Some of the most promising advances in nanoscience technology today have to do with materials. One of the most promising technology discoveries are Quantum dots (QDs). On 4 October 2023, the Royal Swedish Academy of Sciences awarded Moungi G Bawendi of MIT, Alexei I Ekimov of Nanocrystals Technology Inc, and Louis E Brus of Columbia University the Nobel Prize in Chemistry for their discovery and synthesis of quantum dots.
What Are Quantum Dots?
Quantum dots are extremely small atoms or particles of semiconductor material (about 2-10 nm/10-50 atoms). Simply put, quantum dots are semiconductor nanoparticles with conduction band electrons, valence band holes and excitons (electron-hole pair) in all three dimensions. Quantum dots, discovered in 1980, are oddly electronic – somewhere between solid semiconductors and molecules. They're also partly new, because they have such a high surface area to volume ratio. Among those are fluorescence, in which nanocrystals glow in different colours as they grow.
Structure of Quantum Dots
Quantum dots are usually manufactured from II-VI (CdSe, CdTe), III-V (InP, InAs) or IV-VI (PbS, PbSe) semiconductors. Typical quantum dot structures include:
Core/Shell: Quantum dots generally have a core and shell. Its inner layer glows, and its outer layer shields its inner layer from the world to give quantum yield. The best known one is CdSe/ZnS, and the ZnS shell imparts strength and optical clarity.
Modification of the Quantum Dot Surface: Organic molecules, polymers, biomolecules are capable of being modified in quantum dot surfaces to make them dispersible, stable, or biocompatible. These modifications play a significant role in their chemical stability, solubility, and applicability in biological and environmental contexts.
Properties of Quantum Dots
Quantum Confinement Effect
Quantum confinement is a unique phenomenon of quantum dots, where electron and hole movements are restricted on a very small scale, leading to discrete electron energy levels. This effect makes the optical and electrical properties of quantum dots closely related to their size:
Size-dependent Emission: Smaller quantum dot, more emission band goes to blue, bigger dots go to red. When the size is varied, the wavelength of the light can be varied to get different colours.
High Brightness and Thin emission peaks: Quantum dots have very bright and very narrow emission peaks (10-40 nm), great for displays and LEDs where you want high color saturation.
Optical Properties
Quantum dots have great optical properties such as high quantum yield, life span, and photobleaching immunity. After capturing photons, they radiate photons so well that they are commonly employed in fluorescence labelling, sensing and imaging:
Quantum Yield: Quantum dots are highly quantum yield, which is to say that they can produce photons efficiently after collecting them. Proper surface modification and shell coverage can further enhance quantum yield.
Photostability: Quantum dots are more photostable than organic dyes and it won't discolor in the long run.
Electrical Properties
The electric properties of quantum dots, for example, the high electron and hole mobility make them a candidate for solar cells and optoelectronic devices. Furthermore, quantum dots' conductivity and band gap can be controlled by changing their size and material.
Why Quantum Dots are Better Than Organic Dyes?
Fluorescence is one of the most common instrument in biology. As fluorescent probes, QDs have unique optical properties over organic dyes traditionally employed in biological imaging:
Broad Excitation Spectrum
QDs have a long excitation range and, due to this, it is possible to stimulate several QDs of varying sizes with the same wavelength. Organic dye probes need multiple excitation wavelengths, adding complexity and cost.
Tunable Emission Wavelength
By changing the particle size and composition, QDs' emission wavelength can be varied, and hence they can have different fluorescence spectrums.
Larger Stokes Shift
QDs have a large Stokes shift and symmetrical, narrow emission spectra, facilitating easier identification and analysis in multicolor imaging.
High Photostability
QDs exhibit longer fluorescence lifetimes than organic dyes (1-2 orders of magnitude longer), with less photodegradation.
These combined optical properties make QDs highly valuable in multivariate analysis, multicolor imaging, and high-sensitivity diagnostics.
Types of Quantum Dots
Graphene Quantum Dots
Graphene quantum dots (GQDs) are a new type of carbon nanomaterial with novel physics and chemical structure. They have one or several layers of graphene and are typically lateral, smaller than 10 nanometres, and thinner than 10 layers. Graphene quantum dots, which can perform quantum confinement and edge effects (being quasi-zero-dimensional) are already a common material in optoelectronics and biomedicine.
Graphene quantum dots emit fluorescence, exhibit high quantum efficiency and are low in biotoxicity, so are essential for sensors, bioimaging and treatment of cancers. Graphene quantum dots in biomedical applications, for example, are effective ion, biomolecule and free-radical detectors. You can use them to create optoelectronic sensors and optoelectronic semiconductor chips.
Carbon Quantum Dots
Carbon quantum dots (CQDs)– carbon dots, carbon nanodots – are a new zero-dimensional carbon nanostructure. They're typically quasi-spherical, ranging in radius from 10 nanometres to 1000 nanometres, and are filled on the surface with a generous proportion of oxygen, nitrogen and other functional groups. Carbon quantum dots with excellent fluorescence, toxicity, water soluble, environmental and economical properties have become very common in many fields in recent years.
The commercial potential of carbon quantum dots in biomedicine, sensors and optoelectronics is huge. In biomedicine, for instance, they could be used for cell-scanners, drug delivery and cancer treatment. Moreover, carbon quantum dots are used in optoelectronics like light emitting diodes and solar cells, and catalysis and sensors.
Metal-based Quantum Dots
Metal-based quantum dots are optically and electrically different nanomaterials used in a variety of applications. Metal quantum dots tend to be made of metal or metal compounds, with a nanometre-scale size, and so induce huge quantum effects. These quantum dots can be produced by liquid-phase ultrasound or chemical vapor deposition.
Quantum dots of metal have other significant uses in optoelectronics, for example in quantum dot lasers, photodetectors and solar cells. They are also used in LED lighting, chemical catalysis and the life sciences. The optical properties of metal quantum dots can be tuned by doping different metal ions. For example, incorporating Ag2O into perovskite quantum dot glasses can enhance their crystallization and luminescence properties.
References
- Kumawat M K, Thakur M, Gurung R B, et al. Graphene quantum dots for cell proliferation, nucleus imaging, and photoluminescent sensing applications[J]. Scientific reports, 2017, 7(1): 15858.
- Yuan F, Li S, Fan Z, et al. Shining carbon dots: Synthesis and biomedical and optoelectronic applications[J]. Nano Today, 2016, 11(5): 565-586.
- Sukharevska N, Bederak D, Goossens V M, et al. Scalable PbS quantum dot solar cell production by blade coating from stable inks[J]. ACS applied materials & interfaces, 2021, 13(4): 5195-5207.
