Researchers at Japan’s Nagoya University have developed a method to form dome-shaped bumps on underwater nanofilms using a computer-guided electron beam. Bumps form in less than 10 seconds and can be flattened, reshaped, and repositioned as needed.
This method could enable computer-guided manipulation of nanomachines for applications such as microscale touch sensing, induction of cell proliferation, and direct assembly of colloidal particles. The research results were published in the journal ACS Applied Materials & Interfaces.
Each existing approach has its drawbacks. Light-based techniques typically take 60 seconds or more for each shape change, while electrical methods rely on fixed electrodes, which limits where the reshaping can occur and limits the size of the changes.
To overcome these limitations, first author Ken Sasaki and Associate Professor Hisataka Maruyama, along with Professor Takayuki Hoshino of the Nagoya University Graduate School of Engineering, combined two innovative techniques. The first is a “virtual cathode” display in which an electron beam is scanned across a silicon nitride (SiN) film along a computer-defined path to generate a localized electric field with nanoscale precision. Because the pattern is set by the scanning path rather than physical electrodes, its shape and position can change instantly.
The second is a multilayer film of pyrene-bonded graphene oxide, approximately 45 nanometers thick, consisting of approximately 29 stacked layers, anchored to a SiN film. Since the film has a negative surface charge in water, exposure to the charged region of the beam induces electrostatic repulsion towards the SiN layer. This causes the stacked layers to slide apart slightly, then the bottom layer is peeled away from the membrane, causing the film to expand into a dome shape.
Observe nanoscale changes
Graphene oxide typically does not fluoresce. This is because tightly stacked sheets quench each other’s fluorescence. When the beam was applied, the film’s fluorescence turned on and increased in intensity. This indicates that the layers are separated and the extinction is relaxed. As the film expanded, the thickness of the underlying water layer changed, creating an interference pattern that resembled contour lines, allowing the team to measure otherwise invisible changes in height in real time.
Important experimental results
A dome-shaped bump approximately 1,200 nanometers high and 37 micrometers in diameter was formed within 10 seconds. This is significantly faster than light-based methods and comparable to the speed of the fastest reported electrical systems, but with much larger height changes.
The deformity was reversible but asymmetric. The film expanded at 100 to 200 nanometers per second, but subsided to only 40 to 55 nanometers once the beam was turned off, requiring more than 20 seconds for full recovery. The researchers believe this is because the SiN film’s dielectric polarization builds up quickly under the beam, while residual surface charge dissipates much more slowly.
By adjusting the beam exposure time and current, and moving the beam to combine adjacent deformed regions, the researchers reshaped the dome into a larger dome or valley-like depression, and the film retained its structure even after repeated reconfigurations in the same location.
As a proof of concept, the bulge pushed a single 10-micrometer polystyrene bead into water in a controllable direction, with an estimated mechanical pushing force of 0.05 piconewtons and another electrostatic repulsion force of 0.11 piconewtons. This suggests, but has not yet been demonstrated, the potential for moving cells or powering microscope robots.
outlook
“We believe this technology will facilitate the integration of nanomachines and computers,” Hoshino said. “Nano- and microscale irregularities at interfaces are critical for friction and adhesion between objects. We hope that this display technology can generate these irregularities on demand, ultimately allowing us to control the adhesion and aggregation of microscopic cells and objects.”
The researchers note that precisely controlling where the film detaches and demonstrating stable operation in physiological electrolytes rather than pure water will remain open challenges until living cells can be manipulated in this way.
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