as you know, the different studies show that the shell improve the QY of QDs. I want to know that, how the shell could improve the QY of QDs?
Dear Kambiz Hosseinpanahi , of course, shell structure QDs plays a decisive role in the process of photoinduced charge transfer from QDs to sacridine ligand that causes the quenching of photoluminescence QD. It is shown that the shell CdSe-nuclei QDs having multicomponent structure ZnS / CdS / ZnS, while its relatively small thickness allows to largely suppress the decrease of the quantum yield of the photoluminescence quantum dots, compared with both the classical thin shell ZnS, and with extra-thick shells this composition. Thus, QDs type "core / sheath multicomponent" with the structure of CdSe / ZnS / CdS / ZnS may become optimal photoluminescence-labeled to generate diagnostics and cancer therapy using the anticancer compounds based on derivatives of acridine.
Dear Kambiz,
the surface properties of QD (and other nanostructures like nanowires, films) determine essentially the recombination of electron hole pairs. One point is the surface recombination - a nonradiative recombination process. A second property is the position of the quasi-fermi-level. This property determines the density of electrons (holes) in the bands. The intensity of recombination radiation is proportional to the density of free carriers. Furthermore, the number of energy levels taking part in the recombination depends on the position of the Fermi level.
A third property - the boundary between core and shell could influence the recombination negatively if defects are grown.
With regard
R. Mitdank
Dear Kambiz,
As the previous comments already pointed out, the surface properties of QDots and other materials with a high surface area give rise to the need of an epitactically grown shell exhbiting a wider band gap (e.g. CdS onto CdSe or ZnS onto CdS). From a chemical point of view, the surface of QDots is terminated by chemical groups with high phonon frequencies such as hydroxide, hydrosulphide, or carbonate, which foster non-radiative quenching of excitons by multi-phonon relaxation. To visualise the process, please have a look to the attached ppt-file.
Yours truly,
Thomas
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