Researchers demonstrated a curious quantum effect using a diode only a few atoms thick.

The research team discovered a quantum mechanical transport phenomenon in an ultra-thin room temperature material, Penn State Materials Research Institute reported. The finding could lead to the development of unique nanoelectronic circuits.

This quantum transport effect, dubbed negative differential resistance (NDR), was spotted when researchers applied a voltage to structures made from the atomically-thin layered synthetic material. The "van der Waals material" consists of a three-part structure made from: a base of graphene, and atomic layers of either molybdenum disulfide (MoS₂), molybdenum diselenide (MoSe₂), or tungsten diselenide (WSe₂).

NDR is a process in which the wave nature of electrons paves the way for them to tunnel through any material with varying resistance, potentially leading to electronic circuits that could be operated at an extremely high frequency.

"Theory suggests that stacking two-dimensional layers of different materials one atop the other can lead to new materials with new phenomena," says Joshua Robinson, a Penn State assistant professor of materials science and engineering whose student, Yu-Chuan Lin, is first author on a paper appearing online today, June 19, in the journal Nature Communications.

Achieving this novel phenomenon requires a nearly perfect tunneling interface, which is only possible through direct growth techniques. In this recent study, the researchers used oxide vaporization of molybdenum oxide in the presence of sulfur vapor to make the MoS₂ layer, and metal organic chemical vapor deposition to make the WSe₂ and MoSe₂.

"This is the first time these vertical heterostructures have been grown like this," Robinson says. "People typically use exfoliated materials that they stack, but it has been extremely difficult to see this phenomenon with exfoliated layers, because the interfaces are not clean. With direct growth we get pristine interfaces where we see this phenomenon every time."

Through this process, the researchers observed a "sharp peak and valley" in electrical measurements, which was surprising because they had expected a regular upwards slope. To explain these results, the team turned to nanoscale electronic device expert Suman Datta, who suggested they were seeing a 2-D version of a "resonant tunneling diode," which is a quantum mechanical device that operates at low power.

"Resonant tunnel diodes are important circuit components," said Datta, a coauthor on the paper and Penn State professor of electrical engineering. "Resonant tunneling diodes with NDR can be used to build high frequency oscillators. What this means is we have built the world's thinnest resonant tunneling diode, and it operates at room temperature!"