Power/Performance Bits: Dec. 26

2nm memristors
Researchers at the University of Massachusetts Amherst and Brookhaven National Laboratory built memristor crossbar arrays with a 2nm feature size and a single-layer density up to 4.5 terabits per square inch. The team says the arrays were built with foundry-compatible fabrication technologies.

“This work will lead to high-density memristor arrays with low power consumption for both memory and unconventional computing applications,” said Qiangfei Xia, professor of electrical and computer engineering at Amherst. “The working circuits have been made with technologies that are widely used to build a computer chip.”

The 2-nm memristor crossbar array. (Source: UMass Amherst)

Key to the team’s work was using “nanofins,” metallic nanostructures with very high height-to-width ratio and vastly reduced resistance, as the electrodes.

The memristor crossbar array is comparable to the information density achieved using three-dimensional stacking in state-of-the-art 64-layer and multilevel 3D-NAND flash memory, according to the researchers.

Synapse model
Researchers at the University of Michigan developed a memristor that better models the behaviors of a synapse connecting two neurons.

While memristors are good models for mimicking the way that the connections between neurons strengthen or weaken when signals pass through them, the shape of the channels of conductive material within the memristor previously could not be precisely controlled.

The new device is comprised of 2D molybdenum disulfide layers with lithium ions injected between them.

The team found that if there are enough lithium ions present, the molybdenum sulfide transforms its lattice structure, enabling electrons to run through the film easily as if it were a metal. But in areas with too few lithium ions, the molybdenum sulfide restores its original lattice structure and becomes a semiconductor, and electrical signals have a hard time getting through.

The lithium ions are easy to rearrange within the layer by sliding them with an electric field. This changes the size of the regions that conduct electricity little by little and thereby controls the device’s conductance.

“Because we change the ‘bulk’ properties of the film, the conductance change is much more gradual and much more controllable,” said Wei Lu, U-M professor of electrical and computer engineering.

A schematic of the molybdenum disulfide layers with lithium ions between them. On the right, the simplified inset shows how the molybdenum disulfide changes its atom arrangements in the presence and absence of the lithium atoms, between a metal (1T’ phase) and semiconductor (2H phase), respectively. (Source: Xiaojian Zhu, Nanoelectronics Group, University of Michigan)

Additionally, the layered structure let the team link multiple memristors with shared lithium ions. A single neuron’s dendrite, or its signal-receiving end, may have several synapses connecting it to the signaling arms of other neurons. Lu compared the availability of lithium ions to that of a protein that enables synapses to grow.

If the growth of one synapse releases these proteins, other synapses nearby can also grow. Neuroscientists have argued that this cooperation between synapses helps to rapidly form vivid memories that last for decades and create associative memories. If the protein is scarce, however, one synapse will grow at the expense of the other, paring down the number of connections.

This was the behavior the team showed using the memristor device. In the competition scenario, lithium ions were drained away from one side of the device. The side with the lithium ions increased its conductance, emulating the growth, and the conductance of the device with little lithium was stunted.

In a cooperation scenario, they made a memristor network with four devices that can exchange lithium ions, and then siphoned some lithium ions from one device out to the others. In this case, not only could the lithium donor increase its conductance—the other three devices could too, although their signals weren’t as strong.

The team is currently building networks of memristors like these to explore their potential for neuromorphic computing.

Wearable heating
Researchers at Rutgers University and Oregon State University developed thin, durable heating patches by using intense pulsed-light sintering to fuse silver nanowires with polyester. Capable of being powered by coin batteries, its heating performance is nearly 70% higher than similar patches, say the researchers.

The team estimate that 47% of global energy is used for indoor heating, and 42% of that energy is wasted to heat empty space and objects instead of people, the study notes. Personal heating patches, however, could be sewn into clothing to provide directed heat where the body needs it.

This image shows how to make a personal heating patch from polyester fabric fused with tiny silver wires, using pulses of intense light from a xenon lamp. (Source: Hyun-Jun Hwang and Rajiv Malhotra/Rutgers University-New Brunswick)

“This is important in the built environment, where we waste lots of energy by heating buildings – instead of selectively heating the human body,” said Rajiv Malhotra, an assistant professor in the Department of Mechanical and Aerospace Engineering at Rutgers University-New Brunswick.

Next steps include seeing if this method can be used to create other smart fabrics, including patch-based sensors and circuits. The engineers also want to determine how many patches would be needed and where they should be placed on people to keep them comfortable while reducing indoor energy consumption.