Quantum Dots (QDs) are crystals of semiconductor materials. Its diameter is in orders of many nanometres. This is size that results in free carriers of charge to experience “quantum confinement” in all 3 spatial dimensions. Properties electronically of QDs have been intermediate in between that of semiconductors bulk and of discrete molecules. It is related closely to shape and size. It allows the properties like absorption spectrum, band gap, and emission colour being tuneable highly. Size distribution of QDs is controlled in stages of fabrication. Example is, band gap in QD determines emitted light’s frequency range. This is related inversely to size. In applications of fluorescent dye, emitted light’s frequency increases as size of QD decreases. This shifts emitted light’s colour from red to violet.
Fabrication: Self-assembled QDs are sized in between typically 5nm and 50nm. QDs being defined by gate electrodes lithographically patterned or by 2 dimensional electron gas etching in semiconductor heterostructures possess dimensions laterally in between 20nm and 100nm. Few QDs are smaller regions of 1 material being buried to other with large band gaps. This is called core shell structures. Example is CdSe in core and ZnS in shell or from many forms of silica known as ormosil. QDs occur fewer times spontaneously in well quantum structures because of fluctuations monolayer in thickness of well. QDs self-assembled spontaneously nucleate whilst some conditions at times of (MBE) molecular beam epitaxy and (MOVPE) metallorganic vapour phase epitaxy. This is when materials are grown on substrates that are not matched lattice wise. Strains resulting coherently produce islands strained on top of 2 dimensional wetting layers. This specific growth mode is called Stranski- Krastanov Growth. Islands are buried subsequently for forming QDs. Method of fabrication has applications potentiality in quantum computation and quantum cryptography (that is photon sources singly). Important ways limitations are fabrication costs and control lacking over single dots positioning. Viral assembly: Lee et al. in year 2002 stated using of genetically engineered M13 virus bacteriophage for creating QDs biocomposite structures. Viruses genetically engineered recognize semiconductor surfaces specifically by methods of combinational phage displays selections.
Different other processes of making of QDs are Electrochemical Assembly, Bulk Manufacturing, and Heavy metal free QDs.