I don’t start my PhD until October so I won’t be posting much about it for a couple of months. In the mean time, I thought it would be nice to talk about what I did for my final year research project as part of my MSci degree.
The aim was to synthesis quantum dots[^1] using microfluidic reactors. It sounds complicated, but really it’s quite straight forward! An explanation of it all in one post would be rather long so I’m going to break it down into two posts, starting with quantum dots and then moving on to microfluidic reactors.
What are Quantum Dots?
Quantum dots are nanoparticles—particles only a few billionths of a metre in size—made from semiconductors. Semiconductors are materials whose electrical conductivity is midway between that of insulators and conductors. They are the foundation of modern electronics and without them we wouldn’t have components like transistors and diodes which are essential building blocks of the technology we use every day.
All materials have particular physical properties—such as the melting point or density—that are independent of how much of the material you have. For example, if you measured the melting point of a material, cut it in half, then remeasured the melting point, the melting point would not change. Properties like these are called intensive properties.
Imagine you had a piece of semiconductor and repeatedly measured an intensive property, such as melting point, then cut it in half. You would expect intensive properties to stay the same, regardless of the amount of material. However, if you carried on doing this for quite some time—so that your semiconductor was just a few billionths of a metre across—you would find that its properties would start to change: properties which were intensive become extensive and dependent on how much of the material you have. Chemists take can advantage of this phenomenon to tune the properties of semiconductors for particular applications by controlling the particle size.[^2]
Making Quantum Dots
Rather than breaking down macro- or microscopic bits of semiconductor to make nanoparticles (“top-down”), chemists usually make quantum dots from individual atoms (“bottom-up”). This is most commonly achieved by injecting the appropriate reagents into a hot solvent. The quantum dots spontaneously form in the hot solvent and are left to grow to the desired size.
The photo below is of some cadmium selenide quantum dots that I made last year. I think it’s a wonderful example of their size-dependent properties.
Each vial contains quantum dots that were removed from the reaction vessel at regular intervals. The vial on the far left hand side contains quantum dots grown for 30 seconds and the vial on the far right hand side contains quantum dots grown for 3 hours. The mean size of the particles grown for 30 seconds and 3 hours was 2.8 nm and 4.2 nm respectively, so the nanoparticle size increases from left to right.[^3]
The colour arises from a process called fluorescence. The vials are sat on top of an ultraviolet lamp which causes the quantum dots to fluoresce and emit light, the wavelength of which is dependent on the size of the quantum dots.
These unique optical properties make quantum dots very attractive for use in solar cells, displays and even in medical imaging. The trouble is that high-quality quantum dots are quite tricky to make, especially on an industrial scale. In part 2, I’ll talk a bit more about the applications of quantum dots, what microfluidics is and why it’s great for making quantum dots. If anyone has any questions, please don’t hesitate to ask!
[^1]: For the chemists/physicists: I was working on core-shell and ternary quantum dots rather than regular binary quantum dots.
[^2]: This behaviour isn’t unique to semiconductors—it’s just that the change occurs at a much larger particle size for semiconductors than for metals because of differences in the arrangement of electron energy levels in metals and semiconductors. See this very frequently cited paper in Science if you want the details.
[^3]: Analysis was performed using a technique called transmission electron microscopy, if you were wondering. It’s very cool.