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Nanomaterials

Quantum Dots

Quantum dots are called semiconductor nanocrystals and are composed of II-VI or III-V group elements. Nanocrystalline grains with a size between 2-10 nm are soluble in water. At present, CdS, CdSe, CdTe, ZnS, etc. have been studied more frequently. The physical, optical and electrical performance of quantum dots are far better than the ordinary organic fluorescent dyes, and they have high sensitivity, good stability, long shelf life. Quantum dots are a new generation of multifunctional materials.

Figure 1. Luminescent quantum dots.

Applications:

As a two-photon fluorescent material, quantum dots possess various nano-effects, such as quantum size effect, surface effect, and unique photoelectric effect. These characteristics make it have broad application prospects in the fields of biomedicine, luminescent materials, and clean energy.

Biomedical applications: Compared with organic dyes, semiconductor quantum dots have the advantages of broad absorption spectrum, narrow emission band, and higher light stability, making them the best choice for biological monitoring of long-term chemotaxis. Quantum dots have the advantages of stable physical and chemical properties, such as non-crosslinking, biocompatibility, water-solubility, photostability and anti-photobleaching, etc. They can be applied in cell imaging, and can also be used to develop many drug delivery systems that enable drugs to be delivered to specific parts of the body.
Solar cells: Quantum dot solar cells mainly improve the photoelectric conversion efficiency from two aspects: one is that they have photons with sufficient energy to produce multiple excitons; second, an intermediate band can be formed in the energy band gap, and multiple band gaps can be used to generate electron hole pairs. In addition, it can slow down the cooling and lifting of hot electron-hole pairs through other effects. Due to the auger recombination between charge carriers and coulomb coupling and the three-dimensional limitation of the carriers, the transition process does not have to meet the conservation of momentum, thereby improving the conversion efficiency.
Light-emitting diodes: Quantum dot luminescent materials have the characteristics of emission frequency varying with size, narrow emission line width, relatively high luminescence quantum efficiency, and ultra-high light stability and solution processing. As a new type of semiconductor fluorescent material, quantum dots have gradually replaced traditional fluorescent materials and become a research hotspot. As an LED, it has adjustable light emission color, better resistance to water and oxygen erosion, higher color rendering index and color saturation, and lower energy consumption.

Production Processes:

The synthesis methods of quantum dots include epitaxial technology (such as MBE, MOVPE, LPE, etc.) and chemical methods (such as metal organic synthesis method, water phase synthesis method, continuous ion layer adsorption reaction method, sol-gel method, etc.). Among them, the quantum dots synthesized by the metal-organic synthesis method, the aqueous phase synthesis method, the continuous ion layer adsorption reaction method have good growth and high yield.

References:

Rachel S. Selinsky, Qi Ding, Matthew S. Faber, John C. Wright and Song Jin. Quantum dot nanoscale heterostructures for solar energy conversion. Chem. Soc. Rev., 2013, 42, 2963-2985.
Shi Ying Lim, Wei Shen and Zhiqiang Gao. Carbon quantum dots and their applications. Chem. Soc. Rev., 2015, 44, 362-381.
Serena Silvi and Alberto Credi. Luminescent sensors based on quantum dot–molecule conjugates. Chem. Soc. Rev., 2015, 44, 4275-4289.

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