Panel Discussion on 2D Materials
Graphene and 2D Materials Session, MRS Memorial


Millie used the scotch tape trick in the 1960's... but threw away the graphene on the tape!  Photo courtesy of the Kavli Foundation.
Millie used the scotch tape trick in the 1960's... but threw away the graphene on the tape! Photo courtesy of the Kavli Foundation.

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.”

Photo courtesy of the Dresselhaus Family
Photo courtesy of the Dresselhaus Family

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.

Millie watches Paul McEuen speak.
Millie watches Paul McEuen speak.

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?

Millie took apart her graphite models to create graphene.  Photo credit: Shoshi Cooper
Millie took apart her graphite models to create graphene. Photo credit: Shoshi Cooper

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.”

[Laughter]

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.

Millie lectures at the Technion.  Photo courtesy of the Technion
Millie lectures at the Technion. Photo courtesy of the Technion

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.  

At Millie's 80th birthday.
At Millie's 80th birthday.

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!

Millie receives an award from the American Carbon Society.  Photo courtesy of the Dresselhaus Family
Millie receives an award from the American Carbon Society. Photo courtesy of the Dresselhaus Family

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.

Millie shows off a buckyball
Millie shows off a buckyball

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.

Photo credit: Shoshi Dresselhaus-Cooper
Photo credit: Shoshi Dresselhaus-Cooper

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?

[Silence]

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.

Millie takes notes on someone else's lecture
Millie takes notes on someone else's lecture

One is kind of obvious, but it suffers from this problem of being in divided communities — and that is the interface between chemistry and 2D materials. And this problem exists, in particular, in the context of understanding how growth occurs. But there is, for example, a very large community that does catalytic chemistry on the edges of 2D materials. We know there are 10,000 papers on the physics of 2D materials. There are also 10,000 papers on the catalysis of 2D materials. Despite this, there’s been almost no communication between the two fields!  And I think some of the things that we know, in terms of controlling defects but also modifying materials by strain by applying potentials, could really add to that field and — conversely — we could benefit from their field quite a lot.

One obvious area where these will have to come together is in the creation of 2D quantum dots. If you want to make a quantum pancake, then we care about what happens at the edges of the material, because what happens there is completely different from what happens on the basal plane.  I think this is one area that we could all, collectively, consider a little more.

Second is the interface with quantum measurement. I believe 2D materials are almost the ultimate limit of what Paul has been talking about in physical displacements — especially when you have one quantum of vibration. And 2D materials are the ideal way of looking at macroscopic quantum mechanics. By doing these measurements in high Q cavities and things like that, people have shown that you can cool vibrational motion in 2D materials and you can start measuring things at the quantum limit. But I think that’s another facet of 2D materials where our community could probably contribute a lot.

Paul McEuen: So... one comment. I would say that, actually, 1D materials are even better than 2D materials. Right? So I think that the nanotube is even a better choice than the 2D materials.

Pablo Jarillo-Herrero: Another question from the audience?  Yes, please.

Leora Dresselhaus-Cooper: Thank you. This is a wonderful discussion. So, I’m a chemist...

[All panelists freeze]

Pablo Jarillo-Herrero: Eva didn’t actually want to get rid of you!

[Laughter]

Photo courtesy of MIT
Photo courtesy of MIT

Leora Dresselhaus-Cooper: Well, actually, I think Tony brought up a really great point, which is that there are chemists out there that are using these 2D materials in a totally different way, and then there are physicists and materials scientists that are interested in defect engineering and strain engineering and all sorts of interesting properties. And so, as the chemist in the room, my question is: what should chemists know about what we can contribute to 2D materials, and what should the chemistry community be looking at that would actually really be helpful to the materials science community? Does that make sense?

Pablo Jarillo-Herrero: It makes perfect sense.

Eva Andrei: I feel I need to respond to this.

[Laughter]

Eva Andrei: My comment about chemists just reflects my ignorance.

[Laughter]

Leora Dresselhaus-Cooper: It’s fine! I’m not offended.

Eva Andrei: I think that chemists can — and have already started to — create 2D materials from the bottom-up. You start out from molecules and you just build on up….  And people have already been able, at Berkeley and in Europe, to build nanoribbons starting bottom-up from a molecule that contains carbon — and they can actually make nanoribbons that are seven atoms wide! And that is very exciting. The thing is that these ribbons are very fragile, and it’s hard to make electrical contacts to probe their electronic properties — so this is going to be something for the future. And I think that the biggest challenge, at the moment, is how to grow them larger.

Frank Koppens: To add to this, I think we actually benefit greatly from chemists, because we need chemists in order to grow high quality samples of many of these materials we’re analyzing — especially the ones beyond graphene.

Millie at the WITI Hall of Fame
Millie at the WITI Hall of Fame

For graphene, it’s easy — you can just take a piece of graphite and use exfoliant techniques, because they work so well. We have all become very spoiled by that. But actually, with all the other materials — including the TMDs — it’s extremely difficult to get good quality samples. So in that respect, I think the chemists have a great opportunity. And I think we physicists should be a bit careful about getting spoiled and lazy by just exfoliating the materials, because that’s not the way to go. It’s really about growing high quality materials, and, ideally, if you want to make technologies out of any of these materials on a larger scale, you will need chemists.

Philip Kim: The other way chemists help is in terms of modifications of the materials. The advantage of 2D materials is that the surface is bulk while the edges are not — and that means that basic knowledge of surface chemistry will be directly applicable to modifying the surface of these materials. I think that’s kind of exciting. All the intercalation I just mentioned is completely on the chemistry side. I think that, in fact, the layer part of the work that…. In my group, actually, I hired an electrochemist to work in our group, and I learned a great deal from him. At first, I’ll admit, I had a lot of confusion in terms of chemistry and trying to remember what a redox reaction was, or which was a cathode and which an anode.

[Laughter]

Millie reading.  Photo credit: Gene Dresselhaus
Millie reading. Photo credit: Gene Dresselhaus

Philip Kim: And things like that! But he helped me with it, and then I started asking more interesting questions from there, and the project really advanced as a result.  So basically, without this type of help, a lot of the most interesting science will not be possible. That’s one of the things I like most about materials science — it’s a place where physicists, chemists, and materials scientists can meet together, and probably there are fewer boundaries between these common knowledges and common goals than in other fields.  And this kind of cross-fertilization and opportunity to learning different science-languages — I think that is a very useful experience and a very important experience to undertake, in order to meet the goal of really creating the next level of technology. In some sense, I think that in the case of 2D materials and low dimensional materials, the boundaries between the physics and chemistry is even more blurred than usual, there.

Pablo Jarillo-Herrero: I would like to add something. I have the highest of respect for chemists, you know.

[Laughter]

Pablo Jarillo-Herrero: And that’s because of Jing Kong, whom I see sitting over there. She was a postdoc in Delft, where I was doing my PhD, and I would often be working side-by-side with her. Now, the physicist’s approach to a situation where something doesn’t work is to say, “Hm... why doesn’t it work?” And then you spend just hours and hours figuring out why it doesn't work!  So I did this, and Jing was sitting next to me the whole time, and she’d say “No, no, don’t spend all day sitting around thinking about why it doesn’t work! Just make it work!”

Jing and Millie
Jing and Millie

So I watched as she kept working, and sure enough, she made it work. Therefore, just by sitting next to her — she improved my productivity by leaps and bounds! That’s my first comment about chemists. We physicists have a lot to learn from chemists in that respect.

The second thing I wanted to say is related to what Tony mentioned, which is that… I was recently visiting my hometown in Spain, where I got a chance to meet with a very distinguished chemist named Avellino Korma, and I was ready to impress him with all my research into this new and exciting world of 2D materials. So I started to talk about molybdenum disulfide and how great it was. He tells me, “Oh yeah, we’ve been using that for catalysis for a while, now.  We grow a hundred tons at a time.”

And I was like, “No, but this is layered.”

And he said, “Yeah, I know! Mine are, too.  In fact, mine are 1 nm thick.”

So I think there should definitely be more communication between the two communities. If they can already do 100 tons at a time, then maybe we should watch and learn from them. It’s a sad reality of our time that these two communities do not talk very much, however, and I think it would be of great benefit to both fields if they spoke more often in the future, so that they don’t spend forever working on problems that the other field has already solved.

So... are there more questions? I’m going to keep offering the audience the opportunity to talk, and, otherwise I’ll keep steering the conversation.

Millie delivers a lecture. Photo courtesy of the Dresselhaus Family
Millie delivers a lecture. Photo courtesy of the Dresselhaus Family

Question Asker: Sorry, I’m going to ask about a boring and mundane topic. This is a 2D material session, and we all know that carbon nanotubes are basically just rolling up a 2D material — graphene — and creating tubes, single or double-walled tubes. Could any of these other 2D materials be used for something similar?  

Pablo Jarillo-Herrero: Sorry, can you repeat...?

Question Asker: Like could you take molybdenum disulfide, grow a single or double-walled nanotube out of that, instead?  Or perhaps with boron nitride. Could you have a boron-nitride nanotube? Could anyone comment on that? Thank you.

Pablo Jarillo-Herrero: Well, I think at least 4 of the people standing here have worked on nanotubes for a large fraction of their careers. So any of you wants to comment?

Paul McEuen: Well, the way we work on nanotubes is that somebody else makes them, and then we steal them and start studying them. So I have nothing useful to say about whether you could make MoS2 tubes. Are they...?

All: There are, multi-walled.

Tony Heinz: I think the answer to this question is a little analogous to graphene, in the sense that graphene is the most perfect 2D material and it’s also the one that grows the best single-walled nanotubes. So although you technically could grow tubular structures in a lot of these materials, they wouldn’t be quite as controlled as carbon nanotubes.  I think perhaps they just haven’t gotten sufficient attention, though. Perhaps that’s another area where we could crossover between the fields.

Pablo Jarillo-Herrero: I know that boron nitride nanotubes have been investigated quite thoroughly. However, because they’re insulating, their properties have not received so much attention, but there’s a group at Berkeley and I think a few others around the world that have grown boron nitride nanotubes — single walled, multi-walled, etc. — and they’re really interesting objects.

Any more questions from the audience?

No?

Millie used to handwrite notes for her courses.  Photo credit: Shoshi Cooper
Millie used to handwrite notes for her courses. Photo credit: Shoshi Cooper

Well, one last topic I wanted to discuss or ask the panelists is — we have seen, now, that with these 2D materials and especially in the past couple of years, we’ve essentially covered all of the behaviors that traditional condensed matter systems exhibit. We have semi-metals, metals, insulators, semiconductors, superconductors, and recently we even have 2D magnets. Now, some people may argue, “Well, all of those already existed. So it’s fun to make new characteristics and their behavior depends with layered numbers, etc. But what would be really cool is if we could make something new that didn’t exist in 3D, before. A material that displayed a completely new behavior.”

So maybe one of you panelists could to predict or suggest what kind of thing you could create with 2D that does not exist in 3D?

[Silence]

Pablo Jarillo-Herrero: The audience can also offer their own ideas?

[Silence]

Pablo Jarillo-Herrero: Anything?  Any ideas? Big fancy medal from Stockholm for whoever guesses correctly and realizes it!

[Silence]

Pablo Jarillo-Herrero: Anyone?  No?

Paul McEuen: I think you’ve really stumped us.

Audience Member (calling out): Philip should build a room-temperature superconductor!

Pablo Jarillo-Herrero: Okay! A room-temperature superconductor! Philip, there’s your next homework assignment. In the spirit of Millie, you have to get it done by tomorrow morning!

Philip Kim: I’ll get right to that.

Millie gives a lecture.  Photograph courtesy of the Dresselhaus Family
Millie gives a lecture. Photograph courtesy of the Dresselhaus Family

Pablo Jarillo-Herrero: Since it seems there are no answers, let me change the question a little. What kinds of things do you think 2D materials could do or enable you to do, technology-wise or science-wise, for which 3D materials just aren’t up to the job?

Paul McEuen: So going back to what I said earlier, I think that, as a mechanical material, 2D materials really are special. There’s really nothing else quite so stiff and impenetrable as, say, a graphene sheet. So I think either the kind of actuator things I was talking about, or more generally, even as a diffusion barrier between different materials — you can put two materials next to each other that don’t want to live next to each other and put up a graphene sheet in between them and keep them apart. I think there are all kinds of things involving using the two dimensionality — the thinness, roughly speaking — of the material as an essential component in all kinds of future technology.

Tony Heinz: I forgot what the question was.

Pablo Jarillo-Herrero: Last thoughts! Just give your last thoughts.

Philip Kim: There is one thing that I found in the 2D materials community that I thought was quite interesting and very exciting. It was done by Jeehwan Kim at MIT — not related to Philip, by the way. It was just beautiful work. He used a graphene layer on the top of the gallium nitride (or possibly gallium arsenide) and could regrow other materials. The graphene is extremely thin, but nevertheless, somehow, the epitaxial layer appears across the graphene layer. So that’s epitaxial growth, seeing the substrate underneath. And then, using the graphene as a separation layer, he can separate out the grown materials from the substrate and then reuse that substrate — I think that was very beautiful work, and it really shows the strength of the 2D-ness of the materials. Yet that also works as a template for the 3D growth. If you combined this kind of work with Paul’s idea of really using the thinness of 2D materials to your advantage — I think that would be another very exciting direction.

Millie gets ready to mark up papers on the airplane.  Photo credit: Marianne Dresselhaus Cooper
Millie gets ready to mark up papers on the airplane. Photo credit: Marianne Dresselhaus Cooper

Eva Andrei: So I can think of many properties and possible applications of 2D materials which would be difficult in 3D. From the science point of view, 2D materials have unique topological properties, intrinsic to their being 2-dimensional — the quantum Hall effect is an example — that enable electrical transport with minimal losses. This could be extremely useful for miniaturizing electronic components and computer chips. 2D electrons in the fractional quantum Hall regime form so-called anyon quasiparticles that could encode qubits and do simple quantum computations. In terms of more immediate applications, graphene and other 2D materials have an exceptionally wide range of mechanical, thermal, and electrical properties that can be used to improve the performance of existing products or to create new applications. For example, the fact that graphene is almost perfectly impermeable to gases and liquids allows it to be used in filtering and desalination applications. Other applications include supercapacitors for energy storage, scaffolds for bone grafts, cancer therapies and drug delivery, ultra-sensitive sensors, and the list goes on.

Frank Koppens: In the field of microelectronics, following the invention of the transistor, we have seen three Nobel Prizes that were related to heterostructures. Most of those heterostructures were actually based on gallium arsenide and aluminum gallium arsenide. Now, unlike before, we have the flexibility to choose from 2,500 materials, instead of just 2, 3, 4, or 5. And what the expansion of this 2D materials field has enabled (which has already led to a Nobel Prize) is, for example, faster transistors. Just to go from a slow transistor to a fast transistor was a Nobel Prize, and that was a result of developing heterostructures. Making quantum wells for infrared detectors in quantum cascade lasers was a big revolution, too.

Millie's desk in her office
Millie's desk in her office

So in terms of technology, we have the possibility to make, for example, lasers that are even smaller. We can make infrared and terahertz lasers that are extremely small and then integrate them into computer chips. We could make extremely small antiquated data communication systems, and integrate those into computer chips. We could lower the power consumption! The expectation, now, is that in the future, all this technology will be wearable — so we go from smartphones to smartclothes. Silicon technology is not going to be able to do this for us. However, with 2D materials, I think we can solve those problems. If you talk to companies, you’ll find these questions are already out there. Like, can you make a terahertz laser at room temperature? No, we can’t. But maybe with two dimensional quantum wells, we might be able to make it.

So we have to also look at... not just new physical phenomena, but also new capabilities. Remember, the transistor already existed, but just making them faster was a big technological revolution. So to make a laser move from visible infrared to terahertz would also be a revolution.

Eva Andrei: Tony? Last words? Last thoughts?

Tony Heinz: Well, this is a bit more on the fundamental side. It goes back to chemistry and physics, perhaps. Now, one aspect of chemistry is more than forming and controlling new bonds. But I think an aspect that intrigues me is that, in chemistry, the notion that a molecule is modified by its environment is a completely standard thing, and there are many, many terms that are used to describe this. You don’t have to have a reaction — just the presence of the environment modifies its properties and gives rise to changes in electronic structures and spectral shifts.

Photo credit: Shoshi Dresselhaus-Cooper
Photo credit: Shoshi Dresselhaus-Cooper

In our normal way of thinking of materials, they communicate with the environment in very limited and prescribed ways — things like strain, electric fields, charge transfer, etc. But they don’t respond to the details of the local environment in the same way molecules do.  2D materials, on the other hand, definitely do that. Not only do they have the potential to do that — they also show it in experiments! And we need to update our thinking both to fully appreciate that, and also to take use of it in new and creative ways. You could say that, at one level, it’s like a new class of chromophores that are involved periodic structures and that benefit from all the merits of periodic structures but which also have environmental sensitivity. Or you can say that we have the ability to apply strain into things with fields, so why not apply that to molecular structures?  It’s sort of a broad concept, but I think that’s another facet of 2D materials that will lead to a lot of new insights and technologies, as we understand and appreciate them more over the coming years.

Pablo Jarillo-Herrero: Okay. And so, with these words of wisdom, we are going to close this session. It’s been a pleasure and an honor to chair this session in tribute to Millie and to remember her many contributions, her inspiration, and her pioneering work in 1D, 2D, 0D — and all the dimensions of materials! I want to thank the speakers again. And thank you to the audience for the insightful questions.

Photo credit: Shoshi Dresselhaus-Cooper
Photo credit: Shoshi Dresselhaus-Cooper