Question

As the particle size reduces, the optical absorption spectra shifts towards:

• A. Red
• B. Green
• C. Blue
• D. Yellow

Hint:

Remember the relationship: Smaller size \(\rightarrow\) Stronger confinement \(\rightarrow\) Larger band gap \(\rightarrow\) Higher energy absorption \(\rightarrow\) Shorter wavelength \(\rightarrow\) Blueshift.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
This question describes the quantum confinement effect observed in semiconductor nanoparticles (quantum dots). When the size of a particle is reduced to the nanometer scale (comparable to the Bohr exciton radius), its electronic and optical properties change significantly from those of the bulk material.
Step 2: Detailed Explanation:
As the particle size decreases, the movement of electrons and holes becomes confined in all three dimensions. This confinement leads to the quantization of energy levels, similar to the "particle in a box" problem. The result is that the continuous energy bands of the bulk material are replaced by discrete energy levels.
The most important consequence is that the effective band gap (\(E_g\)) of the material increases as the particle size decreases.
The energy of absorbed photons is related to their wavelength (\(\lambda\)) by the equation \(E = hc/\lambda\).
Since the band gap \(E_g\) increases with decreasing size, the material must absorb photons of higher energy to excite an electron across the gap. Higher energy corresponds to a shorter wavelength.
In the visible spectrum, blue light has a shorter wavelength (and higher energy) than red, yellow, or green light. Therefore, the absorption peak shifts towards the blue end of the spectrum. This is known as a "blueshift".
Step 3: Final Answer:
As particle size reduces, the band gap increases, requiring higher energy (shorter wavelength) photons for absorption. This causes the absorption spectra to shift towards blue.