Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 405–414
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Growth and optical, thermal, mechanical and surface morphology studies of semiorganic nonlinear optical material: Dichlorobis (L-proline) zinc (II) crystal D. Anbuselvi a,⇑, D. Jayaraman b, J. Arul Martin Mani a, V. Joseph a a b
Department of Physics, Loyola College, Chennai, Tamil Nadu, India Department of Physics, Presidency College, Chennai, Tamil Nadu, India
h i g h l i g h t s Dichlorobis (L-proline) zinc (II)
g r a p h i c a l a b s t r a c t 1
H NMR spectrum of DCBPZ crystal.
(DCBPZ) is an organometallic compound. DCBPZ single crystal was grown by slow evaporation technique. Thermal stability and meting point were analyzed using thermal studies. Highly optical transparency was observed in the crystal. The NLO result obtained show that the SHG efficiency of DCBPZ is found to be twice that of KDP.
a r t i c l e
i n f o
Article history: Received 25 October 2013 Received in revised form 3 January 2014 Accepted 16 January 2014 Available online 31 January 2014 Keywords: Crystal growth X-ray diffraction NMR NLO material
a b s t r a c t The organometallic nonlinear optical material Dichlorobis (L-proline) zinc (II) (DCBPZ) was crystallized using solution growth technique. XRD data reveal that the grown crystal belongs to orthorhombic system with space group P212121. The crystals were characterized using UV–vis–NIR, FTIR and NMR spectral studies, SEM–EDAX analysis and Atomic force microscopy (AFM), thermal and microhardness studies. Photoconductivity measurements were made to understand the response of the grown material to the visible light. The SHG efficiency of DCBPZ was also measured using Kurtz and Perry powder technique. It is observed that the NLO activity of DCBPZ is found to be twice that of KDP due to improved linear and nonlinear optical properties of the material. Ó 2014 Elsevier B.V. All rights reserved.
Introduction In the recent years, efforts have been made on organic–metal mixed amino acid crystals in order to improve the chemical stability, laser damage threshold and nonlinear optical properties. ⇑ Corresponding author. Address: Department of Physics, Loyola College, Chennai 600 034, India. Tel.: +91 9677545821. E-mail address: [email protected] (D. Anbuselvi). http://dx.doi.org/10.1016/j.saa.2014.01.081 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
Several researchers have carried out a lot of studies on pure and organic and metal ions-doped crystals [1–5]. Recent advances in the fields of optoelectronics and photonics have created enormous interest in nonlinear optical materials. The advantages offered by organics over inorganic systems include high electronic susceptibility through high molecular polarizability, fast response time, facile modification through standard synthetic methods and lower dielectric constant. But organic materials present a difficulty when growing large optical-quality single crystals and the fragile nature
406
D. Anbuselvi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 405–414
of these crystals makes them difficult for processing in device applications. Inorganic materials have high melting point, mechanical strength and degree of chemical inertness, but their optical nonlinearities are poor [6,7]. The inherent limitations in the maximum attainable nonlinearity in inorganic materials and the moderate success in growing device-grade organic single crystals have made scientists to adopt alternate strategies. The obvious approach was to develop hybrid organic–inorganic materials with some trade off in their respective advantages. This new class of materials has come to be known as semiorganic compounds. In recent years, much attention has been focussed on the research of metal – organic coordination complexes in which the organic ligand is usually more dominant to exhibit nonlinear optical (NLO) behaviour. When the organic ligand is ionically bonded to metal ions, it can improve the mechanical and thermal properties. Hence, the combination of organic ligand with metal ions will enhance the NLO efficiency with improved optical quality and mechanical and thermal properties. In view of the improved NLO efficiency and other characterisations of semiorganic material (organometallic compound), we have synthesized and grown the semiorganic material Dichlorobis (L-proline) zinc (II) (DCBPZ) in the present investigation. The characterization studies such as single crystal XRD, UV–vis–NIR, FTIR, NMR, SEM–EDAX, AFM, thermal, microhardness and photoconductivity studies were carried out to analyze the crystalline structure, transmission range, various functional modes, molecular structure, composition, growth mechanism, thermal and mechanical stabilities and photoconducting behaviour of the crystal. The Second Harmonic Generation efficiency (SHG) was specifically evaluated to confirm the improved NLO activity of the grown crystal. Experimental procedure Material synthesis and crystal growth Dichlorobis (L-proline) zinc (II) was synthesized by dissolving Lproline and zinc chloride in deionized water with 2:1 molar ratio. The chemical reaction is given as follows:
2ðNH2 CH2 CH2 CH2 CHCOOHÞ þ ZnCl2 ! ZnðNH2 CH2 CH2 CH2 CHCOOHÞ2 Cl2 The synthesized compound was subjected to repeated crystallization process in order to improve purity. A saturated growth solution was prepared using synthesized compound DCBPZ salt in deionized water at room temperature. The prepared solution was filtered and the solution was kept in a constant temperature bath. The temperature was maintained at room temperature with an accuracy of ±0.01 °C. The solution evaporates at a steady rate to reach the supersaturation for the formation of crystal nuclei. The supersaturation was found to vary between 1.1 and 1.5 from nucleation stage till the completion of the growth process. After a period of 25 days, single crystals of DCBPZ with dimensions of 11 3 4 mm3 were harvested successfully. The photograph of as-grown DCBPZ single crystal is shown in Fig. 1.
Fig. 1. Photograph of as-grown DCBPZ crystal.
for every 25 reflection without significant variations. The crystallographic informations are summarized as follows. The results show that DCBPZ crystal belongs to orthorhombic system with non-centrosymmetric space group P212121. The calculated lattice parameters were found to be a = 6.6155 Å, b = 13.5574 Å, c = 16.2532 Å; a = b = c = 90° and volume = 1457.7 (Å)3. The single crystal XRD results obtained are found to be in good agreement with the already reported values [8]. UV–vis–NIR spectral analysis UV–vis–NIR absorption spectrum of the grown crystal of thickness 2 mm was recorded using the instrument Varian carry 5E spectrophotometer in the range of 200–1100 nm. From the UV– vis NIR spectrum as shown in Fig. 2, the UV cut-off wavelength is found to be 233 nm. The absence of absorption in the region between 233 and 1100 nm clearly indicates that the crystal DCBPZ has wide transparency. According to Tauc’s relation, the absorption coefficient (a) of a material is related to optical band gap (Eg) and photon energy (ht) as [9]
aht ¼ Aðht Eg Þn
where A is a constant for different transitions and n is an index which assumes the values 1/2, 3/2, 2 and 3 depending on the nature of electronic transitions responsible for absorption. The band gap energy value Eg can be estimated from the plot (aht)2 versus photon
Results and discussion Single crystal X-ray diffraction analysis XRD data were collected using a computer – controlled Enraf Nonius – CAD 4 single crystal X-ray diffractrometer. Single crystal of DCBPZ of suitable size 0.2 0.1 0.1 mm3 was selected for the X-ray diffraction analysis. The structure was refined by the matrix least square using SHELXL-97 (WINGX) program. During the course of data collection, one standard reflection was monitored
ð1Þ
Fig. 2. UV–vis–NIR absorption spectrum of DCBPZ.
D. Anbuselvi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 405–414
energy (Fig. 3) and the optical band gap energy value Eg has been estimated as 4.99 eV for DCBPZ. The higher value of optical band gap (4.99 eV) offers more optical conductivity to the medium without optical photons being absorbed in the specific range of wavelengths. This provides dielectric nature to the material. Only dielectric materials will have wide transparent range. The optical photons have energies ranging from 1 eV to 3 eV in the UV– vis–NIR region. This range of energy is not sufficient to excite the electrons for transition from valance band to conduction band in the dielectric material. Therefore, the incident optical photons in the range of UV–vis–NIR region are not absorbed by the material and the material is capable of transmitting the light of wavelengths in the range 233–1100 nm. The higher value of optical band gap thus provides sufficient transmitting power required for the NLO activity of the materials. FTIR spectrum analysis The FTIR spectrum of DCBPZ crystal recorded in the range 400– 4000 cm1 is shown in Fig. 4. The peak corresponding to 3155 cm1 is due to strong NH stretching vibration. The absorption peaks in the region of 2744–2457 cm1 are resulting from the superimposed OAH and NHþ 2 stretching bands. The peak at 1665 cm1 corresponds to the asymmetrical NHþ 2 stretching vibrations. The absorption peaks in the 1557–1461 cm1 regions are due 1 to symmetrical NHþ 2 stretching vibrations. The peak at 1415 cm is assigned to symmetrical COO ion group stretching. The other peaks at 1375 and 1330 cm1 are assigned to wagging of CHþ 2 group of the DCBPZ. The peak due to 1195 cm1 is assigned to twisting NHþ 2 [10]. The CAN stretching is indicated due to the prominent peaks at 1058 and 1035 cm1 respectively. The peaks corresponding to 939 and 848 cm1 are due to CHþ 2 rocking. The in-plane deformation of COO, COO scissoring and CCO rocking vibrations are observed at 780 cm1, 679 cm1 and 551 cm1 respectively. The presence of zwitterions (NHþ 2 and COO ) clearly indicates that an imbalance of charges is provided to the grown material to make the material non-centrosymmetric for initiating induced polarization when a powerful laser beam is incident on the material. The predicted functional groups are found to be in close agreement with the already available report [8]. Thermal analysis Thermogravimetric analysis (TGA), differential thermal analysis (DTA) and differential scanning calorimetry (DSC) of DCBPZ
Fig. 3. Tauc’s plot of DCBPZ crystal.
407
crystals were carried out simultaneously using SDT Q600 V8.0 Build 95 thermal analyser. A powder sample was used for these analyses in the temperature range of 30–1000 °C in nitrogen atmosphere at a heating rate of 20 °C/min. TGA and DTA curves of Dichlorobis (L-proline) zinc (II) are shown in Fig. 5. From the TGA curve, it is observed that the decomposition starts at 228.23 °C due to dissociation of the compound followed by the evaporation of volatile products. This is confirmed by the endothermic peak of DTA at 228.23 °C. In the DSC curve also (Fig. 6), there is also an exothermic peak at 229.96 °C to confirm the onset of decomposition. Hence, the melting point of the crystal is 229 °C which is in good agreement with the earlier reports [8,10]. The exothermic peak corresponding to 77.67 °C in the DTA curve indicates the loss trapped water molecules or dehydration of the compound. The endothermic peak at 131.87 °C following the exothermic peak at 77.67 °C in the DTA curve may be due to the evaporation of CO2, NO2 and NH3 gas molecules. The remaining exothermic peaks of DTA curve represent the various stages of decomposition of the compound. It is observed from the TG curve that the rapid decomposition starts at 255.30 °C which is confirmed by the exothermic peak of DTA curve at 255.34 °C. This is further confirmed by the exothermic peak at 255.34 °C in the DSC curve. This is therefore concluded that the material is thermally stable upto 229 °C which is suitable for the possible applications in lasers to withstand high temperature. SEM–EDAX analysis Fig. 7(a, b and c) shows the SEM images of the grown crystal surface with magnifications 260, 4000 and 31,000 respectively. With lower magnification (Fig. 7a) the surface appears foggy without showing the transparent and smooth surfaces. With higher magnification, Fig. 7b and c show the transparent nature of the grown crystal with smooth surface containing minimum density of defects. Fig. 7b and c indicates the particles of various sizes which predict the presence of various components in the grown crystal. Fig. 8 shows the energy dispersive X-ray analysis (EDAX) of Dichlorobis (L-proline) zinc (II). From EDAX spectrum, the presence of nitrogen, oxygen, zinc and chlorine is confirmed. Table 1 presents the percentage of elements present in the grown crystal. The incorporations of zinc and chlorine in L-proline are clearly understood from the EDAX spectral analysis. The incorporation of metal ions in the sublattices may increase the number of charge carriers for the material to be dipolar. Atomic force microscopy analysis Atomic force microscopy (AFM), a relatively new and rapidly advancing technique, proves to be a unique and excellent approach for observing crystal growth dynamics, imaging and measuring surface features as well as investigating growth mechanism. AFM images of DCBPZ crystals were recorded using a Digital Instrument Multimode Nanoscope IIIa operating in the tapping mode. Figs. 9– 12 show the AFM images of the DCBPZ crystals with scan sizes 5 lm, 10 lm, 15 lm and 25 lm respectively. The images were recorded on the (0 0 1) faces of DCBPZ crystals. Fig. 9(a) and (b) shows the surface features in two and three dimensions respectively with screw dislocations formed on (0 0 1) surface [9]. The black spots in Figs. 9(a) and 10 (a) indicate the points of dislocation. It is understood that the main sources of growth steps for most of the crystals growing at low to moderate supersaturation are growth spirals. The points of dislocations with screw components at crystal surfaces are important because they can provide a continuous source of steps which can propagate across the surface of the crystal. Fig. 10(a) and (b) presents the observation of growth hillocks in two and three dimensions respectively. Fig. 11(a) and (b) depicts
408
D. Anbuselvi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 405–414
Fig. 4. FTIR spectrum of DCBPZ.
Fig. 5. TGA and DTA curves of DCBPZ.
the elementary steps provided by the growth hillocks in two and three dimensions respectively. Fig. 12(a) and (b) shows the adsorption of molecules (white colour) into the kinks on the steps with 2D and 3D views. The spiral growth mechanism was formulated by Frank in 1949. According to Frank, the growth of crystals at low supersaturation could take place due to a screw dislocation emerging at a crystal face providing a continuous step source [11]. Burton, Cabrera and Frank were the first to develop a theory of spiral growth [12,13]. The growth spirals at temperature 0 K will have high concentration of kinks due to thermal roughening. The molecules that are absorbed on the surface diffuse into the kinks. The process continues for ever since the spiral would be renewed and the preferential sites would always be available for new growth. The idea introduced by Frank can explain most of the
features observed in crystal growth at low supersauration. BCF approximated the radius of the growth spiral for a spherical spiral having equispaced steps as
r ¼ 2q h
ð2Þ
where r and h are the polar coordinates of the growth spiral and q⁄ is the radius of the critical two dimensional nucleus. The radius of the two dimensional critical nucleus is given as
q ¼
vV kT ln s
ð3Þ
where m is the interfacial energy, V is the molecular volume, k is Boltzmann constant and T is the temperature (in Kelvin) and s is
D. Anbuselvi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 405–414
409
sources of growth spirals and steps, are provided due to thermal roughing. NMR spectral analysis
yo ¼ r 2 r 1
ð5Þ
The 1H NMR spectrum of DCBPZ is shown in Fig. 13. NMR spectrum was recorded by using Bruker AVANCE III 500 MHz NMR spectrometer and D2O as solvent. The signals corresponding to the CH2 (a) protons of the pyrrolidine ring appear as a triplet centred at d 1.90 ppm. The doublet proton signals appearing at d 2.3 ppm have been assigned to CH2 (b) protons of the same ring. The CH2 (c) protons appear as a multiplet centered at d 3.335 ppm in the aliphatic region of the spectrum. The signals due to CH2 (d) protons are shifted to higher d values as a consequence of the electron withdrawing carbonyl group in the vicinity and appear as a triplet at d 4.083 ppm. The sharp singlet around at d 4.75 ppm is due to the presence of undeuterated water in D2O. The signals due to NAH and COOH protons do not appear because of the exchange of fast deuterium in these two groups, with D2O being used as a solvent. The predicted structure of the compound from 1H NMR spectral data agrees very well with the already available report [8]. Fig. 14 shows 13C NMR spectrum of DCBPZ which contains 5 signals. The resonance signal at 174.69 ppm confirms the free carboxyl (C@O) group. The resonance signal at 61.11 ppm is due to the CH group of the pyrrolidine ring. The other peaks at 46.11 ppm, 28.9 ppm and 23.7 ppm are due to the aliphatic CH2 groups present in the pyrrolidine ring. Hence, NMR spectral analysis confirms the molecular structure of the grown compound.
yo ¼ 2r s ½ðh þ 2pÞ h ¼ 4pr s
ð6Þ
Microhardness
Fig. 6. DSC curve of DCBPZ.
the supersaturation. The value of m was estimated from the following equation [14],
t¼
kT ½0:173 ð0:248 lnðxm ÞÞ a2
ð4Þ
If yo is the distance between two success steps,
The calculated steps height is of the order 5.13 nm which lies closely in the range of 1.5–5.0 nm as reported by Liu et al. [15]. Numerous attempts have been made to understand the mechanism of macrosteps formation. From the images of growth sequence observed on the [0 0 1] face of DCBPZ crystals, the surface initially appears with dislocations followed by growth spirals. When the surface was scanned again, the growth spirals reappears in the form of steps to provide kinks for the adsorption of molecules from the supersaturated solution. It is worth noting that the growing surface is dominated by elementary steps. It is therefore concluded that the microcrystals formed initially should contain dislocations to continue the growth process for the transformation of microcrystals into large size macrocrystals. The dislocations, which are the
Microhardness testing is one of the best methods of understanding the mechanical properties of materials such as hardness number, crack length, fracture toughness, brittle index and elastic stiffness constant. Loads ranging from 25 to 100 g were used for making indentations, keeping the time of indentation constant at 10 s for all the cases. The diagonal lengths of the indentation mark and the crack length were measured, using a micrometer eyepiece. The Vickers hardness value was calculated as
Hv ¼
1:8544P d
2
kg=mm2
ð7Þ
where P is the applied load in kg, 1.8544 is a constant of geometric factor for diagonal pyramid and d is the diagonal length in mm. The
Fig. 7. SEM images of DCBPZ crystal.
410
D. Anbuselvi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 405–414
for the grown crystal yields crystal yields a straight line and its slop gives the work index n. The value of n is greater than 2 (2.39) and the microhardness should increase with increase of load as predicted by Onistch [16]. Since the material takes some time to revert to the elastic mode after every indentation, a correction x is applied to the d value and the Kick’s law is related as 2
P ¼ K 2 ðd þ xÞ
ð9Þ
From Eqs. (8) and (9) n=2
d
¼
12 12 K2 K2 dþ x K1 K1
ð10Þ
The slope of dn/2 versus d yields (K2/K1) and the intercept is a measure of x and is shown in Fig. 16. The fracture toughness (Kc) is given by
Kc ¼
B¼ Table 1 Compositional analysis of DCBPZ crystal from EDAX spectrum. Wt%
At%
C N O Cl Zn
39.54 07.47 03.79 22.72 26.48
64.45 10.44 04.63 12.55 07.93
variation of Hv with the applied load P is shown in Fig. 15 and it is found to increase with increase of load. According to Meyer’s relation,
ð8Þ
where K1 is the standard microhardness value which can be found out from the plot of P versus dn. A plot of ln P versus ln d (Fig. 16)
a
Hv Kc
ð12Þ
The yield strength (rv) of the material can be found out using the relation,
Element
n
ð11Þ
where C is the crack length measured from the centre of indentation mark to the crack tip, P is the applied load and geometrical constant b = 7 for Vickers indenter. The brittleness index (B) is given by
Fig. 8. EDAX spectrum of DCBPZ crystal.
P ¼ K1d
P bC 3=2
rv ¼
Hv 12:5ðn 2Þ f1 ð2 nÞg 1 ðn 2Þ 2:9
ð13Þ
The important microhardness parameters n, Hv, Kc, B and rv of DCBPZ crystal were calculated and compared with those of KDP crystal. The work index calculated is found to be in good agreement with the earlier report of Ramasamy et al. [8]. Since the microhardness of DCBPZ increases with load and its yield strength is found to be 410 MPa, which is comparable with that of KDP crystal, it is confirmed that DCBPZ crystal is having sufficient mechanical strength for device fabrications. Table 2 presents the microhardness parameters of DCBPZ and KDP [17] crystals for comparison (see Fig. 17).
b
Fig. 9. (a and b) 2D and 3D AFM images of DCBPZ crystal with scan size 5 lm showing screw dislocations observed on (0 0 1) face.
D. Anbuselvi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 405–414
a
411
b Points of dislocation
Fig. 10. (a and b) 2D and 3D AFM images of DCBPZ crystal with scan size 10 lm showing hillocks observed on (0 0 1) face.
Fig. 11. (a and b) 2D and 3D AFM images of DCBPZ crystal with scan size 15 lm showing the elemental steps observed on (0 0 1) face.
Fig. 12. (a and b) 2D and 3D AFM images of DCBPZ crystal with scan size 25 lm showing the adsorption of molecules (white colour) into the kinks observed on (0 0 1) face.
412
D. Anbuselvi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 405–414
Fig. 13. 1H NMR spectrum of DCBPZ crystal.
Fig. 14.
13
C NMR spectrum of DCBPPZ crystal.
Photoconductivity study The photoconductivity of DCBPZ was studied using Keithley 485 picoammeter. The applied electric field was changed from 5 V/cm to 100 V/cm without exposing the sample to any radiation and the corresponding dark current was recorded. The sample was then illuminated with a halogen lamp of 100 W power and the resulting photocurrent was measured by varying the applied field in the same range as used in the case of measurement of dark current.
Fig. 18 shows the plots of darkcurrent (Id) and photocurrent (Ip) against the applied field. From the plots it is observed that the photocurrent for DCBPZ is less than the dark current which is termed as negative photoconductivity. Stockmann model explains the tendency of decrease in mobile charge carriers during negative photoconductivity. According to this model, for the DCBPZ crystal, the negative photoconductivity is based on the state between the Fermi level and the valance band. This state contains more traps (Recombination centre) to capture electrons and holes. The
D. Anbuselvi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 405–414
413
Fig. 17. dn/2 versus d.
Fig. 15. Vickers microhardness number with applied load.
Fig. 18. Photoconductivity of DCBPZ crystal. Fig. 16. ln P versus ln d.
Table 2 Microhardness parameters of DCBPZ and KDP crystals. Parameters
For DCBPZ
For KDP
n Hv (kg/mm2) Kc (MN m3/2) B (m1/2) rv (MPa)
3.32 87.6 0.4546 1.89 103 410.0
1.71 153.86 0.6246 2.83 103 697.53
presence of recombination centres reduces the number of mobile charge carrier giving rise to negative photoconductivity [18]. However, using laser beam of wavelength 1064 nm and electric field 40 V/m the photocurrent was measured as 5.2 lA which is greater than the value obtained for the visible light. Hence, the material shows positive response to the intense laser beam. NLO test Second harmonic generation efficiency of the grown crystal DCBPZ was estimated using Kurtz and Perry powder technique with the help of Nd: YAG laser beam of wavelength 1064 nm [19]. The crystalline DCBPZ material was powdered to the particle size in the range 125–150 lm. The SHG was confirmed from the emission of green radiation of wavelength 532 nm and the optical
signal was detected by a photomultiplier tube. The result obtained show that the SHG efficiency of DCBPZ is found to be twice that of KDP which is found to be in close agreement with the report of Ramasamy et al. [10]. The second harmonic generation in the grown material is also due to positive response of the material to the laser beam. Conclusion Good optical quality of DCBPZ crystals were grown using isothermal slow evaporation method. The lattice parameters have been analyzed using single crystal X-ray diffraction technique. DCBPZ has a wide transparency window from 250 to 1000 nm, which is essentially required for the material to exhibit NLO behaviour. Optical properties were analyzed using UV–vis–NIR. The presence of functional groups of the grown crystal sample was confirmed by FT-IR analysis. TGA-DTA and DSC studies reveal that the crystal is thermally stable up to 263.20 °C. From the Vickers microhardness study, it is observed that Hv increases with increasing load and thus DCBPZ satisfies normal indentation size effect. Photoconductivity analysis shows the negative photoconductivity of the material. Microstructure and compositional analyses were discussed using SEM–EDAX studies. From atomic force microscopic study, the surface features and growth mechanism were analyzed. The observations of growth hillocks and steps due to 2D nucleation clearly confirm the BCF theory of growth mechanism. Even though
414
D. Anbuselvi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 405–414
the presence of the defects decreases the quality of the crystals, their role is very important for the growth mechanism of the crystal. Hence, there should be a compromise between the purity and the size of the macrocrystal in order to obtain sufficiently large crystals with minimum dislocation density. The enhanced SHG efficiency was confirmed by Kurtz and Perry technique. Therefore, it is concluded that the grown material DCBPZ is an excellent semiorganic NLO material with enhanced SHG efficiency due to the incorporation of zinc metal ions in the sublattices and improved linear and nonlinear optical properties.
References [1] R. Mohankumar, D. Rajanbabu, D. Jayaraman, R. Jayavel, K. Kitamura, J. Cryst. Growth 275 (2005) e1935–e1939. [2] K. Meera, R. Muralidharan, R. Dhanasekaran, M. Prapun, P. Ramasamy, J. Cryst. Growth 263 (2004) 510–516. [3] P.N. Prasad, D.J. Williama, Introduction to Nonlinear Optical Effects in Organic Molecules and Polymers, Wiley, New York, 1991. [4] Soma Adhikari, Tanusree Kar, Bulk single crystal growth and characterisation of L-leucine – a nonlinear optical material, Mater. Chem. Phys. 133 (2012) 1055–1059.
[5] S.A. Martin Britto Dhas, G. Bhagavannrayana, S. Natarajan, J. Cryst. Growth 310 (2008) 3535. [6] M.R. Suresh kumar, H.J. Ravindra, J. Jayaraman, S.M. Dharmaprakash, Structural characteristics and second harmonic generation in L-threonine crystals, J. Cryst. Growth 286 (2006) 451–456. [7] D. Balasubramanian, P. Murugakoothan, R. Jayavel, Synthesis, growth and characterisation of organic nonlinear optical bis-glycine maleate (BGM) single crystals, J. Cryst. Growth 312 (2010) 1855–1859. [8] G. Anandha Babu, P. Ramasamy, Mater. Chem. Phys. 113 (2009) 727–733. [9] J. Tauc, in: J. Tauc (Ed.), Amorphous and Liquid Semiconductors, Plenum, New York, 1974. [10] M. LydiaCaroline, A. Kandasamy, R. Mohan, S. Vasudevan, J. Cryst. Growth 311 (2009) 1161–1165. [11] X.J. Liu, D. Xu, G.W. Yu, X.Q. Wang, L.Y. Zhu, G.H. Zhang, G. Yu, Appl. Surf. Sci. 253 (2007) 3674–3677. [12] F.C. Frank, Disc. Faraday Soc. 5 (1949) 48–54. [13] W.K. Burton, N. Cabrera, F.C. frank, Philos. Trans. Royal Soc. A 243 (1951) 299– 358. [14] W. Mott, Micro Indentation Hardness testing, Butter-worths, London, 1956. [15] X.J. Liu, P.J. Wang, Z. Zhang, D. Xu, G.H. Zhang, X.Q. Wang, Curr. Appl. Phys. 10 (2010) 715–717. [16] E.M. Onistch, Mikroscopia 2 (1947) 131. [17] R. Robert, C. Justin Raj, S. Krishnan, S. Jerome Das, Physica B 405 (2010) 20–24. [18] V.N. Joshi, in: Photoconductivity, Marcel Dekker, New York, 1990. [19] S.K. Kurtz, T.T. Perry, A powder technique for the evaluation of nonlinear optical materials, J. Appl. Phys. 39 (1968) 3798–3813.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Growth and optical, thermal, mechanical and surface morphology studies of semiorganic nonlinear optical material: Dichlorobis (L-proline) zinc (II) crystal D. Anbuselvi a,⇑, D. Jayaraman b, J. Arul Martin Mani a, V. Joseph a a b
Department of Physics, Loyola College, Chennai, Tamil Nadu, India Department of Physics, Presidency College, Chennai, Tamil Nadu, India
h i g h l i g h t s Dichlorobis (L-proline) zinc (II)
g r a p h i c a l a b s t r a c t 1
H NMR spectrum of DCBPZ crystal.
(DCBPZ) is an organometallic compound. DCBPZ single crystal was grown by slow evaporation technique. Thermal stability and meting point were analyzed using thermal studies. Highly optical transparency was observed in the crystal. The NLO result obtained show that the SHG efficiency of DCBPZ is found to be twice that of KDP.
a r t i c l e
i n f o
Article history: Received 25 October 2013 Received in revised form 3 January 2014 Accepted 16 January 2014 Available online 31 January 2014 Keywords: Crystal growth X-ray diffraction NMR NLO material
a b s t r a c t The organometallic nonlinear optical material Dichlorobis (L-proline) zinc (II) (DCBPZ) was crystallized using solution growth technique. XRD data reveal that the grown crystal belongs to orthorhombic system with space group P212121. The crystals were characterized using UV–vis–NIR, FTIR and NMR spectral studies, SEM–EDAX analysis and Atomic force microscopy (AFM), thermal and microhardness studies. Photoconductivity measurements were made to understand the response of the grown material to the visible light. The SHG efficiency of DCBPZ was also measured using Kurtz and Perry powder technique. It is observed that the NLO activity of DCBPZ is found to be twice that of KDP due to improved linear and nonlinear optical properties of the material. Ó 2014 Elsevier B.V. All rights reserved.
Introduction In the recent years, efforts have been made on organic–metal mixed amino acid crystals in order to improve the chemical stability, laser damage threshold and nonlinear optical properties. ⇑ Corresponding author. Address: Department of Physics, Loyola College, Chennai 600 034, India. Tel.: +91 9677545821. E-mail address: [email protected] (D. Anbuselvi). http://dx.doi.org/10.1016/j.saa.2014.01.081 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
Several researchers have carried out a lot of studies on pure and organic and metal ions-doped crystals [1–5]. Recent advances in the fields of optoelectronics and photonics have created enormous interest in nonlinear optical materials. The advantages offered by organics over inorganic systems include high electronic susceptibility through high molecular polarizability, fast response time, facile modification through standard synthetic methods and lower dielectric constant. But organic materials present a difficulty when growing large optical-quality single crystals and the fragile nature
406
D. Anbuselvi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 405–414
of these crystals makes them difficult for processing in device applications. Inorganic materials have high melting point, mechanical strength and degree of chemical inertness, but their optical nonlinearities are poor [6,7]. The inherent limitations in the maximum attainable nonlinearity in inorganic materials and the moderate success in growing device-grade organic single crystals have made scientists to adopt alternate strategies. The obvious approach was to develop hybrid organic–inorganic materials with some trade off in their respective advantages. This new class of materials has come to be known as semiorganic compounds. In recent years, much attention has been focussed on the research of metal – organic coordination complexes in which the organic ligand is usually more dominant to exhibit nonlinear optical (NLO) behaviour. When the organic ligand is ionically bonded to metal ions, it can improve the mechanical and thermal properties. Hence, the combination of organic ligand with metal ions will enhance the NLO efficiency with improved optical quality and mechanical and thermal properties. In view of the improved NLO efficiency and other characterisations of semiorganic material (organometallic compound), we have synthesized and grown the semiorganic material Dichlorobis (L-proline) zinc (II) (DCBPZ) in the present investigation. The characterization studies such as single crystal XRD, UV–vis–NIR, FTIR, NMR, SEM–EDAX, AFM, thermal, microhardness and photoconductivity studies were carried out to analyze the crystalline structure, transmission range, various functional modes, molecular structure, composition, growth mechanism, thermal and mechanical stabilities and photoconducting behaviour of the crystal. The Second Harmonic Generation efficiency (SHG) was specifically evaluated to confirm the improved NLO activity of the grown crystal. Experimental procedure Material synthesis and crystal growth Dichlorobis (L-proline) zinc (II) was synthesized by dissolving Lproline and zinc chloride in deionized water with 2:1 molar ratio. The chemical reaction is given as follows:
2ðNH2 CH2 CH2 CH2 CHCOOHÞ þ ZnCl2 ! ZnðNH2 CH2 CH2 CH2 CHCOOHÞ2 Cl2 The synthesized compound was subjected to repeated crystallization process in order to improve purity. A saturated growth solution was prepared using synthesized compound DCBPZ salt in deionized water at room temperature. The prepared solution was filtered and the solution was kept in a constant temperature bath. The temperature was maintained at room temperature with an accuracy of ±0.01 °C. The solution evaporates at a steady rate to reach the supersaturation for the formation of crystal nuclei. The supersaturation was found to vary between 1.1 and 1.5 from nucleation stage till the completion of the growth process. After a period of 25 days, single crystals of DCBPZ with dimensions of 11 3 4 mm3 were harvested successfully. The photograph of as-grown DCBPZ single crystal is shown in Fig. 1.
Fig. 1. Photograph of as-grown DCBPZ crystal.
for every 25 reflection without significant variations. The crystallographic informations are summarized as follows. The results show that DCBPZ crystal belongs to orthorhombic system with non-centrosymmetric space group P212121. The calculated lattice parameters were found to be a = 6.6155 Å, b = 13.5574 Å, c = 16.2532 Å; a = b = c = 90° and volume = 1457.7 (Å)3. The single crystal XRD results obtained are found to be in good agreement with the already reported values [8]. UV–vis–NIR spectral analysis UV–vis–NIR absorption spectrum of the grown crystal of thickness 2 mm was recorded using the instrument Varian carry 5E spectrophotometer in the range of 200–1100 nm. From the UV– vis NIR spectrum as shown in Fig. 2, the UV cut-off wavelength is found to be 233 nm. The absence of absorption in the region between 233 and 1100 nm clearly indicates that the crystal DCBPZ has wide transparency. According to Tauc’s relation, the absorption coefficient (a) of a material is related to optical band gap (Eg) and photon energy (ht) as [9]
aht ¼ Aðht Eg Þn
where A is a constant for different transitions and n is an index which assumes the values 1/2, 3/2, 2 and 3 depending on the nature of electronic transitions responsible for absorption. The band gap energy value Eg can be estimated from the plot (aht)2 versus photon
Results and discussion Single crystal X-ray diffraction analysis XRD data were collected using a computer – controlled Enraf Nonius – CAD 4 single crystal X-ray diffractrometer. Single crystal of DCBPZ of suitable size 0.2 0.1 0.1 mm3 was selected for the X-ray diffraction analysis. The structure was refined by the matrix least square using SHELXL-97 (WINGX) program. During the course of data collection, one standard reflection was monitored
ð1Þ
Fig. 2. UV–vis–NIR absorption spectrum of DCBPZ.
D. Anbuselvi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 405–414
energy (Fig. 3) and the optical band gap energy value Eg has been estimated as 4.99 eV for DCBPZ. The higher value of optical band gap (4.99 eV) offers more optical conductivity to the medium without optical photons being absorbed in the specific range of wavelengths. This provides dielectric nature to the material. Only dielectric materials will have wide transparent range. The optical photons have energies ranging from 1 eV to 3 eV in the UV– vis–NIR region. This range of energy is not sufficient to excite the electrons for transition from valance band to conduction band in the dielectric material. Therefore, the incident optical photons in the range of UV–vis–NIR region are not absorbed by the material and the material is capable of transmitting the light of wavelengths in the range 233–1100 nm. The higher value of optical band gap thus provides sufficient transmitting power required for the NLO activity of the materials. FTIR spectrum analysis The FTIR spectrum of DCBPZ crystal recorded in the range 400– 4000 cm1 is shown in Fig. 4. The peak corresponding to 3155 cm1 is due to strong NH stretching vibration. The absorption peaks in the region of 2744–2457 cm1 are resulting from the superimposed OAH and NHþ 2 stretching bands. The peak at 1665 cm1 corresponds to the asymmetrical NHþ 2 stretching vibrations. The absorption peaks in the 1557–1461 cm1 regions are due 1 to symmetrical NHþ 2 stretching vibrations. The peak at 1415 cm is assigned to symmetrical COO ion group stretching. The other peaks at 1375 and 1330 cm1 are assigned to wagging of CHþ 2 group of the DCBPZ. The peak due to 1195 cm1 is assigned to twisting NHþ 2 [10]. The CAN stretching is indicated due to the prominent peaks at 1058 and 1035 cm1 respectively. The peaks corresponding to 939 and 848 cm1 are due to CHþ 2 rocking. The in-plane deformation of COO, COO scissoring and CCO rocking vibrations are observed at 780 cm1, 679 cm1 and 551 cm1 respectively. The presence of zwitterions (NHþ 2 and COO ) clearly indicates that an imbalance of charges is provided to the grown material to make the material non-centrosymmetric for initiating induced polarization when a powerful laser beam is incident on the material. The predicted functional groups are found to be in close agreement with the already available report [8]. Thermal analysis Thermogravimetric analysis (TGA), differential thermal analysis (DTA) and differential scanning calorimetry (DSC) of DCBPZ
Fig. 3. Tauc’s plot of DCBPZ crystal.
407
crystals were carried out simultaneously using SDT Q600 V8.0 Build 95 thermal analyser. A powder sample was used for these analyses in the temperature range of 30–1000 °C in nitrogen atmosphere at a heating rate of 20 °C/min. TGA and DTA curves of Dichlorobis (L-proline) zinc (II) are shown in Fig. 5. From the TGA curve, it is observed that the decomposition starts at 228.23 °C due to dissociation of the compound followed by the evaporation of volatile products. This is confirmed by the endothermic peak of DTA at 228.23 °C. In the DSC curve also (Fig. 6), there is also an exothermic peak at 229.96 °C to confirm the onset of decomposition. Hence, the melting point of the crystal is 229 °C which is in good agreement with the earlier reports [8,10]. The exothermic peak corresponding to 77.67 °C in the DTA curve indicates the loss trapped water molecules or dehydration of the compound. The endothermic peak at 131.87 °C following the exothermic peak at 77.67 °C in the DTA curve may be due to the evaporation of CO2, NO2 and NH3 gas molecules. The remaining exothermic peaks of DTA curve represent the various stages of decomposition of the compound. It is observed from the TG curve that the rapid decomposition starts at 255.30 °C which is confirmed by the exothermic peak of DTA curve at 255.34 °C. This is further confirmed by the exothermic peak at 255.34 °C in the DSC curve. This is therefore concluded that the material is thermally stable upto 229 °C which is suitable for the possible applications in lasers to withstand high temperature. SEM–EDAX analysis Fig. 7(a, b and c) shows the SEM images of the grown crystal surface with magnifications 260, 4000 and 31,000 respectively. With lower magnification (Fig. 7a) the surface appears foggy without showing the transparent and smooth surfaces. With higher magnification, Fig. 7b and c show the transparent nature of the grown crystal with smooth surface containing minimum density of defects. Fig. 7b and c indicates the particles of various sizes which predict the presence of various components in the grown crystal. Fig. 8 shows the energy dispersive X-ray analysis (EDAX) of Dichlorobis (L-proline) zinc (II). From EDAX spectrum, the presence of nitrogen, oxygen, zinc and chlorine is confirmed. Table 1 presents the percentage of elements present in the grown crystal. The incorporations of zinc and chlorine in L-proline are clearly understood from the EDAX spectral analysis. The incorporation of metal ions in the sublattices may increase the number of charge carriers for the material to be dipolar. Atomic force microscopy analysis Atomic force microscopy (AFM), a relatively new and rapidly advancing technique, proves to be a unique and excellent approach for observing crystal growth dynamics, imaging and measuring surface features as well as investigating growth mechanism. AFM images of DCBPZ crystals were recorded using a Digital Instrument Multimode Nanoscope IIIa operating in the tapping mode. Figs. 9– 12 show the AFM images of the DCBPZ crystals with scan sizes 5 lm, 10 lm, 15 lm and 25 lm respectively. The images were recorded on the (0 0 1) faces of DCBPZ crystals. Fig. 9(a) and (b) shows the surface features in two and three dimensions respectively with screw dislocations formed on (0 0 1) surface [9]. The black spots in Figs. 9(a) and 10 (a) indicate the points of dislocation. It is understood that the main sources of growth steps for most of the crystals growing at low to moderate supersaturation are growth spirals. The points of dislocations with screw components at crystal surfaces are important because they can provide a continuous source of steps which can propagate across the surface of the crystal. Fig. 10(a) and (b) presents the observation of growth hillocks in two and three dimensions respectively. Fig. 11(a) and (b) depicts
408
D. Anbuselvi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 405–414
Fig. 4. FTIR spectrum of DCBPZ.
Fig. 5. TGA and DTA curves of DCBPZ.
the elementary steps provided by the growth hillocks in two and three dimensions respectively. Fig. 12(a) and (b) shows the adsorption of molecules (white colour) into the kinks on the steps with 2D and 3D views. The spiral growth mechanism was formulated by Frank in 1949. According to Frank, the growth of crystals at low supersaturation could take place due to a screw dislocation emerging at a crystal face providing a continuous step source [11]. Burton, Cabrera and Frank were the first to develop a theory of spiral growth [12,13]. The growth spirals at temperature 0 K will have high concentration of kinks due to thermal roughening. The molecules that are absorbed on the surface diffuse into the kinks. The process continues for ever since the spiral would be renewed and the preferential sites would always be available for new growth. The idea introduced by Frank can explain most of the
features observed in crystal growth at low supersauration. BCF approximated the radius of the growth spiral for a spherical spiral having equispaced steps as
r ¼ 2q h
ð2Þ
where r and h are the polar coordinates of the growth spiral and q⁄ is the radius of the critical two dimensional nucleus. The radius of the two dimensional critical nucleus is given as
q ¼
vV kT ln s
ð3Þ
where m is the interfacial energy, V is the molecular volume, k is Boltzmann constant and T is the temperature (in Kelvin) and s is
D. Anbuselvi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 405–414
409
sources of growth spirals and steps, are provided due to thermal roughing. NMR spectral analysis
yo ¼ r 2 r 1
ð5Þ
The 1H NMR spectrum of DCBPZ is shown in Fig. 13. NMR spectrum was recorded by using Bruker AVANCE III 500 MHz NMR spectrometer and D2O as solvent. The signals corresponding to the CH2 (a) protons of the pyrrolidine ring appear as a triplet centred at d 1.90 ppm. The doublet proton signals appearing at d 2.3 ppm have been assigned to CH2 (b) protons of the same ring. The CH2 (c) protons appear as a multiplet centered at d 3.335 ppm in the aliphatic region of the spectrum. The signals due to CH2 (d) protons are shifted to higher d values as a consequence of the electron withdrawing carbonyl group in the vicinity and appear as a triplet at d 4.083 ppm. The sharp singlet around at d 4.75 ppm is due to the presence of undeuterated water in D2O. The signals due to NAH and COOH protons do not appear because of the exchange of fast deuterium in these two groups, with D2O being used as a solvent. The predicted structure of the compound from 1H NMR spectral data agrees very well with the already available report [8]. Fig. 14 shows 13C NMR spectrum of DCBPZ which contains 5 signals. The resonance signal at 174.69 ppm confirms the free carboxyl (C@O) group. The resonance signal at 61.11 ppm is due to the CH group of the pyrrolidine ring. The other peaks at 46.11 ppm, 28.9 ppm and 23.7 ppm are due to the aliphatic CH2 groups present in the pyrrolidine ring. Hence, NMR spectral analysis confirms the molecular structure of the grown compound.
yo ¼ 2r s ½ðh þ 2pÞ h ¼ 4pr s
ð6Þ
Microhardness
Fig. 6. DSC curve of DCBPZ.
the supersaturation. The value of m was estimated from the following equation [14],
t¼
kT ½0:173 ð0:248 lnðxm ÞÞ a2
ð4Þ
If yo is the distance between two success steps,
The calculated steps height is of the order 5.13 nm which lies closely in the range of 1.5–5.0 nm as reported by Liu et al. [15]. Numerous attempts have been made to understand the mechanism of macrosteps formation. From the images of growth sequence observed on the [0 0 1] face of DCBPZ crystals, the surface initially appears with dislocations followed by growth spirals. When the surface was scanned again, the growth spirals reappears in the form of steps to provide kinks for the adsorption of molecules from the supersaturated solution. It is worth noting that the growing surface is dominated by elementary steps. It is therefore concluded that the microcrystals formed initially should contain dislocations to continue the growth process for the transformation of microcrystals into large size macrocrystals. The dislocations, which are the
Microhardness testing is one of the best methods of understanding the mechanical properties of materials such as hardness number, crack length, fracture toughness, brittle index and elastic stiffness constant. Loads ranging from 25 to 100 g were used for making indentations, keeping the time of indentation constant at 10 s for all the cases. The diagonal lengths of the indentation mark and the crack length were measured, using a micrometer eyepiece. The Vickers hardness value was calculated as
Hv ¼
1:8544P d
2
kg=mm2
ð7Þ
where P is the applied load in kg, 1.8544 is a constant of geometric factor for diagonal pyramid and d is the diagonal length in mm. The
Fig. 7. SEM images of DCBPZ crystal.
410
D. Anbuselvi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 405–414
for the grown crystal yields crystal yields a straight line and its slop gives the work index n. The value of n is greater than 2 (2.39) and the microhardness should increase with increase of load as predicted by Onistch [16]. Since the material takes some time to revert to the elastic mode after every indentation, a correction x is applied to the d value and the Kick’s law is related as 2
P ¼ K 2 ðd þ xÞ
ð9Þ
From Eqs. (8) and (9) n=2
d
¼
12 12 K2 K2 dþ x K1 K1
ð10Þ
The slope of dn/2 versus d yields (K2/K1) and the intercept is a measure of x and is shown in Fig. 16. The fracture toughness (Kc) is given by
Kc ¼
B¼ Table 1 Compositional analysis of DCBPZ crystal from EDAX spectrum. Wt%
At%
C N O Cl Zn
39.54 07.47 03.79 22.72 26.48
64.45 10.44 04.63 12.55 07.93
variation of Hv with the applied load P is shown in Fig. 15 and it is found to increase with increase of load. According to Meyer’s relation,
ð8Þ
where K1 is the standard microhardness value which can be found out from the plot of P versus dn. A plot of ln P versus ln d (Fig. 16)
a
Hv Kc
ð12Þ
The yield strength (rv) of the material can be found out using the relation,
Element
n
ð11Þ
where C is the crack length measured from the centre of indentation mark to the crack tip, P is the applied load and geometrical constant b = 7 for Vickers indenter. The brittleness index (B) is given by
Fig. 8. EDAX spectrum of DCBPZ crystal.
P ¼ K1d
P bC 3=2
rv ¼
Hv 12:5ðn 2Þ f1 ð2 nÞg 1 ðn 2Þ 2:9
ð13Þ
The important microhardness parameters n, Hv, Kc, B and rv of DCBPZ crystal were calculated and compared with those of KDP crystal. The work index calculated is found to be in good agreement with the earlier report of Ramasamy et al. [8]. Since the microhardness of DCBPZ increases with load and its yield strength is found to be 410 MPa, which is comparable with that of KDP crystal, it is confirmed that DCBPZ crystal is having sufficient mechanical strength for device fabrications. Table 2 presents the microhardness parameters of DCBPZ and KDP [17] crystals for comparison (see Fig. 17).
b
Fig. 9. (a and b) 2D and 3D AFM images of DCBPZ crystal with scan size 5 lm showing screw dislocations observed on (0 0 1) face.
D. Anbuselvi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 405–414
a
411
b Points of dislocation
Fig. 10. (a and b) 2D and 3D AFM images of DCBPZ crystal with scan size 10 lm showing hillocks observed on (0 0 1) face.
Fig. 11. (a and b) 2D and 3D AFM images of DCBPZ crystal with scan size 15 lm showing the elemental steps observed on (0 0 1) face.
Fig. 12. (a and b) 2D and 3D AFM images of DCBPZ crystal with scan size 25 lm showing the adsorption of molecules (white colour) into the kinks observed on (0 0 1) face.
412
D. Anbuselvi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 405–414
Fig. 13. 1H NMR spectrum of DCBPZ crystal.
Fig. 14.
13
C NMR spectrum of DCBPPZ crystal.
Photoconductivity study The photoconductivity of DCBPZ was studied using Keithley 485 picoammeter. The applied electric field was changed from 5 V/cm to 100 V/cm without exposing the sample to any radiation and the corresponding dark current was recorded. The sample was then illuminated with a halogen lamp of 100 W power and the resulting photocurrent was measured by varying the applied field in the same range as used in the case of measurement of dark current.
Fig. 18 shows the plots of darkcurrent (Id) and photocurrent (Ip) against the applied field. From the plots it is observed that the photocurrent for DCBPZ is less than the dark current which is termed as negative photoconductivity. Stockmann model explains the tendency of decrease in mobile charge carriers during negative photoconductivity. According to this model, for the DCBPZ crystal, the negative photoconductivity is based on the state between the Fermi level and the valance band. This state contains more traps (Recombination centre) to capture electrons and holes. The
D. Anbuselvi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 405–414
413
Fig. 17. dn/2 versus d.
Fig. 15. Vickers microhardness number with applied load.
Fig. 18. Photoconductivity of DCBPZ crystal. Fig. 16. ln P versus ln d.
Table 2 Microhardness parameters of DCBPZ and KDP crystals. Parameters
For DCBPZ
For KDP
n Hv (kg/mm2) Kc (MN m3/2) B (m1/2) rv (MPa)
3.32 87.6 0.4546 1.89 103 410.0
1.71 153.86 0.6246 2.83 103 697.53
presence of recombination centres reduces the number of mobile charge carrier giving rise to negative photoconductivity [18]. However, using laser beam of wavelength 1064 nm and electric field 40 V/m the photocurrent was measured as 5.2 lA which is greater than the value obtained for the visible light. Hence, the material shows positive response to the intense laser beam. NLO test Second harmonic generation efficiency of the grown crystal DCBPZ was estimated using Kurtz and Perry powder technique with the help of Nd: YAG laser beam of wavelength 1064 nm [19]. The crystalline DCBPZ material was powdered to the particle size in the range 125–150 lm. The SHG was confirmed from the emission of green radiation of wavelength 532 nm and the optical
signal was detected by a photomultiplier tube. The result obtained show that the SHG efficiency of DCBPZ is found to be twice that of KDP which is found to be in close agreement with the report of Ramasamy et al. [10]. The second harmonic generation in the grown material is also due to positive response of the material to the laser beam. Conclusion Good optical quality of DCBPZ crystals were grown using isothermal slow evaporation method. The lattice parameters have been analyzed using single crystal X-ray diffraction technique. DCBPZ has a wide transparency window from 250 to 1000 nm, which is essentially required for the material to exhibit NLO behaviour. Optical properties were analyzed using UV–vis–NIR. The presence of functional groups of the grown crystal sample was confirmed by FT-IR analysis. TGA-DTA and DSC studies reveal that the crystal is thermally stable up to 263.20 °C. From the Vickers microhardness study, it is observed that Hv increases with increasing load and thus DCBPZ satisfies normal indentation size effect. Photoconductivity analysis shows the negative photoconductivity of the material. Microstructure and compositional analyses were discussed using SEM–EDAX studies. From atomic force microscopic study, the surface features and growth mechanism were analyzed. The observations of growth hillocks and steps due to 2D nucleation clearly confirm the BCF theory of growth mechanism. Even though
414
D. Anbuselvi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 127 (2014) 405–414
the presence of the defects decreases the quality of the crystals, their role is very important for the growth mechanism of the crystal. Hence, there should be a compromise between the purity and the size of the macrocrystal in order to obtain sufficiently large crystals with minimum dislocation density. The enhanced SHG efficiency was confirmed by Kurtz and Perry technique. Therefore, it is concluded that the grown material DCBPZ is an excellent semiorganic NLO material with enhanced SHG efficiency due to the incorporation of zinc metal ions in the sublattices and improved linear and nonlinear optical properties.
References [1] R. Mohankumar, D. Rajanbabu, D. Jayaraman, R. Jayavel, K. Kitamura, J. Cryst. Growth 275 (2005) e1935–e1939. [2] K. Meera, R. Muralidharan, R. Dhanasekaran, M. Prapun, P. Ramasamy, J. Cryst. Growth 263 (2004) 510–516. [3] P.N. Prasad, D.J. Williama, Introduction to Nonlinear Optical Effects in Organic Molecules and Polymers, Wiley, New York, 1991. [4] Soma Adhikari, Tanusree Kar, Bulk single crystal growth and characterisation of L-leucine – a nonlinear optical material, Mater. Chem. Phys. 133 (2012) 1055–1059.
[5] S.A. Martin Britto Dhas, G. Bhagavannrayana, S. Natarajan, J. Cryst. Growth 310 (2008) 3535. [6] M.R. Suresh kumar, H.J. Ravindra, J. Jayaraman, S.M. Dharmaprakash, Structural characteristics and second harmonic generation in L-threonine crystals, J. Cryst. Growth 286 (2006) 451–456. [7] D. Balasubramanian, P. Murugakoothan, R. Jayavel, Synthesis, growth and characterisation of organic nonlinear optical bis-glycine maleate (BGM) single crystals, J. Cryst. Growth 312 (2010) 1855–1859. [8] G. Anandha Babu, P. Ramasamy, Mater. Chem. Phys. 113 (2009) 727–733. [9] J. Tauc, in: J. Tauc (Ed.), Amorphous and Liquid Semiconductors, Plenum, New York, 1974. [10] M. LydiaCaroline, A. Kandasamy, R. Mohan, S. Vasudevan, J. Cryst. Growth 311 (2009) 1161–1165. [11] X.J. Liu, D. Xu, G.W. Yu, X.Q. Wang, L.Y. Zhu, G.H. Zhang, G. Yu, Appl. Surf. Sci. 253 (2007) 3674–3677. [12] F.C. Frank, Disc. Faraday Soc. 5 (1949) 48–54. [13] W.K. Burton, N. Cabrera, F.C. frank, Philos. Trans. Royal Soc. A 243 (1951) 299– 358. [14] W. Mott, Micro Indentation Hardness testing, Butter-worths, London, 1956. [15] X.J. Liu, P.J. Wang, Z. Zhang, D. Xu, G.H. Zhang, X.Q. Wang, Curr. Appl. Phys. 10 (2010) 715–717. [16] E.M. Onistch, Mikroscopia 2 (1947) 131. [17] R. Robert, C. Justin Raj, S. Krishnan, S. Jerome Das, Physica B 405 (2010) 20–24. [18] V.N. Joshi, in: Photoconductivity, Marcel Dekker, New York, 1990. [19] S.K. Kurtz, T.T. Perry, A powder technique for the evaluation of nonlinear optical materials, J. Appl. Phys. 39 (1968) 3798–3813.
