QUANTUM DOTS
A NEW ERA OF TECHNOLOGY
The advent of quantum mechanics is one of the greatest scientific breakthroughs of the 20th century which made the saying ,“In each atom a hundred suns are concealed”,a mere truth. The quantum size effect describes the physics of electron properties in solids with great reduction in particle size, which does not come into action in macro or micro dimensions but becomes dominant when the nanometer size range is reached because quantum effects begin to dominate the behavior of matter at the nanoscale. Quantum dots are one such nanoparticle that are in quantum size regime and hence is the central topic in nanotechnology. They are tiny crystals which are invisible to normal human eyes, but have an underlying capability to become luminous in presence of light and give of a very precise color depending on its size.
WHAT ARE QUANTUM DOTS?
Quantum dots are artificial semiconducting nanocrystalline structures which are a few nanometers in size, typically 2–10 nm wide and may contain 10–50 atoms. These crystals were invented in solids in 1980 by Russian physicist Alexei Ekimov and later in 1982 American chemist Louise E. Brus discovered the same in colloidal solutions. A quantum dot gets the name because it is a tiny speck of matter so small that is effectively concentrated into a single point, as a result the particles inside it carries electrons and holes which are trapped and have well defined energy levels according to the quantum theory. Although they are single crystals they behave more like individual atoms hence the nickname ‘artificial atoms’. They have properties intermediate between bulk semiconductors and discrete atoms or isolated molecules. Their unique spectral properties include broad absorption, narrow emission and wavelength dependent size. Determined by the size and shape of the particles, they show the property of fluorescence which produces distinct colors from a single spectrum of light. Their size, energy levels and emission color can be controlled which makes it useful for a variety of applications.
QUANTUM CONFINEMENT
Quantum dots have varying size, shape and number of atoms present in it. Instead of a continuous energy band unlike the normal semiconductors, they have closely placed individual or discrete energy levels, with band gap between these energy levels. According to quantum mechanics, energy of photons relates to the wavelength or color of photons. The band gap is the energy required for electrons to enter into an excited state. As the number of atoms present or the size of quantum dots change, band gap also change. Thus the color changing property of a quantum dot is due to this difference in band gaps.
Large particles have smaller band gap and require comparatively less energy to become excited, resulting in low energy input and low frequency giving a longer wavelength. This is why bigger dots emit light towards the red end of the spectrum and smaller ones towards the blue end. With decreasing size the nanocrystals exhibit colors from red to blue, an effect known as blue shift. This phenomenon of “quantum confinement” allows the particles to emit specific colors of light, merely by controlling their size. Doping is an important aspect when quantum dots are used for various technological fields like optoelectronic, magnetic, biological and spintronic applications. The impurities added are called activators. It alters the band structures by creating local quantum states within the band gaps. When the dopants are thermally inactive they are found in an auto-ionized state due to quantum confinement.
MANUFACTURING OF QUANTUM DOTS, VARIOUS FORMS AND MATERIALS USED
A variety of methods are employed for the production of quantum dots like lithographic techniques, epitaxial techniques and colloidal synthesis. Whatever be the method, it involves a basic process where temperature remains a critical factor in determining optimal conditions for the nanocrystal growth. Quantum dots are made by a chemical reaction which uses heat to control crystal growth in a solution in presence of a reagent, resulting in solid nanocrystals. The chemicals are combined over heat and are made to react around 255 degrees Celsius. The length of the reaction affects the crystal size. As soon as the mixture is removed from the heat, the reaction will end and the particles will retain their size and color. Quantum dots can be used in various forms, for example as small crystals in liquid solutions or as quantum dust and in bead form. All these existing forms make their range of applications even wider. Since the 1990s, there has been an ever growing interest in the development of colloidal quantum dots.
Quantum dots are made of one or more elements which follows a construction principle of ‘Core and shell’. Many different semiconductor materials could be used to prepare quantum dots like Cadmium selenide (CdSe), Cadmium sulfide (CdS), Lead selenide (PbSe), Lead sulfide (PbS), Indium arsenide (InAs), Indium phosphide (InP). All of these combinations include heavy metals which are regulated by Food and Drug Administration (FDA). This makes it difficult and potentially dangerous to market products with quantum dots and limits the possibility of biological imaging, one of the major applications of quantum dots. The most promising option so far is Copper indium sulfide crystals with Zinc sulfide protective shell. Graphene quantum dots, an unrolled and planar form of carbon nanotube has also become an extremely interesting innovation for nanoscale electronics. Unlike all other known materials, graphene remains highly stable and conductive even when it is cut into devices one nanometer wide. It has excellent bio-compatibility, low toxicity and high stability.
WHY QUANTUM DOTS ?
· A zero dimensional quantum dot has a sharper density of states than higher dimensional structures, as a result of which they have superior transport and optical properties.
· Regardless of its size, a small amount of energy is only required to excite a quantum dot that can be done by a single blue or ultraviolet beam of light, which attributes to reduced cost of sensing quite dramatically.
· Quantum dots can be engineered to fluorescence in different wavelengths based on their physical dimensions. By use of different colloidal quantum dots for different parts of visible spectrum, the entire range of spectrum can be synthesized.
· The ability to functionalize quantum dots has the potential to change the properties, including solubility of quantum dots in a variety of solvents and to bind the quantum dots to targets.
· One of the biggest advantages of quantum dots relates to the availability of multiple methods to develop them easily and cost effectively. Compared to the close substitutes they can be produced at a lower temperature.
· High photo stability and brightness of quantum dots make them suitable for high sensitivity applications like fluorescent tagging and live cell imaging. Their fluorescence properties and their high resistance to metabolic degradation enable a wide range of experiments to be performed, thus ignoring possible time barriers.
· Quantum dots because of their electro luminescent and photo luminescent and unique physical properties will be at the core of next generation displays.
· The smaller size of quantum dots allows them to be dispersed in liquids. Therefore many liquid processing techniques like inkjet printing and spin cooling can be applied while using quantum dots. This allows them to be integrated into products quickly and easily at low cost.
· Another key advantage is that because quantum dots can be deposited virtually on any substrate you can expect printable, flexible and even roll-able quantum dot display of all sizes.
· Quantum dots can contribute to a wider range of applications that substitute bulk, expensive and inefficient materials. Commercial demand for quantum dots is expected to grow significantly in the next decade.
APPLICATIONS OF QUANTUM DOTS
Quantum Dot TVs and displays:
Quantum dots are a way of augmenting existing kind of television technology. Samsung and LG launched their QLED TVs in 2015, followed by few other companies.
Compared to organic luminescent materials used in organic light emitting diodes, quantum dot based materials have pure colors, longer lifetime, low manufacturing cost and lower power consumption. Unlike the normal televisions that use white light to illuminate the red-green-blue (RGB) color filters which could be used to create distinctive color that make up a TV picture, a simple drop in replacement of plane blue LEDs which shine to the RGB filters through a bunch of quantum dots have been made. Since the color produced in the quantum dots are pure, when they fall on the sub-pixels it causes less wastage, hence increasing the efficiency and thus making a better, more colorful picture. This technology is known as QDEF (Quantum Dot Enhancement Film).
The successor version is called QDOG (Quantum Dots on Glass). Here quantum dots are directly coated onto a glass Light Guide Plate (LGP) which reduces the implementation cost and display thickness with no compromise in color volume. As glass used for manufacturing is sturdier than other materials. A QDOG TV can be made thinner than a smartphone. Quantum Dot technology is the future technological platform for all displays including LCDs, OLEDs, micro LEDs and printable electroluminescent displays.
Biomedical domain:
Quantum dots could revolutionize medicine. Scrutinizing various processes in human cells, these can be simultaneously used for imaging applications, targeting applications as well as for the delivery of therapeutic agents. They also enable researchers to study cell processes at the level of a single molecule and may significantly improve the diagnosis and treatment of diseases such as cancer.
In potential cancer treatments quantum dots can be targeted at single organs such as the liver much more precisely than conventional drugs, so reducing the unpleasant side effect that is a characteristic of untargeted traditional chemotherapy. Quantum dots can be designed such that they could be accumulated at a particular part of the body. Organic dyes in biological research are replaced by using quantum dots as nanoscopic light bulbs to light up and color specific cells that need to be studied under a microscope. Unlike the organic dyes quantum dots are brighter and are available in a variety of colors. Quantum dots are used in passive label probes where selective receptor molecules such as antibodies have been conjugated to the surface of the dots. They have antibacterial properties similar to nanoparticles and can kill bacteria in a dose dependent manner by impairing the functions of anti-oxidative system in the cells and down regulating the antioxidant genes.
As the use of nanomaterials for biomedical applications is increasing environmental pollution and toxicity have to be addressed. The development of non-toxic and bio-compatible nanomaterial is becoming an important issue. Quantum dots are made using combinations of tiny heavy metal based crystals, hence there is a significant concern about how these materials are potentially toxic for travel, collect and decay within the body. A study on this in 2013 concluded without any evidence of toxicity in mice and monkeys. But after four months mice showed accumulation of particles in spine and liver but such side effects were not found in monkeys.
There is still a lot of research to be done before we see whether quantum dots become a part of medical imaging. Although the research of quantum dot nanocarriers for drugs has achieved some development, the application of the same is still at the beginning. Using these carriers for drugs in biology is a burgeoning new field which has got a high value.
Photovoltaic cells:
Hailed as an efficient method in light harvesting technology, attractiveness of using quantum dots in photovoltaic has several advantages over conventional approaches. They can be made from abundant, inexpensive materials that do not require extensive purification as silicon and can also be applied to a variety of low cost and flexible substrate materials, such as light weight plastics. In a traditional solar cell, photons of sunlight knock electrons out of a semiconductor into a circuit, making useful electric power, but with less efficiency. Quantum dots produce more electrons for each photon that strikes them, potentially offering a boost in efficiency of perhaps 10 percent over conventional semiconductors.
A promising route for quantum dot solar cells is a semiconductor ink with the goal of enabling the coating of large areas of solar cell substrates in a single deposition step. Absorbing sunlight from the appropriate portion of the solar spectrum and tuning the band gap of semiconductor by tuning the size of quantum dots allow engineers to optimize the performance of the devices. If the performance of the devices is optimized, these devices could make a huge impact in the energy conversion industry across the globe.
Solid state devices:
Ongoing miniaturization of solid state devices often raises the question: “How small can we make resistors and transistors without changing the way they work?”. The quantum dot technology was indeed a solution for this. Due to their particular electronic properties quantum dots can be used as active materials in single electron transistors. The properties depend on the size, shape, composition and structure. Researchers have shown that it is possible to carve out nanoscale transistors from a single graphene crystal.
As quantum dots naturally produce monochromatic light, they can be more efficient than light sources which must be color filtered. Quantum dot screens tend to modify the light that passes them which tackles the color quality problems with LEDs as if it is incandescent while maintaining the same efficiency as that of a conventional LED bulb. The idea behind this is that by being incredibly small a quantum dot confines electrons raising their energy. Smaller the dot, more energetic it becomes and the result lies in its color. LEDs with quantum light optics will be a breakthrough manufacturing that will be hitting the shelves within months.
Quantum computing:
Computers get faster and smaller with growing needs but there will be a situation when physical limits of materials prevent them from advancing any further, unless new promising technologies are developed. One such possibility is to store and transmit information with light instead of electrons, to be precise the photonics.
These optical computers could use quantum dots in much the same way the present computers use transistors as the basic components in memory chips and logic gates. In a quantum computer bits are not stored by transistors but by individual atoms, ions, electrons or photons linked together and acting as quantum bits or qubits. These qubits can store multiple values simultaneously and work on different problems in parallel as quantum dots are much easier to work with.
QUANTUM DOT TECHNOLOGY TO TACKLE COVID-19
The biggest threat during any outbreak is that it takes time to create a new vaccine to fight the disease. Similar is the situation in case of the current world pandemic due to novel corona virus. We are probably a year from a vaccine for the Wuhan outbreak. Proper diagnosis, quarantine and symptom management is necessary in such conditions. Prior to that early detection is the only means by which we could bring the situation under control. Current methods require time and a great expenditure of resources which might appear to be impractical to certain extent. Application of quantum dot technologies takes its role in such situations. Methods for detection are being developed using a quantum dot matrix based on nanomaterials such as zirconium. It is a tool to improve optical detection of coronavirus via biosensors.
A new class of nanoscale materials called chiral zirconium quantum dots (Zr QDs) are created for the purpose of detection. Quantum dots that contain zirconium have promising characteristics such as strong fluorescence emission and optical stability, broad range of excitation length, better quantum yield and a tunable emission peak which ranges from infrared to ultraviolet. These dots could also help improve a variety of other bio-sensing and bio-imaging applications.
CONCLUSION
Increasing demand for quantum dot in display devices and advanced features of quantum dot are creating scope for the quantum dot industry. In the 21st century, the second quantum revolution will shape the science and world economy, re-defining the speed and scale of scientific and technological expansion.
However several stumbling blocks are still needed to be overcome. Several applications like displays, lighting, selective sensors, bio-imaging, MRI contrast agents and bio-labels need attention to improve further and answer questions about quantum dot synthesis, properties, ageing and toxicity . As a major research and development area, quantum technology is what’s going to be all about the future.
REFERENCES
1. Wikipedia
2. Nanowerk
3. ScienceDaily
4. Sigma- Aldrich
5. Nanosys
6. YouTube
7. Researchgate
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