Featured with bendability and deformability, smartness and lightness, flexible materials and devices have wide applications in electronics, optoelectronics, and energy utilization. The key for flexible electronics is the integration of flexibility and decent electrical performance of semiconductors. It has long been realized that high-performance inorganic semiconductors are brittle, and the thinning-down-induced flexibility does not change the intrinsic brittleness. This inconvenient fact severely restricts the fabrication and service of inorganic semiconductors in flexible and deformable electronics. By contrast, flexible and soft polymers can be readily deformed but behave poorly in terms of electrical properties. Recently, Ag2S was discovered as the room-temperature ductile inorganic semiconductor. The intrinsic flexibility and plasticity of Ag2S are attributed to multicentered chemical bonding and solid linkage among easy slip planes. Furthermore, the electrical and thermoelectric properties of Ag2S can be readily optimized by Se/Te alloying while the ductility is maintained, giving birth to a high-efficiency full inorganic flexible thermoelectric device. This chapter briefly reviews this big discovery, relevant backgrounds, and research advances and tries to demonstrate a clear structure-performance correlation between crystal structure/chemical bonding and mechanical/electrical properties.
- flexible electronics
- plastic inorganic semiconductors
- chemical bonds
Flexible electronics endows electronic circuits with novel flexibility, foldability, and scalability, breaking the restriction of wafers and greatly expanding the application in various fields, e.g., flexible displays, electronic textiles, and sensory skins [1, 2, 3]. Consequently, flexible electronics has attracted widespread attention from both academic and industrial communities and has witnessed marvelous breakthroughs in terms of material development [4, 5], device fabrication, packaging, and integration [6, 7, 8].
In flexible electronics, semiconductors and devices are mounted onto flexible substrates, mostly polymers [1, 9]. The key to flexible electronics is realizing both high flexibility and desired physical/chemical properties (mostly electrical performance) of the semiconductors. Due to the intrinsic softness and flexibility, polymer semiconductors have long been a popular candidate for flexible electronics [10, 11, 12, 13]. However, carriers in organic materials are rather localized, leading to poor electrical conduction. Another way to realize “flexibility” is to process inorganic materials into ultrathin format to reduce the stiffness [5, 14], and this is why 2D materials or thin films are widely used [15, 16]. However, the thinning-induced flexibility does not change the intrinsic brittleness and rigidity [17, 18] of the inorganic semiconductors, which are constituted mainly by covalent or ion-covalent bonding .
Regarding this issue, another important mechanical property, plasticity, should be considered. As a matter of fact, however, plasticity is a long-sought target for inorganic materials, e.g., ceramics . On the one hand, plasticity means machinability, that is, plastic ceramics can be mechanically deformed and processed just like metals do. On the other hand, plastic deformation can prevent the sudden, catastrophic, brittle fracture, which is essential to not only structural materials but also functional materials. Hence, the discovery of the room-temperature plastic inorganic semiconductor Ag2S  and the fabrication of full-inorganic Ag2S-based thermoelectric (TE) power generation modules  are ground breaking, opening a new avenue toward next-generation flexible electronics.
This chapter will provide an in-time overview for the newly discovered plastic/flexible inorganic semiconductors. We shall first clarify the concept of flexibility and then illustrate the intrinsic plasticity for metals and brittleness for inorganic materials. Then, we will mention the special plasticity and the chemical bonding origins in a few ionic crystals such as AgCl. After that, we will systematically review the extraordinary mechanical properties of Ag2S and fully flexible thermoelectric devices. Finally, the prospect and challenge for plastic inorganic semiconductors as flexible electronic materials will be discussed.
2. Fundamental concepts: flexibility
As a matter of fact, “flexibility,” unlike “ductility” or “rigidity,” is not a scientifically clear concept. For flexible electronics, it is widely conceived that only elastic deformation is needed or allowed. In this sense, “flexibility” refers to the ability of the material/device to bend easily in an elastic way. According to Peng and Snyder , flexibility
The maximum elastic strain is reached when plastic deformation occurs: ε =
Removing the shape factor (2/
The value of
The above definition treats flexibility as elastic deformability, which is reasonable considering the real application. However, as discussed in Section 1, plasticity is important, particularly for efficient material processing. A plastic material can be easily processed into target geometry with little fracture or waste. In addition, a plastic material gains intrinsic flexibility without the restrict of size, that is, it can be readily deformed without breaking even in the bulk format, which is essentially important for applications requiring energy densities. Nonetheless, plasticity is rarely seen in inorganic semiconductors, which will be discussed in Section 3.
3. Prevalent brittleness for inorganic semiconductors
Ductile and brittle behaviors are schematically shown in Figure 2(a). Inorganic semiconductors are mostly constituted by covalent or ion-covalent bonds , which assure an appreciable electron orbital overlap, dispersive electronic band, and decent carrier mobility . Covalent bonds are directional, saturated, and localized. Therefore, even a trivial bonding distortion will cause a large instability, which is vividly demonstrated by the deep and steep curve in the interatomic potential versus atomic distance diagram (Figure 2(b)) . As a contrast, ionic bonding is unidirectional and nonsaturated. The columbic force is somewhat diffuse and extended within a large space. Consequently, the bonding distortion will induce a smaller variation in energy, as reflected by the shallow and flat potential-distance curve . Nonetheless, charges in ionic materials are localized around anions, leading to a poor electrical conductivity. Metallic bonding strength is mediate between covalent and ionic bonds. Metal atoms use their outer-shell electrons for two- and multicenter polar interactions, which open the possibility to unite charge transfer with a nonnegligible density of states at the Fermi level. This makes up good electrical conductivity and ductility.
Beyond the covalent bonding characteristics, the absence of plasticity and flexibility in inorganic materials can be interpreted in terms of dislocations. Various defects exist in inorganic materials (e.g., ceramics) such as vacancies, interstitials, and voids, strongly inhibiting the movement of dislocations. In addition, grain boundaries can cause dislocation pile-up, which may be a kind of crack source. For metals, the pile-up of dislocations near grain boundaries will cause hardening in mechanical properties. For ceramics, ductility is limited by crack nucleation and glide. In fact, the main deformation mechanism for most ceramics is creep. This process is mainly controlled by dislocation climb and diffusion, both of which are strongly dependent on temperature.
4. Plastic ionic inorganic materials
Although most inorganic materials are generally brittle, some ionic crystals have been found to exhibit some degree of plasticity upon certain deformation even at room temperature, such as AgCl [27, 28], KCl , and LiF . Under a slow strain rate, these crystals exhibit tensile properties like metals and show the phenomenon of neck down . This ductile behavior is attributed to the wavy slip [31, 32]. During the deformation, each grain has its own change and should conform to the distortion of neighbors . The wavy slipping allows to change planes in the vicinity of grain boundaries to permit distortions. The ability of crystals to relax the stress is essential for such a ductile deformation. At higher temperature, dislocations near the grain boundaries are easier to change the slipping planes due to the thermal active diffusion mentioned above, spreading out from boundaries to another grain. Thus, plastic deformation above transition temperature is mainly caused by the increase in slip systems and the dislocate-diffusion-induced creep and cross-slip.
First-principle calculations have been performed to compare the plasticity between AgCl and NaCl . The generalized stacking fault energy (GSFE) and the double of the surface energy 2
The reason for the different plastic behaviors between NaCl and AgCl was further attributed to their fundamental differences in electronic structures (Figure 3). The calculations  show that the smaller band gap and larger atomic-orbital overlap (Ag-5s and Cl-3p) contribute to a weak ionic bonding in AgCl. Crystal Orbital Hamilton population (COHP) analysis shows that the energies of initial Ag-Cl bonds gradually decrease as slipping proceeds. Particularly, in the (110), (111) planes, new Ag-Cl bonds are formed; a strong Ag-Ag bond is also formed when slipping on (100) planes. These newly formed bonds lower the unstable GSF energies in these slipping systems. While in NaCl, no newly formed bonds are observed.
The plasticity in AgCl was also understood from the perspective of dislocations . Electronic structures of dislocations in AgCl exhibit larger bonding interactions between atomic orbitals than that of NaCl. Also, the nearest neighbor distances around the dislocation core are tend to be shorter than that in NaCl. The two properties lead to a lower core energy (
5. Plasticity of ZnS in darkness
Very recently, ZnS, a well-known brittle material, was also reported to exhibit extraordinary “plasticity” in complete darkness . ZnS crystals fractured immediately when they deformed under light irradiation. However, the crystals could be plastically deformed to a compression strain of 45% in complete darkness as shown in Figure 4. It is also found that the optical band gap decreased by 0.6 eV after deformation as also clearly reflected by the apparent colors, which is probably due to the formation of extra energy levels at the bandgap edge in the presence of dislocations.
Based on optical and electronic microscopies, the plastic deformation in complete darkness is caused by glide and multiplication of dislocations belonging to the primary slip system. By contrast, plastic deformation under light irradiation involves deformation twinning. Obviously, the latter corresponds to a much poorer plasticity.
The origins for different deformation types are explained below. The dislocations in ZnS decompose into two partial dislocations. In darkness, the synergetic glide motion of the two partials will cause large slip deformations. On the contrary, under light irradiation, photo-excited electrons or holes can be trapped, thus charging some dislocations. The mobility of the dislocation can be limited by dragging the surrounding charge cloud compensating the dislocation charge. Therefore, the different charge states of the two dislocations in ZnS will lead to the great difference of their mobility, which will lead to the observed deformation twinning.
The work suggests that the mechanical properties are strongly intercorrelated to optical and electronic properties. It also implies that inorganic semiconductors are not necessarily “intrinsically” brittle.
6. Room-temperature plastic semiconductor Ag2S
The plastic ionic materials like AgCl are nearly insulating and cannot be used as semiconductors. Recently, α-Ag2S was discovered as the room-temperature ductile inorganic semiconductor as shown in Figure 5 . The plasticity of Ag2S is extraordinary: the engineering strains are ~4.5% in tension, 50% in compression, and above 20% in three-point bending, typical characteristics of metals as shown in Figure 6. α-Ag2S is a typical nondegenerate
The marvelous plasticity of α-Ag2S comes from its special crystal structure. α-Ag2S adopts a monoclinic symmetry with the space group
The multicentered, diffuse, and relatively weak bonding gives rise to the small slipping energy and large cleavage energy, i.e., plastic material can slip easily without cleavage as shown in Figure 7. As for α-Ag2S, it is assumed that slip plane is (100) and slip direction . According to the calculation, the slipping energy (
The distribution of the electron localizability indicator (ELI-D) shows a local maximum on the outer side of each S atom, and the basin of this maximum is caused by the formation of a lone pair or a strong Ag-S interaction shown in quantum theory of atoms in molecules (QTAIM). Besides, these lone pairs form double layers in the (100) plane. Thus, the
Li et al.  applied
The Ag-S octagon structure would be highly distorted under shear deformation. However, the Ag-S bond lengths change only slightly during shearing. Only when the strain is larger than ~110%, bond breakage would happen. This feature implies that Ag2S would retain its structural integrity even under very large shear deformation. Under (100)  shear load, Ag-Ag metallic bonds would form at 0.671 strain. These newly formed bonds strengthen the Ag-S frameworks and contribute to its structural integrity. Under  tensile load, the processes of breakage and the formation of Ag-S bond happen at the same time. The structural integrity could thus be preserved to a large strain. Based on these discoveries, they proposed that the easy slip pathways with good structural integrity under shear load are the origin of ductility in Ag2S.
Recent research also found that the single-layer Ag2S is a kind of auxetic materials with unusual negative Poisson’s ratio . According to their calculations, single-layer Ag2S has relatively low Young’s modulus with 61.61 N·m−1 along  direction and 2.78 N·m−1 along  direction. This is the lowest value found in various 2D materials such as graphite and MoS2. This means that along  direction, single-layer Ag2S will show extraordinary flexibility. Moreover, single-layer Ag2S shows negative Poisson’s ratio in both in-plane and out-of-plane directions, which is unique among quasi-2D materials.
7. Full inorganic flexible thermoelectric materials
The discovery of plastic Ag2S makes it possible to fabricate full-inorganic flexible devices. Pristine Ag2S exhibits a medium band gap (~1.0 eV), high mobility (~100 cm2/Vs), and extremely low lattice thermal conductivity (~0.5 Wm−1K−1 near room temperature) , which makes it a potential candidate for thermoelectric application. However, the extremely low carrier concentration (about 1.6 × 1014 cm−3 at room temperature) leads to its poor electrical conductivity. Therefore, the thermoelectric performance of Ag2S should be further optimized, and its plasticity should be maintained at the same time.
By alloying with Se/Te, Liang et al. successfully tuned the carrier concentration of Ag2S by virtue of the lowered defect formation energy of Ag interstitial atoms. Consequently, the electrical conductivity and power factor were largely optimized: the electrical conductivities of Ag2S0.5Se0.5, Ag2S0.8Te0.2, and Ag2S0.5Se0.45Te0.05 reach around 104 S m−1 at room temperature, which are comparable to state-of-the-art brittle inorganic TE materials. The power factors of Ag2S-based materials can reach 5 μW·cm−1·K−2 at room temperature. Meanwhile, Ag2(S, Se), Ag2(S, Te), and Ag2(S, Se, Te) have record low thermal conductivities of 0.3~0.6 Wm−1 K−1 at 300–450 K, among the lowest values observed in fully densified inorganic solids. The band gap of Se or Te alloyed Ag2S-based materials is also reduced, shifting the peak value of
More interestingly, alloying does not severely impair the plasticity. According to the mechanical property test, the ductility and flexibility of Ag2S-based materials would be maintained if the Se content is less than 60% or the Te content is less than 70%. Hence, the materials would possess both good ductility and TE performance when the Se/Te content is in the range of 20~60%, as shown in Figure 9(c). To further verify the robustness of Ag2S-based materials under different usage scenarios, bending cycle test was conducted on Ag2S0.5Se0.5 strip with a thickness of about 10 μm. As shown in Figure 9(f), the variation in Seebeck coefficient and electrical conductivity is less than 10% after 1000 bending cycles with a bending radius less than 3 mm. In addition, the relative resistance variation in the strip under different bending radius is also estimated.
Using Ag2S0.5Se0.5 strips as
The conventional strategy toward flexible TE devices is mounting TE thin-film materials onto the intrinsically flexible substrates. Ding et al.  fabricated flexible TE devices based on Ag2Se nanowires and plastic nylon substrate. The hybrid film was hot pressed at 200°C and 1 MPa for 30 min, which endows the film high TE performance and excellent flexibility at the same time. The highest power factor value of as-prepared Ag2Se/nylon film reaches 9.87 μWm−1 K−2 at 300 K, almost the highest value among reported
8. Summaries and outlook
This chapter reviews the newly emerging plastic inorganic semiconductors (e.g., Ag2S) for next-generation flexible electronics. The term “flexibility” is clarified at the very beginning. It should be recognized that plasticity is important for flexible electronics due to the availability of feasible processing and deformability free of size restricts. The intrinsic brittleness for inorganic semiconductors and ceramics is explained from unidirectional and saturated characteristics of covalent bonds. Historically, ionic crystals like AgCl have been found to exhibit certain plasticity but lack decent electrical conductivity. Groundbreakingly, Ag2S was discovered as the room-temperature ductile semiconductor. From the chemical bonding perspective, the multicentered, diffuse, weak interactions induce easy slipping while maintaining the integrity, which holds well not only for Ag2S but also for other plastic materials. The generalized bonding features are useful guidance for developing new flexible/plastic semiconductors. The electrical properties and thermoelectric performance of Ag2S are readily optimized upon Se/Te alloying while maintaining the plasticity and flexibility. Successively, full-inorganic thermoelectric devices are fabricated based on plastic and flexible Ag2S-based semiconductors, yielding much higher output power density than organic counterparts.
The discovery and application demonstration of plastic Ag2S inorganic semiconductor pave a new way toward next-generation flexible electronics. Facing large-scale applications in electronics and energy conversions, several key challenges lie ahead. First and basically, the mechanisms for plastic deformation in Ag2S needs further investigation, especially on the individual and synergetic effects of both chemical bonding and dislocations, which calls for tremendous efforts of both experimentalists and theorists from a variety of disciplines. Second, practical criteria are required to rapidly screen potentially new, plastic/flexible semiconductors. These performance indicators should be easily available yet insightful, and it is better that they can be implemented into the high-throughput calculations. Third, all the techniques are to be renewed including material processing, electrode/substrate selection, device fabrication, and circuit integration.
Facing all these exciting challenges and fascinating opportunities, there is no doubt that the flexible/plastic inorganic semiconductors will bring a revolution to academic communities, electronic/energy industries, and worldwide market. The next-generation flexible electronics is meant to deeply change our life and reshape the world. The future has come.