Deformation Mechanisms of Inorganic Thermoelectric Materials with Plasticity

Flexible inorganic thermoelectric (TE) materials are beneficial to the development of wireless wearable devices due to their plasticity and high performance. Recently, many novel inorganic semiconductors with plasticity, such as ZnS single crystal, Ag2S‐based alloys, as well as single‐crystalline InSe and SnSe2, attract great attention. They are supposed to break the predicament between power output and mechanical deformability for TE systems with plasticity. To better understand the deformation of TE materials with plasticity and explore new systems, different deformation mechanisms for these plastically deformable inorganic TE materials are summarized. First, the concepts and requirements for inorganic TEs are presented with plasticity in wearable devices. Afterward, their mechanical properties and deformation mechanisms are elaborated including dislocation glide, interlayer slippage of atomic stacks, dual slippage of atomic stacks and dislocations, as well as phase transformation. Finally, a general outlook from previous research is suggested for forecasting the future directions. It is anticipated that this review can guide the design of inorganic TE materials with plasticity and provide insight into the wider application of plastically deformable inorganic semiconductors in electronic materials and devices.


Introduction
The design of wireless wearable electronics faces challenges due to the self-charging power supplies. [1]Thermoelectric (TE) materials can directly convert the temperature gradient into electricity by the Seebeck effect without mechanical noise or vibration. [2]nsequently, the technique displays a broad potential for assembling selfpowered and sustainable power supply to realize wireless wearable electronic devices via generating electricity from skin heat (in Figure 1a-d). [1,3]3b] The power density is strongly dependent on the dimensionless figure of merit (ZT, ZT = S 2 σT/(κ eleþ κ lat )), where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ ele and κ lat represent the electronic and lattice thermal conductivity, respectively. [4]ecause flexible devices facilitate a larger skin contact area for higher power generation, ideal TE materials for wearables are expected to possess both high ZT values and excellent plastic deformability. [5]As the key component of flexible devices, plasticized TE materials can be machined into thinner and smaller TE legs without cracks or breakage during device deformation.They can also bear large deflection deformation, which is a basic requirement for flexible devices.In this regard, developing plastically deformable semiconductors with good TE performance is the most critical challenge for wearable electronics.However, the majority of highperformance TE materials are inorganic compounds with intrinsic brittleness such as Bi 2 Te 3 , [6] Mg 3 Sb 2 , [7] SnS, [8] SnTe, [9] and PbTe. [10]They usually fracture with negligible plastic deformation.Such brittleness restricts their applications for wearable electronic devices.Improving the plasticity of these TE materials by constructing inorganic/organic composites [11] and discovering purely organic semiconductors [12] are considered feasible approaches to realize flexible TEs.The typical example for the former is the Bi 2 Te 3 /organic films, [13] which achieved a normalized power density of %1 μW cm À2 K À2 .3b] Despite the significant increase in normalized power density of organic/inorganic flexible TE composites, they still fail to reach the inorganic level [15] (%10 μW cm À2 K À2 ) because of the difficulty of densification.3a] In addition, it has been found that some brittle inorganic materials, such as GaN, [16] InSb, [17] and even Si 3 N 4 , [18] can exhibit more plasticity on a nano-or micrometer scale.Yet, it is difficult to assemble materials with such a small size into macroscopic wearable flexible devices.Thus, the size effect on plasticity will not be thoroughly discussed in this review.3a,19] They are expected to break the predicament between power output and mechanical deformability for plastic TE systems.
Note that metals, as representatives of inorganic materials, exhibit superior plasticity, because dislocations are more active in lattice with weaker bonding. [20]Yet, most of these state-ofthe-art TE materials with plasticity are covalent or ionic compounds with stronger bonds, resulting in more complex and unpredictable deformation mechanisms.For example, although plastic deformation has been observed in InSe and SnSe 2 single crystals, [19b,21] these results did not disclose the innate brittle nature of their polycrystalline forms. [21]herefore, understanding the origin of plastic systems can deepen our theory and develop predictability for inorganic semiconductors with plasticity.
Herein, we aim to summarize and compare the deformation mechanisms for inorganic semiconductors with plasticity in flexible TEs in this review article.We first clarify the concepts and requirements for inorganic semiconductors with plasticity.After that, their mechanical properties and deformation mechanisms are summarized including dislocation glide, interlayer slippage Figure 1.a) The schematics of flexible thermoelectric (TE) devices that could be used in wearable electronics with a cross-plane p-shaped structure.b) Normalized power density (P max /AΔT 2 ) for flexible TE devices obtained by plastic materials with various filling factors ( f ).The results for Bi 2 Te 3 -film-, organic-material-, and traditional Bi 2 Te 3 -bulk-based devices are included for comparison.3b] Copyright 2022, AAAS.d) Elongation versus electrical conductivity for plastic semiconductors including α-Ag 2 S, ZnS, SnSe 2 , and InSe with various other metal and insulator materials.3a,19aÀc] of atomic stacks, dual slippage of atomic stacks and dislocations, as well as phase transformation.We finally provide a general outlook for future research based on previous studies.It is expected that this review can guide the design of plastic inorganic TE materials and promote their application in wearable electronic materials and devices.

The Plasticity of Inorganic TE Semiconductors
Plasticity can be defined as the capacity of materials to show remarkable deformation (strain) without destruction under stress, [22] which is the pivotal factor for flexible devices.3a] The bonding in metals is described by a cloud of delocalized electrons surrounding the positively charged nuclei.In this case, the metal ions can move without fracture when subjected to external force because they are still surrounded by free electrons, showing good ductility.3a] Importantly, the conductivity in traditional semiconductors can be tuned across a wide range from 10 À7 to 10 5 S•m À1 . [23]3a,19aÀc,24] They also show good electrical conductivity and TE performance compared to conventional semiconductors, promising applications in wearable electronics.
Note that ZT for TEs is a very complex parameter coupled to physical parameters such as electrical conductivity, thermal conductivity, and Seebeck coefficient. [25]3b] Importantly, a PF of %10.8 μW cm À1 K À2 at 375 K for plastic single-crystalline SnSe 1.95 Br 0.05 is achieved, [19c] which is the highest value for plastically deformable TE materials reported so far.It needs to be emphasized that a high power factor is crucial for the output power (ω) because ω is directly proportional to the average PF (ω ∝ PF ave ). [27]Although the achieved ZT of inorganic semiconductors with plasticity is significantly enhanced, they are still not comparable with typical lowtemperature TEs, such as Bi 2 Te 3 (ZT = 1.6 at 300 K) [28] and even Ag 2 Se (ZT = 0.9 at 300 K) . [29]Understanding the deformation mechanism of existing inorganic materials with plasticity becomes a key prerequisite to achieving high TE performance with large plasticity simultaneously.
The deformation ability of materials can be described by the deformability factor [19b,19c] , where E c and E s are the cleavage and slipping energy of the slip systems, respectively; E in is the Young's modulus of the slip direction.19b,19c] The aforementioned materials with an E g of 1-2 eV suggest an obvious semiconductor behavior, while possessing good deformation capabilities, especially for SnSe 2 , which is even comparable to metallic Au and Ti.In addition, Huang et al. [30] found that AgCl and AgBr with wide E g over 2 eV exhibit excellent ductility over 60% strain for compression.This peculiar mechanical ductility in semiconductors calls for thorough investigations.The underlying mechanisms could in turn help to design new materials with desired plasticity in combination with other properties.To date, several deformation mechanisms have been considered to be responsible for different material systems, which will be elaborated later.

Deformation Mechanism for Typical Engineering Materials
The classical stress-strain curve reflects the response of a material to an external force.Figure 2a illustrates the schematic stress-strain curves for typical engineering materials including ceramics, mild steel, and polymers.The ceramics tend to brittle fracture when reaching the elastic limit (the blue solid line in Figure 2a).The classical mild steel (red) and polymers (green) display a plastic behavior (circled by a black dotted line) as the strain exceeds the elastic range (gray dotted line).22b,31] Note that the dislocation is a linear defect in the lattice structure where a half-atomic plane is missing from the lattice layers.This causes a slight displacement of the Table 1.3b,15b,19c,26]

Material
Type adjacent layers, which minimizes the strain produced by the defect.3a] The energy needed in this way is much lower than that for all atoms to move together.Consequently, the plastic deformation of metals is mainly performed based on the slip of dislocations.For the polymers, the structure is the crystalline lamellae closely connected by the amorphous curly polymer chains.With the application of force, the folded polymer chains are twisted in the stress direction and the amorphous chains are highly elongated to form a fiber-like structure (Figure 2c). [32] Mechanisms for the Deformation of Inorganic Plastic TE Semiconductors

Slippage of Dislocations
Dislocation is a linear crystallographic defect within a crystal structure in the arrangement of atoms.The movement of dislocations allows atoms to slide over each other at lower stress than theoretical.The accumulation of sliding of atomic layers in the way of movement of dislocations is manifested in plastic deformation macroscopically.22b,31] In some semiconductors, the formation and slippage of dislocations are also major contributors to the plastic deformation of crystals.19a] Yet, this crystal turns immediately into brittle under white or ultraviolet (UV) light, as shown in Figure 3a-h.Scanning transmission electron microscopy (STEM) revealed high-density dislocations in a specimen deformed in complete darkness up to a plastic strain of %25%, as shown in Figure 3i.In sharp contrast, it was found that the deformed specimens under light produce a great number of twins (Figure 3j,k), suggesting that twinning is the major plastic deformation mechanism under light illumination for ZnS.Importantly, it seems that twinning is less effective than dislocations in promoting the plasticity of semiconductors because, in general, the plastic strain induced by twinning is smaller than that induced by dislocation slip.For the deformation of ZnS single crystals, the deformation mechanism in darkness switches at %10% plastic strain from the slippage of isolated partial dislocations to the simultaneous slippage of paired partials inside the major domains. [33]Further, the light-induced carriers interact with existing dislocations, thus suppressing the movement of dislocations. [34]Although the plasticization of ZnS can only occur in dark environments, these studies open a door to investigate the deformation capabilities and mechanisms of plastic semiconductors.

Interlayer Slippage of Atomic Stacks
α-Ag 2 S is a typical layered material at 300 K.The unit cell is composed of four S and four Ag atoms constituting eight atomic ring fragments interlinked with sulfur atoms.The abovewrinkled layers are stacked along the a-axis as shown in Figure 4a.Shi et al. [3a] found that the ingot and the spark plasma sintering (SPS) sample of polycrystalline α-Ag 2 S displays excellent machinability and plasticity (over 50% of strain under compression), as demonstrated in Figure 4b,c.They carried out systematic investigations including experiments and simulation, demonstrating that the deformation mechanism of α-Ag 2 S is interlayer slip.The experimental microstructure of polycrystalline α-Ag 2 S in Figure 4d clearly shows primary slipping bands.
To facilitate the calculation, the interlayer distance having the lowest energy (E L ) is defined as the d x , which is the preferential interlayer distance during slipping (see Figure 4e).The slipping energy barrier ΔE B is the subtraction in energies between the state with the largest E L and initial total energy E 0 during slipping.The ΔE B is the minimum energy to be overcome during sliding.Furthermore, the defined cleavage energy (ΔE C ) is the minimum strength of the interaction in these slip planes during slipping, which is equal to the energy difference between the state of largest E L and the state of infinite interlayer distance E Inf .Namely, the structure of the material to achieve a high plasticity requires that ΔE B must be small enough to produce a low sliding resistance, while ΔE C must be large enough to maintain the stability of the entire material.3a] Meanwhile, the cleavage energy for α-Ag 2 S is quite large, even higher than that of diamond, [3a] maintaining the intact structure without ruptures during interlayer slippage.The dislocation sliding mechanism for mild steel.3a] Copyright 2018, Nature Publishing Group.c) The folded polymer chains mechanism for polymer.Reproduced with permission. [32]Copyright 1996, Oxford University Press.
These results state the outstanding ductility of α-Ag 2 S attributable to the facile slip of the atomic layer.The combination of irregularly distributed Ag-Ag and Ag-S bonds resulting from silver diffusion inhibits the cleavage of structure, resulting in their excellent plasticity.Nevertheless, its electrical conductivity and TE performance are still not satisfactory for many practical applications.
Alloying Ag 2 S with Ag 2 Te provides an effective approach to improve ZT to %0.8 at 600 K. [15b,15c] However, the effect of such an alloying method on plasticity is more complex.For example, the reported compressive strain for Ag 2 S 1-x Te x (x = 0.2-0.6)alloys ranges from 20% to 70%, implying significant unstable plasticity.Wang et al. [35] carefully captured this issue and designed experiments to verify and solve it. [35]In their study, the reversible brittle-plastic transition is found in Ag 2 Te 0.6 S 0.4 .The Ag 2 Te 0.6 S 0.4 with a monoclinic phase exhibits a brittle behavior, while the cubic-crystalline/amorphous structure exhibits a plastic behavior with a compressive strain over 80%, as compared in Figure 4g.Importantly, a proper heat treatment can achieve the reversible plastic-brittle transition in Ag 2 Te 0.6 S 0.4 inorganic semiconductors.Simultaneously, the plastic cubic-crystalline/ amorphous Ag 2 Te 0.6 S 0.4 also possesses a decent ZT of %0.4 at The specimens deformed under c) white light-emitting diode LED) light and d) UV LED light.The specimens deformed up to e) ε t = 11%, f ) ε t = 25%, and g) ε t = 35% in complete darkness.h) A stress-strain curve obtained by a deformation in complete darkness up to ε t = 10% and the subsequent deformation under UV light.i) Typical bright-field scanning transmission electron microscopy (STEM) images of the specimen deformed in complete darkness up to ε t = 25%.j) A bright-field transmission electron microscopy (TEM) image of a twinning region in the specimen deformed up to ε t = 2.0% under UV light.k) A typical high-angle annular dark-field STEM (HAADF-STEM) image of a crystal twin in the same specimen as in (C).19a] Copyright 2018, AAAS.origin of plastic instability for Ag 2 Te 0.6 S 0.4 and enables the manipulation of the plastic-brittle phase, thus promoting the development of high-performance Ag-based semiconductors with plasticity.

Dual Slippage of Layers and Dislocations
The class of binary 2D van der Waals (2D vdW) chalcogenide crystals is the other discovery of inorganic semiconductors that are plastic/ductile at room temperature. [36]β-InSe crystal with a space group of P6 3 /mmc is a typical layered structure in which each layer is constituted by the In-Se honeycomb framework in the ab lattice plane. [37]These In-Se layers are stacked along the c direction in the way of Se-In-In-Se, with a vdW gap of 0.8 nm between Se and Se shown in Figure 5a.Wei et al. [19b] investigated its deformation along the aforementioned two directions.
Figure 5b displays the obvious interlayer gliding revealing that the plastic deformation mechanism of InSe along the ab lattice plane can be attributed to the slippage of layers.19b] The calculated slipping energy barrier (E s ) of InSe %0.058 eV•atom À1 is lower than the cleavage energy (E c ) %0.084 eV•atom À1 .The small E s suggests an easier interlayer gliding in the ab lattice plane, while the larger E c favors strong interlayer integrity during dislocation slip across the vdW gap.Such differences between E s and E c can be directly reflected in the macroscopic anisotropy plastic behavior along the ab-plane and c directions.Indeed, although the experimental stress-strain results show a high compressive strain of %80% along the c-axis (Figure 5d) and ab-plane (Figure 5e), the obvious serrations in the stress-strain curve in Figure 5d indicate the processes of dislocation nucleation, crosslink, and break off.These results highly confirm the different ).e) Schematic map for energy variation as a function of interlayer distance d during slipping.f ) E L À E 0 behavior during slipping.3a] Copyright 2018, Nature Publishing Group.g) Stress-strain curves for compression tests at 300 K of samples after different annealing processes.h) Temperature dependences of ZT for Ag 2 Te 0.6 S 0.4 samples.Reproduced with permission. [35]Copyright 2023, Wiley-VCH.
deformation mechanisms in the c-axis (dislocation slip) and within the ab-plane (interlayer gliding) of the single-crystal InSe.
Importantly, the plastic 2D vdW chalcogenide crystals can be screened by high-throughput calculations. [38]Several candidates such as GaS, GaSe, SnSe 2 , SnS 2 , and MoS 2 have been revealed.Similarly, the largely anisotropic plastic deformations in singlecrystal MoS 2 also result from the slippage of layers and dislocations along different directions (Figure 5f-i). [38]

Phase Transformation
Note that the superior plasticity of 2D vdW materials such as InSe and SnSe 2 can only be found in single crystals, [19b,19c] while their polycrystalline samples completely lose the plasticity and are as brittle as conventional semiconductor materials.Despite that the interlayer gliding and cross-layer dislocation slip in a deformed InSe single crystal have been observed by TEM, [19b] as we described before, these results did not reveal the origin of brittleness in polycrystalline samples intrinsically.Hence, revisiting the origin of the plasticity of 2D vdW single crystals can deepen our knowledge of inorganic semiconductors with plasticity.
SnSe 2 is a typical CdI 2 -type layer structure, where the unit consists of two sheets of hexagonally close-packed Se atoms with the Sn atoms occupying the octahedral voids sandwiched between the Se layers.In general, reported results confirmed SnSe 2 with multiple polytypism.Namely, it can form different crystal structures such as 18R, 4H, and 2H distinguished by increased periodicity along the c-axis shown in Figure 6a.For example, Ge et al. [21] prepared single crystals of SnSe 2 with a pure 18R structure using the Bridgman method.The dense polycrystalline SnSe 2 pellet was prepared by solid-state reaction combined with SPS, [39] exhibiting the 4H hexagonal P 63 mc space group.In contrast, the 2H trigonal P 3 m 1 structure was also observed in the  c) perpendicular to the c axis.Compression engineering stress-strain curves d) along and e) perpendicular to the c axis.19b] Copyright 2020, AAAS.f ) Optical images of a bar-shaped MoS 2 crystal before and after bending.g) Scanning electron microscopy (SEM) images of an Ag bar before and after bending.h) SEM image of area 1 in (f ).i) Inverse Fourier transform of the dark-field scanning transmission electron microscopy (IFT-DF-STEM) image of area 2 in (f ).Reproduced with permission. [38]Copyright 2022, Nature Publishing Group.compacted dense pellet. [40]The deformation of SnSe 2 with the 18R phase was investigated under an applied force on its single crystal. [21]Figure 6b,c show stress-strain curves by compressing the specimens perpendicular and parallel to the cleavage plane, respectively.The results indicated that the SnSe 2 single crystal did not fracture until the engineering strain of %80% along both directions with nearly isotropic plasticity.In sharp contrast, the InSe crystal shows anisotropic plasticity behavior, which reflects the different deformation mechanisms of SnSe 2 from InSe.Also, only brittle behavior was observed in polycrystalline SnSe 2 , which fractures under compression less than 10% of engineering strain.The atomic-level structure of SnSe 2 reveals a dynamic remarkable phase transition from low-symmetry 18R rhombohedral structure into the high-symmetry 4H and 2H-SnSe 2 phases as in situ loading (Figure 6d,f ).The calculated volumedependent energy for each phase revealed that pressure can recede the energy gap of those phases.These results indicate that such a phase transition is a pressure-activated process.They also explain the poor plasticity of the polycrystal SnSe 2 samples showing the 4H and 2H phases because of the impressed pressure due to sintering or grain boundaries leading to lower deformability.The superior plasticity revealed in SnSe 2 single crystal in The right part is deformed showing the 4H structure mixing with a slight 2H structure.The unit cell of the 18R structure, 2H, and 4H structure is signified by the red, green, and blue dash lines, respectively.f ) Elemental distribution map focusing on the 4H structure by STEM-energy dispersive spectrometer (EDS), which is the overlaid image of the direct EDS signals from Sn (red) and Se (green).Reproduced with permission. [21]Copyright 2023, Wiley-VCH.19c,39,41] 5. Summary and Outlook TE semiconductors tend to fail in a brittle way because of their rigid chemical bonds.Searching for semiconductor materials with plasticity is an extremely sought-after goal for wireless wearable electronics.The fully inorganic flexible materials are expected to embrace both high TE properties and superior plasticity.19c] Although some of them are not yet achieving impressive performance, their interesting deformation mechanisms may contribute to the discovery of new systems.To better understand the deformation of TE materials with plasticity, we review the deformation mechanisms underpinning plastically deformable inorganic semiconductors in flexible TEs.The deformation mechanisms include slippage of dislocations for ZnS in the dark, slippage of interlayers for α-Ag 2 S and its alloys, slippage of layers and dislocations in 2D vdW chalcogenide family such as InSe, and phase transformation found in 18R SnSe 2 .
Although previous studies have uncovered the deformation mechanisms of TE materials with plasticity through computational simulations and experimental observations, many aspects are still worth further research and development, as will be discussed later.Especially, the coupling between TE properties and plasticity is difficult to optimize synergistically.Here, we briefly discuss several difficulties and potential solutions for achieving both high plasticity and TE performance.We hope these arguments can provide some guidance for the development of next-generation flexible electronics.1) Note that grain boundaries are proven to be important for the suppression of κ lat due to the scattering of low-frequency phonons near room temperature.However, among all the plastic TE semiconductors presented before, only the polycrystalline α-Ag 2 S exhibits stable plastic behavior.The other systems such as InSe, MoS 2 , and SnSe 2 polycrystals are brittle for ingots or SPSed samples.This implies that grain boundaries, which have been widely employed to improve TE performance, could not be suitable for these TEs with plasticity.In addition, many issues are still not understood.19b,42] 2) For metallic materials, it is well understood that plasticity decreases while hardness increases by forming solid solutions because alloying atoms impede the slip of dislocations in the lattice.These doping or alloying strategies are also routine methods to optimize TE performance.For example, Cu can be inserted into the vdW gap of SnSe 2 to increase the carrier mobility and thus boost its power factor near room temperature. [39,43]However, it is unclear whether this interlayer Cu could inhibit the phase transition of SnSe 2 and destroy its plasticity.Another confirmed example is the monoclinic phase of Ag 2 Te 0.6 S 0.4 , which exhibits brittle behavior by alloying Te in highly plastic α-Ag 2 S. [35] The coordination between the TE parameters and their plasticity is an unavoidable topic in subsequent research.3) The high-efficiency TE devices are supposed to be simultaneously assembled with n-and p-type materials with matched physical and chemical properties.Almost all the wellknown inorganic TE materials with plasticity are intrinsic n-type semiconductors.The lack of corresponding p-type semiconductors significantly restricts the development of all-inorganic highperformance TE devices.3b] Namely, too much alloying element reduced its plasticity significantly, as we discussed before.Therefore, the search for more suitable intrinsic p-type inorganic TE semiconductors with plasticity may be significant for the development of wireless wearable electronics.

Figure 2 .
Figure 2. a) Schematic stress-strain curves for typical materials.b)The dislocation sliding mechanism for mild steel.Reproduced with permission.[3a]Copyright 2018, Nature Publishing Group.c) The folded polymer chains mechanism for polymer.Reproduced with permission.[32]Copyright 1996, Oxford University Press.

Figure 3 .
Figure 3. a) Stress-strain curves of ZnS single crystals under white or UV light or in complete darkness.Inset: an undeformed specimen marked by b).The specimens deformed under c) white light-emitting diode LED) light and d) UV LED light.The specimens deformed up to e) ε t = 11%, f ) ε t = 25%, and g) ε t = 35% in complete darkness.h) A stress-strain curve obtained by a deformation in complete darkness up to ε t = 10% and the subsequent deformation under UV light.i) Typical bright-field scanning transmission electron microscopy (STEM) images of the specimen deformed in complete darkness up to ε t = 25%.j) A bright-field transmission electron microscopy (TEM) image of a twinning region in the specimen deformed up to ε t = 2.0% under UV light.k) A typical high-angle annular dark-field STEM (HAADF-STEM) image of a crystal twin in the same specimen as in (C).Reproduced with permission.[19a]Copyright 2018, AAAS.

Figure 4 .
Figure 4. a) Perspective view of the α-Ag 2 S crystal structure along the [001] direction.b) A machined cylinder for the compression test (top) and its deformations under hammering (bottom).c) Strain-stress curves for compression tests at room temperature.d) Optical micrograph of the α-Ag 2 S ingot after the bending test (polarized light).e) Schematic map for energy variation as a function of interlayer distance d during slipping.f ) E L À E 0 behavior during slipping.Reproduced with permission.[3a]Copyright 2018, Nature Publishing Group.g) Stress-strain curves for compression tests at 300 K of samples after different annealing processes.h) Temperature dependences of ZT for Ag 2 Te 0.6 S 0.4 samples.Reproduced with permission.[35]Copyright 2023, Wiley-VCH.

Figure 5 .
Figure 5. a) Crystal structure of β-InSe and the projection on the (110) and (001) planes.STEM analyses of deformed InSe crystal observe b) along andc) perpendicular to the c axis.Compression engineering stress-strain curves d) along and e) perpendicular to the c axis.Reproduced with permission.[19b]Copyright 2020, AAAS.f ) Optical images of a bar-shaped MoS 2 crystal before and after bending.g) Scanning electron microscopy (SEM) images of an Ag bar before and after bending.h) SEM image of area 1 in (f ).i) Inverse Fourier transform of the dark-field scanning transmission electron microscopy (IFT-DF-STEM) image of area 2 in (f ).Reproduced with permission.[38]Copyright 2022, Nature Publishing Group.

Figure 6 .
Figure 6.a) Illustration of SnSe 2 structure transition upon applied pressure.The arrow indicates the structure consecutively changes from low symmetry 18R phase to the intermediate 4H phase, and finally to the high-symmetry 2H phase.Black dash boxes outline the unit cell of each structure.Engineering stressstrain curves, b) perpendicular, and c) parallel to the cleavage plane.d) Low-magnification TEM image for resulting sample after in situ deformation.e) Medium-magnification HAADF-STEM image during the in situ deformation.The left part of the yellow broken line is undeformed showing the 18R structure.The right part is deformed showing the 4H structure mixing with a slight 2H structure.The unit cell of the 18R structure, 2H, and 4H structure is signified by the red, green, and blue dash lines, respectively.f ) Elemental distribution map focusing on the 4H structure by STEM-energy dispersive spectrometer (EDS), which is the overlaid image of the direct EDS signals from Sn (red) and Se (green).Reproduced with permission.[21]Copyright 2023, Wiley-VCH.