Research On Preparation Of Fluorescent Nano-materials By Millisecond Laser Ablation In Liquid And The Effects Of Surface Modifacation

Controllable preparation and performance optimization are essential for the industrial applications of nanomaterials. Millisecond pulsed laser ablation in liquid can provide more stable growth conditions than the most widely used nanosecond pulse laser and have a higher preparation efficiency. The morphology, size, composition structure and surface states of nanomaterials can be controlled by changing the type of liquid medium, the laser parameters and other factors. So the research on laser ablation of materials is of great importance in optimizing the preparation conditions to control the synthesis of nanomaterials.Silicon and zinc oxide (ZnO) are environmentally friendly fluorescent nanomaterials which have aroused many interests these years. This research work makes great efforts on the controllable synthesis and optimization of the properties of silicon and zinc oxide (ZnO). Taking usage of millisecond pulsed laser ablation in liquid method, various new nanoconstructions with optimized properties can be obtained with the adjustment of the composition of the solid targets, the type of the liquid medium or the parameters of the laser.The technique combines the pulsed-laser ablation in liquid with solid phase separation of a target material. The growth mechanism for Si nanospheres prepared through this method is proved to be a typical solid state phase separation in a lower-temperature zone of the laser-irradiated suspension of SiO in liquid, while the high temperatures and rapid cooling rates generated by pulsed laser are crucial for the formation and quenching of the nanospheres. Following the same procedure, SiC nanospheres have been synthesized by irradiating an amorphous SiOC powder suspension in de-ionized water under magnetic stirring.Laser ablation of Si nanospheres in organic solution can significantly improve the dispersion of Si nanospheres. Increasing the single-pulse energy of laser ablation, we can obtain carbon (graphite) coated Si nanospheres. Laser ablation of Si layer in hexane can generate SiC and carbon coated nanocrystals at the same time. It is believed that laser induced thermal effects can cause the decomposition of organic molecule, the decomposited carbon atoms absorbed on the Si nanospheres to form layers or involved in the formation of nanocrystals directly to form compounds such as SiC and carbon coated SiC. Cu doping of ZnO quantum dots (QDs) was carried out by laser ablation of Zn/Cu composite targets immersed in PVP aqueous solution. Zn/Cu targets were firstly prepared via a chemical replacement route. Under the extremely non-equilibrium conditions generated by the pulse laser used, a large amount of Cu-doped ZnO QDs were produced with ultrafine size, good dispersibility, high stability, and high performance; on the other hand, the doping extent was feasibly controlled by changing the atomic ratio in Zn/Cu composite targets. In comparison with pure ZnO QDs, Cu-doped QDs exhibited intensive blue emission with longer wavelength, which was ascribed to the acceptor levels related to Cu dopants. Changed the components of taget, we fabricated various doped nanocrystals, such as Mn doped, Mg doped and Li doped ZnO nanocrystals..ZnO quantum dots prepared with different proportion of the reactants exhibit distinct optical properties, which is attributed to the difference of the defects obtained in ZnO. The effects of surface modification of amine ligands on the luminescence property of ZnO quantum dots were investigated systematically. The mechanism of the influence of amine ligands on the fluorescence property was also analyzed in detail. It is revealed that both the species of the defects in ZnO and the reaction manner between the modifier and the defects played important roles.