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Following in the footsteps of graphene, stem-cells, and artificial intelligence; Quantum dots are the latest futuristic-sounding technology to emerge from the mysterious realm of research science and enter the public vernacular. Often referred to as zero-dimensional objects due to their distinct lack of length, width and depth; quantum dots (or QDs for short) each consist of a miniscule speck of semiconductor. Measuring only a few nanometres in diameter, QDs have become something of a central theme in nanotechnology: their small size imbues them with a number of weird physical characteristics that open up a catalogue of applications in medicine, imaging, quantum computing, photovoltaics, and many other high-tech frontiers. But quantum dots’ biggest commercial application (and the one to which they owe their newfound fame) lies in the world of display electronics, where their unique optical properties are leading the way to a new generation of screens that are richly-coloured, efficient – and possibly flexible.
In order to understand what’s so special about quantum dots, we must first understand their physical structure. QDs are microscopically small spheres typically made out of zinc selenide, cadmium selenide, or indium phosphide. These materials are all semiconductors, which lie somewhere in between insulators and conductors.1 It turns out that when you make semiconductors very small, their electrons start to behave strangely – an effect known as quantum confinement. This quirk of electron behaviour means that when a QD absorbs light, it re-emits light of a specific wavelength, depending on the QD’s size. By carefully controlling the synthesis of QDs, we can control their size, and therefore finely tune the colour of light that they emit.
This process of absorbing light and re-emitting specific colours of light is not unique: it’s also exhibited by individual atoms. But the colours emitted by atoms are limited by its electron structure. Any two Hydrogen atoms, for example, will emit exactly the same colours. The “tunability” of quantum dots, however, means that we can harness this behaviour and use it to generate colours of our choice, effectively like man-made atoms whose electron structures we can control.
Quantum dots are drastically changing a number of scientific fields. In optics, the tuneable emission characteristics of QDs have enabled the construction of lasers of wavelengths that were previously impossible.2 In biomedical science, QDs have emerged as a brilliant imaging tool, where their broadband absorption and long lifetime make them superior to conventional fluorescent dye markers.3 Perhaps most impressively, studies show that using QDs in solar cells could redefine the maximum theoretical efficiency for photovoltaic devices.4
The application you’re likely to witness first, however, is a new breed of television screens that exploit quantum dots to produce extremely vibrant images.
Quantum Dot Displays: The Future of Screens
The human eye contains only three types of colour-sensitive cells, each sensitive to a different peak wavelength of light. These wavelengths correspond to red, green and blue colours. We exploit this fact when we make screens, dividing each pixel into a red, green and blue subpixel. Varying the relative brightness of each subpixel tricks our eyes into seeing different colours. Switch on the red and green subpixels and you’ll see yellow, even though in fact no yellow light is entering your eyes.
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This is the basis by which all screens operate. In modern LCD screens – you probably have one of these in your living room – each pixel consists of a white LED backlight (which produces a mixture of wavelengths), layered with three coloured filters to act as subpixels. Each of these subpixels is covered by a liquid crystal shutter which controls how much light of each colour gets through. While this does the job well enough, LCD TVs suffer a number of problems, including narrow viewing angle, poor energy efficiency and inaccurate colour reproduction. Even an ultra-HD LCD TV can only replicate around 70% of the visible colours that are produced by the natural world (as defined by the International Telecommunications Union’s BT.2020 standard).5
To enable precise colour mixing, the spectrum of light emerging from each subpixel must be as narrow as possible. Filtering the light is a double-edged sword: while a narrow filter produces a pure spectrum, it also filters out more light, thus decreasing the brightness of the screen. The result of this trade-off between brightness and chromatic accuracy results in screens that are somewhat inefficient and somewhat inaccurate.
Quantum dots offer a neat solution to this problem: their broad absorption and sharp emission characteristics make them ideal for use as subpixels. When QDs are layered over an LED backlight, they effectively purify the light, converting unwanted wavelengths into pure red, green and blue peaks rather than filtering them out.
This has serious advantages: By lowering the amount of energy required to produce an image of a given brightness, QDs can make screens incredibly energy efficient.6 The sharp emission peaks of QD displays mean they offer unprecedented colour reproduction, replicating up to 93% of the colours visible in the natural world.
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Quantum dot displays offer significant advantages in colour reproduction and efficiency even against the other major new screen technology: OLED TVs. In addition, due to the relative ease with which QDs can be manufactured, QD displays are vastly cheaper to produce than their OLED counterparts.1 QDs can also be deposited on flexible substrates; offering a real possibility of flexible, foldable screens.
With early models hitting the market already, the future of quantum dot displays is bright, vibrant and efficient.
Swiss nanomaterials and electronics manufacturer Avanta produces high-performance quantum dots for display applications. Avanta quantum dots are Cadmium free and RoHS compliant – and with production capacity at the multi-ton scale, they will likely play a critical role in the emergence of QD display technology.
References and Further Reading
- Your Guide to Television’s Quantum-Dot Future. Luo, Z., Manders, J. & Yurek, J. (2018).
- Quantum dot laser. Ledentsov, N. N. Semicond. Sci. Technol 26, (2011).
- Semiconductor Nanocrystal Quantum Dots: Synthesis, Assembly, Spectroscopy and Applications. Andrey L Rogach (Ed.). (SpringerWienNewYork, 2008).
- High Efficiency Carrier Multiplication in PbSe Nanocrystals: Implications for Solar Energy Conversion. Schaller, R. D. & Klimov, V. I. doi:10.1103/PhysRevLett.92.186601
- Display Color Gamuts Shoot-Out: NTSC to Rec.2020. Available at: http://www.displaymate.com/Display_Color_Gamuts_1.htm. (Accessed: 11th April 2018)
- JSS FOCUS ISSUE ON NOVEL APPLICATIONS OF LUMINESCENT OPTICAL MATERIALS Review—Quantum Dots and Their Application in Lighting, Displays, and Biology. Frecker, T., Bailey, D., Arzeta-Ferrer, X., Mcbride, J. & Rosenthal, S. J. ECS J. Solid State Sci. Technol. 5, 3019–3031 (2016).
This information has been sourced, reviewed and adapted from materials provided by Avantama Ltd - Nanoparticle Dispersions.
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