Quantum Dots
Quantum dots are tiny semiconductor nano crystals, typically only a few nanometers in size (2-7nm), that have unique optical and electronic properties which are dependent on their size and composition. The size and composition of quantum dots can be precisely controlled, which allows their emission wavelength to be tuned across a wide range of the electromagnetic spectrum. QDs are very bright and can emit light with high efficiency, making them useful for a range of applications in lighting and displays. They are very stable and can maintain their properties for long periods of time, which is important for many practical applications. QDs can be easily incorporated into a range of materials, including polymers, glasses, and semiconductors, which makes them useful for a variety of applications in fields such as medicine, energy, and electronics. Because of these and other advantages, quantum dots are being studied and developed for a wide range of applications, including in solar cells, LEDs, biological imaging, and quantum computing, among others. They are typically synthesized using one of several methods, including colloidal synthesis, epitaxial growth, and lithographic techniques.
Preparation Of QDs Using Colloidal Synthesis Method
The first step in the colloidal synthesis of quantum dots involves the preparation of precursor solutions containing the appropriate chemical compounds. These compounds typically include a metal salt and a stabilizing ligand. The precursor solutions are then heated to a specific temperature, causing nucleation to occur. At this stage, small clusters of atoms begin to form. As the precursor solution continues to heat up, the clusters of atoms grow into larger nuclei. The size and shape of the nuclei can be controlled by adjusting the reaction conditions such as temperature, concentration, and pH. The larger nuclei eventually begin to coalesce with each other, forming larger particles. During this stage, some particles will dissolve, and others will continue to grow by absorbing atoms from the surrounding solution. This process is called ripening. The final step in the synthesis process is to add a stabilizing agent to the solution to prevent the particles from agglomerating or growing any larger. The stabilizing agent, such as an organic ligand, will bind to the surface of the quantum dot, passivating the surface and stabilizing the structure. The resulting quantum dots can be separated by size and purified for use in various applications such as solar cells, bioimaging, and optoelectronic devices.
Optical Properties Of QDs:
As the QDS size is very smaller, the surface defect density reduces, and the band gap broadens (shows blue shift with respect to the bulk material). Emission color of QDs can be easily tuned by adjusting their size, with smaller QDs emitting at shorter wavelengths (blue) and larger QDs emitting at longer wavelengths (red). i.e., quantum size effect.
Have narrow emission linewidth, which makes them ideal for use in imaging and sensing applications. This is due to the discrete energy levels that are characteristic of quantum confinement.
High photoluminescence quantum yield, that is a larger proportion of the absorbed photons are emitted as light which are more useful in the development of efficient optoelectronic devices.
Also, have a broad absorption spectrum, which allows them to absorb light over a wide range of wavelengths. This property is suitable for applications such as solar cells, where broad spectral coverage is required.
Size-dependent extinction coefficient (the measure of their ability to absorb light), which is more important for the design of QD-based devices, as it affects the device performance. Consequently, quantum dots have an attractive wide range of applications, including biological labeling, light-emitting diodes (LEDs), photovoltaics, and quantum computing.
Metal Oxide Quantum Dots
Metal oxide(MO) quantum dots are nanoscale particles made of metal and oxygen atoms (metal cations and O anions) that exhibit unique physical and chemical properties due to their small size and high surface-to-volume ratio. In MO nanostructures, closely packed Oxygen anions form interstitial sites which are occupied by metal cations. The partially d-shells of transition metal ions facilitate enhanced electronic transitions, wide band gaps, superior electrical characteristics, and high dielectric constants. They have been studied and developed for a range of applications in fields such as energy, catalysis, sensing, and electronics. The size and shape of metal oxide quantum dots can be precisely controlled, which allows their properties to be tailored for specific applications such as solar cells. Metal oxide quantum dots are highly stable, and biocompatible, which makes them useful for medical applications such as imaging and drug delivery. Some examples of metal oxide quantum dots include titanium dioxide (TiO2), zinc oxide (ZnO), and iron oxide (Fe2O3) quantum dots. These materials have been studied for a range of applications, such as photocatalysis, gas sensing, environmental remediation, and bioimaging, among others.
Metal Sulfide Quantum Dots (MS-QDs)
A class of quantum dots that are composed of metal cations and sulfur anions. MS-QDs exhibit unique optical and electronic properties due to their quantum confinement effect, which occurs when the size of the QD is smaller than the exciton Bohr radius. Some examples of commonly studied MS-QDs include cadmium sulfide (CdS), zinc sulfide (ZnS), and lead sulfide (PbS) quantum dots. The emission color of MS-QDs can be tuned by varying their size, with smaller QDs emitting at shorter wavelengths and larger QDs emitting at longer wavelengths. They have high photoluminescence quantum yields, making them useful for applications such as biological imaging and sensing. Their broad absorption spectrum makes them useful for solar energy conversion and photocatalysis. In addition, MS-QDs have excellent electron transport properties due to their high electron mobility and large surface area, making them useful in electronic and optoelectronic applications. The other MS-QDs like ZnS and CuS, are relatively non-toxic and biocompatible, making them attractive for biomedical applications. However, the toxicity of some MS-QDs, such as CdS and PbS, remains a concern and efforts are being made to develop alternative, non-toxic materials for use in these applications.