Novel Patterns by Focused Ion Beam Guided Anodization - Langmuir

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Novel Patterns by Focused Ion Beam Guided Anodization Bo Chen, Kathy Lu,* and Zhipeng Tian Department of Materials Science and Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, United States Received September 24, 2010. Revised Manuscript Received November 24, 2010 Focused ion beam patterning is a powerful technique for guiding the growth of ordered hexagonal porous anodic alumina. This study shows that, with the guidance of the focused ion beam patterning, hexagonal porous anodic alumina with interpore distances from 200 to 425 nm can be fabricated at 140 V in 0.3 M phosphoric acid. When the interpore distance is increased to 500 nm, alternating diameter nanopore arrays are synthesized with the creation and growth of new small pores at the junctions of three large neighboring pores. Moreover, alternating diameter nanopore arrays in hexagonal arrangement are fabricated by focused ion beam patterning guided anodization. Interpore distance is an important parameter affecting the arrangement of alternating diameter nanopore arrays. Different types of novel patterns are obtained by designing different focused ion beam concave arrays. The fundamental understanding of the process is discussed.

1. Introduction Highly ordered nanotube1,2 and nanopillar3-8 arrays with monodispersed size distribution and uniform orientation have been synthesized by melt-wetting or solution-wetting of porous anodic alumina templates. The low-energy organic polymer materials spread and wet the high-energy nanopore surface.9 When the polymer liquid completely fills the pores, polymer nanopillars are formed as the replication of the template. On the other hand, if the polymer liquid just wets the pore wall and does not fill the pores, polymer nanotubes are formed. The diameter, length, and density of the polymer nanoarrays are controlled by the porous anodic alumina template. In order to release 1D polymer nanoarrays from the porous anodic alumina template, the surface of the porous template is silanized with low surface energy silane molecules, which reduce the adhesion between the polymer inside the pores and the pore walls.5,6 These polymer nanopillar arrays exhibit a self-cleaning effect with a large water contact angle due to the superhydrophobicity nature of the surface.7,8 Based on the 10% porosity rule,10 self-organized nanopore arrays can only be synthesized under very strict anodization *To whom correspondence should be addressed. E-mail: [email protected]. Telephone: þ1 540 231 3225. Fax: þ1 540 231 8919. (1) Steinhart, M.; Wendorff, J. H.; Greiner, A.; Wehrspohn, R. B.; Nielsch, K.; Schilling, J.; Choi, J.; G€osele, U. Science 2002, 296, 1997. (2) Steinhart, M.; Wehrspohn, R. B.; G€osele, U.; Wendorff, J. H. Angew. Chem., Int. Ed. 2004, 43, 1334–1344. (3) Lu, Q. Y.; Gao, F.; Komarneni, S.; Mallouk, T. E. J. Am. Chem. Soc. 2004, 126, 8650–8651. (4) Lee, W.; Jin, M. K.; Yoo, W. C; Lee, J. K. Langmuir 2004, 20, 7665–7669. (5) Lee, D. Y.; Lee, D. H.; Lim, H. S.; Han, J. T.; Cho, K. Langmuir 2010, 26, 3252–3256. (6) Grimm, S.; Giesa, R.; Sklarek, K.; Langner, A.; G€osele, U.; Schmidt, H. W.; Steinhart, M. Nano Lett. 2008, 8, 1954–1959. (7) Lee, Y. W.; Park, S. H.; Kim, K. B.; Lee, J. K. Adv. Mater. 2007, 19, 2330– 2335. (8) Cho, W. K.; Choi, I. S. Adv. Funct. Mater. 2008, 18, 1089–1096. (9) Wu, S. Polymer Interface and Adhesion; Dekker: New York, 1982; Chapter 6. (10) Nielsch, K.; Choi, J.; Schwirn, K.; Wehrspohn, R. B.; G€osele, U. Nano Lett. 2002, 2, 677–680. (11) Asoh, H.; Nishio, K.; Nakao, M.; Yokoo, A.; Tamamura, T.; Masuda, H. J. Vac. Sci. Technol., B 2001, 19, 569–572. (12) Li, A. P.; M€uller, F.; Birner, A.; Nielsch, K.; G€osele, U. J. Appl. Phys. 1998, 84, 6023–6026.

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conditions and the interpore distance is confined to several fixed values: 65 nm in sulfuric acid at 25 V,11,12 100 nm in oxalic acid at 40 V,13-15 and 500 nm in phosphoric acid at 195 V.16 Nanoindentation lithography is a widely used patterning technique to fabricate highly ordered porous anodic alumina.17-23 Square arrangement of square nanopores has been made using nanoindentation.23-25 Similarly, triangular anodic alumina nanopores with a graphite lattice structure were synthesized with the guidance of nanoindented graphite lattice structure concaves.23 With patterning, the nanopore organization window can be enlarged. The interpore distance of ordered alumina nanopore arrays can √ be reduced to 1/ 3 of the lattice constant of the guiding patterns if the anodization window is such that a new pore forms at the center of three patterned concaves.20-22 Ordered porous anodic alumina with 115 nm interpore distance was obtained by anodizing a 200 nm interpore distance pattern at 46 V in oxalic acid.18 Similarly, an ordered 300 nm interpore distance nanopore arrangement was produced after the anodization of a nanoindented concave pattern with 500 nm interpore distance at 120 V in phosphoric acid.21,22 Up to now, most studies have been directed toward uniform diameter and equal interpore distance porous anodic alumina (13) Sulka, G. D.; Stepniowski, W. J. Electrochim. Acta 2009, 54, 3683–3691. (14) Li, F. Y.; Zhang, L.; Metzger, R. M. Chem. Mater. 1998, 10, 2470–2480. (15) Kashi, M. A.; Ramazani, A. J. Phys. D: Appl. Phys. 2005, 38, 2396–2399. (16) Masuda, H.; Yada, K.; Osaka, A. Jpn. J. Appl. Phys. 1998, 37, 1340–1342. (17) Masuda, H.; Yamada, H.; Satoh, M.; Asoh, H.; Nakao, M.; Tamamura, T. Appl. Phys. Lett. 1997, 71, 2770–2772. (18) Masuda, H.; Yotsuya, M.; Asano, M.; Nishio, K.; Nakao, M.; Yokoo, A.; Tamamura, T. Appl. Phys. Lett. 2001, 78, 826–828. (19) Choi, J.; Luo, Y.; Wehrspohn, R. B.; Hillebrand, R.; Schilling, J.; G€osele, U. J. Appl. Phys. 2003, 94, 4757–4762. (20) Shingubara, S.; Maruo, S.; Yamashita, T.; Nakao, M.; Shimizu, T. Microelectron. Eng. 2010, 87, 1451–1454. (21) Choi, J.; Nielsch, K.; Reiche, M.; Wehrspohn, R. B.; G€osele, U. J. Vac. Sci. Technol. B 2003, 21, 763–766. (22) Choi, J.; Wehrspohn, R. B.; G€osele, U. Electrochim. Acta 2005, 50, 2591– 2595. (23) Masuda, H.; Asoh, H.; Watanabe, M.; Nishio, K.; Nakao, M.; Tamamura, T. Adv. Mater. 2001, 13, 189–192. (24) Asoh, H.; Ono, S.; Hirose, T.; Nakao, M.; Masuda, H. Electrochim. Acta 2003, 48, 3171–3174. (25) Kwon, N. Y.; Kim, K. H.; Heo, J.; Chung, I. J. Vac. Sci. Technol. A 2009, 27, 803–807.

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patterns. Nanopore patterns with alternating diameters have not been achieved yet. Nanoindentation shows potential in creating different interpore distance patterns,26,27 but more work is needed. One example is the Moire pattern, which is a composite pattern created when two identical grids are overlaid at an angle. Since the interpore distance is dependent on the applied voltage with a linear proportional constant of 2.5 nm/V during the anodization,10 the Moire patterns with very different interpore distances are difficult to synthesize. Focused ion beam (FIB) patterning has the advantage of designing any nanoconcave array with different pore arrangement, pore diameter, and interpore distance.28-32 Combining FIB patterning with anodization has the potential to obtain alumina nanopore arrays with unique nanopore arrangements. In this work, FIB patterning is used to create hexagonal and rectangular arrangements and alternating diameter concave patterns with different interpore distances on Al surfaces. The effects of these concaves on guiding the growth of the nanopores in the subsequent anodization are analyzed. The mechanism of forming alternating diameter nanopore patterns is discussed.

2. Experimental Section High purity aluminum foils (99.999%, Goodfellow Corporation, Oakdale, PA) with 8 mm  22 mm  0.3 mm size were used as the starting material. They were first smoothed by a Buehler specimen mounting press with 27.8 MPa pressure and then washed with ethanol and acetone. After that, they were annealed at 500 °C for 2 h in high purity flowing Ar gas with 5 °C/min heating and cooling rates to recrystallize the aluminum foils and remove mechanical stress. For electropolishing, the annealed aluminum foils were degreased in ethanol and acetone for 5 min each, followed by DI water rinsing after each step. The aluminum foils were then immersed in a 0.5 wt % NaOH solution for 10 min with ultrasound in order to remove the oxidized surface layer. After that, the aluminum foils were electropolished in a 1:4 mixture of perchloric acid (60-62%)/ethanol (95%) (volume ratio) under a constant voltage of 20 V at room temperature with 500 rps stirring speed for 3 min. A dual beam focused Gaþ ion bean microscope (FIB, FEI Helios 600 NanoLab, Hillsboro, OR) was employed to create different concave patterns to guide the anodization. The accelerating voltage for the FIB microscope was 30 keV. The beam diameter was ∼30 nm. The beam current was 28 pA. The beam dwell time at each scan was 3 μs. The FIB created patterns were observed in the scanning electron microscopy (SEM) mode, which allowed for in situ monitoring of the surface features of the Al foils at different stages of the ion exposure. An atomic force microscope (Digital Instruments MultiMode SPM, Veeco Instruments Inc., Camarillo, CA) was used to measure the depth and diameter of the FIB patterned concaves. The FIB patterned Al foils were anodized in 0.3 M phosphoric acid under 20 mA constant current at 0 °C for 5 min. The voltage was ∼140 V after a few seconds of anodization. Pore opening was carried out in 5 wt % phosphoric acid at 30 °C for 10 min. The porous anodic alumina patterns were characterized by scanning electron microscopy (Quanta 600 FEG, FEI Company, Hillsboro, (26) Choi, J.; Wehrspohn, R. B.; G€osele, U. Adv. Mater. 2003, 15, 1531–1534. (27) Luchnikov, V.; Kondyurin, A.; Formanek, P.; Lichte, H.; Stamm, M. Nano Lett. 2007, 7, 3628–3632. (28) Liu, N. W.; Liu, C. Y.; Wang, H. H.; Hsu, C. F.; Lai, M. Y.; Chuang, T. H.; Wang, Y. L. Adv. Mater. 2008, 20, 2547–2551. (29) Liu, N. W.; Datta, A.; Liu, C. Y.; Peng, C. Y.; Wang, H. H.; Wang, Y. L. Adv. Mater. 2005, 17, 222–225. (30) Liu, N. W.; Datta, A.; Liu, C. Y.; Wang, Y. L. Appl. Phys. Lett. 2003, 82, 1281–1283. (31) Liu, C. Y.; Datta, A.; Wang, Y. L. Appl. Phys. Lett. 2001, 78, 120–122. (32) Lu, K.; Zhao, J. J. Nanosci. Nanotechnol. 2010, 10, 6760–6768.

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OR). In order to observe the backside of the porous alumina, the unanodized Al layer was removed in a saturated CuCl2 solution (Reagent ACS 99þ%, Acros Organics, 2000 Park Lane Drive, Pittsburgh, PA). The cross sections of the anodized nanopores were obtained in the FIB microscope using 0.28 pA current to cut the patterned porous alumina samples.

3. Results and Discussion 3.1. Enlarged Interpore Distance Range. To examine the effect of the FIB patterning on the anodization process, three hexagonal concave patterns on the same aluminum foil with the same diameter (45 nm) but different interpore distances are created: 350 nm (Figure 1a), 250 nm (Figure 1c), and 200 nm (Figure 1e). The depths of all the FIB patterned concaves are ∼10 nm (inset (2) of Figure 1a, c, and e). After the anodization under the same condition, all three of the anodized nanopore arrays maintain the original FIB patterned interpore distances while the pore diameters grow from 45 to 110 nm (Figure 1b), to 88 nm (Figure 1d), and to 80 nm (Figure 1f). The backside SEM images without pore opening clearly show the barrier layer profiles (inset (1) in Figure 1b, d, and f) and confirm that ordered hexagonal nanopore patterns with 350, 250, and 200 nm interpore distances are obtained. Self-organized anodization at 140 V without the guidance of the FIB patterns only forms disordered nanopores with the interpore distance at ∼350 nm. This enlarged interpore distance range and the different anodized pore sites have not been explicitly addressed before and can be understood as follows. For self-organized anodization, the electrical field balancing the aluminum oxidation rate and the oxide layer dissolution rate follows the following approximations: the barrier layer thickness dB ≈ 1.3 nm/V, the interpore distance dinter ≈ 2.5 nm/V, the pore diameter D ≈ 0.9 nm/V, and the oxide pore wall thickness dw ≈ 0.8 nm/V.33,34 Under the guidance of the FIB patterning, the pattern with 350 nm interpore distance meets the linear proportional relationship with the applied voltage: dinter = 2.5 nm/V  140 V = 350 nm. There is enough space between two pores for the oxide walls to fully expand before impinging each other. The oxide pore wall thickness is ∼120 nm and the barrier layer thickness is ∼200 nm, which can be clearly observed in the surface image (Figure 1b) and the cross section image (inset (2) of Figure 1b). The pore wall thickness satisfies the relation dw ≈ 0.8 nm/V  140 V = 112 nm. As illustrated in Figure 2a, the oxide wall thickness between two neighboring nanopores is 2dw and the barrier layer thickness dB = AA0 is determined by the anodization voltage. The oxidation rate balances the dissolution rate at positions A and B. When the interpore distance is decreased (d < 2dw), the oxide walls of two neighboring nanopores meet each other before they are fully developed according to the equifield strength model (such as in Figure 2b).35,36 However, the FIB patterned concaves, Gaþ implantation in aluminum, and aluminum amorphization during the FIB patterning are significant factors favoring nanopore growth at the FIB patterned sites. During the anodization, pores preferentially grow at the bottoms of the FIB patterned concaves.37 At the same time, the pores have a tendency to expand and the neighboring walls of these pores approach each other until two alumina layers merge. Since all the nanopores develop at the same rate, the neighboring pores restrain the change of the interpore distance. For the 250 nm interpore distance pattern, assuming the diameter of the nanopores stays the same as in (33) (34) (35) (36) (37)

Lee, W.; Ji, R.; G€osele, U.; Nielsch, K. Nat. Mater. 2006, 5, 741–747. Hunter, M. S.; Fowle, P. J. Electrochem. Soc. 1954, 101, 481–485. Su, Z. X.; Hahner, G.; Zhou, W. Z. J. Mater. Chem. 2008, 18, 5787–5795. Su, Z. X.; Zhou, W. Z. Adv. Mater. 2008, 20, 3663–3667. Chen, B.; Lu, K.; Tian, Z. P. Electrochim. Acta 2010, 56, 435–440.

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Figure 1. SEM images of FIB patterns (a,c,e) and the corresponding anodized nanopore patterns in 0.3 M phosphoric acid at 20 mA with a steady state potential of 140 V (b,d,f). The FIB patterns are designed with different interpore distances: (a) 350 nm, (c) 250 nm, and (e) 200 nm. Insets (1) and (2) in (a), (c), and (e) are the AFM image and corresponding surface topology along the line, respectively. Insets (1) and (2) in (b), (d), and (f) are the backside and cross section views of the anodized nanopore patterns, respectively. The scale bars in all the insets are 300 nm.

Figure 2a, then the wall thickness should decrease to 120 - (350 250)/2 = 70 nm and the thickness along BB0 is dB. However, the 802 DOI: 10.1021/la1038393

thickness from B0 to any position in the range of A-B is larger than dB (Figure 2b), leading to a lower field strength. The field-assisted Langmuir 2011, 27(2), 800–808

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Figure 2. Schematic drawing of (a) large interpore distance, (b) decreased interpore distance, and (c) decreased nanopore diameter.

oxide dissolution rate in the area AB becomes slower than that at the position B and below. In order to balance the dissolution rate and the oxidation rate, the wall thickness between the pores increases by decreasing the nanopore diameters, as shown in Figure 2c. The cross section SEM image in the inset (2) of Figure 1b shows that when the interpore distance of the FIB patterned concaves decreases, the anodized nanopore arrays maintain the 250 nm interpore distance while the wall thickness increases to 82 nm and the diameter of the anodized nanopores decreases to 88 nm. The arc segment of the pore bottom also decreases and the barrier layer thickness stays at ∼200 nm for the 250 nm interpore distance pattern as shown in the inset (2) of Figure 1d, just as expected in Figure 2c. The same finding is true for even smaller interpore distance patterns. When the FIB patterned interpore distance further decreases to 200 nm, the diameter of the anodized nanopores is further reduced to 80 nm, the wall thickness increases from 82 (250 - 200)/2 = 57 to 60 nm, and the barrier layer is still around 200 nm (as shown in inset (2) of Figure 1f). This explains why the anodized nanopore diameter from the 200 nm interpore distance FIB pattern (Figure 1f) is smaller than those from the 250 and 350 nm interpore distance FIB patterns (Figure 1b and d). When the interpore distance increases to larger than 350 nm, such as 425 nm, the space among the three neighboring pores is not large enough to develop a new pore; an ordered alumina nanopore Langmuir 2011, 27(2), 800–808

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Figure 3. (a) Alternating FIB pattern with 200 nm interpore distance; the larger concave size is 65 nm, and the smaller concave size is 45 nm. Insets (1) and (2) in (a) are the AFM image and corresponding surface topology along the line, respectively. The scale bar is 500 nm. (b) Alternating FIB pattern after the anodization in 0.3 M phosphoric acid at 20 mA with a steady state potential of 140 V, and the inset is the schematic of the pore shape and oxide wall shape development.

array is still obtained with the guidance of the FIB pattern by increasing the pore diameter and decreasing the pore wall thickness (see Supporting Information, Figure S1). 3.2. Alternating Diameter Nanopore Array. The guiding effect of the alternating-sized concave patterns from the FIB patterning is also studied. Figure 3a shows the alternating-sized concave pattern created by the FIB with 65 and 45 nm pore sizes and 200 nm interpore distance. The depth of the large pores is 10 nm, and the depth of the small pores is 3 nm (the insert of Figure 3a). After the anodization, alternating diameter pores grow and the interpore distance remains at 200 nm. The large pores have 105 nm diameter. The small pores are elongated in the direction perpendicular to the large pore connecting lines. The long axis of the small pores is 95 nm, and the short axis is 50 nm (Figure 3b). The development of these pore sizes and shapes demonstrates unique opportunities in creating novel pore shapes and patterns by FIB guided anodization. DOI: 10.1021/la1038393

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In Figure 3, the interpore distance of 200 nm is smaller than that of the self-organized anodization, 350 nm. The oxide walls of the nanopores developed from the large and small FIB patterned concaves meet and restrict the full growth of each other. Because of the larger diameter, more Gaþ implantation, and aluminum amorphization, the nanopores developed from the FIB patterned large concaves grow faster and have thicker oxide walls. When the oxide wall thicknesses of both large and small pores are less than dw (112 nm), the development of the nanopores is confined by each other; the anodized nanopores increase their oxide wall thickness by decreasing the pore diameters to 105 and 50 nm (in the short axis direction), respectively. As discussed in Figure 2, the nanopores developed from the small concaves have thinner oxide walls, and the pore diameter is confined to a much smaller value in order to maintain the equifield strength. Alternating diameter nanopore arrangement is thus obtained. The large concaves have six small concaves in a symmetrical distribution as neighbors. As a result, these large concaves develop into large circular nanopores. The small concaves, however, have four small concaves and two large concaves in an asymmetrical distribution as neighbors. Because the oxide layers of the large pores grow much faster, the oxide layers of the small pores come in contact with those of the neighboring large pores first. The oxide layer growth in the direction of the large pore connecting line is restricted and thinner than that in the direction perpendicular to the large pore connecting line. Therefore, the small concaves grow into elliptical nanopores. Figure 4a shows the alternating-sized nanopore array by the FIB patterning with three different pore diameters: 80 nm, 65 nm, and 45 nm, respectively. The corresponding pore depths are 10 nm, 6 nm, and 3 nm, respectively (the insert of Figure 4a). The interpore distance is again 200 nm. After the anodization, the pattern arrangement stays unchanged and the diameters of the nanopores grow to 105, 90, and 55 nm (Figure 4b). An alternatingsized nanopore arrangement with three different diameters is thus created. Similar to Figure 3, the shape of the small pores is not circular. The pore development process in Figure 4 can again be understood based on the equifield strength controlled oxide layer growth model.35,36 During the anodization, the large concaves grow faster because of the more extensive FIB patterning (shown as white rings around the concaves in Figure 4a). Subsequently, large pores have a thicker oxide layer and larger pore diameter (105 nm). Even though the FIB patterned concaves are larger than those large concaves in Figure 3, the anodized pore sizes are the same because of the larger size neighbors (65 nm) in Figure 4a. Because of the symmetrical distribution of the neighboring concaves, the large concaves grow into circular pores (pore 1 in Figure 4b inset). The medium-sized concaves also grow faster than the small concaves and thus have a higher tendency to maintain the original pore shape even though their surroundings are not symmetrical (pore 2 in Figure 4b inset, 90 nm). For the small concaves, the surrounding condition is a little complicated as shown in Figure 4b insert. The small pore “3” is surrounded by four small pores and two medium-sized pores and grows into an elliptical shape, 83 nm in the long axis and 51 nm in the short axis, and elongates in the direction without the confinement of the medium-sized pores. The small pore “4” is surrounded by two small pores and four medium-sized pores and also grows into an elliptical shape that elongates in the direction of the small pores, 86 nm in the long axis and 58 nm in the short axis, The small pore “500 , restricted by two medium-sized pores “2” from the same side, grows into a cometary shape. Differently, the small pore “6” is surrounded by six small pores and has no medium or large pores 804 DOI: 10.1021/la1038393

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Figure 4. (a) Alternating FIB pattern with three different nanopore diameters: 80, 65, and 45 nm, the interpore distance is 200 nm. Insets (1) and (2) in (a) are the AFM image and corresponding surface topology along the line, respectively. The scale bar is 500 nm. (b) Alternating FIB pattern after the anodization in 0.3 M phosphoric acid at 20 mA with a steady state potential of 140 V; the inset is the enlarged SEM image of (b) with different pores labeled.

to restrict its growth. Subsequently, it grows into a larger size (86 nm) than other small pores and the shape is circular. A similar guiding function of gradient FIB concave patterns is also studied, and the results are given in the Supporting Information (Figure S2). For the gradient diameter concave arrays produced by the FIB patterning with 200 nm interpore distance, the large concave size is 65 nm and the small concave size is 45 nm. After the anodization, the nanopores remain the hexagonal arrangement and the interpore distance stays at 200 nm. The anodized large and small concaves grow into circular pores with very similar diameters, 85 and 80 nm, respectively. Similar to pore “6” in Figure 4b, when a pore is symmetrically surrounded by six same diameter pores, it grows into a spherical shape and has the same oxide wall thickness. The diameter change can also be explained by the equifield strength model. The interpore distances for the large and small pores are the same. Both the large and small pores have the same oxide wall thickness as their neighbors. Since the thickness of the oxide barrier layer is determined by the Langmuir 2011, 27(2), 800–808

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Figure 5. (a) FIB patterned alternating-sized concaves with 350 nm interpore distance after the anodization in 0.3 M phosphoric acid at 20 mA with a steady state potential of 140 V. (b) FIB patterned alternating-sized concaves with 500 nm interpore distance after the anodization under the same condition.

anodization voltage, the oxide wall thicknesses of the large and small pores are similar. However, the small pores have a smaller size to start with. Subsequently, the anodized pores are slightly smaller than the large pores in the top region. The diameters of the nanopores along the boundary of the large and small pore regions are around 70 nm, and the pore shape is elliptical. This is because the FIB patterned concaves along the boundary are surrounded by four small concaves and two large concaves and the oxide layer growth is restricted in the direction of the large pores. 3.3. Unique Pore Patterns Dictated by Interpore Distance. The pore size and arrangement can also be directly affected by the interpore distance. When the interpore distance of the alternating FIB pattern with 65 nm large concave size and 45 nm small concave size in Figure 3a is increased to 350 nm (see Supporting Information Figure S3a), the diameters of the nanopores developed from the large and small concaves are very close, around 105 nm (Figure 5a). This value is also very close to the pore sizes in Figures 3b and 4b. Ordered hexagonal nanopore arrays with uniform pore diameter have been fabricated. The main difference is the oxide wall thicknesses around the large and Langmuir 2011, 27(2), 800–808

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small concaves, with the large concaves showing thicker oxide walls. This is believed to result from the more extensive growth of the oxide walls for the large concaves before being restricted by their neighbors. Figure 5a also shows that the shape of the oxide walls developed from the large concaves is regular hexagonal while that from the small concaves is hexagonal but elongated in the direction without the confinement of large pores as discussed in Figure 3b. This reaffirms our explanation of the nanopore shape and diameter development during the FIB guided anodization. When the interpore distance of the FIB patterned concaves is increased to 500 nm (see Supporting Information Figure S3b), the diameters of the anodized nanopores developed from the large and small concaves are close to each other, around 125 nm (Figure 5b). The oxide wall shapes for both pore sizes are regular hexagonal. Since the interpore distance is much larger than 350 nm, the value required for self-organized anodization in this study, the oxide walls of each FIB patterned pore fully grow before coming in contact with those of the neighboring pores during the anodization. As discussed in Figure 2, the large interpore distance leads to much thicker oxide walls and much larger pore diameters (125 vs 105 nm). The unanodized space at the junctions of three neighboring pores increases as the interpore distance increases. When the interpore distance is 500 nm, the junction space is large enough to develop a new pore. The triangular shape of the new small pores results from the symmetrical confinement of the oxide layers of the three neighboring pores. Since the junction space of the three neighboring pores is limited and the new pores form later during the anodization, the new pores have a smaller diameter (∼70 nm). Therefore, another type of alternating diameter nanopore arrangement is created. The arrangement of the large and small pores is different from that in Figure 3b. In Figure 5b, the small pore locates at the center of the triangular gravity of three large neighboring pores, while in Figure 3b the small pore locates in the middle of two large pores, which is difficult to obtain through creating new pores in the middle of the FIB patterned concaves.13 In Figure 5b, some small pores randomly form along the hexagonal oxide wall boundaries. This is a direct result of larger interpore area. When the FIB concaves have uniform size (65 nm) in hexagonal arrangement while the interpore distance is kept at 500 nm, small pores regularly distribute at the trijunctions of the large pores. The image is omitted for brevity. The same phenomenon has also been observed in our other work.38 Based on the above understanding, a variety of alternating diameter nanopore arrays with different arrangements of small and large pores can be obtained by designing different FIB concave patterns. Figure 6a shows the FIB patterned concaves with graphite lattice structure, and the interpore distance is 250 nm. The depth of the FIB patterned pores is 10 nm (inset (2) of Figure 6a). After the anodization, the anodized nanopores (large pores) remain the graphite lattice structure (Figure 6b). The shape of the anodized nanopores is not round but somewhat triangular because of the confinement from the three neighboring pores, as discussed in Figure 4b. Small and shallow pores form at the center of six neighboring large pores, but no other new pores are developed. The large pore locates at the center of the triangular gravity of three small neighboring pores. The backside view of the anodized graphite structure FIB pattern with 250 nm interpore distance is shown in the inset of Figure 6b. The barrier layer profile confirms the triangular oxide wall and graphite structure arrangement of the anodized pore arrays. The small (38) Tian, Z. P.; Lu, K.; Chen, B. J. Appl. Phys. 2010, 108, 094306-1–7.

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Figure 6. FIB patterned concaves with graphite lattice structure and the interpore distance at (a) 250 nm and (c) 300 nm. (e) FIB patterned concaves in hexagonal arrangement with some missing sites and the interpore distance is 300 nm. (b,d,f) SEM images of the FIB patterns and (a,c,e) after the anodization in 0.3 M phosphoric acid at 20 mA with a steady state potential of 140 V, respectively. Insets (1) and (2) in (a), (c) and (e) are the AFM image and corresponding surface topology along the line, respectively. Inset (1) in (d) and (f) is the schematic of the alternating nanopore arrangement. Inset (1) in (b) and inset (2) in (d) are the corresponding backside views of the porous alumina. All the insert scale bars are 500 nm.

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√

Figure 7. FIB patterned concaves in rectangular arrangements. The long interpore distance is 3 times the short interpore distance, and the short interpore distance is (a) 200 nm, (c) 300 nm. (b,d) SEM images of the FIB patterns in (a) and (c) after the anodization in 0.3 M phosphoric acid at 20 mA with a steady state potential of 140 V, respectively. Insets (1) and (2) in (a) and (c) are the AFM image and corresponding surface topology along the line, respectively. Inset (1) in (d) is the schematic of the alternating nanopore arrangement, and inset (2) is the corresponding backside view of the anodized porous alumina. All the inset scale bars are 500 nm.

pores in the barrier layer are caused by the removal of the unanodized aluminum among the large pores during aluminum dissolution; they are not newly developed pores during the anodization. Otherwise the barrier layer should show a convex shape. When the interpore distance of the FIB patterned concaves increases to 300 nm (Figure 6c), alternating diameter nanopore arrays are obtained (Figure 6d). Again, the large pore locates at the center of the triangular gravity of three small neighboring pores; the small pore locates at the center of the hexagonal gravity of six large neighboring pores (inset (1) of Figure 6d). The barrier layer of the anodized alumina confirms the newly developed small pores and the hexagonal nanopore arrangement (inset (2) of Figure 6d). The barrier layer shape for the larger pores is truncated triangle, while that for the small pores is hexagon. The diameters of the large and small pores are 120 and 60 nm, respectively. The 120 nm large pore diameter is very similar to that in Figure 5b, considering the large interpore spacing with the missing sites. Different from Figure 6b, the small pores are more developed in Figure 6d. This is because the large interpore distance in Figure 6d allows the small pores to grow more before being confined by the oxide walls from the large pores. Langmuir 2011, 27(2), 800–808

Because of the larger influence of the better developed small pores, the large pores are also more round compared to those in Figure 6b. Another difference is the more visible presence of the oxide wall boundaries shown in Figure 6d. This again results from the larger interpore distance and the delayed joining of the neighboring oxide walls as discussed earlier in this paper and published before.39 Figure 6e shows FIB patterned concaves in hexagonal arrangement with some missing sites. The interpore distance is 300 nm and the concave depths are 10 nm as shown in the insets. After the anodization, the anodized nanopores retain the arrangement of the FIB pattern with new small pores grown at the hexagonal center of six large neighboring pores. The corresponding patterns for a smaller interpore distance of 250 nm are shown in Supporting Information (Figure S4), and the shape of the patterned nanopores is elongated in the direction of the missing sites (Figure S4b). Alternating diameter nanopore arrays are formed; the diameters of the large and small pores are 115 and 80 nm, respectively. Due to the confinement of the four neighboring (39) Tian, Z. P.; Lu, K.; Chen, B. Nanotechnology 2010, 21, 405301.

DOI: 10.1021/la1038393

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Article

pores, the shape of the oxide walls developed from the FIB patterned concaves is diamond. Additionally, the large pore is in the middle of two small pores. This is different from the alternating pore arrangement in Figure 6d where the large pore is at the center of the triangular gravity of three small neighboring pores. The difference in the small pore size, the large pore shape, and the oxide wall boundary in Figure 6f can be explained as for Figure 6b and d. Different from all the hexagonal patterns discussed so far, Figure 7a shows the FIB patterned concaves in rectangular arrangement √ with the short and long interpore distances at 200 and 200 3 nm, respectively. The concave depth is ∼10 nm. After the anodization, the nanopores remain in the rectangular arrangement (Figure 7b), which is fundamentally different from the familiar hexagonal arrangement. Due to the asymmetrical confinement of the neighboring pores, the shape of the nanopores is not round but elongated with the long axis at 110 nm and the short axis at 70 nm. Small and very shallow pores form at the center of four neighboring large pores (darker horizontal lines in Figure 7b). This is because the interpore distance is small, the oxide walls of each FIB patterned pores contact with those of the neighboring pores before full development, and thus there is no space left to grow new pores. When the √ short and long interpore distances are increased to 300 and 300 3 nm, respectively (Figure 7c), new small pores form at the center of the large pores and a new alternating diameter nanopore hexagonal arrangement is obtained (Figure 7d). The small pore locates at the center of the rectangular gravity of the four large neighboring pores, and the large pore locates at the center of the rectangular gravity of the four small neighboring pores. The barrier layer clearly shows new small pores form at the center of the rectangular gravity of four neighboring pores, and the wall shapes of both the large and small pores are elongated hexagons in perpendicular directions (inset (2) of Figure 7d). The diameter of the large pore is 125 nm, and the diameter of the small pore is 80 nm. The 125 nm large pore diameter is similar to those in Figures 5b and 6d. The pores are round because of the symmetrical arrangement of their neighbors.

808 DOI: 10.1021/la1038393

Chen et al.

The new porous alumina nanopore patterns created in this study can serve as excellent templates for the fabrication of various low dimensional nanostructures from all kinds of materials including polymers, semiconductors, metals, and ceramics. The diameter, aspect ratio, feature arrangement, and density of the fabricated nanostructures can be flexibly tuned by designing different pore arrangements for the porous alumina templates. For example, these novel patterns can be used to grow highly ordered alternating diameter nanopillars, nanowires, and nanotubes, and deposit nanodot arrays with unique patterns on a substrate. The nanostructured materials will in turn have numerous potential applications as gas sensors, photonic crystals, solar cells, and so on.

4. Conclusions With the guidance of FIB patterning, the interpore distance range for ordered porous anodic alumina increases. At 140 V in 0.3 M phosphoric acid, ordered hexagonal nanopore arrays with interpore distances from 200 to 425 nm can be fabricated. At 500 nm interpore distance, an alternating diameter nanopore arrangement can be achieved by developing new small pores at the junctions of three neighboring FIB patterned large pores. With the guidance of the FIB patterned alternating-sized concaves, alternating diameter nanopore arrays with two or three different pore diameters are obtained. Different types of alternating diameter nanopore arrangements are obtained by the FIB guidance and self-compensating effect of the hexagonal arrangement. Acknowledgment. The authors acknowledge the financial support from the National Science Foundation under Grant No. CMMI-0824741 and the Institute of Critical Technology and Applied Science of Virginia Tech. Assistance from John McIntosh and Stephen McCartney from the Nanoscale Characterization and Fabrication Laboratory of Virginia Tech is greatly acknowledged. Supporting Information Available: Images of FIB patterned pores at varying interpore distances. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2011, 27(2), 800–808