A study of spatially coupled bipolar electrochemistry on...

Journal of Electroanalytical Chemistry 522 (2002) 75–85

A study of spatially coupled bipolar electrochemistry on thesub-micrometer scale: colloidal particles on surfaces and cylinders

in nuclear-track etched membranes

Jean-Claude Bradley *, Sundar Babu, Beth Carroll, Aditya MittalDepartment of Chemistry, Drexel Uni�ersity, 32nd and Chestnut Streets, Philadelphia, PA 19104, USA

Received 4 August 2001; received in revised form 20 October 2001; accepted 1 November 2001

Abstract

In the present study we explore the feasibility of applying spatially coupled bipolar electrochemistry to the sub-micrometerregime. This is a technique where electrically isolated objects can be interconnected by the application of electric fields and haspreviously been demonstrated on the millimeter and micrometer scales. Three experimental designs were explored: annealed Auand Ag films on glass, annealed Ag films on silicon nitride membranes and Ag tubes immobilized within polycarbonatenuclear-track etched membranes. In the evaporated and annealed film part of this study, particles on the order of 20–50 nm wereexposed to electric fields up to 2 kV cm−1 for periods up to 180 s, in a mixture of toluene and acetonitrile. Plasmon resonancemeasurements and scanning electron microscopy (SEM) were used to characterize the changes following field application. At allfield intensities and times studied, the gold particle sub-monolayers did not appear to be discernibly affected. The plasmonresonance absorption of the silver sub-monolayers displayed significant peak broadening after field application. Furtherexperiments using transmission electron microscopy (TEM) analysis of annealed silver films on silicon nitride membranesdemonstrated particle agglomeration without evidence for particle interconnection or morphological change. This result suggeststhat, under these experimental conditions, physical movement of the Ag particles occurs instead of electrochemical processes. Inorder to prevent particle movement, polycarbonate membranes were used to anchor silver cylinders with diameters of 1 �m,400nm and 200 nm. Results from these experiments demonstrated that for Ag in 1:1 toluene–acetonitrile, spatially coupledbipolar electrochemistry (SCBE) reaches a practical limit for structures between 200 and 400 nm since the width of the depositapproaches the size of the metal particles. At 200 nm the result is electrochemical migration of the particles, where acommensurate amount of Ag is deposited on one side and dissolved on the other. This size limitation is specific only to SCBE,where structures are required to electro-dissolve, not to the application of bipolar electrochemistry to structures below 200 nm.© 2002 Elsevier Science B.V. All rights reserved.

Keywords: Bipolar electrochemistry; SCBE; Sub-micrometer scale; Electric field; Electrodeposition

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1. Introduction

Spatially coupled bipolar electrochemistry (SCBE) isan electric-field based technique to interconnect free-standing metal [1,2] or semiconductor [3] components.The principle relies on applying sufficiently high electricfields to induce electrodissolution from one componentfollowed by electrodeposition onto an adjacent compo-nent until a bridge forms. It has been demonstratedthat circuits [4,5] can be generated by controlling the

direction of the applied electric fields in the SCBEprocess. Although, electrochemical methods allowingthe formation of conductive bridges between structureshave been developed [6,7], SCBE offers the advantageof not requiring contact between the voltage source andthe components to be interconnected. Furthermore, theaddition of supporting electrolyte or metal salts isgenerally not necessary, and even detrimental because itprecludes the application of strong electric fields.

Thus far, SCBE has been applied to millimeter [1]and micrometer [2]-scale objects. The purpose of thepresent study is to investigate the potential applicationof SCBE on the sub-micrometer scale. The manipula-

* Corresponding author. Tel.: +1-215-895-2647.E-mail address: [email protected] (J.-C. Bradley).

0022-0728/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S 0022 -0728 (02 )00662 -9

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J.-C. Bradley et al. / Journal of Electroanalytical Chemistry 522 (2002) 75–8576

tion of structures on this scale is particularly challeng-ing, but the need is evidenced by recent reports utilizingtechniques such as scanning tunneling microscopy [8,9],diffusion [10] and electrochemical approaches exploit-ing edge effects [11]. A method permitting toposelectivecontactless manipulation of structures at the sub-micrometer scale would certainly complement the arse-nal of available techniques.

The scaling down of the SCBE process presents sometechnical challenges that must be addressed. BecauseSCBE is based on bipolar electrochemistry, involvingthe polarization of isolated components, high electricfields are required. To a first approximation, the re-quired field intensity to produce a given potential dif-ference at the solution interface at the opposite poles ofa conductive sphere is expected to be inversely propor-tional to its diameter [12,13]. In the case of SCBE withCu particles, for millimeter sized structures this corre-sponds to fields on the order of tens of V cm−1 [1]. Formicrometer-sized structures, fields on the order ofabout 1000 V cm−1 are needed [2]. In order to main-tain such field intensities, the use of an aqueous systemproves problematic due to excessive conductivity. How-ever, non-aqueous systems such as mixtures of tolueneand acetonitrile can be used very effectively. Because nosupporting electrolyte or metal salts are required inSCBE, such fields are easily applied without excessivecurrent at the feeder electrodes.

Polarizable particulate materials exposed to highelectric fields are known to form chains due to theattraction mediated by the induced dipoles in the parti-cles [14]. In order to avoid this electrorheological effectit is necessary to immobilize strongly the colloidalmaterial onto a support. Several methods exist to affixmetal colloids onto surfaces, the two most commonbeing adsorption of a preformed colloid onto a chemi-cally derivatized surface [15–21] or the annealing of athin metal film [22–24]. The adsorption method has theadvantage of exploiting the ability to form colloids ofdefined size with a controllable spacing [18]. However,it is likely that considerable contamination by ionic andorganic components results even after thorough wash-ing. For this reason, we chose an evaporation andannealing method to prepare colloidal scale particles ofsilver and gold on glass slides. Such a method is knownto produce sub-monolayers of gold [22,23] and silver[24] particles with diameters on the order of tens ofnanometers. This technique also makes the introductionof the feeder electrodes quite straightforward by evapo-ration of thick gold films through a mask. A schematicof the experimental set-up is shown in Fig. 1. Theimmobilization of the colloids onto an open transparentsurface is also very conducive to analytical techniquessuch as scanning electron microscopy (SEM) and UV–vis spectroscopy, which are used in the first part of thepresent study. However, as will be detailed below, the

results obtained from these experiments alone wereinconclusive from the standpoint of answering the ques-tion of whether or not SCBE can be operative at thesub-micrometer scale. Further experimentation onSi3N4 and polycarbonate membrane supports was re-quired to address this issue fully.

2. Experimental

2.1. Annealed Au and Ag films on glass

Thin films of Ag and Au supported on microscopecover slips were prepared by e-beam evaporation of therespective metal. The target metals (Ag–Au) were pur-chased from Aldrich at 99.99% purity. Microscopecover slips (24×40 mm2) were used as received fromFisher Scientific. Following the deposition the sampleswere annealed at 175 °C for 20 min to generate theparticle sub-monolayer. Gold electrodes (100 nm thick)were then introduced by e-beam evaporation with ametal strip used as a mask, giving an electrode separa-tion of 5 mm. Copper wires were then glued to the goldelectrodes using Ag paste. Fig. 1a and b show theschematic of the entire process of sample preparationand the cross sectional view of the fabricated sample,respectively. Samples were then partially immersed asshown in a 1:1 mixture of C6H5CH3 and MeCN(refluxed over CaH2 before mixing), under nitrogen,then a potential was applied across the electrodes.UV–vis spectra of the immersed area were recordedbefore and after field application using a UV–vis spec-trophotometer (Perkin–Elmer Lambda 2). After evapo-ration of a conductive coat of carbon, SEMmicrographs were obtained from a JEOL 400 fieldcompensated scanning electron microscope. As only thebottom halves of the samples were immersed in thesolution during field application, the top portions wereused as the ‘before field’ control for SEM measure-ments. Although, technically the top portion was ex-posed to an electric field in air, we refer to this portionof the slide as the ‘before field’ control since spectraobtained from this region remain unchanged after fieldapplication. Because the evaporation of a carbon coatis necessary for SEM observation, it is not possible toobtain true before and after SEM micrographs on thesame slide. By using the air exposed portion of thesame slide that was exposed to the field in solution,variations in sample preparation could be taken intoaccount much more reliably than if an unexposed slidewas used as a control.

2.2. Silicon nitride experiments

Silicon nitride membrane TEM grids (200 nm filmsof Si3N4 deposited on 3.05×3.05 mm2 square Si

J.-C. Bradley et al. / Journal of Electroanalytical Chemistry 522 (2002) 75–85 77

wafers, with a 0.46×0.46 mm2 working window) wereobtained from Structure Probe, Inc. Ag (99.99% purefrom Aldrich) was sputtered onto the top side of themembranes for 60 s then annealed at 175 °C for 20 minto generate a particle sub-monolayer. The grids werethen placed between two 1 mm diameter Pt electrodesspaced 5 mm apart and immersed in a 1:1 mixture ofC6H5CH3 and MeCN (refluxed over CaH2 before mix-ing), under nitrogen. A potential was applied across theelectrodes resulting in a field intensity of 6000 V cm−1

for 180 s. TEM analysis both before and after the fieldapplication was performed with a JEOL 2010 F 200 kVfield-emission gun TEM/STEM.

2.3. Polycarbonate experiments

The preparation of Ag rods anchored within in thepores of Nucleopore polycarbonate nuclear-tracketched membranes obtained from Structure Probe, Inc.

is depicted in Fig. 7, adapted from a previously re-ported technique [25].

2.3.1. Step 1Nucleopore membranes of various pore diameters (1

�m, 400 nm and 200 nm) were obtained from StructureProbe, Inc. and used as received. Electroless depositionof Ag inside the pores was accomplished by a two-stepprocess (initiation and deposition). All the chemicalsrequired for both the steps were purchased fromAldrich. The initiator solution was prepared by addingtin(II) chloride (0.9858 g) and trifluoroacetic acid(1.0756 ml) to 200 ml of 1:1 water–MeOH. Membraneswere immersed in the initiator solution for 5 min,washed thoroughly with MeOH then immersed into theelectroless plating solution (AgNO3 (0.6749 g)–CoSO4

(1.55 g)– (NH4)2SO4 (9.91 g)–30% NH4OH (16.33 ml),made up to 100 ml with water) for 30 min in an airtight container under slow agitation and finally washedthoroughly with water and air dried.

Fig. 1. (a) Schematic of the sample preparation on glass; (b) cross sectional view of the fabricated sample; (c) schematic of the experimental setup for field application.


Fig. 2. Scanned images of slides and corresponding UV spectra. The vertical axis represents applied field intensity (kV cm−1) and the horizontalaxis represents time of field application (s). (a) Scanned images of gold slides and corresponding UV spectra for each case. (b) Scanned imagesof silver slides and corresponding UV spectra for each case. (c) A magnified version of specific cases for UV spectra of gold and silver slides beforeand after field application. Field applied is 2 kV cm−1, field application time is 180 s. For all the spectra shown, the black line represents the UVspectrum before field application and the red line represents UV spectrum after field application. Absolute color differences between slides are dueto slightly varying scanning conditions. Only the color differences from the top and bottom half of the same slide should be compared.

2.3.2. Steps 2, 3 and 4The membranes were cut into pieces of l×1 cm2

pieces and one side of the membrane was then affixedto a 2×2 cm2 piece of clear adhesive tape. The otherside of the tape was then fixed to the flat surface of apolymer block using double sided tape taking sufficientcare to avoid air bubbles.

2.3.3. Step 5The Ag layer on the exposed side of the membrane

was then removed by polishing using 1 �m alumina

slurry as the abrasive in a metallographic polisher at150 rpm for 30 min. The samples were then washed,air-dried and examined under an optical microscope tofind the degree of removal of Ag and repeated until noAg could be seen on the top layer.

2.3.4. Steps 6 and 7The polished membranes were then carefully re-

moved from the polymer block and affixed to thebottom piece of a LECO Mold Cup (polyethylene)using the double-sided tape. Castolite resin (polyester


resin from LECO) was then poured from the top andcured at 47 °C inside an oven for 60 min. After coolingthe casting to room temperature (r.t.), the double-sidedtape along with the clear tape was removed carefullyfrom the membrane–polymer block. The membrane–polymer block was then washed thoroughly withC3H6O and excess adhesive (from the clear tape) stick-

Fig. 4. High resolution SEM images for silver and gold samplesbefore and after field application. (a) Images taken at the central areaof silver for varying applied field intensities for the maximum fieldtime of 180 s studied. (b) Images of all the three locations of silversamples (i.e. anode, central and cathode areas) at the highest appliedfield intensity of 2 kV cm−1 for 60 s. (c) Images of all three locationsfor gold samples at the highest applied field intensity of 2 kV cm−1

for 180 s.

Fig. 3. Low resolution SEM images for silver samples in the areaexposed to field (referred to as ‘after field’) and not immersed whileexposed to field (referred to as ‘before field’). See text for details. (a)Images taken at the area near the anode. (b) Images taken at thecentral area. (c) Images taken at the area near the cathode.

ing to the membrane was removed by scrubbing gentlywith soap. The exposed Ag layer was removed bypolishing, by repeating step 6. Individual pieces ofmembranes were cut to 1×0.5 cm2 pieces and themembrane was scratched from either end leaving only 3mm of membrane in the center of the 1 cm long piece.Platinum wire (0.25 mm diameter) (99.5%, Aldrich) wasthen wound on either side of the membrane patchspaced by 5 mm. The entire step-up was then immersedin a 1:1 mixture of C6H5CH3 and MeCN (refluxed overCaH2 before mixing) then exposed to field intensitiesranging from 4 to 6 kV cm−1 for various times rangingfrom 60 to 180 s. The membranes were then examinedwith a JEOL 6400 SEM.


3. Results

3.1. Annealed Au and Ag films on glass

3.1.1. Color changes and UV spectroscopyAs discussed in Section 2, since only the bottom half

of the glass slide samples were exposed to the electricfield in the solvent, it is possible to distinguish the effectof the electric field treatment by visually inspecting theupper and lower portions of the slides. Fig. 2 showsscanned images of the slides along with the correspond-ing UV spectra of the experiments after field applica-tion at various durations and intensities. For the Agsamples at higher fields and longer duration, a colorchange can be seen in the immersed area exposed to theelectric field. The broadening in the UV spectra isaccompanied by a decrease in the peak intensity, givingthe appearance of a peak flattening. For the goldcolloids, at all field intensities and duration, no colorshift from the characteristic red is observed.

3.1.2. SEM dataFig. 3 shows SEM images of the silver samples taken

in the immersed area exposed to the field and the area

not exposed for the samples shown in Fig. 2. At thisscale the individual particles appear as small dots. Inregions that are densely packed, no noticeable changescan be seen in this case for the area close to the anodeor the central area. However, in the area close to thecathode, fractal growth can be observed in several fieldduration and time regimes. In the case of gold (data notshown) there is no difference at any field intensity orduration or location at this scale.

Fig. 4 shows SEM data at a higher magnification,where individual particles can be seen. In regions wherethe particle density is unchanged, inspection of theregion immersed and exposed to the electric field doesnot reveal significant differences with the un-immersedregion, in terms of particle morphology, size or align-ment. For the Ag samples, occasionally voided areaswere identified. These are shown in Fig. 5.

3.2. Annealed Ag films on Si3N4 grids

Since SEM investigations of silver films on glass werelimited by resolution of the instrument used, an ap-proach was sought to use TEM, where much higherresolution can be achieved. Experiments on commonlyused copper–Holey carbon TEM grids were not possi-ble because a high conductivity substrate would preventthe induction of a potential drop across the metalparticles, necessary for bipolar electrochemistry. How-ever, minimally conductive Si3N4 membranes on siliconsupport were found to be suitable for this approach. Inaddition, with this setup it was possible to carry outelectron microscopy analysis before and after field ap-plication and superimpose the same area for an ex-tremely sensitive analysis of even subtle morphologicalchanges of individual particles. Using the same evapo-ration and annealing technique used for the glass sub-strates, sub-monolayers of Ag particles with diametersof 20–50 nm were formed on the Si3N4 membranes, asshown in Fig. 6a. After the application of the electricfield, TEM analysis revealed areas where there wasabsolutely no change in particle morphology or posi-tion (Fig. 6a and b) and areas of significant particleagglomeration (Fig. 6c and d). The particles within theagglomerates were undistinguishable in size from theparticles remaining on the glass, as is shown in Fig. 6eand f. This agglomeration behavior was similar to thatfound for the Ag sub-monolayers on glass and analyzedby SEM, with additional information about the lack ofmorphology changes at the individual particle level.

3.3. Ag cylinders in polycarbonate membranes

Ag cylinders were prepared within nuclear-tracketched polycarbonate membrane templates using elec-troless deposition by adapting a previously reportedtechnique developed by Martin [25]. The sequence of

Fig. 5. SEM micrographs of voided areas for the Ag particle sub-monolayer on glass exposed to 1 kV cm−1 for 120 s in 1:1 toluene–acetonitrile.


Fig. 6. TEM micrographs of Ag particle sub-monolayers on Si3N4 membranes: (a, b) same region before and after field application, respectively;(c, d) agglomerates next to voided areas after field application; (e) typical area before field application; and (f) overlapped particles inagglomerated area showing no difference in particle size in those regions. The applied field intensity was 6 kV cm−1 for 180 s in 1:1toluene–acetonitrile.

polymer casting and polishing on both sides of themembranes, as depicted in Fig. 7, resulted in cylindersthat stuck out of pores slightly. With the electric fieldapplied perpendicular to the tubes, the scale-dependentbehavior of SCBE could be studied with the cylindersfirmly anchored within the membranes. All otherparameters being equal, the potential drop across aconductive cylinder perpendicular to an electric field isidentical to a sphere of the same diameter. Thus, withinthe context of a scale-dependence study of the SCBEprocess, the results obtained from Ag cylinders shouldbe comparable to other bipolar electrochemical investi-gations using spheroidal structures. In fact, such atechnique of using cylinders to study bipolar electro-

chemistry has been applied previously on the centimeterscale for copper [13].

Fig. 8 shows the results of the application of electricfields to Ag cylinders anchored in the polycarbonatemembranes. For the 1 �m cylinders, similar growth tothat of micrometer scale copper particles was observed[2]. At the 400 nm diameter scale, the SCBE processwas still operative with a wire growing from one cylin-der towards the next, although the wire was about halfthe diameter of the cylinder. However, at the 200 nmscale, although bipolar electrodissolution and -deposi-tion were still taking place, the size of the deposit wason the same order as the diameter of the cylinder. Theresult was electrochemical migration towards the anode


Fig. 7. Processing of the polycarbonate membranes to produce electrically isolated Ag cylinders extending slightly from the surface of themembranes. See Section 2 for details.

of the portion of the cylinder extending out of themembrane.

4. Discussion

The present investigation focused on determiningwhether or not SCBE can be operative at the sub-micrometer level. As will be discussed below, the obser-vations are consistent with this hypothesis.

Sub-monolayers of Au or Au particles on glass werechosen because of their exploitable optical properties.In the colloidal state, both metals are characterized bystrong absorption bands in the visible region, caused byplasmon resonance [15–24]. Since the position andshape of this band is sensitive to changes in particle size[16,26], morphology [25,27] or inter-particle spacing,[17,20,11,28] it serves as a sensitive measure for field-in-duced changes in the particle sub-monolayer. The com-parison between the behavior of silver and gold alsooffered one means of differentiating between electro-chemical and physical effects induced by the appliedelectric field. In the solvent system used in the present

study (toluene–acetonitrile), silver [29] has been shownto exhibit SCBE behavior while gold has proven to beinert [30]. For this reason also, gold was used as theelectrode material.

The annealed films of silver and gold exhibited theexpected plasmon absorption for particles in the ob-served size ranges. For gold, a peak at about 550 nmwas observed, which is in line with published findingsof evaporated and annealed gold films [22,23]. Forsilver, a broad peak at 420 nm was observed, consistentwith previous reports [24]. The particle sizes obtained of20–50 nm for Ag and 20–50 nm for Au are alsoconsistent with the published values for Ag [24] and Au[22,23] colloids prepared by evaporation and annealingmethods.

After the application of electric fields in the range of500–2000 V cm−1 for durations of 30–180 s, the silverparticle sub-monolayers showed a color change fromyellowish to reddish for the higher field intensities andtimes. This color change corresponded to a broadeningand flattening of the peak. A shift in the plasmonresonance suggests a change in the electromagneticenvironment of the colloid particles, which might be


Fig. 8. SEM micrographs of top view of Ag cylinders in polycarbon-ate membranes: (a) 1 �m pores after 4 kV cm−1 for 180 s; (b) 400 nmpores after 4 kV cm−1 for 120 s; (c) 200 nm pores after 6 kV cm−1

for 90 s. 1:1 toluene–acetonitrile was used as the solvent in all cases.

the densely packed regions, very little difference wasobserved on the particle diameter scale between regionsexposed to the field and those not immersed duringapplication of the field in the central region where theUV spectra were recorded. Another suggestion that apurely physical effect is not dominant concerns the factthat the gold samples, that do not undergo electrodisso-lution under these conditions, show no significantchanges in the plasmon resonance at any field intensityor duration.

Attempts to identify links between colloids were notsuccessful due to the insufficient resolution of the SEMimages. In order to ascertain conclusively whether ornot inter-particle links were formed, an experiment wasset up in such a way that TEM could be used. Conven-tional TEM grids are not suitable because the particlescannot rest on a conductive substrate during the appli-cation of the electric field. However, silicon grids withSi3N4 windows proved suitable for this type of experi-mental design. After evaporation of silver and anneal-ing under the same conditions as was done for the glasssubstrates, particles on the order of 20–50 nm wereformed (Fig. 6A). After application of the electric fieldunder the same conditions as for the glass substrates,the same area was inspected for even slight changes.The result after the field application (Fig. 6B), showsthat not only is no inter-particle bridge formed, but thebefore and after images can be superimposed perfectly,indicating no morphological change or migration of theparticles in that area. However, as was the case onglass, some voided areas could be identified on themembrane and appear to involve particle agglomera-tion at the edge of the voids. Although, it is notpossible to exclude the possibility that these agglomer-ates are formed through an electrodeposition mecha-nism, the fact that the size of the particles within theclumps is very similar to the size of the particles on themembrane suggests that the aggregation is caused byphysical movement of the particles.

However, a purely mechanical explanation of particleagglomeration, either through electrorheological orconvective effects does not explain the differences inbehavior between Au and Ag particles on glass. Thepossibility that the spectral shifts in the case of silverare attributed oxidation, as has previously been re-ported [31], is unlikely, since at least a slight change inparticle morphology would be expected, which is notseen in the Si3N4 membrane experiment (Fig. 6a and b).Based on SEM images alone, it cannot be ruled outthat there are some areas on the surface where theparticles are not perfectly isolated. In this case, bipolarelectrodissolution can be expected to create voids.However, without additional experimental probing, thecause of the difference in behavior between gold andsilver cannot be conclusively determined from theseresults.

caused by particle interconnection or by changes inparticle spacing or ordering.

The SEM data revealed, for the most part, areas ofdensely packed particles (Fig. 4) in addition to occa-sional voids (Fig. 5). Closer inspection of the denselypacked areas did not indicate any significant shift in theparticle size distribution or density changes, comparedto areas not immersed and exposed to the electric field.Furthermore, the particles apparently did not alignwith the electric field, as might be expected if anelectrorheological effect [14] were operative. In fact, in


In the region close to the cathode a fractal type ofgrowth is observed for silver, as is shown in Fig. 3c.Because of its localization it is likely that this growthinvolves direct plating from the cathodic feeder elec-trode. Electrodeposition in the absence of supportingelectrolyte is well known to produce fractal-type de-posits [32–34]. In the present case the deposition ap-pears to progress by successively bridging the particles,reminiscent of the electrodeposition onto non-conduc-tive supports using a colloidal seed monolayer [35] or atsolvent interfaces [36]. The source of silver ions ispresumably linked to bipolar electrodissolution of otherparticles or particle agglomerates in the sub-monolayeror from silver, which may have been in direct contactwith the anodic feeder electrode.

In order to avoid the possibility of particle migration,an experimental design was set up to anchor the metalstructures in place. A convenient substrate for this wasfound with polycarbonate nuclear track-etched mem-branes. This system was very convenient because it canbe obtained commercially with very precise pore sizes atvarious pore densities. Since a cylinder or disc exposedto an electric field behaves identically to a sphere, thisproved to be a convenient set up to test the behavior ofdifferent sized structures under SCBE conditions. Theonly requirements are that the cylinders be electricallyisolated from each other and that a small disk sectionextends beyond the pore. When an electric field isapplied parallel to the membrane surface the exposeddisk-like portion of the cylinder will experience a poten-tial difference at opposite poles. Isolation of the silverrods inside the membrane was accomplished by firstoverfilling the pores with an electroless plating methodfollowed by several steps of polishing and polymercasting, using SEM analysis of the intermediate steps toensure isolation on the back side. The slight extensionof the cylinders out of the membranes was a fortuitousresult of the last polishing step, apparently related toslightly faster removal of polycarbonate membrane ma-terial relative to silver.

Three sizes of pores that were explored ranged from1000, 400 and 200 nm. The micrometer scale poreexperiment was used as a type of control to ensure thatwe could reproduce the behavior of micrometer-scalecopper particles, previously reported [4]. As shown inFig. 8a, wire growth can clearly be seen after exposureto 4 kV cm−1 in 1:1 toluene–acetonitrile. An interest-ing difference observed on this scale is that toposelectiv-ity is reduced compared to the millimeter-scale SCBEexperiments [1]. In other words, multiple branchingwith large contact bases, relative to the circumferenceof the disks are observed. On the millimeter scale, thistype of behavior is typically observed in bipolar elec-trodeposition experiments where metal salt has beenpurposefully added [1]. It appears that, on this scale,

there is not a significant focusing of the silver ion cloudproduced from adjacent disks. The self-focusing charac-teristic on the millimeter scale is no longer present atthe micron and sub-micrometer scale. This can beclearly seen in Fig. 8a, where the growing wires arealigned in the direction of the electric field, with noredirection towards the target disk. Since the relativecontribution of the induced dipole in the adjacent parti-cle relative to the external electric field is expected toremain unchanged for the same induced potential dif-ferential across the particle, the loss of the self-focusingbehavior may be related to the greater diffusion ofsilver ions relative to the size of the conductive objectsand and/or the loss of the convective contribution [37]on this reduced scale.

To a first approximation, an isolated conductivesphere or disk exposed to an electric field in a lowconductivity solvent is expected to induce a potentialdifferential at the solvent interface present at oppositepoles in line with the electric field equal to the exter-nally applied electric field× the diameter [12,13]. Usingthis relationship, we can then calculate a maximumpotential drop across an isolated particle in the glassexperiments at about 10 mV (assuming a 50 nm diame-ter) and in the Si3N4 experiments at about 30 mV(assuming a 50 nm diameter). For the polycarbonatemembrane experiments, using the same calculation, po-tential differences of 400 mV for 1 �m, 160 mV for 400nm and 120 mV for 200 nm structures are obtained.Thus the lowest calculated potential at which we canconfirm coupled bipolar electrodissolution and elec-trodeposition of Ag is 120 mV. Although, the calcu-lated potentials for particles on glass and Si3N4 areconsiderable below this number at 10–30 mV, thesingle particle approximation is not valid in these sys-tems because of the dense packing of the particles onthe substrates. When inter-particle distances are lessthan three particle diameters, significant field enhance-ments are expected [37,38]. Unfortunately, the agglom-eration of the Ag particles prepared by evaporation andannealing at the fields studied here precludes the appli-cation of higher fields.

In conclusion, the practical limit of SCBE for Ag in1:1 toluene–acetonitrile has been found to lie between200 and 400 nm. The limitation is related to the widthof the growing wire, which reaches a lower limit ofabout 200 nm. However, the present study does demon-strate that bipolar electrochemistry can be induced onstructures with diameters of 200 nm, provided that thestructures can be effectively immobilized. Nuclear-tracketched membranes were found to be convenient sub-strates for the immobilization of the conductive struc-tures and application of the electric fields to inducebipolar electrochemistry.


Acknowledgements

Support from NSF grant CHE-9875855 is gratefullyacknowledged.

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