Applied Surface Science 255 (2009) 7713–7718
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Growth of patterned and aligned carbon nanotube bundles on micro-structured substrate Yongsheng Shi *, Yucheng Ding, Hongzhong Liu, Weitao Jiang, Bingheng Lu State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an 710049, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 25 June 2008 Received in revised form 18 February 2009 Accepted 20 April 2009 Available online 3 May 2009
Microstructures are used as inducement for growth of patterned and aligned carbon nanotube (CNT) bundles by pyrolysis of iron phthalocyanine (FePc) under H2/Ar. The flow of mixture gas can be influenced by geometry profile of microstructure, and the distribution density of catalyst will be different related to the different microstructure. Many types of substrates with different microstructures are used in this study, and several different profiles of CNT bundles are achieved under different process conditions, especially an apical dominance like plant growth is observed under specific H2/Ar flow rate. Through using appropriate microstructures and controlling the flow rate, the density of CNT bundles can be adjusted, which is very important for weakening electric field shielding effect. ß 2009 Elsevier B.V. All rights reserved.
PACS: 81.15.Gh Keywords: CNT bundles Pyrolysis Microstructure Electric field shielding effect
1. Introduction Since the discovery of carbon nanotubes (CNTs) by Iijima [1], there have been numerous literatures reporting functional devices based on CNTs including light-emitting diodes [2], transistors [3], field emitters [4], photovoltaic devices [5], fuel cells [6] and hydrogen storage [7] because of its unique structure and properties. For well-aligned CNTs, adjusting its site density is very important for certain applications, such as field emission [8–9], nanoelectrode array [10], etc. because of the electric field shielding effects from closely packed arrays of CNTs [11–12]. The current main methods used to reduce the site density of CNTs array are to reduce the catalyst site density by pre-patterning catalyst for CNTs growth using electron-beam lithography (EBL), photolithography [13–14], microcontact printing [15–16], ink-jet printing [17], electrochemical deposition [18] and self-assembly [19]. However, all these methods either require expensive equipment to pattern the catalyst or cannot control the uniform in large area. We have developed a simple and efficient method to directly grow patterned and aligned CNT bundles on pre-patterned silicon
* Corresponding author. E-mail address:
[email protected] (Y. Shi). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.04.146
substrate by pyrolysis of iron phthalocyanine (FePc) under H2/Ar. Using this method, we can adjust the density of CNT bundles without predeposition of the metal catalyst pattern, which can effectively weaken electric field shielding effect of the dense array by reducing the density of CNT bundles. 2. Experiments The schematic diagram of fabrication process for pattered and aligned CNT bundles on micro-structured substrates is shown in Fig. 1. The photoresist (PR) is coated on a low resistance silicon substrate, and then the opened PR pattern is formed by developer after UV exposure (Fig. 1(a)). Etching of silicon substrate is carried out in a silicon etchant (forming microstructure with slope sidewalls) or using dry etching process (forming microstructures with vertical sidewalls). The micro-structured substrates are prepared after PR removing (Fig. 1(c)). Two types of substrates with different microstructures, including concave holes (50 mm square holes, 50 mm cone-shaped holes), convex surface (50 mm cubes, 3 mm frustums), are used in this study. Then the micro-structured substrates and FePc powder are placed in pyrolysis apparatus for the growth of CNT bundles. The apparatus consists of two furnaces fitted with a quartz tube (inner diameter, 35 mm) and two independent temperature
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Fig. 2. Schematic diagram of apparatus employed for synsesizing pattered and aligned CNT bundles on micro-structured substrates by pyrolysis of FePc.
Fig. 1. Schematic diagram of synsesizing pattered and aligned CNT bundles on micro-structured substrates.
controllers, as shown in Fig. 2. A certain quantity of FePc powder and micro-structured substrates are taken in a quartz boat and placed inside the quartz tube. The distance between the FePc powder and the micro-structured substrates is in the range of 5– 15 cm, which ensures that the FePc powder and the substrates are exactly placed in the two furnaces, respectively. The growth process is programmed to two heating stages: the temperature of the first quartz furnace is increased to 610 8C and held for 2– 5 min for the evaporation of the FePc powder; a blue vapor of
Fig. 3. SEM micrographics of concave hole: (a) 50 mm square holes before pyrolysis; (b) 50 mm cone-shaped holes before pyrolysis; (c) 50 mm square holes after pyrolysis; (d) 50 mm cone-shaped holes after pyrolysis.
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Fig. 4. SEM micrographics of convexities: (a) 50 mm cubes before pyrolysis; (b) 3 mm frustums of pyramid before pyrolysis; (c) 50 mm cubes before pyrolysis; (d) 3 mm frustum of pyramid after pyrolysis (CNT bundles on the lower part of the picture were stripped off using a needle).
FePc is generated inside the first quartz furnace. Then the vapor is carried by the Ar and H2 to the second quartz furnace, at the same time, the temperature of the second quartz furnace is increased again up to 900 8C for the pyrolysis of the FePc vapor. In the pyrolysis period, the FePc vapor is deoxidized to Fe catalyst and carbon source particles by H2. Then, these particles (Fe catalyst and carbon source particles) are carried by Ar gas flow to the micro-structured substrates and deposited on the surface of microstructures (Fig. 1(d)). The temperature of the second quartz furnace is held for 5 min for CNT bundles growth. Fig. 1(e) shows the schematic profile of as-grown CNT bundles. The morphology of as-grown CNTs and the surface of the patterned substrates are examined using scanning electron microscopy (SEM). 3. Results and discussion It is interesting to find that aligned CNT bundles grown on the concave holes are very different from its grown on the convex surface. Fig. 3 shows the SEM micrographs of concave holes before and after pyrolysis process. As we expect, a layer of dense CNT bundles are grown on the surface of the concave holes. The CNT
bundles are well-aligned and perpendicular to every surfaces of microstructures including the vertical (Fig. 3(c)) and slope sidewalls (Fig. 3(d)). Fig. 3(d) shows the details of CNT bundles grown on the 50 mm cone-shaped holes. The high magnification parts on the top of Fig. 3(d) show micrographs of CNT bundles grown on the top corner and bottom corner of 50 mm cone-shaped holes, respectively. Fig. 4 shows the SEM micrographs of convex microstructures before and after pyrolysis process. Unlike the Fig. 3(c) and (d), the profiles of CNT bundles grown on convex microstructure are beyond all expectations. It can be found that aligned CNT bundles only grow on the sidewall of 50 mm cubes, and only few individual CNT bundles grow on the top and the spaces of the cubes, as shown in Fig. 4(c). It is very clear that the density of catalyst (the white dots, in Fig. 4(c)) is very sparse on the space and top of 50 mm cubes. It indicates the preferred location of catalyst is the sidewalls of the cubes, so the aligned CNT bundles cannot be formed at the top and space between the cubes, which will be discussed later. There are obvious differences in density and profile of CNT bundles in Fig. 4(c) and (d). The CNT bundles grown on 3 mm frustums reveal an upward configuration, which looked like apical dominance as plant growth. To investigate the
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Fig. 5. Schematic diagram of catalyst and carbon source particles deposition on (a) a concave substrate; (b) a convex substrate.
further growth mechanism, a small area CNT bundles are stripped off using a needle, as shown in Fig. 4(d). The top of CNT bundles which grow on the sidewall of adjacent frustums touch each other, and shadows are formed in the spaces of frustums. These shadows can obstruct the carbon source particles continually deposit on the catalysts, so only few individual nanotubes grow on the space of frustums, as shown in the higher magnification part of Fig. 4(d). The reasons of different CNT bundles’ profiles showed on different microstructures are not fully understood at this stage. However, it is believed that there are two main reasons for the selective growth of CNT bundles on the micro-structured substrates. The first reason may be that the selective location of catalyst and carbon source particles during deposition. Fig. 5 shows the effect of concave and convex microstructures on the catalyst and carbon source particles deposition. As described in the former part, the Ar gas carries the FePc vapor from the first furnaces to the second furnaces for pyrolysis of FePc. When the mixture gas (Ar, pyrolysis particles, remnant H2) arrive surface of substrate, an abnormal flow will appears because of the effect of microstructure. It is well known that the influence of convex microstructures to the gas flow is much more than that of concave microstructures. When the mixture gas arrives at the surface of concave microstructures, the particles will be deposited on the all surface of the concaves, including the top, the bottom, sidewalls, as shown in Fig. 5(a). So, highly dense CNT bundles grow vertically on all sides of concave structures, as shown in Fig. 3(c) and (d). To convex microstructure, the preferred location of these particles is the sidewalls of the microstructures, which could be related to the effect of convex microstructure on the flow of the mixture gas. Sathaye and Lal have obtained fluid vortices by embedding microstructures in microfluidic channels and developed a microfluidic particle
capture device [20,21]. According to their research, the vortices trap particles traveling in the channel due to pressure driven flow. An intuitionistic and simple illustration (Fig. 5(b)) can be used for demonstrating the capture procedure. Fig. 5(b) shows vortices would be formed because of the obstruction from the convex microstructures. In particular, the obstruction result in the pressure gradients when the fluid (Ar, pyrolysis particles, remnant H2) travels on the convex microstructures in the quartz tube. The pressure gradients near the microstructures are used to obtain fluid vortices which trap pyrolysis particles due to pressure driven flow. In the magnification part of Fig. 5(b), a schematic diagram shows a vortice which consists of pyrolysis particles. The arrows (on the edge of vortice) show the deposition direction (movement direction) of the pyrolysis particles which is influenced by the movement of carrier gas and the obstruct of convex microstructures. Clearly, the deposition direction of pyrolysis particles is approximately horizontal on the top and the space of convex microstructures, while, the deposition direction of most particles is towards to the sidewalls. Thus, the abnormal flow of mixture gas results in congregation of catalyst on the sidewalls of microstructures, and only a sparse catalyst layer is formed at the top and space between the structures. Specially, the catalysts accumulate on the slope sidewall, and a highly dense and aligned CNT bundles will grow on it, which intervenes each other and appear an upward profile, as shown in Fig. 4(d). The second reason is the size effect of microstructures on the growth of CNT bundles. The length of as-grown CNT is 8–10 mm, which is much smaller than the dimension of the concave holes (50 mm square holes, 50 mm cone-shaped holes), 50 mm convex cubes and longer than the dimension of the 3 mm frustums. So the profiles of CNT bundles grown on 50 mm microstructures (Figs. 3(c) and (d) and 4(c)) can show a ‘‘free’’ configuration (well-aligned and perpendicular to the base surface). But the profiles of CNT bundles grown on 3 mm frustums are very different from that grown on 50 mm microstructures. The growth of CNT bundles in Figs. 3(c) and (d) and 4(c) would not be disturbed by near microstructures or other growing CNT bundles because the pitch of these microstructures is big enough. So, the orientation of CNT bundles is vertical to every surface of microstructure, and the boundary was very clear. Well-aligned CNT bundles can grow on micro-structured substrates with different orientation under controlled gas flow rates. Fig. 6 shows tilted SEM images of patterned and aligned CNT bundles grown on 3 mm frustums with different flow rates of H2 while keeping the same total gas flow rate of 200 sccm (standard cubic centimeter per minute, ml/min). At 20 sccm of H2, only few nanotubes can grow. As shown in Fig. 6(a), most of FePc is re-deposited on the substrates because of lack of reducer (H2). With the increment of the flow rate of H2 from 20 to 120 sccm, more and more FePc is deoxidized to Fe catalyst and carbon source particles, resulting in different growing orientations of CNT bundles. When the H2 flow rate increases to 40 sccm, the patterned and aligned CNT bundles begin to form. Fe catalyst particles are congregated on the sidewalls and specially the four corners at the bottoms of frustums. Due to the flow rate of carrier gas (Ar) is still predominant compare to that of H2, most of catalyst particles which deposited on the top of microstructure are blown away by whirlpools caused by frustums. So, few nanotubes are brought up on the top surface as shown in Fig. 6(b). Fig. 6(c) and (d) shows the patterned and aligned CNT bundles profile at 80 and 120 sccm, respectively. We only find that the angles of CNT bundles grown on the sidewalls of 3 mm frustums to the upright direction can be increased with the increment of H2 flow rate, but the reliable reason cannot be given at present.
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Fig. 6. SEM micrographs for patterned and aligned CNT bundles synthesized on 3 mm frustums with the H2/Ar flow rate (a) 20/180; (b) 40/160; (c) 80/120; (d) 120/80.
4. Conclusions In conclusion, we demonstrated the FePc pyrolysis growing method of patterned and aligned CNTs by microstructure inducement. The induced growing method with different process parameters allow CNT bundles developing in some directions, forming many kinds of CNTs patterns. The patterned and aligned CNT bundles can improve the distribution of electric field and weaken electric field shielding effect. The good characteristics of patterned CNTs are well suited for ordered arrays of CNTs emitters. And this CNT bundles growth process is compatible with silicon integrated circuit processing, which can provide a possible way for incorporating patterned and aligned CNT bundles into some photoelectric device. Acknowledgements This work was supported by the National Science Foundation of China (grant no. 50505037 and no. 50775176), China’s 863 Hi-Tech
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