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Quantum Dot Technology

Semiconductor Quantum Dots_110522A
[Semiconductor Quantum Dots - Science]


- Overview

Quantum dots (QDs) are semiconductor particles a few nanometers in size whose optical and electronic properties differ from those of larger particles due to quantum mechanics. They are central topics in nanotechnology. When a quantum dot is irradiated with ultraviolet light, one of the electrons in the quantum dot can be excited to a higher energy state. 

Quantum dots are theoretically described as point-like or zero-dimensional (0D) entities. Most of their properties depend on the size, shape and material of the fabricated QDs. Typically, quantum dots exhibit thermodynamic properties that differ from their bulk materials. One of these effects is a lowering of the melting point. The optical properties of spherical metallic quantum dots are well described by Mie scattering theory.

In the case of semiconductor quantum dots, this process corresponds to the transition of electrons from the valence band to the conduction band. The excited electrons can fall back into the valence band, releasing their energy in the form of light. This light emission (photoluminescence) is shown on the right. The color of this light depends on the energy difference between the conductance and valence bands, or transitions between discrete energy states when the band structure is no longer well-defined in the QD.


- Semiconductor Quantum Dots

Semiconductor materials have optical and electronic properties that can be engineered through their composition and crystal structure. The use of semiconductors such as silicon gallium arsenide has inspired technologies ranging from computers and cell phones to lasers and satellites. 

Semiconductor quantum dots (QDs) provide an additional lever: as their size is reduced to the nanoscale in all three dimensions, confined electron motion results in discrete atom-like electronic structures and size-dependent energy levels. This enables the design of nanomaterials with widely tunable light absorption, bright emission of pure colors, control over electron transport, and widely tuned chemical and physical functions due to their large surface-to-volume ratios.

Quantum dots (QDs) are artificial nanoscale crystals that can transport electrons. When ultraviolet light hits these semiconductor nanoparticles, they emit light of various colors. These artificial semiconductor nanoparticles have found applications in composite materials, solar cells, and fluorescent biomarkers. 

In the case of semiconductor quantum dots, this process corresponds to the transition of electrons from the valence band to the conduction band. The excited electrons can fall back into the valence band, releasing their energy in the form of light. This light emission (photoluminescence) is shown on the right. The color of this light depends on the energy difference between the conductance and valence bands, or transitions between discrete energy states when the band structure is no longer well-defined in the QD.


- Optoelectronic Properties

In the language of materials science, nanoscale semiconductor materials tightly confine electrons or electron holes. Quantum dots are sometimes called artificial atoms, emphasizing their strangeness, with bound discrete electronic states, just like naturally occurring atoms or molecules. The results show that the electron wavefunction in quantum dots is similar to that in real atoms. By coupling two or more of these quantum dots, an artificial molecule can be made that exhibits hybridization even at room temperature. 

Quantum dots have properties intermediate between bulk semiconductors and discrete atoms or molecules. Their optoelectronic properties vary with size and shape. Larger QDs with diameters of 5-6 nm emit longer wavelengths and are orange or red in color. Smaller quantum dots (2-3 nm) emit shorter wavelengths, producing colors like blue and green. However, the exact color will vary depending on the exact composition of the QD. 


- Optical Properties

In semiconductors, light absorption usually causes electrons to be excited from the valence band to the conduction band, leaving a hole behind. Electrons and holes can combine with each other to form excitons. When this exciton recombines (ie the electron returns to its ground state), the energy of the exciton can be emitted as light. This is called fluorescence. In the simplified model, the energy of the emitted photon can be understood as the sum of the bandgap energy between the highest occupied energy level and the lowest unoccupied energy level, the confinement energy of holes and excited electrons, and the binding energy. Excitons (electron-hole pairs): 

Since the confinement energy depends on the size of the quantum dots, the absorption onset and fluorescence emission can be tuned by changing the size of the quantum dots during synthesis. The larger the point, the redder (lower energy) its absorption onset and fluorescence spectrum. Conversely, smaller dots absorb and emit bluer (higher energy) light. Recent articles in Nanotechnology and other journals have begun to suggest that the shape of the quantum dots may also be a factor in coloration, but not enough information is currently available. In addition, studies have shown that the lifetime of the fluorescence is determined by the size of the quantum dots. Larger dots have tighter energy levels in which electron-hole pairs can be trapped. Therefore, the electron-hole pairs in the larger dots live longer, causing the larger dots to show longer lifetimes. 

To improve the fluorescence quantum yield, a shell of semiconductor material with a larger bandgap can be fabricated around the quantum dots. This improvement is thought to be due to the reduced access of electrons and holes to the recombination path of the non-radiative surface in some cases, but also due to the reduction of Auger recombination in other cases.


- Production

There are several ways to fabricate quantum dots. Possible methods include 

  • Colloidal synthesis:
  • Self-assembly:
  • Electrical gating:


- Potential Applications

Quantum dots are man-made nanostructures that can have many different properties depending on their material and shape. For example, due to their special electronic properties, they can be used as active materials in single-electron transistors. 

Quantum dots are particularly promising in optical applications due to their high extinction coefficient and ultrafast optical nonlinearity, with potential applications for developing all-optical systems. They work like single-electron transistors and show the Coulomb blocking effect. Quantum dots have also been proposed as qubit implementations for quantum information processing, and as active elements for thermoelectricity.

The properties of a quantum dot depend not only on its size, but also on its shape, composition and structure, such as whether it is solid or hollow. A reliable manufacturing technique that exploits the properties of quantum dots -- for a wide range of applications in catalysis, electronics, photonics, information storage, imaging, medicine or sensing -- requires the ability to produce large quantities of nanoscale batches according to the exact same parameters. crystal. Since certain biomolecules are capable of molecular recognition and self-assembly, nanocrystals can also be an important component of self-assembled functional nanodevices. 

Potential applications of quantum dots include single-electron transistors, solar cells, LEDs, lasers, single-photon sources, second-harmonic generation, quantum computing, cell biology research, microscopy, and medical imaging. Their small size allows some quantum dots to be suspended in solution, which could lead to their use in inkjet printing and spin coating. They have been used in Langmuir-Blodgett films. These processing techniques lead to cheaper and less time-consuming semiconductor manufacturing methods. 


- Quantum Dot-Light Emitting Diodes (QD-LEDs)

New blue quantum dot technology could lead to more energy-efficient displays.

Quantum dots are nanoscale crystals capable of emitting light of different colors. Quantum dot-based display devices promise higher power efficiency, brightness and color purity than previous generations of displays. Of the three colors (red, green, and blue) typically required to display a full-color image, the last has proven difficult to produce. A new method based on self-organizing chemical structures provides a solution, and cutting-edge imaging techniques for visualizing these new blue quantum dots have proven critical to their creation and analysis. 

If you look closely at your device's screen, you may see individual picture elements, pixels, that make up an image. Pixels can appear in almost any color, but they're not actually the smallest elements on a screen because they're usually made up of red, green, and blue subpixels. The variable intensities of these subpixels allow a single pixel to exhibit a single color from a palette of billions. The underlying technology behind subpixels has evolved from the early days of color TV, and there are now many possible options. But the next big leap is likely to be so-called quantum dot light-emitting diodes, or QD-LEDs.

QD-LED-based displays already exist, but the technology is still maturing, and current options have some drawbacks, especially the blue subpixels in them. Of the three primary colors, the blue sub-pixel is the most important. Blue light is used to generate green and red light through a process called down-conversion. Because of this, blue quantum dots require more tightly controlled physical parameters. This often means that blue quantum dots are very complex and expensive to produce, and their quality is a key factor in any display. But now, a team of researchers led by Professor Eiichi Nakamura from the Department of Chemistry at the University of Tokyo has found a solution.


[More to come ...]

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