What Physicists Do – April 16, 2018 – AKM Newaz
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[ Music ]>>Okay. It is 4 o’clock,
so we shall get started. We are very pleased to
welcome our speaker of the day, who is a fellow CSU professor, up to visit us from
San Francisco. So it’s always great
to have someone from inside the CSU system. While there may be
many campuses, it’s one of those
things that’s great to have these connectivities
between various locations. The other connection we
have is that the background of Professor Newaz’s work
overlaps with a lot of the work that is conducted here
at Sonoma State and some of the experience we
have in the department, so I think there’s some great
overlap between that and some of the projects that
we can offer. So with that said, let’s
give a very warm welcome to Professor Newaz. [ Applause ]>>Well, thanks for inviting me. I think this is a
really beautiful campus. It’s very nice to
see a dream campus. Yeah.>>Can I ask you to
speak up a little bit?>>Oh, I see, yeah. I’ll try. Well, so I’ll
try to be, in this talk, I’ll try to be kind of informal. So I’ll try to convey
the excitement of the research [inaudible]
in this direction. So in this talk what I’ll
talk about this new class of materials called
two-dimensional atomic layered crystal materials. All right? So this materials is
not like we are using that for a long time
without knowing it. Like, for example, in the
graphite in your pencil, you know, how do you
write on a paper, because you have all
those, in graphite, you have all those layer, one
atom thick layer materials. They are just attached
to the next one just by van der Waals force,
which is very weak. And then when you write, those,
those layers get glided through, on top of the page, so that’s
why you can write with graphite. All right? And then, and interestingly,
these materials are, they do one of two things, but most people can make
very, very large size. And then in this talk, I’ll
show you that, you know, that what gives that
excitement there. So, for example, this
is, as I told you, the carbon atom forming
in your graphite is like you have a benzene
ring attached to each other making
a large shape. All right? And then, and you can see, this is a scanning
electron microscope image of one atomic layered material. So, and you can see, these
are surviving in air, and so that tells you
the mechanical strength of the material. All right? So not only that,
you also recently, we know that there
are other materials that also you can
have one atom thick and with different optical,
different physical properties. So some of them can be metals, some of them can
be semi-conductors. So I hope I’ll be able
to convey the excitement at the end of the talk. So let’s see what is the
dimension I’m talking about. All right? So as you know, the
earth size is ten to the power 16 nanometer. Right? And then you go by 10 to
the power 9, 10 to the power 8, you get to the golf ball, not
golf, I’m sorry, baseball. Then if you go more,
10,000 nanometer of 10 micron is your hair,
the width of your hair. And then if you go more,
it’s a virus, right, 100 nanometer you get to the
scale, and the materials we talk about less than a
nanometer thick. For graphene, one atom
is only .35 angstrom for monolayer MoS2 is .7
nanometer or 7 angstrom. So brief history, when we
talk about that graphite, we know that almost 4,000 years
ago people were using graphite to decorate pottery. All right? And that’s the three-dimensional
carbon-based materials. And then it took
almost 6,000 years to get zero-dimensional
materials that was invented at Rice University by
the smallest group, they’re called carbon-60
or fullerene material where you have carbon-60 atoms
in a hexagon and they wind up making a big sphere. And then, then we
got, around 1991, we got one-dimensional
carbon material from Japan by Iijima group. And then we take a long
time, people realize, because they got 0-dimensional,
1-dimensional, 3-dimensional, so the missing is 2-dimensional. Right? The reason was that
because there was lots of theory papers
there that were saying because of the surface energy of
the 2-dimensional carbon sheet and the substrate
would make it crumble. You will never be able
to get a monolayer sheet of carbon-based material. So that’s why lots of people
did not try it because, theoretically, it
was not possible. But there was a person I’ll
talk about more that, you know, didn’t look much on the theory
paper, just gave it a try. All right? So in 2004 that paper
came out from Manchester, is a 2-dimensional, 2-dimensional carbon-based
atoms called graphene. And then in 2010 people
realized it’s not graphene because it’s open, it’s give you
techniques to really study lots of other materials,
lots of layers. So one of them is molysulfide. And there is another one
called boron nitride. We are using them without
knowing it for a long time in the cosmetic industry,
in lipstick, literally. And then you have pyroelectric
materials in your heat sink or when you cool down something
by using [inaudible] there’s, you can have these
other layered materials. So, for example, molysulfide,
if you go to, I think, AutoZone, you can buy this multiple
[inaudible] actually nothing but this layered
material because, you see, you can have a low feature
because it’s a layered material. So you can very easily
take from layer by layer, so the friction is very low. All right? So the Nobel Prize came in
1996 for this carbon nanotube. And then came, sorry, for,
that is came for buckyball or fullerene for
the 0-dimensional, and then came the
2010 for graphene. So, and then we don’t know in the future what
is going to happen. All right? So I want to talk about, this
is the person, Andre Geim, who actually made
up this script. This is because of Andre’s
out-of-box thinking. All right? So he’s a Russian-born
physicist, got his PhD from Moscow
State or Moscow Institute of Physics and Technology. And then he did a couple
of post docs in UK. And because he did
really nice work there, he got his first faculty
position in Netherlands. All right? So he got graphene and or some of the experiment I’ll talk
about, that because of this, all our kind of crazy ideas
are out-of-box thinking. All right? So one of the experiments, he literally said
Friday night experiment for testing his crazy ideas. So you can, so this I
recommend you read this paper. This is a very easy reading. This is not about physics. It’s more about the
story behind the science. Okay? So one of the crazy
ideas is levitating frog. So people knew that for a long
time, when you have water going through a copper pipe, you
have scale build-up over time. All right? And then when you put the
magnet next to the pipe, you don’t see the scale. All those formation of
those scales are there, it’s like you can
see no scaling. So, and there were, they
call it magnetic water. So people, there was
no clear explanation. So Andre Geim was a
professor at, or the faculty at Radboud University, but
that university is famous for their high-powered magnet. That’s a room temperature
magnet you can go to 20 tesla. And so he was thinking,
well, you know. So one Friday when he did
this crazy experiment, he dumped a bucket of water into the 20 tesla
at room temperature. And you can see that the water
droplet hanging in the water, I mean, in the magnetic field. All right? So the physics behind
is not very complicated. It comes from the diamagnetic
property of the atom. When you have a magnetic field,
your reactor motion changes. That creates a magnetic
moment opposite to the magnetic field
you have applied. So, so he explained that
if there was Michael Berry. They both, was the theory guy and Geim was the
experimental guy. And because of that,
they got the Noble Prize. So Geim is the only person who got both Nobel
and Ig Nobel Prize. You know, Ig Nobel Prize is the
prize that is given by Harvard and MIT that makes you laugh
at the beginning some more, but that also make you think. All right? So, but the media attention
he got when he put the frog in the magnetic field,
the frog also levitating because of the diamagnetic
property of the atom. So another case experiment
he did is the gecko test. You know that gecko is
kind of climb the walls, so if you put the
optical image of that, you can see all these
things there. All right? So what he did is he
microfabricated these kind of structures, and he was able to put a toy hanging
from the wall. All right? So that’s also out of
thinking is a great, comes from a crazy experiment. And, you know, opened
a new direction in thinking and doing research. Okay? So now we’ll talk about
he how he comes to the graphene. So he had this research
on, he was interested to metallic electronics
because he know that all these electronics
in your chip is made of semiconductor because you
can make transistor using semiconductors, not metals. The reason is that when you
apply the electric field, you cannot change the
property of the metal because your electric field will
be — because of the scheme, that will not be able to get further penetrated
into the metal. All right? So, but his idea was if you
have a very, very thin metal, maybe you will be
able to get it. All right? So what he did is, using high
density graphite, and he give it to the student, okay,
this is the graphite. And we know it’s a
layered material. So polish it as thin
as possible. So the student spent
two months polishing it and got one flake off of the
graphite with the thinnest one. Under a microscope it looks
like ten micrometer thickness. All right? So it was like, literally,
you know, finding flakes of, you are polishing a mountain
to get a speck of sand. So, so there was a, so there
was another person sitting next to it. They were listening
to their discussion, and he was a scanning
transmission electron microscope specialist. So if you notice, scanning
transmission microscope, what they do, they use graphite for standard, as
a standard sample. And they use scotch tape
to remove the, the clean — so you have a graphite. You start with a graphite, and you have the
sample sit in your box. Over time it will have
accumulated lots of junk. So what they do, they
put a Scotch tape and they remove the Scotch tape
to expose the fresh surface. And that you can have
extremely clean surface. But, so, so that STM person,
and they’re using it from 1960s. So we’re saying, you know, the Scotch tape has
lots of flakes in it. And they are very, very thin. Okay? And then he found when
he look under microscope, it’s really, really, you can see
very transparent thin nanometer or even lower thicker,
thinner flakes. So there is another group I
should mention at the same time. This is Philip King’s group
at Columbia University. They went more vigorous
scientific way. What they put, they
put a graphite on top of a cantilever tip,
made a micron size. They call it nanopencil. And they put it on the
soft strip and slide it. But even using that, they could
not get less than 10 nanometer. All right? So, well, you know,
so in [inaudible] so, but using Scotch tape, I can
show you some of the result that are, you can go really
one, one atom thick materials. Okay? So how you started? So you started with
the Scotch tape, you put in the graphite
flakes on the Scotch tape, and you do like peeling
off layer by layer because it’s nothing but
all these layers attached very weakly. And then if you do sometimes, you’ll see this ugly
Scotch tape, but it contains all
these flakes in it. When you do that, you put that
Scotch tape on a substrate and you very slightly
you scratch it. And then you remove
that Scotch tape. And what will happen, if
you look under a microscope, you’ll see really very
transparent layer, which is actually made
of one carbon atom thick. All right? So this is the first
graphene electronics, electron devices using the
clues by the Geim’s group made in Manchester University. And I think that currently this
paper has over 40,000 citations. So this is the first
paper that came out. All right? That opened the field
studying, mainly they showed how to isolate the materials,
how to make the devices, how you can measure directly
all these physical properties of these materials. So now you can ask me, well,
you know, those materials in the Scotch tape is,
this scaled is five micron. What is five micron? Less than your hair
thickness, right? That is, that is not feasible
for application purpose. So what people have
developed the recipe, now you can make
really larger scale. And they call it chemical
vapor deposition system. So what they do,
they have a furnace, and they have a vacuum system. And they put some metal
like, for example, you can put a copper foil. In my lab you would
put copper foil. And then we inject methane
gas and the hydrogen gas, and we ramp up the thing
to 2000 degrees centigrade. The methane breaks up, and
methane has carbon in it. Those carbon get into the
copper, and then when you cool down very quickly, those
carbon get to the surface and make those bonded and
make the graphene large sheet, as large as your copper. All right? So this is, for example,
this is made by a Korean group with Samsung. This is [inaudible] and you can
[inaudible] I think it is 100 meter long and 2 meter wide. So people can really make,
have, scale up this technology to make one [inaudible]
materials for technological application. All right? So, so here are the
materials and application. You can think about this large
area, larger scale CVD graphene, and you have a graphene
nanocomposite some of the, like, flakes, they will transfer
and a screen for, like, your smartphone screen or you
can have those conductive ink because graphene is metal. So you can make a
conductive ink. Also, you can have
a solar cells, because graphene you
can put a graphene, and a hydrogen atom cannot
pass the graphene thin. Or you can also have it as
a heat dissipation system. You can make composite
for aerospace industries. And there are many other
application you can think about. So, well, now you can ask me,
well what can we buy currently. Right? I believe you can
by several things. You can buy this graphene
record from head.com. What they do, they made this
metal structure made by graphene because it’s very
light and robust. All right? And then you can buy
this 3D printing system where they make those cartridge
made by graphene made mixing up with other polymer. You can buy battery. That’s a recent, that can be
recharged up to 15 minutes and cost you around $100. All right? So in the future is coming
from you can have a flexible if you have a smartphone, a
flexible wearable technologies, or you can maybe have
some medical application. People are, research groups
are working on to use graphene as a DNA sequencing
technologies. All right? So any questions? Feel free to ask. Yeah.>>Yes. If I heard you
correctly, the [inaudible].>>No, 1,000.>>One thousand?>>Yeah.>>Why is the temperature
so high? Is it because the
methane had to, you know, this kind of temperature
to me –>>I think it probably needs
that high temperature to break up the methane to
carbon and hydrogen.>>Is the copper crucial in the?>>The copper works
as a catalyst. Copper is very crucial. So that’s a very good question. Because if you put a copper,
you can make a one layer. But you can also put nickel. That would grow multi-layer.>>Does [inaudible]
copper matter?>>No, those copper
[inaudible] polycrystalline. But the purity matters.>>Okay.>>So the copper film we
all use is 99.99 percent. So, but other than
that, but it does, it is not very important
for nickel, somehow. So, all right? Yeah, if you do nickel you
can make many, many layers. You can make many layers. So that’s the way people
control the thickness. Some copper is [inaudible]. Well, people also figured out
to make up to five layers, but not more than that. Yeah.>>I have a couple
of naive questions, but I think it will
help fill this in. Because it won’t be just me
that finds this interesting. Graphite, you know, is
large, 3-dimensional, structures usually, or
it’s in our pencils. I’ve heard of the tape as
the part of the discovery, the Scotch tape is
part of the discovery. Is it something that that
amorphous form of it or?>>No. You need a —
they call it HOPG, highly organic polycrystalline
graphene.>>Okay. So there’s not, like,
even when you do small amounts, what would be the
layer of carbon atoms? How many layers thick
would pencil markings? Wouldn’t it be large, right? I mean, that. I was trying to look at
versus your thin, thin layers.>>Right. Now, the
pencil graphite is high density graphite. So for that one, if
you do use Scotch tape, you would not get very
— even if it is there, it’s probably less than
a micron, nanometer.>>You could get small features
of it about a micron thick, and that copper pipe
run across, right?>>Even the smaller, it’s people
have been trying pencil graphite because it’s high density. But using HOPG, you can
easily go to 200-micron. It might sometimes you
can get 200-micron, which is pretty large
for research purpose.>>Right. And I was
just wondering about the purity of it. We’re looking for, you
know, local deformations or inclusions or,
you know, I suppose, whatever the appropriate
word is for the, on the atomic scale
individual –>>I’ll talk about
some of those. I’ll show you that, we call
it the substrate matter.>>Thank you.>>All right. Yeah, the quality of your
electromagnetic [inaudible] depends on your graphene
quality. So if you make it using the
HOPG you get [inaudible] of magnitude, better
quality than you do by [inaudible] lots of grain. It’s not a large sheet. It’s a multiple sheet
that’s [inaudible]. Yeah.>>Can you also say a few more
words about one of one, like, the transistor like the circuit. People [inaudible] the graphene and then they basically
made the [inaudible]. But was that graphene
also grown on copper or some other substrate?>>So, well, even if
you do a master scale, you have to do it on copper. And then copper you can
remove it very easily. There are techniques available.>>But once you put
the electrodes on top of the graphene, it didn’t
show that the matter is going to penetrate into the copper.>>No. No.>>No?>>Because graphene you
cannot pass a hydrogen atom. It is a highly [inaudible]. Okay? All right. Feel free to ask, yeah. Well, so, we talked about the
electron [inaudible] all right, so why is so, so
beautiful or so exciting? So I’ll show you, because some of you is just the
electronic band. You need some idea about
the electron bands factor because that will determine
the electronic properties. So let’s see. So this is just a
very close idea of how you have all these
bands forming in the metal. So this is your one atom. And you can have it, from
the [inaudible] you can, for example, you have a
three-layer in energy scale. And you put many of
them, and then you form, because of the interaction
with them, they form a band. So you have some, allow this
state and then you have some for [inaudible] state. All right? And then you can, you can
put electron into the system. What will happen is that you
understand the lower [inaudible] start filling up,
filling up, filling up. All right? Is that’s called
your fermi energies, and this is your allowed band. Okay? So you need to understand
because we are looking at the properties
of the materials which is determine all
these bands factor, quantum properties
of these materials. Okay? Now, so what is graphene? Again, if you look from the top, you have a 6-carbon
atom next to each other. Every copper atom has three
atoms next to each other. All right? So if you can have unique
cell because you will need to do the calculation
or calculation for any materials you
need to have a unique cell because that will make
your life very easy. All right? So one of the unique cells
you can have that way, like, or you can also have, so this
one have it, you can see, is 161616, sorry, 1/3, 1/3, 1/3,
and you have the six of them so there are actually two
atoms in a single unit. So you can draw, you don’t like
this, you can draw it this way, and you still have two
atoms in a single crystal. And then you can
do the calculation, and it’s call tight
binding calculation. And then you can come up
with this band structures. As I showed you before, but this
is a [inaudible] system so we like to present the band
structures in momentum space that will make your life easier. So here you are seeing the band
structures of these carbon atoms of graphene, and you have all,
so these are all allowed state and that these are empty
state is not allowed state. All right? And then if you blow up,
so this is called k-point, this is called k-prime. If you blow it up, it
looks like two cones. All right? And what happens is that when
you have some [inaudible] particle in the materials,
they have, they’re called quasiparticle,
and it moves with some effective mass. And effective mass depends on
the double derivative of this, of this energy versus
momentum, momentum space. So what would be the
effective mass of resistance? If you’re doing the double
derivative, along the line. You take the first
derivative, it will be constant, you take the secondary
[inaudible]. So that means, that means, that
means if you plug the energy of this equation, it looks like
some velocity times momentum. That means the particle versus
the particle in graphene acts as if like it does
not have any mass. But you have to remember,
that means it acts like a, just like a photon, or,
but photon has a speed, but [inaudible] t times 10
to the power [variable] a. But in this particle,
it’s [inaudible], so you do not have the speed-up
photon, but it would be closer to the speed of photon. So if you do, this, indeed, is 300 times the smaller
than the [inaudible]. But the physical property
is very similar to the line. Because if you do the
photon energy, the creation, the energy creation is
c times p, if you do, if you remember the explanation. All right? So that’s why people say, it’s actually much less
[inaudible] the electrons are feeling like they don’t
have mass [inaudible], all right, with this speed. All right. So now we talk about
the electronic property. So you have, you have graphene. The way you make, the way you
make, so, so this is a cartoon where you have a graphene on
top, you connected them by gold or electron or something, and
then you have some insulator. So you have a graphene,
like this is my graphene, I have some insulator
[inaudible], and I put another
metal at the bottom. So what I make, I
make a capacitor. And by making a capacitor, what
happens is that, as you know, in a capacitor you can bury the
charge by applying the voltage. And your charge is
nothing but n times e. That means you can control the
charge on a number of electrons in the system just by
applying these [inaudible]. All right? So that’s the way you, so
that’s the way you can, so look at this graph. So you have to understand
this now. So you did, here I’m
plotting the gate voltage, this gate voltage means,
like, same as your, this is nothing but, like,
your transistor like property of your, of the sample
or your water faucet. What do you do? You know you switch them out,
you change the flow rate. Same as here, you
change this gate voltage, you can change them, you
can change the properties of the materials, so
this is your conductor. You know the conductor, right? Conductance is how the
metal electrons can travel in the system. So the higher the
conductance, the better it is. So, and you can see, when,
so this is, you remember from the previous slide,
you have the cone shape. And you start when you
find the energy is that, it sits in the middle. There is no stakes available. So your conductors
would be lost. And then you change the
bias into negative site, you make it lower the fermi
energy to the full site. So you are creating
the pole in the system. And that’s why you have
it higher conductance. And then also in the other site,
by changing the gate voltage in the opposite direction, you
move the fermi energy outward. So you get a symmetric
conductors plot like that. Right? So if you do the mobility of
this, mobility is a number which people use to
calculate the sample. You have a higher mobility,
you have a better sample. So you can see, I use the
mobility of the graphene, one graphene is very, very high. The materials in your
computer chip is only 2,000. All right? The best one is available
[inaudible] which is 77,000. All right. So now I’ll talk about
two experiments that, that tells you the
quality of the material that is really, really good. What is, one of the
experiments people study, when you have a very
high quality electron, electron moves very
smoothly without scattering, what you can see,
you can see the front and behavior of electrons. So one of the properties is
that, is the Hall measurement. You remember the
Hall measurement? In a Hall measurement you
have a slab of materials, you inject electron, and you
apply the magnetic field, and that will bend the motion
of the charge direction. And that will give you
generate a voltage. And you needed the
voltage, and that will, from the voltage you can
calculate the area concentration of the quality and how many
carriers are available there. Okay? So you will end up
for the classical case, means there is no quantum
mechanical behavior, you will see this
kind of behavior. All right? For, now, on the case of, you have a very high quality
materials, your electron, you apply magnetic
field, what will happen? The magnetic field will, the
electron will start to rotate around the magnetic field. So you create some
of those levels. That means if you have a density
of space, this is your energy, available states, if you
apply magnetic field, you will quantize those states. So like that, you can
see your [inaudible] and as you apply the magnetic
field, all those tend to move up because you will increase
that gap, which depends linearly on the magnetic field. All right? So, and if you measure
the resistance, it looks like you’ll
see the steps. That’s called quantum
Hall effect. That is, but you
remember, in the classical, what you are supposed to get? Just a linear with no step. Right? For graphene, you can
see, you’re exactly those steps. Right? So that’s telling you that really you can see the
quantum mechanical behavior in graphene giving
you that it’s, indeed, electrons have very
high mobility and moving through the those
masses scattered. All right? By the way, when the first
quantum Hall effect was observed around ’80s, I think
that the Nobel Prize for quantum Hall
Effect around ’86. All right? So, not for graphene. That was done by
using other materials. Now I’ll talk about,
as you say, you know, people first started graphene
on the SiO2, silicon dioxide. Right? You have, you’re
putting one atomic materials, you need to have a very
high quality surface. But if you look at
SiO2, you look, it looks like it’s
a ragged surface. And the variations of
the other .4 nanometers. Right? So what you need? You need to, so if you put a
graphene, what will happen? Graphene would be also
sitting on a ragged surface, and the electron will
feel the, will be, the electron will feel lots of resistance coming
from the materials. All right? So what we found, well,
you have a graphene here, you have a brother
here, so boron nitride. It’s exactly the same
structures, the lattice, this is your lattice
[inaudible] that is the number of the separation between
the, it’s very close to this boron nitride. And this is also
layered material. So you can have atom
flat substrate. So that is what they did. So what they did is, that, so this is the next
advancement they did. So they have a graphene here, and they prepared a microscope
slide with boron nitride. And they, what they did,
so they put, and they, under the microscope,
think about this. All these materials
is a few micron. They aligned their samples under
a microscope and put graphene on top of boron nitride. All right? And this is, this is a step,
this is a quite challenging job because you have to, you have to do everything
perfectly to do that. And then you can see, this
is your boron nitride, and on top of it
you have graphene. And if you have that,
and graphene. And if you have that,
and what you are looking at here is the surface
roughness. So this is your surface
roughness in silicon dioxide, and where you put it on
top of boron nitride, it changes by a magnitude. Okay? So, because now you
have a better quality graphene so you can see even more quantum
mechanical behavior properties, they call it fractional
quantum Hall effect. So, which was not
possible to see in, when you have a graphene
on silicon dioxide. So you can see that you have
all these fractional number. All right? So I just want to do, I hope
that answered your question, that in the surface roughness is
very important for this sample. Okay. So now I’ll talk
about the other things. So now we know, I know how to make one materials
on top of other. Right? So what happens
is that, what happens if I do the aligning of two
graphenes on top of each other? All right? You can, you can change
the physical property of the band structures of these
materials, just making, so we, with the 48, with the
preferred orientation. So, for example, here is the
image you can see you have one graphene and the other
graphene on top is moving. You see that? Those structures are appearing. So you have a, you have a carbon
atom with hexagonal symmetry, and then you have,
depending on the angle, you create another
structures on top. Right? That’s called
Moire pattern. All right? So, so this is, if
you, in a slower slide, so you have this graphene,
if you put a certain angle, you can see that the angle,
you can have graphene here, you can also have this
hexagonal pattern. All right? And, and so this is, so you can,
you are painting a superlattice with another, you
have a hexagonal, and then putting another
hexagonal with larger crystal, larger lattice constant. So what will happen is
that, indeed, you remember, I showed you at the
beginning that band structures for field graphene
is a conic shape. Now you another here and another
hexagonal, what happens is that you create more conic shape
but at different energy level. Right? And then you can see, before I had only
one, one structure. Now I create other
structures in conductance as I change the carrier
density in the system. Yes.>>So experimentally speaking, how can you align the
two layers so precisely?>>That’s the magic
of bare hand. You can do that. Because that’s a
very good point. Because I figured out that
you have this breaking point of the graphene. And you have another breaking
point of the graphene. They follow a certain
angle of distribution. So either they can
have [inaudible] or they can have armchair shape. So you can, if you do
those angular orientation of those edges, you
can come to that angle. All right? Very nice question. So, well, so you can
see that, indeed, you can [inaudible]
not only the, you can change the band
structures of the materials by angular reorientation
of the system. Right? So [inaudible] this just
came out last month. There is, there is,
at one point, one they call it magic angle. If you have a magic angle,
what happens is that this, the band structures get
flattened like there. You have a band structure
flattened at 0, what happens is that you cannot change this
new type of superconductivity. So that is, you can see, at 1.5
go to low temperatures, boom, you go to superconductivity. And this is your current
data as a function of both, as you can see all these
superconducting [inaudible]. All right? Well, so now I’m talking
about, a little bit about how, what kind of work we do
in the lab is, you know, one of the work we did
is, at the beginning, what is really causing the
scattering of these materials? All right. So you have this flat land. And in the flat land you
will log all these charts in the system. And then because, if you have
the charts in the system, they will appear to what? [Inaudible] scatter, or you can
have difference in the system. So you can have a
vacancy in the graphene. There it is called short range
scattering, means it does not, the effect does not go very far. All right? Another one can be lethal if
you have a ripple in the system, then also you introduce
scattering in your system. So we, what we did is that we
did some control experiment. We created some standard
graphing monolayer material suspended graphene samples
in dielectric liquid. Because every liquid
has dialectic property, so you can have a sample in
a dielectric environment, you can [inaudible] that liquid,
put a new dielectric materials. That means you can tune that
dielectric constant by using, so as you all remember,
this is Coulomb’s law. And Coulomb’s law has
this dielectric constant. So our goal was to vary this
dielectric constant and see, this is your scattering,
scattering strength. If I increase it very, very
large, what will happen? This scattering information
will go down, and then I have my electron
[inaudible] smaller, scattering. So that is what we did. We made a graphene device
that suspended in liquid, and we varied a, we
flushed the liquid and [inaudible] the dielectric
properties, put a new liquid, and [inaudible] it again, and
then what we did, we showed you, indeed, when you have a
higher dielectric constant, your graphene gets better. So that’s tell you
that you need, it needs the long range
Coulomb scattering that is determining the
dielectrical properties. So the fab is, you
start with the graphene on a silicon dioxide,
then you, we have a way to remove the substrate
underneath it. And then you put, started this
device, put all the electrodes, and put the whole thing
in a magnetic field and measure the dialectic
properties. So this is a scanning
electron microscope of a, this is a Hall-bar shape
graphene device in vacuum. All right? So, so very quickly,
again, we have this. So we varied the, we have
varied the liquid form, because, we varied the [inaudible]
in anisole because all these three
liquids are nonpolar solvent. What does it mean? We don’t have any
stray ion in it. It’s a very clear solid. But it has a different
dielectric environment. All right? And then using that, and you
can see this is the beginning of this, and then when
you put it in hexane, you see it improve a lot. So this is a conductance,
the conductance is very high if my sample is getting better. But it is one sample because
all I’m doing is changing the environment next to it. And then if I do more in total, a dialectic constant 2.3
is getting even better. And then if I do anisole,
it is 4.3, it’s even better. So that was the experiment
we did to resolve that controversy we
had at that time to, what is the scattering
[inaudible]. So what is the application
of that? Well, you can think
about, you can think about, so one of the applications
we did, we extended the experiment
farther because graphene is the most
sensitive charged material. So what we did, we made
a graphene transistor, we put a microfluidic
channel on top, and we float the liquid on top. And it started, can you
measure the flow velocity? Indeed, you can. We were able to sensitivity of our devices was 17
nanometer per minute. And then we did sensitivity
not only the flow velocity, you can also meter the
strength in the material. All right? So, so it has a dual, dual sensing properties
of this center. All right? Now, so we’re done now. Graphene is metal. We all are beyond that, right? It’s a metal because graphene,
it’s not a semiconductor. All right? So now I’ll talk about
what graphene has opened, what graphene field has
opened up this techniques to study other materials. So one of the materials
is called molysulfides, which is a [inaudible]. So what does that mean by this? I mean that, so semiconductor
can have two types of bandgaps. One is the direct bandgap,
one is the indirect bandgap. So the indirect bandgap, again,
this is your conduction band, and this is your [inaudible]. And the bulk of this conduction
band sits exactly on top of the, on top of the balance band. So what happens is that
if you shine the light, light has a very tiny momentum. It will generate electron, photoelectron here,
put a hole there. It’s a very efficient process. But on the other hand, if
you have an indirect bandgap, there seems to have
two different position. So you need to, because your
momentum needs to be conserved, so, like these two [inaudible]
to transfer this process to make this, when you heat
light, when you heat the sample with light, you’ve created. So to get an electron here, a
hole here, you need to have a, it’s a three-particle system. So it’s the very, that is why
you cannot buy a laser pointer that’s made with silicon because
silicon isn’t indirect bandgap. On the other hand, this is an
aluminum gallium arsenide, this, the glazing medium
here, because it’s a [inaudible] semiconductor. So, so this is molysulfide, where in molysulfide you have
a [inaudible] in the middle, and you have a sulfur
on the outside. So they call it tribunal cell, and it has some crystal
symmetry. Because of the crystal
symmetry, what happens is that this is the bandgap looks
like now for a monolayer. And you can see, indeed, this is
a direct bandgap semiconductors on the monolayer level
with this at the k-point. All right? And if you look at more,
because it’s also [inaudible] so you will have a six disband of six different
point in the hexagon. And it also has a very
strong speed of interaction. It means that your balance
bands speed up here. This is all at 116 minutes
so you can observe it, even at room temperature. Okay? Not only that, it also has
these quantum properties called values k and minus k. And note
that, at this minus k point, the blue is sitting on top. On this one blue is
sitting at the bottom. So the electron at this value
cannot just travel to here because it’s sitting
at the bottom. So there is, so you will have
a way to create carriers, electrons, with certain
this quantum behavior. All you need to do, put
some angular momentums so that your electron can
transfer to here but it does, so when you put,
the way we do is, you can polarize
the light this way, and the polarization has
angular momentum plus one, this way angular
momentum minus one. And you can, you can
selectively generate these polarized carrier. They call it [inaudible]
equalization. So that’s a differential
for a new type of electrons called [inaudible]. All right? So again, it’s, yeah,
it’s the [inaudible] at the monolayer level, and
then you have varying degrees of freedom. So you can see here that
when you have this kind of circular motion, you are
talking only the k-point. When it’s going the
other direction, you are talking only
the k-prime. So this is your optics. So this is your [inaudible]. There’s the monolayer, but
it’s hard to see from there. So this is your band gap. You hit it with light, you create an electron
hole, and what happens? You create an electron hole, the
electron in here, pole is here, there’s nothing here,
there’s nothing here. So what will happen? You have a very strong
polar interaction. Because of that,
what do you create? A hydrogen light
[inaudible] called exciton. All right? And that exciton has binding
energies, and this exciton, and then those excitons,
those excitons collapse and a new light will come out. All right? So this is the [inaudible]
same as that these are, I’m heating the sample
with green light and measuring what is
the light coming out. And you can see this is a
room temperature measurement. I can see that epic when the
transition happened from here. I can also see the [inaudible] when the transition happened
from the [inaudible]. All right? So not only that, because it’s a
very strong, strong interaction, I can also create three
body system called trions that can have a two electron one
hole, one electron two holes. The electrical properties
is a very little different than graphene because
you have this band gap when you finally have the
[inaudible] in the middle, there is no carbon
going through the system so your conductance is 0. And when you form the energy
it touches this bottom, your current starts to rise. So this is very similar to your
transistor in your silicon. Yeah.>>I was just wondering, on the
previous slide, I was thinking about new problems with,
you know, particle in a box, you know, quantum
uncertainly for the lifetimes of these excited states
in the hydrogen atom. What are the lifetimes? I mean, this looks like
another problem that could –>>No, no, the exciton has
[inaudible] to nanosecond. It depends on what is
the bond [inaudible].>>You can generate the
same thing maybe by –>>What is the same thing?>>I mean, a lifetime from
delta e delta t and h-bar.>>Yeah, it’s a, well,
I think it, yeah, it’s a very similar behavior. You can do –>>It’s interesting
to do on a –>>Maybe [laughter].>>Okay.>>All right? So the electrical properties
here, you have two connected by two gold electrodes, and
this is a very transistor-like behavior, very similar
to your silicon. And then as you are
already discuss, the graphene looks
like very different. So what is the problem
of the graphene? Because graphene does
not work as a switch. Here you can see, right? You can completely turn it up. Here you cannot turn it up. You change the voltage, so you
change the faucet this way, water flows, go other
way watering flows. So you cannot make
transistor out of graphene. But you can make
transistor out of MoS2. All right? Yeah.>>Isn’t it true your mobility
is only one [inaudible].>>[Inaudible] has to go
to 100 now, 100 centimeter. I think recently they have
even seen quantum [inaudible]. So they are improving,
but it’s a slow. It’s only 80 years now. So what we are seeing now, yeah. One of the reasons is that it
has lots of defects in it, too. It’s very hard to make high
quality compared to graphene. Well, again, people have
figured out to make a lot of this [inaudible] too
because if you want to make it as a device, you need to make it
as a larger scale [inaudible]. So very similar techniques,
they use one is precursor, they use [inaudible] another
one they use transition metal, and put any kind of
substance in with it to around 800 degrees
centigrade, and you will get two
flakes like that. So this is one of the
samples in our lab, but you can see,
this is 50 microns. So we don’t, we don’t work
on [inaudible] but people like already demonstrated all that [inaudible] growth
of these materials. All right? So very quickly, that one of the
study that we do in the lab is, we study the photocurrent. You hit it with different color
of photon and see what kind of light or what kind
of current coming out. And from there you can tell that
electron, it bend structure, so the quasi-particle
behavior of this. So we have a monochromator. We put through a monochromator that can selectively
select photon color from red to all the way to green or UV, and then we [inaudible] what
is the proper, so this is one of the photocurrent as
a function of the color. And you can see this is your,
this is EV, so this is UV to all the way to red color. And you can see all these peaks
coming out as very similar to when you have this transition
happening you see a peak there. You can see when, when
do you see big peaks when this transition happened
and you can also [inaudible]. And this has happened when you
have this transition directly from here to there
if you start that, you can directly measure the
binding energies of this exiton. You know the binding
energy of hydrogen. What is that? This is around 501. All right? Which is very large,
so you can see all the different [inaudible]. All right? So, so very quickly, so now I
will very quickly show you that, because, indeed, this
is one thick material and you can control the,
this luminescence property of this material just by generating different
carrier number in the system. All right? So you can see, this
is a device. All I’m shining this
with light, green light, and measuring what kind
of light coming out. All [inaudible]. If I so that, you can see, my sample was very
bright at the beginning. As I’m changing the gate
voltage, it’s becoming dimmer. You can have it moving, you can
see I’m changing this intensity level just by changing
the carrier density level in the system. So you can think of it as, you
know, motivating this behavior. All right? So now, where is
the field going? Right? Hardly there are almost
200 materials of this kind. So what I’m having you know, and
so some of them can be graphene, some of them have, I showed
you [inaudible] semi-conductors that can be off site, that
can be magnetic materials, that can be electric. And not only that, you can
put one on top of each other. So what do you have? You have infinite possibilities
to make the complex devices. You can have, you can make
whatever preferred is it you want and make the devices
to get the properties. Right? So I get, you can see this
is like, as I told you, you can have this, they call it
Van der Waals Heterostructures because the thing that is
holding it is the Van der Waals force. Right? So this is, you can have many
different, like, for example, boron nitride MS2, boron
nitride graphene, boron nitride, so people figured out
to make many stacked on top of each other. Right? So this is, you know,
one of our lab, in our lab you can see we put
two different semiconductors on top of each other. Not only that, nature
makes these materials with two different
semiconductor, one open at the way. Just called franckeite. There’s it called franckeite. This is, this is
your semiconductor and you have another
type of semiconductor, then this one repeats again. And nature made that. All right? So this is one of our
research recently. We worked on that. And you can see the
layered structures in this. All right? So, but nature made two
semiconductors alternately. All right? So, again, so we make
devices in the lab and we study the
optical properties and photocurrent behavior. So, well, you know, so
what I wanted in this talk to share the excitement in the
field and where is it going. So the possibilities
kind of endless. You can just think about
what materials you want, what kind of stacking you
want, and then you can. So lots of new, exciting
physics has a study coming out. One I have shown you, the superconductor people
have studied other behavior. So people just realizing, and
I’ve showed that variable, it just started moving
forward, graphene, and then new methods
will start to come out, more things are started coming out because there are
[inaudible] complex devices, high quality devices. So, and, yeah, this is
my group at SF State, and thanks for the time. [ Applause ]>>As it’s 5 o’clock, I know
many of you who are here for extra credit have
other things to attend. So if you’re here
for extra credit, you’re welcome to depart. Otherwise, those of us who remain behind will take
some questions for the speaker. So, if you have questions
for the speaker.>>It’s a neat idea. I mean, when you have the
rotation, right, I mean, you were talking about
the permeability. And I just want to know,
is there thoughts and views of molecular series or are
you basically saying nothing is important? It’s impermeable
to most everything. Are any of these
structures have any sizes? Are there any interesting
ways to use this to –>>[Inaudible] university,
they’re working on the property of this [inaudible]
and they showed, you can control them
different types of molecules. You can do, you can do that,
you can also protect, you know, isolate [inaudible]
by applying that. Also people are working on this. I told you that DNA sequencing
like making a tiny hole and passing the DNA and
electrical property. So [inaudible] are
trying all different ways. Rather than putting the
superconductivity, just, because it’s, you can control
the superconductivity behavior. And you can understand why
the superconductivity have any [inaudible] materials. So this is a new field
when controlled this, so band structures
at this angle.>>This is great. I mean, this is a wonderful
material engineered. So one thing that kind
of surprised me was when you showed the
photoelectron spectrum of the disulfide, there was
really no clothe to that 2.5 or 2.6 electron volts. But what [background sounds]
so I was kind of wondering, is it because of the polyfield in the [inaudible] don’t have
any transitioning, you know, from the top or the
bottom to the other side of the band structure.>>So I did a, because that’s,
that’s a very good question. Because the [inaudible] is
that there is a real talk about there is this new
band structures appear because you have
a convention band and your bands, they are flat. So what happens is that
you have a optical density, which is also flat. So you have a single
[inaudible] in the optical lens of your state, and
that’s [inaudible], and that’s why you can see the
[inaudible] which is very next to the [inaudible], but [inaudible] is much
larger than [inaudible].>>Right.>>That’s telling you that [inaudible] absorb
almost 40 percent [inaudible], and that [inaudible] is totally
coming from this singular yield, [inaudible] and that’s kind of
how it’s trying to, you know, to overrun the transition, bigger condition you are
talking about, around 2.5.>>But you didn’t
have the 1.9 edp, and I always suspect this is
due the defects in the band gap.>>No.>>It’s not. It’s definitely –>>That I show you. Those are for suspender
samples, and [inaudible]. So the quality of the
sample is very, very good. Even, yeah, even, you can,
even if you have a cvd sample, you put it in [inaudible],
which is not optical active, you see [variable] a and b. You don’t see others. So it looks like the exciton
behavior, Coulomb interaction is so strong, you find
the slide [inaudible].>>Right. The [inaudible]
usually they have energy. Maybe 100, 150, you know,
but not the band gap, but here we’re talking
1.9 versus 26. So it’s not that huge.>>Yeah, this is [inaudible]. This is very strongly bound. The only reason is
that, you know, you don’t have any materials
on the top or on the bottom, so the Coulomb interaction is
that, is very, very strong. You know, so that’s, you know, it’s very interesting what
you have for optical devices. Yeah.>>Yeah. I think
you were the talk and being the Moire
effect [inaudible] angle and changing the properties. That’s really remarkable. But when will [inaudible]. It doesn’t conduct
heat this way. It does conduct [inaudible]. Is it [inaudible] the
visco the thermal electric material [inaudible]? So when you twist the material
or misalign the two fringes, what happens to the conduction? The conduction is usually
given along the point.>>Correct.>>So it doesn’t get up
at all outside the plane or the two stacks
don’t [inaudible].>>So people have studied
and tried to, you know, they’ve tried to study that two
layers on top of each other. So there is no conduction
this way. It’s mostly conduction this way.>>But when you twist
it, is there?>>No. Because of the
interaction of those two layers, you modify the band structure. The electron is still
remaining the same plant. We now see two different
[inaudible].>>Well, I’m getting, like,
tunable grading, tunable filters through the forbidden way or,
you know, through the wide way. So when you make those
changes to the spacings, it looks like larger pathways
through it were opened up through the two sheets. So no, or, conduction
through them, the forbidden way
is now allowed.>>Yeah. I think, so, I don’t
know the answer for that. So I think that people are
just starting to realize to do this angle,
preferred angle orientation and more of those are coming.>>And what do you see
as the applications?>>Oh, for that one? You know, the first of
all application would be to understand the
superconductivity. People have studied
all these materials, but there is still
lacking understanding of superconductivity. So that would be one.>>That’s the big one.>>That’s the big one. That’s the most exciting one.>>But we’ve got microphones that are pulling
out electron tuning.>>Well, I showed you that
for those you don’t need Moire pattern. Even a single layer of
graphene will do the job. I think some of you would
buy those Galaxy [inaudible] that you can bend it or you
can wear some [inaudible] with graphene on it. So those you can, you don’t
need a complex [inaudible].>>Before we finish,
just a quick note that our speaker is
staying for dinner, if you’d like to join us. Otherwise, let’s thank
our speaker again. [ Applause ] [ Music ]

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