Optical Tweezers for Photonic Nanosoldering

Focused laser light generates an optical tractor beam, which can manipulate and orient semiconductor nanorods (red) with metal tips (blue) in an organic solvent solution. The energy from the laser superheats the metallic tip of the trapped nanorod, allowing the aligned nanorods to be welded together end-to-end in a solution-based nanosoldering process. (Source: V. Holmberg, M. Crane, E. Pandres, P. Pauzauskie)

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Standard tools, even minia­turized, are too bulky and too corrosive to repro­ducibly manufacture components at the nanoscale. Therefore, researchers at the University of Washington have developed a method that could make reproducible manu­facturing at the nanoscale possible. The team adapted optical tweezers to operate in a water-free liquid environment of carbon-rich organic solvents, thereby enabling new potential appli­cations. 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 Peter Pauzauskie, a UW associate professor of materials science and engineering. “There are no chamber surfaces involved in the manu­facturing process, which minimizes 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.” “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 Vincent Holmberg, a UW assistant professor of chemical engi­neering. “Organic solvents also help us to superheat the material we’re working with, allowing us to control material trans­formations and drive chemistry.”

To demons­trate the potential of this approach, the researchers used the optical tweezers to build a novel nanowire hetero­structure, which is a nanowire consisting of distinct sections comprised of different materials. The starting materials for the nanowire hetero­structure were shorter nanorods of crystalline germanium, each just a few hundred nanometers long and tens of nano­meters in diameter. Each is capped with a metallic bismuth nanocrystal. The researchers 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,” said Holmberg. Nanowires that contain junctions between materials such as the germanium-bismuth junctions may even­tually be a route to creating topological qubits for appli­cations in quantum computing. The tractor beam is actually a highly focused laser that creates a type of optical trap. To date, optical traps have been used almost exclu­sively in water- or vacuum-based environ­ments. Pauzauskie’s and Holmberg’s 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 of forces, and we were lucky 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 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. (Source: U. Washington)

Reference: M. J. Crane et al.: Optically oriented attachment of nanoscale metal-semiconductor heterostructures in organic solvents via photonic nanosoldering, Nat. Commun. 10, 4942 (2019); DOI: 10.1038/s41467-019-12827-w

Link: Molecular Engineering & Sciences Institute, University of Washington, Seattle, USA

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