Chemistry XXI M2. Inducing Electron Transitions. M1. Controlling Electron Transfer Analyze 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 8 How do 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. Unit 8 How do we use chemical systems to harness energy?

Chemistry XXI 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). Transformation How do I change it? How can we control these types of transformations?

Chemistry XXI 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E

Chemistry XXI As atoms combine into larger molecules, the energy difference between the available electron energy levels decreases. Energy Bands E # of interacting atoms In solids, with ~10 23 atoms, the energy difference becomes negligible, and continuous “energy bands” are formed. E Valence band ( Lowermost filled) Conduction band (Uppermost empty) Energy Gap (E g )

Chemistry XXI Conductivity 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 E g can be overcome by thermal vibrations or UV-vis-IR light. E g ~ kJ/mol Insulator E g > 300 kJ/mol Very large E g.

Chemistry XXI 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. Let’s Think Which of these composite materials are likely to be semiconductors? GaAsCdS InPGaSe

Chemistry XXI Band Gap The energy gap E g between valence and conduction bands is a critical feature of a given semiconductor. Semiconductor The E g can be overcome by thermal vibrations or UV-vis-IR light. E g ~ kJ/mol E VB CB The E g depends on the types and relatives amounts of the different atoms that compose the system.

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 Let′s Think!

Chemistry XXI Periodic Trends ~atomic size E g increases as the interaction between atoms becomes either more covalent: Smaller size  More electron density overlap  larger E g

Chemistry XXI Periodic Trends ~atomic size E g increases as the interaction between atoms becomes more ionic: Larger   More ionic character  larger E g  Al = 1.5  Ga = 1.6  Mg = 1.2  Cd = 1.7

Chemistry XXI Doping 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

Chemistry XXI 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

Chemistry XXI p-n Junctions E n-type VB p-type CB Mobile 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 E field at the interface stops the flow).

Chemistry XXI 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-e-

Chemistry XXI Diodes Reverse bias No current flows Forward bias Current flows E VB CB Energy in the form of light may be emitted as e - fall to lower E levels.

Chemistry XXI 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

Chemistry XXI Solar Cells

Chemistry XXI Assess what you know Let′s apply!

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

Chemistry XXI Let′s apply! Design a cheap full LED device that emits: Red ( nm) Green ( nm) or Blue ( nm) light. MaterialE g (J) Ge1.06 x Si1.79 x GaAs2.28 x AlGaAs3.06 x GaP3.62 x SiC4.23 x E = h  = hc/ h = x J-s c = 3.00 x 10 8 m/s a)What semiconductor would you use? b)How would you dope it? c)What other materials would you use?

Chemistry XXI Let′s apply! Polyepoxide Lead SiC  Blue GaP  Green AlGaAs  Red

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

Chemistry XXI 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 (E g ) The energy gap E g can be controlled by changing the composition of the semiconductor

Chemistry XXI 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

Chemistry XXI Are You Ready?

Chemistry XXI 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