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Uniaxial growth of 〈1 0 0〉 zinc (tris) thiourea sulphate (ZTS) single crystal by Sankaranarayanan–Ramasamy (SR) method and its characterizations
Spectrochimica Acta Part A 94 (2012) 265–270
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Uniaxial growth of 1 0 0 zinc (tris) thiourea sulphate (ZTS) single crystal by Sankaranarayanan–Ramasamy (SR) method and its characterizations M. Iyanar a , C. Muthamizhchelvan b , J. Thomas Joseph Prakash c,∗ , S. Stephen Rajkumar Inbanathan d , S. Ponnusamy b a
Department of Physics, National College, Trichy 620 001, Tamil Nadu, India Department of Physics, SRM University, Kancheepuram 603 203, Tamil Nadu, India c PG & Research Department of Physics, H.H. The Rajah’s College, Pudukkottai 622 001, Tamil Nadu, India d Department of Physics, The American College, Madurai 625 002, Tamil Nadu, India b
a r t i c l e
i n f o
Article history: Received 1 February 2012 Received in revised form 8 March 2012 Accepted 22 March 2012 Keywords: Characterization Defects Etching X-ray diffraction
a b s t r a c t 1 0 0 directed single crystals of zinc (tris) thiourea sulphate, a semi-organic compound, have been grown at an average growth rate of 2 mm per day by Sankaranarayanan–Ramasamy (SR) method. Transparent ZTS crystal of size 70 mm length and 15 mm diameter was grown. The growth conditions have been optimized. Chemical etching, Vickers microhardness, UV–Vis NIR, dielectric constant and dielectric loss analysis were made on conventional and SR method grown ZTS crystals. Thermo gravimetric and differential thermal analysis was carried out to determine the thermal properties of the grown crystal. The NLO efficiency of the crystal has been confirmed using the Kurtz powder technique. The comparative study indicates that the crystal quality of unidirectional grown ZTS crystal is better compared to conventional slow evaporation method grown crystal. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Thiourea molecule is an interesting inorganic matrix modifier due to its large dipole moment and zinc (tris) thiourea sulphate [Zn(SC(NH2 )2 )3 ·SO4 ] is one such metal–organic nonlinear optical, non-hygroscopic crystal, which can be used for electro-optical applications, laser fusion and frequency doubling of near IR laser radiations [1]. Structurally, ZTS crystal is orthorhombic and belongs to the C2v -mm2 crystal class [2]. The space group is Pca21 with ˚ four molecules per unit cell and the lattice parameters: a = 11.13 A, ˚ Growth and characterization of convenb = 7.77 A˚ and c = 15.49 A. tional method grown ZTS single crystals of dimension up to 10 mm and its morphology have been reported in a number of publications [1–3]. Crystals with different morphology have been grown by conventional slow evaporation solution technique (SEST) [4]. But the phase matchable portion is only useful for applications and other portions will be wasted after cutting the crystals with the phase matching angle. For devices phase matching needs to be achieved, because otherwise the conversion efficiency would be very low [5]. From this point of view, Sankaranarayanan–Ramasamy method [5,6] has given the solution and it is possible to grow bulk size
∗ Corresponding author. Tel.: +91 9842470521. E-mail address: [email protected] (J.T.J. Prakash). 1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.03.075
single crystals along a desired orientation needed for device fabrication. The grown ZTS crystals were subjected to chemical etching, Vicker’s microhardness, UV–Vis, dielectrics, TG–DTA and NLO studies. Identical samples prepared with similar orientation were subjected to all the studies and several samples were analysed. 2. Experimental procedures 2.1. Synthesis and crystal growth Zinc (tris) thiourea sulphate (ZTS) crystal was synthesized by reacting stoichiometric amount of thiourea (CH4 N2 S) and zinc sulphate (ZnSO4 ) in the ratio of 3:1 in Millipore water of resistivity 18.2 M cm on the following reaction: 3(CH4 N2 S) + ZnSO4 → Zn[(CH4 N2 S)]3 ·SO4 Merck-GR grade chemical reagents were used. According to the data of the solubility diagram, 6.3 g of ZTS was dissolved in 100 ml of water at 35 ◦ C [7]. The period of growth ranged from 25 days. Fig. 1(a) shows the as grown crystal of ZTS with an optimized solution pH value of 4.2. Transparent crystals of average size 6 mm × 5 mm × 5 mm were obtained with perfect external morphology and the problem of microbial attack has not been encountered. Two times recrystallized ZTS crystals were used in the SR method of crystal growth. It is essential to increase the purity to a reputable level before proceeding further. The major habit planes
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Fig. 2. (a) 1 0 0 directional ZTS crystal grown by SR method and (b) cut and polished ingots of ZTS single crystal.
crystal is shown in Fig. 2(b). The growth observations are listed in Table 1. 3. Results and discussion 3.1. Single crystal X-ray diffraction The single crystal XRD analysis of ZTS single crystal was carried out using Enraf Nonius MACH3-CAT4 single crystal diffractometer ˚ radiation to identify the structure and to with Mo K␣ ( = 0.717 A) determine the lattice parameter values. The unit cell dimensions ˚ b = 7.979 A, ˚ c = 15.875 A. ˚ The measured cell paramare a = 11.435 A, eters are in good conformity with the previously published values [2–4]. 3.2. Chemical etching
Fig. 1. (a) Single crystal of ZTS grown by slow evaporation solution technique and (b) morphology of as grown ZTS crystal.
of ZTS crystal are (1 0 0), (0 1 0) and (0 0 1) (Fig. 1(b)). Both external conditions and structural factors determine the normal growth rate of a face (h k l) and hence the external shape of the crystal [8]. The growth rate along a-direction is faster than along b- and cdirections. In our present work the growth was carried out along 1 0 0 direction (Fig. 1(b)). 2.2. Growth rate optimization The SR glass ampoule was kept in a water container to avoid the temperature fluctuation of the daily variation. A thick plastic sheet covered the top portion of the ampoule with a hole at the centre to limit the evaporation. The top of the solution in the growth ampoule was maintained at 40 ◦ C to increase the evaporation rate. The temperature around the growth region is maintained at 33 ◦ C (±0.01 ◦ C accuracy) for growing crystals. This is the optimized growth condition of 1 0 0 direction of ZTS crystal. The growth rate was nearly 2 mm/day. The size of the harvested 1 0 0 directed ZTS crystal is 15 mm diameter and 70 mm length and was grown within a period of 35 days (Fig. 2(a)). The cut, polished unidirectional grown ZTS
Chemical etching studies were carried out on the conventional and SR method grown 1 0 0 directed ZTS single crystal for a known duration ranging from 5 to 10 s using water as the etchant. Dislocations influence a number of physical properties like crystal perfection, mechanical strength, optical quality, etc. Hence it is necessary to know the density of dislocations in a ZTS crystal. Fig. 3(a) represents the rectangular shaped well defined etch pits observed in SEST grown ZTS crystal and the calculated etch pit density (EPD) is 12.8 × 102 cm−2 . In utilizing single crystals for devices it is necessary to grow single crystals containing a reduced dislocation density. On increasing the etching time from 5 to 10 s, the chemical etching described above has additionally caused a number of etch pits which became prominent and well-defined. Also on successive etching the rectangular etch pits enlarge in size retaining their geometrical shape and do not disappear suggesting that the etch pits are due to dislocations. Chemical etching studies were carried out Table 1 Different growth temperatures and its observations. Temperature (◦ C) Top portion
Bottom portion
36 38 40
33 33 33
42
33
Growth observations
1 mm/day 2 mm/day 4 mm/day But the quality of the crystal is very poor Multi-nucleation is formed
M. Iyanar et al. / Spectrochimica Acta Part A 94 (2012) 265–270
160
SR grown ZTS Conventional grown ZTS
2
Vickers Hardness Number (kg/mm )
a
267
140
148
120
126
100
80
60
40
20 20
30
40
50
60
70
80
90
100
110
Load (g)
b
SR=1.5 Conventional=1.6
2.0
1.8
Log P
1.6
1.4
1.2
Fig. 3. (a) Surface of conventional method grown ZTS crystal etched by water for 5 s and (b) etch pattern produced on the 1 0 0 plane of SR grown ZTS crystal by water after etching for 5 s.
1.0 1.5
on the conventional slow-cooling method on (1 0 0), (0 0 1) plane of ZTS crystals and the EPD is 102 and 103 cm−2 [7]. Fig. 3(b) represents the etch pits observed for SR method grown ZTS on 1 0 0 direction and the EPD was 4.4 × 102 cm−2 . The reasons for the higher EPD of SEST grown ZTS crystals are growth sector boundaries (GSB). It has also been observed that dislocations can originate from the growth sector boundaries. But in SR method the growth is unidirectional on morphology defined facet. Hence dislocations which are associated with growth sector boundaries are absent in SR method grown ZTS crystal (Fig. 2(b)) [9]. Less EPD in SR method grown ZTS crystal shows that the crystalline perfection of the SR grown ZTS crystal is better than the crystal grown by conventional method. The reduced EPD in SR grown KDP, KAP and TGS crystals have been already reported [5,10,11].
3.3. Vickers microhardness For post growth processes and device fabrication, the mechanical hardness of the crystal is an important parameter. Vickers microhardness measurements were carried out on 1 0 0 direction of conventional and SR grown ZTS crystals using HMV-2Tmicrohardness tester. The distance between any two indentations was maintained to be greater than five times that of the diagonal length in order to avoid any mutual influence of the indentations.
1.6
1.7
1.8
1.9
2.0
Log d Fig. 4. (a) Vickers microhardness analysis and (b) plot of log P vs. log d.
The dwell time was 5 s for all the loads. Hardness number was calculated from the relation: Hv = 1.8544
P kg/mm2 d2
(1)
where P – applied load (kg) and d – diagonal length of indentation (mm). Fig. 4(a) clearly indicates that hardness of the SR grown crystal increases with increase in load up to 100 g and having maximum hardness number at 100 g (148 kg/mm2 ). The hardness of the SEST grown ZTS crystal is of the same order [3] and it is less (126 kg/mm2 ) than SR method grown ZTS crystal. Lesser hardness for SEST grown crystals may be due to entrapped solvent during growth [12]. Solvent inclusions are potentially a major source of growth induced defects and it is extremely difficult to avoid the inclusion of solvent at the seed–crystal interface [13]. Variation in supersaturation during growth, non-uniform growth rates are responsible for the formation of inclusions. But in SR method, depending on the temperature at top and bottom portion the rate of evaporation of solvent is controlled more effectively. So there are no such growth fluctuations or non-uniform growth rates the dislocations of the above causes are avoided. Higher hardness for SR grown ZTS crystal indicates greater stress required to form dislocation thus confirming greater crystalline perfection. An
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a
19
70
18
60
17
50
Dielectric Constant
Transmittance (%)
80
40 30
SR Conventional 1 mm thickness
20 10
SR=18 Conventional=11
16 15 14 13 12 11 10
0
9 -10 200
400
600
800
1000
8
1200
30
Wavelength (nm)
40
70
80
90
100
110
120
Temperature ( C)
b
0.34
SEST=0.32 SR=0.05
0.32 0.30
(2)
where ‘P’ is the load (g), ‘d’ is the diameter of indentation (mm), ‘a’ and ‘n’ are constants for a given material. According to Onitsch, 1.0 ≤ n ≤ 1.6 for hard materials and n ≥ 1.6 for soft materials [14]. The value of ‘n’ obtained for 1 0 0 plane of SR grown ZTS crystal was 1.5 and conventional grown ZTS crystal was 1.6 (Fig. 4(b)). Low work hardening coefficient shows less dislocation [6] in the SR method grown ZTS crystal since work hardening coefficient is caused by the dislocations present in the crystal.
0.28 0.26
Dielectric Loss
increase in the hardness will have significant effect on device fabrication and processing such as ease in polishing and less wastage due to cracking or breakage while polishing [9]. The ratio between the load and size of indentation is given by Meyer’s law as P = ad
60
0
Fig. 5. UV–Vis NIR analysis.
n
50
0.24 0.22 0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 40
60
80
100
120
140
0
Temperature ( C) Fig. 6. Temperature dependence of (a) dielectric constant at frequency 100 Hz and (b) dielectric loss at frequency 1 MHz.
3.4. UV–Vis NIR analysis 3.5. Dielectrics To determine the transmission range UV–Vis NIR spectrum of SEST and SR method grown 1 0 0 directed ZTS crystals were recorded using Perkin-Elmer Lambda-35 spectrometer for the wavelength range 200–1100 nm covering entire near ultraviolet, visible and higher energy part of near IR region to know the suitability for NLO application. Cut and polished SR and SEST method grown ZTS crystals of 1 mm thickness were used. Because of wide optical applications of NLO materials, the transmission range and transparency cutoff are very important parameters, especially for crystals used in SHG [15]. The crystal shows a good transmittance in the visible region. NLO crystals with high conversion efficiencies for SHG and transparent in visible and ultraviolet ranges are required for numerous device applications. The lower UV-cutoff wavelength is 269 nm. The transmittance of SR grown ZTS crystal is 30% higher than the transmittance of the conventional slow evaporation method grown ZTS crystal (Fig. 5). The improvement in the percentage of transmission by 30% may be attributed to a reduced scattering from crystal’s point and line defects. The improvement of optical quality of SR grown KAP, SA, TGS, KDP and DGBC crystals has been already reported [5,6,10,11,16]. The higher transmittance in SR grown ZTS crystal shows that the defect concentration in the grown crystal is less.
The dielectric study for the SEST and SR grown ZTS crystal was carried out using Agilent 4284-A LCR metre. The observations are made in the frequency range 100 Hz–1 MHz at the temperature of 40–130 ◦ C. The dielectric constant decreases with increase in frequency. This effect can be attributed to the effect of charge distribution by mean carrier hopping on defects. At low frequency, the charge on the defects can be rapidly redistributed so that defects closer to the positive side of the applied field become positively charged. This leads to a screening of the field and overall reduction in the electric field. Because the capacitance is inversely proportional to the field, this reduction in the field for a given voltage results in the increased capacitance observed as the frequency is lowered. The dielectric constant and dielectric loss increase with increase of temperature. The dielectric constant in SR method grown ZTS crystal is higher than the conventional method grown crystal and it is shown in Fig. 6(a). The dielectric constant of materials is due to the contribution of electronic, ionic, dipolar and space charge polarizations, which depend on the frequencies. At low frequencies, all these polarizations are active. The space charge polarization is generally active at lower frequencies and high temperatures. In conventional ZTS, the dielectric loss has a high value
M. Iyanar et al. / Spectrochimica Acta Part A 94 (2012) 265–270
269
Table 3 Comparison results of conventional and SR method grown ZTS crystal.
Fig. 7. Thermogravimetric and differential thermal analysis.
of 0.32 at 100 Hz and decreases to 0.18 at 1 MHz. But in SR-ZTS the value of 0.05 at 100 Hz, the dielectric loss decreases to 0.02 at 1 MHz (Fig. 6(b)). Low dielectric loss indicates that the SR grown ZTS crystal contains very low density of defects [17] which is in tune with the chemical etching results with low EPD. The low dielectric loss shows the crystals are of enhanced optical quality [18,19]. This is in good agreement with the result of optical studies and this parameter is of vital importance for various NLO devices. 4. Thermal analysis The thermal properties of solid-state materials have a very important influence on their preparation and application. In laser science (especially for a high-power laser system), thermal properties of NLO crystals are basic and essential parameters [15]. Simultaneous thermo gravimetric and differential thermal analysis of ZTS crystal was carried out between 35 ◦ C and 300 ◦ C at a heating rate 10 ◦ C/min in nitrogen atmosphere. The DTA curve shows that ZTS decomposition starts at 217 ◦ C and it undergo endothermic transition around 250 ◦ C (Fig. 7). The TG curve shows that there is a weight loss of about 10% in the temperature range 200–250 ◦ C due to the liberation of volatile substances possibly carbon dioxide, carbon monoxide and sulphur oxide. The thermal stability of the grown ZTS was compared with the other semi-organic NLO crystals in Table 2 [20–24]. Hence we can conclude that ZTS crystal is suitable for application up to 250 ◦ C. 4.1. Non-linear optical studies Kurtz and Perry powder technique [25] remains an extremely valuable tool for initial screening of materials for second harmonic generation (SHG). Powder SHG measurement was carried out for ZTS with 1064 nm laser radiation. A Quanta Ray of Nd:YAG laser producing pulses with a width of 10 ns and a repetition rate of 10 Hz was used. A photo multiplier tube (Philips Photonics) was
Table 2 Thermal stability of the thiourea based semi-organic NLO crystals. Crystal
Decomposition point (◦ C)
References
Tris thiourea zinc sulphate (ZTS) Bis thiourea ZnCl2 (BTZC) Bismuth thiourea chloride (BTC) Bis thiourea CdCl2 (BTCC) Tris alloyl thiourea ZnBr (TTZB) Bis thiourea CdI (BTCI)
250 115 215 215 167 200
Present study [19] [20] [21] [22] [23]
Studies
Conventional grown ZTS
SR method grown ZTS
Etch pit density (×102 cm−2 ) Vickers microhardness (kg/mm2 ) Meyer’s index Dielectric constant Dielectric loss UV–Vis (%)
12.8 126 at 100 g 1.6 11 at 100 Hz 0.18 at 1 MHz 42
4.4 148 at 100 g 1.5 18 at 100 Hz 0.02 at 1 MHz 72
used as a detector. A KDP sample was used as the reference material and the output power intensity of ZTS was observed. The SHG was confirmed by the emission of green radiation (532 nm) and the optical signal was collected by a photomultiplier tube. The ZTS shows a powder SHG efficiency of 1.2 times greater than that of KDP. Table 3 illustrates the results of EPD, microhardness, UVtransmittance, dielectric constant and dielectric loss on SEST and SR method grown ZTS crystal. Thus, low EPD, higher mechanical strength, good optical transmittance, higher dielectric constant and low dielectric loss of the SR method grown ZTS crystal indicate that it is more suitable for nonlinear optical (NLO) applications. 5. Conclusions Single crystals of ZTS, a potential semi-organic NLO material, have been grown from aqueous solution. Unidirectional 1 0 0 directed bulk ZTS single crystals were grown by Sankaranarayanan–Ramasamy (SR) method. Etch pits of rectangular shape were observed and the EPD is less in SR grown crystal compared to SEST grown ZTS crystal. The Vickers hardness test suggested an increase in hardness values for SR grown ZTS crystal. The SR method grown ZTS crystal has higher transmission compared to SEST grown crystal. The frequency dependence of dielectric constant and dielectric loss is found to be normal in the sense that these values decrease with increase in frequency. The dielectric constant was higher and dielectric loss was less in SR method grown ZTS crystal against conventional grown ZTS crystal. The decomposition temperature of the grown ZTS crystal is 250 ◦ C. The powder SHG efficiency of ZTS was found to be 1.2 times greater than that of KDP. References [1] P.U. Sastry, R. Chitra, R.R. Choudhury, M. Ramanadham, J. Phys. 63 (2004) 257. [2] K. Vasantha, P.A. Angeli Mary, S. Dhanuskodi, Spectrochim. Acta A 58 (2002) 311. [3] S. Moitra, T. Kar, Opt. Mater. 30 (2007) 508. [4] P.M. Ushasree, R. Jayavel, C. Subramanian, P. Ramasamy, J. Cryst. Growth 2197 (1999) 216. [5] M. Senthil Pandian, N. Balamurugan, G. Bhagavannarayana, P. Ramasamy, J. Cryst. Growth 310 (2008) 4143. [6] M. Senthil Pandian, U. Charoen In, P. Ramasamy, P. Manyum, M. Lenin, N. Balamurugan, J. Cryst. Growth 312 (2010) 397. [7] S. Moitra, T. Kar, Mater. Chem. Phys. 106 (2007) 8. [8] X.-Y. Liu, P. Bennema, J. Cryst. Growth 139 (1994) 179. [9] M. Senthil Pandian, P. Ramasamy, J. Cryst. Growth 312 (2010) 413. [10] S. Balamurugan, P. Ramasamy, Mater. Chem. Phys. 112 (2008) 1. [11] M. Senthil Pandian, N. Balamurugan, V. Ganesh, P.V. Raja Shekar, K. Kishan Rao, P. Ramasamy, Mater. Lett. 62 (2008) 3830. [12] N. Vijayan, G. Bhagavannarayana, R. Ramesh Babu, R. Gopalakrishnan, K.K. Maurya, P. Ramasamy, Cryst. Growth Des. 6 (2006) 1542. [13] H.G. Gallagher, R.M. Vrcelj, J.N. Sherwood, J. Cryst. Growth 250 (2003) 486. [14] E.M. Onitsch, Mikroscopia 2 (1947) 131. [15] Z. Sun, G. Zhang, X. Wang, Z. Gao, X. Cheng, S. Zhang, D. Xu, Cryst. Growth Des. 9 (2009) 3251. [16] M. Senthil Pandian, P. Ramasamy, J. Cryst. Growth 311 (2009) 944. [17] S.K. Kushwaha, N. Vijayan, G. Bhagavannarayana, Mater. Lett. 62 (2008) 3931. [18] C. Balavew, R. Dehlew, J. Solid State Chem. 55 (1984) 1. [19] M. Senthil Pandian, N. Pattanaboonmee, P. Ramasamy, P. Manyum, J. Cryst. Growth 314 (2011) 207.
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[20] P.A. Angeli Mary, S. Dhanuskodi, Cryst. Res. Technol. 36 (2001) 1231. [21] R. Sankar, C.M. Raghavan, R. Jayavel, Cryst. Growth Des. 7 (2007) 501. [22] A. Pricilla Jeyakumari, J. Ramajothi, S. Dhanuskodi, J. Cryst. Growth 269 (2004) 558.
[23] H. Sun, D. Yuan, X. Hou, X. Wang, L. Liu, D. Zhang, H. Ye, Cryst. Res. Technol. 42 (2007) 65. [24] M. Lydia Caroline, S. Vasudevan, Mater. Chem. Phys. 113 (2009) 670. [25] S.K. Kurtz, T.T. Perry, J. Appl. Phys. 39 (1968) 3798.
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Uniaxial growth of 1 0 0 zinc (tris) thiourea sulphate (ZTS) single crystal by Sankaranarayanan–Ramasamy (SR) method and its characterizations M. Iyanar a , C. Muthamizhchelvan b , J. Thomas Joseph Prakash c,∗ , S. Stephen Rajkumar Inbanathan d , S. Ponnusamy b a
Department of Physics, National College, Trichy 620 001, Tamil Nadu, India Department of Physics, SRM University, Kancheepuram 603 203, Tamil Nadu, India c PG & Research Department of Physics, H.H. The Rajah’s College, Pudukkottai 622 001, Tamil Nadu, India d Department of Physics, The American College, Madurai 625 002, Tamil Nadu, India b
a r t i c l e
i n f o
Article history: Received 1 February 2012 Received in revised form 8 March 2012 Accepted 22 March 2012 Keywords: Characterization Defects Etching X-ray diffraction
a b s t r a c t 1 0 0 directed single crystals of zinc (tris) thiourea sulphate, a semi-organic compound, have been grown at an average growth rate of 2 mm per day by Sankaranarayanan–Ramasamy (SR) method. Transparent ZTS crystal of size 70 mm length and 15 mm diameter was grown. The growth conditions have been optimized. Chemical etching, Vickers microhardness, UV–Vis NIR, dielectric constant and dielectric loss analysis were made on conventional and SR method grown ZTS crystals. Thermo gravimetric and differential thermal analysis was carried out to determine the thermal properties of the grown crystal. The NLO efficiency of the crystal has been confirmed using the Kurtz powder technique. The comparative study indicates that the crystal quality of unidirectional grown ZTS crystal is better compared to conventional slow evaporation method grown crystal. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Thiourea molecule is an interesting inorganic matrix modifier due to its large dipole moment and zinc (tris) thiourea sulphate [Zn(SC(NH2 )2 )3 ·SO4 ] is one such metal–organic nonlinear optical, non-hygroscopic crystal, which can be used for electro-optical applications, laser fusion and frequency doubling of near IR laser radiations [1]. Structurally, ZTS crystal is orthorhombic and belongs to the C2v -mm2 crystal class [2]. The space group is Pca21 with ˚ four molecules per unit cell and the lattice parameters: a = 11.13 A, ˚ Growth and characterization of convenb = 7.77 A˚ and c = 15.49 A. tional method grown ZTS single crystals of dimension up to 10 mm and its morphology have been reported in a number of publications [1–3]. Crystals with different morphology have been grown by conventional slow evaporation solution technique (SEST) [4]. But the phase matchable portion is only useful for applications and other portions will be wasted after cutting the crystals with the phase matching angle. For devices phase matching needs to be achieved, because otherwise the conversion efficiency would be very low [5]. From this point of view, Sankaranarayanan–Ramasamy method [5,6] has given the solution and it is possible to grow bulk size
∗ Corresponding author. Tel.: +91 9842470521. E-mail address: [email protected] (J.T.J. Prakash). 1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.03.075
single crystals along a desired orientation needed for device fabrication. The grown ZTS crystals were subjected to chemical etching, Vicker’s microhardness, UV–Vis, dielectrics, TG–DTA and NLO studies. Identical samples prepared with similar orientation were subjected to all the studies and several samples were analysed. 2. Experimental procedures 2.1. Synthesis and crystal growth Zinc (tris) thiourea sulphate (ZTS) crystal was synthesized by reacting stoichiometric amount of thiourea (CH4 N2 S) and zinc sulphate (ZnSO4 ) in the ratio of 3:1 in Millipore water of resistivity 18.2 M cm on the following reaction: 3(CH4 N2 S) + ZnSO4 → Zn[(CH4 N2 S)]3 ·SO4 Merck-GR grade chemical reagents were used. According to the data of the solubility diagram, 6.3 g of ZTS was dissolved in 100 ml of water at 35 ◦ C [7]. The period of growth ranged from 25 days. Fig. 1(a) shows the as grown crystal of ZTS with an optimized solution pH value of 4.2. Transparent crystals of average size 6 mm × 5 mm × 5 mm were obtained with perfect external morphology and the problem of microbial attack has not been encountered. Two times recrystallized ZTS crystals were used in the SR method of crystal growth. It is essential to increase the purity to a reputable level before proceeding further. The major habit planes
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Fig. 2. (a) 1 0 0 directional ZTS crystal grown by SR method and (b) cut and polished ingots of ZTS single crystal.
crystal is shown in Fig. 2(b). The growth observations are listed in Table 1. 3. Results and discussion 3.1. Single crystal X-ray diffraction The single crystal XRD analysis of ZTS single crystal was carried out using Enraf Nonius MACH3-CAT4 single crystal diffractometer ˚ radiation to identify the structure and to with Mo K␣ ( = 0.717 A) determine the lattice parameter values. The unit cell dimensions ˚ b = 7.979 A, ˚ c = 15.875 A. ˚ The measured cell paramare a = 11.435 A, eters are in good conformity with the previously published values [2–4]. 3.2. Chemical etching
Fig. 1. (a) Single crystal of ZTS grown by slow evaporation solution technique and (b) morphology of as grown ZTS crystal.
of ZTS crystal are (1 0 0), (0 1 0) and (0 0 1) (Fig. 1(b)). Both external conditions and structural factors determine the normal growth rate of a face (h k l) and hence the external shape of the crystal [8]. The growth rate along a-direction is faster than along b- and cdirections. In our present work the growth was carried out along 1 0 0 direction (Fig. 1(b)). 2.2. Growth rate optimization The SR glass ampoule was kept in a water container to avoid the temperature fluctuation of the daily variation. A thick plastic sheet covered the top portion of the ampoule with a hole at the centre to limit the evaporation. The top of the solution in the growth ampoule was maintained at 40 ◦ C to increase the evaporation rate. The temperature around the growth region is maintained at 33 ◦ C (±0.01 ◦ C accuracy) for growing crystals. This is the optimized growth condition of 1 0 0 direction of ZTS crystal. The growth rate was nearly 2 mm/day. The size of the harvested 1 0 0 directed ZTS crystal is 15 mm diameter and 70 mm length and was grown within a period of 35 days (Fig. 2(a)). The cut, polished unidirectional grown ZTS
Chemical etching studies were carried out on the conventional and SR method grown 1 0 0 directed ZTS single crystal for a known duration ranging from 5 to 10 s using water as the etchant. Dislocations influence a number of physical properties like crystal perfection, mechanical strength, optical quality, etc. Hence it is necessary to know the density of dislocations in a ZTS crystal. Fig. 3(a) represents the rectangular shaped well defined etch pits observed in SEST grown ZTS crystal and the calculated etch pit density (EPD) is 12.8 × 102 cm−2 . In utilizing single crystals for devices it is necessary to grow single crystals containing a reduced dislocation density. On increasing the etching time from 5 to 10 s, the chemical etching described above has additionally caused a number of etch pits which became prominent and well-defined. Also on successive etching the rectangular etch pits enlarge in size retaining their geometrical shape and do not disappear suggesting that the etch pits are due to dislocations. Chemical etching studies were carried out Table 1 Different growth temperatures and its observations. Temperature (◦ C) Top portion
Bottom portion
36 38 40
33 33 33
42
33
Growth observations
1 mm/day 2 mm/day 4 mm/day But the quality of the crystal is very poor Multi-nucleation is formed
M. Iyanar et al. / Spectrochimica Acta Part A 94 (2012) 265–270
160
SR grown ZTS Conventional grown ZTS
2
Vickers Hardness Number (kg/mm )
a
267
140
148
120
126
100
80
60
40
20 20
30
40
50
60
70
80
90
100
110
Load (g)
b
SR=1.5 Conventional=1.6
2.0
1.8
Log P
1.6
1.4
1.2
Fig. 3. (a) Surface of conventional method grown ZTS crystal etched by water for 5 s and (b) etch pattern produced on the 1 0 0 plane of SR grown ZTS crystal by water after etching for 5 s.
1.0 1.5
on the conventional slow-cooling method on (1 0 0), (0 0 1) plane of ZTS crystals and the EPD is 102 and 103 cm−2 [7]. Fig. 3(b) represents the etch pits observed for SR method grown ZTS on 1 0 0 direction and the EPD was 4.4 × 102 cm−2 . The reasons for the higher EPD of SEST grown ZTS crystals are growth sector boundaries (GSB). It has also been observed that dislocations can originate from the growth sector boundaries. But in SR method the growth is unidirectional on morphology defined facet. Hence dislocations which are associated with growth sector boundaries are absent in SR method grown ZTS crystal (Fig. 2(b)) [9]. Less EPD in SR method grown ZTS crystal shows that the crystalline perfection of the SR grown ZTS crystal is better than the crystal grown by conventional method. The reduced EPD in SR grown KDP, KAP and TGS crystals have been already reported [5,10,11].
3.3. Vickers microhardness For post growth processes and device fabrication, the mechanical hardness of the crystal is an important parameter. Vickers microhardness measurements were carried out on 1 0 0 direction of conventional and SR grown ZTS crystals using HMV-2Tmicrohardness tester. The distance between any two indentations was maintained to be greater than five times that of the diagonal length in order to avoid any mutual influence of the indentations.
1.6
1.7
1.8
1.9
2.0
Log d Fig. 4. (a) Vickers microhardness analysis and (b) plot of log P vs. log d.
The dwell time was 5 s for all the loads. Hardness number was calculated from the relation: Hv = 1.8544
P kg/mm2 d2
(1)
where P – applied load (kg) and d – diagonal length of indentation (mm). Fig. 4(a) clearly indicates that hardness of the SR grown crystal increases with increase in load up to 100 g and having maximum hardness number at 100 g (148 kg/mm2 ). The hardness of the SEST grown ZTS crystal is of the same order [3] and it is less (126 kg/mm2 ) than SR method grown ZTS crystal. Lesser hardness for SEST grown crystals may be due to entrapped solvent during growth [12]. Solvent inclusions are potentially a major source of growth induced defects and it is extremely difficult to avoid the inclusion of solvent at the seed–crystal interface [13]. Variation in supersaturation during growth, non-uniform growth rates are responsible for the formation of inclusions. But in SR method, depending on the temperature at top and bottom portion the rate of evaporation of solvent is controlled more effectively. So there are no such growth fluctuations or non-uniform growth rates the dislocations of the above causes are avoided. Higher hardness for SR grown ZTS crystal indicates greater stress required to form dislocation thus confirming greater crystalline perfection. An
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a
19
70
18
60
17
50
Dielectric Constant
Transmittance (%)
80
40 30
SR Conventional 1 mm thickness
20 10
SR=18 Conventional=11
16 15 14 13 12 11 10
0
9 -10 200
400
600
800
1000
8
1200
30
Wavelength (nm)
40
70
80
90
100
110
120
Temperature ( C)
b
0.34
SEST=0.32 SR=0.05
0.32 0.30
(2)
where ‘P’ is the load (g), ‘d’ is the diameter of indentation (mm), ‘a’ and ‘n’ are constants for a given material. According to Onitsch, 1.0 ≤ n ≤ 1.6 for hard materials and n ≥ 1.6 for soft materials [14]. The value of ‘n’ obtained for 1 0 0 plane of SR grown ZTS crystal was 1.5 and conventional grown ZTS crystal was 1.6 (Fig. 4(b)). Low work hardening coefficient shows less dislocation [6] in the SR method grown ZTS crystal since work hardening coefficient is caused by the dislocations present in the crystal.
0.28 0.26
Dielectric Loss
increase in the hardness will have significant effect on device fabrication and processing such as ease in polishing and less wastage due to cracking or breakage while polishing [9]. The ratio between the load and size of indentation is given by Meyer’s law as P = ad
60
0
Fig. 5. UV–Vis NIR analysis.
n
50
0.24 0.22 0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 40
60
80
100
120
140
0
Temperature ( C) Fig. 6. Temperature dependence of (a) dielectric constant at frequency 100 Hz and (b) dielectric loss at frequency 1 MHz.
3.4. UV–Vis NIR analysis 3.5. Dielectrics To determine the transmission range UV–Vis NIR spectrum of SEST and SR method grown 1 0 0 directed ZTS crystals were recorded using Perkin-Elmer Lambda-35 spectrometer for the wavelength range 200–1100 nm covering entire near ultraviolet, visible and higher energy part of near IR region to know the suitability for NLO application. Cut and polished SR and SEST method grown ZTS crystals of 1 mm thickness were used. Because of wide optical applications of NLO materials, the transmission range and transparency cutoff are very important parameters, especially for crystals used in SHG [15]. The crystal shows a good transmittance in the visible region. NLO crystals with high conversion efficiencies for SHG and transparent in visible and ultraviolet ranges are required for numerous device applications. The lower UV-cutoff wavelength is 269 nm. The transmittance of SR grown ZTS crystal is 30% higher than the transmittance of the conventional slow evaporation method grown ZTS crystal (Fig. 5). The improvement in the percentage of transmission by 30% may be attributed to a reduced scattering from crystal’s point and line defects. The improvement of optical quality of SR grown KAP, SA, TGS, KDP and DGBC crystals has been already reported [5,6,10,11,16]. The higher transmittance in SR grown ZTS crystal shows that the defect concentration in the grown crystal is less.
The dielectric study for the SEST and SR grown ZTS crystal was carried out using Agilent 4284-A LCR metre. The observations are made in the frequency range 100 Hz–1 MHz at the temperature of 40–130 ◦ C. The dielectric constant decreases with increase in frequency. This effect can be attributed to the effect of charge distribution by mean carrier hopping on defects. At low frequency, the charge on the defects can be rapidly redistributed so that defects closer to the positive side of the applied field become positively charged. This leads to a screening of the field and overall reduction in the electric field. Because the capacitance is inversely proportional to the field, this reduction in the field for a given voltage results in the increased capacitance observed as the frequency is lowered. The dielectric constant and dielectric loss increase with increase of temperature. The dielectric constant in SR method grown ZTS crystal is higher than the conventional method grown crystal and it is shown in Fig. 6(a). The dielectric constant of materials is due to the contribution of electronic, ionic, dipolar and space charge polarizations, which depend on the frequencies. At low frequencies, all these polarizations are active. The space charge polarization is generally active at lower frequencies and high temperatures. In conventional ZTS, the dielectric loss has a high value
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Table 3 Comparison results of conventional and SR method grown ZTS crystal.
Fig. 7. Thermogravimetric and differential thermal analysis.
of 0.32 at 100 Hz and decreases to 0.18 at 1 MHz. But in SR-ZTS the value of 0.05 at 100 Hz, the dielectric loss decreases to 0.02 at 1 MHz (Fig. 6(b)). Low dielectric loss indicates that the SR grown ZTS crystal contains very low density of defects [17] which is in tune with the chemical etching results with low EPD. The low dielectric loss shows the crystals are of enhanced optical quality [18,19]. This is in good agreement with the result of optical studies and this parameter is of vital importance for various NLO devices. 4. Thermal analysis The thermal properties of solid-state materials have a very important influence on their preparation and application. In laser science (especially for a high-power laser system), thermal properties of NLO crystals are basic and essential parameters [15]. Simultaneous thermo gravimetric and differential thermal analysis of ZTS crystal was carried out between 35 ◦ C and 300 ◦ C at a heating rate 10 ◦ C/min in nitrogen atmosphere. The DTA curve shows that ZTS decomposition starts at 217 ◦ C and it undergo endothermic transition around 250 ◦ C (Fig. 7). The TG curve shows that there is a weight loss of about 10% in the temperature range 200–250 ◦ C due to the liberation of volatile substances possibly carbon dioxide, carbon monoxide and sulphur oxide. The thermal stability of the grown ZTS was compared with the other semi-organic NLO crystals in Table 2 [20–24]. Hence we can conclude that ZTS crystal is suitable for application up to 250 ◦ C. 4.1. Non-linear optical studies Kurtz and Perry powder technique [25] remains an extremely valuable tool for initial screening of materials for second harmonic generation (SHG). Powder SHG measurement was carried out for ZTS with 1064 nm laser radiation. A Quanta Ray of Nd:YAG laser producing pulses with a width of 10 ns and a repetition rate of 10 Hz was used. A photo multiplier tube (Philips Photonics) was
Table 2 Thermal stability of the thiourea based semi-organic NLO crystals. Crystal
Decomposition point (◦ C)
References
Tris thiourea zinc sulphate (ZTS) Bis thiourea ZnCl2 (BTZC) Bismuth thiourea chloride (BTC) Bis thiourea CdCl2 (BTCC) Tris alloyl thiourea ZnBr (TTZB) Bis thiourea CdI (BTCI)
250 115 215 215 167 200
Present study [19] [20] [21] [22] [23]
Studies
Conventional grown ZTS
SR method grown ZTS
Etch pit density (×102 cm−2 ) Vickers microhardness (kg/mm2 ) Meyer’s index Dielectric constant Dielectric loss UV–Vis (%)
12.8 126 at 100 g 1.6 11 at 100 Hz 0.18 at 1 MHz 42
4.4 148 at 100 g 1.5 18 at 100 Hz 0.02 at 1 MHz 72
used as a detector. A KDP sample was used as the reference material and the output power intensity of ZTS was observed. The SHG was confirmed by the emission of green radiation (532 nm) and the optical signal was collected by a photomultiplier tube. The ZTS shows a powder SHG efficiency of 1.2 times greater than that of KDP. Table 3 illustrates the results of EPD, microhardness, UVtransmittance, dielectric constant and dielectric loss on SEST and SR method grown ZTS crystal. Thus, low EPD, higher mechanical strength, good optical transmittance, higher dielectric constant and low dielectric loss of the SR method grown ZTS crystal indicate that it is more suitable for nonlinear optical (NLO) applications. 5. Conclusions Single crystals of ZTS, a potential semi-organic NLO material, have been grown from aqueous solution. Unidirectional 1 0 0 directed bulk ZTS single crystals were grown by Sankaranarayanan–Ramasamy (SR) method. Etch pits of rectangular shape were observed and the EPD is less in SR grown crystal compared to SEST grown ZTS crystal. The Vickers hardness test suggested an increase in hardness values for SR grown ZTS crystal. The SR method grown ZTS crystal has higher transmission compared to SEST grown crystal. The frequency dependence of dielectric constant and dielectric loss is found to be normal in the sense that these values decrease with increase in frequency. The dielectric constant was higher and dielectric loss was less in SR method grown ZTS crystal against conventional grown ZTS crystal. The decomposition temperature of the grown ZTS crystal is 250 ◦ C. The powder SHG efficiency of ZTS was found to be 1.2 times greater than that of KDP. References [1] P.U. Sastry, R. Chitra, R.R. Choudhury, M. Ramanadham, J. Phys. 63 (2004) 257. [2] K. Vasantha, P.A. Angeli Mary, S. Dhanuskodi, Spectrochim. Acta A 58 (2002) 311. [3] S. Moitra, T. Kar, Opt. Mater. 30 (2007) 508. [4] P.M. Ushasree, R. Jayavel, C. Subramanian, P. Ramasamy, J. Cryst. Growth 2197 (1999) 216. [5] M. Senthil Pandian, N. Balamurugan, G. Bhagavannarayana, P. Ramasamy, J. Cryst. Growth 310 (2008) 4143. [6] M. Senthil Pandian, U. Charoen In, P. Ramasamy, P. Manyum, M. Lenin, N. Balamurugan, J. Cryst. Growth 312 (2010) 397. [7] S. Moitra, T. Kar, Mater. Chem. Phys. 106 (2007) 8. [8] X.-Y. Liu, P. Bennema, J. Cryst. Growth 139 (1994) 179. [9] M. Senthil Pandian, P. Ramasamy, J. Cryst. Growth 312 (2010) 413. [10] S. Balamurugan, P. Ramasamy, Mater. Chem. Phys. 112 (2008) 1. [11] M. Senthil Pandian, N. Balamurugan, V. Ganesh, P.V. Raja Shekar, K. Kishan Rao, P. Ramasamy, Mater. Lett. 62 (2008) 3830. [12] N. Vijayan, G. Bhagavannarayana, R. Ramesh Babu, R. Gopalakrishnan, K.K. Maurya, P. Ramasamy, Cryst. Growth Des. 6 (2006) 1542. [13] H.G. Gallagher, R.M. Vrcelj, J.N. Sherwood, J. Cryst. Growth 250 (2003) 486. [14] E.M. Onitsch, Mikroscopia 2 (1947) 131. [15] Z. Sun, G. Zhang, X. Wang, Z. Gao, X. Cheng, S. Zhang, D. Xu, Cryst. Growth Des. 9 (2009) 3251. [16] M. Senthil Pandian, P. Ramasamy, J. Cryst. Growth 311 (2009) 944. [17] S.K. Kushwaha, N. Vijayan, G. Bhagavannarayana, Mater. Lett. 62 (2008) 3931. [18] C. Balavew, R. Dehlew, J. Solid State Chem. 55 (1984) 1. [19] M. Senthil Pandian, N. Pattanaboonmee, P. Ramasamy, P. Manyum, J. Cryst. Growth 314 (2011) 207.
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