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Nine and two thousand and.

Ten.

Orders most notably
this year this year and

also.

A.

Real pleasure.

Thank you Dave thank you very much for the
invitation and thank you all for coming.

To us which leapt up.

OK excellent.

So today I'd like to share with you.

A direction my group is going in with
regard to synthesize the nanoscale

materials and I want to talk about what
will lead us down this path what was

the the spark that started
us down this path and

what we hope to do with it just going to
give you a taste for that and so I call

this expanding the semiconductor nano wire
design space so I'll give it away straight

the beginning we work with one dimensional
materials in my crew primarily you

can think about putting quantum dot type
structures inside nano wire so doesn't

preclude the study of zero dimensional
materials but we do grow no wires and

that's actually the reason why I picked
this photo this is of downtown Atlanta and

I like it because one you can think
about the buildings as nanowires and

actually the way buildings
are are built every building

is built the way we grow nano wires OK in
an additive fashion from the ground up.

We don't do subtractive.

We don't use attractive techniques like
you would see throughout the semiconductor

industry and we want to ideally achieve
a similar level of complexity OK So

in these buildings you pick what is on
each floor you pick where the exits

are you pick where the toilets are OK
you have control of that when you

design the building and you have at the
macroscopic length scale tools that allow

you to put the toilets where you want them
and so can we do that at the nano scale

before I go further let me acknowledge
the people who do this work

primarily today I will be
talking about nature all shinin

work both were my first two graduate
students upon arriving in Georgia Tech and

funding primarily from
the National Science Foundation and

Air Force OK.

So this is what I mean we build buildings
from the ground up and we'd like

to do the same for a nano wires so we can
create different levels of diversity and

complexity I would argue the buildings
are remarkably complex structures.

There's immense complexity in
biological systems this is.

A schematic illustration of Photosystem
two but all throughout living

systems it's remarkably
complex remarkably complex.

I'm always motivated by what organic
chemist can do I think this is remarkable

as a chemical engineer I look
into the chemistry world and

that you can take such simple building
blocks and through a sequence of steps you

can create remarkably functional
molecules OK And this is an example.

Taxil anti-cancer drug and
so I look at all this and

I say wow I wish I could do
that with nano scale materials

I wish I could have the same level of
complexity the same level of diversity.

Unfortunately we're talking about
materials at the nanoscale we have about

a hundred year.

Delay in starting to tackle this problem
so we're going to chemist have been

tackling this for a long long time
probably even longer than that biology is

how even longer so we're starting with
a little bit of a knowledge deficit.

So where do things currently stand I've
picked few things out of the literature

from the bottom up Nano Scale
fabrication literature and so

we can do a number of things we can we can
control morphology we can create cubes or

these pyramidal structures we can even
assemble these types of things there's

quite a bit of nice work done
making superstructures of

nano scale building blocks and here on
the nano wire side one of the exciting

parts about nano wires the ability
to merge materials that in two D.

wouldn't have ever thought to merge
because of the mechanical problems that

interfaces which would have led to really
poor materials properties so here is

an example of silicon in gallium FOSS
fied in Indian FOSS five gallon FOS five.

And there's many other examples where
you can tolerate lattice mismatch up to

ten fifteen percent and then a scale
materials do the strain relaxation

that's just inherent in this
one dimensional structure and

then a bit more recently the idea
that you can direct a nano

wire to grow in whatever direction
you'd like it to kink it so

to speak is also pretty intriguing.

And so this is this is what motivates me
a classic quote from Richard Fineman.

The key point there and white
the principles of physics as far as I can

see do not speak against the possibility
of maneuvering things out and by atom.

We just don't have sufficiently
fine tweezers to do that OK

scalable tweezers not one pair of
tweezers but scalable Twitters and.

Just one toilet all of the toilets and
all the windows so how do we

think about that so this is that's kind of
the crux of what my group thinks about and

so these are kind of the four questions
we can boil everything down to so

how can bottom up synthesis
the chief complexity precision and

functional diversity similar
to nature organic chemistry.

To get at that and to answer the question
I think we need to understand what

are the atomic level phenomena that
underlie the synthetic process.

What's going on at these like scales and

of course then what techniques can
you use to access these links go.

Time scales OK So

can we make new things by understanding
materials at the atomic length so

what insight does that provide and what
can we do that you couldn't previously do

what structures may be possible now that
otherwise would not have been possible and

then what application areas
emerge from these new materials.

So that motivates what I'm
going to talk about today.

I'm going to focus on first I'm going to
give you a little taste for those who

aren't familiar with where one dimensional
materials have been used to date.

Talk a bit about the challenge so this is
a significant challenge understanding and

then a rationally controlling
nanoscale structure.

There's an immense need for fundamental
chemistry structure property relationships

belong to a corresponds to structure B.
and we don't know much about that.

Present status of talk about that which
is largely empirical our approach is

in-situ what I'll talk about today
in situ infrared spectroscopy and

then this initial insight we had
which still to this day I'm.

Really tickled pink about but still I'm
surprised it affects so many aspects of

our growth and
then how we've exploited this insight OK.

What's the potential Well I mean.

Yes you can take any object and try and
throw it into and make make any device

out of it there's lots of stuff like that
but there are really interesting uses for

one dimensional materials OK So if this
pulled a few out of the literature each

week at Stanford it is shown some was the
first to show some interesting properties

in one dimensional silicon wires and
their ability to relax strain and

see breakthroughs in
lithium ion batteries.

Simultaneously there were two studies this
is just one of them looking at how you can

improve the figure of merit and thermal
lots of devices by confining structures

OK maintaining electrical conductivity but
slowing thermal conductivity.

In photonics has a lot of really
interesting work being done in the lazy

in these types of nano
scale materials OK So

there's lots of applications lots of
things to do all of these are what I would

call prototype really beautiful proto
typical demonstrations of what's possible.

And.

Yet in many cases the underlying
chemistry of that structure is still.

Maybe not quite where we'd want it so
I spent a little time talking about D.L.'s

growth which is the technique we use in my
group it's one of several techniques and

we use it primarily because it's one
of the most controllable to date.

So the technique vapor liquid solid or
V.L. last just stems from the fact that

there are three phases of interest in
the system so you have the vapor face and

in our case the vapor will consist of
basically C.V.T. precursors that deliver

atoms to a liquid eutectic
droplets that sits atop the wire

and then below that droplet is
the solid the Nano wire and

as you deliver more silicon to this
drop but eventually you achieve a super

saturation and you'll get a precipitation
event right here at this trip.

Baseline where the liquid solid and
vapor meet and then this

this nucleus that forms undergoes what's
called lead flow across the interface and

that happens over and over and
over again and that's how you grow arrays

that look like this where that catalyst
droplets started on the substrate and

then through off of the substrate.

Interestingly enough one of the great
things about via last is that it's easy to

dope these materials so we can add
dope and species that was deliver for

example phosphorous for this droplet and
doped region of the wire and

then we can turn that off and have another
region of the wire that maybe undocked we

could turn on Germany a merger main and
have a region that mania and

then turn on silicon and make hetero
structures there's a lot of options for

making things this way.

And ultimately we needa
a synthetic toolkit.

All of these properties that I'm
showing up here impact C B All

of these structures impact properties and
this becomes more and more important

more you scale these things OK So diameter
is obviously something that one would

need to control actually is pretty easy
down to a certain point orientation

OK Are you looking at wires that are
oriented along the one one direction or

the one one two direction or
the one one zero etc distinct transport

issues with all those different directions
What's the fasting actually one of

the nice things about bottom up synthesis
as opposed to top down in many cases is

the fact that you can create beautiful
faceted structures OK Very nice

side wall morphologies and then these
are nano scale structural motifs and

here at the atomic scale can
we change the law to us.

Can we take silicon which is usually done
in cubic and can we make it hexagonal for

example and
then doping we deliver impurities where.

They go where do they sit how do we
control where they go and where they said.

And ultimately all this boils down to at
the beginning of the chain is chemistry.

So chemistry of the bulk of a lot of
structure the bonds between the atoms

surfaces and I'll talk about today
play a critical role in this system.

Of course hetero interfaces and
impurities and

opens those lead to these different types
of structures I was mentioning which of

course impact all sorts of properties and
then will dictate the application.

Tell you whether you
want to use something for

an electronic application application
a thermal application etc.

So what we seek to do is provide these
fundamental chemistry structure property

relationships and since starting at Tech
we focused a lot on chemistry structure

there's much less done on that side
of things and much more done and

I have this structure what
are its properties but

dictating that structure is
still streamlined challenging.

And so to some all that up really
shortly we've been trying to

build houses with only a hammer.

OK when what you really need
is your entire tool kit and

we have no tool kit we have
a bunch of hammers and so

we want as a group to expand
this too to have all those

tweezers I was talking about to be able to
do things that otherwise could be done.

OK So then the question becomes what
are these atomic level phenomena

we know some we don't know others so
let me start talking about what's known.

What's known at the atomic length
scale is primarily from in situ.

So Francis Ross that I.B.M.

who's been in the in the materials
field doing some I can talk to work for

a long time and her collaborator Gerry
terse off started looking at wire

growth in situ there's some really elegant
beautiful experiments where they were

able to follow what was happening in
real time and they were able to see.

He for example here on
the left that germanium wires

sometimes will grow with a liquid droplet.

But you can also get them to grow
with a solid droplet as a faceted

dropped on the top there was a lot of
controversy about whether that was

that was possible and they said yes it is.

More recently they were actually the group
to to first show that there is a large

flow in these systems and you can
kind of they were following you can

see this year five point nine nanometers
five point six five point three They were

following individual layers as they formed
and they could follow this in the T.M.

in their showing with these arrows
different layers that are moving from

left to right across the interface
it turns out these were

these were solid catalysts it was
the only way they could capture it their

frame rate wasn't fast enough to see what
how fast it grows when it's a liquid but

still they showed this
ledge flow across there.

And then.

The Ross group also.

Related another group were able to show
this really fast any behavior which is

something I won't dwell on too long but
that there is this little tiny facet at

the corner of the of the tri
Junction which is formed and

then dissolved as the wire grows so
each deposition process

underlying each one of these alleged flows
is this process that's happening on a more

fine time scale whereby First
you form this little facet and

then you fill it in and
once it's filled in.

The super saturation leads to
a large flow across the interface so

really beautiful work has
provided some nice insight

into what's going on during
the growth of these.

Yet.

T.M. doesn't tell you much about chemical
bonds tells you lots about structure.

Yeah you can use things like eels
to get information about atomic.

Species but getting stuff
information about bonding is hard

the bond between atoms.

So that's where we come in so
we take the philosophy that

a lot of that structure is ultimately
influenced by chemistry and so

can we get access to the chemistry on
these nano scale structures while they're

being grown that's that's I think
that's a hard task because.

If you want chemical body information in
real time there are too many techniques

that provide access and this isn't perfect
but this is the way we tackle it so

we use in situ I.R. where we have
our substrate here the substrate

being silicon that we
grow wires off of and.

We simply take the I.R.B.M. from F.
to our spectrometer and we pass it in

a transmission geometry through
the array of wires as they're growing.

And we collect the output
of light of them C.T. and

we are able to follow as
a function of temperature and

pressure the bonds that exist on
these materials in real time and

these end up being transit Vaughn's So
if you were to stop this experiment and

then look at the chemistry after
the fact it's all gone there's no

nothing left to see so you better try and
catch it when it's taking place.

So that's what this first time we'll talk
about real time in real time means kind of

seconds to minutes timescale OK
not tempt us economically but

we're looking for these bonds and

the particular story I want to tell that
got us into this is the story of King.

This is a story that's been around for
a while.

The guest nanowires growth
has been around for

a while it turns out people were growing
whiskers back in the sixty's as a as

an alternative way to make single crystals
of material as you know if you don't want

to use easy What else could you use you
could use whisker growth technology and

that was basically B.L.S. It wasn't till
really the last ten years where people say

let's scale this down can we do
anything interesting at the.

And so Wagner and Ellis.

Were the first to show that this does work
and but they're also the first to show

that sometimes it doesn't work quite like
you might expect and you sometimes get

these kicks the wire goes straight for
a while and then it can't and

for a long time and often times that
can happen due to some perturbation

some instability some fluctuation
in temperature pressure and

it just destabilizes the problem so
that can go in that direction a little bit

more recently actually a lot more recently
it turns out people were also able to show

that kicking can happen in a rational way
and there were certain temperatures and

pressures that led to straight wires and
there were other temperatures and

pressures same precursors that
led to King two wires and

this was first time we got into
understanding that there was a fact and

many times a reason or
there should be a reason why

all of them would turn in
the direction that they're growing.

There's another little subtle thing here
this was one of the early hints to us that

when they go straight you see
a bunch of gold droplets or

gold particles in the sidewall So
gold has left up here

hasn't been confined to the drop what
it shows up here when they grow one to

certain pressure regimes show there's
no gold on the side will come so

that's an interesting difference and then
I showed this already but then Charlie

leaders group at Harvard there are a quite
creative bunch they showed that they

could make these kinky super structures
and then they I love that they use these

to probe biological systems I think it's
just really really interesting work OK but

ultimately why this happens is not
understood I can say it's an instability I

can say a lot of things but the underlying
reason why all of this happens

is not clear where it wasn't clear.

Another piece of information.

Came from my background so
my background in surface science and

I've studied semiconductor
services quite a bit and

I knew that when you deliver dye Siling or
silent to a surface

right you don't just get the silicon which
is what you would want in a growing film

you're also get the baggage you get
the hydrogen with it and if you use other

species you get the other Legan's
associated with it those are not invisible

those those exist OK so in the case of
Siling we deliver six hydrogen is for

every two silicon atoms and those
hydrogens are bonded to the surface and

if you want to deliver another molecule
of the silent you need to get rid of

the hydrogen and that hydrogen is
effectively blocking the surface so

the reason we grow silicon films from
silent about eight hundred Kelvin

is because hydrogen needs
to come off the surface.

Now.

What I thought was interesting was because
of the catalyst which accelerates the rate

of silent decomposition we can grow nano
wires at temperatures that are one hundred

one hundred fifty maybe two hundred
degrees lower in temperature.

And so that's kind of intriguing because
the major direction peak of hydrogen

is here and we're growing wires here
these other peaks are due to defects

on the surface won't really
get into a major peak is here.

So what why should should this be
something we worry about I mean we're not

changing per se the fundamental chemistry
of Siling on silicon surfaces but

we are working in a different regime so
is this something we should worry about.

The answer will be yes but before I can
tell you about that I do need to tell you

a little bit about the measurements we
do so we do diffraction limited by our

spectroscopy So we have to be extremely
careful and just Much of the chagrin of my

students you can ask any of them in the
audience after this is over the fact that.

There are colleagues who do in-situ T.M.

can find a couple nice wires that
are growing and image those for

a period time and then build up statistics
from individual wires they wish they could

do that but we have to have control of the
growth over macroscopic length scales so

that we think we're measuring
what we think we're measuring So

this is actually can which can be
challenging and we tackle this

a couple ways the first thing is that
we want to maintain nano wire density

as a function of temperature and
pressure anyone who's grown nanowires and

very temperature and pressure will
know if you start without wires and

you try and grow at different conditions
you'll get different densities and

the ripening process the film breakup
process all changes how the wires

initially grow so what we do is we say
we're going to have an incubation step

a first step where everything is constant
We're not changing it no matter what we do

with temperature after the fact so
we do this this is constant so that every

time the density in diameter of our wires
is more or less the same which allows us

to compare then between
different temperature and

pressure runs so
then we do the long geisha and

this is the step where we're very
in temperature and pressure.

OK and
trying to follow the infrared signal.

And so just like I said from the prior
literature we saw the same thing we

saw the fact that under certain conditions
which I'll tell you about the minute

you get these one one one gross where

the majority of wires grow vertically off
the surface and in this situation you have

a combination actually of facets on the
water of the wire There's one one one and

one one three subtasks going to
sawtooth fast in the field so they're

not vertical side walls they actually
have quite a bit of structure to them.

And then when we change the conditions you
can see that a lot of them start to change

their growth reaction so many start
to grow in this one one two direction

we lose the fasting that we saw here.

And they all sort of go off in
this direction so so we were seen

the same thing that others have
been seen so that's not new but

I think what was new was our infrared
data so what we started to see and

I'm showing absorbance here is a function
of wavenumber right in the middle you

are right we're stretching is OK so
is the SIE.

And these are two different pressures
while cold low pressure and

whether you agree that that's a low
pressure that's debatable of course

I'll call that high pressure and
whether that's high pressure

I'll leave that to you OK in our world
that's kind of low that's kind of high for

purposes of this argument I'll use that
and then here you can see as we change

the temperature both of these
plots show decreasing temperature.

And what we started to see was the
emergence of these absorbance bands and

bands in the sci region and
they got more and

more intense as the temperature went down
and this happened for both pressures but

it slightly shifted temperatures OK so
you can see if we pick.

Here at four forty at low
pressure showed fewer or

weaker modes than four
forty at high pressure.

But we were seeing this via
these real time experiments and

I guess I'll reiterate if you were to try
and look at the wires in the system not

take it out but look at them in
the system after this growth is over and

you return the pressure back to
the base pressure none of this is seen.

So we looked at this a little bit more
quantitatively we look at what is

the growth direction ratio how many one
one two one one one wires we have as you

go up this axes there's more kinking
relative to the integrated Peak area that

we're seeing in the I.R. and you can see
pretty clearly they match each other so

the dotted lines are the integrated
Peak area and the solid lines or

the growth direction ratio
tracks quite nicely for

to the minus four not quite as nice for
ten to minus three.

But we thought that this was was fairly
strong evidence that there was this

relationship between integrative Peak area
actually per unit nanowires length that's

an important point and growth direction so
the more hydrogen we were seen the more

wires grew in this one two directions so
this was kind of intriguing to us and

actually at first we said this is really
weird because this is a surface chemical

effect and these wires are one hundred
fifteen and a meters or so in diameter

it's quite large you know at the outset
I would have said you know five minute

meters ten minute meters I expect surfaces
to become more important but this is

suggesting the services are quite critical
even at these larger length scales.

So if we look at what I'll
call a phase diagram for

Growth direction pressure as
a function of temperature.

Is it that we think it's
fairly straightforward and

that OK if I have high temperature and
low pressure

the high temperature allows that
hydrogen come off really rapidly.

Because H.

to these option is very fast the higher I
get temperature pressure the lower I get

pressure the lower the amount of hydrogen
I'm delivering in the first place.

So effectively I don't have hydrogen and
that's what I see in the I.R.

for these high temperature
low pressure conditions and

I get one one one growth in those
situations that go straight.

Here at high pressure low
temperature now H two

is these will be much more slowly and
I'm delivering more hydrogen with each

we've each molecule just
delivered a lot of hydrogen and

that's where we see one hydrogen on
the side walls of these materials and

also growth off into
this one two direction.

One thing I'd like to point out is that
those two peaks the two absorbance Benz.

We can attribute to different facets on
the wire so I mentioned we know from T.M.

that there is a combination of one
one one and one one three facets and

if these are planar studies now
these are not nano our studies but

if we take one one three and one one one
substrates and actually one of the great

things about silicon it's you can get lots
of surfaces actually is really a really

good to start with silicon because there's
so much known about the surfaces and

even then these pressures and temperatures
are way different than what people usually

at least in the surface science community
have paid attention to in the past so

we were able to distinguish the blue and
the red between one one one facet and

one one three facets so we think those
absorbance peaks the two combined

in the Nano wire experiments are from
the two different facets that are on these

wires so it's actually pretty intriguing
for future studies in that we may be able

to distinguish which facet Well we can in
this case which facet an individual atom

is sitting on and
that may be useful in the future.

OK I'm going back to this for a second and

the people in the audience are saying
well yeah that's that's great OK So

you see a nice correlation between
hydrogen coverage and growth direction OK

fine but how do you know it's
the hydrogen that does this.

You could argue that there's some
other effect that's happening and

that you're creating something different
in the growth such that you're favoring

the Hydra becomes or

has a different stability after this
happens what we don't we don't think that

was the case because there are still one
one one one one three facets before and

after kicking so it's just a matter of
whether they're sawtooth faceted or

more smooth looking the fast identity
didn't change too much so to us it didn't

seem to make sense it seemed to us to be
that hydrogen was doing this but still

there was no definitive proof that this is
actually happening so we set out and then.

Set out to think OK how
can I prove this and.

This is a great example of how
it doesn't take a lot of money

to have a definitive experiment.

So what we did was we started
with the growth conditions

that would always give us one on one
kind of the straight growth no hydrogen.

And if we just continued that
the wires would continue to grow.

OK fine nothing to earth shattering
there now if we do this a different way

we grew a short part of the wire first
under these one on one conditions.

And then we didn't change temperature and
pressure but

we added a flux of hydrogen atoms and

the only way we could think of that we
didn't deliver another atom with it so

we thought about putting in other
species to manipulate the growth but

this was the cleanest we weren't changing
the super saturation of silicon for

example just adding more hydrogen
atoms now it's still distinct It's not

just silent chemistry
these are radicals now but

we built a two hundred
dollar tungsten filament.

Where we just had a tungsten
filament we flow in past it and

we crack a couple percent of hydrogen and
we deliver that to the substrate and

when we do that all the wires
go off in this direction.

And.

That to us was very nice confirmation that
in fact hydrogen was the root cause and

not just hydrogen but surface chemistry
and that that's an important point surface

chemistry this data suggest that surface
chemistry is a new avenue to manipulate

these structures.

We can do this in another way to try and

convince ourselves that
it still makes sense.

This is not quite as clean but

I still think it suggests very
similar effects so here are.

T.P.D. data very similar to what I showed
before on a clean silicon surface but

here is a silicon germanium surface
where the percent of germanium on

the surface is going up and so what you
see as you ramp the temperature at higher

germanium compositions is that the these
options peak decreases in temperature so

now hydrogen because of the presence
of Jermyn at a lower temperature.

Well so that would suggest if we were
growing wires in this one two direction

and we had a little bit of Germany we
should change the civility of hydrogen on

the surface that was kind of the idea so
you can see here we grew these wires that

one one two we added two percent and
they all came back.

And we think that's effectively the same
effect I was just describing surface

changes in surface energetics that
are favoring different facets.

Not is clear because we are also
putting germanium into the droplet and

that does change those
conditions a little bit but

I think it is suggestive
of the same thing.

So you can start to think about taking
this a step further where you have.

Some simple superstructures where
you can get and then kick it.

And here's one with a Chatham Zen Here's
one with Germany and

I'll point this out the one of the biggest
challenges with with this is the crystal

doesn't care which direction in which of
the many degenerate directions it goes

back to to it they're all the same so
if you want to maintain for

example the same one to the exact same
one two that's a little challenging.

Just to show you what this looks like
across the substrate I mean they all

they all do this.

And the hydrogen atoms is a gradual
process it's not quite as abrupt as you

see with other chemistries but
they all do this.

And so is the engineer.

He then says OK we have some new
fundamental understanding this suggests

surface chemistry and primarily changing
the civility of different facets on

the sidewall favors these
new growth directions.

What can we do with it.

I used to like these when I was a kid and
they must still make them.

So.

My other student Eldar has been playing
with this for a little while now and

he's been doing this in the germanium
system so the nice thing about germanium

is that it's not quite as reactive it
sidewalls or not as reactive a silicon

silicon bonds with or at the world has
lots of silicon in it for a reason.

So germanium is a nice material because
species we may deliver aren't as likely to

completely decompose on us so we get some
flexibility there by studying germanium so

Eldar we would normally grow with
Jermaine instead of silent you go with

German here and grow these straight wires
you can see this little bit of tapering

here that's because there's some German
that deposits on the side wall and

as the wire grows you get this taper.

And you can really get them tapered
if you raise the temperature.

And this is actually been a limiting
factor of really getting robust so

it can germanium hetero interface is
the difference in growth temperature

of these two species.

If I take the German anywhere near
the asylum conditions I start to get this.

One Eldar was able to show was two facts

the first effect is that if I add
another species not not hydrogen now but

a molecular species something that would
be more amenable to normal C.B.D..

After a short growth with just Germany and
the Met the methyl German is out of here

he can get them to kink at low temperature
so this is another route to kinking.

A little bit more robust.

From a process point of view and
that at high temperature he gets

straight wires with no tapering.

We have the same thing here no tapering
but the wires can't so they can't and

stop tapering and here they stop tapering
you can see if you look really closely at

the base we have these tapered bases which
was the German only growth followed by

these tapered structures at the top.

And so really simplistically and this was
actually done X. that you so we don't have

in situ data for this and unfortunately
the difference in pressures between

traditional C.B.D. and what we do in our
you view system are sufficiently different

the flux of species is different that it
changes the temperatures and pressures and

the surface species that you would see so
in the ideal case we want to go back and

look at this and say OK why is it is
that one of the species that exists with

a metal groups or they Methylene does the
metal decompose we'd like to get access

to that it's proven harder than I
would have liked and I think that's

just because there's a widely different
fluxes at a traditional C.B.D. and.

Type system but more lasts OK so
we grow with pure germ ain we add

methyl Jermyn and
at high temperature which was

the straight one well it's decomposing So
we're putting some sheath around.

The wire and in fact we think it's mostly
it's got to be mostly carbon related

carbon so you build it into
a mania is extremely low so

it doesn't really want to be in
a lot of us so the surface and

it's robust enough to
prevent the tapering.

We think at low temperature a similar
effect is happening far from the far from

the growth front but at the growth front
metal German is some species of methyl

German is stable for long enough that it
is like in the case of silicon favoring

new facets so it's favoring
growth in this new direction.

That's our current working theory
about the underlying mechanism here.

And I think that I think it's.

Pretty decent starting point but we'd
really and we still want to get some some

more detailed information and I have
a student who's working on that now.

Again what can you do with this or

started to play with some interesting
superstructures where we turn on and

off metal German and so
rather than those kind of.

Slowly curving silicon wires here
we can do it very sharply on and

off and because the the.

Because we're going now
between one hundred one and

one one zero there's a couple
different angles we can create and

we can create three different
angles thirty five sixty and

seventy which opens up new
opportunities to make or

achieve properties based on the angle
of the different segments now of course

you say it sitting there saying
that this is that's really nice but

again you need to be able to choose
which Crystal a graphic direction

of the degenerate ones it goes back to and
that is a real big challenge and

someone else in my group is working
on that immense challenge there to my

knowledge Charlie has been able to
do it in a very narrow window but

it's it's unclear how to do that
with different angles and so

we're working on that right now as well.

Just briefly I want to mention
another use of surface chemistry so

we started to see this in
the metal German and that was.

The following so here again you start
with just German these are tapered again.

And if I add I'm sorry I'm going to
add a mystery species OK This hasn't

this hasn't been submitted yet so I'm
going to call it mystery species X. but

if you add another species that
interacts with the sidewall but not so

much strongly that it affects
the facets at the triple phase line

you stop the tapering immediately
upon adding that species and

these temperatures are much lower
these are three seventy five

versus what I showed a minute ago
it was more like five hundred.

You can start from the beginning
with germanium and species X..

And this being videotaped too so
definitely species X. and.

You get an tapered wires all the way
through even though you would get

paper wires normally.

The cool thing about this is that like
in the King where you can turn it on and

off you can turn on and
off the side wall terminations and

so it's a little bit difficult to see
here but you can see the undulations in

the wire those undulations or user defined
those are in an instability we can tune

the period this city of them we can
tune their space scenic cetera.

By just turning on and off the species.

And so what we hope to do here is to
make complex super structures that

may interact with light
in different ways or may.

Affect thermal transport
different ways I don't know but

I think there are some interesting
opportunities as we gain a better control

of the Synthesis here.

Missing a salon.

Or.

Check real quick I apologize there's
one slide that I might have moved or

deleted or I don't know to.

OK it's gone.

I want to show picture some more S.C.M.
pictures where we can tune and

get one home to homes three I
mean we can really two in that

the challenge here is how
do we control the extent

how to control the shape of the sidewall
how do we do all of that and

that's again going to be through
understanding of the growth chemistry What

are the species that are there and
we're continuing to unravel that OK so

I must get to this the take home message
is I'd like you to walk away from here

our takeaway from here our
surface chemistry is important.

In a wired world.

It's important in a crystal synthesis we
know that and I think our work has for

the first time going to shown how critical
the same thing is for synthesizing

nano art and I'll point out that these
are three dimensional materials and

I wrote this in the abstract they are not
the same as two dimensional films so for

a long time we have focused on two
dimensional film deposition for

the semiconductor industry but
you need new species to do new things if

you hope to create the three D.
complexity.

We're interested in we leverage in situ
infrared spectroscopy is a workhorse in

our lab for
trying to unravel the chemical bonding.

That's going on on these systems and
now we have some insight that allows us to

ab initio to design more complex crystal
structures and superstructures work is

ongoing as we continue to unravel these
things I'll point out this work which.

I think is pretty great where we have
rationally through control of surface

chemistry put in both stacking
faults into wires and twin planes so

these are four thousand planes in a single
nano wire where the lot of rotates.

Sixty degrees each point so that's pretty
intriguing again another way to control

the properties of these materials
through design of the Synthesis.

OK And I think there's a number of
applications here one the B.L.S. process

is widely used to make all sorts of better
wires three fives group forks cetera so

I think a lot of this insight is important
in the other fields as well the three five

community I'm pretty confident is sees all
these effects but doesn't doesn't know

they're seeing them and there's a lot of
applications it's not clear to me exactly

where this will be useful whether it's
fundamental study or whether it's

ultimately going to be an application but
to us it's pretty intriguing.

Exciting so.

One more reminder Eldar who are both in
the audience did did did this work and

like to thank again
the funny the new season and

you for your attention I'll be
happy to take any questions.

Thanks.

That's right that's right passive
in-service the exact structure of

the passivation that's that's what we want
access to we want to know is it a myth

is that array of methyl groups that's that
they're blocking it do those metal groups

decompose and it's a combination of carbon
and hydrogen or whatever that will provide

really much needed information as far
as what we should pick in the future so

that we're not kind of going through
a blind guessing game about which species

could be interesting we want to know this
may deliver these method routes which

seem to be stable under these conditions.

So if that is your question

yeah.

I don't know you mean on one wire I
mean like sort of back not up and

down you mean near the triple phase line
around it I don't think it's necessarily

favoring one fast I think there's a set
of facets that are becoming favored.

It's not just diaspora ones.

It's a noose a collection of facets that
are favored under the flux of hydrogen and

that we know there's a lot of one one
one there's these big broad one one

one thousand that emerge and we think
that's because an hydrogen is case

it makes it prefers the close packed
surface so hydrogen can make beautiful

terminations on one surfaces and
so hydrogen selecting we think

one to one facets that drive
it off to this new direction.

We.

Welcome you and

I don't think we have any information
directly about how uniform is I.

Mean I think inherent actually in the
hydrogen flux experiment because hydrogen

is reactive that is going to may
have some directionality to it but

we are now we see the same
effect with temperature and

pressure and only die silent as we do
with delivery of hydrogen atoms so

I see your point I see your point
think it's a good point but

we don't have really any information
about the distribution kind of around

the radial fraction of water.

That's this that's a great question so I
think that varies depending on the system

so in the gold's in this you mean the
silicon system I've pointed this out early

on and come back to it I guess was
the goal of loves to wet silicon.

So gold will reconstruct silicon servo
reconstruct on silicon surfaces so.

That's different from germanium for
example were gold doesn't wet it quite

nearly as well the driving force for
Germany is much lower and it really varies

catalyst to semiconductor how strongly
the driving forces I think there's

probably some systems the germanium one
in fact where you see tapering the reason

you see tapering is because there
are bare surface bonds there.

Right even in the case of point it's
not even the case the silicon wires

you bring up a good point.

Here.

Right even in this case I'm under
conditions where there's no hydrogen.

And I see an tapered straight growth.

Whereas in the drain
case I saw taper growth.

And this is because gold West the sidewall
So gold at some level acts as a passive

agent it's not perfect but there are gold
there's a gold wedding layer that

sits there when you add all
the hydrogen it directs the gold.

Now in this remaining case gold doesn't
want to go there nearly as much in

the first place so you have more dangling
bonds that's exactly what they're dangling

bonds and so then the next German
molecule comes in as orbs decomposes and

now you're getting this tapering.

That makes sense OK yeah and it was.

In the picture.

Or you know for.

Sure.

That.

You mean in the face yes yeah that's
not that's not I mean I think I

think about it I mean here is a step.

So yes we do see a gradual shift in.

Yes So actually my student always tells
me don't show that plot he always

says this plot is misleading because you
draw this line through the middle but

I yeah that's a good point so this data
shows that there is an increasing number

of King two wires as
the temperature drops.

Thank you.