Interview: Miquel Salmeron
2008 Medard W. Welch Award Recipient
Interviewed by Paul Holloway, October 22, 2008
HOLLOWAY: My name is Paul Holloway. I’m a member of the AVS History Committee, and today is Wednesday, October 22, 2008. We’re at the 55th International Symposium of the AVS in Boston and I have the honor today of interviewing Dr. Miquel Salmeron from Lawrence Berkeley National Laboratories, the 2008 Welch Award Winner for the AVS. And, his citation reads, “For seminal contributions to the development of surface characterization techniques, useable in a variety of environments and their applications to catalysis, tribology, and related surface phenomenon.” So, Miquel, thank you very much, first of all, for agreeing to the interview and thank you very much for all the work that earned you Welch Award. So, to get us started, how about giving us your date of birth (
SALMERON: Right.) and your place of birth?
SALMERON: Well, first of all, thank you Paul for the interview. That’s very nice and thanks to the AVS for this nice award. I feel very honored, particularly when I look at the long list of people that preceded me. In any case, I was born, you know, in Spain, Barcelona in September 19, 1944. That makes me sixty-four years old.
HOLLOWAY: You and I are the same age then.
SALMERON: And, that happened, actually, last month.
HOLLOWAY: Oh, you’re younger than I.
SALMERON: So, fresh, fresh 64. Right? [Laughter] Yes.
HOLLOWAY: Well, good. Give us some information about your background and educational history.
SALMERON: So anyway, I grew up in the town of Mataro, near Barcelona. I come from a working family and studied in a [Catholic] school, like most everybody at that time. I got very soon fascinated with science because my father always talked to me about wonderful things, astronomy and other things, and I think that there is something about what you learn with your parents that sticks to your mind and sort of dictates in a subtle way.
HOLLOWAY: Sparked the curiosity?
SALMERON: Sparked the curiosity. Yes, that’s right. [Laugh]
HOLLOWAY: Good. So, you went to the university in Barcelona?
SALMERON: Barcelona, yes. I studied physics there and as I remember these years as a student I was really happy because I was doing something that I really enjoyed immensely. I was a very curious boy at the time. I think, and I’d like to believe that I’m still a kid at my age. [Laughter] My curiosity is even bigger now
HOLLOWAY: Well, that is a critical key feature of people who do excellent work and work at it full-time. You’re always asking yourself the question, “Why?”
SALMERON: Exactly.
HOLLOWAY So, you got your degree from the University Autonoma at Madrid?
SALMERON: Yes. But first I went to Toulouse in the south of France to do what we would call here a “Master degree,” the French call it “Thèse de Troisieme Cycle”. After that I went to Madrid to work on my PhD in the Physics Department, directed by Prof. Nicolas Cabrera. Professor Nicolas Cabrera is well known to most people, perhaps, for the famous Cabrera-Mott theory of oxidation. (
HOLLOWAY: Right.) Later on he did a lot of beautiful work on helium scattering, growth of crystals with defects and dislocations as nucleation centers. He did his thesis with a famous scientists, Louis De Broglie in Paris. Nicolas Cabrera father was Blas Cabrera, also very famous. He had left Spain to go to Paris, under the advance of Franco during the Spanish civil war. Nicolas Cabrera came to the States and became a professor at the University of Virginia, where he stayed many years. He later went back to Spain to start something new there in Madrid. And, of course, when I was a young person and I knew that he was coming I knew immediately where I should go. [Laugh]
HOLLOWAY: Wow. That made up your mind?
SALMERON: Yes, I made up my mind. So, I went to see him when he visited in Europe and I told him “I want to work with you.” [Laugh]
HOLLOWAY: So, did you work with him?
SALMERON: And sure, I worked with him. Well, since he was a theorist, for the experimental part I worked with collaborators of Cabrera, like Professor Juan Rojo who was my direct thesis advisor. So in 1972 we started in Madrid a surface science group.
HOLLOWAY: I see.
SALMERON: We bought a Varian system. [Laugh] We started doing Auger spectroscopy at the time, (HOLLOWAY: Right.) which for me was fantastic.
HOLLOWAY: Well, they were really state-of-the-art at the time.
SALMERON: That was really state-of-the-art at the time. That was it. [Laugh]
HOLLOWAY: They were responsible for the boom in alternative surface science activity?
SALMERON: Absolutely. It is always the case that a good person makes a good place, you know, and Madrid really flourished in terms of surface science under the leadership of Cabrera, who surrounded himself with young inexperienced people like me, and more important yet, with high quality people he managed to attract (
HOLLOWAY: Sure.) to the university. So, it was this scientific island in Spain of really great science, that I was fortunate to be part of.
HOLLOWAY: So, did you do surface science for your PhD in physics?
SALMERON: My PhD in physics was the study of Auger spectroscopy structure. Sepcifically, what determines the structure of the Auger peaks. How the electronic density of states, and the coupling between spin and angular momentum determines the line shapes. Pure electron spectroscopy.
HOLLOWAY: So, that was primarily with metals? Or . . .
SALMERON: With metals, yeah, essentially metals, but also Silicon. I studied magnesium deposits on top of Silicon. We studied plasmon excitation and plasmon coupling to the excited electrons. Most of what I did at the time was related to electron spectroscopy. (HOLLOWAY: Right.) So I would call myself an electron spectroscopist at the time. [Laughter]
HOLLOWAY: What time frame was that?
SALMERON: That was from 1971 until I finished my PhD in 1975.
HOLLOWAY: Yeah.
SALMERON: After that I decided to do a Postdoc. And following Professor Cabrera’s advice I went to Berkeley, (HOLLOWAY: Uh huh.). Now, I’m in Berkeley again, but then I came to Berkeley as a Postdoc.
HOLLOWAY: So, you went to Berkeley National Lab?
SALMERON: That’s right, and to the Department of Chemistry. The Department, like other UC Berkeley Campus departments has strong connections to the Lawrence Berkeley National Lab. I was a Postdoc with Professor Somorjai at the time. Right.
HOLLOWAY: I see.
SALMERON: And this for a couple of years.
HOLLOWAY: Somorjai has had many (
SALMERON: Many. Right.) Postdocs, and many students.
SALMERON: He has a very large and prolific [Laugh] group. There I changed subjects completely. Although it’s all surface scienceI started studying and doing more chemistry. (
HOLLOWAY: Yeah.) I worked on molecular beam scattering (
SALMERON: Uh huh.) Helium scattering, and hydrogen deuterium reactive scattering, (
SALMERON: Yes.) and studying the structure of step and flat surfaces. I really had a marvelous time there, you know. For me it was a mind opener, a change and a different way of doing science.
HOLLOWAY: So, it was primarily the study of metals? Or . . .
SALMERON: Metal. Yeah, essentially metals. Everything I did in Berkeley at that time was one metals: Platinum. Step platinum, flat platinum, all kinds of platinum surfaces. This is because platinum is such a wonderful catalyst.
HOLLOWAY: Right.
SALMERON: Many scientists, including Professor Somorjai, have made part of their career studying platinum catalysis in its various manifestations. Yeah.
HOLLOWAY: Yeah. And that’s where he’s using LEED and electron spectroscopy both?
SALMERON: Yes. Exactly. LEED and spectroscopy. I came with my background from my PhD in Auger Spectroscopy. Then I learned atomic and beam scattering techniques and in 1977, after a very good and productive time in Berkeley I decided to go back to Madrid.
HOLLOWAY: So, in ‘77?
SALMERON: And I stayed there for several years. I was a professor in the Physics Department.
HOLLOWAY: At Autonoma?
SALMERON: At the University of Autonoma in Madrid where Cabrera was still a professor. He was the head of the department and so I came back and now became a young associate professor and all that. I became associated also with the National Spanish Research Council, which played a role a bit similar to that of the Department of Energy here. So, in Madrid, I was in a joint center with the university, and teaching in the Physics Department until 1984.
HOLLOWAY: So, you stayed until ‘84?
SALMERON: From ‘77 to ‘84, during this time I was a professor, until I saw this position announcement in Berkeley, and of course I knew Berkeley already, and I liked it very much. [Laugh] (
HOLLOWAY: Yeah. Yeah.) So, when the announcement came I applied and came for an interview. And here I am [Laugh] since that time.
HOLLOWAY: So, now you’re stuck in Berkeley?
SALMERON: And I am stuck in Berkeley. [Laugh] And now I can say that I have spent more years of my scientific career in the States than anywhere else.
HOLLOWAY: Yes.
SALMERON: So, now I’m settled in Berkeley.
HOLLOWAY: So, when you were at the Autonoma University in Madrid did you do research?
SALMERON: Oh yeah. Absolutely. After the experience in Berkeley I think I became more of a chemist at the time, even though I was in the Physics Department and considered myself a physicist, but a physicist doing chemistry or surface science (
HOLLOWAY: Right). The line is sort of vague and diffused between the two fields. Right. [Laugh] (
HOLLOWAY: Right.) Interestingly in Europe you will see more physicists doing surface science, while in the States it’s more the chemists that do surface science. I do not mean one hundred percent, but certainly a lot of them.
HOLLOWAY: Well, it’s a generalization.
SALMERON: Yeah. That is what I thought at the time what In any case, back in Europe I worked on helium scattering and diffraction, as a structural tool to study growth of metals on metals and the evolution of surface structure .
HOLLOWAY: So, what did the helium scattering give you that (
SALMERON: Right.) the electron spectroscopy did not give you?
SALMERON: Well, if I wanted to study the growth of copper on copper for example, Auger or any other electron spectroscopy wouldn’t help me much (
HOLLOWAY: Right.) because you can’t tell the adsorbate from the substrate by spectroscopy alone
HOLLOWAY: On copper?
SALMERON: Right. With helium scattering, if you start with a flat surface or with a stepped surface you could choose the conditions of incidence angle of your He beam such that constructive interference exists between reflections on difference terraces, (
HOLLOWAY: Right.) , or destructive interferences. In destructive interference condition the scattering is very sensitive to the growth, and to the formation of islands on the terraces. So, the helium intensity gave a very good description of the distribution of copper islands. That was one direction of my research in Spain. Another direction is the study of chemisorption. I studied several chemisorption systems during that time.
HOLLOWAY: Did you use x-ray photoelectron spectroscopy?
SALMERON: Yeah, I started right at that time. In Madrid we bought a Leybold XPS machine and we used it to study oxidation. That was a topic very dear to Professor Cabrera (
HOLLOWAY: Naturally). It is strange that it took all these years to finally study something that was really close to his heart
HOLLOWAY: That’s very interesting because I had always wondered if Cabrera had participated in the more detailed understanding of the low-temperature oxidation and the nature of oxide film growth?
SALMERON: Right. Well, he always pushed us to do that and then of course, we didn’t have any instrument at the beginning. It took us a few years before we had the lab built up (
HOLLOWAY: Right.) to do that. You have to understand that although we had the support of Cabrera, who got the money to buy the first instruments, none of us knew any surface science. (
HOLLOWAY: Right.) So, there you have a group of people [Laugh] that had to learn on the go. It was doing experiments and learning at the same time and looking at everybody else in the world and trying to catch up. I remember this as a very interesting time, as we were l,earning experimental science by ourselves.
HOLLOWAY: Right. Yeah. It’s easy to see how to get to where you’re at when you’re looking backwards, but looking forwards it’s very difficult.
SALMERON: In fact, these were very formative years, you know. I mean, we had complete freedom to do what we wanted, because as I said before, nobody in our lab was an expert in anything. And, of course we made many mistakes, often wasting time here and there. But, in the end, I think in retrospect that for my career it was very good, (
HOLLOWAY: Right.). It gave me a way of attacking problems and since then that has been always my philosophy: “Don’t rely on anybody. Just choose a problem and solve it for yourself.” That was my motto. (
HOLLOWAY: Right.) And, of course, once you are on a problem you look at what everybody else did.(
HOLLOWAY: Yeah.) But the ability to choose problems that are important on their own, not because somebody tells you, I think is what always matters. [Laugh]
HOLLOWAY: That’s one of the very most difficult problems to try to educate students in, (
SALMERON: Right.) and that’s the ability to set priorities, make selections of topics to focus on. (
SALMERON: Yes.) And, I’m not sure there’s any good solid way to do that.
SALMERON: Right.
HOLLOWAY: You have to be a curious person.
SALMERON: And I think this combination of natural curiosity is either something you have or you don’t. If you have it, that’s good. It’s a blessing. Then you have to choose problems. Nobody is going to solve things for you. You have to move away from what is the common and fashionable thing of the moment. (
HOLLOWAY: Right. Yeah.) Okay, you can do it but you’ve got to be very good and step in the first wagon of the train immediately, or else you’ll catch a train that is already passed through, [Laugh] and you’ll catch the last wagon. [Laugh]
HOLLOWAY: That’s just about like trying to time the stock market. You never know whether you’re at the bottom (
SALMERON: Precisely.) or whether you’re halfway up to the top. (
SALMERON: Exactly.) Or all the way to the top.
SALMERON: So I think that this has served me quite well. Latching onto the hot topic of the day is a temptation for any young researcher (
HOLLOWAY: Right.) because that’s where you find funding, supposedly. But, in the long term you find yourself fighting against well-established things and people and you don’t have the means. So this is the advice I give to my students, “Just look for something that you think is important and never mind, never mind that it’s in fashion at the moment or not. (HOLLOWAY: Right.) Really try to do something, you know, that’s important“ [Laugh]
HOLLOWAY: And you just have to put together a sales plan for getting this funded.
SALMERON: Of course, of course. This is always the case but everybody’s gone through that, (HOLLOWAY: Yeah.) and difficult periods of time. But Berkeley was very good to me because when I came here, with my hands in my pockets I didn’t have anything. I left all my research instruments in Madrid. I couldn’t take them with me. (HOLLOWAY: Right.) I was given the freedom to do what I wanted. When I came I had just heard about this wonderful thing called scanning tunneling microscopy that had just been discovered a couple years ago, (HOLLOWAY: Yeah.) and I knew immediately what I wanted to do. (HOLLOWAY: Uh huh.) So I said, “That’s what I want to do. I don’t know how I’m going to do it, but I’m going to do it.” [Laugh]
HOLLOWAY: So that was when you came back to Berkeley?
SALMERON: Exactly. And that was my selling project. I developed techniques (HOLLOWAY: Yeah.) to study chemical reactions on surfaces, the growth of materials, and things that I’ve been already doing. . [Laugh] (HOLLOWAY: Right.) At that time there were no commercial companies that would sell you a microscope. If you wanted to get into the field you had to make it yourself. Right? (HOLLOWAY: Yes.) And that’s another thing in my background: I’ve never been afraid to build new things. In the past, I always build instruments, with my own hands. So that was fine with me and I started from scratch, designing the STM and of course making all kinds of mistakes . That’s the price that you have to pay.
HOLLOWAY: So, who did you go to get some of the tricks?
SALMERON: From reading papers, and then figuring it out. In fact I made the mistakeof trying to copy a microscope design that somebody handed me. (
HOLLOWAY: Yeah.) So, of course, when I finally thought it through and made my own design as I should have done in the first place, and then it worked quite well. [Laugh]
HOLLOWAY: First think about it?
SALMERON: Yes, think always about it first. (HOLLOWAY: Yeah.) and then we will understand it, and eventually you can build it. HOLLOWAY: So, you complimented the STM with electron spectroscopy?
SALMERON: Right. Yeah.
HOLLOWAY: What about helium scattering? Did you stay in it as well?
SALMERON: Unfortunately I dropped it. Unfortunately, you know, because in life you cannot do everything.
HOLLOWAY: Yeah. There’s only twenty-four hours.
SALMERON: Exactly. There’s only twenty-four hours and there’s only so much money. So, I ended up by dropping that, but putting all my effort and money and everything into first scanning tunneling microscopy, which eventually I developed in a successful way and got nice data. However as nice as STM is, it provides only part of the answer of your problem. There’s no single tool, no single instrument that will give you all the answers.
HOLLOWAY: That’s right.
SALMERON: STM doesn’t have in its present state, or I should say it doesn’t provide easily spectroscopic capabilities. Right?
HOLLOWAY: Right.
SALMERON: In STM images you just see bumps, which you may call atoms, (
HOLLOWAY: Yeah.) and you see where they are. But, you’re not sure [Laugh] if under the little bumps there is an oxygen, or a carbon atom, etc,.And that’s when we need have to have spectroscopy. But, there is one thing about STM, which is unique, and that is that it doesn’t require ultra high vacuum. (
HOLLOWAY: Yeah.) I mean, we work in ultra high vacuum because we may want surfaces very clean and very well prepared, and secondly, and perhaps more important because the best tools that we have all work in vacuum. (
HOLLOWAY: Right.) LEED, Auger, etc. But STM can work in the presence of gases, and under liquids if you want. As, mentioned in the award it is the development of this capability that constituted my first development: an STM machine that operates inside a reactor cell, so that you could follow a reaction on a crystal and watch in real-time how the images evolve, how atoms absorb, desorb, and move about during a chemical reaction.
HOLLOWAY: Very dynamic.
SALMERON: Very dynamic indeed. As I said, STM doesn’t put names to the atoms in the images. You have to figure it out. So I decided, “spectroscopy is next,” and this became my next development, the “ambient pressure,” photoelectron spectroscopy.
HOLLOWAY: Well, that was very impressive data that you showed this morning in your lecture.
SALMERON: Well, I tried to show the ways that these two techniques are connected. You get the structure with the STM, and then you need the addition of spectroscopy to really understand what it’s going on.
HOLLOWAY: Well, you can get atom resolution at atmospheric pressure with STM?
SALMERON: You can.
HOLLOWAY: By preparing the tip properly?
SALMERON: Of course, you know, you can’t generalize that. The tip is certainly half of the problem in STM. (
HOLLOWAY: Right.) And, unfortunately the tip does not always cooperate. In fact, we are still at a stage where we cannot tell what the tip is, and what its structure and termination is. All we hope is that during the entire image acquisition, whatever the tip is, it stays the same. (
HOLLOWAY: Right.) We need theory to help us explain the images.
HOLLOWAY: To get atom resolution requires some preparation of the tip?
SALMERON: Yeah. Yeah.
HOLLOWAY: Is that becoming more of a science or is it (
SALMERON: No.) still more of an art?
SALMERON: It’s still more of an art, at least under the conditions that I just described, inside a reactor cell in a high-pressure environment. Because the dynamics of atomic motions change with environments (
HOLLOWAY: Right.) ver fast. However if you’re going into another variety of STM, where you cool down in vacuum to low temperatures, (
HOLLOWAY: Yeah.) then the mobility of atoms, the diffusion, is reduced to the point where there’s not a single displacement, even for hours.
HOLLOWAY: Stays in place?
SALMERON: So that then you can do that. You just pick an atom laying on the surface with the tip (
HOLLOWAY: Uhm-hmm.) and use this atom, as your imaging tip. Okay? This has already been demonstrated to be feasible. Right? (
HOLLOWAY: Right.) With this trick then we have absolutely control of the system in all its entirety. But, that only happens under these special circumstances.
HOLLOWAY: Now, LEED and STM both are tools to look at the structural arrangement of atoms on a surface, but I don’t see as much LEED as I once did. Has STM supplanted it?
SALMERON: Well, it has, yes. That’s a very good question, you know. I think there’s a combination of things. The LEED success has been in solving static structures in vacuum, but now people want to solve structures that are relevant to the real world. In the real world surfaces are surrounded by a gas or liquid environment, as in catalysis, biology. (
HOLLOWAY: Right.) and LEED is not well suited for it. Not because of the lack of capability but because the environment doesn’t allow it. So this, in parallel with the fact that today we have x-ray for diffraction, (
HOLLOWAY: Yeah.), with synchrotron x-rays, you can do structural examination in a very precise way, and then of course STM, LEED is at a disadvantage. (
HOLLOWAY: Yeah.) As you know well, the number of laboratories that have LEED as a prime measurement tool for surface structure has decayed. (
HOLLOWAY: Right.) Right? You can count with one hand the laboratories in the world that are active in that area. Right?
HOLLOWAY: Yeah. Now, another technique that compliments STM is AFM?
SALMERON: Correct.
HOLLOWAY: Now what is your attitude about AFM? Was this developed when you were there at Berkeley?
SALMERON: Yes, at about that time. And sure, I’m interested in AFM. I can see that it’s not at the same level as STM. It’s not as developed. Let me qualify what I say. It’s developed in the sense of taking pictures with nanometer resolution. Today you can buy very nicely-working AFM instrument and people use it very extensively on many types of surfaces. But the AFM currently lacks spectroscopic capabilities comparable to the STM where you can study the change of tunnel current with voltage in what is called IV spectroscopy.
HOLLOWAY: IV spectroscopy?
SALMERON: Which gives you not only the density of electronic states but, if you do it in the low voltage regime you can get also vibrational spectroscopy information. So STM has these unique spectroscopy capabilities, right?
HOLLOWAY: Yeah.
SALMERON: It’s not an easy thing but people do it, and we do it. AFM hasn’t reached that point yet, but I’m convinced it’s on its way to do that There’s already been demonstrations. In fact, the AVS Albert Nerken Award this year is exactly for that kind of development that makes AFM into a spectroscopic tool. Professor Morita and his coworkers have shown that by measuring the attractive force atom-by-atom, you can obtain a signature of the identity of the atoms that compose a material. (HOLLOWAY: Right.) Okay? So, AFM is getting to that stage and, of course, this is what we have to use it to study nonconductive materials. STM has this shortcoming that can only be used in conductive materials, be it semiconductor or metallic, (HOLLOWAY: Right.) but not on alumina for example, [Laugh] (HOLLOWAY: Yeah.) or any other insulating material. But, AFM can. Today more and more people are using the technique to produce beautiful images with atomic resolution of insulating surfaces. And I am sure that we’ll see more of it. But, that is not something that is easily done. I don’t know if some commercial instruments sell it already to work in that manner, so that you just buy one off the shelf and plug it in and then it goes. [Laugh]
HOLLOWAY: Now, STM and AFM both can be used at elevated pressures?
SALMERON: Correct.
HOLLOWAY: Is the resolution at atmospheric pressure the same as in ultra-high vacuum?
SALMERON: In principle it is, but of course there are other considerations that affect it. In air one has the problem that the oscillating cantilever or tuning fork, experiences the viscosity of the gas phase. (HOLLOWAY: Uhm-hmm.) This affects the Q-factor of the oscillator, which in turn affects the sensitivity and resolution. So, there are difficulties. I don’t think they are insurmountable, but it is not the same as in vacuum for sure.
HOLLOWAY: But, you showed that the STM works well in atmospheric pressure. But can you really go to atmospheric pressure with photoelectron spectroscopy?
SALMERON: In principle yes. It’s all a matter of the mean-free path of the electrons, which we all know is very short above a few Torr. (HOLLOWAY: Right.) So, and things become a little, more and more difficult as the pressure keeps increasing. Remember that the attenuation of electron current grows exponentially under pressure. (
HOLLOWAY: Uhm-hmm.) So, you have to get your sample very close to the detector, which is separated by a differential pumping stage with a first orifice within a fraction of a millimeter. If the pressure is close to one atmosphere the entrance orifice has to be very small, maybe a hundred micrometers in diameter, which means that your x-ray beam also has to be very well focused. On top of this the alignment of the electron spectrometer, the sample position, and the x-ray beam spot have to coincide at the focal point. You can imagine very quickly how these things end up being difficult. (
HOLLOWAY: Right.) We plan also to modify our spectrometer to work to up to one atmosphere – although we don’t do this yet at the moment. We can also detect the reactants and reaction gas products as they pass through the orifice to the other side of the differential chamber by XPS of the gases.
HOLLOWAY: So, the configuration of the instrument is a little microcell (SALMERON: Yeah.)?
SALMERON: Yes. Exactly.
HOLLOWAY: And a small spacing between the sample surface (SALMERON: Correct.) and the entrance orifice?
SALMERON: And that’s how I see this technique evolving to achive higher operating pressures, with the sample moving closer and closer to the pumping stage and the orifice being smaller and smaller in size. We have to solve the alignment problems. And ideally, by use of zone plates, we should be able to focus the x-ray beam (HOLLOWAY: Right.) into nanometer-sized spots. So, I’m quite confident that if not next year or the other, sooner or later we’ll come up with an instrument (or somebody will come up with an instrument) where you can do scanning at the same time with this kind of resolution, at pressures that are close to one atmosphere. (HOLLOWAY: Uhm-hmm.) I think that the path is open and I’m looking very much forward to its success. [Laugh]
HOLLOWAY: Now, you have applied that to a number of different techniques, materials, classes and properties, but you seem to spread your interest around. Your curiosity extends between catalysts, catalysis and oxidation, etcetera.
SALMERON: Right.
HOLLOWAY: Which of those are your favorite topics?
SALMERON: Yes. [Laugh] It’s like . . .
HOLLOWAY: It’s like asking you which of your children are the favorite?
SALMERON: Right. Exactly. And, you know, I’m like a kid that wants to grab all the toys with two hands, right, and something falls off. It’s true that I’m ambitious. I want to learn more about this and that, because I find these topics fascinating. I think that the two that benefit the most from these techniques are catalysis and environmental science, (HOLLOWAY: Uhm-hmm.) We can now look at a catalyst surface during the reaction, and find out what the structure of the surface is and its chemical structure, (HOLLOWAY: Right.) and we can at the same time and with the same instrument determine also the gas phase composition. As I said this morning in my talk that, for a scientist that wants to understand reactions it’s almost an ideal tool that gives you essentially everything that you want in one shot. [Laugh] (HOLLOWAY: Yeah. Yeah.) In environmental science we can now study the structure of water surfaces and water films at interfaces. Right. (HOLLOWAY: Right.) This topic is, and will be for some time a topic of continuous discussion and interest. And my hope is that these tools can bring spectroscopy to the study of the structure of water-vapor interfaces, and to the study of water films, including how the first layer of water is controlled by the structure of the surface it sits on, and from there to the second layer of water and so on. These are the fundamental questions that we hope to will soon understand.
HOLLOWAY: But, you’ve seen remarkable properties of the surfaces of water and solutions already with metals?
SALMERON: Right. SoOne of this, which I published a few years ago, is the structure of solution interface. People have been dealing with the problem of solutions for years and made theoretical predictions of what is the suface composition compared to that of the bulk liquid, specifically what is the concentration of ions at the surface? (HOLLOWAY: Right.) This is very important in atmospheric science, as I learned from my colleagues in this area. So we ask ourselves, what is the surface composition of an aquesous solution? Is it clean water Do you just see oxygen as a signal or do you see also the ions, and which ion?And are they there in the same concentration that you have in the volume? These are questions that, we could satisfactorily answer. I think is one of the very important topics in molecular sciences. HOLLOWAY: So, can you use AFM or STM to probe those surfaces at the same time you’re using . . . ?
SALMERON: The liquid surface is difficult to study with the AFM. On a liquid the molecules are fluctuating and changing and, the proximity of the tip to the surface of the liquid exerts forces of electrostatic, or van der Waals nature that easily deform the liquid surface. So, to look at the liquid structure with scanning probes the tip has to stay far away, which means that we are giving up on spatial resolution.
HOLLOWAY: But, have people measured force displacement curves for (SALMERON: Yeah.) liquid?
SALMERON: Yeah. You can do that.
HOLLOWAY: With and without segregation, for example?
SALMERON: Not on the case of ions, and that still is a topic that is there. (HOLLOWAY: Yeah.) Sitting there to be tackled, [Laugh] (HOLLOWAY: Yeah.) by some entrepreneurial scientist that wants to really . . .
HOLLOWAY: Someone young and curious?
SALMERON: Some young curious person that wants to jump into it yes. [Laugh]
HOLLOWAY: It was remarkable, in my opinion, if you look at the four major award winners that were announced this year for the AVS, yourself, Professor Morita, and Sergei Kalinin, All three of you work in the scanning probe (SALMERON: Yes.) microscopy area.
SALMERON: That is true. That is true. Yes.
HOLLOWAY: Is this indicating, a maturation of the field?
SALMERON: You know was built in 1982, and people really did not fully appreciate it until 1984 or ‘85, and its popularity and technical development is still growing. People are still finding new ways of using it, and the instruments are getting better and better. You know, I’m still amazed how I see the growth of the use and application of these techniques. So certainly the techniques are mature, in the sense that we know a lot about how they work, but that doesn’t mean that we have exploited all their capabilities. After all, whether the tip-sample interaction is tunnel current, or forces, magnetic force, electrostatic forces, chemical forces it can be used to measure and image at various scales of resolutions. You can also excite the tip with infrared or radiofrequency radiation. Or you can couple light into your tip, (HOLLOWAY: Yeah.) So, the number of opportunities to use the same type of technology of scanning a probe near the surface, (HOLLOWAY: Right.) and couple it to any interaction that you want is almost unlimited. So this is why scanning probe techniques are so versatile and still growing. One area that is growing fast is the coupling of light to scanning probe tips. Visible light has low resolution for imaging because of the diffraction limit. (HOLLOWAY: Right.) But, if you can combine light with an STM probe, where the light excites a plasma oscillation of the tip, (HOLLOWAY: Yeah.) this plasma creates a near field with the same frequency as your light. It is like having a light bulb of the size of your tip probe, which is of nanometer dimensions. So, I think of new developments that can come from that, including optical spectroscopy with nanometer resolution. You can then do Raman spectroscopy (HOLLOWAY: Right.) with nanometer resolution. [Laugh]
HOLLOWAY: It would be tremendous.
SALMERON: I think that this is, as I said before, the vitality of the field, , which has reached maturity but not completion. [Laugh]
HOLLOWAY: Right. There’s still lots of opportunities?
SALMERON: Lots of opportunities to work on it.
HOLLOWAY: Now, you said earlier that you had the equipment in place for atmospheric pressure for electron spectroscopy and you had a number of people who had come banging on your door?
SALMERON: Right.
HOLLOWAY: That means you’ve interacted and collaborated with an awful large (
SALMERON: With many colleagues. Right.) number of people?
SALMERON: Yes.
HOLLOWAY: I wondered if you had any specific recollections or (
SALMERON: Yes.) stories to tell us about those collaborations, for example?
SALMERON: In the field of scanning probe microscopy in Berkeley, one of the persons that has closely interacted with me was Professor Somorjai, who of course became immediatelyvery interested in scanning tunnel microscopy. When I developed thehigh-pressure STM, where the STM operates inside of a reactor cell, he became enthusiastic about it and we started a collaboration, sharing students and postdocs for some number of years, and have been developing and applying the technique to chemical and catalysis problems. (
HOLLOWAY: Right.) So, that has been a wonderful collaboration. He having more experience in chemistry and having more access to very brilliant students that he would share with me during that time. That was wonderful. I still enjoy working with him. In the high-pressure STM, because of the proximity, he has been my primary collaborator in Berkeley. In the high-pressure XPS or ambient pressure photoelectron spectroscopy I had also many collaborators like Anders Nielson, at Stanford and in Sweden, but now in Stanford, Gordon Brown also at Stanford, and John Hemminger at UC Irvine join into my list of collaborators. I have a really long list of people that are joining. I mentioned only the names of the more continued collaborations . In Berkeley, we have two ambient pressure photoelectron spectroscopy machines located in the Advanced Light Source, the LBNL Synchrotron facility (
HOLLOWAY: Right.) Because this is a public facitlity even if I develop the instrument, pay for a large fraction of the cost of the end station, (
HOLLOWAY: Right.) and put my postdocs to work and develop I still don’t ow it. [Laugh] (
HOLLOWAY: Yeah.) I get user time in what is called an “approved project,” or a program. But anybody in the scientific community, can apply to use the instrument (
HOLLOWAY: Yeah.) .
HOLLOWAY: Right. Now, you’ve done a lot of photoelectron spectroscopy. You can either have a standalone laboratory system, with a monochromatic x-ray source, (
SALMERON: Correct.) or nearly monochromatic now, or do the photoelectron spectroscopy with the synchrotron source? Or, what’s your fraction of time allocated?
SALMERON: Yeah. I have the fortunate circumstance that in Berkeley I have a synchrotron essentially a walking distance from my office and my laboratory so I tend to do a lot of experiments there. I also have a commercial XPS machine in the laboratory, that I (HOLLOWAY: Right.) use. Of course I can use it day and night and don’t have to apply for time to use it. [Laugh] (HOLLOWAY: Yeah.) What is going to happen soon is that commercial companies will be developing on a commercial basis these high-pressure instruments, initially for synchrotrons, because that’s where the demand is at this moment. But I’m told that they are building stand-alone instruments, which of course will not have the advantage of the synchrotron where you can change the wavelength of the x-rays. But, in laboratory XPS instruments you can still do high-pressure XPS with fixed energy (HOLLOWAY: Right.). And I think this is a tremendous advantage.
HOLLOWAY: It is. Now, you’ve had a number of students that you’ve mentored. I wondered if you had any recollections and would like to mention anything about mentoring and the techniques and importance of that?
SALMERON: Right. Well, both you and I know that the quality of your work is as good as that of your students and I’ve been fortunate to have really good students and postdocs (HOLLOWAY: Yeah.). They deserve a lot of credit. I see myself as an inspirator, and an instigator. (HOLLOWAY: Right.) That’s what I do now, more than directly sitting in front of a machine. I have really wonderful memories of students that are my friends now, because they really took on these ideas, and this inspiration that I mentioned, and they carry it on. Some of them are professors at various universities and also in industries. I can mention names by dozens, that I mentored during the twenty-four years I’ve been in Berkeley. [Laugh]
HOLLOWAY: I think in many ways the students help our curiosity. They drag us into new areas and they’re not so bound up with ideas that they don’t take chances every once in a while? [Laugh] It makes us take chances too.
SALMERON: You have to calibrate your students. You have to get a sense of how deep you have to mentor them, you know, whether you can let them loose a little bit more or less. But I try to get students thinking by themselves, I always tell them, “Listen, I’ll give you an idea. I’ll push you in a direction. But, don’t be afraid to let me be wrong.” [Laugh] (
HOLLOWAY: Right. Exactly.) You know, “Don’t think, I would be not offended if [Laugh] you tell me it was a stupid idea. And, the rest of it is “try and go in the direction that you want,” . I hope they discover there are better ways of doing things. And when that happens I think this is a happy moments in life. You really understand when you have a collaborator and not only a student, and you learn from them as well. Right?
HOLLOWAY: Exactly. You learn a lot from them.
SALMERON: And that is, I think, is why our profession is one that I consider myself fortunate to be in. It’s like winning the lottery, right? I mean I’m doing something that is profoundly satisfactory to me. I work for what I like and I’m paid for [Laugh] doing something that I enjoy very much. And this interaction with students and postdocs is wonderful because they think and they end up improving your ideas. This is, to me, the best.
HOLLOWAY: Now, I’d like to come back again to a technical area. And, in your talk this morning you showed some photoelectron spectral analysis of nanoparticles.
SALMERON: Right.
HOLLOWAY: The rhodium, I believe, and the platinum rhodium?
SALMERON: Right.
HOLLOWAY: Where you varied the energy and you showed the shell structure on that material, and then you also showed with chemisorption the rearrangements of the composition of the surface. One of the big problems in terms of the nanoparticles that we’re now trying to deal with is characterization. (
SALMERON: Right. Right.) What techniques besides photoelectron spectroscopy are particularly suited for that purpose?
SALMERON: Yes. That is one area where we started with spectroscopy for the characterization, structural characterization, right. And, clearly spectroscopy, as you saw and you mentioned, already provides you with very rich information and we can already see that these nanoparticles rearrange in a profound way due to the adsorption and reactions of gases. And someone in the audience asked me, “Well, hasn’t that been known for years?” And, of course the answer is “Yes.” The difference, and I wanted to repeat that here, is that in a nanoparticle all the material is included in this ten-nanometer diameter.
HOLLOWAY: That’s right.
SALMERON: So, in the nanoparticle, the whole material is re-structured, while in the past studies only the surface regions is re-structured due to the large amount of bulk material that cannot equilibrate. That is the novelty. Now, of course, we need to know more structurally. And I think that the tool of choice is transmission electron microscopy, and we’re trying to follow that. But transmission electron microscopy needs to be modified so that you can image your material inside gases. (HOLLOWAY: Uhm-hmm.) So, if the electron beam goes through the gas and particles it can give you a very high-resolution image, so that you can see now how the nanoparticles evolve. So, that is my next step and I will try to use these instruments, or if necessary develop them further. So, that would provide a powerful tool for the structural characterization. In addition you could do diffraction with the electron beam and thus have a different sense of the distribution and crystallography of the nanoparticles. So, I think that TEM is where I would put my money. [Laugh] Okay?
HOLLOWAY: Okay. We’ve covered a lot of topics and I think that that concludes the topics that I was thinking about. I wondered if you had anything that you’d like to add?
SALMERON: Well, I think that we did cover a lot of topics that I have in mind myself. Perhaps on the matter of curiosity, and going back to our discussion of what young people should be doing, just to repeat one more time, my advice is to really follow your own instinct and your own ideas. And don’t be looking only at what is being done. Always keep your eyes open, that is part of the curiosity, but don’t follow necessarily what has become the fashionable topic of the moment. Just think for yourself and if you think there is something very important that you want to do, really do it. After all you get recognized for something that is your unique contribution.
HOLLOWAY: That’s right. So, you got to prioritize, and evaluate.
SALMERON: Exactly. Exactly.
HOLLOWAY: Follow your own curiosity.
SALMERON: That’s exactly my advice, [Laugh].
HOLLOWAY: The professional societies offer the opportunities to meet people and interact and calibrate yourself relative to where the world is at. Presumably they played an important part in your career as well?
SALMERON: Absolutely. And meet people at scientific meetings, although these days there are so many that one has to be selective. I cannot go to all the meetings.
HOLLOWAY: No, you can’t go to all.
SALMERON: But, you know, the neat thing to me is not only that I can see and listen to topics that my colleagues are working on, but equally important they provide also another complimentary part of that, in that you can see them personally and sit down and discuss topics. And that you cannot substitute with anything. In the one week that a conference lasts you have a snapshot of the field and a very rich environment to interact with your colleagues. I always try to go to one or two, and sometimes even more than three, which may be a little too much. [Laughter]
HOLLOWAY: Yeah. You can’t stay out of the lab completely?
SALMERON: That’s right. That’s right. [Laugh]
HOLLOWAY: Well, Miguel, thank you very much for the interview. I’m grateful for you being willing to participate and I thoroughly enjoyed it.
SALMERON: My pleasure. Thank you very much, Paul.
HOLLOWAY: Thank you.