Transcript Slide 1

Unit 8
How do use chemical systems to
harness energy?
Chemistry XXI
The central goal of this unit is to apply and extend
central concepts and ideas discussed in this course
to design chemical systems to harness energy.
M1. Controlling Electron Transfer
Analyze electron transfer
between coupled systems.
M2. Inducing Electron Transitions
Explore the effect
.
of electron
transitions in solid systems.
Unit 8
How do we use chemical
systems to harness energy?
Chemistry XXI
Module 1: Inducing Electron Transitions
Central goal:
To explore the effect
of electron
transitions in solid
chemical systems.
The Challenge
Transformation
How do I change it?
Chemistry XXI
In many chemical systems, electron transitions
between different energy levels lead to the
transformation of energy into different forms
(heat, light, electrical current).
How can we control
these types of
transformations?
Electronic Levels
Electron transitions between different energy levels
may be induced by providing energy to a
chemical system.
Chemistry XXI
E
In isolated atoms and
molecules, the energy states
in which electrons exist are
clearly quantized.
DE
Transitions between levels only occur when the
appropriate DE is absorbed or released.
Energy Bands
Chemistry XXI
As atoms combine into E
larger molecules, the
energy difference
between the available
electron energy levels
decreases.
In solids, with ~1023
atoms, the energy
difference becomes
negligible, and
continuous
“energy bands”
are formed.
E
1
2
3
4
20
# of interacting atoms
Conduction band
(Uppermost empty)
Energy Gap (Eg)
Valence band
(Lowermost filled)
Conductivity
Electrical conductivity depends on the existence
of empty energy levels that e- can access:
E
CB
Chemistry XXI
VB
Eg ~
60-300
kJ/mol
Metal
Semiconductor
The energy cost
for e- to jump from
the VB to the CB is
negligible.
The Eg can be
overcome by
thermal vibrations
or UV-vis-IR light.
Eg >
300
kJ/mol
Insulator
Very large Eg.
Semiconductors
Chemistry XXI
The metalloids Si and Ge are
semiconductors at room temperature,
and they form the basis for computer
processors and other electronic devices.
Other “composite” semiconductor materials have
been developed by mixing different chemical
elements. However, these composites tend to have
an average number of valence electrons equal to 4,
as Si and Ge.
Which of these composite materials are
likely to be semiconductors?
Let’s Think
GaAs
CdS
InP
GaSe
Band Gap
Chemistry XXI
The energy gap Eg between
valence and conduction
bands is a critical feature of
a given semiconductor.
The Eg depends on the
types and relatives
amounts of the different
atoms that compose the
system.
E
CB
Eg ~
60-300
kJ/mol
VB
Semiconductor
The Eg can be
overcome by
thermal vibrations
or UV-vis-IR light.
Let′s Think!
Chemistry XXI
What periodic trends do you detect for the
band gap of semiconductors? Hint: Analyze
families of compounds with one common element.
~atomic size
Periodic Trends
Chemistry XXI
Eg increases as
the interaction
between atoms
becomes either
more covalent:
Smaller size 
More electron
density overlap
 larger Eg
~atomic
size
Periodic Trends
Chemistry XXI
Eg increases as
the interaction
between atoms
becomes more
ionic:
Larger Dc 
More ionic
character 
larger Eg
cAl = 1.5
cMg = 1.2
cGa = 1.6
cCd = 1.7
~atomic
size
Doping
Adding very small amounts of impurities (ppm)
to an intrinsic semiconductor can increase its
conductivity by a factor of a million.
E
E
CB
CB
Chemistry XXI
Donor
level
VB
Instrinsic
Si, Ge
VB
n-type
Si + P (impurity)
Carriers (e-)
Adding atoms
with
5 valence eintroduces ein donor
levels that are
close to the
conduction
band.
Doping
Conductivity can also be increased using atoms
with fewer valence e- than the host.
E
E
CB
CB
Chemistry XXI
Acceptor
level
VB
Instrinsic
Si, Ge
VB
p-type
Si + Al (impurity)
Carriers (h+)
Adding atoms
with
3 valence eintroduces
empty levels
that e- can
occupy close
to the valence
band.
p-n Junctions
Mobile e- in a n-type
semiconductor are in higher
potential energy states than
mobile e- in p-type systems.
E
CB
VB
Chemistry XXI
What happens if we put them
in contact (p-n junction)?
p-type
n-type
e- flow from the n to the
p side until equilibrium
is reached (the Efield at
the interface stops
the flow).
Let′s Think!
Imagine now that the p-n junction is connected to a
battery as shown:
hole
Chemistry XXI
e-
a) What would you expect to happen? Will e- move? If
yes, in which direction?
b) What would happen if we reverse the connections?
Will e- move? If yes, in which direction?
Diodes
Reverse bias
No current flows
E
Chemistry XXI
CB
VB
Forward bias
Current flows
Energy in the form of
light may be emitted as
e- fall to lower E levels.
LED/Photocells
E
CB
In a Light Emitting Diodes
(LED), electrons emit light in
the UV-vis-IR region when they
transfer from the CB to the VB
in moving across the junction.
VB
Chemistry XXI
p-type
n-type
In a photocell, light photons are
absorbed by electrons in the VB
and transferred to the CB. This
creates an electric field that can
be used to generate a current.
Chemistry XXI
Solar Cells
Chemistry XXI
Let′s apply!
Assess what you know
Let′s apply!
Chemistry XXI
An LED is made with a
combination of different
materials.
Let′s apply!
Chemistry XXI
Design a cheap full LED device that emits:
Red (620-750 nm) Green (495-570 nm)
or Blue (450-495 nm) light.
Material
Eg (J)
Ge
1.06 x 10-19
Si
1.79 x 10-19
GaAs
2.28 x 10-19
AlGaAs
3.06 x 10-19
GaP
3.62 x 10-19
SiC
4.23 x 10-19
a) What
semiconductor
would you use?
b) How would you
dope it?
c) What other
materials would
you use?
E = hn = hc/l
h = 6.626 x 10-34J-s
c = 3.00 x 108 m/s
Let′s apply!
Chemistry XXI
Polyepoxide
AlGaAs  Red
Lead
SiC  Blue
GaP  Green
Chemistry XXI
Explain something that you
learned in this module to other
person in the class.
Exploring Electronic Structure
Summary
Semiconducting systems can be used to transform
light energy into electrical energy, and vice versa, by
inducing e- transitions between energy bands.
E
Conduction band
Chemistry XXI
(Uppermost empty)
Energy Gap (Eg)
Valence band
(Lowermost filled)
The energy gap Eg
can be controlled by
changing the
composition of the
semiconductor
Doping and Junctions
Summary
Semiconductors are normally “doped” with other
substances to change their electric properties.
E
Chemistry XXI
CB
VB
p-type
n-type
Junctions formed with pand n- types are elementary
"building blocks" of almost
all semiconductor
electronic devices such as
diodes, transistors, solar
cells, and LEDs.
Chemistry XXI
Are You Ready?
Electronics
A company interested in producing
semiconductors for diverse electronic devices
wants to know what binary material to produce
to generate a semiconductor with the smallest
band gap given the available resources.
Chemistry XXI
Elements
Available
Binary material with an
average # valence e- = 4,
involving the largest
atoms:
InSb