Researchers at Duke University and North Carolina State University have demonstrated the first custom semiconductor microparticles that can be steered into various configurations repeatedly while suspended in water.

With an initial six custom particles that predictably interact with one another in the presence of alternating current (AC) electric fields of varying frequencies, the study presents the first steps toward realizing advanced applications such as artificial muscles and reconfigurable computer systems.

The study appears online on May 3 in the journal Nature Communications.

“We’ve engineered and encoded multiple dynamic responses in different microparticles to create a reconfigurable silicon toolbox,” said Ugonna Ohiri, a recently graduated electrical engineering doctoral student from Duke and first author of the paper. “By providing a means of controllably assembling and disassembling these particles, we’re bringing a new tool to the field of active matter.”

While previous researchers have worked to define self-assembling systems, few have worked with semiconductor particles, and none have explored the wide range of custom shapes, sizes and coatings that are available to the micro- and nanofabrication industry. Engineering particles from silicon presents the opportunity to physically realize electronic devices that can self-assemble and disassemble on demand. Customizing their shapes and sizes presents opportunities to explore a wide-ranging design space of new motile behaviors.

“Most previous work performed using self-assembling particles has been done with shapes such as spheres and other off-the-shelf materials,” said Nan Jokerst, the J. A. Jones Professor of Electrical and Computer Engineering at Duke. “Now that we can customize whatever arbitrary shapes, electrical characteristics and patterned coatings we want with silicon, a whole new world is opening up.”

In the study, Jokerst and Ohiri fabricated silicon particles of various shapes, sizes and electrical properties. In collaboration with Orlin Velev, the INVISTA Professor of Chemical and Biomolecular Engineering at NC State, they characterized how these particles responded to different magnitudes and frequencies of electric fields while submerged in water.

Based on these observations, the researchers then fabricated new batches of customized particles that were likely to exhibit the behaviors they were looking for, resulting in six different engineered silicon microparticle compositions that could move through water, synchronize their motions, and reversibly assemble and disassemble on demand.

The thin film particles are 10-micron by 20-micron rectangles that are 3.5 microns thick. They’re fabricated using Silicon-on-Insulator (SOI) technology. Since they can be made using the same fabrication technology that produces integrated circuits, millions of identical particles could be produced at a time.

“The idea is that eventually we’re going to be able to make silicon computational systems that assemble, disassemble and then reassemble in a different format,” said Jokerst. “That’s a long way off in the future, but this work provides a sense of the capabilities that are out there and is the first demonstration of how we might achieve those sorts of devices.”

That is, however, only the tip of the proverbial iceberg. Some of the particles were fabricated with both p-type and n-type regions to create p-n junctions — common electrical components that allow electricity to pass in only one direction. Tiny metal patterns were also placed on the particles’ surfaces to create p-n junction diodes with contacts. In the future, researchers could even engineer particles with patterns using other electrically conductive or insulating materials, complex integrated circuits, or microprocessors on or within the silicon.

“This work is just a small snapshot of the tools we have to control particle dynamics,” said Ohiri. “We haven’t even scratched the surface of all of the behaviors that we can engineer, but we hope that this multidisciplinary study can pioneer future studies to design artificial active materials.”

In even the most fuel-efficient cars, about 60 percent of the total energy of gasoline is lost through heat in the exhaust pipe and radiator. To combat this, researchers are developing new thermoelectic materials that can convert heat into electricity. These semiconducting materials could recirculate electricity back into the vehicle and improve fuel efficiency by up to 5 percent.

The challenge is, current thermoelectric materials for waste heat recovery are very expensive and time consuming to develop. One of the state of the art materials, made from a combination of hafnium and zirconium (elements most commonly used in nuclear reactors), took 15 years from its initial discovery to optimized performance.

Now, researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have developed an algorithm that can discover and optimize these materials in a matter of months, relying on solving quantum mechanical equations, without any experimental input.

“These thermoelectric systems are very complicated,” said Boris Kozinsky, a recently appointed Associate Professor of Computational Materials Science at SEAS and senior author of the paper. “Semiconducting materials need to have very specific properties to work in this system, including high electrical conductivity, high thermopower, and low thermal conductivity, so that all that heat gets converted into electricity. Our goal was to find a new material that satisfies all the important properties for thermoelectric conversion while at the same time being stable and cheap.”

Kozinsky co-authored the research with Georgy Samsonidze, a research engineer at the Robert Bosch Research and Technology Center in Cambridge, MA, where both authors conducted most of the research.

In order to find such a material, the team developed an algorithm that can predict electronic transport properties of a material based only on the chemical elements used in the crystalline crystal. The key was to simplify the computational approach for electron-phonon scattering and to speed it up by about 10,000 times, compared to existing algorithms.

The new method and computational screening results are published in Advanced Energy Materials.

Using the improved algorithm, the researchers screened many possible crystal structures, including structures that had never been synthesized before. From those, Kozinsky and Samsonidze whittled the list down to several interesting candidates. Of those candidates, the researchers did further computational optimization and sent the top performers to the experimental team.

In an earlier effort experimentalists synthesized the top candidates suggested by these computations and found a material that was as efficient and as stable as previous thermoelectric materials but 10 times cheaper. The total time from initial screening to working devices: 15 months.

“We did in 15 months of computation and experimentation what took 15 years for previous materials to be optimized,” said Kozinsky. “What’s really exciting is that we’re probably not fully understanding the extent of the simplification yet. We could potentially make this method even faster and cheaper.”

Kozinsky said he hopes to improve the new methodology and use it to explore electronic transport in a wider class of new exotic materials such as topological insulators.