By Dr. Rick Marta
Chemistry Instructor, University of Waterloo
At the University of Waterloo, we are constantly upgrading our undergraduate chemistry labs to keep pace with today’s technology. This isn’t just a matter of honing professional skills. I believe that students genuinely enjoy learning about the science that shapes their everyday lives.
One experiment that I teach in the 2nd year undergraduate materials and nanoscience lab consistently stands out as my students’ favourite. It involves the synthesis and characterization of “capped” cadmium selenide (CdSe) “quantum dots” (QDs), a type of nanoparticle with a diameter of 1 to 100 nanometres.
In this lab, students discover that the colour emitted from a QD is directly related to its particle size. Students can control the particle size of the QDs by removing aliquots of a reaction mixture at different times.
Pictured above are the aliquots of the capped CdSe QDs – all the same compound – being irradiated with a black light. This causes the solutions to strongly fluoresce. The colour of the emitted fluorescent light is determined by the particle size of the QDs.
The longer reaction time (labelled in seconds), the larger the QDs, and therefore the longer the wavelength of light emitted. This is just like the colours of the familiar "rainbow" of visible light corresponding to different wavelengths of light.
It’s this optical property that makes QDs so commercially desirable because they produce tunable and narrow emission wavelengths which result in a very “pure” coloured light.
Although the commercial applications of CdSe QDs are limited by the environmental toxicity of cadmium, QDs can also be made without the use of heavy metals. Such quantum dot devices are physically flexible yet inexpensive to make. For example, Samsung’s current flagship QLEDTM televisions use InP/ZnS QDs to produce beautiful, high-contrast, high-resolution pictures.
As part of the lab report, the students discuss how their devices and high tech screens may involve the use of quantum dots. Their reports also touch on aspects of nanochemistry, metallic compounds, solubility concepts, energy of a photon, and energy states.
The feedback on this experiment has always been positive - students love this lab! It’s exciting for them to make something so interesting from “scratch” and clearly observe that they were successful in doing so.
My next step is to have students run the experiment in a glove box. A glove box is an airtight glass and metal box with gloves integrated into one side. The inert environment inside keeps oxygen and moisture out of the reaction, while providing an additional level of safety that we want students trained in using.
Dr. Rick Marta (BSc Chemistry ’04; PhD Chemistry ‘09) has been an instructor in the Department of Chemistry since 2012. His article with full details of the quantum dots lab experiment appears in the November 2017 issue of Chem13 News.