OAK RIDGE, Tenn., April 15, 2016鈥擡pitaxy, 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鈥檛 match up easily. Then, weak van der Waals forces create attraction but don鈥檛 form strong bonds between unlike layers.
In a study led by the Department of Energy鈥檚 Oak Ridge National Laboratory, scientists synthesized a stack of atomically thin monolayers of two lattice-mismatched semiconductors. One, gallium selenide, is a 鈥減-type鈥� semiconductor, rich in charge carriers called 鈥渉oles.鈥� The other, molybdenum diselenide, is an 鈥渘-type鈥� semiconductor, rich in electron charge carriers. Where the two semiconductor layers met, they formed an atomically sharp heterostructure called a p鈥搉 junction, which generated a photovoltaic response by separating electron鈥揾ole pairs that were generated by light. The achievement of creating this atomically thin solar cell, published in Science Advances, shows the promise of synthesizing mismatched layers to enable new families of functional two-dimensional (2D) materials.
The idea of stacking different materials on top of each other isn鈥檛 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 鈥渓attice match.鈥� 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.
鈥淏ecause the two layers had such a large lattice mismatch between them, it鈥檚 very unexpected that they would grow on each other in an orderly way,鈥� said ORNL鈥檚 Xufan Li, lead author of the study. 鈥淏ut it worked.鈥�
The group was the first to show that monolayers of two different types of metal 鈥攂inary compounds of sulfur, selenium or tellurium with a more electropositive element or radical鈥攈aving such different lattice constants can be grown together to form a perfectly aligned stacking bilayer. 鈥淚t鈥檚 a new, potential building block for energy-efficient optoelectronics,鈥� 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 they showed in the electron microscope. 鈥淲e were surprised that these patterns aligned perfectly,鈥� Li said.
Researchers in ORNL鈥檚 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.
鈥淭hese new 2D mismatched layered heterostructures open the door to novel building blocks for optoelectronic applications,鈥� said senior author Kai Xiao of ORNL. 鈥淭hey 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鈥檚 butterfly 别蹿蹿别肠迟.鈥�
Li first grew a monolayer of molybdenum diselenide, and then grew a layer of gallium selenide on top. This technique, called 鈥渧an der Waals epitaxy,鈥� is named for the weak attractive forces that hold dissimilar layers together. 鈥淲ith van der Waals epitaxy, despite big lattice mismatches, you can still grow another layer on the first,鈥� Li said. Using scanning transmission electron microscopy, the team characterized the atomic structure of the materials and revealed the formation of Moir茅 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.
鈥淲e鈥檝e opened the door to exploring all types of mismatched heterostructures,鈥� Li said.
The title of the paper is 鈥淭wo-dimensional GaSe/MoSe2 misfit bilayer heterojunctions by van der Waals epitaxy.鈥�
Research, including materials synthesis, was supported by the DOE Office of Science. Materials characterization was conducted in part at the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility at ORNL. ORNL Laboratory Directed Research and Development funds supported some of the device measurements in the study.
UT-Battelle manages ORNL for DOE鈥檚 . The single largest supporter of basic research in the physical sciences in the United States, the Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit .鈥�by Dawn Levy
