so semiconductors actually have
electrical conductance they conduct the
electricity much less than actually -
compared to the metal we said but the
explanations of the electrical
conductivity used in the band Theory
simply explained by small energy gap
between the valence band and the
conduction band for the semiconductors
this is again for silicon germanium and
tin they're all similar again and only
the bandgap changes but if the band gap
is much higher than 3 and so on that
means that the compound will not behave
as a semiconductors now there are two
kinds of semiconductors one is called
simply pure element semiconductors and
these are all group for a actually time
when they actually not semiconductor but
you have to think about that consider
that and all have four valence electrons
but the silicon is a semiconductor
germanium is a semiconductor tim is not
almost metal actually but there are
inorganic semiconductors or they are
called compound semiconductors like
arsenic gallium arsenide and others now
these compound semiconductors made of
two different types of element but the
when you find out the number of valence
electrons in these compound per atom it
becomes equal to 4 so that simply means
that these compound semiconductors or
inorganic semiconductors do also make
four bonds to the surrounding atoms
other atoms and all of those four bonds
contains two electrons so there are four
electron
/ methyl or actually the the elements
atom in these inorganic semiconductors
like gallium arsenide now the gallium
has one less electron that means three
valence electrons but arsenic has five
valence electrons so when you have
gallium arsenide as a compound that
means the number of valence electrons
per atom becomes four same as silicon
and germanium and so on
and there are other than the gallium
arsenide in fact conductors for example
you can have the cadmium and the
tellurium for example right collyrium
has six valence electrons cadmium has
two valence electrons and when you
combine one cadmium and tellurium and
make the semiconductor cadmium telluride
then per number of valence electrons per
atom becomes four right and so on now it
is actually possible to adjust the
electrical conductivity of element
semiconductors by introducing impurities
to the pure element consider a silicon
for example and you can simply add very
few number of phosphorus to the silicon
pure silicon actually in parts per
million level you know few parts per
million level now the electrical
conductivity of the silicon changes
dramatically
almost increases billion times million
times just a few atoms and this
controlled way of adding impurities to
another pure element particularly these
semiconductor elements is called doe
so when we say doping means that we add
small amount of impurities control
amount of impurities to the pure element
semiconductor and depending on what
degree of conductance we want we can
adjust that by simply adjusting the
dopant atoms number of dopant atoms to
the impurities now let's think about
here as I said for example when you
simply have a pure silicon and it's very
very important to gain again as I said
since very very small impurities changes
the electrical conductivity tremendously
so when you're making chips from the
silicon that silicon has to be pure
silicon actually it's called nain nain
silicon means that the silicon used to
make chips is ninety nine point nine
nine nine nine nine nine nine nine nine
percent pure because if it is not that
pure than the electrical conductivity of
the silicon is not what is simply
required whatever it's used for so we
have to be very very careful then when
we have that pure silicon then we can
adjust again if we control the amount of
impurity that we add add some basis
actually then we can obtain whatever the
electrical conductivity we want to get
from that mixture again and that is
called doped silicon for example if you
add into the silicon some impurities in
this case suppose that we add phosphorus
to the silicon that means we dope
silicon pure silicon with phosphorus
what really happens in the solid state
the some of the silicon atoms in the
lattice is replaced by the phosphorus
atom
but still that phosphors remember there
are four silicon Iran it will make four
buns right but the silicon has 4 valence
electrons but the phosphorus has 5
valence electrons but when the
phosphorus replaces the silicon in the
lightest it only needs four electron to
make for a coolant four covalent bonds
so there is one excess electron there
now where that electron goes that
electron goes since this is all full
right that electron goes into the
conduction band here free electrons and
that increases the electrical
conductivity of the silicon increasing
electrical conductivity of the silicon
now depending on what degree of
electrical conductivity we want from the
silicon so we adjust the number of
phosphorous atoms accordingly right
accordingly and now we can also dope
silicon with another atom which has less
than four valence electrons for example
if we simply dope the silicon with
aluminium atoms very few aluminium atoms
aluminium has three valence electrons
the aluminium again in the crystal
lattice and the lattice will replace the
silicon and the aluminium will make four
bonds with other silicones around in
that crystal but there's only seven
electrons there right for that four
bonds what does that means that means
that the valence band in that case will
not be all full there's going to be some
empty molecular orbitals here and that
empty molecular orbitals will increase
the electrical conductivity of the
silicon right but in this case the
mechanism of electric conductivity will
be obtained by jumping the electron from
one lattice point from to the other
lattice points as the electron jumps
from one lattice point to the other that
means that the positive hole moves in
one direction right in the opposite
directions so the electrical
conductivity increase
is due to the positive holes that form
in the valence band when we dope silicon
with atoms which has less than four
valence electrons so the semiconductor
which is obtained by adding phosphorus
for example to the silicon is called
n-type semiconductors why it's all
called anti because there is excess
electron that means there's a negative
charge that's why it's called n-type now
the semiconductors which is obtained by
doping aluminium to the silicon and we
have simply partially filled valence
band here and they're positive holes and
it's called p-type that means we have
less electron than required that we need
and it's called p-type now when we
combine this n-type and p-type
semiconductors make a junction then we
can make all kinds of devices actually
and such as the diodes light emitting
diodes and and others actually okay now
the next solid compound that we will
talk about is polymers now the polymers
are actually long-chain molecules and
the molecular weight of polymer
molecules is sometimes goes up to
million now remember again like
molecular weight of water is eighteen
right imagine the million and usually
the polymers are obtained from the
monomers the smallest unit smallest
molecule putting them together as a
matter of fact now there are two primary
types of polymers it says here that
means addition polymers and the
condensation polymers addition polymers
are formed when bond breaks and the
electrons in that bond make two new
bonds condensation polymers are formed
when a small
remove between two large molecules
actually usually this removed molecules
its water molecules now the next slide
shows how we have the addition and
condensation polymers addition polymers
monomer is ethylene double bond here so
when we get the ethylene together this
bond opens up and then this carbon bonds
to this one and that carbon bonds to the
other one and then we have a long chain
of polyethylene that is a polymer right
and what very long chain of polymer
polyethylene molecule here like this now
when we simply combine this is for
example a dybbuk acid and hexamine hexa
diamine to form nylon and here this is
one end of the molecule the amine part
and this is the carboxylic acid acid
part and when they polymerizes this o H
and that H
simply combines to form water and that
water comes out and then we form a bond
between this carbon and that nitrogen
and this continues like this that kind
of polymers or called condensation
polymers or that polymerization is
called condensation polymerization that
means we form a bond between the smaller
molecules by removing another small
molecule in this case water as a matter
of fact now these are the examples of
some common polymers that we use there
almost everywhere in our life and this
is the most common one again
polyethylene like this and this is the
polypropylene again and we have there
there is a double bond here the monomers
is poor polypropylene and the monomers
is properly propene I guess 214 and this
is another polystyrene and you got that
like this and we have also hole even a
vinyl chloride this is where we use
for example polyvinyl chloride in pipe
fitting Plumbing's and things like that
now these are the conversation polymers
most common one I guess is the nylon
here that's poly urethane this one is
polyurethane and polyesters nylon this
is the first conversation polymers
almost and polar carbonates and this is
where we use them as a matter of fact
now remember again the cellulose and
starch which is present in particularly
plants and the proteins which is present
both in plant and the animals and human
beings are also polymers they're
long-chain big molecules very big
molecules so the polymers are really
very important in fact to make Goods out
of them
devices and things like that and they're
also important biologically to because
most of those biological molecules like
proteins are enzymes mostly are polymers
okay now the bulk properties of polymers
again the molecules are not straight
lines the longer the chain the more
twists in the twisting happens it says
chains can have variety of lengths and
therefore a variety of molecular weights
again you see when you polymerize the
monomer to form polymers molecule they
will not be all the same length that
means they will not be all the same
molecular weight so there is always you
know different molecular weight polymer
molecules present there and that also
affects the properties of bulk
properties of the polymers too as a
matter of fact now material they can be
very flexible or can be hard actually
they're usually flexible now there are
actually strong range order and can lead
to crystalline ik crystallinity in solid
now this crystallinity again has
followed
suppose that long-chain molecules
somewhere have hydrogen bonding or some
polarity right so when we have those
chains of polymers they will pack in
this fashion to get very nice packing
like here because there's an attraction
between the chains here through the
intermolecular forces and these points
are crystalline and this is called
crystallinity when he increased the
crystallinity that means the structures
in the polymer which looks like this you
know regional sort of regularities in
orange its short-range regularities the
properties of the polymer changes again
dramatically now this is isoprene poly
isoprene which is natural rubber again
now natural rubber itself is very
elastic is not very strong is not really
that stable but when you simply bind the
natural rubber polymer chains with
cross-linking with sulfur this is called
vulcanization actually and the rubber or
I should say that polymer which is the
rubber as a matter of fact has very very
good properties to be used in industry
like tires for example are all
vulcanized rubber as a matter of fact so
you simply bind the change together and
the bulk properties changes the strength
of those boiling points and other things
that changes drama dramatically also -
you get much much better stronger
polymers by simply cross linking those
polymer chains in this case why sulfur
if you do it by sulfur it is called
vulcanization actually this process is
called organizations now not all
materials these are simply the
substances that has the particle size
between
and hundred nanometers so when the
particle size of solids is between 1 and
hundred nanometers the properties of the
solid again changes drastically changes
drastically and those substances are
called nano materials and they're great
research is going on these nano
materials as a matter of fact because
people are after simply trying to make
devices that molecular size small by
using these nano materials nano
materials now uh for example if we
consider the semiconductors and metals
in fact as nano scale small molecules
have discrete orbitals macro molecule
materials have bends where does it
switch over now the things is again as
follow the bitch simply says when we
have two atoms or small molecule we can
simply form the molecular orbitals and
these molecular orbitals are very
localized only for those if it's made
from two atoms only if on those two
atoms we put the electrons in bonding
anti-bonding molecular orbitals and so
on now when we have a chunk of metal or
semiconductor now we have bends right
that means we have molecular orbitals
which are very closely spaced
energy-wise very closely spaced and we
call them bands and the question here
says when the local molecular orbital
approach shifts over to the bent
approach band Theory what size right
because if you consider single molecule
the properties of that single molecule
is not same as the properties of the
bulk which you have all your molecules
together so and here it says the theory
calculation tells you that when the size
of the crystal or that particle is about
10 nanometers or beyond that 10
nanometers then we go to the bent
structure that means we consider that as
a bulk again right and that that range
again again about 10 nanometer sized
particles there are very unusual
properties that those particle shows and
very useful again in making devices -
now let's see
now let's look at here one thing that is
the determine is this when the size of
the semiconductor nanoparticles get
smaller and smaller the energy band gap
becomes gets higher and higher right now
what does that means when the energy
band gap gets higher and higher that
means it will not absorb for example the
visible light or the visible light
energy visible light photons energy will
not be enough to excite the electron
from the valence band to the conduction
band is again now this is the example
here I guess this one is the phosphides
some yes cadmium phosphide 3 cadmium and
2 phosphorus cadmium phosphide nano
particles again nanoparticles now
remember the size here is less than 10
nanometers the size here is about 10
nanometers these particles even though
they're very large seems like large but
they're very very small now this one is
almost black this one is sort of
brownish and then sort of orange or red
whatever this is and then yellow and
then this becomes a white at the
particle gets smaller the explanations
of these color again is follow when we
have large sized nano particles remember
again it has to be between 1 and hundred
nanometers the band gap is very small
and all of the visible light is absorbed
when all of the visible light is
absorbed the color of the substance will
be what black
colors right because it's all absorbed
visible light now when you go this lower
the size of the particles then the
energy gets get a little bit higher and
higher now in this range here it
observes in the visible region and for
example this one absorbs the blue light
complementary light it would be probably
blue here and then we go down here and
the bandgap increases increases
increases and when it gets to here this
is colorless why it's colorless because
in this case the bandgap is so big that
there will be no absorption of the
visible light now and we simply see all
the colors which is white here total
absorption very small energy band gap
here no absorptions very large band gaps
you see again how the size of the
particles in nano scale changes the
properties of the substance you see in
color now this one is again another this
is a cadmium selenide I guess another
semiconductor but we have the solutions
in this case cadmium selenide solutions
and all have different particles and we
simply eliminate this by UV radiation
this is luminescence actually when you
simply eliminate this by UV radiation UV
radiation as much higher energy than the
visible light the electron jumps into
the conduction band from valence band to
the conduction band but then they drop
back from the conduction band to the
valence band again as they drop down
from conduction to the valence band they
emit light in the visible region just
like the emission spectrum of hydrogen's
is again but different colors tells you
what here different colors tells you
that the band gap between the conduction
band and the valence band because when
the electron dropped from the conduction
band to the valence band the photon
release will have the energy equals to
the band gap so when the band gap
changes that means particle size changes
the color emitted also changes let's
again another example how the size of
the particles changes the properties of
nano material again now metals on the
nano scale also have a lot of
applications one of them is known for
years and years you know even thousand
years before and that is gold if you
take gold and dissolve it in melted
glass and you get all different colors
why because in a glass gold
nanoparticles form and depending on the
size of those nanoparticles we have
different colors these are simply
stained things and this was explained by
Michael Faraday in 1857 actually and he
prepared some gold colloidal gold
particle solutions all they have
different colors even they're very very
stable particularly gold nanoparticles
these solutions are very very stable
very stable they still they say they
still in a museum in London there they
keep those in different colors now this
is again the glass you see stained glass
and those colors are due to the
nanoparticles that present in the glass
metal nanoparticles now carbon on the
nano scale is also important and great
deal of research is done on this nano
tubes of carbon nano scale carbon
materials as a matter of fact now
remember the carbon has three isotopes
one is the graphite layer structure the
other one
Dimond the allotropes actually now
another one is discovered in 1985 that
makes about 30 years 31 years ago and
that is fullerene carbon 60 for example
which has very different structures than
the others as a matter of fact now let's
look at here this is again now then
another allotrope I can probably say off
carbon which is one layer of the
graphite actually if you take one layer
of graphite like this all carbons are
bonded to three other carbons and sp2
hybridization here and that is called
graphene and this was also discovered in
1990s I guess and the Nobel Prize is
given to the people who discover that
physicist in 2010 I I guess the graphene
and they have very very different
properties very interested in different
properties this is the fullerene
structure this is carbon 60 I have 60
carbon atoms and you see again we have
the Pentagon's regular Pentagon's and
regular hexagons as phases this is
simply I guess it has 20 hexagons and 12
Pentagon's as a face 32 faces and the
football the ball that is play football
also has the same structure there
Pentagon's and hexagons as simply on the
surface like this so it's called
fullerene why it's called fullerene
because an american engineer and
architect sometimes they say philosopher
he find out how to make Joe dezik domes
Kubek Big Joe dezik domes and he used
very similar structure there half of
that dome will be the half of that and
when this was discovered
18 1985 actually the name given these
type of compounds of carbon are called
fullerenes actually fullerenes
and you can simply take the fullerenes
and dissolve it in organic solvent you
see again
you can't dissolve graphite in organic
solvent you can desire dissolved
diamonds in organic solvent but you can
dissolve fullerenes in organic solvents
actually and then you can carry out
reactions with this reaction with this
now so we have these carbon-carbon
minute Nano tips this is like you take
those one layer which is called graphene
of the graphite and then roll it up like
this and then on to end you can cap it
with half of the fullerene c60 you know
you take the half of that and then cap
it to here and here and then you have
these nanotubes and these nanotubes are
so strong and they have so unusual
properties actually and they have lot of
applications and that increases every
day and a great amount of research as I
said is devoted to develop these nano
tube carbon nanotubes and other nano
scale carbons now for example this is an
imaginary picture which imagines that
the carbon nanotube this is the carbon
nanotube can be used as a carrier belt
you know the carrier belt to carry one
molecule like this you know that would
be like devices in the nano scale
actually as I said the main purpose is
to use the nano scale knowledge and make
devices which is in nano scale size very
very small sizes you know the molecular
size is it okay I guess
this is all do I have sometimes yes but
I will stop it here and that is the end
of this chapter now please read it from
the book and try to solve the problems
at the end of each chapter please do it
because those are the type of problems
we usually ask in the exam