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Monolayer semiconductors align despite lattice mismatch

ORNL scientists grow high-quality layers of GaSe and MoSe2

Above: Light drives the migration of charge carriers at the juncture between semiconductors with mismatched crystal lattices

Stacking different semiconductor materials on top of each other usually only works when the individual materials have similar crystal lattices. Now, in a study led by the US Department of Energy's Oak Ridge National Laboratory (ORNL), scientists have grown high-quality layers of atomically thin monolayers of two lattice-mismatched semiconductors.

The GaSe (p-type semiconductor) and MoSe2 (n-type) layers formed a p-n junction, which generated a photovoltaic response by separating electron-hole pairs generated by light. According to the researchers, the achievement of creating this atomically thin solar cell, published in Science Advances, shows the promise of synthesising mismatched layers to enable new families of functional 2D materials.

"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."

While characterising 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 visualised 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 MoSe2, and then grew a layer of GaSe 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 characterised 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.

"We've opened the door to exploring all types of mismatched heterostructures," Li said.

'Two-dimensional GaSe/MoSe2 misfit bilayer heterojunctions by van der Waals epitaxy' by Xufan Li et al;  Science Advances 15 Apr 2016: Vol. 2, no. 4, e1501882

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