Surface Plasmons and an Awesome Group

I thought I should discuss surface plasmons a bit because they are a part of our research and just plain cool. So get pumped, it’s time for surface plasmons! Let’s start with the basics. We know surface plasmon resonance is an effect that has to do with light but what exactly is light?

Light is a wave (don’t forget there’s some particle duality too, but I won’t focus on that) that consists of oscillating electric and magnetic fields. So what happens when visible light hits things? Like glass and water and squids? Well, many of these phenomena we see every day: the light can be transmitted through, refracted (Yay Snell!), reflected (which we observe as colors), and lots more. But what happens when we shine light onto something really small, I mean nanoscale small, and what if that something happens to be gold? Say a gold nanoparticle?

Well if the size of the gold nanoparticle is within a certain range compared to the wavelength of the light, those oscillating electric fields can induce a dipole (separation of charge) across the gold nanoparticle, like so:

The Go-To Image for explaining surface plasmon resonance in nanoparticles. Look at those electrons ride that sick wave!

This creates collective oscillations of free electrons in the metal. These oscillations are in resonance with the frequency of the light, hence why this is called surface plasmon resonance. Surface plasmon resonance occurs in other metals as well, just at different frequencies. Silver and copper nanostructures have surface plasmon frequencies within the visible light spectrum as well, while other metals will have plasmon frequencies at longer or shorter wavelengths. The frequency can also be tuned by the size and shape of the nanostructures (they don’t have to be spherical like in the picture above). As the structures become larger, longer wavelengths (like infrared) must be used to excite surface plasmons. Conversely, as smaller structures are used, shorter wavelengths must be used (like ultraviolet). However, at a certain point the structures are too small and quantum effects start to dominate, so surface plasmon resonance ceases. At this point, I have to just throw my hands up in the air, take some quantum physics classes, and let you know that “I’ll get back to you on how that works.”

These oscillating electrons now have some additional energy. We can use these energetic electrons for various purposes, including solar water splitting devices (what first attracted me to the Moskovits group), photovoltaics, and photocatalysis. I’ll save that for a future blog post but let’s bring this back into everyday life. I said that changing the size of the nanoparticles alters the plasmon frequency, so what if we place nanoparticles of different size in different solutions? Well, the nanoparticles will absorb at different frequencies because of the different plasmon frequencies, so that means the reflected visible light will differ for each solution:

Beautiful. Science.

Finally, my mentor, Jose, has been in China for the last couple of weeks (so proud of him by the way, what a boss) and today was my first day back in the lab for a while. I just wanted to add how I really missed everything at the lab the last few weeks or so, not just the research, the surface plasmons, and the frustrating field effect transistors but the people who I get to work with and around. Though I may not always understand the many accomplishments of the Moskovits group, I feel privileged to be a part of the group and contributing to the research. I can’t say enough how amazing our group is, and I’m happy to have been able to learn so much from them and work with such great people!

Look at these beautiful and talented people!

Christopher Siefe

Chris is going into his fourth year at UCSB as a Chemical Engineering major. He is interested in promoting change with science and is currently working with surface plasmons in professor Moskovits’ lab.