Excitons go to the edge on semiconducting pyramid
A new kind of ‘wire’ for moving excitons, developed by researchers at the University of Michigan, could lead to a new class of devices, perhaps including room temperature quantum computers. What’s more, the researchers also observed a dramatic violation of Einstein’s relation, which describes how particles spread out in space, and leveraged it to move excitons in much smaller packages than previously possible.
“Nature uses excitons in photosynthesis. We use excitons in OLED displays and some LEDs and solar cells,” said Parag Deotare, an associate professor of electrical and computer engineering and co-corresponding author of a paper on this work in ACS Nano. “The ability to move excitons where we want will help us improve the efficiency of devices that already use excitons and expand excitonics into computing.”
While an exciton can be thought of as a particle, it’s really an electron linked with a positively charged empty space in the lattice of a material (a ‘hole’) and so is termed a quasiparticle. Because an exciton has no net electrical charge, moving excitons are not affected by parasitic capacitances, an electrical interaction between neighboring components in a device that causes energy losses. Excitons are also easy to convert into and from light, so they open the way for extremely fast and efficient computers that use a combination of optics and excitonics, rather than electronics.
According to Mackillo Kira, a professor of electrical and computer engineering and co-corresponding author of the paper, this combination could find use in room-temperature quantum computing. Excitons can encode quantum information, and they can hang onto it longer than electrons can inside 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 (10-15 seconds) to process information.
“Full quantum-information applications remain challenging because degradation of quantum information is too fast for ordinary electronics,” said Kira. “We are currently exploring light-wave electronics as a means to supercharge 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 used acoustic waves to push excitons through semiconductors. In this study, the researchers showed that a pyramid structure covered in a semiconducting material can achieve more precise transport for smaller numbers of excitons, by confining them to one dimension like a wire.
They used a laser to create a cloud of excitons at a corner of the pyramid’s base, bouncing electrons out of the valence band of the semiconducting material into the conduction band. But the negatively charged electrons are still attracted to the positively charged holes left behind in the valence band. The semiconducting material is a single layer of tungsten diselenide, just three atoms thick, draped over the pyramid like a stretchy cloth. And the stretch in the semiconductor changes the energy landscape that the excitons experience.
If imagining an energy landscape chiefly governed by gravity, it seems impossible for the excitons to ride up the pyramid’s edge and settle at its peak. But the energy landscape is actually governed by how far apart the valence and conduction bands of the semiconductor are. The energy gap between the two, known as the semiconductor’s band gap, shrinks where the semiconductor is stretched. Because the excitons migrate to the lowest energy state, they are funneled onto the pyramid’s edge, where they then rise to its peak.
Usually, an equation penned by Einstein is good at describing how a bunch of particles diffuses outward and drifts. But the semiconductor contains defects, which act as traps that nab some of the excitons as they drift by. Because the defects at the trailing side of the exciton cloud were filled in, that side of the distribution diffused outward as predicted. The leading edge, however, did not extend so far. Einstein’s relation was off by more than a factor of 10.
“We’re not saying Einstein was wrong, but we have shown that in complicated cases like this, we shouldn’t be using his relation to predict the mobility of excitons from the diffusion,” said Matthias Florian, a research investigator in electrical and computer engineering, working under Kira, and co-first-author of the paper.
This story is adapted from material from the University of Michigan, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.
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