Substituting Silicon with TMDs - Improving Computing Devices

The world has now reached a point from where starts the culmination of the silicon age. Since computer chips have attained the physical limits of miniaturization and high-power-requiring processors increase energy costs, researchers are trying to find a new class of exotic materials that could promote an innovative generation of computing devices.

Extreme conditions are used to protect and preserve the TMDs during the experiments. As shown here, all samples are stored and manipulated in a vacuum that is close to the conditions in space. (Image credit: Kyle Mittan/UANews)

In contrast to prevalent silicon-based electronics, which discharge a major portion of the consumed energy in the form of waste heat, the future wholly concerns low-power computing. Referred to as spintronics, this technology is dependent on a quantum physical characteristic of electrons, namely, up or down spin, for processing and storing information, instead of transferring them using electricity; similar to traditional computing technology.

In the endeavor to make spintronic devices real, researchers from the University of Arizona have been analyzing an exotic class of materials called transition metal dichalcogenides (TMDs). TMDs have exotic characteristics, enabling their use in innovative ways for processing and storing information and can offer the foundation of next-generation photovoltaics and transistors, and prospectively even open the door for quantum computing.

According to Calley Eads, a fifth-year doctoral student in the UA’s Department of Chemistry and Biochemistry who investigates specific characteristics of these innovative materials, for instance, prevalent silicon-based solar cells in real time, transform only around 25% of sunlight into electricity; hence efficiency is a problem. “There could be a huge improvement there to harvest energy, and these materials could potentially do this,” she stated.

However, there is a drawback - the majority of the TMDs exhibit exotic properties only if they are in the form of very large sheets, just one to three atoms thin. Atomic layers such as these can be difficult to produce even in the laboratory, apart from industrial mass production.

Oliver Monti, a Professor in the department and Eads’ adviser, stated that several attempts are ongoing to develop atomically thin materials for low-power electronics, quantum communication, and solar cells. Analyzing a TMD comprising interspersed layers of sulfur and tin, his group recently found out a probable shortcut, reported in the Nature Communications journal.

We show that for some of these properties, you don’t need to go to the atomically thin sheets. You can go to the much more readily accessible crystalline form that’s available off the shelf. Some of the properties are saved and survive.

Oliver Monti

Understanding Electron Movement

This certainly can drastically simplify the design of the device.

These materials are so unusual that we keep discovering more and more about them, and they are revealing some incredible features that we think we can use, but how do we know for sure? One way to know is by understanding how electrons move around in these materials so we can develop new ways of manipulating them—for example, with light instead of electrical current as conventional computers do.

Oliver Monti

In order to perform this study, the researchers had to overcome a difficulty that could not be solved earlier — discover a method to “watch” individual electrons when they flow through the crystals.

We built what is essentially a clock that can time moving electrons like a stopwatch,” stated Monti. “This allowed us to make the first direct observations of electrons move in crystals in real time. Until now, that had only been done indirectly, using theoretical models.”

The study is a significant progress toward tapping the exotic properties that make TMDs interesting materials for future processing technology since that mandates an in-depth knowledge of the way electrons behave and travel around in them.

Monti’s “stopwatch” enables tracking of moving electrons at attosecond resolution, where one attosecond is one billionth of a billionth of one second. While tracking electrons in the crystals, the researchers found out one more thing — the charge flow relies on direction, a finding that is against the laws of physics.

In cooperation with Mahesh Neupane, a computational physicist at Army Research Laboratories, and Dennis Nordlund, an X-ray spectroscopy expert at Stanford University’s SLAC National Accelerator Laboratory, Monti’s group used a high-intensity, tunable X-ray source to stimulate individual electrons in their analysis samples and raise them to ever higher energy levels.

When an electron is excited in that way, it’s the equivalent of a car that is being pushed from going 10 miles per hour to thousands of miles per hour,” explained Monti. “It wants to get rid of that enormous energy and fall back down to its original energy level. That process is extremely short, and when that happens, it gives off a specific signature that we can pick up with our instruments.”

The team could achieve this in a manner that enabled them to differentiate whether the stimulated electrons remained inside the same layer of the material, or moved on to adjoining layers in the crystal.

We saw that electrons excited in this way scattered within the same layer and did so extremely fast, on the order of a few hundred attoseconds,” stated Monti.

However, electrons that moved on to adjoining layers took over 10 times longer to get back to their ground energy state. The difference enabled the team to differentiate between the two populations.

I was very excited to find that directional mechanism of charge distribution occurring within a layer, as opposed to across layers,” stated Eads, the lead author of the paper. “That had never been observed before.”

Closer to Mass Manufacturing

According to Monti, although the X-ray “clock” adopted to trace the electrons is not part of the visualized applications, it is a mechanism to analyze the behavior of electrons within them, an obligatory initial step in nearing the achievement of the technology with the desired characteristics that can be mass-manufactured.

One example of the unusual behavior we see in these materials is that an electron going to the right is not the same as an electron going to the left,” he stated. “That shouldn’t happen—according to physics of standard materials, going to the left or the right is the exact same thing. However, for these materials that is not true.”

This directionality is an instance of that which renders TMDs fascinating for researchers as it can be used for encoding information.

Moving to the right could be encoded as ‘one’ and going to the left as ‘zero’,” stated Monti. “So if I can generate electrons that neatly go to the right, I’ve written a bunch of ones, and if I can generate electrons that neatly go to the left, I have generated a bunch of zeroes.”

Rather than applying an electrical current, scientists can regulate electrons in this manner by using light, for example, a laser, to optically read, write, and process information. Moreover, maybe someday it can even be feasible to optically entangle information, opening the door to quantum computing.

Every year, more and more discoveries are occurring in these materials,” stated Eads. “They are exploding in terms of what kinds of electronic properties you can observe in them. There is a whole spectrum of ways in which they can function, from superconducting, semiconducting to insulating, and possibly more.”

Monti stated that the study reported here is only one method of investigating the unanticipated, exotic characteristics of layered TMD crystals.

If you did this experiment in silicon, you wouldn’t see any of this,” he stated. “Silicon will always behave like a three-dimensional crystal, no matter what you do. It’s all about the layering.”

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