Awardee Interviews | Charles Skyes - 2012 Peter Mark Award - Interview

Charles Skyes

2012 Peter Mark Award Recipient

Interviewed by Paul Holloway, October 30, 2012

HOLLOWAY:  Good afternoon. My name is Paul Holloway. I’m a member of the AVS History Committee. Today is Tuesday, October 30, 2012. We’re at the 59th International Symposium of the AVS in Tampa, Florida. Today I have the privilege of interviewing Dr. E. Charles H. Sykes from Tufts University who is the 2012 Peter Mark Award winner. His citation reads, “For pioneering atomic scale studies of chirality, catalysis, and molecular rotation.” So congratulations, Charles, on the award.
 
SYKES:  Thank you very much, Paul.
 
HOLLOWAY:  Could you start by giving us your birth place and birth date?
 
SYKES:  Yes. I was born just outside of Belfast in Northern Ireland on March 16, 1976.
 
HOLLOWAY:  Good. What about your education? You can start as early as you like.
 
SYKES:  At high school, I went to a place called the Royal Belfast Academical Institution. It’s a long name. But it was an all boy’s school. It was particularly strong in sciences and math. From there I did A levels, which you specialize in the U.K. at the age of 16 in math, physics, and chemistry before going on to Oxford University to study chemistry at Queen’s College. I got particularly interested in physical chemistry there. Having Paul Madden, who is a renowned theorist, tutor me on a sort of one-on-one basis every week really sparked my interest in physical chemistry.
 
HOLLOWAY:  So this was a theoretical study, not experiments?
 
SYKES:  Yeah. It was the weekly tutorial system that runs in Oxford and Cambridge where problem sets are given out, and then the professor will actually sit down with you and go through the problems, which is really a great way to really see how their brains work. Sometimes he wouldn't always get them right, and you could see him think through issues. I think that was really a nice way to really get into the details of physical chemistry on that sort of one-on-one basis.
 
HOLLOWAY:  This was for which degree? The second degree or…?
 
SYKES:  This would be B.S., or Bachelor of Science equivalent.
 
HOLLOWAY:  Yeah, good. So he was very interactive with you then under those conditions.
 
SYKES:  Yes, very much so. Then it was actually a combined bachelor’s and master’s degree. So in considering what research to undertake for my master’s degree at Oxford, I looked around and found the now late John Brown, his spectroscopy work was really interesting because he kind of captured my imagination in describing the high-resolution spectroscopy he was doing on radicals, and that if you can understand the different species on Earth you can interpret spectra from space and really tell what chemicals are right there in outer space, which really was a sort of leap of understanding and quite profound, really. I found that very interesting. So that took me into his group for one year for my master’s research.
 
HOLLOWAY:  So he was using optical spectroscopy for this characterization then. [Yes.] Good. So that led you into that line of research then. [Yes.] So where did that take you?
 
SYKES:  So at that point I was interested in pursuing a Ph.D., but I was, at that point, thinking a little bit longer term in that after a Ph.D. I would have to find a sort of career in science. That made me think well, what are the problems in the future? It’s going to be environment, energy, catalytic conversion. Things like that are areas that perhaps a physical chemist can contribute to. So in choosing my Ph.D. group, I looked around at groups in Cambridge University, because I wanted a change of scene, I guess. In the British system, it’s not like the U.S. system where you arrive at grad school and you have a few months to look around and decide. In the British system, you apply to one professor or one group.
 
HOLLOWAY:  Is that right?
 
SYKES:  If you get in, you get into the university. So I was incredibly naïve at that point, so I was just looking at fairly primitive websites and making judgments based on the different types of research described there. So it’s amazing that I ended up in a lab that did a lot of high quality catalysis and surface science work. I think that really suited my interests and also my experimental skills.
 
HOLLOWAY:  So whose lab was that then?
 
SYKES:  Oh sorry. That was Richard Lambert’s lab in Cambridge.
 
HOLLOWAY:  So what sort of problems and materials did you focus on in that lab in those studies?
 
SYKES:  Yeah, so that was in 1998 and there was just beginning the big interest in nano-sized gold particles for catalysis. Wayne Goodman had just made his discovery of quantum size effects in the gold titania system. So Richard Lambert set me up with the project of designing a more realistic model system not based off of single crystals and very high precision stuff like that, but to try and—still in vacuum, but model the system in a more realistic way. So I ended up taking polycrystalline titanium and studying the oxidation of that and then putting gold clusters on top and absorbing hydrogen, oxygen, water, and then reagents like styrene and looking for oxidation and hydrogenation reactions. So it really got my feet wet in surface science and surface chemistry.
 
HOLLOWAY:  So did you use the quantum effect in any of your studies or it was important to your studies?
 
SYKES:  In our system, it actually looked that the smaller particles became charged positive in the interaction with the support, so it was that. My results would tend more that in my system it was the charge that was having an effect. It was an interesting project, because again, being fairly young and naïve at that point, I didn't really realize that when you study a system you have to really understand every single variable that you go through methodically before you can take the next step. I was, you know, at the time very excited to get the result of creating a good model catalyst and getting a reaction to go. I really had to step back and realize to make any progress after the first year or so I really had to go back to the fundamentals and study first the oxidation of titanium and what that produced and how the molecules interacted before even adding the gold or the reactants.
 
HOLLOWAY:  That’s the advantage of being young. You know, sometimes all these things you know you can't do when you’re older you don't know and you still go ahead and do them when you’re younger! [Laughter]
 
SYKES:  That’s true.
 
HOLLOWAY:  A good lesson. Yeah. So you mentioned that Wayne Goodman discovered the quantum effect. Could you describe that quantum effect just briefly? It’s a size versus the density of states in a gold particle?
 
SYKES:  Exactly,  He was one of the first groups applying scanning tunneling microscopy to fairly complex catalytic systems. So he was able to grow different sized gold clusters on titania, and through scanning tunneling spectroscopy measure the density of states and essentially the transition from metallic to semi-metal to insulator and then relate that to the particle’s ability to catalyze. In that case it was CO oxidation.
 
HOLLOWAY:  Okay. Good. So you found that charging was important in your studies.
 
SYKES:  We had pretty strong interactions of the small gold clusters with the support. That’s what actually drove me into trying scanning tunneling microscopy myself. Richard Lambert was good enough to allow me to branch my project out to also include scanning probes. So we did both ambient and vacuum STM on my systems. It was very, very difficult because these were not flat oxide single crystals. This was a piece of titanium metal after many oxidation cycles that we had deposited gold particles on. So when you had this lump of metal in your hand, it was rough to the touch, let alone at the micro scale. So again, it was an incredibly challenging prospect to get any type of high resolution STM out of that. But my real inspiration of looking around at what Don Eigler was doing and Wayne Goodman and others in the high end STM field really drove me to try and get some data off of that system, and we were able to get some particle size and see sintering and things like that, although it was incredibly difficult because the surface was incredibly rough even on the micro scale.
 
HOLLOWAY:  Now if it were gold nanoparticles on a titania film, on a titania substrate, I presume [Yes.], how thick could you tolerate that titania film to be and still use STM?
 
SYKES:  Well, that’s an interesting question, yeah. So the films we used were a couple of nanometers thick. But when we did angular resolved XPS, what you could really see is it wasn't TiO2 and then titanium underneath. It was TiO2 and then all the stoichiometries all the way out on to TiO and then through to titanium. So it was sort of a composition spread as you went into the metal.
 
HOLLOWAY:   And some of those sub oxides are conducting themselves. [Right.] So what year are we talking about now?
 
SYKES:  That was ’94 to ’98, the Ph.D.
 
HOLLOWAY:  What did you do once you finished your Ph.D.?
 
SYKES:  Yeah. So at that point I had really become fascinated by scanning probes and particularly low temperature STM, so I looked around for post-doc doing low temperature STM. At the time there were only two instruments in the whole of the U.K. there was one at a national lab in London in Taddington, and then there was one in Professor Sir David King’s lab. Neither group had any post-doctoral openings, so at that point I said well, I could sort of settle down in the U.K. and maybe get a job somewhere like Johnson Matthey making catalysts, or I could sort of take the leap of faith and see about moving to the States and finding a post-doc in one of the low temperature STM labs in the U.S. That was a kind of surreal experience to go into the travel agent and basically buy a one-way ticket to the States, to sit there and look at that ticket and think, “Yeah, this is potentially definite. You’re going to move to the States and not come back.” So yeah, I got my one-way ticket to the States, which I actually recently found the ticket stub from.
 
HOLLOWAY:  Is that right? [Yeah.] Did it scare you again? [Laughter]
 
SYKES:  It was surreal to look at it, and I guess it made me happy that things had worked out well.
 
HOLLOWAY:  Very happy, I’m sure.
 
SYKES:  And I’m very happy to have moved to the States because I think obviously the opportunities for younger scientists in America are really good in terms of the availability of faculty positions on the startup packages to get going in high end research.
 
HOLLOWAY:  As more and more of us get old enough to retire and go off and do different things, there is more opportunity for young people like yourself. Tell me, relative to your scanning probe instruments, did you buy them yourself, build them yourself, or did you buy them from a vendor?
 
SYKES:  Yeah, that’s an interesting point. When I started my post-doc with Paul Weiss at Penn State, he had homebuilt scanning tunneling microscopes because Paul Weiss had actually worked in Don Eigler’s group at IBM right at the point when they were discovering the atomic manipulation, the famous xenon experiments where they wrote IBM with atoms. Paul had set himself up as an assistant professor at Penn State several years ago and built these IBM-style four Kelvin machines from scratch.
 
HOLLOWAY:  This is in chemistry or in…?
 
SYKES:  Yep. He joined chemistry at Penn State and branched into physics as well. But he was a chemist by training.
 
HOLLOWAY:  Yeah. So what was the timeframe that you spent at Penn State?
 
SYKES:  That was 2002 to 2004. Now I remember actually my undergrad was ’94 to ’98 and my Ph.D. was ’98 to 2002. So that’s a correction.
 
HOLLOWAY:  We can catch some of that. So you studied what in Penn State, what type of materials and problems?
 
SYKES:  We were interested in doing spectroscopy. There was a big race on to do single molecule inelastic electron tunneling spectroscopy. The systems we were interested in were light atoms like hydrogen on palladium, to try and see the palladium hydrogen stretch. We did some of that, but the really interesting discovery we made there is we kept noticing that when we were tunneling at higher voltages, the surface got a sort of ghostlike features on it that never went away. So after reading more of the literature and thinking back to my Ph.D. days where there was a lot of talk in Richard Lambert’s group about subsurface oxygen in silver, I started to think about subsurface hydrogen in palladium. It turns out what we had done is we had brought hydrogen from the bulk of the crystal up into the subsurface level where it had gently rumpled the surface by just fractions of an angstrom. It turns out we could even write words using that subsurface hydrogen.
 
HOLLOWAY:  Is that right?
 
SYKES:  But it was very interesting because no one had been able to manipulate and observe that. We could put hydrogen or absorbates on the surface, and then bring up subsurface hydrogen in specific regions and see how the two interacted.
 
HOLLOWAY:  What was the driving force to bring the hydrogen close to the surface like that? Why did it happen?
 
SYKES:  The current thinking, and the theorists are still working on this right now, but the thinking is that hydrogen is actually more stable in the subsurface region than in the bulk. So if you randomly scatter it, some will end up focused in the subsurface region, and there are electric field effects as well.
 
HOLLOWAY:  But electric fields should be supported in palladium as at platinum. [Right.] But there is still an electric field effect.
 
SYKES:  Yeah, so that’s probably the wrong word to use. The current thinking is that when you’re tunneling into different bands and different orbitals, there are specific effects that will direct this atom transport.
 
HOLLOWAY:  Good. So did you do another post-doc besides Penn State?
 
SYKES:  Yeah. I met my now wife at Penn State. She was a post-doc in a different lab there in Chris Keating’s lab, who is an analytical chemist. My wife got a faculty position at UNC Charlotte, so actually a year later I went to Charlotte for a year to work in the optics center there to solve temporarily the two body problem. Mike Fiddy was the head of the optics center there, so that was an interesting time to really go into a pretty different area. I’d never done any optics experiments before, so I got my feet wet a little bit in some optical surfaces and trying to study those with scanning probes and then collaborate to measure optical transport in microspheres and things like that. Then at that point I was applying for my own academic positions, and that’s when the job at Tufts University came up in 2005.
 
HOLLOWAY:  This is in chemistry. [Yes.] So did your wife take up a faculty position in that area as well?
 
SYKES:  No. We actually had a fairly tricky two-body problem for a number of years because first she had moved to Charlotte and we had time apart and then I had moved there. Then I had moved to Boston and we had time apart as well. At that point, she was interested in transitioning into administration, so she worked both at the Chemistry Department at Tufts as a strategic planner, and she’s now a program director at the Harvard Medical School in Boston.
 
HOLLOWAY:  Well congratulations to her! Certainly very nice. So what was it like to be a young faculty member at Tufts? Presumably you didn't have any labs established and all the equipment needed to be brought in and facilitated, etc. So give us an inkling of how that was handled.
 
SYKES:  Yeah. It was kind of scary because you sort of walk in late August. They had prepared a very nice lab space for me in the basement because the ground is more stable there for scanning probe work. But essentially walk into a very clean, empty room.
 
HOLLOWAY:  Newly painted, but that’s it.
 
SYKES:  Yeah. But very sort of quiet place where you kind of think to yourself wow, I have to turn from this empty room into a fully functional lab. One of the good things about starting at Tufts was that they gave me a very competitive startup package because just the instrument I wanted to buy alone was a half-million-dollar investment. So they supported me very well, and I was able to buy a commercial low temperature scanning tunneling microscope which arrived several months later. I think that was key to getting started quickly was to have a working instrument.
 
HOLLOWAY:  So what do you prefer? The homebuilt ones that you did with your post-doc or the commercial one that you had with the new position at Tufts?
 
SYKES:  I think as a chemist you really want to explore the systems and the problems, so you want a system that is fairly user friendly and works most days. So from that respect, commercial instruments have come a long way and there are some pretty good ones on the market. But you can have students in chemistry. You aren’t experts in electronics or in machine building, but they can really get going very fast and get fantastic data from commercial instruments these days.
 
HOLLOWAY:  Did you immediately inherit students that were already established in the program at Tufts or you had to bring in new personpower?
 
SYKES:  Right. So I recruited students that had just joined the department at the same time as I had. So they typically look at all the groups and then in November they’ll decide which ones to join. So I was lucky to get two students, Ashleigh Baber, Erin Iski my first year at Tufts, and also an undergrad researcher, Stephen Jensen, who has now almost finished his Ph.D. with Cynthia Friend at Harvard. So I was able to get three really good young people by Christmas time in the first year.
 
HOLLOWAY:  That always helps. If you find one that’s good with their hands and their head both, that’s double luck for you. [Yes.] Did you have post-docs come in to help you with this startup effort?
 
SYKES:  I’ve never had a post-doc in the time I’ve been independent so far, yeah.
 
HOLLOWAY:  That’s good. That gives you the privilege and pleasure and duty of working directly with the students then.
 
SYKES:  Yeah. It’s something I really enjoy because you sort of train them for a year or so and then you really see them come on through several years of their Ph.D. to the point they’re incredibly productive and telling me much more than I know about the system.
 
HOLLOWAY:  Well, that's what you want to do. You want to bring in somebody that is not expert in an area and develop an expertise that is greater than your own, let them teach you a little bit of a lesson and then move on and redo the process again. [Laughter]
 
SYKES:  Yeah. Never-ending cycle.
 
HOLLOWAY:  Never-ending cycle. Let’s see. At Tufts, you tended to focus on catalysis?
 
SYKES:  I had several areas I was interested in. One of them which I’ll talk about at the AVS meeting on Thursday is chirality. I’d always been fascinated by chiral molecules and chiral surfaces.
 
HOLLOWAY:  Tell us a little bit about chirality, for some of the people that may read this that are not so familiar.
 
SYKES:  Your hands are chiral. Your left and right hands are pretty much identical, but they’re non-superimposable or in a way they’re mirror images of one another. Just like that, molecules like proteins and DNA are also chiral. That has a big effect on their chemistry and their biology and even some of the drugs we take because all our biological molecules in our body are chiral. It’s important for the pharmaceutical industry to have the correct chirality of drugs. So it was an interest both in the fundamental aspects of chirality, but also in terms of developing heterogeneous processes or catalytic processes that occur on surfaces where you don't have to separate out the catalyst, which is done for many chemicals in industry, but to extend that to chiral reactions to make single anantomers. So inspired by the work of Andy Gellman at Carnegie Mellon, who had shown that you can actually take a regular single crystal of metal and you can cut it and polish it in such a way that the whole surface becomes decorated by chiral kink sites all of the same chirality. Almost 15 years ago now at this point, he was able to show that right- or left-handed molecules stick preferentially on that surface so he could actually do a chiral separation using that chiral surface. So as a surface scientist and also a scanning probe person, I really wanted to start to look at that type of interaction at the atomic scale with STM.
 
HOLLOWAY:  So does that constitute a majority of your work then over the last few years, or that’s just one aspect of it?
 
SYKES:  That’s about a third of the work. The three main projects that evolved in the last seven years are, like I said, chiral surface chemistry, molecular motors are a big one, and then also metal alloys and how their atomic scale geometry relates to their reactivity. Those would be the three main thrusts right now in the group.
 
HOLLOWAY:  So we talked about chirality. Are there aspects of that that you wanted to talk about further?
 
SYKES:  No. I could probably say a thing or two about both the molecular motors and the metal alloys.
 
HOLLOWAY:  Okay, let’s do that. What about molecular motors? Tell us what that boils down to.
 
SYKES:  So we were interested in just fundamentally how molecules rotate on surfaces, and we got to looking at very simple molecules called thioethers where you have a sulfur atom that sticks to the surface and then two alkyl tails that just flop around. When we looked very carefully with a scanning tunneling microscope, we could see that if we heated up the surface these molecules would begin to spin. So the molecule first would look like a banana shape as it was frozen on the surface. Then as we heated it up it would start to look like a flower petal when it was rotating around. But then we really got to thinking how could we go from random rotation that we could measure where it goes equally clockwise and anticlockwise, how can we go from that to something that starts to look more like directed motion? So we had lots of interest and experiments involved in inelastic tunneling where we were exciting specific vibrations within that molecule and seeing how the vibrations coupled to the rotation. So in that way we weren’t subject to the laws of thermodynamics that state that a system of thermal equilibrium can't do useful work. If we added an external source of energy, i.e. electricity, we could potentially get directional motion on it. Then the real key to describing the motor and experimentally demonstrating that was to make that absorbed molecule chiral. So by the symmetry of the molecule of the… One of the rotor arms is longer than the other. The way the sulfur absorbs on the surface leads to left- and right-handed form of that rotor. Through collaboration with theorists, Anatoly Kolomeisky , Feng Wang, and David Sholl who are able—
HOLLOWAY:  Are they at Tufts?
 
SYKES:  No. Feng Wang was at Boston University at the time and David Sholl is at Georgia Tech. They were able to show that the energetic landscape of those chiral rotors is asymmetric, so it looks a little bit like a ratchet potential and combining that with being excited by electrons led to us being able to measure directional rotation. That was quite a challenge because even though we had a computer program that would count these rotational events, we had to check them by hand to get the statistics for our paper report on the first single molecule electric motor. The grad students, undergrads, and even high school interns had to count over half a million rotational events by hand to check the statistics, so that was a huge undertaking to do that.
 
HOLLOWAY:  That’s why we call them students, I guess. We convince them that this is important enough and they’ll go ahead and do that.
 
SYKES:  Yeah. They were very enthusiastic and never complained about the task.
 
HOLLOWAY:  I’d like to ask you speculate a little bit about the future of molecular motors. We’re learning more and more about how they operate, how to control them, and how to make them do what we want if we can decide what we want them to do. How big is that going to be in the future?
 
SYKES:  It’s hard to know. The issue with functional molecules is how do you connect to them? I think there’s potential, especially for electrically driven systems, because even with current lithographic techniques you can get pretty small feature sizes and obviously you can focus electrons down to very small dimensions. So potentially interfacing that with optical inputs or outputs, i.e. having a molecular rotor that has an electric dipole so that when it rotates you have a rotating charge and rotating dipolic potentially emit electromagnetic waves or vice versa. It can be driven to connect that electrically may give rise to very new and interesting nano scale devices to convert between optical and electrical and nanomechanical signals. I don't know what you’d potentially use that for, but it would certainly give you access to new physics.
 
HOLLOWAY:  Right now it’s just developing the fundamental knowledge in the area.
 
SYKES:  Yeah. For us it was more just the challenge of can we get this to do anything that isn’t just random?
 
HOLLOWAY:  Again, a question that is germane to the young people that may look at your interview. That is this is a new area for you. Did you learn it by having exposure to it in the classroom, or you learned it by your own self study, or interacting with people at meetings, or combinations thereof?
 
SYKES:  Combination of everything, really. I’d seen people talk about molecular motors and rotors and describe systems of putting them on surfaces.
 
HOLLOWAY:  And that interested you.
 
SYKES:  It did. And I think as a chemist I always thought is there a simpler
 way to make these systems? Because I think some of the more engineering types would have an organic chemist build a very complicated looking molecule that would have specific functionality. But thinking back to my undergrad days of doing actually quite a lot of organic chemistry as part of the undergrad degree, I thought are there simpler ways to get these functionalities? And we ended up using molecules that you could buy for $20 commercially, thioethers [?], all different types were available, and that really gave us access to all different types of rotational properties and chirality in the absorb systems. So that was a way to get into that field with a very different system that was simple enough we could get a lot of understanding from it and take each step that built on the next of saying, “Well, can we understand how it spins, how fast it spins? Can we measure the direction it spins in? Can we then excite it? Can we make it ratchet like? Can we add chirality?” and basically building that up towards making a single molecule motor. I think one of the people that was a good mentor and also an inspiration was Dean Astumian, who is a physicist at the University of Maine who has written many papers from the hardcore theory behind these systems to popular articles about molecular machines, and really made the point that you can't think of nanoscale mechanics in the way we view the classical world. Molecules don't have inertia like your wheels on your car or your engine. They’re subject to massive friction damping. He always says to do useful work at the nanoscale at room temperature is like trying to walk in a hurricane. It’s practically impossible. It might be that biological machines use their energy not to push against the Brownian motion but just to hang on and use the Brownian motion to push them in the direction they want to go. So applying that same sort of logic to our system, we were essentially randomly exciting the molecule on the surface in an asymmetric environment. That’s what ultimately led us to be able to demonstrate the molecular motor. It’s nothing like an electric motor that we know about that electromagnetic fields create a constant force.
 
HOLLOWAY:  Right. Now you are emphasizing scanning probe microscopies. Is that the only technique you use to look at these molecular motors?
 
SYKES:  In the motor project, yes, because they are so small and we use such a low concentration of them on the surface. They’re not amenable to many other techniques.
 
HOLLOWAY:  Let’s turn to metal alloys then. What did you do in the area of metal alloys?
 
SYKES:  So we’re interested there in turning scanning probes onto somewhat older systems that were known about with all the other catalysis and surface techniques. But the actual atomic geometry wasn't completely well defined. So our approach has been to take very small quantities of atoms like cobalt, platinum, palladium, deposit them on substrates that are fairly inert and unreactive like copper, silver, and gold, and then to build up from the case of having individual atoms to dimers, trimers, all the way up to nanoparticles. Then with the scanning probes really understand the atomic scale structure and then through desorption measurements in the lab, relate that to the chemical reactivity.
 
HOLLOWAY:  So what sort of chemical reactions do you typically look at for these materials?
 
SYKES:  The main one would be hydrogenations. In hydrogenation reactions, one of the rate limiting steps can be just associated in the hydrogen molecule. The trick there is if you can dissociate your hydrogen but still have it weakly bonded to the surface. You can potentially do selective catalysis. Manos Mavrikakis is a theoretician that’s talked a lot about this in the past with what he calls near surface alloys. But that's a slab of atoms on top of another slab of atoms. We took a sort of different approach just to take tiny quantities of pretty reactive elements and alloy them into a more inert substrate to see if you could temper and tune reactivity not by putting a slab on another slab, but just to isolate a single atom in a crystal. We’re now trying to take that through to some sort of practical application with our collaboration with Maria Stephanopoulos in chemical and biological engineering at Tufts who makes real catalysts and tests them in micro reactors. So they’re looking into taking tiny quantities of palladium and say, copper nanoparticles and seeing if that is more active in the atomic state than in a more equal stochiometry.
 
HOLLOWAY:  Now these catalysts are supported, the nanoparticles are supported, and there is this catalyst/nanoparticle support interaction. Do you look at that aspect of the alloys on the supports?
 
SYKES:  In our group, no. We’re looking at the fundamentals of activating for example hydrogen or oxygen on one part of the alloy and then spillover of those atoms or molecules to a different part of the alloy. You’re right that interfaces between metals and oxides are pretty important, but I would also argue that interfaces between different elements in the alloy are important because in a bigger alloy nanoparticle, the reaction could occur from activation to desorption of the product all within a few lattice sites, especially if there is heterogeneous… different atoms or different step edges and so on.
 
HOLLOWAY:  Now I would be interested in knowing whether the idea is still prevalent. But at one time in the binary alloys in the nanoparticle size were demonstrated to segregate one component or the other to the surface, presumably because of strain energy. Is that still an accurate picture or a picture that’s been developed more by experimental data?
 
SYKES:  Very much so. The state of the art on that is just like you said. Certain elements will have a preference to be on the surface or the bulk, depending on their surface free-energy. But of course if you have a reaction occurring on the surface, the element that is traditionally in the bulk, if that binds more strongly to your reactants or products, that will actually bring it out towards the surface, so called reverse surface segregation. With the development of techniques like high-pressure XPS, people have been able to have core/shell particles with two metallic elements and see that surface segregation change as a function of what gases are flowed over the nanoparticles. So it’s a very important effect and it definitely has been measured pretty accurately these days.
 
HOLLOWAY:  Have you studied any of that in your group?
 
SYKES:  Yeah, we look at the atomic structure of these alloys as a function of temperature so we can make metastable arrays of atoms on top of the surface. We can then make more stable arrays with substitution atoms in the surface or if the element has a very high surface free-energy and it wants to go to the bulk, we can study. STM, you can actually see a few layers into the surface so you can see subsurface atoms as well.
HOLLOWAY:  But can you identify the composition of the first layer, for example, and show it enriched in component A in an AB alloy?
 
SYKES:  These days with low temperature STM, we can see every single atom and we can see the differences between two atoms. Even with a similar size, they have a different electronic structure so you can very clearly see the difference between atoms of an alloy.
 
HOLLOWAY:  So this would be correlating the composition with an IV curve, crudely speaking. Is that accurate or not accurate?
 
SYKES:  Even just in the topographic image as the tip scans across the surface, you can see just in the picture of the atoms which ones are different.
 
HOLLOWAY:  So you could tell whether A is different from B. but how do you know then? Do you assume then that you added platinum and rhodium together so one is one and one is the other?
 
SYKES:  So we start with incredibly clean surfaces of copper, silver, and gold. Those elements are nice because you can clean them up to the level of impurity less than one impurity in 10,000 atoms or better. So when we start to add very, very small quantities of platinum or palladium or cobalt, we can instantly see the buildup of the second atom and then study the placement of where it goes and where it alloys.
 
HOLLOWAY:  Now let me ask you whether you’ve looked at the kinetics of this. The segregation driven to the surface is what I would call a thermodynamic effect. But the kinetic effect is how quickly does it reach a steady state or an equilibrium condition? Have you looked at the kinetics of those processes?
 
SYKES:  I’ve done some work on that with palladium, copper, and silver. Copper in collaboration with David Sholl who is a theorist to look at different barriers. For example, palladium atoms that have landed on a copper surface to cross step edges to alloy into the surface, to alloy into step edges. You can explain some of what you see post alloy and in the STM by the pathways that are available to the atoms. But I think our biggest aim is to make up different compositions and then understand which one has the ideal reactivity, and then go to catalysis folks and ask them to try and prepare nanoparticles with similar compositions and test them out in a real system.
 
HOLLOWAY:  Do you have any outstanding persons who mentored you in this area or worked with you collaboratively in this area?
 
SYKES:  Yeah, quite a long list. I think for one, in the last seven years the chance to work alongside theoreticians has been really good. Sometimes with scanning probe work you get these very nice images, but you don't always understand what you’re looking at or know much about the energetics. So in the past working with John Kitchin, David Sholl, Feng Wang, Talat Rahman, Angelos Michaelides, Anatoly Kolomeisky—that’s a sort of short but not exhaustive list of just some of the theoretical collaborations I’ve had on the different projects. Like I mentioned, I also working with the chiral systems with Andy Gellman and David Sholl and collaborating also with my colleague in chemical engineering, Maria Stephanopoulos, on taking it through to real systems. All those collaborations and mentors have been really, really important in getting where I am now.
 
HOLLOWAY:  I understand that you recently were given a career award from NSF. Congratulations! That’s good support and a stable foundation to build on. [Yes.] What other awards have you received?
 
SYKES:  So aside from the career award, which was a five-year award that supported the molecular motor work, that was great because it let us, as I mentioned, go through all the steps from looking at the molecules spinning all the way through to making the motor. That was really a nice grant to work under because it did give us five years to develop that. But alongside that, early funding came from the Beckman Foundation and Research Corporation, which allowed us to have a fairly big budget to build up newer pieces of equipment. So we were able to get a variable temperature scanning tunneling microscope that could work at much higher temperatures and then add different techniques to that and really branch from scanning probe work out into different surface analysis like temperature programmed desorption and reaction and X-ray photoelectron spectroscopy. So they had a big effect on my early career because it’s very hard to get equipment money these days, and that provided a very large chunk of money to basically branch out into different types of equipment to go into more sophisticated projects.
 
HOLLOWAY:  That’s good. I think that covers the topics that I had in mind for today. Do you have any others that you would like to add?
 
SYKES:  No. I think the last thing I would say is probably the most important thing. I’ve been really lucky the whole time to have some really excellent students in the group. The grad students, the undergrads, and also high school students and collaborations with one visiting professor, George Kyriakou. So I think overall I’ve been really blessed to have such good people in the group who were always willing to learn and try new things and had a sort of “never say never” attitude towards the science. So I think I’m always eternally grateful to the contributions everyone’s made in the group in really moving the science forward quickly.
 
HOLLOWAY:  That’s generally true. We hardly do it ourselves if we didn't have the students to drive us. We teach the students, but the student clearly teaches us as well. [Yes.] Well, thank you very much for the interview today, and congratulations again on the Peter Mark Award.
 
SYKES:  I very much appreciate it, so thank you.


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