Nanoscale materials under the (proverbial) microscope

Teri W. Odom — the Charles E. and Emma H. Morrison Professor of Chemistry and Department Chair at Northwestern University — is an expert in designing structured nanoscale materials that exhibit extraordinary size and shape-dependent optical properties. Odom was a plenary speaker at SPIE Optics + Photonics, where she discussed advances and future prospects in manipulating light at the nanoscale with plasmonic nanoparticle lattices.

What are plasmonic nanoparticle lattices and what makes them well suited for lasing and other applications?

Plasmonic nanoparticle lattices are periodic arrays of metal nanoparticles that — when designed optimally — integrate the best of both plasmonic and photonic systems. Localized surface plasmons of the metal particles can hybridize with photonic modes defined by the array spacing to result in collective excitations known as surface lattice resonances (SLRs). The benefit from photonics is that SLRs support high quality factors that are at least an order of magnitude larger than individual metal nanoparticles. The benefit from plasmonics is that the nanoparticles in the lattice show extremely high local field enhancements, several orders of magnitude higher than individual nanoparticles. Moreover, plasmonic nanoparticle lattices exhibit simultaneously short-range and long-range optical properties.

Plasmonic nanoparticle lattice and a lifted portion of the nanohole mask used in the fabrication process. Credit: Teri Odom

This class of periodically structured optical materials has a direct analog in electronic materials. They support optical band structures that are dispersive and with high symmetry points that depend on lattice symmetry. Their energy scales are defined primarily by the lattice periodicity and then finely tuned by nanoparticle material, size, and shape. The SLR modes can also be tailored by their dielectric environment. There is very large flexibility in the lattice design — and hence an expanding range of applications.

One application that has been of most interest to my group is nanoscale lasing. Our earliest work focused on answering fundamental questions: could this open architecture support cavity modes such that losses could be overcome by gain? Could we explain the mechanism of how stimulated emission occurred only within the electromagnetic hot spots of the nanoparticles? Was directional lasing at room temperature possible in a plasmonic system? [Answers: Yes, yes, and yes.]

The next advances were more engineering in nature: can the stability of the lasing device be increased? [Yes: instead of gain in a solid matrix, disperse the molecules in solvent.] Can the lasing wavelength be tuned in real-time, and reversibly? [Yes: integrate the device in a microfluidic channel or on a stretchable substrate.] Most recently, we have been interested in different types of gain materials, including colloidal quantum dots and upconversion nanoparticles-and which has resulted in exquisite lasing properties, from circular and radial polarized emission to continuous wave lasing with ultralow thresholds. These are just some examples of how flexibility in lattice design can drive different sets of questions and solutions for lasing.

Also, we have pursued these lattices for other applications, including as flat lenses by patterning the local dielectric around the nanoparticles, as photo-electrocatalysts for hydrogen evolution reactions, and as photothermal heating elements in programmable, self-regulatory materials systems.

Your work bridges the processes for creating materials and then actually using those materials for various applications. Which is the chicken, and which is the egg, when developing new research directions?

If I had to assign roles, I would say that the egg is the processes to create new structures and materials. Once we understand the fundamental advances and properties, we can then imagine a range of applications [the chicken] commensurate with the physical properties. But because chickens and eggs share the same "genetic material"— if you don't mind extending the analogy — advances in an application can also result in new processing directions. For example, our ability to pattern nanoparticle lattices over large areas (cm²) not only enabled us to access previously inaccessible regions in optical band structures, but also to integrate them into microfluidic platforms for tunable and reversible lasing. Later, we were interested in improving the uniformity of the nanoparticles for lasing. We wondered whether we could simultaneously thermally anneal and grow other materials to avoid adverse oxidation effects from certain plasmonic metals. The net result was the growth of few-layer graphene on Cu nanoparticle surfaces and a general approach to design new hybrid metamaterials.

What new developments or projects in your group are you most excited about?

I'm most excited about our work that shows unexpected results, or at least results that we weren't anticipating. For example, in almost all work published on plasmonic nanoparticle lattices, the optical properties are measured normal or at a slight angle relative to the substrate, but not at 90 degrees [parallel to the substrate]. We recently measured edge emission from in-plane SLRs that exist at Brillouin zone edges. What is very exciting is that simple rotation of the nanoparticle lattice results in different lasing wavelengths emitted from the sample edge, depending on which Brillouin edge was accessed. Other new developments in my group include combining different nanoparticle lattices into devices.

Rolled-up structure of a nanohole mask that emphasizes its large, patterned area and mechanical flexibility. Credit: Teri Odom

Are there particular advances or developments in metamaterials and nanofabrication that you are excited about or anticipating?

What I've appreciated about the metamaterials and nanophotonics community over the last decade or so is their inclusivity. When the concept of artificially structured materials "not found in nature" first appeared, the definition and properties were narrow in scope. However, the current expanded definition has enabled many more groups to make strong contributions that have resulted in new methods, especially regarding the integration of soft materials or biomolecules for assembling the nanostructured building units.

In my lab, I've been very excited about multi-functional nanoparticle lattices. Recently, we found that the properties of plasmonic nanoparticle lattices could be improved by thermal annealing. Not only did the quality factors of the SLRs increase, but also the stability of lattices even made from metals that oxide such as Al and Cu improved over months. Furthermore, besides its plasmonic properties, Cu can function as a catalyst for materials such as graphene. So, subjecting Cu nanoparticle lattices to the chemical vapor deposition conditions for graphene resulted in the growth of few-layer graphene on the nanoparticles, which I think is pretty neat. Also, we have used lattices as templates for core-shell lattices by growing Pt shells around the Cu nanoparticles. Since Pt is catalytic and Cu is plasmonic, we explored how these structures can be used as hydrogen evolution reaction photo-electrocatalysts.

Our next steps are to explore even more ways to take advantage of the materials' properties of the plasmonic lattices.

Your research is very multidisciplinary, do you consider yourself a chemist, an optical engineer, a nano engineer? How do you classify your work, and what skills are necessary to be successful in such multidisciplinary research?

Yes, full stop.

I am the Chair of the Chemistry Department at Northwestern, and I am a chemist — with the caveat that I also have broad and deep expertise in nanoscience. Nanoscience as a field has matured over 20 years such that now the use of the prefix "nano" implies the integration of two (usually more!) disciplines... and so, yes and yes to optical and nano-engineering.

Success can be defined by many metrics. For me and multidisciplinary research, the bar is fairly low. I am a broadly curious person. When I don't understand something, I try and find a way to do so — and I am interested in making many connections among different areas. An unintended consequence of this integrated approach is that often new insight emerges, which I view as success.

If I were to offer advice to students who are new to multidisciplinary research, I would say that success requires a confluence of factors: common scientific language across disciplines; motivation to gain skills outside of formal education and training; dedication to learning; and understanding the greatest impact of the research. The last piece of advice is a bit subtle, but understanding how different communities might appreciate the research is very helpful for framing and communicating the story.