AFM prints patterns on the nanoscale
Image: Copper clusters on the surface of the gold plate form the letters AMOLF
Researchers from AMOLF’s 3D-Photovoltaics group in The Netherlands have used an atomic force microscope to electrochemically print at the nanoscale.
Using an electrochemical AFM to manipulate dissolved copper ions, Mark Aarts and Esther Alarcón-Llado have written 'AMOLF' on the surface of a gold chip, and now hope to harness the method to print nano-architectured solar cells.
To directly write copper nanostructures onto the surface of a gold chip, Aarts and Alarcón-Llado used the EC-AFM to dynamically perturb the electrical double layer at the solid-liquid interface of the chip and a dilute aqueous electrolyte.
The researchers first placed the chip and AFM's platinum tip within the electrolyte, with the tip being the only part of the AFM cantilever exposed to the liquid.
They then applied a potential difference between the gold substrate and AFM tip to control the local growth of copper on the chip
When the voltage is applied across the electrodes, copper is deposited precisely where the tip is located on the gold surface.
If the tip is moved, then the copper is deposited at the new location, and with this approach the EC-AFM can be used to draw arbitrary shapes into the chip.
Direct writing of copper with dynamic EC-AFM. (a) Writing set-up. (b) Ex situ AFM-topography, cross-section height profile, and in-column SEM, of a 1 μm long line. (c) Ex situ AFM-topography and cross-section height profile of the written AMOLF logo. Scale bar is 200 nm in Fig 1b (d) In situ AFM topography, after following a 12 μm-long path across the centre of each image. Scale bar is 2 μm. [Aartsa and Alarcon-Llado, Nanoscale]
During writing, the AFM is operated in the PeakForce tapping mode.
Here, the force exerted by the tip on the sample is used as a feedback signal while the intermittent contact results in minimal lateral forces on the chip.
In this way, Aarts and Alarcón-Llado were able to demonstrate direct electro-deposition of copper on gold with lateral dimensions down to 50 nm and an aspect ratio slightly above 0.5 for a writing speed of 3 nm/s.
“The double layer is one of the most important phenomena in electrochemistry, but we do not yet fully understand it," says Aarts. "This knowledge could be important for the development of improved batteries or electrocatalysis.”
The structures drawn so far are around 50 nm due to the AFM tip dimensions, but the researchers realise that smaller lateral dimensions would be better.
“We think that we could easily use a smaller tip to draw even smaller structures,” says Aarts.
The researchers now hope to use the method to fabricate a new generation of nano-architectured solar cell that will capture sunlight in vertical nanostructures, such as wires, cones, even tree-shaped elements.
And, ultimately, the production of such solar cells will require structures built from materials such as gallium arsenide.
“With electrochemistry, we can easily apply materials simultaneously or in sequence,"says Aarts. "Within the group we are also investigating these processes, and we hope to combine it all in the future.”
Research is published in Nanoscale.