- Email: [email protected]
Characterization of anodic porous alumina by AFM
April 2001
Materials Letters 48 Ž2001. 127–136 www.elsevier.comrlocatermatlet
Characterization of anodic porous alumina by AFM YuCheng Sui, Jose´ M. Saniger ) Laboratorio de Materiales y Sensores, Centro de Instrumentos, UniÕersidad Nacional Autonoma de Mexico, Ciudad UniÕersitaria, Apdo. ´ ´ Postal 70-186, Mexico D.F. 04510, Mexico Received 8 February 2000; received in revised form 16 June 2000; accepted 19 June 2000
Abstract The topography of supported and unsupported anodic aluminum oxide ŽAAO. films was measured by AFM under different testing modes. The supported film, attached onto an aluminum substrate, is mechanically stable and the contact mode shows good resolution for imaging the intrinsic features of the surface. Additionally, the non-contact mode can detect the presence of adsorbed species on the surface, which are not detected by the contact mode. On the contrary, for the unsupported alumina film, the non-contact mode shows better resolution, even for surface pore detection. These results are discussed in view of the different mechanical properties of both types of alumina films, and the magnitude of the sample–tip interaction for each operation mode. In relation with the pore arrangement, a single-step anodization process results in a dense but non-regular pore pattern. Regular pore arrays was obtained with a two-step anodization method using both oxalic and sulfuric acid as oxidation agents. AFM characterization allows the evaluation of the order of the pore array, the pore density and the external shape of AAO films obtained under different experimental approaches. A reliable determination of the internal diameter and shape of the pores is not well accomplished under the present experimental conditions. q 2001 Elsevier Science B.V. All rights reserved. PACS: 61.16.C; 81.05.R; 68.60.B; 81.05.Y Keywords: Porous alumina; Atomic force microscopy ŽAFM.; Thin films; Anodization; Nanostructures; Surface structure
1. Introduction Anodic aluminum oxide ŽAAO. films grown in acid electrolyte possess regular and highly anisotropic porous structure with pore diameter ranging from five to several hundred nanometers, and with a density of pores ranging from 10 9 to 10 11 rcm2 . These characteristics allow the use of AAO as microfilters
)
Corresponding author. Tel.: q52-5-622-8616; fax: q52-5622-8620. E-mail address: [email protected] ŽJ.M. Saniger..
w1x, templates for the growth of metal or semiconductor nanowires w2–5x and carbon or carbon nitride nanotubes w6,7x. Additionally, semicoductor or metal deposited on AAO templates often posses unique electronic, optical and magnetic properties w2,3,5,8,9x. The AAO film can also be used as support for the measurement of the mechanical properties of nanocarbon tube ropes w10x. The production of different kinds of micro- and nano-devices based on the AAO films requires a strict characterization of their topographic features, which is normally accomplished by electron microscopy studies. Nevertheless, this technique is not well suited for this purpose because of the required
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 2 9 2 - 5
128
Y. Sui, J.M. Sanigerr Materials Letters 48 (2001) 127–136
sample preparation and vacuum testing, which may alter the AAO film structure w11x. Because of the insulator behavior of the alumina, its characterization by scanning electron microscopy ŽSEM. requires the covering of its surface with a thin conducting film, which may hide some fine details of its surface features and is not adequate for certain purposes such as the development of microfilter devices. On the other hand, the use of transmission electron microscopy ŽTEM. demands a complicated and time-consuming preparation of the sample. The rapid development of atomic force microscopy ŽAFM. and its applications provide another interesting opportunity for characterization as a non-destructive method, which brings complementary information, especially when a judicious use of different observation modes is accomplished. Nevertheless, to the best of our knowledge, a systematic study of the capabilities and limitations of the AFM for the characterization of AAO has not been published. In this study, AFM was employed for the characterization of porous anodic alumina formed in 10% H 2 SO4 electrolyte. Some problems related to the applicability and reliability of the different AFM observation modes are discussed together with the evaluation of different experimental approaches related to the formation of a hexagonal array of the pores.
2. Experimental methods Ultra-pure Al Ž99.999%. plates with a thickness of 0.5 mm were obtained by cold pressuring 2-mm thick aluminum discs. The plates were then annealed at 5008C for 30 min and mechanically and electrochemically polished Ž5:1 vrv solution of EtOH rHClO4 .. Finally, they were used as the central anodes in a home-made electrochemical cell, vertically suspended between two Pt cathodes. The oxidation or anodization was carried out following two different approaches. In the first case, a one-step method, 10% H 2 SO4 aqueous solution was used together with an operation voltage of 20 V DC at 108C. The anodization time was changed as a function of the desired thickness of the AAO films. A time-controlled pore widening was carried out in 0.1 M H 3 PO4 at 308C. The AAO film obtained on a metallic aluminum substrate following this approach
will be referenced as supported film. In order to obtain a suspended or unsupported aluminum oxide, the oxidized sample was mounted on a device specially designed for this purpose, which allows a chemical etching on a selected area of the samples. A round area, about 6.5 mm in diameter, on one side of the anodized sample was exposed by sequence to 4 M NaOH and 20% HCl aqueous solutions to remove first the alumina and then the metallic aluminum. The HCl attack was stopped when the solution reached the alumina film of the opposite side. With this approach, an unsupported round alumina film was obtained. Both supported and unsupported AAO obtained in this way presented an unordered pore distribution across their surface. The production of highly ordered hexagonal pore arrays has been previously obtained by a two-step anodization method, using oxalic acid as oxidation agent w12x. In this study, we extend this method to the use of sulfuric acid as oxidation agent, which has in principle the advantage of reducing the anodization time and lowering the operation voltage. For this purpose, the polished aluminum plate was first anodized for 1 h in the same conditions as in the one-step method Ž10% H 2 SO4 aqueous solution, 20 V DC and 108C.. Then, the formed AAO film was removed by immersion for 2 h in a phosphocromic acid Ž6% H 3 PO4 , 1.8% H 2 CrO4 . aqueous solution at 608C. After the alumina film was stripped off, the sample was oxidized again for 35 min in the same conditions ŽH 2 SO4 10% aqueous solution, 20 V DC, 108C.. Pore widening was carried out with 0.1 M H 3 PO4 at 358C for 20 min. Before the AFM surface examination, all the samples were mounted on a glass holder and ultra-sonically cleaned in bi-distilled water for more than 1 h to remove the residual debris in the pores. Then they were dried with flowing pressurized gas for 2 min and kept in dry air before testing. AFM topography examinations were carried out with an Auto Probe CPe atomic force microscope, using Ultralevere Si conical tips with a typical radius of curvature of 10 nm and an aspect ratio about 3:1 w13x. The samples were imaged under contact ŽC., intermittent contact ŽIC. and non-contact ŽNC. observation modes. The average force constant of the cantilevers used in the C-mode was 0.4 Nrm, while those used for IC and NC-modes were 17 Nrm. For the NC-mode, the
Y. Sui, J.M. Sanigerr Materials Letters 48 (2001) 127–136
cantilever vibrates at a driving frequency slightly greater than the peak value of its resonant frequency Žaround 320 kHz.. A constant force gradient of the cantilever is achieved by a feedback control of the scanner movements, and sample topographic information is obtained without touching the sample. For the IC-mode, the cantilever vibrates at a driving frequency slightly lower than the peak value of its resonant frequency. Because the vibration amplitude of the cantilever increases when the cantilever is brought close to the sample surface, non-continuous contacts of tip with sample is achieved during scanning at the intermittent mode. As for the contact mode, the amount of scanner z movement necessary to maintain a constant force is used to generate a topographic image. 3. Results and discussion 3.1. Characterization of supported AAO film on aluminum Fig. 1 presents the AFM images of a supported AAO film, prepared by oxidizing the aluminum plate for 1 h in 10% aqueous sulfuric acid solution, at 20 V DC and 108C Žone-step method.. The film was submitted to a pore-widening treatment for 10 min in 0.1 M H 3 PO4 at 308C, and the images were obtained by using different detecting modes, that is, NC ŽFig. 1a., IC ŽFig. 1b. and C ŽFig. 1c. modes. The NC and IC mode images were taken at a scan height of 20 nm. The applied force for the C-mode was 2.5 nN. This figure shows that although the general topographic features are similar for all the NC, IC and C modes images, different topographic information is obtained for each of them. In the NC-mode ŽFig. 1a., small pores Žblack spots of the images. are observed together with some foreign dots almost homogeneously distributed on the AAO surface. In the ICmode, no surface pores can be detected and the foreign dots are enlarged with streak-like artifacts overlapping them. When C-mode is adopted for topographic test, no foreign dots are observed, while the pores are detected with a sharper definition than they appear in the NC-mode. It is well known that the NC-mode can image liquid droplets or films on a sample surface, there are numerous examples in the literature that confirm this
129
fact w14,15x. Following this experience, it seemed logical to assume that the observed foreign white dots are tiny water drops with size range from 40 to 100 nm. This assumption was reinforced by the fact that these droplets disappeared after the sample was slightly heated. The artifacts in the sample surface image using the IC-mode require some kind of discussion on the light of the basic operation principles w16x. Imaging in the IC-mode results in a periodic striking of the tip at certain points of the surface and no lateral displacement of the tip along the scanning direction takes place. In these conditions, the tip comes in and out of the water droplets repeatedly during scanning, which renders them unstable by wetting and capillary interactions with the vibrating tip. This effect tends to enlarge the water drops and causes some streak-like artifacts on them. Contact AFM imaging is then the best option for the characterization of supported films of porous anodic alumina. It eliminates the effects of the covering water droplets or other adsorbed layers, giving the best lateral resolution for the imaging of the intrinsic surface features. It can be concluded that in our case the C-mode gives the best information in relation to the intrinsic features of the AAO surface, while the NC-mode offers complimentary information of the foreign species present on the AAO surface. On the other hand, in our experience the IC-mode is unable to discern between the intrinsic and extrinsic surface features, resulting in the poorest image quality among the three modes. In relation to the pore structure, it is important to note here that their distribution at the surface, while homogeneous, does not show the feature of selforganized hexagonal pore array due to the random pore nucleation on the surface w17x. This is an important point to be discussed later. The diameters of the pores were measured using a line profile, their values ranged between 30 and 40 nm. 3.2. Characterization of the suspended porous anodic film In order to get insight of the sample–tip interaction, a 6-mm thick w18x unsupported aluminum oxide film was produced, using the previously described approach. Both the oxidation and the pore widening
130
Y. Sui, J.M. Sanigerr Materials Letters 48 (2001) 127–136
Fig. 1. AFM images of anodic alumina film supported on aluminum. Ža. NC-mode; Žb. IC-mode; Žc. C-mode. Scan height for NC and IC modes, 20 nm. Applied force for C-mode, 2.5 nN. Sample oxidization 1 h Ž10% H 2 SO4 , 20 V DC, 108C.. Pore widening 10 min in 0.1 M H 3 PO4 at 308C.
times were 10 min. Fig. 2 shows the topography of the surface of such a film in the NC ŽFig. 2a. and C ŽFig. 2b and c. modes. The images 2a and b are quite similar, but contrary to those observed for the supported film, a sharper definition of the surface features is now obtained in the NC-mode. This apparent contradiction with the results of the supported AAO observations could be related with the mechanical properties of the unsupported AAO films. Effectively, these films would behave as a
vibrating drum membrane, which becomes unstable when observed in the C-mode, due to the relatively high sample–tip interaction force Ž10y6 –10y8 N. involved w19x. On the contrary, in the NC-mode observation the interaction force becomes significantly lower Ž10y1 2 N., and the unsupported AAO film would behave then as a stable structure. This assumption was confirmed when a new image of the same sample was taken at lower contact force, 2.5 nN in Fig. 2c. In these softer observation conditions,
Y. Sui, J.M. Sanigerr Materials Letters 48 (2001) 127–136
131
Fig. 2. Topographies of the surface suspended film. Ža. NC-mode; Žb and c. C-mode. Scan height for NC Ža. 20 nm. Applied force Žb. 7 nN and Žc. 2.5 nN. Sample oxidization 35 min Ž10% H 2 SO4 , 20 V DC, 108C.. Pore widening 10 min in 0.1 M H 3 PO4 at 308C.
the surface holes becomes clearly defined, although their average size is smaller than that corresponding to the NC images. 3.3. Characterization of AAO films obtained by the two-step anodization method The density of the pores in the images of Figs. 1 and 2 is sufficiently high to obtain a uniform pore distribution, although an ordered hexagonal array of the pores is absent, showing that under this synthetic
approach a random nucleation of the pores occurs. In this sense, these AAO films may be used as nanotemplates only if a dense packaging of the nanostructures is the main requirement. But frequently, as for the fabrication of nano-devices, an ordered distribution of the nano-structures is also required and then a template with ordered pore array must be obtained. To obtain an ordered hexagonal array of the pores on an AAO film a double oxidation step method has been developed w12x, which uses oxalic acid
132
Y. Sui, J.M. Sanigerr Materials Letters 48 (2001) 127–136
ŽHOOC–COOH. as oxidizing agent and an oxidation voltage of 40 V. In the present work, an analogous two-step method was tested, but substituting the oxalic acid by sulfuric acid, which in principle has the advantage of higher oxidation rate and lower voltage operation values. Fig. 3a presents the image of the AAO film obtained with this approach Žanodization voltage 20 V; pore widening 0.1 M H 3 PO4 for 20 min.. A tendency of the pores to self-organize in a hexagonal array is observed in the figure and is evidenced in the inset showing a typical pore hexagonal array formed in an area of 200 = 200 nm of this sample. For comparison purposes, an additional AAO sample was obtained using the oxalic acid two-step method w12x Ž40 V anodization voltage, 158C, pore widening 0.1 M H 3 PO4 for 50 min. and the result is presented in the Fig. 3b. An improved order of the pore patterns is clear in this image when compared with that of Fig. 3a. It is important to note here that the pore density in an AAO film increases at lower operation voltage w18x, while the pore size is a function of the extension of the pore size treatment. The differences observed in pore density and pore size in Fig. 3a and b originated mainly in the differences in the voltage operation and pore-widening time and not because of the different oxidizing agent.
As an attempt to quantify the hexagonality of the pore array, a segmentation of the 0.5-mm image was made and the angles of the hexagons were measured. Fig. 4a and b present the segmented images of the AAO obtained by the two-step anodization method using sulfuric and oxalic acid, respectively. The insets in these figures are the Fourier transform of the corresponding images, and represent an additional indication of the pore array ordering. Table 1 presents a quantitative comparison of the hexagonality of the pore array and other geometric parameters for both samples. The hexagonal array percentage of the sample Žsecond column, Table 1. means the percentage of the total area of the samples, which present a clear hexagonal pore pattern Žsee Fig. 4.. The reported values were obtained from three separated regions Ž0.5 = 0.5 mm. for each sample and have to be considered only as a rough approximation. Nevertheless, it is clear that the hexagonal array of the two-step sulfuric acid oxidized sample is not extended over its whole surface, instead one third of it still has an irregular pore array. Additionally, the hexagon distortion Ževaluated from the standard deviation of the 1208 angle of the regular hexagon. is also higher for the sulfuric acid anodized sample. These disadvantages could be minimized by additional adjustment of the experimental conditions of
Fig. 3. AFM images Ž2 = 2 mm. of AAO films obtained with the two-step method. Ža. Sulfuric acid, 20 V, pore widening 20 min in 0.1 M H 3 PO4 ; Žb. oxalic acid, 40 V, pore widening 50 min in 0.1 M H 3 PO4 .
Y. Sui, J.M. Sanigerr Materials Letters 48 (2001) 127–136
133
Fig. 4. AFM images Ž0.5 = 0.5 mm. of AAO films obtained with the two-step method using as oxidizing agent Ža. sulfuric acid; Žb. oxalic acid. Same samples as that in Fig. 3. The images were segmented following the hexagonal pore array. Inset: FFT images.
synthesis, and some work is presently being performed in this direction. On the other hand, the areas of the single hexagons are smaller for the sulfuric acid anodized sample and, consequently, its pore density becomes higher. Nevertheless, these features cannot be related only to the anodizing agent used, but mainly with the voltage value used in the anodization process; it is well known w18x that lower anodization voltage results in higher pore density. Finally, a short discussion must be undertaken about the evaluation of the AAO pore size and shape by AFM. From the images of Fig. 4, it seems clear that the sample surface is not perfectly flat and that the pores cannot be regarded as ideal cylinders from base to mouth w18x. Instead of that, a mouth pore may be distinguished surroundings an increasing dark, almost circular, spot in the center of the pore. So the definition of the pore diameter requires the establishment of a criterion indicating if the mouth
of the pore is considered or not in the measurement. A demonstration of the difficulties of measuring the real shape of the pores, even at their upper part, is given in Fig. 5 where some line profiles across the surface of the Fig. 4b image are shown. Each line profile is formed by a set of cones instead of the expected cylinder-shaped internal pore. This deviation of the expected shape results from the inability of the probe tip to go inside the pores due to its conical geometry with a relatively low aspect ratio Žabout 2:1.. In fact, the apparent conical profile of the pore reflects the tips geometry rather that the actual internal pore shape. Typical curvature radius at the end of these probe tips is around 10 nm, but they have a conical shape with a relatively high aperture angle and then a few nanometer apart from its end the cross-section of the tip is comparable with the pore radius. Following the z-sale of Fig. 5 profiles, it is evident the very short penetration of the
Table 1 Geometric parameters of the hexagonal pore array Sample
Hexagonal array percentage
Averaged angle value
Averaged hexagonal area Žmm2 .
Pore density Žporesrcm2 .
Sulfuric acid anodized Oxalic acid anodized
f 66 f 100
120.1 " 9.78 120.0 " 5.68
0.012 0.024
2.6 = 10 10 1.2 = 10 10
134
Y. Sui, J.M. Sanigerr Materials Letters 48 (2001) 127–136
Fig. 5. Line profile across a set of pores of the images from Fig. 4b. Pore distance 99 " 1 nm, mouth diameter 86 " 4 nm, internal pore diameter 56 " 4.
tips into the pore, around 15 nm when the expected deepness of the AAO pores is of several micrometers. Then it must be emphasized that with the probe tip used in this work, the only reliable information is that related with the diameter of the pore mouth. In a general way, it could be established that when the aspect ratio of a pore or hole is larger than that of the tip, the observed profile will not fit the actual pore dimensions. Carbon nanotubes attached to Si com-
mercial tips have been proposed to overcome this difficulty and will be evaluated when available. With the exposed limitations in mind, the diameter of the pore mouth and the pore separation were measured. Additionally, a rough evaluation of the internal pore diameters is presented. For this purpose, the profile marks in Fig. 5 were visually set on different points. For the evaluation of the pore mouth diameter, the marks were set on the upper inflexion
Y. Sui, J.M. Sanigerr Materials Letters 48 (2001) 127–136
point of the conical profile lines, and the pore separation was estimated by putting the mark in two contiguous minima of the same profile lines. Finally, the internal pore diameters were evaluated setting the marks at the edges of the central black spots. Following this criterion for the two-step oxalic acid anodized sample shown in Fig. 5, the pore mouth diameter ranges between 82 and 89 nm, while the pore separation oscillates between 98 and 100 nm. The apparent diameter of the internal cross-section of the pores would be comprised between 52 and 60 nm.
4. Conclusions The AFM has been shown to be a powerful and convenient technique for characterizing anodic porous alumina films. General features of their surface and pore structures and their dependence with different methods of synthesis were shown. The capabilities and some limitations of the technique were presented. The application of different testing modes for imaging both supported and unsupported films provide complimentary information allowing a more complete characterization of the films. The supported films are rigid enough to resist the perturbation of strong testing forces and the C-mode, with a higher lateral resolution than the NC-mode, is better employed for the characterization of the film surface features. Nevertheless, the NC-mode provides important complimentary information about the adsorbed species on the film. On the other hand, the unsupported or suspended film is not strong enough to resist the perturbation of relatively high testing forces, and then NC-mode is recommend for its characterization because of the smaller exerted testing force. A modification of the two-step method was tested in order to obtain hexagonal ordered array of AAO pores. For this purpose, sulfuric instead of oxalic acid was used with the initial advantages of its higher oxidation rate and the lower operation voltages needed. The AAO film obtained with this approach presents a significant self-organization of the pore in a hexagonal array, but still has a higher distortion than that obtained with oxalic acid. It is
135
expected that further refinements of the experimental conditions will result in a higher ordering of the pore hexagonal array. The study of intermediate resolution images Ž0.5 = 0.5 mm. of two samples obtained under different experimental conditions, shows the capability of AFM to evaluate the degree of ordering of the pore array, as well as the AAO pore density and the diameters and external shape of the pores. A substantial experimental improvement is still required in order to have a more reliable measurement of the internal diameter and even the length of the pores in the AAO films. A promising expectation in this direction is the availability of very high aspect ratio tips such as those obtained by attaching a carbon nanotube to commercial Si tips.
Acknowledgements This work was partially supported by CONACyT of Mexico. The authors are very grateful for Raul Ruvalcaba Morales of the Centro de Instrumentos for his valuable technical help.
References w1x N. Itoh, N. Tomura, T. Tsuji, M. Hongo, Microporous Mesoporous Mater. 20 Ž1998. 333. w2x C.A. Huber, T.E. Huber, M. Sadoqi, J.A. Lubin, S. Manalis, C.B. Prater, Science 263 Ž1994. 800. w3x D. Routkevitch, A.A. Tager, J. Haruyama, D. Almawlawi, M. Moskovits, J.M. Xu, IEEE Trans. Electron Devices 43 Ž1996. 1646. w4x S. Kawai, R. Ueda, J. Electrochem. Soc. 122 Ž1975. 32. w5x D.N. Davyddov, J. Haruyama, D. Routkevitch, B.W. Statt, D. Ellis, M. Moskovits, J.M. Xu, Phys. Rev. B 57 Ž1998. 13550. w6x S.L. Sung, S.H. Tsai, C.H. Tseng, F.K. Chiang, X.W. Liu, H.C. Shih, Appl. Phys. Lett. 74 Ž1999. 199. w7x J. Li, M. Moskovits, T.L. Haslett, Chem. Mater. 10 Ž1998. 1963. w8x M. Saito, M. Kirihara, T. Taniguchi, Appl. Phys. Lett. 55 Ž1989. 607. w9x A. Balandin, K.L. Wang, N. Kouklin, S. Bandyopadhyay, Appl. Phys. Lett. 76 Ž2000. 137. w10x J.-P. Salvetat, G. Andrew, D. Briggs, J.-M. Bonard, R.R. Bacsa, A.J. Kulik, T. Stockli, N.A. Burnham, L. Forro, ¨ ´ Phys. Rev. Lett. 82 Ž1999. 944. w11x I. Serebrennikova, P. Vanysek, V.I. Birss, Electrochim. Acta ´ 42 Ž1997. 145.
136
Y. Sui, J.M. Sanigerr Materials Letters 48 (2001) 127–136
w12x H. Masuda, M. Satoh, Jpn. J. Appl. Phys., Part 2 35 Ž1996. L126. w13x Park Scientific Instruments, General Purpose AFM Cantilever Technical Sheet. w14x J. Hu, R.W. Carpick, M. Salmeron, J. Vac. Sci. Technol., B 14 Ž1996. 1341. w15x J. Hu, X.-D. Xiao, D.F. Ogletree, M. Salmeron, Science 268 Ž1995. 267. w16x N.A. Burnham, O.P. Behrend, F. Oulevey, G. Gremaud, P.-J.
Gallo, D. Gourdon, E. Dupas, A.J. Kulik, H.M. Pollock, G.A.D. Briggs, Nanotechnology 8 Ž1997. 67–75. w17x O. Jessensky, F. Muller, U. Gosele, Appl. Phys. Lett. 72 ¨ ¨ Ž1998. 1173. w18x J.W. Diggle, T.C. Downie, C.W. Goulding, Chem. Rev. 69 Ž1969. 365. w19x R. Howland, L. Benatar, A Practical Guide to Scanning Probe Microscopy, Park Scientific Instruments, 1993, Chap. 1, pp. 8–11.
Materials Letters 48 Ž2001. 127–136 www.elsevier.comrlocatermatlet
Characterization of anodic porous alumina by AFM YuCheng Sui, Jose´ M. Saniger ) Laboratorio de Materiales y Sensores, Centro de Instrumentos, UniÕersidad Nacional Autonoma de Mexico, Ciudad UniÕersitaria, Apdo. ´ ´ Postal 70-186, Mexico D.F. 04510, Mexico Received 8 February 2000; received in revised form 16 June 2000; accepted 19 June 2000
Abstract The topography of supported and unsupported anodic aluminum oxide ŽAAO. films was measured by AFM under different testing modes. The supported film, attached onto an aluminum substrate, is mechanically stable and the contact mode shows good resolution for imaging the intrinsic features of the surface. Additionally, the non-contact mode can detect the presence of adsorbed species on the surface, which are not detected by the contact mode. On the contrary, for the unsupported alumina film, the non-contact mode shows better resolution, even for surface pore detection. These results are discussed in view of the different mechanical properties of both types of alumina films, and the magnitude of the sample–tip interaction for each operation mode. In relation with the pore arrangement, a single-step anodization process results in a dense but non-regular pore pattern. Regular pore arrays was obtained with a two-step anodization method using both oxalic and sulfuric acid as oxidation agents. AFM characterization allows the evaluation of the order of the pore array, the pore density and the external shape of AAO films obtained under different experimental approaches. A reliable determination of the internal diameter and shape of the pores is not well accomplished under the present experimental conditions. q 2001 Elsevier Science B.V. All rights reserved. PACS: 61.16.C; 81.05.R; 68.60.B; 81.05.Y Keywords: Porous alumina; Atomic force microscopy ŽAFM.; Thin films; Anodization; Nanostructures; Surface structure
1. Introduction Anodic aluminum oxide ŽAAO. films grown in acid electrolyte possess regular and highly anisotropic porous structure with pore diameter ranging from five to several hundred nanometers, and with a density of pores ranging from 10 9 to 10 11 rcm2 . These characteristics allow the use of AAO as microfilters
)
Corresponding author. Tel.: q52-5-622-8616; fax: q52-5622-8620. E-mail address: [email protected] ŽJ.M. Saniger..
w1x, templates for the growth of metal or semiconductor nanowires w2–5x and carbon or carbon nitride nanotubes w6,7x. Additionally, semicoductor or metal deposited on AAO templates often posses unique electronic, optical and magnetic properties w2,3,5,8,9x. The AAO film can also be used as support for the measurement of the mechanical properties of nanocarbon tube ropes w10x. The production of different kinds of micro- and nano-devices based on the AAO films requires a strict characterization of their topographic features, which is normally accomplished by electron microscopy studies. Nevertheless, this technique is not well suited for this purpose because of the required
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 2 9 2 - 5
128
Y. Sui, J.M. Sanigerr Materials Letters 48 (2001) 127–136
sample preparation and vacuum testing, which may alter the AAO film structure w11x. Because of the insulator behavior of the alumina, its characterization by scanning electron microscopy ŽSEM. requires the covering of its surface with a thin conducting film, which may hide some fine details of its surface features and is not adequate for certain purposes such as the development of microfilter devices. On the other hand, the use of transmission electron microscopy ŽTEM. demands a complicated and time-consuming preparation of the sample. The rapid development of atomic force microscopy ŽAFM. and its applications provide another interesting opportunity for characterization as a non-destructive method, which brings complementary information, especially when a judicious use of different observation modes is accomplished. Nevertheless, to the best of our knowledge, a systematic study of the capabilities and limitations of the AFM for the characterization of AAO has not been published. In this study, AFM was employed for the characterization of porous anodic alumina formed in 10% H 2 SO4 electrolyte. Some problems related to the applicability and reliability of the different AFM observation modes are discussed together with the evaluation of different experimental approaches related to the formation of a hexagonal array of the pores.
2. Experimental methods Ultra-pure Al Ž99.999%. plates with a thickness of 0.5 mm were obtained by cold pressuring 2-mm thick aluminum discs. The plates were then annealed at 5008C for 30 min and mechanically and electrochemically polished Ž5:1 vrv solution of EtOH rHClO4 .. Finally, they were used as the central anodes in a home-made electrochemical cell, vertically suspended between two Pt cathodes. The oxidation or anodization was carried out following two different approaches. In the first case, a one-step method, 10% H 2 SO4 aqueous solution was used together with an operation voltage of 20 V DC at 108C. The anodization time was changed as a function of the desired thickness of the AAO films. A time-controlled pore widening was carried out in 0.1 M H 3 PO4 at 308C. The AAO film obtained on a metallic aluminum substrate following this approach
will be referenced as supported film. In order to obtain a suspended or unsupported aluminum oxide, the oxidized sample was mounted on a device specially designed for this purpose, which allows a chemical etching on a selected area of the samples. A round area, about 6.5 mm in diameter, on one side of the anodized sample was exposed by sequence to 4 M NaOH and 20% HCl aqueous solutions to remove first the alumina and then the metallic aluminum. The HCl attack was stopped when the solution reached the alumina film of the opposite side. With this approach, an unsupported round alumina film was obtained. Both supported and unsupported AAO obtained in this way presented an unordered pore distribution across their surface. The production of highly ordered hexagonal pore arrays has been previously obtained by a two-step anodization method, using oxalic acid as oxidation agent w12x. In this study, we extend this method to the use of sulfuric acid as oxidation agent, which has in principle the advantage of reducing the anodization time and lowering the operation voltage. For this purpose, the polished aluminum plate was first anodized for 1 h in the same conditions as in the one-step method Ž10% H 2 SO4 aqueous solution, 20 V DC and 108C.. Then, the formed AAO film was removed by immersion for 2 h in a phosphocromic acid Ž6% H 3 PO4 , 1.8% H 2 CrO4 . aqueous solution at 608C. After the alumina film was stripped off, the sample was oxidized again for 35 min in the same conditions ŽH 2 SO4 10% aqueous solution, 20 V DC, 108C.. Pore widening was carried out with 0.1 M H 3 PO4 at 358C for 20 min. Before the AFM surface examination, all the samples were mounted on a glass holder and ultra-sonically cleaned in bi-distilled water for more than 1 h to remove the residual debris in the pores. Then they were dried with flowing pressurized gas for 2 min and kept in dry air before testing. AFM topography examinations were carried out with an Auto Probe CPe atomic force microscope, using Ultralevere Si conical tips with a typical radius of curvature of 10 nm and an aspect ratio about 3:1 w13x. The samples were imaged under contact ŽC., intermittent contact ŽIC. and non-contact ŽNC. observation modes. The average force constant of the cantilevers used in the C-mode was 0.4 Nrm, while those used for IC and NC-modes were 17 Nrm. For the NC-mode, the
Y. Sui, J.M. Sanigerr Materials Letters 48 (2001) 127–136
cantilever vibrates at a driving frequency slightly greater than the peak value of its resonant frequency Žaround 320 kHz.. A constant force gradient of the cantilever is achieved by a feedback control of the scanner movements, and sample topographic information is obtained without touching the sample. For the IC-mode, the cantilever vibrates at a driving frequency slightly lower than the peak value of its resonant frequency. Because the vibration amplitude of the cantilever increases when the cantilever is brought close to the sample surface, non-continuous contacts of tip with sample is achieved during scanning at the intermittent mode. As for the contact mode, the amount of scanner z movement necessary to maintain a constant force is used to generate a topographic image. 3. Results and discussion 3.1. Characterization of supported AAO film on aluminum Fig. 1 presents the AFM images of a supported AAO film, prepared by oxidizing the aluminum plate for 1 h in 10% aqueous sulfuric acid solution, at 20 V DC and 108C Žone-step method.. The film was submitted to a pore-widening treatment for 10 min in 0.1 M H 3 PO4 at 308C, and the images were obtained by using different detecting modes, that is, NC ŽFig. 1a., IC ŽFig. 1b. and C ŽFig. 1c. modes. The NC and IC mode images were taken at a scan height of 20 nm. The applied force for the C-mode was 2.5 nN. This figure shows that although the general topographic features are similar for all the NC, IC and C modes images, different topographic information is obtained for each of them. In the NC-mode ŽFig. 1a., small pores Žblack spots of the images. are observed together with some foreign dots almost homogeneously distributed on the AAO surface. In the ICmode, no surface pores can be detected and the foreign dots are enlarged with streak-like artifacts overlapping them. When C-mode is adopted for topographic test, no foreign dots are observed, while the pores are detected with a sharper definition than they appear in the NC-mode. It is well known that the NC-mode can image liquid droplets or films on a sample surface, there are numerous examples in the literature that confirm this
129
fact w14,15x. Following this experience, it seemed logical to assume that the observed foreign white dots are tiny water drops with size range from 40 to 100 nm. This assumption was reinforced by the fact that these droplets disappeared after the sample was slightly heated. The artifacts in the sample surface image using the IC-mode require some kind of discussion on the light of the basic operation principles w16x. Imaging in the IC-mode results in a periodic striking of the tip at certain points of the surface and no lateral displacement of the tip along the scanning direction takes place. In these conditions, the tip comes in and out of the water droplets repeatedly during scanning, which renders them unstable by wetting and capillary interactions with the vibrating tip. This effect tends to enlarge the water drops and causes some streak-like artifacts on them. Contact AFM imaging is then the best option for the characterization of supported films of porous anodic alumina. It eliminates the effects of the covering water droplets or other adsorbed layers, giving the best lateral resolution for the imaging of the intrinsic surface features. It can be concluded that in our case the C-mode gives the best information in relation to the intrinsic features of the AAO surface, while the NC-mode offers complimentary information of the foreign species present on the AAO surface. On the other hand, in our experience the IC-mode is unable to discern between the intrinsic and extrinsic surface features, resulting in the poorest image quality among the three modes. In relation to the pore structure, it is important to note here that their distribution at the surface, while homogeneous, does not show the feature of selforganized hexagonal pore array due to the random pore nucleation on the surface w17x. This is an important point to be discussed later. The diameters of the pores were measured using a line profile, their values ranged between 30 and 40 nm. 3.2. Characterization of the suspended porous anodic film In order to get insight of the sample–tip interaction, a 6-mm thick w18x unsupported aluminum oxide film was produced, using the previously described approach. Both the oxidation and the pore widening
130
Y. Sui, J.M. Sanigerr Materials Letters 48 (2001) 127–136
Fig. 1. AFM images of anodic alumina film supported on aluminum. Ža. NC-mode; Žb. IC-mode; Žc. C-mode. Scan height for NC and IC modes, 20 nm. Applied force for C-mode, 2.5 nN. Sample oxidization 1 h Ž10% H 2 SO4 , 20 V DC, 108C.. Pore widening 10 min in 0.1 M H 3 PO4 at 308C.
times were 10 min. Fig. 2 shows the topography of the surface of such a film in the NC ŽFig. 2a. and C ŽFig. 2b and c. modes. The images 2a and b are quite similar, but contrary to those observed for the supported film, a sharper definition of the surface features is now obtained in the NC-mode. This apparent contradiction with the results of the supported AAO observations could be related with the mechanical properties of the unsupported AAO films. Effectively, these films would behave as a
vibrating drum membrane, which becomes unstable when observed in the C-mode, due to the relatively high sample–tip interaction force Ž10y6 –10y8 N. involved w19x. On the contrary, in the NC-mode observation the interaction force becomes significantly lower Ž10y1 2 N., and the unsupported AAO film would behave then as a stable structure. This assumption was confirmed when a new image of the same sample was taken at lower contact force, 2.5 nN in Fig. 2c. In these softer observation conditions,
Y. Sui, J.M. Sanigerr Materials Letters 48 (2001) 127–136
131
Fig. 2. Topographies of the surface suspended film. Ža. NC-mode; Žb and c. C-mode. Scan height for NC Ža. 20 nm. Applied force Žb. 7 nN and Žc. 2.5 nN. Sample oxidization 35 min Ž10% H 2 SO4 , 20 V DC, 108C.. Pore widening 10 min in 0.1 M H 3 PO4 at 308C.
the surface holes becomes clearly defined, although their average size is smaller than that corresponding to the NC images. 3.3. Characterization of AAO films obtained by the two-step anodization method The density of the pores in the images of Figs. 1 and 2 is sufficiently high to obtain a uniform pore distribution, although an ordered hexagonal array of the pores is absent, showing that under this synthetic
approach a random nucleation of the pores occurs. In this sense, these AAO films may be used as nanotemplates only if a dense packaging of the nanostructures is the main requirement. But frequently, as for the fabrication of nano-devices, an ordered distribution of the nano-structures is also required and then a template with ordered pore array must be obtained. To obtain an ordered hexagonal array of the pores on an AAO film a double oxidation step method has been developed w12x, which uses oxalic acid
132
Y. Sui, J.M. Sanigerr Materials Letters 48 (2001) 127–136
ŽHOOC–COOH. as oxidizing agent and an oxidation voltage of 40 V. In the present work, an analogous two-step method was tested, but substituting the oxalic acid by sulfuric acid, which in principle has the advantage of higher oxidation rate and lower voltage operation values. Fig. 3a presents the image of the AAO film obtained with this approach Žanodization voltage 20 V; pore widening 0.1 M H 3 PO4 for 20 min.. A tendency of the pores to self-organize in a hexagonal array is observed in the figure and is evidenced in the inset showing a typical pore hexagonal array formed in an area of 200 = 200 nm of this sample. For comparison purposes, an additional AAO sample was obtained using the oxalic acid two-step method w12x Ž40 V anodization voltage, 158C, pore widening 0.1 M H 3 PO4 for 50 min. and the result is presented in the Fig. 3b. An improved order of the pore patterns is clear in this image when compared with that of Fig. 3a. It is important to note here that the pore density in an AAO film increases at lower operation voltage w18x, while the pore size is a function of the extension of the pore size treatment. The differences observed in pore density and pore size in Fig. 3a and b originated mainly in the differences in the voltage operation and pore-widening time and not because of the different oxidizing agent.
As an attempt to quantify the hexagonality of the pore array, a segmentation of the 0.5-mm image was made and the angles of the hexagons were measured. Fig. 4a and b present the segmented images of the AAO obtained by the two-step anodization method using sulfuric and oxalic acid, respectively. The insets in these figures are the Fourier transform of the corresponding images, and represent an additional indication of the pore array ordering. Table 1 presents a quantitative comparison of the hexagonality of the pore array and other geometric parameters for both samples. The hexagonal array percentage of the sample Žsecond column, Table 1. means the percentage of the total area of the samples, which present a clear hexagonal pore pattern Žsee Fig. 4.. The reported values were obtained from three separated regions Ž0.5 = 0.5 mm. for each sample and have to be considered only as a rough approximation. Nevertheless, it is clear that the hexagonal array of the two-step sulfuric acid oxidized sample is not extended over its whole surface, instead one third of it still has an irregular pore array. Additionally, the hexagon distortion Ževaluated from the standard deviation of the 1208 angle of the regular hexagon. is also higher for the sulfuric acid anodized sample. These disadvantages could be minimized by additional adjustment of the experimental conditions of
Fig. 3. AFM images Ž2 = 2 mm. of AAO films obtained with the two-step method. Ža. Sulfuric acid, 20 V, pore widening 20 min in 0.1 M H 3 PO4 ; Žb. oxalic acid, 40 V, pore widening 50 min in 0.1 M H 3 PO4 .
Y. Sui, J.M. Sanigerr Materials Letters 48 (2001) 127–136
133
Fig. 4. AFM images Ž0.5 = 0.5 mm. of AAO films obtained with the two-step method using as oxidizing agent Ža. sulfuric acid; Žb. oxalic acid. Same samples as that in Fig. 3. The images were segmented following the hexagonal pore array. Inset: FFT images.
synthesis, and some work is presently being performed in this direction. On the other hand, the areas of the single hexagons are smaller for the sulfuric acid anodized sample and, consequently, its pore density becomes higher. Nevertheless, these features cannot be related only to the anodizing agent used, but mainly with the voltage value used in the anodization process; it is well known w18x that lower anodization voltage results in higher pore density. Finally, a short discussion must be undertaken about the evaluation of the AAO pore size and shape by AFM. From the images of Fig. 4, it seems clear that the sample surface is not perfectly flat and that the pores cannot be regarded as ideal cylinders from base to mouth w18x. Instead of that, a mouth pore may be distinguished surroundings an increasing dark, almost circular, spot in the center of the pore. So the definition of the pore diameter requires the establishment of a criterion indicating if the mouth
of the pore is considered or not in the measurement. A demonstration of the difficulties of measuring the real shape of the pores, even at their upper part, is given in Fig. 5 where some line profiles across the surface of the Fig. 4b image are shown. Each line profile is formed by a set of cones instead of the expected cylinder-shaped internal pore. This deviation of the expected shape results from the inability of the probe tip to go inside the pores due to its conical geometry with a relatively low aspect ratio Žabout 2:1.. In fact, the apparent conical profile of the pore reflects the tips geometry rather that the actual internal pore shape. Typical curvature radius at the end of these probe tips is around 10 nm, but they have a conical shape with a relatively high aperture angle and then a few nanometer apart from its end the cross-section of the tip is comparable with the pore radius. Following the z-sale of Fig. 5 profiles, it is evident the very short penetration of the
Table 1 Geometric parameters of the hexagonal pore array Sample
Hexagonal array percentage
Averaged angle value
Averaged hexagonal area Žmm2 .
Pore density Žporesrcm2 .
Sulfuric acid anodized Oxalic acid anodized
f 66 f 100
120.1 " 9.78 120.0 " 5.68
0.012 0.024
2.6 = 10 10 1.2 = 10 10
134
Y. Sui, J.M. Sanigerr Materials Letters 48 (2001) 127–136
Fig. 5. Line profile across a set of pores of the images from Fig. 4b. Pore distance 99 " 1 nm, mouth diameter 86 " 4 nm, internal pore diameter 56 " 4.
tips into the pore, around 15 nm when the expected deepness of the AAO pores is of several micrometers. Then it must be emphasized that with the probe tip used in this work, the only reliable information is that related with the diameter of the pore mouth. In a general way, it could be established that when the aspect ratio of a pore or hole is larger than that of the tip, the observed profile will not fit the actual pore dimensions. Carbon nanotubes attached to Si com-
mercial tips have been proposed to overcome this difficulty and will be evaluated when available. With the exposed limitations in mind, the diameter of the pore mouth and the pore separation were measured. Additionally, a rough evaluation of the internal pore diameters is presented. For this purpose, the profile marks in Fig. 5 were visually set on different points. For the evaluation of the pore mouth diameter, the marks were set on the upper inflexion
Y. Sui, J.M. Sanigerr Materials Letters 48 (2001) 127–136
point of the conical profile lines, and the pore separation was estimated by putting the mark in two contiguous minima of the same profile lines. Finally, the internal pore diameters were evaluated setting the marks at the edges of the central black spots. Following this criterion for the two-step oxalic acid anodized sample shown in Fig. 5, the pore mouth diameter ranges between 82 and 89 nm, while the pore separation oscillates between 98 and 100 nm. The apparent diameter of the internal cross-section of the pores would be comprised between 52 and 60 nm.
4. Conclusions The AFM has been shown to be a powerful and convenient technique for characterizing anodic porous alumina films. General features of their surface and pore structures and their dependence with different methods of synthesis were shown. The capabilities and some limitations of the technique were presented. The application of different testing modes for imaging both supported and unsupported films provide complimentary information allowing a more complete characterization of the films. The supported films are rigid enough to resist the perturbation of strong testing forces and the C-mode, with a higher lateral resolution than the NC-mode, is better employed for the characterization of the film surface features. Nevertheless, the NC-mode provides important complimentary information about the adsorbed species on the film. On the other hand, the unsupported or suspended film is not strong enough to resist the perturbation of relatively high testing forces, and then NC-mode is recommend for its characterization because of the smaller exerted testing force. A modification of the two-step method was tested in order to obtain hexagonal ordered array of AAO pores. For this purpose, sulfuric instead of oxalic acid was used with the initial advantages of its higher oxidation rate and the lower operation voltages needed. The AAO film obtained with this approach presents a significant self-organization of the pore in a hexagonal array, but still has a higher distortion than that obtained with oxalic acid. It is
135
expected that further refinements of the experimental conditions will result in a higher ordering of the pore hexagonal array. The study of intermediate resolution images Ž0.5 = 0.5 mm. of two samples obtained under different experimental conditions, shows the capability of AFM to evaluate the degree of ordering of the pore array, as well as the AAO pore density and the diameters and external shape of the pores. A substantial experimental improvement is still required in order to have a more reliable measurement of the internal diameter and even the length of the pores in the AAO films. A promising expectation in this direction is the availability of very high aspect ratio tips such as those obtained by attaching a carbon nanotube to commercial Si tips.
Acknowledgements This work was partially supported by CONACyT of Mexico. The authors are very grateful for Raul Ruvalcaba Morales of the Centro de Instrumentos for his valuable technical help.
References w1x N. Itoh, N. Tomura, T. Tsuji, M. Hongo, Microporous Mesoporous Mater. 20 Ž1998. 333. w2x C.A. Huber, T.E. Huber, M. Sadoqi, J.A. Lubin, S. Manalis, C.B. Prater, Science 263 Ž1994. 800. w3x D. Routkevitch, A.A. Tager, J. Haruyama, D. Almawlawi, M. Moskovits, J.M. Xu, IEEE Trans. Electron Devices 43 Ž1996. 1646. w4x S. Kawai, R. Ueda, J. Electrochem. Soc. 122 Ž1975. 32. w5x D.N. Davyddov, J. Haruyama, D. Routkevitch, B.W. Statt, D. Ellis, M. Moskovits, J.M. Xu, Phys. Rev. B 57 Ž1998. 13550. w6x S.L. Sung, S.H. Tsai, C.H. Tseng, F.K. Chiang, X.W. Liu, H.C. Shih, Appl. Phys. Lett. 74 Ž1999. 199. w7x J. Li, M. Moskovits, T.L. Haslett, Chem. Mater. 10 Ž1998. 1963. w8x M. Saito, M. Kirihara, T. Taniguchi, Appl. Phys. Lett. 55 Ž1989. 607. w9x A. Balandin, K.L. Wang, N. Kouklin, S. Bandyopadhyay, Appl. Phys. Lett. 76 Ž2000. 137. w10x J.-P. Salvetat, G. Andrew, D. Briggs, J.-M. Bonard, R.R. Bacsa, A.J. Kulik, T. Stockli, N.A. Burnham, L. Forro, ¨ ´ Phys. Rev. Lett. 82 Ž1999. 944. w11x I. Serebrennikova, P. Vanysek, V.I. Birss, Electrochim. Acta ´ 42 Ž1997. 145.
136
Y. Sui, J.M. Sanigerr Materials Letters 48 (2001) 127–136
w12x H. Masuda, M. Satoh, Jpn. J. Appl. Phys., Part 2 35 Ž1996. L126. w13x Park Scientific Instruments, General Purpose AFM Cantilever Technical Sheet. w14x J. Hu, R.W. Carpick, M. Salmeron, J. Vac. Sci. Technol., B 14 Ž1996. 1341. w15x J. Hu, X.-D. Xiao, D.F. Ogletree, M. Salmeron, Science 268 Ž1995. 267. w16x N.A. Burnham, O.P. Behrend, F. Oulevey, G. Gremaud, P.-J.
Gallo, D. Gourdon, E. Dupas, A.J. Kulik, H.M. Pollock, G.A.D. Briggs, Nanotechnology 8 Ž1997. 67–75. w17x O. Jessensky, F. Muller, U. Gosele, Appl. Phys. Lett. 72 ¨ ¨ Ž1998. 1173. w18x J.W. Diggle, T.C. Downie, C.W. Goulding, Chem. Rev. 69 Ž1969. 365. w19x R. Howland, L. Benatar, A Practical Guide to Scanning Probe Microscopy, Park Scientific Instruments, 1993, Chap. 1, pp. 8–11.