Concepts

Diodes, band structure, thermal expansion

Overview

A yellow LED at room temperature changes colour to green when dipped in liquid nitrogen. This is due largely to the lattice contracting in the cold, resulting in closer electron orbitals, stronger bonds, a greater band gap, and higher energy emitted photons.

Details

Equipment

• [2] LED with twisted pair leads
  
• [1] 6 V lantern battery
  
• [1] 330 ohm resistor
  
• [1] Breadboard and 22 gauge hookup wires
• [2] Banana cable
• [2] Insulated alligator clips
  
• [1] Small (~2L) dewar of liquid nitrogen

Safety Equipment

• [1] Safety goggles
• [2] Safety glove

Classroom Assembly

1. Assemble a series circuit with the LED, 330 ohm resistor, and 6 V lantern battery on the breadboard using the cables, wires, and clips.
   
2. Position the dewar under a document camera, if desired.
   

Important Notes

• The room needs to be quite dark for the demo to be seen easily.
  
• The LED leads can slip out of the breadboard when you're not looking.
• Two LEDs are supplied in case one fails, as can happen when repeatedly dipped in liquid nitrogen.
  
• Liquid nitrogen can cause severe burns and blindness. Use with caution. Wear gloves when handling it.

Script

1. Turn off the room lights and close the curtains, if applicable.
2. Put on the safety goggles and gloves.
   
3. Power the LED with the battery.
4. Show that it is yellow at room temperature.
5. Dip the LED in liquid nitrogen for a few seconds until the nitrogen is no longer bubbling vigorously.
6. Show that it is now green when cooled.

Additional Resources