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?

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Understand the concept of band gap energy, which is the energy difference between the valence band and the conduction band in a semiconductor. In CdS, this energy is 2.4 eV.

Recognize that the emission of light in semiconductors occurs when electrons return from the conduction band to the valence band, releasing energy equal to the band gap.

Identify the energy of blue light, which typically ranges from about 2.5 eV to 3.0 eV.

Consider the effect of quantum confinement in quantum dots, which can cause the band gap to increase as the size of the quantum dots decreases.

Determine if reducing the size of CdS quantum dots can increase the band gap energy to fall within the energy range of blue light, thus allowing the quantum dots to emit blue light.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

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.

Related Practice

Textbook Question

Which statement correctly describes a difference between graphene and graphite? (a) Graphene is a molecule but graphite is not. (b) Graphene is a single sheet of carbon atoms and graphite contains many, and larger, sheets of carbon atoms. (c) Graphene is an insulator but graphite is a metal. (d) Graphite is pure carbon but graphene is not. (e) The carbons are sp2 hybridized in graphene but sp3 hybridized in graphite.

Textbook Question

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. (c) What about red light?

Textbook Question

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. (a) What color is the emitted light?

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