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Many areas of nanotechnology and advanced materials are becoming the basis for the next generation of various technologies because you can miniaturize devices without impacting performance. In fact, in many cases, the performance can be increased simultaneously. Currently, there are many aspects of nanotechnology in electronics, with more becoming a commercial reality each year. This blog looks at one specific arm of nanotechnology, the quantum dot, which is commercially used today in quantum dot light-emitting diode (branded as QLED) TVs, but the scope of technological applications extends far beyond televisions.
Let’s begin by defining quantum dots and exploring their unique properties. Quantum dots are some of the smallest nanomaterials in existence, with most only a few nanometers (nm) in diameter and rarely any above 10nm in size. Their small size means they typically have between 10 and 50 atoms, and as such, display unique electronic properties and quantum effects not seen with larger materials.
The electronic properties of quantum dots are often seen as a middle ground between conventional semiconductor materials and discrete molecules—independent molecules that are not covalently attached to other molecules. The most unique and important property of quantum dots—for practical purposes—is their fluorescence, and more importantly, the versatility in the fluorescence to produce different colors based on the size of the quantum dot.
Perhaps the most noteworthy aspect that quantum dots have over other molecules is that they can produce any color of light required by simply changing the size of the quantum dot. This makes them a highly effective and versatile material for generating colored light—be it for displays, biological imaging, or for being a harvester and absorber of photons in solar cells.
The semiconducting nature of quantum dots means there is a natural electronic bandgap between the valence band and the conduction band in the quantum dot. Naturally, like any semiconductor, this bandgap can be overcome, and the electrons can travel from the valence band to the conduction band with an energy input, such as a photon of light or an electrical input.
Unlike many other semiconductors, the small size of quantum dots means their electrons are confined in a nanosized space known as a quantum box. In this space, the electrons are confined in all three spatial dimensions, and as a result, quantum dots are often referred to as zero-dimensional (0D) materials. It is this confinement that governs the fluorescence in quantum dots.
Fluorescence creates a distinct color in quantum dots and occurs when the energy input across the quantum dot causes the confined electrons to move from the valence band to the conduction band. After excitation, the electrons lose energy, returning to the ground state where they become confined again. It is the relaxation of the electrons to the ground state that causes the excess energy from the excitation to be released in the form of light.
The color of the light emitted solely relies on the size of the quantum dot. The general rule is that the smaller the quantum dot, the further away the valence and conduction bands are from each other (and vice versa for larger quantum dots having the two bands closer together). With larger bandgaps, more energy is requires to promote the electron from the ground state to an excited state. Since more energy is required as the initial input, more energy is then released during the relaxation of the electron, causing a shift towards the blue light region of the visible light electromagnetic spectrum. On the flip side, less energy is required for larger quantum dots, so less energy gets released and the light emitted is towards the red end of the visible light spectrum. Quantum dot sizes in between these two extremes can then be tailored in size to emit any color of the visible light spectrum.
Using quantum dots in QLED TVs has been a commercial reality for a number of years now, with the performance of the TVs surpassing many organic light-emitting diode (OLED), liquid-crystal display (LCD), and light-emitting diode (LED) devices. The ability to produce crisp colors across the whole color spectrum has been the key to the quantum dots’ success in different displays. QLED TVs work in a similar way to most other TVs, but the tuning of the smaller quantum dot size enables narrow band green, blue, and red light to be emitted. This is then used to build up pictures on the screen that are much brighter and clearer, thanks to the bright fluorescence mechanism of the quantum dot.
Designers have been using quantum dots to improve the performance of different solar cells, including cadmium selenide (CdSe), copper indium selenide (CIS), lead sulfide (PbS), lead selenide (PbSe), indium phosphide (InP), and perovskite solar cells. In addition, quantum dots are well-suited for use in multi-junction solar cells because different materials absorb different wavelengths of the electromagnetic spectrum, so the tunable nature of quantum dots can be utilized to meet the absorption demands of different junctions.
The tunability of quantum dots also means they can be tailored to match incident light and extract charge carriers without losing voltage in a process known as thermalization. In this process, electrons move to the band edges when excited and lose a lot of energy in the form of heat. The ability to negate this heat loss means that more efficient light harvesting devices can be created.
Furthermore, quantum dots can also generate multiple charge carriers from a single incident photon, helping generate a higher current per photon hitting the solar cell and increasing the power conversion efficiency (PCE) of the device. Finally, one of the key areas of interest for quantum dots is the potential to enhance the PCE of solar cells beyond the Shockley-Queisser limit (32 percent for single junction silicon cells), which is certainly something to keep an eye out for in the future.
Quantum dots have also found use in biomedical applications, namely bioimaging and biosensing. In bioimaging, quantum dots’ natural fluorescence means they can be used as a fluorescent label to illuminate biological tissues. The inherently small size of quantum dots means that they are small enough to pass through biological membranes into cells, tissues, and organs of interest. They can then be illuminated externally to reveal different biological features. At the cellular level, quantum dots are used to visualize intracellular components and can be used on a more macro level to look at potential issues in tissues and organs.
On the biosensing side, quantum dots can be integrated into the existing active sensing surfaces of a biosensor to improve the selectivity, efficiency, accuracy, and detection sensitivity of the device. Quantum dot enhanced biosensors are used across diagnostic, toxicological, and post-surgical medical applications. Additionally, these advanced biosensors have been used in environmental applications to detect different contaminants in water and soil samples.
Quantum dots exhibit unique properties and are one of the smallest materials in use today. The confinement of the electrons in all three dimensions means that the 0D nature of quantum dots give rise to some unique and functional properties. One of the most important of these is the ability to fluoresce under an external energy stimulus where different wavelengths of bright light are emitted based on the size of the quantum dots. The properties and color emitted are easily tuned by using a different sized quantum dot and this tunability has enabled quantum dots to be used in a wide range of applications, but most notably in QLED displays, solar cells, bioimaging, and biosensors.
Liam Critchley is a writer, journalist and communicator who specializes in chemistry and nanotechnology and how fundamental principles at the molecular level can be applied to many different application areas. Liam is perhaps best known for his informative approach and explaining complex scientific topics to both scientists and non-scientists. Liam has over 350 articles published across various scientific areas and industries that crossover with both chemistry and nanotechnology.
Liam is Senior Science Communications Officer at the Nanotechnology Industries Association (NIA) in Europe and has spent the past few years writing for companies, associations and media websites around the globe. Before becoming a writer, Liam completed master’s degrees in chemistry with nanotechnology and chemical engineering.
Aside from writing, Liam is also an advisory board member for the National Graphene Association (NGA) in the U.S., the global organization Nanotechnology World Network (NWN), and a Board of Trustees member for GlamSci–A UK-based science Charity. Liam is also a member of the British Society for Nanomedicine (BSNM) and the International Association of Advanced Materials (IAAM), as well as a peer-reviewer for multiple academic journals.