CHEMISTRY 2000 Topics of Interest #3: Quantum Dots

Research into quantum dots is poised to take off within the next few years. They are of great interest to manufacturers of energy- efficient displays and lighting. As shown below, the same material can generate practically any desired colour just by changing particle size! Special Report in Nature (2010) 459, Image from

Quantum Dots We saw in the last section of Chemistry 1000 that isomers (same molecular formula; different shape) could be different colours if the difference in energy between the highest energy electron and the lowest energy “empty space” (“hole”) was different for the two isomers. Only the colour(s) of light with energies matching this energy gap would be absorbed and the substance would reflect the rest of the colours of light.

Quantum Dots Earlier in Chemistry 1000, we saw that burning different salts gave rise to different colours. Energy was absorbed, exciting an electron from a lower energy orbital to a higher energy orbital. Light was then released as the electron dropped back into the lower energy orbital.

Quantum Dots The quantum dots have something in common with both examples: Like the co-ordination complex isomers, what is important is the energy gap between the highest energy electron and the lowest energy “hole” Like the flaming salts, the coloured light is released when the electron “falls” back down to the ground state. So, why mention quantum dots in Chemistry 2000? They are semiconductors! Our explanation of band theory explains why different colours are absorbed by different sizes of particles! Image from University of Washington

Quantum Dots Recall the diagram at the right: As the quantum dot gets bigger, it contains more atoms. As the quantum dot contains more atoms, the states get closer and closer in energy. As the states get closer and closer in energy, the colour of light released when an electron falls back from an excited state to a ground state gets redder…