2-D Materials Enable Swarms of Floating Microbots

Electronic devices made from 2-D materials grafted onto microparticles create floating robots

A schematic diagram of a microscopic chemical detection machine depicting a micrometer-sized polymer particle coated with a nanoelectronic circuit.
Illustration: Michael Strano
This diagram shows a microscopic device built by MIT researchers to detect chemicals. It consists of a very small polymer particle with a tiny circuit attached to it.
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Often when science fiction has envisioned nanotechnology, it takes the form of some miniature devices floating around in a swarm performing either a nefarious or miraculous act. Now, this vision is no longer relegated to thinly plotted sci-fi.

Researchers at the Massachusetts Institute of Technology (MIT) have devised a way to graft nanoscale electronic devices onto floating microscale particles to monitor everything from gases in a range of environments to the inner workings of the human digestive system.

Last week at the American Chemical Society National Meeting & Exposition, Michael Strano, a chemical engineering professor at MIT, and Volodymyr Koman, a research fellow in Strano's group at MIT, presented work in which they were able to exploit Van der Waals forces (forces that create a bond between atoms and molecules that are close together) to get electronic devices made from two-dimensional (2D) materials to stick to floating microparticles.

Strano and Koman fabricated three discrete electronic devices from the 2D materials molybdenum disulfide and tungsten diselenide. Equipped with these atomically thin materials, which belong to the family of materials known as transition metal dichalcogenides, the researchers fashioned three devices: a power source capable of converting light into an electrical current; a sensor capable of detecting molecules; and a memory device from which the data that sensor had collected can be retrieved.

For the power source, the researchers combined the molybdenum disulfide and the tungsten diselenide to create a p-n heterojunction that acts as a photodiode. The ubiquitous p-n junction forms the backbone of devices such as solar cells, light-emitting diodes, photodetectors, and lasers.

In explaining how the device converts light into an electrical charge, Koman says that molybdenum disulfide flakes are oxidized in the process of preparation. “A thin oxide layer of the material has the ability to store charge,” said Koman. “When a threshold voltage is applied, molybdenum disulfide flakes trap charges and change its resistance, switching to a different state.”

The sensor device plays right into the strength of 2D materials, which because of their atomic thinness are highly sensitive to changes in their electrical resistance. In this case, the researchers used a single layer of molybdenum disulfide to make a chemristor in which the material changes its electrical resistance to the presence of molecules.

The last electronic element—the memory device—has not always proven to be as easy to get out of 2D materials. However, earlier this year, researchers at the University of Texas, Austin found a way to coax a memory device by sandwiching an atom-thick layer of molybdenum disulfide between two electrodes and found the resulting device displayed memristance. The basic structure of the device the MIT team came up with, in which an atomic layer of molybdennum disulfide is sandwiched between two electrodes, one made from silver and the other gold, resembles this architecture. In this case, the memristive device was modeled after research published in Nature Materials back in 2015, according to Koman.

All three of these devices are discrete but are integrated into one chip, according to Koman. “Modularity is what we strive for so that we are able to interchange, add, and subtract individual components,” he said.

Once the electronic devices were ready to go, the team needed to find the perfect microparticle on which to attach their 2D electronic devices. They settled on the micrometer-sized particle known as SU-8. The key feature of this particle is that it is a colloidal particle, meaning it floats in a suspension.

The researchers found that they could propel the microparticles equipped with their nanoelectronics in an aerosol form, with the particles traveling as far as a meter. In physical experiments, the researchers propelled their microbots into a simulated gas pipeline to detect carbon particulates or volatile organic compounds.

In order to collect the microbots after they had travelled down the gas pipe, the researchers attached small reflectors to them so the microbots could be seen by their reflection of light. Once collected, the microparticles have metallic connections that make it possible to download their memory.

While the simulated gas pipeline was the first test, the researchers are also looking at the potential of using these devices as monitors for the human digestive system.

The aim of future work will be to expand the arsenal of electronic devices that can be integrated on-board the chip, according to Koman.

He added: “We will apply these various machines to different applications, such as the human digestive system, large-area monitoring, extended pipeline monitoring, and geological exploration.”

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