9 Quantum Dots: An Overview

S.S. islam

Controlling individual atoms presents an exciting opportunity to build novel materials. Materials can be built as per the requirements if we can control individual atoms. The atoms could be turned ON or OFF to store informations, different colours can be emitted by the them, etc. However, it is not possible to control and manipulate individual atoms. Due to their dimensions, Quantum dots are closest to atoms than any other nanostructured material, such as nanowires. For this reason, quantum dots are often referred as ‘artificial atoms’. They are essentially the group of atoms made up of semiconductors and hold potential to revolutionize almost all of our existing technologies. In this module, we will learn about quantum dots and properties.

1. Quantum Dots?

Owing to its small size, whole matter or all the atoms effectively seem to be concentrated to a tiny spot or dot (zero-dimensions), the term quantum dot is often used. Consequently, the charge carriers (elec-trons and holes) remain entrapped inside the dot and possess discrete energy levels (somewhat similar to individual atoms). The size of the quantum dots is of a few nanometers, thus they are not single atom structure, rather they are made up of a group of atoms, wherein the number of atoms may vary anywhere between few dozen atoms to a few hundred atoms as well. For most applications, they are made up of semiconducting materials, such as silicon. Additionally, despite being small crystals, they behave much like individual atoms. It is for this reason, that they are also termed as artificial atoms.

1.1 Working of Quantum Dots

Since quantum dots demonstrate size-dependent properties, their behavior can be easily tuned by con-trolling their size. Analogous to the atoms, quantum dots can be engineered to function as per require-ments. For example, an atom can be excited by irradiating it with some energy: that is, an electron from a lower level can be excited to higher energy level. When this excited electron returns back to its original energy level, it emits light having energy equal to the energy difference between the two energy levels involved in the process. The color (or wavelength as well as frequency) of emitted light is characteristic to the atom. For instance, iron appears green when its atoms are excited upon exposure to heat, sodium gives off yellow color. This is attributed to the specific arrangement of energy levels within an atom. Thus, different atoms emit light of different colors or wavelength. This difference comes due to the discretization of energy levels.

Figure 1 Schematics illustration of light emission from an atom.

Figure 1 shows schematics of the mechanism behind light emission from an atom. The steps involved can be summarized as follows:

The atom is in its ground state with no electron present in excited states.
Light of a specific wavelength is incident onto the atom, which is absorbed by the atom, thereby promoting an electron from lower energy level to a higher energy level.
The electron returns to its original energy level (where it was present before excitation) by emitting a light photon. The color of the emitted photon or light depends upon the energy difference between the two energy levels involved in the process. As different atoms have different arrangements of their energy levels, dissimilar atoms emit light of varying colors.

A quantum dot also emits light by similar mechanism, because the charge carriers (electrons and hole) entrapped within it have discrete energy levels. However, the arrangement of energy levels within a quantum dot is also influenced by the dimensions of the dot. Thus a quantum dot made from same material can be tuned to emit light of different wavelengths or colors, depending upon the size of the dot. In this regard, bigger quantum dots emit light of longer wavelengths (or smaller frequencies), while smaller quantum dots produce light of shorter wavelength (higher frequency). This large dots give off red light and smaller dots produce blue light, and intermediate dots produce green light. In this way, by adjusting the size of the quantum dot, almost all the colors of the spectrum can be produced.

This can be explained as follows (see Figure 2): The band gap of a smaller quantum dot is more than that of a bigger quantum dot. Band gap can be described as the minimum energy required by the atom to promote an electron from the valance band to the conduction band. In this respect, a smaller quantum dot requires more energy to excite the electron. Since the frequency of the emitted light is directly proportional to the energy, small dots produce light having higher frequencies (or shorter wavelength). Similarly, larger dots more closely spaced energy levels, thus they produce lower frequency (or higher wavelength) light.

Figure 2 The size of the quantum dots is in nanometer range. Bigger dots emit light of longer wavelength and lower frequency, such that the color of the emitted light is close to red. Simi-larly, smaller dots produce shorter wavelength and higher frequency, and the colour is towards blue.

1.3 Preparing Quantum Dots

Quantum dots are nothing but small sized crystal. Therefore, they can also be produced similar to other crystals. Most popular techniques to produce quantum dots are: molecular beam epitaxy (MBE, wherein a beam of atoms is bombarded onto a substrate and the single crystal gradually builds up), ion implan-tation (wherein accelerated ion beam is incident onto the substrate), and X-ray lithography (atomic scale engraving using X-rays to produce the desired structures). Recently, several researchers have proposed preparing quantum dots by using biological processes, such as, by feeding metals to the enzymes.

Figure 3 Image showing a sensor chip of a webcam which converts the real time images into dig-ital format when light is incident on its grid. The grid contains light detecting pixels. Quantum dots can be used to produce small sized pixels as well as sensors which have higher resolutions.

Quantum dots can also be used in computer screens and other displays, owing to the following important advantages:

Conventional LCD (liquid crystal display) screens use a combination of red, blue and green crystals to show the images. These color crystals are basically color filters which can switch ON or OFF under electronic control. These crystals are illuminated from behind a very bright backlight. Thus the color of the picture are produced by combining tiny crystals of red, blur or green colors. As mentioned earlier, an individual quantum dot can be tuned to produce different colors by adjusting its size. Thus the colors produced by a quantum dot is likely to be more realistic.
Since quantum dots produce light by themselves, they do not need any backlight to show images. Thus, they are more energy efficient. This is their added advantage which can benefit the portable devices which we frequently see around us, since they require less energy to perform.
As the size of quantum dots is very small than that of the liquid crystal used in conventional LCD displays, the use of quantum dot is expected to produce much higher resolution image than possible by the LCDs.
Quantum dots are much brighter than the recently used organic LEDs or the OLEDs. Thus, they can also be used to produce OLED displays.

Applications

Quantum Computing

Quantum dots hold great potential for use in preparing devices for solid state quantum computing. By controlling the applied voltage across the electrodes, the flow of electrons (and, in turn, the flow of current) through the quantum dot can be controlled. This can be highly useful for precise measurements of various properties of the quantum dots (for example, spin). The qubits, or entangled quantum dots, can make quantum computing a reality.

Medical Imaging

Latex beads, when filled with specific quantum dot crystals, can be used to bind to specific DNA se-quences. This can be used to identify specific DNA sequences. Probes can be made to combine different sized dots in these beads which can emit light of particular intensity as well as wavelength. Upon ex-posing to UV illumination, these beads serve as bio-markers to identify specific regions in the DNA sequences. This technique has a variety of applications including cancer research, where affected cells can be easily identified.

Batteries and chargers

Researchers at Vanderbilt University have shown that by using quantum dots produced from iron pyrite, a modern smartphone can be fully charged within 30 seconds, for dozens of charging cycles. While the technology is still relatively new, it shows great promise for another highly accessible technological use.

Solar Energy

Quantum dots can also be used to harness solar radiations. They can absorb the incoming sunlight and create excitons (electron-hole pairs). These excitons should be quickly separated to avoid their sponta-neous recombination. This can be achieved by applying a suitable bias across the end terminals of the device. Electrons travel toward the anode via metal oxide nanorods, while the holes move toward the cathode. This separation of the charge carriers causes current in the semiconductor. Consequently, quantum dots can also be used to produce transparent and semi-transparent films for applications in the solar panels.

Biological and chemical applications

Quantum dots also have applications in medical field, such as in cancer treatment. Quantum dots can be configured to accumulate at specific sites within the body and then release the anti-cancer drugs bound to them. In this manner, targeted drug delivery can become a reality. The most important ad-vantage of using quantum dots is that they can be targeted to single organs, e.g., liver. In this way, they can deliver the drug more efficiently, thereby reducing the undesired side effects, which are inherent problems with conventional untargeted chemotherapy.

Quantum dots can also be used as a substitute for organic dyes in biological research. For instance, they can be used like nanoscopic light bulbs to light up and color specific cells that need to be studied under a microscope. In contrast to the organic dyes which can be used over a limited range of colors and degrade quickly, quantum dyes are brighter and can be designed to produce any color in the visible spectrum of light. In addition to this, the theoretical lifespan of quantum dyes in infinite (that is, they are considered to be photostable). Their applications as sensors to detect chemical as well as biological warfare agents has gained recent attention.

Who invented quantum dots?

Russian physicist, Alexei Ekimov, in 1980, discovered the quantum dots in solids (glass crystals) while working at Vavilov State Optical Institute. Later, in 1982, Louis E. Brus, an American chemist, while working at the Bell Laboratories, observed the same process in colloidal solutions. He found that the wavelength of the emitted or absorbed light by a quantum dot changed over a period of days, as the crystal grew in size. He suggested that the confinement of electrons caused these properties of the par-ticles. Both these scientists shared the Optical Society of America’s 2006 R.W. Wood Prize for their contributions.

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Khaledi-Nasab, Ali; Sabaeian, Mohammad; Sahrai, Mostafa; Fallahi, Vahid (2014). “Kerr nonlinearity due to intersubband transitions in a three-level InAs/GaAs quantum dot: the impact of a wetting layer on dispersion curves”. Journal of Optics. 16 (5): 055004.
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http://nptel.ac.in/courses/104103019/9
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