Preparation And Application Of The Cation-doped Water-soluble ZnS Quantum Dots

In recent years, semiconductor quantum dot luminescent materials,especiallyⅤ-Ⅵ race of wide band-gap ones, have been shown a hugeapplication potential in such areas as biomedicine, environmentalmonitoring and fluorescent security, because of their excellent opticalproperties like wide excitation spectra, narrow emission spectra and highphotochemical stability. Cation can be doped in ZnS quantum dots toform different wide band-gap luminescent materials, and the dopingcation may exert significant effects on the microscopic structure andluminescence properties of the materials. Therefore, it must be of greattheoretical and practical significance to undertake the researches on theregulation and synthesis of doped ZnS quantum dots, and on the influenceof different doping ions on the microscopic structure and luminescenceproperties. In this research, by adopting co-precipitation as the methodand ZnS matrix as the object, we focused on studying the microscopicstructure, luminescent properties and formation mechanism of the cation(Mn²⁺,Co²⁺) doped ZnS quantum dots, and their exploratory applicationin fluorescent security inks. The specific main contents are as follows:（1） With co-precipitation method, and3-mercaptopropionic acid（MPA） as the surface modifier, the water-soluble ZnS quantum dots wereprepared. The results indicated that water-soluble ZnS quantum dots hada cubic sphalerite structure,a shape of irregular sphere, and comparativelyuniform particles whose diameters were mainly concentrated in about4.8nm; that under280nm excitation, ZnS quantum dots had two emissionpeaks which could be attributed to the ZnS surface states trap emissionand band-edge emission respectively; and that under320nm excitation,ZnS performed single peak emission with yellow fluorescence, andsignificantly enhanced luminous intensity. Meanwhile, this paperdiscussed the influence that different S/Zn and MPA/Zn ratios had onthe luminescent properties of water-soluble ZnS quantum dots. It wasfound that they had the best luminescent properties while Zn/S=3:1andMPA/Zn=4:1. Besides, their luminescence mechanism and formationmechanism also have been studied. （2） With co-precipitation method, and3-mercaptopropionic acid（MPA） as the surface modifier, the Mn²⁺doped water-soluble ZnSquantum dots were prepared. The results showed that a mixed peakappears in the （111） crystal plane of the ZnS with traces of Mn²⁺doped,but the ZnS phase wasn’t impacted: the obtained Mn²⁺dopedwater-soluble ZnS quantum dots remained cubic sphalerite structure witha slightly larger diameter than pure ZnS quantum dots, mainlyconcentrated in about9.7nm; and that under320nm excitation, Mn²⁺doped ZnS quantum dots appeared two emission peaks located at587nmand637nm which indicated ZnS surface emitting and Mn²⁺:4T1-6A1levelcharacteristic luminescence respectively. Meanwhile, this paper discussedthe influence that different concentrations of Mn²⁺and MPA/（Zn+Mn）ratios had on the luminescent properties of Mn²⁺doped water-soluble ZnSquantum dots. It was found that they had the best luminescent propertieswhile Mn²⁺doping amount was2%（mole fraction） and MPA/（Zn+Mn）=4.0. Besides, their luminescence mechanism and formation mechanismalso have been studied.（3） With co-precipitation method, and3-mercaptopropionic acid（MPA） as the surface modifier, the Co²⁺doped water-soluble ZnSquantum dots were prepared. The results showed that Co2+was dopedinto the internal ZnS lattice in place of Zn²⁺, and the ZnS phase wasn’timpacted with traces of Co²⁺doped: he obtained Co²⁺dopedwater-soluble ZnS quantum dots remained cubic sphalerite structure withcomparatively uniform particles whose diameters mainly concentrated inabout5.2nm; and that under320nm excitation, Co²⁺doped ZnS quantumdot performed a single peak emission, which can be attributed to therecombination of impurity level (Co²⁺:4A1-4T1) and defects, whose crestwas located at728nm with red fluorescent and significantly increasedluminous intensity. At the same time, this paper discussed the influencethat different concentrations of Co²⁺and MPA/（Zn+Co） ratios had onthe luminescent properties of Co²⁺doped water-soluble ZnS quantum dots.It was found that they had the best luminescent properties while Co²⁺doping amount was4%（mole fraction） and MPA/（Zn+Co）=5.0.Besides, their luminescence mechanism and formation mechanism also have been studied.（4） An aqueous fluorescent security ink formulations were identifiedby conducting a large number of orthogonal experiments, with Mn²⁺doped ZnS quantum dots as fluorescence agent, and titanium dioxide,water-based acrylic resin and acrylic emulsion as pigment and binderrespectively. The results showed that the fluorescent security ink had auniform fineness concentrating in about15μm, good liquidity andpigmenting properties, and a viscosity between20Pa.s and30Pa.s whichwell met the requirements of the aqueous ink flexographic printing; andthat under320nm excitation, the ink performed doublet emission whosepeaks were located at587nm and637nm respectively, and the maximumemission intensity was produced by the characteristic peaks of the Mn²⁺:4T1-6A1at637nm, but its luminous intensity was significantly reducedwhen compared with that of the fluorescence agent at637nm. At thesame time, this paper discussed the influence of different titanium dioxidecontent, fluorescent agent content and ammonia content had on theaqueous fluorescent security ink performance. It was found that thefluorescent ink had the best printing adaptability when the titaniumdioxide content was15%to20%, the fluorescent agent content4%to6%,and the ammonia content0.6%to0.7%.