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Highly Ordered Nanoporous Alumina on Conducting Substrates with Adhesion Enhanced by Surface Modification: Universal Templates for Ultrahigh‐Density Arrays of Nanorods

2010; Volume: 22; Issue: 18 Linguagem: Inglês

10.1002/adma.200903763

1521-4095

Jinseok Byun, Jeong In Lee, Seungchul Kwon, Gumhye Jeon, Jin Kon Kim,

Semiconductor materials and devices

A highly ordered nanoporous anodized aluminum oxide template with excellent adhesion is fabricated on various conducting substrates by surface modification of the substrates. This template can be universally utilized to fabricate laterally long-range-ordered and hexagonally packed arrays of freestanding and vertically aligned metal, semiconductor, and conducting polymer nanorods on various substrates, including flexible substrates (see image). High-density arrays of one-dimensional (1D) nanostructures such as nanorods, nanowires, and nanotubes have attracted a great deal of interest owing to their potential applications as optoelectronics,1, 2 data storage materials,3 artificial actuators,4 surface modifiers with specific wetting behavior,5 and sensors.6 One of the most important aims of the fabrication of high-density 1D arrays is to achieve uniform structural parameters (diameter, height, and center-to-center spacing) over a large area, because this is essential for data storage materials,3 photonic devices with tunable photonic bandgaps (PBGs),7 near-field optical waveguides,7 and devices utilizing surface-enhanced Raman scattering (SERS).8 SERS, for example, requires periodic arrays of plasmon-resonant nanomaterials with uniform diameter and spacing to detect very accurately chemical and biomolecular species.8 Of the various methods for the fabrication of high-density arrays of 1D nanostructured materials,1, 9-15 electrodeposition or electropolymerization in nanoporous templates has been widely used because of its versatility, large scale, low-temperature operation, cost-effectiveness, and rapid processing speed.16-18 Typical nanoporous templates are block copolymer thin films with cylindrical nanopores,18 anodized aluminum oxide (AAO),19 and polycarbonate membranes. Among them, AAO with a high areal density (up to 1011 pores per cm2) and narrow size distribution over a large area has received much attention because of its simple and inexpensive control of structural parameters and excellent thermal and mechanical stability. Many research groups have reported the fabrication of nanorods (or nanotubes) of various metals, semiconductors, and conducting polymers by electrodeposition (or electropolymerization) inside nanopores of AAO.16, 17 However, since most AAO is fabricated by two-step anodization from aluminum sheet, a thin metal electrode (thickness of hundreds of nanometers) should be prepared at the bottom of pores by evaporation of excess metal (for instance, Au) for electrodeposition (or electropolymerization).16, 17 A critical drawback to this method is that once AAO is removed, most fabricated nanorods collapse (or at best become bundles) because the thin electrode cannot act as a rigid support.16, 17 Some research groups simply placed AAO on rigid substrates, and fabricated metal (or semiconductor) nanodot or nanohole arrays.20 However, this method cannot be used for the fabrication of an array of freestanding nanorods on a conducting substrate by electrodeposition (electropolymerization) because of poor adhesion between AAO and the substrate. Other research groups introduced the “direct anodizing” method. For this purpose, a high-purity Al layer was deposited on a rigid substrate using thermal or electron-beam evaporation, and nanopores were fabricated by direct anodization of the Al layer.21-27 However, the first anodization, which is essential in obtaining nanopores with uniform pore size and lateral long-range ordering, could not be carried out for a long time in this method because of a thickness problem. Namely, a relatively thick and high quality Al layer with a thickness of ca. 50 µm would be needed for this step, but it is practically difficult to prepare a thick Al layer by evaporation. Also, a metallic adhesive layer (such as W or Ti) is required between the substrate and Al to prevent cracks, delamination, and the detachment of Al during the anodizing process.26, 27 Furthermore, owing to the existence of a barrier layer that inhibits direct contact between the pores and the underlying substrate, one cannot avoid undesired pore widening during the removal of this layer by wet etching.21-27 Although several pre-texturing processes using soft imprinting,25 focused ion beams,28 and positive ceramic molds29 were performed before the anodization, high-density arrays of nanopores with uniform pore size and lateral long-range ordering on a rigid substrate have not been achieved, in spite of multistep processes. In this study, we achieved excellent adhesion between AAO fabricated by two-step anodization and various conducting substrates (indium tin oxide (ITO)-coated glass, Au, or Au-coated polymer film). For this purpose, we prepared an ultrathin polymer layer of either dihydroxy-terminated polystyrene (PS-dOH) or thiol-terminated PS (PS-SH) on the substrates by utilizing a graft reaction between the functional groups in the polymer chains and the substrate. The graft layer was further exposed to an ozone environment to make a favorable interaction with AAO. Since we used AAO prepared by two-step anodization, the uniformity of the nanopores is dramatically improved compared to that of direct anodizing on rigid substrates. Then we demonstrate that this template is powerful and universal for the fabrication of a laterally long-range-ordered and ultrahigh-density array of freestanding and vertically aligned nanorods of metals, semiconductors, and conducting polymers on various substrates. A particularly important example in this study is the fabrication of an ultrahigh-density array of freestanding and vertically aligned poly(3-hexylthiophene) (P3HT) nanorods on a flexible substrate. This well-ordered array of P3HT nanorods could be employed to achieve high power conversion efficiency of polymer solar cells on flexible substrates. Figure 1 shows schematically the preparation of the highly ordered AAO template on a conducting substrate and the fabrication of a laterally long-range-ordered and ultrahigh-density array of freestanding and vertically aligned nanorods on various conducting substrates. First, a very thin layer of functional polymer (either PS-dOH or PS-SH) was spin-coated on a conducting substrate and thermally annealed (Fig. 1a). PS-dOH was used for substrates with an oxide layer (for instance, ITO-coated glass), while PS-SH was used for Au-coated substrates. During the thermal annealing, the functional groups in the polymer layer were chemically interacted with the surface, graft polymers are generated on the substrate.30 After the ungrafted polymer chains had been washed out, an ultrathin polymer layer with a thickness of 1 nm was formed on the substrate. This thin layer was further treated with ozone to provide a hydrophilic surface (generating hydroxyl groups), which can facilitate the favorable interaction with AAO (Fig. 1b). AAO fabricated by the well-established two-step anodization from Al sheet20 was carefully transferred to the surface-modified substrate (details are in the Experimental section and Supporting Information S1), and dried completely at room temperature. Then the sample was annealed at 120 °C to ensure better contact of AAO to the substrate through favorable interaction of AAO and the hydroxyl groups of the substrate (Fig. 1c). Next, electrodeposition (or electropolymerization) was carefully performed inside the AAO template so that fabricated nanorods should not overgrow outside the pores (Fig. 1d). Because the height of the AAO template employed in this study was ca. 550 nm, the fabrication of nanorods with height less than ca. 350 nm was easily achieved without overgrowing outside the pores. Finally, the AAO template was removed by wet-etching, and residual water was completely removed by freeze drying,31, 32 generating a laterally long-range-ordered and ultrahigh-density array of freestanding and vertically aligned nanorods on various conducting substrates (Fig. 1e). Scheme of the preparation of the highly ordered AAO template on a conducting substrate and fabrication of laterally long-range-ordered and ultrahigh-density arrays of freestanding and vertically aligned nanorods on various conducting substrates. Figures 2a–c show top and cross-sectional scanning electron microscopy (SEM) images of the AAO template with a pore size of 40 nm on two different substrates (Au and ITO-coated glass). It is seen that the adhesion between AAO and the substrates was excellent, irrespective of the nature of the substrate. We also found good adhesion between another AAO template with a pore size of 20 nm and the substrates (Supporting Information S2). Since AAO was prepared by two-step anodization, the lateral long-range ordering and narrow size distribution of pores were well maintained after AAO was placed on the substrates. The areal density of nanopores in Figure 2 was 7.1 × 1010 pores per inch2 (ca. 1.1 × 1010 pores per cm2). It is noted that the areal density and pore size can be easily tuned by the characteristics of the pores in AAO, which depends on the anodization conditions. Figure 2d is a photograph of AAO on ITO-coated glass substrates (25 mm × 25 mm) without and with surface modification, after they had been immersed in deionized (DI) water for 30 min and then dried. When a bare ITO substrate was used, AAO detached from the substrate (right), indicating that the adhesion between AAO and the substrate was poor. In this situation, the wet-chemical process could not be performed. On the other hand, the adhesion between AAO and the substrate was excellent when the surface was modified (left). To evaluate quantitatively the adhesion force between AAO and the substrates without and with surface modification, we carried out a peel test. The adhesion force between AAO and the surface-modified substrate was found to be ca. 20 times that of the unmodified substrate (see Supporting Information S3). Top (a) and cross-sectional (b,c) SEM images of the AAO template with pore size of 40 nm on two different conducting substrates (ITO-coated glass (b) and Au (c)). d) Photograph of AAO on the surface-modified (left) and bare (right) ITO-coated glass after the samples had been immersed in DI water for 30 min. SEM images of ultrahigh-density arrays of CdSe nanorods on two different substrates (ITO-coated glass and Au) are shown in Figure 3a–c. CdSe is an important II–VI semiconductor material because of its high photosensitivity.33 Figures 3a–c show that highly ordered CdSe nanorod arrays with a uniform diameter (40 nm) and height (200 nm) were fabricated on various substrates after removal of the AAO template. We found that lateral long-range ordering in hexagonal packing was obtained, which is confirmed by the analysis of the Voronoi diagram (see Supporting Information S4). The diameter and height of the nanorods could be easily tuned by the pore size of the AAO template and electrodeposition time. Figure 3d shows a transmission electron microscopy (TEM) image and a selected area electron diffraction (SAED) pattern of a single CdSe nanorod. From X-ray diffraction (XRD) patterns, the fabricated CdSe nanorods show a well-defined cubic (zinc blende type33) crystalline structure (see Supporting Information S5). The ultrahigh-density array of freestanding and vertically aligned CdSe nanorods on ITO-coated glass could be used as an electron-accepting material for photovoltaic cells with higher conversion efficiency.34 A Co nanorod array was also fabricated by electrodeposition and characterized. Co has been widely used for data storage materials owing to its ferromagnetic property.35 The TEM image and XRD pattern of fabricated Co nanorods show hexagonally closed packed (hcp) single crystals (see Supporting Information S6). SEM images of laterally long-range-ordered and ultrahigh-density arrays of freestanding and vertically aligned CdSe nanorods on two different substrates (ITO-coated glass (a,b) and Au (c)). d) TEM image of a single CdSe nanorod. Inset in (d): HRTEM image with the SAED pattern. The above method could be also applied to fabricate ultrahigh-density arrays of various nanorods on a flexible substrate with conducting surface. Flexible substrates are especially needed in wearable electronic devices and implants for medical purposes.36, 37 For this purpose, we chose polyarylate (PAR) film with a good chemical resistivity and a high thermal stability because of its high glass transition temperature (330 °C). After evaporation of Au electrode on the PAR film (the details are given in the Supporting Information S7), PS-SH was spin-coated and ungrafted chains were washed out. The Au-coated PAR film was easily bent without cracking, as shown in Figure 4a. After AAO was carefully placed on the surface-modified PAR film, electropolymerization of P3HT, the best electron-donating conducting polymer used in organic photovoltaic cells and organic field effect transistors, was carried out inside the AAO template. Figures 4b and c show top and cross-sectional SEM images of an ultrahigh-density array of P3HT nanorods with a diameter of 60 nm and height of 200 nm on the PAR film, after removal of the AAO template. Laterally long-range-ordered and freestanding P3HT nanorods were fabricated even on the flexible substrate. We also found that even when the sample was very severely bent, as shown in Figure 4d, the freestanding P3HT nanorods on the flexible substrate were not detached from the substrate (see the insets of Fig. 4d). a) Photograph of bent Au-coated PAR film. Top (b) and cross-sectional (c) SEM images of a laterally long-range-ordered and ultrahigh-density array of free-standing and vertically aligned P3HT nanorods on Au-coated PAR film after removal of the AAO template. d) SEM image of the bent substrate over a large area. The insets are high magnification images of the P3HT nanorod array on the bent regions (scale bar = 100 nm). In summary, we have introduced a simple but very efficient method that allows excellent adhesion of AAO prepared by two-step anodization to various conducting substrates by generating an ultrathin graft polymer layer on the substrate. Since the lateral long-range ordering in hexagonal packing of nanopores was well maintained on the substrate, an ultrahigh-density array of freestanding and vertically aligned CdSe and Co nanorods was successfully fabricated on the substrate. Furthermore, we demonstrated that this method can also be used to fabricate an ultrahigh-density array of freestanding and vertically aligned P3HT nanorods on a gold-coated flexible polymer substrate. The highly ordered array of nanorods could be used for optoelectronics, high-density data storage materials, sensors, and photovoltaic cells with high conversion efficiency. A solution of PS-dOH with a weight-average molecular weight Mw of 6500 g mol−1 (Polymer Source, Inc.) in toluene (1 wt%) was spin-coated at a rotating speed of 1500 rpm onto the ITO-coated glass. The thickness was 50 nm. Then it was annealed at 170 °C under vacuum to facilitate the graft reaction of hydroxyl groups in PS-dOH with the oxide surface. Ungrafted PS-dOH was washed out by cyclohexane, and the final thickness of the grafted PS-dOH layer was ca. 1 nm, measured by ellipsometry (EC-400, M-200V). The chemically grafted layer was further exposed to an ozone environment for 20 min. For the Au substrate, PS-SH with Mw of 7700 g mol−1 (Polymer Source, Inc.) was used. After spin-coating, the sample was annealed at 120 °C, and physically adsorbed chains were completely washed out. We found that when the graft layer of PS-SH was further exposed to an ozone environment, the surface became hydrophilic; thus a good contact between this layer and AAO was achieved (see Supporting Information S8). AAO was prepared by two-step anodization (the details are given in the Supporting Information S1) 20. Nail polish was infiltrated into the nanopores to avoid pore widening during removal of the barrier layer 38. After removal of the barrier layer, the nail polish was removed from AAO by immersion in acetone. Then the template was floated onto DI water/acetone (50:50 v/v) solution, which provides an appropriate drying rate for complete contact between AAO and the substrate. Finally, AAO was carefully placed on the modified substrate, and annealed at 120 °C for 20 min (details are given in the Supporting Information S1). Electrodeposition (or electropolymerization) was carried out using a three electrode system: a working electrode of ITO or Au, a counter electrode of platinum foil, and a reference electrode of Ag/AgCl in a 3M NaCl aqueous solution. (The details of experimental conditions for CdSe and Co and P3HT are given in the Supporting Information S9.) After fabrication of the CdSe and Co nanorods, the AAO template was removed by immersing the sample in sodium hydroxide (2 wt%) solution for 1 h, followed by rinsing with DI water several times. Phosphoric acid (H3PO4, 5 wt%) solution was used to remove the AAO template for P3HT nanorods. The remaining water was completely removed by freeze drying to maintain the vertically aligned freestanding nanorod array on the substrate. The morphology of the AAO template and fabricated nanorods was investigated by field emission scanning electron microscopy (FESEM, Hitachi S-4800), TEM (Hitachi 7600) and high-resolution TEM (HRTEM, JEOL, JEM-2100F) at 200 kV operating voltage. The X-ray diffraction was examined with Cu Kα radiation (wavelength is 0.154 nm) by a PANalytical X'Pert PRO Diffractometer. This work was supported by the National Creative Research Initiative Program supported by the National Research Foundation of Korea (NRF). Supporting Information is available online from Wiley InterScience or from the authors. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. 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