Quantum dots are man-made nanostructures that are revolutionizing a number of fields, from television displays to healthcare to solar power. Read on to learn how quantum dots work and their most promising applications.

A quantum dot (QD) is an artificial atom, essentially a small box-like nanostructure about 100 nm wide per side that holds a variable number of electrons. The QD nanostructure, which is crystalline in nature, confines the motion of electrons in all three dimensions. Unlike a natural atom, where electrons are attracted to a positively-charged nucleus, the electrons in a QD are trapped in a bowl-like well and tend to fall inwards towards the bottom of the well. This enables the QD to function as a nanoscale semiconductor.

First theorized in the 1970s, by the early 1980s quantum dot technology advanced to enable the first working units. QDs were conceived on the theory that miniaturing semiconductor particles to a certain degree would introduce quantum effects that could control the energy levels of the electrons passing through. Changing these energy levels affects the optical properties of the particle, thus producing light of different wavelengths. QDs, then can be made to emit or absorb very bright and pure light of different colors based on the particle’s size.

Other properties of a quantum dot can be determined by its shape, composition, and structure – whether it’s solid or hollow, for instance. Designing quantum dots with specific properties results in a variety of applications for the technology.

There are three main types of quantum dots being manufactured today. They’re classified based on composition and structure, in addition to differences in size and shape.

Core-type quantum dots

A core-type quantum dot is composed of a single material. The internal composition is typically a chalcogenide (such as a selenide, sulfide, or telluride) of a metal such as cadmium, led, or zinc. Changing the crystallite size of a core-type QD changes the nanocrystal’s photoluminescence and electroluminescence qualities.

Core-shell quantum dots

A core-shell quantum dot (CSQD) is created by surrounding a core-type material with a second semiconductor material that has a wider band gap. For example, one popular type of CSQD features a core of cadmium and selenium (CdSe) and a shell of zinc sulfide (ZnS).

This type of hybrid nanoparticle has an improved quantum yield compared with core-type QDs. That’s because coating the core quantum dots makes them more robust.

Alloyed quantum dots

The third most common type of QD is composed of multiple materials in a homogeneous mixture. These so-called alloyed quantum dots combine two different semiconductors with different band gaps, resulting in new properties distinct from the original materials.

Instead of tuning optical and electronic properties by changing the crystallite size, alloyed QDs enable properties to be tuned by changing the composition and internal structure of the QD. This makes alloyed QDs the better choice for applications with distinct size restrictions.

The small size of quantum dots and their ability to emit a veritable rainbow of pure, bright light make them ideal for use in a variety of technologies and applications. The most significant current and potential applications include:

• Solar cells
• Flat-screen television displays (such as new QD-OLED models from Samsung and Sony)
• Biomedical imaging and bioanalytics
• Solid state lighting
• Logic gates and quantum computers

Of these applications, solar technology represents one of the most significant potential uses of quantum dot technology. Experts say that QD solar cells have the potential to increase thermodynamic conversion efficiency by up to 66%, which could dramatically reduce the cost of solar energy.

QDs are used in three different solar cell configurations:

• Photoelectrodes with QD arrays
• QD-sensitized nanocrystals
• Perovskite-based QDs dispersed in a blend of electron- and hole-conducting polymers

All of these configurations have their own unique challenges, especially the last. Perovskites are materials with a specific crystalline structure that offers high performance at relatively low cost. Efficiently coating photovoltaic devices with a uniform layer of perovskite quantum dots requires the use of large-scale deposition methods, such as slot-die coating.

To advance the development of this technology, FOM Technologies offers slot-die coating machines that can precisely deposit a thin liquid film of quantum dots onto the surface of any substrate. The result promises to be lower cost, higher efficiency solar cells, thanks to the use of quantum dot technology.