Quantum Dots

What They Are, How They Are Made, And How They Are Used 9 min. read -

Krishna Kowlgi, John T Hartigan, Stephen Banks

 What are quantum dots?  

Quantum dots are tiny particles or nanocrystals of a semiconducting material with diameters in the range of 2-10 nanometers (1000-100,000 atoms).1 They were first discovered in 1980.2 They display unique electronic properties, distinct from those of their bulk form (> μm) or constituent atoms, that are partly the result of the unusually high surface-to-volume ratios for these particles.3,4,5 The most apparent result of this is fluorescence, wherein the nanocrystals when illuminated by a single color can produce distinctive colors that are determined by the size of the particles.6 

Quantum dots are composed of thousands of atoms but due to their small size (relative to bulk), the motion of the electrons in the dots are confined to discrete energy levels as observed in atoms which lead to the dots being nicknamed 'artificial atoms'.7,8 The energy levels and the respective emission colors vary with the size and composition of the dots. Additionally, because of the high level of control possible over the size of the nanocrystals produced, these semiconducting structures can be tuned during manufacturing to emit a specific color of light that is of a lower energy to the illuminating light (down conversion).9 

How are quantum dots made? 

Quantum dots are crystalline #nanoparticles and they are made using a wide range of techniques ranging from chemical-based ones such as colloidal synthesis to physical-based ones such as vapor deposition. Colloidal synthesis techniques are further classified into batch-mode synthesis including hot-injection, heat-up, cluster assisted, and microwave-assisted synthesis methods. 

 How are quantum dots used? 

Quantum Dots Applications

The unique size and composition tunable optoelectronic property of quantum dots make them very appealing for a variety of applications and new technologies.10,11 

Quantum dots are particularly significant for optical applications owing to their bright, pure colors along with their ability to emit rainbow of colors coupled with their high efficiencies, longer lifetimes and high extinction coefficient. Examples include LEDs and solid-state lighting, displays and photovoltaics. 9,12,13 

Being zero dimensional (in nanotechnology terms it means all dimensions being less than 100 nm), quantum dots have a sharper density of states than higher-dimensional structures. Their small size also means that electrons do not have to travel as far as with larger particles, thus electronic devices can operate faster. Examples of applications taking advantage of these unique electronic properties include transistors, solar cells, ultrafast all-optical switches and logic gates, and quantum computing, among many others.12,13,14 

Anti-Counterfeit, Supply Chain, and Track and Trace 

A new and novel use for quantum dots by Quantum Materials Corp. is anti-counterfeit and brand protection applications. Due to the fact quantum dots can create exclusive, unique, and uncopiable, optical signatures that can be read by off the shelf scanning technology they are now at the leading edge of physical and digital security. Because of the dots extremely small size and durability they can be integrated on a tamper proof molecular level into almost any physical material, including metal, glass, fabric, liquids, inks, plastics, and many more. The unique optical signatures are registered on the QDX? Platform blockchain which creates an immutable identification and record of the optical signature and whatever item it is molecularly bonded with creating an end to end tamper proof anti-counterfeit, supply chain, and customer journey (ie returns, warranty) tracking capabilities. 15 

 Optical Applications 

So far, quantum dots have attracted most interest because of their interesting optical properties: they're being used for all sorts of applications where precise control of colored light is important. In one simple and relatively trivial application, a thin filter made of quantum dots has been developed so it can be fitted on top of a fluorescent or LED lamp and convert its light from a blueish color to a warmer, redder, more attractive shade similar to the light produced by old-fashioned incandescent lamps. Quantum dots can also be used instead of pigments and dyes. Embedded in other materials, they absorb incoming light of one color and give out light of an entirely different color; they're brighter and more controllable than organic dyes (artificial dyes made from synthetic chemicals).16 

Solar and Semiconductor Applications 

Still in the world of optics, quantum dots are being hailed as a breakthrough technology in the development of more efficient solar cells. In a traditional solar cell, photons of sunlight knock electrons out of a semiconductor into a circuit, making useful electric power, but the efficiency of the process is quite low. Quantum dots produce more electrons (or holes) for each photon that strikes them, potentially offering a boost in efficiency of perhaps 10 percent over conventional semiconductors. CCDs (charge-coupled devices) and CMOS sensors, which are the image-detecting chips in such things as digital cameras and webcams, work in a similar way to solar cells, by converting incoming light into patterns of electrical signals; efficient quantum dots could be used to make smaller and more efficient image sensors for applications where conventional devices are too big and clumsy. 

 Display Applications 

Quantum dots are also finding their way into computer screens and displays, where they offer three important advantages. First, in a typical LCD (liquid crystal display screen), the image you see is made by tiny combinations of red, blue, and green crystals (effectively color filters that switch on and off under electronic control) that are illuminated from behind by a very bright backlight. Quantum dots can be tuned to give off light of any color, so the colors of a quantum dot display are likely to be much more realistic. Second, quantum dots produce light themselves, so they need no backlight, making them much more energy efficient (an important consideration in portable devices such as cellphones where battery life is very important).Third, quantum dots are much smaller than liquid crystals, so they'd give a much higher-resolution image. Quantum dots are also brighter than a rival technology known as organic LEDs (OLEDs) and could potentially make OLED displays obsolete. 

 Quantum Computing  

Computers get faster and smaller every year, but a time will come when the physical limits of materials prevent them advancing any further, unless we develop entirely different technologies. One possibility would be to store and transmit information with light instead of electrons—a technology broadly known as photonics. Optical computers could use quantum dots in much the same way that electronic computers use transistors (electronic switching devices)—as the basic components in memory chips and logic gates. 

Optical computers may or may not take off, depending on how much progress computer scientists make with rival technologies, including quantum computers. In a quantum computer, bits (binary digits) are stored not by transistors but by individual atoms, ions, electrons, or photons linked together ("entangled") and acting as quantum bits called qubits. These quantum-scale "switches" can store multiple values simultaneously and work on different problems in parallel. Individual atoms and so on are hard to control in this way, but quantum dots (on a considerably larger scale) would be much easier to work with. 

 Biological and Chemical Applications 

Quantum dots are also finding important medical applications, including potential cancer treatments. Dots can be designed so they accumulate in particular parts of the body and then deliver anti-cancer drugs bound to them. Their big advantage is that they can be targeted at single organs, such as the liver, much more precisely than conventional drugs, so reducing the unpleasant side effects that are characteristic of untargeted, traditional chemotherapy. 

The small size of dots allow them to go anywhere in and out of the body making them suitable for different bio-medical applications like medical imaging, biosensors, etc. At present, fluorescence-based biosensors depend on organic dyes with a broad spectral width, which limits their effectiveness to a small number of colors and shorter lifetimes to tag the agents. 

 Quantum dots are also being used in place of organic dyes in biological research; for example, they can be used like nanoscopic light bulbs to light up and color specific cells that need to be studied under a microscope. They're also being tested as sensors for chemical and biological warfare agents such as anthrax. Unlike organic dyes, which operate over a limited range of colors and degrade relatively quickly, quantum dyes are very bright, can be made to produce any color of visible light, and theoretically last indefinitely (they are said to be photostable).17 

 References 

1. Njuguna J., Ansari F., Sachse S., Zhu H., Rodriguez V.M., 1 - Nanomaterials, nanofillers, and nanocomposites: types and properties, Health and Environmental Safety of Nanomaterials, Woodhead Publishing, 2014, 3-27. 
2. Ekimov, A. I.; Onushchenko, A. A. JETP Lett. 1981, 34, 345–349. 
3. Kastner, M. A. Physics Today, 1993, 46(1), 24. 
4. Ashoori, R. C. Nature, 1996, 379(6564), 413. 
5. Collier, C. P.; Vossmeyer, T.; Heath, J. R. Annual Review of Physical Chemistry, 1998, 49, 371. 
6. https://www.sigmaaldrich.com/technical-documents/articles/materials-science/nanomaterials/quantum-dots.html 
7. Reimann, S. M.; Manninen, M. Reviews of Modern Physics, 2002, 74(4), 1283. 
8. Bawendi, M. C.; Steigerwald, M. L.; Brus, L. E. Annual Review of Physical Chemistry, 1990, 41, 477.  
9. Yoffe, A. D. Advances in Physics, 2001, 50(1), 1. 
10. Vastola, G.; Zhang, Y.-W.; Shenoy, V. B. Current Opinion in Solid State & Materials Science, 2012, 16(2),  
11. Vahala, K. J. Nature, 2003, 424(6950), 839. 
12. Nirmal, M.; Brus, L. Accounts of Chemical Research, 1999, 32(5), 407. 
13. Sargent, E. H. Nature Photonics, 2012, 6(3), 133. 
14. Zhao, Y.; Burda, C. Energy & Environmental Science, 2012, 5(2), 5564.Medintz, I. L.; Uyeda, H. T.; Goldman, E. R.; Mattoussi, H. Nature Materials, 2005, 4(6), 435. 
15. Nano Magazine https://nano-magazine.com/news/2019/10/28/combining-blockchain-and-nanotechnology-to-fight-criminal-counterfeiters-and-build-brand-trust 
16. Medintz, I. L.; Uyeda, H. T.; Goldman, E. R.; Mattoussi, H. Nature Materials, 2005, 4(6), 435. 
17. Methods of Synthesizing Monodisperse Colloidal Quantum Dots, Lutfan Sinatra, Jun Pan, Osman M. Bakr, Material Matters, 2017, 12.1 
18. https://www.explainthatstuff.com/quantum-dots.html, Chris Woodford. updated: July 28, 2018.