Tin-selenide as a futuristic material: properties and applications

SnSe/SnSe2 is a promising versatile material with applications in various fields like solar cells, photodetectors, memory devices, lithium and sodium-ion batteries, gas sensing, photocatalysis, supercapacitors, topological insulators, resistive switching devices due to its optimal band gap. In this review, all possible applications of SnSe/SnSe2 have been summarized. Some of the basic properties, as well as synthesis techniques have also been outlined. This review will help the researcher to understand the properties and possible applications of tin selenide-based materials. Thus, this will help in advancing the field of tin selenide-based materials for next generation technology.


Introduction
The fast developing area of applied material science demands materials to be cheap, non-toxic, environment-friendly, easy to synthesize, and well competitive in performing particular applications. Nowadays, materials with versatility have gained massive attention due to their applicability in almost all elds. 1,2 Various materials have been explored and showed promising versatile applications, e.g., graphene (Gr), 1 TiO 2 , 3 ZnO, 4 Cu 2 SnS 3 (CTS), 2 etc. The multifunctional applicability of these materials paves the foundation for interdisciplinary research. Chalcogenide-based materials have also shown such potential and can be seen as the future hope to meet a similar requirement. Among chalcogenides, tin selenide has demonstrated great potential in the applied material science. Tin selenide exists in two phases, i.e., SnSe and SnSe 2 . Some researchers have observed another phase, Sn 2 Se 3 , 5,6 but this phase is the superposition of SnSe and SnSe 2 . 7 Tin selenide has demonstrated versatility in thermoelectric, 8 photodetector, 9 solar cells, 10 photocatalytic, 11 phase change memory, 12 gas sensing, 13 anode material for battery, 14 supercapacitor, 15 and topological insulator (TI). 16 These applications strongly depend upon the properties of SnSe (optical, electrical and microstructural, etc.). Apart from material properties, material synthesis/deposition methods also play an essential role in obtaining high-quality materials. [17][18][19][20][21] Excellent review article exists on thermoelectric materials consisting of fundamental properties to the thermoelectric device's nal design, growth, defects, working environment issues, and applications. 22 Other reviews that focusses on SnSe describes all the aspects mentioned above (like growth, defects, conguration, etc.). 23 The aim of this review is to summarize the ongoing progress on SnSe, SnSe 2 synthesis methods, materials properties, and its possible application in various elds. However, some studies focus on pristine tin-selenide phase and its application 8,24 Another good reviews 8,25,26 that concentrates on SnSe describing all aspects mentioned above (like growth, defects, design, etc.), provides all-round knowledge to researchers. This review gives insights into the phases, structures, synthesis methods, progress in the tin-selenide, and tin diselenide for various applications. Hence, this article aims to review the tin selenide devices, present status, recent progress in the growth process, relatedissues, possible solutions, and their possibility of developing next-generation technology.

Material properties of tin selenide
Tin selenide based SnSe/SnSe 2 are binary compound semiconductor materials having p/n-type conductivity. 20,27 Fig. 1(a) shows a salient feature of tin selenide materials. The SnSe exists in two crystallographic phases, i.e., orthorhombic (a-SnSe) 28 and cubic (p-SnSe), 29 and SnSe 2 30 exists in the hexagonal crystal structure. The p-SnSe material is mechanically stable and energetically comparable to a-SnSe and has already shown potential in the piezoelectric application. 29 The orthorhombic crystal structure of SnSe showed second-order displacive type phase transition above 750 K, from a-SnSe (space group Pnma) to b-SnSe (space group-Cmcm) with (a ¼ 11.49 A, b ¼ 4.44 A, c ¼ 4.135 A) to the (a ¼ 4.31 A, b ¼ 11.70 A, c ¼ 4.31 A), respectively ( Fig. 1(b)). 31,32 The crystal structure of SnSe (violet, Sn atoms;

Synthesis of SnSe and SnSe 2
The material's properties also depend on the growth conditions, 42 size, and morphology of the material, 43 phase purity, 43 defects, 44 etc. Various physical and chemical methods have been established to fabricate SnSe/SnSe 2 materials on different substrates (glass, exible metal foil, polymer, etc.) for multiple applications. The tin selenide based semiconductor materials have been synthesized using atomic layer deposition (ALD), 17 sputtering, 45 thermal evaporation, 46 hydrothermal, 47 spray pyrolysis, 48 chemical vapor deposition (CVD), 49 etc.
Bulk crystals of the SnSe and SnSe 2 can be easily obtained by the physical solid-state reaction method. >99.99 pure elemental powders of the Sn and Se (in the stoichiometric ratio) is taken and sealed in the quartz tube at the pressure of $10 À4 torr. It is placed at the temperature according to the Sn-Se system phase diagram, as shown in Fig. 2(a). The Bridgman method can be used to obtain single crystals. The powder was melted into the furnace and brought in the contacts with the single crystal's seed during its cooling. The material grows along the crystallographic orientation of the seed crystal. Depending upon the geometry it is called the horizontal Bridgman method or vertical Bridgman method. The Bridgman-Stockbarger technique as shown in (Fig. 2(a)) can be used to obtain a single crystal of SnSe and SnSe 2 with an optimized ampule lowering rate of 7 mm h À1 and 4 mm h À1 , respectively, for SnSe and SnSe 2 with an optimized temperature gradient of 15 C cm À1 . 50 Another technique to grow single crystals are the direct vapor transport method, 42 and temperature gradient method. 51 A detailed review of the growth of the SnSe single crystals can be found in an earlier study. 25 Various methods to grow the nanostructure of the SnSe and SnSe 2 is described in this section.

Synthesis of nanostructured SnSe
Li et al. 52 devised the facile cathodic exfoliation method to exfoliate SnSe bulk into quantum dots with high yield. Under the organic electrolyte containing 0.2 M of tetrabutylammonium (at À7.5 V for 30 min) bulk SnSe was exfoliated and downsized up to $10 nm in lateral size, which was further downsized to $4 nm by breaking weakly entangled dots with the help of sonication as shown in Fig. 2(b). To synthesize the SnSe ake up to one layer, Jiang et al. 53 reported the two-step process, in which rst the bulk SnSe akes were obtained by the atmospheric pressure vapor transport deposition method. Flash evaporation of SnSe powder was done by moving the quartz tube position containing SnSe powder at 700 C under Ar/H 2 gas pressure under atmospheric condition. SnSe is deposited on the Si substrate kept in upside-down position. As obtained akes were etched with the N 2 gas. Etching time of 5-20 min gave the single layer SnSe ake (thickness of 6.8 A). 53 The SnSe nanosheet of the best quality can be produced from the SnSe bulk powder by the three-step method. 54 The method comprises the intercalation of Li + ion into the SnSe layers by lithication and then reacting the material with water. The rapid expansion of H 2 gas during reaction with water exfoliated the SnSe nanosheets. To improve the nanosheets' yield, further sonication of the SnSe powders' residuals is carried out (shown in Fig. 2(c)).
Using the precursor diselenoether SnCl 4 [BuSe(CH 2 ) 3 -Se n SeBu]}, SnSe x nanostructures are formed on the Si(100) substrate coated with Au (acted as a catalyst) by the liquid injection chemical vapor deposition method, and growth was studied in the temperature range 450-550 C. Under the 1.1 sccm ow of Ar gas and 1.5 mL h À1 of precursor injection rate, SnSe 2 ake growth occurred at 450 C, and considerable growth of SnSe nanowires occurred at 500 C as shown in Fig. 2(d) and (e). 49

Synthesis of nanostructured SnSe 2
Choi et al. 55 synthesized the nano-plates of the SnSe 2 by dissolving 5 mL oleylamine in 50 mg, 0.26 mmol SnCl 2 at 220 C.
Aer that, a solution containing 1,3-dimethylimidazoline-2selenone (90 mg, 0.53 mmol), dichloromethane (4 mL), and oleylamine (2 mL) were mixed and heated for 2 h at this temperature. Aer that, it was cooled to room temperature, centrifuged, and the residue was washed using hexane to obtain SnSe 2 nanoplates. The transmission electron microscopy (TEM) image of SnSe 2 nanoplates are shown in Fig. 3(a)-(d).
A simple two-step method, i.e., sonication followed by laser ablation, was used by Li et al. to obtain SnSe 2 quantum dots (QD) from bulk powder. 56 Manually grinded SnSe 2 powder (20 mg) was dispersed in 30 mL deionized water, and the solution was sonicated (650 W) for 2 h in an ice bath for 4 s. Sonication resulted in smaller particles and akes of SnSe 2 . The obtained solution kept in quartz cuvette was irradiated with a 1064 nm laser (Nd:YAG, 2.2 W) for 10 min. Irradiated tiny particles were centrifuged for 30 min at 6000 rpm, resulting in SnSe 2 QD, as shown in Fig. 4

(a) and (b).
Ma et al. 58 synthesized the nanorods and nanoplates of the SnSe 2 by organic solution phase route. Two different solutions of 0.4 mmol SnCl 2 $2H 2 O, 2 mmol oleic acid, 8.5 mL liquid paraffin oil, and 0.4 mmol selenium powder, 9 mL liquid paraffin were used. The rst solution was heated at 160 C till lemon yellow color appeared, and the later solution was heated at 240 C till wine color appeared. The latter solution was injected into rst using syringe rapidly and the mixture was heated at 200 C for 20 min. Aer cooling, toluene and methanol was added. Aer centrifugation nanorods and nanoplates' were obtained. Chemical vapor deposition was employed by Zhau et al. to synthesize ultrathin (1.5 nm) SnSe 2 akes of high quality. 57 Low melting point precursor SnI 2 (0.01 g) and Se (0.1 g) powders were placed in two different alumina crucible, and mica substrate were placed $15 cm from the central zone. The central zone was heated at 600 C for 15 min, with a ow of H 2 (5 sccm) and Ar (20 sccm) at ambient pressure. As-synthesized akes are shown in Fig. 4(c)-(g).

Various applications
In recent times, tin selenide has gained signicant research attention for solar cells, thermoelectric, photodetectors, photocatalytic, phase change memory, gas sensing, and anode material for battery, supercapacitor, topological insulator etc. Various reported applications based on SnSe materials are displayed in Fig. 5.

Thin lm solar cells (TFSCs)
Fabrication of low-cost thin-lm solar cells is highly needed as they can also be deposited over exible substrates. The CuInGaSe 2 (CIGSe) and CdTe based solar cells have shown record small cell power conversion efficiency (PCE) of 23.35% and 22.1%, respectively. 66 But in CIGS TFSC, indium is rare, and gallium is very costly. In CdTe TFSC, Cd is toxic, and Te is a rare material. These materials related problems hinder the commercialization of low-cost, earth-abundant TFSC based on CISG and CdTe. In recent time, earth-abundant and low-cost compound semiconductor materials like Cu 2 ZnSnS 4 (CZTS), Cu 2 ZnSnSe 4 (CZTSe), and Cu 2 ZnSn(S,Se) 4 (CZTSSe) have gained massive consideration as alternate materials instead of CIGS and CdTe as they have shown the efficiency of 10%, 11.95%, and 12.62%, respectively. 67-69 But, kesterite material consists of many elements. 70 The presence of many elements in the absorber layer increases the processing cost, and defects are also formed during the processing, which ultimately reduces the overall PCE of solar cells. To overcome this problem, binary element-based materials with similar optical and electronic properties are more suited than quaternary element-based kesterite materials. Tin sulde (SnSe) is also attracting attention of researchers for optoelectronic device applications. In SnSe, only one impurity phase of SnSe 2 is present. 71 Therefore, the formation of phase SnSe is easier. Also, being consists of earth-abundant, inexpensive, and eco-friendly elements, SnSe has attracted signicant attention. The SnSe has an optimum bandgap of 1.1 to 1.3 eV, an absorption coefficient of $10 5 cm À2 , and p-type conductivity with high carrier concentration (10 17 cm À3 ), making it a fabulous material for solar photovoltaic applications. 8 Due to its optimum bandgap (1.3 eV) and high absorption coefficient (10 5 cm À1 ), a thin layer of 300 nm thickness can absorb most of the useful solar spectrum. The tunable bandgap of thin-lm SnSe can help absorb more solar radiation and enable more photon absorption, resulting in more electron-hole pair generation. The theoretical efficiency for material with a bandgap of 1.3 eV is 32%. 72 The SnSe thin lm can be deposited using various chemical and physical routes like chemical bath deposition (CBD), 73 electrodeposition, 74 spray pyrolysis, 75 atomic layer deposition (ALD), 17 thermal evaporation, 46 sputtering. 76,77 The device architecture for SnSe TFSC is SLG/Mo/SnSe/CdS/i-ZnO/TCO/metal grid, similar to CIGS, 78 and CZTS/CZTSe/ CZTSSe TFSCs. 79 Therefore, a lot of modications and optimizations are required. Also, many issues need to be mitigated to improve efficiency. 80 Consequently, it is essential to conduct a study on SnSe material to develop environmentally-friendly, low-cost TFSCs. Any photovoltaic materials should have good optical properties, appropriate band gap, good absorption coefficient, high carrier concentration, and better transport properties. Solar cell efficiency may get affected due to losses, i.e., optical losses, non-absorption, thermalization, reection loss, transmission loss, area loss, collection losses, and resistance losses. 81 However, the formation of pure phase SnSe thin lms has not been reported as the phase lies in a very narrow region, and ne compositional tuning is required to achieve SnSe pure phase. Sn and Se's reaction mechanism plays a crucial role in attaining pure phase SnSe thin lms. 19,82 Reddy et al. studied the effect of selenization temperature, elemental composition, and selenization pressure on SnSe. They observed that the single-phase SnSe thin lms could be achieved during the selenization process in the temperature ranges from 300 to 500 C. 77 The solar cell efficiency depends on various parameters, like deposition condition, the absorber's crystallinity, carrier concentration, and the p-n junction's nature. The lower solar cell efficiency based on SnSe thin lms may be due to SnSe absorber materials' low quality. The parameters that highly inuence SnSe thin lm quality are phase purity, higher crystallinity, and larger grain size with fewer pinholes. It has been observed that during SnSe phase formation, there is a possibility of the formation of the SnSe 2 phase. Thus, the SnSe 2 phase's impact on the solar cell performance of SnSe thin lms needs to be carefully studied and optimized. 8 The electronic properties of p-n junction interfaces are strongly inuenced by the discontinuities between the valence band (VB) maxima and conduction band (CB) minima of each material, which restrict the electron transport across the junction interfaces. Band alignment of SnSe with an n-type buffer layer, the density of defect states at the hetero-junction interface, and the nature of back contact also inuence the solar cell performance. Therefore, band offsets play a crucial role in reducing the dark current in a diode, reducing the photon-generated carrier losses, and improving the overall solar cell conversion efficiency. The n-type buffer layer with a wider band gap is needed to overcome the optical losses and provide better alignment with SnSe absorber material.
In 1990, Singh et al. reported SnSe based solar cells with power conversion efficiency (PCE) of 2.3%. 75 The fabricated solar cell showed an open-circuit voltage (V oc ) of 410 mV, shortcircuit current density (J sc ) of 9.20 mA cm À2 , and a ll factor (FF) of 49%, respectively. Rahman et al. fabricated heterojunction (p-SnSe/n-Si) solar cells and achieved an efficiency of about 6.44%. 83 They observed improved power conversion efficiency due to the improvement in the J sc with a graded junction. 83 Shinde et al. fabricated SnSe thin-lm solar cells using electrodeposition and exhibited device efficiency of 1.4% using an absorber lm with 300-400 nm-sized grains, uniform, and dense lm morphology. 73 Franzman et al. synthesized SnSe/PPV (poly[2-methoxy-5-(3 0 ,7 0 -dimethyl octyl oxy)-1,4phenylenevinylene]) nanocrystals for solar cells and observed improvement in efficiency from 0.03% to 0.06%.
They observed signicant improvement in external quantum efficiency (EQE) and J sc aer adding PPV into the SnSe absorber. . The low device efficiency is mainly due to poor junction quality. 74 Makori et al. 84 fabricated solar cell with device structure glass/Ag/CdO:Sn/ SnSe/Ag by using thermal evaporation and reported device efficiency of 0.59% (V oc ¼ 273 mV, J sc ¼ 0.993 mA, and FF ¼ 69%). The observed lower efficiency is mainly due to the thin absorber (148 nm) layer, which attributed to lower J sc in the device. The above discussion is summarized in Table 1. Table 1 shows that the efficiency of SnSe based solar cells is well below Cu 2 ZnSn(S,Se) 4 . 85 The observed lower efficiency is mainly due to bulk defects, secondary phase formation in SnSe, non-ideal band-alignment at heterojunction interfaces, and back and front electrode recombination. The absorber quality plays a signicant role, and a sound absorber should have a dense, pinhole-free, smooth surface of the lm for highefficiency heterojunction device applications. The low interfacial quality could primarily affect the device performance and is responsible for lower V oc and FF in the solar cells. This is mainly due to CB discontinuity/band offsets at the interfaces, which leads to the CB's energy barrier and restricts carrier transportation across the p-n junction. Apart from this, absorber thickness, grain size, bandgap, and deposition methods also inuence the device performance.

Thermoelectric generators
It requires energy to maintain the sustainability and development of humankind. Energy sources are minimal, and there arises a question about the world's power crisis and its ultimate solution. A thermoelectric device that can convert the waste heat into electricity can contribute to a remarkable extent in this.

SnSe thermoelectric
SnSe single crystal. With the report of a high ZT value of 2.6 along the b axis in single-crystal SnSe, 32 this material became the hot topic of research in the eld of thermoelectric (TE) domains since 2014. SnSe is anisotropic and exhibits the second-order displacive phase transition from Pnma (distorted) to Cmcm (non-distorted) at 750-800 K. It shows a moderate power factor of 10.1 mW cm À1 K À2 at $850 K along the b axis, which is comparable or even lesser than typical thermoelectric materials values. [86][87][88][89] The thermal conductivity of SnSe has astonished the researchers. Using the Gruneisen parameter, it has been estimated that SnSe exhibits the ultra-low thermal conductivity (<0.25 W m À1 K À1 ) due to the strong anharmonicity in bonding. 32 Theoretical studies 90,91 showed that optimized carrier concentration lies in the range 10 19 -10 20 cm À3 for SnSe for high ZT. For n-type SnSe, the estimated ZT is 3.1 along a axis at 770 K at carrier concentration of 2.8 Â 10 19 cm À3 , while the study showed that the n-type SnSe performed better than p-type SnSe. 92 Motivated by the high ZT along the b axis in SnSe, Li et al. 38 conducted the theoretical study on the p-type SnSe using the rst-principle calculation. A gure of merit (ZT) was greatly affected by the carrier concentration and also sensitive to the phases of SnSe (Pnma and Cmcm) (shown in Fig. 7(a) and (b)). Fig. 7(c) shows a comparison of theoretical and experimental data. 32 This study showed that a maximum ZT of 4.33 could be achieved for the Cmcm phase of SnSe at 923 K for carrier concentration of 1.84 Â 10 19 cm À3 , along the b axis.
Through another study, for Na doped SnSe single crystal, Zhao et al. reported the device with ZT device ¼ 1.34 in the temperature range 300-773 K. With ZT max. of 2.0 at 773 K, the device showed the conversion efficiency of $16.7%, which was higher than many Pb-based thermo-electric modules. 93 Various researchers showed different ZT values for the single crystal. For example, Wei et al. 94 showed ZT of $1, $0.8, and $0.25 along b, c, and a axis, respectively for in-house made fully dense singlecrystalline SnSe, Jin et al. 95 showed ZT $ 1 for single crystal grown by a vertical vapor deposition method. Different types of doping are being done to enhance the single-crystal performance. Chang et al. 96 doped bromine to make the SnSe conduction n-type and achieved record high ZT $ 2.8 AE 0.5 at 773 K along out of the plane direction. With the help of density functional theory and scanning tunneling microscopy, they pointed out that delocalized p electrons of Sn and Se near conduction band minima contribute more to orbital overlapping in out of the plane direction. The S doping in single-crystal SnSe decreased the carrier concentration due to charge trapping at grain interface. 97 Hence, S doping decreased the single crystal's thermoelectric performance rather than enhancing as predicted by theoretical calculation. 98 Pb doped n-type single crystal (via facile Sn-ux method) showed a 33% enhanced power factor than its pristine SnSe due to increased carrier concentration. 99 Ag-doped single crystal (Sn 0.97 Ag 0.03 Se) grown in a horizontal Bridgman furnace showed maximum ZT $ 0.95 along a axis at 793 K. 100 Bi doping in single-crystal SnSe resulted in n-type conduction and showed the remarkable ZT $ 2.2 along the b axis at 733 K. 51 Polycrystalline SnSe thermoelectric. Polycrystalline SnSe has comparably higher thermal conductivity and lower electrical conductivity than its single crystal counterpart. Though phonon scattering is reduced mainly by the grain boundaries, 101 it has higher thermal conductivity than its single crystal due to tin oxide layer 102 or absorption of oxygen. 103 Highly discrete values of ZT have been reported for polycrystal SnSe made by different growth techniques. Various researchers have tried to achieve a comparable result to a single-crystal. Many researchers chose to dope (Na, K, Cu, Zn, etc.) in poly-crystal SnSe to enhance the electrical properties and reduce the thermal conductivity effectively. In one study, the polycrystalline SnSe thermoelectric material showed ZT $ 0.5 at 823 K. 104 Several methods improve the device's thermoelectric performance, like large mass uctuations, band gap engineering, alloying, doping, nanostructuring, etc. 105 Gong et al. 47 studied the effect of Cu doping  15 74 in polycrystalline SnSe by varying the concentrations in the Cu x Sn 1Àx Se from x ¼ 0.01 to 0.004. They found that Cu doping enhanced the electrical conductivity due to the increased carrier concentration, as conrmed by Hall measurement. Still, Cu doping acted as a point defect and hence decreased the carrier mobility. The Seebeck coefficient also increased due to doping. Thus, the overall power factor increased due to doping, and the optimized value was obtained for doping x ¼ 0.01. Around 60% reductions in the lattice thermal conductivity as compared to undoped poly-crystal SnSe is observed. This was due to nanoprecipitation and mesoscale grains (as evidenced by highresolution transmission electron microscopy (HRTEM)). Finally, they achieved a ZT of 1.2 at 873 K. Shi et al. 106 enhanced the Cu's solubility limit in the SnSe and obtained the ZT of $1.41 for the doping level of x ¼ 0.118. Lee et al. 102 synthesized polycrystalline SnSe (Na 0.01 (Sn 0.95 -Pb 005 ) 0.99 Se), which exhibited ZT $ 2.5 at 773 K. They noticed that the oxide formation leads to higher thermal conductivity in poly-crystal SnSe (SnO 2 has $140 times higher thermal conductivity than SnSe). 107 They removed the oxides layers of the tin in SnSe by Ball milling followed by a reduction in 4% H 2 /Ar atmosphere for 6 h at 613 K. They pointed out that ball milling (BM) and reduction processes both are critically important to remove the oxides and its residuals from the sample. A remarkably ultralow thermal conductivity (total) of 0.20 W m À1 K À1 (even lower than the single crystal reported by Zhao et al. 32 ) (Fig. 8(a)) and $2 times enhanced power factor than the pristine SnSe (unball-milled and unreduced). The performance of pristine, reduced, ball-milled and reduced (BR) compared to with the single-crystal SnSe 32 and Na doped SnSe, 93 is shown in Fig. 8(b). Recently Gainza et al. 108 reported the highest ZT of 1.8 for the un-doped polycrystalline SnSe, and even with surface oxides layers on the sample.
The pressure-induced effect modies the textures of the polycrystal SnSe. A sintering pressure of 60 MPa leads to improved electrical properties. There was a meager improvement in thermal conductivity, resulting in a ZT of 0.7. 109 There is a vast literature on doping in the polycrystalline SnSe, like Na, 110 K, 111 Cd, 112 Ag, 113 and co-doping Na/Ag, 114,115 Bi/Cl, 116 Na/CNT, 117 etc. which resulted in the enhancement in the thermoelectric performance of the material. Lu et al. 118 enhanced the performance of polycrystalline SnSe by introducing large mass uctuations by doping sulfur, which led to very low lattice thermal conductivity (0.13 W m À1 K À1 at 873 K). Still, this doping enhanced the bandgap of the SnSe, which lowered the electrical conduction and hence the power factor (low carrier concentration due to large bandgap). Pb co-doping improved the power factor, further increasing the carrier concentration by one order of magnitude. Through co-doping a remarkable ZT of 1.85 at 873 K was achieved. The inclusion of micro carbon bers into the polycrystalline SnSe decoupled the thermal and electrical transport in the host matrix. Carbon bers acted as good electrical conductors and simultaneously reduced the lattice thermal conductivity (0.22 W m À1 K À1 ) by enhancing the scattering due to the high density of interface, which led to ZT $ 1.3 at 823 K. This inclusion increased the mechanical stability of the device. 119 The thermoelectric parameters of poly-crystals SnSe are tuned by sintering temperature in the spark plasma sintering (SPS). Zhang et al. 120 varied the sintering temperature from 300 to 650 C in SPS and showed that vacancy defects (Se, Sn, and Se-Sn) were responsible for the performance variation.
The optimized sintering temperature for the best performance of the material was 550 C. The ZT of 0.47 was observed at 430 C. Ge alloying's effect was studied by varying Ge concentration in the range x ¼ 0.01 to 0.03 (Sn 1Àx Ge x Se). The carrier concentration increased from 3.9 Â 10 17 to 4.2 Â 10 19 cm À3 for x ¼ 0.03, which resulted in a high-power factor of $5.10 mW cm À1 K À2 at 873 K. A very low lattice thermal conductivity of $0.18 W m À1 K À1 was achieved due to nanoscale grains, precipitation and anharmonicity due to Ge doping. Finally, high ZT of 1.75 and 2.1 were acquired at 873 K along parallel and perpendicular SPS's pressing direction. 121 The gure of merit (ZT) with different doping elements in single and polycrystal SnSe is shown in Fig. 9.
SnSe thin lm. Low temperature (4-300 K) thermoelectric measurements were carried out by Urmila et al. 122 in SnSe thin-lm obtained by the co-evaporation method. The maximum power factor obtained was 7.2 Â 10 À4 W m À1 K À2 . The thermal conductivity was in the range of 0.023 to 0.045 W m À1 K À1 , which showed a maximum ZT of 1.2 at 42 K, and thus showed a potential application in a low-temperature TE device. Nair et al. 123 fabricated the n-p type thermo-couples with PbSe (ptype), SnSe (p-type), and SnSe 2 (n-type), in the conguration SnSe-SnSe 2 -SnSe-SnSe 2 -SnSe and PbSe-SnSe 2 -PbSe-SnSe 2 -  PbSe using chemical method. A voltage was developed for a temperature difference of 20 C of magnitute 50 mV and 15 mV, respectively (the author did not report the ZT of the device). Thin-lm SnSe deposited by glancing angle (80 ) pulsed laser revealed a striking power factor of 18.5 mW cm À1 K À2 at 478 K (higher than the single crystal along b axis) due to enhanced grain boundaries and out of plane thermal conductivity (highest 0.189 W m À1 K À1 at 340 K). The authors did not report the ZT and in-plane thermal conductivity. 124 For the rst time, Burton et al. 125 reported thin lm SnSe thermoelectric generator on the glass substrate. Very low thermal conductivity of 0.08 W m À1 K À1 in the temperature range 375 to 450 K was observed (4 times lower than single-crystal thermal conductivity along a-axis). Still, it had a low power factor, which resulted in a very low ZT of 0.055 at 501 K. When the hot side was kept at 618 K, an output of 0.09 mW was achieved. Due to the oxide formation and uncontrolled cold side temperature, the performance degraded for a longer exposure. Song et al. 126 synthesized SnSe thin lm of varied thickness in the range of 600-1000 nm by magnetron sputtering and showed effect of post-annealing on the performance. The optimized annealing temperature was 700 K. The power factor for the lm with thickness 700 nm was 2.4 mW cm À1 K À2 at 675 K. The authors used the thermal conductivity data of bulk polycrystalline SnSe to calculate the ZT (0.28 at 675 K). Yan et al. 127 fabricated a exible thermoelectric device based on SnSe composite with PEDOT:PSS that showed 13.73 nW output power at 50 K temperature difference at 353 K, and showed possible application in wearable electronics as well due to stability of device even aer 1000 cycles of bending. The above discussion is summarized in Table 2.
4.2.2 SnSe 2 thermoelectric. A theoretical study on SnSe 2 using the rst principal method revealed its anisotropic nature. Due to the bipolar conduction nature (because of the low bandgap) optimal predicted doping values were (0.86-2.03) Â 10 19 and (1.71-2.47) Â 10 19 cm À3 along with the a and c axes, respectively. A maximum power factor of 11.72 Â 10 À4 W m K À2 Optimum doping in SnCu x Se 2 was found to be x ¼ 0.01, which resulted in an optimized power factor of 1.96 at room temperature and a decrease in thermal conductivity up to 0.81 W m À1 K À1 due to increased interface. 145 A very high ZT of 0.75 than to pristine SnSe 2 was obtained which was about two orders of magnitude larger than pristine SnSe 2 ( Fig. 10(a)).
Thin-lm SnSe 2 thermoelectric. Thermoelectric performance of SnSe 2 thin lms have also been studied. But there are very few reports on this, and the reports are limited to the lm's power factor only. Yin et al. deposited SnSe 2 thin-lm on a glass substrate by spin coating techniques and reported a Seebeck coefficient of $126.3 mV K À1 . The electrical conductivity of the order of 10 3 S m À1 at room temperature was observed. 146 Chen et al. deposited the SnSe thin lm on the Si wafer and annealed it in Se vapor. The lm was reduced to SnSe 2 and showed enhanced Seebeck coefficient 631 mV K À1 for SnSe 2 than 38.6 mV K À1 for SnSe. Around 44 times enhanced power factor (0.2 mW m À1 K À2 ) was obtained. 147 Duong et al. deposited SnSe 2 thin lm on the Al 2 O 3 substrate by the pulsed laser deposition technique. They reported a power factor of 8 mW m À1 K À2 at 220 K. 148 A detailed summary of the above discussions and ZT variation with the doping is provided in Table 3 and Fig. 10(b), respectively. This material (SnSe) has the highest efficiency among the bulk materials without any doping, but it faces commercialization issues like poor mechanical stability and low thermoelectric property below $800 K. Besides its high ZT, low cost, easy fabrication, and earth-abundance, SnSe suffers the real possible application likely due to the quick oxides' formation or thermal instability. 150 It readily forms the oxides in the range >600 C, where it showed maximum ZT. Protective environment costs hamper its effectiveness as a cheap/high ZT. Sassi et al. 129 showed thermal instability above the phase transition temperature of SnSe, and the working efficiency of the device decreased continuously over cyclic performance due to Se loss. Thus, there should be an optimized temperature range over which there is no loss of performance. Continuous research should go on in the direction to improve the thermal stability up to the temperature for which SnSe showed maximum ZT. Also, large phonons scattering centers exists in polycrystalline SnSe than in single crystal SnSe due to grain boundaries, yet single crystal SnSe has lower lattice thermal conductivity than the polycrystalline SnSe. Ibrahim et al. 151 noted this fact and investigated the reason behind this. They systematically discarded the oxide formation and found many native defects in the single crystal. 152 Their results reected higher lattice thermal conductivity than the reported one. 32 They also noted that the single crystal grown by Zhao et al. 32 was not fully dense and hence may be the reason for low lattice thermal conductivity. But it still requires the study to know the exact causes of the very low lattice thermal conductivity, whether it was due to intrinsic defect or not. SnSe single crystal suffers a low mechanical strength and high growth cost as a single crystal. SnSe single crystal thermoelectric results showed inconsistencies in the different group reports' as shown in Table 2. However, the further study required to unravel the SnSe performance, where it seeks the possibilities of enhancement in ZT. 38 Another phase, SnSe 2 , was also predicted by theoretical consideration as the best thermoelectric material that can achieve ZT around 3.5. Till date, ZT $ 1.1 is achieved in this material. Also, both materials (SnSe and SnSe 2 ) showed inferior TE property in thin lm form. Thus, this material demands greater attention and research to achieve predicted ZT. Researchers should pay attention to new approaches to improve the SnSe thermoelectric material's performance like nano-inclusion, 153 decoupling of interrelated parameters, 119 nano-structuring and texturing, etc. Simultaneously, the theoretical study should be carried out to optimize the threshold values of the parameters. Finally, tin-selenide has emerged as a futuristic material that showed the best efficiency. Both materials (SnSe and SnSe 2 ) demand more research in the thin lm and bulk thermoelectric form because of the inferior TE property and device stability.

Photodetectors (PDs)
4.3.1 SnSe based PDs. SnSe act as suitable optoelectronic material due to its high absorption coefficient (>10 4 cm À1 ), 154 low band gap and its tunable band gap over an extensive range. 155 Solvothermal processed SnSe nanorod showed repeatable and stable photoresponse. 156 SnSe thin lm grown on the n-Si substrate showed an ultrahigh response/recovery time of 0.9/17.3 ms in a position-dependent detector. 157 A exible device of SnSe on the mica substrate (grown by PLD) showed high responsive behavior of the device in a longer wavelength region (370-808 nm). Also, it reected its durability in bending test for the exible device. The device showed the highest photoresponse of 5.5 A W À1 which is attributed to its shallow potential barrier with Bi 2 Te 3 contacts (perfect band alignment) and low potential uctuation at the interface. 158 Under white light illumination, SnSe nanoplate and graphene composites showed very high photosensitivity (1110%) and short response time ($1 s). 159 The SnSe lm grown on Si substrate by vapor deposition and dispersed with graphene oxide quantum dots showed improved response time and photocurrent than the SnSe lm in a larger spectrum. 160 Selfpowered photodetection was demonstrated in the SnSe/Si heterostructure device, with very high detection and ultrafast response in ms range. Magnetron sputtered SnSe lm on Si(100) showed a nearly ideal diode property with a reported power exponent of 0.92 and ideality factor of $1, which revealed a trapfree and high-performance of the device. 36 Zhong et al. 161 showed the thin lm's superiority due to the Marangoni effect over other solution-based methods. The exible and vertically fabricated Gr/SnSe/Gr device showed a responsivity of 38 mA W À1 . Magnetron sputtered SnSe thin lm on Si substrate  162 Ouyang et al. 163 harnessed combined photovoltaic and thermoelectrics effect to enhance the self-biased n-type SnSe:Br (ITO/n-SnSe/Ag) photodetector performance. They coupled the thermoelectric property with the material's photovoltaic property via maintaining the temperature difference with Peltier cooler's help at the one contact (Ag contact) of the device and another under the illumination of 760 nm radiation. The thermoelectric diffusive process accelerates photoelectrons generated under photovoltaic conditions. This is due to specic heat capacities of electrons and photonic drags under the thermal gradient resulting in enhanced performance ( Fig. 11(a) and (b)). Simultaneously, in the energy band aspect, the thermoelectric effect reduced the Schottky barrier height at the ITO side and increased at the Ag side, which resulted in the de-acceleration of the photoelectrons. The combined effect enhanced the PD parameters. The light current and voltage were improved by 38.1 and 81.9%, respectively, at a cooling temperature difference of 1.5 K (the Ag side was cooler compared to the ITO side). When the Ag side is heated w.r.t. ITO with 4.5 K difference, the opposite result was observed, i.e., there was a decrease in photocurrent and voltage. Hence authors concluded that the coupling of the effect is polarity dependent. Record high value (comparable to the commercialize Ge and Si photodetectors) of responsivity (R) and detectivity (D) were reported in the nanowire SnSe. 164 Above discussions have been summarized in Table  4.
4.3.2 SnSe 2 based PDs. The tunable bandgap and high absorption coefficient of the SnSe 2 have made it a potential candidate for optoelectronics applications. For the rst time, Zhou et al. 173 reported ultrathin SnSe 2 ake (1.5 nm) grown by a CVD method using SnI 2 as a precursor, which exhibited high responsitivity (R) 1100 A W À1 with meager response time in millisecond. However, the far value of power exponent (0.7), re-ected its impurity and defects. Bilayered SnSe 2 of 1T type structure with D 3d point group symmetry showed high responsivity of   176 (which have deeper conduction minima than these) so efficiently that it improved performance parameters several times better. SnSe 2 with MoS 2 interface resulted in enhanced responsivity from 37.3 to 9.1 Â 10 3 A W À1 under 500 nm illumination. 176 Mukhokosi et al. studied the thickness-dependent optical properties of the SnSe 2 thin lm grown by DC sputtering followed by selenization and also studied the photodetection performance ( Fig. 12(a) and (b)). 30 Films with thickness <140 nm did not show any IR response. The lm with 1200 nm thickness had a slow response (time), and responsitivity was 0.4 mA W À1 . 30 Mukhokosi et al. 177 also reported the self-powered, organic-inorganic hybrid heterostructure PD consisting of poly (3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and SnSe 2 . The device had responsivity in the range of 1.4-2.6 mA W À1 . The response time improved to 1.33 s. Mukhokosi et al. 178 reported highly enhanced response time and enhanced responsivity. The device was fabricated on the ptype Si substrate by DC sputtering, followed by selenization. Kumar et al. 65 deposited SnSe 2 thin-lm on the soda-lime glass substrate and reported the improved responsivity at very low bias and fast response time in the IR range (Fig. 12(c) and (d)). Although the band gap was in the visible region, photodetection was observed in the IR region which was attributed to the defect/trap assisted levels lying in the forbidden region. 65 Usually, SnSe 2 shows high absorption coefficient 30,65 but poor chemical stability against environmental conditions (degrade into SnSe aer prolonged exposure). Therefore, Gao et al. used graphene (Gr) to improve the chemical stability. SnSe 2 was sandwiched between graphene. Monolayer Gr acted as useful ohmic contacts and also had weak absorption in the UV to IR range. 179 This sandwiched structure of SnSe 2 with Gr showed responsivity of 1.09 Â 10 3 A W À1 , and the response time was 30 ms. Murali et al. studied the substrate-induced effect on SnSe 2 ake (i) suspended over contacts and (ii) supported on Si substrate. It showed a vast difference in rise and decay times. 180 The suspended structure showed low gain but quicker decay than the supported one because of the device's interfaceinduced trap states. Field-effect transistors based on multilayer SnSe 2 showed very high responsivity and detectivity. The device showed its potential application comparable to the other commercialized photodetector. 9 The above discussion is summarized in Table 5.

Gas sensors
The gas sensor is a basic need for environment cleaning. A gas sensor is a device, which detects harmful gases and sends an alarm. There are many toxic gases in the environment (CO 2 , NO 2 , NH 3 , SO 2 , H 2 S, CH 4 , etc.). Gas sensing parameters are response time, recovery time, selectivity, gas concentration, temperature, detection limit, etc. Tin based chalcogenide (SnSe and SnSe 2 ) have shown promise for gas sensing applications. A brief discussion of tin chalcogenide (SnSe and SnSe 2 ) materials for gas sensing applications is described. The gas sensing mechanism of the tin chalcogenide (SnSe and SnSe 2 ) gas sensor is based on the adsorption of gas molecule and charge transfer.
Physisorption-based charge transfer in tin chalcogenide (SnSe and SnSe 2 ): To evaluate the interaction efficiency of the SnSe/SnSe 2 monolayer and gas molecules, the absorption energy (E a ), Hirschfeld charge transfer (Q), and the distance (d 0 ) of nearest atoms between the gas molecule and the SnSe/SnSe 2 layer were calculated. The absorbed energy is dened as: E gas , E material , and E total are the gas molecule's total energy, SnSe/SnSe 2 monolayer, and gas molecule-SnSe/SnSe 2 system, respectively. 184 The adsorption properties, including adsorption energy (E a ), equilibrium distance (d 0 ), and Hirshfeld charge transfer (Q) for SnSe, are listed in Table 6. The adsorbed CO, CO 2 , CH 2 O, NO 2 , and SO 2 gas molecules on the b-SnSe monolayer have the E a values À0.202, À0.175, and À0.322, À0.829, and À0.499 eV, respectively, and the Hirshfeld charge transfer values are À0.033, À0.036, À0.085, À0.279, and À0.279 e, respectively. 185 The equilibrium distance of CO, CO 2 , CH 2 O, NO 2 , and SO 2 from the b-SnSe layer is 3.293, 3.617, 3.222, 2.531, and 2.692 A, respectively, which are greater than the C-Sn (2.15 A), O-Sn (2.22 A), and N-Sn (2.11 A) bonds, indicating the process to be physisorption. 184 However, the optimized structure shows some distortion for O 2 on the surface of the b-SnSe sheet. The value of E a and Q c for O 2 are À1.596 and À0.445 eV, respectively, which is much larger than other gas molecules adsorbed on one SnSe layer, indicating that O 2 molecules (CO, CO 2 , CH 2 O, NO 2 , and SO 2 ) are chemically absorbed in the b-SnSe layer. 185 Adsorption of CO, CO 2 , CH 2 O on the b-SnSe layer showed lower adsorption energy and lower charge transfer values, which indicates that one b-SnSe layer is not suitable for the detection of these three molecules. 184 The higher absorption energy of NO 2 (À0.829 eV) and SO 2 (À0.499 eV) indicates that NO 2 and SO 2 molecules' absorption behavior for the b-SnSe layer was more potent than that of the absorption system. 185 The charge transfer values for NO 2 and SO 2 are À0.279 and À0.278 eV, respectively, which show a clear charge transfer between the gas molecule and the b-SnSe layer ( Fig. 13(a)-(f)). 185 The adsorption distances for NO 2 and SO 2 are 2.531 A and 2.692 A, respectively, which is close to the Sn-O bond length range (2.22 A to 2.66 A). 186 For the b-SnSe sheet, it is crucial to detect NO 2 and SO 2 in the gas sensor region. All the most stable energy adsorption centers are located on the Sn side of the atom of the b-SnSe sheet, which indicates that the absorbing properties of the metal atom are more substantial than that of the nonmetallic atom. 187 4.4.1 SnSe based gas sensors. Tin selenide (SnSe) is a member of (IVA-VIA) binary semiconductors family and has a high absorption coefficient. 188 The bandgap of SnSe varies from 0.9 eV to 1.3 eV. 189 SnSe has an orthorhombic crystal structure, and it has p-type conductivity with Sn-vacancies. 190 The majority of charge carriers of SnSe are holes and have a good impact on gas sensing applications. Using Ph 3 PSe as Cr/Au/n-Si/SnSe 2 (single crystal ake)/Cr/Au 450 3 5.11 Â 10 5 2.79 Â 10 13 -9 a SED ¼ single exponential decay, DED ¼ double exponential decay. a precursor in chemical vapor deposition, Assili et al. 21 deposited orthorhombic tin selenide thin lms onto three substrates. A 1% vol concentration of methane gas showed a good sensitivity at the operating temperature of about 200 C. The sensitivity, response, and recovery times were $47%, $52 s, and $220 s, respectively. They observed that when SnSe was mixed with any n-type materials, it showed excellent response and recovery times at, lower operating temperature. In 2013, Wang et al. 191 synthesized SnO 2 nanoparticles decorated SnSe nanosheets via a facile, lost-cost, and safe solution method and had studied the gas sensing properties. For 1000 ppm CO gas, this device's response was 1.9 s at the operating temperature of 260 C (Fig. 14(a)-(c)). Lee et al. 192 prepared the SnSe 2 /SnSe heterostructure lm from a thick Sn layer onto the glass substrate. The lm also showed response for one ppm NO 2 gas at room temperature, and the response value was 75%. In 2020, Wang et al. 193 reported a one-step colloid method for making SnSe/SnSe 2 heterostructures, with doping of SnSe z 30, 50, and 70%, respectively. The SnSe (50%)/SnSe 2 (50%) based sensor with an active layer thickness of 2 mm showed the highest sensitivity of 30% to 0.1 ppm NO 2 gas at room temperature (25 C) with a limit of detection (LOD) down to 69 ppb. The above discussion is summarized in Table 7. 4.4.2 SnSe 2 based gas sensors. SnSe 2 is an n-type material due to the presence of Se-vacancies. The majority of charge carriers in SnSe 2 are electrons. The SnSe 2 lm is an anisotropic binary material having a hexagonal structure arranged in the form of Se-Sn-Se. 194 SnSe 2 has a small bandgap of 1-2 eV. 30,195 SnSe 2 has the same electronegativity and similar structure as MoS 2 , thus it is better suited for the detection of NO 2 . 2D materials have a substantial surface to volume ratio, which signicantly changes the absorption and release of gases, making them an inexpensive gas detection candidate. 196 Given all these properties of SnSe 2 , various researchers have tried to study the properties of SnSe 2 gas. Lee et al. 192 prepared the SnSe 2 /SnSe heterostructure lm on the glass substrate from a thick Sn layer. At room temperature (RT), this lm showed a better response for one ppm NO 2 gas. The response value of this lm was 75%. In 2010, Popescu et al. 197 prepared silver doped SnSe 2 and Ge 2 Sb 2 Te 5 thin lms by pulsed laser deposition. Ag-Doped SnSe 2 thin lm-based gas sensor had a response of 3.26 for 500 ppm of CO gas at 500 C. In 2018, Chen et al. 194 produced SnSe 2 nanoplate arrays and made a sensor that could detect 1% vol. of CH 4   thermal evaporation technique. This sensor had response of 112% for (5 ppm) NO 2 gas at room temperature and the response/recovery times were 15/10 s, respectively. 199 AuPd coated SnSe 2 thin lm based NO 2 sensor showed enhanced response of 117% for 5 ppm NO 2 gas at room temperature. The response/ recovery times of this sensor were 10/18.7 s respectively. 200 Thus, efforts have been made to use SnSe 2 for sensing methane as well as NO 2 . The above discussion is summarized in Table 8. . Furthermore, hydroperoxyl HO 2 and hydroxyl radical OH À are formed aer the protonation process (hydrogen cation). Simultaneously, the holes (h + ) can oxidize hydroxyl radicals OH À and H 2 O molecules to generate OH À and hydroperoxyl HO 2 , which may break the RhB dye molecule to convert them into CO 2 and H 2 O or in other forms as shown in Fig. 15. The dye's degradation rate is dened as Here A 0 is the absorbance of dye in the dark condition. A T is the absorbance of dye at specic time gaps aer irradiation of light of a certain wavelength. Li Cheng et al. used the RhB solution under UV radiation to check the SnO 2 , SnSe, and SnS nano-bers' photocatalytic activity. 62 The degradation curve showed a rst-order kinetics equation type curve. The degradation rate for RhB was 85.90%, 92.55% and 92.86% for SnO 2 , SnS and SnSe, respectively. In another experiment, Jing group used thin lms of Sn and SnSe to analyze the photodegradation Hetero-structure R is called the sensor's response, S is called the sensor's sensitivity, R g is the resistance of the sensor in the presence of target gas, and R a is the sensor's resistance in the air. I g is the current of device in presence of target gas and I o is the current of device in presence of air. of RhB. 11 Two lms SnSe 2 and SnSe 2 /Se were made by the facile solvothermal method. The photocatalytic efficiency was measured from the formula mentioned below, where C is the concentration at time t and C 0 is the concentration at time t. The result showed that SnSe 2 /Se lm showed better degradation, i.e., around 94% compared to Sn lms in 50 min. This increase in efficiency was attributed to the formation of heterojunction and the production of a larger number of electron-hole pairs and a lower recombination rate. 4.5.2 SnSe composites for photocatalytic applications. Zhou Li et al. synthesized a composite of SnSe@SnO 2 nanoparticles and used them in the photothermal-photocatalyst mechanism study. The mechanism of electron-hole pair generation is similar to that mentioned earlier.
The photon falls on the surface, and the elector hole pair is generated. Here, both SnSe and SnO 2 took part in the photothermal and photocatalytic processes synchronously. In the process, the electrons from CB of SnSe travels to the CB of SnO 2 .
This increases the oxidation process of photocatalysis and thus improves the photocatalytic behavior. The inner shell SnSe favors the photothermal heat efficiently. 201 The photocatalytic activity of SnSe@SnO 2 nanocomposite was evaluated through the degradation of methyl orange (MO) experiment under solar light's irradiation. The photocatalytic degradation of MO with SnSe, SnO 2 , SnSe@SnO 2 , and P25 (commercially named as TiO 2 nanoparticles) was compared using the time versus C/C 0 graph. Here, C is the concentration at time t and C 0 is the concentration at time t. It was observed that SnSe@SnO 2 showed a much higher degradation rate than SnSe, SnO 2 , and TiO 2 . In some other methods, blue SE-2R dye degradation was studied by making a composite.
Karamat et al. 202 made SnSe composite with LaNdZr 2 O 7 and compared the photocatalytic efficiency with SnSe and blank dye. The setup composed a 300 W Xe arc lamp (PLS-SXE300, Beijing Trusttech Co. Ltd). The degradation of blue dye was analyzed at 658 nm using a UV-Vis spectrophotometer. Before the experiment, the stability of blue dye was checked under dark and light conditions. It was conrmed that the dye was stable, and the continuous illumination does not show any loss in spectra. The photocatalytic degradation experiment was then carried out, and it was found that LaNdZr 2 O 7 /SnSe showed a better degradation efficiency. The maximum absorption intensity at wavelength 658 nm showed a decrease in absorption as the irradiation time increases. SnSe alone has demonstrated degradation efficiency of just 15.5% while mixing it with LaNdZr 2 O 7 increased it signicantly. 4.6 Storage devices 4.6.1 Battery electrode. Tin-based alloys showed high theoretical capacity and are being considered as a very suitable material for sodium-ion batteries. Tin-based composites have a theoretical storage capacity of nearly 847 mA h g À1 . Still, a few other composites like SnSe and SnSe 2 showed even higher values than theoretical ones, with additional capacity contributions from conversion reactions. 14 Considering that one molar SnSe 2 can accomodate 7.75 molar sodium, SnSe 2 has shown a theoretical capacity of 756 mA h g À1 . 203 But during the alloying process, the volume of the material expands, which is dangerous for the cycle where I g is the current of device in presence of target gas and I o is the current of device in presence of air. R is called the sensor's response, S is called the sensor's sensitivity, R g is the resistance of the sensor in the presence of target gas, and R a is the sensor's resistance in the air. stability; hence some composites of carbon are used to reduce this bottleneck. Tin selenide-based battery application has been divided into two categories, namely Li À ion and Na + ion batteries. The recent advances in SnSe 2 for sodium-ion batteries and their results are explained in the coming section. Zhang et al. synthesized SnSe 2 based two-dimensional (2D) nanosheets using the hydrothermal technique. They achieved a theoretical capacity during the rst cycle and a stable and reversible specic capacity of 515 mA h g À1 at 0.1 A g À1 aer 100 cycles, which exhibited excellent performance. 204 The SnSe and SnSe 2 both have been used for both sodium and lithium-ion battery applications.  introduced Cu in SnSe for sodium-ion battery and exhibited a capacity of 330 mA h g À1 at 20 A g À1 . In a study, Xia et al. used electrospun SnSe with carbon nanobers for lithium and sodium-ion battery applications and showed improved results. Kim et al. investigated SnSe alloy as an anode for Na-ion batteries. They exhibited excellent electrochemical performance with a high reversible capacity of 707 mA h g À1 and stable performance over 50 cycles. 204 In recent time, various studies on SnSe composite has been conducted by multiple research groups to nd better results due to the layered structure of SnSe and SnSe 2 materials. Table 9 shows a summary of the above discussions.
Tin selenide-based composites have shown an excellent theoretical capacity. Aer many cycles, real practical uses of tin-based anodes in both types of batteries are still very limited. Some structural design, preparation advancement, and morphological development are needed to bring these materials to the production level. 4.6.2 Supercapacitor. Supercapacitors have gained colossal consideration due to their longer operational life, higher power densities, and better safety tolerances than batteries. Zhang et al. synthesized tin selenide (SnSe, SnSe 2 )-based 2D nanostructures on exible substrates for supercapacitor applications. 63 Ni et al. synthesized SnSe based anode using a microwave-assisted method. They exhibited a suitable specic capacitance of 214.3 F g À1 at 1 A g À1 and rate capability of 182.8 F g À1 at 20 A g À1 with outstanding cyclic stability. 211 Pandit et al. synthesized binder-free SnSe hexagonal nanosheets using a one-pot colloidal method and achieved high-performing anode material for supercapacitors. It showed a specic capacitance of 617.9 F g À1 at a scan rate of 2 mV s À1 and with fast charge-discharge cycles. 15

Memory devices
The phase change from amorphous to crystalline induces a sharp change in the optical and electrical properties like reectivity and resistivity, respectively. This change in the property is explored to store the data in the form of a bit. As this transition is reversible, this property is utilized in the rewritable data storage. Fast operating speeds, low power consumption, high energy density, large retention time, etc., are the critical parameters for selecting materials for memory applications. Chalcogenides based materials play the central role in this eld due to their very short time of the order of few nanoseconds of phase transition, high resistance ratio of the amorphous to the crystalline phase. Chung et al. investigated all phases of the tin selenide (SnSe, SnSe 2 , Sn 2 Se 3 ) 7 for memory application based on phase change and compared the result with the well-studied GeTe. 212 SnSe 2 is best among all phases, with high crystallization temperature (220 C), high electrical contrast (resistance ratio of amorphous to crystalline) of 8.3 Â 10 5 , T g /T m of 0.52, which showed its potential for the memory application. 212 Wang et al. demonstrated the swi recrystallization time of 20 ns for the solution-processed SnSe 2 thin lm, 213 comparable to the commercialize Ge 2 Sb 2 Te 5 . Sun et al. studied the multilayer Si/ SnSe 2 lm by varying the Si's thickness in steps of 4 nm for 0-20 nm, with the lm's total thickness nearly constant at 100 nm. 214 The varying thickness of the Si did not show any effect on the crystallization temperature. Still, a remarkable change in the resistance of the lm with Si thickness of 12 nm {[Si (12 nm)/SnSe 2 (5 nm)] 6 } was observed, i.e., crystalline resistance increased and amorphous resistance decreased, which led to enough (more than three orders of magnitude) electrical contrast to storing information. Simultaneously increasing crystalline resistance reduced the operating current that led to lower power consumption. Multilayer with Si thickness of 12 nm (of nearly 100 nm total thickness) showed its high potential for the low power operated phase-change random access memory (PCRAM) device because of almost ten year retention time at 83 C and low activation energy. 214 The difficulty in phase-change memory (PCM) application of these materials is due to high amorphous resistivity of SnSe 2 , fast crystallization speed, low thermal stability, retention of Sb rich alloy, and very low phase change temperature of Sb. Optimized design of the super lattice like (SLL) structure of SnSe 2 and Sb ([SS(10)/S(2)] 4 ) (SS stands for SnSe 2 and S for Sb with 10 and 2 nm thickness, respectively) proved to be four times better than the commercialized Ge 2 Sb 2 Te 5 (GST) 215 SLL's resistance with varying temperature and phase switching times for optimized SLL is shown in Fig. 16(a) and (b)). Table 10 shows the comparison of the supremacy of SLL over GST.

Topological insulators
Topological insulators are the materials that have insulating bulk accompanied by the protected conducting surface states. These surface states provide an impurity and defect insensitive scattering path to the electrons. The unusual metallic nature of an insulator's surface due to the changes in the insulator's topology is the signature of a topological insulator. 216 These surface states are induced due to the high spin-orbit coupling, inverse parity of bulk band, band inversion, 217,218 and protected by the time-reversal symmetry. 16,218 When the crystal symmetry protects the topological insulator's surface, instead of the timereversal symmetry protecting it, these insulators are called the topological crystalline insulator (TCI). 217 Sun et al. 16 reported  that SnSe is a TCI in its native phase (without any pressure and alloying) based on the theoretical calculation using the rst principle along with ab initio tight-binding modeling. Chen et al. 219 reported the co-existence of topological states and superconducting phase at around 27 GPa of the CsCl type phase of SnSe. Ab initio calculation showed that this CsCl phase of SnSe has the topological states' stability against the spin-orbit coupling. SnSe provides the correlation among the superconductivity and topological electronic structures. 219

Conclusion
This review summarizes the tin-selenide material's physical properties, phases, defects, growth mechanisms, deposition methods, and various possible applications. Tin-based compound semiconductors have attracted enormous interest in photovoltaic and optoelectronic applications due to favorable bandgap (1.0-1.3 eV) and higher absorption coefficient (10 5 cm À1 ). The reported efficiency of tin selenide based solar cells is very low compared to other compound semiconductor materials like CIGS, CdTe, and CZTSSe. The main limiting factor for tin selenide's relative low efficiency might be defects present in absorber materials, interface trap states, and unfavorable hetero-junction interfaces. Moreover, conduction band offset (CBO) between the absorber and buffer layer plays a crucial role. It needs to be engineered in a precise manner for efficient transport of electrons toward a metal electrode. Thus, ne-tuning of elements, precise control over growth conditions, favorable band alignment between absorber and buffer can pave the way towards improving the power conversion efficiency for realizing the development of environmental-friendly low-cost nextgeneration photovoltaic technology. Tin selenide can be a potential futuristic material due to its versatile applications. It shows leading hands in some applications in its native crystal structure like thermoelectric. Whereas in other elds it shows competitive nature with the other materials. Tin selenide based thermoelectric materials can pave the ways to reuse waste energy. Tin selenidebased photodetector has also exhibited fast response speed with good responsivity and detectivity in the NIR range. Moreover, exible photodetector based on tin selenide have also gained attention due to its easy fabrication and fast device response. The device response can be improved by controlling absorber thickness and carrier collection before recombination. Another application based on tin selenide is photocathode in photoelectrochemical water splitting for green hydrogen generation. Lastly, some studies have shown that tin selenide can also be used in gas sensors, storage devices (battery electrode, supercapacitors), memory devices, topological insulators.

Conflicts of interest
Authors declare no conict of interest exists.