Practice English Speaking&Listening with: Nanowires and Nanocrystals for Nanotechnology

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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.


Let's welcome Professor Cui.

PROFESSOR YI CUI: First of all let me thank Chuck for his


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


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


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


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


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


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


If you look at a phase diagram, when you mix gold and

silicon together, they melt at really, really low


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


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


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.


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


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


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


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


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


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


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


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


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


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


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.


Thank you.

The Description of Nanowires and Nanocrystals for Nanotechnology