Design the boundary between 2-D and 3-D materials

In recent years, engineers have found ways to adjust the properties of some “two-dimensional” materials, which are only one or a few atoms thick, by stacking two layers together and twisting one slightly relative to the other. This creates the so-called moire patterns, where small shifts in the alignment of 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, such two-dimensional materials must at some point join the ordinary world of 3-D materials. An international team led by MIT researchers has now devised a way that is going on 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 consequent changes in the material’s properties.

The new findings are described in the journal today Nature communication, in a paper 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 the University of Victoria in Canada.

Pairs of two-dimensional materials such as graphene or hexagonal boron nitride can show incredible variations in their behavior when the two sheets are only slightly twisted relative to each other. This causes chicken wire-like atomic grids to form moire patterns, the kind of strange bands and stains that sometimes occur when you take a picture of a printed picture or through a window. In the case of 2-D materials, “it looks like anything, every interesting material property you can think of, you can modulate or change in some way by twisting the 2-D materials relative to each other, “says Ross, who is the Ellen Swallow Richards professor at MIT.

Although these 2-D pairings have attracted scientific attention worldwide, she says, little is still known about what happens where 2-D materials meet ordinary 3-D solids. “What made us interested in this topic,” says Ross, “was what happens when a 2-D material and a 3-D material are put together. First, how do you measure the atomic positions at and near the interface? Second, what are the differences between a 3-D-2-D and a 2-D-2-D interface? And thirdly, how can you control it – is there a way to deliberately design the interface structure “around the to produce desired properties?

Finding out exactly what happens at such 2-D-3-D interfaces was a daunting challenge because electron microscopes produce an image of the sample in projection, and they are limited in their ability to extract the depth information. required to analyze the details of the interface structure. But the team devised a set of algorithms that allowed them to extrapolate back from images of the monster, which looked somewhat like a set of overlapping shadows, to figure out which set-up stacked layers would yield that complex “shadow” .

The team used two unique transmission electron microscopes at MIT that enable a combination of capabilities unmatched 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 by deposition processes and fed directly into the imaging system. It is one of only a few such facilities worldwide, which 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 housed in MIT’s new research facility, MIT.nano. This microscope has excellent stability for high-resolution imaging, as well as various imaging modes to gather information about the sample.

In contrast to stacked 2-D materials, the orientation of which can be changed relatively easily by simply picking up one layer, twisting it slightly and placing it back down, the bonds that hold 3-D materials together are much stronger, so the team needed new ways to obtain aligned layers. To do this, they added the 3D material in ultra-high vacuum to the 2-D material and selected to select growth conditions where the layers themselves were rotated in a reproducible orientation with specific degree of rotation. “We had to cultivate a structure that would align in a certain way,” Reidy says.

After growing the material, they had to figure out how to reveal the atomic configurations and orientations of the different layers. A scanning transmission electron microscope actually produces more information than appears in a flat image; in fact, each point in the image contains details of the paths along which the electrons arrived and departed (the process of diffraction), as well as any energy that the electrons lost during the process. All this data can be separated so that the information at all points in an image can be used to decode the actual solid structure. This process is only possible for modern microscopes, such as those in MIT.nano, which generate a probe of electrons that is extremely narrow and precise.

The researchers use a combination of techniques called 4-D STEM and integrated differential phase contrast to achieve the process of extracting the entire structure at the interface from the image. Then, says Varnavides, they ask, “Now that we can represent the complete structure at the interface, what does this mean for our understanding of the properties of this interface?” The researchers showed through modeling that electronic properties are expected to change in a way that can only be understood if the complete structure of the interface is incorporated into the physical theory. “What we have found is that this stacking, the way the atoms are stacked outside the plane, does modulate the electronic properties and charge density properties,” he says.

Ross says the findings could lead to improved types of crosses in some microchips, for example. “Every 2-D material used in a device must exist in the 3-D world, and so it must somehow have a connection with three-dimensional materials,” she says. Thus, with this better understanding of the interfaces, and new ways of studying them in action, “we are well able to create structures with desired properties in a kind of planned way as ad hoc.”

“The methodology used has the potential to calculate the modulation of the local electron momentum from the acquired local diffraction patterns,” he says, adding that “the methodology and research shown here have an excellent future and great interest. for the materials science community. ”

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