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Patterned growth of vertically aligned carbon nanotube arrays using colloidal lithography and plasma enhanced chemical vapor deposition
Accepted Manuscript Patterned growth of vertically aligned carbon nanotube arrays using colloidal lithography and plasma enhanced chemical vapor deposition Yuhong Man, Zhiqian Chen, Yongping Zhang, Peitao Guo PII:
S0925-8388(15)30684-8
DOI:
10.1016/j.jallcom.2015.07.286
Reference:
JALCOM 34982
To appear in:
Journal of Alloys and Compounds
Received Date: 7 March 2015 Revised Date:
29 July 2015
Accepted Date: 30 July 2015
Please cite this article as: Y. Man, Z. Chen, Y. Zhang, P. Guo, Patterned growth of vertically aligned carbon nanotube arrays using colloidal lithography and plasma enhanced chemical vapor deposition, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.07.286. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Patterned growth of vertically aligned carbon nanotube arrays using colloidal lithography and plasma enhanced chemical vapor deposition Yuhong Man a,∗, Zhiqian Chen a ,Yongping Zhang a , Peitao Guo a Faculty of Materials and Energy, Southwest University, Chongqing 400715, China
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Abstract. Patterned carbon nanotube structures were synthesized on silicon substrates by
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plasma enhanced chemical vapor deposition and solution dipping deposition assisted by
monolayer colloidal crystals for the first time. Through the regulation of the concentration
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of the precursor solution and plasma treatment conditions, we have grown carbon nanotube bundles into periodic structures with different growth site and density. The proposed method greatly reduces the fabrication cost and can tune the periodic structure of carbon nanotubes quite flexibly. Furthermore, the synthesis strategy can also be applied to
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synthesize CNT/ Fe3C nanocomposites with excellent magnetic property, indicating a potential application in magnetic devices.
Keywords: carbon nanotubes; plasma enhanced chemical vapor deposition; colloidal
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lithography; monolayer colloidal crystals; patterns
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1. Introduction
Recently, plasma enhanced chemical vapor deposition (PECVD) has been recognized
as one of the viable fabrication techniques of carbon nanotubes (CNTs) for its ability to grow individual, free-standing, vertical CNTs at lower growth temperatures relative to thermal CVD [1]. However, CNTs are generally randomly deposited on plain substrates ∗
Corresponding author at: Faculty of Materials and Energy, Southwest University, Chongqing 400715, China. Tel.: +86 023 68253204 E-mail address: [email protected] (Y. Man)
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with no control or limited control on the growth site by plasma pretreatment during PECVD [2]. Two-dimensionally (2D) patterned carbon nanotube structures have attracted intensive interest because they exhibit unique pattern-dependent properties and show
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promising applications in optics [3], electronics [4] and field emission devices [5]. The
most direct method to achieve patterned carbon nanotube structures is to utilize patterned catalyst seed layers which directly determine where the nanotubes can grow. Up to now,
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significant research effort has been dedicated to patterning the catalyst layers into ordered geometries by various methods, such as photolithography [6], e-beam lithography [7],
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shadow mask lithography [8] and microcontact printing [9] to grow ordered carbon nanostructures. However, these methods usually suffer from high cost and low throughput. Colloidal lithography based on monolayer colloidal crystals (MCCs) has been proven to be a facile, inexpensive, efficient and flexible technique for large-scale fabrication of 2D
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patterned nanostructures [10, 11]. The nanoparticle patterns templated with MCCs are applied as patterned catalysts for growing periodic carbon nanotube arrays by PECVD [1217]. However, the nanoparticles such as Fe, Co or Ni were deposited on the substrate by
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physical vapor deposition such as electron beam evaporation or DC sputtering which requires expensive equipment, thus greatly increases the fabrication complexity and cost.
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Solution dipping deposition assisted by MCC templates is quite simple and can obtain diverse ordered porous structured films, including Ag, Fe2O3, NiO etc [18- 20]. Nevertheless, growing patterned carbon nanotube structures by the simple solution dipping deposition has rarely reported. Herein, we attempt to employ solution dipping deposition instead of physical vapor deposition for fabricating patterned catalyst layers for the patterned growth of CNTs. The
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influence of the concentration of the precursor solution and plasma treatment conditions on the patterned growth of CNTs has been discussed. Moreover, carbon nanotube arrays terminated with Fe3C nanomagnets have been synthesized. The proposed synthesis route
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could facilitate the direct fabrication of more cost-effective and site-selective periodic CNT structures compatible with novel nanoelectronic devices, field emission devices and magnetic devices.
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2. Experimental details 2.1. Material synthesis
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2.1.1. Self-assembly of monolayer colloidal crystal templates
Polystyrene (PS) nanosphere suspensions (2.5 wt %) were bought from Alfa Aesar Corporation, and glass substrates were washed in ethanol and distilled water in an ultrasonic bath, and then sequentially cleaned according to the method proposed by Haynes
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et al [21]. Centimeter square sized hexagonal-close-packed (hcp) MCCs were spincoated on cleaned glass substrates in a custom-built spin-coater. 2.1.2. Preparation of catalyst films
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The MCC template was transferred onto silicon substrates by lifting off in Fe (NO3)3 aqueous solutions according to the method given by Sun et al [18]. The samples were then
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dried at 383K for 10 min by placing horizontally in an oven. Ordered catalyst films were obtained by subsequent heat treatment at 673K for 8 h to burn off the latex nanospheres and to decompose Fe (NO3)3 entirely into Fe2O3. The calcined samples were ultrasonically washed for 20 min before sample characterization and growth of carbon nanotubes. 2.1.3. Preparation of carbon nanotubes Carbon nanotubes were synthesized with the growth temperature of 500 ◦C, a rf power
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of 50W and a 5:2 flow of argon and ethylene gases in a homemade rf- PECVD reactor which is described in detail elsewhere[22]. The growth time was varied from 5 min to 45 min in order to clarify the patterned growth mechanism by colloidal lithography and
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PECVD. 2.2. Material characterizations
The morphology and microstructure of catalyst films and CNTs were performed by a
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field emission scanning electron microscope (SEM, JEOL JSM-7001F) operated at an
accelerating voltage of 15 kV. The composition of the products were identified by X-ray
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diffraction analyzer (XRD, Rigaku D/max- 2500) using Cu Kα radiation (λ = 1.5418Å). A JEOL-2011F high-resolution transmission electron microscopy (HRTEM) operated at 200 Kev was used to determine the crystal structure of CNTs and Fe3C nanoparticles. The magnetic properties of the products were evaluated using a vibrating sample magnetometer
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(Lakeshore 7307) at room temperature. 3. Results and discussions
2D ordered Fe2O3 porous films were synthesized on silicon substrates through solution
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dipping deposition assisted by MCC templates. XRD confirmed that the porous films consist of crystal α- Fe2O3, as shown in Fig. 1. Fig. 2 shows the SEM images of Fe2O3
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porous films obtained by using MCC templates made of 1000 nm colloidal spheres and 0.1 M Fe (NO3)3 aqueous solutions as the precursor. The vertical top view of the Fe2O3 porous films show highly ordered hexagonal closepacked structure with a long-range periodicity, as shown in Fig. 2A. The resulting thin films exhibit a periodic spacing (i.e. the distance between the centers of two neighboring pores) of 1000 nm, which corresponds well to the diameter of the colloidal spheres. Interestingly, each unit takes on a bowl-like structure in
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the array film, and the thickness of the bottom of the bowl is relatively thin compared to the wall of the bowl, as shown in Fig. 2B. The thickness of Fe2O3 film is between 30 nm
catalyst layer for the growth of CNTs by PECVD.
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and 430 nm. The self-assembled ordered Fe2O3 array film was directly applied as the
3.1. Effect of plasma treatment conditions on the patterned growth of CNTs
During the PECVD process, the catalyst film was constantly bombarded and etched by
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plasmas. Therefore, we investigated the morphological and structural evolution of products to clarify the role of plasmas. The morphology of CNTs grown at different time was shown
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in Fig. 3. Fig. 3A and B displays that CNTs preferred to grow at the center of each bowllike structure unit. When the carbon source was pumped in for 15 min, well aligned periodic CNT arrays were obtained, and the CNT bundles were arranged into a unique hexagonal lattice structure, as shown in Fig. 3C and D. Fig. 3E and F exhibits that CNTs
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were entwined without any order at the growth time of 30 min. As the growth time prolonging to 45 min, uniform vertically aligned CNT array film was deposited on silicon substrates, as shown in Fig. 3G and H. From Fig. 3, one can clearly see that with the
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increase of the growth time, the length of carbon nanotubes had slight change while the growth site extended from the center of bowl-like structure units to the entire substrate.
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Meanwhile, the ordered Fe2O3 array film was gradually etched away because of plasmas. The possible patterned growth mechanism of CNTs by colloidal lithography and
PECVD is discussed as follows. The self-assembled ordered Fe2O3 catalyst film consists of bowl-like structure units. During the PECVD process, the catalyst film was constantly bombarded and etched by plasmas leading to the decrease in the size of catalyst nanoparticles. There is a consensus in the literature that larger particles always appear to be
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onionated and so are inactive for catalysis of CNTs [23]. When the 1 µm PS monolayer was used as the template, the catalytically active particles expanded from the center of each bowl-like structure unit to the entire substrate, therefore, the growth site of CNTs gradually
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extended to all over the substrate with increasing the growth time.
In addition to the change of the microstructure, the composition and crystal structure of the products has also changed under the action of plasmas. Fig. 4 shows the XRD
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patterns of the as-prepared samples grown at different time. The original catalyst film was crystal α- Fe2O3. When the carbon source was pumped in within 15 min, the products
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revealed weak diffraction peaks of Fe2O3, indicating that great amount of catalyst particles have been etched away by plasmas, as shown in Fig. 4a and b. While increasing the growth time to 30 min, the catalyst film exhibited intense signals at 2θ= 44.7°, which can be assigned to Fe (JCPDS No. 3-1050), as shown in Fig. 4c. As the growth time increasing to
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45 min, there only remain the diffraction peaks at around 2θ=37.7, 39.8, 40.6, 42.9, 43.7, 44.5, 45.1, 46.0, 48.6 and 49.1°, correspond to (210), (002), (201), (211), (102), (220), (031), (112), (131) and (221) planes of Fe3C nanoparticles (JCPDS No. 89-2867),
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respectively, as the Fig. 4d displays. Thus it can be seen that the catalyst nanoparticles were gradually reduced to Fe by plasmas, and then the dissoluted carbon atoms combined with
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Fe to form a certain amount of Fe3C [24-25]. TEM images have also been used to further analyze the features of the obtained
nanocomposite. Fig. 5A shows well-crystallized structures with lattice fringes of about 0.34 and 0.21 nm, corresponding to an interplanar spacing of graphite (002) and Fe3C (211) crystal plane, respectively. This agreed with the XRD results. The well-aligned CNTs were quasi-continuously filled with Fe3C nanoparticles, as shown in Fig. 5B. And the Fe3C
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nanoparticles were in the range of 5- 20 nm in size. The magnetic properties of the CNT/ Fe3C nanocomposites were measured at room temperature by vibrating sample magnetometer (VSM) and its hysteresis loops are shown in Fig.6. As can be seen that the
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M – H curves taken with the external field both parallel and perpendicular to the nanotube’s axis are strongly sheared, suggesting the presence of a strong magnetostatic interaction in this system. A value of 292 Oe for Hc was obtained when the external field is applied
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perpendicular to the nanotube’s axis. The coercivity shifts to 250 Oe when the field
paralleling to the nanotube’s axis. The experimentally found magnetic anisotropy of the
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aligned Fe3C-filled CNTs may be attributed to the following two factors. Firstly, the quantity of iron carbides along the tube direction is less compared to the surface direction. Therefor, the nanocomposite shows weaker magnetic property when the external field along tube direction.Generally, this factor was neglected when analyzing the magnetic
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anisotropy of Fe or Fe3C filled CNTs[26-27]. Secondly, large shape anisotropy [28] and magnetocrystalline anisotropy contribution[29] should be responsible for the magnetic anisotropy of the CNT/ Fe3C nanocomposites. Which mechanism plays a key role need to
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be further studied. Nevertheless, iron carbides exhibit excellent properties compared with iron or iron oxide nanoparticles [30-32] due to carbon atoms occupying the interstices
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between close-packed iron atoms [33]. Therefore, the proposed synthesis route could facilitate the direct fabrication of CNT/ Fe3C nanocomposites for applications in catalysis, electrochemistry and solar cells. 3.2. Effect of the concentration of the precursor solution on the patterned growth of CNTs We also investigated the effect of the concentration of the precursor solution on the morphology of CNTs. Fig. 7A and B shows the morphology of Fe2O3 porous films
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obtained through a 0.05 M and 0.005 M Fe (NO3)3 aqueous solution as the precursor, respectively. Combined with Fig. 2, it can be seen that with the concentration of the precursor decreasing, the thickness of the Fe2O3 porous film gradually decreased. If the
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solution concentration is reduced to a very level (0.005 M) , Fe2O3 nanoparticles are
mainly present at the interstitial position of the closely packed 3-spheres[18]. Fig. 7C and D shows the morphology of CNTs grown at 15 min in the PECVD reactor. When the solution
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concentration is reduced to 0.05 M, carbon nanotubes were preferentially formed at the center of each bowl-like structure unit, which is consistent with Fig. 3D because the
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morphology of Fe2O3 porous film had no big change. When the solution concentration is reduced to 0.005 M, CNTs preferred to grow at the interstitial position of nanospheres, and thus CNT bundles were arranged into a unique periodic structure, as shown in Fig. 7D. Therefor, the growth site of CNTs can be flexibly adjusted by the concentration of the
4. Conclusions
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precursor solution.
In summary, we have synthesized patterned carbon nanotube arrays by colloidal
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lithography and PECVD on silicon substrates. Catalyst seed layers were one-step selfassembled on silicon substrates by solution dipping deposition assisted by MCCs,
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eliminating the need for expensive lithography and physical vapor deposition. The growth site and density of CNTs were determined by the concentration of the precursor solution and plasma treatment conditions. We also discussed the formation mechanism of patterned carbon nanotube structures. This work further demonstrated that colloidal lithography is a simple, feasible and efficient method to prepare patterned nanostructures. The proposed strategy can also be applied to synthesize CNT/ Fe3C nanocomposites and other CNT
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supported nanoparticle composites, which will have more potential applications. Acknowledgments This work was financially supported by National Natural Science Foundation of China
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(No. 51302227) and Fundamental Research Funds for the Central Universities (No. XDJK2015B017). References
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Figure Captions Fig.1. XRD pattern of the Fe2O3 polycrystalline film on silicon substrates. Fig.2. SEM images of Fe2O3 porous films with a periodic spacing of 1000 nm. (A) vertical
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top view; (B)side view.
Fig.3. (A), (C), (E) and (G) are SEM images with low resolution of CNTs grown at 5 min, 15 min, 30 min and 45 min, respectively; (B), (D), (F) and (H) are the corresponding SEM
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images with high resolution.
Fig.4. XRD patterns of the CNTs on silicon substrates grown at different time in the
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PECVD reactor at: (a) 5 min, (b) 15 min, (c) 30 min and (d) 45 min.
Fig.5. HRTEM image of the Fe3C nanoparticles encapsulated by CNTs. Fig.6. The VSM measurement curves for CNTs encapsulated Fe3C nanoparticles. Fig.7. SEM images of Fe2O3 porous films and CNTs with different concentrations of
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Fe2O3 porous film.
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precursor solution: (A) and (C) 0.05 M; (B) and (D) 0.005 M. The inset is the side view of
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Highlights • Patterned carbon nanotube structures were obtained using colloidal lithography. • The synthesis strategy greatly reduces the fabrication cost. • The proposed method can tune the periodic nanostructures quite flexibly. • CNT/ Fe3C nanocomposites were synthesized with excellent magnetic property.
S0925-8388(15)30684-8
DOI:
10.1016/j.jallcom.2015.07.286
Reference:
JALCOM 34982
To appear in:
Journal of Alloys and Compounds
Received Date: 7 March 2015 Revised Date:
29 July 2015
Accepted Date: 30 July 2015
Please cite this article as: Y. Man, Z. Chen, Y. Zhang, P. Guo, Patterned growth of vertically aligned carbon nanotube arrays using colloidal lithography and plasma enhanced chemical vapor deposition, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.07.286. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Patterned growth of vertically aligned carbon nanotube arrays using colloidal lithography and plasma enhanced chemical vapor deposition Yuhong Man a,∗, Zhiqian Chen a ,Yongping Zhang a , Peitao Guo a Faculty of Materials and Energy, Southwest University, Chongqing 400715, China
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a
Abstract. Patterned carbon nanotube structures were synthesized on silicon substrates by
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plasma enhanced chemical vapor deposition and solution dipping deposition assisted by
monolayer colloidal crystals for the first time. Through the regulation of the concentration
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of the precursor solution and plasma treatment conditions, we have grown carbon nanotube bundles into periodic structures with different growth site and density. The proposed method greatly reduces the fabrication cost and can tune the periodic structure of carbon nanotubes quite flexibly. Furthermore, the synthesis strategy can also be applied to
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synthesize CNT/ Fe3C nanocomposites with excellent magnetic property, indicating a potential application in magnetic devices.
Keywords: carbon nanotubes; plasma enhanced chemical vapor deposition; colloidal
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lithography; monolayer colloidal crystals; patterns
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1. Introduction
Recently, plasma enhanced chemical vapor deposition (PECVD) has been recognized
as one of the viable fabrication techniques of carbon nanotubes (CNTs) for its ability to grow individual, free-standing, vertical CNTs at lower growth temperatures relative to thermal CVD [1]. However, CNTs are generally randomly deposited on plain substrates ∗
Corresponding author at: Faculty of Materials and Energy, Southwest University, Chongqing 400715, China. Tel.: +86 023 68253204 E-mail address: [email protected] (Y. Man)
1
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with no control or limited control on the growth site by plasma pretreatment during PECVD [2]. Two-dimensionally (2D) patterned carbon nanotube structures have attracted intensive interest because they exhibit unique pattern-dependent properties and show
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promising applications in optics [3], electronics [4] and field emission devices [5]. The
most direct method to achieve patterned carbon nanotube structures is to utilize patterned catalyst seed layers which directly determine where the nanotubes can grow. Up to now,
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significant research effort has been dedicated to patterning the catalyst layers into ordered geometries by various methods, such as photolithography [6], e-beam lithography [7],
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shadow mask lithography [8] and microcontact printing [9] to grow ordered carbon nanostructures. However, these methods usually suffer from high cost and low throughput. Colloidal lithography based on monolayer colloidal crystals (MCCs) has been proven to be a facile, inexpensive, efficient and flexible technique for large-scale fabrication of 2D
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patterned nanostructures [10, 11]. The nanoparticle patterns templated with MCCs are applied as patterned catalysts for growing periodic carbon nanotube arrays by PECVD [1217]. However, the nanoparticles such as Fe, Co or Ni were deposited on the substrate by
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physical vapor deposition such as electron beam evaporation or DC sputtering which requires expensive equipment, thus greatly increases the fabrication complexity and cost.
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Solution dipping deposition assisted by MCC templates is quite simple and can obtain diverse ordered porous structured films, including Ag, Fe2O3, NiO etc [18- 20]. Nevertheless, growing patterned carbon nanotube structures by the simple solution dipping deposition has rarely reported. Herein, we attempt to employ solution dipping deposition instead of physical vapor deposition for fabricating patterned catalyst layers for the patterned growth of CNTs. The
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influence of the concentration of the precursor solution and plasma treatment conditions on the patterned growth of CNTs has been discussed. Moreover, carbon nanotube arrays terminated with Fe3C nanomagnets have been synthesized. The proposed synthesis route
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could facilitate the direct fabrication of more cost-effective and site-selective periodic CNT structures compatible with novel nanoelectronic devices, field emission devices and magnetic devices.
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2. Experimental details 2.1. Material synthesis
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2.1.1. Self-assembly of monolayer colloidal crystal templates
Polystyrene (PS) nanosphere suspensions (2.5 wt %) were bought from Alfa Aesar Corporation, and glass substrates were washed in ethanol and distilled water in an ultrasonic bath, and then sequentially cleaned according to the method proposed by Haynes
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et al [21]. Centimeter square sized hexagonal-close-packed (hcp) MCCs were spincoated on cleaned glass substrates in a custom-built spin-coater. 2.1.2. Preparation of catalyst films
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The MCC template was transferred onto silicon substrates by lifting off in Fe (NO3)3 aqueous solutions according to the method given by Sun et al [18]. The samples were then
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dried at 383K for 10 min by placing horizontally in an oven. Ordered catalyst films were obtained by subsequent heat treatment at 673K for 8 h to burn off the latex nanospheres and to decompose Fe (NO3)3 entirely into Fe2O3. The calcined samples were ultrasonically washed for 20 min before sample characterization and growth of carbon nanotubes. 2.1.3. Preparation of carbon nanotubes Carbon nanotubes were synthesized with the growth temperature of 500 ◦C, a rf power
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of 50W and a 5:2 flow of argon and ethylene gases in a homemade rf- PECVD reactor which is described in detail elsewhere[22]. The growth time was varied from 5 min to 45 min in order to clarify the patterned growth mechanism by colloidal lithography and
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PECVD. 2.2. Material characterizations
The morphology and microstructure of catalyst films and CNTs were performed by a
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field emission scanning electron microscope (SEM, JEOL JSM-7001F) operated at an
accelerating voltage of 15 kV. The composition of the products were identified by X-ray
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diffraction analyzer (XRD, Rigaku D/max- 2500) using Cu Kα radiation (λ = 1.5418Å). A JEOL-2011F high-resolution transmission electron microscopy (HRTEM) operated at 200 Kev was used to determine the crystal structure of CNTs and Fe3C nanoparticles. The magnetic properties of the products were evaluated using a vibrating sample magnetometer
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(Lakeshore 7307) at room temperature. 3. Results and discussions
2D ordered Fe2O3 porous films were synthesized on silicon substrates through solution
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dipping deposition assisted by MCC templates. XRD confirmed that the porous films consist of crystal α- Fe2O3, as shown in Fig. 1. Fig. 2 shows the SEM images of Fe2O3
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porous films obtained by using MCC templates made of 1000 nm colloidal spheres and 0.1 M Fe (NO3)3 aqueous solutions as the precursor. The vertical top view of the Fe2O3 porous films show highly ordered hexagonal closepacked structure with a long-range periodicity, as shown in Fig. 2A. The resulting thin films exhibit a periodic spacing (i.e. the distance between the centers of two neighboring pores) of 1000 nm, which corresponds well to the diameter of the colloidal spheres. Interestingly, each unit takes on a bowl-like structure in
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the array film, and the thickness of the bottom of the bowl is relatively thin compared to the wall of the bowl, as shown in Fig. 2B. The thickness of Fe2O3 film is between 30 nm
catalyst layer for the growth of CNTs by PECVD.
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and 430 nm. The self-assembled ordered Fe2O3 array film was directly applied as the
3.1. Effect of plasma treatment conditions on the patterned growth of CNTs
During the PECVD process, the catalyst film was constantly bombarded and etched by
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plasmas. Therefore, we investigated the morphological and structural evolution of products to clarify the role of plasmas. The morphology of CNTs grown at different time was shown
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in Fig. 3. Fig. 3A and B displays that CNTs preferred to grow at the center of each bowllike structure unit. When the carbon source was pumped in for 15 min, well aligned periodic CNT arrays were obtained, and the CNT bundles were arranged into a unique hexagonal lattice structure, as shown in Fig. 3C and D. Fig. 3E and F exhibits that CNTs
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were entwined without any order at the growth time of 30 min. As the growth time prolonging to 45 min, uniform vertically aligned CNT array film was deposited on silicon substrates, as shown in Fig. 3G and H. From Fig. 3, one can clearly see that with the
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increase of the growth time, the length of carbon nanotubes had slight change while the growth site extended from the center of bowl-like structure units to the entire substrate.
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Meanwhile, the ordered Fe2O3 array film was gradually etched away because of plasmas. The possible patterned growth mechanism of CNTs by colloidal lithography and
PECVD is discussed as follows. The self-assembled ordered Fe2O3 catalyst film consists of bowl-like structure units. During the PECVD process, the catalyst film was constantly bombarded and etched by plasmas leading to the decrease in the size of catalyst nanoparticles. There is a consensus in the literature that larger particles always appear to be
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onionated and so are inactive for catalysis of CNTs [23]. When the 1 µm PS monolayer was used as the template, the catalytically active particles expanded from the center of each bowl-like structure unit to the entire substrate, therefore, the growth site of CNTs gradually
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extended to all over the substrate with increasing the growth time.
In addition to the change of the microstructure, the composition and crystal structure of the products has also changed under the action of plasmas. Fig. 4 shows the XRD
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patterns of the as-prepared samples grown at different time. The original catalyst film was crystal α- Fe2O3. When the carbon source was pumped in within 15 min, the products
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revealed weak diffraction peaks of Fe2O3, indicating that great amount of catalyst particles have been etched away by plasmas, as shown in Fig. 4a and b. While increasing the growth time to 30 min, the catalyst film exhibited intense signals at 2θ= 44.7°, which can be assigned to Fe (JCPDS No. 3-1050), as shown in Fig. 4c. As the growth time increasing to
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45 min, there only remain the diffraction peaks at around 2θ=37.7, 39.8, 40.6, 42.9, 43.7, 44.5, 45.1, 46.0, 48.6 and 49.1°, correspond to (210), (002), (201), (211), (102), (220), (031), (112), (131) and (221) planes of Fe3C nanoparticles (JCPDS No. 89-2867),
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respectively, as the Fig. 4d displays. Thus it can be seen that the catalyst nanoparticles were gradually reduced to Fe by plasmas, and then the dissoluted carbon atoms combined with
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Fe to form a certain amount of Fe3C [24-25]. TEM images have also been used to further analyze the features of the obtained
nanocomposite. Fig. 5A shows well-crystallized structures with lattice fringes of about 0.34 and 0.21 nm, corresponding to an interplanar spacing of graphite (002) and Fe3C (211) crystal plane, respectively. This agreed with the XRD results. The well-aligned CNTs were quasi-continuously filled with Fe3C nanoparticles, as shown in Fig. 5B. And the Fe3C
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nanoparticles were in the range of 5- 20 nm in size. The magnetic properties of the CNT/ Fe3C nanocomposites were measured at room temperature by vibrating sample magnetometer (VSM) and its hysteresis loops are shown in Fig.6. As can be seen that the
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M – H curves taken with the external field both parallel and perpendicular to the nanotube’s axis are strongly sheared, suggesting the presence of a strong magnetostatic interaction in this system. A value of 292 Oe for Hc was obtained when the external field is applied
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perpendicular to the nanotube’s axis. The coercivity shifts to 250 Oe when the field
paralleling to the nanotube’s axis. The experimentally found magnetic anisotropy of the
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aligned Fe3C-filled CNTs may be attributed to the following two factors. Firstly, the quantity of iron carbides along the tube direction is less compared to the surface direction. Therefor, the nanocomposite shows weaker magnetic property when the external field along tube direction.Generally, this factor was neglected when analyzing the magnetic
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anisotropy of Fe or Fe3C filled CNTs[26-27]. Secondly, large shape anisotropy [28] and magnetocrystalline anisotropy contribution[29] should be responsible for the magnetic anisotropy of the CNT/ Fe3C nanocomposites. Which mechanism plays a key role need to
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be further studied. Nevertheless, iron carbides exhibit excellent properties compared with iron or iron oxide nanoparticles [30-32] due to carbon atoms occupying the interstices
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between close-packed iron atoms [33]. Therefore, the proposed synthesis route could facilitate the direct fabrication of CNT/ Fe3C nanocomposites for applications in catalysis, electrochemistry and solar cells. 3.2. Effect of the concentration of the precursor solution on the patterned growth of CNTs We also investigated the effect of the concentration of the precursor solution on the morphology of CNTs. Fig. 7A and B shows the morphology of Fe2O3 porous films
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obtained through a 0.05 M and 0.005 M Fe (NO3)3 aqueous solution as the precursor, respectively. Combined with Fig. 2, it can be seen that with the concentration of the precursor decreasing, the thickness of the Fe2O3 porous film gradually decreased. If the
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solution concentration is reduced to a very level (0.005 M) , Fe2O3 nanoparticles are
mainly present at the interstitial position of the closely packed 3-spheres[18]. Fig. 7C and D shows the morphology of CNTs grown at 15 min in the PECVD reactor. When the solution
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concentration is reduced to 0.05 M, carbon nanotubes were preferentially formed at the center of each bowl-like structure unit, which is consistent with Fig. 3D because the
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morphology of Fe2O3 porous film had no big change. When the solution concentration is reduced to 0.005 M, CNTs preferred to grow at the interstitial position of nanospheres, and thus CNT bundles were arranged into a unique periodic structure, as shown in Fig. 7D. Therefor, the growth site of CNTs can be flexibly adjusted by the concentration of the
4. Conclusions
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precursor solution.
In summary, we have synthesized patterned carbon nanotube arrays by colloidal
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lithography and PECVD on silicon substrates. Catalyst seed layers were one-step selfassembled on silicon substrates by solution dipping deposition assisted by MCCs,
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eliminating the need for expensive lithography and physical vapor deposition. The growth site and density of CNTs were determined by the concentration of the precursor solution and plasma treatment conditions. We also discussed the formation mechanism of patterned carbon nanotube structures. This work further demonstrated that colloidal lithography is a simple, feasible and efficient method to prepare patterned nanostructures. The proposed strategy can also be applied to synthesize CNT/ Fe3C nanocomposites and other CNT
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supported nanoparticle composites, which will have more potential applications. Acknowledgments This work was financially supported by National Natural Science Foundation of China
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(No. 51302227) and Fundamental Research Funds for the Central Universities (No. XDJK2015B017). References
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Figure Captions Fig.1. XRD pattern of the Fe2O3 polycrystalline film on silicon substrates. Fig.2. SEM images of Fe2O3 porous films with a periodic spacing of 1000 nm. (A) vertical
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top view; (B)side view.
Fig.3. (A), (C), (E) and (G) are SEM images with low resolution of CNTs grown at 5 min, 15 min, 30 min and 45 min, respectively; (B), (D), (F) and (H) are the corresponding SEM
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images with high resolution.
Fig.4. XRD patterns of the CNTs on silicon substrates grown at different time in the
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PECVD reactor at: (a) 5 min, (b) 15 min, (c) 30 min and (d) 45 min.
Fig.5. HRTEM image of the Fe3C nanoparticles encapsulated by CNTs. Fig.6. The VSM measurement curves for CNTs encapsulated Fe3C nanoparticles. Fig.7. SEM images of Fe2O3 porous films and CNTs with different concentrations of
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Fe2O3 porous film.
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precursor solution: (A) and (C) 0.05 M; (B) and (D) 0.005 M. The inset is the side view of
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Highlights • Patterned carbon nanotube structures were obtained using colloidal lithography. • The synthesis strategy greatly reduces the fabrication cost. • The proposed method can tune the periodic nanostructures quite flexibly. • CNT/ Fe3C nanocomposites were synthesized with excellent magnetic property.