There are two well-known essential ways in order to manufacture materials in a nanoscale which are top-down approach and bottom-up approach. The top-down approach essentially means synthesizing nanomaterials by breaking down the bulk amount of materials into nanoscales. Chemical Vapour Deposition (CVD) is one example of bottom-up approach where nanomaterials are synthesized by assembling the particles together to make a larger structure. Nanomaterials can be produced in several forms or structures depending on the intended use of these materials. The wells, wires and dots are just nanostructure of the materials, but they are called quantum wells, quantum wires and quantum dots because of the nanostructure of them which have explained through quantum mechanics [1]. Moreover, they are made of semiconductor components such as Cadmium Selenide (CdSe) Cadmium Sulfide (CdS).
Fig. 1 Effect of reduced dimensionality. (a) Charge carriers are confined in a semiconductor (red) with a smaller bandgap than the cladding semiconductor (blue). The confinement changes the density of states D(E). Ec is the conduction-band edge of a 3D semiconductor [1]
As shown in figure 1, the bulk is three-dimensional (3D) structure which means that electrons in the conduction band and holes in the valence band are moving freely in all space's dimensions. Quantum well is the two-dimensional (2D) structure where one of the three dimensions will be accomplished the range of nanoscale (1 – 100nm) while the other two dimensions remain on their actual size (i.e. electrons are confined in one dimension). Nanowires, nanorods nanotubes, and thin films are all classified as (2D) structures. Then, the quantum wire which is one-dimensional structure. Here the electrons are confined in two dimensions which simply means that two directions are reduced to nanometer range and one dimension remains. Finally, zero-dimensional structure or quantum dot where all of the dimensions reduced to the nanoscale (i.e. three-dimensional quantization). While electrons and holes are confined in all three directions, surrounding materials have high energy gap (Eg means the energy needed by an electron to leave the atom)[2], this quantum confinement of electrons in the three-dimensional quantum dots gives these dots a good property in dealing with light effectively and making it an ideal candidate to form devices such as solar cells[3,4].
References:
[1] Bimberg, D. and Pohl, U.W., 2011. Quantum dots: promises and accomplishments. Materials Today, 14(9), pp.388-397.
[2] Rossetti, R., et al., J Chem Phys (1983) 79, 1086
[3] Shockley, W., and Queisser, H. J., J Appl Phys (1961) 32, 510
[4] Friedman, D. J., Curr Opin Solid St M (2010) 14, 131.