m2. inducing electron transitions

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M2. Inducing Electron Transitions.

M1. Controlling Electron TransferAnalyze electron transfer

between coupled systems.

Explore the effect of electron transitions in solid systems.

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.

Unit 8How do use chemical systems to

harness energy?

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Module 1: Inducing Electron Transitions

Central goal:

To explore the effect of electron

transitions in solid chemical systems.

Unit 8How do we use chemical

systems to harness energy?

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The Challenge

In many chemical systems, electron transitions between different energy levels lead to the

transformation of energy into different forms (heat, light, electrical current).

TransformationHow do I change it?

How can we control these types of

transformations?

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Electronic Levels

Electron transitions between different energy levels may be induced by providing energy to a

chemical system.

In isolated atoms and molecules, the energy states in which electrons exist are

clearly quantized.

E

Transitions between levels only occur when the appropriate E is absorbed or released.

E

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As atoms combine into larger molecules, the

energy difference between the available electron energy levels

decreases.

Energy Bands

E

# of interacting atoms

1 2 3 4 20

In solids, with ~1023 atoms, the energy

difference becomes negligible, and

continuous “energy bands”

are formed.

E

Valence band(Lowermost filled)

Conduction band(Uppermost empty)

Energy Gap (Eg)

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IConductivity

Electrical conductivity depends on the existence of empty energy levels that e- can access:

E

Metal

The energy cost for e- to jump from the VB to the CB is

negligible.

VB

CB

Semiconductor

The Eg can be overcome by

thermal vibrations or UV-vis-IR light.

Eg ~ 60-300 kJ/mol

Insulator

Eg > 300

kJ/mol

Very large Eg.

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ISemiconductors

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.

Let’s Think

Which of these composite materials are likely to be semiconductors?

GaAs CdS InP GaSe

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Band Gap

The energy gap Eg between valence and conduction

bands is a critical feature of a given semiconductor.

Semiconductor

The Eg can be overcome by

thermal vibrations or UV-vis-IR light.

Eg ~ 60-300 kJ/mol

E

VB

CB

The Eg depends on the types and relatives

amounts of the different atoms that compose the

system.

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What periodic trends do you detect for the band gap of semiconductors? Hint: Analyze

families of compounds with one common element.

~atomic size

Let′s Think!

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Periodic Trends

~atomic size

Eg increases as the interaction between atoms becomes either more covalent:

Smaller size More electron

density overlap larger Eg

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Periodic Trends

~atomic size

Eg increases as the interaction between atoms becomes more

ionic:

Larger More ionic

character larger Eg

Al = 1.5

Ga = 1.6

Mg = 1.2

Cd = 1.7

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IDoping

Adding very small amounts of impurities (ppm) to an intrinsic semiconductor can increase its

conductivity by a factor of a million.

E

Instrinsic

Si, Ge

VB

CB

Carriers (e-)

E

n-type

Si + P (impurity)

VB

CB Adding atoms with

5 valence e- introduces e-

in donor levels that are

close to the conduction

band.

Donor level

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Doping

Conductivity can also be increased using atoms with fewer valence e- than the host.

E

Instrinsic

Si, Ge

VB

CB

Carriers (h+)

E

p-type

Si + Al (impurity)

VB

CB Adding atoms with

3 valence e- introduces

empty levels that e- can

occupy close to the valence

band.

Acceptor level

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p-n JunctionsE

n-type

VB

p-type

CBMobile e- in a n-type

semiconductor are in higher potential energy states than mobile e- in p-type systems.

What happens if we put them in contact (p-n junction)?

e- flow from the n to the p side until equilibrium is reached (the Efield at

the interface stops the flow).

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Imagine now that the p-n junction is connected to a battery as shown:

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?

Let′s Think!

hole

e-

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Diodes

Reverse biasNo current flows

Forward biasCurrent flows

E

VB

CB

Energy in the form of light may be emitted as e- fall to lower E levels.

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LED/Photocells

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.

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.

E

VB

CB

n-typep-type

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Solar Cells

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I Assess what you know

Let′s apply!

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ILet′s apply!

An LED is made with a combination of different

materials.

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ILet′s apply!

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

E = h= hc/h = 6.626 x 10-34J-s c = 3.00 x 108 m/s

a) What semiconductor would you use?

b) How would you dope it?

c) What other materials would you use?

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Let′s apply!

Polyepoxide

LeadSiC Blue

GaP Green

AlGaAs Red

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Explain something that you learned in this module to other

person in the class.

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Semiconducting systems can be used to transform light energy into electrical energy, and vice versa, by

inducing e- transitions between energy bands.

Exploring Electronic Structure

Summary

E

Valence band(Lowermost filled)

Conduction band(Uppermost empty)

Energy Gap (Eg)

The energy gap Eg can be controlled by

changing the composition of the

semiconductor

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Summary

Doping and Junctions

Semiconductors are normally “doped” with other substances to change their electric properties.

Junctions formed with p- and n- types are elementary "building blocks" of almost

all semiconductor electronic devices such as diodes, transistors, solar

cells, and LEDs.

E

n-type

VB

p-type

CB

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Are You Ready?

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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.

Elements Available Binary material with an

average # valence e- = 4, involving the largest

atoms:

InSb