Growth of a Bulk Organic Single Crystal of Benzoylglycine by Unidirectional Crystal Growth Method S. Dinakaran,† Sunil Verma,‡ C. Justin Raj,† J. Mary Linet,† S. Krishnan,† and S. Jerome Das*,†
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 151–155
Department of Physics, Loyola College, Chennai-34, India and LMDDD, Raja Ramanna Centre for AdVanced Technology, Indore-13, India ReceiVed January 23, 2008; ReVised Manuscript ReceiVed September 11, 2008
ABSTRACT: A good optical quality and large size (75 mm) organic single crystal of benzoylglycine has been grown using the unidirectional crystal growth method and characterized for mechanical, dielectric, and photoconductivity properties. The microhardness measurements show a Vickers hardness value as 8.55, which is comparable to well-known organic crystal, urea. The work hardening coefficient, n, is found to be 1.77, satisfying Onitcsch’s concept and Kick’s law. Dielectric loss values were found to be small, signifying that the grown crystal is relatively defect free. The photoconductivity reveals the negative nature of the photocurrent in these crystals. Introduction Extensive investigations have been made on organic nonlinear optical (NLO) materials due to their high nonlinearity, high flexibility in terms of molecular structure, high optical damage threshold, low cost, and short response time to optical excitations.1,2 The microscopic origin of nonlinearity in these molecular NLO materials is due to the presence of delocalized π-electron systems, connecting donor and acceptor groups, responsible for enhancing their asymmetric polarizability.3 In the case of organic materials, significant efforts have been made to grow bulk crystals due to their potential applications in optoelectronics and nonlinear optical applications.4 The growth of bulk single crystals without defects is a challenging task for crystal growers. Also for SHG applications, phase-matchable crystals are needed where the specimen should have optimum size along the particular direction.5 From this point of view, the recently reported unidirectional method can be used to a grow bulk single crystal along a particular direction.6-8 The main advantages of this technique are (a) a simple experimental setup, (b) unidirectional growth, (c) a high percentage of solute-solid conversion, (d) minimum thermal stresses, and (e) prevention of microbial growth.9-11 Benzoylglycine (BG) is an organic NLO material which belongs to the orthorhombic system with noncentrosymmetric space group P212121 having a secondorder harmonic efficiency 1.5 times that of KDP crystals.12 In the present investigation we have grown a good optical quality and large size benzoylglycine single crystal by unidirectional solution growth method. In addition to XRD and transmission studies, the grown crystal was subjected to mechanical hardness, dielectric, SEM, and photoconductivity studies in detail. Experimental Section Material Synthesis. BG was synthesized by reacting glycine and benzoyl chloride in a molar ratio of 1:1. The calculated amount of glycine was dissolved in 10% sodium hydroxide to the appropriate amount of benzoyl chloride in the solution to get benzoylglycine as a white colored precipitate. The precipitate was collected, washed with cold water, and dried well. The product was recrystallized many times from boiling water. The chemical reaction is as follows: * To whom correspondence should be addressed. E-mail: sjeromedas2004@ yahoo.com,
[email protected]. Fax: 044-28175566. † Loyola College, Chennai-34. ‡ Raja Ramanna Centre for Advanced Technology.
Figure 1. Solubility of BG.
CH2NH2COOH + C6H5COCl 98 10 % NaOH
C6H5CONHCH2COOH + NaCl + H2O
(1)
Solubility Studies. Selecting a suitable solvent is a tiresome process as BG is insoluble in water. However, N,N-dimethylformamide (DMF), acetone, and acetic acid are found to be suitable solvents. The solution was kept in a constant-temperature bath maintained at 303 K (accuracy (0.01 K) and continuously stirred using a motorized magnetic stirrer to have a uniform temperature and concentration throughout the volume of the solution. After supersaturation was attained, the equilibrium concentration of the solute was analyzed gravimetrically.13 The same procedure was repeated for the temperatures 308, 313, 318, and 323 K. The solubility of BG with temperature is shown in Figure 1. From the solubility studies, DMF was found to be a better solvent for the growth of BG crystals. Seed crystals have been grown by the slow evaporation technique using DMF as a solvent at a constant temperature of 318 K. A selected seed of (001) plane was used to grow a bulk crystal. Experimental Setup and Crystal growth. The growth setup is shown in Figure 2. It consists of a heating coil, a thermometer, an inner container, a temperature controller, a growth vessel, and a water bath. A cylindrical glass tube of diameter 6 cm and height 40 cm was used in the inner container. This assembly is designed to provide a necessary temperature at the top using a ring heater. The entire setup was placed inside a constant-temperature bath (accuracy 0.01 K) for avoiding temperature fluctuations in the growth region. The seed was fitted at the bottom of the growth ampule and filled with the saturated solution of benzoylglycine. The ampule was placed along the axis of the growth assembly. Several growth experiments were performed to optimize the condition to get a flat interface during growth that would
10.1021/cg8000834 CCC: $40.75 2009 American Chemical Society Published on Web 12/17/2008
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Figure 2. Experimental setup of the unidirectional method. Figure 4. Cut and polished portions of grown crystal.
Figure 3. Photograph of grown crystals. Figure 5. Optical transmission spectrum of BG. allow spreading of atoms across the face of the layer with a constant thickness, and the height of the layer must be very small compared with the distance between the advancing steps.14 The temperature at the top and bottom of the ampule was maintained at 320 and 314 K, respectively. This gradient creates maximum concentration at the bottom of the ampule and minimum at top, thereby avoiding the possibility of a spurious nucleation along the length of the ampule. The excess solute generated by evaporation of the solution is driven down the ampule by the thermal gradient. Thus, the growth commences upon the seed fixed at the bottom of the ampule with the desired orientation (001). The growth rate of the crystal was found to be 2 mm/day. The crystal of 75 mm length has been grown successfully within a period of 37 days. The photograph of the grown crystal is shown in Figures 3 and 4. Recently, Kishore Kumar et al.15 has grown this crystal using a conventional method and achieved a growth rate of 0.5 mm/day (crystal size of 17 × 8 × 10 mm3 in 30 days). Therefore, it is reported that the unidirectional method adopted by us is useful for growing a BG crystal with a high growth rate and good quality.
Results and Discussion Single Crystal X-ray Analysis. The single crystal data of the grown crystal were obtained using an Enraf-Nonius CAD 4 X-ray diffractometer. The unit cell parameters are determined using the method of short vectors. The crystallographic data reveal that the grown crystal belongs to the orthorhombic system with space group P212121 and lattice parameters a ) 8.8514 Å, b ) 9.0842 Å, and c ) 10.5807 Å. Optical Transmission Spectral Analysis. The optical transmission spectrum gives valuable information about the structure of the molecule because the absorption of UV and visible light involves promotion of electrons in σ and π orbitals from the
ground state to a higher energy state. The transmission spectrum is important from the device point of view, as the grown crystal can be used only in the highly transparent region. The transmission spectrum (Figure 5) is recorded in the wavelength range between 200 and 1200 nm using a Varian Cary 5E UV-Vis-NIR spectrometer. The crystal is highly transparent in the entire visible and near-IR region, whereas it has a UV cutoff below 300 nm, which shows that the crystal is transparent in the blue region. The transmission is uniformly high (60%) for light in the visible region of the electromagnetic spectrum, which is useful for device application. Scanning Electron Microscopy. The surface morphology and dislocation on the surface of the grown crystal along (001) was magnified using a JSM 840-A scanning electron microscope. The transparent growth plane of the BG crystal was coated with gold to discharge the charge of particles and scanned at two different magnifications. The resultant images of magnifications 25× and 1500× are shown in Figures 6 and 7. From the image of 25× magnification, it is observed that the surface consists of regular trapezoidal16 morphology habits with a maximum size of 2.4 × 1.2 mm in length and width. Figure 7 shows that, at higher magnification, the trapezoidal morphology shows a perfect growth surface with some microcrystallites on the surface. Dielectric Studies. A study of dielectric response in crystals gives information about the electric field distribution and charge transport mechanism. Hence, the grown crystal was subjected to dielectric studies using a Hioki 3532-50 LCR HITESTER. Transparent polished crystals of rectangular dimensions 1 × 1
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Crystal Growth & Design, Vol. 9, No. 1, 2009 153
Figure 6. SEM image of BG in 25× magnification.
Figure 8. Variation of the dielectric constant as a function of the frequency.
Figure 7. SEM image of BG in 1500× magnification.
cm2 was selected for dielectric studies. Electronic grade silver paste was applied on the surface of the sample to make firm electrical contact. The experiment was carried out for different frequencies starting from 50 Hz to 5 MHz. The dielectric constant was calculated using the relation
ε)
Cd ε0A
(2)
where C is the capacitance, d is the thickness of the crystal, ε0 is the permittivity of free space, and A is the area of the crystal. Figures 8 and 9 show the variation of the dielectric constant and dielectric loss at different frequencies. From the curve, it was observed that the dielectric constant and dielectric loss decrease slowly with increasing frequency and attain saturation at higher frequencies. The low dielectric constant value of the crystal at high frequency is attributed to space charge polarization.17 The low dielectric loss was consistent with a nearly constant level of dielectric constant over the wide frequency range. The low values of dielectric loss indicate that the grown crystal contains minimum defects.18 Comparing our dielectric results with those reported recently by Kishore Kumar et al.,15 it is found that our dielectric values between 3.5 and 6.5 (log f) are significantly higher. Photoconductivity Studies. The photoconductivity studies of grown crystals were carried out by connecting the sample in series with a dc powder supply and a picoammeter (Keithley 480) at room temperature. The setup is similar to that in the
Figure 9. Variation of dielectric loss as a function of the frequency.
work of Ledorux.19 The applied field was increased from 100 to 1800 V/cm, and the corresponding dark current and photocurrent were recorded. Figure 10 shows the dependence of the dark current and photocurrent with respect to the applied field at room temperature. The dark current and photocurrent increase linearly with respect to the applied field. At every instant, the dark current is greater than the photocurrent, which is called negative photoconductivity. This may be attributed due to decrease in either the number of free charge carriers or their lifetime when subjected to radiation. According to the Stockmann model, the forbidden gap in the material contains two energy levels in which one is situated between the Fermi level and the conduction band while the other is located close to the valence band.20 The second state has high capture cross sections for electrons and holes. As it captures electrons from the conduction band and holes from the valence band, the number of charge carriers in the conduction bands gets reduced and the current decreases in the presence of radiation. Comparing the photoconductivity results with those of Kishore Kumar et al.,15 it is observed that while our measurements reveal a negative photoconductivity, they report a positive photoconductivity.
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Figure 12. log P vs log d.
Figure 10. Dark current and photocurrent as a function of the applied field.
Figure 13. Load P vs dn.
which shows the BG crystal has the reverse indentation type and it satisfies the Onistch concept. According to Meyer’s relation Figure 11. Load P vs hardness Hv.
P ) K1dn
Mechanical Properties. The hardness of a material is influenced by various parameters such as the lattice energy, Debye temperature, heat of formation, and interatomic spacing. Microhardness measurement is a general microprobe technique for assessing the bond strength, apart from being a measure of bulk strength. The crystal slices are well polished with a thickness variation of less than 10 µm to avoid the surface defects which influence the hardness value strongly. Microhardness studies were carried out along the growth plane (001) at room temperature using a Shimadzu HMV-2000 fitted with a Vickers pyramidal indentor. The load P is varied between 10 and 50 g, and the time of indentation is kept constant at 15 s for all trials. The BG crystal can withstand a maximum load of 50 g, after which cracks developed around the indentation mark. The diagonal lengths of the indentation are measured. The hardness of the material (Hv) is determined by the relation21
Hv ) 1.8544
P (kg/mm2) d2
(3)
where P is the applied load (kg) and d is the diagonal length of the indentation impression (mm). The variation of Hv with the applied load P is shown in Figure 11, and a plot of log P vs log d for the grown crystal is shown in Figure 12. The plot between log P and log d yields a straight line graph, and its slope gives the work hardening index n, which is found to be 1.77. According to the Onistch concept, if n < 2, Hv decreases with increasing load (reverse indentation type), whereas for n > 2, Hv increases with increasing load.2 In our case n < 2,
(4)
where K1 is the standard hardness value which can be obtained from the plot of P versus dn, which is found to be 8.55. It is equal to that of urea, a well-known organic crystal (Figure 13). 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 Kick’s law is related as
P ) K2(d + x)2 From eqs 4 and 5
dn/2 )
(5)
() () K2 K1
1/2
d+
K2 x K1
(6)
The slope of dn/2 versus d yields (K2/K1)1/2, and x is measured from the intercept as shown in Figure 14. According to Kick’s law, x is positive when n < 2 and negative for n > 2.22 The calculated x value is 4.7 × 10-3 (positive value), which satisfies Kick’s law. From the hardness value, the yield strength (σv) of the material can be found using the relation
σv )
Hv 12.5(2 - n) [1 - (2 - n)] 2.9 1 - (2 - n)
[
2-n
]
(7)
The load-dependent hardness parameters n, K1, and K2 and yield strength σv are calculated for the grown crystal and are given in Table 1. Conclusions A good optical quality and large size (75 mm length) single crystal of benzoylglycine has been successfully grown with a
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Crystal Growth & Design, Vol. 9, No. 1, 2009 155
authoritites of SAIF, Indian Institute of Technology, Chennai for the suggestions.
References
Figure 14. d vs dn/2. Table 1. Hardness Parameters of the BG Crystals parameter
value
n K1 (kg/mm) K2 (kg/mm) x (µm) σv (MPa)
1.77 8.55 14.28 4.7 122.08
growth rate of 2 mm/day by unidirectional solution growth method. Single crystal X-ray diffraction study confirms that the crystal belongs to the orthorhombic system with space group P212121. The Vicker microhardness measurement reveals a hardness value of 8.5, which is comparable to urea crystal. The work hardening coefficient, n, is found to be
