Quantum dots!

If I have to choose the most badass phrase from my experiment, “quantum confinement effect” is the most compelling candidate. Yes, we nanochemists can control the “quantum confinement effect” in our laboratory by our own hands. (Blah!)

Today’s topic is about semiconductor nanocrystals, as known as quantum dots. Quantum dots are another reason why nanochemistry has been one of the hottest topics in science in the past decade. The shininess of the quantum dots and variety of the colors that quantum dots emit are very useful for display. You may hear QLED displays in Target or Walmart. It is the abbreviation of “quantum-dot light-emitting diode.” The superior optoelectronic properties are a perfect fit for solar-cell materials. As of 8/17/2019 when I am writing this post, the efficiency of the state-of-art quantum-dot based solar cell reach 16.6%. The improvement of the efficiency record has never stopped. It is exciting to imagine what happens in, let’s say, 10 or 15 years.

Quantum dots used here refers to nanosized semiconductors of which size is close or less than the Bohr radius, which is a theoretical parameter unique to the material composition. For example, the Bohr radius of CdSe is 5.4nm. CdSe materials smaller than 5.4nm emerge quantum confinement effect, leading to the generation of unique molecular orbitals. There are a lot of things that I can talk about in this process, but practically speaking, this will lead to the size-dependency of fluorescence. 

In the synthesis point of view, you always want to monitor

The absorption and fluorescence evolution

The quantum yield and the surface state (fluorescent quenching can happen when the surface is not good)

The core-shell synthesis and the related shape effect (like nanorods or tetrapods)

Very nice schematics about quantum confiniment effect.  The figure is from this paper.

Quantum dots synthesis is satisfying because when you nail it, the product is shiny and beautiful. And understanding physics behind the beautiful color gives you another satisfaction that you are really controlling the “quantum confinement effect” by our own hands.

Good luck with confining quantum effect.

Gold nanocrystals!

Gold nanocrystals and the size-color relationship.  The picture is from Wikipedia

If you are a nanochemist, and your friend is interested in scientific trivia related to what you research, size dependence of gold nanocrystal color would one of the easiest examples. Gold and silver nanocrystals can change their color by changing their shape, just like when you are skinny, you look red; when you are fat, you look blue.

There are a lot of interesting things going on with nanosized metals and the basic physical events that lead to super-good nanotechnologies. However, the history is pretty long; the emergence of nanosized gold was back to the 1857 paper by Michael Faraday.  He used phosphorus to reduce a solution of gold chloride to make nanosized gold (he called it ‘gold leaf’ in the paper). He found the color was different from bulk gold.

One of the oldest research papers in my laptop.

Another important breakthrough was made by Turkevich, as you can guess, the originator of the famous Turkevich method, where he made gold nanoparticles by reducing HAuCl4 by citrate (aka Vitamine C). This method is still used in the lab to create water-soluble particles (yes, good scientific discovery is immortal!). 

Frens was one of the first researchers to study the nucleation and growth of nanocrystal synthesis, leading to a very famous theory, the LaMar theory. The most monumental work was published in Nature Physical Science (another trivia: The journal Nature was Nature Physical Science before!) in 1973. 

This historical research spurred the flourish of the metal nanocrystal synthesis protocols, notably contributed by C. Murphy, Y. Xia, K. Klaubunde, G. Stucky, and so on. Today, we can make a lot of gold nanocrystals in terms of size, shape (rod shape, spherical shape, a lot of polyhedra shape), and solubility (water-soluble, or oil-soluble). Gold nanocrystals are cool because they can change the physical properties by synthesis that can be useful in the field of medical materials or materials science such as for special sensing techniques or imaging agents through its coloration, Raman response, and the plasmonic features.

Good luck with your “golden” research!

Keep Your Presentation Shorter!

Your presentation must be beneficial to both you and your audience. (It is never for you to show off your data or how hardworking you are!) One factor for effective presentations that sometimes is ignored but always important is the length of your presentation. Keep it short!

You have learned how to conclude. You have learned how to refine your experiment’s plans and presentation. Now it’s time to extract the critical points.

You must select the essential points for your presentation. How do you know what the most critical points are? They are the points that help you reach your presentation goal: (1) for your audience to share your thoughts and (2) for you to make sure that your understanding is right. Once you’ve finalized your most essential points, you can make your presentation.

Your other slides can be in the backup slides. For example, perhaps four pages could be shortened up into one sentence by saying, “Control experiments on concentration, temperature, reaction time proves these are not important factors.”

Your main topic often falls into one of the two ideas below:

1.          Newly learned technique or knowledge

Your benefit: A conscious understanding of new knowledge. Audience’s interest: Can learn something new.

2.          Newly developed thoughts or conclusion (could be from a stack of the experimental results)

Your benefit: Can test your logic and the quality of your experiments’ conclusion. Audience’s interest: Can understand.

Ideally, weekly group meetings become a record of your thinking and learning processes. Making your presentation concise is the best way to organize them both.



Good luck with your short presentation!