SnSe/SnS: Multifunctions Beyond Thermoelectricity

3.1   Thermoelectricity

At first, SnSe and SnS are not suitable for thermoelectric field because of their poor electrical properties, and they are more used in solar cells and photoelectric fields thanks to appropriate band gap and optical properties. Since the high-performance SnSe crystals was discovered in 2014,^[43] people have gradually realized that undoped single-crystal SnSe has extraordinarily low thermal conductivity (< 0.4 W m⁻¹ K⁻¹ at 923 K) along the out-of-plane direction because of its strong anharmonicity from the unique 2D crystal structure. At present, the proposed strategies to enhance the thermoelectric figure of merit ZT include energy band engineering,^[69-71] full-size phonon scattering,^[72] introduction of nanostructures^[73] and microstructure manipulations.^{[74, 75]} After the optimization of the above strategies, the maximum ZT of n-type single-crystal SnSe has reached ~ 2.8 at 773 K.^[19] Several studies have confirmed that the decoupling of electrical and thermal transports can be realized by modifying the energy band structure (Figure 5a shows the multiple available valence bands and conduction bands available of SnSe) and full-scale phonon scattering engineering in SnSe. Zhao et al.^[18] improved the power factor of p-type SnSe by doping Na (~40 μW cm⁻¹ K⁻² at 300 K). On the one hand, Na increases electrical conductivity from 12 S cm⁻¹ to > 1500 S cm⁻¹ by increasing the hole carrier concentration. On the other hand, the increasing carrier concentration facilitates the Fermi level move deeper into the valence bands, activates multiple valence bands transport and enhances the Seebeck coefficient. Qin et al.^[20] further proposed the momentum and energy multiband synglisis in p-type SnSe, as shown in Figure 5b. In particular, alloying small amounts of Pb promotes the alignments of multiple valence bands in both the momentum and energy spaces, so that the exceptional power factor of ~ 75 µW cm^–1 K^–2 and ZT ~ 1.2 at 300 K were reached. This study broke the previous phenomenon that the highest ZT values only appear in a narrow range at high temperatures, and reached an average figure of merit ZT of ~1.90 at 300-773 K. Chang et al.^[19] suggested that Br doped n-type SnSe has excellent thermoelectric properties along the out-of-plane direction, breaking the common sense of thermoelectric properties in materials with 2D layered structures. This phenomenon can be attributed to the facts that electrons can travel due to the overlap of electron orbits along the out-of-plane direction. Therefore, electrons can transport in a three-dimensional way, while phonons tend to transport in a two-dimensional way (see the Figure 5c in details). The 3D charge and 2D phonon transports contribute greatly to the relatively high power factor with extremely low thermal conductivity along the out-of-plane direction in SnSe. In addition, it was reported that Bi is also an effective n-type dopant in SnSe, and the carrier concentration can reach ~ 2.1 × 10¹⁹ cm⁻³ at 773 K.^[76] Compared with high-performance single crystals, polycrystalline SnSe, which has better mechanical properties and higher possibility to be used in devices, possesses much lower carrier mobility because of the strong scattering from the enormous grain boundaries. Theoretically, the thermal conductivity of polycrystal SnSe should be much lower than that of single crystal because of the extra scattering of phonons. However, the thermal conductivity of polycrystal SnSe seems to be unstable and is sometimes much higher than that of single crystal because of the existence of the second phase SnO₂ on the surface.^[77] Inhibiting the oxidation during the preparation of polycrystalline SnSe can effectively reduce the thermal conductivity. The thermal conductivity of hole-doped polycrystalline SnSe can be lower than 0.07 W m^–1 K^–1 and the ZT value reaches ~ 3.1 at 783 K by purifying the raw materials and removing surface oxides, which is even higher than that of single crystal SnSe.^[49]

As an analog of SnSe, the thermoelectric properties of SnS become a focus to be increasingly noticed. Intrinsic SnS with poor electrical conductivity was not considered to be an excellent thermoelectric material. However, it has been proved both experimentally and theoretically that SnS also has intrinsic low thermal conductivity owing to the strong anharmonicity. Therefore, appropriate doping can effectively enhance the thermoelectric properties of SnS.^{[78, 79]} He et al.^[22] introduced Se on the basis of SnS doped with 2% Na. With the increase of temperature, the energy bands were gradually sharpened, and the introduction of Se further sharpens the energy bands, which reduce the effective mass and improve the carrier mobility. Compared with undoped SnS, the introduction of Se reduces the energy gap between VBM 1 and VBM 3, increases the band degeneracy and improves the Seebeck coefficient (Figure 5d). As a result, ZT_max ~ 1.6 at 873 K and ZT_ave ~ 1.25 from 300 to 873 K were obtained. The excellent thermoelectric properties and the reserves of S element abundance further verify the development potential of SnS. The efficiency of SnS-based thermoelectric devices can be further enhanced by jointly improving the performance of p-type and n-type SnS. He et al.^[80] proposed that Br doped SnS crystal can realize high-performance n-type behavior. Although the carrier concentration after Br doping is still low as ~ 4.46 × 10¹⁷ cm⁻³, this lower carrier concentration reduces ionized impurity scattering and causes the high carrier mobility approaching ~ 1268 cm² V⁻¹ s⁻¹ and weighted mobility of ~ 2.7 × 10³ cm² V⁻¹ s⁻¹ along the in-plane direction at 300 K. This in-plane carrier mobility is much higher than those along the out-of-plane direction and the previously reported values in polycrystals. A larger Seebeck coefficient can also be obtained at a lower carrier concentration. Therefore, power factor reaches ~ 28 µW cm⁻¹ K⁻² at 300 K in n-type SnS. Great electrical performance and extremely low thermal conductivity caused by layered structures together determined a ZT value of ~ 0.4 at 300 K in Br doped n-type SnS single crystals.

Fig. 5  (a) Schematic of multivariate valence band of SnSe. Reproduced with permission.^[35] Copyright 2016, Royal Society of Chemistry; (b) Schematic illustration of multiband synglisis in SnSe. Reproduced with permission.^[20] Copyright 2021, American Association for the Advancement of Science; (c) Schematic 3D-charge and 2D-phonon transports in n-type SnSe, the p-type SnSe is also plotted for comparison. The colored dots represent the charge densities. The gray blocks represent the two-atom-thick SnSe slabs along the out-of-plane direction (a-axis) of SnSe. Reproduced with permission.^[19] Copyright 2018, American Association for the Advancement of Science; (d) Schematic diagram of variation trend of three valence bands with the increase of temperature for SnS. Reproduced with permission.^[22] Copyright 2019, American Association for the Advancement of Science.

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3.2   Solar cells

Solar cell is a semiconductor equipment that applies solar energy to generate electricity, which directly converts light into electric energy by photovoltaic effect without thermal process. Thin-film techniques of novel generation of solar cells which satisfy the requirements of low cost, high efficiency and environmental protection have captured growing attention. The corresponding efficiency of commercial Cu(In, Ga)Se₂ and CdTe absorbing materials are 20.9% and 20.4%,^[5] but In and Ga elements are expensive, toxic and polluting. Whereas, it remains a significant challenge to synthesize homogeneous and pure single-phase Cu₂ZnSn(S, Se)₄, Sb₂Se₃, CuSbS₂ and CuSbSe₂ materials with non-toxic and environmentally friendly. 2D SnSe and SnS thin films have gradually applied as absorption materials for solar cells due to the characteristics of easy preparation, environmental protection, non-toxicity, low cost, outstanding optical and electronic properties. On the basis of Shockley-Queisser criteria, the limiting efficiency of SnSe and SnS-based photovoltaic cells reach 32% and 33% respectively. Although, there is a sketchy calculation, showing that the actual efficiencies of SnSe and SnS-based sollar cells is less than one tenth of the theoretical values. The differences between theoretical values and experimental values can be attributed to two major factors: firstly, depositing high purity, compact and non-porous single-phase thin film materials is difficult, and yet to be inadequately resolved; secondly, inappropriate n-type buffer layer causes the band-off.

In 1990, J. P. Singh et al.^[81] initially prepared the SnSe heterojunction solar cells with efficiency of 2.3%. Over the past few years, abundant researches were conducted on thin film SnSe solar cells.^{[10-13, 15, 16, 82, 83]} And then, it was found that the most efficient device was p-SnSe/n-Si heterojunction solar cells, which has exhibited an efficiency of 6.44%.^[13] In the field of SnS-based solar energy, the first study was reported in 1998.^[84] But the transfer efficiency of most SnS-based solar cells has not exceeded 4%.^{[4, 5, 9, 56, 85, 86]} Recently, massive researches have revealed that annealing after film deposition is a shared strategy to improve solar energy efficiency. Annealing contributes to promote grain growth and reduce defect density, which reduces hole-electron recombination, dark saturation current density and shunt conductance.

As show in Figure 6a, Dipak V. Shinde et al.^[15] constructed thin film CdS-SnSe heterojunction solar cells, with SnSe film as absorption layer and CdS as n-type buffer layer. Firstly, CdS and SnSe films were deposited on Indium tin oxide (ITO) substrates by chemical bath deposition method, then annealed in air at 573 K for 30 min. Finally, Au electrodes were deposited on the top of SnSe films. The films were well contacted with each other and uniformly deposited on ITO substrate, which avoids the leakage current at the heterojunction interface and effectively inhibits the recombination of electron-hole pairs. The efficiency of solid-state ITO-CdS-SnSe-Au solar cells reached 0.8%. In order to further elevate the efficiency of CdS-SnSe heterojunction, they constructed ITO-CdS-SnSe-polysulfide-Pt-FTO liquid junction cell, and the solar energy conversion efficiency achieved 1.4% due to the existence of polysulfide reduced the loss caused by carrier recombination.

The annealing process can also be applied to SnS-based solar cells. The power conversion efficiency of TiO₂/SnS heterojunction solar cells prepared by SnCl₂-TU (thiourea) complex solution has reached 4.8%.^[8] High purity orthogonal SnS was prepared by spin-coating SnCl₂-TU solution on ns-TiO₂/TiO₂ blocking layer (BL)/fluorine-doped tin oxide (FTO), annealing in Ar atmosphere for 3 min at 375 °C, and then depositing Au electrode to form FTO/BL-TiO₂/ns-TiO₂/SnS/Au. Figure 6b shows that cross-sectional image of SnS-based solar cells observed by FESEM (field emission scanning electron microscope). The efficiency of solar cells reached 3.9%. The annealing process ensured the orthorhombic phase SnS with high purity, which prevented the Au electrode from directly contacting ns-TiO₂ or FTO. And the reason for short circuit of the device was the inferior crystallization of SnS. Through additional SnCl₂ post-treatment, the efficiency reached 4.8%. Excessive SnCl₂ helps SnS to form a smooth and dense film, because Cl^-1 was introduced into SnS lattice to decrease lattice defects.

In the future, more researches should enhance the properties of SnSe/SnS-based solar cells through optimizing the material preparation methods, modulating the lattice mismatch of composite materials, and selecting appropriate n-type buffer layers.

Fig. 6  (a) Schematic of SnSe-based solid solar cell device structure, with SnSe film as absorption layer and CdS as n-type buffer layer; (b) Schematic of cross-sectional FESEM (field emission scanning electron microscope) image of a complete SnS-based solar cell device. Reproduced with permission.^[8] Copyright 2019, Wiley-VCH.

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3.3   Lithium/sodium ion batteries

Lithium-ion batteries (LIBs) and Sodium-ion batteries (SIBs) are regarded as a green energy storage device to replace fossil energy. During charge and discharge, LIBs/SIBs utilized Li⁺/Na⁺ to be repeatedly embedded and removed in the positive and negative electrodes to transfer charges and generate electric energy. High energy density and long cycle life are important parameters to evaluate the performance of batteries. Despite that LIBs have occupied a large market share, low-cost SIBs have more development prospects.^[87] Comparing with LIBs, poor cycle performance and insufficient charge capacity are the fatal problems of tin-based SIBs anode materials. What is surprising is that 2D SnSe/SnS with unique electronic properties can solve these problems. SnSe/SnS-based electrodes allow more Na⁺ to be stored, and the theoretical charge capacity can achieve 1022 mA h g⁻¹ and 780 mA h g⁻¹, respectively. Furthermore, the weaker SnSe/SnS forces between layers are beneficial to the embed and remove Na⁺. Consequently, SnSe/SnS-based SIBs has high energy/power density and superior dynamics properties.^[88-97] Nevertheless, the radius of Na⁺ is 55% larger than that of l Li⁺, the diffusion rate within electrodes is slow in application. Electrode phase transition is large in the process of solidification/de-solidification, and the electrodes are easy to aggregate or crush after multiple cycles. Pristine SnSe and SnS have poor electrical conductivity, which led to the decline of the battery efficiency. Recently, research studies have shown that the use of SnSe/SnS composite conductive materials,^[98-105] doping SnSe/SnS-based electrodes and controlling electrode materials morphology^[106-114] are common methods to improve the electrochemical performance. When SnSe/SnS is composited with conductive carbon materials to form the nanocomposite electrode, the weak Sn-C bonding causes a large charge energy barrier and voltage hysteresis between SnSe/SnS and carbon during the solidification/de-solidification process, and deteriorates the performance of the battery.

As shown in Figure 7a, Ren et al.^[115] proposed to directionally grow vertical SnSe nanoplates on N-doped graphite by the cation-exchange method. Meanwhile, the first-principles calculations indicated that N-doped graphite is beneficial to enhance the Sn-C bonds between SnSe and N-doped graphite. Strengthening Sn-C bonds accelerates the transfer of electrons between graphite layer and SnSe layer and the diffusion rate of Na⁺ between SnSe sheets, which also prevent the aggregation of SnSe nanoplates during multiple cycles process. Accordingly, the SnSe nanocomposite electrodes exhibit large capacities of 723 mA h g⁻¹ at 25 mA g⁻¹, and capacity retention of 82% for over 200 cycles at 2 A g⁻¹.This study has promoted the development of the high-performance SIBs.

Xiong et al.^[116] deposited SnS nanoparticles (NPs) electrodes on chemically modified graphene to form composite electrodes, which improved the conductivity of the electrodes and offered SnS a larger phase transition buffer space between layered graphite (Figure 7b). Thus, the composite anode indicates excellent capacity retention rate of 87.1% for 1000 cycles at 2.0 A g⁻¹ with a high capacity of 509.9 mA h g⁻¹. For SIBs, traditional SnS with graphene composite anode produce strong van der Waals forces in the cycle process, which causes graphene re-deposition. Additionally, polar SnS associates with nonpolar graphene through weak chemical bonds, resulting in the aggregation or fragmentation of SnS after multiple cycles. Modified graphene is an effective method to make up for the shortcomings of traditional graphene. Figure 7b shows the structure diagram of 3D SnS electrode, in which spherical material represents SnS nanoparticles and lamellar material represents modified graphene. Poly (diallyldimethylammonium chloride) was coated on graphene oxide as colloidal molecules to fix SnS nanoparticles, and then nitrogen was doped in graphene oxide to form chemically modified graphene. Chemically modified graphene is superior to traditional graphene, which enhances the affinity for active materials and discharge substances, and the electrostatic assisted construction of 3D graphene materials also prevents the re-deposition of graphite. This research implies that high performance nanocomposite electrodes with activity and durability provide a new design strategy.

At present, compared with LIBs, SIBs have no great advantages except that the reserves of sodium are richer than lithium-ions. Therefore, the optimization of the performance of sodium-ion batteries is far from enough.

Fig. 7  (a) Schematic illustration of the vertical SnSe nanoplates on N-doped graphite. Reproduced with permission.^[115] Copyright 2018, Elsevier; (b) Stereoscopic diagram of SnS/3D N-doped graphene hybrid anode material, in which orange spheroids represent SnS and gray lamellar structures represent 3D N-doped graphene. Reproduced with permission.^[116] Copyright 2017, Royal Society of Chemistry.

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3.4   Supercapacitors

With the development of renewable energy such as solar energy, it is extremely urgent to develop efficient energy storage equipment. Supercapacitors are energy storage device by separating charges on the interfaces between electrode and electrolyte. Compared with ion batteries, supercapacitors have greater power density and longer cycle life.^[117-122] Nanostructured electrode is an ideal electrode material, which can decrease the diffusion length of ions/electrons and increase the active sites and alleviate the volume change of electrode.^{[123, 124]} The optimization strategy of supercapacitor electrode material is alike to that of LIBs/SIBs. So far, the common electrode materials are metal oxide,^[125-129] 2D metal sulfide nanomaterials,^[130-133] carbon/carbide nanomaterials^[134-138] and so on. In recent years, SnSe and SnS are gradually emerging in the field of supercapacitors because of unique electronic properties and redox reversibility.

Ni et al.^[139] suggested that a simple way for microwave-assisted synthesis of SnSe electrode. As shown in Figure 8a, the tin source solution and selenium source solution were mixed and stirred, and then heated in a microwave oven for 15 minutes to form SnSe particles. Compared with the previous hydrothermal synthesis of SnSe electrode material,^[140] microwave assisted synthesis can quickly transfer microwave energy to the reaction mixture to realize internal heating and rapid nucleation. It has the advantages of energy saving and uniform heating.

Himani chauhan et al.^[141] synthesized surfactant-free SnS nanorods by solvothermal method , which can be used as candidate electrode materials. As can be seen from Figure 8b, Liu et al.^[27] reported that SnS composite graphene formed 3D porous electrode, which inhibits the re-aggregation of SnS after multiple cycles. This report offers a novel idea for improving the performance of 2D electrode materials. In the following researches, improving the energy density of capacitors and maintaining the high compatibility between electrodes, electrolytes and device systems are the core development direction.

Fig. 8  (a) Diagram of the SnSe synthesized by the microwave-assisted synthesis. The Sn raw material solution and Se raw material solution are mixed and stirred, and then heated by microwave for 15min. After cleaning, SnSe was dried in vacuum. Reproduced with permission.^[139] Copyright 2018, Elsevier; (b) Schematic of 3D porous SnS/S-doped graphene hybrid nanoarchitectures (HNAs). Reproduced with permission.^[27] Copyright 2017, Wiley-VCH.

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3.5   Memristors

Recently, extensive studies have shown that memristor is most likely to become the core electronic component of artificial intelligence according to its working principle. Memristors can simulate the process of learning, memorizing and processing information of human brain by continuously adjustable conductance, but the core issue is to seek materials that can store prodigious information and process information efficiently. At present, oxides, sulfides and organic compounds are mainly involved in memristors,^[142-144] and utilizes oxygen vacancies, silver-ions and copper-ions to simulate ions diffusion in neurons.^{[142, 145-148]} With the improvement of the requirements for the running speed of memristors and information storage capacity, the device is gradually developing towards miniaturization and integration. In the out-of-plane direction, the 2D material is connected by van der Waals forces. According to this characteristic, the excellent properties of 2D memristors can be hardly influenced when reducing the volume of the devices. Excitatory and inhibitory postsynaptic potentials are two basic synaptic responses of human nervous system under different functions. Recent studies have pointed out that the co-release of glutamate and GABA neurotransmitters at the end of a single axon in the ventral tegmental area can lead to the reconfiguration of postsynaptic potential between excitability and inhibition. As shown in Figure 9a-9b, He et al.^[149] prepared black phosphorus-SnSe heterojunction to simulate the reset process of postsynaptic potential after synaptic simultaneous release of excitatory and inhibitory transmitters. The appropriate bandgap of black phosphorus forms the heterostructure with SnSe, resulting in tunable rectifying electrical characteristics. In addition, the charge transfer between the natural oxide of black phosphorus and black phosphorus channel is used to realize synaptic behavior, which can be reconfigured between excitatory and inhibitory responses. This potential reconfigurable artificial synaptic device can simplify the device structure and can be applied to artificial neural networks in the future. Subsequently, they^[150] applied Markov chain algorithm to realize the machine learning on SnO_x/SnSe/SnO_x heterojunction devices. Wang et al.^[151] suggested that high-performance Pd/SnSe/NSTO devices can simulated the transfer of Na⁺, K⁺, Ca²⁺ between synapses of human neurons by the formation of Pd filaments and the diffusion of Pd²⁺ in SnSe films, and realized simple algorithm operation.

Recently, Au/SnSe/graphene/SiO₂/Si two-terminal memristors has been demonstrated to be a candidate material for simulating human neural network.^[152] In Figure 9c, the memristors are realized by intrinsic Sn vacancies in SnSe. Au electrodes and graphene electrodes were used as presynaptic and postsynaptic, respectively. SiO₂ is used as an insulating layer to prevent current leakage. The high thermal conductivity of graphene can enhance the thermal diffusion performance of memristors. Under the external electric field, Sn vacancies are arranged into conductive filaments connected with Au and graphite electrodes, indicating that the storage resistance is in low resistance state (SET). When reverse voltage is imposed, Sn vacancy conductive filaments are broken and memristors are in high resistance state (RESET). The memristors has the characteristics of stability, uniformity, high density storage and low power, which are the best device so far to realize the function of biological synapse.

Fig. 9  (a) Schematics of black phosphorus-SnSe heterojunction synaptic device. Presynaptic input is applied at the silicon bottom gate terminal. The electrode in contact with SnSe is grounded. Postsynaptic output is measured on electrodes in contact with black phosphorus. V_bias is applied between black phosphorus and SnSe, and voltage V_g is applied between input terminal and SnSe; (b) Schematics of biological synapses that can jointly release excitatory and inhibitory neurotransmitters. Reproduced with permission.^[149] Copyright 2017, American Chemical Society; (c) Schematics of the resistance switching mechanism for the SnSe-based memristor devices in their initial, high-resistance, and low-resistance states. Reproduced with permission.^[152] Copyright 2021, Science China Press and Springer-Verlag GmbH, Germany.

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3.6   Gas sensors

The main sources of NO₂ and SO₂ emissions are fossil fuel combustion, automobile exhaust and industrial production, which not only cause enormous harm to the natural environment, but also causes serious damage to human respiratory system and kidney.^{[153, 154]} A challenging problem which arises in this domain is to develop devices for detecting NO₂ and SO₂. In previous studies, it has found that metal oxides and metal oxide heterojunction gas sensors have lots of disadvantages, such as, undistinguished gas selectivity and high operating temperature.^[155-161] 2D materials can effectively solve the above problems due to their thin atomic layer and large specific surface areas. Gas molecules (NO₂, SO₂, etc.) can be adsorbed on the film substrate and transfer electrons to change the band structure, which can influence the electronic properties of the material (such as conductivity) and can be detected immediately. Graphene, phosphorene and MoS₂ have been proved to be applied as gas sensors experimentally and theoretically.^[162-167] However, the poor stability of phosphorus in air limits its further development.^[168]

Recently, the first-principles calculations have revealed the feasibility of adsorption of NO₂ and SO₂ on SnSe and SnS monolayers.^[169-172] Theoretical calculations showed that SnSe and SnS have moderate adsorption energy and large charge transfer for NO₂ and SO₂ molecules. Recently, Wang et al.^[173] synthesized p-SnSe/n-SnSe₂ 2D heterojunction by one-step colloid method to detect NO₂ gas. At room temperature, pure SnSe₂ has poor gas adsorption capacity and conductivity, which affect the sensing response. As a result of narrower band gap and superior intrinsic conductivity, SnSe can promote the adsorption capacity and conductivity of SnSe₂ at room temperature. And the SnSe(50%)/SnSe₂(50%) heterojunction exhibits n-type conductivity. As shown in Figure 10a, when the SnSe₂ part of the heterojunction contacts NO₂ molecules, the following reactions occur:

Additional electron-hole pairs are produced at the heterojunction interface when illumination is applied. In Figure 10b, the electrons of SnSe₂ react with NO₂ molecules, which destroy the balance of PN junction and widen the width of built-in electric field. The increase of built-in electric field width and the decrease of electron concentration in heterojunction lead to the increase of resistance and improve the detection ability of NO₂ gas. When 405 nm laser irradiation and 0.1 ppm NO₂ gas are applied simultaneously, the response performance of the gas sensor has reached 103% due to the additional photogenerated electrons react with NO₂ gas on the surface. Sun et al.^[174] suggested a high performance SnS₂/SnS p-n heterojunction NO₂ gas sensor, and the sensing response value was 660% at 4 ppm NO₂. In addition to acting as a gas detector, M. F. Afsar et al.^[175] reported SnS nanosheets as sensors for volatile compounds alcohol and propanol. The feasibility of SnSe/SnS as detectors for other harmful gases and volatile compounds is still a valuable research direction.

Fig. 10  (a) Schematic of the sensing principle of SnSe(50%)/SnSe₂(50%) heterostructures was responsed to NO₂ at room temperature under laser illumination; (b) Schematic of the band structure of SnSe(50%)/SnSe₂(50%) in N₂ or NO₂ atmosphere. E_vac, E_F, E_C and E_V denote the energy level for vacuum, Fermi level, conduction band and valence band, respectively. Reproduced with permission.^[173] Copyright 2020, Chemistry Europe.

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3.7   Memories

Modern information storage and processing technology are accomplished by magnetic moment and carrier transport in magnetic materials. Metal and ferrite ferromagnetic materials do not possess semiconductor characteristics, and the Curie temperature is low, in which the magnetism has disappeared in practical applications at room temperature. Semiconductor materials with high mobility and signal processing are incompatible with ferroelectric and ferromagnetic properties. Therefore, doping magnetic elements into non-magnetic semiconductors can form diluted magnetic semiconductors with ferromagnetic, ferroelectric and ferroelastic properties. Through this method, some semiconductor materials with superior performance can also be used for non-volatile memories. B. Parveen et al.^[176] doped magnetic Ni in SnS nanocrystals to prepare diluted magnetic semiconductor, which showed ferromagnetism at room temperature.

Ferroelectric tunnel junctions have a heterostructure with ferroelectric ultrathin film as barrier layer and electrode lead sandwiched on both sides, which have tunnel electrical resistance effect. It solves the problems of short service life, limited reading time, low storage capacity and difficult miniaturization of traditional ferroelectric memories. It has a wide application prospect in nonvolatile memories. In many ferroelectric tunnel junctions, 3D lattice materials are used as the barrier layers. But with the reduction of the material size, the film will appear depolarization effect and the ferroelectric property will disappear.^{[177, 178]} Therefore, it is difficult for the 3D ferroelectric tunnel junctions to minimize the device size.^{[177, 179-181]} 2D materials with ferroelectric properties break the critical thickness problem of three-dimensional materials and greatly reduce the size of devices.^[182-188]

Wu et al.^[64] predicted through the first-principles calculations that SnSe and SnS with high mobility and anisotropic physical properties can also have ferroelectric and ferroelastic properties, and have high Curie temperature, which is expected to maintain the ferroelectric properties of materials at room temperature. Shen et al.^[25] doped In and Sb elements in SnSe to prepare In: SnSe/SnSe/Sb: SnSe ferroelectric tunnel junctions (see Figure 11a in detail). Compared with the previous ferroelectric tunnel junctions, this report used pure SnSe as the ferroelectric barrier, and uses In-doped p-type SnSe and Sb doped n-type SnSe as the two ends of electrodes respectively. The ferroelectric tunnel junctions with the same structure can avoid the lattice mismatch and reduce the difficulty of device preparation. Since the polarization of SnSe in the plane is the same along the b- and c-axis, in Figure 11b, they defined the polarization directions as P_+x and P_-x, and the original width of the 2D ferroelectric barrier is d. When the polarization direction is P_+x, the left side of the ferroelectric barrier is negative and the right side is positive. Electrons and holes gather on both ends of the ferroelectric barrier, and the width of the ferroelectric barrier becomes d₁. When the polarization direction is P_-x, the electrons and holes on both ends of the ferroelectric barrier are neutralized, leaving only ionized donors and acceptors, and the ferroelectric barrier width becomes d₂(d₁ < d₂) (Figure 11c). In the direction of P_+x polarization, the carriers converge at both ends of the ferroelectric barriers, which make the aggregation zone conductive and reduces the effective barrier width. In the direction of P_-x polarization, the carriers are consumed at both ends of the ferroelectric barriers, and the interface region changes from a conductive state to an insulating state, increasing the width of the effective barrier. As show in Figure 11c, the average barrier height in two polarization directions is φ_R and φ_L. Because the conductivity of tunnel electrons is affected by the height and width of the barrier, the resistance in the P_+x polarization direction is small, which is regarded as the "ON" state, while the resistance in the P_-x polarization direction is large and is called the "OFF" state. In the previous ferroelectric tunnel memories, only the height of ferroelectric barrier is changed when the polarization direction changes. However, the memories reported in this paper utilized the unique properties of semiconductor materials to achieve high tunneling electroresistance effect of 1460% by dynamically adjusting the barrier width and barrier height.

Phase change memories (PCM) are other devices to realize the nonvolatile memory. The storage state is determined by the resistance value of the resistor, and the data are stored by the resistance difference in mutual transformation of crystalline state and amorphous state. Campbell et al.^[189] verified the potential of Ge₂Se₃/SnSe device as phase change memories.

Fig. 11  (a) Schematic of the 2D ferroelectric tunnel junction device based on homostructure. The pristine SnSe semiconductor with in-plane ferroelectricity acts as a ferroelectric barrier. The electrodes on both sides are p-type and n-type SnSe respectively; (b) Switching mechanisms of the 2D ferroelectric tunnel junction p-SC/FE/n-SC (SC represents single crystal and FE represents ferroelectric barrier, and P_+x and P_-x represent the polarization direction). The black “+” and “−” symbols in the FE area represent positive and negative charges, respectively. The red “+” in p-SC and blue “−” in n-SC electrodes represent hole and electrons, respectively. The “⊕” and “⊖” represent ionized donors and acceptors, respectively; (c) Overall potential energy profiles with corresponding band diagrams. The barrier width in either P_+x or P_−x state is given by the green line. The average potential barrier height is showed by the orange line. Reproduced with permission. ^[25] Copyright 2019, American Chemical Society.

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3.8   Photodetectors

Photodetectors are devices thattransfer optical radiation signals into electrical signals. According to its absorption spectrum, photodetectors can be applied in many fields such as commerce, military and so on. So far, however, there was little research about a wide spectrum with detection range from infrared to ultraviolet in photodetectors. The appearance of 2D materials brought a breakthrough for the development of photodetectors. Compared with the traditional semiconductor materials, they have high in-plane mobility, stable chemical properties, good mechanical flexibility, exceptional photoelectric conversion properties. More importantly, there is no chemical bonds in the vertical direction of the plane, which is convenient for constructing 2D heterojunction and no-dangling bonds interlayer structure.^[190-192] Therefore, 2D materials may become candidate materials for the next-generation flexible optoelectronic devices. Graphene is widely used in infrared photoelectric detection, but graphene has zero-band gap and low absorbance, which are easy to oxidize and adsorb impurities.^[193] The features of transition metal chalcogenides (TMDs)-based photodetectors (MoS₂, WS₂, and WSe₂) are high photo-responsiveness and photo-sensitivity, however, they are only sensitive to visible light and weak to infrared light.^[194-196] Comparing with graphene and transition metal chalcogenides, SnSe/SnS possess high absorption coefficients and tunable band gaps, which are expected to be candidate materials for wide spectrum photodetectors.^{[197, 198]} When 2D semiconductor materials are used alone, their electronic and optical properties often cannot meet the application conditions of photodetectors. Previously, the traditional optimization strategy was to improve the synthesis methods of SnSe/SnS thin films and nanocrystallines^{[31, 52, 140, 199-210]}, or composite other materials.^[211-216] In recent years, researches provided new insights to improve the performance of SnSe/SnS-based photodetectors through superimposing a variety of effects on traditional optoelectronic devices.^{[206, 209, 211, 217-221]} For example, photoelectric effect is coupled with photothermal effect,^{[217, 220, 222-225]} piezoelectric effect^[226-228] and ferroelectric effect.^{[229, 230]} Whereas, the large band gaps of ferroelectric materials narrow the spectral detection range, the photothermal effect cannot continuously enhance the photoelectric signal, and the piezoelectric strain inevitably destroys the device structure. Thus, people began to use thermoelectric effect to couple with photoelectric effect.

Recently, Ouyang et al.^[231] suggested an efficient self-powered photodetector using the extraordinary thermoelectric properties of SnSe: Br and SnS: Na single crystals. Figure 12a is the structural diagram of the proposed photodetector. The device structure is ITO/SnSe: Br/Ag. Under the light of 760 nm, ΔT = 1.5 K along the (100) direction is applied to the single crystal SnSe, the light energy and heat energy are converted into electrical signals at the same time. And the most interesting finding was that the output voltage and current are increased by 38.1% and 81.9% compared with those without temperature difference. This study verified that the combined action of thermoelectric effect and photoelectric effect enables to improve the performance of photodetectors. The work functions of ITO and Ag are 4.8 eV and 4.26 eV, respectively, without temperature difference. The electron affinity and band gap of SnSe: Br are 3.4 eV and 0.9 eV. When SnSe: Br contacts with ITO electrode and Ag electrode, the contact surface forms the Schottky barrier. In Figure 12b, under the condition of illumination only, SnSe: Br produces photo-generated electron-hole pairs. Under the action of built-in electric field, photo-generated electrons transfer to Ag electrode and holes transfer to ITO electrode. The current direction is ITO electrode to Ag electrode in the external circuit, and the device forms a closed loop.

As shown in Figure 12d, under the condition of no light, the temperature difference is applied to Ag electrode and ITO electrode, the external circuit current flows from ITO electrode to Ag electrode, which is consistent with the current direction under illumination. The thermoelectric effect promotes the transfer of electrons from ITO electrode (high temperature end) to Ag electrode (low temperature end). Meanwhile, phonons also transfer to the low temperature end, dragging electrons to move rapidly. Both actions accelerate the electron transfer. However, the thermoelectric effect simultaneously causes the energy band bias, which causes the decrease of the Schottky barrier of ITO and SnSe: Br (see the Figure 12e in detail). The increase of Schottky barrier between Ag and SnSe: Br decreases the transfer speed of electrons to Ag electrode and intensifies the recombination of electrons and holes. Thus, the thermoelectric effect suppresses the photoelectric effect. Nevertheless, when light and temperature difference are applied simultaneously, they observed that the directions of photo-current and thermoelectric-current were consistent, and then the current increased, as shown in Figure 12f, which indicates that thermoelectric effect elevates the response ability of photodetectors. Based on the above phenomenon, the thermoelectric effect has two functions: 1) thermoelectric temperature difference and phonon diffusion can promote the transfer of photogenerated electrons. 2) the tilt of energy band shift caused by thermoelectric effect reduces the transfer speed of photo-generated electrons and increases the recombination rate of electrons and holes. In fact, the former played a dominant role. This suggested that the coupling of thermoelectric effect and photoelectric effect improves the performance of photodetectors, and the continuous application of temperature difference will not cause damage to the device. The above two reports also further verified the excellent thermoelectric properties of SnSe and SnS.

Fig. 12  (a) Schematic of photodetector structure; (b, c) Schematic working state and energy band structure under the light illumination condition; (d, e) Schematic working state and energy band structure under the cooling condition; (f, g) Schematic working state and energy band structure under the combined action of light illumination and cooling. Reproduced with permission.^[24] Copyright 2019, Elsevier.

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3.9   Photocatalysis

In recent years, the environmental problems caused by industrialization have posed a serious threat to global biosafety. Semiconductor photocatalytic technology is a critical part for solving the environmental pollution problems efficiently, which uses solar energy to treat environmental pollutants and decompose water to produce hydrogen.^[232-235] It has the advantages of low cost, low energy consumption and less secondary pollution. Photocatalysis absorbs photons through semiconductors and produces photo-generated electron-hole pairs. Some electrons and holes transfer to the surface of the material, and react with surface substances by oxidation or reduction. The other part is electron-hole pairs recombination. In the prevailing view, TiO₂, Bi₂WO₆, and other materials have superior photocatalytic performance, however, the absorption range of visible light is narrow, which hinders the exertion of photocatalytic ability. On the contrary, SnSe and SnS with suitable band gaps and large optical absorption coefficients are expected to be promising photocatalytic materials. In fact, the two materials have not been adequately showed photocatalytic activity, and the recombination rate of photo-generated electrons is fast. Manufacturing SnSe/SnS-based heterojunction is an effective approach to improve the photocatalytic activity. Heterojunction photocatalytic materials can adjust the light absorption range compared with the inherent energy band structure of single-phase materials. By adjusting the energy band structure, the separation of photo-generated carriers can be accelerated and their recombination can be inhibited.^[236-238] A. Ghorban shiravizadeh et al.^[239] prepared SnSe/rGO (reduced graphene oxide) nanocomposites photocatalytic materials by co-precipitation method. Bi₂WO₆/SnS heterostructure prepared by Tang et al.^[240] through surface functionalization approach showed stronger catalytic activity for Rhodamine B dye than single phase Bi₂WO₆ and SnS. Xia et al.^[241] prepared novel metal-organic framework/stannous sulfide nanocomposite photocatalysts by one-step position process, which can efficiently degrade Cr element under visible light.

As show in Figure 13, Li et al.^[26] reported a novel water treatment strategy combining photodegradation and photothermal evaporation. Photothermal effect depends on the recombination of photogenerated electrons and holes to generate heat, while efficient photocatalytic effect needs to suppress the recombination of photogenerated electron-hole pairs. Generally, the two effects cannot be applied in the same device. Li et al.^[26] prepared SnSe-SnO₂ nanocomposites by exposing SnSe nanoparticles to air at 413 K for 1 h. The outer layer is n-type SnO₂ and the core is p-type SnSe. SnSe-SnO₂ nanocomposites exhibited excellent photocatalytic and photothermal properties. The surface thickness of SnO₂ was less than 5 nm, which reduced the influence on the photothermal effect of SnSe in the core and acted as the active layer of the photocatalysis. SnSe cores performing photothermal effect have large absorption coefficient and visible light absorption range, which promote the photocatalytic effect. Photocatalytic effect of SnO₂ is also promoted by photo-generated carriers on the surface of SnSe-SnO₂ heterojunction. The photocatalytic mechanism of this device is shown in Figure 13. During the photothermal-photocatalytic production, light firstly promotes the generation of electrons and holes in semiconductor materials. Then, these charges are transferred to the surfaces of the materials and catalyze the reactions with the surrounding substances, while the remaining unreacted electrons will transfer from the conduction band to the valence band and release heat, which can produce the photothermal effect.

As posted before, Bi₂WO₆ has outstanding photocatalytic performance under visible light irradiation, but its light absorption range is narrow.^[235] Li et al.^[242] prepared SnS/Bi₂WO₆ Z-scheme heterostructure by bath sonication method, which can be used as an efficient photocatalytic material for wastewater treatment. The light absorption range of SnS covered almost the whole visible spectrum. So, SnS extended the light absorption range of SnS/Bi₂WO₆ and hindered the recombination of photo-generated carriers. However, SnS and SnSe nanoparticles tended to aggregate in the photocatalytic process, which led to the decrease of specific surface area and light absorption efficiency. Therefore, it is a current challenge to select materials with large specific surface area, suitable energy band structure and good photocatalytic performance to composite SnSe and SnS.

Fig. 13  Schematic of the synthesis of the SnSe-SnO₂ core–shell nanocomposites and photothermal and photocatalytic mechanisms of degradation of sewage under light conditions. Reproduced with permission.^[26] Copyright 2019, Royal Society of Chemistry. During operation, light promotes the generation of electrons and holes, which are transferred to the surfaces of the materials and catalyze the reactions with the surrounding substances. And the unreacted electrons will transfer from the conduction band to the valence band and release heat, producing photothermal effect.

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3.10   Topological insulators

Since topological insulators were discovered in 2007, they have gradually become the new focus in condensed matter physics. Topological insulators (TIs) are materials with internal insulation that allow charge movement at the interface. In 2011, Fu et al. proposed the concept of topological crystal insulator (TCI).^[243] Different from topological insulators, topological crystal insulators are new-type topological material. Its outstanding gapless conductive surface state (topological surface state) is protected by both crystal symmetry (point group, space group, etc.) and internal symmetry (time inversion, chirality, etc.), which reduces the difficulty of improving the topological properties of materials and makes them more promising.^[243-245] In recent years, 2D topological insulators with quantum spin effect have attracted extensive attention with the improvement of preparation technology for 2D materials. 2D topological insulators are also called quantum spin insulators. The quantum spin Hall effect was firstly discovered in graphene and HgTe quantum Wells. However, graphene has a small energy gap, and the quantum spin Hall effect of HgTe demands to be realized at low temperature. In order to adapt the actual demand, researchers turned to the semiconductor of IVA-VIA elements. So far, the reported topological crystalline insulators include SnTe and Pb_1−xSn_xSe/Te.^[244-248] PbSe, PbTe and PbS have been predicted to be topological insulators through alloying and applying pressure.^[249] Sun et al. verify that SnSe and SnS are intrinsic topological crystal insulators through first principle calculations.^[250] This TCI is characterized by an even number of Dirac cones on the highly symmetrical (001), (110) and (111) surfaces, which are protected by reflective symmetry, perpendicular to the ($\stackrel{-}{1}$10) mirror symmetry plane.

Wang et al.^[30] proposed to use molecular beam epitaxy technology to prepare SnSe (111) thin films on Bi₂Se₃ substrate. As Figure 14a, the surface topological state of SnSe had four Dirac cones, which were located in the time reversal invariant momenta of the (111) surface Brillouin region, namely one $\stackrel{-}{\Gamma }$ point and three $\stackrel{-}{M}$points, respectively. The results of ARPES (Angle-resolved photoemission spectroscopy) spectra showed that the two energy bands cross to form Dirac cone. According to the calculation results, it was confirmed that the Dirac cone was the surface state. There were even number Dirac cones on the highly symmetric surfaces, which was an obvious feature of topological crystalline insulators (see theFigure 14b-14ein details). The first-principles calculations further verified the above experimental phenomena. At present, most of the relevant articles on topological insulators are about the theoretical calculations of SnSe and SnS, which is still a long way from practical applications.

Fig. 14  (a) The bulk and (111) surface Brillouin zones of SnSe; (b-e) The energy band structures of a 16 nm SnSe (111) film. The bandmap around $\stackrel{-}{\varGamma }$ point obtained by (b) ARPES (Angle-resolved photoemission spectroscopy) and (c) first-principles calculations. The bandmap around $\stackrel{-}{M}$ point obtained by (d) ARPES and (e) first-principles calculations. Reproduced with permission.^[30] Copyright 2015, Wiley-VCH.

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3.11   Flexibility

In the above, other applications of high-performance SnSe/SnS thermoelectric materials have been elaborated. However, the rigid SnSe/SnS are not adequate for the needs of flexible wearable devices. Advances in the performance of novel flexible fiber/textile/film provide a promising path for the development of flexible wearable devices. In recent years, flexible thermoelectric generators, flexible energy storage materials and flexible photodetectors as core devices are in the new frontier of researches.

The electrode materials with superior mechanical flexibility can replace the traditional rigid electrodes as the core components of the flexible energy storage devices and provide continuous energy supply. Xia et al.^{[29, 251]} reported that SnSe/C and SnS/C nanofibers were synthesized by electrospinning and calcination as flexible anode electrodes. Liang et al.^[252] fabricated SnS porous films as flexible electrodes for Al-ion batteries. Compared with rigid photodetectors with large thickness and poor mechanical performance, portable flexible photodetectors are more suitable for wearable electronic devices. Recently, attention has been geared toward SnSe/SnS-based flexible photodetectors.^{[28, 253-255]} Zhong et al.^[208] indicated that high quality polycrystalline textured films synthesized by one-step chemical method can be applied to wearable photothermoelectric(PTE) detectors. In Figure 15a, different from the traditional photodetectors based on photovoltaic principles, PTE detectors have the function of thermoelectric generator. Flexible thermoelectric devices can be extensively used in various fields because of their high reliability, superior mechanical flexibility and good high temperature stability. A crucial question with regard to flexible thermoelectric material is how to maintain the excellent properties after multiple bending. In 2014, Xu et al.^[256] proved that the flexibility of SnS thin films and SnS-based heterojunctions can be realized by using flexible polyimide substrates. Subsequently, Yin et al.^[257] also successfully prepared SnSe/SnS thermoelectric thin films through controllable colonial synthesis approach. Nicolas Augustus Rongione et al.^[258] reported that SnSe flexible films for waste heat recovery near room temperature were synthesized by heating hydrothermal synthesis temperature difference method and they worked well after 1000 bends (Figure 15b). Liu et al.^[259] pointed out that conducting polymers poly (3,4-ethylenedioxythiophene): poly (4-styrenesulfonate) (PEDOT: PSS) with excellent mechanical flexibility and intrinsic thermal conductivity can be widely used as matrix for the preparation of inorganic/conductive polymer composite thermoelectric materials. Flexible SWCNT/SnSe NS/PEDOT: PSS nanocomposite films have been successfully fabricated via vacuum filtration technology, and the thermoelectric performance hardly changes after 1000 bends.

Excitingly, Zhang et al.^[260] utilized laser-induced directional crystallization process to synthesize SnSe single crystal fibers (see the Figure 15c in details), which provided a novel idea for the application of rigid high-performance single crystal in flexible devices. The overlength single crystal SnSe fiber with high thermoelectric properties was realized by "two-step method". Firstly, the fiber with SnSe polycrystalline material as the fiber core is prepared by hot drawing process. Secondly, the CO₂ laser is used to melt and recrystallize the SnSe core, and the single crystal SnSe structure is realized over the whole fiber length by laser thermal scanning, the ZT value increases significantly from about ~ 0.58 to ~ 2 at 862 K.

Fig. 15  (a) Schematic diagram of the thermoelectric generators based on SnSe films. Reproduced with permission.^[208] Copyright 2020, Elsevier; (b) Schematic diagram of SnSe films under mechanical bending. Reproduced with permission.^[258] Copyright 2019, Wiley-VCH; (c) Schematic diagram of thermal drawing SnSe fibers and post-draw laser recrystallization process. The insets show the cross-sectional microscope image of SnSe fibers and the crystal structure $Fm\overline 3 m$ of SnSe along c and b-axis directions. Reproduced with permission.^[260] Copyright 2020, Wiley-VCH.

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