American Scientist

Combining materials science, physical chemistry, solid-state physics, and engineering, David Flannigan is using electron microscopy to make movies instead of pictures—probing materials at the nanoscale at femtosecond (one quadrillionth of a second) time scales. Director of undergraduate studies for materials science and engineering at the University of Minnesota, Flannigan is the recipient of the 2018 Sigma Xi Young Investigator Award, an annual honor established in 1998 for members within 10 years of their highest earned degree at the time of nomination. American Scientist’s Robert Frederick spoke with Flannigan about the growing field of imaging fundamental behaviors of materials with the use of what is now called ultrafast electron microscopy.

What was the process in developing this technology?

In [Ahmed] Zewail’s group at Caltech—even before he received the Nobel Prize for his work in spectroscopy—they had been working for some time on generating very brief electron pulses to interrogate materials. Around the time that I joined his lab at Caltech, they were transitioning to using microscopy to try to do it. So I think it was a natural extension of the work that had been done in the group—and even in other groups before that time—of using these very brief electron pulses to probe the atomic nature of materials.

Electron microscopes are exceptionally expensive devices. What was the decision process involved in taking this device and modifying it to try to do something new with it?

This gets to the heart of one of several aspects that were so wonderful about working in Professor Zewail’s group at Caltech: He had such a vision and a great group of people working with him that it really was not much of a stretch to think that it could be achieved. Yes, the investment is substantial. But the payoff is enormous. And if the vision is there and if the physics seems to make sense, why wouldn’t you go for it? Money is a concern. But if we can do it, we should and we should get past this idea of, “Well, it costs too much.”

Now, it is truly remarkable that we can do this kind of imaging. So the investment in terms of the money and then the modifications is secondary in that sense. Provided that we can convince others and show them that we have a proven track record, then the barrier becomes much lower.

As for the materials that you are imaging, what is driving those choices?

There is a large class of materials that are under study. Layered materials are really interesting to a lot of people right now: Not only do they have interesting fundamental properties—fundamental behavior that can be changed just by changing the amount of material or the types of atoms that are in that material—but they also may be useful in practical applications and devices, whether solar cells, transistors, or what have you. Because they are layered materials, they can be made very thin, and can be made flexible. I mean, your imagination takes over.

When I think about having an application that is new and provides access to an experimental parameters space, if you will, that largely has not been explored in the past [that is, making videos instead of pictures], I start getting excited about materials that we think we fully understand—that we have studied extensively for decades—and about going back and taking another look with these new techniques. Are there things we have missed? Is there some dogma in the science literature that we can maybe overturn? Are there new physical insights to be made—if you can do that—on a material such as silicon, germanium, or something that is just “run of the mill,” and can we make new discoveries? That is profoundly exciting. Not only does it further illustrate that we still have a lot that we can learn and understand, but it also drives home the power of these new techniques and the importance of technique, method, and instrument development.

Hopefully, that excitement translates into funding to help us build those tools that—you are right—are expensive. So you could extend this technique even into biological materials, and on and on: There is a whole host of explorations that could be made with these techniques. When they are new, they are largely unexplored. And that is the fun. We are just beginning.

To probe these materials, it sounds like you really have to understand a lot about the material you are probing and why, so that you can understand the results. You have to be much more than a tool maker.

That is absolutely right. There are two things I can say about that. One is that benchmarking—if you want to call it that—is critically important. These are new types of measurements. We are getting data that we just have not gotten before. These are new things we are seeing. How do we deal with that? How do we handle those data? How do we get the important information out, and then what does it mean? Can we tie a new measurement or a new observation—at least in terms of the data—to a known phenomenon? For example, we understand from these types of measurements that this behavior will occur, but how might that connection show up in our measurements, and can we look for those signatures in our measurements? Once we have a basic understanding of the signals that we are measuring, then we can explore new areas.

That being said, a lot of discovery is involved. It seems almost every day I talk to my students and we have something new that we just do not know: We do not know what is going on; we do not really understand it; we do not know what is generating the signal, this response. So that becomes a fun challenge, trying to tie what we know to what we do not and coming up with some good physical explanations for the new data that we see.

It sounds like there is no shortage of problems to study coming from your students, but how about from potential collaborators?

I do not know if I should say this, but we have more people wanting to collaborate with us than we can possibly accommodate. It is just—if I had a few more instruments, maybe. But it is just not possible, I think.

And that all speaks to the excitement about the technique and the possibilities.

National labs are becoming interested in developing these sorts of techniques and allowing their user base access to these types of measurements. I think that is a direct indicator of the excitement. It will be wonderful if we can get more people involved in doing these types of measurements. It will be really exciting.