Applied Surface Science 254 (2008) 5612–5617

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Applied Surface Science

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Electrodeposition and characterization of thin selenium films modifiedwith lead ad-atoms

Murilo F. Cabral a, Hugo B. Suffredini b, Valber A. Pedrosa c, Sonia T. Tanimoto a, Sergio A.S. Machado a,*a Instituto de Quımica de Sao Carlos, Universidade de Sao Paulo, Cx. P. 780, 13560-970 Sao Carlos, SP, Brazilb Universidade Federal do ABC (UFABC), Centro de Ciencias Naturais e Humanas, Rua Santa Adelia, 166, Bairro Bangu, 09210-170 Santo Andre, SP, Brazilc Instituto de Quımica, Universidade de Sao Paulo, Cx. P. 26077, 05513-970 Sao Paulo, SP, Brazil

A R T I C L E I N F O

Article history:

Received 7 August 2007

Received in revised form 4 March 2008

Accepted 4 March 2008

Available online 16 March 2008

Keywords:

Selenium

Semiconductor films

UPD

Band gap

Flat band potential

Donors

Acceptors

Se/Pb films

A B S T R A C T

The deposition and characterization of Se films doped with Pb underpotentially deposited (UPD) ad-

atoms was studied in this work. The employed experimental techniques were cyclic voltammetry,

chronoamperometry, electrochemical impedance spectroscopy, UV–vis spectroscopy and atomic force

microscopy. The initial deposition of Se film by chronoamperometry yielded a thin film composed of

approximately 700 layers. The Pb UPD on Se was achieved by chronoamperometry in a potential value

previously determined in voltammetric experiments. This deposition yielded a deposition charge of

approximately 7.5% of the total one. The film resistance altered from 320 V cm�2 for Se to 65 V cm�2 for

the Se/Pb one. Flat band potential values and number of acceptors and donors were also calculated for

both films and the values obtained were +0.95 and �0.51 V for Se and Se/Pb, respectively. The Se coating

presented 1.2 � 1017 cm�3 acceptors while the Se/Pb one presented 3.2 � 1017 cm�3 donors. The band

gap values for both films were 2.4 eV and 1.9 eV, correspondingly.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

The changes of structural and optical properties of chalcogen-ides, presenting a band gap comparable to the visible light photonenergy, have attracted a great deal of interest in recent years. Thephoto-induced changes commonly observed in this kind ofcompounds are basically attributed to the amorphous state andhave been scarcely detected in crystalline semiconductors [1].Among the available chalcogenides, selenium has played afundamental role in the production of the so-called metal selenides(ZnSe, CdSe, PbSe, etc.). All of them have attracted extensiveinterest in technological applications, since they present interest-ing properties in different uses, such as thermoelectric coolingmaterials, optical filters, optical recording materials, solar cells,superionic materials and sensor and laser materials [2–4].Particularly, the interest in Se films is associated to theirsemiconductor properties and light sensitivity. Several yearsago, it was shown that cathodic reduction of selenious acid wasthe best way to obtain Se films either in the so-called metallic form[5] or in the amorphous insulating one [6].

* Corresponding author.

E-mail address: sasmach@iqc.usp.br (Sergio A.S. Machado).

0169-4332/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2008.03.038

Aiming to obtain an amorphous layer of semiconductor metal-selenide compound, electrodeposition is one of the most adequatetechniques, since it allows the formation of the film at lowtemperature, besides several other advantages [7]. However, Seelectrodeposition is always a side reaction in the cathodic synt-hesis of metal selenides [8]. This reaction has been extensivelyinvestigated as an inexpensive method for polycrystalline materialthin films obtention – suitable for solar energy conversion or otheroptical applications. Indeed, when MSe (where M2+ is a divalentcation) is deposited according to the general reaction H2SeO3 + M2+

+ 6e� + 4H+!MSe + 3H2O by a mechanism involving Se2� (as anintermediate), selenide and selenious species chemically react toyield elemental selenium [8]. Therefore, the stoichiometry depositcontrol is a delicate subject, which has been extensively debated. Forexample, it has been proposed for CdSe deposition: (i) the use of verylow Se(IV)/Cd(II) concentration in the electrolyte [8], (ii) the use ofother selected soluble Se compounds as a Se source (e.g.,selenosulphate, which is unable to undergo chemical reactionwith selenides [9]), (iii) sequential monolayer electrodeposition[10,11], etc.

Since the late 90s Streltsov and co-workers [12–16] have beenpublishing a number of papers concerning the electrodeposition ofamorphous Se and Te films doped with lead from a Pb under-potential deposition (UPD) process. According to the authors, the

Fig. 1. Cyclic voltammogram of the Se film deposited from 0.1 mol L�1 in

HNO3 + 0.02 mol L�1 SeO2 in 0.1 mol L�1 in HNO3. v ¼ 0:02 V s�1 .

M.F. Cabral et al. / Applied Surface Science 254 (2008) 5612–5617 5613

UPD process, onto Se surface, is a very convenient tool to promotethe simultaneous electrodeposition of PbSe, following themechanism:

Pb2þ þ2e� ! Pb0ðUPDÞ (1)

H2SeO3þ4Hþ þ4e� ! Se0þ3H2OðOPDÞ (2)

Pb0þ Se0 ! PbSe (3)

The authors reported that the PbSe films produced under theseconditions are well crystallized and contain randomly orientedcrystallites, with a size range of 40–80 nm [14].

In those papers, the authors deal with underpotential deposi-tion (UPD) of metals ad-atoms. This term refers to the potentialdifference between the oxidation potential of submonolayeramounts of metals deposited on ‘‘inert’’ foreign metal substrateand the reversible Nernst potential of the depositing metal in thesame electrolyte [17]. The theory and applications of UPDprocesses are very well documented and are reported in extensiverevision works [17].

In this work, the investigation of amorphous selenium thinfilms electrodeposition, their modification with Pb atoms inclusion(obtained by underpotential deposition onto Se surface) as well asthe characterization of electrochemical, morphological and elec-trical aspects changes of such modified layers have been carried out.

2. Materials and methods

2.1. Preparation of the working electrode surface

An Au foil electrode (10 mm � 10 mm � 0.1 mm) was used forSe electrodeposition. This electrode was previously prepared byimmersion in a piranha etch solution (1:3 (v/v) H2O2:H2SO4) andthoroughly washed in Milli-Q purified water. Before eachelectrodeposition experiment the Au electrode was conditionedby cycling the potential in the range of 0.0–1.65 V, in 1.0 mol L�1

H2SO4. A diagnostic criterion of the surface purity was provided bythe classic voltammetric profile of Au in acid medium [18]. Theelectrochemical area of this electrode was obtained following themethod presented by Trasatti and Petrii [19]. In this method, thecharge reduction of Au oxide, formed at 1.3 V during 60 s, isevaluated and compared to the theoretical value of 390 mC cm�2,considering a two electrons transfer for each Au atom in thepolycrystalline surface. Hence, a roughness factor of 2.17 wasfound. The electrochemical area was used throughout this work tonormalize the experimental current and charge values.

2.2. Se and PbSe films preparation

The Se films were deposited onto polycrystalline Au surface bypotentiostatic polarization (�0.40 V) in 0.1 mol L�1 HNO3 +0.02 mol L�1 SeO2 (98% purity, Merck) for 30 min under a 100 Whalogen lamp lighting and magnetic stirring. The Pb doping wasconducted at �0.10 V in an electrolyte containing 0.1 mol L�1

HNO3 + 1 � 10�3 mol L�1 Pb(NO3)2 (98%, Merck) for 10 min inabsence of light and stirring.

2.3. Electrochemical measurements

All electrochemical measurements were performed with theRadiometer Inc. Voltalab PGZ402 potentiostat/galvanostat coupledto an IBM type microcomputer containing the Voltamaster 4software.

Experimental set-up was carried out in a standard electro-chemical cell with three electrodes, i.e., the Ag/AgCl (3.0 mol L�1

KCl) system as the reference, Pt foil with 1.0 cm2 geometric area asauxiliary and the working electrodes which were gold surfacesmodified either with Se or PbSe films.

The electrolytes were prepared with water purified in anAcademic Milli-Q system from Millipore Inc. All reagents wereMerck PA used without any further purification.

The electrochemical impedance spectroscopic measurementswere obtained from 100 mHz to 40 kHz frequency range and 0.01 V(p/p) amplitude. The impedance spectra were obtained within thepotential range for the Fe3+/Fe2+ couple (0.40 to �0.46 V), in0.1 mol L�1 HNO3 + 1 � 10�3 mol L�1 K4Fe(CN)6 on Se and PbSefilms, in Nyquist plots format.

Atomic force microscopy (AFM) measurements were conductedin the contact mode with an Explorer equipment from TopometrixInc.

Optical studies were carried out at normal incidence at roomtemperature using a 5G UV–vis CARY model spectrophotometer,on a slide of conductive glass (SnO2 coated glass).

3. Results and discussion

The Se thin films were obtained in chronoamperometricexperiments where the Au electrode was polarized at�0.4 V during1800 s in an electrolyte composed of 0.1 mol L�1 HNO3 +0.02 mol L�1 SeO2. The correspondent deposition charge wasevaluated from the area under the curve. After several experiments,the average deposition charge was 276 mC cm�2. Considering that(i) the theoretical charge value for one Se monolayer deposition onAu surfaces is (195� n) mC cm�2 [19], where n is the number ofelectrons transferred during the electrodeposition (4, herein), (ii)each Se atom occupies, initially, two sites on the electrode surfaceand (iii) that the film grows in an ordered mode, the achieved chargeshould be approximately 700 layers.

The cyclic voltammogram of gold electrode covered with the Sefilm in 0.1 mol L�1 HNO3 at 0.1 V s�1 is shown in Fig. 1. It isconcluded that in the potential range more positive than 0.35 V, anumber of anodic peaks are evident, which are related to theoxidation of several phases in the Se film. On the other hand, duringthe negative scan between 0.3 and�0.4 V, a well defined reductionpeak is observed, with its equivalent anodic on in the reverse scan.This couple of peaks is associated firstly with the reduction of Se toSe2� in the negative scan, and the reaction with H+ generatinggaseous H2Se species, which diffuses away from the electrodesurface. Finally, the oxidation of H2Se to Se generates the anodicpeak at approximately �0.35 V in the reverse scan [20].

Fig. 2. AFM topography image of Se film. The periodicity is 1.71 � 0.44 mm and the amplitude varies from 747.0 � 164.6 nm: (A and B) 2D representation and (C and D) 3D

representation.

M.F. Cabral et al. / Applied Surface Science 254 (2008) 5612–56175614

The topography of the obtained Se films was analyzed byatomic force microscopy (AFM). The visual analysis of the surfacespresented in Fig. 2 indicated an organized layer composedof pyramidal shape structures presenting dimensions of 1.71 �0.44 mm and 747.0 � 164.6 nm height. Several measurements indifferent locations on the electrode surface showed a homogeneousdeposition, generating the same pattern in the entire surface.

Such Se films were further modified by the underpotentialdeposition of Pb. In order to demonstrate the UPD behavior of Pb onSe films, Fig. 3 shows the first cycle voltammogram for Se electrodein 0.1 mol L�1 HNO3 + 1 � 10�3 mol L�1 Pb(NO3)2 at 0.02 V s�1. Thepair of peaks A1 and C1 corresponds to the deposition/redissolutionof the UPD Pb on the Se surface. The formation of H2Se, as shown inFig. 1, was inhibited by the Pb layer formed on the Se film at�0.4 V.In Fig. 4, a schematic diagram showing the organization of such Au/Se/Pb layers are presented. The relationship between the cathodicand anodic peak areas was 1.10, indicating that approximately 10%of the deposited Pb was not recovered in the anodic sweep. As aconsequence, this suggests a diffusion of such amount of Pbads into

Fig. 3. Cyclic voltammogram (first cycle) in 0.1 mol L�1 in HNO3 + 1.0 �10�3 mol L�1 Pb(NO3)2 for Se film UPD on Se film. v ¼ 0:02 V s�1.

the Se phase, for this short period of time, during the voltammetricexperiment. That difference, in voltammetric peak areas, is morepronounced for lower scan rate values.

Larger amounts of incorporated Pb into the Se phase can beachieved in a chronoamperometric experiment. For this purpose,the Se electrode should be immersed in a 0.1 mol L�1

HNO3 + 1 � 10�3 mol L�1 Pb(NO3)2 electrolyte, with an initialand a final potential of 0.1 V and �0.1 V, respectively, for 600 s.Although the deposition potential is far from that related to Pb bulkdeposition, it is shown that an increasing cathodic (deposition)current is obtained, which is only associated to Pb UPD. Thus anincreasing diffusion of surface Pb to the inner phase of Se must beoccurring. The total charge density for Pb deposition is20.69 mC cm�2. Considering that each Pb ad-atom occupies twoSe active sites and receives two electrons in the reduction process,a total of nearly 106 monolayers of incorporated Pb to the Se filmare obtained. Considering the 276 mC cm�2 charge for Se deposi-tion on Au surface (see the text above), Pb incorporationcorresponds to 7.5% of Se total deposition charge.

The Se and Se doped with Pb ad-atoms films were analyzed withelectrochemical impedance spectroscopy, in an electrolyte con-taining 0.1 mol L�1 HNO3 + 1 � 10�3 mol L�1 K4Fe(CN)6. The vol-

Fig. 4. Adsorption model for Se atoms and Pb ad-atoms on polycrytalline Au surface.

Deposition scheme for Pb UPD on Se film.

Fig. 5. Cyclic voltammograms of 1.0 � 10�3 mol L�1 [Fe(CN)6]2+/3+ in 0.1 mol L�1

HNO3 solution at pollycrystaline Au electrode (dotted line), Se film (full line), PbSe

modified electrode (dashed line). Scan rate: 0.05 V s�1.

M.F. Cabral et al. / Applied Surface Science 254 (2008) 5612–5617 5615

tammetric profiles for such system are shown in Fig. 5 at0.05 V s�1. The Fe2+/Fe3+ profile is very well defined for the Ausurface, while the Se and doped Se/Pb films present distorted redoxpeaks, probably due to the semi-conductive characteristic of suchsurfaces, which introduce a resistive behavior to the electro-chemical response. Based on such responses, a potential regionbetween 0.4 and 0.5 V was selected for electrochemical impedanceexperiments.

Fig. 6. AC impedance plots in 1.0 � 10�3 mol L�1 [Fe(CN)6]2+/3+ + 0.1 mol L�1 HNO3

within a frequency range of 0.1–40 kHz for Se film (A) and PbSe modified electrode

(B). Potential range of 0.40–0.46 V vs. Ag/AgCl, 3.0 mol L�1 KCl.

Fig. 7. Mott–Schottky plots determined at Se film (A) and PbSe modified electrode

(B) in acid media, 0.1 mol L�1 in HNO3. The potencial sweep is from anodic to

cathodic potencial, and the measuriing frequency is 5 and 10 kHz. V-axis

intersection point indicate the shift of the extrapolation point in the anodic (A)

and cathodic (B) potencial range.

Fig. 6A demonstrates the Nyquist graph for Au/Se electrodewhile in Fig. 6B the same graph is demonstrated for Au/PbSe films.The potential values previously selected for such measurementswere 0.40, 0.42, 0.43, 0.44 and 0.46 V. All curves in Fig. 6A show avery similar and practically constant behavior with a filmresistance value of 320 V cm�2, indicating the stability of thefilm in this potential range. On the other hand, the behavior of PbSefilms shown in Fig. 6B is quite different. The curves for the twolower potentials (0.40 and 0.42 V) showed the behavior of a muchless resistive film, with a film resistance of approximately65 V cm�2, which is expected for the doped semiconductor layer.However, the three higher potentials presented an increasedresistance, reaching the characteristic value of a non-doped Se film.This is attributed to the slow dissolution of Pbads in such anodicpotential region, yielding non-doped Se film. Apparently, for thetwo first potentials, although the values are positive enough tocause the Pbads dissolution, this is not observed during thescanning frequencies, probably due to the strong interactionbetween Pb and Se surfaces.

The Mott–Schottky plots for impedance data are shown in Fig. 7,with the same conditions as those of Fig. 6, using a frequency of5 kHz. Here, ‘‘A’’ stands for the Se film while ‘‘B’’ is related to the Se/Pb film. The observed linear relationship determines the flat-bandpotential (Vfb) for a n-type semiconductor by [21]:

C�2 ¼ 2

ee0eNDA2

!V � V fb � kT

e

� �(4)

Table 1Determination of Se film and PbSe modified electrode parameters Mott–Schottky

plots

Parameters/films Se PbSe

e0 (kg cm2 s�2) 1.60 � 10�19 1.60 � 10�19

e 206 280

e0 (F cm�1) 8.85 � 10�14 8.85 � 10�14

(d(C�2)/dE) �5.7 � 109 +1.6 � 1010

Na,d (cm�3) 1.2 � 1017 3.2 � 1017

Eg (eV) 2.40 1.90

M.F. Cabral et al. / Applied Surface Science 254 (2008) 5612–56175616

where e is the dielectric constant of the semiconductor (e0 for thevacuum permittivity), A is the interface surface area, e is theelectron charge, ND is the number of acceptors or donors, V is thepotential, k is the Boltzmann constant and T is the temperature.Based on this relationship, Vfb can be determined from V0, theV-axis intersection point of the C�2 vs. V plot. The values obtainedfor Se and Se/Pb films are presented in Table 1. The slopes of therelationships were used to calculate the number of acceptors ordonors in the films, which are also included in Table 1. These valuesare in good agreement with those obtained in the literature[22,23].

Finally, the optical band gap was calculated from the opticaltransmission spectra of the thin electrodeposited films [24–26].Here the optical spectra of electrodeposited Se and Se/Pb films ontin oxide coated glass slides were recorded as a function ofwavelength in the range 350–800 nm. If substrate absorption wasobserved, a correction was done by introducing an uncoated SnO2

glass substrate in the reference beam. The mathematical treatmentis detailed in reference [24]. Fig. 8 shows the UV–vis spectra for (A)

Fig. 8. Variation of (a2) with photon energy (hy) for Se (A) and PbSe (B) thin film.

Se and (B) Se/Pb films. The energy gap was determined using therelationship [24]:

a/ðhy� EgÞ1=2 (5)

where a = �ln(T/d), T being the transmittance and d the filmthickness. Thus, a plot of a2 against hy was constructed and thelinear portion was extrapolated to the energy axis as shown inFig. 8 (A for Se and B for Se/Pb films). The intersection point givesthe direct material band gap, which were 2.40 and 1.90 eV for Seand Se/Pb, respectively. The former is very close to a previouslypublished one [27], while the latter shows the influence of dopingin films optical properties and is much more appropriated to beused in solar energy conversion.

4. Conclusions

The electrochemically deposited smooth thin layers of Seobtained on Au surfaces were found to be conveniently modifiedwith Pb UPD in order to improve its semiconductor character-istics. After long time electrodeposition, up to 7.5% of the dopingagent can be incorporated, as measured by its depositioncharge, if the adsorbed layer is allowed to diffuse towards thebulk Se. Here, the incorporation of Pb into the whole Se phasewas proposed, instead of a mere surface modification, generat-ing one or little more monolayer of PbSe, due to the significantchange in optical properties of the Se layer after incorporationof Pb.

The UPD technique was suitable to promote such modificationsince it avoided the potential incursion, during Pb electrodeposi-tion, to reach the negative potential region associated with theH2Se formation, thus preserving the thickness and smoothness ofthe Se layers. Moreover, UPD provided a very useful technique tocontrol the amount of doping metal to be incorporated into thefilm.

The modification with Pb ad-atom reduced the Se filmresistance from 320 to 65 V cm�2 and promoted a significantreduction in the band-gap value. The experimental conditionsdescribed in this work resulted in the variation of band gapvalues, from 2.40 to 1.90 eV for the Se and the Se/Pb films,respectively. Such lower value of band-gap can suggest theutilization of such electrochemical modified Se films in the solarenergy conversion.

Acknowledgements

The authors would like to thank FAPESP (Fundacao de Amparo aPesquisa do Estado de Sao Paulo) for the financial support andstudent grant (Proc. no. 04/09906-3) used in this work.

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