Our work as scientists, as any other human activity, is often influenced and even shaped by personal taste, character attitudes, cultural background and inclinations, and in some case (unfortunately) by preconstructed ideas. The network of teachers, students, friends, collaborators, that each one of us builds around over time is unique, and forms the very basis of our ability to learn and progress in science. I am often confronted with others about our working styles and preferences. For example, I have a very good friend who constantly points at my attitude of periodically switching research subjects, and entire scientific domains, which he considers detrimental. He purports the advantage and integrity of pursuing a stricter career path, by focusing on just a few related subjects and trying to go as deep as possible in the investigation, whereas my own attitude is rather deemed “horizontal”. But that’s just me, nobody ever taught or advised me to work in this way, nor I took inspiration from some “role model”; rather I believe this is but the reflection of my personal life history and character. I get easily bored.

Hence, during the second part of my Argonne stint, around 1997, I started getting utterly bored with cracks and dislocations in metals, and with the help of my best friend in science Luciano Colombo, I started working on the correlations between the atomic structure and electronic properties of covalent materials. Leveraging on the enormous knowledge available at Argonne on grain-boundary physics and chemistry, most notably in the person of our group leader Dieter Wolf and his lifelong experimental partner Karl Merkle, we thus started to investigate the electronic structure of grain boundaries in pure silicon and carbon with tetragonal diamond coordination. At that time, DFT calculations were still too heavy for the system sizes we needed, not to speak of doing molecular dynamics, so I adopted the excellent order-N tight-binding technique (TB) developed by Luciano and Stefan Gödecker a few years before. It was at this time that I made the acquaintance with one mostly extraordinary scientist and an exceptional man, who was then a group leader in the Materials Chemistry Division, named Dieter Gruen. He had just developed a revolutionary synthesis technology to obtain nanostructured diamond at nm-sizes, and after a brief discussion together, he’d immediately catch up the possibility of applying our theoretical models to try to explain some of the mechanical and electrical properties of his fully-dense nanodiamond mats, which they called “ultra-nano-crystalline diamond”, or UNCD.

I worked out large-scale TB molecular dynamics simulations of high-energy grain boundaries in diamond nanocrystals at finite temperature, including about 1,000 carbon atoms described by 4 localized electronic orbitals each, which for the year 1998 was indeed quite a challenge. The simulations showed that C-C bonds across the grain boundaries bring about a complex electronic structure that can be quantitatively described in terms of sp2 partial hybridization. Our idea was that, with its about 10-15% of the atoms lying at the boundaries of 5-7 nm nanocrystals, UNCD could lower its total free energy by bond rehybridization at grain boundaries, rather than by bond-bending and distortion as is the case with nanocrystalline silicon grain boundaries, that preserve sp3 hybridization completely. In two subsequent papers, I also calculated electron energy-loss spectra, and constructed a theoretical model of electrical conductivity by multi-phonon assisted hopping across the sp2-bonded network of grain-boundary atoms, which fitted nicely several experimental data of D. Gruen’s group. For me as a physicist, the problem was solved: case closed, I was ready to move on to something else. The two papers got a decent citation record, and were followed by several similar studies, which Dieter endeared to pursue with other more materials-oriented simulators, such as Larry Curtiss, Peter Zapol, and others. In retrospect, I think in this case I was too lighthearted in not pushing forward the collaboration with Dieter (we just co-authored a brief communication to the 2000 MRS Fall Meeting). That’s life, when you are a “subject-switcher” like myself (maybe my friend above, the one who regularly scolds me, has a point in this case).

However, my friendship with Dieter was to last, and I am extremely proud of humbly counting him among my friends, one who left an important mark in my path in science. I developed the greatest admiration for his life story, both as a scientist and as a person. The list of his achievements in science and of the awards he received is impressive (you can find it in his MRS Medal address, reported in the MRS Bulletin of October 2001). He was part of the Manhattan project in 1944-45, and by now, he is likely one of the only 2 or 3 survivors of that incredible scientific adventure, which shaped our modern world. We continued discussing the most varied subjects in science at every possible occasion whenever we met, and I always found him incredibly far-sighted and illuminating. Last time I met him in person was at the Argonne cafeteria, in the summer of 2015, and we had a long chat about the future of thermoelectric materials. This extraordinary man named Dieter Gruen will turn the age of 100, just this Monday, that’s why I wanted to dedicate him this ‘special issue’ of my Sunday letter. I hope you will now enjoy hearing (ehm… reading) Dieter to speak in first person, from two interviews he gave to The Atomic Heritage Foundation, and to the American History TV on C-SPAN, a few years ago.

“I was born Nov. 21, 1922 in Walldorf, Germany, a small village near Meiningen and Thuringia, not far from Weimar. My childhood was very pleasant, I went to the primary school there and did all the things little boys do. Son of the school principal, by 1936 I was no longer able to continue school because the Hitlerjugend would beat me up after school. Being Jewish, it soon became clear that if I had to survive, I had to leave. Fortunately, I had relatives in Little Rock, Arkansas. My uncle was a personal friend of Sen. Joseph Taylor-Robinson, the majority leader of US Senate during the first Roosevelt administration. My uncle wrote a letter to him on my behalf, and he in turn wrote a letter to the American ambassador in Berlin. So, when I went to pick up my visa in the summer of 1937, they rolled out the red carpet for me! Then I came to the US, went to school in Little Rock, and when my parents got a visa and left Germany, in the early 1939, we reunited in Chicago. And that’s where I have been ever since.

I entered as a student at Northwestern University [ Evanston, north of Chicago ] in 1941. To complement the war effort, it was the policy of the US government to encourage students which were in their final years, to continue their studies in view of useful applications. So, I could continue to study physics and chemistry until 1944, when I graduated cum laude under Irving Klotz. Then, I was supposed to go work at the Metallurgical Laboratory of the University of Chicago, which was part of the Manhattan Project. But in 1944 the Met Labs had already achieved their objectives: the first one, under Fermi, was to create the first self-sustaining chain reaction, which was done in December 1942 [ and that’s a story for next Sunday ]; and the second one, was to develop the separation process for plutonium (the process discovered in 1940 by Glenn Seaborg, at Berkeley) that could be scaled to kilograms production, and that was achieved by mid-1943. So, by the time I finished my studies the Met Labs were no longer in need of manpower. Therefore, I was asked to move to Oak Ridge, to work on the uranium project. I jumped on a train and went from Chicago to Knoxville, and then took a bus to Oak Ridge. It was September 1944, and I was barely aged 22.

I didn’t know what would be the subject of our research (well… it was top secret!) but when I arrived, I said that I wanted to be in the research division rather than in engineering. I was located in the Y-12 campus, and used to sleep in a dormitory together with the construction workers of building K-25. I was assigned to work with the director of the chemistry programs, so I went to his office, and he took off from a safe a book of chemical physics. He opened it at the page on uranium and told me: “This is what we’re working on.” However, we were not allowed to mention the word ‘uranium’; actually, uranium had the code name of ‘tube alloy’, because the uranium produced in Great Britain was made by the Tube Alloy Company. So, be it lunch conversations or the title of a document, we would always call it ‘tube alloy’. I was involved in different areas of the chemical studies, for example how to synthesize large amounts uranium tetrachloride, that was the first step to U-235 separation. I also studied the problem of removing U-235 oxide from the graphite getters, burned in oxygen. For this, I had to synthesize an indicator, a color-dye, that could be stable in nitric acid. In fact, in the final stages, uranium peroxide was dissolved in nitric acid, and the resulting uranium nitrate was then extracted into an organic solvent. It was then necessary to determine the interface between the aqueous phase and the organic phase. I invented an entirely new material for this purpose, a blue-colored sulfonated copper phthalocyanine.

The electromagnetic separation process was done by the ‘calutron’ (a shorthand for CALifornia University Cyclotron), a kind of mass-spectrometer element obtained from recycled parts of the 4,7m cyclotron magnets. In 1944, there were more than 1,000 such calutrons built at the Y-12, working in series and parallel lines in the ‘alpha’ and ‘beta’ processes. We thus could produce several kilograms of U-235, between September 1944 and about May 1945, which was shipped to Los Alamos to find its way into the Hiroshima bomb. Differently from the Nagasaki one (that was a plutonium bomb, like the one of the Trinity test in Alamogordo), this was a uranium bomb, and was never to be tested before the launch over Hiroshima. That was the end result of our labors. However, I note that the Smyth report of 1946 does not ever speak of chemistry or chemists. Only the physics parts of the project were declassified at that time, so the physicists took all the ‘credit and merits’… so to speak… This is one big untold part of the story. However, the chemistry reports I wrote myself were later declassified, and I was able to obtain them from Oak Ridge, after some years.

At that time, we knew we were working for a kind of atomic bomb, but I knew nothing about what was going on in Los Alamos, the bomb design, and all the problems of turning our kilograms of uranium into a weapon. No one ever told us the final scope; we could only kind of figure it out on our own. But we had no thinking about the consequences, before the bomb was dropped. It’s only after the war that I became very involved into the moral reflection about the events. One of the things that we learned was that there are no secrets about the atoms, and there is no defense against atomic weapons. Hence, we started an organization called The Oak Ridge Engineers and Scientists, to create public awareness and prevent this to happen again. Together with J. Balderstone and D. Waymire, we sent a letter to more than a hundred outstanding personalities of the time, including many scientists, and even Einstein, asking: what could be done to prevent nuclear weapons to be ever used again? We received responses from nearly everybody… Einstein said this could only happen with a united world government. I am still waiting…

In April 1946 I went back to Northwestern, to finish my master work and start my PhD, but in the meantime was hired at Argonne. So, I ended up with the University of Chicago, where I did my thesis on the magnetic susceptibility of neptunium compounds. When I was still a graduate student, one day Enrico Fermi showed up in our lab, because my mentor there was using a new technique of spin resonance spectroscopy, which Fermi wanted to know better. So, he walked around the experiment for about five minutes, attentive and in silence, then he said “OK, I see how it works!” and left! Actually, our apartment was at 53rd street and University Avenue: Fermi would ride his bike past my window every morning at 8am, so I knew when to get up and go to his lecture. He was the most marvelous teacher… he and his grad students had lunch in the commons every day. There I got in contact and became great friend with Glenn Seaborg, we worked on the electronic structure of the actinides.

Those were great days at University of Chicago, we had such professors as Fermi, and Edward Teller, and Harold Urey, and Willard Libby, so many Nobel laureates. The Argonne laboratory, established in 1946, became the national center for developing peaceful uses of atomic energy. My early work at Argonne was strongly influenced by my experience on the Manhattan Project, and the thinking that atomic energy could be also used for the benefit of humanity, and not just for its destruction potential. The first reactor built there was for the naval submarine, the Nautilus, which had lots of interesting chemistry problems to solve. For a first work, I developed a process to remove hafnium impurities from the zirconium used in cladding of fuel elements. I spent many years studying nuclear fission reactors, and was a US delegate at the Second Conference on Peaceful Uses of Atomic Energy, in Geneva, 1958. Later, I became interested in fusion energy and many of the chemistry issues therein. But after all these years, I don’t see either of these sources as technologically viable solutions for the next future. That is why in the last part of my career I moved to solar energy.”

I wrote an e-mail to Dieter last week, to prepare for the celebration of his 100th birthday, and asked him to kindly write a conclusion for my ‘special issue’ letter, dedicated to him. This is what he replied:

“Dear Fabrizio [ … ] Since I retired some ten years ago, I have stayed busier than ever pursuing the goal of a “hot carrier” solar cell with a theoretical conversion efficiency of 86%. Using my own savings, I was able, with the help of a university team, to create a graphene/ZnO nanowire prototype with a single junction open circuit potential of 2 V and a short circuit current of about 1 ma/cm2. After much toil, investors were found to continue this effort in order to optimize the performance of the cell. Success in this venture would make it possible to create a global grid for the production of plentiful and economical PV electricity. “Subtle is the Lord”, as Einstein used to say! Light/matter interactions that depend on the remarkable quantum electrodynamic properties of graphene are at the heart of this activity. […]  Hurrah for the inputs to our knowledge of the workings of the Universe made possible by science in all its marvelous manifestations!”

Last boy standing

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