17760
2008, 112, 17760–17763 Published on Web 10/25/2008
In Situ Fabrication of Density-Controlled ZnO Nanorod Arrays on a Flexible Substrate Using Inductively Coupled Plasma Etching and Microwave Irradiation Seungho Cho,† Semi Kim,‡ Nam-Hyo Kim,† Ung-Ju Lee,† Seung-Ho Jung,† Eugene Oh,† and Kun-Hong Lee*,† Department of Chemical Engineering, National Center for Nanomaterials Technology (NCNT), Department of Mechanical Engineering, Pohang UniVersity of Science and Technology (POSTECH), San 31, Hyoja-Dong, Nam-Gu, Pohang, Gyungbuk, Korea 790-784 ReceiVed: September 12, 2008
We have fabricated density-controlled ZnO nanorod arrays on a flexible substrate (Teflon) using inductively coupled plasma (ICP) etching with anodic aluminum oxide (AAO) membranes and microwave irradiation without using a catalyst at 90 °C. The average interpore distances of Si surface etched with an AAO membrane anodized in 0.3 M oxalic acid (Mask 1) and an AAO membrane anodized in 0.1 M phosphoric acid (Mask 2) were ∼100 nm (Si pore density, ∼1.86 × 1010/cm2) and ∼450 nm (Si pore density, ∼7.16 × 108/cm2), respectively. During the microwave irradiation, ZnO nanorods grew from the Si pores. Thus, we could control the ZnO nanorods density by changing the interpore distance of the AAO membrane mask (Mask 1 case, ∼1.34 × 1010/cm2 and Mask 2 case, ∼6.12 × 108/cm2). Flexible electronics is a longstanding dream that is now under rigorous development. The advantages of flexible substrates, among many, are lower cost and the ability to construct devices of any geometry and shapes.1 A critical obstacle to achieving flexible electronics is the low transition or melting temperatures of flexible materials. The poor thermal stability of these materials restricts the use of high temperature procedures in the fabrication of flexible devices. Zinc oxide (ZnO) has a wide direct band gap of 3.37 eV at room temperature and a large exciton binding energy of about 60 meV. In addition, ZnO has the electrical and optical properties of a II-VI semiconductor and shows useful characteristics, such as a large piezoelectric constant and easy electrical conductivity modification. Direct synthesis of quasi onedimensional (1D) ZnO nanostructures on a substrate is highly desirable and the way forward for various electronic applications, including electron emitters,2 ultraviolet light emitting devices,3 and field-effect transistors.4 Many studies have examined the direct synthesis of aligned quasi 1D ZnO on a substrate.5,6 If such substrates are to be applied in advanced high-quality devices, researchers must be able to control the density, location, size, and orientation of the 1D nanostructures on the substrate. To this end, photolithography and electronbeam lithography procedures have been used to control the spatial positioning and size of 1D ZnO structures;7,8 however, such techniques require expensive and sophisticated devices. Previous studies have demonstrated the feasibility of sitespecific nucleation of ZnO nanorods by spatially and nonlithographically selective deposition of a catalyst.9,10 Conventional * To whom correspondence should be addressed. E-mail: ce20047@ postech.ac.kr. † Department of Chemical Engineering and National Center for Nanomaterials Technology (NCNT). ‡ Department of Mechanical Engineering.
10.1021/jp808117q CCC: $40.75
vapor-solid-liquid (VLS) methods were often used to synthesize ZnO nanorods using a metal catalyst. However, the synthetic temperature was sufficiently high that some materials, such as a polymer, could not be used as substrates. In addition, the catalyst material on the tips of the ZnO nanorods may act as defects, thereby modifying the properties of the nanorods and thus affecting the utility of pure ZnO nanorods for electronics application. Herein, we report a novel method for the low-temperature fabrication of flexible and density-controlled ZnO nanorods arrays. The key ideas of our fabrication method are to expose the Zn layer through regular hexagonal spacing Si pores formed by inductively coupled plasma (ICP) etching with an anodic aluminum oxide (AAO) mask and to synthesize ZnO nanorods selectively on the Zn layer in a solution that contains the growth units of ZnO. Figure 1 shows the experimental procedures for the fabrication of the flexible ZnO nanorod arrays. To prepare the substrate, a 15 nm thick Ti film (an adhesion layer), a 150 nm thick Zn film and a 150 nm thick Si film were successively deposited on a 200 µm thick Teflon sheet by magnetron sputtering in a vacuum chamber (∼10-6 torr). The Ti film was used to enhance the adhesion between the substrate and grown ZnO nanorods. To obtain two types of AAO membrane as ICP etching masks, bulk AAO was fabricated via the 2-step anodization of a high-purity (99.999%) bulk Al sheet in a 0.3 M oxalic acid solution at 40 V and 15 °C (Mask 1) or in a 0.1 M phosphoric acid solution at 195 V and 0 °C (Mask 2). AAO membranes of thickness ∼1.5 µm were obtained by removing the bulk Al part using a saturated HgCl2 solution. The AAO membrane mask was placed on the substrate and then the substrate was etched by ICP etching with Cl2 and Ar gas. Since the AAO membrane was attached to the substrate by weak attractive force, it could be easily removed using an air blowing gun after ICP etching. The pores created by ICP etching yielded 2008 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 112, No. 46, 2008 17761
Figure 1. Schematic illustration of the experimental procedures for the fabrication of the flexible density-controlled ZnO nanorod arrays.
a periodic arrangement of exposed areas of the Zn layer. For the growth of the ZnO nanorods, zinc acetate dihydrate (Zn(CH3COO)2 · 2H2O, 99%, Samchun) and ammonia-water (28.0∼30.0 wt %, Samchun) were used as zinc cation and hydroxide anion precursors, respectively. A 100 mL aqueous solution of zinc acetate dihydrate (0.02 M) and ammonia (∼0.16 M) was prepared at room temperature (pH ) 11.2). The substrate was transferred into the solution and the substratecontaining solution was irradiated using a temperature-controlled microwave synthesis system (2.45 GHz, single-mode, Greenmotif, IDX, Japan) at 90 °C for 40 min. The substrate was washed by ultrasonication in deionized water for 5 min, after which it was dried in an oven at 60 °C for 12 h. Microwave irradiation plays an important role in the fast synthesis method. It has been observed that significantly different results can be obtained by conventional heating and microwave heating.11-14 Recently, our group has been reported that various ZnO structures could be obtained by the homogeneous nucleation using microwave irradiation.11 We can accelerate the ZnO crystal growth by material-microwave interactions leading to thermal effects and specific (not purely thermal) effects,15,16 which can reduce the reaction time. The microwave field applied to dielectric materials mainly induces the rotation of polarized dipoles in molecules which generates heat due to molecular inner friction. The presence of an electric field also leads to orientation effects of dipolar molecules and hence reduces the activation energy (entropy term) in the Arrhenius equation.17,18 We believe that due to the lower activation energy, faster nucleation and growth took place in the reaction solution. The Mask 1 has a hexagonal arrangement of pores, one of which is marked by a circle in Figure 2a; the average pore diameter was ∼60 nm and the average interpore distance was ∼100 nm. Figure 2b shows the surface of the Si layer after ICP etching and removal of the AAO membrane mask. The Si layer also has regularly distributed pores, one of which is marked by a circle in Figure 2b; the average pore diameter of the Si surface is ∼80 nm, which is larger than the average AAO pore diameter because ICP etching with Cl2 and Ar is not perfectly
Figure 2. The experiment with Mask 1 (an AAO membrane anodized in 0.3 M oxalic acid): (a) SEM image of the surface of Mask 1 (plan view); (b) Si surface etched with Mask 1 (plan view); (c) surface of the substrate after microwave irradiation (oblique view). The inset in (c) is the plan view image of ZnO nanorods from the Si pores.
anisotropic etch process. ZnO nanorods were then grown from the pores under microwave irradiation (Figure 2c). ZnO nanorods are almost vertically aligned and physically separated due to the presence of the pores. In rare cases, two nanorods grew from one pore. ZnO nanorods were not detached from the substrate by the ultrasonication for clearing because of the good adhesion between the grown ZnO nanorods and the substrate. The zinc seed layer deposited on the substrate is crucial for the spatially selective growth of the ZnO nanorods on the substrate. In the reaction solution, hydroxide anions are provided very quickly by hydration of the ammonia-water (pH ) 11.2). Under the reaction conditions used in these experiments, solution-exposed zinc metal initially reacts with hydroxide ions to form soluble zincate ions (ZnO22-). These zincate ions then react with water and are deposited as solid-phase ZnO.19,20
Zn + 2OH- f ZnO22- + H2
(1)
ZnO22- + H2O f ZnO + 2OH-
(2)
Through reactions 1 and 2, the solution-exposed areas of the Zn metal film at the bottom of the pores are converted to ZnO, which provides good nucleation sites for a solution-phase ZnO precursor at the solution temperature of 90 °C. The reactions involved in the growth of ZnO on the ZnO seed layer are believed to be as follows:
NH3 + H2O f NH4+ + OH-
(3)
Zn2+ + 4OH- f Zn(OH)42-
(4)
Zn2+ + 4NH3 f Zn(NH3)42+
(5)
Zn(OH)42- f ZnO + H2O + 2OH-
(6)
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Zn(NH3)42+ + 2OH- f ZnO + 4NH3 + H2O 2-
Letters
(7)
2+,
The growth units, Zn(OH)4 and Zn(NH3)4 are incorporated into the ZnO crystal lattice by reactions 6 and 7.21,22 The ZnO crystal structure consists of hexagonally close packed oxygen and zinc atoms. ZnO crystals grow preferentially along the [0001] direction to form 1D rodlike structures due to the higher growth rate along this direction (c-axis).21 We found that some ZnO nanorods did not emerge from the Si pores. This occurs when the direction of the c-axis of the ZnO nanorod points toward the wall of the Si pore, leading to hindrance of the growth of the ZnO (see illustration in Figure 1). In addition, because the average interpore distance of Si pores etched with Mask 1 was relatively small, at some locations neighboring ZnO nanorods merged into a single larger ZnO rod. Through the use of Mask 2 (the AAO membrane anodized in phosphoric acid), the number of ZnO nanorods per unit area was reduced because the average interpore distance of this mask was larger than that of the Mask 1. The other experimental conditions were identical to those used in the Mask 1 experiments. Mask 2 had an average pore diameter of ∼100 nm and an interpore distance of ∼450 nm (Figure 3a). Thus, the pore density is ∼7.16 × 108/cm2 which is much smaller than that of Mask 1 (∼1.86 × 1010/cm2). Figure 3b shows an SEM image of the surface of the Si layer, which has hexagonally ordered pores. The dark parts of the image correspond to the exposed Zn layer; these features are distributed with the average interpore distance of ∼450 nm, the same as that of the Mask 2. The average pore diameter is ∼125 nm, which is bigger than that of the AAO membrane (Mask 2) due to the reasons outlined above for the case of Mask 1. During microwave irradiation, ZnO nanorods grew from the Si pores (Figures 3c and 3d). The variation in the diameters of the ZnO nanorods is greater than that of the Mask 1 case. The average length of the exposed parts of the nanorods is ∼500 nm. It was also observed that, as marked by the circles in Figure 3c, ZnO nanorods did not emerge from some of the Si pores due to the physical hindrance of the wall of the pore. Because the average diameter of the Si pores etched with Mask 2 is larger than that of the pores etched with Mask 1, coalescence of neighboring ZnO nanorods to form
Figure 3. The experiment with Mask 2 (an AAO membrane anodized in 0.1 M phosphoric acid): (a) SEM image of the surface of Mask 2 (plan view); (b) Si surface etched with Mask 2 (plan view); (c) surface of the substrate after microwave irradiation (plan view); (d) oblique view image of the Si pores with ZnO nanorods.
Figure 4. (a) TEM image of a ZnO nanorod. (b) HRTEM image of the point marked by the center of the circle in panel a. The inset shows the SAED pattern. (c) XRD pattern of the ZnO nanorods.
large-diameter rods was not observed in the case of Mask 2. The areal density of ZnO nanorods on the surface is ∼6.1 × 108/cm2 which is again much lower than the ZnO nanorods density of the Mask 1 experiment (∼1.34 × 1010/cm2). Figure 4a shows a TEM image of a single ZnO nanorod synthesized and detached from a substrate fabricated through the Mask 1 experiment. An HR-TEM image (Figure 4b) shows that the ZnO nanorod is highly crystalline with a lattice spacing of about 0.52 nm, corresponding to the distance between the (0001) planes in the ZnO crystal lattice. In addition, the selectedarea electron diffraction (SAED) pattern of the ZnO sample (inset of Figure 4b) confirms that it has a single crystalline structure along the [0001] direction. Figure 4c displays the X-ray diffraction (XRD) pattern of the ZnO nanorods. To record this pattern, the ZnO nanorods were separated from the substrate and gathered as a powder to remove the substrate signal from the XRD data. The XRD pattern can be indexed as the hexagonal wurtzite ZnO structure with calculated lattice constants of a ) 0.325 nm and c ) 0.521 nm. No other diffraction peaks were observed in the XRD pattern. In summary, using ICP etching with an AAO membrane mask, we directly fabricated ZnO nanorod arrays on a flexible substrate. The areal density of the ZnO nanorods could be controlled by simply changing the interpore distance of the AAO membrane masks. The low synthetic temperature of this method allows the use of materials with low transition temperatures as the substrate. We expect that this method can be readily generalized to fabricate other metal oxide arrays with controlled areal densities on a variety of substrates. Acknowledgment. This work was supported by the second phase BK21 program of the Ministry of Education of Korea and the Korea Foundation for International Cooperation of Science & Technology (KICOS) through a grant provided by the Korean Ministry of Science & Technology (MOST) in NANO/BIO/INFO Technology (NBIT) Symbiosis.
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