Sciences : histoire orale

STENSGAARD Ivan, 2001-03-08

Sophie Jourdin — 2011-11-11T21:40:34Z

Ivan Stensgaard received a PhD in physics from the University of Aarhus, Denmark, in 1977. He subsequently spent two years at Bell Labs in Murray Hill, New Jersey. From 1979 to 1998 he was Associate Professor at the Institute of Physics and Astronomy of the University of Aarhus. From then on, he has been a Research Professor at the Center for Atomic-Scale Materials Physics (CAMP).

From the 1970s onwards, Stensgaard has investigated surfaces. His primary tool before the advent of the Scanning Tunneling Microscope was ion scattering. His first publication investigated radiation damage in reactor materials, and soon he moved on to investigate other surfaces, such as platinum, tungsten, and silicon. He used both backscattering and channeling of ions to infer the reconstructions of crystal surfaces. Most of his research employed ions in the MeV range, but he has also worked with low and high energies.

Because of his extensive knowledge of surface structures and the tools used to gain knowledge about them, he was in a good position to judge the efficacy of the scanning tunneling microscope when it came upon the scene in the mid-1980s. In collaboration with Flemming Besenbacher and Erik Lægsgaard, he built up a research project (CAMP) that relies extensively upon this new tool.

In our interview, Stensgaard outlines the history of CAMP and describes some of the many uses to which the STM can be put.

IVAN STENGSAARD (IS) : In the early the days, the way I recall it at least, in the STM field, most people worked on semiconductors, where the advantage is that the corrugation revealed by the STM is generally much larger than on metal surfaces. That means that the demands on the equipment are much more modest. So it was easier to enter the field of semiconductors - and of course also the technological implications were thought to be greater in that field. But when we entered the field, which must have been around 1987, there were few groups working on metal surfaces. So although the STM was really recognized as an excellent tool, there had been some delay in really using this as a common tool in studying metal surfaces.

ARNE HESSENBRUCH (AH) : But during the period of '82-'87 you would have known about the STM ?

IS : Yes.

AH : And it was something that you perhaps watched ? You watched the development of this instrumentation and you were critical to see whether it would prove its mettle for you to begin to use it ?

IS : Yes. Well we were not, as I recall it, completely convinced that it would add a lot to the insight into metal surfaces. But as soon as we did enter the field ourselves, placed the instrument in vacuum and seen the results, we were immediately convinced. I'd say from then on it has been fantastic : it has been quite easy to achieve good results with just about any topic of investigation. One saw new phenomena almost regardless of what one studied—new and completely unanticipated phenomena.

AH : So you're saying "we" now, this is presumably the CAMP group. When did that gel ? Was that in '87 ?

IS : Yes, I do mean the CAMP group. But we began collaborating earlier. Historically, I think it was like this : I returned from the US and received this grant for building up an ion scattering facility for studying surfaces [referring to an earlier mention that is not on the tape – this part of the interview will be repeated in the near future]. Some years later, Flemming Besenbacher and I worked together on ion scattering from surfaces. We looked at small molecules, and specialized on the examination of hydrogen on metal surfaces. Then in the 80's, probably around '85 or so, we began to discuss in more detail this question of building a scanning tunneling microscope. Flemming took the initiative in those days and contacted Erik Lægsgaard. Erik was really the key person in building the STM here. And from when he started until he had a table-top version, giving atomic resolution on graphite, lasted only a few months. Of course it took longer to make an instrument fitting into a vacuum system, and so on.

AH : Did you make it locally ?

IS : Yes, everything here has always been homemade.

AH : You've never bought instruments from the outside ?

IS : No. We tend to claim here that our instrument is better than what we can buy. It may sound like boasting, but I think our results help prove the contention. The fact that we can work with extremely rudimentary vibration isolation, proves that we have an highly stable instrument.

AH : So what were the characteristics of the design – the Aarhus design ?

IS : Well, there are two basic ingredients in the construction : one is the scanner tube scanning the tip across the surfaces, and I think that was copied simply from the Binnig and Rohrer design.

AH : The empty cylinder.

IS : Yes. So, the larger problem was how to approach this scanner tube tip to the surface in a controlled way so that you would come close enough to draw a tunnel current without actually colliding. To begin with we had many ideas about some mechanical way of doing it with micrometer screws, but soon we focused upon piezoelectric materials. And I think, again, that it was Erik's idea to make an inchworm. This is a really strange story. We had a very small inchworm made : only a few millimeters in diameter - maybe 10-12 mm long. It worked well and a company called DME in Copenhagen wanted to commercialize our setup. During the discussions and negotiations with them, they found out that the inchworm principle was already known, and what was worse, it was already patented. That was a something of a setback - not for our in-house microscopes but for the efforts to commercialize it. The commercially available inchworms were much bigger and not nearly as elegant as ours. But the principle was patented, and the patent had a few years left before expiration.

AH : Oh, so the patented inchworm was not intended for the STM ?

IS : No. The patent was about 10-15 years old and had been developed for something else. Whether it was developed for one of the topografiners and what they were called, the instruments used prior to STMs, I don't know. We can check the patent.

AH : Well, the topografiner was invented at the National Bureau Standards now NIST and I don't think they would have taken a patent.

IS : We could check it.

AH : So you designed the inchworm in '87, just when you were making your first STM ?

IS : Yes.

AH : But your first STM did not have an inchworm.

IS : Yes, it was a mechanical approach.

AH : Where did you get all the information for making an STM ? Did you look at the publications of Binnig and Rohrer, did you go and see them ? Did you go to the IBM labs ?

IS : No, the way I recall it, we had no personal contact. I think everything was done just using the publications.

AH : And it wasn't particularly difficult ?

IS : Not if you had a clever man like Erik. The short time required to make the first STM also indicates that it was fairly easy. Of course in retrospect many aspects of building an STM seem easy, because we know just how piezos work and what to pay attention to. The game nowadays is really to make them as compact as possible, to make them work even with vibrations in the surroundings and so on.

AH : Yes, you were saying you had very elementary vibration damping. This is another characteristic of this design ?

IS : Yes. Exactly. When you visit other STM labs, you're frequently told that they have to work at night when there is no traffic. They have all kinds of vibration damping, entire vacuum systems hanging in rubber — springs from the ceiling or something like that. They have the microscope damped with magnetic or eddy current damping. And people had to stay quiet and not move about in the room and so on. That remained characteristic until at least ten years ago. By contrast, from the beginning we hung our microscope on three or four springs, or viton-o rings, which are really rubber springs. That's all ; we have no further vibration damping. And people can even touch the UHV chamber without interfering with the tunneling. So that has been the main characteristics of the instrument here : its ability to scan fast and under adverse conditions.

AH : Fast and robust. You have told us about the robustness, could you tell us about the speed ?

IS : Fast scanning is interesting also because it has opened up new opportunities compared to other microscopes. There are more fast-scanning microscopes today but to begin with we scanned faster than most others. We scanned at the rate of one second per image, where many others spent one minute per image. This gives you the opportunity to follow dynamic processes on surfaces by sequentially recording images and then replaying the images at a faster rate - in the form of a movie. That gives you a very vivid impression of what happens on the surface. You might object that a movie yields no new information, but it turns out to be a very good means of attracting attention to dynamic processes. Such phenomena miss easily when just examining static pictures. The eye is well-suited to spotting change, so that even when there is a lot to keep track of, say 20 to 40 entities on a surface, your eye will immediately pick it up a movement of any one entity. So this question of scanning fast and taking movies, is not just a question of presenting results in a nice way, it also gives you a better insight into what is going on.

AH : But you weren't capable of making movies off the bat in 1987 were you ? How long did you need to scan in 1987, roughly ?

IS : I don't recall it really. I think we could record images in seconds, but the problem was probably rather that saving and replaying images requires a lot of data storage, and in those days the hard disks were not very big. It was slow process simply to get an image from the hard disk onto the screen.

AH : Was your ability to scan quickly a desideratum in 1987 ?

IS : Early on we probably didn't focus on the time you spent recording. It probably didn't matter whether it was 1 or 20 seconds or so. But I see now [checking on the computer] that taking movies and things like that, must have started in the late 80's. Because we have publications at least from '91 where we have included, not movies because that wasn't possible, but results based on these techniques.

AH : And what sort of thing did you image ? Metal surfaces with adsorbed atoms ?

IS : Yes, of course the standard sample in those days (as it still is for testing an STM in air) was graphite. Of course we used that as the test that our STM was working properly. Having done that we moved rather swiftly in the surface science direction again. We examined some gold-plated plastics, simply to investigate the micro-roughness. The purpose was to test the coating of large mirrors of an x-ray telescope to be used in a satellite. We also investigated some biological samples. But we were really implementing the STM in a vacuum.

AH : You say you were immediately convinced, when you saw the images, that this was a useful tool. It was seeing the images that convinced you almost instantly ?

IS : Yes

AH : Why ? There are many problems with the imaging process of course, with the noise and the interpreting just what the tunneling current actually is referring to. On metals this is less of a problem than semiconductors. Was that a help to you ?

IS : Yes, to some extent it was. Let me just mention the first system we examined, to maybe make references to that. Once we got the microscope into a vacuum we continued our work on small molecules on a metal surface. The first system we examined was one I had investigated earlier using ion scattering. We had absorbed oxygen on a particular copper surface, a copper (110) surface. And it was known what happened to the oxygen, how the surface reconstructed. What was not known was how the process took place, and that was revealed immediately almost with the STM, because we could look upon the surface while the reconstruction took place, and we could make a movie and watch the process.

AH : That must have been exciting.

IS : Yes, and then I was at least totally convinced at that time that here was an amazing tool.

AH : So the problems that there might be with interpretation were really secondary to you, you said that even with all those problems, "it's still exactly what I need ?"

IS : Yes, you said that in the case of semiconductors it is more difficult to relate the STM-produced image to the surface corrugation. That is correct of course, but you could say that we had a similar problem on our hands with oxygen on a copper surface : were we imaging the copper or the oxygen ? And it turned out that we were usually imaging the copper. This posed the question : why not the oxygen ? And why is it sometimes just the other way around, that we image the oxygen and not the copper. So scanning on metals was not completely free of problems, at least not when you have the adsorbates on them. But if you image one clean surface with only one element present, then the interpretation is straightforward.

AH : What I'm trying to understand is that when you use a completely new tool, you cannot know whether it works.

IS : I think the problems of instrumentation were so insignificant compared to the insight gained, that we were absolutely convinced of the amazing utility of the instrument. That's the way I recall it, really. There was a trend in those days—probably through the 90's—from static to dynamic problems. Other methods, such as low-energy electron diffraction, high-energy ion scattering, x-ray diffraction, are better suited for static problems. You can induce a reaction on the surface, and then you can take your tool and find out how the surface has changed. But with the STM you had a tool with which to see the transformation from one state to the next. Later on, you could even follow single atoms moving on the surface. That was really amazing, and almost impossible with other techniques. Field ion microscopy has been able to do it in some cases.

AH : Before the STM, you were using tools like ion-scattering, but it was not the tool that defined your research. Now it looks like the STM defines the group.

IS : I don't really think that's correct. No matter what you investigate,
you have to take both things into account. You have to find an interesting problem amenable to investigation with your background, its relevance and so on. Then you have to consider the available techniques. You have to find a topic amenable to the techniques available to you. I think that also describes what we did with the STM. It is a question of resources. The microscope does not define the activities ; a natural development has rather taken place. I mentioned before that we first investigated small molecules on surfaces, and we have continued to do that. Our research has now diversified but it is still to a large extent coupled to chemical reactions and the like. We have studied surfaces in greater detail – also clean surfaces although clean surfaces are less interesting for STM research because one can get the information by other means. But when you grow metal on metal for example, what happens ? How do these atoms of one metal, influence the properties of a surface of another metal. How does the surface restructure ? And, to come back to my initial point, how do the chemical properties change ? What happens if we now adsorb oxygen, what happens if we carry out a chemical reaction, let's say oxygen and CO reacting on the surface. In all these cases there has been a connection to the initial questions.

AH : Have you expanded the number of chemical reactions you worked with ?

IS : Over the years we have worked with a number of chemical reactions, but in the last few years we have focused more on catalysis. We have for many years collaborated with Haldor Topsøe AS, one of the leading companies in the catalytic world. We have collaborated on a number of issues, in once case a new catalyst was developed on the basis of our results. We proved scientifically that the catalytic conversion works. Whether that catalyst in the end will be commercially available now depends on other considerations, such as the cost of producing a catalyst, its lifetime and so on.

AH : But you discovered new catalysts with the STM, is that what you're saying ?

IS : We discovered a system that turned out to have possibilities in catalysis. We could talk in more detail about that, but it's a story in itself.

AH : Okay. What can you do now with the STM that you couldn't do in 1990 ?

IS : There has been constant improvement in speed : the rate at which you record and replay images. And new designs have made the microscope usable under new conditions. It now works over a large range of temperatures and pressures. To begin with experiments could only be carried out at room temperature, whereas we now can work from 20K or so and up to maybe 150ºC (500K). This increase in the temperature range is important for the investigation of dynamic processes because the rate at which they proceed is governed by temperature. And if you wish to study a certain reaction or a certain diffusion property, you have to have the events happen at a rate you can follow. That means that you shouldn't have too many events per second, preferably no more than one event every few seconds. And you can normally do that by adjusting the temperature.
We can also work at many different pressures now. Early on we had to work either at atmospheric (ambient) pressure or else in a very high vacuum. We have recently become able to work in the full range of pressures from ultra high vacuum to atmospheric pressures and could in principle make the system work at even higher pressures, if it weren't for some windows unable to withstand atmospheric pressure. And that again is advantageous for catalytic research, since all catalytic processes run at high pressure and high temperature. In the surface science community there has always been arguments that their field would help improve catalysts and understand catalytic reactions. That is to some extent true, but there has also been a lingering skepticism whether the processes taking place at atmospheric pressure remain the processes governing chemical reactions while raising the pressure from 10-10 Torr to atmospheric pressure. So that has been an important result : that at least in the one case that we have done so far, the system behaves identically at high pressure and in an ultrahigh vacuum, it's exactly the same things that govern the processes.

AH : What do you have to change in the STM to make it amenable to the range of pressure and temperature ?

IS : Actually not much was changed ; it's basically the same setup. The only major change is that it is located in a minor vessel ; it's in a minor chamber with a smaller volume, which is the volume that is brought up to high pressure. The suspension system has changed also, but the basic STM is the same. The suspension system is still based on a few springs, and looks differently, but the system as such is really the same. And of course minor things like gold coating surfaces to avoid reactions of those, but the heart of the microscope is completely unchanged.

AH : Why didn't you do these things 10 years ago ?

IS : I think in the field of ultrahigh vacuum there have been so many extremely interesting topics that there simply hasn't been the capacity to look in other directions. We should bear in mind that when CAMP was established in '93, much more funding became available than before. And the group grew much bigger. And although to begin with we also focused very much on processes at low to high vacuum, then at least during the second period of CAMP (from 1998 onwards) we have been able to diversify. High pressure is simply a natural development out of our original research. When I refer to new fields, I mean investigating organic molecules and other biology-related issues. So our research program has really developed quite naturally.

AH : So you started CAMP in 1993 - you, Besenbacher and Lægsgaard.

IS : The history of CAMP is more complicated – it is related to changes in the funding of Danish research. A large insurance company, Statens Livsforsikringsanstalt, was sold and a fund, Grundforskningsfonden, was set up with the money from there : 1 or 2 billion Danish kroners. Interest from the capital there was to be spent on basic research in all fields. In those days the interest rate was high and leading to a fair amount of interest each year. They requested and received many proposals for research centers. I don't recall exactly but it was a very large number. And in the end they funded 23 centers in all kinds of science : medicine, humanities – Søren Kierkegaard. We got a center too, a collaboration between our group here at Aarhus and Jens Nørskov's group at the Danish Technical University at Lyngby. The group there focused mainly on theory, and Nørskov was the director of the center, as he still is technically. The reason the two groups joined in this project was that we had had a prior collaboration within another program called FTU, research for technological development. In addition Flemming had collaborated with Jens Nørskov on another topic. So we knew each other. A proposal was made based on the joint resources of these two groups and after some external referring procedures it was eventually funded.

AH : And the project was focused on the STM ?

IS : I think the scope was broader : the interaction between small molecules and surfaces, to some extent chemical reactions. Our tool of investigation was STM but the center as such was not a STM center, it was a center doing research in a specific field. And our tool of research was the STM.

AH : You mentioned that you had contacts with Topsøe and other companies. Have you had contact with chemists, people in chemistry departments ?

IS : We have, but not much.

AH : Are they not interested in this ?

IS : I think they are in many cases, but we work in surface physics, as it is called in Danish. The English term, "surface science", is more neutral. Of course what we do is to some extent surface chemistry. But our background is just physics. You could say that a center like this might just as well have been placed at an institute of chemistry. It just so happens that we had this collaboration, and that we had not had the opportunity to interact much with chemists.

AH : So it's not the barriers between physics and chemistry that are particularly high, it just hasn't happened ?

IS : It just hasn't happened. It might have worked out differently, had we had a common institute instead of a separate physics and chemistry institute. Some of the activities going on here might as well have been placed in the institute of chemistry and vice versa. Nowadays it's not that natural to have this division into two institutes.

AH : How far is it from here to chemistry ?

IS : It's just across the parking lot, 100 meters. We're even connected underground to them, although it takes a specific key.

AH : But you don't go to each other's seminars ?

IS : Very infrequently.

AH : This does seem surprising and in need of an explanation. No ?

IS : I think much of what we're doing is of interest to chemistry as such, but our institute is comparatively small. Chemistry and physics are amazingly broad fields and our small institute is only able to cover certain areas. It has just so happened that we work in a field without a natural counterpart in the institute of chemistry. There are no ill feelings on the other side of the parking lot ; it's not based on that.

AH : And what about chemists in general out there in the world ? Do you get resonances ? Where do you publish, for instance ?

IS : We publish in papers with as high an impact as possible. We have published very large number of Physical Review Letters during the period of CAMP, on very few occasions in Nature and Science also. But that is not that easy, and sometimes you feel it is more difficult being European than if you had been American, but that's another story. We publish of course also in other journals ; some material will go to Surface Science. Surface Science does not have that high an impact, but it is obviously a very natural journal for people working in surface science. In my opinion the quality is rather mixed, but these are the main journals that we choose. We do not publish much in European journals, or journals that are particularly European such as European Journal of Physics and European Physics Letters.

AH : For any particular reason ?

IS : Simply because impact and visibility are low.

AH : When you say that in Physical Review Letters the visibility is high, is it not limited to the physics community ? Isn't your work broader ?

IS : It's probably correct, but we have colleagues in other places working in chemistry departments. And some of them will also publish in Physical Review Letters. I think it is recognized as some natural place for good work, also. But we have also published in Chemical Physics Letters, there are also a number of publications there, but they're not quite as prestigious as Physical Review Letters.

AH : Are you under pressure to produce results of commercial interest and if so, does this conflict with your own interest in pure science ?

IS : I don't' think so. I don't think we are under much pressure in CAMP to produce results of direct relevance to industry. I do think that we in CAMP all see an extra benefit or satisfaction in producing something of relevance to the "real world” and not just to pure science.

AH : So when you get around to renewing CAMP for another five years, the grant-giving bodies will not look to commercial applications ?

IS : It won't come to that. The funding will stop after ten years. It's the general policy that centers such as ours may reapply only once, so that they run for a maximum of two periods. After two periods, they ought to find alternative funding and in some way become a more integral part of the institute. What will happen in two to two and a half years from now, I don't know.

AH : You are not worried ?

IS : Not terribly, I think some kind of funding will come. I think that the government has set aside some funding for integrating centers into their host institutions.

AH : The reason I'm asking these things, is that Jens E. T. Andersen at the DTU was telling me about tremendous pressures. The DTU has now a patent office, and renewal of academic contracts is subject to results in terms of patents rather than scientific publications.

IS : The emphasis on patents has definitely increased here also, but in my opinion it has not yet become a pressure. There is a greater emphasis on it, and some of that emphasis originates from the government. A new law was introduced in Denmark. Up until maybe a year ago, you had the rights to exploit your own inventions, even as an employee and civil servant. That has now changed so that when you develop anything patentable or otherwise exploitable, you have to first contact the university. Your invention will be assessed. The university decides whether it wants to patent your invention and exploit it. If it declines, then you are free to do it yourself. So you could say that maybe the timing is a little bit strange, because on the one hand you remove some carrots from the researcher and on the other hand you apply the stick to encourage work resulting in patents. I think the general political perspective is that a fair amount of knowledge of interest to industries is generated at the universities, but no good forum for the transferal of that knowledge exists. Maybe the thinking was that having the knowledge documented in patents would facilitate the transferal.

AH : Is it possible to pinpoint advances made in the field of catalysis over the last ten years related to the possibilities of the STM ?

IS : I think this story about developing a catalyst from scratch based on pure research is rare, almost unique. There are no more than one or two other examples. Normally catalysis has been a highly empirical field. Developments have mostly been trial and error - of course guided by different kinds of knowledge. But in the end it came down to trying something, modifying it, changing the stoichiometry or the ratios, optimizing and so on. The understanding of the catalyst emerged gradually out of this process. By contrast, ours is a clear case of going directly from scientific insight into the working of a catalyst to its development.

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« Entretien avec Ivan Stensgaard », par Arne Hessenbruch, 8 mars 2001 Sciences : histoire orale, https://sho.spip.espci.fr/spip.php?article131.

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Entretien avec Ivan Stensgaard, par Arne Hessenbruch, 8 mars 2001

Lieu : CAMP Aarhus : The Scanning Tunneling Microscopy Group, Aarhus University, Denmark.

Support : enregistrement non précisé

Transcription : Arne Hessenbruch

Edition en ligne : Sophie Jourdin

ANDERSEN Jens E.T., 2001-03-06

2009-12-21T23:53:02Z

Jens E. T. Andersen est chercheur au Département de Chimie de la Technical University of Denmark à Lyngby. Dès 1992, il utilise une technique inaugurée par Dieter Kolb et Richard Nichols un an auparavant : le microscope à effet tunnel en milieu liquide, appelé également « STM in situ » ou « microscopie électrochimique à balayage » (SECM pour Scanning ElectroChemical Microscopy). À partir de 1995, Jens E. T. Andersen a étendu la gamme de ses usages du STM in situ de l'électrochimie à la biologie, en imageant des protéines adsorbées sur des surfaces. Il a organisé trois conférences sur la technique du STM in situ en 1994, 1996 et 2000.


Autorisation de diffusion

ARNE HESSENBRUCH (AH) : Could you give us an overview of your academic career ?

JENS E.T. ANDERSEN (JA) : I studied chemistry and physics. I majored in chemistry and got a bachelor's degree in physics at the University of Copenhagen in 1987. I did the Danish equivalent of the PhD for three years in surface science at Preben Juul Møller's laboratory at the University of Copenhagen, doing metal on insulator surfaces. I finished in 1991, after which I got a post-doc at the University of Cambridge, with Dr. Richard Lambert - he is now a professor at the Chemical laboratory. I worked for a year and a half on a catalytic effect denoted as NEMCA, Non-Faradic Electrochemical Modification of Catalytic Activity (figure 1). We wanted to investigate this effect, invented by a Greek scientist in the late 80's, and we wished to check this under UHV conditions. Then I saw this advertisement for a Danish position to implement a technique called in situ STM. Not just STM, but in situ STM, or electrochemical STM. You see in 1990, as far as I recall, or 1991, Prof. Kolb (University of Ulm) and Richard Nichols (University of Liverpool) found that you were able to operate an STM while the atoms were submerged in a conducting liquid, in an electrolyte, like water. This was a sensation. We used LEED (Low Energy Electron Diffraction) imaging to study electronic details of electronic diffraction, but it was very difficult, especially because we needed UHV conditions. It seemed absolutely sensational that one could image something like that under ambient conditions in air in a liquid, just by electrochemical control of the STM tip. That was brilliant, and this really aroused some interest in me. I took this position. I applied with Per Møller at the Institute of manufacturing and engineering here at the Danish Technical University in 1992 to get the job and I was successful. We continued for two years developing the instrument together with DME. So the purpose with this project...

Figure 1. Non-faradic electrochemical modification of catalytic activity (NEMCA) Principle.

AH : So when you started out with surface science, did you know about STM in the late 80s. It was not of very great interest ?

JA : Well it was interesting as such, because we all speculated : is this technique really able to image atoms. What is it that we really see in the images ? Are these blobs really atoms ? But I think in the mid 80's it became evident that you could image a silicon 7x7 reconstruction, and this convinced me that this was some kind of atomic resolution. So it was all a technique that was developing.

AH : So you even knew about the STM in the early 80's. You'd heard about Binnig and Rohrer and you kept an eye on it, but you were not convinced. The blobs had to be...

JA : Had to be something to do with metal conduction bands and the band structure of metals. Semiconductor structures, and maybe more subtle to interpret. But with the silicon 7 x 7 reconstruction it seemed sort of convincing.

AH : And was this common in surface science, or maybe even in chemistry ?

JA : I think it's more or less a part of the education that you keep an eye on some of the new techniques. You see, the question always arises : what is an atom in reality ? At that time Transmission Electron Microscopy was the prominent technique of atomic studies. But it has become less prominent in the light of STM, because the latter is fairly easy to use and also less expensive. But then again one asks, is it really atoms ? At that time lots of people were speculating and still there is not a convincing theory describing all the details of the tunneling experiments.

AH : I presume that by the time Binnig and Rohrer had gotten the Nobel prize in 1986, then there was no doubt that these blobs were atomic resolution. Everyone agreed about this. But there was still not commercial STM's on the market, you couldn't have used STM's yourself.

JA : I'm not really sure, the Nanoscope, was that in 1980 ?

AH : Late 80's. Digital Instruments was only founded after the Nobel prize.

JA : But when they received the Nobel prize I presume they invented Atomic Force Microscopy in the same year ?

AH : 1986, right. But you would have had to build one yourself, and Joergen Garnaes over there [across the road] in the early 90's he was building his own. And he could only buy one off the shelf in 1992, and there was no longer a point in building one.

JA : In 1991 DME was putting the instrument on the market, but I think Digital Instruments, they were much earlier. I think Besenbacher, who was the Danish pioneer, built his own instruments in the early 80's, mid 80's. He's recognized as a real scientist of developing STM also to convince scientists that this is really a technique of the future. I think he made a really impressive contribution there. So I think he did that in the mid-80's, he must have because I started out doing surface science in 1987, and he was well known in the field at that time. [Actually, Besenbacher, Lægsgaard, and Stensgaard built their first STM in 1987.]

AH : So in your own work in 1987, what tools did you use, what characterizing tools did you use ?

JA : I used the most common tools, low energy electron diffraction and electron spectroscopy, Auger electron spectroscopy, electron deposition for building up atomic layers, and high resolution EELS (Electron Energy Loss Spectroscopy).

AH : The whole palette ?

JA : Not x-rays. X-rays is a common surface technique of elemental analysis, but we analyzed by AES (Auger Electron Spectroscopy). I think I was also one of the only ones in Denmark doing high-resolution ELS and analyzing surface optical phonons on insulators. This is a fairly difficult technique, and Preben Juul Møller, I think, was the only one who had the instrument at the time. And together with 2 Chinese guys we made it work properly. So we got some interesting surface optical phonon studies of metallic insulator surfaces. The interface of insulators and metals.

AH : Would you describe how that works ? The instrument, and what you had to do to get it work.

JA : The difficulty is, that it is a very narrow electron beam of low energy and you have to focus it delicately - it's a very delicate focusing problem. You have to optimize with an electrometer where you measure all the currents at the equipment and then you try to focus into the detector, which is a fairly difficult process, and the manufacturer couldn't really tell you how to do it. Just told us optimize, use optimizing procedures. And we were three people, working on this for three months, and then we made it work by a systematic optimizing approach. You can never do anything by trial and error. Optimizing by statistical methods, experimental planning basically. We used the instrument for a fortnight each, and we did as well as we could, a sort of competitive method of achieving the best signals, and suddenly we got the same signals as the manufacture's best signal. But I think it's quite a struggle, and too much of a struggle in comparison to the information you get out of it. But it was fun trying to make this very delicate instrument working. This is interesting for low energy electron studies, when you can do spectroscopy of something like phonons, that's quite interesting I think. And one of the Chinese guys, Guo Qinlin he's now a professor in China with a group of 30 people, and the other one, Dr. Wu Mingcheng, he went to the States and was employed in Texas A&M, College Station and he is still living there. So he did quite a good job and produced very exciting results, it was really good experimentally, brilliant. But then the technique you can study surface optical phonons, and overtones of these optical phonons, it can give you an idea about the electronic structure of the surface of insulators.

AH : So, is this a common thread for you to use new techniques try to stay abreast in most recent technology and use them to find out about surfaces ?

JA : In the sense that they can give us new information inside of how Nature is working, then it's interesting. Some new techniques emerged, I think, that are not that much of a help to something that we know already. When it's complementary, they can be the sort you need to advance only slightly. Some of the really new exciting technologies can do something that was not possible before, such as the in situ STM, where you can study atoms in an electrolyte medium.

AH : And you started using the in situ STM when you got here in 1994 ?

JA : 1992.

AH : But you had an interlude in Cambridge, and you didn't work on the STM in Cambridge. What tools did you use then ?

JA : Mainly mass spectrometry and Auger. I think we may have used electron energy loss spectrometry. Oh yes, we also used thermal desorption spectrometry together with the mass spectrometer to study molecules desorbing off a surface. This was quite an interesting project and we had a close collaboration with the Greek group who invented this technique.

AH : In Athens ?

JA : It may have been the University of Athens. Constantinos Vayenas was the head of the group. And Ioannis Yentakakis was our collaborator at the time. He went to Cambridge and we started this phenomenon under UHV conditions. We succeeded and I was happy to see what was going on, and we went as far as we could interpreting their method. They have brilliant articles in many famous journals where they show how this effect was working. When we got it running under UHV conditions it was not a significant effect, we could not be convinced that this was as brilliant, as they told us it should have been. To this day, a lot of work is going on to study what exactly this NEMCA effect is. So we didn't really finish the project completely because I left for another position, but I got enough information as to decide not to build a career on it. It was something really exciting, a new science and a new idea, a new and novel effect you can study in detail and resolve all the chemical mechanisms. But I think it was fairly simple. It was an increase in oxygen production by the material that was used for the electrochemistry - it was sort of solid state electrochemistry. And a zirconium dioxide ionic conductor where you can pass oxygen ions through a material by electrochemical potential differences. You can also desorb oxygen off the surface, and of course if you got an oxygen consuming process at the surface you will consume the oxygen produced by the material. And in my mind it was all coming from the material, but my colleagues were not quite convinced, so we disagreed a little.

AH : So you agreed to disagree in the end.

JA : Yes.

AH : How did you get to work in Cambridge in this period ?

JA : Well I had been working with Dr. Preben Juul Møller at University of Copenhagen in surface science and it was sort of a natural continuation of the career. You have to apply for vacant positions abroad, and I was maybe overdoing it a little applying for a position at Cambridge. Richard Lambert knew Preben Juul Møller very well. He was familiar with his work on insulators. So I had been working on insulators and metallic surfaces contacts for 3 years and I knew what it was about. And I think also very quickly, within half a year, we constructed the necessary UHV improvements to study this NEMCA effect together with Ian Harkness, a scottish PhD student who made a brilliant job of constructing this electrochemical cell for UHV conditions. It's not trivial to make this sort of system, but we did it within half a year. And then we got a whole year of studying the NEMCA effect which worked quite well. But of course I was extremely surprised and happy that I could get this post-doc position at Cambridge.

AH : The laboratory life at Cambridge is very similar to that of the University of Copenhagen ?

JA : Yes.

AH : Same kinds of equipment same level of funding ?

JA : The funding is better at Cambridge. More students available for the studies, but equipment is similar. The level of science is also basically similar, teaching is probably a little better at Cambridge. I haven't followed the graduate courses, but to my knowledge some of the people there were really brilliant. The young PhD students were really brilliant scientists.

AH : Were you a member of a college ?

JA : No, I was not affiliated through the colleges. I did try to become a member of some of the colleges, but as a post-doc you are not supposed to interfere with college life.

AH : So the people you talked to were people in your field, in your lab.

JA : Yes it was a highly professional and competitive working environment and I think a very good time.

AH : Was it also a lot of fun ?

JA : Well, it was hard work – I am not sure about fun. I had my family with our daughter aged six months. Also my wife was not too happy, it was one of the reasons why we left a little early. We were supposed to stay for 2 years, we left after 1.5 years. My wife also wanted to go back because it was tough when I was working from 9am until late in the evening. She didn't really have anything to do apart from taking care of our daughter. And she has a Masters in music and rhetoric and wanted to use her education. It was a little too boring for her, I think. But for me it was excellent.

AH : And so you went back to the in situ STM ?

JA : Yes, I started with the STM when I arrived back in Denmark. I had never worked with the STM before.

AH : But you had kept an eye on STM all the while.

JA : Oh yes, as a surface scientist you always look out for interesting techniques that are competitive to what you have in your field. And when I saw this atomic resolution in an electrolyte, I couldn't believe it, because you can't imagine the struggles you have to go through to make a metallic surface clean under UHV conditions. And once it's clean you have only approximately half an hour to make all your studies because it adsorbs all the gases even under UHV conditions. Approximately one hour you can keep your surface clean under UHV conditions. By contrast, with the arrival of the STM you suddenly were able to study atoms under brilliant conditions (ambient conditions) with a low cost instrument and you can even deposit atoms electrochemically. You can study the metallic overlayer at atomic resolution while it is being constructed atom by atom. I just couldn't believe my own eyes. My colleagues and the PhD students had a good laugh and they said : “So, you're going into in situ STM submerged surfaces. Well, you will need a diving suit.” I assured them that “No, no, no, it's atomic resolution.” They didn't believe me. We discussed the article and I have to say that even Richard Lambert was really surprised that this was possible.

AH : So you were one of the people that convinced others that this was possible, that this was real.

JA : Well it seemed real. If there is a paper in Physical Review Letters my attitude is always to take it seriously ? And you can study also the references in the particular paper by Richard Nichols. You can see in the references that they had been struggling with various modifications of STM to get this result. So it seemed very convincing to me.

AH : But if someone like Lambert was critical, he must have had a good reason. And you say that...

JA : It's so difficult to get a clean metallic surface and it's so highly reactive towards gases in the background and it will destroy the clean surface in a matter of minutes and then suddenly you can have water on top of the metal adsorbing to the metallic surface and you can still observe your atoms. At first nobody really understood, and they said okay it was an aqueous monolayer or something like that, we observed under liquid conditions. Of course this is not what Nichols told us - It's really metallic atoms deposited electrochemically, you can see atom by atom building up. No doubt !

AH : So Richard Lambert didn't develop a new STM really, he just used it ?

JA : Well he did acquire an STM for UHV studies at a later stage. Maybe he got it while I was there, but he was convinced that this was a technique he also had to consider for catalytic studies.

AH : But he didn't build his own STM. You take an STM off the shelf and use it for new studies.

JA : But it wasn't much off the shelf for UHV conditions. I think there was one Swiss company building UHV equipment for the STM.

AH : So a UHV chamber with an STM inside as a package ?

JA : Yes.

AH : And the job here was advertised for in situ STM you said ?

JA : It was advertised in a Danish union journal. Magisterbladet, I think. Or maybe it was in Ingeniøren.

AH : So the people here, they must have trusted the new technique also.

JA : Well you see, Danish Micro-Engineering produced the instrument already before I arrived. I think they sold 10 or 20 air STM equipments around Europe. So they wished to develop the STM. They considered this to be a minor problem and they employed me to do the job. As a matter of fact, Per Møller who advertised this position wasn't really familiar with science as such, but the purpose of implementing this technique was to use it for bulk electrochemical studies. So he would more easily be able to study corrosion processes under the real conditions in an electrolyte. He also wanted to use the instrument to automate electrochemical deposition. You know, he is an expert in plating. So we would like to optimize plating processes, let's say pulse plating and things like that. We wanted to study such things with the instrument. He also wanted to build small metallic machinery, something like submarines swimming about in your veins, depositing medicals at the right spots.

AH : Nanomachines ?

JA : Nanomachines. Within 10 months we got the desired atomic resolution and we finished development of the instrument. I'm saying we because Carl-Erik Foverskov, an instrument maker at the institute, he made the design for a bi-potentiostat. Actually, I must mention that I visited Richard Nichols. I think that without his help we would have taken much longer.

AH : Where was Richard Nichols ?

JA : Richard Nicols was employed at Schering, in Berlin. Schering is a pharmaceutical company, and they were taken over by a French company, Atotech. He stayed in Berlin for a number of years doing in situ STM. But he helped me out with some of the problems. I visited him in Berlin and he participated in our conferences. He was invited to help us out, and he also delivered some of the gold samples that were necessary to obtain atomic resolution. So he was very helpful. Within 10 months, we improved the DME instrument, got atomic resolution, and it was ready for distribution, ready for sale. They needed maybe six more months before they were ready to sell the instrument.

AH : What was the connection between the Danish Technical University and the DME in your job ? You said there was a DME that started the job how did they influence the DTU ?

JA : The connection was Per Møller, the center leader. He was head of the Center for advanced electroplating at the DTU. In Denmark, such a center must be a collaboration between DTU and a company. I think there must be something like 3 institutions involved. I don't know which was the third, but I think it's Danish Plating Industry, something like that. So it was a joint venture. I think we succeeded. But I remember that Møller wasn't too happy about the outcome, because he wasn't really interested in the scientific achievements. And we even made this instrument work for pulse plating, varying electrochemical potentials up and down very quickly. This influences the instrument, so you lose the imaging, but we could maintain the imaging while doing pulse plating.

AH : What did you do next ?

JA : Then I was employed with Professor Ulstrup, who's a bioinorganics scientist. I never expected to be involved in bio-something. But then suddenly I found myself in the imaging of proteins. I still don't know what a protein is. [Laughter] Today I do know a little about this field. We started a joint venture with Per Møller, and we applied for the [Danish] National Technical Science Foundation and got a three-year project. We were supposed to image proteins, and yet we uncertain about what we saw in the STM images (figure 2). But we knew some other group in Europe was grappling with proteins because if you can image proteins, you can image the molecule of life. That's really a big thing. It took quite a while but we succeeded.

Figure 2. In situ STM image of azurin adsorbed on gold(111).

From Esben P. Friis, Jens E. T. Andersen, Yu. I. Kharkats, A. M. Kuznetsov, R. J. Nichols, J.-D. Zhang, and Jens Ulstrup, 1999, « An approach to long-range electron transfer mechanisms in metalloproteins : In situ scanning tunneling microscopy with submolecular resolution », Proceedings of the National Academy of Sciences of the United States of America, vol. 96, n° 4, pp. 1379-1384 (figure 7). Quoting the paper : « individual molecules and a submolecular central feature of brighter contrast are clearly visible ». Copyright © 1999, The National Academy of Sciences.

AH : Which groups ?

JA : Professor Kolb in Ulm. Well, he is working not on proteins as such. Dr. Davis at Oxford and his group. Prof Allen Hill, also Oxford. They are the prime investigators in Europe I think. They have come up with some of the best results, including the imaging of proteins. So we've been working in parallel towards the same goal and we arrived there sort of simultaneously.

AH : And when did you succeed ?

JA : 1996 I think.

AH : Were in touch with Hansma ?

JA : Not as such, but I asked him for some preprints, and he sent me some piles of preprints and said "I'm happy you're working in this important field." He encouraged us very much and we invited him to our conference last year in 2000, but he was not able to attend. But he's done very good work and I think encouraged us to continue what we were doing.

AH : So these workshops. How did they start. Who took the initiative ?

JA : That's professor Ulstrup. 100%. He took all the initiative. When he saw this instrument and that we were able to image proteins he became very excited because he's a theoretician doing electrochemistry on metalloproteins. He has developed protein electrochemistry together with Allen Hill but from a more theoretical point of view. And then he's got me and the group to do the experiments. This way he could develop his theory. He took the initiative and then we've got lists of Danish companies and foundations who supported the first workshop. It's no problem to obtain funding for this purpose, I think. We even got funding from the Danish National Research Foundation. You know the Carlsberg Foundation in Copenhagen, they've got a house of science, the Royal Danish Academy of Science and Letters. And they supported us so we could use the premises of their Foundation. There was I think 60 people attending the workshop, in 1994.

AH : Was it a conference on STM in general ?

JA : No it was on in situ STM, electrochemical STM, also called scanning electrochemical microscopy. Allen Bard at the University of Texas, Austin, invented a similar method, and he denoted it as SECM - scanning electrochemical microscopy. So there are various denotations of this technique. We say in situ because we image while it's occuring.

AH : What sort of people came in 1994 ? What fields did they come from ?

JA : Well we invited people to display their results in STM in general and we did get some contributions that were not really in situ. It's definitely not under water, it is not in an electrolyte. We accepted more or less a wide range of contributions because there were overlaps also with in situ in many fields : solid state depositions by vacuum evaporation can also be considered as in situ, and can be made as comparison to some of our in situ experiments. But the purpose was to promote in situ STM. We still believe that in situ STM is superior to regular STM. The availability, information, methods, and scientific results you can obtain are more varied, there are many things you can do. So we would like to promote this technique and of course you can also image proteins in air, but we would like to image molecules in their own environment, which is electrolyte environments. And I've also considered to image viruses and things like that. At the present stage we have imaged proteins satisfactorily, and we have been promoting in situ STM also in the year 2000 to make people understand that this is really something worthwhile.

AH : Have you had a conference every year since 1994 ?

JA : No, only in 1994, 1996, and 2000.

AH : Were the people who came in 1996 all in situ STM people ?

JA : Almost yes.

JA : It was absolutely the most important groups in Europe represented at this congress and some invited speakers from the States also present. I think there were a couple of students from Japan, but no speakers from there.

AH : How many people came in '96 ?

JA : There was around 60 people – same as in 1994.

AH : And in 2000 ?

JA : The same

AH : Any change in composition of attendees in 2000 ?

JA : No, the same groups were represented. The focus was more bio-oriented in 2000 because now the issue was protein superstructures. Bio-inorganic imaging, that was the purpose, so there were some new groups.

AH : Is that because you are now more involved in the bio-side, or because the field in general has moved towards bio.

JA : I don't think the field in general has moved towards bio. The prime investigations are still made in superstructures and under-potential deposition, which is mono-layer studies, because in situ STM is the only technique where you can obtain atomic resolution. Subatomic resolution can be obtained even with other techniques than in situ STM. Some groups may consider imaging of proteins as subatomic. It's not really atomic resolution. Superatomic I'd say, not really at a higher resolution but at a lower resolution than atomic resolution. And they don't consider this very interesting because you get some difficulties in STM theory : how does it work when you see a protein ? People are a little anxious that they may be studying artifacts rather than real molecules.

AH : Yes of course.

JA : There are a number of artifacts in STM, it's known for artifacts.

AH : Can we back up a little ? I have difficulties imagining just how you would scan something like a protein where you don't have a surface, right ? You have something more unwieldy - how does it work ?

JA : It's a little surprising when you try to study large features with STM because the tunneling distance is some 10 Ångstroms, and a protein is at least 40 Ångstroms in diameter. Obviously, the tunneling current itself ought to deform the protein completely. And something like the deformation of a protein is observed : it's flattened considerably. The measured thickness of a protein is approximately 20 Ångstroms so it's probably flattened while in scanning. But it needs to be immobilized at a surface, otherwise you cannot get the image.

AH : How do you do that ?

JA : We choose specific enzymes and proteins where you have a sulfide bridge to a gold surface. If you react sulfides or thiols with gold you form a covalent bond that is relatively strong. Sulfides react readily with gold surfaces, and our problem in the first instance was to show that we were able to image proteins that were immobilized properly and the question was : Is one sulfide bridge from the protein to the gold surface sufficient linkage for imaging ? And it turned out it was adequate. There was just one molecular bond to the gold surface from the protein, through this sulfide bridge.

AH : Thank you !

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« Entretien avec Jens E.T. Andersen », par Arne Hessenbruch, 6 mars 2001, Sciences : histoire orale, https://sho.spip.espci.fr/spip.php?article4.

Lieu : Danish Technical University, Lyngby, Danemark.

Support : enregistrement cassette.

Transcription : Arne Hessenbruch.

Édition en ligne : Sacha Loeve