Improved stability of quantum dots

Research into quantum dots, semiconductor nanocrystals that are sufficiently small as to exhibit quantum mechanical properties, and which are being increasingly used as photoluminescent materials in bio-imaging, photonics and optoelectronic applications, has seen an exciting new development. Scientists, led by a team from Toyohashi University of Technology along with researchers at the National Institute of Technology, Kurume College in Japan, have managed to improve on the stability of the quantum dots by chemically altering their surface in an environmentally friendly way.

The study presented a new supercritical fluid-assisted, environmentally benign process for producing stabilized luminescent materials. In their paper, ‘Formation of poly(methyl methacrylate)-ZnO nanoparticle quantum dot composites by dispersion polymerization in supercritical CO2’, which has recently been published in The Journal of Supercritical Fluids [Matsuyama et al. J. Supercrit. Fluids (2015) DOI: 10.1016/j.supflu.2015.04.025], the team were able to immobilize these highly luminescent semiconductor nanoparticles in a polymer matrix that uses supercritical CO2 to help ensure the stability of their photoluminescence properties necessary for these important applications. Embedding quantum dots in polymeric matrices can enhance their stability and also stop agglomeration.

Up to now, the problem has been that any surface chemical modification would usually need a large amount of organic solvents, which can potentially cause a great deal of damage to the environment. To get round this issue, many studies have tried to synthesize polymer–nanoparticle composites with supercritical fluid (SCF)-based technology. Supercritical CO2 has received a great deal of focus as a SCF medium since it is cheap, easily available and nonflammable, as well as being known to be safe for the environment.

The Japanese team looked into the formation of nanostructured material using supercritical CO2, and showed the formation of composite nanoparticles of luminescent ZnO quantum dots and polymer by dispersion polymerization in supercritical CO2. As a result of the supercritical-CO2-assisted surface modification of quantum dots, the quantum dots were well dispersed in the polymer matrix and also managed to demonstrate the necessary high luminescence.

There has also been an issue concerning that the photoluminescence properties of pristine luminescent quantum dots were quenched in supercritical CO2, while the surface structure of the quantum dots was wrecked by supercritical CO2. However, as researcher Kiyoshi Matsuyama states, they “found that the quenching of ZnO quantum dots could be prevented by coating with silica to obtain PMMA-ZnO composite quantum dots with high luminescence using a supercritical-CO2-assisted surface modification with polymer.”

"the quenching of ZnO quantum dots could be prevented by coating with silica to obtain PMMA-ZnO composite quantum dots with high luminescence using a supercritical-CO2-assisted surface modification with polymer"Kiyoshi Matsuyama