Synthesis, Shell Modification And Optical Properties Of CdS: Mn Semiconductor Nanocrystals

Due to their unique physical and chemical properties, semiconductor nanocrystals showed their potential applications in various technological fields, such as biological labels, wavelength-tunable laser, sensors, and solar cells. Therefore, semiconductor nanocrystals have attracted enormous attention in recent years. Among various semiconductor nanocrystals, transition metal-doped II-VI semiconductor nanocrystals are a research hotspot in recent years. The transition metal Mn ion dopant has been widely investigated in view of its long wavelength emission, high PL quantum yield, minimized self-absorption and higher photostability. Mn dopants can strongly influence and improve properties of CdS semiconductor nanocrystals. However, the nanocrystal cannot still meet the requirements of technological material sometimes because the specific surface area of nanocrystals is too larger. Large surface areas result in more surface defects, which led to nonradiative couplings and reduce the value of nanocrystals. Thus, surface modification is necessary. Among all the surface modification methods, core/shell structure nanocrystals is proved to be a very effective method to enhance optical properties.In this paper, Mn doped CdS:Mn nanocrystals were synthesized by a co- precipitation technique in aqueous phase. And then CdS:Mn/ZnS core/shell nanocrystals were prepared by ZnS epitaxial coated on the surface of CdS:Mn nanocrystals. CdS:Mn/SiO₂ core/shell composite nanoparticles were also synthesized by reverse micro-emulsion method.The main work of this paper is listed as follows:(1) With 3-Mercaptopropionic acid as stabilizer, water-soluble Mn-doped CdS nanocrystals were synthesized by an aqueous colloidal synthetic technique. Their structures, morphologies, optical properties were characterized by X-ray diffraction, transmission electron microscopy, UV-Vis absorption and photoluminescence spectroscopy respectively. Results identify that Mn-doped CdS nanocrystal are cubic structure and have a narrow size distribution. Moreover, CdS:Mn nanocrystals have a distinct Mn²⁺ ion ⁴T₁–⁶A₁related emission well-separated from the trap state emission. Reaction variables such as the 3-Mercaptopropionic acid/Cd (MPA/Cd) ratio, reaction temperature and Mn²⁺ doped concentration were systematically investigated to evaluate the impact of the variable on particle size and optical property. Results suggest that the optical properties of CdS:Mn nanocrystals are strongly influenced by the variables. The optimum reaction conditions in this system are obtained. The photostability of CdS:Mn nanocrystals with different pH values and storage time were also researched.(2) CdS:Mn/ZnS core/shell nanocrystals were prepared by ZnS epitaxial coated on the surface of CdS:Mn nanocrystals. The crystal structure, morphology and optical property of CdS:Mn/ZnS nanocrystals were characterized by X-ray diffraction, transmission electron microscopy and UV-Vis absorption spectroscopy. The results show that nanocrystals are nearly monodisperse and confirm that ZnS is epitaxial grown on the surface of CdS:Mn nanocrystals. Luminescence and photostability of the nanocrystals were researched with photoluminescence emission spectroscopy. Luminescence intensity of the core/shell nanocrystals is significantly enhanced up to eight times, and the Mn²⁺ photoluminescence peak is gradually red-shifted as the ZnS shell increases. Furthermore, a strongly improved photostability of the optical properties is observed.(3) CdS:Mn/SiO₂ core/shell composite nanoparticles were synthesized by reverse micro-emulsion method and the properties were also characterized. TEM results suggest that particle size of the SiO₂ nanoparticle is about 100nm and has a uniform size distribution, furthermore, CdS:Mn nanocrystals disperse in SiO₂ nanoparticles. PL results show that the emission intensity of CdS:Mn/SiO₂ composite nanoparticles decreases due to the light scattering of SiO₂. And the photostability study shows that the photostability of CdS:Mn/SiO₂ core/shell composite nanoparticle is significantly enhanced.