Silicon hosts quantum hares in the form of electrons, while the novel Re6Se8Cl2 semiconductor hosts quantum tortoises in the form of acoustic exciton-polarons. Image: Jack Tulyag, Columbia University.
Silicon hosts quantum hares in the form of electrons, while the novel Re6Se8Cl2 semiconductor hosts quantum tortoises in the form of acoustic exciton-polarons. Image: Jack Tulyag, Columbia University.

Semiconductors – most notably, silicon – underpin the computers, cellphones and other electronic devices that power our daily lives. But as ubiquitous as semiconductors have become, they come with limitations. This is because the atomic structure of any material vibrates, creating quantum particles called phonons. Phonons in turn cause the particles that carry energy and information around electronic devices – either electrons or electron-hole pairs called excitons – to scatter in a matter of nanometers and femtoseconds. This causes energy to be lost in the form of heat and places a speed limit on information transfer.

The search is thus on for better options. Now, in a paper in Science, a team of chemists at Columbia University led by Jack Tulyag, a PhD student working with chemistry professor Milan Delor, reports the fastest and most efficient semiconductor yet: a superatomic material made of rhenium, selenium and chlorine (Re6Se8Cl2).

Rather than scattering when they come into contact with phonons, excitons in Re6Se8Cl2 actually bind with phonons to create new quasiparticles called acoustic exciton-polarons. Although polarons are found in many materials, those in Re6Se8Cl2 have a special property: they are capable of ballistic, or scatter-free, flow. This ballistic behavior could lead to faster and more efficient devices.

In experiments run by the team, acoustic exciton-polarons in Re6Se8Cl2 moved fast – twice as fast as electrons in silicon – and crossed several microns of the sample in less than a nanosecond. Given that polarons can last for about 11 nanoseconds, the team thinks the exciton-polarons could cover more than 25µm at a time. And because these quasiparticles are controlled by light rather than an electrical current and gating, processing speeds in theoretical devices have the potential to reach femtoseconds – six orders of magnitude faster than the nanoseconds achievable in current Gigahertz electronics. All at room temperature.

“In terms of energy transport, Re6Se8Cl2 is the best semiconductor that we know of, at least so far,” Delor said.

Re6Se8Cl2 is a superatomic semiconductor created in the lab of collaborator Xavier Roy. Superatoms are clusters of atoms bound together that behave like one big atom, but with different properties to the elements used to build them. Synthesizing superatoms is a specialty of the Roy lab, and they are a main focus of Columbia’s Material Research Science and Engineering Center on Precision Assembled Quantum Materials.

Delor is interested in controlling and manipulating the transport of energy through superatoms and other unique materials developed at Columbia. To do this, his team builds super-resolution imaging tools that can capture particles moving at ultrasmall, ultrafast scales.

When Tulyag first brought Re6Se8Cl2 into the lab, it wasn’t to search for a new and improved semiconductor – it was to test the resolution of the lab’s microscopes with a material that, in principle, shouldn’t have conducted much of anything. “It was the opposite of what we expected,” said Delor. “Instead of the slow movement we expected, we saw the fastest thing we’ve ever seen.”

Tulyag and his peers in the Delor group spent the next two years working to pinpoint why Re6Se8Cl2 showed such remarkable behavior. This included developing an advanced microscope with extreme spatial and temporal resolution that can directly image polarons as they form and move through the material. Theoretical chemist Petra Shih, a PhD student working in Timothy Berkelbach’s group, also developed a quantum mechanical model that provides an explanation for the observations.

The new quasiparticles are fast, but, counterintuitively, they accomplish that speed by pacing themselves – a bit like the story of the tortoise and the hare, Delor explained. What makes silicon a desirable semiconductor is that electrons can move through it very quickly. But like the proverbial hare, they bounce around too much and don’t actually make it very far, very fast in the end.

Excitons in Re6Se8Cl2 are, comparatively, very slow, but it’s precisely because they are so slow that they are able to meet and pair up with equally slow-moving acoustic phonons. The resulting quasiparticles are ‘heavy’ and, like the tortoise, advance slowly but steadily along. Unimpeded by other phonons along the way, acoustic exciton-polarons in Re6Se8Cl2 ultimately move faster than electrons in silicon.

Like many of the emerging quantum materials being explored at Columbia, Re6Se8Cl2 can be peeled into atom-thin sheets, a feature that means they can potentially be combined with other similar two-dimensional (2D) materials in the search for additional unique properties. Re6Se8Cl2, however, is unlikely to ever make its way into a commercial product, as rhenium is one of the rarest elements on Earth, making it extremely expensive.

But with the new theory from the Berkelbach group in hand, along with the advanced imaging technique that Tulyag and the Delor group developed to directly track the formation and movement of polarons in the first place, the team is ready to see if there are other superatomic contenders capable of beating Re6Se8Cl2’s speed record.

“This is the only material that anyone has seen sustained room-temperature ballistic exciton transport in. But we can now start to predict what other materials might be capable of this behavior that we just haven’t considered before,” said Delor. “There is a whole family of superatomic and other 2D semiconductor materials out there with properties favorable for acoustic polaron formation.”

This story is adapted from material from Columbia University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.