Authors: Daniel K. Harris, Peter M. Allen, Hee-Sun Han, Brian J. Walker, Jungmin Lee, and Moungi G. Bawendi
Journal: Journal of the American Chemical Society
Quantum Dots are three dimensional semiconductors, which have electrical conductivity between that of insulators and conductors. The semiconductor has an energy gap that only those electrons with sufficient energy would go through. This conductivity is related to various factors such as the composition of material, temperature, size and etc. The bulk material property is not a simple sum of individual molecular property. The tunable property of quantum dots via size control makes quantum dots very attractive for various biological and industrial applications.
The band gap is inversely proportional to the size of the crystal. This means you need more photon energy to excite the dot and more energy is released when the excited crystal returns to its ground state. So the control over the size of quantum dots is very important yet it is synthetically most difficult. The quantum dot synthesis is achieved in a colloidal condition so a lot of variables such as temperature, time, compositions etc affect the crystal greatly. Usually it is harder to achieve smaller quantum dots. Ostwald ripening mechanism explains the current view on the quantum dot synthesis.
Colloidal quantum dots luminescent in the IR are important due to their ability to emit at the telecommunications wavelengths of 1.3 and 1.5 μm and to their application for in vivo imaging. The authors used two step procedure that uses an initial fast injection of tris(trimethylsilyl) arsine (TMS3As) into a solution containing cadmium(II) myristate at 175 C to form small nuclei followed by the slow, continuous addition of additional TMS3As to promote growth. This strategy is similar to previous reports of nanocrystal growth by an initial hot injection and subsequent slow injection. However, the narrow distribution and high quantum yield of quantum dots are noteworthy.
The paper represents the success in achieving Cd3As2 quantum dots luminescent from 530 to 2000 nm (IR). After achieving the series of quantum dots with same composition but different sizes to yield corresponding photophysical properties, the authors confirmed that the each synthesis condition yields small variation of crystal size. The authors used Transmission electron microscope (TEM) to support the narrow size distribution.
After crystal growth, quantum dots were coated with various polymers to achieve desirable solubility and functionalization. This process does not disturb the photophysical properties of quantum dots because the photophysical properties are governed only by the core. The authors tested Cadmium phosphide as potential shell material. The deterioration of the shell results in decrease of quantum yield. Therefore they are trying to investigate on Cd3P as a passivating and stabilizing shell material.