The optical properties of quantum dots are known to vary between different types and can be predicted by certain factors.
The material that the quantum dot is constructed from plays a role in determining the intrinsic energy signature of the particle, but the most important factor that affects the optical properties is the size of the dots. Different sized quantum dots change the color emitted or absorbed by the crystal, due to the energy levels within the crystal.
Energy Levels on Fluorescence Spectrum
In the fluorescence spectrum, the color of the light differs according to the energy emitted by the crystal. Red light is associated with lower energy and blue light with higher energy.
The band gap energy of a quantum dot is the difference in energy level between the dot’s excited energy state and its resting state. The quantum dot can absorb fluorescent light at the frequency of its band gap to become excited, or emit the same frequency of light to return to its resting state.
Effect of Size
The size of a quantum dot is inversely proportional to the band gap energy level, and therefore alters the frequency light emitted and has an effect on the color. Smaller dots emit higher energy light that is bluer in color, whereas larger dots emit lower energy red light.
It is also possible for larger quantum dots to posses several energy levels that are more closely aligned. This allows for the absorption of photons with different frequency levels, such as those on the red end of the light spectrum. Additionally, due to these additional energy levels electron-hole pairs can become trapped inside larger quantum dots. Over the long term, this causes larger quantum dots to have a longer lifespan that that of small quantum dots.
Effect of Shape
Recent research has also suggested that the shape of quantum dots may play a role in the band level energy of the dots and, as a result, affect the frequency of fluorescent light emitted or absorbed.
However, there is insufficient evidence to support this hypothesis and the currently available information does not aid the construction of quantum dots to optimize their shape for specific optical properties.
Effect of Structure
The crystal lattice of the quantum dot semiconductor has an effect of the electronic wave function. As a result, a quantum dot has a specific energy spectrum equal to the band gap and a specific density of electronic state on the outside of the crystal.
Quantum dots can also be synthesized with a protective shell to lengthen its lifespan and increase frequency of fluorescent emission. For example a quantum dot composed of cadmium selenide may have a thicker protective shell made of cadmium sulfide.
Optimizing Optical Properties for Imaging
The most important aspect of the quantum dot that affects the optical properties it displays is its size. The size of the dot can be manipulated in manufacturing processes to create a quantum dot suitable for the purposes of optical imaging.
The shape and structure of the quantum dot should also be considered, as well as the material used in the construction process. However, as the size has a direct effect on the optical properties and the frequency of fluorescent light emitted or absorbed by the crystal, it should be given appropriate consideration.