Fabrication and Thermoelectric Properties of Single-Crystal Argyrodite Ag 8 SnSe 6

The discovery of a record high figure of merit (ZT) of ≈2.6 associated with bulk SnSe has stimulated considerable enthusiasm in searching for 2D systems with similar high ZT. However, previously reported 2D thermoelectric (TE) materials generally possess very low ZT due to the high lattice thermal conductivity (κ L ) and/or small power factor (PF). Herein, a very high ZT (≈2.08) value associated with atomically thin 2D KAgSe nanosheet is reported, which also exhibits an unprecedented low intrinsic κ L (≈0.03 Wm −1 K −1 at 700 K for trilayer) and fairly large PF. The low κ L mainly stems from the high lattice anharmonicity induced by both the "interfacial shear slip" vibrations and the asymmetric "AgSe pair" vibrations from distorted AgSe 4 tetrahedrons. Meanwhile, the complete band-extrema alignment and coexistence of heavy and light bands result in an optimal Seebeck coefficient and electrical conductivity, thereby a large PF. This work suggests not only an alternative way to acquiring high lattice anharmonicity but also a highly competitive 2D TE candidate for wide applications.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.202001200.devices, recently much attention has been devoted to the development of 2D highefficiency TE materials with controllable thickness. [5][6][7] In general, the TE efficiency is characterized by the dimensionless figure of merit ZT = S 2 σT/(κ L + κ e ), where S, σ, T, κ L , and κ e are Seebeck coefficient, electrical conductivity, absolute temperature, lattice thermal conductivity and electronic thermal conductivity, respectively. Apparently, the high TE efficiency can be achieved by increasing the power factor (PF = S 2 σ) together with suppressing the sum of thermal conductivity (κ L + κ e ).Currently, the TE efficiency can be improved by two approaches: i) Tuning effective mass to improve the electronic transport and ii) increasing phonon scattering to suppress the lattice thermal transport. [8][9][10][11] For a given carrier concentration, a large carrier effective mass (m*) contributes to a large S, and the m* is proportional to the band effective mass (m b *) according to m* = N V 2/3 m b *, where N V refers to the band degeneracy. [10,12] A large m b *, however, will reduce the carrier mobility μ since m b 1/ * 2 µ ∝ (2D systems). [13][14][15][16] Hence, to optimize S 2 σ, it is important to attain high N V and moderate m b * simultaneously. A high N V can be achieved either by mixing two types of low-symmetric compounds into a reconstructed highly symmetric structure or by alloying/doping to converge different bands in the Brillouin zone. [8,[17][18][19] For electronic bands at the band edge, a moderate m b * can be realized in principle through element doping or strain engineering. [20] However, in practice, many compounds have limited doping capability and are easily damaged by strain; and the crystal symmetry of the reconstructed structure is uncontrollable. Regarding increasing phonon scattering, intro...

Section: Resultsmentioning

confidence: 77%

High ZT 2D Thermoelectrics by Design: Strong Interlayer Vibration and Complete Band‐Extrema Alignment

Zhang

Liu

Tao

et al. 2020

“…Copyright 2017, Cell Press. f) Room‐temperature carrier mobility and g) maximum zT for selected argyrodite‐type liquid‐like materials: Cu 8 GeSe 6 ,10a,d Cu 7 PSe 6 ,10b,c Ag 9 AlSe 6 ,11g Ag 8 SnSe 6 ,11a,b,66 Ag 9 GaSe 6 ,11e,f Ag 9 GaS 6 , Ag 8 SiSe 6 ,11d and Ag 8 GeSe 6 11h…”

Section: Typical Liquid‐like Te Materialsmentioning

confidence: 99%

“…A series of argyrodite compounds, such as Cu 8 GeSe 6 ,10a,d Cu 7 PSe 6 ,10b,c Ag 9 AlSe 6 ,11g Ag 8 SnSe 6 ,11a,b,66 Ag 9 GaSe 6 ,11e,f Ag 9 GaS 6 , Ag 8 SiSe 6 ,11d and Ag 8 GeSe 6 11h have been developed and demonstrated as promising TE materials with high zT s (cf. Figure g).…”

Section: Typical Liquid‐like Te Materialsmentioning

confidence: 99%

“…By alloying Ag and Te in Cu 8 GeSe 6 , the zT values are further improved to above unity at 800 K in Cu 7.6 Ag 0.4 GeSe 5.1 Te 0.9 10a. Besides, Ag 8 SnSe 6 single crystals, with a highly preferred orientation of (110), were grown by a vertical Bridgman technique . Compared to the polycrystals, Ag 8 SnSe 6 single crystals exhibit higher Hall mobility but a very similar lattice thermal conductivity.…”

Section: Typical Liquid‐like Te Materialsmentioning

confidence: 99%

See 1 more Smart Citation

Recent Advances in Liquid‐Like Thermoelectric Materials

Zhao

Qiu

Shi

et al. 2019

177

124

Thermoelectric technology has attracted great attention due to its ability to recover and convert waste heat into readily available electric energy. Among the various candidate materials, liquid‐like compounds have received tremendous research interest on account of their intrinsically ultralow lattice thermal conductivity, tunable electrical properties, and high thermoelectric performance. Despite their complex phase transitions and diverse crystal structures, liquid‐like materials have two independent sublattices in common: one rigid sublattice formed by immobile ions for the free transport of electrons and one liquid‐like sublattice consisting of highly mobile ions to interrupt the thermal transports. This review first outlines the common structural features of liquid‐like thermoelectrics, along with their unusual electron and phonon transport behaviors that well satisfy the concept of “phonon‐liquid electron‐crystal.” Next, some commonly adopted strategies for further improving their thermoelectric performance are highlighted. The main progress achieved in the typical liquid‐like TE materials is then summarized, with an emphasis on their diverse crystal structures, common characteristics, and unique transport properties. The recent understandings on the stability issue of liquid‐like TE materials are also introduced. Finally, an outlook is given for the liquid‐like materials with the aim to boost further development in this exciting scientific subfield.

“…[ 3 ] In the past few decades, many strategies have been employed to improve the zT value by improving α 2 σ through band engineering [ 4–8 ] and introducing resonant levels in the electron density of states by doping. [ 9,10 ] Meanwhile, nanostructuring, [ 11 ] nanocomposites, [ 12 ] complex crystal structuring, [ 13,14 ] and the phonon softening technique [ 15 ] have been employed to reduce κ tot . For power generation and refrigeration, various thermoelectric materials have been reported to have high zT ≥ 1, including Bi 2 Te 3 , [ 16,17 ] PbTe, [ 18,19 ] SnTe, [ 20,21 ] SnSe, [ 22 ] and SiGe.…”

Section: Introductionmentioning

confidence: 99%

High Thermoelectric Performance of ZnO by Coherent Phonon Scattering and Optimized Charge Transport

Acharya

Hwang

et al. 2021

ZnO is identified as a potentially attractive n‐type oxide thermoelectric material due to its abundance, nontoxicity, and a high degree of stability. However, working with ZnO is challenging due to its high thermal conductivity from its strong ionic bonds and low electrical conductivity due to its low charge concentrations. Here, it is demonstrated that the electrical and thermal transport properties of ZnO can be simultaneously improved via the successful doping of Al and ZnS coating. The ZnS coating in Al‐doped ZnO is observed and analyzed through microstructure and spectroscopic studies. The power factor for 1% ZnS‐coated Zn0.98Al0.02O is increased to ≈0.75 mW m−1 K−2 at 1073 K, 161% higher than pure ZnO. This enhancement in the power factor can be explained by the aliovalent Al3+ doping and modifications in intrinsic defects, leading to an increased carrier concentration. Interestingly, ZnS coating significantly reduces lattice thermal conductivity to ≈2.31 W m−1 K−1 at 1073 K for 2% ZnS‐coated Zn0.98Al0.02O, a 62% decrease over pure ZnO. This large reduction in lattice thermal conductivity can be elucidated based on coherent phonon scattering via Callaway's model. Overall, the figure of merit, zT, increases to 0.2 in 2% ZnS‐coated Zn0.98Al0.02O, which is 272% higher than pure ZnO at 1073 K.