Quantum dots, also known as semiconductor nanocrystals, are nanoscale materials composed of a small number of atoms. The number of atoms in a quantum dot is usually between a few and a few hundred, and their size in all three dimensions are less than 100 nm. The movement of carriers in the three dimensions of quantum dots is limited by the size effect. Due to the quantum confinement effect, the energy levels of carriers in quantum dots are similar to those of atoms having a discontinuous energy level structure, so quantum dots are also called artificial atoms. Due to their special energy level structure, quantum dots exhibit unique physical properties. 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. The energy after quantification of the semiconductor material is: E(R)=Eg+h²π²/2uR²-1.8/εR. In the formula, Eg is the bulk band gap, u is the mass of electrons and holes, ε is the dielectric constant of the quantum dot material, R is the radius of the particle, and E(R) is the lowest excitation energy. The value obtained by subtracting Eg from E(R) is the amount of increase in kinetic energy.
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. As R decreases, the quantum confinement energy increases more than the Coulomb interaction energy, resulting in a blue shift of the spectrum.
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. Surface defects lead to trapped electrons or electron holes, which in turn affect the luminescent properties of quantum dots, causing nonlinear optical effects.
4 Luminescence property
The principle of luminescence of quantum dots is similar to that of conventional semiconductor luminescence, that is, carriers in a material reach an excited state after receiving external energy, and release energy when carriers return to the ground state, and this energy is usually released in the form of light. 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 quantum dots of different sizes or materials are in different bands. For example, the luminescence spectra of ZnS quantum dots covers the ultraviolet region, and the luminescence spectra of CdSe quantum dots 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.
4.3 Large stokes movement
The peak of the emission spectrum of a quantum dot material is usually red-shifted relative to the peak of the absorption spectrum. The difference between the peak of the emission and absorption spectrum is called the Stokes shift. The Stokes shift of quantum dots is larger than conventional materials. Stokes shift is widely used in the detection of fluorescence spectral signals.