One of the most current and intensively studied nanoparticles within the QD family is the (ZnO) n nanoclusters. QDs have other interesting features as well, such as broad absorption spectra, high quantum yields, and photochemical robustness. The degree of the band gap shift can be described by the Brus quantum mechanics model, taking advantage of the well-known “particle in a box” paradigm. The band gap can be tuned via passivation with a ligand or by doping, so as to emit any desired colour of light. This effect can be observed as a colour difference that arises in changing the dot size in a given material. Thus, a smaller QD ought to require more energy to excite an electron into the latter band. In general, decreasing the size of the QD crystal increases the difference in energy between the highest valence band and the lowest conduction band, the so-called band gap. The designation as “quantum dots” is derived from the notion that their optoelectronic properties are strictly connected with principles of quantum mechanics, for example the effect of size on their properties. QDs are defined as “containing a variable number of electrons that occupy well-defined, discrete quantum states and have electronic properties intermediate between bulk and discrete fundamental particle”. Due to their size, the applications of QDs are limited to not only industrial but also biomedical purposes, with possible applications in medical imaging, drug delivery, and biosensing. These particles are well adapted to play the role of advanced semiconductors and other similar nanoscale devices, and highly useful in a wide range of processes in new technology development, including optical sensing, photocatalysis, energy storage (solar cells), ultrafast optical switches and logic gates, transistors, and even membrane fabrication. Discovered in 1980, intriguing small nanoparticles (NPs) of diameters in the range of 2–10 nm are characterized by unique physicochemical, electric, and optical properties such as large surface-to-volume ratio, tuneable band gap, diameter-dependent absorption spectrum, and high carrier mobility. Quantum dots (QDs) represent one of the primary frontier subjects of research in the field of nanotechnology.
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