Precision Interface Engineering of an Atomic Layer in Bulk Bi2Te3

Jun 5, 2019 - When the number of ALD cycles increases, the peak approaches 1021.7 eV .... We synthesized well-designed BST thermoelectric materials of...
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Precision Interface Engineering of an Atomic Layer in Bulk Bi2Te3 Alloys for High Thermoelectric Performance Downloaded via NOTTINGHAM TRENT UNIV on July 19, 2019 at 06:24:59 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Kwang-Chon Kim,† Sang-Soon Lim,†,‡ Seung Hwan Lee,§ Junpyo Hong,§ Deok-Yong Cho,∥ Ahmed Yousef Mohamed,∥ Chong Min Koo,§ Seung-Hyub Baek,†,⊥ Jin-Sang Kim,*,† and Seong Keun Kim*,† †

Center for Electronic Materials, Korea Institute of Science and Technology (KIST), Seoul 02792, Korea School of Materials Science and Engineering, Yonsei University, Seoul 03722, Korea § Materials Architecturing Research Center, Korea Institute of Science and Technology, Seoul 02792, Korea ∥ Institute of Photonics and Information Technology (IPIT) and Department of Physics, Chonbuk National University, Jeonju 54896, Korea ⊥ Division of Nano and Information Technology, KIST School, Korea University of Science and Technology, Seoul 02792, Korea ‡

S Supporting Information *

ABSTRACT: Grafting nanotechnology on thermoelectric materials leads to significant advances in their performance. Creation of structural defects including nano-inclusion and interfaces via nanostructuring achieves higher thermoelectric efficiencies. However, it is still challenging to optimize the nanostructure via conventional fabrication techniques. The thermal instability of nanostructures remains an issue in the reproducibility of fabrication processes and long-term stability during operation. This work presents a versatile strategy to create numerous interfaces in a thermoelectric material via an atomic-layer deposition (ALD) technique. An extremely thin ZnO layer was conformally formed via ALD over the Bi0.4Sb1.6Te3 powders, and numerous heterogeneous interfaces were generated from the formation of Bi0.4Sb1.6Te3−ZnO core−shell structures even after high-temperature sintering. The incorporation of ALDgrown ZnO into the Bi0.4Sb1.6Te3 matrix blocks phonon propagation and also provides tunability in electronic carrier density via impurity doping at the heterogeneous grain boundaries. The exquisite control in the ALD cycles provides a high thermoelectric performance of zT = 1.50 ± 0.15 (at 329−360 K). Specifically, ALD is an industry compatible technique that allows uniform and conformal coating over large quantities of powders. The study is promising in terms of the mass production of nanostructured thermoelectric materials with considerable improvements in performance via an industry compatible and reproducible route. KEYWORDS: thermoelectric, bismuth antimony telluride, ZnO, atomic layer deposition, p-type, heterogeneous interface

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conflicts between the material parameters impose a limitation on the improvement of the thermoelectric efficiency. Tremendous efforts have focused on overcoming the fundamental limitation to improve the thermoelectric performance.3−5 A promising and widely examined strategy for the improvement corresponds to nanostructuring of thermoelectric materials.6−10 Phonon waves, which carry heat flow, are scattered at structural defects. The introduction of nanostructured thermoelectric materials creates numerous interfaces and point defects to enhance phonon scattering. Recent

hermoelectric materials that allow direct conversion of heat to electric energy and vice versa attracted significant attention in terms of applications for sustainable energy involving scavenging from waste heat.1,2 The performance of thermoelectric materials is determined via figure of merit, which is defined as zT = S 2σ T/κ, where S denotes the Seebeck coefficient, σ denotes the electrical conductivity, T denotes the absolute temperature, and κ denotes the total thermal conductivity of electronic (κe) and lattice (κL) contributions. In order to maximize the thermoelectric performance, it is necessary to tune the material parameters. In principle, three material parameters (S, σ, and κ) are strongly coupled with each other, and S is inversely proportional to σ, while κe increases with σ. These types of © 2019 American Chemical Society

Received: April 3, 2019 Accepted: June 5, 2019 Published: June 5, 2019 7146

DOI: 10.1021/acsnano.9b02574 ACS Nano 2019, 13, 7146−7154

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Figure 1. Creation of heterogeneous interfaces via ALD approach. (a) A schematic of the BST/ZnO nanostructure including heterogeneous interfaces. (b) Schematic process diagram for the formation of BST/ZnO heterogeneous interfaces.

The introduction of the heterogeneous interfaces composed of Bi2Te3 and an extremely thin layer with a high melting point can resolve the instability issue and eventually enable the control and design of the grain boundaries. A key challenge in introducing the heterogeneous interfaces in the thermoelectric materials involves methods to adequately disperse the heterogeneous ultrathin layers in the thermoelectric matrix. Atomic layer deposition (ALD) constitutes a promising approach. Specifically, ALD is a thin-film growth technique based on self-limiting mechanism and exhibits distinct features of precise thickness control at an atomic scale and excellent conformality even on a complex-shaped substrate.25,26 Furthermore, ALD is widely used in industry due to its mass production suitability.27,28 The predominant capabilities of ALD allow almost perfect conformal coating of nanometerthick layers even over large quantities of fine powders.29 In the present study, we demonstrate high thermoelectric performance via the design of heterogeneous grain boundaries of Bi0.4Sb1.6Te3(BST)/ZnO (Figure 1a) by using a mass production compatible technique of ALD. The fabrication procedure is shown in Figure 1b. In these types of configurations, κL is significantly decreased due to phonon scattering at heterogeneous grain boundaries, and σ can be modulated by controlling the number of cycles of ZnO ALD as well. The simultaneous combination of the decrease in κL and increase in σ achieves a high zT of 1.50 ± 0.15 in a temperature range corresponding to 329−360 K. The high thermoelectric performance through the interface engineering via the ALD is also proved in the thermoelectric module fabricated with p-type BST/ZnO legs and commercial zonemelt n-type legs.

studies on nanostructured materials reported significant improvements in thermoelectric performance (zT > 1) due to the decrease in κL. However, over the past decades, zT of bulk Bi2Te3 thermoelectric material has remained at approximately 1 near room temperature.11 Among the suggested nanostructures of thermoelectric materials, heterogeneous nanocomposites such as nanoinclusion and nanoprecipitates in the thermoelectric matrix were extensively examined with respect to high thermoelectric performance.7,12−17 The heterogeneous nanocomposite approach was verified to significantly improve the zT value via decreasing the κL and increasing the power factor via electronic carrier filtering.12,15,18 Additionally, bulk-scale nanocomposite thermoelectric materials are facilely fabricated through the conventional fabrication techniques such as ball-milling and subsequent hot pressing or spark plasma sintering (SPS). This is definitely advantageous because nanocomposite thermoelectric materials can be fabricated in a commercially available form in large quantities. However, it is not easy to uniformly synthesize nanocomposites with the optimized structure via conventional fabrication techniques.19−22 Additionally, further loading of heterogeneous precipitates for the enhancement of phonon scattering is limited owing to the deterioration in thermoelectric properties such as S and σ. Another strategy for high thermoelectric performance involves employing nanoscale grains.9,23 Materials composed of fine grains contain a high density of grain boundaries and lead to more efficient phonon scattering. A critical challenge in the synthesis of thermoelectric materials with nanoscale grains corresponds to the thermal instability of nanostructures. High energies associated with numerous grain boundaries make the nanostructure thermodynamically unstable.18,24 Nanoscale grains can spontaneously dissolve during the sintering process and long-time operation at elevated temperatures. Specifically, the grain boundary engineering is more challenging for the Bi2Te3-related materials due to their low melting point (approximately 600 °C).

RESULTS AND DISCUSSION Design of Heterogeneous Interfaces. A prerequisite to forming the heterogeneous grain boundaries is to synthesize BST powders uniformly coated with an ultrathin ZnO layer (i.e., core−shell structured powders). Using the exquisite 7147

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Figure 2. Structural analysis of ultrathin ZnO-coated BST materials. (a) HRTEM images of the pristine BST (top left panel) and the 10cycle-ZnO-coated BST powders (bottom left and right panels). (b) HAADF STEM images of the SPS pellet of the 10-cycle-ZnO-coated BST. TEM EDS elements maps corresponding to the area observed in the HAADF STEM image. (c) The magnified TEM images corresponding to the square region in (b). Right figures show the HAADF STEM image and the EDS element maps of Zn and O for the indicated area. (d) SEM images of the fractured surfaces of the uncoated (left panel) and 10-cycle-ZnO-coated (right panel) BST pellets.

observed. The density of the BST SPS pellets is not significantly changed by the ALD coating of ZnO (Figure S5, Supporting Information). This indicates that the ALD of ZnO over the BST powders does not destroy the structure of BST even after the SPS process. A peak corresponding to ZnO is also not detected owing to the small fraction of the ZnO layer and its thinness. A TEM analysis confirms the presence of ZnO/BST heterogeneous grain boundaries in the SPS pellets. Figure 2b shows high-angle annular dark-field (HAADF) scanning TEM (STEM) images of the SPS pellet of a 10-cycle-ZnO-coated BST. The TEM EDS element maps corresponding to the area observed in the HAADF STEM image reveal that the ZnO layers mainly exist at the boundaries of submicrometer-sized BST grains and thus encircle the BST grains (Figure 2b and Figure S6, Supporting Information). In the magnified TEM images corresponding to the square region in Figure 2b (Figure 2c), the ZnO layer is approximately 8 nm thick, and this is consistent with the thickness expected from the growth per cycle (Figure S1, Supporting Information). A concern in the study using oxide ALD corresponds to the possibility of oxidation of BST due to the incorporation of oxygen during the SPS process. The oxidation of BST should be suppressed because it deteriorates the thermoelectric performance.30 Despite the concern of oxidation, severe diffusion of Zn and O into BST grains is not observed in the EDS element maps in Figure 2c. This can be attributed to the high reactivity of Zn toward oxygen when compared to that of Bi and Sb. The ALD coating of the ZnO layer significantly influences the microstructure and specifically the grain size of the SPS BST pellets. Figure 2d shows SEM images of the fractured surfaces of the SPS pellets of the uncoated and ZnO-coated BST. The fractured surface of the uncoated BST pellet displays plate-like shaped grains due to the layer structure of

control of the growth conditions provided by ALD, the ZnO layers are uniformly and conformally formed over the BST powders. The ALD process was performed at room temperature to suppress the oxidation of BST. The thickness of ALDgrown ZnO films is verified as linearly proportional to the number of ALD cycles (Figure S1, Supporting Information), thereby indicating the excellent thickness controllability at subnanoscale. Figure 2a shows high-resolution transmission electron microscopy (HRTEM) images of the pristine BST and ZnO-coated BST powders. The pristine BST powder does not exhibit a distinct layer on the surface. Conversely, the ALD-processed BST powder is enclosed with a thin layer composed of small grains. This type of a core−shell structure is observed even for an extremely small powder with a size of approximately 20 nm (right image in Figure 2a). The fast Fourier transform (FFT) pattern on the selected area of the thin surface layer corresponds to that of a ZnO (002) plane, thereby indicating that the thin layer corresponds to ZnO. Additionally, the results of the TEM energy dispersive X-ray spectroscopy (EDS) element map of Zn indicates that the ZnO layer is spaciously distributed over the BST powders (Figure S2, Supporting Information). The results support that the ZnO ALD process can successfully synthesize core−shell type powders uniformly coated with ultrathin ZnO film. In the study, BST thermoelectric pellets are fabricated via SPS at 450 °C for 5 min with the prepared powders. The EDS analysis results (Figure S3, Supporting Information) indicate that the Zn element is spaciously distributed in the ZnOcoated BST SPS pellet. However, the Zn signal is not detected in the uncoated BST SPS pellet. The X-ray diffraction (XRD) patterns of the pellets indicate that all of the diffraction peaks match well with Bragg peaks of the rhombohedral structure of Bi0.4Sb1.6Te3 irrespective of the number of ALD cycles (Figure S4, Supporting Information). A secondary phase is not 7148

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Figure 3. Modulation of electrical properties through interface engineering. The variations in the value of (a) resistivity, (b) Hall mobility, and (c) hole concentration of SPS pellets measured at room temperature as a function of the number of ZnO ALD cycles. Each value was obtained from an average of 10 samples fabricated under the same conditions. The electrical properties were measured along the direction perpendicular to the pressing direction of SPS. (d) The amplitude and real part of the Fourier-transformed EXAFS of the 10-cycle-ZnOcoated BST SPS pellet. (e) XPS spectra of Zn 2p3/2 core level in 0, 1, 3, 5, 10, and 15 cycles ZnO-coated BST SPS pellets.

Bi0.4Sb1.6Te3. Large grains exceeding 10 μm are observed in the fractured surface. Conversely, the results indicate that the ZnO-coated BST pellet is composed of significantly smaller grains despite the identical conditions of the SPS process. This type of inhibition in the grain growth is attributed to the uniform dispersion of ZnO layers in the BST matrix. Specifically, the low movement energy of ZnO at the sintering process temperature (which is attributed to the large difference in the melting points between ZnO (1975 °C) and BST (614 °C31)) further impedes the grain growth of BST. The results indicate that the ALD approach desirably designs the microstructure composed with numerous heterogeneous interfaces even after the SPS process at high temperatures. Engineering of Electronic Carriers at Heterogeneous Grain Boundaries. In order to understand the effect of ZnO coating on the electronic conductivity of BST SPS pellets, we varied the number of ZnO ALD cycles. Figure 3a shows the variation in the average resistivity of 10 SPS pellets measured at room temperature as a function of the number of ZnO ALD cycles. The electrical properties were measured along a direction perpendicular to the pressing direction of SPS. The change in the resistivity in Figure 3a exhibits two distinct regions, exhibiting opposite trends in terms of the number of ALD cycles. Up to the 5 ALD cycles, the resistivity decreases with increases in the number of ZnO ALD cycles. Conversely, above the 5 ALD cycles, the resistivity increased with increases in the number of ZnO ALD cycles. In order to elucidate this interesting phenomenon, the variations in the electronic mobility and carrier concentration were examined via the Hall measurement. As shown in Figure 3b, the mobility monotonically decreases in the entire range when the number of ZnO ALD cycles increases. This suggests that the decrease in the resistivity up to 5 ALD cycles in Figure 3a does not result from the change in the electronic mobility. Although both the resistivity and the mobility decrease with increases in

the number of cycles exceeding 5 ALD cycles, the extent (approximately 16%) of the decrease in the mobility in the range of 5−15 ALD cycles is not sufficient to cause the large difference (approximately 63%) in the resistivity in the corresponding region. With respect to the hole concentration in Figure 3c, the increase in the number of ZnO ALD cycles leads to the increase in the carrier concentration from 2.3 × 1019/cm3 at 0 cycles to 3.5 × 1019/cm3 at 5 cycles and further ZnO deposition significantly decreases the hole concentration to 2.8 × 1019/cm3 at 15 cycles. This indicates that the change in the carrier concentration is the main reason for the significant change in resistivity in Figure 3a. The changes in the local atomic structures of the SPS BST pellets are examined via Zn K-edge extended X-ray absorption fine structure (EXAFS) analysis to understand the variation in carrier concentration. Figure 3d shows the amplitude and real part of the Fourier-transformed EXAFS of the 10-cycle-ZnOcoated BST SPS pellet. The results indicate that two different shell features corresponding to Zn−O (atomic distance of approximately 0.208 nm) and Zn−Te (atomic distance of approximately 0.23 nm) bonds optimally reproduced the experimental spectra. This suggests that Zn ions in the ZnOcoated BST pellet exhibit bindings with O ions and also with Te ions. Details of the fitting results are described in the Note S1 (Supporting Information). In order to further elucidate the change in the local structure near Zn ions in terms of the number of ZnO ALD cycles, the chemical binding states of Zn ions in 1, 3, 5, 10, and 15 cycles of ZnO-coated BST pellets are examined via XPS as shown in Figure 3e. The XPS spectra of the other elements are shown in Figure S7 (Supporting Information). A peak is not observed in the uncoated BST pellets, while a non-negligible peak of Zn 2p3/2 core level is observed even with only 1-cycle deposition of ZnO. Interestingly, the peak shifts to a higher binding energy with increases in the number of ZnO ALD cycles. The peak of the 7149

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Figure 4. Thermoelectric characterization. Temperature dependence of the (a) electronic conductivity, (b) Seebeck coefficient, (c) power factor, (d) thermal conductivity, (e) lattice contribution of the thermal conductivity, and (f) zT of the SPS BST with various ZnO thicknesses. The thermal conductivity and the other data of the 10-cycle-ZnO-coated BST were obtained from the average of 8 and 16 samples, respectively.

1-cycle-ZnO-coated BST pellet is located at 1021 eV and corresponds to Zn−Te bonding. When the number of ALD cycles increases, the peak approaches 1021.7 eV and corresponds to ZnO. The EXAFS and XPS results indicate that Zn ions are preferentially bonded with Te in BST within a few ZnO cycles, and the grown layer changes into ZnO when the number of ALD cycles increases. The ALD reaction shows a transient growth mode for ALD of ZnO on BST. Consequently, it can be summarized that the grown layer changes with increasing the number of the ZnO ALD cycles because of the easy interaction between diethylzinc and BST, and the properties of the initially formed interfacial layer and the subsequently formed ZnO layer are different. We previously reported on the ALD growth behavior of ZnO films on a Bi2Te3 epitaxial layer.32 The previous study reported that Zn−Te bonds were formed at the very initial growth stage due to the interaction between diethylzinc and Bi2Te3, and ZnO was then grown on the interfacial layer,32 and this is consistent with the results of the present study. The Zn ions exhibit a valence state of +2, and this can act as an acceptor in BST matrix when Zn exhibits a bond with Te by substituting the Bi or Sb site. Hence, the Zn−Te bonds formed at the heterogeneous interfaces potentially contribute to the increase in the hole concentration of the BST. In contrast, ZnO is a ntype semiconductor with a wide band gap, and this contributes to decreasing the hole concentration in the BST pellets. The opposite influences of the Zn−Te bonds and ZnO on the variation in the hole concentration can aid in understanding the change in the carrier concentration in terms of the number of ZnO ALD cycles, as shown in Figure 3c. Thermoelectric Properties. The microstructure of the SPS BST was engineered to include heterogeneous grain boundaries via the introduction of the ultrathin ZnO layer using the ALD technique, and this significantly affects both the electronic carrier density and phonon transport. The above results indicated that the electronic carrier density was changed by the variation in the ZnO layer thickness. In order to

determine the optimal thermoelectric properties of the ZnOcoated BST materials, the temperature dependence of thermoelectric properties including σ, S, and κ was examined for SPS BST with various thicknesses of ZnO (Figure 4). The thermoelectric properties were measured both perpendicular and parallel to the SPS pressing direction (Figure S8, Supporting Information). For the ZnO-coated BST pellets, no significant difference in the thermoelectric properties was observed for both directions. So, we mostly measured the thermoelectric performance along the perpendicular to the SPS pressing direction. All the properties in Figure 4 were measured perpendicular to the SPS pressing direction. As shown in Figure 4a, the σ of all the samples decreases with increases in temperature, and this corresponds to a typical characteristic of highly degenerated semiconductors. The σ value increased with increases in the number of ZnO cycles up to 5 cycles, while the further growth of ZnO decreases the σ above the 5 ALD cycles. This is consistent with the conductivity results obtained from the Hall measurement in Figure 3a. The S value increases with the temperature, exhibits a maximum value near 400 K, and then decreases with the temperature above 400 K (Figure 4b), which is attributed to bipolar conduction (Figure S9, Supporting Information). Near 300 K, the S reveals an inverse trend of the σ with increases in ZnO cycles. It is known that S in thermoelectric materials exhibits an inverse relationship with the electronic carrier density. The variation in the hole carrier density in Figure 3c supports the change in the S with ZnO cycles. Figure 4c shows the variation in the power factor of the BST pellets as a function of the temperature. The power factor of the uncoated BST pellet corresponded to 4 mWm−1 K−2 at 300 K. Given the enhanced σ and slightly decreased S by the ZnO coating, the power factor increased, and the value for the 10-cycle-ZnOcoated BST pellet was 4.3 mWm−1 K−2 at 300 K. The κ of the SPS BST pellets was also measured along the same direction as the σ and S (perpendicular to the SPS 7150

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Figure 5. Thermoelectric performance of modules. (a) The thermoelectric module fabricated with 127 pairs of p−n legs (Crediting the KIST logo to correct source). The variations in the (b) maximum output power and the (c) conversion efficiency of the modules as a function of temperature difference across the module. Several data from literatures are shown for comparison.18,37

The decrease in the κ while maintaining high σ led to a significant improvement in the figure of merit zT (Figure 4f). The zT value increased approximately to 45% by 10 cycles of ZnO ALD in the entire temperature range corresponding to 329−490 K (zTavg = 1.32 for the 10-cycle-ZnO-coated BST and 0.91 for the uncoated BST in the corresponding temperature range). The zT value was obtained from the average of 16 SPS pellets of 10-cycle-ZnO-coated BST for reliability (Figure S11, Supporting Information). This type of improvement in a wider temperature range is definitely favorable for energy harvesting applications in the lowtemperature region. Specifically, the 10-cycle-ZnO-coated BST pellet shows the highest zT value of 1.50 ± 0.15 in a temperature range of 329−360 K. The value corresponds to an increase of approximately 40% over the highest value (1.05) of the uncoated BST in the same temperature range. This type of performance is highly promising and competitive when compared to other p-type nanostructured Bi2Te3-related materials.15,18,37−39 Such a significant improvement in the thermoelectric performance is also proved for the other compositions of BST (Figure S12, Supporting Information). This indicates that the performance improvements via the proposed approach are even valid for a wide composition range of Bi2Te3-based materials. The superior thermoelectric performance of the p-type 10cycle-ZnO-coated BST was further verified via a thermoelectric module. Figure 5a shows the thermoelectric module fabricated with 127 pairs of p−n legs. A zone-melt Bi2Te2.7Se0.3 single crystal was used as the n-type counterpart leg. The thermoelectric module with the ZnO-coated BST exhibited a high maximum output power and a high conversion efficiency when compared to the other module fabricated only via zone-melt single crystal p−n legs (Figure 5b,c). When the temperature difference across the module reached 100 K (temperature of the cold side is 300 K), the maximum output power and conversion efficiency of the module with the ZnO-coated BST were as high as 2.33 W and 3.64%, respectively, while the value of the module with zone-melt single crystal legs only

pressing direction). Figure 4d shows the temperature dependence of the total κ of the ZnO-coated BST with various thicknesses of ZnO. All of the BST pellets exhibited a minimum κ value near 370 K. Increases in the number of the ZnO cycles decreased κ up to the 10 cycles. Conversely, it increased above the critical cycle of 10. The minimum value of κ corresponded to 1.12 and 0.85 Wm−1 K−1 for the uncoated and 10-cycle-ZnO-coated BST pellets, respectively. The κ value significantly reduced by approximately 25% via only the ZnO ALD of 10 cycles, although the σ of the 10-cycle-ZnOcoated BST pellet even exceeded that of the uncoated BST pellet. As mentioned above, the introduction of ZnO into BST matrix via ALD formed fine BST grains enclosed with an ultrathin ZnO layer and created numerous heterogeneous interfaces. This structure significantly decreased κL through boosting phonon scattering at the heterogeneous interfaces. In order to further understand the significant decrease in κ, the κL is calculated based on Wiedemann−Franz formula, κL = κ − κe (= LσT), where L denotes the Lorenz number. The L value that is calculated using Snyder’s approach33 was estimated in a range from 1.66 × 10−8 to 1.69 × 10−8 V2 K−2 in the study (see Note S2, Supporting Information, for details). As shown in Figure 4e, the κL value at 345 K for the 10-cycle-ZnO-coated BST is 0.35 Wm−1 K−1, and this is extremely close to the theoretical minimum κL (κmin = 0.31 Wm−1 K−1 as calculated by the Cahill’s model).34,35 This demonstrates that the design of BST/ZnO core−shell structures via the ALD technique is an effective strategy to reduce κL. The increase in the κL value was observed above 10 cycles of ZnO ALD. It should be noted that Zn−Te bonds are dominantly formed at the very initial ALD growth, and the further increase in the ALD cycles changes the grown layer into ZnO. By increasing the number of ZnO ALD cycles, thus, the contribution of the ZnO layer to the thermal transport is enhanced. The thermal conductivity (37−147 Wm−1K−1)36 of ZnO is much higher than that of BST. Hence, the increase in the κL value observed above 10 cycles of ZnO ALD might be attributed to the contribution of the ZnO layer with a high thermal conductivity. 7151

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Bi0.4Sb1.6Te3 and were sealed in a steel vial with steel balls under ambient Ar to prevent oxidation of the materials. Mechanical alloying was performed at a milling speed of 1200 rpm for 6 h. The formed BST was ground by ball-milling with zirconia balls for 24 h in ambient Ar to reduce and control the size of powders. Subsequently, ZnO thin films were grown over the BST powders via ALD at room temperature to minimize the oxidation of the powders. Diethylzinc and H2O were used as the Zn and O sources, respectively. The chamber containing the BST powders was ultrasonically vibrated during the growth to enhance the conformality of the ZnO layers with respect to the whole powders. The ZnO thickness was controlled via the change in the ALD cycles. A SPS process was performed in a vacuum chamber for 5 min at 723 K with a heating rate of 150 K min −1 under an applied pressure of 40 MPa. Structural and Chemical Characterization. X-ray diffraction (XRD, D8 ADVANCE, BRUKER) was used to identify the crystal phase. The microstructure was examined via scanning electron microscopy (SEM, Inspect F, FEI) and transmission electron microscopy (TEM, Titan 80−300 and Talos F200X, FEI). The element maps were obtained using STEM equipped with energydispersive X-ray spectroscopy (EDS). An X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe, ULVAC-PHI) technique was employed to analyze the chemical binding states of the materials. Furthermore, Zn K-edge X-ray absorption spectroscopy was performed in a 10 C beamline at the Pohang Light Source. The absorption coefficients of the samples were collected in the transmission mode. Fourier analyses of the EXAFS were processed via the UWXAFS package.40 Thermoelectric Characterization. The electrical conductivity and the Seebeck coefficient were measured from 300 to 490 K via a standard four-probe method (TEP 600, SEEPEL instrument). The Seebeck coefficient was measured by a method based on the slope of a voltage versus temperature difference curve. The thermal conductivity was calculated from the equation of κ = DCpρ, where D, Cp, and ρ denote the thermal diffusivity, specific heat capacity, and density, respectively. Additionally, D was obtained via a laser flash method (LFA 457, Netzsch), and the Cp was measured by a differential scanning calorimeter (DSC, Q20, TA Instruments) (Figure S14, Supporting Information). Furthermore, ρ was determined via the Archimedes method (Figure S5, Supporting Information). The carrier concentration and electronic mobility were measured by the Hall measurement system (HMS-3000, ECOPIA). Module Fabrications and Measurements. Thermoelectric modules with a size of 40 × 40 × 3.4 mm3 were fabricated with 127 pairs of p−n legs. The p-type TE legs correspond to materials prepared in the study. The counterparts, n-type legs, are commercial zone-melt Bi2Te2.7Se0.3 single crystal provided by MI-Seojin, Inc. A nickel/tin bilayer was used as a diffusion barrier between the thermoelectric legs and solder. The size of the legs corresponds to 1.4 × 1.4 × 1.6 mm3. The output power and conversion efficiency of the modules were measured via a commercial measurement system (TEGBlue-1500, BlueSys Co.).

corresponded to 2.06 W and 2.58% (Figure 5b and Figure S13 (Supporting Information) and Figure 5c). The conversion efficiency of the module was also compared with those of modules in extant studies, as shown in Figure 5c. The module with the ZnO-coated BST exhibited a higher conversion efficiency than that of the other modules by utilizing nanostructured Bi2Te3-related materials.18,37 The results evidently support the enhancement in the thermoelectric performance of the p-type BST engineered with the ALD approach.

CONCLUSIONS We synthesized well-designed BST thermoelectric materials of fine grains enclosed with an ultrathin ZnO layer for high thermoelectric performance. The ALD enables conformal coating over a complex structure and realizes the microstructure of fine BST grains with heterogeneous grain boundaries. The introduction of the ZnO layer into the BST matrix via the ALD approach affects the thermoelectric properties in two critical ways. First, the κL significantly decreases. The ZnO-coated BST includes small grains and abundant heterogeneous interfaces. The structural defects block the phonon propagation. Specifically, 10-cycle-ZnOcoated BST exhibited an extremely low κL of 0.35 Wm−1K−1, and this is extremely close to the theoretical limit. Second, the σ was modulated via the incorporation of the ALD ZnO layer. At the very beginning of ZnO ALD on BST, the Zn ions are preferentially bound with Te ions in the BST as opposed to O ions. The Zn ions near the interface with BST led to creation of holes as acceptors. This effect is dominant up to a certain number of ALD cycles (5) of ZnO, and the hole carrier density increases. Further growth of ZnO significantly decreases the carrier density. It is potentially attributed to the n-type nature of ZnO. The synergetic combination of the decrease in the κL and the increase in the σ through optimization of the number of ALD cycles demonstrates an extremely high thermoelectric performance of zT = 1.50 ± 0.15 at a temperature range of 329−360 K. The study shows the possibility of mass production of the nanostructured thermoelectric materials beyond the realization of the high thermoelectric performance. Specifically, ALD is widely used in industry and makes it possible to form extremely uniform and conformal films even over a large quantity of powders.29 It should also be noted that the number of ALD cycles required for the optimized performance only corresponds to 10 cycles in the study. The low growth rate is a crucial disadvantage of ALD in mass production and is nullified via the use of the aforementioned small number of ALD cycles. Furthermore, the proposed approach for high thermoelectric performance is effective for the other thermoelectric materials beyond Bi2Te3-related materials. Along with a decrease in the κ via the structure engineering, a variety of materials grown by ALD offer possibilities to modulate the electronic carrier density of the thermoelectric material based on the selection of the ALD-grown material. Thus, the approach used in the study allows a variety of core−shell type nanostructures that provide excellent opportunities to maximize thermoelectric performance in an industry compatible manner.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b02574. ALD growth per cycle of ZnO; TEM and EDS images; SEM images and EDS results; XRD patterns; density; EXAFS analyses; XPS spectra; thermoelectric properties depending on the measurement direction; Hall measurement; Lorenz number calculation; thermoelectric properties for reproducibility; thermoelectric properties of the BST with different composition; output performance of the modules; thermal diffusivity and specific heat capacity (PDF) (PDF)

EXPERIMENTAL SECTION Materials synthesis. High-purity Bi (99.999%), Sb (99.9999%), and Te (99.9999%) granules (Alfa Aesar Co. Ltd.) were used as starting materials. The materials were weighed for a composition of 7152

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ACS Nano

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AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Deok-Yong Cho: 0000-0001-5789-8286 Chong Min Koo: 0000-0002-8674-9236 Seong Keun Kim: 0000-0001-8712-7167 Author Contributions

The manuscript was prepared through the contribution of all authors. K.C.K. and S.S.L. produced the thermoelectric materials and performed the thermoelectric characterization. S.H.L., J.H., and C.M.K. performed measurements of the thermal conductivity. D.Y.C. and A.Y.M. performed the EXAFS analysis. S.H.B., J.S.K., and S.K.K supervised the work. K.C.K. and S.K.K. prepared the manuscript. S.K.K. designed and directed the research. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We would like to acknowledge the financial support from the R&D Convergence Program of NST (National Research Council of Science and Technology) of Republic of Korea. D.Y.C. acknowledges the support from Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (grant no. 2018R1D1A1B07043427). REFERENCES (1) Snyder, G. J.; Toberer, E. S. Complex Thermoelectric Materials. Nat. Mater. 2008, 7, 105. (2) DiSalvo, F. J. Thermoelectric Cooling and Power Generation. Science 1999, 285, 703−706. (3) Pei, Y.; Wang, H.; Snyder, G. J. Band Engineering of Thermoelectric Materials. Adv. Mater. 2012, 24, 6125−6135. (4) Kim, S. I.; Lee, K. H.; Mun, H. A.; Kim, H. S.; Hwang, S. W.; Roh, J. W.; Yang, D. J.; Shin, W. H.; Li, X. S.; Lee, Y. H.; Snyder, G. J.; Kim, S. W. Dense Dislocation Arrays Embedded in Grain Boundaries for High-Performance Bulk Thermoelectrics. Science 2015, 348, 109− 114. (5) Heremans, J. P.; Wiendlocha, B.; Chamoire, A. M. Resonant Levels in Bulk Thermoelectric Semiconductors. Energy Environ. Sci. 2012, 5, 5510−5530. (6) Vineis, C. J.; Shakouri, A.; Majumdar, A.; Kanatzidis, M. G. Nanostructured Thermoelectrics: Big Efficiency Gains from Small Features. Adv. Mater. 2010, 22, 3970−3980. (7) Liu, W.; Yan, X.; Chen, G.; Ren, Z. Recent Advances in Thermoelectric Nanocomposites. Nano Energy 2012, 1, 42−56. (8) 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. (9) Poudel, B.; Hao, Q.; Ma, Y.; Lan, Y.; Minnich, A.; Yu, B.; Yan, X.; Wang, D.; Muto, A.; Vashaee, D.; Chen, X.; Liu, J.; Dresselhaus, M. S.; Chen, G.; Ren, Z. High-Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys. Science 2008, 320, 634−638. (10) Lan, Y.; Minnich, A. J.; Chen, G.; Ren, Z. Enhancement of Thermoelectric Figure-of-Merit by a Bulk Nanostructuring Approach. Adv. Funct. Mater. 2010, 20, 357−376. (11) Shi, X.; Chen, L.; Uher, C. Recent Advances in HighPerformance Bulk Thermoelectric Materials. Int. Mater. Rev. 2016, 61, 379−415. 7153

DOI: 10.1021/acsnano.9b02574 ACS Nano 2019, 13, 7146−7154

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DOI: 10.1021/acsnano.9b02574 ACS Nano 2019, 13, 7146−7154