How Quantum Dots Work

A Special Class of Semiconductors

Quantum dots, also known as nanocrystals, are a special class of materials known as semiconductors, which are crystals composed of periodic groups of II-VI, III-V, or IV-VI materials. Semiconductors are a cornerstone of the modern electronics industry and make possible applications such as the Light Emitting Diode and personal computer. Semiconductors derive their great importance from the fact that their electrical conductivity can be greatly altered via an external stimulus (voltage, photon flux, etc), making semiconductors critical parts of many different kinds of electrical circuits and optical applications. Quantum dots are unique class of semiconductor because they are so small, ranging from 2-10 nanometers (10-50 atoms) in diameter. At these small sizes materials behave differently, giving quantum dots unprecedented tunability and enabling never before seen applications to science and technology.

The usefulness of quantum dots comes from their peak emission frequency's extreme sensitivity to both the dot's size and composition, which can be controlled using Evident Technologies' proprietary engineering techniques. This remarkable sensitivity is quantum mechanical in nature, and is explained as follows.

Bands and Bandgaps

The electrons in bulk (much bigger than 10 nm) semiconductor material have a range of energies. One electron with a different energy than a second electron is described as being in a different energy level, and it is established that only two electrons can fit in any given level. In bulk, energy levels are very close together, so close that they are described as continuous, meaning there is almost no energy difference between them. It is also established that some energy levels are simply off limits to electrons; this region of forbidden electron energies is called the bandgap, and it is different for each bulk material. Electrons occupying energy levels below the bandgap are described as being in the valence band. Electrons occupying energy levels above the bandgap are described as being in the conduction band.

Electrons and Holes

In natural bulk semiconductor material, an extremely small percentage of electrons occupy the conduction band the overwhelming majority of electrons occupy the valence band, filling it almost completely. The only way for an electron in the valence band to jump to the conduction band is to acquire enough energy to cross the bandgap, and most electrons in bulk simply do not have enough energy to do so. Applying a stimulus such as heat, voltage, or photon flux can induce some electrons to jump the forbidden gap to the conduction band. The valence location they vacate is referred to as a hole since it leaves a temporary "hole" in the valence band electron structure.

Bulk Semiconductors - A Fixed Range of Energies

A sufficiently strong stimulus will cause a valence band electron to take residence in the conduction band, causing the creation of a positively charged hole in the valence band. The raised electron and the hole taken as a pair are called an exciton. There is a minimum energy of radiation that the semiconductor bulk can absorb towards raising electrons into the conduction band, corresponding to the energy of the bandgap. It is established that because of the continuous electron energy levels as well as the number of atoms in the bulk, the bandgap energy of bulk semiconductor material of a given composition is fixed.

It is also established that electrons in natural semiconductor bulk that have been raised into the conduction band will stay there only momentarily before falling back across the bandgap to their natural, valence energy levels. As the electron falls back down across the bandgap, electromagnetic radiation with a wavelength corresponding to the energy it loses in the transition is emitted. It is established that the great majority of electrons, when falling from the conduction band back to the valence band, tend to jump from near the bottom of the conduction band to the top of the valence band- in other words, they travel from one edge of the bandgap to the other. Because the bandgap of the bulk is fixed, this transition results in fixed emission frequencies. Quantum dots offer the unnatural ability to tune the bandgap and hence the emission wavelength.

Quantum Dots - Quantum Confinement

Quantum dots are also made out of semiconductor material. The electrons in quantum dots have a range of energies. The concepts of energy levels, bandgap, conduction band and valence band still apply. However, there is a major difference. Excitons have an average physical separation between the electron and hole, referred to as the Exciton Bohr Radius this physical distance is different for each material. In bulk, the dimensions of the semiconductor crystal are much larger than the Exciton Bohr Radius, allowing the exciton to extend to its natural limit. However, if the size of a semiconductor crystal becomes small enough that it approaches the size of the material's Exciton Bohr Radius, then the electron energy levels can no longer be treated as continuous - they must be treated as discrete, meaning that there is a small and finite separation between energy levels. This situation of discrete energy levels is called quantum confinement, and under these conditions, the semiconductor material ceases to resemble bulk, and instead can be called a quantum dot. This has large repercussions on the absorptive and emissive behavior of the semiconductor material.

Quantum Dots - A tunable range of energies

Because quantum dots' electron energy levels 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. The bandgap in a quantum dot will always be energetically larger; therefore, we refer to the radiation from quantum dots to be "blue shifted" reflecting the fact that electrons must fall a greater distance in terms of energy and thus produce radiation of a shorter, and therefore "bluer" wavelength.

Size Dependent Control of Bandgap in Quantum Dots

As with bulk semiconductor material, electrons tend to make transitions near the edges of the bandgap. However, with quantum dots, the size of the bandgap is controlled simply by adjusting the size of the dot. Because the emission frequency of a dot is dependent on the bandgap, it is therefore possible to control the output wavelength of a dot with extreme precision. In effect, it is possible for Evident Technologies to tune the bandgap of a dot, and therefore specify its "color" output depending on the needs of the customer.