Thermoelectric performance of monolayer Bi2Te2Se of ultra low lattice thermal conductivity

Physics Letters A 383 (2019) 125864

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Physics Letters A www.elsevier.com/locate/pla

Thermoelectric performance of monolayer Bi2 Te2 Se of ultra low lattice thermal conductivity Bin Xu a,∗ , Liangong Song a , Gaohui Peng a , Jing Zhang a , Shanshan Ma a , Yusheng Wang a , Yuanxu Wang b a b

North China University of Water Resources and Electric Power, Zhengzhou 450011, China Institute for Computational Materials Science, School of Physics and Electronics, Henan University, Kaifeng 475004, China

a r t i c l e

i n f o

Article history: Received 12 June 2019 Received in revised form 17 July 2019 Accepted 30 July 2019 Available online 5 August 2019 Communicated by R. Wu Keywords: Monolayer Bi2 Te2 Se Thermoelectric property Electronic structure Phonon dynamics Boltzmann transport theory Density functional theory

a b s t r a c t The electronic structure and thermoelectric properties of monolayer Bi2 Te2 Se were studied by density functional theory and semi-classical Boltzmann transport equation. The band gap with TB-mBJ can be improved for monolayer Bi2 Te2 Se. Monolayer Bi2 Te2 Se have ultra-low thermal conductivity comparing with other well-known two-dimensional materials. The monolayer Bi2 Te2 Se can improve electrical conductivities. ZT increases with increasing temperature for monolayer Bi2 Te2 Se. Comparing to GGA, TB-mBJ has larger ZT value in p-type doping. Monolayer Bi2 Te2 Se have larger ZT comparing with other well-known two-dimensional materials. Our calculated results show that our calculation greatly underestimates ZT value, therefore, monolayer Bi2 Te2 Se should have a higher ZT value. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Thermoelectric (TE) materials are of great interest because they represent a technology that can directly utilize the vast amounts of heat that are currently underutilized in an environmentally friendly manner. The conversion efficiency of thermoelectric materials depends on the figure of merit Z T = S 2 σ T /(κ L + κe ), where S, σ , T , κ L , and κe are Seebeck coefficient, electrical conductivity, temperature, lattice thermal conductivity, and electron thermal conductivity. Therefore, an excellent thermoelectric material should have a high Seebeck coefficient, a high electrical conductivity, and a low thermal conductivity. However, the interdependence between these transport coefficients makes optimization a challenging task. In recent years, various types of thermoelectric materials have been developed, for instance, filled-skutterudites [1,2], layered cobalt oxides [3,4], half-Heusler alloys [5–7], Zintl compounds [8], clathrates [9,10], Bi-Te-Sb family [11,12], 2D structures of layered materials [13–18], and so on. The large-scale application of the Bi2 Te3 -based alloys have been limited due to their relatively low efficiency. Over the past few decades, various measures have been taken, such as textur-

*

Corresponding author. E-mail address: [email protected] (B. Xu).

https://doi.org/10.1016/j.physleta.2019.125864 0375-9601/© 2019 Elsevier B.V. All rights reserved.

ing [19–21] and nanostructures [22–24] to obtain improved ZT. Thermoelectric materials based on Bi2 (Te,Se)3 have long been the best materials for Peltier coolers because they have the highest ZT near and below room temperature [25–27]. However, these materials are still under investigation as researchers continue to strive for higher thermoelectric performance. The lattice parameters Bi2 Te2 Se is a = b = 4.298 Å and c = 29.774 Å [28], which is consisted of layers of five atomic planes as quintuple layers (QLs). Thermoelectric properties can be enhanced by mechanically stripping single QL samples [29,30]. In recent years, by chemical vapor deposition, ultra-films of non-encapsulated layered Bi2 Te2 Se have been explored [31,32]. Bi2 Te2 Se with a band gap of 0.8 eV has excellent high maneuverability and air stability. In Bi2 Te3 , partial substitution of Te atoms with Se atoms results in the formation of the ternary compound Bi2 Te2 Se [33]. Bi2 Te2 Se has outstanding thermal and dynamic stability [34]. More importantly, Bi2 Te2 Se have high electron mobility and moderate band gaps, and their optical absorption covers almost the entire incident solar spectrum. Layer-dependent band gaps have been also observed, indicating that they are highly tunable in future electronic applications [34]. Because Bi-Te-Sb family have excellent thermoelectric properties near room temperature, therefore they were widely used in thermoelectric devices [35, 36]. The thermoelectric properties of Bi-Te-Sb family have studied extensively experimentally and theoretically [12,27,37,38], so the

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Fig. 1. Top (a) and side (b) views of monolayer Bi2 Te2 Se. The black lines show the primitive unit cell.

thermal transport of two-dimensional Bi-Te-Sb has attracted the interest of many researchers [39,40]. However, we have not found out any research on the thermoelectric properties of monolayer Bi2 Te2 Se, and the electronic structures and thermoelectric properties of monolayer Bi2 Te2 Se will be systematically investigated with the generalized gradient approximation (GGA) [41] and the Tran–Blaha modified Becke–Johnson (TB-mBJ) [42]. 2. Computational methods Our calculations were performed by using the full potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K code [43]. The exchange-correlation potentials are GGA and TB-mBJ. We take Rmt Kma equal to 9.0 and make the expansion up to l = 10 in the muffin tins spheres (MT). Nonoverlapping MT sphere radii of 2.5 a.u. were used for Bi, Te and Se. We have used 100000 k-points in the first Brillouin zone. Selfconsistency is considered to be achieved when the total energy difference between succeeding iterations is less than 10−5 Ry per formula unit. The calculated method for the transport properties of crystalline solids is based on the semi-classical Boltzmann theory [44] and the rigid band method. We use the semi-classical Boltzmann theory to perform transport calculations from electronic structures, as implemented in the BoltzTraP code [45]. The relaxation time τ is inserted as a constant, and doping is treated within the rigid band approximation [46]. This ab initio approach has been successful in rationalizing compounds [4–6,14,15,17,18,47–51]. Using the finite displacement method, phonon spectrum are obtained by using the 6 × 6 × 1 supercell in the Phonopy package [52]. We use the phonon Boltzmann transport equation to obtain the lattice thermal conductivity κl , as implemented in ShengBTE [53]. This method successfully predicted the phonon thermal conductivity [54–57]. 3. Results and discussion The monolayer Bi2 Te2 Se is cut through the (0 0 1) plane of the Bi2 Te2 Se crystal with the R-3M space group (No. 166), and we add a vacuum slab of 15 Å in the direction perpendicular to the nanoplate (z direction). Te atoms are located in the first and fifth layers, Bi atoms are located in the second and fourth layers, and Se atoms are located in the third layers. For monolayer Bi2 Te2 Se, the optimized lattice parameters are given as a = b = 4.298 Å, which is very close to the experimental bulk values [28,33]. For monolayer Bi2 Te2 Se the side and top views of are shown Fig. 1. Bi, Se and Te atomic layers alternate along the z axis. The calculated electronic band structure, total density of states (TDOS) and partial density of states (PDOS) of monolayer Bi2 Te2 Se are plotted in Fig. 2. The band structures were first calculated with GGA for Bi2 Te2 Se monolayer, which is known to produce correct band shapes, but underestimates the bandgap of semiconductors.

In order to remedy such a potential deficiency, the more accurate the Tran–Blaha modified Becke–Johnson (TB-mBJ) method [42] was further employed to obtain the accurate electronic band structures. The results show that the band structure is in agreement with TDOS in Fig. 2(b). Near the Fermi level TDOS with TB-mBJ are larger than those with GGA, which caused a larger Seebeck coefficient. The results show that the main forms of the valence band structure are similar under the different potentials. We find that the conduction band minimum (CBM) of monolayer Bi2 Te2 Se appears at  point, while the valence band maximum (VBM) locates between  and . The results show that the band gap is 1.018 eV and 0.801 eV with TB-mBJ and GGA, respectively, which shows that the band gap with TB-mBJ can be improved for monolayer Bi2 Te2 Se. The stronger repulsive TB-mBJ potential did not affect the electronic orbit by comparing the band structure. The study found that a group of bands above the Fermi level shifted to higher energy with the TB-mBJ scheme. TB-mBJ potential reproduces very well the step structure and derivative discontinuity of the exact exchange potential by using only semi local quantities. Therefore, the calculated material band gap is more in agreement with the experimental results [58,59]. It is worth noting that the shape of the band structure we calculated is in good agreement with the previously reported monolayer Bi2 Te2 Se structures [34], which proves the reliability of our calculation method. Our calculated band gap values with GGA are very close to the other theoretical results for monolayer Bi2 Te2 Se with GGA [34], however, our calculated band gap values with TB-mBJ is lower than the other theoretical results for monolayer Bi2 Te2 Se at the HSE06 level [34]. Our calculated band gaps of monolayer Bi2 Te2 Se values are much higher than the bulk experimental and theoretical values of Bi2 Te2 Se [60–63], which can be attributed to the well-known quantum confinement effect [64]. The calculated band gaps of monolayer Bi2 Te2 Se values are much higher than monolayer Bi2 Te3 and monolayer Bi2 Se3 [65–67]. Due to the lack of sufficient experimental and theoretical data for monolayer Bi2 Te2 Se, we can’t compare with each other. The results of the above contradictions need further research by other experiments and theories. In Fig. 2(c) we also calculated PDOS for monolayer Bi2 Te2 Se to further clarify the nature of the band structure. DOS of valence band maximum mainly originate from Se 4p and Te 5p states with admixtures from Bi 6s states, furthermore, there is a strong hybrid between Se 4p and Te 5p states. DOS of the conduction band minimum mainly originate from Te 4p and Bi 6p states, and there is a strong hybrid between Te 4p and Bi 6p states. From the above results, we can see that the increase of bandgap is mainly due to the upward movement of Te 4P and Bi 6p states for TB-mBJ scheme. Fig. 3(a) shows the calculated phonon spectrum along highsymmetry lines as obtained from the aforementioned harmonic IFCs. There is no imaginary frequency for the phonon band structure of monolayer Bi2 Te2 Se, which guarantees its dynamic stability. There are fifteen vibrational modes for the five atoms in a primitive unit cell, which includes three acoustic phonon modes (color lines LA, TA, and ZA) and twelve optical phonon modes. Comparing to the Bi2 Te3 monolayer [11,66], some important differences are found out. The maximum frequency of acoustic phonon modes for monolayer Bi2 Te2 Se is larger than that for monolayer Bi2 Te3 [15], which illustrates a higher group velocity of acoustic phonon modes. The main profiles of the calculated phonon spectrum are similar to other computational results [34,63]. In contrast to monolayer Bi2 Se3 and monolayer Bi2 Te3 , we find some significant differences. On one hand, the maximum frequency of optical phonon modes is 154.1 cm−1 for monolayer Bi2 Te2 Se, which is close to other computational result 152.7 cm−1 and 154.5 cm−1 [34,63]. Our calculated maximum frequency of optical phonon modes for monolayer Bi2 Te2 Se is lower than that 183.7 cm−1 for monolayer

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Fig. 2. Calculated band structures (a), total (b) and partial (c) density of states for monolayer Bi2 Te2 Se.

Fig. 3. Calculated phonon dispersion relations (a) along high symmetry directions for monolayers Bi2 Te2 Se. The red, black, blue, and olive lines represent the ZA, TA, LA, and optical modes, respectively. Temperature dependence of phonon thermal conductivity with other experimental results (b). (For interpretation of the colors in the figures, the reader is referred to the web version of this article.)

Bi2 Se3 and higher than 136.0 cm−1 , 144.7 cm−1 and 146.2 cm−1 for monolayer Bi2 Te3 [14,68,66]. The above results show that the maximum frequency of optical phonon modes for that monolayer Bi2 Te2 Se is between monolayer Bi2 Se3 and monolayer Bi2 Te3 , which is considered that monolayer Bi2 Te2 Se κl should be between monolayer Bi2 Se3 and monolayer Bi2 Te3 . Our calculated acoustic modes 54.9 cm−1 of monolayer Bi2 Te2 Se values are in good agreement with other theoretical results 54.1 cm−1 [63]. However, our calculated acoustic modes of monolayer Bi2 Te2 Se values are lower than other theoretical results 61.1 cm−1 [34], which are mainly because we have taken more K points and larger supercell, and our calculated phonon spectrums have no imaginary frequency, comparing with Wang et al.’s calculation results [34]. Fig. 3(b) plots calculated lattice thermal conductivity κl of monolayer Bi2 Te2 Se as a function of temperature, comparing to previous results. The calculated lattice thermal conductivity κl of monolayer Bi2 Te2 Se is shown in Fig. 3(b) as a function of temperature, comparing to previous results. Our calculations lattice thermal conductivity of monolayer Bi2 Te2 Se is 1.961 W m−1 K−1 , 1.465 W m−1 K−1 and 1.755 W m−1 K−1 at 300 K, 400 K and 500 K respectively, which are much lower than those of bulk Bi2 Te3 in molecular dynamics simulations [69], and the monolayer Bi2 Te3 [66]. Our calculations lattice thermal conductivity of monolayer Bi2 Te2 Se is higher than Other experimental results [70]. However, our calculated lattice thermal conductivity of monolayer Bi2 Te2 Se is very close to Bi2 Te3 films in molecular dynamics simulations [34]. In the whole temperature region considered, we see that the lattice thermal conductivity of monolayer Bi2 Te2 Se decreases monotonically with temperature T, suggesting that Umklapp phonon scattering dominates three-phonon interactions [71]. The lattice thermal conductivities of the monolayer Bi2 Te2 Se are low comparing with other well-known two-dimensional materials [72–76]. The low lattice thermal conductivities of the monolayer

Bi2 Te2 Se are due to their low phonon velocities, strong anharmonicity. As shown in Fig. 2(b), the calculated κl at room temperature for monolayer Bi2 Te2 Se is about 1.961 W/m K, which is close to 1.654 W/m K for monolayer Bi2 Te3 [77], 1.2 W/m K for Bi2 Te3 [78], and 1.5 W/m K for PbTe [79]. Furthermore, our calculated κl at room temperature for monolayer Bi2 Te2 Se is lower than the most excellent thermoelectric materials SnSe 3.27 W/m K [16], however, our calculated κl for monolayer Bi2 Te2 Se is higher than the experimental bulk material Bi2 Te3 [70] and Bi2 Te2 Se [80]. Low-dimensional Bi2 Te2 Se should reduce the lattice thermal conductivity, which indicates that our calculations overestimate the lattice thermal conductivity. With the increase of temperature, the phonon thermal conductivity of monolayer Bi2 Te2 Se monolayer becomes more low, e.g. 0.649 W m−1 K−1 at 900 K. Therefore, like Bi2 Te3 family, monolayer Bi2 Te2 Se would be promising TE materials due to low lattice thermal conductivity. The relaxation time τ depends on the temperature, electronic energy and carrier concentration and generally is treated as a constant for convenience. Due to lack of available data for the relaxation time of monolayer Bi2 Te2 Se, furthermore, there is also no available relaxation time for bulk materials Bi2 Te2 Se. Therefore, for monolayer Bi2 Te2 Se we have adopted the monolayer Bi2 Te2 value as a reliable approximation by noting that a Te atom is replaced by a Se atom in monolayer Bi2 Te3 [66], and Zhang’s fitted exponential function: τ = 25.05 exp(−T/93.65) + 1.11 [66]. Fig. 4(a) and 4(b) show that Seebeck coefficient and electrical conductivities as a function of temperature for monolayer Bi2 Te2 Se. For monolayer Bi2 Te2 Se, the maximum ZT value is obtained between carrier concentration 5 × 1019 cm−3 and 10 × 1019 cm−3 . A carrier density of ∼ 1019 cm−3 is commonly regarded as a better concentration for the optimal ZT [27]. So monolayer Bi2 Te2 Se should be a promising thermoelectric material. Therefore, we studied the thermoelectric properties of monolayer Bi2 Te2 Se between

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Fig. 4. Seebeck coefficient and electrical conductivities as a function of temperature for monolayer Bi2 Te2 Se.

carrier concentration 5 × 1019 cm−3 and 10 × 1019 cm−3 . Within the constant scattering time approximation the Seebeck coefficient is directly determined by the electronic structure with no adjustable parameter. The Seebeck coefficient of promising thermoelectric materials should be greater than 200 μV K−1 [16]. Furthermore, the maximum values of monolayer Bi2 Te2 Se are larger than 200 μV K−1 , which indicates that the value of ZT may be higher than other thermoelectric materials. It is found that with the increasing of temperature the absolute values of S increase at the same carrier concentration, indicating no bipolar conduction, however, with the increasing of carrier concentration the decrease at the same temperature. For monolayer Bi2 Te2 Se, the valence band edge is much flatter than the conduction band edge in Fig. 2(a), indicating that the effective mass of the holes is greater than that of the electron. Therefore, p-doping produces a higher s value than n-doping, and S of p-type is higher than those of ntype below 6 × 10−19 cm−1 . From Fig. 4, the Seebeck coefficient increases from 117.312 μV/K at 300 K to 340.759 μV/K at 1500 K at carrier concentration 6 × 1019 cm−3 . S of p-type monolayer Bi2 Te2 Se is higher than bulk Bi2 Te2 Se [81] and Bi2 Te2.7 Se0.3 [81] Within a certain range of carrier concentrations, which indicates that monolayer Bi2 Te2 Se has good thermoelectric properties. The main profiles of GGA are in consistent with those of TB-mBJ. S of TB-mBJ is much higher than GGA for p-type doping around 800k, and S of TB-mBJ is higher than GGA on n-type doping. The Seebeck coefficient of TB-mBJ is far higher than that of GGA for p-type doping near 800 K, however, the Seebeck coefficient of TB-mBJ is higher than that of GGA for n-type doping. The electrical conductivities σ as a function of temperature for monolayer Bi2 Te2 Se is potted in Fig. 4(c) and 4(d) for monolayer Bi2 Te2 Se. It is found that σ of monolayer Bi2 Te2 Se is much higher than that of the bulk Bi2 Te2 Se [80] and Bi2 Te2.7 Se0.3 [81], and the maximum values of σ reach 1.547 × 105 (W m)−1 μV/K at the carrier concentrations of 1.0 × 1020 cm−3 at room temperature, indicating that the single layer can improve thermoelectric performance. σ of monolayer Bi2 Te2 Se decreases as the temperature increases. However, regardless of the doping type, σ increases as the carrier concentration increases for monolayer Bi2 Te2 Se. For

monolayer Bi2 Te2 Se, in the carrier concentrations 6 ∼ 9 × 1019 , σ of n-type is higher than that of p-type in Fig. 4(c) and 4(d). The conduction band is much delocalized than the valance band around the Fermi level (see Fig. 2(a)), which results in higher σ for n-type doping [16]. When the doping concentration is 1020 cm−3 , the difference is large between σ of p-type doping. σ of TB-mBJ is larger than that of GGA, showing that the large band gaps increase the electrical conductivities within a certain range of energy gap. σ of monolayer Bi2 Te2 Se are low comparing with other wellknown two-dimensional materials [67], which shows that monolayer Bi2 Te2 Se is the excellent thermoelectric materials. Electronic thermal conductivity κe as a function of temperature is plotted in Fig. 5(a) and 5(b) for monolayer Bi2 Te2 Se. κe as a function of temperature is shown in Fig. 5(a) and 5(b) with different carrier concentration for monolayer Bi2 Te2 Se. κe increases rapidly with increasing temperature. Regardless of n-type or p-type doping, κe increases with increasing carrier concentration. κe for p-type is lower doping than that for n-type doping. In addition to the tractor, TB-mBJ has higher value of κe comparing to GGA, indicating that TB-mBJ can’t reduce κe . It is noted that κe increases rapidly with increasing temperature and increases slowly with the increasing doping. Our calculated κe of monolayer Bi2 Te2 Se is higher than that of the bulk Bi2 Te2 Se [81] and Bi2 Te2.7 Se0.3 [81], which shows that our calculations overestimate κe . ZT as a function of temperature is plotted in Fig. 5(c) and 5(d) with GGA and TB-mBJ scheme. The Shengbte method [53] estimates the lattice thermal conductivity more accurately, comparing to the method of Cahill’s model [82], and in turn, our estimated ZT value is more accurate estimated monolayer Bi2 Te2 Se ZT is more accurate for monolayer Bi2 Te2 Se. It is found that ZT increases with increasing temperature for monolayer Bi2 Te2 Se. The above results are mainly due to the increase in electron thermal conductivity and Seebeck coefficient with increasing temperature. The results show that p-doping produces a higher ZT value than n-doping below 900 K. It is shown that p-type doping is better than n-type doping below 900 K, the p-type optimal ZT value is about 0.853 and the n-type optimal ZT value is about 0.695 at 900 K. For p-type doping, TB-mBJ has a larger ZT value than GGA, mainly be-

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Fig. 5. Electronic thermal conductivity and ZT as a function of temperature for monolayer Bi2 Te2 Se.

cause TB-mBJ scheme has a higher Seebeck coefficient and a higher thermal conductivity. However, comparing to GGA, TB-mBJ has a small change in ZT for n-type doping, which is mainly caused by the Seebeck coefficient and thermal conductivity. At 1200 K the optimal ZT value of monolayer Bi2 Te2 Se is 0.9 in the carrier concentrations of 9.0 × 1019 cm−3 with TB-mBJ in Fig. 5(d), and at 1500 K the optimal ZT value is 0.97 in the carrier concentrations of −1.0 × 1020 cm−3 with GGA. The ZT value of monolayer Bi2 Te2 Se we calculated is lower than that of the bulk Bi2 Te2 Se [81] and Bi2 Te2.7 Se0.3 [81], which is due to our overestimation of the sum. ZT values of monolayer Bi2 Te2 Se we calculated are lower than that of the bulk Bi2 Te2 Se [81] and Bi2 Te2.7 Se0.3 [81], which is due to our overestimation of κe and κl . ZT values we calculated are much higher than the available theoretical values of monolayer MoS2 and MoSe2 [83,84], but very close to those of monolayer MoS2 and MoSe2 [84]. Due to much lower κι , monolayer Bi2 Te2 Se has an obviously higher ZT. For the GGA and TB-mBJ functional at 323 K, our calculated ZT values is 0.202 and 0.151 in the hole concentration of 6.27 × 1019 cm−3 respectively, however, ZT values appear 0.388 and 0.427 respectively at 523 K. For the GGA and TB-mBJ functional at 323 K, ZT values achieve 0.507 and 0.4004 in a hole concentration of 6.27 × 1019 cm−3 respectively by employing thermal conductivity κ of Ref. [81], however at 523 K, ZT values appear 0.585 and 0.674 respectively. ZT value by employing thermal conductivity κ of ref 67 for the GGA and TB-mBJ functional is 6.15 and 6.46 times that of our calculated ZT value at 300 K at a carrier concentration of 6.0 × 1019 cm−3 respectively; nevertheless, for the GGA and TB-mBJ functional, ZT value is 4.79 and 5.18 times that of our calculated ZT value at 1500 K respectively. The above results show that our calculation greatly underestimates ZT value, therefore, monolayer Bi2 Te2 Se should have a higher ZT value. 4. Conclusion We present the calculations of the transport properties on the Heusler-type compound Bi2 Te2 Se using the full-potential linearized augmented plane-wave method and the semi-classical Boltzmann

theory. The band gap with TB-mBJ can be improved for monolayer Bi2 Te2 Se. There is no imaginary frequency for the phonon band structure of monolayer Bi2 Te2 Se, which guarantees its dynamic stability. Monolayer Bi2 Te2 Se have ultra-low thermal conductivity comparing with other well-known two-dimensional materials. Our calculation shows that the monolayer can improve electrical conductivities. TB-mBJ can’t reduce the electronic thermal conductivity. Our calculated κe of monolayer Bi2 Te2 Se is higher than that of the bulk Bi2 Te2 Se and Bi2 Te2.7 Se0.3 , which shows that our calculations overestimate κe . It is found that ZT increases with increasing temperature for monolayer Bi2 Te2 Se. Comparing to GGA, TB-mBJ has larger ZT value in p-type doping. Monolayer Bi2 Te2 Se have larger ZT comparing with other well-known two-dimensional materials. Our calculated results show that our calculation greatly underestimates ZT value, therefore, monolayer Bi2 Te2 Se should have a higher ZT value. Acknowledgements This research was sponsored by the National Natural Science Foundation under Grant No. U1404108, Innovative Talents of Universities in Henan Province (17HASTIT013), Basic and frontier technology research program of Henan (162300410056). References [1] T. Dahal, Q. Jie, Y.C. Lan, C.F. Guo, Z.F. Ren, Thermoelectric performance of Ni compensated cerium and neodymium double filled p-type skutterudites, Phys. Chem. Chem. Phys. 16 (2014) 18170–18175. [2] M.M. Mallick, S. Vitta, Giant enhancement in high-temperature thermoelectric figure-of-merit of layered cobalt oxide, LiCoO2 , due to a dual strategy co-substitution and lithiation, Inorg. Chem. 56 (2017) 5827–5838. [3] S.D. Chen, Y. He, A. Zong, Y. Zhang, M. Hashimoto, B.B. Zhang, S.H. Yao, Y.B. Chen, J. Zhou, Y.F. Chen, S.K. Mo, Z. Hussain, D.H. Lu, Z.X. Shen, Large thermopower from dressed quasiparticles in the layered cobaltates and rhodates, Phys. Rev. B 96 (2017) 081109(R). [4] Vikram, J. Kangsabanik, Enamullaha, A. Alam, Bismuth based half-Heusler alloys with giant thermoelectric figures of merit, J. Mater. Chem. A 5 (2017) 6131–6139.

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