Researchers at Lawrence Berkeley National Laboratory have developed a new method to directly observe how electrons behave around defects in ultrathin semiconductors. The approach combines high-resolution imaging with computer simulations and could help scientists better understand materials for future miniaturized electronic devices.
The research team was led by scientists at Berkeley Lab, with researchers from UC Berkeley and several other institutions. The study was published in Nature and focused on two-dimensional, or 2D, semiconductor materials that are only one or a few layers of atoms thick.
The team studied molybdenum diselenide, a 2D semiconductor where electrons can enter an unusual state called a Wigner solid. In this state, electrons strongly repel one another and arrange themselves into an organized pattern instead of moving more freely through the material.
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To see this behavior, the researchers used a scanning tunneling microscope. The instrument scans a material’s surface with an extremely small metal tip and measures electrical current, allowing scientists to observe features at very small scales.
The researchers built a specially designed device to make these measurements possible. The molybdenum diselenide was placed between layers of graphite, silicon and boron nitride, with tiny openings in the graphite allowing the microscope tip to reach the semiconductor.
The team created samples with different numbers of defects and then changed the electron density inside the material. This allowed them to watch how the electrons changed from a Wigner solid into a more fluid state known as a Fermi liquid.
The results showed that defects can have a major effect on electron behavior. When many defects were present, the electrons formed a stable but irregular Wigner solid pattern. With fewer defects, they formed more orderly, triangular patterns and changed more easily into the Fermi liquid state.
Mike Crommie, a senior faculty scientist in Berkeley Lab’s Materials Sciences Division and a UC Berkeley physics professor, said the researchers could see the Wigner solid begin to behave like a liquid around defects. The team compared some defects to large potholes and others to smaller bumps that affected electron movement in different ways.
The researchers then used Quantum Monte Carlo simulations to test whether their microscope images matched theoretical predictions. The simulations produced patterns that closely resembled the experimental images, supporting both the imaging method and the computer model.
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This agreement was important because earlier studies often had to infer electron behavior from electrical measurements. The new approach allows researchers to see electrons and defects together, providing a more direct way to study their interaction.
The findings could become useful as semiconductor devices continue to shrink. Understanding how the location and type of defects affect electrons may help researchers design future 2D semiconductors for electronics, sensors and other small devices.
The technology is still at the research stage, and Wigner solids are not used in today’s mainstream computer chips or smartphones. The researchers are now studying how electrons behave when confined to very small channels and how reducing defects changes their behavior.
The new combination of direct imaging and quantum simulations gives scientists another tool for studying advanced semiconductor materials. As devices become smaller, understanding electron behavior at this scale could help guide the development of new semiconductor technologies.
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