Introduction of Quantum Dots
Quantum dots (QDs) are a kind of semiconductor nanomaterial with a particle size of about 2~10nm. Owing to quantum confinement effect, QDs have unique optical and electronic properties different from traditional semiconductor materials, which are also known as "artificial atom" like properties, which have broad application prospects in a variety of high-tech fields. Due to the size of QDs, the mobility of electrons and holes is restricted, which causes a discrete energy level.
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Properties of Quantum Dots
The main factors affecting the properties of QDs are the particle size, shape, material, and surface modification, and so on. These properties provide QDs with irreplaceable advantages in many fields:
Optical Properties
Optical properties are one of the most prominent features of quantum dots. Due to the quantum confinement effect, the wavelength of the emitted light by the quantum dots can be finely tuned by simply adjusting their size. For example, quantum dots with particle size of 2-3nm are more blue or green, while those with particle size of 5-6nm are more red or orange. Quantum dots have different excitation/emission characteristics, which make them widely used in the field of display, bioimaging, and photocatalysis. In addition, quantum dots also exhibit high quantum yields and narrow emission bandwidths, which make them suitable for high-sensitivity detection and high-color saturation displays.
Electronic Properties
Quantum dots have electronic behavior that is intermediate between bulk materials and molecules, with a distinct electronic energy level structure. In addition, their high surface-to-volume ratio significantly increases the localization of electrons and holes, thus improving the electrical and optical properties. The electronic structure of quantum dots can also be modulated by external stimuli (electric field or photon), thus having potential applications in electronic devices, sensors, and quantum computing.
Other Properties
In addition to optical and electronic properties, quantum dots also have nonlinear optical and electrical properties. This provides it with a broad application in electronic devices, sensors, and photocatalysis. For example, quantum dots can be used to develop efficient solar cells, LEDs, lasers, and also used in the biomedical fields such as environmental remediation and drug delivery.
Quantum Dots Types
Quantum dots can be divided into different types based on their material composition, structure, and synthesis methods, each with its own specific application scenarios.
Classification by Material
Zinc oxide (ZnO) quantum dots: They have good electrical properties and biocompatibility, and can be used in electronic devices and biomedical applications.
Cadmium sulfide (CdS) quantum dots: They have high photostability and good optical properties, and are widely used in display technology and photocatalysis.
Lead sulfide (PbS) quantum dots: They have excellent infrared absorption, and are suitable for near-infrared imaging and solar cells.
Silicon-based quantum dots: They are non-toxic and biocompatible, and are suitable for biomedical applications and environmental monitoring.
Graphene-based quantum dots: They have high conductivity and good chemical stability, and are suitable for electronic devices and sensors.
Classification by Structure
Core-shell structures: By adding a high-bandgap semiconductor material on the outside of the core, the stability and optical properties of quantum dots can be improved.
Alloy structure: Alloying can achieve a wider range of spectral control, and is suitable for applications that require multicolor emission.
Janus structure: They have two distinct surface regions and are suitable for catalysis, sensing, and bioimaging.
Doped structure: By adding impurities or dopant atoms, the electronic and optical properties of a structure can be modified, and are suitable for optoelectronic devices such as solar cells and transistors.
Classification by Synthesis Method
Top-down method: Mechanically or chemically decomposes bulk materials into nanoparticles, suitable for large-scale production.
Bottom-up method: Quantum dots are synthesized by atomic or molecular self-assembly, and are suitable for the synthesis of quantum dots with high purity and narrow size distribution.
Hydrothermal and solvothermal methods: Suitable for the synthesis of water-soluble quantum dots, with the advantages of environmental friendliness and high reproducibility.
Microwave irradiation method: Microwave irradiation accelerates the reaction and improves the efficiency and uniformity of the synthesis.
Applications of Quantum Dots
Optoelectronics
Quantum dots are applied in optoelectronic devices:
- LED and Display Technology: Quantum dot light-emitting diodes (QLEDs) obtain high color purity (full width at half maximum as narrow as 20–30 nm) and wide color gamut (covering >100% of NTSC standard) by size-tuning the emission wavelength (2–10 nm). Compared to traditional LEDs, QLEDs have 30% lower power consumption and can display more accurate colors.
- Solar Cells: Quantum dot solar cells (QDSCs) achieve theoretical conversion efficiency as high as 66% (over twice as high as that of silicon-based solar cells, 29%) with their wide spectral absorption range (covering the ultraviolet to infrared). The current experimental efficiency is only ~18%, with major breakthroughs expected using multiple exciton generation (MEG) technology.
- Lasers and Photodetectors: Quantum dot lasers have a low threshold current (<100 A/cm2) and high-temperature stability, suitable for optical communication applications. Quantum dot photodetectors exhibit 10 times higher sensitivity than traditional photodetectors in infrared imaging and gas detection.
Biomedicine
Quantum dots have high application potential in biomedical areas in the use of their optical properties:
- Cell Imaging and Diagnosis: Quantum dot-antibody complexes can label cell surface receptors (e.g., glycoprotein receptors) and realize real-time tracking at the single-molecule level. The fluorescence lifetime of quantum dots is as long as 20 ns (only 1–5 ns for organic dyes) and the photostability is 100-fold improved.
- In Vivo Imaging and Tumor Targeting: Near-infrared quantum dots (700–900 nm wavelength) can penetrate tissue to a depth of 5–10 cm and be used for real-time localization of lymph nodes during surgery. The tumor targeting efficiency is 40% higher than that of traditional contrast agents.
- Drug Delivery: Quantum dots with water solubility electrostatically adsorb drug molecules and achieve targeted release of drug to cancer cells, micron-level precision for drug delivery.
Sensing Technology
Quantum dot sensors can be designed to have exceptional sensitivity:
- Electrochemiluminescence (ECL) sensors can achieve ppb-level detection of pollutants (e.g., pesticide residues) with a response speed of<5 s; biosensors can be modified to recognize specific DNA sequences with a detection limit as low as 10-18 M.
Energy Technology
- Catalysis and Hydrogen Production: Quantum dot photocatalysts achieve an efficiency of 8.6% (at a wavelength of 420 nm) in water splitting to produce hydrogen, which is three times higher than that of traditional catalysts. As an electrode material in lithium-ion batteries, the charge and discharge speeds are increased by 50%.
Electronic Devices
- Quantum Computing: Quantum dots can be used as quantum bits (qubits) with a coherence time of 100 μs, applicable in building quantum processors. In nanocircuits, quantum dot switching response speeds can be in the picoseconds range.
