Scanning tunneling microscopy image of an oxygen substituting sulfur (left), and a sulfur vacancy (right) in tungsten disulfide. In comparison, a strand of human DNA is 2.5 nanometers (nm) in diameter, and a strand of human hair is about 100,000 nm wide. (Credit: Berkeley Lab)
Like any material, atomically thin, 2D semiconductors known as TMDs or transition metal dichalcogenides are not perfect, but their imperfections can actually be a good thing.
Understanding how defects are structured at the atomic scale, how they are created, and how they interact with electrons are the first steps to designing new advanced materials. However, no one has been able to link useful properties like optical absorption and emission, conductivity, or catalytic function to specific defects in TMDs.
Now, two studies led by scientists at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have revealed surprising details on how some atomic defects emerge in TMDs, and how those defects shape the 2D material's electronic properties. Their findings could provide a more versatile yet targeted platform for designing 2D materials for quantum information science and smaller, more powerful next-generation light-based electronics (optoelectronics).
A quantum tip for 2D materials
In the world of materials science, many researchers assumed that the most abundant defects in TMDs were the result of missing atoms or "vacancies" of sulfur in tungsten disulfide (WS2), or selenium vacancies in molybdenum diselenide (MoSe2).
But as reported in Nature Communications, the researchers found that the defects previously observed with other methods were actually created by oxygen atoms replacing sulfur or selenium atoms, said D. Frank Ogletree, a staff scientist at Berkeley Lab's Molecular Foundry and a co-author of the two studies.
Oxygen, like sulfur and selenium, is part of the oxygen or "chalcogen" family of elements. And since chalcogens share similar properties, there isn't much change in conductivity when an oxygen atom takes the place of a sulfur or selenium atom in a TMD crystal structure, he said.
Atomic force microscopy image of sulfur vacancy in tungsten disulfide. (Credit: Berkeley Lab)
"In other words, it's like exchanging one kind of apple for another," explained co-lead author Bruno Schuler, a postdoctoral researcher at Berkeley Lab's Molecular Foundry. "So when an oxygen atom fills in for a missing sulfur or selenium atom, it effectively restores the TMD's electronic properties."
Co-lead author with Schuler is Sara Barja, who was a postdoctoral researcher in Berkeley Lab's Materials Sciences Division at the time of the Nature Communications study.
Key to their finding was the use of the Molecular Foundry's atomic force microscope (AFM), with a single carbon monoxide (CO) molecule acting as an ultrasharp "tip" or probe, and scanning tunneling microscope (STM). They also benefited from state-of-the-art calculations carried out by scientists from Berkeley Lab's Center for Computational Study of Excited State Phenomena in Energy Materials (C2SEPEM).
When used with AFM, the CO-tip images the surface atoms at a very high resolution that's not possible with conventional techniques, and precisely pinpoints the defect's atomic site; STM provides the defect's unique electronic fingerprint.
A Deep Dive Into the Imperfect World of 2D Materials
Previously, scientists at Berkeley Lab demonstrated how easily defects can be created in TMDs to engender useful properties.
