The source of importance, supramolecular interaction

A chick in a vial with a biological organization.  You can buy this cute thing in here.

You have a chick in a vial. Then, you smash the chick into liquid. If you compare it before and after the process, what was lost? This is called the “Paul Weiss thought experiment.” His answer is “biological organization.” The point is the importance of the organization or superstructure. Even if you have the same components, if there is functionally constructed organization, it does not work as it should.

Noteworthy, this is an “imaginary” thought experiment. If you actually do the experiment, not only will a chicken lose the biological organization, but you will also lose something important: humanity!

The point of this story is how important superstructures are important for us. Without properly constructed structures, things don't work as they should. 

In chemistry, you can also create beautiful superstructures from molecules and nanocrystals, and the forces to create superstructures are called "supramolecular interaction." Given the very importance of superstructures, you may call supramolecular interaction as the source of the importance.

"Organization" is important in our human society too!

The representing supramolecular interactions are van der Waals forces, electrostatic forces, hydrogen bonding, and magnetic forces.  Such interactions create various materials ranging from a biological molecule such as DNA to nanocrystal superlattices. 

A picture of a computer model of a nanocrystal superlattices.

Wanna study intermolecular interaction?  Here is an excellent textbook by Prof. Israelachvili.  This is a good book. I learned a lot from it.

https://www.sciencedirect.com/book/9780123919274/intermolecular-and-surface-forces

The number of citation for this book is more than 32000!

Good luck with making your molecules interacted nicely!

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!