Synthesis And Optical Properties Of Water-soluble Zn-based Ⅱ-Ⅵ Semiconductor Nanocrystals

Semiconductor nanocrystals or quntum dots （q-dots） have attracted considerableinterests over the past decades due to their unique optical properties and enormousapplications. In recent developments, Zn-based II-VI semiconductor nanocrystals,such as: ZnS, ZnSe and their doped or core/shell nanocrystal without heavy metal ions,are being explored as viable alternatives to the Cd-based nanocrystal. In this thesis，several methods were developed to synthesize series of Zn based nanocrystal. Themethods were simple and controllable, the obtained nanocrystals not only hadwater-solubility but possessed good optical property. By doping, controlling theparticle size and changing the composite of Zn based nanocrystal，the emiss range ofthe nanocrystal can be tuned in visible emission range （from blue to orange）. Themain contributions are as follows:1. High quality Mn doped ZnS nanocrystal （ZnS:Mn） were successfullysynthesized via an alternative route based on the solvothermal method using oleic acidas stabilizing agent. The ZnS:Mn nanocrystal had two emission bands, includingsurface defect emission （blue） and Mn emission （orange）, which were both sensitivityto the Mn doped concentration. By varying the Mn doped concentration, the relativePL intensities of dual-color emissions could be well manipulated, and the color hue ofthe Mn doped ZnS d-dots could be tuned from blue to orange. A subsequent ligandexchange had been introduced to give the ZnS:Mn nanocrystal hydrophilic surfaces,in order to improve the photoluminescence （PL） property of the obtainedwater-soluble ZnS:Mn nanocrystal, ZnS shell were deposited on the sureface ofZnS:Mn. After overcoating ZnS shell on the ZnSe:Mn core，the Mn emission intensityof obtained ZnS:Mn/ZnS was obviously improved, the surface defect emission wasquenched.2. Silica-coated ZnS:Mn nanoparticles （ZnS:Mn/SiO₂） were synthesized by coatinghydrophobic ZnS:Mn nanoparticles with silica shell through microemulsion. Compared to uncoated ZnS:Mn nanoparticles, the silica-coated ZnS:Mn nanoparticleshad the improved PL intensity as well as good photostability. The obtainedwater-soluble silica-coated ZnS:Mn nanoparticles not only had good PL property butalso exhibited fluorescence sensitivity to Cu2+ions. By the further study, theZnS:Mn/SiO₂nanoparticles were firstly used as fluorescent probe for detection ofCu2+based on the quenching of fluorescence intensity, thus affording a very sensitivedetection method for Cu2+ions.. The possible quenching mechanism was discussed indetail.3. New water-soluble and highly luminescent ZnSe/ZnS core/shell nanocrystal withtunable emission range between390and460nm were synthesized via a two-stepmethod. Compared to the organometallic approach and its alternative routs, thistwo-step method used low-cost and no-toxic chemicals. In addition, the reactiontemperatures are low. Furthermore, the obtained core/shell nanocrystal had not onlyhigh qutunam yeilds but also good water-solubility. The ZnSe/ZnS core/shellnanocrystal prepared by our two-step method also shown their superiorities to onesprepared by the conventional aqueous solution synthesis, such as narrow emission,good monodispersity and board emission range. Moreover, this method was used toprepare water-soluble ZnSe:Mn/ZnS core/shell doped nanocrystal with dual emission（ZnSe host emission and Mn emission）.4. A modified two-step method based on our previous report was used to synthesizeZnSe:Mn/ZnS core/shell doped nanocrystal with pure dopant emission: ZnSe:Mncores firstly were prepared in two-phase system （water-toluene） under solvothermalcondition and then ZnS shell were deposited on the ZnSe:Mn core surface in theaqueous solution. Finally the water-soluble ZnSe:Mn/ZnS d-dots with high PLqutuam yield （QY） and good stability were obtained. The pure dopant emission can beoptimized with varying reaction parameters, and it has been found that the layers ofZnS shell as well as the Mn doped concentration in the reaction mixture were keyfactors for controlling the Mn emission position and PL QY. Remarkably, the PLintensity of the ZnSe:Mn/ZnS d-dots was found to be stable under intense UVirradiation, possessed better chemical stability than CdSe q-dots, and could be kept stable in water for several months,5. A facile approach was develpoed for the preparation of Cd_xZn_1-xSe alloynanocrystal via cation exchange reaction of Cd2+with the pre-prepared ZnSenanocrystal which were firstly synthesized in two-phase system （water-toluene） undersolvothermal condition. The obtained Cd_xZn_1-xSe alloy nanocrystal possessed high PLintensity and tunable emission. By controlling the Cd composite, the emission rangeof Cd_xZn_1-xSe alloy nanocrystal could be tuned from460nm to570nm. Furthermore,ZnS shell overcoating was adopted to improve the PL property of Cd0.4Zn0.6Se alloynanocrystal.