Quantum dots (QDs) are extremely small semiconducting nano-sized particles, ranging between 1 – 10 nanometers wide. To give you some idea of how tiny they are, 1 nanometer is the same as 1/10,000th the thickness of a single strand of human hair. This means that Quantum dots are extremely small that you cannot see them with your naked eye.
Quantum dots are very interesting because of their photoactive property. When you shine a light on them, they absorb the light in the form of energy. Then they can use the absorbed energy to produce their own light with a specific, pure color. The color emitted by quantum dots is primarily determined by their size and shape.
Quantum dots are quantum confined materials meaning that their physical size is smaller than their Bohr exciton radius. The smaller the QD, the shorter the wavelength emitted. This means that, for example for CdSe, which has a bulk band gap of 1.74 eV at 300 K, quantum confinement can produce photoluminescence ranging from 1.74 eV to 3.1 eV. This is equivalent to the wavelength range of 712 nm (deep red) down to 400 nm (violet), i.e. the entire visible spectrum! The smallest CdSe quantum dots emit blue light while the larger ones emit red light. And by simply tuning the nanoparticle size of the quantum, it can be made to absorb and emit energy in the entire visible spectrum. This light emission process is called photoluminescence (PL) or more specifically is called, fluorescence.
Since the physical size of quantum dots determines the wavelength at which they will emit light, variability in the nanoparticle size produces variability in color emissions, which can be measured as broadening of the full width of the emission peak at half its maximum height (FWHM). The narrower the bandwidth FWHM, the wider the color gamut achieved resulting to more pure and vivid colors emitted.