This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Communication Cite This: J. Am. Chem. Soc. 2019, 141, 8025−8029 pubs.acs.org/JACS Ligand-Mediated Band Engineering in Bottom-Up Assembled SnTe Nanocomposites for Thermoelectric Energy Conversion Maria Ibań ̃ez,*,#,†,‡ Roger Hasler,†,‡ Aziz Genç,∥,§ Yu Liu,#,⊥ Beatrice Kuster,†,‡ Maximilian Schuster,† Oleksandr Dobrozhan,⊥ Doris Cadavid,⊥ Jordi Arbiol,§,¶ Andreu Cabot,⊥,¶ and Maksym V. Kovalenko*,†,‡ # Institute of Science and Technology Austria, Am Campus 1, 3400 Klosterneuburg, Austria † Institute of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1, Zürich CH-8093, Switzerland ‡ Empa-Swiss Federal Laboratories for Materials Science and Technology, , Ü berlandstrasse 129, Dübendorf CH-8600, Switzerland ∥ Department of Metallurgy and Materials Engineering, Faculty of Engineering, Bartin University, 74100 Bartin, Turkey § Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Catalonia, Spain ⊥ Catalonia Energy Research InstituteIREC, Sant Adrià del Besòs, 08930 Barcelona, Spain ¶ ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain * S Supporting Information are nanocomposites produced via spinodal decomposition ABSTRACT: The bottom-up assembly of colloidal during solid-state synthesis, resulting in a crystalline matrix nanocrystals is a versatile methodology to produce containing nanoinclusions of a second phase.3 Alternatively, the composite nanomaterials with precisely tuned electronic bottom-up assembly of colloidally synthesized, solution- properties. Beyond the synthetic control over crystal processed nanocrystals (NCs) has recently emerged as a domain size, shape, crystal phase, and composition, convenient methodology to produce high-ZT materials.4,5 solution-processed nanocrystals allow exquisite surface Using this strategy, materials such as Cu3Sb1−x−ySnxBiySe4,6 engineering. This provides additional means to modulate (Ag2Te)5(Sb2Te3)5,7 AgBi0.5Sb0.5Se2,8 PbTe-Bi0.7Sb1.3Te3,9 the nanomaterial characteristics and particularly its Bi2Te2.7Se0.3,10 and Bi0.5Sb1.5Te311 have reached ZT above 1. electronic transport properties. For instance, inorganic Despite these promising results, further progress requires precise surface ligands can be used to tune the type and surface engineering of the NCs and methods to control the NC concentration of majority carriers or to modify the assembly and consolidation into bulk nanocrystalline materials. electronic band structure. Herein, we report the thermo- Most colloidal syntheses deliver NCs with organic molecules electric properties of SnTe nanocomposites obtained from (ligands) coordinated at their surfaces.12 These molecules are the consolidation of surface-engineered SnTe nanocrystals highly insulating and thus need to be removed prior to NCs into macroscopic pellets. A CdSe-based ligand is selected consolidation. Usually, such organic ligands are thermally to (i) converge the light and heavy bands through partial decomposed, chemically displaced, or exchanged with inorganic Cd alloying and (ii) generate CdSe nanoinclusions as a ligands to produce fully inorganic nanomaterials. However, secondary phase within the SnTe matrix, thereby reducing beyond avoiding detrimental effects of bulky organic ligands, the thermal conductivity. These SnTe-CdSe nanocompo- surface modification can be envisioned as a powerful tool to sites possess thermoelectric figures of merit of up to 1.3 at further adjust the final nanocomposite composition and 850 K, which is, to the best of our knowledge, the highest therefore its functional properties. Recent work has demon- thermoelectric figure of merit reported for solution- strated the possibility to tune the type and concentration of processed SnTe. majority carriers with aliovalent electronic impurities, incorpo- rated through surface ligands.13,14 In addition, ligands have been proposed as molecular solders to facilitate charge transport T he search for thermoelectric materials, able to efficiently convert heat into electricity (and vice versa), has been a longstanding challenge. A material’s thermoelectric performance between the grain boundaries.15,16 Here, we demonstrate that ligands can be also used to modify the electronic band structure of the final nanocomposite, while is usually evaluated through a dimensionless figure of merit, ZT, simultaneously acting as precursors for the formation of defined as ZT = σS2Tκ−1 where T is the absolute temperature, σ secondary phases during NCs consolidation. the electrical conductivity, S the Seebeck coefficient or In particular, we focus on the development of SnTe-based thermopower, and κ the thermal conductivity.1 All these nanocomposites from SnTe NCs (Figure 1). While very few parameters are governed by the electronic structure of the material and its charge and phonon scattering properties.2 Received: February 5, 2019 Several of the current best-performing thermoelectric materials Published: April 19, 2019 © 2019 American Chemical Society 8025 DOI: 10.1021/jacs.9b01394 J. Am. Chem. Soc. 2019, 141, 8025−8029 Journal of the American Chemical Society Communication Figure 1. TEM images of the as-prepared SnTe NCs. reports have centered on the synthesis and assembly of SnTe NCs,17−19 the highest ZT value (ca. 0.5 at 800 K) obtained from bottom-up assembled SnTe was reached using rather large (ca. 170 nm) particles consolidated via spark plasma sintering. Such nanomaterials showed lower lattice thermal conductivities, 0.52 W m−1 K−1, than bulk SnTe, 1.6 W m−1 K−1.20 However, they featured lower electrical conductivities (500 S cm−1 for NC based solid18 compared to 1000 S cm−1 for bulk SnTe20) and lower Seebeck coefficients (90 μV K−1 versus 130 μV K−1). Figure 2. (a) Electrical conductivity, σ; (b) Seebeck coefficient, S; (c) Consequently, the power factor (PF = σS2) was approximately power factor, PF; (d) schematic of the electronic band structure; (e) 4 times smaller than that of bare SnTe solids. In order to thermal conductivity, κ; and (f) figure of merit, ZT, for SnTe overcome such limitations, we explored the use of SnTe NCs as nanomaterial prepared with SCN (black) and CdSe (blue) surface- building blocks to produce SnTe-based nanomaterials. modified SnTe NPs. SnTe NCs were prepared by reacting tin bromide with trioctylphosphine telluride in the presence of oleylamine and offset (0.3−0.4 eV)24,25 between the light valence band (L) and trioctylamine (experimental details can be found in the the heavy valence band (Σ) that prevents the contribution of the Supporting Information (SI)). The obtained NCs were purified heavier holes to the Seebeck coefficient.26 Additionally, the off- inside an argon-filled glovebox to prevent surface oxidation. stoichiometric nature of SnTe with a large number of Sn lattice Figure 1 shows representative transmission electron microscopy vacancies yields very high intrinsic doping levels (p = 1021 (TEM) images of the obtained SnTe NCs, exhibiting cubic cm−3).27 These characteristics are far from the optimum for morphology and an average side length of 14 ± 1 nm. thermoelectric applications, what translates into low Seebeck In order to evaluate thermoelectric properties of SnTe-based coefficients and high thermal conductivities, and makes SnTe a nanomaterials obtained from the assembly of these NCs, we first poor thermoelectric material.26 Overall, the success toward displaced native organic ligands from the NC surface. For this producing SnTe with high ZT should rely on the carrier purpose, we mixed an ammonium thiocyanate (NH4SCN) concentration optimization, a proper band structure engineering, solution in anhydrous methanol with SnTe NCs suspended in and the introduction of all-scale (atomic, nano, and meso) anhydrous tetrahydrofuran (see SI). NH4SCN efficiently hierarchical scattering centers.28 replaced organic ligands21,22 and allowed the production of Previous reports show that the hole carrier concentration fully inorganic SnTe nanosolids (further referred to as SnTe- could be reduced by introducing donors such as I,29 Bi,30 or Sb,30 SCN) through hot pressing of the organic-free NCs. Details of as well as by Sn self-compensation (Sn1+xTe).31,32 However, the the NC consolidation can be found in the SI. Figure 2 displays the band structure could be modified to introduce resonant states in thermoelectric properties of SnTe-SCN nanomaterial. Hall the valence band near the Fermi level via In doping20 or to charge carrier concentrations of 2 × 1020 cm−3 and electrical promote band convergence and open the band gap via conductivities up to 720 S cm−1 were measured at room Cd,20,31,33−35 Hg,36 Mg,37 Ca,38 and Mn39,40 alloying. Both temperature. However, low Seebeck coefficients (ca. 110 μV strategies allowed for higher Seebeck coefficients. Further K−1) and high thermal conductivities (ca. 2.25 W m−1 K−1) strategies to reduce the lattice thermal conductivity are based provided maximum ZT values of just 0.35 at 850 K. on the introduction of nanoscale secondary phases such as SnTe has a high symmetry cubic crystal structure and a SrTe,30 AgBiTe2,41 AgInTe2,42 In2Te3,43 GaTe,44 CdS,31 and complex valence band structure with a small band gap (0.18 eV at ZnS31 within the SnTe matrix via solid state precipitation. room temperature), which favors bipolar charge transport at Inspired by the enhancement of the Seebeck coefficient relatively low temperatures.23 It is also characterized by a large achieved in SnTe through band convergence and widening of the 8026 DOI: 10.1021/jacs.9b01394 J. Am. Chem. Soc. 2019, 141, 8025−8029 Journal of the American Chemical Society Communication Figure 3. (a) General TEM micrograph of the SnTe@CdSe core−shell like nanoparticles, revealing the homogeneity in size and shape. (b) Atomic resolution HAADF STEM micrograph of a core−shell nanoparticle and its corresponding power spectrum. (c) ADF STEM micrograph of several SnTe@CdSe NPs and the corresponding STEM−EELS elemental composition maps: Sn (red), Te (green), Cd (blue), and Se (yellow). band gap via alloying with transition metals, and specifically with structure modification of SnTe with the incorporation of Cd Cd,20,31,33,34 we designed a ligand exchange strategy using (Figure 2d).31 Substituting Sn atoms with isovalent Cd soluble CdSe-compounds as inorganic ligands. The CdSe-based significantly increased the band gap and diminished the energy ligand was prepared by dissolving stoichiometric amounts of separation between the light-hole band and the heavy-hole band elemental Cd and Se in a thiol-amine mixture45 at room (Figure 2d).31 The larger band gap and the reduced carrier temperature and in inert-atmosphere (SI). This molecular concentration (9 × 1019 cm−3) of SnTe-CdSe also reduced solution (referred to as CdSe ink) allowed the recovery of pure bipolar contributions. Additionally, the convergence of the hexagonal CdSe upon mild annealing at 350 °C (Figure S1). We valence bands provided a higher hole density at the Fermi level. hypothesize that the ligand could be a variety of chalcogenido- Both phenomena translated into much higher Seebeck cadmates such as (Cd2Se3)n2n− or CdSe22− as previously reported coefficients in the whole temperature range, and especially at for hydrazinum based CdSe solutions.46,47 In order to replace the high temperatures.20,31 Thus, much higher power factors of ca. organic ligands on the NC surface by the CdSe-based ligand, a 1.3 mW m−1 K−2 were reached. Finally, thermal conductivities of phase transfer reaction was used: a solution of SnTe NCs in 0.85 W m−1 K−1 were systematically acquired at 850 K, which is hexane was layered on top of a solution of the CdSe ink in N- approximately 2.7 times lower than for SnTe-SCN, and methylformamide. The change of colloidal stabilization from corresponded to a lattice thermal conductivities of 0.48 W m−1 steric to electrostatic was evidenced by the color change of the K−1 (SI), close to the theoretical amorphous limit (0.5 W m−1 solutions, reaching a colorless hexane phase and colored polar K−1).36 phase. The top hexane phase was then discarded and the SnTe To understand the observed low lattice thermal conductivities, NCs were precipitated by centrifugation, rinsed twice with the mesoscale structure of SnTe-CdSe nanocomposites was acetone, and dried under vacuum for future use. investigated (Figures 4 and S7−S9). HRTEM analysis showed Figure 3a shows TEM images of CdSe-functionalized SnTe the presence of CdSe precipitates, both with cubic or hexagonal NCs (SnTe@CdSe). Atomic resolution high-angle annular dark- phases, and with sizes between 3 and 10 nm, within the SnTe field (HAADF) scanning TEM (STEM), demonstrated that the matrix. In some cases, both crystal domains, SnTe and CdSe, NCs were composed of a ∼1 nm thick amorphous shell around a were found to be in perfect epitaxy due to their low lattice perfectly crystalline ∼14 nm core (Figure 3b and SI). The power mismatch of 1.8% (Figure S8). Overall, the alloy formation, the spectrum (FFT) obtained from these nanoparticles revealed presence of nanoscale CdSe secondary phases, and the mesoscale their core to have a crystal structure matching an fcc SnTe phase SnTe domains provided scattering mechanisms to block visualized along its [001] zone axis. To unravel the composition propagation of phonons on all relevant length scales, thereby of the amorphous shell, STEM electron energy loss spectroscopy reducing the lattice thermal conductivity to the amorphous limit. (EELS) analyses were conducted. EELS compositional maps In conclusion, a scalable high-yield colloidal synthetic route to indicated the presence of amorphous CdSe shells at the surface of prepare SnTe NCs with narrow size distribution was presented. SnTe cores. More examples can be found in Figures S5 and S6. In order to optimize the thermoelectric performance of bottom- Subsequently, SnTe@CdSe nanoparticles were used as up SnTe-based nanosolids, a new NC ligand-exchange process building blocks to produce SnTe-CdSe nanocomposites under using CdSe-based compounds was presented. Upon thermal the same conditions as SnTe-SCN nanomaterials. A 6.4% consolidation, the surface-deposited CdSe units partially alloyed content of Cd was estimated considering the ligand surface with SnTe to modify its band structure and partially crystallized coverage and verified by ICP (Table S1). Figure 2 displays the in CdSe nanodomains within the SnTe matrix. Alloying resulted thermoelectric properties of these materials. Compared to SnTe- in an increase of the Seebeck coefficient due to the influence of SCN nanomaterials, the electrical conductivity of SnTe-CdSe Cd in the electronic band structure of SnTe. Besides, the nanocomposite was reduced from 720 to 380 S cm−1, while the mesoscale structuration allowed reducing the thermal con- Seebeck coefficient increased from 110 up to 185 μV K−1 at 850 ductivity to the amorphous limit. Overall, SnTe-CdSe nano- K. These results are in agreement with the predicted band composites with ZT of ca. 1.3 at 850 K were produced. 8027 DOI: 10.1021/jacs.9b01394 J. Am. Chem. Soc. 2019, 141, 8025−8029 Journal of the American Chemical Society Communication research and innovation programme under the Marie Sklodow- ska-Curie grant agreement No. 754411. IREC acknowledges funding from Generalitat de Catalunya (2014SGR1638). J.A. acknowledge funding from Generalitat de Catalunya 2017 SGR 327 and the Spanish MINECO coordinated projects between IREC and ICN2 ENE2017-85087-C3. ICN2 acknowledges support from the Severo Ochoa Program (MINECO, Grant SEV-2017-0706) and is funded by the CERCA Programme/ Generalitat de Catalunya. ■ REFERENCES (1) Snyder, G. J.; Toberer, E. S. Complex Thermoelectric Materials. Nat. Mater. 2008, 7 (2), 105−114. (2) Pei, Y.; Wang, H.; Snyder, G. J. Band Engineering of Thermo- electric Materials. Adv. Mater. 2012, 24 (46), 6125−6135. (3) Tan, G.; Zhao, L.-D.; Kanatzidis, M. G. Rationally Designing High- Performance Bulk Thermoelectric Materials. Chem. Rev. 2016, 116 (19), 12123−12149. (4) Ortega, S.; Ibáñez, M.; Liu, Y.; Zhang, Y.; Kovalenko, M. V.; Cadavid, D.; Cabot, A. Bottom-Up Engineering of Thermoelectric Nanomaterials and Devices from Solution-Processed Nanoparticle Building Blocks. Chem. Soc. Rev. 2017, 46, 3510−3528. Figure 4. (a) Schematic of the bottom-up process. (b) HRTEM (5) Ibáñez, M.; Luo, Z.; Genc, A.; Piveteau, L.; Ortega, S.; Cadavid, D.; micrograph obtained from the cross-section of the SnTe-CdSe pellet Dobrozhan, O.; Liu, Y.; Nachtegaal, M.; Zebarjadi, M.; Arbiol, J.; sample with the corresponding power spectrum (FFT) from the marked Kovalenko, M. V.; Cabot, A. High-Performance Thermoelectric yellow squared area. In the bottom line, phase filtered images correspond Nanocomposites from Nanocrystal Building Blocks. Nat. Commun. to a CdSe precipitate (red); a SnTe crystal domain is visualized along the 2016, 7, 10766. [001] zone axis (green) and another SnTe crystal domain is also (6) Liu, Y.; Garcia, G.; Ortega, S.; Cadavid, D.; Palacios, P.; Lu, J.; visualized along the [001] zone axis (blue). The inset below corresponds Ibáñez, M.; Xi, L.; De Roo, J.; Lopez, A. M.; Marti-Sanchez, S.; Cabezas, to a phase-filtered RGB composition map showing the different I.; Mata, M. d. l.; Luo, Z.; Dun, C.; Dobrozhan, O.; Carroll, D. L.; Zhang, superposed phases found. W.; Martins, J.; Kovalenko, M. V.; Arbiol, J.; Noriega, G.; Song, J.; ■ Wahnon, P.; Cabot, A. Solution-Based Synthesis and Processing of Sn- and Bi-doped Cu3SbSe4 Nanocrystals, Nanomaterials and Ring-Shaped ASSOCIATED CONTENT Thermoelectric Generators. J. Mater. Chem. A 2017, 5 (6), 2592−2602. * S Supporting Information (7) Xu, J.; Li, H.; Du, B.; Tang, X.; Zhang, Q.; Uher, C. High The Supporting Information is available free of charge on the Thermoelectric Figure of Merit and Nanostructuring in Bulk AgSbTe2. J. Mater. Chem. 2010, 20 (29), 6138−6143. ACS Publications website at DOI: 10.1021/jacs.9b01394. (8) Xiao, C.; Xu, J.; Cao, B.; Li, K.; Kong, M.; Xie, Y. Solid-Solutioned Details of the chemicals used, NCs synthesis; ligand Homojunction Nanoplates with Disordered Lattice: A Promising exchange reactions; pellet fabrication, materials character- Approach toward “Phonon Glass Electron Crystal” Thermoelectric ization (X-ray diffraction patterns, HRTEM, electrical and Materials. J. Am. Chem. Soc. 2012, 134 (18), 7971−7977. thermal characterization); stability measurement and (9) Xu, B.; Agne, M. T.; Feng, T.; Chasapis, T. C.; Ruan, X.; Zhou, Y.; additional notes (PDF) Zheng, H.; Bahk, J.-H.; Kanatzidis, M. G.; Snyder, G. J.; Wu, Y. ■ Nanocomposites from Solution-Synthesized PbTe-BiSbTe Nano- heterostructure with Unity Figure of Merit at Low-Medium Temper- AUTHOR INFORMATION atures (500−600 K). Adv. Mater. 2017, 29 (10), 1605140. Corresponding Authors (10) Liu, Y.; Zhang, Y.; Lim, K. H.; Ibáñez, M.; Ortega, S.; Li, M.; *mibanez@ist.ac.at David, J.; Martí-Sánchez, S.; Ng, K. M.; Arbiol, J.; Kovalenko, M. V.; *mvkovalenko@ethz.ch Cadavid, D.; Cabot, A. High Thermoelectric Performance in Crystallo- graphically Textured n-Type Bi2Te3−xSex Produced from Asymmetric ORCID Colloidal Nanocrystals. ACS Nano 2018, 12 (7), 7174−7184. Maria Ibáñez: 0000-0001-5013-2843 (11) Liu, Y.; Zhang, Y.; Ortega, S.; Ibáñez, M.; Lim, K. H.; Grau- Yu Liu: 0000-0001-7313-6740 Carbonell, A.; Martí-Sánchez, S.; Ng, K. M.; Arbiol, J.; Kovalenko, M. V.; Oleksandr Dobrozhan: 0000-0001-9238-7596 Cadavid, D.; Cabot, A. Crystallographically Textured Nanomaterials Jordi Arbiol: 0000-0002-0695-1726 Produced from the Liquid Phase Sintering of BixSb2−xTe3 Nanocrystal Building Blocks. Nano Lett. 2018, 18 (4), 2557−2563. Andreu Cabot: 0000-0002-7533-3251 (12) De Roo, J.; De Keukeleere, K.; Hens, Z.; Van Driessche, I. From Maksym V. Kovalenko: 0000-0002-6396-8938 Ligands to Binding Motifs and Beyond; the Enhanced Versatility of Notes Nanocrystal Surfaces. Dalton Trans 2016, 45 (34), 13277−13283. The authors declare no competing financial interest. (13) Ibáñez, M.; Hasler, R.; Liu, Y.; Dobrozhan, O.; Nazarenko, O.; ■ ACKNOWLEDGMENTS This work was financially supported by the European Union Cadavid, D.; Cabot, A.; Kovalenko, M. V. Tuning p-Type Transport in Bottom-Up-Engineered Nanocrystalline Pb Chalcogenides Using Alkali Metal Chalcogenides as Capping Ligands. Chem. Mater. 2017, 29 (17), 7093−7097. (EU) via FP7 ERC Starting Grant 2012 (Project NANOSOLID, (14) Ibáñez, M.; Korkosz, R. J.; Luo, Z.; Riba, P.; Cadavid, D.; Ortega, GA No. 306733). M.I. was supported by IST Austria, and by S.; Cabot, A.; Kanatzidis, M. G. Electron Doping in Bottom-Up ETH Zurich via ETH career seed grant (SEED-18 16-2). Y.L. Engineered Thermoelectric Nanomaterials through HCl-Mediated acknowledges funding from the European Union’s Horizon 2020 Ligand Displacement. J. Am. Chem. Soc. 2015, 137 (12), 4046−4049. 8028 DOI: 10.1021/jacs.9b01394 J. Am. Chem. Soc. 2019, 141, 8025−8029 Journal of the American Chemical Society Communication (15) Zhang, H.; Son, J. S.; Dolzhnikov, D. S.; Filatov, A. S.; Hazarika, (33) He, J.; Xu, J.; Liu, G.-Q.; Shao, H.; Tan, X.; Liu, Z.; Xu, J.; Jiang, H.; A.; Wang, Y.; Hudson, M. H.; Sun, C.-J.; Chattopadhyay, S.; Talapin, D. Jiang, J. Enhanced thermopower in rock-salt SnTe-CdTe from band V. Soluble Lead and Bismuth Chalcogenidometallates: Versatile Solders convergence. RSC Adv. 2016, 6 (38), 32189−32192. for Thermoelectric Materials. Chem. Mater. 2017, 29 (15), 6396−6404. (34) Tan, X. J.; Shao, H. Z.; He, J.; Liu, G. Q.; Xu, J. T.; Jiang, J.; Jiang, (16) Son, J. S.; Zhang, H.; Jang, J.; Poudel, B.; Waring, A.; Nally, L.; H. C. Band engineering and improved thermoelectric performance in M- Talapin, D. V. All-Inorganic Nanocrystals as a Glue for BiSbTe Grains: doped SnTe (M = Mg, Mn, Cd, and Hg). Phys. Chem. Chem. Phys. 2016, Design of Interfaces in Mesostructured Thermoelectric Materials. 18 (10), 7141−7147. Angew. Chem. 2014, 126 (29), 7596−7600. (35) Dong, X.; Yu, H.; Li, W.; Pei, Y.; Chen, Y. First-Principles Study on (17) Zhang, X.; Zhou, Y.; Pei, Y.; Chen, Y.; Yuan, B.; Zhang, S.; Deng, Band Structures and Electrical Transports of Doped-SnTe. J. Y.; Gong, S.; He, J.; Zhao, L.-D. Enhancing thermoelectric performance Materiomics 2016, 2 (2), 158−164. of SnTe via nanostructuring particle size. J. Alloys Compd. 2017, 709, (36) Tan, G.; Shi, F.; Doak, J. W.; Sun, H.; Zhao, L.-D.; Wang, P.; Uher, 575−580. C.; Wolverton, C.; Dravid, V. P.; Kanatzidis, M. G. Extraordinary role of (18) Li, Z.; Chen, Y.; Li, J.-F.; Chen, H.; Wang, L.; Zheng, S.; Lu, G. Hg in enhancing the thermoelectric performance of p-type SnTe. Energy Systhesizing SnTe nanocrystals leading to thermoelectric performance Environ. Sci. 2015, 8 (1), 267−277. enhancement via an ultra-fast microwave hydrothermal method. Nano (37) Banik, A.; Shenoy, U. S.; Anand, S.; Waghmare, U. V.; Biswas, K. Energy 2016, 28, 78−86. Mg Alloying in SnTe Facilitates Valence Band Convergence and (19) Han, G.; Zhang, R.; Popuri, S.; Greer, H.; Reece, M.; Bos, J.-W.; Optimizes Thermoelectric Properties. Chem. Mater. 2015, 27 (2), 581− Zhou, W.; Knox, A.; Gregory, D. Large-Scale Surfactant-Free Synthesis 587. of p-Type SnTe Nanoparticles for Thermoelectric Applications. (38) Al Rahal Al Orabi, R.; Mecholsky, N. A.; Hwang, J.; Kim, W.; Materials 2017, 10 (3), 233. Rhyee, J.-S.; Wee, D.; Fornari, M. Band Degeneracy, Low Thermal (20) Tan, G.; Shi, F.; Hao, S.; Chi, H.; Zhao, L.-D.; Uher, C.; Conductivity, and High Thermoelectric Figure of Merit in SnTe−CaTe Wolverton, C.; Dravid, V. P.; Kanatzidis, M. G. Codoping in SnTe: Alloys. Chem. Mater. 2016, 28 (1), 376−384. Enhancement of Thermoelectric Performance through Synergy of (39) Roychowdhury, S.; Shenoy, U. S.; Waghmare, U. V.; Biswas, K. An Resonance Levels and Band Convergence. J. Am. Chem. Soc. 2015, 137 enhanced Seebeck coefficient and high thermoelectric performance in p- (15), 5100−5112. type In and Mg co-doped Sn1-xPbxTe via the co-adjuvant effect of the (21) Fafarman, A. T.; Koh, W.-k.; Diroll, B. T.; Kim, D. K.; Ko, D.-K.; resonance level and heavy hole valence band. J. Mater. Chem. C 2017, 5, Oh, S. J.; Ye, X.; Doan-Nguyen, V.; Crump, M. R.; Reifsnyder, D. C.; 5737. Murray, C. B.; Kagan, C. R. Thiocyanate-Capped Nanocrystal Colloids: (40) Wu, H.; Chang, C.; Feng, D.; Xiao, Y.; Zhang, X.; Pei, Y.; Zheng, Vibrational Reporter of Surface Chemistry and Solution-Based Route to L.; Wu, D.; Gong, S.; Chen, Y.; He, J.; Kanatzidis, M. G.; Zhao, L.-D. Enhanced Coupling in Nanocrystal Solids. J. Am. Chem. Soc. 2011, 133 Synergistically optimized electrical and thermal transport properties of (39), 15753−15761. SnTe via alloying high-solubility MnTe. Energy Environ. Sci. 2015, 8 (22) Koh, W.-k.; Saudari, S. R.; Fafarman, A. T.; Kagan, C. R.; Murray, (11), 3298−3312. C. B. Thiocyanate-Capped PbS Nanocubes: Ambipolar Transport (41) Tan, G.; Shi, F.; Sun, H.; Zhao, L.-D.; Uher, C.; Dravid, V. P.; Enables Quantum Dot Based Circuits on a Flexible Substrate. Nano Lett. Kanatzidis, M. G. SnTe-AgBiTe2 as an efficient thermoelectric material 2011, 11 (11), 4764−4767. with low thermal conductivity. J. Mater. Chem. A 2014, 2 (48), 20849− (23) Joffé, A. F. Heat Transfer in Semiconductors. Can. J. Phys. 1956, 20854. 34 (12A), 1342−1355. (42) Banik, A.; Shenoy, U. S.; Saha, S.; Waghmare, U. V.; Biswas, K. (24) Kafalas, J. A.; Brebrick, R. F.; Strauss, A. J. Evidence that SnTe is a High Power Factor and Enhanced Thermoelectric Performance of Semiconductor. Appl. Phys. Lett. 1964, 4 (5), 93−94. SnTe-AgInTe2: Synergistic Effect of Resonance Level and Valence (25) Littlewood, P. B.; Mihaila, B.; Schulze, R. K.; Safarik, D. J.; Band Convergence. J. Am. Chem. Soc. 2016, 138 (39), 13068−13075. Gubernatis, J. E.; Bostwick, A.; Rotenberg, E.; Opeil, C. P.; Durakiewicz, (43) Tan, G.; Zeier, W. G.; Shi, F.; Wang, P.; Snyder, G. J.; Dravid, V. P.; T.; Smith, J. L.; Lashley, J. C. Band Structure of SnTe Studied by Kanatzidis, M. G. High Thermoelectric Performance SnTe−In2Te3 Photoemission Spectroscopy. Phys. Rev. Lett. 2010, 105 (8), 086404. Solid Solutions Enabled by Resonant Levels and Strong Vacancy (26) Brebrick, R. F.; Strauss, A. J. Anomalous Thermoelectric Power as Phonon Scattering. Chem. Mater. 2015, 27 (22), 7801−7811. Evidence for Two-Valence Bands in SnTe. Phys. Rev. 1963, 131 (1), (44) Al Rahal Al Orabi, R.; Hwang, J.; Lin, C.-C.; Gautier, R.; Fontaine, 104−110. B.; Kim, W.; Rhyee, J.-S.; Wee, D.; Fornari, M. Ultralow Lattice Thermal (27) Brebrick, R. F. Deviations from stoichiometry and electrical Conductivity and Enhanced Thermoelectric Performance in SnTe:Ga properties in SnTe. J. Phys. Chem. Solids 1963, 24 (1), 27−36. Materials. Chem. Mater. 2017, 29 (2), 612−620. (28) Zhou, M.; Snyder, G. J.; Li, L.; Zhao, L.-D. Lead-free tin (45) McCarthy, C. L.; Brutchey, R. L. Solution Processing of chalcogenide thermoelectric materials. Inorg. Chem. Front. 2016, 3 (11), Chalcogenide Materials Using Thiol-Amine ″Alkahest″ Solvent 1449−1463. Systems. Chem. Commun. 2017, 53 (36), 4888−4902. (29) Zhou, M.; Gibbs, Z. M.; Wang, H.; Han, Y.; Xin, C.; Li, L.; Snyder, (46) Dolzhnikov, D. S.; Zhang, H.; Jang, J.; Son, J. S.; Panthani, M. G.; G. J. Optimization of thermoelectric efficiency in SnTe: the case for the Shibata, T.; Chattopadhyay, S.; Talapin, D. V. Composition-matched molecular “solders” for semiconductors. Science 2015, 347 (6220), 425. light band. Phys. Chem. Chem. Phys. 2014, 16 (38), 20741−20748. (47) Hudson, M. H.; Dolzhnikov, D. S.; Filatov, A. S.; Janke, E. M.; (30) Zhao, L.-D.; Zhang, X.; Wu, H.; Tan, G.; Pei, Y.; Xiao, Y.; Chang, Jang, J.; Lee, B.; Sun, C.; Talapin, D. V. New Forms of CdSe: Molecular C.; Wu, D.; Chi, H.; Zheng, L.; Gong, S.; Uher, C.; He, J.; Kanatzidis, M. Wires, Gels, and Ordered Mesoporous Assemblies. J. Am. Chem. Soc. G. Enhanced Thermoelectric Properties in the Counter-Doped SnTe 2017, 139 (9), 3368−3377. System with Strained Endotaxial SrTe. J. Am. Chem. Soc. 2016, 138 (7), 2366−2373. (31) Tan, G.; Zhao, L.-D.; Shi, F.; Doak, J. W.; Lo, S.-H.; Sun, H.; Wolverton, C.; Dravid, V. P.; Uher, C.; Kanatzidis, M. G. High Thermoelectric Performance of p-Type SnTe via a Synergistic Band Engineering and Nanostructuring Approach. J. Am. Chem. Soc. 2014, 136 (19), 7006−7017. (32) Zhang, X.; Wang, D.; Wu, H.; Yin, M.; Pei, Y.; Gong, S.; Huang, L.; Pennycook, S. J.; He, J.; Zhao, L.-D. Simultaneously Enhancing the Power Factor and Reducing the Thermal Conductivity of SnTe via Introducing its Analogues. Energy Environ. Sci. 2017, 10 (11), 2420− 2431. 8029 DOI: 10.1021/jacs.9b01394 J. Am. Chem. Soc. 2019, 141, 8025−8029