Bulk SnTe Research Articles

Enhanced Thermoelectric Performance of Nanostructured 2D Tin Telluride.

A lot of experimental studies are conducted on theoretically predicted thermoelectric 2D materials. Such materials can pave the way for charging ultra-thin electronic devices, self-charging wearable devices, and medical implants. This study systematically explores the thermoelectric attributes of bulk and 2D nanostructured Tin Telluride (SnTe), employing experimental investigations and theoretical analyses based on semiclassical Boltzmann transport theory. The bulk SnTe is synthesized through flame melting, while the 2D SnTe is produced via liquid phase exfoliation. The comprehensive assessment of thermoelectric properties integrated experimental measurements utilizing a Physical Property Measurement System and theoretical calculations from the BoltzTraP code. Experimental thermoelectric studies show a high ZT of 0.17 for 2D SnTe when compared to bulk (0.005) at room temperature. This rise in ZT is due to the high Seebeck coefficient and low thermal conductivity of nanostructured 2D SnTe. Density functional theory (DFT) studies reveal the contribution of the density of states (DOS) and energy bandgap in enhancing the Seebeck coefficient and lowering thermal conductivity by interface scattering.

Reproduced out-of-plane ferroelectricity in monolayer SnTe van der Waals heterostructures

Due to the shrinking in size of nonvolatile memory devices, the two-dimensional ferroelectric van der Waals (vdW) heterostructures have received huge attention. However, it is still difficult to maintain the out-of-plane (OOP) ferroelectricity. In this work, we have theoretically investigated the relationship between the ferroelectricity and the strain of bulk and few-layer SnTe by first-principles calculations. The results indicate that the α-SnTe can exist stably within the strain between −6% and 6%, and the complete OOP polarization occurs within the strain between −4% and −2%. Unfortunately, the OOP polarization disappears while the bulk α-SnTe is thinned to a few layers. However, the complete OOP polarization recurs in monolayer SnTe/PbSe vdW heterostructures, which is due to the strong interface coupling. Our findings provide an effective way to enhance ferroelectric performance, which is beneficial for the design of ultra-thin ferroelectric devices.

Structural phase transition of monochalcogenides investigated with machine learning

As machine learning becomes increasingly important in science and engineering, it holds the promise to provide a universal approach applicable to various systems to investigate their crystalline phase transitions. Here, we build and train accurate artificial neural networks that can distinguish tiny energy difference, which is crucial to predict the crystalline phase transitions. Employing the trained artificial neural networks in Monte Carlo simulations as the surrogate energy function, we apply this approach to monochalcogenides, including bulk and two-dimensional monolayer SnTe and GeTe, investigating their crystalline phase transitions. The machine-learning approach, when viewed as providing a universal mathematical structure, can be transferred to the investigation of other materials when the training data set generated with ab initio methods are available. Moreover, the machine-learning approach resolves the difficulties associated with constructing the effective Hamiltonian for monochalcogenides, showing great potential with its accuracy and efficiency.

One-step fabrication of a bulk SnTe thermoelectric material with excellent performance through self-propagating high-temperature synthesis under a high-gravity field

One-step fabrication of bulk SnTe thermoelectric material with excellent performance through self-propagating high-temperature synthesis under high-gravity field.

Low temperature CVD of thermoelectric SnTe thin films from the single source precursor, [nBu3Sn(TenBu)

This work has demonstrated that the single source precursor [nBu3Sn(TenBu)], bearing n-butyl groups and containing the necessary 1 : 1 Sn : Te ratio, facilitates growth of continuous, stoichiometric SnTe thin films. This single source CVD precursor allows film growth at significantly lower temperatures (355-434 °C at 0.01-0.05 Torr) than required for CVD from SnTe powder. This could be advantageous for controlling the surface states in topological insulators. The temperature-dependent thermoelectric performance of these films has been determined, revealing them to be p-type semiconductors with peak Seebeck coefficient and power factor values of 78 μV K-1 and 8.3 μW K-2 cm-1, respectively, at 615 K; comparing favourably with data from bulk SnTe. Further, we have demonstrated that the precursor facilitates area selective growth of SnTe onto the TiN regions of SiO2/TiN patterned substrates, which is expected to be beneficial for the fabrication of micro-thermoelectric generators.

Anion-exchanged porous SnTe nanosheets for ultra-low thermal conductivity and high-performance thermoelectrics

Thermoelectric materials with efficient heat-energy conversion have been widely considered as potential electrical energy sources. Herein we propose an anion-exchanged porous SnTe nanosheets (NSs) as a high-performance thermoelectric material with ultra-low thermal conductivity. The solution-phase reaction mechanisms for porous SnTe NSs are proposed, and the reaction conditions are carefully manipulated in order to determine the optimal morphology and excellent thermoelectric performance. The porous SnTe NSs, which consist of extremely small nanoparticles of only ~3 nm in size with defective shapes, lead to a substantial reduction in the lattice thermal conductivity and a higher Seebeck coefficient compared to bulk SnTe, caused by the introduced nanointerfaces, pores and defects. An impressive ZT of ~1.1 at 923 K is achieved without atomic doping process, which is as many as ~2.8 times higher than that of bulk SnTe. This proposed strategy provides an insight to represent an unconventional method for the production of the porous SnTe NSs that can potentially be employed in practical high-performance thermoelectric applications.

Enhanced Spontaneous Polarization in Ultrathin SnTe Films with Layered Antipolar Structure.

2D SnTe films with a thickness of as little as 2 atomic layers (ALs) have recently been shown to be ferroelectric with in-plane polarization. Remarkably, they exhibit transition temperatures (Tc ) much higher than that of bulk SnTe. Here, combining molecular beam epitaxy, variable temperature scanning tunneling microscopy, and ab initio calculations, the underlying mechanism of the Tc enhancement is unveiled, which relies on the formation of γ-SnTe, a van der Waals orthorhombic phase with antipolar inter-layer coupling in few-AL thick SnTe films. In this phase, 4n - 2 AL (n = 1, 2, 3…) thick films are found to possess finite in-plane polarization (space group Pmn21 ), while 4n AL thick films have zero total polarization (space group Pnma). Above 8 AL, the γ-SnTe phase becomes metastable, and can convert irreversibly to the bulk rock salt phase as the temperature is increased. This finding unambiguously bridges experiments on ultrathin SnTe films with predictions of robust ferroelectricity in GeS-type monochalcogenide monolayers. The observed high transition temperature, together with the strong spin-orbit coupling and van der Waals structure, underlines the potential of atomically thin γ-SnTe films for the development of novel spontaneous polarization-based devices.

Intrinsic Origin of Enhancement of Ferroelectricity in SnTe Ultrathin Films.

Previous studies showed that, as ferroelectric films become thinner, their Curie temperature (T_{c}) and polarization below T_{c} both typically decrease. In contrast, a recent experiment [Chang etal., Science 353, 274 (2016)SCIEAS0036-807510.1126/science.aad8609] observed that atomic-thick SnTe films have a higher T_{c} than their bulk counterpart, which was attributed to extrinsic effects. We find, using first-principles calculations, that the 0-K energy barrier for the polarization switching (which is a quantity directly related to T_{c}) is higher in most investigated defect-free SnTe ultrathin films than that in bulk SnTe, and that the 5-unit-cell (UC) SnTe thin film has the largest energy barrier as a result of an interplay between hybridization interactions and Pauli repulsions. Further simulations, employing a presently developed effective Hamiltonian, confirm that freestanding, defect-free SnTe thin films have a higher T_{c} than bulk SnTe, except for the 1-UC case. Our work, therefore, demonstrates the possibility to intrinsically enhance ferroelectricity of ultrathin films by reducing their thickness.

Finite element analysis of temperature and stress fields during the selective laser melting process of thermoelectric SnTe

A three dimensional nonlinear transient thermo-mechanically coupled finite element model (FEM) is introduced to analyze the temperature and stress fields during the selective laser melting (SLM) of thermoelectric (TE) SnTe. Considering the powder-to-solid transition and the latent heats of melting and vaporization, we simulate the temperature distribution near the molten pool and the residual stress inside the printed material at room temperature under different laser processing parameters. The calculated results indicate that vaporization gradually weakens as the laser power decreases and the scanning speed increases. Within the optimum forming window, the residual stress rises with the increasing laser power and decreasing scanning speed. The thermal stress mainly concentrates in the middle of the first scanning track, the terminal of each scanning track, and the edges of the forming surface, which is well consistent with the experimental results. This work has laid a significant foundation for rapid manufacturing of bulk SnTe thermoelectric materials using the selective laser melting process and will serve as a guide for laser processing of other thermoelectrics and perhaps even complete thermoelectric modules.

Enhanced visible light photodegradation of pharmaceutical pollutant, warfarin by nano-sized SnTe, effect of supporting, catalyst dose, and scavengers

Improvement of new nanophotocatalysts enable to decompose the pharmaceutical pollutants with the aid of solar energy is of particular importance. In this research, the ability of SnTe photocatalyst for degradation of warfarin was enhanced and the separation difficulties of the used photocatalyst, from solutions was removed by immobilization of the photocatalyst on a suitable porous support. A novel nano-sized photocatalyst was prepared by coupling of SnTe on the surface of SBA-15 support. Characterization of the synthesized photocatalyst (SnTe@SBA-15) was performed by different methods including XRD, TEM, TGA, FT-IR, EDS and BET techniques. The map of constituent elements was also prepared. The results indicated that the activity of SnTe photocatalyst was significantly enhanced after immobilization on the support and lower catalyst dose was needed. The visible light irradiation was more effective than UV irradiation. The degradation process was kinetically fast, and the equilibrium was established within 10 min. Separation of the synthesized photocatalyst from the solution was much easier than the bulk SnTe. The regenerated photocatalyst retained more than 90% of its initial efficiency.

Enhanced thermoelectric performance of SnTe thin film through designing oriented nanopillar structure

SnTe, an ideal lead-free substitute of PbTe, recently has attracted much attention for thermoelectric heat to electricity generation. Previous reports mainly focused on band engineering and all-scale phonon scattering for enhancing the thermoelectric performance of bulk SnTe. Herein, we adopt strategy of interface engineering to design oriented nanopillar structure in SnTe film via one-step thermal evaporation method. The analysis of the electrical and thermal properties illustrates that the SnTe film with highly oriented growth shows enhanced thermoelectric performance. The excellent properties are mainly attributed to the special nanopillar structure, which may facilitate the electrons' transport while significantly scatter the phonons. The maximum power factor of 19.8 μW cm−1 K−2 and the thermal conductivity of 3.54 W m−1 K−1 are obtained. This work provides a promising approach to enhance the thermoelectric performance of SnTe film, which can be easily extended to other thermoelectric films.

Experimental evidence for topological surface states wrapping around a bulk SnTe crystal

We demonstrate that the metallic topological surface states wrap on all sides the 3D topological crystalline insulator SnTe. This is achieved by studying oscillatory quantum magneto-transport and magnetization at tilted magnetic fields which enables us to observe simultaneous contributions from neighbouring sample sides. Taking into account pinning of the Fermi energy by the SnTe reservoir we successfully describe theoretically the de Haas-van Alphen oscillations of magnetization. The determined \pi-Berry phase of surface states confirms their Dirac fermion character. We independently observe oscillatory contributions of magneto-transport and magnetization originating from the bulk SnTe reservoir of high hole density. It is concluded that the bulk and surface Landau states exist in parallel. Our main result that the bulk reservoir is surrounded on all sides by the topological surface states has an universal character.

Observation of the inverse spin Hall effect in the topological crystalline insulator SnTe using spin pumping

Topological crystalline insulator SnTe is a promising material for future spintronics applications because of the strong spin-orbit coupling and surface states protected by the mirror symmetry of the crystal. In this paper, using a high-quality epitaxial (001)-oriented Fe/SnTe/CdTe/ZnTe heterostructure grown on GaAs, we successfully observe the inverse spin Hall effect in SnTe induced by spin pumping, which is confirmed by detailed analyses of the dependence of the electromotive force on the microwave power, magnetic-field angle, and temperature. By a rough estimation, a relatively large spin Hall angle of \ensuremath{\sim}0.01 is obtained for bulk SnTe at room temperature. This large value may be partially caused by the surface states. Our result suggests that SnTe can be used for efficient spin-charge current conversion.

Topological crystalline insulator SnTe nanoribbons

Topological crystalline insulators are systems in which a band inversion that is protected by crystalline mirror symmetry gives rise to nontrivial topological surface states. SnTe is a topological crystalline insulator. It exhibits p-type conductivity due to Sn vacancies and Te antisites, which leads to high carrier density in the bulk. Thus growth of high quality SnTe is a prerequisite for understanding the topological crystalline insulating behavior. We have grown SnTe nanoribbons using a solution method. The width of the SnTe ribbons varies from 500nm to 2µm. They exhibit rock salt crystal structure with a lattice parameter of 6.32Å. The solution method that we have adapted uses low temperature, so the Sn vacancies can be controlled. The solution grown SnTe nanoribbons exhibit strong semiconducting behavior with an activation energy of 240meV. This activation energy matches with the calculated band gap for SnTe with a lattice parameter of 6.32Å, which is higher than that reported for bulk SnTe. The higher activation energy makes the thermal excitation of bulk charges very difficult on the surface. As a result, the topological surfaces will be free from the disturbance caused by the thermal excitations

Enhancement of the thermoelectric performance of bulk SnTe alloys via the synergistic effect of band structure modification and chemical bond softening

Seebeck coefficient of SnTe is largely enhanced by large band effective mass or decrease of energy separation through synergistic effect including resonance level and band convergence.

Diameter dependent thermoelectric properties of individual SnTe nanowires.

The lead-free compound tin telluride (SnTe) has recently been suggested to be a promising thermoelectric material. In this work, we report on the first thermoelectric study of individual single-crystalline SnTe nanowires with different diameters ranging from ∼218 to ∼913 nm. Measurements of thermopower S, electrical conductivity σ and thermal conductivity κ were carried out on the same nanowires over a temperature range of 25-300 K. While the electrical conductivity does not show a strong diameter dependence, the thermopower increases by a factor of two when the nanowire diameter is decreased from ∼913 nm to ∼218 nm. The thermal conductivity of the measured NWs is lower than that of the bulk SnTe, which may arise from the enhanced phonon - surface boundary scattering and phonon-defect scattering. Temperature dependent figure of merit ZT was determined for individual nanowires and the achieved maximum value at room temperature is about three times higher than that in bulk samples of comparable carrier density.

Synthesis of SnTe nanoplates with {100} and {111} surfaces.

SnTe is a topological crystalline insulator that possesses spin-polarized, Dirac-dispersive surface states protected by crystal symmetry. Multiple surface states exist on the {100}, {110}, and {111} surfaces of SnTe, with the band structure of surface states depending on the mirror symmetry of a particular surface. Thus, to access surface states selectively, it is critical to control the morphology of SnTe such that only desired crystallographic surfaces are present. Here, we grow SnTe nanostructures using vapor-liquid-solid and vapor-solid growth mechanisms. Previously, SnTe nanowires and nanocrystals have been grown [Saghir et al. Cryst. Growth Des. 2014, 14, 2009-2013; Safdar et al. Cryst. Growth Des. 2014, 14, 2502-2509; Safdar et al. Nano Lett. 2013, 13, 5344-5349; Li et al. Nano Lett. 2013, 13, 5443-5448]. In this report, we demonstrate the synthesis of SnTe nanoplates with lateral dimensions spanning tens of micrometers and thicknesses of a few hundred nanometers. The top and bottom surfaces are either (100) or (111), maximizing topological surface states on these surfaces. Magnetotransport on these SnTe nanoplates shows a high bulk carrier density, consistent with bulk SnTe crystals arising due to defects such as Sn vacancies. In addition, we observe a structural phase transition in these nanoplates from the high-temperature rock salt to a low-temperature rhombohedral structure. For nanoplates with a very high carrier density, we observe a slight upturn in resistance at low temperatures, indicating electron-electron interactions.

Topologically nontrivial narrow bands in ultrathin SnTe films with defect superstructures

It is shown that a two-dimensional topological insulator can be realized and the band topology (equivalently, the edge states) may be further controlled by charge doping in an ultrathin SnTe film with a defect superstructure. Based on first-principles density functional theory (DFT), we predict that a Sn-Te bilayer, if exfoliated from three-dimensional bulk SnTe in the (1 1 1) direction, has a trivial band topology in its pristine form, but is made topologically nontrivial by introducing an appropriate array of defects. The emergence of the topological state is ascribed to the formation of topologically nontrivial narrow bands near the Fermi level by spin–orbit splitting of defect-induced bands. In addition, we demonstrate that a transition between a topological insulator and a normal insulator is possible under the electron or hole doping which can be useful for controlling the topological edge states.

Phonons and periodons in IV–VI semiconductor superlattices

We have calculated the phonon and periodon dispersion relations in IV–VI semi-conducting bulk PbTe and SnTe and their superlattice structure. The model used here is a one-dimensional lattice which includes harmonic interactions up to second neighbours as well as on-site nonlinear electron-ion interactions at the anion site. We calculate the phonon and periodon dispersion relations in bulk and PbTe-SnTe superlattice for the transverse optic and acoustic modes using the transfer matrix method. Our analysis has predicted correct nature of the folding of acoustic and confinement of optical phonons at various frequency intervals corresponding to pass and stop bands of the superlattices.

Strained-layer epitaxy of SnTe on CdTe(110)

Bulk SnTe is a IV–VI compound which crystallizes in the NaCl structure. However, deposition of SnTe on CdTe(110) can force very thin films of SnTe to grow in the zinc-blende structure, leaving an ‘‘extra’’ electron per atom not involved in tetrahedral bonds. We present an angular-resolved photoemission study of this exotic heterostructure, and show that indeed the first ∼4 monolayers of SnTe do grow in the zinc-blende structure. For films of more than ∼10 layers in thickness, the film relaxes into the more favorable NaCl structure. The valence-and conduction-band offset between NaCl structured SnTe and CdTe(110) are ΔEν=1.01±0.05 eV, and ΔEc=0.37±0.05 eV, respectively. The Fermi level lies at the midpoint of the SnTe band gap.