Soluble Telluride-Based Molecular Precursor for Solution-Processed

Jun 11, 2019 - Bong-Seo Kim .... Experimental details, ZT comparison table, absorption spectra, SEM images, SEM-EDS analysis, TGA, FT-IR, XRD, DSC, ...
0 downloads 0 Views 6MB Size
Download PDF
Letter Cite This: ACS Appl. Energy Mater. 2019, 2, 4582−4589

www.acsaem.org

Soluble Telluride-Based Molecular Precursor for Solution-Processed High-Performance Thermoelectrics Seungki Jo,† Sun Hwa Park,‡ Hosun Shin,‡ Inseon Oh,† Seung Hwae Heo,† Hyeong Woo Ban,† Hyewon Jeong,† Fredrick Kim,† Seungjun Choo,† Da Hwi Gu,† Seongheon Baek,† Soyoung Cho,† Jin Sang Kim,§ Bong-Seo Kim,∥ Ji Eun Lee,∥ Seungwoo Song,‡ Jung-Woo Yoo,† Jae Yong Song,‡ and Jae Sung Son*,† Downloaded via GUILFORD COLG on July 31, 2019 at 05:26:17 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

School of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea ‡ Center for Convergence Property Measurement, Korea Research Institute of Standards and Science (KRISS), Daejeon 34113, Republic of Korea § Center for Electronic Materials, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea ∥ Energy Conversion Research Center, Korea Electrotechnology Research Institute (KERI), Changwon 51543, Republic of Korea S Supporting Information *

ABSTRACT: The recent interest in wearable electronics suggests flexible thermoelectrics as candidates for the power supply. Herein, we report a solution process to fabricate flexible Sb2Te3 thermoelectric thin films using molecular Sb2Te3 precursors, synthesized by the reduction of Sb2Te3 powder in ethylenediamine and ethanedithiol with superhydride. The fabricated flexible Sb2Te3 thin films exhibit a power factor of ∼8.5 μW cm−1 K−2 at 423 K, maintaining the properties during 1000 bending cycles. FePt nanoparticles are homogeneously embedded in the Sb2Te3 thin film, reducing the thermal conductivity. The current study offers considerable potential for manufacturing high-performance flexible thin film devices. KEYWORDS: Sb2Te3, thin film, thermoelectrics, flexible device, molecular precursor, solution process

T

electronics.8 However, the fabrication of high-performance TE thin films remains a technological challenge as most TE materials are prepared on a bulk scale by conventional melting or sintering processes. While vacuum deposition processes, including chemical vapor deposition (CVD) and molecular beam epitaxy (MBE), have been reported to enable the fabrication of TE thin films of moderate performance,9,10 these fabrication processes are complicated and costly, limiting the widespread application of TE thin films. Solution-phase deposition of TE materials can serve as a convenient route for fabricating nanostructured thin films in a cost-effective and scalable manner.11 Soluble chalcogenidometallates are promising candidates for use as the source ink for high-performance TE thin films as these compounds are fully dispersible in polar solvents and can be recovered into crystalline phases by moderate heat treatment.12 Furthermore, the nanostructuring to enhance ZT can be optimized using these precursors by embedding nanoparticles or controlling the grain size.4,5,13 So far, several chemical routes for the synthesis of chalcogenidometallate precursors have been suggested.

hermoelectric (TE) energy conversion between heat and electricity has been of significant interest in various research fields because of its simple, sustainable, and environment-friendly features.1 However, the market size of TE modules is limited by the low performance of typical TE materials, as well as their limited practical applications.2 The TE efficiency of materials is generally estimated by the dimensionless figure of merit ZT = (S2σT/κ), where S, σ,T, and κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively. So far, considerable research efforts have been dedicated to enhancing ZT by decoupling the trade-off relationship among S, σ, and κ. One available strategy is to embed nanoparticles in bulk TE materials, which can significantly intensify phonon scattering to reduce κ by the formation of numerous interfaces3 or to engineer electronic band structures to enhance the power factor (S2σ).4,5 The recent rapid growth of portable and wearable electronics has led to considerable interest in the use of microscale or millimeter-scale generators that harvest electricity from light, mechanical, or thermal sources for the construction of self-powered systems.6,7 In this field, power generation from waste heats by thin film thermoelectrics has found new niche applications, including as a flexible and microscale TE power supply for low-power-consumption © 2019 American Chemical Society

Received: April 2, 2019 Accepted: June 11, 2019 Published: June 11, 2019 4582

DOI: 10.1021/acsaem.9b00685 ACS Appl. Energy Mater. 2019, 2, 4582−4589

Letter

ACS Applied Energy Materials

Figure 1. (a) Schematic illustration of the procedure for the synthesis of the molecular telluroantimonate precursor and thin film fabrication. (b) DLS analysis showing the size distribution of the Sb2Te3 solute. SEM images of thin films fabricated by spin-coating of (c) polymeric Sb2Te3, (d) molecular Sb2Te7, and (e) molecular Sb2Te3 solutions, and subsequent annealing at 300 °C. (f) Cross-sectional SEM image of the Sb2Te3 thin film corresponding to panel e.

Mitzi et al. opened the field of solution-phase synthesis of metal chalcogenide compounds using a hydrazine-reducing route, which is applicable to a variety of semiconductors.14 However, hydrazine is not desirable for practical use because of its explosiveness and toxicity issues. Recently, Brutchey et al. have developed an alternative method that uses less toxic synthetic routes, in which a mixture of amine and thiol is used to dissolve powdered metal chalcogenides.12,15 Although this approach offers a viable route for the synthesis of soluble inorganic precursors, telluride-based precursors such as Sb2Te3, Bi2Te3, and PbTe have rarely been studied with this chemistry for practical TE applications. This limitation originates in the underlying challenge of synthesis of molecule-level precursors as a prerequisite to fabricate high-quality TE thin films. As a solution for this challenge, in this work, we developed the synthesis of fully reduced molecular telluroantimonate precursors by using a superhydride reducing agent in a solvent mixture of ethanedithiol and ethylenediamine for the solutionprocessed fabrication of high-performance TE thin films. Sb2Te3 was chosen as a model system for this study because Sb2Te3 is known to be one of the best TE materials, showing a peak ZT near room temperature, and its structural and

compositional characteristics are well-established.16 We found that the superhydride treatment for Sb2Te3 precursor considerably decreased the size of Sb2Te3 precursors to the molecular level, allowing extremely uniform Sb2Te3 thin films fabricated by spin-coating. Furthermore, the versatility of this TE precursor was validated by embedding FePt nanoparticles, increasing a ZT value up to 0.72 at 423 K by the reduction of thermal conductivity. Finally, the TE thin films fabricated on polyimide substrates withstood 1000 bending cycles without significant performance degradation. Figure 1a illustrates the entire process for the Sb2Te3 thin film fabrication. The initial solution was synthesized by dissolving powdered Sb2Te3 in a mixture of ethylenediamine and ethanedithiol. Dynamic light scattering (DLS) analysis revealed that this Sb2Te3 solute existed not as molecules but as colloidal structures with sizes of several tens of nanometers (Figure 1b). This suggests that Sb2Te3 compounds exist as nanoparticulates, which might be the forms of polymeric Sb− Te structures that were not fully reduced to molecule-level species, as the chalcogenidometallate complexes are sometimes found to have extended polymeric structures.17 In contrast, the precursors synthesized with the superhydride contain solutes 4583

DOI: 10.1021/acsaem.9b00685 ACS Appl. Energy Mater. 2019, 2, 4582−4589

Letter

ACS Applied Energy Materials

was attributed to the thermal decomposition of molecular precursors into a crystalline Sb2Te3 phase (Figure S7). This result agrees with the Fourier transform infrared (FT-IR) spectra, displaying a ν(N−H) stretch around 3300 cm−1 which should originate in the ν(N−H) stretching mode of the ethylenediammonium cation bound to Sb2Te3 chalcogenidometallate anion for the sample dried at 100 °C and removing the stretching band of all organic species after heat treatment at 250 °C (Figure S8). Accordingly, to obtain crystalline Sb2Te3, the deposited thin films were further annealed at temperatures higher than 220 °C. At annealing temperatures at 300 °C, peaks corresponding to a rhombohedral Sb2Te3 structure appeared in the XRD pattern of the thin films. It is noteworthy that the observed crystallographic orientation in these thin films was negligible in the XRD patterns, and the relative peak intensities along the c-axis and in the ab-plane were identical to those of the reference (JCPDS 00-015-0874). To quantitatively estimate the degree of orientation along the c-axis, the orientation factor (F) of the current thin film was calculated by Lotgering’s method.18 The detailed calculation is described in the Supporting Information. The estimated F of the fabricated Sb2Te3 thin film marked 0.0831, close to 0, indicating almost randomly oriented grains (Figure S9). To further understand the thermal decomposition behavior of the telluroantimonate precursor, we carried out X-ray photoelectron spectroscopy (XPS) analysis for Sb2Te3 thin films dried and heat-treated at 300 °C to identify the chemical bonding evolution of Sb2Te3 during heat treatment. The XPS spectrum of the dried thin film in the Sb 3d region shows multiple peaks of 3d5/2 at 528.2 and 529.8 eV that are indexed to Sb−Sb homopolar and Sb metallic bondings (Figure S10).19 The peak at 531.6 eV may indicate the surface oxidation of the specimen. Also, in the Sb 3d3/2 region, two peaks for Sb−Sb homopolar and Sb metallic bondings are clearly observed. This result suggests that the telluroantimonate molecular precursor consists of Sb−Sb bonding as well as Sb−Te bonding, which is in line with the reported molecular structure of the Sb6Te94− Zintl anion.20 On the other hand, the heated Sb2Te3 sample shows only the peaks for the Sb−Te bonding at 529.8 and 539.0 eV in the XPS spectrum, indicating the decomposition of the molecular precursor into crystalline Sb2Te3. As with Sb 3d peaks, the Te 3d5/2 of homopolar bonding (Te−Te) at 571.7 and 582.1 eV for the molecular precursor shifts to the Te metallic peaks at 572.4 and 582.7 eV after heat treatment at 300 °C (Figure S7). On the basis of these results, we speculate that the unidentified peaks in the XRD pattern (Figure S6) of the dried sample could originate in the local bonding formation of Sb−Sb clusters or amorphous Sb−Te. Another benefit of the current molecular Sb2Te3 precursor solution is the possibility to embed nanoparticles inside the Sb2Te3 matrix in thin films because this inorganic anion readily attaches to the surface of nanoparticles to generate allinorganic nanoparticles,21 which are homogeneously miscible with the Sb2Te3 precursor solution. We chose FePt for the embedded nanoparticles because FePt nanoparticles have high thermal stability and remain as nanoscale structures during the annealing process without undergoing a solid-state reaction with Sb2Te3.22 Generally, it has been reported that metal nanoparticles as guest additives are easily reacted with and are integrated into host semiconductors of thermoelectric materials such as Bi2Te3 and Sb2Te3 by atomic diffusion. For example, Zhang et al. reported that Ag nanoparticles reacted with the Sb2Te3 matrix and formed intermetallic phases of

that were less than 1 nm in size, and no nanoscale structures such as polymeric or colloidal structures were detected in DLS measurements, indicating that Sb2Te3 precursor was fully reduced to molecular species (Figure 1b). UV−vis absorption spectra of the Sb2Te3 precursors before and after the reaction with superhydride showed identical transitions at 529 nm (Figure S1), suggesting that these precursors possessed the same basic molecular units. The elemental analysis revealed that the Sb:Te ratio of precursors changed from ∼2:3 to ∼2:7 after the superhydride reaction, confirmed by scanning electron microscopy energy dispersive X-ray spectroscopy (SEM-EDS) and inductively coupled plasma optical emission spectroscopy (ICP-OES) (Figure S2). It seems that the formed Sb-thiolates could be removed during the purification step, leading to the reduction of the content of Sb in the resulting molecular Sb2Te3 precursor.15 One possible form of this cluster is the reported molecular Sb2Te74− cluster synthesized in hydrazine,13 which has a composition identical to that of the current Sb2Te3 precursor. The molecular Sb2Te7 precursor was further treated with tri-n-octylphosphine (TOP) to control the ratio of Sb/Te (Figure 1a) since the excess Te could deteriorate the microstructures of the fabricated thin films upon heat treatment due to the evaporation of Te. TOP binds easily to Te to form a TOP−Te complex, which is soluble in nonpolar solvents, allowing removal of TOP-Te by simply washing with an organic solvent. The Sb2Te3 thin films were fabricated with three different inks of as-synthesized polymeric Sb2Te3 solution, superhydride-treated molecular Sb2Te7 solution, and superhydrideand TOP-treated molecular Sb2Te3 solutions. These ink solutions were deposited on different substrates (glass, silicon, and polyimide) by spin-coating and were subsequently annealed at 300 °C. The as-synthesized polymeric Sb2Te3 solution and molecular Sb2Te7 solution generated films that appeared rough and hazy. SEM analysis of these thin films (Figure 1c,d) shows numerous voids that should hinder the charge carrier transport. The poor continuity in this film should originate in the nanoparticulated feature of the assynthesized Sb2 Te3 precursor solution and excess Te evaporation in the molecular Sb2Te7 precursor, as evidenced by the SEM and EDS analysis (Figure S3 and Table S1). Under the same condition, the deposition of the superhydride- and TOP-treated molecular Sb2Te3 solution formed a continuous thin film that showed a mirror-like reflection (Figure 1a), indicating its highly uniform thickness and negligible light scattering by surface roughness. This thin film showed continuous and smooth coverage on the substrate with minimal pinholes (Figure 1e) and had a uniform thickness of ∼200 nm, as shown in cross-sectional SEM images (Figure 1f). Furthermore, EDS analysis reveals that this film possessed a stoichiometric composition of Sb2Te3 (Figure S4), thereby enabling us to avoid the loss of TE performance resulting from the compositional deviation from Sb2Te3. In addition, the elemental mapping images of surface and cross-sectional area of Sb2Te3 thin film also reveal that Sb and Te elements are homogeneously distributed over the entire area without the local aggregation (Figure S5). Figure S6 shows the X-ray diffraction (XRD) patterns of the thin films annealed at various temperatures. The as-deposited film shows broad and featureless peaks in its XRD pattern, indicating rather amorphous or molecular structures. Thermogravimetric analysis (TGA) of the molecular Sb2Te3 precursors shows remarkable weight loss at 220 °C, which 4584

DOI: 10.1021/acsaem.9b00685 ACS Appl. Energy Mater. 2019, 2, 4582−4589

Letter

ACS Applied Energy Materials

Figure 2. (a) Photographs showing the two-phase ligand exchange procedure of FePt nanoparticles with the ligands of Sb2Te3 chalcogenidometallate anions. TEM images of 4.0 nm sized FePt nanoparticles capped with (b) organic ligands and (c) Sb2Te3 chalcogenidometallate anions. (d) FT-IR spectra of organic- and Sb2Te3-chalcogenidometallate-capped FePt nanoparticles. (e) SEM image of FePt-nanoparticle-embedded Sb2Te3 thin film. (f) Bright field STEM image and (g) SAED patterns of FePt-nanoparticle-embedded Sb2Te3 matrix annealed at 300 °C.

Ag5Te3, AgTe3, and Ag2Te at temperatures as low as 150 °C.5 Ko et al. reported that Pt nanoparticles started to react with Sb2Te3 at above 200 °C.4 Such solid-state reactions are sometimes helpful for the realization of the modulation doping and the formation of the coherent phases, but the major benefit of nanostructuring on thermoelectrics, the increase of the density of metal−semiconductor interfaces, cannot be observed. The 4.0 nm sized FePt nanoparticles capped with long-chained oleylamine and oleic acid (Figure S11) were mixed in a molecular Sb2Te3 solution by a typical two-phase

ligand exchange process (Figure 2a).23 After the ligand exchange, FePt nanoparticles retained their sizes and shapes, as manifested in the transmission electron microscopy (TEM) images (Figure 2b,c). Moreover, FT-IR spectra of FePt nanoparticles show the complete disappearance of the ν(C− H) stretching modes after the ligand exchange, further supporting surface capping with Sb2Te3 chalcogenidometallate anions (Figure 2d). This mixed ink solution of FePt nanoparticles in Sb2Te3 molecular solution was used to fabricate thin films under the 4585

DOI: 10.1021/acsaem.9b00685 ACS Appl. Energy Mater. 2019, 2, 4582−4589

Letter

ACS Applied Energy Materials

Figure 3. (a) Photographs showing the flexible Sb2Te3 thin film deposited on a polyimide substrate. (b) Relative change in resistance of the Sb2Te3 thin film on polyimide substrate with the number of bending cycles at a bending radius of 10 and 15 mm. Temperature dependence of (c) electrical conductivity (σ), (d) Seebeck coefficient(S), (e) power factor (S2σ), and (f) thermal conductivity (κ, solid lines) and lattice thermal conductivity (κL, dotted lines).

high annealing temperature of 300 °C, which allows us to observe the nanostructuring effect on TE properties. To investigate the versatility of the molecular Sb2Te3 precursor solution for the flexible thin films, we fabricated Sb2Te3 and FePt-embedded Sb2Te3 thin films on polyimide substrates by spin-coating of precursor solutions and subsequent annealing at 300 °C. The specularly reflective surface of the fabricated Sb2Te3 thin film on polyimide in Figure 3a reveals its high degree of microstructural uniformity and continuity. The flexibility of the Sb2Te3 thin film was examined by measuring the change in the two-probe resistance of the film with the number of bending cycles at a bending radius of 10 and 15 mm (Figure 3b and Figure S12). The increase in resistance after 1000 bending cycles was less than 9% and 28% at a bending radius of 10 and 15 mm, respectively. Considering that the resistance of indium tin oxide (ITO) on polyimide substrates was recently reported to increase by 600% after 1000 bending cycles,24 our result clearly demonstrates the outstanding durability and the potential applicability of the Sb2Te3 molecular precursors for flexible electronic devices. The temperature-dependent TE properties of these flexible Sb2Te3 and FePt-nanoparticle-embedded Sb2Te3 thin films on polyimide substrates were measured in the temperature range 303−423 K, as shown in Figure 3c−f. The Sb2Te3 thin film exhibited an electrical conductivity (σ) of 46 492 S m−1 at room temperature. The Seebeck coefficient (S) was 105.6 μV

same condition as that for Sb2Te3 thin films. The added amounts of FePt nanoparticles were investigated from 1 to 7 wt % in the Sb2Te3 matrix while the microstructural uniformity of the annealed thin films was maintained at an amount of FePt up to 3 wt %. The fabricated film with 3 wt % of FePt nanoparticles exhibits highly uniform and continuous microstructures as shown in the SEM image (Figure 2e). The XRD pattern of this thin film exclusively shows the peaks to be indexed to a rhombohedral Sb2Te3 structure, and the peaks related to FePt nanoparticles were not detected due to tiny amounts of FePt. The orientation factor, F, was estimated to 0.0885, almost identical to that estimated from the XRD pattern of the pristine Sb2Te3 thin film, indicating the isotropic crystallographic orientation (Figure S10). To further explore the conservation of FePt nanoparticles upon heating, the dried molecular Sb2Te3 compound with ∼7 wt % of FePt nanoparticles was annealed under the same condition as that for the Sb2Te3 thin films. The bright field scanning TEM (STEM) image of the annealed sample (Figure 2f) shows welldispersed nanostructures embedded in the host grains, indicating that FePt nanoparticles were not agglomerated in the Sb2Te3 matrix during annealing. The selected area electron diffraction (SAED) analysis clearly shows both patterns of Sb2Te3 and FePt (Figure 2g), demonstrating the preservation of FePt nanoparticles without alloying with Sb2Te3 despite the 4586

DOI: 10.1021/acsaem.9b00685 ACS Appl. Energy Mater. 2019, 2, 4582−4589

Letter

ACS Applied Energy Materials K−1 at room temperature and increased up to 135.5 μV K−1 at 423 K. The electrical conductivity of the FePt-nanoparticleembedded Sb2Te3 thin film was slightly lower than those of pristine Sb2Te3 thin films over the entire temperature range while the Seebeck coefficients were similar to or higher than those of pristine Sb2Te3 thin films. The resulting power factor (S2σ) of the Sb2Te3 thin film increased from ∼5 μW cm−1 K−2 at room temperature to ∼8.5 μW cm−1 K−2 at 423 K (Figure 3e). The FePt-nanoparticle-embedded Sb2Te3 thin film shows similar values over the entire temperature range, because of the trade-off between the electrical conductivity and the Seebeck coefficient. We further checked the stability of the electrical conductivity and the Seebeck coefficient of the Sb2Te3 thin films by measuring the properties over heating and cooling cycles. As manifested in Figure S13, the electrical properties of the Sb2Te3 thin film were identically maintained on both heating and cooling, demonstrating the thermal stability of the thin films. To understand the electrical behaviors, we measured the carrier concentration (nh) and mobility (μh) by Hall effect measurement (Table S2) at room temperature. The Sb2Te3 thin film exhibited an nh of 1.66 × 1020 cm−3, higher than that of the bulk Sb2Te3 crystal, and the nh of the FePt-nanoparticleembedded Sb2Te3 thin film was slightly increased to 1.77 × 1020 cm−3, resulting from an increased energy level difference between the Fermi level and the valence band at the FePt− Sb2Te3 interfaces (Figure S14).4 Accordingly, the current Sb2Te3 thin films show the typical behaviors of heavily doped semiconductors both in electrical conductivity and Seebeck coefficient. The Seebeck coefficient continuously increased with increasing temperatures, indicating that the peak value can be obtained at above the measurement temperature range. The μh was 17.5 cm2 V−1 s−1, one of the highest values for the reported Sb2Te3 thin films fabricated by a solution process that demonstrates a high degree of structural continuity.4,5,25,26 The μh was slightly decreased to 16.3 cm2 V−1 s−1, which is understandable, considering the effects of increased charge carrier scattering at the interfaces between Sb2Te3 and FePt nanoparticles. Such boundary scattering of charge carriers at the interfaces is less affected by temperature, leading to weaker temperature dependence in electrical conductivity of FePtnanoparticle-embedded Sb2Te3 thin films. The impurity scattering which is induced by the diffusion of FePt into Sb or Te phases during heat treatment also could contribute to the reduction of carrier mobility. Moreover, it is noteworthy that the increased nh could not explain the Seebeck coefficients that are higher than those obtained from pristine Sb2Te3 thin films, as the Seebeck coefficient is known to be inversely proportional to nh.16 The increased Seebeck coefficients might be attributed to the energy filtering effect since the FePt nanostructure−Sb2Te3 host combination forms the interfacial energy barrier at the metal−semiconductor junction (Figure S14). There have been many reports of metallic nanostructureembedded thermoelectric materials showing the energy filtering effect. For example, the Pt nanoparticle−Sb2Te3 junction has been cited as the benchmark combination for the energy filtering effect in many papers by the formation of the Schottky barrier.4 Such barriers can obstruct the flow of low-energy charge carriers in Sb2Te3 host grains,27 which increase the Seebeck coefficient because the low-energy charge carriers are known to negatively affect the Seebeck coefficient. The temperature-dependent thermal conductivity (κ) of the thin films was measured in the through-plane direction by

time-domain thermoreflectance (TDTR) (Figure 3f) with the specific heat capacity measured by the differential scanning calorimetry (DSC) and the density measured by the X-ray reflectivity (XRR) (Figure S15). The Sb2Te3 thin film exhibited the κ value of 0.673 W m−1 K−1 at room temperature, and this value gradually decreased to 0.573 W m−1 K−1 at 423 K. These values are about 3 times lower than the reported κ of the polycrystalline Sb2Te3 bulk,28 which is attributed to the boundary scattering of phonons at the nanoscale grain boundaries as well as the surface and the interface with a substrate.29 Interestingly, embedding FePt nanoparticles in the thin film leads to a dramatically reduced κ of 0.473 W m−1 K−1 at 423 K. To further understand the thermal properties of the thin films, the lattice thermal conductivity (κL) of these samples was calculated by subtracting the electronic contribution of thermal conductivity (κE) from the total thermal conductivity, where κE was calculated by the Wiedemann− Franz Law, κE = L0σT, where L0 is the Lorenz number, σ is the electrical conductivity, and T is the absolute temperature. The L0 was calculated by the equation based on the single parabolic band (SPB) model reported by Kim et al.30 The calculated κL values of the FePt-nanoparticle-embedded Sb2Te3 thin film were lower than those of Sb2Te3 thin films in the entire measurement ranges, demonstrating that 4.0 nm sized FePt nanoparticles act as phonon scattering sites to reduce κL. Furthermore, the impurities created by diffusion of a small amount of FePt into Sb or Te sites upon annealing might contribute to the reduction of κL. Although the current thin films exhibit nearly isotropic crystallographic orientation close to the reference, as manifested by ultralow Lotgering’s factors of 0.08−0.09, the material with a minimal Lotgering’s factor can have strong anisotropy in electrical and thermal properties. Since the thermoelectric properties of the current thin films were characterized along the in-plane direction for electrical properties and the through-plane direction for thermal properties, the calculated ZT values could be somewhat exaggerated. Accordingly, here, the ZT values were calculated and referred to only for the comparison with the reported values in Sb2Te3-based thin films, where the electrical and thermal properties were characterized along the perpendicular directions. Owing to its fine electrical properties and low thermal conductance, the ZT values of the Sb2Te3 and FePtnanoparticle-embedded Sb2Te3 thin films exhibited 0.23 and 0.30 at room temperature and the peak ZT values of 0.64 and 0.72 were obtained at 423 K, respectively (Figure S16). The enhanced ZT values by the FePt-nanoparticle-embedded Sb2Te3 thin film should be attributed to lower thermal conductivity than that of the pristine Sb2Te3, demonstrating the effect of nanostructuring by FePt nanoparticles. It is noteworthy that these maxima are much higher than the reported values in the fully solution-processed Bi2Te3- or Sb2Te3-based thin films or vacuum-filtered layers, which estimate ZT values from the electrical and thermal properties measured in different directions (Table S3). These results validate the feasibility of the molecularly designed Sb2Te3 precursor solution for fabricating high-performance nanostructured TE thin films. In summary, we developed a method for synthesizing a versatile molecular Sb 2Te 3 precursor ink solution for fabricating TE thin films with high efficiency. Such a high performance was achieved on the basis of the fundamental chemical principles for the synthesis of inorganic molecular 4587

DOI: 10.1021/acsaem.9b00685 ACS Appl. Energy Mater. 2019, 2, 4582−4589

Letter

ACS Applied Energy Materials

(2) DiSalvo, F. J. Thermoelectric cooling and power generation. Science 1999, 285, 703−706. (3) Sumithra, S.; Takas, N. J.; Misra, D. K.; Nolting, W. M.; Poudeu, P. F. P.; Stokes, K. L. Enhancement in thermoelectric figure of merit in nanostructured Bi2Te3 with semimetal nanoinclusions. Adv. Energy Mater. 2011, 1, 1141−1147. (4) Ko, D. K.; Kang, Y.; Murray, C. B. Enhanced thermopower via carrier energy filtering in solution-processable Pt-Sb2Te3 nanocomposites. Nano Lett. 2011, 11, 2841−2844. (5) Zhang, Y.; Snedaker, M. L.; Birkel, C. S.; Mubeen, S.; Ji, X.; Shi, Y.; Liu, D.; Liu, X.; Moskovits, M.; Stucky, G. D. Silver-based intermetallic heterostructures in Sb2Te3 thick films with enhanced thermoelectric power factors. Nano Lett. 2012, 12, 1075−1080. (6) Lee, M.; Chen, C. Y.; Wang, S.; Cha, S. N.; Park, Y. J.; Kim, J. M.; Chou, L. J.; Wang, Z. L. A hybrid piezoelectric structure for wearable nanogenerators. Adv. Mater. 2012, 24, 1759−1764. (7) Bahk, J.-H.; Fang, H.; Yazawa, K.; Shakouri, A. Flexible thermoelectric materials and device optimization for wearable energy harvesting. J. Mater. Chem. C 2015, 3, 10362−10374. (8) Kim, C. S.; Yang, H. M.; Lee, J.; Lee, G. S.; Choi, H.; Kim, Y. J.; Lim, S. H.; Cho, S. H.; Cho, B. J. Self-powered wearable electrocardiography using a wearable thermoelectric power generator. ACS Energy Lett. 2018, 3, 501−507. (9) 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. (10) Kim, Y.; DiVenere, A.; Wong, G. K. L.; Ketterson, J. B.; Cho, S.; Meyer, J. R. Structural and thermoelectric transport properties of Sb2Te3 thin films grown by molecular beam epitaxy. J. Appl. Phys. 2002, 91, 715−718. (11) Panthani, M. G.; Korgel, B. A. Nanocrystals for electronics. Annu. Rev. Chem. Biomol. Eng. 2012, 3, 287−311. (12) Heo, S. H.; Jo, S.; Kim, H. S.; Choi, G.; Song, J. Y.; Kang, J.-Y.; Park, N.-J.; Ban, H. W.; Kim, F.; Jeong, H.; et al. Composition change-driven texturing and doping in solution-processed SnSe thermoelectric thin films. Nat. Commun. 2019, 10, 864. (13) Kovalenko, M. V.; Spokoyny, B.; Lee, J. S.; Scheele, M.; Weber, A.; Perera, S.; Landry, D.; Talapin, D. V. Semiconductor nanocrystals functionalized with antimony telluride zintl ions for nanostructured thermoelectrics. J. Am. Chem. Soc. 2010, 132, 6686−6695. (14) Mitzi, D. B. Solution processing of chalcogenide semiconductors via dimensional reduction. Adv. Mater. 2009, 21, 3141− 3158. (15) Buckley, J. J.; Greaney, M. J.; Brutchey, R. L. Ligand exchange of colloidal CdSe nanocrystals with stibanates derived from Sb2S3 dissolved in a thiol-amine mixture. Chem. Mater. 2014, 26, 6311− 6317. (16) Snyder, G. J.; Toberer, E. S. Complex thermoelectric materials. Nat. Mater. 2008, 7, 105−114. (17) Kanatzidis, M. G.; Huang, S.-P. Coordination chemistry of heavy polychalcogenide ligands. Coord. Chem. Rev. 1994, 130, 509− 621. (18) Lotgering, F. K. Topotactical reactions with ferrimagnetic oxides having hexagonal crystal structuresI. J. Inorg. Nucl. Chem. 1959, 9, 113−123. (19) Cheng, H.-Y.; Jong, C. A.; Chung, R.-J.; Chin, T.-S.; Huang, R.T. Wet etching of Ge2Sb2Te5 films and switching properties of resultant phase change memory cells. Semicond. Sci. Technol. 2005, 20, 1111−1115. (20) Warren, C. J.; Dhingra, S. S.; Ho, D. M.; Haushalter, R. C.; Bocarsly, A. B. Electrochemical synthesis of new Sb-Te Zintl anions by cathodic dissolution of Sb2Te3 electrodes: Structures of Sb2Te54‑ and Sb4Te94‑. Inorg. Chem. 1994, 33, 2709−2710. (21) Nag, A.; Zhang, H.; Janke, E.; Talapin, D. V. Inorganic surface ligands for colloidal nanomaterials. Z. Phys. Chem. 2015, 229, 85−107. (22) Nakaya, M.; Kanehara, M.; Teranishi, T. One-pot synthesis of large FePt nanoparticles from metal salts and their thermal stability. Langmuir 2006, 22, 3485−3487.

precursors of telluride materials by introducing a strong reducing environment. The synthesized molecular Sb2Te3 precursor fulfilled all expected functions as an ink solution for the fabrication of thin films, demonstrated by the structural uniformity, stoichiometric composition, and high TE performance of the thin films. Furthermore, the versatility of this Sb2Te3 precursor solution was validated by embedding nanoparticles in the thin film to realize the phonon scattering, and the mechanically durable flexible thin films on polymer substrates without the loss of TE properties. The current synthetic route contributes to advances in the chemical synthesis of telluride-based inorganic precursors for solution processing of compound semiconductors, which shed light on the fabrication of high-performance TE thin films for flexible and portable electronics.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.9b00685. Experimental details, ZT comparison table, absorption spectra, SEM images, SEM-EDS analysis, TGA, FT-IR, XRD, DSC, XRR, carrier concentration and mobility, and XPS (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ji Eun Lee: 0000-0002-5812-6680 Seungwoo Song: 0000-0002-8524-3444 Jung-Woo Yoo: 0000-0001-7038-4001 Jae Sung Son: 0000-0003-3498-9761 Author Contributions

S.J. and J.S.S. designed the experiments, analyzed the data, and wrote the paper. S.J., S.H.H., S.Cho, and J.S.K. carried out the experiments. S.J., H.W.B., H.J., F.K., S.Choo, D.H.G., S.B. and S.S. carried out the characterization of the materials. S.J., S.H.P., H.S., I.O., J.S.K., J.E.L., J.-W.Y, B.-S.K., and J.Y.S. performed a measurement of TE properties. All authors discussed the results and commented on the manuscript at all stages. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was supported by the R&D Convergence Program of National Research Council of Science and Technology (NST) of the Republic of Korea, the Nano· Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF2018M3A7B8060697) of the Republic of Korea, and LowDimensional Materials Genome Development by the Korea Research Institute of Standards and Science (KRISS-201717011082).

■

REFERENCES

(1) Bell, L. E. Cooling, heating, generating power, and recovering waste heat with thermoelectric systems. Science 2008, 321, 1457− 1461. 4588

DOI: 10.1021/acsaem.9b00685 ACS Appl. Energy Mater. 2019, 2, 4582−4589

Letter

ACS Applied Energy Materials (23) Jeong, H.; Yoon, S.; Kim, J. H.; Kwak, D.-H.; Gu, D. H.; Heo, S. H.; Kim, H.; Park, S.; Ban, H. W.; Park, J.; et al. Transition metalbased thiometallates as surface ligands for functionalization of allinorganic nanocrystals. Chem. Mater. 2017, 29, 10510−10517. (24) Lin, Z.; Chen, Y.; Yin, A.; He, Q.; Huang, X.; Xu, Y.; Liu, Y.; Zhong, X.; Huang, Y.; Duan, X. Solution processable colloidal nanoplates as building blocks for high-performance electronic thin films on flexible substrates. Nano Lett. 2014, 14, 6547−6553. (25) Dun, C.; Hewitt, C. A.; Li, Qi.; Xu, J.; Schall, D. C.; Lee, H.; Jiang, Q.; Carroll, D. L. 2D chalcogenide nanoplate assemblies for thermoelectric applications. Adv. Mater. 2017, 29, 1700700. (26) Zhang, Y.; Bahk, J.-H.; Lee, J.; Birkel, C. S.; Snedaker, M. L.; Liu, D.; Zeng, H.; Moskovits, M.; Shakouri, A.; Stucky, G. D. Hot carrier filtering in solution processed heterostructures: A paradigm for improving thermoelectric efficiency. Adv. Mater. 2014, 26, 2755− 2761. (27) Shakouri, A. Recent developments in semiconductor thermoelectric physics and materials. Annu. Rev. Mater. Res. 2011, 41, 399− 431. (28) Zhang, C.; Peng, Z.; Li, Z.; Yu, L.; Khor, K. A.; Xiong, Q. Controlled growth of bismuth antimony telluride BixSb2−xTe3 nanoplatelets and their bulk thermoelectric nanocomposites. Nano Energy 2015, 15, 688−696. (29) Park, N.-W.; Lee, W.-Y.; Kim, J.-A.; Song, K.; Lim, H.; Kim, W.D.; Yoon, S.-G.; Lee, S.-K. Reduced temperature-dependent thermal conductivity of magnetite thin films by controlling film thickness. Nanoscale Res. Lett. 2014, 9, 96. (30) Kim, H.-S.; Gibbs, Z. M.; Tang, Y.; Wang, H.; Snyder, G. J. Characterization of Lorenz number with Seebeck coefficient measurement. APL Mater. 2015, 3, 041506.

4589

DOI: 10.1021/acsaem.9b00685 ACS Appl. Energy Mater. 2019, 2, 4582−4589