aao nanopore arrays

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AAO nanopore arrays: A practical entree to nanostructuresS. A. Knaack, M. Redden, and M. Onelliona)

Physics Department, University of Wisconsin, Madison, Wisconsin 53706

Received 8 August 2003; accepted 23 January 2004

We discuss the fabrication of anodized aluminum nanopore arrays by electrochemical means using

inexpensive and readily available equipment. The arrays of pores exhibit short-range hexagonal

order, with diameters ranging from 7 nm to hundreds of nanometers. The pore diameter and

spacing are varied by changing the anodization conditions. We have used nanopore arrays to

fabricate arrays of bismuth nanowires and as masks for x-ray lithography. © 2004 American Associationof Physics Teachers.

DOI: 10.1119/1.1677354

Since 1995 anodized altered by electrochemical expo-sure aluminum has been known to exhibit hexagonally or-dered pores on the nanoscale.1 The methods for fabricating

such nanopore arrays of alumina (Al2O3) are nowwell-established.1– 8 The alumina nanopore arrays exhibitconsiderable structural strength and can be used at high tem-peratures. These qualities make the material an attractivetemplate for nanowires,8–11 and, more recently, as masks for

projection x-ray lithography.12

Several different anodization electrochemical solutions,including sulfuric acid, oxalic acid, and phosphoric acid,have been reported in the literature.1– 8 The smallest porediameter reported to date is 7 nm, using a sulfuric acidsolution,9 while the largest, using a phosphoric acid solution,is 300 nm.7 By using oxalic or sulfuric acid solutions, wehave fabricated nanopore arrays with pore diameters as smallas 8 nm and as large as 50 nm. We have fabricated nan-opore arrays with lateral areas as large as 12 cm2.

The experimental apparatus used to fabricate nanopore ar-rays is illustrated in Fig. 1. There are several critical steps weused to obtain nanopore arrays with the highest long-range

pore–pore order and uniform pore density:1

use high pu-rity aluminum foil and treat the foil by annealing and elec-trochemical polishing as discussed in the following; 2control the solution temperature to 0.1 °C during the anod-ization; 3 maintain the solution temperature below 10 °Cduring the anodization; 4 stir the solution at a rate of 1rev/s to obtain a uniform solution concentration and uniformfoil temperature along the foil surface.

The graduated cylinder, cooling bath, stirring apparatus,voltage source, and multimeter are the main components seeFig. 1. The aluminum foil we used was 99.998% pure and0.5 mm thick.13 We purchased 1010 cm2 sheets of foil and

cut them into 12.5 cm2 samples for the anodization. Thegraphite cathode is typically of comparable size to the Al

foil. Graphite rods, of 0.5–1 cm diameter, are narrowed onone end to be secured by an alligator clip, and the other endis immersed in the acid solution during anodization.

The preparation of the Al foil involves two steps: 1 the12.5 cm2 samples are annealed at 350 °C in room air forapproximately 1 h; 2 the sample is electropolished in a2:2:4 wt solution of phosphoric acid: sulfuric acid: distilledwater.14,15 The aluminum foil is the anode and the graphite isthe cathode in the electrochemical cell. A voltage between 25and 30 V is applied at room temperature. The acid solution isplaced in a glass beaker inside a fume hood to reduce anyeffect of vapors. Rubber gloves were worn for electrical

safety. The foil is placed into the solution and the voltageincreased from zero immediately after immersing the foil.Approximately 0.5 A/cm2 of current runs through the foil. If the polishing proceeds properly, the aluminum foil willbrighten as bubbles form on the foil surface after a short time10–15 s in the electropolishing solution. Once the foilhas developed a mirror-like finish, it should be removed fromthe electropolishing solution and placed immediately into a

bath of distilled water, to prevent acid from further eating thesurface. The time needed varies slightly, but should take littlemore than 1–2 min. The voltage should be turned off as thefoil is removed from the electropolishing solution, and be-fore immersing in the water.

The electrochemical anodization solution for the AAOgrowth usually 0.3 M oxalic but also sulfuric acid is cooledto 0 °C before the aluminum foil is placed in the solution. Itis critical that the electrochemical solution be below 10 °Cbefore placing the aluminum foil anode and graphite cath-ode into the solution, so that the acid does not stronglycorrode the surface of the foil. The foil and graphite are thenplaced into the reaction cell, with alligator clips securingthem. The clips themselves should be a few millimeters

above the surface of the electrochemical solution. The volt-age source is then increased to 40 V for a 0.3 M oxalic acidsolution. We varied both the voltage and oxalic acid concen-tration to obtain pores of slightly different diameters. We alsovaried the time of the reaction; a typical first anodizationlasts 24 h. The anodization will exhibit pores that are in-creasingly well-ordered as the film grows thicker, so it isdesirable to anodize long enough to achieve good pore–porelong-range order that is, regions with the same hexagonalorder, especially during the first anodization. If the tempera-ture of the solution is low enough, the current should remainnegligible compared to 1.0 A when increasing the voltage to40 V. The solution should be stirred continuously during an-odization. Stirring the solution is a critical step in obtaining

ordered pore arrays. A standard magnetic stirrer is accept-able. In our case, the cooling bath geometry meant we couldnot use a magnetic stirrer. Instead, a glass rod, bent on oneend, was created to fit into a mechanical drill. The drill wasset up above the apparatus for the rod to go through theTeflon™ cap on the reaction cell a modified graduated cyl-inder and the rod was immersed in the solution, stirringbelow the anode and cathode. The rate of rotation was ap-proximately 45–60 rpm. The stirring should be gentle so thatthe solution is not turbulent and the anode and cathode arenot moved.

When using a sulfuric acid solution, we employed a 20%

856 856Am. J. Phys. 72 7, July 2004 http://aapt .org/ajp © 2004 Americ an Association of Physics Te achers



wt acid solution and applied 15 V across the anodizationcell. Everything else is the same as in the oxalic acid reac-tions.

The total cost to set up this facility, including equipmentsee Fig. 1, aluminum foil, and chemicals, is between $3000and $5000, depending on choice of components. This cost islow enough to make such a facility suitable for undergradu-ate research or laboratory work. The cost does not includethe scanning electron microscopes used to characterize thenanopores.

When the growth of the AAO film has been completed, theAl and graphite are removed and soaked in distilled water.There are then two options. One is to dissolve the alumina(Al2O3) film using a solution of 85% phosphoric acid by

weight, and use the now textured Al foil as a template togrow another alumina film with pores that are mutually par-allel along the entire thickness of the alumina film. The otheroption is to dissolve the remaining aluminum foil in HgCl2

saturated aqueous solution leaving the oxide films. Figure 2shows a top and bottom view of a single-stage anodizationnanopore. The lateral long-range order is comparable to ourbest nanopores of each type, although some reports in theliterature6,8 argue that two-stage anodization yields nanop-ores with intrinsically better order. Figure 3 shows a sideview of a two-stage nanopore with pore diameters of 40nm. Note that in the left panel the pores are not all perfectlyparallel for the entire 6 m thickness the thinnest nanop-ore we have fabricated. However, there are portions of the

nanopore for which the pores are perfectly parallel over theentire nanopore thickness see the right panel. We have suc-ceeded in fabricating nanopores as thick as 150 m. Ourexperience indicates that it is possible to fabricate nanoporesas thick as 1 mm. All that would be needed is a longeranodization time and corresponding thicker 1.5 mm alu-minum foil. It is estimated that as the aluminum foil is con-verted to alumina nanopore film, the thickness expands by1.4.4

As in other areas of physics, Fourier transformations alsoare useful in analyzing AAO nanopore arrays. A Fouriertransformation gives information on the spacing/ordering of

pores in an image see Fig. 2. Reference 8 provides signifi-

cant discussion of the usefulness of this tool. The Fouriertransformation of an ideal hexagonally ordered array shouldbe another hexagon. As depicted in Ref. 8, the result often isseen as a hexagon of small dots with one in the center. Theinset Fourier transformation in Fig. 2b shows this patternand is from a 128 by 128 pixel square of the upper left sidecorner of the original image. In Fig. 2a, the inset Fouriertransformation is of a center 256 by 256 pixel square.

By using oxalic, sulfuric, or phosphoric acid solutions, wewere able to vary the nanopore diameter over a considerationrange. Figure 4 shows a top view of a nanopore made usingthe oxalic acid solution, and a top view of a nanopore made

Fig. 1. A schematic of the anodization setup, with the reaction cell a modi-

fied graduated cylinder placed into a cooling bath to control the electro-

chemical acid solution temperature during the anodization. The multimeter

and voltage source are connected to the anode and cathode in parallel. A

glass rod stirs the solution just below the prepared Al foil anode and graphitecathode. The anode and cathode are set up with a voltage source and mul-

timeter in parallel.

Fig. 2. a A 150 k SEM Hitachi 1685 image of the outer surface of a

first anodization in 0.3 M oxalic acid. The pores here are 50 nm in

diameter, yet are not as well ordered. The upper right hand inset is a Fourier

transform of a 256256 square of the image center the original image was

550475 pixels. b A 45k SEM Hitachi 1685 image of the pores on the

bottom, Al side, of the AAO naopore. Each pore is 70 nm in diameter.

Note the domains of hexagonally ordered pores. The large domains indicate

the pores are increasingly ordered as the reaction proceeds. The upper right

inset is a Fourier transform of the upper left 128128 pixel corner from a

500475 pixel image. This area is especially uniformly ordered, and we

can see the hexagonal FFT pattern, as predicted by Ref. 8.

857 857Am. J. Phys., Vol. 72, No. 7, July 2004 Knaack, Redden, and Onellion



three-dimensional rendering of the AFM data, and covers a0.50.5 m2 area. The total vertical variation is 26 nm. Italso is noteworthy that the average feature diameter is35 nm3.8 nm, smaller than the 423.5 nm average di-ameter of the pore in the AAO mask itself.

Alumina is not an obvious material to use as an x-raymask, because the atomic numbers of aluminum and oxygenare low. Yet with a reasonably small thickness, 10.41 m

in our work, we find good contrast (T hole / T alumina33.3)with an acceptable transmission from the alumina of this

thickness 0.03 at 0.8 nm, the average wavelength of ra-diation used in the exposure.18

In summary, we have reported on a simple and inexpen-sive method for fabricating alumina nanopore templates thatare suitable for nanostructure investigations. The nanoporediameter can be made as small as 7–8 nm, and the nanoporethickness ranged from 6 to 150 m. As reported in Ref.9, we have confirmed that it is possible to use these nanoporearrays to fabricate nanowires of Bi, and there are severalreports in the literature of other nanopore materials.8–11 Us-ing such nanopores, scientists have, for example, investi-gated quasi-one-dimensional magnetic wires and found sub-stantially different properties compared to bulk materials.19,20

We have used the nanopores as masks for x-ray lithography,

which appears promising as a noncontact method for fabri-cating arrays of structure with feature sizes significantly be-low 50 nm.21,22 The techniques described herein are suitablefor undergraduate research or even an advanced undergradu-ate laboratory project.

ACKNOWLEDGMENTS

We benefited from financial support provided by the Uni-versity of Wisconsin and an undergraduate Hilldale Fellow-ship for undergraduate research work SAK. We also ben-efited from access to the facilities at the University of Wisconsin Center for Nanotechnology, and from discussionswith F. Cerrina CNTech, J. Eddington CNTech, D. Men-doza Universidad National de Mexico, Cuidad de Mexico,Mexico, R. Metzger Chemistry Dept., University of Ala-bama, and Q. Leonard CNTech.

aAuthor to whom all correspondence should be addressed. Electronic mail:

[email protected]. Masuda and K. Fukuda, ‘‘Ordered metal nanohole arrays made by a

two-step replication of honeycomb structures of anodic alumina,’’ Science

268, 1466–1468 1995.2O. Jessensky, F. Muller, and U. Gosele, ‘‘Self-organized formation of hex-agonal pore arrays in anodic alumina,’’ Appl. Phys. Lett. 72, 1173–1175

1998.3K. Nielsch, J. Choi, K. Schwirn, R. B. Wehrspohn, and U. Go sele, ‘‘Self-

ordering regimes of porous alumina: The 10% porosity rule,’’ Nano Lett.

2, 677–680 2002.4A. P. Li, F. Muller, A. Birner, K. Nielsch, and U. Go sele, ‘‘Hexagonal pore

arrays with a 50–420 nm interpore distance formed by self-organization in

anodic alumina,’’ J. Appl. Phys. 84, 6023–6026 1998.5O. Jessensky, F. Muller, and U. Gosele, ‘‘Self-organized formation of hex-

agonal pore structures in anodic alumina,’’ J. Electrochem. Soc. 145,

3735–3740 1998.6F. Li, L. Zhang, and R. M. Metzger, ‘‘On the growth of highly ordered

pores in anodized aluminum oxide,’’ Chem. Mater. 10, 2470–2580 1998.7H. Masuda, K. Yada, and A. Osaka, ‘‘Self-ordering of cell configuration of

anodic porous alumina with large-size pores in phosphoric acid solution,’’

Jpn. J. Appl. Phys., Part 2 37, L1340–L1342 1998.8S. Shingubara, O. Okino, Y. Sayama, H. Sakaue, and T. Takahagi, ‘‘Or-

dered two-dimensional nanowire array formation using self-organized

nanoholes of anodically oxidized aluminum,’’ Jpn. J. Appl. Phys., Part 1

36, 7791–7795 1997.9Z. Zhang, D. Gekhtman, M. S. Dreselhaus, and J. Y. Ying, ‘‘Processing

and characterization of single-crystalline ultrafine Bismuth nanowires,’’

Chem. Mater. 11, 1659–1665 1999.10Z. Zhang, X. Sun, M. S. Dresselhaus, J. Y. Ying, and J. Heremans, ‘‘Elec-

tronic transport properties of single-crystal bismuth nanowire arrays,’’

Phys. Rev. B 61, 4850–4861 2000.11J. Choi, G. Sauer, K. Nielsch, R. B. Wehrspohn, and U. Gosele, ‘‘Hexago-

nally arranged monodisperse silver nanowires with adjustable diameter

and high aspect ratio,’’ Chem. Mater. 15, 776–779 2003.12S. A. Knaack, J. Eddington, Q. Leonard, F. Cerrina, and M. Onellion,

‘‘Dense arrays of nanopores as x-ray lithography masks,’’Appl. Phys. Lett.

submitted.13Alfa Aesar Co., http://www.alfa.com.14W. D. Guo and D. Johnson, ‘‘Role of interfacial energy during pattern

formation of electropolishing,’’ Phys. Rev. B 67, 075411–075415 2003.15W. Konovalov, G. Zangari, and R. M. Metzger, ‘‘Highly ordered nanoto-

pographies on electropolished aluminum single crystals,’’ Chem. Mater.

11, 1949–1952 1999.16M. Kahn, G. Han, G. Tsvid, T. Kitayama, J. Maldonado, and F. Cerrina,

‘‘Can proximity x-ray lithography print 35 nm features? Yes,’’ J. Vac. Sci.

Technol. B 19, 2423–2426 2001.17R. Cole, P. Anderson, G. M. Wells, E. Brodsky, K. Yamazaki, and F.

Cerrina, ‘‘Performance of the CXrL x-ray beamlines,’’ Proc. SPIE 1671,

461–470 1992.18http://www-cxro.lbl.gov/optical–constants/ . This is found under x-ray

transmission of a solid.19

M. Zheng, L. Menon, H. Zeng, Y. Liu, S. Bandyopadhyay, R. D. Kirby,and D. J. Sellmyer, ‘‘Magnetic properties of Ni nanowires in self-

assembled arrays,’’ Phys. Rev. B 62, 12282–12286 2000.20Y.-G. Guo, L.-J. Wan, C.-F. Zhu, D.-L. Yang, D.-M. Chen, and C.-L. Bai,

‘‘Ordered Ni-Cu nanowire arrays with enhanced coercivity,’’ Chem. Mater.

15, 664–667 2003.21J. O. Choi, A. I. Akinwande, and H. I. Smith, ‘‘100 nm gate hole openings

for low voltage driving field emission display applications,’’ J. Vac. Sci.

Technol. B 19, 900–903 2001.22C. A. Ross, H. I. Smith, T. Savas, M. Schattenberg, M. Farhoud, M.

Hwang, M. Walsh, M. C. Abraham, and R. J. Ram, ‘‘Fabrication of pat-

terned media for high density magnetic storage,’’ J. Vac. Sci. Technol. B

17, 3168–3176 1999.

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aao nanopore arrays

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