Realizing high thermoelectric performance below phase transition


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Functional Inorganic Materials and Devices

Realizing high thermoelectric performance below phase transition temperature in polycrystalline SnSe via lattice anharmonicity strengthening and strain engineering Guodong Tang, Jiang Liu, Jian Zhang, Di Li, Rui Xu, Wenqi Lu, Jizi Liu, Yongsheng Zhang, and Zhenzhen Feng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10056 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 8, 2018

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Realizing high thermoelectric performance below phase transition temperature in polycrystalline SnSe via lattice anharmonicity strengthening and strain engineering Guodong Tang,1,∗∗ Jiang Liu,1 Jian Zhang,2 Di Li,2 Rui Xu,1 Wenqi Lu,1 Jizi Liu,3,∗∗ Yongsheng Zhang,2,4, ∗ Zhenzhen Feng,2,4

Abstract We report the high thermoelectric performance of p-type polycrystalline SnSe obtained below phase transition temperature by harnessing Pb doping and introducing Sn vacancies. The enhanced carrier concentration induced by Pb doping and introducing Sn vacancies contributes to enhancements of electrical conductivity and power factor of polycrystalline SnSe. We demonstrate that the lattice anharmonicity is strengthened by Pb substitution and Sn vacancies through forming weaker bonds. We find that Pb substitution introduce huge stress field in the interior of SnSe grains. The thermal conductivity can be greatly reduced by lattice anharmonicity strengthening and applying huge stress field. κL is reduced to as low as 0.18 Wm-1K-1 in Sn0.92Pb0.03Se sample at 773 K. As a result, a remarkable high ZT of ∼1.4 was achieved at 773 K in Sn0.93Pb0.02Se sample through lattice anharmonicity strengthening and strain engineering. KEYWORDS: SnSe, thermal conductivity, thermoelectric properties, anharmonicity, strain engineering

1

MIIT Key Laboratory of Advanced Metallic and Intermetallic Materials Technology, School of Materials Science and Engineering,

Nanjing University of Science and Technology, Nanjing 210094, China. 2Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China. 3Herbert Gleiter Institute of Nanoscience, School of Materials Science and Engineering, Nanjing University of Science and Technology, Jiangsu 210094, China.4Science Island Branch of Graduate School, University of Science and Technology of China, Hefei 230026, China. E-mail: [email protected] (G.T.); [email protected] (J.L.); [email protected] (Y.Z.).

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1. Introduction Thermoelectric materials attract great interest due to their potential applications in power generation and electronic cooling. The performance of thermoelectric materials is described by the dimensionless figure of merit ZT=S2σT/ κ, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, κ is the total thermal conductivity consisting of the lattice thermal conductivity (κL) and the carrier thermal conductivity (κe).1 Advanced thermoelectric material should possess large Seebeck coefficient, high electrical conductivity and low thermal conductivity. It is a challenging to develop effective approaches to decouple these parameters and improve the overall ZT due to complex interdependence of S, σ and κ.2 The power factor can be improved by band structure engineering (resonant levels and band convergence)3-5 and energy filtering effect.6 The lattice thermal conductivity can be reduced through nanostructuring1, 7-11 and all-scale hierarchical architectures.12-14 Binary compound SnSe consisting of abundant-earth and low-toxicity elements had been discovered to exhibit excellent thermoelectric properties in undoped15 and doped crystals,16-18 which inspired the thermoelectric community for a while. SnSe adopts a highly anisotropic layered orthorhombic (Pnma space group) crystal structure at room temperature. As the temperature increases to ~ 793 K, lower symmetry phase (Pnma space group) SnSe undergoes a structural transition into a higher symmetry Cmcm space group.15,

19

Anharmonic bonding in the layered

structure are responsible for its low thermal conductivity.20,

21

However, poor

mechanical properties and time consuming of synthesis of single crystals limit their 2

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practical application.22-25 Thus, polycrystalline SnSe materials attract great attention due to facile processing and machinability.26 Chemical doping of Ag,22 alkali-ions,27 In,28 Cu,29 Zn,30 S,31 I,25 Br,32 and BiCl333 were used to tune the carrier concentration and enhance ZT. Recently, Na, Pb co-doping approach achieved the maximum ZT of ~1.2 at 773K via tuning phase transition temperature in polycrystalline SnSe.23 We recently demonstrated that high performance can be achieved in Pb doped polycrystalline SnSe through optimizing power factor and depressing thermal conductivity via phase-separations and hierarchical architecturing.24 It suggests that Pb doping is an effective approach to enhance thermoelectric properties of polycrystalline SnSe.23,

24

We recently found that Sn vacancies can lead to an

enhancement of electronic density of states in the Fermi level, which improves the electrical conductivity and thermoelectric power factor.34 Meanwhile, Sn vacancies introduce strong phonon scattering, leading to low thermal conductivity. It suggests that introducing Sn vacancies is a promising solution to enhance thermoelectric performance.34 This motivates us to further optimize electrical and thermal transport properties of polycrystalline SnSe by simultaneous harnessing Pb doping and introducing Sn vacancies. Here, we fabricated Sn-vacancies containing Pb doped SnSe samples by hydrothermal synthesis method. It is found that Pb doping and Sn vacancies simultaneously leads to significantly enhanced power factor thanks to enhanced carrier concentration. More importantly, the lattice thermal conductivity is reduced to as low as 0.18 Wm-1K-1 at 773 K due to the lattice anharmonicity strengthening and 3

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the presence of huge strain field. As a result, a high ZT of 1.4 was achieved in Sn0.93Pb0.02Se below the phase transition temperature by enhancing electrical transport properties while reducing lattice thermal conductivity. This value is a record high ZT reported so far for SnSe polycrystals below the phase transition temperature ~ 793 K. 15,19

In view of practical application, polycrystalline SnSe materials with high ZT

values below the phase transition temperature are greatly desired.

2 Experimental 2.1. Sample fabrication SnCl2·2H2O ( 99% ), PbCl2 ( 99.999% ), Se powder ( 99.9% ) were used as raw materials to prepare polycrystalline samples of Sn0.95-xPbxSe by hydrothermal method. SnCl2·2H2O and PbCl2 powders were put into beaker and dissolved deionized water. The mixed solution was magnetic stirring for 10 min. Then put NaOH into the mixture followed by stirring for 10 min. Above solution was transferred into a stainless Teflon lined autoclave. After adding Se powders, the autoclave was heated at 403 K for 36 h. After cooling to room temperature, we used deionized water and absolute ethanol to wash the synthesized powders for several times. Finally, the black powders were dried under vacuum at 333 K for 4 h. Spark plasma sintering (HPD 10, FCT System GmbH) was used to consolidate the powders into the dense bulk samples at 693 K for 7 min under a 50 MPa auniaxial pressure. The heating and cooling rates of SPS sintering were 150 K min-1 and 70 K min-1.

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2.2. Characterization A Bruker D8 Advance instrument with Cu Kα radiation was used to obtain the XRD diffraction patterns. Chemical composition of the sample was determined by electron probe micro analysis (EPMA). Atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and transmission electron microscopy (TEM) investigations were conducted on a FEI Titan G2 60−300 image-corrected transmission electron microscope with operating voltage 300 kV. The samples were prepared by focused ion beam (FIB) milling using a in situ liftout technique on a FEI Nova Nanolab DualBeam instrument. The thickness of the TEM lamellae is about 50 nm.

2.3. Thermoelectric property measurements Both the electrical and thermal transport properties of all samples were measured parallel to the SPS pressure direction. An Ulvac-Riko ZEM-3 instrument system was used to measure the electrical conductivity (σ) and Seebeck coefficient (S) simultaneously in a helium atmosphere at 300-773K. The thermal diffusivity coefficient (D) of the samples were measured on a laser flash apparatus (Netzsch LFA-457 instrument). The differential scanning calorimetry (DSC) was used to measure the heat capacity (Cp), and sample with the dimensions of 6 x 6 x 1.5mm was prepared to meet the test requirements. The density ρ was measured by the Archimedes drainage method. Thermal conductivity was calculated by the relation k = DCpρ. Temperature-dependent Hall coefficient was performed by the van der Pauw 5

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technique using a home-built system under a reversible magnetic field of 1.5 T. The measurement uncertainty for the electrical conductivity and Seebeck coefficient is about 5%. The measurement uncertainty for the total thermal conductivity is 12%, which comprises uncertainties of 5% for the thermal diffusivity, 5% for the specific heat and 2% for the density. The combined uncertainty of all measurements for the experimental determination of ZT is approximately 20%.

3. Results and discussion XRD diffraction patterns of Sn0.95-xPbxSe and pure SnSe are shown in Fig. 1a. All samples show the single-phase Pnma orthorhombic structure without impurity peaks. Compared with pristine SnSe, (011), (111) and (400) diffraction peaks of Sn0.95-xPbxSe shift from a low angle to a high angle (Fig. 1b), indicating that lattice parameters decrease by the introduction of Sn vacancies. As the amount of vacancies is constant (5%), the diffraction peaks shift toward smaller angles with the increasing Pb content, which is consistent with the fact that Pb2+ ion has a larger ionic radius than Sn2+ ion in these samples (Fig. 1b).23 The refined lattice parameters are given in Table S1. EPMA composition detection was adopted to further validate the vacancy feature of our samples. EPMA results reveal the presence of a high density of Sn vacancies in these samples (Table S2). Bulk polycrystalline SnSe sample shows anisotropy in their transport properties due to the anisotropic crystal structure of SnSe20, 35 and the uniaxial pressure of SPS treatments.34 We measured transport properties of Sn0.99Pb0.01Se perpendicular and 6

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parallel to the pressing direction (Fig. S1). It indicates that our samples show anisotropy in thermoelectric data. The anisotropic thermoelectric data of pure SnSe can be found elsewhere.36 All results suggest that ZT along the pressing direction is higher than that perpendicular to the pressing direction. Therefore, in this study, the thermoelectric properties of polycrystalline Sn0.95-xPbxSe are investigated along the pressing direction. Fig. 2a shows the temperature dependence of electrical conductivity (σ) for polycrystalline Sn0.95-xPbxSe and pure SnSe. σ of all samples exhibits the similar temperature-dependence trend. Above 673 K, σ increases with the increasing temperature, which is ascribed to the thermal excitation of minority carrier.24, 37 It is found that σ of Sn0.95-xPbxSe samples exhibit a significantly enhanced σ compared to that of pure SnSe and phase separated polycrystalline Sn0.99Pb0.01Se.24 Among the series samples, σ of Sn0.93Pb0.02Se is evidently larger than that of other samples in the whole temperature range. A high σ value of 41.8 S·cm-1 is obtained at 773 K in this sample, which is about two times larger than that of pristine SnSe. Hall carrier concentration (n) was measured at the range from 300 K to 650 K, as shown in Fig. 2b. As compared to pure SnSe, n of Sn0.95-xPbxSe increases among the whole investigated temperature range. At room temperature, n is 6.64 x 1018 cm-3 for Sn0.93Pb0.02Se and increases to 6.98 x1018 cm-3 at 650 K. Sn vacancies can produce extra holes to enhance carrier concentration, which leads to higher σ. Fig. 2c shows the variation trend of the Seebeck coefficient (S) with the increasing temperature for polycrystalline Sn0.95-xPbxSe and pure SnSe. Data of 7

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phase separated polycrystalline Sn0.99Pb0.01Se 24 is shown for comparison. S increases from 300 K to ~623 K, then gradually decreases with further increasing temperature. This type of variation was also found in previous reports, which is a hint of bipolar conduction onset.24,

38-40

All polycrystalline Sn0.95-xPbxSe show lower S than pure

SnSe due to the enhanced n confirmed by Hall measurements. As compared with phase separated polycrystalline Sn0.99Pb0.01Se,24 Sn0.95-xPbxSe exhibit enhanced S. The introduction of Sn vacancies can increase the electronic density of states in the Fermi energy level and effective mass,34 contributing to enhanced S of Sn0.95-xPbxSe. Fig. 2d shows the temperature dependence of power factor (PF) for polycrystalline Sn0.95-xPbxSe and pure SnSe. The similar trend was observed for all samples. PF of Sn0.95-xPbxSe is enhanced as compared with pristine SnSe and Sn0.99Pb0.01Se.24 Among this series, Sn0.93Pb0.02Se exhibit the largest PF in the whole temperature range. A high PF of about 4.32 µWcm-1K-2 was obtained at 773 K in polycrystalline Sn0.93Pb0.02Se, being 70% higher than that of pure SnSe prepared by the same process. Fig. 3a shows the temperature dependence of total thermal conductivity (κT) for polycrystalline Sn0.95-xPbxSe and pure SnSe. κT of reported Sn vacancies containing polycrystalline Sn0.95Se is shown for comparison. The total thermal conductivity of all samples decreases with the increasing temperature, and the lowest value of 0.219 Wm-1K-1 is achieved in Sn0.92Pb0.03Se sample at 773 K. Low κT is achieved in all Sn0.95-xPbxSe samples, which is significantly lower than those of pure SnSe and other reported Ag-doped,22 Zn-doped30 and alkali-ion doped SnSe materials.27 It is worthy 8

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to note that κT of Sn0.95-xPbxSe is smaller than that of Sn vacancies containing polycrystalline Sn0.95Se.34 The lattice thermal conductivity (κL) is obtained by using the formula κL= κT –κe, in which the carrier thermal conductivity is estimated by the Wiedemann–Franz law (κe = LTσ). The Lorenz number was calculated by derivation from Fermi energy with a simple assumption of a single band model and acoustic phonon scattering, as shown in supplementary Fig. S2.41 The discrepancy between the

κT and κL is not remarkable, indicating that phonon transport dominates in κT (Fig. 3b). Compared with pure SnSe and Sn vacancies containing polycrystalline Sn0.95Se,34 κL of Sn0.95-xPbxSe reduces significantly. κL is achieved as low as 0.18 Wm-1K-1 in Sn0.92Pb0.03Se sample at 773 K. Microstructure analysis of Sn0.95-xPbxSe using atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and transmission electron microscopy (TEM) was performed to comprehend the reduced

κL. The results obtained for typical Sn0.93Pb0.02Se sample are presented. Fig. 4 shows the overview of the microstructures in Sn0.93Pb0.02Se sample. Fig. 4a shows the layered grains with different contrast. The typical grains were enlarged in Fig. 4b and c. There is a prevalent “dark” phase in between SnSe grains as shown in Fig. 4b. The elemental map (Fig. 5) indicates that there is no Sn element in this “dark” phase, and it is Se crystal. The atomic number of Se is the smallest in this SnSe(Pb) system, so Se crystal has the weakest contrast (dark intensity). Existence of Se-crystals would lead to a significant decrease of Sn vacancies.34 As a result, the Hall carrier concentration is lower than expected due to Sn vacancies. In addition, another typical 9

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feature is the bright-dark wave-like structures in the interior of SnSe grains (Fig. 4b and c). Composition analysis (Fig. 5) shows the bright wave-like structures contain more Pb atoms, and the dark wave-like structures are poor of Pb element, which is consistent with the image contrast because the atomic number of Pb is larger than that of Sn and Se. Although Pb atoms were detected in SnSe system here, no change occurs for the structure of SnSe (Fig. 6b) and also no Pb precipitate is observed in the present work. Obviously, most of Pb atoms were solid solution in the SnSe compounds. The other defect is vacancy clusters, which are popular in this system and located in the interior of grains (Fig. 4e) or at the phase boundaries (Fig. 4f). In HAADF-STEM mode, they have darkest intensity, even weaker than Se crystals. According to the HAADF-STEM observations and the experimentally suggested Sn-vacancy and Pb concentrations, we set up corresponding simulation models to investigate the origins of the low thermal conductivity in the defected SnSe system. From the theoretically calculated elastic parameters (the bulk modules B and shear modules G in Table 1), we find that the point defects (vacancies or atomic substitutions) lower the bulk modules and shear modules, which indicates that the defect systems have weaker mechanic properties and possible stronger lattice anharmonicity. A smaller cell volume would lead to a higher bulk module when the geometry structure of a material keeps intact during the volume change. In our investigated system, SnSe with Pb doping and Sn vacancies, the defects “destroy” the SnSe original structure. The different bonding environments around the defects play an important role in the strength of bonds and the bulk modulus. Since the bonds 10

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around the defects are weaker than the original Sn-Se bond, the modulus of the defected material should be softer than that of the pristine SnSe. Consequently, the calculated sound velocities and Debye temperatures of Sn0.95Se (with a Sn divacancy) and Sn0.925Pb0.025Se (one Sn vacancy plus one Pb substituting Sn) are lower than those of the pristine SnSe compound. The Grüneisen parameter represents the lattice anharmonicity or the lattice thermal conductivity. The calculated Grüneisen parameter of Sn0.95Se (2.2) is close to that of SnSe (2.3), indicating that the two compounds exhibit the similar lattice anharmonicity. Thus, only considering the Umklapp (U) and normal (N) phonon scatterings, we find that the calculated lattice thermal conductivities of SnSe and Sn0.95Se are similar (e.g. at 300 K in Table 1). However, when taking the point-defect (PD) phonon scatterings into account, the lattice thermal conductivity of Sn0.95Se is decreased from 1.43 to 0.98 Wm-1K-1 at 300 K (Table 1). This means that defects play an important role in reducing the lattice thermal conductivity. For the Pb substituted compound (Sn0.925Pb0.025Se), the introduced Pb atoms can raise the Grüneisen parameter to 2.5. The large Grüneisen parameter originates from both the Sn vacancy and the weak Pb-Se bonds, (interatomic interactions becoming weaker as the metal cation goes down in the periodic table along a given column). Thus, the lattice thermal conductivity of Sn0.925Pb0.025Se considering the Umklapp and normal phonon scatterings is 0.98 Wm-1K-1 at 300 K (Table 1), lower than those of SnSe and Sn0.95Se. Moreover, including the point-defect effects (both the Sn vacancy and Pb substitution) leads to a lower thermal conductivity of 0.68 W m-1K-1 at 300 K. Inserting the DFT calculated Grüneisen 11

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parameters, phonon velocities and Debye temperatures (Table 1) in the Debye-Callaway model (Equation (1). of the Computational Method Section), we can calculate temperature-dependent lattice thermal conductivities (considering the three phonon scatterings) of SnSe, Sn0.95Se and Sn0.925Pb0.025Se (Fig. 7). We find that the defect compounds (Sn0.95Se and Sn0.925Pb0.025Se) have the lower thermal conductivity than that of the pristine SnSe compound, and the dual-defect compound (Sn0.925Pb0.025Se) has the lowest lattice thermal conductivity. The trend from the theoretical calculations is in good agreement with the experimental measurements. Our results imply that different defect types (Pb substitution and Sn vacancies) could not only strengthen the lattice anharmonicity through forming weaker bonds but also enhance the phonon scattering by abruptly changing local bonding environment, and finally lead the lattice thermal conductivity to reduce significantly. We also found that Pb atoms introduce huge stress in the interior of SnSe grains, which suggests significantly different bonding environment (Pb-Se) than that of Sn-Se. The strain contrast is easily presented in the bright-field and dark-filed TEM images (Fig. 6a and c). Huge stress field is identified by TEM experiments. The strain field fluctuation resulting in increasing phonon scattering when substituting Pb on the Sn sublattices. The thermal conductivity of Pb doped samples can be greatly tuned by the presence of huge strain field.42, 43 The boundaries between the bright wave and dark wave (the boxed area in Fig. 4c) were enlarged in Fig. 6d, the lattice distortion is observed in the bright wave and there is a small misorientation between the bright and dark waves. Besides the huge stress field, antiphase boundary (APB)44 defects are 12

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commonly observed in this system, as show in Fig. 4d. Such planar defects were usually reported in layered structures and believed to be as scattering centers to lower the thermal conductivity. Huge strain field and defects can contribute to the effective scattering of phonon, further suppress the lattice thermal conductivity. This leads to a low lattice thermal conductivity in Sn0.95-xPbxSe samples. Fig. 8 shows the temperature dependence of ZT for polycrystalline Sn0.95-xPbxSe and pure SnSe. Benefiting from the enhanced power factor and low thermal conductivity by lattice anharmonicity strengthening, the highest ZT of ∼1.4 is achieved in Sn0.93Pb0.02Se below phase transition temperature (773 K). Polycrystalline Sn0.95-xPbxSe show enhanced ZT as compared with recently reported high performance (Na, Pb) co-doped SnSe,23

phase-separated Sn0.99Pb0.01Se.24 We achieved a high ZT

value so far in bulk SnSe polycrystals22, 23, 25, 27-33, 37, 44-46 below the phase transition temperature ~793 K15,19 by Pb doping and introducing Sn vacancies. Good repeatability for this high ZT value was achieved, as evidenced by the measurements on several independently prepared samples (Fig. S4).

4. Conclusions Here we achieve high thermoelectric performance of p-type polycrystalline SnSe obtained below phase transition temperature by harnessing Pb doping and introducing Sn vacancies. It is found that electrical conductivity and power factor is enhanced due to enhanced carrier concentration induced by Pb doping and Sn vacancies. The different defect types (Pb substitution and Sn vacancies) could not only strengthen the 13

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lattice anharmonicity through forming weaker bonds but also enhance the phonon scattering by abruptly changing local bonding environment. Furthermore, Pb substitution leads to huge stress field in the interior of SnSe grains. The stain field greatly tune the thermal conductivity by increasing phonon scattering. Lattice anharmonicity strengthening and strain engineering lead to extremely low lattice thermal conductivity. κL is reduced to as low as 0.18 Wm-1K-1 in Sn0.92Pb0.03Se sample at 773 K. These two favorable factors lead to achieve remarkably high thermoelectric performance ZT ∼ 1.4 below phase transition temperature (at 773 K). These finding provide a new perspective to realize high thermoelectric performance in polycrystalline SnSe below phase transition temperature for practical application.

Conflicts of interest There are no conflicts to declare.

ASSOCIATED CONTENT Supporting Information. Computational method; Lattice parameters; EPMA results; Anisotropic

thermoelectric

properties;

Lorenz

number;

Crystal

structures;

Reproducibility of thermoelectric properties; The heating and cooling measurements for typical high performance Sn0.93Pb0.02Se sample.

Acknowledgments

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The authors wish to thank the National Natural Science Foundation of China ((No. U1732153, 11474283 and 11774347) for funding, would like to acknowledge Natural Science Foundation of Jiangsu Province (No. BK20161495) for partial support, wish like to thank support from the Fundamental Research Funds for the Central Universities (No. 30917011206). The authors also wish to acknowledge characterization facilities (TEM and HAADF-STEM experiments) provided by Materials Characterization Facility of Nanjing University of Science and Technology. The authors would like to thank Prof. Y. Pei from Tongji University for high temperature Hall measurements.

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References (1) Yang, L.; Chen, Z.-G.; Dargusch, M.S.; Zou, J. High Performance Thermoelectric Materials: Progress and Their Applications. Adva. Energy Mater. 2018, 8, 1701797. (2) Yang, L.; Chen, Z.-G.; Han, G.; Hong, M.; Zou, J. Impacts of Cu Deficiency on the Thermoelectric Properties of Cu2 − XSe Nanoplates. Acta Mater. 2016, 113, 140-146. (3) Hong, M.; Chen, Z.-G.; Pei, Y.; Yang, L.; Zou, J. Limit of zT Enhancement in Rocksalt Structured Chalcogenides by Band Convergence. Phys. Rev. B 2016, 94, 161201. (4) Moshwan, R.; Yang, L.; Zou, J.; Chen, Z.-G. Eco-Friendly SnTe Thermoelectric Materials: Progress and Future Challenges. Adv. Funct. Mater. 2017, 27, 1703278. (5) Pei, Y.; Wang, H.; Snyder, G. Band Engineering of Thermoelectric Materials. Adv. Mater. 2012, 24, 6125-6135. (6) Zide, J.; Vashaee, D.; Bian, Z.; Zeng, G.; Bowers, J.; Shakouri, A.; Gossard, A. Demonstration of Electron Filtering to Increase the Seebeck Coefficient in In0.53Ga0.47As⁄ In0.53Ga0.28Al0.19As Superlattices. Phys. Rev. B 2006, 74, 205335. (7) Hsu, K.F.; Loo, S.; Guo, F.; Chen, W.; Dyck, J.S.; Uher, C.; Hogan, T.; Polychroniadis, E.; Kanatzidis, M.G. Cubic AgPbmSbTe2+m: Bulk Thermoelectric Materials with High Figure of Merit. Science 2004, 303, 818-821. (8) Wu, H.; Carrete, J.; Zhang, Z.; Qu, Y.; Shen, X.; Wang, Z.; Zhao, L.-D.; He, J. Strong Enhancement of Phonon Scattering through Nanoscale Grains in Lead Sulfide 16

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Thermoelectrics. NPG Asia Mater. 2014, 6, e108. (9) Xie, W.; Weidenkaff, A.; Tang, X.; Zhang, Q.; Poon, J.; Tritt, T.M. Recent Advances in Nanostructured Thermoelectric Half-Heusler Compounds. Nanomater. 2012, 2, 379-412. (10) Zhou, M.; Li, J.-F.; Kita, T. Nanostructured AgPbmSbTem+2 System Bulk Materials with Enhanced Thermoelectric Performance. J. Am. Chem. Soc. 2008, 130, 4527-4532. (11) Minnich, A.; Dresselhaus, M.; Ren, Z.; Chen, G. Bulk Nanostructured Thermoelectric Materials: Current Research and Future Prospects. Energy Environ. Sci. 2009, 2, 466-479. (12) Biswas, K.; He, J.; Chu, S.; Majumdar, A.; Janssen, R.A.; Mitzi, D.B.; Sargent, E.H.; Logan, B.E.; Elimelech, M.; Debe, M.K. Materials for Energy 2012. Nature 2012, 489, 414-418. (13) Lee, Y.; Lo, S.-H.; Androulakis, J.; Wu, C.-I.; Zhao, L.-D.; Chung, D.-Y.; Hogan, T.P.;

Dravid,

V.P.;

Kanatzidis,

M.G.

High-performance

Tellurium-free

Thermoelectrics: All-scale Hierarchical Structuring of p-Type PbSe–MSe Systems (M= Ca, Sr, Ba). J. Am. Chem. Soc. 2013, 135, 5152-5160. (14) Hu, L.; Wu, H.; Zhu, T.; Fu, C.; He, J.; Ying, P.; Zhao, X. Tuning Multiscale Microstructures

to

Enhance

Thermoelectric

Performance

of

n-Type

Bismuth-Telluride-Based Solid Solutions. Adv. Energy Mater. 2015, 5, 1500411. (15) Zhao, L.D.; Lo, S.H.; Zhang, Y.; Sun, H.; Tan, G.; Uher, C.; Wolverton, C.; Dravid, V.P.; Kanatzidis, M.G. Ultralow Thermal Conductivity and High 17

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Thermoelectric Figure of Merit in SnSe Crystals. Nature 2014, 508, 373-377. (16) Zhao, L.D.; Tan, G.; Hao, S.; He, J.; Pei, Y.; Chi, H.; Wang, H.; Gong, S.; Xu, H.; Dravid, V.P.; Uher, C.; Snyder, G.J.; Wolverton, C.; Kanatzidis, M.G. Ultrahigh Power Factor and Thermoelectric Performance in Hole-doped Single-crystal SnSe. Science 2016, 351, 141-144. (17) Peng, K.; Lu, X.; Zhan, H.; Hui, S.; Tang, X.; Wang, G.; Dai, J.; Uher, C.; Wang, G.; Zhou, X. Broad Temperature Plateau for High ZTs in Heavily Doped p-Type SnSe Single Crystals. Energy Environ. Sci. 2016, 9, 454-460. (18) Duong, A.T.; Nguyen, V.Q.; Duvjir, G.; Duong, V.T.; Kwon, S.; Song, J.Y.; Lee, J.K.; Lee, J.E.; Park, S.; Min, T.; Lee, J.; Kim, J.; Cho, S. Achieving ZT=2.2 with Bi-doped n-Type SnSe Single Crystals. Nat. Commun. 2016, 7, 13713. (19) Chattopadhyay, T.; Pannetier, J.; Vonschnering, H.G. Neutron Diffraction Study of the Structural Phase Transition in SnS and SnSe. J. Phys. Chem. Solids 1986, 47, 879-885. (20) Zhao, L.-D.; Chang, C.; Tan, G.; Kanatzidis, M.G. SnSe: A Remarkable New Thermoelectric Material. Energy Environ. Sci. 2016, 9, 3044-3060. (21) Heremans, J.P. Thermoelectric Materials: the Anharmonicity Blacksmith. Nat. Phys. 2015, 11, 990-991. (22) Chen, C.-L.; Wang, H.; Chen, Y.-Y.; Day, T.; Snyder, G.J. Thermoelectric Properties of p-Type Polycrystalline SnSe Doped with Ag. J. Mater. Chem. A 2014, 2, 11171-11176. (23) Lee, Y.K.; Ahn, K.; Cha, J.; Zhou, C.; Kim, H.S.; Choi, G.; Chae, S.I.; Park, 18

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J.H.; Cho, S.P.; Park, S.H.; Sung, Y.E.; Lee, W.B.; Hyeon, T.; Chung, I. Enhancing p-Type Thermoelectric Performances of Polycrystalline SnSe via Tuning Phase Transition Temperature. J. Am. Chem. Soc. 2017, 139, 10887-10896. (24) Tang, G.; Wei, W.; Zhang, J.; Li, Y.; Wang, X.; Xu, G.; Chang, C.; Wang, Z.; Du, Y.; Zhao, L.D. Realizing High Figure of Merit in Phase-Separated Polycrystalline Sn1-xPbxSe. J. Am. Chem. Soc. 2016, 138, 13647-13654. (25) Zhang, Q.; Chere, E.K.; Sun, J.; Cao, F.; Dahal, K.; Chen, S.; Chen, G.; Ren, Z. Studies on Thermoelectric Properties of n-Type Polycrystalline SnSe1-xSxby Iodine Doping. Adv. Energy Mater. 2015, 5, 1500360. (26) Sassi, S.; Candolfi, C.; Vaney, J.B.; Ohorodniichuk, V.; Masschelein, P.; Dauscher, A.; Lenoir, B. Assessment of the Thermoelectric Performance of Polycrystalline p-Type SnSe. Appl. Phys. Lett. 2014, 104, 212105. (27) Wei, T.R.; Tan, G.; Zhang, X.; Wu, C.F.; Li, J.F.; Dravid, V.P.; Snyder, G.J.; Kanatzidis, M.G. Distinct Impact of Alkali-Ion Doping on Electrical Transport Properties of Thermoelectric p-Type Polycrystalline SnSe. J. Am. Chem. Soc. 2016, 138, 8875-8882. (28) Kim, J.H.; Oh, S.; Kim, Y.M.; So, H.S.; Lee, H.; Rhyee, J.-S.; Park, S.-D.; Kim, S.-J. Indium Substitution Effect on Thermoelectric and Optical Properties of Sn1− xInxSe

Compounds. J. Alloys Compd. 2016, 682, 785-790.

(29) Singh, N.K.; Bathula, S.; Gahtori, B.; Tyagi, K.; Haranath, D.; Dhar, A. The Effect of Doping on Thermoelectric Performance of p-Type SnSe: Promising Thermoelectric Material. J. Alloys Compd. 2016, 668, 152-158. 19

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(30) Li, J.C.; Li, D.; Qin, X.Y.; Zhang, J. Enhanced Thermoelectric Performance of p-Type SnSe Doped with Zn. Scripta Mater. 2017, 126, 6-10. (31) Han, Y.-M.; Zhao, J.; Zhou, M.; Jiang, X.-X.; Leng, H.-Q.; Li, L.-F. Thermoelectric Performance of SnS and SnS–SnSe Solid Solution. J. Mater. Chem. A 2015, 3, 4555-4559. (32) Chang, C.; Tan, Q.; Pei, Y.; Xiao, Y.; Zhang, X.; Chen, Y.-X.; Zheng, L.; Gong, S.; Li, J.-F.; He, J.; Zhao, L.-D. Raising Thermoelectric Performance of n-Type SnSe via Br Doping and Pb Alloying. RSC Adv. 2016, 6, 98216-98220. (33) Wang, X.; Xu, J.; Liu, G.; Fu, Y.; Liu, Z.; Tan, X.; Shao, H.; Jiang, H.; Tan, T.; Jiang, J. Optimization of Thermoelectric Properties in n-Type SnSe Doped with BiCl3. Appl. Phys. Lett. 2016, 108, 083902. (34) Wei, W.; Chang, C.; Yang, T.; Liu, J.; Tang, H.; Zhang, J.; Li, Y.; Xu, F.; Zhang, Z.; Li, J.F.; Tang, G. Achieving High Thermoelectric Figure of Merit in Polycrystalline SnSe via Introducing Sn Vacancies. J. Am. Chem. Soc. 2018, 140, 499-505. (35) Yang, J.; Zhang, G.; Yang, G.; Wang, C.; Wang, Y.X. Outstanding Thermoelectric Performances for Both p- and n-Type SnSe from First-principles Study. J. Alloys Compd. 2015, 644, 615-620. (36) Tang, G.; Wen, Q.; Yang, T.; Cao, Y.; Wei, W.; Wang, Z.; Zhang, Z.; Li, Y. Rock-salt-type Nanoprecipitates Lead to High Thermoelectric Performance in Undoped Polycrystalline SnSe. RSC Adv. 2017, 7, 8258-8263. (37) Chen, Y.-X.; Ge, Z.-H.; Yin, M.; Feng, D.; Huang, X.-Q.; Zhao, W.; He, J. 20

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Understanding of the Extremely Low Thermal Conductivity in High-Performance Polycrystalline SnSe through Potassium Doping. Adv. Funct. Mater. 2016, 26, 6836-6845. (38) Chere, E.K.; Zhang, Q.; Dahal, K.; Cao, F.; Mao, J.; Ren, Z. Studies on Thermoelectric Figure of Merit of Na-doped p-Type Polycrystalline SnSe. J. Mater. Chem. A 2016, 4, 1848-1854. (39) Wei, T.R.; Wu, C.F.; Zhang, X.; Tan, Q.; Sun, L.; Pan, Y.; Li, J.F. Thermoelectric Transport Properties of Pristine and Na-doped SnSe1-xTex Polycrystals. Phys. Chem. Chem. Phys. 2015, 17, 30102-30109. (40) Leng, H.-Q.; Zhou, M.; Zhao, J.; Han, Y.-M.; Li, L.-F. The Thermoelectric Performance of Anisotropic SnSe Doped with Na. RSC Adv. 2016, 6, 9112-9116. (41) 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. (42) Li, X.; Maute, K.; Dunn, M.L.; Yang, R. Strain Effects on the Thermal Conductivity of Nanostructures. Phys. Rev. B 2010, 81, 245318. (43) Fu, C.; Xie, H.; Zhu, T.J.; Xie, J.; Zhao, X.B. Enhanced Phonon Scattering by Mass and Strain Field Fluctuations in Nb Substituted FeVSb Half-Heusler Thermoelectric Materials. J. Appl. Phys. 2012, 112, 124915. (44) Ge, Z.H.; Song, D.; Chong, X.; Zheng, F.; Jin, L.; Qian, X.; Zheng, L.; Dunin-Borkowski, R.E.; Qin, P.; Feng, J.; Zhao, L.D. Boosting the Thermoelectric Performance of (Na,K)-Codoped Polycrystalline SnSe by Synergistic Tailoring of the Band Structure and Atomic-Scale Defect Phonon Scattering. J. Am. Chem. Soc. 2017, 21

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139, 9714-9720. (45) Leng, H.; Zhou, M.; Zhao, J.; Han, Y.; Li, L. Optimization of Thermoelectric Performance of Anisotropic AgxSn1-xSe Compounds. J. Electron. Mater. 2016, 45, 527-534. (46) Gao, J.; Xu, G. Thermoelectric Performance of Polycrystalline Sn1-xCuxSe (x=0-0.03) Prepared by High Pressure Method. Intermetallics 2017, 89, 40-45.

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Table 1. Calculated bulk modules (B, in GPa), shear modules (G in GPa), average sound velocity (߭௠ in ms-1), Grüneisen parameters (γ), Debye temperatures (Θ஽ , in K), and the lattice thermal conductivities (ߢ௅ in Wm-1K-1) both considering the Umklapp (U) and normal (N) phonon scatterings and the ones with additional point-defect (PD) scatterings at 300 K in SnSe, Sn0.95Se and Sn0.925Pb0.025Se. ࣄ૜૙૙ࡷ ࡸ,ࢁାࡺ

ࣄ૜૙૙ࡷ ࡸ,ࢁାࡺାࡼࡰ

B

G

V



SnSe

37.9

25.2

2208

2.3

111

1.47

Sn0.95Se

36.1

22.7

2105

2.2

106

1.43

0.98

Sn0.925Pb0.025Se

36.3

21.5

2042

2.5

102

0.98

0.68

દࡰ

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Fig. 1 (a) XRD patterns of polycrystalline Sn0.95-xPbxSe and pure SnSe samples; standard peaks of SnSe were given for comparison. (b) XRD patterns measured by focusing on a narrow angular range.

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Fig. 2 Temperature dependence of (a) electrical conductivity (σ), (b) carrier concentration (n), (c) Seebeck coefficient (S) and (d) power factor (PF) for polycrystalline

Sn0.95-xPbxSe

and

pure

SnSe.

Data

for

high-performance

phase-separated polycrystalline Sn0.99Pb0.01Se is shown for comparison.

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Fig. 3 Temperature dependence of (a) total thermal conductivity (kT), (b) lattice thermal conductivity (kL) for polycrystalline Sn0.95-xPbxSe and pure SnSe. Data for high-performance

Sn

vacancies

containing

Sn0.95Se

polycrystalline Sn0.99Pb0.01Se is shown for comparison.

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and

phase-separated

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Fig. 4 HAADF-STEM images showing the microstructures in Sn0.93Pb0.02Se sample (The thickness of the lamellae is about 50 nm): (a) the overview of layered grains, (b, c) the enlargement of grains in (a), (d) antiphase boundary (APB) and corresponding atomic structure model projected along with 1/2[010] shift, (e) vacancy cluster (the circled dark area) in the interior of grains, (f) the phase boundary between elemental Se and SnSe compound and vacancy clusters (the circled dark area) observed at the boundary. The boxed areas in (b, c) were taken to do element analysis (Fig. 5) and enlarge in Fig. 6d, respectively.

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Fig. 5 The elements distribution map taking from the boxed area in Fig. 4b.

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Fig. 6 (a) low-magnification bright-field TEM image of grain as shown in Fig. 4c, (b) the corresponding selected area electron diffraction (SAED) pattern, (c) the corresponding dark-field image of (a), (d) the high-resolution HAADF-STEM image of the boxed area in Fig. 4c.

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Fig. 7 Calculated lattice thermal conductivity ( ߢ௅ ) of SnSe, Sn0.95Se, and Sn0.925Pb0.025Se

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Fig. 8 Temperature dependence of ZT for polycrystalline Sn0.95-xPbxSe and pure SnSe.

ZT

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

Sn0.94Pb0.01Se Sn0.93Pb0.02Se Sn0.92Pb0.03Se SnSe Sn0.99Pb0.01Se (ref. 24) Na, Pb Codoped (ref. 23)

300

400

500 600 T (K)

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700

800

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