Interview: Mohan Sankaran
2011 Peter Mark Memorial Award Recipient
Interviewed by Paul Holloway, November 2, 2011
HOLLOWAY: Good afternoon, my name is Paul Holloway, and I’m a member of the AVS History Committee. Today is Wednesday November 2, 2011. We’re at the 58th International Symposium of the AVS in Nashville, Tennessee. This afternoon I have the privilege and pleasure of interviewing Dr. Mohan Sankaran from Case Western Reserve, who is the 2011 Peter Mark Memorial Award winner. His citation reads, “For the development of a tandem plasma synthesis method to grow carbon nanotubes with unprecedented control over the nanotube properties and chirality.” So Mohan, it’s a pleasure to congratulate you on the Peter Mark Award.
SANKARAN: Thank you, Paul, I appreciate that.
HOLLOWAY: How about getting us started by giving us your birth date and birth place.
SANKARAN: Sure. I was born on October 2, 1976. It’s a famous date in Indian history because it’s the birth date of Mahatma Gandhi. But I wasn’t born in India; I was born in Palo Alto, California.
HOLLOWAY: But you were born under the proper sign, then.
SANKARAN: That’s right! [Laughter]
HOLLOWAY: Tell us a little bit about your educational background.
SANKARAN: Sure. I’m going to start with my college, undergraduate. I did my bachelor’s degree in Chemical Engineering at UCLA, and that’s when I first started some research. I worked in the Chemical Engineering Department with Prof. Harold Monbouquette. And then after doing my bachelor’s degree, I went on to get my Ph.D. nearby Los Angeles in Pasadena at California Institute of Technology, also in Chemical Engineering. I stayed there for a very short stint as a postdoc after I defended my thesis and finished my Ph.D. for about six months, and I had a job lined up before I finished my Ph.D. and I went straight to Case Western Reserve University to start my faculty position.
HOLLOWAY: What year was it that you got your bachelor’s degree?
SANKARAN: I got my bachelor’s degree in 1998 and my Ph.D. in 2004.
HOLLOWAY: And all of them in chem engineering?
SANKARAN: That’s right.
HOLLOWAY: So you did a postdoc there at UCLA.
SANKARAN: Actually at Caltech.
HOLLOWAY: Caltech, sorry. And where did you go after that?
SANKARAN: I went straight to start up as an assistant professor at Case Western.
HOLLOWAY: So you went in fresh? [Chuckles]
SANKARAN: I did. It was maybe a decision I had to make, whether I wanted to do a postdoc to get a little bit more experience. But I had the job interview before I finished my Ph.D., and the folks at Case Western Reserve convinced me that I could start as soon as I wanted, and making a real salary sounded good to me. [Laughs]
HOLLOWAY: Yeah, making some real money rather than spending it.
SANKARAN:That’s right.
HOLLOWAY: That’s good. So which department did you go into there?
SANKARAN: Also Chemical Engineering.
HOLLOWAY: How many faculty are in the Chem Engineering Department at Case Western?
SANKARAN: So we’ve fluctuated between about 10 and 13. When I first started I think we had about 13, and at the moment right now we’re at 10.
HOLLOWAY: Let’s see, so you’ve won some awards from NSF as well. Is that true?
SANKARAN: Yes, that’s right. I received an NSF Career Award in 2008.
HOLLOWAY: Those are very convenient and very nice, and very difficult to achieve.
SANKARAN: Well, it’s nice to not have to compete with everyone. At least I’m only competing with people in my starting position or age group [laughs]. So it was very nice, and it’s been very instrumental for me to start my career with some of those types of grants.
HOLLOWAY: Tell us a little bit about your work. What is your focus in your research area?
SANKARAN: Sure. So as a graduate student I developed a plasma source that works at atmospheric pressure, and we call it a micro plasma because it’s also formed over very small length scales. The reason it works at atmospheric pressure is pd scaling—it’s a well-known property for electrical discharges that if you scale d down you can scale p up, and the pd scaling is preserved. So we are able to go to atmospheric pressure and not have to use any vacuum pumps to operate a plasma by going to real small scales. So we form a plasma over length scales of microns or so, 100 microns actually, and this allows us to have a plasma at atmospheric pressure. That in itself is really important because in lighting applications it’s really important to be able to have a plasma without any vacuum seals and things like that. My interest as a chemical engineer has always been in processing, especially material processing, so we developed the micro plasma for a potential source for material processing at atmospheric pressure without any vacuum reactors. So I started that work as a grad student, and then by the end of my Ph.D. and into the postdoc that I did a very short stint with another faculty, I started getting interested in using my micro plasma to make nanomaterials, particularly nanoparticles, and that’s what I started at Case Western is a lab that was focused on using micro plasmas for nanomaterials synthesis and processing. And so some of the things that we do is we use this plasma to disassociate different precursors that can then nucleate nanoparticles in the gas phase. And then after we nucleate these particles, we can deposit them as a film or we can collect them and put them in a solution. The advantage is that we have a lot of versatility because the particles are made sort of in a very generic scheme, and so we have lots of different materials that we can play with that normally aren’t as easy by chemical routes. And we don’t have any coatings on the particles, no surfactants, and so they’re made very clean, and they have some good properties because of that.
HOLLOWAY: So you can make them in nanometer scale sizes, then?
SANKARAN: Yes. So we’ve gone as small as about 2 nanometers. Anything smaller than that, we have trouble characterizing them very easily. But yeah, we get down to 2 nanometers, and truly nanoscale.
HOLLOWAY: What’s your particle size distribution?
SANKARAN: So that’s the other thing, we think we have a process that has a pretty good dispersion in terms of how mono-disperse it is. So if we make particles that are 2 nanometers, they’re usually plus or minus about 0.2 to 0.3 nanometer.
HOLLOWAY: That’s a very tight distribution.
SANKARAN: Yes. So standard deviations are usually about 15 to 20%.
HOLLOWAY: Let me back up just a moment and ask you about Caltech and who your mentors were there. Did you mention that already?
SANKARAN:: No I didn’t. So I mentioned at UCLA I worked with Harold Monbouquette. I’ll tell you a little bit about him, too. So as an undergraduate, that was the first time I was exposed to nanoparticles. I worked with Harold Monbouquette, who is really more of a biochemical person in chemical engineering, so he does a lot of things with enzymes. We had a smaller project that I worked on with another Ph.D. student where he was using phosphatidylcholine vesicles. These sort of mimic cells. They’ve got the bilayer membrane, but you can make them synthetically. And we use that as a reactor to grow cadmium sulfide nanoparticles. So the cadmium and the sulfide would react inside the vesicle, and you can control the volume of the reaction, so you can control the particle size that it would end up making. And these cadmium sulfide particles are semiconductors, and when you make them down at the small sizes they’re what’s known as quantum dots, so they have this fluorescence that you can shift based on the size. So that was a project that I worked on as an undergrad, so it wasn’t my own project; I worked with a Ph.D. student. But I got pretty involved in that, and I was sort of fascinated by these kinds of materials.
HOLLOWAY: What was the material you were working with?
SANKARAN: Cadmium sulfide. And so when I decided on graduate school I wanted to continue some work with nanoparticles, but in Caltech in the Chemical Engineering Department they didn’t have someone that actually worked on nanoparticles. But I was able to find someone who worked on materials, and some of the materials that I was interested in also, like other semiconductors like silicon, and so I ended up working with Konstantinos Giapis, and he is also a plasma person, so he was working on semiconductors but he was working on plasma processing of semiconductors. I think this turned out to be kind of a good thing in the long term, because it wasn’t exactly what I was intending to do in graduate school, but it opened up my eyes to other things. During my Ph.D., as I said, I developed this micro-plasma source, which was a smaller project in this group, but I got exposed to all kinds of things related to plasmas, etching, deposition of films, things related to AVS. I became involved in AVS for the first time because Giapis was always heavily involved in AVS. And all these things ended up maybe indirectly or subconsciously affecting my future career because I started thinking a lot about how plasmas can be used for nanoparticles, which I think is still a pretty small community, and that’s what I ended up doing now as my career.
HOLLOWAY: Yeah, most people think about either lithography for scaling from the top down or solution growth from the bottom up.
SANKARAN: Exactly.
HOLLOWAY: But plasma is a new technique that hasn’t been exploited that much.
SANKARAN: Yeah, you could have done the introduction for my talk, because that’s exactly what I said! [Laughter] I said that we were working, trying to take the best of both worlds—that’s how I put it—the solution phase approaches with plasmas.
HOLLOWAY: So you mentioned earlier pd scaling. So d is the distance between the probe tip and the substrate, and p is pressure?
SANKARAN: Yeah, that’s a good question. D becomes a little complicated especially in the plasmas that we work in. We actually have a spacing between the two electrodes that we apply the DC voltage to, but we also have an electrode that has a hollow cathode. And it turns out that the hollow cathode is really the scale, in our case, that matters more because that’s where the electrons are generated. So we scale the hollow cathode down appropriately with the pressure, so that the pressure time the diameter of the hollow cathode scales appropriately. And the other electrode doesn’t matter as much, actually, the distance, although it has to be somewhere on the order of the diameter of the hollow cathode as well.
HOLLOWAY: So for atmospheric pressure, what sort of d’s are you working with?
SANKARAN: 100 microns does the trick, so at about 100, if you get down to 100 microns atmospheric pressure becomes stable.
HOLLOWAY: Now, you in a very nice presentation this morning mentioned that you can do plasma deposition under a liquid. Is the scaling different under a liquid?
SANKARAN:: So the liquid acts as sort of our other electrode, just like a metal anode would in the case of a DC plasma. So I would say that no it doesn’t, because again, the hollow cathode that we use is preserved in that case, and the liquid is just sort of another electrode that extracts the plasma. But there could be situations where what you’re saying is absolutely true, because there are some people that are working on having liquids that, for example, ionize, or maybe contribute electrons into the discharge. And I could see that in those cases there might be an effect of scaling and how the plasma is stabilized.
HOLLOWAY: The other thing I was wondering about is the pressure. When you’re taking the molecular density of gas, it is much less than that of a liquid, even at atmospheric pressure.
SANKARAN: Yes, absolutely.
HOLLOWAY: Does that come into scaling at all?
SANKARAN: So again, in our case we do form the plasma in the gas phase, and then it interacts with the liquid at a boundary. So at the boundary you’re absolutely right. But there are other people that are working on plasmas inside the liquid, and they’ve been able to break down the liquid and form a discharge. In some cases they form a gas bubble, and the gas bubble is where the discharge forms. There are situations where what you’re saying is even more relevant. I would say not as related in our case because the plasma is formed mostly in the gas phase.
HOLLOWAY: Let me take you back again, and you said, if I’m correct, that you had another mentor during your brief postdoc. Could you mention that?
SANKARAN:Yes. Thanks for reminding me. So at the end of my Ph.D. I was working on plasmas, and my advisor was very open to allowing me to do some things that were sort of my own interests, and one of them was trying to make nanoparticles. We collaborated with another faculty in the same department, Richard Flagan, who is a well-known person in the aerosol community. He also had projects where he was making nanoparticles in the laboratory. So a lot of his work was studying aerosols in the atmosphere, environmental type things. But he had a project where he was making nanoparticles in the lab, and he was doing measurements on nanoparticles that were made as an aerosol in the gas phase. He and I and my advisor Costas came up with this idea of trying to make the nanoparticles in our plasma, and then characterizing the particles using his aerosol instrumentation. And this exposed me to another community, which is the aerosol community, but we had a plasma technique to make the nanoparticles. Usually in the aerosol community they use things like thermal pyrolysis and stuff like that. So it opened up an area for me with aerosol instrumentation, which I use heavily in my lab now; and also in things related to aerosols, how they form, nucleation, how they can be deposited. So you have to use electric fields to deposit these things because they almost act like atoms—they sort of flow with the gas flow rather than just depositing by impaction because they’re so small. And so all these kinds of things were opened up to me, and it’s become a big part of our laboratory. So I think I balance between plasmas and aerosols in everything that I do.
HOLLOWAY: So are all the plasmas that you use at atmospheric pressure? Or do you span a range of pressures?
SANKARAN: I used to do some low pressure, but since I started at Case Western it’s been fully atmospheric pressure. It’s made my life a lot easier to develop my lab. [Laughter]
HOLLOWAY: Yeah, you don’t have to mess around with that vacuum stuff. [Laughter]
SANKARAN: Yeah! So I probably shouldn’t say that here! [Laughter] But AVS has changed so much. I think it continues to change every year.
HOLLOWAY: It’s necessary. If you’re not changing, then you’re stagnant, and stagnant means that you’re making buggy whips and trying to sell them.
SANKARAN: [Laughs] Right. And I was in the atmospheric pressure plasma session today, so I think that was still appropriate, even at AVS.
HOLLOWAY: What’s known about atmospheric pressure plasmas? Are they well understood and well characterized? Or there’s a lot to be learned about them yet?
SANKARAN: There’s a lot to be learned. So I think that there was an obvious motivation for it I would say back to the ’70s. There was work I know done by Ulrich Kogelschatz. He was developing atmospheric plasmas for ozone generation, and he was at a company in I believe in Switzerland that pushed this research a lot. And a lot of the initial things that I learned about atmospheric plasmas were from his work. He did a lot of research, so he was in a company. They developed DBDs, dielectric barrier discharges, which I would say was one of the earliest atmospheric systems. There has also been a lot of work done on coronas, and even arcs there’s been a lot of work. But even despite all that, there are a lot of challenges with atmospheric plasmas. It’s always hard to sustain, so a lot of these discharges can have spatial and temporal inhomogeneities that make it hard to characterize compared to a low-pressure plasma where at least you have the positive column that is relatively uniform. In a low pressure plasma, the sheath is hard to characterize, but the rest of the plasma is not so hard. The other issue is you have heating, so it’s hard to do things with the plasma, and the electrodes aren’t stable and things like that. The other issue is a lot of these plasmas, because of that pd scaling, are scaled down. In the case of micro plasmas, what makes them really hard to characterize is that you can’t stick probes in them because the probe affects the plasma where the probe is bigger than the plasma itself. And so you’re limited to optical methods, and optical methods are always indirect. So that was done early on, but they’ve had to develop a lot of advanced optical methods to really characterize different things. And then of course modeling, once that’s become powerful enough now within the last decade, that has really helped also in advancing things. But we’re still learning new things. In fact I was at a meeting just a few weeks ago, and there was work presented by David Go, who is at Notre Dame, and he’s shown that these plasmas at high pressures and microscales deviate from Paschen’s breakdown. This is theory that was done over 100 years ago, and now we’ve found that Paschen’s law doesn’t hold at these scales. I found that to be fascinating, that something that everybody knows, there’s something new there. So I absolutely think there are a lot of new things to be learned from these systems.
HOLLOWAY: Now, I keep thinking about the analogy of micro plasma and atmospheric plasma as arc welding. That’s inducing a plasma to heat exclusively.
SANKARAN: Yes.
HOLLOWAY: Probably not promoting the chemistry. You’re trying to control and promote the chemistry at the same time.
SANKARAN: Right. So the biggest difference between the arc welding and our plasma would be that an arc welding plasma is really at equilibrium, so it’s a thermal plasma and everything is pretty hot. And you want that, in that case. In our case we don’t want things to get hot. We try to prevent that. Number one, we don’t want to destroy our electrodes if we don’t need to because we like to keep the plasma on and stable and everything. The other thing is if we’re doing material processing, we’re probably working with materials that could get damaged if there’s heating around. So we try to keep the gas temperature as low as possible, so they’re referred to as non-equilibrium plasma. Now there’s another advantage to that, which is that in a thermal plasma the electron energy gets depressed, since the electron energy is mostly from thermionic emission, which is not going to create high electron energies. Whereas in a non-equilibrium plasma you have electrons created by secondary electron emission and other electron excitation mechanisms. And so we like those energetic electrons because we use them to drive a lot of chemistry. So we want electron energies over 1 eV, which is really only possible in a non-equilibrium plasma.
HOLLOWAY: So how high will the electron energies actually go?
SANKARAN: That’s the other neat thing about our plasmas. Even though they’re atmospheric pressure where you would think collisions dampen the electron energy, the hollow cathode geometry that we use actually can give you very high electron energies. We have mostly electrons that are probably very low energy, so if you look at the electron energy distribution it might be like a Maxwellian, or slightly from Maxwellian they call it a non-Maxwellian. In some cases it might be slightly different. But essentially it will be peaked at about fractions of eV, but the tail will have electrons as high as 10 eV or even higher, and it’s those tail electrons that really drive a lot of the chemistry that we do.
HOLLOWAY: And they get accelerated by the fields in the plasma?
SANKARAN: Yes. Mostly by the sheath.
HOLLOWAY: Now, you’ve used the plasma to make nanoparticles. I must confess, I don’t understand how a plasma discharge at atmospheric pressure would wind up generating two nanometer particles. Could you give me a brief education in that area?
SANKARAN: Sure. So there has been a lot of evidence of this that was mostly accidental, so I’ll maybe start with that as an observation of how this works. So there are lots of people in the plasma community that are interested in using chemistries, gases that can create radicals and things like that, to deposit films. One example would be in the silicon film deposition that’s used for depositing polycrystalline silicon as gate material, or in silicon photovoltaics to make silicon films. You take silane gas and you use a plasma to break the silane, you form radicals like SiH, SiH2 and so on, and these things deposit to form your film on the substrate. Well people found that if you weren’t at the right conditions, like the pressure was a little too high or your concentration of silane was a little high, because you usually have another gas like argon to dilute it, but if you’re at the wrong conditions, they found that these radicals would nucleate particles in the gas phase. And then you get particles contaminating the device. So people really worried about it. They called it a dusty plasma, and it was considered a contaminant and they wanted to avoid it. So then some very smart people realized that is was also a way to purposely make particles. This happened all about the same time when people were interested in nanoscale materials and devices with nanomaterials and things like that. I couldn't tell you exactly who the first person was, but I can tell you one of the people that contributed to this work was Pere Roca I Cabarrocas in France. I remember following his work as a grad student, thinking about a lot of these things. So now if we talk about my plasmas, we’re pushing the pressure even higher. This is moving beyond just low-pressure plasmas to atmospheric, where gas phase collisions are really important. Your density is higher. So any time you form any reactive spaces in the gas phase, they’re going to find each other. And so the idea that once they find each other, you’ve got now a chance to nucleate because these silicon atoms or these metal atoms want to get together and form a little cluster. We’re just trying to promote that. So I would say it’s sort of a nice history, because we went from not wanting it to now wanting it [chuckles].
HOLLOWAY: Yeah. It must be analogous to the evaporation out of a metal boat…
SANKARAN: Absolutely.
HOLLOWAY: …at high-pressure argon where you get the metal smokes.
SANKARAN: Yes, it is. And people have done that also to make nanoparticles. They either evaporate metals by heating a powder or laser ablation of a solid target, and when you get the metal atoms evaporating you then have an area where they cool and they find each other, and you get the cluster formation that way.
HOLLOWAY: You’ve used your nanoparticles for a variety of purposes, including catalysis for growth of nanotubes. Could you tell us a little bit about that work?
SANKARAN: Sure. I really consider myself more of a plasma and nanoparticle person more than a carbon nanotube person, but we sort of stumbled upon something that led to a really interesting story. So initially when we were making nanoparticles we were picking out different types of materials to make, and we decided to try some different metals, and then eventually we made nickel and iron particles, and we started getting some really neat results for those particles, where they were very uniform in size and so on. And we wanted to demonstrate that our materials could be used for something useful, and so when we were looking at applications of nickel and iron particles, we found that they are very often used for growing carbon nanotubes. The obvious reason is that nickel and iron have very high carbon solubilities, and so they work really well to nucleate nanotubes. So we thought we would try growing nanotubes and demonstrate that our particles could be used as a catalyst for nanotubes, and that’s how it all started. We initially got some positive results. We were able to grow nanotubes; we were able to control some different properties of the nanotubes. And then eventually the way that story ended up developing is that we decided that we could tune these catalysts and we can make these bi-metallic catalysts where we mix nickel and iron together, and we wanted to know how that would affect the nanotube growth. And so we developed a very systemic experiment where we tried to just compare nickel, iron, and different compositions of these bi-metallic nanoparticles that we were making in the lab to what happens with nanotube growth. That sort of became a really important achievement in our group.
HOLLOWAY: You talked about separating a multi-wall and single-wall and the chirality of the nanoparticles. All that came from that research.
SANKARAN: That's right. So once we were able to grow nanotubes and once we were able to relate the nanotubes to the catalyst, this happened very fast. We were able to get lots of interesting data. So we were first able to show that if you changed the size of the particle you would change the type of nanotube you would get—you would go from multi-walled to single-wall nanotubes by reducing the catalyst size. That was the first thing we tried. That had been done by other methods, but in some ways our method was a little bit cleaner because we didn’t have any substrates and our process was more scalable. But it had been demonstrated by others. So the other one that we were perhaps more interested in, in terms of its uniqueness and potential impact was trying to control the chirality of the nanotubes. The way we did that is by changing the composition of a bi-metallic catalyst. So there we fixed the diameter of the particles and we just changed the metal composition, and we found that we could start to preferentially grow certain types of chiralities.
HOLLOWAY: What other types of applications have you investigated for your nanoparticles?
SANKARAN: So we have looked at other similar catalytic growth. Carbon nanotubes are grown from a nanoparticle, and there is a very similar mechanism for how semiconductor nanowires grow. Semiconductor nanowires have lots of applications as well, just like nanotubes. In some ways there’s an advantage because they’re all semiconductors, so you don’t have this problem with metallic and semiconducting tubes having to be separated. So we’ve done some work where we’ve used our nanoparticles as catalysts for nanowires.
HOLLOWAY: If you grow semiconductor nanowires at atmospheric pressure plasma, do you have an abnormally large incorporation of impurities and/or development of traps?
SANKARAN: So we actually don’t grow the nanowires at atmospheric pressure. We use the catalyst that we made at atmospheric pressure and we either deposit it on a substrate or find another way to grow the nanowires. That’s the way we’ve done it so far. I think if we grew them at atmospheric pressure we could still reduce the impurities because we’d be using clean gas, but you’d have to be sure that you’re using very clean gas. And then you could vacuum pump maybe before to remove water and things. But you’re right, that could be an issue.
HOLLOWAY: Are you alone in working with miniature plasmas at atmospheric pressure at Case Western, or do you have colleagues there that you collaborate with in the area?
SANKARAN:: I would say that not only am I alone in working with atmospheric plasmas and micro plasmas, but I would generalize that to say I’m really the only one that does a lot of heavy work with plasmas. There are other people that use plasmas on campus, but they may have a plasma system that’s a commercial instrument that they might use for some small things like sputtering metal or removing polymer using an oxygen etch, or something like that. But I’m pretty much the only one that has a research lab that’s really focused on plasmas.
HOLLOWAY: Have you had a mentor there at Case Western that helped you through the development of a research program, comprehension of requirements for teaching, and organization of your life so that you can go to sleep a little bit at night?
SANKARAN: Yeah, I don’t think I would have succeeded as an assistant professor if I didn’t have some really, really important mentors. We have I think a very collegial department, maybe because it’s a little bit smaller. I would say I’ve had two really instrumental mentors in my career at Case Western thus far. One is John Angus. John Angus retired soon after I got there, and I actually took over his lab. I’m in what used to be known as the Diamond Lab. John Angus did most of his career on low-pressure CVD growth of diamond. I’ve done a little bit of work with diamond, so I’ve gotten a chance to interact with him on research, and we still chat a lot and we have some collaborations and things. But beyond that, he has always been a sounding board for me. Like I’ve bounced ideas off him, I’ve had him review my grants, I’ve had him review my manuscripts that we’ve published, and he’s just been a great source for me to talk about anything. Another mentor in the Department C. C. Lu. His name is Chung-Chin Lu, but everybody calls him C. C. He works on microfabrication of sensors, and so there is some overlap between him and me in some of our research. But again, he’s someone that has really just been sort of a person that I could talk to about anything. And we’ve recently collaborated on some research, too. I’ll mention one other person who’s not really a mentor; he’s really more of a collaborator for me in the Department, Daniel Lack. We have another project that is not related to plasmas. It’s on triboelectric charging of materials and electrostatics. I’ve done a lot of work through this collaboration; we’ve published I think over ten papers. That’s been a real important thing for me in my career to have these publications with him. He has been I would say really helpful and bringing me along in this project that was outside for me. It’s an opportunity to collaborate with him, and having sort of a successful research that’s complementing everything else I do has been really important.
HOLLOWAY: That’s a really fascinating area. It always fascinated me, the fact that you can have the atmospheric imbalance, and thunderstorms build up and you have charge separation large enough to have dielectric breakdown of air called lightning. Are there any similarities between what you do and that?
SANKARAN:Absolutely. That’s a great example of what we do. There are lots of really good examples of it. Lightning is definitely one of them. More specifically, what we work on is how materials charge, and then lead to things like breakdown. And so in volcanic eruptions, the ash that forms is believed to erupt together, and that charging leads to a high propensity of lightning over a volcanic eruption. You get these beautiful displays of lightning above volcanic eruptions. Similarly in dust storms, which is something that he and I have actually gone out to the field and studied, in dust storms you have maybe wind erosion and other effects causing the sand particles to rise up, and those particles can rub and also create lightning and other types of breakdown. So those are examples that we don’t really focus on because we do most of our experiments in the lab, but we use those examples all the time to help support our work and motivate our work.
HOLLOWAY: Tom Dickinson at Washington State did a lot of work in that area, as I recall. So what’s the mechanism of tribocharging? Is it bond breaking and charge trapping at surface states?
SANKARAN: Dan, my collaborator, and I just wrote a review paper, and the short answer is it’s still not known. There are three proposed models that have been out there in the literature, and some of them were just recently proposed. The first model that has always kind of been the one that everyone has generally accepted is based on electron transfer. So if you have two metals, the driving force is difference in work functions, and the electrons will move from one metal to the other. If you have insulators, people talk about these trapped electrons. So even though an insulator doesn’t have electrons within the band gap, you may have some trapped states that are there because of either deformation caused by the rubbing, or maybe the material isn’t perfect to begin with. But more recently, the one that you just suggested is the one that has been proposed, where you actually have either mechanical deformation or some type of bond breaking and transfer of a material from one material to another. And that could be activated by some type of mechanical deformation, chemical reaction; it’s not really well known. People talk about that also in terms of ion transfer, so if you have two materials and you have some type of mobile ion around, the ion can move between the two materials.
HOLLOWAY: Truly a fascinating area.
SANKARAN: Yeah. And it’s actually considered one of the oldest areas of scientific study, because even the Greeks were doing it to understand charging and how it could be used for different things.
HOLLOWAY: Do you study triboluminescence as well?
SANKARAN: We don’t. Most of our experiments are done by measuring the electrostatic charge on the material and doing a lot of chemical characterization of the materials. But there are others in the field doing that. We don’t have the setup for that.
HOLLOWAY: Let me turn your comments to another subject, and that is as a young professor and a Peter Mark Award winner, what would you advise other young graduate students who are about to enter the academic world as to how to be successful?
SANKARAN: What mistakes that I’ve made that I would tell them to avoid? [Laughter] So that’s an important question. I’m trying to think. There are a lot of things that I could probably think of. Well, these are my own words, but I think this has come through from conversations that I’ve had with different mentors. I would say first and foremost, probably don’t worry about what other people are doing or other people tell you. Find good problems that interest you—good problems that you think are worth tackling, and go after them. I think the success will come from what you do, not from maybe what’s popular or what a reviewer or a program manager or someone like that tells you. I would say that probably is the most important thing: learn to find good problems. Something maybe more practical that I would say is that I know that when I started as a faculty, I felt a lot of pressure to write proposals and get grants. I think this is becoming more and more common, at least from my conversations. A lot of universities are really pushing their faculty to bring in more money. And I think it’s obvious with all the things going on in our economy now. I did this after a little while, but I think that it’s more important in starting a research career to set up a lab and find a way to make a mark by publishing papers and doing good work. I didn’t realize this early, and I think I probably spent too much time on grants, and I think it’s really important—obviously without the money you’re not doing anything. But I think you should never put off the doing the work and publishing the papers, which I have found that some people tend to do. I think that that should be the highest priority because it helps yourself because you put your name out, and I think papers are the most important thing because that’s what people see from your work. Number two, it helps grad students, and I think a grad student should never go through a Ph.D. and have to publish their work afterwards or not have any publications. So I think that’s a priority that really should be emphasized. I don’t know if it is enough, because it wasn’t for me until I figured it out.
HOLLOWAY: Yeah, it’s one of those things where I call it the Golden Rule: If you have gold, you can rule. But if you have all your time spent chasing, as you say, the pursuit of grants, you don’t pay attention to your primary customer, and your primary customer is your students.
SANKARAN: Absolutely. That’s right. And I think it’s obviously important to have money, but I think if you have a little bit of money, there are other ways to be creative and to do research. That’s important too, to be able to get your work done.
HOLLOWAY: How important is it for students to interact with professional societies like the AVS?
SANKARAN: I think it’s becoming more and more important, because with everything that’s happening in the world and all the impact of the Internet and everything, I think it’s easy to get lost in doing things remotely in a vacuum. Because you can do those things now. You don’t have to present a paper at a conference because you can just email and get everything done. But what’s missing in that whole part in being able to do things electronically or however people are doing it, teleconferencing, all this stuff, is this personal interaction, and this ability to kind of critically differentiate good work and bad work, and what someone is actually doing and what’s being written up, and all these kinds of subtleties. And I learned so much at conferences—I still learn so much at conferences that it sparks all of the ideas that I go back to in my lab. So it might be because I’m meeting someone and generating a collaboration; it might be hearing how something is discussed or presented, or having a conversation in the hallway; whatever it is, to me, it’s everything in all of our ideas that we generate in our lab.
HOLLOWAY: Well, the deans and administrators at the university like to talk about distance learning and television broadcasts. But every time I’ve had an experience where the students could select to come to a personal class or to a telecast class, they go to the personal class, because there is a lot of communication that exists in the classroom besides the oral and visual shot of your face on a television broadcast.
SANKARAN: Yes. Another example of this would be that we had a seminar speaker a few weeks ago, and he went through my lab. He had seen my work before; I think he had even seen me present. I heard about this from my grad student. He apparently told my grad student that when he saw our experimental setup, it was so much cooler than having seen it in a schematic in a paper or in a conference or whatever. So I think there’s the value. I mean you learn so much more. There’s nothing like seeing things in person.
HOLLOWAY: Now you’ve had a number of successes. For example, you mentioned earlier you’d received a career award. I believe you also received an Air Force award. Is that true?
SANKARAN: Yes, that’s right.
HOLLOWAY: You’ve received a Glennan and Learning Fellowship at Case Western. What’s that all about?
SANKARAN: At our University we have an office that helps with teaching on campus, so they interact with faculty to improve teaching. It’s called UCITE – the University Center for Innovation in Teaching and Education. They also interact with students, but they’re mostly there for the faculty. They have two programs that they run to support teaching activities of faculty, which I think is great because it’s not that easy to get funding that’s often needed to create a new class or create a new educational program. NSF does support it, but that’s usually always with the main research proposals that you’re submitting. So the Glennan Fellowship was for a proposal that I made to work on a Chem-E Car. The idea is from our AIChE, American Institute of Chemical Engineering, which is our professional organization. They have a competition that they run where students have to build a small vehicle. So it’s not a vehicle that you get in and drive, but just a very small vehicle. And it has to be powered by some type of alternative energy; it’s can’t be powered by conventional fuel. It has to be a chemical reaction, so it can’t be a battery that you just buy off the shelf either. It has to be something that you put a little bit of design and building into. So the Glennan Fellowship was given to me to support a team of students that I put together to build the vehicle. We’ve been doing this for the last two years, we’re building a car and have been working on that. So I have undergraduates anywhere within their first four years who are interested in doing this as an extracurricular activity, and it sort of complements their coursework. The Learning Fellow program is a little bit different. It was a program that we were asked to participate in where we discussed approaches to teaching. I did this last year after I’d already been teaching a little bit, but it really kind of made me look at how I am as a teacher in the classroom. The main theme that we learned about was passive learning versus active learning, so what it’s like to have a class where you just have lecture and you listen versus interact somehow. It could be discussions or it could be hands-on experiments or whatever it is. And I’ve taken that really seriously and I’ve tried to incorporate that into my teaching.
HOLLOWAY: Is that the undergraduate and graduate level both?
SANKARAN: Yes. I would say it’s easier on the graduate level because our classes are smaller and the students are more, let’s say, confident and mature to discuss things. Undergraduate participation is always an issue, but I’m trying to get students to participate more because that way I think they really enhance their learning by discussing these things and debating them and questioning them and so on.
HOLLOWAY: I notice you’ve done some outreach to high school as well. Could you tell us about that?
SANKARAN: Sure. Actually I’ve had a few high school students come through my lab. The first program that I was involved in is at a high school in the Cleveland area, it’s in Shaker Heights. Hathaway Brown, it’s a private women’s school. They actually go from kindergarten all the way to high school, but I mainly worked with the high school level from 9 to 12. They have a program there where students who are interested in doing research in their 9th grade they join this program called the Student Research Program, and part of the program is that they have to then go find a research mentor either at the University or the medical school or somewhere—whatever they’re interested in—and then they have to spend four years doing research, mostly after school and then during the summers also. So I’ve had now two students do this with me, so one student already did her four years and I have another student who is in her fourth year. Each of them has contributed to research either by working with another student or in some cases even working independently. The undergrad student I have right now is extraordinary. She spent three-plus years. She is going to be a co-author on a paper; it’s in review now.
HOLLOWAY: Wonderful.
SANKARAN: She’s participated in science fairs as an undergraduate, some of these real prestigious ones like the Intel Science Fair and so on, and she’s won prizes and done very well both at the local level and even the national level. This has been something that I’ve taken seriously. I think the fact that it’s a women’s school is important, to get more women interested in science and engineering. But in general, I feel like engineering for me was an unknown thing in high school, and I want to try to expose students to engineering, so that when they go to college they can make a decision hopefully to choose engineering.
HOLLOWAY: Have you anecdotal at least evidence that that effort is paying dividends?
SANKARAN: Small statistics, but I think it’s positive. I’ve had these students go on to college and most of them are choosing engineering.
HOLLOWAY: Good! I notice that you also have outreached to international programs like the University of Botswana.
SANKARAN: So I should really credit my colleague Dan, because he’s the one that really had this idea, but he and I are working together on this. This started about two years ago or so that we had a connection in Botswana. We had met a faculty at the University of Botswana through our research in tribocharging, and we had decided that we would go visit him, and we got a grant through NSF as a travel supplement to one of our other NSF grants to support a trip there to try to initiate collaborations between their University and ours. It was initially mostly research that we were planning. But when we got to the University, they were very happy to have people from the US, and we started talking about all kinds of things that we could do as potential collaborations between the universities. And so we came up with a type of research collaboration, and we have an NSF grant that’s similar to a REU (Research Experience for Undergraduates), where we send US undergrad students to spend a summer to do research with mentors that are at the University of Botswana. So it’s essentially an international REU program. But in addition to that, something else that we came up with is we decided it would be neat for students at our university to take a class there, in Botswana, so this would be like a study-abroad program. But my colleague and I are teaching it, because we wanted to make sure that it didn’t disrupt their academics. So they can take this class that’s already offered at Case, but we teach it in Botswana as a study-abroad course. We did this for the first time this past May. We had 21 students go. We taught thermodynamics to our students in Botswana. You might wonder, you know, why are you teaching this class in Botswana when you could just teach it in the US. The reason that we are doing this is that study-abroad courses have some other things that you can do. For example, you’ve got the students there for full days, so you can actually integrate a lot of activities into the course. Some of the activities were social, cultural, but some of them were actually for the course. So we, for example, went out to local villages in Botswana and learned how water is pumped, and this was part of the thermodynamics course, and we got values from their water authority, and they use that to calculate cost of water pumping and stuff like that. And then we went out to a diamond mine. Botswana has the largest diamond mine in the world, and we learned about the thermodynamic transition of graphite to diamond, and how diamond is actually created in the ground and then brought up by volcanos and stuff like that. So we would try to incorporate that into the class to really make that something that was useful to them. And then overall, having a study-abroad experience we think as an undergraduate can really broaden perspectives and things. So they learned about a country that’s not as westernized as Western Europe and the US, and got to see all these things.
HOLLOWAY: I think that’s extremely valuable. For many years I’ve had a collaboration with the University of the Free State in South Africa.
SANKARAN: Oh, great!
HOLLOWAY: And I share your opinions in terms of the collaboration and the benefit to me and to the students at the University of Florida as well as to the students at the University of the Free State. In fact, there’s a young faculty member from there that made a presentation here this year at the poster session. He found an interaction with AVS so beneficial that he’s come back several years now. So, we covered a lot of topics, Mohan. Anything you’d like to add to the interview?
SANKARAN:: I actually just want to thank the Nomination Committee. I’m very lucky to have colleagues in the plasma community that have been very supportive of my work, and mentors. In addition to some of the other mentors we talked about, I also just want to mention Eray Aydil, University of Minnesota, who was the chair of my nomination package. He’s been a mentor for many years. I’ve known him since I was a graduate student, and I met him at AVS. I also want to mention the other people that were on the committee. Jane Chang from UCLA, David Ruzic from University of Illinois, Mark Hersam from Northwestern, Vince Donnelly from University of Houston, and Mark Kushner from University of Michigan. I think five of the six are in the plasma community. Mark Hersam is in the nanotube community. So that’s how that committee was sort of put together. But I’ve known all of them for many years, and not just with this award. They have been important mentors and supporters to me for a long time, and it’s really been a big part of the success that I’ve had.
HOLLOWAY: Well that’s certainly one of the important lessons the young students need to learn is networking and connection, connectivity is important for you no matter what you’re trying to do. Whether it’s trying to improve your teaching or trying to improve your research or trying to develop new directions for research, it’s input from high-class people that makes it successful.
SANKARAN:: And I think we talked about this before, but these connections can only be made if you attend these conferences and you meet these people face-to-face.
HOLLOWAY: Absolutely. Well good. Anything else?
SANKARAN: No, that’s it.
HOLLOWAY: Well thank you very much for participating.
SANKARAN:: Thank you, Paul.