On the Spontaneous Structural Reconstructions of Chalcogen-Depleted MoS$_2$, MoSe$_2$, and Janus MoSSe Monolayers

Imagine a sheet of material so thin it is only one atom thick, yet it holds the promise of revolutionizing everything from faster electronics to more efficient solar cells. These are two-dimensional crystals, specifically a family known as transition-metal dichalcogenides. Think of them as a sandwich: a single layer of metal atoms, like molybdenum, is tightly sandwiched between two layers of non-metal atoms, called chalcogens, which in this case are sulfur or selenium. Under normal conditions, this sandwich is stable and orderly. However, nature is rarely perfect. In the real world, these sheets often lose some of their outer atoms, leaving the metal layer exposed and unstable. When this happens, the remaining atoms do not just sit there; they scramble to find a new, more comfortable arrangement. This process, known as structural reconstruction, determines whether the material remains a useful, orderly crystal or collapses into a messy, disordered state. Understanding exactly how and why this happens is crucial for engineers who hope to build devices from these fragile sheets, as the material's behavior depends entirely on how it recovers from damage.

A team of researchers set out to solve a specific puzzle regarding this recovery process. They wanted to know if the type of atom left behind after the damage matters more than the original shape of the crystal. To test this, they looked at three different starting materials: a standard sheet of molybdenum disulfide, a sheet of molybdenum diselenide, and a "Janus" sheet, which is a hybrid where one side is sulfur and the other is selenium. By simulating the removal of an entire outer layer of atoms from each of these sheets, they created four distinct scenarios to observe. In two cases, sulfur was left behind on the metal; in the other two, selenium remained. The scientists used powerful computer simulations to watch these atomic sheets evolve over time, tracking how the remaining atoms moved and how the metal lattice responded.

What they discovered was a clear and decisive rule: the identity of the remaining atom dictates the outcome, while the original parent material plays only a minor role. When the remaining atoms were sulfur, the sheet behaved with remarkable patience. At room temperature, the sulfur atoms stayed put, and the metal lattice underneath remained a perfect, crystalline grid. It was only when the researchers heated the simulation to 900 Kelvin that the sulfur atoms began to move, crossing over the metal plane to find new positions. Even then, the metal atoms managed to stay organized, forming small, ordered islands that persisted for a long time, though they eventually began to break down as the temperature rose and tiny holes appeared in the sheet.

The story was entirely different when selenium was the remaining atom. In these simulations, the sheet lost its order almost instantly. As soon as the simulation began, even at room temperature, the selenium atoms started crossing over the metal plane. Within the first trillionth of a second, the metal lattice, which had been a perfect hexagonal grid, completely lost its crystalline structure and became disordered. This chaotic rearrangement happened so quickly that the temperature of the simulation barely mattered; the selenium atoms crossed over regardless of whether the environment was cool or hot. The researchers found that this process released a significant amount of energy, driving the atoms into a new, denser configuration that was fundamentally different from the orderly state seen with sulfur.

To ensure these findings held up beyond the tiny, limited view of a small computer model, the team also ran simulations on much larger sheets, measuring 50 by 50 nanometers. These larger models confirmed the pattern: sulfur left behind meant a sheet that could hold onto its crystal structure for nanoseconds or even longer, while selenium meant immediate disorder. The researchers also tested a hybrid scenario, creating a single sheet that was half sulfur-based and half selenium-based. The result was striking. The two halves behaved exactly as they would have if they were separate sheets. The sulfur side stayed orderly while the selenium side fell into chaos, proving that the reconstruction mode is a local property determined solely by the atom present on the surface.

The study also revealed how the physical shape of the sheet changes during this process. As the atoms crossed over the metal plane, the sheet did not just rearrange vertically; it also shrank horizontally. This contraction was not uniform; the sheet squeezed more in one direction than the other, breaking its original symmetry. The researchers noted that this shrinking happened after the atoms had already moved, suggesting that the structural reconstruction itself forces the lattice to contract. In the larger simulations, they observed that the remaining sulfur atoms tended to cluster together in specific ways, while the selenium atoms spread out more randomly.

Ultimately, the work suggests that scientists can control the fate of these damaged materials by choosing which atoms are left behind. If a material is designed to keep its crystalline structure after losing atoms, it should retain sulfur. If the goal is to create a disordered, amorphous region, selenium is the key. This insight opens a potential path for "chalcogen patterning," where engineers could deliberately arrange different atoms on a single sheet to create specific regions of order and disorder within the same material. The researchers emphasize that while these results come from computer simulations, the clear distinction between the behavior of sulfur and selenium offers a reliable guide for understanding how these atomic sheets will react when they are damaged or altered in the real world.