Question

What happens to the optical absorption spectrum of a nanoparticle as its size is reduced?

• A. It shifts toward longer wavelengths (red shift)
• B. It shifts toward shorter wavelengths (blue shift)
• C. It remains unchanged
• D. It disappears completely

Hint:

Smaller nanoparticle → Larger band gap → Blue shift in absorption spectrum

Answer 1

The Correct Option is B

Concept:
Nanoparticles exhibit unique optical properties due to the quantum confinement effect. When the size of a particle approaches the nanoscale, the motion of electrons becomes confined, altering the electronic energy levels of the material.
Step 1:Understanding quantum confinement.
As the size of a nanoparticle decreases, the energy levels become more widely spaced because electrons are confined in a smaller region.
Step 2:Effect on energy transitions.
Larger spacing between energy levels means that a higher amount of energy is required for electronic transitions from the valence band to the conduction band.
Step 3:Effect on wavelength.
Since energy and wavelength are inversely related: \[ E = \frac{hc}{\lambda} \] an increase in transition energy leads to a decrease in wavelength. This results in a blue shift in the absorption spectrum. Conclusion:
Therefore, when the size of a nanoparticle decreases, its optical absorption spectrum shifts toward shorter wavelengths (blue shift).