Panel Discussion on 2D Materials
Graphene and 2D Materials Session, MRS Memorial
Pablo Jarillo-Herrero: So now, I’m going to ask all the speakers to come up front, here. We don’t have wireless mics, so we’re going to stand up here with a couple of mics and do our best to share. I’ll start the discussion, and then I’ll open it up to questions from the audience.
So when I was brainstorming how to moderate this discussion, I remembered one of my conversations with Millie from 10 years ago. As always, Millie was very kind and very considerate — and I remember that one day, she wrote me an email: “Pablo, I have this event at the New York Academy of Sciences, but I think it would be great if you came and spoke.”
She addressed this email to both Jing Kong and myself — inviting both of us young researchers to accompany her and speak. I can’t speak for Jing, but for me, it was like, “Oh, wow! New York Academy of Sciences! That’s so exciting.”
So we went, and after the actual talks, Millie and I were chatting together and she asked me, “So, Pablo, what do you see as being up-and-coming in the future? What do you think is the next big thing?”
I was like, “Ooh… I don’t know, Millie. What do you think is the next big thing?”
And I remember she told me, “I think it would be interesting to look into non-equilibrium physics.”
Now, as I was listening to Paul, Tony, Frank, Philip, Eva, and all the speakers today, I realized that Millie might have been onto something. Especially in Paul’s talk — there are a lot elements of non-equilibrium physics go into the extreme of actuating things, and so on.
So in that vein, one of the things that I wanted to ask the speakers today is: we have seen some glimpses of the future, especially in the last talk, but what do you see as the future hot-topic in this kind of research? What’s the next big thing? And, of course, what’s the next big thing in 2D materials, specifically?
Paul McEuen: It looks like I’m the first to speak, which means we’re going to start with the weird. As indicated by my talk, I think one of the things that’s really fascinating is…. If you go back to this famous talk that Feynman gave fifty years ago, he said there’s plenty of room in the bottom. And he said that there’s lots of cool stuff to do there. He said, therefore, you should miniaturize information, and you should miniaturize computing. I don’t know if that was good or bad advice, but it worked, and we’re now all slave to our information and computing tools.
But Feynman also said that we should also miniaturize machines — and we really haven’t done that, yet. As you could tell from my talk, we’re just at the beginning of miniaturizing machines. So I think that’s a real interesting area. And, if you take it deeper, that becomes a question about... basically, life is a set of miniaturized machines that do something. So basically, we’re going to be trying to reinvent life. And there are two pieces to that. One is what you saw here — what you might call ‘metabolism’ — which means trying to build things that do stuff. So that’s the only-slightly-crazy thing to do. The much crazier thing to do is to try to make things that know how to make copies of themselves and do stuff — in other words, to produce self-replicating machines. And that is the real fun thing, or maybe a terrible thing. I don’t know which, but it’s going to be cool. So over the next 50 years, I think making simple, self-replicating machines is going to be fascinating.
Pablo Jarillo-Herrero: I just wanted to interject here and say that actually, when Millie asked me what I thought the next big thing in science would be, the first thing that popped into my head was life and biophysics. Non-equilibrium seems related to quantum mechanics of living objects and things like that, and I’ve been putting it into all the outlook sections for my proposals to NSF ever since that day with Millie, because I thought — if Millie said this was the next big thing, it’ll sound really good if I write it into the proposal! And it seems to have worked; I’ve been lucky with the proposals, so I can’t complain. But I was wondering if anyone else had any thoughts about this?
Tony Heinz: Paul is an extremely hard act to follow, and I’m tempted answer this question with a quote from him from an earlier conference — once, when Paul was once asked a tough question, I remember he said (and I believe I’m quoting correctly), “The short answer is, I don’t know. The long answer is, I really, really don’t know.”
Tony Heinz: Millie would probably tell us, following your advice, to keep alert for opportunities. But taking a much narrower focus, something that is broadly recognized (but will still be very exciting over the next 10 years) is this: moving our building blocks over to a slightly smaller scale — a more physics scale — and creating new materials by combining phases that previously didn’t coexist. So I think that’s a very exciting opportunity. We’ve done some of the first steps in that, and we’ve seen Hofstadter’s butterfly and things, but obviously, putting magnetism and superconductivity in intimate contact is bound to lead to a lot of very exciting new physics.
Pablo Jarillo-Herrero: To follow up on that, I was recently at a meeting funded by a private foundation — the Moore Foundation. Philip was there and, I believe, Tony was there, too. And some of us were discussing a possible platform that’d use robots to create an assembly of 2D heterostructures. And one of the materials growers in the audience mentioned that this might actually work, because you're working under non-equilibrium conditions. There’s no way, if you wanted to put materials together and then heat them up and have them realize that structure, that it should not work — because you are working against thermodynamics and equilibrium. But thinking of this robot assembly of all these heterostructures under non-equilibrium conditions, that might actually work. It reminded him of some of the synthetic chemistry things that the pharmaceutical companies have been doing for a while. So maybe, if some of you wanted to comment on this field of 2D heterostructures — what kind of crazy, futuristic things could we do with them? Or perhaps, what’s the thing that most captures your imagination?
Philip Kim: We’re not into nanobots yet, but some of the technology is out there. There are technologies out there that would allow for automatic or semi-automatic assembly of the materials. There are some wafer scale technologies where one can take some of the materials and put them one on top of another using flip-chip technique.
I heard a rumor that… I was not there, but I heard that at the Graphene 2017 conference, the groups from Japan showed that they could have 30 layers of semi-robotic assemblies happening at the same time. So I think it’d be really exciting to see those kinds of technologies continue to advance in this field. And tied in with that, perhaps having some more theoretical guides would help us out quite substantially; that way we’re not just randomly searching for these new and interesting interfaces — we can perform guided searches and do test screening. I think that, alone, would probably revolutionize this research field and would germinate increased discoveries of new types of interfaces in new materials.
Frank Koppens: I agree. So the question now is: what do I add to that? Like Paul said, we should miniaturize electronics, but I think that we also have the opportunity to miniaturize photons, now… bring photons onto the chip, make them move in the nanoscale dimensions, make the spins move. That field has already been in your life for a while, but I think that now, there’s a real opportunity to bring this all together. After all, you can now do this on the wafer scale — so what you said was not just done on small scale, you can do it on a wafer! And that’s incredible; that’d make a real technological revolution. So I think we’ll see that emerging over the next... well, maybe not 50, but probably 30 years.
Eva Andrei: I can remember a time when 2D materials were a glimmer in a theorist’s eye, and they only existed on paper but not in real life. Then, all of a sudden, we had all these 2D materials at our disposal, even though we initially thought they could never exist. So this is one of the great things about the progress in this field — we started with exfoliating graphene from bulk graphite, and this taught us how to make or isolate other 2D materials, so that now, we have a whole family of them! You can create electronic or material properties just by stacking these layers one on top of the other, and somehow, you seem to have eliminated the need to rely on chemists!
For example, we can change electronic properties of a 2D layers by using strain, or by doping them with a gate voltage and so on and so forth. Because all their atoms lie on the surface, these 2D materials are extremely sensitive to the environment. And that’s interesting, too, because it can lead to new applications. As we’ve seen in this afternoon’s talks, you can take all this research we’ve been doing on graphene and other 2D materials, and you can use it create sensors or actuators or all sorts of things!
Now, I was really very excited by Paul’s talk. These nanobots… okay, if you want to replicate life or if you want them to replicate themselves, you also want them to be able to get the energy to power themselves without having to plug them into the wall. So you need to build robots able to generate their own energy. Now, we know that many of the 2D materials have very high Seebeck coefficients. So maybe that’s another very important direction to go in. Perhaps we could combine thermoelectrics and the work Paul is doing to realize self-powering nanobots. I think that is going to be a very exciting direction to go in.
And, finally, I was also excited by Philip’s talk about getting to carrier densities that are in the 2 x 1014 carriers per cm2. And, if you remember, I think that back in 2010, there was a paper predicting that graphene could become superconducting once you go to 2 x 1013. Now, we went there, and nothing happened. So those same theorists insisted it would still happen, but they pushed the limit to 1014. So now, it looks like we’ve finally gotten to 1014, so.… Are your samples superconducting?
Philip Kim: No, not yet.
Eva Andrei: Someone should look into that, perhaps.
Pablo Jarillo-Herrero: This is very interesting, but I do want to make sure I give the audience a chance to participate in the discussion as well — either to ask questions or to make comments. So please, if you have a question, do please come up and ask.
Question Asker: Thank you so much for organizing this symposium in honor of Millie. I love the awesome, outstanding speakers. I had one question about the microbotics. From your report, the nanobots are driving over a few hundred millivolts and over a few nanometers increase — 3 nm. So I would just say, your electric effect is very high rate as compared to other materials. Is that correct?
Paul McEuen: Just to be clear, the voltage is — roughly speaking — between the bimorph and the electrolytes surrounding it. So the voltage drop is across the divide double layer from that to the surrounding electrolyte. That's where the voltage is applied. Not between the graphene and the platinum.
Question Asker: I see. Got you. Is it possible to incorporate a smart polymer or something? So that would normally drive only a few millivolts — only 0.5 millivolts. That will make the actuation very large.
Paul McEuen: So, yeah, in fact, there’s a group at Johns Hopkins that we collaborate with, and they have made graphene and then a smart polymer bimorph that can change. And in that case, it’s not doing it with respect to voltage. But, yes, I think it’s very exciting. Basically, take graphene and anything else, and it’s exciting. All the smarts is actually in the ‘anything else’; it knows how to do stuff in response to external signals. The graphene’s job, here, is just to be the absolute most boring thing that you can imagine. It’s a structural material. And I know, again, that saying this is kind of like heresy to this crowd — but it turns out, graphene really is an excellent structural material.
Question Asker: One more question. I wanted to ask if this type of microbot could, later on, be nanobotic, implanted in the body, and if it could use light to drive itself instead of using an electric field. So let me give an example: by shining red light, you could make it activate and do something that’d be of great help to the healthcare industry. Thank you.
Paul McEuen: Yes and absolutely. Our goal is to run everything, more or less, off visible light — or actually, infrared light. So that’s how we’ll get information into the system and also how we’ll get power into the system. You can imagine other ways, but this is the easiest thing, because every biologist has a microscope in their lab. Basically, if you use light-in to power the nanobot and you use light-out to look at it, it’s just a big fancy fluorophore from a communications point of view.
Pablo Jarillo-Herrero: Are there more questions or comments from the audience?
Okay, while we wait for some of you to get inspired…. Oh, wait a second. One of our panelists has a question.
Tony Heinz: Yes, I do. Just in your challenge to think about the future and what’s the next big thing… I think we identified some frontiers in the direction of developing a new class of materials that interface with the life sciences. I’d just like to mention two more that, I think, are sort of latent but maybe not discussed so explicitly.