nanotechnology series · nanotechnology series vol. 1: fundamentals and applications author: naveen...

NANOTECHNOLOGY SERIES VOL. 1: Fundamentals and Applications Author: Naveen Kumar Navani, Shishir Sinha & J.N. Govil Year: 2013 ISBN: 1-62699-001-8 Pages: 499 Binding: HB Language: English Publisher: Studium Press LLC Price: US$ 110

About the Volume

This first volume in the nanotechnology series “Fundamentals and Application” gathers and presents the principal concepts of nanotechnology and its diverse and multidisciplinary field of research and emerging applications. The initial chapter bestows us with the introduction to nanoscience and nanobiotechnology which deals with the brief history of naturally existing nanomaterial as an inspiration of nature to design engineered nanomaterials with broad-spectrum properties. Owing to its nano size, this technology has fashioned enormous products offering advantages over the conventional ones; ranging from the materials (nanofuel cells, catalyst, lubricant, aeronautics, automobiles, telecommunications, energy production, mechanics, biology, medicine, etc.); electronics and information technology (nanoelctronics, solar cell, single electron transistors, high sensitivity sensors, etc.); machines (nano engines, nano pumps, nanopropeller, pharmaceutical processes, in space technology, defense and ships, etc.); Life Science (molecular medicine, bioprocessing, agricultural systems, medical surgery, neural surgery, ecotoxicology, molecular imaging, delivery systems, etc). Many scientists and professionals have extensively admired and got inspired by the

characteristics of nanomaterials, their broad range of existing applications and future prospects for improved lifestyle. While some of them have also addressed the risk of nanomaterials being hazardous to human and environment owing to its “NANO” nature, making them highly reactive and in some cases even capable of crossing the blood brain barrier. So as it true for any new and “promising” technology; before being used for any human welfare; they should be tested for its non-target harmful effects. This can be achieved by employing risk/safety and management studies of nanomaterials. The target of this volume is to cultivate interest amongst inter disciplinary students, researchers, scientist and professional in academics not limited to the area of physics, chemistry, medicine, biology, defence, civil, energy and environment, information technology, healthcare drug discovery and electronics.

Contents VOLUME 1: FUNDAMENTALS AND APPLICATIONS: 1.The Nanotechnologies World: Introduction, Applications and Modelling: PAOLO DI SIA (ITALY); 2. Natural and Engineered Nanomaterials: Fundamental Concepts and Applications: H.C.VASCONCELOS AND M. CLARA GONÇALVES (PORTUGAL); 3. Applications of Nanofluids: SANJEEVA WITHARANA (U.K); 4. Applications of Nanomaterials: PRASANTA DHAK AND DEBASIS DHAK (INDIA); 5. Risk Management and Nanomaterials: PAUL SWUSTE AND DAVID ZALK (NETHERLANDS, USA); 6. Nanotechnology Products: RUCHITA JAISWANI, KHOMENDRA KUMAR SARWA, MANABENDRA DEBNATH AND BIPLAB DE (INDIA): 7. Fabrication, Properties, and Application of Porous Anodic Alumina Templates: KAILIN LONG, XIANZHONG LANG, TENG QIU AND PAUL K. CHU (CHINA); 8. Applications of Ultra/Nanocrystalline Diamond Films: A.F. AZEVEDO, M.R. BALDAN AND N.G. FERREIRA (BRAZIL); 9. Application of Nanofluid in Thermosyphon (TPCT) A–Review: T. PARAMETTHANUWAT AND N. BHUWAKIETKUMJOHN (THAILAND); 10. Non-Volatile Memory Devices: SUBARNA MITRA AND SOUMEN DAS (INDIA, KOREA); 11. Growth and Modification of Nanostructured Thin Films: Fundamental and Application Aspects: SANTANU GHOSH (INDIA); 12. The Theory of Quasi One-Dimensional and Two-Dimensional Polaron Structures: VLADIMIR K. MUKHOMOROV (RUSSIA); 13. Second Order Sliding Mode Control of MIMO Coupled Uncertain System: BENAMOR ANOUAR, LARBI CHRIFI ALAOUI, HASSANI MASSAOUD AND MOHAMED CHAABANE (FRANCE, TUNISIA); 14. Synthesis and Applications of Electrospun Nanofibers-A review: P.K. PANDA AND B. SAHOO (INDIA); 15. Applications of Nanotechnology in Plant Nutrition: SOMEN ACHARYA, B.C. VERMA, A.K. BAJPAI AND R.B. SRIVASTAVA (INDIA); 16. In-vitro Applications of Nanomaterials for Plants: VINOD SAHARAN, AJAY PAL, RAM C. YADAV, SAVITA BUDANIA AND R.A. KAUSHIK (INDIA); Appendix-I: Table of Contents of Other Volumes of the Series Vols. 2 to 10; Subject Index: Preface of Volume 1: Fundamentals and Applications Nanotechnology is delivering its benefits to society in different sectors like information technology, defence, healthcare, energy and environment, and civil/construction among many others. This volume 1 of a 12 volumes series on nanotechnology comprises articles on fundamentals of nanotechnology and its applications in various areas of life. The volume 1 begins with an introduction to the applications of nanotechnology in the actual technological and scientific world, considering also the theoretical modelling, with which it is possible to test the experimental data of the sector and to predict new features. The chapter 2 presents fundamental concepts and applications of natural and engineered nanomaterials. The chapter covers the fundamentals of nanoscience and nanotechnology, general features of nanomaterials such as size, law of the scales, size dependent properties, brief history of nanotechnology, top-down and bottom-up approaches; an overview of natural nanomaterials, their properties and functions; engineered nanomaterials; applications of the nanomaterials in medicine, environment, electronics and communication technologies; and a brief reflection about nanomaterials and their toxicity in five well structured sections. The chapter 3 is primarily concerned with the applications of nanofluids. Nanofluids are stable solid - liquid mixtures containing suspended solid particles of size less than 100 nm in one dimension. Nanofluids have favorable thermal properties at negligibly small nanoparticle concentrations and, thus, may lead to massive energy savings in heating and cooling applications. The chapter discusses the different issues, like motion of suspended particles and formation of particle clusters that require attention of researchers before successful commercial applications of nanofluids. The chapter 4 discusses the applications of nanomaterials from nanoscale electronics and optics to nanobiological systems and nanomedicine. The chapter also discusses few applications of most significant nanomaterials like gold nanoparticles, iron oxides, quantum dots, fullerenes, carbon nanotubes, nanowires, nanosized metal oxides etc. which are currently under intensive research. The chapter 5 covers a very crucial issue of risk management associated with emission and exposure to these nanomaterials. Risk management of nanomaterials is a field with many uncertainties mainly because of absence of much quantitative information. Still, there are few qualitative tools and methods to assist risk management decisions. The control banding nanotool and

the method design analysis has been discussed in details. The chapter 6 provides an extensive coverage of sciences currently covered by nanotechnology. It also covers various nanotechnology based products currently available in market. Due to their small size, nano-products offer several advantages over conventional products, like effective targeting of difficult–to–reach sites, improved solubility and reduced adverse effects. The possible future developments and safety issues have also been discussed. The chapter 7 presents a detailed discussion on the fabrication, properties and applications of porous anodic alumina (PAA) templates for synthesis of nanostructured materials with big areas, large aspect ratios, and uniform dimensions. The chapter discusses the comparative edge of the technology over the conventional lithographic method. Recent work on the structure of PAA templates and use of PAA to fabricate alumina–based and embedded nanostructures has also been covered. The chapter 8 offers an overview on the current development status of ultra/nanocrystalline diamond films for a variety of applications. The author has summarized various nanodiamond preparation methods, followed by a comparison between the un-doped, boron-doped and nitrogen-doped ultra/nanocrystalline diamond films. Finally, the practical applications of these films have been discussed. The chapter 9 focuses on thermal performance of nanofluids applied as a working fluid in a two-phase closed thermosyphon. The theories for investigating nanofluid properties such as the thermal conductivity, rheological behavior, etc. have been highlighted. The chapter 10 provides a review on the basic operation, electrical characterization, and applications of nanomaterials as the active components for non-volatile memory devices. The many impediments that are hindering the growth of such memory devices and the future progress and development of such devices have also been discussed. The chapter 11 describes the growth and modification of various oxide, nitrides, and metal insulator nanocomposite thin films. These nanostructured films have significantly improved the physical properties of ‘thin films’ and a rapid development has taken place in almost all industrial areas including biology and medicine. Various physical properties emphasizing magnetotransport properties of these films have been discussed in a comprehensive manner. The chapter 12 delves into theory of quasi one-dimensional and two-dimensional polaron structures. V. K. Mukhomorov has shown that specific interpolaron interaction into nano-gap leads to results that are basically different from those observed for electron systems. Quasi one- and two- dimensional electron structures are of applied interest. For example, in one-dimensional nanocapillary electroneutral metal–ammonia systems, electrical conductivity resembles the superconductivity transition with decreasing temperature. In chapter 13, the second order sliding mode control (SOSMC) to coupled multi-input multi-output (MIMO) system is presented. The proposed method can be applied to a large class of nonlinear coupled MIMO processes affected by parameter uncertainties and disturbances. The chapter 14 presents a detailed review on synthesis and applications of electrospun polymer and ceramic nanofibers. These nanofibers have attracted the attention of researchers due to high surface area to volume ratio leading to enhanced micro and nanostructural characteristics. The chapter 15 discusses the role of nanotechnology in agriculture sector by increasing productivity through soil and water conservation. The smaller size, higher specific surface area and reactivity of nano-fertilizers compared to bulk one may increase the solubility, diffusion and hence availability to plants and enhance crop productivity. The environmental risks of nanoparticles demand thorough understanding of the fundamental reaction pathways and proper toxicity study so as to decide their optimum dose to ensure safer and more sustainable use in agriculture. The chapter 16 mainly throws information regarding possible use of nanomaterials in plant tissue culture. Plant tissue culture is playing a vital role in making agriculture an industry. The author argues that in-vitro contamination, uncontrolled exposure of media components, in-vitro imaging tools, delivery of nucleic acids in-vitro are some of the major area where novel tools of nanotechnology could be utilized. This volume gives an introduction of the nano world along with articles on natural and engineered nanomaterials, applications of nanofluids and nanomaterials, risk management and nanomaterials, electrospun nanofibers, and nanotechnology based products. We are much indebted to all contributing academicians and researchers who enthusiastically accepted our request, and made great efforts to write chapters for a wide audience. The length of these chapters varies considerably depending on the topic. Some of them have the appearance of a small book. Their authors deserve special thanks for their painstaking efforts and generosity in choosing to publish their work in this series. We also thank the referees for their hard work to ensure the high quality of the chapters. The NANOTECHNOLOGY is a comprehensive compilation of research and review articles that pertain to nanomaterials, from a consideration of their methods of preparation, their novel properties, and areas of their utilization. The series is believed to be of interest to engineers, scientists, and technologists in academic institutions, research laboratories, and industry. It is a befitting introduction of the subject of nanotechnology to the students as well as a mean to provide them an up-to-date review of recent innovations in the field, all in one place.

1 Department of Physics, Southeast University, Nanjing 211189, China,2 Department of Physics and Materials Science, City University of Hong Kong, Tat Chee

Avenue, Kowloon, Hong Kong, China*Corresponding author: E-mail: [email protected], [email protected]

7

Fabrication, Properties and Applicationsof Porous Anodic Alumina TemplatesKAILIN LONG1, XIANZHONG LANG1, TENG QIU1* AND PAUL K. CHU2*

ABSTRACT

There has been increasing attention to the fabrication of nanometer–sizedstructures and materials because of their potential applications to electronics,optics, optoelectronics, and biotechnology. Many methods have been designedto fabricate nanostructured materials and one simple and convenientapproach is to employ a porous anodic alumina (PAA) membrane consistingof intrinsic nanometer–sized channels. In comparison with conventionallithographic means, the low–cost PAA which can be used to synthesizenanomaterials with big areas, large aspect ratios, and uniform dimensionsis very promising. In this chapter, recent work on the structure of PAAtemplates and use of PAA to fabricate alumina–based and embeddednanostructures is reviewed.

Key words: Porous anodic alumina, Structure, Properties, Two–stepanodization, Nanostructures, Applications

1. INTRODUCTION

Porous anodic alumina (PAA) has attracted the interest of researchersworking in nanoscale science and technology and many methods have beendesigned to fabricate nanostructured materials[1–10]. From the viewpoint ofcommercial applications and fundamental research, PAA membranes witha highly ordered nanopore array have many advantages. In this respect,

212 Nanotechnology Vol. 1: Fundamentals and Applications

efforts have been made to improve the arrangement of the pore patterns,[9,11]

for instance by optimizing the experimental conditions such as temperatureand pre–texturing to initiate pore growth on the aluminum surface andenhance the order of the patterns[12,13]. To produce highly ordered PAA films,a two–step anodization process in conjunction with pre–texturing have beenproposed by Masuda and co–workers[6,11] .

PAA membranes are quite versatile and can be used to fabricate varioustypes of low–dimensional nanostructures including nanodots[6] nanowires[14],and nanotubes[15]. These nanostructures with the appropriate dimensionsexhibit unique optical[16] and magnetic properties. Moreover, PAA has beenwidely used as anticorrosion or decoration coatings to improve themechanical properties of aluminum[17] and spurred by the rapid developmentof biotechnology, PAA membranes have recently been explored from theperspective of biosensing[18]. In this chapter, recent work on the fabrication,properties, and application of PAA templates is reviewed.

2. STRUCTURES OF POROUS ANODIC ALUMINA MEMBRANES

2.1. Geometrical Structure

The structure and formation mechanism of PAA membranes have beeninvestigated since 1932[19]. Using electron microscopy, Keller et al. foundthat PAA membranes consisted of close–packed cells of aluminapredominately hexagonal in shape and each cell contained a singlenanopore[1]. This structure is schematically displayed in Fig. 1[20]. The basepattern consists of spherical sections less than a hemisphere and therefore,the radius of curvature and location of the center of curvature can bedetermined. The height and width of the pore base can be measured by thefollowing formula: R=H/2+W2/8H, where R, H, and W are the radius of thecurvature, height, and width of the nanostructure, respectively[1] Kellerand co–workers studied the structure of the PAA membranes formed atdifferent anodic voltages and found that the cell width depended on theapplied voltage[1]. The anodizing ratio of Å/V was determined to be about 1nm/V and only small changes were observed from different electrolytes.They also investigated another parameter, the pore volume defined as thepercentage volume of the film occupied by pores, in different PAAmembranes. If the nanopores are considered to be perfect cylinders, it shouldnot depend on the membrane thickness, electrolyte concentration,temperature, electrolyte types, and anodic current density. However, ithas been demonstrated that the pore volume is a function of all four ofthese parameters.

Later experiments conducted by Wood and O’Sullivan[21] confirm thegeometrical structure presented by Keller[1]. According to their results, the

213Applications of Porous Anodic Alumina Templates

pore wall thickness is always 0.71 of the barrier layer thickness and so thepore diameter is given by p = c–2 0.71d, where p is the pore diameter, dthe barrier layer thickness, and c the cell diameter. They found that thepore diameter and all the pertinent parameters were directly proportionalto the applied anodic voltage by neglecting the effects of subsequent porewidening by prolonged contact between the PAA membranes and electrolyte.This simplified matters leading to a simple, integrated model of film growthwhich had been verified by others[22,23]. Li et al. found that although the celldiameter depended mainly on the applied anodic voltage, the electrolytetype had only a small influence on the cell diameter[8]. The electrolyteconcentration and temperature also affected the dimensions of the PAAmembranes. In the constant voltage anodizing process, the barrier layerthickness decreased slightly at high temperature. In fact, a larger electrolyteconcentration affected the barrier layer more so than the temperature.However, very little change in the cell diameter could be detected withtemperature[21]. The diameter of the nanopore appeared to vary with depths,and deeper penetration into the PAA was observed only in a highlyconcentrated electrolyte due to pore enlargement[21].

The structural features of PAA such as the cell diameter, interporedistance, and pore arrangement depend very much on the chosenelectrolyte[24]. A two–step anodization process is usually performed in a

Fig. 1: Schematic diagram of fabricating PAA films. Reprinted with permission fromRef. [20]. Copyright 2010 by Elsevier


sulfuric[25–27] oxalic[11], or phosphoric acid solutions[28,29] and nanostructureswith fine pores and the highest pore density can be obtained by anodizing insulfuric acid[25] Anodization is carried out in phosphoric acid to produce PAAmembranes with the largest pore diameters and interpore distances[30]. Byselecting the appropriate electrolyte and applied potential, anodizationproduces PAA with a pore diameter from 10 to over 300 nm[24] and thedepth of the nanoporous channels in the PAA membranes can be adjustedto be between a few to hundreds of micrometers by changing the anodizationtime.

2.2. Pore Formation Mechanism

Many models have been proposed to account for the pore formationmechanism in PAA. The most acceptable model at present is the field–assisted dissolution model[21,31,32]. When the power is switched on, an electricfield builds up on the surface of the aluminum sheet. Oxygen containingions (O2–/OH–) migrate from the electrolyte to the surface of the membraneto form alumina[7,21,32,33]. The thickness of the evolving barrier layer is notuniform and nuclei or spots of oxide can be observed on the aluminumsurface at the beginning of anodization[21]. Afterwards, the size of the nucleiincreases and finally the spots merge. In the ensuing steady–state nanoporeformation stage in which an equilibrium exists between field–assisteddissolution of the alumina at the electrolyte/alumina interface and aluminaformation at the alumina/aluminum interface, nanopores grow basicallyperpendicular to the membrane surface[21]. A schematic of the processes is

Fig. 2: Elementary processes in nanopore growth showing the ionic species movementin the alumina barrier layer. Reprinted with permission from Ref.[31]. Copyright1992 by IOP Publishing Ltd.


presented in Fig. 2. The alumina grows at the electrolyte/oxide interface asa result of the outward migration of Al3+ ions which drift through the oxidelayer and react with oxygen containing ions[32]. However, oxide dissolutiontakes place at this interface via a field–assisted reaction between theelectrolyte and alumina surface because the dissolution rate is faster thanthat of formation.

As aluminum is anodized, a nonporous layer forms in a near–neutralelectrolyte such as boric acid or ammonium tetraborate. Al2O3 is dissolvedaccompanied by dissociation of water at the electrolyte/oxide interface asindicated by A, B, C in Fig. 3(a).

Al2O3 + nH2O ® Al3+ + O2 –+ OH– + H+ (1)

Here, the ratio of O2– to OH– cannot be determined and n is used toindicate the molar ratio of dissolution of Al2O3 and dissociation of water[34].

In an electric field, protons and Al3+ leave the solid surface into theelectrolyte immediately, while the O2– and OH– anions migrate to the oxide/metal interface to form Al2O3, as indicated by A, B, and C in Fig. 3(a).Reaction (1) reduces the thickness of the oxide layer, while oxidation ofaluminum increases the layer thickness. The layer thickness is determinedby the electrolyte types on which the dissolution rate of Al2O3 depends strongly.In reaction (1), dissolution of Al2O3 is very slow in a near–neutral solutionand hence, the thickness of the oxide layer increases continuously as long asfield–assisted anion migration occurs. On the other hand, the electric fieldstrength can be written as E = U/d, where U is the voltage applied to theoxide layer with a thickness d. Therefore, E diminishes as d increasesconsequently reducing the migration rate of anions. Oxidation ceaseseventually when d approaches a critical value, dC while the correspondingelectric field strength, EC = U/dC, is too weak to drive the anions through theoxide barrier layer. Finally, a uniform thickness (dC) and constant electricfield strength (EC) result on the entire area of the barrier layer[34]. Theexperimentally observed U/dC value is about 0.7 V nm–1 for a near–neutralelectrolyte[17]. This is the simplest concept of the equifield strength model.

When aluminum is anodized in an acidic solution, dissolution of Al2O3is fast. When the oxide layer is thick, reaction (1) dominates and thethickness of the oxide layer tends to decrease. When the oxide is thin, theoxidation rate increases due to enhancement by the electric field. Finally,dissolution and anodization reach a balance and the equilibrium oxidelayer has a constant thickness (dB) which is smaller than dC. Thecorresponding field strength EB is greater than EC. However, the dissolutionrate of Al2O3 on the entire surface is normally different at nanometerscale lengths. Defects such as impurities, dislocations, grain boundaries,nonmetallic inclusions, and the rough surface of the original oxide layercan induce growth of pits in the initial step of pore growth (Fig. 3(b))[35]. It


is well known that when the oxide layer on the pit bottom is thinner, theoxidation rate is larger. The shape of the interface A B C tend to replicatethat of A B C and the hemispherical shape, as often observed from PAAand it is the only morphology which can achieve a uniform thickness andconsequently equal field strength in the whole area (Fig. 3(c))[34]. Sincethe field strength along D D or E E on the wall of the pore is the same asthat on the pore bottom, the oxide layer can migrate not only downwardsbut also along the side. When two pores are separated as shown in Fig.3(e), they will expand and neighboring walls will move towards each otheruntil the two walls merge becoming a combined one with a thickness of2dB (Fig. 3(f)). The pores then further move to each other because theoxidation rate at the joint position B of the hemispheres must be muchhigher than that at any other position because oxygen anions migratefrom both sides. The joint position of the pore base then moves down to‘‘D’’ as shown in Fig. 3(f) and in this case, the wall thickness 2dW betweenthe pores is smaller than 2dB (Fig. 3(g)).

Fig. 3: Schematic diagrams of the electric-field strength distribution in some typicaloxide barrier layers with the electrolyte/oxide interface marked by A, B, C andthe oxide/metal interface marked by A, B, C: (a) Planar oxide layer with auniform thickness. (b) Planar layer with a corrosive pit. (c) Surface of a corrosivepit at the electrolyte/oxide interface is replicated at the oxide/metal interface.(d) Formation of the hemispherical pore base and a cylindrical wall of a singlepore. (e) Two pores have a separation larger than 2dB. (f) Pores move towardseach other to achieve a wall thickness of 2dB. (g) Pores move closer with abalanced curvature of 2<180º. (h) Two pores are too close to each other and (i)Self-adjustment to increase the wall thickness. Reprinted with permissionfrom Ref.[34]. Copyright 2008 by WILEY-VCH Verlag GmbH & Co. KGaA,Weinheim


When two pores are too close, i.e., where d < 2dW, the pores will moveapart to increase the wall thickness as illustrated in Fig. 3(h) and (i). Thethickness of any positions in the range of A B to B is larger than dB, leadingto weaker field strength. The field–assisted oxide dissolution rate at thesepositions becomes smaller than that at positions B and below. Accumulationtakes place in this area and the wall between pores increase until the wallthickness approaches the equilibrium value of 2dW.

2.3. Order and Arrangements of Pores

Characterization of the morphology of PAA shows that under theappropriate conditions, hexagonal nanopore arrays are obtained[8], as shownin Figs. 4 and 5. A perfect hexagonal arrangement is observed in thedomain, and each pore bottom is surrounded by six white apexes as shownin Fig. 4[36]. The hexagonal nanopore arrays are formed in domains of afew micrometers separated from neighboring domains of the pore latticewith different orientation and by domain boundaries as shown in Fig. 5.That is, the nanopore arrays are analogous to the two–dimensionalpolycrystalline structures[37]. Some defects can be observed from thesegrain boundaries. Compared to PAA with disordered nanopores, PAA withordered nanopore arrays have attracted increasing attention in recentyears due to the use as templates for nanostructures in magnetic,electronic, and optoelectronic devices[38–40]. Therefore, the self–organizationprocess which forms the ordered nanopore arrays has been intensivelyinvestigated in order to fathom the ordering mechanism of the nanoporearrays and identify methods to improve the ordering. The results aresummarized in this section.

Fig. 4: Top view of the scallop shaped aluminum template seen after removal of theapproximately 30 m thick porous oxide layer by selective etching. Perfecthexagonal order is observed in the domain, and each pore bottom is surroundedby six white apexes. Reprinted with permission from Ref.[36]. Copyright 2011by American Institute of Physics.


To quantitatively evaluate the ordering quality of different porousalumina patterns, the coordinates of the pore centers in each pattern arecaptured by the Image J software as proposed by Hillebrand et al.[41] Threeanalytical methods have been used to quantify the pore ordering as follows[42].

1. Radial distribution function (RDF)The two–dimensional RDF is defined as

dr

rdn

rN

SRDF pattern )(

2

where Spattern is the pattern area, r is the distance between the centers ofany two pores in the pattern, N is the total number of pore pairs, and n(r) isthe number of pore pairs in which the pores are separated by a distance £ r.The RDF yields the probability of finding a neighboring pore at a distance raway from any given pore in the pattern.

2. Angle distribution function (ADF)In a porous pattern, three nearest neighbor pores form one triangle, and sothe whole pattern can be represented as a mesh of triangles with the porecenters as the mesh nodes. In order to avoid unwanted side effects, thenearest neighbors of a given pore are found in a region around that porecenter which is less than 1.8 times of the first RDF peak position, and eachangle of a triangle should be in the range of 30° to 90° otherwise a triangle

Fig. 5: SEM image of PAA with ordered nanopore array. The PAA is formed in 0.3 Msulfuric acid at 25 V. Reprinted with permission from Ref.[8]. Copyright 1998 byAmerican Institute of Physics


is not formed. Sometimes, in a disordered pattern, edges from two differenttriangles may intersect each other and in this case, the triangle whosethree edges have a smaller standard deviation (Dev_d) is chosen. For atriangle, Dev_d is defined as[41,43]

23

13

11_

i i dd

ddDev

where di (i = 1, 2, 3) represents the edge lengths of that triangle. Theangles of all the formed triangles are statistically evaluated to give theADF which represents the probability of finding a particular angle value inthe pattern[41].

3. Angular orientation distribution (AOD)RDF and ADF are helpful when the difference in the ordering quality of theporous patterns is large. However, they are not sensitive enough to discernpatterns with close but different ordering quality. A more sensitive methodbased on the orientation of the triangles formed in the ADF representationis proposed. For any given triangle in the ADF representation as describedabove, its orientation q in the scanning electron microscopy (SEM) imageas the reference direction can be calculated as

180tantantan

3

1

13

13

32

32

21

21

xx

yya

xx

yya

xx

yya

where (xi, yi), i = 1, 2, 3, are the coordinates of three vertices of the triangle.Most of the triangles in a mildly disordered pattern usually have qcalculated from the equation in the range [–30°,30°], but if q exceeds thisrange, it is reduced to be within [–30°,30°] by adding or subtracting 60°,because the mesh structure comprises a hexagonal arrangement of thetriangles.

There is evidence that ordered nanopore arrays can only be obtainedusing the appropriate experimental conditions. The anodic voltage andconcentration of the electrolyte are the most important parameters inanodization. Different research groups have attempted to identify the mostappropriate parameters and some of the optimal anodization parametersare summarized in Table 1[8].

In addition to the effects of the applied anodic voltage and electrolyteconcentration, the existence of a smooth etching front and grain boundariesin the Al sheets can influence self–organization. Therefore, a pretreatmentsuch as annealing are commonly adopted prior to anodization. Thistreatment can yield alumina sheets with a smooth surface and grain sizeson the order of 100 m. Larger grain sizes improve the homogeneity and


preventing self–organization. It is thus difficult to obtain ordered nanoporearrays on aluminum sheets without electrochemical polishing[9]. It is alsonecessary to stir the electrolyte during oxidation because the electrolytecomposition in the nanopores depends on stirring which promotes exchangeof the electrolytes along the nanopores and removes bubbles formed on thesurface of the PAA thus providing a spatially homogenous etchingenvironment. Masuda et al.[6,12,45] found that the anodization time impactedthe ordering of the nanopore array. Self–organization becomes morepronounced with time and as a result, the size of the defect–free domainsincreases with time. Prolonged anodization rearranges the cells and reducesthe number of defects and dislocations and this tendency does not saturateeven after 60 h[46], although the size of the defect–free domain is notproportional to the anodization time.

Ordering of the nanopores is generally thought to be related to stress atthe alumina/aluminum interface arising from volume expansion[8,9,46]. Whenaluminum is oxidized to form alumina, the volume expands since the atomicdensity of aluminum in alumina is smaller than that in metallic aluminum[8].Jessensky et al.[44] investigated the volume expansion of PAA formed atdifferent anodic voltages during oxide growth by measuring the step heightbetween the aluminum surface and that at the edge of the anodized regionwith a mechanical profiler. The magnitude of volume expansion increaseswith higher anodic voltages. Based on the morphology of the PAA fabricatedunder different conditions, the best ordered nanopore arrays are producedwhen expansion is moderate. On the contrary, in cases of both large volumecontraction and expansion, no ordered structures can be obtained. Duringvolume contraction, there is no repulsive force between the nanopores,

Table 1: Some optimal experiment parameters for PAA templates with ordered nanoporearray. Ratios of the thickness of formed alumina layer to the thickness ofconsumed aluminum under these experiment conditions are also indicated.Reprinted with permission from Ref.[8]. Copyright 1998 by American Instituteof Physics

Electrolyte Concentration Voltage Temperature Thickness(acid) (wt. %) (V) (ºC) ratio

Oxalic 2.7 40 1 1.42Sulfuric 20.0 19 1 1.41

1.7 25 10 1.401.7 25 1 1.36

Phosphoric 10.0 160 3 1.45

promote ordering[9]. Surface roughness is another important factor as arough surface expedites the formation of the alumina barrier layer andnanopores at the concave regions on the surface. The rough surface istransferred to the etching front at the alumina/aluminum interface thereby


whereas excessively large volume expansion may produce structural defectsand irregular pore growth. A detailed analysis was conducted by Li et al.[8].As shown in Table I, in spite of the different conditions, the relative aluminathickness ratios, i.e., volume expansion, should be very close to 1.4 in orderto obtain ordered nanopore arrays. Therefore, the best ordering can beachieved at a moderate expansion factor of 1.4 and it is independent of thetype of electrolytes used.

In addition, external tensile stress applied to the aluminum sheet mayalso influence the nanopore ordering. A regular arrangement of thenanopores has been observed at a relatively low external stress, but a hightensile stress completely destroys the ordered arrangement as manifestedby huge holes and pits on the PAA surface. The amount of defects increaseswith applied tensile stress[47].

Generally, self–organization can produce defect–free domains with sizesover 5 m[45]. In order to achieve a larger and perfectly ordered domain,pre–treatment or pre–texturing techniques have been proposed[6,11,13,48–51].Development of the nanopores can be initiated by the appropriate surfacetexture in the initial stage of anodization. Masuda and co–workers proposeda pre–texturing process using textured SiC with a hexagonal array to produceordered patterns on the aluminum sheet by mechanical indentation priorto anodization[11]. The long–range–ordered nanopore array with dimensionson the order of millimeters or even centimeters and a pore density of 1010

cm–2 has been produced. This process is sufficient for the mass productionof a restricted ordered nanopore array with a large aspect ratio.

These pre–texturing treatments are generally expensive and Masuda etal.[13]. have developed a two–step anodizing process. The first anodizationstep is carried out for a long time to improve the pore arrangement on thegrowth front. The alumina layer formed in this first step is then removedin a mixture of phosphoric acid (6 wt. %) and chromic acid (1.8 wt. %) at60 °C. Afterwards, a textured concave pattern is produced on the aluminumsubstrate and it is anodized again under the same conditions. Nanoporesare produced on the bottom of the concave features and finally the orderednanopores. This technique which is essentially a pre–texturing process isvery simple and suitable as a template for the preparation of nanodot/nanowire arrays in a large area. The technique has thus been widely adopted.

2.4. Influence of Conditions

PAA is a widely used template in nanofabrication. The structural features ofPAA such as the pore diameter, interpore distance, porosity, pore density canbe controlled by selecting the proper anodization conditions. The most importantfactor that determines the type of anodic alumina membranes formed is theelectrolyte[17]. An electrolyte in which alumina is insoluble can produce a barrier–


type anodic alumina membrane. Examples of this kind of electrolytes includeboric acid, aqueous ammonium borate or tartrate (pH 5–7), ammoniumtetraborate in ethylene glycol, and several organic electrolytes such as citric,malic, and glycolic acids. However, in a strongly acidic environment, theseelectrolytes do not form nonporous barrier membranes completely[17]. On theother hand, electrolytes in which the alumina membrane is slightly solublelead to the formation of porous–type anodic alumina membranes. Examples ofthis type of electrolytes include sulfuric, phosphoric, chromic, and oxalic acids.Anodization of an aluminum film in an alkaline solution such as NaOH alsoproduces a porous anodic alumina membrane but PAA templates formed inacidic solutions usually have better quality[52]. Therefore, an acidic solution isadopted in most experiments. The anodic porous alumina templates are usuallyobtained by self–organized two–step anodization in sulfuric acid[14–16,18,53–56] oxalicacid[57–60], or phosphoric acid[17,57,58] (see Fig. 6).

Fig. 6: Summary of pore diameters and corresponding interpore distances in theordered PAA anodized under mild conditions. Reprinted with permission fromRef.[61].Copyright 2012 by Elsevier.

The electrolyte concentration imposes a larger effect. The barrier layerthickness is reduced from 100 nm in a 0.4 M solution to 30 nm in a 2.5 Msolution. In fact, increasing the electrolyte concentration has a much greatereffect on the barrier layer than temperature. The trend stems from thehigher dissolution rate under the electric field at the electrolyte/aluminainterface with increased electrolyte concentration and temperature[21]. Abigger increase in the nanopore diameter is observed at the film surfacedue to chemical dissolution of the PAA. The diameter of the nanopore appearsto vary with depth, and deeper penetration into the PAA template isobserved only in a highly concentrated electrolyte due to pore enlargement[21].Quantitative analysis of the experimental results shows that at eachtemperature, there is an optimal oxalic acid concentration which produces


the best self–ordering porous pattern of anodic porous alumina. Both theoptimal acid concentration and resulting average grain size in the porouspattern increase approximately linearly with temperature[42]. The oxidegrowth rate increases approximately exponentially with acid concentrationand temperature. The results suggest that fast fabrication of self–orderedanodic porous alumina can be realized by performing anodization at a hightemperatures using the optimal acid concentration[42].

The relationship between the pore dimension and applied anodic voltagein the PAA membrane with ordered nanopore arrays have been investigated.Li et al.[8] fabricated PAA membranes in three types of aqueous solutions,namely sulfuric, oxalic, and phosphoric acids, at different constant voltages.The interpore distance (i.e., cell diameter) increases more or less linearlywith the anodic voltage as shown in Fig. 7. The linear relationship derivedfrom the disordered nanopore arrangement describes the results from theordered nanopore array reasonably well[8,46]. However, there are some smalldeviations. The interpore distances are a little bit larger than the fittedvalues in the case of phosphoric acid but slightly smaller for oxalic andsulfuric acids. This means that although the cell diameter depends mainlyon the applied anodic voltage, the electrolyte type has only a small influenceon the cell diameter. Microscopy reveals that the cell nm/V values are 2.77,2.78, and 2.65 for phosphoric, oxalic, and sulfuric acids, respectively[21]. Thecell size of the anodic porous alumina, that is, the pore interval, isdetermined by the applied voltage used in anodization.

Fig. 7: Interpore distance d in self-organized porous alumina versus anodic voltageUa in sulfuric, oxalic, and phosphoric acid solutions. Reprinted with permissionfrom Ref. [8]. Copyright 1998 by American Institute of Physics.


The heat generated during anodization, especially at a high voltage, influencesgrowth of the nanochannels. Heat generation is harmful to nanostructuresbecause it accumulates randomly over the PAA structure, especially thediscontinuous geometry, to enhance dissolution. Both a magnetic stirrer andcooling system are frequently added to anodic systems. Therefore, the electrolytetemperature is usually kept at 0–5 °C to reduce dissolution of alumina duringanodization. However, a low temperature decreases the growth rate and therehave been some research activities on conducting anodization at hightemperature. A higher temperature increases the growth rate and enlargesthe pore size of PAA films. In the constant voltage anodizing process, thebarrier layer thickness decreases slightly at a high temperature, but very littlevariations in the cell diameter can be observed with temperature. Chung et al.have demonstrated that two–step hybrid pulse anodization can reduce theresistive heating effect of the anodizing current and form nanopores at roomtemperature[62–64]. Pulse technology has widely been applied to increase thewear resistance and corrosion resistance in barrier type anodic oxidation ofaluminum[65] and fabricated novel three dimensional nanostructures in PAA[66].

Electrochemical polishing at a high temperature shows preferentialetching behavior in specific boundaries[67], leading to different dissolutionrates from top to bottom of original fiber–like structures on the aluminumsheet. Besides, during polishing, the high temperature softens the aluminumelectrode and rearranges the particles by changing the particle size anddistribution[68]. It may also roughen the surface and affects the structures.

Chemical etching conducted between the two anodization steps to removethe disordered aluminum oxide layer formed in the first anodization step iscrucial to the morphology of PAA templates. Increasing the etchingtreatment time is favorable and it is imperative that the etching cycle beemployed in the first anodization treatment. It must be at least half of theanodization treatment time to ensure the production of an ordered porestructure. Etching solutions based only on H3PO4 fail to remove aluminumoxide in the first anodization step but addition of as much as 2% CrO3 offersa marked improvement in the PAA pore features. Increasing the H3PO4concentration in the H3PO4–CrO3 mixture improves the pore features whilea larger CrO3 concentration has no effect on the PAA structure. The mostcommon etching solution is a mixture of H3PO4 (6 wt%) and H2CrO4 (1.8wt%). The temperature during etching also affects the morphology of thePAA pores remarkably and the temperature of the process is usually between45 °C and 80 °C. An etching solution temperature of 55 °C has been identifiedto be the optimum one in the etching step.

3. FABRICATION OF POROUS ANODIC ALUMINA MEMBRANES

Fabrication of PAA films by electrochemical anodization is a well knownprocess which has been studied since 1950s. PAA templates generally


fabricated on aluminum have received significant attention due to theirdiverse applications such as dielectrics in aluminum capacitors as well asretention of organic coatings and protection of aluminum substrates fromthe working environment.

Anodization of aluminum usually takes place in an aqueous acidic solutioncomprising sulfuric/phosphoric/oxalic acid at a constant voltage or morecommonly at a constant current. With regard to the PAA templates, thepore and cell dimensions as well as pore regularity are strong functions ofthe applied voltage. For example, the growth rate for the cell size, or thepore (center–to–center) distance, is 2.5 nm/V. Since the discovery of theoptimal conditions for the self assembly of highly ordered pores after 1995,these ordered porous films have been employed as templates for a myriadof one–dimensional and composite nanomaterials.

3.1. Preparation of Ordered Porous Alumina Membranes byTwo–Step Anodization

Generally, high–purity (99.999%) aluminum sheets are used as the startingmaterials. Before anodization, the aluminum foil is degreased withacetone[8,37,69,70] and annealed at 400 or 500 °C in nitrogen for severalhours[8,9,11,37] in order to increase the grain size in the metal and to improvethe homogeneity over a large area. Afterwards, the aluminum sheets areelectrochemically polished. This treatment can be performed in a mixtureof perchloric acid and ethanol (1:5 in volume) at a constant direct currentvoltage of 18 V for 3 min[45,71] or in a 4:4:2 by weight H3PO4, H2SO4, and H2Osolution[9]. Other solutions are sometimes used in electrochemicalpolishing[72]. However, the pre–treatment processes are not necessary toobtain PAA membranes, even though they can improve the ordering of thenanopore pattern. If a PAA membrane with a perfectly ordered nanoporearray is needed, more pre–treatments should be considered, for instance, amolding process using a SiC mold[11,73], holographic lithography[74], atomicforce microscopy nanoindentation[75], focused ion beam lithography[76,77], andtwo–step anodization[12], Masuda and Fukuda found that mild anodizationconducted in 0.3 M oxalic acid at 40 V at 0°C gave rise to self–orderingpatterns.

In this chapter, we describe two–step anodization primarily. A two–stepanodization method, in which the second anodization step starts from thepre–textured patterns formed by the first, can improve self–ordering of thepore–channel growth. As the pores on the aluminum surface have a tubularshape, removal of the first oxide film leaves the aluminum surface withhighly ordered semi–spherical etch pits, which thereupon act as seeds forthe ordered pore formation during the second anodization step.Consequently, non–intersecting, uniform pore channels running straightthrough the PAA membranes can be produced. From the perspective of


template synthesis applications, two–step anodized PAA membranes arepreferable over single–anodized or commercially available PAA membranesas the latter ones lack long–range uniformity concerning the pore size andshape.

Anodization of the aluminum sheet is conducted in an electrolytic celland Fig. 8 displays a schematic diagram of the typical apparatus[78]. During

Fig. 8: Schematic diagram of the electrolytic cell. Reprinted with permission fromRef.[78]. Copyright 2005 by Elsevier

the entire process, the electrolyte should be vigorously stirred andmaintained at a relatively low temperature to reduce Joule heat. Anodizationcan be carried out in the constant current mode or constant voltage mode[31].Fig. 9(a) shows the corresponding current–time curves of the anodizationprocess of aluminum in the constant voltage mode and several stages canbe easily discerned. In the initial stage, the current diminishes suddenlywhen the anodic voltage in turned on indicating that the aluminum oxidebegins to grow. The isolated barrier layer composed of nonporous aluminaforms quickly as the anodic voltage is applied to the aluminum sheet,resulting in a sharp increase of the resistance. After the thickness of thealumina barrier layer reaches a certain value, the field–assisted effectpromotes local dissolution of the barrier layer giving rise to nanopore nucleiformation[8,17,74] Relatively fine channels are formed in the outer regions ofthe barrier layer. In this stage, the anodic current increases slowly due toalumina growth and alumina dissolution. Further anodization causespropagation of individual channels through the barrier layer with enlargedfronts and the anodic current reaches a constant value after a certain time.At this point, growth of the nanopores becomes stable[31] because dissolutionand formation of alumina at the electrolyte/alumina interface are in


equilibrium. If the anodizing process is conducted in the constant currentmode, the situation is similar. Here, the voltage–time curve rather thanthe current–time curve is monitored, as shown in Fig. 9(b). The processincludes three stages as well. The anodic voltage increases initially, implyingthe formation of an initial alumina barrier layer, decreases due to nanoporeformation, and finally reaches a stable value (see Fig 9(b)). Since thenanopore dimension is mainly determined by the applied anodic voltage[8,46],the constant voltage mode is more common.

Fig. 9: (a) Current-time curve of the anodization process. (b) Voltage-time curve ofthe anodization process. Reprinted with permission from Ref.[31]. Copyright1992 by IOP Publishing Ltd.

The self–ordered PAA is obtained by two–step anodization in threeregimes using sulfuric acid at 25 V, oxalic acid at 40 V, and phosphoric acidat 195 V to obtain an interpore distance of 63, 100, and 500 nm, respectively.A schematic representation of the procedure is depicted in Fig. 10. In thefirst step, a clean aluminum sheet is anodically oxidized to form an aluminamembrane (A). This preformed membrane is subsequently removedcompletely in a phosphocromic acid solution to form a textured pattern ofconcave substrate (B) for the second anodic oxidation process.[12,79,80] Afteranother anodic oxidation of B, a well–ordered PAA membrane (C) withordered pores is formed. This final film is then detached from the barrierlayer by a voltage pulse of about 5 V for a time shorter than that for anodicoxidation to form a freestanding PAA membrane (D) and an alumina barrierlayer to cover the aluminum substrate (E). The porosity can be remarkablydifferent outside the optimal range of the anodization potential. For example,reducing the potential from 195 to 160 V in phosphoric acid increases thefilm porosity to about 40%[28]. On the other hand, increasing the potentialfrom 40 to 120–150 V in oxalic acid under extreme cooling of the aluminum/electrolyte concentration has only minor effects on the template parameters


which are usually set to 0–20 °C and 0.1–0.4 M, respectively. For example,increasing the concentration of sulfuric acid from 1 to 10 wt%, i.e., fromabout 0.1 to 1 M, results in a mere 20% reduction in the cell parameter[28].The volume expansion factor (R = VAl2O3/VAl) of the ordered PAA having10% porosity is determined to be about 1.2[44].

The as–anodized PAA remains on the aluminum substrate and the porebase is closed by an oxide barrier layer. The structure is useless and evendetrimental to certain applications[81]. In order to get rid of the surplusaluminum and barrier layer, several methods have been developed. Acommon technique is a chemical one in which the PAA with the aluminumsubstrate is immersed in a saturated HgCl2 solution, and the aluminumsubstrate is replaced by Hg. After removing the liquid Hg, the free standingPAA can be obtained[6,8,45,70,82]. The saturated CuCl2 solution may also beadopted to avoid the poisoning effects of Hg[83] but removal of Cu is somewhatdifficult. Besides, these chemical methods invariably introduce impuritiesinto the free standing PAA. Furneaux et al.[4] have found that progressivereduction of the applied anodic voltage causes perforation of the barrierlayer and separation of the PAA. The thickness of the barrier layer can bereduced by decreasing the anodic voltage[21]. However, a large voltagedecrease in a single step thins the barrier layer only on the bottom of a fewnanopores[84]. Therefore, the voltage is typically reduced in small steps in

Fig. 10: Schematic representation of the fabrication procedure for the formation ofordered and through-hole porous alumina membrane. (a) Formation of theporous alumina layer after the first anodic oxidation process; (b) Removal ofthe porous alumina layer; (c) Formation of the ordered porous alumina layerafter the second anodic oxidation process; (d) Free-standing PAA; and (e) Thebarrier layer structure on aluminum base after electrical detachment of thePAA. Reprinted with permission from Ref.[85]. Copyright 2004 by AmericanChemical Society


order to maintain the electric field at a relatively high level to accomplishuniform barrier layer thinning[4]. As a result, the PAA without the aluminumsubstrate is obtained when the anodic voltage reaches zero. This method isnonetheless time consuming, and so Yuan et al.[85] have explored an easierelectrochemical method to prepare through–hole PAA. After the regularanodization process, the sample is further anodically oxidized in a solutionof HClO4 and (CH3CO)2 (1:1 in volume) at a voltage of 5 V higher than thatin the preceding anodizing process for 3 s and then detached from the barrierlayer immediately[85–87]. The advantage of this simple electrochemical methodis that the solution is environmental friendly and free of heavy metal ions.Consequently, problems created by the metal residues can be avoided.

Open–circuit chemical etching of PAA in an acidic solution such as dilutedH3PO4 is a commonly used post treatment[70]. It is generally adopted toadjust the nanopore diameter or to remove the barrier layer. Theconcentration and temperature of the acidic solution has a significantinfluence on the chemical etching rate and therefore should be consideredcarefully in the experiments.

3.2. Preparation of Highly Ordered Porous Alumina Membranesby Aluminum Pre–Texturing

The size of the ordered pore domains in two–step anodization of PAA can beextended to a maximum of about 5 m. In order to obtain ideally orderedfilm surface, pre–texturing of aluminum has to be carried out. Severalmethods have been proposed, and formation of even mono–domain PAAfilms has been demonstrated. As predicted, despite the high cost andadditional complexity, lithography aided aluminum pre–texturing obviatesthe need form double/multiple anodization since high pore ordering is readilyobtained by a single anodization process. In the case of the self–organizedtwo–step process, the first anodization step is equivalent to the pre–texturingprocess in other techniques and relies on the formation of periodic concavestructures on the aluminum during the first anodization step.

Masuda et al.[88] have studied the development of pores in pretexturedaluminum, where the shallow concave structure formed by nanoindentationinitiates the development of the pores during anodization[11]. It is furtherobserved that even at the deficiency site of the concave structure, the porescan be compensated automatically and an almost perfect arrangement of thepore configuration can be recovered under the appropriate anodizing conditions.Introduction of the self–repair system in nanofabrication is promising, especiallyin the preparation of large–scale fine patterns in which reduction of defects isessential to the improvement of the final production yield[88].

The experimental procedure is described schematically in Fig. 11. Thestarting pattern is a hexagonal array of dots (convexes) with nanometer


dimensions on a SiC wafer which has periodic defects in the array. The SiCmolds are fabricated using conventional lithography. Molds with such anarray of convex structures on the substrates have recently become a subjectof increasing interest in nanoimprinting and fabrication of fine patterns onpolymers[1,21,89] metals[6,90], and semiconductors[70]. The pattern of the SiCmold is transferred to an aluminum sheet using nanoindentation to producea negative structure of the mold of the array of concave structures onaluminum. Anodization of aluminum is conducted at a constant voltage of80 V in 0.5 wt% oxalic acid at 17°C[88]. In the subsequent anodizing process,each concave indent produces a pore in the oxide.

The layout of the initiation sites for hole development in anodic aluminais produced by a process based on nanoindentation. Fig. 12 shows the SEMmicrographs of the cells and openings observed from the bottom side of theanodic porous alumina prepared using the initiation sites in triangularpatterns. Fig. 13 depicts the SEM micrographs of the cells from the bottompart of the oxide and openings in the anodic alumina resulting from initiationsites in a square pattern. Fig. 14 displays the SEM micrographs of thetriangular cells and openings on the bottom side of the anodic aluminaformed by initiation sites with the graphite structure lattice[91]. Theseprocesses are based on the artificial layout of the initiation sites of holes inthe square and graphite structure (hexagonal) lattices, thus allowing the

Fig. 11: Schematic of the experimental process for self-repair of defects in the pattern:(a) Starting pattern of a dot array with defects on a SiC mold, (b) Preparationof an array of concaves on aluminum by indentation, (c) Transferred patternof array of concaves on aluminum, (d) Pattern in the porous alumina afteranodization, and (e) Pattern in the replicated Ni using the self-compensatedporous alumina as a template. Reprinted with permission from Ref.[88]. Copyright2001 by American Institute of Physics


Fig. 12: SEM micrographs of (a) cells and (b) openings on the bottom side of the anodicalumina formed by indentation of the triangular lattice. Reprinted withpermission from Ref.[91]. Copyright 2001 by WILEY-VCH Verlag GmbH & Co.KGaA, Weinheim

production of the corresponding shapes based on nanotiling through theclose packing of the cells.[88] The hole array has a highly ordered architectureand is used as a template to fabricate small metal particles with a well–defined shape. These new types of hole array architecture are powerfultools to synthesize several kinds of nanostructures such as two–dimensionalphotonic crystals[92,93]. and carbon nanotubes with a triangular or squarecross section[94].

It should be noted that the obtained alumina is free of defects. Ratherthan a hexagonal arrangement, square or triangular nanohole arrays canbe obtained. Major drawbacks of the aforementioned methods include limitedsurface area of the pre–texturing and high costs of manufacturing. Theseproblems can be easily avoided by the application of two step self–organizinganodization. The self–organized two–step anodization process produces ahighly ordered structure over a relatively large surface area. In this case,


however, alumina is not free of defects which are present mostly in thearea of the domain boundaries, but the domains are defect free.

Fig. 13: SEM micrographs of (a) cells, (b) openings on the bottom side of the anodicalumina and (c) cross section of anodic alumina formed by indentation of thesquare lattice. Reprinted with permission from Ref.[91]. Copyright 2001 byWILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


4. PROPERTIES OF ANODIC ALUMINUM OXIDE TEMPLATES

4.1. Photoluminescence and Optical Properties

Kennedy et al.[95] have made advances in aluminum mirrors protected byAl2O3 using various substrates. The approach is to use new reflector materialsbased on porous alumina with high reflectance compared to a conventionalaluminum foil. At present, most of the research focus is on thephotoluminescence (PL) properties of PAA films formed in oxalic acid, butrelatively little research has been done on the PL properties of PAA filmsformed in sulfuric and phosphoric acids. The intensity of the emitted PLand shape of the PL excitation and emission spectra depend on many factorssuch as the wavelength of the incident radiation[96], electrolytes, conditionsof anodization,[97] thickness of the oxide films, as well as thermal treatmentconditions.[98] Many models have been postulated to describe the mechanismsof the PL and the nature of PL centers in the PAA on aluminum[99]. There

Fig. 14: (a) Triangular cells and (b) openings on the bottom side of the anodic aluminaformed by initiation sites with the graphite structure lattices. Reprinted withpermission from Ref.[91]. Copyright 2001 by WILEY-VCH Verlag GmbH & Co.KGaA, Weinheim.


are two main schools of thoughts about the nature of the PL centers in thePAA formed in organic electrolytes. The first one proposes that the PLcenters are related to oxygen vacancies whereas the second one suggeststhat the PL is attributed to oxalic impurities in the oxide films introducedduring anodization[100].

Ghrib et al.[101] have studied the PL and optical properties of PAA insulfuric acids. They correlate between the annealing temperature and boththe PL and optical properties of the porous alumina films. Thecrystallographic structure depends strongly on the annealing temperature.Ellipsometric analysis of the porous alumina shows the evolution of opticalproperties (refractive index, extinction coefficient) with thermal treatment.The refractive index increases and optical loss decreases with annealingtemperature. This behavior seems to be related to the decrease in thealumina porosity and structure modification. As a consequence thereflectivity increases progressively to 97% when the porous alumina filmbecomes amorphous at 650 °C.

Garabagiu et al.[102] have developed a new method to control the dissolutionof the barrier layer of PAA and to thin the film using a simple electrochemicalsetup. By thinning the barrier layer and alumina membrane, some changesappear in the optical transmission spectra experimentally but are alsoobserved in theoretical calculation. Simulation shows that increasing thefilm thickness increases the number of interference fringes in thetransmission spectra and it is in good agreement with both experimentaland analytical results. Reducing the barrier layer thickness increases theoptical transmission through the membrane due to the smaller overallthickness[102].

4.2. Mechanical Properties

The mechanical properties such as the modulus and hardness of anodicaluminum oxide structures can be measured by nano–indentation which isthe simplest technique[103–105] Ko et al.[106] found that the indentation modulusand hardness decreased monotonically as the hole diameter increased. Xiaet al.[107] measured the Young’s modulus, hardness, and fracture toughnessof highly ordered nanoporous alumina and the mechanical properties forthe barrier were measured by Alcala et al.[108] Lu et al.[109] determined thechanges in the elastic modulus and hardness by monitoring the changes inthe indentation depth of the nanoporous materials.

The specimens sometimes possess residual stress due to phasetransformation, deposition, and absorption during production. It has alsobeen shown that micro–electromechanical systems prepared by etching ofthin films are influenced by residual stress such as film cracking and plasticbending[110]. If the materials are subjected to too much residual stress, film


delamination may occur. The residual stress can affect materials propertiesas shown in Fig. 15(a). The tensile stress shifts the loading curve to theright whereas compressive stress shifts it to the left. Fig. 15(b) depicts theschematics illustrating the residual stress evaluation[106]. In order to calculatethe contact area, Ko et al.[106] used the change in the relative position of theindenter to determine the indentation depth. Under the assumption thatthere is no residual stress, the total indentation depth, ht, can be derived.The hardness can be calculated after modifying the indentation depth bythe residual stress, as shown in Fig. 15(a). Fig. 15(c) explains the effect ofthe depth change due to the residual stress. There is a ‘pile–up’ phenomenonin the compressive residual stress and a ‘sink–in’ phenomenon in the tensileresidual stress. The flat punch theory developed by Doerner et al.[104] isused in this study. The indentation test can be modeled by the contactcondition of the flat punch, as shown in Fig. 15(b). The specimen preparationprocess induces residual tensile stress which diminishes when the hole sizeincreases because the specific volume of the alumina decreases as the holesize increases.

4.3. Tribological Properties

The tribological properties of the self–lubricating structure have beeninvestigated and the friction and wear performance are enhanced to a certaindegree[111–115]. PAA templates are useful to tribological applications as thenanoporous structure can be utilized as a reservoir or template for solidlubricants[111–113] and nano–tubes, –rods, or –fibers[114,115] to form self–

Fig. 15: Schematic representation of (a) change of indentation depth for differentresidual stress state; (b) release of elastic sink-in depth; (c) modeled surfacemorphologies: (i) compressive stress, (ii) stress-free, (iii) tensile stress.Reprinted with permission from Ref.[106]. Copyright 2006 by Elsevier.


lubricating structures. Kim et al.[20] have investigated the PAA films withvarious pore sizes to better understand the tribological behavior in slidingcontact with steel balls showing 4 orders of magnitude range in the normalload (from 1 mN to 1 N). It is found that the larger the pore size, the higheris the friction coefficient. In other words, the friction coefficient related tothe pore size (pore diameter) of the PAA template is dominant at relativelyhigh loads (0.1 N and 1 N). However, with increasing loads applied to allPAA films, the friction coefficient diminishes considerably. Smooth and thicktribolayer patches are formed on the worn surface of the PAA underrelatively high loads (0.1 N and 1 N) due to tribochemical reactions andcompaction of the wear debris. The smooth and thick tribolayer patchescontribute to the lower friction and wear under high loads[20].

5. APPLICATIONS OF POROUS ANODIC ALUMINAMEMBRANES

5.1. Materials Fabrication

PAA has attracted significant practical interest due to applications pertainingto nanoscale science and technology. Novel structures such as nanoparticles,nanotubes, nanowires, and so on have been developed on PAA templatesand widely used in optics, micro/nano electronics, and microfluidic systems.

5.1.1. Nanodots

Current electron beam machines can readily write thousands of nanodotpatterns on photoresists with great accuracy. However, the process is timeconsuming if millions or billions of nanodots must be processed. It is limitedby the microscopic field of view if stitching errors are to be avoided. Inaddition, it does not account for the complex proximity issues when decidingon the exposure time which can dramatically alter the dimensions of thedesired patterns, especially nanodots spaced by about 100 nm or less. Byadopting the PAA template, billions of nanodots with a uniform size andspacing can be formed in parallel often in a single or a few steps.

Fabrication of metal nanodots starts with the PAA with the barrier layerremoved leaving a through–pore template. The template is placed on thesubstrate usually in a solution as the membrane which should be as thinand as large as possible. The materials must be handled carefully becausethey are easily fractured. The template adheres well to a flat substratebecause of Van der Waals force. Afterwards, a thin metal layer is evaporatedthrough the pores onto the substrate surface. Evaporation is typicallyperformed in an e–beam evaporator using a small deposition rate as fasterrates tend to clog the nanopores. After peeling off or etching away the PAAtemplate, the undesired metal is lifted off using a conventional semiconductorprocessing technique and a perfect hexagonal array of metal nanodots isformed naturally on the host (e.g., semiconductor) substrate. The mean


diameter and spacing of the nanodots which can be varied are limited bythe pore diameter and spacing of the PAA. The dot diameter can be reducedfurther after lifting off using wet chemical etching.

Masuda and co–works have fabricated a highly ordered gold nanodotarray by vacuum evaporation using a PAA mask with nanometer holes[12].Different metal nanodot arrays such as Ni, Co, and Fe have been depositedin a similar fashion on substrates such as Si, GaAs, and GaN[116,117] . Theability to form metallic nanodots on a substrate provides the platform toetch nanopillars and synthesize nanowires. On the other hand, the nanodotsare not confined to be just metallic although evaporation of metals is simpler.GaAs nanodot arrays have been grown on GaAs (001) substrate using thenanopore template by molecular beam epitaxy[118]. Disk–shaped GaAsnanodots with a slightly convex surface are shown in Fig. 16. Larger growthrates produce disk–like nanodots indicating a layer–by–layer growthmechanism.

Fig. 16: GaAs nanodot arrays on GaAs substrate with (a) top view and (b) obliqueangle view. Reprinted with permission from Ref.[118]. Copyright 2003 by Elsevier


5.1.2. Nanowires and nanorods

Arrays of metallic and magnetic nanowires are attractive to radiation andmagnetic sensors, high–density magnetic devices, surface–enhanced Ramanspectroscopy, as well as for fundamental studies of nano–magnetics. Theability to produce highly ordered nanowire arrays economically andefficiently is imperative. PAA template synthesis using electro–depositionis stable at high temperature and in organic solvents. It has been proven tobe a low–cost, high–yield, and large–throughput technique to produce largearrays of nanowires.

An alternative method to prepare metallic nanowires involves theremoval of the barrier layer of the PAA followed by evaporation of a metallayer onto the surface of the through–pore template providing a contact forelectrodeposition. Various metals can be deposited into the nanochannelsby a simple direct–current deposition technique. However, the drawback isthe requirement for a freestanding PAA template that must be mechanicallyrobust enough since the membrane is already separated from the supportingaluminum substrate. On the other hand, a thicker template makes it difficultto fill each nanopore uniformly.

Instead of making nanostructures by electrochemical deposition as inthe case of metallic nanowires, nanostructures can be fabricated by etchingexisting materials by using the PAA template as an etching mask. Intraditional lithography, a mask typically made of metal or silicon dioxide/nitride is deposited to protect the desired areas from etching. The materialsin the unprotected areas are etched anisotropically leaving a pillar–likestructure protruding from the substrate. A similar approach can be adoptedto fabricate highly ordered nanopillars. The procedure begins with theformation of a perfect hexagonal array of metal nanodots on the substratewhere the nanopillars are to be formed. A deep reactive ion etching processis employed to obtain high–aspect–ratio semiconductor nanopillars usingthe metal nanodots as the mask.

PAA templates are particularly useful to optical studies of nanoparticlestructures prepared within their pores because the host oxide is transparentthroughout much of the visible and infrared spectral regions. Furthermore,polarization dependent optical properties can be determined from the nanowiresdue to the parallel orientation of the cylindrical nanopores. Orderedsemiconductor ZnO nanowire arrays are grown in the PAA template along thenanochannels[119]. After electrodeposition of Zn onto the bottom of the PAAtemplate, the sample is heated in air at 300 ºC for 35 h transforming the metalinto polycrystalline ZnO with various orientations. The diameters of thenanowires range from 15 to 90 nm depending on the nanopore size and height.

In many applications, the nanowires need to extend beyond the templatesurface or be completely removed from the PAA membrane. Unfortunately,it cannot be accomplished using the electrodeposition and oxidation method.


There has been renewed interest in a technique proposed in the 1960s,namely, the vapor–liquid–solid growth method[120]. In this technique, metalnanodots are used as the catalysts to promote nucleation at each site toform the nanowires. Huang et al.[121] have demonstrated the ability to usean PAA–patterned metal nanodot array as a masking template to etch siliconnanowires (SiNWs). This has practical importance since mastering of thepore size of the PAA template allows one to control the diameter of thenanowires. Huang et al. have developed a simple and economical method tofabricate high–density and well–aligned SiNWs by metal–assisted chemicaletching using metal nanodots as a hard mask. By fine–tuning the pore sizeof the PAA template used in the metal nanodot fabrication, the diameter ofthe SiNWs can be controlled to a precision of 10 nm in the range of 40 to 80nm as shown in Fig. 17. The use of Cr/Au as a hard mask blocking materialis also of tremendous value in other patterning methods for Si nanostructurefabrication using catalytic etching.

Fig. 17: SEM images of PAA with average pore diameters of (a) 40 nm, (b) 50 nm, (c) 60nm, (d) 70 nm, and (e) 80 nm. The corresponding SiNWs produced using thesePAA templates are shown in panels (f) to (j), respectively. The insets in eachimage are the respective top-view SEM images of the SiNWs shown in themicrograph. The mean diameters and standard deviations, in nanometers(nm) are (41.3, 4.4), (49.7, 3.7), (60.5, 4.3), (69.1, 3.9), and (80.2, 3.3), respectively.The scale bars are 200 nm for all SEM images and 100 nm in all the insets.Reprinted with permission from Ref.[121]. Copyright 2010 by American ChemicalSociety

Sol–gel is a technique that has been employed to fabricate LiNiO2,LiMn2O4, LiCoO2, and LiNi0.5Co0.5O2 in–template nanowires. The processtypically involves hydrolysis of a solution containing the precursor moleculesto obtain a suspension of colloidal particles. This is the sol part of the process.The gel is composed of aggregated sol particles thermally treated to yieldthe desired materials. To obtain the in–template nanowires, the PAAtemplate is dipped in the sol allowing the solution to diffuse into the


nanochannels. After a certain time, the template is removed from the soland heated. Highly ordered zirconia nanowire arrays have been fabricatedby this PAA template method using the sol–gel technique[122].

Cheng et al.[123] synthesized in–template GaN nanowires using a gas–phase reaction involving Ga2O vapor and flowing NH3. Mei et al.[124] produceda large quantity of monocrystalline germanium nanorods on a PAA templateutilizing saturated vapor adsorption. During this process, the Ge gas pressurewas saturated at a high temperature in an airtight quartz tube. Martín etal.[125] prepared polymer–based nanorods and nanotubes using the self–ordered anodic aluminum oxide templates. By replicating the PAA templatesand using infiltration methods, hollow and solid one–dimensional polymericnanostructures such as nanotubes, nanorods, and nanocolumns could besynthesized. Hence, the PAA templating method is a versatile tool to fabricatehighly ordered and densely packed nanowires composed of various types ofmaterials.

5.1.3. Nanotubes

Carbon nanotubes (CNTs) have unique electrical and mechanical properties.Arrays of parallel carbon nanotubes with uniform diameters and periodicarrangements have been synthesized with the aid of the PAA template[94,126–130].The typical fabrication procedure begins at the same starting point asdescribed in section 3.1. After the second anodization step, the PAA templateconsists of uniform, parallel, hexagonally packed nanochannels. The nextstep is to electrochemically deposit a small amount of cobalt, nickel, or ironcatalyst onto the bottom of the template channels as illustrated in theschematic in Fig. 18(a). The ordered arrays of nanotubes are produced byfirst reducing the catalysts by heating the cobalt–loaded templates in atube furnace under flowing CO. The CO is then replaced by a mixture of10% acetylene in nitrogen using the same flow rate and the samples areannealed in nitrogen. Fig. 18(b) shows the SEM image of the CNT arrayswhich have been ion–milled to remove residual amorphous carbon fromthe template surface. The nanotubes can subsequently be partially exposedby etching the PAA in a mixture of phosphoric and chromic acids. Theresulting nanotubes are uniform in length, parallel to each other, andperpendicular to the template.

Synthesis of the connections between two or more CNTs is an importantstep in the development of CNT–based electronic devices and circuits.However, this is difficult to achieve using conventional methods becausethe straight tubular structure cannot be controllably altered along its length.Various post–growth modification methods have been suggested, but theytend to be difficult to implement and also are prone to defects. The PAAtemplate method to fabricate uniform, highly ordered, and straight CNTsdescribed above can be extended to grow uniform, highly ordered, and Y–junction CNTs[131]. The synthesis of Y–junction CNTs follows the same


procedure as straight CNTs by means of pyrolysis of acetylene in the presenceof cobalt catalysts. The difference is in the preparation of the PAA template.Because the pore diameter is proportional to the anodization voltage,reducing the voltage by a factor of 2–1/2 results in twice as many nanoporesand nearly all nanopores branching into two smaller diameter nanopores.Therefore, by changing the voltage during anodization, a larger nanopore(stem) evolves gradually into two smaller diameter nanopores (branches).

5.1.4. Nanopores

In this section, some representative nanopores fabricated using the PAAtemplating are presented. The fabrication of nanopores is relatively simple.Inthea positive transfer technique, the nanopore array pattern is replicatedby chemical and/or physical etching of the substrate to yield lateralsuperlattices. However, it is difficult to form nanohole arrays with sufficientlystraight holes when the aspect ratio of the holes (depth divided by diameter)is large[6]. Recently, Yanagishita and co–workers reported a new strategyto fabricate metal nanohole arrays with straight holes and high aspect ratiosbased on the preparation of polymer pillars supported on both sides bylayers[132]. Fig. 19 shows the cross–sectional SEM image of the Ni hole arraydemonstrating the retention of the straight–hole–array feature after

Fig. 18: (a) Schematic of fabrication process and (b) SEM image of the resultinghexagonally ordered array of carbon nanotubes fabricated using the methodin (a). Reprinted with permission from Ref.[127]. Copyright 1999 by AmericanInstitute of Physics


replication. The holes are straight and parallel to each other. The thicknessof the metal hole array membrane formed by the process is expected tocorrespond to the PAA used as a template.

Nakao et al.[133] reported the formation of nanopore arrays in III–Vsemiconductors using the PAA template method. GaAs and InP nanoporearrays were formed using a reactive beam etching technique with Br2–N2gases at an elevated substrate temperature. Nanopores of up to 1 µm indepth were etched, but as pointed out by the authors, the holes at thisdepth partly collapsed.

In transferring the PAA nanopore pattern onto the substrate, reactiveion etching is typically used due to the directive etching nature, smoothetched surface morphology, large etching rates, and vertical sidewalls. Inthis process, any gap between the template and substrate should beminimized to prevent unintentional etching along the gap. To circumventthe difficulty in placing the ultra–thin PAA on the substrate, an aluminumfilm can be sputtered or evaporated onto the substrate as suggested byCrouse et al.[134] After evaporating 2 m of aluminum onto the conductingsilicon substrate, anodization by using similar recipes is carried out. Onelimitation of this process is that the substrate must be conducting as theanodization procedure requires a voltage drop on the aluminum surface.After anodization, the PAA film is thinned from 2 to 300 nm by 1 h argonion milling to transfer the reactive ion etched pattern to the silicon substrate.

Pu and co–workers have reported an electrochemical lithographystrategy to fabricate silicon nanotip arrays by PAA as a pattern–transferrednanomask for directional anodizing of a silicon wafer[135]. The tip density is

Fig. 19: Cross-sectional SEM image of a Ni hole array membrane. Reprinted withpermission from Ref.[132]. Copyright 2005 by WILEY-VCH Verlag GmbH & Co.KgaA, Weinheim


larger than the pore density of the mask and every tip has the same crystalorientation as that of the starting wafer.

5.2. Biological Science and Engineering

Alumina has been used extensively as a substrate in bone tissue engineering.The biocompatibility of alumina has been demonstrated and researcherscontinue to explore new applications. The use of alumina with a smoothsurface has been reported and but more recent studies have focused ontextured alumina which shows better bone in–growth possibility. Favorableresults about using microporous and nanophase alumina with osteoblastshave been reported. The osteoblast response is extremely sensitive to thesurface roughness and porosity[136]. In one study, pore sizes up to 200 nmwere investigated and shown to promote bone growth. In another study,osteoblasts cultured on alumina with pore sizes up to 200 nm showed thenormal growth patterns and phenotype[137]. More recently, Swan et al.[138]

demonstrated that PAAs fabricated by a two–step anodization process withpore sizes between 30 and 80 nm showed favorable osteoblasts adhesionand short–term phenotype. Based on these results, long–term studies havebeen conducted to investigate the osteoblast matrix production and cellviability on PAA. These studies are important to disclose the ability of thenanoporous membranes to enhance osteoblast functionality thus spurringthe fabrication of next generation bone implant surfaces.

Recently, PAA templates have been used as templates to construct highlyordered structures of biological molecules such as proteins, DNA, andantibodies on the nanometer scale. This has become an important subjectbecause these methods can lead to improved biodevices[139]. Patterns ofbiological molecules observed by conventional optical microscopy in anaqueous environment are important to the evaluation of the functions ofbiological molecules. Masuda et al.[140] fabricated an ordered array of DNAnanopatterns with a controlled pattern interval formed on a Au–nanodiskarray with a controlled disk interval using PAA. As patterns of biologicalmolecules on the array become miniaturized, the process is expected tobecome more important in the optical evaluation of the functionality ofsingle biological molecules.

A primary goal of biotechnology research is to develop devices for real–time detection of small concentrations of biomolecules. Detection of antigen–antibody reactions in real time is a particularly important goal in thebiomedical field. Among the various biosensor detection methods[141–143],optical biosensing offers high accuracy, sensitivity, and rapid response. Yeomet al.[144] fabricated uniform periodic nanopore lattice PAA templates by two–step anodizing and assessed their suitability as biosensors by characterizingthe change in the optical response upon addition of biomolecules onto thePAA template (see Fig. 20). Their results confirm that the PAA–based


biosensor is highly selective towards detection of CRP antigen and canmeasure a change in the CRP antigen concentration of 1 fg/ml[144]. Thismethod can provide a simple, fast, and sensitive analysis in terms of proteindetection in real time.

5.3. Chemistry

Porous materials are widely used as adsorbents and catalyst carriers becauseof their large surface area and specific surface properties. They absorb watermolecules which change the effective refractive index and so the materialsare useful to chemical sensing.

PAA Bragg stacks were fabricated by modified two–step anodization, andthe optical transmittance spectra of the PAA Bragg stacks soaked in variedanalytes (air, series of alcohols and alkanes) were collected[145]. As shown inFig. 21, there are obvious variations in the wavelength and intensity of thereflected light indicating that the PAA Bragg stack has high color sensitivityand transmittance sensitivity. In comparison with alkanes, alcohols caninfiltrate the soaked PAA stack more effectively. They take up more spaceand have larger volume ratio, thus yielding enhanced sensitivity to refractiveindex change[145]. Besides, the depth of the transmittance dip is sensitive torefractive index changes due to the change in the refractive index contrastbetween a pair of layers in the PAA Bragg stack. Consequently, the PAA

Fig. 20: Schematic diagram of fabricated PAA chip and antigen detection method.Reprinted with permission from Ref.[144]. Copyright 2011 by OSA


Bragg stacks not only have potential applications as Bragg reflectors butalso provide a simple means to organic chemical reactions in situ bymeasuring the intensity of the reflected light with a photodiode.

Kumeria et al.[146] demonstrated the use of nanoporous PAA inreflectometric interference H2S gas sensing and the practical application tomalodour measurement. A schematic of reflectometric interferencespectroscopy (RIfS) device with a PAA sensing platform assembled with amicrochip device, light source, optical detection and data processing unit is

Fig. 21: Photographs of PAA Bragg stack taken by reflection (a, b). (a) Stack is exposedin air. (b) Stack is soaked in ethanol. (c) Transmittance spectra of the stackexposed to air and soaks in ethanol, respectively. (d) Wavelength of thetransmittance dip of PAA Bragg stack as a function of the refractive index ofthe analyte when the stack is exposed to air (1) and soaks in a series of alcoholsand alkanes, respectively. The alcohols include anhydrous ethanol (2), 2-propanol (3), 1-butanol (4), and 1-hexanol (5). The alkanes include n-hexane(6), n-octane (7), and n-decane (8). Reprinted with permission from Ref.[145].Copyright 2008 by American Chemical Society


illustrated in Fig. 22. To achieve sensitivity and selectivity for H2S andvolatile sulfur compound (VSC) detection, the PAA surface is coated withgold which has good affinity to SH groups[147]. The gas detection is based onthe change in the interference signal from the porous structure as a resultof adsorption of gas molecules onto the gold–coated PAA surface. Theperformance of this system concerning VSC detection and real oral malodourmonitoring has been demonstrated.

5.4. Spectroscopy

Surface–enhanced Raman scattering (SERS) is one of the best techniquesto study interfacial effects. It has recently been reported that even singlemolecule spectroscopy is possible by SERS, suggesting that enhancementfactors as high as 1014 to 1015 can be achieved[148–152]. The technique has beenwidely used to study the orientation and behavior of adsorbed molecules onsurfaces and to analyze the interphase tropism, configuration andconformation of biological molecules. These molecules generally hold uniquevibrational energy levels and in practice, the Raman spectra also containfingerprint information pertaining to the molecular structures. In addition,SERS boasts several potential advantages over other spectroscopictechniques such as measurement speed, high sensitivity, portability, andsimple operation. The most effective SERS substrates to date are colloidalsilver or gold clusters that are widely used in the bulk–volume solution–based detection[153–156].

High–quality SERS spectra have been obtained from perylene originatingfrom the ordered arrays of Ag–perylene core–shell nano–pillars by Luo etal.[157] The SERS from the nano–pillar arrays differs from that of individualAg–perylene nanorods and Ag colloidal nanoparticles. The high–densityordered arrays of nano–pillars produce surface plasmon resonance and large

Fig. 22: Schematic of the RIfS device for gas sensing and scheme of detection of VSCsusing nanoporous Au-PAA. Reprinted with permission from Ref.[146]. Copyright2011 by Springer


enhancement in the Raman intensity. This kind of physically assembledcore–shell one–dimensional structure may be an ideal SERS substrate forfluorescent molecules which tend to produce weak Raman spectra usingconventional means.

Qiu et al.[158] have fabricated highly ordered hemispherical silver nanocaparrays templated by PAA membranes which are robust and cost effectiveSERS substrates yielding significant SERS enhancement. The use of PAAmembranes as templates in the fabrication of SERS substrates is especiallypromising considering the easy fabrication, excellent reproducibility, modestcost, and large area production. The materials and technique are thusapplicable to sensing[159]. This design of the silver nanocap array with uniformand highly reproducible SERS–active properties may enable the fabricationof robust, economical, and large–area SERS–based sensors.

Wong–ek et al.[160] have produced low–cost, highly–sensitive SERSsubstrates by magnetron–sputtering deposition of silver nanoparticles onPAA templates. By adopting the optimized deposition conditions, a highdensity of silver nanoparticles (approximately 1 1010 cm–2) is embeddedalong the sidewalls and top surface of the PAA nanoholes[160]. As shown inFig. 23, the enhancement factor is 8.5 107, which suggests promisingpotential for the direct application to chemical detection and analysis.

Fig. 23: Raman spectra of dried methylene blue droplets on several types of the samplesurfaces: (a) Blank silicon surface, (b) Blank PAA template on Si, (c) 16 s silvernanoparticles deposited on Si, (d) 8 s silver nanoparticles deposited on Si, (e) 16s silver nanoparticles deposited on PAA template, and (f) 8 s silver nanoparticlesdeposited on PAA template. Reprinted with permission from Ref.[160]. Copyright2010 by Elsevier


6. SUMMARY

Nanostructured materials have many applications in microelectronics,optoelectronics, magnetic storage, and sensors because they possess novelphysical and chemical properties. Since Masuda and Fukuda’s study of self–ordered porous alumina membranes by a two–step replicating process, PAAfilms have become one of the most common template materials in thepreparation of nanostructured materials and SEM reveals that the poresare usually enlarged in phosphoric acid.

The increasing interest in PAA as a template is mainly due to easy andeconomical processing. The arrangement and shape of the alumina poresare determined by the applied voltage, temperature, as well as type andconcentration of the electrolyte. It is now possible to fabricate well–definedself–ordered porous alumina with 50, 60, 100, 420, and 500 nm interporedistances. PAA templates with interpore distances between 100 and 420nm may be achieved using pre–patterning methods such as imprintlithography or e–beam lithography. However, lithographic methods are costlyand time consuming and may not be the techniques of choice in some templateapplications.

Self–ordered nanoporous PAA is a versatile platform in sensing, storage,separation, and synthesis of one–dimensional nanostructures. PAA templateswith complex architectures are fabricated and used in combination withother fabrication methods thereby widening the controlled synthesis of avariety of nanostructured materials with advanced electrical, magnetic,and optical properties.

7. ACKNOWLEDGEMENTS

This work was jointly supported by the National Natural Science Foundationof China under Grant No. 51071045, the Program for New Century ExcellentTalents in University of Ministry of Eduction of China under Grant No.NCET–11–0096, Southeast University, Hong Kong Research Grants Council(RGC) General Research Funds (GRF) Nos. CityU 112510 and 112212, aswell as City University of Hong Kong Research Grant Nos. 9360110 and9220061.

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