Recent Progress on Layered Sn and Pb-Based Mono Chalcogenides: Synthesis, Structure, Optical, and Thermoelectric Properties and Related Applications

The research on two-dimensional materials has gained significant traction due to their potential for thermoelectric, optical, and other properties. The development of two-dimensional (2D) nanostructured-based TE generators and photodetectors has shown promising results. Over the years, researchers have played a crucial role in advancing this field, enhancing the properties of 2D materials through techniques such as doping, alloying, and various growth methods. Among these materials, black phosphorus, transition metal dichalcogenides, graphene, and IVA-VIA compounds stand out for their remarkable electronic, mechanical, and optical properties. This study presents a comprehensive review of the progress in the field, focusing on IVA-VIA compounds and their applications in TE and photodetector technologies. We summarize recent advancements in enhancing these materials' TE and optical properties and provide an overview of various synthesis techniques for their fabrication. Additionally, we highlight their potential applications as photodetectors in the infrared spectrum. This comprehensive review aims to equip researchers with a deep understanding of the TE and optical properties of 2DMs and their potential applications and to inspire further advancements in this field of research.

Identification and Manipulation of Atomic Defects in Monolayer SnSe

SnSe, an environmental-friendly group-IV monochalcogenide semiconductor, demonstrates outstanding performance in various applications ranging from thermoelectric devices to solar energy harvesting. Its ultrathin films show promise in the fabrication of ferroelectric nonvolatile devices. However, the microscopic identification and manipulation of point defects in ultrathin SnSe single crystalline films, which significantly impact their electronic structure, have been inadequately studied. This study presents a comprehensive investigation of point defects in monolayer SnSe films grown via molecular beam epitaxy. By combining scanning tunneling microscopy (STM) characterization with first-principles calculations, we identified four types of atomic/molecular vacancies, four types of atomic substitutions, and three types of extrinsic defects. Notably, we have demonstrated the ability to convert a substitutional defect into a vacancy and to reposition an adsorbate by manipulating a single atom or molecule using an STM tip. We have also analyzed the local atomic displacement induced by the vacancies. This work provides a solid foundation for engineering the electronic structure of future SnSe-based nanodevices.

Fluidized Bed Chemical Vapor Deposition on Hard Carbon Powders to Produce Composite Energy Materials

Herein, we report a general route for the uniform coating of hard carbon (HC) powders via fluidized bed chemical vapor deposition. Carbon-based fine powders are excellent substrate materials for many catalytic and electrochemical applications but intrinsically difficult to fluidize and prone to elutriation. The reactor was designed to achieve as much retention of powders as possible, supported by a computational fluid dynamics study to assess the hydrodynamic behavior for varying gaseous flow rates. Solutions of the tin seleno- and thio-ether complexes [SnCl4{nBuSe(CH2)3SenBu}] and [SnCl4{nBuS(CH2)3SnBu}] were used as single source precursors and injected at high temperature into a fluidized bed of HC powders under nitrogen flow. The method allowed for the synthesis of HC-SnSx-SnSe2 composites at the gram scale with potential applications in electrocatalysis and sodium-ion battery anodes.

Local Sn Dipolar-Character Displacements behind the Low Thermal Conductivity in SnSe Thermoelectric

The local atomic structure of SnSe was characterized across its orthorhombic-to-orthorhombic structural phase transition using x-ray pair distribution function analysis. Substantial Sn displacements with a dipolar character persist in the high-symmetry high-temperature phase, albeit with a symmetry different from that of the ordered displacements below the transition. The analysis implies that the transition is neither order-disorder nor displacive but rather a complex crossover. Robust ferrocoupled SnSe intralayer distortions suggest a ferroelectriclike instability as the driving force. These local symmetry-lowering Sn displacements are likely integral to the ultralow lattice thermal conductivity mechanism in SnSe.

Solution phase growth and analysis of super-thin zigzag tin selenide nanoribbons

Tin selenide (SnSe), a highly promising layered material, has been garnering particular interest in recent times due to its significant promise for future energy devices. Herein we report a simple solution-phase approach for growing highly crystalline layered SnSe nanoribbons. Polyvinylpyrrolidone (PVP) was used as a templating agent to selectively passivates the (100) and (001) facets of the SnSe nanoribbons resulting in the unique growth of nanoribbons along theirb-axis with a defined zigzag edge state along the sidewalls. The SnSe nanoribbons are few layers thick (∼20 layers), with mean widths of ∼40 nm, and achievable length of >1μm. Nanoribbons could be produced in relatively high quantities (>150 mg) in a single batch experiment. The PVP coating also offers some resistance to oxidation, with the removal of the PVP seen to lead to the formation of a SnSe/SnO_xcore-shell structure. The use of non-toxic PVP to replace toxic amines that are typically employed for other 1D forms of SnSe is a significant advantage for sustainable and environmentally friendly applications. Heat transport properties of the SnSe nanoribbons, derived from power-dependent Raman spectroscopy, demonstrate the potential of SnSe nanoribbons as thermoelectric material.

Ultrathin Twisted Germanium Sulfide van der Waals Nanowires by Bismuth Catalyzed Vapor-Liquid-Solid Growth

1D nanowires of 2D layered crystals are emerging nanostructures synthesized by combining van der Waals (vdW) epitaxy and vapor-liquid-solid (VLS) growth. Nanowires of the group IV monochalcogenide germanium sulfide (GeS) are of particular interest for twistronics due to axial screw dislocations giving rise to Eshelby twist and precision interlayer twist at helical vdW interfaces. Ultrathin vdW nanowires have not been realized, and it is not clear if confining layered crystals into extremely thin wires is even possible. If axial screw dislocations are still stable, ultrathin vdW nanowires can reach large twists and should display significant quantum confinement. Here it is shown that VLS growth over Bi catalysts yields vdW nanowires down to ≈15 nm diameter while maintaining tens of µm length. Combined electron microscopy and diffraction demonstrate that ultrathin GeS nanowires crystallize in the orthorhombic bulk structure but can realize nonequilibrium stacking that may lead to 1D ferroelectricity. Ultrathin nanowires carry screw dislocations, remain chiral, and achieve very high twist rates. Whenever the dislocation extends to the nanowire tip, it continues into the Bi catalyst. Eshelby twist analysis demonstrates that the ultrathin nanowires follow continuum predictions. Cathodoluminescence on individual nanowires, finally, shows pronounced emission blue shifts consistent with quantum confinement.

Inorganic Thermoelectric Fibers: A Review of Materials, Fabrication Methods, and Applications

Thermoelectric technology can directly harvest the waste heat into electricity, which is a promising field of green and sustainable energy. In this aspect, flexible thermoelectrics (FTE) such as wearable fabrics, smart biosensing, and biomedical electronics offer a variety of applications. Since the nanofibers are one of the important constructions of FTE, inorganic thermoelectric fibers are focused on here due to their excellent thermoelectric performance and acceptable flexibility. Additionally, measurement and microstructure characterizations for various thermoelectric fibers (Bi-Sb-Te, Ag₂Te, PbTe, SnSe and NaCo₂O₄) made by different fabrication methods, such as electrospinning, two-step anodization process, solution-phase deposition method, focused ion beam, and self-heated 3ω method, are detailed. This review further illustrates that some techniques, such as thermal drawing method, result in high performance of fiber-based thermoelectric properties, which can emerge in wearable devices and smart electronics in the near future.

Te Nanoneedles Induced Entanglement and Thermoelectric Improvement of SnSe

Chalcogenide-based materials have attracted widespread interest in high-performance thermoelectric research fields. A strategy for the application of two types of chalcogenide for improved thermoelectric performance is described herein. Tin selenide (SnSe) is used as a base material, and Te nanoneedles are crystallized in the SnSe, resulting in the generation of a composite structure of SnSe with Te nanoneedles. The thermoelectric properties with various reaction times are investigated to reveal the optimum conditions for enhanced thermoelectric performance. A reaction time of 4 h at 450 K generated a composite Te nanoneedles/SnSe sample with the maximum ZT value, 3.2 times larger than that of the pristine SnSe. This result is attributed to both the reduced thermal conductivity from the effective phonon scattering of heterointerfaces and the improved electrical conductivity value due to the introduction of Te nanoparticles. This strategy suggests an approach to generating high-performance practical thermoelectric materials.

Controllable growth of two-dimensional SnSe2 flakes with screw dislocations and fractal structures

We report on the controllable synthesis of 2D SnSe₂ flakes with screw dislocations and fractal structures on mica and graphite substrates using a molecular beam epitaxy technique.

Growth of vertical heterostructures based on orthorhombic SnSe/hexagonal In2Se3 for high-performance photodetectors

Vertical heterostructures based on two-dimensional (2D) layered materials are ideal platforms for electronic structure engineering and novel device applications. However, most of the current heterostructures focus on layered crystals with a similar lattice. In addition, the heterostructures made by 2D materials with different structures are rarely investigated. In this study, we successfully fabricated vertical heterostructures by combining orthorhombic SnSe/hexagonal In₂Se₃ vertical heterostructures using a two-step physical vapor deposition (PVD) method. Structural characterization reveals that the heterostructures are formed of vertically stacked SnSe on the top of the In₂Se₃ film, and vertical heterostructures possess high quality, where In₂Se₃ exposed surface is the (0001) plane and SnSe prefers growing along the [100] direction. Raman maps confirm the precise spatial modulation of the as-grown SnSe/In₂Se₃ heterostructures. In addition, high-performance photodetectors based on the vertical heterostructures were fabricated directly on the substrate, which showed a broadband response, reversibility and stability. Compared with the dark current, the device demonstrated one order magnification of photocurrent, about 186 nA, under 405 nm laser illumination and power of 1.5 mW. Moreover, the device shows an obvious increase in the photocurrent intensity with the changing incident laser power, where I _ph ∝ P ^0.7. Also, the device demonstrated a high responsivity of up to 350 mA W^-1 and a fast response time of about 139 ms. This study broadens the horizon for the synthesis and application of vertical heterostructures based on 2D layered materials with different structures and further develops exciting technologies beyond the reach of the existing materials.

Thermodynamics, Electronic Structure, and Vibrational Properties of Sn n (S1-x Se x ) m Solid Solutions for Energy Applications

The tin sulfides and selenides have a range of applications spanning photovoltaics and thermoelectrics to photocatalysts and photodetectors. However, significant challenges remain to widespread use, including electrical and chemical incompatibilities between SnS and device contact materials and the environmental toxicity of selenium. Solid solutions of isostructural sulfide and selenide phases could provide scope for optimizing physical properties against sustainability requirements, but this has not been comprehensively explored. This work presents a detailed modeling study of the Pnma and rocksalt Sn(S_1-x Se _x ), Sn(S_1-x Se _x )₂, and Sn₂(S_1-x Se _x )₃ solid solutions. All four show an energetically favorable and homogenous mixing at all compositions, but rocksalt Sn(S_1-x Se _x ) and Sn₂(S_1-x Se _x )₃ are predicted to be metastable and accessible only under certain synthesis conditions. Alloying leads to a predictable variation of the bandgap, density of states, and optical properties with composition, allowing SnS₂ to be "tuned down" to the ideal Shockley-Queisser bandgap of 1.34 eV. The impact of forming the solid solutions on the lattice dynamics is also investigated, providing insight into the enhanced performance of Sn(S_1-x Se _x ) solid solutions for thermoelectric applications. These results demonstrate that alloying affords facile and precise control over the electronic, optical, and vibrational properties, allowing material performance for optoelectronic applications to be optimized alongside a variety of practical considerations.

Thermoelectric properties of SnSe nanowires with different diameters

Tin selenide (SnSe) has been the subject of great attention in the last years due to its highly efficient thermoelectricity and its possibilities as a green material, free of Pb and Te. Here, we report for the first time a thermoelectricity and transport study of individual SnSe micro- and nano-wires with diameters in the range between 130 nm and 1.15 μm. X-ray diffraction and transmission electron microscopy analyses confirm an orthorhombic SnSe structure with Pnma (62) symmetry group and 1:1 Sn:Se atomic ratio. Electrical and thermal conductivity and the Seebeck coefficient were measured in each individual nanowire using a specialized suspended microdevice in the 150-370 K temperature range, yielding a thermal conductivity of 0.55 Wm-1 K-1 at room temperature and ZT ~ 0.156 at 370 K for the 130 nm diameter nanowire. The measured properties were correlated with electronic information obtained by model simulations and with phonon scattering analysis. The results confirm these structures as promising building blocks to develop efficient temperature sensors, refrigerators and thermoelectric energy converters. The thermoelectric response of the nanowires is compared with recent reports on crystalline, polycrystalline and layered bulk structures.

High-Performance Near-Infrared Photodetectors Based on p-Type SnX (X = S, Se) Nanowires Grown via Chemical Vapor Deposition

Because of the distinct electronic properties and strong interaction with light, quasi-one-dimensional nanowires (NWs) with semiconducting property have been demonstrated with tremendous potential for various technological applications, especially electronics and optoelectronics. However, until now, most of the state-of-the-art NW photodetectors are predominantly based on the n-type NW channel. Here, we successfully synthesized p-type SnSe and SnS NWs via the chemical vapor deposition method and fabricated high-performance single SnSe and SnS NW photodetectors. Importantly, these two NW devices exhibit an impressive photodetection performance with a high photoconductive gain of 1.5 × 10⁴ (2.8 × 10⁴), good responsivity of 1.0 × 10⁴ A W^-1 (1.6 × 10⁴ A W^-1), and excellent detectivity of 3.3 × 10¹² Jones (2.4 × 10¹² Jones) under near-infrared illumination at a bias of 3 V for the SnSe NW (SnS NW) channel. The rise and fall times can be as efficient as 460 and 520 μs (1.2 and 15.1 ms), respectively, for the SnSe NW (SnS NW) device. Moreover, the spatially resolved photocurrent mapping of the devices further reveals the bias-dependent photocurrent generation. All these results evidently demonstrate that the p-type SnSe and SnS NWs have great potential to be applied in next-generation high-performance optoelectronic devices.

Tin Selenide (SnSe): Growth, Properties, and Applications

The indirect bandgap semiconductor tin selenide (SnSe) has been a research hotspot in the thermoelectric fields since a ZT (figure of merit) value of 2.6 at 923 K in SnSe single crystals along the b-axis is reported. SnSe has also been extensively studied in the photovoltaic (PV) application for its extraordinary advantages including excellent optoelectronic properties, absence of toxicity, cheap raw materials, and relative abundance. Moreover, the thermoelectric and optoelectronic properties of SnSe can be regulated by the structural transformation and appropriate doping. Here, the studies in SnSe research, from its evolution to till now, are reviewed. The growth, characterization, and recent developments in SnSe research are discussed. The most popular growth techniques that have been used to prepare SnSe materials are discussed in detail with their recent progress. Important phenomena in the growth of SnSe as well as the problems remaining for future study are discussed. The applications of SnSe in the PV fields, Li-ion batteries, and other emerging fields are also discussed.

Tin(iv) chalcogenoether complexes as single source precursors for the chemical vapour deposition of SnE2 and SnE (E = S, Se) thin films

Distorted octahedral complexes of Sn(iv) with thio- and seleno-ether ligands have been used as single source precursors in low pressure CVD experiments under various conditions to deposit tin mono and dichalcogenide thin films.

Ionic liquid bifunctionally modulated aggregation-coalescence mechanism to synthesize SnSe single-crystal nanorod/nanoparticle core shell nanostructures and single-crystal nanorods for optoelectronics

Ionic liquid-bifunctional modulated synthesis of SnSe nanorafts and nanorods for optoelectronics.

Structural investigations of SnS1−xSexsolid solution synthesized from chalcogeno-carboxylate complexes of organo-tin by colloidal and solvent-less routes

Tin chalcogenides are important semiconducting materials due to their non-toxic nature, cost effectiveness and layered structure.

Hydrothermal transformation of SnSe crystal to Se nanorods in oxalic acid solution and the outstanding thermoelectric power factor of Se/SnSe composite

The present work demonstrates the synthesis of one-dimensional (1D) Se nanorods with ~50 nm diameter by hydrothermal transformation of SnSe crystals in oxalic acid solution and suggests the reaction mechanism for this chemical transformation. SnSe particles react with oxalic acid to generate numerous Se nuclei, which crystallize into Se nanorods due to the intrinsic character of the 1D growth of Se. The resulting Se/SnSe composite exhibits outstanding thermoelectric power factor without the aid of any rare dopants, which is higher than both undoped polycrystalline SnSe and SnSe doped with Pb and Cu.

Chemically Exfoliated SnSe Nanosheets and Their SnSe/Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) Composite Films for Polymer Based Thermoelectric Applications

Tin selenide (SnSe) nanosheets (NSs) are prepared by hydrothermal lithium-intercalation and a subsequent exfoliation process from a SnSe ingot. Conducting polymer poly(3,4-ethylenedioxythiohene):poly(styrenesulfonate) (

PEDOT

PSS)-based thermoelectric composites are fabricated with varying SnSe NSs content, and the thermoelectric properties of the composites are examined at 300 K. The exfoliated SnSe particles show thin two-dimensional sheet-like structures that are evenly distributed into the

PEDOT

PSS matrix. The significantly enhanced power factor (S(2)·σ) of the SnSe NS/PEDOT:PSS composites with increasing SnSe NSs content can be explained by the potential difference at the interface between the SnSe and

PEDOT

PSS. The fabricated SnSe NS/PEDOT:PSS composites show a maximum figure of merit (ZT) of 0.32 at a SnSe NSs loading of 20 wt %. The mixing of inorganic nanoparticles with the conducting polymer matrix forms products with extremely low thermal conductivities, which is a promising strategy for the realization of polymer based efficient thermoelectric applications.

Crystal structure and phase transition of thermoelectric SnSe

Tin selenide-based functional materials are extensively studied in the field of optoelectronic, photovoltaic and thermoelectric devices. Specifically, SnSe has been reported to have an ultrahigh thermoelectric figure of merit of 2.6 ± 0.3 in the high-temperature phase. Here we report the evolution of lattice constants, fractional coordinates, site occupancy factors and atomic displacement factors with temperature by means of high-resolution synchrotron powder X-ray diffraction measured from 100 to 855 K. The structure is shown to be cation defective with a Sn content of 0.982 (4). The anisotropy of the thermal parameters of Sn becomes more pronounced approaching the high-temperature phase transition (∼ 810 K). Anharmonic Gram–Charlier parameters have been refined, but data from single-crystal diffraction appear to be needed to firmly quantify anharmonic features. Based on modelling of the atomic displacement parameters the Debye temperature is found to be 175 (4) K. Conflicting reports concerning the different coordinate system settings in the low-temperature and high-temperature phases are discussed. It is also shown that the high-temperatureCmcmphase is not pseudo-tetragonal as commonly assumed.

Porous nanostructures of SnSe: role of ionic liquid, tuning of nanomorphology and mechanistic studies

Porous SnSe nanoparticles have been synthesized in imidazolium based RTILviaelectron beam irradiation. RTIL provides a stabilizing environment as well as anin situsource of reducing radicals for the reduction of precursors.

One-step synthesis of high-quality homogenous Te/Se alloy nanorods with various morphologies

A convenient method was developed for controllable synthesis of homogeneous trigonal Te/Se alloy nanorods (t-Te/Se ANRs) with diverse morphologies, aspect ratios and compositions at room temperature in aqueous solution.

Aqueous phase one-pot green synthesis of SnSe nanosheets in a protein matrix: negligible cytotoxicity and room temperature emission in the visible region

A rapid, facile, reproducible and green method for synthesizing SnSe nanosheets in aqueous media is reported. Cyclic voltammetry studies indicate better thermodynamic feasibility for reducing SnSe, while the nanomaterial is nontoxic up to a 100 μM concentration in CHO cells.

Wide range photodetector based on catalyst free grown indium selenide microwires

We first report the catalyst free growth of indium selenide microwires through a facile approach in a horizontal tube furnace using indium and selenium elemental powders as precursors. The synthesized microwires are γ-phase, high quality, single crystalline and grown along the [112̅0] direction. The wires have a uniform diameter of ∼1 μm and lengths of several micrometers. Photodetectors fabricated from synthesized microwires show reliable and stable photoresponse exhibiting a photoresponsivity of 0.54 A/W, external quantum efficiency of 1.23 at 633 nm with 4 V bias. The photodetector has a reasonable response time of 0.11 s and specific detectivity of 3.94 × 10(10) Jones at 633 nm with a light detection range from 350 to 1050 nm, covering the UV-vis-NIR region. The photoresponse shown by single wire is attributed to direct band gap (Eg = 1.3 eV) and superior single crystalline quality. The photoresponsive studies of single microwires clearly suggest the use of this new and facile growth technique without using catalysts for fabrication of indium selenide microwires in next-generation sensors and detectors for commercial and military applications.