April 20, 2024

Hard-to-move quasiparticles slide along the edges of pyramids

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Colored atomic force microscopy image of a silicon dioxide pyramid with a single layer of tungsten diselenide on it. The green line is a graph of the exciton distribution and the red arrow shows their path from the base of the pyramid. The colors of the surface and the pyramid indicate the height at that location. Credit: Excitonics and Photonics Laboratory and Quantum Science Theory Laboratory, University of

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Colored atomic force microscopy image of a silicon dioxide pyramid with a single layer of tungsten diselenide on it. The green line is a graph of the exciton distribution and the red arrow shows their path from the base of the pyramid. The colors of the surface and the pyramid indicate the height at that location. Credit: Excitonics and Photonics Laboratory and Quantum Science Theory Laboratory, University of

A new type of “wire” for moving excitons, developed at the University of Michigan, could help enable a new class of devices, perhaps including room-temperature quantum computers.

What’s more, the team observed a dramatic violation of the Einstein relation, used to describe how particles propagate in space, and took advantage of it to move excitons into much smaller packets than was previously possible.

“Nature uses excitons in photosynthesis. We use excitons in OLED displays and in some LEDs and solar cells,” said Parag Deotare, co-author of the study in ACS Nano supervising experimental work and associate professor of electrical and computer engineering. The study is titled Enhanced Exciton Drift Transport Using Suppressed Diffusion in One-Dimensional Guides.

“The ability to move excitons wherever we want will help us improve the efficiency of devices that already use excitons and expand excitons into computing.”

An exciton can be thought of as a particle (hence a quasiparticle), but it is actually an electron bound to a positively charged empty space in the material’s lattice (a “hole”). Because an exciton has no net electrical charge, moving excitons are not affected by stray capacitances, an electrical interaction between neighboring components in a device that causes energy losses.

Excitons are also easy to convert to and from light, paving the way for extremely fast and efficient computers that use a combination of optics and excitonics, rather than electronics.

This combination could help enable quantum computing at room temperature, said Mackillo Kira, a co-corresponding author of the study overseeing the theory and a professor of electrical and computer engineering.

Excitons can encode quantum information and can retain it longer than electrons within a semiconductor. But that time is still measured in picoseconds (10-12 seconds) at best, so Kira and others are figuring out how to use femtosecond laser pulses (10-fifteen seconds) to process information.

“Full applications of quantum information remain challenging because the degradation of quantum information is too rapid for ordinary electronics,” he said. “We are currently exploring light wave electronics as a means to boost excitonics with extremely fast processing capabilities.”

However, the lack of net charge also makes excitons very difficult to move. Previously, Deotare had led a study that drove excitons through semiconductors with acoustic waves. Now, a pyramidal structure allows more precise transport for a smaller number of excitons, confined to one dimension like a wire.

It works like this

The team used a laser to create a cloud of excitons in one corner of the base of the pyramid, bouncing electrons from a semiconductor’s valence band into the conduction band, but the negatively charged electrons are still attracted to the charged holes. positively that remain in the pyramid. the valence band. The semiconductor is a single layer of tungsten diselenide semiconductor, just three atoms thick, which covers the pyramid like an elastic cloth. And stretching in the semiconductor changes the energy landscape that the excitons experience.

It seems counterintuitive that excitons climb up the edge of the pyramid and settle at the top when we imagine an energy landscape governed primarily by gravity. But instead, the picture is governed by the distance between the valence and conduction bands of the semiconductor. The energy gap between the two, also known as the semiconductor’s bandgap, narrows when the semiconductor stretches. The excitons migrate to the lowest energy state, funneled toward the edge of the pyramid, where they then rise to their peak.

An equation written by Einstein is usually good for describing how a group of particles diffuses outward and moves. However, the semiconductor was imperfect and those defects acted as traps that trapped some of the excitons as they tried to pass through. Because defects on the back side of the exciton cloud were filled in, that side of the distribution diffused outward as predicted. The avant-garde, however, did not go that far. Einstein’s relationship was off by more than a factor of 10.

“We are not saying that Einstein was wrong, but we have shown that in complicated cases like this, we should not use his relationship to predict exciton mobility from diffusion,” said Matthias Florian, co-first author of the study and a researcher in electrical and computer engineering, who works with Kira.

To measure both directly, the team needed to detect single photons, emitted when bound electrons and holes spontaneously recombined. Using time-of-flight measurements, they also discovered where the photons came from with enough precision to measure the distribution of excitons within the cloud.

The pyramidal structure was built at Lurie’s nanofabrication facility. The team has filed for patent protection with the help of UM Innovation Partnerships and is seeking partners to bring the technology to market.

More information:
Zidong Li et al, Enhanced Exciton Drift Transport by Suppressed Diffusion in One-Dimensional Guides, ACS Nano (2023). DOI: 10.1021/acsnano.3c04870

Magazine information:
ACS Nano

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