to fabricate efficient field emitters with triode structures. The leakage current would be large if the heights of the CNT emitters are lower than the gate position, while the control of anode current would be difficult when the heights of the CNT emitters are higher than the gate position. The distance between the tip of an emitter and the gate is only a few microns in modern vacuum microelectronic devices, so that control of the dimensions of CNTs is an important technological element in the fabrication of microelectronic devices. For example, if the CNTs of the same length were attached to the hole of the gold bottom with the self-assembled monolayer technique, the triode structure with the emitter tips located at the surface of the gate electrode can be fabricated [22]. 4. Conclusions CNTs with an identical dimension and a sub-micrometer length were synthesized using the AAO templates on Si wafers. Two ends of the CNTs were distinctively different in morphology and possibly in chemical nature, so that selective functionalization of one end is likely to be achieved. Proposed technique is simple, straightforward, yet an evolutionary step toward the fabrication of vacuum microelectronic devices. Acknowledgements This work was supported by the National R&D Project for Nano Science and Technology and by the Brain Korea 21 Project in 2003.
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