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Today we start
the section of our course
in which we will study the possible
molecules that we can use for building
our Nanodevices for sensing
or for nano and molecular electronics.
We will start with two lessons
in which we will analyse
the conductive molecules. My first lecture
will start with an introduction and a recover
of some concepts that
were explained in the previous lecture from Professor Piccinini
and we will talk about the
again the metal molecule metal structure
and then we spend some words
talking about the contacts
and we will talk about Nanogaps, and about these
contacts with the Nano distance
will be also a matter of laboratory work
we will do later.
Then we will go deeply
in understanding which
of molecules we could
need for our application and maybe
we will talk about organic molecules, so
the hydrocarbons, in particular
Benzine and we will see
how we can take these molecules for building
molecular wires.
Then the handling of this first lecture
will be focused on analyzing
the diffrent families that are now used
in literature and the applications.
Well, I want to have some
points that were already touched by Professor Piccinini
why we need to put together
electronics and molecules.
From an application point of view
the very interesting one of the first interesting points is that
we can produce them at low-cost.
Low-cost means that now we are in the
step of development of this application
that is related to research so
it is correct to say now that the applications and devices that we
develop now
are prototypes, but
the technologies that we use, the processes that we use
are simple at the end, and so
it is true to say and it's possible to say now
that when we will reach a final device
it will be applicable using simple
not too many
processes, not too many complex processes
and so this means they can be produced
in a low-cost way. The application of
more electronics plus molecules
are several we can talk about
electronics optics already here
all are already in the market, and also sensing.
The technologies
that we will see are derived by
the standard microelectronics
technologies, because it is a perfect
choice to take, the
already well-known microelectronics applications
to be used as an interface
to contact and to manage
the information that you have from your nano device
and it
is clear that one of the
great advantages is that we are talking
about nano dimensions so we reduce our
dimensions and
this was a concept already touched by Professor Piccinini.
We must think that it's important what we lose maybe
and try to forget something about the
the old, I can say applications based on gating based on transistor
concepts. They are important
of course. We must continue to think about that but
we are opening and now it's open a new possibility
using nano dimensions, so
the effects in physics that these nano dimensions have
as impact in the
properties, in the functionalities that we can obtain.
So it's time also to think
to set up completely new
ideas, completely new structures.
For understanding
the topics in which I want to
go into details today we have to consider mainly
four steps, four
things that we have to
exploit and to to study.
First of all you already started in the previous lectures
to study which are the basic principles
that leads and generates inside
a structure the molecular conduction.
Then we must decide which are the good
molecules for us for building molecular wires,
and then it's important and I will push
on this concept the importance of the
contacts, so to study and to find
good solutions for building
effective contacts for our molecules,
and, last but not least,
all the studies and the analysis related
to the simulation, but then passing to understanding better
what is happening in our nano device.
We start from a structure that you
already know, so the metal molecule, metal structure,
because this is the brick that
we need to set up for building
let's say our house, in which we will develop
our sensors,
our new nano electronic and molecular electronic
devices.
This slide is well known to you because you already
had it in the previous lectures from Professor Piccinini
and you see now I want to recall
the basic concepts that are useful for us
to continue and to apply them
for creating molecular wires and molecular
devices. You remember that
this structure is based on on two metal electrodes.
One can be considered the donor, the other can be considered the acceptor
and
in the middle you put your molecule.
In the equilibrium
you have all the energy levels, the electrochemical
potential, Mu 1 and Mu 2
the fermi levels that are all at the same level
and so in this case at the equilibrium you do not have a current inside your
structure. But
one of the first important points that we have to touch
is what is related to the Gamma
that you consider that was defined before
using a Tau, so
the H bar on Gamma is the time that the electron needs to pass
in the interface molecule metal.
I took these slides from a nano hub.
There's a very interesting website
so I suggest to you to dip inside it,
where Professor D'Atatab(?) use also
this concept so
the inverse of the what Piccinini called the Tau
is Gamma on H bar, but if it was the time to pass the
it becomes a rate,
so it becomes what is named the escape rate
Gamma on H bar over
electrons inside our structures.
I'll show you this now because
this Gamma will be
one of the key points for having and obtaining the currents
and will be one of the parameters
who will the give us
the numbers, give us how
the conductance and of course the
current will behave inside our structure.
You know that for inducing a current
inside the MMM structure
you must move
the level of the Mu's. In this case
we are applying a negative voltage to the right electrodes
so apply a negative voltage, the Mu level goes up
and in this case we have the first
simple example that if
the energy level, one of the
free energy level of the molecule
it is inside the
levels Mu 1 and Mu 2
in this case we start, you see here,
to have a current. This is a very simplified
model, just to start to understand what happened.
You know that
after it we must consider
that in the structure, the model
and the situation is more complex, so we have a broadening
of the energy levels, and about this broadening of the energy levels
I want
to put my finger on these two
equations, that give us the possibility later to
use them for understanding
the current and the conductance in our
molecules and devices. The first,
this one, says to us what
I already started to tell you
you see in this
equation we have that our
current is related to the states
that we have in our molecule, to
the Gamma's of the two contacts and the difference of the Fermi levels
of the two metal electrodes.
what I want to point out is that you see
that the current, it depends
directly to the Gamma's, so, it depends
directly to the quality
of the contacts, it depends directly
to how you build and you create
your contacts. The second
equation on which I want to do some comments
is better, that becomes from the Landauer, from Landauer studies.
Here you see that
Professor Piccinini introduce you
the transmission spectrum
that is related to the probability to have
a transmitted electrode between the two
electrodes. And in this case you see that the current
is proportional to the integral
of the transmission spectrum
we will see, integral means area,
so, studying
some areas, and we will do it later, we
will have also an idea and direct
information of how much is the current
inside our structure.
Last thing that I want to
recover and recall from what you did before
is that applying of the equation I show you
you obtain what is named
the quantum conductance that is terse
that becomes from 2q squared to h
that if we do a calculation
has a value of 77.6
Micro Siemens, that means a resistance of about
13 Kilo Ohms, 12.88.
This is the basic conductance
that you have your last, we can say,
molecule inside your
electrodes, and so we will use this
in the lab when we will build
the nano gap inside a wire
because if we reach,
if we reach this value
means that we reached the last
possibility to have something that interconnects the two
electrodes. And so it's a sort of threshold.
After it means that if we
break it we put out the last
molecule inside our electrode, so means that
we create our gap.
We are talking about, already started to talk about Nanogaps, just
few words about them
because now it is, I hope, more clear
that the contacts are important, so the way
how to do the contacts when you make
molecules is fundamental.
A Nanogap is defined as a structure
with a gap within, between
the two electrodes, of less than 10 Nano Meter.
you can imagine if you want to introduce your
molecules, and the length of your molecules we will see,
depending on the application, is from 1 Nano Meter
to 10, also if we talk about sensor
and so we pass to bigger molecules, to larger molecules
we can reach also larger dimensions, but
it's important to build
breaks, to build gaps inside the electrodes
of the most controllable dimension.
and, now I can think that you
already start to have idea of where
we can use these Nanogaps, mainly
in Nanoelectronics, because
they will help us to build our new Nanodevices
for going,
going farther, going
to advance from what the technologies
we have now, but mainly the CMOS technology
and they are also useful for having,
for using them as
sensors, because if I create a gap
of a dimension of a molecule that I want to detect
it's clear than I can detect the presence of the molecule
also because we have seen and we will see some
AFM images in the gap
there is a sort of good point of lower energy where
the molecules
have a higher probability
to go and to stay inside.
And so, as you see, here we have our two electrodes
here we continue to call them source and drain but I told you,
if you want to, you can also a little bit leave out
these concepts of source and drain, and
here inside we put our molecule.
in the literature and several applications now that
you can find
around the research in the world
there are mainly two possibilities to build them.
One is to build them vertically,
so in this case I took these slides from a very interesting seminar
made by Professor Lugli of
University of Munich
well you see here, you build a
multilayer structure but the at the end for
our purpose the point is here, where
you create, doing
an etching, a standard etching process
you etch
the Gallium Arsenide layer you see here
the point
here, you etch the Gallium Arsenide layer
for having here the gap,
vertical, in which you can insert your molecules.
You can do them also horizontally
in this case one of the interesting applications
is this one, where
you create mechanically
your gap, so, using a piezo actuator
on the bottom you can press up to
break with the Nano dimension
your, you have in this case you see
you have a wire in which you create the breaker
OK, pushing from the bottom with your piezo actuator
and in this application they reach about
100 and 250 Nanometers
or so
they started to
to use wires of these dimensions,
the width, OK, the width of the wire
is about 100 and 250 Nanometers
what we do here in Politechnico, we
are producing horizontal Nanogaps
and we produce them using electromigration.
This means that electromigration was,
and it is a problem in microelectronics because it breaks
the wires inside the chips.
In this case we exploit it because
inserting the current inside the gap
in a controlled manner
we can break the wire.
In fact here you have some example.
Here you have a wire in which doing a
good control of the process you can break it,
obtaining the Nano distance between
the two gold electrodes.
And it is, you can imagine, a very
simple and low-cost technique.
Simple means, I'm working on it so I can tell you,
not simple in doing it and obtaining a result, because we
are studying this problem for almost two years,
because the problem is not to have to
create big holes but to create small gaps, so
it's very important to control. But after that
you have found the good parameters
of your process, at the end the instrumentation that you need is not so
complicated.
OK, now I gave you
the starting points, starting concepts,
from which we can now we leave
a little bit the introduction
to the problem and start to think about molecules.
The molecules that we use
inside the our
electrodes for building these
new Nanodevices are mainly hydrocarbons.
Hydrocarbons are compounds
mainly based
on Hydrogen and Carbon atoms.
They are classified in two different
classes. They are classified as saturated
and unsaturated hydrocarbons.
The saturated hydrocarbons
are named Alkanes
and the structure of Alcanes, from this example of a
Pentane molecule, you see, they are
open chain, OK, made of
Carbon atoms, the grey ones here,
and the Hydrogen atoms around
of
Hydrogen, so they build
these chains. And in
this case there one important point to remember
is that each hydrocarbon
has the maximum number of Hydrogen atoms
that we can have per Carbon atom. This is
because each Carbon has
a single bond to
to the other Carbon atoms.
So it means that it has one, you see,
in this case, one to this Carbon, one to this Carbon
and you see the balance of Carbons
who are completed by
two Hydrogen, you see, two
Hydrogen atoms,
in this case in the left and in the right.
There is no Carbon atom here so the balance is completed by
another Hydrogen atom.
A general formula of Alkanes
is CnH2n+2.
Which are the characteristics
and the properties of Alkanes that we
want to understand
for having the possibility to use them in our structure.
At the end they are almost
isolants, so they have a very
low conduction and we will,
you already had some course talking about Benzine, we will go a little bit
in detailing the Benzine molecule
because the
possibility to have an exchange of electrons inside
your chain, inside your molecules, it is based on more,
on different structures. So,
in this case to have this long chain means that we do not have a very
high conduction. They are also used for some purposes
and in this case I introduce you
the concept of the Thio,
so you see here you have the your,
this is a Decane, an Arcane
based on ten Carbon
atoms, so it's a Decane, and in this case it is a Decane
Di-Thio, means that on the left and on the right
you have to solve for atoms
that make the Thio
bonding, that it is a very strong contacts
with gold. So you can imagine
we will find these Thiolated molecules
everywhere and we will find almost
in each of the molecules, we will
we will analyze
the possibility to have a bonding between the molecule
and the gold electrodes using Thiodes.
Just to start to put down some numbers
we can say about that Alkanes have
an ***-Lumo energy gap of about
8 electron volts and now we will
compare this number with the other molecules, then
we will analyze during this lecture.
The unsaturated hydrocarbons
are Carbon,
sorry, hydrocarbons in which
the Carbons between them and more than one
bond, at least in one place, one or
more than one bond, and
we have Alkenes that
in which the Carbons have
one or more double bonds
between them, and the general formula is here
CnH2n
Cn, sorry, H2n.
We have also a, sorry, and here
I show you as example you see an Ethene molecule where you see here
we have a double bond
or a propene molecule you see here the double bond
between the 2 Carbon atoms.
We have Alkynes. The Alkynes are
hydrocarbons in which we have a triple
bond, at least one, one or more triple bonds
and you see you have here Propyne where you have also
a Chloride,
Chloro, in this proposition and here
is a Pentyne and where you have here the
3 bonds in the Propyne, the 3 bonds are here.
So this is why these 2 molecules
are Alkynes.
And the general formula of them is
CnH2n-2.
Let's pass to the most interesting structures for us.
The most interesting hydrocarbons that are the Cyclic structures.
The Cyclic structures we can
have: CycloAlkanes.
So, sort of interconnection,
connection between the two ends of an Alkane, so you build
these Alkanes.
But, more interesting even,
are what are named the Aromatic hydrocarbons, or
Arenes, that
they are
aromatic ring where
you have an alternation,
and this is one of the key points, of double
and single bonds between the different
Carbon atoms. And I
show you here some example, the Benzene, we will
go in detail soon in this molecule.
Toluene, you see Naphthalene and others, you see, you have
these rings with
single, here for example, double, single, double, single, double and
Benzene has this structure.
OK, so the Arenes are Aromatic
hydrocarbons. Benzene,
Professor Piccinini alright told you and examined,
and analyze the structure. I just want to recall some points.
The formula is C6H6
and the Benzene molecule
has this alternation in the
double and single C bonds.
And the special property
of Benzene to have what
you have seen, these pie bonds
inside the structure as a strictly,
of course, relation with the distance you have
between Carbon and Carbon.
In fact the distance
is in the middle, between
a single bond and a double bond.
This means that effectively
we cannot say that between 2 Carbons we have 1
bond. But we cannot also say
that we have 2 bonds. We have
a situation in the middle, and this is
the key point.
The key point, as you have seen,
becomes from the fact that you have these
sp2 orbitals,
between Carbon and Hydrogen.
On the plane between the C atoms
you have sigma bonds.
The p orbitals that are perpendicular
to the plane of the ring are
interconnected together using
these Pi bonds.
And, these Pi bonds
generate a sort of
resonance structure in which you have
two equivalent structures that are in resonance.
One you see with the double bonds in this direction
and the single in these positions, and the single in the others
and, very complimentary we can say,
situation. These bonds
create this sort of circle,
because again, I told you, the distance is in the middle
between one and two bonds, so it means that they are not one nor two.
But, instead of having separated orbitals here,
at the end, the final
orbital structure of your molecule is a ring
inside, and now we have the key point,
the electrons can move around. So
you have this resonance
becoming from this hybrid situation
where that electrons can move.
And so, you have delocalized
electrons that that generates
a sort of cloud around
the ring of Benzene
and, if they can move
equal, we can easily
have a current. The are ready
to move and to give us the current
that we are looking for.
So, the Benzene molecule
is the perfect candidate for us, to become
the basic brick of our
molecular wires. In fact,
for example, I give you here and now we
will enter in detail soon, of
different families of molecules that we can
build for our applications.
Here I show you where you have this
chain of free
Benzene rings, here you have detail,
so the Sulphur on the left and on the right,
and so if you put this molecule,
these molecules inside 2 Gold
electrodes, you can generate a current
inside your molecule and inside your
M - metal, Mem -
molecule, M- metal structure.
Which are the characteristics of the currents
that we have inside the structure? We have
2 possibilities, and I will explain a few words later also on it,
we can have the possibility to have a current
based on a tunnel effect, so
you have your electrons that passes
inside, through a barrier, and so
you have your tunnel current. But,
with these molecules we can also have
Hopping, so electrons that take enough
energy to jump, we can say,
from one ring to the other,
to move inside
our molecular wire.
OK, now these are the basic concepts -
Benzene, rings and delocalized electrons.
Let's go in details in showing you
which are there most used families of molecules
we can have. The first family
is what is named the oligo(phenylene-ethynylene)
molecules, where we have
more than 3 rings
of this basic structure
that is based on
Phenylene, so
Benzene without 2 Hydrogen
atoms, is one, plus an Ethynylene,
so, a chain based on
2 Carbons that have between them
free bonds. And so they have the fourth
bond free on the right and the left, for
adding the interconnection bond
with the outside world. And,
this is a derivation from the
Ethyne, because the Ethyne is based on this
molecule plus the two Hydrogens
on left and right.
So, we exploit the fact that
these chains are named
conjugated chains because of these delocalized electrons
that we have in the Benzene, and
so we can create, as you see here,
which is a
OPE with free Benzene
rings, Thiolated in the middle, and
here you have Sulphur plus another
group. The other group is Acetyl.
Acetyl is not needed for the functionalities
of the device but the Thiol group are very reactive.
So, you cannot the have your molecule
with the Thiol groups free
to react with outside world, else you have immediate
reactions. And so you protect
these Thyiol groups using Acetyl.
Before having the bonding
of your molecule all your Gold electrodes
you use some chemical solutions
for detach the Acetyl
in place close to electrodes, so you put out the Acetyl, so in this case
the Thiols are ready to attach,
to bond to the electrodes.
Thanks to these delocalized electrons
the OPE are good conductors and you see
the ***-Lumo gap in this case, you remember for the Alkane
was 8, here is
3.5 Electron Volts.
The second family that we analyze are the OPVs of the Oligo
Phenylene
Vinylene. These are based
as a basic repetitive structure on again
Phenylene, but here we have the
Vinylene. The Vinylene is obtained
taking out 2 Hydrogen atoms
from the Ethene.
So, we can build
chains based on this structure, again you see
you have the Phenylene, the Vinylene here,
which is a chain again, based on
3 rings, and
again the Sulfur for the Thiolated
bond, and here the Acetyl for
protection. One interesting
property of OPV is the fact that
thanks to the Vinylene structure
they are more planar.
And the triple bond of the OPE,
triple bonds of the OPE,
blocks this alternation of single, double, single, double
bonded bonds. That is
the key of having this delocalization of electrons.
So, we can conclude that
becoming from these 2 properties
of OPV, the OPV are more
conductive than OPE.
And in fact you can immediately see
from the ***-Lumo gap that is about 3.1
3.1 Electron Volt and we can say that
having similar structure, having similar
length, so number of rings of OPV
or OPE, we obtain
one order of magnitude of increase
in conductance.
Last but not least, the Oligo
Thiophenes, Oligo Thiophenes
are molecules based not on Benzene but
on a similar structure that is the
Thiophene. The Thiophene is a ring
in which you have 4 Carbon atoms
and 1 Sulphur atom all bonded to create
this basic structure that is a ring, and
again, for the Thiophene,
the game, we can say,
is the same. So we have again the P orbitals,
we have Pi bondings, and so
this structure has again a ring where we have delocalized
electrons.
And so, in this case, I show you this
ring with some other molecules and chain,
Alkane chains in this case is a molecule that I show you because
it is the one that we will use doing some
analysis of some experiments that we have here.
We use this molecule,
so this Oligo of Thiophene chain.
And, at the end, we can say that this
structure is even more reactive
than, and stable than the Benzene
molecule. And so we obtain again
an increase in conductance.
Just to summarize what we have seen up to now,
we have now
the possibility ????
the Alcane so you see
the length, and the length is important because we know
which must be that this much dimension
of the gap between, the distance between
the electrodes that must use
the molecule. So you see we are talking about
10 to 20 Angstrom.
And here is the ***-Lumo gap, about 7-8
Electron Volts or maybe a little bit longer.
Of course, we can also, the basic structure, but we can also create
long chains, but, this is a point that we will touch
in the next lecture, the length
of the chain, it's strictly related and is proportional
to the conductance and to the current we can obtain in our
device. So starting from the basic
OPE so with the
3 chains, 3 rings if the distance is about the 19
Angstrom, with the a 3.5
gap, 24 the OPV, we are having
3.1 and the Terthiophene, so
for doing this comparison of same structure same length in
3 rings, we take 3 rings of
Thiophene, so the third Thiophene
the length is 1.4 Nanometers, so 14 Angstrom
and the energy gap, the ***-Lumo gap
the ***-Lumo gap is 2.9 Electron Volts.
Now I quickly show you some
molecules that we found in literature,
showing that, I just
touch some basic example, but
these molecules can be change and we will soon,
we will see soon the possibility to create a sort of PN junction
between molecules, so
they are also changed, adding some external
change or some other molecules for
giving them different properties. So, you see here
some
OPE,so
some different OPE, types of OPE
again here we have other OPE,
or here we have, for example,
a third example of Vinylene, so OPV, so you see, with a different...
Well usually the termination, you see, are
again Sulphur and Acetyl, and in the middle
you have these different molecular wires. I did not
talk about ?????, that is another
interesting structure to be used
in our molecular wires, and here
you have some examples.
The interest of these
molecules, it raised in the last years, and in fact now
I just give you an example
of a catalog. In this case we are talking of Sigma-Aldrich
where you can find and buy directly
these molecules, ready to be used in your
Nanodevices. And,
in the catalog, I took these examples because
is interesting because you see you can find
P-type chains.
So, sort of P-type semiconductors
and so, you have here the, you see
here the Thiol groups
and the different structures, different mobility, different
characteristics.
And also you have the complimentary structures of
the N-type structures, N-type molecules.
So, starting from this
you can, and I give you this as first example,
we will have later some lectures specific
for giving you coverage of possibilities
in creating devices and applications. So,
starting from these basic molecules
you can set up
PN junctions, so a diode based
on these molecules, where you put
a molecule that can be the doner.
The molecule that can be your receptor
you put in the middle with R group
that at the end is an insulating group
that is the barrier between the P and
N molecule, and so, you obtain,
at the end, the structure in which you have
your band gaps, and so on.
For thinking to build inside
the 2 electrodes, with a chain
of different molecules your first, we can say,
PN diode, based on molecules.
In the next lecture
we will pass to analyze
the possibilities that we have
in changing conductance in our structure
changing length, rotation of the molecules, so
we will analyze together which are the characteristics
of our molecular wires
that influence the conduction and the current
we can have in our devices.