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There are many types of nanomaterials that have emerged and established themselves over the years, and one area of growing interest is the use of quantum dots. While their commercial applications are mostly limited to anti-counterfeiting packaging at the moment, they have the potential to be used in applications ranging from electronics and optoelectronics to disinfectant surfaces and many more in between.
If you ask most people about what types of nanomaterials they know about, many will mention graphene, many will also mention carbon nanotubes, and many will understand the premise of there being many different types of nanoparticles. However, while it is a material of growing interest, and many people have heard the name, there are a lesser number of people who know about quantum dots (or quantum dot nanoparticles as they are otherwise called), even sometimes within the nanotechnology community. The main possible reason for this is that they have not been as widely documented, have not been hyped up like other materials, and when they are mentioned in the news, there is often no description of what they are, just their name.
Basics of Quantum Dots
So, what are quantum dots or quantum dot nanoparticles? In short, they are nanoparticles which are very, very small in size, and smaller than most other nanoparticles that are manufactured/synthesized. On average, the typical quantum dot is anywhere between 2 and 10 nm, but they can be larger if required. Quantum dots are typically (and traditionally) made of semiconductor materials, such as cadmium selenide (CdSe) or gallium arsenide (GaAs), but developments in the graphene field have also led to the creation of graphene quantum dots. Some quantum dots are also core-shell nanoparticles, in which multiple layers of semiconductors are constructed, where each layer generally has differing properties.
The small size of quantum dots means that they have some very interesting properties because quantum effects come in to play at this scale, which changes the optical, magnetic and electronic properties compared to their bulk equivalent. But, it’s not just the size, as the shape and composition also influence the properties of the quantum dot. In terms of their quantum structure, quantum dots are classified as 0D materials, which means that the movement of electrons is restricted in all three dimensions outside of the quantum dot (which means the electrons can’t tunnel outside of the quantum dot). One key property of a quantum dot is their ability to fluoresce when illuminated with a light source, and it is this property that is being exploited for many applications.
Fluorescence of Quantum Dots
The fluorescence is due to the small diameters of the quantum dots themselves and the semiconductor materials that they are made of—as semiconductors have energy bands (valence and conduction) which are relatively close to each other and electrons can move between them with a small energy input. Because quantum dots are so small, when the electrons are excited by light, electrons that absorb the light will escape the quantum dot structure and become delocalized. This leaves holes in the quantum dot where the electrons used to be, and the moving electrons enable the quantum dot to conduct electricity. Because the electrons are moving around such a small space, it reduces the energy difference between the valence and conduction band to where the other electrons can move freely between the two energy bands. When the electrons that have moved up to the conduction band move back to the valence band, they reduce their energy as they are going from an excited to a ground state. The excess energy that is left over is then emitted as fluorescent light.
The color that is emitted by a quantum dot is highly dependent upon its size, and by changing the size of the quantum dots, you can tune the color of the quantum dot when they fluoresce. The reason for this is due to the energy gap between the valence and conduction bands within the quantum dot. In short, the smaller the quantum dot, the further away from each other these energy bands are, and therefore, the larger the quantum dot, the nearer the energy bands are to each other. This leads to small quantum dots having a blue color and larger quantum dots to emit a red color. Quantum dots between the two extremes can then be tuned to have a specific color by using their size as a guide.
Summary
Given that quantum dots have such a small diameter, and have specific compositions, there are only a select number of methods that can be used to fabricate them. The two most common methods to realize quantum dots are the top-down nanofabrication techniques, electron beam lithography (EBL) and molecular beam epitaxy (MBE). EBL typically ‘writes’ the quantum dot on to the surface of a material, whereas MBE uses precursor gases (containing the desired atoms/ions) and deposits them directly on a surface so that they form into a quantum dot.
Sources and Further Reading
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