A jaunt through the world of nanotechnology - Part 1; quantum dots
In a game of sci-fi buzzword bingo, nanotechnology is almost always present. Next to “quantum” it’s usually meant to mean “sci-fi magic”, but nanotechnology is real and far from magic. The field of nanotechnology is incredibly broad and far reaching. Almost every aspect of our lives can and will be affected by it, if it hasn’t been already. This means there are lots of area of it to explore, each with it’s own challenges and rewards. While the idea of nanotechnology has been around for a long time, it’s only in recent years that we’re really starting to make headway. In this series I’d like to take you on tour of some of my favorite nanotechnologies, some of my work on them, and where I see each field going.
One of my first experiences with any nanomaterial was with something called a quantum dot. Before I continue I should explain a bit about tiny things. At the quantum level of atoms and electrons, our ideas about traditional Newtonian physics go out the window. Things like the uncertainty principle and the wave/particle duality of matter become the governing rules. Quantum dots exist on the line between this world of quantum weirdness and the world we’re more familiar with. Consisting of only a few hundred to a few thousand atoms, these particles have some very weird properties. The particles behave as if the whole unit was 1 single atom. So rather than their properties being defined by the material they’re made of, their properties are determined by their size and shape. The most common way this manifests is in the color of the particles, specifically the color (or wavelength of light) that the particles will absorb and also the color they emit. If you shine a light on larger quantum dots they’ll emit a longer wavelength that looks more red, whereas if you do the same to a smaller quantum dot, they’ll emit a wavelength that’s shorter and looks more blue. Using this method, one can precisely tune the particles to have specific properties simply by changing their size. With quantum dots you don’t need to engineer a dozen different molecules to make a dozen colors, you can simply change the size.
When quantum dots were first discovered they were made out of semiconductors like cadmium sulfide and cadmium selenide. These quantum dots glow brightly and are relatively easy to synthesize. They just have one real downside in that they’re fairly toxic. However in recent years, much safer alternatives have become more common. The first quantum dots I ever made weren’t made from cadmium, but instead from ordinary table sugar. I used a technique called hydrothermal synthesis to make them, which is a fancy way for saying I heated up a dilute sugar solution and I kept it all under pressure. My pressure vessel that let me keep the solution hot under pressure was made out of a couple of pipe fittings I got from the local hardware store. Now, I wouldn’t suggest repeating this as high pressure can be very dangerous, but surprisingly I never had any catastrophic failures.
I baked my sugary liquid for 8 hours at 180 degrees in my kitchen oven and then took it out to cool. I cracked open my reactor and was left with this dark brown solution. At first I was skeptical that anything had really happened, but then I turned on the UV light. The surface of the liquid was glowing a beautiful light blue. I had successfully made carbon based quantum dots. It took some experimentation but I eventually managed to make yellow and green dots, at first using sugar but then using other carbon sources like gelatin. Of course, all this was replicating papers and not ground breaking research, but it was proof that the technology can work outside of some expensive lab. You don’t need nanopure water, or expensive reagents, just a carbon source, heat, and pressure.
So why is all this important? Quantum dots tunable properties make them ideal for anything that requires the absorption or emission light. It’s not hard to imagine just how many technologies quantum dots will have an impact on. Scientists are already finding their way into everything from LEDs to solar panels and are making both more efficient. For LEDs it means you can tune the color to exactly what you want. For solar panels it means you can make a material that absorbs lots of different wavelengths of light by mixing different sizes of quantum dot.
Mostly this is being done with the old cadmium based dots, but more recently carbon and graphene based quantum dots are starting to compete. Another application would be in dye based lasers. Dye lasers are great because by switching out which dye you’re using, you get a different color laser beam. However normally the dyes are expensive and only come in a few colors so options are limited. With quantum dots, you could make a laser in almost any color simply by changing the size of your particles. Finally, we get to biology, where the need for color is greater than ever. It wasn’t until the first synthetic dyes were produced that the hidden world of the cell started to come into view. Suddenly scientists could see the individual pieces and what they do. But again, biologists are limited to a small number of compounds and colors. Quantum dots are small enough that they can be used for many of the usual staining procedures and they can be engineered to stick to whatever the biologist needs to see. With the huge variety of possible colors this means a biologist can stain lots of different things at once and see how they all interact. From all this, it’s clear that quantum dots will play a major role in future technologies, and we’re only just starting to scratch the surface. Some groups are even considering trying to use them to build a quantum computer, but that’s a discussion for another time. Quantum dots will always have a place in my nano-engineering toolkit. There are so many exciting opportunities, and I’m excited to explore them all.
