Transcript Polymer Physics Ph.D. Course

Behzad Pourabbas
Sahand University of Technology
Faculty of Polymer Engineering
[email protected]
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IT works with DATA
Creates or collects DATA (Direct Input, Through
Sensors, CCD or CMOS, Receive DATA).
 Process DATA (Computers: CPU).
 Stores DATA (Hard Discs, Data Storage devices,
CD, DVD).
 Reviews DATA (Paper, Display devices).
 Transmit DATA (Internet, Wires and Cables,
Optical Fibers).
 Consumes Energy (Electricity: Power, Batteries,
Solar Cells , etc).


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IT works with DATA
Creates or collects DATA (Direct Input, Through
Sensors, CCD or CMOS, Receive DATA).
 Process DATA (Computers: CPU).
 Stores DATA (Hard Discs, Data Storage devices,
CD, DVD).
 Reviews DATA (Paper, Display devices).
 Transmit DATA (Internet, Wires and Cables,
Optical Fibers).
 Consumes Energy (Electricity: Power, Batteries,
Solar Cells , etc).


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IT works with DATA
Sensors (Sensitive Materials, CCD or CMOS
(transistors, Receive DATA, Cable or Optical
Fibers).
 Computers: CPU => Transistors.
 Hard Discs, Data Storage devices, =>
Semicinductors Patterning, CD, DVD, Blue
Rays=> Laser Sources (Diods) and Patterning.
 Display devices, LCD, LED.
 Optical Fibers=> Glass or Polymeric Fibers.
 Batteries, Solar Cells=> Combination of
Semiconductors, Patterning and polymers.
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Materials for IT
Polymers
Conductive Polymers
 Conductivity Concept in Materials
 Conductivity Concept in Polymers
 Electrically Conductive Polymers

Liquid Crystal Polymers
 Basic Concepts & History
 Main Features of LCs

Advanced Applications of Polymers
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LCD
TFT
LED and OLED
CCD and CMOS
Sollar Cells
Optical Fibers
Patterning
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
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Electrical conductivity of a MATTER is its
ability to conduct electrons.
We can measure it by measuring Resistivity
very easily by several methods, Using one
OHM meter for example.
There are more complicated and standard
method to do this.
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
Range of Conductivity in Materials
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Electrical Conductivity () may occur by Electrons or
Ions.
 What is the Charge of Carriers? How many Carriers?
And with what speed (mobility)?

  qn 
The ease with which the charged species will move
under the influence of the applied electric field E and
is usually expressed as a velocity per unit field (m2V-1s-1)
 What happens in the absence of an electric field for
the charge carriers?

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
There is a drift velocity: and an average for it:
: time between
scattering events
qE
 
.
m

+
+
+
+
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q
whence  

E
m
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
Mobile Species:
 Electrons and Holes: (Electronic Conductors)
▪ Electron is an Electron (Negative Charge)
▪ Hole is the an empty place of a moved electron
(Positively Charged).
 Cations and Anions, (Ionic Conductors).
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Theories of Conductions are aimed to explain
how n and  depend on molecular structure, T
and E the applied field.
In Polymers, the mobility depends on
morphology as well.
There is a large range of mobity values for
different materials. (The next Slide).
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 Mobility values for different materials
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
Conductivity in POLYMERS:
 They are usually insulators if:
▪ There is no charged species : (polymers are composed of covalent
bonds)
▪ Careful separation of any ionic species from for example: catalysts
residues; Initiators, Ionic End groups, Oxidation Products.

Conversely,
 One insulating polymer can made conductive by adding
conductive fillers such as carbon black or metallic particles
(Gold, Silver, Nickel,..) (Conductive Composites).

There are substantially conductive polymers as
well!!!!!
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
Order of conductivity:
superconductors, conductors,
semiconductors, insulators
 conductors: material capable of carrying electric current, i.e.
material which has “mobile charge carriers” (e.g. electrons,
ions,..) e.g. metals, liquids with ions (water, molten ionic
compounds), plasma
 insulators: materials with no or very few free charge carriers; e.g.
quartz, most covalent and ionic solids, plastics
 semiconductors: materials with conductivity between that of
conductors and insulators; e.g. germanium Ge, silicon Si, GaAs,
GaP, InP
 superconductors: certain materials have zero resistivity at very
low temperature.
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.

some representative resistivity ():

R = L/A, R = resistance, L = length, A = cross section area; resistivity at 20o C
resistivity in  m
▪
▪
▪
▪
▪
▪
▪
▪
▪
▪
▪
▪
aluminum 2.8x10-8
brass
8x10-8
copper
1.7x10-8
platinum 10x10-8
silver
1.6x10-8
carbon
3.5x10-5
germanium 0.45
silicon
 640
porcelain 1010 - 1012
teflon
1014
blood
1.5
fat
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resistance(in )(L=1m, diam =1mm)
3.6x10-2
10.1x10-2
2.2x10-2
12.7x10-2
2.1x10-2
44.5
5.7x105
 6x108
1016 - 1018
1020
1.9x106
3x107
Why
materials
have different
conductivities
?
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
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In solid materials, electron energy levels form bands of allowed energies, separated by
forbidden bands
valence band = outermost (highest) band filled with electrons (“filled” = all states
occupied)
conduction band = next highest band to valence band (empty or partly filled)
“gap” = energy difference between valence and conduction bands, = width of the
forbidden band
Note:
▪ electrons in a completely filled band cannot move, since all states occupied (Pauli
principle); only way to move would be to “jump” into next higher band - needs
energy;
▪ electrons in partly filled band can move, since there are free states to move to.
Classification of solids into three types, according to their band structure:
▪ insulators: gap = forbidden region between highest filled band (valence band) and
lowest empty or partly filled band (conduction band) is very wide, about 3 to 6 eV;
▪ semiconductors: gap is small - about 0.1 to 1 eV;
▪ conductors: valence band only partially filled, or (if it is filled), the next allowed
empty band overlaps with it
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Atoms form a solid  valence electrons interact  two quantum mechanical effects.
Heisenberg's uncertainty principle: constrain electrons to a small volume  raises
their energy called promotion.
Pauli exclusion principle limits the number of electrons with the same energy.
Result: valence electrons form wide electron energy bands in a solid.
Bands separated by gaps, where electrons cannot exist.
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 Fermi Energy (EF) - highest filled state at 0 K
 Conduction band -partially filled or empty band
 Valence band – highest partially or completely
filled band
Semiconductors and insulators, valence band is filled, and no more electrons can
be added (Pauli's principle).
Insulators
> 2 eV
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
Intrinsic silicon:

DOPED SEMICONDUCTORS

:
“doped semiconductor”: (also “impure”, “extrinsic”) = semiconductor with small
admixture of trivalent or pentavalent atoms;
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
donor (n-type) impurities:
▪ dopant with 5 valence electrons (e.g. P, As, Sb)
▪ 4 electrons used for covalent bonds with surrounding Si atoms, one
electron “left over”;
▪ left over electron is only loosely bound only small amount of energy
needed to lift it into conduction band (0.05 eV in Si)
▪  “n-type semiconductor”, has conduction electrons, no holes (apart from
the few intrinsic holes)
▪ example: doping fraction of 10-8 Sb in Si yields about 5x1016 conduction
electrons per cubic centimeter at room temperature.
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 acceptor (p-type) impurities:
▪ dopant with 3 valence electrons (e.g. B, Al, Ga, In)  only 3 of
the 4 covalent bonds filled  vacancy in the fourth covalent
bond  hole
▪ “p-type semiconductor”, has mobile holes, very few mobile
electrons (only the intrinsic ones).
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▪ Can “tune” conductivity by choice of doping fraction
▪ can choose “majority carrier” (electron or hole)
▪ can vary doping fraction and/or majority carrier within
piece of semiconductor
▪ can make “p-n junctions” (diodes) and “transistors”
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
p-n JUNCTION:
▪ p-n junction = semiconductor in which impurity changes abruptly from p-type to n-type ;
▪ “diffusion” = movement due to difference in concentration, from higher to lower
concentration;
▪ in absence of electric field across the junction, holes “diffuse” towards and across boundary
into n-type and capture electrons;
▪ electrons diffuse across boundary, fall into holes (“recombination of majority carriers”);
 formation of a “depletion region” (= region without free charge carriers) around the
boundary;
▪ charged ions are left behind (cannot move):
▪ negative ions left on p-side  net negative charge on p-side of the junction;
▪ positive ions left on n-side  net positive charge on n-side of the junction
▪  electric field across junction which prevents further diffusion.
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 diode = “biased p-n junction”, i.e. p-n junction with voltage applied
across it
 “forward biased”: p-side more positive than n-side;
 “reverse biased”: n-side more positive than p-side;
 forward biased diode:
▪ the direction of the electric field is from p-side towards n-side
▪  p-type charge carriers (positive holes) in p-side are pushed
towards and across the p-n boundary,
▪ n-type carriers (negative electrons) in n-side are pushed towards
and across n-p boundary
 current flows across p-n boundary
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
Depletion region and potential barrier
reduced
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 reverse biased diode: applied voltage makes n-side more positive than
p-side  electric field direction is from n-side towards p-side
 pushes charge carriers away from the p-n boundary
 depletion region widens, and no current flows

diode only conducts when positive voltage applied to p-side and
negative voltage to n-side
 diodes used in “rectifiers”, to convert ac voltage to dc.
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
Depletion region becomes wider,
barrier potential higher
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Shockley, Brattain and Bardeen start
working with p- and n- type germanium
and silicon semiconductors in 1946
Bardeen and Brattain put together the first
transistor in December 1947:
 a point-contact transistor consisting of a single
germanium crystal with a p- and an n- zone.
Two wires made contact with the crystal near
the junction between the two zones like the
“whiskers” of a crystal-radio set.
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 (bipolar) transistor = combination of two diodes that share middle portion,
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called “base” of transistor; other two sections: “emitter'' and “collector”;
usually, base is very thin and lightly doped.
two kinds of bipolar transistors: pnp and npn transistors
“pnp” means emitter is p-type, base is n-type, and collector is p-type
material;
in “normal operation of pnp transistor, apply positive voltage to emitter,
negative voltage to collector;
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 if emitter-base junction is forward biased, “holes flow” from battery into
emitter, move into base;
 some holes annihilate with electrons in n-type base, but base thin and lightly
doped  most holes make it through base into collector,
 holes move through collector into negative terminal of battery; i.e. “collector
current” flows whose size depends on how many holes have been captured by
electrons in the base;
 this depends on the number of n-type carriers in the base which can be
controlled by the size of the current (the “base current”) that is allowed to
flow from the base to the emitter; the base current is usually very small; small
changes in the base current can cause a big difference in the collector
current;
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 transistor acts as amplifier of base current, since small changes in base
current cause big changes in collector current.
 transistor as switch: if voltage applied to base is such that emitter-base
junction is reverse-biased, no current flows through transistor -- transistor is
“off”
 therefore, a transistor can be used as a voltage-controlled switch; computers
use transistors in this way.
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
“field-effect transistor” (FET)
 in a pnp FET, current flowing through a thin channel of n-type material is
controlled by the voltage (electric field) applied to two pieces of p-type
material on either side of the channel (current depends on electric field).
 This is the kind of transistor most commonly used in computers.
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
Due to improvements in manufacturing, integrated circuits became
smaller and smaller

Gordon Moore observed that “the number of transistors on a chip
seems to double every year….”
 Moore’s Law: the number of transistors on a chip seems to double every 18
months, while the price remains the same.
 Grosch’s law for mainframes: every year, the power of computers doubles
while the price is cut in half
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