Preparation of Bismuth Telluride Films with High Thermoelectric

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Preparation of Bismuth Telluride Films with High Thermoelectric Power Factor Jongbeom Na, Younghoon Kim, Teahoon Park, Chihyun Park, and Eunkyoung Kim* Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Korea S Supporting Information *

ABSTRACT: Highly conductive n-type Bi2Te3 films on a flexible substrate were prepared via electrodeposition followed by a transfer process using an adhesive substrate. The growth of the Bi2Te3 crystals was precisely controlled by an electrochemical deposition potential (Vdep), which was critical to the preferred orientation of the crystal growth along the (110) direction and thus to the properties of a flexible thermoelectric generator (FTEG). A Bi2Te3 film prepared under Vdep of 0.02 V showed high electrical conductivity (691 S cm−1) with a maximum power factor of 1473 μW m−1 K−2, which is the highest among the Bi2Te3 films prepared by the electrodeposition methods. As-prepared FTEG was bendable, showing only a small resistance change after 300 repeated bending cycles. Combined with the n-type Bi2Te3 FTEG, a prototype p-n-type flexible thermoelectric (pn-FTEG) was prepared using p-type poly(3,4-ethylene dioxythiophene)s. The pn-FTEG (5-couples) generated an output voltage of 5 mV at ΔT = 12 K with high output power of 56 nW (or 105 nWg−1). These results indicate that the FTEG can reproducibly work well in a bent state and has high application potential for harvesting thermal energy from curved sources such as human body temperature. KEYWORDS: electrodeposition, bismuth telluride (Bi2Te3), thermoelectric generator, flexible device, nanogenerator

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sputtering,26 evaporation,27 and metal-organic chemical vapor deposition (MOCVD),28 the electrochemical deposition of TE materials enables preparation of thin film TEGs. In this context, various electrodeposition methods have been reported, such as deposition at constant potential,16,29 constant current,30 as well as pulse potential,31,32 on a metallic33−35 or silicon substrate with a Bi2Te3 seed layer.32 However, Bi2Te3 films deposited on these substrates are rigid in general due to the low flexibility of the substrate. On the other hand, the polymer substrate provides lighter and more bendable TEGs, which could be applied in IoT and also for harvesting energy from variously curved heat sources including the human body. Thus, it is challenging to prepare Bi2Te3 films on a polymer substrate. Herein, we report the facile preparation of highly conductive flexible n-type Bi2Te3 films via electrodeposition followed by transfer onto a flexible substrate, yielding bendable organic− inorganic hybrid TEGs. For providing flexible TEGs with a high power factor, the composition, morphology, crystallinity, and electrical properties of the Bi2Te3 were precisely controlled through deposition potential. Combined with a flexible p-type TE film, a p-n-type flexible thermoelectric generator (pn-FTEG) was fabricated. As a p-type TE film, poly(3,4-ethylenedioxythiophene)s (PEDOTs) was used due to its solution processability for the preparation of large-area TE patterns.7,36

INTRODUCTION In the upcoming areas of the Internet of things (IoT) and artificial intelligence (AI), self-powered electronics including sensors are essential for autonomous systems. Such self-powered electronics have been integrated with renewable energy harvesters, such as photovoltaic,1−4 triboelectric,5,6 thermoelectric,7 or their hybrids,8 and so forth. Among these, thermoelectric generators (TEGs) have been considered as one of the promising energy sources to power electronics because of the high reliability in thermoelectric conversion and surplus heat source around electronics, living environment, and human body.9−12 Thus, many thermoelectric (TE) materials have been developed and optimized to obtain high TE efficiency by improving material properties, such as electrical conductivity, Seebeck coefficient (S), and thermal conductivity (k). Concurrent with these efforts is the use of bismuth telluride (Bi2Te3) as a TE material because of its high electrical conductivity and low thermal conductivity.13−15 The TE conversion of Bi2Te3 is dependent on the stoichiometry of its chemical composition and structure: although B-rich materials show a positive S (p-type), Te-rich materials show a negative S (n-type).16−21 Traditionally, bulk Bi2Te3 materials for TEGs have been prepared via solid-state reactions at high temperature. However, it is difficult to control the stoichiometry and structure of Bi2Te3 materials using these methods. On the other hand, the electrochemical deposition at room temperature provides a cost-effective method, offering easy control of the crystallinity, composition, and morphology of TE materials.22−25 Furthermore, compared to other methods such as © XXXX American Chemical Society

Received: August 15, 2016 Accepted: November 1, 2016 Published: November 1, 2016 A

DOI: 10.1021/acsami.6b10188 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Table 1. TE Properties, Formula, and Crystalline Properties of FTEG at Various Potentials

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(bottom side before transfer) with a distance of 4 mm. The size of the prepared n-FTEGs was 1.8 cm × 2.5 cm (area = 4.5 cm2). Fabrication of p-n-Type Flexible Thermoelectric Generator (pn-FTEG). The PEDOT films were prepared by following a previous report.7 Pyridine (13.54 mg) and PEPG triblock copolymer (200 mg) were added to 1 g of the oxidative solution containing 40 wt % of iron(III) tris-p-toluenesulfonate (Fe-Tos) in n-butanol. After stirring for 6 h, the solution was sonicated for 10 min to make a homogeneous solution. The coating solution consisted of 52.8 wt % of the mixture solution of pyridine, PEPG, Fe-Tos, monomer, and 47.2 wt % of n-butanol. The molar ratio of pyridine:iron(III) tosylate:monomer was fixed at 0.55:2.25:1. Then, it was cooled before adding the EDOT monomers. The oxidative solution containing EDOT was spin-coated onto the fluorine-doped tin oxide (FTO) glass substrates. All of the samples were polymerized at 70 °C for 2 h. After cooling to room temperature, the samples were washed with ethanol to remove the residual oxidant, low molecular weight oligomers, and impurities. Then, the obtained films were dried under N2 flow and annealed on a hot plate at 70 °C for 10 min. The polymerized PEDOT films were transferred to an AFS. As prepared Bi2Te3 and PEDOT films (0.5 cm × 1.5 cm at one leg) were combined on the AFS to fabricate pn-FTEG (1.8 cm × 9 cm at five legs). Thermally evaporated Au (100 nm) was used as an electrode with a distance of 5 mm between the hot and cold sides of the pn-FTEG. Characterization of Thermoelectric Properties. The electrical conductivity of Bi2Te3 films on AFS, not on SSS (Figure 1), was determined using the four-point probe (Jandal, M86). The needle radius of the four-point probe was 100 μm, and the spacing between needles was 1 mm. The film area was 1.5 cm2. The average resistance of the film was obtained by the linear sweep volammetry method. The conductivity was determined by passing current through the outside two points of the probe and measuring the voltage across the inside two points. The thickness of the films was measured by an Alpha step profilometer

EXPERIMENTAL SECTION

Fabrication of an n-Type Flexible Thermoelectric Generator (n-FTEG). Bi2Te3 films have been grown by pulsed electrochemical deposition in a solution consisting of 8 mM Bi3+, 10 mM HTeO2+, and 1 M HNO3. The solution was prepared from bismuth (Sigma-Aldrich, 99.999%), tellurium powder (Sigma-Aldrich, 99.997%), and 70% nitric acid (Sigma-Aldrich). Nitric acid was used because H+ acts as the supporting electrolyte of the solution and NO3− as counterion. The preparation of the solution required several steps. Initially, both species, Bi3+ and HTeO2+, were dissolved in nitric acid separately. Then, deionized water was added to both solutions. Finally, in the last step, both solutions were mixed. A conventional three electrode cell configuration was used in these studies with the following electrodes: a working electrode using a stainless steel substrate (SSS, 1 cm × 3 cm), a Pt wire as the counter electrode, and a 3 M Ag/AgCl electrode as reference electrode. A bipotentiostat (Eco Chemie, Model AUT302.0) was used to control the cell with the customized software Nova 1.7 to obtain the cyclic voltammograms and the constant and pulsed electrodeposition between potentiostatic and galvanostatic modes. All experiments were carried out at room temperature. The potentials applied during deposition were determined by analysis of the cyclic voltammograms. The all-Bi2Te3 films were prepared by pulsed deposition method at the constant concentration of electrolyte. The Bi2Te3 films were applied with the potential described in Table 1 for 5 s (on time), and then the current was dropped to zero for 5 s (off time). The pulse procedure was repeated during the deposition until it reached the target thickness (2−3 μm). For the films grown with pulsed deposition, the pulse consisted of the same potential value. The deposited Bi2Te3 films were transferred to an adhesive film substrate (AFS, 3 M Scotch Magic Tape). The size of transferred n-FTEGs was 1 × 1.5 cm2. After transferring the Bi2Te3 film from SSS to AFS, the Au electrode was thermally evaporated onto the transferred Bi2Te3 film B


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ACS Applied Materials & Interfaces

Figure 1. Preparation and photographic images of an n-type flexible thermoelectric generator (n-FTEG). (a) An electrodeposited Bi2Te3 film on stainless steel substrate (SSS). (b) Attaching the adhesive tape on the Bi2Te3 film and (c) detaching the Bi2Te3 adhered tape from the stainless steel substrate. (d) FTEG with thermally evaporated gold electrode. (f) Electrodeposited Bi2Te3 film on SSS and the PEDOT film on FTO glass. (g) Attaching the AFS on the Bi2Te3 film and PEDOT film using finger pressure. (h) Detaching the Bi2Te3 and PEDOT with AFS from the SSS and FTO glass. (i) pn-Type flexible thermoelectric generator (pn-FTEG) with thermally evaporated gold electrode. The structure of prepared (e) n-FTEG and (j) pn-FTEG with measurement points of temperature and Seebeck coefficient. (Tencor Instruments, Alpha-step IQ). The Seebeck voltage and the temperature gradient of the Bi2Te3 films were determiend from a homemade shielded setup using an Agilent 34410A Multimeter and Agilent 34970A, respectively. Au lines (thickness of 120 nm) were depsoited on the Bi2Te3 films as electrode with a spacing between the lines of 4 mm (Figure S12). For general measurements, two Peltier devices attached on an aluminum heat sink, linked by a thermal paste between the two devices (∼4 mm apart), were used to generate the temperature gradient. The temperature gradient was controlled by applying various input currents, from +0.5 to −0.5 A, on the two Peltier devices using a Keithley 2400 Multimeter. Temperature gradients along the edge of the sample were deternined by two T-type thermocouples, which were on the z-direction controllable stage. The end of the thermocouples was connected to the electrode using an Ag thermal paste to ensure thermal contact. The average temperature was measured by the two thermocouples from the hot and cold regions at room temperature (18−20 °C). The voltage difference was obtained at the same point (Au electrode) as the thermal contact to minimize error. We used a pair of thin Cu wires as a T-type thermocouple, which has a low Seebeck coefficient (1.94 μV K−1) (Figure 1e and j). The temperature sensitivity of the thermocouple was 0.001 °C. In each experiment, six points of ΔV and ΔT were determined three times with various source currents, and the ΔV were plotted linearly against ΔT to afford a Seebeck coefficient of the sample. The error of the Seebeck coefficient measurement was determined as 4%, which may arise from the error in the geometry of the Au electrodes.37,38 Characterization. Film morphology and EDS mapping were observed by field-emission scanning electron microscope (FE-SEM, JSM-7001F, JEOL). X-ray diffraction (XRD) measurements were carried out using a Rigaku (Ultima IV) wide-angle goniometer using Cu Kα radiation. The flexural strain (εf) could be calculated by the equation (εf = 6Dd/L2), where D, d, and L are the maximum deflection of the center of the beam, thickness (Figure S4), and support span, respectively. The εf and angle (θ) were determined for the bending radius of 1−10 cm.

The SSS was used as the working electrode, whereas a platinum plate was used as a counter electrode during the deposition. The electrodeposition of the bismuth telluride film from the Bi and Te ions in solution can be represented as a reduction reaction32,33 3HTeO+2 + 2Bi 3 + + 9H+ + 18e− = Bi 2Te3 + 6H 2O

(1)

In this condition, we confirmed the reduction potential range of Bi3+ and HTeO2+ to Bi2Te3 and optimized the electrodeposition potential for high (110) orientation, which is a favorable orientation for high thermoelectric performance of Bi2Te3 films.32,39,40 The pH of the solution could be determined at room temperature from the solution containing 8 mM Bi3+, 10 mM HTeO2+, and 1 M HNO3 using the Pourbaix diagram to show that the selected deposition potential (Vdep) range was optimal for the electrodeposition of Bi2Te3 in the above concentrations. These conditions are consistent with previous work in acidic solutions.31 For increasing crystallinity with target orientation, Bi2Te3 was deposited in pulse potential mode, as described in the Experimental Section, to give highly dense Bi2Te3 films deposited on the SSS. Interestingy, the Bi2Te3 films were easily detached from the electrode and transferred onto an adhesive film substrate (AFS) to produce flexible Bi2Te3 films, as described in Figure 1. This allowed the fabrication of an n-type flexible thermoelectric generator (n-FTEG) via a simple transfer process. Figure 1 shows a schematic illustration and photographs of the n-FTEG fabrication process using an AFS. First, the Bi2Te3 film was grown on SSS through the pulse potential deposition method, which improves the quality of deposited Bi2Te3 film (Figure 1a). After washing with water and drying, the Bi2Te3 film on SSS was covered firmly with AFS. As shown in Figure 1b, the area of the AFS-covered Bi2Te3 film appeared dark. Next, the Bi2Te3 film was detached from the SSS and transferred to an AFS by simple finger pressure. The transferred Bi2Te3 film was uniform and flexible on the AFS without cracks. During the detaching step, the Bi2Te3 film was completely transferred

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RESULTS AND DISCUSSION Bi2Te3 films were first deposited on a stainless steel substrate (SSS) using an acidic nitric electrolyte8,21,22 solution consisting of 8 mM Bi3+, 10 mM HTeO2+, and 1 M HNO3 solution. C


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Figure 2. SEM image of electrodeposited Bi2Te3 film at various potentials of (a) −0.3, (b) −0.2, (c) −0.15, (d) −0.1, (e) −0.01, (f) 0.01, (g) 0.02, (h) 0.04, (i) 0.06, (j) 0.08, (k) 0.1, and (l) 0.2 V (scale bar: 2 μm) on a stainless steel substrate (SSS). Magnified SEM image of Bi2Te3 film (inset; scale bar: 500 nm).

along the (110) direction with higher electrical conductivity.31,42 The surface morphology and crystallinity of the Bi2Te3 film were highly dependent on the deposition potential (Vdep). Figure 2 shows scanning electron microscopy (SEM) images of the deposited Bi2Te3 film on the SSS with a different Vdep from −0.3 to 0.2 V. The surface morphology of the Bi2Te3 film deposited by the application of a negative Vdep was rough with inhomogeneously distributed large grains (Figure 2a−d). The SEM image for the Bi2Te3 film deposited at −0.3 V (BTN0.3) and −0.2 V (BTN0.2) showed rough morphologies with very large grain size. The grain size of these films was dozens of micrometers in range, and thus, the films showed low density of Bi2Te3. When deposited at −0.15 V (BTN0.15) and −0.1 V (BTN0.1), the morphologies of Bi2Te3 films looked like aggregated particles to form a rough film with poor connection between Bi2Te3 crystals. The films obtained under the application of a negative Vdep from −0.3 V (BTN0.3) to −0.1 V (BTN0.1) consisted of large grain sizes (micrometer scale) with large gaps between grains. Therefore, the electrical conductivity of BTN0.3−BTN0.1 films was low because of the poor connection between Bi2Te3 crystals. However, the Bi2Te3 film deposited at −0.01 V (BTN0.01) was different from those obtained under negative Vdep. The BTN0.01 film showed a smaller grain size (Figure 2e) and higher electrical conductivity (198 S cm−1) compared to BTN0.3−BTN0.1. This could be attributed to the well-connected film morphology of BTN0.01. When the Vdep was shifted toward positive potentials, the films showed a highly crystallographic texture of stoichiometric Bi2Te3. The films deposited with a positive Vdep from

without leaving any residue on the SSS. As shown in Figure 1c, the transferred Bi2Te3 film was metallic in appearance because it was originally prepared on a smooth stainless steel face with very low roughness (Figure S1). Such a transfer of the Bi2Te3 film to the AFS could be attributed to the large difference in surface energy between the surfaces. The average surface energies of the Bi2Te3 film and stainless steel were determined as 42.8−52.2 (top−bottom) and 27.9 mN m−1, respectively, by a contact angle experiment. Because the surface energy difference between the two materials is quite large (14.9−24.2 mN m−1), their adhesion is rather weak. Therefore, the Bi2Te3 film is easily detached from the stainless steel substrate by AFS, which consists of a cellulose acetate backing with acrylic adhesive. The adhesive energy of the AFS to Bi2Te3 is 2.5 × 105 mN m−1, which is much stronger than that between the Bi2Te3 film and SSS, thus allowing the transfer of the Bi2Te3 film to the AFS. The Bi2Te3 films were electrodeposited by pulse potetial method (Figure S11). In the pulse potential deposition, the ion concentration at the electrode surface decreases while the voltage is applied (on-time) as the ions near the electrode are converted into the film. When the current rests at zero (off-time), the ions (bismuth and tellurium) could be refilled at the film deposited electrode−electrolyte interface through their diffusion from the elecrolyte. During the duty cycles, such film growth-rest (diffusion of ions) cyles are repeated, and thus, it is possible to achieve precisely controllable deposition with a homogeneous and defined stoichiometry.41 A Bi2Te3 film deposited at an optimized pulse potential condition has shown a higher crystallinity D


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Figure 3. (a) X-ray diffraction patterns of Bi2Te3 films electrodeposited at various potentials of −0.3−0.2 V. (b) Schematic of the layered structure of Bi2Te3 with the direction of electron transfer. (c) Corresponding electrical conductivity (black circle), Seebeck coefficient (red circle), and power factor (blue circle) of the FTEGs with Bi2Te3 films deposited at different potentials (V vs Ag/AgCl). (d) Plot of the Seebeck coefficient (red circle) and power factor (blue circle) against electrical conductivity proving the Mott’s relation. (e) Plots of the electrical conductivity (black), Seebeck coefficient (red), and power factor (blue) against (110) crystallite size.

low density. The ratio of Bi and Te was determined from energydispersive X-ray spectroscopy (EDS) along with a mapping analysis. The semiconductor type of bismuth telluride is dependent on the chemical stoichiometry and B-rich shows p-type whereas Te-rich shows n-type.16−21 The EDS analysis for the prepared Bi2Te3 films showed a Te-rich stoichiometry, which indicates the formation of an n-type semiconductor. Furthermore, the stoichiometric ratio between Bi and Te was decreased as the Vdep was more positive, as determined by EDS analysis (Figure S2 and Table S2). The Bi2Te3 films deposited at a positive potential (BTP) presented a preferred orientation along the (110) orientation; the intensity of the (110) diffraction peak (2 θ = 41.15°) was stronger than that of the other peaks (Figure 3a and Figure S5). Furthermore, the electrical conductivities of BTP (0.01−0.2 V) were much higher than those deposited at negative potentials (BTN, −0.3 to −0.01 V) with weak (110) orientation. The films

0.01 V (BTP0.01) to 0.2 V (BTP0.2), respectively, showed highly dense and well-connected film morphologies. Furthermore, the films deposited under a positive potential showed small grain size, low roughness, and high electrical conductivity (Table 1), which means that the (110) crystal plane is preferably oriented parallel to the SSS. This resulted in low roughness and uniform crystal size distribution in the Bi2Te3 films. Figure 3a shows the X-ray diffraction (XRD) spectra of the Bi2Te3 films deposited under different potentials. On the basis of the reported assignment for Bi2Te3 (JCPDS 015-0863), several preferential orientations were observed: (015), (1010), (205), and (110). The electrodepostied Bi2Te3 films were formed of polycrystalline structure with several orientations. The diffraction peak with (015) orientation for BTN0.3−BTN0.15 was stronger than the peaks from other samples obtained using a more positive Vdep (Figure 3a). Films with a strong (015) orientation showed a rougher surface and were coarsely connected with E


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was greatest in BTP0.04 (36.6 nm), which showed the highest electrical conductivity (708 S cm−1). On the other hand, BTN0.3 showed the lowest electrical conductivity (121 S cm−1) with the smallest τ for the (110) orientation (Table 1). Interestingly, the films with small τ for the (110) orientation correspond to the films with large grain boundaries in SEM (Figure 2), which showed low electrical conductivities. Conversely, the films with highly dense structures and high electrical conductivity showed a large τ for the (110) orientation. This strongly indicates that τ for the (110) orientation is critical for electrical conductivity of the Bi2Te3 thin films. Therefore, the overall electrical conductivity is increased for the (110)-oriented Bi2Te3 films due to the much larger nanocrystallite size of the (110) orientation. As shown in Table 1 and Figure 3e, the electrical conductivity of Bi2Te3 films was linearly increased as the (110) nanocrystallite size was increased.40 Therefore, it is clear that the increased ordering onto the (110) direction is favorable for conductivity. In addition, the enhanced electrical conductivity was also related to the composition of the Bi2Te3 film. The (110) orientation of Bi2Te3 films was enhanced by increasing Te atomic percentage at the stoichiometric value.33,40 This led to an increase in the thermoelectric power. Thus, the effect of applied potential on the electrical conductivity was matched to that on Te as well as crystallinity of Bi2Te3 films. It is well-known that the electrical conductivity is proportional to the carrier concentration. However, the Seebeck coefficient is inversely proportional to the electrical conductivity (and thus carrier concentration) as shown in Figure 3d. Thus, the thermoelectric power factor (S2σ) was maximized for BTP0.02. Gold (120 nm) was deposited on the Bi2Te3 film on AFS as an electrode to prepare an n-type flexible TEG (n-FTEG). Bi2Te3 film on AFS kept its original morphology without cracks after gold deposition (Figure 1d). For determining the Seebeck voltages, the n-FTEGs were heated from either side while simultaneously measuring the temperature across the Bi2Te3 films (4 mm of distance between gold electrodes). At the same position, the Seebeck voltages were measured by those of the two probes, and this confers an advantage that the temperature measurement occurs precisely at the point of the electrical contact to the Bi2Te3 films (Figure 1e). The slope of the plot for Seebeck voltage for Bi2Te3 films against the temperature gradient was negative, resulting in a negative Seebeck coefficient, which is evidence of an n-type semiconductor (Figure S9) The Seebeck coefficients of the Bi2Te3 thin films were also correlated to the Vdep and orientation of Bi2Te3. The Seebeck coefficient dramatically decreased from BTN0.3 (−186 μV K−1) to BTP0.04(−132 μV K−1). Then, the Seebeck coefficient was increased in the film deposited with a more positive Vdep above 0.04 V (Figure 3c and Table 1). The highest Seebeck coefficient was acquired with BTN0.3, which displayed a strong (1010) and (015) orientation (Figure 3a). On the other hand, the BTP0.04 film showed the lowest Seebeck coefficient (−132 μV K−1) with a weak (1010) and (015) orientation. As shown in Figure 3d, the Seebeck coefficients of the films were well correlated with the electrical conductivity in accordance with the predictions of the Mott’s relation.46 The high Seebeck coefficient of n-FTEGs can be ascribed to the lower electrical conductivity, which means lower carrier concentration in the films. The reason for the low Seebeck coefficient is likely due to the high carrier concentration because the Seebeck coefficient is inversely proportional to the carrier concentration, which is related to the electrical conductivity increase.42,47 Furthermore, the charge carrier can face the grain

with a compact dense structure showed strong (110) orientation, which is consistent with previous studies on the electrodeposition of Bi2Te3 thin films on metallic substrates.22,23 The film grown at 0.04 V (BTP0.04) showed strong orientation along the (110) orientation and presented a small diffraction (1010) peak (Figure S6). On the other hand, when the Vdep was decreased from −0.01 to −0.3 V (BTN0.01−BTN0.3), the (1010) diffraction peak was increased, and crystals were grown along other orientations, such as (015) and (205) orientation (Figure S3). Because the deposition is based on the reduction of HTeO2+ and Bi3+ ions, the deposition become very fast when a negative potential was applied, resulting in crystal growth with random orientations such as (015), (1010), and (205). On the other hand, when the applied potential was changed toward positive (beyond 0.02 V), the growth became slow and Bi2Te3 would adopt their crystallographic orientation to the metal substrate at the BTNs so that they grow along the (110) orientation with decreasing random orientation. It has been wellknown that the growth speed is important for the crystallographic orientation of bismuth telluride:31,32 the growth speed of BTN0.3 was 136 nm min−1 whereas that of BTP0.02 was 33.3 nm min−1. The duty cycles for films were different based on the applied potential for their growth because the film growth was highly dependent on the applied potential, which is directly related to the total charge for the deposition. Thus, the duty cycles for BTN0.3 was 90 whereas that for BTP0.02 was 360 to reach 2.1 and 2.6 μm thick films, respectively. For this reason, the rate of deposition was different from BTN0.3 to BTP0.2 (Figure S11). The (110) nanocrystallite size was increased from BTN0.3 (21.6 nm) to BTP0.04 (36.6 nm) and then decreased as the deposition potential was higher than 0.04 V. The orientation in the (110) direction corresponds to the basal plane, which is perpendicular to the substrate. BTPs (BTP0.01−BTP0.08) showed stronger intensity for the (110) orientation, which means that the five atomic planes in the order Te(1)−Bi-Te(2)− Bi-Te(1) are parallel to the substrate in these films.43 In other words, these Bi2Te3 films are oriented with the c-axis parallel to the surface of the stainless steel substrate (SSS) (Figure 3b). The film with increased intensity of (110) orientation from this work coincides with the higher thermoelectric performance (Table 1), which matches that of previous studies.35,39−41,44,45 Figure 3c illustrates the effect of Vdep on the electrical conductivity, Seebeck coefficient, and power factor of the n-FTEG with Bi2Te3 films. The electrical conductivities of the BTNs were low compared with those of the BTPs. Furthermore, the conductivities of the BTNs were decreased to 121 S cm−1 from BTN0.01 to BTN0.3. On the other hand, the conductivity increased dramatically from BTP0.01 to BTP0.04 and then decreased when the Vdep was beyond 0.04 V (BTP0.04) (Figure 3c) because the (110) orientation of the crystal growth became smaller at Vdep > 0.04 V (Figure 3a). The preferred orientation of the crystal growth along (110) has an influence on the nanocrystallite size of the Bi2Te3, which could be determined by the Scherrer equation τ=

Kλ β cos θ

(2)

where τ, K, λ, β, and θ are the size of the nanocrystallite domain, dimensionless shape factor, X-ray wavelength, full width at halfmaximum (FWHM), and Bragg angle, respectively. The τ was determined from the FWHM of the (110) orientation of crystals in XRD (Figure 3a and Table 1). The τ for the (110) orientation F


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Figure 4. (a) Electrical conductivity, Seebeck coefficient, and power factors of Bi2Te3 thin films prepared in this work (BTP0.02) compared with those reported in the literature by using the electrodeposition method. (b) Internal resistance stability under bending stress with different bending radii (black circle) and bending cycles with a bending radius of 5 cm (blue circle). (c) Output power per weight of 1-couple (filled black circle) and 5-couples (open black circle) and output power of 1-couple (red circle) and 5-couples (blue circle) of pn-FTEGs over a temperature gradient (ΔT).

boundaries that are more present in a film with small nanocrystallite size.42 Therefore, the film with a strong (110) orientation showing large nanocrystallite size may have a small amount of grain boundaries to increase the carrier concentration but decrease the Seebeck coefficient of Bi2Te3 films. Because of the conflicting effects between the electrical conductivity and Seebeck coefficient of Bi2Te3,48,49 the power factor (S2σ) of n-FTEG was optimized by precisely controlling the deposition potential. As shown in Figure 4a, BTP0.02 showed a maximum power factor of 1473 μW m−1K−2, which is the highest value among the Bi2Te3 films reported so far.32,34,42,50−54 Figure 4b shows the resistance change of the n-FTEG under bending stress as a function of the bending radius. The increase in the resistance of the n-FTEG was less than 3% by bending the generator into a bending radius from 10 to 5 cm but was >10% when the bending radius was below 1 cm as compared to that of a flat n-FTEG (Figure 4b). In the cyclability experiment for bending stability at different bending radii, the resistance increases of the n-FTEG were observed as 2.6% (R = 10 cm) to 12.5% (R= 2 cm) after 200 repeated bending cycles (Figure S7). Furthermore, the resistance change after 300 repeated bending cycles was 6% with a fixed bending radius of 5 cm (Figure 4b). These results indicate that the n-FTEG can reproducibly work well in a bent state and has high application potential for harvesting thermal energy from curved sources, such as human body temperature. The thermoelectric properties of conducting polymers such as PEDOT have been reported by many research groups.7,55

In particular, the pristine PEDOT film prepared by the solution casting polymerization (SCP) in the presence of polymeric surfactant showed not only a high electrical conductivity but also high p-type thermoelectric properties with a high power factor.25 Thus, PEDOT films were prepared by SCP on FTO glass and transferred to AFS, taking advantage of the large surface energy difference between FTO glass and PEDOT film. The average surface energies of the FTO glass and PEDOT were determined as 84.9 and 51.3−53.1 (top−bottom) mN m−1, respectively, through the contact angle experiment. Because the surface energy difference between the FTO glass and PEDOT is large (31.8−33.6 mN m−1), the PEDOT film could be transferred from the FTO glass to AFS (Table S1). As shown in Table 1, the prepared PEDOT films on AFS showed a high electrical conductivity (1264 S cm−1). The PEDOT film showed a high Seebeck coefficient (72.1 μV K−1) and power factor (657 μW m−1 K−2). Combined with n-FTEG, a prototype pn-FTEG was prepared using the above PEDOT film. As shown in Figure 1(f−j), the Bi2Te3 and PEDOT films deposited on SSS and FTO, respectively, were transferred to an AFS to fabricate a pn-FTEG. Then, a Au electrode was thermally deposited in a transverse direction of the thermoelectric films. The internal resistance of the pn-FTEG was 20 Ω and 110 Ω for 1-couple and 5-couples, respectively. The 1-couple pn-FTEG generated an output voltage of 1.2 mV (Figure S8) and output power of 12 nW at ΔT = 12 K. As a result, the maximum output power generated by G


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(NRF) funded by the Ministry of Science, ICT & Future Planning (2016K1A1A2912753). This work was supported by The Next-generation Converged Energy Material Research Center (CEMRC) of the Agency for Defense Development (ADD).

5-couple pn-FTEG was 56 nW, which is one of the highest values among n-FTEGs with organic p-type TE materials. Moreover, in terms of output power per weight (nW g−1), 5-couple pn-FTEG provides 105 nW with low weight (0.54 g) under a low temperature difference (ΔT = 12 K) (Figure 4c). Furthermore, we also applied the 5-couple pn-FTEG to harvest human body heat by verically contacting the TEG on an arm as shown in Figure S10. This vertical-type TEG generated an output voltage of 0.93 mV upon contact with the arm. The temperature gradient between air and the arm was 2.2 K. These results demonstrate the advantages of pn-FTEGs such as their low weight, high flexibility, and ability to work under low temperature difference, which should be crucial for TEG applications in various ubiquitous heat sources including human body temperature.

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(1) O'regan, B.; Gratzel, M. A low-cost, High-efficiency Solar Cell Based on Dye-sensitized Colloidal TiO2 Films. Nature 1991, 353, 737. (2) Heo, J. H.; Im, S. H.; Noh, J. H.; Mandal, T. N.; Lim, C.-S.; Chang, J. A.; Lee, Y. H.; Kim, H.-j.; Sarkar, A.; Nazeeruddin, M. K.; Gratzel, M.; Seok, S. I. Efficient Inorganic-organic Hybrid Heterojunction Solar Cells Containing Perovskite Compound and Polymeric Hole Conductors. Nat. Photonics 2013, 7, 486−491. (3) Na, J.; Kim, Y.; Park, C.; Kim, E. Multi-layering of a Nanopatterned TiO2 Layer for Highly Efficient Solid-state Solar Cells. NPG Asia Mater. 2015, 7, e217. (4) Kim, S.; Koh, J. H.; Yang, X.; Chi, W. S.; Park, C.; Leem, J. W.; Kim, B.; Seo, S.; Kim, Y.; Yu, J. S.; Kim, J. H.; Kim, E. Enhanced Device Efficiency of Bilayered Inverted Organic Solar Cells Based on Photocurable P3HTs with a Light-Harvesting ZnO Nanorod Array. Adv. Energy Mater. 2014, 4, 1301338. (5) Lin, Z.-H.; Xie, Y.; Yang, Y.; Wang, S.; Zhu, G.; Wang, Z. L. Enhanced Triboelectric Nanogenerators and Triboelectric Nanosensor Using Chemically Modified TiO2 Nanomaterials. ACS Nano 2013, 7, 4554−4560. (6) Kim, Y.; Na, J.; Park, C.; Shin, H.; Kim, E. PEDOT as a Flexible Organic Electrode for a Thin Film Acoustic Energy Harvester. ACS Appl. Mater. Interfaces 2015, 7, 16279−16286. (7) Park, T.; Park, C.; Kim, B.; Shin, H.; Kim, E. Flexible PEDOT Electrodes with Large Thermoelectric Power Factors to Generate Electricity by the Touch of Fingertips. Energy Environ. Sci. 2013, 6, 788− 792. (8) Park, T.; Na, J.; Kim, B.; Kim, Y.; Shin, H.; Kim, E. Photothermally Activated Pyroelectric Polymer Films for Harvesting of Solar Heat with a Hybrid Energy Cell Structure. ACS Nano 2015, 9, 11830−11839. (9) Sales, B. C. Smaller Is Cooler. Science 2002, 295, 1248−1249. (10) Dresselhaus, M. S.; Chen, G.; Tang, M. Y.; Yang, R. G.; Lee, H.; Wang, D. Z.; Ren, Z. F.; Fleurial, J. P.; Gogna, P. New Directions for Low-Dimensional Thermoelectric Materials. Adv. Mater. 2007, 19, 1043−1053. (11) Bell, L. E. Cooling, Heating, Generating Power, and Recovering Waste Heat with Thermoelectric Systems. Science 2008, 321, 1457− 1461. (12) Martín-González, M.; Caballero-Calero, O.; Díaz-Chao, P. Nanoengineering Thermoelectrics for 21st Century: Energy Harvesting and Other Trends in the Field. Renewable Sustainable Energy Rev. 2013, 24, 288−305. (13) Satterthwaite, C. B.; Ure, R. W. Electrical and Thermal Properties of Bi2Te3. Phys. Rev. 1957, 108, 1164−1170. (14) Wright, D. A. Thermoelectric Properties of Bismuth Telluride and its Alloys. Nature 1958, 181, 834−834. (15) Kim, S. J.; We, J. H.; Cho, B. J. A Wearable Thermoelectric Generator Fabricated on a Glass Fabric. Energy Environ. Sci. 2014, 7, 1959−1965. (16) Martín-González, M. S.; Prieto, A. L.; Gronsky, R.; Sands, T.; Stacy, A. M. Insights into the Electrodeposition of Bi2Te3. J. Electrochem. Soc. 2002, 149, C546−C554. (17) Del Frari, D.; Diliberto, S.; Stein, N.; Boulanger, C.; Lecuire, J.-M. Comparative Study of the Electrochemical Preparation of Bi2Te3, Sb2Te3, and (BixSb1−x)2Te3 Films. Thin Solid Films 2005, 483, 44−49. (18) Martín-González, M.; Snyder, G. J.; Prieto, A. L.; Gronsky, R.; Sands, T.; Stacy, A. M. Direct Electrodeposition of Highly Dense 50 nm Bi2Te3-y Sey Nanowire Arrays. Nano Lett. 2003, 3, 973−977. (19) Michel, S.; Diliberto, S.; Stein, N.; Bolle, B.; Boulanger, C. Characterisation of Electroplated Bi2(Te1−xSex)3 Alloys. J. Solid State Electrochem. 2008, 12, 95−101.

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CONCLUSIONS In conclusion, we demonstrated the facile preparation of highly conductive n-type Bi2Te3 films via electrodeposition followed by transfer onto a flexible substrate, thus creating a flexible organic− inorganic hybrid thermoelectric generator using Bi2Te3 film and PEDOT. The composition, morphology, nanocrystallite size, and electrical properties of the Bi2Te3 were precisely controlled by electrochemical methods. The formation of n-type flexible thermoelectric generators (n-FTEGs) by optimized electrodeposition and transfer to adhesive film substrate (AFS) was successfully demonstrated. Using this new method, the Bi2Te3 and PEDOT films were transferred well to an AFS with high flexibility and low weight. Combined with a flexible p-type thermoelectric film, the p-n-type flexible thermoelectric generator (pn-FTEG) was fabricated with a Au electrode. As a result, a 5 mV output voltage was generated by pn-FTEG (ΔT = 12 K) from 5-couples of n-type (Bi2Te3) and p-type (PEDOT) thin films. In addition, output power per weight (nW g−1) of pn-FTEG (0.54 g) provides 105 nW g−1. These results are expected to have practical applications, such as hybrid pn-FTEGs in the human body that can generate electricity anywhere to activate portable electronic devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b10188. SEM images, EDS analysis, schematic of layered crystal structure of Bi2Te3, cross-cut SEM images, X-ray diffraction patterns, FWHM ratios vs deposition potentials, schematic bending properties of n-FTEG along with resistance changes, temperature gradients and Seebeck voltages relative to applied temperatures, output voltage generation on an arm, current profile and deposition rate of Bi2Te3 films, schematic for thermoelectric measurement of n-FTEG, and contact angle, surface energy, and atomic percentage values (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Global Research Laboratory (GRL) through the National Research Foundation of Korea H


Research Article


(40) Michel, S.; Diliberto, S.; Boulanger, C.; Bolle, B. Effect of Electrochemical Deposition Conditions on the Crystallographic Texture of Bismuth Telluride Alloys. J. Cryst. Growth 2006, 296, 227− 233. (41) Glatz, W.; Durrer, L.; Schwyter, E.; Hierold, C. Novel Mixed Method for the Electrochemical Deposition of Thick Layers of Bi2+xTe3−x with Controlled Stoichiometry. Electrochim. Acta 2008, 54, 755−762. (42) Ma, Y.; Ahlberg, E.; Sun, Y.; Iversen, B. B.; Palmqvist, A. E. C. Thermoelectric Properties of Thin Films of Bismuth Telluride Electrochemically Deposited on Stainless Steel Substrates. Electrochim. Acta 2011, 56, 4216−4223. (43) Federici, J. A.; Norton, D. G.; Brüggemann, T.; Voit, K. W.; Wetzel, E. D.; Vlachos, D. G. Catalytic Microcombustors with Integrated Thermoelectric Elements for Portable Power Production. J. Power Sources 2006, 161, 1469−1478. (44) Maas, M.; Diliberto, S.; de Vaulx, C.; Azzouz, K.; Boulanger, C. Use of a Soluble Anode in Electrodeposition of Thick Bismuth Telluride Layers. J. Electron. Mater. 2014, 43, 3857−3862. (45) Chaouni, H.; Bessières, J.; Modaressi, A.; Heizmann, J. J. Texture Prediction of Bi2Te3 Electroplated Layers Using Hartman’s Theory of Crystal Growth. J. Appl. Electrochem. 2000, 30, 419−427. (46) Sootsman, J. R.; Chung, D. Y.; Kanatzidis, M. G. New and Old Concepts in Thermoelectric Materials. Angew. Chem., Int. Ed. 2009, 48, 8616−8639. (47) Snyder, G. J.; Toberer, E. S. Complex Thermoelectric Materials. Nat. Mater. 2008, 7, 105−114. (48) Shen, J. J.; Hu, L. P.; Zhu, T. J.; Zhao, X. B. The Texture Related Anisotropy of Thermoelectric Properties in Bismuth Telluride Based Polycrystalline Alloys. Appl. Phys. Lett. 2011, 99, 124102. (49) Caillat, T.; Carle, M.; Pierrat, P.; Scherrer, H.; Scherrer, S. Thermoelectric Properties of (BixSb1−x)2Te3 Single Crystal Solid Solutions Grown by the T.H.M. Method. J. Phys. Chem. Solids 1992, 53, 1121−1129. (50) Takahashi, M.; Kojima, M.; Sato, S.; Ohnisi, N.; Nishiwaki, A.; Wakita, K.; Miyuki, T.; Ikeda, S.; Muramatsu, Y. Electric and Thermoelectric Properties of Electrodeposited Bismuth Telluride (Bi2Te3) Films. J. Appl. Phys. 2004, 96, 5582−5587. (51) Yoo, B. Y.; Huang, C. K.; Lim, J. R.; Herman, J.; Ryan, M. A.; Fleurial, J. P.; Myung, N. V. Electrochemically Deposited Thermoelectric N-type Bi2Te3 Thin Films. Electrochim. Acta 2005, 50, 4371− 4377. (52) Li, S.; Toprak, M. S.; Soliman, H. M. A.; Zhou, J.; Muhammed, M.; Platzek, D.; Müller, E. Fabrication of Nanostructured Thermoelectric Bismuth Telluride Thick Films by Electrochemical Deposition. Chem. Mater. 2006, 18, 3627−3633. (53) Wang, W.-L.; Wan, C.-C.; Wang, Y.-Y. Composition-dependent Characterization and Optimal Control of Electrodeposited Bi2Te3 Films for Thermoelectric Application. Electrochim. Acta 2007, 52, 6502−6508. (54) Caballero-Calero, O.; Díaz-Chao, P.; Abad, B.; Manzano, C. V.; Ynsa, M. D.; Romero, J. J.; Rojo, M. M.; Martín-González, M. S. Improvement of Bismuth Telluride Electrodeposited Films by the Addition of Sodium Lignosulfonate. Electrochim. Acta 2014, 123, 117− 126. (55) Bubnova, O.; Khan, Z. U.; Malti, A.; Braun, S.; Fahlman, M.; Berggren, M.; Crispin, X. Optimization of the Thermoelectric Figure of Merit in the Conducting Polymer Poly(3,4-ethylenedioxythiophene). Nat. Mater. 2011, 10, 429−433.

(20) Martín-González, M.; Prieto, A. L.; Knox, M. S.; Gronsky, R.; Sands, T.; Stacy, A. M. Electrodeposition of Bi1‑xSbx Films and 200-nm Wire Arrays from a Nonaqueous Solvent. Chem. Mater. 2003, 15, 1676− 1681. (21) Prieto, A. L.; Martín-González, M.; Keyani, J.; Gronsky, R.; Sands, T.; Stacy, A. M. The Electrodeposition of High-Density, Ordered Arrays of Bi1‑xSbx Nanowires. J. Am. Chem. Soc. 2003, 125, 2388−2389. (22) Xiao, F.; Hangarter, C.; Yoo, B.; Rheem, Y.; Lee, K.-H.; Myung, N. V. Recent Progress in Electrodeposition of Thermoelectric Thin Films and Nanostructures. Electrochim. Acta 2008, 53, 8103−8117. (23) Boulanger, C. Thermoelectric Material Electroplating: a Historical Review. J. Electron. Mater. 2010, 39, 1818−1827. (24) Lei, C.; Burton, M. R.; Nandhakumar, I. S. Facile Production of Thermoelectric Bismuth Telluride Thick Films in the Presence of Polyvinyl Alcohol. Phys. Chem. Chem. Phys. 2016, 18, 14164−14167. (25) Manzano, C. V.; Abad, B.; Muñoz Rojo, M.; Koh, Y. R.; Hodson, S. L.; Lopez Martinez, A. M.; Xu, X.; Shakouri, A.; Sands, T. D.; BorcaTasciuc, T.; Martin-Gonzalez, M. Anisotropic Effects on the Thermoelectric Properties of Highly Oriented Electrodeposited Bi2Te3 Films. Sci. Rep. 2016, 6, 19129. (26) Kim, D.-H.; Byon, E.; Lee, G.-H.; Cho, S. Effect of Deposition Temperature on the Structural and Thermoelectric Properties of Bismuth Telluride Thin Films Grown by Co-sputtering. Thin Solid Films 2006, 510, 148−153. (27) Zou, H.; Rowe, D. M.; Min, G. Growth of P- and N-type Bismuth Telluride Thin Films by Co-evaporation. J. Cryst. Growth 2001, 222, 82−87. (28) Venkatasubramanian, R.; Colpitts, T.; Watko, E.; Lamvik, M.; ElMasry, N. MOCVD of Bi2Te3, Sb2Te3 and Their Superlattice Structures for Thin-film Thermoelectric Applications. J. Cryst. Growth 1997, 170, 817−821. (29) Prieto, A. L.; Sander, M. S.; Martín-González, M. S.; Gronsky, R.; Sands, T.; Stacy, A. M. Electrodeposition of Ordered Bi2Te3 Nanowire Arrays. J. Am. Chem. Soc. 2001, 123, 7160−7161. (30) Michel, S.; Diliberto, S.; Boulanger, C.; Stein, N.; Lecuire, J. M. Galvanostatic and Potentiostatic Deposition of Bismuth Telluride Films from Nitric Acid Solution: Effect of Chemical and Electrochemical Parameters. J. Cryst. Growth 2005, 277, 274−283. (31) Manzano, C.; Rojas, A.; Decepida, M.; Abad, B.; Feliz, Y.; Caballero-Calero, O.; Borca-Tasciuc, D.-A.; Martin-Gonzalez, M. Thermoelectric Properties of Bi2Te3 Films by Constant and Pulsed Electrodeposition. J. Solid State Electrochem. 2013, 17, 2071−2078. (32) Cao, Y.; Zeng, Z.; Liu, Y.; Zhang, X.; Shen, C.; Wang, X.; Gan, Z.; Wu, H.; Hu, Z. Electrodeposition and Thermoelectric Characterization of (00L)-Oriented Bi2Te3 Thin Films on Silicon with Seed Layer. J. Electrochem. Soc. 2013, 160, D565−D569. (33) Miyazaki, Y.; Kajitani, T. Preparation of Bi2Te3 Films by Electrodeposition. J. Cryst. Growth 2001, 229, 542−546. (34) Naylor, A. J.; Koukharenko, E.; Nandhakumar, I. S.; White, N. M. Surfactant-Mediated Electrodeposition of Bismuth Telluride Films and Its Effect on Microstructural Properties. Langmuir 2012, 28, 8296− 8299. (35) Lei, C.; Ryder, K. S.; Koukharenko, E.; Burton, M.; Nandhakumar, I. S. Electrochemical Deposition of Bismuth Telluride Thick Layers onto Nickel. Electrochem. Commun. 2016, 66, 1−4. (36) Kim, J.; You, J.; Kim, B.; Park, T.; Kim, E. Solution Processable and Patternable Poly(3,4-alkylenedioxythiophene)s for Large-Area Electrochromic Films. Adv. Mater. 2011, 23, 4168−4173. (37) Hamedi, M.; Elfwing, A.; Gabrielsson, R.; Inganäs, O. Electronic Polymers and DNA Self-Assembled in Nanowire Transistors. Small 2013, 9, 363−368. (38) Wang, H.; Ail, U.; Gabrielsson, R.; Berggren, M.; Crispin, X. Ionic Seebeck Effect in Conducting Polymers. Adv. Energy Mater. 2015, 5, 1500044. (39) Fan, X. A.; Yang, J. Y.; Zhu, W.; Bao, S. Q.; Duan, X. K.; Xiao, C. J.; Li, K. Preferential Orientation and Thermoelectric Properties of P-type Bi0.4Sb1.6Te3 System Alloys by Mechanical Alloying and Equal Channel Angular Extrusion. J. Alloys Compd. 2008, 461, 9−13. I


Preparation of Bismuth Telluride Films with High Thermoelectric

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