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The brilliance of quantum dots: Why does their color change with size?
Quantum dots (QDs) are semiconductor nanocrystals in size between a few nanometers whose optical and electronic properties differ from those of larger particles due to quantum mechanical effects. These tiny semiconductor particles are currently an important topic in nanotechnology and materials science. When a quantum dot is illuminated by ultraviolet light, the electrons in the quantum dot can be excited to a higher energy state. This process corresponds to the transition of electrons from the valence band to the conduction band for semiconductor quantum dots. Excited electrons can be driven back to the valence band again, releasing their energy and emitting as light, which is called photoluminescence.
The color of light depends on the difference in discrete energy levels between the conduction and valence bands of the quantum dots.
The color change of a quantum dot is closely related to its size. Typically, quantum dots with a diameter of 5 to 6 nanometers emit longer wavelength light, which is usually orange or red in color. Quantum dots with a diameter of 2 to 3 nanometers emit shorter wavelengths of light, such as blue and green. However, changes in specific colors are also affected by the precise composition of the quantum dots.
The characteristics of quantum dots are between large semiconductors and independent atoms, and their optoelectronic properties change with changes in size and shape.
With the advancement of technology, quantum dots have demonstrated their potential in many applications, including single-electron transistors, solar cells, light-emitting diodes (LEDs), lasers, single-photon sources, secondary harmonic generation, quantum computing, cell biology research, microscopy and medical imaging. Furthermore, due to the tiny size of the quantum dots, some can even be suspended in solution, which creates the potential for applications in inkjet printing and rotary coating. Nevertheless, the technology of the core/shell structure is also important in terms of improving the luminescence efficiency of quantum dots. Quantum dots are often coated with organic ligands with long hydrocarbon chains to control growth, avoid aggregation and promote dispersion in solution, however these organic coatings can lead to a “nonradiative recombination” phenomenon of photon emission, reducing light quantum yield.
Quantum dots with core/bivalve structures can improve the emission wavelength of photoluminescence by adjusting the thickness of each layer as well as the overall size of the quantum dots.
There are currently various methods for preparing quantum dots, among them colloidal synthesis, self-assembly and electrical gating. Among them, colloidal synthesis is a method of synthesizing semiconductor nanocrystals from solution, in which a light-colored solution is first heated to prompt the precursor to depolymerize and generate nanocrystals. The growth process of nanocrystals is closely related to the concentration, temperature, and time of the precursor.
However, the preparation of quantum dots is not limited to colloidal synthesis, but can also be produced by gas-phase methods such as plasma synthesis. This process not only allows us to precisely control the size, shape and composition of quantum dots, but also introduces doping elements into the process to improve performance. This improves the tunability and functionality of quantum dots, and the future application prospects in consumer electronics and optoelectronic equipment are bright.
With the advancement of quantum dot manufacturing technology, which is expected to be more widely used in consumer goods in the future, how to ensure the safety of these materials in terms of environmental and health?
In today's society, with the emphasis on environmental protection, many regions have imposed restrictions on substances using heavy metals, which has also led to the impact of many traditional quantum dot applications. Therefore, many enterprises and research institutions are working on developing heavy metal-free quantum dot materials, which not only have bright luminous properties, but also avoid the potential harm to health and environment of traditional heavy metals.
In short, quantum dots are gradually becoming an important topic in the technology community due to their unique optical characteristics, showing great application potential in both the fields of blue LED, medical imaging or quantum computing. With the continuous advancement of inductive quantum dot technology, we can look forward to wider applications in the future, but at the same time we have to face the safety issues of these materials. Are we ready to meet this challenge?
