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Thin films, surfaces and nanomaterials

How does the color of emission change when the size of semiconductor nanocrystals (quantum dots) such as CdSe is reduced?

  1. AThe band gap becomes smaller and the emission color shifts to longer wavelengths (toward red)
  2. BThe emission is the same color as the bulk crystal, regardless of size
  3. CThe band gap becomes larger and the emission shifts to shorter wavelengths (toward blue)
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Answer: C. The band gap becomes larger and the emission shifts to shorter wavelengths (toward blue)

When the particle size becomes comparable to or smaller than the exciton Bohr radius, a measure of the spatial extent of the electron and hole (about 5–6 nm for CdSe), the carriers are confined in a small space and the energy levels become discrete and more widely spaced. This is the quantum size effect: the smaller the particle, the larger the effective band gap, and the emission shifts to shorter wavelengths (toward blue). The emission color of a single material can therefore be tuned just by changing the size, which is used for color conversion in displays. The choice saying the gap gets smaller is the reverse. The same color as the bulk is obtained when the particles are much larger than the Bohr radius.

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