So graphene is a kind of graphite? How, what's the... graphite, graphite looks like that
That's a big slab of graphite. I can by using the scotch tape method peel pieces off
The way I think of it
Is that a single atomic layer of graphite is graphene. I knocked it around a bit this be?
Single molar mono layer by layer graphene where you've got two layers held together by these Van der Waals forces
That's got slightly different electrical properties and then you've got tri layer and when you get up to 50 layers the graphene starts looking black
It's you seem to note that
one model layer of graphene over a wide part of the electromagnetic spectrum
Absorbs about 2% of the light intensity
incident on it
So it's very transparent and that makes it an interesting material as well because you got a transparent conductor
Amazingly the absorbance of graphene is given by the fine-structure constant
Times pi. Now, if any of you up to sixty symbols, you know, I'm fascinated with the fine-structure constant
You could see a sixty symbols video on that Andrei Guymon costume Novoselov isolated
graphene from a lump of graphite and
For some electrical contacts on and so that had some very interesting
Electrical properties they made a little transistor out of it
What we're waiting for now is to make lots of transistors
Out to reverse a much more difficult job that it's very difficult to switch off the channel
because of a bit of quantum mechanics that the
Electrons in graphene of this curious band structure graphene does not have a bandgap
like silicon or germanium or gallium
Arsenide a band gap is a gap in energy
If you want to get the electrons flowing, you've got to put the Fermi energy which tells you the energy of
the energy of the highest energy electron
you
Don't want that in in the middle of in the middle of the gap because then you don't get any free electrons or free holes
So you guys are gonna push the chemical potential down into the valence band or up into the conduction band
they stand bandgap is crucial for being able to turn off your silicon or gallium arsenide field effect transistor because in the
band gap between the conduction band and the valence band
Any electronic state so you could get in there and you can't put them in there by putting in some impurities. Those states are
Evanescent they decay very rapidly in space. So the electrons can't propagate through it when you're in the conduction band
About above the bandgap of the valence band below the bandgap the electrons or the missing electrons call the hole called the holes
Can move very very freely and had pretty good velocities, especially in something like indium phosphide or gallium arsenite that's faster than silicon
But silica has got this fantastic property
You can oxidize its surface and make silicon oxide now in addition to that, of course in the latest bunch of transits
He's not using silicon oxide
As the barrier you want a higher dielectric material
So you got like a hafnium salts and oxides that can that can push up that and you get more more electrons in your channel
For a given gate voltage. What's so exciting about graphene from my kind of looking forwards point of view. Well whereas silicon
or germanium
Or indium phosphide or gallium arsenide has a structure like this
It's a very rigid structure
Basically, the diamond structure for silicon silicon is just like diamond except the silica is replacing the carbon atoms. It's a very rigid structure
And the electrons can move freely in all three direct dimensions
The surface for MOSFETs the interface between the silicon and the oxide or the hafnium oxide
At that surface
you've got to be very careful because once you break these bonds and you're changing the chemistry at the atomic level and
You can have charge on those on those surface States that charge can be very adverse in principle. You can scatter electrons
scattering electrons randomize your momentum
so if you're trying to get them to move down a channel with a high current as quickly as possible to make if you wanna
switch a device class off if they zigzagging and knocking off impurities and so on till
They take longer to go down the device their mobility as it says is reduced
So you want it's nice to have a clean surface now the point about graphene. Is that graphene?
This is a lump of graphite
It looks very black. Of course
But you could take graphite and thin it down by exfoliation
Using the scotch tape method and I I just did exfoliate a little bit of graphene
Earlier this surface underneath the sellotape. It's not beginning to look a bit dirty. And those are layers of
Multi-layer graphite it's not necessarily single layer graphite. They may be five six ten 20 layers, but amongst all that
There will be a single model layer of
Graphene, so having it thin that's really important
Well having it thin is is interesting from a physical point of view because now we've got the electrons moving in
a single mono layer and they move
first of all at a very high speed
About five times faster than they would in gallium arsenide or indium
Phosphide three of three to five times faster anyway at a 10 to the 6 meters per second
Which is actually one 300 through the speed of light curiously enough, which is pretty fast. The problem is
That things like silicon germanium gallium arsenide and so on
These have a energy gap between the conduction band and the valence band
Any electron in that gap which are many because there not many states in the gap those electrons tell you get stuck now with graphene
There's no energy gap. It's a problem for switching off a transistor. You can't you can't switch it off with a conventional potential so
What the Manchester group decided to do?
They decided to go vertical so by combining what you need to do is two switches current off
so this you make a vertical tunnel transistor and a key element of the vertical tunnel transistor is
By getting a sister material to graphi and colleagues agonal boron nitride
It's got exactly the same crystal structure as graphene except that alternate atoms
if all of these are carbon atoms in Boran light or this one it would be
Boron, this will be nitrogen. This will be boron
this will be the nitrogen and so on going around here and that creates a very very strong barrier very effective and
powerful barrier a barrier in fact at six electoral votes
so six times the value of the electron gap in the energy gap in in gallium arsenide and so
in this device the
Graphene layer has put on top of it boron nitride and then on top of that we have another
graphene layer and we mount that on silicon silicon oxide and use the silicon as a gate electrode and you use that to
Control the tunnel current through the layer and using that device we were able to make a transistor in which you could switch off
So we were very excited about that
That work was published in in science and we were even more surprised and pleased that six weeks off
I think six weeks after our paper came out the Samsung group came up with a very similar concept
And they managed to put about a hundred of these transistors on a chip
so I think at that point everybody thought wow, these vertical devices are gonna take off but
Things do seem to have that doesn't seem to have happened. I think the technology is very
Difficult we to remember that it took
You know many beers decades for silicon to go from just an interesting lab based material
Into a field effect transistor and then into these incredible integrated circuits with all these transistors on a single chip
It takes a lot of time and it takes understanding the physics and understanding the material science
but in parallel with with all this work on graphene, I want to emphasize the graphene is one of a large family of
similar
two-dimensional materials in which you clinics which can be
Exfoliated and there are about I believe it around the thousand of these
Materials and one material that we are working on here in Nottingham and Manchester interesting as well is indium selenide
Molybdenum disulphide is none materials
Remember your Mali slip used to probably put on the gears of your bicycle on the gear wheels
That's a similar material because the molybdenum disulphide sheets can slide over each other with almost no friction
So this isn't that very hot material as well
Now those materials have got an energy gap and an energy gap that depends on
the number of atoms you make the crystal out of the great thing about
These layered compounds. Is that when you break them apart you
Break the band of ours bonds
these these bonds
don't get charged don't trap and don't become defects it looks as if
You know
the electron just doesn't get scattered off that so that's that's an interesting aspect of these devices that people are playing around with and
indium selenide
This is again a
Manchester knot in collaboration
we produced a feel effect transistor that has some really very good very very nice properties and that
Was published a few years ago and people that that indium selenide 'sister coppers are making quite a big impact at the moment
people are working away on it because it's like gap semiconductor and
you can combine it you can switch it off with using a field effect and you can also put make or a nitride barriers and
Not only that but then take one band of ours crystal
And then it take another exfoliation one on top of it and make a multiple sandwich structure with lots of layers and bread and butter
and so on and
They're all sleeping between properties that these materials could have good ab, so could be the EM facility, but not no no, no
No silicon has got years to go heist
I quite like when I my PhD I was spent ages trying to put good electrical contacts on silicon
And eventually the French colleague put very good contacts on and we were a very nice paper on the electronic properties in bulk silicon
But that's forgotten in the history of silicon, but back in
1971-72
We looked at bonito photon resonance. Oh, I love silicon. I've got very got me interested in the field really and and curiously enough
I yes. I've almost forgotten this GC and Plessy were making very primitive
Feel effect transistors they were blimburn and reliable and we got our hands on some of these and we did some quite interesting
quantum experiments on the very first UK feel about transistors we couldn't get them from Germany or America, but
DC and Plessy were making material that materials. So no silicon is a great material. I'm not going to do it done
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