SnSe System Research Articles

Advancing room temperature NO2 gas sensing performance through high-energy mechanical milling of Tin-dichalcogenides

Eco-friendly gas sensing devices with simple architecture, reduced cost, and high performance at room temperature are being sought to replace the traditional metallic oxide materials, aiming to address environmental concerns. Lamellae semiconducting materials have shown promising detection properties for this purpose. Here, we investigated the sensing response of SnS2, Sn(S0.5Se0.5)2, and SnSe2 tin-dichalcogenides prepared by high-energy mechanical milling. High RNO2/Rair response signals, from 102 to 106, for 2–100 ppm of NO2 were observed for temperatures between 30 °C and 300 °C. The materials were not sensitive to CO, while H2 detection could only be observed above 200 °C, implying high NO2 selectivity. Additionally, we investigated the influence of samples suspension in water and isopropanol on grain size and morphology. We found that isopropanol crystallizes amorphous selenium phase dispersed in the Sn-Se system and increase the agglomeration in the Sn(S0.5Se0.5)2 system. Deformed and defective particles were observed regardless the preparation methodology. This unique defect-rich morphology might increase surface reactivity for selective NO2 detection by physisorption, owing a high adsorption/desorption rate at room temperature.

P-type Sn0.98Ag0.02Se with low thermal conductivity synthesized by hydrothermal method

Tin selenide exhibits outstanding thermoelectric properties. Among its various forms, multicrystalline SnSe stands out as a more practical choice compared to single-crystal SnSe, owing to its straightforward chemical synthesis process, enhanced processability, and scalability. Herein, we synthesized p-type polycrystalline Sn1−xAgXSe bulk materials (x = 0, 0.01, 0.015, 0.02, 0.025, 0.03) using a simple hydrothermal and hot pressing method. Characterization methods such as XRD, Raman and XPS were used to demonstrate the successful doping of Ag+ into the SnSe matrix. When the doping concentration reached Sn0.98Ag0.02Se (x = 0.02), Ag+ reached the optimal concentration in the SnSe matrix, causing significant lattice distortion which increased phonon scattering and lowered the thermal conductivity, resulting in Sn0.98Ag0.02Se achieving ZTmax= 1.14. However, when the doping concentration exceeded Sn0.98Ag0.02Se (x > 0.02), excess silver ions precipitated, partially acting as charge carriers, leading to a significant increase in electrical conductivity, resulting in Sn0.97Ag0.03Se having an electrical conductivity of 6860 s/m at 773 K. Compared to the pure phase, the electrical conductivity of Sn0.97Ag0.03Se increased by 259 %, and the ZT value of Sn0.98Ag0.02Se increased by 178%. This work demonstrates that Ag+ is an effective p-type dopant in the SnSe system and indicates that high ZT values in SnSe can be achieved through a simple, low-cost, and environmentally friendly method.

Enhanced thermoelectric performance of p-type PbTe thin films deposited by magnetron sputtering via incorporating SnS

Incorporating second phase or solid solution into the thermoelectric (TE) material matrix has been proven effective to promote its performance. Recent investigations manifest that the synergistical optimization of the electrical and phonon transport properties could be achieved in the PbTe–SnSe system. Being an analogue of SnSe and more environmental, here, SnS was introduced into p-type PbTe film through intermittent magnetron co-sputtering technique. Small amount SnS was observed to induce the shift of predominant orientation from the (200) plane to the (222) plane as well as distinct change in the surface morphology. After the quite possible SnS solid solution and appropriate subsequent annealing, the electrical conductivity and the power factor (PF) have been optimized prominently. In comparison with that of the pristine PbTe film, the maximum PF has been increased by 217% in the annealed film with the intermediate SnS content.

Optoelectronic, mechanical, and thermoelectric properties of Na/I co-doped SnSe via ab initio calculations

Tin selenide-based materials have attracted much attention recently because of their unique properties. This study investigates the effect of Sodium and Iodine co-doping on the optoelectronic, mechanical, and thermoelectric properties of orthorhombic SnSe via First-principles calculations. Na/I co-doped crystal was found to have a triclinic structure, and its electronic bandgap is 0.325 ​eV, whereas the calculated band gap of the pristine SnSe is 0.824 ​eV using SCAN functional. Na/I co-doping alters the Fermi level of SnSe up to its conduction bands, resulting in an n-type system. Furthermore, the static dielectric constant shows that the doped system could be suitable for capacitors and solar cell applications. According to the calculated elastic constants, the doped system is stable. Moreover, it has a negative Poisson's ratio value suggesting it's auxetic sensor material. The thermoelectric performance is examined from 300 ​K to 800 ​K across a broad range of carrier concentrations for the doped and undoped SnSe systems. We have found that Na/I co-doping enhances the electrical conductivity and the Seebeck coefficient of SnSe. The highest power factor calculated for the doped system was 27.9μV/Kcm at carrier concentration of n≅−3×1020cm−3.

Effect of Sn oxides on the thermal conductivity of polycrystalline SnSe

SnSe is a promising thermoelectric material, with intrinsically low lattice thermal conductivity, κL. Surprisingly, in several reports, polycrystalline samples are found to have a higher thermal conductivity than single crystals. This disparity has been attributed to trace amounts of thermally conductive Sn oxides at the grain boundaries of polycrystalline samples. The same culprit was recently proposed to explain the reduction of κL in purified, oxide-free, SnSe polycrystals. Here, we test this hypothesis by: (i) tuning the type of oxide in SnSe by exploiting thermodynamic stability regions, since Sn-rich or Sn-poor compositions favour the formation of SnO or SnO2, respectively; and (ii) varying the quantity of SnO2 by intentionally oxidizing SnSe powder before consolidation, to obtain samples with quantifiable amounts - up to 15% - of SnO2. We find that the κL of SnSe is impervious to changes in the type or the amount of Sn oxide present in the samples. Our results show that a simple “rule of mixtures” cannot be used to estimate the effect of grain boundary oxides on the thermal conductivity of SnSe. These results call for an improved understanding of the intriguing thermal transport mechanisms in SnSe and numerous other systems where a two-phase transport is presumed.

Structural and optical investigations on amorphous SnSe[formula omitted] alloy

The structural and optical properties of an amorphous SnSe9 alloy made using mechanical alloying were investigated through Extended X-ray Absorption Fine Structure and photoacoustic spectroscopy techniques. Average interatomic distances, average coordination numbers, disorder and asymmetry of the gij(r) functions, and also optical parameters such as Urbach and weak absorption tail energies and the optical gap were obtained. At 300 K, we found rSe–Se=2.388±0.002 Å and NSe–Se=1.96±0.04, for Se–Se pairs, and rSe–Sn=2.65±0.02 Å and NSe–Sn=0.35±0.04 for Se–Sn pairs. We determined a direct gap at Eg=1.51±0.02eV for SnSe9. Results indicated that the structure of SnSe9 is very different from those found in crystalline Sn–Se alloys, and the optical gap is the largest one considering amorphous alloys, being even larger than those found in some crystalline alloys of Sn–Se system.

Tuning of the electronic bandgap of SnSe compound by oxygen and sulphur doping and their optical characteristics for solar cell applications

Based on the ab-initio calculations, we have investigated the electronic, optical, and thermal properties of pristine SnSe and doped with oxygen (O) and sulphur (S) at the Se-site at concentration of 3.125%. The calculated negative formation energy indicates the stability of the pristine SnSe and doped system. We find that pristine SnSe possesses a direct bandgap value of 0.90 eV having good comparable to experimental value. The doping of O and S tunes the bandgap of SnSe, which can be promising for photovoltaic devices. Further, for the technological applications of O-/S-doped SnSe, we have analyzed optical properties in terms of absorption of photon energy and polarization. Strong absorption peaks confirms the direct transition of electrons from valence band to conduction band. Moreover, the thermal properties are calculated in terms of electrical/thermal conductivities, Seebeck coefficient, and figure of merit (ZT). At room temperature the ZT values are calculated round about 0.41 for O- and S-doped SnSe, respectively. We expect that our calculated thermoelectric parameters of the O- and S-doped SnSe could pave a new route for the application in the thermoelectric devices.

Enhancement of the power factor of SnSe by adjusting the crystal and energy band structures.

Adjusting the crystal and band structures is of great significance for phonon or carrier transport in functional materials. Lattice contraction was designed to adjust the crystal and energy band structures, which bring about energy valley degeneracy and a band gap decrease in the SnSe system. The Seebeck coefficient of the rapid cooling sample increases significantly due to the energy valley degeneracy. The multichannel effect based on the energy valley degeneracy weakens scattering among the carriers and maintains a high σ along the "⊥" direction. The PF for the rapid cooling sample at 773 K reaches 686 μW m-1 K-2, which is boosted by 66.13% compared with that of the slow cooling sample. Due to the contribution from the high power factor, the ZT value of the rapid cooling sample reaches 0.86, which is an increase of 26% compared with the 0.69 of the slow cooling sample.

PHASE EQUILIBRIA AT THE ISOTHERMAL SECTIONS OF THE QUASI-TERNARY SYSTEMS Ag(Cu)2X–PbX–SNX2 (X=S, Se) AT ROOM TEMPERATURE

The isotermal sections of the quasi-ternary systems Ag(Cu)2X–PbX–SnX2 (X=S, Se) at room temperature were investigated by X-ray phase analysis. No intermediate quaternary phases were found in the systems. Intermediate alloys on the PbSe–SnSe2 side are three-phase, the alloys in the triangles bounded by this side are four-phase. The reason for this is that the PbSe–SnSe2 section is non-quasi-binary causing an unusual tetrahedration of the Ag(Cu)–Pb–Sn–Se systems that is different from the sulfide systems.

Strong lattice anharmonicity exhibited by the high-energy optical phonons in thermoelectric material

Thermoelectric material SnSe has aroused world-wide interests in the past years, and its inherent strong lattice anharmonicity is regarded as a crucial factor for its outstanding thermoelectric performance. However, the understanding of lattice anharmonicity in SnSe system remains inadequate, especially regarding how phonon dynamics are affected by this behavior. In this work, we present a comprehensive study of lattice dynamics on Na0.003Sn0.997Se0.9S0.1 by means of neutron total scattering, inelastic neutron scattering, Raman spectroscopy as well as frozen-phonon calculations. Lattice anharmonicity is evidenced by pair distribution function, inelastic neutron scattering and Raman measurements. By separating the effects of thermal expansion and multi-phonon scattering, we found that the latter is very significant in high-energy optical phonon modes. The strong temperature-dependence of these phonon modes indicate the anharmonicity in this system. Moreover, our data reveals that the linewidths of high-energy optical phonons become broadened with mild doping of sulfur. Our studies suggest that the thermoelectric performance of SnSe could be further enhanced by reducing the contributions of high-energy optical phonon modes to the lattice thermal conductivity via phonon engineering.

From microstructure evolution to thermoelectric and mechanical properties enhancement of SnSe

Defect existing form and its evolution play an important role in the thermoelectric transport process. Here different forms of Pb into the SnSe system were introduced in order to improve the thermoelectric and mechanical properties of SnSe. Pb/SnSe samples were fabricated by vacuum melting, solid phase diffusion, spark plasma sintering and annealing treatment. The element valence mapping diagram and the X-ray photoelectron spectra (XPS) characteristic peaks of Pb show that a certain amount of elemental Pb exists in the initial state, and evolves into Pb2+ ion after annealing treatment. The micro-structure evolution leads to significant enhancement of the power factor and the ZT value. The power factor (PF) and the ZT value for Pb/SnSe increases to 623 μW/m/K2 and 1.12 at 773 K after annealing treatment, respectively. Compared with SnSe matrix, the hardness and fracture toughness of Pb/SnSe samples increased by about 40% and 10%, respectively. Reasonable control of microstructure evolution is expected to be a design idea to improve thermoelectric and mechanical properties of SnSe.

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.

Ag–Sn–Se SYSTEM: PHASE DIAGRAM, THERMODYNAMICS AND MODELING

Ag–Sn–Se SYSTEM: PHASE DIAGRAM, THERMODYNAMICS AND MODELING

Anisotropy thermoelectric and mechanical property of polycrystalline SnSe prepared under different processes

In this work, the polycrystalline SnSe bulk samples were prepared by the spark plasma sintering (SPS) using the SnSe powders, which were synthesized by the hydrothermal reaction (HR) or the melting reaction (MR), respectively. X-ray diffraction (XRD) patterns reveal strong orientation along the [l00] direction for both wafer samples, but there is obvious different micro-morphologies in the powder and the bulk fracture as shown in scanning electron microscopy (SEM) images. The zigzag type texturing characteristic appears in the entire interior of the bulk sample prepared by the process (HR + SPS), while random arrangement occurs in interior of another bulk sample prepared by the process (MR + SPS).The formation mechanism of the zigzag type texturing come mainly from control of initial powders morphology and pressing into pre-pressed blocks by layers. The special zigzag type texturing characteristic improve the thermoelectric properties of samples, and the highest ZT value (~ 0.67) at 773 K were found in the SnSe bulk sample prepared by the process (HR + SPS) in the direction parallel to the pressing axis. The thermoelectric compatibility factor of samples prepared by the process (HR + SPS) are less than 2 at 773 K. The fracture toughness in the plane that vertical or parallel to the pressing direction was calculated to be 4.0 MPam1/2 and 4.2 MPam1/2, respectively. The proper thermoelectric compatibility factor and fracture toughness make the SnSe system better application prospect.

Synergistically optimized electrical and thermal transport properties of polycrystalline SnSe via alloying SnS

SnSe crystals were reported to possess extraordinary thermoelectric performance, while the polycrystals were less marked due to the inferior carrier and phonon transport properties. Herein, we fully take advantage of the complex band structure and strong point scattering via alloying SnS. The carrier concentration and Seebeck coefficient were synergistically optimized via activating multiple valance bands and enlarging band effective mass, which contribute to a maximum power factor ∼7.53 μWcm−1K−2 at 793 K. Meanwhile, the lattice thermal conductivity was greatly reduced to ∼0.92 Wm−1K−1 due to the effective phonon scattering from point defects, as well demonstrated by the Callaway model. Combining a high power factor with low thermal conductivity, relatively high ZT of 1.2 at 793 K was obtained in polycrystalline Sn0.98Na0.02Se0.9S0.1. Our work demonstrates that alloying is simple yet effective approach for enhancing thermoelectric performance for polycrystalline SnSe, and SnSe system is one of promising thermoelectric candidates.

Experimental Study and 3D Modeling of the Phase Diagram of the Ag–Sn–Se System

In connection with the contradictoriness of literature data, phase equilibria in the Ag–Sn–Se system were restudied by differential thermal analysis and X-ray powder diffraction analysis. A number of polythermal sections and the isothermal section at room temperature of the phase diagram were constructed, and a projection of the liquidus surface was built. The primary crystallization fields of phases and the types and coordinates of in- and monovariant equilibria were determined. It was demonstrated that, in the system, two ternary compounds, Ag8SnSe6 and AgxSn2 – xSe2 (0.84 < x < 1.06), form. The former melts congruently at 1015 K and undergoes a polymorphic transformation at 355 K, and the latter melts with decomposition by a peritectic reaction at 860 K. The formation of the compound Ag2SnSe3, which was previously reported in the literature, was not confirmed. Based on the phase diagrams of boundary binary systems and the results of the differential thermal analysis of a limited number of samples of the ternary system, equations were obtained for calculation and 3D modeling of the liquidus and phase-separation surfaces.

Realizing High Thermoelectric Performance in n‐Type Highly Distorted Sb‐Doped SnSe Microplates via Tuning High Electron Concentration and Inducing Intensive Crystal Defects

AbstractIn this study, a record high figure of merit (ZT) of ≈1.1 at 773 K is reported in n‐type highly distorted Sb‐doped SnSe microplates via a facile solvothermal method. The pellets sintered from the Sb‐doped SnSe microplates show a high power factor of ≈2.4 µW cm−1 K−2 and an ultralow thermal conductivity of ≈0.17 W m−1 K−1 at 773 K, leading a record high ZT. Such a high power factor is attributed to a high electron concentration of 3.94 × 1019 cm−3 via Sb‐enabled electron doping, and the ultralow thermal conductivity derives from the enhanced phonon scattering at intensive crystal defects, including severe lattice distortions, dislocations, and lattice bent, observed by detailed structural characterizations. This study fills in the gaps of fundamental doping mechanisms of Sb in SnSe system, and provides a new perspective to achieve high thermoelectric performance in n‐type polycrystalline SnSe.

Boosting the thermoelectric performance of p-type heavily Cu-doped polycrystalline SnSe via inducing intensive crystal imperfections and defect phonon scattering.

In this study, we, for the first time, report a high Cu solubility of 11.8% in single crystal SnSe microbelts synthesized via a facile solvothermal route. The pellets sintered from these heavily Cu-doped microbelts show a high power factor of 5.57 μW cm-1 K-2 and low thermal conductivity of 0.32 W m-1 K-1 at 823 K, contributing to a high peak ZT of ∼1.41. Through a combination of detailed structural and chemical characterizations, we found that with increasing the Cu doping level, the morphology of the synthesized Sn1-x Cu x Se (x is from 0 to 0.118) transfers from rectangular microplate to microbelt. The high electrical transport performance comes from the obtained Cu+ doped state, and the intensive crystal imperfections such as dislocations, lattice distortions, and strains, play key roles in keeping low thermal conductivity. This study fills in the gaps of the existing knowledge concerning the doping mechanisms of Cu in SnSe systems, and provides a new strategy to achieve high thermoelectric performance in SnSe-based thermoelectric materials.

α-SnSe thin film solar cells produced by selenization of magnetron sputtered tin precursors

The temperature-pressure-composition phase diagrams of Sn-Se system were calculated using the CALPHAD (CALculation of PHase Diagram) models. The phase diagrams showed the formation of α-SnSe phase at selenium-rich side with pressures lower than atmospheric pressure and in the temperature range of 300–500°C. As a first step, the effect of Sn/Se ratio on the phase formation was studied experimentally by selenization of tin metal precursor films using effusion cell evaporation. The Sn/Se ratio was varied by changing the selenium weight in the range of 0.5–1.5g. The physical properties of the films were studied with suitable characterization techniques and the obtained results showed the formation of single phase α-SnSe at 1.0g of selenium. Further, α-SnSe/CdS interface was studied by photoelectron yield spectroscopy (PYS), which showed a ‘type-I’ band alignment with a valence-band offset (∆Ev) of 1.3eV and a conduction-band offset (∆Ec) of 0.2eV. Finally, α-SnSe solar cells with a device structure of soda-lime glass (SLG)/Mo/α-SnSe/CdS/i-ZnO/Al:ZnO/Ni/Ag were fabricated and a power conversation efficiency of 1.42% was achieved at 1.0g of selenium.

Influence of Sodium Chloride Doping on Thermoelectric Properties of p-type SnSe

We investigated the effect of NaCl doping on the thermoelectric properties of p-type Sn 1−x Na x SeCl x (x = 0, 0.005, 0.01, 0.02, 0.03 and 0.04) prepared by a method which combines rapid induction melting and rapid hot pressing. After introducing the NaCl into the SnSe system, the carrier concentration of SnSe is significantly increased from ∼4.55 × 1017 cm−3 to ∼3.95 × 1019 cm−3 at 300 K. An electrical conductivity of ∼102.5 S cm−1 was obtained at 473 K by addition of 2 mol.% NaCl. It was found that Cl was effective in reducing the thermal conductivity by inducing abundant defects. A maximum ZT value of 0.84 was achieved in the Na0.005Sn0.995SeCl0.005 sample at 810 K. This suggests that doping with NaCl is a facile and cost-effective method in optimizing the thermoelectric properties of SnSe materials.