'Odd couple' monolayer semiconductorsalign to advance optoelectronics15 April 2016

Light drives the migration of charge carriers (electronsand holes) at the juncture between semiconductors withmismatched crystal lattices. These heterostructures holdpromise for advancing optoelectronics and exploringnew physics. The schematic's background is a scanningtransmission electron microscope image showing thebilayer in atomic-scale resolution. Credit: Oak RidgeNational Laboratory, US Dept. of Energy. Image byXufan Li and Chris Rouleau

Epitaxy, or growing crystalline film layers that aretemplated by a crystalline substrate, is a mainstayof manufacturing transistors and semiconductors. Ifthe material in one deposited layer is the same asthe material in the next layer, it can beenergetically favorable for strong bonds to formbetween the highly ordered, perfectly matchedlayers. In contrast, trying to layer dissimilarmaterials is a great challenge if the crystal latticesdon't match up easily. Then, weak van der Waalsforces create attraction but don't form strong bondsbetween unlike layers.

In a study led by the Department of Energy's OakRidge National Laboratory, scientists synthesized astack of atomically thin monolayers of two lattice-

mismatched semiconductors. One, gallium selenide,is a "p-type" semiconductor, rich in charge carrierscalled "holes." The other, molybdenum diselenide,is an "n-type" semiconductor, rich in electroncharge carriers. Where the two semiconductorlayers met, they formed an atomically sharpheterostructure called a p-n junction, whichgenerated a photovoltaic response by separatingelectron-hole pairs that were generated by light.The achievement of creating this atomically thinsolar cell, published in Science Advances, showsthe promise of synthesizing mismatched layers toenable new families of functional two-dimensional(2D) materials.

The idea of stacking different materials on top ofeach other isn't new by itself. In fact, it is the basisfor most electronic devices in use today. But suchstacking usually only works when the individualmaterials have crystal lattices that are very similar,i.e., they have a good "lattice match." This is wherethis research breaks new ground by growing high-quality layers of very different 2D materials,broadening the number of materials that can becombined and thus creating a wider range ofpotential atomically thin electronic devices.

"Because the two layers had such a large latticemismatch between them, it's very unexpected thatthey 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 oftwo different types of metal chalcogenides—binarycompounds of sulfur, selenium or tellurium with amore electropositive element or radical—havingsuch different lattice constants can be growntogether 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 buildingblock, the researchers found that the two

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mismatched layers had self-assembled into arepeating long-range atomic order that could bedirectly visualized by the Moiré patterns theyshowed in the electron microscope. "We weresurprised that these patterns aligned perfectly," Lisaid.

Researchers in ORNL's Functional HybridNanomaterials group, led by David Geohegan,conducted the study with partners at VanderbiltUniversity, the University of Utah and BeijingComputational Science Research Center.

"These new 2D mismatched layeredheterostructures open the door to novel buildingblocks for optoelectronic applications," said seniorauthor Kai Xiao of ORNL. "They can allow us tostudy new physics properties which cannot bediscovered with other 2D heterostructures withmatched lattices. They offer potential for a widerange of physical phenomena ranging frominterfacial magnetism, superconductivity andHofstadter's butterfly effect."

Li first grew a monolayer of molybdenumdiselenide, and then grew a layer of galliumselenide on top. This technique, called "van derWaals epitaxy," is named for the weak attractiveforces that hold dissimilar layers together. "Withvan der Waals epitaxy, despite big latticemismatches, you can still grow another layer on thefirst," Li said. Using scanning transmission electronmicroscopy, the team characterized the atomicstructure of the materials and revealed theformation of Moiré patterns.

The scientists plan to conduct future studies toexplore how the material aligns during the growthprocess and how material composition influencesproperties beyond the photovoltaic response. Theresearch advances efforts to incorporate 2Dmaterials into devices.

For many years, layering different compounds withsimilar lattice cell sizes has been widely studied.Different elements have been incorporated into thecompounds to produce a wide range of physicalproperties related to superconductivity, magnetismand thermoelectrics. But layering 2D compoundshaving dissimilar lattice cell sizes is virtually

unexplored territory.

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

The title of the paper is "Two-dimensionalGaSe/MoSe2 misfit bilayer heterojunctions by vander Waals epitaxy."

More information: Two-DimensionalGaSe/MoSe2 misfit bilayer heterojunctions by vander Waals epitaxy, Science Advances, dx.doi.org/10.1126/sciadv.1501882

Provided by Oak Ridge National Laboratory

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APA citation: 'Odd couple' monolayer semiconductors align to advance optoelectronics (2016, April 15)retrieved 7 November 2021 from https://phys.org/news/2016-04-odd-couple-monolayer-semiconductors-align.html

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