in situ observation of electrode surfaces using scanning maxwell-stress microscopy under water

SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 27, 317È323 (1999)

In situ Observation of Electrode Surfaces UsingScanning Maxwell-stress Microscopy under Water

Yoshiki Hirata,1,* Fumio Mizutani1 and Hiroshi Yokoyama21 National Institute of Bioscience and Human-Technology (NIBH), 1-1 Higashi, Tsukuba, Ibaraki 305-8566, Japan2 Electrotechnical Laboratory (ETL), 1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan

We present the results of a recent attempt to extend scanning Maxwell-stress microscopy (SMM) to aqueousenvironments. Scanning Maxwell-stress microscopy is a type of scanning probe microscopy designed to imagemicroscopic electrical properties, such as the local surface potential distribution. In particular, the non-resonantbehaviour of SMM makes it fundamentally suitable for underwater operations, in contrast to other resonant-typeelectric force microscopes. From underwater operations of SMM, the high viscosity of an aqueous solution can bemade immaterial by making the driving frequency sufficiently low, whereas the large dielectric permitivity of wateris always quite advantageous in enhancing the electric forces. Furthermore, the electrochemical potentials of stylusand sample are controlled independently and simultaneously during SMM by using a dual potentiostat. This makesit possible for in situ observation of charge on the electrode surface during an electrochemical reaction. Copyright

1999 John Wiley & Sons, Ltd.(

KEYWORDS: Scanning Maxwell-stress microscopy (SMM); surface potential ; electric double layer ; modiÐed electrode

INTRODUCTION

Electrostatic interactions play an extremely importantrole in forming the structure and appearance of its func-tions in organic molecular assemblies and a widevariety of biological systems. Recently a number ofscanning microscopes have been developed to observeelectrical properties such as isolated charges,1,2 dielec-tric constant of insulating thin Ðlms3 and surfacepotentials4h7 with submicron resolution. They o†er thepossibility to measure a variety of physical propertieswith high spatial resolution. These techniques were usedmainly in media with low viscosity, i.e. in atmosphericor vacuum conditions. The high viscosity of an aqueoussolution is not beneÐcial for operational stability andsensitivity of resonant-type electrostatic microscopy.

In this paper we present the results of our recentattempt to extend scanning Maxwell-stress microscopy(SMM) to aqueous environments. Scanning Maxwell-stress microscopy is a dynamic electric force microscopyoperated at the “o†-resonanceÏ frequency of the cantile-ver and has been demonstrated to be useful for imagingstatic and high-frequency charge distributions on sur-faces under atmospheric conditions.8h18 The o†-resonant feature of the SMM makes it suitable for usein aqueous solution, in contrast to other resonant-typedynamic electric force microscopes.

* Correspondence to : Y. Hirata, National Institute of Bioscienceand Human Technology (NIBH), 1-1 Higashi, Tsukuba, Ibaraki 305-8566, Japan. E-mail : hirata=nibh.go.jp.

We investigated basic features of the operation ofSMM in aqueous solution by analysing the frequencydependence and forceÈdistance relationships and foundan optimal condition for under-water SMM. Further,the e†ect of the electrolyte for SMM measurement wasexamined.

PRINCIPLES OF SMM

The principles of SMM are well described else-where.9,12,13,15 In SMM, an alternating voltage isapplied to a conductive stylus placed on the end of can-tilever, and the resultant forced oscillation of the cantil-ever is detected by the usual optical position-sensingdetection. To simplify treatment, we assume that thestylus sample conÐguration can be regard as a planeelectrode set parallel against the sample. The electro-static force acting on the stylus is given by

F\ 12V 2 LCLd

(1)

where C and d are the capacitance and distancebetween metal-coated stylus and sample, respectively.To vibrate the stylus, a d.c.-biased sinusoidal voltagewas applied between them

Vt \ VDC] VAC sin ut (2)

The oscillation contains not only the fundamental butalso the higher harmonics of the applied a.c. voltage fre-quency. Resultant frequency components are given as

CCC 0142È2421/99/050317È07 $17.50 Received 15 September 1998Copyright ( 1999 John Wiley & Sons, Ltd. Accepted 14 December 1998

318 Y. HIRATA, F. MIZUTANI AND H. YOKOYAMA

Figure 1. (a) Schematic diagram of the scanning Maxwell-stress microscope. (b) Detailed circuit of electrochemical scanning Maxwell-stress microscope.

Eqs (3) and (4) by eliminating the vibration displace-ment of the stylus (d \ constant)

Fu \ ed2 VAC(VDC] VS)sin ut (3)

F2u \ [ e4d2 V AC2 cos 2ut (4)

The fundamental component is due to the interactionbetween the oscillating charge on the stylus induced bythe applied a.c. voltage and the charge and, therefore,polarization in the sample. On the other hand, thesecond-harmonic term represents the force oscillating

charge on the stylus and the conductor. Using these fre-quency analyses, SMM has made it possible for simulta-neous imaging of local electrical properties and surfacetopography.

EXPERIMENTAL SET-UP

SMM in air atmosphere

A silicon nitride cantilever with a pyramidal stylus(OMCL-RC800, Olympus, spring constant D0.05 N

Surf. Interface Anal. 27, 317È323 (1999) Copyright ( 1999 John Wiley & Sons, Ltd.

SMM OF ELECTRODES UNDER WATER 319

m~1, resonant frequency D10 kHz) was used aftercoating the surface with a 50 nm thick Pt Ðlm by sput-tering to make it conductive. The scanning Maxwell-stress microscope consists of a commercial atomic forcemicroscope (Nanoscope IIIa with multimode AFMhead, Digital Instruments), dual lock-in ampliÐers, analternative voltage source and a d.c. voltage source. Theoscillating force signal appearing from the AFM headpreampliÐer is led to a couple of lock-in ampliÐers,which extract the fundamental and second-harmoniccomponents and The preampliÐed signal isFu F2u .picked up from a signal breakout box placed betweenthe AFM head and the controller.

The amplitude signal of which gives informationFu ,about the surface potential and/or isolated charges, wasfed back to the d.c. bias voltage source to determine the

which eliminates the fundamental amplitude ; thisVDC ,is then ampliÐed and either measured directly or fedback to give a d.c. bias voltage to compensate for thefundamental oscillation. The resultant voltage ([V DC),which gives the surface potential of the sample, is sentto the auxiliary input of the Nanoscope to compose thesurface potential image.

The amplitude of the second-harmonic signal (2u) isused to drive the z-piezo to control the tipÈsurface dis-tance to almost constant, as describe below. The 2uamplitude is compared to an externally given set pointvalue. The error from this value was ampliÐed to dis-place the z-piezo scanner by a high-voltage ampliÐer.The feedback gain and time constant can be set manu-ally. Throughout SMM, the situation of feedback statuswas monitored by a voltmeter or oscilloscope, etc.

Scanning Maxwell-stress microscopy under water andelectrochemical SMM (EC-SMM)

Figure 1(b) shows a diagram of the electrochemicalscanning Maxwell-stress microscope. The harmonicsanalyses block and feedback circuit was in commonwith SMM in air. For easy to access to the electro-chemical cell, a stylus scan-type AFM head (Bioscope,Digital Instruments) was introduced. The AFM headand electrochemical cell were mounted on an invertedoptical microscope (Zeiss Axiovert 100TV). To controlthe potential of the stylus and sample separately, a dualpotentiostat (HD-502D, Hokuto Denko) was employed.To apply an alternating voltage between the stylus andthe sample electrode, the alternating voltage source wascombined with an electrically isolated condition. Toavoid the alternating voltage being inÑuenced by thestability of the potentiostat, a low-pass Ðlter is insertedbetween the voltage source and the potentiostat.

Sample preparation

2-Dodecanylthio-1,4-naphthoquinone was synthesizedin our laboratory and used as a surface modiÐer for agold electrode by a self-assembling technique. Otherchemicals were of extrapure grade and were used asreceived. Milli-Q water was used for preparing the elec-trolyte. A gold electrode was prepared by vacuum depo-sition using a high vacuum deposition system. Agold-plate electrode was cleaned thoroughly by an elec-

trochemical oxidation/reduction cycle treatment ([0.3V to ]1.5 V vs. Ag AgCl for 30 min) in 0.05 M H2SO4solution before force measurements and before surfacemodiÐcation. The auxiliary electrode and reference elec-trode used were Pt wire and Ag/AgCl, respectively. Tomeasure the redox behaviour, cyclic voltammogramswere recorded using a potentiostat in 0.1 M KCl solu-tion. The sample Ðlms were prepared by dipping in amixed chloroform/ethanol (1 : 1) solution of 2-dodecanylthio-1,4-naphthoquinone and dodecanethiole(molar ratio 1 : 2) for 6 h. After treatment, the electrodewas rinsed well in pure solvent and then water.

RESULTS AND DISCUSSION

Scanning Maxwell-stress microscopy in air

Figure 2 shows the frequency dependence of the electricÐeld-induced oscillation amplitude of the cantilever,together with its phase. One can see the resonance fre-quency peak at D11 kHz in air. At a resonance fre-quency of [1 kHz, the oscillation amplitude wasgradually decreased to zero. Therefore, the operationfrequency of SMM should be set at less than half theresonance. As mentioned previously, SMM detects 2uoscillation to control the stylusÈsurface distance.

Figure 3 shows the force curves for oscillation at 2utaken against a vapour-deposited gold Ðlm electrode inair. The alternating frequency was set at 4.0 kHz andthe amplitude, was changed from 0.2 V to 4.0 VVAC ,(peak to peak). In order to operate SMM in a stablecondition, the forced oscillation should be set at D2È3V. Amplitude response was increased logarithmicallyfrom [10 lm to a few nanometres before directcontact. The shape of the force curve seems to reÑectthe geometry of the stylus and sample system. This rela-tion can be written as Eqn. (1). The deviation from 1/d2dependence expected from the parallel capacitor modelis inÑuenced by stylus shape and Van der Waals gravi-tational force.

Scanning Maxwell-stress microscopy under water

Figure 4 shows the frequency dependence of the electricÐeld-induced oscillation amplitude of the cantilever,together with its phase, in pure water. The electrochemi-cal potentials of the stylus and the sample electrodewere regulated at 0.0 V vs. RHE (reversible hydrogenelectrode) as a reference by using a dual potentiostat.This di†ers from operation in air because there is noamplitude peak around the resonance frequency of thecantilever due to the large viscosity of water. This isdetrimental for other resonant-type scanning probemicroscopes but is not a problem for SMM andEC-SMM because these microscopes do not need to beoperated at resonant frequency, as mentioned above.When we choose an applied frequency that is very muchlower than the resonance frequency, an oscillationamplitude sufficient for controlling the stylusÈsamplesurface distance can be obtained. The amplitude of theforced oscillation in the mode of EC-SMM is several

Copyright ( 1999 John Wiley & Sons, Ltd. Surf. Interface Anal. 27, 317È323 (1999)


Figure 2. Frequency dependence of second-harmonic amplitude and its phase in air. Applied voltage between stylus and sample is V ¼2 V(peak to peak).

times larger than that of operation in air. This largedi†erence is attributable to the di†erence in dielectricpermitivity between air and aqueous medium (eair D 1and This is quite advantageous for under-ewater D 79).water SMM of high resolution.

Next, the e†ect of the electrolyte for SMM measure-ment was examined. In Fig. 5, relationships between thesecond-harmonic amplitude and the stylusÈsurface dis-tance at di†erent KCl concentrations are shown: VACand the frequency were set at 0.2 and 400 Hz,Vpvp

Figure 3. The second-harmonic amplitude change during variation of tip–surface distance in air. The applied voltage between stylus andsample was: (a) 4 V; (b) 3 V; (c) 2 V; (d) 1 V; (e) 0.2 V.



Figure 4. The frequency dependence of the second-harmonic amplitude and its phase in pure water. The applied voltage between stylusand sample is V ¼0.2 V (peak to peak).

respectively. The potentials of the stylus and sampleelectrode were kept at 0.0 V vs. RHE. The addition ofKCl brought about the force against the stylus, owingto shielding of the electric repulsion between the stylus

and the electrode surface by charged ions. This pheno-menon is known as the “Debye lengthÏ.19 Most of thespecimens and electrodes give a net surface charge inaqueous medium. The magnitude and sign of the

Figure 5. The second-harmonic amplitude change during variation of tip–surface distance in an aqueous environment : (a) measured inpure water ; (b,c,d) containing 1, 10 and 100 lM KCl as electrolyte, respectively.



surface charge depend on the electrode potential andpH of the solutions. These net charges are compensatedby counter-ions from the bulk solution. The Debyelength can be attributed to the thickness of the di†usedelectric double layer formed by the counter-ions. If thestylus is placed relatively distant from the electrodesurface (a few hundred nanometres), the force seems tobe small. On the other hand, the stylus makes an inva-sion upon the electrode surface if the force acting uponthe stylus is higher. From these measurements, we canestimated the thickness and distribution of the electricdouble layer at the electrode/electrolyte solution inter-face.

Imaging of charged electrode surface using EC-SMM

An example image of EC-SMM is demonstrated in thefollowing experiments. A mixed self-assembled mono-layer of 2-dodecanylthio-1,4-naphthoquinone and dode-canethiole was attached to the vapour-deposited goldelectrode surface by a self-assembling process.20h22 Themonolayer adsorption of the electroactive quinonemoiety was checked by cyclic voltammetry. Reductionand oxidation peaks were observed at [70 mV and]240 mV vs. Ag/AgCl, respectively. Because a baregold electrode shows no redox peak in this potentialrange, these current peaks correspond to the redox reac-tion of a surface-modiÐed quinine moiety.

Plate 1(a) shows a feedback topographic image of theelectrode surface modiÐed with the 2-dodecanylthio-1,4-naphthoquinone/dodecanethiole Ðlm. There are nosurface structures of self-assembled Ðlms unless there

are large grains of vapour-deposited gold, because thesize is much larger than the height of the self-assembledÐlm (D2 nm) covering the grainÏs surfaces. Plate 1(b,c)presents the EC-SMM potential images at di†erent elec-trode potentials for the 2-dodecanylthio-1,4-naphthoquinone/dodecanethiole Ðlm. Two kinds of dif-ferent potential islands can be seen in the potentialimage. Note that the potentials of these two domainsare the reverse of each other : (b) was measured at ]200mV vs. RHE and (c) at [200 mV vs. RHE at the stylusalso Ðxed same potential as sample. Domain A in Plate1 (b,c) seems to consist mainly of the quinone moiety ;the surface charge density was drastically changed dueto the electrochemical reactions take place. Thus, thedi†erence in the surface charge density could be visual-ized by using EC-SMM. Figure 6 shows the forceÈdistance curve at di†erent electrode potentials. If theelectrode potential is kept at ]200 mV vs. RHE, thesurface-attached quinone moiety is mainly the oxidizedform (the quinone form), which has no net charges,whereas an electrode potential of [200 mV vs. RHEcauses a reduction of the quinine moieties to the hydro-quinone form. These surface charge di†erences mayreÑect the force proÐles shown in Fig. 6.

CONCLUSION

We have demonstrated that SMM can operate not onlyin air but also in aqueous solutions. Despite the largeviscosity of water, we demonstrate that a sufficient levelof forced oscillations could be induced on the cantileverby applying an a.c. voltage with an amplitude as small

Figure 6. The fundamental amplitude change during variation of tip–surface distance at different electrode potentials in the potentiostaticcondition.


Copyright ©

1999 John Wiley &

Sons,Ltd.

Su

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Plate 1. The SMM feedback topography (a) and potential images at the sample potential controlled at +200 mV vs. RHE (b) and -200 mV vs. RHE (c) of the mixedself-assembled film of 2-dodecanylthio-1, 4-naphoquinine and dodecanethiole formed on gold electrode surface. Scan area is 5 x 5 µm.


as 0.2 V. By making the driving frequency sufficientlylow, one can eliminate the problem caused by the highviscosity of aqueous media. On the other hand, thelarge dielectric permitivity of aqueous medium is alwaysquite advantageous in enhancing the electric forces.Furthermore, the electrochemical potentials of thestylus and sample were controlled independently andsimultaneously with SMM by using a dual potentiostat,making it possible for in situ observation of charge onthe electrode surface during electrochemical reaction.

These results suggest that SMM (and EC-SMM) is anovel tool for understanding electrochemical and bio-chemical dynamic phenomena.

Acknowledgement

This study was partly supported by the Original Industrial Tech-nology R&D Promotion Programme from the New Energy andIndustrial Technology Development Organization (NEDO) of Japan.

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in situ observation of electrode surfaces using scanning maxwell-stress microscopy under water

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