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Electrochimica Acta 63 (2012) 256– 262

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

j ourna l ho me pag e: www.elsev ier .com/ locate /e lec tac ta

ewfound capability of focused ion beam patterning guided anodization

athy Lu ∗

epartment of Materials Science and Engineering, Virginia Tech, Blacksburg, VA 24061, USA

r t i c l e i n f o

rticle history:eceived 8 November 2011eceived in revised form0 December 2011ccepted 22 December 2011vailable online 30 December 2011

a b s t r a c t

This study addresses focused ion beam (FIB) patterning guidance effect on the nanopore developmentduring the subsequent anodization on uneven surfaces. Different shape, uneven features are first createdon aluminum surfaces. After that, hexagonally arranged nanoconcaves with different inter-concave dis-tances are patterned across the surfaces. The pore development in different anodization electrolytes aswell as with different interpore distances and feature surface curvatures are studied. The results show

eywords:rdered nanopore patternocused ion beamnodization

nterpore distance

that FIB guided anodization can produce ordered nanopore arrays on various uneven surfaces. In thenanopore depth direction, pores grow perpendicularly to the tangential direction of the surface, whichleads to pore splitting or termination based on the surface curvature.

© 2011 Elsevier Ltd. All rights reserved.

urvature

. Introduction

Since the discovery of self-ordered porous alumina in 1995 [1],nodic aluminum oxide has attracted great interest as templatesor the fabrication of various low dimensional nanostructures, suchs nanodots [2,3], nanowires [4–8], and nanotubes [9–12]. Thereas been the well-known 10% porosity rule (self-ordering requires

porosity of 10%, independent of the specific anodization con-itions) for aluminum anodization [13]. However, self-organizednd ordered alumina nanopores occur only in narrow anodiza-ion windows and with hexagonal arrangement. Also, the anodizedores have fairly limited interpore distances. For sulfuric acid elec-rolyte, the anodization voltage is 25 V and the resulting interporeistance is ∼63 nm [14,15]. For oxalic acid electrolyte, the anodiza-ion voltage is 40 V and the resulting interpore distance is ∼100 nm16,17]. For phosphoric acid electrolyte, the anodization voltage is95 V and the interpore distance is ∼500 nm [18,19]. In addition,he ordered nanopore region size is limited to a few microns evenhen the surface is defect (grain boundary, twining, and disloca-

ion) free. The fundamental cause for this limitation has not beenlearly addressed but has prompted the research to explore newechniques for longer range nanopore ordering.

To widen the applications of anodic porous alumina, differ-

nt patterning techniques are used to fabricate feature arrays onnodized aluminum surfaces with large, ordered domain sizes andontrollable interpore distances. Using nanoimprint lithography

∗ Corresponding author. Tel.: +1 540 231 3225, fax: +1 540 231 8919.E-mail address: klu@vt.edu

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.12.095

(with SiC, Si3N4, or Ni mold) [20–24], focused ion beam (FIB) lithog-raphy [25–33], and holographic lithography [34], highly orderedconcave patterns on the aluminum surface are created; and theconcaves are in turn used to guide the subsequent anodization ofaluminum to generate highly ordered anodic alumina templates.These approaches can eliminate the influence of defects and allowthe creation of large, organized anodic alumina.

Combining nanoimprint lithography or FIB lithography withself-organized anodization can synthesize alumina nanoporearrays with hexagonal, triangular, or square arrangement [35–37].Moreover, with the guidance of the FIB created gradient-sized andalternating-sized concave patterns, hexagonally ordered gradient-sized and alternating-sized nanopore arrays are produced [30].With even further advancement, square nanopore patterns areobtained by the guidance of FIB patterned concave arrays withsquare arrangement [33]. In addition, the shape of the nanoporescan be adjusted by changing the arrangement of the guidingpatterns. Nanopores with square and triangular shapes are devel-oped using square and triangle concave arrangements during thenano-indentation patterning, respectively [23,37]. When the FIBpatterned concave arrays are created by overlapping two periodicpatterns, the subsequent anodization creates porous anodic alu-mina with Moiré patterns, which have a wide range of interporedistances and area specific pore densities. The periodicity of theMoiré patterns can be predicted by the interpore distance of theinitial patterns and the rotation angle [38].

Even though great progress has been made in the guidedanodization on flat aluminum surfaces, the full potential of anodiza-tion with patterning guidance is yet to be explored. This is especiallytrue for the FIB guided anodization, which offers well controlled,

K. Lu / Electrochimica Acta

Fig. 1. Different designs used to create features on aluminum surfaces: (a) circle,wd

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in 0.3 M oxalic acid under 50 V constant voltage at 0 ◦C for 5 min.

ith contrast change in the radial direction, (b) square, with contrast change in theiagonal direction and (c) stripes, with contrast change from one side to the other.

ery small size guiding concaves and can pattern a vast array ofuiding patterns on many different surfaces. There are some studiesbout the growth of anodic alumina nanopores on curved surfaces29,39–41]. One interesting yet untested area is patterning orga-ized pore arrays on uneven aluminum surfaces. This will addresshe critical needs that require uneven surfaces, such as sensors andptical devices that will have a higher efficiency if curved surfacesan be more effectively utilized. If organized nanopores can be cre-ted on curved surfaces, the follow-up question would be the poreepth growth. The surface curvature on nanopore growth directionecomes an important aspect that needs to be examined.

In this study, different shape and uneven features are first cre-ted on aluminum surfaces by aluminum material removal through

FIB microscope. These features are designed as a few microns inize but the region size can be easily enlarged or shrunk by changinghe feature designs. After that, hexagonally arranged nanoconcavesith different interpore distances are patterned across the surfaces,

till using the FIB microscope. The micron features provide a uniquepportunity to study the patterned nanopore growth during theubsequent anodization. The pore development in different elec-rolytes as well as with different interpore distances and featureurface curvatures is studied.

. Experimental procedure

.1. Pattern design

For the feature creation using the FIB microscope, Serif Draw-lus 4.0 software was used to design the features first. As shown

n Fig. 1, circle, square, and stripe images were designed with vary-ng contrast changing radially (Fig. 1a), diagonally (Fig. 1b), or fromne side to the other (Fig. 1c). During the feature creation, the fea-ure shapes were produced on the aluminum surfaces after theIB microscope imported the feature designs and directed the ioneam movement accordingly. The image contrast difference dic-

ated the material removal amount on the aluminum surfaces. Theluminum material in the bright area was removed. The materialn the dark area was left intact. The material in the gray area had

63 (2012) 256– 262 257

partial material removal in-between. This created different featureswith varying curvature on the aluminum surfaces.

2.2. FIB patterning

High purity aluminum foils (99.999%, Goodfellow Corporation,Oakdale, PA) with 8 mm × 22 mm × 0.3 mm size were used as thestarting material. They were washed with ethanol and acetone, andthen annealed at 500 ◦C for 2 h in high purity flowing Ar gas with5 ◦C min−1 heating and cooling rates to recrystallize the aluminumfoils and remove mechanical stresses.

For electropolishing, the annealed aluminum foils weredegreased in ethanol and acetone for 5 min, respectively, followedby DI water rinsing after each step. The aluminum foils were thenimmersed in a 0.5 wt% NaOH solution for 10 min with ultrasoundin order to remove the oxidized surface layer. After that, the alu-minum foils were electropolished in a 1:4 mixture of perchloricacid (60–62%):ethanol (95%) (volume ratio) under a constant volt-age of 12 V at room temperature with 500 rpm stirring speed for5 min.

A dual beam focused Ga+ ion beam microscope (FIB, FEI Helios600 NanoLab, Hillsboro, OR) was employed to create different fea-tures on the aluminum surfaces first, using the designs illustratedin Fig. 1 and other designs to be explained later. The acceleratingvoltage for the FIB microscope was 30 keV. The beam diameter was∼30 nm. The beam current was 93 pA and the beam dwell time ateach scan was 30 �s. For the feature designs in Fig. 1, the FIB gen-erated features were shown in Fig. 2. As shown, the circle designtransformed into a hemisphere; the square design transformed intoa square pit, and the stripe design transformed into a stair-case withan overall square shape. The diameter of the hemisphere (Fig. 2a)as well as the edge lengths of the square pit (Fig. 2b) and the stair-case (Fig. 2c) were all 3 �m. The largest depth for each pattern was∼1.2 �m with local regions varying in depth based on the contrastdifferrence in Fig. 1. The total FIB time used for the feature creationwas 84 s, 216 s, and 150 s for Fig. 2a–c, respectively.

After the feature creation, the FIB microscope was used to pat-tern different concave arrays across the entire features. For theconcave array patterning across the uneven aluminum surfaces, theFIB patterning current was 28 pA, the beam dwell time was 3 �s,and the ion beam patterning time for each concave was 40 ms. Thenanoconcaves were designed in hexagonal arrangement. However,the approach reported in this study is general and can be appliedto all types of nanoconcave arrangements, such as square, alter-nating nanoconcave size, or concave arrays with missing sites. Theinter-concave distance was set at 200 nm, 350 nm, or 425 nm asexplained later. The concave diameter and depth were designedat 75 nm and 50 nm, respectively. The patterned concave arrayswith 200 nm interpore distance were shown in Fig. 3 for the fea-tures shown in Fig. 2. The concaves along the feature walls wereelongated.

During both the feature and concave pattern array creation pro-cesses, the aluminum surfaces were observed in the SEM mode,which allowed for in situ monitoring of the surface changes of thealuminum foils at different stages of the ion exposure.

2.3. Anodization

Some of the FIB patterned aluminum samples were anodized in0.3 M phosphoric acid under 20 mA cm−2 constant current densityat 0 ◦C for 5 min. The voltage was ∼140 V after a few seconds ofanodization. Other FIB patterned aluminum samples were anodized

The current density became stable at ∼25 mA/cm2 after 3 min. Inboth cases, pore opening was carried out in 5 wt% phosphoric acidat 30 ◦C for 10 min after the anodization.

258 K. Lu / Electrochimica Acta 63 (2012) 256– 262

Fig. 2. FIB generated smooth aluminum surface with different feature shapes: (a)h

nOttss

Fig. 3. FIB patterned aluminum surfaces with nanoconcave arrays in hexagonal

Fig. 4 shows the anodized nanopore arrays on flat aluminum

emisphere, (b) square pit, and (c) stair-case.

The porous anodic alumina patterns were characterized by scan-ing electron microscopy (Quanta 600 FEG, FEI Company, Hillsboro,R). The cross sections of the anodized nanopores were obtained in

he FIB microscope by using 0.28 pA current to cross section the pat-erned aluminum samples. Before cutting, the anodized aluminum

urfaces were coated with a layer of Pt to protect the anodizedtructures.

arrangement. The inter-concave distance is 200 nm: (a) hemisphere, (b) square pitand (c) stair-case.

3. Results and discussion

3.1. Electrolyte effect on pore patterns

surfaces and across the patterned features shown in Fig. 3. As itshows, hexagonally ordered nanopore arrays are created on both

K. Lu / Electrochimica Acta 63 (2012) 256– 262 259

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ig. 4. Anodized nanopore arrays on flat aluminum surface and across the patterneda), (c) and (e) have 200 nm interpore distance, (b), (d) and (f) have 350 nm interpo

he flat aluminum surfaces as reported before [32,33] and acrosshe uneven features. For Fig. 4a, c and e, the interpore distance is00 nm. For Fig. 4b, d and f, the interpore distance is 350 nm. Aseported before, with FIB patterned concave guidance, the inter-ore distance can be varied within a large range while maintaininghe hexagonal arrangement [32,33]. In Fig. 4, the nanopore arraysre well maintained across different uneven surfaces, a newly foundapability for FIB guided anodization. The pore sizes are the same,95 nm, about 30% growth after the anodization (considering theatterned concave size at 75 nm). This newfound capability ofIB patterning guided anodization across varying uneven surfacespens numerous possibilities to pattern non-ideal surfaces for var-ous applications. Another observation is about the pore shape. Inll the images, when a pore is on a flat surface, the pore shape isound; when a pore is situated at a sloped location, it transitionsnto an elongated, tear-drop shaped pore. The elongation of theore is dependent on the slope. This phenomenon is most obviouscross the feature edges. This is because the pore is stretched in theepth direction (also shown in Fig. 3). If the pore is projected on

two dimensional surface, the pore shape should still be round asesigned.

It should be noticed that around each nanopore, no cell bound-ries (the oxidized layer around the anodized pores) are observedor the pores in Fig. 4a, c and e; the anodized surfaces appearmooth; the feature edges have a higher contrast. In Fig. 4b, d and f,owever, hexagonal cell boundaries are clearly visible around eachanopore. When a pore is stretched to become elongtaed becausef the sloped aluminum surface, the oxide cell boundaries are alsotretched. Similarly, the feature edges also have a higher contrast.

he cell boundary phenomenon was explained before and is relatedo the interpore distance and oxide layer growth [32,33]. Here, its important to point out that the pore development on unevenurfaces has no effect on the oxide layer growth. Pores might be

en features: (a) and (b) hemisphere, (c) and (d) square pit, and (e) and (f) stair-case.ance.

elongated but the oxide layers around them still form. The vast FIBpatterning abilites developed for flat aluminum surfaces [25–33]can be equally applied to curved surfaces. This drastically increasesthe ability of FIB patterning guided anodization and novel tem-plates can be created using this approach. As a result, many newnanorod, nanowire, nanotube, and nanodot arrays can be gener-ated. From a different perspective, this result also demonstratesthe effectiveness of FIB patterning guided anodization. For self-organized anodization, ordered pores cannot be maintained evenon flat surfaces.

Comparing the edge and flat locations in Fig. 4, it appears thata sloped surface has more room for the alumina layer growth dur-ing the anodization and thus more protruded surfaces are formed toaccommendate the volume expansion. In addition, the larger inter-pore distance patterns (Fig. 4b, d and f) show less feature patterningeffect after the anodization. For example, the pores in the diagonaldirections for the square pit are less distinguishable from others(Fig. 4d). The lower stair-case edge for the stripe pattern is hardlyvisible (Fig. 4f).

So far, FIB patterning guided anodization has been mainly con-ducted in phosphoric acid electrolyte. To understand electrolyteeffect on the ordered nanopore array development during the FIBguided anodization, the aluminum foils have also been anodized inoxalic acid electrolyte. The results are shown in Fig. 5. To demon-strate different abilities of ordered nanopore array development,the hemisphere feature is replaced with a square feature with agradually changing slope. The feature in Fig. 5a is a square with3 �m edge length. The largest depth on the right side of the squareis ∼0.3 �m, so the slope is 0.1. The time used to create the fea-

ture is 111 s. For the diagonally gradient square feature (Fig. 5b),the edge length is 3 �m, the depth is ∼0.3 �m, and the FIB time is128 s. For the stair-case feature (Fig. 5c), the edge length is 3 �m,the depth is ∼0.3 �m, and the FIB patterning time is 104 s. In all

260 K. Lu / Electrochimica Acta 63 (2012) 256– 262

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earlier for the 425 nm interpore distance pattern, at ∼200 nmdepth, vs. 1.5 �m for the 250 nm interpore distance pores. Thisis consistent with the interpore distance effect. From a different

ig. 5. Hexagonally arranged nanopore arrays across different features after the anit with slope change in diagonal directions and (c) stair-case.

ases, the depths of the features are purposely designed to be shal-ower in order to illustrate the FIB patterning ability. For the oxaliccid electrolyte, the interpore distance for FIB guided anodiza-ion can be expanded to a wider range (100–200 nm) even thought cannot be as large as that for the phosphoric acid electrolyte250–500 nm) [30–33]. In Fig. 5, the interpore distance is 125 nmnd the FIB patterned concave diameter and depth are again 75 nmnd 50 nm, respectively. The FIB patterning time for each feature is4 s.

Fig. 5 shows that for the oxalic acid electrolyte, the FIB pat-erning guided anodization can still effectively guide the nanoporeevelopment across uneven aluminum surfaces. The pore sizesre again 90 nm. Different from the phosphoric acid electrolyte,he smaller interpore distance in Fig. 5 leads to much thinneralls between the pores. As a result, no cell boundaries are vis-

ile. Compared to the flat aluminum surface, the pores on theatterned features seem to be more developed. Beneath the top

ayer, the pores appear to be not straight because the local sur-ace curvature leads to the bending of nanopore growth direction39–42]. With the small slopes across these features and a lessxtent of anodization from the oxalic acid electrolyte, the poresre generally round; the feature characteristics are more visi-le (diagonal direction depth, stair case edges). Nonetheless, theifferences between Figs. 4 and 5 are mainly a result of the elec-rolyte difference. The FIB patterning guidance effect remains theame.

.2. Interpore distance effect on pore depth growth

While the FIB patterning guidance has demonstrated desirableapabilities during the anodization, the effect on the nanoporeepth growth direction needs to be examined. In this study, theesults are shown for the phosphoric acid anodization samplesecause the large interpore distance allows clearer observation ofhe nanopore cross sections. The interpore distance effect on theore depth direction development is shown in Fig. 6. The two hemi-pheres have the same diameter in the horizontal direction, 5 �m,nd the same depth, ∼1.5 �m. The interpore distances are 250 nmnd 425 nm, respectively. After the anodization, the aluminum sur-ace has been coated with a Pt layer in order to preserve the integrityf the porous sample during the FIB cross-sectioning. As a result,he top surfaces in Fig. 6 appear differently from those in Fig. 4.

As shown in Fig. 6, all the nanopores grow vertically to theocal surface. For the regions away from the hemisphere features,

he pores grow in parallel and vertically from the top surface. The50 nm interpore distance sample shows better ability to main-ain the parallel growth of the pores. This is easy to understandince the smaller interpore distance hinders pore tilting or new

tion in oxalic acid electrolyte: (a) square pit with gradual slope change, (b) square

pore formation. For the nanopores in the hemisphere concaves,the growth direction is perpendicular to the tangential directionof the local surface. As a result, the interpore distance increaseswith the nanopore depth increase. When the pore depth is largeenough to reach the critical value for maintaining the organizedpores from the FIB patterning (∼500 nm), some nanopores splitinto two pores during the anodization. The pore splitting occurs

Fig. 6. Interpore distance effect on the pore depth development under concavesurfaces. The interpore distance is (a) 250 nm and (b) 425 nm.

K. Lu / Electrochimica Acta 63 (2012) 256– 262 261

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ig. 7. Effect of surface curvature on nanopore depth growth direction: (a) uniformoncave depth, (b) cross section image of anodized alumina nanopores of (a).

erspective, pore depth direction interaction at the hemisphereoundaries can be understood as follows. Since the pores out-ide of the patterned features grow vertically, the vertical poresnd the tilted pores inevitably meet in the pore depth direction.or the 250 nm interpore distance, this phenomenon occurs at aeeper location (4.4 �m). For the 425 nm interpore distance pat-ern, this occurs much earlier (2.1 �m). Again, this can be tracedack to the different degree of pore tilting for the concave features.ores with larger interpore distance have a higher degree of tilt-ng and thus earlier pore impingement. This also means that onhe same curvature surface, interpore distance affects pore split-ing, tilting, impingement, and thus interactions with neighboringores.

.3. Surface curvature effect on pore depth growth

Fig. 7 shows the effect of the aluminum surface curvaturen the nanopore depth growth. The nanopores are in hexago-al arrangement. Fig. 7a shows the top surface of a ridge with

�m × 8 �m feature size, and the largest depth of the feature is.2 �m. All the nanopores have the same size and depth at 75 nmnd 50 nm, respectively. Fig. 7b shows the corresponding anodizedanopore cross sections of Fig. 7a. Fig. 7b shows that for the curved

Fig. 8. Effect of surface curvature and patterned concave depth on nanopore growthdirection: (a) larger concave depth on the ridge, (b) cross section image of anodizedalumina nanopores of (a).

surface, the nanopores grow in the direction perpendicular to thetangential direction of the local surface under examination. Thismeans the nanopores bend outwards for the concave surface butinwards for the convex surface. As a result, the nanopores mergeunderneath the convex surfaces and split underneath the concavesurfaces.

There is an interplay between the guiding nanoconcave depthand the surface curvature. Fig. 8a has the same feature geometry asFig. 7a but with different nanoconcave depths. The nanoconcavesacross the ridge in Fig. 8b have a larger depth before the anodiza-tion, 50 nm. For the rest of the patterned concaves, the depth is10 nm. This feature is specifically created to understand whetherdeeper guiding nanoconcaves can effectively hinder the pore depthdirection tilting because of different surface curvatures. As seen inFig. 8b, even though the patterned nanoconcaves on the ridge isdeeper than the nanoconcaves at the groove bottoms, the nanoporegrowth direction still tilts. In future studies, it would be meaning-ful to find the threshold surface curvature for parallel nanopore

depth growth for a given feature with biased nanoconcave depth.Conversely, it would be interesting to understand how the guidingnanoconcave depth should be varied in order to overcome the poredepth growth direction tilting for known uneven surfaces.

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62 K. Lu / Electrochimic

. Conclusions

FIB patterning guided anodization has been used to produceexagonally ordered nanopore arrays with different interpore dis-ances on various uneven surfaces. The micron features provide anique opportunity to study the patterned nanopore growth duringhe subsequent anodization. Pore development in different elec-rolytes as well as with different interpore distances and featureurface curvatures show the same pore directing ability. In theanopore depth direction, pores grow perpendicularly to the tan-ential direction of the surface, which leads to pore splitting orermination based on the surface curvature.

cknowledgment

The authors acknowledge the financial support from Nationalcience Foundation under grant no. CMMI-0824741 and no. CMMI-969888.

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