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Two-Dimensional Materials Tackle the Diode

The p-n heterojunction diode, one of the most common building blocks in electronics, can be made from a 2-D material

2 min read
Two-Dimensional Materials Tackle the Diode
Image: Northwestern University

While graphene by itself has been generating enormous interest in both the research community and outside of it, what many are still missing is that we are in the midst of a two-dimensional material explosion that goes beyond just graphene.

Molybdenum disulfide (MoS2) is beginning to take center stage right behind graphene in the cast of 2-D materials that includes silicene (a single layer of silicon) and boron nitride. Nearly three years ago, MoS2 was revealed as a possible 2-D replacement for three-dimensional silicon in transistors.

Even though MoS2 had an advantage over graphene in that it has an inherent band gap, it was really first imagined as a complement to graphene in applications such as optoelectronics and energy harvesting, where thin, transparent semiconductors are required.

Now researchers at Northwestern have gone back to MoS2's complementary role and combined it with carbon nanotubes to create p-n heterojunction diode. The p-n junction forms the backbone of devices such as solar cells, light-emitting diodes, photodetectors, and lasers.

“The p-n junction diode is among the most ubiquitous components of modern electronics,” said Mark Hersam, director of the Northwestern University Materials Research Center, in a press release. “By creating this device using atomically thin materials, we not only realize the benefits of conventional diodes but also achieve the ability to electronically tune and customize the device characteristics. We anticipate that this work will enable new types of electronic functionality and could be applied to the growing number of emerging two-dimensional materials.”

In a paper published in the journal Proceedings of the National Academy of Sciences (“Gate-tunable carbon nanotube–MoS2 heterojunction p-n diode”), the Northwestern team used single-walled carbon nanotubes as the p-type semiconductor and the MoS2 as n-type semiconductor.

The researchers discovered that when they stacked the two semiconductors vertically on top of each other they formed a heterojunction that allowed for the tuning of the device’s electrical characteristics with an applied gate bias.

In addition to its tunability, the p-n heterojunction diode is highly light sensitive. The researchers exploited this capability by making an ultrafast photodetector with the diode that displayed an electronically tunable wavelength response.

With 2-D materials already proving capable of making field-effect devices, it is hoped that this latest addition of a p-n junction diode made from one will mark an important step in the next generation of electronics.

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Two Startups Are Bringing Fiber to the Processor

Avicena’s blue microLEDs are the dark horse in a race with Ayar Labs’ laser-based system

5 min read
Diffuse blue light shines from a patterned surface through a ring. A blue cable leads away from it.

Avicena’s microLED chiplets could one day link all the CPUs in a computer cluster together.


If a CPU in Seoul sends a byte of data to a processor in Prague, the information covers most of the distance as light, zipping along with no resistance. But put both those processors on the same motherboard, and they’ll need to communicate over energy-sapping copper, which slow the communication speeds possible within computers. Two Silicon Valley startups, Avicena and Ayar Labs, are doing something about that longstanding limit. If they succeed in their attempts to finally bring optical fiber all the way to the processor, it might not just accelerate computing—it might also remake it.

Both companies are developing fiber-connected chiplets, small chips meant to share a high-bandwidth connection with CPUs and other data-hungry silicon in a shared package. They are each ramping up production in 2023, though it may be a couple of years before we see a computer on the market with either product.

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