In this interview, Dr Ventsislav Valev talks to AZoNano about his research on "hotspots" in electrical fields on nanostructured surfaces, which have applications in catalysis, sensors and analytical science.
Can you give us some background to your research on interactions on nanostructured surfaces?
Hi Will, and thanks for your question. You probably know that the optical properties of natural materials depend on their building blocks, such as atoms and molecules. Well, very similarly, artificially engineered nanomaterials derive their properties from those of nanoengineered unit cells.
When light shines on such unit cells, its electromagnetic oscillations drive the electron density within the nanostructures and, in turn, these variations of electron density constitute the source of a very inhomogeneous surface electric field – the local field.
If you consider the local electric field lines, you would see those lines become very crowded in two sorts of regions on the nanostructures. First, in the sharp regions, such as corners or tips, where the field lines are pulled together by geometric constraints. And second, in the regions of highest electron density, where the electron charges constitute strong sources for the local field.
These regions of crowded field lines, at the nanostructured metal surface, correspond to local field enhancements and are commonly referred to as "hotspots". All of these processes can have some fascinating consequences.
What attracted you to working in this field?
To be honest, I was a science fiction fan long before I became a scientist. As a child, I remember dreaming that I would live long enough to see real Star Trek handheld communicators; and now we have smartphones. What attracted me to the field of nanophotonics is that it pursues so many ideas that seem to come straight out of science fiction.
Colleagues within my field are pursuing things like invisibility cloaking, levitation with light, quantum levitation, teleportation, Bose-Einstein condensates at room temperature, optical computing, super-resolution microscopes, etc. My only regret is that, for practical reasons, I can't work on everything that is so exciting within my field.
In ring-shaped nanostructures, the "hotspots" that normally form on nanostructured surfaces are delocalized around the whole structure, providing much more attractive properties for catalysis and analytical applications.
Your recent work was published in Advanced Materials in September - tell us about the results published in that paper.
Our work so far has been focused on imaging the hotspots on nanostructured materials with a second harmonic generation microscope. We have studied many nanostructured designs and consequently observed a great number of hotspots. Then we turned our attention to ring-shaped nanostructures and, for the first time, we saw no hotspots. Instead, the entire rings became visible. We wondered what had happened to the hotspots.
It is important to point out that in this experiment the light shining on the ring-shaped nanostructures was circularly polarized. This means that, as the light propagates, a nanostructure that is immobile in space will experience a rotating electric field from the light wave. As this electric field drives the charge density of the nanostructures, we though that perhaps the charge density was rotated along the rings. If that were true, it would mean that the local field enhancements, or "hotspots", were still there, only they were no longer confined to spots, but instead were distributed over the entire surface of the rings.
In order to test our hypothesis, we performed two sets of independent numerical simulations and both demonstrated that, indeed, the rotation of the electric field of circularly polarized light is imparted on the charge density of the ring-shaped nanostructures. This behaviour could lead to significant applications.
What applications could your new discoveries have in the wider science community?
Generally speaking, the interactions between molecules and hotspots are of great interest because they allow localized photochemical reactions, catalytic reactions and extreme enhancement of the molecules' optical properties.
Hotspots however suffer two intrinsic limitations: they can become too hot and they are confined to small areas. In other words, the heat from the local field enhancements can destroy the nanostructures and, as for the molecules, before they can have their optical properties enhanced, first, they need to find the hotspots on the surface of the material.
By distributing the local field over the surface of the rings, our results seem to address both limitations: the heat is more uniformly distributed over the surface and that whole surface becomes available for interactions with molecules.
These discoveries will no doubt lead on to more research in the area. What is the next step in your work?
So far, our nano rings were made by electron beam lithography, which is a very precise but very expensive method. While it is excellent for laboratory studies of the underlying principles of future materials, the method itself is unlikely to be used for actually making these materials. Therefore, next, we will turn our attention to other types of methods for producing nano rings. This is one of the main reasons why I have recently moved to the group of Prof. Jeremy Baumberg at the Cavendish laboratory, in Cambridge.
Where can we find more information about your work?
My personal website www.valev.org is perhaps the best source of information on our current work. In the future, more information could also be found on the page of the NanoPhotonics Centre at Cambridge.
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