Monolayer Semiconductors to Advance Optoelectronics

Light drives the migration of charge carriers at the juncture between semiconductors with mismatched crystal lattices (Source: ORNL)

Epitaxy, or growing crystalline film layers that are templated by a crystal­line substrate, is a mainstay of manu­facturing transistors and semi­conductors. If the material in one deposited layer is the same as the material in the next layer, it can be energe­tically favorable for strong bonds to form between the highly ordered, perfectly matched layers. In contrast, trying to layer dissimilar materials is a great challenge if the crystal lattices don’t match up easily. Then, weak van der Waals forces create attrac­tion but don’t form strong bonds between unlike layers.

In a study led by the Department of Energy’s Oak Ridge National Laboratory, scientists synthesized a stack of atomically thin monolayers of two lattice-mismatched semi­conductors. One, gallium selenide, is a p-type semiconductor, rich in charge carriers called holes. The other, molybdenum diselenide, is an n-type semi­conductor. Where the two semiconductor layers met, they formed an atomically sharp hetero­structure called a p-n junction, which generated a photo­voltaic response by separating electron-hole pairs that were generated by light. The achievement of creating this atomically thin solar cell shows the promise of synthe­sizing mismatched layers to enable new families of functional two-dimensional materials.

The idea of stacking different materials on top of each other isn’t new by itself. In fact, it is the basis for most electronic devices in use today. But such stacking usually only works when the individual materials have crystal lattices that are very similar, i.e., they have a good lattice match. This is where this research breaks new ground by growing high-quality layers of very different 2D materials, broadening the number of materials that can be combined and thus creating a wider range of potential atomically thin elec­tronic devices.

“Because the two layers had such a large lattice mismatch between them, it’s very unex­pected that they would grow on each other in an orderly way,” said ORNL’s Xufan Li, lead author of the study. “But it worked.” The group was the first to show that monolayers of two different types of metal chalcogenides having such different lattice constants can be grown together to form a perfectly aligned stacking bilayer. “It’s a new, potential building block for energy-efficient opto­electronics,” Li said.

Upon charac­terizing their new bilayer building block, the researchers found that the two mismatched layers had self-assembled into a repeating long-range atomic order that could be directly visualized by the Moiré patterns they showed in the electron microscope. “We were surprised that these patterns aligned perfectly,” Li said. Researchers in ORNL’s Functional Hybrid Nano­materials group, led by David Geohegan, conducted the study with partners at Vanderbilt University, the University of Utah and Beijing Compu­tational Science Research Center.

“These new 2D mismatched layered hetero­structures open the door to novel building blocks for optoelectronic applications,” said senior author Kai Xiao of ORNL. “They can allow us to study new physics properties which cannot be discovered with other 2D hetero­structures with matched lattices. They offer potential for a wide range of physical phenomena ranging from interfacial magnetism, super­conductivity and Hofstadter’s butterfly effect.”

Li first grew a monolayer of molybdenum dise­lenide, and then grew a layer of gallium selenide on top. This technique, called van der Waals epitaxy, is named for the weak attractive forces that hold dissimilar layers together. “With van der Waals epitaxy, despite big lattice mismatches, you can still grow another layer on the first,” Li said. Using scanning trans­mission electron microscopy, the team characterized the atomic structure of the materials and revealed the formation of Moiré patterns.

The scientists plan to conduct future studies to explore how the material aligns during the growth process and how material composition influences properties beyond the photo­voltaic response. The research advances efforts to incorporate 2D materials into devices. For many years, layering different compounds with similar lattice cell sizes has been widely studied. Different elements have been incorporated into the compounds to produce a wide range of physical properties related to super­conductivity, magnetism and thermo­electrics. But layering 2D compounds having dissimilar lattice cell sizes is virtually unexplored territory. “We’ve opened the door to exploring all types of mismatched hetero­structures,” Li said. (Source: ORNL)

Reference: X. Li et al.: Two-dimensional GaSe/MoSe2 misfit bilayer heterojunctions by van der Waals epitaxy, Sci. Adv. 2, e1501882, online 15 April 2016, DOI: 10.1126/sciadv.1501882

Link: Functional Hybrid Nanomaterials group (D. Geohegan), Oak Ridge National Laboratory, Tennessee, USA

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