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International audience; In this work, to improve the ZT value of Sb2Te3, a novel concept of inorganic-organic hybridization is presented and Sb2Te3/CH3NH3I hybrid thin films have been prepared via a post-annealing involved sequential sputtering/evaporation method. The results show that the crystallinity of the thin films is greatly improved after post-annealing with preferential orientations of (006) and (009) at an appropriate temperature, leading to an obvious enhancement of electrical conductivity. Additionally, the hybrid thin films show uniform nano-sized grainy structures and the organic component CH3NH3I has been successfully incorporated into the hybrid system in a stable state, thus resulting in an increase of the Seebeck coefficient and decrease of the thermal conductivity based on the energy filtering effect. As expected, a maximum room-temperature ZT value of 0.56 for the Sb2Te3/CH3NH3I hybrid thin film annealed at 573 K can be obtained. This outcome further demonstrates great potential for the inorganic-organic hybrid thin film in low temperature application scenarios.

Highlights A comprehensive review on the recent development of two-dimensional (2D) nanomaterials for bulk or thin-film thermoelectric materials, as well as composite filler, has been extensively presented.Development of micro-device platform and its application to study the inherent thermoelectric properties of individual single- and few-layer 2D nanomaterials., Thermoelectric generators have attracted a wide research interest owing to their ability to directly convert heat into electrical power. Moreover, the thermoelectric properties of traditional inorganic and organic materials have been significantly improved over the past few decades. Among these compounds, layered two-dimensional (2D) materials, such as graphene, black phosphorus, transition metal dichalcogenides, IVA–VIA compounds, and MXenes, have generated a large research attention as a group of potentially high-performance thermoelectric materials. Due to their unique electronic, mechanical, thermal, and optoelectronic properties, thermoelectric devices based on such materials can be applied in a variety of applications. Herein, a comprehensive review on the development of 2D materials for thermoelectric applications, as well as theoretical simulations and experimental preparation, is presented. In addition, nanodevice and new applications of 2D thermoelectric materials are also introduced. At last, current challenges are discussed and several prospects in this field are proposed.

Tin telluride (SnTe), with the same rock salt structure and similar band structure of PbTe alloys, was developed as a good thermoelectric material. In this work, SnTe quasi crystal was grown by vertical Bridgman method, with texturing degree achieved at 0.98. Two sets of samples, perpendicular and parallel to the growth direction, were cut to investigate thermoelectric properties. As a result, a carrier concentration (pH) of ~9.5 × 1020 cm−3 was obtained, which may have originated from fully generated Sn vacancies during the long term crystal growth. The relatively high Seebeck coefficient of ~30 μVK−1 and ~40 μVK−1 along the two directions was higher than most pristine SnTe reported in the literature, which leads to the room temperature (PF) for SnTe_IP and SnTe_OP achieved at ~14.0 μWcm−1K−2 and ~7.0 μWcm−1K−2, respectively. Finally, the maximum dimensionless figure of merit (ZT) values were around 0.55 at 873 K.

International audience; β-In2-xCuxS3 (0 ≤ x ≤ 0.40) bulks were prepared by solid-state reaction followed by pulsed current sintering. For x ≤ 0.20, X-ray diffraction indicated the formation of single phase, while the secondary phase, Cu2S, was observed for the samples prepared at x = 0.30 and x = 0.40. The Cu-doping allowed us to optimize the power factor in range of whole measured temperature by reducing the low temperature electrical resistivity. With the significant reduction in lattice thermal conductivity, a maximum ZT of 0.51 at 700 K and an average ZT of 0.31 from 300 to 700 K were obtained for the sample with x = 0.05, which values are 0.3 and 1.2 times higher than that of pristine sample.

β-In2S3 was developed as a potential thermoelectric material both experimental and computational. In this work, Mg doped In2S3 samples were prepared by conventional solid-state reaction method followed by pulsed current sintering. For In2-xMgxS3 (0 ≤ x ≤ 0.20), the prepared samples maintains tetragonal β-In2S3 phase while α-In2S3 with cubic structure were obtained at x ≥ 0.30. The both Mg and S concentrations were less than the nominal values, and the lack of S naturally lead to an effective increase of electron concentration in the bands crossing the Fermi level. The lattice thermal conductivity was reduced by the Mg-substitution, and the value of ZT reached 0.53 at 700 K for x = 0.05, which is 1.4 times larger than that of non-doped sample.

We report that K2Bi8Se13 exhibits multiple conduction bands that lie close in energy and can be activated through doping, leading to a highly enhanced Seebeck coefficient and a high power factor with elevated temperature. Meanwhile, the large unit cell, complex low symmetry crystal structure, and nondirectional bonding lead to the very low lattice thermal conductivity of K2Bi8Se13, ranging between 0.42 and 0.20 W m–1 K–1 in the temperature interval 300–873 K. Experimentally, we further support the low thermal conductivity of K2Bi8Se13 using phonon velocity measurements; the results show a low average phonon velocity (1605 ms–1), small Young’s modulus (37.1 GPa), large Gruneisen parameter (1.71), and low Debye temperature (154 K). A detailed investigation of the microstructure and defects was carried out using electron diffraction and transmission microscopy which reveal the presence of a K2.5Bi8.5Se14 minor phase intergrown along the side of the K2Bi8Se13 phase. The combination of enhanced power factor an...

P-type polycrystalline SnSe and K0.01Sn0.99Se are prepared by combining mechanical alloying (MA) and spark plasma sintering (SPS). The highest ZT of ≈0.65 is obtained at 773 K for undoped SnSe by optimizing the MA time. To enhance the electrical transport properties of SnSe, K is selected as an effective dopant. It is found that the maximal power factor can be enhanced significantly from ≈280 μW m−1 K−2 for undoped SnSe to ≈350 μW m−1 K−2 for K-doped SnSe. It is also observed that the thermal conductivity of polycrystalline SnSe can be enhanced if the SnSe powders are slightly oxidized. Surprisingly, after K doping, the absence of Sn oxides at grain boundaries and the presence of coherent nanoprecipitates in the SnSe matrix contribute to an impressively low lattice thermal conductivity of ≈0.20 W m−1 K−1 at 773 K along the sample section perpendicular to pressing direction of SPS. This extremely low lattice thermal conductivity coupled with the enhanced power factor results in a record high ZT of ≈1.1 at 773 K along this direction in polycrystalline SnSe.

In this study, we, for the first time, introduce p-type two-dimensional (2D) WSe2 nanoinclusions in n-type Pb/I co-doped SnSe matrix to form WSe2/SnSe p-n junctions. These p-n junctions act as energy barriers and effective phonon scattering sources, leading to a high figure-of-merit (ZT) of ∼1.35 at ∼790 K in n-type polycrystalline SnSe. First-principles density functional theory calculation results indicates that I-doping shifts Fermi level up into conduction bands of SnSe, making the system n-type behavior, while both Pb and I dopants act as point-defect-based short-wavelength phonon scattering centers. The introduced p-type 2D WSe2 nanoinclusions induce high-density WSe2/SnSe interfaces that act as p-n junctions, which block the electron carriers and rationally tune the carrier density, contributing to a high absolute Seebeck coefficient of ∼470.7 μV K-1 and a high power factor of ∼5.9 μW cm-1 K-2. Meanwhile, the dense phase boundaries and considerable lattice strains by the introduced 2D WSe2 nanoinclusions significantly strengthen the mid- and long-wavelength phonon scattering, giving rise to a much low thermal conductivity of 0.35 W m-1 K-1 and in turn a high ZT of ∼1.35. This study provides a new strategy to achieve high thermoelectric performance in n-type polycrystalline SnSe.

This study has investigated the thermoelectric property of a new sulfide compound Bi2SeS2, which is as an analogy of the traditional compound Bi2Te3 but with earth-abundant elemental composition. By investigating the electrical and thermal transport properties of 3 mol% Ag, Cu, AgBr and CuBr doped Bi2SeS2, it is revealed that Bi2SeS2 has the potential as a good thermoelectric material used at mid-temperature, benefiting from the lower thermal conductivity below 0.6 W/mK and higher power factor over 500 μW/mK2 at 723 K. A high ZT value of 0.71 can be finally obtained for the sample Bi2SeS2–3%CuBr at 723 K, which is two times higher than that of the undoped sample and also larger than those of most reported Bi2S3 and Bi2Se3 at the same temperature. The present work also suggests that Cu and CuBr are much more effective to improve the thermoelectric properties of Bi2SeS2 among all the dopants due to the obviously enhanced electrical conductivity. But the lattice thermal conductivity can be reduced significantly for the Ag doped sample, which indicates higher ZT value can be further expected via dual-doping to synergetic tuning of the electrical and thermal transport properties.

Zirconium pentatelluride (ZrTe5) single crystal has recently received significant attention because of its quantum electronic transport properties and is regarded as a promising candidate for low-temperature thermoelectric cooling and spintronic applications. However, single crystal of ZrTe5 has generally small sizes and can only be produced in small quantities using a complicated process, whereas ZrTe5 polycrystals are easily produced and their properties are easily adjusted. In this study, we focus on the magneto-transport properties at low temperatures of nanocomposites of ZrTe5 produced using both hand-milling and ball-milling processes to investigate the impact of the microstructure. The ball-milled sample shows a low thermal conductivity of 1 W m−1 K−1, which is almost a constant below 300 K. However, due to its small grain sizes, the electron mobility is significantly decreased, thus their thermoelectric performances are not as good as that of the hand-milled sample. Also, below 25 K, the resistivity and the Seebeck coefficient of the ball-milled sample are decreased, which is associated with the energy barrier at their grain boundaries. Due to the larger grain sizes and fewer defects in the hand-milled sample, the external magnetic field shows a significant influence on its thermoelectric properties at low temperatures. These results indicate that polycrystalline ZrTe5 with large grain sizes may exhibit similar quantum properties as those of single crystals.

In this work, stoichiometric Bi2Te3 thin films were successfully prepared on a polymer substrate by using single-source evaporation method. Rapid annealing was applied to improve the crystallinity and control the grain size of the thin films, thereby conferring them with high thermoelectric performance. As expected, the prepared thin films revealed a pure Bi2Te3 phase with nanosized grains. Consequently, a substantially enhanced power factor and reduced lattice conductivities resulting in a high ZT value of 0.5 was achieved. The results of the present work provide novel insights into the development of high-performance thermoelectric thin films through thermal control of crystallinity and grain size.

Polycrystalline p-type Cu

High ZT value of ∼1.2 at 773 K was achieved in n-type polycrystalline SnSe. The high thermoelectric performance derives from the low thermal conductivity of SnSe and enhanced electrical conductivity induced by Br doping and Pb alloying.

SnSe based materials have attracted much attention as high performance thermoelectric materials recently. In this work, polycrystalline SnSe1±x bulk samples were prepared by mechanical alloying (MA) combined with spark plasma sintering (SPS). When the Se stoichiometric ratio is above 1, the samples are p-type, the thermal conductivity of the samples decreased with increasing Se contents. When the Se stoichiometric ratio is under 1, the turning point from p-type to n-type was observed for all the samples, the SnSe0.8 bulk sample showed a total negative Seebeck coefficient at the measured temperature ranges from 323 to 773 K. Increases in the content of Se, lead to a considerable improvement of the power factor and a significant decrease of the thermal conductivity. Maximum ZT of ∼0.65 at 773 K was achieved for the p-type SnSe1.05 sample. n-Type SnSe0.8 polycrystals were also obtained with a peak ZT of 0.05 at 773 K.

We present in this manuscript that enhanced thermoelectric performance can be achieved in polycrystalline SnSe prepared by hydrothermal reaction and spark plasma sintering (SPS). X-ray diffraction (XRD) patterns revealed strong orientation along the [l 0 0] direction in bulk samples, which was further confirmed by microstructural observation through transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). It was noticed that the texturing degree of bulk samples could be controlled by sintering temperature during the SPS process. The best electrical transport properties were found in the sample which sintered at 450 °C in the direction vertical to the pressing direction, where the highest texturing degree and mass density were achieved. Coupled with the relatively low thermal conductivity, an average ZT of ∼ 0.38, the highest ever reported in pristine polycrystalline SnSe was obtained. This work set up a forceful example that a texture-control approach can be utilized to enhance the thermoelectric performance effectively.

Lead-free IV–VI semiconductors SnQ (Q = Te, Se, S) are deemed as promising thermoelectric (TE) materials. In this work, we designed a hydrothermal route to selectively synthesize single phase SnTe, SnSe and SnS nanopowders. For all three samples, the phase structure were characterized by x-ray diffraction, SnTe particles with octahedron structure and SnSe/SnS particles with plate-like shape were observed by field emission scanning electron microscopy and transmission electron microscopy, the formation mechanism was discussed in detail. Then, SnTe, SnSe and SnS nanopowders were densified by spark plasma sintering for investigating TE properties. It was noticed that SnSe and SnS exhibited remarkably anisotropy in both electrical and thermal properties attributed to the layered crystal structure. The highest ZT values 0.79 at 873 K, 0.21 at 773 K, and 0.13 at 773 K were achieved for SnTe, SnSe and SnS bulk samples, respectively.

We have prepared single crystals of type-VIII clathrate Ba8Ga15.9−xInxSn30.1 for x ≤ 0.60 by the Sn-flux method. As x is increased from 0 to 0.60, the lattice parameter increases by 0.2%, which is consistent with the larger covalent diameter for In than for Ga. The Seebeck coefficient α, electrical resistivity ρ, and thermal conductivity κ were measured in the temperature range from 300 K to 600 K. For all samples, α is negative, indicating the dominant charge carriers are electrons. With increasing x from 0 to 0.20, ρ and \(\left| \alpha \right|\) decrease by 50% and 30%, respectively. As a result, the lattice thermal conductivity at 300 K decreases from 0.58 W/Km to 0.41 W/Km, which is ascribed to enhancement of rattling of the guest atoms. It is found that the maximum of the dimensionless figure of merit ZT reaches 1.05 at 540 K for x = 0.20.

P-type SnSe compositing with 2D MoSe

We revisited and investigated a layer-structured thermoelectric material AgCrSe2, which has an extremely low thermal conductivity. After using both differential scanning calorimetry and a comparative laser flash method, we realized that the specific heat of this material, the main contributor to the reported low thermal conductivity, is unlikely to be way below the Dulong–Petit limit as revealed in the literature. Besides, our in situ X-ray diffraction pattern up to 873 K indicated the instability of AgCrSe2 over 723 K, where it begins to decompose into Cr2Se3 and Ag2Se. This unexpected decomposition phenomenon resulted in the gradual increment of specific heat and thermal diffusivity, hence the deterioration of the overall thermoelectric performance. We deliberately introduced Ag and Cr vacancies into the lattice for carrier concentration optimization and could achieve an optimal figure of merit of ZT ∼ 0.5 at 723 K in the nominal composition Ag0.96CrSe2 in the direction perpendicular to the sintering press. Our findings suggest that more thorough investigations are necessary to ensure that AgCrSe2 is a promising thermoelectric material.

Transition metal chalcogenide CuCrS2 powder was synthesized by mechanical alloying (MA) and then consolidated by spark plasma sintering (SPS) technique at 673–1073 K. The phase structure, microstructure and thermoelectric properties of samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and Seebeck coefficient/electrical conductivity measuring system, respectively. All the bulks indicated a single phase CuCrS2, while the high relative density over 90% were attained for the samples sintered at 873–1073 K. The electrical conductivity of bulk samples displayed a typical characteristic of semiconductor. With increasing measuring temperature, the conductive behaviour of bulk samples sintered over 973 K showed a semiconductor transformation from n-type to p-type due to the changes of main carrier type. The sample obtained by applying SPS at 873 K got the highest power factor 83.2 μW m−1 K−2, and the largest ZT value 0.11 at 673 K.

Single orthorhombic phase Bi 2− x Ag x S 3 ( x = 0–0.07) bulk materials were prepared by combining mechanical alloying (MA) with spark plasma sintering (SPS) technique. The microstructure and electrical transport properties were investigated with a special emphasis on the influence of Ag content. All the samples have a high density (91.1–95.9%) and homogeneous grains of about 100–500 nm. Optimizing the Ag content greatly improves the electrical conductivity and leads to a peak value of 2.8 × 10 3 S m −1 at 523 K for the Bi 1.99 Ag 0.01 S 3 composition. The absolute value of Seebeck coefficient shows an inversely varying trend to the electrical conductivity with Ag content, whereby a power factor at 523 K by adjusting Ag content is enhanced from 91 μW mK −2 for Bi 2 S 3 to 235 μW mK −2 for Bi 1.99 Ag 0.01 S 3 . The thermal conductivity of Bi 2− x Ag x S 3 ranges from 0.48 to 0.73 W mK −1 , which is lower than the reported values of Bi 2 S 3 compounds. An enhanced maximum ZT value 0.25 is achieved at 573 K for the Bi 1.99 Ag 0.01 S 3 sample, which is the highest value in Bi 2 S 3 system in literature. The present work reveals that the non-toxic and cheap Bi 2 S 3 based compound is a promising candidate for the thermoelectric applications.

Polycrystalline p-type Cu1.8S composites with WSe2 nanoparticles were fabricated by the mechanical alloying method combined with the spark plasma sintering technique. The Seebeck coefficient was significantly enhanced by the optimized carrier concentration, while the thermal conductivity was simultaneously decreased due to the refined grain and WSe2 nanoparticles. An enhanced Seebeck coefficient of 110 μV K−1 and a reduced thermal conductivity of 0.68 W m−1 K−1 were obtained for the Cu1.8S + 1 wt% WSe2 sample at 773 K, resulting in a remarkably enhanced peak ZT of 1.22 at 773 K, which is 2.5 times higher than that (0.49 at 773 K) of a pristine Cu1.8S sample. The cheap and environmentally friendly Cu1.8S-based materials with enhanced properties may find promising applications in thermoelectric devices.

P-type SnSe compositing with 2D MoSe2 materials have been prepared by the solid solution method followed by the spark plasma sintering technique. The total thermal conductivities of SnSe/MoSe2 composites were found to be higher than for pristineSnSe at room temperature; and the disparity between them becomes smaller at higher temperatures, where the low thermal conductivities remained. Both the carrier concentration and the carrier mobility were significantly improved after MoSe2 was introduced into the SnSe matrix along the direction perpendicular to the pressing direction, leading to an extraordinary enhancement in electrical transport performance. The maximum ZT of 0.5 was obtained at 773 K for SnSe + 1.5%MoSe2 along the direction perpendicular to the pressing direction; this value is 1.5 times as large as that of the pristine SnSe.

Single phase CuCrS2 powder is synthesized by ball milling of the elements (425 rpm, 40 h) followed by spark plasma sintering at 673—1073 K.

Type-VIII clathrate Ba8Ga16Sn30 is a promising thermoelectric material in the 400?600?K range. We report on the effects of Ge substitution for Sn on both crystal growth and thermoelectric properties. In a series of samples prepared from the initial composition of Ba8 Ga40 Sn30?X GeX (0???X???6), the size of single crystals reaches the maximum of 8?mm at X?=?0.5. The atomic composition of single crystals is found to be described as Ba8Ga15.9Sn30.1?xGex (0???x???4.73). Ge substitution results in the increase in the positive Seebeck coefficient and electrical resistivity by 1.3 and 2 times at the maximum, respectively, whereas the hole carrier density does not change substantially. Taken together with the thermal conductivity data, the maximum of the dimensionless figure of merit ZT reaches 0.87 at 540?K for x?=?0.07.