The Shanghai Institute of Ceramics has discovered a semiconductor material with ductility comparable to that of metals

Metals and ceramics/semiconductors exhibit vastly different mechanical properties: metals possess excellent ductility, plasticity, and ease of machining, whereas ceramics and semiconductors are characterized by brittleness, poor plasticity, and difficulty in processing. The study of these fundamental materials is indispensable to human survival and development. Today, metals and ceramics/semiconductors have permeated every aspect of our production and daily life. However, their contrasting mechanical properties have led to nearly opposite application domains for the two materials. In particular, due to the significant difference in ductility, the preparation science and processing technologies for metals and ceramics/semiconductors are entirely distinct. For instance, metals typically undergo melting combined with mechanical processing, stamping, and precision casting to form components, while ceramics/semiconductors, owing to their brittleness, generally rely on methods such as powder sintering to produce bulk materials. In applications that demand special shapes or forms, as well as deformability, only metals and organic materials are currently suitable; ceramics/semiconductors, because of their brittleness, simply cannot meet these requirements.

In recent years, flexible electronics have attracted widespread global attention and have been rapidly developing, with the potential to trigger a revolution in electronic technology. Flexible electronics is an emerging technology that involves fabricating organic and inorganic material-based electronic devices on flexible substrates. Thanks to its unique deformability as well as efficient and low-cost manufacturing processes, it holds great promise for wide-ranging applications in fields such as information, energy, healthcare, and national defense. However, current inorganic materials—especially semiconductors—are brittle; under conditions of large bending, significant deformation, or tensile stress, they are highly prone to fracture, leading to device failure. Moreover, organic semiconductors have lower carrier mobility compared to their inorganic counterparts, and their electrical performance is less tunable, making it difficult for them to meet the booming demands of the semiconductor industry. Therefore, developing inorganic semiconductor materials with excellent ductility and bendability, and achieving breakthroughs in the integration equipment and manufacturing processes for flexible electronics, have become pressing needs for the advancement of flexible electronics.

Recently, Researcher Shi Xun and Researcher Chen Lidong from the Shanghai Institute of Ceramics, Chinese Academy of Sciences, in collaboration with Professor Yuri Grin from the Max Planck Institute in Germany, discovered a semiconductor material that exhibits ductility comparable to that of metals at room temperature. The study found that α-Ag2S is a typical semiconductor yet possesses remarkably unusual mechanical properties akin to those of metals—specifically, it demonstrates excellent ductility and flexibility, making it promising for wide-ranging applications in flexible electronics. The relevant research was published in the journal Nature Materials and was featured in "news & views" by nat mater. In their commentary, Dae-Hyeong Kim and Gi Doo Cha, experts in flexible electronics from Seoul National University in South Korea, noted: “While both metals and insulators have suitable mechanical properties, today’s potential flexible semiconductors—such as carbon nanotubes, graphene, and molybdenum disulfide—cannot simultaneously achieve mechanical ductility, high carrier mobility, and an appropriate bandgap. High-quality inorganic semiconductors that meet all these requirements simply do not yet exist. Now, this research team has reported in Nature Materials an outstanding semiconductor material—α-Ag2S—that displays metal-like ductility at room temperature.”

At room temperature, α-Ag2S exhibits a monoclinic layered structure with zig-zag corrugated layers. Four S atoms and four Ag atoms form an 8-atom ring, and these rings are connected to each other via S atoms. α-Ag2S is a typical semiconductor with a bandgap of approximately 1 eV. Undoped α-Ag2S is primarily electron-conductive, featuring a low electron concentration and relatively low electrical conductivity—around 0.01 S m⁻¹—and a high electron mobility—about 100 cm² V⁻¹ s⁻¹. The electron concentration and electrical conductivity of α-Ag2S can be increased by several orders of magnitude through elemental doping, allowing its electrical properties to be freely tuned within the semiconductor regime.

Compared to other semiconductors or ceramics, α-Ag2S exhibits exceptionally unusual and unique mechanical properties. It possesses ductility and deformability akin to metals—under external forces and large strains, it does not suffer material failure or fracture. The machining debris of α-Ag2S also resembles metal in that it forms thin, elongated, filament-like strands, whereas the debris from conventional ceramics and semiconductors typically consists of fine particles or powders. Further characterization of its mechanical properties reveals that α-Ag2S can undergo compressive deformation up to more than 50%. In three-point bending tests, its maximum bending strain exceeds 20%, and tensile tests show that α-Ag2S can sustain tensile strains as high as 4.2%. All these values significantly surpass those of known ceramics and semiconductors, closely resembling the mechanical performance of certain metals.

The research team further investigated the mechanisms underlying these anomalous mechanical properties of α-Ag2S. For a material with good slipability and ductility, two fundamental conditions must be met: First, there must be slip planes with low energy barriers that can slide under external forces; second, during the sliding process, the material must not decompose and must maintain its overall integrity and completeness.

We performed first-principles computational simulations of the slip processes in a series of materials, including α-Ag2S, NaCl, graphite, diamond, metallic Mg, and Ti. We found that α-Ag2S, NaCl, graphite, metallic Mg, and Ti all have slip planes with relatively low energy barriers. Among these, the slip plane of α-Ag2S is the (100) plane. In contrast, diamond exhibits an excessively high energy barrier during slip, and thus no slip plane exists for it. We also discovered that the interaction forces among the slip planes of α-Ag2S, metallic Mg, and Ti are relatively strong, making it difficult for cracks to form or for the materials to undergo dissociation during slip, thereby maintaining their overall integrity and structural continuity. On the other hand, the interaction forces between the slip planes of NaCl, graphite, and diamond are too weak, allowing cracks to easily develop and leading to material dissociation during slip. Furthermore, quantum chemical calculations were used to reveal the origin and mechanism of the interaction forces between the slip planes of α-Ag2S. We found that within a single crystal period, in addition to intermolecular forces, the (100) slip planes are connected only by bonding interactions between two yellow sulfur atoms and six gray silver atoms. During slip, the two sulfur atoms move along the glide path formed by the six silver atoms; at the same time, old Ag-S bonds continuously weaken or even break, while new Ag-S bonds strengthen or even form. As a result, the interaction forces between the (100) slip planes remain consistently in the Ag-S bonded state, leading to minimal energy fluctuations during slip and consequently resulting in a low slip energy barrier. Moreover, this bonded state ensures strong interactions among these slip planes, effectively preventing crack formation and even material dissociation during the slip process.

For applications in flexible electronics, the team also prepared α-Ag2S thin films and found that they exhibit greater deformability than bulk materials. They also characterized the electrical properties of α-Ag2S after deformation and discovered that after dozens or even hundreds of repeated bending cycles, its electrical performance remained essentially unchanged or changed only slightly.

Unlike the well-known brittle ceramics and semiconductor materials, the α-Ag2S semiconductor exhibits metal-like mechanical properties, maintaining both its structural integrity and electrical performance under bending and deformation. Its widely tunable electrical properties, suitable bandgap, and high carrier mobility make it promising for broad applications in the field of flexible electronics. Meanwhile, this work will also pave the way for research into identifying and discovering other semiconductor materials with similar metal-like mechanical properties.

The research was supported and funded by the National Natural Science Foundation of China (51625205 and 51632010), the Key Deployment Project of the Chinese Academy of Sciences (KFZD-SW-421), the Shanghai Municipal Major Basic Research Project (15JC1400301), and the Discipline Leader Program (16XD1403900).

Article link: https://www.nature.com/articles/s41563-018-0047-z

. Tensile properties (left figure) and crystal structure (right figure) of α-Ag2S semiconductor material

. Mechanical properties of α-Ag2S semiconductor material. Figure a: Physical photograph of α-Ag2S under compression; Figure b: Compressive performance; Figure c: Flexural performance; Figure d: Tensile performance

. Resistance changes during the bending process of α-Ag2S semiconductors