M2. Inducing Electron Transitions

1. Unit 8How do use chemical systems to harness energy? 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. Analyze electron transfer between coupled systems. M1. Controlling Electron Transfer . Explore the effect of electron transitions in solid systems. M2. Inducing Electron Transitions

2. Unit 8How do we use chemical systems to harness energy? Module 1: Inducing Electron Transitions Central goal: To explore the effect of electron transitions in solid chemical systems.

3. TransformationHow do I change it? How can we control these types of transformations? 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).

4. 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. Electronic Levels Electron transitions between different energy levels may be induced by providing energy to a chemical system.

5. 2 3 4 20 In solids, with ~1023 atoms, the energy difference becomes negligible, and continuous “energy bands” are formed. E Conduction band(Uppermost empty) Energy Gap (Eg) Valence band(Lowermost filled) Energy Bands As atoms combine into larger molecules, the energy difference between the available electron energy levels decreases. E 1 # of interacting atoms

6. E Eg ~ 60-300 kJ/mol CB Eg > 300 kJ/mol VB Semiconductor Metal The Eg can be overcome by thermal vibrations or UV-vis-IR light. The energy cost for e- to jump from the VB to the CB is negligible. Insulator Very large Eg. Conductivity Electrical conductivity depends on the existence of empty energy levels that e- can access:

7. Which of these composite materials are likely to be semiconductors? GaAs CdS InP GaSe Let’s Think Semiconductors 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.

8. E CB Eg ~ 60-300 kJ/mol VB Semiconductor The Eg can be overcome by thermal vibrations or UV-vis-IR light. Band Gap 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.

9. ~atomic size Let′s Think! What periodic trends do you detect for the band gap of semiconductors? Hint: Analyze families of compounds with one common element.

10. Smaller size More electron density overlap  larger Eg ~atomic size Periodic Trends Egincreases as the interaction between atoms becomes either more covalent:

11. ~atomic size cAl= 1.5 cGa = 1.6 cMg= 1.2 cCd = 1.7 Periodic Trends Eg increases as the interaction between atoms becomes more ionic: Larger Dc  More ionic character  larger Eg

12. E Adding atoms with 5 valence e- introduces e- in donor levels that are close to the conduction band. CB Donor level VB n-type Si + P (impurity) Doping Adding very small amounts of impurities (ppm) to an intrinsic semiconductor can increase its conductivity by a factor of a million. E CB VB Instrinsic Si, Ge Carriers (e-)

13. E E Adding atoms with 3 valence e- introduces empty levels that e- can occupy close to the valence band. CB CB Acceptor level VB VB p-type Instrinsic Si + Al (impurity) Si, Ge Doping Conductivity can also be increased using atoms with fewer valence e-than the host. Carriers (h+)

14. e-flow from the n to the p side until equilibrium is reached (the Efield at the interface stops the flow). p-n Junctions E Mobile e- in a n-type semiconductor are in higher potential energy states than mobile e- in p-type systems. CB VB What happens if we put them in contact (p-n junction)? p-type n-type

15. Let′s Think! Imagine now that the p-n junction is connected to a battery as shown: hole e- • What would you expect to happen? Will e-move? If yes, in which direction? • What would happen if we reverse the connections? Will e-move? If yes, in which direction?

16. E CB VB Energy in the form of light may be emitted as e- fall to lower E levels. Forward biasCurrent flows Diodes Reverse biasNo current flows

17. 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. 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 p-type n-type

18. Solar Cells

19. Let′s apply! Assess what you know

20. Let′s apply! An LED is made with a combination of different materials.

21. Let′s apply! Design a cheap full LED device that emits:Red (620-750 nm) Green (495-570 nm)orBlue (450-495 nm) light. • What semiconductor would you use? • How would you dope it? • What other materials would you use? E = hn = hc/lh =6.626 x 10-34J-s c =3.00 x 108 m/s

22. Polyepoxide Lead Let′s apply! AlGaAs Red SiC Blue GaP Green

23. Explain something that you learned in this module to other person in the class.

24. E Conduction band(Uppermost empty) Energy Gap (Eg) Valence band(Lowermost filled) 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. The energy gap Egcan be controlled by changing the composition of the semiconductor

25. E 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. CB VB p-type n-type Doping and Junctions Summary Semiconductors are normally “doped” with other substances to change their electric properties.

26. Are You Ready?

27. 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