CRYSTAL GROWTH & DESIGN
Twin-free and High-Quality DAST Crystals - Effected through Solutions of Lower Supersaturation Coupled with Isothermal Solvent Evaporation S. Brahadeeswaran,*,† S. Onduka,† M. Takagi,† Y. Takahashi,† H. Adachi,† T. Kamimura,§ M. Yoshimura,† Y. Mori,† K. Yoshida,§ and T. Sasaki†
2006 VOL. 6, NO. 11 2463-2468
Department of Electrical, Electronic and Information Engineering, Osaka UniVersity, 2-1 Yamada-oka, Suita 565-0871, Osaka, Japan, and Department of Electronics, Information and Communication Engineering, Osaka Institute of Technology, 5-16-1 Ohmiya, Asahi-ku, Osaka, 535-8585, Japan ReceiVed October 28, 2005; ReVised Manuscript ReceiVed August 25, 2006
ABSTRACT: Growth of twin-free and high-quality 4-N,N-dimethylamino-4′-N′-methyl stilbazolium tosylate (DAST) crystals, with a surface roughness in the range of 1.7-3.8 nm (99.8%) was used as a solvent. The equilibrium concentration corresponding to the temperature of 43 °C is found to be 35 g/L (Figure 1). The initial cooling rate was 0.5 °C/day until nucleation of few crystals and was later reduced to 0.2 °C/day for the growth. The detailed report on growth by SNM can be found elsewhere.9 To explore the possibilities to avoid twinning in DAST crystals, we made certain modifications in the growth conditions. At first, we prepared the DAST solutions with a concentration of 35 g/L (same as that of SNM) in Teflon vessels with a Teflon lid and screw cap. These vessels were then housed in the water bath of the growth chamber whose temperature is controlled to the accuracy of (0.01 °C. The temperature of the solution was increased to 60 °C and kept at this temperature for about 6 h. The solution was stirred continuously during the heating and dwelling period. Subsequently, the solution was cooled to 41 °C with a cooling rate of 3 °C/h, and the stirring was halted during cooling. After the solution was kept for about 1 day, the Teflon lids and screw caps were replaced with holed ones whose diameters were in the range of 1-3 mm. This provision allowed different levels of evaporation through various holes at a fixed temperature. Although Zhang et al.2 have already reported on DAST growth by the evaporation method, the details about the experimental conditions are not available. It has been shown that crystal plates with varying thickness in the range of 0.2-1.7 mm were obtained by this method. In our experiments, we performed growth at two different equilibrium concentrations. We had initially fixed the growth temperature at 41 °C (for the solution concentration of 35 g/L) and later the DAST solutions were prepared with lower concentrations in the range of 14-16 g/L and the corresponding growth temperature also was fixed at 20 °C, the room temperature. For the latter case, saturation temperatures were in the range of 24-26 °C, respectively. The Teflon flask used for the growth of DAST crystals by the evaporation method is schematically shown in the picture (Figure 2c) and the flask used for SNM (Figure 2a,b) also is given for comparison. The as-grown crystals obtained by the evaporation method at 20 °C only were characterized for their optical and surface properties (for the reason stated in section 3), and the results were compared with those obtained by SNM. They were subjected to optical studies such as transmission and EO sensitivity measurements and surface studies such as X-ray rocking curve (XRC) analysis and Mirau interferometry (MI) to evaluate their bulk and surface qualities, respectively. The optical transmission spectra were recorded using a Shimadzu UV 3100 spectrophotometer. Samples with no visible inclusions and nearly equal thickness (about 0.5 mm) were chosen for this purpose. To measure the EO sensitivity of the crystals, we used indigenously developed experimental setup.10 Since the largest tensor of EO coefficient in DAST crystals is r11, the EO effect is expected to be largest along the dielectric principal x-axis, which is almost parallel to the crystallographic a-axis. The DAST crystals with a-b plane as a large face can be used in transverse electric-field probing. In addition, a commercial KTP crystal
Figure 2. Schematic of Teflon growth flasks used for SNM (a, b) by solution cooling and (c) for solvent evaporation. with dimensions of 3.0 × 3.0 × 0.56 mm3 (polished a-c plane) was used as a standard crystal for comparison. The DAST (or KTP) crystal was placed on the electrode with a space of 0.5 mm, so that the electric field was applied along the a-axis for the DAST crystal. The electric field applied to an EO crystal induces a birefringence effect. The polarization of a laser passing through the crystal is changed according to the intensity of an electric field. Since the EO crystal is placed between crossed polarizers, the intensity of the probe beam changes as a function of electric field or voltage. In the case of a DAST crystal, the phase shift (δ) representing the change in polarization is given by
π VL δ ) (r11nx3 - r21ny3) λ d
(1)
where λ is the wavelength of the probe beam (Nd: YAG laser, λ ) 1064 nm), V is an applied a sinusoidal voltage [200 V (peak to peak)] with the signal frequency of 20 kHz, and L is the thickness of the crystal. Under these experimental conditions, the EO figure of merit (n13r11 n23r21) is calculated to be 457 pm/V. The electric field was applied throughout the crystal because the distance between the electrodes was as wide as 0.5 mm. EO sensitivity is expected to be proportional to optical length L (i.e., thickness of the crystal) in this experiment from eq 1, provided the crystallinity is uniform. DAST crystals with varying thickness were used for this purpose. The XRC was measured using a Rigaku Double Crystal Monochromatic system. We employed the two beam MI method to measure the surface roughness of the DAST crystals. The Zygo NewView optical profiling system (model 200 3D) was used for this purpose. This instrument had a lateral resolution of 0.25 µm and vertical resolution of 0.1 nm. The as-grown DAST crystals were selected to study their surface roughness.
3. Results and Discussion 3.1. Crystal Growth. The DAST crystals grown by SNM by slow cooling in the range 43-39 °C were both twinned and twin-free with typical dimensions in the range of 3-7 × 3-8 × 0.4-1 mm3 along the a, b, and c axes, respectively. Sufficient care was necessary while separating twin-free crystals since twinned crystals also look similar, if observed by the naked eye. However, the twinned DAST crystals are easily distinguishable by a polarizing microscope, as they appear as a pair of DAST crystals (with well-developed 001 faces) that share a common plane along the c-axis, with a small shift (Figure 3b). In addition, the yield of twin-free crystals was generally low (about 30%), and they exhibited typical growth morphology as shown in the figure (Figure 3a). From the figure it can be seen
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Figure 4. Twin-free (a) (dimension 1.5 × 2.5 × 0.2 mm3) and twinned (b) (dimension 4.0 × 4.0 × 0.330 mm3) crystals of DAST obtained from SNM-SE at 41 °C.
Figure 3. Twin-free (a) (dimension 6.0 × 7.0 × 0.7 mm3) and twinned (b) (dimension 3.5 × 4.0 × 0.6 mm3) DAST crystals grown from SNM and their respective cross-sectional view.
that the 001 face is well developed, whereas the 001h face has a tendency to grow out, suggesting that the growth rate is relatively faster along the 001h direction. Consequently, this necessitates tedious surface polishing to obtain parallel (001) and (001h) faces, so that measurements such as EO coefficients and terahertz generation can be made more reliably. It is also possible to obtain as-grown DAST crystals occasionally with well-developed (001) and (001h) faces by SNM. The nature of twinning in the DAST crystal could be classified as a growth twin, which is generally expected to originate during nucleation, as explained in the following discussion. In a classic article by Buerger on the genesis of twins in crystals,11 it was stated that the growth twins are driven mainly by supersaturation of the solution, which leads to an “accident” in the growing layers (i.e., the formation of growth twins) during nucleation. If supersaturation of the solution is higher, then the rate at which molecules arrive at the surface is greater and they see less time to orient themselves in relation to molecules already there. This leads to random accretion of molecules and eventually to twinning. This shows that higher supersaturation could be detrimental for the growth of twin-free crystals and an efficient control over solution supersaturation and low growth rate are essential for their growth and stresses a need for suitable modifications in the growth method. Similar behavior has been observed for the case of saccharine crystals when they are grown in ethanol solvent.12 When the growth was performed using the isothermal evaporation method at 41 °C the DAST crystals started nucleating after 30 h after replacing the caps with holes. The growth by this method took place for nearly 15-20 days. During the growth process, we observed crystals on both Teflon slope and on the bottom of the Teflon vessel. At this point, we classify the SNM utilizing the solution cooling as SNM-SC and that utilizing isothermal solvent evaporation as SNM-SE for convenience. The crystals grown by SNM-SE were separated intact using a high density and immiscible liquid. There were many small crystals with typical dimensions in the range of 1-2 × 1.7-3.0 × 0.15-0.2 mm3. However, for the case of SNM-SE, when these crystals were carefully examined under a polarized optical microscope it was found that nearly 60% of the grown crystals were twins (Figure 4b). Moreover, the twin-free crystals that were studied under the microscope showed that they had developed nearly parallel 001 and 001h faces in contrast to SNMSC. A typical crystal grown by this method is shown in Figure 4a. Although this approach led us to grow a relatively higher yield (about 60%) of twin-free crystals with a well-developed 001h face as compared to SNM-SC, their size did not improve
considerably. It is possible that the higher growth temperature could have triggered faster evaporation of methanol solvent (which was about 3.8 g/day) and thus resulted in the multinucleation and growth of many tiny crystals. This observation prompted us to reduce the equilibrium concentration so that the crystal growth can take place at lower temperatures. When the solutions prepared at lower concentrations (between 14 and 16 g/L) were observed periodically, it was found that the onset of nucleation was greatly dependent on the equilibrium concentration of the solution. The solution prepared with 16 g/L concentration nucleated after 2 days after it was kept at 20 °C, whereas those prepared with 15 and 14 g/L concentrations nucleated after 11 and 25 days, respectively. From the studies on metastable zone width of DAST solutions during cooling, it is understood that the zone width was relatively wider at lower concentrations (at temperatures below 27 °C) as compared to that at higher solution concentrations.13 This observation appears to be true for the case of the isothermal evaporation method also. It shows that the DAST solution is sensitive to even small changes in its concentration, especially at lower temperatures. We chose the concentration of 16 g/L for our growth purposes since the nucleation is relatively faster. Like SNM-SE at 41 °C, the nucleation was observed on the Teflon slope as well as on the bottom of the Teflon vessel. However, the number of nucleated crystals was considerably lower as compared to that obtained at 41 °C. The growth was terminated after 30-40 days, and the evaporation rate of methanol solvent was calculated to be about 1.8 g/day. It is important to mention here that the both the crystals growth and harvesting temperatures are same (i.e., 20 °C). This experimental condition is favorable since it avoids the nucleation of numerous tiny crystals, which are commonly noticed for SNM-SC and SNM-SE at 41 °C and are caused by the crystallization of residual DAST solution after the rapid evaporation of the solvent, on the DAST crystal extracted, due to differences in growth and harvesting temperatures. When the DAST crystals grown by SNM-SE at 20 °C were observed under the microscope, it was found that most of them were, surprisingly, twin-free (nearly 95% of yield and we seldom noticed twinned crystals), and they also exhibited well-developed 001h faces (Figure 5a-d). This development showed that the growth rate along 001h and was nearly the same as that along the 001 direction and resulting in the growth of nearly parallel 001 and 001h faces (Figure 5e). The growth of crystals at this lower equilibrium concentration appeared to facilitate the accretion of DAST molecules in an ordered manner, thus ensuring the yield of twin-free crystals. The typical dimensions of the crystals grown by this modified method were in the range of 3-7 × 3-7 × 0.2-0.8 mm3 and the growth rates were about 0.050.20 mm/day. The sizes of the as-grown crystals are, as mentioned in the introductory section, expected to be sufficient for the purposes for which they are intended. The morphology of the crystals grown by the spontaneous nucleation method at
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Figure 7. Electrooptic (EO) sensitivities of SNM-SC and SNM-SE grown DAST crystals. The EO sensitivity of potassium titanyl phosphate (KTP) crystal of 0.5 mm thickness also is given for comparison.
Figure 5. Twin-free DAST crystals (a-d) (with dimensions 3.0 × 4.5 × 0.40, 3.5 × 3.5 × 0.32, 3.0 × 3.0 × 0.29, and 3.0 × 3.5 × 0.3 mm3, respectively) obtained from SNM-SE at 20 °C, their crosssectional view (e), and the typical growth morphology of the DAST crystal grown by seeded method (f).5
Figure 6. Transmission spectra recorded for SNM-SC and SNM-SE at 20 °C grown DAST crystals.
relatively lower concentration were similar to those obtained by the seeded method (Figure 5f5). Thus, the crystal growth by SNM-SE at lower equilibrium concentration has been proven to be suitable for the growth of twin-free and nearly flat DAST crystals. Subsequently, we characterized the crystals grown by SNM-SE at 20 °C and compare the results with the previously available data on SNM-SC grown crystals. 3.2. Optical Transmission. The results of optical transmission spectra are shown in the Figure 6. From the figure, it can be seen that the overall percentage of transmission has increased for SNM-SE crystal grown at 20 °C as compared to SNM-SC. This result shows that the crystal grown by SNM-SE at relatively lower concentration and temperature are more transparent, which might be due to the reduction of solvent inclusion, and hence less scattering centers, in the crystal. It has already been shown that, for the case of organic-ionic crystals such as sodium p-nitrophenolate dihydrate (NPNa dihydrate)14,15 and L-histedine tetrafluoroborate (L-HFB),16 modifications in growth conditions
Figure 8. The X-ray rocking curve (XRC) for DAST obtained from SNM-SE at 20 °C.
have considerable influence on the optical quality of the grown crystals. While the NPNa dihydrate grown by solution cooling method in the range between 35 and 29 °C yielded highly transparent crystals as compared to those grown by solvent evaporation performed at 45 °C, the L-HFB crystals grown by the solvent evaporation method were better than those obtained by the solution cooling method. Hence, it is obvious that optimization of growth conditions specific to the crystal being studied is necessary for obtaining high-quality crystals and the method of achieving supersaturation is vital to achieve the purpose. 3.3. EO Sensitivity. The EO sensitivity of the twin-free DAST crystal obtained by SNM-SC has been reported to be about 30 times higher than that of KTP, a well-studied inorganic NLO material, and the EO sensitivity measured for the case of SNM-SC grown DAST crystals exhibited linear dependence with their thickness.10 The higher the EO sensitivity, the better will be the ability to detect the low electric field signals and the better will be the response while testing IC circuits.7 The EO sensitivity for SNM-SE grown crystals was also measured to compare with those grown by SNM-SC. The results are shown in the Figure 7. From the figure, it can be seen that the sensitivity of the SNM-SE grown crystals are relatively higher as compared to those obtained from SNM-SC, and they also exhibit a linear trend with respect to their thickness. The higher EO sensitivity of SNM-SE grown crystals also could be attributed to a higher bulk quality of the grown DAST crystals. It was also observed that the EO sensitivity measured at various points throughout the sample had nearly identical values, thus exhibiting the uniform crystallinity in the bulk of the grown crystal. 3.4. Surface Mosaicity. The ability to withstand the highpower THz waves for longer duration depends on the surface quality of the crystals also. The XRC measurements analyze the crystal mosaicity, which is expected to yield information on the angular dispersion of the crystal blocks characterizing
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Figure 9. The surface roughness measured for DAST grown by SNM-SE at 20 °C.
the crystal, and in turn to reflect the intrinsic crystal order.17 The full width at half maximum (fwhm) of the rocking curves measured for the DAST crystals were in the range of 21-40 arc-sec. Figure 8 shows the typical XRC curve of an SNM-SE grown DAST crystal with a fwhm of 23.4 arc-sec. These values are comparable with those obtained from SNM-SC.6 We also examined the fwhm of both 001 and 001h faces of the DAST crystals to compare their surface qualities. During this investigation, we found that the fwhm of both the faces were nearly comparable and that the 001h face was indeed relatively better than that of 001 face. The improvement of the 001h face and its surface quality in the SNM-SE grown DAST is significant considering fact that this face has the tendency to grow out in SNM-SC grown crystals. 3.5. Surface Roughness Measurements. The surface roughness of the as-grown DAST crystals was measured to obtain information on the finest level of irregularities on their surfaces and on their suitability for surface coating. It has already been shown that the polished surfaces of DAST crystals possessed the surface roughness of
