US scientists observe electrons transitioning from solid to liquid state inside a two-dimensional semiconductor

US scientists have developed a new method that allowed them to directly observe how electrons transition from a solid state into liquid-like waves, in a process that takes place inside an ultrathin two-dimensional semiconductor. The research was conducted at Lawrence Berkeley National Laboratory (Berkeley Lab) of the US Department of Energy. The team used scanning tunneling microscopy to image electrons and defects in a semiconductor made of molybdenum diselenide (MoSe2). The researchers found that the number of defects can significantly alter electron behavior, an insight that could inform the design of increasingly miniaturized electronic devices.

Berkeley Lab scientist and University of California, Berkeley professor of physics Mike Crommie said the study provides deep insights into electron behavior in two-dimensional semiconductors. He added: "Our approach opens the door to discovering previously unseen electron behaviors that can be used in the development of new semiconductor capabilities."

Deeper insights into electron behavior

Electron state transitions Traditional semiconductor devices, such as transistors, computer chips and sensors, are typically made from silicon-based materials. Silicon atoms are bound internally in a rigid three-dimensional diamond cubic lattice, allowing billions of transistors to be packed onto a single chip. More recently, however, scientists have increasingly turned their attention to two-dimensional semiconductors, ultrathin materials consisting of only one or a few atomic layers. Their extreme thinness alters how electrons behave, potentially opening up new capabilities for future semiconductor devices. The team is now focused on an electron state known as a Wigner solid, an exotic state of matter in which free electrons freeze into a rigid, crystal-like lattice rather than flowing like a fluid.

In conventional semiconductor devices, electrons behave more like individual particles moving through the material, a state known as a Fermi liquid. In a Wigner solid, however, mutual repulsion between electrons becomes dominant, causing the electrons to separate and become nearly immobile, organizing into a relatively ordered pattern.

Challenges in observing Wigner solids

Scientists have previously struggled to directly observe how defects influence these strongly interacting electrons. Now the researchers built a custom device that places MoSe2 between a graphite layer and layers of silicon and boron nitride. Tiny holes in the graphite allow the metallic tip of the microscope to scan the semiconductor beneath. They then examined samples with varying levels of defects and adjusted electron density to track the transition between Wigner solid and Fermi liquid states. The images revealed dramatic differences in electron behavior depending on the number of defects. When defects were abundant, electrons locked into a surprisingly stable Wigner solid, forming irregular and disordered patterns, while with fewer defects, electrons arranged themselves into more orderly, triangular crystal-like patterns and transitioned more readily into a Fermi liquid state.

Crommie described his reaction to seeing the unexpected images: "It was exciting to see a Wigner solid melt into liquid-like waves and slam into defects." He went on to explain: "We could see that electrons responded so differently to defects." He noted that some defects resembled large potholes, while others acted like small speed bumps. The team also needed to confirm that these unusual structures reflected real electron behavior rather than artifacts created by the microscope. To do so, they used quantum Monte Carlo simulations to calculate how strongly interacting electrons should respond to random defects according to quantum mechanics.

The simulation results closely matched the microscope images. Crommie believes that as electronic components continue to shrink, controlling the location and type of defects will become increasingly important. He concluded in a press release: "This also helps us better understand the physics of the behaviors we are seeing in our images." The study has been published in the journal Nature.

The importance of defects in electron behavior

As semiconductor technology continues to advance, a deeper understanding of electron behavior has become increasingly important. This study shows that defects in two-dimensional semiconductors can significantly affect electron states, thereby altering their behavior in different environments. This carries important implications for the design and manufacture of future electronic components, since controlling the type and location of defects will be a key factor in improving device performance. The scientists' findings not only provide a new perspective on understanding electron behavior in novel materials, but also pave the way for the development of new semiconductor technologies.