A Quantum Dot (QD) is a chunk of semiconductor material with quantum-like properties whose size is on the order of a few nanometers to a few hundred nanometers. QD's confine electron-hole pairs, the excitons. The separation between the electron in the conduction band and the hole it leaves behind in the valence band is called the Exciton Bohr Radius. Quantum confinement is the set of conditions under which the size of the crystal is smaller than or in the order of the Exciton Bohr Radius. Under quantum confinement the energy levels of the crystal may be treated as discrete. By definition, quantum dots are in a state of quantum confinement.4
Because the QD has discrete energy levels, much like an atom, they are sometimes called ‘artificial atoms’. The emission frequency of a QD depends on the size of the bandgap; the distance between the valence band and conduction band. Therefore it is possible to change the output wavelength of the QD simply by changing its size or, more difficult, by tuning the bandgap itself. The larger the bandgap, the more towards the red end of the spectrum the fluorescence is. The smaller the bandgap, the more towards the blue end it is.
Larger quantum dots have more energy levels which are more closely spaced. However, the larger and more red-shifted the QD is, the less the quantum properties are.3
Figure 5 shows the photoluminescence (PL) spectrum of the QD ensemble used by Yoshie et al. The samples were optically pumped by an 770 nm Ti:sapphire continuous wave laser. The lowest transition line at ~1200 nm and the first excited transition line at 1125 nm are visible.
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