This review paper is a summary and introduction to core/shell quantum dots, as well as discussing their current and perhaps future role in modern-age electroluminescent devices. Quantum dots, with their extremely large color variety and tunable properties are becoming more and more the next “Big Thing” in the liquid crystal display (LCD) industry, and are predicted to have rapid development from 2014 to 2023. Thus, this review paper delves into their notable properties, as well as the reason that these quantum dots are becoming the next big source of interest for the display industry.
Quantum dots (QD) are tiny, zero-dimensional particles or nanocrystals, also known as artificial atoms, made of semiconductors with diameters between 10-50 atoms (2-10 nm). They are extremely small structures so tiny that only single electrons are allowed through. However, because the space between the structures is so limited, the electron’s kinetic energy can assume only a certain allowed value which can be changed based on the size, shape, and material of the dot that they fit through. This value is a color, and thus, it is possible to manipulate the structures so that they produce a desired color. The tunable electronic properties of these quantum dots makes them very appealing for various technologies and experiments, such as electronic devices, sensors, and catalysis. In 2013 only, it was predicted that the quantum dot display component market surpassed $70 million. In this review paper, core/shell quantum dots, a specific type of quantum dots, will be reviewed. According to Fatemeh Mirnajafizadeh, professor of the University of New South Wales, “Core/shell QDs are a special class of nanoparticles with unique optical properties such as narrow emission, wide absorption and photo-stability as found in quantum dots, but the specific structure of core/shell QDs promotes their optical properties over simple QDs”.
Within a core/shell quantum dot is an almost Earth-like structure, with its many layers and core. Most of these quantum dots usually have about one to two layers, called shells, around the core, which can vary in material but ultimately have the same basic functions.
The core, which is the basis of the quantum dot, controls the overall properties of the QD as well as filters the light that shines through the quantum dot. Some of the most commonly used cores are Indium-Phosphide (InP) or Cadmium Selenide (CdSe) cores, which have varying properties that make them more desirable to use than other materials. For instance, CdSe quantum dots are efficient and have a high quantum yield, but are highly toxic and could possibly have severe consequences if exposed to human skin or malfunction. Quantum yield (QY) is the measure of the efficiency of photon emission as defined by the ratio of the number of photons emitted to the number of photons absorbed. Due to these consequences, the InP QDs are being researched and prepped to be a replacement for CdSe QDs, as they are more environmentally-friendly but still perform as desired.
Valence bands are the outermost electron orbital of an atom of any specific material that electrons actually occupy, while conduction bands are the band of electron orbitals that electrons can jump up into from the valence band when excited.
We use the term ‘quasi fermi level’ to mathematically describe the potential energy of electrons in the valence (outer shell) and conduction bands in nonequilibrium operations of semiconductor electronics, optoelectronic, and electrochemical devices. Shells change the positioning of the quasi-fermi-level in a quantum dot, which determines the density of electrons (holes).
Here is a visual representation. Cores have a specific pattern on their outsides, which come in the form of small “holes” that are the “band gap” of the cores, or distance between the conductive and valence bands of the core. These band gaps determine the shape and thus kinetic energy of the electrons that pass through, but with only one gap for the electrons to pass through, it is not possible to generate a wider range of products or more accurately manipulate the results of the quantum dot. Thus, shells come into play by matching up with the structure of the core, almost like a Grecian computer puzzle, adding their own band gaps on top of those of the core’s and changing the shape in which the electrons pass through. This is best shown in Figure 2, where the different possible combinations of band gaps are shown.
This positioning varies depending on the type of shell or overall combination of the core and shell. These differences are organized into four main types of core/shell quantum dots, which are Type I, Inverse Type 1, Type II, and Inverse Type II.
Type I QDs have a smaller band gap of the core compared to the shell, which leads to a higher quantum yield and long-term stability. Examples include CdSe/ZnS and CdSe/CdS QDs, which completely confine excited electrons in the core region. On the other hand, Inverse Type I QDs have a smaller band gap for the shell in comparison to the core, leading to a lower QY and poor stability. Examples are CdS/HgS or CdS/CdSe QDs, and in these QDs the excited electrons are completely or partially confined in the shell.
In comparison, Type II QDs are when the valence band edge of the core is within the band gap of the shell, or the conduction band edge of the shell is within the band gap of the core. This type of QD also has a lower Qy and poor stability, and examples include CdTe/CdSe, and CdSe/ZnSe QDs. The opposite of these QDs, Inverse Type II QDs have the valence band edge of its shell within the band gap of the core, or the conduction band edge of the core within the band gap of the shell. Examples are InP/CDS or PbS/Cds QDs, and these QDs have a relatively higher QY and fair stability.
Furthermore, core/shell quantum dots are known for their large surface-to-volume ratio. Because the dots are so small, compared to a standard cube with side length E and a surface-to-volume ratio of 6/E, their surface-to-volume ratios are in the millions, such as , where E is 10 nanometers. This large ratio is extremely helpful in sensing things, because usually sensing happens on the surface and not within the particle. It is also helpful to replace large particles with quantum dots to get the maximum surface area. This can be explained through the example of perhaps a Rubix cube. If the Rubix cube is split into its 9 parts of equal size and volume, it has a consistent volume but a greater surface area. Now, when applied to quantum dots, this Rubix cube can be split into millions of tiny particles of equal size and volume, thus making the surface area much larger than what it started out with. As a result, having quantum dots as a replacement of larger particles can create better sensing and faster reaction rates, wherein if they are used in devices or machines, they can be used to detect substances or function much faster than before.
Another one of the most notable properties of the quantum dot is the fluorescence different particle sizes can emit. Quantum dots can create color when an electron inside the dot undergoes a transition from a higher energy (excited) state to a lower energy (ground) state. This change in kinetic energy is created after the electron squeezes through the band gaps of both the core and shell of the QD, as seen in sections above. The kinetic energy assumes the form of color, light being bent and shaped in ways that can form a much wider and saturated range of colors. It is even estimated that quantum dots increase the color gamut (the entire range of colors and tones achievable by an imaging system) of an LCD display upward of 50 percent more color than the average display, in the range shown in Figure 3.
It is due to these properties that quantum dots are piquing the interest of technology companies, most notably Samsung, who wish to advance the quality of their displays. In the present, Samsung has presented their QD-OLED TV, which utilizes a “color layer” of quantum dots over an OLED, or organic light-emitting diodes, display that changes the color of light first emitted by a blue light backlight that is the OLED display (Figure 4). Average LED TVs which use a white backlight and a LCD that is typically composed of two sheets of polarizing material with a liquid crystal solution between them. In comparison, the QD-OLED TV’s are able to use both properties of a QD and OLED display, containing the self-emissive (meaning they emit their own light) pixels of OLEDs, and the rich variety of color that the QD filter can provide. This would also correct the dim output of OLED displays, creating a television that not only contains the most colors possible in the display as of now, but also one that can provide immaculate control of brightness and contrast. Although this display hasn’t been released yet, it is probable that it could be released within the next few years. Other companies such as LG may show interest in quantum dots in the future as well, but for now the main developer of these displays seems to be Samsung. In the future, quantum dots are also predicted to be developed into technologies such as QD Glass on LGP, Quantum dot colour filters (QDCF) for microLEDS and displays, Active Matrix Quantum-dot Light Emitting Diode (AMQLED), Perovskite QDs, and Inkjet printed QDs.
Figure under CC BY Creative Commons License Samsung Group
However, quantum dots face some notable problems, such as the toxicity of the quantum dots. Because the coating of quantum dots can be cytotoxic such as mercaptoacetic acid, quantum dots provide the risk of environmental harm or possibly injuring buyers in the case of malfunctions or exposure to the quantum dots themselves. Examples include CdSe quantum dots, and as shown above in the review paper as well, scientists are working to find ways to make the QDs less toxic and also find alternative QDs that work just as well but are ultimately not toxic. Other properties that must be taken into consideration are overall quantum yield, as well as the absorbance rates – how much light the QD absorbs – of the QDs. Scientists work to increase the total output of these quantum dots, and in many instances try new combinations of shells and cores, as well as increase the number of shells to detect any changes in results that may be beneficial.
From the information gathered and after noting the unique properties of quantum dots, it is evident that these devices may provide a very useful and worldwide development into the future of displays. QDs can provide a whole color spectrum previously unheard of, and due to their extremely small structures can also form displays that are not necessarily “flat”. Of course, core/shell quantum dots need much development to get to the point where they may one day replace LCD or LED screens, but they seem to show much promise. It is predicted in this review paper that in the case that the QD-OLED TVs to be released by Samsung are a success, we may one day see QD displays appearing all around the world, not just through TV’s but other displays as well. In conclusion, core/shell quantum dots are slowly becoming more acknowledged in the display industry and may possibly become the next “Big Thing”, but ultimately, it is up to their development in addressing toxicity and eventual cost-benefit analysis that will determine the extent to which QDs will make waves upon modern-age electroluminescent devices.
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About the Author
Gina is a chemistry, science-liking individual who enjoys learning new things and asking questions. She first got into liking chemistry and science at the age of 12 after watching videos of Schoolhouse Rock and Science is Real. She is also a dramatic and happy person who likes to listen to music and take naps around 2 PM. If you have any music or food recommendations, she would love to hear them!