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♪ [music playing-- no dialogue]♪♪.
Okay, so today I'm talking about nanotubes and electronics
of small scale structures.
I should say that I have tried to gear this
talk toward undergraduates.
So if you're an undergraduate in the lab, in the audience, please
ask me questions as we go along if it doesn't make sense, and if
you're a faculty, you especially have to ask me questions if it
doesn't make sense and I've really done something wrong.
Or if you want to know more about it, I can say a lot more
than I'm showing here, about anything.
So I'm going to start by just giving a general background
on mesoscopics research.
I'm a condensed matter experimentalist and I focus on
small scale structures.
I'll define what those are, talk about why it's interesting, and
then talk about my specific research on carbon nanotubes,
what the history of those are, why they're interesting, how we
make those devices, and then we go on more specifically into
some data we have taken on these materials.
Looking specifically at low dimensional behavior, one
dimensional behavior, and even what we call zero dimensional
behavior of these objects.
Oh dear, okay, so that's.
Ha ha, look at that.
That was interesting.
Okay, so I just want to start by showing the people who actually
do the work on this, some of it I did at postdoc but a lot of it
is done by my current research group at University of Illinois
and a lot especially from my postdoc.
And some students in some students in collaboration with
other groups at Michigan State and at Harvard.
And of course, it's funded, you can't do any of it without
funding from the NSF and the DOE.
Okay, so the basic, for all of the work that I do,
we really ask one big question.
You know, what happens electronically when we shrink
things down, right?
If you take a piece of metal, you know, a spoon, you know, a
faucet and make it really really small, down to the nano scale
even, you know, billionths of a meter, how do
the electronic properties change?
Right, what's different about a very, very small
object compared to a very, very large object?
In terms of the physical principles, right?
How is the physics different when things are
small versus when they're big?
And you can think of this if you just first consider a piece of
gold that's, you know, a few millimeters, and you can hold in
your hand And that obeys Ohm's laws, right, you guys have seen
this, V = I R, you sort of know what goes on there.
Then on the other hand, you take a single atom and if you've had
quantum mechanics, you sort of know what goes on there too, it
obeys quantum mechanics.
So you have particle wave duality, you know, it can go
through barriers without stopping, you know an elephant
can't do that, but an atom can.
You know, it has phase coherence, it fills, the
electrons fill atomic shells, so you sort of know what goes on on
either side of this, big things and very very small things, and
the question I approach, that I ask, is what happens in between?
What happens when you have something
that's a few atoms long.
You know, here is a nanotube that's about
ten atoms in diameter.
Okay, that's small enough that you can still see quantum
properties, but big enough that you can actually measure it.
You can't measure a single atom with probes, but something like
this you can actually measure with probes.
So this is the area called mesoscopics, it's something
between macro and microscopic, it's small enough to see quantum
properites, but big enough that you can still define a bulk
property, you can still study it in the way that you do bigger
things and think about what the physics there is.
What really interests me is how do you get
from here to here, right?
How do you build up your physical understanding, how do
you go from quantum mechanics to big things, and what interesting
science is there to learn along the way?
So this mesoscopics research is what we call a bottom-up
approach, you know, starting from small things
and making them bigger.
A bottom-up approach to understanding physics and to
making devices, you know, at the end of the day, as electronics
get smaller, it's more and more interesting to look at smaller
devices, what happens in small structures.
And I've written here just three big questions that you'll see
keep coming up throughout this talk.
What are the questions we ask when we make things small?
Well, one is, how do low dimensional systems, you know, I
say one dimensional or zero dimensional systems, differ from
two or three dimensions?
Right, it turns out that, here's a picture of Alice in Wonderland
as she found out, things are different
when you're smaller, right?
You guys know, okay, anyway, so, you know in small systems in
physics, you see stronger interactions among the
particles, electrons interact much more strongly you can see
interesting phenomenon due to that.
You get unusally correlated states due to these strong
interactions, you see quantum mechanical behavior, You do see
different things when things are very small
and confined to small spaces.
We also might see and we can study quantum properties, right?
How does quantum mechanics manifest itself if it's not one
atom, but it's not a big thing, but it's a few atoms across, how
do you see quantum mechanics in these things?
And also, can we make devices out of these?
What, you know, can we make something novel in
these really small structures and make something
useful and interesting?
So these are the basic questions that we ask in this research.
Now as I mentioned, there's a practical motivation, and that's
to make really small circuits.
Now you guys have probably seen Moore's law before.
Moore's law says that the number of transistors on a chip, the
transistor density on any given chip, is increasing
exponentially every year, right?
So if the density is increasing exponentially, it means
that the size is decreasing.
Right, and this seems to work out amazingly since the 1970's,
and computer manufacturers are really
desperate to stay on this curve.
Alright, but as things get smaller, we're already at the
size where the size of a transistor is the order of 65
billionths of a meter, you know, across.
That's really really small, that's nano scale already, you
know, that's a couple hundred atoms across right now.
So already, just practical considerations in electronics of
the things we use every day forces us to try to understand
and work with nanoscale objects.
And again, you can see that, as you pattern things down, you can
get to about 65 nanometers, you can use advanced technology to
get a little bit smaller, but if you start moving to things like
nanotubes you can get things even smaller if you work with
these sort of structures.
This sort of nanoscale mesoscopic research is also key
to using quantum mechanics to make more advanced devices than
you can make with classical mechanics, and one example of
that is a quantum computer.
Now a quantum computer is something that uses quantum
mechanics to get many more bits than you can
get with classical mechanics.
The idea here is that you use things like spin, which is a
quantum property of things, of electrons, and these quantum
mechanical states, these spin states, can not only be an up or
a down, right, that's one bit, up or down, but they can be in a
superposition of both up and down at the same time.
So you don't only get two states out of that, you get, you know,
up, down, plus up-down, plus down-up.
You get four states out of just two bits in that case for a
quantum computer versus a classical computer.
So that's just one example, so in general, for classical
computers you need two to the n calculations from end bits, you
can do two to the n calculations, and for a quantum
computer you only need n calculations.
Okay, so you can get many much more powerful parallel
processing out of a quantum computer than out of a classical
computer, but in order to do this, you have to use quantum
mechanics in small structures, you need to manipulate spins,
you need to define qubits, you need coherence in small
structures, you need to harness the quantum mechanical power of
small structures to implement these sort of quantum devices.
And finally, you guys have heard of nanotechnology, mesoscopics
is smack in the middle of nanotechnology, it's researching
on small scale structures.
In general, nanotechnology is just making and studying things
on the nanometer length scale, that's basically
what I'm talking about here.
There's a lot of interest in all different areas in chemistry, in
physics, in material science, from putting colloids into paint
to working on carbon nanotubes, like I do.
And there's also just been a lot of funding for this area, you
can see that the investments in nanotechnology have been in the
orders of $1.5 billion for the past few years, so there's
funding and a lot of interest for working in this area.
So this is the motivation for doing this sort of research.
Now, in physics there are a lot of examples of nanotech research
or mesoscopics research in physics, here's just three to
give you a sense of the work that I'm talking about.
People can work on, you know, I said mesoscopics is taking and
going from one atom to, you know ten atoms.
People do that explicitly by using a special probe to put
individual atoms in a line and see how they can build up from
one atom to lots of atoms, and see when it starts getting
properties of a wire versus a single atom, they do this using
a scanning tunneling microscope.
People also can fabricate nanostructures, here's an
example of a nanostructure, that uses an optical track that only
lets through certain wavelengths of light, alright used for
understanding how you can track certain wavelengths of light,
also for all-optical switches.
Again the scale of these holes here is sort of
hundred nanometer holes.
Or you can study things like molecules, DNA, molecular
transistors, nanotubes, there's just different examples
of this sort of research.