Quantum dots (QDs) are a type of nanomaterial. They can be defined as semiconductor nanocrystals with sizes that can range from 1mm to 20mm, which makes them quite small and almost microscopic! They are so small, that they contain only a few hundreds or thousands of atoms, which are impossible to see with our own eyes.
Due to their relatively small size, quantum dots tend to confine electrons. To understand this better, try to imagine a fish in an aquarium: the fish will be able to move, but will be limited on all 3 dimensions, in terms of how wide, tall, and deep the aquarium space is. As the aquarium gets smaller, there is less space for the fish to move around in freely, ultimately making it more difficult for them to move.
The same thing happens to electrons on a quantum dot; they are totally confined and have nowhere to go! When the size of the material is reduced, the motion of electrons is also reduced.
The Properties of Quantum Dots
The electronic properties of QDs are determined by their size and shape. This means we can manipulate these properties to produce the result we want.
When QDs are excited by a photon of energy, the larger ones (5-6 nm) will emit light in the orange or red wavelength. Smaller QDs emit light in the blue or green wavelength. This way, QDs of the same material, but different sizes, produce different colors. This property of QDs is known as “photoluminescence.”
What Are Quantum Dots Made Of?
Quantum dots can be made from single materials such as silicon or germanium, or from compound materials such as CdSe, PbSe, CdTe, and PbS.
What Are Quantum Dots Used For?
The applications of quantum dots are unlimited. QDs can be used in image sensors, solar cells, LEDs, displays, lasers, computing, and also in medical imaging. Here, you can see some of these applications and how they have been used to change our lives:
Quantum Dot Displays:
Over the last number of years, we´ve seen dramatic improvements in screen technology. LCD and OLED TVs opened our eyes to a new world of viewing with enhance image quality across the areas of sharpness, contrast, and color. From there, displays have become more about picture quality than simply entertainment. The display that can produce the best and most accurate depiction of the world around us is almost always the one that is most desired by consumers.
Many quantum dot manufacturers have already began introducing QD display materials into the market, and by applying this technology to existing LED and LCD displays, they are revealing more visible colors and adding brightness in a way that was never possible before.
To achieve this, QDs are applied to layers of films, filters, glass, and electronics that make up these displays. Generally, you´ll see a Q or Quantum in the product name, to distinguish them from other TVs.
How does it work?
To backlight the display screen, conventional LCD TVs use white LEDs, which produce a broad light spectrum, ultimately decreasing its efficiency to produce saturated colors. When QDs are incorporated, they are capable of directly converting the light from blue LEDs into highly pure narrow-band green and red colors. That way, the colors appear much brighter and vivid to the viewer.
The pharmaceutical and biotech industries depend on fluorescent dyes for many of its applications. They are labeled with antibodies and used to detect a specific target molecule in a sample.
Because of their superiority in color contrast and brightness, and also due to their high stability, QDs can be used to achieve more reliable and reproducible results.
The technology is promising because QDs can absorb photons from a wide range of wavelengths, starting from ultraviolet waves to long, visible wavelengths. In this way, a single light source can excite QDs of various sizes, with no signal overlap. When you use an organic dye, the absorption band is usually narrow, requiring an excitation wavelength for each dye. Also, QDs are more photostable and resistant to photographic bleaching.
Despite its advantages, more research is needed to assess the possible toxicity caused by heavy metals.
Quantum Dot Solar Cells
Solar cells (photovoltaic cells) are devices capable of converting the energy of light into electrical energy using the photovoltaic effect. They are commonly made of silicon, but QDs have received a lot of attention due to their optical properties, depending on the particle size. Several research groups are studying the potential of applying QDs to solar cells, and so far, the greatest efficiency in converting solar energy has been 16.6%.
How does a QD solar cell work?
A photon of light enters the cell and reaches the QD. This leads to a burst of energy within some of the electrons in the QD. These excited electrons are injected into titanium dioxide, where they travel to the surface of the electrode. This process leaves holes in the QD that need to be filled by other electrons. To fill these holes, QDs remove electrons from the electrolyte. The electron-depleted electrolyte, in turn, removes electrons from the counter electrode. This process creates a voltage across the cell and induces a current.