Quantum dots are extremely tiny nanoparticles typically between 10 to 10,000 atoms (1 to 9 nanometers) in diameter; they are thus smaller than 1/10,000th the width of human hair. Quantum dots are so small that they cannot be seen under a conventional microscope. To put it simply, quantum dots are incredibly small particles with sizes on an atomic scale. However, the effects of this extreme small size cannot be ignored. Quantum dots are actually very powerful materials, and their size gives them unique abilities, including highly efficient conversion of light to nearly any colour in the visible spectrum.
The electronic characteristics of quantum dots are determined by the quantum confinement effect, in accordance with their chemical composition, size, and shape. This feature means that we can control the colour of light given off by a quantum dot just by changing its size. Generally, larger dots emit light of longer wavelengths, such as red, while smaller dots emit light of shorter wavelengths, such as green or blue. The tune of a quantum dot is the wavelength of the light that it emits; this unique property can therefore be utilized for the following applications:
TV displays and smartphone displays
Security tags, security inks, and counterfeit protection devices
Quantum dot lasers
Quantum dot transistors
Bio-imaging, biomarkers, medical display applications (cancer cell imaging, protein analysis, and cell tracking)
High-density solid-material-based memory
Quantum dot computers
Visible-light-response-type photocatalytic materials
Various types of quantum dots currently exist, but in general, they are made of semiconductor materials such as CdSe, CdS, InP, ZnS, InP/ZnS, CIS (CuInS2, CuInSe2), AgInS2 CIS/ZnS, and PbS, etc. Moreover, there are carbon, graphene, and silicon quantum dots. Fuji Pigment Co., Ltd. synthesizes and supplies all of these quantum dots. However, Cd-based quantum dots are toxic. In addition, indium is expensive and requires complete exclusion O2 and H2O, to synthesize InP. Furthermore, set up of a manufacturing facility for these quantum dots is expensive.
In this regard, Dr. Ryohei Mori at Fuji Pigment Co., Ltd. (Kawanishi City, Japan) has been diligently researching and developing a new type of quantum dots, perovskite quantum dots. The half width of their emission spectra is substantially narrower than that of InP; this property is very beneficial to the application of the dots in display materials, LED, bio-imaging. The chemical composition of perovskite quantum dots are either CsPbX3 or CH3NH3PbX3 (X= Cl, Br, I). Their quantum efficiency is 50–80 per cent and their half width is 15–39 nm. Since the half width of the dots is small, they can be utilized in displays, lasers, LED and bio-imaging. Their base solvent is either hexane or toluene. However, finding alternative solvents is a challenge that is now being addressed. Furthermore, replacing Pb (lead) with another element such as Sn is under study, despite its difficulty; this entails widening of the half width and a decrease in quantum efficiency. Moreover, Fuji Pigment is trying to coat perovskite quantum dot surface with either polymer or inorganic materials which would enhance the quantum dot resistance against water, light and heat. The Fuji Pigment Co., Ltd. is suggested to be the first company, to commercialize perovskite quantum dots in the world.