A team of mechanical engineers at Georgia Tech say that they have demonstrated a new process to rapidly fabricate complex three-dimensional nanostructures from a variety of materials, including metals. The new technique, they say, uses nanoelectrospray to provide a continuous supply of liquid precursor, which can include metal ions that are converted to high-purity metal using a focused electron beam.
The engineers explain that the new process generates structures that would be impossible to make using gas-phase focused electron beam-induced deposition (FEBID) techniques, and allows fabrication at rates up to five orders of magnitude faster than the gas-phase technique. And, since it uses standard liquid solvents, the new process could take advantage of a broad range of precursor materials. Multiple materials can also be deposited simultaneously.
“By allowing us to grow structures much faster with a broad range of precursors, this technique really opens up a whole new direction for making a hierarchy of complex three-dimensional structures with nanoscale resolution at the rate that is demanded for manufacturing scalability,” says Andrei Fedorov, a professor in the George Woodruff School of Mechanical En gineering. “This could provide a fundamental shift in the way this field will go.”
How It Works
In the established FEBID process, an electron beam is used to write structures from molecules adsorbed onto a solid surface that provides support and nucleation sites for deposit growth. The precursors are introduced into the high-vacuum electron microscope chamber in gas phase. High-energy electrons in the beam interact with the substrate to produce the low-energy secondary electrons, which dissociate the adsorbed precursor molecules, resulting in deposition of solid material onto the substrate surface. (See Figure 1)
Although it enables extremely precise fabrication of nanostructures, the process is very slow because the low density of adsorbed gas molecules in the vacuum environment limits the amount of material available for fabrication. And structures must be fabricated from the substrate surface up at continually decreasing growth rate and from a limited number of precursor gases available.
Fedorov and his team have accelerated the process dramatically by introducing electrically-charged liquidphase precursors directly into high vacuum of the electron microscope chamber. Liquid-phase precursors had been demonstrated before, but the materials had to be enclosed in a tiny capsule where the reaction took place, limiting fabrication flexibility, capacity, and utility of the approach for 3D nanofabrication.
The research team used low volatility solvents such as ethylene glycol, dissolving a salt of silver in the liquid. In solution, the salt dissociates into silver cations, allowing production of silver metal deposits by electrochemical reduction reaction using solvated secondary electrons rather direct molecular decomposition.
The solvent containing the desired material ions is introduced into the chamber using a nanoelectrospray system composed of a tiny nozzle just a few microns in diameter. By applying the focused electric field to the nozzle, the fluid jet is drawn and delivers to the substrate forming a precisely controlled thin liquid film. (See Figure 1)
The electrospray produces nanometer-scale charged droplets from a Taylor cone jet just 100 nanometers in diameter, which coalesce upon impingement and form a thin film of the precursor on the solid substrate.
The team the used the electron beam itself to visualize the Taylor cone jet in the vacuum environment, the first time this has ever demonstrated, as well as to measure the thickness of the liquid film in situ by using a nanoscale “ruler” prefabricated on the deposition substrate. The electron beam then scans over the liquid film following a desired pattern, producing suitable energy electrons which solvate and reduce the cations, writing structures in precise formation from the precursor delivered by the electrified jet. Though evaporation of the solvent does occur, the nanoelectrospray can maintain a stable film long enough for the structures to form.
The combination of a denser precursor, reduction in material surface transfer problems and elimination of the need to break chemical bonds with the electron beam allows fabrication up to five orders of magnitude—a factor of 5,000—faster than the earlier gas-phase technique.
Varying the precursor type, film thickness, concentration of ions and the energy and current of the electron beam controls the kinds of structures that can be made, Fedorov said. Structures such as bridges connecting posts become possible because material can be written atop the thin films.
The new process allows considerable flexibility in fabrication, opening the possibility of depositing more than one material simultaneously. That could allow production of alloys and composites, such as combinations of silver and gold. Or, one material could be used as a template to be coated by another material with the simple substitution of precursor materials.
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