Power/Performance Bits: March 31

Tellurium transistors; combinatorial optimization; safer perovskite solar.


Tellurium transistors
Researchers from Purdue University, Washington University in St Louis, University of Texas at Dallas, and Michigan Technological University propose the rare earth element tellurium as a potential material for ultra-small transistors.

Encapsulated in a nanotube made of boron nitride, tellurium helps build a field-effect transistor with a diameter of two nanometers.

“This tellurium material is really unique. It builds a functional transistor with the potential to be the smallest in the world,” said Peide Ye, professor of electrical and computer engineering at Purdue.

Previously, the team investigated tellurene, a 2D material derived from tellurium. They found that transistors made with this material could carry significantly more electrical current, making them more efficient.

The researchers started out by growing one-dimensional chains of tellurium atoms and synthesizing bare tellurium nanowires for comparison. Another team simulated how tellurium might behave.

Under TEM imaging, the atoms in these one-dimensional chains wiggle. The wiggles were the atoms strongly bonding to each other in pairs to form DNA-like helical chains, then stacking through van der Waals interactions to form a tellurium crystal.

“Silicon atoms look straight, but these tellurium atoms are like a snake. This is a very original kind of structure,” Ye said.

These silver, wiggling lines are strings of atoms in tellurium behaving like DNA. Researchers have not seen this behavior in any other material. (University of Texas at Dallas/Qingxiao Wang and Moon Kim)

These van der Waals interactions would set apart tellurium as a more effective material for single atomic chains or one-dimensional nanowires compared with others because it’s easier to fit into a nanotube, Ye noted.

Since the opening of a nanotube can’t be any smaller than the size of an atom, tellurium helices of atoms could achieve smaller nanowires and, therefore, smaller transistors.

The researchers built a transistor with a tellurium nanowire encapsulated in a boron nitride nanotube, provided by physics professor Yoke Khin Yap’s lab at the Michigan Technological University. A high-quality boron nitride nanotube effectively insulates tellurium, making it possible to build a transistor.

“This research reveals more about a promising material that could achieve faster computing with very low power consumption using these tiny transistors,” said Joe Qiu, program manager for the U.S. Army Research Office, which funded the work.

Combinatorial optimization processor
Researchers at Tokyo Institute of Technology, Hitachi Hokkaido University Laboratory, and the University of Tokyo designed a processor architecture for solving combinatorial optimization problems.

The ‘traveling salesman’ is probably the most famous combinatorial optimization problem – how does one salesperson move between multiple cities in the shortest possible distance. But the same techniques are applicable to any area where the number of variables is high, like financial trading, machine learning, and drug discovery.

The researchers processer architecture is designed to specifically solve combinatorial optimization problems expressed in the form of an Ising model. The Ising model was originally used to describe the magnetic states of atoms (spins) in magnetic materials. However, this model can be used as an abstraction to solve combinatorial optimization problems because the evolution of the spins, which tends to reach the so-called lowest-energy state, mirrors how an optimization algorithm searches for the best solution. In fact, the state of the spins in the lowest-energy state can be directly mapped to the solution of a combinatorial optimization problem.

The proposed processor architecture, called STATICA, is fundamentally different from existing processors that calculate Ising models, called annealers, according to the researchers. One limitation of most reported annealers is that they only consider spin interactions between neighboring particles. This allows for faster calculation, but limits their possible applications. In contrast, STATICA is fully connected and all spin-to-spin interactions are considered. While STATICA’s processing speed is lower than those of similar annealers, its calculation scheme uses parallel updating.

According to the researchers, in most annealers, the evolution of spins (updating) is calculated iteratively. This process is inherently serial, meaning that spin switchings are calculated one by one because the switching of one spin affects all the rest in the same iteration. In STATICA, the updating process is carried out in parallel using what is known as stochastic cell automata. Instead of calculating spin states using the spins themselves, STATICA creates replicas of the spins and spin-to-replica interactions are used, allowing for parallel calculation. This saves a tremendous amount of time due to the reduced number of steps needed.

“We have proven that conventional approaches and STATICA derive the same solution under certain conditions, but STATICA does so in N times fewer steps, where N is the number of spins in the model,” remarked Masato Motomura, visiting professor at Hokkaido University. The research team also implemented an approach called delta-driven spin updating. Because only spins that changed in the previous iteration are important when calculating the following one, a selector circuit is used to only involve spins that flipped in each iteration.

The researchers argue that STATICA offers reduced power consumption, higher processing speed, and better accuracy than other annealers.

Safer perovskite solar
Researchers at Northern Illinois University and the National Renewable Energy Laboratory found a way to make perovskite solar cells less damaging to the environment and human health.

Perovskite solar technology is promising for its low cost and high efficiency – but many of the top performing contain water-soluble lead. Lead is a particularly damaging toxin to human development, and contamination of groundwater with lead is a major concern for large-scale deployment of perovskite-based solar panels.

The researchers developed a technique to sequester the lead used to make perovskite solar cells and minimize potential toxic leakage by applying lead-absorbing films to the front and back of the solar cell.

“The lead toxicity issue has been one of the most vexing, last-mile challenges in the perovskite solar cell field,” said Tao Xu, an NIU professor of chemistry. “We think we have a highly promising remedy to this problem–and it could be a game-changer. In the event of a damaged cell, our device captures the great majority of the lead, preventing it from leaching into groundwater and soils. The films that we use are insoluble in water.”

A transparent lead-absorbing film is applied to a conducting glass on the front of the solar cell. The sequestration film contains strong lead-binding phosphonic acid groups but does not hinder cell capture of light. A less expensive polymer film blended with lead-chelating agents is used on the back metal electrode, which has no need for transparency.

Under conditions of severe solar cell damage in a lab setting, the lead-absorbing films sequestered 96% of lead leakage, the scientists said. Their experiments further indicate the lead-absorbing layers do not negatively impact cell performance or long-term operation stability.

The newly developed “on-device sequestration approach” can be readily incorporated with current perovskite solar cells configurations, according to Xu.

“The materials are off-the-shelf, but they were never used for this purpose,” Xu said. “Light must enter the cell to be absorbed by the perovskite layer, and the front-side film actually acts as an anti-reflection agent, improving transparency just a bit.”

Tests for lead leakage included hammering and shattering the front-side glass of 2.5-x-2.5 cm cells, and scratching the backside of the solar cells with a razor blade, before submerging them into water. The films could absorb the vast majority of the lead in severely damaged cells due to water ingress.

Kai Zhu, senior scientist at NREL, said, “It is worth noting that the demonstrated lead-sequestration approach is also applicable to other perovskite-based technologies such as solid-state lighting, display and sensor applications.”

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