'Tractor beam' manipulates and fabricates at nanoscale

The researchers at the University of Washington have adapted optical traps or optical tweezers, which are normally used by biologists, to operate in a water-free liquid environment of carbon-rich organic solvents.

In a paper published in Nature Communications, the team said the optical tweezers act as a light-based ‘tractor beam’ that can assemble nanoscale semiconductor materials precisely into larger structures.

"This is a new approach to nanoscale manufacturing," said co-senior author Peter Pauzauskie, a UW associate professor of materials science and engineering, faculty member at the Molecular Engineering & Sciences Institute and the Institute for Nano-engineered Systems, and a senior scientist at the Pacific Northwest National Laboratory. "There are no chamber surfaces involved in the manufacturing process, which minimises the formation of strain or other defects. All of the components are suspended in solution, and we can control the size and shape of the nanostructure as it is assembled piece by piece."

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"Using this technique in an organic solvent allows us to work with components that would otherwise degrade or corrode on contact with water or air," said co-senior author Vincent Holmberg, a UW assistant professor of chemical engineering and faculty member in the Clean Energy Institute and the Molecular Engineering & Sciences Institute. "Organic solvents also help us to superheat the material we're working with, allowing us to control material transformations and drive chemistry."

To demonstrate this approach, the researchers used the optical tweezers to build a novel nanowire heterostructure. The starting materials for the nanowire heterostructure were shorter nanorods of crystalline germanium that were capped with a metallic bismuth nanocrystal.

According to UW, the team then used the light-based tractor beam to grab one of the germanium nanorods. Energy from the beam also superheats the nanorod, melting the bismuth cap. They then guide a second nanorod into the tractor beam and - thanks to the molten bismuth cap at the end - solder them end-to-end. The researchers could then repeat the process until they had assembled a patterned nanowire heterostructure with repeating semiconductor-metal junctions that was five-to-ten times longer than the individual building blocks.

"We've taken to calling this optically oriented assembly process 'photonic nanosoldering' - essentially soldering two components together at the nanoscale using light," Holmberg said in a statement.

Nanowires that contain junctions between materials - such as the germanium-bismuth junctions synthesised by the UW team - could eventually be a route to creating topological qubits for applications in quantum computing.

The tractor beam is a highly focused laser that creates a type of optical trap, a method pioneered by Arthur Ashkin in the 1970s. To date, optical traps have been used extensively in water- or vacuum-based environments.

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

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

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

"We hope that this demonstration results in researchers using optical trapping for the manipulation and assembly of a wider set of nanoscale materials, irrespective of whether or not those materials happen to be compatible with water," said Holmberg.