Linking properties to defects in 2D materials

(Nanowerk News) Scientists measured the atomic and electronic structure of a two-dimensional semiconductor to understand defects in the crystal structure. The measurements were made at the same time and at the same location. The scientists visualized quantum orbitals associated with the defects. They accomplished this by visualizing the quantum orbitals using an ultra-sharp probe made from a single carbon monoxide molecule.
This is the first time that 2D material properties have been linked to specific defects in the materials’ crystal structure. This approach enables systematic defect engineering in 2D materials. These materials are critical for next-generation quantum information science. They are also essential for “Beyond Moore’s Law” electronics that move us past the limits of current technology.
Scanning tunneling microscopy topography of an oxygen substituting sulfur (left) and sulfur vacancy (right) in tungsten disulfide
Scanning tunneling microscopy topography of an oxygen substituting sulfur (left) and sulfur vacancy (right) in tungsten disulfide. Bottom: Corresponding atomically resolved non-contrast atomic force microscopy image. (Image: Berkeley Lab)
Like any material, atoms-thick 2D semiconductors known as transition metal dichalcogenides (TMDs) are not perfect. While some defects may interfere with a material’s electronic conductivity, other defects can actually help to control its conductivity, optical properties, or catalytic functionality. But no one has been able to link these useful properties to specific defects at the atomic level in the crystal structure of TMDs.
Now, two new studies led by scientists at DOE’s Molecular Foundry user facility have revealed surprising new details on atomic defects in TMDs and how those defects shape their electronic properties. Their findings could provide a more versatile yet targeted platform for designing 2D materials for quantum information science. The findings could also lead to smaller and more powerful next-generation optoelectronics devices.
In the world of materials science, most researchers assumed that the most abundant defects in TMDs were missing atoms or “vacancies” in specific 2D semiconductor materials. Specifically, researchers attributed defects to sulfur vacancies in tungsten disulfide or selenium vacancies in molybdenum diselenide. But as reported in Nature Communications ("Identifying substitutional oxygen as a prolific point defect in monolayer transition metal dichalcogenides"), the scientists found that there is more to this mystery than sulfur or selenium atoms gone missing.
Using atomic force microscopy (AFM) and scanning tunneling microscopy (STM) techniques and a single carbon monoxide molecule as an ultrasharp “tip” or probe, the scientists surprisingly found no evidence of selenium or sulfur vacancies. Instead, they found that oxygen atoms had taken their place.
When used with AFM, the carbon monoxide-tip images the surface atoms at a very high resolution that is not possible with conventional techniques and precisely pinpoints the defect’s atomic site. STM then provides the defect’s unique electronic fingerprint.
In their second study, published in Physical Review Letters ("Large Spin-Orbit Splitting of Deep In-Gap Defect States of Engineered Sulfur Vacancies in Monolayer WS2"), the scientists demonstrate how to deliberately create these elusive chalcogen vacancies by heating a sample of tungsten disulfide in a vacuum up to 600 degrees Celsius (1,112 degrees Fahrenheit), resulting in a thermal energy that causes the atoms to vibrate. At some point, the vibrations kick out one of the sulfur atoms, creating an atomic “hole” known as a defect state in the material’s crystalline structure.
The scientists found that these defect states are shaped in part by spin-orbit coupling. This coupling involves the interaction between different aspects of an atom’s movement. These interactions can change an atom’s energy levels, but the degree of change is unusually strong in the tungsten disulfide material.
Source: U.S. Department of Energy, Office of Science
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