| Today, mankind has entered the information era totally. The non-linear optical(NLO) crystals which belong to information materials play an increasingly important role. The metal-organic complexes which combine the advantages of inorganic and organic compounds are introduced into the research field of non-linear optical materials and zinc tris(thiourea) sulfate crystals(Zn[CS(NH2)2]3SO4, ZTS) was one of it. ZTS crystals, whose frequency-doubled effect is 2-4 times as that of potassium dihydrogen phosphate(KDP) crystal, belong to a non-centrosymmetric orthorhombic and exhibit low angle sensitivity. Besides, ZTS crystal has a high laser damage threshold, good mechanical hardness and wide optical transparency. In view of the above advantages, ZTS crystals are widely used in laser technology, optical communications, data storage technology and optical computing et al. Currently, the studies of ZTS crystal mainly focused on the growth of large-size single crystals from solutions and the effects of additives on the optical properties, but seldomly on its growth mechanism and microscopic morphology. To a certain extent, the lack of microscopic experimental data restricts the preparation and application of high-quality single crystals. With the advent of Atomic Force Microscopy(AFM), it is possible to explore the microscopic field. In addition, as an important complexing agent, ethylene diamine tetra-acetic acid(EDTA) can complex with trace metal ions in solution which are harmful to the crystal growth, thereby improving the optical performance of ZTS crystals. Unfortunately, the microscopic studies of the effect of EDTA on the growth of ZTS crystal have not been reported. Now, the growth of ZTS crystal can be observed and investigated in real-time by AFM, through which we can not only understand how a growth unit entries into the lattice position through the growing interface, but also obtain the microscopic data closely related to the crystal growth and know the microscopic mechanisms during the growing process so as to guide researchers to control every aspect of crystal growing effectively and produce single crystals of high-quality rapidly. Moreover, further studies should be conducted on how the additive(EDTA) affects the solution properties and growing process of ZTS crystal.In this paper, based on the theory of thermal science, the effect of different doping concentration and supersaturated on stability, growth mechanisms and microscopic surface morphology were deeply discussed from the views of thermodynamics and kinetics. The main contents can be summarized as following:(1) The solubility, width of metastable zone and induction period of ZTS crystal experiments were conducted. The solubility curve with different supersaturated and doping concentration of ZTS crystal were measured, as well as the width of metastable zone under different temperature, doping concentration and cooling rate. It was found that both of the solubility and width of metastable zone increased with the increasing of EDTA doped, which indicated the promotion of stability of solution. Meanwhile, the width of metastable zone also enhanced with the increase of cooling rate. The nucleation process of ZTS solution under different doping(EDTA) concentration and supersaturation ratio was investigated by induction period experiment. The results showed that the nucleation mode was different under different supersaturation ratio. In addition, a series of the thermodynamic parameters were calculated with different doping concentration and supersaturation ratio.(2) The growth mechanisms of(100) face of ZTS crystal under different growth condition was studied by in situ AFM. In the two-dimensional(2D) nucleation mechanism, it is easier to nucleate at the edge dislocations and defects positions. The surface growth is primarily layer-by-layer growth. The spreading velocities of 2D nuclei shows a significant anisotropy in the <001> and <010> direction. The 2D nuclei can be generated at a lower driving force and developed into the hillock due to the reduction of the nucleation barrier resulting from the addition of EDTA. Besides the above, the growth characteristics of 2D nucleus with the addition of EDTA were also discussed.(3) The growth mechanism of screw dislocation was observed on the(100) face of ZTS crystal under lower supersaturation after adding EDTA. Under this growth mechanism, the different growth characteristics of single dislocation spiral, the spirals of same sign and opposite sign were discussed in detail.(4) The microscopic morphology of the(100) of ZTS crystals is mainly manifested as advancement of steps and the space of steps is not uniform. The step bunching occurs as steps moves in the same direction. Instead, the steps moving in the reverse direction separate if they meet each other, thus the step bunching decreases. The single step trains perpendicular to the original steps are generated on the wider terrace, and the bunching steps with different height are formed along the edge of original steps. According to microscopic data of step moving, the growth kinetics of steps with different doping(EDTA) concentration at supersaturations(0.03???0.08) at 25? was studied, by which the relationship between the kinetic coefficients, edge energies of steps and the doping(EDTA) concentration were obtained. Moreover, the diffusion mechanism of mass transfer was bulk diffusion under this condition.(5) It was found that the defect of the(100) face of ZTS crystal were mainly including hollow cores, liquid inclusions and solid inclusions by in situ AFM. The mechanism of the formation of defects and its impact on the surface topography were discussed.The innovation points of this paper are mainly including:(1) The stability of solution of ZTS crystals with different doping concentration, supersaturation, cooling rate and temperature was studied by polythermal and isothermal method. It was shown that the stability of solution was enhanced by doping EDTA and the reason for the improvement was also found.(2) The growth of(100) face of ZTS crystal with different doping concentration in real time was studied by AFM. The growth characteristics, both under two-dimensional(2D) nucleation and screw dislocation mechanisms, were obtained. In the two-dimensional(2D) nucleation mechanism, a new phenomenon was found for the first time that the rapid spread of 2D nucleus cannot proceed until finish an “incubation period” with the addition of EDTA. Moreover, it was also found that the 2D nuclei can be generated at a lower driving force with doping EDTA.(3) Polytype growth of dislocation was observed under screw dislocation mechanism and the growth of these was analyzed. By measuring the step length and advancing rate of single dislocation, it was found that the relationship between them is not completely consistent to the Gibbs-Thomson law, which indicated that this law is not applicable for every condition. Meanwhile, the interaction effect between dislocations was also researched, from which it was found that the motion of spirals of same sign will affect each other during the growth process resulting in the decline of advancement velocities of steps and the growth of steps produced by the spirals of opposite sign is not symmetry. In addition, if there are several dislocation sources, the one which grows more slowly will be swallowed and disappear ultimately. What mentioned above has not been reported in the studied of melt-organic crystals.(4) The different growth form of steps on the surface under different conditions was summarized and the effect of doping EDTA on the step kinetic and edge energy was discussed. It was deduced that the diffusion mechanism of mass transfer was bulk diffusion.(5) The formation of defects and its impact on the advancement of steps were discussed. It was put forward that if there are hollow cores on the surface, the formation of liquid inclusions is related to the height of steps. This finding has not been seen in relevant report according to the accessible information. |
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