Nanoring Arrays on Fe Coated Substrate: Formation and Guidance for

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Nanoring Arrays on Fe Coated Substrate: Formation and Guidance for the Growth of Hierarchical CNTs Tianchan Luo,† Can Du,† Aijuan Zhang,† Laisen Wang,† Hua Bai,† and Lei Li*,†,‡ †

College of Materials, Xiamen University, Xiamen 361005, China State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, China

‡

S Supporting Information *

ABSTRACT: In this article, we report the formation of nanoring structures on Fe coated substrate and their application in guiding the growth of carbon nanotube (CNT) patterns with hierarchical structures. The formation of nanorings involves the etching of polystyrene (PS) monolayer colloidal crystals (MCCs) under reactive ion etching (RIE), and the redeposition and cross-linkage of the active degradation products at the contact line between the MCCs and the substrate. After washing out the MCCs, insoluble nanorings with hexagonal order on the substrate are developed. The RIE process can control the morphology of the nanorings, as well as the distribution of the Fe element on the substrate; thus, a continuous Fe layer and separated Fe discs on the substrate are created on substrate after washing, depending on the etching time and the shield of MCCs. The surviving Fe element can work as the catalyst to initiate the in situ growth of aligned CNTs in the following chemical vapor deposition (CVD) process, while the Fe element underneath the nanorings keep its inactivity. Eventually, CNT patterns with hierarchical structures are formed. One level originates from the surviving Fe layer; the other level is templated from the nanoring structures, which cause the blank area in the CNT bundles.

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by CL technique.18 A thin layer of metal was first evaporated onto a colloidal-crystal-coated substrate, and the metal film was treated by plasma etching, during which secondary sputtering of material created a metal shell around the sides of the PS colloidal crystal. Eventually, after the removal of the residual polymer particles by a UV-ozone treatment and subsequent water rinse, free-standing metal nanorings were obtained. Compared with solid metal particles of similar size, nanorings exhibit a red-shifted localized surface plasma that can be tuned over an extended wavelength range by varying the ratio of the ring thickness to its radius, and thus has promising application in ultrasensitive sensor. However, one of the disadvantages that limits the application of the CL technique is its ineffectiveness in tuning the size and shape of a structure, which are usually dependent on the original particle size. Then several technologies involving angleresolved deposition and reactive ion etching (RIE)19,20 have been suggested to adjust the size and the shape of nanostructures, and control the surface morphology and roughness of the template. In fact, many kinds of active species have been detected during the RIE process, such as radicals, atoms, and ions, which can react easily with the substrate.21,22 Therefore, a delicately designed substrate can be further functionalized after the RIE process and have ingenious application, which has been neglected in the previous research.

INTRODUCTION The fabrication and adjustment of nanostructures has received substantial attention based on the development of building blocks with desired functions and compositional complexity.1−4 The invention of simple and reliable methodology also plays an important role in the nanotechnology improvement.5 Template-directed methods for producing well-ordered surface patterns and structures with variable sizes and tunable properties continue to evolve due to their important applications in areas such as catalysts,6,7 sensors,8 and nanoelectronics.9−11 Especially, various advanced lithographic technologies have been applied to create microscale and nanoscale structures.12,13 Many of these preparation methods use self-assembled colloidal crystals as masks or templates to fabricate multifarious patterns and functional materials, due to the advantage of its low cost, high reproducibility, large-area fabrication, and good controllability on the chemical composition and structural parameters. Such processes involving colloidal crystals as the template are known as colloidal lithography (CL).14,15 The first step of CL is selfassembling colloids into two-dimensional or three-dimensional periodic arrays on the substrate. The following steps are diverse, depending on the different practical applications. For example, nanohole array was fabricated by selective etching of a monolayer colloidal crystal (MCC) partially embedded in an electrochemically deposited metal layer.16 Furthermore, these kinds of structures can be employed to fabricate functional composite particles, such as Pt-incorporated polystyrene (PS)Pt composite particles.17 Metal nanorings were also prepared © 2015 American Chemical Society

Received: October 20, 2015 Revised: November 19, 2015 Published: November 24, 2015 13327

DOI: 10.1021/acs.langmuir.5b03886 Langmuir 2015, 31, 13327−13333

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Langmuir Here, we demonstrate a simple and versatile preparation strategy combining CL with RIE process for the fabrication of nanoring structures on Fe coated substrate, and the nanorings can be further used as masks to construct aligned carbon nanotube (CNT) arrays. By simply changing the etching conditions, the nanoring morphology and the CNT patterns can be adjusted, which is difficult for the conventional lithography.

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Figure 1. Schematic illustration of the formation of CNT patterns with hierarchical structures based on nanorings on Fe coated substrate. (a) PS MCC obtained from the needle tip flow method; (b), (e) modified MCC template after the short-term and long-term etching; (c), (f) developed nanoring arrays after the removal of the MCC arrays obtained from (b) and (e); (d) continuous CNTs bundles with hexagonally arranged cavity based on the nanoring template of (c); (g) isolated CNT bundles with ring-like blank based on the template of (f).

EXPERIMENTAL SECTION

Preparation of the Substrates. In the present work, silicon was employed as the substrate. Pieces of Si wafers were washed in a Piranha solution [H2SO4 (96%):H2O2 (30%) = 3:1] for 30 min followed by 10 min deionized water rinse in an ultrasonic bath. (Caution: Piranha solution is highly reactive and corrosive, and should be handled with extreme care.) Then the Si wafers were dried with dry nitrogen gas. For the growth of CNTs, the Si substrate with a ∼500nm-thick SiO2 layer was used as the substrate. Then, a 10 nm Al2O3 layer as the buffer layer and a 2 nm Fe layer were deposited by RF magnetron sputtering (RFMS). Preparation of the Polystyrene Monolayer Colloidal Crystals. Monodisperse PS spheres with diameter of 3, 5, or 10 μm dispersed in water (10%, w/v) were purchased from Nano-Micro (Suzhou, China). MCCs were prepared according to a previously reported method.23 Briefly, the PS dispersion was first diluted with water and propanol (1:1:1, v/v/v). Then, the obtained PS dispersion was loaded into a syringe and layered on water surface using a syringe pump (WZ-50C2, Zhejiang University Medical Instrument Co., LTD, China), forming MCC. The flow rates were controlled at 2−3 mL h−1. Finally, the MCCs on the water surface were transferred onto the silicon substrates. Preparation of the Nanorings. MCCs were used as the templates for the preparation of the periodic nanorings on the substrate. Inductively coupled plasma reactive ion etching (ICP-RIE) (ICP-98A, Institute of Microelectronics of Chinese Academy of Sciences), with a CF4 flow rate of 30 standard cubic centimeters per minute (sccm), a pressure of 1.0 Pa, and a source power varied from 30 to 50 W, was performed to etch the MCCs. After a certain period of etching, the remaining MCC templates were removed by ultrasonic treatment in deionized water, and the nanoring arrays were left on the substrate. Growth of the Patterned CNTs. Patterned CNTs were grown by chemical vapor deposition (CVD) technique. Nanoring-patterned catalyst substrates were placed on a heating plate in the center of a quartz reaction tube. Acetylene (C2H2), Ar, and H2 gas were then admitted into the tube to exclude air. After the substrate was electrically heated to 750 °C, the CNTs began to grow. The flow rate of C2H2, Ar, and H2 gas maintained at 66, 200, and 500 sccm, respectively. The growth period was varied from 5 to 30 min. The length of carbon nanotubes was controlled by the growth time. After the growth process was done, the heating plate was cooled slowly to room temperature in Ar gas ambient. Characterization and Apparatus. Scanning electronic microscopy (SEM) images were obtained using a LEO 1530 and a SU-70, Hitachi scanning electron microscopy. The chemical compositions of the samples were measured by energy dispersive X-ray spectroscopy (EDX) microanalysis attached to SEM. The surface topography was measured by atomic force microscope (AFM, SII Nano Technology, Inc., Nanonavi Probe Station). X-ray photoelectron spectroscopy (XPS) was carried out using a PHI Quantum 2000 scanning ESCA microprobe instrument.

arrays of the MCC. Then two kinds of nanoring structures are formed on the substrate using RIE technique with different etching time and power (Figure 1c and f). It is worth noting that the Fe is also etched during the RIE process but with a slow etching rate. Therefore, the etching time and etching power could control the thickness of the Fe layer and the pattern of the Fe on the substrate. With the two types of Fe patterns as the catalyst, two different patterned CNTs are fabricated by the CVD process, as shown in Figure 1d and g. The needle tip flow method is a well-developed strategy to prepare large area, high quality MCC arrays.23 A wafer sized MCC array on a substrate was demonstrated in Figure 2,

Figure 2. Digital photo of the MCC arrays on 2 in. Si wafer.

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showing that the prepared MCC is highly ordered and defectfree. The magnified MCC made from 3 μm PS spheres before and after RIE are shown in Figure 3a and b. During the RIE, many kinds of active species have been detected in CF4 plasma, such as radicals (e.g., CF3, CF2, CF) and ions (e.g., CF3+, CF3−, and F−).24 Such radicals and ions can decompose a polymer in distinct ways including random chain scission and decomposition of side group from the material surface. Therefore, the

RESULTS AND DISCUSSION The formation of nanorings from MCC and the growth of CNT patterns guided by the nanorings are schematically illustrated in Figure 1. First, the highly ordered PS MCCs are prepared on a Fe-coated silicon substrate by a needle tip flow method, and Figure 1 shows the hexagonally close-packed 2D 13328

DOI: 10.1021/acs.langmuir.5b03886 Langmuir 2015, 31, 13327−13333

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template by washing, nanoring arrays were observed on substrate by SEM and AFM, as shown in Figure 3c and d. The periodicity of the nanoring arrays (∼3 μm) correlates with that of the MCC template, indicating that the formation of nanorings is guided by the PS MCC. The prepared nanorings are uniform and have bowl-like shape, with a width of 650 nm, inner diameter of 250 nm, and height of ∼38 nm (Figure 3e, f). It is should be noted that this kind of nanoring structure has hardly been reported in previous publications. Therefore, additional experiments and detailed spectrum characterization are carried out to elucidate the formation mechanism of the novel nanostructures. The influence of the etching time and power on the nanoring morphology is investigated by SEM and AFM, as shown in Figure 4. At an etching power of 30 W, after 6 min etching, isolated nanoring structures with a period of 3 μm are found on the substrate (Figure 4a and e). By further increasing the etching time to 9 min, the height of the ring increases from 8 to 12 nm while the period remains constant (Figure 4b and f). When the etching time is extended to 12 min, the nanoring structures become blurred. The AFM images suggest that the height of the ring decreases to only 4 nm (Figure 4c and g). The etching power also has significant influence on the height of the nanorings: by increasing the etching power from 30 to 50 W, we find that the height of the nanorings increases from 8 to 38 nm after 6 min etching. Moreover, three MCCs with different sizes of PS spheres (3 μm, 5 μm, and 10 μm) were used as the templates to investigate the influence of sphere diameter on the morphology of the nanoring structures. After etching and washing, definite nanorings are found on all the substrates. Similarly, the nanoring arrays have the same periodicity as the MCC arrays (Figure S2, SI). The surface chemical compositions of the substrate with nanoring structures are identified by microscopic energy dispersive X-ray spectroscopy (micro-EDX) and XPS. The micro-EDX results shown in Figure 5a and b verify the elemental composition on the nanoring and the substrate areas, respectively. Obviously, carbon element is selectively enriched around the edge of the nanoring. The chemical composition is further confirmed by XPS measurement (Figure S3, SI). The core level scan of carbon indicated two main components at

Figure 3. SEM images of MCCs made from 3 μm PS colloidal crystals (a) before and (b) after the RIE process; the inset: the corresponding cross section; (c−f) SEM images and AFM images of nanorings. Scale bars: (a), (b), (c), and (e) 5 μm; (d) 1 μm; the inset 2 μm.

diameter of spheres was slightly reduced (Figure 3b) to ∼2.8 μm and the shape of the individual sphere became ellipsoidal after the RIE process (Figure 3b inset), due to the removal of the upper parts of the spheres. Besides, some protrusions were found on the upper hemisphere (the magnified SEM showed in Figure S1, SI), and they were the aggregates of the polar lowmolecular-weight fragments, which would dewet the PS surface as a result of Rayleigh instabilities.25 After removing the MCC

Figure 4. SEM images and topographic images (insets) on different etching conditions; the CF4 flow rate is 30 sccm, the power and the time are (a) 30 W, 6 min, (b) 30 W, 9 min, (c) 30 W, 12 min, (d) 50 W, 6 min. (e−h) Topographic images corresponding to the insets in (a−d). Scale bars: (a− d) 5 μm; the insets 2 μm. 13329

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Figure 5. Micro-EDX of (a) nanorings and (b) substrate; XPS spectra of the core scan of individual element on substrate: (c) C 1s, (d) O 1s, (e) F 1s, (f) Fe 2p.

low-molecular-weight etching fragments. On the other hand, the fragments are active and can react with each other to form a new macromolecular structure. The produced fragments sputter toward the substrate along with the plasma, and hit the substrate. Some of the fragments reflecting toward the bottom of the PS spheres react with each other and form a deposition, which is shielded by the spheres so that its further etching was avoided. Such a sputter-redeposition process has also been observed in other systems.34 Min et al. investigated the redeposition of fragments on sidewalls during RIE and concluded that the redeposition rate gradually increased as the position on the sidewall approached the bottom.35 Therefore, in our case, the reactive fragments are inclined to redeposit at the bottom of PS spheres, near the contact line of the spheres and substrates. Moreover, under the RIE, the underlying Fe film can also be fluorinated and Fe may react with active fragments to form Fe−C bonds, which may contribute to the strong adhesion of nanorings toward the Fe film. The evolution of nanoring structures with etching time also supports this mechanism. As described in Figure 4, isolated but blurry nanoring structures with a period of 3 μm are found on the substrate after 6 min etching under 30 W, and after 9 min etching under 30 W power, the nanoring structures become more clear. In the beginning, the low-molecular-weight fragments will increase as the time goes by, which means the cross-linkage and redeposition of the fragments are predominant. However, with further elongation of the etching time, the amount of fragments sputtered from the PS spheres decreases

284.69 and 287.5 eV, which were assigned to C−C (and/or C− H) and C−CFn bonds (Figure 4c).26,27 The F 1s spectrum (Figure 5d) also confirmed the existence of C−F bonds, whose peak was located at around 688 eV.28 Another component in the F 1s peak was attributed to F−Fe.29 The oxygen element (Figure 5e) originated from the surface oxidation of Fe and the organics, and the Fe 2p peak (Figure 5f) demonstrated the oxidation of the Fe layer. From the above results, we can confirm that the nanorings are mainly composed of carbon and fluoride elements, and the substrate was covered by Fe2O3 and FeF3.30,31 Considering the chemical compositions of the etchant and the colloidal crystals, it can be inferred that the carbon and fluorine elements are supplied by CF4 and PS, and both the PS spheres and the Fe layer are fluorinated and oxidized during the RIE process. In addition, the nanorings cannot be dissolved in polar organic solvents, even with the ultrasonic treatment. We further calcined the sample at 800 °C for 8 h, and found that the nanoring structures totally disappear (Figure S4, SI), indicating that the nanorings are composed of instability of the evaporating thin films.16,32,33 All the facts indicate that the nanorings are composed of cross-linked carbon−fluorine components. Based on these results, the mechanism for the formation of nanorings is proposed. The arrangement and periodicity of the nanorings are the same as that the MCC arrays, definitely indicating that the nanoring structures originate from the MCC template. During the RIE process, the active species in CF4 plasma causes the chain scission and decomposition, producing 13330

DOI: 10.1021/acs.langmuir.5b03886 Langmuir 2015, 31, 13327−13333

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CNT bundles initiated by the Fe catalyst underneath the bottom of the PS spheres, which is consistent with the distribution of Fe on the substrate as shown in the inset of Figure 6a. For the long-term etching substrate, the surviving Fe discs are activated. Similarly, the nanoring areas remain intact (the inset of Figure 6b). Thus, isolated hexagonal CNT bundles are created on the substrate after H2 reduction and CVD process. Close examination reveals that the diameter of the individual CNT bundles is identical to that of PS spheres after RIE, and the CNT bundles are hollow and filled by a thinner bundle, as shown in Figure 6b and d, in good agreement with the Fe distribution shown in the Figure 5b inset.

since the top layer of the PS spheres is also cross-linked. Meanwhile, the nanoring structures formed are slightly decomposed again by the reflected plasma. In other words, the degradation reaction has become dominant at this stage.36 Therefore, the nanorings formed are partially destroyed and became blurred under SEM and AFM (Figure 4c and g). Increasing the etching power produces more active fragments, which means that when the etching time is constant, the crosslinkage and redeposition are still dominant. This is why the edges of the nanorings became thicker with 50 W power at the same etching time (Figure 4d and h). Patterned CNTs have attracted increasing attention due to their application in scanning probe microscopy,37 supercapacitor,38 field-emission flat panel display,39 and gecko-footmimetic dry adhesives.40,41 Based on our previous research,42 the Fe element distribution on the substrate has significant influence on the growth of CNTs. Since the RIE process is also stripped with the Fe element, the distribution of Fe on the substrate can be adjusted by simply changing the etching time. After washing out the MCCs, undissolved nanorings with hexagonal order on the substrate are developed. Therefore, two kinds of Fe element distribution are created on the substrate. One is the continuous Fe layer covered by nanorings (shorttime etching) and the other one is the separated Fe discs covered by nanorings (long-time etching), depending on the etching time and the shield of MCCs. In the following CVD proofs, two kinds of patterned CNT arrays with hierarchical structures are constructed on the substrate. Before the CVD process, H2 was charged into the chamber to activate the Fe catalyst. For the short-term etching substrate, the whole surface area is active, except the areas covered by the nanorings (inset of Figure 6a). Therefore, continuous CNTs perpendicular to the substrate with hexagonally packed dimples are formed, templating from this substrate (Figure 6a). A magnified SEM image reveals that the diameter of the dimples is close to that of nanorings (Figure 6c). The dimples are filled with isolated

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CONCLUSION In conclusion, we have developed a novel method for the preparation of nanoring arrays on Fe coated substrate in large area, combining RIE with CL techniques. The formation mechanism of the nanostructures is elucidated by the microscopy and spectroscopy investigation in detail. It is believed that the redeposition of the degraded fragments at the contact lines between the MCC and the substrate is the critical step for the formation of nanorings. After washing out the MCC template, the nanoring arrays are developed. Since Fe element on the substrate is also stripped by the RIE process, the distribution of Fe element on the substrate can be adjusted by simply changing the etching time. With the guidance of the nanorings, two kinds of CNT patterns with hierarchical structures are in situ initiated, which is difficult for the conventional lithography. We believe that this strategy will find more promising applications in different areas, which is currently under investigation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03886. SEM, AFM images of the MCCs or the nanorings, and the XPS survey-wide scan of the substrate; additional information on the nanorings after calcination at 800 °C for 8 h in air (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.L. gratefully acknowledges the National Natural Science Foundation of China (Nos. 51373143 and 21174116), the Natural Science Foundation of Fujian Province (No. 2014J0105), and the Fundamental Research Funds for the Central Universities (Nos. 2013SH003 and 201312G004).

Figure 6. SEM images of patterned-CNT arrays. RIE periods: (a) and (c) 5 min; (b) and (d) 30 min. Scale bars: (a), (b), and (d) 5 μm; (c) 1 μm. 13331

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ABBREVIATIONS AFM, atomic force microscope; CL, colloidal lithography; CNT, carbon nanotube; CVD, chemical vapor deposition; EDX, energy dispersive X-ray spectroscopy; MCC, monolayer colloidal crystals; PS, polystyrene; RIE, reactive ion etching; SEM, scanning electronic microscopy; XPS, X-ray photoelectron spectroscopy

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DOI: 10.1021/acs.langmuir.5b03886 Langmuir 2015, 31, 13327−13333

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DOI: 10.1021/acs.langmuir.5b03886 Langmuir 2015, 31, 13327−13333