Characteristics of Quantum Dots

NEW YORK - Nov. 5, 2019 - PRLog -- Quantum dots, also known as semiconductor nanocrystals, are nanoscale materials composed of a small number of atoms. This paper mainly discusses some properties of quantum dots, including quantum confinement effect, quantum size effect, surface effect and luminescence property.

1 Quantum confinement effect

Generally, the smaller the volume, the greater the bandwidth, so the optical and electrical properties of the quantum dots are highly dependent on the size of the material. Generally, when the size of the quantum dot is equal to or smaller than the exciton Bohr radius of the corresponding bulk material, the movement of the carrier electron-hole pair is in a strongly restricted state. When the energy gap increases as the particle size becomes smaller, the semiconductor material is quantified.

2 Quantum size effect

It can be seen from the above formula that the quantum confinement energy and the coulomb interaction energy are proportional to 1/R2 and 1/R, respectively, the former can increase the band gap energy (blue shift), and the latter can reduce the band gap energy (red shift). When R is small, the quantum confinement can be more sensitive to R.

3 Surface effect

Surface effect means that the specific surface area of quantum dots increases with the decrease of particle size, resulting in insufficient coordination of surface atoms and increased number of unsaturated bonds and dangling bonds, thus the atoms on the surface of quantum dots are extremely unstable and easily bind to other atoms. This surface effect gives the quantum dots a large surface energy and high activity, which not only causes changes in the atomic structure of the quantum surface, but also causes changes in the surface electron energy spectrum.

4 Luminescence property

Unlike conventional luminescent materials, the luminescent materials of quantum dots have the following characteristics.

4.1 Adjustable emission spectrum

Semiconductor quantum dots are mainly composed of elements in IIB-VIA, IIIA-VA or IVA-VIA group. The luminescence spectra of ZnS quantum dots (https://www.alfa-chemistry.com/zncuins-zns-quantum-dots-i...) covers the ultraviolet region, and the luminescence spectra of CdSe quantum dots (https://www.alfa-chemistry.com/cdse-zns-quantum-dots-item...) covers the visible region, while the luminescence spectra of PbSe quantum dots covers the infrared region. Even for the same quantum dot material, the luminescence spectrum is different if the size is different.

4.2 Wide excitation spectrum and narrow emission spectrum

The range of the spectrum that triggers the quantum dot to reach the excited state is wide, and the quantum dot can be excited as long as the excitation light energy is higher than the threshold value. Regardless of the wavelength of the excitation light, as long as the material and size of the quantum dots are not changed, the emission spectrum of the quantum dots is fixed, and the emission spectrum range is narrow and symmetrical.