ORIGINAL PAPER

Thermoelectric properties of Bi2Te3 films by constantand pulsed electrodeposition

Cristina V. Manzano & Adriana A. Rojas & Michelle Decepida &

Begoña Abad & Yazmin Feliz & Olga Caballero-Calero &

Diana-Andra Borca-Tasciuc & Marisol Martin-Gonzalez

Received: 31 January 2013 /Revised: 12 March 2013 /Accepted: 14 March 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract Bi2Te3 films have been grown by constant andpulsed electrochemical deposition. The pulsed depositionwas carried out by alternating between a constant potential(potentiostatic mode) and an open circuit potential(galvanostatic mode, where current density is fixed at0 mA/cm2). The Harris texture analysis was performed todetermine the degree of preferred orientation. The resultsshowed that the films were strongly oriented along (110)direction. The morphology and compositions of the filmswere then analyzed. Finally, their Seebeck coefficient andelectrical resistivity were measured and used to determinethe thermoelectric Power Factor of the films for a tempera-ture range between 57 and 107 °C.

Keywords Semiconductors . Films . Pulsedelectrodeposition . Constant electrodeposition . Bi2Te3 .

Thermoelectric properties

Introduction

In the last decades, thermoelectric materials have producedgreat interest due to the necessity of fabricating high-performance devices for energy harvesting [1]. Given thatthese materials produce electrical energy from a temperaturedifference, they can be used in a wide variety of applica-tions. In the case of devices that operate around roomtemperature, the principal candidate is bismuth telluride;this semiconductor is one of the most efficient thermoelec-tric materials at room temperature due to its good thermo-electric properties in bulk: Seebeck coefficient −240 μV/K(S), electrical conductivity 0.1 (μΩ m)−1 (σ), and thermalconductivity 2.02 W m−1 K−1 (κ) [2]. With these values, thefigure of merit (ZT=S×σ×T/κ) of Bi2Te3 at room temper-ature results≈1.

Bi2Te3 is a semiconductor with a small band gap of0.15 eV and belongs to the space group R-3m. One of theadvantages of this semiconductor is that depending on itsstoichiometry its behavior varies: when the material is bis-muth rich, its Seebeck coefficient is positive, resulting in ap-type semiconductor; when the material is tellurium rich, itsSeebeck coefficient turns to negative values and it becomes ann-type semiconductor. Ternary and binary compounds ofBi2Te3 [3] such as (BixSb1−x)2Te3 [4] and Bi2Te3−xSex[5, 6], (for p-type and n-type) or Bi1−xSbx [7, 8] alsohave high thermoelectric figure of merit.

One of the most appealing aspects of this thermoelectricmaterial is the fact that it can be grown by inexpensiveelectrochemical deposition in potentiostatic and/orgalvanostatic modes. The first films were fabricated withelectrodeposition procedures carried out at constant poten-tial [3, 9] or at constant current [10]. Recently, new electro-chemical approaches were used in order to improve theirthermoelectric efficiency, which was still far from the

C. V. Manzano :A. A. Rojas :M. Decepida : B. Abad :Y. Feliz :O. Caballero-Calero :M. Martin-Gonzalez (*)IMM-Instituto de Microelectrónica de Madrid (CNM-CSIC),Isaac Newton 8 PTM,Tres Cantos 28760 Madrid, Spaine-mail: Marisol@imm.cnm.csic.es

A. A. RojasChemical Engineering Department, Rensselaer PolytechnicInstitute, Troy, NY, USA

M. Decepida :D.-A. Borca-TasciucMechanical, Aerospace and Nuclear Engineering Department,Rensselaer Polytechnic Institute, Troy, NY, USA

Y. FelizDepartment of Mathematics, Rensselaer Polytechnic Institute,Troy, NY, USA

J Solid State ElectrochemDOI 10.1007/s10008-013-2066-7

performance of the bulk material. One of these approacheswas to use pulsed potential deposition to grow the films.

A type of pulsed electrodeposition, consisting of changesin the current or in the potential during time, was carried outby C. Boulanger’s group to improve the morphology of bis-muth telluride films [11, 12]. Later, Glatz et al. [13] presenteda method to obtain electrodeposited Bi2+xTe3−x films whichalternated pulses of voltage with current pulses for the firsttime. This new method was used in 2011 by Ma et al. [14] todeposit Bi2Te3 films on stainless steel substrates, maintainingthe ratio between on and off-time fixed at 0.1 s.

In this study, we report the growth of Bi2Te3 films byelectrochemical deposition at constant and pulsed electrode-position. In the first part, the films were grown at differentconstant potentials in order to obtain films with a certainpreferential crystallographic orientation. In the second part,in an attempt to improve the quality of Bi2Te3 films, theelectrodeposition conditions were changed to performpulsed deposition. The applied pulses were alternated be-tween a certain constant potential (on) and zero currentdensity (off), according to a previously reported study[13]. In our case, the on and off deposition times were equal.

The electrical resistivity and the Seebeck coefficient werethen measured for all films, and the power factor wascalculated and reported as a function of temperature.

Experimental section

Bi2Te3 films have been grown by pulsed electrochemical de-position in a solution consisting of 0.75×10−2 M Bi3+, 1×10−2 M HTeO2

+, and 1 M HNO3 [3]. The solution was pre-pared from Aldrich(R) Bismuth pieces (99.999 %), FlukaChemika Tellurium powder, and Panreac 65 % nitric acid [3].Nitric acid is used because H+ acts as the supporting electrolyteof the solution and NO3

− as counter ion. The preparation of thesolution required several steps. Initially, both species, Bi3+ andHTeO2

+, were dissolved in nitric acid separately. Then de-ionized water was added to both solutions. Finally, in the laststep, both solutions were mixed. A conventional three-electrode cell configuration was used in these studies with thefollowing electrodes: a working electrode composed of 150 nmof platinum evaporated on silicon (100) by electron-beamevaporation, a platinum mesh as the counter electrode, and anAg/AgCl 3M electrode as reference electrode. A bi-potentiostat(Eco Chemie, Model AUT302.0) was used to control the cellwith the customized software Nova 1.7 to obtain the cyclicvoltammograms and the constant and pulsed electrodepositionbetween potentiostatic and galvanostatic modes. All experi-ments were carried out at room temperature. The potentialsapplied during deposition were determined with the analysis ofthe cyclic voltammograms. For the films grown with pulseddeposition, the pulse consisted in the same potential value

applied for a certain time, and then at the same time at zerocurrent density. The pulse sequence was then repeated for aspecified period of time.

The structural characterization of the films was thenanalyzed through X-ray diffraction (XRD) using a PhilipsX’Pert four-circle diffractometer system with CuKα radia-tion. The scan rate and time for step was the same for all thefilms, 0.02° and 1 s, respectively. The morphological char-acterization was performed using a commercial scanningelectron microscope (SEM; Zeiss SUPRA 55). The chemi-cal composition was studied using the energy dispersive X-ray spectrometry (EDS; JEOL JSM-6335) with a field emis-sion SEM system equipped with a Si/Li detector and EVEXAnalytical software. Finally, the thermoelectric properties,which are, electrical resistance (ρ) and Seebeck coefficient (S) ofthe bismuth telluride films were measured with a LSR-3 Linseisequipment, a commercial measurement system. Before themeasurement was performed, the films were taken off fromtheir substrates using an epoxy high-temperature adhesive,being then transferred to a glass slide. The electrical resistivitywas measured by means of the four-point probe method, andthe Seebeck coefficient was obtained from two temperaturedifferences of approximately 1 and 8 °C, respectively, asmeasured between the two ends of the film. These two prop-erties were characterized over a temperature range between∼57 and 107 °C. Multiple measurements were carried out ateach temperature point in order to ensure the stabilization ofthe temperature gradient at the set temperature.

Results and discussion

Cyclic voltammograms and electrodeposition of Bi2Te3films

In order to find the appropriate potential range for theelectrochemical deposition, a cyclic voltammogram wasperformed on a platinum working electrode in 0.75×10−2 M Bi3+, 1×10−2 M HTeO2

+, and 1 M HNO3 aqueoussolution over the potential range −1.1 to 1.1 V vs. Ag/AgCl(3 M KCl). The scan rate was 10 mV s−1.

Figure 1 shows the cyclic voltammogram where threereduction peaks and three oxidation peaks can be seen.Among them, the reduction peak found at −0.02 V vs.Ag/AgCl is due to the reduction of HTeO2

+ and Bi3+ toBi2Te3. The origin of these peaks was explained by Martín-González et al. [3]

In order to obtain films grown with a preferential crys-tallographic orientation, the applied constant potential waschosen to be around the minimum of the Bi2Te3 reductionpeak found in the cyclic voltammetry. Different films werefabricated at constant potential for 1 h, with the followingapplied potentials: −0.04, −0.02, −0.00, and +0.02 V. The

J Solid State Electrochem

charge density of the films has been calculated from thechronoamperometry, obtaining values of 12.4, 10.7, 9.6,and 8.2 C/cm2 for the films grown at −0.04, −0.02, −0.00,and +0.02 V, respectively. The lowest charge density wasobtained for the films grown at +0.02 V, which indicates aslower growth rate.

Then, a second set of films were grown by pulsed elec-trochemical deposition, alternating between a constant po-tential (potentiostatic mode) and a constant current density(galvanostatic mode, with J=0 mA/cm2). It is important tonote that this does not correspond to the open circuit poten-tial measured at the beginning of the experiment, since the

surface of the electrode is modified due to deposition ofBi2Te3 materials). The reason why this procedure improvesthe quality of the deposit is that it balances the loss of ions atthe surface of the electrode that occurs when a constantpotential is applied for a certain time. In the case of pulsedelectrodeposition, the ion concentration at the surface of theelectrode decreases during the on-time, while the constantpotential is being applied. Then during the off-time (whichcorresponds to a zero current density) the bismuth and tellu-rium ions are replenished in the diffusion layer and a change inthe electrical double layer at the metal-electrolyte interface isproduced [15]. These factors enhance the ad-atoms populationand nucleation rate during the on-time. Different time regimeswere studied, varying the on/off period: 5/5, 0.1/0.1, and0.01 s/0.01 s. In these cases, the charge density of the filmscould not be calculated due to the limitations of the softwareof the potentiostat. Nevertheless, the thickness of the samplescan be measured afterwards. Moreover, according to the workcarried out by Ma et al. [14], the thickness of the films grownby pulsed deposition has a small effect in the electrical resis-tivity and Seebeck coefficient when the measurements arecarried out in plane.

Structural properties of Bi2Te3 films

Figure 2 shows the XRD of the films grown at constantpotential deposition. Different peaks can be seen, which canbe associated to the components of the substrate: Pt (JCPDS

-1.2 -0.9 -0.6 -0.3 0.0 0.3 0.6 0.9 1.2-8.1x10-3-7.2x10-3-6.3x10-3-5.4x10-3-4.5x10-3-3.6x10-3-2.7x10-3-1.8x10-3-9.0x10-4

0.09.0x10-41.8x10-32.7x10-33.6x10-34.5x10-35.4x10-3

Inte

nsity

(A

)

Applied potential (V) vs Ag/AgCl (3M KCl)

OCP = 0,763 V

Fig. 1 Cyclic voltammogram of Bi3+ (0.75×10−2 M) and HTeO2+ (1×

10−2 M) in 1 M HNO3. Scan rate=0.01 V/s, room temperature

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

-0.04V

Inte

nsity

(ar

b. u

nits

)

-0.02V

Si(001)

0.00V

Bi2Te

3

(0 1 5) Bi

2Te

3

(1 0 10)

Bi2Te

3

(1 1 0)

Bi2Te

3

(1 2 5)

Bi2Te

3

(2 2 0)

Pt (111)

Si(004) Pt

(220)

2 (degree)

+0.02V

Fig. 2 XRD of filmselectrodeposited at constantpotential

J Solid State Electrochem

004-0802) and Si (JCPDS 271402), along with others thatcorrespond to Bi2Te3 (JCPDS 015-0863). All the filmspresent a preferred orientation along the (110) directiongiven that the intensity of the (110) diffraction peak (foundat 2θ=41.148°) is stronger than that of the other peaks, andthe second diffraction order of (110), that is (220) located at2θ=89.278°), is also present in all these films. In the case ofthe films grown at −0.04, −0.02, and 0 V constant potential,some other diffraction peaks were also present, especiallythose corresponding to (015), (1010), and (125). Actually,the diffraction peaks corresponding to (015) and (1010) arestronger in the standard diffraction pattern of powder Bi2Te3than the plane family of (110) direction, with relative inten-sities of 100 % in the case of (015), 25 % for (1010) and25 % for (110). In this case, these proportions are changed,showing a strong orientation along the (110) direction.Among the films grown at constant potential, the one thatpresents a higher orientation along the (110) direction wasthe one grown at a reduction potential of +0.02 V, whichpresents a small diffraction peak (1010) along with strong(110) and (220) peaks. This was the reason why the poten-tial used for pulsed electrodeposition was +0.02 V, whichcan be found just before the reduction peak in the cyclicvoltammetry of Bi2Te3 (Fig. 1).

XRD of films grown at pulsed electrodeposition are shownin Fig. 3. The film grown with on-time and off-time of 0.1 spresents a preferred crystalline orientation along (110) direc-tion, because only the two diffraction maxima are observed,(110) and (220). However, when the on and off-time in-creased to 5 s, (1010) diffraction peak is observed. Moreover,when the on and off-time decreased to 0.01 s, (1010) diffrac-tion peak is also observed, however the intensity is higher thanin the films grown with on and off-time of 5 s.

In order to obtain a degree of preferred orientation quan-titatively, the Harris texture analysis was performed [16].The equation of the texture coefficient is:

TC hklð Þ ¼I

hklð ÞI 0

hklð Þ

1N

P Ihklð Þ

I 0hklð Þ

ð1Þ

where I(hkl) and I0hklð Þ are the intensity of a generic peak

observed in the experimental XRD and the literature valuefrom the data base (JCPDS=15-0863), respectively and N isthe number of reflection considered in the analysis. Thestandard deviation (σ) indicates the deviation intensity ofthe experimental XRD from published values of JCPDS andis calculated as:

σ ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiP

TC hklð Þ � 1� �2

N

s

ð2Þ

The values of the texture coefficient and its standard devi-ation were calculated for the films grown at constant potentialof +0.02 V and at pulsed electrodeposition with on/off timesequal to 5/5, 0.1/0.1, and 0.01 s/0.01 s, respectively. Thesevalues are shown in Table 1. The (220) maxima was notconsidered because it is the second order of (110) maxima.Only (1010) and (110) diffraction peaks were considered.

From Table 1, it can be observed that the texture coeffi-cient is higher in (110) maxima than in (1010) maxima forall the films electrodeposited. The texture coefficient in (110) maxima is higher than 1 in all the cases indicating that thefilms have preferential orientation along (110) direction.The highest texture coefficient in (110) maxima is observedfor the film grown at pulsed potential with on and off-times

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

Bi2Te

3

(2 2 0) Pt(111)

Si(004)

Bi2Te

3

(1 0 10)

Bi2Te

3

(1 1 0)

Inte

nsity

(ar

b. u

nits

)

+0,02V 5s on 5s off

2 (degree)

+0,02V 0,1s on 0,1s off

+0,02V 0,01s on 0,01s off

Fig. 3 XRD of films grown at+0.02 V by pulsedelectrodeposition

J Solid State Electrochem

of 0.1 s, which is also the film that presents the higheststandard deviation.

In conclusion, the film electrodeposited at pulsed depo-sition with on-time=off-time=0.1 s is strongly orientedalong (110) direction. This means that the Te–Bi–Te layersare parallel to the substrate [3], or in other words, that thisfilm is oriented with the c-axis parallel to the surface of theplatinum substrate.

Morphological and composition properties Bi2Te3 films

Figure 4 shows SEM images of the Bi2Te3 films grown atconstant potential (+ 0.02 V) and at pulsed potential (5, 0.1,and 0.01 s pulse times). In the SEM micrographs, one canobserve that all films present large grain size, in the micronrange, and that all the films have similar type of growth.

The chemical composition of the films was analyzed byEDS. The measurements were performed at two different

locations on the surface for each of the four films studied.The atomic percent values of bismuth and telluriumaccording to the EDS analysis for the films grown at con-stant and pulsed electrodeposition with on/off times equal to5/5, 0.1/0.1, and 0.01 s/0.01 s are shown in Table 2. TheEDS analysis indicates that the surface of the films has acomposition rich in Bi. It should be noted that these resultsare not quantitatively accurate due to the inherent inaccura-cies of this technique.

Thermoelectric properties Bi2Te3 films

The electrical resistivity, ρ, and the Seebeck coefficient, S,were measured as a function of temperature, from 57 to107 °C. It is the first time that the Seebeck coefficient andthe power factor have been measured at 107 °C in Bi2Te3films obtained by pulsed electrodeposition. In Chen et al.[17], the Seebeck and electrical resistivity of bismuth tellu-ride films grown by constant electrodeposition were mea-sured at 77 °C, which is the highest temperature reportedpreviously.

Figures 5 and 6 illustrate the in-plane electrical resistivity(ρ) and the Seebeck coefficient (S) respectively plotted as afunction of temperature for all films grown at +0.02 V, bothconstant and pulsed electrodeposition. These thermoelectricproperties were measured in-plane, due to the availability ofthe characterization methods. Ideally, the cross-plane prop-erties (perpendicular to the surface of the films) should beassessed in order to understand if grain size orientationproduces any unusual enhancement in thermoelectric powerfactor. However, in-plane properties are also useful to de-termine the quality of the deposited films and could be

Table 1 Harris texture coefficient and standard deviation of Bi2Te3films grown at constant and pulsed electrodeposition

Films Peak IntensityXRD

IntensityJCPDS

Texturecoefficient(TC(hkl))

Standarddeviation(σ)

h k l

+0.02 V 1 0 10 16 25 0.0125 0.96271 1 0 2,550 25 1.9575

5/5 s 1 0 10 22 25 0.0113 0.98871 1 0 3,864 25 1.9887

0.1/0.1 s 1 0 10 0 25 0 1.00001 1 0 3,926 25 2.0000

0.01/0.01 s 1 0 10 40 25 0.0187 0.98131 1 0 4,246 25 1.9813

+0.02 V +0.02 V5s on 5s off

+0.02 V0.1s on 0.1s off

+0.02 V0.01s on 0.01s off

Fig. 4 SEM images of Bi2Te3films. a Constantelectrodeposition and b 5 s, c0.1 s, and d 0.01 s pulsedelectrodeposition(electrodeposition conditionsare shown in each figure).Total deposition time for allfilms is 1 h

J Solid State Electrochem

readily compared with other measurements reported in lit-erature for electrodeposited Bi2Te3 films. Although the datafor the four films are represented in the same graph, they arenot directly comparable because the charge produced duringthe electrodeposition is not the same in all cases.

The electrical resistivity of the films is represented inFig. 5, where it can be observed that the electrical resistivityis maintained relatively constant with the temperature. Forthe film grown at constant potential, the electrical resistivityis around 9 μΩm and thus the electrical conductivity (whichis the inverse of the electrical resistivity) is 0.11 (μΩ m)−1.The values of electrical resistivity obtained in Chen et al. [17]was 8.3 μΩ m after annealing treatment of the film grownabove ITO substrate and in Wang et al. was 7 μΩ m in foras-deposited film grown above platinum substrate [18]; theresistivity in both studies was measured at room temperature.In conclusion, the electrical resistivity of the film grown atconstant potential has the same order of magnitude of bismuthtelluride films grown in similar conditions.

In the case of the films grown with pulsed electrodepo-sition, the value of the electrical resistivity is very similar tothe previous one, around 11 μΩ m. This value is also withinthe range of those reported in the literature for pulsedelectrodeposition (9.65 and 12 μΩ m, in [12, 14]).

The absolute value of the Seebeck coefficient increasesslightly with temperature for all bismuth telluride films asshown in Fig. 6. All Seebeck coefficients have negative values,which show that the films present n-type conductivity probablydue to an excess of tellurium in the films composition. This isin clear contradiction with the composition measured with

Table 2 The atomic percent values of bismuth and tellurium accordingto the EDS analysis of Bi2Te3 films grown at constant and pulsedelectrodeposition

Films Atomic %bismuth

Atomic %tellurium

Formula

Constant potential 42±2 58±2 Bi2.1Te2.9Pulsed electrodepositionon-time=off-time=5 s

42±2 58±2 Bi2.1Te2.9

Pulsed electrodepositionon-time=off-time=0.1 s

46±2 54±2 Bi2.3Te2.7

Pulsed electrodepositionon-time=off-time=0.01 s

42±2 58±2 Bi2.1Te2.9

55 60 65 70 75 80 85 90 95 100 105 110

7.5

8.0

8.5

9.0

9.5

10.0

10.5

11.0

11.5

12.0

12.5

13.0

13.5

14.0

14.5

15.0

Temperature (oC)

V = 0.02V V = 0.02V, t

on=t

off=5s

V = 0.02V, ton

=toff

=0.1s

V = 0.02V, ton

=toff

=0.01s

Res

istiv

ity (

.m)

Fig. 5 Electrical resistivity measured as a function of temperature forconstant electrodeposition (E=+0.02 V (squares)) and pulsed electro-deposition alternating between constant potential (E=+0.02 V) andconstant current density (J=0 mA/cm2). Pulse time was 5 (circles),0.1 (triangles), and 0.01 s (diamonds)

55 60 65 70 75 80 85 90 95 100 105 110-80

-75

-70

-65

-60

-55

-50V = 0.02V V = 0.02V, t

on=t

off=5s

V = 0.02V, ton

=toff

=0,1s

V = 0.02V, ton

=toff

=0,01s

See

beck

coe

ffici

ent (

V/K

)

Temperature (oC)

µ

Fig. 6 Seebeck coefficient as function of temperature for constantelectrodeposition (E=+0.02 V (squares)) and pulsed electrodepositionalternating between constant potential (E=+0.02 V) and constant cur-rent density (J=0 mA/cm2). Pulse time was 5 (circles), 0.1 (triangles),and 0.01 s (diamonds)

55 60 65 70 75 80 85 90 95 100 105 110320

340

360

380

400

420

440

460

480

500

520

540

560

580

600

620

V = 0.02V V = 0.02V, t

on=t

off=5s

V = 0.02V, ton

=toff

=0.1s V = 0.02V, t

on=t

off=0.01s

Pow

er F

acto

r (

W/K

·m2 )

Temperature (oC)

µ

Fig. 7 Power factor as function of temperature for constant electrode-position (E=+0.02 V (squares)) and pulsed electrodeposition alternat-ing between constant potential (E=+0.02 V) and constant currentdensity (J=0 mA/cm2). Pulse time was 5 (circles), 0.1 (triangles) and0.01 s (diamonds)

J Solid State Electrochem

EDS, which indicate an excess of bismuth in all cases. Suchdiscrepancy has been previously reported [3, 19, 20] and hasbeen attributed to a difference in chemical composition be-tween the surface, which was analyzed by EDS, and the wholebulk of the film. Nevertheless, given that the measurementshave been performed with EDS technique (see Table 2), themeasured compositions lie within the stoichiometric composi-tion should the inaccuracy of the technique is accounted for,except in the case of the film grown at pulsed electrodepositionon-time=off-time=0.1 s.

As far as the Seebeck coefficient is concerned, the mea-surement is shown in Fig. 6. It can be observed that thisthermoelectric coefficient decreases slightly with tempera-ture, with the maximum values for the Seebeck coefficientfound at around 100 °C. For the film grown at constantvoltage, the Seebeck coefficient is −73 μV/K at 107 °C.

The films grown at pulsed deposition exhibit a Seebeckcoefficient of −66, −72, and −70 μV/K at 107 °C for onand off-times of 5, 0.1, and 0.01 s respectively. Themaximum Seebeck coefficient for the films grown atpulsed deposition is found for the on and off-times equalto 0.1 s. This value of Seebeck coefficient (−72 μV/K) isthe highest value found in the literature for films grown atpulsed electrodeposition by alternating two differentpulses, one at fixed voltage and the other at 0 current.In Glatz et al. [13] and in Ma et al. [14], the Seebeckcoefficients measured were −40 and −58.3 μV/K at roomtemperature, respectively.

The power factor of the films, which is defined as S2σ,was calculated from the electrical conductivity and theSeebeck coefficient obtained previously. Given that in thiscase the electrical conductivity was almost independent ofthe temperature and that the power factor is directlyproportional to the square of the Seebeck coefficient, itcan be immediately concluded that for our films thepower factor increases with the temperature, as it can beseen in Fig. 7. Thus the maximum Power Factor for thefilm grown at constant deposition is found at 107 °C andhas a value of around 600 μW m−1 K−2. The films grownat pulsed deposition also show their maximum powerfactor at 107 °C, with values of power factor of around400, 440, and 420 μW m−1 K−2 for on and off-times of 5,0.1, and 0.01 s, respectively. In Fig. 7, it can be seen thatthe highest power factor among the films grown at pulseddeposition corresponds to the film with on and off-timesequal to 0.1 s. In Diliberto et al. [12] and Ma et al. [14],power factors of 352 and 352.2 μW m−1 K−2at roomtemperature respectively were reported. It is important tonote that this value of power factor is the highest onefound in the literature for films grown at pulsed electro-deposition. Moreover, it is the first time that the powerfactor has been measured in Bi2Te3 films grown by pulsedelectrodeposition at 107 °C.

Conclusions

Bi2Te3 films have been electrochemically grown at constantand pulsed applied potential of +0.02 V vs. Ag/AgCl on aplatinum substrate. All of them presented a preferentialcrystallographic orientation along the (110) direction.Among the different pulses studied, the film grown with apulse of on-time=off-time=0.1 s presents the best qualitiesfor thermoelectric applications, namely the highest crystal-lographic orientation, the highest Seebeck coefficient(−72 μV/K) and a power factor of around 440 μW m−1 K−2

at 107 °C. These values are the highest Seebeck coefficientand Power Factor measured in the literature for pulsed depo-sition bismuth telluride films. The film grown at constantvoltage presents a Seebeck coefficient value of −73 μV/K at107 °C and due to its higher electrical conductivity (0.11(μΩ m)−1) in comparison with the pulsed ones, the powerfactor obtained is around 600 μW m−1 K−2 at 107 °C.

Acknowledgments The authors would like to acknowledge partialfinancial support from ERC StG NanoTEC 240497. D.A.B.T. ac-knowledges support from NSF IRES-1028071. C.V.M acknowledgesa JAE Ph.D. grant, O.C.C. acknowledges JAE post-doctoral positionsfrom CSIC, and the European Social Fund and B.A.M. recognizes theNANOTHERMA project (FCCI), MICINN project MAT2008-06330.This work had equal contributions from Adriana A. Rojas, MichelleDecepida and Cristina V. Manzano.

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