Synthesis, Growth, and Characterization of Novel Nonlinear Optical Active Dichloridodiglycine Zinc Dihydrate Single Crystals† S. Mary Navis Priya,⊥ B. Varghese,‡ J. Mary Linet,⊥ G. Bhagavannarayana,§ C. Justin Raj,⊥ S. Krishnan,⊥ S. Dinakaran,⊥ and S. Jerome Das*,⊥
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 5 1663–1667
Department of Physics, Loyola College, Chennai-600 034, India, SAIF, Indian Institute of Technology, Chennai-600 036, India, and Materials Characterization DiVision, National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi- 110 012, India ReceiVed NoVember 25, 2007; ReVised Manuscript ReceiVed January 30, 2008
ABSTRACT: The nonlinear optical photonic crystal of dichloridodiglycine zinc dihydrate was grown from a mixture of glycine and zinc chloride by a slow evaporation technique. The morphology and the structure of the grown crystal were characterized using single crystal X-ray diffraction analysis. The crystalline perfection of the grown crystal was analyzed using highresolution X-ray diffractometry rocking curve analysis. Fourier transform infrared spectral analyses confirmed the presence of the functional groups in the crystal lattice. The second harmonic efficiency of the crystal was tested by the Kurtz powder technique. The optical property and thermal stability of the grown crystal were studied using UV absorption spectral and thermogravimetric/differential thermal analysis. Microhardness studies revealed that the hardness of the grown crystal increases with an increase in load. The presence of the elemental composition such as zinc and chlorine present in the title compound was confirmed by energy dispersive X-ray analysis. Introduction Semiorganic nonlinear optical (NLO) materials are reputed candidates for device fabrication technology, owing to their large nonlinear coefficient, high laser damage threshold, and exceptional mechanical and thermal stability. Semiorganic materials are metal-organic coordination compounds in which the organic ligand plays a dominant role in the NLO effect. As for the metallic part, the focus is on group IIB metals (Zn, Cd, and Hg) as these compounds usually have a high transparency in the UV region, because of their closed shell.1,2 In the organometallic complexes, the metal center is engaged in π-bonding with the organic ligating groups and can be involved in strongly allowed metal to ligand charge transfer (MLCT), which leads to excellent second harmonic generation (SHG) devices. Efficient SHG has been mostly limited to noncentrosymmetric crystals. However, SHG in some centrosymmetric crystals would be allowed because of the breaking of inversion symmetry at the surface of the particle. Hence, second harmonic light may be reflected from the interface of the two centrosymmetric media.3,4 Recently, several complexes of glycine were reported5–10 of which glycine sodium nitrate11 and glycine lithium sulfate12 have promising NLO properties. In the present work, high quality single crystals of dichloridodiglycine zinc dihydrate have been grown by a slow evaporation technique. The structure of organo metallic dichloridodiglycine zinc dihydrate has been reported by us,13 and it was found to possess SHG, which was confirmed by the Kurtz powder technique. Further, the grown crystals were characterized by single crystal X-ray diffraction analysis, high-resolution X-ray diffractometry (HRXRD), FTIR
* Corresponding author. E-mail:
[email protected]; jerome@ loyolacollege.edu. Tel: 009144 2817 5662. Fax: 009144 2817 5566. † PACS: 81.10.Dn; 61.10.Nz; 81.70 P; 78.30.Aj. ⊥ Loyola College. ‡ Indian Institute of Technology. § National Physical Laboratory.
Figure 1. Photograph of the title crystal.
analysis, thermal analysis, optical absorption, energy dispersive X-ray (EDX) analysis, NLO studies, and microhardness tests. Experimental Procedures A supersaturated solution of glycine and zinc chloride prepared in equimolar proportions was stirred continuously using a magnetic stirrer for 3 days. The prepared solution was filtered and kept undisturbed at room temperature. Initially some crystals of irregular polymorphs were harvested and identified as dichloro-bis glycine-O)-zinc glycine, and after a few days seed crystals of dichloridodiglycine zinc dihydrate with an excellent morphology were observed, and it was found to be a centrosymmetric system. The chemical reaction may be represented as
2C2H5NO2 + ZnCl2 + 2H2O f [ZnCl2(C2H5NO2)2]·2H2O (1) The identity of grown crystal was confirmed from the lattice parameters and the crystalline perfection being studied from diffracting rocking curve. The crystal grown to a dimension of 30 × 10 × 10 mm3 obtained over a period of 45 days is shown Figure 1.
Characterization Morphology. The morphology of the grown crystal was identified using an Enraf Nonius FR 590 single crystal XRD diffractometer. The major flat face along a-axis is indexed as
10.1021/cg701162j CCC: $40.75 2008 American Chemical Society Published on Web 04/17/2008
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Figure 4. Packing diagram of the title crystal. Figure 2. Morphology of the grown crystal of dichloridodiglycine zinc dihydrate.
Figure 5. Diffraction curve recorded for dichloridodiglycinc zinc dihydrate single crystal for (010) diffracting planes by employing multicrystal X-ray diffractometer with Mo KR1 radiation.
Figure 3. ORTEP diagram of the crystal.
(100) plane and other planes are indexed as (010), (1j02), (1j11), (01j0), (11j1j), and (102j). The growth of the crystal is found to be along the (11j1j) (102j) plane, which is considered as the growth plane, and the morphology of the crystal is shown in Figure 2. Single Crystal X-ray Diffraction Analysis. The X-ray diffracting data were collected using an automatic diffractometer (MESSRS, Enraf Nonius, The Netherlands). The structure was solved by the direct method using the SHELXL program. From the analysis, the cell parameters were found to be a ) 14.4167 Å, b ) 6.9068 Å, c ) 12.9531 Å, β ) 117.94, space group C2/c, and point group 2/m. The ORTEP diagram and the packing diagram of the grown crystal are shown in Figures 3 and 4, respectively. High-Resolution X-ray Diffractometry Study on Dichloridodiglycine Zinc Dihydrate. To reveal the crystalline perfection of the grown crystals, a multicrystal X-ray diffractometer (MCD) developed at NPL14 was used to record high-resolution diffraction curves (DCs). In this system, a fine focus (0.4 × 8 mm; 2 kW Mo) X-ray source energized by a well-stabilized Philips X-ray generator (PW 1743) was employed. The wellcollimated and monochromated Mo KR1 beam obtained from the three monochromator Si crystals set in dispersive (+,-,-)
configuration has been used as the exploring X-ray beam. This arrangement improves the spectral resolution (∆λ/λ , 10-5) of the Mo KR1 beam. The divergence of the exploring beam in the horizontal plane (plane of diffraction) was estimated to be , 3 arc sec. The specimen crystal is aligned in the (+,-,-,+) configuration. Because of the dispersive configuration, though the lattice constant of the monochromator crystal(s) and the specimen are different, the unwanted dispersion broadening in the diffraction curve of the specimen crystal is insignificant. The specimen can be rotated about a vertical axis, which is perpendicular to the plane of diffraction, with a minimum angular interval of 0.5 arc sec. The diffracted intensity is measured by using a scintillation counter and is mounted with its axis along a radial arm of the turntable. The rocking or diffraction curves were recorded by changing the glancing angle (angle between the incident X-ray beam and the surface of the specimen) around the Bragg diffraction peak position θB starting from a suitable arbitrary glancing angle (denoted as zero). The detector was kept at the same angular position 2θB with wide opening for its slit, the so-called l scan. Before recording the diffraction curve to remove the noncrystallized solute atoms remaining on the surface of the crystal and to ensure the surface planarity, the specimen was first lapped and chemically etched with a mixture of water and acetone in a volume ratio 1:2. This process also ensures removing surface layers, on the surface of the crystals due to organic additives.15 Figure 5 shows the highresolution diffraction curve (DC) recorded for the dichlorido-
Dichloridodiglycinezinc Dihydrate Single Crystals
Figure 6. FTIR spectrum of dichloridodiglycine zinc dihydrate crystal.
diglycine zinc dihydrate specimen using (010) diffracting planes in symmetrical Bragg geometry by employing the multicrystal X-ray diffractometer described above with Mo KR1 radiation. As seen in the figure, the curve does not contain a single diffraction peak. The solid line, which follows well with the experimental points (filled circles), is the convoluted curve of two peaks using the Lorentzian fit. The additional peak depicts an internal structural low angle (tilt angle >1 arc min but less than an arc degree) boundary16whose tilt angle (misorientation angle between the two crystalline regions on both sides of the structural grain boundary) is 7.7 arc min from its adjoining region. The full width at half maximum (fwhm) of the main peak and the very low angle boundary are, respectively, 2.6 and 3.2 arc min. Though the specimen contains a low angle boundary, the relatively low angular spread of around 14 arc min of the diffraction curve and the low fwhm values show that the crystalline perfection is reasonably good. The segregation of impurities or the entrapment of solvent molecules at the boundaries during the growth process could be responsible for the observed low angle boundary. It may be mentioned here that such a low angle boundary could be detected with the well-resolved peak in the diffraction curve only because of the high-resolution of the multicrystal X-ray diffractometer used in the present studies. The influence of such minute defects on the optical properties is very insignificant. However, a quantitative analysis of such unavoidable defects is of great importance, particularly in the case of phase matching applications. FTIR Analysis. The FTIR analysis is carried out in the region 400–4000 cm-1 and is shown in Figure 6. The peak at 3450 cm-1 is due to the OH stretch of water. The corresponding OH2 bonding vibration is seen at 1634 cm-1. The presence of H2O is also confirmed in our structure reports.13 The peaks at 3199 cm-1, 2734 cm-1, 2645 cm-1, 2433 cm-1 are due to N-H vibration. All the vibrations are due to hydrogen bending of N-H grouping. The CH2 vibration occurs at 2955 cm-1 and the bending modes occurs at 1445 cm-1, whereas the asymmetric and symmetric NH3+ vibration occurs at 1634 and 1522 cm-1. The asymmetric and symmetric vibration of COO– occurs at 1630 and 1396 cm-1. The COO- and OCC vibrations lies at 1342 cm-1 and on comparison of the spectra with glycine illustrates the shift in peak position as well as the change in the intensity of the peak below 1140 cm-1, supporting that the
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Figure 7. UV absorption spectrum of the grown crystal.
Figure 8. Thermogram of dichloridodiglycine zinc dihydrate crystal.
spectrum is different from that of pure glycine, and hence glycine interacts with ZnCl2 which is also evident from the XRD analysis. UV Absorption Spectral Analysis. The UV absorption spectrum of dichloridodiglycine zinc dihydrate was obtained by using a Varian Cary 5E spectrophotometer, and the resultant spectrum is shown in Figure 7. The absorbance is found to be very low in the entire visible and near IR region which is one of the most desired properties for the materials possessing SHG. From the obtained spectrum, the UV cutoff wavelength is found to be at 220 nm. Thermal Analysis. The thermogravimetric and differential thermal analysis shown in Figure 8 was carried out using NETZSCH STA 409C/CD for a sample weight of 15.450 mg in the temperature range 20–1000 °C at a heating rate of 10 °C in nitrogen atmosphere. There is a minute weight loss starting at 95.8 °C, due to a loss of physically adsorbed water on the crystal surface. This is followed by another sharp weight loss starting close to 110.3 °C, which is assigned to a loss of water of crystallization. The presence of water in the unit cell is also revealed in the XRD analysis, as discussed above. There is major weight loss starting close to 205 °C, due to decomposition. The DTA trace illustrates three endotherms at 110, 125.6, and 202.5 °C. The second and third endotherm coincides with a weight loss shown in Figure 8. But the first endothermic peak at 110 °C does not correspond to any weight loss in TGA. Hence, it is assigned to melting. The melting point of the crystal was also determined separately by the capillary method, and it was found to be 103 ( 1 °C; hence, this endotherm is assigned to the
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Figure 10. Variation of hardness versus P. Figure 9. EDX spectrum of dichloridodiglycine zinc dihydrate single crystal.
melting point of the crystal. On the basis of the above result of TGA and DTA, it is concluded that the crystal retains its texture up to 110 °C. EDX Studies. The elemental percentage of the zinc and chlorine was detected by using energy dispersive X-ray (EDX) analysis [ISIS LINK, Oxford Instrument, U.K]. Figure 9 illustrates the EDX spectra of dichloridodiglycine zinc dihydrate, which reveals the elemental percentage of zinc and chlorine, and the remaining elements are not identified due to their low atomic number. NLO Studies. In order to confirm the NLO property, the grown specimen was subjected to a Kurtz powder test using a Q-switched, mode locked Nd:YAG laser of 1064 nm and a pulse width of 8 ns (spot radius of 1 mm) on the powder sample of dichloridodiglycine zinc dihydrate.17 The input laser beam was directed on the as-grown crystal powder to get maximum powder SHG. The emitted light passed through an IR filter was measured by means of a photomultiplier tube and oscilloscope assembly. The SHG efficiency of the dichloridodiglycine zinc dihydrate crystal was evaluated by taking the microcrystalline powder of KDP as the reference material. The powder SHG efficiency output was found to be 0.5 greater with respect to KDP. Microhardness Test. Microhardness measurement is a general microprobe technique for assessing the bond strength, apart from being a measure of bulk strength. The crystal slices are well polished with a thickness variation less than 10 µm18 to avoid the surface defects which influence the hardness value strongly. Microhardness studies are carried out at room temperature using Shimadzu HMV-2000 fitted with Vickers pyramidal indentor. The load P is varied between 10–200 g, and the time of indentation is kept constant at 15 s for all trials. The diagonal lengths of indentation are measured. The hardness of the material HV is determined by the relation.
HV ) 1.8544
P kg/mm2 d2
(2)
where, P is the applied load in kg, and d is the diagonal length of indentation impression in mm. A graph was plotted for hardness versus P (Figure 10) and log P versus log d (Figure 11). The plot of log P versus log d yields a straight line, and its slope gives the work hardening coefficient n. The value of n is found to be 3.57 for titled crystal; since the value of n is greater than 2, the hardness of the material is found to increase with
Figure 11. Plot of log P versus log d.
the increase of load. It confirms the prediction of Onitsch19 and also the reverse indentation size effect (RISE).20,21 Conclusions Optical quality single crystals of dichloridodiglycine zinc dihydrate have been grown by the slow evaporation technique at room temperature. The structure and morphology of the grown crystal were characterized by single crystal XRD analysis. The high crystal perfection of the grown crystal was confirmed using HRXRD rocking curve. The presence of various functional groups were confirmed by FTIR analysis. From the TGA/DTA, the crystal was thermally stable up to 110 °C. The presence of zinc and chlorine in the crystal was confirmed by EDX spectral analysis. The SHG efficiency was found to be 0.5 times that of KDP. The microhardness test shows that the hardness value increases with load, which confirms the reverse indentation size effects of the crystal. From the above characterizations, the grown crystal is considered as a potential candidate for the fabrication of optoelectronic and photonic devices.
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