Wiggling nanowires

Imagine an electrical wire the size of around one millionth of a hair wiggling as you shine light upon it

My experience at Seki lab of Osaka University has been just amazing. It’s been a year already working full-time with these great people. SUPER-DUPER smart chemists in the field of condensed matter and physical chemistry. In article I take you through my work with Dr.Kazuyuki Enomoto, Cola (who is now enjoying chicken rice while doing PhD at NUS in Singapore) and Michael (then an exchange student from UCSD).

I think it started about almost two years ago when Michael came to Osaka University to do an internship and Dr.Enomoto was assigned to take care of him. They set out to fabricate this photo-responsive nanowires via a very awesome nanofabrication technique called SPNT or Single Particle Nanofabrication Technique) perfected by the chief of the lab, Dr. Shuhei Seki.

The fabrication scheme of SPNT technique perfected by our laboratory. The grey images on the left are images from a scanning electron microscope while the orange ones are from atomic force microscopes.

The concept of SPNT was relatively simple yet powerful (literally, and you will get this later). To avoid a diffraction problem found in normal lithography techniques and to overcome the barrier which keeps us from fabricating nanostructures with high aspect ratios, Seki sensei proposed that we shoot a high energy particle like Osmium ion (By means of a particle accelerator, yeah! hence the “powerful” part) through polymer films. As the particle travels through the film, it dissipates a bunch of energy along its trajectory. By tinkering with the amount of the energy and the design of the polymer, it allowed us to get the energy just right that it can “cross-link” the polymers and hence special regions which polymers are stuck together and immiscible with normal solvents. Since the particle is spherical and it travels linearly through the film, the “cross-linked” regions are therefore cylindical and that’s how we arrived to the “wire” part of the nanowires.

Now, allow me to paint the big picture of how necessary we need to create these tiny wires.

Richard Feynman gave a talk in 1959 at Caltech, I don’t remember much of what he said. I wasn’t there. But the lecture was named “There’s a plenty of room at the bottom” hinting that when the technology enables us to manipulate things so tiny quatum effects start to take place, wonderful discoveries and applications are just around the corner.

This is one of those works to fill the gap. This is definitely not too scientifically novel. But it damn sure is significant for engineering. With the aforementioned method, it’s possible to fabricate nanowires. Now, we want them to perform something.

Because of the Azobenzene units, the polymers bend its backbone upon light irradiation.

This time we make it electrically conductive and wiggle when shone a light on it.

We start off with designing the building block of the nanowires. Remember the polymer film we use as the precursor for the nanowire? Since it’s essentially what the nanowires are made of, we go back and re-engineer it.

While the dark part on the left helps absorbing light particles, the blue unit on the right (Azobenzene) is what actually functions as the source of mechanical response. You can actually see that there’re double lines linking atoms all over the entire structure. Those are double bonds and when they are all interconnected, electrons can flow from one atom to another. And Voila! It’s now electronically conductive.

Then we applied this polymer with the technique. We obtained something which looked like this.

Look at the scale! That black bar on the right is approximately a size of a small bacteria.

This image is taken from an atomic force microscope or AFM, which is really awesome. The nanowires are so small that even lightwave would be too big they won’t reflect our nanowires properly and hence unable to produce any observable image.

Anyway, that was a lot of Osmium ions (one hundred million ions per square centimeter) shot into a polymer film incase you were wondering why they were so many of them.

To see whether they actually under the promised photo-responsive mechanical change, we shine light on it and again we use AFM to observe them.

The nanowires went from flat and uniform to distorted and wavvy. They survive solvent treatments which means that the crumpled appearance wasn’t due to the light destroying our nanowires.
Topological crosssection of the nanowires show great reduction of nanowires’ radii when irradiated. They also swell back when we use an “inverse” wavelength which means that the shrinkage wasn’t due to evaporation of solvent.

The change was obvious although not entirely as we expected.

But Hey! Scientists just don’t expect, aren’t we trying so hard to be unbiased?

We saw a massive shrinkage of the wires and also the deformation of them. These effects were reversible, that’s how we concluded that it’s due to the also reversible cis-trans isomerization while other phenomena like solvent evaporation or photo-damaging the nanowires are not.

Hold on! Did we just fabricate nanowires capable of mechanically response to the light applied on them? I think we did.

The work was selected to be published in the journal Advance Material Interfaces by Wiley. Here goes the link.

Reversible Control of Radius and Morphology of Fluorene-Azobenzene Copolymer Nanowires by Light Exposure http://onlinelibrary.wiley.com/doi/10.1002/admi.201400450/abstract

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