SnSe Crystals Research Articles

Giant Room-Temperature Power Factor in p-Type Thermoelectric SnSe under High Pressure.

Materials that can efficiently convert heat into electricity are widely utilized in energy conversion technologies. The existing thermoelectrics demonstrate rather limited performance characteristics at room temperature, and hence, alternative materials and approaches are very much in demand. Here, it is experimentally shown that manipulating an applied stress can greatly improve a thermoelectric power factor of layered p‐type SnSe single crystals up to ≈180 µW K−2 cm−1 at room temperature. This giant enhancement is explained by a synergetic effect of three factors, such as: band‐gap narrowing, Lifshitz transition, and strong sample deformation. Under applied pressure above 1 GPa, the SnSe crystals become more ductile, which can be related to changes in the prevailing chemical bonding type inside the layers, from covalent toward metavalent. Thus, the SnSe single crystals transform into a highly unconventional crystalline state in which their layered crystal stacking is largely preserved, while the layers themselves are strongly deformed. This results in a dramatic narrowing in a band gap, from E g = 0.83 to 0.50 eV (at ambient conditions). Thus, the work demonstrates a novel strategy of improving the performance parameters of chalcogenide thermoelectrics via tuning their chemical bonding type, stimulating a sample deformation and a band‐structure reconstruction.

Realization of adjustable electron concentration and its effect on electrical- and Seebeck-property of n-type SnSe crystals

Manipulation of carrier types in SnSe crystals is quite advantageous to fabricate SnSe-based homojunction devices such as thermoelectric modules. However, tuning the n-type charge carrier at an optimal level of SnSe is quite challenging because of its natural p-type without intentional doping. Here, we report the realization of the n-type SnSe through halogens or Ce doping. Importantly, heavily electron doped SnSe single crystals (∼1019 cm−3) can be obtained by Ce-doping through the Bridgeman method. The electrical conductivity of as-grown SnSe crystals evolves from thermally activated behavior to the metallic one when the electron concentrations are increased from 1016 to 1019 cm−3. Remarkably, the power-factor and electronic quality factor of heavily electron Ce-doped SnSe crystals can reach 1.59 and 0.44 μW cm−1 K−2 at 300 K, respectively, which is one of the best thermoelectric n-type SnSe. This work suggests that Ce-doping through the Bridgeman method is an ideal route for further improving the thermoelectric property of n-type SnSe crystals.

Realizing High Thermoelectric Performance in p-Type SnSe Crystals via Convergence of Multiple Electronic Valence Bands.

SnSe crystals have gained considerable interest for their outstanding thermoelectric performance. Here, we achieve excellent thermoelectric properties in Sn0.99-xPbxZn0.01Se crystals via valence band convergence and point-defect engineering strategies. We demonstrate that Pb and Zn codoping converges the energy offset between multiple valence bands by significantly modifying the band structure, contributing to the enhancement of the Seebeck coefficient. The carrier concentration and electrical conductivity can be optimized, leading to an enhanced power factor. The dual-atom point-defect effect created by the substitution of Pb and Zn in the SnSe lattice introduces strong phonon scattering, significantly reducing the lattice thermal conductivity to as low as 0.284 W m-1 K-1. As a result, a maximum ZT value of 1.9 at 773 K is achieved in Sn0.93Pb0.06Zn0.01Se crystals along the bc-plane direction. This study highlights the crucial role of manipulating multiple electronic valence bands in further improving SnSe thermoelectrics.

Understanding the electrical transports of p-type polycrystalline SnSe with effective medium theory

SnSe crystal is one of the most potential thermoelectric materials due to its excellent transport properties. The electrical conductivity of p-type SnSe crystal gradually decreases with increasing temperature, while that of the polycrystalline sample shows a completely different trend. We revealed that below 400 K, the existence of plentiful grain boundaries dominates the carrier scattering and determines the electrical transport of p-type polycrystalline SnSe, while at high temperatures, from 400 to 800 K, the electrical transport and conductivity curve still requires a clear understanding. In this study, by conducting the high-temperature synchrotron radiation x-ray diffraction (SR-XRD) measurements and refining the patterns, we obtained the phase fractions of Pnma and Cmcm phases in this temperature range. Using the derived single-phase theoretical electrical conductivity, combined with the self-consistent effective medium theory, the electrical conductivity of p-type polycrystalline SnSe between 400 and 800 K was simulated. This study provides a perspective and simulation method to understand the electrical transport of p-type polycrystalline SnSe.

Synthesis and thermoelectric properties of Bi-doped SnSe thin films* *Project supported by the National Key Research and Development Program of China (Grant No. 2017YFB0405702) and the National Natural Science Foundation of China (Grant No. 51672179).

Bi doped n-type SnSe thin films were prepared by chemical vapor deposition (CVD) and their structure and thermoelectric properties were studied. The x-ray diffraction patterns, x-ray photoelectron spectroscopy, and microscopic images show that the prepared SnSe thin films were composed of pure SnSe crystals. The Seebeck coefficients of the Bi-doped SnSe were greatly improved compared to that of undoped SnSe thin films. Specifically, Sn0.99Bi0.01Se thin film exhibited a Seebeck coefficient of –905.8 μV⋅K−1 at 600 K, much higher than 285.5 μV⋅K−1 of undoped SnSe thin film. Further first-principles calculations reveal that the enhancement of the thermoelectric properties can be explained mainly by the Fermi level lifting and the carrier pockets increasing near the Fermi level due to Bi doping in the SnSe samples. Our results suggest the potentials of the Bi-doped SnSe thin films in thermoelectric applications.

Theoretically comparative study of spectrally selective solar absorbers in concentrated solar-thermoelectric generators working at high temperature

A spectrally selective solar absorber (SSA) made from alternating tungsten and SiO2 layers is an important component in a concentrated solar-thermoelectric generator (CSTEG). However, the highly efficient SSA consisting of a high number of tungsten and SiO2 layers may require complicated fabrication procedures, thereby raising the overall cost of the CSTEG. In this study, we have theoretically optimized SSAs consisting of a different number of layers, and the performances of CSTEGs installed with different optimized SSAs and thermoelectric materials (Si80Ge20 alloys, SnSe crystals) are theoretically investigated. It is found that two SSAs, one with only one layer of SiO2 glass (88 nm) coated on a tungsten reflective plane, and the other with three alternating layers of SiO2(92nm)/tungsten(7nm)/SiO2(78nm) coated on a tungsten reflective plane, have the highest average solar absorptance equal to 0.7050 and 0.9999, respectively. To reach the same steady-state temperature (Th), the one-layer SSA requires higher optical concentrations (Copt) than the three-layer SSA. The CSTEG with Si80Ge20 alloys requires much higher optical Copt than the CSTEG with SnSe crystals in order to reach the same Th because of much a higher rate of heat flux. At Th=800deg⁡C, the CSTEG with Si80Ge20 alloys have the output electrical power per unit cross-sectional area of TEG about 6 times higher than that of the CSTEG with SnSe crystals. The net efficiency of CSTEG with SnSe crystals could reach 25.6% if the electrical conductivity of its p-type thermoelectric leg could be enhanced to be comparable to the electrical conductivity of its n-type thermoelectric leg.

Lattice Strain Leads to High Thermoelectric Performance in Polycrystalline SnSe.

Polycrystalline SnSe materials with ZT values comparable to those of SnSe crystals are greatly desired due to facile processing, machinability, and scale-up application. Here manipulating interatomic force by harnessing lattice strains was proposed for achieving significantly reduced lattice thermal conductivity in polycrystalline SnSe. Large static lattice strain created by lattice dislocations and stacking faults causes an effective shortening in phonon relaxation time, resulting in ultralow lattice thermal conductivity. A combination of band convergence and resonance levels induced by Ga incorporation contribute to a sharp increase of Seebeck coefficient and power factor. These lead to a high thermoelectric performance ZT ∼ 2.2, which is a record high ZT reported so far for solution-processed SnSe polycrystals. Besides the high peak ZT, a high average ZT of 0.72 and outstanding thermoelectric conversion efficiency of 12.4% were achieved by adopting nontoxic element doping, highlighting great potential for power generation application at intermediate temperatures. Engineering lattice strain to achieve ultralow lattice thermal conductivity with the aid of band convergence and resonance levels provides a great opportunity for designing prospective thermoelectrics.

As-doped SnSe single crystals: Ambivalent doping and interaction with intrinsic defects

We performed ambivalent doping study on single crystals of two sets, $\mathrm{Sn}{\mathrm{Se}}_{1\ensuremath{-}x}{\mathrm{As}}_{x}$ and ${\mathrm{Sn}}_{1\ensuremath{-}x}{\mathrm{As}}_{x}\mathrm{Se}$, with the aim to explore the interaction of doping species with intrinsic defects. We found that As atoms substitute preferentially for Se atoms in both sets forming the extrinsic substitutional point defect ${\mathrm{As}}_{\mathrm{Se}}$. In the first set, As lowers the concentration of Sn vacancies, ${V}_{\mathrm{Sn}}$, by an order of magnitude compared to undoped stoichiometric SnSe crystal. The remaining Sn vacancies are preferentially coordinated with As atoms. Importantly, a very low concentration of As led to healing process of hosting structure in terms of intrinsic point defects and eventual $\mathrm{Sn}{\mathrm{Se}}_{2}$ inclusions. This is reflected in an increase of the Hall mobility and drop of the Hall concentration. In the second set, the concentration of Sn vacancies markedly increases upon doping in contrast to the first set. Additionally, the coordination of Sn vacancies by As atoms is less evident due to the high concentration of vacancies. The substitutional defect ${\mathrm{As}}_{\mathrm{Se}}$ is a deep-level defect that produces no free carriers at room temperature. Moreover, the coupling of ${V}_{\mathrm{Sn}}$ to ${\mathrm{As}}_{\mathrm{Se}}$ defects increases their activation energy. This results in an unprecedentedly low Hall concentration in SnSe which stays below ${10}^{16}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{--3}$ for $x=0.0075$. The present study indicates that doping of SnSe is a rather complex process that generally includes a strong interaction of doping atoms with the hosting structure. On the other hand, such doping allows adjustment of the type and concentration of defects. The present study reveals a general tendency of point defects to clustering, which modifies the properties of point defects markedly.

Oxidation-induced thermopower inversion in nanocrystalline SnSe thin film

Given the growing demand for environmentally friendly energy sources, thermoelectric energy conversion has attracted increased interest as a promising CO2-free technology. SnSe single crystals have attracted attention as a next generation thermoelectric material due to outstanding thermoelectric properties arising from ultralow thermal conductivity. For practical applications, on the other hand, polycrystalline SnSe should be also focused because the production cost and the flexibility for applications are important factors, which requires the systematic investigation of the stability of thermoelectric performance under a pseudo operating environment. Here, we report that the physical properties of SnSe crystals with nano to submicron scale are drastically modified by atmospheric annealing. We measured the Seebeck effect while changing the annealing time and found that the large positive thermopower, + 757 μV K−1, was completely suppressed by annealing for only a few minutes and was eventually inverted to be the large negative value, − 427 μV K−1. This result would further accelerate intensive studies on SnSe nanostructures, especially focusing on the realistic device structures and sealing technologies for energy harvesting applications.

Influence of pressure on phase transition, electronic and thermoelectric properties of SnSe

Single-crystalline Tin Selenide (SnSe) is a good thermoelectric material with record-breaking figure of merit. Here, we reveal the influence of pressure on structures, electronic and thermoelectric properties of SnSe crystals by combining CALYPSO and first-principle calculation. In our results, the experimental synthetic phase Pnma-SnSe is the most stable structure at ambient pressure. However, a phase transition from Pnma to Cmcm occurs at about 22 GPa. More than 22 GPa, some new crystal structures (Pm3¯m-, C2/m- and Cmmm-SnSe) are revealed. The calculated phonon spectrum indicates their dynamic stabilities. From the energy band structures and density of states, it is found that only Pnma-SnSe phase is a semiconductor with bandgap of 0.79 eV which is benefit to suppress thermal activation of electrons in the conduction band, while the other phases are metallic. Moreover, our calculated Seebeck coefficient, conductivity versus time ratio and thermal conductivity indicate that SnSe in Pnma and Cmcm phases possess good thermoelectric properties with the ZT values of 0.9 and 0.8, respectively.

Thermal decomposition study of SnSe single crystals

Single crystals of SnSe are grown by direct vapor transport technique. The elemental composition of as-grown crystals determined by dispersive X-ray energy analysis showed the crystals to be slightly selenium rich and tin deficient. The X-ray powder diffraction showed the crystal to possess orthorhombic unit cell structure. The lattice parameters are in good match with reported data. The surface micro-topography showed flat surfaces with layer edges, stating crystal growth to happen by mechanism of layer growth. The thermo-curves of the as-grown single crystals are recorded in temperature range of 308 K and 1053 K in inert nitrogen atmosphere. The curves are recorded for three thermal heating rates of 10, 15 and 20 K/min. The recorded thermogravimetric curves showed single-step decomposition. The single peak in differential thermogravimetric analysis further confirmed single-step decomposition. The differential thermal analysis of the samples showed initial exothermic followed by endothermic and at last exothermic nature. The thermodynamic variables determined from the thermo-curves data employing Kissinger relation stated absorption of heat leading to non-spontaneous decomposition.

Estimation of the potential performance in p-type SnSe crystals through evaluating weighted mobility and effective mass

It has been proved that the thermoelectric performance of p-type SnSe crystals can be optimized through enhancing carrier concentration. The calculations of electronic band structure elucidate that this approach can be interpreted by including multiple valence bands. To better estimate the potential performance, we proposed the transport properties for p-type SnSe crystals and analyzed the weighted mobility from the experimental results. The weighted mobility approaches ∼600 cm2V−1s−1 when the carrier concentration is as high as ∼6.3 × 1019 cm−3. Combined with obtained lattice thermal conductivity, through rising carrier concentration, the quality factor B possesses significant improvements of ∼235% and 138% at 300 K and 773 K, respectively. Through comparing weighted mobility and Hall mobility, two effective mass values ∼0.9 me and 1.8 me can be derived using carrier concentrations. It is expected that the ZT ∼1.0 at 300K and ZT ∼2.9 at 773 K can be obtained when the carrier concentration of ∼8.0 × 1019 cm−3 and the effective mass ∼1.8 me were selected. This work provides an alternative way to comprehend the performance optimization in thermoelectric community.

Dynamic Epitaxial Crystallization of SnSe2 on the Oxidized SnSe Surface and Its Atomistic Mechanisms.

Surface oxidation of SnSe sharply reduces its thermoelectric properties though the bulk single-crystalline materials of SnSe claim the record high zT values. Investigation on the oxidation behaviors of SnSe together with the subsequent phase transition and element migration is fundamentally important to maintaining the ultrahigh zT values, with a potential for further improvement. In this work, we disclose the dynamic epitaxial crystallization of SnSe2 on the amorphous surface of partially oxidized SnSe crystals and the corresponding atomistic mechanisms via transmission electron microscopy (TEM). It is revealed that the thermally annealed amorphous surface crystallized to SnO2 and SnSe2 in the outermost and secondary layers, respectively, forming distinctive SnSe/SnSe2/SnO2 multilayer heterostructures with specific orientation relationships between the two selenides. By means of in situ scanning TEM (STEM), the dynamic epitaxial crystallization process of SnSe2 was revealed when the oxidized SnSe surface was subjected to electron beam irradiation. Through the atomic-scale characterization and modeling analysis, we find that the exposed dangling Se diatoms on the SnSe surface serve as nucleation sites for lateral epitaxial crystallization of SnSe2. The same valence and similar coordination configuration of Se atoms in these two phases are supposed to facilitate the sharing of Se atoms, with lattice distortions in the SnSe2/SnSe interface. These findings are valuable for understanding the surface oxidation behavior of SnSe and revealing the interface structures of SnSe2/SnSe heterojunctions and also offering new routes for SnSe-related multilayer or heterostructure system design.

Effect of Te doping on thermoelectric properties in vast off-stoichiometric tin-rich SnSe crystals

As a promising candidate for next generation thermoelectric material, SnSe has attracted extensive attentions due to its intrinsic ultralow thermal conductivity and adjustable electrical transport properties. However, unavailability of Sn-rich crystal leaves a gap in the Sn-deficient, stoichiometric, and Sn-rich series to study the effects of point defects and bonding nature evolution on thermal transport properties in SnSe compound. Also, historic references recorded contradicting electrical transport properties in polycrystalline Te-doped SnSe samples. This study shifted the research object from polycrystalline sample to single crystal to avoid the disturbance of grain boundary scattering and texture on the effects of Te doping. Vast off-stoichiometric Sn-rich Te-doped SnSe crystals were obtained by a self-flux method. The evolution of phase structure and Sn/Se atomic ratio were recorded and correlated. Our results confirmed that the Te atoms are incorporated in the Se sub-lattice, and have the potential to change the cation-anion atomic ratio in Sn-rich crystals. Crystal Sn1·17Se0·97Te0.027 with low concentration of Te shows rather robust thermodynamic properties, while crystal Sn1·10Se0·88Te0.115 is unstable and prone to decompose above 297 °C. Te incorporation and aroused defects are capable of decreasing the hole concentration as other donor dopants, at least at low doping level. Te doping has the potential to change both the band gap and the band structure, and may lead to convergence of multiple valence bands. An increase of thermal conductivity with increase of cation-anion atomic ratio Sn/(Se + Te) was recorded in Te-doped crystals, along with much lower thermal conductivity compared with the pristine one. The results indicates that the excess Sn in Sn-rich samples may cause a change in the bonding nature between cross-plane Sn–Se slabs and makes the material behaves less layer-like and shows more three dimensional thermal transport characterization as in Br-doped SnSe crystals.

Ultrahigh Average ZT Realized in p-Type SnSe Crystalline Thermoelectrics through Producing Extrinsic Vacancies.

Crystalline SnSe has been revealed as an efficient thermoelectric candidate with outstanding performance. Herein, record-high thermoelectric performance is achieved among SnSe crystals via simply introducing a small amount of SnSe2 as a kind of extrinsic defect dopant. This excellent performance mainly arises from the largely enhanced power factor by increasing the carrier concentration high as 6.55 × 1019 cm-3, which was surprisingly promoted by introducing extrinsic SnSe2 even though pristine SnSe2 is an n-type conductor. The optimized carrier concentration promotes a deeper Fermi level and activates more valence bands, leading to an extraordinary room-temperature power factor ∼54 μW cm-1 K-2 through enlarging the band effective mass and Seebeck coefficient. As a result, on the basis of simultaneously depressed thermal conductivity induced from both Sn vacancies and SnSe2 microdomains, maximum ZT values ∼0.9-2.2 and excellent average ZT > 1.7 among the working temperature range are achieved in Na doped SnSe crystals with 2% extrinsic SnSe2. Our investigation illustrates new approaches on improving thermoelectric performance through introducing defect dopants, which might be well-implemented in other thermoelectric systems.

Modulating electrical transport properties of SnSe crystal to improve the thermoelectric power factor by adjusting growth method

The SnSe crystal is a promising candidate in the field of thermoelectric materials. In order to elucidate basic physics in the SnSe system, here we report the heavily hole doping SnSe single crystals by the flux method (using alkali halide as solvent). Compared to bad-metal behavior of SnSe grown by the Bridgeman method, the flux-grown SnSe crystals show the metallic conductive behavior consistent with the Landau Fermi liquid (resistivity ρ ∼ T2) with temperatures ranging from 2 to 300 K. Combined angle-resolved photoemission spectroscopy and empirical Landau Fermi liquid theory, screening lengths λ of Coulomb electron–electron interaction U of SnSe grown by the flux method are 6.6 Å and 6.1 eV, which are much higher than those of normal metals. Remarkably, the excellent electrical conductivity (870 S/cm) of the SnSe crystal grown by the flux method at room temperature is attributed to the higher hole concentration (∼3.8 × 1019 cm−3) and large mobility (152.2 cm2 V−1 s−1). Meanwhile, these SnSe crystals still have large Seebeck coefficients (∼190 μV/K). Thus, the SnSe crystals grown by the flux method have an ultrahigh power factor [∼31.5 μW/(cm K2)] at room temperature, which is ten times larger than that of SnSe crystals grown by the Bridgeman method and as best as currently reported results. Our work shows a method for growing heavily hole-doped SnSe crystals, which provides a platform for understanding the electrical properties and improving its thermoelectric performance.

Growth of large size SnSe crystal via directional solidification and evaluation of its properties

SnSe crystal is an attractive Ⅳ-Ⅵ thermoelectric (TE) material. In this work, a Ø25 × 50 mm3 SnSe crystal was prepared using a directional solidification method. The as-grown crystal has standard Pnma structure at room temperature. The band gap Eg is determined to be 0.96 eV which agrees well with the theoretical value. Thermal expansion measurements show that SnSe crystal has significant anisotropic expansion behaviors. The linear thermal expansion along <010> and <100> directions are positive but is negative in <001> direction. Pnma-Cmcm phase transition has remarkable influence on thermal expansion performance. The electronic and thermal transport properties of SnSe crystal along (100) plane are investigated. It is found the highest power factor PF = 6.20 μWcm−1K−2 and the lowest total thermal conductivity ktot = 0.46 W/m-K both occur near Pnma-Cmcm phase transition temperature. Finally, the maximum ZT = 1.05 appears around 800 K suggesting SnSe crystal is a candidate for middle-temperature TE material.

The crystallization, thermodynamic and thermoelectric properties of vast off-stoichiometric Sn–Se crystals

The higher thermal conductivity in Sn-rich samples suggests that the enhancement of the covalent bonding across Sn–Se slabs along a direction dominated the phonon scattering process in SnSe.

Fermi-surface dynamics and high thermoelectric performance along the out-of-plane direction in n-type SnSe crystals

Fermi-surface dynamics drives the thermoelectric performance of n-type SnSe along the out-of-plane direction.

Optimization of sodium hydroxide for securing high thermoelectric performance in polycrystalline Sn1 − xSe via anisotropy and vacancy synergy

AbstractThe morphology and composition are two key factors to determine the thermoelectric performance of aqueously synthesized tin selenide (SnSe) crystals; however, their controlling is still under exploring. In this study, we report a high figure‐of‐merit (ZT) of ∼1.5 at 823 K in p‐type polycrystalline Sn1 − xSe resulted from a synergy of morphology control and vacancy optimization, realized by carefully tuning the sodium hydroxide (NaOH) concentration during solvothermal synthesis. After a comprehensive investigation on various NaOH concentrations, it was found that an optimized NaOH amount of 10 mL with a concentration of 10 mol L−1 can simultaneously achieve a large average crystal size and a high Sn vacancy concentration of ∼2.5%. The large microplate‐like crystals lead to a considerable anisotropy in the sintered pellets, and the high Sn vacancy level contributes to an optimum hole concentration to the level of ∼2.3 × 1019 cm−3, and in turn a high power factor of ∼7.4 μW cm−1 K−2 at 823 K, measured along the direction perpendicular to the sintering pressure. In addition, a low thermal conductivity of ∼0.41 W m−1 K−1 is achieved by effective phonon scattering at localized crystal imperfections including lattice distortions, grain boundaries, and vacancy domains, as observed by detailed structural characterizations. Furthermore, a competitive compressive strength of ∼52.1 MPa can be achieved along the direction of high thermoelectric performance, indicating a mechanically robust feature. This study provides a new avenue in achieving high thermoelectric performance in SnSe‐based thermoelectric materials.image