Introduction of Quantum Dots
Quantum dots (QDs) are nanometer-size semiconductor crystals. Quantum dots range in size from about 1-10 nanometers (nm) in diameter. To put this in perspective, a human hair is 10,000 times thicker than a quantum dot. This small size leads to unique optical and electronic properties. The unique properties of QDs lead to vast application possibilities.
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The Physics of Quantum Dots
Quantum Confinement Effect
Quantum confinement effect is one of the essential properties of quantum dots. When a quantum dot size is smaller than the Bohr radius of the electron-hole pair, the electron and the hole will be confined to three-dimensional space and become discrete energy level. This means that its optical property like emission color will be strongly related to the size of quantum dot, due to this quantum confinement effect. For instance, small quantum dots will emit blue, and large ones will emit red.
The relationship between quantum confinement effect can be simply expressed by the following formula: The change of the energy level of quantum dots is inversely proportional to the square of size, where is the size of quantum dots. The quantum confinement effect provides controllability of quantum dots for electronic or optical devices.
Energy Level Structure of Quantum Dots
Quantum dots show discrete energy levels and the energy level structure of quantum dots is similar to the energy level structure of atoms. The electrons in quantum dots can only exist at certain energy levels, and they cannot be in between these energy levels. By controlling the size or material of the quantum dot, we can modify the energy level of the quantum dot and change its optical and electrical properties.
The excitation and emission of a quantum dot consist of two processes, which are photon absorption and light emission. First, a quantum dot will absorb a photon with a high energy higher than the band gap of the quantum dot. This will lead to an electron jumping from the valence band to the conduction band. The excited electron will relax and fall back to the valence band and this process will release energy in the form of a photon. The photon energy is equivalent to the band gap energy and this can lead to emission colors that are related to the band gap energy of the quantum dot.
Quantum Dot Synthesis and Manipulation
Quantum Dot Synthesis Methods
Quantum dots can be prepared by various methods. The most common method is colloidal synthesis. The reaction conditions in the solution are adjusted to precisely control the size, shape, and composition of quantum dots. For example, CdE (E = S, Se, Te) semiconductor nanocrystals with high purity and controllability can be prepared by thermal decomposition of organometallic precursors. Reaction temperature, precursor concentration and reaction time are adjusted to obtain quantum dots with narrow size distribution (monodispersity). In addition to colloidal synthesis, physical vapor deposition (PVD) method and solution method can also be used for synthesis. Surface modification is also an important step in the synthesis of quantum dots. The surface properties of quantum dots are modified to improve their stability and biocompatibility.
Quantum Dot Operation Methods
Quantum dot operating methods are mainly divided into photoexcitation and electrical excitation. Photoexcitation refers to that quantum dots are irradiated with an external light source (usually a laser), and then absorb photons and emit light of a certain wavelength. This operating method is mostly used in quantum dot displays (QLEDs). By controlling the size and composition of quantum dots, it can achieve high-color gamut and high-brightness displays.
Electrical excitation refers to that the electric field is applied to quantum dot light-emitting diodes (QLEDs), and then the electrons and holes recombine in the quantum dots to produce luminescence. This operating method is of great significance in quantum dot display technology. It can achieve low power consumption and high-efficiency luminescence.
In recent years, researchers have also explored new operating methods to control the luminescence of quantum dots through strain-induced electric fields. For instance, by growing a thick shell on the quantum dot, the core in the middle is squeezed to generate a strong internal electric field and change its luminescence characteristics. In addition, the introduction of alloy quantum dots also provides a new way to control the luminescence properties of quantum dots. For example, by doping zinc, manganese or mercury into a CdSeS alloy, the effective masses of electrons and holes in the quantum dot can be tuned to change its spectral properties.
Applications of Quantum Dots
Display Technology
Quantum dots have some of the largest market shares when it comes to applications. The luminescence properties of quantum dots are used to produce high color gamut, high brightness and low power consumption quantum dot displays (QLEDs). QLEDs also have a higher brightness and wider color gamut than conventional organic light-emitting diodes (OLEDs). QLEDs are a major force in the high-end TV and monitor market.
Medical Imaging
Quantum dots have a broad range of applications in medical imaging. Quantum dots have been used as fluorescent markers for bioimaging, cell tracking, and disease diagnosis. Quantum dots can be encapsulated with an organic receptor shell that mimics the shape of specific diseases, viruses, or other things in the human body. The mimicking shell-bound quantum dots will then search for the appropriate site and attach to the disease site making the problem area more clearly visible. The brightness and stability of quantum dots also make them a promising substitute for traditional organic dyes.
Quantum Computing
Quantum dots show a lot of promise in the area of quantum computing. Quantum bits (qubits) can be controlled by controlling the electron spin state in quantum dots. Coupling between quantum dots also enables quantum gate operations and can serve as a basic model for quantum computers.
Energy
Quantum dots have a broad range of applications in the energy industry, such as in solar cells, photocatalysis, and energy storage devices. Quantum dots are used in many optoelectronic devices, as they have a high light absorption rate and a band gap that is tunable to specific wavelengths. There has been a lot of development made in the area of quantum dot white LEDs. White light can be achieved by combining quantum dots of multiple sizes.
