In recent years, engineers have discovered ways to modify the properties of certain “two-dimensional” materials that are only one or a few atoms thick by stacking two layers together and rotating them slightly relative to the other. This produces the so-called moiré pattern, in which small changes in the arrangement of atoms between the two sheets produce a larger pattern. It also changes the way electrons move through materials in potentially useful ways.
But for practical applications, such two-dimensional materials must be connected to the ordinary world of 3-D materials at some point. An international team led by MIT researchers has now come up with a method that can image what happens on these interfaces down to the level of a single atom, and remove the moiré at the 2-D-3-D boundary. Streaks are associated with the results obtained. Changes in material properties.
The new discovery was described in the magazine today Nature CommunicationsMIT graduate students Kate Reidy and George Varnavides, Professor of Materials Science and Engineering Frances Ross, Jim LeBeau and Bo Polina Anikeeva and five other professors from MIT, Harvard University and Victoria University in Canada.
When two pieces of material are slightly twisted relative to each other, paired two-dimensional materials (such as graphene or hexagonal boron nitride) may exhibit surprising performance changes. This causes the atomic lattice like chicken wire to form a moiré pattern. These odd bands and spots sometimes appear when taking photos of printed images or passing through window screens. For two-dimensional materials, “it seems that everything, every interesting material property you can think of, can be modulated or changed in some way by twisting two-dimensional materials relative to each other,” Ross said. Ellen Swallow Richards is a professor at the Massachusetts Institute of Technology.
She said that although these 2-D pairings have attracted scientific attention worldwide, little is known about what happens when 2-D materials meet conventional 3-D entities. Ross said, “What makes us interested in this topic is,” what happens when 2-D materials and 3-D materials are put together. First, how do you measure the position of atoms at and near the interface? Secondly, what is the difference between 3-D-2-D and 2-D-2-D interfaces? Third, how do you control it-there is a way to deliberately design the interface structure to produce the desired characteristics. ?
Knowing exactly what happens on such a 2-D-3-D interface is a daunting challenge, because electron microscopes produce images of the sample in the projection, and their ability to extract depth information is limited and cannot analyze the interface. Detail structure. But the research team came up with a set of algorithms that would allow them to infer images that look like a set of overlapping shadows from the sample images, thereby figuring out which configuration of the stacked layers would produce complex “shadows.”
The team used two unique transmission electron microscopes from the Massachusetts Institute of Technology to achieve an unparalleled combination of functions in the world. In one of these instruments, the microscope is directly connected to the manufacturing system, so samples can be produced on-site through a deposition process and immediately sent directly to the imaging system. This is one of only a few such equipment in the world. They use an ultra-high vacuum system, even if a 2-D-3-D interface is prepared, it can prevent the smallest impurities from contaminating the sample. The second instrument is a scanning transmission electron microscope at MIT.nano, a new research institution at the Massachusetts Institute of Technology. The microscope has excellent stability for high-resolution imaging and has multiple imaging modes to collect information about the sample.
Unlike stacked 2D materials, whose orientation can be changed relatively easily by simply picking up a layer, twisting it slightly and putting it down again, the bonds that hold the 3D materials together are much stronger, so the team must develop new materials. How to get the alignment layer. To this end, they added 3-D materials to 2-D materials in an ultra-high vacuum, and selected growth conditions under which the layers self-assembled in a reproducible direction with a specific degree of torsion. Reidy said: “We must develop a structure that must be consistent in some way.”
After growing the material, they must figure out how to reveal the atomic configuration and orientation of the different layers. Scanning transmission electron microscopy actually produces more information than what is shown in a flat image. In fact, each point in the image contains detailed information about the path (diffraction process) the electron arrives and departs, as well as any energy lost by the electron in the process. All these data can be separated out so that the information of all points in the image can be used to decode the actual physical structure. This process is only possible with state-of-the-art microscopes like MIT.nano, which produces extremely narrow and precise electron probes.
The researchers used a combination of a technology called 4-D STEM and integrated differential phase contrast technology to achieve the process of extracting the complete structure at the interface from the image. Then, Varnavides said, they asked: “Now we can image the entire structure on the interface, what does this mean for us to understand the characteristics of the interface?” The researchers showed through modeling that only the complete structure of the interface is included in the physical theory. Under the circumstances, it is hoped to understand the electronic properties in a certain way. He said: “We found that, in fact, this stacking (the way atoms are stacked out of plane) does modulate the electron and charge density characteristics.”
Ross said these findings may help improve the connection types of certain microchips. She said: “All the 2-D materials used in the device must exist in the 3-D world, so it must be combined with the three-dimensional materials in some way.” Therefore, there is a better understanding and research on these interfaces. After their new approach, “we are already in a good condition to construct structures with the required characteristics in a planned way rather than a temporary way.”
He said: “The method used makes it possible to calculate the modulation of local electron momentum based on the obtained local diffraction pattern.” He added, “The methods and research shown here have broad development prospects and strong prospects for the material science community. interest.”
A two-dimensional heterostructure composed of layers with slightly different lattice vectors
Kate Reidy and others performed direct imaging and electronic structure modulation on the Moiré superlattice on the 2D/3D interface, Nature Communications (2021). DOI: 10.1038 / s41467-021-21363-5
Provided by the Massachusetts Institute of Technology
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