The secret glow of nanobits from the kitchen

What are carbon quantum dots (C-dots)? Why do they behave so differently when exposed to different light sources? How can we observe or change their properties?

Note: This lesson is good for physical science class in middle school, high school chemistry class, AP chemistry class when using extensions provided.

Some topics that can be discussed before these lessons are implemented:

• The metric system
• Size scales
• Light, color, and the electromagnetic spectrum
• Absorption and emission of light

More than any other element, carbon has the capability of forming bonds to itself, which can lead to very large molecules that are predominantly carbon, while the other elements tend to ‘boil’ away, typically as water molecules (H2O). In the case of carbon quantum dots (C-dots), the molecules can reach the nanoscale or larger. (Of course, carbon particles can be made to truly macroscopic scale, in the form of graphite or diamond.) But it is the “nanoparticle” quantum dots that behave in the most curious way.

As the name suggests, nanomaterials are very small particles, characterized by a size less than 100 nm in diameter. They can assume a variety of shapes such as sheets, tubes, fibers, and spheres. They are relatively non-toxic, able to be produced using inexpensive raw materials, and they are fairly easy to synthesize. They were discovered in the early 2000s as a by-product of the purification of carbon nano-tubes4. Many synthetic methods and various carbon sources (even fresh plant leaves!) can be used to make C-dots. The negative aspect of that ubiquity is that the resultant particles are not well characterized at all. A generic representation of C-dots involves an aromatic carbon core surrounded by an outer skin including functional groups left over from the starting material, or formed during the reaction process.

The presence of some functional groups in the starting material (e.g., amines) has been shown to change the luminescent yield of the particles, but the theory is not as well developed.

Luminescent properties in quantum dots:

A unique aspect of quantum dots is that their fluorescent emission can be “tuned” simply by changing the size of the particle, or its chemical composition. Larger particles emit at longer (redder) wavelengths, while smaller particles emit at shorter (bluer) wavelengths. A crude picture of this is that because the particle size is roughly comparable to the size of the wavelength of light. Providing energy to the particle can “excite” an electron from the ground state to an excited state, much as providing electricity to a neon light can excite the neon atoms in the tube. For the C-dots in this exercise, we provide the energy in the form of light from a UV flashlight or green laser. A red laser does not have sufficient energy to cause the excitation. Once excited, the or C-dot particles can glow. Nature, in general,l dictates that systems prefer to be at their lowest energy level (think water running downhill). The C-dot particles can de-energize to emit their excess energy as light. Neon tubes glow red because the energy of red light corresponds to the energy difference between the excited state energy level and the lower (ground) state level. 

In this activity, we find that de-excitation of the C-dots leads to broad emission across the blue-green part of the spectrum, and in some cases, includes red emission as well. And, just as changing gases in neon lamps alters the emitted colors, incorporating other atoms in the C-dots can change the absorption and emission characteristics. It is worth noting that the energy of the light emitted will always be equal to or less than the energy of excitation – that is, the emission will be at longer wavelengths (blue, green or red) than the ultraviolet originally absorbed. One further point to note is that some excited molecules may not emit visible light at all, or emit light for a long time after the excitation is turned off. In the former case, energy could be emitted as heat (i.e., infrared light, which we cannot see.) The latter case will be familiar to almost all children, since most have seen or played with “glow-in-the-dark” toys. Laundry “brighteners” work by this process – the UV we are exposed to all the time keeps them glowing, at least while in the light. Remove the exciting ultraviolet light and the brightening effect on the clothes goes away. But glow-in-the-dark toys work by “phosphorescence, “ where the energy gets trapped in the molecule, and only ‘leaks’ out slowly (i.e. glow-sticks). The specific energy level structure of the molecules causes the difference.

The C-dots made from sucrose or Tic-Tac® candies cannot be easily distinguished in terms of intensity or color of emission under UV light. However, subtle differences in response to a green laser beam can be distinguished, in some samples. For example, C-dot suspensions made from table sugar show a red-orange color and were illuminated by a green laser beam, while C-dots from Tic-Tacs® show no change from the green beam. Note that the structures of the starting materials in these cases are quite similar (mostly sucrose), but Tic-Tacs® also include sugar polymers, coloring and flavoring agents, etc. If you use Urea to make a sample, you will see a difference in color emitted as well.

Teacher tips:

• Suggested STEP UP Everyday Actions to incorporate into the activity.
• Consider using whiteboards during discussions, so students have time to brainstorm and work through their ideas before saying them out loud.
• As students experiment, roam around the room to listen in on discussion and notice experiment techniques. If needed, stop the class and call over to a certain group that has hit on an important concept.
• Consider these responsive tools and strategies and/or open ended reflection questions to help push student thinking, and to help students track their thinking during the activity.
• Connect to students’ lives and create opportunities to develop STEM identity using these suggested extensions.
• Allow the work of physicists to come alive by signing up for a virtual visit from a working physicist using APS’ Physicist To-Go program. You can request a quantum physicist to talk about the concepts students learned in this activity!

Real world connections

• Sign up for Physicists To-Go to have a scientist talk to your students.
• Career and Workforce Connections: Quantum Careers lesson (1-2 class periods)
• Use Elisa Torres Durney’s scientists profile to spark conversations about who does quantum physics

Real world situations/connections can be used as is, or changed to better fit a student’s own community and cultural context.