Mesoscopic Physics - Quantum Confinement Effects

Quantum Confinement Effects

Quantum confinement effects describe electrons in terms of energy levels, potential well, valence bands, conduction band, and electron energy band gaps.

Electrons in bulk dielectric material (larger than 10 nm) can be described by energy bands or electron energy levels. Electrons exist at different energy levels or bands. In bulk materials these energy levels are described as continuous because the difference in energy is negligible. As electrons stabilise at various energy levels, most vibrate in valence bands below a forbidden energy level, named the band gap. This region is an energy range in where no electron states exist. A smaller amount have energy levels above the forbidden gap, and this is the conduction band.

The quantum confinement effect can be observed once the diameter of the particle is of the same magnitude as the wavelength of the electron's wave function. When materials are this small, their electronic and optical properties deviate substantially from those of bulk materials. As the material is miniaturized towards nano-scale the confining dimension naturally decreases. But the characteristics are no longer averaged by bulk, and hence continuous, but are at the level of quanta and thus discrete. In other words, the energy spectrum becomes discrete, measured as quanta, rather than continuous as in bulk materials. As a result the bandgap asserts itself: there is a small and finite separation between energy levels. This situation of discrete energy levels is called quantum confinement.

In addition, quantum confinement effects consist of isolated islands of electrons that may be formed at the patterned interface between two different semiconducting materials. The electrons typically are confined to disk-shaped regions termed quantum dots. The confinement of the electrons in these systems changes their interaction with electromagnetic radiation significantly, as noted above.

Because the electron energy levels of quantum dots are discrete rather than continuous, the addition or subtraction of just a few atoms to the quantum dot has the effect of altering the boundaries of the bandgap. Changing the geometry of the surface of the quantum dot also changes the bandgap energy, owing again to the small size of the dot, and the effects of quantum confinement.