CdS has a band gap of 2.4 eV. If large crystals of CdS are illuminated with ultraviolet light, they emit light equal to the band gap energy. (b) Would appropriately sized CdS quantum dots be able to emit blue light?

Key Concepts

Band Gap Energy

The band gap energy is the energy difference between the valence band and the conduction band in a semiconductor. It determines the wavelengths of light that a material can absorb and emit. For CdS, a band gap of 2.4 eV means it can absorb photons with energy equal to or greater than this value, leading to the emission of light corresponding to this energy when excited.

Quantum Dots

Quantum dots are nanoscale semiconductor particles that have quantum mechanical properties. Their electronic and optical properties can be tuned by changing their size, which affects the band gap. Smaller quantum dots have larger band gaps, allowing them to emit light at shorter wavelengths, such as blue light, when excited.

Photoluminescence

Photoluminescence is the emission of light from a material after it absorbs photons. In semiconductors like CdS, when the material is excited by ultraviolet light, electrons are promoted to the conduction band and, upon returning to the valence band, they release energy in the form of light. The wavelength of this emitted light is determined by the band gap energy of the material.