Research Article www.acsami.org
Thermoelectric Properties of SnS with Na-Doping Binqiang Zhou, Shuai Li, Wen Li, Juan Li, Xinyue Zhang, Siqi Lin, Zhiwei Chen, and Yanzhong Pei*
ACS Appl. Mater. Interfaces 2017.9:34033-34041. Downloaded from pubs.acs.org by RMIT UNIV on 10/30/18. For personal use only.
Interdisciplinary Materials Research Center, School of Materials Science and Engineering, Tongji University, 4800 Caoan Road, Shanghai 201804, China ABSTRACT: Tin sulfide (SnS), a low-cost compound from the IV−VI semiconductors, has attracted particular attention due to its great potential for large-scale thermoelectric applications. However, pristine SnS shows a low carrier concentration, which leads to a low thermoelectric performance. In this work, sodium is utilized to substitute Sn to increase the hole concentration and consequently improve the thermoelectric power factor. The resultant Hall carrier concentration up to ∼1019 cm−3 is the highest concentration reported so far for this compound. This further leads to the highest thermoelectric figure of merit, zT of 0.65, reported so far in polycrystalline SnS. The temperature-dependent Hall mobility shows a transition of carrier-scattering source from a grain boundary potential below 400 K to acoustic phonons at higher temperatures. The electronic transport properties can be well understood by a single parabolic band (SPB) model, enabling a quantitative guidance for maximizing the thermoelectric power factor. Using the experimental lattice thermal conductivity, a maximal zT of 0.8 at 850 K is expected when the carrier concentration is further increased to ∼1 × 1020 cm−3, according to the SPB model. This work not only demonstrates SnS as a promising low-cost thermoelectric material but also details the material parameters that fundamentally determine the thermoelectric properties. KEYWORDS: thermoelectric, SnS, carrier concentration, SPB model, zT Polycrystalline SnSe33,34 was found to show a promising zT as well. As an analogue to SnSe, SnS crystalizes in the same Pnma structure and shows a very similar band structure,35 receiving increasingly attentions for its potential thermoelectric applications. However, the existing literatures mainly focus on its optical properties,36,37 and an indirect band gap of 0.9−1.2 eV is estimated. Theoretical calculations on the band structure also indicate the band gap (∼1.05 eV) to be indirect.35 Calculations on the phonon transport properties35 reveal a comparable low lattice thermal conductivity as that of SnSe resulting from its strong lattice anharmonicity.18,38 The calculated low lattice thermal conductivity shows a good agreement with the experimental measurements.39 These features indicate SnS as a potential thermoelectric material with cheap constituent elements. Unfortunately, peak zT > 0.6 has not yet been realized experimentally so far in either single- or polycrystalline SnS.39−41 This is due to the low carrier concentrations achievable. It is known that both power factor (PF = S2/ρ) and zT can only be maximized in a certain narrow energy range of Fermi level for thermoelectrics,42,43 which corresponds to a certain carrier concentration range. Therefore, a full assessment of a thermoelectric material and its critical material parameters determining the thermoelectric performance fundamentally requires an investigation of the transport properties in a broad range of carrier concentration. Therefore, the discovery of an
1. INTRODUCTION Thermoelectric materials, enabling a direct conversion between heat and electricity based on either Seebeck or Peltier effect, have attracted extensive interests due to the energy crisis and environmental issue.1 A high conversion efficiency of thermoelectric materials is required for large-scale applications. The performance of thermoelectric materials is determined by dimensionless figure of merit, zT = S2T/ρ(κE + κL), where S, ρ, κE, κL, and T are the Seebeck coefficient, electrical resistivity, electronic thermal conductivity, lattice thermal conductivity, and absolute temperature, respectively.2 Due to the strong coupling effect among S, ρ, and κE, it is difficult to improve zT through an individual optimization of these parameters. Therefore, numerous efforts have been focusing on the reduction of lattice thermal conductivity, the only independent parameter for the thermoelectric properties. This strategy has been successfully realized in various materials for enhancing zT through various phonon-scattering sources, such as nanostructures,3−5 alloy defects,6−13 dislocations,14,15 liquid phonons,16,17 lattice anharmonicity,18,19 and low sound velocity.20 Alternatively, band engineering approaches, such as band convergence21,22 and nestification,13,23 have been found to effectively decouple S, ρ, and κE for an enhanced power factor by increasing the band degeneracy (Nv). These strategies have been demonstrated in various materials, including PbTe,21,24,25 SnTe,11,12,26 GeTe,27,28 Mg2Si,29 half-Heusler,30,31 and Te,13,23 for a significantly enhanced zT. Due to a strong lattice anharmonicity, single-crystalline SnSe was reported to show a low lattice thermal conductivity and thus a high zT,18,32 thereby attracting increasing attentions. © 2017 American Chemical Society
Received: June 18, 2017 Accepted: September 12, 2017 Published: September 12, 2017 34033
DOI: 10.1021/acsami.7b08770 ACS Appl. Mater. Interfaces 2017, 9, 34033−34041
Research Article
ACS Applied Materials & Interfaces
Figure 1. Crystal structure of SnS, with red and blue balls for Sn and S atoms, respectively (a). Powder X-ray diffraction (XRD) patterns (b) and scanning electron microscopy (SEM) images for Sn1−xNaxS (c−f).
lower-lying valence bands in SnS,35 an even higher zT can be expected when these bands are engineered to contribute to the charge transport as well. At T < 400 K, the scattering of charge carriers can be understood due to a grain boundary potential. This work not only demonstrates polycrystalline SnS as a promising low-cost thermoelectric material but also provides a better understanding of its material parameters.
effective dopant, enabling such a control in carrier concentration, is the key not only for understanding this material as a thermoelectric but also for guiding the performance enhancement. In this work, Na-doping on the Sn site is found to increase the hole concentration from 8 × 1017 in pristine SnS to 2 × 1019 cm−3 in Na0.02Sn0.98S. This is so far the largest range of carrier concentration achieved in this compound, which enables a systematic assessment of the electronic and thermal transport properties. The resultant highest carrier concentration indeed enables the highest zT of ∼0.65 to be achieved in this work in polycrystalline SnS with a lattice thermal conductivity as low as ∼0.4 W/m/K−1 at high temperatures. The electronic transport properties can be well understood by a single parabolic band (SPB) model with an acoustic scattering at T > 400 K, enabling an expectation of zT up to 0.8 in this material when the carrier concentration is further increased. Moreover, considering the
2. MATERIALS AND METHODS Polycrystalline Sn1−xNaxS (0 ≤ x ≤ 0.03) samples were synthesized by melting the stoichiometric amount of high-purity elements (>99.99%) at 1193 K for 4 h, quenching in cold water, and then annealing at 923 K for 3 days. Na is used to tune the carrier concentration. The obtained ingots were ground into powder for X-ray diffraction and hot pressing. The dense pellets (>95% of theoretical density) of ∼12 mm diameter were prepared by induction heating at 823 K for 30 min under a uniaxial pressure of ∼80 Mpa.44 34034
DOI: 10.1021/acsami.7b08770 ACS Appl. Mater. Interfaces 2017, 9, 34033−34041
Research Article
ACS Applied Materials & Interfaces
Figure 2. SEM images with the corresponding mapping on compositions by energy-dispersive spectrometer (EDS) for Sn0.98Na0.02S.
Figure 3. Na concentration dependent expected carrier concentration assuming each Na releases one hole (a), composition-dependent lattice parameters (b), and carrier concentration dependent Seebeck coefficient and resistivity (c) for Sn1−xNaxS at room temperature. The electrical transport properties, including electrical resistivity, Seebeck coefficient, and Hall coefficient, were simultaneously measured in the temperature range of 300−850 K under helium. The Hall coefficient and electrical resistivity were measured through the van der Pauw technique under a reversible magnetic field of 1.5 T. The Seebeck coefficient was obtained from the slope of the thermopower versus temperature difference within 0−5 K.45 Thermal diffusivity (D) was measured by a laser flash technique (Netzsch LFA457 system). The thermal conductivity was calculated via κ = dDCp, where d is the measured density through the mass and geometric volume of the pellets and Cp is the heat capacity from literature.39 The measurement uncertainty for S, ρ, and κ is about 5%. The microstructure was characterized by scanning electron microscope (Phenom Pro) equipped with an energy-dispersive spectrometer. Infrared Fourier transform spectroscopy (Bruker Tensor 2) with a diffuse reflectance attachment was used in this work for optical
measurements. The optical band gap was estimated by the Tauc method.46
3. RESULTS AND DISCUSSION The crystal structure of SnS shown in Figure 1a is an orthorhombic structure with a space group of Pnma at room temperature (as that of SnSe). Being similar to SnSe, SnS also shows a transition to a high-temperature Cmcm structure, and the phase transition temperature is 858 K.47 This leads the current work to focus on the thermoelectric properties of SnS in the low-temperature orthorhombic structure (T < 850 K). Powder XRD patterns for Sn1−xNaxS (0 ≤ x ≤ 0.03) are shown in Figure 1b. All of the peaks can be well indexed to the orthorhombic structure of SnS, indicating the high purity of the 34035
DOI: 10.1021/acsami.7b08770 ACS Appl. Mater. Interfaces 2017, 9, 34033−34041
Research Article
ACS Applied Materials & Interfaces
Figure 4. Temperature-dependent Hall carrier concentration (a) and Hall mobility (μH) (b), with a comparison to those of Sn0.95Ag0.05S,39 normalized optical absorption between the conduction and valence band edges vs photon energy (c), temperature-dependent deformation potential coefficient (Edef) and density of state effective mass (m*) (d), the Hall carrier concentration dependent Seebeck coefficient (e) and Hall mobility (f) for Sn1−xNaxS (0 ≤ x ≤ 0.03) at different temperatures.
concentration. Therefore, the higher carrier concentration achieved by Na-doping can be understood by its higher solubility in SnS. It is frequently observed that Na is more soluble than Ag in many IV−VI thermoelectrics, including PbTe,48,49 PbSe,50,51 and SnSe.32 The broad carrier concentration achieved enables a systematic discussion on the electronic transport properties to fundamentally understand the material parameters. Assuming each Na releases one hole, when Na substitutes Sn in SnS, we found the expected carrier concentration to be much higher than that actually obtained (Figure 3a). This indicates that the carrier concentration easily saturates at a very low value (400 K), the Hall mobility shows a temperature dependence approximately via μH ∼ T−1.5, indicating a dominant scattering of charge carrier by acoustic phonons,56 which is typically observed in the IV−VI semiconductors.57,58 The bands of SnS can be approximated as parabolic due to a wide band gap of ∼1.1 eV,37 which leads to a weak interaction between the conduction and valence bands.59 Such a wide band
gap is further confirmed by the optical measurements, which shows a strong absorption of photons with an energy close to 1.1 eV (Figure 4c). Therefore, a single parabolic band (SPB) model with acoustic phonon scattering is used to understand the transport properties at T > 400 K. The details about the SPB model can be found elsewhere.42,60 According to this model, the density of the state effective mass (m*) for Sn1−xNaxS (x ≤ 0.03) is estimated, and its temperature dependence is shown in Figure 4d. The m* increases slightly from ∼0.9me at 400 K to ∼1.2me at 850 K. Similar temperature dependence on the effective mass has been observed in lead chalcogenides61,62 and CuGaTe2.60 Use of the average experimental m* further enables the SPB model to predict the Hall carrier concentration dependent Seebeck coefficient and Hall mobility at different temperatures for Sn1−xNaxS (0 ≤ x ≤ 0.03), which agree well with the measurements as shown in Figure 4e,f at both 500 and 700 K. It is known from the band structure calculations35 that the first valence band maximum is located along the Γ−Z direction in the Brillouin zone; therefore, the band degeneracy Nv is 2. Other valence bands (along the Γ−Y direction and Z point) are at least 0.2 eV (8kBT at 300 K) lower in energy and, therefore, are not sufficiently involved to contribute to the transport of charge carriers because the samples obtained here all show a weakly degenerated conduction band (i.e., a high Seebeck coefficient). Use of the band degeneracy Nv of 2 enables the SPB model to estimate the band effective mass mb* via m* = Nv2/3mb* at any temperature. Further assuming the band to be isotropic, the deformation potential coefficient, Edef, measuring the strength of carrier scattering by acoustic phonons, can be 34037
DOI: 10.1021/acsami.7b08770 ACS Appl. Mater. Interfaces 2017, 9, 34033−34041
Research Article
ACS Applied Materials & Interfaces
Figure 6. Temperature-dependent total thermal conductivity (a), lattice thermal conductivity (b), with a comparison to those of Sn0.95Ag0.05S,39 thermoelectric figure of merit (zT) (c), and the predicted zT vs Hall carrier concentration and temperature (d) for Sn1−xNaxS (x ≤ 0.03).
determined from the measured mobility. Edef is found to decrease with the increasing temperature as shown in Figure 4d. It should be noted that Edef at temperatures slightly higher than 400 K can be overestimated due to the existence of carrier scattering by the boundary potential (a lower mobility as compared with that purely by acoustic scattering) as discussed above. The measured Seebeck coefficient and electrical resistivity for Sn1−xNaxS (x ≤ 0.03), as a function of temperature, are shown in Figure 5a,b, respectively. The Seebeck coefficient for all of the samples is positive, indicating a p-type conduction. The Seebeck coefficient and the electrical conductivity significantly decrease with an increase in nH due to Na-doping. It is found that the much higher nH achieved in the Na-doped samples causes the resistivity to be much lower than that of the Agdoped samples.39 The temperature-dependent power factor (PF) for Sn1−xNaxS (0 ≤ x ≤ 0.03) is shown in Figure 5c. Compared with pristine SnS, Na-doping significantly enhances the PF in the entire temperature range due to the increased carrier concentration (Figure 4a). Moreover, the nominal content (≥0.5%) of Na in this work is much higher than what is soluble (∼0.1%) at room temperature; therefore, the rest dopant in the material likely continues to be dissolved and leads to a further increase in the carrier concentrations at high temperatures (∼700 K in this work). Such an increase in solubility with an increase in the carrier concentration at high temperatures is frequently observed in the thermoelectrics, such as PbTe:Ag,63 PbTe:Na,64 and PbSe:Cu.65 It should be noted that the intrinsic excitation of minority carriers is unlikely to be strong in SnS because its band gap of ∼1.1 eV is as large as 15 kBT even at the
highest measured temperature in this work, whereas the Fermi level estimated by the SPB model at 650 K is only about 2 kBT above the valence band edge. Otherwise, comparable power factors for both doped and undoped materials would be expected, which is not supported either by this work with Nadoping or by the literature work with Ag-doping.39 In addition, the continuous decrease in temperature-dependent lattice thermal conductivity (Figure 6b) suggests a weak bipolar conduction in this work as well because the intrinsic excitation of the minority carriers usually increases the thermal conductivity. Use of the average m* and Edef (Figure 4d) further enables the SPB model to predict the carrier concentration dependent power factor at any temperature. Figure 5d shows the cases for 500, 700, and 850 K. It is seen that both power factor and carrier concentration required for maximizing power factor increase with increase in temperature, which is typically observed for thermoelectrics.66 Temperature-dependent total thermal conductivity and its lattice component for Sn1−xNaxS (x ≤ 0.03) are shown in Figure 6a,b, respectively. The lattice thermal conductivity is obtained by subtracting the electronic thermal conductivity (κE) from the total thermal conductivity via the Wiedemann− Franz Law, κE = LT/ρ, where L is the Lorenz factor determined by the SPB model. The lattice thermal conductivity is comparable for all of the samples due to the low concentration of impurities. It is found that Umklapp processes dominate the scattering of phonons in all of the samples because κL decreases with increase in temperature via a T−1. It is interesting to note that SnS intrinsically shows a κL as low as its amorphous limit estimated by the Cahill model67 when the literature sound 34038
DOI: 10.1021/acsami.7b08770 ACS Appl. Mater. Interfaces 2017, 9, 34033−34041
Research Article
ACS Applied Materials & Interfaces
Figure 7. Temperature-dependent Seebeck coefficient (a), electrical resistivity (b), thermal conductivity (c), and zT (d) for Sn1−xNaxS during heating and cooling, where Sn0.98Na0.02S is measured twice.
4. SUMMARY In this work, Na-doping realizes the highest carrier concentration reported for SnS, which leads to a significantly enhanced power factor in the entire temperature range. Being similar to that reported in the analogue compound SnSe, SnS was found to have an intrinsically low lattice thermal conductivity. The synergic effects of carrier concentration optimization and intrinsically low lattice thermal conductivity lead to a peak zT as high as ∼0.65, which is the highest reported for this compound. The broad range of carrier concentration achieved by Na-doping enables a meaningful assessment of this compound as a thermoelectric material and validates a single parabolic band approximation to understand the critical transport parameters. This leads to an expectation of a peak zT up to 0.8 once the carrier concentration is further increased to ∼1020 cm−3, with possibilities for an even higher zT provided the lower-lying valence bands are engineered to contribute to the transport properties. This work demonstrates SnS as a promising low-cost thermoelectric material.
velocities (3368 m/s for longitudinal and 1537, 2368 m/s for transverse branches)68 are used. Such an intrinsically low κL has been reported in both SnS35 and its analogue compound SnSe, with the same origin from a strong lattice anharmonicity.18,38,69 The temperature-dependent figure of merit (zT) for Sn1−xNaxS (x ≤ 0.03) is given in Figure 6c. zT of all of the samples increases with increase in temperature. A significantly enhanced zT up to 0.65 is obtained in Na-doped SnS in this work, which is a result of the synergy of both PF enhancement and the intrinsic low κL. The SPB model further enables a prediction of the carrier concentration dependent zT, where an average measured lattice thermal conductivity is used. As shown in Figure 6d, the predicted zT agrees well with the experimental results. It is further expected, according to the model, that a peak zT up to 0.8 can be achieved at 850 K once the carrier concentration is further increased to ∼1020 cm−3 through doping by more soluble dopants, strongly indicating SnS to be a promising low-cost thermoelectric material. To investigate the thermal stability of Sn1−xNaxS, the thermoelectric properties are measured during both heating and cooling for all of the samples and Sn0.98Na0.02S is measured twice. The results are shown in Figure 7. Although the Seebeck coefficient, resistivity, and thermal conductivity show hysteresis particularly at T < 700 K, the figure of merit zT is found to be consistent between the heating and cooling measurements. However, utilization of a protection surface coating, such as sodium silicate,70 is believed to be an effective approach for improving the stability of thermoelectric SnS at high temperatures.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yanzhong Pei: 0000-0003-1612-3294 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant Nos. 51422208, 11474219, and 34039
DOI: 10.1021/acsami.7b08770 ACS Appl. Mater. Interfaces 2017, 9, 34033−34041
Research Article
ACS Applied Materials & Interfaces
(19) Morelli, D. T.; Jovovic, V.; Heremans, J. P. Intrinsically Minimal Thermal Conductivity in Cubic I-V-Vi2 Semiconductors. Phys. Rev. Lett. 2008, 101, No. 035901. (20) Li, W.; Lin, S.; Ge, B.; Yang, J.; Zhang, W.; Pei, Y. Low Sound Velocity Contributing to the High Thermoelectric Performance of Ag8SnSe6. Adv. Sci. 2016, 3, No. 1600196. (21) Pei, Y.; Shi, X.; LaLonde, A.; Wang, H.; Chen, L.; Snyder, G. J. Convergence of Electronic Bands for High Performance Bulk Thermoelectrics. Nature 2011, 473, 66−69. (22) Pei, Y.; Wang, H.; Snyder, G. J. Band Engineering of Thermoelectric Materials. Adv. Mater. 2012, 24, 6125−6135. (23) Lin, S.; Li, W.; Chen, Z. W.; Shen, J. W.; Ge, B. H.; Pei, Y. Z. Tellurium as a High-Performance Elemental Thermoelectric. Nat. Commun. 2016, 7, No. 10287. (24) Pei, Y.; Wang, H.; Gibbs, Z. M.; LaLonde, A. D.; Snyder, G. J. Thermopower Enhancement in Pb1-xMnxTe Alloys and Its Effect on Thermoelectric Efficiency. NPG Asia Mater. 2012, 4, No. e28. (25) Pei, Y.; LaLonde, A. D.; Heinz, N. A.; Shi, X. Y.; Iwanaga, S.; Wang, H.; Chen, L. D.; Snyder, G. J. Stabilizing the Optimal Carrier Concentration for High Thermoelectric Efficiency. Adv. Mater. 2011, 23, 5674−5678. (26) Li, W.; Chen, Z.; Lin, S.; Chang, Y.; Ge, B.; Chen, Y.; Pei, Y. Band and Scattering Tuning for High Performance Thermoelectric Sn1−XMnxTe Alloys. J. Materiomics 2015, 1, 307−315. (27) Li, J.; Zhang, X.; Lin, S.; Chen, Z.; Pei, Y. Realizing the High Thermoelectric Performance of GeTe by Sb-Doping and Se-Alloying. Chem. Mater. 2017, 29, 605−611. (28) Li, J.; Chen, Z. W.; Zhang, X. Y.; Sun, Y. X.; Yang, J.; Pei, Y. Z. Electronic Origin of the High Thermoelectric Performance of GeTe among the P-Type Group Iv Monotellurides. NPG Asia Mater. 2017, 9, No. e353. (29) Liu, W.; Tan, X. J.; Yin, K.; Liu, H. J.; Tang, X. F.; Shi, J.; Zhang, Q. J.; Uher, C. Convergence of Conduction Bands as a Means of Enhancing Thermoelectric Performance of N-Type Mg2Si1-xSnx Solid Solutions. Phys. Rev. Lett. 2012, 108, No. 166601. (30) Fu, C. G.; Zhu, T. J.; Pei, Y. Z.; Xie, H. H.; Wang, H.; Snyder, G. J.; Liu, Y.; Liu, Y. T.; Zhao, X. B. High Band Degeneracy Contributes to High Thermoelectric Performance in P-Type Half-Heusler Compounds. Adv. Energy Mater. 2014, 4, No. 1400600. (31) Fu, C.; Bai, S.; Liu, Y.; Tang, Y.; Chen, L.; Zhao, X.; Zhu, T. Realizing High Figure of Merit in Heavy-Band P-Type Half-Heusler Thermoelectric Materials. Nat. Commun. 2015, 6, No. 8144. (32) 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. (33) 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. (34) 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. (35) Guo, R.; Wang, X.; Kuang, Y.; Huang, B. First-Principles Study of Anisotropic Thermoelectric Transport Properties of Iv-Vi Semiconductor Compounds SnSe and SnS. Phys. Rev. B 2015, 92, No. 115202. (36) Gao, C.; Shen, H.; Sun, L. Preparation and Properties of Zinc Blende and Orthorhombic SnS Films by Chemical Bath Deposition. Appl. Surf. Sci. 2011, 257, 6750−6755. (37) Albers, W.; Haas, C.; Vink, H. J.; Wasscher, J. D. Investigations on SnS. J. Appl. Phys. 1961, 32, 2220−2225. (38) Li, C. W.; Hong, J.; May, A. F.; Bansal, D.; Chi, S.; Hong, T.; Ehlers, G.; Delaire, O. Orbitally Driven Giant Phonon Anharmonicity in SnSe. Nat. Phys. 2015, 11, 1063−1069. (39) Tan, Q.; Zhao, L.-D.; Li, J.-F.; Wu, C.-F.; Wei, T.-R.; Xing, Z.-B.; Kanatzidis, M. G. Thermoelectrics with Earth Abundant Elements: Low Thermal Conductivity and High Thermopower in Doped SnS. J. Mater. Chem. A 2014, 2, 17302−17306.
51401147) and the national Recruitment Program of Global Youth Experts (1000 Plan).
■
REFERENCES
(1) Bell, L. E. Cooling, Heating, Generating Power, and Recovering Waste Heat with Thermoelectric Systems. Science 2008, 321, 1457− 1461. (2) Snyder, G. J.; Toberer, E. S. Complex Thermoelectric Materials. Nat. Mater. 2008, 7, 105−114. (3) Hsu, K. F.; Loo, S.; Guo, F.; Chen, W.; Dyck, J. S.; Uher, C.; Hogan, T.; Polychroniadis, E. K.; Kanatzidis, M. G. Cubic AgPbmSbTe2+m: Bulk Thermoelectric Materials with High Figure of Merit. Science 2004, 303, 818−821. (4) Biswas, K.; He, J.; Blum, I. D.; Wu, C. I.; Hogan, T. P.; Seidman, D. N.; Dravid, V. P.; Kanatzidis, M. G. High-Performance Bulk Thermoelectrics with All-Scale Hierarchical Architectures. Nature 2012, 489, 414−418. (5) Poudel, B.; Hao, Q.; Ma, Y.; Lan, Y.; Minnich, A.; Yu, B.; Yan, X.; Wang, D.; Muto, A.; Vashaee, D.; Chen, X.; Liu, J.; Dresselhaus, M. S.; Chen, G.; Ren, Z. High-Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys. Science 2008, 320, 634−638. (6) Shen, J. W.; Zhang, X. Y.; Chen, Z. W.; Lin, S. Q.; Li, J.; Li, W.; Li, S. S.; Chen, Y.; Pei, Y. Z. Substitutional Defects Enhancing Thermoelectric CuGaTe2. J. Mater. Chem. A 2017, 5, 5314−5320. (7) Pei, Y. Z.; Morelli, D. T. Vacancy Phonon Scattering in Thermoelectric In2Te3-InSb Solid Solutions. Appl. Phys. Lett. 2009, 94, No. 122112. (8) Shen, J. W.; Zhang, X. Y.; Lin, S. Q.; Li, J.; Chen, Z. W.; Li, W.; Pei, Y. Z. Vacancy Scattering for Enhancing the Thermoelectric Performance of CuGaTe2 Solid Solutions. J. Mater. Chem. A 2016, 4, 15464−15470. (9) Li, W.; Lin, S. Q.; Zhang, X. Y.; Chen, Z. W.; Xu, X. F.; Pei, Y. Z. Thermoelectric Properties of Cu2SnSe4 with Intrinsic Vacancy. Chem. Mater. 2016, 28, 6227−6232. (10) Pei, Y.; Zheng, L.; Li, W.; Lin, S.; Chen, Z.; Wang, Y.; Xu, X.; Yu, H.; Chen, Y.; Ge, B. Interstitial Point Defect Scattering Contributing to High Thermoelectric Performance in SnTe. Adv. Electron. Mater. 2016, 2, No. 1600019. (11) Li, W.; Zheng, L.; Ge, B.; Lin, S.; Zhang, X.; Chen, Z.; Chang, Y.; Pei, Y. Promoting SnTe as an Eco-Friendly Solution for P-PbTe Thermoelectric Via Band Convergence and Interstitial Defects. Adv. Mater. 2016, No. 1605887. (12) Zheng, L. L.; Li, W.; Lin, S. Q.; Li, J.; Chen, Z. W.; Pei, Y. Z. Interstitial Defects Improving Thermoelectric SnTe in Addition to Band Convergence. ACS Energy Lett. 2017, 2, 563−568. (13) Lin, S. Q.; Li, W.; Zhang, X. Y.; Li, J.; Chen, Z. W.; Pei, Y. Z. Sb Induces Both Doping and Precipitation for Improving the Thermoelectric Performance of Elemental Te. Inorg. Chem. Front. 2017, 4, 1066−1072. (14) Chen, Z.; Ge, B.; Li, W.; Lin, S. Q.; Shen, J.; Chang, Y.; Hanus, R.; Snyder, G. J.; Pei, Y. Vacancy-Induced Dislocations within Grains for High-Performance PbSe Thermoelectrics. Nat. Commun. 2017, 8, No. 13828. (15) Chen, Z.; Jian, Z. Z.; Li, W.; Chang, Y. J.; Ge, B. H.; Hanus, R.; Yang, J.; Chen, Y.; Huang, M. X.; Snyder, G. J.; Pei, Y. Z. Lattice Dislocations Enhancing Thermoelectric PbTe in Addition to Band Convergence. Adv. Mater. 2017, 29, No. 1606768. (16) He, Y.; Day, T.; Zhang, T.; Liu, H.; Shi, X.; Chen, L.; Snyder, G. J. High Thermoelectric Performance in Non-Toxic Earth-Abundant Copper Sulfide. Adv. Mater. 2014, 26, 3974−3978. (17) Snyder, G. J.; Christensen, M.; Nishibori, E.; Caillat, T.; Iversen, B. Disordered Zinc in Zn4sb3 with Phonon-Glass and Electron-Crystal Thermoelectric Properties. Nat. Mater. 2004, 3, 458−463. (18) 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 Thermoelectric Figure of Merit in SnSe Crystals. Nature 2014, 508, 373−377. 34040
DOI: 10.1021/acsami.7b08770 ACS Appl. Mater. Interfaces 2017, 9, 34033−34041
Research Article
ACS Applied Materials & Interfaces (40) Chaki, S. H.; Chaudhary, M. D.; Deshpande, M. P. Effect of Indium and Antimony Doping in SnS Single Crystals. Mater. Res. Bull. 2015, 63, 173−180. (41) Nassary, M. M. Temperature Dependence of the Electrical Conductivity, Hall Effect and Thermoelectric Power of SnS Single Crystals. J. Alloys Compd. 2005, 398, 21−25. (42) Goldsmid, H. J. Introduction to Thermoelectricity; Springer: Heidelberg, 2009. (43) Ioffe, A. F. Semiconductor Thermoelements, and Thermoelectric Cooling; Infosearch: London, 1957. (44) LaLonde, A. D.; Ikeda, T.; Snyder, G. J. Rapid Consolidation of Powdered Materials by Induction Hot Pressing. Rev. Sci. Instrum. 2011, 82, No. 025104. (45) Zhou, Z.; Uher, C. Apparatus for Seebeck Coefficient and Electrical Resistivity Measurements of Bulk Thermoelectric Materials at High Temperature. Rev. Sci. Instrum. 2005, 76, No. 023901. (46) Gibbs, Z. M.; Lalonde, A.; Snyder, G. J. Optical Band Gap and the Burstein-Moss Effect in Iodine Doped PbTe Using Diffuse Reflectance Infrared Fourier Transform Spectroscopy. New J. Phys. 2013, 15, No. 075020. (47) Chattopadhyay, T.; Pannetier, J.; Von Schnering, H. G. Neutron Diffraction Study of the Structural Phase Transition in SnS and SnSe. J. Phys. Chem. Solids 1986, 47, 879−885. (48) Pei, Y.; LaLonde, A.; Iwanaga, S.; Snyder, G. J. High Thermoelectric Figure of Merit in Heavy-Hole Dominated PbTe. Energy Environ. Sci. 2011, 4, 2085−2089. (49) Noda, Y.; Orihashi, M.; Nishida, I. A. Thermoelectric Properties of P-Type Lead Telluride Doped with Silver or Potassium. Mater. Trans., JIM 1998, 39, 602−605. (50) Wang, H.; Pei, Y.; LaLonde, A. D.; Snyder, G. J. Heavily Doped P-Type PbSe with High Thermoelectric Performance: An Alternative for PbTe. Adv. Mater. 2011, 23, 1366−1370. (51) Wang, S.; Zheng, G.; Luo, T.; She, X.; Li, H.; Tang, X. Exploring the Doping Effects of Ag in P-Type PbSe Compounds with Enhanced Thermoelectric Performance. J. Phys. D: Appl. Phys. 2011, 44, No. 475304. (52) Seto, J. Y. W. The Electrical Properties of Polycrystalline Silicon Films. J. Appl. Phys. 1975, 46, 5247−5254. (53) Li, Y. W.; Li, F.; Dong, J. F.; Ge, Z.; Kang, F. Y.; He, J. Q.; Du, H. D.; Li, B.; Li, J. F. Enhanced Mid-Temperature Thermoelectric Performance of Textured SnSe Polycrystals Made of Solvothermally Synthesized Powders. J. Mater. Chem. C 2016, 4, 2047−2055. (54) Zhang, X. Y.; Shen, J. W.; Lin, S. Q.; Li, J.; Chen, Z. W.; Li, W.; Pei, Y. Z. Thermoelectric Properties of GeSe. J. Materiomics 2016, 2, 331−337. (55) Martin, J.; Wang, L.; Chen, L.; Nolas, G. S. Enhanced Seebeck Coefficient through Energy-Barrier Scattering in PbTe Nanocomposites. Phys. Rev. B 2009, 79, No. 115311. (56) Ravich, Y. I.; Efimova, B. A.; Smirnov, I. A. Semiconducting Lead Chalcogenides; Plenum Press: New York, 1970; pp 1−34. (57) Wang, H.; Schechtel, E.; Pei, Y. Z.; Snyder, G. J. High Thermoelectric Efficiency of N-Type PbS. Adv. Energy Mater. 2013, 3, 488−495. (58) Pei, Y.; LaLonde, A. D.; Wang, H.; Snyder, G. J. Low Effective Mass Leading to High Thermoelectric Performance. Energy Environ. Sci. 2012, 5, 7963−7969. (59) Smirnov, I. A.; Moizhes, B. Y.; Nensberg, E. D. The Effective Carrier Mass in Lead Selenide. Sov. Phys.-Solid State 1961, 2, 1793− 1804. (60) Shen, J.; Chen, Z.; Lin, S.; Zheng, L.; Li, W.; Pei, Y. Single Parabolic Band Behavior of Thermoelectric P-Type CuGaTe2. J. Mater. Chem. C 2016, 4, 209−214. (61) Rogers, L. M. Electron Scattering in Some II-IV-VI Alloy Semiconductors. J. Phys. D: Appl. Phys. 1971, 4, 1025−1033. (62) Zhitinskaya, M.; Kaidanov, V.; Chernik, I. Nonparabolicity of the Conduction Band of Lead Telluride. Sov. Phys.-Solid State 1966, 8, 246−247.
(63) Pei, Y.; May, A. F.; Snyder, G. J. Self-Tuning the Carrier Concentration of PbTe/Ag2Te Composites with Excess Ag for High Thermoelectric Performance. Adv. Energy Mater. 2011, 1, 291−296. (64) Yamini, S. A.; Mitchell, D. R. G.; Gibbs, Z. M.; Santos, R.; Patterson, V.; Li, S.; Pei, Y. Z.; Dou, S. X.; Snyder, G. J. Heterogeneous Distribution of Sodium for High Thermoelectric Performance of PType Multiphase Lead-Chalcogenides. Adv. Energy Mater. 2015, 5, No. 1501047. (65) Fedorovich, N. Sov. Phys.-Solid State, USSR 1965, 7, 1289. (66) Pei, Y.; Gibbs, Z. M.; Gloskovskii, A.; Balke, B.; Zeier, W. G.; Snyder, G. J. Optimum Carrier Concentration in N-Type PbTe Thermoelectrics. Adv. Energy Mater. 2014, 4, No. 1400486. (67) Cahill, D. G.; Watson, S. K.; Pohl, R. O. Lower Limit to the Thermal Conductivity of Disordered Crystals. Phys. Rev. B 1992, 46, 6131. (68) Ding, G.; Gao, G.; Yao, K. High-Efficient Thermoelectric Materials: The Case of Orthorhombic IV-VI Compounds. Sci. Rep. 2015, 5, No. 9567. (69) 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-xSx by Iodine Doping. Adv. Energy Mater. 2015, 5, No. 1500360. (70) Shi, X. Y.; Huang, F. Q.; Liu, M. L.; Chen, L. D. Thermoelectric Properties of Tetrahedrally Bonded Wide-Gap Stannite Compounds Cu2ZnSn1-xInxSe4. Appl. Phys. Lett. 2009, 94, No. 122103.
34041
DOI: 10.1021/acsami.7b08770 ACS Appl. Mater. Interfaces 2017, 9, 34033−34041
