Question

What is a quantum dot? Name 2 applications of them. What are the main differences between semiconductor and metallic nanoparticles.

Answer 1

Quantum dots (QD) are very small semiconductor particles, only several nanometers in size, so small that their optical and electronic properties differ from those of larger particles. These semiconductor nanostructure confines the motion of conduction band electrons, valence band holes, or excitons (bound pairs of conduction band electrons and valence band holes) in all three spatial directions.

Applications:

1.Quantum dots are valued for displays because they emit light in very specific Gaussian distributions. This can result in a display with visibly more accurate colors.

  The ability of Quantum dots (QDs) to precisely convert and tune a spectrum makes them attractive for LCD displays. Previous LCD displays can waste energy converting red-green poor, blue-yellow rich white light into a more balanced lighting. By using QDs, only the necessary colors for ideal images are contained in the screen. The result is a screen that is brighter, clearer, and more energy-efficient. The first commercial application of quantum dots was the Sony XBR X900A series of flat panel televisions released in 2013.

2. Quantum dots also function as photocatalysts for the light-driven chemical conversion of water into hydrogen as a pathway to solar fuel. In photocatalysis, electron-hole pairs formed in the dot under band gap excitation drive redox reactions in the surrounding liquid. Generally, the photocatalytic activity of the dots is related to the particle size and its degree of quantum confinement.

Difference between the semiconductor and metallic nanoparticles:

semiconductor quantum dots have low numbers of individual trapped charge carriers (electrons, holes, excitons) while metallic nanoparticles have collective electron excitations (surface plasmons and bulk plasmons) with resonances often extending into the optical range.

In semiconductor nanoparticles, confinement leads to the formation of discrete levels instead of the valence and conduction bands. The phenomenon causes several effects, for example, an increase in the band gap, thus the possibility of modulating the absorption with the particle size. Also, multiple excitons can be formed between other effects due to the discretization of the energy levels. Confinement in metals leads to an increase in the electric field as all the atoms are on the surface. Metallic nanoparticles show a higher plasmon resonance compared to bulk metals.Applications are for example Surface-Enhanced Raman Spectroscopy, among other effects.