US scientists watch electrons melt from crystal solids to liquid in 2D semiconductors

Scientists in the US have developed a new approach that allowed them to directly observe how electrons move from a solid-like state into liquid-like waves inside a 2D ultrathin semiconductor.

The study was carried out by the US Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab). For the project, the team used a scanning tunneling microscope and imaged electrons and defects inside a semiconductor made from molybdenum diselenide (MoSe2).

The researchers discovered that the number of defects could significantly change electron behavior. The finding could reportedly offer new insights for increasingly miniaturized electronic devices.

Mike Crommie, PhD, a Berkeley Lab scientist and UC Berkeley physics professor, said the study offered insights into electron behavior in 2D semiconductors. “Our methods open the door to the discovery of never-seen-before electron behaviors that can be used for new semiconductor capabilities,” he added.

Electrons switch states

Conventional semiconductor devices, like transistors, computer chips and sensors, are usually made from silicon-based materials. Inside, silicon atoms bond together in a rigid, 3D diamond cubic lattice. This allows billions of transistors to be packed onto a single chip.

However, in recent years scientists have increasingly explored 2D semiconductors, ultra-thin materials made from just one or a few layers of atoms. Their super-thin structure changes how electrons behave, potentially enabling new capabilities in future semiconductor devices.

Experimental and simulated images show electrons transitioning from a Wigner solid to a Fermi liquid. Credit: Berkeley Lab

The team now focused on electrons in a state known as a Wigner solid, an exotic state of matter in which free electrons freeze into a rigid, crystal-like grid instead of flowing like a fluid. In conventional semiconductor devices, electrons behave more like independent particles moving through a material. This state is known as a Fermi liquid.

In a Wigner solid, however, repulsion between electrons becomes dominant. The electrons separate, become largely immobile, and also organize themselves into relatively orderly patterns. Scientists have previously struggled to directly observe how defects affect these strongly interacting electrons.

Now, the researchers created a specialized device that placed MoSe2 between a graphite layer and layers of silicon and boron nitride. Tiny holes in the graphite allowed the microscope’s metal tip to scan the semiconductor underneath. They then studied samples with different defect levels and adjusted electron density to track the shift between Wigner solid and Fermi liquid states.

Defects alter electron behavior

The images revealed massive differences depending on the number of defects present. When many defects were present, the electrons became locked into an unexpectedly stable Wigner solid with irregular and disordered patterns.

With fewer defects, the electrons instead formed more orderly triangular, crystal-like patterns. These transitioned more readily into the Fermi liquid state.

Crommie described his first reaction to seeing the unexpected images. “It was exciting to see the Wigner solid melt into liquid-like waves splashing up against defects,” he elaborated. “We could see how electrons respond to defects in very different ways.”

He explained that certain defects acted like large potholes while others acted like tiny speed bumps. The team also needed to confirm that the unusual structures were genuine electron behavior rather than artifacts created by the microscope.

For this, they used Quantum Monte Carlo simulations to calculate how strongly interacting electrons should respond to random defects according to quantum mechanics. The simulations closely matched the microscope images.

Crommie believes that controlling the location and type of defects could become increasingly important as electronic components shrink. “It also helped us better understand the physics underlying the behaviors we observed in our images,” he concluded in a press release.

The study has been published in the journal Nature.