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Electrochimica Acta 53 (2008) 3879–3888

Multiparametric characterisation of metal-chalcogen atomic multilayerassembly by potentiodynamic electrochemical impedance spectroscopy

G.A. Ragoisha a,∗, A.S. Bondarenko a,b, N.P. Osipovich a, S.M. Rabchynski c, E.A. Streltsov c

a Physico-Chemical Research Institute, Belarusian State University, Minsk 220050, Belarusb Faculty of Science and Technology, University of Twente, P.O. Box 217, 7500 AE, Enschede, The Netherlands

c Chemistry Department, Belarusian State University, Minsk 220050, Belarus

Received 1 June 2007; received in revised form 29 August 2007; accepted 3 September 2007Available online 14 September 2007

bstract

An approach to multiparametric characterisation of variable electroactive interfaces based on potentiodynamic electrochemical impedancepectroscopy (PDEIS) [G.A. Ragoisha, A.S. Bondarenko, Electrochim. Acta 50 (2005) 1553] has been extended to atomic multilayer assemblyonitoring. The multilayers were formed by successive underpotential deposition of Te, Se and Pb or Cu adlayers on Au, and also by cadmium

dlayer deposition on tellurium underlayer supported by gold. These multilayers were characterised in potentiodynamic mode by dependences ofc equivalent circuit parameters on electrode potential. The dependences disclose variations of interfacial double electric layer, charge transfer andiffusion. The dependencies of the characteristic parameters of the Au/Tead/Sead/Pbad composite three-layer have been found to be significantlyifferent from the corresponding dependences of Au/Tead/Pbad and Au/Sead/Pbad bilayers, while Au/Tead/Sead/Cuad has shown much similarity withu/Sead/Cuad in Faradaic part of ac response. Upd of Pb, Cu and Cd on the chalcogen adlayers has shown irreversibility with especially strongotential shift of adlayer oxidation potential in the case of lead deposition on bi-chalcogen Au/Tead/Sead underlayer. Unlike Pb adlayer, which

s formed locally on top of tellurium–selenium bilayer and could be fully dissolved in the anodic scan in the potential range of stability of thehalcogen composite underlayer, copper penetrated into the Au/Tead/Sead bilayer and dissolved incompletely at Cu adlayer oxidation potential. Theelf-descriptiveness of potential dependences of circuit parameters suggests PDEIS to be a handy tool for layer-by-layer deposition monitoring inlectrochemical nanotechnologies.

2008 Published by Elsevier Ltd.

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eywords: Underpotential deposition; Adlayer; Metal-chalcogen multilayer; Po

. Potentiodynamic electrochemical impedancepectroscopy of atomic layer growth

Surface limited electrochemical deposition at electrodeotential more positive than the potential of bulk phaseormation (underpotential deposition, upd [1–3]) exploitsominating adatom/substrate interactions to form atomic adlay-rs, which can be used further as building blocks forlectrochemical atomic layer epitaxy [4–8], preparation ofemiconductor heterostructures [9] and nanocolloids [10], etc.

pd enables deposition of atomic layers on substrates ofarious geometries. However, practically feasible monitoringechniques for atomic layers are available only at atomically

∗ Corresponding author.E-mail address: ragoishag@bsu.by (G.A. Ragoisha).

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013-4686/$ – see front matter © 2008 Published by Elsevier Ltd.oi:10.1016/j.electacta.2007.09.017

odynamic electrochemical impedance spectroscopy; Layer-by-layer deposition

mooth surfaces and this limits the technological potentialf upd.

The monitoring of upd by direct current can be self-escriptive only in simple systems. Classical examples of updhow perfect current peaks in cathodic and anodic scans [11,12].owever, the monitoring of underpotential deposition turns to beore complicated when metal ion is reduced on a pre-deposited

tomic layer of another element. For instance, copper upd giveserfect current peaks on bare gold electrode but shows no updeak on gold electrode covered with tellurium atomic adlayerTead) [13]. In spite of this, Cuad can be still deposited in surfaceimited mode and the Au/Tead/Cuad bilayer formation can beerified by characteristic peaks of successive anodic oxidation

f Cuad and Tead adlayers.

Atomic layers and multilayers are electroactive materials.he atomic layer deposition changes dramatically chemical andlectric status of the electrochemical interface, so more than a

3880 G.A. Ragoisha et al. / Electrochimica Acta 53 (2008) 3879–3888

Fig. 1. Typical equivalent electric circuits of upd in PDEIS: (a) impedance of irreversible upd is in parallel with the double layer capacitance charging; (b) the samea se eleu rrelat

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s (a) but with nonideal capacitance of double layer represented by constant phapd and (d) double layer capacitance in parallel with impedance of mutually co

ingle dependent variable monitoring is required for compre-ensive characterisation of the upd dynamics. The alterationf chemical nature of the electrochemical interface in atomicayer deposition affects the capacity of double layer, so theharging current depends on the potential, but this is oftennderestimated in upd investigations with a single dependentariable monitoring in cyclic voltammetry (CV). Examinationf various upd processes with potentiodynamic electrochemicalmpedance spectroscopy (PDEIS) [14–16] has shown that thehanges in double layer capacitance (Cdl) are often very sig-ificant in upd [15,17–20]. Additionally, other processes, e.g.,o-adsorption of anions [15,16], interaction between adlayersf different elements [13,19] can affect the total current mea-ured with CV, and this also calls for a multiparametric approachn potentiodynamic monitoring of electrochemical atomic layereposition.

The potentiodynamic mode used in cyclic voltammetry isuitable for upd but the classical CV often fails in discrimi-ating between several concurrent processes as it monitors aingle dependent variable. Due to two-dimensional nature ofc response and different frequency dependences of real andmaginary impedance [21], multifrequency ac probing is suitableor simultaneous characterisation of different processes on theariable interface [14]. PDEIS acquires the potential-dependentmpedance spectra in quasi-linear potential scans [16,22] andhis approach turned out to be self-descriptive in characterisa-ion of various atomic layers in upd [13,15–19]. PDEIS spectranalysis in terms of equivalent electric circuits (EEC) normallyives several functions of the potential that characterise sep-rately variations of double layer, interfacial charge transfer,iffusion of electroactive particles [17,20], and (depending onystem properties) coadsorption of anions [15,16], capacitance

f adsorption [19].

Fig. 1 shows examples of equivalent circuits of electrochem-cal systems with upd. The elements of these circuits have veryransparent physical meaning.

ment; (c) a model for the impedance of electrochemical system with reversibleed reversible adsorption of cations and anions.

. Serial resistance Rs normally results from resistance of solu-tion. This element does not belong to the interface, but takingthis element into account is required for calculation of otherEEC elements.

. Double layer capacitance, Cdl is one of the most sensitiveindicators of upd, due to the effect of upd on the interfacestatus. Cdl is usually the main contributor to the phase shift inthe ac response, and the phase shift can be normally measuredin PDEIS more precisely than the ac amplitude.

. An ideal capacitor contributes to the ac response with 90◦phase difference between voltage and current, however, inpractice of nonstationary interfaces the double layer oftenshows slightly different behaviour—its contribution is inde-pendent on frequency in several orders of frequency but thephase shift may be somewhat smaller than 90◦. The EEC ele-ment with this kind of behaviour is called a constant phaseelement (CPE) and its impedance ZCPE is defined by thefollowing equation:

ZCPE = Q−1dl (jω)−n,

where ω is circular frequency, j is imaginary unit, the physicalmeaning of Qdl is dependent on exponent n. When n → 1,Qdl transforms into capacitance, so the values of Qdl can betreated as pseudo-capacitance. The dependence of Qdl on theelectrode potential can be used as indicator of the doublelayer status in the same way as the Cdl(E), when n is close tounity [18].

. The part of the equivalent circuit, which is in parallel to Cdlor Qdl, characterises Faradaic part of ac response and thispart can be represented by different combinations of EECelements, which depends, first of all, on reversibility of the

upd. In reversible upd the oscillation of the potential producesthe oscillation of adatom coverage on the electrode surface,and the effect of the oscillations on the resulting ac currentis equivalent to the presence of a capacitor in the equivalent

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circuit [15,17,23]. The resulting adsorption capacitance Cadsis also very helpful parameter for upd characterisation, asit originates in kinetics, e.g., the EEC with Cads shown inFig. 1c was used for characterisation of phase transition inPb upd on Au [19]. Mutually correlated adsorption of cationsand anions, e.g., Cu [15] and Bi [16] upd on Au in sulphuricand perchloric acids give two adsorption capacitances in par-allel, and this allows simultaneous monitoring of the twointerrelated adsorption processes in a single potential scan[15,22,24,25].

In this work, irreversible upd will be considered, which doesot have capacitance in the Faradaic branch of equivalent circuit.he EEC of irreversible upd is similar to a common Randlesircuit (Fig. 1a) but the double layer is generally representedy CPE (Fig. 1b) [17–19,22]. Rct is charge transfer resistancend ZW is impedance of diffusion which usually complies in updith the model of semi-infinite diffusion that gives the followingependence of ZW on circular frequency:

W = Aw

(jω)0.5

here Aw is Warburg coefficient, a helpful parameter for moni-oring diffusion controlled part of Faradaic ac response.

The possibility of decomposing the total ac response of updnto contributions of few fairly simple and easily interpretableomponents in PDEIS spectra analysis is due to a rather plainative structure of the interfacial layer. Though the upd alters theouble layer, it does not disturb noticeably the additivity of dou-le layer charging and Faradaic current, and diffusion is ratherimple to be represented by Warburg element. The equivalentircuits are different in different upd systems but the differ-nces are expectable and may be easily interpreted [12–19].hus, PDEIS in combination with EEC analysis demonstratedromising results as a tool for simultaneous separate monitoringf double layer, charge transfer, diffusion and adsorption. Thether important point is that PDEIS spectra are acquired usinghe same equipment (potentiostat) which is commonly usedor controlling electrochemical deposition. In potentiodynamicmpedance spectroscopy, the small ac perturbation (usually fromto 10 mV) is overlaid on a quasi-linear potential scan, so that thexperimental procedures are very similar in PDEIS and cyclicoltammetry.

Based on the previous experience in multiparametric char-cterisation of different atomic layer deposition [15–19], thisrticle extends the approach to the electrochemical layer-y-layer assembly and characterisation of composite atomicayers of two or three elements. In contrast to bimetallicu/Tead/Cuad/Pbad three-layer, which was reported earlier [13],

n this work, we have used a Au/Tead/Sead building block com-osed of the two chalcogen adlayers. This block was used as thenderlayer in Pb and Cu upd. Separate examination of Faradaicnd double layer responses has disclosed a variety of specific

halcogen–chalcogen and chalcogen–metal effects in each partf ac response. The multidimensional view of the electrochemi-al interface response presented by PDEIS appears to be highlyharacteristic and self-descriptive in multilayer assembly.

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a Acta 53 (2008) 3879–3888 3881

. Experimental

Polycrystalline gold wire (ChemPur) electrode was flamennealed in each experiment and after cooling in air cycledor reproducible surface status in 0.1 M HClO4 between −400nd 900 mV before adlayer deposition. Au/Tead/Sead/Pbad andu/Tead/Sead/Cuad multilayers were formed by successive sur-

ace limited deposition of the elements on 0.024 cm2 Aulectrode from the following solutions: 1 mM TeO2 + 0.1 MClO4 (Te upd), 1 mM SeO2 + 0.1 M HClO4 (Se upd), 1 mMb(ClO4)2 + 0.1 M HClO4 (Pb upd), 10 mM Cu(ClO4)2 + 0.1 MClO4 (Cu upd). Au/Tead/Cdad bilayer was deposited on.02 cm2 Au electrode by successive deposition of Tead frommM TeO2 + 0.1 M HCl + 0.1 M KCl and Cdad from 3 mMdCl2 + 0.1 M HCl + 0.1 M KCl. Au electrode surface area wasvaluated by oxygen electrochemical adsorption as described inef. [26].

After each deposition step, the working electrode was rinsedith the corresponding blank solution (0.1 M HClO4 or 0.1 MCl + 0.1 M KCl) and placed into the next working solution. In

he earlier works [13,19], Te and Se atomic layers were foundo remain intact in similar procedures.

The working solutions were deaerated with nitrogen. Theeference electrode (Ag,AgCl/KClsat) was placed in an isolatedompartment of a three-electrode electrochemical cell to preventorking solution from contamination.Tead/Sead bilayer was formed on Au by surface limited depo-

ition of selenium on tellurium atomic underlayer at 240 mV.e have found that the deposition proceeded in a surface lim-

ted mode under a potentiostatic control in the range from 150o 250 mV, which overlapped with the range of limited bulkelenium deposition on bare Au [27]. In contrast to bare Au,he electrode covered with Tead did not exhibit Se upd peak, asell as peak of limited bulk selenium deposition in CV; how-

ver, bulk Se deposition occurred below 150 mV. Due to slowinetics of selenium deposition on Au/Tead, the anodic peakf Sead increased gradually with the electrode holding timep to approximately 1 min in the potential range from 150 to50 mV. We used a 2 min deposition at 240 mV to reach theustained amount of Sead. The completeness of Sead deposi-ion on Tead and the absence of bulk Se phase were testedy CV.

PDEIS spectra [16] were obtained in the frequency rangeetween 20 and 900 Hz by real-time analysis of 19 waveletst each 3 mV step of a staircase potential ramp. The analysisf the spectra was performed with a built-in spectrum analyser28,29] of the PDEIS spectrometer, which gave a best-fit EECnd presented its parameters as functions of the potential. Due torreversibility of the upd processes, the equivalent circuit shownn Fig. 1b fitted all the spectra and provided separate examinationf double layer, which was represented by pseudo-capacitancedl, interfacial charge transfer, represented by charge trans-

er resistance Rct, and diffusion, the latter was represented

y Warburg coefficient Aw—a potential-dependent parame-er of Warburg impedance ZW. For further details of PDEISxperiment and fitting procedure see Ref. [13] and referencesherein.

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. PDEIS spectra and preliminary analysis

Fig. 2a–c shows PDEIS spectra of Pb and Cu upd onu/Tead/Sead composite underlayer and PDEIS spectrum of Cdpd on Tead. Each PDEIS spectrum consists of two surfacesormed by hundreds of common impedance spectra. Four exam-les of the latter are shown in Fig. 2d in Nyquist coordinates.ach coloured line in the PDEIS spectrum connects points ofqual frequency in the set of Nyquist plots attributed to the samecan direction (red: cathodic; blue: anodic).

The first conclusion that can be easily drawn from Fig. 2s the significant difference of the spectra that correspond to

he cathodic and anodic scans. Similarly to cathodic and anodicranches in CV, the cathodic and anodic parts of PDEIS spec-ra characterise different states of the interface at differenttages of atomic layer deposition and oxidation. The poten-

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ig. 2. PDEIS spectra of (a) Pb and (b) Cu upd on Au/Tead/Sead underlayer and (cections of the PDEIS spectra (circle and square symbols), solid lines show spectra capectrum corresponds to cathodic scans; blue, anodic scans (For interpretation of thersion of the article.)

a Acta 53 (2008) 3879–3888

iodynamic mode is essential in voltammetric investigation ofpd, since no upd peak could be observed in stationary voltam-ogram. The same is valid for the potentiodynamic frequency

esponse examination—the PDEIS spectrum should be treateds a multidimensional characteristic of nonstationary states onhe trajectory of the process during the atomic layer formationnd destruction, not just a set of distorted stationary impedancepectra. Correspondingly, the parameters obtained from thepectra, which will be further presented and discussed in thisrticle, characterise the components of the ac response relatedo certain interfacial object in different points of the processrajectory. The spectra shown in Fig. 2 allow characterisation

f the variation of the three interfacial objects represented byhree EEC elements in the best fit EEC shown in Fig. 1b: chargeransfer (resistance Rct), diffusion of electroactive particles, andouble layer represented in the EEC by impedances ZW and

) Cd upd on Tead in the bidirectional scan, (d) examples of constant potentiallculated with equivalent electric circuit shown in Fig. 1b. Red colour in PDEISe references to colour in this figure legend, the reader is referred to the web

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tmtapproximately 330 mV. The potentials of oxidation of Au/Tead(curve 1 in Fig. 3) and Au/Sead differ in 200 mV, in agreementwith data presented in literature [27,30–38]. The anodic chargeswere 280 �C cm−2 for Tead and 230 �C cm−2 for Sead. A charge

G.A. Ragoisha et al. / Electroc

CPE. The variation of the two latter constituents has beenescribed by dependences on the potential of Warburg coeffi-ient Aw and parameter Qdl of the constant phase element. Thether parameter of ZCPE—exponent n was in the range from.9 to 1, so the constant phase element was close to a capacitory its physical meaning. With n being somewhat smaller than.0 the phase shift is a little bit smaller, therefore, we desig-ate the parameter Qdl as pseudo-capacitance. Note: there is alsonother physical quantity called pseudo-capacitance–adsorptionapacitance, which has chemical origin but is actually a trueapacitance from the point of view of ac response (it gives 90◦hase shift). Qdl has a truly capacitive origin (it originates inhe double layer capacitance) but is pseudo-capacitance for thether reason—it describes the EEC element with a phase shiftlightly smaller than of true capacitor. The reason for n beingot exactly equal to 1.0 in most upd processes is probably thenterface nonstationarity, which makes the double layer behavelightly different from ideal capacitor. One more note on theircuit elements: the EEC in PDEIS is derived from frequencyesponse analysis in a more restricted frequency range than intationary impedance spectroscopy, so in principle there is a pos-ibility to ‘overlook’ some element of the EEC if the elementontributes to the response only in infralow or in high frequen-ies, which are not tested by PDEIS. The circuit analysis inDEIS is intended to decompose the acquired multifrequencyc response into meaningful components originating from differ-nt interfacial objects (interfacial processes, e.g., charge transfer,nd interfacial structures, e.g., double layer) and characteriseshe objects individually by variation of their inherent parame-ers in the potential scan. PDEIS does not scrutinize whetherhe EEC would remain unchanged in the unlimited frequencyange, but it accurately analyses the available part of spectrumy a procedure [28] of complex nonlinear regression. In thisespect, PDEIS does not overlook any part of response, on theontrary, the EEC analysis on hundreds of 2D impedance spec-ra during a single PDEIS spectrum processing has a potential of

ore reliable estimation of the equivalent circuit validity than aingle, even perfectly recorded spectrum could give. Robustnessf circuit analysis in PDEIS is due to the EEC testing at differentombinations of circuit parameters in the potential scan, whichay be not available for a single 2D spectrum analysis.The values of real and imaginary impedance in the spectra

hown in Fig. 2a–c sum up effects of the three above-mentionednterfacial objects and the effect of solution resistance. Despitehe complexity of the impedance origin, a preliminary exam-nation of the interface variation in upd can be performed byxamining images of the PDEIS spectra. All the three systemshow strong decrease in magnitude of imaginary impedance |Z′′|n a certain range of potential in the cathodic scan and increaset more positive potentials in the reverse scan. One of the rea-ons of this is very transparent, since the significant contributoro Z′′ in irreversible upd is the double layer capacitance, and theatter is much higher for metallic electrodes than for chalcogen

lectrodes. So one can expect Au crystal structure terminated athe surface by Tead/Sead to give smaller capacitance and, corre-pondingly, higher magnitude of imaginary impedance than thelectrode surface terminated by metal adlayer. The picture of

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a Acta 53 (2008) 3879–3888 3883

he spectrum complies with this expectation and shows stronghanges of Z′′ at potentials of deposition and oxidation of adlay-rs. Quantitative data presented in the next section confirm theeduction made from appearance of the spectra and disclosendividual behaviour of each of the three major contributors tohe variable interface ac response in the multilayer assembly.

. Dependences of characteristic parameters onlectrode potential

.1. Au/Tead/Sead/Pbad

Fig. 3 presents the potentiodynamic voltammograms charac-erising different stages of Au/Tead/Sead/Pbad three-component

ultilayer assembly on gold. Voltammograms of selenium andellurium underpotential deposition showed cathodic peaks at

ig. 3. Voltammograms characterising the stages of Au/Tead/Sead/Pbad atomichree-layer assembling. Anodic stripping voltammograms were recorded in theame solutions that were used for deposition of the corresponding adlayers.E/dt = 78 mV/s.

3 himica Acta 53 (2008) 3879–3888

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ihaoserpeffect is not characteristic for Pbad stripping from a composite

884 G.A. Ragoisha et al. / Electroc

f 280 �C cm−2 is typical for Te adlayer formed in the poten-ial range of its main upd peak. Though some additional Teould be deposited in a surface limited mode at potential closeo Te bulk deposition [32], we limited the deposition potentialy 120 mV to secure from bulk deposition. Voltammograms ofu/Tead/Sead assembled by Sead deposition at 240 mV exhibited

wo anodic peaks, which corresponded to Tead and Sead oxida-ion with the tellurium peak shifted to more positive electrodeotentials and selenium peak slightly shifted to more nega-ive potentials against individual adlayers. The anodic chargeas 505 �C cm−2 for Au/Tead/Sead. The additive charge wasbserved in our experiments in layer-by-layer Tead and Seadeposition on Au at different scan rates. On the contrary, anodicharges of composite Se-Te adlayers, which were formed fromerchloric acid solutions that contained equal amounts of botheO2 and SeO2, appeared to be much smaller (a typical chargef a composite adlayer in our experiments was 330 �C cm−2).he charges obtained from voltammetric data depend on scan

ates and composition of solution and are not very suitable forharacterisation of atomic multilayers. However, the additivityf selenium and tellurium charges in the bi-chalcogen struc-ure deposited layer-by-layer suggests the selenium depositionas not accompanied by replacement reaction with tellurium

dlayer.To check whether bulk selenium deposition could affect the

u/Tead/Sead structure formation, the selenium deposition wasonducted at different potentials with the subsequent recordingf anodic stripping voltammogram. Curves 3 and 4 in Fig. 3ahow the change in the anodic voltammogram that resulted fromelenium deposition potential shift from 240 to 120 mV. Bulkhase deposition of selenium on Au can proceed at the latterotential progressively according to Refs. [27,30,31]. Additionalnodic peak at approximately 620 mV indicates the addition ofD selenium phase to the bilayer, while the anodic peak of Teadhifts further to more positive potentials obviously due to tel-urium protection by Se overlayer. Thus, the anodic peak ofellurium adlayer is not fixed and can shift with selenium deposi-ion but the anodic peak still allows Tead distinguishing from bulkelenium, the latter shows up only when selenium depositionotential was much below the potential used for Au/Tead/Seadi-chalcogen structure preparation.

Pb underpotential deposition on Au/Tead/Sead proceedstrongly irreversibly (Fig. 3b). The anodic oxidation of Pbad onhe composite bilayer (curve 4 in Fig. 3b) shifts to more positiveotential compared to Pbad anodic oxidation on Au/Tead that waseported in Ref. [13]. Anodic stripping shows three successiveeaks attributed to lead, tellurium and selenium oxidation (curvein Fig. 3b). Lead ions in the solution affect the potentials of

ellurium and selenium adlayer oxidation, but the anodic chargef the Au/Tead/Sead bilayer is not effected significantly by theyclic lead adlayer deposition and stripping.

A more detailed view of the interface variation in Pb upd andtripping on the bi-chalcogen underlayer is given by the depen-ences of EEC parameters (Fig. 4) obtained from corresponding

DEIS spectra. Fig. 4a presents double layer pseudo-capacitancedl variation in the potential cycles of lead monolayer deposition

nd stripping on a composite Au/Tead/Sead underlayer and on

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f (b) charge transfer resistance and (c) Warburg coefficient. Curves 1, 2 and 3n (a) characterise Au/Tead, Au/Sead and Au/Tead/Sead electrodes, respectively,n 0.1 M HClO4. dE/dt = 2.7 mV/s.

ndividual tellurium and selenium underlayers. Double layer isighly sensitive to Pb adlayer deposition on individual telluriumnd selenium adlayers and can be used for the deposition andxidation monitoring. Pbad deposition lifts the Qdl, while theubsequent anodic stripping restores the original low-capacitivelectrode status. Similarly to Pbad stripping from bulk tellurium,eported in Ref. [17], Pbad stripping from chalcogen adlayers isreceded by an additional increase in Qdl, however, the latter

ilayer. Au/Tead/Sead underlayer keeps the reduced status of Qdlore tightly, so the amplitude of Qdl variation in Pb upd cycles

hows almost a threefold reduction compared to the Qdl varia-

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ion in Pb upd on individual tellurium and selenium adlayers.he irreversible character of Pb upd on Au/Tead/Sead is explic-

tly demonstrated by a pronounced hysteresis in the Qdl variationn the potential cycle.

The parameters of the Faradaic part of impedance (Fig. 4and b) are also very self-descriptive in Pb upd on a bilayer andndividual chalcogen adlayers. Rct(E) is especially sensitive tob deposition on Au/Tead/Sead, while Aw(E) shows its most pro-ounced feature at the stripping stage. Charge transfer resistanceas a small contribution to the interface impedance in Pb updn selenium adlayer, therefore, the corresponding curve is miss-ng in Fig. 4b. The inverse charge transfer resistance in Pb updn the bi-chalcogen underlayer is an order higher than in Pb

pd on individual tellurium adlayer. Selenium in the adlayeravours the faster kinetics of Pb upd and this can be exploitedn multilayer assembly monitoring. The predominant peak in−1w (E) (Fig. 4c) is also very characteristic to the selenium

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ig. 5. Cyclic voltammograms of Cu upd on (a) Au/Sead, (b and c) Au/Tead and (dnd stripping of copper in upd and opd range while curve 3 characterises subsequenurve 4 characterises stripping of Au/Tead/Sead directly after the bilayer assembling. Slectrolyte: 10 mM Cu(ClO4)2 + 0.1 M HClO4.

a Acta 53 (2008) 3879–3888 3885

resence in the underlayer and this part of ac response can bespecially useful in the anodic scan. Thus, different componentsf the multifrequency ac response in Pb upd on composite bi-halcogen adlayers are specialised in disclosing different aspectsf the multilayer behaviour, so the multiparametric nature ofDEIS data can be exploited in designing monitoring tools forultilayer assembly.

.2. Au/Tead/Sead/Cuad

Fig. 5 shows the cyclic potentiodynamic voltammograms ofu upd on Au/Sead, Au/Tead and Au/Tead/Sead. Cu upd on all

hese substrates exhibits considerable irreversibility. Cu upd on

u/Sead starts at approximately 150 mV, which is about 65 mV

bove Nernst potential of bulk Cu deposition, shows cathodiceak at 110 mV and anodic peak at 270 mV (Fig. 5a). Cu updn Au/Tead shows only a shoulder at different scan rates in

) Cu upd and opd on Au/Tead/Sead. Curves 1, 2 in (d) characterise formationt stripping of composite deposit formed in three cycles in Cu upd–opd range.can rate is 78 mV/s, except (c), where the scan rates are indicated in the figure.

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he cathodic scan and a peak in the anodic scan (Fig. 5b and). In a series of voltammograms shown in Fig. 5c the anodicharge increases from 196 to 360 �C cm−2 with the scan rateecrease from 50 to 5 mV/s. A CV for Cu on Cu is also shownn Fig. 5c to indicate the more positive position of Au/Tead/Cuadnodic peak than of bulk copper. The less pronounced Cu updn Au/Tead than on Au/Sead and also the lower potential ofu anodic peak on Au/Tead can be explained by less negative

ree energy of tellurides formation compared to selenides whichimilarly affected Zn upd potential on Au covered with differenthalcogen adlayers [39].

Cu upd on the composite bilayer has shown only a shoul-er in the cathodic scan before the transition to bulk deposition.here was no discrimination in anodic oxidation between cop-er deposited at conditions of upd and overpotential depositionopd). The similar effect is characteristic for Cu deposition onulk Te [40] and is due to copper atoms penetration into thehalcogen support. As a result of Cu interaction with chalcogen,he peaks of Te and Se anodic stripping are dependent on coppereposition conditions (Fig. 5d) and differ significantly from theorresponding stripping voltammograms in Pb upd (Fig. 3b).

Qdl(E) shows characteristic hysteresis loops in cyclic scans inu deposition on Au/Sead and Au/Tead/Sead and a more complexariation in upd on Au/Tead. In the latter case a splash of capaci-ive response in the range of upd reveals effects of fast interactionetween Cu adatoms and the underlayer. The upd peaks observedn the dependences of EEC parameters on electrode potentialFig. 6) compensate for the inexpressive CV, which shows nopd peak in the cathodic scan. Cu interaction with Te resultslso in higher contribution to admittance of Faradaic part of acesponse (Fig. 6). On the contrary, in the range of copper oxi-ation the presence of selenium determines the Faradaic part ofdmittance. This example shows advantages of multiparametricc response over the one-dimensional dc response in charac-erising electroactive multilayers. With the multiparametric ac

onitoring, interfaces that appear quite inexpressive in cyclicoltammetry can show a lot of hidden dependences, which can besed for better understanding and controlling the nonstationaryrocesses therein.

Chalcogen atomic layers and bilayer with copper adatomsre a primitive electroactive material with a quite complex inter-elation of electrochemical, chemical, charging and diffusionalffects. Because of copper mobility in the chalcogen compos-te layer, this system can hardly give stable structures of fewtomic layers. On the other hand, fast interaction of adatoms withhalcogen can be still of interest for controlling electrochemicalynthesis of interfacial nanostructures.

.3. Au/Tead/Cdad

Cd upd on atomic layer of tellurium is one of the ECALE stepsn layer-by-layer assembly of CdTe nanomaterials [27,41–45].

ultiparametric monitoring of ac response in ECALE may give

dditional information to control the deposition. Fig. 7a showshe variation of Qdl in tellurium adlayer deposition on gold andn the subsequent Cdad deposition onto Au/Tead. The originallyigh capacitance of gold electrode decreases by an order dur-

tttc

ig. 6. Equivalent circuit parameters variation in Cu upd and opd on atomicayers of Se and Te and on Te–Se atomic bilayer (cyclic scan). Electrolyte:0 mM Cu(ClO4)2 + 0.1 M HClO4; dE/dt = 2.7 mV/s.

ng tellurium adlayer deposition. Cdl or, more generally, Qdlppears to be a convenient parameter for monitoring the variablenterface status. During Cd upd, in the next step of multilayerssembly, the change in Qdl is not thus strong as in Tead depo-ition but is quite sufficient to monitor the dynamic status ofhe interface. The information on kinetic parameters—charge

ransfer resistance and Warburg coefficient is obtained simul-aneously with Qdl (Fig. 7b) and also can be used to monitorhe interfacial processes. In this system, the changes in both theharge transfer resistance and Warburg coefficient were well cor-

G.A. Ragoisha et al. / Electrochimic

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ig. 7. (a) Qdl, (b) R−1ct , and A−1

w variation in cyclic potential scan in Cd upd onu/Tead. (a) Shows also Qdl in Te upd on Au and in ‘blank’ scan of Au/Tead in.1 M HCl + 0.1 M KCl in Cd upd range. dE/dt = 2.3 mV/s.

elated and could be used both in a similar way for the interfaceonitoring.

. Conclusions

Potentiodynamic electrochemical impedance spectroscopyntroduces into a potentiodynamically controlled experiment

set of parameters that traditionally belonged to stationarympedance spectroscopy. Similarly to direct current in CV, thearameters of PDEIS spectra analysis are functions of the elec-rode potential. Each of these supplementary potentiodynamicurves, which come from the PDEIS spectra, characterise sepa-ately their specific part of ac response. In the irreversible upd,DEIS normally gives three variables that characterise double

ayer, interfacial charge transfer, and diffusion of electroactivearticles. As has been shown in this article in several exam-les, all the parameters can be obtained simultaneously in thelectrochemical atomic layer-by-layer deposition by analysis ofmpedance data acquired by the same instrument (potentiostat)hat controls the potential scan. No significant interference into

he system is required to get information about various aspectsf the interface dynamics. Analysis of PDEIS spectra is imple-ented after the scan in the present version of PDEIS; however,

here is no fundamental restriction for moving the analysis to the

[

[[

a Acta 53 (2008) 3879–3888 3887

eal-time mode. We expect that the opportunity of the multipara-etric monitoring of interfacial processes in the potential scanill significantly facilitate control of electrochemical multilayer

ssembly.

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