Pressure turns Ångström-thin semiconducting bismuth into a metal, expa…

Two-dimensional (2D) materials, sparked by the isolation of Nobel-prize-winning graphene in 2004, has revolutionized modern materials science by showing that electrical, optical, and mechanical behaviors can be tuned simply by adjusting the thickness, strain, or stacking order of such 2D materials. From transistors and flexible display to neuromorphic chips, the future of electronics is expected to be significantly empowered by 2D materials.

In a new study published in Nano Letters titled “Pressure-Driven Metallicity in Ångström-Thickness 2D Bismuth and Layer-Selective Ohmic Contact to MoS₂,” researchers led by SUTD have discovered that a gentle squeeze is enough to make bismuth—one of the heaviest elements in the periodic table—switch its electrical personality.

Using state-of-the-art density functional theory (DFT) simulations, the team showed that when a single layer of bismuth, only a few atoms thick, is compressed or “squeezed” between surrounding materials, the atoms reorganize from a slightly corrugated (or buckled) structure into a perfectly flat one. This structural flattening, though subtle, has dramatic electronic consequences: it eliminates the energy band gap and allows electrons to move freely, turning the material metallic.

“Once the bismuth sheet becomes completely flat, the electronic states overlap, and the material suddenly conducts electricity like a metal. The transformation is fully driven by mechanical pressure,” said Dr. Shuhua Wang, a postdoctoral research fellow at SUTD.

Explaining a recent experimental surprise

Earlier in 2025, a landmark Nature paper reported that when bismuth was squeezed between two layers of molybdenum disulfide (MoS₂) down to the Ångström-thickness limit, it behaved as a metal, in sharp contrast to the semiconducting character predicted by decades of theoretical studies and previous experiments on freestanding monolayers.

That unexpected observation posed an open question: Why does confined bismuth conduct electricity when its unconfined counterpart does not?

This research provides the missing theoretical explanation. By linking pressure, structure, and electronic behavior, the team demonstrated that van der Waals squeezing flattens the atomic lattice of bismuth, thus triggering the precise structural and electronic transition needed for metallicity.

A new way to rewire current

The researchers further proposed a MoS₂-Bi-MoS₂ trilayer heterostructure, where the atomically thin bismuth acts as a metallic bridge sandwiched between two semiconducting layers.

Their simulations revealed a striking asymmetry: one MoS₂ layer forms a low-resistance (Ohmic) contact with the metallic Bi, while the other forms a higher-resistance (Schottky) barrier. By applying an external electric field perpendicular to the stack, the team showed that this Ohmic contact can be switched between the top and bottom layers, thus allowing electrical current to be steered between layers on demand.

This mechanism, termed a layer-selective Ohmic contact, marks a new milestone in 2D electronics. It generalizes the familiar metal–semiconductor interface into a layer-dependent, field-controllable contact—the essence of layertronics, a device concept that exploits the layer degree of freedom in 2D materials for data processing and storage.

“Traditional circuits are wired once and fixed forever,” said Assistant Professor Yee Sin Ang, the project lead and Kwan Im Thong Hood Cho Temple Early Career Chair Professor in Sustainability at SUTD. “In MoS₂-Bi-MoS₂ trilayer heterostructure, we can reconfigure where the current flows simply by tuning an electric field. That means the same device can perform multiple functions without any physical rewiring. It’s a key step toward reprogrammable, energy-efficient nanoelectronics.”

Such advances may help address one of the greatest challenges in modern electronics: integrating ultrathin transistors and interconnects without sacrificing contact performance. The ability to fine-tune contact behavior via mechanical or electrical fields provides a powerful, sustainable pathway toward the next generation of flexible, low-power, and reconfigurable computing chips.