Just as realtors stress three important factors in home value (location, location, location!), the nanomaterial community understands the three most important things to consider when designing and using a nanomaterial product:
surfaces, surfaces, surfaces!
As many of my fellow scientists and clients can attest, I strongly believe that the problems/solutions/innovations in nanomaterials rely on the surface chemistry of these wonderfully complex particles.
Surface:Volume (a ratio worth rationalizing)
Perhaps many of you are familiar with the concept of surface to volume ratio, but I feel it’s worth reiterating. Since we often deal with particles in the sub-micron size range (sometimes <10 nm) it is of utmost importance to internalize the concept of surface-to-volume ration. As you can see from the following figure, nanomaterials (let’s classify that as <100 nm for now) are FULL of surface species. (Disclaimer, this is a quick and dirty model that I created for a specific system, not applicable for all nanomaterials.)
So based on this model, nearly half of the atoms of a 5 nm particle are surface atoms! For a 10 nm particle, it’s 23%. Even at 20 nm a particle has >10% surface atoms. That’s more than enough to dominate many of the important parameters you might care about.
Why is this important? Surfaces are where the fun stuff (or not fun stuff) happens. Surfaces are imperfect. Surfaces are dirty. Surfaces are complex.
Where does catalysis happen? Surfaces!
What determines the solubility of nanoparticles? Surfaces!
Where do the imperfections exist that degrade optical properties of quantum dots? Surfaces!
How can one tune the energy levels of quantum dots (other than size)? Surfaces!
And don’t think that just because a particle is 100 nm or larger means it does not fall into the same wormhole of surface domination. Let’s plot the graph another way, out to 1,000 nm (or 1 micron).
Here you see that even up to 1000 nm (no longer “nano” sized) a particle can have a few tenths of a % surface atoms. You may be thinking “so what if my particles are 0.5% surface atoms, that’s 1 of every 200 atoms!” Correct you are, but 1/200 is still significant, very significant.
Stick with me for a minute as I rationalize…
Think of surface atoms like dopants. They can control the photophysics of a semiconductor much the same way dopants do in bulk semiconductors. Now let’s take for example a heavily doped silicon semiconductor at a doping level of 1×10^17 atoms/cubic cm. Pure silicon contains ~5×10^22 atoms/cubic cm. This means that 0.0002% of atoms are dopant atoms (1 out of every 500,000 atoms) and this is considered HEAVILY doped! For a particle in my model to reach 0.0002% surface atoms, you would have to have a particle that is 1,000,000 nm (1000 microns or 1 mm)! In the world of nanomaterials and semiconductors, it doesn’t take much to make a difference.
Surfaces as a tuning lever
Now that I’ve convinced you that the surfaces are important, I’d like to share a few examples of what can be done with them. What follows are examples of recent literature that demonstrates how nanoparticle surfaces (quantum dots in particular) can be tuned to impact the electronics and behavior of the material. The result: better understanding and improved properties.
Custom QD surface can tune energy levels. Work from NREL (my old stomping ground) has demonstrated that one can tune the energy levels of a quantum dot by extraordinary amounts just by changing the surface. This should lead to more efficient solar cells and other quantum dot devices.
QD surface ligands impact trap states. Contrary to popular belief, the authors in this paper point out that only certain types of ligands and dangling bonds contribute to mid-gap states in quantum dots. This work will impact the way in which we design future QD systems.
Metal nanoparticle catalysis. Metal nanoparticles can make great catalysis, that’s nothing new. What is still not completely understood is the role of nanoparticle surface states in catalytic systems. Slight changes to nanoparticle structure, composition, and surface species can result in huge different in catalytic activity and specificity. The line between heterogeneous and homogeneous catalysis continues to blur, and it’s becoming increasingly apparent that the surface is what dictates the reaction.
I hope this article has given you an appreciation for the important role that surfaces play in nanomaterial science and development of products that rely on the amazing properties of nanoparticles. I have only “scratched the surface” (pun intended) in this article. There area many more examples and important properties that can be customized by the tailoring of surface chemistry.
So remember, next time you are up against a challenge in developing a nanomaterial technology, the three most important things to consider are…
surfaces, surfaces, surfaces.
Thanks for reading.