Unlike conventional inorganic semiconductors, which are typically brittle, α-Ag2S exhibits room-temperature ductility with favourable electrical properties, offering promise for use in high-performance flexible and stretchable devices.
Flexible and stretchable electronics technologies have bestowed conventional devices with novel mechanical properties, such as bendability, foldability and stretchability. They have attracted significant attention and driven breakthroughs in electronics1, optoelectronics2,3, biomedicine4,5 and energy storage6. These flexible and stretchable devices are poised to exploit soft electronic materials, that is, polymeric insulators for gate dielectrics and encapsulation, soft semiconductors for active channels, and ductile conductors for gate, source, drain and interconnection (Fig. 1a). Although there are both metals7 and insulators8 with such mechanical properties, current candidates for soft semiconductors (for example, carbon nanotubes, organics, nanostructured graphene and MoS2)9 do not simultaneously satisfy the requirements of mechanical ductility and high carrier mobility with an appropriate bandgap. A high-quality bulk inorganic semiconductor that can meet all of these requirements is not yet available (Fig. 1b). Now, writing in Nature Materials, Lidong Chen and co-workers report the synthesis of an inorganic material with favourable semiconducting properties, α-Ag2S, which shows metal-like ductility at room temperature7.
Chen and co-workers synthesized α-Ag2S via two methods, ingot casting and spark plasma sintering, in the form of bulk materials and/or thin films. The macroscopic ductility was evaluated by deforming the bulk material through hammering or by winding machine-processed α-Ag2S wires. The stress–strain curves of α-Ag2S were measured through compression, bending and stretching tests. Its deformation limit was found to be superior to that of other typical semiconductors and ceramics by several orders of magnitude, and even higher than some alloys10. In addition to being mechanically ductile, α-Ag2S showed typical semiconducting properties, with an electrical conductivity of 0.09–0.16 S m−1, a bandgap of 1.03 ± 0.1 eV and a carrier mobility of ~100 cm2 V−1 s−1. When compared with other materials (Fig. 1c), α-Ag2S was found to have unique material features not included in any typical material classification, with a combination of semiconducting conductivity and metal-like ductility. Even after several deformation cycles, there was no significant change in the electrical properties of the α-Ag2S thin film, which corroborates its potential for use as an active layer in flexible and stretchable electronic devices.
The ductility of α-Ag2S was explained by two physical properties at the atomistic level: low threshold energy of certain interlayer slippage and relatively strong interatomic forces interlinking these slip planes. These imply the existence of atomic slipping planes in α-Ag2S and structural retention during material deformation. At room temperature, α-Ag2S has a monoclinic crystal structure, which is maintained up to 451 K, and changes to body-centred cubic and then face-centred cubic crystal structures with increasing temperature. At room temperature, the unit cell is composed of four S and four Ag atoms constituting eight atomic ring fragments, and the units are stacked perpendicular to the planar direction (Fig. 1d). The ductility of α-Ag2S originates from the interlayer slippage of these atomic stacks. The clear existence of cleavage planes after compression confirms this explanation (Fig. 1e).
The mechanism of ductility was also numerically explained by density functional theory calculations. The slipping event was divided into 12 substeps, and the energy of each step was calculated and compared with that of other materials. The maximum value of the energy barrier for slipping was 0.166 eV atom−1, which is comparable to that of other metals and much lower than that of diamond. The minimum value of the cleavage energy was 0.148 eV atom−1, which is much higher than that of NaCl, graphite and diamond. These results explain the facile interlayer slippage and the stability of the whole structure without cleaving. According to the quantum theory of atoms in molecules, while weak van der Waals interactions are the main force between the layers, the formation of additional bonds due to the movement of Ag atoms in the structure results in the increase of cleavage energy and maintains the structure intact without any ruptures during interlayer slippage. These simulation results help explain the ductile mechanism of α-Ag2S at the atomic scale.
The room-temperature ductile inorganic semiconductor α-Ag2S is a promising material for use as soft deformable active layers in flexible and stretchable electronics. Its unique mechanical and electrical properties are quite stable, even under external stress, which is particularly attractive. However, certain challenges still remain for the development of high-performance flexible and stretchable devices based on α-Ag2S. In particular, device fabrication on plastic substrates is one key topic that should be explored further. For example, there could be unexpected issues at interfaces with other layers, such as ductile metals and polymeric insulators, and low-temperature processes are also supposed to be developed for device fabrication on plastic substrates. Integration for advanced circuits using α-Ag2S, a necessary step towards commercialization, as well as other factors, such as long-term stability and large-area uniformity, must be investigated for its practical applications in the future. Despite all the aforementioned challenges, the high ductility and excellent electrical properties of α-Ag2S clearly differentiate it from conventional semiconducting materials, and it is likely to be helpful in overcoming the overarching challenges of next-generation flexible and stretchable devices.
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Kim, DH., Cha, G.D. Deformable inorganic semiconductor. Nature Mater 17, 388–389 (2018). https://doi.org/10.1038/s41563-018-0066-9
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