Researchers give insight about how to create defect-free 2D materials.
Two-dimensional materials refer to solids containing a single layer of atoms. These materials are promising for developing new ultra-compact electronic devices. However, the defect-free 2D materials are difficult to implement.
A perfect direct bandgap in semiconductors is ideal for electrons to get excited to a conduction state. Therefore, semiconductor relies on perfection of band gaps for reliable operation. Moreover, spin and valley degrees of freedom have also shown promise in 2D materials and can be manipulated to enable new types of devices. Orientation of multiple spins in the material can lead to magnetization, and distributing electrons into valleys can enable new ways to process and store information. But all of this relies on defect-free 2D materials.
Penn State University researchers have suggested how to create 2D materials without imperfections. The team studied defects in tungsten disulfide, which belongs to a class of 2D crystals known as transition metal dichalcogenides, which are three-atom-thick crystals that have properties that make them ideal for the development of future electronics. The study has been published in Nano Letters.
“2D materials are exciting new materials for electronics, and because they are so thin, they make it possible to shrink devices to very small sizes,” said Danielle Reifsnyder Hickey, Penn State assistant research professor of materials science and engineering. “This is critical for making electronics more powerful so that they can handle more data. However, it is a huge challenge to grow perfect 2D materials over areas large enough to be able to make large arrays of high-quality devices.”
“2D material monolayers have different properties than bulk crystals,” Reifsnyder Hickey said. “For example, they have direct band gaps and can therefore be used as very small transistor materials, and their crystal symmetry enables new types of devices based on increased degrees of freedom relative to their bulk counterparts.”
Team discovered translational grain boundaries, which occur at the interface between two crystallites that have the same orientation but a translational offset. Generally, grain boundaries connect grains with dissimilar orientations and can affect electrical conductivity of the materials, lessening their value for electronics. For investigating this, the team used a combination of scanning transmission electron microscopy imaging and a ReaxFF reactive force field simulation.
“Through a synergistic approach, we were able to explain our experimental findings using simulations and uncover the growth mechanism that leads to such microstructure,” said Nasim Alem, Penn State associate professor of materials science and engineering and the study’s corresponding author. “This is an important step, because by learning the underlying physics of growth and defect formation, we can learn to modify and control them, and this will have a profound effect of the electronic properties of the crystal.”