Tin Chalcogenides Research Articles

Thermoelectric Properties of Pnma and Rocksalt SnS and SnSe

Thermoelectric materials convert waste heat to electricity and are part of the package of technologies needed to limit global warming. The tin chalcogenides SnS and SnSe are promising candidate thermoelectrics, with orthorhombic SnSe showing some of the highest figures of merit ZT reported to date. As for other Group IV chalcogenides, SnS and SnSe can form rocksalt phases under certain conditions, but the thermoelectric properties of these phases are largely unexplored. We have applied a fully ab initio modelling protocol to compare the ZT of the orthorhombic and rocksalt phases of SnS and SnSe. Electronic structures from hybrid density-functional theory were used to calculate the three electrical transport properties, including approximate models for the electron relaxation times, and lattice dynamics calculations were performed to model the phonon spectra and lattice thermal conductivities. We obtained good estimates of the ZT of the well-studied orthorhombic phases. The rocksalt phases were predicted to show larger electrical conductivities and similar Seebeck coefficients to the orthorhombic phases, resulting in higher thermoelectric power factors, but these were offset by larger thermal conductivities. These results therefore motivate further investigation of the recently discovered “π-cubic” phases of SnS and SnSe, which are based on distorted rocksalt supercells, to establish their thermoelectric performance.

General strategies to improve thermoelectric performance with an emphasis on tin and germanium chalcogenides as thermoelectric materials

Figure of merit, and strategies to improve it in materials.

Enhanced Charge Carrier Separation and Improved Biexciton Yield at the p-n Junction of SnSe/CdSe Heterostructures: A Detailed Electrochemical and Ultrafast Spectroscopic Investigation.

Tin chalcogenides (SnX, X = S, Se)-based heterostructures (HSs) are promising materials for the construction of low-cost optoelectronic devices. Here, we report the synthesis of a SnSe/CdSe HS using the controlled cation exchange reaction. The (400) plane of SnSe and the (111) plane of CdSe confirm the formation of an interface between SnSe and CdSe. The Type I band alignment is estimated for the SnSe/CdSe HS with a small conduction band offset (CBO) of 0.72 eV through cyclic voltammetry measurements. Transient absorption (TA) studies demonstrate a drastic enhancement of the CdSe biexciton signal that points toward the hot carrier transfer from SnSe to CdSe in a short time scale. The fast growth and recovery of CdSe bleach in the presence of SnSe indicate charge transfer back to SnSe. The observed delocalization of carriers in these two systems is crucial for an optoelectronic device. Our findings provide new insights into the fabrication of cost-effective photovoltaic devices based on SnSe-based heterostructures.

One-Pot Synthesis of High-Performance Tin Chalcogenides/C Anodes for Li-Ion Batteries.

Tin chalcogenides are considered as promising anode materials for lithium-ion batteries (LIBs) due to their high theoretical lithium-storage capacity. Herein, we have successfully synthesized the composites of tin chalcogenides and graphite, that is, SnS/C, SnSe/C, and SnS0.5Se0.5/C, via a simple one-pot solid-state method. During the electrochemical test, they exhibit excellent lithium-storage ability and cyclic performance as the anode electrodes of LIBs due to the introduction of carbon. In particular, (i) SnS/C displayed a high specific capacity of 875 mAh g–1 at 0.2 A g–1 over 200 cycles; (ii) SnSe/C presents 734 mAh g–1 at 0.2 A g–1 after 100 cycles, and it delivers 690 mAh g–1 at 1.0 A g–1 over 300 cycles; and (iii) the SnS0.5Se0.5/C composite electrode delivers a specific capacity of 643 mAh g–1 at 0.5 A g–1 over 150 cycles. Furthermore, another series of tin-based composites have also been successfully fabricated (i.e., Sn/C, SnS2/C, SnSe2/C, and SnTe/C), showing the general applicability of the synthetic route applied here. Our synthetic approach demonstrates a promising route for the large-scale production of high-performance tin chalcogenides/C anode materials for LIBs and other battery systems (e.g., Na-ion and K-ion batteries).

Miniband engineering and topological phase transitions in topological-insulator–normal-insulator superlattices

Periodic stacking of topologically trivial and non-trivial layers with opposite symmetry of the valence and conduction bands induces topological interface states that, in the strong coupling limit, hybridize both across the topological and normal insulator layers. Using band structure engineering, such superlattices can be effectively realized using the IV-VI lead tin chalcogenides. This leads to emergent minibands with a tunable topology as demonstrated both by theory and experiments. The topological minibands are proven by magneto-optical spectroscopy, revealing Landau level transitions both at the center and edges of the artificial superlattice mini Brillouin zone. Their topological character is identified by the topological phase transitions within the minibands observed as a function of temperature. The critical temperature of this transition as well as the miniband gap and miniband width can be precisely controlled by the layer thicknesses and compositions. This witnesses the generation of a new fully tunable quasi-3D topological state that provides a template for realization of magnetic Weyl semimetals and other strongly interacting topological phases.

Thermal conductivity and enhanced thermoelectric performance of SnTe bilayer

Tin chalcogenides (SnS, SnSe, and SnTe) are found to have improved thermoelectric properties upon the reduction of their dimensionality. Here we found the tilted AA + s stacked two-dimensional (2D) SnTe bilayer as the most stable phase among several stackings as predicted by the structural optimization and phonon transport properties. The carrier mobility and relaxation time are evaluated using the deformation potential theory, which is found to be relatively high due to the high 2D elastic modulus, low deformation potential constant, and moderate effective masses. The SnTe bilayer shows a high Seebeck coefficient, high electrical conductivity, and ultralow lattice thermal conductivity. High TE figure of merit (ZT) values, as high as 4.61 along the zigzag direction, are predicted for the SnTe bilayer. These ZT values are much enhanced as compared to the bulk as well as monolayer SnTe and other 2D compounds.

Effects of annealing temperature on electrochemical performance of SnSx embedded in hierarchical porous carbon with N-carbon coating by in-situ structural phase transformation as anodes for lithium ion batteries

Tuned tin chalcogenides rooted in hierarchical porous carbon (HPC) with N-carbon coating layers are prepared by thermal shock under various temperatures (denoted as HPC-SnS2-PAN-Various T). With the increase of annealing temperature, the morphology and phase structure of SnS2, as well as the cyclization degree of polyacrylonitrile (PAN), are significantly changed, which leads to the formation of rod-like SnS and ordered structure of conductive N-carbon layer. By combining HPC, N-carbon coating derived from the cyclization of PAN, with 1D SnS nanorods generated from structural phase transformation of SnS2, the optimized composite (HPC-SnS2-PAN-500) as anode for lithium ion batteries (LIBs) provides buffer space for volume changes during alloying/dealloying process, builds a highly conductive network as well as decreases irreversible capacity from solid electrolyte interphase and enhances the ion/electron transport. Attributed to the above merits from composition regulation and architecture modification by sulfur depletion and PAN cyclization, this target anode exhibits an extraordinary cycling stability with a high specific capacity of 652.5 mA h/g at 0.5 A/g after 900 cycles. It suggests that rod-like SnS embedded in HPC with cyclized PAN layers by thermal treatment approach renders a potential structural design of anode materials for LIBs.

Atomic observation of phase transition in layered SnS2 driven by in situ heating and electron beam irradiation

The phase transition in transition metal dichalcogenides has a significant effect on the electrical conductivity and thermal conductivity. Herein, the SnS2 phase transition was observed on the atomic scale by in situ scanning transmission electron microscopy. The 1T and 1H mixed phase was formed under in situ heating and electron beam irradiation. First principles calculations demonstrate that the phase transition between 1H and 1T occurs through multiple layers of phase transition rather than layer by layer. The bandgap of the mixed SnS2 phase was found to be much smaller than the reported pure 1T phase. Our results provide microscopic insights into the transformation mechanism and highlight how heating and irradiation can be used to tune the electrical transport property of layered tin chalcogenides.

Thermoelectric and lattice dynamics properties of layered MX (M = Sn, Pb; X = S, Te) compounds

Lead and tin chalcogenides are extensively studied due to their promising thermoelectric (TE) properties. They show an enhancement in the TE efficiency upon the reduction ofdimension, which is an important feature in device fabrications. Using density functional theory combined with the semi-classical Boltzmann transport formalism, we studied the structural, electronic, and TE properties of two-dimensional (2D) MX (M = Sn, Pb; X = S, Te) monolayers (MLs). Spin-orbit coupling played a significant roleon their electronic structure, particularly for the heavy compounds. Structural optimization followed by phonon transport studies prevailed that the rectangular (γ-) phaseis energetically the most favorable for SnS and SnTe MLs, whereas the square structure is found the most stable for PbS and PbTe MLs.These 2D materials exhibit high Seebeck coefficients (1000–1500 µV/K)and power factors ((33–77.3) × 10−4W m−1 K−2) along with low lattice thermal conductivities (<3Wm−1 K−1)—these are the essential features of good TE materials. The maximum figure of merits (ZT) of 1.04, 1.46, 1.51, and 1.94 are predicted forn-type SnS, SnTe, PbS, andp-type PbTe MLs, respectively at 700 K, which are higher than their bulkZTvalues. Hence, these MLs are promising candidates for TE applications.

Tin(II) Aminothiolate and Tin(IV) Aminothiolate Selenide Compounds as Single-Source Precursors for Tin Chalcogenide Materials.

Tin aminothiolate compounds SnII(dmampS)2 (1) and SnIV(dmampS)2Se (2), where dmampS = 1-(dimethylamino)-2-methylpropane-2-thiolate, were synthesized. The molecular structures of 1 and 2 reveal a seesaw and distorted trigonal-bipyramidal geometry, respectively. The 1H NMR spectrum of 1 shows two types of resonances for the methyl groups of the α-carbon, methyl groups of the amino groups, and methylene groups at room temperature. On the other hand, the 1H NMR spectrum of 2 exhibits a single resonance for each of these groups. According to variable-temperature 1H NMR analysis, each of these two types of resonances occurring at relatively low temperatures (under 223 K for 1 and under 333 K for 2) are merged as a single resonance with increasing temperature. By using thermogravimetric and thermal decomposition analyses, the residual materials of compounds 1 and 2 are confirmed to be SnS and SnSSe, respectively. Compound 1 was subjected to a metal-organic chemical vapor deposition process, which allowed for the deposition of a dense and well-faceted orthorhombic phase SnS thin film on a SiO2/Si substrate with ∼200 nm thickness.

Theoretical model for predicting thermoelectric properties of tin chalcogenides

The global energy crisis demands the search for new materials for efficient thermoelectric energy conversion. Theoretical predictive modelling with experiments can expedite the global search of novel and ecoconscious thermoelectric materials. The efficiency of thermoelectric materials depends upon the thermoelectric figure of merit (ZT). In this perspective, we discuss the theoretical model to calculate thermoelectric properties. Different scattering mechanisms of electrons and phonons are calculated using a simple model for the fast prediction of thermoelectric properties. Thermoelectric properties based on the simple model have shown more than 90% agreement with the experimental values. Possibility to optimize the figure of merit by alloying, defects, nanostructuring and band convergence is also discussed for layered chalcogenides of tin. In the case of doped materials, ion-impurity scattering is found to be dominating over electron-phonon scattering and the power factor can be optimized by tuning the former scattering rate. For phonon transport, alloy scattering is found to be the most dominating among all other scattering mechanisms. Theoretically, it is found that in the temperature range between 300 K and 800 K, SnSe0.70S0.30 has the highest ZT with an efficiency of 17.20% with respect to Carnot efficiency. There could be 53.8% enhancement of the device efficiency in SnSe0.70S0.30 compared to experimentally reported SnSe0.50S0.50 in the medium temperature range (300 K to 800 K). Possible routes to achieve the best ZT in the medium temperature range are also discussed in this perspective.

One-pot synthesis of tin chalcogenide-reduced graphene oxide-carbon nanotube nanocomposite as anode material for lithium-ion batteries.

In this study, a ternary tin chalcogenide (TC)-reduced graphene oxide (RGO)-carbon nanotube (CNT) nanocomposite was synthesized as a lithium-ion battery (LIB) anode by a simple one-step protocol. The nanocomposite was prepared through a hydrothermal method using tin chloride as the tin precursor, thiourea as the sulfur source and reducing agent, and GO-CNT hybrid as the carbonaceous nanostructure. The structure, morphology, and phase analysis of the synthesized nanocomposite powder were investigated using Raman spectroscopy, field-emission scanning electron microscopy (FESEM), and X-ray diffraction (XRD). The results show that GO is reduced while SnS and SnS2 nanosheets along with SnO2 nanoparticles are simultaneously formed within the RGO-CNT hybrid framework throughout the hydrothermal process. During the first lithiation-delithiation process, the discharge capacity and the columbic efficiency for the ternary TC-RGO-CNT nanocomposite electrode at a current density of 50 mA g-1 are 1401 mA h g-1 and 50%, respectively. The TC-RGO-CNT electrode gives an improved capacity of 197 mA h g-1 at 500 mA g-1 while the corresponding value for the bare TC, and binary TC-CNT and TC-RGO nanocomposite electrodes was only 5, 18, and 41 mA h g-1, respectively. Meanwhile, the ternary nanocomposite anode indicates outstanding stability after 150 cycles with a reversible capacity of 100 mA h g-1 at 500 mA g-1. The excellent electrochemical performance of the ternary TC-RGO-CNT nanocomposite is ascribed to the synergistic effect of the high capacity of electrochemically-active TC nanostructures along with the large surface area, porous structure, and exceptional conductivity of the 3D RGO-CNT framework.

Hollow/porous-walled SnO2 via nanoscale Kirkendall diffusion with irregular particles

Hollow/porous structured SnO2 nanoparticles were synthesized by simple oxidation of dense metal chalcogenide precursors via nanoscale Kirkendall diffusion effect. First, tin chalcogenide (SnS, SnSe) nanoparticles were synthesized by mechanochemical method, which is considered a facile, scalable, and eco-friendly process. Hollow/porous-walled SnO2 nanoparticles were synthesized by simple oxidation of the prepared Sn chalcogenide precursors, for which the transformation mechanism was verified in detail. Nanoscale Kirkendall diffusion process was thoroughly investigated by morphological, crystallographic, and elemental analyses performed at various oxidation temperatures and times. To examine the morphological effect of hollow/porous-walled SnO2 nanoparticles on the electrochemical performance, the synthesized nanoparticles were applied as anode material in a lithium-ion battery. Anode material showed highly improved electrochemical properties compared to its dense counterpart, with 83% capacity retention from the second cycle at the 400th cycle and capacity of 302 mA h g−1 at a high current density of 30 A g−1.

Efficient and stable hydrogen evolution based on earth-abundant SnSe nanocrystals

Tin chalcogenide nanocrystals (NCs) exhibit a wide absorption spectrum with excellent optoelectronic properties. Both Sn and Se are relatively less harmful and less expensive compared to heavy-metals and precious elements. However, it is still a challenge to directly synthesize SnSe NCs without using toxic and sensitive precursors. Here, we use a cation exchange assisted approach to synthesize SnSe NCs that are optically active in the near infrared (NIR). The SnSe NCs have a cuboid shape obtained thorough an orientated growth path. A typical indirect bandgap was found in these NCs ranging from 0.9 to 1.2 eV. As a proof-of-concept, the SnSe NCs were used as light absorbers for photoelectrochemical (PEC) hydrogen evolution, leading to a saturated photocurrent density of 8.2 mA/cm2, a record value for PEC devices using heavy-metal-free NIR NCs. Our results indicate that SnSe NCs of cuboid shape have great potential for use in emerging optoelectronic and photovoltaic devices.

Hollow/Porous-Walled SnO₂ via Nanoscale Kirkendall Diffusion with Irregular Particles

Hollow/porous structured SnO₂ nanoparticles were synthesized by simple oxidation of dense metal chalcogenide precursors via nanoscale Kirkendall diffusion effect. First, tin chalcogenide (SnS, SnSe) nanoparticles were synthesized by mechanochemical method, which is considered a facile, scalable, and eco-friendly process. Hollow/porous-walled SnO₂ nanoparticles were synthesized by simple oxidation of the prepared Sn chalcogenide precursors, for which the transformation mechanism was verified in detail. Nanoscale Kirkendall diffusion process was thoroughly investigated by morphological, crystallographic, and elemental analyses performed at various oxidation temperatures and times. To examine the morphological effect of hollow/porous-walled SnO₂ nanoparticles on the electrochemical performance, the synthesized nanoparticles were applied as anode material in a lithium-ion battery. Anode material showed highly improved electrochemical properties compared to its dense counterpart, with 83% capacity retention from the second cycle at the 400th cycle and discharge capacity of 302 mA h g⁻¹ at a high current density of 30 A g⁻¹.

DFT modeling of thermoelectric and optical features of novel MgxSn1-xSe (x = 6%, 12% & 18%)

We explore influence of Mg alloying effect on electronic band structure dispersion and thermoelectric properties of tin chalcogenide materials. Based on density functional theory (DFT) within a framework of full potential linearized augmented plane wave method (FP-LAPW), we evaluate the energy band structure and optical properties of MgxSn1-xSe (x = 6%, 12% and 18%) materials. Moreover, we extend our calculations to simulate the electrical transport properties using Boltzmann transport theory. Within the approximations employed in our calculations the theoretically predicted band energy gap values and the temperature dependence of electrical transport properties of MgxSn1-xSe compounds revealed that the Mg-alloying have enhanced thermoelectric features. To verify the quality of calculations the comparison with the experimental absorption spectra are presented. The better thermoelectric performance in MgxSn1-xSe is expected to occur for all doping concentrations, however 18% Mg-doped material exhibits higher value of Seebeck coefficient and lower thermal conductivity which is suggestive that at higher Mg concentration the holes become dominant over electrons and hence make these materials to be more promising candidates for their use in thermoelectric power generation and in cooling devices.

Aqueous-Based Binary Sulfide Nanoparticle Inks for Cu2ZnSnS4 Thin Films Stabilized with Tin(IV) Chalcogenide Complexes

Cu2ZnSnS4 (CZTS) is a promising semiconductor material for photovoltaic applications, with excellent optical and electronic properties while boasting a nontoxic, inexpensive, and abundant elemental composition. Previous high-quality CZTS thin films often required either vacuum-based deposition processes or the use of organic ligands/solvents for ink formulation, which are associated with various issues regarding performance or economic feasibility. To address these issues, an alternative method for depositing CZTS thin films using an aqueous-based nanoparticle suspension is demonstrated in this work. Nanoparticles of constituent binary sulfides (CuxS and ZnS) are stabilized in an ink using tin(IV)-based, metal chalcogenide complexes such as [Sn2S6]4−. This research paper provides a systematic study of the nanoparticle synthesis and ink formulation via the enabling role of the tin chalcogenide complexing power, the deposition of high-quality CZTS thin films via spin coating and annealing under sulfur vapor atmosphere, their structural characterization in terms of nanocrystal phase, morphology, microstructure, and densification, and their resultant optoelectronic properties.

Wrap-Around Core-Shell Heterostructures of Layered Crystals.

Engineered heterostructures create new functionality by integrating dissimilar materials. Combining different 2D crystals naturally produces two distinct classes of heterostructures, vertical van der Waals (vdW) stacks or 2D sheets bonded laterally by covalent line interfaces. When joining thicker layered crystals, the arising structural and topological conflicts can result in more complex geometries. Phase separation during one-pot synthesis of layered tin chalcogenides spontaneously creates core-shell structures in which large orthorhombic SnS crystals are enclosed in a wrap-around shell of trigonal SnS2 , forcing the coexistence of parallel vdW layering along with unconventional, orthogonally layered core-shell interfaces. Measurements of the optoelectronic properties establish anisotropic carrier separation near type II core-shell interfaces and extended long-wavelength light harvesting via spatially indirect interfacial absorption, making multifunctional layered core-shell structures attractive for energy-conversion applications.

The effects of ion implantation on rhenium and tin dichalcogenide ultrathin films

Investigations were carried out on the effects of rhenium and tin chalcogenide ultrathin films using an ion implantation method for specific characteristics and wide applications. ReS2, ReSe2, SnS2, and SnSe2 films were implanted with 300 keV O ions at a fluence of 1 × 1014 ions/cm2. The research focused on the evolution of the morphology, crystallinity and structure of the films through atomic force microscopy, X-ray diffraction, Raman scattering and photoluminescence before and after ion implantation. The results showed diverse changes in different films, such as thickness decreases in the ReS2, ReSe2, and SnS2 films and smoothing effects in the ReS2, ReSe2, and SnSe2 films. In terms of crystallinity, small decrements were observed only in ReSe2 and SnSe2 films. The Raman modes of ReS2 film exhibited redshifts which also appeared in the photoluminescence peaks of ReS2, ReSe2, SnS2, and SnSe2 films. The results indicate feasible modification of film properties by ion implantation, which also bring new possibilities and options to ultrathin material research.

New two-dimensional phase of tin chalcogenides: Candidates for high-performance thermoelectric materials

Tin-chalcogenides SnX (X = Te, Se and S) have been arousing research interest due to their thermoelectric physical properties. The two-dimensional (2D) counterparts, which are expected to enhance the property, nevertheless, have not been fully explored because of many possible structures. Generating variable composition of 2D Sn$_{1-x}$X$_{x}$ systems (X = Te, Se and S) has been performed using global searching method based on evolutionary algorithm combining with density functional calculations. A new hexagonal phase named by $\beta'$-SnX is found by Universal Structure Predictor Evolutionary Xtallography (USPEX), and the structural stability has been further checked by phonon dispersion calculation and the elasticity criteria. The $\beta'$-SnTe is the most stable among all possible 2D phases of SnTe including those experimentally available phases. Further, $\beta'$ phases of SnSe and SnS are also found energetically close to the most stable phases. High thermoelectronic (TE) performance has been achieved in the $\beta'$-SnX phases, which have dimensionless figure of merit (ZT) as high as $\sim$0.96 to 3.81 for SnTe, $\sim$0.93 to 2.51 for SnSe and $\sim$1.19 to 3.18 for SnS at temperature ranging from 300 K to 900 K with practically attainable carrier concentration of 5$\times$10$^{12}$ cm$^{-2}$. The high TE performance is resulted from a high power factor which is attributed to the quantum confinement of 2D materials and the band convergence near Fermi level, as well as low thermal conductivity mainly from both low elastic constants due to weak inter-Sn bonding strength and strong lattice anharmonicity.