“The many top international scientists YUCOMAT has welcomed as participants over the years, and the excellent science presented and discussed here, send the signal out to the world: science knows no borders.”
Professor Dr. Knut Urban, the laureate of the 2023 MRS- Serbia Award for a Lasting and Outstanding Contribution to Materials Science and Engineering talks to YUCOMAT about great leaps in science and nanoworld
Professor Urban, you are one of the pioneers of aberration-corrected electron optics. And you have received a number of scientific awards for it together with Harald Rose and Max Haider. Why is it of such great importance and at the same time so difficult to build electron microscopes with aberration-corrected optics?
According to the theory of lens optics and diffraction of waves, the dimension of the smallest details that one can just resolve in an object inspected in a microscope is at best of the order of the wavelength of the radiation used for imaging. In light optics, Ernst Abbe showed in the seventies of the nineteenth century that such quasi-ideal microscopic imaging can be achieved even with real, i.e., aberration-prone, lenses, if one uses systems of lenses, converging lenses and diverging lenses, calculated in such a way that their errors as a whole largely compensate each other. This marked the triumph of classical light microscopy, whose resolution limit today is actually wavelength-limited, at about 200 nm.
Since the wavelength of fast electrons is about 5 orders of magnitude shorter, it was hoped that after the invention of the electron microscope by Ernst Ruska, 1931, and Manfred von Ardenne, 1937, there would be a corresponding increase in resolving power and thus access to atomic dimensions. However, these hopes were disappointed. Despite the greatest efforts in academia and industry, by the end of the nineteen-eighties the resolution had reached at best about 0.5 nm. Resolving atomic distances in solids (< 0.1 nm) was out of reach, which seemed particularly tragic at a time whennanoscience (my field of work) was experiencing its first flowering. The reason for this was due to the failure to construct aberration-corrected electron optics. In fact, the insurmountable obstacle to this was a law of nature, Gauss's law of magnetism. According to this law, it is fundamentally impossible to use round magnetic fields to construct the diverging lens that according to Abbe is needed to realize corrected lens systems. To overcome a law of nature is indeed a formidable task. And although Otto Scherzer had already shown in 1947 that - in theory - it should be possible to construct a diverging lens by means of systems of non-circular lenses, multipoles, until 1991, when we started our project, one had made progress, but was still very far from success.
How was it possible that – against all odds – you then succeeded? This was in 1997, more than 60 years after the invention of the electron microscope and 50 years after Scherzer's pioneering paper?
Harald Rose's concept of correction was far better than any other design that had been proposed before. And we formed a team, a brilliant theoretician, Harald Rose, a student of Otto Scherzer, a brilliant experimental electron optician, Max Haider, and a materials scientist, that was me, who was eager to work with this novel instrument and thus to do materials science in single-atomic dimensions for the first time.
What were the first investigations that you did with this microscope from 1997 on? Were the 7 years worth it?
Yes, here we meet again the problem I am used to by now. The layman thinks, here we have the microscope, now we put a sample "under it", and then hey presto we "see" the atoms. Far wrong! What this great microscope does is something extraordinary, really extraordinary, it opens the door to the world of quantum physics. And every student learns in their first semester that you can't intuitively understand the quantum world. This is serious, and it is by no means different in atomic electron microscopy. You get all kinds of “images", some look like atomic structures, but others, most, do not. Can you tell me what an atom looks like? In particular, how electrons see it? So the first thing was to understand how atomic images are formed, and what kind of information they contain. Actually our first publications dealt with that; in fact we discovered a novel imaging technique nobody had even thought about. Since that time hundreds of papers have been published (by us, but most by other authors, of course) dealing with the nature of the atomic signal.
That this was a great approach to the nanoworld became clear when we studied the first materials science problems in superconductors and in oxides, atom by atom. Actually, by example, nobody had been able to see oxygen directly in materials before, but we did. This was a real breakthrough. And this is confirmed by the many thousands of papers that were published in the past 25 years – on atoms and atomic structures in solids.
When your team started, could you have imagined that you would have such success? After all, atomic images are an almost everyday part of materials science today?
To be honest: no, we could not imagine this. We had high hopes, but they were not very concrete. Microscopic access to atomic dimensions is a typical change of paradigm, entirely in the sense of Thomas Kuhn, who coined this term. Afterwards, one can no longer imagine what it was like before, i.e. before the innovation.
After this big step, what is next in microscopy?
This is a question I am asked again and again. Such great leaps are a rare phenomenon in science. Now it is a matter of painstaking, careful and comprehensive work to elicit further secrets from nature and to work them out quantitatively. Even today, new fields of application for atomic electron microscopy are being discovered. I consider particularly exciting the progress in picometer microscopy, the integration of computer-based image interpretation algorithms up to artificial intelligence and the involvement of aberration correction to achieve the grand goal of today, atomic resolution in biology.
Before you started studying physics at the university, you spent a significant amount of time at Siemens company as an apprentice to learn some of the skills in mechanical and electrical engineering. Do you think this was useful for your later career, for example also for the project to achieve atomic resolution in electron microscopy?
I have always considered this time to be particularly important and, in a way, formative. What is called "hands on" is of great importance for a physicist. One should have a feeling for materials and their processing, and above all a realistic idea of what is technically feasible. In addition, I think it is extremely important to know what the everyday life of a normal industrial worker is like.
Your great mentor was Professor Alfred Seeger and you also worked with Professor Ernst Ruska for quite some time. Both were famous scientists. Being a professor yourself for decades now, how do you observe the relationship between mentors and students today?
I'm glad you brought up this point. As a doctoral student and in the early stages of your postdoctoral work, you are already working in your profession, but you are still in a critical personal formation phase. During this time, the professor is particularly addressed as a personal teacher! And I was lucky that my supervisors were aware of this task and dedicated their valuable time to it. Unfortunately, in the age of third-party funding, young people are often regarded primarily as readily available labor, and the professor, for the sake of scientific achievement, not infrequently neglects his task as a teacher. I should add that difficult and challenging work topics are important for young scientists, because only in this way can they qualify themselves early and also distinguish themselves.
For a number of years, you held the office of president of the German Physical Society, one of the largest societies of its kind in the world, which has seen many famous physicists as presidents. This was certainly a great challenge, not least in terms of time. How can one manage to hold this demanding office and continue to be a successful scientist at the same time?
Yes, those were exceedingly busy times, and I had to learn to be efficient. On the other hand, the contact with other physics fields I had to deal with brought quite a lot to my own field. In this sense, electron optics was just one of the areas I worked on with my group. Other areas of work were scanning tunneling microscopy of semiconductors, and there was of course the physics of superconductors and oxides, for which we also built our own thin-film preparation techniques. We still hold the world record in high-Tc SQUID field sensitivity, and our superconducting microwave resonators flew on a space mission. And being a metals scientist by background, I found it particularly exciting to be able to perform (via the astronauts) microgravity experiments on melts of our quasicrystal alloys in Spacelab. I had excellent collaborators, and this interdisciplinarity in our own group also helped us in the advancement of electron microscopy applications. I firmly believe that interdisciplinary collaboration, as we also saw in our project to realize aberration-corrected electron optics, is indispensable in modern science.
Parallel with your research career you are and were active in knowledge sharing and promotion of science all over the world, including the participation in YUCOMAT conferences for years now. What does YUCOMAT represent for you?
In my speech at the banquet on occasion of the twentieth conference I appreciated the initiative of Professor Dragan Uskokovic and his colleagues to found the Materials Research Society of Serbia, to found YUCOMAT and to invite scientists from all over the world to Herceg Novi. This was an extraordinarily early moment in terms of political developments and the war on the territory that was formerly Yugoslavia. And it was a commitment to a common international culture and to common principles of humanity. Today again we have war in Europe. Yes, I support the idea that science can build bridges between nations, and this appears the more necessary at times when the politicians are obdurate and unable to talk to each other. The many top international scientists YUCOMAT has welcomed as participants over the years, and the excellent science presented and discussed here, send the signal out to the world: science knows no borders. I would like to express my heartfelt gratitude to all those who invite us to Herceg Novi every year and who take the burden to organize this wonderful and very personal meeting.
This year, I consider it a special honor to be presented with the MRS Serbia Award 2023.
Thank you all very much.
Prof. Dr. Knut Urban’s plenary lecture will be presented during the Opening Ceremony of
the 24th MRS-Serbia Annual Conference YUCOMAT 2023, at 9.00 am on September 04, 2023
