Light-Based "Tractor Beam" Assembles Materials At The Nanoscale

Light-Based Tractor Beam Assembles Materials At The Nanoscale
Modern construction is an effort of precision. Manufacturers must use components manufactured to meet specific standards - such as beams of a desired composition or rivets of a specific size. The building industry relies on manufacturers to create these components reliably and reproducibly to build secure bridges and healthy skyscrapers.

Now imagine a construction on a smaller scale - less than 1 / 100th of the thickness of a piece of paper. It's the nanoscale. It is at this scale that scientists are working to develop potentially revolutionary technologies in areas such as quantum computing. It's also a scale where traditional manufacturing methods simply will not work. Our standard tools, even miniaturized, are too bulky and too corrosive to make nano-scale reproducible components.

Researchers at the University of Washington have developed a method for reproducible nanoscale fabrication. The team adapted light-based technology widely used in biology, known as an optical trap or optical tweezer, to operate in a water-free liquid environment containing carbon-rich organic solvents, thus enabling potential new applications.

As the team reports in an article published Oct. 30 in Nature Communications, optical tweezers act as a light-based "pulling beam" capable of assembling nanoscale semiconductor materials into larger structures. Unlike sci-fi tractor bundles, which catch spaceships, the team uses optical tweezers to trap materials nearly a billion times less than one meter.

"This is a new approach to manufacturing at the nanoscale," said Peter Pauzauskie, associate author and associate professor of materials science and engineering, faculty member of the Molecular Engineering & Sciences Institute and from the Institute for Nano-engineering systems. scientist at the Pacific Northwest National Laboratory. "The manufacturing process does not involve any chamber surface that minimizes the formation of deformations or other defects." All components are suspended in solution and we can control the size and shape of the nanostructure when assembled piece by piece . "

"The use of this technique in an organic solvent allows us to work with components that would deteriorate or corrode on contact with water or air," said Vincent Holmberg, co-lead author, assistant professor of chemical engineering at the University of Washington, professor at Clean Energy. Institute and Institute of Engineering and Molecular Sciences. "Organic solvents also help us to overheat the material we work with, allowing us to control material transformations and stimulate chemistry."

To demonstrate the potential of this approach, researchers used optical precelles to construct a new nanowire heterostructure, which is a nanowire consisting of discrete sections made of different materials. The starting materials of the heterostructure of nanowires were shorter "nanorodes" in crystalline germanium, each one hundred nanometers long and several tens of nanometers in diameter, about 5,000 times thinner than a hair human. Each is capped with a metallic bismuth nanocrystal.

The researchers then used the light-based "tractor beam" to capture one of the germanium nanorods. The beam energy also overheats the nanodod, melting the bismuth plug. They then guide a second nanoref in the "tractor beam" and weld them end to end, thanks to the molten bismuth cap. The researchers could then repeat the process until they assembled a patterned heterostructure of nanowires with repeating semiconductor metal junctions five to ten times longer than the different building blocks.

"We started to call this optically-oriented" photon nanosocket "assembly process, which is to weld two components together at the nanoscale using light," said Holmberg.

Nanowires that contain junctions between materials, such as the Germanium-bismuth junctions synthesized by the UW team, could possibly be a means of creating topological qubits for applications in quantum computing.

The tractor beam is actually a highly focused laser that creates a type of optical trap, a Nobel Prize winning method developed by Arthur Ashkin in the 1970s. To date, optical traps have been used almost exclusively in water-based or vacuum-based environments. The Pauzauskie and Holmberg teams adapted optical trapping to work in the more volatile environment of organic solvents.

"Generating a stable optical trap in any type of environment is a delicate balancing act and we were fortunate to have two very talented graduate students working together on this project," said Holmberg.

The photons that make up the laser beam generate a force on the objects in the immediate vicinity of the optical trap. Researchers can adjust the properties of the laser so that the force generated can trap or release an object, be it a germanium nanorode or a longer nanowire.

"This is the type of precision needed for reliable and reproducible nanofabrication methods without chaotic interaction with other surfaces or materials that can introduce defects or deformations into nanomaterials," said Pauzauskie.

The researchers believe that their nanosoldering approach could allow the additive manufacturing of nanoscale structures with different sets of materials for other applications.

"We hope this demonstration will inspire researchers to use optical trapping for the manipulation and assembly of a broader set of nanoscale materials, whether or not these materials are compatible with water," he said. Mr. Holmberg.

The lead co-authors of the paper are Elena Pandres, a UW graduate student in chemical engineering, and Matthew Crane, a UW PhD and postdoctoral researcher in UW's Department of Chemistry. Co-author is E. James Davis, Professor Emeritus of Chemical Engineering at the University of Washington. The research was funded by the National Science Foundation, the UW Molecular Engineering Materials Center, the UW Molecular Engineering & Sciences Institute, the UW Institute for Nano-engineering Systems, the UW Clean Energy Institute, Washington State, Washington Research Foundation and the Air Force's Scientific Research Office.

Journal Reference:
Matthew J. Crane, Elena P. Pandres, E. James Davis, Vincent C. Holmberg, Peter J. Pauzauskie. Optically oriented attachment of nanoscale metal-semiconductor heterostructures in organic solvents via photonic nanosoldering. Nature Communications, 2019; 10 (1) DOI: 10.1038/s41467-019-12827-w

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