Crystal Growth, Properties And Surface Morphologies Of LATF Crystals

Nonlinear optics (NLO) crystal materials has become forefront of current research in view of its vital applications in areas such as high-speed optical communication, optical signal processing, optical modulation, optical switching, frequency shifting and high-density data storage. Organic NLO crystals have already become a hotspot in the field of nonlinear optical crystal material research, as they have large nonlinear coeffieicents, inherent ultrafast response times, high optical thresholds for laser power and easy processability as compared with inorganic crystals.L-arginine phosphate monohydrate (LAP) is discovered by Shandong University as a very promising UV organic NLO crystal material and has been given more emphasis by scientists. Due to the incomparable high laser damage threshold and more than 90% conversion efficiency, LAP crystal behaves important value in developing intense laser frequency shifting and phase conjugating mirrors and attracts more and more worldwide researchers's attention to develop rapidly and widely.Stimulated by the discovery of LAP crystal, our group has synthesized L-arginine trifluoroacetate (LATF) crystal as another NLO crystal material exhibiting high damage threshold for the fisrt time. LATF is synthesized by dissolving equimolar quantities of L-arginine and trifluoroacetate acid in deionized water. Primary experimental results indicate that LATF belongs to the monoclinic system, P2₁ space group, powder SHG efficiencies is 2.5 times higher than that of KDP, ultraviolet transparency cutoff is 232 nm, its specific heat is 2 times larger than that of LAP, and the optical damage threshold of LATF at low repeat frequency strong laser is higher than that of LAP and DLAP. Therefore, LATF crystal is a new valuable and potential NLO crystal material. In this dissertation, the bulk crystal growth, structure, defects, surface morphologies, mechanisms, characterization and appraisal properties of LATF crystal are systemically investigated. The outline of this dissertation is as follows:1. Investigation of bulk crystal growth of LATF single crystals The growth velocity of LATF crystal oriented various directions has large anisotropy, c-axis is much slower than that of b-axis and a-axis, which bring difficulty to provide crystal samples for experimental measurement. Through adjustment of the solution composing, the rule of prevention sheet growth of crystals has been discovered, and combined with a continuous optimized seed-crystal method, the key technical question of obtaining excellent bulk LATF crystal has been resolved. In addition, through the investigations of nucleation kinetics, the metastable zonewidth and induction period are determined, the nucleation parameters such as interfacial energy of crystal-solution, critical radius and critical free energy barrier are all calculated. Buck LATF crystals with maximum size up to 50×27×7 mm³ and high optical quality have been grown by using the temperature lowering method. The crystal morphology was also analyzed, the optimal growth direction is oriented along b axis and the major flat facets are indexed as (101), (100) and (001).2. Investigations of crystal structure and properties of LATF crystalsThe structure of LATF crystal is determined and characterized by X-ray diffraction, Fourier transform infrared (FT-IR), Fourier transform Raman and Fourier transform nuclear magnetic resonance (FT-NMR) techniques. The introduction of CF₃COOH optimizes the orientation of L-Arginine, namely L-Arginine and CF₃COOH arrange in order along c-axis. The optically active L-Arginine with the asymmetrical guanidyl and carboxyl groups to combine with the alkyl radical possessing F₃C tetrahedra of CF₃COOH and forms the high nonlinearity of the crystal. The NMR studies found that there is hydrogen-bonding and static interactions exist between L-Arginine and CF₃COOH, and may result in the conformational change of L-Arginine.Through the investigations of optical, thermal and electric properties, LATF crystal has been proved to be an excellent new NLO crystal material.The SHG efficiency of LATF crystal powder has been estimated as 2.5 times higher than that of KDP; the transparent region is in optical range of 232-2000 nm, with 232 nm being the UV cutoff wavelength, besides negative solvatochromism, i.e., hypsochromic (blue) shift occurs with increasing solvent polarity which might indicate a reduction in dipole moment on excitation; the principal refractive indices of crystal were measured by classical V-prism method and the dispersion curves obtained from the fit in terms of the Sellmeier analytical equation were plotted; the phase-matching curves of crystal were simulated and calculated theoretically and the equation for the effective nonlinear coefficients were presented; the laser damage threshold is 2 GW/cm² for laser wavelength 1053 nm, pulse width 20 ns and 1064, 19, 3.5 GW/cm² for laser wavelength 1064 nm, pulse width 20 ps, 1 ns, 10 ns, the essential and external factors that affect the laser damage threshold were both studied. For low repeat frequency strong laser, the electric field effect plays a great role in damage process and the laser damage threshold of crystal is correlated with its thermal (specific heat and thermal expansion coefficient) and mechanical properties.LATF crystal is thermally stable up to 217℃, which is substantially higher than that of LAP (140℃); the specific heat of LATF is larger than that of LAP and the thermal expansion coefficients of LATF are less anisotropic than that of LAP, Therefore, LATF has a higher laser damage potential than LAP; According to the method obtaining the values of principal expansion coefficients, projection of thermal expansion quadric along the b-axis is presented.The dielectric constant and dielectric loss were recorded and analyzed both as function of frequency and temperature. The dielectric constant is relatively high in the lower frequency region and decreases with increasing frequency and becomes almost saturated beyond 100 kHz. This may be due to the interfacial polarization, in which the mobile charge carriers are interdicted by a physical barrier which restrains generating a localized polarization of the material. The variation of dielectric constant with temperature is small, which infers that the crystals are of good chemical homogeneity. The higher values of dielectric loss at low frequencies originates from space-charge polarization mechanism of molecular dipoles, and the characteristic of low dielectric loss at high frequencies clarifies that the grown samples possess enhanced optical quality with lesser defects. 3. Investigations of surface morphologies and growth mechanisms of LATF crystalsThe surface morphologies and growth mechanisms of LATF crystals grown from various conditions were investigated using by atomic force microscopy (AFM). Crystals grow mainly by layer mode including dislocation-controlled mechanism and two-dimensional (2D) nucleation growth which dominates during the crystal growth.(1) The growth step patterns consist of three types, namely the elementary steps, macrosteps and quasi-macrosteps. The step height of elementary steps is about 0.64 nm, matching the interplanar distance of d₁₀₁ and the macrosteps's height is multiple as that of elementary steps. The step bunching is always introduced by foreign impurities or particles. However, the impurities have a selective affect on the step movement. The straight step trains will show ultimately if the growth component is ample and the influence by the impurities is not evident. Quasi-macrosteps are practically formed by highly dense steps but remain the properties of elementary steps and its formation can be explained by Chernov's kinematic waves of steps theory. Steps with low density usually advance faster than steps with narrow separations, the elementary steps behind gradually overtake and pile up at the step bunches and finally the highly dense steps come into being.The ubiquity of straight steps oriented along b axis is one of the primary growth characteristics. Its formation is correlated with the microcomponent of crystal and their arrangement pattern. Since large polarized field and binding energy exist along the b axis, crystal growth oriented this direction advances rapidly. Once a critical nucleus forms, it will engender the formation of b-oriented straight steps inevitably, and then the step trains spread along the tangent direction and overspread the whole crystal facet.(2) Most of the spiral dislocation growth hillocks are elliptical, rectangular shaped hillocks which indicate the anisotropy of step advancement are seldom seen only when the driving force surroundingthe the spiral dislocation differs slightly. The b-axis is the preferential growth direction, which contributes to the formation of rectangular shaped hillocks, crystal growth is confined not only by growth conditions but also by crystal structure. The steps generated by spiral dislocation have dissimilar heights and spacing, however, under few growth conditions, the Berg's effect which indicates the stability of step movement can also be discovered.(3) The sequence of nucleation, growth, coalescence of islands and expansion of a "stack" has been observed. A new birth and spread (terraced nuclear) model is put forward to explain this new phenomenon. Previously, this new birth and spread phenomenon was not found during LAP studies, a majority of the hillocks were primarily generated by spiral dislocation.In the multiple nuclei growth, the 2D nuclei can gradually merge into a uniform and perfect growth layer when they meet with one another. No defects exist at the junction of the 2D nuclei. The height in the center of the 2D islands is lower than that of far from the center and maximal height difference is 50 nm, which shows that these 2D islands extend inward to the center and expand outward simultaneously. Thus, impurities were easily absorbed and deposited at the bottom of the hillocks and defects will appear ultimately.The correlation between step speed and terrace width can be explained by surface-diffusion-controlled growth coupled with the up-step diffusion bias model. The growth rate of a step with a wider terrace is faster than that of a step having a narrow terrace and the steps with larger terraces are the preferential growth sites. At the same time, the emergence of the 2D islands provides step sources for further growth and growth of the wide steps is expedited when several 2D nuclei grow on top of them. Thus, the segments proceed to grow faster than other segments, and the overhang morphology of steps appears in the end.(4) The morphologies of 2D nuclei depend both upon supersaturation and upon temperature. More specifically, at the low temperature, 2D nuclei often have circular and elliptical shape. With the increase of temperature and decrease of supersaturation, the islands change from circular to elliptical shape, and further become sector-shaped.4. Investigations of LATF crystal defectsMake use of opton optical microscope and atomic force microscopy, combined with chemical etching method, the results of the observations and discussions of growth defects and its formation mechanisms were presented in the dissertation. The main defects of LATF crystals are growth dislocations, hollow cores, inclusions, cracks, tapering. Growth dislocation defects are mainly influenced by disturbance of growth conditions and distribution of impurities; Hollow cavities result from the adsorbed impurities that block up crystal gowth; the formation of liquid inclusions is correlated with the macrosteps, once the growth surface loses its stability, mother solution will be trapped resulting in liquid inclusions; cracks in crystals are related with the disturbance of temperature, large diversity of supersaturation, anisotropic thermal expansion properties and the presence of inclusions; impurity particles, the pH value, supersaturation and growth temperature of the solution may cause tapering defects. In order to avoid or decrease these defects, corresponding measurements such as a newly disposing high temperature solutions method were taken according to the distinct formation mechanisms results.