SnSe/SnS: Multifunctions Beyond Thermoelectricity

Schematic of the multifunctional applications of SnSe/SnS beyond thermoelectricity, typically including photodetector, solar cell, photocatalysis, topological insulator, gas sensor, ion battery, flexibility, and memristor, etc.^[23-30]

Schematic of the Pnma and Cmcm crystal structures of SnSe/SnS, the claret ball represents Sn atom and the orange ball represents Se(S) atom. (a) Crystal structure of Pnma phase along b-axis; (b) Crystal structure of Cmcm phase along b-axis; (c) SnSe₇/SnS₇ 3D structure, the dotted line represents the weak covalent bonding.

(a) Structure diagram of SnSe energy band. Red solid and dash lines represent the positions of Fermi level with different carrier concentrations. Reproduced with permission.^[18] Copyright 2016, American Association for the Advancement of Science; (b) Angle resolved photoelectron spectroscopy (ARPES) band dispersion of VBM1 and VBM2 along Γ-Z and Γ-Y directions. Reproduced with permission.^[54] Copyright 2017, American Physical Society; The energy band structures of SnSe with different number of layers. (c) single-layer; (d) double-layer; and (e) bulk SnSe. Reproduced with permission.^[10] Copyright 2015, American Chemical Society.

(a) Schematic of SnS Pnma energy band structures. The number on the energy band diagram represents the maximum value of the valence band (1, 2, 3, 4). The horizontal lines of different colors represent the position of Fermi level when the carrier concentration is ~ 9 × 10¹⁷ cm⁻³, ~ 5 × 10¹⁹ cm⁻³, ~ 9 × 10¹⁹ cm⁻³, ~ 4 × 10²⁰ cm⁻³, respectively. Reproduced with permission.^[21] Copyright 2018, Royal Society of Chemistry; (b) Brillouin zone of SnS and high symmetry points; (c) Angle resolved photoelectron spectroscopy (ARPES) band structure along the X-U direction; (d) ARPES band structures of SnS along the Γ-Y, Γ-Z, and X-U directions. Three parts show the band dispersion of the three VBMs in SnS. Reproduced with permission.^[22] Copyright 2019, American Association for the Advancement of Science.

(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.

(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.

(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.

(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.

(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.

(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.

(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.

(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.

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.

(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.

(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.