Progress in Micro- and Nanopatterning via Electrochemical

Sep 23, 2009 - He received his Ph.D. in electrochemistry at the University of Ulm (D). Simeone has been a researcher at CNR-Institute for Nanostructur...
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J. Phys. Chem. C 2009, 113, 18987–18994

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FEATURE ARTICLE Progress in Micro- and Nanopatterning via Electrochemical Lithography Felice C. Simeone, Cristiano Albonetti, and Massimiliano Cavallini* CNR-ISMN Bologna, Via P.Gobetti, 101 40129 Bologna, Italy ReceiVed: April 16, 2009; ReVised Manuscript ReceiVed: August 10, 2009

In this paper we present a critical overview on recent progress in electrochemical methods for nanopatterning and nanofabrication. In the first part we consider recent advancements in serial methods, which are mostly based on scanning probe microscopy. We also show applications for nanopatterning, electropolymerization, and dots fabrication. In the second part, we consider the up-grading from serial to parallel with particular attention to the most recent and promising progress. We show the most interesting results highlighting the problems, limits, and future directions. 1. Introduction Ordered arrays of nanometric-sized dots and stripes, fabricated on metals and semiconductors, are largely used to exploit magnetic,1-5 optical,6-10 and electrical11,12 properties of functional materials. This approach, generally called patterning, exploits the most important characteristic of nanotechnology that is the ability to tailor the physical-chemical properties of systems by Euclidean dimensionality. Thanks to this property, patterning techniques have been successfully applied in several fields of science and technology, the most important being nanoelectronics,13,14 information storage,15-17 catalysis,18,19 and sensing.20-22 The establishment of nanotechnology requires the development of novel, low-cost, and user-friendly nanopatterning approaches. The demand of new patterning techniques is attended to overcome the restrictions of conventional fabrication techniques, such as photo- and electron-beam lithography, that are limited in spatial resolution and low throughput, respectively. With that intent, several new lithographic methods23,24 have been developed in the last 20 years, including methods based on scanning probe microscopy (scanning probe lithography,25 nanostencil lithography,26,27 dip-pen nanolithography28,29) and methods based on stamps or masks (soft-lithography,30 nanoimprinting,31 nanosphere lithography,32-34 and unconventional wet lithographies35,36). All of these methods possess important advantages: high efficiency (soft and nanosphere lithography), suitability for large areas (soft, unconventional lithography and nanoimprinting), a few nanometers resolution (nanoimprinting, scanning probe lithography, nanostencil and dip-pen nanolithography), high versatility (soft unconventional lithography and nanoimprinting), and direct processability of solutions (soft and unconventional lithography). Hence, they are suitable to fabricate nanometric structures on the surface assisting, and in some cases driving, selforganization of the materials during the deposition (fabrication) process. * Corresponding author. Phone: +39 0516398516. Fax: +39-0516398540. E-mail: [email protected].

A further advancement in nanopatterning methods has been introduced by adding chemical transformations during the processes, thereby increasing the number of possible applications and prospects for these approaches. This advancement involves the integration of unconventional lithographies with chemical and electrochemical processes. Here we present an overview of the most important methods of integrating electrochemical processes and unconventional lithographies (hereafter called electrochemical lithography (EL)) emphasizing recent progress in nanoscale fabrication. Starting from the most interesting results found in the literature we discuss the problems, limits and prospective of electrochemical lithographies. All of the EL methods are based on the building of an electrochemical cell, which, in the simplest configuration, consist in two conductive surfaces (electrodes) separated by either a liquid or solid conductive phase (electrolyte). By applying an appropriate bias, an electrochemical reaction, namely a chargetransfer process localized at the electrode/electrolyte interface, occurs. In EL, the electrodes are, respectively, the substrate and a conductive structure that could be the tip of a scanning probe microscope or the protrusion of a stamp. In principle, EL offers a broad spectrum of possible applications such as the local control of the reactions (electrochemical oxidation or electrochemical reduction), to change the chemical nature of materials in confined and specific places, local deposition of materials, and the fabrication of chemical patterns. Moreover, unlike all of the other techniques for nanofabrication, in EL the substrate can be directly exploited as the reactive layer. This capability allows us to overcome the concepts of bottom-up and top-down nanofabrication and represents an example of the so-called “third way” for nanofabrication37 (see Figure 1). In the “third way” process, a chemical pattern is created within a homogeneous medium without morphological modifications (Figure 1c). This chemical transformation changes in precise places and in a controlled manner the chemical nature of the medium and, therefore, its physical-chemical properties (e.g. electrical conductivity, wettability, color, and chemical reactivity).

10.1021/jp903494e CCC: $40.75  2009 American Chemical Society Published on Web 09/23/2009

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Felice Carlo Simeone, Laurea degree in Chemistry at the University of Florence. He received his Ph.D. in electrochemistry at the University of Ulm (D). Simeone has been a researcher at CNR-Institute for Nanostructured Materials-Bologna since 2008. His research interests are in scanning probe microscopy, in particular scanning tunneling microscopy under electrochemical control, electrochemistry, and integration of electrochemistry with unconventional lithographies.

Cristiano Albonetti, Laurea in Physics at University of Bologna. He received his Ph.D. in Physics in 2005. He has been a researcher at CNRInstitute for Nanostructured Materials-Bologna since 2001. His research interests are on new SPMs, development of ultrahigh vacuum systems, growth of organic and inorganic materials thin films, and surfaces nanostructuring.

Massimiliano Cavallini, EURYI Awardee in 2006. Ph.D. in Chemistry. He has been a senior researcher at CNR-Institute for Nanostructured Materials (ISMN)-Bologna since 2000, working in the interdisciplinary field of nanotechnology. He is the author of more than 72 papers in highly qualified peer-reviewed international journals, several book chapters, and 12 international patents. He has been principal investigator and coordinator of several EU projects. In 2005 he founded the spin-off company named Scriba Nanotecnologie SrL operating in the field of nanofabricated identification tags.

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Figure 1. (a) Scheme of top down nanofabrication process. In topdown nanofabrication a macroscopic block of material is manufactured into an object by removing part of it. (b) Bottom-up nanofabrication proceeds via atom by atom or molecule by molecule assembly of nanostructures. (c) “Third way” for nanopatterning. In the “third way” process the pattern is created within a homogeneous medium without morphological modifications).

2. Serial Methods The concepts and capabilities of electrochemical methods in the fabrication of patterns were initially developed using Scanning Probe Microscopy (SPM) techniques,38 since they allow for a highly precise control and confinement of the electrochemical process.39 2.1. Electrochemical Scanning Tunneling Microscopy Based Methods. Scanning Tunneling Microscopy (STM)40 is one of the methods intensively used in surface science41,42 to investigate the topography and electronic structures of conductive surfaces. Its electrochemical control has been a breakthrough in the development of electrochemical surface science, which is one the leading topics of contemporary electrochemistry.43,44 After its invention, it soon became clear that STM offers the possibility to have much more information than simple imaging and characterization,40 since its setup allows the use of the tip as a tool for nanofabrication.45-47 Although the first successful experiments were conducted in ultra high vacuum,48 it is in electrochemistry that the use of STM for nanofabrication is widely used49 because allows it to work at room temperature in ionic solutions, which are an infinite reservoir of electro-active species. Based on the well-known behavior of surface defects as nucleation centers for metal deposition, Penner et al.50 used an STM for the local generation of nanosized defects just by touching the tip against the surface at a desired position. The process can be repeated at will and the surface can be decorated with predesigned patterns of metal nanoclusters. In 2005 Homma et al.51-53 applied the same method for the decoration of Si substrates just by exposing the samples patterned with nanodefects to solutions containing metal ions. Examples of Cu, Ag, and Au clusters on silicon were reported. In an STM the tip can be brought in proximity to the surface and made to penetrate a tarnishing film causing a high overpotential for the metal deposition. When the tip is moved laterally on the surface, the film is locally removed and, if the sample is held at a slightly negative potential, i.e., near to the Nernst equilibrium, the metal deposition occurs only on the cleaned portion. In this way Cu has been deposited on CuO layers54 and on Au(111) covered with a self-assembled monolayer.55 Bringing the STM tip in proximity to the substrate causes the merging of the double-layers of the STM tip and substrate. In terms of electrochemical potential, both sample and tip potentials are no longer independent, so that the local electrochemical conditions are not well-defined. This configuration is known as “double layer cross talk”, a particular electrochemical

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Figure 2. STM image of an Au(111) surface covered by approximately 400 Pd clusters generated via jump-to-contact mechanism between an STM tip and the substrate (for description see ref 45).

Figure 3. (a) General schemes of serial local oxidation induced by an electric field, using a conductive tip. In serial processes the nanostructures are fabricated one by one. Using a conductive SPM tip, the microscope can be operated in contact, noncontact, and tapping mode. The applied sample bias can be either static or dynamically modulated. (b) Schemes of corresponding parallel process, using a stamp instead a probe all the protrusions work at the same time. The difficulty to control the tip-surface distance limits these parallel methods to contact mode, therefore they can be applied to semiconductors or self-assembled monolayers.

configuration useful both for local dissolution and for local deposition of metals. In the first application, Xie et al.56 have reported a tip induced local dissolution of a Cu surface while the electrode was held at a known potential, where no dissolution is expected. The phenomenon has been explained by assuming direct tunneling from the Cu/Cu2+ redox couple to the STM tip. This tip-induced local dissolution has then been used for local modification of semiconductor surfaces, where the same mechanism seems to be active.57 An example of local deposition, induced by local polarization of the electrode by the STM tip, was reported by Widmer et al.58 In this case, after the tip was brought into proximity to the surface, a pulse of anodic overpotential59 was applied. The cluster deposition occurs in a local cathodic process at the surface via a local tip-induced negative image charge. Schindler et al. proposed an analogous method where the STM tip is loaded by metal electrodeposition from the solution. Afterward, it is moved toward the surface and then the metal is redissolved in a burst-like way, briefly setting the tip potential positive with respect to the Nernst value.45,60 In this way the high local concentration of the cations causes a more positive Nernst potential for the surface region immediately underneath the tip that results in the local formation of a nanosized cluster. A further strategy for the local formation of nanosized metal clusters on an electrode surface is based on short-range forces between tip and substrate which give rise to the so-called “jumpto-contact” phenomenon,61,62 an instability of the STM tunnel gap causing a displacement of atoms into the gap. Controlling this phenomenon, Kolb et al.45 have fabricated a highly ordered array of metal clusters. Figure 2 shows an example of an Au(111) surface covered by approximately 400 clusters of palladium deposited via the jump-to-contact mechanism between an STM tip and the substrate. Schuster et al.56 have investigated the time-charging properties of the electrochemical double-layer to control the etching process of surfaces. This can be done by applying a nanosecond voltage pulse between a tool electrode and a work piece immersed in an electrolyte solution.63 Due to the conductivity of the solution, in such a short time only the double layers of the region around the tip and substrate can be charged. An external potentiostat holds the potential at a value such that no reactions occur outside

this region. In such way, the local etching of the substrate can be performed with high spatial resolution, the latter being a direct function of the pulse duration.64 2.2. Atomic Force Microscopy Based Methods. Most of the serial electrochemical methods for nanopatterning and nanofabrication involve the use of atomic force microscopy (AFM) as a tool for local anodic oxidation. Local anodic oxidation (or field-induced oxidation) of metallic, semiconductor and organic surfaces by AFM is the most robust, reliable and versatile lithographic method with which to fabricate nanometric scale structures and devices (for extensive reviews see references38,65). In particular, the work of the past few years has been devoted to the patterning of thin metal films66-69 with the goal of fabricating devices for the study of single electron phenomena, the generation of charge accumulation layers in heterostructures,70-72 magnetic tunnel junctions,73-79 fabrication of nanowires by successive oxidation/ etching processes80-83 and fabrication of quantum dots.71,84-86 Lately, technological interest in the fabrication of masks for lithographic processes87,88 has diminished within the scientific community, although it was proposed as the first application of nanolithography. This lack of interest is due to the difficulties involved in up-scaling the AFM oxidation process to a large area. The strong activity developed around this nanolithography technique takes advantage of the simplicity required to achieve nanometer range resolution and to image the resulting patterns with the same equipment. Local oxidation occurs by applying bias voltage between the sample and the AFM tip. Figure 3a shows a typical experimental scheme illustrating the process of scanning probe oxidation on silicon with a native layer of SiOx. A positive sample bias provides the strong electric field (typically ∼107 V/cm) for ion migration (mainly H+ and OHions) within the water meniscus formed between the tip (cathode) and the sample surface (anode), where the electrochemical reaction occurs. Local oxidation by AFM has the typical limitations of scanning probe techniques, viz. maximum scan size of the piezoelectric actuator (generally no more than a few hundred

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Figure 4. Example of more complex metal-organic nanoarchitecture fabricated via the template-guided hierarchical self-assembly using gold nanoparticles as reported in ref 104. (Copyright American Chemical Society 2004).

micrometers) and the serial fabrication dependent on the scan frequency. These limitations have been partially overcome by using multiple probes89 and high-speed AFM.90,91 Due to its well-understood mechanism and the optimized control of the process, silicon is considered the benchmark substrate for the study of nanometric size oxidation. Recently silicon has been used by Ramses et al.92 to study the time evolution of oxide dots, Lee et al.93 have electrically and thermally characterized SiOx lines, and Miyake et al.94 have investigated the influence of pressure and applied voltage on SiOx nanostructures. The same approach adopted for Si has begun for other inorganic surfaces of technological interest such as SiN95 and 6H-SiC.96 The oxidation process has been applied also to surfaces coated with SAM of organic molecules grown on different substrates.97-100 The double layer is the structured part of the liquid phase of the interface where the potential drop is localized.55b In this last case the electrochemical modified SAM is used as base for further chemical reaction as described in the so-called constructive nanolithography (CNL), originally developed by Sagiv’s group101 in 2000. In constructive nanolithography a conductive AFM tip operating in contact mode is used to write chemical information on the exposed surface of a self-assembled monolayer (SAM). Typically in the oxidative configuration, the -CH3 end group of the SAM is oxidized to -COOH. So the modified surface is more reactive, and it is used for further chemical functionalization102 (see also the section on constructive microlithography). Compared to other methods, CNL allows the control of the chemical species produced by tuning the applied bias.101 Schmid et al.103 have demonstrated that a layer of Au nanoparticles can be assembled on bilayer template patterns functionalized by thiol (-SH) from a water-soluble precursor. As an unusual and beautiful example, Figure 4 shows the reproduction by CNL of the famous poster of “world without weapons” by Pablo Picasso as published by Liu et al. in ref 104. The combination of CNL with chemical or plasma etching leads to a new technique suitable for the fabrication of nanometric sensors.105,106 In 2001, Li et al.107 introduced the so named electrochemical dip-pen nanolithography. Here molecules, previously deposited onto an AFM tip, are transferred to a surface upon the effect of an electric field generated applying a voltage between the tip and substrate. During this process molecule can be oxidized (reduced) at the surface. In their experiments Li et al. have used H2PtCl6 that was electrochemically reduced to metallic Pt during the deposition. By electrochemical dip-pen nanolithography, patterns with 30 nm size were produced on silicon.

Simeone et al. 2.3. Scanning Electrochemical Microscope. Scanning electrochemical microscopy (SECM) has been an important evolution of electrochemical scanning probe microscopy108 because it was developed with the aim to get more insight into the local reactivity of electrodes, giving information complementary to that obtainable with the STM.63 Very similar to an STM for its hardware setup, SECM measures the faradaic current flowing through the SECM-tip which depends on the reduction or the oxidation of chemical species on the sample’s surface. The tip is usually an ultramicroelectrode made from a metal wire insulated with a glass shield, which also ensures well-defined diffusion conditions. The spatial resolution of the SECM depends on the active area of the tip, for this reason Heinze et al. developed a new method for the construction of ultramicroelectrodes with an active surface diameter of about 20 nm called “nanodes” that allows precise measurements of reaction rates.109,110 As for the STM, the nanodes are potential tools for local electrochemical modification of surfaces at a nano scale, which in principle could compete with the electrochemical STM in the near future. 3. Parallel Methods Although several attempts have been made to directly parallelize the SPM fabrication processes,35,89,91 the techniques based on scanning probes keep the typical limits of the serial modes; that is, they write the nanostructures one by one and are limited by the scan size of the piezo-system. Therefore, the most promising up-grade with which to increase the fabrication rate by EL concerns the use of stamps (or molds) made from a prefabricated array of protrusions. This concept was introduced by Whitesides group in 2000.111 In their patent they proposed the oxidization of a large surface area by using a metallized stamp and fabricating several nanostructures contemporaneously. Figure 3b shows the scheme of the upgrading from serial (Figure 3a) to parallel (Figure 3b). Potentially, the use of a stamp instead of an SPM tip has several advantages with respect to serial methods, and it can be directly used to up-scale the processes. However, some technical problems have prevented its extensive use, as proved by the limited literature. The most important problem is the fine adjustment of the stamp-surface distance, which, in parallel methods, is often obtained by micromanipulators (often by simple micrometric screws). Although new instruments with piezo-electric positioning have been developed, the stampsurface distance control is not comparable to scanning probe methods (where you have to control the distance of a single tip). The new configuration allow a fine control of the position in the plane, however, due to the deformability of the elastomeric stamps, the control of stamp-surface distance is still a major problem.112 It is a matter of fact that stamps are always used in mechanical and electrical contact with substrates (apart from the adsorbed electrolytes present between the stamp and the substrate). This configuration limits the application of parallel EL to semiconductors or conductive substrates with ultrathin insulating layers such as native SiOx or metals covered by SAMs. 3.1. Parallel Oxidation. Several works deal with the upscaling of local silicon oxidation by means of using a conductive stamp with multiple protrusions as a cathode. Mu¨hl et al.113 used a stamp with few protrusions to demonstrate the feasibility of the parallel oxidation, Cavallini et al.114 transferred a logic pattern (i.e., a pattern containing information in binary code) to the silicon surface, Martinez et al.115 used silicon stamps,

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Figure 6. Scheme of electrochemical lithography in a microchannel application. The counterelectrode can be a simple metal wire in contact with the electrolyte.

Figure 5. Friction contact mode AFM images of printed monolayer patterns produced by the grid for TEM as stamp (image adapted from ref 104, copyright American Chemical Sosiety 2004).

modified by electron beam lithography, to generate arrays of parallel lines separated by 100 nm over cm2 regions, and Yokoo et al.116 have transferred a nanoelectrode pattern onto a Si substrate by fabricating a multiple patterning after an etching process. In these experiments, the oxidized areas are as large as 1 × 1 cm2, and the height of the oxide nanostructures are limited to a few nm, viz. comparable to those fabricated by local anodic oxidation due to the fact that the process occurs only under the apex of the stamp protrusions. Parallel oxidation has been successfully used also to pattern zirconium nitride thin films with features 70 nm high, as demonstrated by Farkas et al.117 3.2. Constructive Microlithography. As described in a previous section, the concept of constructive lithography is based on the fabrication of an initial chemical pattern, which can be further developed by applying a postpatterning surface chemical functionalization. This concept is exploited in the constructive nanolithography that in the parallel version is named constructive microlithography (CML) due to reduced resolution. CML was introduced by Sagiv’s group in 2003118 as an up-grading of constructive nanolithography. Moreover, Hoeppener, et al.104 have reported monolayer template patterns spanning the micrometer-millimeter dimension range, directly produced by onestep electrochemical printing on the top surface of a high-quality n-octadecyltrichlorosilane self-assembled monolayer. In their work the authors have reported on a micrometric pattern created using a metal grid as a stamp. In order to set- up the local electrochemical cell, the grid was first exposed to a saturated water-vapor atmosphere and then immediately pressed against a monolayer-coated silicon wafer surface while applying a suitable electrical bias between the metal grid and the silicon substrate. The printed pattern is the result of a water bridgemediated electrochemical oxidation process selectively converting surface exposed -CH3 groups of the monolayer to more reactive -COOH groups. After this first oxidative step the modified sites were used to react with other n-octadecyltrichlorosilane (e.g., by exposing the surface to environment containing free molecules) to selectively fabricate a second layer. An AFM friction image of a hydrophilic pattern of oxidized n-octadecyltrichlorosilane monolayer supported on silicon is shown in figure 5. The pattern is obtained using a grid for transmission electron microscopy as the stamp. Furthermore, the patterned self-assembled monolayer can be directly used as a mold for the so-called contact electrochemical

replication.119 Contact electrochemical replication is a pattern replication methodology that offers the capability of direct onestep reproduction of monolayer surface patterns. It is based on the direct electrochemical transfer of information, through aqueous electrolyte bridges acting as an information transfer medium, between two organosilane monolayers self-assembled on smooth silicon surfaces. Upon the application of an appropriate bias voltage between an oxidized monolayer of n-octadecyltrichlorosilane prepared by parallel local oxidation, which plays the role of the “stamp”, and a second monolayer playing the role of the “target”, the hydrophilic features of the stamp are copied onto the hydrophobic surface of the target. 3.3. Electrochemistry in Micro- and Nanochannels. In EL the electrochemical reaction occurs in the gap between the stamp protrusions and the surface, which is filled with the layer of electrolyte. The proximity of the tip to the substrate limits the thickness of the structures that can be fabricated (usually to a few nanometers) in between the electrodes. This limit was overcome by applying EL in a microfluidic configuration (i.e., using a micrometric channel as an electrochemical cell). Figure 6 shows a schematic view of a submicrometric channel in which the electrochemical process occurs. In this configuration, microfluidic channels control the delivery of electrolyte to selected regions. The micro channels are fabricated using the same configuration as in micromolding in capillaries:35 an elastomeric stamp, whose motif consists of grooves, is placed in contact with the surface. The grooves between the protrusions in contact with the surface delimit the micrometric (or submicrometric) channels. When a solution containing an electrolyte is poured into the open end of the stamp, it spontaneously fills the microchannels under the effect of capillary pressure. Once an electrical potential is applied to the substrate, only the zones exposed to the electrolyte solution within the microfluidic channels are involved in the electrochemical reaction. It must be noted that in this configuration it is not necessary to use a conductive stamp as an electrode. A conventional counterelectrode can be used provided that it is in contact with the solution. This scheme was successfully applied by Westcott et al. to hydroquinone self-assembled monolayer for the precise immobilization of ligands and cells in patterns120 and by Downard et al. for patterning of photoresist films on carbon surfaces.121 In a similar configuration, but with the stamp coated by a conductive layer, very recently Albonetti et al.112 have grown oxide nanostructures on a silicon surface up to 15 nm high, i.e. five times higher than those obtained with rigid or semirigid conductive stamps. This has been possible because the oxidation process occurs both under the stamp protrusions (in the classic configuration as described in the section 3.1) and inside nanometric channels. The stability of the process grants a high

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Figure 7. (a) Oxidation process performed by a stamp containing parallel lines. (b) The second oxidation, over a silicon sample previously patterned. It produces a square pattern of crossed oxide lines just by turning the stamp 90 deg with respect to the oxide lines obtained from the first oxidation. (c) AFM image of the silicon sample after the first oxidations. (d) AFM image of the silicon sample after two sequential oxidations. The oxide lines were made by applying a bias of 36 V for 2 min and are homogeneous for both oxidations.

reproducibility and can be repeated in the same sample (multistep process). In Figure 7 is shown a square pattern of lines obtained by printing parallel lines of SiOx then by simply rotating the stamp 90° and repeating the process. Similar to this approach Barlett et al. used so-called nanosphere lithography.32-34 In nanosphere lithography a template formed by polystyrene latex spheres self-assembled on metallic electrodes is used to form a 3D network of nanochannels (i.e., the cavities in between the latex spheres). Appling appropriate voltages, electroactive species are deposited on the electrodes forming a highly periodic hexagonal pattern. After the metal deposition the polystyrene spheres can be fully removed by washing in toluene to leave an interconnected network of spherical pores within the deposited film. Nanosphere lithography was successfully applied to Pt, Pd, and Co32 thin films. 3.4. Parallel Methods Based on Solid-State Electrolytes. As already mentioned, the use of liquid electrolytes can limit the applications of parallel EL. Another approach for the direct electrochemical patterning of metal surfaces was introduced in 2007 by Schultz et al.122,123 based on the solid-state superionic stamping process. In this case the cathode is made of a superionic material with mobile ions that is prepatterned by a focused ion-beam milling. The pattern transfer from the stamp to Cu and Ag surfaces has been demonstrated. In a recent work, Hsu et al.122,123 used this technique for surface reproduction of geometric features smaller than 50 nm with well-defined acute angles. Indeed, in contrast with methods where a liquid electrolyte is used, in the solid-state superionic stamping approach this is possible because the oxidation with further dissolution is restricted to the areas of the substrate in direct contact with the stamp. Furthermore the limits of spatial resolution due to ion diffusion using a liquid electrolyte are avoided. Figure 8 shows a SEM image of a patterned Ag surface obtained with a solid-state superionic stamping process where an array of geometric patterns and letters are fabricated. 4. Conclusions and Perspectives In this paper we have shown that ELs can be effectively used to fabricate small and regularly spaced nanostructures and patterns. Fabrication of micro and nano structures takes prominence in a wide range of areas ranging from microelec-

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Figure 8. SEM images of a patterned Ag surface obtained with solidstate superionic stamping process where an array of geometric patterns and letters can be seen. The radius of the circles ranges from 2.4 µm to 200 nm; the squares are patterned with a range of width from 300 nm to 1.7 µm; the triangles are patterned with varying angles from 15° to 60° with 15° steps, and the radius of the acute corner is as small as 50 nm. Each of the concentric rings is made with uniform pitch, from left to right, the pitch measures 500, 390, and 240 nm, respectively. The rectangles are patterned with a width varying from 60 nm to 1.3 µm. To form the letters, the line width is set to be 200 nm and the depth is nearly 300 nm. (b) Solid-state etching results in a 250 nm thick evaporated silver film with 10 nm seed Cr layer on top of a glass substrate, showing a complementary nanopattern to that on the stamp. (c) Perspective view of the generated pattern. (d) Close-up view of the alphabet with 200 nm line width, showing an aspect ratio better than 1. (Image from ref 123. Copyright American Chemical Society 2007.)

tronics to biodiagnostics, through photonics, sensing, and information storage technology. In all these scientific fields there is an urgent need to develop a common technological platform able to process materials on large area by means of electrochemical methods. ELs offer advantages in applications where other lithographic methods usually fail, in particular ELs allow the use of the third process, i.e., the local transformation of the substrates without altering the topography. Furthermore it offers the possibility to build other structures by chemical reactions in a controlled manner, to change the chemical nature of materials in confined and specific places, and to deposit functional materials in confined space. Using EL methods, large area (>1 × 1 cm2) patterns can be easily achieved in a routine way, in a few seconds and in a single step. ELs are simple and do not require complex tools or facilities such as a clean room or vacuum chamber, so they can be proposed as methods for laboratory prototyping. In terms of fundamental studies, EL techniques provide an alternative method to investigate chemical-physical phenomena confined into nanometric volumes, such as electrochemical reactions and electrochemical assisted self-organization. Nowadays, serial methods are the most developed and used; therefore, the major potential of ELs is expected to be in parallel methods. Up to now the electrochemical control in EL is not comparable with the traditional system for electrochemistry, because only a two-electrode configuration (work-electrode and counterelectrode) and bias pulse are used to induce the electrochemical process. In the near future, improvements are expected to allow a more efficient patterning with electrochemical processes. Among them, the introduction of the three-electrode configu-

Feature Article ration (reference, work-electrode and counter-electrode) should ensure better control of the electrochemical process and allow traditional electrochemistry to be applied to nanofabrication. Furthermore, the use of bias voltage ramps modulated in time and amplitude will enable successive electrochemical transformations on the same sample. This approach will allow the integration of ELs with electrodeposition processes such as metal deposition,124 electrodeposition of organic active layers,44,125 local polymerization, and electrochemical atomic layer deposition.126 Electrochemistry applied to nanofabrication is in an early stage of activity; thus, future growth, also in view of technological issues, is expected. We believe that significant work must still be done on the systematic optimization of existing processes and on the development of new strategies. ELs work at ambient conditions without high-technological instrumentation such as clean rooms or large facilities; these features make EL methods extremely attractive for the future of nanotechnology. Acknowledgment. The authors thank John Stephen for a careful reading of the manuscript. This work, as part of the Eurocores/ESF - European Young Investigators Awards Scheme, was supported by funds from the National Research Council of Italy and other National Funding Agencies participating in the 3rd Memorandum of Understanding, as well as from the EC Sixth Framework Programme and the project PRRIITT-Misura 4.A PROMINER. References and Notes (1) (a) Cavallini, M.; Biscarini, F.; Gomez-Segura, J.; Ruiz, D.; Veciana, J. Nano Lett. 2003, 3, 1527. (b) Cavallini, M.; Gomez-Segura, J.; Albonetti, C.; Ruiz-Molina, D.; Veciana, J.; Biscarini, F. J. Phys. Chem. B 2006, 110, 11607. (2) (a) Chou, S. Y. Proc. IEEE 1997, 85, 652. (b) Cavallini, M.; Facchini, M.; Massi, M.; Biscarini, F. Synth. Met. 2004, 146, 283. (3) Fu, L.; Liu, X. G.; Zhang, Y.; Dravid, V. P.; Mirkin, C. A. Nano Lett. 2003, 3, 757. (4) Ji, R.; Lee, W.; Scholz, R.; Gosele, U.; Nielsch, K. AdV. Mater. 2006, 18, 2593. (5) McClelland, G. M.; Hart, M. W.; Rettner, C. T.; Best, M. E.; Carter, K. R.; Terris, B. D. Appl. Phys. Lett. 2002, 81, 1483. (6) Alivisatos, A. P.; Johnsson, K. P.; Peng, X. G.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P.; Schultz, P. G. Nature 1996, 382, 609. (7) Alkaisi, M. M.; Blaikie, R. J.; McNab, S. J.; Cheung, R.; Cumming, D. R. S. Appl. Phys. Lett. 1999, 75, 3560. (8) Ma, H.; Jen, A. K. Y.; Dalton, L. R. AdV. Mater. 2002, 14, 1339. (9) Notzel, R.; Niu, Z. C.; Ramsteiner, M.; Schonherr, H. P.; Tranpert, A.; Daweritz, L.; Ploog, K. H. Nature 1998, 392, 56. (10) Yu, M.; Lin, J.; Wang, Z.; Fu, J.; Wang, S.; Zhang, H. J.; Han, Y. C. Chem. Mater. 2002, 14, 2224. (11) (a) Greco, P.; Cavallini, M.; Stoliar, P.; Quiroga, S. D.; Dutta, S.; Zachini, S.; Lapalucci, M. C.; Morandi, V.; Milita, S.; Merli, P. G.; Biscarini, F. J. Am. Chem. Soc. 2008, 130, 1177. (b) Serban, D. A.; Greco, P.; Melinte, S.; Vlad, A.; Dutu, C. A.; Zacchini, S.; Iapalucci, M. C.; Biscarini, F.; Cavallini, M. Small 2009, 5, 1117. (12) (a) Cavallini, M.; Stoliar, P.; Moulin, J. F.; Surin, M.; Leclere, P.; Lazzaroni, R.; Breiby, D. W.; Andreasen, J. W.; Nielsen, M. M.; Sonar, P.; Grimsdale, A. C.; Mullen, K.; Biscarini, F. Nano Lett. 2005, 5, 2422. (b) Leclere, P.; Surin, M.; Brocorens, P.; Cavallini, M.; Biscarini, F.; Lazzaroni, R. Mater. Sci. Eng. R 2006, 55, 1. (13) (a) Goldhaber-Gordon, D.; Montemerlo, M. S.; Love, J. C.; Opiteck, G. J.; Ellenbogen, J. C. Proc. IEEE 1997, 85, 521. (b) Melucci, M.; Barbarella, G.; Zambianchi, M.; Benzi, M.; Biscarini, F.; Cavallini, M.; Bongini, A.; Fabbroni, S.; Mazzeo, M.; Anni, M.; Gigli, G. Macromolecules 2004, 37, 5692. (c) Leclere, P.; Surin, M.; Lazzaroni, R.; Kilbinger, A. F. M.; Henze, O.; Jonkheijm, P.; Biscarini, F.; Cavallini, M.; Feast, W. J.; Meijer, E. W.; Schenning, A. J. Mater. Chem. 2004, 14, 1959. (14) (a) Tian, B. Z.; Zheng, X. L.; Kempa, T. J.; Fang, Y.; Yu, N. F.; Yu, G. H.; Huang, J. L.; Lieber, C. M. Nature 2007, 449, 885. (b) Menozzi, C.; Corradini, V.; Cavallini, M.; Biscarini, F.; Betti, M. G.; Mariani, C. Thin Solid Films 2003, 428, 227. (c) Corradini, V.; Menozzi, C.; Cavallini, M.; Biscarini, F.; Betti, M. G.; Mariani, C. Surf. Sci. 2003, 532, 249.

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