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Mixing electrochemistrywith microscopy

The integration of electrochemical measurementswith scanning-probe microscopy imaging has ledto various interrelated instruments.James P. Smith

Scanning-probe microscopy (SPM)imaging has become an indispensa-

ble tool for direct high-resolution stud-ies of surfaces and surface forces. Buteven these “views” are limited. Afterall, the production and performance ofmany biological and nanotechnologicalsystems are founded in electrochemicalprocesses. Thus, the full characterizationof many processes requires topographicimages and chemical information. Onesuch approach is to combine electro-chemical (EC) measurements with SPMimaging.

SPM was developed during the 1980s,and some of the first applications of thisnew technology involved EC measure-ment. (For a product review on SPM,see Anal. Chem. 11999966,, 68, 267 A–273A.) Until recently, EC measurement andimaging were separate. The limitationwas that the SPM probe was not anelectrode. Today, topographic imagesand simultaneous EC analyses are com-monly obtained on surfaces submergedin electrolyte solutions.

The arsenal of EC techniques thatare applied to the study of surfaces isbroad and versatile. Commercial SPMsystems equipped for EC studies use integrated software and signal-process-ing capabilities, allowing a variety of ECtechniques that complement the topo-graphic data, such as sweep voltamme-try, chronoamperometry, potentiometry,and differential pulse voltammetry.

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In many cases, the EC cell is thestandard three-electrode system consist-ing of a working, reference, and counterelectrode. The working electrode is usu-ally the sample, and the reference elec-trode is a nonpolarizable electrode ofknown potential, such as Ag|AgCl orcalomel. When EC measurements aremade with an SPM probe, both thesample and the probe become workingelectrodes; the EC cell is then a four-electrode system, and a bipotentiostat/galvanostat is required to independentlycontrol both working electrodes.

Table 1 lists selected features of com-mercial SPM instruments that are avail-able with complete EC capabilities. Thetable is not intended to be a completelist of products, but it provides a start-ing point for those interested.

MMiiccrroossccooppyy cchhooiicceessScanning tunneling microscopy (STM)was one of the first new microscopies,appearing in 1981. STM applies a biasvoltage between a sharp conductive tipand a conductive sample. At a distanceof a few nanometers, tunneling currentflows between the sample and tip. Thiscurrent indicates the tip-to-sample prox-

imity with very high accuracy. STM pro-duces true atomic resolution on somesamples even at ambient conditions, andcan be used with conductive surfaces orthin nonconductive films and small ob-jects deposited on conductive substrates.

Atomic force microscopy (AFM) isanother particular type of SPM that canbe modified for EC analysis. The key element of AFM is the cantilever, whichhas a sharp tip at its end. The force ex -perienced between the tip and sampledeflects the cantilever and generatesthe data for a topographical map ofthe surface. “AFM tips, which are nor-mally nonconductive, can be made ofconducting materials and then coatedwith an insulator except for the tip,”says David Wipf of Mississippi StateUniversity. “This creates an electro-chemical tip.”

Like STM or AFM, the scanningelectrochemical microscope (SECM)scans or rasters a small probe tip overthe surface to be imaged. Imaging occurs in an electrolyte solution withan electrochemically active tip. In mostcases, the SECM tip is an ultramicro-electrode, and the tip signal is a Fara -daic current resulting from electrolysis

of solution species. Moreover, someSECM experiments use an ion-selectiveelectrode as a tip.

Although the spatial resolution isnot as good as that produced by STM,SECM can probe the topography andreactivity of a surface with a resolutiongoverned by the dimensions of the elec-trochemical tip, which typically has a di-ameter on the order of 0.2–2 µm. Twofeatures distinguish SECM from relatedEC methods based on STM: the chemi-cal sensitivity of the SECM tip, and theuse of solution-phase ions or moleculesto produce the imaging signal.

“In traditional scanning-probe meth-ods, you are trying to image a surface,so you are looking at surface topogra-phy, and there is no reasonable chemicalspecificity,” says Allen Bard of the Uni-versity of Texas–Austin, a pioneer in developing SECM. “SECM is different.You can study reactions in solution, sur-face chemistry, insulators, and conduc-tors. SECM is not as good an imagingtool as other SPM methods, but SECMis much more chemically selective.”

On the other hand, a scanning tun-neling microscope modified to run ECexperiments may be sufficient. “You can

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CH Instruments, Inc.3700 Tennison Hill Dr.Austin, TX 78738512-402-0176

www.chinstruments.com

SECM

40,000

Imaging is produced by EC currentpassing through the probe elec-trode; imaging resolution is nor-mally >50 nm.

CHI holds the license to buildSECMs, the only SPM instrumentdeveloped exclusively for EC stud-ies, which provides quantitativechemical species data at selectedspots on the surface.

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Digital InstrumentsVeeco Metrology Group112 Robin Hill Rd.Santa Barbara, CA 93117800-873-9750

www.di.com

STM, AFM

60,000

Imaging is independent of the ECcell; electrolyte volume is <100 µL;atomic imaging resolution.

The AFM Tapping Mode is patent-ed by DI and features low-noiseoperation and ease of use. BothSTM and AFM can be used for insitu studies.

PPiiccoo SSPPMM

Molecular Imaging Corp.9830A S 51st St., Ste. 124Phoenix, AZ 85044480-753-431

www.molec.com

STM, AFM

70,000

Imaging is independent of the ECcell; electrolyte volume is <100 µL;atomic imaging resolution.

The AFM magnetic ac mode ispatented by MIC. Top-down scan-ner allows easy access and ma-nipulation of samples. STM andAFM scanners can be run by thesame microscope.

EExxpplloorreerr SSPPMM

ThermoMicroscopes99 Kingwood-Stockton Rd.Rosemont, NJ 08556-0139609-773-0456

www.thermomicro.com

STM, AFM

70,000

Imaging is independent of the ECcell; electrolyte volume is <100 µL;the STM includes both tunnelingcurrent and EC current together;atomic imaging resolution.

Electrostatic force, scanning ca-pacitance, scanning thermal, andpulsed force modes are available.

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insulate an etched STM tip, except forthe very end, and use that as an elec-trode as well as a tunneling tip,” saysWipf. Lift the tip to stop the tunnelingcurrent and you are ready for EC analy-sis. “This is a well-established area; forexample, these electrodes can be usedto modify surfaces, deposit polymers, oroxidize surfaces by generating voltagedifferences across the surface,” he adds.

“However, those doing this type ofexperiment are generally not doing thesame things that are done with SECM,”cautions Wipf. “Equipment makers maysay they are doing SECM with theirelectrochemical STM, but such state-ments are due to a misunderstanding ofwhat SECM actually is.” According toWipf, the key difference is that SECM’simaging signal is generated by a chemi-cal species, so that the informationabout the surface is controlled by thediffusion of chemicalspecies in solution.

Michael Mirkin ofQueens College agrees.“These techniques arenoncompeting becausethey do different things,”he says. “The idea is toget chemical informationwith SECM, specificallyelectrochemical reactivi-ties, and more recentlyion-transfer. This informa-tion is complementary to topographicinformation obtained by STM orAFM.”

CCoommmmeerrcciiaall llaannddssccaappeeCH Instruments, Inc. (CHI), is the onlycompany currently producing a SECMinstrument, say the experts. Accordingto CHI manager Peixin He, “Althoughsurface imaging of EC chemical reactivi-ty is an important function of SECM,the approach curve is also [useful] basicdata.”

The approach curve provides quanti-tative information on the electrical prop-erties, chemical species, and EC reactionkinetics at selected points on the sam-ple. This curve is obtained at a selectedspot by lowering the probe from farabove the surface while simultaneouslymeasuring the EC current (distance vs

current). The current is limited by thediffusion rate of the reactant to the elec-trode from the bulk electrolyte. As theprobe approaches the surface, the diffu-sion from the solution is hindered. Ifthe surface is an insulator, then the cur-rent decreases toward zero as the probemoves to the surface. If the surface is aconductive working electrode, then thecurrent increases as the probe approach-es the surface.

SECM can also supply kinetic data.“Suppose you have an enzyme on thesurface that can support a slow oxida-tion process,” says Wipf. “Then the relationship between feedback currentand distance can determine the kineticparameters of the enzyme. The ap-proach curve current will become con-stant below a certain probe-to-surfacedistance, because the EC reaction be-comes limited by the enzyme reaction

on the surface. This technique can beapplied to a heterogeneous surface witha range of different rate constants. Sincethe tip is very small, the kinetics at vari-ous small areas can be studied on thesurface—the kinetics of a 10-micronspot rather than the whole surface. Andit will be quantitative.”

How do other companies tackle ECand microscopy? According to MichaelSerry, applications scientist for DigitalInstruments/Veeco Metrology Group,their system combines EC and STM.“The STM tip is used only for imagingand has nothing to do with the ECmeasurements. A reference and a count-er electrode are inserted into the elec-trolyte. So, the system has two separatecomponents: one is the STM, whichcharacterizes the sample topography;the other is the electrochemical cell,

which is monitored in a very traditionalfashion. Events in the EC data correlatewith changes in the topography of thesample surface.” The same thing is donewith AFM. “The AFM tip does not haveto be conductive and is less susceptibleto the EC process occurring in the elec-trolyte” says Serry. “As the EC parame-ters are controlled, you can watch indi-vidual atoms being deposited or lost atthe surface.”

Shijie Wu, product manager of Mo-lecular Imaging Corp. describes a simi-lar strategy with their SPMs. His com-pany boasts environmental chambers ofvariable sizes and functions that cancontrol the experimental atmosphere,such as oxygen level and humidity.“One can also use environmental cham-bers for exposure of samples to haz-ardous gases,” Wu said. “Heating andcooling stages allow one to study a sam-

ple in a wide temperaturerange, from –30 to +200°C.”

According to Ray Eby,applications manager withThermomicroscopes, inthe STM–EC mode, “Thesample is a working elec-trode. And you can op-tionally run the tip as asecond working electrode.So you can have eitherthree- or four-electrode

systems.” Their STM can flip from ECto topographic imaging, as well.

The Thermomicroscopes AFM sys-tem uses the three-electrode configura-tion, in which the sample is a workingelectrode. The AFM probe is not con-ductive; it only performs imaging. TheEC experimental setup is in the bulk solution and is independent of the im-aging process. The STM and the AFMhave the same software interface forsweeping, cyclic voltammetry, and con-trolling all EC parameters.

Do-it-yourselfers may be interestedin Quesant Instruments Corp., whichsells an AFM system that can be modi-fied for EC experiments. “We provideour users with an optional package ofall the hardware schematics and all thesoftware, so they get the source code,the schematics, and 22 BNC connectors

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on the back of the interface units,” saysGeorge McMurtry, vice president ofQuesant. “They have direct access toall the input and output signals. Severalof our customers have built their ownEC system.” The basic unit starts at$30,000 and can cost up to $60,000.

CCoommiinngg aattttrraaccttiioonnssOne of the most interesting of the newmicroscopic techniques is near-fieldscanning optical microscopy (NSOM),which provides high-resolution opticalimages by scanning a small spot of lightover a specimen and detecting the re-flected light. NSOM uses apertureswith typically 50- to 100-nm diameters,which is much smaller than visible lightwavelengths. Image resolution is de-fined by the size of theaperture, and the opticalresolution limit for NSOMis governed by the lightintensity passing throughthe aperture. A practicalaperture limit is between80- and 200-nm diame-ters, but in ideal cases, itcan be <20 nm.

Bill Smyrl at the Uni-versity of Minnesota hasconverted an NSOMshear-force feedback system to obtaintopography with an SECM probe. “Wereplaced the normal optical fiber tipwith a scanning electrode,” he says.“We can now scan the microelectrodewith the shear-force mode. You getboth topography and electrochemicalimages. The diameter of the metalprobe is below a micron. We expect toobtain resolution below 100 nm andhope to approach 10 nm.” For example,the new system has been useful in thestudy of flaws in 100 Å-thick diamond-like carbon film on computer read/write heads.

Bard has also continued to improveSECM. “In some cases, the resolutionhas been down to the point of beingable to image DNA molecules or otherlarge biological molecules” he reports.“We can do this by using a very smalltip and a very thin layer of solution— essentially the layer that forms in ahumid atmosphere. In such a small

surface layer, the diffusion of speciesaway from the surface is very limited.The best spatial resolution occurs invery thin films of fluid–on the orderof a few nanometers.”

What could be the best of bothworlds, a combined SECM and AFMinstrument, has been developed by JulieMacpherson and Patrick Unwin at theUniversity of Warwick (U.K.). Theyshaped a 50-µm-diam. platinum wireinto a tip and coated it all with an insu-lator except for a tiny area at the end.This probe becomes the SECM elec-trode as well as the force-sensing detec-tor. “The present limitations of SECMare largely associated with the probesavailable,” says Macpherson. “Our goalhas been to marry the key attributes of

SECM and AFM by developing newprobes which are capable of unambigu-ous measurements of topography andEC activity at condensed-phase inter-faces, with enhanced resolution. The resulting improved resolution allowscloser examination of diffusional trans-port in the electrolyte,” she adds.

The characterization of surfaces bya combination of high-resolution topo -graphy and EC measurements is an ac-tive area, and new probe designs and instrumentation promise to expand theapplications for these measurements.With the availability of more and moresophisticated instrumentation and soft-ware and the development of new ap-proaches to higher-resolution topogra-phy, the electroanalytical chemist hasmoved into the atomic and molecularworld.

James P. Smith is a freelance science writerbased in western Massachusetts.

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