Sushi-rolled 2D heterostructures could lead to new miniaturized electronics

The recent synthesis of one-dimensional van der Waals heterostructures, a kind of heterostructure made by applying two-dimensional materials that are one atom thick, could lead, according to a team from Penn State, to new, miniaturized electronics that are currently not possible. not. University of Tokyo researchers.

Engineers usually manufacture heterostructures to obtain new device properties that are not available in one material. A Van der Waals heterostructure is one made from 2D materials stacked directly on top of each other such as Lego blocks or a sandwich. The van der Waals force, which is an attraction between uncharged molecules or atoms, holds the materials together.

According to Slava V. Rotkin, professor of engineering and mechanics at Penn State Frontier, the one-dimensional van der Waals heterostructure manufactured by the researchers differs from the van der Waals heterostructures that engineers have produced so far.

“It looks like a stack of 2D layers of material rolled up into a perfect cylinder,” Rotkin said. “In other words, when you roll up a sandwich, you keep all the good stuff in it where it should be and do not move around, but in this case you also make it a thin cylinder, very compact like a sausage sandwich or ‘ a long sushi roll.In this way, the 2D materials still contact each other in a desired vertical heterostructure sequence, while one does not have to worry about their side edges, all rolled up, which is a big thing for super-small devices to make. “

The team’s research, published in ACS Nano, suggests that all 2D materials can be rolled in these one-dimensional heterostructure cylinders, known as hetero-nanotubes. Researchers from the University of Tokyo recently manufactured electrodes on a hetero-nanotube and showed that, despite its size, it can work as an extremely small diode with high performance.

“Diodes are an important type of device used in optical electronics – they are the core of photodetectors, solar cells, light emitting devices, and so on,” Rotkin said. “In electronics, diodes are used in several specialized circuits; although the main element of electronics is a transistor, two diodes, which are connected back-to-back, can also serve as a switch.”

This opens up a potential new class of materials for miniaturized electronics.

“It takes device technology of 2D materials to a new level, potentially enabling a new generation of electronic and optoelectronic devices,” Rotkin said.

Rotkin’s contribution to the project was to solve a particularly challenging task, to ensure that they could make the one-dimensional van der Waals heterostructure cylinder of all required material layers.

“Using the sandwich analogy again, we had to know if we had a shell of ‘roast’ over the entire length of a cylindrical sandwich, and if there were regions where we only had ‘bread’ and ‘salad shells’,” said Rotkin. . “The absence of a middle insulating layer would mean that we could not fail the synthesis of devices. My method explicitly showed that the middle shells exist over the entire length of the device.”

In regular, flat van der Waals heterostructures, the confirmation of the existence or absence of some layers can be easily done because it is flat and has a large surface area. This means that a researcher can use different types of microscopies to collect a lot of signal from the large, flat areas so that it is easily visible. When researchers roll it up, as in the case of a one-dimensional van der Waals heterostructure, it becomes a very thin wire-like cylinder that is difficult to characterize because it gives little signal and becomes practically invisible. To prove the existence of an insulating layer in the semiconductor-insulator-semiconductor terminal of the diode, you must not only dissolve the outer sheath of the hetero-nanotube, but the middle, which is completely covered by the outer shells of a molybdenum sulfide semiconductor.

To solve this, Rotkin used an optical field scattering survey in the near field that is part of the Materials Research Institute’s 2D Crystal Consortium, which can “see” objects of nanoscale size and the optical properties of their material. determined. He also developed a special method of analyzing the data known as nanometer-resolution hyperspectral optical imaging, which can distinguish different materials and thus test the structure of the one-dimensional diode over its entire length.

According to Rotkin, this is the first demonstration of the optical resolution of a hexagonal boron nitride (hBN) shell as part of a hetero-nanotube. Much larger pure hBN nanotubes, consisting of many shells of hBN without any other types of material, have been studied in the past with a similar microscope.

“However, the imaging of these materials is completely different from what I did before,” Rotkin said. “The beneficial result is the demonstration of our ability to measure the optical spectrum from the object, which is an inner sheath of a wire that is only two nanometers thick. It is comparable to the difference between a block of wood and a being able to recognize a graphite stick inside the pencil through the pencil walls. ‘

Rotkin plans to expand its research to expand hyperspectral imaging to better dissolve other materials, such as glass, various 2D materials, and protein tubes and viruses.

“This is a new technique that will hopefully lead to future discoveries,” Rotkin said.