1998 preparation and characterization of electrodeposited bi,se, thin films

7/30/2019 1998 Preparation and Characterization of Electrodeposited Bi,Se, Thin Films

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MATERIALSCHEM;STRYiFDELSEVIER Mater ials C hem istry and Physics 55 ( 1998 1 5 1-54

Preparation and characterization of electrodeposited Bi,Se, thin filmsA.P. Torane , C.D. Lokhande b, P.S. Patil b, C.H. Bhosale ,*

Received 25 July 199 7; received in revised form 24 November 1997; accepted 23 January 1998

AbstractThin f i lms of BizSel have been prepared from an aqueousacid ic bath at room temperatureusing selenium dioxide as a selenium on sourceby a simple, nexpensiveelectrodeposition echnique. The electrodepositionpotentials for different bath compositions and concentrationsare

estimated rom polarization curves. t has been ound that Bi( NO, j .? nd SeO, n the volumetric proportion as7:3 and their equimolar solu tionsof 0.025 M form good qu al i ty f i lms. The f i lms are annealed in a ni trogen atmosphere at 200C for 2 h. The f i lms are character ized by scanningelectron microscopy, X-ray di ffractionand optical absorption techniques. Studies reveal that aa-deposi ted andannealed thinfilmsarecontinuousand polycrysta lline n nature. The optical band gap has been found to be 0.55 eV for the above-mentionedcomposition and concentration ofthe film. 0 1998 Elsevier Science S.A. All r ights reserved.Keylvrds: Bismuth selenide: Electrodeposi tion; Polar ization curves

1. IntroductionGroup V-VI binary compounds and their pseudobinarysolid solutions are highly anisotropic and crystallize in

homologous layered structures parallel to the growth d irec-tion [ 11. Over the last two decades, many experimental datahave been gathered on electrical, optical and thermoelectricproperties of these materials [ 21, owing to their applicationssuch as precise temperature control of laser diodes [ 31, opti-cal recording system [ 41, electromechanical devices [ 51,strain gauges [6] and thermoelectric devices [7-g]. Thesethin film s have great technological importance owing to theirpotential applica tions in photoelectrochemical devices, solarselective, optoelectronic devices, thermoelectric coolers anddecorative coatings [ lo]. In recent years considerable atten-tion has been focused on glassesof Bi and Se because of theiuse in optical and photosensitive devices [ 111. Co-depositedBi-Se thin films of different composition and thickness wereprepared by Nikam and Aher for studying the elect rical trans -port properties [ 121. Pramanik et al. have prepared Bi,Se,thin films by the chemica l deposition method [ 131.Among various deposition methods, electrodeposition is asimple and inexpensive method fo r the preparation of thinfilms. The growth rate can be easily controlled throughelect rical quantities such as current density and deposition

* Corresponding author: Tel .: + 91 655 571 5228; F ax: + 91 23 1 6561330254-0584/98/$19.00 0 1998 Elscvier Science S.A. Al l r ights reserved.PIlSO254-0584(98)00085-6

potenpotential. In rhe present investigation, we report theelectrodeposition of Bi,Se, thin films on the stainless steeland fluorine doped tin oxide (FI O) coated glass substratesas deposition on copper and brass substrates is hard to obtain.The electrodeposition potentials for different bath com-positions and concentrations are estimated from polarizat ioncurves. It has been found that equimolar (0.025 M) solutionsof B i( No~)~~H ~O and SeO, in rhe volumetric proporation7:3 form good quality films. The films have been studied fortheir structural and optical properties. An effect of annealingon the properties of the films has also been discussed.

2. Experimental2.1. Prepnration qf thir~j2ms

The Bi$e, thin film s were prepared on stainless steel sub-strate and fluorine doped tin oxide (FTO ) coated glasssubstrates (area 4 X 2 cm) by electrodeposition technique.The stainless steel substrate was polished by fine zero numberpolish paper, then washed by laboline and finally by anultrasonic cleaner. The back of the substrate was coveredwith an insulating tape. The FTO coated glass substrate wascleaned by 50% diluted HCI and then by double distilledwater. The economical and inert polished graphite plate(4 X 2 X 0.2 cm3) was used as a counter electrode. The dep-


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52 A, P. Torane et al. / Materids Chrwist~~ cd Physics 55 (1998) 51-54

osition was carried out from an unstirred solution at roomtemperature with a potentiostatic mode of conditions. A threeelectrode system was used and the potentials were measuredwith respect to saturated calomel electrode (SCE). Thedistance between anode and cathode was 0.4 cm.Bi(N03)3 .5Hz0 was dissolved in concentrated nitr ic acidand then diluted by double distilled water to a desirednormality. Selenium dioxide (SeO,) was dissolved indouble distilled water. The equimolar (0.1 M) solutions ofBi( N03)3. 5H10 and SeO, with bath compositions in thevolumetric proportion lO:O, 9:1, 8:2, 7:3, 64, 5:5, 4:6, 3:7,28, 1:9, 0: 10 were used as electrolytes for determining thedeposition potentials. The electrical and optical studies of thefilms reveal that a good quality film formation occurs at 7:3composition. By keeping thecomposition fixed the equimolarconcentration of the bath of Bi( N03) 3. 5Hz0 and SeOl wasvaried from 0.005 M, at the interval of 0.025 M, to 0.01 M.The deposition time and deposition potential were keptconstant at 30 min and - 0.09 V (SCE) respectively.The as-deposited films were washed with double distilledwater. The Bi,Se, thin films deposited on stainless steel sub-strate were dark grey in colour, well adherent to the substratesand uniform in appearance. They were annealed in a nitrogenatmosphere at 200C for 2 h.The thickness of the deposited film was determined byweight difference method.2.2. Cizaracterizntion

For microstructure investigation with SEM (CambridgeStereoscan 250-MK-3 unit) the films were coated with gold-palladium by a polaron sputter coating unit E-2500. Thecoating thickness was 15 nm.The XRD patterns of the as-deposited and annealed Bi$e,thin films were investigated using a Phillips X-ray diffracto-meter (PW 1710) with CuKa target.Optical absorption studies were carried out using a UV-VIS-NIR spectrophotometer (Hitachi Model 330, Japan)within the wavelength range of 900 to 1500 nm.

3. Results and discussionThe polarization curves were plotted to estimate deposition

potentials of B&Se, deposited on to the stainless steel (Fig.1) and FTO coated glass (Fig. 2) for the baths of variouscompositions. The estimated deposition potentials for Bi,Se,on stainless steel and FTO coated glass substrates are shownin Table 1, It is seen that the deposition potentials for Bi2Se3thin films deposited on FTO coated glass substrates are higherthan that for deposition on stainless steel substrates. This maybe ascribed to the higher resistivity of FTO coated glasssubstrates than that of the stainless steel substrates.The electrodeposition of Bi,Se, had been carried out froman aqueous acidic solution contain ing bismuth and seleniumions.

Volt (SCE)cl - 0.08 -0.16 - 0.24- 0

2E

JFig. 1. Polarisation curves of Bi$e, on stainless steel substracts for com-positions of 3i:Se . (a) Bi, (b) 9:1, (c) 8:2, (d) 7:3, (e) 6:4, (f) 5:5, (g)3:6, th) 3:7, (i) 2:8, (j) 1:9, (k) Se.

Volt ( SCE)0 -1 -2 -3 -4 -5

Fig. 2. Polarisation curves of Bi,Se, for FPO coated glass substrates forcompositions ofBi:Se. (a) 36, (b) 3:7, (c) 2:8, (d) 1:9, (e) O:IO(Se).Table 1Estimated deposition potentials of equimolar (0.1 M) B&Se, from the polar-isation curves for different compositions for stainless steel and FTO coatedglass substratesDepositedfilm

Bath composition0.1 M (equimolar)PHZ

Potentials V vs. (SCE)Stainless steel FTO coated glass

BiBi$e,Bi,Se,Bi$e,Bi,Se,Bi,Se,Bi$e;Bi,Se,B&Se,BiSe,Se-

IO:09: I8:27136:45:54:63:72:sI:90:lO

- 0.059- 0.06- 0.07- 0.09- 0.094-0.15-0.12-0.1-C-0.102-0.081- 0.07

No depositionNo depositionNo depositionNo depositionNo depoaitionNo deposition- 0.326-0.67- 0.3-0.9- 0.802

Se02+Se++20-Se+ is converted into Scads.

71)The electrons react with Serldras

3Se,,,+&--+3Se-A complex of Bi3+ reacts with Se- to give

(2)


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A.P. Torane et al. /Materials Chemistry and Physics 55 (1998) 51-54

I0.0 I I I I0 0.4 0.8 1.2hY (eV)

Fig. 6. Plot of (~yhv)> against (hv) for B&Se, thin films.

cient is of the order of lo4 cm- supporting the direct bandtransition of the material and the transitions are allowed[ 17-221.The nature of the transition involved can be determined onthe basis of the dependence of absoption coefficient a onphoton energy hv. For allowed direct trans ition, 01 s givenbyaa(hv-E,)2 (5)where Eg is the energy gap between the bottom of the con-duction band and the top of the valence band at the samevalue of wave vector K. The variation of (ah v) 2 with 11 sshown in Fig. 6 which is a straight line indicating the transi-tion involved has direct band gap energy, determined byextrapolating the straight portion to the energy axis at cr= 0and is found to be 0.55 eV, which is greater than the value ofEg = 0.35 eV reported earlier [ 231, This could be due to dif-ferent preparative conditions of measurements [ 241.4. Conclusions

A good quality Bi2Se3 thin film deposition is possible usingthe electrodeposition technique. The films are polycrystalline

in nature and the crystallinity increases after annealing. Theoptical band gap (direct) energy is 0.55 eV.

References111121131

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[lOI11111121[I31Cl31Ll51[ 16111711181[I91WI1211[=It231

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1998 preparation and characterization of electrodeposited bi,se, thin films

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