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The research team – including graduate student and first author Jeffrey Fisher, postdoctoral fellow Songkil Kim and senior research engineer Peter Kottke – 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.
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 research team 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.
“By changing the energy of the beam and current, we can preferentially grow nanostructures in 3D at much faster rate,” Fedorov said. “All of a sudden, there are a whole host of different applications that were not possible before.”
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.
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