What is Quantum size effect?

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.

Answer: A. More atoms sit at the surface, and the surface energy raises the free energy of the solid, so the liquid becomes stable at a lower temperature

Surface atoms have lost some of their bonds, so a surface energy term is added to the free energy of the particle. The smaller the particle, the larger the fraction of surface atoms and the more the free energy of the solid is pushed up, so the temperature at which the free energies of solid and liquid become equal, the melting point, falls (the Gibbs–Thomson effect). Considering the solid–liquid interfacial energy, the melting point depression is roughly inversely proportional to particle size. The quantum size effect concerns electron energy levels and is not the main cause of melting point depression in metals. Also, the heat of fusion actually tends to decrease in nanoparticles, so the reasoning that “a larger heat of fusion makes melting easier” runs backward.