Nanoscale Topography: A Tool to Enhance Pore Order and Pore Size

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Nanoscale Topography: A Tool to Enhance Pore Order and Pore Size Distribution in Anodic Aluminum Oxide D. C. Leitao,*,† A. Apolinario,† C. T. Sousa,† J. Ventura,† J. B. Sousa,† M. Vazquez,‡ and J. P. Araujo† †

IFIMUP and IN - Institute of Nanoscience and Nanotechnology, Department of Physics and Astronomy, Faculdade de Ci^encias da Universidade do Porto, Rua do Campo Alegre, 678, 4169-007, Porto, Portugal ‡ ICMM-CSIC, Campus Cantoblanco, 28049 Madrid, Spain ABSTRACT: In nanoporous alumina, surface pretreatment prior to the anodization process plays a crucial role in the nanopore formation, strongly influencing their organization and size uniformity. In this work, an effective control of the anodic aluminum oxide (AAO) hexagonal order and nanopore size uniformity was achieved by simply changing the initial aluminum electropolishing pretreatment time. A drastic change in the nanoscale topography of the treated aluminum foils was observed, and intrinsic features were revealed. The effect of such distinct landscapes on the AAO surface after first and second anodization processes was studied in detail. Quantitative analysis of the AAO surface after the second anodization enabled establishing a close correlation between aluminum surface roughness and AAO pore order and regularity.

’ INTRODUCTION In the last 20 years, anodic aluminum oxide (AAO) membranes emerged as an extremely popular template-based method for the fabrication of a wide range of nanostructures.1
4 The compatibility of these porous films with existing semiconductor technology along with their simple and low-cost fabrication makes AAO a viable candidate for large-scale industrial processing and a suitable alternative to conventional nanolithography techniques. In fact, highly ordered self-organized hexagonal lattices of AAO have been successfully prepared by the two-step anodization process developed by Masuda and Fukuda5 with pore diameters (Dp) in the 24
158 nm range and interpore distances (Dint) between 66 and 500 nm.6,7 In addition, square and triangular nanopore lattices have also been obtained by means of hybrid nanoimprint and self-assembling processes.8 The versatility of AAO arises from the possibility of obtaining nanometer-sized pores disposed naturally in an ordered hexagonal lattice, just by adequately selecting the anodization voltage and electrolyte type.6,7 However, smearing of the hexagonal order occurs when deviations from the optimum anodization conditions take place.9 For many applications, such as high-density magnetic recording media,10 photonic crystals,11 or pattern-transfer masks,12 the ordering and organization of the nanopores is a crucial factor and a high degree of regularity and uniformity is required. Several intrinsic factors of the starting aluminum (Al) foil can affect the structure of AAO, namely, impurities, defects,13 and Al grain size.14 The latter is also a limiting factor when anodizing Al thin films for etching masks.15,16 In addition to the distinct anodization parameters (electrolyte type and concentration, temperature, applied potential) that influence considerably the porous structure (Dp and Dint) and r 2011 American Chemical Society

organization of AAO,7,17
21 the surface roughness of the initial Al foil also plays a crucial role on the onset of pore nucleation.22 Previous studies showed the influence of surface pretreatment on the AAO self-ordering.19,23
26 The work of Ono et al. (using malonic and tartaric organic acids as electrolytes)19 showed faster barrier oxide layer and pore formation in high-roughness surfaces. However, the rough etching front prevented self-organization. Moreover, phenomenological theories27 have addressed the influence on pore ordering of the pH dropping at the pore bottom, temperature and concentration gradients, and cracks on the surface.28 Although these available studies cover a broad set of conditions, no systematic study regarding a particular pretreatment and its consequent effect on AAO characteristics was so far reported. Electropolishing (EP) is a well-established and understood technique,29 used currently at both industrial and research levels. Therefore, several reports on EP of different metals can be found.26,30
33 It was shown that, besides working as a surface pretreatment, EP can also be used to create nanopatterns in the surface of aluminum,31 titanium, and tungsten.32 However, it was not until 1996 that Yuzhakov et al. showed that, depending on the applied voltage and exposition time, stripe structures or hexagonally ordered dimples could be obtained with dimensions between 50 and 150 nm.32
36 Other parameters, such as temperature, stirring, distance to the electrode, and type of electrolyte, were also found to play an important role in the process.34,35 Furthermore, when used as a pretreatment of Al foils for the subsequent growth of AAO26 and due to the preferential pore nucleation in the troughs Received: March 11, 2011 Published: April 12, 2011 8567

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Figure 2. (a) ÆRsurfæ dependence on tEP measured from AFM images and (b) spatial order parameter (γ) dependence on tEP. As an upper threshold, we adopt the value obtained for an optimized AAO sample. Lines are guides to the eye.

Figure 1. Selected topography images of the Al surface obtained with AFM after the EP pretreatment: (a) without electropolishing (tEP = 0 s), (b) tEP = 30 s, and (c) tEP = 120 s. Notice the different height scales in the images.

of the pretextured features,15,37 ordering can be achieved in a wider range of anodization potentials, thus improving the selforganization conditions of AAO.15,37,38 Nevertheless, the physical mechanisms behind these nanopatterning self-assembling techniques (anodization and electropolishing) are very distinctive. The first (anodization) depends strongly on the topography of the sample as a consequence of the local modulation of the electrical field strength, whereas the latter (electropolishing) relies first on ion migration through the double layer and thus shows almost no sensitivity to topography. In this work, we describe how one can improve pore order and pore size distribution in AAO membranes by tailoring the surface roughness of the initial Al foil. We show that, through the control of the Al topography at the nanoscale, simply by changing the electropolishing pretreatment time (tEP), a noticeable improvement of the AAO surface quality and organization (upon anodization) is observed. Our analyses allowed us to extract quantitative information not only on the Dp and corresponding distribution but also on the Dint and pore order parameter (γ).

’ EXPERIMENTAL DETAILS After standard cleansing processes, the high-purity Al foils (99.997%) were submitted to an EP treatment using a 75% ethanol and 25% perchloric acid solution (% in volume) at ∼10
C with an applied potential of 20 V. This is the suitable potential for EP with the selected electrolyte.36 The distance between the sample and the Pt electrode was kept constant at 4 cm.

Distinct tEP were employed: a first series of samples with tEP = 0, 30, 60, 90, and 120 s were subjected to a first anodization only; a second series with the same tEP detailed above was subjected to a two-step anodization process.5,39 After EP but prior to the anodization procedure, the Al surface was analyzed with a Nanoscope multimode atomic force microscope (AFM) from Veeco Instruments operating in tapping mode. Such a study allowed us to establish the dependence of surface roughness (Rsurf) on tEP. The anodizations were then performed (mild anodization) in 0.3 M oxalic acid solution at ∼4
C and under an applied potential of 40 V.17 The first anodization was carried out for 2 h, whereas the second lasted 1 h. For comparison, we also fabricated an optimized AAO sample with tEP = 120 s and a first anodization of about 3 days. The current density transients [j(t)] were monitored during the anodization procedure using a Keithley 2004 sourcemeter remotely controlled. Low-vacuum scanning electron microscopy (SEM) imaging of the surfaces was performed with an FEI Quanta 400FEG, for all samples. From the SEM images, fast Fourier transforms (FFT) and image statistical analysis were performed using an open-source image treatment program,40 enabling a quantitative evaluation of the organization pattern and pore structure dimensions.

’ EXPERIMENTAL RESULTS Figure 1 shows representative 500  500 nm2 images of the Al surface with different tEP, enabling a visual comparison. From these images, we extracted the average surface roughness (ÆRsurfæ), defined as the root-mean-square value of the images' pixel height (hpixel) value: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 n 2 ð1Þ ÆR surf æ ¼ ðhpixel
hmean pixel Þ n i¼1

∑

Figure 2a displays the ÆRsurfæ dependence on tEP. A sharp decrease is observed after the pretreatment is employed for just tEP = 30 s (from a local ÆRsurfæ = 4.2 nm for the unpolished sample to ÆRsurfæ = 1 nm). As the tEP is increased, a significantly smoother 8568

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Figure 3. j(t) monitored during first (a) and second (b) anodizations. Inset: dependence of tmin on tEP. The depicted curves concern normalized j(t). [jn(t)] is given by jn(t) = [j(t)
jmin(tmin)]/[jst(tst)
jmin(tmin)], where jst is the value obtained after a 5 min anodization and jmin the value at the minimum of the curve.

surface is attained, reaching ÆRsurfæ = 0.4 nm for tEP = 120 s. Furthermore, with increasing tEP, (almost) periodical rippled structures are revealed (Figure 1b,c). These features are typical of EP treatment and appear due to electrostatically packed polar (ethanol) molecules on the Al surface, thus reducing the ion transport close to the electrode interface and protecting the crests from field-assisted dissolution.31
35 In fact, it is known that, by combining adequate EP voltage and time, dimple patterns from striped to hexagonal arrangements can be obtained.32,33 Figure 3 shows first and second anodization current density [j(t)] curves of selected samples (tEP = 0, 30, and 120 s), exhibiting the characteristic behavior of porous structure formation.27 Interestingly, an increase in the valley width of j(t) minimum (attributed to nanopore formation) with increasing tEP is present in all cases. This behavior was studied using the time indicative of the onset of pore nucleation (tmin), that is, the minimum value of the j(t) curves. Notice that, although the pores start to nucleate near the initial stage of anodization,27 tmin still stands as a clear indication of earlier (or delayed) nucleation onset, which also translates into a narrower (larger) broadening of the j(t) valley. The inset of Figure 3b displays the tmin dependence on tEP. Overall, one observes an increase in tmin with increasing tEP or, equivalently, with decreasing ÆRsurfæ. For the first anodization step (inset of Figure 3b, blue dots), the slightly higher values of tmin indicate a delayed start of pore nucleation. During the anodization process, pores start to grow where the oxide layer is thinner. At such sites, the density of electrical field lines is enhanced by a focusing effect due to the accentuated topography, leading to chemical dissolution promoted by local heating.27 Therefore, deeper valleys (present in higher ÆRsurfæ samples) will induce higher field-assisted-dissolution rates and, consequently, early pore nucleation. Figure 4 depicts AAO top-view SEM images after the first anodization for samples with different tEP. The expected porous structure inferred from j(t) curves monitored during the anodization process was confirmed (Figure 3). For the unpolished sample (tEP = 0 s; Figure 4a), we observed an irregular and rough surface and nanopores with very dissimilar sizes. The other samples displayed considerable smoother surfaces, but still a random porous structure. This characterization emphasizes the importance of a good surface pretreatment, prior to the anodization. Noticeably, an almost periodical structure is seen for sample

Figure 4. Top-view SEM images of the samples with a tEP of (a) 0, (b) 30, and (c) 120 s, after the first anodization. (d) Cross section of image (c) showing the preferential pore nucleation in the topographic valleys.

tEP = 120 s (Figure 4c). These stripelike structures have an average separation of 66 nm, which correlates well with those observed in the AFM images (Figure 1). The size of these features is determined by the concentration of ethanol in the EP solution and the applied potential.41 Moreover, such valleys act as preferential pore nucleation sites, as supported by the SEM cross-sectional image shown in Figure 4d. This effect is only superficial, and no reminiscences were observed after the second anodization was performed (Figure 5c). This result suggests that oxide dissolution occurs preferentially in between the ripples; at such sites, the topography is more pronounced, thus enhancing the local electrical-field-assisted chemical dissolution of Al oxide. After removing the first porous alumina layer, the templates, having a patterned nanohole structure, were again anodized. Analysis of the j(t) curves during the anodization process mainly points to the direct growth of pores guided by the first anodization nucleation sites. Figure 5 (left images) shows SEM micrographs of the AAO surface after the second anodization step. As expected, a 8569

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Figure 7. (a) ÆDporeæ and corresponding σpore dependence on tEP extracted from the statistical analysis of SEM images. Lines are guides to the eye. (b, c) Pore size (Dpore) distributions for tEP = 0 and 120 s.

Figure 5. SEM images (left) and respective FFT (right) after the second anodization of samples with (a) tEP = 0 s, (b) tEP = 30 s, and (c) tEP = 120 s.

Figure 6. (a) SEM image of the selected hexagonal monodomain region and (b) corresponding FFT profile. The important parameters for the image treatment are also shown: b represents the reciprocal lattice vector magnitude, A the amplitude of the first peak, and w its width at half-maximum.

significant improvement in the pore structure organization is observed when compared with the first anodization surfaces, also a consequence of an effective local focusing of the electrical field. To provide a systematic route to evaluate the organization, quality, and lattice constant (Dint) of the AAO surfaces, we performed a detailed FFT analysis of the second anodization SEM images (Figure 5, right images). For a monodomain region (Figure 6a), the FFT evidences the 6-fold symmetry of this pattern, characterized by well-defined six central spots, hexagonally disposed. A profile taken from one direction of the FFT pattern of Figure 6a shows, at least, eight distinct peaks, stressing

the long-range order of the structure (Figure 6b). This pattern corresponds to a triangular Bravais lattice. The magnitude of the primitive lattice vectors (|a to the reciprocal B| = a) are related √ lattice vectors (|b B| = b) through b = 2/( 3a).42 In this case, for AAO, a corresponds to Dint. In addition, a Dint size distribution is seen as the displacement of the central frequencies of the FFT peaks, allowing us to obtain, from Figure 6b, an average ÆDintæ = 103.8 nm. Contrary to the monodomain region, the low-tEP samples display smooth ringlike images, indicating a large number of hexagonal domains with different orientations (Figure 5a). However, as tEP increases, a better definition in the FFT hexagonal lattice is observed, pointing to a more organized pore structure (Figure 5c). The study of the FFT profiles also allows us to infer quantitatively the periodicity of the porous structure, enabling an easy and straightforward way to evaluate its long-/short-range order. In fact, the profiles obtained from the FFT images hold information on the spatial order parameter γ defined as43 γ¼

A w

ð2Þ

where A represents the peak intensity and w its width at halfmaximum (Figure 6b). Figure 2b reports the dependence of γ on tEP. A clear increase of γ with tEP is visible toward the maximum value established by the optimized sample, emphasizing the improvement in AAO order by increasing the ordered domain size. Figure 7 shows the representative Dp size distribution for samples with tEP = 0 and 120 s, obtained from the statistical analysis of the SEM images (Figure 5). First, one observes a noticeable increase in the sharpness of the pore size Gaussian distribution when tEP increases from 0 to 120 s (Figure 7c,d). Such a trend is translated in the decrease of the standard deviation (σp) with increasing tEP (Figure 7a), corresponding to a better definition of the pore sizes and improved AAO pore uniformity. Also, an increase of the average pore size [ÆDpæ] with 8570

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The Journal of Physical Chemistry C increasing tEP (or alternatively, decreasing ÆRsurfæ) was observed (Figure 7a). In fact, due to the nanoscale topography tailoring upon EP treatment, better-defined concave Al patterns occur after the first anodizations with larger tEP. Therefore, AAO templates with improved order (higher γ and better-defined FFT) and uniformity (lower σp) are obtained, resulting also in a ÆDpæ close to the one observed in the optimized sample. Notice that an improvement of AAO ordering with EP pretreatments has been indicated in previous studies.19,26,44 Nevertheless, our results clearly establish for the first time a quantitative dependence of γ and σp on ÆRsurfæ. With this work, we stressed the usefulness of a widely implemented procedure, such as EP, as a capable tool to control and tune the Al surface at the nanoscale, leading to a more uniform and ordered AAO. Additionally, our detailed study focusing on ÆRsurfæ complements previous analyses on the dependence of γ with other anodization conditions,7,17,18 thus extending the knowledge of the influence of external parameters on the organization of AAO.

’ CONCLUSIONS In conclusion, we emphasized the crucial role played by the initial Al surface topography on AAO template organization and regularity, highlighting the possibility of using ÆRsurfæ (via EP pretreatment) as a control order parameter. We were able to accurately modify the roughness of the Al substrates, altering significantly the landscape characteristic of the surface, only by controlling the time during which the samples were electropolished. Samples with longer tEP revealed the best-quality first anodization surfaces. Moreover, the dimple structures observed acted as preferential sites for pore nucleation due to higher fieldassisted dissolution rates, showing, afterward, major repercussions on the pore order and uniformity. Quantitative analysis of SEM images after the second anodization revealed an increasing narrowing of the Dp distribution with tEP, which, in turn, correlates extremely well with γ(tEP) dependence. We were thus able to improve the AAO order and pore dimension uniformity and regularity only by tailoring the ÆRsurfæ of the Al initial substrate. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported, in part, by the project FEDER/ POCTI/n2-155/94. D.C.L. and C.T.S. are thankful to FCT for grants SFRH/BD/25536/2005 and SFRH/BD/38290/2007. A. A. acknowledges the Faculty of Sciences of Oporto University for the financial support under the FCT project NANA/NMed-SD/ 0156/2007. J.V. and J.P.A. acknowledge financial support through FSE/POPH and Fundac-~ao Gulbenkian (“Programa Gulbenkian de Estímulo a Investigac-~ao Científica”), respectively. ’ REFERENCES (1) Xiao, Z. L.; Han, C. Y.; Welp, U.; Wang, H. H.; Vlasko-Vlasov, V. K.; Kwok, W. K.; Miller, D. J.; Hiller, J. M.; Cook, R. E.; Willing, G. A.; Crabtree, G. W. Appl. Phys. Lett. 2002, 81, 2869–2871. (2) Son, S. J.; L., S. B.; Bai, X. Drug Discovery Today 2007, 12, 650.

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