In recent years, engineers have found ways to modify the properties of some “two-dimensional” materials, which are only one or a few atoms thick, by stacking two layers together and rotating one slightly relative to the other. This creates what are known as moiré patterns, where small changes in the alignment of the atoms between the two sheets create larger-scale patterns. It also changes the way electrons move through the material, in potentially useful ways.
But for practical applications, these two-dimensional materials must connect at some point with the ordinary world of three-dimensional materials. An international team led by MIT researchers has devised a way to image what happens at these interfaces, down to the level of individual atoms, and to correlate the moire patterns at the 2-D-3-D boundary with the result. changes in material properties.
The new findings are described today in the journal. Communications from nature, in an article by MIT graduate students Kate Reidy and Georgios Varnavides, materials science and engineering professors Frances Ross, Jim LeBeau, and Polina Anikeeva, and five others at MIT, Harvard University, and Victoria University in Canada.
Two-dimensional material pairs like graphene or hexagonal boron nitride can exhibit astonishing variations in their behavior when the two sheets are slightly twisted from each other. That causes chicken wire-shaped atomic networks to form moiré patterns, the kind of strange bands and spots that sometimes appear when taking a photograph of a printed image, or through a window screen. In the case of 2-D materials, “it seems that any interesting property of materials you can think of, you can somehow modulate or change by rotating the 2-D materials around each other,” says Ross, who is Professor Ellen Swallow. Richards at MIT.
While these 2-D pairings have drawn scientific attention around the world, he says, little is known about what happens when 2-D materials meet regular 3-D solids. “What got us interested in this topic,” says Ross, was “what happens when a 2-D material and a 3-D material come together. First, how are the atomic positions at and near the interface measured? Secondly, what are the differences between a 3-D-2-D interface and a 2-D-2-D interface? And thirdly, how can it be controlled? Is there a way to deliberately design the structure interfacial “to produce the desired properties? ?
Figuring out exactly what happens at such 2-D-3-D interfaces was a daunting challenge because electron microscopes produce an image of the projected sample and have limited ability to extract the depth information needed to analyze the details of the interface. structure. But the team discovered a set of algorithms that allowed them to extrapolate the sample images, which look a bit like a set of overlapping shadows, to determine what configuration of stacked layers would produce that complex “shadow.”
The team made use of two unique transmission electron microscopes at MIT that enable a combination of capabilities that is unrivaled in the world. In one of these instruments, a microscope is connected directly to a manufacturing system so that samples can be produced on-site using deposition processes and immediately fed directly into the imaging system. This is one of the few facilities of its kind in the world that uses an ultra-high vacuum system that prevents even the smallest impurities from contaminating the sample while preparing the 2-D-3-D interface. The second instrument is a scanning transmission electron microscope located at MIT’s new research facility, MIT.nano. This microscope has outstanding stability for high resolution images, as well as multiple imaging modes to collect information about the sample.
Unlike stacked 2-D materials, whose orientations can be changed relatively easily simply by lifting a layer, turning it slightly, and repositioning it, the bonds that hold 3-D materials together are much stronger, so the team had to develop new ways to get layers aligned. To do this, they added the 3-D material over the 2-D material in ultra-high vacuum, choosing growing conditions where the layers self-assembled in a reproducible orientation with specific degrees of twist. “We had to develop a structure that aligned in a certain way,” says Reidy.
Having cultivated the materials, they had to figure out how to reveal the atomic configurations and orientations of the different layers. A transmission scanning electron microscope actually produces more information than is seen in a flat image; in fact, each point in the image contains details of the pathways that the electrons arrived and left (the process of diffraction), as well as any energy the electrons lost in the process. All of this data can be separated so that information at all points in an image can be used to decode the actual solid structure. This process is only possible for state-of-the-art microscopes, such as MIT’s nano, which generates an electron probe that is unusually narrow and precise.
The researchers used a combination of techniques called 4-D STEM and integrated differential phase contrast to accomplish that process of extracting the entire structure at the image interface. Then, says Varnavides, they asked, “Now that we can visualize the entire structure in the interface, what does this mean for our understanding of the properties of this interface?” The researchers demonstrated through models that the electronic properties are expected to change in a way that can only be understood if the complete structure of the interface is included in the physical theory. “What we found is that in fact this stacking, the way the atoms are stacked out of plane, modulates the electronic and charge density properties,” he says.
Ross says the findings could help improve joint types on some microchips, for example. “Every 2-D material that is used in a device has to exist in the 3-D world, so it has to have a bond in some way with three-dimensional materials,” he says. So with this better understanding of those interfaces and new ways of studying them in action, “we are in good shape to make structures with desirable properties in a planned rather than ad hoc way.”
“The methodology used has the potential to calculate local electronic moment modulation from acquired local diffraction patterns,” he says, adding that “the methodology and research shown here have an outstanding future and great interest for the scientific community of materials “.
Two-dimensional heterostructures made up of layers with slightly different lattice vectors
Kate Reidy et al, Direct imaging and modulation of the electronic structure of moiré superlattices in the 2D / 3D interface, Communications from nature (2021). DOI: 10.1038 / s41467-021-21363-5
Provided by the Massachusetts Institute of Technology
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