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Quantum dots (QD) are artificial nanostructures that are semiconductor nanocrystals which exhibit quantum mechanical behavior. The properties of a quantum dot are determined by size, shape, composition, and structure. The interesting electronic properties of quantum dots arise from the specific size of their energy band gaps.
The Properties of Quantum Dots
Quantum dots were discovered by the Russian Physicist Alexey I. Ekimov in 1981. The tiny nanoparticles have diameters that range from 2 nanometers to 10 nanometers. The particles differ in color depending on their size. Quantum dots can be classified into different types based on their composition and structure, which includes core type, core-shell, and alloyed quantum dots.
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Quantum dots emit light when excited, with smaller dots emitting higher energy light. Manufacturers can accurately control the size of a quantum dot and as a result, they are able to ‘tune’ the wavelength of the emitted light to a specific color.
They find applications in several areas such as solar cells, transistors, LEDs, medical imaging and quantum computing, thanks to their unique electronic properties.
Optical Applications
Quantum dots can emit any color of light from the same material by changing the dot size. They have bright, pure colors along that can emit a rainbow of colors coupled with their high efficiencies, longer lifetimes and high extinction coefficient. The high extinction coefficient of a quantum dot makes it perfect for optical uses. Quantum dots of very high quality can be ideal for applications in optical encoding and multiplexing due to their narrow emission spectra and wide excitation profiles. Examples of optical applications of quantum dots include light-emitting diodes (LEDs), solid-state lighting, displays, and photovoltaics.
Light Emitting Diodes
Quantum dot light emitting diodes (QD-LED) and ‘QD-White LED’ are very useful when producing the displays for electronic devices because they emit light in highly specific Gaussian distributions. QD-LED displays can render colors very accurately and use much less power than traditional displays.
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Photodetectors
Quantum dot photodetectors (QDPs) can be produced from traditional single-crystalline semiconductors or solution-processed. Solution-processed QDPs are ideal for the integration of several substrates and for use in integrated circuits. These colloidal QDPs find use in machine vision, surveillance, spectroscopy, and industrial inspection.
Photovoltaics
Quantum dot solar cells are much more cost-effective when compared to silicon solar cells. Quantum dot solar cells can be produced using simple chemical reactions and can help to save manufacturing costs as a result.
Operational efficiency is also greatly improved by using quantum dots. In traditional silicon p-n junction solar cells, when a photon with energy less than the bandgap of silicon hits the solar cell it is transmitted and does not contribute to the power output. This results in a trade-off in design, if the bandgap is lower more incoming photons can excite electrons (meaning a higher current) but the electrons have lower energy (thus lower voltage) and vice versa for a higher bandgap. Theoretical peak solar efficiency for a silicon p-n solar cell is 33.7%. Researchers at the Los Alamos National Laboratory have developed a solar cell that uses copper indium selenide sulfide quantum dots, which are non-toxic and cheaper than quantum dots containing lead or cadmium.
Quantum dots can offer a significant increase in efficiency, by using dots of varying sizes top of each other with the largest bandgaps on top. Incoming photons will be transmitted until reaching a layer with a bandgap smaller than the photon energy. With enough layers, each photon will excite an electron with a bandgap close to its own energy and thus waste a small amount of energy. When the number of layers approaches infinity, the efficiency approaches a theoretical thermodynamic limit of 86%.
Perovskite quantum dots are an area of photovoltaics that shows a lot of promise but needs more research as they suffer from instability issues.
Biological Applications
The latest generation of quantum dots has great potential for use in biological analysis applications. The small size of quantum dots allows them to go anywhere in the body making them suitable for biological applications such as medical imaging and biosensors. They are widely used to study intracellular processes, tumor targeting, in vivo observation of cell trafficking, diagnostics and cellular imaging at high resolutions.
Quantum dots have proven to be far superior to conventional organic dyes because of their high quantum yield, photostability, and tunable emission spectrum. They are 100 times more stable and 20 times brighter than traditional fluorescent dyes.
The extraordinary photostability exhibited by quantum dots makes them ideal for use in ultra-sensitive cellular imaging. This allows several consecutive focal-plane images to be reassembled into three-dimensional images at very high resolution.
Quantum dots can target specific cells or proteins using peptides, antibodies or ligands and then observed to study the target protein or the behavior of the cells. Researchers have found out that quantum dots are far better at delivering the siRNA gene-silencing tool to target cells than currently used methods.
Researchers at the University of Colorado Boulder investigated the use of quantum dots to treat antibiotic-resistant infections. Adding light-activated particles to antibiotics combats the increasing problem of drug-resistant infections. The kinds of chemicals created after the light has hit the quantum dot can be modified by changing the size. The team at the University of Colorado developed antibiotics that release a superoxide enzyme with quantum dots. This stresses the bacteria, making it more vulnerable to antibiotics which it had previously been immune to. This could be incredibly important for the future, with the amount of drug-resistant infections continually rising.
Quantum Computing
Quantum dots have paved the way for powerful ‘supercomputers’ known as quantum computers. Quantum computers operate and store information using quantum bits or ‘qubits’, which can exist in two states – both on and off simultaneously.
This remarkable phenomenon enables information processing speeds and memory capacity to both be greatly improved when compared to conventional computers.
The Future of Quantum Dots
Quantum dots are zero-dimensional and exhibit a sharper density of states than structures of higher dimensions. This explains their excellent optical and transport properties, which are currently being studied for potential uses in amplifiers, biological sensors, and diode lasers.
The broad range of real-time applications of quantum dots in the field of biology is expected to be very useful in many research disciplines such as cancer metastasis, embryogenesis, lymphocyte immunology, and stem cell therapeutics. In the future researchers also believe that quantum dots can be used as the inorganic fluorophore in intraoperative tumor detection when performed using fluorescence spectroscopy.
In September this year, Osaka University fabricated the first nanoelectric device that detects single-electron events in a target quantum dot, using a second dot as a sensor. The device was made using two indium arsenide (InAs) quantum dots that were connected to electrodes deliberately narrowed to minimize any undesirable screening effects. The discovery is important due to the ability to achieve electrical readout of single-electron states and combining them, with photonics for quantum communications.
Outside of the laboratory, quantum dots are thought to be the future of next-generation displays due to their photoluminescent and electroluminescent unique physical properties. When compared to the organic luminescent materials used in organic light-emitting diodes (OLEDs), quantum dot-based materials have a longer lifetime, purer colors, longer power consumption, and a lower manufacturing cost. Quantum dots can also be deposited on most substrates substrate, meaning you can get printable and flexible quantum dots of varying sizes.
A company called Store Dot is also using quantum dot technology to assemble a new battery. The new battery can charge a phone from flat to full battery in less than one minute. The quantum dots used are peptides modified to contain optical properties that can generate a charge. The battery uses a quantum dot nanocrystal instead of the traditionally used electrolytes.
Quantum Dots
Sources and Further Reading
This article was updated on 19th December, 2018.