Study On The Nucleation Characteristics,Step Morphology And Its Thermodynamics And Kinetics Of Zinc Tris(Thiourea) Sulphate Crystal

Zinc tris(thiourea) sulphate (Zn[CS(NH2)2]3SO4, ZTS) crystal is an excellentmetaorganic coordination compound of the nonlinear optical crystal, which has highnonlinear optical coefficient, high laser damage threshold, wide range of lighttransmission and low angular sensitivity, eta. Due to its excellent properties, it can bebroadly used in many high technology fields, such as secondary harmonic lasergeneration, electro optics modulated, high-power laser frequency conversion and so on.In addition, ZTS crystal has some other significant advantages, such as stablephysicochemical properties, high mechanical strength, and the low expensive for rawmaterial synthesis, thus, the growth and preparation of ZTS crystal have been paid moreattention.Currently, the emphasis focus on analysising and testing of the performance ofZTS crystal grown under in variety of growth conditions (including kinds of inorganicor organic materials doping and different pH value and so on), while the micro growthprocesses of ZTS crystal was few studied. Although some performances of ZTS crystalcan be improved by doping, there are still many difficulties for growing large-size ZTScrystal with high-quality at a rapid growth speed. Therefore, how to improve the qualityand increase the growth rate of ZTS crystal should arouse attention of researchers. It iswell known that crystal growth process has significant influences on crystal quality andresultantly affect the performance of the crystal. The crystal growth process is indeed aprocess of growing units entering into the lattice position via the growth interface.Therefore, the study of crystal growth interface micromorphology should enableresearchers to achieve more information about the crystal growth process and deepenthe understanding of microscopic phenomena associated with crystal growth andunderstand the microscopic mechanism of the growth process so as to guide researchersto optimize the crystal growth process and ultimately produce high-quality ZTS crystals.The atomic force microscopy (AFM) is just a relatively new and powerful instrumentthat can help people to investigate the microscopic phenomenon of crystal growth. Theoperational principle of AFM is that a sensitive tip scanning over the surfaces to test theatomic force between the tip and the surfaces. For its ability of working in liquids, itprovides us an opportunity to observe in-situ the growth surfaces morphology andunderstand the growth mechanisms at a nanometer scale. In this paper, the nucleation and real-time growth process of ZTS crystal werestudied under different conditions based on the theory of thermal science. The solutionstability was discussed and the surface morphology of ZTS crystal (100) surface beinggrown under different conditions were recorded and analyzed. The main works can besummarized as flows:①The effects of different supersaturations on the nucleation of ZTS solution werestudied. The induction periods with different supersaturations were measured.According to the classical homogeneous nucleation theory, nucleation parameters arecalculated by using the experimental results. By the calculation of the surface entropyfactor, the crystal surface growth model of ZTS in these experiments is confirmed.②Through the investigating on the growth of (100) face of ZTS crystal by in situAFM under different growth conditions, the micro morphologies of the interface wereobtained and the movement of steps were analyzed. The (100) surface exhibits steppedsurface characteristics. The growth step patterns consist of three types, namely theelementary steps, macrosteps and quasi-macrosteps. Steps derived from differentsources have different heights and the step speed inversely proportional to the stepheight. A fan-shaped steps phenomenon which was never reported before was observedby in situ AFM. The growth process of recovering from the “dead zone” and stepbunching phenomenon were recorded. A phenomenon of step advancement showstypical anisotropy was observed. In addition, the stability of the parallel step trains wasanalyzed.③Growth morphology of the (100) face of ZTS crystals, grown under differentsupersaturations at30℃were investigated by in situ AFM. It is found that at lowersupersaturations, nucleation easily occurred at the outcrops of edge dislocations whichcould act as persistent sources of monomolecular growth steps, and the edge dislocationsites appeared to exhibit a “memory function”. At higher supersaturations, however,poly-nucleation becomes the primary growth mechanism. The rates of step advancementin the [010] and [001] directions of the two-dimensional nuclei were different, as itensues from the symmetry anisotropy. The activation energy of the two-dimensionalnuclei along with the step kinetic coefficients in the [010] and [001] directions werecalculated. The result also showed that the position on the step edge where the edge isdiscontinuity is the preferential sites for growth.④Through investigating on the kinetics of step flow of the (100) face of ZTScrystal by using in situ AFM at lower supersaturations at30℃, the horizontal direction growth velocities of the (100) face were estimated. The real step flowing rates of ZTSare also calculated, and we find that the mass transfer is controlled by bulk diffusion inthe growth of steps.⑤The growth morphologies of the (100) face of ZTS crystals under differentconcentrations of L-alanine doped were investigation by using in situ AFM at the samesupersaturations at30℃. The effects of the dopant on step morphology and stepmovement were analyzed. The step velocity and step kinetic coefficients under differentdoping concentrations were calculated. The results of in situ AFM study on ZTS crystal(100) surface with supersaturation at5%with the doping concentration of2mol%showed that preferential nucleation sites sequentially formed which lead to thenucleation appears alternating, and the preferential nucleation sites still able to maintainits activity after it be covered.⑥By optical microscope and atomic force microscopy，we found that there aremany kinds of defects in ZTS crystals. The main defects of ZTS crystals are growthdislocations, hollow cores, inclusions, cracks and tapering. The real-time process of theformation of pits and inclusions were recorded and the formation mechanisms werediscussed. According to the distinct formation mechanisms results, some methods wereproposed to decrease these defects.