Quantum dots are small assemblies of metal, metal oxide or semiconductor materials with novel electronic, optical, magnetic and catalytic properties of size 2 -10 nm. Quantum dots also known as artificial atoms are considered to be neither an extended solid structure nor a single molecular entity. They absorb light, and then re-emit the light at a different wavelength. An example is cadmium selenide. When illuminated, the quantum dot emits a particular colour: smaller dots fluoresce at shorter wavelengths, such as blue, while larger dots emit longer wavelengths, like red . Researchers have studied quantum dots in transistors, solar cells, LEDs, and diode lasers. They have also investigated quantum dots as agents for medical imaging and hope to use them as qubits.

Working of quantum dots

Quantum dots are a special class of semiconductors, which are crystals composed of periodic groups of II-VI, III-V, or IV-VI materials. Semiconductors are used by modern electronics industries to make Light Emitting Diode and personal computer. Quantum dots due to their very small size behave differently, giving quantum dots unprecedented tunability and enabling never before seen applications to science and technology. An important property of quantum dot is that it can easily transfer energy. When a laser shines on it energy can pass to a nearby molecule, which in turn emits a fluorescent glow that is visible under a microscope.

Several routes are used to synthesize them, the most common being wet chemical colloidal processes. Lot of research is going on about the semiconductor quantum dots, as they exhibit distinct ‘quantum size effects’. The light emitted can be tuned to the desired wavelength by altering the particle size through careful control of the growth steps. Quantum dots can operate in a liquid environment and therefore can be applied to biological imaging.
In the fabrication of quantum optoelectronic devices, semiconductor quantum dots (QD) are integrated as active elements. This requires a precise control on their shape, size and location over the substrate. For this purpose different strategies are followed. Patterned substrates is one such method with good results on the growth selectivity and photoluminescence (PL) emission of single QD. Another strategy is based on droplet epitaxy growth technique which has emerged as an optimal strategy for obtaining different nanostructures complexes with a great shape and size control. An example is on obtaining size controlled low density InAs QD nucleated inside previously formed GaAs nanoholes.

Quantum dots can be made using pyrolysis method by injecting organometallic reagents into hot coordination fluid at 300˚C such as triocytlphosphine (TOP) and triocytlphosphine oxide (TOPO), which are used as capping reagents and the reaction medium. They prevent the bulk semiconductor formation and go into the inner surface for maintaining the optical properties.
Uses of QD

Many industries can profit from the use of quantum dots. The following applications illustrate the many ways that quantum dots' advantages may be exploited.

• Displays

• LEDs

• Life Sciences

• Thermoelectrics

• Photonics & Telecommunications

• Security Inks

• Solar Cells & Photovoltaics
Demerits

Quantum dots can have surface defects which can affect the recombination of electrons and holes by acting as temporary traps resulting in blinking of the quantum dots which deteriorates the quantum yield of the dots. Quantum dots when placed into live cells exhibit aggregation which can interfere with cell function making delivery into cells more difficult. The toxicity of quantum dots to cells is a major issue and metabolism and degradation within the body is still largely an unknown issue.