MALE SPEAKER: Hi everyone.
I'd like to welcome Professor Yi Cui from Stanford to give
us a talk today.
Professor Cui is one of their most famous young scientists
working on nanotechnology area.
He got his bachelor's degree from the University of Science
and Technology of China, and got his Ph degree in chemistry
from Harvard University.
Professor Cui is right now an assistant professor at
Stanford University.
Yeah.
Let's welcome Professor Cui.
PROFESSOR YI CUI: First of all let me thank Chuck for his
invitation.
It's a pleasure to be here.
The first impression I have after I got here is that
Google is a really dynamic place.
That's probably the most impressive part I have seen
after visiting so many companies.
It's also great to see several alumni from University of
Science and Technology of China sitting in the front.
That's a big plus for me.
So in return I'd like to tell you what I'm doing in the last
well years also using nanowires and nanocrystals for
nanotechnology.
This is the title.
But I do recognize this is a talk for--
it's like general public talk.
It would be a good idea to give you some introduction
about the history of nanotechnology.
First of all, what's nanotechnology?
At the most general level, the nano science and technology--
it's defined by the size scale.
It's defined by these nanometer unit.
Usually it defined as a science engineering on the
100's nanometer scale or below.
So it's the size solution between the atomic level and
the box level you have seen.
So one nanometer is around 1 out of 10 to the 9 billion
meters also.
So to give you an idea how small it is, if you like
soccer, you probably are very familiar with this picture.
So he does still have some hairs.
So the size of the hair is around 50,000 nanometer also.
That give you an idea.
A nanometer unit is really, really small.
And how small it is.
Now let me give you some example to let you build up
the intuition how small a nanometer is.
You probably all learn about atoms. Atom have the size
around 0.1 nanometer also.
So going up, you start to see nanometer solution.
The size of the nanometer solution includes typically
molecules, DNAs for example, proteins, and currently really
active research area carbon nanoteams, quantum dots,
protein cell, and these nanometer illusion.
Going up slightly to 10 nanometer also, semi conductor
and other nanowires I'm going to talk about
in just a few minutes.
And viruses has the size close to 100 nanometers also.
And a Pentium 4 chip have the size around this 100 nanometer
to a micron also.
And going up, a single biological cells has 10
microns also.
So one micron equals to 1000 nanometer.
10 micron is 10,000 nanometers.
And microfluidics laying on a chip, that size scale is in
100 micron scale also.
And a human being will be all the way to this
end, right hand side.
So nanometer scale is really a small size solution.
But it's not the smallest size human being ever studied.
Chemists and physicists used to study the size scale and
the atomic size.
So that's smaller than one nanometer.
And we are also very familiar with the very,
very big size solution.
So nanometer solution is really between the atomic size
and the box size.
That's something you will expect new properties happen.
You can take advantage of these new properties to come
up new applications.
So look at the history of nanotechnology.
People working in this field always use Faynman's
encouraging speech as the starting of the
nanotechnology.
So in 1959, Richard Feynman, a physicist in Caltech give this
famous speech.
He described the importance when you decrease the size of
material, size of objects to really, really small.
You can do something new.
You can do something important.
So I highlight this sentence right here. "You can decrease
the size of things in a practical way.
I now want to show that there is plenty of room."
What he means "plenty of room" is there's plenty of things
you can do in this size scale.
So after he gave that speech, Feynman used his own money to
set up two Feynman prizes.
So anyone who can do one of these two things
will win this prize.
So the first one is-- that was in 1959.
Anyone who can build a model that will fit into the size of
1/64 inch by 164 x 164 box.
And the second one is anyone who can write a page of text
with letter that are small enough for the Encyclopedia
Britannica to be printed on a pinhead size.
As soon as Feynman announced these two prizes--
and the first one got claimed right away.
Bill McLellans did a really good job just by using hands
to made a small motor which is small enough to fit into the
size to win the Feynman prize right away.
But the second Feynman prize didn't got claim until very
late, until 1985.
So that's some 20, 30 years later.
Tom Newman of Fabian Pease-- fortunately Fabian Pease is a
professor at Stanford.
I got to meet him a couple times a month.
So he used e-beam lithography to lie a part of A Tale of Two
Cities in the length scale required by Feynman.
So they claimed the second Feynaman prize
after a couple decades.
So what's the big deal about decreasing the size of object?
So what's the big deal of this nanometer scale size?
Let me use one most important example to illustrate
importance of decreasing the size of the object.
We all know about the first transistor.
Transistor was invented in Bell Lab on the
Christmas Eve of 1947.
So the inventors won the Nobel Physics Prize in 1956.
The first transistor is really, really big.
So you can just use your hands to catch it.
So over so many decades, people keep pushing down the
size of transistors.
Now the state of the art transistor fabricated by Intel
has the gates length around 30 nanometer or below.
So all the important length scale right here like the
gates that electric thickness a couple nanometer.
And the source and during contact all
gets very, very small.
So it's due to this development, due to the
decreasing of the size, you can make more and more
powerful transistor, more and more powerful computer chips.
And decreasing the size allow you to pack a lot of things
within a tiny areas.
So along the line of electronics integrated circuit
is the most famous example what important things you can
do when the size get small.
So you can integrate things together into tiny areas.
Jack Kilby won the Nobel Physics Prize in 2000 for the
invention of integrated circuit.
So it's the size decrease which motivate, which drive
the transistor smaller and smaller, and the computer
chips become powerful and powerful.
This comes to most famous Moore's Law.
It describes about every two years also, you can double the
number of transistor within the same chip.
So given this eample, now what does nanotechnology do?
So people have developed the transistors get down to about
30 nanometer size scale also.
So what nanotechnology do is it would like to develop the
new tools to study, to see the size and the deep nanometer
scale, even atomic scale.
And also the second way to produce the structure in the
nanometer scale size.
You can use photolithography to pattern computer chip down
to 30 nanometer scale also.
But that is not enough for nanotechnology.
So what are the new applications given you can
produce these nanostructures?
You have the tool to see the nanostructures.
Now let me give you some of the research people have been
doing in these three areas.
Some of the example just come from my own research.
The first areas to which we're starting in nanometer scale.
One of the most important tool is scanning tunneling
microscopy.
So this tunneling is like this.
If you want to look at a sample of substrate, you can
take a really, really sharp metallic tip,
scan over this sample.
So the tip is sharp enough only a single atom terminates
at the tip.
So you apply a voltage between the tip and the substrate.
You have current flowing through between the tip and
the substrate.
And by moving the tip, scanning the tip, since you
have atomic size, a single atom at the tip.
So you have the resolution to look at the substrate down to
atomic scale.
So people use this technology to look at--
for example, this is a solid state substrate.
You can clearly resolve single atoms. And you can even use
this tip to do something like medical operation.
So you can apply a voltage to pick up an atom from the
substrate and then move it, transport this atom to a new
place, and deposit into a new place.
By doing this, you can use atom to manipulate to form the
special pattern you would like to study what happen in the
nanometer scale.
The inventors of STM won the Nobel Physics Prize in 1986.
In the same family of scanning probe, there is another
technique that is atomic force microscopy.
This also take advantage of a very sharp tip.
So you use the laser to monitor this
the cantilever position.
So by scanning the tip, you can maintain their force
interaction between the tip and the substrate as constant
while you are scanning your tip.
If you have surface topology change, it's going
to change the force.
So you have a feedback loop to pull your tip up and down to
monitor the surface topology.
But doing this way and having a really sharp tip, you can
look at the structures, for example that
proteins, like DNAs.
The difference became atomic force microscopy and scanning
tunneling microscopy is scanning tunneling microscopy
use the current as the signal.
And atomic force microscopy is the force interaction between
the tip and the substrate.
So they have different usage in different areas.
The inventor of atomic force microscopy is Binning Quate
and Gerber.
Among all these inventors, Quate is a
professor at Stanford.
Another technique to look at the size scale and the
nanometer is the transmission and scanning electron
microscopy.
So you have an electron gun shooting electron out, passing
a lot of electromagnetic lengths and
focused onto your samples.
Since the wavelengths of the electrons is very small, you
know the imaging resolution is limited by the wavelengths of
the lights of the electron waves.
So using sharp wavelengths of the electron beam, you can
look at the structure.
For example, you have quantum dots right here, down to the
atomic size resolution.
So this technique are really important for the development
of nanotechnology.
Not let's come to the second areas.
How do you produce the structures in nanometer scale?
There are two philosophies in making things smaller.
One philosophy is you can take a piece of a big object and
then cut it down into a small piece.
This is so-called top-down approach, so going from big
size to small size.
Second approach is so-called bottom-up approach.
You use molecular precursors starting from molecules.
And then you do chemical synthesis.
These precursors start a nuclea and grow, grow into the
nanometer scale size object.
These two philosophies represent different ways in
doing almost the same thing.
The industry has been motivated by
the top-down approach.
The whole transistor's development is really relying
on the top-down approach.
It's the bottom-up approach in the last 10 years also, really
make the nanotechnology really, really exciting.
You can do something new using this bottom-up approach.
So I'm going to skip how people do
the top-down approach.
You're probably all familiar with photolithography idea.
Now let me come to the bottom-up approach, how people
produce nanoscale structures, nanoscale materials a using
bottom-up approach.
One example is the Carbon 60.
Carbon 60 was discovered by these three people.
They are the winner of a Nobel Chemistry Prize in 1996.
So taking a graphite structures--
graphite has a layer by layer crystal structure.
If you use laser to evaporize graphite, you have certain
catalysts around.
You can produce this beautiful--
about five nanometer size also--
a ball of--
people call it a buckyball.
So it 12 pentagons and 20 hexagons.
It is exactly like a soccer ball.
This structure, this buckyball, has a beautiful
structure, which is beautiful enough to get awarded a Nobel
Prize to these people.
And second important
nanomaterials is carbon nanotubes.
Carbon nanotubes is a structure
also based on graphite.
If you take a piece of graphite sheet and then roll
it up, it would become a nanotube.
Depends on how you roll it up.
You can get different chirality of tubes.
And you can have semi-conductive behavior.
You can have metallic behavior.
So carbon nanotube becomes a very active research areas
about 10, 15 years ago also.
So another type of nanomaterials I have been
working on in the last three years also while I was still a
postdoc at Berkeley, are semiconductor nanocrystals.
So these tiny crystals have the size between about 1 to 10
nanometer also.
It's based on purely chemical synthesis, and a very hot
organic surfactant you can inject molecular precursor
into this flask.
And these precursors decompose to nuclea and grow into these
so-called 0 dimensional nanostructures.
And depending on what kind of molecule and surfactant you
have in a solution, you can produce a spherical shape of
quantum dots, nanorods, one-dimentional structure, or
even branches structure.
We call it-- in this case it has four arms as a tetrapod.
Now one more type of nanostructures produced by
chemical synthesis, produced by a bottom-up approach is
semi-conductant nanowires.
There's many ways to make one-dimentional nanowires.
One particular powerful way I have been working on is the
so-called vapor-liquid-solid growth of nanowires.
They come from the ideas you can take tiny, tiny, seed
particles, nanoscale particles, function as a
catalyst.
For example, if you want to grow silicon nanowires, you
can take a gold nanoparticles and heat it up about, say, 400
degrees C also.
And pass silicon precursors-- it could be silane.
And these precursors decompose and deposit into gold
nanoparticles.
If you look at a phase diagram, when you mix gold and
silicon together, they melt at really, really low
temperature.
Gold alone, the melting point is about 1000 degrees C. And
silicon alone, the melting point is about 1400 degrees C.
But when you mix these two things together, the melting
point is 360 degrees C. It's really, really low.
So using these properties at about 400 degrees C also, you
can add silicon into gold and then make a liquid droplet.
So if you keep adding silicon into this droplet, that
silicon supersatura--
it's going to nucleate and blow into one dimension, the
nanowire structures.
It's due to this tiny size which defines
the nanowire structure.
Here you see a transmission electron microscopy image is
showing you a gold particle at the end of silicon nanowires.
And you can produce beautiful single crystal nanowires.
So there are other nanoscale structures made by nature in
addition to this artificial nanostructure
made by human being.
But DNA is one type of powerful nanostructure
proteins and viruses and organic molecules.
So given all these structures, what kind of
things you can do?
What useful things you can do to the human being?
So you have carbon nanotubes, semiconductor nanowires,
semiconductor quantum dots, DNAs, proteins.
You have Carbon 60s.
So over the last 15 years, people have been really
creative in using these materials for different type
application.
Now let me give you several examples.
So I give you a list right here.
The top two cosmetics and catalysis.
It's not something new.
It happened for really a long time.
It's there.
And the new things is in nanoscale electronics,
molecular electronics, biosensors for healthcare, and
bio-warfare detection, and renewable energy, and battery
technology for example.
So I don't need to say too much about cosmetics.
Just by pointing out the sunscreen you use has
nanoparticles in it.
So the semiconductant nanoparticles has the
semiconductor bank large enough which can absorb UV
light, can be a good materials as sunscreen.
So remove the UV light and protect your skin.
And for catalysis, the whole oil industry relies on this
technology.
They use metallic particles function as catalysts for
cranking up the oil and decompose them and into
different components.
So it's a big industrial already.
Now for better and faster transistors, people have been
demonstrating using carbon nonochip.
One of my colleagues at Stanford University, Hongjie
Dai, in Stanford chemistry have show using carbon
nanotubes, you can produce the transistors which is way more
better than the transistors Intel can make.
Since carbon nanotube has these graphite structures, it
roll up as a tube structure, there is no surface dangling
bonds on the surface.
So that means when electron is moving along the nanotubes,
they won't get trapped.
That also mean electron can move very,
very fast in nanotubes.
So using nanotubes contact by two electrodes.
One is [UNINTELLIGIBLE]
The other is source.
And you can have a gate electrode whether it's using
substrate as a gate or you can fabricate so-called top gate.
People make really high quality transistors and
demonstrate that mobility is like infinitely high.
Because the transport of electrons doesn't get
scattered along the nanotube.
It's like molesic transport.
And in many companies are pushing very hard.
Especially IBM tried to use nanotubes for the next
generation of transistors.
And another example coming out of my own research work is to
use semiconductant nanowires for making better transistors.
Carbon nanotubes are nice.
Their performance are very rare.
But carbon is new materials for the
whole silicon industry.
So what don't we just use the silicon nanowires, because
anything related to silicon has been developed, all the
action process, all the chemistry, all the
fabrication.
So during my work in PhD I had to demonstrate using the
silicon nanowire contact by two electrodes.
You can fabricate a transistor which is also better than the
common Intel transistor.
So the mobility of the charge it carries becomes higher.
That also suggests you can have faster transistors.
The reason for that is it's based on the same material.
It's silicon nanowire.
The reason it can be better is really due to when the size of
these nanowires get smaller along the cross section.
So the corner confinement changed the electronic
structure of silicon.
So it might be a continuous energy band become a discrete
energy band.
This decrease the probability of electrons got scattered
when they are transport along these nanowires.
And we are developing this in this area try to make better
transistors for the next generation of computer chip.
So I mentioned to you for all the nanostructures, molecules
is one type of nanostructure.
They have a size scale along a nanometer also.
In the last 10 years, also, several research group,
especially Jim Heath's group in Caltech and Stoddart
Williams' group in UCLA.
Williams Stoddart's group, Stan Williams' group, and HP--
they are developing molecular electronics.
They're trying to use really tiny size of molecules
function as a switch.
And this structure--
this yellow color indicates the metallic electrodes.
So this indicates the molecule.
So they sandwich molecules between two sets of
electrodes, try to apply a voltage to induce the change
in the molecules.
They can store informations right there.
They can make the switch.
Also this very active research area.
So due to the work in the area of nanowires and molecules,
and this work made it to the cover of Science in 2001.
So this is right as the breakthrough of the years,
using nanowires and molecules as transistors, as switches,
to make better electronics.
So I would hope someday you can really get these
transistors, make them small enough, make them powerful
enough, make them reproducible enough, make them dense enough
to produce better and better electronics.
It should go beyond iPod Nano.
So he has a Nano in it.
But it's not really the nano we are looking for yet.
So after giving you the examples of the application in
electronics, if you ask me how long would you predict the
nano can do something for the transistors, to replace
Intel's transistor, I would tell you I don't know.
It's going to be very long.
Doing electronics is probably the hardest things in
nanotechnology.
It's the major driving force 10 years ago, but now the
driving force come from all the directions.
So electronics probably is the latest areas you can impact in
nanotechnology.
Other areas of technology might be possible, might be
earlier than the electronics.
One area is in biosensing for medical
cares, for human health.
Now let me give you one example coming
out of my own research.
So we made this one-dimensional nanowires
structure, semiconductant nanowires.
You all know semiconductors has field effect.
So if you apply a voltage, you apply electric field close to
the semiconductor, you can change the charge carry
concentration in semiconductor.
You can change its conductivity.
Using these ideas, if you take the nanowires contacted by two
electrodes and then you put certain receptors which
function as a recognition reagent to recognize certain
molecules you want to detect--
if this molecule comes in and binds onto these receptors, if
these molecules have charges, it's going to
function as gate voltage.
It will have electric field interact with the nanowires,
change the carry concentration in nanowires.
I mentioned the conductors.
You will see the conductors change.
You would be able to detect the existence of these red
color charged molecules.
So based on these ideas, we think we can develop a new
type of sensors.
Since the nanowires-- they are so small, they must be really,
really sensitive to any charged molecules.
So we the proteins, DNAs--
they have charges.
So this is the idea for detecting something in the
human body.
The other one piece of this technique is since the wire is
small, it's sensitive as I mentioned.
And second, this is purely based on
the electrical current.
In conductance measurement you don't need
to label any molecules.
This is really different from the existing current
technology.
So they need to go through extensive radioactive labeling
for example.
So a nanowire sensor doesn't need to do that.
And imagine if you can bring a lot of nanowires together, you
can do a array-based sensing.
If you have 1000 of these nanowire, each one has
different receptor.
You can detect different molecules at the same time.
Here's one example.
We were doing the cancer marker detection.
So for many type of cancers, before the cancer really
develop to the late stage, the human body already have some
change in the blood.
For example, for prostate cancer--
prostate cancer is a killer in US for the men.
It has a specific cancer marker.
It's called PSA, prostate specific antigen.
It's a protein.
If a man gets prostate cancer, in the early stage this
protein concentration will go up.
So if we can find the antibody, the labeled nanowire
reads the antibody of these cancer markers.
Then you can detect it in the really early stage.
So here I show you one example,
having three nanowires.
Each nanowire modified by different antibody, they can
detective of cancer marker antigen.
They're all proteins.
And you can do that quite successfully.
Here you can just simply match the carbon or say conductors
versus time, and different time.
And you flow in the different cancer
markers onto the nanowires.
Nanowire number one-- it has a PSA antibody.
So when PSA goes in, you see the conductance goes up.
But the other two nanowire doesn't have
response right here.
And you can do the same thing for the other two nanowire
have different cancer markers, like CEA comes in,
and MUC comes in.
They call response to different type of cancer.
So you can detect them really well.
Indeed, we can go down to the concentration about picogram
per mil, and even close to femtogram per mil.
It's like 100 times more sensitive than the carbon
technology.
So using this nanowire sensor detection scheme, it's
possible to discover cancer in really early stage.
So biosensor is a big areas.
Another big areas just coming up become hotter and hotter is
the energy related areas.
As the gasoline price goes up, you'll all understand why this
area is important.
The key things for making solar cells, collect sunlight
energy and make it into electricity, is to come up of
way you can collect the sunlight and covert it into
electricity in very high efficiency with very, very low
cost. That's something nano can do.
So we made this nanowires.
We made this nanocrystals, especially nanocrystals that
dissolve an organic solvent.
They can be processed as chemicals.
If you want to make solar cells, what you do is you take
a substrate of glass.
And then you just Spinco these nanocrystals or
just simply by printing.
Spinco imprinting is very, very cheap technology.
So you can cover really large areas, and for example making
solar cells for all these windows
converted into solar cells.
And that's what nano can do.
In these examples you mix nanoparticles, nanocrystals
together with organic polymer.
So organic polymer and
nanocrystals will form a junction.
If you look at its electronic structures, this junction can
supply electrons in holes to create electricity.
Electron in holes are created by sunlight.
When sunlight comes in, the photon energy create electron
in whole pairs.
Excite them.
And then you separate them.
You can collect as electricity.
So my post-doc advisor, Paul Alivisatos, was working really
hard at Berkeley in these areas.
In my research group, I also have program in doing that by
this different idea from Paul.
And my colleague, Mike McGehee at Stanford Material Science,
is an expert in using an organic nanostructure bring
together this polymer to make better solar cells.
So what about for battery?
You're all familiar with lithium battery now.
The laptop you have-- it's powered
by lithium-ion battery.
So all your cell phones are powered
by lithium-ion battery.
Lithium ion battery really dominate part of electronics.
Now we are thinking very hard.
Can we use lithium-ion batteries for doing something
even bigger?
One area is electrical vehicles.
Can you use battery to power vehicles as the sole power
source instead of using gasoline?
Gasoline is really polluting the whole world.
About a quarter of greenhouse gas comes from the
transportation consumption and gasoline.
So it would be nice if you can use battery to power the
electrical vehicles.
But what's the critical problems in batteries?
One critical problem is you need high
energy and power density.
Higher energy density means you want to have a battery in
your car, and then you can drive for 300 miles without
charging it.
You don't want your car to be charged every 15 miles.
That won't be good.
And higher power density means if you want your car to
accelerate to the certain speed, you really, really need
to crank up the power a lot.
The current battery technology is not good enough
for doing that yet.
Of course you want it to be cheap.
And the content battery technology is not cheap.
Current lithium-ion battery technology it's using lithium
cobalt oxide.
Cobalt is very expensive.
It's costly.
And also there is not enough cobalt in the world to go into
electrical vehicles.
So we are coming from the nanotechnology standpoint.
We want to impact these areas a lot, especially to improve
the higher energy density and power density.
When you look at the problems in lithium-ion batteries, how
can you increase the power for sample?
If you have higher carbon, you get higher power.
That also means if you can get lithium very fast into the all
the battery electrodes, then your power gets higher.
So the key thing is you want to maximize the surface
counter areas between the battery electrodes and the
electrolyte.
So that's what nano can do.
Nanomaterials have really, really high
surface to volume ratios.
So surface area is high.
So you can have high interfacial area with
electrolytes.
So in my group we are using nanowires in content by this
red color electrolyte.
So you can see the surface area get increased by a lot
compared to the current lithium-ion battery
technology.
But doing that, we hope the power density goes higher.
We also have the reason why the energy
density can go higher.
I probably just don't have time to mention that to you.
So we are also looking into the areas trying to make a
better nonvolatile memory.
And I'm going to just skip this and come to the
conclusion.
So I gave you the examples in three
areas of active research.
Develop the tools for nanotechnology.
Produce nanostructure and the structures
in nanometers scales.
So the first two areas are more or less mature.
So we know how to make nanostructures.
We have really powerful tools to look at the nanostructure.
What we need can be really impact application with
nanomaterials.
That's the key thing.
The public has been looking into
nanotechnology for a while.
They are eager to see some meaningful, some critical
application coming out of the nanomaterials.
So this application can range from transistors, non-volatile
memory, battery technology, solar cells, and biosensors.
It's hard to predict which one will stand out first. But all
of them all have the potential.
So I thank you for your attention.
And I would be happy to entertain any questions.
AUDIENCE: Do you have any idea what the relative efficiency
of nanotechnology-based solar cells are versus the
conventional cells that [INAUDIBLE]?
PROFESSOR YI CUI: The nanotechnology-based solar
cell-- it's low at this moment.
So conventional solar cell-- if you look at a single
crystal of silicon, you go up to 18, 19%.
All the crystal of silicon about, say, about 12% also.
And current nanotechnology solar cell, based on the
organic polymer blend together with an organic structure, go
up to 5%, 7%.
There is still a long way to go for
nanotechnology solar cells.
But coming back to the history of the nanotechnology solar
cell-- it's not that long.
It's only about five to seven years also.
It certainly still has some potential to go higher.
OK.
Thank you.
