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Colloidal quantum dots irradiated with a UV light. Different sized quantum dots emit different color light due to quantum confinement. They are a central theme in nanotechnology. In the language of materials science, nanoscale semiconductor materials tightly confine either electrons or electron holes. Quantum dots exhibit properties that are intermediate between those of bulk semiconductors and those of discrete molecules.
Their optoelectronic properties change as a function of both size and shape. Because of their highly tunable properties, QDs are of wide interest. There are several ways to prepare quantum dots, the principal ones involving colloids. Colloidal semiconductor nanocrystals are synthesized from solutions, much like traditional chemical processes. The main difference is the product neither precipitates as a bulk solid nor remains dissolved. There are colloidal methods to produce many different semiconductors. Large batches of quantum dots may be synthesized via colloidal synthesis.
Due to this scalability and the convenience of benchtop conditions, colloidal synthetic methods are promising for commercial applications. Plasma synthesis has evolved to be one of the most popular gas-phase approaches for the production of quantum dots, especially those with covalent bonds. Self-assembled quantum dots are typically between 5 and 50 nm in size. Quantum dots defined by lithographically patterned gate electrodes, or by etching on two-dimensional electron gasses in semiconductor heterostructures can have lateral dimensions between 20 and 100 nm. Some quantum dots are small regions of one material buried in another with a larger band gap.
Quantum dots sometimes occur spontaneously in quantum well structures due to monolayer fluctuations in the well’s thickness. Individual quantum dots can be created from two-dimensional electron or hole gases present in remotely doped quantum wells or semiconductor heterostructures called lateral quantum dots. The sample surface is coated with a thin layer of resist. A lateral pattern is then defined in the resist by electron beam lithography. The energy spectrum of a quantum dot can be engineered by controlling the geometrical size, shape, and the strength of the confinement potential.
Also, in contrast to atoms, it is relatively easy to connect quantum dots by tunnel barriers to conducting leads, which allows the application of the techniques of tunneling spectroscopy for their investigation. The quantum dot absorption features correspond to transitions between discrete, three-dimensional particle in a box states of the electron and the hole, both confined to the same nanometer-size box. Genetically engineered M13 bacteriophage viruses allow preparation of quantum dot biocomposite structures. Highly ordered arrays of quantum dots may also be self-assembled by electrochemical techniques. A template is created by causing an ionic reaction at an electrolyte-metal interface which results in the spontaneous assembly of nanostructures, including quantum dots, onto the metal which is then used as a mask for mesa-etching these nanostructures on a chosen substrate. This reproducible production method can be applied to a wide range of quantum dot sizes and compositions.