Article pubs.acs.org/JACS
Enhancing p‑Type Thermoelectric Performances of Polycrystalline SnSe via Tuning Phase Transition Temperature Yong Kyu Lee,†,‡ Kyunghan Ahn,‡ Joonil Cha,†,‡ Chongjian Zhou,‡ Hyo Seok Kim,‡ Garam Choi,‡ Sue In Chae,†,‡ Jae-Hyuk Park,†,‡ Sung-Pyo Cho,§ Sang Hyun Park,∥ Yung-Eun Sung,†,‡ Won Bo Lee,‡ Taeghwan Hyeon,†,‡ and In Chung*,†,‡ J. Am. Chem. Soc. 2017.139:10887-10896. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 10/12/18. For personal use only.
†
Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826, Republic of Korea School of Chemical and Biological Engineering and Institute of Chemical Processes, and §National Center for Inter-University Research Facilities, Seoul National University, Seoul 08826, Republic of Korea ∥ Advanced Materials and Devices Laboratory, Korea Institute of Energy Research, Daejeon 34129, Republic of Korea ‡
S Supporting Information *
ABSTRACT: SnSe emerges as a new class of thermoelectric materials since the recent discovery of an ultrahigh thermoelectric figure of merit in its single crystals. Achieving such performance in the polycrystalline counterpart is still challenging and requires fundamental understandings of its electrical and thermal transport properties as well as structural chemistry. Here we demonstrate a new strategy of improving conversion efficiency of bulk polycrystalline SnSe thermoelectrics. We show that PbSe alloying decreases the transition temperature between Pnma and Cmcm phases and thereby can serve as a means of controlling its onset temperature. Along with 1% Na doping, delicate control of the alloying fraction markedly enhances electrical conductivity by earlier initiation of bipolar conduction while reducing lattice thermal conductivity by alloy and point defect scattering simultaneously. As a result, a remarkably high peak ZT of ∼1.2 at 773 K as well as average ZT of ∼0.5 from RT to 773 K is achieved for Na0.01(Sn1−xPbx)0.99Se. Surprisingly, spherical-aberration corrected scanning transmission electron microscopic studies reveal that NaySn1−xPbxSe (0 < x ≤ 0.2; y = 0, 0.01) alloys spontaneously form nanoscale particles with a typical size of ∼5−10 nm embedded inside the bulk matrix, rather than solid solutions as previously believed. This unexpected feature results in further reduction in their lattice thermal conductivity.
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INTRODUCTION
materials with no Te have been highly sought after. SnSe that consists of no scarce and toxic elements has been ignored for TE applications mainly because of seemingly high electrical resistivity. In 2014, SnSe single crystal was reported to show a record high ZT of 2.6 at 923 K along the crystallographic b-axis. Its exceptionally high ZT originates from unique crystal chemistry.31 The corrugated layered structure coupled with highly distorted [SnSe7] polyhedra provides giant anisotropic and anharmonic bonding characteristic, consequently giving rise to one of the lowest κlatt values known for crystalline materials ( 0.1 at 300 K. Plausibly, PbSe alloying considerably suppresses the generation of intrinsic Sn vacancies (Figure S8a). The μH value lessens from 46 cm2 V−1 s−1 for SnSe to 11 cm2 V−1 s−1 for Sn0.8Pb0.2Se because of enhanced point defects and alloy scattering with increasing Pb amounts (Figure S8b). The temperature-dependent Seebeck coefficient (S) of Sn1−xPbxSe sharply decreases above ∼573 K (Figure 5b). Such behavior is correlated with the sudden upturn of the σ. The maximum S (Smax) values range between 515 and 550 μV K−1, which is similar to that of undoped SnSe single crystals.31 The power factors (PFs) of Sn1−xPbxSe remain almost constant up to ∼523 K and suddenly rise afterward (Figure 5c). 10891
DOI: 10.1021/jacs.7b05881 J. Am. Chem. Soc. 2017, 139, 10887−10896
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Journal of the American Chemical Society Despite boosting the nH, Na doping seriously deteriorates the μH: ∼ 6 cm2 V−1 s−1 for the sample with x = 0.05 in comparison with 28 cm2 V−1 s−1 of undoped SnSe (Figure S9b). Similar behavior is also observed when alkali-ion is doped to polycrystalline pristine SnSe.40 This result implies that improving the μH would be a next task for further advance in heavily doped polycrystalline SnSe thermoelectrics. The enhanced nH remarkably improves the σ to 59 S cm−1 for Na0.01(Sn0.95Pb0.05)0.99Se from 0.8 S cm−1 for Sn0.95Pb0.05Se at 300 K (Figure 6a). The σ increases from 300 to 373 K, subsequently decreases up to 673 K, and rises quickly afterward due to bipolar conduction. Very importantly, the σ upturn in the third region is driven by the Pnma−Cmcm transition as discussed in the previous section. Shifting the Tc downward by PbSe alloying reduces the onset temperature of bipolar conduction and resultantly the upturn temperature of the σ value. Earlier activation of bipolar conduction contributes to the enhanced σ and PF at operating temperature of TE conversion of SnSe-based materials and accordingly improved average ZT. For example, the σ upturns at 673, 623, 623, and 573 K for samples with x = 0.05, 0.1, 0.15, and 0.2, respectively. This effect is not obviously observed for Sn1−xPbxSe due to their too low σ values. The S of Na0.01(Sn1−xPbx)0.99Se increases with temperature from RT to 600−673 K and subsequently decreases afterward (Figure 6b). The sample with x = 0.05 exhibits the largest Smax of 378 μV K−1 at 673 K. The temperature of the Smax shifts downward with increasing x, consistent with a trend of the σ. To understand the effect of PbSe alloying on the S, we calculated the Pisarenko relation between the S and the nH for SnSe, and compared the results with the experimental S values at 300 K for Sn0.96Pb0.04Se and Na0.01(Sn0.95Pb0.05)0.99Se in this work as well as for the previous report.31,37,40 (see Supporting Information and Figure S10 for details) The results demonstrate that PbSe alloying marginally affects S values, which is consistent with the results of our electronic structure calculations showing that it negligibly disturbs the VBM. The maximum PF value for Na0.01(Sn0.99Pb0.05)0.99Se is 7.39 μW cm−1 K−2 at 823 K, which is remarkably improved from that of Sn0.95Pb0.05Se and Na0.01Sn0.99Se, mainly due to the gain in the σ value (Figure 6c). The κtot and the κlatt of the Na0.01(Sn1−xPbx)0.99Se show the similar trend to those of Sn1−xPbxSe with the comparable value at 300 K (Figure 6d). The minimum κtot of the former is lower than that of the latter: ∼ 0.44 W m−1 K−1 at 773 K of Na0.01(Sn0.95Pb0.05)0.99Se compared to ∼0.60 W m−1 K−1 at 773 K of Sn0.95Pb0.05Se. The degree of PbSe alloying fraction influences on a temperature dependence of the κlatt. The κlatt of samples with x = 0, 0.05, and 0.1 follows a power series of T−1.2, T−1.1, and T−0.7, respectively, revealing that the Umklapp phonon scattering is dominant for x < 0.1 (Figure 6e). The sample with x = 0.05 exhibits the highest ZT of 1.16 at 773 K among the series, which is attributed simultaneously from the remarkably enhanced σ and the lowered κtot with the least loss of the S compared to Sn1−xPbxSe. (Figure 6f). After obtaining the high ZT of 1.16 from the sample of x = 0.05, we finely tuned the Pb level in the range of x = 0.03−0.06 for optimizing TE performance. The samples generally show a similar trend for TE properties versus temperature (Figure S11). The x = 0.04 sample displays the highest PF of 6.7 μW cm−1 K−2 and the lowest κtot of ∼0.43 W m−1 K−1, resulting in the highest ZT of 1.18 among this family of Na0.01(Sn1−xPbx)0.99Se, slightly larger than 1.16 of the x = 0.05 sample
(Figure 7). Despite the polycrystalline nature of the samples, their TE properties are highly anisotropic. The ZT taken
Figure 7. Temperature-dependent figure of merit ZT of Na0.01(Sn1−xPbx)0.99Se (x = 0, 0.03−0.06).
parallel to the press direction of SPS is lower at 0.88 (Figure S12). The achieved ZT is improved by a 2-fold magnitude from ZT = 0.6 of pristine SnSe polycrystals40 as well as by 43% from ZT = 0.8 of Na0.01Sn0.99Se40 because of the simultaneous contributions from enhanced PF due to tuning the Tc and the reduced κlatt due to effective point defect scattering by PbSe alloying. Importantly, PbSe alloyed and Na-doped SnSe in this work shows remarkably enhanced average ZT from RT to 773 K: ZTave ∼ 0.5 for the sample with x = 0.05 in comparison with ∼0.35 for Na0.01Sn0.99Se40 and phase-separated Sn1−xPbxSe.21 Recently, ball-mill-processed K-doped SnSe was reported to show the ZT of 1.1 and ZTave ∼ 0.5.41 However, we could not enhance ZT of the Sn1−xPbxSe system by doping either K or Na and K together. Temperature-Dependent Hall Effect Measurements. We performed the temperature-dependent Hall effect measurements for SnSe, Na 0.01 Sn 0.99 Se, Sn 0.96 Pb 0.04 Se, and Na0.01(Sn0.96Pb0.04)0.99Se to better understand the effect of Pb and Na incorporation on electrical transport properties. The Hall coefficient RH (T) for all samples is temperaturedependent. Above ∼475 K, the RH (T) of Na-doped samples of Na0.01Sn0.99Se and Na0.01(Sn0.96Pb0.04)0.99Se increases gradually while that of the others declines rapidly. For undoped samples, the carrier concentration (nH) is the order of ∼1017 cm−3 at RT and slowly increases until 523 K, followed by rapid increase afterward because of the intrinsic carrier excitation (Figure 8b). This trend is related with the rapid increment of the σ above 523 K. The nH for Na-doped samples is the order of ∼1019 at RT with a decrease above 523 K. This is due to scattering between major carriers of high concentration.40 Pb alloying lowers the nH because Pb substitution for Sn reduces intrinsic Sn vacancies that are the major source of charge carriers in p-type SnSe. Figure 8c represents the hole mobility (μH) as a function of temperature. For all samples, it increases from RT to 373 K. This trend is attributed to the barrier-like scattering due to defects in grain boundaries as reported earlier.40,62 Afterward, the μH for all samples follows a temperature dependence of T−1.5 from 373 to 523 K. This is the characteristic behavior given by acoustic phonon scatterings.62,63 The μH of Na-doped samples continuously decreases above 523 K because scattering between carriers of high concentration becomes significant at high temperatures.64 In contrast, the μH of undoped samples increases with rising temperature, attributed to excitation of 10892
DOI: 10.1021/jacs.7b05881 J. Am. Chem. Soc. 2017, 139, 10887−10896
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Figure 8. (a) Hall coefficient, (b) carrier concentration and (c) mobility of SnSe, Sn0.96Pb0.04Se, Na0.01Sn0.99Se, and Na0.01(Sn0.96Pb0.04)0.99Se as a function of temperature.
Figure 9. Cross-sectional STEM images and elemental mapping of Na0.01(Sn0.96Pb0.04)0.99Se. (a) Typical low-magnification bright-field (BF) STEM image. Inset: selected area electron diffraction pattern along the [010] axis. (b) Annular BF(ABF)-STEM image focused on a nanostructure in (a). Inset: fast Fourier transform (FFT) image along [010] axis. (c) Inverse FFT image of (b). Red symbols indicate edge dislocations. (d) High-angle annular dark-field (HAADF) STEM (upper) and ABF-STEM (lower) atomic-resolution images focused on a nanostructure. Insets: Signal intensity of Z-contrast across a nanostructure taken by a line profile. (e) Magnified atomic-resolution HAADF-STEM image taken on a matrix, showing crystal structure of Sn1−xPbxSe at the atomic scale. White arrows indicate spaces between two atom-thick Sn1−xPbxSe slabs. (f) Elemental mapping of Se, Sn, and Pb atoms depicted in red, yellow, and blue color, respectively, by STEM energy dispersive X-ray spectroscopy scanned on the region of a red rectangle in (e) (lower panels), confirming the location of respective atoms in crystal structure of Sn1−xPbxSe. These three panels jointly create the image in the upper panel, verifying Sn and Pb atoms are completely disordered in Sn1−xPbxSe. (g) STEM-electron energy loss spectroscopy (EELS) signals for samples of Sn0.96Pb0.04Se and Na0.01(Sn0.96Pb0.04)0.99Se. The black arrow indicates an energy of Na L2,3 edge. (h) EELS signal intensity of Na L-edge across a nanostructure recorded by a line profile.
the first nanostructured bulk thermoelectric material reported, opening a new paradigm of thermoelectrics, of which nanostructuring is a key aspect of high TE performance due to substantial reduction in κlatt.18,66,67 This example underscores the importance of atomic resolution investigations for properly understanding thermal and electrical transport properties of TE materials. We investigated detailed nanostructures for the SPS Na0.01(Sn0.96Pb0.04)0.99Se sample employing atomic resolution
carriers. Note that PbSe alloying moderately suppresses the μH because of the point defect scattering.46 Scanning Transmission Electron Microscopy (STEM). The AgPbmSbTe2+m (LAST-m) system had long been regarded as a solid solution between isostructural PbTe and AgSbTe2.65 However, its members with m > 10 were revealed to be a bulk material that spontaneously forms a considerable amount of nanostructures even though they appear single phase by conventional XRD, following Vegard’s law. The LAST-m is 10893
DOI: 10.1021/jacs.7b05881 J. Am. Chem. Soc. 2017, 139, 10887−10896
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which is absent from that of Sn0.96Pb0.04Se. The line scan profile of the Na EELS spectrum across the nanostructure and the matrix demonstrates that Na concentration is lower in the former (Figure 9h). Overall, combined analysis of STEM, EDS, and EELS results reveals that PbSe alloying spontaneously induces a considerable amount of nanostructures of Sn1−xPbxSe despite appearance as single phase by conventional XRD. The nanostructures contain relatively more Pb and less Sn and Na concentrations than the matrix. This observation is in striking contrary to the published phase diagram and literature46,47 that describe Sn1−xPbxSe as solid solution. The slightly different chemical composition and indistinguishable unit cell dimension of the nanostructure from the matrix give rise to highly coherent interface between them. Sn and Pb atoms are totally disordered. As a result, point defects, lattice strains, and mass fluctuation between the nanostructures and the matrix caused by PbSe alloying and Na doping can synergistically interrupt short-wavelength phonon transfer with simultaneously enhanced hole carrier transport, plausibly explaining high TE performance of Na0.01(Sn0.96Pb0.04)0.99Se.65 To support this, we performed theoretical calculations on lattice thermal conductivities for the samples of pristine, Pb alloyed, and Na-doped and Pb alloyed SnSe with consideration of various well-known scattering mechanisms based on the models of Umklapp (U),69,70 Umklapp combined with point defects (U+PD),71,72 U+PD with interfaces, and U+PD with interfaces and dislocations73 (see Supporting Information for further details). Figure 10 shows the calculated κlatt curves in comparison with
STEM, EDS, and EELS. Typical cross-sectional bright-field (BF) STEM image viewed down the [010] axis demonstrates the presence of nanostructures with a size of ∼5−10 nm, evenly distributed in the matrix. The Na0.01(Sn0.96Pb0.04)0.99Se sample (Figure 9a) contains more distinct and denser nanostructures than the Sn0.96Pb0.04Se (Figure S13). The selected area electron diffraction (SAED) pattern (inset, Figure 9a) on the former is clearly indexed as the SnSe structure along the [010] zone axis, indicating the nanostructures and the matrix are isostructural. Absence of extra diffraction spots excludes the presence of second-phase precipitates such as PbSe and its structural analogues. High-magnification annular BF STEM (ABFSTEM) image focusing on the individual nanostructure reveals a nearly coherent interface with the matrix (Figure 9b). The corresponding fast Fourier transform (FFT) image does not show extra spots deviating from the SnSe structure (inset, Figure 9b). The inverse FFT (IFFT) image of the (101) atomic planes clearly shows edge dislocations marked by the red symbols in Figure 9c. Afterward, we analyzed the compositional difference between the nanostructures and the matrix using the STEM-energy dispersive X-ray spectroscopy (EDS). We observed that Pb is more abundant in the nanostructures, whereas Sn is richer in the matrix with similar abundance of Se in both the regions (Figure S14). Both high-magnification atomic resolution highangle annular dark-field (HAADF) and ABF-STEM images in Figure 9d support the STEM-EDS results. Examined along the yellow dashed line in Figure 9d, the nanostructure region gives a stronger signal intensity of the Z-contrast in ABF-STEM (vice versa in HADDF-STEM image) than the matrix due to the higher degree of Pb content. A high-magnification ABF-STEM image focusing on the nanostructure shows severely distorted atomic arrangements compared with that on the matrix probably because of compositional discrepancy and edge dislocations around the former (Figure S15), consistent with the result in Figure 9c. The cross-sectional HAADF-STEM image taken on the nanostructure viewed down the b-axis clearly shows layered crystal structure of Na0.01(Sn0.96Pb0.04)0.99Se (Figure 9e). Because the contrast of HAADF-STEM image is approximately proportional to the square of the atomic number (Z),68 apparently bigger and brighter spheres can be assigned to heavier Sn or Pb atoms and weaker to Se atoms. Individual double atomic layers consisting of short (Sn, Pb)−Se bonds are stacked along the a-axis. The slabs are bound by long (Sn, Pb)···Se interactions. Figure 9f displays atomic resolution elemental mapping by STEM-EDS for the area marked by a red rectangle shown in Figure 9e. Signals of Se, Sn, and Pb atoms are depicted in red, yellow and blue colors, respectively, in the lower panels in Figure 9f, jointly creating the image in the upper panel. In the latter, Sn and Pb atoms in whitish color are verified to occupy the same atomic sites and be totally disordered while Se atom in red color takes its own positions (Figure 9f). The atomic arrangement generated by elemental mapping given here is consistent with crystal structure of SnSe. Note that segregated PbSe precipitates are not found therein. Because Na is much lighter than other constituent elements, its position could not be determined by the STEM-EDS analysis. Accordingly, we performed a STEM-EELS elemental scan profile at the atomic level to confirm the presence and the relative location of Na atoms in the Na0.01(Sn0.96Pb0.04)0.99Se sample. Figure 9g shows the Na L2,3 edge of the EELS spectrum at 31 eV verifying the presence of Na atoms in the sample,
Figure 10. Calculated κlatt with application of various scattering mechanisms in comparison with the experiment values for the samples of pristine SnSe, Pb alloyed Sn0.95Pb0.05Se, Na doped and Pb alloyed Na0.01(Sn0.95Pb0.05)0.99Se.
experimental values. The calculation results clearly demonstrate that experimental κlatt values for Na doped and Pb alloyed samples are located well below the κlatt curve based on the U +PD model over the entire temperature range, indicating multiple phonon scatterings occur. Considering both nanostructuring and dislocations observed in STEM studies in this work as the significant source of phonon scattering, our theoretical model fits well with experimental values. As a consequence, the synergistic effect of nanostructuring and dislocations could be the main reason for the reduced κlatt observed for Na-doped and Pb-alloyed samples, rather than single point defect scattering. However, reduction in κlatt is not as dramatic as shown in representative nanostructured bulk 10894
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Journal of the American Chemical Society thermoelectric materials20,24,66,74,75 and hydrothermally prepared Sn1−xPbxSe nanoparticles containing PbSe precipitates.45
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CONCLUSIONS We demonstrate that PbSe alloying causes phase transition of SnSe to occur at low temperatures, thereby decreasing the onset temperature of bipolar conduction. Interestingly, its effect on a ZT is ambivalent. When carrier concentration is too low as Sn1−xPbxSe (nH ∼ 1016−1017 cm−3), its earlier initiation predominantly contributes to increasing thermal conductivity at elevated temperatures and deteriorates a ZT. On the other hand, raising the nH to the level of ∼1019 cm−3 by 1% Na doping, dramatically enhances electrical conductivity, giving remarkably improved power factor. Simultaneously, lattice thermal conductivity is reduced by Na doping due to point defect scattering. As a result, the highest ZT of ∼1.2 among bulk-processed, polycrystalline SnSe thermoelectrics is achieved for Na0.01(Sn0.96Pb0.45)0.99Se, which is 2-fold and ∼40% magnitude larger than that of pristine SnSe and Na0.01Sn0.99Se, respectively. Na0.01(Sn0.95Pb0.05)0.99Se exhibits significantly improved average ZT of ∼0.5 in the temperature range from RT to 773 K, which is also the highest reported for polycrystalline SnSe thermoelectrics. Importantly, employing atomic resolution scanning transmission electron microscope, we also discover that PbSe alloying spontaneously generates substantial amounts of nanostructures of Sn1−xPbxSe with compositional fluctuation and lattice strains across the interfaces, rather than forming solid solutions as long believed, further reducing lattice thermal conductivity. Accordingly, impurity alloying can favorably affect both electrical and thermal transport behaviors and thereby serve as a new platform to enhance a ZT of SnSe thermoelectrics.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b05881. Data of elemental analyses, XRD, optical absorption, DFT calculations, Hall effect measurements, TE properties, and additional STEM images (PDF)
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AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
Kyunghan Ahn: 0000-0002-7806-8043 Chongjian Zhou: 0000-0002-2245-3057 Yung-Eun Sung: 0000-0002-1563-8328 In Chung: 0000-0001-6274-3369 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MSIP) (NRF-2015R1A5A1036133) and Institute for Basic Science (IBS-R006-D1 and IBS-R006-G1).
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REFERENCES
(1) Goldsmid, H. Thermoelectric Refrigeration; Springer: New York, 1964. 10895
DOI: 10.1021/jacs.7b05881 J. Am. Chem. Soc. 2017, 139, 10887−10896
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DOI: 10.1021/jacs.7b05881 J. Am. Chem. Soc. 2017, 139, 10887−10896
