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Power/Performance Bits: Dec. 7

Logic-in-memory with MoS2; graphene memristors; thermoelectric polymer film.

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Logic-in-memory with MoS2
Engineers at École Polytechnique Fédérale de Lausanne (EPFL) built a logic-in-memory device using molybdenum disulfide (MoS2) as the channel material. MoS2 is a three-atom-thick 2D material and excellent semiconductor.

The new chip is based on floating-gate field-effect transistors (FGFETs) that can hold electric charges for long periods. MoS2 is particularly sensitive to charges stored in FGFETs, allowing the researchers to develop circuits that work as both memory storage units and programmable transistors.


A computer chip that combines two functions – logic operations and data storage. (Source: © 2020 EPFL / LANES)

“This ability for circuits to perform two functions is similar to how the human brain works, where neurons are involved in both storing memories and conducting mental calculations,” said Andras Kis, the head of EPFL’s Laboratory of Nanoscale Electronics and Structures (LANES). “Our circuit design has several advantages. It can reduce the energy loss associated with transferring data between memory units and processors, cut the amount of time needed for computing operations and shrink the amount of space required. That opens the door to devices that are smaller, more powerful and more energy efficient.”

The team demonstrated the concept with a programmable NOR gate. They said that the design can be simply extended to implement more complex programmable logic and a functionally complete set of operations.

Graphene memristors
Researchers at Pennsylvania State University are using graphene memristors for neuromorphic computing.

“We are creating artificial neural networks, which seek to emulate the energy and area efficiencies of the brain,” said Thomas Schranghamer, a doctoral student at Penn State. “The brain is so compact it can fit on top of your shoulders, whereas a modern supercomputer takes up a space the size of two or three tennis courts.”

“We have powerful computers, no doubt about that, the problem is you have to store the memory in one place and do the computing somewhere else,” added Saptarshi Das, assistant professor of engineering science and mechanics at Penn State.

Inspired by how synapses connecting neurons in the brain can be reconfigured, the artificial neural networks the team is building can be reconfigured by applying a brief electric field to a sheet of graphene. In this work they show at least 16 possible memory states, as opposed to the two in most oxide-based memristors.

“What we have shown is that we can control a large number of memory states with precision using simple graphene field effect transistors,” said Das.

The team thinks that ramping up this technology to a commercial scale is feasible and believe the work will be of interest to companies pursuing neuromorphic computing.

Thermoelectric polymer film
Researchers at King Abdullah University of Science and Technology (KAUST) are working to improve the thermoelectric properties of polymer thin films.

The thermoelectric effect, where electron migration from hot to cool areas generates a current, is typically taken advantage of using semiconductors with rigid ceramic structures. But using flexible polymers, instead, could improve energy harvesting capabilities of wearable devices.

The team focused on a conducting polymer containing a blend of poly(3,4-ethylenedioxythiophene) and polystyrenesulfonate (PEDOT:PSS) chains. PEDOT:PSS  is relatively inexpensive and easy to process for applications, including inkjet printing. It also happens to be one of the top-performing thermoelectric polymers thanks to its ability to take in dopants.

Typically, thermoelectric PEDOT:PSS thin films are often exposed to dopants in the form of strong acids. This process washes away loose PSS chains to improve polymer crystallinity and leaves behind particles that oxidize PEDOT chains to boost electrical conductivity.

“We use nitric acid because it’s one of the best dopants for PEDOT,” said Diego Rosas-Villalva, a researcher at KAUST. “However, it evaporates rather easily, and this decreases the performance of the thermoelectric over time.”

After the doping step is completed, the PEDOT:PSS film has to undergo a reverse procedure to neutralize or “dedope” some conductive particles to improve thermoelectric power generation.

The team found that PEDOT:PSS films dedoped with polyethylenimine retained twice as much thermoelectric power after one week compared with untreated specimens. The treatment encapsulated the films to prevent nitric acid escape. Additionally, the coating modified the electronic properties of the thermoelectric polymer to make it easier to harvest energy from sources, including body heat.

“We were not expecting that this polymer would improve the lifetime of the device, especially because it’s such a thin film–less than 5 nanometers,” says Villalva. “It’s been incorporated into other organic electronics before, but barely explored for thermoelectrics.”



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