Professor Gaim gave us an excellent discussion of the basic physics of graphene.

I'm going to focus on applications, applications of graphene in electronics and optoelectronics. From the physics that we've heard, they're all important. But I wanted to stress one important point for electronic devices.

That involves the Klein tunneling that you heard. Because it affects the most important property of today's electronic devices, namely the confinement of electrons by electrostatic means. And we start with graphene. Let's say it's n-doped electron carriers.

And then we apply a gate field. So we produce a p-type section of the graphene. And then on the other side, we have the electron, again, n-type graphene. So we have an n-p, n-type of junction.

We heard about pseudospin, which, like real spin, is conserved. In this situation, we have zero-bond gap graphene. So it's a case of tunneling without a gap.

And it conserves pseudospin, which is, again, you don't actually need the pseudospin. It is based on the orthogonality of the pi and pi star wave functions of graphene. So an electron from the conduction band on the n-side

can very nicely tunnel conserving pseudospin into the valence band of this section and exit, again, in the conduction band on the other electrode. So we go back to this gapless inter-band tunneling has been described a long time ago

and used, actually, in semiconductors. Incident electrons on a barrier that's very sharp, as indicated here, would lead to full transmission. This is the basis, also, of the absence of backscattering in graphene. Other directions of electrons incident on the barrier

will be reflected. And what the actual transmission coefficient would be depends very much on the shape of this barrier. The sharper the barrier, the easier the transmission, up to 100% transmission. As a result, you cannot confine electrons

by electrostatic means, by a long-range potential. So if I look at the next slide, if we look at the current versus voltage,

gate voltage of a field-effect device, we get the characteristic V-shaped by V. This is because graphene is an anti-polar device. It is depending on what kind of bias we apply on the gate. We can either have hole or electron conduction.

But the most important consequence is because of the client tunneling, the gate can never turn off completely the current. There will always be a leakage. And of course, this is very essential because if you cannot turn off the device,

you cannot have a digital switch. So you cannot do what, say, silicon does, or gallium arsenide, or other conventional semiconductors. Typically, the neutrality point is shifted

from zero gate bias. And that's, again, because, as Professor Gai mentioned, the existence of electron hole panels, the fact that the insulator supporting the graphene has trapped charges, and that gives rise to the minimum conductance,

which, again, is a function of the trapped charges in the substrate. So the substrate has a very profound effect on negating characteristics of graphene. A different view that engineers have

in describing graphene is actually that of a material with a band gap. And that is based on the fact that there is an angular dependence on the scattering. So you can describe graphene as having,

on the average, a band gap. And this is a trick that electrical engineers and device physicists use to apply all the developed theories for conventional semiconductors to graphene. You consider that you have a valence, a conduction band with an effective gap.

I just bring it up just so that there are other ways of thinking about graphene. So we already heard about the fantastic properties of graphene, and here is a set of properties that are of particular interest in electronics.

High mobility, the fact that you can tune the current, as we saw by a gate field, ultra-thin body of the device, the ability to carry very high carrier densities, excellent thermal conductivity, that's very important,

and large optical absorption. But I should stress that when we are interested in the intrinsic properties of a material, we try to decouple as much as possible from the environment. Now, graphene is a single atomic layer.

It's all surface. So it interacts very strongly with the environment. When you do basic physics, you use low temperatures, you suspend the graphene, and so on. When you want to have applications, the application itself would determine what the environment should be.

Of course, almost all devices work at room temperature or higher. They are encapsulated inside complex structures. So it is important to understand that interactions with the environment can drastically change most of these properties,

and Professor Gaim stressed that point. Here are just list types of scattering mechanisms, for example, that are important in transport. Sorry. Scattering by impurities,

and impurities typically are not in graphene. They are on the substrate that you place graphene on. These are charged impurities. We use insulators, which are notorious for trapping charges, and they are long-range scatterers. They tend to dominate transport in graphene.

Neutral defects, short-range scatterers, can be important, and of course, invalidate a lot of the things we said about absence of backscattering and so on, because they distort the structure, the local structure of graphene. Charge transfer between a controlled data mode

to the substrate and so on. Can it use doping? Change the chiral concentration and scattering. Surface roughness can be very important. And then we have inelastic scattering mechanism. Of course, phonons of the graphene itself. Something that is usually neglected in discussions

is the graphene has a very high optical phonon frequency. Sounds like five times higher than that of gallium arsenide. So typically, the scattering in electronic devices as dominated by optical phonons

is not very strong in graphene. However, we have new scattering mechanisms, such as coupling of the conduction electrons with the surface phonons of the polar substrate. They are always present, even in conventional devices,

but the field is generated, decays exponentially away from the surface. And in a finite thickness electronic device, that field has decayed. But graphene is a one atomic layer, so it's right smack at that surface of the polar insulator that leads to doping.

So at the end, the mobility that you measure is determined by all, by a sum of all these scattering mechanisms. So if you want to have applications, especially in a complex structure, you have to evaluate and control

all these different scattering mechanisms to get a reproducible, as you need for an application to resolve. The other very important aspect is bringing in the carriers and taking them out.

Transporting graphene may be great, but how are we going to bring in those carriers and take them out? And that is the issue of contacts. The traditional way of measuring the resistance of contacts is shown here. You just make a channel of different,

channels of different length, and you measure the resistance, then plot the resistance as a function of channel length. And you extrapolate to zero channel length. This is called the transfer length method. And you find that for graphene, the zero length resistance depends very strongly

on the gate voltage you apply. And you can see here, contact resistance versus gate bias. Of course, the resistance is highest near the direct point. But on top of that,

you also have a temperature dependence. Contact resistance in graphene is temperature dependent. The key conclusion is, this is for a palladium metal contact. And as far as I know, actually, this is the smallest contact resistance observed so far,

which is 200 ohms micron. That's a very large resistance. And if you go closer to the direct point, it can be kiloohms. So the contact resistance is comparable to the resistance of the channel itself. It can dominate the performance.

So let's see where this resistance comes from. We have graphene on an insulator. We deposit the metal contact. So the first thing that happens is, graphene and the metal have different work functions.

So it would be a change of charge, charge transfer. So the graphene would be doped. As a result of that doping, there will be a dipole layer formation that the carriers have to cross. It's one bar. They also have to propagate a finite distance

under the contact. That's the transfer length. And then exiting in the channel, they will find graphene that is differently doped because it hasn't been doped by the metal. So you will have a doping junction akin to a p-n junction in a semiconductor device.

So at the end, these carriers encounter a number of blocks. And you can understand that looking at the Rolfandauer's resistance, contact resistance, which would be proportional to a transmission coefficient.

This is the sum of these processes. And the number of modes, one of the modes that carry the current, and that's determined by doping. And of course, the doping is here and different from there. So whichever is the smaller number of modes will dominate.

The other important aspect is, depending on what this transfer length is, will determine how small you can make device. Because in electronics, you have a pitch for the device spacing. And if your contacts are required to be big,

you don't gain much by making the active channel small. So you have to minimize both of them. And one can generate a simple model that will describe the propagation under the metal

and the transmission in terms, describe it in terms of two lens, the mid-free path under the graphene and a coupling lens that depends on the interaction between graphene and the metal. And sometimes this actually, you want to minimize that kind of ratio.

Of course, in electronics, the game is scaling, making things smaller and smaller. And we talked about the biostructural graphene.

We have a symmetry between electron and whole states. But if you take some graphene, this case, the sample was from chemical or they put the position graphene palladium contacts.

You measure the resistance versus gate bias at a large channel length, 240 nanometers, you get an almost symmetric resistance voltage curve. And then you can start scaling it and you see that it becomes more and more asymmetric.

And eventually you get a 40 nanometers of various symmetric shape. And on top of it, you start seeing oscillations. As the length changes, of course the transport mechanism change.

And at this point, actually we are in the ballistic regime. So now electrons, actually I shouldn't say ballistic, we are in the coherent regime and now electrons can interfere and give us this Fabry-Perot like oscillations.

The behavior can be understood very easily considering that palladium which has high work function, P-dops graphene. So when we apply a positive bias, we create a P and P junction. And what I saw you before, that will lead to resistance.

And the asymmetry comes from the contacts. So the electron characteristics are really dominated by the contact. And you can see that visually

using near field source circuit for conductivity of graphene palladium system. You measure for the conductivity bringing in with a tip, light source and you scan it along the channel. You see the contacts here for negative gate bias.

We have a bond structure that looks like these. So you have a P, P prime, P kind of bond structure. Apply a positive voltage and you can see the formation of P and P junctions.

Of course, depending on the detailed transport mechanism inside the graphene, the properties, the dependence of properties changes. For example, here, I saw dependence of mobility on carrier density, ballistic of course,

we're talking about ballistic conductance proportional to the square root or columbic barrier scatterers. We have a mobility that's independent of the carrier density, but for impurity scattering, we have one over N. In general, the diffusive regime,

the properties depend on what kind of impurities or scatterers you have. That brings to again, the electrical engineer's point of view. Electrical engineers have been trying for a long time

to understand shorter and shorter devices, particularly in three-fives. And they have come very close to ballistic regime. There's always some scattering. And we know about mobility,

the normal mobility we all talk, which is proportional to the mean-free path. But once you start having ballistic mobility, then they introduce a somewhat artificial term called ballistic mobility. And this is based on the fact

that the mean-free path can never be smaller than the channel length, because you have the contacts, which are scattering. So, and they write the effective mobility in terms of Matheson's rule. So, in that formulation of the problem,

in scale devices, mobility, high mobility doesn't play at all. Everything saturates. So for example, if you start with a mobility, the normal mobility of 10,000,

the mean-free path is 160 nanometers. So the ballistic mobility will be only 600. And the effective mobility would be about 570. If you go to 100,000, you still get 600 mobility.

So this is something to consider. The effect of the contacts is paramount. Taking a graphene that's made by CVD and has poor properties,

say a mobility about 1,000, and looking at the minimum conductivity as a function of the aspect ratio of the device width over length, you see that already at the aspect ratio of 25, which is about 50 nanometers, or even earlier,

we are very close to a theoretically predicted ballistic limit. So if we are talking about scale devices, we don't have to worry that much about mobility. Okay, and there are some other reasons that we get to it.

What we have to worry is, of course, getting good graphene, large area wafer-scale graphene. And there have been several approaches in the growth of graphene. One introduced back in 75 at the village labs

involves taking silicon carbide, heating it at high temperature, 15, 1600 degrees, where silicon evaporates, and the carbon atoms that are left behind reorganize to form a graphene layer.

And this is an AFM image of such a graphene type of sample. At the high temperatures used, typically you get bunching of steps, and flat terraces.

And you can measure the whole mobility of such a sample. Here's mobility versus carrier density, and the whole mobility curve can be understood in terms of a combination of Coulomb and short-range scattering.

The point is that we have a strong carrier density dependence, and you can get high mobilities at low carrier densities. But for electronic devices, you have to work at higher densities because you need to have at least milliamp per micron currents.

Topography plays a very important role. And this is, again, the same image that I saw you. And the point is that if you, by making devices in different orientation within the steps,

you find that a single step, this is about 10 nanometers high, this is a bunched step, can introduce resistances of the order of 10 kilo-ohm micron. So topography is important.

Also, point defects in graphene play a role. They affect the temperature dependence of the carrier density. I won't discuss, it's too complicated. But there are specific defects in silicon carbide with an energy of about 70 milli-electron volts.

Another big technical problem is that graphene is inert. It's a polar, inert. So if you want to deposit, as in other normal semiconductors, an insulator, it's very difficult.

It doesn't nucleate properly. So if you want to put, for example, half-milli-dioxide, here's 10 nanometers of half-milli-dioxide, and you can see there are many, many gaps. Initially nucleates at steps and defects.

It's never very homogeneous unless you go to very high coverage. And of course you want the thinnest possible films. So as a result of that, you need to do something to the surface, usually sort of prime it with something. This is one example. Early on we used a polymer,

a very thin seed layer of polymer, which covered uniformly the surface, and then use atomic layer deposition to complete, and then you get a good surface. So the question now is, how do you use graphene in electronics?

We know it's a zero-gap semiconductor. We cannot completely confine them. A typical on-off ratio of the current in graphene is of the order of 10, 20, depending on the quality of the graphene, but to make digital devices, you need at least 10 to the four.

So at this point in time, pristine graphene cannot be used for the kinds of things silicon can do. However, we have a finite current ratio. We have high carrier mobilities

and drive currents and thrust conductance, which suggest that graphene can be ideal for analog applications, particularly fast applications, RF. And these applications are very widespread, especially when they are growing these days, because everything that involves wireless communications,

from cell phones to radars, sensors, biomedical imaging, security at airports, involves very high frequency transistors and amplifiers. So there is a big demand for this.

And high-end devices in that regime are not like silicon devices. They are very expensive. They cannot be produced massively, except the lower frequencies. They are hand-selected and very, very pricey. Okay, so here's an example of a structure of a transistor

for RF application based on graphene. We draw graphene on a full-wave person. To make the devices, this was made out of silicon carbide, as I mentioned. And here are some typical results of graphene devices.

At this point, we're operating at high frequency, so we cannot use resistances or other things to describe the material, because everything is mixed in, capacitance, inductance, resistance.

So we describe the operation of the device in terms of wave propagation, so-called S-matrix approach. And we can define a number of metrics, for example, the current gain. And another important metric

is the so-called cutoff frequency, f sub t, which is the frequency at which the current gain becomes one, that is, we stop having current gain. And if we measure the current gain versus frequency, for here are two devices, one is 550 nanometer channel length,

the other is smaller, 240. You see first that there is scaling, that is, as you decrease the channel length, the cutoff frequency increases. This is at room temperature, with a rather modest mobility of about 1500.

And already at 240, you can reach 100 gigahertz. That was significant because if you were trying to do that with silicon, with the same channel length, you will probably get no more than 40 gigahertz.

Now, the typical procedure, of course, is to start decreasing the channel length, and already silicon is at 22 nanometers, an order of magnitude smaller.

But instead of doing that, I will go back, this is the same foil that I had before, and remind you this, that there is a strong topography dependence, and try to make devices a little bit more carefully.

So here we make the device to make sure that lies on the signal terrace, that no steps are crossed. And when you do that, then you look again at the scaling, and at the same length, 210 was 240 before,

so as close as possible, we see that now we have doubled the f sub t, so we have cut off now 210 gigahertz. And as I said, the currents are very good. We have drain currents of over two milliamps per micron,

and looks promising. Another thing that needs to be done to increase the performance is to decrease these spaces,

the ungated regions, they add resistance, but we cannot overlap the gate, the source, and drain because the capacitance, parasitic capacitance will come in and slow the device down. So the fact that the graphene doesn't, insulators don't nucleate in graphene

can be used to make something called self-aligned gating. And R1 is the technique. Another approach that can be used, and we started using, is graphene by CVD. We saw examples already.

The advantage here is that unlike silicon carbide, it saturates at monolayer. Only very small portions around defects can become double layers. And the great thing about it is that you can now grow single graphene crystals,

like this called graphene snowflake. It's inexpensive, the most important thing is you just use copper foil. You can get any copper foil, put it with a hydrocarbon, methane, ethylene, whatever, and make the graphene.

The key advantage is that you can peel that graphene and paste it on anywhere you want. For example, you can put it on a transparent polymer, or in our case, you can put it on silicon wafers up to eight inch wafers.

And I think Professor Hong will discuss this technique probably very extensively. The next worry is heating, which is currently the major roadblock in silicon electronics.

Power dissipation limits the growth. These days you don't get more powerful chips because of the heat dissipation. As the density increases, you cannot cope with the power. And we have used Raman thermometry to look at these effects on graphene.

And as you can see, as you increase the electrical power, you get very large temperature increases. You can see it in a color mark here, as we increase the drain bias, how hot the graphene gets.

And there's also some degree of anisotropy in the heating because obviously the middle gets hotter than the edges. And if you try to simulate the heat dissipation, you see, of course, near the contacts,

you have efficient dissipation, but for a long device, there is extreme heating. And for this particular geometry, we find that about 70% of the power is dissipated with the substrate. So that is a key question then,

where do you want to place graphene if you have the ability to peel it off and put it on anything you want? So the properties of the substrate, of course, you want something for commercial applications

to be readily available, compatible with wafer-scale coverage. You don't want to have charged traps, which are the main problem with silicon dioxide. You don't want to have something hydrophilic because water, oxygen system on SiO2

and other polar insulators are the causes of doping. And you want, of course, high thermal conductivity. That made us conclude that maybe diamond type of films, SP3 carbon, which like diamond itself

is known to have very high thermal conductivity being non-polar and having all this other properties is used extensively to cover memory, hard disks and so on is a possibility. So we try that.

Here again results with CVD graphene. We are not very good at growing CVD graphene yet, so mobility is over the order of a thousand. But here we saw the scaling for CVD graphene. 550 nanometers, we get 26 gigahertz, 140, 70 gigahertz, all of course at room temperature

and at 40 nanometers, 155 gigahertz. So you can see that we had no problems whatsoever with doping or moisture or traps or anything.

And proving the quality of the graphene, this looks extremely promising. The other thing is we looked at the temperature dependence of these devices.

So we measured for the first time the frequency, the F sub T from room temperature to liquid helium. And as you can see, there is hardly any difference in these devices for all lengths,

which implies that graphene has another excellent property. There's no carrier freeze out. So you can use it, for example, even in space without having any deterioration of properties.

So that looks very encouraging. Of course, kind of frequency is not the only one, the only interesting thing. You have to have power. The applications that graphene has to

displace generate power. There are amplifiers. For internet, we need stations that will be used to transmit movies, big files. And for that, you need power. And that is more difficult for reasons not intrinsic to graphene,

but it's getting there. We started getting unilateral power gain that is comparable to F sub T. How do we optimize the graphene devices and circuits? Basically what you need is self gain

for graphene to be useful. And self gain is the ratio of transconductance and output conductance. Unfortunately, these two quantities go in opposite direction. You want the output conductance to tend to zero. That is, you want this curve to be flat.

You need the transconductance to go to infinity. So you want maximum slope. It turns out that this goes faster. So as in other semiconducting devices,

you need current saturation. That is, the current has to reach a certain value and remain flat as a function of grain bias. I'll discuss this. So what we need is good mobility,

good gating to increase the transconductance. We need current saturation to increase the output conductance. We need good dielectrics, high K dielectrics to affect the current saturation. And as I mentioned, self-aligned structures to minimize the full axis resistance.

For circuits, you also require to minimize the contact resistance that enters in the gain, power gain. If you optimize the gating,

then you can start getting current saturation that you need, and you also increase a lot the transconductance. This is just some simulations

that show how critical the contact resistance is. If you have f sub t as a function of channel length for different contact resistance, you see how fast the performance drops as a function of the contact resistance, really dominates the performance.

Then, of course, the next step is to try to integrate devices. Graphene is planar material, so it's easy to fabricate, well, easier than some other material to fabricate individual devices.

But then, in circuits, you have to fabricate not only the transistors, but all the passive elements, inductors, capacitors, and so on, on graphene. So there, you have to develop patterning techniques that can operate on graphene, and there are artesian problems and other things.

So just one example of making a unipolar frequency mixer that involves inductors and other components on graphene. And the frequency mixer is, of course,

you put two frequencies in, and you generate some and different frequencies in every radio and every place, because you want to have a high frequency but to process it at a lower frequency. So this is our graphene transistors,

our symbol for graphene transistors. So you bring in one frequency and another. Here they are, the local oscillator and the RF, and you generate the sum and the difference, and now you have taken a four gigahertz frequency

and made it 200 megahertz, you can easily process. The advantage of graphene in this case, this is not optimized by far, but the advantage of this particular design is that first it works even in heavily doped samples that don't show direct point, and it has superior thermal stability.

It operates even at higher temperature without loss, unlike conventional semiconductors. So probably I'm running late. I should jump to a little bit of the optical properties and how we can use it.

Graphene, the many body effects are not very strong, so we can imagine the optical absorption, signal parking transition and interbound transition. We already heard about the universal optical absorption,

which is, we'll see to some extent, frequency independent. It's about 2.3% for normal incidence for free graphene. For multi-layers, for energies above about half a volt, it's additive, so by looking at the absorption spectrum,

you can get how many layers. And then we have also interbound transition, which is through the type of transition in the very far IR. That can directly provide you the carrier density of graphene

and then you have an additional property that first described by Professor Kim, the poly-blocking, which is simply the fact that if you don't have states, your thermal level is there, there's no absorption. So by tuning the thermal level by a gate,

you can tune what wavelengths will be absorbed. The absorption spectrum of graphene over a very wide energy range provided by Tony Hines of Columbia is shown here.

You see it's, of course, it's a semi-metal, so it absorbs over the entire region. Indivisible and near IR, the absorption is relatively constant, about 2.3%, as we said. Up here, you have a plasmonic effects to complicate the picture,

but in general, it's high. For us, the most interesting part is the far IR, where we have Drude absorption that can get very high. If we take graphene, we see the characteristic

Drude behavior with frequency, or we can chemically dope it and increase it up, in this case, up to 40% of the incident light. And you can use it diagnostically if you want.

Here is over in the mid IR, the absorption. From this type of spectrum, you can get the Fermi energy by the poly-blocking, as I mentioned. You can dope it chemically, and you see that the poly-blocking moves far out of the region that we can probe.

So that leads to an increased absorption in the Drude regime and a decrease in the mid IR visible, because we have an oscillator strength sum rule.

For us, the first application we explore in photonics is that of photodetectors. Photodetectors, of course, look a little bit unusual for graphene, given that it doesn't have a band gap.

But when we did photoconductivity experiments, where we put graphene, source and drain electrodes, but didn't apply a bias between them, brought in the light with a optical tip in near-field microscopy,

and scanned the light over the surface of graphene and detected the photo current. We saw that the photo current was localized around the contacts. And if you think about it,

we measured at the very beginning that metal in contact with graphene will create charge transfer, and that charge transfer, of course, will lead to band bending. And the band bending will create a local electric field. So near the contacts,

if we radiate near the contact, there will be a band bending, which we can manipulate with a gate without applying a drain bias. So if we excite one contact, the electrical hole pairs that are produced

will be separated by this intrinsic field, built-in field, and we get a photocard. And if we excite the entire device, we get nothing, because the field acts in opposite directions at the two contacts,

so there is no net field. But if you excite near one contact, then you get a photocard. So we decided to use this as a photodetector. Advance, of course, of course, is because we don't need to apply a drain bias,

we have no dark current, and therefore, no shock noise. So we did that in measuring photoconductivity. These are results of the photoresponse and photoresponsivity for 1.5 microlight,

which is used in optical communications, as a function of the modulation frequency of the light. And the highest frequency we can measure now loud with equipment we have is 40 gigahertz.

And as you see, the photoresponse is essentially flat, and the little decrease that you see comes actually from the cables, not the graphene. And if we measure the photoresponsivity of DC at high frequency, essentially, they coincide. This tells us that the graphene

photodetectors are very fast. My postdoc went back to Austria and tells me that through an optical technique now, he can measure up to 270 gigahertz with no problem. So the advantage here, of course,

is that you have a universal type of photodetector that is responsive to just about every frequency. The disadvantage is that because of the single atomic layer,

the current you get to photoresponse is not very high, cannot compete, say, with three fives. But then also, we have the problem that we have to radiate near the contact. So the first thing we wanted to do is correct that problem.

So typically then, if we have two electrodes, or identical metal, the potential will look like this, the red, symmetric. And as I said, left and right will cancel out. But if you use two metals with different work functions,

they will lead to different buck bending. And so we made structures like this, and they're digitated electrodes, one with a high and low work function metal, like titanium, another with high work function, like palladium. And then we have the gate, no applied bias.

For an arbitrary gate, we have cancellation, again, of the photocurrents. By tuning the gate, we can get to a regime where we have positive contributions only. And as a result, you can first irradiate the whole area,

so you have a large detection area, and you get a 15 times enhancement in the photorexistivity. This is not the end, of course, because one could first make more layers,

but also you can enhance the absorption of graphene coupling to plasmas, for example. There are many ways of doing that with silicon waveguides, noble metals, even to plasmas of graphene itself. But you can get very high absorbers in performance.

And to test this, we applied it to optical communications so we used the graphene photodetector to detect an optical signal, again, at the communications frequencies,

and use eye diagrams to check how faithfully the interpretation of the optical data is done. And we, again, limited only by our ability

to measure high frequencies, we could measure up to 10 gigabits per second internet rate with the photo, graphene photodetector. So these are some of the things we are doing.

I wanted to, some conclusions. I really feel that graphene can have many important applications in both electronics and photonics. The key advantages, in my opinion, are the excellent transport and optical properties, the thinness.

Something that is not stressed enough, in my opinion, is the possibility of low price and ease of fabrication. People always compare with their fives and so on, which are established technologies, but expensive require MBE or other difficult systems,

expensive systems. I think particularly CVD graphene can bring a tremendous advantage there. Another great advantage, which, again, conventional semiconductors don't share, is integration with silicon technology. Graphene cannot be used in isolation because you don't have the digital part,

but it can be integrated with silicon technology. The critical issues is material. We need high quality, large area, homogeneous and low-priced graphene. We need control toppings. We need low-resistance contacts and gates

and good insulators. And also, I believe that we need more science, particularly science of graphene and interaction under what I would call real-life conditions. Anyway, I would like to thank all of my collaborators,

so remain unnamed, and also IBM and DARPA for the support of the project. Thank you for your attention. You mentioned that mobility of electrons in graphene

can be measured at small distances. What, circa 50 nanometers. Is it isotropic or may depend on the direction? I think it would be isotropic. I mean, first, I didn't say that, really, it would be basically small.

It doesn't have physical meaning because in very small dimensions, basically, it's ballistic. The concept of ballistic mobility is sort of artificial, but I don't think where you can measure mobility

that would be an isotropic. Unless, there is one report at least, but it's a very structured surface. In silicon carbide, people have done measurements with tips, metallic tips. So put down the tip at one point and circle with the other

and define anisotropy, but that is because the impurities in the silicon carbide are anisotropically distributed. They tend to collect near steps and things, so there's no,

but intrinsically, graphene should not be anisotropy. Yeah, I'm Novitlan, Imperial College, London. The FT values you were reporting for the transistors are very impressive forever. They're still well below the values you can achieve in gallium, arsenide, indium, phosphide. What is the physical limitation? I mean, you said that the mobility doesn't play a role.

It's limited by the geometrical dimension anyways. It's a matter of time. You're comparing a technology that developed over 40 years versus one that's less than a year old. Nothing is optimized and there are still issues

with the substrate, the gates. I think the insulator is a major problem still, and the quality of graphene is not reproducible and up to par. So individual devices,

I don't see any intrinsic limitation. But that's why I stress, in my opinion, the goal is not to go to 400, 500 gigahertz. There are reports of germanium devices going to 700.

The applications there are very minimal. May I add one more question? Is it a realistic vision to think about, let's say RFID, plastic electronic with graphene or that?

Yes, yes. My feeling is that the major advantage of graphene is simplicity and price. And the sweet spot, if you want, for applications is in the range of 50 to 60 gigahertz.

There is a market for very high frequencies for military applications. Don't forget that the propagation of electromagnetic waves is limited by the atmosphere. For point-to-point secret communications, you can use very high frequencies.

But for long distance, you can't. And the frequencies are controlled. So I won't say that graphene has a future in cell phones because you cannot improve anything, really. Everything is controlled. The frequency, the power you can use because of health effects.

So it is primarily internet, diagnostics, terahertz for imaging. I can't tell you all our plans, but I don't think the search for higher and higher

frequencies is the ultimate goal because companies have to make money. And if the market is small, although needs develop as potentials disappear. What is special about palladium as a contact? Is it just high density of states? Well, you need a metal that is stable.

You don't want it to be oxidizable. You cannot use copper or anything like that. So it has, it's noble, it has a high work function. It sticks well to.

or graphene. I mentioned about charge transfer and so on, but most metals will not stop at charge transfer. We'll actually hybridize with graphene and make it an insulator. So by sort of trial and error, we came up with as the best metal for us.

People have also used titanium. We have used titanium too. There are other possibilities, but I think for us, the optimal metal or surprise. Gold is not good.

It is, these days, extremely expensive. So palladium is the best compromise. Daniel Rehm from university. You mentioned a lot of potential applications in your talk so far. Which one excites you personally the most at the moment? Which one are you most excited about at the moment?

I don't know. I came from the optics field, so maybe interested more in the optics intrinsically, but that's the, I work for IBM and IBM is not into optics. And I'm supported by DARPA, which

interested only high frequencies. So what I'm interested in, what reality is. Working in industry is different than in university. Mark Fox, University of Sheffield.

I'm trying to understand the optical absorption spectrum you showed from Tony Hines. So if I remember the one which was measured in Manchester, it was more or less completely flat across the visible spectrum. I don't know if Andre can confirm that. The one you were showing from Tony Hines clearly has a big absorption in the blue.

So what's the difference? Very large energy rates. The flatness appears in the visible and near IR. The spectrum you showed was clearly in the blue. It's already, the absorption is going up quite a lot. You don't have a single part of the transitions over there.

It has been described by some theorists as a result of excitonic interactions. They may be there. But traditionally, that part of the spectrum has been considered as plasmonic. Yeah, we did measure before Tony Hines.

It's in T0FB or white range. And we see the same, essentially. Some samples show flat and visible frequencies. Other, broader peak, but the peak is there, sometimes narrow. Then it's gives flat, sometimes go a little.

Crabbets and talsis are big, just before this paper by Tony Hines. That peak has been traditionally used as a probe of how doped graphite is. It shifts very strongly. And there are sensors based on that absorption.

And as I mentioned, also the intensity by some rule of conservation is a measure of the density, carrier density, in both graphite and graphene. And photoelectron spectroscopy is used that peak all the time to test graphinization and so on.

In the old days, within surface science, always you see it in eels and other techniques. It's always there from carbon separated from transition metals and so on.