Interview: Beatriz Roldan Cuenya
2009 Peter Mark Memorial Award Recipient
Interviewed by Paul Holloway, November 11, 2009
HOLLOWAY: Good afternoon, my name is Paul Holloway. I'm a member of the AVS History Committee. We are at the 56th International Symposium of the AVS at San Jose, California. Today is Wednesday November 11, 2009. I have the pleasure of interviewing Dr. Beatriz Roldan of the University of Central Florida. Dr. Roldan is the 2009 Peter Mark Memorial Award winner, and her citation reads, "For pioneering contributions to the understanding of processes taking place in metal nanocluster catalyzed chemical reactions." So Beatriz, congratulations on the award.
ROLDAN: Thank you very much.
HOLLOWAY: Could you start by giving us your birth date and place of birth?
ROLDAN: I was born in Oviedo, Spain, on May 6, 1976.
HOLLOWAY: To continue, could you give us some education background, starting as early as you want.
ROLDAN: I started in Spain. My Bachelor of Science was at the University of Oviedo in northern Spain. I went for a Master's there in Physics and a minor in Material Science. So my bachelor's is in physics, but I also have a minor in materials.
HOLLOWAY: What year was that?
ROLDAN: That was 1998. From there I moved to Germany. I was at the University of Duisburg-Essen, and did my PhD in solid-state physics investigating structural, vibrational and magnetic properties of thin films, surfaces and nanostructures. The samples were prepared in ultra-high vacuum and I used Mossbauer spectroscopy, magneto-optic Kerr effect and nuclear resonant inelastic X-ray scattering to investigate their properties.
HOLLOWAY: Who was your mentor there?
ROLDAN: It was Prof. Werner Keune. I graduated in 2001.
HOLLOWAY: So you were using ultra-high vacuum for that study?
ROLDAN: Yes. It was there when I started working in vacuum and in topics related to the American Vacuum Society. I grew my samples using molecular beam epitaxy, and characterized their structural properties in UHV via low-energy electron diffraction and reflection high energy electron diffraction, their chemical composition via Auger electron spectroscopy, and their magnetic properties using ex-situ and in-situ Mossbauer spectroscopy.
HOLLOWAY: And your MBE growth was to produce epitaxial layers or clusters?
ROLDAN: We worked with different systems. I started looking at multi-layers. For example, we looked at different structures that you can grow to obtain phonon confinement, so we had thin layers of iron encapsulated by layers for example of silver, and we were looking at how the phonon density of states at those interfaces is modified. Those are also magnetic materials. We measured the vibrational properties of those samples at the Advanced Photon Source in Chicago, and studied their magnetic properties in Germany. I also looked at systems that were important for spintronics applications at the time. So for example, I tried to understand interfaces of ultrathin iron films with gallium arsenide and indium gallium arsenide. I used Mossbauer spectroscopy to look at the magnetic properties of such interfaces, and gain insight into the possible formation of magnetically dead layers. I also investigated the stability of metastable phases and crystalline structures, as for example FCC iron versus BCC iron, and the associated changed in the magnetic moment. I also investigated the stability of a semiconducting phase of tin (α-Sn) that has diamond structure with a very small bandgap. All of those structures were grown by MBE.
HOLLOWAY: So you were doing low-temperature studies at the same time?
ROLDAN: At the time we did a lot of low-temperature measurements to investigate the magnetic properties of some systems. For example, we prepared iron clusters supported on gallium arsenide, also by MBE, and we used in-situ (UHV) Mossbauer spectroscopy at low temperature to determine the blocking temperature of these clusters as a function of the particle size. Also during my PhD I went for six months to the Argonne National Laboratory. I was working with Dr. Sam Bader and his group, and we were looking also at the magnetic properties of low-dimensional systems, as for example Fe nanowires grown on stepped Pd surfaces, in that case using magneto-optic Kerr effect.
HOLLOWAY: Who were you working with at Argonne?
ROLDAN: Samuel Bader. I think he was one of the previous AVS awardees a few years ago.
HOLLOWAY: Are there other names of people that you remember from Argonne that you worked with?
ROLDAN: Within Sam Bader's group, Dongqi Li was my direct supervisor. She was working in Sam Bader's group at that time. She had an unfortunate accident afterwards. She was my mentor there. I work right now with people at the Advanced Proton Source, in particular, with Wolfgang Sturhahn, Ercan Alp and Jiyong Zhao.
HOLLOWAY: So how much time do you have on APS?
ROLDAN: Normally almost every beam cycle we get beam time, so about a week per beam cycle. I'm also using the facilities at Brookhaven National Lab to look at the structure of small metal nanocatalysts as well as structure-reactivity relationships using X-ray absorption fine-structure spectroscopy.
HOLLOWAY: So you do primarily XAFS on the beam line?
ROLDAN: Yes. At Brookhaven we do XAFS. At Argonne National Laboratory we use nuclear resonant inelastic Z-ray scattering. It's like the inelastic version of Mossbauer spectroscopy.
HOLLOWAY: Which beam line is that on?
ROLDAN: This is 3ID beamline at Argonne.
HOLLOWAY: Tell me what your work is involving at the University of Central Florida.
ROLDAN: We have a wide research program there. We are working on topics that are related to understanding of structural, electronic, magnetic, vibrational and catalytic properties of nanostructures. Part of my group is doing solid-state physics, looking at how the magnetic properties of nanostructures change as a function of the particle size and as a function of chemisorption. Some of the people are interested in understanding new vibrational properties of nanostructures. That's the work that we do at Argonne. However, the main core of my research deals with catalysis and trying to find structure-activity relationships with the ultimate goal of improving the understanding of the parameters that control the catalytic activity of nanoscale systems.
HOLLOWAY: What year did you join UCF?
ROLDAN: 2004 was my first year.
HOLLOWAY: So we missed a post-doc in between there, right?
ROLDAN: Yes. Between 2001 and 2003 I was at the University of California Santa Barbara. I I moved from a Physics Department in Germany (Duisburg) to a Chemical Engineering Department. There is where I started working in the field of catalysis, trying to combine chemical engineering tools and chemistry tools to synthesize nanostructures with basic solid-state physics and surface science to characterize the structures and their activity in situ (UHV) and ex-situ via electrochemistry.
HOLLOWAY: What was your mentor at UCSB?
ROLDAN: Eric McFarland.
HOLLOWAY: Did you work with other post-docs that you considered to be your colleagues?
ROLDAN: At UC Santa Barbara I had kind of two independent projects, so there was not really anyone I was closely related to.
HOLLOWAY: How much equipment building did you do, or did you just take on already built systems?
ROLDAN: When I started at the University of Central Florida I did the design of the UHV system together with a company in Germany (Specs GmbH), and I didn't do the building myself. I was fortunate enough that they did the construction for me. So I could start doing science from the very beginning of my tenure-track appointment. After a year I had an up and running system.
HOLLOWAY: So what's the capability? What analytical techniques do you have on that SPECS system?
ROLDAN: Right now I have four chambers interconnected. I have a load-lock chamber, MBE chamber with electron beam evaporation, Knudsen-cell evaporation, quartz microbalance for monitoring film thickness, Ar+ sputtering for cleaning, etc. So anything that you imagine for sample preparation. We also have a plasma source that works in UHV that we use to remove organic ligands from our nanoparticles in situ. The main analytical chamber has XPS, UPS, Auger, and temperature programmed desorption (TPD) capabilities. Attached to the analysis chambers there is a variable temperature scanning tunneling microscope. When I did the design, I wanted a flexible multipurpose system to be able to do in situ nanostructure preparation, to clean ex-situ prepared nanostructures (micellar particles) and make them UHV compatible, and then do all the electronic and chemical characterization and chemical reactivity in-situ.
HOLLOWAY: So do you transfer samples from station to station?
ROLDAN: Everything is done in UHV. I started with a system one-third of the size that I have right now. We have been building up over the years. In terms of building new things, once I got tenure it was easy for me to dedicate time to start building new systems. I couldn't afford spending that time at the beginning of my independent research career. Right now we're building an in situ magneto-optical Kerr effect system. That's the design of one of my PhD students together with my advice, and this new UHV system is now almost ready.
HOLLOWAY: What's your objective in the study of magnetic materials? Just a fundamental understanding?
ROLDAN: For many years there was always the question of how do the magnetic properties of nanomaterials change as a function of their size and shape. But there is always the limitation, if you prepare these materials for example by evaporation in vacuum, that you can't really control their size or shape. You can do annealing treatments to try to change the nanoparticle morphology, but you cannot control the inter-particle distance. We have developed chemical synthesis methods in which we can control the distance between the particles, the size, and the shape. In the last years, I was able to make sure that we can clean those particles from any ligands that we use during the synthesis, put them on different surfaces, like single crystal surfaces, and create the perfect model system to look at their magnetic properties as a function of the different geometrical parameters, one parameter at a time. I can decouple changes in the shape and changes in the inter-particle distance and inter-particle interactions from changes in the size. I want to make a link with catalysis there, so I'm also interested in looking at how chemisorption affects the magnetic properties of these small structures. Like how CO and ammonia absorption can reduce the magnetic moments at specific surface sites within nanoparticles.
HOLLOWAY: Do you have some preliminary results from the studies?
ROLDAN: No because we just finished buying the individual equipment and UHV parts and we are building right now. This is the latest project that we are undertaking.
HOLLOWAY: But you reported some very interesting data in your talk yesterday about the colloidal synthesis of nanoparticles. Could you tell us about that?
ROLDAN: Yes. We are using the PS-PVP diblock-copolymers to make micelles. The idea is that you have a polymer that has one part that is polar and the other part that is non-polar. When you dissolve that polymer in a non-polar solvent, it will make those reverse micelles. Then you can take those micelles that are like nano-cages and you can fill them with a metal salt. Any metal will work, and also some semiconductors, so it's a very flexible synthesis method. You can also make isotopically-enriched nanoparticles that we use for the Mossbauer spectroscopy and nuclear resonant inelastic X-ray scattering studies by loading the micelles with for example a 57Fe salt. We can also make bimetallic nanoparticle systems by just adding two metal salts to the polymeric solution at the same time. Then we dip-coat the nanoparticles on a surface, or you can also take a solution of nanoparticles and impregnate them in a nanocrystalline powder support and then make real-word nanocatalysts. So they are flexible to be used in UHV or also in high-pressure applications. Then the main challenge is to remove the ligands without affecting the properties of the particles, affecting the size, the shape, the distribution. For that purpose we use plasma treatments in a high vacuum or annealing treatments in oxygen.
HOLLOWAY: So in most of the cases, the ligands are removed by an oxygen plasma?
ROLDAN: Yes. You can use an oxygen or hydrogen plasma treatment. Oxygen plasma is more efficient. But if you work with systems that can oxidize, like iron, you will do first the oxygen plasma treatment to get the particles clean (polymer-free), and then will reduce them with an hydrogen plasma treatment. So you use a combination of both. There are many methods to prepare nanoparticles. I think that there are two things that make this method special. One is that the length of the polymer tail that we use can control the distance between the particles, which is very useful for many applications. For example, in catalysis if you have particles very close to each other and you anneal or you have any reactants that will destabilize the particles, you can coarsen them, and with increased particle size the reactivity might change and normally will decrease. If we can keep the particles far apart enough, this won't happen. Also, if you are interested in magnetic properties, you might be interested in seeing what happens as a function of the distance between the particles. The other advantage of this synthesis is that you can control the size of the nanoparticles either by changing the length of the polymer head that will give you the size of the micelle, or by tuning how much metal you load. So those are the advantages.
HOLLOWAY: Now you showed a difference in the behavior of dispersed clusters of gold between micelle generation versus evaporation. Is that true?
ROLDAN: Yes. One of the things that to me was most interesting is that for many years people were working with single crystal TiO2 in UHV, evaporated metal seeds in UHV, and then they annealed them in-situ to make clusters, and then looked at the activity of those structures. This is normally done on TiO2(110). The main problem is that when you carry out any of the reactivity experiments at elevated temperatures, those clusters like to form two-dimensional islands, very large, 100 to 200 nanometers in seze, and those are not very active. They are also always supported on very flat model single crystal surfaces. With our synthesis method, we were able to make smaller particles. It can go from 0.4 nanometers, this are kind of our smallest size, to 30 nanometers. We can tune that. But you can make small sizes, and at the same time you can anneal those particles and see then how they melt and how they reconstruct, without having them sinter to each other. So we can keep the particle size constant up to annealing temperatures of at least 1060 degrees C. What we see is that depending on the melting temperature of the particular metal, the stability of the particles of course is going to be different, but it's always much higher for the micelle-synthesized particles than for the UHV-evaporated particles. One of the reasons is that when you evaporate the particles in UHV, you get a couple of cluster seeds, but there are also loose isolated atoms or dimers in between the cluster seeds and particles. And there is a size distribution that favors coarsening processes such as Ostwald ripening. In my case, I have the particle size selected. They are farther away from each other, and there are no loose atoms in between, which does not make Ostwald-ripening based coarsening processes favorable.
HOLLOWAY: So you did a size selection on the colloidal micelle?
ROLDAN: Yes. The micelles that we prepare encapsulate the metal nanoparticles, making them size selected.
HOLLOWAY: Very nice. You made a lot of advance and accomplishment in a short time for your career. Tell me what you consider to be the most important attribute in order to make those advances.
ROLDAN: I think it's perseverance. I was working in UHV. When I moved from Spain to Germany, I didn't have any experience with ultra-high vacuum. Coming in as a PhD student, it is not the easiest field to work in because many times you need to do bake-outs, the system breaks down, so there are a lot of things that will challenge you and will test your patience. The first thing that I learned during my PhD is that if you are working in vacuum, you need to become patient. It has certain advantages. It makes you not give up so easily because you know that sometimes you are going to need to be chasing experiments for a long time, so it's not like some of the other fields, you do a quick measurement and in a month you can have a publication. For vacuum people, we are used to really spending a year doing measurements before we can have something that will could become a complete story. So I think it's perseverance—working hard, that's all.
HOLLOWAY: So where do you see yourself going in the future?
ROLDAN: That's a difficult question. I don't think that far in advance. Right now I kind of still live by the deadline. I enjoy very much the work that I do, so I see myself at the university, I see myself teaching, and mentoring PhD students is something I enjoy. In terms of research, there are so many advantages and so many opportunities that you have right now with the advent of nanotechnology that I believe will allow us to reach major breakthroughs in the field of catalysis. For energy generation purposes, for example. So this is something that I'm very excited about. I'm a physicist with some projects that are solid-state physics related, some projects that are physical chemistry related, and some projects in the field of chemical engineering. I work in the interdisciplinary field of nanoscience, and right now I think that we have the right scientific atmosphere for this type of research, because funding agencies are encouraging this type of multidisciplinary research. I am very excited to be working in this field.
HOLLOWAY: So you sort of work at the junction and boundary between physics and chemistry and materials, chemical engineering. Is that why you're in AVS, because we're sort of an interdisciplinary society ourselves?
ROLDAN: That's very important, actually, because sometimes when you are working in these fields, it is very difficult to know where you fit in. So I am a member of the American Chemical Society, American Physical Society, and also the AVS. But to tell the truth, the meetings that I attend the most often are the AVS meetings because here you can meet your physics colleagues, your chemical engineering colleagues, and your chemistry colleagues, so there are sessions that are oriented to all the fields that have in common either characterization techniques or certain approaches. I think that this is a major advantage of the AVS, that you have reunite interdisciplinary fields just by the nature of the society.
HOLLOWAY: You not only come to the meetings, you actually participate in planning and organization. Give us some indication of the positions you are occupying now, the committees, and why.
ROLDAN: This year I was nominated to be part of the Surface Science Executive Committee, and I was elected, so that was a great honor for me, and I think it is a very good opportunity to make an impact in trying to bring new views, new topics to sessions, new speakers. Yesterday was our first meeting, and it was very exciting because we are really given the chance to plan for the future and to make sure that all the fields are represented. Everyone on the committee has a different field of expertise, so I think that is something exciting. I also taught short courses for the AVS before. I was sent to Mexico, for example, to teach a nanophysics course, so I had that experience. As a student, the first time I came to the AVS I came thanks to one of the student awards, so I have been related to the AVS since I was in Germany.
HOLLOWAY: So who nominated you to do the nanophysics short course?
ROLDAN: Gary McGuire and Joe Greene.
HOLLOWAY: So it's important for the people to make sure that you are nominated and actively participate.
ROLDAN: Yes, I think that when you start as an assistant professor, you don't know very many people in the community. But I think the AVS community has been really very supportive for young people, and I am very appreciative of that. This Peter Mark award is one example. Also the student awards of this society are very encouraging. When the opportunity for the short courses appeared, I was very happy to be able to participate and contribute to the national and international student training and recruitment to AVS-related fields. Actually Joe Greene was involved in that process, but it was Gary McGuire the person that suggested me for that. He was also the person that nominated me for this award, so I'm very thankful to him.
HOLLOWAY: He has been active in the society.
ROLDAN: Yes. I'm also part of the Editorial Board of the Journal of the Vacuum Science and Technology B under his supervision; he is the main editor. We are thinking about strengthening my involvement. Right now I think he is taking the major load, and we are planning for the next one or two years to have me more involved in that.
HOLLOWAY: How important is it for young people to become active and give back through services like that?
ROLDAN: I think it goes both ways. For the younger people it's a great opportunity because you can get to know senior people in the community, and you can have new collaborations or you gain some visibility. At the same time, for example for the planning of the program, sometimes you go to meetings and you always see the same people over and over giving the same talks. When some younger people come into these planning meetings and commitees, they might know different people. You know, we are a different generation. So it might be easier that you can refresh the program by bringing new views, new people. So I think that's something that we can contribute. Also, I think in terms of mentoring the students, younger students might be encouraged if they see younger people also involved in these committees, because they might think that you don't need to be really famous to be able to be a participant of these activities, you can be just a junior person.
HOLLOWAY: It's important to make sure that message is conveyed. It's critical to the society and for advances. I noticed that you were also active in high school programs, getting high school and K through 12 involved with science. Could you tell us a little bit about that?
ROLDAN: When I got my NSF Career Award, it was one of the things that I promised, that I will start a program to involve K-12 students in university-level research. What we are doing is called the ASPIRE program. It provides opportunities for local high school students to come to research labs, and I have had many students over the years already in my lab through this program. Right now I have one. I have normally between one and three students in my group at a time. It has advantages for the students because they get to know how research is done, they get to know sophisticated equipment that they have never seen in their lives. For them it's very exciting. For the people in my group, first you learn to interact with younger people, how to teach, how to mentor younger students. I have the help of my undergraduate students to mentor my K-12 students. There is a very small age gap, so it is easy for them to interact. At the same time, they provide a very good atmosphere to the group because they are very enthusiastic. At present, most of my sample preparation and AFM work is being done by K-12 students. We prepare presentations for science fairs and things like that. I think this mix of some senior people in the group, some PhD students, and students from K-12, it provides a nice atmosphere where people are happy to be working together.
HOLLOWAY: That's remarkable. You're paying back. We've covered some territory and your career. Is there anything else you would like to add to the interview?
ROLDAN: I would like to thank the AVS members for this award, and the people that nominated me for the award, especially Gary McGuire. I wouldn't have applied to this award if it wasn't for him. I would like to thank the encouragement of scientists in the AVS field, including people like Charlie Campbell that has been a model for me as a surface scientist. My PhD mentor, Prof. Werner Keune is also a person that I am in debt to. My family I think is very important, especially if you are female and you come from Spain, it's not very common to become a physicist. I think that the encouragement of the family to pursue a scientific career is very important. This is something I'm working on with the families of my K-12 students. I don't just talk to the students; I also talk to their parents because many times, the parents don't even consider that there are opportunities in science. Still scientists and physicist are regarded by many people in society like strange creatures. My family was very open to that, and my father in particular. He was very supportive of my career choice.
HOLLOWAY: I'm sure they're proud of you. Anything else? Well thank you very much for participating in the interview, Beatriz. It was really good. Congratulations again on the Peter Mark Award.